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

Abstract

Although TiO 2 is generally considered to be an oxygen deficient n-type compound, the role of oxygen vacancies and Ti 3+ ions on its photocatalytic activity is not fully understood. In this study, we investigated the effects of high-temperature calcination and H 2 reduction treatment on the water oxidation activity of rutile TiO 2 under ultraviolet irradiation. Calcination above 900 °C decreased the photocatalytic activity of the TiO 2 owing to strong oxidation, but its initial activity was restored by H 2 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 H 2 reduction treatment created Ti 3+ 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 H 2 treatment temperature. Measurements of the sheet resistance and space charge layer capacitance of the thermally oxidized TiO 2 films indicated that the H 2 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 TiO 2 was attributed to donor doping by H 2 reduction.