Structural and optical properties of Cu-doped ZnAl2O4 and its application as photocatalyst for Cr(VI) reduction under sunlight

Abstract Cu-doped ZnAl2O4 spinel oxides (Zn(1-x)CuxAl2O4, 0.0 ≤ x ≤ 1.0) were synthesized by co-precipitation method at 800 °C. The X-ray diffraction analysis of the as-prepared powders confirmed a spinel structure with a space group Fd-3 m. XPS was used to investigate the state of the material surface and the elemental composition. The SEM image confirmed the presence of nano spherical particles. The optical properties were characterized by UV–Vis diffuse reflectance and all band gap values, in the range of 1.71 to 3.54 eV, indicate a semiconductor character of our compounds. The energy bandgap (Eg) values of Zn(1-x)CuxAl2O4 decreased with increasing of the copper content. The photocatalytic activity was evaluated for Cr(VI) reduction under sunlight irradiation. The highest photoreduction efficiency was obtained for Zn0.2Cu0.8Al2O4 with a removal ratio of 75% after 150 min. The increase of surface area increases the photocatalytic activity as it implies larger contact surfaces exposed to Cr(VI). High surface area and pore volume are useful in the formation of photogenerated electron and hole pairs. The kinetics of photoreduction follow the modified Langmuir-Hinshelwood model. The obtained results indicate a good photocatalytic activity, in particular for compounds with a high content of Cu.