Role of Sn2+ in the Band Structure of SnM2O6 and Sn2M2O7(M = Nb and Ta) and Their Photocatalytic Properties

The photocatalytic properties of metal oxides consisting of Sn 2+ with a 5s 2 configuration were studied to find new visible light responsive photocatalysts. The band gaps of Ca 2 Ta 2 O 7, Sn 2 Ta 2 O 7, and Sn 2 Nb 2 O 7 were found to be 4.8, 3.0, and 2.3 eV, respectively. Density functional calculations revealed that the valence band levels formed with Sn 5s orbitals were more negative than those with O 2p orbitals. The result is that niobates and tantalates containing Sn 2+ have narrow band gaps compared with typical niobates and tantalates. SnNb 2 O 6 showed photocatalytic activity for H 2 and O 2 evolution from aqueous solutions containing sacrificial reagents (methanol and Ag + ) under visible light irradiation (λ > 420 nm). Moreover, the photocatalytic activity of SnNb 2 O 6 depended on the material used as the Sn source. SnNb 2 O 6 synthesized from Sn 3 O 2 (OH) 2 showed higher activity for H 2 and O 2 evolution than that synthesized from commercial SnO, which included a small amount of SnO 2 . SnNb 2 O 6 synthesized from Sn 3 O 2 (OH) 2 showed activity for O 2 evolution under visible light, even without IrO 2 as a cocatalyst, whereas that synthesized from commercial SnO showed activity under irradiation only when an IrO 2 cocatalyst was loaded. The activity of SnNb 2 O 6 synthesized from SnO was improved by IrO 2 cocatalyst loading.