Facile synthesis of ZnO nanospheres by co-precipitation method for photocatalytic degradation of azo dyes: optimization via response surface methodology

Abstract

Zinc Oxide (ZnO) has been deemed as an excellent material for photocatalytic processes due to its high photosensitivity, suitable bandgap and low cost. This paper aims to optimize the synthesis process of ZnO nanoparticles via co-precipitation for the enhanced photodegradation of Methyl Orange (MO) textile. Two sets of experiments were conducted; each with different metal hydroxides (NaOH and KOH) used for the synthesis process. For each set, three factors were investigated: temperature, mixing time, and molar ratio of zinc salt to hydroxide salt. Response surface methodology (RSM) using centered central composite design (CCD) was employed to model and optimize the synthesis process with respect to the aforementioned factors. The success of the nanoparticles synthesis was confirmed by X-ray Diffraction (XRD) and Scanning Electron Microscope (SEM). A hexagonal wurtzite crystal structure of as-synthesized ZnO NPs was observed with crystalline size of 24.2 nm. For 20-min UV illumination time, the optimization results revealed maximum degradation efficiency of 32.48% and 32.44% using NaOH and KOH, respectively. The optimum operating conditions for NaOH-based experiments were: temperature of 70 °C, 2.5 h of mixing time, and molar ratio of 1:8.5; while those for the KOH-based experiments were temperature of 60 °C, 1.5 h of mixing time, and molar ratio of 1:6. At optimum condition, these nanoparticles successfully achieved 94.82% degradation in 120 min. The reaction kinetics well fitted pseudo first-order model with a rate constant of 0.0227 min−1 and correlation value R2 of 0.9031.