Preparation of ZnO nanoparticles using a rotating packed bed

Zinc oxide (ZnO) nanoparticles can be prepared by the precipitation method in a rotating packed bed (RPB). In this work, precursors were firstly prepared in a continuous mode using the liquid-liquid reaction of zinc chloride (ZnCl₂) and sodium hydroxide (NaOH). The effects of the molar ratio of ZnCl₂ to NaOH, the flow rates of aqueous ZnCl₂ and NaOH, the rotational speed, and the concentrations of ZnCl₂ and NaOH on the size of the precursors were examined. Experimental results demonstrate that larger flow rates of the reactant and precipitant, a higher rotational speed, and higher concentrations of ZnCl₂ and NaOH were associated with smaller precursors. The smallest precursors were obtained at a ZnCl₂ concentration of 0.8 mol/L, an NaOH concentration of 1.6 mol/L, a rotational speed of 2000 rpm, and flow rates of 0.5 L/min. Then, the aforementioned precursors were calcined at 450 °C for 1 h to yield ZnO nanoparticles with a mean size of 68 nm and a rather narrow size distribution. The photocatalytic activity of ZnO nanoparticles was evaluated by the photocatalytic decolorization of methyl orange. After 3 L of aqueous methyl orange had been photodegraded by ZnO nanoparticles for 200 min, the decolorization efficiency of methyl orange was found to be 84%.