摘要:

TNew preparation of superconducting alkali-metal fullerides utilizing monomethylamine as solvent S. Cooke, S. Glenis, X. Chen, C. L. Lin and M. M. Labes* Departments of Chemistry and Physics, Temple University, Philadelphia, PA 19122, USA Monomethylamine is employed as a solvent for the preparation of alkali-metal (K, Rb, Cs and mixtures) doped C60lfullerene (c60). Not only is the initial solution homogeneous, but also the extended temperature range of this solvent, as compared to ammonia, provides favourable conditions for the preparation of single phase, well defined materials with high superconducting volume fractions. The materials were characterized by dc magnetization and X-ray diffraction measurements. Since the first report in 1991 of superconductivity in alkali- metal fullerides,' substantial effort has been devoted to developing appropriate procedures for the intercalation of alkali-metal, alkaline-earth-metal and lanthanide ions into [60lfullerene (c60).It is known that superconductivity occurs in M3C60 (M =K and Rb), which crystallizes in a face-centred cubic lattice. In this structure there are three sites associated with each c60 molecule: one is a relatively large, octahedrally coordinated position, while two smaller sites are tetrahedrally coordinated. There are a number of techniques that have been investigated in an attempt to prepare this structure with high phase purity and a high volume fraction of superconductivity. In this work, the solvent monomethylamine is used in the preparation, and considerable success is achieved in obtaining well defined materials.The synthesis of superconducting alkali-metal-doped c60 can be achieved by a number of solid-state and solution methods. One of the most recent is the rapid synthesis of superconducting alkali-metal fullerides using a microwave-induced argon plasma method.2 Among the solid-state tech- niques, perhaps the most efficient are: (a) the direct reaction of stoichiometric amounts of c60 with metal vapo~r,~.~ and (b)a method which uses M6C60 as a starting material and then mixes an appropriate amount of c60 with it to obtain the desired stoichiometry.' Although the solid-state techniques provide a useful method for producing single-phase supercon- ducting compounds, there are some difficulties with the hetero- geneous nature of the starting materials leading to limited accuracy of stoichiometric control in method (a), and possible disruption of the lattice during the mixing process in method (b).Thus, there is a wide variation in the volume fraction of superconductivity observed in products of these solid-state reactions. Alternative solution techniques using toluene6,' and ammonia*>' have also been developed to comple- ment the solid-state techniques. The samples prepared using toluene as a solvent for c60 typically show quite small diamag- netic fractions (1-5%). When ammonia is employed as a solvent, the samples frequently show some depression in the superconducting transition temperature, T,.In some cases, dynamic pumping for a few days is required in order to remove any residual ammonia, presumably resident at both octahedral and tetrahedral sites, and to obtain similar T, values to those achieved for samples prepared by solid-state reaction." One of the limitations of the use of ammonia as a solvent is that the reaction is restricted to being conducted in its liquid temperature range of -78 to -33 "C. Alkylamines share the property of being solvents for both alkali-metals and c60. It is for this reason that we undertook, for the first time, the preparation of Rb3C60, Rb2CsC, and K3C60 from monomethyl- amine solutions. Monomethylamine extends the operating temperature range for conducting the reaction from -93 to -6 "C.The monomethylamine technique is found to furnish an efficient method for producing a variety of intercalation compounds which typically have large superconducting volume fractions as well as the same T, and lattice parameters achiev- able by other solid-state and solution techniques. Dissolving alkali-metals in liquid amines creates a reductive environment, the well known Birch reduction." There is, therefore, concern that preparation of the alkali-metal fullerides in amine solvents may be accompanied by protonation of c60 if any adventitious source of protons, such as a trace of water, is available. The rates of such reactions are undoubtedly very slow at the temperatures involved, and there has been no analytical evidence of substantial protonation occurring. In this work, the criteria for 'purity' are the values of the super- conducting parameters.Experimental The c60 used was Hoechst's 'Gold Grade' and was used without further purification. All alkali-metals were obtained from Ae_s_ar with purities of >99% and were used without further purification. The monomethylamine (98%) was obtained from Matheson & Co. and required further purifi- Cation as described below. Nitrogen (99.999% Grade 5) was obtained from Airco. c60 (75mg) was weighed into a glass reactor containing a quartz-encapsulated stirring bar. The reactor was then evacuated and brought into a nitrogen atmosphere glove box, where appropriate masses of alkali- metals were added. Potassium (25 mg) was added to a second reactor containing only a quartz-encapsulated stirring bar.Both reactors were then closed, removed from the glove box, and connected to a manifold system to which methylamine and nitrogen could be added. The system was purged of oxygen and moisture. The amine was then first condensed into the reactor containing the potassium. The potassium was allowed to react with the liquid monomethylamine until a permanent dark blue colour appeared, characteristic of solvated electrons. The amine was then distilled over into the second reactor containing both c60 and the alkali metal. This reactor was kept at -10 to -6 "C for 1-3 h. In the case of the ternary Rb,CsC, sample, rubidium and caesium will react completely with 75 mg of c60 in methylamine in 1-3 h, rates that are much slower than those in ammonia.Sodium and potassium require much longer times to react completely in methylamine. Additional amine was distilled over when necessary. Note that the reaction process is accompanied by a change of colour from blue to brown for Na, K and Rb and to red-brown in the case of Cs. After the J. Muter. Chem., 1996, 6(1), 1-3 reaction was complete, the amine was allowed to boil off, leaving the solid intercalation compound on the reactor walls. The reactor was sealed and vacuum-annealed in a sand bath for 1 h at 100°C. The reactor was then returned to the glove box where the product was scraped out and packed into glass or quartz ampoules for further vacuum-annealing in a Thermolyne Type 21 100 tube furnace.Rb& and Rb2CSC6, were annealed at 375 "C for 1-2 days. X-Ray diffraction (XRD) measurements were performed using an INEL powder diffractometer. The radiation used was generated from a fixed target Cu anode operated at 40 kV and 20 mA. A flat HOPG (004) monochromator selects the charac- teristic Cu-Ka,,, radiation (1.5424 A). Estimates for grain (particle) size were made using the Scherrer formula. The temperature dependence of the magnetization was measured using a Quantum Design SQUID magnetometer. Results and Discussion Fig. 1 shows the room-temperature XRD profile of Rb& prepared from monomethylamine solution. The pattern con- sists of a number of reflections, all of which can be indexed on the basis of a face-tentred cubic (fcc) unit cell with a lattic? parameter of 14.43 A, slightly larger than the value of 14.39 A obtained in a solid-state preparation.' Unreacted c60 appears to be minimal.If one examines the XRD pattern carefully, one can find shoulders on the 200, 220 and 311 reflections, which may arise from the presence of RblC60. Further annealing of the sample results in reduction of the size of these shoulders, but Goes not remove them completely. A particle size of ca. 220 A can be deduced from the Scherrer formula. Particle sizes vary with sample preparation and annealing conditions, and constitute another important variable in defining the super- conducting properties. A preparation with a Rb2CsC6, stoichiometry p!oduced a single phase with a lattice constant of ca.14.45A (Fig. 2), .......1 .........J . . .... J 10 20 30 40 50 2Bldegrees Fig. 1 Powder XRD pattern of the Rb3C6,, sample - cn which is comparable with the latticeo parameter obtained from the solid-state preparatjon (14.49 A)." The particle size is estimated to be ca. 205 A for this sample. It is known that in Rb2CSC60, the Rb ions mainly occupy the tetrahedral sites while the Cs ion occupies the octahedral interstitial site.13 Fig. 3 shows the temperature dependence of the dc magnetiz- ation of the Rb3C60 sample which was first cooled to 6 K in zero magnetic field. The zero-field-cooled (ZFC) curve was obtained by applying a magnetic field of 10G to the sample and warming it slowly to 35 K.The field-cooled (FC) curve was obtained by cooling the sample again to 6 K in the same magnetic field. The superconducting transition temperature of both ZFC and FC curves of 28-29 K is comparable to that obtained for samples prepared by direct solid-state reaction. The shielding (ZFC) and Meissner (FC) diamagnetic fractions, estimated by comparison to the ideal value of -1/4n: for a long cylinder, are 90 and 8%, respectively. Annealing the sample for 2 days at 375°C is required to produce the high diamagnetic fraction. The reaction temperature does not appear to have a significant effect on T,. In this work, Rb& was reacted at temperatures varying from -78 to -6 "C, and T,remains at 28-29 K over the entire temperature range.Fig. 4 shows the temperature dependence of the magnetiz- ation for a sample of Rb2CSC60. Superconductivity occurs with an onset temperature of 30-31 K, similar to the T, value obtained from preparation via solid-state rea~ti0n.l~The shielding and Meissner diamagnetic fractions were found to 0 0 0 H=lOG 0 Fig. 3 Temperature dependence of the zero-field-cooled (ZFC) and field-cooled (FC) magnetic susceptibility of Rb&, measured at an applied field of 10 G ma -0.006t 0 0 00 O FC DOe oDo.*o.~o*O 0 0 D 0 H=lOG 0 -0.018 e ZFC ooDI e DO* ** I.).. . . . . . . . . . I I ......... 10 20 30 40 50 2Bldegrees Fig. 2 Powder XRD pattern of the RbzCSC6, sample Fig.4 Temperature dependence of the zero-field-cooled (ZFC) and field-cooled (FC) magnetic susceptibility of RbzCSC6, measured at an applied field of 10 G 2 J. Muter. Chem., 1996, 6(1), 1-3 be 28 and 14%, respectively. There is an extended tail below T, in the magnetization curve, instead of the expected sharp drop. The same phenomenon is also observed in the shape of the curve of R~,C~C,,samples prepared by sohid-sfate reac-tions. This extended-tail behaviour may be associated with the small particle size of the powder sample and the existence of intergranular voids. Conclusions Alkali-metal fullerides can be easily obtained by precipitation from liquid monomethylamine which, because of its extended temperature range, appears to be a very useful medium for such syntheses.The technique works particularly well with the more reactive alkali metals rubidium and caesium, whereas reactions with potassium and sodium will require extended reaction times. The high volume fraction of superconductivity observed in Rb3C60 reflects the accuracy of the end-product stoichiometry.T,and lattice constan+ If the prepared materials agree quite closely with publisLd values using solid-state techniques. Helpful discussions with Professor J. E. Fischer are gratefully acknowledged. Research sponsored by the Air Force Office of Scientific Research, Air Force Systems Command, USAF, under Grant AFOSR F49620-93-1-0018. References 1 A. F. Hebard, M. J. Rosseinsky, R.C. Haddon, D. W. Murphy, S. H. Glarum, T. T. M. Palstra, A. P. Ramirez and A. R. Kortan, Nature (London), 1991,350, 600. 2 R. E. Douthwaite, M. L. H. Green and M. J. Rosseinsky, .I. Chem. Soc., Chem. Commun., 1994, 2027. 3 M. J. Rosseinsky, A. P. Ramirez, S. H. Glarum, D. W. Murphy, R. c. Haddon, A. F. Herard, T. T. M. Palstra, A. R. Kortan, S. M. Zahurak and A. V. Makhija, Phys. Rev. Lett., 1991,662830. 4 K. Holczer, 0. Klein, S-M. Huang, R. B. Kaner, K-J. Fu, R. L. Whetten and F. Diederich, Science, 1991,252, 11 54. 5 J. P. McCauley, Q. Zhu, N. Coustel, 0. Zhou, G. Vaughan, S. H. J. Idziak, J. E. Fischer, S. W. Tozer, D. M. Groski, N. Bykovetz, C. L. Lin, A. R. McGhie, B. H. Allen, W. J. Romanow, A. M. Denenstein and A. B. Smith, J. Am.Chem. Soc., 1991, 113, 8537. 6 H. H. Wang, A. M. Kini, B. M. Savall, K. D. Carlson, J. M. Williams, K. R. Lykke, P. Wurz, D. H. Parker, M. J. Pellin, D. M. Gruen, U. Welp, W-K. Kwok, S. Fleshler and G. W. Crabtree, Inorg. Chem., 1991,30,2838. 7 H. H. Wang, A. M. Kini, B. M. Savall, K. D. Carlson, J. M. Williams, M. W. Lathrop, K. R. Lykke, D. H. Parker, P. Wurz, M. J. Pellin, D. M. Gruen, U. Welp, W-K. Kwok, S. Fleshler, G. W. Crabtree, J. E. Schirber and D. L. Overmyer, Inorg. Chem., 199 1,30,2962. 8 D. R. Buffinger, R. P. Ziebarth, V. A. Stenger, C. Recchia and C. H. Pennington, J. Am. Chem. Sac., 1993,115,9267. 9 W. K. Fullagar, I. R. Gentle, G. A. Heath and J. W. White, J. Chem. Soc., Chem. Commun., 1993, 525. 10 R. D. Boss, J. S. Briggs, E. W. Jacobs, T. E. Jones and P. A. Mosier-Boss, Physica C, 1991, 243, 29. 11 A. J. Birch and G. Subba Rao, Advances in Organic Chemistry, ed. E. C. Taylor, Wiley-Interscience, New York, 1972, vol. 8, p. 1. 12 R. M. Fleming, A. P. Ramirez, M. J. Rosseinsky, D. W. Murphy, R. C. Haddon, S. M. Zahurak and A. V. Makhija, Nature (London), 1991,352,787. 13 I. Hirosawa, K. Tanigaki, J. Mizuki, T. W. Ebbesen, Y. Shimakawa, Y. Kubo and S. Kuroshima, Solid State Commun., 1992,82,979. 14 K. Tanigaki, T. W. Ebbesen, S. Saito, J. Mizuki, J. S. Tsai, Y. Kubo and S. Kuroshima, Nature (London), 1991,352,222. Paper 5105304H; Received 8th August, 1995 J. Muter. Chem., 1996, 6( l), 1--3

ISSN:0959-9428    

DOI:10.1039/JM9960600001

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Structure and non-linear optical properties of phosphine oxide derivatives Minh Lequan," Rose Marie Lequan," Kathleen Chane-Ching," Pierre Bassoul," Georges Bravic,b Yvette Barransb and Daniel Chasseaub "Laboratoire de Chimie et d 'Electrochimie des Mathiaux moliculaires CNRS, URA 429, ESPCI 10 rue Vauquelin, 75231 Paris Cedex 05, France bLaboratoire de Cristallographie et de Physique cristalline, Universitk Bordeaux I, 351 Cours de la Libhation, 33405 Talence Cedex, France The crystal structures of three different phosphine oxides derivatives possessing high quadratic susceptibilities, p, in solution have been investigated by X-ray diffraction. Slight modification of the transmitter chain leads to an acentric molecule; however, it is shown that these polar molecules have specific arrangements in the crystalline state, leading to a weak non-linear optical (NLO) response.The crystallographic data were entered into the MOPAC program to calculate the quadratic hyperpolarizability of each structure. lhe results were compared to those obtained by the use of standard internal data of the MOPAC program. The large non-linear optical (NLO) effects found in some organic crystals make these materials attractive for applications in the field of non-linear optics. It is well known that these crystals require a non-centrosymmetric structure to exhibit second-order optical non-linearity. To this purpose, crystal engineering has been developed to obtain acentric crystals. Various chemical procedures have been used for the production of such crystals, for example the introduction of chiral or the creation of a dissymmetry in the principal molecular the substitution of small donor groups by bulky one^,^.^ or the formation of intermolecular bonds in the crystal^.^-^ We have used two of these methods to induce a non-centrosymmetric crystalline space group.(4-[4-(Dimethylamino)phenylazo]phenyl} diphenylphos- phine oxide (PONA), recently investigated for its NLO proper- ties, exhibits a high quadratic hyperpolarizability'' [b,(O) = 45 x lo-,' esu] which is comparable to that of 4-[4-(dibutylamino)phenylazo] phenylmethylsulfone;" its efficiency lies between those of the cyano and nitro analogues. This result evidences the ability of the P=O entity to withdraw an electron from a donor group in an internal transfer process.In this paper, we present a full crystallographic characteriz- ation and the optical properties of PONA, and those of two of its derivatives obtained by introducing a methyl substituent on one of the phenyl groups of the unsaturated chain (2'MPONA), or by replacing the (pheny1azo)phenyl group by the trans-stilbenyl one (PONS-E). We have complemented this study by performing semi- empirical calculations using the crystallographic data on one hand, and internal standard data provided by the MOPAC program on the other hand, to minimize the energy of the molecules and to calculate their physical properties, and we have compared the results obtained from the two methods of calculation. Experimenta1 Synthesis The 2'MPONA was prepared according to the method employed for the formation of PONA.'' By a similar method, PONS-E was prepared; its synthesis is summarized in Scheme 1.The experimental procedure for the synthesis of PONS-E was as follows. In a 250 cm3 three-necked flask equipped with a condenser under an argon atmosphere, 3.7 g (12 mmol) of the synthon bromide I were dissolved in 50cm3 THF at -60 "C. BuLi (7 cm3) in hexane (1 1 mmol) was then added. After 15 min, 2 cm3 of Ph2PCl (11 mmol) was introduced into the flask. The reaction was kept under stirring overnight while the reaction was allowed to warm to room temperature. The solvent was evaporated under vacuum to dryness and the crude residue was dissolved in 150cm3 diethyl ether.The organic layer was washed twice with water (20 cm3), then the diethyl ether was evaporated off to yield an oil which was then dissolved in 20 cm3 of acetone and oxidized at 5-10 "C with H202 (3 cm3) over 20 min (a longer oxidation time could give rise to the formation of explosive acetone peroxide). The solution was poured into water (50 cm3) and the product extracted with CH2C12. The crude phosphine oxide I1 was purified by column chromatography over silica using pentane- dichloromethane as the eluent. The crude product, containing 92% cis isomer, was heated at reflux in 80cm3 of toluene in the presence of a crystal of iodine for 1 h. The trans isomer crystallised from the solution after cooling.Mp 255 "C; Elemental analysis: Found: C, 78.78; H, 6.18; N, 3.19; P, 5.83%. Calc. for C2,H2,NOP: C, 79.43; H, 6.15; N, 3.31; P, 5.33%; 'H NMR (300 MHz, CDCl,): 6 6.70 (HA, JHAHB=8.79Hz), 6.90 (HJ, 7.14 (HD, JH,H,= 16.3Hz), 3.00 (HE); others: 7.4-7.64 (multiplets). X-Ray diffraction Crystals of PONA were prepared by slow evaporation at 6 "C from a dichloromethane-octane (1:1) solution. Crystals of 2'MPONA were recrystallized from a toluene-ethanol mixture, and crystals of PONS-E from dichloromethane-methanol. X-Ray diffraction (XRD) experiments were performed at room temperature with an Enraf-Nonius CAD4 single-crystal diffractometer equipped with a 6raphite monochromator, using Mo-K, radiation (1=0.71069 A). The structures were solved by direct methods using the MITHRIL prograrnl2 and were refined by a block-diagonal least-squares method based on F(hkl)minimizing the term Zw(lFol -IFc1)2,where w is G(F,)-~.The crystallographic data of PONA and its derivatives are given in Table 1. Calculations Calculations were performed on an IBM RS/6000 computer using the AM1 semi-empirical Hamilt~nian'~ of the MOPAC program (version 6). The molecules were designed by the use of standard data or by introducing the Cartesian coordinates J. Muter. Chem., 1996, 6(l), 5-9 1) BuLi, -60 "C, THF Ph,P 0I c CH= CH2) Ph2PCI; 3) H@2 II -0 cis + trans I1 E Scheme 1 Table 1 Crystallographic data PONA 2'MPONA PONS-E experimental linear absorption/mm-' 0.155 0.141 0.145 crystal data crystal system sp!ce group a14 bl+ CIA a/degrees jldegrees yldegrees Z v/A3 triclinic Pi 21.838(5) 6.327( 3) 8.452(2) 108.58( 3) 98.78(2) 90.97( 3) 1093.3( 6) 2 orthorhombic Pn~2~ 15.243( 3) 9.682( 2) 33.039( 6) 90 90 90 4876( 3) 8 monoclinic p218.685( 3) 43.31 (2) 6.258( 2) 90 104.30( 3) 90 2281(2) 4 DJg cmP3 1.295 1.197 1.23 3 data collection 8 limitldegrees h limits 25 -24, 24 25 -1, 18 25 -10,lO k limits -1, 7 -1,ll -1, 51 1 limits -10, 10 -1, 39 0, 7 number of reflections: unique observed (I > 34 2997 2170 2936 1705 2847 2347 corrections: lorentz polarization absorption Yes semi-empirical Yes no Yes no refinement R 0.047 0.083 0.060 Rw S 0.040 0.992 0.106 0.996 0.067 1.165 derived from the X-ray determination. Optimization was achieved according to the general procedure already described." Results and discussion XRD performed on a parallelepiped pale-red crystal of PONA (0.04 x 0.18 x 0.28 mm3) shows a triclinic system, space group Pi;the unit-cell parameters are shown in Table 1.The molecule is practically planar along the charge-transfer P-N axis. The crystal structure of PONA reveals an 'herringbone' arrange- ment, with two molecules arranged in a head-to-tail configur- ation in the unit cell (Fig. 1 and 2). The molecules are stacked in columns, with the phenyl rings of the aromatic chain parallel in an overlapping face-to-face configuration. Two adjacent Fig. 1 (a) The molecule PONA: the torsion angle 6 is small (ca.5"),molecules within a column have antiparallel charge-transfer then a represents (practically) the torsion angle between the two axes. The P=O bonds, which point along the b axis, are also aromatic rings. (b) PONA after optimization with the inertial axes are the calculated dipole moment components. in opposite directions for adjacent molecules. A dipolar inter- oriented along x, y, z. F,,~,~ J. Muter. Chem., 1996,6(l), 5-9 b Fig. 2 Crystal structure of PONA: (a)head-to-tail arrangement of two molecules in the unit cell; (b) projection of the structure on a plane orthogonal to the b axis action of the P=O bond with another molecule in the neigh- bouring column is observed. Thus, in this centrosymmetric structure the two main components of the dipole moment contribute to molecular interactions inside each column and between the columns, respectively.The strong polarization of the P=O bond frees the oxygen atom to interact with another atom to induce a pseudo-reticulation in a polymer matrix, which may increase the stability of the polymer without affecting the electron-transfer process. Modification of the PONA molecule by introducing a methyl substituent to aromatic ring 4 leads to a non-centrosymmetric crystal belonging to the space group Pna2,. The unit cell contains eight molecules, and the asymmetric unit contains two molecules in a head-to-tail configuration. They are not face-to-face overlapped as was seen previously for PONA; the planes of the two molecules are practically orthogonal to each other, as shown in Fig.3. Because of these features, although 4 Fig.3 Relative orientation of two molecules of 2'MPONA in the asymmetric unit b the crystal of 2'MPONA is acentric, it exhibits a weak second harmonic generation (SHG) response. When the azo bond is replaced by the trans ethylenic group in (PONS-E) the molecule crystallizes in a non-centrosym- metric P2, space group with four molecules in the unit cell. The asymmetric unit, shown in Fig. 4, is made up of two molecules which form an angle of 110" between the two P-N charge-transfer axes. This arrangement is different from the previous cases. b ily Fig. 5 The two molecular P-N axes of the PONS-E asymmetric unit referred to the xyz frame of hyperpolarizability coefficients bxyz C C18b 4 Fig.6 Crystallographic data of (a) PONA, (b) 2'MPONA and Fig. 4 Asymmetric unit of the PONS-E crystalline structure (c) PONS-E J. Muter. Chem., 1996, 6(l), 5-9 7 Table 2 Comparison of the bond lengths (in A) of the three phosphine oxide derivatives studied, as determined by calculation and from XRD experimental results calculated XRD bond" PONA 2'MPONA PONS-E PONA ~'MPONA~ PONS-E~ 1.473 1.473 1.473 1.483 1.49/1.47 1.48 111.472 1.629 1.629 1.630 1.818 1.8311.82 1.82811.8 17 1.630 1.630 1.630 1.798 1.78/1.81 1.816/ 1.822 1.630 1.630 1.628 1.803 1.80/1.80 1.801/1.8 15 a Parameters defined in Fig.1. Bond lengths given for the two molecules of an asymmetric unit. Table 3 Comparison of experimental and calculated parameters for PONA obtained using both crystallographic (X-ray) and stardard data P -ca,,"/A 1.803 1.803 ab/degrees 1.26 37 AH,/kcal' 230.1 229.6 CLID 5.9 5.8 p,(0)/10-~O esu 24.2 23.3 " (wo), without optimization; (o),optimized; (c), constrained (a= 37"). The crystal point group 2 generally exhibits the highest crystalline SHG yield, so we have calculated its theoretical yield, bijk, for SHG, according to the method of Zyss and Oudar.14 If we assume the crystallographic axis is y parallel to the b axis of the unit-cell (Fig. 5), the 8 angles between y and the charge-transfer axis lying in the yx plane allow us to calculate the coefficients by,, and byxxrelative to the crystal.bijk is related to dijk, the second-order NLO susceptibility of the crystal, by the expression dijk= Nf20%oifokbijk where N is the number density of molecules andfijk are the local factors at the indicated frequencies. The molecular hyper- polarizability coefficients, 8, of the two molecules inside the asymmetric unit are assumed to be identical. The charge- transfer axes of the two molecules are practically parallel to a plane such as the xy reference plane (Fig. 5). Therefore, the following equations are obtained: by,, =(cos3 el +cos3 e2)p by, =(cos 8, sin2 el +cos e2 sin2 e2)8 Crystallographic data yield values of 8, =35.77', and O2 = 145.84'. Therefore, by,, =0.03258, i.e.the yield of the crystalline SHG is 1.6% of the molecular hyperpolarizability; and by, = 0.0163, the yield is 0.8%. We attempted to measure the second harmonic generation by irradiation of the powder, but the signal was so weak that no quantitative results could be obtained. Fig. 1 shows the PONA molecule after geometrical optimiz- ation by the AM1 Hamiltonian. The principal inertial axes of the molecule are orientated along the x,y,z axes so the x component of the dipole moment is nearly parallel to the charge-transfer axis, whereas the z component is roughly in the direction of the P=O bond. The dipole moment of this molecule is the combination of two dipoles along the x and z axes, the y component being small with respect to the two other components; their contributions can be related to the stacking of the molecules in the crystal. To test the validity of the calculations in the cases of the phosphine oxides, we compared some geometrical parameters, such as interatomic distances, obtained from the theoretical and crystalline molecular conformations.Calculations making use of internal standard data show that the P-Car bonds are approximately 10% shorter than those found by XRD, whereas J. Muter. Chern., 1996, 6(l), 5-9 experimental -1.630 1.630 -37 38 -91.7 91.7 6.7 7.2 6.2' 34.0 31.0 45 Parameters defined in Fig. 1. '1cal=4.184 J. In toluene. the other bond lengths are in agreement with the experimental results within 3% (Table 2). In order to select the best input data for the determination of the physical properties of the molecules investigated in this work, such as the enthalpy of formation, AHo, the dipole moment, p, or the first hyperpolarizability coefficient, #I,we performed two sets of calculations: (i) by introducing Cartesian coordinates of PONA derived from the crystallographic data; and (ii) by making use of internal standard data supplied by the MOPAC program.The results in Table 3 show that the solid structure without optimization exhibits a very high enthalpy and a weak quad- ratic susceptibility in spite of the quasi-planarity of the struc- ture. The same structure when optimized yields an enthalpy value which is very similar to that obtained in making use of standard data, and a quadratic susceptibility value which is slightly larger.Constraining the torsion angle, M to be 37" (the value obtained from optimization of the X-ray structure) gives rise to no significant change in the value of 8,(0). In contrast, the longer P-Car bonds have a marked effect on the quadratic susceptibility. The best results determined by calculations are obtained by the use of crystallographic data with optimization; nevertheless, calculations give rise to smaller values than those determined by the experiments, in solution. This difference can probably be attributed to the experimental accuracy (cu. 20%) and to the role of local factors due to the presence of a solvent, which does not exist for a molecule in the gas phase.Whereas the structure of PONA in the solid state is practi- cally planar, calculations by AM1 with optimization show an important torsion angle between the two aromatic rings of the transmitter of approximately 38". In solution the mean value of torsion angle is probably situated between these two limits. Table 4 shows the variation of AHo and llpll us. the constrained torsion angle a, for the optimized structure of PONA by a Table4 Variation of AHo and llpll versus the constrained torsion angle, a aldegrees AH,/kcal 0 92.15 15 91.93 35 91.69 45 91.76 60 92.07 ~~~~~/10-30esu 49.5 49.4 47.9 46.5 45.2 single point calculation. The enthalpy value reaches a minimum value at an angle of ca. 37" between the two aromatic rings.The value of llpll is not significantly dependent on the torsion angle in the 15-45' range, but depends rather on the length of the valence bonds. Conclusion We have investigated the crystal structures of three phosphine oxide derivatives: PONA, 2'MPONA and PONS-E. The crys- tal of PONA is centrosymmetric. In contrast, the slight modifi- cation of the unsaturated chain gives rise to acentric space groups Pna2, and P2,; nevertheless, the yield in p is weak due to a head-to-tail arrangement of pseudo-symmetric molecules, which are not able to generate strong second harmonic light. The introduction of crystallographic data allows one to test the reliability of calculations by the use of standard data. The P-0 and P-CA, bond lengths are slightly shorter with respect to the distances determined by X-ray diffraction; never- theless, the values obtained for physical properties are similar when optimization is used.However, the use of crystallographic data gives rise to values of ,u and p which are closer to those determined by experimental measurements. References 1 J. L. Oudar and R. Hierle, J. Appl. Phys., 1977,48,2699. 2 J. Zyss, J. F. Nicoud and M. Coquillay, J. Chem. Phys., 1984, 81,4166. 3 C. C. Teng and A. F. Garito, Phys. Rev. B, 1983,28,6766. 4 W. Tam, B. Guerin, J. C. Calabrese and S. H. Stevenson, Chem. Phys. Lett., 1989, 154, 93; J. D. Bierlin, L. K. Cheng, Y. Wang and W. Tam, Appl. Phys. Lett., 1990,56,423. 5 J. F. Nicoud, Mol. Cryst. Liq. Cryst., 1988, 156,297. 6 S. Tomaru, S. Matsumoto, T. Kurihara, M. Suzuki, N. Ooba and T. Kaino, Appl. Phys. Lett., 1991,58,2583. 7 J. M. Lehn, M. Pascal, A. De Cian and J. Fisher, J. Chem. SOC., Chem. Commun., 1990,479. 8 M. C. Etter, J. Phys. Chem., 1991,95,4601. 9 M. C. Etter and K. S. Huang, Chem. Mater., 1992,4824. 10 K. Chane-Ching, M. Lequan, R. M. Lequan, C. Runser, M. Barzoukas and A. Fort, J. Mater. Chem., 1995, 5, 649; M. Lequan, R. M. Lequan, K. Chane Ching, A. C. Callier, M. Barzoukas and A. Fort, Adv. Mater. Opt. Electron., 1992, 1,243. 11 A. Ulman, C. S. Wiland, W. Kohler, D. Robello, D. J. Williams and L. Handley, J. Am. Chern. SOC.,1990,112,7083. 12 C. J. Gilmores, J. Appl. Crystallogr., 1984, 17,42. 13 M. J. S. Dewar, E. G. Zoebisch, E. F. Heary and J. J. P. Stewart, J. Am. Chem. SOC., 1985,107,3902. 14 J. Zyss and J. L. Oudar, Phys. Rev. A, 1982,26,2028. Paper 5/03274A; Received 22nh May, 1995 J, Muter. Chem., 1996, 6(1), 5-9

