3266
Langmuir 1997, 13, 3266-3276
Feature Article Nanosized Particles Made in Colloidal Assemblies M. P. Pileni Laboratoire SRSI, URA CNRS 1662, Universite P. et M. Curie Baˆ t F, 4 Place Jussieu, 75005 Paris, France, and CEA, DSM-DRECAM Service de Chimie Mole´ culaire CE Saclay, 91 191 Gif sur Yvette Cedex, France Received April 3, 1996. In Final Form: February 12, 1997X In this feature article, syntheses of nanosized particles by using colloidal assemblies as a template are described. We asked ourselves the following question: What parameters play a role in the control of the size, shape, and polydispersity? We know that parameters such as the shape of colloidal assemblies, the hydration of the head polar group, the water molecules bounded to the interface, etc. play a major role. However, there are a number of exceptions preventing any generalization. It is shown that the chemical mechanism in nanoparticles production in colloidal assemblies can differ from those usually observed in homogeneous solution. This shows that the solution chemistry cannot always be transferred to colloidal systems. It is possible to select the size and markedly reduce the polydispersity by a surface treatment of the nanoparticles. This favors formation of mono- and multilayers made of nanoparticles, and it is found that these particles form crystals organized in a three-dimensional face-centered cubic superlattice.
I. 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. It 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 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 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, leading to spherulites. In term of particle growth, some analogies between surfactant self assemblies and natural media can be proposed. In both cases, this growth needs a supersaturated medium where the nucleation can take 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, by definition, are colloidal and since processing of advanced materials involves reactions at solid-solid, solid-liquid or solid-gas interfaces.3-5 In solution, surfactant molecules self assemble to form aggregates. At low concentration the aggregates are generally globular micelles,6 but these micelles can grow on increasing surfactant concentration and/or upon addition of salt, alcohols. etc. In this case, micelles have X
Abstract published in Advance ACS Abstracts, June 1, 1997.
(1) Addadi, L.; Weiner, S. Angew. Chem., Int. Ed. Engl. 1992, 31, 153. (2) Mann, S. Inorganic Materials Bruce, D. W., O’Hare, D., Eds.; J. Wiley & Sons Ltd: 1992. (3) Pileni, M. P. J. Phys. Chem. 1993, 97, 6961. (4) Fendler, J. H.; Meldrum, F. Adv. Mater. 1995, 7, 607. (5) Fendler, J. H. Chem. Mater. 1996, 8, 1616.
S0743-7463(96)00319-8 CCC: $14.00
been shown to grow to elongated more or less flexible rodlike micelles,7-10 in agreement with theoretical predictions for micellization.11,12 In addition to their academic interest, the ordering transitions might well be at the origin of mechanical instabilities in dispersion of mesoscopic aggregates subjected to high constant shear. Owing to the remarkable polymorphism of surfactant aggregates, quite a number of these transitions have been observed in amphiphilic systems. As illustrative examples, we can mention the sponge (isotropic) to lamellar (smectic) transition or the onion (liposome) to lamellar (smectic) transition.13 The amphiphilic molecules spontaneously self assemble to form highly flexible locally cylindrical aggregates with the average size reaching several micrometers.14,15 The details of the morphology (curvature of the film) of the mixture at a local scale fluctuate strongly. 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 is toward the water, and by convention we describe this as negative mean curvature. Phases with negative mean interfacial curvature are known as type II or inverse.16 The fabrication of assemblies of perfect nanometer-scale 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 (6) Tanford, C. The hydrophobic effect; Wiley: New York, 1973. (7) Chen, S. J.; Evans, D. F.; Ninham, B. W.; Mitchell, D. J.; Blum, F. D.; Pickup, S. J. Phys. Chem. 1986, 90, 842. (8) Evans, D. F.; Mitchell, D. J.; Ninham, B. W. J. Phys. Chem. 1986, 90, 2817. (9) Barnes, I. S.; Hyde, S. T.; Ninham, B. W.; Derian, P. J.; Drifford, M.; Zemb, T. N. J. Phys. Chem. 1988, 92, 2286. (10) Porte, G.; Appell, J.; Poggi, Y. J. Phys. Chem. 1980, 84, 3105. (11) Mitchell, D. J.; Ninham B. W. J. Chem. Soc., Faraday Trans. 2 1981, 77, 601. (12) Safran, S. A.; Turkevich, L. A.; Pincus, P. A. J. Phys. Lett. 1984, 45, L69. (13) Diat, O.; Roux, D. J. Phys. II 1993, 3, 9. (14) Berret, J. F.; Roux, D. C.; Porte G.; Lindner, P. Europhys. Lett. 1994, 25, 521. (15) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc. Faraday Trans. 2 1976, 72, 1525. (16) Reactivity in Reverse Micelles; Pileni, M. P., Ed.; Elsevier: New York, 1989.
