Electronic structure, growth mechanism, and sonophotocatalytic properties of sphere-like self-assembled NiWO4 nanocrystals

Abstract: In this communications, we report the synthesis of nickel tungstate (NiWO₄) nanocrystals by controlled co-precipitation at 95 °C for 2 h, followed by heat treatment at 600 °C for 2 h. The structure of the NiWO₄ nanocrystals was characterized using X-ray diffraction (XRD) and Rietveld refinement analysis. Field emission-scanning electron microscopy (FE-SEM) was employed to observe the shape, average size and propose a growth mechanism for the synthesized NiWO₄nanocrystals. The optical behavior was investigated by ultraviolet–visible (UV–Vis) spectroscopy and first-principles quantum mechanical calculations based on the density functional theory at the B3LYP level to obtain their electronic band structureand density of states. We investigated the sonophotocatalytic (SPC) properties of NiWO₄ nanocrystals for degradation of remazol brilliant violet 5R (RBV5R) anionic dye using a violet light emitting diode of power 10 W. The XRD patterns indicate that the NiWO₄ nanocrystals heat-treated at 600 °C for 2 h have a wolframite-type monoclinic structure. The FE-SEM images showed the presence of irregular sphere-like crystals formed by self-assembly of several NiWO₄ nanocrystals. The experimental optical band gap energy (E_gap(exp)) was found to be 2.77 eV using UV–Vis spectroscopy and theoretical calculations indicate an indirect band gap with E_gap3.91 eV, which the (O 2p orbitals) are predominant in the valence band and the (W 5d orbitals) in the conduction band and inhomogeneous electronic distribution into the lattice with the electron density map. We demonstrate for the first time that SPC activity can be enhanced after 120 min by approximately 32% for the degradation of the RBV5R anionic dye by using a NiWO₄ nanocatalyst.