摘要:

J. MATER. CHEM., 1994,4(8), 1259-1262 Improvement of Copper Oxide-Tin Oxide Sensor for Dilute Hydrogen Sulfide Tomoki Maekawa, Jun Tamaki, Norio Miura and Noboru Yamazoe Department of Materials Science and Technology, Graduate School of Engineering Sciences, Kyushu University, Kasuga-shi, Fukuoka 816, Japan The influence of the loading and dispersion of CuO on the H,S-sensing properties of CuO-SnO, sensors has been investigated. The response rates to H,S depend on various factors such as CuO loading, specific surface area (or size) of SnO, grains, method of loading the SnO, grains with CuO and the operating temperature. For the CuO-SnO, sensor prepared by a chemical fixation method, the response rates to dilute H,S (1 -1 0 ppm) increased with decreasing CuO loading, while the sensitivity to H2S decreased monotonically with decreasing amount of CuO dispersed per unit surface area of SnO,.By optimizing these factors, it was possible to obtain a CuO-SnO, sensor which responded to H,S above 1 ppm at 160 "C with sufficient sensitivity and fairly good response kinetics. The sensing of odours has become increasingly important for the control of living environments and food processing. Hydrogen sulfide (H2Sj is a typical bad-smelling component, in addition to its rather strong toxicity. High performance sensors for H2S are demanded for the purposes of auto- ventilation, diagnosis in dentistry and safety. It has been attempted to develop semiconductor-type sensors for H2S. The sensitivity and/or selectivity of Sn0,-based sensors to H,S are reportedly improved by adopting unusual operation modes such as quick cooling' and thermal cycling,2 or by modifying the sensors with hydrophobic silica,, ZrO,,'' basic oxides5 or Ag,6 but further improvements seem to be necessary.Recently an Au-doped WO, film sputtered on LiNb0, was reported to respond to sub-ppm levels of H2S.7 An Sn0,-based sensor impregnated with a small amount (typically 5 wt.%) of CuO is extremely sensitive and selective to 50 ppm H,S in air.' However, the rate of response is highly dependent on the concentration of H2S and becomes extremely sluggish in dilute H,S conditions (<20 ppm). A clue to improving this problem seems to exist in the fact that the response rate also depends strongly on the amount of CuO as well as the choice of the copper salt used as the starting reagent for the impregnation of CUO.~ This suggests that the dispersion of CuO particles on the surface of SnO, grains strongly affects the H,S-sensing properties of the sensor.From this viewpoint, the influence of the specific surface area (or grain size) of SnO, and the CuO loading were investigated in more detail in this paper. A chemical fixation method was partly adopted to attain a fine dispersion of CuO. As a result, it was possible to extend the lower detection limit of the CuO-SnO, sensor down to ca. 1ppm H,S. Experimental Preparation of Sensing Material The powder of SnO, was prepared from SnC1, by a wet process' and was calcined at 400°C for 1h (Sn0,-A), 600°C for 5 h (Sn0,-B) or 900°C for 4 h (Sn0,-C).The specific surface area of SnO, was 62 (A), 16 (B), and 6.4 (C)m2 g-'. Each sample was loaded with CuO by one of the following two methods. (1) Impregnation method: SnO, powder was impregnated with a solution of basic copper carbonate, followed by calcination in air at 400 "C for 1h (Sn02-A) or at 700 "C for 4 h (Sn0,-B and C). The CuO loading was fixed at 5 wt.%. (2) Fixation method:" SnO, powder was suspended in an aqueous solution of CuC1, (0.05mol dmF3j and CH3CO2NH4 (1mol dm-3) at room temperature for 24 h under agitation, to obtain copper chloro complexes that are chemically fixed onto the SnO, surface as shown in Fig. 1. The suspension was then filtered off, washed with deionized water and calcined in air at 400°C for 1h.The CuO loading was determined using an X-ray fluorescence analyser. When the fixation reaction was carried out at pH 6, the CuO loading on SnO, (A), (Bj and (C) was 3.3, 1.3, and 0.42 wt.%, respectively, which is equivalent to 0.53,0.81 and 0.66 mg CuO rn-, of SnO, surface in surface loading density. The CuO loading on SnO, could be reduced by decreasing the pH of the CuC1,-containing solution with the addition of HC1. With Sn02-C, for example, CuO loadings were 0.15 and 0.091 wt.%, or 0.23 and 0.14 mg CuO mP2 of SnO, surface, at pH 4 and 2, respectively. Measurement of Sensing Properties Sensors of a sintered block type were fabricated as described elsewhere.8 The electrical resistance of each sensor was meas- ured in a flow (200cm3 rnin-') of dry air as well as H2S ( 1-48 ppm) in dry air at given temperatures (typically 200 "C).Prior to the acquisition of electrical resistance data, each sensor was exposed to H,S (48 ppm) in an air stream at 200°C for 1 h. The gas sensitivity (S)was defined as the ratio (R,/R,) of the electrical resistance in air (R,) to that in the sample gas (RJ. H Hoi oi. I I + [CuCh(OAc),f--Sn-0-Sn-I I 2 NH4'I CI, ,CI 0 I I -Sn-0-Sn-I1 I Fig. 1 Chemical fixation of the Cu complex on the surface cif SnO, J. MATER. CHEM., 1994, VOL. 4 Results and Discussion Sensing Properties of CuO-impregnated SnOz Sensors Fig. 2 shows the response transients of three CuO(5 wt.%)-SnO, sensors to H,S at 200°C.The response on turning on H2S was in many cases sluggish in contrast to the extraordinarily fast recovery on turning off H2S, which is one of the unique characteristics of the CuO-SnO, sensor. Up to 48 or 40ppm H,S in air, the response rates were very dependent on the SnO, powder used, resulting in the order of A >I3 >C; The sensor using Sn02-A had a 90% response time as short as 1 min, but that using Sn0,-C could not reach steady state within 30 min. The sensors using Sn0,-B and C have an induction period of a few minutes in the response transients [Fig. 2(d) and (e)]. The sensitivity to H2S at the steady state is seen to be unusually high, i.e. 16200 (48 ppm H,S) for A and MOO0 (40 ppm H2S) for B, although the value for C could not be determined.This is another unique character- istic of the CuO-SnO, sensor. The order in the response rate coincided with that of specific surface area. With the CuO loading fixed at 5 wt.%, an increase in specific surface area of SnO, means an increase in the dispersion of CuO. In this connection, the amount of CuO per unit surface area of SnO, (surface loading density, Y) appear to be important, which are 0.85, 3.2 and 8.2 mg CuO of SnO, surface, for the sensors using Sn0,-A, B and C, respectively. These results indicate that the response rate to H2S tends to increase with decreasing although the sensitivity to H2S can behave in the opposite manner. The response rates were also highly dependent on the H,S concentration, decreasing drastically with H2S concen-tration.As is seen from Fig. 2, for example, the sensor using Sn02-A could not respond to 15 or 9 ppm at the rates that are acceptable practically. It is suggested that a lower Y will be more advantageous for detecting dilute H,S. Sensing Properties of Chemically Fixed CuO-SnO, Sensors In order to attain a finer CuO dispersion, CuO was dispersed on the SnO, grains by means of the chemical fixation method. In this case, Cu complexes were first supported on the SnO, surface up to monolayer coverage or less and then converted to CuO particles during the subsequent calcination. In prin- ciple, this method should be superior to the impregnation method for attaining high CuO dispersion, especially at small CuO loadings. Fig.3 shows the response transients of the resulting sensors to 9 ppm H2S at 200 "C. The two sensors, CuO( 1.3 wt.%)-Sn0,-B and CuO(O.42 wt.%)-Sn0,-C, could respond sharply to 9 ppm H2S, with 90% response times of ca. 2 min, in marked contrast to Fig. 2. On the other hand, CuO( 3.3 wt.%-Sn0,-A could not attain a steady state within 30 min, despite the fact that it should have a surface loading density of CuO (Y)comparable to the other sensors. This H2S off (a) 48 ppm S= 16200 + tv HZS on H2Son Fig. 2 Response transients of CuO(5 wt.%)-SnO, sensors to H2S in air at 200 "C (impregnation method). (a)-(c) SnO, (A) (surface area 62 m2 g-'), H2S 48 ppm (4, 15 ppm (b),9 ppm (c); (4 SnO, (B)(16 m2 g-'), H2S 40 ppm; (e)SnO, (C)(6.4 m2 g-'), H,S 40 ppm (b)S=47100 Fig.3 Response transients of CuO-SnO, sensors to 9 ppm H,S in air at 200°C (fixation method): (a) CuO(3.3 wt.%)-SnO, (A), (b)CuO(1.3 wt.%)-SnO, (B), (c) CuO(O.42 wt.%)-SnO, (C) suggests that factors other than such as porosity of the sensor, are also important for the response rates. As estimated from the specific surface area of SnO,, the mean grain size of Sn0,-A is 4.8 nm while those of Sn0,-B and C are 11 and 27 nm, respectively. Thus the micropores as well as macro- pores of the sensors can be significantly different, depending on the crystalline state of the SnO,. The diffusion of H2S molecules into and/or inside the pores is thus affected by the crystalline state of Sn02, probably causing the slower response of the CuO-Sn0,-A sensor.Even with the CuO-Sn0,-B and C sensors. the response became increasingly' sluggish as H,S was diluted further. Lowering the operating temperature from 200 to 160°C increased the response rates slightly. Fig. 4 shows the response transients of the CuO(O.42 wt.%)-Sn0,-C sensor to varying concentrations of H2S at 160°C. The detection of H,S at 6ppm and above could be achieved in a few minutes, while that of 2ppm H,S took ca. 20min, indicating that further improvement is still necessary for detecting such dilute concen- trations of H,S. It was possible to increase the rate of response to dilute H2S by decreasing the CuO loading further. For example, Fig.5 shows the response transient of the CuO(O.091 wt.%)-Sn02-C sensor on turning on and off 1.2 ppm H2S at 160"C. On exposure of the sensor to H,S, the output voltage reached 90% of the full change in 4 min. The gas sensitivity of the sensor was reduced significantly because of the smaller CuO loading, but it was still as high as 130. Fig. 6 shows the stationary electrical resistances (R,) of three sensors having different CuO loadings under exposure to H,S as a function of H,S concentration. The logarithm of R, is linearly correlated with the logarithm of H2S concentration for each sensor, assuring that each is applicable for the detection of H,S in the respective concentration range. So far attention was mainly focused on the response rate. H2S Off H~S9 ppm S=49000 6 ppm S = 37000 al 0, ,c > c 2a c 0 4 H2Son Fig.4 Response transients of CuO(O.42 wt.%)-SnO, (C) sensor to H2S at 160 "C (fixation method) J. MATER. CHEM., 1994, VOL. 4 8.7 x lo3 12 5 min--Qc 0 1.1 x 106R Fig. 5 Response transient of CuO(O.091 wt.%)-SnO, (C) sensor to 1.2 ppm H,S at 160°C (fixation method) 106f 1 23 5 10 H2Sconc. (ppm) Fig. 6 Electrical resistances of CuO-Sn02 (C) sensors at 160 "C as a function of H,S concentration (fixation method). CuO loading (wt.%): 0,0.091; A, 0.15; I?, 0.42 It was found that the sensitivity to H,S (S),another important characteristic, correlated well with the surface loading density of CuO (Y).Fig.7 shows the sensitivity to 9 ppm H,S at 200 "C as a function of Y. S increases sharply with Y,obeying the following equation: log S=2.34 log Y +4.68 The correlation is seen to hold for both Sn0,-B and C, indicating that S is independent of the grain size of SnO,. In this way, an increase in Y brings about a sharp increase in S, while it is not favourable for the response kinetics to dilute 'O2I10' 1000 0.1 0.2 0.5 1 Y/mg m-2 Fig. 7 Sensitivity of CuO-SnO, sensors to 9 ppm H2S at 200°C as a function of the amount of CuO per unit SnO, surface area (Y) (fixation method) H2S as mentioned previously. Thus the optimum Y value as a compromise of these two factors depends on the range of H,S concentration to be detected. Influenceof CuO Dispersion on H2SSensitivity The H2S-sensing mechanism of CuO-SnO, sensors involves the reaction between CuO and H2S,'y9 as schematically shown in Fig.8. In air, SnO, grains with CuO dispersed on their surface are strongly depleted of electrons due to p-n junctions at CuO(p)/SnO,(n) interfaces. On exposure to H,S. the CuO is converted into CuS according to the reaction CuO +H,S +CuS +H,O resulting in the destruction of the p-n junctions and the reduction of the electron depletion layer. When H,S is turned off, CuS is quickly oxidized to CuO by cus +3/20, +cue +So2 thus restoring the p-n junctions. The whole electrical resist- ance (R)of the sensor is assumed to be mainly determined by the height of the potential barrier at the grain bchundaries.The conversion between CuO and CuS is thus accompanied by a drastic change of resistance between R, and Rg,giving rise to the extraordinarily high sensitivity of the (hO-pro- moted sensor to H,S. Based on such a sensing scheme, the dispersion of CuO particles on the SnO, surfacc will be critically important for the H2S sensitivity; CuO should be dispersed as finely as possible to increase the density of p-n junctions on the SnO, surface. Generally speaking, the chemi- cal fixation method can give finer CuO dispersion than the impregnation method; this is why the former method is advantageous for obtaining high H,S sensitivity at reduced CuO loadings. When the former method was adopted, the sensitivity to a fixed concentration of H2S was shown to correlate well with the amount of CuO per unit surface area of SnO, (Fig.7). This can be understood as reflecting the increase in the density of p-n junctions. H2SConcentratio2-dependent Resistance The electrical resistance of the sensor (R,)correlated well with the H,S concentration (Fig. 6). It is not very obvious how R, varies with H2S concentration on the basis of the above sensing mechanism. A possible interpretation is that the degree Fig. 8 Schematic drawings for CuO dispersed SnO, under :xposure to air (u) and H,S-air (b) J. MATER. CHEM.. 1994, VOL. 4 Fig. 9 Part of the microstructure of a sintered block type sensor using CuO-SnO, under exposure to H,S in air. CuO-dispersed SnO, grains coagulate to form secondary particles.CuO in the outer region of each secondary particle is sulfurized while CuO in the inner region is not of sulfurization of CuO particles on all SnO, grains changes with the H2S concentration. However, this fatally ignores the microstructure of the H,S-sensing entity. In the actual sensor, SnO, grains are coagulated to form large clusters (secondary particles) which are in contact to each other, as depicted in Fig. 9. H,S molecules should gain access to Sn02 grains by diffusion through macropores and micropores which are developed among and inside the secondary particles, respect- ively. Since H2S is very reactive with CuO, its concentration should decrease when H,S diffuses from the outer to the inner part of the microstructure.Such a concentration gradient of H,S would be especially steep inside the micropores. In other words, the sulfurization of CuO proceeds favourably on the SnO, grains that are located at the outer region of the secondary particles, the sulfurized region in the steady state becoming thicker with increasing H,S concentration. Such a heterogeneous progress of sulfurization depending on the H2S concentration is a more plausible cause for the H2S concentration-dependent resistance of the sensor. However, further investigations are necessary to explain the observed correlations quantitatively. Rates of Response to H2S The rate of response of the CuO-SnO, sensor to H,S was shown to deteriorate sharply when the H,S concentration dropped below a certain level which depended on the CuO loadings as well as on the specific surface area (or mean size) of the SnO, grains.To illustrate these kinetic properties more quantitatively, the times for 70% of the full response were evaluated from the response transients to H2S depicted in terms of the logarithm of electrical resistances for the three CuO-Sn0,-C sensors (Fig. 10). For the same CuO loading, the 70% response time is seen to increase almost linearly with a decreasing logarithm of H2S concentration, and the slope tends to become steeper as the CuO loading increases. This figure explains well why reduced CuO loading is necessary for detecting dilute H2S. What then determines the rate of response to H,S? Based on the scheme depicted in Fig.8 and 9, the rate of response involves not only the rate of sulfur- ization of CuO (or H,S consumption) but also the rate of 1 2 5 10 H2Sconc. (ppm) Fig. 10 70% response time of CuO-SnO, (C) sensors as a function of H,S concentration at 160"C. (fixation method 1. CuO loading (Wt.Yo): 0,0.091; A,0.14; 0,0.42 diffusion of H2S inside the sensor microstructure. The rate of response appears to reflect how quickly the H,S concentration profile inside the microstructure reaches a steady state. It seems that this process becomes quicker as the rate of diffusion of H,S increases relative to the rate of consumption of H2S. The former rate increases with H2S Concentration while the latter decreases with CuO loading, both changes resulting in the reduction of the 70% response times as observed in Fig.10. These considerations suggest the importance of sensor structure other than the sintered block type investigated in the present study. For example, thick or thin film type sensors may be more advantageous with regard to the detection of dilute H2S, because the diffusion path for H2S can be made far shorter. Conclusions The H2S-sensing properties of CuO-SnO, sensors are influ- enced strongly by the CuO loading amongst other factors. The response rates to dilute H2S increase with decreasing CuO loading, while the sensitivity to H2S decreases sharply with the amount of CuO per unit surface area of SnO,. Such characteristics can be understood well from the H2S-sensing scheme which involves the sulfurization of CuO with H2S and the diffusion of H2S inside the macro- and micro-pores of the sensors. References 1 V. Lantto and P. Romppainen, J. Electrochem. SOC., 1988, 135, 2550. 2 V. Lantto, P. Romppainen, T. S. Rantala and S. Leppavurori, Sensors Actuators B, 1991,4,451. 3 S. Kanefusa, M. Nitta and M. Haradome, J. Electrochem. Soc., 1985,132,1770. 4 S. Kanefusa, M. Nitta and M. Haradome, 7ech. Digest, 7th Chemical Sensor Symp., Saitama, Japan, 1988, p. 145. 5 T. Nakahara, Proc. Symp. Chem. Sensors, Honolulu, 1987, p. 55. 6 V. Lantto and J. Mizsei, Sensors Actuators B, 1991,5, 21. 7 P. J. Smith, J. F. Vetelino, R. S. Falconer and E. L. Wittman, Sensors Actuators B, 1993,13-14,264. 8 T. Maekawa, J. Tamaki, N. Miura and N. Yamazoe, Chem. Lett., 1991,575. 9 J. Tamaki, T. Maekawa, N. Miura and N. Yamazoe, Sensors Actuators B, 1992,9, 197. 10 T. Kaji, H. Ohno, T. Nakahara, N. Yamazoe and T. Seiyama, Nippon Kagaku Kaishi, 1980, 1088. Paper 4/02042A; Received 6th April, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401259

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994,4(8), 1263-1265 Porosity of Pyrolysed Sol-Gel Waveguides Jeremy J. Ramsden Department of Biophysical Chemistry, Basel University, Klingelbergstrasse 70, 4056 Basel, Switzerland The pyrolysis of a sol-gel film dip-coated on a substrate is a convenient and inexpensive way of preparing optical waveguides with a wide range of refractive indices. Such waveguides are, however, microporous. For many uses, in particular sensing applications such as the chemical analysis of solutions, the porosity is one of the key parameters determining the response of the waveguide to its environment. In this paper, the porosity of a planar waveguide is accurately determined by measuring the mode indices for two modes with the pores filled alternately with liquid H20 and D,O.Planar dielectric waveguides can be fabricated by a simple, inexpensive process by first preparing a sol of the precursor of the dielectric material, typically a metal oxide, in an organic solvent,? e.g. isopropyl alcohol. A planar substrate is vertically lowered into and withdrawn from the sol. This procedure results in a thin layer of the sol remaining on the substrate. The solvent is then allowed to evaporate, whereupon a semirigid gel is formed. Finally the waveguiding layer is pyrolysed in an oxidizing atmosphere usually in a first stage at 300°C and in a second at 500°C; water and CO, are expelled and a pure metal oxide is formed.' The optimal temperature of the second stage depends on the substrate. During the second stage the layer becomes more compact, i.e.it becomes thinner and its refractive index increases. Films produced in this way are isotropic, homogeneous and show low losses, provided the thermal expansion coefficients of film and substrate are well matched. The process is applicable to many different materials, yielding waveguiding films over a wide range of refractive indices. During drying but before pyrolysis, i.e. in the gel-like state, microstructuring of the waveguide surface can readily be carried out, e.g. gratings can be embossed into When the waveguides are placed in water or other liquids, it has been empirically established that the thickness (by ca. 1-2%) and refractive index of the films increase, over a period of several hours. The process is reversible, and appears to be qualitatively consistent with the waveguide being com- posed of fine channels embedded in a dense matrix, initially filled with air, which is gradually replaced by the liquid in which the waveguide is immersed.A recent and potentially important application of these waveguides is as sensing elements. Optical switches and gas sensors based on grating couplers fabricated out of such coatings have been rep~rted.~ The waveguides also respond to changes in the concentration of ions in the solution bathing the waveguide,' and can thus function as sensors for small soluble ions. In this type of application, it is important to know the porosity of the waveguide, which is a key parameter determin- ing the response of the waveguide to a given ambient.In this report, an accurate method of determining the porosity in situ is described, and results from a commercially available planar waveguide are presented. Experimental Planar optical waveguides fabricated by dip-coating and pyrolysing a sol-gel film were obtained from AS1 AG, Zurich, Switzerland (type 2400). The given specifications of the wave- t Aqueous sols tend to be rather unstable and aggregate. guides were: material, Si, -xTi,02, x x0.4, refractive index of the waveguiding film nF, 1.8; thickness dF, 160-200nm; substrate, AF45 glass (Schott DESAG) with refractive index ns =1.525 781. The waveguides incorporated a grating coupler of period A =4165 nm. A waveguide was first equilibrated in H20,$ purified using a Barnstead (Dubuque, Iowa) Nanopure installation (conductivityx 18 MQ cm), for at least 48 h.It was then mounted on a goniometer scanning instrument (10s-1,AS1 AG, Zurich, Switzerland) which allows the angle of incidence of a linearly polarized external beam (He-Ne laser, A= 632.8 nm) to be varied while measuring the incoupled power by means of photodiodes positioned at the ends of the waveguide. The angular resolution of the instrument is 1.25x rad. Incoupling occurs when the angle a fulfills the following ~ondition:~ N =nai,sin a +LA/A (1) where N is the effective refractive index of the waveguide and L the diffraction order. a was determined for the zeroth-order transverse electric (TE) and transverse magnetic (TM ) modes.During the measurement, a small water-filled cuvette was sealed over the grating region with an '0'-ring, the waveguide thus forming one wall of the cuvette. The temperature was monitored by a Pt-100 resistance thermometer embedded in the goniometer head. Typically 20-30 readings were taken and averaged. The standard error in N did not exceed 4 x The mode equations for zeroth-order modes in a three-layer waveguide 2n: -A d, J(ni -N2)= with p equal to 0 and 1, respectively, for TE and TM modes. n, is the refractive index of the solution covering the wave- guide. Since the pyrolysed sol-gel material can be considered to be isotropic,' both TE and TM modes have the same nF. Hence eqn. (2) can be solved simultaneously for the two unknowns nF and dF using the measured N.n, for water was taken from the work of Tilton and Taylor." $, The porosity in the presence of water was investigated because of the author's interest in the response of waveguides to aqueous ions. The metal oxides which are often used for fabricating high-refractive- index waveguiding layers become hydrated in contact with aqueous The hydrated oxides often form complexes with other metal iom8 J. MATER. CHEM., 1994, VOL. 4 H20 and D20 are chemically and structurally very similar molecules, and it can be assumed that the waveguide structure is not changed when one is substituted for the other. Their refractive indices differ appreciably from each other, however.Hence, the waveguide was then soaked in D,O, 99.9% pure (Isotec, Miamisburg, Ohio) for a further 48 h, and NT, and N,, again determined. The sequence of soaking in H,O, measuring N, soaking in D,O and measuring N again was repeated several times. The refractive index of D20 was measured in a Rayleigh interferometer (L13, Carl Zeiss Jena, Germany), calibrated with an He-Ne laser. Results and Discussion Table 1 shows the measured values of N, together with nF calculated using eqn. (2). The refractive index of the micropo- rous waveguiding film measured in the presence of a liquid 1 can be written as: (3) where 8 is the porosity, defined as the fraction of film volume occupied by pores. The pores are assumed to be uniformly distributed within the film, and to be connected with one another and the external medium.The close similarity of H20 and D,O allows it to be assumed that 8 is unchanged by the substitution of one for the other. Eqn. (3) solved simultaneously with successively I =H20 and D20 (data from Table 1) gave 8=0.1418 &0.0015 and nSi(Ti)O, = 1.8896-t 0.0016. Measurement of the porosity of several wave- guides of the same type gave values of I3 in the range 0.14-0.17. Sources of Uncertainty In eqn. (l),the main sources of uncertainty are in a, A and A. Owing to slight mechanical instabilities, a is measured to an accuracy of ca. 0.3". The variation in the refractive index of air12 can be neglected. Hence the uncertainty in the first term is ca.2 parts in lo6. Fluctuations in the wavelength of the He-Ne laser are of the order of 1 part in lo6. From the average thermal expansion coefficients of quartz13 and rutile,I4 5.5 x and 9.5 x 'C-', respectively, we calculate a weighted mean of 4.7 x "C-'; the thermal expansion coefficient of the AF45 glass used as the waveguide support is almost identical to 4.5 x "C-' (data from DESAG). Hence the uncertainty in A due to temperature variations is ca. 0.2 parts in lo6, and the overall uncertainty in the second term 1.2 parts in lo6. Combining the uncertainties from the first two terms gives an uncertainty in the measured N of ca. 2 parts in lo6. Measurements (the standard deviation of a long series of points recorded under unchanging conditions) corroborated this figure.The uncertainties given for I3 and nSi(Ti)O, may be slightly overestimated since some of the given uncertainties are due to small fluctuations in the temperature, the consequences of which are correlated in the quantities combined to calculate 6' and nSi(Ti)O,. Table 1 Measured waveguide parameters in H,O and D20 1 n, =nl NTE NTM nF ~ ~ H2O 1.331 556 1.618 582 1.572 960 1.810470 (uncertainty) (0.000005) (0.000004) (0.000004) (0.000015) D20 1.325 641 1.617 630 1.571 898 1.809 535 (uncertainty) (0.000005) (0.000004) (0.000004) (0.000015) ~~~~~ i=632.8 nm. The waveguide thickness dF did not change according to the ambient liquid and had a value of 178.342k0.072 nm. Measurements were carried out at 22.05 0.05 "C.Rapid Determination of Waveguide Porosity If the chemical composition of the waveguide is known, then a single determination of nF suffices to determine the porosity, as can be seen by rearranging eqn. (3): %(Ti)O, -nF (4)%i(Ti)O, -Validity of the Model Since the waveguides are too thin to allow more than one mode to be excited for each of the two polarizations, TE and TM, it is not possible to verify that the waveguides remain isotropic when water diffuses into the microchannels. The most likely source of anisotropy would be a structure approaching that of a system of rods, filled with the ambient medium, perpendicular to the plane of the waveguide, embed- ded in the Ti(Si)O,. Wiener" has shown that such a system will exhibit positive birefringence, the magnitude of which depends on the difference between the two media, i.e.-ncl. Since this difference will diminish when water replaces air in the channels, the birefringence must necessarily decrease, and hence since the waveguides were found to be isotropic in air,' they must remain so in water. On the other hand, the exact nature of the change, in particular the increase in apparent thickness when previously unimmersed wave-guides are immersed in water, is not clear. It is assumed that after the lengthy period (ca. 48 h) of equilibration required the F layer is uniformly hydrated, but this has not been indepen- dently verified. Interpretation of B (Microstructure of the Waveguide) The packing density D, of randomly packed spheres in three dimensions lies between 0.61 and 0.64,16 i.e.8 is predicted to be ca. 0.37, but the measured value is significantly less. Two possible reasons for the discrepancy are: (i ) the particles constituting the waveguide are not spheres, but irregularly shaped objects. They are hard and no significant deformation is expected to take place during drying or pyrolysis. The density of randomly packed irregular objects has not been systematically investigated but for certain shapes could be higher than for spheres; (ii) since D, (for spheres) exceeds the percolation threshold not all voids may be accessible to the surface. The volume fraction of inaccessible voids is 13inacc.pore --1-D3-Orneas.=0.21. In this case the computed refractive index of 1.8896 must apply to Si(Ti)02 plus the inaccessible pores, which are presumably filled with vapour with a refrac- tive index of ca. 1. Reapplying eqn. (3) gives the result that the refractive index of Si(Ti)O, is practically equal to that of pure TiO,, an inference ruled out by the composition of the starting materials. It is doubtless incorrect to think of the waveguide as a percolating cluster of hard spherules. During drying pyrolysis solvent vapour and CO, escaping through the gel may render most of the voids accessible from the surface. The porosity strongly depends on the pyrolysis temperature. Films fired at higher temperatures are more compact. For example, waveguides fired at 800 "C (compared with 500 "C used for the type 2400 waveguides) have a porosity I3 of only a few per cent.17,18 Interpretation of nsivi)o, The measured value of 1.8896 reflects the chemical composi- tion of the waveguiding film.The sol-gel system used to coat J. MATER. CHEM., 1994, VOL. 4 the glass substrate was a Liquicoat (Merck) solution composed of premixed SiO, and TiO, precur~ors.~~ The ratio of the Si and Ti is not specified precisely by the manufacturer and may, moreover, change during dip-coating and drying, both because of the differing volatilities of the organometallic precursors, and because Ti may be preferentially leached from the film.,' Therefore film composition was analysed with X-ray photo- electron spectroscopy (XPS) at the Institute for Experimental Physics of Condensed Matter of the University of Basel.,' Samples were analysed using a Leybold EA11/100 spec- trometer using Mg-Ka radiation (1253.6 eV).The residual gas pressure in the spectrometer was <lo-'' mbar. Fortunately the material was conductive enough to prevent the samples charging up sufficiently to vitiate the determination of the mole fractions of titanium, silicon and oxygen. Apart from the surface region, 1-2 nm in depth, the material was slightly substoichiometric with respect to oxygen [i.e. Si(Ti)01.87]. the ratio of Ti to Si was 38 :62, i.e. x =0.38. We can write %i(Ti)O, =xnTiO, +( -x)nSiO, (5) Taking the value of nSiOz (fused quartz) as 1.458,,, we obtain nTiO,=2.56.This value corresponds to the refractive index of TiO, films produced by reactive ion plating,23 which is known to produce films with a density between 95 and 100% of bulk material. Conclusion A convenient way of measuring the porosity and refractive index of optical waveguides has been devised, based on measuring their refractive indices in the presence of two liquids, chemically similar but with differing refractivities. Results for pyrolysed sol-gel dip-coated waveguide show that the porosity is much lower (about half) than that expected from sintered spheres. The fabrication process appears to result in a denser, more compact structure with all remaining pores accessible to the external medium. The method is useful for determining the porosity of thin film materials in situ.Other available methods for porosity characterization require the material to be present in powder form (e.g. ref. 24.). The present method can be used with any pore-filling liquid, provided that a pair of chemically similar molecules can be found. Deuteriated solvents are especially convenient for this purpose. The author thanks the Kommission zur Forderung der wissen- schaftlichen Forschung, Bern for support, Dr P. P. Herrmann for a fruitful discussion and a critical reading of an earlier version of the manuscript, and Mr. R. Gampp of the Institute for Experimental Physics of Condensed Matter of the University of Basel for the X-ray photoelectron spectrometry. References 1 P. P. Herrmann and D.Wildmann, IEEE J. Quantum Electron., 1983,19,1735. 2 W. Lukosz and K. Tiefenthaler, Opt. Lett., 1983,8, 53'. 3 K. Heuberger and W. Lukosz, Appl. Opt. Lett., 1986,25.1499. 4 K. Tiefenthaler and W. Lukosz, Opt. Lett., 1984, 10, 1.17. 5 J. J. Ramsden, D. U. Roemer and J. E. Prenosil, PrIc ECIO 6, 1993,12-38. 6 T. W. Healy and L. R. White, Adc. Colloid Interfact Sci., 1978, 9, 303. 7 D. N. Furlong, D. E. Yates and T. W. Healy, Fundamental Properties of the OxidelAqueous Solution Interface, in Electrodes of Conductive Metal Oxides, ed. S. Trasatti, Part 13, Elsevier, Amsterdam, 1981, p. 367. 8 R. J. Hunter, Zeta Potential in Colloid Science, Acadcmic Press, London, 1981. 9 K. Tiefenthaler and W. Lukosz, J. Opt. Soc. Am. B, 19h9,6,209.10 A. Ghatak and K. Thagarajan, Optical Electronics, Cambridge University Press, Cambridge, 1989. 11 L. W. Tilton and J. K. Taylor, J. Res. Natl Bur. StIind., 1938, 20,419. 12 H. Barrel1 and J. E. Sears, Philos. Trans. R. Soc. Lonam, Ser. A, 1939,238, 1. 13 Handbook of Chemistry and Physics, ed. R. C. Weast, RC Press, Boca Raton, 56th edn., 1975-6, p. F-78. 14 F. A. Grant, Rev. Mod. Phys., 1959,31,646. 15 0. Wiener, Abh. Math.-Phjls. K1. Kgl. Sachs. Ges. Mb., 1912. 32, 503. 16 K. Gotoh and J. L. Finney, Nature (London), 1974,252,202. 17 Ch. Stamm and W. Lukosz, Sensors Actuators B, 1993, 11, 177. 18 Ph. M. Nellen and W. Lukosz, Biosensors Bioelectro~iics, 1993, 8, 129. 19 K. Tiefenthaler, V. Briguet, E. Buser, M. Horisbt-rger and W. Lukosz, Proc. SPIE, 1983,401, 165. 20 K. Shingyouchi, A. Makishima, M. Tutumi, S. Takenlwchi and S. Konishi, J. Nun-Cryst. Solids, 1988, 100, 383. 21 P. Oelhafen, D. Ugolini, S. Schelz and J. Eitle, in Diamond and Diamond-like Films and Coatings, ed. R. E. Clawing, L. I . Horton, J. C. Angus and P. Koidl, Plenum Press, New York, 1991,p. 377. 22 Handbook of Chemistry and Physics, ed. R. C. Weast, CMC Press, Boca Raton, 56th edn., 1975, p. E-224. 23 K. Bange, C. R. Ottermann, 0. Anderson, U. Jerchowski, M. Laube and R. Feile, Thin Solid Films, 1991, 197, 279. 24 J. H. Strange, M. Rahman and E. G. Smith, Phys. Rev. LTtt., 1993, 71, 3589. Paper 4/02688H; Received 6th May, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401263

