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

Abstract

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 Egap(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 (Egap) confirmed the existence of direct electronic transitions (Γ↔Γ points in Brillouin zone). The optimized structure exhibited an Egap(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.