Control of the Shape of Copper Metallic Particles by Using a

Langmuir 1997, 13, 639-646

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Control of the Shape of Copper Metallic Particles by Using a Colloidal System as Template J. Tanori and M. P. Pileni* Laboratoire S.R.S.I., U.R.A.C.N.R.S. 1662, Universite´ P. et M. Curie (Paris VI), B.P. 52, 4 Place Jussieu, F-75231 Paris Cedex 05, France, and C.E.A.-C.E.N. Saclay, DRECAM-S.C.M.. F-91191 Gif-sur-Yvette, Cedex, France Received June 19, 1996. In Final Form: September 26, 1996X In this paper we show that the use of colloidal assemblies as templates favors the control of the shape of nanoparticles. Cylindrical copper metallic particles having same size can be obtained in various parts of the phase diagram when the template is made of interconnected cylinders. A very low amount of cylinders (13%) is formed when the synthesis is performed in cylindrical reverse micelles. When the colloidal self-assembly is a mixture of several phases, various types of shapes can be obtained. In some cases, the polydispersity in size is so low that metallic particles are able to self-assemble in a hexagonal network. Multilayers can be observed and are arranged in a face centered cubic structure.

1. Introduction Nature has taken advantage of the rich liquid-crystalline behavior of amphiphilic liquids to create ordered yet fluid biomembrane structures and to modulate dynamic processes in cells.1,2 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.3 The space is usually delineated by cellular membranes, vesicles, or predeposited macromolecular matrix frameworks. Filling up these spaces with amorphous minerals would appear to require a quite different strategy as compared to filling space 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 state of lower energy by crystallization, while at the same time removing the excess solvent. This situation is observed in spherulites of calcium carbonate which spontaneously form in metastable supersaturated solutions. “Droplets” of calcium carbonate and water separate from solution and with time, or upon drying, crystallize to lead to spherulites.4 In terms of growth of particles, some analogies between surfactant self-assemblies and natural media can be proposed. In both cases, the growth of particles needs a supersaturated media 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. Recently, selforganizations have been used to make calcium carbonate material.5,6 In solution, surfactant molecules self assemble to form aggregates.7 At low concentration, the aggregates are generally globular micelles, but these micelles can grow upon an increase of surfactant concentration and/or upon addition of salt, alcohols, etc. In that case, micelles have been shown to grow to elongated more or less flexible * All correspondence to this author. X Abstract published in Advance ACS Abstracts, December 1, 1996. (1) Addadi, L.; Weiner, S. Angew. Chem., Int. Ed. Engl. 1992, 31, 153. (2) Mann, S. In Inorganic Materials; Bruce, D. W., O’Hare, D., Eds., J. Wiley: New York, 1992; p 238. (3) Simkiss, Biomineralization in Lower Plants and Animal; Leadbeat, B. S. C., Riding, R., Eds.; p 19. (4) Lengyel, E. V. Z. Kristallogr. 1937, 97, 67. (5) Waish, D.; Hopwood, J. D.; Mann, S. Science 1994, 264, 1576. (6) Waish, D.; Mann, S. Nature 1995, 377, 320. (7) Larson, R. G. Rheol. Acta 1992, 31, 497.

