Direct Observation of Surface Hole Accumulation Under Water Oxidation Conditions By Efish and Impedance Measurements

Abstract

Water oxidation reaction is considered the bottleneck and rate-determining step in photoelectrochemical water-splitting process. To achieve efficient water oxidation on photoanode, sufficient built-in potential on semiconductors should be maintained. Herein, we introduced in-situ Electric Field Induced Second Harmonic Generation (EFISH) technique to probe the change of built-in potential under water oxidation reaction process of a single-crystal rutile TiO2 photoanode. The band flattening effect caused by the reduction of built-in potential on photoanode was investigated by EFISH. Up to 600 mV built-in potential screening on TiO2 was observed at 1 V vs Ag/AgCl applied bias under 15mW/cm2 on TiO2 photoanode, in accordance with the anodic shift of flat-band potential. This phenomenon can be explained by the contribution of hole accumulation at surface states and local pH variation in an insufficiently buffered electrolytes, indicating that surface OH· is the rate determining step in water oxidation process. Built-in potential under excitation can be well maintained in a sufficiently buffered electrolytes, suggesting a different rate determining step in water oxidation process.