ISSN:0959-9428    

DOI:10.1039/JM9960600005

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

~~ Preparation of photochromic spiropyrans linked to methyl cellulose and photoregulation of their properties Kenichiro Arai," Yasutada Shitara and Takeshi Ohyama Faculty of Engineering, Gunma University, Kiryu, Gunma, 376 Japan Spiropyran linked to methyl cellulose (SP-MC) is prepared by the treatment of methyl cellulose with 1'-(2-~arboxyethyl)-3',3'-dimethyl-6-nitrospiro [2H-benzopyran-2,2'-indoline] as a photofunctional material and photoregulation of some of its properties under irradiation with UV light are investigated. The spiropyran moiety in SP-MC is shown to undergo reversible isomerization from the spiropyran form to the merocyanine form under irradiation with UV light of 1=300-400 nm and from the merocyanine form to the spiropyran form under subsequent irradiation with visible light of A>550 nm, both in solution and in film, although repeated cycles of the alternate irradiation with UV and visible light resulted in considerable fatigue of the spiropyran moieties over up to 20 cycles.The solubility of SP-MC in benzene and the contact angle of the SP-MC film with water are reversibly regulated under alternate irradiations with UV and visible light. A benzene solution of the SP-MC yielded turbidity under irradiation with UV light and the turbidity disappeared under subsequent irradiation with visible light. The contact angle of the SP-MC film with water decreased under irradiation with UV light and then increased under subsequent irradiation with visible light, the reversible change in contact angle is shown to result from the isomerization of the spiropyran moiety in SP-MC.Photoregulation of the properties of polymers containing photochromic moieties has been the subject of many investi- gations. We have investigated cellulose derivatives containing photochromic moieties such as azobenzene,1.2 cinnamate3 and ~tilbene.~.~ the trans to cis For cellulose-stilbene-4-carboxylate, isomerization of the stilbene moieties in the cellulose derivative was found to be irreversible, probably due to the coexistence of side reaction^.^ Cellulose-2-methylstilbene-5-carboxylate (MSB-cellulose), which was designed to inhibit side reactions in the photoisomerization of the stilbene moiety, was then prepared and the methylstilbene moieties in the cellulose derivative were confirmed to be isomerized reversibly under alternate irradiation with UV and visible light5 The swelling ability of MSB-cellulose in water and in benzene was reversibly regulated under the alternate irradiations due to a change in polarity of the 2-methylstilbene moiety induced by the iso- merization, although the extent of the changes in the swelling ability was not great.In the present experiment, spiropyran linked to methyl cellulose (SP-MC), in which the spiropyran moiety can be isomerized to a zwitterion type merocyanine form under irradiation with UV light and give a greater change in polarity (Scheme 1),637 is prepared and the reversible changes in the properties of the SP-MC under irradiation with UV light and subsequently with visible light, as well as the isomeriz- ation behaviour of the spiropyran moiety in the derivative, are examined.Experimental Materials 1'-(2-Carboxyethy1)-3',3'-dimethyl-6-nitrospiro [2H- benzo- pyran-2,2'-indoline] (SP-COOH) was prepared according to spiropyran merocyanine Scheme 1 the method reported by Aizawa et al.,' by the treatment of 2,3,3-trimethylindolenine with 3-iodopropionic acid to obtain l-(2-carboxyethyl)-2,3,3-trimethylindolenium iodide (CE-TMI-I), and subsequent treatment of the CE-TMI-I with 5-nitrosalicylaldehyde in the presence of piperidine. The prod- uct was identified by 'H NMR, IR and mass spectra, although the melting point of the product (198-199°C) was different from that reported by Aizawa et al.(110-111 "C). Commercially available methyl cellulose (Wako Pure Chemical Ind., 1OOcP) with a degree of substitution (DS) of 1.52 was dehydrated in uucuo over diphosphorus pentaoxide. Butan-2- one ando tetrahydrofuran (THF) were dried over molecular sieves 3 A. All other reagent-grade chemicals were used without further purification. SP-MC Methyl cellulose (0.91 g, 5.0 x mol of anhydroglucose units) and SP-COOH (3.80 g, 1.0 x mol) were dissolved in THF (80 cm3) and then N,N'-dicyclohexylcarbodiimide (2.06 g, 1.0 x lov2mol) and p-dimethylaminopyridine (0.24 g, 2.0 x lop3mol) were added. The mixture was kept at 5 "C for 2 h and then at 25°C for a given time with stirring; the antecedent reaction at lower temperature favours dehydration- dimerization of SP-COOH9,10 and then the dimer reacts with methyl cellulose at 25 "C to give SP-MC (Scheme 2).The reaction mixture was filtered to remove insoluble material and then the filtrate was poured into methanol. The precipitate was recovered and purified by reprecipitation three times from THF solution into methanol and once into n-hexane. The degree of substitution (DS) by the spiropyran moiety of the SP-MC obtained was determined by elemental analysis: e.g. C, 62.7; H, 5.8; 0,26.1; N, 5.4%; DS=1.1. Irradiation UV and visible light from a high-intensity projecting lantern (Ushio Electric Inc., Tokyo; 200 W) were used for the irradiation of SP-MC in solution and in film using colour filters (Toshiba UV-D36A and Toshiba 0-55 for A= 300-400 nm and A> 550 nm, respectively).J. Muter. Chem., 1996, 6(l), 11-14 methylcellulose f COOH SP-COOH I + SP-MC Scheme 2 SP-MC film An SP-MC film with thickness of 3 pm was prepared by casting from the SP-MC THF solution and dried in vucuo. The film was used for the examination of the isomerization behaviour of the spiropyran moiety in the film and the contact angle of the film with water. IR and UV absorption spectra IR absorption spectra of SP-COOH, methyl cellulose and SP-MC were obtained with a double-beam grating FT-IR spectrometer (Japan Spectroscopic Co., Tokyo; FT-IR-5300) using the KBr disk technique. UV absorption spectra of SP-MC in solutions and in films were obtained with a Hitachi 200 spectrometer. Turbidity of SP-MC benzene solutions Turbidity of SP-MC benzene solutions was monitored using a Hitachi 200 spectrometer at 750 nm under irradiation with UV and subsequently with visible light.Contact angle of SP-MC film The contact angle of the SP-MC film with water was measured using a contact angle meter (Kyowa Kaimenkagaku Co., Ltd, Tokyo; CA-P) at ambient temperature. Results and Discussion Fig. 1 shows the IR absorption spectra of SP-COOH, methyl cellulose and SP-MC. An absorption peak at around 3400cm-', which is assigned to the hydroxy group, in the spectrum of methyl cellulose is found to decrease with the introduction of the spiropyran moiety. An absorption peak at 3069 cm-', which is assigned to aromatic C-H, those at 1520 and 1338 cm-', assigned to N-0 in the nitro group, and those at 810 and 750cm-', assigned to substituted benzene, are found in the spectrum of SP-MC instead. Furthermore, an absorption peak at 1711 cm-' in the spectrum of SP-COOH, which is assigned to the C-0 in carboxylic acid, shifts to 1745 cm-' in that of the SP-MC, and is assigned to an ester group.These results confirm the presence of spiropyran moiet- ies in SP-MC. Fig. 2 shows a plot of the DS by spiropyran moieties versus 12 J. Muter. Chem., 1996, 6(1), 11-14 -1 1 1I....I.t..I . . . I , I 4000 3000 2000 1000 wavenumber/ cm-I Fig. 1 IR absorption spectra of (a) 1'-( 2-~arboxyethy1)-3',3'-dimethyl-6-nitrospiro[2H-benzopyran-2,2'-indoline](SP-COOH); (b) methyl cellulose [degree of substitution (DS)= 1.521 and (c)spiropyran linked to methyl cellulose (SP-MC, DS by spiropyran moiety= 1.1) 1.2 1.o 0.8 I 10.6 I I I I I 0.4 II 0.2 0 0 12 reaction time 1 h Fig.2 Changes in DS by spiropyran moiety of SP-MC obtained from the treatment of methyl cellulose with SP-COOH at 25 "C, preceded by reaction at 5 "C for 2 h, with reaction time. Reaction mixture:methyl cellulose (0.91 g), SP-COOH (3.80 g), N,N'-dicyclohexylcarbodiimide (2.06 g), p-dimethylaminopyridine (0.24g), tetrahydrofuran (THF) 80 cm3). reaction time in the reaction of methyl cellulose with SP- COOH at 25 "C, preceded by reaction at 5 "C for 2 h. The DS is found to increase initially with reaction time steeply and then show a tendency to level-off.The maximum value of DS obtained in the present experiment is 1.1 at a reaction time of 96 h. In the following examinations, the SP-MC with DS of 1.1 was used. Changes in the UV absorption spectra of SP-MC in pyri- dine, dioxane and dichloroethane under irradiation with UV light of A =300-400 nm and then subsequently with visible light of A >550 nm were examined. Fig. 3 shows the changes in the UV absorption spectrum of SP-MC in dioxane under irradiation with UV light for 1min and subsequently with visible light for 3 min. A new absorption peak is found to appear at 589 nm in the spectrum under irradiation with UV light, and is assigned to the merocyanine form.The peak disappears under subsequent irradiation with visible light, and the spectrum returns almost completely to the original one. The absorption spectra of SP-MC in pyridine and in dichloro- ethane showed similar changes to that in dioxane under 0.8 0.6 i0.4 0.2 0 500 700 wavelength / nrn Fig. 3 Changes in the UV absorption spectrum of SP-MC in dioxane under irradiation with UV light of A =300-400 nm and subsequently with visible light of L> 550 nm. (a) Before irradiation; (b) after irradiation with UV light for 1 min; (c) after subsequent irradiation with visible light for 3 min; [SP-MC] =0.030 g dm-3. irradiation with UV light and subsequently with visible light, but the isomerization rates, especially the disappearance rates of the absorption peak in the solvents were faster than that in dioxane.These changes in the spectrum prove the reversible photoisomerization of spiropyran moieties in SP-MC in the solvents. Alternating irradiation with UV and visible light of SP-MC in the solvents were repeated and the changes in the absorbance at 589 nm in the spectrum of SP-MC with increasing number of repetitions were observed. Fig. 4 shows the changes in the absorbance at 589 nm in the spectrum of the SP-MC pyridine solution, which is assigned to the merocyanine form, under repeated alternating irradiation with UV light for 1 min and with visible light for 3 min uersus the number of repetitions. The absorbance at 589nm is found to decrease with the number of repetitions to about half that after the first cycle after 10 repetitions and to about one third after 20 repetitions.This result shows that there is a significant fatigue of the spiropyran moieties in SP-MC under repeated alternating irradiation with UV light and visible light in the solvent. The same experiments were carried out in dichloroethane and in dioxane, and a greater fatigue of the spiropyran moieties by the repeated alternate photoisomerization was found than in pyridine; the absorbance at 589 nm in dichloroethane decreased to 17% of that after the first cycle after 20 repetitions and that in dioxane almost disappeared. More polar solvents seem to lead to less fatigue of the spiropyran moieties in SP-MC in the repeated alternating photoisomerizations.The benzene solution of SP-MC was found to yield turbidity E c1.5 Q,m m c.a8 1.0 C-3! %00.5 0 0 0 5 10 15 20 number of repetitions Fig. 4 Changes in absorbance at 589 nm in the UV absorption spec- trum of SP-MC pyridine solution under repeated alternating irradiation with UV light for 1min and subsequently with visible light for 3 min, with number of repetitions; [SP-MC] =0.20 g dm-3 under irradiation with UV light. This phenomenon is believed to be due to a decrease in solubility of SP-MC in the solvent, resulting from the photoisomerization of the spiropyran moiety to the more polar merocyanine form. Thus, the changes in the turbidity of the benzene solution of SP-MC under irradiation with UV light and subsequently with visible light were exam- ined.Fig. 5 shows the changes in transmittance of the benzene solution of SP-MC under irradiation with UV light and subsequently with visible light, uersus irradiation time. It is found that the transmittance decreases under irradiation with UV light with irradiation time and almost levels-off at 40 min and then the turbidity disappears under subsequent irradiation with visible light, although the transmittance does not com- pletely return to the original level. This result suggests the possibility of regulating the solubility of SP-MC in solvents under irradiation with UV and visible light. An SP-MC film was prepared by casting from the THF solution and changes in the UV absorption spectrum of the film under irradiation with UV light and subsequently with visible light were examined.The results are shown in Fig. 6. It was found that an absorption peak at 585 nm, which is assigned to the merocyanine form, increases under irradiation with UV light with the irradiation time and then decreases under 100. I 0 10 20 30 0 30 60 90 120 irradiation time / s Fig. 5 Changes in transmittance of SP-MC benzene solution moni- tored with 750nm light under irradiation with (a) UV light and (b)subsequently with visible light, with irradiation time; [SP-MC] = 3.70 g dm-3 2.0 r 1 irradiation 1 time / min 1.o Q)VC n 400 500 600 700 wavelength I nrn Fig. 6 Changes in the UV absorption spectrum of SP-MC film under irradiation with (a) UV light and (b) subsequently with visible light, with irradiation time.Film thickness =3 pm. J. Mater. Chem., 1996, 6(1), 11-14 2.0IT 0 5 10 15 20 number of repetitions Fig. 7 Changes in absorbance at 585 nm in UV absorption spectrum of an SP-MC film under repeated alternating irradiation with UV light for 10min and subsequently with visible light for 4 h, with number of repetitions Absorption2.01 I I Contact angle I 0 2 4 6 0 60 120 180 240 irradiation time / min Fig. 8 Changes in the contact angle with water (below), together with those in absorbance at 585 nm in the UV absorption spectrum (above), of SP-MC film under irradiation with (a)UV light and (b)subsequently with visible light, with irradiation time subsequent irradiation with visible light, although it takes a longer time for the peak to completely disappear.This result proves that the photoisomerization of spiropyran moieties in the SP-MC film is reversible. Furthermore, the merocyanine formed under irradiation with UV light in the film, as compared with that formed in solvents (Fig. 3), is found to be more stable. This larger stability of the merocyanine form may be due to the spatial restrictions on the isomerization of the planar merocyanine structure to the spiropyran form with a perpendicularly crossed planes structure, imposed by the poly- mer matrix. Fig. 7 shows the changes in the absorbance at 585 nm in the spectrum of the SP-MC film under repeated alternating irradiations of the film with UV light for 10min and sub- sequently with visible light for 4 h, with the number of rep- etitions.The absorbance assigned to merocyanine form is found to decrease with the number of repetitions to about 15% of that after the first repetition after 20 repetitions. The larger decrease in the absorbance compared with that in solvents is thought to be due to the longer irradiation time needed for the isomerization, in particular this is due to the greater stability of merocyanine in a film than in a solvent. Changes in the contact angle of the SP-MC film with water under irradiation with UV light and subsequently with visible light were examined. The results are shown in Fig.8, together with the change in absorbance at 585 nm in the UV spectrum of the SP-MC film under the same conditions. The contact angle is found to decrease under irradiation with UV light and then increase with subsequent irradiation with visible light, with the irradiation time. These changes in the contact angle correspond well to those in the absorbance at 585 nm in the UV absorption spectrum. This correspondence proves that the changes in the contact angle result from photoisomerization of the spiropyran moiety to the more polar merocyanine form and from the merocyanine form to spiropyran form in SP-MC. Conclusions SP-MC with DS by spiropyran moiety of 1.1 was prepared by the treatment of methyl cellulose (DS by methyl group 1.52) with SP-COOH and the spiropyran moieties in SP-MC were confirmed to undergo reversible isomerization under irradiation with UV light and subsequently with visible light, both in solvents and in a film, although the repeated cycle of alternating irradiation with UV light and with visible light resulted in considerable fatigue of the spiropyran moieties in SP-MC. The solubility of SP-MC in benzene and the contact angle of the SP-MC film with water were found to be reversibly regulated under irradiation with UV light and subsequently with visible light, the regulation being deduced to result from the reversible isomerization of the spiropyran moieties in SP-MC. References 1 K.Arai and H. Udagawa, Makromol. Chem., Rapid Commun., 1988, 9, 797. 2 K. Arai and H. Udagawa, Sen-i Gakkaishi (J. SOC. Fiber Sci. Tech. Jpn), 1990,46, 150,491. 3 K. Arai and H. Satoh, J. Appl. Polym. Sci., 1992,45,387. 4 K. Arai, S. Sano and H. Satoh, J.Mater. Chem., 1992,2, 1257. 5 K. Arai and S. Sano, J. Mater. Chem., 1994,4,275. 6 R. Heiligman-Rim, Y. Hirshberg and E. Fischer, J. Phys. Chem., 1962,66,2477. 7 M. Gehrtz, Chr. Brauchle and J. Voitlander, J. Am. Chem. SOC., 1982,104,2094. 8 M. Aizawa, K. Namba and S. Suzuki, Arch. Biochem. Biophys., 1977, 180,41. 9 H. G. Khorana, J. Chem. SOC., 1952,2081. 10 J. C. Sheehan and G. P. Hess, J. Am. Chem. SOC., 1955,77, 1067. 11 J. Kolc and R. S. Becker, J. Phys. Chem., 1967,71,4045. Paper 5/04862A; Received 24th July, 1995 14 J. Muter. Chem., 1996,6(l), 11-14