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4(8), 1267-1270 Sol-Gel Synthesis of Superconducting YBa,Cu,O, using Acetate and Tartrate Precursors Aivaras Kareiva,+ Maarit Karppinen and Lauri Niinisto Laboratory of Inorganic and Analytical Chemistry, Helsinki University of Technology, FIN-02 150 Espoo, Finland A simple sol-gel route starting from an aqueous mixture of Y, Ba and Cu acetates and tartrates has been refined to prepare superconducting YBa,Cu,O, at an oxygen pressure of 1 atm. Thermal decomposition of the fabricated gels was studied by means of thermogravimetry. In order to interpret the decomposition mechanisms, both individual and double gels of Y, Ba and Cu were studied. The synthesis products were characterized by X-ray diffraction for phase purity, by thermal analysis for oxygen stability and by SQUID measurements for superconducting properties.High-T, superconducting YBa,CU,O, (124) is a member of the homologous series of compounds of general formula Y2Ba,Cu6 +n (n=0, 1, 2). The thermodynamically fav- oured 124 phase differs from YBa,Cu,O, (123) in having a double, instead of single, Cu-0 chain running parallel to the h axis. It exhibits a transition temperature, T,, at around 80 K, and unlike the 123 phase its oxygen content has excellent thermal stability. With decreasing oxygen content in 123, T, decreases gradually and the compound loses its supercon- ducting properties. On the other hand, all oxygen atoms in the 124 phase are stable even at high temperatures; the orthorhombic-to-tetragonal phase transition and the macro- scopic twins observed in the 123 structure are also absent.All these properties make the 124 phase very interesting from a theoretical point of view as a slightly different model system for testing the general trends of high-T, materials and also in view of possible applications. The YBa2Cu,08 phase was first isolated under high oxygen pressures,' but synthesis under ambient conditions is also p~ssible.~-~A major problem encountered in the preparation of the 124 phase under normal pressure is the difficulty in obtaining a single-phase material. At higher temperatures the 124 phase decomposes into 123 and CuO, while at lower temperatures the synthesis does not proceed to completion. The sol-gel meth~d~,~ of synthesizing multicomponent cer- amics from metal alkoxides or salts provides many advantages over conventional solid-state methods.Homogeneous prod- ucts are easily obtained by mixing the metal precursors in solution in a molecular scale, and synthesis temperatures can be lowered noticeably. Furthermore, the rheological properties of the precursor gel allow the preparation of film^.^,^ In the sol-gel process the most critical step is the choice of starting materials since the greatest difficulty lies in the preparation of a stable precursor sol. Two types of route have been described in the literature for the preparation of the superconducting powders, depending on whether the precur- sor is an aqueous solution of inorganic saltsg-'l or a non- aqueous solution of metal-organic corn pound^.^^^^^^^ Most aqueous sol-gel processes employ tartaric acid for metal complexation and gel formation, but recently a novel synthetic route has been developed where the yttrium and barium oxides are dissolved into an aqueous solution of copper acetate.' The purpose of the present work was to refine a simple and reliable sol-gel route for preparing superconducting YBa2Cu,08 under low oxygen pressure.An acetate solution t Permanent address: Department of General and Inorganic Chemistry, Vilnius University, Lithuania. of Y, Ba and Cu was selected for the precursor. Irr order to prevent partial crystallization of copper acetate hydriite during the gelation process the ability of tartaric acid to form soluble complexes with copper was investigated.The synthesis param- eters optimized included pH and temperature profile during the sol-gel process as well as the annealing conditions during the final heat treatments. Special attention was dso paid to the thermal decomposition mechanism of the precursor gel. Individual and double gels of Y, Ba and Cu were prepared and investigated to assist the interpretation of the reactions leading to the formation of the ternary 124 system. Synthesis products were characterized by X-ray diffraction (XKD) ther- mal analysis and SQUID measurements. Experimental The gels were prepared using stoichiometric amounts of analytical-grade Y203,Ba(CH,C0,)2 and Cu(CH3C( )2)2.H20 as starting materials.Yttrium oxide (3.125 mmol) was first dissolved in 100 ml of 0.2 mol 1-' CH,C02H. After stirring the mixture for 10h at 55-60 "C in a beaker covered with a watch-glass a clear solution was obtained. Next, 50ml of 0.5mol 1-' Cu(CH,CO,),.H,O was added and the solution was stirred for 2 h at the same temperature. Finally, 25 ml of 0.5mol 1-' Ba(CH,CO,), was added and the solufion was stirred for another 2 h at room temperature. The pli of the metal acetate solution was 6.1. In most experiments small amounts (1-2 g) of tartaric acid, dissolved in 20 ml of distilled water, was added to the above solution, decreasing the pH value to 5.6. After concentrating the solution for 8 h at 65°C in an open beaker under stirring the acetate-tartrate solution turned into a blue gel while the acetate gels were light green.The gels were dried in a furnace at 80 "C. The synthesis of the individual and double gels of Y, Ba and Cu were carried out under the same conditions. To check the metal contents in the dried gel powders photometric (Y, Cu)15 and gravimetric (Ba) methods were applied. The gel powders were ground in an agate mortar and preheated for 10 h at 780 "C in flowing oxygen. Since the gels are very combustible slow heating ( 1 "C min- ') especially between 150 and 350°C was found to be essential. After an intermediate grinding the powders were sintered at various temperatures from 750 to 820°C in air or flowing oxygen. The annealing times varied between 10 and 45 h.For the thermogravimetric ( TG) analyses a Perkin-Elmer System 7 thermobalance was used. The thermal decomposition of the individual, double and ternary acetate-tartrate gels of Y, Ba and Cu was studied up to 1200°C in a dynamic oxygen atmosphere using a heating rate of 2°C min-'. The sample J. MATER. CHEM., 1994, VOL. 4 weight was 30-60mg. Also, the oxygen stability of the final products under oxygen and argon atmospheres was confirmed by TG measurements (heating and cooling rates 5 "C min-'; sample weight 25-50mg). The phase purity and the crystal- linity of the products were studied by XRD analyses using a Philips MPD 1880 diffractometer equipped with a graphite secondary monochromator and a Cu tube. The reported critical temperatures of superconductivity are onset tempera- tures of the diamagnetic signal determined by a SQUID magnetometer (Quantum Design MPMS2).Results and Discussion Optimization of the Sol-Gel Process To ascertain the homogeneity of the acetate gel, the pH of the starting solution has to be adjusted carefully. If the solution is too acidic (pH<5.6), copper acetate hydrate is crystallized during the concentration process, while in solu- tions that are too basic (pH>6.1) copper hydroxide may floc~ulate.~In the acetate solution of Y, Ba and Cu a considerable quantity of CU(CH,CO~)~*H,O was precipitated when the evaporation process was started from pH 6.1. This could be seen, for example, in the resemblance of the TG curves recorded for the Y-Ba-Cu acetate gel and for the pure copper acetate hydrate (Fig.1). The decomposition of anhy- drous acetate occurs around 250 "C, which coincides with the decomposition temperature of the gel. However, when tartaric acid was added to the acetate solution a transparent gel was easily obtained even when the pH of the starting solution was 5.7. Evidently, tartrate ligands form stable16 and sufficiently soluble complexes with Cu2+ ions to prevent the crystalliz- ation of Cu(CH3C02),-H20. The observed critical limits for the C4H606:Cu ratio are 0.3 and 0.6, restricted by the solubilities of copper acetate and the metal tartrates, respect- ively. For the further experiments a C4H606:Cu ratio of 0.44" was selected. Thermal Decomposition of the Gels The mechanism of the thermal decomposition of the dried gels was investigated in an oxygen atmosphere by means of TG measurements in the temperature range 40-1200 "C.Fig. 2 shows the TG curve for the ternary yttrium, barium and copper acetate-tartrate gel. The decomposition occurs in several steps. The weight loss below 200°C (8.5 %) is due to the evolution of water and to the initial decomposition of the copper constituent. The final decomposition of the Y-Ba-Cu acetate-tartrate precursor above 200 "C proceeds uia horno-40 -I CUO -+20 -----. -, -. ----. -. -. -20 -I I I I I I II Fig. 1 TG curves for the ternary Y-Ba-Cu acetate gel (solid line) and Cu(CH,CO,),.H,O powder (broken line; theoretical weights of the decomposition products are indicated) samples recorded in a flowing oxygen atmosphere.The heating rate was 2 "C min-l. r-"or----7 60 \ 200 400 600 800 TI'C Fig. 2 Thermal decomposition of the ternary Y-Ba-Cu acetate-tartrate gel under a flowing oxygen atmosphere. The heating rate was 2 "C min-l. geneously distributed intermediate species, e.g. BaCO,. Since the structures of the gel complexes are not known, a more detailed interpretation of the decomposition mechanism is complicated. In order to clarify the possible reactions, individ- ual and double Y, Ba and Cu acetate-tartrate gels were studied. The TG curves for the individual nietal gels are shown in Fig 3-5. The decomposition of the acetate-tartrate gels differs considerably from that of the corresponding solid acetates.CUO -cue------- - ---------20 u 200 400 600 800 T1°C Fig. 3 TG curves for the copper acetate-tartrate gel (solid line) and Cu(C,H,O,) powder (broken line; theoretical weights of the decompo- sition products are indicated) samples recorded in a flowing oxygen atmosphere. The heating rate was 2 "C min-l. LuJ 200 400 600 800 1000 TI'C Fig. 4 TG curves for the barium acetatetartrate gel (solid line) and Ba(CH,CO,), powder (broken line; theoretical weights of the decomposition products are indicated) samples recorded in a flowing oxygen atmosphere. The heating rate was 2 "C min-'. J. MATER. CHEM., 1994, VOL. 4 40 II 200 400 600 800 T1"C Fig. 5 TG curves for the yttrium acetate-tartrate gel (solid line) and Y (CH,CO,),H,O powder (broken line; theoretical weights of the decomposition products are indicated) samples recorded in a flowing oxygen atmosphere.The heating rate was 2 "C min-'. A comparison between the TG curves recorded for Cu(CH,CO,),-H,O (Fig. l), Cu(C4H406) and the copper acetate-tartrate gel (Fig. 3) confirms that the main part of the gel is decomposed at the same temperature as the tartrate but at a much lower temperature than the acetate. In the temperature range 40-175 "C (weight loss 10.0%)the residual solvent and the coordinated water molecules are evolved, followed by a rapid decomposition of the gel above 175°C (46.9%) to a mixture of Cu20 and CuO. With increasing temperature the oxidation of Cu' takes place.Judging from the TG data and the analysed copper content the probable composition of the gel is either CU,(C4H2O,)(H20), or Cu2(OH)2(C4H40,)(H20)2.17The decomposition of Ba(CH,C02)2 to BaO proceeds via BaCO,, but is incomplete up to 1100°C owing to the thermal stability of the carbonate (Fig. 4). On the other hand, the decomposition of the barium gel occurs in three steps. By assuming that all the available C,H,O,,-ligands (C,H,O, :Ba =0.88) form barium tartrate, the TG curve obtained for the Ba gel (Fig. 4) could be explained by the following reactions: (i) between 160 and 280°C (weight loss 1.8%) Ba(OH),.H,O is dehydrated to BaO; (ii) between 280 and 345°C (28.7%) the tartrate is decomposed to BaCO,; and (iii) above 1000°C decomposed further to the oxide.However, in the presence of copper (Ba-Cu and Y-Ba-Cu gels) barium tartrate was not precipitated any longer, but a gelatinous product with a complicated decomposition scheme was formed. In the case of Y(CH,C02)3-H,0 the dehydration starts at 80cC, and the resulting anhydrous yttrium acetate is stable up to 300"C, at which temperature Y,02C03 is formed (Fig. 5). Further heating leads to the oxide as the final product above 600°C. The TG data of the yttrium acetate-tartrate gel (Fig. 5) reveal weight losses of 4.0% (40-160°C), 6.6% (160-215 "C), 14.4% (215-265 "C), 15.9% (265-330°C) and 28.1 O/O (330-365 "C), which could be associated, respect- ively, with the desorption of residual solvent, the escape of water molecules from the coordination sphere of yttrium and with a multi-step decomposition of the yttrium hydroxo and/or acetato tartrate complex.According to charge-balance considerations and the above assumptions, the gel might be composed of Y,(OH)(C,H,O,),(H,O), or Y,(CH,COO)(C4H,06),( H,O), (z=4-8). In the Y-Cu double and Y-Ba-Cu ternary gels the tartrate ligands tend to form complexes with copper, and the situation may be different. Optimization of the Annealing Conditions The 124 phase can be grown in a flowing oxygen atmosphere above 750 "C. The growth is promoted by increasing annealing temperature and time. XRD data of the powder heated for 10 h at 780 "C indicated the presence of various oxide phases such as CuO, BaCuO,, YBa2Cu307 and YBa,Cu,O,.Further heat treatment with intermediate grinding improved the phase purity of YBa,Cu40,. Essentially single-phase 124 was obtained at 780°C after annealing for 30 h. In the XRD spectrum shown in Fig. 6 the reflections of the 124 phase are indicated; the only unidentified peaks seen at the 2t' values of 29" and 31" are most probably due to small amounts of BaCuO, impurities. Above 800 "C the 124 phase already starts to decompose to 123 and CuO, and these phases wcre clearly present in the XRD spectrum recorded for the sample annealed at 820 "C for 15 h. Characterizationof the Products Because of the structural similarities, the d values of the strongest peaks in the XRD patterns of YBa2Cu3O7 and YBa,Cu,O, coincide, which makes it difficult to distinguish between these two phases.However, in order to detect small amounts of 123 impurities the differences in the oxygen stability or in the superconducting transition temperature may be utilized. The 123 phase starts to lose oxygen around 330-400 "C depending on the atmosphere, while the 124 phase should be stable up to 800-900°C11~12~'8~19in oxygen and up to 670-700"C1918 in an inert atmosphere. Fig. 7 shows the I I I I I I I I lu 20 30 40 50 60 70 28ldegrees Fig. 6 Powder X-ray diffraction pattern (Cu-Ka radiation) for the ternary Y-Ba-Cu acetate-tartrate system annealed in flowing oxygen at 780 "C for 10+30 h. The reflections of the 124 phase are mdicated. I I I I I I I IL 200 400 600 800 TI'C Fig.7 TG heating and cooling curves for the synthesized YBa,Cu,O, material showing sufficient thermal stabilitities under flowing argon (solid line) and oxygen (broken line) atmospheres. The heating and cooling rates were 5 "C min-l. J. MATER. CHEM., 1994, VOL. 4 The authors thank Prof. K. V. Rao and Ms. T. Turkki, Royal Institute of Technology, Stockholm for carrying out the SQUID measurements. Prof. S. Pejovnik, National Institute of Chemistry, Slovenia is thanked for stimulating discussions. Financial support by the Nordic Council of Ministers to A.K. in form of a scholarship administrated by the Finnish Center for Mobility (CIMO) is gratefully acknowledged. References 1 J. Karpinski, E. Kaldis, E.Jilek, S. Rusiecki and B. Bucher, Nature (London),1988,336,660.1R. J. Cava, J. J. Krajewski, W. F. Peck Jr., B. Batlogg, L. W. Rupp2 0 20 40 60 80 100 TIK Fig. 8 SQUID data for the synthesized YBa,Cu,O, material TG curves of subsequent heating and cooling cycles under oxygen and argon atmospheres for one of the synthesized 124 samples (annealed at 780 "C for 10+ 30 h). No indication of weight loss could be seen below 690°C even in the argon atmosphere, and in the oxygen atmosphere the oxygen content was stable up to 880°C. Finally, the SQUID measurement of the same sample showed the onset of superconductivity at 78 K (Fig. 8), but owing to the low density of the non-pelletized powder sample the transition was found to be rather broad.Nevertheless, the TG and SQUID results above con- firm the absence of the other superconducting impurity phases, and together with the XRD data verify sufficient phase purity of the synthesized 124 material. Conclusions A sol-gel method, starting from an aqueous acetate solution of the metals, was investigated for synthesizing super-conducting YBa,Cu,O, under low oxygen pressure. Homogeneous gels are achieved by complexing copper ions with tartaric acid before the gelation process. The thermal decomposition scheme of the Y-Ba-Cu ternary system is complicated but it can be understood through studies on the individual and double metal precursor gels. Annealing conditions can be optimized to produce virtually single phase 124 powders from the acetate-tartrate gel.The synthesized YBa,Cu,O, material exhibited excellent thermal stability under oxygen and argon atmospheres and showed a superconductivity transition at 78 K. The sol-gel process can also be applied easily to produce YBa,Cu,O, samples doped with various metal ions, e.g. Eu3+ and Fe3+ for Mossbauer studies.20,21 Jr., R. M. Fleming, A. C. W. P. James and P. Marsh, Nature (London), 1989,338,328. 3 S. Ohara, M. Matsuda, Y. Watanabe and M. Takata, Appl. Phjx Lett., 1991,59,603. 4 L. Bonoldi, M. Sparpaglione and L. Zini, Appl. Plzys. Lett., 1992, 61, 964. 5 J. Livage, M. Henry and C. Sanchez, Prog. Solid State Chem., 1988, 18,259. 6 G. R. Lee and J. A. Crayston, Adv. Mater., 1993,5,434. 7 S. Katayama, M.Sekine, H. Fudouzi and M. Kuwabara, Appl. Phys. Lett., 1992,60, 118. 8 Y. Masuda, R. Ogawa, Y. Kawate, K. Matsubara, T. Tateishi and S. Sakka, J. Mater. Res., 1992,7, 819. 9 P. Catania, N. Hovnanian, L. Cot, M. Pham Thi, R. Kormann and J. P. Ganne, Mater. Res. Bull., 1990,25,631. 10 S. Fujihara, H. Zhuang, T. Yoko, H. Kozuka and S. Sakka, J. Mater. Rex, 1992,7,2355. 11 P. L. Steger and X. Z. Wang, Physica C, 1993,213,433. 12 H. Murakami, S. Yaegashi, J. Nishino, Y. Shiohara and S. Tanaka, Jpn. J. Appl. Phys., 1990, 29, 2715. 13 S. Koriyama, T. Ikemachi, T. Kawano, H. Yamauchi and S. Tanaka, Physica C, 1991,185-189,519. 14 S. Pejovnik and M. Bele, Slovenian Pat. Appl., 9300578. 15 A. N. Turanov, Zavodskaya Laboratoriya, 1990,56( 8), 9. 16 Critical Stability Constants, ed. A. E. Martell and R. M. Smith, Plenum Press, New York, 1977, vol. 3, p. 128. 17 R. J. Missavage, R. L. Belford and I. C. Paul, J. Coord. Chem., 1972, 2, 145. 18 J. Mullens, A. Vos, A. DeBacker. D. Franco, J. Yperman and L. C. Van Poucke, J. Thermal And., 1993,40, 303. 19 T. Wada, N. Suzuki, A. Ichinose, Y. Yaegashi, H. Yamauchi and S. Tanaka, Jpn. J. Appl. Phys., 1990,29, L915. 20 J. LindCn, M. Lippmaa, J. Miettinen, I. Tittonen, T. Katila, A. Kareiva, M. Karppinen, L. Niinisto, J. Val0 and M. Leskela, Phys. Rev. B, in press. 21 M. Karppinen, A. Kareiva, J. LindCn, M. Lippmaa and L. Niinisto, 2nd International Conference on $Elements, Helsinki 1994, to be presented. Paper 4/00765D; Received 8th February, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401267

