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

Luminescence properties of nanosized zinc oxide (ZnO) colloids greatly depe nd on their surface properties. These surface properties in turn are largel y determined by the method of preparation. The procedure for producing ZnO colloids consists of two major steps: (I) preparing the precursor by reacti ng 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 concentr ations of Zn2+ with different concentrations of lithium hydroxide. The lumi nescence properties were evaluated by the energy difference, Delta E, betwe en 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 the se procedures are not pure but have acetate (CH3COO-) groups on their surfa ce. These groups originate from the reagent, zinc acetate. These acetate gr oups consist of a mixture of unidentate, chelate, and bridging type structu res. The amount and structure of the acetate groups depend on the concentra tions of Zn2+ in the precursors and the amount of lithium hydroxide used to hydrolyze these precursors. The luminescence properties, Delta E, also cha nged with the concentrations of Zn2+ and lithium hydroxide. Our results, ho wever, could be normalized to the concentration ratio of zinc acetate to li thium hydroxide, ZnAc/LiOH. The amount of acetate groups on the ZnO particl es prepared at the same concentration ratio of ZnAc/LiOH were the same, and the IR spectra were also coincident with each other. Furthermore, the visi ble luminescence properties, Delta E, are also the same, for the ZnO colloi ds prepared at the same concentration ratio of ZnAc/LiOH, and increased wit h increasing ratios. These results suggest that the visible luminescence pr operties of ZnO particles depend on their surface properties which are in t urn determined by the concentration ratio of ZnAc/LiOH.