Defect landscape in TiO2 after ms-range annealing and resulting photocatalytic efficiency

Transition metal oxides, particularly TiO2, are photoactive materials, which can be utilized to clean the air from pollutants and to produce green hydrogen for clean energy at the same time. They are one of the most promising candidates for high-performance photocatalysis. In this work, we investigate the effect of 20 ms flash lamp annealing (FLA) of sputter deposited TiO2, where arising from the light irradiation structural modifications of the films facilitates the photocatalytic (PC) degradation of two chemical compounds, namely methyl blue and methyl orange, once they interact with the surface of TiO2. The precise control of the flash energy input enables tuning of the TiO2 phase formation starting from pure anatase to mixed anatase/rutile phases, the latter associated with increased PC effect. Scanning electron microscopy and X-ray diffraction studies show that the crystal size and film quality increase with increasing annealing temperature. Further, the evolution of structural defects after FLA has been assessed by positron annihilation spectroscopy (PAS) and photoluminescence. Positron annihilation lifetime characteristics, i.e., positron lifetimes and their relative intensities profoundly sketch a transition between pure anatase and emerging rutile phase as a function of flash energy. Vacancy complexes close to the size of trivacancy dominate the anatase phase, whereas in the mixed anatase/rutile phase smaller open volume is evidenced, likely as a direct consequence of annealing. Finally, Doppler broadening PAS indicates the overall reduction of defect density exhibiting a similar transient phase region at the intermediate flash energies.