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4(8), 1271-1274 Preparation of Single-phase Pb(Mg,,,Nb,,,)O, Samples utilizing Information from Solubility Relationships in the Pb-Mg-Nb-Citric Acid-H,O System Jin-Ho Choy,*" Yang-Su Han? Seung-Wan Songa and Soon-Ho Changb a Department of Chemistry, Seoul National University, Seoul 151-742, Korea Nectronics and Telecommunications Research Institute, P.0. Box 8, Daeduk Science Town, Daejeon, Korea The optimum pH for preparing single-phase Pb(Mg,,3Nb2i3)03 (PMN) powder can be estimated from the solubility vs. pH diagrams for the corresponding metal ions in hydroxide, carbonate and citrate media. The homogeneous and stoichiometric citrate PMN precursor could be prepared from the system Pb-Mg-Nb-citric acid-H,O by adjusting the pH of the solution to 6.Ultrafine (0.05-0.3 pm) and stoichiometric PMN powder could be obtained through the thermal decomposition of the citrate precursor at the relatively low temperature of 900 "C. A ferroelectric relaxer material with the chemical composition Pb( Mgli3Nb2,,)O3 (PMN) was first synthesized in the late 1950s.' Since that time, PMN has been extensively studied because of its high relative permittivity and high electrostric- tive However, difficulty has been experienced in preparing X-ray monophasic samples of PMN.7-9 Depending upon the synthetic conditions, the thermodynamically stable pyrochlore phase can be formed as a second phase during the initial stages of reaction and consequently reduces the relative permittivity of the final material.In addition, the processing temperature should be kept as low as possible in order to reduce the evaporation of Pb0.l' In the recent literature, many attempts have been made to prepare single-phase PMN: ( 1) the conventional ceramic route, which requires repeated calcination at high temperature (>900 "C) and a long reaction period (ca. 24 h);' (2) two-step synthesis, i.e. reaction PbO with the oxide precursor MgNb206;7 and (3) sol-gel processes using the alkoxide" and citrate as precursors." The first two methods require a high reaction temperature and a long reaction period and consequently suffer from a lack of reproducibility of physical properties and of control of PbO content and particle size. The third method, however, can potentially overcome these difficulties.Since the sol-gel method can lead to highly pure, homogeneous and stoichiometric PMN powders with finer particle size, which can be processed at comparatively low temperature, they may offer significant advantages over the conventionally processed powders. In addition, the lower processing temperature in the sol-gel method enables minim- ization of PbO evaporation upon heat treatment of the sol-gel-processed material. However, in the conventional sol-gel method using citric acid," a polyhydroxy alcohol (ethylene glycol) was used as the esterification agent, this causes the formation of a dense and rigid intermediate of citric acid and ethylene glycol. In this case, additional extensive post-calcination grinding is usually required to break down the hard agglomerates formed during the calcination.Thus, in order to obtain a soft and porous intermediate, our attention was focused on the devel- opment of an improved citrate gel process utilizing only the chelating ability of citric acid without using any esterification agent. As previously pointed 0ut,I2-l5 the chelating ability and the complex formation of citric acid with various metal ions are highly dependent upon the pH of the solution. It is, therefore, necessary to study the behaviour of ionic species present in an aqueous solution. For this purpose, an attempt was made to draw the solubility diagrams based on the theoretical calculation of solubilities of individual metal hydroxides, carbonates and citrates.Theoretical Calculation of Solubilities It is necessary to predict theoretically the fc jrmation and dissolution of chemical species present in the Pb-Mg-Nb-citric acid-H,O system with respect to pH and metal-ion concentrations. Since the solubility of each ionic species for hydroxides, carbonates and citrates is given directly as a function of pH, the solubility diagram can ciasily be obtained by plotting the logarithmic molar concentration calculated from the values of the thermodynamic equilibrium constants for hydroxides, carbonates and citrates.12-' Solubility of Metal Hydroxides Fig. 1 shows the log ci (log molar concentration of metal ions) dependence of lead hydroxide as a function pH at cbf 25°C.The slope of this solubility line is determined by rearranging and taking logarithms to the hydrolysis reactions of the possible lead species [eqn. (1)-(4) in Tablt: 11. As shown in Fig. 1, the precipitation of lead hydroxide can be expected to begin at ca. pH 7 and reaches to a maximum at ca. pH 10 (when [Pb2+]=0.1 mol dm-3). The log c, us. pH diagram (25 "C) of magnesium hydroxide derived fr( )m eqn. 0 2 4 6 81012 PH Fig. 1 Solubility diagram for the lead hydroxide system at 25 "C J. MATER. CHEM.. 1994, VOL. 4 Table 1 Reactions and equilibrium constants used in the solubility calculations eqn. reaction symbol for constants log K ref. ~~~ ~ Pb2'+H20=PbOH'+H' -7.7 16-18 Pb2+ +2H20 =Pb(0H),'(aq)+2Ht -17.1 Pb2' +3H20=Pb(OH),-+3H+ -28.1 Pb(OH),(s)+2H+ =Pb2+ +2H20 12.7 Mg2++H20=MgOH++H+ -11.4 16,18 Mg(OH),(s) +H' =MgOH' +H20 5.4 Nb5+ +4H20=Nb(OH)4f +4H' -0.9 16,18 Nb5++6H20 =Nb(OH),- +6H' -7.6 Nb(OH),(s)=Nb5+ +50H--71.0 COZ(g)+ HZO=H2C03 -1.5 19 H2C03*=H++HC03--6.3 HC0,-=H+ +CO,'--10.2 PbC02(s)=Pb2' +C032--13.1 MgC03(s)=Mg2++C0,'--7.5 -3.1 18 -4.8 -6.4 16.9 -0.4 Pb2++Cl-=PbCl' 1.6 17 Pb2' +2C1-=PbCl,"(aq) 1.3 Pb2' +3C1- =PbC13- 1.7 Pb2+ + 4C1-=PbC142-1.4 PbCl,(s) =PbC12(aq) -4.8 P*, overall stability constants representing the protolytic equilibria; P, overall stability constant; K, normal stability; KH, Henry's constant; K,,, solubility constant.I(5) and (6) in Table 1 is shown in Fig. 2. The precipitation of magnesium hydroxide can occur only in basic condition (pH>9, when [Mg2'] =0.1 mol dm-3).Fig. 3 shows the solubility curve of niobium hydroxide, which was derived from eqn. (7)-(9) in Table 1. Niobium hydroxide can be formed in the pH range <8.5 ([Nb"] =0.1 mol dm-3) and is most stable at ca. pH 3. Solubility of Metal Carbonates From the equilibrium constants for characterizing the solu- bility and the equilibria of carbonates [eqn. (10)-(14)], the log ci us. pH diagrams of metal carbonates are plotted in Fig. 4. Thermodynamic data for niobium carbonate are not 0 2 4 6 8 10 12 PH Fig. 3 Solubility diagram for the niobium hydroxide system at 25 "C available in the literature because of its low formation con- stant, which is, therefore, neglected in this work.The solubility curves of lead and magnesium carbonates are shown in Fig. 4. Solubility of Metal Citrates Taking into account the dissociation constants for citric acid and the pertinent equilibria [eqn. (15)-(19) in Table 11, the log ci us. pH diagrams of metal citrates are plotted in Fig. 5 and 6. The solubility of niobium citrate is not considered 0 2 4 6 8 1012 here, because no solubility product (Ksojis available probably because of strong tendency of this citrate to form soluble PH species, i.e. it has a high solubility. Fig. 5 and 6 show that the Fig. 2 Solubility diagram for the magnesium hydroxide system at optimum pH domains for the complex formation of lead and 25 "C magnesium citrates are pH 6-7 and pH >6, respectively.J. MATER. CHEM., 1994, VOL. 4 / 0 2 46 81012 PH Fig. 4 Solubility diagram for the lead carbonate (-) and magnesiumcarbonate (---) systems at 25 "C 11111,l IIIII 0 2 4 6 8 10 12 PH Fig. 5 Solubility diagram for the lead citrate system at 25 "C 0 2 4 6 8 1012 PH Fig. 6 Solubility diagram for the magnesium citrate system at 25 "C Solubility of Lead Chloride Fig. 7 shows the predominance area diagram of the Pb2+-C1--H20 system as a function of pH and log molar concentration of chloride at 25"C, as derived from eqn. m r"9. I)Q 12 Fig. 7 Predominance area diagram for the Pb2+-CI--H:0 systemat 25°C (20)-(24) in Table 1. Lead hydroxy species are predominant at pH>9, but lead chloride species predominate at pH<9 (when [Cl-] =1.0 mol dm-3).Also, the difference in equilib- rium constants between PbC1, (s) and other chloride species is so large that rapid formation of PbCl, (s) is expected below ca. pH 9 Experimenta1 The starting reagents were NbCl,, Pb(N03), and Mg(NO3),-6H,O with high purity. First, niobium chloride was dissolved in 30% H202 aqueous solution to prevent the hydrolytic precipitation of niobium hydroxide, and lead and magnesium nitrates were dissolved in HNO, solution. Citric acid was then added to the nitrate mixture, and finally niobium chloride solution to this mixed (nitrate and citrate) solution. At this stage the pH of the solution was 1.5, which could be adjusted up to 6 by adding NH40H solution.Evaporating the water slowly from the solution at SO'C, a colourless colloidal suspension was first obtained, followed by bulky gel with high viscosity. The gel product was prefired at 500 "C for 2 h, ground and reheated at 900 "C for 40 min by the thermal shock technique to minimize the form,ition of the undesired pyrochlore phase and to diminish the PbO vaporization by a short reaction time.20 Results and Discussion In order to determine the optimum pH for preparing the homogeneous sol, it is necessary to simply overlap each solubility diagram for hydroxide, carbonate and citrate. As shown in Fig. 1-3, the hydroxide precipitation of Pb2+ and Mg2+ takes place in the pH range 8-12, while niobium hydroxide precipitation occurs at pH 1-6, implying that homogeneous and stoichiometric PMN precursors cannot be obtained by hydroxide coprecipitation in any pH domain.The carbonate coprecipitation method is also unsuitable due to the unstability of niobium carbonate. If we overlap Fig. 1, 4 and 5, we find that the optimum pH for the formation of the lead citrate complex is 6. where the solubility of citrates is minimized, indicating that the citrate complex is more stable than the hydroxide or carbon- ate. From the predominance area diagram for the system Pb2+-C1--H20 (Fig. 7), PbC1, precipitation would be expected below pH 9. In our experiment, however, lead dichloride was not formed at ca. pH 6 in the presence of citric acid. This result can be explained by the retarding eEect of I L 20 30 40 50 2Bldegrees Fig.8 X-Ray diffraction patterns of the PMN powders: (a)precursor, (b)prepared at 500 "C, (c)prepared at 900 "C. a,PMN; 'I,pyrochlore Fig. 9 Scanning electron micrograph of the PMN powders prepared at 900 "C (40 min) nucleation and crystal growth of lead dichloride in the pres- ence of a complex-forming agent such as citric acid in aqueous solution.21 Similarly, the optimum pH for Mg2+ solution is determined to be 6-8 from Fig. 2, 4 and 6. According to the solubility curve for Nb(OH)5 (Fig. 3) in the absence of citrate, niobium hydroxide precipitation would be very likely in the pH range 3-4. However, in the presence of citric acid as a chelating agent,22.23 the niobium ions would form soluble complexes such as Nb(OH)3(C6H,0,)-and Nb(OH),(C6H40,)-in this pH range, which retard the formation of niobium hydroxide.According to the solubility diagrams considered so far, it could be concluded that the optimum pH is ca. 6 for the formation of metal citrate complexes without forming any secondary phases such as hydroxide, carbonate or chloride. This theoretical consideration was confirmed experimentally, since we could obtain the stoichiometric PMN powder suc- cessfully through the thermal decomposition of a citrate precursor, which was prepared from the improved citrate gel J. MATER. CHEM., 1994, VOL. 4 process by adjusting the pH of the solution to the optimum pH of 6. Fig. 8 shows the diffraction patterns of samples prepared by the thermal shock technique at the given temperatures.The pyrochlore phase is formed less as the reaction tempera- ture is increased from 500 t? 900 "C. The lattice constant was determined to be a=4.039 A by the least-squares method. At 900 "C no pyrochlore phase was observed in the XRD pattern. The particle size and its distribution in the PMN powder, as measured by scanning electron microscopy, is shown in Fig. 9. The powder consists of nearly homogeneous particles with a size of 0.05-0.3 pm. From the results of electron probe microanalysis, the chemical composition of the sample synthe- sized at 900°C for 40min could be formulated as Pb0.93Mg0.36Nb0.6402.89. This research was supported in part by the Korean Science and Engineering Foundation (92-00-25-02).References 1 G. A. Smolenskii and A. T. Agranovskya, Sov. Pltys. Tech. Phys., 1958,3, 1380. 2 V. A. Bokov and I. E. Mylinikova, Sou. Phys. Solid State (Engl. Transl.), 1961,3,613. 3 G. A. Smolenskii, A. I. Agranovskya and S. N. Popov, Sov. Phys. Solid State (Engl. Transl.), 1959, 1, 147. 4 G. A. Smolenskii and A. I. Agranovskya, Son Phys. Solid State (Engl. Transl), 1960,1, 1429. 5 S. L. Swartz, T. R. Shrout, W. A. Schulz and L. E. Cross, J. Am. Ceram. SOC., 1984,6, 311. 6 K. Uchino, S. Nomura, L. E. Cross, S. J. Jang and R. E. Newnham, J. Appl. Phys., 1980,7,1142. 7 S. L. Swartz and T. R. Shrout, Muter. Res. Bull., 1982,17, 1245. 8 M. Inada, Jpn. Natl. Tech. Rept., 1977,27,95. 9 J. P.Guha and H. U. Anderson. J. Am. Ceram. SOC., 1986, 69, C-287. 10 P. Ravindranathan, S. Komarneni, A. S. Bhalla, R. Roy and L. E. Cross, Ceramic Transactions; Ceramic Powder Science II, A, 1988, 1, 182. 11 H. V. Anderson, M. J. Pennell and J. P. Guha, Advances in Ceramics: Ceramic Powder Science, 1987,21,91. 12 J. H. Choy, J. S. Yo0 and S. G. Kang, 1st Int. Symp. on High- T, Superconductivity and Ionic Character in Lajured Compounds, Tokyo Institute of Technology, Yokohama, 1989. 13 J. H. Choy, J. S. Yoo, Y. S. Han, J. Kim, H. K. Lee and H. N. Kim, J. Korean Chem. Soc., 1991,35,275. 14 J. S. Yoo, J. H. Choy, K. S. Han and Y. S. Han, J. Korean Chem. Soc., 1991,35422. 15 J. H. Choy, J. S. Yoo, S. Y. Yoon, T. S. Park. D. Y. Jung and G. Demazeau, Mater. Lett., 1992, 13, 232. 16 M. M. Morel, Priciples of Aquatic Chemistry, John Wiley, New York, 1983. 17 D. R. Turner, M. Whitfield and A. G. Dickson, Geochim. Cosmochim. Acta, 1981,45, 855. 18 W. F. Link, Solubilities of Inorganic and Metal Organic Compounds, American Chemical Society, Washington, DC, 1958, vol. 2. 19 W. Stumm and J. J. Morgan, Aquatic Chemistry, John Wiley, New York, 1981. 20 J. H. Choy, J. S. Yoo, S. G. Kang, S. T. Hong and D. G. Kim, Muter. Res. Bull., 1990,25283. 21 Y. Kitano and D. W. Hood, Geochim. Cosmochim. Acta, 1965, 29,29. 22 V. V. Grigoreva and I. V. Golubeva, Zh. Nrorg. Khim., 1975, 20,941. 23 V. V. Grigoreva and I. V. Golubera, Ukr.Khim. Zh., 1979,45,327. Paper 4/00703D; Received 4th February, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401271

