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 Sn2+ with a 5s2 configuration were studied to find new visible light responsive photocatalysts. The band gaps of Ca2Ta2O7, Sn2Ta2O7, and Sn2Nb2O7 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 Sn2+ have narrow band gaps compared with typical niobates and tantalates. SnNb2O6 showed photocatalytic activity for H2 and O2 evolution from aqueous solutions containing sacrificial reagents (methanol and Ag+) under visible light irradiation (λ > 420 nm). Moreover, the photocatalytic activity of SnNb2O6 depended on the material used as the Sn source. SnNb2O6 synthesized from Sn3O2(OH)2 showed higher activity for H2 and O2 evolution than that synthesized from commercial SnO, which included a small amount of SnO2. SnNb2O6 synthesized from Sn3O2(OH)2 showed activity for O2 evolution under visible light, even without IrO2 as a cocatalyst, whereas that synthesized from commercial SnO showed activity under irradiation only when an IrO2 cocatalyst was loaded. The activity of SnNb2O6 synthesized from SnO was improved by IrO2 cocatalyst loading.