Experimental studies provide additional information on the influence of holes on the reaction. Durrant et al. investigated photo-OER for a series of metal oxides (BiVO4, anatase TiO2, Fe2O3, WO3) by transient absorption spectroscopy (TAS), which enabled the dynamics of photogenerated carriers leading to water oxidation on these oxides to be investigated. They revealed a high-order dependence of photocurrent on surface hole density when the hole density is greater than some specific value. This observation led to a proposal that sequential trapping of up to 3 holes directly controls the reaction kinetics. For anatase TiO2, the relevant hole concentration range is reported to be 0.05~1 h + nm -2 , a value that integrates the hole populations through the entire thickness (1000 nm) of the sample. Studies of the rate of O2 photodesorption from single crystal TiO2(110) surfaces during UV illumination by Yates and coworkers provides information on formation and transport of holes to the crystal surface. It was found that desorption abruptly increases at a critical photon flux, which was attributed to the saturation of deep hole traps.