Self-Modification of Defective TiO₂ under Controlled H₂/Ar Gas Environment and Dynamics of Photoinduced Surface Oxygen Vacancies

In recent years, defective TiO₂ has caught considerable research attention because of its potential to overcome the limits of low visible light absorption and fast charge recombination present in pristine TiO₂ photocatalysts. Among the different synthesis conditions for defective TiO₂, ambient pressure hydrogenation with the addition of Ar as inert gas for safety purposes has been established as an easy method to realize the process. Whether the Ar gas might still influence the resulting photocatalytic properties and defective surface layer remains an open question. Here, we reveal that the gas flow ratio between H₂ and Ar has a crucial impact on the defective structure as well as the photocatalyic activity of TiO₂. In particular, transmission electron microscopy (TEM) in combination with electron energy loss spectroscopy (EELS) revealed a larger width of the defective surface layer when using a H₂/Ar (50 %-50 %) gas mixture over pure H₂. A possible reason could be the increase in dynamic viscosity of the gas mixture when Ar is added. Additionally, photoinduced enhanced Raman spectroscopy (PIERS) is implemented as a complementary approach to investigate the dynamics of the defective structures under ambient conditions which cannot be effortlessly realized by vacuum techniques like TEM.