PROGRESS ARTICLE

MARIE-PAULE PILENILaboratoire des Materiaux Mésoscopiques et Nanomètriques,LM2N,(UMR 7070),Université Pierre et Marie Curie,BP 52,4 Place Jussieu,75005 Paris,Francee-mail: pileni@sri.jussieu.fr

In the past few years, research on inorganic nanocrystalshas developed tremendously because of their potentialapplications1 in fields such as biology,electronics,transport and information technology.Because theelectrical,optical and magnetic properties of inorganicnanomaterials vary widely with factors such asnanocrystal size, shape and crystallinity,methods formanipulating these factors must be found.Althoughseveral approaches for producing these nanomaterialshave been investigated in the past decade2–15, a generalmethod for fabricating large amounts of well-definednanocrystals has not yet been found.

In this paper,we discuss the efficacy of surfactant-based templates in controlling the size, shape andcrystallinity of inorganic nanocrystals,and the variousrelevant parameters affecting the size and shape.Let usfirst describe the way surfactants self-assemble,and theproperties of these colloidal solutions.

THE STRUCTURE OF COLLOIDAL SELF-ASSEMBLIES MADEOF SURFACTANTS AND USED AS TEMPLATES

Surfactants are molecules with a polar hydrophilic head(attracted to water) and a hydrophobic hydrocarbonchain (attracted to oil). If a surfactant is added to water,the hydrocarbon chains tend to self-associate tomimimize contact with the water molecules, resulting inthe formation of various aggregates16. If the water is ableto solubilize the polar head and the alkyl chains equally,no aggregates are formed. The most commonly usedsurfactants are the single-chained,anionic sodium

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The role of soft colloidal templates in controllingthe size and shape of inorganic nanocrystalsIn the past decade, colloidal solutions have been assumed to be very efficient templates for controlling particle

size and shape. A large number of groups have used reverse micelles to control the size of spherical

nanoparticles. This makes it possible to determine the various parameters involved in such processes, and

demonstrates that nanoparticles can be considered to be efficient nanoreactors. However, some

discrepancies arise. There are few reports concerning the control of particle shape, and it is still rather difficult

to determine the key parameters, such as the adsorption of salts and other molecules, and the synthesis

procedure. Here, we discuss these controls of the size and shape of inorganic nanomaterials.

a b

c d

e f g

Water

Figure 1 Surfactant shapes and various self-assemblies in colloidal solution.a,Cone-shaped surfactantresulting in b,normal micelles.c,Champagne cork shaped surfactant resulting in d, reverse micelles withcontrol of their size by the water content.e, Interconnected cylinders. f,Planar lamellar phase.g,Onion-likelamellar phase.

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dodecylsulphate (SDS) and the cationiccetyltrimethylammonium chloride (CTAC) or bromide(CTAB),and the double-chained,anionic sodiumbis(2-ethylhexyl) sulphosuccinate (Na(AOT)).Theshape of the surfactant plays an important role informing the assembly.

If the surfactant molecules have a very large polarhead and a small chain forming a cone shape(Fig.1a),the chains tend to self-associate to form a sphericalaggregate called a normal micelle, in which the chainsform the inner core and the headgroups the outersurface (Fig.1b).At low surfactant concentration thenormal micelle is spherical,with a diameter fixed by thelength of the hydrocarbon chain and the size of thepolar head.The system is dynamic, in that thesurfactants involved in formation of micelles leave theaggregate and are replaced by others that freely move inthe aqueous bulk phase.However the spherical micellarstructure is always retained.After a few microseconds,all the surfactant molecules constituting a normalmicelle are replaced by others.

If the surfactant has the shape of a champagne cork(small polar head and branched hydrocarbon chains,

Fig.1c) spherical reverse micelles are formed17, in whichthe inner core is formed by the head groups and thechains form the outer surface (Fig. 1d).This is termed awater-in-oil droplet. If two reverse micelles meet theyexchange their water content and reform into twodistinct micelles. In contrast to normal micelles, the sizeof reverse micelles increases linearly with the amount ofwater added to the system,and can increase, forexample, from 4 nm to 18 nm diameter with 0.1 MNa(AOT) surfactant18.

