Obtenção de nanoestruturas de Zn1-xMxO (M= Mn2+ e Co2+) pelo método hidrotérmico: estrutura e propriedades ópticas e magnéticas

Abstract

In this work were prepared pure ZnO and Zn1-xMxO (M= Mn2+ e Co2+) samples by microwave hydrothermal method and analyzed the effect of metal ions on the structure, morphology and the optical properties of the oxide. The powders were obtained at 100ºC with a heating rate of 5ºC/min for 8 min. The material structure was characterized by X-ray diffraction (XRD), Rietveld refinement, and Raman spectroscopy. The particle size and morphology were investigated by scanning electron microscopy by field emission (FEG-SEM). The optical properties were analyzed by photoluminescence spectroscopy (PL) and ultraviolet visible absorption spectroscopy (UV-vis). The results of XRD and Rietveld refinement showed the formation of wurtzite-type structure related to Zn1-xMnxO e Zn1-xCoxO (x=0,0; 0,005; 0,010 e 0,020 mol de Mn2+ e Co2+) samples, indicating a periodic structural organization at long range. For the 0,040 mol Mn2+ and Co2+ doped ZnO samples the secondary phases ZnMn2O4 and ZnCo2O4 were detected, respectively. The Raman active modes that characterize the hexagonal structure of ZnO were observed for all materials. The increasing concentrations of Mn2+ and Co2+ ions in the ZnO matrix generated local distortions leading to structural disorder at short range. UV-vis spectra showed absorption bands from the d-d transition of Mn2+ and Co2+. The microscopy images revealed the nanostructures formation with homogeneous distribution for the Mn doped ZnO and nanostructures with irregular shapes for samples of ZnO doped with Co2+ when compared to pure ZnO sample. The average particle size decreased with the insertion of Mn2+ and Co2+ ions, wherein the sizes were around 89, 47 and 83 nm for samples of pure ZnO Zn0,96Mn0,04O e Zn0,96Co0,04O, respectively. All samples showed photoluminescent properties with maximum emission in the green region of the visible spectrum, which are related to structural defects in the medium and short ranges in the ZnO crystal lattice. The incorporation of metal ions in the oxide matrix caused a decrease in photoluminescence intensity compared to pure ZnO. The EPR spectra show lines corresponding to Mn2+ ions when incorporated into two different sites located in the core and the surface of the ZnO nanostructures.