Photocatalytic hydrogen generation on low-bandgap black zirconia (ZrO₂) produced by high-pressure torsion

Photocatalysis on semiconductors using solar energy sources provides a clean technology to produce hydrogen from water splitting. Although zirconia (ZrO₂) is a semiconductor oxide, it is not generally considered as a photocatalyst owing to its poor light absorbance and wide bandgap (over 5 eV). In this study, black ZrO₂ with a large concentration of lattice defects such as oxygen vacancies, dislocations and nanograin boundaries is stabilized by high-pressure torsion (HPT) straining. The black ZrO₂, which experiences monoclinic-tetragonal phase transformations during the HPT process, shows large light absorption, a small bandgap, reduced conduction band energy and high photocatalytic activity for hydrogen evolution due the presence of oxygen vacancies. These results confirm that the introduction of strain-induced oxygen vacancies is a potential method to produce low-bandgap photocatalysts.