PH Dependent Behavior and Effects of Photoinduced Surface States during Water Photooxidation at TiO2/Solution Interface: Studied by Capacitance Measurements

Abstract

By performing photocurrent and capacitance measurements in pH 6–14 solutions with using a nanotubular TiO2 photoanode, we have studied the pH-dependent behavior and properties of photoinduced surface states. Two types of surface states (i.e. the short- and long-lived surface states) were in situ detected at TiO2 surface. It was demonstrated that the density and response potential of these surface states are two essential factors influencing the recombination process and hence determining the overall photoconversion efficiencies for water splitting. With increasing pH, significant decrease in the estimated surface-state densities was observed at pH around 11–12, which was found well inline with the variation with pH both for the measured apparent recombination probabilities and for the maximum photoconversion efficiencies. For effectively inhibiting the occurrence of surface recombination process, and for obtaining maximum photo-conversion efficiencies, the most suitable Schottky barrier heights were revealed to be 0.9 V at pH <11–12 and 0.7 V at pH > 11. Based on the results of this work, some mechanistic aspects for the water photooxidation at TiO2 surface were revealed and discussed.