Size and Morphology Dependence of ZnO Nanoparticles Synthesized by a Fast Continuous Flow Hydrothermal Method

Abstract

We have investigated the formation of ZnO nanoparticles using a fast continuous flow hydrothermal synthesis method. The synthesis conditions have been varied with respect to temperature, pH, and concentration of the Zn(NO3)2·4H2O + NaOH aqueous precursor. The different conditions affect the size, morphology, and crystallinity of the produced ZnO nanoparticles. The nanoparticles were investigated by Rietveld refinement of powder X-ray diffraction data, transmission electron microscopy, and scanning electron microscopy. The particle size and morphology are highly temperature dependent: Anisotropic particles of a maximum length of 1 μm are produced at a reactor temperature of 122 °C, while isotropic particles of around 25 nm are produced at temperatures above 200 °C. The crystallinity of the particles increases from 90(1)% at 122–200 °C to 99(1)% at 390 °C. Variation of the pH of the precursor results in different morphologies: (1) acidic conditions produce large rods, (2) neutral conditions give isotropic particles, and (3) alkaline conditions result in large plates. Finally, it was found that the particle size increases with the precursor concentration.