Synthesis by Sol-Gel and Coprecipitation of Zn1−xFexO Nanoparticles for the Adsorption of Congo Red Dye

The influence of synthesis method on the properties of Zn1−xFexO nanoparticles with different Fe doping levels (x = 0, 0.01, 0.03, and 0.05) for Congo Red (CR) adsorption was investigated. Nanoparticles were prepared by sol–gel and coprecipitation and characterized by XRD, SEM-EDS, FTIR, and BET analyses. Sol–gel synthesis produced smaller particles (~13 nm) than coprecipitation (~35 nm), and both the method and calcination temperature strongly affected crystallite size. Sol–gel nanoparticles showed significantly higher adsorption efficiency (~90%) due to their larger BET surface area, greater BJH pore volume, and smaller particle size, which increased the number of accessible active sites. In contrast, coprecipitation nanoparticles exhibited a much lower adsorption capacity (~24%). Fe incorporation further enhanced performance by introducing lattice distortions and oxygen vacancies, as evidenced by XRD peak broadening and increased lattice strain. SEM images displayed particle growth and compaction after adsorption, particularly in doped samples. Temperature-dependent experiments indicated that undoped ZnO lost efficiency at 60 °C due to weak physical interactions, whereas Fe-doped nanoparticles maintained high adsorption, due to improved stability of the adsorbent-adsorbate bond. The combination of Fe doping and sol–gel synthesis significantly improved the properties of ZnO, yielding highly efficient adsorbents suitable for environmental remediation.