p-Type BiVO4 for Solar O2 Reduction to H2O2

Photoelectrochemical cells (PECs) can directly utilize solar energy to drive chemical reactions to produce fuels and chemicals. Oxide-based photoelectrodes in general exhibit enhanced stability against photocorrosion, which is a critical advantage for building a sustainable PEC. However, most oxide-based semiconductors are n-type, and p-type oxides that can be used as photocathodes are limited. In this study, we report the synthesis, characterization, and application of p-type BiVO4 with a monoclinic scheelite (ms) structure. ms-BiVO4 is inherently n-type, and it has been investigated only as a photoanode to date. In this study, we prepared p-type ms-BiVO4 (bandgap of 2.4 eV) via atomic doping of Ca2+ at the Bi3+ site under an O2-rich environment and examined its performance as a photocathode. We then demonstrated that the Ca-doped ms-BiVO4 photocathode can be used for solar O2 reduction to H2O2 when coupled with appropriate catalysts. Our computational investigation using hybrid density functional theory revealed that holes are stable as polarons in ms-BiVO4 and have a low self-trapping energy, that may lead to free carriers in the valence band at finite temperature. Our calculations also show that Ca is an effective shallow acceptor dopant with low formation energy and thermal ionization energy leading to p-type conductivity. Our joint experimental and computational results provide critical insights into the design of p-type ms-BiVO4, enabling its use as a polaronic oxide photocathode.