Defective titanium dioxide has recently attracted large attention, particularly for its unique co-catalyst-free heterogeneous catalytic application for H₂ generation. The enhanced photocatalytic activity of the defective 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. Additionally, we evaluate the influence of defects type and density on the electrocatalytic H₂ generation from a TiO_2-x nanotubes electrode. To introduce different types of defects to TiO₂ powder, thermal annealing in air (oxidative), Ar (inert), Ar/H₂ (reducing), and H₂ (reducing) atmospheres was performed [1]. To generate defects in TiO₂ nanotubes electrode the sonochemical reduction approach was followed. Then, the samples 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. Interestingly, an optimum sonochemical reduction of TiO₂ nanotubes provides an electrode that requires 30 mV less H₂ generation overpotential compare to pristine TiO₂ nanotube. Such reduced electrode is a potential scaffold for noble metal single atom decoration for various electrodics applications.

[1] Sh. Mohajernia et al. J. Mater. Chem. A, 2020, 8(1432)