Influence of Precursor Concentration in the Synthesis of ZnO Nanoparticles on their Morphological, Structural, and Photocatalytic Properties

Metal oxide semiconductors are usually employed as catalysts for organic pollutant degradation, where zinc oxide (ZnO) is one of the most promising materials. Here, ZnO nanoparticles (NP’s) were synthesized with three molar concentrations of the same zinc precursor to know the precursor concentration’s influence on the morphology, structural, optical, and photocatalytic properties of the resulting nanoparticles. Structural and morphological characterization was carried out employing X-Ray diffraction, scanning and transmission electron microscopy. Specific surface area was made with nitrogen adsorption–desorption analysis. The optical properties were measured using UV–Visible in diffuse reflectance mode and photoluminescence spectroscopy. The colloidal stability was carried out by dynamic light scattering and electrophoresis measurements. All synthesized materials display wurtzite, hexagonal structure with different crystalline sizes and morphologies and have absorbance in the UV region, with different crystal defects (observed by photoluminescence). Additionally, it was found that the ZnO NPs are colloidally stable at neutral and basic pH. However, they lose stability at very acidic pH (close to pH2). The photocatalytic activity of the synthesized materials was evaluated with the degradation of reactive black 5 azo dye (RB-5) and the mineralization degree by the total organic carbon (TOC). During the degradation reaction, dye concentration was followed by UV–Visible spectroscopy. In order to describe the kinetic behaviour a kinetic model was developed. This model considers the influence of light irradiation, adsorption, and mass transfer on the photodegradation of RB-5 azo dye. In particular, the ZnO nanoparticles synthesized with a precursor concentration of 0.2 M yield to a higher degradation and mineralization degree of the azo dye. Our proposed kinetic model describes properly the experimental results of the RB-5 photodegradation, with less than 1% error.