pH Dependent Behavior and Effects of Photoinduced Surface States during Water Photooxidation at TiO 2 /Solution Interface: Studied by Capacitance Measurements

Abstract

By performing photocurrent and capacitance measurements in pH 6–14 solutions with using a nanotubular TiO 2 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 TiO 2 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 TiO 2 surface were revealed and discussed. During the last decades, photoelectrochemical (PEC) hydrogen generation from water splitting using a semiconductor photoelectrode has been an important subject of research for utilizing solar energy. 1–3 Metal oxides (such as TiO 2 , Fe 2 O 3 , WO 3 , ZnO, SrTiO 3 and BiVO 4) have continuously attracted particular interest due to their high chem-ical stability and low cost. Most metal oxides are n-type semicon-ductors and used as the photoanode in PEC cells for the oxidation of water (O 2 evolution) by photogenerated holes. Considerable progress is currently being made in the development of suitable semiconductor materials. 4–7 For a given semiconductor under AM 1.5 global solar illumination condition, the theoretical maximum photoconversion ef-ficiency for water splitting depends on its light adsorption property (i.e., bandgap energy, E g), e.g., 1.3% for anatase TiO 2 with E g = 3.2 eV, and 12.9% for Fe 2 O 3 with E g = 2.2 eV. 8 However, the real photoconversion efficiencies are well below the theoretical values for various semiconductor materials. 9–12