PbBi₃O₄X₃ (X = Cl, Br) with Single/Double Halogen Layers as a Photocatalyst for Visible-Light-Driven Water Splitting: Impact of a Halogen Layer on the Band Structure and Stability

Abstract

Layered oxyhalides with a Silleń(−Aurivillius) structure have attracted considerable attention as promising photocatalysts for efficient visible-light-induced water splitting due to the elevation of the O-2p valence band maximum (VBM) derived from the strong s−p interaction between Pb/Bi-6s and O-2p electrons and the electrostatic destabilization of oxygen anions in the fluorite layers. However, the role of the halogen layer, consisting of single or double layers, was not known. In this study, we examined a Silleń-type layered oxyhalide PbBi 3 O 4 X 3 (X = Cl, Br) with alternating stacks of single/double halogen layers and skeletal Bi-based oxide layers. Density functional theory (DFT) calculations of PbBi 3 O 4 X 3 revealed that the two inequivalent halogen layers contribute differently to the valence band density of states; the halogen p orbitals in double layers are higher in energy and responsible for the VBM, which is understood in terms of Madelung site potentials, where the electrostatic repulsion between halide ions is at play. Another finding from the DFT calculation is the spatial separation of the conduction band minimum and the VBM based on the asymmetry of halogen layers, which may promote the charge separation. While both PbBi 3 O 4 Cl 3 and PbBi 3 O 4 Br 3 possess band levels appropriate for visible-light-induced water splitting, only the former showed photocatalytic O 2 evolution activity. The stability of PbBi 3 O 4 Cl 3 is due to the significant contribution of the O-2p orbitals to its VBM, in stark contrast to PbBi 3 O 4 Br 3 , where Br-4p predominately contributes to the VBM. PbBi 3 O 4 Cl 3 functions as a stable O 2 -evolving photocatalyst in Z-scheme water splitting under visible light irradiation. These results show that the electrostatic repulsion of halogen layers is a key factor for tuning the valence band structure of oxyhalides while maintaining sufficient stability for water-splitting photocatalysis.

A Sillén oxyhalide SrBi3O4Cl3 as a promising photocatalyst for water splitting: impact of the asymmetric structure on light absorption and charge carrier dynamics

“…6), which is regarded as an effective strategy to promote charge separation, [60][61][62] as also suggested in analogous PbBi 3 O 4 Cl 3 . 56 The present result shows that the coexistence of single and double halogen layers in SrBi 3 O 4 Cl 3 not only provides visible light response but also improved carrier separation and suppression of charge recombination. SrBi 3 O 4 Cl 3 showed a negligibly low H 2 evolution activity (Fig.…”

First-Principles Calculations on Narrow-Band Gap d10 Metal Oxides for Photocatalytic H2 Production: Role of Unusual In2+ Cations in Band Engineering

“…with the ( n – 1)­d 10 n s 2 electronic configuration is a promising strategy to create a narrow-band gap metal oxide. The new VBM is made up of occupied Pb/Bi-6s orbitals or Sn-5s orbitals, which raise the VBM and narrows the band gap. − Generally, LP cations tend to form multicomponent oxides by combining with d 0 metal (Ti 4+ , Nb 5+ , Mo 6+ ), and the CBM constructed by the empty d orbitals is unfavorable for electron migration. Beyond that, LP cations occasionally incorporate into the structural framework constructed by d 10 metal MO 4 (M = Ga, Zn, etc.)…”

Deciphering the photocatalytic hydrogen generation process of Fresnoite Ba2TiGe2O8 by electronic structure and bond analyses

“…Most interestingly, the electronic structure analyses by density functional theory (DFT) calculations allow us to decipher the photocatalytic process in detail. [28][29][30] The photo-excited electrons originate from the O 2p non-bonding orbitals, where some are excited to Ge 4s and O 2p anti-bonding orbitals, followed by a two-dimensional migration to the catalyst surface; some are excited to the Ti 3d and O 2p anti-bonding orbitals, however, these are quite localized, and thus those electrons are mostly forced to quench with holes in the valence band. In other words, the Ge-O covalent network is the key component endowing Ba 2 TiGe 2 O 8 with its photocatalytic activity.…”