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rodlike micelles8-15 in agreement with theoretical prediction on micellization.16,17 In previous papers we demonstrated that reverse micelles can be used to control the size of copper metallic particles. The increase in the water content induces an increase in the particle size which varies from 1 to 12 nm.18 The control of the particle size is regulated by the intermicellar potential between droplets.19 When spherical water-in-oil droplets are replaced by cylindrical droplets, a small amount of cylinders is obtained. The size of the cylinders depends on the number of droplets in which the chemical reaction takes place.18 The addition of a very small amount of surfactant (cetyltrimethylammonium chloride) having a charge opposite from that of the surfactant used to form reverse micelles (AOT), induces drastic changes in the particle diameter and in the amount of material formed.20 Whatever the system is, the largest size of spherical particles made in reverse micelles reaches the value of 10-12 nm. Rods can be observed in lamellar phases.21 In the present paper we show that, as in nature, the shape of the particles can be partially controlled by the shape of the template used. It is demonstrated that the shape and the number of copper metallic particles strongly depend on the colloidal structure in which the chemical reduction of Cu(II) takes place. Self-organization of copper metallic particles can be obtained from syntheses in colloidal assemblies. (8) Chen S. J.; Evans, D. F.; Ninham, B. W.; Mitchell, D. J.; Blum, F. D.; Pickup, S. J. Phys.Chem. 1986, 90, 842. (9) Evans, D. F.; Mitchell, D. J.; Ninham, B. W. J. Phys. Chem. 1986, 90, 2817. (10) Barnes, I. S.; Hyde, S. T.; Ninham, B. W.; Deerian, P. J.; Drifford, M.; Zemb, T. N. J. Phys. Chem. 1988, 92, 2286. (11) Mazer, N.; Benedek, G.; Carey, M. C. J. Phys. Chem. 1976, 80, 1075. (12) Blankschtein, D.; Thurston, G. M.; Benedek, G. Phys. Rev. Lett. 1986, 85, 7268. (13) Porte, G.; Appell, J.; Poggi, Y. J. Phys. Chem. 1980, 84, 3105. (14) Hoffmann, H.; Platz, G.; Ulbricht, W. J. Phys. Chem. 1981, 85, 3160. (15) Mishic, J. R.; Fisch, M. R. J. Chem. Phys. 1990, 92, 3222. (16) Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1981, 77, 601. (17) Safran, S. A.; Turkevich, L. A.; Pincus, P. A. J. Phys. Lett. 1984, 45, L69. (18) Lisieki, I.; Pileni, M. P. J. Am. Chem. Soc. 1993, 115, 3887. (19) Lisieki, I.; Pileni, M. P. J. Phys. Chem. 1995, 99, 5077. (20) Lisieki, I.; Borjling, M.; Motte, L.; Ninham, B. W.; Pileni, M. P. Langmuir 1995, 11, 2385. (21) Tanori, J.; Pileni, M. P. Adv. Mater. 1995, 7, 862.

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2. Experimental Section 2.1. Compounds. Copper(II) bis(2-ethylhexyl)sulfosuccinate (Cu(AOT)2) has been described previously.22 Water contents solubilized in Cu(AOT)2 in isooctane solution are analyzed by Karl Fischer titration using a Mettler automatic titrator. The concentrations of Cu(AOT)2 are determined by adding a sample to a ≈0.03 M hydrochloric acid, ≈0.3 M ammonium acetate solution and subsequently titrating for copper(II) using 0.01 M sodium EDTA with 4-(2-pyridylazo)resorcinol as indicator. Isooctane was supplied by Fluka (99.5% puriss), hydrazine (100%) by Merck, ammonium acetate (98%), sodium EDTA, and 4-(2pyridylazo) resorcinol (99%) by Prolabo. Single distilled water was passed through a Millipore MilliQ system cartridge until its resistivity reached 18 MΩ cm. All chemicals were used without further purification. 2.2. Apparatus. Electron micrographs are obtained with a JEOL electron microscope (Model Jem.100 CX.2). 2.3. Histograms. Histograms are obtained by measuring the diameter Di of all the particles from different parts of the grid (magnification 160 000) for an average number of particles close to 500. The standard deviation, σ, is calculated from the following equation:

∑[n (D - D) ]/[N - 1]}

σ){

2

i

1/2

i

where D and N are the average diameter and the number of particles, respectively.

