Assembling Synthesis of ZnSe Orthohexagonal Slices through Emulsion Liquid Membrane System of Gas-li

Chinese Chemical Letters Vol. 16, No.3, pp 375-378, 2005375http://www.imm.ac.cn/journal/ccl.htmlAssembling Synthesis of ZnSe Orthohexagonal Slices throughEmulsion Liquid Membrane System of Gas-liquid TransportLu LIU', Qing Sheng WU'*, Ya Ping DING, Hua Jie LIU''Department of Chemistry, Tongji University, Shanghai 200092^Department of Chemistry, Shanghai University, Shanghai 200436Abstract: Orthohexagonal slices assembled by ZnSe quantum dots were synthesized throughemulsion liquid membrane system. These orthohexagonal slices were 1.5-3.5 pum in side lengthand were self-assembled by ZnSe quantum dots of 2-3 nm. It was proposed the surfactantmolecules on ZnSe quantum dots played a key role in the self-assembly process.Keywords: ZnSe, emulsion liquid membrane, orthohexagonal slices, self-assembly.II-VI semiconductor quantum dots (QDs) are ideal materials to be applied in electronics,biology, and so on, because of their unique size-dependent electronic and opticalproperties'However, biomimetic method on synthesis of these QDs has not beenreported. Moreover, when we assemble these semiconductor QDs into 2D or 3D nano-assemblies with regular morphologies and novel superlattice structures, it will exhibitpotential applications in practical nano-devices based on semiconductor arrays and showimportant scientific values in structure and morphology'. However, up to date, thereports on synthesis of these regular assemblies are still few .In this paper, we report a biomimetic and assembling way to synthesize ZnSe QDs .and prepare 2D regular orthohexagonal ZnSe slices. In the first stage, ZnSe QDs havebeen synthesized through emulsion liquid membrane (ELM) system, which is differentfrom well-known microemulsion, by gas-liquid transport. And subsequently, ortho-hexagonal nano-slices are successfully assembled via self-organization of surfactantmolecules and self-assembly of ZnSe QDs on the basis of their interaction.Typically, the water-n-oil-in-water (w/o/w) emulsion liquid membrane wasprepared in the emulsification step by initially dissolving the surfactant (Span80, 8%)and carrier (N7301, trialiphaticamine R;N R=Cg-C10, 20%) in the kerosene, then adding0.1 mol/L ZnCl2 solution. The emulsification was carried out for 8-10 min whenagitating at 3000 rpm.Appropriate amounts of Se powder and K BH4 powder were put into a three-neckedflask, on which a separating funnel, an inlet device for N2 gas and an outlet device forH2Se gas were installed. Then 100 mL of emulsion liquid membrane was sealed in a*E-mail: qswu@mail.tongji.edu.cn中国煤化工MHCNMH G.376Lu LIU et al.conical flask to compose a set of reaction system. With the N2 protection, a dilutesolution of HCI (pH 4) was dropped into the flask at the rate of 2 mL/min undermagnetic agitation, and gentle stream N2 took off H2Se gas from the reaction system,which was absorbed by NaOH solution. After Se powder was dissolved, the reactionwas kept for 10 min and then ceased.The emulsion liquid membrane was demulsified by centrifuge. The obtainedprecipitate was washed several times with absolute ethanol until CS determinator (LecoCS-400) showed the surfactant remains were about 0.3-0.4%. The sample was kept inethanol.The mechanism of reaction of Se(I) and Zn(I) in ELM is demonstrated as follows.(1) Se powder is reduced by KBH4 powder in acid solution under the protection ofN2 atmosphere. H2Se gas is generated in this step.(2) As shown in Figure 1, organic base N7301 is chosen as carrier in ELM systemto form complex compound (R;N)2:H2Se with H2Se. That is to say that with the help ofcarrier N7301, Se(I) can be transported from external-aqucous phase to internal-aqueousphase.