Strain induced by temperature evolution of mesoporous ZnO nanopowders and its effect on structural, optical and electrical impedance properties

Abstract

The structural and optical properties of ZnO nanopowder samples prepared by a novel green hydrothermal method have been studied. The prepared ZnO nanopowder samples were annealed (200, 250, 300, 400, 500 °C) and characterised through XRD (X-ray diffraction), FE-SEM (field emission scanning electron microscopy), BET (Brunauer–Emmett–Teller), FT-IR (Fourier transform infrared spectroscopy), UV–Vis (Ultraviolet–visible) spectroscopy, PL (Photoluminescence) spectroscopy and EIS (Electrochemical Impedance spectroscopy). The XRD Study revealed that the prepared samples were crystalline with ZnO hexagonal wurtzite structures. The crystallite sizes were determined using the Debye–Scherrer and Williamson–Hall methods and found to increase from 20.5037 to 25.9157 nm and 22.3261 to 31.4388 nm respectively with an increase in annealing temperature. The FE-SEM revealed that the prepared ZnO nanopowder samples were of spherical shape which corroborated the XRD results. The BET results showed that the prepared nanopowder samples were mesoporous with the average pore size varying between 15.125 and 25.583 nm and surface area varying between 15.337 and 23.959 m2/g. The presence of functional groups and chemical bonding were confirmed by the FTIR measurements. The energy band gaps were determined using the Kubelka-Munk function applied to UV–Vis spectra and found to be decreasing from 3.243 to 3.160 eV with an increase in annealing temperature. PL results revealed a clear dependence of the emission with annealing temperature which was caused by the decrease of free carrier concentration as the annealing temperature increased. EIS measurements indicated an increase in electrical resistance with an increase in annealing temperature with the sample annealed at 300 °C having the smallest resistance.