Surfactant-free Colloidal Crystal of ZnO Quantum Dots
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1207-N12-03
Surfactant-free Colloidal Crystal of ZnO Quantum Dots Xi Zhang1, Dazhi Sun1 and Hung-Jue Sue1, 2 Polymer Technology Center 1 Materials Science and Engineering Program 2 Mechanical Engineering, Texas A&M University College Station, TX 77843, U.S.A. ABSTRACT Monodisperse ZnO quantum dots (QDs) with a particle size of about 5 nm have been synthesized. Isopropanol together with hexane were utilized to precipitate ZnO nanoparticles to form condensed phases, ranging from white flocculation, to gel-like fluid, and to transparent solid. The morphology and structure in the transparent ZnO solid was characterized by UV-vis, X-ray diffraction, small-angle X-ray scattering, transmission electron microscopy, and scanning electron microscopy. The mechanisms for the formation of transparent ZnO QDs close-packed structure were monitored via UV-vis spectra, and found likely to be a colloidal crystal. The colloidal crystal is transparent and absorbs UV light efficiently. Possible conditions for the formation of the ZnO QDs colloidal crystal are discussed. INTRODUCTION Colloidal crystals are similar to atomic crystals in many aspects, and can serve as a model system to study classical condensed phase phenomena [1]. Many research teams have prepared various nanoparticles-based colloidal crystals via dip coating [2], spin coating [3], lightcontrolled interaction of ligands-decorated nanoparticles [4], lithography templating [5], and gravity precipitation tuned by solvent composition [6]. Most of the colloidal crystals obtained in the past are in micrometer range and the nanoparticles as a building block are usually capped or “protected” by ligands or surfactants. Here, we demonstrate the 1st synthesis of colloidal crystal of ZnO QDs without surfactants by rearranging flocculated ZnO nanoparticles toward equilibrium state of ordering via carefully adjusting the ionic concentration in the surrounding medium. ZnO colloidal crystals synthesized with this method have lateral dimensions in centimeters and are almost 100% transparent to the visible light, while maintaining capability to block ultraviolet (UV) light. EXPERIMENTAL Colloidal ZnO QDs with a particle size of about 5 nm were prepared by hydrolyzing zinc acetate dihydrate in the basic methanol solution as we previously reported [7]. After reaction for 2 hours, the solution was concentrated by 10 times at ~ 60 °C via rotary evaporation under vacuum. The concentrated ZnO sol was obtained and the concentration of [K+] was 0.8M. Ions and zinc layered double hydroxide (Zn-LDH) Zn5(OH)8(Ac)2_2H2O in the concentrated ZnO sol can be removed by the precipitation redispersion procedure introduced by Meulenkamp [7, 8]. A considerable amount of ZnO QDs were precipitated and two phases were formed after overnight at 4 °C. One is the solid phase of ZnO QDs and the other one is the supernatant, a diluted liquid
phase containing mostly [K+] ions and a small quantity of ZnO QDs. In order to obtain purified ZnO QDs, this precipitation and redispersion procedure should be repeated until
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