In situ Sn²⁺-incorporation synthesis of titanate nanotubes for photocatalytic dye degradation under visible light illumination

Sn²⁺-incorporated titanate nanotubes, prepared by washing a layered sodium titanate with a SnCl₂ solution for tube formation, exhibit noticeable photocatalytic activity under visible light irradiation. This in situ synthesis results in a Sn/Ti ratio of approximately 0.6. Because of the introduction of Sn²⁺ ions, the Sn 5s orbital replaces the O 2p orbital as the top level of the valence band of titanate nanotubes. Optical absorption analysis shows that Sn doping reduces the bandgap of titanate nanotubes from 3.5 to 2.6 eV. Oxidation of the Sn²⁺-incorporated titanate nanotubes leads to oxidation of Sn²⁺ to Sn⁴⁺, hence, widening the bandgap. Under visible light irradiation, Sn ²⁺-incorporated titanate nanotubes effectively degrade methylene blue in an aqueous solution, whereas the bare titanate nanotubes exhibit substantially lower photocatalytic activity. Photoluminescence analysis demonstrates that the induced charges from excitation of the Sn²⁺ ions tend to be relaxed through chemical interactions, rather than irradiative recombination.