Salt-Induced Block Copolymer Micelles as Nanoreactors for the Formation of CdS Nanoparticles
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SALT-INDUCED BLOCK COPOLYMER MICELLES AS NANOREACTORS FOR THE FORMATION OF CdS NANOPARTICLES Hanying Zhao, Elliot P. Douglas Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, USA ABSTRACT A novel preparation method of CdS nanoparticles in the core or corona of micelles is presented. Poly(styrene-block-2-vinylpyridine) (PS-b-P2VP) and cadmium ions form aggregates of single micelles, called compound micelles, upon addition of the cadmium acetate salt into a solution of the block copolymer in tetrahydrofuran. The growth of CdS nanoparticles is confined to the core of single micelles after introduction of hydrogen sulfide gas into the solution. UVvisible spectroscopy, fluorescence spectroscopy, and transmission electron microscopy were employed to characterize the prepared core-embedded CdS nanoparticles. Corona-embedded CdS nanoparticles were prepared by dropping the core-embedded CdS nanoparticles into water with a low pH value. The location change of the CdS nanoparticles was accompanied by a structural change of the micelles, a change from compound micelles to single micelles. In a single micelle, CdS nanoparticles distribute randomly in the corona. The size of the nanoparticles increases slightly after the transition. INTRODUCTION The synthesis of semiconductor nanoparticles or clusters is emerging in the fields of materials science and colloid science because of their wide range of optical and electronic properties [1-5]. Many people have prepared nanoparticles in micelles formed by block copolymers [6-7]. A micelle core can be regarded as a nanosized reaction vessel, and the growth of colloidal particles can be limited to the micelle core. Recently in our group we prepared CdS nanoparticles in the core (core-embedded) and corona part (corona-embedded) of salt-induced micelles [8-9]. The size of CdS nanoparticles can be controlled by changing 2VP:Cd2+ molar ratio and block copolymer concentration in THF etc. EXPERIMENTAL DETAILS The synthesis of the PS-b-P2VP block copolymer was performed using sequential anionic polymerization. The molecular weight of the block copolymer is 25.5 × 103 g/mol (Mn) with a polydispersity of 1.30, measured by GPC relative to PS standards. The content of 2-vinylpyridine units in the block copolymer is about 35 mol% as measured by 1H-NMR. Preparation of core-embedded CdS nanoparticles: PS-b-P2VP block copolymer was dissolved in THF at different concentrations under vigorous stirring for 1 hour. Cd(Ac)2.2H2O dissolved in a mixture of methanol and THF (1:2 V/V), was added to the polymer solution. The light yellow solution of core-embedded CdS nanoparticles was prepared by introducing H2S gas into the complex solution.
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Preparation of corona-embedded CdS nanoparticles: Corona-embedded CdS nanoparticles were prepared by dropping varying amounts of core-embedded CdS nanoparticles solution into water with a low pH value and then removing THF in the solution under vacuum. UV-Visible absorption spectra were recorded on a Shimadzu UV-Vis spec
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