Incorporating W cations into ZnO nanosheets: an efficient method towards ZnO/ZnWO4 photocatalysts for highly effective degradation of organic compounds under UV and visible-light irradiation

Abstract

In this study, novel ZnO/ZnWO₄ nanocatalysts were synthesized for the removal of organic compounds under ultraviolet (UV) and visible-light irradiation. ZnO was prepared by a homogenous precipitation process followed by an impregnation technique for the formation of ZnO/ZnWO₄ photocatalysts with different percentages of tungsten. The resulting photocatalysts formed a mesoporous network of ZnO nanoparticles, with a size of 18 nm, and modified by tungsten, according to X-ray diffraction and N₂ sorption analyses. Their morphology and composition were studied by transmission electron microscopy, energy-dispersive X-ray spectroscopy, and scanning electron microscopy. According to optical studies, the absorption edge of the ZnO nanomaterials revealed that the red-shift with augmented tungsten concentration resulted in a decrease in the forbidden energy gap (E_g) from 3.25 eV to 2.95 eV. Therefore, the ZnO/ZnWO₄ compound absorbed light in the visible-light region, which probably indicates that the developed photocatalysts may be active in this region. The presence of surface hydroxyl groups was confirmed by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy, and these hydroxyl groups play a significant role in the photocatalytic activity of the compounds by catching the formed holes in the valence band under irradiation to create the hydroxyl radical, thus improving the photocatalytic property. Zn–ZnW_0.2 showed the best photodegradation of rhodamine B (RhB), with a rate constant K_app = 0.095 min⁻¹ (98.5% degradation) and K_app = 0.04 min⁻¹ (91% degradation) after 30 and 60 min of irradiation under UV and visible light, respectively. This improvement was rationalized by the effective separation of photogenerated electron–hole (e⁻/h⁺) pairs due to the formation of a heterojunction structure between ZnWO₄ (E_CB = +0.165 eV; E_VB = 2.885 eV) and ZnO (E_CB = −0.365 eV; E_VB = 3.115 eV). In addition, results confirmed that holes (h⁺) and ˙OH radicals are the major reactive species involved in the photodegradation of RhB dye. A possible reaction mechanism was proposed for explaining the photodegradation of RhB over the ZnO/ZnWO₄ photocatalyst. The Zn–ZnW_0.2 photocatalyst retained its high photodegradation rate and good photostability over four consecutive cycles with no changes in its morphology.