Two Novel Bi-Based Borate Photocatalysts: Crystal Structure, Electronic Structure, Photoelectrochemical Properties, and Photocatalytic Activity under Simulated Solar Light Irradiation

Through the combination of Bi³⁺ and a large negative charge ion (BO₃)^3–, two novel Bi-based borate photocatalysts Bi₄B₂O₉ and Bi₂O₂[BO₂(OH)] with layered structure have been successfully developed. For the first time, the borates were investigated as photocatalysts. They were synthesized by solid-state reaction and hydrothermal method, respectively, and further characterized by XRD, SEM, TEM, HRTEM, and DRS. Bi₄B₂O₉ and Bi₂O₂[BO₂(OH)] possess direct and indirect transition optical band gaps of 3.02 and 2.85 eV, respectively. Density functional calculations revealed that the valence band (VB) and conduction band (CB) of both borates were composed of hybridized states of the O 2p and Bi 6p or 6s orbitals, and a large dispersion was observed in the energy band of Bi₂O₂[BO₂(OH)]. The photodecomposition experiments demonstrated that Bi₄B₂O₉ and Bi₂O₂[BO₂(OH)] can be used as effective photocatalysts under simulated solar irradiation, and Bi₂O₂[BO₂(OH)] exhibits the high photocatalytic activity, which is 2.5 and 3.2 times compared with that of P25 and Bi₂O₂CO₃, respectively. Moreover, the photocurrent conversion further confirmed that Bi₄B₂O₉ and Bi₂O₂[BO₂(OH)] were potential photofunctional materials. The layered structure with (Bi₂O₂)²⁺ layer, hybridized and dispersion energy band, and large negative charge of (BO₃)^3– ion should be responsible for the high photocatalytic activity of Bi₂O₂[BO₂(OH)]