Gas phase synthesis of zinc oxide nanocrystals and their surface modification using small and large acidic ligands
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Gas phase synthesis of zinc oxide nanocrystals and their surface modification using small and large acidic ligands Sankhanilay Roy Chowdhury1, Moazzam Ali1, Daniela Sudfeld2, and Markus Winterer1 1 Nanoparticle Process Technology, Faculty of Engineering and Cenide, University of DuisburgEssen, Duisburg, 47057, Germany 2 Experimental Physics, Faculty of Physics and Cenide, University of Duisburg-Essen, Duisburg, 47057, Germany ABSTRACT Zinc oxide nanocrystals are synthesized using the chemical vapor synthesis (CVS) technique. Diethylzinc is used as zinc precursor. Synthesized zinc oxide nanocrystals are characterized by X-ray diffraction (XRD), nitrogen adsorption and desorption, transmission electron microscopy (TEM) and dynamic laser scattering. As-synthesized zinc oxide has a primary particle size of about 10 nm and an isoelectric point at pH 9.5. The surface of zinc oxide nanocrystals can be modified using short chain organic acids like formic acid and glycine as well as low molecular weight polyacrylic acid (PAA). The isoelectric point shifts towards acidic pH when glycine is used as surface modifier whereas it shifts towards basic pH with formic acid. The use of PAA generates a very high value of zeta potential over the range of pH studied in these experiments and thereby eliminates the isoelectric point. INTRODUCTION Zinc oxide has become an important materials in the field of semiconducting and optoelectronic devices. Synthesis of ZnO nanocrystals (ZnO NCs) can be carried out in the liquid or in gas phase [1,2]. Synthesis of ZnO NCs by chemical vapor synthesis (CVS) from zinc-tetramethyl-heptanedionate or zinc-acetylacetonate, which are solid precursors, have been reported in literature [3,4]. In the present study we used diethylzinc (DEZ), a liquid precursor, for the synthesis of ZnO NCs. DEZ has certain advantages over solid precursors. It can easily be evaporated and the amount of the precursors can be controlled by the combination of a bubbler and a controlled mass flow of carrier gas. The precursor evaporation is an important parameter in CVS as it helps to control the growth of NCs in desired shapes and sizes. For several applications it is necessary to have ZnO powders dispersed in solvents. Zinc oxide powders may be dispersed in aqueous and nonaqueous media using suitable dispersing agents [5,6]. Stabilization of ZnO dispersions for sufficiently long time is still a technological challenge. In this communication we describe synthesis of ZnO NCs from a liquid precursor using the CVS method and its aqueous dispersion characteristics using various acidic ligands. EXPERIMENTS Fig. 1 shows the schematic drawing of the experimental setup of the reactor. A bubbler with DEZ is used as a precursor delivery system. Helium is used as a carrier gas for DEZ and its flow is controlled by a mass flow controller. Oxygen is used as oxygen source and it is added just before the reactor to prevent preliminary oxidation of DEZ. The flow of Helium and oxygen are maintaind at 50 and 1000 sccm, respectively. Te
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