ISSN:0959-9428    

DOI:10.1039/JM9960600011

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Examples of amphitropic polymers: monolayer film, Langmuir-Blodgett film and liquid-crystalline properties of some polymeric amphiphiles containing cholestanol moieties and those of some closely related non-polymeric amphiphiles Ziad Ali-Adib," Andrea Bomben,b Frank Davis: Philip Hedge,*' Pietro Tundob and Ludovico Valli" "Departmentof Chemistry, University of Manchester, Oxford Road, Manchester, UK M13 9PL bDepartment of Environmental Chemistry, University of Venice, Venice, Italy 'Department of Material Sciences, University of Lecce, Lecce, Italy A range of alternating copolymers were prepared by free-radical-initiated copolymerizations of maleic anhydride with a series of a-alkenes containing cholestanyl moieties. Derivatives of these copolymers were prepared by reacting the anhydride residues with methanol, water, dimethylamine and/or morpholine.A related series of non-polymeric amphiphiles containing steroid moieties was also prepared. Isotherms were measured for monolayers of the various polymers and the various non-polymeric amphiphiles on water and, where possible, Langmuir-Blodgett (LB) multilayers were prepared. The majority of the materials gave good isotherms (relatively steep with collapse pressures >40 mN m-') indicating that the monolayers were ordered and, as determined by the detection of Bragg peaks by X-ray diffraction, Y-type LB films with regular layer structures. Appropriate materials were also examined, by optical microscopy and differential scanning calorimetry (DSC), for possible liquid-crystalline properties.Four polymers and one non-polymeric amphiphile exhibited smectic A mesophases. Another non-polymeric amphiphile exhibited a cholesteric mesophase. Thus, examples were found of amphitropic polymers and non-polymeric amphiphiles which can form organised molecular arrangements both because they are amphiphilic and because they contain mesogens. There has been great interest in recent years in monolayers of amphiphiles at air-water interfaces and, in favourable cases, in the deposition of such monolayers onto solid supports (Langmuir-Blodgett Often thick LB multilayers can be prepared by the repeated deposition of monolayers. Such structures are not only of interest as examples of organised systems, but they also have potential applications in microelec- tronics, optoelectronics and molecular electronic^.^ Although the LB films of many non-polymeric amphiphiles are highly ordered, unfortunately they are prone to molecular reorganis- ati~n.~,~This prompted us several years ago to study mono- layers of preformed polymers and the derived LB The polymeric LB films are usually more stable and physically more robust than those of non-polymeric amphiphiles.Even though in the best case^^,^^,'^ polymeric LB films are generally less ordered, they can nevertheless be sufficiently ordered for many applications. Liquid crystals, including polymeric liquid crystals,13 are another major family of organized systems that has been studied extensively in recent years.Currently there is growing interest in compounds which have both amphiphilic character and contain mesogens, i.e. compounds which contain the two types of ordering effect. Such compounds have been called amphitropic14 or amphotropic.15 Examples are the cyano- biphenyl derivative 114 and the cholesteryl derivative 2.'' We considered it of interest to study some amphitropic polymers which, on the basis of previous w~rk,'-~~*'~ could be expected to form excellent LB films but which also contained liquid crystal mesogens, and to determine how the interplay of the two ordering effects influences their properties. For example, do the ordering effects tend to reinforce each other or do they act in opposition? Do some polymers form ordered monolayers and LB films as well as being liquid crystals? Our initial aim is to identify amphitropic polymers which form good monolayers and LB multilayers and which are also liquid crystals.(2) In this paper we describe the synthesis of some a-alkene- maleic anhydride copolymers and their derivatives, a type of polymer which is known to give excellent LB films in many ca~es,~.''where in the present work the a-alkene contains 5a-cholestanyl mesogens. The monolayers, LB films and liquid- crystalline properties of these amphiphilic polymers, and the same properties of some closely related non-polymeric analogues, have been studied. Monolayers of steroids at the air-water interface have been studied before on several occasion^^^^^^-^^ but very little work has been carried out on steroidal LB Such films are also of interest because Y-type LB films can be considered as assemblies of bilayers, and derivatives of cholesterol (3)are present in many biological bilayers.22 Experimenta1 Synthesis of amphiphiles Cholesterol (3), 5a-cholestane-3P-01 (4), cholesteryl hemi- succinate (5) and cholesteryl hemiphthalate (6) were obtained J. Mater.Chem., 1996, 6(1), 15-22 Scheme 1 from the Aldrich Chemical Company Limited and were recrys- tallized before use. Other amphiphiles were prepared from compounds 3 and 4 using the reactions described below. The key synthetic steps are outlined in Scheme 1. Tetrahydrofuran (THF) and benzene were dried by distillation over calcium hydride.Organic solutions were dried over sodium sulfate. Mps were determined using a Kofler hotstage apparatus and are uncorrected. IR spectra were measured for KBr discs (unless indicated otherwise) using a Nicolet MX1 instrument. 'H NMR spectra were recorded on a Varian Gemini 200 MHz instrument for solutions in deuteriochloroform containing tetramethylsilane as an internal standard. High-pressure liquid chromatography (HPLC) was carried out using Waters 6000A equipment with a R401 refractive index detector, a silica column coated with octadecylsilane and THF as the eluent. (3) R =-OH (4) R =-OH (5) R =-OCO(CH2)2CO,H (16) R = -O(CIIJ9C0211 (6) R =-OCOaCOZH /\ (7) R =-OCO.CH~CH.CO~H Cholesterol hemimaleate (7).A mixture of cholesterol (3) (5.02 g; 13.0 mmol), sodium hydride (80% in mineral oil, 0.50 g; 16.6 mmol) and maleic anhydride (5.01 g; 51.1 mmol) in dry benzene (25 ml) was stirred vigorously and heated under reflux. After 2 days the mixture was cooled and treated carefully with water (2 ml). The solvents were evaporated off and the residue recrystallized from aqueous ethanol. Yield 3.98 g (64%), mp 130-132°C. Satisfactory IR and 'H NMR spectra were obtained. Found: C, 76.6; H, 9.8. Calc. for C31H4804: C, 76.8; H, 10.0%. Ally1 5a-cholestan-3p-yl ether (8). A two-phase mixture of 5a-cholestan-3P-01 (4) (12.26 g; 3 1.54 mmol), allyl chloride (96.85 g; 1.23 mol), an aqueous solution of sodium hydroxide (40 g of a 50% solution) and tetrabutylammonium hydrogen sulfate (2.47 g; 7.22 mmol) was stirred vigorhsly under reflux for 12 h.Excess allyl chloride was then distilled off, water (150ml) was added and the mixture extracted with diethyl ether. The extracts were washed with saturated brine (2 x150 ml), dried and the solvent evaporated, The residue was recrystallized from hot ethanol to give the desired product 8 (12.5 g, 93%), mp 67-68.5 "C (lit.,23 62 "C); IR v,,, (Nujol mull)/cm-' 1635; 'H NMR 6: 0.7-2.4 (46 H, m, steroid nucleus), 4.51 (2 H, m, =CHCH,O-), 3.60 (1 H, m, 3a-H), 5.08 (2 H, m, vinyl CH,), 5.84 (1 H, m, vinyl CH). 5a-cholestan-3~-yl undec-10-enyl ether (9). Undec-1 0-en-1-01 (17.03 g; 0.10 mol), pyridine (40 ml) and toluene-p-sulfonyl chloride (20.97 g; 0.1 1mol) were stirred together at 15 "C for 1 h, then at ambient temperature for 1 h.The mixture was added to ice and concentrated hydrochloric acid (40 ml). After the mixture reached ambient temperature it was extracted with 16 J. Muter. Chem., 1996, 6(1), 15-22 diethyl ether (2 x50 ml). The extracts were washed successively with dilute hydrochloric acid (50ml of 5%), aqueous sodium hydrogen carbonate (50 ml of 5%), water (2 x50 ml), dried and then the solvent evaporated off. Distillation of the oily residue at 0.2 mmHg gave undec-10-en-1-yl tosylate (29.53 g; 91%) as an oil. A satisfactory 'H NMR spectrum was obtained. A suspension of sodium hydride (0.31 g; 12.9 mmol) in dry THF (20 ml) was prepared. 5a-Cholestan-3P-01 (4) (2.00 g; 5.15 mmol) was added.When effervescence of hydrogen ceased the above tosylate (2.00g; 6.16mmol) in THF (lOml) was added to the mixture. It was then heated under reflux for 24 h, cooled and added to water (100 ml) acidified with hydrochloric acid. The product was extracted with diethyl ether-light pet- roleum [bp 40-60 "C; 2 x50 ml (1 :15 v/v)]. The extracts were washed with hydrochloric acid (25 ml of 5%), dried and the solvent evaporated off. Recrystallization of the residue from acetone gave the desired product 9 (1.20g; 43%), mp 42544°C; IR v,,x/cm-l 1638; 'H NMR 6: 0.7-2.4 (62 H, m, steroid nucleus and 8 side-chain CH,), 3.62 (1 H, m, 3a-H), 4.55 (2 H, m, CH,O), 5.09 (2 H, m, vinyl CH,), 5.90 (1 H, m, vinyl CH). Found: C, 84.6; H, 12.9. Calc.for C3&680: C, 84.4; H, 12.7%. 5a-cholestan-3~-yl2-( prop2-eny1)ethyl ether (10). A mixture of ethylene glycol (81.0 g; 1.31 mol), potassium carbonate (18.0 g; 0.13 mol) and allyl chloride (15.0 g; 0.2 mol) was stirred and heated at 100°C for 8 h. Fractional distillation of the mixture at 0.2 mmHg gave 2-(prop-2-enyloxy)ethanol (12.1 g; 60% yield) with a satisfactory 'H NMR spectrum. This was converted into the corresponding tosylate (47% yield) which was then reacted with sodium hydride and 5a-cholestan-3P-01 (4) using the reaction procedures described previously for the preparation of compound 9. This gave the desired product 10 (46% yield), mp 45-46°C; IR v,,/cm-' 1642; 'H NMR 6: 0.7-2.4 (46 H, m, steroid nucleus), 3.55 (1 H, m, 3a-H), 3.60 (4 H, s, OCH2CH20), 4.22 (2 H, m, =CHCH20), 5.25 (2 H, m, vinyl CH,), 5.80 (1 H, m, vinyl CH).Found: C, 81.0; H, 11.7. Calc. for C32H5602: C, 81.3; H, 12.0%. 5a-cholestan-3~-yl 1,4,7,10,13-pentaoxahexadec-l5-enyl ether (11). Tetraethylene glycol was converted into the corre- sponding monoallyl ether (40% yield), which was converted into the corresponding tosylate (63% yield) and this was then reacted with 5a-cholestan-3P-01 (4) using the reaction pro- cedures described for the preparation of compounds 9 and 10. This gave the target compound 11 (81% yield) as a pale yellow viscous oil. IR v,,x/cm-l 1637; a satisfactory 'H NMR spec- trum was obtained. Found: C, 75.1; H, 11.0. Calc. for C38H6805: C, 75.4; H, 11.3%. Copolymerizations. Alkenes 8, 9, 10 and 11 were separately copolymerized with maleic anhydride in anhydrous THF at 60" using azoisobutyronitrile as the initiator and the products were isolated and characterized using the procedures given previously for the copolymerization of hexadec- 1-ene and maleic anhydride.' This gave copolymers 12a-l5a, respectively, in yields of 45-70%.All of these polymers gave satisfactory IR spectra, i.e., they had bands at vmaX 1850 (m) and 1780 (s) cm-characteristic of carbonyl groups in five-membered ring anhydrides and they had no carbonyl bands due to maleic anhydride, a possible impurity. By gel permeation chromatog- raphy (GPC), using the methods given before,' 12a had M,= 4800, fi,=8600; 13a had &?,=5100, fi,=9700; 14a had M, =3400, M, =6800; 15a had fin=4300, a,=7400.There was no evidence for non-polymeric impurities. Derivatives of copolymers. The following procedures are typical. Long reaction times were used simply to ensure virtually quantitative reactions. Half methyl ester 12b. Polymer 12a (1.00 g) was treated with methanol (50 ml) at reflux temperature for 14 days. Excess methanol was then evaporated off and the residue dissolved in a minimum of THF (10 ml). The solution was added slowly to ice-cold light petroleum (100 ml) to precipitate the polymer. The polymer was collected and dried (0.84 g, 79%); IR v,,, 1728 and 1711 cm-', no carbonyl bands attributable to anhy- dride residues were detected. Found: C, 77.2; H, 10.5%.Calc.: C, 77.5; H, 10.8%. The half methyl esters 14b and 15b were prepared similarly from the appropriate anhydride copolymers. Diacid 12c. Polymer 12a (1.60 g) was treated with sodium hydroxide (2.0 g) in water (50 ml) for 1 week at reflux tempera- ture. The cooled solution was then added to aqueous hydro- chloric acid (2 mol lF1, 500 ml). The precipitate was collected, dissolved in THF, and reprecipitated into water (500 ml). The polymer was collected, washed with water and dried (1.20 g, 72%); IR v,,,/cm-' 3800-2200 (br), and 1708, no carbonyl bands attributable to anhydride residues were detected. Found: C, 76.5; H, 10.4%. Calc.: C, 77.2; H, 10.7%. The anhydride residues of polymers 13a, 14a and 15a were hydrolysed to the corresponding diacid derivatives by allowing monolayers on the water surface at a surface pressure of 20 mN m-' to stand for 20 h at 20°C.' Acid-dimethylamide 12d.Polymer 12a (0.50 g) was dissolved in a solution of dimethylamine in ethanol (33%, 50ml), and the mixture was heated under reflux for 4 days. The cooled mixture was acidified (2mol 1-l HCl). This precipitated the polymer, which was collected and reprecipitated into water and then into light petroleum. The dried product (0.33 g, 61%) gave IR v,,,/cm-' 1709 (strong, carboxy), 1652 (strong, amide), no bands attributable to anhydride residues were detected. Found: C, 77.4; H, 11.5; N, 2.4%. Calc.: C, 77.8; H, 11.1; N, 2.5%. The acid-amides 15d, 12e and 14e were prepared similarly from the appropriate anhydride copolymers. lo-(5a-cholestan-3~-yloxy)decanoicacid (16).A mixture of ether 9 (2.50 g; 4.63 mmol), potassium peunanganate (2.46 g; 15.57 mmol), water (26.3 ml), benzene (26.3 ml), tetrabutylam- monium hydrogen sulfate (13 mg; 3.83 x lo-' mmol) and acetic acid (4.66 ml) was stirred vigorously at 0 "Cin a conical flask for 5 h. Sodium sulfate (2.10 g; 16.67 mmol), hydrochloric acid (2.1 ml; 12 mol 1-') and water (2.1 ml) were added, and the dark colour rapidly disappeared. The mixture was then extracted with diethyl ether (2 x 75 ml). The combined extracts were washed with water (2 x 10 ml), dried and evaporated to dryness. Recrystallization of the residue from pentane gave the desired product 16 (1.51 g; 46%), mp 85-100°C (see Table 3; later).Satisfactory IR and 'H NMR spectra were obtained. Found: C, 79.8; H, 11.7. Calc. for C3&&3: C, 79.5; H, 11.9%. Langmuir isotherms and film deposition Isotherms of monolayers and, in appropriate cases, their deposition properties were studied for compounds 3-7 and 16 and for polymers 12a-l2e, 13a and 13c, 14a-14c and 14e, and 15a-1%. All the non-polymeric amphiphiles were shown by HPLC analysis to be >99.5% pure. Isotherms were measured for monolayers on water at pH 5.4-5.6 with no added ions using the apparatus and procedures described previously.' For the non-polymeric amphiphiles, the spreading solvent was ethyl acetate, while for the polymers a mixture of ethyl acetate and THF (9: 1 v/v), or a mixture of chloroform and THF (9: 1 v/v) proved to be satisfactory (see Table 2; later).LB films were deposited onto hydro-phobic" silicon wafers from over water containing CdCl' (2.5 x mol 1-I) using the apparatus and procedures described previo~sly.~ The presence of the CdC1, did not noticeably affect the isotherm. In general, transfer was carried out at a surface pressure of 30 mN m-l and at a dipping speed of 8 mm min-l. The results are summarized in Tables 1 and 2. X-Ray studies These were carried out as described previously.' The results are summarised in Tables 1 and 2. Studies of polymerization of Langmuir-Blodgett multilayers These studies were carried out using the procedure as previously described for crosslinking LB multilayers.1° Liquid crystal studies The thermal properties of the various samples were examined using a Nikon Optiphot-2 optical microscope equipped with a hot stage controlled by a Mettler FP80 HT apparatus. The results are summarised in Table 3.Differential scanning calor- imetry (DSC) was carried out using a Perkin-Elmer DSC4 instrument linked to a data station with heating and cooling rates of 5 "C min-l. Results and discussion Synthesis of amphiphiles We have previously shown' that the amphiphilic polymers obtained (see Scheme 1) by (a) the copolymerization of equi- molar amounts of long-chain vinyl compounds such as hexa- dec- l-ene with maleic anhydride, followed by (b)reaction of the anhydride residues in the products with nucleophiles such as water, methanol or small amines, give excellent monolayers on water and good Y-type LB multilayers which display up to three orders of Bragg peaks in X-ray reflection experiments.In the present work, polymers of the same general family have been prepared using vinyl compounds containing cholestanyl Table 1 Properties of monolayers at air-water interface and Langmuir-Blodgett films of the non-polymeric amphiphiles 3-7 and 16 monolayer area per molecule/A2" X-ray data at at collapse number of layers number of d-spacing per compound 0 mN m-lb 30 mN m-l pressure, nJmN m-l in LB film" Bragg peaks bila yer/A 3 47 43.0 37 - 4 46 42.0 39 - - - 5 50 42.0 39 282 3 42.5 6 62 47.0 32 220 1 44.6 7 43 39.0 44 242 3 45.5 16 41 34Sd 29 25 3 41.7 a Monolayers prepared using solutions of the amphiphiles in ethyl acetate.By extrapolation of the 'solid' section of the isotherm to zero pressure. Transferred onto hydrophobic silicon at a surface pressure of 30 mN m-' from over water containing CdCl, (2.5 x mol I-'). Deposition occurred on both up and down strokes with a deposition ratio of 0.95-1.05. At 20 mN m-'. J. Muter. Chem., 1996, 6(l), 15-22 Table 2 Properties of monolayers at air-water interface and Langmuir-Blodgett films of the amphiphilic polymers ~~ ~ monolayer area per repeat unit/AZ X-ray data collapse at at pressure, number of layers number of d-spacing per polymer 0 mN ,-la 30 mN m-' nJmN m -' in LB filmb Bragg peaks bilayer/A 12a' 50 39.0 53 12bd 47 41.3 55 100 1 43.1 12c' 45 40.0 54 100 4 45.6 1 2dd 45 40.8 57 100 1 44.0 12e' 63 50.1 38 100 1 41.7 13a' 48 40.2 54 13c' 56 47.6 50 50 1 50.0 14a' 52 44.5 53 14b' 54 47.2 52 100 2 49.3 14c' 57 40.2 48 50 2 48.6 14e' 50 44.2 49 100 2 50.4 -15a' 62 43.5 48 15b' 52 49.0 45 -2 54.1 15c' 62 50.3 43 30 2 55.7 1 5d' 41 34.1 47 100 2 56.6 See footnote b in Table 1.See footnote c in Table 1. Monolayers prepared using solutions of the amphiphiles in a mixture of chloroform-THF 9 : 1 v/v. Monolayers prepared using solutions of the amphiphiles in a mixture of ethyl acetate-THF (9:1 v/v). Table 3 Liquid-crystalline properties of various amphiphiles" determined using DSC transition temperatures ("C) and enthalpy changes (kJ mol-') determined by DSCb glass to liquid liquid crystal' to compound to cry star isotropic liquid polymer T/"C AH/kJ mol-' TI"C AHlkJ mo1-I 6 95d 0.75 155d 0.38 16 68 1.oo 85 14.65 12a decomposed at 235 "C 12d clearing point 295 "C 12e decomposed at 260 "C 13a 137 0.38 194 3.16 14a 125 0.27 163 1.17 14e decomposed at 250 "C 15a 142 0.03 252 0.12 15d 69 17.12 173 39.42 a No liquid-crystalline phase observed for either of compounds 5 and 7.Cooling rate 5°C min-'. 'Except where indicated otherwise the mesophase was smectic A as determined by optical microscopy. Cholesteric mesophase as determined by optical microscopy. moieties. The products are a series of amphiphilic polymers Monolayer and LB film properties of compounds 3-7 and 16 containing cholestanyl moieties which differ in the head groups Monolayers of compounds 3-7 and 16 were prepared (by and the type and length of the 'spacer' unit linking the mesogen using solutions in ethyl acetate) on a subphase of water at pH to the polymer backbone.5.4-5.6 at 20°C and their isotherms were recorded. These are Vinyl compounds 8-11 were prepared by reacting the appro- shown in Fig. 1 and 2. When possible the monolayers were priate chloride or tosylate with the sodium alkoxide of 5a- transferred onto hydrophobic silicon wafers24 by verticalcholestan-3P-01 (4). Each vinyl compound in THF was then dipping and the multilayers were studied by X-ray reflectome- copolymerized with an equimolar amount of maleic anhydride try.The results obtained are summarized in Table 1.using azoisobutyronitrile as the free-radical initiator (see The isotherms of cholesterol (3) and cholestanol(4) are very Scheme 1). This gave polymers 12a-15a. Their molecular similar to those reported previously.' They have a good shape masses were estimated by GPC (relative to polystyrene stan- with collapse pressures, n,>35 mN m- and, by extrapolation dards) and were found to be in the range M,=4300-5100. of the 'solid' portion of theo isotherm tc zero pressure, cross- Polydispersities were in the range 1.7-2.0. The anhydride sectional areas, A, of 47 A2 and 46 A2, respectively. CPK copolymers were reacted with appropriate n~cleophiles~ to moleculat models of these steroids have cross-sectional areas give acid-methyl ester derivatives 12b, 14b and 15b, the diacid of ca.40 A2. All attempts to prepare LB multilayers from these derivatives 12c-l5c, acid-dimethylamides 12d and 15d, and two alcohols failed. The monolayers deposited well on the 12e and 14e (see Scheme2). For the synthesis of the diacid down strokes but they rapidly transferred back to the water derivatives 13c-15c the anhydride residues were hydrolysed surface on the upstrokes. Langmuir et al. reported similar by leaving monolayers of the corresponding anhydride copoly- problems with these two compounds but found that cholestanol mers on the water surface for 20 h.9 (4) could be deposited as Y-type films on a base of barium Oxidation of vinyl compound 9 with potassium perman- stearate.'* On the same base, however, cholesterol (3) only ganate gave lo-(5a-cholestan-3~-yloxy)decanoicacid (16).deposited as an X-type film and even then only for a few Reaction of cholesterol (3) with maleic anhydride gave the half layers, but it did deposit Y-type if the subphase contained maleate 7. cupric chloride.I8 18 J. Muter. Chem., 1996, 6(1)' 15-22 501 O=5a-cholestan-3fi-ylC Scheme 2 50 40 -I E '.. [IIjI I Irri 0 10 20 30 40 50 60 70 80 90 100 area per repeat unit/A2 Fig. 2 Isotherms of various non-polymeric amphiphiles. All isotherms were measured at 20 "C and pH 5.3-5.6. Cholesteryl hemisuccinate (5) (-); cholesteryl hemiphthalate (6) (-- - ); cholesteryl hemimaleate (7) (-*-.-) and compound 16 (-.--).showed three orders of Bragg peaks whilst that of the half phthalate showed only one. Given that stearic acid multilayers usually show at least 12 orders,25 it is evident that the present LB multilayers are not particularly well ordered. However, it is important to note that these simple cholesterol derivatives readily give LB multilayers whereas cholesterol (3) itself does not. Using CPK molecular mo$els tbe maximym lengths of the half esters 5, 6 and 7 are 25 A, 27 A and 23 A, respectively. This, and the d-spacings given in Table 1, suggest that the molecules in the bilayers are probably in a simple Y-type pattern and tilted from the normal by 32", 22" and 5", respectively. A multilayer (20 layers) of the half maleate ester 7 was deposited in the usual way.Exposure to UV irradiation through the small holes of a simple metal mask brought about polymerization." A quick rinse of the exposed film in 95% ethanol resulted, due to the lower solubility of the polymer than the monomer, in the formation of the expected pattern in the films. The best results were obtained with 30min exposure and a 10 s rinse. Longer exposures led to the polymer becoming more soluble, presumably due to chain scission and photodesorption. As a control a multilayer of the half succinate 5 was treated similarly. It did not become less soluble on 0 10 20 30 40 50 60 70 80 90 100 area per repeat unit/A2 Fig. 1 Isotherms of cholesterol (3) (---) and cholestanol (4) (-) measured at 20 "C and pH 5.3-5.6 The half esters 5-7 gave good isotherms (see Fig.2), though they were less satisfactory than those of the steroidal alcohols 3 and 4. Thus, the isotherms either had lower collapse pressures, were less steep, and/or showed a more gradual transition from the 'gaseous' to the 'solid' region. The isotherm of the phthalate 6 was, not surprisingly in view of the relative bulk of the head group, the least satisfactory. All the half esters 5-7 could be deposited readily from a subphase of 2.5 x mol 1-1 aque- ous CdC1, solution. The multilayers of half esters 5 and 7 each irradiation, indicating that it is the maleate C=C rather than 10 the cholesteryl C= C which polymerizes. Polymerization of ester 7 therefore provides a simple route to a polymeric LB film containing steroid moieties.0 The is0 therm obtained for lo-(5a-cholestan-3fl-yloxy) decan- ,,,, L,,i,oic acid (16), was poor (see Fig. 2). It did not display a clear 'solid' region and had a relatively low collapse pressure. Even so, the monolayer at 20 mN m-l over aqueous CdCl, solution could be deposited to give a Y-type multilayer which exhibited three orders of Bragg peaks. Surprisingly, the d-spacing of the multilayer was slightly less than that of the multilayer of the half succinate 5. Since the methylene chain is long and flexible it is difficult to predict with confidence how the multilayers of compound 16 are packed. Monolayer and LB film properties of the amphiphilic polymers Monolayers of the various polymers were prepared [by using solutions in chloroform- or ethyl acetate-THF (9: 1 v/v); see Table 21 on a subphase of water at pH 5.4-5.6 at 20 "C and their isotherms were recorded.When possible the monolayers J. Muter. Chem., 1996, 6(l), 15-22 were transferred onto pieces of hydrophobic silicon and the multilayers obtained were studied by X-ray reflectome- try. The results obtained are summarized in Table 2. The family of amphiphilic polymers 12a-12e have just one CHI group as a ‘spacer’ between the steroid moiety and the polymer backbone. As shown in Fig. 3, all these polymers give excellent isotherms. The isotherm of the parent anhydride polymer 12a was steep, indicating that the film was well ordered. In view of the reactivity of the anhydride moieties no attempts were made to prepare LB films from this polymer.The isotherms of the half methyl ester derivative 12b, the diacid derivative 12c and the acid dimethylamide derivative 12d were very similar. They fll showed significant ‘solid’ regions, had A, values of ca. 45 A’, a value $lose to the cross- sectional area of the steroid nucleus (40 A2), and collapse pressures >50 mN m-’. The isotherm of derivative 12e had a lower collapse pressure and a somewhat greater value of A,. All these polymers deposited well as Y-type multilayers which, in X-ray reflectivity studies, displayed one or *more Bragg peaks corresponding to d-spacings of 41.7-45.6 A. The diacid derivative 12c was particularly well ordered and displayed four Bragg peaks, the highest number we have observed for deriva- tives of a-alkene-maleic anhydride copolymers.Given the data available, it is possible to suggest a likely structure for these multilayers. Thus, given: (a) that the experimental values for the d-spacings are significantly less than those expected for a Y-type multilayer in which the side chains are perpendicular 40 30 to the planes of the films; (b)that the areas per repeat unit on the water surface and, since the deposition ratios wfre 1.00f.0.05, also in the LB films are too small (i.e., <2 x 40 A2) for a structure with fully interdigitated side chains; and (c) that the steroid nucleus is essentially rigid, the most likely structure is a Y-type film with the side chains essentially straight but significantly tilted and interpenetrating, if at all, only at the ends.Such an arrangement is shown schematically in Fig.4 for polymer 12c. Taking polymer 12c as an example and assuming that the dhickness of the ‘headgroups’ in the bilayer is the same (13.1 A) as was as deduced in oud earlier work,’ then the space left for the side chains is 32.5 A. From a CPK molecular model, the length of an entire side chain is 22.3 A. If there is no interdigitation at all, this suggests that all the side chains are tilted from the vertical at an angle 8 where cos 8=[32.5/(2x 22.3)]; i.e., 8=43”. If the C,,-C, side chains interdigitate, since they have a length of ca. 7.? A, and could reasonably overlap with each other by ca.5.0A,as shown in Fig. 4, 8 will be 35”. The amphiphilic polymers 13a and 13c are generally similar in structure to the polymers discussed above, except that they have a much longer ‘spacer’ group between the steroid nucleus and the polymer backbone: ten methylene groups instead of one. The parent anhydride polymer 13a gave a reasonably steep isotherm (see Fig. 5), indicating that the monolayer was well organised. The diacid derivatives 1% a!so gave a steep isotherm (see Fig. 5), the value of A, being 56 A2. This polymer deposited to give a Y-type multilayer which displaxed only one Bragg peak corresponding to a d-spacing of 50.0 A. Given the presence of the long flexible ‘spacer’ group, it is difficult, with the data available, to suggest with confidence how the multilayer may be organised, but it could well be similar to that proposed for polymers 1242, shown in Fig. 4, but with the side chains more tilted.The amphiphilic polymers 14a-14c and 14e only differ from those of polymer 12a and its derivatives in having a ‘spacer’ that is -OCH2CH20CH2-instead of -OCH2-, i.e. a ‘spacer’ that is a little larger and somewhat more flexible, The isotherm of polymer 14a (see Fig. 6), indicates that here, as in the other series, the parent anhydride-containing polymer forms a well ordered monolayer on water. Polymers 14b, 14c /O’0 IIllilJlJ~ 0 10 20 30 40 50 GO 70 SO 90 100 area per repeat unit/A2 +wCO,H CO,H Fig.3 Isotherms of copolymer 12a and its derivatives measured at 20°C and pH 5.3-5.6.(a) Polymer 12a (-); polymer 12b (---). Fig. 4 Scheme showing probable arrangement of repeat units of (b) Polymer 12c (-); polymer 12d (-*-.-) and polymer 12e (---). polymer 12c in the LB multilayer 20 J. Mater. Chern., 1996, 6(l), 15-22 60--\ 50 -\ \ \-I E 40-z & Q)2 30-v) & Q)0 E ?O-v) 10-\ 0 L1111 Fig. 5 Isotherms of copolymer 13a (---) and its derivative polymer 13c (-) measured at 20°C and pH 5.3-5.6 6o 1 0 10 20 30 40 50 60 70 80 90 100 area per repeat unit/A2 Fig. 6 Isotherms of polymer 14a and its derivatives measured at 20 "C and pH 5.3-5.6. Polymer 14a (-); polymer 14b (.--); polymer 14c (---) and polymer 14e (-.---).and 14e gave isotherms (Fig. 6) with A, values of 50-57 A2 and collapse pressures>48 mN m-'. They deposited well to give Y-type multilayers which displayed two ordFrs of Bragg peaks corresponding to d-spacings of 48.6-50.4 A. The same arguments for the plausible structure shown in Fig. 4 for the polymers derived from polymer 12a apply here, except that in this case the side chains are tilted a little further from the vertical. Similar calculations to those given above indicate values for 0 of 39" and 45" according to whether there is or is not interdigitation, respectively. The final series of polymers studied, polymers 15a, 15c and 15d, differ from the series prepared from polymer 12a in that they have as a 'spacer' the moiety -(-CH,CH,O-)),-.This 'spacer' was the longest used in the present work and it has substantial hydrophilic character. Of this series, polymers 15a, 15b and 15d gaye good isotherms (see Fig. 7). The values of A, were 41-62 A2. Polymers 15b-15d deposited well to give Y-type multilayers which displayed tFo Bragg peaks corre- sponding to d-spacings of 54.1-55.7A. The structure of the 0 10 20 30 40 50 60 70 80 90 100 110120 area per repeat unit/A2 Fig. 7 Isotherms of polymer 15a and its derivatives measured at 20 "C and pH 5.3-5.6. Polymer 15a (--); polymer 15b (-); polymer 15c (-.-,-) and polymer 15d (---). LB multilayers might well be a more highly tilted analogue of that shown for polymer 12c in Fig. 4. Investigation of various amphiphiles for liquid-crystalline properties Several of the amphiphiles were investigated by optical microscopy and DSC for their liquid-crystalline properties.The results are summarised in Table 3. As expected, neither of the half esters 5 and 7 displayed a liquid-crystalline phase. However, the half phthalate 6 dis-played a smectic A phase and the long-chain acid 16 a cholesteric phase. Investigation of the anhydride copolymers revealed that copolymer 12a, with only a -0CH2-moiety as a 'spacer' between the steroid moiety and the polymer backbone, did not display liquid-crystalline properties, but that the other anhydride copolymers 13a-l5a, which have longer 'spacers', all showed a smectic A phase on cooling from the liquid phase. Skoulios et al.previously reported that the closely related polymer 17 shows liquid-crystalline phase behaviour. On the basis of X-ray diffraction studies the phase was identified as smectic E.26 Acid-ester derivatives formed by reacting the anhydride residues of polymer 17 with alcohols also showed liquid-crystalline phases.27 The acid-methyl ester (series b) and diacid (series c) amphiphilic polymer derivatives were not studied for their liquid-crystalline properties in the present work because these derivatives begin to revert to the parent anhydride copolymers on heating to ca. 150-200°C. This is, however, not a problem with the acid-N,N-dimethylamide derivatives 12d and 15d and the acid-morpholinoamide derivatives 12e and 14e because they contain tertiary-amide moieties.In the event neither polymer 12d, polymer 12e nor polymer 14e displayed liquid- crystalline properties. In the first two cases, this is not surprising in view of the fact that the parent anhydride copolymer 12a is not a liquid crystal, but in the case of polymer 14e it suggests J. Mater. Chern., 1996, 6(l), 15-22 that when mesogenic properties are desired the acid-tertiary- mesogenic group actually facilitates the formation of ordered amide head group is not as satisfactory as an anhydride head group. The final polymer of this set investigated, Ed, proved to be a liquid crystal and to exhibit a smectic A phase. Conclusions Although cholesterol (3) and cholestanol (4) give isotherms which indicate that the monolayers are well ordered, it is not easy to prepare LB multilayers from these compounds by the vertical lifting method.However, LB multilayers are prepared easily from the half succinate 5, half maleate 7 and half phthalate 6 esters of cholesterol and from compound 16, all of which contain a carboxylic acid head group. The half phthalate ester 6 and compound 16 also show liquid-crystalline behav- iour. The former exhibits a smectic A mesophase, the latter a cholesteric phase. LB multilayers of the half maleate 7 can be polymerized by UV irradiation to give a polymeric LB film. The anhydride copolymer 12a and the various derivatives 12b-12e formed by reacting the anhydride residues with appro- priate nucleophiles give well ordered monolayers and excellent LB films, but none of the polymers of this series that were investigated were liquid crystals.The LB multilayers of the various ring-opened derivatives are probably arranged essen- tially as shown for polymer 12c in Fig.4, with or without interdigitation of the steroid side chains. The anhydride copolymers 13a-l5a, all of which have longer spacers than anhydride copolymer 12a, form well ordered monolayers at the air-water interface and are also liquid crystals. The mesophases are smectic A. The carboxylic acid-N,N-dimethylcarboxamidepolymer 15d forms well ordered monolayers, good LB multilayers (two Bragg reflections) and is also a liquid crystal, again exhibiting a smectic A structure. The LB multilayers of the various derivatives of the anhy- dride copolymers 13a-15a probably have a structure similar to that shown in Fig.4but with a larger tilt of the side chains. Polymers 13a-15a form well ordered monolayers and exhibit liquid-crystalline properties. Compound 16 forms ordered LB multilayers and exhibits liquid-crystalline properties. Polymer 15d and compound 6 form well ordered monolayers and LB multilayers also exhibit liquid-crystalline properties. Thus, examples of amphitropic polymers and further examples of amphitropic non-polymeric amphiphiles have been found which are able to form organised molecular arrangements both because they are amphiphilic and because they contain meso- gens. The results indicate that with the present materials the longer the ‘spacer’ moiety between the hydrophilic group(s) and the mesogenic group the more likely the materials are to exhibit amphitropic properties. However, further examples of amphitropic molecules need to be studied before the inter- actions, if any, between the two types of ordering effects can be fully understood.It will, for example, be interesting to determine whether there are cases where the presence of a monolayers and ordered LB multilayers. We thank the SERC/ESPRC for financial support and Professor John W. Goodby for helpful discussions. References 1 G. Gaines, Insoluble Monolayers at Liquid-Gas Interfaces, Interscience, New York, 1966. 2 Langmuir-Blodgett Films, ed. G. G. Roberts, Plenum, New York, 1990.3 An Introduction to Ultrathin Organic Films: From Langmuir- Blodgett to Self Assembly, ed. A. Ulman, Academic, Boston, 1991. 4 P. S. Vincent and G. G. Roberts, Thin Solid Films, 1980, 68, 135; G. G. Roberts, Adu. Phys., 1985,34,475; M. Sugi, J.Mol. Electron., 1985,1, 3. 5 M. J. Grundy, R. J. Musgrove, R. M. Richardson, S. J. Roser and J. Penfold, Langmuir, 1990,6, 519. 6 P. Stroeve, J. F. Rabolt, R. 0. Hilleke, G. P. Felcher and S. H. Chen, Mater. Res. SOC. Symp. Proc., 1990, 166, 103; M. Shimomura, K. Song and J. F. Rabolt, Langmuir, 1992,8,887. 7 P. Hodge, E. Khoshdel, R. H. Tredgold, A. J. Vickers and C. S. Winter, Br. Polym. J., 1985, 17, 368. 8 P. Hodge, F. Davis and R. H. Tredgold, Philos. Trans. R. SOC., London, 1990,330,153.9 F. Davis, P. Hodge, C. R. Towns and Z. Ali-Adib, Macromolecules, 1991,24,5695. 10 F. Davis, P. Hodge, X-H. Liu and Z. Ali-Adib, Macromolecules, 1994,27,1957. 11 I. P. Aspin, P. Hodge, C. R. Towns and Z. Ali-Adib, Polymer, 1995, 36, 1707. 12 T. Miyashita, Y. Mizuta and M. Matsuda, Br. Polym. J., 1990, 22, 327. 13 Thermotropic Liquid Crystals,. ed. G. W. Gray, Wiley, Chichester, 1987; H. Finkelmann and G. Rehage, Adu. Polym. Sci., 1984,60/61, 99; V. P. Shibaev and N. A. PlatC, Adu. Polym. Sci., 1984, 60/61, 173. 14 S. Fuller, J. Hopwood, A. Rahman, N. Shinde, G. J. Tiddy, G. S. Attard, 0.Howell and S. Sproston, Liq. Cryst., 1992,12,521. 15 G. Decher and H. Ringsdorf, Liq. Cryst., 1993,13,57. 16 P. Hodge, Z. Ali-Adib, D. West and T. A. King, Thin Solid Films, 1994,244,1007. 17 N. K. Adam, F. A. Askew and J. F. Danielli, Biochem. J., 1935, 29, 1786. 18 I. Langmuir, V. J. Schaefer and H. Sobotka, J. Am. Chem. SOC., 1937,59,1751. 19 W. W. Davis, M. E. Krahl and G. H. A. Clowes, J. Am. Chem. SOC., 1940,62,3080. 20 B. A. Pethica, Trans. Faraday Soc., 1955,51, 1402. 21 K. Naito, A. Miura and M. Azuma, J. Am. Chem. SOC., 1991, 113,6386. 22 G. F. Gibbons, K. A. Mitropoulos and N. B. Myant, The Biochemistry of Cholesterol, Elsevier Biomedical Press, New York, 1982. 23 G. M. Janini, R. J. Lamb and T. J. Shaw, Makromol. Chem. Rapid Commun., 1985,6, 57. 24 R. Jones, C. S. Winter, R. H. Tredgold, P. Hodge and A. Hoofar, Polymer, 1987,28, 1619. 25 Z. Ali-Adib and P. Hodge, unpublished results. 26 Y. Frere, F. Yang, P. Gramain, D. Guillon and A. Skoulios, Makromol. Chem., 1988,189,419. 27 T. Ikeda, S. Hasegawa, T. Sasaki, T. Miyamoto, M.-P. Lin and S. Tazuke, Makromol. Chem., 1991,192,215. Paper 5/02127H; Received 3rd April, 1995 22 J. Mater. Chern., 1996, 6(l), 15-22