© 1997 American Chemical Society
Nanosized Particles Made in Colloidal Assemblies
mainly due to the unusual dependence of the electronic properties on the particle size, either for metal3,17 or semiconductor3,18,19 particles, in the range 1-10 nm. Relatively little work has been carried out on magnetic materials having sizes smaller than 10 nm.20 The preparation and characterization of these colloids have thus motivated a vast amount of work.21 Various colloidal methods are used to control the size and/or the polydispersity of the particles, using reverse3 and normal22,23 micelles, Langmuir-Blodgett films,4,5 zeolites,24 two-phase liquid-liquid systems,25 or organometallic techniques.26 The achievement of accurate control of the particle size, their stability, and a precisely controllable reactivity of the 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. It must be noted that manipulating nearly monodispersed nanometer size crystallites with an arbitrary diameter presents a number of difficulties. Moreover the ability to assemble particles into welldefined two- and three-dimensional spatial configurations should produce interesting properties such as new collective physical behavior.27 The development of general procedure for the fabrication of a “quantum” crystal is a major challenge for future research. The fabrication of such crystals of particles would, for example, lead to the production of optical gratings,28,29 optical filters,30 antireflective surface coating,31,32 selective solar absorbers,33 data storage, and microelectronic devices.34 Several approaches have been used to obtain 2D and 3D structures: (i) Inorganic-organic superlattices have been synthesized by using multilayer cast films.35 Asher et al.36-38 developed a method for creating new submicron periodic structures of both organic and inorganic materials. By combined synthesis in reverse micelles with the precipitation of other materials, highly ordered nanocomposites are formed.39 (ii) Another approach to obtaining 2D and 3D structures is to assemble nanoparticles in ordered arrays. This requires only a hard sphere repulsion, a controlled size distribution, and the inherent van der Waals attraction between particles and dispersion forces. The polydisper(17) Lisiecki, I.; Pileni, M. P. J. Amer. Chem. Soc. 1993, 115, 3887. (18) Brus, L. E. J. Chem. Phys. 1983, 79, 5566. (19) Rossetti, R.; Ellison, J. L.; Bigson, J. M.; Brus L. E. J. Chem. Phys. 1984, 80, 4464. (20) Leslie-Pelecky, D.; Rieke, R. D. Chem. Mater. 1996, 8, 1770. (21) Clusters and Colloids Schmid, G., Ed.; VCH: Weinheim, 1994. (22) Lisiecki, I.; Billoudet, F.; Pileni, M. P. J. Phys. Chem. 1996, 100, 4160. (23) Moumen, N.; Veillet, P.; Pileni, M. P. J. Magn. Magn. Mater. 1995, 149, 42. (24) Herron, N.; Wang, Y.; Eddy, M.; Stucky, G. D.; Cox, D. E.; Moller, K.; Bein, T. J. Amer. Chem. Soc. 1989, 111, 530. (25) Brust, M.; Walker, D.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (26) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (27) Heitmam, D.; Kotthaus, J. P. Phys. Today 1993, 56. (28) Xia, Y.; Kim, E.; Mrksich, M.; Whitesides, G. M. Chem. Mater. 1996, 8, 601. (29) Kumar, A.; Whitesides, G. M. Science 1994, 263, 60. (30) Asher, S. A. U. S. Patents 4,627,689, and 4,632,517. (31) Yoldas, B. E.; Partlow, D. P. Appl. Opt. 1984, 23, 1418. (32) Hinz, P.; Dislich, H. J. Non-Cryst. Solids 1986, 82, 411. (33) Hahn, R. E.; Seraphin, B. O. In Physics of Thin Film; Academic: New York, 1978. (34) Kastner, M. A. Phys. Today 1993, 24. (35) Kimizuka, N.; Kunitake, T. Adv. Mater. 1996, 8, 89. (36) Tse, A. S.; Wu, Z.; Asher, S. A. Macromolecules, 1995, 28, 6533. (37) Asher, S. A.; Holtz, J.; Liu L.; Wu, Z. J. Am. Chem. Soc. 1994, 116, 4997. (38) Kesavamoorthy, R.; Tandon, S.; Xu, S.; Jagannathan, S.; Asher, S. A. J. Colloid Interface Sci. 1992, 153, 188. (39) Chang, S. Y.; Liu, L.; Asher, S. A. J. Am. Chem. Soc. 1994, 116, 6739.
