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

Abstract

The Ag(I) and Bi(III) tantalates Ag 2 Ta 4 O 11, BiTa 7 O 19, and Bi 7 Ta 3 O 18 were prepared by solid-state methods at 1000 °C for 24–48 h. The Pb(II)-containing tantalate PbTa 2 O 6 was prepared at 1100 °C for 24 h, whereas Pb 3 Ta 4 O 13 and PbTa 4 O 11 were synthesized from a reaction of A 2 Ta 4 O 11 (A = Na, Ag) precursors with a PbCl 2 flux (at 1:1, 5:1, and 10:1 molar ratios) at 700 °C from 24 to 96 h. The PbTa 2 O 6, Pb 3 Ta 4 O 13, and Bi 7 Ta 3 O 18 structures consist of TaO 6 layers and TaO 6 chains/rings with Pb(II) ions located within the cavities. The structures of Ag 2 Ta 4 O 11, PbTa 4 O 11, and BiTa 7 O 19 consist of layers of TaO 7 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 Ag 2 Ta 4 O 11 (∼3.9 eV), PbTa 4 O 11 (∼3.8–3.95 eV), Pb 3 Ta 4 O 13 (∼3.0 eV), PbTa 2 O 6 (∼3.6 eV), BiTa 7 O 19 (∼3.6 eV), and Bi 7 Ta 3 O 18 (∼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 H 2 ·g –1 ·h –1 in aqueous methanol and from ∼42 to ∼213 μmol O 2 ·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 TaO 7 pentagonal bipyramid layers within the A 2 Ta 4 O 11 (A = Na, Ag), PbTa 4 O 11, and BiTa 7 O 19 structures. Nearly all of the tantalates exhibit significant water oxidation photocatalytic activity. However, higher activity for water reduction was found for tantalates consisting of TaO 7 pentagonal bipyramid layers that can serve as charge-migration pathways.