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 V_O) are smaller than those of the other two types of Bi and V vacant BiVO₄ (denote as V_Bi and V_v) respectively, and V_1O BiVO₄ is most easily formed. The indirect band gaps of V_1Bi, V_2Bi, V_1V, V_2V, V_1O, V_2O, V_3O and V_4O 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 V_1Bi, V_1V and V_1O 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 V_1O BiVO₄, while they decrease for V_1Bi and V_1V BiVO₄. Moreover, compared with ideal BiVO₄, defective BiVO₄ will absorb more energy, while V_1O and V_1V 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.