ISSN:0959-9428    

DOI:10.1039/JM9960600015

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

2,4-Bis [4-(N,NOdibutylamino) phenyl] squaraine: X-ray crystal structure of a centrosymmetric dye and the second-order non-linear optical properties of its non-centrosymmetric Langmuir-Blodgett films Geoffrey J. Ashwell,*" Gurmit S. Bahra,b Christopher R. Brown,b Darren G. Hamilton,a Colin H. L. Kennard' and Daniel E. Lynch" aCentrefor Molecular Electronics, Cranjield University, Cranjield, UK MK43 OAL bDefence Research Agency, Fort Halstead, Sevenoaks, Kent, UK TN14 7BP 'Department of Chemistry, The University of Queensland, Brisbane, Australia Qld 4072 Crystals of the title compound exist as a green monoclinic phase [space group P21/c with u=9.046( l), b= 19.615(2), c =9.055( 1) A, p =116.107(5)", 2=21 and a purple triclinic phase. The chromophore is both planar and centrosymmetric and its dimensions indicate a tendency towards a quinoidal structure with extensive delocalisation.The Langmuir-Blodgett (LB) films show two types of aggregation with absorption maxima at 655 nm and 540 nm. These monolayers also exhibit strong second harmonic generation (SHG) comparable to the intensity from films of hemicyanine dyes. The anomalous non-linear optical properties are attributed to a serendipitous non-centrosymmetric packing arrangement within the films and to an intermolecular charge transfer contribution to the bulk second-order susceptibility. Current interest in organic materials for non-linear optics'-' stems from the high molecular hyperpolarizabilities of donor-(z-bridge)-acceptor molecules and the potential appli- cations of such materials in the electronics and communications industries.The non-linearities arise from the dependence of the polarization on the electric field of the incident radiation: p =aE +PE2+yE3 + . . . (1) P =EO {x"'E +x'~'E~ + . . . 1 (2)+x'~'E~ x is the bulk susceptibility of the material and, hitherto, has been attributed to the tensor sum of the individual molecular components (a, p, y) corrected for orientation and local field effects. As significant SHG can only occur in non-centrosym- metric media the studies to date have mainly concerned dipolar molecules and their alignment by an electric field' or LB dep~sition.~.' Thus, it has been widely accepted that the molecule, the smallest building block of any organic structure, should be acentric but this assumes that intermolecular inter- actions are weak.Contrary to this belief, we have recently discovered that centrosymmetric squaraine? dyes, previously studied as third-order materials,*-" can give rise to strong second-order effects when deposited as LB films.'' The molecu- lar hyperpolarizability (p) is zero but SHG can arise if there is an intermolecular charge transfer contribution to the suscep- tibility, x(~),and if the molecules aggregate in a manner that satisfies the symmetry requirements. This recent discovery" (reviewed by Bredas and MeyersI2) has focused attention on the influence of intermolecular effects on the second-order properties. As part of our continuing work, we have observed SHG from anilinium" and indolinium13 derivatives of the squaraine dyes and, in this paper, we report the synthesis and X-ray crystal structure of 2,4-bis [4-(N,N-dibutylamino)-phenyl] squaraine (Fig.1) and the non-linear optical properties of its LB films. Experimenta1 Synthesis The method employed in the squaraine synthesis is that developed by Law and Bailey14 for their syntheses of unsym- t Squaraine =cyclobutene-1,3-dione. metrical squaraine derivatives. We have found this method to be superior to the original general proced~re'~ which employs a Dean-Stark apparatus to drive the condensation process. Thus, 2,4-bis[4-(N,N-dibutylamino)phenyl]squaraine was obtained from the condensation of N,N-dibutylaniline ( 1.80 g, 2.00 cm3, 8.8 mmol) with squaric acid (0.5 g, 4.4 mmol) in a refluxing mixture of propan-2-01 (35 cm3) and tributyl ortho- formate (3.00 cm').After 4 h the intense blue solution was cooled depositing green crystals with a metallic sheen. This material was collected by filtration and the filtrate reduced in volume and cooled to afford a further crop of crystals. After drying in vucuo a total yield of 1.90 g (88% yield) was obtained, mp 188-19OoC, (Found C, 78.9; H, 9.1; N, 5.6. C32H44N202 requires: C, 78.64; H, 9.07; N, 5.73%); GH(CDCl,; J/H,) 1.00 (12 H, t, J 7), 1.40 (8 H, sextet, J 7), 1.64 (8 H, quintet, J 7), 3.42 (8 H, t, J 7), 6.70 (4 H, d, J 8), 8.46 (4 H, d, J 8); Gc(CDC13) 14.04 (3), 20.40 (2), 29.74 (2), 51.34 (2), 112.40 (l), 119.55 (0), 133.38 (l), 153.52 (0), 183.52 (0), 187.47 (0); m/z (FAB; NBA) 488.The 'H NMR (300 MHz) and I3C NMR (90 MHz) spectra were recorded on a Bruker AM300 spectrophotometer and the FAB mass spectrum on a VG Analytical 70-250-SE double- focusing mass spectrometer. X-Ray crystallographic analysis Single crystals were obtained by partial evaporation of a dilute solution of the dye in ethanol and, as previously reported16 for anilinium squaraines, two crystal polymorphs were observed. Crystals of a green monoclinic phase were separated and employed in the structural determination whereas those of the purple triclinic phase were deemed unsuitable for X-ray structural analysis. 9-6' Fig. 1 Molecular structure of 2,4-bis[4-(N,N-dibutylamino)phenyl]-squaraine J.Muter. Chem., 1996, 6( l), 23-26 X-Ray intensity data were collected on an Enraf-Nonius four-circle diffractometer at 298 K using graphite crystal mono- chromated Mo-Ka radiation (1=0.71073 A) to 28 =50". Unit cell parameters were determined from the least-squares refinement for 25 reflections with 28 <25". 2536 independent reflections with I >241) were used for the structural analysis. Data were corrected for both Lorentz and polarization effects and empirical absorption correction was applied. The structure was solved by using SHELXS-86,17 and refined to residuals R1=0.0395 and wR, =0.1234 (SHELXL-93l8) using full-matrix least-squares with anisotropic thermal parameters for all non- hydrogens.Hydrogens were located by difference methods and both positional and thermal parameters refined. The crystallographic data and atomic coordinates are listed in Tables 1 and 2 while selected bond lengths are given in Fig. 2. Bond distances and angles, anisotropic thermal param- eters, hydrogen atom coordinates, and observed and calculated Table 1 Crystallographic data for 2,4-bis(4-[N,N-dibutylamino)-phenyl] squarainea empirical formula C32H44N202 formula weight 488.69 temperature 298(2) K wavelength 0.71073 A crystal system monoclinic space group P2Jc unit cell dimensions u =9.0460( 10)oA b =19.615(2)AD c =9.0550( 10) A /?=116.107(5)" volume 1442.8( 3) A3 z 2 density (calc) 1.125 Mg mP3 absorption coefficient 0.069 mm -' F(000) 532 crystal size 0.40 x 0.32 x 0.20 mm3 8 range for data collection 2.08 to 24.96' index ranges O<h<lO, O<k<23, -10<1<9 reflections collected 2702 independent reflections 2536 [R(int)=0.0376] absorption correction Psi scan refinement method full-matrix least-squares on F2 data/restraints/parameters 2 5 3 61012 52 goodness-of-fit on F2 0.676 final R indices [I >2a(1)] R, =0.0395, wR, =0.1234 R indices (all data) R1 =0.1042, wR2=0.?350 largest diff. peak and hole 0.124 and -0.139 e Ae3 Residuals R, and wR, are defined as R, =X(IF, I -IF, l)/X IF, 1 and wR, =[Xw(FO2-F,2)2/Cw(F,z)z]"2.Weighting scheme w = [0~(F,)~+(0.2084P)~+ 1.85P]-' where P= [max(FO2,0)+2F:]/3. Table 2 Atomic coordin+es (x lo4)and equivalent isotropic displace- ment parameters (x lo3 A').U(eq) is defined as one third of the trace of the orthogonalized Uij tensor -1578(3) 9216( 1) 477(4) 5975(3) 7704( 1) 2156(4) -723(4) 9640(2) 220( 4) 934(4) 9645(2) 312(4) 2189(4) 9158(2) 728(4) 1994(4) 8496(2) 1236( 4) 3221(4) 8024( 2) 1681 (4) 4750(4) 8177( 2) 1674(4) 4941(4) 8841 (2) 1161(4) 3705(4) 9306(2) 706(4) 7584( 4) 7830(2) 2196(5) 8930( 5) 7978(2) 3897(5) 8718( 6) 8640( 2) 4620( 6) 10023( 10) 8758(5) 6379(8) 5712(5) 7009(2) 2594(4) 4721(5) 6566(2) 11 10( 5) 4458(7) 5854(2) 1582(5) 3281 (9) 5436( 3) 125(7) C(16) '(15)-x Fig. 2 Molecular conformation, atomic numbering scheme and selec- ted bond distances (A ) and angles (") of the chromophore.Squarate group: O(1)-C( 1) 1.226(4), C( 1)-C(2) 1.464( 5), C( 1')-C(2) 1.466(4); O(1)-C( 1)-C(2) 135.2(3), C(2)-C(l)-C(2') 89.7(3), C(l)-C(2)-C(l') 90.3(3). Six-membered ring: C(3)-C(4) 1.414(4), C(4)-C(5) 1.364(5), C(5)-C(6) 1.418(5), C(6)-C(7) 1.417(4), C(7)-C(8) 1.360(4), C(S)-C(3) 1.411(4); C(3)-C(4)-C(5) 121.3(3), C(4)-C(5)-C(6) 1213 3), C( 5)-C(6)-C(7) 117.2( 3), C(6)-C( 7)-C(S) 120.9(3), C(7)-C(8)-C(3) 122.2(3), C(S)-C(3)-C(4) 117.0(3). Exocyclic bonds and angles: C(2)-C(3) 1.402(4), C(6)-N(1) 1.361(4); C(l)-C(2)-C(3) 134.3(3), C( l')-C(2)-C(3) 135.4(3), C(2)-C(3)-C(4) 120.7(3), C(2)-C(3)-C(8) 122.2(3), C(5)-C(6)-N( 1) 120.8(3), C(7)-C(6)-N( 1) 122.1 (3). The symmetry transformation used to generate the equivalent atoms is -x, -y+2, -2.structure factors are provided as supplementary material. All data have been deposited at the Cambridge Crystallographic Data Centre (refer to Information for Authors, J. Muter. Chem., Issue No. 1). Film deposition The squaraine dye was spread from a 0.1 mg cmP3 chloroform solution onto the pure water subphase (MilliQ) of an LB trough (Nima Technology, Model 622), left for 10 min at cu. 25 "C and then compressed at 0.5 cm2 s-'. Films were deposited at 5 to 25 mNm-l by passing a hydrophilically treated glass slide vertically through the floating monolayer at 80 pm s-' on the upstroke. Second-harmonic generation SHG measurements were performed in transmission using a Nd:YAG laser (A= 1.064 pm) with the beam at 45" to the film.The apparatus is described in ref. 19. Results and Discussion X-Ray crystal structure The molecule is centrosymmetric and with the exception of the four butyl groups is essentially planar. The asymmetric unit consists of half the molecule with the other half being symmetry generated across an inversion centre, centralized in the four-membered ring. The bond lengths are similar to those reported for other anilinium squaraines'0,16 with a tendency towards a quinoidal structure with extensive delocalisatiop (Fig. 2). The edrocyclic C-C and C-N bonds are 1.402(4) A and 1.361(4)A, respectively, whereas the contracted C-$ bonds of the six-memberet ring are C(4)-C(5)= 1.364(4) A and C( 7)-C( 8)= 1.360(4)A. Thesc may be compared with averaged C-C lengths of 1,415(8) A for the remainder of the donor group and 1.465( 6) A, for the four-membered ring.The C-0 bond lengt!, 1.226(4)A, is significantly shorter than the 1.244A to 1.253 A reported for other anilinium squaraines,10.16 but the carbonyl length in these cases is probably influenced by neighbouring hydroxy groups in the ortho positions of the six-membered ring. The packing arrangement consists of parallel stacks inclined 24 J. Muter. Chem., 1996,6( l), 23-26 5 Fig. 3 Packing within the unit cell viewed down a to the ac plane and a herringbone-type arrangement parallel to the b axis (Fig. 3). Common to other anilinium squa- raines,'2*16 the parallel stacking shows the molecules overlap- ping with the amino group (donor) aligned over the central ring (acceptor) yith a mean perpendicular plane-to-plane spac- ing of ca.3.9( 1) A. However, the closest intermolecular contac!s are lateral interactions between O(1)..C(9) [3.382(4) A; 1+x,y, z], O(l)...H(9a) [2.479(4) A; 1+x,y, z] and O(1)*-H(7) [2.618(4) A; 1+x, y, z]. The crystal structure is centrosymmetric and thus, unlike the LB films below, the monoclinic crystals show no SHG. The same applies to the purple triclinic phase but we have been unsuccessful in growing crystals suitable for structural analysis. LB films The dye is an unconventional material for LB deposition as it has four short legs rather than a long hydrophobic chain. Nonetheless, the initial part of the FA isotherm rises steeply before undergoing a structural rearrangement, qr collapse, at 9 mN m-' with a corresponding area of ca.82 A2 molecule-' (Fig. 4). This is consistent with the chromophore residing on its long edge as the dimensions, including the first CH, of each of the four butyl groups, obtain:d from the structural analysis are approximately 19 x 7 x 4A3. A second transition is observed at 16 mN m-' with a corresponding area of ca. 50 A2 molecule-' and, once more, the isotherm rises steeply above 20 mNm-'. The chromophores may adopt a vertical align- ment at higher pressures although the reduction in area may also be attributed to partial collapse and bilayer formation. The LB film spectra are significantly shifted from the absorp- tion in chloroform (1,,,=633 nm, half width at half maxi- mum= 13 nm) and the film colour is dependent upon the deposition pressure.Deposition below 9 mN m-', the first transition pressure, gives rise to an absorption maximum at 665 to 670 nm whereas, at higher pressures, deposition results in the emergence of a second peak at ca. 540 nm (Fig. 5; Table 3). This is characteristic of the formation of different types of aggregate and this type of behaviour has been exten- sively documented for various squaraine Aggregation Fig.4 Surface pressure versus area isotherm of the dye spread from chloroform solution at ambient temperature (ca. 18 "C)and compressed at 0.5 cm2 s-' 400 500 600 700 800 wavelength/nm Fig. 5 Absorption spectra of LB monolayers deposited at 5 mN rn-' (lower spectrum) and 25 mN rn-l (upper spectrum) Table 3 Dependence of the spectra and non-linear optical properties of freshly prepared LB films on the deposition pressure n/mN m-' ,l,,/nm absorbance/layer - SHG"/au 5-8 665-670' 0.026 0.5 20-25 540 0.060 0.5 665 0.037 The SHG is relative to the intensity obtained from monolayer films of the hemicyanine dye, (15)-4-[2-(4-dimethylaminophenyl)ethenyi]-N-docosylpyridinium bromide.'Weak shoulder at ca. 540 nm. the spectral differences attributed to changes in the molecular orientation and intermolecular overlap. By comparison with the published work of Law and Chen23 the band at 540nm probably corresponds to a face-to-face arrangement with dipole-dipole interactions between the central groups of adjac- ent chromophores whereas the absorption at ca.665nm involves charge transfer between the donor and acceptor parts of neighbouring molecules. For films deposited above 16 mN m-', the second transition pressure, both bands result and the absorbance of each is stronger than those reported for similar dyes.23*24 There may be a mixture of phases and the possibility of multilayer formation in this high pressure regime. LB films of this centrosymmetric molecule have unusual non-linear optical properties in so far as they exhibit strong and SHG, similar in magnitude to that obtained from films of in LB films has been studied by Law and co-~orkers~~*~~ J. Muter. Chern., 1996, 6(l), 23-26 hemicyanine dyes.25 The second harmonic intensity from freshly prepared films, deposited at 5-8 mN m-l, is reproduc- ible but decays to 50% of the original value within ca.4 h and to only 5% within 24 h. This is accompanied by a decrease in absorbance at 667nm with the gradual emergence of the second absorption band at 540nm. Thus, it is assumed that the optical properties are influenced by the formation of an alternative aggregate with loss of the non-centrosymmetric packing arrangement. Surprisingly, the SHG is independent of the deposition pressure although the spectra of films deposited at 20-25 mN m-' are very different (Fig. 5; Table 3). In all cases the second harmonic intensity diminishes with time whereas films of the previously reported N-hexyl-N-methyl- amino analogue have remained stable over a period of several months." The phenomenon of SHG has been traditionally associated with donor-(n-bridge)-acceptor materials1V2 but, as explained previously," the structural criteria for SHG may be satisfied if adjacent chromophores adopt a non-parallel arrangement and if the repeating dimer motif does not pack centrosymmet- rically.For conventional donor-(n-bridge)-acceptor materials the intramolecular charge transfer contribution to the molecu- lar hyperpolarizability (b)is larger than the sum of all other effects.26 Since our observed SHG is as strong as that obtained for conventional second-order materials it is clear that there must be a significant intermolecular charge transfer contri- bution to the bulk susceptibility and that dimers, or higher aggregates, are responsible for the unusual non-linear optical behaviour.Conclusions In this work we have reported the crystal structure of a centrosymmetric molecule and SHG from its LB films. These results are significant as they conclusively show that inter- molecular charge transfer has a profound effect on the second-order non-linear optical properties of organic dyes. Additionally, the magnitude of the SHG arising from such intermolecular charge transfer is comparable with that observed from films of donor-(n-bridge)-acceptor molecules. In calculations which utilise SHG data (the determination of the chromophore tilt angle from the polarization dependence and pfrom f2)) it has been widely assumed that the intermol- ecular contribution is insignificant compared with that of the intramolecular component.27 However, it is apparent that polar materials may be susceptible to strong intermolecular interactions and this should be taken into account.In addition, the existing design rules should be broadened to include intermolecular complexes as well as donor-(n-bridge)-acceptor molecules as promising candidates for SHG. This is particularly relevant to LB film structures where alternate layers of amphi- philic donors and amphiphilic acceptors may be deposited to give significant SHG at the layer boundaries. We are grateful to the EPSRC for support of the non-linear optics programme at Cranfield and to the Ramsay Memorial Fellowships Trust and Defence Research Agency for co-sponsoring a research fellowship (to D.G.H.).We also thank Mrs. J. Street and Dr. G. J. Langley (Southampton University) for performing the NMR and mass spectral analyses. References Nonlinear Optical Properties of Organic Molecules and Crystals, eds. D. S. Chemla and J. Zyss, Academic Press, Orlando, 1987. S. Allen, in Molecular Electronics, ed. G. J. Ashwell, Wiley, Research Studies Press, Taunton, 1992, pp. 207-265. R. G. Denning, in Spectroscopy of New Materials, ed. R. J. H. Clark and R. E. Hester, Wiley, 1993, pp. 1-60. Principles and Applications of Nonlinear Optical Materials, ed. R. W. Munn and C. N. Ironside, Blackie A&P, Chapman and Hall, Glasgow, 1993.5 Organic Materials for Nonlinear Optics 111, ed. G. J. Ashwell and D. Bloor, Royal Society of Chemistry Press, Cambridge, 1993. 6 G. J. Ashwell, E. J. C. Dawnay, A. P. Kuczynski, M. Szablewski, I. M. Sandy, M. R. Bryce, A. M. Grainger and M. Hasan, J. Chem. SOC., Faraday Trans., 1990, 86, 1117; G. J. Ashwell, G. Jefferies, E. J. C. Dawnay, D. E. Lynch, A. P. Kuczynski, G. Yu and D. G. Bucknall, J. Muter. Chem., 1995,5,975. 7 G. J. Ashwell, P. D. Jackson and W. A. Crossland, Nature (London), 1994, 368, 438; G. J. Ashwell, G. Yu, D. Lochun and P. D. Jackson, Polym. Prepr., 1994,35,185. 8 Q. L. Zhou, R. F. Shi, 0. Zamani-Khamari and A. F. Garito, Nonlinear Optics, 1993,6, 145. 9 J. H. Andrews, J. D. V. Khaydarov and K.D. Singer, Polym. Prepr., 1994,35, 112. 10 C. W. Dirk, W. C. Herndon, F. Cervantes-Lee, H. Selnau, S. Martinez, P. Kalamegham, A. Tan, G. Campos, M. Velez, J. Zyss, I. Ledoux and L-T. Cheng, J. Am. Chem. SOC., 1995, 117, 2214. 11 G. J. Ashwell, G. Jefferies, D. G. Hamilton, D. E. Lynch, M. P. S. Roberts, G. S. Bahra and C. R. Brown, Nature (London), 1995,375,385. 12 J. L. Bredas and F. Meyers, Nature (London), 1995,375,362. 13 G. J. Ashwell, T. Handa and D. G. Hamilton, to be published. 14 K-Y. Law and F. C. Bailey, J. Org. Chem., 1992,57,3278. 15 H. E. Sprenger and W. Zeigenbein, Angew. Chem., Znt. Ed. Engl., 1967,6, 553. 16 J. Bernstein and E. Goldstein, Mol. Cryst. Liq. Cryst., 1988, 164, 213. 17 G. M. Sheldrick, SHELXS-86, Structure Solution Package, University of Gottingen, 1986. 18 G. M. Sheldrick, SHELXL-93, Program for Crystal Structure Determination, University of Gottingen, 1993. 19 G. J. Ashwell, P. D. Jackson, D. Lochun, P. A. Thompson, W. A. Crossland, G. S. Bahra, C. R. Brown and C. Jasper, Proc. R. SOC. London, ser. A, 1994,445, 385. 20 E. Buncel, A. McKerrow and P. M. Kazmaier, J. Chem. SOC., Chem. Commun., 1992,1242. 21 S. Das, K. G. Thomas, M. V. George and P. V. Kamat, J. Chem. SOC., Faraday Trans., 1992,88,3419. 22 S. Das, T. L. Thanulingam, K. G. Thomas, P. V. Kamat and M. V. George, J. Phys. Chem., 1993,97,13620. 23 K-Y. Law and C. Chen, J. Phys. Chem., 1989,93,2533. 24 K. Liang, K-Y. Law and D. G. Whitten, J. Phys. Chem., 1994, 98,13379. 25 I. R. Girling, N. A. Cade, P. V. Kolinski, J. D. Earls and G. H. Cross, Thin Solid Films, 1985, 132, 101. 26 D. J. Williams, Angew. Chem., Znt. Ed. Engl., 1984,23,690. 27 S. Allen and R. T. Murray, Phys. Scr., 1988, T23,275. Paper 5/04648C; Received 14th July 1995 26 J. Muter. Chem., 1996, 6(l), 23-26

