High-performance zinc tungstate photocatalyst: tailoring crystallinity, bandgap, and charge separation via controlled calcination

Zinc tungstate (ZnWO 4 ) nanoparticles were successfully synthesized via a simple co-precipitation method and calcined at a temperature range of 400–700 °C to optimize their catalytic performance. Structural characterization confirmed the formation of monoclinic wolframite-phase ZnWO 4 , with crystallite size increasing from 9.6 to 17.5 nm with the calcination temperature. Optical studies revealed that the bandgap can be tuned from 2.5 (as-prepared) to 3.2 eV (500 °C), with a decrease to 2.6 eV at 700 °C. ZnWO 4 calcined at 500 °C exhibited optimal properties, including balanced crystallinity, surface area (8.58 m 2 /g), and charge separation efficiency, leading to exceptional photocatalytic degradation of methylene blue under UV irradiation (96.5% removal in ~ 4.3 h). Kinetic analysis followed a pseudo-first-order model, with the highest rate constant (~ 1.24 × 10 −2 min −1 ) observed for the 500 °C-calcined sample. Scavenger tests identified hydroxyl radicals (·OH) as the dominant reactive species, with holes contributing moderately and superoxide radicals playing a negligible role. Photoelectrochemical measurements confirmed enhanced charge carrier separation, and cycling tests demonstrated remarkable stability (> 95% efficiency retention after five uses). XRD analysis of the used catalyst confirmed structural stability with negligible phase changes. This work highlights the critical role of calcination temperature in tailoring the structural, optical, and photocatalytic properties of ZnWO 4 , establishing it as a highly efficient, stable, and cost-effective photocatalyst for wastewater treatment.