Hydrogen doping in the bulk of anatase TiO2 was studied by density functional theory approach. Our results show that hydrogen atom prefers the interstitial sites which are closer to the oxygen atoms. Investigation of the electronic structures of different hydrogen doped systems indicates the increment of Fermi energy and bond length, affecting the instability of the system. Hydrogen doping in the anatase TiO2 structure injects electron in the lower region of the conduction band, around the Ti states, leading Ti4+ toward Ti3+, while having no effect on the band gap energy value of TiO2. Therefore, the observed hydrogen induced band gap narrowing in TiO2 samples could be due to the structural disorders generated in the hydrogenation process. The hydrogen storage capacity of approximately 3 H atoms per supercell (Ti32O64) is also obtained, which is promising for renewable energy storage issue.