3. Syntheses and Characterization of Copper Metallic Particles The copper particles are prepared by reduction of the Cu(AOT)2 in isooctane solution containing various amounts of water molecules. Hydrazine is used as the reducing agent. The reaction takes place under N2 atmosphere. It starts immediately after hydrazine addition to Cu(AOT)2 in isooctane. The reaction takes 3 h. The ratio of Cu(AOT)2 to hydrazine is kept equal to 1/3. The water content, w, is defined as the ratio of [H2O]/[AOT]. The concentration of Cu(AOT)2 is kept constant and equal to 5 × 10-2 M. At the end of the reaction a drop of the solution is placed on a carbon film supported by a copper grid and the sample is examined by transmission electron microscopy (TEM) and electron diffraction (ED). The electron diffraction shows concentrical circles characteristic of a face-centered cubic phase with a lattice dimension equal to 3.61 Å as it is observed with bulk copper metallic material. 4. Phase Diagram Progression The progression of the phase diagram of 5 × 10-2 M Cu(AOT)2-isooctane with increasing the water content is extensively described in a full article.23 Figure 1 summarize the phase diagram with increasing water content. The overall concentration of Cu(AOT)2 remains constant. However, phase transitions take place and the amount of Cu(AOT)2 differs from one phase to the other. At low water contents (1 < w < 5.6) a homogeneous reverse micellar solution (the L2 phase) is formed. In this range the shape of the 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. The increase of the water content, (5.6 < w < 11), destabilizes the solution, and the L2 phase separates into a more concentrated reverse micellar solution (L2*) and an almost pure isooctane phase. Structural studies indicate that L2* is characterized by a bicontinuous network of cylinders with an increase in the number of (22) Petit, C.; Lixon, P.; Pileni, M. P. J. Phys. Chem. 1990, 94, 1598. (23) Tanori, J.; Gulik, T.; Pileni, M. P. Langmuir, previous paper in this issue.

connections with increasing w. The persistence length of the cylinders measured by SAXS does not change (〈l〉 ) 3 nm) with the increase of w. Upon further addition of water (11 < w < 15), the coacervate becomes more and more concentrated, and subsequently a turbid, birefringent, lamellar phase (LR) starts to coexist with the L2* and isooctane phases. Formation in equilibrium of planar lamellar phase and spherulites has been observed. As more water is added, the proportion of spherulites increases and finally they coexist alone with isooctane (15 < w < 20). Further addition of water molecules leads to appearance of a probable mixture of a L3 (sponge) and a L2* (interconnected cylinder) phases in coexistence with the LR and isooctane phases (20 < w < 30). The increase in the water content from w ) 20 to 30 induces a progressive disappearance of LR phase. Above w ) 30, the L2* is in equilibrium with isooctane. The increase of the water content induces an increase in the volume of the L2* phase and a decrease in the isooctane volume. This corresponds to a dilution of the interconnected cylinders. At w ) 35, the upper phase totally disappears and remains an isotropic region attributed to water-in-oil droplets. 5. Results Syntheses are performed in the various parts of the phase diagram described above. Most of the syntheses have been performed by increasing the water content by an unit. However, when the results obtained were similar, we choose to give only one example. At low water content from w)2 to 5.5 formation of isolated water-in-oil droplets, usually called the L2 phase, is observed. The shape of the droplets evolves from spheres (w ) 3) to cylinders (w > 4, Figure 1). Syntheses performed in this region of the phase diagram show formation of a relatively small amount of copper metallic particles. Most of the particles are spherical (87%) with a very few percentage (13%) of cylinders (Figure 2A). The average size of spherical particles is characterized by a diameter equal to 12 nm (Figure 2B) with a polydispersity in size equal to 14%. Histogram performed on the cylinders shows that the average ratio of the length over the width of the particle is equal to 2.3. The length and width of the cylinders are 18.5 ( 2.2 nm and 8.2 ( 0.6 nm. Spherical particles characterized by same size self assemble on the TEM grid as shown on Figure 2A. By increasing the water content from 5.6 to 11, a phase transition takes place with the formation of an interconnected network of cylinders (Figure1). Syntheses performed at w ) 6 show (Figure 3) formation of a relatively large amount of copper metallic cylinders (32%) in coexistence with 68% of spheres. The average diameter of the spherical particles is 9.5 nm, and a rather large polydispersity (27%) is obtained. A histogram shows the formation of large cylinders (Figure 3C). The average length over width ratio of the cylinder is found equal to 3.5 with 40% in polydispersity. The length and width of the cylinders are equal to 22.6 ( 5.4 and 6.7 ( 1.4 nm respectively. Because the number of connection and then the local concentration increases by increasing the water content, syntheses performed at water content higher than w ) 6 (w ) 8, 10) induce formation of a too large amount of material which makes them impossible to be observed by TEM. At the phase transition (w ) 11), a LR phase composed by 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. Figure 4A shows the TEM