In surfactant systems containing large amounts ofboth oil and water, the shape and dimension of theaggregates changes,with the formation ofinterconnected water channels,or cylinders (Fig. 1e).In these systems, the space is divided into two separatevolumes of oil and water by the surfactant layer,whichforms a continuous surface having a saddle shape.The structures are therefore doubly continuous — alsoknown as bicontinuous or co-continuous — in bothwater and oil19.Adding more water induces a new phasetransition,with the surfactant molecules reorganizingas a planar or lamellar film (Fig. 1f),and the systembecomes opalescent and birefringent (the light is not

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A

a b w = 1 w = 3 w = 5 w = 20

14 nm 21 nm 21 nm 21 nm

B

a b c

21 nm 5 nm

Additional intermediate planes

C

a b c d

Fivefold centre

(111)

[01-1]

[1-10]

[-101]

5 nm 10 nm5 nm

Figure 2The different shapes ofcopper nanocrystals produced incolloidal self-assemblies ofsurfactant-H2O-isoctanesolution.The surfactant,S,usedis either Na(AOT) or Cu(AOT)2.The water content w = [H20]/[S]is related to the droplet radius byR (nm) = 0.15 × w.A [AOT] = 0.1 M; [Cu(AOT)2] =10–2 M; [N2H4] = 2 × 10–2 M.a,Reverse micelles.b,Control ofnanocrystal size with w, that is,control of the size of water-in-oildroplets.B [Cu(AOT)2] = 5 × 10-2 M,[N2H4] = 1.5 × 10-1 M,w = 10.a, Interconnectedcylinders.b, formation ofspherical and cylindricalnanocrystals.c,cylindricalparticle composed of a set ofdeformed f.c.c. tetrahedrabounded by (111) faces parallelto the fivefold axis with additionalplanes.C, [Cu(AOT)2] = 5 × 10-2 M, [N2H4] = 1.5 × 10-1 M,w = 20.a,Supra-aggregates.b,Various nanocrystals.c,Particle composed of fivedeformed f.c.c. tetrahedralsbounded by (111) planes.d,Large, flat nanocrystals [111]-oriented and limited by (111)faces at the top,bottom and edges.

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extinguished between two crossed polarizers).Recently,it was demonstrated that a stable emulsion could bemade by the formation of a ‘supra-aggregate’, from asystem containing two functionalized surfactantmolecules that were capable of being strongly bound20.The surfactants self-organize into a lamella-like phase,which is no longer planar but has an onion (orspherulite) shape (Fig.1g). In the external and internalphases of the spherulites, the surfactants self-organizeinto interconnected cylinders as shown in Fig.2c—such an assembly is called a supra-aggregate.Formation of the different structures shown in Fig.1,is explained by geometrical factors related to the shapeof the surfactant16,19.

The colloidal self-assemblies described above arevalid for dilute solutions.These structures change withincreasing surfactant concentration and are rathercomplicated.Not all the structures are described here,because very few are used to produce the nanomaterialsthat are the subject of this paper.

CONTROL OF NANOCRYSTAL SIZE

Some years ago21,we discovered that reverse micelles aregood candidates for templates.The two propertiesdescribed above — the droplet size and exchange ofaqueous contents — enable their use as variably sizednanoreactors.Let us consider A and B solubilized in twomicellar solutions.On mixing them,and because of theexchange process,A and B are in contact and react. It isthus possible to fabricate a very wide range of sphericalnanomaterials5 such as semiconductors,metals,oxidesand alloys (some alloys cannot be produced at thenanoscale even though they are formed in the bulkphase22).The control of the template size by changingthe water content enables control of the sphericalnanocrystal size (Fig. 2A)5. It is of interest to note thatthis type of nanoreactor makes it possible to producemetal nanocrystals without any detectable oxide.Several groups have claimed that these nanoparticles areamorphous23 but it has been clearly demonstrated thatthis is incorrect24.Figure 3 shows silver nanocrystalshaving several crystalline structures that were producedsimultaneously.The key parameter in obtaining highlycrystalline nanomaterials is the use of a functionalizedsurfactant as one of the reactants.Otherwise,when thetwo highly hydrated reactants are solubilized in the twodroplets,amorphous nanomaterials are formed andmetals are produced in their oxide form.

