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Colloidal self-assemblies used as templates to control size, shape and self-organization of nanoparticles
PII: S0968-5677(98)00026-1
Supramolecular Science 5 (1998) 321—329 1998 Elsevier Science Limited Printed in Great Britain. All rights reserved 0968-5677/98/$19.00
Colloidal self-assemblies used as templates to control size, shape and self-organization of nanoparticles M.P. Pileni* Laboratoire SRSI, URA CNRS 1662, Universite P. et M. Curie Baˆ t F, 4 Place Jussieu, 75005 Paris, France In this paper we show that the use of colloidal assemblies as templates favors the control of the size and shape of nanoparticles. As expected theoretically, the change in size and shape of copper metal nanosized particles induces changes in their optical properties. Cylindrical copper metal particles having the same size and shape can be obtained in various regions of the phase diagram when the template is made of interconnected cylinders. Self-assembly of silver metal nanoparticles is reported. Monolayers of particles organized in a hexagonal network are formed over very large domains. Small or large aggregates can also be produced, and, in these aggregates, the particles are highly organized and form pseudo-crystals with a face-centered cubic structure for various particles sizes. The optical properties of the silver nanoparticles isolated in micellar solution or self-assembled in 2D or 3D supperlattices are reported. Syntheses of magnetic fluids differing in their particle size are presented. The magnetic properties differ with the particle size. 1998 Elsevier Science Limited. All rights reserved.
1. INTRODUCTION A key step in the control of mineralization employed by almost all organisms is the initial isolation of a space . Then, under controlled conditions, minerals are induced to form within the space. Filling up these spaces with amorphous minerals would appear to require a quite different strategy from that in filling spaces with crystalline material. The simplest way to fill a space with crystals is to create as high a local supersaturation as possible, and then induce nucleation or let the system spontaneously reach a lower-energy state by crystallization, while at the same time removing the excess solvent. In terms of particle growth, a number of analogies between surfactant self-assemblies and natural media can be proposed. In both cases, this growth needs a supersaturated medium where the nucleation takes place. Increasingly, chemists are contributing to the synthesis of advanced materials with enhanced or novel properties by using colloidal assemblies as templates. Colloid chemistry is particularly well suited to this objective since nanoparticles . In solution, surfactant molecules self-assemble to form aggregates. At low concentrations the aggregates are generally globular micelles but these micelles can grow by increasing the surfactant concentration and/or upon addition of salt, alcohols, etc. In this case, micelles grow to elongate, more or less flexible rod-like micelles\, in agreement with theoretical predictions for *Also at: CEA, DSM-DRECAM Service de Chimie Mole´culaire CE Saclay, 91 191 Gif sur Yvette Cedex, France
micellization . The amphiphilic molecules spontaneously self-assemble to form highly flexible locally cylindrical aggregates with the average size reaching several micrometers . There are large fluctuations in details of the morphology (curvature of the film) of the mixture at a local scale. The contribution of the entropy of the folded film is predominant in the free energy of the solution, while the morphology has little influence. The interfacial curvature towards water is, by convention, described as negative mean curvature and is known as type II or inverse curvature. The fabrication of assemblies of perfect nanometerscale crystallites (quantum crystal) identically replicated in unlimited quantities in such a state that they can be manipulated and understood as pure macromolecular substances is an ultimate challenge in modern materials research with outstanding fundamental and potential technological consequences. These potentialities are mainly due to the unusual dependence of the electronic properties on the particle size, either for metal\ or semiconductor \ particles, in the 1—10 nm range. The preparation and characterization of these nanomaterials have motivated a vast amount of work. One of the methods to control the particle size is the use of reverse micelles as microreactors . The achievements of an accurate control of the particle size, their stability and a precisely controllable reactivity of small particles are required to allow attachment of the particles to the surface of a substrate or to other particles without leading to coalescence and hence losing their size-induced electronic properties. There are several reasons for forming films of inorganic particles attached to or
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Colloidal self-assemblies of nanoparticles: M.P. Pileni embedded just under the surface. Moreover, the ability to assemble particles into well-defined two- and three-dimensional spatial configurations should produce advantageous properties such as new collective physical behavior. The synthesis of inorganic—organic supperlattices has been achieved using the multilayer casting of films. Asher et al. have developed a method for creating both organic and inorganic submicrometer periodic materials. Recently, in our laboratory spontaneous arrangements either in a monolayer organized in a hexagonal network or three-dimensional FCC arrangements of particles have been observed\. Similar arrangements have been reported\.
2. CONTROL OF THE PARTICLE SIZE Reverse micelles are well known to be spherical water in oil droplets stabilized by a monolayer of surfactant. The phase diagram of the surfactant sodium bis(2-ethylhexyl) sulfosuccinate, called Na(AOT), with water and isooctane shows a very large domain of water in oil droplets and often forms reverse micelles . The water pool diameter is related to the water content, w" [H O]/[AOT], of the droplet by D(nm)"0.3w. From the existing domain of water in oil droplets in the phase diagram, the droplet diameters vary from 0.5 to 18 nm. Reverse micelles are dynamic\ and attractive interactions between droplets take place. The intermicellar potential decreases either by decreasing the number of carbon atoms of the bulk solvent or by increasing the number of droplets. This is due to the discrete nature of solvent molecules and is attributed to the appearance of depletion forces between two micelles (the solvent is driven off between the two droplets). When the droplets are in contact forming a dimer, they exchange their water contents. This exchange process is associated with the interface rigidity which corresponds to the bending elastic modulus of the interface. Hence, in collisions, the droplets exchange their water contents and again form two independent droplets. This process has been used to make nanosized material by either chemical reduction of metallic ions or coprecipitation reactions. These various factors (water content, intermicellar potentials) control the size of the particles. Syntheses in reverse micelles induce formation of nanoparticles dispersed in the solution. This can be followed by measuring the absorption spectra of the colloidal particles dispersed in the solvent. The average size of the particles can be determined by depositing a drop of this solution on a carbon grid and obtaining the TEM patterns. Several types of material such as +CdS \ , Cd Zn S , ZnS, Cd Mn S , W \W W \W PbS, for (Ag) , (Cu) 18,19,50 and Co B alloys, have L L been used. As an example, the synthesis of copper metal particles is described. Reverse micelles are good candidates for making nanosized copper metal particles . To form this material, a functionalized surfactant such as copper bis(2-ethylhexyl)sulfosuccinate, Cu(AOT) , is needed.
When this is replaced by copper sulfate (Cu>), oxidized copper instead of copper metal particles are formed. Furthermore, a few minutes after starting the reaction, the particles flocculate. Mixed micelles made of sodium and copper bis(2-ethylhexyl)sulfosuccinate are prepared. The water content, w, is defined as the ratio of water over the sum of Na(AOT) and Cu(AOT) concentrations. The water content, w"[H O]/+[Na(AOT)]#[Cu(AOT) ],, is fixed at a given value. This solution is mixed with another Na(AOT) micellar solution in hydrazine, having the same water content. The copper bis(2-ethylhexyl)sulfosuccinate is reduced with the formation of copper metal particles, characterized by TEM, electron diffraction and absorption spectroscopy. Figure 1 shows an increase in the particle size and a marked change in the absorption spectrum with appearance of a plasmon peak with increasing water content. As expected, from expanded versions of Mie’s theory\, the absorption spectra vary with the particle sizes below 5 nm. The direct relationship between the particle size and the absorption spectrum allows obtaining a calibration curve relating the average particle diameter to its absorption.
