Synthesis and Optical Properties of Ag(I), Pb(II), and Bi(III) Tantalate-Based Photocatalysts

Abstract

The Ag(I) and Bi(III) tantalates Ag2Ta4O11, BiTa7O19, and Bi7Ta3O18 were prepared by solid-state methods at 1000 °C for 24–48 h. The Pb(II)-containing tantalate PbTa2O6 was prepared at 1100 °C for 24 h, whereas Pb3Ta4O13 and PbTa4O11 were synthesized from a reaction of A2Ta4O11 (A = Na, Ag) precursors with a PbCl2 flux (at 1:1, 5:1, and 10:1 molar ratios) at 700 °C from 24 to 96 h. The PbTa2O6, Pb3Ta4O13, and Bi7Ta3O18 structures consist of TaO6 layers and TaO6 chains/rings with Pb(II) ions located within the cavities. The structures of Ag2Ta4O11, PbTa4O11, and BiTa7O19 consist of layers of TaO7 pentagonal bipyramids that alternate with Ag(I), Pb(II), and Bi(III) cations, respectively. UV–vis diffuse reflectance data were used to measure bandgap sizes for Ag2Ta4O11 (∼3.9 eV), PbTa4O11 (∼3.8–3.95 eV), Pb3Ta4O13 (∼3.0 eV), PbTa2O6 (∼3.6 eV), BiTa7O19 (∼3.6 eV), and Bi7Ta3O18 (∼2.75 eV). A decrease in the band gap was observed with an increase in the Pb(II) or Bi(III) content. Photocatalytic activities of the platinized samples in aqueous solutions under ultraviolet irradiation were found to range from ∼7 to ∼194 μmol H2·g–1·h–1 in aqueous methanol and from ∼42 to ∼213 μmol O2·g–1·h–1 in aqueous silver nitrate. Electronic-structure calculations based on density functional theory show the highest-energy valence band states consist of the respective Ag 4d orbital/Pb 6s orbital/Bi 6s orbital and O 2p orbital contributions, and the lowest-energy conduction band states arise from the Ta 5d orbital contributions. The latter are delocalized over the TaO7 pentagonal bipyramid layers within the A2Ta4O11 (A = Na, Ag), PbTa4O11, and BiTa7O19 structures. Nearly all of the tantalates exhibit significant water oxidation photocatalytic activity. However, higher activity for water reduction was found for tantalates consisting of TaO7 pentagonal bipyramid layers that can serve as charge-migration pathways.