Nearly Barrierless Four-hole Water Oxidation Catalysis on Semiconductor Photoanodes with Highly Accumulated Surface Holes

Abstract

The sluggish water oxidation reaction (WOR) is considered the kinetic bottleneck of artificial photosynthesis, due to the complicated four-electron and four-proton transfer process. Herein, we find that the WOR can be kinetically nearly barrierless on four representative photoanodes (i.e., α-Fe₂O₃, TiO₂, WO₃, and BiVO₄) under the concentrated light irradiation, wherein the rate-limiting O − O bond formation step is driven by highly accumulated surface photo-generated holes that exhibit a superior fourth-order kinetics. The activation energy is quantitatively estimated by combining the population model with Eyring-like equation and is further confirmed by density functional theory (DFT) calculations. The WOR rate under this condition shows more than one order of magnitude enhancement compared with that has 1st, 2nd or 3rd -order kinetics. Focusing on α-Fe₂O₃, the highly accumulated surface holes form adjacent Fe^V=O intermediates that effectively activate surface-adsorbed H₂O molecules via hydrogen bonding effect as revealed by operando Raman measurements and ab initio molecular dynamics (AIMD) simulations. This work discloses a systematic understanding of the internal relations between activation energy and reaction orders of surface holes for future WOR study.