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 (γ(L)-Bi₂MoO₆), and (ii) a high temperature phase (γ(H)-Bi₂MoO₆). UV-vis spectroscopy showed the optical band gaps of γ(L)-Bi₂MoO₆ and γ(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-O₂ conversion efficiency for PEC water splitting under UV illumination and, importantly, exhibited excellent photostability as a consequence of the unique synthesis method.