摘要:

J. MATER. CHEM., 1991 1(3), 357-359 Characterization of Epitaxially Grown ZnS :Mn Films on a GaAs(lO0) Substrate prepared by the Hot-wall Epitaxy Technique Takato Nakamura,*a Hitoshi Muramatsu,a Yoji Takeuchi,b Hiroshi Fujiyasub and Yoichiro Nakanishi" a Department of Applied Chemistry and Material Technology, Faculty of Engineering, Shizuoka University, Hamamatsu 432, Japan Department of Electronics, Faculty of Engineering, Shizuoka University, Hamamatsu 432, Japan Research Institute of Electronics, Shizuoka University, Hamamatsu 432, Japan Epitaxially grown ZnS: Mn films on a GaAs(l00) substrate prepared by a hot-wall epitaxy technique have been studied using electron paramagnetic resonance (EPR) spectroscopy. The lowest-energy transition assigned to M,= -512 splits into either a triplet or a quintet depending on the thickness of the film when the magnetic field is applied normal to it.Computer simulation confirmed that the EPR parameters obtained correspond to those for a single crystal of cubic structure, although the apparent lineshape is different. There is no significant difference between the hyperfine coupling constants and splitting parameters. The apparent difference in the EPR lineshape is ascribed to the change in linewidths of the fine structure involved. In particular, linewidths for the transitions Ms= +_5/2-+_3/2 decrease markedly with increasing thickness of the ZnS: Mn film on the GaAs(100) substrate, confirming that uniaxial deformation of the crystal field surrounding manganese@) is derived from the lattice mismatch between them.Keywords: Thin film; Hot-wall epitaxy; Electron paramagnetic resonance spectroscopy Electron paramagnetic resonance (EPR) spectroscopy has been applied to the study of paramagnetic ions in crystals because the resonance observed reflects the surroundings of the ions. Manganese@) ions doped in single crystals and powders of the cubic and hexagonal structures of zinc sulphide have already been examined in order to elucidate the sym- metry and strength of the crystal field surrounding the manga- nese@) in the host.'-4 Recently, Kreissl et ~l.'-~have not only re-investigated the EPR spectra of ZnS :Mn powders with different crystal structures, but also examined those of thin films deposited on glass substrates, prepared by evapor- ation and chemical vapour deposition.This is of interest since thin films of ZnS :Mn (M =metal ion) are electroluminescent materials. According to Mitsui et ~l.,~there has been lattice distortion in ZnS films on GaAs and GaP substrates due to the lattice mismatch between the epilayer and the substrate. In ZnS :Mn films on a GaAs substrate this should cause distortion of the crystal field surrounding the manganese@), so that anomalous EPR resonance should be observed. Therefore, we consider EPR spectra of the ZnS :Mn films with a variety of thicknesses grown on a GaAs(l00) substrate prepared by the hot-wall epitaxy (HWE) technique. Experimental Thin films of ZnS :Mn were grown on a GaAs( 100) substrate under a background pressure of lov6 Torrt by means of the HWE technique.Precise control of the temperatures of the ZnS and Mn-metal sources, the GaAs substrate and the wall was required. In particular, the temperature of the Mn-metal source, which determined the amount of manganese@) doped in the ZnS host, was of importance because too much doping causes a broad signal due to spin exchange, which gives little information about the surroundings. Preliminary experiments showed that the optimum temperature for the manganese metal was 873 K. The ZnS source was evaporated at 973- 993 K, while the substrate was kept at 523 K during the ~ ~~ t 1 Torr ~133.322Pa. deposition. The deposition rate of the film was in the range 1.3-2.7 A s-I.EPR spectra were measured at 10 and 293 K using JEOL JES-RE3X and JES-FE- 1 XG spectrometers, respectively, the former being equipped with a variable-temperature apparatus, ES-LTRSX. Films on the GaAs substrate were attached to a quartz rod with glue, and the rod was set in a cylindrical cavity operating in the TE,,, mode. Manganese doped in MgO and an ECHO NMR field meter were used for the calibration of the magnetic field. Simulation of the EPR spectra was carried out with the aid of an NEC PC-9801 personal computer with Turbo-Pascal 5.0 language. Results and Discussion For manganese@) in single-crystal cubic ZnS the resonance fields of respective hyperfine lines rotated about the [Oll] axis have been worked out theoretically as' H(Ms,MI)=H,-AM1-A2/2H,[35/4 -MI2+MI(2Ms-I)] -(a/64)(35cos46-30cos26+3-5sin46) x (56Ms3-84Ms2-134Ms+81) (1) in which H,=hv/gpB (his Planck's constant, v is the frequency, g is the g-value and pB the Bohr magneton), Ms and MI are the electron and nuclear spin quantum numbers, respectively, A is the hyperfine coupling constant, a is the splitting param- eter and 6 is the angle between the [loo] axis and the applied magnetic field.Fig. 1 shows the EPR spectrum of a film of ZnS :Mn with thickness 0.98 lm on a GaAs(l00) substrate at 6=0, in which the magnetic field is applied normal to the film. Also shown is a calculated spectrum; the EPR parameters are those for a single crystal of cubic ZnS :Mn obtained by Matarrese et al.,' the linewidth is 0.5 mT, and the lineshape is described in terms of a Lorentz function, since the fine-structure lines observed in this study fit a Lorentzian better than a Gaussian.It is immediately noticed that for the observed spectrum the hyperfine lines split into a poorly resolved quintet. Even in I111l lllll Ill I1 I I1 I 1 I) I I1 I,IrI,I,I,I 310 320 330 340 350 360 HImT Fig. 1 Comparison of the EPR spectrum of a ZnS :Mn film, thickness 0.98 lm, on a GaAs(l00) substrate (top) with a calculated spectrum using the EPR parameters of A=-64.1 ~10-~cm-' and a= 1.30 x cm-' for a single crystal with cubic structure (bottom), in which 0 is 0" the EPR spectrum measured at 10K they do not give well resolved fine structure because of the g-strain.It has been reported that there is lattice distortion in the ZnS films grown on GaAs and GaP substrates by metal- organic chemical vapour deposition because of the lattice mismatch between the epilayer and substrate, and that the misfit strain is mostly eliminated by 1 pm.8 If this is true, the surroundings of the manganese@) atoms in the ZnS :Mn films will depend on the thickness of the film. Therefore ZnS layers of 1 pm without manganese were grown prior to the deposition J. MATER. CHEM., 1991, VOL. 1 of the ZnS: Mn, and the EPR spectrum was compared with those of the ZnS:Mn films deposited directly on the GaAs substrate. The EPR spectrum of the ZnS :Mn/ZnS film contains fine structure and a quintet at the resonance assigned to the lowest-energy transition MI= -5/2 was observed, although the resolution corresponding to the transition Ms= -90 -80 70 -60 -so --40 -30 20 --10 n--3 -2 -1 0 1 2 relative HImT Fig.3 Angular dependence of the resonance field of fine-structure lines of the lowest-energy transition MI= -5/2 for a ZnS:Mn film with thickness 1.4 pm on a GaAs(100) substrate (open circle); A, B, C, D and E denote the calculated angular dependence of the resonance field of the transitions M, = +1/2* +3/2, -5/2*-3/2, -1/2* +1/2,+3/2* + 5/2 and -3/2--1/2, respectively, based on eqn. (l),using the EPR parameters of a single cubic crystal 100 mT-300 310 320 330 340 350 360 370 Fig. 2 EPR spectra of a ZnS :Mn film deposited on a ZnS/GaAs( 100) HImT substrate (top) and deposited directly on a GaAs(100) substrate (bottom).In the former the thicknesses of the ZnS:Mn and ZnS Fig. 4 EPR spectral simulation of ZnS :Mn films with thicknesses of layers are 0.6 and 1.0 pm, respectively, and in the latter that of the 0.7 (A, B) and 1.4 pm (C, D) on a GaAs(100) substrate, in which A, ZnS :Mn film is 1.4 pm and C are observed spectra and B and D are calculated J. MATER. CHEM., 1991, VOL. 1 11;, field strength, elucidated theoretically by Watanabe,g it is presumed that the crystal-field strength itself, surrounding manganese@) in the ZnS: Mn film, does not change signifi- cantly with increasing thickness. It is found that some of the linewidths of the fine structure vary with the thickness of the film.In Fig. 5, the linewidths for the transitions Ms= f5/20 & 3/2, f3/20 f1/2 and +1/2--1 /2 are plotted against the thickness of the ZnS :Mn film. A marked decrease in the linewidth as the film thickness increases up to 1.5 pm is observed for the transition between Ms = & 5/2 and & 3/2. Similar phenomena have been observed in the study of the EPR spectra of single-crystal cubic MgO :Mn under external uniaxial stress." The distortion lifts the degeneracy of the spin manifold. This splitting is not resolved, and manifests OO*itself as a linewidth increase in the anisotropic region relative to the isotropic one. The above are consistent with the phenomena for manga- nese@) under uniaxial stress." It is therefore concluded that the anomalous resonance showing three-line fine structure is due to the distortion of the crystal field surrounding manga- Q,5 1,o 1,5 nese@) in the ZnS film near the GaAs substrate. The distortion thickness of film/pm Fig.5 Variation of the linewidths of fine structure with the thickness of the ZnS: Mn film on a GaAs(100) substrate; 0, A and 0 denote the transitions M, = k5/2w f3/2, f3/2w f1/2 and +1/2w-1/2, respectively f512-& 3/2 is poor (Fig. 2). Similar fine structure appears in the EPR spectra of the ZnS: Mn films with a thickness of >1 pm (Fig. 2). Also, Fig. 3 shows that the angular dependence follows eqn. (1). Introducing the linewidth parameters for the transitions M, = f5/2-f3/2, f3/20 f1/2 and +1/20-1/2, calculated spectra were fitted with observed spectra (Fig.4). Obtained values of the hyperfine coupling constant and splitting param- eters are -63.8f0.2+10-4 and 1.30fO.l +lop4 cm-', respectively, independent of the thickness of film. A compari-son of the obtained parameters with those of the single cubic crystal determined by Matarrese et al.' indicates that there is no significant difference between them. Taking account of the parabolic dependence of the splitting parameter on crystal- is almost relaxed at a film thickness of 1.5 pm. We thank Mr. K. Fujii and Mr. K. Mituda, R&D Department, JEOL Ltd., for their help in the EPR measurement at 10 K. References 1 L. M. Matarrese and C. Kikuchi, J. Phys. Chem. Solids, 1956, 1, 117. 2 S. P. Keller, I. L. Gelles and W. V. Smith, Phys. Rev., 1958, 110, 850. 3 H. D. Hershberger and H. N. Leifer, Phys. Rev., 1952, 88, 714. 4 T. Buch, B. Cherjaud, B. Lambert and P. Kovacs, Phys. Rev. B, 1973, 7, 184. 5 J. Kreissl and W. Gehlhoff, Phys. Status Solidi A, 1984, 81, 701. 6 J. Kreissl and D. Backs, Phys. Status Solidi A, 1987, 99,K117. 7 J. Kreissl, Phys. Status Solidi A, 1986, 83, 191. 8 I. Mitsui, H. Mitsuhashi and H. Kukimoto, Jpn. J. Appl. Phys., 1988, 27, L15. 9 H. Watanabe, Prog. Theor. Phys., 1957, 18,405. 10 E. R. Feher, Phys. Rev. A, 1964, 136,145. Paper 0/04860G; Received 29th October, 1990

ISSN:0959-9428    

DOI:10.1039/JM9910100357

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(3), 361-364 361 Evolution of Structural Changes during Flash Calcination of Kaolinite A 29Siand *'AI Nuclear Magnetic Resonance Spectroscopy Study Robert C. T. Slade"" and Thomas W. Davied a Department of Chemistry, Universiiy of Exeter, Exeter EX4 4QD, UK Department of Chemical Engineering, University ofExeter, Exeter EX4 4QF; UK Kinetically frozen samples of flash calcined kaolinite (rapidly heated to 1000 "C,maintained at that temperature for a variable residence time and then rapidly cooled) have been produced in a laboratory calciner. Time resolution of the structural changes occurring has been achieved by following 27AI (78.15 MHz) and %i (59.58 MHz) magic angle spinning nuclear magnetic resonance (MAS NMR) spectra as a function of residence time.29Si spectra can be deconvoluted into a minimum of four Gaussian components with shifts ranging from mullite-like (-90 ppm) to Q4 (-110 ppm) Si environments. 27AI spectra show peaks for four-co-ordinate and six- co-ordinate Al. The derived picture of flash calcination is progressive transformation of kaolinite to a single product which undergoes little further chemical reaction during its short time in the calciner. Keywords: Flash calcine; Kaolinite; Magic angle spinning nuclear magnetic resonance spectroscopy; Deh ydroxyla tion Kaolinite [china clay, A12Si205(OH),, sometimes written as A1203.2Si02 *2H20] is a raw material of considerable indus- trial significance on an international scale.' The kaolinite structure is built by stacking lamellae composed of a pair of silica and alumina sheets.The silica sheets contain vertex- shared SiO, tetrahedra, while the alumina sheets contain edge-shared A106 octahedra. Kaolinite calcination (dehydroxylation) to form the ther- mally stable compound metakaolin (A12Si207, A1203*2Si02) is an important step in the manufacture of clay products. Reorganisation of the structure during dehydroxylation is dominated by forces within the alumina sheets. Three of the four hydroxyl groups associated with the alumina sheets lie in the interlamellar space between successive sheet pairs, while the fourth is intralamellar (between the silica and alumina sheets). Dehydroxylation is a rate process and calcines can be kinetically frozen at various stages of structural reorganis- ation.In the absence of structural collapse, an idealised kaolinite would lose 13.95% of its mass on complete dehy- droxylation and its density would drop from 2.64 to 2.27 g cm-3.2 Two very different methods for dehydroxylation are used, soak calcination and flash calcination. Soak Calcination In industrial soak calcination the dehydroxylation is achieved by holding the clay at a sufficiently high temperature (600< T/ "C<1000) for a sufficient length of time (ca.1 h) in an oil- or gas-fired furnace. The rate at which the clay is brought to the calcination temperature is low and is not used as a process variable; the resulting metakaolin has a density of ca. 2.74 g cm-3.3 The properties of metakaolin formed by soak calcination have been the subject of extensive studies (see e.g.ref. 4) and are a useful benchmark against which the properties of flash calcines (see below) can be compared. Overheating kaolinite results in the formation of mullite (A16Si2013)and cristobalite (SO2), generally considered unde- sirable (as the abrasiveness of the calcine is increased). Much effort has been devoted to studies of the physical and chemical changes associated with this reaction e.g. ref. 5-9. The use of analytical techniques such as MAS NMR has revealed more detail about structural changes accompanying the dehydroxyl- ation step e.g. ref. 10. Flash Calcination If kaolinite particles are heated at such a speed that the steam released within them is generated faster than it can escape by diffusion, structural disruption is likely.Such structural dis- ruption may endow the resulting calcine with desirable or interesting properties, such as internal voids. ''The diameters of such voids are comparable to the wavelengths for visible light, therefore producing light scattering and imparting opac- ity to the material (then usable as an effective paper covering). Construction at Exeter of a furnace to allow kaolinite particles to be subjected to thermal histories comparable to those in industrial flash calciners has been described elsewhere." In industrial flash calcination, cold powdered clay is passed through a gas or oil flame and then quenched by injection of cold air.The laboratory simulation of this process involves plunging a stream of clay particles into a co-flowing stream of hot He(g) (which is in downward laminar flow) in a vertical electrically heated reaction tube. Flash calcines produced in this way have quite different properties from corresponding soak calcine^."^'^-'^ We have previously demonstrated the utility of MAS NMR techniques in probing the structural consequences of introduc- ing water vapour into the calciner atmosphere (variation of H20 content of the He carrier gas) in production of flash calcines of similar densities.16 We now report the use of NMR absorption spectra to gain insight into the evolution of structural changes during the flash calcination of kaolinite, with time resolution being achieved via variation of the residence time at the reaction temperature.Experimental The kaolinite feedstock was commercial grade SPS clay (English China Clays, St. Austell, Cornwall). 90% of the powder was <2 pm particle size. XRF analysis gave Si02 46.2%, Al2O3 38.7%, Fe203 0.56%, Ti02 0.09%,CaO 0.20%, MgO 0.20%, K20 1.01%, Na20 0.07% and the loss on ignition ('H20 content') was 13.14%. Partially dehydroxylated flash calcines were prepared using the laminar-flow furnace described previously. ' This allows powdered clay to be heated rapidly (in a few ms) from room temperature to a controlled temperature (ca. 1000 "C) and then cooled back to room temperature by quenching with cold N2(g). The residence time at the reaction temperature is controllable and was the only process variable explored in this study.Residence times (at 1000 "C in dry He) were varied in the range 0.2-0.8 s. Fig. 1 presents the results of density determinations (p, by water displacement according to BS 190-304) and degree of dehydroxylation (a, by subsequent thermogravimetric dehy- droxylation to completion on a Stanton STA-780 instrument) for the calcines produced. X-ray powder diffraction patterns (Ni-filtered Cu-Ka radi-ation) were recorded using a computer-controlled Philips PW 1050 goniometer incorporating accumulation of multiple scans. In calcines with residence times up to 0.3 s little change from the kaolinite pattern was seen. At longer residence times, scattering from 'amorphous' (transformed) material became dominant, with progressive loss of intensity from a residual kaolinite component.At no stage was any other phase (e.g. mullite or cristobalite) observed in the diffraction pattern. Proton-decoupled high-resolution MAS NMR spectra for 27Al (78.15 MHz) and 29Si (59.58 MHz) were recorded at ambient temperature using a Varian VXR300 spectrometer. For 29Si spin rates of ca. 5 kHz and n/2 radiofrequency pulses were used. For 27Al a high-spin-rate (Doty) probe was used with spin rates of 9-10 kHz and employing 46 radiofrequency pulses. A small signal due to A1 in the probe was subtracted from the recorded 27Al spectra. Relaxation delays were varied to ensure the absence of saturation effects from reported 2.6 6 I 1 I I I I \ 2.55- \ 2.5- \ 5 2.45-2.4-(1 I a Q \ \ \Q \ \ t 2.35- \ / t 2.3/ 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 residence time/s 80 o/ / czj 404 /I / I '0 204 4' Fig.1 Effect of particle residence time on the density (p) and degree of dehydroxylation (a) of the resulting calcine J. MATER. CHEM., 1991, VOL. 1 spectra. In the case of 29Si spectra necessary relaxation delays depended on the calcine (e.g. 60s for kaolinite and short residence-time calcines, 2.0 s for calcines at the longest resi- dence times), while delays of 1.0 s were employed in obtaining 27Al spectra. Spectra are referenced to tetramethylsilane (2gSi) and Al(H20)z+(aq) (27Al). Results Fig.2 shows the 29Si spectrum as a function of residence time. At residence times longer than 0.3 s the kaolinite contribution (line at ca. 91.5 ppm) decreases progressively, indicating (in accord with the X-ray studies above) progressively less unchanged material, and the (broad) absorption due to trans- formed material appears. As with soak calcines," the latter absorption extends from mullite-like (ca. -85 ppm) through to Q" (ca. -1 10 ppm) Si chemical shifts. With increasing residence time the maximum in the broad absorption appears to move progressively upfield, but this is consequential on the observation of superimposed spectra from kaolinite and transformed material (see below). Fig. 3 shows the 27Al spectrum as a function of residence time.No significant or systematic variations in shifts with residence time were seen. Peaks assignable to octahedral A1 were observed at -3.1 k0.3 ppm, with those assignable to tetrahedral A1 at 55.5 & 0.4 ppm. At short residence times a dominant peak assignable to octahedral A1 is observed. At residence times longer than 0.3 s a peak assignable to tetra- hedral A1 and a broad underlying signal (-50-75 ppm) arising from A1 in distorted environments appear, both increasing in intensity with increasing residence time. Discussion Relationship to Spectra of Soak Calcines A number of high-resolution NMR studies of soak calcines have been reported. "7 17-22 II -80 -100 -120 c, -80 -100 -120 -80 -100 -120 6 (PPm) 6 (PPm) Fig.2 29Si MAS NMR spectra of flash-calcined kaolinite as a function of residence time 7 at lo00 "C in He z/s: (a)0.2, (b) 0.3, (c) 0.4, (d) 0.5, (e)0.6, (f)0.7, (g)0.8 J. MATER. CHEM., 1991, VOL. 1 ------L 100 ' 76 100 A -1 00 ----qA%100 100 0 -100 c_?v/ 8 (PPm) 100 0 -100 6 (PPm) Fig. 3 "A1 MAS NMR spectra of flash-calcined kaolinite as a function of residence time z at 1000 "C in He z/s: (a) 0.2, (b) 0.3, (c) 0.4, (d) 0.5, (e) 0.6, (f) 0.7, (g) 0.8 29Si spectra of soak calcines are broad (as in this study of flash calcines) with features extending from Si shifts character- istic of mullite-like environments through to Q" Si (superscript denotes number of Si-0-Si bridges per tetrahedral Si, Si in kaolinite itself is Q3). Spectra are consistent with the presence of a range of environments Q", each of which includes sites with geometries grossly distorted from those in parent kaolinite.Lambert et ~1.~'deconvoluted spectra into the minimum number of overlapping Gaussians, this giving suggestive information concerning abundances of different Q" types as a function of (soak) calcination conditions. Higher numbers of Gaussians have been mooted,21.22 but the choice then becomes somewhat arbitrary. Discussions of 27Al (nuclear spin I=3) spectra commonly neglect field-dependent second-order quadrupole shifts (aqs)and line-broadenings (the true field-independent chemical shift ~cs=~c~I&where aCG is the centre of gravity of the observed lineshape22).Consideration of these effects would be problematic in these systems. 27Al spectra of soak calcines show peaks at ca. 0 ppm (octahedral Al), at ca. 55 ppm (tetra- hedral Al) and at ca. 30 ppm. The peak at 30 ppm is now believed to arise from pentaco-ordinated Al, this assignment being unambiguous in the case of andal~site~~ and also being made in the case of a variety of thermally/hydrothermally treated alumino~ilicates.~~ No spectral feature unambiguously assignable to pentaco-ordinated A1 was observed in this study (of flash calcines). The picture of soak calcination that has emerged from such studies is as follows: (1) transformation of kaolinite to meta- kaolin is accompanied by disorder and distortions in Si environments; (2) treatment to higher temperatures leads to segregation of regions of 4" Si, mullite and a spinel phase (Sanz et a1.,I9 Lambert et aL2' and Rocha and Klinowski" are contradictory as to whether this could be y-A1203).29SiSpectra of Flash Calcines The broad absorption characteristic of transformed material is indicative (as in the case of soak calcines) of a range of (~3,)~ Gaussian components characterised by their chemical shifts distorted Si environments of differing Q". The spectra in Fig. 2, which contain the broad absorption (residence time >0.3 s) can each be deconvoluted into a minimum of four Gaussian components, the results of this deconvolution being given in Table 1. Fig. 4 shows the resulting fit and contributory Gaussians for the calcine with residence time 0.4s.The Gaussians are assignable (in order of increasingly negative 6,) to mullite-like, residual kaolinite-like (in untransformed material and possibly within the product particles also), metakaolin and Q" Si environments. In accord with this assignment (i) the kaolinite peak is by far the narrowest and its intensity decreases with increasing residence time (as would be predicted from the X-ray studies above) and (ii) the relative intensities of the mullite-type and Q" peaks are approximately in the ratio 1 :2 (which would be that observed in the event of phase segregation). Table 1 Deconvolution of 29Si spectra for flash calcines into four widths (FWHM) and intensities residence time/s 6, (ppm) FWHM/Hz intensity (YO) Gaussian 1 (mullite-like) 0.4 -90.9 825 18 0.5 -91.0 754 20 0.6 -90.7 762 13 0.7 -88.9 852 18 0.8 -88.8 798 15 0.4 Gaussian 2 (kaolinite) -93.1 122 11 0.5 -92.5 121 9 0.6 -93.4 142 6 0.7 -92.6 185 5 0.8 -92.6 202 3 0.4 Gaussian 3 (metakaolin) -98.6 768 34 0.5 -99.1 806 36 0.6 -99.2 768 40 0.7 -100.2 794 45 0.8 -100.2 823 43 0.4 Gaussian 4 (Q4, cristobalite-like) -109.5 909 36 0.5 -109.6 856 35 0.6 -110.1 830 41 0.7 -109.6 765 33 0.8 -1 10.0 862 38 -40.2 6 (PPm) -166.2 Fig.4 Deconvolution of the 29Si MAS NMR spectrum of flash- calcined kaolinite with a residence time r=0.5 s (at lo00 "Cin He) into four contributing Gaussian components (see text).The experimen- tal data, the final fit (smooth bold line) and their difference are shown The chemical shifts of the peaks associated with changed Si environments remain approximately constant with increas- ing residence time and degree of conversion. It follows that the apparent upfield shift of the maximum in the broad absorption with increasing residence time is a consequence of the decrease in kaolinite-like sites present, rather than of a change in the transformed material. The proportion of Si in new sites that are assigned to metakaolin-like environments appears to increase slightly with increasing residence time (39,40, 43, 46 and 45% of new sites at 0.4,0.5,0.6,0.7 and 0.8 s, respectively).This could be linked with the increasing density of the calcine evident in Fig, 1. *'A1 Spectra of Flash Calcines Quantitative deconvolution of 27Al spectra is not particularly meaningful because of (i) the contribution from A1 in highly distorted environments being quadrupolar broadened beyond observation (there is a corresponding loss in intensity) and (ii) second-order quadrupolar shifts, linewidths, unknown lineshapes and overlapping contributions. The lack of a spectral feature assignable unambiguously to pentaco-ordinate A1 in this study is a significant difference from the spectra of metakaolin produced by soak calci- nation.20g22A broadened contribution to the underlying signal (-50-75 ppm) from distorted pentaco-ordinate A1 cannot be ruled out however.It should be emphasised that the use of high spin rates is essential in this work. Use of a lower spin rate (such as the more usual ca. 3 kHz at the Larmor frequency in this study) would result in a spinning sideband close to the anticipated location of a signal from pentaco-ordinate A1 and consequent difficulty in deriving any conclusion as to the occurrence of such a feature. The peak shown by detectable A1 atoms at 56ppm is attributable to tetrahedral Al. This supports the formation of some three-dimensionally connected aluminosilicate-like (tetrahedral Al) regions during dehydroxylation. The suppo- sition that the silica and alumina sheets are preserved during dehydr~xylation'~is therefore too simplistic, The proportion of observed A1 in tetrahedral sites increases with increasing residence time, this variation correlating with the decreasing proportion of untransformed material evident in X-ray pat- terns and 29Si spectra.The increasing relative intensity assign- able to tetrahedral A1 arises from the increasing proportion of transformed material. Conclusion The observed variations in densities, degrees of dehydroxyl- ation and NMR spectra suggest the following temporal evol- ution of structural changes during flash calcination: (1) At short times (<0.3 s) most of the kaolinite is largely unchanged (as evident from NMR spectra), while the bulk density decreases (and degree of dehydroxylation increases) owing to transformation of surface layers.(2) At longer times (>0.3 s) dehydroxylation within the particles commences, with accompanying 'bubbling' (internal void formation) and density loss. At this stage, Si within the material migrates increasingly to three non-kaolinite environ- ments (termed mullite-like, metakaolin-like and Q").Some A1 migrates to tetrahedral sites. (3) The relative proportion of Si in transformed regions that is assigned as metakaolin-like environments increases slightly at the longest residence times. (4) As residence time increases and the amount of untrans- formed material decreases, the relative proportion of A1 in tetrahedral environments increases (correlating with the pro- portion of product). J. MATER. CHEM., 1991, VOL. 1 The derived composite picture of flash calcination is that of a material transforming increasingly from kaolinite to a single product.Once formed, the product appears to undergo little further chemical reaction during its short time in the calciner. Prolonged (soak) thermal treatment of flash calcines pro- duces materials comparable (e.g.densities) to those from soak calcination. The transformations towards those products are much slower than the initial step giving the flash calcine. It should be noted, however, that slightly increasing densities at the longest residence times in this work could be evidence for shrinkage of internal voids (by atom migrations in the trans- formed material at the calciner temperature), this process being accompanied by a slightly increasing proportion of metakaolin-like Si environments.The detailed interpretation of 29Si and 27Al NMR spectra for calcines is likely to be highly complex. For soak calcines detailed models of thermal transformations of structures have been pr~posed,'~.'~ but understanding remains incomplete. For the flash calcines produced in this study, further NMR investigations are in hand using cross-polarisation ('H-29Si CP-MAS) and nutation (27Al)techniques. We thank SERC for supporting this study under grant GR/E 81999 and for access to the National Solid State NMR Service (University of Durham). We thank the staff of that service for recording high-resolution spectra and for subsequent computations. References 1 W.D. Keller, Geology Today, 1985, 109. 2 G. W. Brindley and M. Nakahira, J. Am. Ceram. SOC., 1959, 42, 31 1. 3 R. E. Grim, Clay Mineralogy, McGraw-Hill, New York, 2nd edn., 1968. 4 M. C. Gastuche, F. Toussaint, J. J. Fripiat, R. Touilleaux and M. van Meersche, Clay Miner. Bull., 1963, 5, 227. 5 G. W. Brindley and G. L. Millhollen, Science, 1966, 152, 1385. 6 G. W. Brindley, personal communication, 1984. 7 C. Otero-Arean, M. Letellier, B. C. Gerstein and J. J. Fripiat, in Proc. Znt. Clay Con5 1981-BoIogna, ed. H. Van Olphen and F. Veniale, Elsevier, Amsterdam, 1982, p. 73. 8 G. W. Brindley, J. H. Sharp, J. H. Patterson and B. N. Narahari Achar, Am. Miner., 1967, 52, 201. 9 J. M. Criado, A. Ortega, C. Real and E. Tores de Torres, Clay Miner., 1984, 19, 653.10 R. H. Meinhold, K. J. D. Mackenzie and I. W. M. Brown, J. Mater. Sci. Lett., 1985, 4, 163. 11 D. Bridson, T. W. Davies and D. P. Harrison, Clays Clay Miner., 1985, 33, 258. 12 T. W. Davies, High Temp. Technol., 1984, 2, 141. 13 T. W. Davies, Chem. Eng. Res. Des., 1985, 63, 82. 14 T. W. Davies and R. M. Hooper, J. Mater. Sci. Lett., 1985, 4, 39. 15 T. W. Davies, J. Mater. Sci. Lett., 1986, 5, 186. 16 R. C. T. Slade and T. W. Davies, Colloids SurJ, 1989, 36, 119. 17 K. J. D. Mackenzie, I. W. M. Brown, R. H. Meinhold and M. E. Bowden, J. Am. Ceram. SOC., 1985, 68, 293. 18 I. W. M. Brown, K. J. D. Mackenzie, M. E. Bowden and R. H. Meinhold, J. Am. Ceram. SOC., 1985, 68, 298. 19 J. Sanz, A. Madani, J. M. Serratosa, J. S. Moya and S. ha, J. Am. Ceram. SOC., 1988, 71, 418. 20 J. F. Lambert, W. S. Millman and J. J. Fripiat, J. Am. Chem. SOC.,1989, 111, 3517. 21 J. Rocha and J. Klinowski, Angew. Chem., Znt. Ed. Engl., 1990, 29, 553. 22 J. Rocha and J. Klinowski, Phys. Chem. Miner., 1990, 17, 179. 23 E. Lipmaa, A. Samoson and M. Magi, J. Am. Chem. SOC., 1986, 108, 1730. 24 J. Gilson, G. C. Edwards, A. W. Peters, K. Rajagopalan, R. F. Wormsbecher, T. G. Roberie and M. P. Shattock, J. Chem. SOC., Chem. Commun., 1987,9 1. Paper 01049 17D; Received 1st November, 1990

