bestbuyControlled Synthesis of Multi-armed CdS Nanorod Architectures Using Monosurfactant System
Young-wook Jun,Sang-Min Lee,Nam-Jung Kang,and Jinwoo Cheon*
Department of Chemistry and School of Molecular Science BK21,Korea Ad V anced Institute of Science and
胳臂
Technology(KAIST),Taejon305-701,Korea
Recei V ed March5,2001
Re V id Manuscript Recei V ed April16,2001 Recently,nanomaterials have drawn interests owing to their special characteristics which differ from bulk crystals.1-7The synthesis of colloidal inorganic nanocrystals especially with respect to the control of their shape,however,is under developed and still complicated.Since novel properties of nanomaterials depend on their size and shape,a new direction for synthetic methods and an understanding of the mechanisms by which the size and shape of the nanocrystals can be easily varied are key issues in nanochemistry.8-10怪胎英文
For the past few years,various methods have been developed for the synthesis of1-dimensional(1-D)nanomaterials including template-assisted,10vapor-liquid-solid(VLS)-assisted,11col-loidal micellar,
12and electrochemical process.13Until very recently,however,the synthesis of complex structures of rod-bad CdSe ,arrow,teardrop,and tetrapod)has been unprecedented.The new structures were formed during thermal decomposition of two precursors in a mixture of binary surfactants.14,15This synthetic scheme requires the u of rather complicated procedures including delicate control of surfactant ratios and inert reaction conditions due to the toxic and unstable nature of the precursors.Until now,it has been prerequisite to have at least two different surfactants for the formation of well-defined colloidal nanorods.冬季长裙搭配
In this paper,we demonstrate a new method for simple and systematic control over CdS nanorod architectures using a monosurfactant system under atmospheric benchtop condition. The shape of our obtained nanocrystals can be easily varied between the monorod,bipod,tripod,and tetrapod as well as pencil-type rod by simply changing either the growth temperature or the precursor concentration.Without further size-and shape-sorting,the obtained nanorods are moderately monodisperd with unique forms.
Thermal decomposition of an air-stable single-source molecular precursor,Cd(S2CNEt2)2,in hexadecylamine(HDA)leads to the formation of anisotropic nanocrystal morphologies.16,17At a given growth temperature,varying the precursor concentration resulted in systematic control of both t
he width and the length of the nanorods.Figure1shows transmission electron micrographs (TEM)of CdS nanorods grown at300°C.The CdS obtained from a precursor concentration of30mg18are rod structures of 6.3(0.6nm in width and26.4(5.1nm in length.As the precursor concentration is incread to50and300mg,an increa in both the width and length is obrved(10.5(1.2,25.0(7.3 nm in width and45.0( 6.2,101.4(20.7nm in length, respectively).HRTEM,X-ray diffraction(XRD),and lected area electron diffraction(SAED)analys indicate that the CdS nanorods have single crystallinity in the wurtzite pha and the measured interplanar distance(3.36Å)of the(002)direction parallel to the long axis of the rod is consistent with known values.14,15
In contrast,varying the growth temperature at a fixed precursor concentration of50mg leads to the formation of a tetrahedral geometry-bad architecture of CdS nanorods(Figure2).19While the exclusive formation of1-D nanorods is obrved at high temperature(∼300°C),mixtures of two(∼45%)or three(∼40%) armed rods(bipods or tripods)are obtained as the growth temperature is decread to∼180°C(Figures2B,3A,B).Finally,
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(16)Cd(S2CNEt2)2is synthesized according to known literature method. See:Coucouvanis,D.Prog.Inorg.Chem.1970,11,233.
(17)In a typical synthesis of the CdS nanorods,a warm solution of Cd-(S2CNEt2),50mg dissolved in∼0.3g of HDA at∼70°C)is injected into a hot solution of∼0.5g of HDA.After mixing,the colorless solution rapidly changes to orange-yellow.After1h,the solution was cooled to70°C and treated with ethanol to generate orange-yellow flocculates,which were parated by centrifugation.The resulting nanocrystals were obtained as orange-yellow powders and readily redisperd in dichloromethane.Modifica-tion of this procedure in terms of the temperature or precursor concentration results in size and shape changes.The substitution of HDA with other solvents such as octylamine did not generate any noticeable changes in the resulting nanorods.
(18)The total amount of solvent(HDA)is fixed at0.8g throughout all the experiments and we only specify the precursor amount afterwards.
(19)The synthesis is carried out exactly the same way as that described in ref17except the temperature is
varied.my vacation英语作文
Figure1.TEM images of different sized CdS nanorods.(A)6.3,(B) 10.5,and(C)25.0nm in width with aspect ratio of∼
vortex
4.
Figure2.Variation of the shapes of CdS nanocrystals by changing of growth temperature.(A)300,(B)180,and(C)120°C.