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( 8), 1275-1282 Synthesis and Characterization of Ni,Sb,(OEt),, and its Hydrolysis Products Gunnar Westin and Mats Nygren* Department of Inorganic Chemistry, Arrhenius Laboratory, Stockholm University, S-1069I Stockholm, Sweden The hydrolysis pathways of Ni,Sb,(OEt),, in toluene, ethanol and toluene-ethanol mixtures have been studied. Five intermediate compounds have been isolated from solutions having different H20/Ni2Sb,(OEt),, ratios (h values) with h<6. A detailed description of the routes used in connection with the preparation of Ni,Sb,(OEt),, and of the five intermediates, Ni5Sb30p(OEt),5(HOEt),, Ni7Sb404(OEt)18, Ni6Sb404(0Et)16(HOEt),, NiSb303(0Et), and Ni6Sb,,0,8(OEt),8(HOEt)2are given. The compositim of three of the intermediates compounds have been established from single-crystal structural studies.The suggested compositions of the two others, Ni7Sb404(OEt)18 and NiSb 303(OEt)5, are based on elemental analysis. The chemical compositions of the sol and of the gel formed for h>6, NiSb,OS(OEt),, are estimated from chemical considerations. A brief investigation of the hydrolysis pathways of Sb(OEt), in the same solvents is also included. The great interest in bimetallic alkoxides stems from the possibility of utilizing them as catalysts and in sol-gel pro- cessing of advanced If, in the sol-gel processing of the alkoxides, the hydrolysis is controlled carefully one can obtain gels of high homogeneity, and by applying different preparation routes gels possessing various physical properties and morphologies are f~rmed.~,~ Knowledge about the reac- tions that occur in the precursor solution during the hydrolysis process, which finally leads to a gel, is thus of great importance in designing the properties of the product obtained.Such information is scarce, however, especially for bimetallic alkox- ide systems. Studies of monometallic systems such as Ti(OEt), show that many different molecular 0x0-alkoxides are formed as intermediates before the gelling process takes place.9-" The molecular structures of these intermediates are often very complicated. In bimetallic systems it can be expected that the intermediates have metal ratios different from the precursor alkoxide because the metal ions involved have different affin- ities for oxygen and different coordination requirements. Such shifts in stoichiometry have indeed been observed for the first hydrolysis product in some of the bimetallic alkoxide systems studied.', If oxoalkoxides with various metal ratios are formed in the hydrolysis process, these compounds can find use as precursors for further processing.In a previous article we reported the sol-gel processing of M-Sb ethoxides (M=Mn, Fe, Co and Ni) with an M:Sb ratio of 1:2.13 These samples were hydrolysed using the humidity in the atmosphere, but the hydrolysis pathway was not evaluated. In this article, however, we describe the hydroly- sis pathways of Ni,Sb,(OEt),, dissolved in toluene, ethanol and toluene+thanol mixtures.Six intermediates have been isolated and the preparation routes and some characteristic properties of Ni2Sb4(OEt),6 and the intermediates formed are described. For some of these compounds it has been possible to prepare single crystals and accordingly the crystal structures of these have been The preparation aspects of Ni,Sb,(OEt),, and of the intermediates formed are emphas- ized in this article, while a forthcoming article will deal with the structural relationships between compounds formed in the hydrolysis processes." During the course of this work it was found necessary to make a brief investigation of the hydrolysis pathway for Sb(OEt),, and these results are also included here. This study has been conducted within a research pro- gramme aiming at understanding the reactions involved in sol-gel processing of bimetallic alkoxides.Experimental Fourier-transform infrared (FTIR) spectra were obtained using a Mattson Polaris FTIR instrument with deuteriated triglycine sulfate (DTGS) detectors and KBr or polyethylene (PE) beam splitters for the mid- and far-IR studies. respect- ively. The IR spectra of the solids were recorded with KBr or PE tablets, and the dissolved alkoxides were investigated with a KBr cell. The spectra in the far-IR region were scaled to fit the mid-IR spectra and connected with them at 400-450 cm-l. Owing to the strong background absorption from the solvents, significant spectra of the dissolved alkoxides could be obtained only in the region 690-390 cm-' (there is only one relative strong solvent peak in the region around 460 cm-I).Elemental analysis of the Ni :Sb ratio of hydrolysed samples were routinely performed using a scanning electron micro- scope (SEM) equipped with an energy-dispersive specs rometer (EDS)(JEOL 820 and Link AN 10000)with a detect ion limit of ca. 1 wt.% while the accuracy of the EDS measurements was of the order of 6-8% depending on the morphology of the sample. Two of the bimetallic alkoxides were analysed for their Ni, Sb, 0,C and H contents at Analytische Laboratorien, Gummersbach, Germany. The melting behaviour of samples, sealed in glass capillaries, was studied in a solid-block Gallenkamp melting-point apparatus. Anhydrous NiC1, was used as the Ni source, and Sb(OEt), was prepared from SbCl, using the ammonia route" and then distilled in vacuum. Three solvents were used: (i) toluene, which was dried with Na chips; (ii) ethanol, which was distilleg over CaH,; and (iii) acetonitrile, which was dried with 3 A molecular sieves.After they had been dried, the solvents were saturated with dry argon. The KBr and the PE used in the spectroscopic studies were dried at 240 and 110"C, respect-ively, at a pressure of 0.1 Torr. Synthetic Procedure All syntheses and preparations of samples for spectroscopic studies were performed in a glove-box provided uith dry oxygen-free argon. All glassware was kept at 150"C for >3 h before use and was then transferred into the glove-box while it was still hot.Preparation of Ni2Sb4(OEt)16 Typically, Na (0.855 g, 37.2 mmol) was dissolved ilk 40 ml ethanol and Sb(OEt), (9.58 g, 37.3 mmol) was then added. After 16 h NiCl, (2.41 g, 18.6 mmol) was added under stirring, and the stirring was continued for 24 h. The NaCl formed in the reaction was then allowed to precipitate and after ca. 24 h the green solution was separated from the precipitate. A green crystalline mass was formed upon evaporation of this solution. The yield was in the range 85-90%. The material formed was characterized structurally and found to have the composition Ni2Sb,(OEt)16,'4 SEM-EDS studies also revealed a Ni :Sb ratio close to 1 :2. Preparation of Ni,Sb,O,(OEt),,( HOEt), Ni,Sb,(OEt),, (0.5 g, 0.377 mmol) was dissolved in 2.0 ml of a toluene: ethanol (4: 1) solvent and 0.30 ml of a toluene-ethanol solvent containing H,O (1 mol I-' with respect to H20) was added slowly under vigorous stirring, yielding a yellow solution.The H,O :Ni,Sb,(OEt),, ratio (h value) in the final solution was thus 0.8. Square yellow crystals were formed in a yield of 75-80% upon evaporation of this solution. Single-crystal structure analysis has shown that the composi- tion of this compound is Ni,Sb,02(OEt),,( HOEt),.', Ni,Sb,0,(OEt)l,(HOEt)4 also is formed in high yield by spontaneous decomposition of Ni,Sb,(OEt),, in dilute ethanol solutions. Thus, when a solution formed by dissolving Ni,Sb,(OEt),, (0.5 g, 0.377 mmol) in 10 ml ethanol was kept for 2 days at room temperature, well formed crystals of Ni,Sb302(0Et),,(HOEt), were obtained in a yield of 80%.However, if this solution was kept for longer times, Ni,Sb,O,(OEt),,( HOEt), (see below) was formed, which later decomposed into an unidentified oily product. Preparation of Ni7Sb404(OEt)18 Ni,Sb,O,(OEt)l, was prepared by dissolving Ni,Sb,(OEt),, (0.5 g, 0.377 mmol) in 2.0 ml toluene followed by the addition of 0.43 ml of a tolueneeethanol(4: 1) solution (1 mol I-' with respect to H20 content). This corresponds to an h value of 1.14 in the final solution. After a few minutes this solution was evaporated to half of its original volume and acetonitrile was added intermittently under stirring. Yellow-green crystals were formed in a yield of 75-80%.The SEM-EDS studies revealed that the crystals formed had an Ni: Sb ratio of ca. 7 :4. Elemental analysis of the crystals (expressed in wt.%) yielded the following content [data given in parentheses refer to the calculated content according to the formula Ni,Sb,O,(OEt),,]: C, 24.24 (24.39); H, 5.05 (5.12); 0, 19.94 (19.85); Ni, 22.90 (23.17); Sb, 27.65 (27.47)%. The observed and calculated element contents agree within 1.3%. Preparation of Ni6Sb,04(OEt),6( HOEt), Ni,Sb,O,(OEt),,( HOEt), was formed by dissolving Ni,Sb,(OEt),, (0.5 g, 0.377 mmol) in 2.0ml of a toluene-ethanol (1 :2) solvent followed by slow addition of 0.50 ml of a 1 moll-' H20 solution of toluene-ethanol (1 :2). An h value of 1.33 was thereby achieved in the final solution.After 2 h, this solution was intermittently evaporated till a green crystalline mass was formed. The precipitate was washed with ethanol [in order to remove remaining Sb(OEt),]. The single-crystal structure investigation showed that the composition of this phase, formed with a yield of 75'/0, is Ni,Sb,0,(OEt),,(HOEt)4.'6 As described above, Ni6Sb404(OEt)16(HOEt), can also be formed by spontaneous decomposition of dilute solutions of Ni,Sb,(OEt),,. Crystals of Ni,Sb,O,(OEt),,(HOEt), in a yield of 75% were thus formed when a 10ml ethanol solution containing Ni,Sb,(OEt),, (0.5 g) was aged for 4 days at room tempera- ture. If this solution was kept for longer times the J. MATER. CHEM..,1994, VOL. 4 Ni,Sb,04(0Et)16( HOEt), crystals redissolved and evapor- ation of this solution yielded an oily product.Preparation of NiSb,O,(OEt), NiSb,O,(OEt), was formed by dissolving of Ni,Sb,(OEt),, (0.200g, 0.151 mmol) and 0.08 g of Sb(OEt), in 1.0ml of tolueneeethanol (4: 1) whereupon 0.53 ml of ;z solution of toluene-ethanol (4: 1) (1 moll-' with respect to H20) was slowly added under vigorous stirring. These amounts corre- spond to an Ni: Sb ratio of 1:3 and an h value of 3.5 in the final solution. After 2 h the solvent was evaporated and 0.2 ml of the solvent was immediately added. The amount of solvent was then gradually reduced by evaporation with, as above, small additions of a solvent richer in ethanol to keep the ratio of toluene :ethanol at 4 :1. Green crystals were then formed in a yield of 30-50%.The SEM-EDS studies of these crystals revealed an Ni:Sb ratio of ca. 1:3, and elemental analysis of the compound gave [data given in parentheses refer to calculated content according to the formula NiSb,O,(OEt),]: Ni, 8.28 (8.42); Sb, 52.35 (52.39); 0, 17.29 (18.36); C, 18.12 (17.23); H, 3.68 (3.61)%. These amounts fit the proposed formula to within 6.2%. Preparation of Ni6Sb14018(OEt)18(HOEt), Ni6Sb,4018(0Et)18( HOEt), was formed by slow hydrolysis of Ni,Sb,(OEt),, (0.500g, 0.377 mmol) dissolved in 2.0 ml etha- nol with 1.51 ml aqueous ethanol (1 moll-' with respect to H20) under stirring. This yielded an h value of 4 in the final solution. After 2 h, 3 ml of the solution was evaporated off.Crystallization was performed by intermittent evaporation of the remaining solution over a period of normally <1 week, yielding green crystals. The crystal structure of this com-pound has been determined17 and its composition was found to be Ni6Sb,,018(OEt),,(HOEt),. The green crystals rapidly become opaque in the absence of ethanol. Ni6Sb14018(0Et)18(HOEt)2was formed for 3 <h <5 in pure ethanol in rather low yields with a maximum at h =4 (ca. 30%). Preparation of NiSb,O,(OEt),, Sb,O(OEt), and Sb,O, When Ni,Sb,(OEt),, was dissolved in ethanol-containing solvents green sols were formed rather sharply at h=6. If all the added water is consumed in the hydrolysis, the formed compound ought to have a composition of NiSb,O,(OEt),.Sb,O(OEt), was prepared by hydrolysing a mixture of Sb(OEt), (0.30 g, 1.2 mmol) and 1 ml solvent (toluene-ethanol 4: 1, 1:2 or 0:l) by adding 0.40ml of the corresponding solvent containing H20 (1 moll-' with respect to H,O, yielding h=0.3 in the final solution) drop by drop. After 2 h the mixture was rapidly evaporated to dryness and a white insoluble residue was formed. Assuming that all the added water was consumed in the hydrolysis, the precipitate ought to have the composition Sb,O(OEt),. Sb(OEt), (0.30 g, 1.2 mmol) was dissolved in 1 ml solvent (toluene-ethanol with volume ratios of 4: 1, 1 : 2 and 0: 1) and hydrolysed by addition of 1.78 ml of the corresponding aque- ous solvent (1 moll-' yielding h= 1.5 in the final solution), drop by drop.After 2 h the solvent was evaporated off. The compound formed was identified by its IR spectrum (see below) to be Sb203. Results and Discussion Hydrolysis of Ni,Sb, (OEt),, in Toluene-Ethanol Ni,Sb,(OEt),, was hydrolysed in toluene-thanol mixtures with volume ratios of 1:0, 4: 1, 1 :2 and 0: I. The alkoxide J. MATER. CHEM., 1994, VOL. 4 was dissolved to form 1moll-' solutions with respect to Ni and Sb, and then the same solvent mixture containing water (1 mol I-' with respect to H20) was added. (As water is poorly soluble in toluene the hydrolysis in this case was performed with a toluene-ethanol (4: 1) solution, but after a few minutes most of the ethanol was azeotropically removed by evaporation.) The H20 solutions were added at a rate of ca. 1 equivalent of H20 per Ni2Sb,(OEt)16 per 5 min, drop by drop, with vigorous stirring.After 2 h the solvent was rapidly evaporated off until most of the solvent had been removed. Immediately afterwards, the same solvent was added to form ca. 3 moll-' metal solutions. Crystallization was achieved by slow evaporation of the solvent. When toluene- ethanol solvents are used one might expect a shift of the composition of the solvent during evaporation as the formed gas phase has a different toluene-thanol composition (the azeotrope has a 30:70 composition2') from the original solvent. Even if efforts were made to keep the toluene :ethanol ratio constant during the crystallization process (by intermit- tent addition of a solvent richer in ethanol) some drift away from this ratio might still have occurred.With the precursor Ni2Sb4(0Et)',, the yield of crystals of the various bimetallic ethoxides was high for h<1.5 [h= H,O/Ni,Sb,(OEt),,], while for 1.5 < h -= 5 it was normally < 50% and for h 36 sols were obtained, which formed gels upon evaporation. The hydrolysis experiments were repeated several times and were, with few exceptions, quite reproducible. The present study has focused on the preparation of crystal- line compounds, but non-crystalline materials were also stud- ied. The main tools used to differentiate one intermediate from another were IR studies and SEM-EDS analyses, but we also utilized the fact that the intermediates formed have slightly different colours.The compositions of the compounds formed were determined either from their crystal structures, elemental analysis or from chemical considerations. We will first present some characteristic properties of Ni,Sb,(OEt),, and its hydrolysis products. Next we will discuss the different hydrolysis pathways of Ni,Sb,(OEt),, when dissolved in toluene, ethanol and toluene-ethanol mixtures. Characteristic Properties of Ni,Sb,(OEt),, IR spectra of the solid and dissolved Ni,Sb,(OEt),, [0.17 moll-' toluene-thanol (4: 1) solution] are shown in Fig. 1. Characteristic peaks of the solid compound in the C-0 and M-0 regions (1200-50cm-') are: 1096, 1053, 1036, 879, 604, 553, 497, 493, 457, 425, 374, 292, 271 and 135cm-'.The spectrum of the dissolved compound has an M-0 band maximum at 515 cm-'. The spectra of concen-trated toluene-ethanol solutions also contain a broad band with a maximum at 515 cm-', but in addition they contain weak shoulders at positions where the solid sample exhibits strong and sharp peaks (see also below). Note also that the spectra of a 0.17 moll-' ethanol solution and a 0.17 mol I-' toluene-ethanol (4 : 1)solution are identical. When the compound was heated in the melting point apparatus a change in colour from green to purple was observed at 63-65"C, and the material melted to form a viscous liquid at 96-98 "C. The purple compound formed by heat treatmeni at 70°C for 10min reverted to the original green modification of Ni,Sb,(OEt),, in a few hours.Furthermore, a green solution was formed when a toluene-thanol solvent was added to the purple compound. Subsequent evaporation of thc solvent yielded crystals of Ni,Sb,(OEt),,. Ni,Sb,(OEt),, is very soluble in mixtures of ethanol and toluene C0.4 moll-' both in solutions of pure ethanol and of toluene-thanol(4 : 1)I, in both cases yielding green solutions. In less concentrated solutions Ni,Sb,02(OEt),,( HOEt), and later Ni6Sb404(0Et)16(HOEt)4 are formed in high yields within a few days (see also above). The chemical reactions leading to the formation of these compounds are unknown, but it is known that alkoxides can decompose to yield oxoalkoxides and ethers.2'-22 Ni,Sb,(OEt),, is stable for months in more concentrated solutions and in the solid state.Dissolving Ni2Sb,(OEt),, in toluene initially yielded a solution with a very strong purple colour, but after ca. 1h the solution became faintly green-brown and a purple precipi- tate was formed. The purple precipitate reverted to the green modification of Ni2Sb4(OEt)16 within 1 week when it was stored. Addition of toluene-ethanol solvent to thc purple precipitate resulted in partial dissolution, yielding a green solution which, as above, upon evaporation yielded Ni2Sb4(0Et),,. Examining the purple precipitate in the SEM revealed it to have an Ni: Sb ratio of ca. 1: 2. The IR spectrum of the purple precipitate is shown in Fig. 2. The spectrum is composed of a rather broird band containing a number of peaks, some of which can be ascribed to Ni2Sb4(OEt),, (see Fig.1). The IR spectrum of the purple compound formed by heat treatment is similar, but the contribution from Ni,Sb,(OEt),, is more pronounced. This is in agreement with the latter compound reverting to Ni,Sb,(OEt),, much faster than the former (see above). The two spectra are, however, sufficiently similar to suggest that the purple precipitate and the compound formed by heat treatment are the same. The similarity in the chemical behav- iour of the two samples also supports this conclusion. The IR spectra of Ni2Sb,(OEt),, dissolved in toluene- ethanol solutions (4: 1 and 0: 1) and that of solid Ni,Sb,(OEt),, differ greatly, suggesting that the molecular -% 5 es:13 d --%, 4 Fig.1 IR spectra of Ni,Sb,(OEt),,: (a) solid and (b) dissolved in Fig. 2 IR spectrum of the purple compound formed by precipitation toluene-ethanol (4 : 1) of Ni,Sb,(OEt),, in toluene structure of Ni,Sb,(OEt),, is not the same in solution and in the solid state. However, the IR spectra of concentrated ethanol solutions contain weak shoulders which coincide with the strongest peaks occurring in the spectrum of the solid compound. SnSb2(OR),23 and PbSb,(OR)824 have been reported to occur as monomers in refluxing parent alcohol or in benzene. Ni alkoxides are known to have either tetrahedral or octa- hedral coordination around the Ni ions, depending on the bulk and electron-donating properties of the ligands.The Ni ions in Ni,Sb,(OEt),, are octahedrally c00rdinated.l~ Ni(OR),," NiNbz(OR),2 and NiTa2(OR)1226 with ethoxy ligands exhibit octahedrally coordinated Ni ions, while isopro- poxy ligands yield tetrahedral coordination. However, Ni-isopropoxy compounds dissolved in electron-donating sol- vents like pyridine and THF yield alkoxides with octahedrally coordinated Ni ions. On the other hand, the Ni ions in NiAl,(OEt), are tetrahedrally coordinated in the solid state but octahedrally coordinated in ethanol solution, while the corresponding isopropoxy compound contains tetrahedrally coordinated Ni ions both in the solid state and in isopropyl alcohol solutions.27 These observations seem to suggest that the Ni ions are always tetrahedrally coordinated by isopro- poxy ligands but can be either octahedrally or tetrahedrally coordinated by ethoxy ligands.In an excess of ethanol the Ni ions seem to prefer octahedral coordination, but at some concentration level a small shift in the concentration of ethanol may cause a change in coordination from octahedral to tetrahedral or vice versa. Note also that the colour of compounds with Ni in an octahedral environment is typically green.25-28 Based on these considerations, it seems plausible that: (i) the Ni,Sb,(OEt),, occurs mainly as monomeric units in solution, but in concentrated solutions (see above) minor amounts of dimeric units might also be present; (ii) the Ni ions are octahedrally coordinated in the solution. An octahedral environment around the Ni ions in the monomer is achieved by the addition of two ethanol units. This implies that the composition of the monomeric unit in the solution ought to be NiSb,(OEt),(HOEt),.Considering the proposed composition of the monomer and recalling that Ni ions can be tetrahedrally coordinated and that the purple colour is typical of Ni in tetrahedral c~ordination,~~-~~it is tempting to suggest that the purple compound, which is formed in the absence of ethanol, contains tetrahedrally coordinated Ni ions. Characteristic Properties of Ni5Sb302(OEt)15(HOEt)4 The IR spectra of Ni5Sb302(0Et)15(HOEt), as a solid and dissolved in a toluene-ethanol (4: 1) solvent (0.07 moll-') are shown in Fig. 3. Characteristic peaks of the solid com- pound are: 1104, 1060, 894, 594, 534, 490, 414, 389 and 261 cm-'.A broad band ascribed to OH stretching is observed around 2500 cm-', with a maximum at 2545 cm-'. The spectrum obtained for the solution exhibited M-0 peaks at 590, 532,488,420 and 390 cm-' and the OH stretching band has its maximum at 2540cm-'. The similarities of the IR spectra of the solid and dissolved samples, as well as the observation that both the solid and the-solution are yellow indicate that the molecular structures in the solid staie and in solution are the same. The colour might be ascribed to the occurrence of five- and six-coordinated Ni ions in the str~cture.'~ When heated, Ni5Sb30,(OEt),,( HOEt), became dark and decomposed around 100-105 "C, yielding a grey-green solid product and liquid Sb(OEt),.Ni,Sb,02(OEt),,(HOEt), is soluble in toluene-ethanol J. MATER. CHEM., 1994, VOL. 4 '1200 . 1000 800 600 400 200 wavenurnber/cm-' Fig. 3 IR spectra of Ni,Sb,O,(OEt),,(HOEt),: (a) solid and (b)dis-solved in toluene-thanol(4 :1) (4: 1, 0.07 mol l-'), yielding an orange solution, but is very sparingly soluble in ethanol, giving a very faintly yellow solutions. The compound is stable for months in the solid state. Characteristic Properties of Ni,Sb404(OEt)18 The IR spectra of solid Ni,Sb,O,(OEt),, and of the substance dissolved in toluene and toluene-thanol (4 :1) solutions are shown in Fig.4. Characteristic peaks in the spectrum of the solid compound are: 1110, 1096, 1062, 895, 636, 515,458 and 259cm-'.In this case no OH band was observed. Characteristic peaks in the 690-390 cm-' region of the spectra of the dissolved samples in toluene solution were found at 635, 598 and 516 cm-' and at 633, 596 and 507 cm-' for the toluene-ethanol solution. Although the spectra of the solid and the dissolved compound exhibit some differences, the similar overall appearance of the spectra suggests that the molecular structure of the solid and dissolved samples are similar. No distinct melting point could be determined, but the compound became black at 195-200 "C. Ni7Sb,0,(OEt),8 is very soluble both in pure toluene and in toluene-thanol (4:l) and 0.2moll-' solutions can be prepared in both cases. Ni7Sb40,(0Et)18 is very sparingly soluble in acetonitrile.Although the crystal structure of this compound has not yet been determined, its yellow-green colour suggests that some of the Ni ions might be five-coordinated. The structure of a related compound, Mn,Sb,O,(OEt),,( HOEt)2,29 has been determined by single-crystal X-ray diffraction. The structure analysis revealed that this compound contains both five- and six-coordinated Mn ions. It has the same M :Sb :0:OEt ratio as Ni,Sb,O,(OEt),,, but contains two additional ethanol ligands. Furthermore, the IR spectra of the two solid com- 1200 1000 800 600 400 200 wavenumberlcm-' Fig. 4 IR spectra of Ni,Sb,O,(OEt),,: (a)solid,(b)dissolved in toluene and (c) dissolved in toluene-ethanol (4 : 1) J. MATER.CHEM., 1994, VOL. 4 pounds are similar. These observations suggest that the struc- tures of the two compounds ought to have similar features. Characteristic Properties of Ni,Sb,O,(OEt),,( HOEt), The IR spectra of Ni,Sb,O,(OEt),,(HOEt), in the solid state and dissolved in a toluene-ethanol(4: 1,O.l moll-l) are given in Fig. 5. Characteristic IR peaks of the solid compound are: 1108,1060,893,646,634,610,579,522,471,278and 261 cm-'. A broad OH stretching band around 2500cm-' with a maximum at 2520cm-' was observed. The spectrum of the dissolved sample exhibits M-0 peaks at 646, 609, 576, 541, 505 and 460cm-' and a broad OH stretching band with a maximum at 2570 cm-l. The main features of the M-0 bands of the solid and the dissolved compound are similar, but there are some differences in the region 400-550 cm-'.Thus the peak at 521 cm-l in the spectrum of the solid seems to be shifted to higher wavenumbers (541cm-') for the dissolved compound, and the peak at 471 cm-' seems to be split in two; one with a maximum at 505 cm-' and another at 460 cm-'. As mentioned above, the solvent exhibits an IR peak at the latter position. However, studying the IR spectra of solutions containing different amounts of solvent confirmed that the dissolved compound has a peak at 460cm-'. The differences between the spectra of the solid and the dissolved compound may be explained by some rearrangement of ethoxy groups in the molecule or by dissociation of the molecule. The latter reaction mechanism has been observed for Pb,Nb,O,(OEt),,, which has a structure resembling that of Ni6Sb404(OEt)16(HOEt),, containing dimers of Nb(OEt)5.30If a similar dissociation mechanism occurs in this case, Sb(OEt), ought to be formed.However, the IR spectrum does not reveal the presence of Sb(OEt),. On the other hand, Sb(OEt), is not easily detected in the presence of Ni,Sb,O,(OEt),,( HOEt), since the characteristic bands of Sb(OEt), overlap with the M-0 band of Ni6Sb,04(0Et),,(HOEt)4. However, the general features of the IR spectra of solid and of dissolved Ni6Sb404(OEt)16(HOEt), are similar enough to indicate that some structural features of the molecule ought to be present both in the solid state and in solution. It was not possible to determine a distinct melting point.A change of colour from green to yellow occurred in the range 50-80 "C, and the sample became dark around 150-170 "Cin connection with a loss of Sb(OEt),. Ni,Sb,O,(OEt),,( HOEt), is very soluble in toluene-etha- no1 (4:1, 0.36moll-l) but only sparingly soluble in ethanol. The compound is stable for months in the solid state in the presence of ethanol. In ethanol-free environments, however, the crystals turn yellow and decompose within a few minutes. Ni,Sb,0,(OEt)l,(HOEt)4 is formed in high yields when r prepared as described above. In solvents containing <20 vol% ethanol Ni,Sb,O,(OEt),, is formed instead. Attempts to prepare Ni7Sb4O4(0Et),, from Ni,Sb,O,(OEt),,( HOEt), and vice zlersa by dissolving the former in toluene or the latter in toluene-ethanol I 1 :2) and then crystallizing the compounds were unsuccessful and non- crystalline materials were formed, Characteristic Properties of an Ni-Sb Ethoxide having an Ni :Sb Ratio of ca.1 :3 Hydrolysis of Ni,Sb,(OEt),, in toluene-ethanol (4 1) in the range 1.5<h <3 yielded pale green crystals after more than 1 week; according to their IR spectra they were different from all other compounds identified in this system. The maximum yield (20-30Y0)was obtained at h =2.5. The SEM-EDS studies revealed that the crystals had an Ni:Sb ratio of ca. 1 :3. This implies that the remaining solution ought to be more Ni-rich than the starting one. In this connection note that crystallization from a solution containing Ni and Sb in a ratio of 1 :3 did not improve the yield very much.An IR spectrum of this solid compound is given in Fig. 6 and contains characteristic peaks at 1098, 1053, 891, 690, 650, 627,570,550,501,355and 275 cm-'. No OH stretching band was observed. This compound changed colour to yellow and became opaque at 55-60°C, and at higher temperatures a slow decomposition was observed. The compound is very soluble in toluene-ethanol (4: l), yielding a green solution. Characteristic Properties of NiSb,O, (OEt), The IR spectra of solid NiSb,O,(OEt), and of the dissolved compound in toluene+thanol (4:1) are shown in Fig. 7. Characteristic peaks of the solid sample are: 1098,1053, 891, c 1200 1000 800 600 400 200 wavenum ber/cm-' Fig.6 IR spectrum of an Ni-Sb ethoxide with an Ni: Sb ratio of 1:3 formed in toluene-ethanol (4 : 1) at 1.5<h<3 r 1200 1000 800 600 400 260 ' 1200 1000 800 600 400 200 wavenum ber/cm-' waven umber/cm-' Fig. 5 IR spectra of Ni6Sb,04(OEt)16(HOEt)4:(a)solid and (b) dis-Fig. 7 IR spectra of NiSb,O,(OEt),: (a) solid and (b) dissolved in solved in toluene-ethanol (4:1) toluene-ethanol(4 :1) J. MATER. CHEM.. 1994, VOL. 4 706, 653, 613, 529, 500, 367 and 270 cm-'. No OH stretching band was observed. The M-0 peaks of the dissolved sample were found at 654, 613 and 528 cm-'. The spectra of the dissolved and solid samples are very similar, suggesting that the molecular structure of the solid persists in solution.The compound melts at 136-150°C and becomes black at 195-200 T.NiSb,O,(OEt), is very soluble in toluene-ethanol (4:l),yielding a green solution. The crystallization rate and the yield are low, which might be explained by the very high solubility of this compound in toluene-ethanol. Thus the crystallization does not start until the mother liquor has become highly viscous. When the starting solution contained Ni and Sb in a ratio of 1:2, NiSb,O,(OEt), is formed throughout the hydrolysis region 3 <h <5, but the crystallization rates and the yields are lower compared with the findings above. It thus seems that an excess of Ni ions in the solution inhibit the crystallization process, possibly due to formation of more Ni-rich Ni-Sb ethoxides.Characteristic Properties of Ni,Sb,,018 (OEt),,( HOEt), An IR spectrum of the solid modification of Ni,Sb14018(0Et)18(HOEt)2is given in Fig. 8, with character- istic peaks at 1098, 1054, 892, 708, 670, 649, 588, 541, 477, 388,357,297,252 and 151 cm-'. A weak, broad OH stretching band with a maximum around 3000-2500cm-' was also observed in the spectrum. The compound melts around 140 "C to form a green liquid. The Ni :Sb ratio in Ni6Sb14018(OEt)18( HOEt), is some- what lower than that in the starting solution. The solution thus ought to contain minor amounts of another alkoxide, richer in Ni than Ni,Sb,,O,,(OEt),,( HOEt),. However, we have not been able to obtain crystals of any composition other than Ni,Sb,,O,,(OEt),,( HOEt), in the region 3 <h<5.Characteristic Properties of NiSb,O,(OEt), The IR spectra of the gels of nominal composition NiSb,O,(OEt), produced in 0 :1,1: 2 and 4 :1toluene-ethanol solutions were similar and exhibited very broad M-0 bands (the IR spectrum of the gel produced in a 4: 1 solvent is shown in Fig. 9). Characteristic IR peaks of the gels were found at 1096, 1050, 888 and 656 cm-'. No OH stretching band was observed, neither were any peaks stemming from toluene. The absence of OH groups indicates that the peaks at 1096, 1050 and 888 can be attributed to ethoxy groups bonded to Ni and/or Sb ions. Hydrolysis of Sb(OEt), in Toluene-Ethanol Sb(OEt), solutions (1moll-') in toluene+thanol with volume ratios ranging from 0: 1 to 4: 1 were hydrolysed by #-'I 1 8 C (d-2 za (d I wavenurnber/crn-' Fig.8 IR spectrum of solid Ni,Sb,,O,,(OEt),,( HOEt), 8 c a 42 0u) n (d 1200 1000 800 600 400 200 wavenurnberkrn-' Fig.9 IR spectrum of a gel formed by hydrolysis of Ni,Sb,(OEt),, dissolved in toluene-ethanol (4 : 1) at h =6 slow addition of a 1moll-' solution of H,O in toluene- ethanol. The hydrolysis of Sb(OEt), followed the same pathway in all toluene-ethanol mixtures studied. Insoluble hydrolysis products were formed for all h values. Two products were identified, one at h=0.3 and one at h= 1.5. The first product coexists with Sb(OEt), (Fig. 10) up to h =0.3 and for 0.3 <h <1.5 two hydrolysis products coexist.For h> 1.5 only Sb203 was formed, and this hydrolysis seems to be quantitative. Characteristic Properties of Sb,O(OEt), and Sb,O, The IR spectrum of Sb,O(OEt), (Fig. 11)shows characteristic peaks at 1098, 1045, 892, 630, 555, 307, 196 and 159cm-'. No OH stretching band was found. Corresponding studies for O<h<0.33 showed that the hydrolysis product in this h range is composed of two phases, namely liquid Sb(OEt), and the solid phase formed at h=0.33. The IR spectrum of the compound formed at h = 1.5 is identical with that of senarmontite (Sb203), which is built up from Sb406 units.,' Corresponding studies for 0.33<h <1.5 r 1200 1000 800 600 400 200 wavenurn ber/cm-' Fig. 10 IR spectrum of liquid Sb(OEt), 1200 1000 800 600 400 200 waven umbe r/cm-' Fig.11 IR spectrum of Sb,O(OEt), J. MATER. CHEM., 1994, VOL. 4 r 1200 1000 800 600 400 200 waven umbedcm-' Fig. 12 IR spectrum of Sb20, revealed that the precipitates consisted of mixtures of senarmontite and Sb,O(OEt),. Hydrolysis Pathway of Ni2Sb4(OEt),,in Toluene These studies were limited to h<1.5. As mentioned above, a purple precipitate starts to form within 1h when Ni,Sb,(OEt),, is dissolved in toluene, implying that the hydrolysis studies had to be performed immediately after the purple solution was formed. Unreacted Ni,Sb,(OEt),, was found in the range h< 1.14 and in larger amounts at lower h values. In addition, Ni,Sb,O,(OEt),, and liquid Sb(OEt), were found for 0.5 <h< 1.5.When a stoichiometric amount of water was added (h= 1.14), Ni7Sb404(OEt)18 and Sb(OEt), were the only species present, which suggests that the following reaction occurs: 7Ni,Sb,(OEt),, +8H20+2Ni,Sb,04(0Et),, + 20Sb(OEt), +16EtOH Hydrolysis Pathway of Ni2Sb4(OEt)16in Toluene-Ethanol (4:1) Ni,Sb,O,(OEt),,( HOEt), was found along with Ni,Sb,(OEt),6 and liquid Sb(OEt), for 0.1<h<0.8, while only Ni,Sb,02(OEt),,(HOEt), and liquid Sb(OEt), were formed at h =0.8. This observation suggests that the following reaction occurs at h =0.8: 5Ni,Sb,(OEt),, +4H,O -+2Ni,Sb,O,(OEt),, (HOEt), + 14Sb(OEt), Between h=0.8 and 1.33, one obtains a mixture of Ni,Sb,O,(OEt),,(HOEt), Ni,Sb,0,(OEt),,(HOEt)4 and Sb(OEt),, while for h=1.33 the solution contains only Ni6Sb40,(0Et),,( HOEt), and Sb(OEt),.The reaction that occurs at h= 1.33 can thus be written: 3Ni,Sb,(OEt),6 -I-4H2O +Ni$b404(OEt)16( HOEt), + 8Sb(OEt),+4EtOH Ni,Sb40,(0Et),,(HOEt), was found for h values up to ca. 1.5. Between h= 1.5 and h= 3 the crystallization yield was low (20-30%) with a maximum around h =2.5. The composi- tion of the crystals formed in this interval is not known, but SEM-EDS analysis indicates that they have an Ni: Sb ratio of 1:3. The comparatively large Sb content of these crystals suggests that one or more Ni-rich compounds ought to occur within this h range. The IR studies of non-crystalline materials formed within the range indicate the presence of another compound in relatively large quantities. So far we have not been able to isolate and characterize it.In the range 3 <h <5, NiSb,O,(OEt), could be crystallized with maximum yield at 1281 h=3.5. At h36, a sol is formed which yields a gel upon evaporation of the solvent. The hydrolysis pathway of Ni2Sb,(0Et),, in toluene-etha- no1 (2: 1) is identical to that found for toluene-ethanol (4: 1) solutions, but in the former case no crystals are formed for 1.5<h<3. In conclusion, our findings so far indicate that the hydrolysis pathway for Ni,Sb,(OEt),, in toluene-ethanol solutions involves the following species: Ni,Sb,(OEt),,+Ni,Sb302(OEt)15(HOEt), +Sb(UEt), -+ Ni,Sb,O, (OE t ),, (HOE t ), +Sb (OEt ), +NiS b303 (OE t)5 + NiSb,O,(OEt), (sol) Hydrolysis Pathway of Ni,sb,(OEt),, in Ethanol The first part of the hydrolysis pathway of Ni,Sb,(OEt),, in ethanol resembles that found for the toluene-thanoh solvents.Thus, the hydrolysis of Ni,Sb,(OEt),, in ethanol in the range 0 <h<1.5 yielded Ni,Sb,O,(OEt),,( HOEt), at lower h values, with a maximum yield at h=0.8, and at higher water contents Ni6Sb40,(0Et),,(HOEt), in a maximum yield at h=1.33. In the range 1.5<h<3 no compounds could be isolated or identified. In the range h<3 the Ni-containing alkoxides are very sparingly soluble. For 3.0 <h <5 Ni6Sb140,,(OEt),,( HOEt), is formed, with maximum yield at h=4. At even higher h values, ha6, a sol of the nominal composition NiSb,O,(OEt), is formed. The hydrolysis pathway for Ni,Sb,(OEt),, in ethanol involves the following species: Ni,Sb,(OEt),,+Ni,Sb,02(OEt)15(HOEt), +Sb(OEt),+ Ni,Sb,O,(OEt),,( HOEt), +Sb(OEt),+ Ni,Sb,,O,, (OEt),, (HOEt), -+NiSb,O, (OEt), (sol I Conclusions The hydrolysis pathways of Ni,Sb,(OEt),, in toluene.ethanol and toluene-thanol mixtures have been surveyed. Six inter- mediate compounds were identified. It was found that Ni-rich Ni-Sb oxoethoxides were formed in almost quantitative yields together with Sb(OEt), for small additions of H,O. This indicates that Ni2+ ions are more susceptible to hydrolysis than Sb3+ ions. When more water was added (h> 1.5) free Sb(OEt), was no longer observed, neither were ariy of its hydrolysis products. Instead, various Ni-Sb oxoethoxides were formed.It was also found that the hydrolysis pathways differ somewhat depending on the solvent used, but sols were formed for ha6 in all solvents used. Note also that in most cases the molecular structures of the Ni-Sb oxoethoxides in the solid and dissolved state appeared to be similar. The formation of different Ni-Sb oxoethoxides upon hydrolysis makes new compounds with different Ni :Sb ratios accessible for further processing. Many of these corn pounds, especially those formed at low h values, are obtained in high yield. Single-crystal structural studies of some of the intermediates formed have already been accomplished, while corresponding studies for the others are in progress. The structural features of the Ni-Sb oxoethoxides, obtained in conjunction with the hydrolysis pathways described above, will be discussed in a forthcoming article.', The stimulating and fruitful collaboration with Prof.R. Norrestam and Dr. U. Bemm is gratefully acknowledged. 1282 J. MATER. CHEM.. 1994, VOL. 4 This study was financially supported by the Swedish National Research Science Council. 15 16 U. Bemm, R. Norrestam, M. Nygren and G. Westin, Inorg. Chem., 1992,31,2050. U. Bemm, R. Norrestam, M. Nygren and G. Westin, Inorg. Chem., 1993,32,1597. References 17 18 19 U. Bemm, R. Norrestam, M. Nygren and G. Westin, manuscript in preparation. U. Bemm and R. Norrestam, manuscript in preparation. Haslam, U.S. Pat. 2 839 554, 1958; Chem. Ahstr., 1959,53, 1144e. 1 2 3 4 5 6 7 8 9 10 11 12 K.G. Caulton and L. G. Hubert-Pfalzgraf, Chem. Rev., 1990, 90,969. M. Guglielmi and G. Carturan, J. Non Cryst. Solids, 1988,100, 16. L. G. Hubert-Pfalzgraf, New J. Chem., 1987, 11,663. C. Sanchez and J. Livage, New J. Chem., 1990,14,513. G. Yi and M. Sayer, Ceram. Bull., 1991,70,1173. D. C. Bradley, Chem. Rev., 1989,89,1317. S. Livage, M. Henry and C. Sanchez, Prog. Solid State Chem., 1988, 18,249. C. J. Brinker and G. W. Scherer, Sol-Gel Science, Academic Press, New York, 1990. V. W. Day, T. A. Eberspacher, W. G. Klemperer, C. W. Park and F. S. Rosenberg, J. Am. Chem. SOC., 1991,113,8190. Y. N. Chen, W. G. Klemperer and C. W. Park, Better Ceramics through Chemistry V, Proc. Material Research SOC. Symp. 1992, in the print. R. Schmid, A. Mosset and J. Galy, J. Chem. Soc., Dalton Trans., 1991,1999. C. D. Chandler, C. Roger and M. J. Hampden-Smith, Chem. Rev., 1993,93,1205. 20 21 22 23 24 25 26 27 28 29 30 Handbook of Chemistry and Physics, ed. R. C. Weast, CRC Press, Boca Raton, 67th edn., 1986-1987, p. D-15. A. I. Yanovsky, M. I. Yanovskaya, V. K. Limar. V. G. Kessler, N. Ya. Turova and Y. T. Struchkov, J. Chem. SOC., Chem. Commun., 1991,1605. N. Ya Turova, V. G. Kessler and S. I. Kucheiko, Polyhedron, 1991, 10, 2617. T. Athar, R. Bohra and R. C. Mehrotra, Main Group Met. Chem., 1987, 10, 399. T. Athar, R. Bohra and R. C. Mehrotra, Synth. React. Inorg. Met.- Org. Chem., 1989,19,195. B. P. Baranwal and R. C. Mehrotra, Aust. J. Chem., 1980,33, 37. R. Jain, A. K. Rai and R. C. Mehrotra, Z. Nutuforsch., Teil B, 1985,40,1371. R. C. Mehrotra and J. Singh, Can. J. Chew., 1984.62,1003. G. Garg, R. K. Dubey, A. Singh and R. C. Mehrotra, Polyhedron, 1991,10,1733. U. Bemm, R. Norrestam, M. Nygren and G. Westin, manuscript in preparation. R. Papiernik, L. G. Hubert-Pfalzgraf, J-C. Daran and Y. Jeannin, J. Chem. SOC., Chem. Commun., 1990,695. 13 G. Westin and M. Nygren, J. Muter. Sci., 1992,27, 1617. 31 C. Svensson, Acta Crystallogr., Sect. B, 1975,31,2016. 14 U. Bemm, R. Norrestam, M. Nygren and G. Westin, Acta Crystallogr., Sect. C, submitted. Paper 4/01289E; Received 3rd March, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401275

