Mechanisms of Zinc Oxide Nanocrystalline Thin Film Formation by Thermal Degradation of Metal-Loaded Hydrogels

Thermal degradation of metal-loaded hydrogel films is a powerful tool to synthesize high-quality metal oxide thin films with nanometric grain size, but the formation mechanism is still poorly known. We exploited fluorescence X-ray absorption fine structure as a short-range probe to investigate the first stages of nucleation and the following development of ZnO nanoparticles in Zn-loaded hydrogels, annealed for 20 min at temperatures between 150 and 500 °C. The experimental results evidenced that the first nuclei of ZnO begin to form at 300 °C, whereas at 350 °C the ZnO nanoparticles are already well crystallized. By coupling the results with those obtained by thermal analysis, profilometry, HR-TEM, diffraction and μ-FTIR, a formation mechanism was suggested. The optical and electrical properties of the samples confirm that the polymer forms an insulating shell around the nanoparticles up to 450 °C: the intimate hybridization promotes stress relaxation during the annealing, yielding crack-free metal oxide films. Prolongation of the annealing time allowed the removal of the organic shell at 350 °C, yielding fully conductive, transparent ZnO films with particle size reduced to 7 nm.