J. MATER. CHEM., 1994, 4(10), 1581-1584 Preparation of Gold-dispersed Vanadium Oxide Thin Films by an Alternate Spin-coating Method for Electrochromic Applications Katsumi Nagase, Seigo Izaki, Youichi Shimizu,+ Norio Miura and Noboru Yamazoe* Department of Materials Science and Technology, Graduate School of Engineering Sciences, Kyushu University, Kasuga-shi, Fukuoka 876, Japan Thin films of V205 with dispersed gold particles were prepared from ethanol solutions of vanadyl isopropoxide and HAuCI, by means of an alternate spin-coating method. The Au content was maintained at 12-36 atom% by selection of the appropriate concentration of the alkoxide solution. After calcination at 400 "C,the mean diameter of dispersed Au particles was 7.0 nm, as observed for the film containing 24 atom% Au.Violet (cathodic)-green (anodic) electrochro- mism was exhibited by the Au (24 atom%)-V,O, film. Owing to the plasma resonance of the Au particles, the film had a strong optical absorption band centred at 570 and 61 0 nm under cathodic and anodic polarization, respectively. Electrochromism has been observed in a number of inorganic and organic materials, among which transition-metal oxides such as W03,1-5M003,6,7Ti02,8,9 NiOx,'*,'' IrOx12313and COO,'^-'^^: appear to be promising from the viewpoint of long-term stability. However, these oxides cannot give a coloration other than blue, brown or grey, with the exception of V20j16-20 which shows blue-green-yellow multichro-mism. Obtaining new colorations, especially red, is an import- ant aim in electrochromic research.So far investigations have been from two directions. One is the use of oxide-oxide composite systems. New reddish colours have been reported with CuOx-W03 ,21 v205-wo322 and V205-Ti02.23 The other is to utilize the coloration due to the plasma resonance absorption of ultrafine particles of metals such as Au, Ag and Cu, as observed with the oxide- or fluoride-based dispersions prepared by e~aporation,~~sputtering25 or the sol-gel An Au-WO, film has been reported to show a reddish coloration when polarized ~athodically.~~"' We are particularly interested in V205-based composites because of the multichromic nature of V205. We reported previously that the Au-V205 thin films, prepared by an evaporation method, show a new electrochromism of reddish ~iolet-green.~~ Subsequently we found that such an Au-dispersed film could be fabricated by a wet process using an alternate spin-coating technique.This paper deals with the new fabrication process and the electrochromic properties of the Au-V20, thin films thus obtained. Experimenta1 Gold-dispersed V205 thin films were prepared by means of an alternate spin-coating method shown in Fig. 1. The starting reagents, vanadyl isopropoxide [VO(OPr'), , High Purity Chemicals Co. Ltd.] and hydrogen tetrachloroaurate(rI1) (HAuCl,.4H20, Kishida Chemicals) were dissolved separately in ethanol to concentrations of 0.1-0.4 and 0.02mol dm-3, respectively. The vanadyl alkoxide solution was first spin- coated onto an indium tin oxide (ITO) glass (20 mm square, 10 s1 per square, Kinoene Kougaku Kogyo), at 3000 rpm at room temperature in a dry box. The coating layer was dried at room temperature for 10 min, the aurate solution was then spin-coated onto it under the same conditions.Then, the deposit was calcined in air at 400 "C for 30 min, in order to decompose the vanadyl alkoxide to crystalline V205 and the f Present address: Department of Chemistry, Faculty of Engineering, Kyushu Institute of Technology, Kitakyushu-shi, Fukuoka 804, Japan.1It has been reported