Electronic structure, optical and sonophotocatalytic properties of spindle-like CaWO4 microcrystals synthesized by the sonochemical method

Article abstract of DOI:10.1016/j.jallcom.2020.157377

In this letter, the electronic structure, optical, and sonophotocatalytic properties of calcium tungstate (CaWO₄) microcrystals synthesized by the sonochemical method are reported. Structural and morphological characterization techniques revealed that CaWO₄ has a tetragonal structure and composed of several spindle-like microcrystals. The ultraviolet–visible spectroscopy showed an optical band gap energy E_gap(exp) of 4.69 eV. The theoretical calculations were performed to describe the electronic band structure, the density of states, and Infrared/Raman vibrational modes. The theoretical models were based on optimized and defect-based structures. The theoretical optical band gap energy (E_gap) confirmed the existence of direct electronic transitions (Γ?Γ points in Brillouin zone). The optimized structure exhibited an E_gap(theo) value of 5.70 eV due to the participation of energy levels arising from O and Ca atoms in the valence band as well as W and O atoms in the conduction band. A decrease from 5.70 to 4.29 eV was observed for the defect-based structure. The sonophotocatalytic properties of CaWO₄ microcrystals were investigated for the first time with respect to degradation of Rhodamine B dye and they revealed a degradation capacity of approximately 96% after 200 min. Finally, our electron density maps indicate that the presence of structural defects induces a polarization phenomenon and inhomogeneous distribution of electron charge between the [CaO₈]–[WO₄] clusters.