ISSN:0959-9428    

DOI:10.1039/JM9960600023

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Growth of gallium oxide thin films from gallium acetylacetonate by atomic layer epitaxy Minna Nieminen," Lauri Niinisto"" and Eero Rauhalab "Helsinki University of Technology, Laboratory of Inorganic and Analytical Chemistry, FIN-021 50 Espoo, Finland bUniversity of Helsinki, Department of Physics, Accelerator Laboratory, FIN-0001 4 Helsinki, Finland Gallium oxide thin films have been deposited by atomic layer epitaxy (ALE) using Ga(acac), (acac =pentane-2,4-dionate) and either water or ozone as precursors. Films were grown on silicon (loo), soda lime and Corning glass substrates. The influence of the deposition parameters (e.g. pulse duration, growth and source temperatures) on film growth were studied and by a proper choice of the parameters a self-controlled growth was demonstrated around 370 "C.Spectrophotometry, X-ray diffraction (XRD), Rutherford back-scattering spectroscopy (RBS) and X-ray photoelectron spectroscopy (XPS) were used to determine the refractive index, thickness, crystallinity and stoichiometry of the films. All the films were amorphous and highly uniform with only small thickness variations. The films deposited with water contained a considerable amount of carbon as an impurity whereas ozone as an oxidizer gave stoichiometric Ga203 films. Gallium oxide is a thermally and chemically stable material which is insulating at room temperature but semiconducting at higher temperatures. The n-type semiconducting property of gallium oxide is due to a slight oxygen deficit in the crystal lattice. Ga203 is the single stable oxidation state of gallium under normal conditions.It exists in several crystalline forms, of which the monoclinic, high temperature P-Ga203 modifi- cation is the most stable one. Because of its optical and electrical properties, gallium oxide has found applications in metal-insulator structures on GaAs' and facet coatings for GaAs-based lasers. Recently, the use of gallium oxide thin films as gas sensors has attracted increasing intere~t.~-~ At high temperatures (800-1000°C) the films can be used as oxygen sensors while at lower temperatures (<7OO"C) they can be used to make sensors for reducing gases. A small number of articles concerning the growth of gallium oxide thin films has been published so far. Fleischer and co- workers 2,6 have produced stoichiometric Ga203 thin films with a high-frequency sputtering process using an ultrapure Ga203 ceramics target.Macri et uL5 also used the same target when they studied the growth of films by reactive radio- frequency magnetron sputtering. Homogeneous Ga203 films have been deposited by electron-beam evaporation using a single-crystal high-purity Gd,Ga5012 source7 which, however, introduces a few per cent of gadolinium into the films, especially on the surface region.8 A spray pyrolysis process was used to grow stable P-Ga203 and a-Ga20,:Co9 thin films on glass substrates. Hariu et al." deposited gallium oxide by reactive vapour evaporation of gallium in an oxygen atmosphere. To our knowledge, the use of chemical methods [chemical vapour deposition (CVD) and atomic layer epitaxy (ALE)] in the growth of gallium oxide thin films has not been reported. The ALE process has certain unique features when compared to conventional thin film deposition techniques." The growth rate is not dependent on the rate of the reactions provided that the dose in each reaction step is high enough to give a monolayer coverage of the surface.Accordingly, growth is uniform over large areas and no thickness monitoring is needed because it is determined by the number of reaction cycles. The relatively high substrate temperature eliminates any weak bonds from the surface by re-evaporation. Together with the lack of vapour-phase reactions, this results in highly stable and stoichiometric films.Nucleation phenomena are appar- ently modified to allow predominantly two-dimensional (2D) nucleation ensuring very uniform layers even in ultra-thin structures or conformal coatings. ALE has been mainly applied to the growth of 111-V and 11-VI semiconducting thin films and oxide layers.12 This work is part of our studies of using modified aluminium oxide thin films as catalyst supports.13 One possible way to modify alumina is to replace some of the cations by other trivalent cations. Because the properties of gallium are similar to those of aluminium, we have chosen gallium to replace some of aluminium in the A120, structure. In this paper we have studied the growth of gallium oxide thin films in a flow- type atomic layer epitaxy (ALE) reactor using Ga(acac), and water, oxygen or ozone as precursors.In the ALE method the source materials are alternately pulsed into the reactor chamber.14-16 Between the reactant pulses the excess of reactant and gaseous side-products are purged out with an inert gas pulse. In an ideal case, one monolayer of the first reactant is chemisorbed on the substrate and this monolayer reacts with the second precursor pulsed onto the substrate resulting in the formation of a solid film. A controlled layer-by-layer growth is achieved by repeating this reaction cycle. The influence of the deposition parameters such as pulse duration, pressure, substrate and source temperature on the film growth were studied in detail.Experimental Gallium oxide thin films were deposited in a flow-type ALE reactor as described el~ewhere.'~,~~ The reactants were alter- nately introduced into the reactor and nitrogen with a purity of 99.999% was used as a carrier and purging gas. The source material for gallium was Ga(acac), (acac =pentane-2,4-dionate) which was synthesized from 99.999% GaC1, (Strem Chemicals, Inc.) using the method described by Belcher et al." Water, oxygen or ozone were used as an oxygen source. Ga(acac), was evaporated from an open aluminium crucible held at 130°C in the source furnace. Water was contained in thermo- statted glass reservoir held at 20 "C and it was introduced into the reactor through a capillary by means of its own vapour pressure.Ozone was produced by feeding oxygen gas (99.998%) to the reactor through an ozone generator (Fischer model 502). The concentration of ozone was cu. 10% (60 g mP3) and the gas flow rate during the pulse was about 60cm3 min-' (measured for the oxygen gas). The film deposition took place at a reduced pressure (approx. 1.5 mbar) in the temperature J. Muter. Chem., 1996, 6(l),27-31 range 350-400°C. The effect of the source temperature of Ga(acac), (120-150 "C)as well as the duration of the precursor pulses C2000-2750 ms for Ga(acac),, 2000-3500 ms for H20 and 600-1200 ms for O,] and the purge pulses (2000-4500 ms) on the film growth were studied. Soda lime glass (5 x5 cm2), Corning 7059 glass (5 x5 cm2) and silicon( 100) were used as substrates.Thermal analysis was used to study the thermal behaviour of Ga(acac),. Simultaneous thermogravimetry (TG) and differential thermal analysis (DTA) curves were recorded in a Seiko TG-DTA instrument of the SSC 5200 series. A pressure of 1-2 mbar and nitrogen atmosphere were chosen to simulate the growth conditions in the ALE reactor. The heating rate was 10 "C min-' and sample weight was ca. 10 mg. Thicknesses and refractive indices of the films were evaluated by fitting the transmittance and reflection spectra1* measured with a Hitachi U-2000 double-beam spectrophotometer in the region of 370-1100 nm. The crystallinity of the films was determined by XRD measurements with a Philips MPD1880 powder diffractometer using Cu-Ka radiation.Film composition and stoichiometry were determined by Rutherford back-scattering spectroscopy (RBS) using ions from the 2.5 MV Van de Graaff accelerator of the University of Helsinki. Both 4He and 'H ions with a scattering angle of 170" were used. By varying the 'H beam energy, both oxygen and carbon could be detected with good sensitivity due to nuclear potential and nuclear resonance scattering-enhanced cross- sections. Film thicknesses and possible heavier impurities, on the other hand, are readily revealed by the 4He spectra. X-Ray photoelectron (XP) spectra were recorded with a Kratos Analytical XSAM 165 spectrometer using a monochromatic aluminium X-ray source. The intention of the analysis was to determine whether the films were stoichiometric throughout the bulk and also to detect any carbon remaining in the films.Therefore, the films were sputtered until there was no change in the intensities of theo C Is, 0 1s and Ga 3d peaks. This depth was cu. 100-150 A. The charge-up shifts, depending on the insulating sample were corrected using the C 1s signal (284.6 eV) from the unsputtered sample. Results and Discussion Optimization of growth parameters Deposition with water. In the preliminary studies the subli- mation behaviour of Ga(acac), was studied by varying the source temperature. The experiments were performed using a growth temperature of 385°C and a reduced pressure of cu. 1.5 mbar. The pulse durations of Ga(acac), and H,O were 600 and 2000 ms, respectively.When the source temperature was above 120°C a constant growth rate was observed. The sublimation behaviour of Ga(acac), was verified by TG-DTA studies which indicated a sharp and complete volatilisation at around 130 "C under a reduced pressure of 1-2 mbar (Fig.1). The dependence of the growth rate on the growth tempera- ture is shown in Fig. 2. As a result of preliminary studies the pulse durations of Ga(acac), and ,H,O were chosen to be 2250 and 2500 ms, respectively. The growth ?ate increased with increasiq temperature, namely from 0.25 A per cycle at 350 "C to 0.55 A per cyclc at 400°C. A temperature-independent growth rate of 0.33 A per cycle was obtained between 365 and 380 "C, indicating a very narrow plateau of self-controlled ALE growth.When the growth temperature was 380°C or lower, the films were transparent and had a green colour of inter- ference, but at temperatures over 380°C the films became less transparent and also the interference colour was changed to brown-green. In order to verify the self-controlled growth of the films, the effect of source and purge pulse durations on the film growth rate at 370°C were studied. The dependence of the growth 28 J. Muter. Chew., 1996,6( l), 27-31 I"'" .-F-5fL 3 E 1' I -10 55 110 163 220 275 330 385 440 495 5SO TIT Fig. 1 Thermoanalytical curves for Ga(acac), recorded with a heating rate of 10°C min-' in flowing nitrogen at a pressure of 1-2mbar. The sample mass was 13 mg. 0.8 0.7-I-a, 0.6 02 0.53 0.4 0.3 sg 0.2 L UJ0.1 04 340 350 360 370 380 390 400 410 depositiontemperaturePC Fig.2 Dependence of the growth rate on the deposition temperature. Pulse durations were 2250 ms for Ga(acac), and 2500 ms for H20. rate on H20 pulse [Ga(acac), 2250 ms] is presented in Fig. 3(u). The saturation of the growth rate at a constant level is seen when the water pulse duration is longer than 2500 ms. This indicates that a certain minimum pulse duration is needed to obtain a full coverage of the substrate. The dependence of the growth rate on the Ga(acac), pulse (H20 pulse 3000 ms) is shown in Fig.3(b). The growth rate was found to be indepen- dent of the Ga(acac), pulse dutation between 2000 and 2800 ms.This confirms that the growth is self-controlled. The purge gas pulse durations had no effect on the growth rate, indicating that the constant surface state was achieved. The effect of the total pressure on the growth rate was also studied between 0.5 and 2.5 mbar. The consumption of the precursor was the same at all pressures and no effect on the film growth rate was observed. Deposition with oxygen. The optimized parameters obtained for gallium oxide film growth using water as an oxygen source were then used in experiments where oxygen was studied as an oxidizer. However, when oxygen was used no film growth was detected. Varying the substrate temperature (350-390 "C) also had no effect on film growth. This indicates that oxygen is not reactive enough to facilitate film growth at the tempera- tures studied.Deposition with ozone. The dependence of the growth rate on the growth temperature is presented in Fig. 4. The source temperature for the gallium precursor was 130 "C and the pulse durations of Ga(acac), and 0, were 2250 and 600 ms, respectively. Molsa and co-workers studied the growth of Ce021g and Y20320using ozone as an oxygen source, and according to their results the growth rate saturated to a c 0.1o'2 1"/;::;I go 50 lsoo 2000 2500 3ooo 3500 4ooo water pulse durationjms 0 -0.3 O+ I 180 2OOO 2200 2400 2600 2800 3000 Ga(a~ac)~pulse duration/ms Fig. 3 Dependence of the growth rate at 370°C on the H20 (a) and Ga(acac), (b)pulse durations constant value when the pulse duration of ozone was longe? than 400 ms.A temperature-independent growth rate of 0.22 A per cycle was obtained between 350 and 375°C. When the growth temperature was raised above 375°C the growth rate increased as a consequence of decomposition of the precursor, i.e. the growth took place via CVD. In order to compare better the results obtained using water as an oxidizer, a substrate temperature of 370 "C was chosen for further experiments. The dependence of the growth rate on the ozone pulse duration [Ga(acac),, 2250 ms] is shown in Fig. 5(u). When the ozone pulse is longer than 800 ms saturation of the growth rate at a constant level of 0.28 A per cycle is indicated. Similarly, the growth rate was found to be independent of the Ga(acac), pulse duration between 2000 and 2500 ms [Fig.5(b)]. These results verify that the thin film growth is self-controlled ALE growth. Dependence of the film thickness on reaction cycles The dependence of the film thickness on the reaction cycles at 370 "C is shown in Fig. 6. When water was used as .an oxygen source a linear relation with a growth rate of 0.33 A per cycle was observed. Similarly, using ozone a! an oxidizer gave a linear relation with a growth rate of 0.28 A per cycle. However, these growth rates are much less than one monolayer per cycle, which is probably caused by steric hindrance due to the 0.8 0.7 + II 340 350 380 370 380 390 400 410 deposition temperaturePC Fig.4 Growth rate as a function of temperature. Ga(acac), and 0, pulse durations were 2250 and 600 ms, respectively. $-600 800 la00 1200 1400 53 0.6 ozone pulse duration/ms I LI I 0 0 0.2 0-3 1 0 is00 1750 2OOO 2250 2500 2750 3OOO Ga(a~ac)~pulse duration/rns Fig.5 Dependence of the growth rate at 370°C on the O3 (a) and Ga(acac), (b)pulse durations bulky Ga(acac), molecule. The higher growth rates of films grown with water may be due to the formation of films which contain some GaO(0H) in analogy with A10(OH),21 since more hydroxy residues are probably left in the films grown with water than in those films grown with ozone. Other properties XRD measurements made on films grown on all substrates and with both oxidizers revealed that films were amorphous. The thickness measurements indicated that films were uniform, showing only small thickness variations (typically < 1%) in the gas flow direction, within the substrate length of 5 cm.The films deposited on soda lime glass and Corning 7059 glass had the same growth rate, whereas the growth rate ?f the films deposited on silicon decreased to yalues of 0.23 A per cycle (water as an oxidizer) and 0.21 A per cycle (ozone). The refractive index at a wavelength of 580nm had a constant value of 1.8 when water was used as the oxygen source. The films grown using ozone as the oxygen source were highly transparent in the visible region and the refractive index of the films at a wavelength of 580 nm was 1.9.Neither film thickness nor deposition temperature was observed to have an effect on the refractive indices. The increase in refractive index for films deposited with ozone as compared with films grown with water 400 350 1 300 250 \2 200 5 150 .-5 100 so 0 0 2oM) 4000 6OOO 8000 10000 12000 number of cycles Fig.6 Dependence of the film thickness on the number of cycles at 370 "C:0,water; A,ozone. The pulse durations for Ga(acac),, H20 and O3were 2250, 3000 and 1000 ms, respectively. J. Mater. Chem., 1996, 6( l), 27-31 29 may be due to higher film densities in the former case. Passlack et uL7 reported refractive indices between 1.84 and 1.89 for thin gallium oxide films grown with electron-beam evaporation, which agrees with our results.The refractive index of bulk Ga203 is between 1.92 and 1.95.22 Stoichiometry and carbon residues were quantitatively deter- mined by RBS. The back-scattering spectra of films grown on silicon( 100) using water and ozone as the oxygen source are illustrated in Fig. 7 and 8, respectively. The solid lines represent the theoretical simulations by a computer program.23 A small linear background has been added to the simulated silicon signal in Fig. 8 to account for the low energy tail. Helium ion scattering at 2000 keV was mainly used for determining the layer thickness and possible impurities. In addition, the 12C('H, 'H)12C resonance at 1735 keV24 and proton nuclear potential scattering at 2500 keV25 were used to determine the carbon and oxygen contents, respectively. The same sample structure was, however, indicated by all energies and by both He and H ions.Fig. 7 shows the proton scattering yields from C, 0 and Ga at 2000 keV. At 1735 keV the carbon signal is enhanced by about 10 times relative to that shown in Fig. 7. The films grown using water as the oxidizer had a carbon impurity content of ca. 30 atom% (25 atom% Ga, 45 atom% 0 with a total of 2.26 x 10'' atom cm-2) as determined by RBS. The XPS analysis discussed below gave a lower value 'H ion energy/keV Fig.7 Back-scattering spectrum for 2.0MeV 'H ions incident on a 320 nm thick Ga,,,O,.,C, sample on a silicon substrate (-, experimen-tal data; -, theoretical spectrum). The precursors used were Ga(acac), and water.The theoretical Ga, 0,Si and C signals are shown. 4He ion energylkev Fig. 8 Rutherford back-scattering spectrum for 2.0 MeV ,He ions incident on a 197 nm thick Ga203 film on a silicon substrate (-a, experimental data; -, theoretical spectrum). The precursors used were Ga(acac), and ozone. The theoretical Ga, 0 and Si signals are shown. 30 J. Muter. Chem., 1996, 6(l), 27-31 (< 9 atom%) but nevertheless indicated a significant amount of carbon in the film. The effect of preferential sputtering of carbon at the surface is believed to account for a lower XPS quantification of carbon compared with RBS results. The gallium precursor is responsible for the carbon impurity of the films. Since the growth of films with water was found to be very controllable at 370°C it is not likely that carbon is due to the thermal decomposition of the Ga(acac), ligand in the gas phase.Obviously the adsorbed Ga(acac), molecule reacts with the H20 pulse with a different mechanism than with 03, resulting in carbon residues in the former case. The reactor temperature is too low (370 "C) and water as the precursor is not reactive enough to drive off the carbon residues. Recently, Yoshida et al. found in CVD-deposited SrTiO, a carbon content of 7 atom% even at 6OO0C,but this could be reduced by increasing the oxygen partial pressure.26 The films deposited with ozone were stoichiometric, almost pure Ga203 films with ca. 1 atom% carbon impurity (total area density 1.78 x lo1* atom cm-2) as determined by proton back-scattering at 2500 and 1735 keV.There was no difference in the atomic composition between films grown on soda lime glass and silicon. No impurities other than carbon were detected. Chlorine, for example, with the possible signal located between 4He ion energies 1100 and 1300 keV, (Fig. 8) had an impurity content below the detection level of 0.2 atom%. The amount of atoms (expressed in atom cm-2) were used with the thicknesses obtained by the optical transmission or reflection spectroscopy to estimate the densities of the films. The calcu- lated densities for films grown on silicon and soda lime glass were 5.6 and 5.3 g ~m-~, respectively. These values are lower than that of bulk P-Ga203 (5.88 g ~m-~).~' The XP spectrum of a 170nm thick Ga,03 film deposited using ozone as the oxidizer on a silicon substrate is presented in Fig.9(a) (before sputtering) and (b)(after sputtering). Since the ratio of gallium :oxygen atoms in the film was 3959, the film was shown to be close to stoichiometry using the standard sensitivity factors. The carbon concentration in films deposited with ozone was, after sputtering, close to zero (around 1 atom% but dominated by noise in the spectrum). No other binding energylev Fig.9 XP spectrum of a 170nm thick Ga,O, film on a silicon substrate. (a) XPS survey (before sputtering), (b) Ga 3d peak (after sputtering). impurities were detected. On the other hand, the films grown with water contained a higher concentration of carbon (around 9 atom%) which was not removed by sputtering. The Ga 3d peak was found at 20.5 eV and the full width at half-maximum (FWHM) was 1.6 eV [Fig.9(b)]. The binding energy of Ga 3d 3 4 5 6 M. Fleischer and H. Meixner, Sensors Actuators B, 1991,4,437. W. Hanrieder and H. Meixner, Sensors Actuators B, 1991,4,401. P. P. Macri, S. Enzo, G. Sberveglieri, S. Groppelli and C. Perego, Appl. Surf. Sci., 1993,65166,277. M. Fleischer, W. Hanrieder and H. Meixner, Thin Solid Films, 1990, 190,93. photoelectrons corresponded to that given in ref. 27 for Ga203 powder, indicating that gallium is present as Ga3+ in the films. 7 M. Passlack, E. F. Schubert, W. S. Hobson, M. Hong, N. Moriya, S. N. G. Chu, K. Konstadinidis, J.P. Mannaerts, M. L. Schnoes and G. J. Zydzik, J. Appl. Phys., 1995,77,686. 8 M. Passlack, M. Hong, E. F. Schubert, J. R. Kwo, J. P. Mannaerts, Conclusions S. N. G. Chu, N. Moriya and F. A. Thiel, Appl. Phys. Lett., 1995, 66, 625. The present study demonstrates that by using a chemical deposition method (ALE), Ga203 thin films can be grown on glass and silicon substrates. The precursors used were Ga(acac), and either water or ozone. No film growth was detected when oxygen was used as the oxidizer. The growth rate of the films was dependent on the pulse durations of the 9 10 11 12 13 H-G. Kim and W-T. Kim, J. Appl. Phys., 1987,62,2000. T. Hariu, S. Sasaki, H. Adachi and Y. Shibata, Jpn. J. Appl. Phys., 1977, 16, 841. T. Suntola and J. Hyvarinen, Ann.Rev. Muter. Sci., 1985, 15, 177. L. Niinisto and M. Leskela, Thin Solid Films, 1993,225, 130. M. Nieminen, L. Niinisto and R. Lappalainen, Mikrochim. Acta, 1995, 119, 13. precursors and on the growth temperature, but a narrow plateau of self-controlled ALE growth was observed around 370°C. The growth rate was much less than one monolayer per cycle and this is probably caused by steric hindrance due to the bulky Ga(acac), molecule. The films were amorphous and highly uniform with only small thickness variations over 14 15 16 17 T. Suntola, Muter. Sci. Rep., 1989,4,261. M. Leskela and L. Niinisto, in Atomic Layer Epituxy, ed. T. Suntola and M. Simpson, Blackie and Son, Ltd., Glasgow, 1990,p.1. T. Suntola, in Handbook of Crystal Growth, vol. 3, ed. D. T.J. Hurle, Elsevier, Amsterdam, 1994, p. 601. R. Belcher, C. R. Jenkins, W. I. Stephen and P. C. Uden, Tulunta, the substrate area of 5 x 5 cm2. RBS and XPS measurements 1970, 17,455. indicated that the films deposited with water contained a considerable amount of carbon as an impurity, whereas the films grown with ozone were stoichiometric and had a carbon content of ca. 1 atom%. No other impurities were detected. 18 19 20 M. Ylilammi and T. Ranta-aho, Thin Solid Films, 1993,232, 56. H. Molsa and L. Niinisto, Muter. Res. SOC. Syrnp. Proc., 1994, 335, 341. H. Molsa, L. Niinisto and M. Utriainen, Adv. Mater. Opt. Electr., 1994,4,389. 21 L. Hiltunen, H. Kattelus, M. Leskela, M. Makela, L. Niinisto, The authors wish to thank Dr. Kevin S. Robinson (Kratos Ltd., Manchester, UK) for XPS measurements. The assistance of Ms. Heini Molsa and Mr. Pekka Soininen is gratefully acknowledged. This work was suppmkd in part by the 22 23 E. Nykanen, P. Soininen and M. Tiitta, Muter. Chem. Phys., 1991, 28, 379. CRC Handbook of Chemistry and Physics, CRC Press, Boca Raton, FL, 69th edn., 1988, p. B-92. J. Saarilahti and E. Rauhala, Nucl. Instrum. Methods Phys. Res. B., Academy of Finland. 24 1992,64,734. E. Rauhala, Nucl. Instrum. Methods Phys. Res. B., 1985, 12,447. 25 M. Luomajarvi, E. Rauhala and M. Hautala, Nucl. Instrum. References 1 A. Callegari, P. D. Hoh, D. A. Buchanan and D. Lacey, Appl. Phys. Lett., 1989,54, 332. 26 27 Methods Phys. Res. B., 1985,9,255. M. Yoshida, H. Yamaguchi, T. Sakuma, Y. Miyasaka, P-Y. Lesaicherre and A. Ishitani, J. Electrochem. Soc., 1995,142,244. R. Carli and C. L. Bianchi, Appl. Surf. Sci., 1994,74,99. 2 M. Fleischer, L. Hollbauer and H. Meixner, Sensors Actuators B, 1994,18-19, 119. Paper 5/03276H; Received 22nd May, 1995 J. Muter. Chem., 1996, 6(1), 27-31

