Growth of well-defined ZnO microparticles by hydroxide ion hydrolysis of zinc salts

The morphology of the microcrystalline zinc oxide formed by reaction of zinc salts with sodium hydroxide depends critically on the reaction conditions. To understand this, the nature of the solid product has been probed by scanning electron microcopy (SEM) and X-ray diffraction (XRD) and the concentration of zinc remaining in the solution has been determined at regular intervals throughout the reaction. Two general preparative procedures have been followed. The first, which involves simply heating an aqueous solution containing Zn(NO₃)₂ (0.025 M) and sodium hydroxide (0.375 M) to 101°C and maintaining it at that temperature for periods of up to 8 hours, produces star-like microcrystals. The star-like morphology is apparently caused by multiple crystal twinning at the onset of growth, and the size and shape of the microcrystals have been found to be affected by both the reaction stoichiometry and the type of zinc salt counterion. In the second method, which produces needles of zinc oxide (lengths up to 9.5 μm - aspect ratio > 6), the reagents are stirred at room temperature for two hours before heating the mixture to 101°C and maintaining it at that temperature for up to 24 hours. Pre-stirring allows the formation of Zn(OH)₂, which is shown by XRD and SEM to transform to ZnO upon heating.