(Invited) Bismuth-Based Ternary Oxide Thin Film for Solar Water Oxidation

Abstract

Bismuth based ternary oxide such as BiVO4, Bi2WO6 and Bi2MoO6 have been reported to oxidise water into oxygen under visible light. These materials have sufficient absorption within the solar spectrum (UV and visible region) and stability against photocorrosion. The hybridized valence band of these ternary oxides made them active towards water oxidation. In this presentation, we investigate the performance of these bismuth based ternary oxides in photoelectrochemical water oxidation. A few aspects will be discussed to highlight 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. 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 photoelectrochemical performance is observed.