Synthesis of new visible light active photocatalysts of Ba(In _1/3Pb_1/3M′_1/3)O₃ (M′ = Nb, Ta): A band gap engineering strategy based on electronegativity of a metal component

We have synthesized new, efficient, visible light active photocatalysts through the incorporation of highly electronegative non-transition metal Pb or Sn ions into the perovskite lattice of Ba(In_1/3M _1/3M′_1/3)O₃ (M = Sn, Pb; M′ = Nb, Ta). X-ray diffraction, X-ray absorption spectroscopic, and energy dispersive spectroscopic microprobe analyses reveal that tetravalent Pb or Sn ions exist in the B-site of the perovskite lattice, along with In and Nb/Ta ions. According to diffuse UV-vis spectroscopic analysis, the Pb-containing quaternary metal oxides Ba(In_1/3Pb_1/3M′_1/3)O₃ possess a much narrower band gap (E_g ∼ 1.48-1.50 eV) when compared to the ternary oxides Ba(In_1/2M′_1/2)O ₃ (E_g ∼ 2.97-3.30 eV) and the Sn-containing Ba(In _1/3Sn_1/3M′_1/3)O₃ derivatives (E_g ∼ 2.85-3.00 eV). Such a variation of band gap energy upon the substitution is attributable to the broadening of the conduction band caused by the dissimilar electronegativities of the B-site cations. In contrast to the ternary or the Sn-substituted quaternary compounds showing photocatalytic activity under UV-vis irradiation, the Ba(In_1/3Pb _1/3M′_1/3)O₃ compounds induce an efficient photodegradation of 4-chlorophenol under visible light irradiation (λ > 420 nm). The present results highlight that the substitution of electronegative non-transition metal cations can provide a very powerful way of developing efficient visible light harvesting photocatalysts through tuning of the band structure of a semiconductive metal oxide.