ISSN:0959-9428    

DOI:10.1039/JM9960600027

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Structural change of the LiMn,O, spinel structure induced by extraction of lithium Kiyoshi Kanamura,* Hidetoshi Naito, Takeshi Yao and Zen-ichiro Takehara Division of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto 606-01, Japan A structural change of the Li,Mn,O, spinel induced by extraction of lithium was investigated using the Rietveld refinement method for its X-ray diffraction pattern change. Two cubic phases of the spinel Li,Mn,04 were observed in the range 0.5 >x >0.13 and their lattice parameters were found to decrease with decreasing x. If both phases were thermodynamically stable, the lattice parameters should not change during the extraction of lithium. Therefore, our X-ray diffraction (XRD) patterns suggest the destruction of the rigid Li,Mn,O, spinel structure which provides a high reversibility for the insertion and extraction of lithium.The possible mechanisms corresponding to this structural degradation are the compositional change of Mn or 0 atoms in Li,Mn,O, through the extraction of lithium. On the other hand, the separation of Mn2+ ions from Li,Mn,04 (0.5>x >0.13) was detected by electron paramagnetic resonance (EPR). From these results, it is concluded that Li,Mn,O, decomposes through the extraction of lithium to form Mn2+ compounds as a separate phase. Various kinds of transition-metal oxides have been investigated Japan; Mo-Ka radiation) equipped with a curved-crystal as materials for lithium insertion and extraction processes, which are very important electrochemical reactions for active materials in rechargeable lithium batteries;lP8 for example, LiCoO, has been utilized in a practical rechargeable lithium battery.'-* Recently, alternative cathodes, in particular the LiMn,O, spinel, have also been investigated as good candi- dates for electrochemical lithium insertion materials. Since the electrochemical characteristics of LiMn,O, depend on its crystal nature, size and shape, various kinds of preparation methods have been developed to improve the characteristics of the LiMn,O, spinel.'^^.^-^^ Fundamental studies on this structure have been performed using X-ray diffraction (XRD) and electrochemical methods to examine the electrochemical characteristics associated with its structural changes.1*2,9*10.18v21 Several studies have proposed that the structural changes take place when lithium ions are inserted into or extracted from LiMn204.These changes are: the phase transition of Li,Mn,O, from a cubic phase to a tetragonal phase in the region from x=2 to x =1, and the homogeneous phase reaction of the Li,Mn,O, spinel with a continuous lattice parameter change in the region from x= 1 to In the region x<O.5, the presence of two cubic phases has been proposed, but the detailed structural change has not yet been reported. In this study, the Rietveld method was used to refine the structural changes of Li,Mn,O, in the region from LiMn,O, to Lio.13Mn,0,. Experiment a1 Several different preparation methods of LiMn,O, have been rep~rted;'.~.~-~'Li,C03 and MnO, were the most commonly used starting materials.Recently, Momchilov et ul. suggested that LiMn,O, prepared from LiNO, and chemical manganese dioxide (CMD) has a large specific surface area and a high electrical conductivity.21 This preparation method was adopted in this study in order to realize a more uniform electrochemical reaction and to obtain equilibrium states of Li,Mn,O,. A mixture of LiNO, and CMD in a mole ratio of 1:2 was heated at 450°C for 36 h and then at 750°C for 72 h in air, according to ref. 21. The crystal structure of the product was determined by XRD using Mo-Ka (or Cu-Ka) radiation, to confirm the formation of a well characterized LiMn,O,.For the Rietveld analysis, a powder XRD pattern was collected using a Model RAD-B powder X-ray diffractometer (Rigaku Co., Tokyo, graphite monochromator at the High Intensity X-ray Laboratory, Kyoto University. The step-scanning technique with steps of 28=0.01" and a stepping time of 15 s was used over the range 5"<28<60". The Rietveld calculation was performed on the vector processor (Cray Y-MP2E/264) at the Institute for Chemical Research, Kyoto University, by using the 'Rievec' computer program for Rietveld refinement.,,-,' The atomic ratio of Li :Mn was determined by atomic adsorp- tion spectroscopy (AAS) to be 1:2. Electrochemical extraction of lithium from LiMn204 was performed using a pellet-type electrode to prepare several Li,Mn,O, samples (0.13 <x ~0.5).The cathode pellet was prepared by pressing a mixture (100 mg) of the prepared LiMn204, and acetylene black in a mass ratio of 70: 30 at a pressure of 5 x lo5 Pa for 15 min. The diameter of the cathode pellet was 1.0 cm. Propylene carbonate contain- ing 1.0 mol dm-3 LiClO, was used as the electrolyte. Lithium metal was used for the reference and counter electrodes. After the electrochemical extraction, the pellet electrode was washed with pure propylene carbonate and then with tetrahydrofuran. The prepared sample was dried in vacuum for 1 h. The current used for the electrochemical extraction of lithium was 0.03 mA, which was low enough to attain an equilibrium state within the sample.After the electrochemical extraction process, the samples were kept in electrolytes for several days until the electrode potential became constant. The crystal structures of the Li,Mn,O, samples were also analysed with the Rietveld method to determine the lattice parameters. Electron paramagnetic resonance (EPR) was also used to analyse the reaction products of the extraction of lithium from LiMn,O,; the EPR signal was obtained at room temperature using JES-RE equipment (JEOL). An X-band microwave was used, with an output power of 1mW. The amplitude of the magnetic field modulation was 0.5 mT and its frequency was 100 Hz. The response time was 0.3 s. Results and Discussion Fig. 1 shows the XRD patterns of Li,Mn20, (x= 1.0, 0.5, 0.3 and 0.13) in the 28 region from 43" to 47" (Cu-Ka). At x= 1.0, 0.5 and 0.13, only a single peak was observed at 28=43.87, 44.34 and 44.93", respectively.At x=0.3, two peaks were observed, at 20 =44.55 and 44.82". When a two-phase reaction takes place during the extraction of lithium, the peak positions should not change with changing x value in Li,Mn,O,. J. Muter. Chem., 1996, 6(1), 33-36 0.3 Phase I + Phase I1A I 'i 0.25 x=0.5 v) Y5 0.2 0 0 $I 0.15 \ .-a v)c a, 0.1 c. c.-0.05 0 40 42 44 46 48 50 281degrees (Cu-Ka) Fig. 1 XRD patterns of Li,Mn,O, (x= 1.0, 0.5, 0.3 and 0.13) prepared by the electrochemical extraction of lithium from LiMn,O, using the LiMn,O, disc cathode with an apparent electrode area of 0.785 cm2 under galvanostatic conditions at 0.03 mA in propylene carbonate containing 1.0 mol dm-3 LiClO,.LiMn,O, was prepared by heating a mixture of LiN03 and CMD. XRD patterns were obtained with an X-ray diffractometer using Cu-Ka radiation. However, both peak positions in the XRD pattern at x=O.3 were different from those of Li0.,Mn,O4 and Li0.,,Mn,O4, which are two different single phases of Li,Mn,O,. These XRD patterns suggest that the electrochemical reaction of LiMn,O, is not explained by the extraction of lithium via a two-phase reaction. More detailed analyses on the XRD patterns were performed using the Rietveld method to clarify the structural change undergone by Li,Mn204 (0.5 >x > 0.13). The lattice parameters, oxygen positional parameters, frac- tions of two cubic phases, Rwp, R,, RF and R, values for the Rietveld analysis on the XRD patterns of Li,Mn,O, are summarized in Table 1.Fig. 2 shows the observed and simu- lated XRD patterns of Li0.,Mn204, which consist of two different phases. The observed pattern agreed well with the simulated pattern obtained from the Rietveld method; such good agreement between the observed and simulated patterns was also obtained for the other x values in Li,Mn20,. The LiMn,O, prepared in this study possessed a cubic structure with Fd3m symmetry, and with the lattice parameter calculated as 0.824219 nm. The electrochemical extraction of lithium from this LiMn,O, was performed under galvanostatic conditions at 0.03mA.The crystal structure of Li,Mn20, in the region from x= 1.0 to x=O.5 was also determined to be cubic. The lattice parameter decreased from 0.824219 nm to 0.815041 nm during the extraction of lithium from the LiMn,O,. This structural change indicates that the electrochemical extraction 5 10 20 30 40 50 2t)(Mo-Ka)/degrees Fig.2 XRD pattern for Li,.,Mn,O, prepared under the same conditions as those in Fig. 1; the solid curve is the simulated pattern and the broken curve is the observed one. The XRD pattern was obtained with an X-ray diffractometer using Mo-Ka radiation. of lithium from LiMn,O, in the region from x= 1.0 to x=O.5 proceeds via a homogeneous phase reaction with a decrease in the host matrix size. This result was in good agreement with those reported previously,2,18-20 but we have now provided more precise lattice parameter changes of Li,Mn,O, in the region from x=1.0 to x=O.5.Fig. 3 shows the electrode potential change during the electrochemical extraction of lithium from LiMn,O, in the region from x = 1 to x =0.1 at 0.03 mA in propylene carbonate containing 1.0 mol dmP3 LiClO,. When homogeneous lithium extraction takes place, the free energy of Li,Mn,O, changes with changing x value, which causes the electrode potential change. This is typical behaviour for a solid solution. The electrode potential change in the region from x = 1 to x =0.5 was in good agreement with the reaction scheme for the homogeneous extraction of lithium from LiMn,O,.Similar results have been reported elsewhere.2,18-20 When the electro- chemical extraction of lithium was performed in the region of 0.5 >x, two cubic phases were observed in the XRD pattern. The lattice parameter of one cubic phase (phase I) was slightly larger than that of the other phase (phase 11). The relative amounts of the two phases changed according to the amount of lithium extracted from LiO.,Mn2O4, as shown in Table 1. At x =0.13, phase I almost disappeared and phase I1 only was observed, as shown in Fig. 1, suggesting that the extraction of lithium from Li,.,Mn,O, proceeds via a two-phase reaction in Table 1 Lattice parameters, oxygen positional parameters and R-factors of the Li,Mn,O, spinel (space group Fd3m) obtained from the Rietveld refinement method for the X-ray diffraction patterns lattice parameters (phase I) (phase 11) oxygen positional (phase I) parameters (phase 11) RWP RP RF (phase I) (phase 11) RB (phase I) (phase 11)4, (phase 1)(phase 11) fraction of phase I fraction of phase I1 x= 1.0 x=0.5 x =0.4 x =0.3 x=0.15 0.8242 19 ( 8) 0.38701 (6) 0.0895 0.815041( 1) 0.3871 1( 13) 0.0775 0.8 12510( 2) 0.810254( 3) 0.38730( 24) 0.38750( 15) 0.0563 0.8 10779( 4) 0.807956( 2) 0.38750( 35) 0.38765( 50) 0.0548 0.804834( 9) 0.38770(7) 0.0781 0.1181 0.0875 0.0738 0.0726 0.0936 0.0393 0.0309 0.0273 0.0256 0.0305 0.0341 0.0319 0.0372 0.0392 0.0341 0.03 19 0.0310 0.0199 0.0249 0.17( 1) 0.56(2) 0.42 ( 1) 0.30(2) 0.697 0.52(8) 0.14( 8) 0.412 0.57( 1) 0.303 0.588 Rwp, R-weighted pattern; R,, R-pattern; R,, R-structure factor; RB, R-Bragg factor; Bsm, temperature factor.34 J. Muter. Chem., 1996, 6(l), 33-36 4.50 4.25 h ?-2 -! Lu 4.00 3.750 10 0.20 0.40 0.60 0.80 1.00 x in Li,Mn204 Fig. 3 Electrode potential change of the LiMn20, disc cathode with an apparent electrode area of 0.785 cm2 during the charge process at 0.03 mA in propylene carbonate containing 1.0 mol dm-3 LiClO, in the region of 1.O >x >0.1 which the phases are Li,.,Mn,O, and Li,~,,Mn,O,. If the electrode reaction proceeds uiu a two-phase reaction without any lattice parameter changes, the electrode potential should be independent of the concentration of lithium in Li,Mn,O,. The flat potential change (no free-energy change) has been reported in several previous paper^.'.'^^*^^ This is typical thermodynamic behaviour for a two-phase reaction.Our results also show a relatively flat potential change, which differs from that in the region of 1>x >0.5. However, from our XRD pattern at x=O.3 in Li,Mn,O,, it can be seen that both peaks corresponding to the two cubic phases move with changing x in Li,Mn,O,. This result is inconsistent with the flat potential curve in this region. Note that a flat potential change (no free-energy change) is not necessary for a two-phase reaction because, when some unexpected physical prop- erty changes occurs during the extraction of lithium from Li,.,Mn,O,, the electrode potential change can be compen- sated by the unexpected physical property change. In the case of LiMn204, the flat potential change might be obscured by some physical property change which is related to our XRD pattern change.Since the Rietveld refinement method enables one to deter- mine the precise lattice parameter, the structural change of Li,Mn,O, in the region of OS>x was analysed using this method. Two phases appeared in the region from x=O.5 to x=O.13 with cubic structures, and their space groups were assigned to Fd3rn, as shown in Table 1. The lattice parameters of phase I at x=0.4 and 0.3 were 0.812510 and 0.810779 nm, respectively; those of phase I1 at x=0.3 and 0.15 were 0.807956 and 0.804834nm7 respectively.From these results, it is clear that the lattice parameter decreases during the electrochemical extraction of lithium, indicating that the host matrix of Li,Mn,O, is influenced by the extraction of lithium. The decrease in the lattice parameters of the two cubic phases is probably explained by a chemical composition change in the host matrix of Li,Mn,O,. If both cubic phases have different chemical compositions from Li,,,,Mn,O, and Lio.,Mn,O,, the structural change and electrode potential change in the region from x=OS to x=0.13 can be understood. Such chemical composition changes must result in the formation of a third acetylene black/ "VJ \ I I I I 3 178 278 378 478 magnetic field / mT Fig. 4 EPR signals for (a) Lio.3Mn20, and (b) LiMn20, phase, corresponding to the degradation of the host matrix.No third phase could be detected by XRD in this study. The third phase may have an amorphous nature, or it may dissolve in an electrolyte solution. If this is correct, the third phase prevents the reversible insertion and extraction of lithium. Since the lattice parameter change is very small, it can be expected that the amount of the third phase formed during the first extraction of lithium is very small. Fig. 4 shows the EPR signals for (a) Li,,,Mn,O, and (b) LiMn,04 at room temperature. A broad peak was observed and was assigned to the electron spins of Mn3+ or Mn4+ ions in LiMn,O,. Simultaneously, six sharp signals were observed. The same signal pattern were observed for an Mn2+/Mg0 marker sample, indicating that Mn2+ ions are formed in Lio.,Mn,O,.This result is a direct indication that the reaction of Li,Mn,O, in the region x=O.5-0.13 cannot be explained by a two-phase reaction. The EPR signal corresponding to Mn2+ ions shows that Mn2+ ions are present in a phase separated from the two cubic systems. Conclusion The Rietveld refinement method for the X-ray diffraction patterns proved the existence of a very fine structural change of LiMn,O,. We can conclude that an irreversible structural change takes place during the extraction process in the region of x <0.5. This result was also confirmed by EPR spectrometry. This study was supported by a Grant-Aid for Scientific Research from the Society for Promotion of Space Science and a Grand-in-Aid for Scientific Research (C) from the Ministry of Education Science and Culture of Japan (no.06650745). Computation time was provided by the Supercomputer Laboratory, Institute for Chemical Research, Kyoto University. References J. M. Tarascon and D. Guyomard, Electrochim. Acta, 1993, 38, 1221. T. Ohzuku, M. Kitagawa and T. Hirai, J. Electrochem. Soc., 1990, 137,769. T. Ohzuku, A. Ueda and M. Nagayama, J. Electrochem. Soc., 1993, 140,1862. J. R. Dahn, U. von Sacken, M. W. Juzkow and H. Al-Janaby, J. Electrochem. Soc., 1991, 138,2207. J. N. Reimers and J. R. Dahn, J. Electrochem. Soc., 1992,139,2091. E. Plichta, S. Slane, M. Uchiyama and M. Salomon, J. Electrochem.Soc., 1989, 136, 1865. M. Antaya, J. R. Dahn, J. S. Preston, E. Rossen and J. N. Reimers, J. Electrochem. SOC., 1993, 140, 575. T. Ohzuku and A. Ueda, J. Electrochem. Soc., 1994,141,2972. J. Muter. Chern., 1996, 6(l),33-36 9 J. M. Tarascon, E. Wang and F. K. Shokoohi, J. Electrochem. SOC., 20 M. M. Thackeray, W. I. F. David, P. G. Bruce and J. B. 10 1991,138,2859. D. Guyomard and J. M. Tarascon, J. Electrochem. SOC., 1992, 21 Goodenough, Muter. Res. Bull., 1983,18,461. A. Momchilov, V. Manev and A. Nassalevska, J. Power Sources, 139, 937. 1993,41,305. 11 J. M. Tarascon and D. Guyomard, J. Electrochem. SOC., 1991, 138,2864. 22 23 H. M. Rietveld, J. Appl. Crystallogr., 1969,2, 65. Y. Oka, T. Yao and N. Yamamoto, J. Solid State Chem., 1990, 12 J. C. Hunter, J. Solid State Chem., 1981,39, 142. 86, 1 16. 13 W. J. Macklin, R. J. Neat and R. J. Powell, J. Power Sources, 1991, 24 Y. Oka, T. Yao and N. Yamarnoto, J. Solid State Chem., 1990, 34,39. 89, 372. 14 M. H. ROSSOUW, A. de Kock, L. A. de Picciotto, M. M. Thackeray, W. I. F. David and R. M. Ibberson, Muter. Res. Bull., 1990,25, 173. 25 26 T. Yao, Y. Oka and N. Yamamoto, Mater. Res. Bull., 1992,27,669. Y. Oka, T. Yao, N. Yamamoto, Y. Ueda and A. Hayashi, J. Solid 15 L. Li and G. Pistoia, Solid State Ionics, 1991,47,231. State Chem., 1993, 105,271. 16 L. Li and G. Pistoia, Solid State Ionics, 1991,47,241. 27 T. Yao, Y. Oka and N. Yamamoto, J. Mater. Chem., 1992,32,331. 17 18 G. Pistoia, G. Wang and C. Wang, Solid State Ionics, 1992,58,285. A. Mosbah, A. Verbaere and M. Tournoux, Muter. Res. Bull., 1983, 18, 1375. Paper 5/04666A; Received 17th July, 1995 19 M. M. Thackeray, P. J. Johnson, L. A. de Picciotto, P. G. Bruce and J. B. Goodenough, Muter. Res. Bull., 1984,19, 179. 36 J. Mater. Chem., 1996, 6(l), 33-36

ISSN:0959-9428    

DOI:10.1039/JM9960600033

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Low-temperature mixed spinel oxides as lithium insertion compounds Luis Sanchez,"' Jacqueline Farcy," Jean-Pierre Pereira-Ramos,a Lourdes HernPn,b JuliPn Moralesb and Jose Luis Tiradob 'CNRS, LECSO, UMR 28, 2 rue Henri-Dunant, 94320 Thiais, France bLaboratorio de Quimica Inorganica, Facultad de Ciencias, Universidad de Cbrdoba, Cbrdoba 14004, Spain Cation-deficient mixed oxides, Mn-Co, Mn-Fe and Co-Fe, with the spinel structure have been synthesized by a solution technique. Preliminary results on electrochemical Li insertion into these compounds are discussed in relation to their structural and chemical characteristics, and emphasize the strong effect of cation vacancies on their electrochemical properties. The best results were obtained with the Mn-Co system.Owing to the three-dimensional lattice of spinel materials which offers pathways for lithium transport, topotactic lithium insertion-extraction reactions have been considered in various phases including Fe,04,' Mn304,2 CO,O~,~ LiMn,04,2i4-7 LiTi2048-10 and LiV204 .'*I1 However, only the spinel electrode of the Li-Mn-0 system shows promising re~ults,'~?'~ while the stoichiometric pure compounds A304 (A =Mn, Co, Fe) do not give rise to any re~hargeabi1ity.l~ The elements manganese and cobalt are known to have different oxidation states. In systems containing these elements, the stability of the oxides depends strongly on the partial pressure of oxygen. We have prepared, via a solution technique, new cation-deficient mixed oxides Mn-Co, Mn-Fe, and Fe-Co with the spinel structure and evaluated some of their electro- chemical properties.Experiment a1 Mixed carbonates of general formula M, -xM',C03 (M =Mn; M'=Co, Fe) were prepared by the addition of a 1 mol dmP3 solution of NaHCO, to a 0.5 mol dm-3 solution of the divalent ions M" and MI1* in the desired proportions, under a continu- ous flow of C02.M:(M+M') ratios and composition homo- geneity in the carbonate and oxides samples were determined by electron diffraction (ED) with a Philips SEM 501 B apparatus. The stoichiometry was confirmed by atomic absorp- tion spectrometry (AAS). The average oxidation state of the metal ions in the samples was determined by the following proced~res.'~-'~ About 50 mg of the sample was dissolved in 5 ml of 0.1 mol dm-3 Fe2+, 5ml of H2S04, 5ml of HC1 and 10ml of H20 under a continuous flow of argon and heated until complete dissolution. After cooling to 20-25 "C, 10 ml of H,PO4,25 ml of H20 and 1.5 ml of indicator (0.3% diphenylamine in ethanol) were added.The solution was titrated against standard 0.05 mol dmP3 K2Cr04. Previously, a blank titration was carried out under the same conditions. The difference between the titrations was assigned to the total content of the oxidizing species and was used to calculate the oxygen-to-metal (0:M) ratios. X-Ray diffraction (XRD) data were collected with a Siemens D5000 diffractometer provided with Co-Ka radiation and a graphite monochromator. For the electrochemical studies, the working electrode consisted of a stainless steel grid of area 1cm2 on which the spinel oxide mixed with graphite (90 mass%) was pressed.The electrolyte was dried anhydrous LiC104 in doubly distilled propylene carbonate. For cycling experiments the cathode was made of a mixture of active material (80 mass%) with graphite (7.5 mass%), acetylene black (7.5 mass%) and PTFE (5 mass%). Results and Discussion The mixed spinel oxides were obtained from the thermal decomposition of the corresponding carbonates in air at 400 or 300°C. From the XRD data the spinel single phases show a cubic structure except for those with high Mn contents where a tetragonal structure is observed. An example XRD pattern is shown for an Mn-Co spinel oxide (Mn2.15C00.3704)in Fig.1. The chemical and crystallographic data of the mixed spinel oxides are summarized in Table 1, and are compared to those of the corresponding stoichiometric materials. The spinel skeleton of >he pristine oxides is practically maintained (a, = 8.1-8.4 A), but the main interest provided by this carbon- ate precursor method is that it results metal ions with high oxidation state 2.76 d 2 < 3.22 (compared with only 2.66 in Table 1 Chemical composition and crystallographic data of mixed Mn, Fe, Co spinel oxides prepared by thermal decomposition of carbonate precursors at 300 and 400°C T/"C composition O:M structure' Zb vacancies/mol- 400 Mn2.15C00.3704 Mn0.93C01.6904 Mn1.43Fe1.0804 Mn1.66Fe0.8204 300 c00.4Fe2.3904c01.OSFel.80°4 Mn304 C0@4 Fe203 1.587 1.527 1.544 1.574 1.492 1.454 1.333 1.333 1.500 a, = 5.805 4; ct =9.49 A a,=8.153 4 8.434 4a, = a, = a, = a, = a, = a, = a, = 8.448 4 8.365 4 8.376 4 5.762 4; c, =9.47 A 8.069 4 8.343 A 3.17 3.05 3.18 3.22 2.86 2.76 2.66 2.66 3.00 0.48 0.38 0.49 0.52 0.21 0.11 't: tetragonal phase; c: cubic phase.Z: mean oxidation state of metallic ions. J. Mater. Chern., 1996, 6(l), 37-39 Ill I I II I I I I I I 20 30 40 50 60 28/degre e s Fig. 1 XRD pattern of Mn2,15C00,3704 spinel oxide L 3.5 -3-A-?--2-A--$ 2.5 Y 2:Lu-2-1.5 -i 0 0.2 0.4 0.6 0.8 1 faradaic yield/F mol-' Fig. 2 Chronopotentiometric curves for the reduction at constant current (100 pA cmW2) of mixed spinel oxides in 1 mol dmP3 LiC104/PC at 20 "C: 0, Mn2~l,Coo,3704;a, Mno,,3Co,,6,04;.YO, Mn1.43Fe1.0804; Mn1.66Fe0.8204; O, c00.4Fe2.3904;+, col.09Fe1.8004; Mn304Y stoichiometric oxides), large cation vacancies and various compositional cation-deficient spinel-related phases.Such characteristics cannot be achieved with stoichiometric phases. For the Mn-Co system, electron micrographs reveal the ultrafineonature of the solid with size doqain an average of cu. 120A for Co-rich samples, and 230A for the Mn-rich materials.'* Chronopotentiometric curves for the reduction-oxidation of mixed spinel oxides at constant current as described in Table 1 are shown in Fig. 2 and 3. In contrast to what occurs for stoichiometric oxides for which an insertion step is located between 2.5 and 1.5 V without any rechargeable process (Fig.2),18 a well defined single reduction step occurs with the Mn-Co oxides in the potential range 3.5-2.OV. A mean faradaic yield of around 0.6 F mol-' of oxide is reached for a 2.0 V cut-off voltage. For a greater discharge depth, the discharge curve was very close to that obtained for the stoichiometric spinel oxides Mn,O, or Co304. Moreover, the charge process is highly efficient and the discrepancy between the charge-discharge voltages is limited when the d.0.d. (depth of discharge) is restricted to 0.5 Li per mole of oxide (Fig. 3). This indicates 38 J. Muter. Chem., 1996, 6(l), 37-39 3.5 3: 2.5; t. A Fig. 3 Chargeedischarge profiles (100 pA cm-2) for the first cycle of spinel oxides: a, Mn2~l,Coo.3704; c, Mnl,43Fe,,0804; b, Mno,93Col~6,04; d, Mn1.66Fe0.8204; e, c00.4Fe2.3904; f, c01.09Fe1.8004 that the spinel structure of these host lattices is maintained for x <0.5/0.6,18as shown in Table 2.The electrochemical perform- ances of the mixed spinel oxides can be ranked according to the sequence: Mn-Co (Z=3.17/3.05)>> Mn-Fe (2= 3.22/3.18)>Co-Fe (Z=2.86/2.76). The very low oxidation state, Z, in Co-Fe, combined with the lower vacancies content (0.11) could explain the poor results observed for these phases. The worst results provided by the Co-Fe materials are consist- ent with the reduction of iron(m) ions in a-Lil-,Fe5O8 as a continuous decrease of voltage us.x up to 0.6 Li per mole of oxide from 3.0 to 1.5 V" or as a small voltage plateau which does not exceed 0.3 Li/M,04 as indicated by Chen et all4 In the case of the Mn-Co samples, the voltage plateau ex-pected near 3.0V corresponding to the reduction of Mn4+ in Lil +,FeMn,0,,14 Lil +,Mn,O,' or Mn3+ reduction in ZnMn,0,20 does not appear. Owing to the possible formation of Co3+(2,,=3.20/3.15), and the faradaic yield (0.6 F mol-') which is higher than the maximum content available for Mn", the simultaneous reduction of Mn" and Mn"' seems to occur in one single process between 3.0 and 2.0 V. On the other hand, the poorer results of Mn-Fe compounds compared to that of Mn-Co are surprising. In spite of their similar chemical and structural characteristics, the discharge curves observed for the Mn-Fe compounds could suggest a distribution of metallic ions which is quite different from that found in the Mn-Co system.Galvanostatic cycling experiments performed in the poten- tial window 4-2 V for the Mn,,,,Co,,,,O4 oxide (Fig. 4) Table 2 Powder XRD data for Li,Mn2.15Coo,,704 hkl x =0.0 x =0.65 01 1 112 020 4.8 17( 2) 3.043 (2) 2.853( 5) 4.8 15( 3) 3.032( 2) 2.850( 8) 013 121 004 2.71 7( 4) 2.455(4) 2.333 (2) 2.716( 3) 2.453 (2) 2.332( 2) 220 015 2.037( 1) 1.772( 2) 2.037( 2) 1.773( 1) 1.5 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 faradaic yield/F rnol-' Fig. 4 Chargeaischarge (I = 100 PA) profiles for the first five cycles of Mn,,,,Co,,,,O, spinel oxide indicate a promising behaviour in comparison with the poor properties observed for the Mn-Fe and Co-Fe systems.Indeed, a specific capacity corresponding to ca. 0.47 mol-I (62 A h kg-l) is recovered for the Mn-Co oxide after 5 cycles. These preliminary results on cation-deficient spinel oxides indicate the interest in obtaining Mn-based systems, which combine a high manganese oxidation state, a large vacancy content and also a particular distribution of reducible cations in specific sites of the spinel structure. In this way, new high- performance cathodic materials for secondary lithium batteries may be prepared. Further structural and electrochemical work is in progress to improve the specific capacity of such materials, their cycle life and to explain the strong effect of cation distribution on their electrochemical properties.We express our gratitude toward DGICYT of Spain for financial support. The Spanish group also acknowledges financial support from CICYT (Project MAT 93-1204). References 1 M. M. Thackeray, W. I. F. David and J. B. Goodenough, Muter. Res. Bull., 1982, 17, 785. 2 M. M. Thackeray, W. I. F. David, P. G. Bruce and J. B. Goodenough, Muter. Res. Bull., 1983, 18,461. 3 M. M. Thackeray, S. D. Baker, K. T. Adendorff and J. B. Goodenough, Solid State Zonics, 1985,17, 175. 4 J. C. Hunter, J. Solid State Chem., 1981,39, 142. 5 A. Mosbach, A. Verbaere and M. Tournoux, Muter. Res. Bull., 1983,18,1375. 6 M. M. Thackeray, P. J. Johnson, L. A. de Piciotto, P.G. Bruce and J. B. Goodenough, Muter. Res. Bull., 1984, 19, 179. 7 J. B. Goodenough, M. M. Thackeray, W. I. F. David and P. G. Bruce, Rev. Chim. Miner., 1984,21,435. 8 D. W. Murphy, M. Greenblatt, S. M. Zahurak, R. J. Cava, J. V. Waszczak, G. Hull and R. S. Hutton, Rev. Chim. Miner., 1982, 19,441. 9 D. W. Murphy, R. J. Cava, S. M. Zahurak and A. Santoro, Solid State lonics, 1983,9-10,413. 10 R. J. Cava, D. W. Murphy and S. Zahurak, J. Solid State Chem., 1984,53,64. 11 L. A. de Picciotto and M. M. Thackeray, Muter. Res. Bull., 1985, 20, 187. 12 D. Guyomard and J. M. Tarascon, Solid State Zonics, 1994,69,222. 13 M. M. Thackeray, A. de Kock, M. H. Rossouw, D. Liles, R. Bittihn and D. Hoge, J. Electrochem. SOC.,1992,139,363. 14 C. J. Chen, M. Greenblatt and J. V. Waszczak, J. Solid State Chem., 1986, 64, 240. 15 J. M. Jimenez Mateos, J. Morales and J. L. Tiyado, J. Solid State Chem., 1989,82,87. 16 J. M. Jimenez Mateos, J. Morales and J. L. Tirado, React. Solids, 1989, 7, 235. 17 J. M. Jimenez Mateos, W. Jones, J. Morales and J. L. Tirado, J. Solid State Chem., 1991,93,443. 18 J. Farcy, J. P. Pereira-Ramos, L. Hernan, J. Morales and J. L. Tirado, Electrochim. Acta, 1994,39, 339. 19 L. A. de Picciotto and M. M. Thackeray, Muter. Res. Bull., 1986, 21, 583. 20 C. J. Chen, M. Greenblatt and J. V. Waszczak, Muter. Res. Bull., 1986,21, 609. Paper 51042871; Received 3rd July, 1995 J. Muter. Chem., 1996, 6(l), 37-39