Langmuir, Vol. 13, No. 13, 1997 3267
sity in particle size prevents contruction of such welldefined two- or three-dimensional structures. Hence, external forces are used to induce formation of monolayers made of particles. For example, electrodeposition40 and Langmuir-Blodgett4 techniques favor the formation of a submonolayer. Recently, in our laboratory, we have demonstrated that reverse micelles can be used to produce silver sulfide particles with sizes ranging from 2 to 10 nm.41 By dodecanthiol addition, the particles are coated and extracted from reverse micelles. The coated particles are dispersed in heptane, giving an optically clear solution. Placing a drop of this solution on a carbon grid induces the formation of a monolayer of particles.42 Such monolayers have been obtained for various particle sizes. It is worth pointing out that the monolayers with the particles organized in a hexagonal network are formed without any external force. In these monolayers, the particles are organized in a hexagonal network. A 3D aggregate42 has been observed. Very recently, colloidal self organization of nanocrystallites has been observed with both cases of metal and semiconductor particles. In all cases, surface passivation with coordinating ligands such as alcanethiol,39,43 alkylphosphine44 or dithiols45 has been used. We will show below that various parameters, such as water structure and intermicellar potential, control the particle size. However, some exceptions are found, indicating that unknown parameters can also be involved. As in nature, the shape of the particles can be controlled by the shape of the template used. Self organization of nanoparticles into three-dimensional quantum dot superlattices is observed. The phase diagram for formation of nanosized particles can be different from that obtained in the bulk phase, showing that the chemical mechanisms in colloidal assemblies and in homogeneous solution differ. Colloidal assemblies used to control the size and the shape of particles are formed by using either functionalized surfactants (i.e. the counterion of the surfactant is one of the reactants) or mixed surfactants (i.e. a mixture of a functionalized surfactant and its sodium derivative). In most cases, increasing functionalized surfactant concentration reduces flocculation and induces formation of better defined particles. II. Control of the Particle Size II.1. Control of the Particle Size by Using Reverse Micelles. Reverse micelles are well known to be spherical water in oil droplets.16 By collisions these 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 metal ions or coprecipitation reactions. By using Aerosol OT (sodium bis(2-ethylhexyl)sulfosuccinate), Na(AOT), as the surfactant, it has been demonstrated that the water pool diameter is related to the water content, w ) [H2O]/[AOT], of the droplet by46
D(nm) ) 0.3w Various materials have been synthetized in reverse micelles.3 Cadmium sulfide and cadmium selenium (40) Giersig, M.; Mulvaney, P. Langmuir 1993, 9, 3408. (41) Motte, L.; Billoudet, F.; Pileni, M. P. J. Mater. Sci. 1996, 31, 38. (42) Motte, L.; Billoudet, F.; Pileni, M. P. J. Phys. Chem. 1995, 99, 16425. (43) Ohara, P. C.; Leff, D. V; Heath, J. R.; Gelbart, W. M. Phys. Rev. Lett. 1995, 75, 19, 3466. (44) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 271, 1335. (45) Brust, M.; Bethell, D.; Schiffin, D. J.; Kiely, C. J. Adv. Mater. 1995, 9, 797. (46) Pileni, M. P.; Zemb, T.; Petit, C.; Chem. Phys. Lett. 1985, 118, 414.