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994,4(8), 1283-1287 Hydrothermal Modification of Electrocatalytic and Corrosion Properties in Nanosize Particles of Ruthenium Dioxide Hydrate H. Neil McMurrayt Chemistry Department, University of Wales, Swansea, Singleton Park, Swansea, UK SA2 8PP Preformed hydrosols comprising 38 nm diameter particles of amorphous ruthenium dioxide hydrate are subject to hydrothermal modification at temperatures between 100 and 225 "C. Hydrothermally induced changes in sol composition have been characterised by transmission electron microscopy (TEM), UV-VIS spectrophotometry, thermogravimetric (TG) analysis and X-ray powder diffraction (XRD). Hydrothermally modified sols show substantial reductions in oxide hydration but show no evidence of increased crystallinity, Ostwald ripening, particle aggregation or agglomeration.Hydrothermally induced changes in electrocatalytic and corrosion behaviour have been characterized by using sols to catalyse the oxidation of water to oxygen by cerium(iv) ions, and measuring both evolved oxygen and ruthenium tetraoxide produced by oxidative corrosion of the catalyst. Sols hydrothermally modified at 3 200 "C show a 20-fold decrease in corrosion and a >5-fold decrease in electrocatalytic rate; these changes are attributed to increased oxygen bridging between surface Ru atoms resulting from hydrothermally induced condensation reactions between surface hydroxy groups. There is considerable contemporary interest in the technologi- cal potential of ceramic and composite materials prepared from particles in the nanometre size range (1-100 nm).'-' These 'nanophase' and 'nanocomposite' materials have been forecast to exert a major impact in the fields of catalysis, sensing, optics, electroceramics and structural ceramic mate- rial~.~,~,~However, there is significant difficulty in preparing and processing nanosize ceramic particles without the irrevers- ible formation of particle aggregates and agglomerates.' A method has recently been reported for the preparation of stable hydrosols of amorphous ruthenium dioxide hydrate (RuO,.xH,O) comprising substantially monodisperse, spheri- cal and pristine submicrometre particles6 In this paper a hydrothermal treatment is described which dramatically modi- fies hydration, electrocatalytic activity and susceptibility to oxidative corrosion in preformed RuO,.xH,O sols, without altering particle size or the state of particle dispersion.Both Ru02-xH20 and anhydrous Ru02 are technologically important ceramic metal oxides7 which have found extensive application as electronic conductors8 and electrocatalytic anode materials for the electrolytic evolution of chlorine and oxygen from aqueous solution^.^ Ru02.xH20 exhibits superior electrocatalytic properties for anodic oxygen evol- ution in terms of minimum overpotential and Tafel slope," but suffers the serious drawback of undergoing anodic cor- rosion to water soluble higher valence Ru It is known that anodic corrosion in RuO,.xH,O powder samples may be controlled by heating the powders in air (calcination) to limit the extent of hydration, but calcination results in particle coarsening and irreversible particle agglomeration through sintering and crystalline growth, thus reducing the effective surface area for electrocatalysis.10911 Electrocatalytic and corrosion performance in untreated and hydrothermally modified RuO,.xH,O sols were compared by exploiting the observation that individual particles in aqueous RuO,-xH,O dispersions behave as microelectrodes and as such will catalyse the oxidation of water by suitably oxidizing redox couples.'3-16 Sols were used to catalyse the oxidation of water by cerium@) ions and measurements were made of evolved oxygen and of ruthenium tetraoxide (RuO,) produced through anodic corrosion of the catalyst.The ability -f Current address: Department of Materials Engineering, University of Wales, Swansea, Singleton Park, Swansea. to compare electrocatalysis by RuO,-xH,O samples in a constant state of dispersion makes possible a more exact study of the influence of oxide hydration upon electrocatalytic activity. Experimental RuO, was prepared by a literature method;17 all other chemi- cals were supplied by BDH in AnalaR purity. The formation of RuO,*xH,O sol was initiated by adding 90 cm3 of mol dmA3 aqueous sodium nitrite (NaNO,) to 1.91 dm3 of 7.92 x mol dmP3 aqueous RuO,. Rapid partial reduc- tion of RuO, by NO, ions results in the precipitation of Ru0,-xH,O nuclei which then grow by catalysing the reduction of the remaining RuO, by water over ca.16 h. The kinetics and mechanism of sol development are discussed in detail elsewhere.6 Hydrothermal modification of the as-prepared Ru0,-xH,O sol at temperatures 2 125 "C was carried out using a Linsey and Baskerville high-pressure autoclave. Aliquots of sol (200cm3) were sealed in 300cm3 Pyrex glass bulbs, and the autoclave two-thirds filled with water to ensure equalization of pressures across the bulb wall. Treatments were carried out over 16 h, with ca. 10 h at the specified maximum temperature. Treatments at 75 and 100°C were conducted simply by placing sealed aliquots of sol in a temperature-controlled oven overnight. UV-VIS spectra of as-prepared and hydrothermally modi- fied Ru0,-xH,O sols were recorded using a Perkin-Elmer Lambda 3 spectrophotometer.TEM photographs of colloidal RuO,.xH,O particles were obtained using a JEOL 120C TEM-SCAN instrument.6 Solid samples of RuO,.xH,O for X-ray studies and TG were obtained by the coagulation of untreated and hydrother- mally modified sols using aqueous magnesium sulfate, fol- lowed by washing to remove salt and drying in air at 20°C. TG was performed using a Stanton Redcroft TG-750 instru- ment. Ru02.xH20 samples (typically 6 mg) were heated from ambient temperature to 900 "C at a rate of 20°C min-'in a flow of N, gas (40 cm3 min-I). Powder XRD patterns were recorded with Cu-Ka radiation using a Phillips PW 1720 generator and Debye-Scherrer camera.deb ye-Scherrer films were analysed using a CCD camera and Vilber Lourmat digital microdensitometry system. 1284 The catalysis by Ru02-xH20 sols of the oxidation of water by Ce" ions was studied using the atmospheric pressure gas- line apparatus shown in Fig. 1. The reaction vessel (A) was initially charged with 90 cm3 of sol and the RuO, trap (B) filled with 100 cm3 of 0.1 mol dm-3 sodium hypochlorite (NaOC1) in 1 mol dmP3 NaOH. Nitrogen was passed continu- ously through the gas line at 200 cm3 min-I. The test reaction was initiated by injection, via septum B, of 10 cm3 of 0.5 mol dmP3 (NH,),Ce(NO,), in 5mol dm-, HNO,. Evolved oxygen was measured directly using a Rank Brothers oxygen membrane polarographic detector (0,-MPD) (D).18 The volatile RuO, corrosion product was reduced to involatile perruthenate (RuO,-) in the RuO, trap." RuO,-was subse- quently determined spectrophotometrically (&298=2162 dm3 rno1-I cm-1).20 The detailed operation of the gas-line appar- atus is described elsewhere.12 All experiments were conducted at 20°C.Results The as-prepared RuO,.xH,O sol exhibited a monotonic decrease in UV-VIS absorbance with increasing wavelength which is typical of colloidal RuO~.XH,O.'~*'~ Visualized by TEM, sol particles appeared substantially spherical and monodisperse.6 The number-average mean particle diameter calculated from 200 TEM image diameter measurements was 38 +4 nm. Fig. 2 shows the TG weight loss curve for solid Ru02*xH20 samples derived from the as-prepared sol.The TG weight loss between ambient and 600°C is given in Table 1, no further weight loss was observed at temperatures Fig. 1 Schematic diagram of the atmospheric pressure gas-line apparatus used to evaluated the electrocatalytic performance of RuO,xH,O sols. A, Reactor (250 cm3 dreshel bottle); B, rubber septum; C, RuO, trap (125 cm3 dreshel bottle); D, 0, membrane polarographic detector; E, chart recorder. 2o th8 15-cn -0 E .-10-03 5-I I I I I I 25 100 200 300 400 500 600 TI'C Fig. 2 TG weight-loss curves for RuO,.xH,O samples derived from untreated and hydrothermally modified RuO,.xH,O sols. (a) Weight loss for untreated sol. Other curves show weight loss for sols hydrothermally modified at (b) 125, (c) 150, (d) 175, (e) 200 and (f) 225 "C.J. MATER. CHEM., 1994, VOL. 4 Table 1 TG and catalytic characteristics of as-prepared and hydro- thermally modified colloidal RuO, * xH,O hydrothermal TG wt. loss oxygen yield fractional T/"C 20-600 "C (%) ( mol) corrosion' as prepared 14.5 1.20 0.38 75 15.5 1.21 0.38 100 15.0 1.23 0.30 125 13.5 1.23 0.13 150 12.8 1.23 0.08 175 11.5 1.23 0.05 200 10.2 1.23 0.02 225 9.8 1.25 0.02 "Calculated as (mol RuO, evolved)/(mol RuO, * xH,O initial). >6OO"C. Similar samples were found to be essentially amorphous to X-rays. Two extremely weak and broad diffrac- tion lines were visible at positions consistent with reflections from the [211] and [loll crystal planes of tFtragona1 RuO,, which exhibit d-spacings of 1.685 and 2.55 A, but there wa,s no evidence of the intense [1lo] diffraction line at 3.17 A which would be expected for crystalline RuO,.The observed diffraction lines were insufficiently clear for the measurement of linewidths by microdensitometry. Hydrothermal treatment at temperatures up to 225 "C produced no significant changes in the UV-VIS absorbance-scattering spectrum of Ru02.xH20 sols, nor was there any increase in Tyndal effect, or any other indication of particle coarsening or loss of colloidal stability. Both the as-prepared and hydrothermally modified sols remained without any sign of coagulation or sedimentation for over six months. TEM investigation of the hydrothermally modified sols revealed no measurable changes in the size or shape of the RuO,-xH,O particles.Table 1 lists the total TG weight loss between ambient temperature and 600 "C for solid RuO,-xH,O samples derived from each hydrothermally modified sol, and Fig. 2 shows the corresponding weight-loss curves. Weight-loss curves for sols modified at 75 and 100°C essentially overlie the curve for as prepared sol and have been omitted from Fig. 2 for the sake of clarity. Hydrothermal modification produced no quantifiable changes in the XRD patterns obtained from sol-derived RuO,.xH,O. The diffraction lines observable in the as-prepared sol were unchanged by eye, and remained too indistinct for the estimation of linewidth by microdensitometry.No new diffraction lines appeared. When evaluated using the gas-line apparatus, all the sols catalysed the reduction of the test quantity of Ce" ions within 10 min (determined visually by the disappearance of intensely yellow CeIV), with the simultaneous evolution of ca. 30cm3 of 02.Fig. 3 shows the time-dependent 0,--MPD response curves for various sols, and Table 1 lists the molar 0, yields for each sol calculated from the area under the corresponding response curve. 0,-MPD response curves for sols hydrother- mally modified at 75-150 "C lie between the as-prepared and 175 "C curves shown in Fig. 3 and have been omitted for the sake of clarity. Table 1 also lists values for the molar fraction of RuO,-xH,O oxidatively corroded to RuO, in the course of the test reaction for each sol.Discussion Ru0,-xH,O is thermodynamically unstable with respect to anhydrous RuO,, which is a crystalline solid iso-morphous with r~tile.~ However, precipitation of amorphous RuO,.xH,O is typical when aqueous solutions of upper-valence Ru species are reduced, or when aqueous Ru" species are hydrolysed at room temperat~re.~The observation of structure, albeit very broad and indistinct, in the XRD patterns J. MATER. CHEM., 1994, VOL. 4 I I I 1 1 I I -1 0 0 10 20 T/min Fig.3 0,-MPD response curves showing O2 evolution profiles for water oxidation by Ce" ions catalysed by: (a)as-prepared Ru02.xH,0 sols, and sols hydrothermally modified at (b)175,(c) 200 and (d)225 "C obtained from as-prepared sol RuO,.xH,O suggests that small ordered regions do exist in this material.,' For the purposes of discussion these regions of short-range order will be referred to as crystallites.21 Particle Size and Dispersion The as-prepared RuO,-xH,O sols are intrinsically stable with respect to particle aggregation, although coagulation may be induced by relatively low concentrations of salt (ionic strength > mol dm-3).6 This behaviour is characteristic of electro- static stabilization, i.e.colloidal stability arising from repulsive interactions between the similarly charged electrical double layers of sol particles.22 Coagulation of RuO,xH,O hydrosols is known to result in dramatic reductions in optical extinction and absorbance- wavelength slopes in the UV-VIS absorbance-scattering spec- tra.14 Consequently, the observation the UV-VIS spectrum of the as-prepared RuO,*xH,O sol is practically unchanged by hydrothermal modification indicates that hydrothermal treat- ment up to 225 "C does not result in appreciable aggregation or coarsening of sol particles.This conclusion is corroborated by the absence of sedimentation or increased Tyndal effect in the hydrothermally modified sols. The unchanged TEM par- ticle size confirms the absence of Ostwald ripening in the hydrothermally modified sols. These findings are in sharp contradistinction to the case of RuO,.xH,O powders heated to ca. 200 "C in air, where interparticle sintering results in the formation of large (0.1-0.3 pm) agglomerates." Simply drying a fine aqueous metal oxide dispersion in air at room tempera- ture can lead to irreversible agglomeration of particles through 'pressure sintering', where the surface tension of the receding meniscus between particles promotes sintering at points of contact, and 'olation', where condensation reactions between surface hydroxy groups on neighbouring particles result in inter-particle oxygen bridging.23 It is assumed that the immun- ity of hydrothermally modified RuO,.xH,O sols to particle aggregation and agglomeration results from the electrostatic stability of these dispersions.Thus particles are maintained in mutual isolation, individually surrounded by water, through- out the hydrothermal treatment.Hydration and TG TG weight loss from RuO,.xH,O samples is predominantly due to thermally induced dehydration." For samples heated in air, irreversible dehydration commences at ca. 75 "C and becomes complete at ca. 600°C. Samples heated in air to 22200°C give powder XRD patterns Characteristic of the RuO, rutile structure." Exact identification of modes of water loss is difficult in IR-opaque RuO,.XH,O,~ but probable modes may be identified by analogy with IR-transparent amorphous oxide hydrates, such as tin@) oxide hydrate (SnO,.xH,O), which similarly develop the rutile structure on heating.24 Combined TG and IR studies of SnO,.xH,O have shown that weight lost reversibly at lower temperatures is due to physically adsorbed H,O whilst weight lost irrcversibly at higher temperatures is predominantly due to 'chernisorbed water', which is water produced by condensation 1 eactions occurring between hydroxy groups initially presen t at the oxide surface.24 Given the above, the TG weight loss data shown in Table 1 imply that solid RuO,.xH,O samples derived from the as-prepared sol contain ca.14% water by weight and that hydrothermal modification of sols at temperatures 3 125 "C results in a progressive reduction in water content, with Ru0,-xH,O derived from sols hydrothermally modified at 225 "C containing only 9.8% water. Studies of polycrystalline RuO, have shown that loss of physically adsorbed water is mostly complete at TG temperatures of llO"C.ll If the same is assumed to be true for RuO,-xH,O, Fig.2 implies that the ratio of chemisorbed to physically adsorbed water decreases with increasing hydrothermal modification temperature. Comparison of curves (a) and (f)in Fig. 2 show that hydro- thermal modification at 225 "C results in a 40% reduction in weight lost between 100 and 600°C (mostly chemisorbed water) but only a 25% reduction in weight lost between ambient temperature and 110 "C (mostly physically adsorbed water). One implication of this observation is that, if the quantity of physically adsorbed water is assumed to be proportional to surface area, hydrothermal treatment does not appear to reduce the specific surface area of sol derived Ru02-xH20 by more than 25%. XRD Behaviour Even when hydrothermally modified at 225"C, the Ru02-xH,0 sols fail to show any changes in their powder XRD pattern indicative of significant development of the RuO, rutile structure.This finding is in contradistinction to the case of calcined Ru02.xH,0 powders, for which clear and measurable diffraction lines corresponding to the [1101, [1011 and [211] RuO, crystal planes appear after calcination at temperatures >200 "C.ll The hydrothermal inhibition of crystallization was not anticipated, as it is that known hydro- thermal conditions frequently allow phase changes and crys- tallization in solid materials to take place at relatively low temperature^.,^ However, hydrothermally induced phase changes often result from a process of dissolution and repre- ~ipitation,,~and the solubility of RuO,-xH,O in neutral water is known to be very small.For freshly precipitated RuO ,*xH,O measurement of the equilibrium K. 4RuO2*2H2O,,, 6Ru4(OH);',+(,,, +4 OH;, , (3) gives K,M 5 x mo15 dm-5, implying a room-temperature solubility for Ru,(OH),,~+ of 5 x mol dm-3 at pH 7.7 Aged Ru02-xH20 precipitates show reduced solubility and none of the Ru0,-xH20 sols described herein showed any sign of room-temperature solubility even at pH 0. The insolu- bility of Ru0,-xH,O also explains the absence of Ostwald ripening in the hydrothermally modified sols. The loss of chemisorbed water from heated metal oxide hydrates occurs either as a result of condensation reactions between adjacent hydroxy groups at the surface of a single crystallite (two-dimensional condensation) or condensation reactions between hydroxy groups present at the surfaces of J.MATER. CHEM., 1994, VOL. 4 least five times as fast as that for the sol hydrothermally modified at 225 "C. The above observations closely parallel the findings of both macroelectrode" and dispersed microelectrode' studies using calcined RuO2-xH,O powders, which have shown that pro- gressive reduction of chemisorbed water content by calcination increases resistance to anodic corrosion but eventually neighbouring crystallites (three-dimensional c~ndensation).~~ produces Condensation in three dimensions implies an effective coalesc- ence of crystallites, and it is this process which eventually leads to long-range order and the appearance of XRD patterns characteristic of the anhydrous oxide.Given the observation that crystallinity does not appear in the hydrothermally modified Ru02.xH20, it would seem that hydrothermally induced dehydration is largely restricted to two-dimensional OH condensation processes. It is possible that the inhibition of three-dimensional condensation results from condensation reactions, generally, being thermodynamically disfavoured under hydrothermal conditions, in accordance with Le Chatalier's principle. Alternatively, it is possible that the intrusion of water between crystallites interferes with the approximation of surfaces necessary for three-dimensional condensation. Electrocatalytic Behaviour When RuO2-xH20 dispersions are mixed with aqueous solu- tions of redox couples, such as Cerv/Ce"', which are thermo- dynamically but not kinetically capable of oxidizing water, catalytic oxygen evolution may proceed as a result of RuO,.xH,O particles adopting a mixture potential such that the currents at the particle surface, due to electrochemical water oxidation and electrochemical CeIV reduction, are equal and opposite.13 The anodic corrosion of dispersed Ru02.xH20 has similarly been studied using the microelectrode approach.26 When exposed to CerV ions in 1mol dm-, HNO, the RuO,.xH,O sols may either catalyse the oxidation of water to 02 or undergo anodic corrosion to Ru04 The formal potential of the Ce'v/Ce''' couple in 1mol dm-, HNO, is 1.61 V us.NHE.27 Comparing this value with E' for the H,O/O, couple (1.229 V us. NHE at pH )28 and E' of the RuO,/RuO, couple (1.401 V us. NHE at pH 0)29shows that ample overpotential exists for both reactions (1) and (2) under the specified experimental conditions. It may be seen from Table 1that the fraction of RuO,.xH,O consumed through reaction (2) is dramatically influenced by hydrothermal modification of the Ru02-xH20 sols. Table 1 also shows that the yield of evolved O2 is substantially independent of hydrothermal sol modification, remaining practically constant at 96-100% of the maximum theoretical yield of reaction (1) (1.25 x lo-, mol 0,).However, Fig. 3 shows that the rate at which 0, is evolved in the test reaction decreases markedly for sols hydrothermally modified at tem- peratures > 175 "C.The dead volume of the gas-line apparatus (ca. 200cm3) makes the shape of the 0,-MPD response curves an imperfect measure of the rate of reaction (l),but measurement of the initial slopes of curves (a) and (d) in Fig. 3 suggests that initial 0, evolution for the unmodified sol is at loss of electrocatalytic activity. Calcination of RuO,.xH,O powders at 140°C has been shown to produce an X-ray amorphous material which will catalyse the oxi- dation of water by CeIv ions without measurable corrosion, but powders calcined at > 140 "C give reduced rates of 0, evolution." Formulation of an exact explanation for the increase in corrosion resistance observed in Ru02.xH20 hydrothermally modified at 2125°C is made difficult by the amorphous nature of the material.However, the formation of oxygen bridges between surface Ru atoms due to two-dimensional condensation between surface OH groups would certainly be expected to increase the energy with which individual Ru atoms are bound into crystallites, with the result that such atoms become more difficult to volatilize through anodic corrosion. Increased corrosion resistance in calcined Ru02.xH20 powders has similarly been attributed to a reduction in surface defects.', Decreased electrocatalytic activity in RuO,.xH,O powders calcined at temperatures >14O"C has been attributed to a decrease in effective surface area due to the crystallisation and sintering of particles." This seems entirely reasonable in the case of calcined powders but fails to explain the observed decrease in 0, evolution rates in RuO,.xH,O sols hydrother- mally modified at >175"C which do not exhibit crystalliz- ation, sintering or any change in their state of dispersion.One possible explanation is that hydrothermal treatment influences the surface concentration of electrocatalytically active sites. Electrocatalytic O2 evolution at Ru0,-xH,O anodes is believed to proceed uia the formation of covalent bonds between surface Ru atoms and water-derived 0 atom^.^ It follows that the surface Ru atom must lie in a lattice, edge or defect,,' position which permits the necessary changes in oxygen c~ordination.~~,~~ It is possible that extensive oxygen bridging of surface Ru atoms through two-dimensional con- densation of surface OH groups reduces the number of Ru atoms for which facile changes in oxygen coordination are feasible.Conclusions Hydrothermal treatment of electrostatically stabilized disper- sions of amorphous nanosize RuO,.xH,O particles at pro- gressively increasing temperatures between 100 and 225 "C results in progressive reduction in oxide hydration without significant changes in the crystallinity, size or dispersity of RuO,.xH,O particles. The predominant mode of oxide dehy- dration appears to be hydrothermally induced condensa- tion reactions occurring between hydroxy groups initially present at the oxide surface.Hydrothermal modification of RuO,.xH,O dispersions at temperatures 3 175 "C sharply reduces susceptibility of particles to oxidative (anodic) cor- rosion but simultaneously reduces electrocatalytic activity for anodic water oxidation. It is proposed that both of these changes in electrochemical properties result from increased oxygen bridging of surface Ru atoms. References 1 S. Komarneni, J. Mater. Chem., 1992,2, 1219. 2 G. W. Kriechbaum and P. Kleinshmit, Adv. Mater., 1989,28,1416. J. MATER. CHEM., 1994, VOL. 4 1287 3 4 5 E. Matijevic, Pure Appl. Chem., 1988,60, 1479. A. L. Robinson, Science, 1986,233,25. J. Karch, R. Birringer and H. Gleiter, Nature (London), 1987, 21 22 B. D. Cullity, in Elements of X-ray DifSraction, Addison Wesley, Mass., 2nd.edn., 1978, p. 321. D. J. Shaw, in Introduction to Colloid and Surface Chc.mistry, 330, 556. Butterworths, London, 3rd. edn., 1980, p. 183. 6 7 8 9 H. N. McMurray, J. Phys. Chem., 1993,97,8039. J. A. Rard, Chem. Rev., 1985,85, 1, and references therein. H. C. Angus and P. E. Gainsburg, Electronic Components, 1968, 84. S. Trasatti and G. Lodi, in Electrodes of Conducting Metal Oxides, 23 24 R. D. Nelson, in Handbook of Powder Technology, Dispersing Powders in Liquids, ed. J. C. Williams and T. Allen, Elsevier, Amsterdam, 1988, vol. 7, pp. 1-20. P. G. Harrison and A. Guest, J. Chem. SOC., Faraday Trans. I, 1987,83,3383. 10 ed. S. Trasatti, Elsevier, Amsterdam, 1981, pp. 521-626. C. Iwakura, K. Hirao and H. Tamura, Electrochim.Acta, 1977, 24 A. R. West, in Solid State Chemistry and its Applications, J. Wiley, Chichester, 1984, pp. 41-43. 22, 329; 335. 25 A. Mills and H. Davies, J. Chem. SOC., Faraday Trans. I, 1991, 11 A. Mills, S. Giddings, I. Patel and C. Lawrence, J. Chem. SOC., 87, 473. Faraday Trans. I, 1987,83,2331. 26 A. A. Noyes and C. S. Gardener, J. Am. Chem. SOC., 1936,58,1264. 12 A. Mills, S. Giddings and I. Patel, J. Chem. SOC., Faraday Trans. I, 1987,83,2317. 27 M. Pourbaix and N. de Zoubov, in Atlas of Electrochemical Equilibria in Aqueous Solutions, ed. M. Pourbaix, Pergamon 13 A. Mills and N. McMurray, J. Chem. SOC., Faraday Trans. 1,1989, 85,2047; 2055. 28 Press, Oxford, 1966, p. 343. M. Pourbaix and N. de Zoubov, in Atlas of Electrochemical 14 A. Mills and H. N. McMurray, J. Chem. SOC., Faraday Trans. I, 1988,84, 379. Equilibria in Aqueous Solutions, ed. M. Pourbaix, Pergamon Press, Oxford, 1966, p. 97. 15 17 18 19 20 A. Harriman and M.-C. Richoux, J. Chem. SOC., Faraday Trans. 1, 1987,83, 3001. R. E. Connick and C. R. Hurley, J. Am. Chem. Soc., 1952,74,5012. A. Mills and C. Lawrence, Analyst (London), 1984,109, 1549. R. P. Larson and L. E. Ross, Anal. Chem., 1959,31, 176. J. L. Woodhead and J. M. Fletcher, J. Chem. SOC., 1961,5039. 29 30 31 G. Lodi, E. Silvieri, A. De Battisti and S. Trasatti, J Appl.Electrochem., 1978,8, 1011. G. C. A. Schuit, Int. J. Quantum Chem., 1977,12,43. M. Boudart, Am. Sci., 1969,57,97. Paper 4/02267J;Received 18th Aprir',1994

