Abstract
M.Sc.
This dissertation describes the synthesis and characterization of CdSe/ZnO and Ag/ZnO core/shell nanoparticles using water as a solvent. The effects of the concentration of the shell precursor, pH, and stabilizing agents on the properties of the nanoparticles were investigated. In general, the type of capping agents had an influence on the crystallite size of the core nanocrystals. The particle size distributions which were calculated from TEM images show that thioglycolic acid as a capping agent produced larger particle sizes compared to thioglycerol. For example, thioglycolic acid produced on average 5.0 nm CdSe nanoparticles while 4.0 nm average CdSe particle size was obtained when using thioglycerol as stabilizer. This observation was confirmed by UV/Vis absorption results which showed that thioglycerol capped CdSe NCs exhibited excitonic peaks positioned at lower wavelength than thioglycolic capped CdSe NCs. XRD results showed that the capping agents used in the current work had no influence upon crystal structure of the CdSe nanocrystals as the cubic structure was obtained with both stabilizers.
The effect of shell precursor concentration at pH 12 showed heterogeneous formation of ZnO nanoparticles at high shell precursor concentrations. Various ZnO morphologies including nanoflowers and nanotriangles were observed on TEM images when 50 mL and 25 mL of zinc nitrate solution were used as shell precursors. UV/Vis absorption results also confirmed the presence of ZnO absorption peaks for samples prepared with 50 mL and 25 mL of zinc nitrate solution as shell precursors. PL results showed an increase in peak intensity as a function of precursor volumes. XRD results showed diffraction patterns due to the wurtzite structure of ZnO. CdSe diffraction patterns in these samples were not detected by the XRD instrument.
At low volume of the shell precursor, which was 5 mL of 0.05 M zinc nitrate, TEM results showed that spherical CdSe/ZnO core/shell nanoparticles were obtained at pH 12 with 5 mL of 0.05 M zinc nitrate solution and 0.1 M sodium hydroxide used as shell precursor solutions, and thioglycerol as a stabilizer. XRD analysis of the
v
sample exhibited ZnO diffraction patterns and the CdSe patterns were not detected owing to their low peak intensities compared to those of the ZnO. Similar results were obtained when thioglycolic acid was used a stabilizing agent. However, TEM images showed the hexagonal shape of the CdSe/ZnO core/shell nanoparticles.
The pH level was found to influence the photoluminescence properties of the CdSe/ZnO core/shell nanoparticles. The enhanced PL intensity was obtained with CdSe/ZnO core/shell nanoparticles with pH 9 in comparison with CdSe/ZnO core/shell nanoparticles prepared at pH 12. TEM images showed the presence of a thin ZnO shell on the surface of CdSe cores for CdSe/ZnO prepared at pH 9 which could be attributed to the PL enhancement. Although EDS results confirmed the presence of elements such as Cd, Se, Zn and O for both the CdSe core and ZnO shell, the XRD results confirmed the presence of only CdSe diffraction patterns, which suggested the ZnO shell had low peak intensity or was amorphous.
A similar effect of these capping agents was observed for Ag nanoparticles as thioglycerol produced average particle size of 16.0 nm whereas 31.0 nm as the average particle size was obtained with thioglycolic acid as stabilizing agent. The crystal phase of Ag NPs was independent of the type of stabilizer. The Ag NPs precipitated in face centred cubic phase. Core/shell Ag/ZnO nanoparticles were produced with 5 ml of zinc nitrate as shell precursor. Higher volumes (50 ml and 25 ml) of zinc nitrate shell precursor induced heterogeneous precipitation of ZnO nanoparticles. XRD patterns confirmed peaks due to ZnO and Ag for Ag/ZnO core/shell nanoparticles.