Crystallization

DOI: 10.1002/smll.200700297

CdSe Nanorod Synthesis: A New Approach**

Sameer Sapra,* Jan Poppe, and Alexander Eychm�ller

The shape control of colloidal nanocrystals has been pur-sued actively for the past few years after it was first demon-strated that CdSe nanocrystals could be crystallized as elon-gated rods with the possibility of formation of other linearand branched structures, such as arrows, nanorice, and tetra-pods.[1, 2] Recently, some approaches have focused on themechanism of anisotropic growth and precise shape controlof these systems.[3–5] However, all the present approachesare based upon the kinetics of the reaction favoring nano-rod formation as opposed to spherical nanocrystal forma-tion. Until now strong binding ligands and high concentra-tions have been deemed necessary in order to produce non-spherical particles.[1–5] In this Communication we present amethod that favors growth of nanorods under nonextremeconditions. Our method uses neither strong ligands, such ashexyl-, tetradecyl-, or octadecyl- phosphonic acids (HPA,TDPA, or ODPA) nor high concentrations of the precur-sors; both of which have been assumed necessary conditionsfor nanorod formation.[3] Other ligands such as hexadecyla-mine (HDA) have also been shown to induce nanorod for-mation, though these rods generally have diameters greaterthan �6 nm for CdS and therefore lie outside the quantum-confinement regime.[6]

CdSe crystallizes in the wurtzite phase and it has beenestablished that nanocrystals grow preferentially along the<002> direction or the c-axis.[3] However, most of the usedsynthesis conditions yield spherical nanocrystals rather thanany other shapes. Most colloidal synthesis methods, basedon organometallic precursors, or the “greener methods” usebulky stabilizers such as trioctylphosphine (TOP), its oxideTOPO, HDA, or oleic acid, to name but a few.[7–9] The firstnanorod synthesis was achieved by adding HPA to the reac-tion mixture.[1] Phosphonic acids, like HPA, bind strongly toCd, thereby reducing the growth rate of the nanocrystalsand allowing more monomers to be present in the reactionmixture. This slow growth allows the crystallization of thenanocrystals along a preferred growth direction, which hap-pens to be the c-axis in the case of wurtzite crystals. In laterreports,[3] Peng and Peng have observed that the use ofHPA, a 6-carbon phosphonic acid, readily yields nanorodswhereas TDPA, a phosphonic acid with 14 carbon atoms,produces spherical nanocrystals. Only after multiple injec-

tions of the reactants was it possible to obtain nanorods.The smaller Cd-HPA complexes diffuse faster through thegrowth solution compared to the larger Cd-TDPA com-plexes and this results in rod formation in the former case.[3]

The argument presented above can be extended to li-gands other than phosphonic acids. It may be possible touse other shorter ligands to induce anisotropic growth. Onepossibility is to reduce the size of the carboxylic acid usedto bind Cd. Another parameter that can be tuned is the sizeof the Se ligand. Here we have only two options, namelyTOP and tributylphosphine (TBP). One should bear inmind that the Cd stabilizers can be approximated to belinear whereas the Se stabilizers have three carbon chainsattached to the P atom thereby rendering them conical andin such a sense are more bulky compared to the longerlinear chains.

The present synthesis scheme is a slight modification ofthe earlier reported methods for CdSe synthesis[10] in thenoncoordinating solvent 1-octadecene (ODE) and is de-tailed in the Experimental Section. To test the proposed hy-pothesis we chose 2-octenoic acid (OA) as the short-chaincapping agent for Cd since long-chain carboxylic acidsalways yield spherical particles. Further, TOP and TBPwere used to bind Se. Figure 1 shows the transmission elec-tron microscopy (TEM) images for the synthesis carried outwith Cd-OA/TOPSe (Figure 1a) and Cd-OA/TBPSe (Fig-ure 1b) at the same temperature (270 8C). TOP has three 8-carbon chains projecting from the phosphorus atom, whichshould considerably slow down the diffusion process. As a

Figure 1. Nanocrystals synthesized with different ligands at 270 8C:a) TOP and OA, b) TBP and OA.

[*] Dr. S. Sapra, J. Poppe, Prof. A. Eychm�llerPhysikalische Chemie, TU DresdenBergstr. 66b, 01062 Dresden (Germany)Fax: (+49)351-463-37164E-mail: sameer.sapra@chemie.tu-dresden.de

[**] We are thankful to Dr. Stephen Hickey and Dr. Nikolai Gaponikfor useful discussions.

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result, we observe spherical particles as seen in Figure 1a.Such a situation was overcome by using TBP, which hasthree chains but with four carbon atoms each. This shouldincrease the diffusion coefficient and we observe the forma-tion of nanorods. The shorter chain lengths, both for cadmi-um and selenium, allow natural growth along the c-axis tooccur.