3268 Langmuir, Vol. 13, No. 13, 1997
Pileni
semiconductors46-56 were the first materials prepared by this method. This has been extended to semiconductor alloys such as Cd1-yZnyS57,58 or dilute magnetic semiconductors like Cd1-yMnyS.59 Metal particles17,60-69 have also been synthetized, which allows us to observe the optical properties of colloidal metal particles in the range of size and for catalysis. Silica70-73 and silica-cadmium sulfide nanocomposites have been produced.74 Relatively little work has been carried out on magnetic75-78 and superconducting79-81 and oxide82-84 nanosized particles. Latexes differing by their sizes were produced in reverse micelles.85-96 (47) Meyer, M.; Walberg, C.; Kurchara, K.; Fendler, J. H. J. Chem. Soc., Commun. 1984, 90, 90. (48) Lianos, P.; Thomas, K. Chem. Phys. Lett. 1986, 125, 299. (49) Lianos, P.; Thomas, K. J. Colloid Interface Sci. 1987, 117, 505. (50) Petit, C.; Pileni, M. P. J. Phys. Chem. 1988, 92, 2282. (51) Stigerwald, M. L.; Alivisatos, A. P.; Gibson, J. M.; Harris, T. D.; Kortan, R.; Muller, A. J. J. Am. Chem. Soc. 1988, 110, 3046. (52) Petit, C.; Lixon, P.; Pileni, M. P. J. Phys. Chem. 1990, 94, 1598. (53) Robinson, B. H.; Khan-Lodhi, A. N.; Towey, T. F. J. Chem. Soc., Faraday Trans. 1990, 86, 3757. (54) Motte, L.; Petit, C.; Lixon, P.; Boulanger, L.; Pilenim M. P. Langmuir 1992, 8, 1049. (55) Karayigitoglu, C.; Tata, M.; John-Vivay, T.; McPherson, G. L. Colloids Surf., A 1994, 82, 151. (56) Suriki, K.; Harada, M.; Shioi, A. J. Chem. Eng. Jpn. 1996, 109, 245. (57) Hirai, T.; Sato, H.; Komasawa, I. Ind. Eng. Chem. Res. 1994, 33, 3262. (58) Cizeron, J.; Pileni, M. P. J. Phys. Chem. 1995, 99, 17410. (59) Levy, L.; Hochepied, J. F.; Pileni, M. P. J. Phys. Chem. 1996, 100, 18322. (60) Boutonet, M.; Kizling, J.; Steinius, P.; Maire, G. Colloid Surf. A, 1982, 5, 209. (61) Petit, C.; Lixon, P.; Pileni, M. P. J. Phys. Chem. 1993, 97, 12974. (62) Lisiecki, I.; Pileni, M. P. J. Phys. Chem. 1995, 99, 5077. (63) Lisiecki, I.; Borjling, M.; Motte, L.; Ninham, B.; Pileni, M. P. Langmuir, 1995, 11, 2385. (64) Kishida, M.; Fujita, T.; Umaxoshi, K.; Ishiyama, J.; Nagata, H. J. Chem. Soc., Chem. Commun. 1995, 763. (65) Aliotta, F.; Arcoleo, V.; Buccoleri, S.; Manna, G.; Turco-Liveri, V. Thermochim. Acta 1995, 265, 15. (66) Meldrum, F. C.; Kotov, N. A.; Fendler, J. H. J. Chem. Soc., Faraday Trans. 1995, 91, 673. (67) Gan, L. M.; Chan, H. S. O.; Zhang, L. H.; Chew, C. H.; Loo, B. H. Mater. Chem. Phys. 1995, 10, 17. (68) Arcoleo, V.; Turco-Liveri, V. Chem. Phys. Lett. 1996, 258, 223. (69) Sangregorio, C.; Galeotti, M.; Bardi, U.; Baglioni, P. Langmuir 1996, 12, 5800. (70) Osseo-Asare, K.; Arrigade, F. J. Colloids Surf. 1990, 50, 312. (71) Amagada, F. J.; Osseoasare, K. J. Colloid Interface Sci. 1995, 170, 17. (72) Espinard, P.; Guyot, A.; Mark, E. Inorg. J.: Organomet. Polym. 