Synthesis of ZnO nanoparticles with tunable size and surface hydroxylation

Abstract

Zinc oxide (ZnO) is an important metal oxide for hybrid inorganic-organic devices in which the surface properties can dictate the overall charac- teristics of the system. The particle size and the amount of hydroxyl groups' density at the surface are key parameters to promote further bonding of organic phase on metal oxide. The precipitation method was used to successfully prepare ZnO nanoparticles at room temperature with a wurtzite structure and a controlled surface hydroxylation. Spherical nanopar- ticles with diameters around 6-8 nm were synthesized in ethanolic solutions whereas the addition of water in the reaction mixture led to bigger particles within the range of 20-50 nm together with a change in mor- phology. The X-ray diffraction data revealed that a high crystal quality of ZnO with hexagonal (wurtzite- type) crystal structure could be obtained with increas- ing the amount of water and the annealing tempera- ture. Transmission electronic microscopy images demonstrated the presence of two populations of particles size synthesized in an ethanol/water reaction mixture together with the presence of a zinc dihydroxide amorphous layer surrounding the well- crystallized grains in water solution. The amount of physically and chemically adsorbed water on to ZnO particles was determined through thermogravimetric analysis. The surface hydroxylation of ZnO particles and the hydrophilic character of the particles surface were shown to be modulated by the solvent, the time, and the annealing temperature of the precipitated particles. In ethanol/water solutions, the use of a reactive silane capping agent such a 3-(trimethoxysi- lyl)propylmethacrylate was shown to limit the growth of ZnO particles with diameters around 5 nm to switch their wetting characteristics from a hydrophilic to a hydrophobic surface.