Studies of crystallization kinetics and optical properties of ZnO films prepared by sol–gel technique

Abstract

The crystallization of ZnO prepared by sol–gel technique was investigated under nonisothermal conditions by differential scanning calorimetry (DSC) and prediction isothermal method. The transformation from the amorphous to crystalline state was investigated by X-ray diffraction (XRD). The influence of the annealing temperature on the structure of the ZnO was studied by Filed Emission Scanning Electron Microscopy (FESEM) and Atomic Force Microscopy (AFM). Two exothermic changes were reported between the temperature ranges 590–710 K and 645–750 K for first and second transition, respectively. The isoconversional methods of Friedman, Ozawa and Flynn and Wall (FWO), Kissinger–Akahira–Sunose and the Vyazovkin (minimizing) were used to determine the variation of the effective activation energy with the extent of crystallization, Eα (α) and hence, with temperature Eα (T). In addition, the activation energy was calculated from predicted isothermal results. Analysis of the obtained data shows that the activation energy of crystallization is not constant but varies with the degree of conversion and, hence, with temperature. The value of the local Avrami exponent, n (α), varies within 1.88–1.97 for isothermal study and within 2.35–2.4 for nonisothermal process. By using the Johnson–Mehl–Avrami (JMA) model, a good fit was achieved between experimental and theoretical calculation of the reaction (crystallization) rate. The optical energy gap decreased with increasing annealing temperature.