Shape of Copper Metallic Particles

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Figure 1. Evolution of the phase diagram with the increasing in the water content, w, and keeping Cu(AOT)2 concentration constant. [Cu(AOT)2] ) 5 × 10-2 M.

pattern obtained at w ) 12. 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 nm and 7.3 ( 1.4 nm, respectively. From the histogram given in Figure 4C is observed a slight increase in the size distribution. The average length over width ratio of the cylinders is equal to 3.7 with 44% in polydispersity. In some regions of the carbon grid, the particles having similar sizes self-assemble (insert Figure 5). The average distance between particles is kept constant and equal to 1 nm. High-resolution of such self-

assembly (Figure 5) shows that the interlattice distance is similar to that obtained for the 111 phase in bulk material (3.6 Å). This indicates the high crystallinity of the material synthetized in self-organized assembly. From w ) 15 to w ) 19, spherulites remain in equilibrium with isooctane (Figure 1D). The size of the spherulites strongly differs (from 100 to 8000 nm). Syntheses performed in this phase region (15 < w < 20) show the formation of particles having a higher polydispersity in size and in shape (Figure 6A) than those observed at low water content. As a matter of fact, Figure 6 shows the formation of triangles, squares, cylinders, and spheres. However, in some regions of the carbon grid self-assemblies made of spherical particles are observed

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Figure 2. TEM pattern obtained after synthesis at w ) 4, [Cu(AOT)2] ) 5 × 10-2 M (A), histogram of diameter of spheres (B), and ratio of the length over the width of cylinders (C).

Tanori and Pileni

Figure 4. TEM pattern obtained after synthesis at w )12, [Cu(AOT)2] ) 5 × 10-2 M (A), histogram of diameter of spheres (B), and ratio of the length over the width of cylinders (C).


Figure 5. High resolution of self-assembly made of cylindrical copper metallic particles. (insert self assemblies). w ) 12.

(insert in Figure 6A, 24% of particles are characterized by a spheroid shape). The average diameter of the spherical particles, is equal to 10.4 nm with a size distribution equal to 31%. Because of the strong change in the shape of the particles, it is difficult to produce histograms. The size distribution has been measured as the following: triangular and tetrahedrical particles have been assimilated to spheres. When the difference in length

and width of particles were higher than 3 nm, it has been assumed that the particles were cylinders. Table 1 confirms the very high polydispersity in size and shape. At w ) 20, an isotropic phase appears. By increasing the water content from w ) 20-29, the lamellar phase progressively disappears to reach two phases made of isooctane and the isotropic phase. The latter is attributed to a mixture between a sponge and interconnected cylinder phases.


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Table 1. Variation with the Water Content, w, of the Average Diameter of Sphere, 〈ds〉, Polydispersity of Spheres,σs, the Percentage of Spheres, %s, the Percentage of Cylinders, %c, the Percentage Expressed in Weight of Copper Metallic Cylindrical Particles, %c (Weight), Average Length, 〈Lc〉, and width, 〈lc〉, of the Cylinders, Polydispersity in the Average Length, σLc (%), and Width σlc (%), of the Cylinders, the Average Ratio of the Cylinder Axis, 〈Lc/lc〉, and Polydispersity in This Ratio, σLc/lc w 〈ds〉 (nm) ss (%) %s %c %c (weight) 〈Lc〉 (nm) sLc (%) 〈lc〉 (nm) slc (%) 〈Lc/lc〉 sLc/lc (%)

