Effect of annealing on defect formations in ZnO nanoparticles

This study reports on the effect of annealing on the structural, defect formation and optical characteristics of ZnO nanoparticles (NPs) synthesized via self-combustion reactions. For investigating the effect of annealing, one part of as-synthesized ZnO NPs was kept as such and other two parts were annealed at 450ºC & 700ºC (hereafter referred as ZCA, ZCA450 and ZCA700) for six hours in ambient environment. Rietveld refinement of XRD patterns approved phase pure formation of ZCA, ZCA450 & ZCA700 in the Wurtzite type hexagonal symmetry along with a monotonic increment in the unit cell volume and mean crystallite size with annealing temperature (AT). In the same line, observation of three characteristic Raman modes (2E2H-2E2L, E2 (high) and E1 (LO)) validated hexagonal symmetry of these ZnO NPs. Blue shifting in the E2 (high) and E1 (LO) modes and red shifting in the 2E2H-2E2L mode and, ESR analysis corroborated formation of oxygen and zinc vacancies in these ZnO NPs along with annihilation of surface defects with AT. UV-Vis-NIR data exhibited considerable absorption of light below 500 nm by these ZnO NPs with a maxima noted in the blue color region. The obtained band gap Eg = 2.74 eV for ZCA NPs is found to be higher than ZCA450 (Eg = 2.50 eV) and ZCA700 (Eg = 2.68 eV). Notably, the obtained Eg for these ZnO NPs are considerably lower than the reported values in literature. These ZnO NPs seems to be useful for making UV & blue light protection layers and UV absorbers for sunscreen lotion etc.