ISSN:0959-9428    

DOI:10.1039/JM9940401283

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4(8), 1289-1291 1289 Determination of the Potential Limits for W03 Colouration Peikang Shen and Alfred C. C. Tseung* Chemical Energy Research Centre, Department of Chemistry and Biological Chemistry, University of Essex, Wivenhoe Park, Colchester, CO4 3SQ UK The determination of the maximum permissible degree of colouration for an electrochromic film in a particular device is technically important for long-term reversible modulation and safe operation. Chronopotentiometry combined with cyclic voltammetry have been used to examine the degree of colouration for WO, films on various conducting substrates and electrolytes. The results revealed that an overreduced WO, film with high amount of intercalated ions will lose its reversibility, lowering its electrochromic performance.WO, is the main optically functioning film in almost all electrochromic devices. It is able to change its optical proper- ties in a reversible and persistent way under a potential control.' W0,-based electrochromic devices are much better than those of the liquid-crystal-based displays in terms of colour and viewing properties. WO, films appear blue in colour during the electrochemical reduction in acid solution or alkali-metal ion-containing solution^.^^^ The reaction can be expressed as WO,+xM+ +xe- =M,W03 (M=H, Li, K,Na, ...) (1) The degree of colouration depends on the amount of proton or alkali-metal ion inserted in the film. It has been shown that the amount of ion (x) in the coloured film is a function of the electrode potentials a~plied.~ However, the limiting amount of inserted ion in the WO, film is crucially dependent on the electrode substrate and the electrolyte solution.This limit means that the WO, films can be used safely at the corresponding potential without any side-reactions taking place. Since the practical electrochromic devices rely on the long-term stability of the electrochromic film, it is important to control the appropriate operating conditions. This paper describes a simple and rapid method for measuring the limiting potential for WO, colouration and the corresponding value of x in MxW03. This method should provide a good guideline for the operational parameters for various WO, based electrochromics under world-wide development.Experimental WO, films were prepared on Au, Pt or indium-doped tin oxide coated (ITO) glass substrates. The Au and Pt substrates were cut into circular disks of 0.5 cm2 and were embedded in PTFE holders. They were then polished using sandpaper. All substrates were degreased with methanol in an ultrasonic bath for 5 min followed by a distilled water rinse. WO, films were prepared by constant potential deposition at -0.35 V (on Pt or Au) or -0.4 V (on ITO) for 30-45 min in a homogeneous solution of 50 mmol dm-, tungsten and 30% discharge the coloured film. Fig. l(u)shows a typical chrono- potentiogram of a WO, film on Pt substrate in 0.5 mol dmP3 H2S04at 20°C. The amount of intercalated ions in the film can be calculated by using the following relationship: x =izM/FW where i is the discharge current, z is the time used for complete oxidation of MxW03,M is the molecular weight of tungsten trioxide, F is the Faraday constant and W is the weight of tungsten trioxide determined by UV-VIS ~pectrophotometry.~ Since x is a function of conditioning potential, the amount of intercalated ions is different at different potentials, the oxi- dation charge should be varied. The procedure for examining the colouration behaviour of the WO, films was as follows: the W0,-coated electrodes were held at 1.0 V us.SCE for 60 s and then a negative constant current of 150 pA cm- was applied for varying times, dependent on the experimental conditions [Fig.1(b)]. Results and Discussion Chronopotentiometric measurements of a WO, film on Pt for colouration and bleaching are show in Fig. 1. As indicated in the figure, after complete oxidation of H,W03 the potential rapidly rises to the oxidation potential of Pt (ca 0.8 t') and then to oxygen evolution potential (1.45 V). In the reverse process, WO, is reduced after the reduction of Pt oxides. When the potential reached the potential where hydrogen is evolved, the main reaction is hydrogen evolution arid the potential remains constant. Under such a potential, the pro- tons are removed from the film by the formation of hydrogen PtO + H20+ Pt02+ 2H+ + 2e-Pt + H20+ PtO + 2H+ + 2e-1.6 72H20-+O2 + 4H+ + 46flw 0.8 -I 1PtO + 2H++ 2e-+ Pt + H,O v/v propan-2-01.The details have been reported el~ewhere.~>~ Electrochemical measurements were carried out on an EG&G PAR 273A potentiostat/galvanostat controlled by 270 Research Electrochemistry Software. A three-electrode cell with a Pt counter electrode and a saturated calomel electrode (SCE) reference electrode was used. Before the measurements, the solutions were sparged with nitrogen for 30min. All the chemicals were supplied by BDH and used as received. The intercalation of small ions into the WO, films was conducted by applying a fixed potential (conditioning poten- tial) for 60 s. A constant current of 150 p.A cm-2 was used to + xe-WO3 + +I+ + xe--+ HY03I -0.8 2H' + 2e--3 H2 I I._2 0.0 0.1 0.2 0.3 0.4 t/ks Fig.1 Chronopotentiograms of a WO, film on Pt in 0.5 ma1 dm-3 H,SO, solution at 20 "C: (a) under 150 PA cmP2 oxidation current after 60 s held at -0.25 V and (b) under -150pA cm-2 reduction current after 60 s held at 1.0 V 1290 J. MATER. CHEM., 1994, VOL. 4 r 0.8 - 2--II-L-LJ 0.0 0.10 0.20 _ 0.30 tlks Fig.2 Chronopotentiograms of WO, films on (a) Au, (b) Pt and (c) IT0 in 0.5 mol dmP3 H2S04solution at 150 pA crn-', 20 "C gas and this limits the increase in the amount of inserted ion. It is worth noting that gas formation is dangerous for an electrochromic device since it will insulate the film from electrolyte in the cell and increase the hydrogen pressure inside the pores, which may cause rupture of the electroch- romic film.From the measured z value at a given condition potential, x can be calculated by using eqn. (2). The potential just before the hydrogen evolution is the limiting potential for a WO, film operating in aqueous acid solution. Therefore, the limit, xL,for WO, colouration can be determined from the value of zL. xLis the maximum amount of ions inserted in coloured film. This is a simple method but is very sig- nificant in determining the safe operating potential. For a practical electrochromic device, one can use this method to determine the operating potential for protecting the device from the damage due to hydrogen evolution. Fig. 2 compares the chronopotentiograms of the WO, films colouration on different substrates in 0.5 mol dm-, H2S04 solution.It is seen that the potential decreases monotonically on H intercalation (in a two-electrode electrochromic device, + the potential decrease on WO, coated electrode will result in a decrease in the electromotive force of the device). Monotonic potential variations indicate that the ion intercalation takes place without major structural rearrangements.6 The param- eters measured from these curves are summarised in Table 1. The results show that the limiting degree of colouration is strongly dependent on the substrate in aqueous media. WO, films have higher limiting degree of colouration on substrate that has higher overpotential for hydrogen evolution. In addition, the effect of pH on the degree of colouration was also examined.Changing the concentrations of the sulfuric acid solutions from 0.01 to 1mol drn-,, the limiting potentials shifted to more positive values that obey the Nernst equation. However, the limiting amount of inserted ions are unchanged. The maximum value of x in H,WO, on IT0 glass substrate at corresponding limiting potential is 0.157 in this study. In non-aqueous solutions, the colouration process of WO, film is more complex. Fig. 3 shows a response curve of a WO, film on Au under -150 pA cmV2 charging in 1mol dmP3 LiC10,-propylene carbonate (PC) solution at 20 "C. It is obvious that there are several transition points on the curve Table 1 Limiting potentials and limiting x values for WO, colouration on different substrates in 0.5 mol dm-, H,S04 solution at 20 "C limiting potential/ electrode type V us.SCE XL WO, on Pt -0.27 0.07 1 WO, on Au -0.28 0.075 W03 on IT0 -0.55 0.157 -3.0t Ii -uiI____L -I 0.0 1.o 2.0 3.0 tlks Fig. 3 Chronopotentiogram of a W03 film on Au in 1 mol dm-3 LiC104-PC solution at 150 pA cm--, 20 "C at about -1,- 1.5 and -2 V. The same behaviour has also been observed for WO, film on a Pt substrate. Since both Li+ ion and PC are stable at potentials less negative than -3 V us. NHE,7v8 there could not be any side-reactions during the charging process except the reduction of WO,. Fig. 4 shows the cyclic voltammograms of a WO, film on Au in 1rnol dmP3 LiC10,-PC at a scan rate of 50 mV SKI. There is a pair of quasi-reversible peaks between -1.0 and -0.6 V.When the potential sweeps towards potentials more negative than -1.2 V, the film loses its reversibility perma- nently. The broad cathodic peaks at -1.0 and -1.5V corre-spond to the transition points on the chronopotentiogram in Fig. 3. The values of x calculated using eqn. (2) are 0.33 at -1.0 V and 0.79 at -1.5 V. It has been shown that the X-ray photoelectron spectroscopy (XPS) applied to core levels can give information on the valence states of the tungsten ions in WO, film at different degrees of colouration. In highly dis- ordered tungsten trioxide, the amount of W6 transformed+ into W5 + determines the optical response. For an evaporated WO, film with different amounts of intercalated H+ , the deconvoluted XPS spectra showed that W6+ and W5+ co-existed in the film at x=O.O9; however, there are three valence states of W6+, W5+ and W4+ at x =0.42.9This means the tungsten ion could be reduced to lower valence states by the increased amount of intercalated ions.It is possible that some of the tungsten ions were reduced to W4+ at -1.5V with an increased amount of Li+ in the film. However, such lower-valence tungsten ions might be difficult to reoxidise (Fig. 4).Li+ ions may be unable to deintercalate after a high degree of intercalation. In fact, Li+ ion is found both in the intercalated and in the deintercalated state by using secondary- -2.0 0.0---2.0 --4.0 -2.0 -1.2 -0.4 0.4 EN vs. SCE Fig. 4 Cyclic voltammograms of a WO, film on Au in 1 rnol dmP3 LiC104-PC solution at 20 "C,scan rate, 50 mV s-': (a) -0.5-0.8 V, (b) -0.13-0.8 V and (c) -2.0-0.8 V J.MATER. CHEM., 1994, VOL. 4 ion mass spectrometry (SIMS)." It is clear that the limiting potential should be no more negative than -1.2 V for WO, colouration in 1mol dmW3 LiC10,-PC solution to maintain its reversible property. This gives a limiting xL of 0.46. Conclusion As shown above, the limiting degree of colouration for WO, films can be readily examined by chronopotentiometry com- bined with cyclic voltammetry. The electrochromism of W03 hinges on ion intercalation and deintercalation from an adjac- ent electrolyte and, of course, the electron insertion/extraction occurs jointly with the ionic movement.The limiting amount of intercalated ions is strongly dependent on the conducting substrate and the electrolyte. In aqueous solutions, the colouration process is limited by hydrogen evolution. In non-aqueous solutions, it is limited by overreduction of the WO,, resulting in an electrochemically irreversible film. References C. G. Granqvist, in Physics of Thin Films, Mechanic and Dielectric Properties, ed. M. H. Francombe and J. L. Vossen, Academic Press, San Diego, 1993, vol. 17, p. 302. 2 P. K. Shen, J. Syed-Bokhari and A. C. C. Tseung, J. Elecrrochem. Soc., 1991, 138, 2778. 3 P. K. Shen and A. C. C. Tseung, J. Muter. Chem., 1992,2, 1141. 4 P. K. Shen, K. Y. Chen and A. C. C. Tseung, J. Electrochtrm. Soc., 1994,141,1758. 5 A. C. C. Tseung, P. K. Shen and J. Syed-Bokhari, PCTbrt. Appl. WO 92 16,027, 17 Sep. 1992; GB Appl. 91/4,377, 1 March. 1991. 6 R. S. Crandall, P. J. Wojtowicz and B. F. Faughnan, So/;d State Commun., 1976,18,1409. 7 A. J. Bard, R. Parsons and J. Jordan, Standard Poteiitials in Aqueous Solution, Marcel Dekker, New York, 1985,p. 727. 8 R. Jasinski, in Advances in Electrochemistry and Electroc hemical Engineering, ed. P. Delahay and C. W. Tobias, John Wilcy, New York, 1971, vol. 8, p. 253. 9 A. Ternmink, 0. Anderson, K. Bange, H. Hantsche and X. Yu, Thin Solid Films, 1990,192,211. 10 Q. Zhang, S. A. Wessel, B. Heinrich and K. Colbow, Solar Energy Muter., 1990,20, 289. Paper 4/01957A; Received 31st March, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401289