4 12.3 13 87 13 13 18.5 23.5 8.2 16 2.3 43

6 9.5 27 68 32 45.5 22.6 24 6.7 21 3.5 40

12 10,9 17 62 38 48.6 25 32 7.3 29 3.7 44

18 10.4 31 86 14 44.7 20.7 34 13.4 57 1.9 51

20 7.8 11 97 3 4.7 10.9 10.9 6.8 16 1.7 31

24 9.4 26 87 13 19.9 19.3 29.8 6.9 34 2.9 35

26 9.2 22 91 9 13.8 17.1 25 7.0 22 2.5 26

34 9.5 19 58 42 51.4 19.8 27 6.5 24 3.2 33

38 8.6 22 86 14 28.2 16.8 27 7.8 29 2.4 39

40 7.5 16 93 7 12.3 12.0 21 6.6 17 1.9 32

44 7.4 19 86 14 28 12.8 22 7.1 17 1.9 37


Syntheses performed at w ) 20 show very surprising data. Observed is the formation of spherical particles having an average diameter equal to 7.8 nm and characterized by a very low polydispersity in size (11%). Because of that, the particles self-arrange with a very high organization. In all the various parts of the TEM patterns mono- and multilayers made of spherical particles are observed (Figure 7). Figure 7A shows monolayers made of 7.8 nm spherical particles. They are arranged in a hexagonal network with an average distance between particles equals to 11.00 nm. In some regions several layers can be observed (Figure 7B). Figure 7B shows addition of a second layer and a third layer. This of course drastically changes the contrast. This is attributed to packing of hexagonal layers in a face centered cubic structure. At such water content (w ) 20), only 3% of cylinders are formed. They are characterized by a length and width equal to 10.9 ( 1.7 and 6.8 ( 1.0 nm respectively. This result is rather surprising. As matter of fact, below and above w ) 20, a large polydispersity in size and shape are observed (Table 1). Syntheses, performed at various water content from w

Figure 7. TEM pattern obtained after synthesis at w ) 20, [Cu(AOT)2] ) 5 × 10-2 M. On the same carbon grid, monolayers (A) are in coexistence with self-assemblies of nanoparticles (B).

) 21 to 29, induce formation of roughly 10% of cylinders and 90% of spheres. The percentage of cylinders decreases with increasing the water content from 13 to 7%, whereas the length and the width of the cylinders remain unchanged. The length and width of the cylinders are found equal to 19.0 ( 2.5 and 6.7 ( 0.8 nm, respectively. The diameter of the spheres slightly decreases and the polydispersity increases with increasing the water content from 9.7 ( 0.7 to 8.1 ( 1.2 nm at w ) 22 and w ) 28, respectively. At w ) 30, the isotropic phase remains in equilibrium with isooctane. It is attributed to L2* phase similar to that obtained at lower water content (in the range w ) 5.6-11). By increasing the water content from w ) 30 to 35, the interconnected cylinders network is diluted with

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decrease in the number of connection. In all this water content range, formation of spherical and cylindrical copper metallic particles is observed. Figure 8 shows a TEM pattern obtained at w ) 34. As at lower water content, cylindrical (42%) and spherical (58%) nanoparticles are observed. The average size of spherical particles remains the same as observed in most of the cases with a diameter equal to 9.5 ( 0.9 nm. The length and width of the cylinders are equal to 19.8 ( 2.7 and 6.5 ( 0.8 nm, respectively. At w ) 35, an isotropic solution formed by water in oil droplets is obtained. From SAXS it has been observed,23 that the reverse micelles are cylindrical water in oil droplets with a gyration radius equal to 4.8 and 5.8 nm at w ) 35 and w ) 40 respectively. Syntheses have been performed at w ) 35 and above. Most of the particles formed are spherical. The percentage of cylinders remains small (Table 1). No drastic changes in the particle sizes and in the percentage of cylinders are observed by increasing the water content from w ) 35 to 40. Figure 9 shows the TEM pattern obtained at w ) 40. The particle size is found equal to 7.5 ( 0.6 nm. 4. Discussion The phase diagram described on Figure 1 shows various colloidal structures. By increasing the water content, the colloidal system evolves from reverse micelles to bicontinuous systems formed first by interconnected cylinders and then by a lamellar phase. Such behavior has been already observed in many systems.10-16 However, opposite from what it is well-known, the increase in the water content does not induce and increase in the average curvature to form oil-in-water aggregate. In the present case, the increase in the water content induces formation of multiphases having spherulites, sponge structure, interconnected cylinders, and then reverse micelles. Hence this phase diagram shows presence of reverse micelles in two various water content domains (below w

