Structural, optical and vacancies investigations of Li-doped ZnO

We have synthesised undoped and Li-doped ZnO by co-precipitation method and solid-state route resulting in ZnO (Zn1-xLixO; x = 0, 0.03, 0.05, 0.08) nanostructures (NSs) and bulk ZnO (Zn1-xLixO; x = 0, 0.05, 0.1), respectively. To examine the structural and optical properties, oxygen vacancies and local disorder, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), diffuse reflectance spectroscopy (DRS), photoluminescence spectroscopy (PL), X-ray absorption fine structure (XAFS) spectroscopy, X-ray photoelectron spectroscopy (XPS) and valence band spectra techniques were utilized. XRD confirms wurtzite structure for both undoped and Li-doped ZnO bulk and NSs samples. The undoped ZnO NSs show granular structures, while Li-doped ZnO NSs showed flake-like structures as elucidated by SEM. The flake dimension increases with increasing the Li content in the ZnO NSs, whereas the thickness of the flakes seems to be independent of the Li concentration. A reduction in particle size with Li doping was observed from TEM for Li-doped ZnO NSs samples, which is consistent with the average crystallite size deduced from XRD. Bulk undoped ZnO shows nearly cuboid shape particles and the size of the particles increases with increasing Li content in ZnO. A blue shift in the optical band gap was obtained for Li-doped ZnO NSs, whereas a red shift is observed for Li-doped ZnO bulk. Band gap decreases with the incorporation of Li in ZnO bulk, while no further increase in band gap was observed for bulk ZnO after higher doping of Li. The oxygen vacancies in Li-doped ZnO bulk and NSs were observed by PL spectra, which were further corroborated by XAFS and oxygen vacancies are increased with an increase in the Li concentration in ZnO for both bulk as well as NSs samples. From the XPS study, it is confirmed that the oxygen vacancies for Li-doped ZnO NSs samples are increasing more as compared to Li-doped ZnO bulk samples. From DRS and valence band studies, it has been revealed that Li-doped bulk ZnO has band gap narrowing, whereas Li-doped ZnO NSs show band gap widening. Graphical abstract