Synthesis and Growth of ZnO Nanoparticles

ZnO nanoparticles in the size range from 2 to 7 nm were prepared by addition of LiOH to an ethanolic zinc acetate solution. This method [Spanhel, L.; Anderson, M. A. J. Am. Chem. Soc. 1991, 113, 2826] was modified and extended at several points. The synthesis of very small ZnO nuclei was simplified. It was found that aging of particles was governed by temperature, the water content, and the presence of reaction products. Water and acetate induced considerably accelerated particle growth. Growth could almost be stopped by removal of these species ("washing"). Washing consisted of repeated precipitation of ZnO by addition of alkanes such as heptane, removal of the supernatant, and redispersion in ethanol. The aging characteristics are interpreted in terms of the concentration of dissolved ZnII species and reactions well-known in sol−gel chemistry. These findings present a better-defined and more versatile procedure for production of clean ZnO sols of readily adjustable particle size. Such sols are of particular interest for studies of electrical and optical properties of ZnO nanoparticle films. For example, films exhibiting >99% transparency in the visible region could only be obtained by deposition from a washed sol.