Visible luminescence and surface properties of nanosized ZnO colloids prepared by hydrolyzing zinc acetate

Luminescence properties of nanosized zinc oxide (ZnO) colloids greatly depend on their surface properties. These surface properties in turn are largely determined by the method of preparation. The procedure for producing ZnO colloids consists of two major steps: (I) preparing the precursor by reacting zinc acetate with ethanol and (2) hydrolyzing the precursor to form the colloid by using lithium hydroxide. The sample colloids in this study were prepared by hydrolyzing zinc acetate precursors containing various concentrations of Zn2+ with different concentrations of lithium hydroxide. The luminescence properties were evaluated by the energy difference, Delta E, between the band gap and the emission energy which were obtained from the onset of the absorption spectrum and the peak wavelength of the emission spectrum, respectively. The surface properties of ZnO particles were studied using thermal gravimetric and infrared techniques. ZnO particles produced via these procedures are not pure but have acetate (CH3COO-) groups on their surface. These groups originate from the reagent, zinc acetate. These acetate groups consist of a mixture of unidentate, chelate, and bridging type structures. The amount and structure of the acetate groups depend on the concentrations of Zn2+ in the precursors and the amount of lithium hydroxide used to hydrolyze these precursors. The luminescence properties, Delta E, also changed with the concentrations of Zn2+ and lithium hydroxide. Our results, however, could be normalized to the concentration ratio of zinc acetate to lithium hydroxide, ZnAc/LiOH. The amount of acetate groups on the ZnO particles prepared at the same concentration ratio of ZnAc/LiOH were the same, and the IR spectra were also coincident with each other. Furthermore, the visible luminescence properties, Delta E, are also the same, for the ZnO colloids prepared at the same concentration ratio of ZnAc/LiOH, and increased with increasing ratios. These results suggest that the visible luminescence properties of ZnO particles depend on their surface properties which are in turn determined by the concentration ratio of ZnAc/LiOH.