Hydrogen Treated Anatase TiO2: A New Experimental Approach and Further Insights from Theory

Here, we demonstrate the effect of vacuum hydrogen annealing on TiO2 nanocrystals (H:TiO2) to modify the structural, optical and electrical properties for significantly improved photocatalytic and photoelectrochemical performance. The vacuum hydrogen annealed TiO2 nanocrytals contained paramagnetic Ti3+ centers and exhibited a higher visible light absorption cross-section as was confirmed by electron paramagnetic resonance and diffuse reflectance spectra measurements. The vacuum hydrogen annealed samples showed a noticeable improvement in photocatalytic activity under visible light (λ > 380 nm) which was demonstrated by degrading methylene blue dye and improved photoelectrochemical response in term of high photocurrent density. Ab-initio simulations of TiO2 were performed in order to elucidate the conditions under which localized Ti3+ centres rather than delocalized shallow donor states are created upon the reduction of TiO2. Randomly distributed oxygen vacancies in general lead to localized deep donor states while the occupation of the oxygen vacancies by atomic hydrogen favours the delocalized shallow donor solution. Furthermore, it was found that localization is stabilized at high defect concentrations and destabilized under external pressures. In those cases where localized Ti3+ states are present, the DFT simulations showed a considerable enhancement of the visible light absorption as well as a pronounced broadening of the localized Ti3+ energy levels with increasing defect concentration