Photocatalytic property and electronic structure of lanthanide tantalates,LnTaO(4) (Ln = La, Ce, Pr, Nd, and Sm)

Photocatalytic activity of lanthanide tantalates, LnTaO(4) (Ln = La, Ce, Pr , Nd, and Sm), for water splitting was studied in connection to the effect of Ln 4f levels on the electronic structure. Valence band XPS, UV-vis. and first-principle electronic calculations suggested that the position of Ln 4 f levels becomes lowered monotonically across the series of Ln. The empty L a 4f 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, Fo r 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 a ctivity was attained by LaTaO4, which solely can decompose water into stoic hiometric H-2/O-2 mixtures. This is largely due to the overlap between Ta 5 d and O 2p orbitals, which produces a wide density-of-states distribution i n the conduction band. On the other hand, the tantalates containing from on e (Ce3+) to five (Sm3+) 4f electrons were much less active, because unoccup ied 4f levels lying on or below the conduction band edge would play as a tr apping center for photoexcited electrons. The interactions between the empt y and filled Ln 4f levels and carriers generated under UV irradiation are p ossible reasons for the Ln-dependent photocatalytic activity.