ISSN:0959-9428    

DOI:10.1039/JM9910100361

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(3), 365-370 Preparation of Ternary Composite Hydrogels of Agarose, Concanavalin A and a Glycolipid Monolayer, and their Permeation Properties Nobuyuki Higashi, Minoru Takematsu and Masazo Niwa* Department of Applied Chemistrx Faculty of Engineering, Doshisha University, Kamikyo-ku, Kyoto 602, Japan Synthetic glycolipids (2Cl,-de-glu and 2C,,-de-gal) form condensed surface monolayers, which have been successfully deposited on the agar hydrogel by the Langmuir-Blodgett (LB) technique. The monolayer-deposited gel membranes were satisfactory for use in the permeation experiments of ammonium salts such as tetramethyl- ammonium bromide or hexyltrimethylammonium bromide over a wide temperature range. The permeability of the salts through the modified gel membranes depends strongly upon the hydrophobicity of the permeants and could be controlled by the phase transition of the deposited monolayers.Concanavalin A (ConA)-bound agar gels were modified with the 2Cl,-de-glu monolayer, having the a-D-glucopyranosyl head group, but not with the 2C,,-de-gal, having the B-D-galactopyranosyl head group, owing to the specific binding ability of ConA. The permeability of the 2Cl,-de-glu monolayer-deposited ConA-bound agar gel was selectively controlled by the addition of monosaccharides: the permeability was enhanced by the addition of D-mannose, whereas it was hardly changed by the addition of D-galactose, resulting from the high binding constant of D-mannose with ConA. Keywords: Glycolipid; Agar hydrogel; Monolayer; Langmuir-Blodgett film; Permeation The permeation of a hydrogel by a water-soluble solute has received a great deal of attention from the standpoint of a controlled-release system of hydrophilic drugs, owing to the fact that hydrogels provide high biocompatibility based on their large surface free energy.' It is well known that an artificial lipid membrane has a variety of peculiar physico- chemical properties (e.g.crystal-liquid crystal phase transition and phase separation) which can control the substrate per- meability through the membrane in a similar way to biomem- brane function.2 Thus, the combined study of hydrogels and lipid membranes should provide a new promising area of permeability-controllable membranes. The LB technique3 transferring lipid monolayers from an air/water interface is useful for preparing oriented thin films on a substrate.LB multilayer films supported on a porous substrate have been used recently to control the permeability of and gases.g-" The LB deposition of a phospholipid bilayer on a poly(acry1amide) hydrogel has been reported by Arya et al.12 However, in the paper they mention that the experiment has not been successfully repeated. Hongyo et ~1.'~showed the applicability of a phospholipid black lipid membrane (BLM) painted onto an agar hydrogel for conductometric devices. In a previous report,I4 we presented preliminary results of the preparation of a glycolipid monolayer-coated agar hydro- gel and of the substrate permeability through the modified hydrogel membranes.The present paper describes the results of our more intensive study of the substrate permeability through the monolayer-coated hydrogel membrane. Also, we report on the preparation of a novel ternary composite hydrogel membrane composed of agar, ConA and a glycolipid monolayer, and its controlled permeability upon addition of monosaccharides. ConA, isolated from Canaualia ensifomis, is a lectin, and a tetrameric protein with four carbohydrate- binding sites which specifically binds a-D-glucopyranosyl or a-D-mannopyranosyl moieties. ConA has been widely used for model studies of the molecular recognition on cell surfaces; liposomes are agglutinated by addition of ConA if they contain the glycolipid with the a-D-glucopyranosyl moi- ety.15-" These agglutinations are reversed by the addition of an excess of low-molecular-weight saccharides.We employed such glycolipids as 2C18-de-glu or 2Cls-de-gd having the a-D-glucopyranosyl or /?-D-galactopyranosyl unit as hydro- philic head group, respectively, as monolayer components. A schematic illustration of the composite hydrogel membranes is shown in Fig. 1 in which the permeants are also included. Experimental Materials Glycolipid 2C18-de-glu containing a-D-glucopyranosyl-D-gluconamide as a hydrophilic head group was prepared as follows. 3-Azapentamethylene bis(octadecan0ate) was syn-thesized by condensation of octadecanoic acid (14.7 g, 52 mmol) with bis(2-hydroxyethy1)amine(2.4 g, 22 mmol) in toluene in the presence of toluene-p-sulphonic acid (10.7 g, 56 mmol): pale-yellow powder (8.0 g, 56%), m.p.57 "C from acetone. Maltose [O-a-D-ghcopyranosyl-(1-+4)-~-glucopy-ranose] (12.0 g, 33 mmol) was oxidized in methanol in the presence of 17.1 g of iodine (67 mmol) to maltonolactone (8.2 g, 73 YO)according to the literature.20 Maltonolactone (1.7 g, 5 mmol) was allowed to react with 4.1 g of 3-azapenta- methylene bis(octadecan0ate) (5 mmol) in refluxing methanol for 4 h. After cooling the mixture to room temperature, a pale-yellow powder precipitated. The powder was filtered and recrystallized from methanol (2.5 g, 52%), m.p. 73.5 "C (Found: C, 63.55; H, 10.09; N, 1.49. Calc. for C52H99015N: C, 63.84; H, 10.20; N, 1.43%); TLC (CHC13) RfO single spot; v,,,/cm-' 1600 (C=O, amide), 1740 (C=O, ester); dH (C,D,) 0.90 (6 H, t, CH3), 3.08-4.22 (21 H, m, glucopyranose).Glycolipid 2C1 ,-de-gal containing /?-D-galactopyranosyl- D-gluconamide as head group was prepared from 4.1 g of 3-azapentamethylene bis(octadecan0ate) (5 mmol) and the oxi- dized lactose [0-B-D-galactopyranosyl-(1+4)-~-glucopy-ranose] (1.7 g, 5 mmol) in the same way as above: pale-yellow powder (2.7 g, 55%), m.p. 75.5 "C from methanol (Found: C, 63.63; H, 10.00; N, 1.39. Calc. for C52H99015N: C, 63.84; H, agar glycoli pid acJ+aT---ConA glycolipid .glycolipid per meant I+ -I+ -Et+ -N-Br , -N-Br QN-Et Ci I I Et CgN+ C1 N+ BTAC Fig. 1 A schematic illustration of glycolipid-deposited agar gel membranes 10.20; N, 1.43%); TLC (CHC13) RfO single spot; v,,,/cm-' 1600 (C=O, amide), 1740 (C=O, ester).ConA was purchased from Sigma. Other reagents were analytical grade used without further purification. Spreading Experiments The monolayers were obtained by spreading benzene-ethanol (8:2 in volume) solutions of glycolipids on purified water ('Milli-Q' system, Millipore). The concentration of the spread- ing solution was 1.0mgcm-3. 10 min after spreading the gaseous monolayer was compressed continuously. The com- pressional velocity was 1.20 cm2 s-'. Below this value, the effect of compression rate on the monolayer area was within experimental error. Wilhelmy's plate (filter paper plate) method and a PTFE-coated trough with a microprocessor- controlled film balance (FSD-20, San-Esu Keisoku), having a precision 0.01 mN m- ', were used for surface pressure measurements.Measurements of the surface pressure(n)- area(A) curves for all samples were repeated several times to check their reproducibility. Preparation of Composite Hydrogel Membranes A 4%-agar gel was loaded into a glass plate of 2 mm thickness with a pore size of 6 mm diameter as described previou~ly.'~ Deposition of the surface monolayer onto one side of the 4%- agar gel-loaded glass plate thus prepared was performed at a constant surface pressure with a microprocessor-controlled film balance (FSD-21, San Esu Keisoku). The bare gel sub- strate had been immersed in the subphase before spreading the monolayer and was withdrawn at a speed of 15 mm min-' at a constant surface pressure of 30 mN m- '.The ternary composite hydrogels were prepared as follows. J. MATER. CHEM., 1991, VOL. 1 The 4%-agar gel-loaded glass plates were immersed in a ConA aqueous solution (1 mg ~m-~) for 12 h to give ConA- modified agar gels. Subsequently, the glycolipid monolayers were deposited onto one side of the ConA-modified agar gels in the same way as mentioned above. Permeation Measurements Permeation experiments were performed between pure water (14 cm3) and aqueous CIN+, CsNf, or BTAC (50 cm3) with a conventional, thermostatically controlled H-shaped cell. The membrane area was 0.28 cm2, and both sides of the cell were stirred at a constant speed.Permeation of the salts (C,N+, C6N+, and BTAC) was followed by detecting increases in the electrical conductance in the water side, since good linear correlations between the conductance and the concentration of salts were obtained in the range 0.1-10mmol dm-3. Portions of the solutions of D-mannose or D-galactose were directly added to the salt solution side ([monosaccharide] = 1.5 x mol dm-3). Apparent permeation rates, P (cm s-I), were calculated from P =kv/aC, (1) where k, v, a, and Co are the initial slope of a permeant transport, the volume of the water side of the cell (14 cm3), the membrane area (0.28 cm2) and the concentration of per- meants in the salt solution side (50 mmol dm-3), respectively. Results and Discussion Glycolipid Monolayer-deposited Agar Gels Fig.2 shows surface pressure(n)-area(A) isotherms of mono- layers of 2C18-de-glu and 2C18-de-gal amphiphiles on pure water at 20 "C. Both monolayers showed a similar shape in their n-A isotherms and a condensed solid phase alone with a limiting area per molecule extrapolated at zero pressure (A,) of 0.64 nm2 (2C18-de-glU) or 0.66 nm2 (2C18-de-gal). These values are relatively large compared with those for other dialkyl amphiphiles such as phospholipids (0.40- 0.50 nm2),'l owing to the steric effect of the large sugar head group. In order to obtain information about the phase change of monolayers, n-A isotherms of 2C1p-de-glU monolayers were measured at various temperatures (10-40 "C), and the limiting area per molecule (A,) is shown plotted against temperature in Fig.3. The value of A, increased drastically near 30°C. At the melting point the isobar of a three-area/nm2 per molecule Fig. 2 Surface pressure (71)-area (A) isotherms of (a) 2Cl,-de-gal and (b) 2Cl,-de-glu on pure water at 20 "C J. MATER. CHEM., 1991, VOL. 1 layer-deposited gel membrane was apparently enhanced relative to that across the bare gel membrane when the -Q, o-68t I I I I I I I 10 20 30 40 Tl "C Fig. 3 A typical temperature dependence of the limiting area (A,) for the 2C18-de-glu monolayer dimensional system (crystal) shows an increase in volume. Similarly an increase in area is displayed in a two-dimensional monolayer system when an increase in temperature causes a phase change from the solid analogue to the liquid analogue orientation.Thus, the drastic increase in the A. value near 30 "C (T,)observed in Fig. 3 is ascribed to the phase transition from the gel state to the liquid-crystalline state of the 2C18- de-glu monolayer. It is important to estimate the phase- transition behaviour in an aqueous bilayer state of the lipid and to compare it with that of its monolayer state. However, the lipids prepared in this study could not be dispersed at all in water by sonication. Deposition of the 2Cl8-de-glU monolayer onto one side of a 4%-agar gel-loaded glass plate was performed at a surface pressure of 30 mN m-' with the monolayer in condensed phase.The permeation experiment through the monolayer- deposited agar gel thus obtained was carried out between pure water and aqueous ammonium salt. Fig. 4 shows typical time courses of ammonium salts [(a) C6N+; (b) CIN+] through the monolayer-deposited gel membranes at 20 "C. The membranes were set in two ways: the monolayer faced either the pure-water side or the salt-solution side as shown in Fig.4 (inset). For the permeation of the relatively hydro- phobic permeant C6N+, the permeability across the mono- monolayer was set to face the salt-solution side. In contrast, when the monolayer was set to face the water side, the permeability was markedly reduced, and was close to that of the bare gel membrane. The reason why the membrane is more permeable when the monolayer faces the salt-solution side is probably because the hydrophobicity of C6N+ causes it to penetrate and concentrate in the monolayer, so that the concentration gradient across the hydrogel layer becomes greater than that for the bare hydrogel layer.In the permeation of the hydrophilic probe C,N+ [Fig. 4(b)],the permeability was suppressed compared with that of the bare gel membrane, in contrast to the results of C6N+. When the monolayer faced the pure-water side, the permeability was close to that of the bare gel membrane. These results suggest that the surface monolayer of 2C18-de-gh was definitely deposited onto the gel membrane and its hydrophobic surface covered with alkyl chain would provide a higher permeability for C6N+ than CIN+.In order to confirm that such a stable deposition of the 2Cl,-de-glu monolayer onto the agar gel is due to a sugar-sugar interaction between the &D-glUCOpyranOSyl head group of 2Cl,-de-glu and the sugar residues of agar, a lipid that has a trimethylammonium group in place of the sugar residue of 2C18-de-glU was deposited on the agar gel and used for the same permeation experiment. The permeation rate of C6Nf was initially the same as that of the 2Cl,-de-glu- deposited gel and then approached that of the bare gel within 1 h, suggesting that the ammonium monolayer flaked off into the aqueous solution from the gel surface. Therefore, the sugar-sugar interaction between the monolayer and the agar surface can be concluded to play an important role in stabiliz- ation and facilitated deposition of the 2C18-de-gh monolayer on the agar gel.Fig.4 shows that when the monolayer- deposited gel membranes were set to face the pure water side, permeation data for neither permeant showed any significant difference between the monolayer-deposited gel membranes and the bare gel membranes. The monolayer was thus set to face the salt-solution side in the following permeation experiments. Effect of the Phase Transition of Monolayer-deposited Gels Arrhenius plots of apparent permeation rate P are shown in Fig. 5. The original bare agar gel membrane gave straight Arrhenius plots for both permeants, although the permeability 20 40 60 80 100 20 40 60 80 100 timelmin timelmin Fig.4 Time courses of (a) C6N+ or (b) CIN+ permeation through the 2CI8-de-glu monolayer-deposited agar gel membranes at 20°C.The membranes were set in two ways: the monolayer faced either pure-water side or salt-solution side. Dashed lines show data for the bare gel membranes 368 J. MATER. CHEM., 1991, VOL. 1 TI "C TI "C 40 30 20 40 30 20 I I -8.5 h -I v)E -9.0 0c 0)--9.5 -lo/ I I I I I -3.1 3.2 3.3 3.4 3.5 3.1 3.2 3.3 3.4 3.5 103 KIT 103 KIT Fig. 5 Arrhenius plots of permeation coefficient (P)for (a)C6Nf and (b)CIN+ across the 2C1,-de-glu monolayer-deposited agar gel membranes (a)and the bare agar gel membrane (0).The membranes were set to face salt-solution side.Arrows indicate the phase-transition temperature (T,)of the monolayer determined by the temperature dependence of the n-A curve for CIN+ was always larger than that for C6N+, probably owing to the difference in steric bulkiness and/or hydro- phobicity between the permeant molecules. In the permeation through the monolayer-deposited gel membrane, two types of Arrhenius curve were obtained, which were different from those of the bare membrane. For the permeation of the hydrophilic CINf, the Arrhenius plot was lower than that of the bare gel membrane in the whole temperature range, and gave a discontinuous inflection at ca.30 "C, which corresponds to the T, obtained from the temperature dependence of 7~-A curves (Fig. 3). At temperatures above T,, the permeability was suppressed compared with that for below T,.We do not have any conclusive evidence so far to explain this phenom- enon, but one possibility, which will require further explo- ration, is as follows.The monolayer deposited on the hydrogel seems to produce defective pores in the gel state, and the permeation of a smaller and more hydrophilic molecule such as C,N+ relative to C6Nf below T, is not suppressed greatly compared with that of the bare hydrogel. Since these pores may disappear in the liquid-crystalline state of the deposited monolayer above T,, CIN+ permeation is drastically decreased above T,. Similar situations were observed in the substrate release from a non-ionic lipid-coated capsule membrane." In the case of the hydrophobic permeant C6N+, the per- meability was remarkably enhanced.The Arrhenius plot was always higher than that of the bare gel membrane and was clearly inflected at T,. Such an enhancement of the per- meability for C6N+ may be considered as due to a significant increase in hydrophobicity of the gel surface by deposition of the monolayer. The slope of the Arrhenius plot below T, was steeper than that above T,. Activation energies (E,) for the permeation were calculated on the basis of Arrhenius plots and are listed in Table 1. In the crystalline monolayer below T,,the permeant must diffuse with a high activation energy (90 kJ mol-'). At temperatures above T,,the permeant could easily pass through the liquid-crystalline monolayer with a relatively small activation energy (8 kJ mol- I).These results clearly demonstrate that even on the agar gel the glycolipid monolayer shows a phase transition from gel to liquid crystal, which can control the substrate permeability. Table 1 Comparison of activation energy (E,) for the permeation through the monolayer-deposited hydrogel membranes between permeants E,/kJ mol-' monolayer-deposited gel permeant bare gel T< T, DT, 27 32 90 8 To demonstrate further that the glycolipid monolayer located at the gel surface plays a key role in controlling the substrate permeability, we prepared an agar gel membrane containing 2C18-de-glu in bulk and examined the permeability of C6N + through it. The glycolipid-included gel membrane was prepared by mixing the 4%-agar solution and 2Cl,-de- glu (1 wt.% to agar) and then by loading the mixture into the glass plate in the same way as described in the Experimen- tal.The resulting gel membrane included a larger amount of 2C1 ,-de-glu than that of the 2C18-de-glu monolayer-modified gel membrane. Arrhenius plots of the apparent permeation rate are shown in Fig. 6. The glycolipid-included gel mem- brane gave only the straight Arrhenius plot without an inflection point as had been observed in case of the glycolipid monolayer-deposited gel membranes. Furthermore, the Arrhenius plots fell on the same line as the bare gel membrane. These results imply that the permeability of the glycolipid- included gel membrane is not affected by the phase transition; in other words, the glycolipid molecule is homogeneously dispersed in the gel matrices, or the domain size of the glycolipid would be too small to cause a phase change even if the glycolipid could form a domain in the gel matrices.The above considerations lead to the following conclusions. The glycolipid molecules are stably immobilized at the surface of the agar gel in a monolayer state (mainly by a sugar-sugar interaction) and the monolayer on the gel has a physicochemi- cal property (phase transition) similar to the monolayer on water, which results in controlling the substrate permeability. J. MATER. CHEM., 1991, VOL. 1 -8.5 --v) E2 -9.0 !5 0) '-9.5 3.1 3.2 3.3 3.4 3.5 lo3 KIT Fig.6 Arrhenius plot of permeation coefficient (P) for C6N+ across the 2C1,-de-glu-included (1 wt.% to agar) agar gel membrane (e)and the bare agar gel membrane (0) Ternary Composite Hydrogels Fig. 7 shows time courses of BTAC transport from the salt- solution side, when the bare gel was treated with ConA solution and then the ConA-modified gels were coated with a 2CI8-de-gal or 2C18-de-glu monolayer. These modified-gel surfaces were set to face the salt-solution side. BTAC was employed as a permeant; it has a relatively bulky benzyl group but not a long alkyl chain. The permeation of BTAC through the ConA-modified gel membrane was slightly reduced compared with the bare-gel membrane; this results from an increase of the barrier capability to BTAC permeation due to the binding of ConA with sugar residues in the agar surface.When the monolayer of 2Cl,-de-gal having the B-D-galactopyranosyl moiety was deposited on the ConA-modified gel membrane, the BTAC permeability was hardly changed. This result strongly suggests that the 2C18-de-gal monolayer could not be deposited on the ConA-bound agar surface owing to its poor binding ability with ConA, whereas it could be stably deposited on the pure agar gel (without ConA). In contrast, when the monolayer of 2C1,-de-glu, carrying 3.4 -/ ,(a) / / r U l-I I I 1 1 1 a-D-glucopyranosyl moiety was deposited on the ConA-modi- fied gel membrane, the BTAC permeability was markedly suppressed by a factor of ca.10 compared with that of the bare gel membrane. This means that vacant binding sites of the ConA immobilized on the agar gel surface recognize and strongly bind with the a-D-glucopyranosyl head group of 2C1,-de-glu monolayer. These results show good correlation with the specific bind- ing ability of ConA: ConA binds specifically with a-D-glucopy- ranosyl, but not with /3-D-galactopyranosyl, in glycolipids and polysaccharides on the cell ~urface.'~-'~ It is well known that when ConA is added to an aqueous dispersion of liposomes containing a-D-glucopyranosyl lipids, liposomes are agglutin- ated with each other owing to the specific binding between ConA and the E-D-glucopyranosyl head group of the bilayer surface^.'^-'^ ConA-induced agglutination is inhibited in the presence of an excess of low-molecular-weight sugars such as D-mannose, because the monosaccharide, which has a high binding constant with ConA, expels the glucopyranosyl resi- due of lipids from the binding sites resulting in the dissociation of liposome agglutination.To test the effect of the addition of monosaccharides to our ternary composite gel systems, we employed the 2C18-de-glu monolayer-deposited, ConA-agar gel membrane which had been found to have a barrier to BTAC permeation. Fig. 8 displays the time course of BTAC transport from the salt- solution side. When a large excess of D-galactose (1.5 x 10-mol dm -3, was directly added to the salt-solution side, BTAC permeability did not change, which is as expected since ConA cannot bind D-galactose.On the other hand, in the case of the addition of D-mannose, which has an extremely high binding constant with ConA, BTAC permeability was apparently enhanced. This means that D-mannose expels either the glucopyranosyl residues of the lipid or the sugar residues of agar from the binding sites of ConA, resulting in the formation of a defect at the gel surface as illustrated schematically in Fig. 9. Hence, the permeability increases. Concluding Remarks The glycolipid (2C ,-de-glu and 2C ,-de-gal) monolayer films were successfully formed on the agar hydrogel by the LB technique. The monolayer-deposited gel membranes were satisfactory for the permeation experiments over a wide temperature range.The permeability of the ammonium salts through these modified gel membranes depended strongly -D-mannose 0 ____ __--------30 min w time + Fig. 8 Permeation changes of BTAC through the 2C18-de-glu mono- layer-deposited, ConA-modified agar gel membrane by the addition of D-galactose or D-mannose (1.5 x mol dm-7 agai D-galactose Fig. 9 A schematic representation for effect of the addition of mono- saccharides on the substrate permeability through the ternary com- posite gel membrane upon the hydrophobicity of permeants and could be controlled by the phase transition of the deposited monolayers. The ConA-bound agar gels were modified only with the 2CI8-de- glu monolayer having the a-D-glucopyranosyl head group but not with the 2C18-de-gal having the /3-D-galactopyranosyl head group owing to the specific binding ability of ConA.The permeability of the 2C18-de-glu monolayer-deposited ConA-agar gel was selectively controlled by the addition of monosaccharides: the permeability was enhanced by the addition of D-mannose, whereas it was hardly changed by the addition of D-galactose which resulted from the high binding constant of D-mannose with ConA. The combined properties of hydrogels and lipid membranes could be interesting, not only for a variety of biomedical studies, including the development of new controlled drug- release systems but also for an applicability to conductometric devices. J. MATER. CHEM., 1991, VOL. 1 References 1 Hydrogels for Medical and Related Application, ed. J.D. Andrade, American Chemical Society, Washington DC, 1976. 2 For a recent review, see H. Ringsdorf, B. Schlarb and J. Venzmer, Angew. Chem., 1988, 100, 117 and references cited therein. 3 K. B. Blodgett and I. Langmuir, Phys. Rev., 1937, 51, 964. 4 T. Kajiyama, A. Kumano, M. Takayanagi and T. Kunitake, Chem. Lett., 1984, 915. 5 K. Hechmann, C. Strobl and S. Bauer, Thin Solid Films, 1983, 99, 265. 6 Y. Okahata, K. Ariga, H. Nakahara and K. Fukuda, J. Chem. SOC., Chem. Commun, 1986, 1069. 7 T. Kunitake, N. Higashi, M. Kunitake and Y. Fukushige, Mucro-molecules, 1989, 22, 485. 8 M. Niwa, E. Matsuyoshi and N. Higashi, Langmuir, 1989, 5, 1256. 9 G. D. Rose and J. A. Quinn, J. Colloid Interface Sci., 1968, 27, 193. 10 0. Albrecht, A. Laschewsky and H. Ringsdorf, Macromolecules, 1984, 17, 937. 11 N. Higashi, T. Kunitake and Y. Kajiyama, Polym. J., 1987, 19, 289. 12 A. Arya, U. J. Krull, M. Thompson and H. E. Wong, Anal. Chim. Acta, 1985, 173, 331. 13 K. Hongyo, J. Joseph, R. J. Huber and J. Janata, Langmuir, 1987, 3, 827. 14 N. Higashi, M. Takematsu and M. Niwa, Chem. Lett., 1990,675. 15 H. Bader, H. Ringsdorf and J. Skura, Angew. Chem. Znt. Ed. Engl., 1981, 20, 305. 16 T. Williams, N. R. Plessas and I. J. Goldstein, Arch. Biochem. Biophys., 1979, 19, 145. 17 J. Slama and R. R. Rando, Carbohydr. Rex, 1981, 88, 213. 18 G. A. Orr, R. R. Rando and F. W. Bangerster, J. Biol. Chem., 1979,254,4721. 19 R. Y. Hampton, R. W. Halz and I. J. Goldstein, J. Biol. Chem., 1980,255,6766. 20 K. Kobayashi, H. Sumitomo and Y.Ina, Polym. J., 1985, 17, 567. 21 M. C. Philips and D. Chapman, Biochim. Biophys. Acta, 1968, 163, 301. 22 Y. Okahata, H-J. Lim and G. Nakamura, J. Membrane Sci., 1984, 19, 237. Paper 0/04935B; Received 2nd November, 1990

