Mo-doped BiVO4 thin films – high photoelectrochemical water splitting performance achieved by a tailored structure and morphology

Abstract

The n-type semiconductor bismuth vanadate (BiVO₄) is one of the most promising ternary oxide materials for visible light-induced water oxidation, offering a theoretical solar-to-hydrogen efficiency of 9.1%. However, several factors strongly limit its actual efficiency and among these, poor charge transport has been identified as one major limitation to be overcome. Many efforts have been made to improve charge transport and charge separation in BiVO₄ including various doping strategies as well as nanostructuring the absorber to the dimension of its charge carrier diffusion length. In this article, we present a new wet chemical synthesis approach to fabricate pristine and Mo-doped BiVO₄ thin film photoanodes. Starting from a solution containing metalorganic Bi, V and Mo precursors, homogeneous and reproducible thin films can be fabricated by a simple procedure of dip-coating and subsequent calcination in air. Structural and morphological characterization reveals that the polycrystalline BiVO₄ thin films crystallize in the monoclinic scheelite structure in micrometer-sized randomly oriented, porous domains. The small band-gap scheelite structure is maintained upon Mo doping into the BiVO₄ lattice, yielding optimized light harvesting and improved charge carrier transport properties. As a result, the photoelectrochemical performance regarding water oxidation of the as-synthesized Mo-doped BiVO₄ thin film photoanodes is highly improved with respect to their pristine counterparts. Photocurrent densities of 1.9 mA cm⁻² and 4.6 mA cm⁻² at 1.23 V vs. RHE under visible light illumination (100 mW cm⁻²) are measured for unmodified and CoPi-modified Mo-doped BiVO₄ photoanodes respectively, both of which are amongst the highest values reported for modified BiVO₄ single-layer design photoanodes so far.