Effect of reaction parameters on the formation and properties of ZnO nanocrystals synthesized via a rapid...

ZnO nanocrystals were successfully prepared under mild conditions by solochemical method from sodium hydroxide and zinc nitrate hexahydrate in 3 h refluxing. In this process the dependence of the morphologies, sizes and formation mechanisms of the ZnO nanocrystals on the reaction temperature was investigated. The samples were analyzed by X-ray diffraction (XRD), including Rietveld analyses, transmission electron microscopy (TEM), Raman spectroscopy and UV-Visible absorption spectroscopy. The X-ray diffraction patterns revealed that the formed products are pure ZnO with hexagonal (wurtzite) structure. Rietveld analyses of the XRD patterns showed anisotropic effects on the size and microstrain of ZnO nanocrystals. The anisotropy on the crystallite size decreases as the reaction temperature increases. The microstrain remains constant up to 90 degrees C when reached highest values in all directions. The transmission electron microscopy images showed short ZnO nanorods and rounded shape nanoparticles for all samples. The average size (length by diameter) ratio of the ZnO nanorod increased from 1.5 to 2.4 when reaction temperature was raised from 50 degrees C to 80 degrees C and decreased to 1.4 as the temperature was further increased to 90 degrees C. The HRTEM image of the sample prepared at 90 degrees C showed that ZnO nanocrystals have their c-axes as the primary growth direction. The wurtzite structure in ZnO nanorods has been verified by its characteristic E2 mode in the Raman spectra of all samples. All Raman peaks (especially for E2 mode) observed for the sample prepared at 90 degrees C showed huge intensity reduction and linebroadening, except for the second order Raman mode at about 1068 cm(-1) which presented a very huge and unusual increase on intensity. All samples presented a blue shift in the excitonic absorption compared to ZnO bulk that increases alongside with reaction temperature. In addition a mechanism for the synthesis of ZnO nanocrystals using zinc nitrate hexahydrate and sodium hydroxide by the solochemical method has also been proposed.