that electrochromic cobalt oxide switches between olive green and brown in colour. VO(OPS), + C2H50H (Room temp.) HAuC14~4H20+ C2HSOH Drying(Room temp.) (400"C) VO(OPi), + CPH50H Calcination (400"C)dhAu-V,05 thin film Fig.1 Alternate spin-coating method for the preparation of Au-V,O, films HAuC14-4H20 to metallic Au. The latter decomposition takes place at 3300 0C.26 These serial procedures of spin-coating, drying and calcination were repeated 10 times. Filially the vanadyl alkoxide was spin-coated on top of the slack and calcined at 400°C. The total thickness of the films obtained was in the range of 150-600 nm as estimated from scanning electron microscopy (SEM, JEOL, JSM-840F). The composi- tions of the films were determined by electron probe micro- analysis (EPMA, JEOL, JXA-8621SX/MX), while the structure and morphology were examined by X-ray diffraction analysis (XRD, Rigaku Denki 4011)and transmission electron microscopy (TEM, JEOL, JEM 2000 FX).For electrochemical measurements, each Au-V205 film on IT0 was paired with a Pt counter electrode in a 1 mol dmP3 LiCIO,-propylene carbonate solution, with its potential being referred to a saturated calomel electrode (SCE). Cyclic voltammetry was performed with a potentiostat (Hokuto Denko, HA-303) and a function generator (Hokuto Denko, HB-105). Optical absorption was measured in situ on a spectrophotometer (Hitachi, 200-10, 330) in the wavelength range 400-800 nm. Results and Discussion Deposition of Au-V,O, Thin Films It was reported that a sol-gel process could be applied to the preparation of noble metal-silicate composite^.^^-^^ In this case, the starting reagents silicon alkoxide and noble-metal chlorides, were mixed in the same coating solution.In the present case, however, it was not possible to prepare such a coating solution because the vanadyl alkoxide was not suffic- iently stable in the presence of the aurate. This is why the solutions of vanadyl alkoxide and HAuC1,-4H20 were spin- coated alternately. Attempts to stack a sufficient number of alternate spin-coated layers without the calcination step in between were unsuccessful. The vanadyl alkoxide solution could not be spin-coated well onto the uncalcined aurate layer. Moreover, the whole stack turned strongly opaque after the final calcination. Thus calcination was introduced after each step of the aurate spin-coating.The concentration of VO(OPri)3 in the coating solution was varied, using a fixed aurate concentration of 0.02mol dmP3. Fig. 2 shows the relationship between the Au content (or V:Au ratio) in the composite films and the VO(OPr'), concentration. The Au content (atom%) decreased and the V :Au atomic ratio increased linearly with alkoxide concen- tration. This shows that the composition of the Au-V20, thin film can be controlled well by the concentration of the starting vanadyl alkoxide solution. The total thickness of the com- posite film containing 24 atom% Au was 270 nm as estimated from the SEM photograph of its cross-section (Fig. 3). Characterization of Deposited Films Fig. 4 shows XRD patterns of three composite films having different Au contents after calcination at 400°C. The (001) diffraction peak of V205 was very strong for the Au (12 atom%)-V,O, film, (a), suggesting that the crystal orien- tation was along the c axis perpendicular to the IT0 substrate.The (001) peak intensity was drastically weakened for the 30t \ concentration of VO(0Pr')dmol dm-3 Fig. 2 Au content and V:Au ratio of the composite thin film as a function of VO(OPr'), concentration in the coating solution (0.02 mol