Reverse micelle synthesis and characterization of ZnSe nanoparticles

Reverse micelle synthesis and characterization of ZnSe nanoparticles

Materials Letters 62 (2008) 3694–3696 Contents lists available at ScienceDirect Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i ...

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Materials Letters 62 (2008) 3694–3696

Contents lists available at ScienceDirect

Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t

Reverse micelle synthesis and characterization of ZnSe nanoparticles Zhongli Lei a,⁎, Xiangyu Wei a, Shuxian Bi b, Rongjian He a a b

Key Laboratory of Applied Surface and Colloid Chemistry (Shaanxi Normal University), Ministry of Education, School of Chemistry and Materials Science, Xi'an, 710062, PR China School of Chemistry and Chemical Engineering, Ningxia University, Yinchuan, 750021, PR China

A R T I C L E

I N F O

Article history: Received 30 October 2007 Accepted 9 April 2008 Available online 16 April 2008 Keywords: Composite materials Semiconductors ZnSe Reverse micelles PEA

A B S T R A C T Novel spherical assemblies of ZnSe-containing block copolymer reverse micelles in aqueous solution have been formed by the addition of HSe-solution to mixtures of Zn2+and the reverse micelles soft template Polyacrylonitrile-block-poly (ethylene glycol)-block-Polyacrylonitrile. The products were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and UV-Vis spectrometer. The results of XRD and TEM analyses demonstrated that the products were spherical and homogeneous with cubic structure, which are about 60 nm in diameter. The corresponding UV-Vis absorption peaks showed a blue shift compared to bulk ZnSe. From the position of the absorption edge (λe), we calculate an actual ZnSe particle diameter of ~5.0 nm. This approach represented a simple method in the synthesis of the polymer micelles shelled with inorganic materials based on using amphiphilic block copolymers. And studies to improve the uniformity of size, shape, and crystal domain will be a focus of future work. © 2008 Elsevier B.V. All rights reserved.

1. Introduction The self-assembly of amphiphilic diblock copolymers in selective solvents generally results in micelles of a core-shell structure. These micelle systems have the potential and practical application such as drug delivery [1,2], microreactors [3], and general detergents [4]. A further advancement of these techniques is to prepare nanoparticles to produce hybrid materials [5–7] made up of polymer and inorganics. In recent years, the study on ZnSe nanoparticles with quantum size (b10 nm) has attracted more interest since ZnSe has wide band gap (2.58 eV) and transmittance range (0.5-22 Am), high luminescent efficiency, low absorption coefficient and its excellent transparency to infrared [8,9]. Therefore, designing of new polymeric micelle system for the synthesis of well-defined nanomaterials with controlled dimensions and morphology will be one of the most important challenges both for scientific and applicational reasons. Various methods have been used to synthesize nanoparticles with controlled morphology and structure, and the use of reverse micelles seems especially suited for tailoring particle size at the nanolevel [10– 12]. And it is known that the composition conditions of reverse micelles largely influence the size and shape of the final nanocrystals [13]. The reverse micelle technique often requires specifically generated surfactants or expensive reagents. Several goups have used the reverse micelle technique to make nanostructure of semiconductor and many copolymers reverse micelles have been used as templates for preparing inorganic nanoparticles [14–18].

⁎ Corresponding author. Tel.: +86 29 85303952; fax: +86 29 85307774. E-mail address: [email protected] (Z. Lei). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.04.043

Here, we designed and synthesized amphiphilic block copolymer Polyacrylonitrile-block-poly (ethylene glycol)-block-Polyacrylonitrile (PAN-b-PEG-b-PAN, PEA) with low polydispersity index (PDI) by atom transfer radical polymerization (ATRP) [19]. PEA forms reverse micelles with PEG as the core and PAN as petal-like shell in DMF solutions. The ZnSe-block copolymer composites are characterized by transmission electron microscope (TEM) and X-ray diffractogram (XRD). 2. Experimental section The designed amphiphilic block copolymer, PAN-b-PEG-b-PAN (PEA) was synthesized by atom transfer radical polymerization (ATRP) as described in literatures [19]. The water used in all experiments was prepared by a Milli-Q Academic purification system. All of the other regents and solvents were analytical grade, and used without further purification. NaHSe aqueous solution was prepared freshly by dissolving Se powder in NaBH4 solution under nitrogen atmosphere at room temperature for about 4 h. Power X-ray diffraction (XRD) patterns of all samples were measured on a Rigaku model D/max2000 PC X-ray powder diffractometer with Cu Kα radiation (λ = 1.5418 Å). TEM samples were prepared by sonicating particles in solution 10 min prior to transfer to a carbon coated copper mesh grid. The optical properties of the ZnSe nanoparticles had been determined by UV-Vis adsorption using a Perkin-Elmer Lambda 950 spectrometer. 3. Results and discussion The amphiphilic copolymers PEA were dissolved in a nonselective solvent (DMF). During the dissolution of the block copolymers, the insoluble blocks form the core while the soluble blocks form the corona of the micelles. Then, deionized water as a nonsolvent for polyacrylonitrile blocks but as a solvent for poly (ethylene glycol) blocks was added slowly at a rate of one drop per 10 seconds to the stock solution under vigorous

