| Research Abstract |
1) The correlation of electronic structure with photocatalytic properties of layered perovskite tantalates, RbLnTa_2O_7 (Ln=La, Pr, Nd, and Sm), containing the partly-occupied Ln4f-shell is studied by means of valence band photoemission (XPS) measurement and first-principles calculations of the band structure based on the all-electron full-potential linear augmented plane-wave (FLAPW) method. The unoccupied La4f level is located on the bottom of the conduction band, whereas the partially occupied 4f levels become lowered with an increase of 4f electrons down to the position above the valence band edge (Ln=Pr) and finally fall into the valence band mainly comprised of O2p orbitals (Ln=Nd and Sm). It was suggested that unoccupied as well as occupied 4f orbitals are not completely localized but partly contribute to the hybridization with O2p and Ta5d orbitals. The different photocatalytic activity of the present for the overall water splitting is closely related to the degree of the Ln-O-
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Ta hybridization, which affects not only the positions of the conduction band and valence band edges, but also the density of states (DOS) distribution in both bands.
2) Photocatalytic activity of lanthanide tantalates, LnTaO_4 (Ln=La, Ce, Pr, Nd, and Sm), for the water splitting was studied in connection to the effect of Ln4f levels on the electronic structure. Valence band XPS, UV-vis, and first-principle electronic calculations suggested that the position of Ln4f levels becomes lowered monotonously across the series of Ln. The empty La4f level is supposed to be higher than the conduction band edge, whereas the filled 4f levels of the Nd and Sm compounds lie in the valence band. For Ln=Ce and Pr, occupied 4f levels lying within the forbidden gap lead to the lower band gaps (E_g) compared to others. The highest photocatalytic activity was attained by LaTaO_4, which solely can decompose water into stoichiometric H_2/O_2 mixtures. This is largely due to the overlap between Ta5d and O2p orbitals, which produces a wide density-of-states (DOS) distribution in the conduction band. On the other hand, the tantalates containing from one (Ce^<3+>) to five (Sm^<3+>) 4f electrons were much less active, since unoccupied 4f levels lying on or below the conduction band edge would play as a trapping center for photoexcited electrons. The interactions between the empty and filled Ln4f levels and carriers generated under UV irradiation are possible reasons for the Ln-dependent photocatalytic activity. Less
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