Effect of Ti³⁺ Ions and Conduction Band Electrons on Photocatalytic and Photoelectrochemical Activity of Rutile Titania for Water Oxidation

Abstract

Although TiO2 is generally considered to be an oxygen deficient n-type compound, the role of oxygen vacancies and Ti3+ ions on its photocatalytic activity is not fully understood. In this study, we investigated the effects of high-temperature calcination and H2 reduction treatment on the water oxidation activity of rutile TiO2 under ultraviolet irradiation. Calcination above 900 °C decreased the photocatalytic activity of the TiO2 owing to strong oxidation, but its initial activity was restored by H2 treatment at above 500 °C. Electron spin resonance (ESR) spectra showed that the high-temperature calcination created O•– radicals (trapped hole in oxygen lattice site), while the H2 reduction treatment created Ti3+ ions (trapped electron in titanium lattice site) with oxygen vacancies. Diffuse reflectance ultraviolet–visible–near-infrared (UV–vis–NIR) spectroscopy indicated an increase in the amount of electrons in shallow traps and the conduction band with H2 treatment temperature. Measurements of the sheet resistance and space charge layer capacitance of the thermally oxidized TiO2 films indicated that the H2 treatment improved the electrical conductivity owing to an increase in donor density (electron density). Thus, the increase in the photocatalytic and photoelectrochemical activities of the rutile TiO2 was attributed to donor doping by H2 reduction.