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4(8), 1293-1301 Phases in the ZrxTal -x(O,N),, System, formed by Ammonolysis of Zr-Ta Gels: Preparation of a Baddeleyite-type Solid Solution Phase ZrxTal -xO1+xN1 0 <x <1 Jekabs Grins,* Per-Olov KaII and Gunnar Svensson Department of lnorganic Chemistry, Arrhenius Laboratory, Stockholm University, S-10697 Stockholm, Sweden Phase formation in the system ZrxTa, -x(O,N)y has been studied by ammonolysis of Zr-Ta gels, prepared by the sol-gel technique, at temperatures between 700 and 1000 "C. The starting gels and observed phases were characterised by X-ray powder diffraction (XRPD), scanning and transmission electron microscopy (SEM and TEM) and thermogravimetric (TG) analysis. Oxynitride phases of compositions Zr,Ta, -,O, +,Nl -,, 0 <{ d 1, with the baddeleyite-type structure, were prepared at 800 "C.The unit-cell volume increased linearly from 127.8 A3 for TaON (x =0) to 140.9 A3 for ZrO, (x=1). The structure was verified for the composition Zr,,,Tao,01.6No., (x =0.4) by a Rietveld refinement (R, =3.5%) using Cu-Ka, XRPD data. An orthorhombic oxynitride phase was observed in preparations at 700°C for 0.26dxdO.90 in Zr,Ta, -,01+,N, -,. Unit-cell parameters and powder X-ray reflection intensities agree with an orthorhombic ZrO, type structure. According to X-ray data, a cubic solid solution phase with a fluorite related subcell is present in materials prepared at 900 O0Cfor 0.26 5x d 0.68. However, electron microdiffraction patterns suggest a metrically monoclinic unit cell with a =6.1 A, b =14.1 A, c =7.1 A and p= 125".The Ta,N, type of structure was found to incorporate up to ca. 18 atomo%Zr at 900 and 1000 "C. Transition-metal nitrides with metallic properties and com- positions M,N, with n 61 are relatively well characterised with respect to structure and properties, while considerably less is known about more covalently or ionically bonded nitrides and oxynitrides containing one or more transition metals. A series of oxynitride perovskites have, however, been prepared, including ATa0,N with A=Ba, Sr and Ca,' BaNb0,N' and LnWO,N,-, (Ln=La and Nd).2 Compounds LnW0,N (Ln =Nd, Sm, Gd and Dy) having the CaWO, scheelite structure have also been ~ynthesized,~ as well as LnA1203N compounds (Ln=Nd, La and Sm) with the K,NiF,-type ~tructure.~Other rather recently reported oxynitride phases are LnA11,02,N compounds with the PbFe1,Ozg magnetoplumbite structure5 and BaCeLn(O,N), compounds (Ln=La and Ce) possessing the CaFe,O, structure.6 The increased interest in these types of compounds emanates from the obvious possibility to prepare new compounds with interesting thermal, electrical and magnetic properties.Two Ta compounds are presently known which contain nitrogen and Ta5+, viz. Ta,N, and TaON. The red Ta,N5 was reported in 18767 and can be prepared by ammonolysis of Ta205 at temperatures <1000oC.8 The crystal structure was determined by Strahle' by single-crystal X-ray diffraction and was later refined from neutron powder diffraction data obtained at 16 K." The orthorhombic structure is isotypic with pseudo-brookite Fe2Ti0511 and contains irregular TaN, octahedra sharing corners and edges. The yellow oxynitride TaON is formed as an intermediate phase when Ta205 is ammonolysed.8 The crystal structure was determined from powder neutron diffraction data recorded at 4.2 K', and was subsequently confirmed by single-crystal X-ray diffraction.', The structure is homeotypic with monoclinic ZrO,', and shows a complete ordering of 0 and N atoms in alternating layers perpendicular to the c axis.Ta nitrides with Ta in oxidation states below +5 are obtained when Ta205 is ammonolysed at temperatures above ca. 1000"C;15 Ta4N5 has an N?Cl-related tetragonal structure with a =6.831 A, c 7 4.269 A an! Ta,N, a hexagonal structure with a =5.176 A, C= 10.353 A.The brown non-conducting zirconium nitride Zr,N, was prepared by ammonolysis of ZrX, (X =C1, Br, I) at 700 OC;', its crystal structure is not known. Three Zr oxynitrides have been reported by Gilles and co-worker~;'~~'~ Zr,ON, (y), Zr708N4(p) and Zr701,N, (p'). The compounds were pre- pared at temperatures near 1000, 1100 and 2000°C, rcspect-ively, by ammonolysis of ZrO,, or by heat treating mixtures of Zr0, and ZrN in an NH, atmosphere. The crystal structure of Zr20N2 is allegedly isotypic with the C-type rare-earth- metal structure." Recent studies by Ohashi et showed, however, that Zr,ON, has a variable composition acci brding to the formula Zr02-2xN4x,3, with 0.5dxd0.8 at 950°C. Zr708N, and Zr,O,,N, are isotypic with Zr,Sc,O, and Zr5Sc2013,2' respectively, with structures that can both be derived from the fluorite structure by ordered omission of oxygen atoms.No Zr-Ta oxynitrides containing Zr4+ and Ta5+ havc been reported. The present work is a part of a programme aimed at synthesis of new transition-metal oxynitrides and nitrides. This study is an investigation of the possibility of synthesizing non-metallic oxynitrides containing both Zr and Ta by tiitrid- ation of appropriate oxide mixtures with NH, gas, i.e. by ammonolysis. Because the oxides ZrO, and Ta205 are unreac- tive at low temperatures, Zr-Ta xerogels were used as starting materials for the ammonolysis. The results from ammonolysis experiments on Ta-Zr xerogels of different compositions at temperatures between 700 and 1100"C are described below.Experimental A JEOL JSM-880 scanning electron microscope and a JEOL JEM-2000 FX-I1 transmission electron microscope with EDX (energy-dispersive X-ray) microanalysis systems LINK AN 10000 and LINK QX200, respectively, were used in character- isations of Ta-Zr starting gels and ammonolysed materials. Metal compositions were determined by averaging 15-20 EDX point analyses, The statistical errors in each EDX analysis were ca. 1 atom% in the SEM and ca. 4 atom% in the TEM. Approximate dimensions sampled in the analyses were ca. 0.5 pm in the SEM and G0.05 pm in the TEhL TG recordings were obtained with a Perkin-Elmer TGS-2 TG analyser, operated in air with a heating rate of 10"C min-'.The synthesized materials were characterised by their XRPD, recorded with a Guinier-Hagg camera, using Cu-Kq radiation and Si as an internal standard. The patterns were evaluated with a film-scanning system.22 The relative pro- portions of the phases present were estimated visually from reflection intensity ratios. XRPD data from baddeleyite-type were collected on a STOE STADI/P diffractometer, using Cu-Ka, radiation, a rotating sample in symmetric transmission mode and a linear position-sensitive detector covering 4.6" in 28. The step-length was 0.02" and the 20 range 15-92" was covered over a period of 72 h, yielding an intensity of ca. 8500 counts for the strongest Zro,4Tao&1.4No,6 peak.Preparation and Characterisation of Zr-Ta Gels Xerogels containing Ta and Zr were prepared with TaC1, and Zr n-propoxide as precursors in a dry-box containing an oxygen- and water-free atmosphere. Appropriate amounts of TaC1, (Merck p.a.) and a calibrated 70 wt.% solution of Zr n-propoxide in propanol (Merck) were weighed into 100 ml beakers. The beakers were sealed with Parafilm and removed from the dry-box. Dry ethanol (ca. 25 ml) was then added to each beaker and the clear solutions were stirred for 10min. The solutions were then hydrolysed by rapid injection of water. The amount of water corresponded to twice the amount of alkoxide present. Hydrolysis was carried out both under prevailing acidic conditions and under basic conditions.The latter was achieved by adding NH, to the water. The basic solutions gelled rapidly, whereas viscous oils were produced under acidic conditions. Xerogels were then obtained by evaporation of the solvent on a hot-plate. An SEM image of a typical gel, hydrolysed under basic conditions, is shown in Fig. 1. The gel exhibits a large surface area and is composed of agglomerates of granules of ca. 0.1 pm. The higher magnification image shows that these granules are composed of particles of ca. 50-100 A. This fine structure is more distinctly revealed in TEM images, as shown in Fig. 2(a).Gels hydrolysed under acidic conditions exhibited fragmen$ resembling crystallites, ranging from ca. 1 pm down to 100A.These fragments did not show the kind of fine structure observed in gels hydrolysed under basic conditions, as seen in Fig. 2(b). The gel morphology thus depends, as expected, on the hydrolysis procedure applied. The two types of gel produced no electron microdiffraction patterns and were thus amorphous. The materials formed when ammono- lysing the gels were in some instances conditioned by the type of gel used, as described below. Gels with the nominal compositions Zro.18Tao.82, zr0.68Tao.32 (made under acidic hydrolysis conditions) and Zro.60Tao.40 (made under basic hydrolysis conditions) were analysed by EDX in the SEM. The analyses showed that the gels were homogenous and that they contained substantial amounts of C1 from the use of TaCl, as precursor.No EDX signal from possible residual C1-could, however, be detected after ammonolysis of the gels. Elemental analysis of each gel yielded the compositions Zro.19~l~Tao.81~l~ ( 19 atom% 'l), zr0.70(1)Ta0.30(1) (28 C1) and zr0.63(1)Ta0.37(1) (21 atom% CI), respectively, with standard deviations in parantheses. The compositions agree well with the nominal ones. Corresponding EDX analyses of the last two gels in the TEM yielded the compositions: Zro.63(33Tao,37(3) and Zro.71(4)Tao.29(4),in good agreement with the SEM analysis. The TEM studies also showed that Zr and Ta were homogen- ously distributed. Typical TG curves showing the decomposition of the Zr-Ta J. MATER. CHEM., 1994, VOL. 4 Fig. 1 SEM micrographs of a typical Zr-Ta gel hydrolysed under basic conditions (composition Zro.6Tao,4) gels are shown in Fig.3. The gels hydrolysed under basic conditions showed weight losses of up to 55%, decreasing with Zr content. Gels hydrolysed under acidic conditions showed smaller weight losses, up to 20%, increasing with Zr content. The weight loss was found to take place at lower temperatures for the gels hydrolysed under basic conditions, up to ca. 5OO0C, compared with the gels hydrolysed under acidic conditions, up to ca. 800°C. Ammonolysis of Zr-Ta Gels The Ta-Zr xerogels were heat-treated in flowing NH, gas at temperatures between 700 and 1100"C. The reaction chamber consisted of a silica tube, length 1.5 m, id 3 cm, placed in a horizontal tube furnace.Gas connections for NH, and N2 were provided by glass fittings. The quartz tube was flushed with N, for ca. 1h before the ammonolysis. The ammonia gas was dried by passing it through a molecular sieve and CaH,. The gels were placed in small Pt or Al,O, cups, each J. MATER. CHEM., 1994, VOL. 4 Fig. 2 TEM micrographs of a Zr-Ta gel hydrolysed under (a) basic conditions (composition Zro,,Tao.4) and (b)acidic conditions (composition ZrO.52Tao.48) containing 20-50 mg. The temperature was measured with a Pt/Pt-Rh thermocouple near the sample. An NH, gas flow of 10-20ml s-' was used and the ammonolyses were ended by cooling the samples to ca. 100cC,in the furnace. Results The Ta-Zr gels were ammonolysed at 700, 800, 900, 1000 and 1100'C.Ammonolysis times of 12 and 96 h in most cases yielded similar results. The results from the 96 h runs are described below. Comparisons with runs using shorter ammonolysis times are given in cases where significantly different results were obtained. The nominal gel compositions used were ZrxTalPx with x=0,0.13, 0.18,0.26, 0.33,0.40,0.52, 0.60, 0.68, 0.80, 0.90 and 1: the sloping x values correspond to gels hydrolysed under acidic conditions. The samples in the 96 h runs were heated in an NH, atmosphere at a rate of ca. 200°C h-' to 7OO0C, prereacted at this temperature for 24 h, and then heat-treated at the various final tempcratures for 72 h. The XRPD patterns in general exhibited broad Bragg peaks with half-widths of 0.3-0.8" in 26, at 28 =50".TEM studies of the ammcnolysed gels revealed the presence of grain sizes down to 50 A, implying that the broad peaks can be attributed mainly to the presence of small crystallites. The phase relations observed at different preparation temperatures are summar- ised in Fig. 4. rI uu h80 8 \v E .cn 1P 40 I I I I I I 0 200 400 600 800 1000 T/"C Fig. 3 TG curves for the thermal decomposition of the Zr-Ta gels in air. Acid hydrolysis conditions: (a) x =0.18, (b) x =0.68; basic hydrolysis conditions: (c) x =0.80, (d)x=0.13. 00to 000 9$) Oooooo Oo0 11~1111111111 0.0 0.2 0.4 0.6 0.8 1.o x in Zr,Ta,-, Fig. 4Schematic illustration of observed phases at different prep- aration temperatures.Estimated relative amounts of the phases are indicated by the sizes of the symbols. 0, Ta,N,; A, Zr,O,N,; V,Ta4N,; A,Ta,N,; 0, baddeleyite; 0, cubic phase; 0,orthor-hombic phase; +, cubic phase. Phase Analysis 700 "C The samples with x G0.18 were biphasic, containing a baddele- yite-type phase, i.e. iso- or homeo-typic with monoclinic ZrO,, and small amounts of a phase isotypic with Ta,N,. Both the baddeleyite- and Ta,N,-type phases showed a cell expansion with increasing nominal Zr content and are thus solid solution (ss) phases. Ammonolysis performed for shorter times, e.g. 12 h, yielded materials containing only the baddeleyite phase. This indicates that the baddeleyite phase is formed first and then partly transformed to the Ta,N,-type phase.Preparations with 0.26 <x <0.90 consisted of mixtures of the baddeleyite phase and an ss phase, whose powder pattern could be indexed on bas@ of an orthorhombic unit cell with all cell axes around 5 A. Further characterisation of the orthorhombic phase is given below. The relative amounts of the two phases varied unsystematically with x. Preparations with x =0.52 and 0.68 were green-grey and contained mostly the orthorhombic phase, while the other preparations were light yellow and contained the baddeleyite phase as the major phase. The samples which contained mainly the orthorhombic phase emanated from Ta-Zr gels which had been hydrolysed under acidic conditions. The sample with x= 1.0 contained baddeleyite.J. MATER. CHF,M., 1994, VOL. 4 800 "C Preparations with xG0.18 consisted of mixtures of the baddeleyite-and Ta,N,-type phases. The amount of Ta,N,-type phase was larger than in the 700°C series but decreased with increasing x. Shorter amnionolysis times yielded, as above, monophasic samples of the baddeleyite phase. Monophasic samples of the baddeleyite phase were obtained for 0.26 <x < 1.00. The powder patterns for s=0.52 and 0.68 exhibited, however, one faint reflection from the orthorhombic phase appearing at lower preparation temperatures. The samples were of different colours, ranging from yellow or beige for 0.26 <x <0.40, green-grey for x =0.52 and 0.68, dark brown for x =0.60 to grey for x >,0.80.900 "C Preparations with x 60.18 were monophasic and yielded powder patterns from an Ta,N,-type phase. The powder patterns for 0.26 <x <0.80 showed the presence of two phases: an ss phase with apparent cubic symmetry and small amounts of the Ta,N,-type phase. The reflections of the cubic phase were considerably broader than those of the other observed phase, with half-widths of ca. 1" in 20. The :trong reflections could be indexed using a cell with az5 A. The intensities of these reflections showed the structure to be related to fluorite. Additional superstructure reflections were, however, present for compositions with 0.26 ,<x <0.68. This phase is further characterised below. The unit-cell dimensions of the Ta,N,-type phase were found to be constant in the biphasic materials.The sample with x =0.90 contained two phases, a baddeley- ite phase and a smaller amount of the cubic phase. The unit- cell volume of the baddeleyite phase showed that its composi- tion was close to x= 1. The unit-cell volume of the cubic phase could not be reliably assessed owing to overlap of all reflections with those of baddeleyite. The sample with x= 1.0 contained only baddeleyite. 1000"C The results obtained at this temperature were similar to those at 900°C. For xG0.18, the materials were thus found to be X-ray monophasic and consist of an Ta,N,-type phase. Biphasic materials were obtained for 0.26 dx <0.68: they contained an Ta,N,-type phase and the same cubic phase as described above.The colours of the biphasic materials showed no systematic variation with x.The majority of these samples had a grey to green-grey bulk colour and a brown surface colouration. The powder reflexions of the cubic phase were even broader than those found for the preparations at 900 "C. The apparent amount of the cubic phase did not increase systematically with x. Preparations with starting gels hydro- lysed under acidic conditions contained higher fractions of the cubic phase. The unit-cell volume of the Ta,N,-type phase increased with the nominal content of Zr in the single-phase region x G0.18, whereas approximately constant unit-cell volumes were observed for the biphasic materials with 0.26 6 x<0.68. The unit-cell volumes of the cubic phases were larger than those recorded for the 900 "C preparations.The sample with x=0.80 was orange-red and contained, in addition to the Ta,N,-type phase and the cubic phase, a small amount of a baddeleyite phase. The baddeleyite phase had a unit-cell volume corresponding to an estimated com- position near x= 1, i.e. ZrO,. The powder pattern for x =0.90 was the same as that observed in the 900°C series, showing the presence of a baddeleyite phase and a small amount of a cubic phase. The powder pattern for x= 1 showed only baddeleyite. J. MATER. CHEM., 1994, VOL. 4 1100 "C All preparations were black or very dark brown. The sample with x=O contained Ta,N, and a smaller amount 9f Ta,N,. The pbserved cel! parameters are a =5.178(1)A, c = 10.346( 6)0A, V =240.! A3 for Ta,N, and a =6.842( 2) A, c =4.266( 4) A, V=199.7A3 for Ta,N,, in fair agreement with the reported values,', given above.Preparations with 0.13<x 60.68 contained four phases: comparatively large amounts of Ta,N, and a baddeleyite phase, plus smaller amounts of Ta,N, and the cubic phase observed at lower temperatures. The unit-cell parameters of Ta,N, and the baddeleyite phase showed no variation with x. The unit-cell parameters of the baddeleyite phase were close to those of ZrO, and the parameters of Ta,N, were negligibly larger than those observed for x=O. The amount of the cubic phase varied unsystematically with x and was, as for preparations at 900 "C, higher for gels hydrolysed under acidic conditions.The samples with x =0.80 and 0.90 contained the baddeley- ite phase. a small amount of Ta,N, and a cubic phase with a face-centred cell with a =4.313(2)A. This unit-cell parameter and observed reflection ipensities are similar to those reported for cubic TaN, a=4.33 A (JCPDS no. 32-1283). The sample andwith x= 1 contained Zr708N4 (/I)a small amount of baddeleyi te. The observe! hexagonal c~ll parameteTs of Zr,O,N, are a=9.549(2) A, c=8.815(4) A, V=696.1 A3, in general agreement with the parameters given in ref. 18. A schematic illustration of the phases observed at different preparation temperatures is given in Fig. 4. Baddeleyite-type Phase Zr,Ta, -xO1+ xN,-0 d x <1 A phase with the baddeleyite (monoclinic ZrO,) type structure was obtained essentially X-ray monophasic for all composi- tions between the end-members TaON and ZrO, by ammon- olysis of the Ta-Zr gels at 800°C for 12 h.The stryngest reflection from the orthorhombic phase, 11 1 at dz 2.95 A, was present in the powder patterns for x=O.52 and 0.68. As described above, increasing the ammonolysis time resulted in the formation of the Ta3N5-type phase for compositions with x60.18. The unit-cell parameters obtained for the end-members TaON and ZrO, agreed well with literature data.l43l5 A linear increase in cell volume of ca. 10% is observed when going from TaON to ZrO, (Fig. 5). The increase is a consequence of the replacement of Ta5+ ions by larger Zr4+ ions, albeit counteracted to a small extent by the simul- taneous substitution of N3-ions by smaller 0,-ions.The Shannop-Prewitt ionic radii23 for the ions ar:: Ta5+ VII)=0.78 A, 0,-(IV) = 1.38 A and(VII)=0.69 A, 0Zr4+( N3-(IV)= 1.46 A. The a, h and c axes increase in a similar, essentially linear, manner with Zr content, as shown in Fig. 6. Deviations from linear behaviour are, however, discernible for 0.7<x <0.9; they might arise from different ordering of N3-and 02-ions (see below). The monoclinic B angle showed only a slight linear variation between the end compositions. The Ta:Zr ratios in the baddeleyite phase preparations were verified for two compositions, with nominal x values of 0.68 and 0.52, by EDX(SEM) analysis. The analyses yielded the experimental values x =0.66( 1) and 0.53( l), respectively, in good agreement with the nominal values.The thermal stability of the baddeleyite compounds in air was examined by TG heating runs (Fig. 7). The compounds were oxidised at elevated temperatures according to: Zr,Ta, -xO1 -x +3/4( 1 -x) 0, =ZrxTa1-x05,2-x,2+1/2( 1 -x)N2 1201 / II I I I II 0.0 0.2 0.4 0.6 0.8 1.0 x in Zr,Ta,-, Fig. 5 Unit-cell volumes of ss phases us. x in Zr,Ta,-,; e,baddele-yite-type phase; V,orthorhombic phase; 0,cubic phase; V. ortho-rhombic Zr0,;27 N, cubic Zr0214 5.4 5.3 5 c.$ 5.2 E E! (dQ --8 5.1 5.0 4.9 I I 1 I I I. 0.0 0.2 0.4 0.6 0.8 1.0 x in ZrxTal -fll+PI-cFig. 6 Cell parameters a (O), b (H), (V) t's.x for baddelFyite ZrxTa, -,O, +,N-,. EstimatFd standard deviations are <0.004 A for 0.26dx d 0.80 and <0.001 A for the remaining compositions. 6rc I I I __I 0 300 600 900 1200 TIOC Fig. 7 TG curves for the oxidation of baddeleyite phases Zr,Tal-,Ol+xN1-x in air. Heating rate 10°C min-'. (a) 'TaON, (h)x =0.34, (c) x =0.52, (d) x =0.68. For TaON (x=0) the oxidation effectively takes place in the temperature range 760-880 "C, although a small weight increase is noted to commence at cu. 620°C. For higher x values the oxidation is observed to begin at lower tempera- tures, cu. 500"C, and to occur over an increasingly broad temperature range with increasing Zr content. The observed increase in weight agrees well with values expected for complete oxidation of the nominal oxynitride compositions.The 0 and N contents for x=0.40 and 0.60 were also determined by the combustion method. Assuming nominal Ta: Zr ratios, the results of the analyses were con- J. MATER. CHEM., 1994, VOL. 4 The Rietveld refinement demonstrates that the Zro,4Tao,601.4No,6compound has a structure similar to the MX2 baddeleyite type. The baddeleyite structure is illustrated in a (100) projection in Fig. 9, with atomic coordinates for Zro~4Tao,601.4N0,6.Each M atom is coordinated by seven x atoms, a triangle of X( 1)atoms and a planar square group of X(2) atoms. The two planes defined by the X(1) and X(2) atoms, respectively, are roughly parallel and the structure displays layers of X( l)-M-X(2)-M parallel to (100).The X( 1) atoms are triangularly, and X(2) tetrahedrally, coordinated by M atoms. In the homeotypic structure of TaON, the 0 and and N atoms are completely ordered, with 0 atoms on X( 1)sistent with the compositions ~ro~40~ao,60~1~50~1~~o~5~~l~ Zro,60Tao,4001.69~1~No~37(1~,respectively. The N contents agree well with those obtained from the TG experiments, while the 0 contents are higher than expected. However, since the nominal and experimentally determined Ta :Zr ratios are found to agree well; the combustion analyses thus yield a total anion content which is too high. The determined 0 contents hence seem to contain a systematic error. Rietveld Refinement of Zro.,Ta,60,,No, having the Baddeleyite-type Structure The baddeleyite-type structure of Zro,4Ta,,60,.4No,6 was refined, from XRPD data, using a Rietveld package.24 The final refinement was carried out with a total of 22 parameters and using the pseudo-Voigt profile function (refined Lorentzian fraction =0.66).The number of theoretical Bragg reflections for 28<92" was 127 and the half-width of the peaks was 0.50" in 28 at 28=48". The X-ray data did not allow any distinction between different possible orderings of 0 and N atoms, and these were accordingly statistically distributed over the two available anion sites. Fig. 8 shows the fit between the calculated and observed patterns. A list of atomic coordinates is given in Table 1, with the esds multiplied by 5.5 in order to account for serial ~orrelation.~~An absorption correction, of the form exp(-pR sin O), yielded no significant changes in atomic coordinates and thermal parameters 1.7(1) and 1.3(2)A for the (Ta,Zr) and (0,N) atoms, respectively.10000 8000 v) -c2 6000s-.-4000 a,+ (I._ 2000 0 I 20 I 30 I 40 I 50 1 60 I 70 I 80 LJ 90 2Wdegrees Fig. 8 Observed and difference intensity X-ray patterns of zr0.4Ta0.601 .4N0.6 Table 1 htomic coordin$es for Tao.6Z~o.401,,No.6; monoclinic, (I= 5.043(2) A, b=5.101(2) A, ~=5.243(2)A, fi=99.49(2)" P2,/c, Z=4 atom x (Ta,Zr) 0.287( 1) (O,N)(1) 0.07(1) (O.N)(2) 0.44( 1) R, =4. 7 Yo, R,, =6.0Yo, R, Y Z B/A 0.0417(6) 0.33(1) 0.76( 1) 0.213( 1) 0.34( 1) 0.47 ( 1) LO( 1) 0.3(8) 0.3(8) =4.1Yo, R, =3.5Yo.sites and N atoms on X(2) sites, resulting in a layer sequence of N-Ta-O-Ta. The atomic array formed by the X(2) layer and the M atoms directly above and below this plane is the same as the atom arrangement in the fluorite (CaF,) or cubic zirconia structure. The major dissimilarity between the badde- leyite and fluorite structures is thus a different arrangement of atoms in the X( 1) layer. The M-X distances in the Tao,6Zro,401,4No.6 baddeleyite phase, M =(Fao,6Zro,4)X =(Oo,7No.3)2range between 2.02( 5) and 2.21(5) A, the average being 2.12 A. The average distances to the three X( 1) and four X(2) atoms can be compared with corresponding average distances in monoclinic Zr0,14 and TaON:" TaON (x =0) Zr,,,Ta, 6014No6 ZrO, (x = 1) average M-X/A average M-X( 1)/A average M-X( 2)/A 2.09 2.07 2.11 2.12 2.09 2.15 2.16 2.09 2.21 The average M-X( 1) distance is comparatively constant in the three compounds, while the average M-X( 2) distance increases with x.This implies a relative displacement of the M atoms towards the X(2) layer with decreasing x. For TaON this displacement has, together with observed short N-N distances, been interpreted12 as evidence for considerable covalency in the Ta-N bonds. The distances observed in ~ao,6~ro~4~l~4~o,6accord with a linear variation of the average M-X(2) distance with x, which would indicate that the N atoms in ~ao,6~ro~4~l~4~~~6 are located only on the X( 2) sites.This possibility cannot, however, be verified by the Rietveld analysis. Neutron data would help to distinguish possible ordering of 0 and N atoms and also increase the precision of the anion positions. Fig. 9 Baddeleyite (monoclinic ZrO,) structure projected on (100). The numbers represent the x coordinate in %. Anion positions X( 1) and X(2) are illustrated by unfilled and shaded circles, respectively. J. MATER. CHEM., 1994, VOL. 4 Orthorhombic Zr,Ta, -xO1+,N,-x Phases Materials obtained by ammonolysis of xerogels at 700°C were biphasic for 0.26 d x d 0.90, containing the baddeleyite- type phase and a phase exhibiting a powder pattern that could be, indexed with an orthorhombic unit cell with a,b,c=5 A. The orthorhombic phase was observed as the major phase only for +=0.52 and 0.$8, with unit-c$l parameters a=4.908(2) A, b=5.265(2) A, c=5.138(2) 4, 1/=132.8 A”for ~=0.52~anda=4.960(1)A, b=5.267(1)A, c =5.110(1)A, V= 133.5 A3 for x =0.68.The indexed powder pattern for x =0.68 is given in Table 2. The unit-cell volumes are smaller than those of corresponding baddeleyite phases (see Fig. 5). Two similar, alternative, structural models, an ortho-rhombic ZrO, type structure and an a-PbO, type structure, were both found to yield good agreement between calcu- lated and observed intensities for the orthorhombic phase. Orthorhombic ZrO, is a high-pressure modification, found in considerable quantities in transformation-toughened ceramics cooled to low temperatures.,, The unit-cell volume of this ZrO, modification is 3.3% smaller than that of the monoclinic modification.The orthorhombic ZrO, structure27 is very similar to the monoclinic baddeleyite modification; in both of them there is a coordination of Zr atoms by seven 0 atoms. The spaceogroup is Pbc2, ?nd the unit-Celt parameters are a =5.068( 1) A, b =5.260(1)A, c =5.077(1)A and V= 135.34A3. Similar unit-cell volumes and parameters are also found for oxides adopting the a-Pb0,-type structure.28 The a-Pb0,-type structure, with space group symmetry Pbcn, is commonly idealised as an hcp structure, with M atoms in edge-sharing MX, octahedra that form zigzag strings in the c direction. The structure can, however, be topologically trans- formed into the fluorite structure29 and consequently com- pounds with a-Pb0,-type structures are frequently found to be intermediate between the idealised a-Pb0, and the fluorite structure. Three sets of XRPD data were collected for the orthorhom- bic type phases.Rietveld refinements of the orthorhombic ZrO, structure model converged to RF=4%o for all three sets of data. However, only slightly higher RF values, ca. 5%0,were obtained using the alternative a-PbO, structure model. The two structure models are, in principle, distinguishable by hkO Table 2 Observed and calculated 28 values for the Guinier-Hagg diffraction pattern of orthorhombic Zro,,,Tao,320L,8No,32 up to the 20th observed line [A8 =28,,, -28,,,, i= 1.54098 A the corresponc- ing cell parameters are a =4.960( 1) A, b =5.267(1)A, c =5.1 10( 1) A] hkl 2O,,,/degrees A0/degrees dobs/A If10 110 24.607 -0.027 3.615 4 111 30.284 -0.027 2.952 100 020 34.039 0.022 2.632 12 002 35.069 -0.028 2.557 13 200 36.172 -0.016 2.48 13 10 02 1 38.419 -0.006 2.3412 3 112 43.335 -0.015 2.0863 2 022 49.707 0.026 1.8327 12 220 50.517 0.008 1.8052 16 202 51.258 -0.040 1.7809 14 22 1 53.820 0.012 1.7020 3 130 55.501 0.023 1.6544 4 131 58.589 0.006 1.5743 9 113 59.990 -0.017 1.5408 11 311 61.469 0.005 1.5073 12 222 63.015 0.024 1.4739 3 023 132 65.151 67.362 -0.028 -0.020 1.4307 1.3894 1 004 74.197 0.023 1.2770 3 04 1 74.328 -0.006 1.2751 2 reflections with h +k # 2n, which are allowed in Pbc2, for orthorhombic ZrO,, but systematically absent in Pbcii for an a-Pb0,-type structure.These reflections are, however, very weak for an orthorhombic Zr0,-type structure, and the presence or absence of them could not be unambiguously established. The correct structure could thus not be singled out on the basis of the X-ray intensity data. Further structural studies are in progress to settle the correct structure of the orthorhombic phases. A strong argument favouring the orthorhombic ZrO, model is provided by the observed unit-cell parameters. The observed unit-cell parameters of the orthorhombic phase are depicted in Fig. 10, together with the reported cell parameters for a number of a-Pb02-type structures.3w35 For the a-Pb0,-type structures the orthorhombic cell parameters decrease in the order b>c>a.Cell axis ratios reported in the litcrature range from 1.09 to 1.12 for b/c and 1.05 to 1.08 for c, a. The orthorhombic Zr,Ta, -,01+,N1 -,phases exhibit cell axis ratios which are much closer to unity, b/cz 1.02-1.03 and c/a~~l.02-1.05,and are similar to the axis ratios for ortho- rhombic ZrO,, b/c =1.04 and c/aFZ 1.00. Additional studies currently in progress, indicate that this orthorhombic phase is stabilised by additions of small amounts, 3-7 mol%, of Co, Cr and Fe. The unit-cell param- eters of these stabilised phases conform well with those given above for the Zr,Ta, -xO1+,N1-,phases. Cubic and Pseudocubic Zr,Ta, -x(O,N)y(y d 2) Phases For 0.26 d x <0.80, ammonolysis at 900 “C yielded biphasic materials.According to X-ray analysis these contarned a apparently cubic ss phase as major phase, together with small amounts of an Ta,N,-type phase. The same phase was also found, in smaller amounts, together with a baddeleyite phase in samples with x=O.9, prepared at 900 and 1000°C. The powder patterns of the phase were basically the same ;is that of the cubic fluorite type. The reflections were broad, which hampered the evaluation of the film data. The volumes of the fluorite type cells are shown in Fig. 5. No superstructure reflections were observed for x =0.80 and 0.90, and the structure thus appears to be of the cubic zirconia type for these compositions.The weight increase upon oxidation for the sample with x =0.80 corresponded, however, to a metal anion composition MX1.71. This suggests that the phase may be a disordered relative of the rhombo- hedral Zr oxynitride p phase, Zr,08N,. 4.41 1 I I J 0.60 0.65 0.70 0.75 average cationic radius/A Fig. 10 Cell parameters us. average Shannon-Prewitt cationic radius for orthorhombic ss Zr-Ta oxynitride phases (U), orthorhombic ZrOZ2’ (E) and selected cc-PbO, type phases; (i) Ti02,30 (ii) Taq. 9Feo.9Zn0. 0,.*F0.2,3 (iii) (Zr0.33Tio.67 )204,32 (iv) Hfli04, 33(v) ZrT10,,3~ (vi) ZrSno~,Ti,,,0,35 The powder patterns for 0.26 dx d0.68 contained five or six weak superstructure reflections, which could be indexed using a body-centred cubic cell with a doubled cell axis.The indices and observed intensities in YOof the strongest reflection are; (211) 2-4%, (411) 1-3%, (332) 1%, (541) 1-3% and (631) 1%. Powder patterns of compounds with the C-type rare-earth-metal structure, e.g. Zr,ON,, exhibit medium-intensity reflections at corresponding reflection positions. The observed intensities are, however, apparently much too weak in order for the Zr,Ta, -x(O,N)yphases to have this structure. The weight increase upon oxidation for x= 0.33,0.52 and 0.68 was found to correspond to a metal anion composition of MX1,85-1.87.These anion contents are likely to be somewhat too low, owing to the presence of small amounts of the Ta,N,-type phase. Electron microdiffraction patterns of this phase suggested two orthogonal cell axes of ca.14 and 7 A. Similar axes have been reported for the unit cell of the reduced and metastable rare-earth-metal oxide Tb,,0,036 or Tb01.875. The unit cell of Tb1,030 is metrically monoclinic and related to the fluorite cell a,, bf, cf by; a = a, + bf/2-cf/2 b=2bf+2cf c=-b f+Cf If a, = 5.00 A,the monoclinic cell parameters become a = 6.12, b= 14.1, c=7.07 A and b= 125.3". Electron microdiffraction patterns for the Zr,Ta, -x(O,N)y phase with x = 0.52 were found to be compatible with this type of unit cell. It appears plausible that the Zr,Ta,-,(O,N), phase has a metal anion composition MX1.875,as does Tb,6030, since the same type of unit cell is indicated for the phases.This implies that the unit cell contains two anion vacancies. A structural model may be arrived at by assuming that the two vacancies form pairs across metal atoms (along 1/2 [ ill],), a common vacancy arrangement in reduced rare-earth-metal oxides.37 Such a model cannot be verified presently, however, for lack of sufficient XRPD data. Ta,N,-type Phases X-Ray monophasic samples containing an Ta,N,-type phase were obtained at 900 and 1000°C for xG0.18. The colours changed from vermilion for x=O to red-brown for x=O.18. The unit-tell parameters qf Ta,N, were qbtained as a,= 3.8900(4) A, h= 10.224(1)A, C= 10.273( 1) A, V=408.6 A3. The unit-cell parameters were found to increase, as expected, with Zr content, but also to depend on the preparation temperature.When prepared at 900 "C, thc composition x; 0.18 thus yielfled a cell wiih u=3.919( 1) A, b= 10.258(2) A, c= 10.338( 1) A, V=415.6 A,, whereas the preparation at 1000 "C agave significanLly smaller celj parameter?: a = 3.909( 1) A, b= 10.244(2) A, c= 10.317(2) A, V=413.2 A,. The recorded weight gain upon oxidation for the latter sample was 8.0 wt.%. This agrees well with a calculated value of 7.9 wt.% for a composition Zro,53Ta2,47N4,4700,53,which accords with a replacement of Ta+N for Zr+0. The prep- aration at 900°C yielded a smaller weight gain, however, 7.5 wt.%. These observations suggest that the Ta,N,-type phases have varying anion compositions (cJ:Discussion). Discussion Ammonolysis studies of materials with mixed Zr-Ta composi- tions have not been reported earlier and the present results can thus only be compared with previous findings for com- pounds containing exclusively Ta or Zr.Considering the J. MATER. CHEM., 1994, VOL. 4 various parameters that may influence the formation of phases during ammonolysis, it is not surprising that somewhat differ- ent observations are made in different studies. Brauer et a!.,' when ammonolysing Ta205, found that the formation rate of Ta3N5 is dependent on temperature, flow rate of NH,, the efficiency of the removal of 0, and H20 from the reaction zone, and the amount and presynthesis of Ta,O,. During ammonolysis, the oxygen activity in the sample is, furthermore, expected to change continuously as oxygen is withdrawn.In the present study, the ammonolysed materials were cooled inside the furnace at a comparatively slow rate, introducing the possibility of phase transformations upon cooling. Brauer et al. prepared Ta,N, by ammonolysing Ta,O, at 800-900 "C. Nitrides with higher Ta : N ratios were found to form above 900 "C. Fontbonne and Gilles" prepared Ta,N, in the same way at 850°C. At 975 "C they obtained Ta,N,. Above 975 "C changes in the X-ray powder pattern of Ta,N, were observed, together with the appearance of peaks ascribed to E-TaN and d-TaN0.8,.,. Treatment of Ta,N, in argon at 775"C/12 h was found to yield a compound deficient in nitrogen, having the composition Ta3N4.66(2). The authors found, furthermore, that a cation-deficient Ta,N,-type phase formed when mixed Ta-Nb oxides were ammonolysed.Phases of the Ta,N, type may accordingly exhibit anion as well as cation deficiencies. In the present study we obtain Ta,N, as a dark red and X-ray monophasic material at 900 and 1000 'C, and a mixture of Ta,N, and a small amount of Ta,N, at 1100°C. Our finding that the Ta,N, type phase with metal composition Zro,18Tao,,2 exhibits a varying 0: N content, depending on preparation temperature, conforms with the previous report of non-stoichiometry in Td,N,-type phases. Brauer et al. prepared pure TaON by alternating and successively smaller nitridations and oxidations in atmos-pheres with different oxygen contents. It is not clear from studies made hitherto if the oxynitride TaON is stable in NH, or if it is formed as an intermediate phase, in which case the formation of monophasic samples may be connected with an oxygen buffering by the sample.Gilles et al. obtained Zr,08N4 (b)by ammonolysis of ZrO, at 1100"C.'7 At 950°C Zr20N2 (y) was obtained, with forming as an initial intermediate phase. Ohashi et a/.prepared both and y at 900-1000 "C, by reacting mixtures of b-ZrNC1 and ZrO, in an NH, atmosphere." In the present study, only baddeleyite was observed at 1000 "C,and at 1100"C a mixture of Zr708N, and a small amount of baddeleyite. Conclusions Four different solid solution phases are found in materials prepared by ammonolysis of Zr-Ta gels at temperatures between 700 and 1100°C: (i) a pure baddeleyite-type phase is obtained at 800°C between TaON (x=O) and ZrO, (x=1) for Zr,Ta, -,O, +xN1-,.(ii) An orthorhombic oxynitride phase (probably of the orthorhombic ZrO, type) is observed in preparations at 700 "Cfor compositions with 0.26 < x < 0.68 for ZrxTal-xOl+xN1-x. (iii) A cubic phase with a fluorite- type subcell is present in preparations with 0.26 < x< 0.68 at 900 "C. The observed weight increase upon oxidation corre- sponds to compositions MX, ,85-1 .s7.Electron microdiffraction patterns indicate a monoclinic unit cell with a = J6/2 af,b = J2af, c = 2,/2af and = 125.3", with a, = equivalent cubic fluorite cell edge. (iv) A monophasic Ta,N, type phase, which contains up to 18 atom% Zr, is obtained at 900 and 1000°C.The phase compositions obtained by ammonolysis are found to be dependent on the hydrolysis conditions for the Zr-Ta gels. Preparations made with gels hqdrolysed under acidic conditions were biased towards phases that form at lower temperatures. J. MATER. CHEM., 1994, VOL. 4 1301 Our results for x =0 and x =1 generally agree with previous studies of compounds formed by ammonolysis of pure Zr and Ta oxides. 16 17 18 R. Juza, A. Rabenau and I. Nitschke, Z. Anorg. Allg. Chem., 1964, 332, 1. J-C. Gilles, Bull. SOC. Chim. Fr., 1962,2118. R. Collongues, J.-C. Gilles, A. M. Lejus, M. Perezy Jorba and D. Michel, Muter. Res. Bull., 1967,2, 837. The authors thank Prof. M. Nygren for support and valuable 19 20 R. Norrestam, Ark.Kemi, 1968,29,343. M. Ohashi, H. Yanamoto, S. Yamanaka and M. Mattoti, Mater. discussions and Mr. G. Westin for advice concerning the preparation of the Zr-Ta gels. 21 Res. Bull., 1993, 28, 513. M. R. Thornber, D. J. M. Bevan and J. Graham, Acta Crj~stullogr., Sect. B, 1968,24, 1183. 22 K. E. Johansson, T. Palm and P-E. Werner, J. Phys. E, 1980, 13, 1289. References 23 24 R. D. Shannon, Acta Crystallogr., Sect. A, 1976,32, 751. D. B. Wiles, A. Sakthivel and R. A. Young, Users Guide to 1 2 3 4 5 6 7 8 9 10 11 R. Marchand, F. Pors and Y. Laurent, Rev. Int. Hautes Temp. Rkfruct., Fr., 1986, 23, 11. P. Antoine, R. Marchand, Y. Laurent, C. Michel and B. Raveau, Muter. Res. Bull., 1988,23, 953. P. Antoine, R. Marchand and Y. Laurent, Rev.Int. Hautes Temp. Rifract., Fr., 1987, 24,43. R. Marchand, C.R. Acud. Sci. Paris, Ser. C., 1976,282, 329. X. H. Wang, A. M. Lejus, D. Vivien and R. Collongues, Muter. Res. Bull., 1988,23,43. G. Liu and H. A. Eick, J.Solid State Chem., 1990,89,366. A. Joly, C.R. Hehd. Siances Acad. Sci., 1876,82,1195. G. Brauer, J. Weidlein and J. Strahle, Z. Anorg. Allg. Chem., 1966, 348,298. J. Strahle, Z. Anorg. Allg. Chem., 1973,402,47. N. E. Brese, M. O’Keeffe, P. Rauch and F. J. DiSalvo, Acta Crystallogr., Sect. C, 1991,47,2291. P. Tiedemann and H. K. Muller-Buschbaum, Z. Anorg. Allg. Chem., 1982,494,98. 25 26 27 28 29 30 31 32 33 Program DB W3.2S for Rietveld Analysis of X-ray and Neutron Powder Diffraction Data Patterns (Version 8804), School of Physics, Georgia Institute of Technology, Atlanta. J-F. BCrar and P. Lelann, J. Appl. Crystallogr., 1991,24, 1. C. J. Howard, E. H. Kisi, R. B. Roberts and R. J. Hiil, J. Am. Ceram. SOC., 1990,73,2828. E. H. Kisi, C. J. Howard and R. J. Hill, J. Am. Ceram. SIX, 1989, 72, 1757. B. G. Hyde and S. Andersson, lnorganic Crystal Structurta, Wiley, New York, 1989, p. 69. B. G. Hyde, L. A. Bursill, M. O’Keeffe and S. Andersson, Nature (London), 1972,237,35. I. E. Grey, C. Li, I. C. Madsen and G. Braunshausen, Mder. Res. Bull., 1988,23,743. G. Pourroy, E. Lutanie and P. Poix, J. Solid State Chem., 1990, 86,41. A. Willgallis and H. Hartl, Z. Kristallogr., 1983, 164, 59. A. Harari, J-P. Bocquet, M. Huber and R. Collongues, C R. Acad. Sci. Paris, 1968, 267, 1316. 12 D. Armytage and B. E. F. Fender, Acta Crystallogr., Sect. B, 1974, 34 P. Bordet, A. McHale, A. Santoro and R. S. Roth, J. Solid State 30,809. Chem., 1986,64,30. 13 M. Weishaupt and J. Strahle, Z. Anorg. Allg. Chem., 1977, 429, 261. 35 36 A. Siggel and M. Jansen, Z. Anorg. Allg. Chem., 1990,582,93. R. T. Tuenge and L. Eyring, J. Solid State Chem., 1982,41, 75. 14 C. J. Howard, R. J. Hill and B. E. Reichert, Acta Crystallogr., Sect. B, 1988,44,116. 37 E. Schweda, D. J. M. Bevan and L. Eyring, J. Solid State Chem., 1991,90, 109. 15 A. Fontbonne and J-C. Gilles, Rev. lnt. Hautes Temp. Rkfract., Fr., 1969,6, 181. Paper 3/07020D; Received 25th November, 1993

