Hydrothermal micro continuous-flow synthesis of spherical, cylinder-, star- and flower-like ZnO microparticles

The application of micro segmented flow for the formation of ZnO micro- and nano-particles under the condition of hydrothermal synthesis was investigated. The method involves mixing of zinc acetate solutions with sodium hydroxide solutions at room temperature by a static micro mixer (Statmix 6, IPHT Jena). After mixing, micro fluid segments were formed by injecting the reaction solution into a stream of carrier. These fluid segments were then led to PTFE tube coils inside a thermostat. The segment-internal convection caused by the motion of fluid segments was used for acceleration of heat transfer between the tube walls and the reaction liquid. This process was intensified by increasing flow rates. Constant compositions, sizes and distances of segments ensured equal reaction conditions in all segments and narrow residence time distributions. The segment quality was monitored by fast working micro flow-through photometers. All experiments confirm an increasing product homogeneity with increasing flow rates. However, the applicability of flow rate enhancement is limited by the stability of segment formation process. Best conditions were found in the total flow rate range between 0.5 and 5 mL/min.