Impact of Varying the Photoanode/Catalyst Interfacial Composition on Solar Water Oxidation: The Case of...

Photoanodes used in a water-splitting photoelectrochemical cell are almost always paired with an oxygen evolution catalyst (OEC) to efficiently utilize photon-generated holes for water oxidation because the surfaces of photoanodes are typically not catalytic for the water oxidation reaction. Suppressing electron-hole recombination at the photoanode/OEC interface is critical for the OEC to maximally utilize the holes reaching the interface for water oxidation. In order to explicitly demonstrate and investigate how the detailed features of the photoanode/OEC interface affect interfacial charge transfer and photocurrent generation for water oxidation, we prepared two BiVO₄(010)/FeOOH photoanodes with different Bi:V ratios at the outermost layer of the BiVO₄ interface (close to stoichiometric vs Bi-rich) while keeping all other factors in the bulk BiVO₄ and FeOOH layers identical. The resulting two photoanodes show striking differences in the photocurrent onset potential and photocurrent density for water oxidation. The ambient pressure X-ray photoelectron spectroscopy results show that these two BiVO₄(010)/FeOOH photoanodes show drastically different Fe²⁺:Fe³⁺ ratios in FeOOH both in the dark and under illumination with water, demonstrating the immense impact of the interfacial composition and structure on interfacial charge transfer. Using computational studies, we reveal the effect of the surface Bi:V ratio on the hydration of the BiVO₄ surface and bonding with the FeOOH layer, which in turn affect the band alignments between BiVO₄ and FeOOH. These results explain the atomic origin of the experimentally observed differences in electron and hole transfer and solar water oxidation performance of the two photoanodes having different interfacial compositions.