Impact of annealing on the structural and optical properties of ZnO nanoparticles and tracing the formation of clusters via DFT calculation

In this report, nanoparticles (NPs) of zinc oxide (ZnO) were synthesized through auto-combustion route and annealed in air at different temperatures: 200, 400, 600 and 800 °C. The single phase nature has been confirmed via X-ray diffraction (XRD) and selected area electron diffraction (SAED) analysis. The average of crystallite sizes increased progressively as the thermal annealing increase and ranging between 13.8 and 39.7 nm. Raman spectra resembled to that of hexagonal ZnO wurtzite structure, the narrowing in E2H (438 cm−1) Raman phonon mode in sequence with further annealing indicates improvement in the crystallinity and reduction in the local atomic defects of oxygen vacancy (Vo2+). The defects create energy deep bands within the band gap region and diminish the UV emission efficiency that has been assessed through photoluminescence (PL) spectroscopic. The donor band of oxygen vacancy (Vo2+) was ∼1 eV above valence band (VB), whereas the acceptor band of zinc vacancy (VZn2−) was at ∼0.85 eV above the VB. The ZnO NPs that was annealed at lowest temperature of 200 °C exhibited different behavior trend in which the UV-PL band was diminished clearly, blue shifted to lower wave length and appeared as small plateau at the range of 380–270 nm corresponding to high band gap energy (3.8–4.6 eV), which is indicative to low crystal quality and presence of clusters. We used density function theory (DFT) calculation for computing the HOMO-LUMO band gap of supposed clusters ZnnOn (n = 2, 3, 5, 10). The high band gap energy of the simulated clusters was agreed with that of ZnO NPs annealed at 200 °C that gives indirect evidence on the formation of clusters.