ISSN:0959-9428    

DOI:10.1039/JM9910100365

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(3), 371-374 37 1 High-surface-area Resins derived from 2,3=Epoxypropyl Methacrylate cross-linked with Trimethylolpropane Trimethacrylate P. D. Verweij" and David C. Sherringtonb a Department of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands Department ofPure and Applied Chemistry, University of Strathclyde, Glasgow G1 IXL, UK Suspension copolymerization of 2,3-epoxypropyl methacrylate (glycidyl methacrylate, GMA) and 2-ethyl-2- (hydroxymethyl)-propan-l,3-diol trimethacrylate (trimethylolpropane trimethacrylate, TRIM) has been performed in several solvents as porogens, i.e. cyclohexanol-dodecan-1-01 9/1 v/v, octan-2-one, n-butyl acetate, pxylene, toluene, ethyl acetate, benzonitrile, cyclohexanone and dodecan-1-01.The GMA :TRIM ratio in the monomer mixture and the monomer: porogen ratio have been varied. The B.E.T. surface area, pore volume and number of unreacted carbon-carbon double bonds are strongly dependent on the solvent used. Both the B.E.T. surface area and the pore volume decrease rapidly with increasing GMA :TRIM ratio when the cyclohexanol-dodecan- 1-01 mixture is used as the porogen. In contrast, the monomer: porogen ratio hardly affects the B.E.T. surface area of the resulting copolymers. The pore volume decreases with increasing monomer: porogen ratio in the interval studied. A very large pore volume, i.e. 1.86 cm3 g-', was found when octan-2-one was used as the porogen, with GMA :TRIM =1:1 and monomer :porogen 1:3.Similarly a substantial B.E.T. surface area of ca. 175 m2 g-' was achieved using the same GMA:TRlM ratio and octan-2-one as the porogen with monomer :porogen of 1:2. Keywords: 2,3-Epoxypropyl methacrylate; Copolymer; Cross-linking; Surface area Cross-linked 2,3-epoxypropyl methacrylate-ethylene glycol dimethacrylate (GMA/EDMA) resins containing the epoxy group have been extensively studied. They have been used as ion exchangers after modification of the epoxy group with amines.'S2 Alternatively, diol-containing resins derived from these polymers can be applied as protecting groups for aldehydic functions through the formation of acetal linkage^.^ Horak et al. investigated the effect of a number of polymeriz- ation variables, e.g.the concentration of the porogen in the dispersed phase, the concentration of the cross-linking agent in the monomer mixture and the polymerization temperature, on the specific B.E.T. surface area, the pore size, the porosity and the mechanical properties of GMA/EDMA polymer^.^ These properties are expected to be of great importance in the kinetic performance of ion exchangers. In general terms, rather high levels of cross-linker are required in order to produce resins with high surface areas,' and as a result the content of functional comonomer has to be restricted. In order to allow the maximum content of GMA simultaneously with a high surface area it was decided to investigate resins in which the trifunctional cross-linker TRIM was used in place of EDMA.Rosenberg et aL6 studied the physical properties of the products of the polymerization of TRIM and the copolymeriz- ation of TRIM and methyl methacrylate (MMA). B.E.T. surface areas were found to vary with the solubility parameter of the inert solvent used in the dispersed phase. High B.E.T. surface areas were found for the TRIM homopolymers; how- ever, for the TRIMiMMA copolymers the B.E.T. surface area decreases rapidly with the amount of MMA in the monomer mixture. Recently, Walenius and Flodin reported the synthesis of a GMA/TRIM polymer and the reaction of the epoxy substitu- ents with aliphatic amino compound^.^ In this paper the effect of some polymerization variables in the copolymerization of TRIM and GMA on the physical properties of the products is described.The polymers have been studied by surface area (B.E.T.) measurements, mercury porosimetry and solid-state I3Ccross-polarization and magic- angle spinning nuclear magnetic resonance spectroscopy (CP MAS 13C-NMR). Experimental Starting Materials All monomers and solvents were commercially available and were used without further purification. Polymerization The suspension polymerization mixtures consisted of GMA and TRIM and an inert solvent or solvent mixture as a porogen. The following solvents were used: cyclohexanol- dodecan- 1-01 9/1 v/v, octan-2-one, n-butyl acetate, p-xylene, toluene, ethyl acetate, benzonitrile, cyclohexanone and dodecan-1-01. The aqueous phase was a solution (0.1 wt.%) of Biozan Gum R (xanthan gum, Hercules Powder) in water.The GMA :TRIM ratio was varied from 1:3 to 3 :1 and the monomer :porogen ratio from 1 :1to 1 :3. The polymerization took place at 80 "C for 8 h and the products were purified by extraction in a Soxhlet apparatus with acetone and dried in UQCUO at 60 "C. More details of the polymerization procedure have been published already.2" B.E.T. Surface Area Measurements, Mercury Porosimetry and CP MAS 13C-NMR The B.E.T. surface area was obtained from N2 absorption measurements according to the B.E.T. method' using a Micro- meritics Accusorb 2100E. The pore volume and pore size were determined by mercury porosimetry using a Micromeritics Autopore I1 9220. Solid-state CP MAS 13C-NMR (cross- polarization and magic-angle spinning) was applied to deter- mine the amount of unreacted carbon-carbon double bonds in the polymers.The spectra were obtained on a Bruker MSL 400 spectrometer, operating at 100.6 and 400.1 MHz for I3C and 'H, respectively. The sample spinning rate was 5000 Hz. 3 72 J. MATER. CHEM., 1991, VOL. 1 The cross-polarization contact time was 0.8ms,6 with 3 s recycle delays between successive scans. Generally 1200 scans were employed. Results and Discussion Physical parameters of the polymers are listed in Table 1. Fig. 1 shows the B.E.T. surface area and the pore volume as functions of the amount of GMA in the monomer mixture with monomer :porogen = 1:2 and the cyclohexanol-dodecan-1-01 mixture as the porogen.Both the B.E.T. surface area and the pore volume were found to decrease rapidly with increasing GMA concentration, i.e. falling TRIM content. The pore-size distribution for P4 (GMA :TRIM =3 :1) (Fig. 2) shows a sharp maximum at 0.035 pm, which was found to broaden and to be displaced towards larger pore sizes when the amount of GMA in the monomer mixture decreased. Furthermore, the distributions indicate that a considerable number of pores with diameters of ca. 0.015 pm is mainly responsible for the large surface areas of P2 (GMA :TRIM = pore diametedpm Fig. 2 Pore size distribution for values of GMA (%) in the monomer mixture: (a) 25%; (b) 50%; (c) 75%; monomer :porogen = 1 :2; por- ogen =cyclohexanol-dodecan- l-ol 9/1 v/v 1 :3) and P3 (GMA:TRIM= 1 :l), while the large pore vol- umes of these polymers are mainly due to pores with diameters larger than 0.030nm.Fig. 3(a)and (b)show the B.E.T. surface area and the pore volume as a function of the monomer concentration in the organic phase for cyclohexanol-dodecan- l-ol 9/ 1 and octan- 2-one as the porogen, respectively, and GMA :TRIM = 1 :1. I I I I 20 40 60 80 100 The monomer concentration was found to have a large effect GMA in monomer mixture (YO) on the pore volume. When octan-2-one was used as the porogen a very large pore volume, i.e. 1.86 cm3 g-', was Fig. 1 B.E.T. surface area (0)and pore volume (0)us. GMA (%) found with monomer :porogen = 1:3. The effect of the in the monomer mixture; monomer :porogen = 1:2; porogen =cyclo-monomer :porogen ratio on the B.E.T.surface area was much hexanol-dodecan- 1-01 9/ 1 v/v less pronounced, suggesting that mainly large pores are formed Table 1 Physical and structural parameters of the GMA/TRIM polymers polymer GMA:TRIM M: P" solvent pore vol./cm3 g-' surface areab/m2 g-' surface areac/m2 g-' NMR unreacted C=C (%) P1 1:l 1:1 cycl-dod 9/1 0.38 121 144 3 P2 1:3 1:2 cyd-dod 9/1 1.28 339 267 6 P3 1:l 1 2 cycl-dod 9/1 1.13 1 40 223 2 P4 3: 1 1:2 cycl-dod 9/1 0.57 41 130 0 P5 1:1 1:3 cycl-dod 9/1 1.12 128 120 2 P6 I:1 1:I octan-2-one 0.65 127 173 5 P7 1:1 I :2 octan-2-one 1.31 174 245 3 P8 3: 1 1:2 octan-2-one 0.97 39 73 0 P9 1:l 1:3 octan-2-one 1.86 149 225 1 P10 1:1 1:2 n-butyl acetate 1.27 170 199 3 P11 1:l 1 :2 p-xylene 1SO 139 266 2 -P12 3: 1 1:2 p-xylene 1.47 2 51 P13 1:l 1 :2 toluene 1.02 145 192 4 P14 1:l 1:2 ethyl acetate 0.66 110 176 4 P15 1:l 1:2 benzonitrile 0.07 <1 44 4 P16 1:l 1:2 cyclohexanone 0.16 0.2 82 4 d d d 5P17 1:l 1:2 dodecan- l-ol 'M :P =monomer :porogen; determined by N2 adsorption according to the B.E.T.method; determined by mercury porosimetry; not determined, very fine powder. J. MATER. CHEM., 1991, VOL. I -I I I 1 I 1 I I 20 30 40 50 20 30 40 50 monomer in organic phase (%) Fig. 3 B.E.T. surface area (0)and pore volume (0)us. monomer (%) in the organic phase; GMA :TRIM = 1 : 1; porogen =cyclohexa-nol-dodecan-1-01 9/l v/v (a),octan-2-one (b) when the monomer concentration is low.This is confirmed by the relatively large average pore diameter when monomer :porogen = 1 :3, i.e. 37 and 33 nm for cyclohexanol- dodecan- l-ol 9/ 1 v/v and octan-2-one as the solvent, respect- ively, and also by the pore-size distributions (Fig.4A and B for the cyclohexanol-dodecan- 1-01 mixture and octan-2-one as the solvent, respectively). Solid-state CP MAS 13C-NMR spectroscopy was used in order to determine the amount of unreacted carbon-carbon double bonds. Carbonyl groups conjugated with a double bond have a lower chemical shift (166 ppm) than the unconju- gated, reacted ones (176ppm).6 In Table 1 the amount of unreacted methacrylate groups is listed for the polymers. The number of unreacted double bonds was found to be low and dependent on the porogen used during the polymerization. The values are lower than those normally found with styrene/ divinylbenzene (DVB) resins,' and confirm the observations of Rosenberg et aL6 The difference may be associated with the enhanced flexibility of the TRIM cross-linker versus DVB.Fig. 5 shows the dependence of the amount of unreacted double bonds on the amount of GMA in the monomer mixture. For GMA :TRIM =3 :1 no double bonds remain in the resin after polymerization, while the number of double 0 25 50 75 GMA in monomer mixture (%) Fig.5 Number of unreacted double bonds us. GMA (YO)in the monomer mixture; monomer :porogen = 1:2; porogen =cyclohexa-nol-dodecan- l-ol 911 v/v bonds increases with increasing amount of TRIM, as found for the copolymerization of TRIM and methyl methacrylate (MMA) in ethyl acetate,6 where the number of unreacted double bonds reaches a maximum of 16.7% in the case of TRIM= 100 vol.%.In Fig. 6 the number of unreacted double I I I I 1 1 0.1 0.01 1 0.1 0.01 20 30 40 50 pore diameteripm monomer in organic phase (%) in the Fig.6 Number of unreacted double bonds us. monomer (YO)Fig. 4 Pore size distribution for values of the monomer (YO) in the organic phase: (a) 25%; (b) 50%; (c) 75%; GMA: TRIM = 1: 1; por-organic phase; GMA :TRIM = 1:1; porogen =cyclohexanol-ogen =cyclohexanol-dodecan- 1-01 9/1 v/v (A), octan-Zone (B) dodecan-1-01 9/1 v/v [0,(a)] or octan-2-one [O, (b)] bonds is drawn as function of the amount of monomer in the organic phase for cyclohexanol-dodecan- 1-01 (a)and octan- 2-one (b)as the solvents.In both cases the number of unreacted double bonds was found to increase with increasing monomer concentration. This was previously found by Rosenberg et aL6 for the polymerization of TRIM, and is probably due to the effect that a larger amount of solvent introduces an improved mobility of the methacrylate substituents during the polymerization. Conclusions It can be seen from the results that the copolymerization of GMA and TRIM in different solvents yielded a wide variety of polymers. Some of the polymers, i.e. those made in octan- 2-one, n-butyl acetate, p-xylene, toluene and cyclohexanol- dodecan-1-01 9/1 v/v, show high porosity, while the beads made in benzonitrile and cyclohexanone were found to be non-porous.Both the B.E.T. surface area and the pore volume were found to decrease with increasing GMA:TRIM ratio in the monomer mixture when the cyclohexanol-dodecan- 1-01 mixture was used as the porogen. The pore volume also decreases with increasing monomer :porogen ratio in the organic phase. The B.E.T. surface area is a maximum at monomer :porogen = 1 :2. Using n-butyl acetate and octan-2- one as the porogen resins, substantial surface areas are achievable (170-175 m2 g-') while maintaining a GMA con- tent of 50%. We are indebted to the Netherlands Organization for Scientific Research (NWO) for their financial support and to Mr. C.Erkelens for his assistance with the NMR experiments. J. MATER. CHEM., 1991, VOL. 1 References (a) J. Kalal, F. Svec, E. Kalalova and Z. Radova, Angew. Makromol. Chem., 1976, 46, 93; (b) E. Kalalova, Z. Radova, J. Kalal and F. Svec, Eur. Polym. J., 1977, 13, 293; (c) E. Kalalovi, J. Kalal and F. Svec, Angew. Makromol. Chem., 1976, 54, 141; (d) F. Svec, H. Hrudkova, D. Horak and J. Kalal, Angew. Makromol. Chem., 1977, 63, 23; (e) F. Svec, D. Horak and J. Kalal, Angew. Makromol. Chem., 1977, 63, 37; (f)E. Kalalova, V. Beiglova, J. Kalal and F. Svec, Angew. Makromol. Chem., 1978, 72, 143; (g) F. Svec, J. Kalal, E. Kalalova and M. Marek, Angew. Makromol. Chem., 1980, 87, 95; (h) F. Svec, E. Kalalova, M. Tlusthkova and J. Kalal, Angew.Makromol. Chem., 1980,92, 133; (i) F. Svec and A. Jehlickova, Angew. Makromol. Chem., 1981, 99, 1I; (j)J. Kalal, E. Kalalovi, L. Jandova and F. Svec, Angew. Makromol. Chem., 1983, 115, 13; (k) F. Svec and A. Jehlickova, Angew. Makromol. Chem., 1984, 121, 127; (I) F. Svec, E. Kalalovi and J. Kalal, Angew. Makromol. Chem., 1985, 136, 183. (a) D. Lindsay and D. C.Sherrington, React. Polym., 1985, 3, 327; (b)D. Lindsay, D. C. Sherrington, J. Greig and R. Hancock, J. Chem. Soc., Chem. Commun., 1987, 1270; (c) D. Lindsay, D. C. Sherrington, J. Greig and R. Hancock, React. Polym., 1990, 12, 59; (d) D. Lindsay, D. C. Sherrington, J. Greig and R. Hancock, React. Polym., 1990, 12, 75. J. M. J. Frkchet, E. Bald and F. Svec, React. Polym., 1982, 1, 21. (a)D. Horik, F. Svec, M. Bleha and J. Kalal, Angew. Makromol. Chem., 1981, 95, 109; (b) D. Horak, F. Svec, M. Ilavsky, M. Bleha, J. Baldrian and J. Kalal, Angew. Makromol. Chem., 1981, 95, 117. R. L. Albright, React. Polym., 1986, 4, 155. (a)J-E. Rosenberg and P. Flodin, Macromolecules, 1986,19, 1543; (b)J-E. Rosenberg and P. Flodin, Macromolecules, 1987,20, 1518; (c) J-E. Rosenberg and P. Flodin, Macromolecules, 1987,20, 1522. M. Walenius and P. Flodin, Br. Polym. J., 1990, 23, 67. S. Brunauer, P. H. Emmett and E. Teller, J. Am. Chem. Soc., 1938, 60, 309. A. Guyot and M. Bartholin, Prog. Polym. Sci.,1982, 8, 277. Paper 0/05020B; Receiued 8th November, 1990