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as the temperature is decread to 120°C,the formation of four armed rods (tetrapods)dominates (∼82%)(Figures 2C,3C).1-D rods and tetrapods of CdSe have previously been reported,but this is the first report of bipods and tripods.The formation of tetrapods is almost exclusive in our ca,whereas previous attempts resulted in the formation of a mixture of structures with tetrapods comprising around 15-40%.14
The new structures provide insight into the formation of the multiarmed rods.Our HRTEM studies of bipods indicate that the single crystals of the multiarmed rods consist of a zinc blende core and epitaxially grown wurtzite arms.The lattice fringes of two {111}faces clearly show that the core is of zi
nc blende structure with tetrahedral geometry with a paration angle of ∼109.5°between the arms of the bipods (Figure 4A).14
The studies suggest that the structures of the CdS nanorods can be controlled by simply varying either the growth temperature or the precursor concentration.Under high-temperature conditions (∼300°C),1-D nanorod formation with purely wurtzite-phad CdS is obtained from wurtzite-phad nucleation eds.In general,the (001h )face of the wurtzite crystalline form is more reactive than other faces,and under the high growth rate regime,the formation of rods is favored over that of spherical-shaped nanocrystals.14,15Higher precursor concentrations also generate nucleation eds with a larger average size,and subquent growth steps result in nanorods with larger size and a relatively constant aspect ratio.Our result is consistent with the “3-dimensional diffusion controlled rod growth”en previously.15As the growth temperature decreas,two different phas of nucleation eds coexist in certain temperature regimes (∼180°C),and at lower temperature range (∼120°C)zinc blende-phad eds dominate (Figures 4,5).20
Different growth rates between the crystallographic surfaces results in either bi-,tri-,or tetrapods from zinc blende-phad eds.Tetrapods are formed at the mildest growth conditions of ∼120°C,where the formation of the four arms proceeds evenly on the four different {111}surfaces of CdS zinc blende
core to
form (001h )faces of the wurtzite-phad arms.Bi-or tripods are obtained when the growth becomes relatively fast at higher temperature (∼180°C).It is possible that once the formation of two or three reactive (001h )wurtzite faces on the {111}faces of the zinc blende core occurs,the growth rate on the (001h )face is fast compared to that of the remaining {111}surface(s)of the zinc blende core becau the resulting (001h )faces have more surface area and defects during crystal growth.Finally,under fast growth conditions (i.e.,∼300°C),wurtzite eds are favored,and only 1-D nanorods are formed.
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The homogeneous growth state which results in tetrapod formation can also be interrupted by increasing the precursor concentration.At 120°C,as we increa the concentration from 10to 50mg,an overall increa in tetrapod size is also obrved as similarly en during 1-D growth above.At higher precursor concentrations (∼100mg),however,pencil-shaped nanorods appear as well as the tetrapods (Figure 5B).Finally,at very high concentrations (∼300mg),pencil-shaped nanorods are obrved as a major product (Figures 5C,4B).At the very high precursor concentrations,the equi-directional growth on the four different {111}surfaces is completely interrupted,and only 1-D growth results with a high growth rate along the (001h )face which induces shrinking of the (001h )face and pencil-shaped nanocrystal forma-tion.
Under a particular growth condition,the crystalline pha during nucleation and also the growth rate difference between the surfaces of the crystal determine the overall structure of the nanorods.The hexadecylamine (HDA)ems to increa both the growth rate in the (001h )direction and the overall growth rate as HPA (HPA )hexylphosphonic acid)similarly reported previously during CdSe rod formation.14,15While HPA requires the prence of another stabilizing ligand such as TOPO,our HDA acts not only as a shape-controlling ligand but also as a stabilizing ligand.We are currently investigating the full mechanistic studies for the shape-controlled formation of CdS nanorods.
In conclusion,a novel route for the control of the various sizes and shapes of the nanocrystals has been developed using thermal decomposition of a single-source precursor in a monosurfactant system.Temperature and precursor concentration provide us with a mechanism by which it is possible to control the various architectures.The growth conditions are simple and also can easily be done under atmospheric benchtop condition for large-quantity preparations.It is highly possible that this approach can be extended as a general synthetic method for other metallic and miconducting nanocrystals if their nanocrystal structures show the existence of hexagonal and cubic phas during nucleation and subquent rapid growth of the hexagonal pha.
Acknowledgment.This work was supported by the Tera Level Nanodevices National Program of KIS
TEP and we thank KBSI for the TEM analys.
JA0157595
(20)For CdS,zinc blende is the preferred structure at low temperature (<∼250°C),whereas wurtzite pha is dominant at high temperature (g 300°C).See:Zelaya-Angel,O.;Alvaradi-Gol,J.J.;Lozada-Morales,R.;Vargas,H.;Ferreira da Silva,A.Appl.Phys.Lett.1994,64,
291.
Figure 3.HRTEM image of various shapes of CdS情人节快乐用英语怎么说
nanocrystals.
北京百特英语Figure 4.Structural analysis of various shaped CdS nanocrystals (A)bipod-(B)pencil-shape.It is clearly en that wurtzite arms with (001h )direction grown out of {111}faces of zinc blend core or
end.
Figure 5.Variation of the shapes of CdS nanocrystals by changing of precursor concentration at 120°C.(A)50,(B)100,(C)300mg in 0.8g of HDA.The hexagons and circles are in fact rods parallel to the electron beam under TEM.
Communications to the Editor J.Am.Chem.Soc.,Vol.123,No.21,20015151
