Facile sol-gel synthesis of ZnAl₂O₄ spinel nanoceramics for photocatalytic applications: H₂ production and MB dye degradation

ZnAl₂O₄ spinel nanoceramic was successfully synthesized by a sol-gel combustion method and subsequent annealing at temperatures as low as 500 °C. The photocatalytic activity of the synthesized spinels was evaluated for the H₂ photocatalytic production and photodegradation of methylene blue dye (MB) under UV light irradiation. ZnAl₂O₄ spinels were characterized by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Fourier Transform Infrared (FTIR), UV-vis spectroscopy, and Photoluminescence (PL). XRD revealed that the low-temperature calcination ZnAl₂O₄ spinel nanoceramics have a cubic crystalline structure and nanocrystallites sizes between 10–21 nm. The presence of carbon dot (CDs) nanoparticles is suggested by FTIR, UV-vis, and EDS characterization. Band gap values, estimated from diffuse reflectance spectra, are between 2.00 eV and 4.26 eV, for ZnAl₂O₄ calcined between 500 °C and 900 °C. The best H₂ evolution was achieved with ZnAl₂O₄ treated at 500 °C reaching a 145 μmol g⁻¹ production after 2 h. The highest percentage of MB dye degradation of 84%, after 120 min of UV irradiation, was achieved with ZnAl₂O₄ spinel treated at 900 °C.