ISSN:0959-9428    

DOI:10.1039/JM9910100371

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(3), 375-379 High-resolution Solid-state and "'Sn Magic-angle Spinning Nuclear Magnetic Resonance Studies of Amorphous and Microcrystalline Layered Metal(iv) Hydrogenphosphates Michael J. Hudson* and Andrew D. Workman Department of Chemistry, University of Reading, Whiteknights, P.0. Box 224, Reading, Berkshire RG62AD, UK High-resolution solid-state 31Pand "'Sn magic-angle spinning nuclear magnetic resonance (MAS NMR) and Mossbauer spectroscopies have been used to probe the structure of amorphous and microcrystalline layered hydrogenphosphates and some of their intercalation compounds. A linear relationship between 6('lP) and the Allred-Rochow electronegativity of the metal atom was observed. In the case of both a and y forms of titanium and zirconium hydrogenphosphates, a linear relationship was seen between the isotropic chemical shift (aiso) and the number of bridging P-0-M oxygens (connectivity) of the phosphate groups.In a further study of the intercalation compounds of tin(1v) hydrogenphosphate monohydrate (SnP), the NMR data indicated there to be more electron transfer in the propylamine intercalation compound than is the case with €-N,N'-diethylbut-2- ene-l&diamine (NNBD) and that there is more electron transfer in the case of NNBD than in the ammonium intercalation compound. A linear relationship was observed when 6(11'Sn) was plotted as a function of 6(31P). The Sn Mossbauer spectrum of the host material was indicative of ionic character around the metal atom and similar to that of SnF,.Keywords: Magic-angle spinning nuclear magnetic resonance spectroscopy; Mossbauer spectroscopy; Intercal- ation; Metal( !v) hydrogenphosphate There is currently much interest in layered hydrogenphos- phates and their intercalation compounds. Unfortunately, detailed structural studies are difficult to carry out because the materials are often microcrystalline or amorphous. X-Ray diffraction gives information principally concerning the inter- layer spacing (dOo2).Clearly other probes for structural analy- sis are urgently needed. High-resolution solid-state 31Pand I19Sn MAS NMR and Mossbauer spectroscopies offer such additional technique^.'^^ Previous studies4 on a-tin(Iv) hydrogenphosphate mono- hydrate [a-Sn(HP04)2.H20] have indicated that it is a layered compound which has an interlayer spacing of 0.78 nm and is structurally similar to a-zirconium hydrogenphosphate, (ZrP).'g6 The presence of an undissociated P-0-H group has been established from incoherent inelastic neutron scattering data.7 Similarly, studies on y-Zr(H2P04)(P04) 2H20 (y-ZrP)8*9 indicate that this is indeed the molecular formula and that the compound possesses a layered structure with two different types of phosphorus.Studies on the iso- morphous compound, y-Ti(H2P04)(P04)-2H20 (y-Tip) con- firm that this has the same layered structure as y-ZrP." Metal hydrogenphosphates have been of interest in the past for their proton conducting behaviour," ion-exchange behav- iour, catalytic properties and their use in sensors.Various mono- and poly-amines have been intercalated and studied for their ion-exchange properties and use as proton conduc- tors. The amine intercalation compounds in question here have been studied in this respect and are well ~haracterised,~ but they have not been studied previously with both 31Pand '19Sn MAS NMR. The structures of the NNBD and propyl- amine intercalates (SnP-NNBD and SnP-PrA) are represented by Fig. 1 and 2: though recent work concludes that the alkyl chains of propylamine abut one another and do not overlap.12 Solid-state 31PMAS NMR provides a probe into the elec- tronic environment of the phosphorus and hence the relative degree of deprotonation of the phosphate group by the guest amine, a matter which has previously been an item of di~pute.~ A new intercalation compound of Ru(NH~)~C~~ with SnP is Fig.1 Idealised structure of the SnP-NNBD intercalation compound presented here, the result of work on removing the radio- nuclide lo6Ru from power-station effluent stream^.'^ The new material contains a single phase and is microcrystalline. The dis0 observed by MAS NMR is related to the electronic configuration of the metal centre, and so the presence of "'Sn provides a probe for observing the changing interactions within the layers upon intercalation of the guest species. Multinuclear solid-state MAS NMR can also be used for the comparison of structurally related materials.' Tin Mossbauer provides another method of obtaining complementary infor- mation concerning layered structures related to SnP.Experimental SnP was prepared by the method of Costantino and Gas- peroni,l4 ZrP by the method of Clearfield and Stynes" and Fig. 2 Idealised structure of the SnP-PrA intercalation compound TiP by that of Alberti et ~2.'~y-ZrP was prepared by the method of Clearfield et a/.,'' and y-TiP was prepared by the method of Alberti et ul." The amorphous tin, zirconium, and titanium hydrogenphosphates were prepared using the first step of the respective methods for preparing the crystalline compounds, that is by precipitating the metal hydrogenphos- phate by addition of the metal(1v) chloride to orthophosphoric acid and then drying in uucuoat 60 "C.The amine intercalation compounds were made by the method of Hudson and Rodrig- uez-Castellon.l8 The ammonium intercalation compound was prepared by contact of the host material with concentrated aqueous ammonia, containing three times the maximum exchange capacity of the ion exchanger. The product was then dried at 60 "C in uacuo and stored in a desiccator. Analysis using powder X-ray diffraction (using a Spectrolab series 3000 X-ray diffractometer) and MAS NMR showed no evidence of hydrolysis. CHN analysis gave the formula to be Sn(HP04)o .2( PO,) 1.8(NH2)1.8. Hexaammineruthenium(I1) dichloride was prepared by the method of Fergusson and Love.l9 Its intercalation compound was prepared by shaking SnP for 20 or 180 min with a degassed aqueous solution containing 300% of the total ion-exchange capacity of SnP.The final product was dried under vacuum at 110 "C. This compound was shown by powder X-ray diffraction using the doo2 reflection and by thermogravimetric analysis to be a single phase. The initial and final ruthenium concentrations in solution were determined by atomic absorption spectropho- tometry (using a Perkin-Elmer 1 lOOB atomic absorption spectrophotometer) with a diluent containing 10% HC1 and 0.5% lanthanum trichloride as masking agents, and showed that 100% of the exchange sites had been used. The 31PMAS NMR spectra were recorded at 121.4 MHz using a cross-polar pulse sequence (spectral bandwidth 100 kHz, relaxation delay 60 s, contact time 1.0 ms, spin rate cu.5000 Hz) and gated decoupling. The 'I9Sn MAS NMR spectra were recorded at 11 1.87 MHz, a cross-polar pulse sequence (spectral bandwidth 200 kHz, relaxation delay 0.5 s, contact time 5.0 ms, spin rate cu. 5000 Hz) with flip-back. These were run by the SERC NMR service of the Industrial Research Laboratories at Durham. Peak widths were meas- ured throughout as FWHM. The "'Sn NMR spectra were referenced to Sn(CH3)4, while the 31P NMR spectra were referenced to orthophosphoric acid. Sn Mossbauer analysis was carried out at the Demokritos National Research Centre for Physical Sciences, Athens, Greece. J. MATER. CHEM., 1991, VOL. 1 Results and Discussion Connectivity of Phosphate Groups The 31PNMR were clear and well resolved in the crystalline materials.Fig. 3 shows the linear relationship between the connectivity of the phosphorus (Q") and the isotropic chemical shifts for a-and y-zirconium (ZrP) and titanium hydrogen- phosphates (Tip). The nomenclature is the same as that used to describe condensed silicates and is characterised by the number of bridging oxygens of the phosphate group, n. In the case of a-ZrP, each phosphorus is connected to three metal centres via bridging oxygens and so has a connectivity of three. In the case of y-ZrP, there are equal amounts of two different phosphate groups, one connected to four Zr ions, Q4, while the other is a dihydrogenphosphate connected to two Zr centres and so is Q2." The direction of the gradient in Fig.3 is indicative of increasing summed P-0 bond strength2 with increasing connectivity. This technique was used to confirm that y-TiP has the same type of layered structure as y-ZrP. Electronegativity of the Metal Ion The isotropic chemical shifts of amorphous and a-structured metal(rv) hydrogenphosphates (M =Sn, Ti, Zr) plotted as a function of Allred-Rochow electronegativity of the metal centre are shown in Fig.4. It can be seen that there is a correlation between the electronegativity of the metal centre and isotropic chemical shift. The direction of the gradient shows increasing deshielding of the phosphorus attributable to growing .n electron density and the summed phosphate bond strength'i2 with decreasing electronegativity. The large differences in the chemical shifts of the phosphate groups with different connectivities have already been established in pre- vious =I \\ ~ -30 I 0 -35 1 2 3 4 5-Q Fig.3 6i,0(31P)expressed as a function of the connectivity of the phosphate groups for zirconium (+) and titanium (0)hydrogenphos-phates -251 4 I I I I 1 1 ,1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 electronegativity Fig.4 Si,0(3'P) expressed as a function of the electronegativity of the metal ion for a-(+) and amorphous (0)hydrogenphosphates J. MATER. CHEM., 1991, VOL. 1 Amorphous Compounds Using the above method of assignments for the connectivities of the phosphate groups, the amorphous compounds consist of predominantly Q3 phosphate groups, which are associated with the a-type structure.There are differences between the spectra, however, which can be divided into two categories, the width of the signal and the number of signals. The tin hydrogenphosphate spectrum (Table 1) consists of a single peak, ca. 26 ppm wide. The breadth is indicative of a low degree of crystallinity rather than hydrolysis. There was no evidence of a y-form of SnP. Only one resonance was observed in the "'Sn MAS NMR spectrum, indicating that there was interestingly no hydrolysis to Sn02 and this was confirmed using Mossbauer spectroscopy as discussed later. The spectrum of amorphous zirconium hydrogenphosphate (Fig. 5) consists of two peaks and a shoulder. The first (smaller) peak lies at -12.71 ppm, the second (and principal peak) at -20.35 ppm and the shoulder at -26 ppm.These values suggest that the material consists principally of the a-form with some y-phase as the minor component. The estimated breadth of the central peak is 17ppm. Again, the spectrum of amorphous titanium hydrogenphosphate (Fig. 6) consists of three resonances, -16.02, -18.76 and -30.22 ppm, though the bandwidth is now much reduced to ca. 5 ppm, indicating a higher degree of crystallinity. The peak at the highest resonance, -16.02 ppm is unusual, as it does not appear low enough for a Q2 phosphate. It may be that this is a polyhyd- rated hydrated Q3 phosphate with hydration leading to a reduction in the chemical shift. Conversely, as discussed later with the NNBD intercalation compound, the chemical shift in hydrophobic regions is greater than that in hydrated regions.The X-ray powder diffraction pattern of this com- Table 131P MAS NMR of some tin(1v) hydrogenphosphate com-pounds compound isotropic chemical shift (ppm) amorphous SnP -13.74 (26") SnP-Pra -7.35 SnP-NH: -6.94 -11.17 -14.19 SnP-NNBD -10.28 -15.35 Denotes width in ppm taken as FWHM. -Tv~l-n-r-~-~--10 -20 -30 -40 6 (PPm) Fig.6 31P MAS NMR spectrum of amorphous titanium hydrogen- phosphate pound shows no structure at This can be rationalised on the basis of the compound having a high degree of order but only over a very short range, such that is undetectable by X-ray diffraction. This, therefore, is a truly microcrystalline compound. In the two spectra of multiple resonances, there- fore, most peaks can be assigned to a particular phosphate connectivity, the closest to zero being Q2, the next Q3 and the last Q'.31PMAS NMR of some Intercalation Compounds of SnP The 31PNMR spectrum of crystalline SnP is shown in Fig. 7 to be clear and well resolved, as is the case with the other spectra, which are listed in Table 1. The principal peak occurs at -13.6 ppm and is assigned to phosphorus with an undis- sociated P-0- H group.7 No other phosphate connectivities are seen within this spectrum and comparison with the spectrum of the amorphous compound shows a decrease in linewidth which is related to the increase in crystallinity of the material. The intercalation compound with propylamine analyses as Sn(CH3CH2CH2NH2)2(HP04)2and has a similar structure to SnP, but the dOo2interlayer spacing has been increased to 1.672 nm. The 31PMAS NMR spectrum shows only one type of phosphorus with a chemical shift of -7.34 ppm.The single peak confirms that all of the Q3phosphate groups are involved with bonding to the guest molecule as implied by the 2: 1 amine to tin ratio. It has been shown1V2 that in 31P MAS NMR the isotropic chemical shift moves upfield as the P-0 I""I""l""~""I"I 1 I I I 0 -10 -20 -30 100 0 -100 6 (PPm) 6 (PPW Fig. 531P MAS NMR spectrum of amorphous zirconium hydrogen- Fig.7 31P MAS NMR spectrum of crystalline a-tin hydrogen-phosphate phosphate bond strength increases, corresponding to a decreased para- magnetic term.There is some degree of proton transfer from the phosphate group to the amine, the decrease of 6.5ppm being associated with an increase in negative charge on P-0"-(~21).However, if the free electron density were to result in an increase in the double-bond character of the phosphate group, an opposite movement in the chemical shift would be seen. The movement observed can possibly be explained on the basis of charge localisation on the oxygen, encouraged by hydrogen bonding between the protonated amine and the layer. The phosphorus is, therefore, more shielded and the peak moves accordingly. The spectrum of the ammonium intercalation compound (SnP-Amm) shows three resonances with the major peak at -1 1.1 7 ppm.The small peak at -6.95 ppm is attributable to either a phosphate bonded to both an ammonium ion and water or a diammoniated phosphate, while that at -14.19 ppm is due to an unexchanged phosphate group. The latter has a chemical shift lower than that of the parent tin hydrogenphosphate. This is due to a different extent of hydration with the associated change in hydrogen bonding. The pK, of the ammonium ion is 9.3.21 The extent of proton transfer from the host compound to the guest is larger than is the case with water and the resonance moves for the same reasons as given above. The move, however, is not as great as is the case of SnP-PrA, which is a stronger base (pK,= 10.71 for propylamine).21 The "N MAS NMR spectrum of the ammonium intercalation compound shows only one peak, indicative of only one nitrogen environment.With respect to the intercalation compound with the diamine NNBD as guest molecule, structural studies have already been published.22 The compound analyses as Sn(NNBD)o,73(HP04)2.Like the propylamine intercalation compound, the structure is closely related to that of SnP but with an interlayer spacing of 1.35 nm. Since the ratio of NNBD to Sn is less than unity, some of the 0-H groups are not involved with binding to the amine groups. There are two separate peaks in the spectrum with chemical shifts of -10.3 and -15.4 ppm, confirming the two phosphate environ- ments. This is intermediate between the ammonium intercal- ation compound and SnP, suggesting that the protonation is greater than in SnP but less than in the ammonium or propylamine intercalation compounds, but not as expected from the pK, of the base (the first pK, of NNBD is greater than 11.8).This could be connected to the relatively bulky groups on the diamine holding the nitrogen away from the active site on the layer (area 21.4 A2).23There are parallels in solvent extraction studies and similar phenomena are observed in the active site of enzyme^.^^,^' The second peak at -15.4 ppm is interesting as it is lower than the shift in SnP itself. This low chemical shift is probably due to the shielding of the phosphate active site by the bulky alkyl groups of the diamine keeping water away from the 0-H group by the creation of a hydrophobic environment, as shown in Fig.2. Metal hydrogenphosphates were found26 to exchange radioactively labelled phosphate rapidly from aque- ous solution and the pendant P-0 bonds bend out to accommodate large guest species leaving some hydrophobic regions. l19Sn MAS NMR of SnP Intercalation Compounds Fig. 8 is a plot of 6iso(119Sn) as a function of C~~,,(~~P). It can be seen that there is a linear correlation between these two terms. As the charge on the phosphate group increases so 6(l19Sn) falls. This movement in the isotropic chemical shift is attributable to increasing shielding of the tin nucleus by J. MATER. CHEM., 1991, VOL. 1 -a "'Sn isotropic chemical shift (ppm) m Fig.8 6,,(119Sn) expressed as a function of 6i,,(31P) in reference to Sn1V(P04)2X2,[X =H, Ru(NH,),, protonated amines]: (a) SnP; (b) SnP-Ru(NH,),, 20 min intercalation time; (c) SnP-Ru(NH,),, 180 min intercalation time; (d) SnP-hexylamine intercalate; (e) SnP-Pra; (f) SnP-octylamine intercalate increasing electron density.A previous has shown that there are correlations between 6(lI9Sn) and the amount of electron density around the Sn nucleus and confirms the direction of the change. Mossbauer Spectra The Mossbauer spectrum for SnP is shown in Fig. 9; the spectra for all the intercalation compounds are identical. It can be seen that the value of 6 is -0.361 f0.005 mm s-' with a half-height width of 1.05 mm s-'. These values resemble those for SnF,28 which implies that the tin is ionic in character.By comparison, the Sn Mossbauer of Sn02 (67 0.0015 mm s-') has a larger half-height width of 2.52 mm s-. The tin Mossbauer spectrum of the hexaammineruthenium(1r) intercalation compound of SnP shows little difference from the spectrum of the host material, except a slight movement in 6 towards that of the more covalent Sn-0 bond in Sn02. The spectra for all of the other compounds were identical indicating that the principal changes may be better studied using the NMR techniques. Conclusions There are linear correlations between the chemical shift of the phosphorus and both the connectivity of the phosphate group and the electronegativity of the metal ion. The amorphous compounds are shown to consist of predominantly Q3phos-phate groups.A linear correlation was observed with 6iso('19Sn) was plotted as a function of 6i,0(31P). C .-.i! 98.47-5 97.69-96.92 .. I I I I96.141 '-' I , I , 11 J. MATER. CHEM., 1991, VOL. 1 We are grateful for the funds provided by the Department of the Environment for A. D .W. The results may be used in the formulation of Government policy, but at this stage do not necessarily represent Government policy. We also wish to thank Dr. D. C. Apperley at the SERC NMR facilities of the University of Durham. Professor G. Alberti kindly gave us the sample of y-ZrP and the a-ZrP, a-Tip, y-TiP and their amorphous counterparts were donated by Mr. R. J. W. Adams. Amorphous tin hydrogenphosphate was provided by Mr.P. Sylvester. The Sn Mossbauer spectra were run by Dr. D. Petridis of Demokritos in Athens. 10 11 12 13 14 15 16 17 G. Alberti, U. Costantino and M. L. Luciani Giovagnotti, J. Inorg. Nucl. Chem., 1979, 41, 643. M. Casciola and D. Bianchi, Solid State Ionics, 1985, 17, 287. G. Alberti, personal communication, 199 1. Directorate of Fisheries Research, Lowestoft (Annual Sub-missions), 1986. U. Costantino and A. Gasperoni, J. Chromatogr., 1970, 51, 289. A. Clearfield and J. A. Stynes, J. Inorg. Nucl. Chem., 1964, 26, 117. G. Alberti, P. Cardini Galli, U. Costantino and E. Torracca, J. Inorg. Nucl. Chem., 1967, 29, 571. A. Clearfield, R. H. Blessing and J. A. Stynes, J. Inorg. Nucl. Chem., 1968,30, 2249. 18 M. J. Hudson and E. Rodriguez-Castellon, J.Incl. Phenom., 1989, 7, 301. References 19 J. E. Fergusson and J. L. Love, Inorg. Synth., 13, 1971, 208. F. Taulelle, C. Sanchez, J. Livage, A. Lachagar and Y. Piffard, J. Phys. Chem. Solids, 1988, 49, 229. A. K. Cheetham, N. J. Clayden, C. M. Dobson and R. J. B. Jakeman, J. Chem. SOC., Chem. Commun., 1986, 195. I. L. Mudrakovskii, V. P. Shmackova, N. S. Kotsarenko and V. M. Mastikhin, J. Phys. Chem. Solids, 1986,47, 335. M. J. Hudson, E. Rodriguez, P. Sylvester, A. Jiminez-Lopez and P. Olivera-Pastor, Hydrometallurgy, 1990, 24, 77. A. Clearfield and G. D. Smith, Inorg. Chem., 1969, 8,431. A. Clearfield and J. M. Troupe, Inorg. Chem., 1977, 16, 331 1. D. J. Jones, J. Penfold, J. Tomkinson and J. Roziere, J. Mol. Structure, 1989, 197, 113. N. J. Clayden, J. Chem. SOC., Dalton Trans., 1987, 77. 20 21 22 23 24 25 26 27 28 R. J. W. Adams, personal communication, 1991. Handbook of Chemistry and Physics, ed. R. C. Weast, CRC Press, Boca Raton, p. 33431. M. J. Hudson and P. Sylvester, Solid State Ionics, 1989, 35, 73. I. Fotheringham, M.Sc. Thesis, 1986, University of Reading, UK. M. J. Hudson, Hydrometallurgy, 1982, 9, 149. L. Stryer, Biochemistry, W. H. Freeman, San Francisco, 1989. A. Clearfield, personal communication, 1991. S. J. Blunden, D. Searle and P. J. Smith, Inorg. Chim. Acta, 1986, 116, L31. V. T. Goldanskii, V. Ya. Rochew, V. V. Khraphov, B. E. Dzevitskii and V. F. Sukhoverkhov, Isv. Sib. Otd. Akad. Nauk., Ser. Khim. Nauk., 1968, 22. C. Y. Oritz-Avila and A. Clearfield, J. Chem. Soc., Dalton Trans., 1989, 1617. Paper 01050361; Received 9th November, 1990

ISSN:0959-9428    

DOI:10.1039/JM9910100375

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991,1(3), 381-386 38 1 Electrochromic Nb,O, and Nb,O,/Silicone Composite Thin Films prepared by Sol-Gel Processing G. Rob Lee and Joe A. Crayston* Department of Chemistry, Purdie Building, University of St Andrews, Fife KY16 9ST, UK Alcoholysis of NbCI, in EtOH gives a viscous solution of NbCI,(OEt),-, which may be used to spin-coat optically transparent electrodes (indium tin oxide, ITO). The electrode is converted to Nb205 by immersion in 1 mol dm-3 H2S04.Electron microscopic examination of the resulting 5-10 pm thick films revealed extensive shrinkage and cracking on gel drying. These films proved to be electrochromic (A,,, ca. 800 nm) but unstable to voltammetric cycling in MeCN-(0.5 mol dm-3)LiCI04. Greater stability (>30 cycles) was achieved with a composite Nb205/ silicone electrode, prepared from a solution containing Nb :Si :H20 in the molar ratio 5 : 13:82.This increase in durability was achieved with no sacrifice in electrochromic efficiency. The electrochromic colouration efficiency (6 cm2 C-') compares favourably with that for sputtered Nb205 films. The colouration-bleaching cycle of the composite is complete in <40s, and the electrochromical kinetics of colouration and bleaching follow a diffusional model with DLi=6 x lo-' cm2s-' for Nb20, and 3.2 x lo-' cm2s-' for the composite. The mechanism of the electrochromic response and its decay are discussed. Keywords: Thin film; Electrochromism; Sol-gel processing; Niobium oxide An electrochromic material is one in which a visible absorp- tion band can be introduced reversibly by the passage of current through the material, or by application of an electric field. These materials are of interest for display devices.' A major requirement of an electrochromic material is that it exhibits mixed conduction, i.e. both electronic and ionic conductivity.Typically, a metal redox centre is generated by injecting electrons, and for charge neutrality to be preserved cations must be admitted into the material. Since it is usually the movement of ions which is rate-limiting, materials with fast-ion conduction at room temperature are desired (usually with H+, Li' or Na' ions).' Often such materials are amorphous and of low density so as to provide an open framework for rapid ionic diffusion.Amorphous materials also have the advantages of flexibility of composition, high concentrations of vacancies for ionic motion and are generally more easily prepared as the thin coatings necessary for display devices. Materials which have been investigated include Wo3,3 Prussian blue,4 and organic molecules such as alkyl viologen~.~ Colouration in transition-metal oxides such as W03 is believed to arise from intervalent charge-transfer bands at W5+ sites, and whilst W03 has been extensively studied, other oxides, such as Nb205 have received much less attention. Thin Nb205 films have been prepared by thermal6 (500 "C) or anodic7 oxidation of the metal, and by oxidation of sputtered NbN,.' Unfortunately, the metal oxidation routes preclude the use of the electrochromic films in most display devices where the metal oxide is sandwiched between optically transparent electrodes; however, they can be used in reflec- tance mode.Finally, it is also possible to sputter Nb205 thin fiims.9 In this paper we describe a new route to electrochromic Nb205 films using the sol-gel method. This method involves the controlled hydrolysis of high-purity metal alkoxides. It has the advantages of control of porosity and structure, convenience and the ability to coat large objects with adherent The method has already been employed successfully in the synthesis of V205, TiO2I2 and Wo3I3 electrochromic devices. The sol-gel process can only be used to make crack- free films of thickness 10.5 pm.Typical electrochromic efficiencies are 10 mC cm-' for V205, corresponding to an absorbance of ca. 0.5 in a film of thickness 0.5 pm." For greater display densities a method for producing thick (5 pm), crack-free electrochromic films with fast response times would be desirable. We report the fabrication of thick-film Nb205 electrodes and their stabilisation by overcoating with a sili- cone-based gel to give a more durable composite electrode. This composite is related to the organically modified ceramics or silicates (ORMOCERS or ORMOSILS) introduced by Schmidt.I4 Experimental Electrode Fabrication Nb2O5/I??3 Electrode A5cmxlcmx0.3cm IT0 glass electrode was ultrasonically cleaned then coated with a 50 mm3 drop of 1 mol dm-3 NbC15 (Aldrich) in spectroscopic EtOH (Aldrich), and this was then spun at 1250rpm for 3s on an upturned Pine Electrodes Rotator, so that 2 cm2 was covered. The electrode was then placed in 1 mol dm-3 HzS04 for 1 min for complete hydrolysis and gelation.This gave an even coating ca. 5 pm thick as measured by micrometer and scanning electron microscopy (SEM). The edges of the film were ca. 5-10 pm thicker than the central part. Excess water was very gently blown off with a low-velocity air stream at an angle of ca. 10" to the gel. The edges were stripped off and replaced by silicone grease in order to mask the exposed underlying ITO. The final Nb205/IT0 electrode area was 1.2 cm'. NbzOs/Silicone Composite Electrode An Nbz05/IT0 electrode was prepared as above, but without the silicone grease applied to the exposed ITO, and was then immersed for 3 s in a composite Si-containing sol.The com- posite sol contained 10.0 cm3 0.4 mol dm-3 NbCIS (Aldrich) in spectroscopic-grade EtOH (Aldrich), 2.3 cm3 3-(2,3-epoxy- propoxy)propyl(trimethoxy)silane (Glymo) (96% Aldrich) and 1.17 cm3 deionised water. The resulting solution thus con- tained Nb :Si :H20in the molar ratios 5 :13:82. The electrode was then spun at 1250 rpm for 30 s to remove excess sol and dried at 60 "C for 3 h. Electrode Characterization SEM was performed on a JEOL JSM 35CF (15 keV) at the Gatty Marine Lab., which had a scanning EDAX attachment. Further characterization included Fourier transform infrared (FTIR) (Perkin-Elmer 1710) and Raman spectra (Spex Rama- log).Chloride analysis was performed according to a standard digestion and potentiometric titration method." Niobium analysis was performed according to a standard combustion met hod. ' Electrochemical and Optical Measurements Cyclic voltammetry and chronoamperometry were carried out using a combined pulse/scan generator and potentiostat (Pine Instruments RDE4 or homebuilt) and a Graphtec XY recorder. The three-electrode cell configuration is shown in Fig. l(a). Electrolytes used were, 1 mol dmd3 H2S04 or 0.5 mol dm-3 BuiNPF6 (Aldrich recrystallised) in HPLC grade acetonitrile (Aldrich). All potentials are reported us. SCE (standard calomel electrode).The cell was N2 purged before use. Chronoabsorptiometry data and spectra were recorded using the cell depicted in Fig. l(b), which could be inserted directly into the cell block of a Philips PU8720 UV-VIS Spec- trometer. Here a Pt quasireference was used instead of the SCE. Results and Discussion Nbz05Films Nb205 films were prepared by the spin coating onto IT0 optically transparent electrodes of 1 mol dm- NbC1,-EtOH solution, which was subsequently hydrolysed in acid. The NbC1,-EtOH solution has been shown to contain chiefly NbCl(OEt), and NbC12(OEt)3.'6 Thus the hydrolysis follows: NbCl,(OEt),-,+ 5H20++Nb205 +(5 -x)EtOH+xHCl X-Ray diffraction, EDAX, C1- and Nb analysis confirmed that the resulting film was hydrated amorphous Nb,O,; no evidence could be found for remaining Nb-Cl bonds after hydrolysis.The FTIR spectrum of the gel, after drying at 150°C, showed that all the EtOH and EtO-had been removed by the hydrolysis reaction and subsequent drying. A very broad band (600-700cm-') due to antisymmetric Nb-0 stretching in the distorted Nb06 octahedral7 is observed. A shoulder at ca. 900cm-' may be assigned to v(Nb=O). Three weak bands observed at 1180, 1120 and 1050 cm-' correspond to bands claimed to be diagnostic of ageing of Nb oxide precipitates.I8 Specifically, the 1180 and 1050cm-' bands are assigned to the Nb-(OH)-Nb group.'* The Raman spectrum of the as-formed gels contain bands due to the EtO group, for example v(C0) at 1090 cm-l. Weak bands below 1000 cm -(932,760 465 cm- I) correspond to those of B-Nb2O5.I7 No bands due to Nb-C1 stretching insulator -cuvette (a) light (b1 Fig. 1 Cells used to study sol-gel-processed Nb,05 films: (a)electro-chemical measurements; (b) absorbance measurements using the same electrode assembly J.MATER. CHEM., 1991, VOL. 1 vibrations were observed. After heating the gel to 900 "Cthe Raman spectrum is identical to that of H-Nb205. After these thick films (5-10 pm) had been dried at room temperature it was found that substantial cracking and peeling occurred, pulling the film from the electrode. Electron microscopy revealed that the gels had suffered shrinkage and cracking to give isolated islands of ca. 10 pm in diameter [Fig. 2(a)]. It is known that such shrinkage in dried sol-gel processed films is caused by capillary stresses due to surface tension." For this reason the soft hydrated amorphous Nb205 films proved unstable to potential cycling in cells containing non-aqueous solvent.SEM upon cycled films that failed [Fig. 2(b)J shows a similar set of cracking and islands of ca. 10 pm diameter to those found in the dried films. The films proved to be a little more stable when 1 mol dm-3 H2S04 (aq.) was used as the solvent/electrolyte. In order to increase the longevity of the electrochromic films, several modifications of the procedure were attempted. (a)Using the method of Alquier et a!.," 1 mol dm-3 NbC15 in ethylene glycol was used as the sol precursor to gelation. Although electrochromic films were produced by this method, their durability was actually less than that of the Nb205*xH20 films described above.(b) Mixed Nb-Si gels were produced by the addition of the trialkoxysilane, 3-(2,3-epoxypropoxy)propyl(trimethoxy)silane(Glymo) to the niobium chloroalkoxide precursor, followed by hydrolysis. Glymo is known to form flexible rubbery gels suitable for applications such as soft contact-lens material^.'^ However, as the niobium chloroalkoxides have a widely different hydrolysis rate to Glymo it was found that at Nb :Si ratios of >I hydrolysis led to particulate Nb205 formation, coated in a cracked hydrolysed Glymo film, whereas at Nb :Si ratios of I1, transparent gels were obtained with dramatically reduced electronic and ionic conductivities.No electrochromic response was observed for any of these films. (c) Finally the composite system, described below, gave more robust electro- chemically active films. Nb205Films with Glymo Composite A 0.05 cm3 drop of 1 mol dm-3 NbC15 in spectroscopic-grade EtOH was spread over 2cm2 of IT0 glass, spun on an upturned rotating-disc electrode, typically at 1250 rpm for 3 s. The IT0 electrode was then immersed in 1 mol dmm3 H2S04 for 1 min to catalyse the hydrolysis and gelation of the sol. The second coating of Nb :Si :H20 solution of molar ratio 5 :13 :82 was applied by dipping the electrode in the solution, spinning for 3 s at 1250 rpm and drying at 60 "C for 3 h. These films showed significantly greater durability in non- aqueous solvents but broke up on immersion in water/acid.The films were rougher in morphology [Fig. 2(c)] than the uncoated Nb205 films but did not break up on drying. EDAX revealed that the Si in the second coating had penetrated throughout the full 5 pm thickness of the film; thus, we prefer to describe the electrode as a composite rather than a hetero- geneous two-layer structure. Electrochemical failure was accompanied by the same type of microcracking as in the uncoated Nb205 films [Fig. 2(d)]. During drying the role of the Glymo may be one or both of the following: (1) it provides flexibility during the drying process thus preventing large capillary forces from cracking the material; (2) the epoxide chains on the silicon may act as a hydrophobic barrier, forming less porous pore walls and thus trapping H20 in the metal oxide and hence slowing the drying process.A full range of composite molar ratios were tested for the second coating, as were spinning speeds and times; however, most of the films thus produced failed for a number of reasons: reproducibility, film cracking and loss of electrochromic properties. J. MATER. CHEM., 1991, VOL. 1 Fig. 2 SEM photographs of (a) Nb205*xH20 film after drying at ambient temperature for 4 h; (b) Nb20S*xH20 film after failure during electrochemical potential cycling (five cycles) in 0.5 mol dm-3 LiC10,MeCN; (c) Nb20=,*xH20 film with Nb/Si/H20 (8: 12:80 mole ratio) composite after drying at 60"C for 3 h; (d) film as in (c) after failure during prolonged potential cycling in 0.5 mol dm-3 LiClO,/MeCN Cyclic Voltammetry Cyclic voltammograms (CVs) run at 100 mV s-l of both the coated and uncoated films on IT0 electrodes in 0.5 mol dm-3 LiC104-MeCN are shown in Fig.3(a)and 3(b).The potential range covers the potentials controlling the colouration and bleaching processes. The onset of the blue colouration is observed when the potential reaches -0.4 V. Both CVs exhibit a very large cathodic current at potentials less than -0.4V but this is presumably due to H2 evolution at the ITO/ 'L'& .-Nb205 *xH20 interface rather than colouration current. On +1 .o +1 .o the reverse sweep an anodic peak is observed at -0.5 V vs.SCE , Icassociated with the bleaching of the electrode. Roughly 50% of the composite electrodes required 2-3 cycles of 'breaking in' or 'condhbning' before maximum contrast was observed between the bleached and coloured absorption.This is a common phenomenon seen in electrochromic films due to the solvent opening of the pores. There' appeared to be no apparent morphological differences between the samples; how- ever, the composite sol has to diffuse through the pores and undergo gelation within them. This would give rise to smaller pore sizes and presumably partly closed pores, which would need to be broken into. Similar CVs are observed for the uncoated Nb205 films in 1 mol dm-3 H2S04(aq); however, the coated films are unstable in aqueous media. The optical absorption changes on colouration of the Fig.3 Cyclic voltammetry (100mVs-') of (a) Nb205 film inNb205 films at -0.875 V (Fig.4) are dominated by the broad 0.5 mol dm-3 LiClO,/CH,CN; (b) composite Nb,O,/silicone elec- structureless band which peaks at A >800 nm. This absorption trode. The cross indicates zero current and potential 384 0.6 h .-8 0.4 C3 -? Y 0.3 C 9 sntu '0.2 0.1 10.0 400 450 500 550 600 650 700 750 800 Ilnm Fig.4 UV-VIS spectral changes in NbzO5/ITO absorbance in (a) the bleached state (0 V us. SCE) and (b) the coloured state (-0.875 V us. SCE). Medium 1.0 mol dm-3 H2S04, film thickness ca. 5 pm is probably associated with Nb bronze formation xLi +xe-+Nb2OSS Li,Nb20S Simultaneous injection of electrons and cations is necessary to maintain charge neutrality.The reaction is related to the same process in crystalline Nb bronzes.20 The kinetics of intercoversion between bleached and coloured states is described below. Durability For the uncoated Nb20s films the absorbance maximum of the coloured state (-0.875 V) decays very rapidly with the number of cycles (100 mV s-') as shown in Fig. 5, while simultaneously the background absorbance of the bleached 0.29 0 Oa30!0.28 + 0.27..-E 0.26. C 0.25. x E 0.24' a, 0.231 /.A n / 12345678910. cycles Fig. 5 Durability of the electrochromic response. Dependence of the absorbance at 800nm in the coloured state (upper traces) and the bleached state (lower traces) as a function of the number of cycles: pure Nb205 electrode (--a); composite Nb,O,/silicone electrode (-sample 1; ----sample 2).Media: Nb205, 1 moldm-' H2S04; composite Nb,O,/silicone, 0.5 mol dm-3 LiClO,/MeCN J. MATER. CHEM., 1991, VOL. 1 state increases, possibly as a result of incomplete bleaching, or more likely owing to the increased light-scattering proper- ties of the cracked film. The two states become indistinguish- able after only four cycles, and the electrode suffers the dramatic peeling referred to earlier [Fig. 2(b)]. As already noted, different behaviour is observed for the Glymo Nb2OS composite film (Fig. 5). Here, there is often a rise in absorbance of the coloured state to reach a maximum at about the third cycle (breaking in) followed by a steady decay. Alternatively, a steady decay from the initial absorbance maximum on the first cycle is observed.For both cases after prolonged cycling (typically 30 cycles) the two states are indistinguishable and the electrode is deemed to have failed. Visually, at this point, only the outer edges of the electrode have been observed to retain any of the blue bronze formation. The morphology of the film suggests cracking and peeling [Fig. 2(d)] in a similar way to that of the uncoated electrode. We propose two possible mechanisms for the rapid failure of these films: (i) peeling due to H2 evolution on the cathodic sweep; (ii) rapid dehydration of the hydrated oxide layer, either by H20 diffusion into the non-aqueous solvent, or by H20 binding as water of hydration to the Li+ ions cycling in and out of the film.It is interesting to note that uncycled Nb20S films left in MeCN for a few hours will, upon removal, show similar cracking patterns to those shown in Fig. 2(b), lending credence to (ii). Those left in sulphuric acid tended to peel in large area pieces from the IT0 implying that here dehydration of the oxide film was not so much of a problem as the formation of a water layer between the film and the ITO. That the composite dramatically increases cycling lifetime can thus be explained. If (i) is operating then the composite may be slowing down the diffusion of H+ (however, we show later that the diffusional coefficients for Li' in the two electrodes are comparable); if (ii) is operating then the com- posite may be serving as a barrier to H20/MeCN exchange and dehydration, while still allowing Li' into the film.If this is so then it may be possible to eliminate film failure during cycling by the predrying of the films at a reduced drying rate. This is achieved (and commonly in industry) by drying the films in a stream of air saturated with the solvent used in the sol. Kinetics of NbzOSElectrocbromic Films in LiCIO,/MeCN The kinetics of colouration and bleaching may be studied by stepping the potential between the bleached state (OV us. SCE) and the coloured state (-0.8 V us. SCE) and back, whilst monitoring the time dependence of the current (chronoamper- ometry) and the absorbance at 800 nm (chronoabsorptio-metry).Chronoamperometric results for the Nb2OS and the Nb205 silicone composite electrodes [Fig. 6(a) and (b)] revealed that for both electrodes the full cycle is complete in ca. 40 s. In each case the bleaching current decays considerably faster than the colouration current. Furthermore, the colour- ation charge (area under the i-t curve) is much larger than the bleaching charge. This is probably due to hydrogen evolution on the cathodic potential excursion. Note also that the colouration current rise time of the composite is very sluggish, owing to the high initial electronic resistance of the bleached composite film. In addition, chronoabsorptiometric data are shown for the composite electrode [Fig.6(c)].The Coulombic efficiency (q)for electrodes is given as: J. MATER. CHEM., 1991, VOL. 1 -0.8 V Fig.6 Current-time curve for (a) pure Nb205 and (b) composite Nb,O,/silicone electrodes on stepping the potential from the bleached state (0 V us. SCE) to the coloured state (-0.8 V us. SCE) as in the left- hand side of the diagram, and the back to the bleached state (right- hand side). 0.5 mol dm -LiClO,/MeCN; (c) chronoabsorptiometric curve (absorbance us. time) for the composite Nb,O,/silicone electrode where AA is the difference in absorbance between bleached and coloured states, a is the area of the electrode (cm2) and Q is the total charge passed during colouration. For the composite electrode the charge passed during colouration is 4.9 mC.Thus the Coulombic efficiency for this 0.6 cm2 electrode is 6 cm2 C-'. This value is comparable to the ca. 10 cm2 C-' reported earlier for sputtered Nb205.21 Colouration Kinetics Fig. 7 shows for both the Nb205 and the composite films that the decaying part of colouration i-t curve follows a t-f. dependence. For the composite electrode, during the initial few seconds when the colouration current rise-time is sluggish owing to high initial resistance, the curve deviates from this t-f. dependence. If small overvoltages were employed the t-f. 0:O 0:2 0.4 0:s 0:s 1:O 1.2 1.4 116 f1/2/S-1/2 Fig. 7 Typical i-t-* plots for the decaying portion of the colouration current transients of pure Nb20, (m) and composite electrodes (0) dependence is explained by taking Li' injection at the film/ electrolyte interface as the rate-determining step." However, since we used large overvoltages the current is limited by the diffusion of Li' within the film. The diffusion coefficient (DLi) may be obtained from the slope of the i us. t-f.graphs because in diffusion-controlled systems the Cottrell equation applies: Q Df.d2i( t)=tot.tl-dtf.' D Qtot is the total charge (in C) passed during the colouration step, t is the time taken (s) and d is the thickness of the film (cm). Such plots should extrapolate to the origin; however, the plot for the composite electrode (Fig. 7) intercepts the negative current axis at infinite times. This may reflect lower than expected current values owing to the resistance of the electrode.For each of the two types of electrode the derived diffusion coefficients are independent of the particular cycle taken. These results (Table 1) suggest that diffusion is slower in the com- posite than in the pure Nb205 film. A possible explanation for this is that the silicone layer has diffused and gelled in the very same pores and grain boundaries through which the Li' is trying to move. Finally, the diffusion coefficients for both elec- trodes compare favourably (Table 1) with those obtained for Li20/Nb205 films' and sputtered Nb205 films.g Absorbance us. time transients for the composite electrode may be analysed by assuming that the absorbance is pro- portional to the charge passed.Thus by integrating eqn.(2) we obtain A= 2vSt' + A0 (3) where q is the electrochromic efficiency, S is the slope of i (t-*) graph and A. is the absorbance at t =0. However, plotting A us. tf.gives a sigmoidal curve indicating a more complex depen- den~e,~~similar to that obtained for sputtered films.' Kinetics of Bleaching According to Faughnan et bleaching of W03 films should be enhanced by migration of ions through the field developed across the electrochromic film, and thus the current is expected to decay as t3 rather than as the tf.of the diffusional model. log i us. log t plots for bleaching of our uncoated and composite- coated films appear to show that for the uncoated Nb205 a diffusional process is occurring [Fig.8(a)], whereas for the composite film a migratory process is the dominant form [Fig. 8(b)].The reason for this difference is not clear and further work will be directed at trying to answer this point. The log-log plots should indicate the point (tf) at which the film is completely bleached,24 but for our electrodes the tfpoint is not clearly defined. However, we may obtain a minimum value for tffrom the point at which the curve deviates from the -3 slope. This will give a maximum value for D. In the migration model this point is given by2'*24 where p is the volume charge density of the cations (obtained from the charge per cm3passed during colouration), d is film thickness, E, is the relative permittivity of Nb205, E, is the Table 1 Derived Li' diffusion coefficients for the colouration cycle DLi+/cm2s-' pure Nb20, composite Nb,O,/silicone 5050 Li20/Nb205* sputtered Nb205' 6.0x 3.2 x 3.4 x 1.5 x lo-* J.MATER. CHEM., 1991, VOL. 1 Table 2 Derived Li+ diffusion coefficients for the bleaching cycle in cm2 s-pure Nb205 diffusion model migration model 1.0x 10-7 3.5 x -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 log (t/s) -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 log (Us) Fig. 8 log-log plots of the bleaching current vs. time for: (a)the pure Nbz05 electrode (0, 1st cycle; +, 2nd cycle); (b) the composite electrode (0,1st cycle; + is 3rd and is 5th) permittivity of free space, p is the ion mobility in the film (D=RTp/F) and Vis the applied bleaching voltage across the film.For example, using p = 1.75 C cm- 3, d =5 x cm, E, = 1 15, E, =8.85 x 10-l4 F cm-V= 0.8875 V, tf=2.5 s we obtain the values of D shown in Table2. These are compared to values obtained by a simple diffusional model: d2 tf =5 Clearly the diffusional model appears to be more in keeping with the data obtained from the colouration measurements. Assuming absorbance is proportional to the charge passed and integrating the current-time relations for both models, we should find for the diffusional model that Acct* and for the migration model that Acct*. However, a log (A-A,) us. log t plot does not reflect either of these simple relationship^,^^ and must await a better model. Conclusions We have shown that it is possible to make large-area thick- film displays from sol-gel-processed Nb205, but that these composite Nb,O,/silicone diffusion model migration model 1.0 x 10-7 5.8 x cells are extremely unstable to cycling and thus their durability is limited.It is possible to create more durable cells by the application of a mixed Nb/Si second coating. This coating does not appear to detract from the electrochromic properties of the Nb205 films, as these composite films show similar Li colouration diffusion properties to the uncoated film and + similar Coulombic efficiencies to earlier reported Nb205 films.2' This would indicate that the durability of thick sol- gel-processed films can be increased without the loss of their electrochromic properties, i.e. Coulombic efficiencies and response times are similar to those of Nb205 films created by sputtering techniques.We thank BP for a research studentship (G.R.L.), Irwin Davidson for the SEM photographs, Dr. T. J. Dines (Dundee University) for providing the Raman spectra and Dr. Ian Thompson (BP)for helpful discussions. References 1 M. Green and K. Kang, Displays, 1988, 9(4), 166. 2 W. A. England, M. G. Cross, A. Hamnett, P. J. Wiseman and J. B. Goodenough, Solid State Ionics, 1980, 1,231. 3 B. Reichman and A. J. Bard, J. Electrochem. SOC., 1979,126, 583. 4 K. Itaya and I. Uchida, Acc. Chem. Res., 1986, 19, 162. 5 R. E. Malpas and A. J. Bard, Anal. Chem., 1980,52, 109. 6 B. Reichman and A. J. Bard, J. Electrochem. SOC., 1980,127,241. 7 (a) S.Rigo and J. Siejka, Solid State Commun., 1974, 15, 259; (b) T. Hurless, H. Bentzen and S. Homkjol, Electrochim. Acta, 1987, 32, 1613; (c) R. M. Toresi and F. C. Nant, Electrochim. Acta, 1988,33, 1015; (d) C. K. Dyer and J. S. L. Leach, J. Electrochem. Soc., 1978, 125, 23. 8 R. Cabanet, J. Cliaussy, J. Mazuer, G. De la Bouglise, J. C. Jubert, G. Barral and C. Montella, J. Electrochem. SOC., 1990, 137, 1444. 9 e.g. N. Machida, M. Tatsumisaga and T. Minami, J. Electrochem. SOC.,1986, 133, 1963. 10 C. J. Brinker and G. Scherer, Sol-Gel Science, Academic Press, New York, 1989. 11 J. Livage, Prog. Solid State Chem., 1988, 18, 259. 12 M. Nabari, S. Doeff, C. Sanchez and J. Livage, Mater. Sci. Eng., 1989, 133, 203. 13 (a) A. Chemseddine, R. Monneau and J. Livage, Solid State Zonics, 1983, 5, 357; (b) C. Sanchez, J. Livage, M. Henry and F. Babonneau, J.Non-Cryst. Solids, 1988,100,65; (c)0.Kamaguchi,D. Tonihisa, H. Kawabata and K. Shimizer, J. Am. Ceram. SOC., 1987, 70, C94; (d)K. Yamanaka, J. Appl. Phys., 1981, 20, L307. 14 H. Schmidt, Inorganic and Organometallic Polymers, ACS Sym- posium Series 360, Washington DC, 1988, ch. 27, p. 333. 15 A. I. Vogel, Textbook of Quantitative Inorganic Analysis, Long-man, London, 1971. 16 G. R. Lee and J. A. Crayston, to be submitted for publication. 17 A. A. McConnell, J. S. Anderson and C. N. R. Rao, Spectrochim. Acta, Part A, 1976, 32, 1067. 18 M. Dartiguenave and Y. Dartiguenave, Bull. SOC.Chim. Fr., 1968, 171. 19 C. Alquier, M. T. Vandenborne and M. Henry, J. Non-Cryst.Solids, 1986, 79, 383. 20 R. J. Cava, D. W. Murphy and S. M. Zahurak, J. Electrochem. SOC.,1983, 130, 2345. 21 R. D. Rauh and S. F. Cogan, Solid State Ionics, 1988, 28-30, 1707. 22 R. S. Crandall and B. W. Faughnan, Appl. Phys. Lett., 1976, 28, 95. 23 G. R. Lee and J. A. Crayston, 1990, unpublished results. 24 B. W. Faughnan, R. S. Crandall and M. A. Lampert, Appl. Phys.Lett., 1975, 27, 275. Paper 0105 1201; Received 14th November, 1990