1995, 5, 391. (73) Gan, L. M.; Zhang, K.; Chew, C. H. Colloid Surf., A 1996, 110, 199. (74) Chang, S.; Liu, L.; Asher, S. A. J. Am. Chem. Soc. 1993, 116, 6739. (75) Pillai, V.; Kumar, P.; Multani, M. S.; Shah, D. O. Colloid Surf., A 1993, 80, 69. (76) Chen, J. P.; Lee, K. M.; Sorensen, C. M.; Klabunde, K. J.; Hadjipanayis, G. C. J. Appl. Phys. 1994, 75, 5876. (77) Chen, J. P.; Sorensen, C. M.; Klabunde, K. J.; Hadjipanayis, G. C. J. Appl. Phys. 1994, 76, 6316. (78) Petit, C.; Pileni, M. P. J. Magn. Magn. Mater., in press. (79) Kumar, P.; Pillai, V.; Bates, S. R.; Shah, D. O. Mater. Lett. 1993, 16, 68. (80) Kumar, P.; Pillai, V.; Shah, D. O. Appl. Phys. Lett. 1993, 62, 765. (81) Wang, L. M.; Zhang, Y.; Muhaammed, M. J. Mat. Chem. 1995, 5, 309. (82) Chhabra, V.; Pillai, V.; Mishira, B. K.; Morrone, A.; Shah, D. O. Langmuir 1995, 11, 3307. (83) Gan, L. M.; Zhang, L. H.; Chan, H. S. O.; Chew, C. H.; Loo, B. H. J. Mater. Sci. 1996, 31, 1071. (84) Kawai, T.; Fujino, A.; Konno, K. Colloid Surf., A 1996, 109, 264. (85) Stoffer, J. C.; Bone, T. J. Polym. Sci. Polym. Chem. Ed. 1980, 18, 2641. (86) Stoffer, J. C.; Bone, T. J. Dispersion Sci. Technol. 1980, 1, 37. (87) Leong, Y. S.; Candau, F. J. Phys.Chem. 1982, 86, 2269. (88) Voormans, G.; Verbeek, A.; Jackers, C.; De Schryver, F. C. Macromolecules 1988, 21, 161. (89) Gan, L. M.; Chew, C. H.; Lye, I.; Ma, L.; Li, G. Polymer 1993, 34, 3860. (90) Hammouda, A.; Gulik, T.; Pileni, M. P. Langmuir 1995, 11, 3656. (91) Chieng, T. H.; Gan, L. M.; Chew, C. H.; Ng, S. C. Polymer 1995, 36, 1941.
Figure 1. TEM patterns of copper metallic particles prepared in reverse micelles at various water contents: [AOT] ) 0.1 M; [Cu(AOT)2] ) 10-2 M; [N2H4] ) 2 × 10-2 M; (A) w ) 1; (B) w ) 3; (C) w ) 4; (D) w ) 5; (E) w ) 10; (F) w ) 20.
II.1.1. Variation of the Water Content. Syntheses carried out at various water contents induce formation of nanosized particles with a range of sizes. As an example, Figure 1 shows an increase in the particle size of copper metal particles, (Cu0)n, with increasing water content.17,62 Above a water content (w ) [H2O]/[AOT]) of 10-15, the particle size does not drastically change and an increase in the polydispersity is observed. To compare the data obtained with various materials, syntheses have been carried out under the same experimental conditions (i.e. same reactants and surfactant concentrations). The various materials studied are CdS,17,56,57 CdyZn1-yS,58 ZnS, CdyMn1-yS,59 and PbS, for (Ag0)n,61 (Cu0)n,17,62 and Co2B alloys.78 However, it must be noted that the increase in the particle size depends on the produced material. With semiconductors, the size varies from 1 to 4 nm whereas with silver particles it varies from 1 to 7 nm and with copper metal particles it vaires from 1 to 12 nm. The increase in the particle size with the water content can be attributed to the water structure inside the water pool. At low water content, the number of water per surfactant molecules is too small to hydrate the counterions and the head polar groups. This induces strong interactions between water molecules and the head polar groups. The water molecules can be considered as “bound”. (92) Loh, S. E.; Gan, L. M.; Chew, C. H.; Ng, S. C. J. Macromol. Sci.: Pure Appl. Chem. 1995, A32, 681. (93) Chieng, T. H.; Gan, L. M.; Chew, C. H.; Lee, L.; Ng, S. C.; Pey, K. L. Langmuir 1995, 11, 3321. (94) Gan, L. M.; Li, T. D.; Chew, C. H.; Tea, W. K.; Gan, L. H. Langmuir 1995, 11, 3316. (95) Chieng, T. H.; Gan, L. M.; Chew, C. H.; Ng, S. C.; Pey, K. L. Langmuir 1996, 12, 319. (96) Dacresse, C.; Granafils, C.; Jerome, R.; Teyssie, P. Colloid Polym. Sci. 1996, 274, 482.