3. CONTROLLING THE SHAPE OF THE PARTICLES Syntheses are performed either in self-assemblies having oil as the bulk phase and differing by their local arrangements or in dilute normal (oil in water) micelles. 3.1. Controlling the shape by using water in oil self-assemblies Copper ions have been reduced in colloidal assemblies differing by their structures . In all cases, copper metal particles are obtained. Figure 2 shows syntheses in various template structures. No oxide is detectable and the particles are single crystals. The colloidal system consists of 5;10\ M Cu(AOT) — isooctane—water. The colloidal structure is changed by increasing the amount of water in Cu(AOT) —isooctane solution . Syntheses are performed in various regions of the phase diagram. (i) At low water content from w"2 to 5.5, a homogeneous reverse micellar solution (the L -phase) is formed. In this range, the shape of the water droplets changes from spheres (below w"4) to cylinders. At w"4, the gyration radius has been determined by SAXS and found equal to 4 nm. Syntheses in isolated water in oil droplets show the formation of a relatively small amount of copper metallic particles. Most of the particles are spherical (87%) with a low percentage (13%) of cylinders (Figure 2A). The average size of the spherical particles is characterized by a diameter of 12 nm with a size polydispersity of 14%. (ii) The increase in the water content (5.6(w(11), destabilizes the solution and the L -phase separates into a more concentrated reverse micellar solution (L*) and an almost pure isooctane phase. Structural studies
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Colloidal self-assemblies of nanoparticles: M.P. Pileni
Figure 1 Absorption spectra and TEM pattern of copper metal particles with their sizes
indicate that L* is characterized by a bicontinuous net work of cylinders with an increase in the number of connections with increasing w. The persistence length of the cylinders measured by SAXS does not change (1l2" 3 nm) with increasing w. Syntheses at w"6 show (Figure 2B) the formation of a relatively large amount of copper metal cylinders (32%) in coexistence with 68% spheres. The average diameter of the spherical particles is 9.5 nm. The size distribution is rather large (27%). The average length over width ratio of the cylinder is found to be 3.5 with 40% polydispersity. The length and width of the cylinders are equal to 22.6$5.4 and 6.7$1.4 nm, respectively. (iii) At the phase transition (w"11), a L -phase made ? up of a mixture of planar lamellae and spherulites appears. Syntheses performed in this region of the phase diagram show the formation of a very large amount of cylinders in coexistence with spheres. Very few large rods can be observed (Figure 2C). Their sizes vary from 0.1 to 1 lm. In this region of the phase diagram, a slight increase in the number of cylinders is observed (38% of the particles are cylindrical whereas 62% are spherical). The average diameter of the spherical particles is equal to 10.9 nm with 17% of polydispersity in size distribution. The length and width of the cylinders are 25$4 and 7.3$1.4 nm, respectively. A slight increase in the size
Figure 2 TEM patterns of metallic copper particles obtained in Cu(AOT) —isooctane solution at various water content: w"4, (A), w"6 (B), w"12, (C), w"18 (D), w"34 (E) and w"44 (F), [Cu(AOT) ]"5;10\M
distribution is seen. The average length to width ratio of the cylinders is 3.7 with 44% polydispersity. (iv) From w"15 to w"19, spherulites remain in equilibrium with isooctane. The spherulite size differs markedly (from 100 to 8000 nm). Syntheses in this phase region (15(w(20) show the formation of particles having a higher polydispersity in size and in shape (Figure 2D) than those observed at low water content. As a matter of fact, Figure 2D show the formation of triangles, squares, cylinders and spheres. (v) At w"20, an isotropic phase appears. By increasing the water content w from 20 to 29, the lamellar phase progressively disappears so as to reach two phases consisting of isooctane and the isotropic phase. The latter is attributed to a mixture of a sponge and interconnected cylinder phases. Syntheses at various water contents w from 21 to 29, induce formation of roughly 10% cylinders and 90% spheres. The percentage of cylinders decreases with increasing the water content from 13 to 7%, whereas the cylinder length and width remain unchanged. These values are found to be 19.0$2.5 and
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Colloidal self-assemblies of nanoparticles: M.P. Pileni 6.7$0.8 nm, respectively. The diameter of the spheres slightly decreases and the polydispersity increases with increasing water content from 9.7$0.7 to 8.1$1.2 nm at w"22 and w"28, respectively. (vi) At w"30, the isotropic phase remains in equilibrium with isooctane and is attributed to the L*-phase similar to that obtained at lower water content (in the range of 5.6—11). By increasing the water content w from 30 to 35, the interconnected cylindrical network is diluted with a decrease in the number of connections. Over all this water content range, the formation of spherical and cylindrical copper metallic particles are observed. Figure 2E shows a TEM pattern obtained at w"34. As at lower water content, cylindrical (42%) and spherical (58%) nanoparticles are observed. The average diameter of spherical particles observed in most of the cases is 9.5$0.9 nm. The length and width of the cylinders are 19.8$2.7 and 6.5$0.8 nm, respectively. (vii) At w"35, an isotropic solution formed by water in oil droplets is obtained. On increasing the water content w from 35 to 40, no drastic changes in the particle sizes and in the percentage of cylinders are observed. Figure 2F shows the TEM pattern obtained at w"40. The particle size is 7.5$0.6 nm. From these data it is concluded that the size, shape and size distribution of nanoparticles depend critically on the colloidal structure in which the synthesis is performed. This is well demonstrated when, by changing the water content, similar colloidal structures (reverse micelles or interconnected cylinders) are obtained: (i) Reverse micelles are formed below w"5.5 and above w"35. Most of the copper metal particles obtained in this region of the phase diagram are spherical (Figure 2A and F). The percentage of spheres varies from 86 to 93%. At low water content (w"4), the diameter of the particles is larger than that obtained at higher water content (w'35). Hence, a slight decrease in the particle diameter with increasing water content is observed. In the reverse micellar regions, the length and width of the cylinders are larger at low water content and decrease with increasing w. Furthermore, the number of particles formed markedly increases with increasing water content. These differences could be mainly attributed to hydration of the head polar group. (ii) Interconnected cylinders (L*) are formed in two water content range (5.5(w(11 and 30(w(35). Syntheses in these two regions of the phase diagram show very strong correlation and similar data. Spherical and cylindrical particles are formed in both cases. No other particle shapes have been observed. In both regions, the average diameter of spherical particles is the same (9.5 nm). Similarly, the size of the cylinders remains identical with an average length and width of 21.2 and 6.6 nm, respectively. The same average diameter and same ratio of cylindrical axis (O3.3) are observed at low (5.5(w(11) and high (30(w(35) water content. Because of this large similarity in various experimental conditions, this phenomenon is attributed to the structure of the colloid used as a template. This control of the particle shape could be explained as in nature where
the key step in the control of mineralization is the initial isolation of a space. As in the present case, a local supersaturation of reactants is needed to induce nucleation and let the system reach a lower-energy state. In such local supersaturation regime, the chiral molecules used to form the colloidal template (Cu(AOT) ) impose an orientation of each reactant. The control of the crystal morphology could be due to the presence of high surfactant concentration (or compounds resulting from the chemical reaction) which specifically adsorbs on certain crystal faces. The same concept has been used to control the crystal morphology by the adsorption of impurities from solution onto specific crystal surfaces. Mechanisms of inhibition involved in the control of crystal morphology have been elucidated at the molecular level in studies on organic crystals and tailor-made inhibitors . 3.2. Controlling the shape by using oil in water self-assemblies Copper dodecyl sulfate is made by mixing an aqueous solution of sodium dodecyl sulfate with copper sulfate, as described elsewhere. An aqueous solution of 0.1 M sodium docedylsulfate is mixed with 0.1 M copper sulfate. The solution is kept at 2°C and the precipitate which appears is washed several times with a 0.1 M copper sulfate solution and recrystallized in distilled water. Copper dodecyl sulfate, Cu(DS) , forms micellar aggregates above the critical micellar concentration (c.m.c.). The shape and size of these aggregates have been determined by SAXS and by light scattering and are found to be prolate ellipsoidal micelles with a hydrodynamic radius of 2.7 nm. The micellar size and shape are in good agreement with that observed with Cd(DS) . Copper(II)dodecylsulfate, Cu(DS) is reduced by so dium borohydride, NaBH with a NaBH /Cu(DS) ratio of 2 and copper metal aggregates are obtained for any of the experimental conditions described below. The synthesis is performed at the Cu(DS) critical micellar concentration, c.m.c."1.2;10\M. The TEM measurements made using a drop of this solution deposited on a carbon grid show the formation of large domains of aggregates arranged in an interconnected network (Figure 3A). Enhancement of this aggregate shows that the interconnected network corresponds to a change in the particle shape and not to the aggregation of strongly interacting small particles (Figure 3B). Comparison of the absorption spectra for different shaped particles and spherical particles with 10 nm diameter, shows a red shift in the plasmon peak when the shape of the copper metal particles changes from spheres to rods (Figure 4). Such changes in the absorption spectra of these particles can be related to those predicted at various r values, where r is defined as the ratio of the length to diameter of a cylinder\. The plasmon peak due to the rod particles is centered at 570 nm (Figure 4). According to simulated absorption spectra, this peak corresponds to an r value of 2.5. From image analysis of the skeleton of the interconnected network corresponding to a plasmon peak centered at 570 nm, the average length of
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Colloidal self-assemblies of nanoparticles: M.P. Pileni particle shape is in good agreement with those predicted. Thus, the shift and the ripening in the absorption band, compared to spherical and elongated particles indicate that the interconnected network, observed by TEM, exists in solution. It is due to a change in the effective mean free path of conduction electrons and not evaporation of the solutions.
