Role of Calcination Temperature on the Photocata-lytic Activity of Zinc Tungstate Nanoparticles Syn-thesized by a Novel Route

Abstract

The studies on photocatalysts such as TiO2, ZnO, ZnWO4 have suggested that the crystallinity, surface area, phase purity, electron – hole recombination rate etc. play key roles in their catalytic activity. In the present work, the effect of calcination temperature on the photocatalytic activity of ZnWO4 nanoparticles synthesized by calcining ZnO and WO3 nanoparticle mixture is explored. The structural characterization of the samples were carried out using X-Ray Diffraction (XRD) and Raman techniques. Photocatalytic activity of the samples were studied on the photodegradation of Methylene Blue. The pseudo-first-order rate constant, kapp(min-1),was calculated from the slope of ln (Co/Ct) versus irradiation time t. From the study of photodegradation of MB in the presence of ZnWO4 nanoparticles, it was observed that the samples calcined at 8000C displayed more photocatalytic activity compared to other samples.

Introduction

Background and Significance

• Environmental pollution, driven by the industrial revolution and population explosion, has worsened globally.

• Organic dyes from textile wastewater are particularly hazardous due to their toxicity, carcinogenic nature, and low biodegradability.

• Conventional filtration is ineffective, prompting the need for green, cost-effective wastewater treatment methods.

• Nanomaterials, especially semiconductor-based photocatalysts, offer promise in degrading organic pollutants under light exposure due to their:

  • High surface area

  • Reactive surface chemistry

  • Short diffusion distances

2. ZnWO? as a Promising Photocatalyst

• Zinc tungstate (ZnWO?) is a potential photocatalyst due to:

  • High light yield and photosensitivity

  • Chemical stability and non-toxicity

  • Short decay time and moderate refractive index

• ZnWO?'s photocatalytic activity under light makes it suitable for removal of dye pollutants from water.

3. Synthesis Methods

• Multiple methods exist to synthesize ZnWO?, including:

  • Mechano-chemical synthesis, sol-gel, hydrothermal, solid-state, etc.

• In this study:

  • ZnO and WO? nanoparticles were synthesized via wet chemical precipitation.

  • These were mixed in a 1:3 molar ratio, milled, and calcined at 400°C, 600°C, 800°C, and 1000°C.

  • The goal was to optimize phase purity and photocatalytic efficiency through calcination temperature control.

4. Experimental Procedures

• ZnO was prepared from zinc acetate and NaOH with EDTA as a capping agent.

• WO? was synthesized from sodium tungstate and nitric acid.

• The photocatalytic ability of ZnWO? was tested by degrading methylene blue dye using UV-Vis spectroscopy.

• 80W mercury vapor lamp was used for illumination during the photocatalytic experiment.

5. Key Results

• XRD Analysis:

  • ZnO was in hexagonal (wurtzite) phase; WO? was orthorhombic.

  • ZnWO? samples calcined at 800°C and 1000°C showed pure monoclinic phase—ideal for photocatalysis.

  • Lower calcination temperatures retained WO? phases, indicating incomplete reaction.

  • Crystallite size increased with higher calcination temperatures, suggesting improved crystallinity.

• HRTEM Analysis:

  • Nanoparticles calcined at 800°C were ~50 nm in size.

  • Interplanar spacing of 0.27 nm matched the (111) plane of ZnWO?.

  • SAED patterns confirmed phase purity.

Conclusion

Photocatalytic reaction is a green technique for waste water treatment, performed in the presence of light and suitable catalyst which degrade the pollutant in water. In the present work, nanostructured ZnWO4 were synthesized by calcining ZnO and WO3 nanoparticles at 4000 C, 6000 C, 8000 C and 10000 C. From the XRD pattern of ZnWO4, it was observed that the samples calcined at 8000 C and 10000C exhibited pure monoclinic phase of zinc tungstate while that at 4000 C and 6000 C exhibited reflection peaks corresponding to WO3 also. The grain sizes of nanoparticles of ZnWO4 were calculated using Debye-Scherrer equation. It was observed that the grain size of zinc tungstate nanoparticles increases with increase in calcining temperature. Results of Raman spectra analysis suggested that the ZnWO4 nanocrystals calcined at 8000 C and 10000C were structurally ordered in the short range and correspond to a wolframite-type monoclinic structure. The samples calcined at 4000 C and 6000 C exhibited Raman peaks corresponding to WO3 phase. Photocatalytic activities of the samples were studied on the photodegradation of Methylene Blue. The photocatalytic efficiency and pseudo-first-order rate constant, kapp(min-1) were calculated. It was observed that the samples calcined at 8000 C show more photocatalytic activity and the samples calcined at 10000 C exhibited poor catalysis. The ZnWO4 samples calcined at 8000 C has phase purity compared to that calcined at 6000 C and less grain size compared to that calcined at 10000 C. This may be the reason for the substantially high photocatalytic activity of ZnWO4 samples calcined at 8000 C compared to others.

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Copyright © 2025 Remya M, Sreelekshmi S, S Saravana Kumar. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.