Effects of different exchanging ions on the band structure and photocatalytic activity of defect pyrochlore oxide: a case study on KNbTeO6†
The effects of different exchanging ions on enhancing the photocatalytic activity of KNbTeO6 are investigated where Ag, Cu, and Sn are substituted for K in an attempt to reduce the materials' band gap energy. The KNbTeO6 parent compound has been successfully prepared by a solid state method. The Ag, Cu, and Sn doped samples were obtained by a facile ion exchange technique. Although only 20% of K+ is exchanged with the guest ions, the band gap energy is significantly reduced from 3.38 eV in KNbTeO6 to 2.76 (Ag-doped), 3.21 (Cu-doped), and 2.51 eV (Sn-doped). The detailed investigation based on the XPS spectra in the valence band region, O K-edge XANES spectra, and theoretical calculation indicates that the hybridization of Ag 4d, Cu 3d, and Sn 5s states with O 2p in the ion exchanged samples creates extra states at the top of the valence band. As a consequence, the valence band maximum shifts and the band gap energy is reduced. The magnitude of such band gap reduction is different for each guest ion and is related to the nature of the hybridization with O 2p. Here, the effects of each guest ion on the electronic structure, morphology, and surface properties are discussed. In addition to having the largest effect on band gap reduction, Sn2+ also increases the dispersion of the conduction band. As a result, Sn-doped KNbTeO6 exhibits the highest photocatalytic activity. The mechanism of methylene blue photodegradation is studied by using different quenchers in the presence of the Sn2+-doped KNbTeO6 photocatalyst. The schematic band diagrams of each sample are proposed based on the experimental and theoretical results.