A green approach to the microwave-assisted synthesis of flower-like ZnO nanostructures for reduction of Cr(VI)

Zinc oxide nanostructures with different morphologies were prepared via a microwave-assisted method and used as photocatalysts for reduction of chromium(VI) under ultraviolet-B illumination. X-ray diffraction revealed that as-made ZnO samples have wurtzite hexagonal structure. Scanning electron microscopy demonstrated that the morphology of the ZnO crystals could be easily modified from star-shaped to chrysanthemum flower-like structure by changing the ratio of zinc nitrate and sodium hydroxide. X-ray photoelectron spectroscopy and energy dispersive X-ray analysis confirmed the stoichiometry of pure ZnO. Its optical properties were also investigated by UV-Vis diffuse reflectance spectroscopy, which indicated that the prepared ZnO possessed a band gap value of 3.2 eV. Photocatalytic activity studies revealed that the ZnO (1:6) (1 × 10⁻³mol zinc nitrate: 6 × 10⁻³mol sodium hydroxide) catalyst exhibited a significantly faster reduction rate of Cr(VI) compared to ZnO (1:5) and ZnO (1:4). More than 95% of Cr(VI) at a concentration of 10 mg/L has been reduced to Cr(III).