There is a rather large consensus that reversemicelles are good nanoreactors for obtaining sphericalnanomaterials.The size of the nanomaterial producedis not the same as that of the droplet used as a template6.In fact, for II-VI semiconductors25 (CdS,CdTe,CdMnS), the particle size varies from 2 to 4 nm,whereas for metals6 it increases from 2 to 6 nm for silverand 2 to 10 nm for copperand silver sulphide.Thiscontrol of the particle size is obtained only for therelatively small water-in-oil droplets (varying from 0.6 to 6 nm).For larger template sizes (from 6 to12 nm),no changes in the particle size are observed.This has been explained in terms of water structure inthe droplet6.An exception is found for silver sulphidenanocrystals,which show a linear increase in theparticle size with that of the template.

Some discrepancies are observed: in one study26

two reverse micelles made of CTAB–butanol–octanewere used,one containing MnCl2 in the water dropletsand the other KF.By mixing the two micellar solutions,cubic KMnF3 nanocrystals were produced.Similarly,on bubbling H2S into a system of spherical reversemicelles made of functionalized surfactant —containing Cd2+ associated with the negative polar headgroup — flat, equilateral, triangular CdS nanocrystalswere formed27.However, spheres were produced whenthe water added to fix the droplet size contained Na2S.With a functionalized surfactant having barium as thecounter ions28, reverse micelles are formed.Byincreasing the relative ratio of barium to chromate ionsfrom 1 to 5, first spherical, then elongated and finallyrod-shaped nanocrystals were obtained.Severalgroups29–31 produced various shapes of either BaCrO4

or CaCO3 nanomaterials from spherical reversemicelles made of surfactant functionalized with eitherBa2+ and Ca2+.This could be due to specific adsorptionof impurities and/or residues from the chemicalreaction taking place in reverse micelles.Thepreparation of highly pure functionalized surfactants israther difficult, and the presence of a residual salt couldhave two results: one is a marked change in the phasebehavour32, and the other is incorporation of thisresidue as an impurity in crystal growth.

From this, it is reasonable to conclude that reversemicelles can be used as nanoreactors to producenanoparticles. In most cases,a spherical templateproduces nanospheres.The particle sizes can becontrolled by increasing the volume of the water pool.However,production of various species during thechemical reaction and/or presence of impurities play animportant role in the nanocrystal growth, inducingformation of particles having various shapes.

FACTORS AFFECTING THE CONTROL OF NANOCRYSTAL SHAPE

The control of the shape of the nanocrystals is a realchallenge,and more data are needed to ascertain thegeneral principles that determine it. It is probable thatanisotropic materials such as these are not in theirthermodynamically stable state.

INFLUENCE OF THE COLLOIDAL TEMPLATE IN THE CONTROL OF NANOCRYSTAL SHAPE

Because the results described above with reversemicelles are quite convincing,and because the basicthree-component surfactant–oil–water system

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2 nm2 nm2 nm

a b c

Figure 3 Silver nanocrystalsmade in reverse micellarsolution. Mixed reverse micellesmade of 3 × 10–2 M Ag(AOT) and7 × 10–2 M Na(AOT) were formedin isooctane solution.Hydrazinewas added to the solution toreach a water (polar) contentw = 2 and an overall hydrazineconcentration 7 ×10–2 M.Severalstructures were produced:a,Decahedron.b, Icosahedron.c,Cubo-octahedron.

