In-Situ Measurement of Charge Transfer at Cocatalyst/Semiconductor Interfaces in BiVO4 Particle Photocatalyst

Abstract

Semiconductor photocatalyst particles are known to transform solar energy to useful chemicals such as H2 or methanol. However, the efficiency of semiconductor photocatalysts often suffers from the quick charge carrier recombination. However, there is no real physical reasons for semiconductors to display this low performance. Multiple strategies have been proposed and applied to modify the semiconductor particles and enhance their solar-to-hydrogen efficiencies. It is often assumed that in semiconductor particles with well-defined crystal facets, the photogenerated charge carriers undergo facet-dependent separation. Thus, a common strategy is to deposit a HER electrocatalyst on ‘electron-selective’ facets and an OER electrocatalyst on ‘hole-selective’ facets. Although this strategy have showed plausible results in previous literatures, a precise understanding of how charge-carrier-selective contacts between the deposited electrocatalyst and semiconductor particles emerge and how the semiconductor photocatalysts are rationally designed is still missing.In this work, we used both ex-situ and in-situ conducting AFM experiments and a new ionomer/catalyst–semiconductor test structures to characterize the charge transfer between semiconductor and deposited electrocatalysts at individual nanoparticle scale. We showed that the presence of the water/electrolyte interface is critical to induce hole selectivity between the CoOx OER cocatalyst and the BiVO4 semiconductor light absorber. We demonstrated that the negative surface charge on the semiconductor is central to induce hole selectivity. The work also demonstrates a facile strategy to control local pH and introduce water using thin-film ionomers for conductive AFM measurements. These new insights can be broadly applied to the rational design of semiconductor photocatalysts for overall water-splitting systems to control charge-carrier selectivity and improve their performance.