With the same experimental conditions as describedabove, we also used 2-decenoic acid (DA), which is twocarbon atoms longer, and 2-hexenoic acid (HA), which istwo carbon atoms shorter than OA. The synthesis with DAresults in shorter rods whereas that with HA gives longerrods under the same synthesis conditions. Thus, even car-boxylic acids, which generally do not form strong bonds, canlead to nanorod formation. An important point to note isthe concentration of the reactants. In the present synthesisscheme we use typically 20 times less concentrated solutionscompared to the previously reported syntheses with phos-phonic acids.[1–5] Thus, apart from diffusion of the reactants,there could be other factors controlling the growth of nano-rods. At this stage we must point out that the size of the li-gands and the binding strength do not always dictate theshape of the particles, although it seems evident from thepresent study and the earlier work with phosphonic acids.[3]

An example that proves the contrary is that of the water-based CdSe nanocrystals stabilized with thioglycolic acid(TGA).[11] TGA is a 3-carbon-atom chain, much smallerthan any of the ligands that have been used for nanorod for-mation and with very strong binding to Cd due to the pres-ence of the thiol group. Still it gives spherical particles.

The present observations might be explained by the fol-lowing two mechanisms. The first one is based on the differ-ences in binding energy of the ligands to differentfacets.[12–14] The facets with strong ligand binding do notgrow as fast as those with the weaker ligand-binding energy,thereby resulting in nanorods. Here again a diffusion-con-trolled mechanism is at play that allows for longer nanorodswith shorter ligands. However, there is no need for highconcentrations in such a model.

The second possibility is that the present synthesis meth-ods give less-faceted nanocrystals. In other words the parti-cles are more spherical and cylindrical. In this case it is pos-sible that steric hindrance between the ligands plays an im-portant part. The longer and more bulky ligands wouldprefer to stay as far apart as possible making the curvatureof the particle high, yielding spheres. With smaller chainsthe favored crystallization along the c-axis would win fromthe smaller steric strains resulting in rods. Interligand inter-actions could also possibly explain the formation of spheri-cal CdSe-TGA particles. TGA leaves a free acid group atthe surface so these particles are negatively charged and theligands repel each other to a much larger extent comparedto uncharged ligands. The repelling behavior would cause aseparation of the ligands as far apart as possible and wouldresult in spheres. However, we do not yet have any evidenceto support such a claim.

One could now ask the question whether the differencesin binding to the facets and the steric effects due to theshort chains are strong or whether the crystallization energy

along the c-axis is more favorable, for a given set of condi-tions. In other words, is there a possibility to control thelength of the rods by tuning the reaction conditions? Itwould also be interesting to study the conditions that favoranisotropic growth.

In order to answer these questions we studied the effectof temperature on nanorod formation. Figure 2 shows the

TEM images for particles synthesized at 220, 270, and300 8C using OA and TBP, as described in the ExperimentalSection. A clear trend is observed from this figure and wesee that the particles become more spherical as the synthesistemperature is increased. The average lengths decreasefrom 17.9�7.0 nm to 11.5�3.8 nm and 4.4�0.6 nm (spheri-cal particles). Also, there is an increase in the average diam-

Figure 2. Nanocrystals synthesized with the ligands TBP and OA andat different temperatures: a) 220 8C, b) 270 8C, and c) 300 8C.

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eter of the particles from 1.9�0.5 nm to 2.1�0.6 nm and4.4 nm (spherical particles) with temperature. A higher tem-perature allows the particles to grow faster. The particles atelevated temperatures have enough energy such that theydo not discriminate between the thermodynamics of thecrystallization along the various axes. The diffusion of reac-tants has little importance at higher temperatures. Also, theligand binding to the different facets should not be discrimi-natory at higher temperatures; sterics always favor sphericalparticles. The length of the nanorods can thus be controlledby merely controlling the reaction temperature. To the bestof our knowledge this has not been reported so far.

In conclusion, we have demonstrated the formation ofnanorods without the need for strong ligands such as a phos-phonic acid. It is worth mentioning that octenoic acid is acheaply available chemical compared to the commonly usedphosphonic acids. Also, high concentrations, which werethus far believed to be essential, are not required for aniso-tropic growth with the present synthesis scheme. Furtherstudies are underway to control the size and aspect ratio ofthe rods and to study the kinetics of these reactions. At-tempts to induce the formation of other shapes such as tet-rapods are also being pursued. Theoretical calculations areunderway to gain an insight into the mechanism of rod for-mation.

Experimental Section

All reactions were carried out under argon atmosphere. TOPand TBP were handled in a glove box. CdO (0.2 mmol) and OA(1.6 mmol) were added to a 3-neck round-bottom flask with8 mL ODE and heated to 200 8C to dissolve. The typical concen-trations used for a phosphonic acid synthesis are 20 timeshigher than those reported here. The temperature was then setas desired for the synthesis. A 2 mL (0.08m) TBPSe (TBP/ODE=

1:1) solution was injected into the hot cadmium solution andthe heating was continued for 5 min. The flask was immediatelycooled down by immersion in a water bath and 5 mL hexanewas added to the reaction mixture at �70 8C, followed by an

excess of methanol. Nanocrystals were recovered from thehexane layer by precipitating with acetone. These were redis-solved in toluene and 10 mL of the diluted solution was put on acarbon-coated copper grid. TEM images were recorded on a Phi-lips EM208 microscope.

Keywords:anisotropy · CdSe · nanocrystals · nanoparticles ·nanorods

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Received: April 27, 2007Revised: August 10, 2007Published online on October 12, 2007

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