Effective increasing of optical absorption and energy conversion efficiency of anatase TiO2 nanocrystals by hydrogenation

Disorder-engineered nanophase anatase TiO2 through hydrogenation has been demonstrated to exhibit substantial solar-driven photocatalytic activities [X. Chen, L. Liu, P. Y. Yu, S. S. Mao, Science, 2011, 331, 746], while the detailed image of the disorder is unclear, and the role of the hydrogenation as well as the mechanism of high photoactivity is still ambiguous. Based on first-principles calculations, we find by taking into account the synergic effect of Ti-H and O-H bonds that hydrogen atoms can be chemically absorbed both on Ti-5c and O-2c atoms for (101), (001), and (100) surfaces, while previous studies predicted that chemical absorption of H on both Ti-5c and O-2c only takes place on the (001) surface due to overlooking the synergic effect. The hydrogenation induces obvious lattice distortions on (101) and (100) surfaces of nanoparticles enhancing the intraband coupling within the valence band, while the (001) surface is not largely affected. Different from the previous understanding that the lattice disorder accounts for the induced mid-gap states while the hydrogen only stabilizes the lattice disorders by passivating their dangling bonds, we find that the adatoms not only induce the lattice disorders but also interact strongly with the Ti 3d and O 2p states, resulting in a considerable contribution to the mid-gap states. The optical absorption is dramatically red shifted due to the mid-gap states and the photogenerated electron-hole separation is substantially promoted as a result of electron-hole flow between different facets of hydrogenated nanoparticles, which may account for the exceptional high energy conversion efficiency under solar irradiation. Even more interestingly, we find that hydrogenation reverses the redox behavior of different surfaces of nanoparticles, which provides new hints that one can tune the photoexcited electron-hole flow between different surfaces of nanoparticles in accordance to one's request by appropriate chemical surface treatment. We believe that band-offset-engineering between different facets of nanocrystals can be an effective way to facilitate energy conversion efficiency and should be applicable to other nanophase materials.