Synthesis and evaluation of Fe-doped zinc oxide photocatalyst for methylene blue and congo red removal

Zinc oxide is one of the most common photocatalysts utilized for the photocatalytic degradation of synthetic dyes aside from titanium dioxide. However, the application of ZnO in the treatment of wastewater containing synthetic dyes is limited due to the high energy band gap which allows ZnO to be efficient upon irradiation with ultraviolet radiation only. This study aims to evaluate the photocatalytic degradation efficiency of the zinc oxide photocatalyst and its derivatives, specifically 0.25, 0.5, 2.5 and 5 mol% Fe(II)-doped ZnO, 0.25, 0.5, 2.5 and 5 mol% Fe(III)-doped ZnO and 2.5 mol% Fe(II)-Fe(III)-doped ZnO. The performance of the photocatalysts was evaluated based on the effect of solution pH, effect of photocatalyst loading and nature of dye. The synthesis of photocatalysts were done using sol-gel synthesis method, and photodegradation tests were carried out under visible light exposure for 60 minutes. The photocatalysts were characterized with SEM, FTIR, and UV-Vis spectroscopy. The optical characterization results show that 2.5 mol% Fe(II)-Fe(III)-doped ZnO has the lowest band gap energy of 3.401 eV which was estimated using Tauc’s plot. This further validated the degradation performance of the 2.5 mol% Fe(II)-Fe(III)-doped ZnO photocatalyst where it displayed the highest photocatalytic degradation efficiencies at all pH and photocatalyst loading. The highest degradation achieved using methylene blue was 94.21% and 32.97% using congo red as model solute at optimum pH and 300 mg/L photocatalyst loading. In overall, the present study has proven that Fe-doped photocatalysts have the potential for the degradation of various synthetic dyes upon irradiation with visible light.