Structural study of amorphous WO 3 thin films prepared by the ion exchange method
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The structure of amorphous tungsten trioxide films was investigated with IR and Raman spectroscopic analyses and an XRD method. The films were prepared by ion exchange from sodium tungstate as a starting material. Films consisted of microclusters of 10-30 A diameter, in which the networks are formed with WO 6 octahedra sharing their corners and edges. The networks in the as-prepared samples consisted of WO 6 units with low symmetry, in which termination by W ^ O and W—OH2 groups was common. As the post-annealed temperature became higher, the symmetry of WO 6 was improved and the edge-sharing octahedra disappeared.
I. INTRODUCTION Amorphous tungsten trioxide (a-W0 3 ) thin films show electrochromism and photochromism, which have been extensively studied.1 A-WO3 films can be prepared by various techniques not only in the vapor phase but also in the liquid phase. Chemseddine et al.2 have reported that a-W0 3 films, which are prepared by ion exchange from sodium tungstate, are comparatively stable. However, structural information concerning ion exchanged prepared a-W0 3 has not been published. We have previously examined the structure of a-W0 3 films prepared by electron beam vacuum evaporation,3 where the films are constructed with microclusters and the networks in the clusters are formed by three-, four-, and six-membered rings of WO 6 octahedra sharing their corners. We think that the electrochromic property has a close relationship with the six-membered rings, which has also been suggested by Delichere et a/.1(d) In this paper we investigate the structure of a-W0 3 films prepared by an ion exchange and compare it with that of vacuum evaporated a-W0 3 film. By means of IR and Raman spectroscopic studies, the structural units are examined and the network structure is discussed from radial distribution analyses. II. EXPERIMENTAL The sample preparation procedure is almost the same as that described in the literature2; sodium tungstate (Na 2 WO 4 • 2H 2 O) was used as the starting material. A 0.5 M solution was passed through an ion exchanging resin in which Na+ was replaced with H + . A colloidal solution was then obtained, and a glass substrate was *> Current address: Faculty of Engineering, Okayama University, Tsushima naka, Okayama-shi 700, Japan. b) Current address: Japan CMK Co. Ltd., Naganuma-cho 1744-1, Isezaki-shi, Gunma 372, Japan. 1324
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J. Mater. Res., Vol. 6, No. 6, Jun 1991
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dipped in the solution to obtain WO 3 film (IE-WO3). We prepared three kinds of specimens to vary the content of the trapped water in the film by means of changing a drying condition. Series A was dried in the air and series B and C were dried in vacuum (2 • 10~ 2 Torr) for 1.5 h and 20 h, respectively. Crystallinity was determined by x-ray diffraction. The obtained films were amorphous and a crystalline phase began to appear at 200 °C due to the post-annealing treatment. The films were completely crystallized at 350 °C. The obtained films were removed from the substrates, and the
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