4. SELF-ORGANIZATION IN 2D AND 3D STRUCTURES
Figure 3 Transmission electron microscopy of the copper network prepared in a pure copper dodecyl sulfate solution: [Cu(DS) ]"1.2 ;10\ M, [NaBH ]"2.4;10\ M. A and B correspond to various scales of the TEM pattern
Figure 4 Absorption spectra of interconnected (22), elongated (*) and spherical (E) colloidal particles
linear strands is 22 nm. The average minor diameter is 6.5 nm. Hence, the r value is 3.3. From the theoretical predictions such an r value (r"3.5) corresponds to a higher shift in the plasmon peak (620 nm) than that obtained (570 nm). This difference between r and the maximum of the plasmon peak is explained by the fact that the simulation is related to one size of cylindrical particles. In the present experiment, the particles are interconnected and not isolated cylinders. Furthermore, the polydispersity has to be taken into account. A qualitative shift in the maximum of the plasmon peak with the
The procedure to fabricate colloidal silver, (Ag) , spheriL cal nanoparticles is similar to that described above (see Section 2): The Cu(AOT) is replaced by the silver deriva tive. The relative concentration of Na(AOT)], Ag(AOT) and the reducing agent remain the same. Control of the particle size is obtained from 2 to 6 nm. To stabilize the particles and to prevent their growth, 1 ll ml\ of pure dodecanethiol is added to the reverse micellar system containing the particles. This induces a selective reaction at the interface, with covalent attachment, between thio derivatives and silver atoms. The micellar solution is evaporated at 60°C and a solid mixture of dodecanethiol-coated nanoparticles and surfactant is obtained. To remove the AOT and excess dodecanethiol surfactant a large amount of ethanol is added and the particles are dried and dispersed in heptane. A slight size selection occurs and the size distribution drops from 43 to 37%. The size distribution is reduced through the size-selected precipitation (SSP) technique. A drop of a dilute solution of 4.5 nm coated particles in hexane ([(Ag) ]"2;10\ M) is deposited on a graphite L surface. Figure 5A shows a large area of closed packed monolayers made of nanoparticles organized in a hexagonal network. The optical spectrum of the nanosized particles coated on graphite support is recorded by reflectivity mode and is compared with the spectrum corresponding to free particles in hexane. The optical spectra are normalized to unity. The plasmon peak is shifted towards an energy lower than that obtained in solution (Figure 5B). The coverage support is washed with hexane and the nanoparticles are redispersed in the solvent. The absorption spectrum of the latter solution is similar to that used to cover the support (free particles in hexane). This clearly indicates that the shift in the absorption spectrum of nanosized silver particles is due to their self-organization on the support. The bandwidth of the plasmon peak (1.3 eV) obtained after deposition is larger than that in solution (0.9 eV). This can be attributed to a change in the dielectric constant of the composite medium. Taking into account the variation of the simulated plasmon peak with the dielectric constant, it is concluded that the shift toward lower energy observed is due to an increase in the dielectric constant of the composite medium of the particles which is the superposition of several factors as the spherical particles, the support, particle—particle interactions and the air. Similar behavior is observed for a diluted solution made of 5.2 and 6.2 nm nanosized particles.
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Colloidal self-assemblies of nanoparticles: M.P. Pileni
Figure 5 (A) TEM micrograph of 4.5 nm silver particles deposited on graphite. (B) Absorption spectra of free 4.5 nm silver nanoparticles dispersed in hexane before (black) leaving a drop on the support, after wasking the support with hexane (green), and deposition on the surport forming a 2D superlattices (red). ([(Ag) ]"2;10\M) L
When the particles are dispersed in hexane until complete evaporation of the solvent, it results in the formation of large aggregates. Figure 6A shows a rather high aggregate orientation around a large hole or ring. The average distance between the oriented aggregates varies from 20 to 60 nm. The aggregate magnification confirms that it consists of (Ag) nanoparticles and its average size L is in the range of 0.03—0.55 mm. High magnification of one of these aggregates shows that the particles are arranged in two different symmetries. Figure 6B shows the formation of a polycrystal. Its magnification (Figure 6C) shows either a hexagonal or a cubic arrangement of nanoparticles. The transition from one structure to another is abrupt and there is a strong analogy with ‘‘atomic’’ polycrystals with a small grain called nanocrystals. Each domain or grain has a different orientation clearly showing that the stacking of nanoparticles is periodic and not random. The ‘‘pseudo-hexagonal’’ structure corresponds to the stacking of a +110, plane of the f.c.c. structure. A fourfold symmetry is observed on the same pattern (Figure 6C) which is again characteristic of the stacking of +001, planes of the cubic structure. This cannot be found in the hexagonal structure. As a matter of fact, there is no direction in a perfect hexagonal compact structure for which the projected positions of the particles can take this configuration. This is confirmed by TEM experiments performed at various tilt angles where it is always possible to find an orientation for which the stacking appears to be periodic. Hence, by tilting a sample having a pseudo-hexagonal structure, a fourfold symmetry is obtained. From these data, it is concluded
Figure 6 Progressive enhancement of (Ag°) aggregates L
that the large aggregates of silver particles are formed by stacking of monolayers in a face centered cubic arrangement. Similar self assemblies of silver sulfide\, silver and gold nanoparticles have been obtained by using different experimental modes\. By increasing the concentration of 4.5 nm nanoparticle in hexane solution ([(Ag) ]"4;10\ M), the aggreL gates are so large that they cannot be investigated by TEM. By scattering electron microscopy, aggregates differing by their size and shape are observed. They are well-faceted, as observed for the smaller aggregate by TEM. We choose to investigate one of these aggregates. Enhancement of one face of the aggregate shows welldefined defects in the crystal phase. The 60° tilt of aggregate confirms a 3D structure (Figure 7A). This permits to estimate the size of the aggregate, which is 200 lm high and 100 lm large. Figure 7A shows the presence of smaller aggregates surrounding the larger one. Furthermore, layers at the bottom of the aggregate are observed. To make sure that these aggregates are made of silver nanoparticles, EDX analyses were performed on the top of the aggregate and on the bottom (in the layers region). The analysis is made on a 0.786 lm volume. The
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Figure 7 (A) Scanning electron microscopy on concentrated solution of 4.5 nm silver particles ([(Ag) ]"4;10\M). Large aggregates on silver multilayers are present. (B) Absorption spectra of free 4.5 nm silver nanoparticles dispersed in hexane before (black) leaving a drop on the support, after washing the support with hexane (green), and deposition on the support forming 3D superlattices (blue) ([(Ag) ] L "4;10\ M)
aggregate consists of silver, carbon and sulfur atoms. This is fully consistent with a crystal made of silver nanosized particles coated by dodecanethiol. The UVVisible spectrum (Figure 7B) of the aggregates described above shows a 0.25 eV shift toward lower energy of the plasmon peak with a slight decrease in the bandwidth (0.8 eV) compared to that observed in solution (0.9 eV). As observed above with monolayer, by washing the support, the particles are redispersed in hexane and the absorption spectrum remains similar to that of the colloidal solution used to make the self-assemblies. For isolated particles, the increase in the dielectric constant induces a shift to the lower energy and an increase in the bandwidth of the plasmon peak. For particles organized in a FCC structure, each silver nanoparticle is surrounded by 12 other particles, whereas in a monolayer it has six neighbors. So in 3D superlattices, the dielectric constant is the superposition of several factors as the external field, the dipole fields of all the other particles and the surrounding due to the support. This induces an increase in the total dielectric constant which, as for isolated particles, induces a shift toward the low energy of the plasmon peak. This is confirmed by comparing Figures 6 and 7: The shift of the plasmon peak of particles arranged in 2D and 3D superlattice
compared with the spectrum corresponding to free coated particles in hexane is 0.12 and 0.25 eV, respectively. Hence, the increase in the total dielectric constant induces an increase in the plasmon peak bandwidth. The bandwidth is of 0.8 eV, whereas for particles organized in 2D (Figure 6) it is 1.3 eV. The value obtained for a 3D superlattice (0.8 eV) is close to that observed for free nanoparticles in hexane (0.9 eV). These data show the effect of the medium dielectric constant of the particle when organized in 2D and 3D. The decrease in the bandwidth plasmon peak could be due to an increase in the mean free path conduction electrons of silver particles through a barrier of 2 nm. This is rather suprising, because the average distance between silver nanoparticles is 2 nm and we would not expect a tunneling electron effect through such a large barrier. However, a recent paper published by Ung et al. claims that a 1—2 nm distance between two metal surfaces is enough for tunneling of electrons across the double layers. Because of the fact we keep the same absorption spectrum and TEM pattern after washing the support; fusion between particles during the coverage can be excluded. No predictions have been given in literature on the variation of the electromagnetic coupling of the particles when they are organized in 2D with a hexagonal network and 3D superlattice with a FCC structure. This does not permit us to definitively conclude on a collective effect, due to the transport of the conduction electrons through the barrier due to the coating or due to changing in the dielectric medium.