ISSN:0959-9428    

DOI:10.1039/JM9940401293

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4(8), 1303-1305 Li,Ni,TaO,: A Novel Rock Salt Superstructure Phase with Partial Cation Order James G. Fletcher," Glenn C. Mather," Anthony R. West," Maria Castellanosb and Maria Pilar Gutierre? a University of Aberdeen, Department of Chemistry, Meston Walk, Aberdeen, UK AB9 2UE Universidad Nacional Autonoma de Mexico, Facultad de Quimica, Mexico DF 04510, Mexico The new phase Li,Ni2Ta06 has a novel rock salt superstructure, a =8.4259(3) A, b =5.9073(3) A, c =17.7329 A, Fddd. Within the cubic-close-packed oxide array, Ta occupies one set of octahedral sites giving isolated TaO, octahedra which edge-share with Li/NiO, octahedra. Li and Ni are distributed non-randomly over three other sets of octahedral sites with Li :Ni occupancy ratios of 0.73 :0.27, 0.59 :0.41 and 0.55 :0.45, respectively.The higher Li :Ni ratio of the first set, 0.73:0.27 may be associated with cation-cation repulsion effects since this site is closest to Ta. Li,Ni,Ta06 is a very modest semiconductor with conductivity of ca. 4 x lop6R-' cm-' at 300 "Cand activation energy 0.77 eV; the disorder in the Li' site occupancy is, therefore, static and does not yield significant levels of Li+ ion conductivity. Several examples of complex oxides with rock salt superstruc- tures are known. They usually have fully ordered cation arrangements and different ordering sequences are possible, depending on the cation ratios, e.g. Li,TiO,, Li3Nb04, LiFeO, and LiCoO,. Recently, a family of phases Li,MXO,: M =Mg, Mn, Fe, Co, Ni, Cu, Zn; X=Zr, Hfl,, was discovered with partial cation disorder.The pairs of ions MX are disordered over one set of octahedral sites, giving a crystal structure which is essentially the same as that of ~r-LiFeo,.~ Here we report a second kind of rock salt superstructure with partial cation order but in this case, the disordered cations, Li+ and Ni2', exhibit a most unusual non-random distribution over three sets of octahedral sites. L13Ni2Ta06 was synthesized by solid-state reaction of Li,C03, NiO and Ta,O, in Pt crucibles. Samples were fired over the range 600-11OO0C, initially to expel CO, at 600-700 "C and then at higher temperature to complete reaction. After an intermediate regrinding, final firing was at 1100°C for 2 days.The green product was characterised by X-ray powder diffraction (Siemens D500, Cu-Kq radiation) and found to be very similar to the previously reported Li,Mg,XO,: X =Nb, Ta, Sb,4 whose patterns were indexed but whose structures were unknown. X-Ray data suitable for accurate lattice-parameter determination and Rietveld refinement were collected on a Stoe Stadi P powder diffractometer with a linear position-sensitive detector, Cu-Kcr, radiation, Ge monochromator; for cell-parameter determination, Si internal standard was added, collection time 3 h; for Rietveld refinement, Si was not added and the collection time was 14h over the range 1O"C<2~<11O"C. Various programs in the Stoe software package were used for data handling, processing and refinement. The powder diffraction data were indexed by analogy with those of the phases Li,Mg,X0,4 on an orthorhombic cell, space group Fddd [no.70, second setting]. By assuming a rock salt superstructure, structure models were generated using the software program THEO, taking into account the most likely cation ordering sequences and differences in intensities of certain reflections for different members of the Li,M,XO, family. Before the calculated pattern was allowed to refine, the profile of the observed pattern was fitted to a squared Lorentzian function to describe the shape of the Bragg reflections. Thirteen parameters describing the calculated pat- tern were initially refined, including halfwidths, 28 zero-point, unit-cell dimensions, scale factor and background coefficients.Structural parameters were then refined. Comparison of calcu- lated and observed patterns had indicated the probable cor- rectness of a rock salt superstructure model in which Ta sites are fully occupied but Li and Ni are disordered over three sets of crystallographic sites. The positional parameters and thermal vibration parameters of the three Ni/Li sites were refined initially as rigid units with the occupancy factors set initially to 0.6 and 0.4 for Li and Ni, respectively, according to the chemical composition Li,Ni,TaO,. Positional param- eters of Li/Ni were refined first followed by oxygen positions. Attempts to refine both the nickel and lithium site occupancies and their Uiso values simultaneously were unsuccessful.As a result, it was necessary to choose sensible invariant values for the thermal vibration parameters and allow occupancies to vary with the restriction that total individual site occupancies must be unity. The occupancy factors obtained for Li and Ni corresponded approximately to the total expected number of ions per unit cell. Finally, isotropic thermal vibration param- eters of Ta and 0 were refined. The final observed and difference patterns are shown in Fig. 1; R,=7.56%, R,,=10.25% andR,=6.81%. Ashortened list of indexed powder X-ray data for Li,Ni,TaO6 is shown in Table 1 and crystallographic data obtained by Rietveld refinement in Table 2. The structure may be considered as a rock salt superstruc- 10all7500 i! 25 50 75 100 28ldegrees Fig.1 (a) Observed and (b) difference powder diffraction pattern of Li,Ni,Ta06 J. MATER. CHEM., 1994, VOL. 4 Table 1 X-oRay powder $iffraction data (I for Li,Ni,TaO,: a = Table 3 Ta-Li/Ni distances/A 8.4259( 3) A, b =5.9073( 3) A, c = 17.7329( 6) A distance to 4.6658 4.6665 100 111 Li/Ni ( 1) 2.974(x 2) Li/Ni (2) 3.015(x 4) Li/Ni( 3) 2.954( x 2) 4.4339 4.4332 33 004 2.957(x 4) 3.8052 3.8052 51 022 3.7436 3.7434 57 113 2.8601 2.8024 2.5110 2.4 192 2.3310 2.2441 2.2167 2.1230 2.1067 2.0893 2.8601 2.8023 2.51 10 2.4195 2.4185 2.3310 2.2441 2.2166 2.1231 2.1065 2.0892 32 22 19 60 13 12 6 17 33 67 115 202 131 026 220 133 117 008 224 040 206 ture with eight formula units per unit cell.The relationship of supercell and subcell is: a =J2aSub, b =2aSub,c =3420,"~. The superstructure arises as a consequence of Ta ordering on the 8a site in an attempt to minimise Ta-Ta repulsions. The other octahedral sites are filled non-randomly by Li,Ni. The Li/Ni( 3) site shows a strong preference for Li whereas Li/Ni( 1) shows a slight preference for Ni. The occupancy of Li/Ni(2) is very close to the statistical ratio 0.60 :0.40. Fig. 2 shows a projection along the [loo] zone axis. The 2.0635 1.9064 1.8243 1.7925 1.6840 2.0632 1.9063 1.8247 1.8239 1.7925 1.6838 11 9 13 5 11 135 311 119 313 137 242 TaO, octahedra are isolated from each other but share edges with various Li/Ni octahedra. Average bopd distances are Ta-0, 1.96(2) A; Li/Ni(l)-0, 2.11(3)A; Li/Ni(2)-0, 2.13(4) A;Li/Ni(3)-0, 2.17(8) A.Cubic-close-packed oxide layers in two orientations can be seen in Fig.2. Between any pair of close packed layers, the octahedral sites are occupied by Ta and Li/Ni in the ratio 1:5. The higher Li content of the Li/Ni( 3) site may be associated b -11.36 38.88 39.86 38.88 with cation<ation repulsion effects since the distance from this site to Ta, 2.954A, is the shortest of all the Li/Ni to Ta distances, Table 3. These diffraction results do, of course, give only an average structure and it is possible that a more complex structure, with local cation order, is in fact present. C Conductivity measurements were made on a sintered pellet of Li,Ni,TaO, with Au electrodes, using Solart ron 1250/1286 A / 39'86 14.61 and Hewlett Packard 4192 impedance instrumentation.The impedance data showed an essentially bulk response, with relatively small grain-boundary impedances. Little evidence of electrode polarisation effects was seen, indicating the charge carrier to be electronic. A conductivity Arrhenius plot is B / -21 C/ A 0,-0 -/ I -5 B 0 Ta 0Li/Ni 0oxygen -61 1.0 1.2 1.4 1.6 1.8 Fig. 2 Crystal structure of Li3Ni,Ta0, projected onto bc plane 1O~WT Fig. 3 Conductivity Arrhenius plot (E, =0.77 eV) Table 2 Structure refinement data for Li,Ni,TaO, atom site xla Ylb c/z uiso occupancy (n) ~ Ta(1) 8(4 118 1/8 1/8 0.0064( 7) 1.oo Li( l)/Ni( 1) 16k) 1/8 1.8 0.2928( 7) 0.0225 0.550(9)/0.450( 9) Li(2)/Ni(2) 1m) 1/8 5/8 0.2864( 7) 0.0225 0.%6( 9)/0.414( 9) Li(3)/Ni( 3) 8(b) 1/8 518 1/8 0.0225 0.73( 1)/0.27( 1) O(1) 16(f) 1/8 0.357( 2) 118 0.02 1 (6) 1.oo O(2) 32(4 0.1 10( 2) 0.378( 2) 0.2969( 9) 0.024 (4) 1.oo J.MATER. CHEM., 1994, VOL. 4 shown in Fig. 3, which was reversible on heat/cool cycles; the data indicate a very low level of electronic conductivity. Thus, in spite of the fact that the crystal structure contains considerable disorder in positions of Li' ions, this disorder is static and does not lead to significant levels of ionic conductivity. We thank SERC for financial support (A.R.W.) and the British Council for supporting the Aberdeen-Mexico exchange pro- gramme and we gratefully acknowledge financial support for project UNAM IN 101893 (M.C.) from the Mexican authorities. References 1 M. Castellanos, A. R. West and W. B. Reid, Acta Crystallogr., Sect. C, 1985,41,1707. 2 M. Castellanos, M. Chavez Martinez and A. R. West, 2. Kristallogr., 1990, 190, 161. 3 E. Posnjak and T. F. W. Barth, Phys. Rev., 1931,38,2234. 4 M. Castellanos, J. A. Gard and A. R. West, J. Appl. Crystallogr.,1982,15, 116. Paper 4/01963F; Received 31st March, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401303

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4(8), 1307-1308 Synthesis and Properties of a New p Polymorph of Li,CrO, M. A. K. L. Dissanayake, Susana Garcia-Martin, Regino Saez-Puche,+ H. H. Sumathipala and Anthony R. West Department of Chemistry, University of Aberdeen, Meston Walk, Aberdeen, UK AB9 2UE D-Li,CrO,, isostructural with p-Li3P04, has been synthesized by two routes. Reaction of Li,CO, and Cr203 in a 5 :1 mole ratio in flowing Ar at 800°C yields j3-Li,Cr04 with a small amount of Li,O. The Li2C0, seems to have two functions. It is an essential reactant and acts as a novel oxidant, promoting conversion of Cr"' to Crv. Reaction of Li,CO, and CrO, in a 3 :1 mole ratio in flowing Ar also gives phase-pure P-Li,CrO,. Magnetic measurements show Curie-Weiss behaviour in the range 20-300 K with a magnetic moment of 1.70 pB consistent with Crv in tetrahedral coordination.Conductivity measurements show a modest level of electronic conduction. Stable oxides of CrV are relatively rare. Li,CrO, containing CrV has, however, been prepared by stoichiometric reaction of Li2Cr0, and Li,O.' Subsequently, a single crystal was grown and shown to be isostructural with the higher-temperature y polymorph of Li,P0,.2 Li,PO, is a poly-morphic oxide and the crystal structures of both the high-and low-temperature forms have been determined.,,, The low- temperature p structure has the basic wurtzite structure in which only one set of tetrahedral sites is occupied, but with cation ordering. The high-temperature y structure also has hexagonal close-packed oxygen layers, although the layers are more buckled than in the p structure; in y-Li,PO, the cations are distributed over both sets of available tetrahedral sites.During unsuccessful attempts to synthesize 'Li,CrO,' contain-ing Cr'", we have instead prepared a new p polymorph of Li,CrO, containing CrV. The reaction involved oxidation of Cr"' to CrV in an 0,-free atmosphere, with Li,C03 as the oxidising agent. Subsequently, we have prepared the same p-Li3Cr0, polymorph in a reaction involving reduction of CrV* to CrV. These results are reported here. Experimental P-Li,CrO, was prepared first by reacting powder mixtures of Li2C03 and Cr203 (both analytical grade) in a 5: 1 molar ratio in Au foil boats in flowing Ar.Initial firing was at 500-700°C for a few hours to expel CO,, after which the reaction mixture was reground and heated at 800°C for 12-24 h to give a dark green, somewhat moisture-sensitive product. Subsequently, /?-Li,CrO, was also prepared by reac- tion of a mixture of Li2C03 and CrO, in a 3: 1 molar ratio at 700 'C in Au foil boats in flowing Ar. Exhaust-gas analyses were performed by gas chromatography using a Perkin-Elmer F33 gas chromatograph. Powder X-ray diffraction was carried out using a STOE STAD1,'P diffractometer, with position sensitive detector (PSD) and Cu-Ka, radiation. Silicon was added as an internal standard for accurate d-spacing measurements. Rietveld refinement of the Li3Cr04 structure was not feasible due to the high level of background radiation associated with Cr fluorescence.Pellets for conductivity measurements were cold- pressed at 2 ton cmP2 and sintered at 800 "C for 2-3 h in order to increase their mechanical strength. Electrodes were usually made by coating opposite pellet faces with Engelhard gold paste and gradually heating to 700 "C in air. Conductivity was determined by ac impedance measurements from 0.10 Hz to 13 MHz using Solartron 1250/1286 and HP 4192A impedance analysers in air on a heating cycle. Phase changes t Departamento de Quimica Inorganica, Facultad de Ciencias Quimicas, Universidad Complutense de Madrid, Madrid-28040, Spain. were studied using a Stanton Redcroft DTA 675 instrument, heating rate 5 "C min-'. Magnetic susceptibility measurements in the temperature range 4.2-300 K were made using a DSM-5 pendule magnet- ometer.The set-up was calibrated with Hg[Co(SCY),] and Gd,( S04)3.8H20. Results The synthesis of Li,CrO, from Li2C0, and Cr203 has some unusual features. In flowing Ar, a nearly phase-pure product is obtained provided an excess of Li,CO, (5:1 ratio instead of 3 : 1)is present in the reaction mixture. If a similar reaction is attempted in air or O,, the mixture melts at 700-800°C and no Li,CrO4 forms. If instead a 3: 1 reaction mixture is used, in flowing Ar, then a mixture of Li,CrO, and LiCrO, results. It is clear therefore, that an excess of Li,CO, is required, whose prime function is to act as an oxidant for Cr"' to CrV. Two possible mechanisms for this may be considered.First, Li,O may be the oxidant, giving Li metal which vapourises. We found no direct evidence for this; for instance, on bubbling the exhaust Ar through H,O, no alkalinity developed. Secondly, C02 or carbonate may be the oxidant, giving CO as by-product. Clear evidence for CO formaiion was obtained by gas chromatography of the Ar exhaust gas (a blank experiment, on the synthesis of Li,SiO, from Li2C03 and Si02, in flowing Ar showed no detectable CO). In addition, the excess of Li,O should remain in the reaction mixture and, in fact, a small amount was detected hy XRD. It appears, however, that only C02 generated in situ can act as an oxidant since, on heating 3: 1 mixtures of Li2C0, and Cr203 in flowing CO,, no oxidation to give Li,Cr04 was detected.We conclude that CO, or carbonate rather than Li20 is the principal oxidant, but that the mechanism is not fully understood. Li,Cr04 was also prepared by reaction of Li2C:O3 and CrO, in Ar. In this case, simple reduction of CrV' to CrV occurred during heating; also, the expected 3: 1 ratio of starting materials could be used to obtain a phase-pure product. Since Li3Cr04 can be obtained by two pathways, which involve oxidation and reduction, respectively, of the Cr starting materials, it is concluded that the lattice energy of Li,CrO, must be particularly favourable for stabilising the unusual +V oxidation state of Cr. Of the two methods, that involving reduction of CrO, is preferred since this gives a clean, phase-pure product.Indexed powder X-ray diffraction data for P-Li,C'rO4 are given in Table 1; apart from d-spacing shifts associated with a larger unit cell, the powder data are very similar to those of p-Li3P04, indicating an isostructural relationship. The structure is, therefore, a wurtzite superstructure with the cations ordered over one set of tetrahedral sites within a hexagonal close-packed oxide ion array.5 Table 1 Powder X-rciy diffraction data for P-Li,CrO, [~=5.4305 (0.0009) A, b=6.3136 (0.0008) A, ~=4.9413 (0.0008) A] 28 (obs)/degrees ' ~~ h k 1 int. d (obs)/A d (calc)/A 16.296 100 19.3 5.4348 5.4305 21.555 110 73.9 4.1 192 4.1171 22.828 011 100 3.8923 3.8912 24.325 101 46.3 3.6561 3.6548 28.223 111 35.5 3.1593 3.1630 32.787 120 43.5 2.7292 2.7292 32.948 200 52.8 2.7163 2.7153 35.971 210 7.7 2.4946 2.4944 36.321 002 48.0 2.4714 2.4707 37.624 121 22.7 2.3887 2.3890 37.772 201 43.2 2.3797 2.3797 40.036 102 4.5 2.2502 2.2489 40.484 211 8.8 2.2263 2.2267 42.644 112 11.6 2.1184 2.1185 46.225 130 5.4 1.9623 1.9623 46.664 022 7.7 1.9449 1.9456 46.695 031 7.4 1.9358 1.9362 47.828 221 7.7 1.9002 1.9002 49.740 122 12.8 1.8316 1.8316 49.907 202 7.4 1.8258 1.8274 52.080 212 6.5 1.7546 1.7553 55.181 230 5.7 1.663 1 1.6634 57.785 013 7.1 1.5942 1.5938 58.435 040 13.6 1.5780 1.5784 58.755 320 14.8 1.5702 1.5703 60.139 132 6.3 1.5373 1.5366 60.454 113 4.3 1.5301 1.5293 63.665 023 4.5 1.4604 1.4603 302 1.4602 64.220 141 5.4 1.449 1 1.4490 66.225 123 15.9 1.4100 1.4102 67.827 232 32.4 1.3806 1.3798 68.133 213 32.4 1.3751 1.3745 70.790 042 7.1 1.3299 1.3301 71.050 322 9.1 1.3256 1.3253 71.264 331 6.3 1.3222 1.3223 71.656 241 6.5 1.3159 1.3154 The pentavalent oxidation state of Cr was confirmed by magnetic susceptibility measurements.The magnetic suscepti- bility of Li,CrO, follows a Curie-Weiss behaviour in the temperature range 300-20 K with a magnetic moment of 1.70 pB, Fig. 1. This fully agrees with that expected for the ground term 2E derived from CrV in tetrahedral coordination.The positive value of the Weiss constant, 8 =2.6 K, is indicative of the existence of ferromagnetic interactions in the chromium sublattice, which is confirmed by an increase in the XT value observed below 20 K, Fig. 1 inset. Y "'"E 0 0 0.2h O$ 600 100 200 0-to0f--4001 TIK 0E 0x 0 0 200c no * oo Fig. 1 Temperature dependence of the reciprocal molar susceptibility for Li,CrO,. The inset shows the ~Tvs.T plot. J. MATER. CHEM., 1994, VOL. 4 --3 --5 --7 1-9 Fig. 2 Variation of conductivity with temperature for Li,CrO, The variation of conductivity with temperature, determined by ac impedance measurements is shown in Fig. 2. Modest levels of conductivities, typically 2 x lo-* R-' cm-' at 25 "C rising to 4 x lop5 at 300°C with an activation energy of 0.44 eV were observed.The nature of the impedance -plots, which did not show much evidence of low-frequency electrode polarization effects, indicates the conduction to be primarily electronic. This is consistent with electron hopping associated with the d' electronic state of CrV. In addition, significant numbers of Li' interstitials or vacancies in the a-Li,CrO, structure are not expected and therefore, high Li' ion conduc- tivity in stoichiometric Li,CrO, is not observed. DTA results on /?-Li,CrO, showed an endotherm on heating at 750 "C which was reversible on cooling at 705 "C.This may be associated with the polymorphic phase transition $ y but we could not isolate the high-temperature phase by quenching from e.g.800 "C. The crystal structure report,2 which is for the high-temperature y polymorph, was carried out on a crystal that had been stabilised, by an unknown mechanism, to room temperature. Conclusions A new polymorph of Li,CrO, has been prepared. It appears to be isostructural with P, As and V analogue^,^.^ all of which show a p+y transition with increasing temperature. Li,CrO, may be prepared by two routes, involving either oxidation of Cr"' by Li,C03 or reduction of CrV' in flowing Ar. The structure of Li,CrO,, its magnetic and electrical properties are consistent with the pentavalent oxidation state of Cr. We thank the EC (Human Capital and Mobility Project) for both the grant which supported S.G.M. and for providing financial support (CEC contract C11-CT91-948) to H.H.S. and M.A.K.L.D.; also Dr. I.L. Marr for assistance with the gas chromatography. We are indebted to Dr. Paul Attfield, who suggested that COz or carbonate may be the oxidant for the Cr"' to CrV conversion. References R. Scholder and H. Schwartz, 2.Anorg. Allg. Chem., 1963,1,326. G. Meyer, D. Paus and R. Hoppe, Z. Anorg. Allg. Chem., 1974, 408, 15. J. Zemann, Actu Crystallogr., Sect. B,1960, 13, 863. C. Keffer, A. Mighell, F. Mauer, H. Swanson and S. Block, Inorg. Chem., 1967,6,119. A. R. West, 2.Kristallogr., 1975, 141,422. A. R. West and F. P. Glasser, J. Solid State Chem., 1972,4,20. R. D. Shannon and C. Calvo, J. Solid State Chem.. 1973,6, 538. Paper 4/02012J; Received 5th April, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401307

出版商:RSC    

年代:1994

数据来源: RSC