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Photoelectrochemical Processing for Oxide Films Yasumichi Matsumoto

Introduction Metal oxides are important materials in industry. For example, there are many kinds of functional oxides: dielectric oxides such as BaTiO3 (perovskite), semiconductor oxides such as ZnO, ionic conductors such as Bi2O3, magnetic oxides such as Fe3xMxO4 (spinel), and so on. These oxide materials must be prepared as films, nanodots, and layered films to increase their functionality for a wide variety of applications. Recently, high-performance oxide films, such as perovskites, spinels, and nanosized dots, have been prepared using an electrochemical process. The electrochemical process has some advantages compared with a gas-phase process from an industrial point of view, including low cost, possibility of coating on substrates with complex shapes, control of film thickness by electrolysis charge, and composition by electrode potential. Moreover, the electrochemical process is a “soft solution process” (SSP),1 because the processing is carried out under environmentally benign conditions with relatively low energy (at relatively low temperatures, i.e., 200C). Therefore, I call the present electrochemical process for high-performance materials “soft solution electrochemical processing” (SSEP).2 There are two interesting points from a scientific viewpoint regarding materials production using SSEP. One is the low interdiffusion, which may easily bring about the formation of nanoscale and/or nanophase materials because of the lowtemperature process. The other is the effect of the intermediates produced via the electrochemical reaction on oxide formation. This effect will have the possibility of producing a new material that can never be produced by other processing meth-

MRS BULLETIN/SEPTEMBER 2000

ods. In this article, some unique soft solution electrochemical processes for oxide materials as well as the fundamental electrodeposition mechanisms are reviewed.

Electrodeposition Using Electrochemical Oxidation There are two types of oxide electrodeposition. One involves a reaction between the electrode substrate and the solution species, where the deposited oxide consists of the metal of the substrate. In this case, the metal of the substrate electrochemically dissolves into the solution under anodic bias in the initial step: M (electrode substrate) l Mn  ne. (1) The dissolved cation Mn is immediately and chemically deposited as an oxide or hydroxide onto the substrate surface when the concentration of Mn reaches saturation: Mn  n2H2O l MOn2  nH acid solution. (2) The film growth will proceed by Reactions 1 and 2 through cracks and/or pores in the film until the film completely covers the substrate surface. The film will sometimes grow by oxygen-anion migration in the film when a high electrical bias is applied after covering the substrate surface completely. This case sometimes occurs for oxide film growth on titanium, tantalum, and niobium substrates. The other type of oxide electrodeposition deposits cations in the soluti