Influence of Ti3+ defect-type on heterogeneous photocatalytic H2 evolution activity of TiO2

Abstract

Reduced titanium dioxide has recently attracted large attention, particularly for its unique co-catalyst-free heterogeneous photocatalytic application for H₂ generation. The enhanced photocatalytic activity of the reduced TiO₂ was previously ascribed to the introduction of point crystal defects (mainly Ti³⁺ centers), which result in the formation of intrinsic co-catalytic centers and enhanced visible light absorption. In this work, we systematically investigate the effect of different defects in the TiO_2−x lattice on photocatalytic H₂ evolution. To introduce different types of defects, thermal annealing in air (oxidative), Ar (inert), Ar/H₂ (reducing), and H₂ (reducing) atmospheres was performed on commercially available anatase nanopowder. Then, the powders were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM) to clarify the effect of treatment on material properties. Furthermore, the defect types were characterized by electron paramagnetic resonance (EPR) spectroscopy. We show that thermal annealing in different atmospheres can form different amounts of different defect types in the TiO₂ structure. The highest photocatalytic activation is achieved by annealing the anatase powder in a reducing atmosphere for an appropriate temperature/annealing time. By combining the results from H₂ generation and EPR analysis we show that the simultaneous presence of two types of defects, i.e. surface exposed Ti³⁺ and lattice embedded Ti³⁺ centers in an optimum low concentration is the determining factor for an optimized photocatalytic H₂ evolution rate. In fact, annealing anatase powder under the so-reported optimized conditions in reducing atmosphere leads to the generation of a considerable amount of H₂, with rates as high as 338 μmol h⁻¹ g⁻¹.