5. MAGNETIC FLUID OF COBALT FERRITE NANOSIZED PARTICLES By using oil in water micelles cobalt ferrite nanoparticles are formed and the particle size is controlled. Syntheses of CoFe O particles have been extensively studied in homogeneous solutions. The procedure used is the following: very high ('2 M) concentrations of Fe(II), Fe(III) and Co(II) salts are dissolved in aqueous solution. A base is added and the solution is heated, during 1 h, at 100°C. The precipitate that appears is washed and the magnetic particles are dispersed in aqueous solution. This is a magnetic fluid. Syntheses of cobalt ferrite nanosized particles need specific experimental conditions: (i) high reactant concentrations, (ii) presence of Fe(III) ions, and (iii) high temperature (100°C). The particle size is controlled by drastic changes in pH, ionic strength and addition of polymers or surfactants. These chemical changes strongly perturb the surface of the particles (with adsorption of macromolecules or hydroxides, etc.). As it has been well demonstrated, changes in the surface of the particle induces changes in their magnetic properties. So the use of this procedure makes rather difficult to derive consistent relationship between particle sizes and magnetic properties. By using oil in water micelles made of functionalized surfactants, the reactant concentrations are lower than those usually used (almost two orders of magnitude). At
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Colloidal self-assemblies of nanoparticles: M.P. Pileni room temperature, addition of base to the Co(DS) and Fe(DS) mixed micelles induces formation of a precipi tate made of stoichiometric CoFe O nanoparticles . The magnetic fluid is formed by washing this precipitate. Its characterization indicates the formation of CoFe O with an inverse spinel structure as in the bulk phase. At the starting point, this reaction does not need any Fe(III) ions. Hence, in micelles, the synthesis is performed at Co(DS) and Fe(DS) concentrations in the 10\ M range, whereas in aqueous solution, the reaction does not take place at such low concentrations. An increase in concentration by three orders of magnitude is needed. This could be attributed to the fact that formation of micelles induces high local concentration of the reactant at the interface (Co> and Fe>) and then favors the chemical reaction. No obvious explanation can be given for the formation of CoFe O in micelles at room tem perature whereas the solution needs to be heated at 100°C in homogeneous solution. As said above, in homogeneous solution, the particle size is controlled by drastic changes in the experimental conditions which induces large perturbations in the magnetic properties of the nanoparticles. In micellar solution, the particle size is controlled by keeping the ratio of reactants ([Fe(DS) ]/[CH NH OH] and [Fe(DS) ]/ [Co(DS) ] constant and by increasing the Fe(DS) con centration by a factor of 4(6.5;10\M([Fe(DS) ]( 1.2;10\M) indicate an increase in the particle size from 2 to 5 nm. This control of the particle size is rather surprising. A possible partial explaination is given by XANES data obtained at the same concentrations as in the syntheses. An increase in the oxidation numbers of Co and Fe ions when the Fe(DS) concentration in creases is derived from these data. This could increase the number of nuclei formed and then induce an increase in the particle size. Furthermore, the amount of oxygen dissolved in the solution depends on the micellar concentration. This could play a role in controlling the particle size. Controlling in the particle size does not need large changes in the experimental conditions. This allows, to a first approximation, to assume that the surface of the particle remains unchanged. Hence, it is possible to find a relationship between size and magnetic properties. The study of the magnetic properties of CoFe O nanosized particles shows, from magnetization curves and Mo¨ssbauer spectroscopy, an increase in the anisotropy with a decrease in the particle size. For particles having an average size of 5nm, the anisotropy remains cubic as in the bulk phase. Conversely, when the size of the particle decreases from 5 to 3 or 2 nm, the anisotropy of the nanomaterial changes from cubic to axial. Similar behavior has been observed with Fe O and c-Fe O nanosized particles. The chemical reaction takes place at low reactant concentration, room temperature and in the absence of Fe(III) ions. In homogeneous solutions, syntheses in the absence of Fe(III) ions induce the formation of very well-defined particles in the micrometer range.
ACKNOWLEDGMENTS I would like to sincerely thank my coworkers who have been in charge of these studies. Special thanks are due to A. Filakembo, N. Feltin, Dr I. Lisiecki, Dr N. Moumen, Dr C. Petit, A. Taleb and J. Tanori who participated in this work.
REFERENCES 1 Addadi, L. and Weiner, S. Angew. Chem. Int. Ed. Engl. 1992, 31, 153 2 Mann, S. ‘Inorganic Materials’ (Ed. D.W. Bruce and D. O’Hare), J Wiley, New York, 1992 3 Pileni, M.P. J. Phys. Chem. 1993, 97, 6961 4 Fendler, J.H. Chem. Mater. 1996, 8, 1616 5 Tanford, C. ‘The Hydrophobic Effect, Wiley, New York, 1973 6 Chen, S.J., Evans, D.F., Ninham, B.W., Mitchell, D.J., Blum, F.D. and Pickup, S. J. Phys. Chem. 1986, 90, 842 7 Evans, D.F., Mitchell, D.J. and Ninham, B.W. J. Phys. Chem. 1986, 90, 2817 8 Barnes, I.S., Hyde, S.T., Ninham, B.W., Derian, P.J., Drifford, M. and Zemb, T.N. J. Phys. Chem. 1988, 92, 2286 9 Porte, G., Appell, J. and Poggi, Y. J. Phys. Chem. 1980, 84, 3105 10 Mitchell, D.J. and Ninham, B.W. J. Chem. Soc. Faraday Trans. 2, 1981, 77, 601 11 Hyde, S.T., Anderson, S., Larsoon, K., Landn, T., Idin, S.L., Blum, Z. and Ninham, B. (Eds) The Shape Language, Elsevier, Amsterdam, 1996 12 Berret, J.F., Roux, D.C., Porte, G. and Lindner, P. Europhys. ¸ett. 25, 1994, 521 13 Israelachvili, J.N., Mitchell, D.J. and Ninham, B.W. J. Chem. Soc. Faraday Trans. 2, 1976, 72, 1525 14 Pileni M.P. (Ed.) ‘Reactivity in Reverse Micelles’ Elsevier, Amsterdam, 1989 15 Pileni, M.P. New J. Chem. (in press) 16 Wokaun, A., Gordon, J. P., Liao, P.F., Phys. Rev. ¸ett. 1982, 48, 957 17 Petit, C., Lixon, P. and Pileni, M.P. J. Phys. Chem. 1993, 97, 12974 18 Lisiecki, I. and Pileni, M.P. J. Amer. Chem. Soc. 1993, 115, 3887 19 Lisiecki, I. and Pileni, M.P. J. Phys. Chem. 1995, 99, 5077 20 Brus, L.E. J. Chem. Phys. 1983, 79, 5566 21 Petit, C. and Pileni, M.P. J. Phys. Chem. 1988, 92, 2282 22 Petit, C., Lixon, P. and Pileni, M.P. J. Phys. Chem. 1990, 94, 1598 23 Motte, L., Petit, C., Boulanger, L. and Pileni M.P. ¸angmuir 1992, 8, 1049 24 Suriki, K, Harada, M. and Shioi, A. J. Chem. Eng. Jpn. 1996, 109, 245 25 Hirai, T., Sato, H. and Komasawa, I. Ind. Engng Chem. Res. 1994, 33, 3262 26 Cizeron, J. and Pileni, M.P. J. Phys. Chem. 1995, 99, 17410 27 Cizeron, J. and Pileni, M.P. J. Phys. Chem. 1997, 101, 44, 8887 28 Levy, L., Hochepied, J.F. and Pileni, M.P. J. Phys. Chem. 1996, 100, 18322 29 Levy, L., Feltin, N., Ingert, D. and Pileni, M.P. J. Phys. Chem. 1997, 101 (in press) 30 Special issue Nanostructured Materials, Chem. Mater. 1996, 8 (5) 31 Pileni, M.P. ¸angmuir. 1997, 13, 3266 32 Kimizuka, N. and T. Kunitake, Adv. Mater. 1996, 8, 89 33 Tse, A.S., Wu, Z. and Asher, S.A. Macromolecules 1995, 28, 6533 34 Motte, L., Billoudet, F. and Pileni, M.P. (in preparation) 35 Motte, L., Billoudet, F. and Pileni, M.P. J. Phys. Chem. 1995, 99, 16425 36 Motte, L., Billoudet, F., Lacaze E., Pileni M.P. Adv. Mater. 1996, 8, 1018 37 Motte, L., Billoudet, F., Lacaze, E., Douin, J. and Pileni, M.P. J. Phys. Chem. 1997, 101, 138 38 Taleb, A., Petit, C. and Pileni, M.P. Chem. Mater. 1997, 9, 950 39 Whetten, R.L., Khoury, J.T., Alvarez, M.M., Murthy, S., Vezmar, I., Wang, Z.L., Cleveland, C.C., Luedtke, W.D. and Landman, U. Adv. Mater. 1996, 8, 429 40 Brust, M., Bethell, D., Schiffrin, D.J. and Kiely, C.J. Adv. Mater. 1995, 7, 9071 41 Harfenist, S.A., Wang, Z.L., Alvarez, M.M., Vezmar, I. and Whetten, R.L. J. Phys. Chem. 1996, 100, 13904