Nanosized Particles Made in Colloidal Assemblies
Figure 2. Relative variation of the relative particle diameter, Dm, with water content. The particles are CdS (2), PbS (9), CdyZn1-yS (b), CdyMn1-yS (+), ZnS (×), (Cu0)n (O), and (Ag0)n (solid octagon).
A progressive increase in the water content induces, inside the droplets, a progressive change from “bound” to “free” water in the water pool.3 The bulk water phase is reached for a water content, w ) [H2O]/[AOT], around 10-15. For each material studied, we normalize the largest particle size to 1 and thus derive the relative variation of the particle size with the water content. An example of the correlation between the particle size and the water content is shown in Figure 2. In most cases, the particle size increases with increasing water content and reaches a plateau above w ) 15. Below w ) 10-15, the polydispersity in size is rather low (less than 10%). At higher water content, the average particle size does not change markedly and an increase in the polydispersity is observed. With copper particles, above w ) 15, formation of copper oxide, (Cu2O)n, is observed instead of the metal (Cu0)n. This indicates that “free” water molecules in the pool are very efficient in oxidizing nanosized particles. Furthermore, it confirms that the interface plays an important role (redox potential, ionization degree, etc.) in particle nucleation and growth. To demonstrate that the control of the particle size is related to the water structure’s physical properties, we use the results of FTIR, SAXS, and pulse radiolysis experiments performed in the same system (Na(AOT)isooctane-water):97 (i) The FTIR spectra of AOT reverse micelles are recorded at various water contents. The percentages of “bound” and “free” water were evaluated from the O-H band of water molecules. Furthermore, the absorption spectrum of the two carboxyl groups of AOT changes with the water content. The area of the spectrum obtained at high water content is normalized to unity, and the variation of the carboxyl group area then show a behavior similar to that of the particle diameter with the water content. (ii) The size of the droplets is determined by SAXS. Assuming the droplet is spherical, the surfactant molecules are at the interface and the volume of water molecules controls the droplet size, the apparent volume of water molecule is deduced. The variation of the apparent volume of water with the increase in the water content is in good agreement with direct experiments performed by Khan et al.98 Below w ) 15, the water molecule relative apparent volume increases with increasing water content, and it reaches a plateau above this value. (iii) Hydrated electrons can be formed in the water pool by pulse radiolysis, and their yield increases with increasing water content, reaching a plateau at w ) 15. Similarly the hydrated electron lifetime drastically decreases with increasing the water content and reaches a plateau at the same w value (w ) 15).99,100 (97) Motte, L.; Lisiecki, I.; Pileni, M. P. In Hydrogen Bond Networks; Dore, J., Bellisan, M. C., Eds.; NATO: 1994; p 447. (98) Kahn. Private communication.
