Effect of water content on growth and optical properties of ZnO nanoparticles generated in binary solvent mixtures by micro-continuous flow synthesis

The technique of micro-segmented flow was used for continuous synthesis of ZnO nanoparticles. In the experimental setup, a static micro-mixer was applied for mixing the precursor solutions at room temperature. Then micro-fluid segments were formed by injecting the mixture into a stream of carrier. The formation of ZnO particles started after the fluid segments were led in PFTE tube coils inside a thermostat at enhanced temperature. Two different experimental conditions were applied to prepare ZnO nanoparticles, where Zn(Ac)2 and NaOH in ethylene glycol (EG) were mixed with water (at lower pH, pH = 12.3, 100 °C) or water/EG mixing solvent (higher pH, pH = 12.7, 90 °C) to achieve varied water content in the final mixture solution. The formation of homogeneous quasi-spherical particles at lower water content was proven by TEM and SEM. In the first case, a stronger dependence of particle size on water content (diameters between 18 and 436 nm in a water content range between 15 v% and 90 v%) was observed. In the second case, a lower solvent effect (diameters between 33 nm and 168 nm in a water content range between 15 and 60 v%) was observed. The strong effect of water content on the size of the resulting particles was also reflected by the optical properties of the nanoparticles. So, the water-to-solvent-ratio can be used for tuning the optical absorption of ZnO nanoparticles. The characteristic UV absorption peak was shifted between 317 nm and 373 nm by an enhancement of the water content from 15 v% to 90 v% if the particle formation was initiated by mixing Zn(Ac)2 and NaOH in ethylenglycol with water.