Calcination-dependent microstructural and optical characteristics of eco-friendly synthesized ZnO nanoparticles and their implementation in analog memristor application

Using Camellia Sinensis leaf aqueous extract as a friendly reducing, capping, and stabilizing agent, ZnO nanoparticles (NPs) are synthesized via the green process. The plausible mechanism for the ZnO NPs formation is proposed as the chelation of Zn, or metal complexation, under the thermally assisted energy from the annealing process. An inclusive study on the effect of the annealing process on the ZnO NPs is conducted from room temperature to 850 degrees C. The structural, morphological, thermal, and optical characteristics and their intercorrelation with annealing temperature are thoroughly studied. In particular, the change of crystalline texture coefficient contributed to the systematic perspective of the shape changes of the ZnO NPs. The (002)-oriented growth becomes preferred at 400-700 degrees C, corresponding to the increase of conical-shaped NPs from 150 to 250 nm in average size. With further temperature, the preferred growth transfers from (002) to (100) orientations, and polygonal spherical-shaped NPs can be obtained. Photoluminescence spectra reveal that singly and doubly ionized oxygen vacancies are the most dominant in the NPs. With the optimized green materials, implementing an eco-friendly memristor based on ZnO NPs found promising for electronic devices.