Green Synthesis of Zinc Oxide-Graphene Oxide Composite via <i>Ex Situ</i> and <i>In Situ</i> Methods for the Photoassisted Removal of Congo Red Dye: A Comparative Study

Industrial chemical pollutants, such as Congo red (CR) dye, are released into water in large amounts, posing a serious threat to the ecosystem. To deal with this issue, we aimed to synthesize an environmentally friendly nanocatalyst, especially zinc oxide (ZnO) and <i>in situ</i> ZnO-decorated graphene oxide (I-ZnO@GO), using <i>Aloe vera</i> extract as a reducing agent, graphene oxide (GO) was synthesized by a modified hummer method, while <i>ex situ</i> ZnO-decorated-GO (E-ZnO@GO) was prepared via a simple coprecipitation method. These nanocomposites were then used for the photoassisted removal of Congo red dye from wastewater. The optical properties, chemical composition, morphology, thermal stability, and crystallinity of the synthesized nanocomposites were characterized by employing ultraviolet-visible spectrophotometry, Fourier transform infrared (FTIR) spectroscopy, field emission scanning electron microscopy with energy-dispersive X-ray spectroscopy (FESEM-EDX), thermogravimetric analysis (TGA) with differential scanning calorimetry (DSC), and X-ray Diffraction (XRD) analysis. The photoassisted dye removal efficiency of the synthesized nanocomposites was assessed using batch processing methodology across multiple experimental parameters to eliminate CR dye from wastewater. Notably, I-ZnO@GO exhibited higher photocatalytic degradation of CR (84.78%) compared to ZnO (54.9%), GO (62.17%), and E-ZnO@GO (74.78%) after 100 min under identical experimental conditions. Additionally, controlled experiments were performed in the absence of UV light for I-ZnO@GO, which showed lower CR removal efficiency compared to the UV-assisted degradation. The linear and nonlinear fitting curves for CR degradation by ZnO, GO, E-ZnO@GO, and I-ZnO@GO showed high <i>R</i> ² values (>0.99), indicating that the photoassisted removal of CR follows pseudo-second-order kinetics. The thermodynamic parameters exhibited that the photoassisted dye removal process is spontaneous, feasible, and endothermic. In conclusion, this study highlights the potential of a new and ecofriendly I-ZnO@GO nanocomposite, which exhibits outstanding photocatalytic efficiency and promising applications in the field of photocatalysis.