The Synthesis of Photocatalysts Using the Polymerizable-Complex Method
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of Photocatalysts Using the PolymerizableComplex Method
oxide photocatalysts in their pure form at reduced temperatures (700–900C). The principal aim of this article is to demonstrate the feasibility of a simple polymerizable-complex (PC) technique, known originally as the Pechini method,15 for the low-temperature synthesis of photocatalysts.16 The PC method is capable of producing multicomponent oxides with high compositional homogeneity at reduced temperatures. Although the PC method, like other sol-gel routes, requires heat treatments at high temperatures (typically 400–900C) for the synthesis of crystalline oxides, it may be classified as a kind of “soft solution processing” from the viewpoint that the PC method allows synthesis
Masato Kakihana and Kazunari Domen Introduction Fine powders of semiconducting oxides loaded with deposited metal and/or metaloxide particles have been widely used as heterogeneous photocatalysts for innumerable chemical reactions. Among the many photocatalytic reactions, the splitting of water assisted by light has become one of the most active areas in heterogeneous photocatalysis, since it can be a promising chemical route for energy renewal and energy storage. The photocatalytic splitting of water on TiO2 electrodes, discovered by Fujishima and Honda in 1972,1 is a prototypic example of this technique, and there is a vast body of literature describing the potential application of TiO2based photocatalysts for water decomposition. This has brought about a burst of research related to the development of many other photocatalytic systems.2 In recent years, a new class of photocatalysts for water decomposition has emerged, that is, ion-exchangeable layered compounds such as K4Nb6O17,3–7 KCa2Nb3O10,8 K2La2Ti3O10,9–12 and KTiNbO5.13 It is also known that their photocatalytic activity for water decomposition was greatly enhanced when the host compounds were combined with either Ni or Pt. Ionexchangeable layered compounds have several advantages, with respect to their photocatalytic application to water splitting, over the so-called bulk-type photocatalysts such as TiO2 and SrTiO3. This is due to their structural features: while bulk-type photocatalysts decompose water only on the external surface, layered photocatalysts can use each individual layered
MRS BULLETIN/SEPTEMBER 2000
surface for water splitting.2,14 Figure 1 schematically shows the reaction mechanism of H2O decomposition proposed for three typical photocatalysts:2 (a) Ni(NiO)SrTiO3, as a representative of the bulk-type photocatalyst; (b) Ni-K4Nb6O17, a layered photocatalyst with two different types of interlayer space (I and II); and (c) Ni(NiO)K2La2Ti3O10, a layered perovskite photocatalyst with only one type of interlayer space. The remarkable ion exchangeability and the potential ability for hydration to occur within the alkaline-metal layers common to these layered compounds have been considered as key factors in their unusually high photocatalytic ability for water decomposition.2–12 One of the big problems generally
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