Operando Contactless EFISH Study of the Rate-Determining Step of Light-Driven Water Oxidation on TiO₂ Photoanodes

For many slow solar-fuel-forming reactions, the accumulation of photogenerated minority carriers on the photoelectrode surface leads to light-induced band edge unpinning, affecting the junction properties by decreasing band bending in the semiconductor space charge layer and increasing the driving force of surface reactions in the electric double layer. In this study, we demonstrate a contactless operando electric field-induced second harmonic generation (EFISH) method for measuring the band bending change (δΔΦ_SCR^L) on photoelectrodes upon photoexcitation. For n-doped rutile TiO₂ water oxidation photoanodes at pH 7, δΔΦ_SCR^L increases at more positive potentials or higher illumination power density until it reaches saturation values. We show that under fast mass transport conditions, δΔΦ_SCR^L is exclusively attributed to the accumulated charged rate-determining species that can be regarded as temporary surface states, and the relationship between the photocurrent and δΔΦ_SCR^L can be well modeled by assuming that hole trap states function as the reaction center. Kinetic isotope experiments identify proton-coupled electron transfer as the rate-determining step and suggest a possible chemical nature of the key intermediate. We demonstrate that light-induced band edge unpinning is a beneficial feature under high illumination conditions for oxygen evolution reaction on TiO₂ because it maintains the photon-to-current conversion efficiency by enhancing the surface reaction driving force, shedding light on the actual device application.