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Colloidal self-assemblies of nanoparticles: M.P. Pileni 42 Heath, J.R., Khobler, C.M. and Leff, D. J. Phys. Chem. B, 1997, 101, 189 43 Harfenist, S.A., Wang, Z.L., Whetten, R.L., Vezmar, I., Alvarez, M.M. Adv. Mater. 1997, 9, 817 44 Safran, S.A., Turkevich, L.A. and Pincus, P.A. J. Phys. ¸ett. 1984, 45, L69 45 Towey, T.F., Khan-Lodl, A. and Robinson, B.H. J. Chem. Soc. Faraday ¹rans. 2 1990, 86, 3757—3762 46 Robertus, C., Joosten, J.G.H. and Levine, Y.K. J. Chem. Phys. 1990, 93, 10, 7293 47 Cassin, G., Badiali, J.P. and Pileni, M.P. J. Phys. Chem 1995, 99, 12941 48 Jain, T.K., Cassin, G., Badiali, J.P. and Pileni, M.P. ¸angmuir 1996, 12, 2408 49 Stigerwald, M.L., Alivisatos, A.P., Gibson, J.M., Harris, T.D., Kortan, R. and Muller, A.J. J. Am. Chem. Soc. 1988, 110, 3046 50 Lisiecki, I., Borjling, M., Motte, L., Ninham, B.W. and Pileni, M.P. ¸angmuir, 1995, 11, 2385 51 Petit, C. and Pileni, M.P. J. Magn. Magn. Mater. 1997, 166, 82 52 Fragstein, C.V. and Roemer, H. Z. Physik 1958, 151, 54 53 Roemer, H. and Fragstein, C.V. Z. Physik 1961, 163, 27 54 Fragstein, C.V. and Schoenes, F.J. Z. Physik 1967, 198, 477 55 Petit, C., Lixon, P. and Pileni, M.P. ¸angmuir 1991, 7, 2620 56 Tanori, J. Gulik, T. and Pileni, M.P. ¸angmuir 1997, 13, 633 57 Tanori, J. and Pileni, M.P. ¸angmuir 1997, 13, 639 58 Tanori, J. and Pileni, M.P. Adv. Mater. 1995, 7, 862
59 Weissbuch, I., Addadi, L., Berkovitch-Yellin, Z., Gati, E., Lahav, M. and Leiserowitz, L., Nature 1984, 310, 161 60 Weissbuch, I., Frolow, F., Addadi, L., Lahav, M. and Leiserowitz, L. J. Amer. Chem. Soc. 1990, 112, 7718 61 Moroi, Y., Motomura, K.and Matuura, R. J. Colloid Interface Sci. 1974, 46, 111 62 Petit, C., Jain, T.K., Billoudet, F. and Pileni, M.P. ¸angmuir 1994, 10, 4446 63 Zeman, E.J. and Schatz, G.C. J. Phys. Chem. 1987, 91, 634 64 Cline, M.P., Barber, P.W. and Chang, R.K. J. Opt. Soc. Am. B 1986, 3, 15 65 Wang, D.S. and Kerber, M. Phys. Rev. B 1981, 24, 1777 66 Lisiecki, I., Billoudet, F. and Pileni, M.P. J. Phys. Chem. 1996, 100, 4160 67 Bain, C.D., Troughton, E.B., Tao, Y.T., Evall, J., Whitesides, G.M. and Nuzzo, R.G. J. Am. Chem. Soc. 1989, 111, 7155 68 Ninham, B.W. and Sammut, R.A. J. ¹heor. Biol. 1976, 56, 125 69 Ung, T. Giersig, M. Dunstan, D. and Mulvaney, P. ¸angmuir, 1997, 13, 1773 70 Charles, S.W. J. Magn. Magn. Mater. 1987, 65, 350 71 Moumen, N. and Pileni, M.P. Chem. Mater. 1996, 8, 1128 72 Moumen, N. and Pileni, M.P. J. Phys. Chem. 1996, 100, 1867 73 Moumen, N., Lisiecki, I., Briois, V. and Pileni, M.P. Supramolecular Sci. 1995, 2, 161 74 Moumen, N., Bonville, P. and Pileni, M.P. J. Phys. Chem. 1996, 100, 14410 75 Feltin, N. and Pileni, M.P. ¸angmuir, 1997, 13, 3927 76 Matijevic, E. Chem. Mater. 1993, 5, 412
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Supramolecular Science 5 (1998) 321—329 1998 Elsevier Science Limited Printed in Great Britain. All rights reserved 0968-5677/98/$19.00
Colloidal self-assemblies used as templates to control size, shape and self-organization of nanoparticles M.P. Pileni* Laboratoire SRSI, URA CNRS 1662, Universite P. et M. Curie Baˆ t F, 4 Place Jussieu, 75005 Paris, France In this paper we show that the use of colloidal assemblies as templates favors the control of the size and shape of nanoparticles. As expected theoretically, the change in size and shape of copper metal nanosized particles induces changes in their optical properties. Cylindrical copper metal particles having the same size and shape can be obtained in various regions of the phase diagram when the template is made of interconnected cylinders. Self-assembly of silver metal nanoparticles is reported. Monolayers of particles organized in a hexagonal network are formed over very large domains. Small or large aggregates can also be produced, and, in these aggregates, the particles are highly organized and form pseudo-crystals with a face-centered cubic structure for various particles sizes. The optical properties of the silver nanoparticles isolated in micellar solution or self-assembled in 2D or 3D supperlattices are reported. Syntheses of magnetic fluids differing in their particle size are presented. The magnetic properties differ with the particle size. 1998 Elsevier Science Limited. All rights reserved.
1. INTRODUCTION A key step in the control of mineralization employed by almost all organisms is the initial isolation of a space . Then, under controlled conditions, minerals are induced to form within the space. Filling up these spaces with amorphous minerals would appear to require a quite different strategy from that in filling spaces with crystalline material. The simplest way to fill a space with crystals is to create as high a local supersaturation as possible, and then induce nucleation or let the system spontaneously reach a lower-energy state by crystallization, while at the same time removing the excess solvent. In terms of particle growth, a number of analogies between surfactant self-assemblies and natural media can be proposed. In both cases, this growth needs a supersaturated medium where the nucleation takes place. Increasingly, chemists are contributing to the synthesis of advanced materials with enhanced or novel properties by using colloidal assemblies as templates. Colloid chemistry is particularly well suited to this objective since nanoparticles . In solution, surfactant molecules self-assemble to form aggregates. At low concentrations the aggregates are generally globular micelles but these micelles can grow by increasing the surfactant concentration and/or upon addition of salt, alcohols, etc. In this case, micelles grow to elongate, more or less flexible rod-like micelles\, in agreement with theoretical predictions for *Also at: CEA, DSM-DRECAM Service de Chimie Mole´culaire CE Saclay, 91 191 Gif sur Yvette Cedex, France
micellization . The amphiphilic molecules spontaneously self-assemble to form highly flexible locally cylindrical aggregates with the average size reaching several micrometers . There are large fluctuations in details of the morphology (curvature of the film) of the mixture at a local scale. The contribution of the entropy of the folded film is predominant in the free energy of the solution, while the morphology has little influence. The interfacial curvature towards water is, by convention, described as negative mean curvature and is known as type II or inverse curvature. The fabrication of assemblies of perfect nanometerscale crystallites (quantum crystal) identically replicated in unlimited quantities in such a state that they can be manipulated and understood as pure macromolecular substances is an ultimate challenge in modern materials research with outstanding fundamental and potential technological consequences. These potentialities are mainly due to the unusual dependence of the electronic properties on the particle size, either for metal\ or semiconductor \ particles, in the 1—10 nm range. The preparation and characterization of these nanomaterials have motivated a vast amount of work. One of the methods to control the particle size is the use of reverse micelles as microreactors . The achievements of an accurate control of the particle size, their stability and a precisely controllable reactivity of small particles are required to allow attachment of the particles to the surface of a substrate or to other particles without leading to coalescence and hence losing their size-induced electronic properties. There are several reasons for forming films of inorganic particles attached to or
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Colloidal self-assemblies of nanoparticles: M.P. Pileni embedded just under the surface. Moreover, the ability to assemble particles into well-defined two- and three-dimensional spatial configurations should produce advantageous properties such as new collective physical behavior. The synthesis of inorganic—organic supperlattices has been achieved using the multilayer casting of films. Asher et al. have developed a method for creating both organic and inorganic submicrometer periodic materials. Recently, in our laboratory spontaneous arrangements either in a monolayer organized in a hexagonal network or three-dimensional FCC arrangements of particles have been observed\. Similar arrangements have been reported\.