(3) ZnCl2 in intemal-aqueous phase of ELM system reacts with (RzN)2H2Se toobtain ZnSe sample.In this stage, HCl solution is used to promote the reaction between Se and KBH4.The faster the HCI solution is dropped into, the faster the output speed of H2Se is.However, if output speed of H2Se is too fast, ELM system can be destroyed. Theexperimental results revealed that the best acidity is pH 4 and addition rate of HCl is 2mL/min.An XRD (X-ray diffraction, Philips Pw 1700) pattern of the as-prepared sample isshown in Figure 2, all the peaks in the pattern can be identified to cubic stilleite structureof ZnSe (PDF 37-1463). The measured lattice parameter is a= 5.715 A, which isconsistent with reported value.Figure 1 Coupled processes of transporting (R;N)2: H2Se and forming ZnSe QDsexternal phase| membrane phase internal phaseR.NZnCijH2Se(R:N2rH,Se|)ZnSeFigure 2 XRD pattern of as- prepared sample02030 40507020(°)中国煤化工MHCNMH G.Assembling Synthesis of ZnSe Orthohexagonal Slices377Figure 3 TEM images of as- prepared productsb)100 nm2um[c)d) .(a) initial ZnSe QDs (b) several orthohexagonal ZnSe slices (C) one orthohexagonal ZnSe slice(d) ED pattern of ZnSe slices (e) high-magnification image of ZnSe slicesFigure 3a shows the TEM (tansmission electron microscope, Hitachi H-800)image of fresh products just separated from ELM, indicating the initial as-preparedsample only consists of well dispersed ZnSe QDs with diameters of 2-3 nm. Togenerate the 2D ZnSe structures, we disperse these QDs adsorbing 0.3-0. 4% surfactant inabsolute ethanol to form a ZnSe sol, then aging for 24 h. Surprisingly, Figure 3b and3c reveal that in the final products there are a lot of regular orthohexagonal slices with1.5-3.5 um in side length and their thickness are about 10 nm according to semi-transparent appearance and the transmittable result of routine electron-beam, and someunassembled ZnSe QDs still existed. The ED (electron diffraction) pattern (Figure3d) shows single crystalline diffraction dots. To learn more about the microstructure ofthese slices, magnification TEM image is shown in Figure 3e. The self assemblynature of these ZnSe nanoparticles is obviously observed from the slices assembledregularly by nanoparticles. However, these ZnSe nanoparticles with diameters of 15-30nm are larger than the initial ZnSe QDs, which implies the self-assembly is multi-stepprocess.It seems that the formation process of orthohexagonal ZnSe slices is complicated.Nevertheless, it is clear that at least we can divide this process into two main stages: theformation of ZnSe QDs and the self-assembly of ZnSe QDs to orthohexagonal slices.Previously, it has been demonstrated that ELM system is efficient in limiting the particlesize in nanometer scale' s, attributed to the unique biomimetic double-layer membranestructure and the interaction between original particles and surfactant molecules. So,we are reasonable to depict the first stage of forming ZnSe QDs as follows: (1) H2Se gascomplex with N7301 to form (R;N)2H2Se to be transported into interal-aqueous phase; .(2) Zn(I) reacts with Se(II) to form ZnSe nuclei; (3) limited by the ELM system, ZnSenuclei grow into ZnSe QDsIn the past decade, organic-inorganic interaction has been emphasized in thegeneration of novel ordered inorganic materialsu-is and biomineralization'tR ecentlv.中国煤化工YHCNMH G.378Lu LIU et al.single crystalline porous vaterite hexagonal prisms have been reported throughself-oriented attachment of nanoparticles in gelatin'Our contrast experiments alsoconfirmed that orthohexagonl slices cannot form without surfactant. Therefore,obviously, in addition to stabilizing ELM structure, organic additives may play a crucialrole in the self-assembly. Here, we speculate the proposed mechanism of self-assemblyof ZnSe QDs: (1) the remnant surfactant on ZnSe QDs dissolved gradually in ethanol,then ZnSe QDs grow into nanoparticles of 15-30 nm to reduce surface energy; (2)according to widely accepted micelle theory", surfactant molecules self-organize intohexagonal liquid crystalline template through interaction with ZnSe nanoparticles inethanol; (3) at the same time, ZnSe nanoparticles self assemble into orthohexagonalslices under direction of surfactant liquid crystalline template.In summary, ZnSe QDs and 2D orthohexagonal ZnSe slices have been obtainedthrough ELM system. The overall mechanism is preliminarily discussed, multi-stepself-assembly of initial ZnSe QDs may account for the formation of final regular slices.This method may be promising in preparation of novel 2D nano-structures and futurenano-device based on these structures.AcknowledgmentsThis work was supported by the NNSFC and Nano-foundation of Shanghai.References1. A. 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