Influences of vacancies on the structural, electronic and optical properties of monoclinic BiVO4

Monoclinic BiVO4 (m-BiVO4) is a potential photocatalyst for environmental protection. Based on density functional theory (DFT) calculations, the influences of vacancies on the structural, electronic and optical properties of 2 × 2 × 2 BiVO4 are investigated. The results indicate that the lattice parameters of BiVO4 change with vacancy types slightly. The formation energies of four different types of O vacant BiVO4 (denote as VO) are smaller than those of the other two types of Bi and V vacant BiVO4 (denote as VBi and Vv) respectively, and V1O BiVO4 is most easily formed. The indirect band gaps of V1Bi, V2Bi, V1V, V2V, V1O, V2O, V3O and V4O BiVO4 are 1.994 eV, 1.989 eV, 1.792 eV, 1.793 eV, 0.465 eV, 0.521 eV, 0.515 eV and 0.465 eV, respectively, while the direct band gap of ideal BiVO4 is 2.108 eV. Theoretical calculated band gaps are slightly smaller than experimental values owing to the well-known limitation of DFT method. Taking V1Bi, V1V and V1O BiVO4 as an example, the calculated optical properties show that the static dielectric, refractive index and reflectivity of defective BiVO4 are larger than those of ideal crystal. In the low energy range, ε1,n and R increase with increasing energy for ideal and V1O BiVO4, while they decrease for V1Bi and V1V BiVO4. Moreover, compared with ideal BiVO4, defective BiVO4 will absorb more energy, while V1O and V1V BiVO4 dissipate the highest and lowest energy respectively. The optical absorption spectra indicate that the optical band gaps of defective BiVO4 are reduced and will effectively enhance the optical absorption in visible light range.