2. CONTROL OF THE PARTICLE SIZE Reverse micelles are well known to be spherical water in oil droplets stabilized by a monolayer of surfactant. The phase diagram of the surfactant sodium bis(2-ethylhexyl) sulfosuccinate, called Na(AOT), with water and isooctane shows a very large domain of water in oil droplets and often forms reverse micelles . The water pool diameter is related to the water content, w" [H O]/[AOT], of the droplet by D(nm)"0.3w. From the existing domain of water in oil droplets in the phase diagram, the droplet diameters vary from 0.5 to 18 nm. Reverse micelles are dynamic\ and attractive interactions between droplets take place. The intermicellar potential decreases either by decreasing the number of carbon atoms of the bulk solvent or by increasing the number of droplets. This is due to the discrete nature of solvent molecules and is attributed to the appearance of depletion forces between two micelles (the solvent is driven off between the two droplets). When the droplets are in contact forming a dimer, they exchange their water contents. This exchange process is associated with the interface rigidity which corresponds to the bending elastic modulus of the interface. Hence, in collisions, the droplets exchange their water contents and again form two independent droplets. This process has been used to make nanosized material by either chemical reduction of metallic ions or coprecipitation reactions. These various factors (water content, intermicellar potentials) control the size of the particles. Syntheses in reverse micelles induce formation of nanoparticles dispersed in the solution. This can be followed by measuring the absorption spectra of the colloidal particles dispersed in the solvent. The average size of the particles can be determined by depositing a drop of this solution on a carbon grid and obtaining the TEM patterns. Several types of material such as +CdS \ , Cd Zn S , ZnS, Cd Mn S , W \W W \W PbS, for (Ag) , (Cu) 18,19,50 and Co B alloys, have L L been used. As an example, the synthesis of copper metal particles is described. Reverse micelles are good candidates for making nanosized copper metal particles . To form this material, a functionalized surfactant such as copper bis(2-ethylhexyl)sulfosuccinate, Cu(AOT) , is needed.
When this is replaced by copper sulfate (Cu>), oxidized copper instead of copper metal particles are formed. Furthermore, a few minutes after starting the reaction, the particles flocculate. Mixed micelles made of sodium and copper bis(2-ethylhexyl)sulfosuccinate are prepared. The water content, w, is defined as the ratio of water over the sum of Na(AOT) and Cu(AOT) concentrations. The water content, w"[H O]/+[Na(AOT)]#[Cu(AOT) ],, is fixed at a given value. This solution is mixed with another Na(AOT) micellar solution in hydrazine, having the same water content. The copper bis(2-ethylhexyl)sulfosuccinate is reduced with the formation of copper metal particles, characterized by TEM, electron diffraction and absorption spectroscopy. Figure 1 shows an increase in the particle size and a marked change in the absorption spectrum with appearance of a plasmon peak with increasing water content. As expected, from expanded versions of Mie’s theory\, the absorption spectra vary with the particle sizes below 5 nm. The direct relationship between the particle size and the absorption spectrum allows obtaining a calibration curve relating the average particle diameter to its absorption.
3. CONTROLLING THE SHAPE OF THE PARTICLES Syntheses are performed either in self-assemblies having oil as the bulk phase and differing by their local arrangements or in dilute normal (oil in water) micelles. 3.1. Controlling the shape by using water in oil self-assemblies Copper ions have been reduced in colloidal assemblies differing by their structures . In all cases, copper metal particles are obtained. Figure 2 shows syntheses in various template structures. No oxide is detectable and the particles are single crystals. The colloidal system consists of 5;10\ M Cu(AOT) — isooctane—water. The colloidal structure is changed by increasing the amount of water in Cu(AOT) —isooctane solution . Syntheses are performed in various regions of the phase diagram. (i) At low water content from w"2 to 5.5, a homogeneous reverse micellar solution (the L -phase) is formed. In this range, the shape of the water droplets changes from spheres (below w"4) to cylinders. At w"4, the gyration radius has been determined by SAXS and found equal to 4 nm. Syntheses in isolated water in oil droplets show the formation of a relatively small amount of copper metallic particles. Most of the particles are spherical (87%) with a low percentage (13%) of cylinders (Figure 2A). The average size of the spherical particles is characterized by a diameter of 12 nm with a size polydispersity of 14%. (ii) The increase in the water content (5.6(w(11), destabilizes the solution and the L -phase separates into a more concentrated reverse micellar solution (L*) and an almost pure isooctane phase. Structural studies
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Colloidal self-assemblies of nanoparticles: M.P. Pileni
Figure 1 Absorption spectra and TEM pattern of copper metal particles with their sizes
indicate that L* is characterized by a bicontinuous net work of cylinders with an increase in the number of connections with increasing w. The persistence length of the cylinders measured by SAXS does not change (1l2" 3 nm) with increasing w. Syntheses at w"6 show (Figure 2B) the formation of a relatively large amount of copper metal cylinders (32%) in coexistence with 68% spheres. The average diameter of the spherical particles is 9.5 nm. The size distribution is rather large (27%). The average length over width ratio of the cylinder is found to be 3.5 with 40% polydispersity. The length and width of the cylinders are equal to 22.6$5.4 and 6.7$1.4 nm, respectively. (iii) At the phase transition (w"11), a L -phase made ? up of a mixture of planar lamellae and spherulites appears. Syntheses performed in this region of the phase diagram show the formation of a very large amount of cylinders in coexistence with spheres. Very few large rods can be observed (Figure 2C). Their sizes vary from 0.1 to 1 lm. In this region of the phase diagram, a slight increase in the number of cylinders is observed (38% of the particles are cylindrical whereas 62% are spherical). The average diameter of the spherical particles is equal to 10.9 nm with 17% of polydispersity in size distribution. The length and width of the cylinders are 25$4 and 7.3$1.4 nm, respectively. A slight increase in the size
Figure 2 TEM patterns of metallic copper particles obtained in Cu(AOT) —isooctane solution at various water content: w"4, (A), w"6 (B), w"12, (C), w"18 (D), w"34 (E) and w"44 (F), [Cu(AOT) ]"5;10\M
distribution is seen. The average length to width ratio of the cylinders is 3.7 with 44% polydispersity. (iv) From w"15 to w"19, spherulites remain in equilibrium with isooctane. The spherulite size differs markedly (from 100 to 8000 nm). Syntheses in this phase region (15(w(20) show the formation of particles having a higher polydispersity in size and in shape (Figure 2D) than those observed at low water content. As a matter of fact, Figure 2D show the formation of triangles, squares, cylinders and spheres. (v) At w"20, an isotropic phase appears. By increasing the water content w from 20 to 29, the lamellar phase progressively disappears so as to reach two phases consisting of isooctane and the isotropic phase. The latter is attributed to a mixture of a sponge and interconnected cylinder phases. Syntheses at various water contents w from 21 to 29, induce formation of roughly 10% cylinders and 90% spheres. The percentage of cylinders decreases with increasing the water content from 13 to 7%, whereas the cylinder length and width remain unchanged. These values are found to be 19.0$2.5 and
