Eco-friendly synthesis of large band gap ZnO nanoparticles by trisodium citrate: investigation of annealing effect on structural and optical properties

In this study, large band gap zinc oxide nanoparticles (ZnO-NPs) were prepared by a facile, cost-effective, and eco-friendly co-precipitation method in aqueous solutions. Trisodium citrate was employed as a green and bio-safe complexing agent for zinc ions without using ammonia and/or any organic solution. The annealing effect on crystal structure, morphology, particle size, composition, UV absorption, and fluorescence (FL) properties of the synthesized ZnO-NPs was evaluated using various techniques. The UV-Visible absorption spectra of the ZnO-NPs showed a strong absorption peak in the UV-C region with a wide band gap energy of 4.2-4.4 eV. Field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) images demonstrated fine spherical particles with approximate diameters of 20-50 nm. The average hydrodynamic diameter and zeta potential values of the as-prepared and the annealed ZnO-NPs in deionized water were 28.2 nm, - 32.4 mV, 38.9 nm, and - 16.1 mV, respectively. The FL and UV-Visible results illustrated that the annealing could considerably influence the emission and absorption spectra of the samples in the UV region. The FL and EDX results revealed that the annealing affects the composition, defect points, and states population in the crystal structure of the ZnO-NPs. Practical experiments of exposing the samples to UV radiation and the FL measurements indicated that the ZnO-NPs were sensitive to the excitation wavelengths, therefore, they have potential application in fabricating various types of UV sensors and filters (blockers), as well as photocatalytic activities.