The effect of group-I elements on the structural and optical properties of ZnO nanoparticles

Undoped and group-I elements doped ZnO nanoparticles (NPs) (Zn_1?yX_2yO, X=Li, Na, K, and y=0.05) were synthesized by a sol–gel method. Structural and morphological studies of the resulting products were carried out by X-ray diffraction analysis (XRD) and transmission electron microscopy (TEM). The XRD results revealed that the sample products were crystalline with a hexagonal wurtzite phase. The TEM images showed ZnO NPs with nearly spherical shapes with particle size distributed over the nanometer range. In addition, the XRD and TEM results showed a decrease in crystallite and particle sizes of NPs from Li-doped to K-doped ZnO NPs. Crystalline development in the ZnO NPs was investigated by X-ray peak broadening. The size-strain plot (SSP) method was used to study the individual contributions of crystallite sizes and lattice strain on the peak broadening of the undoped and doped ZnO NPs. The effect of doping on the optical band-gap and crystalline quality was also investigated by using photoluminescence (PL) and Raman spectrometers. The Raman spectra of the all ZnO NPs showed a strong E₂(high) peak. The PL spectra exhibited a strong peak in the ultraviolet (UV) region of the electromagnetic spectrum for the all ZnO NPs. The UV peak of the doped ZnO NPs was red-shifted with respect to that of the undoped ZnO NPs.