Photoelectrochemical water oxidation using a Bi2MoO6/MoO3 heterojunction photoanode synthesised by hydrothermal treatment of an anodised MoO3 thin film

An effective Mo-based ternary oxide Bi2MoO6/MoO3 thin film for photoelectrochemical (PEC) water oxidation was prepared via a simple and direct thin film synthesis route. The synthesis strategy involved subjecting a MoO3 thin film, prepared by anodising a Mo foil, to hydrothermal treatment in the presence of a bismuth salt solution. The new approach removes the need for preformed particles to prepare the film, in turn providing a robust and stable anode for PEC water splitting. X-ray diffraction of the Bi2MoO6/MoO3 thin film revealed the film possessed two photocatalytically active phases of Bi2MoO6: (i) a low temperature phase (gamma(L)-Bi2MoO6), and (ii) a high temperature phase (gamma(H)-Bi2MoO6). UV-vis spectroscopy showed the optical band gaps of gamma(L)-Bi2MoO6 and gamma(H)-Bi2MoO6 were 2.75 eV and 3.1 eV, respectively. In contrast, the neat MoO3 thin film had a wider band gap of 3.4 eV. Transforming the surface MoO3 into Bi2MoO6 to construct the Bi2MoO6/MoO3 composite electrode leads to an improvement in photoelectrochemical (PEC) performance. The Bi2MoO6/MoO3 electrode exhibited a 79% enhancement in anodic photocurrent density compared to the unmodified MoO3 thin film under a positive bias of 0.4 V vs. Ag/AgCl. The better performance was attributed to: (i) the narrow optical band gap of Bi2MoO6, which extended the absorption of light by the film into the visible range and (ii) the well-aligned band structure of MoO3 and Bi2MoO6. The Bi2MoO6/MoO3 thin film electrode was subsequently utilised as a photoanode for PEC water splitting. The Bi2MoO6/MoO3 thin film electrode provided close to 100% faradic photocurrent-to-O-2 conversion efficiency for PEC water splitting under UV illumination and, importantly, exhibited excellent photostability as a consequence of the unique synthesis method.