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.