(Invited) Engineering Bismuth-Based Ternary Oxide Thin Films for Photoelectrochemical Water Oxidation

Abstract

Bismuth based ternary oxide powders (such as BiVO4, Bi2WO6 and Bi2MoO6) have been reported to be active for oxidation of organic substances and water under visible light. In general, these materials have sufficient absorption within the solar spectrum and stability against photocorrosion. The preparation of bismuth based ternary oxide is inexpensive, environmentally benign and can be made using a number of different facile methods. In this presentation, we investigate the performance of these bismuth based ternary oxides in photoelectrochemical water splitting. A few aspects will be highlighted to substantiate the differences when they were employed in photocatalytic water splitting or through photoelectrochemical means. It is generally acknowledged that surface area and crystallinity of photocatalysts are critical factors regulating performances in powder-type suspension reactions. When they were made into thin film, quality of the contact between bismuth based ternary oxides and the charge collecting substrate becomes another crucial factor. In this work, we also discuss a few strategies formulated to directly synthesise these ternary oxide thin films without having the powder oxide as the intermediate. BiVO4, Bi2WO6 and Bi2MoO6 thin films are synthesised in this work. BiVO4 with controlled exposed facet is engineered to study the dynamic of charge interactions between their respective facets. It is revealed that a balance of charge transfer and charge trapping are occurring to govern the overall performance of BiVO4. From that point of view, specific interface is engineered to boost the higher charge transfer efficiency, for example, by using graphene as electron mediator. Bi2WO6 and Bi2MoO6 thin films are synthesised by a direct conversion of anodised WO3 and MoO3 films. The adhesion of the resultant Bi2WO6 and Bi2MoO6 on the substrate is highly stable. With the control of synthesis parameter, a concentration gradient can be introduced to form the composite of Bi2WO6-WO3 and Bi2MoO6-MoO3 with desired composition. With the favourable band alignment in this composite material, an improved activity in photoelectrochemical water oxidation is observed.