Microwave-Assisted Synthesis of ZnO Nanoparticles

ZnO NPs were obtained in accordance with the procedure described in our previous publications [12 (link),18 (link),19 (link)]. The solution (900 mL) of zinc acetate dihydrate (0.3037 mol/dm³) dissolved in ethylene glycol was prepared using a hot-plate magnetic stirrer (70 °C, 450 rpm, SLR, SI Analytics GmbH, Mainz, Germany). Upon the complete zinc acetate dissolution, the solution was immediately poured into a bottle (1000 mL, polypropylene), which was then sealed. The water (H₂O) content test in the precursor was carried out once the solution temperature reached room temperature (RT). A properly calculated amount of H₂O was added to the precursor such that the content was 1.5 wt. %, which was verified.
The synthesis reaction was carried out in a stop-flow microwave reactor, MSS2 model (3 kW, 2.45 GHz, IHPP PAS (Warsaw), ITeE-PIB (Radom), ERTEC (Wroclaw), Poland) [63 (link)]. The synthesis parameters are provided in Table 1.
The reactions of ZnO NPs synthesis in ethylene glycol are described by the following general Equation (1): ${\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2\mathrm{Zn}+2{\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2\stackrel{{\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2,{\mathrm{H}}_2\mathrm{O},.\mathrm{T},.\mathrm{P}}{\to }\mathrm{ZnO}+{\mathrm{H}}_2\mathrm{O}+2{\mathrm{CH}}_3{\mathrm{COOC}}_2{\mathrm{H}}_4\mathrm{OH}$
After the synthesis, the obtained suspensions were sedimented and the supernatants were decanted. The sediments were rinsed three times with water and centrifuged (MPW-350, MPW Med Instruments, Warsaw, Poland). As a result of vigorous stirring of moist powders with water in 100 mL containers, water suspensions of ZnO NPs were obtained from these powders, and subsequently their concentration was determined. Half of the volume of ZnO NPs suspensions was frozen in liquid nitrogen and dried in a freeze dryer (Lyovac GT-2, SRK Systemtechnik GmbH, Riedstadt, Germany).