Tanori and Pileni


) 5.5 and above w ) 35). Similarly, interconnected cylinders can be observed in two different water content domains (5.5 < w < 11 and 30 < w < 35). At other w values several phases in equilibrium can be obtained. To try to find any correlation between the colloidal structure used as a template and the size and shape of the copper metallic particles, we will discuss the data obtained in the various colloidal structures: 4.1. Reverse Micellar Solution. Reverse micelles are formed below w ) 5.5 and above w ) 35. From the data described above and given on Table 1, most of the copper metallic particles obtained in this region of the phase diagram are spherical. 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 (Table 1). In the reverse micellar regions, the length and width of the cylinders are larger at low water content and decrease with the increase w. Furthermore, the number of particles formed strongly increases with increasing the water content (Figures 2 and 9). These differences could be mainly attributed to hydration phenomena. At low water content, the number of water molecules is too small to hydrate the polar head groups of the surfactant and the counterions. This induces a relative high rigidity of the water content with a low efficiency in the intermicellar exchange constant. The chirality of the surfactant molecules could favor formation of nuclei having a boat configuration and induce cylindrical particles. The local rigidity, due to the low w value, could favor this process. At higher water content, “free” water molecules are present in the microphase and the dynamic process between micelles increases. This could favor the growth of the particles in various directions with the decrease in the size of the cylinders with increasing water content. Comparison between Figures 2 and 9 shows a strong increase in the number of particles formed when the water


content increases. At low water content (w ) 4), the local environment cannot be assimilated to aqueous solution. The local confinement of water molecules in the internal microphase creates a lower dielectrical media as in the presence of “free water”. This could strongly change the redox potential of Cu(AOT)2. The increase in the water content induces a better solvation of Cu(AOT)2 and then an increase in the amount of material formed. It can be noticed that the size of spherical metallic particles is not controlled by the water content, as has been demonstrated previously in Na(AOT)-Cu(AOT)2isooctane-water reverse micelles.18-20 This can be explained as following: In the previous study, the concentration of Cu(AOT)2 was very small (10-3 M) compared to that used in the present study (5 × 10-2 M). At a given water content, the number of nuclei formed by chemical reduction of Cu(AOT)2 is much higher in the present study than in mixed micelles. This increase in the number of nuclei induces an increase in the particle size. Hence at w ) 4, in pure Cu(AOT)2 reverse micelles the average size of copper metallic particle is equal to 12.3 nm whereas in mixed micelles it is 7.3 nm.18 Furthermore mixed micelles form spherical water-in-oil droplets with an average radius equal to 0.6 nm whereas pure Cu(AOT)2 reverse micelles are cylindrical with a gyration radius equal to 4 nm. 4.2. Interconnected Cylinders. Interconnected cylinders having similar persistance length are observed in Cu(AOT)2-iosoctane-water solution in two various water content domains (at 5.5 < w < 11 and 30 < w < 35). Syntheses performed in these two domains 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 equal to 21.2 and 6.6 nm, respectively. The polydispersity is a little higher at low water content (Table 1). The number of cylinders increases with increasing the water content. In percentage of weight more than 50% of copper metallic particles are cylinders with a rather low polydispersity in size. Hence a very small increase in the water content (from w ) 4 to w ) 6) induces a strong change in size and shape of nanoparticles. Such an increase cannot be attributed to changes in the hydration of the polar head group. In fact, in reverse micelles (see above), a strong increase in the water content does not induce a drastic change in the particle morphology. Furthermore a very high similitude in the size and shape of nanoparticles is observed in the two parts of the phase diagram differing by their water content and forming by interconnected cylinders. Hence the same average diameter and same ratio of cylinder axis (≈3.3) are observed at low (5.5 < w < 11) and high (30 < w < 35) water content. Because of this high similarity in various experimental conditions, this phenomenon is attributed to the structure of the colloid used as a template. This could be explained as in nature1-4 where the key step in the control of mineralization is the initial isolation of a space. The chiral molecules used to form the colloidal template (Cu(AOT)2) impose orientation of each reactant in a local supersaturation regime which is needed to induce nucleation and let the system reach a state of lower energy. Most of the cylinders shown in TEM patterns present a difference in the contrast. Along the main axis (111), usually in the middle of the cylinder, a dark color is observed. This indicates a change in the density under the electron beam. This can be attributed to the formation of a twin which could be imposed to the formation of boat configuration of the nuclei induced by the chirality of the surfactant molecules. Hence, when a

