Influence of Ti$_3^+$ Defect-type on Heterogeneous Photocatalytic H$_2$ Evolution Activity of TiO$_2$

Reduced titanium dioxide has recently attracted large attention, particularly for its unique co-catalyst-free H$_2$ heterogeneous photocatalytic application. The enhanced photocatalytic activity of the reduced TiO$_2$ was previously ascribed to the introduction of point crystal defects (mainly Ti$_3^+$ 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 H$_2$ evolution. To introduce different types of defects, thermal annealing in air (oxidative), Ar (inert), Ar/H$_2$ (reducing), and H$_2$ (reducing) atmospheres were 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 TiO$_2$ 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 H$_2$ generation and EPR analysis we show that the simultaneous presence of two types of defects, i.e. surface exposed Ti$_3^+$ and lattice embedded Ti$_3^+$ centers, in an optimum low concentration, is the determining factor for an optimized photocatalytic H$_2$ 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 H$_2$, with rates as high as 338 $\mu$molh-1g-1.