Langmuir, Vol. 13, No. 13, 1997 3269
These three techniques97 show a drastic change in the physical parameters of water molecules with the water content. The similar behavior of such different relative parameters as relative distribution of nanosized particles, apparent water volume, O-H band area, hydrated electron yield, and kinetics allows us to conclude that water structure inside the water pool plays an important role in controlling the particle size. II.1.2. Intermicellar Potential. In reverse micelles, the intermicellar exchange process is governed by101 (i) the dimers formed by contacting two micelles and (ii) the exchange process between two water pool droplets. The first factor is related to the attractive interactions between droplets and involves the probabilities of collision of two micelles. The intermicellar potential decreases either by decreasing the number of carbon atoms in the bulk solvent101-105 or by increasing the number of droplets.102 The latter is due to the discrete nature of solvent molecules and is attributed to the appearance of depletion forces between two micelles. The second factor is associated with the interface rigidity and is related to the dynamical properties of a water-surfactant-oil interface. This corresponds to the bending elastic modulus of the interface. The control of these two parameters allows the intermicellar exchange process to be changed. In water-AOTisooctane micellar solution, the replacement of isooctane by cyclohexane induces a decrease in the intermicellar exchange rate constant by one order of magnitude. Similarly, the increase in the number of droplets induces a decrease in the intermicellar rate constant. Nanoparticles are synthetized by varying the intermicellar potential. This is obtained by changing either the bulk solvent or the number of droplets (i.e. the polar volume fraction). The following data show that the particle size is controlled by the intermicellar potential: (i) Effect of the Bulk Solvent on the Particle Size. Addition of sodium tetraborohydrate to Ag(AOT)-Na(AOT)-water-isooctane solution induces formation of silver particles, (Ag0)n.61 This takes place in a few minutes, in the presence of air. As described above, the particle size depends on the water content: At w ) 7.5, the average particle size is 4.9 nm with an absorption spectrum characterized by a plasmon peak centered at 420 nm (inset of Figure 3). Replacing isooctane by cyclohexane and keeping the same experimental conditions as described above (w ) 7.5, [Ag(AOT) ) 3 × 10-2 M, [Na(AOT)] ) 7 × 10-2 M, [NaBH4] ) 2.5 × 10-4 M), induces a marked change, compared to what is shown in the inset of Figure 3, in the absorption spectrum observed, immediately after addition of the reducing agent. This spectrum, which is characterized by a maximum centered at 275 nm (Figure 3), is attributed to Ag42+ clusters.106,107 With time, the size of the clusters increases to reach particles having an average diameter of 3.3 nm. Hence, Ag42+ is stabilized during several hours in reverse micelles. Usually such (99) Pileni, M. P.; Hickel, B.; Ferradini, C.; Puchauld, J. Chem. Phys. Lett. 1982, 92, 30. (100) Pileni, M. P.; Brochette, P.; Hickel, B.; Lerebours, B. J. Colloid Interface Sci. 1984, 98, 549. (101) Jain, T. K.; Cassin, G.; Badiali, J. P.; Pileni, M. P. Langmuir 1996, 12, 2408. (102) Cassin, G.; Badiali, J. P.; Pileni, M. P. J. Phys. Chem. 1995, 99, 12941. (103) Robertus, C.; Joosten J. G. H.; Levine, Y. K. J. Chem. Phys. 1990, 93, 10, 7293. (104) Towey, T. F.; Khan-Lodl, A.; Robinson, B. H. J. Chem. Soc., Faraday Trans. 2 1990, 86, 3757. (105) Fletcher, P. D.; Robinson, B. H.; Tabony, J. J. Chem. Soc., Faraday Trans. 1 1986, 82, 2311. (106) Henglein, A.; Taush-Treml, R. J. Colloid Interface Sci. 1981, 80, 84. (107) Mostafavi, M.; Keghouche, N.; Delcourt, M. O.; Belloni, J. Chem. Phys. Lett. 1990, 167, 193.
3270 Langmuir, Vol. 13, No. 13, 1997
Pileni
Figure 5. Variation of the diameter of copper particles synthesized in 10-2 M Cu(AOT)2-8 × 10-2 M NaAOT-waterisooctane reverse micelles with the water contents in the absence (s) and in the presence of 4 × 10-4 M CTAC (- - -).
clusters are short lived, at room temperature, excepted when they are formed in glass or in silver films produced by reduction under surfactant monolayers.108 In the case of CdS semiconductor and copper metallic particles, (Cu0)n, the kinetics of particle formation markely decreases compared to what it is observed by using isooctane as the bulk solvent. However, it has not been possible to identify clusters made of a few atoms as observed with silver. At given water content, the obtained particle size is smaller when the bulk phase is cyclohexane instead of isooctane (Figure 4). The decrease in the particle size by this replacement is explained in terms of exchange processes which induce a decrease in the particle growth rate. This is evidence the surfactant acts as an agent that protects the particles from growth. Conversely, the increase in the solvent alkyl chains induces an increase in the particle size. (ii) Change in the Number of Droplets Keeping the Micellar Diameter Constant. Copper metal particles have been synthetized at fixed water content and various number of droplets. A decrease in the particle size is observed with increasing number of droplets.62 Hence, the decrease in the intermicellar potential by either changing the bulk solvent or increasing the number of droplets induces a decrease in the average particle size. This behavior has been observed for CdS, CdyZn1-yS, ZnS, and PbS and for (Ag0)n and (Cu0)n nanosized particles. However, this is not the case for silver sulfide particles (see below). This indicates that the intermicellar potential is one of the very important parameters in controlling the size.