Langmuir, Vol. 13, No. 4, 1997 645

twin is formed, the crystal growths in the 111 direction taking into account the symmetry of the nuclei. A similar approach has been recently proposed5,6 in which selforganizing media is used to make calcium carbonate material. 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.24,25 It has to be noticed that the amount of cylinders increases with the increase in the water content. This can be attributed to hydration phenomena as it has been observed above with reverse micelles. 4.3. Multiphase Regions. As described in the phase diagram, the increase in the water content induces the formation of several colloidal structures in equilibrium. 4.3.1. Lamellae and Interconnected Cylinders. At w ) 11 appears a birefrigent phase made of regular lamellae and spherulites in equilibrium with interconnected cylinders. By increasing the water content from 11 to 15, a progressive disappearance of interconnected cylinders takes place. Syntheses at w ) 12 show formation of spheres and cylinders. A very small amount of particles (less than 1%) has a triangular shape. The average diameter of spheres (10.9 nm) is similar to that observed in the other cases, and their percentage is 62%. 38% of cylinders are formed and are characterized by a relatively high polydispersity (Table 1). Hence the presence of various colloidal assemblies used as templates induce an increase in the polydispersity. However, an increase in the number of cylindrical particles can be noticed. A relatively high weight of copper material are cylinders. 4.3.2. Spherulites Aggregates. In the water content range (15 < w < 19) spherulites are in equilibrium with isooctane. Syntheses performed in this region show the formation of particles characterized by various shapes. A strong variation in the contrast can be observed in Figure 6. As has been mentioned, this is due to changes in the electron density under the electron beam. This again indicates the presence of twins. It can be noticed that the twins are not aligned as observed in cylindrical particles (Figure 5), and they are characterized by various orientations. This phenomenon could explain the formation of various shapes: Because of the onion structures, nuclei could be formed in various orientations. The progressive growth of the particles could evolve in various directions. In some cases, the growth of the particles in two directions could cost too much energy. This would prevent against the growth. The shape of the particles would depend on the number of nuclei formed and on the number of directions. 4. Conclusion With this work we demonstrate that the size and shape of copper metallic particles can be partially controlled by the shape of the colloidal assemblies used as template. In cylindrical reverse micelles, most of the copper metallic particles remains spherical and 13% are cylinders. Because of the large amount of nuclei formed in each droplet, the size of copper particles is rather large and does not strongly vary with the water content. However, the number of particles synthetized strongly increases with the hydration of the polar head group of the surfactant. (24) Weissbuch, I.; Addadi, L.; Berkovitch-Yellin, Z.; Gati, E.; Lahav, M.; Leiserowitz, L. Nature 1984, 310, 161. (25) Weissbuch, I.; Frolow, F.; Addadi, L.; Lahav, M.; Leiserowitz, L. J. Am. Chem. Soc. 1990, 112, 7718.

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By using interconnected cylinders as template, spherical and cylindrical copper metallic particles are formed. The size of the cylinders and the polydispersity remain unchanged in all the domain. A rather large increase in the water content does not change the size of the cylinders. As in reverse micelles the number of copper metallic cylinders increases with the water content. This has to be related to hydration of the polar head group and the changes in the Cu(AOT)2 redox potential by increasing the water content. Of course, the system used is dynamic, and this again prevents the formation of 100% of cylinders. However, the consistency of the data obtained in the two regions of the phase diagram (at low and high water content) in which interconnected cylinders are formed

Tanori and Pileni

strongly supports the fact that as in nature, the shape of the template controls the shape of the particles. When several phases are in equilibrium, a strong change in shape and polydispersity of the particles can be observed. In some cases, the polydispersity in size is small enough to induce self-assembly made of nanoparticles. The particles arrange in an hexagonal network. Formation of multilayers made of particles can be observed. They are arranged in a face centered cubic supperlattices. Acknowledgment. J.T. thanks C.O.N.A.C.Y.T for financial support. LA9606097

Control of the Shape of Copper Metallic Particles by Using a

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