Controlled reduction and development of in-gap states in anatase TiO2 under low-energy hydrogen bombardment

In this study, we investigate the reduction mechanism and modifications of the electronic structure of anatase TiO₂ thin films induced by low-energy H₂⁺ ion bombardment at room temperature, focusing on hydrogen-induced changes in the oxidation states of Ti atoms, local chemical bonding of oxygen and the formation of defect states within the band gap. During the initial stages of bombardment, the energetic hydrogen reacts with lattice oxygen to form hydroxyl groups, followed by the reduction of Ti⁴⁺ to Ti³⁺. For the higher doses, the formation of H − O − H bridge bonds causes splitting of O − Ti bonds and formation of H₂O gas molecules, accompanied by a reduction of Ti³⁺ and Ti⁴⁺ states to Ti²⁺. At the same time, the reduction of Ti⁴⁺ to lower oxidation states introduces some defect states within the band gap of TiO₂, associated with Ti³⁺ defects or the formation of double-oxygen-vacancy clusters at Ti²⁺ sites.