The influence of methanol and NH4Cl on solvothermal ZnO synthesis and properties

Powders composed of ZnO sub-microspheres were obtained in two stages: solvothermal synthesis and thermal annealing at 125, 300, 450 or 600ºC in atmospheric conditions. The synthesis was carried out with methanol as solvent, zinc acetate dihydrate as Zn2+ source and ammonia chloride as complexing agent. The as-grown and annealed samples were studied through photoluminescence, microRaman and reflectance spectroscopies, scanning electron microscopy and X-ray diffraction (XRD). After the solvothermal synthesis stage, a white precipitate was obtained composed of a flower-like multiphase assembly of layers identified mainly as a layered basic zinc salts (LBZS) and Zn(NH3)2Cl2. After the annealing treatments, the LBZS and Zn(NH3)2Cl2 transformed into ZnO, while the powder morphology changed from the layered flower-like to polycrystalline ZnO spherical particles with sub-micrometer diameters. With increasing annealing temperature, the ZnO spheres size remained unchanged, while the mean crystallite size and wurtzite lattice parameters decreased as a result of tensile stress relaxation. Concomitantly, a blueshift of the defect-related ZnO emission was observed. The combined analysis of emission, vibrational and reflectance spectra and XRD suggests that the annealing treatments result in the formation of ZnO crystallites with oxygen vacancies and oxygen vacancy-zinc interstitial complexes whose densities increase as the annealing temperature increases. The results and analysis reported in this work contribute to the understanding of growth mechanisms relevant for the tailoring of ZnO powder properties through solvothermal synthesis in non-aqueous media.