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Fig. 1. Schematic representation of the formation mechanism of ZnSe composites.

stirring to induce morphological evolution, and the stirring was continued for 12 h to yield homogeneous dilute solution. After that, the zinc acetate solution was added into the copolymer solution and the stoichiometrical ratio of HSe- was injected into the reaction flask via syringe quickly under vigorously stirring. The products were washed several times with deioned water using centrifugation to remove the chemicals involved in nanoparticle synthesis, and the detailed fabrication procedure of the ZnSe/ PEA composites is outlined below (shown in Fig. 1). We also observed that the micelles were very stable; this is presumably due to kinetic stabilization of the insoluble PEO core at high DMF content and the interactions between the formed nanoparticles. The ZnSe nanoparticles are not formed inside the main solution because that the block copolymers are excessive compared with Zn2+ ions. The phase structure of the samples was investigated by XRD analysis, as shown in Fig. 2. All of the diffraction peaks in the XRD patterns could be readily indexed to the cubic phase of ZnSe with a lattice parameter α = 5.66876 Å, which is close to the

reported data (Joint Committee for Powder Diffraction Studies (JCPDS) No. 37-1463 for ZnSe, cubic). The broad diffraction peaks indicate that the colloidal particles are formed by small nanoparticles which agree with the high-magnification TEM results in Fig. 4. On the basis of the full width at half-maximum (fwhm) of ZnSe (111) diffraction peaks, the crystallite size of the sample was determined to be approximately 6~ –7 nm using the Debye-Scherrer formula [20]. The optical properties of the semiconductor nanomaterials are dependent on the size and the shape of the particles. Fig. 3 shows the UV-Vis spectra of the ZnSe colloidal particles. The absorption peaks of the resulted colloidal particles are all centered around 440 nm (as indicated by the arrow) and excitonic absorption features are not very sharp but appear as broad humps. Some size distribution may be responsible for this. The absorption onset (λ) is obtained from the intersection of the tangent slopes as shown in Fig. 3. From the position of the absorption edge (λe), 470 nm (2.64 eV), we calculate an actual ZnSe particle diameter of ~ 5.0 nm. Absorption for the prepared sample is blue shifted with respect to the bulk ZnSe (~ 2.58 eV), as would be expected for the smaller particle size. TEM image of ZnSe particles immobilized on the copolymer micelle was shown in Fig. 4. Clearly, all of the ZnSe nanoparticles are immobilized on the surface of the micelles and all the micellar aggregates are uniform hollow sphere, with the dimension of ca. 60 nm. In DMF, the whole amphiphilic block copolymer molecules remain well extended, because both of the hydrophobic and hydrophilic blocks have good solubility in DMF. During the process of phase inversion in an aqueous medium, the PAN blocks associate hydrophobically and precipitate out from the aqueous medium, while the PEG blocks remain fully extended in the aqueous medium. The strong repulsion arising from the incomparability of the two blocks in the block copolymer molecule forces the macromolecules to undergo partial phase separation between the hydrophobic blocks and the hydrophilic blocks in an aqueous medium. Due to the good solubility of PEG blocks in an aqueous medium, a large amount of water was trapped within the hydrodynamic volume of the self-assembled PEG chains in the reverse micelles within the polymer matrix of increasing hydrophobicity. Thus, the hydrophobic PAN blocks formed the corona and the hydrophilic PEG blocks formed the core, forming the reverse micelle. In the process of ZnSe particles formation, the Zn2+immobilized on the PAN blocks by complex effect reacted with HSe-in the solution, resulted in the hollow structure observed in Fig. 4.

Fig. 2. XRD pattern of as-prepared ZnSe composites.

Fig. 3. UV-Vis spectra of prepared ZnSe composite micelle.

Fig. 4. TEM image of resulted ZnSe nanoparticles immobolized on reverse micelle.

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4. Conclusion In summary, the uniform and well-dispersed core-shell composite micelle consisting of cubic ZnSe crystals immobilized on PEA reverse micelle were prepared in N,N-dimethylformamide/aqueous solutions. The size of the composite micelle was about 60 nm. This approach represented a simple method in the synthesis of the inorganic-shell composite micelle basing on the copolymer template and a range of possibilities for further development due to the self-assembled structure afforded by amphiphilic block copolymer could be exploited as nanocontainers, as three-dimensional templates for hybrid metal/ polymer order nanomaterials. References [1] Savic R, Luo L, Eisenberg A, Maysinger D. Science 2003;300:615. [2] Kakizawa Y, Kataoka K. Adv Drug Deliv Rev 2002;54:203.

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