Phase transformations, vacancy formation and variations of optical and photocatalytic properties in TiO₂-ZnO composites by high-pressure torsion

TiO₂ and ZnO, two semiconductors with promising optical properties, are considered as potential candidates for solar and photocatalytic applications. Although chemical methods have been primarily used to enhance the optical properties of these oxides, the current authors recently reported enhanced photocatalytic performance of pure TiO₂ and ZnO by plastic straining due to the generation of high-pressure phases and oxygen vacancies. In this study, to improve the optical properties further, large fractions of ZnO/TiO₂ interphase boundaries are also introduced by application of high-pressure torsion (HPT) straining to a mixture of anatase-TiO₂ and wurtzite-ZnO powders. It was found that the amounts of oxygen vacancies and nanograined high-pressure TiO₂-II and rocksalt-ZnO phases increase with increasing plastic strain. Moreover, due to the plastic strain effect, the rutile-TiO₂ phase is formed at room temperature, which is at least 600 »K below the reported anatase-to-rutile transition temperature. These structural features, together with the formation of large fraction of interphase boundaries, lead to electron spin resonance, optical bandgap narrowing, diminishing of the band-to-band photoluminescence and thus, improvement of photocatalytic hydrogen generation. Despite improvements in the photocatalytic activity of TiO₂-ZnO composites after large straining, photocatalytic activity becomes poor by processing at ultra-large strains due to the significant reduction in crystallinity.