H molecule generation from dissociatively adsorbed water on TiO through photoexcitation

加藤 弘一*; 長塚 直樹*; 福谷 克之

Kato, Koichi*; Nagatsuka, Naoki*; Fukutani, Katsuyuki

The dissociative adsorption of water, migration of hydrogen, and molecular hydrogen generation on anatase and rutile TiO surfaces were studied by first-principles calculations. Whereas dissociated H and OH are adsorbed strongly on both TiO 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 H 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 H desorption on both anatase and rutile surfaces. The atom of the oxazole forms a more stable 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 H molecule. It is also shown that an HO molecule possibly desorbs from the same 2H=O configuration.