Amorphous TiO2-coated ZnO nanoparticles were prepared by the solvothermal synthesis of ZnO nanoparticles in ethanol and the followed by sol-gel coating of TiO2 nanolayer. The analyses of X-ray diffraction (XRD) and transmission electron microscopy (TEM) revealed that the resultant ZnO nanoparticles were hexagonal with a wurtzite structure and a mean diameter of about 60 nm. Also, after TiO2 coating, the TEM images clearly indicated the darker ZnO nanoparticles being surrounded by the lighter amorphous TiO2 layers. The zeta potential analysis revealed the pH dependence of zeta potentials for ZnO nanoparticles shifted completely to that for TiO2 nanoparticles after TiO2 coating, confirming the formation of core-shell structure and suggesting the coating of TiO2 was achieved via the adhesion of the hydrolyzed species Ti-O- to the positively charged surface of ZnO nanoparticles. Furthermore, the analyses of Fourier transform infrared (FTIR) and Raman spectra were also conducted to confirm that amorphous TiO2 were indeed coated on the surface of ZnO nanoparticles. In addition, the analyses of ultraviolet-visible (UV-VIS) and photoluminescence (PL) spectra revealed that the absorbance of amorphous TiO2-coated ZnO nanoparticles at 375 nm gradually decreased with an increase in the Ti/Zn molar ratio and the time for TiO2 coating, and the emission intensity of ZnO cores could be significantly enhanced by the amorphous TiO2 shell.
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