H₂ Molecule Generation from Dissociatively Adsorbed Water on TiO₂ through Photoexcitation

The dissociative adsorption of water, migration of hydrogen, and molecular hydrogen generation on anatase and rutile TiO2 surfaces were studied by first-principles calculations. Whereas dissociated H and OH are adsorbed strongly on both TiO2 surfaces, the adsorption energies were substantially decreased when H and OH were combined with polarons, which can be generated through photoexcitation. The H atom coupled with a polaron was found to undergo codiffusion. While a large polaron smoothly follows the H atom migration in anatase, a small polaron in rutile stochastically moves and abruptly jumps between neighboring Ti sites during the H atom migration, exerting strong retardation to the H atom migration. The favorable path for two H atoms coupled with polarons to form an H2 molecule was examined. It was found that the energy barrier is low when an H atom migrates to the H-adsorbed O site to form a 2H-coordinated O atom, followed by H2 desorption on both anatase and rutile surfaces. The atom of the oxazole forms a more stable sp3 electronic configuration by the adsorption of two H atoms. The two H atoms on the same O atom, coupled with polarons, possibly tunnel through the energy barrier to a desorbing H2 molecule. It is also shown that an H2O molecule possibly desorbs from the same 2H═O configuration.