The aerosol-assisted chemical vapour deposition of Mo-doped BiVO4 photoanodes for solar water splitting: an experimental and computational study

Abstract

BiVO₄ is one of the most promising light absorbing materials for use in photoelectrochemical (PEC) water splitting devices. Although intrinsic BiVO₄ suffers from poor charge carrier mobility, this can be overcome by Mo-doping. However, for Mo-doped BiVO₄ to be applied in commercial PEC water splitting devices, scalable routes to high performance materials need to be developed. Herein, we propose a scalable aerosol-assisted chemical vapour deposition (AA-CVD) route to high performance Mo-doped BiVO₄. The materials were characterised using X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM), UV-visible absorption spectroscopy, and a range of PEC tests. By studying a range of Mo-precursor doping levels (0 to 12% Mo : V), an optimum precursor doping level was found (6% Mo : V); substituting V⁵⁺ sites in the host structure as Mo⁶⁺. In PEC water oxidation the highest performing material showed an onset of photocurrent (J_on) at ∼0.6 V_RHE and a theoretical solar photocurrent (TSP) of ∼1.79 mA cm⁻² at 1.23 V_RHE and 1 sun irradiance. Importantly, Mo-doping was found to induce a phase change from monoclinic clinobisvanite (m-BiVO₄), found in undoped BiVO₄, to tetragonal scheelite (t-BiVO₄). The effect of Mo-doping on the phase stability, structural and electronic properties was examined with all-electron hybrid exchange density functional theory (DFT) calculations. Doping into V and Bi sites at 6.25 and 12.5 at% was calculated for t-BiVO₄ and m-BiVO₄ phases. In accord with our observations, 6.25 at% Mo doping into the V sites in t-BiVO₄ is found to be energetically favoured over doping into m-BiVO₄ (by 2.33 meV per Mo atom inserted). The computed charge density is consistent with n-doping of the lattice as Mo⁶⁺ replaces V⁵⁺ generating an occupied mid-gap state ∼0.4 eV below the conduction band minimum (CBM) which is primarily of Mo-4d character. Doubling this doping level to 12.5 at% in t-BiVO₄ resulted in the mid-gap state merging with the CBM and the formation of a degenerate semiconductor with electrons distributed over the 3d orbitals of V ions residing in the [001] plane. In conjunction with our experimental findings, this strongly suggests that it is the increased electron conductivity due to Mo doping of BiVO₄ that produces a more active photoanode for water splitting, and that this maximises between 6.25 to 12.5 at% doping.