dmP3 HAuCI,) J. MATER. CHEM., 1994, VOL. 4 Fig.3 SEM photograph of a cross-section of an Au (24 atom%)-V,O, film calcined at 400 'C I 1-110 20 30 40 50 2fYdegrees Fig.4 XRD patterns of various Au-V,O, films calcined at 400°C: (a) 12, (b)24, (c) 36 atom% Au films containing 24 and 36 atom% Au, (h) and (c).The presence of crystalline Au was obvious from its (111) and (200) diffraction peaks. The composite films were subjected to TEM observation to investigate the state of Au particle dispersion. Fig. 5 (a) shows such a TEM photograph for the Au (24 atom%)-V,O, film after calcination at 400-C. Tiny, nearly spherical Au particles were dispersed well in the matrix of V205. The diameter of the Au particles showed a distri- bution [Fig. 5(b)],from which the mean diameter was esti- mated to be 7.0nm. The optical absorption spectra of pure and Au-dispersed V205 films and Au-V,O, composite films are shown in Fig. 6. The Au-containing films exhibit a strong ab5orption band centred at 618-638 nm ascribable to the plasma resonance of ultrafine Au particles, as was observed in the Au-V20, films prepared by the evaporation method.The peak position shifted to longer wavelength with increasing Au content in accordance with the dielectric theory of Ma~well-Garnett~~ for an insulating compound in which metal particles are dispersed. The Au-related absorption band became markedly broader and weaker as the Au content increased from 30 to 36 atom%, probably due to the grain growth of Au particles. From these results, the optimum Au content was estimated to be 20-30 atom%. Electrochromism of Au-V,O, Thin Films Fig. 7 shows the cyclic voltammograms (CV) for the thin films of pure V,O, (a) and Au (24 atom%~)-V,O, (6). Unlike other V205 films that have been investigated, the pure V,O, film prepared by the present method showed a rather broad CV curve in the potential range between -0.8 V and + 1.2 V us.SCE, the appearance of which is attributed to the lower degree of crystallization of the pure V205 films obtained here. A very J. MATER. CHEM., 1994, VOL. 4 (a1 H20nm (b) r 1 -8 20-v c .-0 c mf 10-a,Ll5 0 0 4 a 12 16 particle diameterhm Fig. 5 TEM photograph of Au particles (u) and their size distri-bution (bl for the Au (24 atom%-V,O, film calcined at 400 "C 2.0 a, C m 42 1.05:LI m --___---0-400 5b0 660 760 8bO ?Jnm Fig. 6 Optical absorption spectra of various Au-V,O, films calcined at 400'C: (a) pure V,O,, (b) 12, (c) 24, (d) 36 atom% similar CV curve was exhibited by the Au-V205 composite film, indicating that the Au-dispersed film undergoes essen-tially the same reaction as V205.Nevertheless, the electrochro-mism of the composite film was totally different from that of the pure V205 film.The film, which was green in the oxidized state, was violet when it had been polarized cathodically. The electrochromism was completely reversible and could be repeated with great stability. Fig. 8 shows the optical absorption spectra of the pure V205 film (a) and the Au (24 atom%-V205 film (b) each being polarized anodically and cathodically. For the pure V205 film, with a change in polarization from +1.2V to -0.4 V, the absorption edge shifted from ca. 500 nm to ca.400nm together with a slight increase in optical absorbance over a wide wavelength range above ca. 500nm. In the case of the Au-V,05 film, on the other hand, the absorption band ascribed to the dispersed Au particles shifted from 610nm -0.8 EN vs. SCE blue green'-2.4 N green -0 EN vs. SCE Fig. 7 Cyclic voltammograms of pure V205 film (u) .tnd Au (24 atom%-V,O, film (b) calcined at 400 "C 1.o 0.5 0 1 