ISSN:0959-9428    

DOI:10.1039/JM9910100381

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(3), 387-391 Nature of Dangling-bond Sites in Native Plasma-polymerized Films of Unsaturated Hydrocarbons, and Electron Paramagnetic Resonance Kinetics on Heat Treatment of the Films Masayuki Kuzuya,*" Masanao Ishikawa," Akihiro Noguchi," Hideki Ito," Kaoru Kamiyab and Tohru Kawaguchib a Laboratory of Pharmaceutical Physical Chemistry, Gifu Pharmaceutical University, 56-I, Mitahora-Higashi, Gifu 502, Japan Tomei Sangyo Co. Ltd. Noritake-Sinmachi, Nishiku, Nagoya 467, Japan We describe the electron paramagnetic resonance (EPR) study of dangling-bond sites (DBS) of plasma- polymerized films prepared from three unsaturated hydrocarbons, phenylacetylene, styrene and hex-3-yne. It has been shown that all the EPR spectra of DBS in such native films (before exposure to air) are much more intense than those after exposure to air.The DBS are quite stable at room temperature under anaerobic conditions, but their number begins to decrease towards a limiting value on heat treatment. However, when exposed to air the DBS decay rather rapidly; this decay tends to level off , indicating that there exist two different types of DBS, reactive and non-reactive. We discuss the difference in the nature of dissipation of DBS between oxidative and non-oxidative conditions. We also present the EPR spectral interpretations of the DBS and the reactions of several plasma-polymerized films previously reported. Keywords: Plasma polymerization; Dangling-bond site; Electron paramagnetic resonance spectroscopy It is a well known fact that glow-discharge plasmolyses of nearly all organic vapours produce unique plasma-poly- merized films on the surface of glass reaction vessels, although the rate of plasma polymer deposition depends on the nature of the organic compounds used.This process is referred to as plasma-state polymerization.ls2 One of the special features of the plasma-state polymerization is the fact that the resulting polymers contain large amounts of stable trapped free radicals at room temperature, all of which show a broad single line in their EPR ~pectra.~-'~ For instance, the EPR spectrum of styrene plasma-polymerized films (PPST) is shown in Fig. 1 as a representative example.' Note the isotropic broad single line with a peak-to-peak width (AHms,)(msl =maximum slope) of cu.1.7-1.8 mT. The pioneering studies of Yasuda indicated more than a decade ago that there is a definite correlation between the radical concentration and the chemical structure of organic compounds used for plasmolysis.2 Plasma-polymerized films of organic vapours that contain triple bonds and aromatic rings, including hetero-aromatic rings, produce the highest level of trapped free radicals, whereas saturated hydrocarbons produce the lowest number of trapped free radicals. These trends roughly parallel those for the rate of plasma-state polymerizations. In view of the fact that plasma-polymer deposition occurs directly on the surface of solid materials from the vapour Fig.1 EPR spectrum of trapped free radicals of styrene plasma- polymerized film' phase through 'atomic polymerization'2 involving a variety of plasma-fragmented species, it can be considered that the higher level of trapped free radicals observed in the plasma- polymerized films from unsaturated organic compounds could result from the involvement of the unsaturated n-bond of the monomer; the regenerated radicals thus formed are immobil- ized in the polymer matrices as the plasma-polymer deposition proceeds. 'Trapped free radicals' in the plasma-pol ymerized films can be considered not to be discrete organic free-radical species, but to be immobilized dangling-bond sites (DBS). These incorporate a variety of chemical structures including conju- gated and non-conjugated radical centres, and are of no structural significance." Therefore, the EPR spectra show only outlines of overlapping multicomponent radical centres, which are not represented by either Lorentzian or Gaussian curves.This is consistent with the assignment of broad single- line spectra observed in plasma-irradiated polymers to compo- nent Most of the spectra of plasma-polymerized films previously reported, however, have been recorded using films (or powders scraped from the glass surface) that had been exposed to air by withdrawal from the plasma reactor, even for those measurements described as in uucuo. When the films were exposed to air, some of the DBS in the surface layer could have reacted rapidly with oxygen to give the corresponding peroxy radicals followed by the termination reactions.We believe that detailed EPR study on the DBS of native plasma-polymerized films without exposure to air can provide more definitive information concerning the nature of the DBS as well as the characteristics of the resulting films. Thus, we have undertaken EPR spectral measurement of DBS in native plasma-polymerized films prepared from unsaturated hydro- carbons such as phenylacetylene (PA), styrene (ST) and hex-3-yne (3H). Furthermore, we have also conducted EPR kinetics of DBS on the heating of such films to gain further insight into the physicochemical properties of the DBS. We report that the spectral intensities of DBS thus observed were much greater than those observed after exposure to air, and J.MATER. CHEM., 1991, VOL. 1 vacuum line T er inlet sealed -0-Fig. 2 Schematic representation for plasma polymerization and EPR spectral measurement of the plasma-polymerized films that they can be assigned to two different types of DBS: reactive sites and non-reactive sites. Thus, we discuss the difference in the nature of DBS decay under oxidative and non-oxidative conditions. We also present the interpretations of EPR spectra of DBS reported in several previous papers relevant to the present work. Experirnental Materials Organic compounds, phenylacetylene (PA), styrene (ST), and hex-3-yne (3H) used for plasma-polymerization are all com- mercially available and were used without further purification.Method of Plasma Polymerization A neutral glass capillary (1 mm i.d., 65 mm long) was placed in a specially designed ampoule with a side branch (30mm i.d., 100 mm long) connected to a capillary tube (2 mm i.d.) at the upmost part of the ampoule. The ampoule was connec- ted to a vacuum line and degassed (0.001 Torrt). The plasma polymerization was carried out using inductively coupled plasma in the region encircled by the radiofrequency discharge coil at 13.56 MHz. The supplied power was 40 W over a period of 10 min for PA, and 1 h for ST and 3H for EPR study, with a monomer flow rate of 3.5 x cm3 min-' for PA and ST and 2.5 x cm3 min-' for 3H. After the plasma irradiation was discontinued, the ampoule reactor was kept in uucuo for 30 min to remove the remaining low-molecular- weight materials.The EPR spectral measurements of plasma-polymerized films formed on the glass capillary were performed by turning the ampoule upside down (Fig. 2). The EPR spectral intensity was determined by double integration. EPR spectra were recorded with a JES-RE1X spectrometer (JEOL) with X-band and 100 kHz field modu- lation, and extra care was taken to ensure that the observed spectra were not saturated by keeping the microwave power level below 0.01 mW. IR Spectral Measurement the IR spectra were measured on a JASCO A-102 spec- trometer. Results EPR Spectra Fig. 3 shows the EPR spectra of immobilized DBS observed with three plasma-polymerized films, PPPA, PPST and PP3H, formed from PA, ST and 3H, respectively, before and after exposure to air.It can be seen that the spectral features are more or less the same and they were unchanged at any stage of plasma polymerization. The spectra are characterized by an isotropic broad single line with AHmslof 1.76 mT for PPPA, 1.90 mT for PPST and 1.89 mT for PP3H. Note that the broad single-line spectrum of PPST is also essentially identical with that of the radicals produced in plasma-irradiated polystyrene (PST), with longer plasma dur- ation.16 A notable feature in Fig. 3(u) is that the spectral intensity of PPPA is markedly larger than for the other two films, PPST and PP3H.t This trend parallels the report of Yasuda.2 Thus, a large number of the DBS in PPPA result t The spectral intensity of PP3H is still much larger than that of other compounds such as methyl methacrylate (MMA), methyl is0 butyrate (MIB) and hexa-1,Sdiene.A B C (b) (b)A-7 (& Fig. 3 EPR spectra of dangling-bond sites of (A) plasma-polymerized Plasma-polymerized films for IR spectral measurements were films of phenylacetylene (PPPA), (B) styrene (PPST) and (C) hex-3-yneprepared on KBr discs in a manner similar to the above, and (PP3H): (a) before exposure to air, (b) after exposure to air. Values of AH,,,/mT: (A) (a) 1.76; (b) 1.47; (B) (a) 1.90, (b) 1.33; (C) (a) 1.89, t 1 Torr x 133.322 Pa. (b) 1.13 J. MATER. CHEM., 1991, VOL. 1 from the presence of both effective structural features for DBS formation: an aromatic ring and a triple bond in PA.The spectral intensities observed after exposure to air are appreciably reduced relative to those prior to exposure as shown in Fig. 3(b). This indicates that a considerable number of DBS react with oxygen and are terminated to stable diamagnetic molecules at room temperature (uide infru). It is known that even a brief plasma-exposure of various kinds of glass substrate produces intense paramagnetic centres, i.e. 'glass radicals', evidenced by the EPR spectroscopic meas~rements,~~~*~~~~~although the EPR spectral features vary depending on the nature of glass substrate. Yasuda and Hsu have shown, however, that glass radicals have not been formed in the plasma polymerizations of unsaturated organic vap~urs.~It was also confirmed by separate experiments that the EPR spectra of the films formed under the present flow plasma conditions shown in Fig.3 were not contaminated with those of the glass radicals. EPR Kinetics Fig. 4 shows the progressive changes in EPR spectral intensity (determined by double integration) of the DBS in PPPA, PPST and PP3H in the course of plasma polymerization. It is clear in all cases that the spectral intensity increases linearly as the reaction proceeds, but the rate varies with the com- pounds used for plasmolysis. The rate of plasma-polymeriz- ation was also evaluated by monitoring the growth of the characteristic peak of the IR spectra in each film formed on KBr discs with various plasma duration.Thus, as shown in Fig. 5, the rate of DBS formation was found to be well correlated to the rate of plasma-polymerized film formation (linear relationship). Fig. 6 shows the EPR spectral changes of DBS in PPPA, PPST and PP3H on standing at room temperature under aerobic conditions. It is seen that the spectral intensity decreases quite rapidly with time under aerobic conditions toward a limiting value. After a few hours the spectral intensity tends to level off and persists essentially unchanged for a long period of time at room temperature. Note that the decay of the spectral intensity under aerobic conditions is accompanied by a decrease in AH,,, of EPR spectra in all cases (from cu. 1.8-1.9 mT to cu. 1.1-1.5 mT) (uide infru).These results demonstrate that the DBS are quite stable at room tempera- ture so long as the films are kept under anaerobic conditions in all cases, indicating that all the DBS are immobilized and 0 20 40 60 plasma duratiodmin Fig.4 Progressive changes in EPR spectral intensity of the DBS in the course of plasma-polymerization: @, PPPA; 0,PPST; @, PP3H P plasma duratiodmin Fig. 5 Progressive changes in characteristic band absorbance (2920- 2930 an-') of IR spectra in the course of plasma-polymerization:0, PPPA; 0,PPST; @, PP3H 1.o 0.8 .-5 0.6 v) ac .-0.4 0.2 0 1 2 3 standing time/h Fig.6 Progressive changes of relative EPR spectral intensity of the DBS on standing in air at room temperature: @, PPPA 0,PPST; 0, PP3H do not readily undergo termination at room temperature.However, the fact that dissipation of the DBS in air tends to level off indicates the presence of unreactive DBS. Thus, it is considered that immobilized DBS can be divided into two different DBS, reactive and non-reactive sites. It is apparent that the former lie in the surface layer, where oxygen can diffuse readily to react with the DBS, producing the corre- sponding peroxy radicals followed by well known complex chain-termination reactions. The latter sites would appear to lie in the bulk where oxygen can not penetrate readily. Thus, most of the DBS previously reported by EPR spectra were probably only the non-reactive DBS located in the bulk of the plasma-polymerized films.Furthermore, as will become more apparent later, the surface layer of such films is less cross-linked than the bulk, this being equivalent to the pres- ence of a larger quantity of lower-alkyl carbons. The bulk layer is of a higher cross-linked network containing a larger quantity of higher-alkyl carbons. Since oxygen was observed to react only with those DBS in the surface layer, it can be reasonably assumed that the remaining DBS consist of more higher-alkyl carbon-centred radicals, which possess fewer hydrogens capable of coupling with free-radical electrons. We believe this is the essential reason for the decrease in AHm1 when films have been left to stand in air. Heat Treatment With a view to improving the dielectric properties of styrene plasma-pol ymerized films, several authors have conducted annealing experiments under a variety of condition^.^*'^^" A detailed kinetic study, however, has not been undertaken.Since we believe that the kinetic study of the native DBS on heat treatment under anaerobic conditions could provide important information of intrinsic properties of DBS, includ-ing the neighbouring structure and the degree of cross-linking of such films, we have conducted the kinetic study in more detail. Fig. 7 shows the progressive changes in the spectral intensity of three films, when heated at various temperatures under anaerobic conditions. It is seen that, although such DBS persisted unchanged in intensity at room temperature, heat treatment caused a decay of the spectral intensities.The nature of the decay depends on the type of film and the temperature, but the intensity tends to level off gradually in all cases. It was also found that all the decay curves at earlier stages could be described by second-order kinetics, indicating that the dissipation of DBS follows a diffusion-controlled bimolecular reaction. Comparison of the three parts of Fig. 7 disclosed several interesting features. The DBS of PPPA appear to be the most stable, so the progressive changes in intensity at 125 "C were nearly the same as those at lOO"C, and both spectral intensities remained at the highest level. This indicates that, of the three films, PPPA is the most rigid in the polymer matrix, probably owing to the presence of both a triple bond and an aromatic ring.On the other hand, the DBS of PPST is much less stable so that the DBS is hardly detectable at 125 "C. This indicates that PPST has the least cross-linked network in the matrix structure. Results of spin-trapping reactions of styrene plasma-polymerized ultrathin films sup- ports this.21 Although the spectral intensity gradually decreased, the AH,,, of the spectra persisted unchanged during heat treat- ment in all cases,t which is in sharp contrast to the case of progressive changes of the spectra under aerobic conditions at room temperature. These results indicate that the surface layer should be less cross-linked than the bulk and may be mobile enough to undergo the termination reaction at higher temperatures even in the absence of oxygen.This view is also consistent with the results of spectral decay under aerobic conditions at room temperature, as described above. Note that heat treatment at a higher temperature than 125 "C accelerated the rate of dissipation of DBS, and after most of 7 This fact differs from that reported previo~sly.~*'' This discrep- ancy may stem from the fact that our PPST had never been exposed to air. 1.o 0.5 -2 0 0 0 J. MATER. CHEM., 1991, VOL. 1 the radicals had dissipated a new EPR signal with much smaller AH,,, appeared at temperatures higher than 200 "C, as has been reported previously (see below). Discussion As stated in the introduction the EPR spectra of stable free radicals in plasma-polymerized films have been.observed by a number of author^.^-'^ In connection with our present work, we wish to discuss the EPR spectral interpretation of the structure of radicals involved in plasma-polymerized films in several previous reports. In the EPR study on the annealing of PPST, several authors have observed that the heating of PPST in air at a higher temperature than ca. 250 "C produced a new EPR signal with a much smaller AH,,, value (ca.0.5-0.8 mT) after the original DBS had nearly disappeared, and the spectral intensity con- tinued to increase as the temperature was raised f~rther.~*'~*" The relatively sharp single-line spectrum with small AHms, thus observed is indicative of the presence of far fewer kinds of component spectra.In the above reports it was postulated that the radical initially observed was due to carbon-centred radicals, while at higher temperatures a new radical was formed as a result of reaction with oxygen in the system. However, there was no further structural characterization. The comparison of elemental analyses before and after heat treatment indicates that hydrogens have been eliminated oxidatively to form a more cross-linked network. In fact, such films have been shown to carbonize on heat treatment resulting in the formation of dark-coloured films. Such a small AH,,, is a spectral feature that is strikingly similar to those of DBS in amorphous carbon films obtained by arc evapor- ation of pure graphite rods22 and diamond thin films formed by microwave plasma chemical vapour depo~ition.~~ More-over, the calculated g value of the new single-line spectrum (g=2.002 for PPST) is consistent with carbon-centred, and not oxygen-centred, radicals.Based on these facts, it seems logical to consider that the dominant formation of tertiary carbon-centred DBS is responsible for a new isotropic single- line spectrum, which possesses essentially no hydrogen capable of coupling with the free-radical electron. Likewise, the observed EPR spectra with a small value of AH,,, (0.6-0.8 mT) in plasma-polymerized films of siloxane compounds7 could be explained similarly in terms of an absence of hydro- gens capable of coupling with the silicon-centred free-radical electron.One should remember that this view is totally consistent with the foregoing interpretation of the tendency for AH,,, to decrease when the films are exposed to air. Several authors have reported that the EPR spectra of plasma-polymerized perfluorinated compounds show excep- 1.o 0.5 1 2 0 1 2 0 1 2 heating time/h heating time/h heating time/h Fig. 7 Progressive changes in relative EPR spectral intensities of DBS on heat treatment in uucuo for (a) PPPA, (b)PPST and (c)PP3H: 0, 70; 0,100; @, 125°C J. MATER. CHEM., 1991, VOL. 1 tionally large values of AHms,of ca.4.0mT, which is much broader than those of non-fluorinated organic polymers (1.5- 2.0 mT).6,7,12,13 Millard et al. have explained that this kind of feature probably results from a combination of inhomo- geneous broadening and exchange narrowing.6 We believe, however, that all these broad single-line spectra are assignable to the same type of DBS as those in other plasma-polymerized films without invoking any special property, and the large AHmslof the spectra can be interpreted by the fact that the hyperfine splitting of fluorine atoms is more than three times larger than that of hydrogen atoms (e.g.ca. 7.5 mT for a- fluorinez4us. ca. 2.0 mT for a-hydrogen coupling). Among previous work dealing with the DBS of plasma- polymerized films, we also wish to comment on the result reported by Venugopalan and co-w~rkers.~ The authors have observed that heat treatment of xylene plasma-polymerized film at 100 "C in air gave a new peak, a doublet separated by 12.5 mT.This new doublet has speculatively been assigned to hydroxybenzyl-type radicals resulting from the oxidation of xylene. As described above, however, the hyperfine splitting of a-hydrogen is normally ca. 2.0 mT and such a large splitting of 12.5 mT for a-hydrogens is unrealistic in any interatomic hydrogen coupling in any organic compound. We have already undertaken a number of plasma-irradiation studies on various kinds of inorganic materials including glass substrates such as soft glasses, neutral glasses, Pyrex and quart^.^'*^' We have similarly observed the doublet separated by 12.0-12.5 mT on quartz by Ar plasma irradiation." Thus, the feature observed by Venugopalan is undoubtedly assignable to the paramag- netic centres generated by plasma irradiation in the quartz substrate, which we believe became observable in the spectrum owing to a significant reduction in intensity of the other peaks by heat treatment.In summary, the results reported here combined with the reinterpretations of the previously reported EPR spectra demonstrate that only the DBS in the surface layer of plasma-polymerized films dissipate oxidatively in air at room tempera- ture, and the broad single-line spectrum is caused by aniso- tropic hyperfine splitting with the superposition of various kinds of radical centres, i.e. DBS. Heat treatment of the films caused further dissipation of the radicals, either oxidatively or non-oxidatively, with oxidative heating (under aerobic conditions) being more prone to dissipation than non-oxidat- ive heating (under anaerobic conditions). Further heat treat- ment at higher temperatures is distinct from the reaction of the original DBS in plasma-polymerized films, since, appar- ently, it incorporates the carbonization of the film.Based on these considerations, the value of AHmslof EPR spectra may 39 1 be taken as an indication of the structure of DBS and the rigidity of plasma-polymerized film matrices. References 1 M. Hudis, Techniques and Applications of Plasma Chemistry, ed. J. R. Hollahan and A. T. Bell, Wiley, New York, 1974. 2 H. Yasuda, Macromolecular Reviews, Wiley, New York, 198I, vol.16. 3 F. J. Vastola and J. P. Wightman, J. Appl. Chem., 1964, 14, 69. 4 J. P. Wightman and N. J. Johnston, Adv. Chem. Ser., 1969, 80, 322. 5 S. Morita, T. Mizutani and M. Ieda, Jpn. J. Appl. Phys., 1971, 10, 1275. 6 M. M. Millard, J. J. Windle and A. E. Pavlath, J.Appl. Polym. Sci., 1973, 17, 2501. 7 H. Yasuda and T. Hsu, J. Polym. Sci., Polym. Chem. Ed., 1977, 15, 81. 8 T. W. Scott, K. Chu and M. Venugopalan, J. Polym. Sci., Polym. Chem. Ed., 1978, 16, 3213. 9 T. W. Scott, K. Chu and M. Venugopalan, J. Polym. Sci., Polym. Chem. Ed., 1979, 17, 267. 10 S. Yamaguchi, S. Nakamura and G. Sawa, Purazuma Jugo, Tokubetsu Toronkai, 1979, 252. 11 S. Nakamura, S. Yamanaka, S. Yamaguchi and G. Sawa, Kobun-shi Ronbunshu, 1981, 38,681.12 K. Hozumi, K. Kitamura and T. Kitade, Bull. Chem. SOC. Jpn., 1981,54, 1392. 13 G. Legeay, J. J. Rousseau and J. C. Brosse, Eur. Polym. J., 1985, 21, 1. 14 M. Kuzuya, S. Nakai and T. Okuda, J. Chem. SOC., Furaday Trans. 1, 1987, 83, 1579. 15 M. Kuzuya, T. Kawaguchi, M. Nakanishi and T. Okuda, J. Chem. SOC. Furaday Trans. 1, 1986,82, 1441. 16 M. Kuzuya, A. Noguchi, H. Ito, S. Kondo and N. Noda, J. Polym. Sci., Polym. Chem. Ed., in the press. 17 M. Kuzuya, A. Noguchi, M. Ishikawa, A. Koide, K. Sawada, A. Ito and N. Noda, J. Phys. Chem., in the press. 18 M. Kuzuya, M. Ishikawa, A. Noguchi, K. Sawada and S. Kondo, J. Polym. Sci., Polym. Chem. Ed., to be published. 19 N. Morosoff, B. Crist, M. Bumgarner, T. Hsu and H. Yasuda, J. Macromol. Sci. Chem., 1976, 10, 451. 20 M. Kuzuya, A. Noguchi, S. Ito, R. Itatani and A. Hatta, to be published. 21 M. Kuzuya, S. Nakai and A. Ito, Chem. Lett., 1987, 1083. 22 S. Orzeszko, W. Bala, K. Fabisiak and F. Rozploch, Phys. Status Solidi A, 1984, 81, 579. 23 I. Watanabe and K. Sugata, Jpn. J. Appl. Phys., 1988,27, 1808. 24 R. J. Rontz and W. Gordy, J. Chem. Phys., 1962,37, 1357. 25 M. Kuzuya, T. Kawaguchi, Y. Yanagihara and T. Okuda, Nippon Kaguku Kuishi, 1985, 1007. Paper 0/05121G; Received 14th November, 1990

