Systematic study of hydrothermal crystallization of zinc oxide (ZnO) nano-sized powders with superior UV attenuation

Zinc oxide (ZnO) powders were synthesized by the hydrothermal method at 160–280 °C for 6–24 h, under 90–930 psi pressure, from concentrated precursors without stirring. The precursors were formed by mixing aqueous solutions of zinc acetate, chloride, sulfate, or nitrate with KOH (aq) under either controlled or rapid precipitation conditions. The precursor quantities were chosen to yield zinc concentration of 0.3–4.0 m, KOH concentration of 0.6–8.0 m, and pH between 4 and 14 after precipitation had been completed. Several of the synthesized powders exhibited equiaxed morphology, low aggregation levels, narrow crystallite size distributions in the range 60–200 nm, and excellent dispersability. Some of the ZnO powders consisted of nano-sized equiaxed crystallites mixed with large, mostly elongated crystals, 0.5–4.0 μm in size. Based on this systematic study encompassing a wide range of synthesis conditions, temperature-controlled growth rate and Ostwald ripening were found to dominate the hydrothermal crystallization of ZnO powders. Comparison with several commercially available ZnO powders revealed superior purity, particle size distributions, defect structure, and ultimately the UV attenuation, in selected powders synthesized hydrothermally. Results of this study enable the use of hydrothermally made ZnO powders in sunscreens or other UV absorption applications and make their hydrothermal production commercially feasible.