ISSN:0959-9428    

DOI:10.1039/JM9960600037

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Lithium intercalation and copper extraction in spinel sulfides of general formula Cu,MSn,S, (M =Mn, Fe, Co, Ni) Pedro Lavela," JosC Luis Tirado," Julian Morales," Josette Olivier-Fourcadeb and Jean-Claude Jumas*b "Laboratorio de Quimica Inorgcinica, Facultad de Ciencias, Universidud de Cbrdoba, ES-1404 Cbrdoba, Spain bLaboratoire de Physicochimie des Mutkriaux Solides (URA 407 CNRS), Universitk Montpellier II, Place E. Bataillon, 34095 Montpellier Ckdex 5, France Cu,MSn,S, (M =Mn, Fe, Co, Ni) and Cu, -,MSn,S, (M =Fe, Co) spinel compounds have been studied and used as cathode materials in Li/LiClO,( PC)/spinel sulfide cells. The pristine compounds and electrochemically inserted products have been characterized by X-ray powder diffraction (XRPD) and 'I9Sn, 57Fe Mossbauer spectroscopy (MS).Rietveld analysis and 57Fe MS of the copper extracted phases show that copper extraction takes place from the 8a sites of the spinel structure with oxidisation of the M element. The lithiation process involves a loss of crystallinity and a slight reduction of the host materials. Discharge<harge curves of lithium cells using pristine and deintercalated spinels as cathode materials have been compared. A better performance for the Ni- and Co-containing spinels is observed. After extraction of copper, a significant increase in cell voltage and reversibility is obtained for Co. Since the early report of Eisenberg,' chalcogenide spinels have proven adequate host lattices for the intercalation of lithium ions, which show some advantages over other chalcogenide frameworks.The occurrence of a non-negligible number of empty sites is particularly favourable to guest ions. Although a detailed single-crystal X-ray analysis has shown a very weak tetragonal distortion (not distinguishable from the X-ray powder patterns) for compounds with Cu,MSn,S8 (M =Mn, Fe, Co, Ni) stoichiometry,2 these compounds can be considered as belonging to the Fd3m space group. The sulfur atoms are arranged in a cubic close-packing (ccp) arrangement. The M2+ and Sn4+ ions are randomly distributed in 16d octahedral sites and Cu+ ions occupy the 8a tetrahedral sites defined by the anion array. Thus, the 16c octahedral, 8b and 48f tetrahedral interstices are The 'bottleneck' formed by the S atoms is large enough to allow lithium diffusion through the structure.In addition, the cubic structure allows an isotropic lattice expansion on lithium insertion, and a three-dimensional (3D) electronic and ionic conductivity. Another advantage is that the rigidity of the spinel framework prevents the insertion of water and/or solvent molecules from the electrolyte. The diffusion path of lithium ions can be improved by removing certain ions from the spinel structure. Copper extrac- tion by treatment with mild oxidising agents was first reported in CUT~,S,.~*~ Further work has shown that copper can also ionic distribution can be written (Cus)ga [M,Sn,,],,,S32, have been studied. The complementary use of X-ray powder diffrac- tion and Mossbauer spectroscopy allowed us to characterize the pristine sulfides and to follow the evolution of the electro- chemically inserted products in order to establish the mechan- ism of lithiation.In addition, copper extraction experiments have been performed to study the modified spinels and their lithiation products. Experimental Cu,MSn,Sg thiospinels (M =Mn, Fe, Co, Ni) were synthesized as follows. Stoichiometric amounts of the constituent elements were mixed and sealed in evacuated quartz tubes. The mixture was heated at 300 "C for 24 h, after which the temperature was increased to achieve a final constant value of 750 "C for 8 days. The occurrence of impurities for the phase containing manga- nese led us to change the final temperature to 670°C to improve the purity of the spinel phase.The products obtained were reground and kept under an Ar atmosphere in a dry box. In order to extract Cu+ ions from the structure, the thiospinels were suspended in an excess of 0.1 mol dm-3 1,-CH3CN solution at 50 "C with magnetic stirring for different periods of time (one, two and three weeks). The electrochemical lithium insertion was carried out in Oxidative extraction of copper was carried out in CuZr2S4 by using strong oxidising agents." Also, treatment of CuZr,S4 with a large excess of concentrated butyllithium resulted in the expulsion of almost all the Cu ions as metallic copper, and subsequent treatment with iodine in acetonitrile allowed the preparation of the defect thiospinel The electro- chemical behaviour of the resulting solids in lithium cells was found to improve significantly.",'2 This process has also been carried out with success in CuMS, (M=Cr and Ti) where Cu' ions have been removed by oxidation with an I,-CH,CN s01ution.l~ This method makes evident the presence of the cation M"+ whose oxidation state can be increased in order to equilibrate the net charge into the stoichiometric compound.In all cases, the electrochemical results show a considerable improvement. In the present work, a group of thiospinels of general stoichiometry Cu2MSn,Sg (M =Mn, Fe, Co, Ni), in which the be easily extracted from Chevrel phases such as CU,MO,S~.~-~~ two-electrode cells. The cathode pellets were prepared by pressing at 200 MPa a mixture containing 90% sample and 10% PTFE to improve the mechanical properties of the electrode.The anode consisted of a metallic lithium disk. The electrolyte solution ( 1 mol dmP3 LiClO,/PC; PC =poly-propylenecarbonate) was supported by porous glass-paper disks. The purity and structure of the pristine and modified phases and their corresponding lithiated products were checked by X-ray powder diffractometry (XRPD) with a Siemens D500 diffractometer furnished with Cu-Ka radiation and a graphite monochromator. Rietveld refinements were carried out with the aid of the computer program DBWS-9006 developed by Saktivel and Young.', Mossbauer spectra of the polycrystalline samples were meas- ured with an ELSCINT AME 40 constant acceleration spec- trometer. The y-ray sources were 119mSn in a BaSnO, matrix and 57C0 in a Rh matrix, used at room temperature.The velocity scale was calibrated by using a 57C0 source and a J. Muter. Chem., 1996, 6( l), 41-47 metallic iron foil as absorber. The spectra were fitted with Lorentzian profiles by a least-squares method. The goodness of fit was controlled by chi-squared and misfit tests. All isomer shifts reported here are given with respect to the centre of a BaSnO, and an a-Fe spectrum, respectively, for each source. Results and Discussion Cu2MSn3S8 and LixCU2MSn3s8 (M =Mn, Fe, Co, Ni) The indexation of XRD patterns of the pristine compounds in the Fd3m space group of a cubic spinel shows the purity of the phases obtained by direct synthesis.The "'Sn Mossbauer spectra are characteristic of Sn4+ in a slightly distorted octa- hedral environment. Thus, XRPD and "'Sn Mossbauer data are in good agreement with previous results." Intermediate lithiated phases with nominal stoichiometry Li,Cu,MSn,S, were obtained by discharging a lithium cell at a constant current of 100 pA cm-, for different periods of time. I 1 Cu2MnSn3S8 1 4 F mol-'b-*r 3 1 F mol-' AA . 2 F mol-' 3 F mol-' 4 F mol-' 7 After cutting off the current, the cell voltage was allowed to subside for several days in order to achieve a constant potential and, consequently, a high homogeneity of the lithiated samples. The XRPD patterns of the lithiated spinels [Fig.l(u)-(d)] show a gradual decrease of intensity of the diffraction lines as a consequence of loss of long-range ordering. This fact is less marked for the spinel containing Mn [Fig. 1 (a)].A high degree of crystallinity remains in this sulfide by comparison with the other highly lithiated compounds. In addition, a weaker modi- fication of the a parameter (Table 1) has also been observed in the Mn sulfides with respect to the amount of inserted lithium. For the other compounds, the lithiation process gives way to an increase of the initial u parameter (Table 1).For Co and Ni compounds [Fig. l(c) and (41,these changes are more marked and, additionally, three new effects are clearly observed. First, a new group of wide and weak signals (*) appears when the lithium concentration increases.The low resolution does not allow a reliable indexation of the diffraction planes, but their positions to the left of the main lines seem to indicate 4 F mol-' 28/degrees Fig. 1 XRD patterns of pristine and electrochemically lithiated samples at different depths of discharge: (a) Cu,MnSn,S,, (b) Cu2FeSn3S8, (c) Cu,CoSn,S, and (d) Cu,NiSn,S, Table 1 Cubic cell parameters of the pristine compounds Cu,MSn,S, and after electrochemical lithiation M Mn Fe co Ni pristine compound 10.416( 7) 10.312(5) 10.272(5) 10.276(5) electrochemical lithiation: 2 F mol-' 10.405( 9) 10.322( 7) 10.295(6) 10.284( 6) 3 F mol-' 10.324(7) 10.274(5) 4 F mol-' 10.41 (1) 10.323(5) amorphous 10.293( 6) 6 F mol-' 10.41 (1) amorphous amorphous amorphous 42 J.Muter. Chem., 1996, 6(l), 41-47 that lithium insertion originates in a quasi-amorphous phase with a large spacing which corresponds to highly lithiated domains. Secondly, a very weak signal at 28=43.16" was observed. This peak can be indexed as belonging to Cu metal and is due to de-intercalation of reduced Cu0.l6 Thus, the general stoichiometry can be expressed as Li,Cu2 -,MSn,S8 (O<y<x). The removal of the Cu' ion would give rise to an important distortion of the structure and a reduction of the rigidity of the framework, thus explaining the origin of the low crystallinity phases. Finally, two additional small peaks (@) often occur at 28 = 18.28" and 20.20" for lithiated samples.These lines can be ascribed to the occurrence of a tetragonal superstructure as a consequence of the ordering of the remain- ing Cu atoms in the 8a sites. .-1.24 91 .-1.20 1.16 1.35 1.33 1.30 2.48 2.46 2.44 2.42 2.30 2.28 2.25 -6-4-2 0 2 4 6 '19Sn Mossbauer spectroscopy was demonstrated to be an important tool for studying the behaviour of tin atoms during the redox reaction,17 even where the XRPD patterns show an important loss of long-range ordering at high concentrations of lithium in the structure. The gradual appearance of a new signal at ca. 3.00 mm s-l, corresponding to Sn2+ ions, signals the reduction of tin atoms during the process [Fig. 2(a)-(d)]. However, the evolution was not homogeneous for every com- pound (Table2).Thus, the Sn atoms in Co and Ni sulfides [Fig. 2(c) and (d)] show a higher tendency to be reduced by lithium insertion. The behaviour of the isomer shift, 6, us. depth of discharge is displayed in Fig. 3, where small changes and a non-linear evolution are observed for all compounds. Nevertheless, a 1.24 1.20 1.16 1.12 I .05 I .01 1.04 1.02 I.65 I .64 I.62 2.79 2.75 VJ2.70 E 35 W .z 1.50 $ 1.76 2 5 I .72 I .68 1.50 1.48 I.46 1 .A4 2.34 2.31 2.28 2.25 2.22 1.72 1.70 3' (4 1.68V 4-4-2 0 2 4 t velocity/mm s-1 Fig. 2 l19Sn Mossbauer spectra of pristine and electrochemically lithiated samples at different depths of discharge: (a) Cu,MnSn,S,, (b)Cu,FeSn,S,, (c)Cu2CoSn,S, and (d) Cu,NiSn,S, J.Mater. Chem., 1996, 6(l), 41-47 Table 2 Il9Sn Mossbauer parametersa of the pristine compounds Cu2MSn,S8, and after electrochemical lithiation M Mn Fe co Ni pristine 6 1.102(2) 1.246( 1) 1.202( 1) 1.136( 1) compound d 0.31( 1) 0.321(6) 0.318(6) 0.312(8) r 0.88(1) 0.863(6) 0.863(6) 0.863(8) electrochemical lithiation: 2F mol-' 6 1.104(1) 1.243(4 1.220( 3) 1.163(4) d 0.26(2) 0.29(3) 0.35( 1) 0.28(2) r 0.8qi) 0.92(2) 0.95( 1) 0.85( 2) c 100 100 94( 1) 96( 1) 3.1(1) 0.24(4) A LO( 1) 0 r 1.9(4) 0.8(2) 4F mol-' 6 1.121(3) 1.245( 2 1.205( 4) 1.179( 2) A 0.29(2) 0.31(2) 0.36( 2) 0.303(8 r 0.86(2) 0.79(2) 0.90(1) 0.863(4 c 100 81(1) 86( 1) 84(1)6 2.9( 1) 3.16(5) 2.81 (2) A 1.1(1) 0.6(2) 0.2(2) r 1.6(3) 1.3(2) 1.2( 1) 6 F mol-' 6 1.113(5) 1.224(2 1.214(4) 1.162(4 A 0.32(1) 0.30( 1) 0.36( 1) 0.35 (2) r oq2) 0.85(2) 0.85 (2) 0.84(2) c 81(1) 87(1) 77(1) 76(1)6 2.57(9) 2.97(5) 3.OO (2) 2.76(2) A 0.8(1) 0.87(7) 0.70(3) 0.46(8) r i.6(2) 1.2( 1) 1.01(7) 1.1(1) a 6=isomer shift (mm s-l) relative to BaSnO,; A =quadrupole splitting (mms-I); r=full width at half maximum (mms-I); C= contribution (YO).o CupMnSnsS8 Cu,FeSn3S8 v Cu,CoSn,S, 'I Cu2NiSnSSe1.28 1.24 1.16 'T 1.12 u) E 0 Cu,MnSn,S, 3.6 CulFeSn3S, v Cu,CoSn,S, 3.4 3.2 ' Pi3.0 2.8 ' 2.6 -0 I 1 I I 1 I Fig. 3 Variation of isomer shift (6) versus concentration (x) for Sn4+ and Sn2+ signals 44 J.Mater. Chem., 1996, 6(l), 41-47 small tendency to increase the 6(Sn4+) and decrease the 6(Sn2+) values seems apparent. This fact can be interpreted by the increase of the covalent character of the bonding. The sharing of electrons increases the s electron population around the Sn4+ and gives rise to the opposite effect for Sn2+ ions. cU2-,MSn3S8 and Li,cU, -,MSn3S8 (M =Fe, CO) New phases with a low copper content (CUl.8FeSn& and CU,.~COS~~S~)[Fig. 4(u) and (b)] have been identified by XRPD from the reaction of the pristine spinels with Fe or Co in their compositions with a 0.1 mol dm-3 12-CH3CN solution. A diminution of the cell parameter and the intensity of re- flection lines such as (220) and (422), which was proportional to the Cu concentration, was observed.These peaks originate from the tetrahedral Cu sites in a perfect spinel. A detailed Rietveld analysis (Fig. 5) of the diffraction data shows that copper extraction effectively takes place from the 8a sites without significant redistribution of the metal ions in octa- hedral coordination (Table 3). Moreover, the site occupancy values obtained by profile fitting agree well with the values obtained by energy dispersive X-ray microanalysis (EDXA; Table 3). In addition, the higher width of the above-mentioned peaks demonstrates that the copper extraction process causes a decrease in crystallinity. This preferential broadening may be taken as indicative of incomplete displacement of copper ions from 8a sites.In order to achieve the Cu reduction, an increase of the transition-metal oxidation state is necessary to maintain elec- troneutrality, and this was examined by 57Fe Mossbauer spectroscopy. The spectrum obtained (Fig. 6)shows two signals 1 AIk 1\ A A. S roekiI ..-A &.. A A-LI1 1 10 20 30 40 50 60 28 /degrees Fig. 4 XRPD patterns of pristine samples and those after several periods of treatment with 1,-CH3CN solution: (a) Cu2 -,FeSn,S, (x =0, 0.2) and (b) Cu2-,CoSn3S8 (x =0, 0.9) Table 3 Results of the Rietveld analysis of XRPD data of Cu,FeSn,S, ,Cu,-,FeSn,S, ,Cu,CoSn,S, and Cu,-,CoSn,S," 8a site time of occupancy I, treatment/ Y y in weeks (EDXA) a/W CU, -,MSn,S, ZS s=R,/Rex, Cu,FeSn,S, 10.303(5) 2.00 0.255(3) 2.41 Cu, -,FeSn,S, 2 1.84(5) 10.303(5) 1.91 0.254(3) 2.38 Cu,CoSn,S, 10.266(5) 2.00 0.257(3) 2.20 Cu, -,CoSn,SB 1 1.11(3) 10.150(5) 1.43 0.249(3) 2.79 Cu, -,CoSn,S, 2 1.16(3) 10.150(3) 0.99 0.257(3) 2.54 "Space group Fdjm; Cu site 8a: 1/8, 1/8, 1/8; Fe/Co site 16d: 1/2, 1/2, 1/2; S site 32e: z, z, z.which can be considered as due to the overlapping of a pair 1.204(1) mm s-l for Cu1.,CoSn3S8] that can be interpreted as of quadrupole split signals. At room temperature the spectrum due to a small increase in the covalent character of the of Cu,FeSn3S8 corresponds to a hyperfine interaction which is Sn-S bonding. a population-weighted average of the low-spin (LS) and high- Electrochemically lithiated Li,Cul.,FeSn3S8 and spin (HS) states.18 The study of the Mossbauer parameters for Li,Cu1~,CoSn3S8 products have been obtained.In both cases, Cu1.8FeSn& allows us to estimate the decrease of the isomer the lower Li' concentration gives rise to the occurrence of shift values (Fig. 6). It can be explained by a slight reduction double lines in XRPD patterns, more conspicuously for the of the screening effect caused by the loss of d electrons. It Co spinel (Fig. 7). After considering the u p?rameters corre-favours an increase of the interaction of 4s electrons with the sponding to these domains (10.16 and 10.26 A), these phases atomic nucleus. In addition, a slight decrease of the quadrupo- can be ascribed to the heterogeneity of the lithiation reaction. lar splitting values has been ascribed to the presence of a d5 At a discharge depth of 6 F mol-l, complete loss of crystallinity electronic configuration which homogenizes the electronic dis- was observed.tribution around the Fe atoms. The decrease of 6 and d values Once again, "'Sn Mossbauer spectroscopy shows a very expresses a partial oxidisation of Fe" to Fe"'. The treatment slight modification of the parameters (Table 4), except for of experimental data by the superposition of two doublets is L~,CU~~~COS~,S,(6 F mol-l) where a non-negligible decrease a simplifying calculation. In fact, the spectrum for CUl.8FeSn3S8 of the isomer shift and the presence of SnO, were observed, is probably a population-weighted average of Fell, FeI'I, LS probably caused by the highly amorphous nature of the and HS states.A study of spectra recorded at different tempera- compound. Thus, a similar behaviour of the Sn atoms in these tures would be necessary to distinguish these different states. compounds to that of Sn in Li,Cu2MSn3S8 can be expected. In the same way, these conclusions can be applied to Co atoms. Discharge<harge curves of lithium cells using pristine and ll'Sn Mossbauer spectra show a very slight increase of the Cu deintercalated spinels as cathode materials are shown in isomer shift [S= 1.272(2)mm s-l for Cul.8FeSn& and 6 = Fig. 8. For the pristine samples, the following reaction is 8 60 I I II L. I r~ r I Ir ....' ... ..-L..'... 10.00 2o:oo 30:00r60:00 7 w 2 8 /degrees Fig. 5 Selected results of the Rietveld refinement of XRPD data: (a)Cu,FeSn,S,, (b)Cu,-,FeSn,S,, (c) Cu,CoSn,S8 and (d)Cu2-,CoSn,S8 J.Muter. Chem., 1996, 6(l),41-47 45 Table 4 l19Sn Mossbauer parameters' of pristine and electrochemically lithiated Cu,.,FeSn,S, and Cu,.,CoSn3S, pristine compound 6 1.275( 1) 1.204( 2) A 0.32 1 (9) 0.34( 1) r 0.924(8) 0.95( 1) 2 F mol-' 6 1.25 1 (3) 1.192( 3) A 0.27(2) 0.33(2) r 0.94(2) 0.94(2) C 100 100 4 F mol-' 6 1.244(3) 1.191(3 A 0.31(1) 0.32( 3) r 0.830( 6) 0.97(2) C 92( 1) 95( 1) 6 3.09(7) 3.28 (9) A 0 0.6(3) r 1.5(6) 1.1(4) 6 F mol-' 1.250(6) 1.178( 3 0.34( 1) 0.32(2) 0.80(2) 0.97(2) 80( 1) 85(1)2.7( 1) 3.04(4) LO( 1) 0.72( 7) 1.5(2) 1.0(1) ~ '6=isomer shift (mm s-l) relative to BaSnO,; A=quadrupole splitting (mm s-'); T=full width at half maximum (mm s-'); C= contribution (%).0.o 2.44 0.2 2.43 0.4 0.6 2.42 -s v E0.8 Y YL 1.290g0.0 .-0 n v)m 9 .,. 1.zee.g a21 Fe 1I CU,,~F~S~~S~ 1.286 (1 week) I -1.284 -1.282 -1.280 V I ~~~ ~ -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 velocity/mm s-1 Fig. 6 57Fe Mossbauer spectra of (a) Cu,FeSn,S, [low spin: 6 = 0.810(4), A=O.96(2), r=0.38( l), C=69.5%; high spin: 6=0.850(6), A= 1.35(2), r=0.28(3), C=30.5%] and (b) Cu,,,FeSn,S, [low spin: 6=0.742(8), d=O.604(2), r=0.43(3), C=64.1%; high spin: 6= 0.82(1), A= i.i7(2), r=o.35(3), c=35.9%1 J. Mater. Chern., 1996, 6(l), 41-47 4 P mo1-' I 10 20 30 40 50 60 2O/degrees Fig.7 XRPD patterns of pristine and electrochemically lithiated samples at different depths of discharge: (a) Cu, -,FeSn,S8 (x =0.2) and (h)Cu,-,CoSn,S, (x=0.9) possible during the first steps of cell discharge: xLi +Cu2MSn3S8+Li,Cu2 -.MSn3S8 +xCu (x d 1, E/V us. Cu'/Cu) The extent of this reaction for each transition metal, M, under the dynamic conditions of the discharge experiments affects the reversibility of the process for discharge depths below 1F mol-'. This is higher for Co and Ni compounds than for Fe and Mn. Copper is not reabsorbed into the spinel on cell charging as evidenced by the presence of Cuo reflections in the XRPD patterns of the recharge product. For larger discharge depths: yLi +Li,Cu2 -,MSn3S8 +Li,+,Cu,-,MSn,S8 (EIVus.Sn4+/Sn2+) This latter reduction plus lithium-ion insertion process takes place with a progressive amorphization of the reaction product. The tin reduction process was evidenced by the Mossbauer data of samples obtained at discharge depths of ca. 2 F mol-' and is accompanied by the amorphization of the solid as evidenced by the XRPD patterns of the samples. For those samples submitted to chemical copper extraction, according to: CU2MSn3S8+(X/2)12+cU2 -,MSn3S8 +XCUI the first part of the discharge curve of the lithium cells is: xLi +Cu2_,MSn3S8 +Li,Cu2-,MSn3S8 (Epus. M3+/M2+) This reaction implies that larger cell voltages are expected under open circuit voltage (OCV) conditions, as a result of the 5.0 -(a) Cu,MnSn,S, Cu,Fe Sn,S, 4.0 -3.0 -\ 1.o 1.0 tkt 5.0 -CuzFeSn,S, 5.0 1 Cu,,,FeSn,S, 4.0 r 1 .o 5.0 -Cu,CoSn,S, 1.0 11 1.0 \I{ Cu,NiSn,S, 5.0 1 CU,,~C~S~,S~ 2 .o 1.o 1.0' I I I I I 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 depth of discharge/F mol Fig.8 Galvanostatic discharge-charge experiments of lithium cells using (a)pristine and (b)Cu-deintercalated compounds as cathodic materials reduction pair involved (E/V us. M3+/M2+),as compared with pristine samples. This is shown by a difference of 2.2 us. 1.8 V in the plateau observed under continuous galvanostatic dis- charge. Thus the strategy presented here allows control of the cell voltage. Moreover, the better diffusivity of Li+ ions into the copper-extracted products decreases the polarization of the cells during discharge.Further lithiation will result in a similar reaction to the pristine solid and with similar progressive amorphization: yLi +Li,Cu, -.MSn,S8 4Li, +,Cu2 -,MSn3S8 Thus, this behaviour makes copper extraction an interesting procedure to increase the performance of lithium cells using cathodic materials with a spinel structure, and to control the cell voltage. The authors acknowledge the financial support of EC (Contract JOU2-CT93-0326) and the Ministries of Education (Spain) and Foreign Office (France) (Picasso Program). References 1 M. Eisenberg, J. Electrochem. SOC., 1980,127,2382. 2 J. C. Jumas, E. Philippot and M. Maurin, Acta Crystullogr., Sect. B, 1980,36, 1993. 3 P.de la Mora and J. B. Goodenough, J. Solid State Chem., 1987, 70, 121. 4 A. C. W. James, J. B. Goodenough and N. J. Clayden, J.Solid State Chem., 1988,77,356. 5 J. Morales, J. L. Tirado, M. L. Elidrissi Moubtassim, J. Olivier-Fourcade and J. C. Jumas, J. Alloys Comp., 1995,217,176. 6 R. Schollhorn and A. Payer, Angew. Chem., Int. Ed. Engl., 1985, 24, 67. 7 S. Sinha and D. W. Murphy, Solid State lonics, 1986,20, 81. 8 W. R. McKinnon and J. R. Dahn, Solid State Commun., 1984,52,254. 9 E. Gocke, R. Schollhorn, G. Aselmann and E. Muller-Warmuth, Inorg. Chem., 1987,26,1805. 10 R. Kanno, Y. Takeda, Y. Oda, H. Ikeda and 0.Yamamoto, Solid State Ionics, 1986, 18/19, 1068. 11 A. C. W. P. James, B. Ellis and J. B. Goodenough, Solid State Ionics, 1988,27,45; 1989,24, 143. 12 N. Imanishi, K. Tnoue, Y. Takeda and 0. Yamamoto, J. Power Sources, 1993,43-44,619. 13 A. C. W. James, J. B. Goodenough, N. J. Clayden and P. M. Banks, Mater. Res. Bull., 1989,24, 143. 14 A. Saktivel and R. A. Young, Program DBWS-9006, Georgia Institute of Technology, 1993. 15 J. Padiou, J. C. Jumas and M. Ribes, Rev. Chim. Mindr., 1981,18,33. 16 T. Jacobsen, B. Zachau-Christiansen, K. West and S. Atlung, Electrochim. Acta, 1989,34, 1473. 17 M. L. Elidrissi Moubtassim, J. Olivier-Fourcade, J. Senegas and J. C. Jumas, Muter. Res. Bull., 1993, 28, 1083. 18 M. Womes, J. C. Jumas, J. Olivier-Fourcade, F. Aubertin and U. Gonser, Chem. Phys. Lett., 1993,201, 555. Paper 5/03947I; Received 19th June, 1995 J. Mater. Chew., 1996, 6(l),41-47 47