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Colloidal self-assemblies of nanoparticles: M.P. Pileni 6.7$0.8 nm, respectively. The diameter of the spheres slightly decreases and the polydispersity increases with increasing water content from 9.7$0.7 to 8.1$1.2 nm at w"22 and w"28, respectively. (vi) At w"30, the isotropic phase remains in equilibrium with isooctane and is attributed to the L*-phase similar to that obtained at lower water content (in the range of 5.6—11). By increasing the water content w from 30 to 35, the interconnected cylindrical network is diluted with a decrease in the number of connections. Over all this water content range, the formation of spherical and cylindrical copper metallic particles are observed. Figure 2E shows a TEM pattern obtained at w"34. As at lower water content, cylindrical (42%) and spherical (58%) nanoparticles are observed. The average diameter of spherical particles observed in most of the cases is 9.5$0.9 nm. The length and width of the cylinders are 19.8$2.7 and 6.5$0.8 nm, respectively. (vii) At w"35, an isotropic solution formed by water in oil droplets is obtained. On increasing the water content w from 35 to 40, no drastic changes in the particle sizes and in the percentage of cylinders are observed. Figure 2F shows the TEM pattern obtained at w"40. The particle size is 7.5$0.6 nm. From these data it is concluded that the size, shape and size distribution of nanoparticles depend critically on the colloidal structure in which the synthesis is performed. This is well demonstrated when, by changing the water content, similar colloidal structures (reverse micelles or interconnected cylinders) are obtained: (i) Reverse micelles are formed below w"5.5 and above w"35. Most of the copper metal particles obtained in this region of the phase diagram are spherical (Figure 2A and F). The percentage of spheres varies from 86 to 93%. At low water content (w"4), the diameter of the particles is larger than that obtained at higher water content (w'35). Hence, a slight decrease in the particle diameter with increasing water content is observed. In the reverse micellar regions, the length and width of the cylinders are larger at low water content and decrease with increasing w. Furthermore, the number of particles formed markedly increases with increasing water content. These differences could be mainly attributed to hydration of the head polar group. (ii) Interconnected cylinders (L*) are formed in two water content range (5.5(w(11 and 30(w(35). Syntheses in these two regions of the phase diagram show very strong correlation and similar data. Spherical and cylindrical particles are formed in both cases. No other particle shapes have been observed. In both regions, the average diameter of spherical particles is the same (9.5 nm). Similarly, the size of the cylinders remains identical with an average length and width of 21.2 and 6.6 nm, respectively. The same average diameter and same ratio of cylindrical axis (O3.3) are observed at low (5.5(w(11) and high (30(w(35) water content. Because of this large similarity in various experimental conditions, this phenomenon is attributed to the structure of the colloid used as a template. This control of the particle shape could be explained as in nature where
the key step in the control of mineralization is the initial isolation of a space. As in the present case, a local supersaturation of reactants is needed to induce nucleation and let the system reach a lower-energy state. In such local supersaturation regime, the chiral molecules used to form the colloidal template (Cu(AOT) ) impose an orientation of each reactant. The control of the crystal morphology could be due to the presence of high surfactant concentration (or compounds resulting from the chemical reaction) which specifically adsorbs on certain crystal faces. The same concept has been used to control the crystal morphology by the adsorption of impurities from solution onto specific crystal surfaces. Mechanisms of inhibition involved in the control of crystal morphology have been elucidated at the molecular level in studies on organic crystals and tailor-made inhibitors . 3.2. Controlling the shape by using oil in water self-assemblies Copper dodecyl sulfate is made by mixing an aqueous solution of sodium dodecyl sulfate with copper sulfate, as described elsewhere. An aqueous solution of 0.1 M sodium docedylsulfate is mixed with 0.1 M copper sulfate. The solution is kept at 2°C and the precipitate which appears is washed several times with a 0.1 M copper sulfate solution and recrystallized in distilled water. Copper dodecyl sulfate, Cu(DS) , forms micellar aggregates above the critical micellar concentration (c.m.c.). The shape and size of these aggregates have been determined by SAXS and by light scattering and are found to be prolate ellipsoidal micelles with a hydrodynamic radius of 2.7 nm. The micellar size and shape are in good agreement with that observed with Cd(DS) . Copper(II)dodecylsulfate, Cu(DS) is reduced by so dium borohydride, NaBH with a NaBH /Cu(DS) ratio of 2 and copper metal aggregates are obtained for any of the experimental conditions described below. The synthesis is performed at the Cu(DS) critical micellar concentration, c.m.c."1.2;10\M. The TEM measurements made using a drop of this solution deposited on a carbon grid show the formation of large domains of aggregates arranged in an interconnected network (Figure 3A). Enhancement of this aggregate shows that the interconnected network corresponds to a change in the particle shape and not to the aggregation of strongly interacting small particles (Figure 3B). Comparison of the absorption spectra for different shaped particles and spherical particles with 10 nm diameter, shows a red shift in the plasmon peak when the shape of the copper metal particles changes from spheres to rods (Figure 4). Such changes in the absorption spectra of these particles can be related to those predicted at various r values, where r is defined as the ratio of the length to diameter of a cylinder\. The plasmon peak due to the rod particles is centered at 570 nm (Figure 4). According to simulated absorption spectra, this peak corresponds to an r value of 2.5. From image analysis of the skeleton of the interconnected network corresponding to a plasmon peak centered at 570 nm, the average length of
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Colloidal self-assemblies of nanoparticles: M.P. Pileni particle shape is in good agreement with those predicted. Thus, the shift and the ripening in the absorption band, compared to spherical and elongated particles indicate that the interconnected network, observed by TEM, exists in solution. It is due to a change in the effective mean free path of conduction electrons and not evaporation of the solutions.
4. SELF-ORGANIZATION IN 2D AND 3D STRUCTURES
Figure 3 Transmission electron microscopy of the copper network prepared in a pure copper dodecyl sulfate solution: [Cu(DS) ]"1.2 ;10\ M, [NaBH ]"2.4;10\ M. A and B correspond to various scales of the TEM pattern
Figure 4 Absorption spectra of interconnected (22), elongated (*) and spherical (E) colloidal particles
linear strands is 22 nm. The average minor diameter is 6.5 nm. Hence, the r value is 3.3. From the theoretical predictions such an r value (r"3.5) corresponds to a higher shift in the plasmon peak (620 nm) than that obtained (570 nm). This difference between r and the maximum of the plasmon peak is explained by the fact that the simulation is related to one size of cylindrical particles. In the present experiment, the particles are interconnected and not isolated cylinders. Furthermore, the polydispersity has to be taken into account. A qualitative shift in the maximum of the plasmon peak with the
The procedure to fabricate colloidal silver, (Ag) , spheriL cal nanoparticles is similar to that described above (see Section 2): The Cu(AOT) is replaced by the silver deriva tive. The relative concentration of Na(AOT)], Ag(AOT) and the reducing agent remain the same. Control of the particle size is obtained from 2 to 6 nm. To stabilize the particles and to prevent their growth, 1 ll ml\ of pure dodecanethiol is added to the reverse micellar system containing the particles. This induces a selective reaction at the interface, with covalent attachment, between thio derivatives and silver atoms. The micellar solution is evaporated at 60°C and a solid mixture of dodecanethiol-coated nanoparticles and surfactant is obtained. To remove the AOT and excess dodecanethiol surfactant a large amount of ethanol is added and the particles are dried and dispersed in heptane. A slight size selection occurs and the size distribution drops from 43 to 37%. The size distribution is reduced through the size-selected precipitation (SSP) technique. A drop of a dilute solution of 4.5 nm coated particles in hexane ([(Ag) ]"2;10\ M) is deposited on a graphite L surface. Figure 5A shows a large area of closed packed monolayers made of nanoparticles organized in a hexagonal network. The optical spectrum of the nanosized particles coated on graphite support is recorded by reflectivity mode and is compared with the spectrum corresponding to free particles in hexane. The optical spectra are normalized to unity. The plasmon peak is shifted towards an energy lower than that obtained in solution (Figure 5B). The coverage support is washed with hexane and the nanoparticles are redispersed in the solvent. The absorption spectrum of the latter solution is similar to that used to cover the support (free particles in hexane). This clearly indicates that the shift in the absorption spectrum of nanosized silver particles is due to their self-organization on the support. The bandwidth of the plasmon peak (1.3 eV) obtained after deposition is larger than that in solution (0.9 eV). This can be attributed to a change in the dielectric constant of the composite medium. Taking into account the variation of the simulated plasmon peak with the dielectric constant, it is concluded that the shift toward lower energy observed is due to an increase in the dielectric constant of the composite medium of the particles which is the superposition of several factors as the spherical particles, the support, particle—particle interactions and the air. Similar behavior is observed for a diluted solution made of 5.2 and 6.2 nm nanosized particles.