ISSN:0959-9428    

DOI:10.1039/JM9910100387

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(3), 393-399 Dispersion of Silicon Carbide Whiskers and Powders in Aqueous and Non-aqueous Media Timothy P. O’Sullivan and Spencer E. Taylor BP Research Centre, Chertsey Road, Sunbury-on-Thames, Middlesex TW16 7LN, UK The dispersion behaviour of two commercial Sic whisker samples and one Sic powder has been investigated in aqueous and non-aqueous media using equilibrium sediment density measurements. In water, the pH range 9-1 1.O was identified as being optimum for the dispersion of the SIC materials. The colloidal stability of aqueous Sic suspensions was broadly consistent with their silica-like surface chemistry, although the SIC whiskers appeared to be more basic than the powder. The dispersion behaviour of the SIC materials was found to be similar in a range of non-aqueous media.Formic acid was found to be the best intrinsic non-aqueous dispersion medium for the materials, although to obtain the best possible dispersion in non-aqueous media it was found necessary to use suitable dispersing agents. On the basis of sedimentation density measurements, it was found to be possible to determine the aspect ratio of the whisker samples. Keywords: Silicon carbide; Dispersion; Sedimentation density As a result of its high-temperature strength, low density and resistance to corrosion, silicon carbide has attracted much interest for advanced structural ceramic applications. In the form of whiskers, silicon carbide is also of interest as a reinforcement for ceramic matrix composite (CMC) mater- ials.1-3 Commercial Sic whiskers are often received as highly agglomerated, matted products (of the form shown in Fig.l), making dry-powder processing routes undesirable, both through non-uniformity of mixing and the likely disintegration of the brittle whiskers. The processing of whisker-based CMCs in liquid media offers potential advantages for effectively dispersing the components by control of colloidal stability, leading to composites possessing more homogeneous micro- structures.2 Aqueous-based systems offer an attractive route for the processing of water-tolerant ceramics owing to their low cost and toxicity. Hydrolysable ceramic components, on the other hand, will require dispersion in suitable non-aqueous media.Fig. 1 Scanning electron micrograph of as-received Tokamax silicon carbide whiskers (bar =5 Pm) The present study therefore addresses the dispersion of Sic materials in water as well as in a number of organic liquids, chosen on the basis of their likely use in ceramic processing as well as for their chemical functionality. This latter aspect permits a fundamental study of the chemical basis of Sic dispersion behaviour to be undertaken through the identifi- cation of optimum conditions for its aqueous and non-aqueous colloidal processing. The effectiveness of some com- mon industrial dispersing agents for non-aqueous processing has also been assessed. Experimental Materials Samples of two commercial /I-silicon carbide whiskers were used.These were Tokamax whiskers supplied by Tokai Car- bon, Japan, and Versar (VClA) whiskers from Versar Manu- facturing, Virginia, USA. These materials were used without further treatment. Particle sizing was performed on well dispersed dilute whisker suspensions, with whisker length (I) being obtained from optical microscopy and diameter (d) from scanning electron microscopy. Results are shown in Table 1, and the average aspect ratio (I/d)in the samples and its polydispersity was obtained from the length and diameter values. From the data given in Table 1, the whiskers can be seen to be very polydisperse in terms of length and aspect ratio, but somewhat more uniform in diameter. Versar whiskers have a larger aspect ratio than Tokamax whiskers.The b-silicon carbide powder used was obtained from Starck. It has a B.E.T. surface area of 16.7 m2 g-’ and an average (spherical) particle diameter of 0.25 & 0.03 pm [obtained on dilute suspensions from photon correlation spectroscopy (PCS) measurements]. All organic solvents used were AnalaR grade from B.D.H. or Aldrich, and were used as received. The dispersing agents used are given in Table 2 and were used as recommended. Methods The surface compositions of the silicon carbide samples used in the present study were determined by XPS (VG Escalab, J. MATER. CHEM., 1991, VOL. 1 Table 1 Physical characteristics of the silicon carbide whiskers used in the present study source average length/pm average diameter/pm average aspect ratio (and range)/km Versar Tokamax 14.0f9.3 10.2 & 7.4 0.52 k0.17 0.55f0.14 27.0 (7-70) 18.5 (4-43) Table 2 Commercial dispersants used in the present study name manufacturer tY Pe Hypermer KDI ICI block copolymer Oloa 1200 Chevron low mol.wt. polymer using Al-Ka radiation). For each sample C Is, 0 Is, N Is, Si 2p, Mg 1s and Fe 2p spectra were recorded and quantified using sensitivity factors given by Wagner et aL4 The binding energies were referenced to C 1s at 285 eV. Results of broad XPS scans to give the surface elemental composition of silicon carbide materials are given in Table 3. The results of high-resolution XPS scans with bond-assign- ment energies are given in Table 4. Electrophoretic mobility (U,)measurements as a function of equilibrium pH were made on dilute aqueous samples using a Malvern Zetasizer IIc instrument.Isoelectric points (IEPs) were obtained from the point at which the UEoersus pH plots cross the pH axis. The quality of Sic dispersion in the various liquids was assessed from measurements of equilibrium sediment vol- ume~.~The volume occupied by a sediment depends on the extent of flocculation in the dispersion. Deflocculated particles pack more efficiently to give a dense sediment, whereas flocculated particles bridge readily to give a loose, low-density sediment. A minimum in sediment volume is thus associated with a maximum in colloidal stability. In ceramic applications, sediment densities, rather than sediment volumes are usually quoted and expressed as a percentage of the bulk solid density.Observation of sedimentation behaviour as a function of time is also commonly used to obtain a further assessment of the state of dispersion.6 In this work the Sic whisker suspensions were classified in the manner suggested originally by Bell and Crowl,' and are illustrated diagramatically in Fig. 2. A flocculated (F) suspen-sion sediments rapidly leaving behind a clear supernatant phase, whereas in a dispersed (D) system the sediment builds up from an initially turbid supernatant layer. Owing to the suggested solvent composition ketones unknown non-polars amine-terminated poly(a-methylpropene) large size polydispersity in the present whiskers, differential settling of fines occurred, resulting in a layered sediment.Sediments formed from flocculated dispersions are more uni- form in appearance. In many cases, behaviour intermediate between the two extremes shown in Fig. 2 was observed, such systems being termed partially flocculated (PF). In the present study, sediment volume tests were performed in graduated measuring cylinders of 25 mm diameter, since tests performed in vessels less than 20 mm in diameter showed a significant apparent increase in sediment volume due to wall effects. Buscall' has also found strong wall effects in sediments of anisotropic attapulgite particles. Tests were performed as follows. For ease of measurement, 1 g test quantities of the particles were used.These were placed in 50 cm3 measuring cylinders to which 50 cm3 of the dispersion medium was then added. After they had been shaken (by hand), the cylinders were placed in a 38 kHz ultrasonic bath for 30-40 min to ensure maximum dispersion. The cylinders were then left at ambient temperature until a steady sediment volume was observed, indicating that sedimentation was com- plete. For the whiskers, complete sedimentation usually occurred within 1 week, although sediments were formed more rapidly from flocculated suspensions. In these latter cases consolidation of the floc structure was observed after formation of the initial sediment over a period of several days. For the smaller-sized powder, complete sedimentation took up to 1 month.For the best dispersed powder suspensions, equilibrium sediment volumes were obtained by centrifugation at 3000 rpm. Sediment volumes were converted to sediment density values using a value of 3.2 g cm-3 for the bulk density of Sic. In addition, the appearance of the sediments formed was noted, and each system classified according to Fig. 3. Table 3 XPS analysis of commercial silicon carbide particles surface composition (at.%) Versar whiskers Tokamax whiskers Starck powder carbon 47.5 47.1 56 oxygen silicon 18.0 33.3 15.4 37.0 I1 30.0 nitrogen 0.3 0.5 1 magnesium iron 0.4 0.6 - fluorine 2.0 Table 4 Carbon and silicon peak synthesis analysis (at.%) element peak position/eV Versar whiskers Tokamax whiskers Starck powder carbon C-H/C-CSi-C 285.0 283.0 5 0.1 15.1 32.4 12.0 35.1 26.7 29.7 silicon Si- 0 103.150.1 3.6 4.3 4.8 Si-C 100.8+0.1 29.7 32.7 26.1 J.MATER. CHEM., 1991, VOL. 1 rnclear clearI Iclear time increasing -+ Icomwsitionil Kcompositionj variable composition time increasing --f Fig. 2 Classification of sedimenting colloidal systems. (a) Sedimentation of a flocculated dispersion; (b) sedimentation of a deflocculated dispersion -3.5 1 234561891011 equilibrium pH Fig. 3 Electrophoretic mobility uersus pH for silicon carbide particles. 0,Starck P-SiC powder; W, Versar Sic whiskers; A,Tokamax Sic whiskers. Arrows indicate positions of IEPs The effectiveness of dispersing agents for non-aqueous media was assessed by comparing the sediment volumes of silicon carbide suspensions in 0.1 wt.% solutions of the dispersants, with those obtained in identical dispersions in the pure liquids.Results Surface Analysis of the Particles The XPS results shown in Tables 3 and 4 suggest that all three commercial Sic materials possess similar surface chemis- tries. The Starck powder showed a higher level of hydrocarbon contamination than the whiskers, together with the presence of a trace amount of fluorine (ca. 2 at.%). All three samples contained small quantities of basic nitrogen. In addition, the Versar whiskers contained trace levels of magnesium and iron. The present findings shown in Table 3 for the Sic whiskers are similar to the XPS results obtained by Adair et a1.' with these materials. The results obtained for the Sic samples are consistent with the 'oxidised shell' model, whereby the silicon carbide is overlaid by an oxidised surface layer of silica." The detection of mainly Sic with lower levels of oxidised silicon species suggests that the oxidised layer on the materials is consider- ably thinner than the analysis depth of the XPS technique (up to ca.50 A). Electrokinetic Properties of the Particles Fig. 3 shows the variation of electrophoretic mobility with pH for the Sic samples. The data indicate that the P-SiC powder has an isoelectric point occurring at a pH of ca. 3. This value agrees reasonably well with that found by other workers, and is a reflection of the acidic silica-like surface of the powder." The whiskers, however, possess much higher IEPs, in the pH range 5-5.5, suggesting a less acidic surface chemistry.In the case of the Versar material at least, this may be connected with the small amounts of metallic contaminants detected by XPS in the surface, which, as cations, would be expected to increase the pH of the IEP. Effect of pH on the Dispersion of Silicon Carbide in Aqueous Media Fig.4 and 5 show the dispersion behaviour of the silicon carbide whiskers and powder, respectively, as a function of equilibrium pH. Fig. 4 indicates that both sets of Sic whiskers exhibit similar dispersion behaviour, in that a pronounced dependency of sediment density upon pH was observed. Versar whiskers were flocculated below pH 7, with a slight minimum in sediment density occurring between pH 6 and 6.5.A sharp maximum in sediment density was observed in the pH range 9-10, followed by another decrease in sediment density for pH >ca. 10.5. Tokamax whiskers behaved simi- larly, except that the minimum in sediment density which J. MATER. CHEM., 1991, VOL. 1 11 -10 -9-DA 0 2 4 6 8101214 equilibrium pH Fig. 4 Dispersion of silicon carbide whiskers as a function of pH: D =dispersed; F =flocculated; PF =partially flocculated. W, Versar;A,Tokamax 42 -38 -34 -h z30-0 Yi2622 1 I I I I I I 1 0 2 4 6 810 12 14 equilibrium pH Fig. 5 Dispersion of Starck P-silicon carbide powders as a function of pH; see Fig.4 for explanation of symbols.Range indicates the zone of long-term colloid stability occurred at ca. pH 7 was more pronounced, and the maximum sediment density is observed at ca. pH 11. The maximum sediment densities obtained for Versar and Tokamax whiskers were 9.2 and 10.7% bulk Sic density, respectively. Fig. 6 shows a typical optical micrograph of a Tokamax whisker suspension of pH 10.5 emphasising the high degree of disper- sion obtained in this pH region. Fig. 5 shows that, as with the whiskers, the powder suspen- sions were flocculated under acidic conditions and showed a maximum in sediment density in the pH range 8-11. In this Fig.6 Optical micrograph of an aqueous suspension of Tokamax silicon carbide whiskers at pH 10.6.Width of micrograph corresponds to 133.5 pm pH range, equilibrium sediment densities were measured after centrifugation, as the powder suspension showed long-term colloidal stability. A maximum sediment density of ca. 43% bulk Sic density was obtained in this case, which compares well with previously quoted literature values for well dispersed ceramic powder s~spensions.~ Fig. 7 shows a scanning electron micrograph of the sediment obtained from the Sic powder at pH 11.0. The micrograph illustrates the well packed nature of the powder sediments obtained in the optimum pH region. Dispersion of Silicon Carbide in Non-aqueous Media Tables 5 and 6 contain results of the dispersion experiments of the Sic materials in a range of organic solvents.The following important points emerge. (i) Large variations in sediment density were observed for identical suspensions in different liquids. However, the same overall trends in disper- sion behaviour across the range of solvents were observed in the three Sic materials studied. (ii) The lowest sediment densities were observed in the hydrocarbon solvents n-heptane and toluene, in which the Sic suspensions were flocculated. (iii) The highest sediment densities were observed in formic acid where dispersion was best. The maximum sediment density values obtained in formic acid were similar to the maximum sediment densities obtained in aqueous media for Versar whiskers and the Sic powder, but were much higher Fig.7 Scanning electron micrograph of the sediment obtained at pH 11.0 with silicon carbide powder (bar =5 pm) J. MATER. CHEM., 1991, VOL. 1 Table 5 Dispersion of Sic whiskers in non-aqueous media Non-aqueous Dispersions in the Presence of Dispersing Agentsequilibrium sediment densityb quality of In the preceding section, it was suggested that ketones such solvent dispersion" Versar Tokamax as acetone and methyl ethyl ketone were possibly the best common polar solvents for the dispersion of silicon carbide formic acid D 10f1 23f2 formamide F 4 &0.4 5 f0.5 whiskers. However, the sediment densities observed in these n-butylamine F 4 f0.4 5f0.5 solvents were too low to be acceptable for the successful methyl ethyl ketone PF 4.5 f0.5 5 f0.5 processing of, e.g., whisker-reinforced CMCs (cj aqueous acetone PF 4 f0.4 5 f0.5 systems). Table 7 shows the effect of commercial dispersants isopropyl alcohol PF 3 k0.3 4 f0.4 on silicon carbide sediment densities in ketones and n-hexane.ethanol F 3 f0.3 4 f0.4 This latter solvent was included for study as it is a typical toluene F 0.7 f0.1 1.3f0.1 n-hexane F 0.5k0.1 1.3f0.1 non-polar solvent which has previously been used in ceramic processing.12 A large increase in sediment density was appar- "D =dispersed; PF =partially flocculated; F =flocculated; expressed ent in the presence of the commercial dispersants, indicating as percent of bulk Sic density. that they were effective dispersing agents for silicon carbide in both whisker and powder form.In particular, the sediment densities obtained in whisker suspensions using the Hypermer Table 6 Dispersion of 8-Sic powder in selected organic solvents KDl dispersant were much higher than those obtained in a variety of pure organic solvents (as shown in Table 5). quality of solvent dispersion" equilibrium sediment densityb Discussion formic acid D 40f3 Dispersion of Silicon Carbide in Aqueous Media formamide F 10f1 methyl ethyl ketone PF 12f1 The observed dispersion behaviour of Sic in aqueous media acetone PF 23f2 can be rationalised using DVLO theory for charge-stabilised isopropyl alcohol F 7f1 ethanol F 6f1 colloids, together with the known silica-like surface chemistry methanol F 6+1 of this rnate1ia1.I~ Surface charge is likely to arise in these toluene F 4fl systems through the hydrolysis of surface silanol groups,14 n-hexane F 511 thus See Table 5.Si-OH+H,O~Si-O-+H,O+"vb A minimum in colloidal stability, and hence a minimum in sediment density, is expected close to the IEP. For the powder, than found for Tokamax whiskers in aqueous media. (iv) All a good agreement between the IEP and pH of minimum Sic suspensions appeared to be flocculated in n-butylamine sediment density is observed. For the whiskers, this agreement and, consequently, low sediment densities were observed. (v) is more qualitative, with the IEPs occurring at pH 5-55 and Dispersion in polar organic solvents was intermediate between the minimum sediment densities at ca.pH 6-6.5. the non-polar solvent and formic acid cases. Ketones have The minimum in sediment density is more pronounced in been suggested as the best common solvent class for the the whisker suspensions, which is consistent with the results dispersion of silicon carbide materials. l2 This was confirmed given in Fig. 2 as whiskers acquire substantial positive charges in the present work by the observation that whisker and at pHs less than the IEP. Improving charge stabilisation in powder suspensions were partially flocculated in ketones, as this pH region accounts for the increased sediment density opposed to being flocculated in the other polar liquids studied. observed. The more acidic nature of the powder means that For the powder in acetone this was reflected in a large increase substantial positive charges are not observed in the powder in sediment density in this solvent compared with the other suspensions over the pH range studied, and consequently no polar solvents.However, for the whiskers the better dispersion substantial increase in sediment density at low pH was found. in ketones was reflected in only a marginal increase in Charge stabilisation would be expected to be at its most sediment density. (vi) In any given solvent, silicon carbide effective for Sic materials at a basic pH, close to that powder dispersions produced much higher sediment densities corresponding to the pK, of the surface silanol group. This than whisker suspensions. Similarly, Tokamax whiskers gave has been measured by other workers14 to be in the range consistently higher sediment densities than the Versar 9-10, which is in reasonable agreement with the observed whiskers.maxima in sediment density at pH 8.5-1 1 for the powder and Table 7 Effects of dispersing agents for silicon carbide in organic solvents equilibrium sediment density" dispersing medium dispersing agent Starck powder Versar whiskers Tokamax whiskers n-hexane -5.0 f0.5 0.5fO.l 1.0 f0.1 Oloa 1200 43f4 llfl 22f2 methyl ethyl ketone -12f1 4.5 f0.5 5.0 f 0.5 Hypermer KDI 40f4 18f2 27f3 acetone -23+3 4 f0.5 4.5 f0.5 Hypermer KDI 4524 18f2 28f3 'Expressed as percent of the bulk Sic density. at 10-11 for the whiskers. The optimum pH range for the successful aqueous colloidal processing of the whiskers and powders is therefore between 9 and 11.At pHs greater than ca. 12, a decrease in sediment density may occur as a result of flocculation brought about by the high ionic strength in the medium. However, it is also possible that dissolution of the stabilising surface silica layer at the high pH may contrib- ute to the floc~ulation.'~ The observed narrower optimum pH range for the whiskers is probably a reflection of the greater sensitivity of the sediment density test to flocculation in suspensions of anisotropic particles. Dispersion of Silicon Carbide in Non-aqueous Media The trends in dispersibility shown in Tables 5and 6 are very similar to those reported recently by Okuyama et ~1.'~for a laser-synthesised Sic powder. These workers found that the dispersibility of the powders in organic solvents was dependent on the degree of oxidation of the powder surface and the chemical, rather than physical, nature of the solvent.Extensive heat treatment of the powders led to a thicker, more coherent silica layer, resulting in a more acidic surface chemistry. A brief comparison between the results of Okuyama et al. and those obtained in the present work are shown in Table 8. The previous workers used a poor (P)/good (G)/very good (VG) notation to describe their dispersions, which is analogous to the F/PF/D colloid notation used in the present work. The correlation between the dispersion behaviour of Okuy- ama's unoxidised silicon carbide powder and the materials studied in the present work is consistent with the XPS data shown in Tables 3 and 4.This indicates that the present materials have a relatively low degree of surface oxidation compared with that found in typical heat-treated powders16 or whisker^.^ The similar dispersion behaviour found in the present study for the whiskers and the powder in non-aqueous media reflects their very similar surface chemistries. The finding that formic acid is an effective dispersion medium, whilst n-butylamine is not, suggests the Lewis acid/base properties of a solvent are important in dictating dispersion behaviour, and accordingly, indicates that the formic acid may be operating via a charge- stabilisation me~hanism.'~ This is supported by the poor dispersibility in solvents with very low relative permittivity, in which charge separation is unfavourable.The commercial Sic materials, both whiskers and powders, appear to possess a Lewis base-type surface chemistry in non-aqueous media, which is in marked contrast to their essentially acidic behav- iour in water. The reason for this amphoteric behaviour is J. MATER. CHEM., 1991, VOL. 1 unclear at present, although it is possibly linked to the surface contaminants detected in the materials, including basic nitro- gen species and metallic cations. Although the present work has demonstrated that, in principle, it is possible to obtain good dispersion of silicon carbide in simple liquids, it is apparent from Table 7 that, in order to achieve the optimum degree of dispersion necessary for ceramic composite fabrication, suitable (polymeric) disper- sants are required.This is particularly pronounced in the case of the effective dispersion of Sic whiskers in non-aqueous media, as considerably higher sediment densities were obtained in the presence of dispersants than was achieved in aqueous whisker suspensions at the optimum pH (cf:Fig. 4). It is conceivable that such dispersing agents can function by a combination of electrostatic (Lewis acid-base interactions) and steric stabilisation mechanisms. l7 Semiquantitative Analysis of Aspect Ratios from Sediment Density Measurements In the foregoing, sediment density measurements were used to indicate trends in dispersion behaviour.For suspensions of spherical particles, sediment densities may be readily com- pared with the theoretical maximum value (of 63%) for the random packing of uniform spheres, to give a more quantitat- ive assessment of dispersion. However, for suspensions of anisotropic particles such as Sic whiskers, sediment density measurements may also be used to provide a measure of the aspect ratio of the particles from a consideration of the state of dispersion together with Milewski's findings' for the ran- dom packing of dry powders comprising rod-shaped particles (shown in Fig. 8). As with spherical particles, these random close-packing conditions should correspond to the theoretical maximum sediment density obtainable in a perfectly dispersed whisker suspension.Thus, for Versar whiskers with an average aspect ratio of 27, the Milewski model predicts a maximum sediment density of ca.19%, and for Tokamax whiskers with an aspect ratio of 18.5 it predicts a sediment density of ca. 23%. The maximum sediment densities found experimen- tally were 18% for Versar and ca.27% for Tokamax (see Table 7). Thus, as indicated in Fig. 8, good agreement exists between the model and the experimental results. As also found by Milewski for glass fibres,' the Sic whiskers apparently pack in sediments according to their average aspect ratio even though they are very polydisperse in this parameter (Table 1). Vold" has calculated the sediment densities in suspensions of strong flocculated anisotropic particles as a function of aspect ratio.Vold's model may therefore be used to estimate Table 8 Dispersibility of Sic particles in organic solvents dispersibility solvent oxidised Sic powderaVb as-received Sic powdef present powders and whiskers' n-hexane P P F toluene P P F acetone G G PF methyl ethyl ketone methanol G VG G P PF F ethanol VG P F n-propyl alcohol isopropyl alcohol n-propylamine n-but ylamine formic acid VG VG --- P P --- F F D -- propionic acid P G - octanoic acid G G - oleic acid G VG - a Ref. 15; heat treated at 600 "Cfor 24 h; taken from Tables 5and 6. J. MATER. CHEM., 1991, VOL. 1 70 -60 --50 -s Y .-540 -rn $ 30 -Y2 20 -10 -0 -' 0246810 20 30 40 SO 60 70 80 100 average axial ratio (Ild) Fig.8 Theoretical bulk densities for anisotropic particles as a function of aspect (axial) ratio. (a) Milewski model for dispersed systems; (b) Vold model for flocculated systems. Numbers (1 -4) indicate maximum (1,2) and minimum (3,4) sediment densities for Versar (1,3) and Tokamax (2,4) whiskers the minimum sediment density expected in whisker suspen- sions, and, even though this model had to be extrapolated to include the aspect ratio range of interest in the present work, Fig. 8(b) shows a reasonable agreement between the predic- tions of the model and the lowest experimental sediment (determined in n-hexane, Table 5). The finding that sediment density is a rapidly decreasing function of aspect ratio in packed sediments of anisotropic particles, regardless of their state of dispersion, explains the consistently higher sediment densities found for Tokamax whiskers.Similarly, the observation of higher sediment densi- ties in the Sic powder suspensions is understandable by the behaviour represented in Fig. 8, since l/d =1. Conclusions On the basis of sediment volume and density measurements, conditions for the effective dispersion of commercial Sic powders and whiskers have been identified for both aqueous and non-aqueous media, in which the respective adjustment of pH or the utilisation of dispersing agents provide the necessary control. The optimum pH range for aqueous processing of colloidal silicon carbide is 9-1 1.For non-aqueous dispersions, formic acid was found to provide the greatest stability, although the presence of dispersing agents, in general, maximises the effect. The compaction of spherical and highly anisotropic par- ticles alike is governed by intrinsic packing constraints for both flocculated and deflocculated systems. Use of suitable models has been shown to provide a good predictive capability for the present systems. We would like to acknowledge the assistance of Miss J. M. Bowles who provided the XPS analysis of the silicon carbide samples, and BP for permission to publish. References 1 J. V. Milewski, Adv. Ceram. Muter., 1986, 1,36. 2 H. K. Bowen and T. Ishii, in Ceramic Powder Science 11, ed.G. L. Messing, E. R. Fuller and H. Hauser, American Ceramic Society, Westerville Ohio, 1987, p. 452. 3 S. J. Barclay, J. R. Fox and H. K. Bowen, J. Muter. Sci., 1987, 22, 4403. 4 C. D. Wagner, L. E. Davis, M. V. Zeller, J. H. Taylor, R. H. Raymond and L. H. Gale, Surf Interface Anal., 1981, 3, 21 1. 5 H. Lee, R. Pober and P. Calvert, J. Colloid Interface Sci., 1986, 110, 144. 6 S. Mizuta, M. Parish and H. K. Bowen, Ceram. Znt., 1984, 10, 43. 7 S. H. Bell and V. T. Crowl, in Dispersion of Powders in Liquids, ed. G. D. Parfitt, Applied Science Publishers, London, 1973, ch. 7. 8 R. Buscall, Colloid Surf, 1982, 5, 269. 9 J. H. Adair, B. C. Mutsuddy and E. J. Drauglis, Ado. Ceram. Muter., 1988, 3, 231. 10 E. Fitzer and R. Ebi, in SiZicon Carbide, ed. R. C. Marshall, J. W. Faust Jr. and C. R. Ryan, Univ. S. Carolina Press, Columbia SC, 1973, p. 320. 11 P. K. Whitman and D. L. Feke, Adv. Ceram. Muter., 1986,1,366. 12 F. Takao, R. Cannon and S. C. Danforth, Ceram. Eng. Sci. Proc., 1986, 7, 990. 13 E. J. W. Verwey and J. Th. G. Overbeek, in The Theory of the Stability of Lyophobic Colloids, Elsevier, Amsterdam, 1948. 14 T. W. Healy and L. R. White, Adv. Colloid Interface Sci., 1978, 9, 303. 15 R. K. Iler, in The Chemistry of Silica, John Wiley, New York, 1979, p. 75. 16 M. Okuyama, G. J. Garvey, T.A. Ring and J. S. Haggerty, J. Am. Ceram. SOC., 1989,72, 1918. 17 R. J. Pugh, T. Matsunaga and F. M. Fowkes, Colloid Surf, 1983, 7, 183. 18 M. J. Vold, J. Phys. Chem., 1959, 63, 1608. Paper 0/05242F; Received 21st November, 1990