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produces self-assemblies of surfactants of variousshapes (Fig. 1),a number of groups have tried todemonstrate that they can be used as templates.The firstresults that have been obtained are rather persuasive.Our work demonstrates that the shapes of colloidalsolutions made of functionalized surfactants partiallycontrol those of the nanocrystals7.Figure 2b showscylindrical and spherical copper nanocrystals that havebeen formed using interconnected cylinder templates inthe Cu(AOT)2-isooctane-water system. As shown inFig.2, the nanoparticles produced are characterized byhigh crystallinity.For example, the cylindricalnanocrystals shown in Fig.2B,b are characterized by afourfold symmetry33. In the region of the phase diagrammade up of an onion phase containing both internaland external interconnected cylinders,a large variety ofshapes is observed (Fig. 2c).This control of thenanocrystal shape by that of the template has beenrecently confirmed34. It is well known that addition ofphosphatidylcholine to Na(AOT) reverse micellesinduces a structural change of the self assembly,with achange of the spontaneous curvature and formation ofworm-like,cylindrical, reverse micelles that entangle toform gel-like systems35.Syntheses of CdS nanoparticlesin such colloidal assemblies make it possible to vary themorphology of the nanocrystals from spheres tonanorods with a switch in the crystal structure fromcubic to hexagonal.However the role of the template isnot as obvious as described above.Adsorptions of ions,salts and molecules have to be taken into account.

INFLUENCE OF THE ANIONS IN NANOCRYSTAL GROWTH

Consider the Cu(AOT)2-isooctane-water systemdescribed above.The shape of the copper metalnanocrystals produced from this system has been foundto change drastically with the type of salt added to thetemplate36 (Fig. 4).Addition of less than 2 × 10–3 Mchloride ions (in the form of NaCl) to the solutioncontaining small interconnected cylinders (Fig. 2b)results in the production of long copper nanorods(Fig. 4)37 with an aspect ratio (defined as the ratio of thelong to short axis of nanocrystals) controlled by theamount of chloride ions in the microphase.Thecrystalline structures of these nanorods are similar tothat of small cylinders.On replacing Cl– by Br–,nanocubes are formed (A.Filankembo,S.Giorgio andM.P.Pileni,manuscript in preparation).However,conversely to what is observed with Cl–, the aspect ratiodoes not change with Br– concentration.Various sizesand shapes are obtained with NO3

– and small cubes areproduced with F–.The reader could argue that these

shape changes on salt addition are due to changes in theaggregate structure.However, this cannot be the casebecause the template keeps the same structure in thepresence of 2 × 10–3 M of various salts36.

The control of the nanocrystal morphology can beexplained in terms of selective ion adsorption on facetsduring the crystal growth.This explains the preferentialformation of cubic KMnF3 nanocrystals from a mixtureof two types of reverse micelles having different salts inthe droplets26.Cubic cadmium sulphide nanocrystals38

have been obtained by using self-assembled monolayersas a template.The template is first immersed in asolution containing cadmium ions,which bind to thesurfactant by electrostatic interaction,and then in asolution containing CTAB and Na2S.Cubicnanocrystals (Cu,KMnF3,CdS) have therefore beenformed in the presence of bromide ions in differentenvironments — when starting from just the self-assembled monolayer,and from the full colloidal mix.

It cannot be claimed,however,as a generalprinciple for growing cubes, that bromide ions have tobe present during the process. In fact,bubblinghydrogenthrough an aqueous solution containingPtCl4

2– produces well-defined and faceted cubicplatinum nanocrystals39.Coalescence is prevented byselective adsorption of H+ or Cl– on the facets.The sizeof these cubic nanocrystals is controlled by addition ofacetate ions and/or polymers40.Similarly,goldnanorods41 are produced by ultraviolet–visibleirradiation of a gold salt (HAuCl4) solubilized in thebulk aqueous phase of normal micelles made of CTAC.The authors claim that normal micelles play the role ofthe template. It is difficult to understand this role: goldions do not interact with the micellar solution and arereduced photochemically.However, the chloride ionsfrom CTAC could play the role of an impurity in thenanocrystal growth.From this, it can be concluded thatanisotropic nanocrystal formation is more related toselective adsorption of ions during the crystal growththan to the nature of the templates.

INFLUENCE OF MOLECULE ADSORPTION ON NANOCRYSTAL GROWTH

Other examples clearly indicate that selectiveadsorption of molecules during the crystal growth isalso a key parameter in controlling the nanocrystalshape.As already discussed,spherical micelles producenanospheres24.Addition of a very large amount ofhydrazine to a reverse micellar solution induces a changein the template structure.In such conditions nanodisksare produced42 (Fig.5).The nanodisk size is controlled bythe amount of hydrazine added during the synthesis43.

