Remarkable photocatalytic activity of Zr doped ZnO and ZrO2/ZnO nanocomposites: Structural, morphological and photoluminescence properties

Industrial effluents containing synthetic organic dyes impose a significant level of threatening to the environment and to the health of living organisms. In the context to develop active materials able to degrade synthetic textile dyes, we report highly efficient photocatalysts composed of Zr doped ZnO and ZrO2/ZnO heterostructures synthesized via modified sol-gel Pechini method. The crystal structure, morphology and the optical properties of the synthesized powders were characterized using XRD, Rietveld refinement, SEM, EDX and diffuse reflectance. A single phase of ZnO hexagonal structure was found in pure and 5 wt% Zr doped ZnO, whereas multi-phases consist of hexagonal ZnO plus either tetragonal or monoclinic ZrO2 were detected using higher concentrations of Zr (10-50 wt %). Pure ZnO shows nanospherical-shaped particles with average size of 36 nm ZrO2/ZnO nanocomposites with ZrO2 content of 30, 40 and 50 wt% show similar spherical grains with homogeneous particle size distribution. EDX spectrum of ZrO2/ZnO nanocomposite confirms the existence of Zn, Zr and O elements and the absence of any impurities. ZrO2/ZnO nanocomposites exhibit two band gaps related to their individual components. The results of photocatalytic experiments revealed that both Zr doped ZnO and ZrO2/ZnO nanocomposites demonstrate remarkable photocatalytic activities for indigo carmine (IC) and methylene blue (MB) degradation. The highest degradation rate was realized by ZrO2/ZnO (50:50) nanocomposite with degradation efficiencies of 99% and 97% for IC and MB in 300 and 150 min, respectively. The superior activity of ZrO2/ZnO nanocomposite was attributed to the increases in the charge separation due to strong interfacial surface interaction. The fluorescence studies based on conversion of coumarin to 7-hydroxycoumarin showed similar results to the photocatalytic activity and proved the effective role of the hydroxyl radicals in the degradation process.