ISSN:0959-9428    

DOI:10.1039/JM9910100393

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(3), 401-407 40 1 Radiation Chemical Yields and Lithographic Performance of Electron-beam Resists based on Poly(methy1styrene-co-chlorostyrene) Richard G. Jones,*' Philip C. Miller Tate' and David R. Brambleyb 'Centre for Materials Research, Chemical Laboratory, University of Kent at Canterbury, Canterbury, Kent CT2 7NH, UK bAdvanced Lithography Research Initiative, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX1 1 OQX, UK Copolymers of methylstyrene and chlorostyrene cross-link when irradiated with 20 keV electrons and hence act as negative-working electron-beam resists. eMethylstyrenelgchlorostyrene and pmethylstyrenelpchlorosty-rene copolymers have been prepared by a free-radical mechanism over the entire composition range and the lithographic performance of the materials has been evaluated.Radiation chemical yields for cross-linking and chain scission have also been estimated. None of the materials undergoes significant chain scission upon irradiation. In contrast to the corresponding methylstyrene/chloromethylstyrene copolymer systems, the resist sensitivities maximize at compositions containing ca. 30% chlorostyrene. A cross-linking mechanism involving an excited-state charge-transfer interaction of adjacent methylstyrene and chlorostyrene chain units is proposed. The copolymers of optimal composition display sufficiently high lithographic sensitivities and contrasts to commend their application as electron-beam resists. Keywords: Lithography; Electron-beam resist; Polflmethylstyreneco-chlorostyrene) ; Copolymer The use of substituted polystyrene derivatives as negative- working electron-beam resists has been well established for more than a decade.Polystyrene itself was demonstrated to be a high-resolution resist when the technology was compara- tively young, but it lacks sensitivity to the electron beam and does not display the high lithographic contrast obtainable from comparably sensitive positive-working resists based on poly(methy1 methacrylate). However, the aromatic structure imparts a high dry-etch durability to resists of this class, which, together with their good film-forming capability, has caused their use in the manufacture of application-specific VLSI devices and in mask-making to endure.It was soon recognised that polychlorostyrene displayed a 10-fold greater sensitivity than polystyrene.' This has been attributed* simply to the increased likelihood of cleavage of the weaker aromatic carbon-chlorine bond when compared with the aromatic carbon-hydrogen bond. Another property to be considered in this context is the greater electron capture cross-section of the chlorine atom. It is important to recognise, however, that the mechanisms of energy transfer from the incident beam to the resist are too complex to allow a simple quantitative analysis of sensitivity in these terms alone. Ring substitution with a chloromethyl group further enhances the inherent sensitivity by at least a factor of four, again owing to a weaker carbon-chlorine bond but further aided by the resonance stability of the resulting benzyl radical.Unfortu- nately, both polychlorostyrene and polychloromethylstyrene undergo significant chain scission upon irradiation.* This suppresses the inherent sensitivity and, worse still, leads to a reduced contra~t.~ However, it has been shown many times that it is not necessary to have a chlorine content as high as one atom per monomer unit in order to realise useful improve- ments in resist sensitivity4*' and lower chlorine contents permit such improvements without greatly compromising lithographic contrast and resolution. Although both exper- imental and commercial resists consisting of either partially chlorinated or chloromethylated polystyrene have been pro- duced, far better overall performance has been achieved from materials that are essentially copolymers of methylstyrene and chloromethylstyrene, usually containing ca.10-20% of the latter. The radiation chemical processes leading to cross-linking in these resists are now well understood.6 Such resists can be produced in a variety of ways. These include non-specific chlorination of polymethyl~tyrene,~ site-specific monochlorination of polymethylstyrene' and through copolymerization of a methylstyrene with chloromethylstyr- ene.'p9 The first two methods have the advantage that the precursor polymer can be synthesized by an anionic mechan- ism to produce a narrow molecular-weight distribution, a feature which enhances the attainable contrast con~iderably.~ However, they both have their disadvantages.The former results in main-chain as well as substituent methyl chlori- nation, thus enhancing undesirable radiation-induced chain- scission reactions, and the latter is restricted to maximum degrees of chlorination of CQ. 20%. Nonetheless, it has recently been demonstrated" that the presence of 20% or more of o-methylstyrene units in a range of chlorinated polymethyl- styrene resists enhances their lithographic performance by suppressing chain scission. The mechanism of this effect is not entirely clear but is postulated as being due to a steric interaction of the chain and the ortho methyl group which raises the energy of chain-centred benzylic radicals, thus inhibiting their formation.Another possible explanation involves intramolecular hydrogen-atom abstraction from an adjacent ortho methyl group by any benzylic radicals formed at chain-centred a-carbon atoms, thus offering a deactivation pathway as an alternative to Q-scission. Preparations based on the copolymerization of methyl- styrenes with chloromethylstyrene have the advantage of enabling the synthesis of materials with the content of chlor- ine-containing units continuously variable from 0-100%. Backbone chlorination is precluded, although the molecular weights and polydispersities are confined to those attainable: through free-radical polymerization. This limits the resist contrast but it has been advocated by Ledwith et al.as a method of achieving substituent positional specificity for investigations of the radiation chemistry of such materials.' It was subsequently applied in lithographic studies of copoly- mers of o-, rn-and p-methylstyrene with chloromethylstyrene.9 Chloromethylstyrene monomer is only available commercially as vinyl benzyl chloride (VBC), a 2: 1 mixture of the rneta and para isomers. Complete positional specificity of the chlor- omethyl substituents was not therefore achieved. Copolymers of methylstyrene and chlorostyrene are not so constrained as all three isomers of monochlorostyrene are readily available. The poly(o-methylstyrene-co-p-chlorostyrene)and poly(p-methylstyrene-co-p-chlorostyrene)systems that are the subject of this paper have been chosen for study to elucidate further the mechanisms of radiation-induced chain scission and cross- linking in chlorine-containing styrene-based resists.Experimental Materials p-Methylstyrene (pMS) and o-methylstyrene (oMS) were sup- plied by Lancaster Synthesis and p-chlorostyrene (pCS) by Koch-Light Laboratories. The monomers were distilled at 40-50 "C under reduced pressure prior to use. Polymerizations were carried out in stoppered boiling tubes. A total of 6 cm3 of monomers together with 10cm3 of dry toluene and an appropriate amount of 2,2'-azobis(isobutyronitri1e) were deaereated with dry argon for 15 min before the tubes were immersed for ca. 5 h in a water bath maintained at a tempera- ture of 70 "C.Polymers were precipitated in a large excess of methanol, reprecipitated from toluene solution, filtered and dried at 80 "C. With one exception, all copolymer yields were maintained within the range 14-25%. Apparatus and Procedures Copolymer compositions were established from NMR spectra obtained using a JEOL JNM-GX270 spectrometer operating at 67.8 MHz. Chemical shifts were relative to TMS. Peak-area measurements were achieved using inverse-gated decoupling with a 5 s pulse delay. Monomer reactivity ratios were estimated by curve-fitting to the feed composition- polymer composition data using non-linear regression analysis in accordance with the 'terminal' model of monomer reactivity. Molecular weights were obtained as linear polystyrene equivalents using HPLC equipment supplied by Polymer Laboratories and equipped with a 5 pm PLgel dual column bank of lo4 and 500A.Glass transition temperatures were determined using a Perkin-Elmer DSC-7 differential scanning calorimeter.Thin films of the polymers ca.0.5 pm thick were spin-coated from 10% solutions in chlorobenzene onto 3 int silicon wafers using a Headway EC-101 spinner, and prebaked at 120°C for 30 min. The wafers were cut into quadrants for density measurements and lithographic assessments of the resists. Film thicknesses before and after exposure were measured using a Taylor Hobson Talystep. Film densities were deter- mined by calculation from the film thicknesses, the areas of the quadrants and their masses before and after resist removal.Masses were measured to 10 pg accuracy. Lithographic assessment was accomplished using a Cam- bridge Instruments EBMF-2 electron-beam lithography tool operating at 20 kV accelerating potential. Exposed resists were developed in methyl isobutyl ketone (MIBK) for 60s, rinsed in isopropyl alcohol for 30 s and dried in a stream of nitrogen. Sensitivities were estimated as the dose correspond- ing to 50% thickness remaining after development (D:.').All thicknesses were normalised to the original spun thickness. 7 1 in=2.54 cm J. MATER. CHEM., 1991, VOL. 1 Lithographic contrasts (y) were calculated from D:.' and the gel dose (D:)using Radiation chemical yields were determined in accordance with the method described by Hartney' and using the approxi- mations to radiation dose and gel fraction developed by Novembre and Bowmer." This involves obtaining a best fit of Inokuti's equation [eqn.(2) below] to the lithographic data by minimising the sum of the squared differences between normalised thicknesses remaining (assumed equivalent to the gel fraction) and those values of gel fraction generated by the equation, over an array of possible Gs and Gx values: 1 -g=A-3(A2A+4Ag[1 -(1 +Ay/Q)-']/y +4g2A[1 +Ay/B]-@+')) (2) where g is the gel fraction, A =Gs/Gx, j7=l/(Mw/Mn-l), A = A+2g, y= 1.04 x 10-6GxMnr,r is the radiation dose in Mrad, Gsand Gx are the radiation chemical yields for scission and cross-linking, respectively, M,and Mw are the number- and weight-average molecular weights respectively.There are a number of considerations pertinent to the application of Hartney's method for the determination of G values and the reader is referred to the original paper2 for clarification. Results Reactivity ratios were calculated from the data of Fig. 1 and 2 which show the variations in the compositions of the copolymers with those of the feedstocks for the pCS/oMS and pCS/pMS series respectively. For the pCS/oMS system with pCS designated as monomer A, rA= 1.406+0.075 and rB= 0.353 & 0.014 (rArB =0.496). Similarly for the pCS/pMS system rA=0.919 & 0.055 and rB =0.543 +0.030 (rArB =0.499). The lat- ter pair of values compares favourably with those reported in the Polymer Handbook12 (rA= 1.150, rg=0.610).These figures indicate that in both cases pCS is the preferred monomer but that the copolymer structures are essentially statistical. The relevant physical and lithographic properties of the two series, together with those of polystyrene, are presented in Table 1.The reasonable substitution of normalised film thickness for gel fraction in Inokuti's equation relies upon uniform gelation occurring throughout the thickness of the film and also upon complete removal of the soluble fraction 1 .o h c 0.8 .-4-E .c-0.6 Y L Q)E5 0.L 9. K.-v)u Q 0.2 0.0 0-0 0.2 0.L 0.6 0.8 1.o pCS in feed (mol fraction) Fig. 1 Composition plot for pCS/oMS copolymers J.MATER. CHEM., 1991, VOL. 1 Table 1 Physical and lithographic properties of pCS/MS copolymer resists pCS(%) M, M, M,/M, 7JK D,"/pC cm-' D:.'/pC cm-' y G, Gx C/(Mrad/pC cm-') (I/R0.') M,/106 Mrad-' OMS 0 7700 11300 1.5 402 405 638 2.5 0.00 0.05 2.12 0.07 8 16000 27700 1.7 412 28.1 pCS/oMS 46.6 2.3 0.05 0.32 2.05 0.38 14 16400 30000 1.8 412 21.1 32.1 2.7 0.05 0.44 2.08 0.50 19 23600 43900 1.9 41 1 11.4 16.9 2.9 0.00 0.56 2.09 0.64 23 24000 44400 1.9 410 10.9 17.2 2.5 0.04 0.51 2.17 0.60 28 24100 46600 1.9 408 9.2 14.3 2.6 0.09 0.62 2.18 0.69 35 6300 10100 1.6 398 46.0 73.0 2.5 0.00 0.59 1.98 0.69 54 7300 12300 1.7 398 40.0 63.5 2.5 0.00 0.53 2.02 0.63 73 8700 14600 I .7 397 37.0 61.0 2.3 0.05 0.49 2.03 0.55 87 10100 17500 1.7 400 36.5 62.0 2.2 0.13 0.43 2.00 0.46 PCS 100 10700 18100 1.7 402 40.0 69.5 2.1 0.18 0.40 2.04 0.39 0 34100 58000 1.7 388 48.6 77.:MS 2.5 0.02 0.09 2.18 0.10 PCSlPMS 3 17100 29400 1.7 386 18.1 28.8 2.5 0.02 0.47 2.04 0.58 3 15900 26900 1.7 386 15.0 23.5 2.6 0.10 0.64 2.14 0.74 9 15500 26200 1.7 388 12.5 19.9 2.5 0.11 0.75 2.17 0.88 28 39600 69500 1.8 396 3.9 6.1 2.6 0.03 0.85 2.21 1.07 47 41400 74800 1.8 398 4.0 6.2 2.6 0.00 0.73 2.39 0.90 64 42900 78200 1.8 40 1 4.4 6.7 2.7 0.02 0.75 2.17 0.88 80 46900 82700 1.8 406 4.4 6.5 3.0 0.09 0.75 2.24 0.83 0 41200 70900 1.7 357 97 styrene 155 2.5 0.02 0.04 2.03 0.04 spun polymer films may differ significantly from those of bulk 1.o samples, a series of density measurements were made on some of the spun samples.Fig. 3 shows a plot of density against composition for the copolymers of the pCS/pMS series. Even oa .-0 though the plot is not perfectly linear, it is reasonable to c. E estimate the density of a given copolymer by way of a linear c-interpolation between the points corresponding to the homo- 0 0.6 E polymers since the direct measurement technique is tedious v L and of limited accuracy. This method has been employed for (u both series of copolymers. The calculated values of C deviateg0.L significantly from 2 and by interpreting the lithographic data n in terms of absorbed radiation dose (R) rather than charge .-c dose (D),this has been taken into account.(I)0 Q. 02 The final column of Table 1 is the reciprocal of the reactivity of the resist, where R0.5is the absorbed energy dose in Mrad corresponding to D:.'. RO.' is employed rather than the / absorbed radiation dose at the gel point, Ro, as the former I I I I 1 00 02 OL 06 08 10 may be more accurately determined, so giving rise to less pCS in feed (mol fraction) experimental scatter when correlated with other parameters. Fig. 2 Composition plot for pCS/pMS copolymers The reciprocal reactivity (1 /RO.'MW)is an excellent indication of the intrinsic sensitivity of a resist in that it provides a by the developing solvent.Neither increased immersion time numerical parameter which increases with increasing suscepti- in MIBK nor a post-development bake at 120 "Cfor 30 min, bility to cross-linking by radiation. It is independent of which was applied to a few samples taken at random, had Tany measurable effect on thickness remaining after develop- 1l5 ment. Hence, the necessary conditions are met and the use of normalised film-thickness measurements as a measure of gel fraction for use in the Inokuti equation is justified. These observations are also in accordance with those published recently regarding free volume in resist layers, in which polystyrene was used for purposes of compari~on.'~ The dose conversion factor, C, referred to in Table 1, is used to convert the charge dose of the incident electron beam in pC cm-2 to the equivalent absorbed energy dose in Mrad in accordance with the method of ref.11. C depends on the density of the resist film and also upon its thickness and for a wide range of polymers of this type it can be approximated to 2 if the 00 c2 OL 36 oe 1c film thickness is of the order of 0.5 pm and its density is pCS in polymer (mol fraction) ca. 1.1 g cmA3. However, bulk poly-p-chlorostyrene has a den- Fig. 3 Variation of the density of spun films with composition in the sity significantly greater than this, and since the density of pCS/pMS copolymer series 404 molecular weight and therefore is determined solely by poly- mer chain microstructure.Its correlation with radiation chemi- cal yields has previously been reported for the chlorinated polymethylstyrenes.lo A typical characteristic exposure curve is shown in Fig. 4. The radiation chemical yields, together with the number- and weight-average molecular weights, completely determine the theoretical response of the polymer to the radiation by way of Inokuti’s model. It is this response that is represented by the line on the figure. The different representation depicted in Fig. 5, in accordance with the Charlesby-Pinner eq~ation,’~ is more revealing: s +,/s =Gs/2Gx+9.65 x 105/MwGxr (3) where s (= 1 -g) is the soluble fraction. Here the Inokuti fit is almost linear (theory predicts that it should be linear for materials with a polydispersity of 2) but the deviation of the data points from the theoretical curve follows a distinctive pattern; rather than being randomly distributed about the curve they appear to follow a contour defined by two intersecting curves which are represented on the plot as broken lines.This behaviour is observed for nearly all of the materials assessed during the course of this study but, as yet, no explanation can be offered; it may just be an artefact. What is clear, however, is that any lithographic contrast derived from a linear-regression analysis of the almost linear section of the plot between the gel point and the point at which 50% normalised thickness remains can give rise to a value rather different from that derived from Inokuti’s model, which for a material with a polydispersity of 1.8 is 2.45.In the case of the system of Fig. 5, the measured contrast is 2.6. In Table 1, values up to 3.0 are recorded. A limitation of the Inokuti model is that it will always provide a gel dose appropriate to the polydispersity and G values of the resist, c3, ’0 0 .-C L c 0.0 1 3 L 6 10 20 LO 60 106’ 1000 dose/pC cm-‘ Fig. 4 Characteristic exposure curve for the pCS/pMS copolymer containing 28% pCS (the curve has been fitted using the Inokuti equation and derived G values) I pGO II 000 0 05 010 0 15 R-’/M rad-’ Fig. 5 Charlesby-Pinner plot for the system of Fig. 4 J. MATER. CHEM., 1991, VOL.1 and in some cases this can lead to the ‘best fit’ deviating markedly from the data points at the low-dose end of the plot. Another feature that is apparent in Fig. 4 is a point at very high dose (1000 pC cm-*) at which there is a significant soluble fraction and a concomitant loss in resist thickness. This was again observed for the majority of samples and is believed to arise when saturation of cross-linking has been achieved. Under these circumstances, although no further cross-linking occurs (either because all possible sites for cross- linking have undergone reaction or because the rigidity brought about by cross-linking prevents sufficient molecular motion to permit further cross-link formation), scission pro- cesses continue.This can lead only to an increase in the soluble fraction and is a situation that is commonly evident when negative-working resists are exposed to doses some 200 times greater than the gel dose. For a polymer of initial number-average molecular weight of ca. 40 000, this dose would correspond to a cross-link density that would involve about one in two of the repeat units, and is a situation in which the polymer is most unlikely to undergo further cross- linking. However, the point is made more to draw attention to the effect than to explain it, for it has been common practice to normalise resist thicknesses to the thickness remaining after exposure to very high doses rather than to the initial thickness. This can lead only to erroneous analysis of the data. Fig.6 depicts the variation of intrinsic sensitivity with copolymer composition. With the exception of one pCS/oMS copolymer of low pCS content, the intrinsic sensitivities of all of the copolymers are greater than those of either of the relevant homopolymers. Both series follow the same general trend, with the pCS/pMS resists being more sensitive than the pCS/oMS materials of the same pCS content. The plots are asymmetric, with maxima at compositions corresponding to a pCS content of between 25 and 35%. A similar variation has been reported by Whipps” for copolymers of styrene and pCS used as electron-beam resists (the so-called ‘HSN/HRN’ series). Fig. 7 depicts the variation in radiation chemical yields with composition and it is clear that the trends in intrinsic sensitivity arise from a similar trend in Gx values. The corresponding Gs values are small for copolymers containing <75% pCS units and only become significant at even higher pCS contents.Though low, the Gs values appear to reach a minimum of zero at 50% pCS content, and, more significantly, they do not differ greatly between the two copolymer series.? ?Note that Gs values cannot be determined as accurately as Gx values since a small error in film thickness measurement, particularly of the original spun thickness to which all other values are normalised, will lead to a much larger error in Gsthan in G,. > lo 00 0 0 0 0 0 00 c 20 LO 60 80 100 pCS (mol%) Fig. 6 Intrinsic sensitivity uersus composition plots: (0)pCS/pMS system; (0)pCS/oMS system J.MATER. CHEM., 1991, VOL. 1 10 OF3 / \ 0 D 06 G OL C 2G LO 60 80 130 pCS (rnol%) Fig. 7 Radiation chemical yields versus composition plots: (m) G, and (0) G,Gx values for the pCS/pMS system; (0)Gs and (0) values for the pCS/oMS system Fig. 8 and 9 are plots that relate intrinsic sensitivity to the opposing influences of cross-linking and scission in the resists. They are based on derivations from the Charlesby-Pinner equation which have been described elsewhere:" Gx -0.250Gs =4.8 x 105/R0Mw (4) Gx-0.414Gs=8.0 x 10'/RO~S~w Despite the Charlesby-Pinner equation being valid only for polymers of polydispersity 2, to date all negative-working resists that cross-link by a single-stage mechanism correlate with these equations.There is a small degree of scatter, which is apparently not dependent on polydispersity.'O In the case of the present systems, the contribution from Gs is negligible, so the correlation is essentially one of intrinsic sensitivity with Gx. However, the deviation of the data points from the line I ! 1 1 1" c5 13 15 (R0M,)-'/1O6Mrad-' Fig. 8 Correlation of G values with 'gel' intrinsic sensitivity: (0)pCS/ pMS system; (0)pCS/oMS system 22 O6OL I1I ux "2 0,' 0 00 ,' 1 1 00 05 10 (R0.5M,) -'/ 1O6 Mrad-' Fig. 9 Correlation of G values with intrinsic sensitivity: (0)pCS/ pMS system; (C)pCS/oMS system is less pronounced in Fig.9 than in Fig. 8 which is an indication of the greater accuracy in the determination of RO.' as opposed to Ro. This further underpins the previously recommended use of the former in resist sensitivity assessment, despite the theoretical premise that the latter is the more fundamental parameter. Discussion The notable observations from the lithographic results are (i) that, contrary to expectation, both the pCS/oMS and pCS/ pMS systems demonstrate negligible scission over a wide range of compositions, and (ii) the synergy that arises from the mutual incorporation of pCS and MS in these resists. Although there are at least two possible explanations for the absence of radiation-induced chain scission in styrene-based polymers containing a significant amount of ortho methyl groups, no ready explanation is forthcoming for the inhibition of scission in the pCS/pMS system.However, it is now apparent that significant scission is observed to occur only in those substituted polystyrene resists that are sensitized by chloromethyl groups. Thus it is possible that it is the presence of a chloromethyl group on a ring which facilitates the abstraction of the hydrogen atom from the chain a-carbon atom which is required for chain scission to ensue. If chloro- substitution of the ring does not activate the a-carbon atom in a similar way, then there is no reason why radiation- induced chain scission should be greater for the copolymers than for the homopolymers. The Gs values for polystyrene, poly-pMS and poly-oMS are virtually zero, and for poly-pCS it is only 0.18.The enhanced intrinsic sensitivities of the copolymers of both series over those of the relevant homopolymers means that the intrinsic sensitivity of a pCS/pMS resist of ca. 30% pCS content is not significantly less than those of resists based on the corresponding VBC/MS copolymers. The intrinsic sensitivity of the optimal pCS/pMS material (1.07 x Mrad-') is half that of comparable CMS resists [i.e. poly-(pMS-co-VBC) or chlorinated poly-pMS] which is a consider- able improvement over the four-fold to five-fold difference in the intrinsic sensitivities of poly-pCS and poly-VBC. It is possible to produce resists based on VBC which display intrinsic sensitivities approaching 3 x Mrad-but such materials have large Gs values and consequently offer poor contrast.The Inokuti model predicts that contrast is invariant with molecular weight3 so, in theory, the pCS/pMS copolymer resists, as with all similar materials, can be tailored to a particular working sensitivity simply by synthesizing them with the appropriate molecular weight. With the current generation of electron-beam machines there is less emphasis on very high sensitivity than on high resolution, and the D:.' values of ca. 6 pC cm-' demonstrated here are acceptable for many types of production lithography. The predominant limitation to performance will be resist swelling during devel- opment which, as for all solvent-developed negative-working polymeric resists, inevitably worsens with increasing molecular weight.However, it has been demonstrated previously,16 through work on related systems, that this effect can be minimised by judicious selection of the developer. The existence of the maxima in the intrinsic sensitivity and Gx uersus composition plots is to be the subject of further investigation. That all the copolymers exhibit a higher intrinsic sensitivity than either of the relevant homopolymers indicates that the radiation-induced cross-linking requires the adjacency of dissimilar chain units and is related to sequence distri- butions. Assuming that this effect operates over and above the cross-linking mechanisms that are inherent to the homo- polymers, it can be rationalised in terms of the involvement CH2-CH-CH,-CH--CH2-CH-CH2-CH-I CH3 c1bIQjCH3 1 pCH,-CH-CH,-CH-I I -CH2-C -CH,-CH-I I -CH-CH,-Scheme 1 of an intramolecular excited-state charge-transfer interaction of adjacent pCS and MS chain units.The following mechanism (Scheme l), in which the charge-transfer complex reacts with a nearby MS unit of another chain to form two comparatively stable benzylic radicals as cross-link precursors, is provision- ally proposed; it is assumed that in the absence of such a reactive pathway, the exciplex would simply deactivate. One of the radicals has been represented as being chain-centred at the or-carbon atom and the other at what was formerly a substituent methyl group. The a-radical is considered to be the more probable structure for the first of these, as proton elimination from the methyl group would result in a mar- ginally less stable radical.Although this less stable structure is the chosen representation for the second radical, depending upon which group (chain methine or substituent methyl) was the most favourably placed for H-atom abstraction, either structure might result. On this basis it is also possible to rationalize the similar behaviour observed by Whipps for the poly( pCS-co-styrene) system. That cross-linking by this mechanism should occur in the vicinity of the track of an electron (either primary or second- ary) depends on three probabilities: (i) the probability (P1) that the electron encounters chlorine atoms of pCS units; (ii) the probability (P2)that the pCS units have adjacent MS units on the same chain with which to form an exciplex, and (iii) the probability (P3) that there are MS units of other chains sufficiently close as to be available for intermolecular hydrogen atom abstraction by the resultant exciplexes within their lifetime.P1 is simply proportional to mA, the mole fraction of chlorostyrene in the copolymer and P3 is similarly pro- portional to mB, the mole fraction of methylstyrene in the copolymer. Pz on the other hand, is compounded from PBAA, PAAB and PBAB, the respective probabilities that a given chlorostyrene unit has a methylstyrene unit to its left, to its right, and on both sides, and is given by P2 =PBAA+PAAB +2PBAB (6) The multiplying factor, 2, of the last term is introduced because there will be twice the probability of exciplex forma- tion if the chlorostyrene unit is bounded by methylstyrene units on both sides.Harwood and Ritchey ”have derived expressions for these sequence probabilities in terms of an index called the run J. MATER. CHEM., 1991, VOL. 1 number (R),which is defined as the average number of both A and B monomer sequences (runs) occurring in a copolymer per 100 monomer units: R(100 mA -R/2) 2(100 mA)2PBAA=PAAB = (7) PBAB =(R/2OOmA)’ (8) P2 =R/1OOmA (9) R and the overall probability of cross-link formation (P= P1P2P3)are given by Assuming that the cross-linking mechanisms inherent to the homopolymers leads to a linearly varying contribution to Gxover the whole composition range, then Gxwill be given by an equation of the form of where K, A and B are constants.For the pCS/pMS system A=0.31 and B=0.09, and for the pCS/oMS system A=0.35 and B =0.05. It is thus possible to determine optimum values of K by the application of a least-squares fitting routine to the data of Fig. 7. The lines represented on Fig. 7 have been determined in this way. The fit is quite acceptable for the pCS/oMS system for which the data points are more abundant. In the range 9-80% pCS content in the pCS/pMS system, it might well be argued that the observed variation in G, is solely within the bounds of experimental error. Nonetheless, the data points follow a similar trend and a maximum is discernible in the same region as that observed for the pCS/ OMS system.Thus, it is not unreasonable to conclude that the same arguments apply to the pCS/pMS system, even though the observed fit is markedly poorer. We thank SERC together with Plessey Research Caswell Ltd. for financial support. We also thank Mr. R. H. Bennett of Plessey Research for his help with the lithographic evaluations, and Professor J. M. G. Cowie and Dr. R. Ferguson of Heriot-Watt University for providing the computer program for the estimation of reactivity ratios. References E. D. Feit, L. F. Thompson, C. W. Wilkins, M. E. Wurtz, E. M. Doerries and L. E. Stillwagon, J. Vac. Sci. Technol., 1979, 16, 1997.M. A. Hartney, J. Appl. Polym. Sci., 1989, 37, 695. D. R. Brambley, R. G. Jones, Y. Matsubayashi and P. C. Miller Tate, J. Vac. Sci. Technol. B, 1990, 8, 1412. S. Imamura, T. Tamamura, K. Harada and S. Sugawara, J. Appl. Polym. Sci., 1982, 27, 937. R. G. Tarascon, M. A. Hartney and M. J. Bowden, Materials for Microlithography, American Chemical Society, Washington DC, 1984, p. 361. (a) Y. Tabata, S. Tagawa and M. Washio, Materials for Microli-thography, American Chemical Society, Washington DC, 1984, p. 151. (b) S. Tagawa, Polymers for High Technology, American Chemical Society, Washington DC, 1989, p. 37. (c) R. G. Jones, Y. Matsubayashi and N. J. Haskins, Eur. Polym. J., 1989, 25, 701. R. G. Jones and Y. Matsubayashi, Polymer, 1990, 31, 1519. A. Ledwith, M. Mills, P. Hendy, A. Brown, S. Clements and R. Moody, J. Vac. Sci. Technol B, 1985, 3, 339. J. MATER. CHEM., 1991, VOL. 1 407 9 10 11 12 13 L1. G. Griffiths, R. G. Jones, D. R. Brambley and P. C. Miller Tate, Makromol. Chem. (Macromol. Symp. Ser.), 1989, 24, 201. R. G. Jones, Y. Matsubayashi, P. C. Miller Tate and D. R. Brambley, J. Electrochem. SOC., 1990, 137(9), 2820. A. Novembre and T. N. Bowmer, Materials for Microlithography, American Chemical Society, Washington DC, 1984, p. 241. Polymer Handbook, ed. J. Brandrup and E. H. Immergut, Wiley- Interscience, New York, 3rd edn., 1989, p. 11230. P. Paniez, M. Pons and 0.Joubert, Microcircuit Engineering 11, Elsevier, Amsterdam, 1990, p. 469. 14 15 16 17 A. Charlesby and S. H. Pinner, Proc. R. SOC. London, Ser. A, 1959, 249, 367. P. W. Whipps, Proceedings of the Microcircuit Engineering '79 Conference (Microstructure Fabrication), 1980, p. 1 18. J. M. G. Cowie, personal communication. H. J. Harwood and W. M. Ritchey, Polymer Lett., 1964, 2, 601. Paper 0/05368F; Received 28th November, 1990

