Acceptor Doping of Bismuth Vanadate to Realize Photocathodes for Hydrogen Evolution: Joint Experimental and Computational Investigations

Bismuth vanadate (BiVO₄) is a promising photoelectrode for water splitting due to its ease of synthesis, stability in aqueous conditions, and high electron-hole separation yield. Its favorable band edge alignment with the water oxidation potential has spurred extensive research into designing n-type BiVO₄ photoanodes for water oxidation [1]. However, relatively less attention has been given to the development of p-type BiVO₄ for water reduction to hydrogen gas. In this work, we show that calcium-doped BiVO₄ can serve as a photocathode that reduces water to hydrogen gas. Using density functional theory at the +U and hybrid levels, we report that Ca doping can shift the conduction band minimum (CBM) of BiVO₄ by 0.15 eV, moving it closer to the water reduction potential. This is consistent with experiments, which observe hydrogen evolution in Ca-doped BiVO₄ under illumination and a shift in the CBM towards vacuum. We show that changes to the electronic density of states and Bi-O bond lengths cause a sizeable shift in CBM. We also analyze compensation of the acceptor dopant by oxygen vacancies and hole polarons. Our work provides insight into the use of BiVO₄ as a photocathode, moving us towards the fabrication of p-n homojunctions based on BiVO₄ that could further increase charge separation efficiency, and development of electrodes for other important technological processes, like N₂ reduction to NH₃.<br/><br/>[1] Lee, D., Wang, W., Zhou, C., Tong, X., Liu, M., Galli, G., & Choi, K. S. (2021). The impact of surface composition on the interfacial energetics and photoelectrochemical properties of BiVO₄. <i>Nature Energy</i>, <i>6</i>(3), 287-294.