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Figure 5 (A) TEM micrograph of 4.5 nm silver particles deposited on graphite. (B) Absorption spectra of free 4.5 nm silver nanoparticles dispersed in hexane before (black) leaving a drop on the support, after wasking the support with hexane (green), and deposition on the surport forming a 2D superlattices (red). ([(Ag) ]"2;10\M) L
When the particles are dispersed in hexane until complete evaporation of the solvent, it results in the formation of large aggregates. Figure 6A shows a rather high aggregate orientation around a large hole or ring. The average distance between the oriented aggregates varies from 20 to 60 nm. The aggregate magnification confirms that it consists of (Ag) nanoparticles and its average size L is in the range of 0.03—0.55 mm. High magnification of one of these aggregates shows that the particles are arranged in two different symmetries. Figure 6B shows the formation of a polycrystal. Its magnification (Figure 6C) shows either a hexagonal or a cubic arrangement of nanoparticles. The transition from one structure to another is abrupt and there is a strong analogy with ‘‘atomic’’ polycrystals with a small grain called nanocrystals. Each domain or grain has a different orientation clearly showing that the stacking of nanoparticles is periodic and not random. The ‘‘pseudo-hexagonal’’ structure corresponds to the stacking of a +110, plane of the f.c.c. structure. A fourfold symmetry is observed on the same pattern (Figure 6C) which is again characteristic of the stacking of +001, planes of the cubic structure. This cannot be found in the hexagonal structure. As a matter of fact, there is no direction in a perfect hexagonal compact structure for which the projected positions of the particles can take this configuration. This is confirmed by TEM experiments performed at various tilt angles where it is always possible to find an orientation for which the stacking appears to be periodic. Hence, by tilting a sample having a pseudo-hexagonal structure, a fourfold symmetry is obtained. From these data, it is concluded
Figure 6 Progressive enhancement of (Ag°) aggregates L
that the large aggregates of silver particles are formed by stacking of monolayers in a face centered cubic arrangement. Similar self assemblies of silver sulfide\, silver and gold nanoparticles have been obtained by using different experimental modes\. By increasing the concentration of 4.5 nm nanoparticle in hexane solution ([(Ag) ]"4;10\ M), the aggreL gates are so large that they cannot be investigated by TEM. By scattering electron microscopy, aggregates differing by their size and shape are observed. They are well-faceted, as observed for the smaller aggregate by TEM. We choose to investigate one of these aggregates. Enhancement of one face of the aggregate shows welldefined defects in the crystal phase. The 60° tilt of aggregate confirms a 3D structure (Figure 7A). This permits to estimate the size of the aggregate, which is 200 lm high and 100 lm large. Figure 7A shows the presence of smaller aggregates surrounding the larger one. Furthermore, layers at the bottom of the aggregate are observed. To make sure that these aggregates are made of silver nanoparticles, EDX analyses were performed on the top of the aggregate and on the bottom (in the layers region). The analysis is made on a 0.786 lm volume. The
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Colloidal self-assemblies of nanoparticles: M.P. Pileni
Figure 7 (A) Scanning electron microscopy on concentrated solution of 4.5 nm silver particles ([(Ag) ]"4;10\M). Large aggregates on silver multilayers are present. (B) Absorption spectra of free 4.5 nm silver nanoparticles dispersed in hexane before (black) leaving a drop on the support, after washing the support with hexane (green), and deposition on the support forming 3D superlattices (blue) ([(Ag) ] L "4;10\ M)
aggregate consists of silver, carbon and sulfur atoms. This is fully consistent with a crystal made of silver nanosized particles coated by dodecanethiol. The UVVisible spectrum (Figure 7B) of the aggregates described above shows a 0.25 eV shift toward lower energy of the plasmon peak with a slight decrease in the bandwidth (0.8 eV) compared to that observed in solution (0.9 eV). As observed above with monolayer, by washing the support, the particles are redispersed in hexane and the absorption spectrum remains similar to that of the colloidal solution used to make the self-assemblies. For isolated particles, the increase in the dielectric constant induces a shift to the lower energy and an increase in the bandwidth of the plasmon peak. For particles organized in a FCC structure, each silver nanoparticle is surrounded by 12 other particles, whereas in a monolayer it has six neighbors. So in 3D superlattices, the dielectric constant is the superposition of several factors as the external field, the dipole fields of all the other particles and the surrounding due to the support. This induces an increase in the total dielectric constant which, as for isolated particles, induces a shift toward the low energy of the plasmon peak. This is confirmed by comparing Figures 6 and 7: The shift of the plasmon peak of particles arranged in 2D and 3D superlattice
compared with the spectrum corresponding to free coated particles in hexane is 0.12 and 0.25 eV, respectively. Hence, the increase in the total dielectric constant induces an increase in the plasmon peak bandwidth. The bandwidth is of 0.8 eV, whereas for particles organized in 2D (Figure 6) it is 1.3 eV. The value obtained for a 3D superlattice (0.8 eV) is close to that observed for free nanoparticles in hexane (0.9 eV). These data show the effect of the medium dielectric constant of the particle when organized in 2D and 3D. The decrease in the bandwidth plasmon peak could be due to an increase in the mean free path conduction electrons of silver particles through a barrier of 2 nm. This is rather suprising, because the average distance between silver nanoparticles is 2 nm and we would not expect a tunneling electron effect through such a large barrier. However, a recent paper published by Ung et al. claims that a 1—2 nm distance between two metal surfaces is enough for tunneling of electrons across the double layers. Because of the fact we keep the same absorption spectrum and TEM pattern after washing the support; fusion between particles during the coverage can be excluded. No predictions have been given in literature on the variation of the electromagnetic coupling of the particles when they are organized in 2D with a hexagonal network and 3D superlattice with a FCC structure. This does not permit us to definitively conclude on a collective effect, due to the transport of the conduction electrons through the barrier due to the coating or due to changing in the dielectric medium.
5. MAGNETIC FLUID OF COBALT FERRITE NANOSIZED PARTICLES By using oil in water micelles cobalt ferrite nanoparticles are formed and the particle size is controlled. Syntheses of CoFe O particles have been extensively studied in homogeneous solutions. The procedure used is the following: very high ('2 M) concentrations of Fe(II), Fe(III) and Co(II) salts are dissolved in aqueous solution. A base is added and the solution is heated, during 1 h, at 100°C. The precipitate that appears is washed and the magnetic particles are dispersed in aqueous solution. This is a magnetic fluid. Syntheses of cobalt ferrite nanosized particles need specific experimental conditions: (i) high reactant concentrations, (ii) presence of Fe(III) ions, and (iii) high temperature (100°C). The particle size is controlled by drastic changes in pH, ionic strength and addition of polymers or surfactants. These chemical changes strongly perturb the surface of the particles (with adsorption of macromolecules or hydroxides, etc.). As it has been well demonstrated, changes in the surface of the particle induces changes in their magnetic properties. So the use of this procedure makes rather difficult to derive consistent relationship between particle sizes and magnetic properties. By using oil in water micelles made of functionalized surfactants, the reactant concentrations are lower than those usually used (almost two orders of magnitude). At
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Colloidal self-assemblies of nanoparticles: M.P. Pileni room temperature, addition of base to the Co(DS) and Fe(DS) mixed micelles induces formation of a precipi tate made of stoichiometric CoFe O nanoparticles . The magnetic fluid is formed by washing this precipitate. Its characterization indicates the formation of CoFe O with an inverse spinel structure as in the bulk phase. At the starting point, this reaction does not need any Fe(III) ions. Hence, in micelles, the synthesis is performed at Co(DS) and Fe(DS) concentrations in the 10\ M range, whereas in aqueous solution, the reaction does not take place at such low concentrations. An increase in concentration by three orders of magnitude is needed. This could be attributed to the fact that formation of micelles induces high local concentration of the reactant at the interface (Co> and Fe>) and then favors the chemical reaction. No obvious explanation can be given for the formation of CoFe O in micelles at room tem perature whereas the solution needs to be heated at 100°C in homogeneous solution. As said above, in homogeneous solution, the particle size is controlled by drastic changes in the experimental conditions which induces large perturbations in the magnetic properties of the nanoparticles. In micellar solution, the particle size is controlled by keeping the ratio of reactants ([Fe(DS) ]/[CH NH OH] and [Fe(DS) ]/ [Co(DS) ] constant and by increasing the Fe(DS) con centration by a factor of 4(6.5;10\M([Fe(DS) ]( 1.2;10\M) indicate an increase in the particle size from 2 to 5 nm. This control of the particle size is rather surprising. A possible partial explaination is given by XANES data obtained at the same concentrations as in the syntheses. An increase in the oxidation numbers of Co and Fe ions when the Fe(DS) concentration in creases is derived from these data. This could increase the number of nuclei formed and then induce an increase in the particle size. Furthermore, the amount of oxygen dissolved in the solution depends on the micellar concentration. This could play a role in controlling the particle size. Controlling in the particle size does not need large changes in the experimental conditions. This allows, to a first approximation, to assume that the surface of the particle remains unchanged. Hence, it is possible to find a relationship between size and magnetic properties. The study of the magnetic properties of CoFe O nanosized particles shows, from magnetization curves and Mo¨ssbauer spectroscopy, an increase in the anisotropy with a decrease in the particle size. For particles having an average size of 5nm, the anisotropy remains cubic as in the bulk phase. Conversely, when the size of the particle decreases from 5 to 3 or 2 nm, the anisotropy of the nanomaterial changes from cubic to axial. Similar behavior has been observed with Fe O and c-Fe O nanosized particles. The chemical reaction takes place at low reactant concentration, room temperature and in the absence of Fe(III) ions. In homogeneous solutions, syntheses in the absence of Fe(III) ions induce the formation of very well-defined particles in the micrometer range.
ACKNOWLEDGMENTS I would like to sincerely thank my coworkers who have been in charge of these studies. Special thanks are due to A. Filakembo, N. Feltin, Dr I. Lisiecki, Dr N. Moumen, Dr C. Petit, A. Taleb and J. Tanori who participated in this work.
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