ISSN:0959-9428    

DOI:10.1039/JM9910100401

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991,1(3), 409-413 Electron Microscopic Study of the Morphology of Lead Sulphide and Silver Sulphide Crystals obtained by the Silica Gel Crystal Growth Technique Pilar Aragon-Santamaria, Maria Jesus Santos-Delgado,* Amalia Maceira-Vidan and Luis Maria Polo-Diez Departamento de Quimica Analitica, Facultad de Ciencias Quimicas, Universidad Complutense, 28040 Madrid, Spain An electron microscopic study has been performed of the morphological changes in PbS and Ag,S crystals during their growth by the silica gel technique in different inorganic acidic gel media using U-tubes. The formation and growth of the PbS and Ag,S crystals were followed by a binocular lens and an optical microscope. The morphological changes of the PbS crystals obtained in an HCI gel medium were the same as those observed for the Ag,S crystals in HCIO, and HNO, gels.The sequence began with the formation of single crystals showing dendritic growth, and was followed by growth along the [loo] direction. The changes were remarkably different for PbS crystals obtained in HCIO, and HNO, gel media. Whereas the first steps also involved the formation of dendritic crystals, subsequently the crystals grew along [lll]. Therefore, morphological changes are due to different nucleus formation and growth rates, which depend on the nature and concentration of the acidic medium. The most perfect cubic structure is obtained for the slowest rate. These changes were confirmed by electron microscopy, and the compounds obtained were identified by X-ray diffraction.Using these techniques it was observed that PbS and Ag,S crystals grew independently in gels which diffused thioacetamide and a mixture of Pb and Ag ions, instead of yielding PbS-containing Ag' ions as an impurity. Keywords: Electron microscopy; Crystal growth; Crystal morphology; Lead sulphide; Silver sulphide The crystal growth technique in gels has become very import- ant because it is straightforward and can be used at room temperature in similar conditions to those under which crys- tals grow naturally.' Metal sulphides, especially lead and silver sulphides, are of potential interest in (a)the development of ion-selective electrodes,2 and (b) the sensitization of other conductors, e.g. exploiting the photovoltaic properties of PbS in solar cells.3 The potential applications increase the import- ance of procedures able to produce large single crystals.In previous ~ork~,~ we established the optimum conditions for obtaining single PbS and Ag,S crystals by precipitation in a homogeneous phase using thioacetamide (TAA) as a sulphide reagent; this technique yielded high-purity crystals. Lead sulphide (galena) presents a cubic structure of the NaCl type, its unit-cell parameter according to the ASTM standard 5-0592 is a =5.92 A. Silver sulphide is found in two forms, argentite (a-phase) which belongs to the cubic system, and acantite (b-phase) of the monoclinic system. According to 4-0774 and 14-72 ASTM standards, the Ag2S cubic system has a=4.899 A and the Ag2S monoclinic system has a=4.229 A, b =6.931 A,c =7.862 A and /3 =99.61'. In this paper we describe an electron microscopic study of the morphological changes of PbS and Ag,S crystals during their growth by the silica gel twin-diffusion technique in different inorganic acidic gel media using U-tube devices.The crystals have also been characterized by X-ray diffraction. Experimental Apparatus A Zeiss D-7082 optical microscope with Zeiss DCR and SR binocular lenses was used. An MUC-83 scanning electron microscope with a 25 kV acceleration voltage was also used. A Philips PW 1729 X-ray generator, a PW 2253-00 X-ray tube of 2 kW and a 6 cm diameter Laue chamber (Unican) were employed for the single-crystal technique, and a Siemens D-500 X-ray diffractometer was used for powder diffraction studies; in all studies Cu-Karadiation was employed.Glass U-tubes measuring 14 or 27cm between their arms and of 1 cm internal diameter were used for crystal growth. Reagents The following reagents were employed: Lead@) nitrate; sil- ver(1) nitrate; TAA; sodium silicate (p =1.37 g cm-3); hydro- chloric, sulphuric, perchloric and nitric acids; sodium hydroxide; the disodium salt of ethylenediaminetetraacetic acid (Na,-EDTA). All the reagents used were of analytical reagent grade. The solutions were prepared by weighing or titration against standards. Inert silica gel was prepared by mixing an Na2Si03 solution (p = 1.06 g cm-3), initially at pH 11.20, with a suitable volume of 3 mol dm-3 acid (HCl, HzS04, HC104 or HN03) to obtain the desired pH of 0.5-2.0.Procedures Preparation of PbS and Ag,S Crystals by the Silica Gel Twin-difusion Growth Technique Glass U-tubes of 1 cm internal diameter and 14 or 27 cm between their arms were used to prepare the PbS or Ag2S crystals, respectively. The central part of the U-tube was filled with the silica gel and gelation was allowed up to complete solidification at room temperature. To obtain the PbS crystals 1% TAA and 1.0 x lo-' mol dm-3 Pb(N03), solutions were added to each tube arm; when the gel contained HCl the Pb(N03), solution was prepared in EDTA. To obtain Ag,S crystals, 1.O x 10-'mol dm- AgN03 solution was added to the cation arm and allowed to diffuse about two-thirds of the way across the central part of the U-tube before adding the TAA solution to the other arm.The above procedure was followed to prepare PbS crystals in the presence of Ag', the Pb(N03)2 solution being added at the same time as the TAA. The Pb(N03)2, AgN03 and TAA solutions were renewed from the U-tube arms every 8 and 15 days, respectively (after the appearance of the first crystal nucleus). Crystals were recovered by dissolving the gels in 1 mol dm-3 NaOH and then washing the crystals with distilled water to completely remove the gel and NaOH. Characterization of PbS and Ag2S Crystals by X-Ray Diflraction (a) Single-crystal Method. This method was used to charac- terize the lead sulphide crystals obtained in the HCl gel medium at pH 1.3 using a chamber sample of 30 mm radius.(b) Powder Method. The PbS and Ag2S crystals obtained in HC104 and HN03 gel media at pH 1.52 were characterized by the powder method using a nickel filter. The working conditions were: goniometer speed 2" min-l, sensitivity 1 x 10 and silicon as internal standard. Electron Microscopy The PbS crystals obtained in HC104 and HCl gel media at pH 0.7 and 1.3, respectively, were placed on the alumina sample holder, adhered with a sticking film and metallized with gold. They were observed under the scanning electron microscope at a 25 kV acceleration voltage. Results and Discussion Morphological Development of Lead and Silver Sulphide Crystals in Different Acidic Media First, the effect of different acidic media (HCl, HC104 and H2S04) on solidification of the silica gel was studied. It was found that the gelation time rose from 3 to 10 days when the acid concentration was increased from 1.0 x 10-to 2.0 x lo-' mol dm-3, independent of the acid anion.Optical Microscopy The formation and growth of the PbS and Ag2S crystals were examined every 24 h over the first 12 weeks using the optical microscope and the binocular lens. In all cases the crystals obtained were black-grey with a metallic sheen. PbS crystals were visible after 8 days in HCl gel media, and after 7 days in HC104 and HN03 media. The nucleation density in the HCl gel medium was similar over the whole pH range (0.7-2.0) studied, because Pb" was complexed by EDTA which was added to prevent PbC1, precipitation.The complexing effect seemed to be predominant at higher pH over the increasing hydrolysis rate of TAA. In aqueous solution a sudden increase in nucleation rate was observed when the pH was increased from 1 to 2.4 However, in HC104 and HN03 gel media the nucleation density began to increase at pH 1.0, because of the increasing hydrolysis of TAA as the acid concentration decreased. In the case of the Ag,S crystals, nucleation units appeared after 10 days in the HN03 or HC104 media at pH 1.52. It was necessary to allow the AgN03 solution to diffuse two-thirds of the way across the central part of the tube before adding the TAA to the other arm of the U-tube; this was required because the S2-ion diffuses at a higher rate than the Ag' ion and the gel turned black, preventing crystal growth and observation. In HzS04 gels, no germ growth appeared during the observation period of 12 weeks.The morphological changes of the PbS crystals obtained in HCl gel medium were different from those observed in HC104 and HN03 media, as indicated in Table 1. In all these J. MATER. CHEM., 1991, VOL. 1 gel media, single dendritic crystals appeared first, but whereas the crystals obtained in the HCl medium grew along the [loo] direction, those obtained in HC104 and HN03 media grew along [ll 1). In the last two media the growth went on to form crystals with eight vertices, while in the HCl medium the octahedral [1 1 13 direction was also developed, yielding cubo-octahedral crystals with (1 1l} and { 100) faces.The growth of the { 11 l} was followed by a decrease in the relative growth rate of the (loo}. Finally, single crystals of cubic morphology (100) with up to 0.6 mm edge were obtained; owing to their compact structure they were easy to handle when the experiment finished after 8 weeks. By contrast, in HC104 and HN03 gel media hopper-shaped single crystals of cubic morphology {loo} were obtained, whose empty sites continued to be filled by the addition of new growth units. Although the crystals obtained in the HC104 medium were very large (edge length up to 1.5 mm), their structural growth was incomplete and they were fragile and disintegrated easily. In the HN03 medium the crystal size was smaller (0.6-0.8 mm edge) because the gel aged more quickly than in the other media studied, especially at high acid concentrations.The Ag,S crystals showed the same morphological changes as observed for the PbS single crystals in the HCl gel medium; they had cubic morphology {loo}, up to 0.1 mm edge length, and were difficult to handle. Electron Microscopy The above observations of the sequence of morphological changes during PbS and Ag2S crystal growth was confirmed by electron microscopy. The growth over time of a PbS single crystal was followed. To study the sequence in HCl gels, their pHs were adjusted to 1.30. Fig. 1 shows the initial dendritic morphology with the six-tip crystals growing preferentially along [1001, i.e.normal to { 100) F faces (in contradiction to the Periodic Bond Chain Theory of Hartman6). The next morphological stage (Fig. 2) shows several secondary branches, which grow along directions normal to the axis and which, because the available interdendritic spaces are very small and fill rapidly, apparently have a continuous structure. Consequently, the cross-section normal to [100) was cross-shaped. After reaching a 'critical' size, the preferential directions of growth changed drastically from [loo] to [lll]. Fig. 3 shows the base of each dendrite branch, with the cross-shaped axis appearing in the centre of the picture. The morphology obtained by preferential growth along [111) can be distinguished clearly on the outside.With continued growth, the (111) faces of the small crystals that form the dendrite disappear. Further growth yields a cube- like crystal with centred flat faces (plateaux), Fig. 4. The plateau on each face of the cube shows macro-step growth layers, which are slightly depressed at the centre. All the edges are very sharp and the crystal does not show cubo-octahedral faces. In the next morphological step (Fig. 5), {loo} and (1 1 l} faces can be distinguished as well as (100) edges which show serrated profiles due the presence of (111) surfaces. Sub- sequently a cubo-octahedral shape is observed in Fig. 6, having smooth {loo}faces and rough { 11 l} surfaces. Growth continues by the piling of blocks on the (1 1l} surfaces, until finally cubes with slightly concave {loo} faces, are obtained (Fig.7). Fig. 8-10 show a sequence of morphological changes observed in PbS single crystals obtained in the HC104 medium at pH 0.70. Fig. 8 shows a cross-section of a dendrite whose branches consist of small crystals which increase in size towards the end of the primary branches. These small crystals show {loo), { 1 lo} and { 11l} faces, and a further increase in size gives rise to only (100) faces, i.e. the final equilibrium shape begins to predominate. Fig. 9 shows hopper-shaped J. MATER. CHEM., 1991, VOL. 1 411 Table 1 Morphological changes of PbS crystals observed by optical microscopy initial phase medium face direction 'Incomplete hopper shape.Fig. 1 Dendritic PbS crystal growing preferentially along [loo] Fig. 2 Intermediate morphology of a growing PbS single crystal Fig. 3 Internal base of the dendrite branch in Fig. 2 faces, indicating that two-dimensional nucleation along the edges is the main growth mechanism. Fig. 10 is a typical hopper-shaped crystal clearly showing its growth units. The differences observed between the above two growth sequences in HC1 and HC104 media confirm the influence of intermediate phase final phase face direction face Fig. 4 Plateau. Morphology during PbS single-crystal growth Fig. 5 Intermediate morphology of PbS single crystal Fig. 6 Intermediate morphology of growing PbS single crystal the growth rate on crystal morphology. In HCl gels the PbS growth rate is slower than in HC104 gels because Pb" is complexed with EDTA.In HC104 gels this growth rate yields imperfect crystals which do not complete their structure. Because of the slow diffusion of Ag' ions in HN03 or HC104 J. MATER. CHEM., 1991, VOL. 1 Fig. 7 Final morphology of PbS single crystal Fig. 8 Cross-sectionof PbS dendrite gels, the growth pattern of Ag2S is similar to that observed for PbS in HCl gels. Effect of Ag' Ions on PbS Crystal Morphology Crystals of PbS in the presence of Ag' ions were obtained under optimum experimental conditions for both sulphides [pH 1.52 in HN03 or HC104 using a 1: 1 (v:v) Pb(N03)2:AgN03 solution, obtained from 1.0 x lo-' mol dmA3 solutions]. Three growing zones were observed with the binocular lens in the U-tube. In the first zone, close to the TAA solution, the crystals presented the same morphological sequence as the PbS crystals obtained in HN03 and HC104 gels; growth along [111] resulted in dendritic crystals with eight tips, whose edges grew up to 0.5 mm.A black precipitate appeared in the central zone (see Fig. 9 Intermediate phase of PbS single-crystal growth Fig. 10 Single hopper-shaped crystal changes as Ag2S crystals obtained in HNOJ and HC104 media. From these results it may be supposed that pure PbS and Ag2S crystals were formed in the first and the third zones, respectively. Characterization by X-Ray Diffraction Lead Sulphide Some crystallographic parameters of the lead sulphide crystals obtained in HCl gels were determined by the rotatory crystal method.The value of the unit-cell parameter was 5.936 A, which according to ASTM Standard 5-0592 is due to PbS (galena). The lead sulphide crystals obtained in HC104 and HN03 media are difficult to handle because they break easily; these crystals were characterized using the powder method. The diffraction data allow these crystals to be identified by ASTM below in the X-ray study). In the third zone some very small Standard 5-0592 as PbS (galena), which belongs to the cubic crystals appeared, which showed the same morphological system. Table 2 Unit-&ll parameters obtained by X-ray diffraction simp!e crystal cubic system, a/A acid medium PbS Ag2S HCl 5.936 - HNOJ 5.937 4.892 5.942" 5.937b HC104 5.931 4.991 5.94" 5.934b precipitate monoclinic system Ag,S -~=4.17A, b=6.92 A c=7.95 A, B= 99.8" a=4.22 A, b=6.92 A c=7.83 A, j3=99.9" a=4.21 A b =6.94 A ~=7.86A, B=99.69" a=4.19 A, b=6.93 A ~=7.83A, B=99.76" " PbS in the presence of Ag' ions in zone I; PbS in the presence of Ag' ions in zone 11.J. MATER. CHEM., 1991, VOL. 1 Silver Sulphide The powder diffraction data from silver sulphide single crystals obtained in the HNOJ and HC104 gels at pH 1.52 indicate that these crystals are ol-Ag2S which belong to the cubic system, ASTM Standard 4-0774. From diffractograms of the silver sulphide precipitate, the d-spacing and relative intensities obtained indicate that the compound obtained is /3-Ag2S (acantite) which belongs to the monoclinic system, according to ASTM Standard 14-72.Lead Sulphide in the Presence of Ag' Ions The diffraction data for the crystals which appeared in the first zone mentioned above, showed that the compound formed in both acidic media was PbS (galena) (cubic system, ASTM Standard 5-0592). Diffraction data for the precipitates which appeared in the second zone indicate a mixture of PbS (cubic system) and Ag2S (monoclinic system), according to ASTM Standard 5-0592 and 14-72, respectively. The value of the relative intensities indicates that PbS predominates. Table 2 shows the unit-cell parameters for the compounds obtained in different acidic media. These results confirm that Ag' ions are not incorporated into the PbS crystals and do not appear as an impurity in these crystals, but that crystals of both sulphides, PbS and Ag2S, grow independently.The conductivity of PbS does not change with the presence or absence of Ag' ions, which supports this concl~sion.~ The authors thank Dr. J. M. Garcia-Ruiz and the Department of Crystallography and Mineralogy, Geology Faculty and Dr. J. M. Gonzalez-Calbkt of the Inorganic Chemistry Depart- ment, Chemistry Faculty, Complutense University, for their invaluable collaboration in the present work. References J. M. Garcia-Ruiz and J. L. Amoros, Bol. R. SOC.Espafiola Hist. Nut. (Geol.), 1979, 77, 101. R. K. Paramguru, S. K. Bose and S. C. Sircar, Trans. Inst. Min. Metall., Sect. C, 1979, 88, 197. H. Hirata and K. Date, Anal. Chem., 1971, 43, 279. P. Aragon-Santamaria, M. J. Santos-Delgado, A. Maceira-Vidan, R. Gallego-Andreu and J. M. Garcia-Ruiz, Ann. Quim., 1984, 80B, 134. M. J. Santos-Delgado, P. Aragon-Santamaria, J. A. Quiroga- Estevez, A. Maceira-Vidan and R. Gallego-Andreu, XX Reunibn Bienal R.S.E.Q. Castellon, 1984, pp. 8-178. P. Hartman, Structure and Morphology in Crystal Growth, An Introduction, North Holland, Amsterdam, 1973. P. Aragon-Santamaria, Doctoral Thesis, Complutense Univer- sity, Madrid, 1987. Paper 0/05508E; Receioed 6th December, 1990

ISSN:0959-9428    

DOI:10.1039/JM9910100409

出版商:RSC    

年代:1991

数据来源: RSC