Supercritical hydrothermal synthesis of nano-ZnO: Effects of key parameters and reaction mechanism

In this paper, ZnO particles with the particle size as low as 23 nm were successfully prepared by supercritical hydrothermal synthesis technique. The particle size of nano-ZnO decreased with the increase of temperature and pressure. Nano-ZnO particles in ZnSO4 system were spherical with smaller particle size. The discrepancy in nano-ZnO produced by different precursor of Zn(NO3)2, Zn(CH3COO)2 or ZnSO4 is attributed to the anion effects and supersaturation. The particles in the KOH system (29 nm) were smaller than those in the NaOH system (44 nm). For precursor concentration, intermediate Zn(OH)2 was generated at lower concentration, while Zn4SO4(OH)6·4H2O was produced at higher concentration. For alkali concentration, as the gradual increase of KOH concentration, Zn(OH)2 began to decrease and gradually transformed into Zn(OH)3– and [Zn(OH)4]2−. When the KOH concentration reached a certain value, [Zn(OH)4]2− occupied the dominance in the mixed solution.