Photocatalytic properties of layered perovskite tantalates, MLnTa2O7 (M = Cs, Rb, Na, and H; Ln = La, Pr, Nd, and Sm)

Layered lanthanide tantalates and their ion-exchanged phases (MLnTa2O7, M = Cs, Rb, Na, and H; Ln = La, Pr, Nd, and Sm) were prepared to evaluate their photocatalytic activity for water splitting under UV irradiation. The optical band gap energy was dependent on the lanthanide, Ln, but negligibly affected by the monovalent interlayer cations, M. By contrast, the photocatalytic activity was strongly affected by not only Ln but also M; the highest activity was attained by a series of M = Rb with a sequence of Ln: Nd > Sm > La > Pr. The effect of Ln can be explained by the energy level and hybridisation of the Ln 4f band, which is discussed based on the results of previous electronic structure calculations. The ion exchanged phases (M = Na and H) having a deformed hydrous layer structure were less active compared to anhydrous Rb and Cs phases, whereas the NiOx loading of the Na phase greatly enhances the activity. It seems that the high mobility of water molecules between the layers allows the interlayer sites to behave as active sites for the photocatalytic reaction over NiOx-loaded catalysts. 18 refs.