Facile synthesis and tailoring the structural and photoluminescence properties of ZnO nanoparticles via annealing in air atmosphere

Abstract

Herein, bulk synthesis of high quality wurtzite structured zinc oxide (ZnO) nanoparticles (NPs) by a facile wet chemical colloidal method is reported along with a detailed investigation on the effect of air annealing (up to 800 °C) on their structure, morphology, composition, optical and photoluminescence (PL) properties and their co-relation, rarely reported in such a wide temperature range. The as-prepared ZnO samples consisted of low sized quasi-spherical NPs (~25 nm) which agglomerated to form random shaped bigger NPs (100–500 nm) after annealing at 800 °C. The Zn/O ratio (wt%) approached to theoretical value for the samples annealed at 800 °C. Structural characteristics like crystallite dimensions, lattice parameters, strain, dislocation density, oxygen vacancy etc. have been evaluated. Further, structural conformation and co-existence of initial strain-free and the strained phases in the high temperature (≥800 °C) annealed ZnO NPs have been discussed with possible mechanism, an aspect not addressed before. Band gap narrowing was also observed from ~3.23 eV for the as-synthesized ZnO NPs to ~2.64 eV after annealing at 800 °C, which has been attributed to the decrease in the dislocation density, increase in the oxygen vacancies as well as the increase of dimensions and hence the number of grain boundaries of the ZnO NPs with annealing temperature. Significantly enhanced PL emission in the visible range was achieved by virtue of tailoring the structural and optical properties via high temperature annealing in the air atmosphere. Emission mechanism and their relation with different defects such as oxygen (VO), and zinc (VZn) vacancies, oxygen (Oi) and zinc (Zni) interstitials, etc. of the ZnO NPs is presented as function of annealing temperature in the air. Such an extensive study presents a better understanding and co-relation of the structure, morphology, composition, defects, optical and PL properties of the ZnO NPs prepared by a facile method with regards to annealing temperature and may find potential applications in optoelectronic devices.