ISSN:0959-9428    

DOI:10.1039/JM9960600041

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Sol-gel-derived vanadium and titanium oxides as cathode materials in high-temperature lithium polymer-electrolyte cells Andrew Davies? Richard J. Hobson,*a Michael J. Hudson,*a William J. Macklinb and Robin J. Neatb aDepartment of Chemistry, University of Reading, Whiteknights, Reading, UK RG6 2AD bApplied Electrochemistry Department, AEA Industrial Technology, Harwell Laboratory, Oxfordshire, UK OX11 ORA Binary and ternary vanadium- and titanium-containing xerogels have been prepared by hydrolysis of the metal isopropoxides and subsequent condensation. Powder X-ray diffraction (XRD) has been used to show that pure gel-derived titanium(1v) oxide possesses a structure resembling poorly crystalline anatase, whereas for all the vanadium-containing materials prepared in isopropyl alcohol solution the data are consistent with some two-dimensional (2D) order and a similar short-range arrangement of V05 moieties to that in crystalline V205.In contrast, th: vanadium oxide xerogel obtained from an aqueous milieu shows evidence only of one-dimensional order, interlayer distance 14.2 A. Thermal analysis and XRD have been used to show that all the vanadium-containing gels lose water in three stages and that no structural change occurs until the third stage of water loss, which occurs simultaneously with crystallisation. The oxides have been employed as the active component of the cathode in lithium polymer-electrolyte cells operating at 120 "C and their cycling performance has been investigated. The binary oxides showed no improvement in performance over similar crystalline materials whereas the ternary materials, whether chemical or physical mixtures, showed good reversibility and gave observed energy densities which compared favourably with that of V6013 in a similar cell.This improvement in performance has been attributed to the preferential reduction of the Ti" over VIV near the low-voltage limit which prevents a reorganisation of the microstructure of the material. The discovery by Armand et a!.' that poly(ethy1ene oxide) (PEO) and certain lithium salts (e.g. LiC104, LiCF3S03) reacted to give polymeric solid electrolytes has led to the development of high-energy-density lithium polymer-electro- lyte batterie~~-~ in which the cathode-active material was a lithium insertion or intercalation compound.Previously we have shown5 that vanadium phosphate glasses may be employed as reversible cathode-active materials in secondary lithium polymer-electrolyte cells. The glasses gave higher open- circuit voltages than V6013and exhibited comparable average cell voltages on cycling (2.1-2.4 V) but their specific capacities were lower than that of V6OI3 (ca. 330mA hg-l) at the capacity plateau. This resulted in lower observed energy densi- ties (500-600 W h kg-' for glasses containing 278 mol% V205)after 25 cycles than those observed for V6013(ca.770 W h kg-I). The poorer performance of the glasses can be attri- buted to the presence of the electrochemically inert phosphate component. This suggested that an amorphous V205-based material in which all the components were electrochemically active might give a utilisation comparable to that of V6013.This has previously been achieved in liquid-electrolyte cells with a cathode containing amorphous V205 prepared by the fast-roller quenching method.6 There has, in recent years, been a great deal of interest in the preparation of amorphous materials, particularly oxides, using the sol-gel pro~ess.~The technique is based on the preparation of a colloidal suspension, a sol, and its transition into a gel from which the amorphous material can be obtained. Sols are commonly prepared by the hydrolysis of alkoxides, the rate of which is controlled either by the amount of solvent present, usually an alcohol, or by the addition of a catalyst such as acetic acid, whereas the extent of hydrolysis depends upon the controlled addition of water.Hydrolysed species link t Present address: Cookson Technology Centre, Sandy Lane, Yarnton, Oxfordshire, UK OX5 1PF. together via a condensation reaction, (l), ( RO), -M -OR' + HO-M (OR), --S (RO),-lM-O-M(OR),-,+R'OH; R'=H or R (1) and as the hydrolysis-condensation or polycondensation pro- ceeds larger molecules are formed which ultimately make up the colloidal particles of the sol. Gelation is said to occur7 when a solid skeleton is formed enclosing a liquid phase. Subsequent evaporation of the solvent at room temperature gives the xerogel, the first completely solid material obtained without any heat treatment. The xerogel frequently contains water or solvent and needs heating in order to obtain a pure amorphous oxide, during which the occupied pores may collapse leading to considerable shrinkage of the material.Vanadium oxide gel-derived materials have been studied by Livage and co-workers,8-12 who prepared their gels by poly- condensation of decavanadic acid, obtained from sodium meta- vanadate using cation exchange. The materials were found to possess a layered structure with 1D order, the interlayer separation depending upon the degree of The local ordering within the layers was to that of orthorhombic V205. The subject area has been thoroughly reviewed.13 Hirashima et a1.14-16 used alkoxide precursors to prepare vanadium and mixed vanadium-titanium oxide gels containing up to 20 mol% Ti02.The crystallisation tempera- ture increased and dc conductivity of the xerogels decreased with increasing TiO, ~ontent.'~?~~ On the basis of XRD evi- dence it was suggestedl6 that these materials also possessed layered structures but with some regularity within the layers, with the titanium component possibly acting as a network former within the structure.15 Some studies of sol-gel-derived materials as cathodes in rechargeable lithium polymer-electrolyte cells have been made. For example, V205 xerogel, prepared by polyconden-sation of decavanadic acid, has been shown to give promising re~ersibility'~when a lower limit of 2.2 V was employed during cycling, which corresponds to an Li/V molar ratio of 0.55.J. Muter. Chern., 1996, 6(l), 49-56 However, this depth of discharge led to low capacities and hence only modest energy densities; to achieve a similar performance to V6013 it is necessary for cathodes containing amorphous vanadium oxide to show reversibility following discharges down to 1.7 V. Minnett and Owen'' reported such a study of V,05-TiO, cathode-active materials derived from alkoxide precursors. The materials tested, most of which were annealed at 400°C and consequently were of low crystallinity, gave disappointing capacities in lithium polymer-electrolyte cells. This included V6013, which gave 225 mA h g-' on first discharge compared with 417 mA h g-' for Li8V6013, which suggests that the low capacities may have been a function of cell design or discharge current density" rather than deficiencies in the materials.The cycling data" suggested that better reversibility was obtained with the mixed materials than for V205 alone, an interesting effect which was noted but not considered in any detail. Clearly, additional studies are required in order fully to evaluate the potential of amorphous alkoxide-derived V205 materials as alternatives to V6013,employing secondary lith- ium polymer-electrolyte cells of a design and with a charge- discharge regime which allows high utilisations to be obtained (e.g. 80% theoretical capacity5 for V6013). The effect of Ti02 on the reversibility of lithium insertion into vanadium-based materials is also of particular interest and warrants further investigation.It is unclear whether the titanium acts as a structure modifier or whether its effect is electronic in origin because, unlike phosphorus5 for example, Ti" may be reduced and thus participate in the lithium-insertion process. This paper describes the preparation and characterisation of a series of sol-gel-derived V205-Ti02 materials, including the binary oxides, and their evaluation as cathode-active materials in secondary lithium polymer-electrolyte cells. The synthetic methods were based on the alkoxide hydrolysis- polycondensation method described by Hirashima et u1.'4-16 which is reported to give single-phase material in which the titanium dioxide modifies the vanadium oxide str~cture.'~ Experimenta1 Syntheses The syntheses described in this section give typically 1-2 g of product which were used for initial investigations.For batches of material used in the cell-testing experiments the amounts were scaled-up about sixfold. Vanadium oxide: 'alcohol route'. The method was based on that described previo~sly.'~,~~ Vanadyl triisopropoxide (Alfa) (10 cm3) was added to isopropyl alcohol (20 cm3) with stirring. To the stirred mixture a solution of water (10.6cm3) in isopropyl alcohol (177 cm3) was added dropwise. The stop- pered flask was allowed to stand and a yellow gelatinous precipitate formed within + h which turned green after 24 h. The green precipitate was filtered off, washed with isopropyl alcohol and air dried.No attempt was made to prepare a monolithic gel because it has been shown previously16 that the isopropoxide does not give a monolithic gel when hydrolysed. Vanadium oxide: 'aqueous route'. Vanadyl triisopropoxide (10 cm3) was added with vigorous stirring to distilled water (100 cm3). The inhomogeneous red gel which formed immedi- ately was dispersed by shaking the flask vigorously for 5 min. The resulting red sol was left to stand for 24 h with the flask stoppered. The dark-red gel was then removed from the flask and air dried in order to obtain the solid xerogel which was green. Titanium(1v) oxide. The sol-gel route described by Yoldasl' was used as the basis for the preparation of amorphous TiO, except that Ti(OPri)4 (Aldrich) was employed instead of Ti(OEt),.Titanium(1v) isopropoxide (8.0 g, 0.028 mol) was added with stirring to isopropyl alcohol (115 cm3, 1.53 mol). Nitric acid (0.1 cm3 conc., 0.0016 mol) was diluted with water (2.5 cm3, 0.14 mol) and added to the stirred mixture, then the flask was stoppered and allowed to stand for 24 h to give a transparent, monolithic gel. Mixed vanadium-titanium oxides. Vanadyl triisopropoxide and titanium(1v) isopropoxide were mixed together in the appropriate amounts (Table 1) and added to isopropyl alcohol (20 cm3). Water (5 cm3) as a solution in isopropyl alcohol (85 cm3) was then added. The flasks were stoppered and left for 24 h, after which the resulting gel was removed and air dried in order to obtain the xerogel.Details of the various gel compositions that were prepared are given in Table 1. Techniques Reduced vanadium (V'") and total vanadium contents in the xerogels were determined by potassium permanganate titration and atomic absorption spectroscopy (AAS), respectively. The titanium content of the pure TiO, xerogel was also determined using AAS but this technique was unsatisfactory for analysis of Ti in the mixed gels owing to interference from vanadium. The vanadium-titanium molar ratios of the mixed oxide samples were established using quantitative energy-dispersive X-ray analysis (EDXA) using a JEOL JXA 840 scanning electron microscope. Simultaneous thermogravimetric and differential thermal analyses (TG and DTA) were carried out at a heating rate of 10°C min-l in atmospheres of static air and flowing nitrogen using a Stanton Redcroft STA 1000 instrument equipped with data manipulation software.Powder XRD patterns were recorded using graphite-monochromated Cu-Kcr radiation on a Philips PW1710 diffractometer controlled by a Citrons Cray 112 system running Sietronics 112 software. Data were col- lected for the 20 range 4-64" at 2" min-' in steps of 0.04". IR spectra were measured between 1400 and 300cm-' as KBr discs using a Perkin-Elmer 1720-X FTIR spectrometer. Densities were obtained by pycnometry using toluene or cyclohexane. Table 1 Compositions of the mixed V,O,-TiO, xerogels used found mass/g molar ratio amount (mol%) amount (mol%) VO(OP+), Ti (OPr' ),H,O/( V +Ti) V,O, TiO, V,O, TiO, V(mass%) v'v/vto' 1 5.00 0.15 13.2 95 5 94 6 46.3 0.18 2 5.00 0.32 12.8 90 10 89 11 45.8 0.18 3 5.00 0.73 12.0 82 18 80 20 44.2 0.19 4 5.20 1.01 11.1 75 25 76 24 42.7 0.17 5 4.00 1.oo 13.9 70 30 73 27 42.5 0.20 50 J.Muter. Chem., 1996, 6(l), 49-56 Cell fabrication and cycling The xerogels were ground and sieved to give a maximum particle size of 53 pm and then heated at 200°C for 2 h in order to remove all loosely bound water. The efficacy of the heat treatment was tested using TG; a material was accepted for use in a cathode when it showed an overall mass loss of < 1% below 200°C. For the purposes of calculating the amount of material to be included in the composite cathode the oxides were considered to have the stoichiometries V205 and Ti02.Solid-state cells were constructed and cycled as described previously' using PEO-LiC10, (CEO units]/[Li] = 12) as the electrolyte. Cycling was carried out galvanostatically between 3.5 and 1.7V for all cells except those containing only Ti02 for which limits of 2.5 and 1.2 V were used. To allow direct comparisons to be made with crystalline materials studied previously the theoretical capacities of the cells containing V205 were calculated using the theoretical capacity of V6013, 417 mA h g-l, whereas for those cells containing only TiOz the value for LiTi02, 335 mA h g-', was adopted.Results and Discussion Appearance and characterisation of xerogels and gels Vanadium oxide. A monolithic gel could not be obtained from VO (OPr'), using water-propan-2-01 mixtures, confirming the results of previous work.16 An aqueous medium gave a red monolithic gel after 24 h, showing that a large excess of water is required for gelation," and evaporation of the solvent from this gel gave a green xerogel. A greater proportion of vanadium was reduced (Table 2), during the preparation in alcohol solution, providing evidence for reduction by the solvent, I t 0 100 200 300 400 500 600 100 95 n be-903 6 85 80 Table 2 Compositions of the V,O, xerogels source V,,, (mass%) V'"/v,,, alcohol route 46.3 0.19 aqueous route 47.2 0.11 although the xerogel prepared by the aqueous route shows that reduction may also be effected either by the alcohol liberated during gel formation or via the mechanism suggested by Hirashima et ~1.~'~which relates the reduction to the process of gel formation.The simultaneous TG and DTA curves of the alkoxide- derived V205 xerogels (Fig. 1) were all similar. When the nitrogen purge was replaced by static air the only difference was a slight gain in mass accompanied by a very weak exotherm at about 430°C due to the oxidation of VIV to Vv. The TG curves show three stages of mass loss, (Table 3 and Fig. l), the boundaries and mid-points of which were delineated using the first derivatives. Stage 1, ca. 2O-15O0C, is accompanied by a large endotherm and can be attributed to loss of loosely bound, probably hydrogen-bonded water.Stage 2, ca. 150-270°C, involves too gradual a mass loss to be detected by DTA. However, data collected at a slower ramp rate, 6°C min-', showed no significant change in the relative mass lost in this region, suggesting that it is not simply the result of the overlap of the low- and high-temperature pro- cesses. It is likely that propan-2-01, formed from residual isopropoxide groups from the alkoxide precursor, is lost at this stage. Stage 3 is characterised by a rapid loss of mass which occurs almost concurrently with crystallisation (T,,= 315-325 "C) to give orthorhombic VzOs, see below. The endo- therm associated with the mass loss 'interrupts' the exotherm 100 95 nE!M 1fi 85 80 0 100 200 300 400 500 100 95 0 100 200 300 400 500 600 0 100 200 300 400 500 600 TPC Fig.1 TG (i)and DTA (ii) curves of the V,O, xerogels. (a)Aqueous route; (b) alcohol route; both recorded at a ramp rate of 10°C min-' in an atmosphere of flowing nitrogen; (c) the same as (a)except that it was recorded in static air; (d) is identical to (b)except that a ramp rate of 6 "C min-' was used. J. Muter. Chern., 1996,6( l), 49-56 51 Table 3 Thermal analysis of the Vz05 xerogels ~~~ ~~ ~~ alcohol route T range/ mass "C loss (Yo.)" assignmentb ~~~ ~ ~~~~ ~ stage 1 20- 160 16.2 2.1 HzO stage 2 160-280 2.4 0.3 H20 stage 3 280-325 2.6 0.3 H20 total 20-325 21.2 2.7 H,O Percentages given are averages of two runs and have an absolute estimated aqueous route T range/ "C mass loss (%)o assignmentb 20-145 14.3 1.8 H20 145-265 2.5 0.3 H20 265-330 3.4 0.4H20 20-330 20.2 2.6 H20 error of &0.3%.To one decimal place. The figures quoted represent the maximum amount of bound water assuming that none of the observed mass loss is due to organic material. assigned to the crystallisation (Fig. 1). When the ramp rate was decreased, both the loss of water and the concomitant crystallisation occurred at a slightly lower temperature (cu. 10°C), a change which can be ascribed to a kinetic effect: that both events were equally affected confirms that they are linked. Therefore, the water lost at this stage is either very strongly bound and associated with the structure of the material (for example, water derived from hydroxy groups), or trapped within the amorphous oxide.The amounts of each type of water associated with the two xerogels are similar with the largest relative difference being observed for the third stage. The endotherms associated with this loss of tightly bound water are also different, with the material prepared by the aqueous route showing evidence that the strongly bound water exists in two distinct sites within the structure (at least one of which is thought to be between the layers), whereas for the xerogel prepared in alcohol solution only one type of strongly bound water exists. Three stages of mass loss have also been described for the xerogel prepared by the ion-exchange meth~d.~ However, in that case the loss at stage 2 was better defined and coincided with a structural change, see below.The XRD pattern of the xerogel prepared by the aqueous route [Fig. 2(a)] indicates a lack of long-range order in tke structure. There is a very intense peak at 28 =6.2", d =14.2 A, 1. .l....l....l....l....l,.r,l. 10 20 30 40 50 60 281degrees Fig.2 XRD patterns of V20, gel (aqueous route): (a) as prepared; (b) after heating in air at 200 "C followed by further very broad peaks of low in!ensity centred around 28= 12.5" and 25.1", d= 7.07 and 3.54 A, respectively, which are probably the second- and fourth-order reflections relative to the first intense peak and indicate a degree of 1D order in the structure of the gel.Previous workers,'-13 who examined vanadium oxide gels and xerogels prepared by the ion-exchange method, suggested that the gels possessed layered structures but lacked long-range order within the layers. They assigned c as the interlayer axis and the peaks in Fig. 2 are labellFd accordingly. The interlayer spacing in this case is 14.2 A. Upon heating to 200°C (near the mid-point of stage 2) the XRD pattern of the aqueous xerogel becomes better resolved [Fig. 2(b)] with all axial peaks up to (006) clearly visible, indicating that the ordering along the c axis becomes better defined. However, there is still no evidence of any long-range order within the layers.Additionally, no shift in peFk positions is observed so that the interlayer spacing, 14.2A, does not change during this heat treatment. Therefore, it appears that the water lost at stage 2 for the alkoxide-derived materials is within, rather than between, the layers. This suggests that there is a fundamental difference between this material and that prepared by ion exchange, which exhibits a decrease in inter- layer spacing as loosely bound water 1,s lost upon heating.' Clearly, an interlayer distance of 14.2 A is too large for the layers to be connected by chains of V...O-V bonds, as in orthorhombic V,05, so the most strongly bound water, which is released when the temperature is raised to around 300°C, probably has a structural role since its loss leads to a concomi- tant crystallisation of the material to give highly crystalline orthorhombic V20,.The XRD pattern of the xerogel prepared in alcoholic solution [Fig. 3(a)] has a poorer signal-to-noise ratio than that of the xerogel prepared by the aqueous route [Fig. 2(a)], suggestive of a more amorphous material. The diffraction pattern of the former [Fig. 3(a)] is similar to that obtained by Hirashima et uE.16 from a material prepared using VO(OEt),, which it was suggested had a similar layer structure to the gel prepared by the ion-exchange method. There are, however, some features in the pattern which were not explained, in particular, the range of broadness and intensity for different (001) peaks, and two peaks which could not be indexed, the relatively intense peak at 20=2$0", d=3.43 A, and a broad peak around 28=61.0", d= 1.52 A.A comparison with the hkO reflections of crystalline V,05 [Fig. 3(c)] suggests that the xerogel may have some 2D order rather than a 1D layer structure, with a similar short-range arrangement of V05 moieties2' to that of crystalline V205.Heating the xerogel at 200°C for 2 h has little effect on the diffraction pattern [Fig. 3(b)], so the loss of the loosely bound water does not affect the structure, whereas the most strongly bound water, which is released at around 300"C, may have a structural role since its loss leads to crystallisation to give orthorhombic V205[Fig. 3(d)]. The differences observed between the xerogels prepared via the aqueous and alcohol routes and their heat-treated deriva- 52 J.Muter. Chem., 1996, 6(l), 49-56 l..I....I....I....l...,1....1. 10 20 30 40 50 60 29ldegrees Fig. 3 XRD patterns of V205gel (alcohol route): (a) as prepared; and after heating in air at (b) 200 "C; (d) 450 "C. (c) Calculated hkO peaks of V20,. (a) and (b) have been smoothed using a simple three- point algorithm. tives are interesting. Chemical and thermochemical analyses show that the only major difference between the two xerogels is their VIV content (Table 2). It is possible, therefore, that this difference is responsible for the difference in structure, since the presence of V" implies either a deficit of oxygen or the presence of some protons, both of which could lead to greater intralayer ordering.The bands in the IR spectra of both vanadium oxide xerogels are at lower frequencies and less well defined than those of crystalline V205 (Table 4). The retention of the V=O stretch- ing band in the gel spectra around lOOOcm-' indicates that the vanadium is present in a similar distorted square-pyramidal environment to that in orthorhombic V205, which suggests that the microstructure of the xerogels may be similar to that of the crystalline material. The shift to lower frequencies of the V=O and V-0-V stretching bands may be connected with a decrease in the average vanadium oxidation state. The spectra are better defined than those of the V205-P205 glasses' of highest V205 content, which have a V20s-like microstruc- ture, suggesting a greater degree of short-range ordering in the gels than in the glasses, which is consistent with the XRD data.Titanium oxide. Ti02 gels have been obtained previo~sly'~ using Ti(OEt), as a precursor. Very similar conditions were employed in this work with Ti(OPr'), as starting material except that an acid/alkoxide ratio of 0.057 rather than 0.2 was found to be required. A translucent xerogel was obtained from this system after evaporation of the solvent at room tempera- ture. The simultaneous TG and DTA curves of the xerogel are shown in Fig. 4. The large initial endotherm is coincident with Table 4 IR spectra of V205xerogels and crystalline V205 sample wavenumber/cm- cryst. V20, 1023 828 599,478 xerogel (alc.) 1005 762 535 xerogel (aq.) 997 759 534 100 95 75 70 65 I 0 100 200 300 400 500 600 TPC Fig.4 TG (i) and DTA (ii) traces of TiO, xerogel a mass loss of ca. 31% up to about 200 "C which is, presumably, mainly due to the loss of loosely bound water and residual organic material. Between 200 and 400°C there is a region of slow mass loss of cu. 5% which is probably due to the loss of chemically bound water. The exotherm, onset at 414 "C,corre-sponds to increased ordering in the material, see below. The total observed mass loss of 36% is equivalent to a formula of Ti02.2.6H20 for the xerogel, with the chemically bound water accounting for 0.34 mol of H,O per TiOz formula unit.XRD patterns of the TiO, gels before and after heat treat- ment are shown in Fig. 5. All show broad peaks in the same positions, which are coincident with those of anatase (Fig. 5), with the exception of an additional low-intensity peak at 28= 30.8'. Even the sample heated above the exotherm around 430°C shows poorly resolved peaks, although the degree of definition does increase with the temperature of heat treatment. Thus all the materials, including the xerogel, have poorly crystalline anatase-type structures. A. IAl A 10 20 30 40 50 60 20/degeea Fig. 5 XRD patterns of TiO, gel: (a) as prepared; and after heating in air for 2 h at (b) 200°C; (c) 450°C. (d) XRD pattern of anatase. J. Muter. Chem., 1996,6(l), 49-56 Mixed vanadium-titanium oxides.Green monolithic gels were obtained for all of the V205-Ti02 mixtures with no significant changes in composition during the process (Table 1). The 30mol% TiOz xerogel appeared to be slightly inhomo- geneous, which is probably an indication that complete gelation did not occur at this composition, particularly since a gel could not be obtained from a mixture containing, on mixing, 40 mol% TiO,. The fraction of reduced vanadium (V'v/V,,,) was relatively constant with, presumably, the reduction being brought about by a similar mechanism to that of the Vz05 gel, see earlier. The simultaneous TG and DTA curves of the V20s-Ti02 gels (Fig. 6) are similar to those of the Vz05 gels (Fig. 1). Single-crystallisation exotherms are observed, at least for the gels containing less than 20mol% TiOz, which is strong evidence for a series of single-phase materials. In addition, as the TiO, content increases so the crystallisation temperature increases and becomes less well defined, and the loss of the strongly bound water at stage 3 becomes spread over a wider temperature range (Table 5). The trend of increasing crystal- lisation temperature with increasing TiOz content was also observed by Hirashima et However, the overall amount ~1.~~7~~ of associated water (19.3-21.1%) is similar to that of the V205 loo. 95* E,.3 a 85.80. 0 100 200 300 400 500 600 100 95 n s '90 85 80 0 100 200 300 400 500 600 TIT Fig. 6 TGA (ij and DTA (ii) traces of mixed V,O,-TiO, gels: (a) 6 mol% TiO,; (b)20 mol% TiO, Table 5 Thermal analysis of mixed V205-Ti02 xerogels stage 3 TiO, (mol%) T rangePC T,*/"c 6 300-350 347 11 300-365 371 20 290-405 409 24 290-425 -27 290-440 -I""I""I""I""I""l' I....l....l....I....I.... I. 10 20 30 40 50 60 WdW-Fig. 7 XRD patterns of a V,O,-TiO, gel: (a) 20 mol% TiO,; (b) the same gel after heating in air at 450°C. (a) has been smoothed using a simple three-point algorithm. *, (101j reflection of anatase Ti02. gels, which is not consistent with the suggestion" that TiO, replaces water in the structure. The XRD patterns of the V205-TiO, xerogels (Fig. 7) are identical to that of the Vz05 xerogel prepared in alcohol (Fig. 3), implying an identity of structure.No peaks belonging to the TiOz gel (Fig. 5) were observed in any of the mixed-gel patterns which provides further evidence that the mixed gels are single-phase materials. The lack of dependence of the structure on TiO, content suggests that the Ti atoms replace V atoms within the 2D layers without inducing interlayer ordering. Xerogels heated at 200°C showed unchanged XRD patterns, like their Vz05 counterparts, whereas XRD patterns of samples heated past their crystallisation temperatures (Fig. 7) showed peaks due to orthorhombic V205 and, for the gels containing >20mol% TiOz, a peak at 28=25.3" corre- sponding to the (101) reflection of anatase TiOz. Thus crystal- lisation gave the expected two products. The anatase component in the other crystallised gels is presumably too small to detect using XRD. Performance in high-temperature lithium polymer-electrolyte cells Vanadium oxides.The discharge profiles of cells containing binary vanadium oxides showed monotonic curves on all cycles, characteristic of a single-phase lithiation of an amorph- ous material. The oxide prepared by the aqueous route gave virtually 100% of the theoretical capacity on the first cycle, whereas that prepared by the alcohol route discharged to only 80% of the theoretically expected value. This is consistent with the greater extent of reduction of Vv to V" during preparation of the latter material (Table 2). However, the absolute capacit- ies achieved by the 20th cycle are very similar (cu. 60% of the theoretical) so that a similar VIV/Vv ratio has been achieved in both materials by this stage.All the cells showed the same cycling behaviour (Fig. 8), with a steady capacity decline over the first ten cycles followed by a region of slower decline. This behaviour is in contrast to 54 J. Muter. Chem., 1996, 6(l), 49-56 6 12 18 24 30 cycle number Fig. 8 Cycling performance of a cell containing sol-gel-derived V205 (alcohol route) the results obtained in similar cells for vanadium phosphate glasses5 which showed a region of steadily increasing capacity after the initial decline. The initial decrease for the gel-derived materials may be due to some lithium becoming immobilised in the structure but this is unlikely to continue over many cycles.Recent ~ork~~,~~ has shown that lithium may be inserted into V205 reversibly, although not topotactically, to give an Li/V molar ratio of between 0.9 and 1.0, and that irreversibility past this value may be due to the slow reoxidation of the V"' formed after a structural rearrangement. The theoretical capacity of the cells studied here is based on that of V6013 (417mA hg-') and is equivalent to an Li/V molar ratio of 1.33; so to achieve this capacity some reduction to V"' is necessary. The decline in capacity for the gel-derived vanadium oxides observed in this study is similar to that previously shown for crystalline V205,6 so it is likely that a similar mechanism operates. The cells all gave open-circuit voltages of cu. 3.5 V and the average voltages on discharge were consistently in the region of 2.3 V.This gives a maximum theoretical energy density of 960 W h kg-', which is superior to that of cells containing V6013(880 W h kg-I). However, the decline in capacity with cycling means that the observed energy density consistently decreased with continued cycling. Titanium oxide. The cells containing the gel-derived TiO, gave monotonous discharge curves, unlike crystalline anatase, in which no discharge occurs until a plateau24 at 1.8 V, with a capacity on the first discharge in excess of 100% of the theoretical value based on LiTi02. This is consistent with previous reports that poorly crystalline anatase-type structures allow free diffusion of lithium ions.25 In contrast, Minnett and Owen1* were able to contain a capacity of only 2 mA h g-' on the first cycle, which is less than 1 YOof the theoretical value. A large loss of capacity was observed on the second dis- charge, followed by further small losses before steady cycling was observed at around 10% of the theoretical capacity.The cycling behaviour is similar to that observed for anatase, which has been shown to retain lithium when discharged to 1.2V forming, irreversibly, a new phase.24 Significantly, anatase has been shown to be reversible along the plateau at 1.8V, becoming irreversible only when discharge proceeds beyond this. The average cell voltage on the first discharge was 1.7V, giving a theoretical energy density of 570 W h kg-', which is the same as that of anata~e.,~ The observed energy density (650 W h kg-') exceeded the theoretical value by virtue of the higher than estimated capacity (see earlier).However, sub- sequent discharges gave average voltages of only 1.4 V which, when coupled with the low capacities achieved, gave very low observed energy densities, for example 110 W h kg-' (second discharge) and 53 W h kg-' (20th discharge). Chemically mixed vanadium-titanium oxides. The cycling performances of representative cells with cathodes containing mixed oxides are shown in Fig. 9. An initial drop in capacity was observed over the first few cycles in each case which was complete by the sixth cycle. The capacity achieved on this cycle was then retained on further cycling up to an experimental limit of 30 cycles.The initial capacity decline was smallest with the 6mol% TiO, cathode, such cells eventually cycling at above 80% of theoretical capacity. This behaviour is in contrast to that of the cells with only V205 as the cathode-active material, see earlier, which continue to lose capacity (Fig. 8). This strongly suggests that it is the TiOz component of the cathodes that is influencing the reversibility. The initial capacity decline observed for all of the cells may be due to the immobilisation of some lithium ions, a view which is consistent with the increasing loss in capacity with increasing titanium content (Fig. 9). The interruption of the V-0-V chains in the structure by Ti" would be expected 100 80 60 40 20 0 4 a 12 16 20 100 80 0 6 12 18 24 30 100 (c> " 10 20 30 cycle number Fig.9 Cycling performances of cells with ternary sol-gel-derived V,05-TiO, cathode-active materials: (a) 6 mol% TiO,; (b) 20 mol% TiO,; (c)physical mixture containing 10 mol% TiO, J. Muter. Chem., 1996,6(l),49-56 55 to hinder the electron transfer along these which, in turn, would reduce lithium ion mobility. Additionally, the titanium atoms may physically restrict the lithium ion passage by inducing disruptions to the local microstructure. The second region of capacity decline (Fig. 8) is slower and does not occur when Ti0, is also present in the cathode active materials [Fig.9(u) and (b)].This effect could be due to the preferential reduction of Ti at low voltages or a structural modification induced by the Ti. To investigate this phenomenon some cells were constructed using physical mixtures of sol-gel-derived V205 and TiO,. Physically mixed vanadium and titanium oxides. The cycling performance of just such a cell is shown in Fig. 9(c). After an initial sharp drop in capacity the cell shows an excellent capacity retention from the fifth cycle onwards. In fact, the mixed cathode is reversible at close to 80% of theoretical capacity (330 mA h g-') which is very similar to the behaviour of V6OI3in the same type of cell. This synergism is the same as was observed for the chemically prepared ternary oxides, demonstrating that the contribution of the Ti-containing com- ponent is not of a structural nature.The most probable protective mechanism is the preferential reduction of the Ti" over V" near the low-voltage limit, which prevents the 'over- reduction' of vanadium to V"' with a concomitant reorganis- ation of the microstructure of the material, see earlier. Overall performance and energy densities The open-circuit voltages observed for all of the cells containing chemically or physically mixed V,O, and Ti02 sol-gel-derived oxides was around 3.5 V, compared with 2.8 V for cells contain- ing V6013,and the average voltages observed on discharge remained steady at around 2.3 V for each successive discharge. This gives a theoretical energy density of cu.960 W h kg-' for each material if the theoretical capacity is based on that of V6013. This value is greater than that of V6OI3(880 W h kg-') because of the improved average cell voltage. The energy densities for some of the reversible cells studied in this work with mixed V205 and TiO, cathode active materials are given in Table 6. The tenth discharge is chosen as a typical later discharge when the cells had all settled into a period of steady reversible cycling and a comparison with the first discharge shows the drop in energy density caused by the initial capacity decline. The observed energy densities compare favourably with that of V6OI3,with the cells using the oxides containing 6mol% TiOz showing a significant improvement. This is due to improved average voltages on discharge combined with high reversible cell capacities.Table 6 Observed energy densities of the reversible cathode materials energy densityfw h kg-' V205 content first tenth material (mol%) discharge discharge V205-Ti02 94 1005 800 V205-Ti02 73 850 570 V20, +TiO," 90 880 690 V6013 - 880 610 " Physical mixture. Conclusion The ternary materials showed good reversibility when employed as cathode components in lithium polymer-electro- lyte cells up to the experimental test limit of 30 cycles. Since a similar performance was obtained from a cell containing a physical mixture of oxides it appears that the preferential reduction of Ti" over V" near the low-voltage limit is a key factor.Observed energy densities were, in many cases, higher than have been obtained with similar cells incorporating V6OI3. The mechanism of protection against cathode over- reduction observed in this work shows great promise for future developments in lithium battery research. Materials previously thought unsuitable because of irreversibility at low voltages might still find favour as cathodes if a second material can be found with suitable electrochemical behaviour. We thank SERC and AEA Industrial Technology, Harwell Laboratory, for a CASE award to A.D. References 1 M. B. Armand, J. M. Chabagno and M. J. Duclot, in Fast Ion Transport in Solids, ed. P. Vashishta, J. N. Mundy and G. K. Shenoy, North Holland, New York, 1979,p. 131.2 M. B. Armand, Solid State Ionics, 1983,9110, 745. 3 A. Hooper and B. C. Tofield, J. Power Sources, 1984,11,33. 4 M. Gauthier, D. Fauteux, G. Vassort, A. Belanger, M. Duval, P. Ricoux, J-M. Chabagno, D. Muller, P. Rigaud, M. B. Armand and D. Deroo, J. Electrochem. SOC.,1985,132,1333. 5 A. Davies, R. J. Hobson, M. J. Hudson, W. J. Macklin and R. J. Neat, J. Muter. Chem., 1994,4, 113. 6 Y. Sakurai, S. Okada, J. Yamaki and T. Okada, J. Power Sources, 1987,20, 173. 7 C. J. Brinker and G.W. Scherer, Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing, Academic Press, London, 1990. 8 P. Aldebert, N. Baffier, N. Gharbi and J. Livage, Muter. Res. Bull., 1981,16, 669. 9 J. J. Legendre and J. Livage, J.Colloid Interface Sci., 1983,94,75. 10 J. J. Legendre, P. Aldebert, N. Baffier and J. Livage, J. Colloid Interface Sci., 1983,94, 84. 11 P. Aldebert, H. W. Haesslin, N. Baffier and J. Livage, J. Colloid Interface Sci., 1984,98,478. 12 J. Livage, P. Barboux, J. C. Badot and N. Baffier, in Better Ceramics Through Chemistry III, ed. C. J. Brinker, D. E. 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Dexpert, Electrochim. Acta, 1990,35,889. 24 W. J. Macklin and R. J. Neat, Solid State Ionics, 1992,53-56,694. 25 K. Kanamura, K. Yuasa and Z. Takehara, J. Power Sources, 1987, 20. 127. Paper 51024276; Received 18th April, 1995 J. Muter. Chem., 1996, 6(l), 49-56

ISSN:0959-9428    

DOI:10.1039/JM9960600049

出版商:RSC    

年代:1996

数据来源: RSC