ZnO Nanoparticles Synthesized via a Supercritical-CO2-Assisted Method as Photocatalysts for the Degradation of Water Pollutants

Zinc oxide (ZnO) is a widely studied photocatalyst for the degradation of organic pollutants in water, yet its conventional sol–gel synthesis often suffers from low yield and produces materials with low specific surface area. In this study, we tackled these limitations by synthesizing ZnO nanoparticles using a supercritical-CO2-assisted sol–gel method (ZnO-scCO2). The influence of the calcination temperature, precursor concentration, and solvent type on the synthesis of ZnO was systematically investigated, and the materials were characterized with a combination of techniques (XRD, SEM, N2 physisorption, UV-Vis-DRS spectroscopy). The photocatalytic performance of the ZnO-scCO2 materials was evaluated in the degradation of two probe pollutants (phenol and rhodamine B, 200 ppm), under UV and visible radiation. The scCO2-assisted method in ethanol as the solvent allowed achieving at least a four-fold higher ZnO yield and two-fold higher surface area compared to the materials prepared with a conventional sol–gel route without scCO2. These ZnO-scCO2 nanoparticles consistently showed enhanced photocatalytic activity in the removal of phenol and rhodamine B compared to their counterparts synthesized without scCO2 and compared to commercial ZnO. Among the screened synthetic parameters, the solvent in which ZnO was prepared proved to be the one with the strongest influence in determining the ZnO yield and its photocatalytic activity. The optimum results were obtained using 0.50 M zinc acetate as the precursor in 1-butanol as the solvent, and calcination at 300 °C.