Sínteses e caracterizações de nanocristais de óxido de zinco

Abstract

In this work, Zinc oxide (ZnO) nanocrystals (NCs) were synthesized by chemical precipitation method by way of aqueous solution, adopting three synthesis temperatures (0°C, 25°C, and 100°C). After, the samples were annealed at 250°C, 500°C, and 750°C, for two hours. In order to study the effects of synthesis and annealing temperatures in the thermal, structural, morphological, and optical properties of these NCs were performed following characterizations: Differential thermal analysis (DTA), X-ray diffraction (XRD), Raman spectroscopy (Raman), scanning electron microscopy (SEM), optical absorption (OA), and fluorescence (FL). The DTA thermograms gave evidence that both the synthesis temperature as the annealing temperature influenced on the density of Zn(OH)2, crystallinity, density of defects, and morphology of ZnO NCs. The XRD patterns gave strong indications of the majority formation of ZnO NCs with wurtzite structure (JCPDS 36-1451). Next, the Zn(OH)2, in the samples, according to the DTA thermograms, probably was in the amorphous phase. It was found that both the synthesis temperature as the annealing temperature did not change the crystalline structure of the ZnO NCs, however, influenced the crystallinity and the size. The Raman spectra reinforced the results obtained by XRD, with respect to the crystal structure of ZnO NCs, and it was observed that both the synthesis temperature as the heat treatment influenced the degree of disorder and density of oxygen vacancies. It was observed in the SEM pictures that both the synthesis temperature as the annealing temperature changed the morphology and influenced both the size dispersion as the size of the NCs ZnO. The behavior of NCs size is acording with the XRD results. The AO spectra gave indications that the ZnO NCs presented quantum confinement properties, being in agreement with the results of XRD and SEM. The FL spectra reinforced the indications, given from the AO spectra, that the NCs showed quantum confinement properties and that both the synthesis temperature as the annealing temperature influenced the densities of Zn(OH)2, according to the DTA thermograms, and oxygen vacancies, according to the Raman spectra. Moreover, it was possible to establish the relationship between the synthesis and annealing temperatures with the density of energy levels due to various defects in the structure of the NCS. Thus, according to these results, it was possible to control the thermal, structural, morphological, and optical properties of ZnO NCs, synthesized by chemical precipitation method in aqueous solution, from the varying the synthesis and annealing temperatures. It is believed that these results can arouse great interest in scientific research, as the synthesis and processing of ZnO NCs from the methodology adopted in this work, enabling their use in various technological applications.