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Br– Cl– F– NO3–

42 nm 110 nm 67 nm 61 nm

Figure 4Various shapes ofcopper nanocrystals producedin interconnected cylinders inthe presence of different saltions having the sameconcentration.5 × 10–2 MCu(AOT)2 was solubilized inisooctane in presence of varioussalts.Hydrazine was added toreach w = 10 and a 10–1 Moverall concentration [N2H4],[NaCl] = [NaBr] = [NaF] =[NaNO3] = 10–3 M.

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The optical properites of the solution containingnanodisks are size dependent.The solution colourevolves from red to grey with increasing particle size(Fig. 5b).Because the amount of surfactant remainsunchanged,to explain the disk formation,we have totake into account the adsorption of hydrazine,hydrogenor hydroxyl ions on the faces.

Another convincing experiment on the influence ofmolecules in the control of nanocrystal shape is theproduction of ZnTe and CdTe nanowires44,45 by thesolvothermal process.Metals are solubilized in hydratedhydrazine,which is not only an electron transfermedium but also a strong electron donor.These authorsclaim that hydrazine plays the role of a template in thismedium.Associations of molecules used as additivesand salts can also selectively adsorb on the facets ofnanocrystals to control the particle shape.Polymer andsalts are needed to produce PbS nanorods46 fromfunctionalized surfactants solubilized in chloroform,whereas spheres are produced in their absence. In theabsence of polymers and salts, the surfactants do notself-assemble in chloroform (the polar head group andalkyl chains are both soluble in this solvent and act asreactants) indicating that the system cannot beconsidered as a template.Similarly,control of particleshape has been obtained by using inorganic synthesesinvolving a mixture of two surfactants47–49.This hasbeen well demonstrated with various materials such asCdSe,Co and Fe nanorods, in which the ratio ofsurfactants controls their aspect ratio.However, suchclaims are specific to the chemical method used.In fact,the presence of a single surfactant is enough to controlthe dimensions of silver and gold nanorods50,51. In thelatter systems, the ratio of reactant concentrations is thekey parameter in controlling the nanorod aspect ratios.The surfactant plays a minor role and acts as astabilizing agent and not as a template.

CONCLUSION

It can be concluded that,with some discrepancies,reverse micelles are rather good candidates forcontrolling the size of spherical nanocrystals5,6.However, they are not the determining factor incontrolling the shape of inorganic materials.Crystalgrowth on the nanoscale seems to follow behavioursimilar to that of the bulk phase52 with a markeddependence on pH.The pH is particularly importantwhen some impurities are present in the growthmedium because it influences, for example, theformation of either zwitterions (ions having bothpositive and negative charges) or complex ions.Thepresence of these various species during the nanocrystalgrowth modify the growth of certain crystal faces.Mostof the changes are based on the existence of a more orless epitaxial adsorption layer on the crystal.This layer iscomposed of solvent, impurity or salts,whose preciseroles are as yet uncertain.The changes in shape are dueto the differences between the growth rates of thevarious crystallographic faces.From this it can beconcluded that the template is not the key parameter inthe shape control.

We can ask why templates made of surfactants arequite effective in controlling formation of nanosphereswhereas rather large exceptions are observed for

anisotropic shapes. This is probably due to the factthat colloidal templates are highly dynamic. Theenergy needed to produce spherical nanocrystals isless than that for producing anisotropic nanocrystals.A general method for controlling nanocrystal shapesthrough soft chemistry has not yet been found.

To determine if such a method exists we needmore data, and to compare the fabrication ofanisotropic nanocrystals from various types ofmaterials. For example, the fabrication of elongatedferrite nanocrystals in biological media and in vitro53

are completely different. To produce nanomaterials invitro that are similar to those made in biologicalmedia, a very high base concentration is needed, andthe corresponding pH would induce the destructionof the biological media. It is clear, therefore, that otherapproaches are required.

doi:10.1038/nmat817

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Competing financial interestsThe author declares no competing financial interests.

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