Synthesis of ZnO Crystal with Micro-Nano Structures by a Continuous Co-Precipitation Method: Morphological Evolution and Mechanism Analysis

ZnO crystals are employed in numerous industries, but precisely controlling their morphologies is still challenging. In this work, a continuous flow process for synthesizing ZnO crystals is developed based on the co-precipitation method. ZnO crystals at different morphologies, including needle-like, rod-like, flower-like, spindle-like, and nanospheres, are successfully prepared at low temperature. The influence of reaction temperature, pH value, ZnCl2 feeding concentration, and reaction time on the morphology, size, and purity of ZnO crystals is systematically investigated. The results of X-ray diffraction show that the high-purity ZnO can only be produced when the reaction temperature is above 75 degrees C. The band gap energies of ZnO evaluated by UV-vis diffuse reflectance spectroscopy increase with the synthesis temperature. According to scanning electron microscope (SEM) findings, the size of ZnO crystals is regulated by controlling the reaction time. Interestingly, the morphology of ZnO crystals is changed from 1D to 3D structure by controlling the feeding concentration of ZnCl2. Moreover, a concentration difference mechanism of ZnO crystal morphology evolution is proposed by monitoring the real-time concentration of free Zn2+ ions.