I I J I a 400 600 800 0c (d 425: a 2.0 m 1.O I I I I 400 600 800 Unm Fig. 8 Optical absorption spectra of pure V,O, film (a) and Au (24 atom%-V,O, film (b) calcined at 400 "C under cathodic and anodic polarization (+1.2V) to 570nm (-0.8V), together with a shift of the absorption edge of V205.This indicates that the new violet coloration is associated with a blue shift of both the plasma resonance absorption band and the absorption edge of V205 on cathodic polarization. It has been suggested that the reddish colour of Au -W0330 films on cathodic polarization results from a changt:: in the effective relative permittivity of metal grains surrounded by a cloud of protons. A similar explanation may be possible for the blue shift of the plasma resonance absorption observed in the present system. References 1 S. K. Deb, Philos. Mug., 1973,27, 801. 2 B. W. Faughnan, R. S. Crandall and M. A. Lampert, Appl. Phys. Lett., 1975, 27, 275. 3 0.F. Schimer, V. Witter, G. Baur and G. Brandt, J. Ele&rochem. SOC.,1977,124, 749.1584 J. MATER. CHEM., 1994, VOL. 4 4 T. Yoshimura, J. Appl. Phys., 1985,57, 911. 20 K. Nagase, Y. Shimizu, N. Miura and N. Yamazoe, Appl. Phys. 5 6 K. Yamanaka, H. Okamoto, H. Kidou and T. Kudo, Jpn. J. Appl. Phys., 1986, 25, 1420. 0. Z. Angel, C. Menezes, F. S. Cinencio and G. F. L. Ferreira, 21 Lett., 1992,61, 243. H. Suiyang, Z. Jikai and C. Jinyi, Proc. SPIE-1987,823,159. int. Soc. Opt. Eng., J. Appl. Phys., 1980,51, 6022. 22 S. Saito and Y. Seino, J. Electron. Commun. Soc . 1982, J65-C, 629 7 N. Baba, S. Morisaki and N. Nishiyama, Jpn. J. Appl. Phys., 1984, 23. 638. 23 (in Japanese). K. Nagase, Y. Shimizu, N. Miura and N. Yamazoe, Appl. Phys. 8 T. Ozuku and T. Hirai, Electrochim. Actu, 1982, 27, 1263. Lett., 1992,61, 243.9 M. Nabavi, S. Doeuff, C. Sanchez and J. Livage, Muter. Sci. Eng. 24 T. Yamaguchi, M. Sakai and N. Saito, Phys. Reu. B. 1985, 32, B, 1989,3.203. 2126. 10 M. Fantini and A. Gorenstein, Sol. Energy Muter., 1987,16,487. 25 R. W. Cohen, G. D. Cody, M. D. Coutts and B. Abeles, Phys. Rec. 11 S. I. Coldoba-Torresi. A. H. Goff and S. Joiret, J. Electrochem. B, 1973,8,3689. Soc., 1991, 138, 1554. 26 M. Ohtaki, Y. Ohsihima, K. Eguchi and H. .4rai, Chem. Lett., 12 S. Gottesfeld, J. D. E. McIntyre, G. Beni and J. L. Shay, Appl. 1992,1992,2201. 13 Phjx Lett., 1978, 33, 208. Y. Sato, K. Ono, T. Kobayashi, H. Wakabayashi and 27 J. Matsuoka, R. Mizutani, S. Kaneko, H. Nasu, K. Kamiya, K. Kadono, T. Sakaguchi and M. Miya, J. Ceram.Soc. Jpn., H. Yamanaka, J. Electrochem. Soc., 1987,134,570. (Japan), 1993, 101, 53. 14 L. D. Burke and 0.J. Murphy, J. Electroanul. Chem., 1980, 112, 379. 28 29 H. Kozuka and S. Sakka, Chem. Muter.. 1993,s. 222. E. K. Sichel and J. I. Gitteleman and J. Zeiez. Appl. Phy~. Lett., 15 C. N. Polo da Fonseca, M-A. De Paoli and A. Gorenstein, Adv. 1977,31, 109. Muter., 1991,3, 553. 30 E. K. Sichel and J. I. Gitteleman, J. Electron. Muter., 1979, 8, 1. 16 17 S. F. Cogan, N. M. Nguyen, S. J. Perrotti and D. Rauh, J. Appl. Phys.. 1989,66, 1333. A. M. Andersson, C. G. Granqvist and J. R. Stevens, Appl. Opt., 1989,28,3295. 31 32 P. V. Ashrit, G. Bader, F. E. Girouard, Von-Van Tuong and T. Yamaguchi, Physica A (Amsterdam), 1989,157,333, K. Nagase, Y. Shimizu, N. Miura and N. Yamazoe, Appl. Phys. Lett., in the press. 18 Y. Shimizu. K. Nagase, N. Miura and N. Yamazoe, Jpn. J. Appl. Phys., 1990, 29, L1708. 33 J. C. Maxwell-Garnett, Philos. Trans. R. SOC.London, 1904, 203, 385. 19 Y. Shimizu, K. Nagase, N. Miura and N. Yamazoe, Solid State Ionics, 1992,53-56,490. Paper 4/01470G; Received 14th March, 1994