II.1.3. Influence on Particle Size of Addition of a Small Amount of Positively Charged Surfactant to AOT Reverse Micelles. Addition of 4 × 10-4 M cetyltrimethylammonium chloride (CTAC) to 0.1 M of AOT reverse micelles containing Cu(AOT)2 does not change the average size of the micelles. However, it induces large changes in the copper metallic particle size:63 Formation of particles having an average size similar to that obtained without CTAC at low water content is shown in Figure 5. By increasing the water content, a large increase in the particle size compared to what is obtained without CTAC is observed (Figure 5). The maximum in the particle diameter is similar to that obtained in its absence, and the production yield of nanoparticles markedly increases in the presence of CTAC. A random distribution of CTAC per micelle cannot be derived from these data. Assuming a Poisson distribution, one in ten AOT reverse micelles contains a CTAC molecule. Thus it is difficult to assume that such a low CTAC concentration induces a large change in the particle size. To explain these data, we assume a cooperative solubilization of CTAC in micelles. That is to say, the CTAC molecules are solubilized in one given droplet and not randomly as usual. Hence, few droplets would contain all the CTAC molecules with most of the droplets being empty. This would induce changes in the local composition with a change in the redox potential, bringing about an increase in the number of nuclei and then an increase in the particle size.63 Syntheses of CdS in the presence of a small amount of CTAC induce large changes in the particle size.109 However, this size depends on the way the syntheses are performed: mixing CTAC in micelles containing Cd(AOT)2 with micelles containing sodium sulfide, S2-, induces formation of smaller size whereas when CTAC is solubilized in micelles containing S2-, large particles are obtained. These data indicate how accurately the particle size can be controlled. Addition of small amounts of CTAC induces large perturbations in the particle production and size. II.1.4. Case of Silver Halide. II.1.4.1. Silver Sulfide. Syntheses of silver sulfide nanoparticles in reverse micelles show a linear increase in the particle size with an increase in water content (Figure 6). Furthermore, increasing the water content decreases the size polydispersity.41 By replacing isooctane by cyclohexane as the bulk solvent, the size of the particles remains the same with a decrease in polydispersity. These behaviors differ markedly from those described above, where (i) an increase in the particle size with the water content takes place only at low water content, below w ) 10-15) (Figure 2), with an invariance in size at higher water content, and (ii) a decrease is observed on replacing isooctane by cyclohexane.
(108) Yi, K. C.; Horvolgyi, Z.; Fendler, J. H. J. Phys. Chem. 1994, 98, 3872.
(109) Pileni, M. P.; Motte, L.; Cizeron, J.; Petit, C.; Moumen, N.; Mackay, R. Kumar, P., Mittal, K. L., Eds.; Marcel Dekker, Inc.: 1996.
Figure 3. Absorption spectrum of Ag42+ obtained by reduction of 3 × 10-2 M Ag(AOT)-7 × 10-2 Na(AOT)-cyclohexane. Inset: absorption spectrum of 4.9 nm (Ag)n particles obtained from reduction of 3 × 10-2 M Ag(AOT)-7 × 10-2 Na(AOT)isooctane. The water content is fixed, w ) 7.5, and [NaBH4] ) 10-4 M. These spectra are obtained when the syntheses are performed in the presence of air.
Figure 4. Variation of the diameter of copper particles synthesized in Cu(AOT)2-NaAOT-water-solvent reverse micelles at various water contents: [AOT] ) 0.1 M; [Cu(AOT)2] ) 10-2 M; [N2H4] ) 3 × 10-2 M; by using isooctane (O) and cyclohexane (2) as the bulk solvent.
Nanosized Particles Made in Colloidal Assemblies
Figure 6. Variation of the average radius of silver sulfide particles with the water content: [Na(AOT)] ) 10-1 M; [Ag(AOT)] ) 4 × 10-4 M; [Na2S] ) 4 × 10-4 M.
Figure 7. TEM patterns obtained from synthesis of AgI in 10-3 M Ag(AOT)-10-1 M NaAOT-cyclohexane, w ) 2, before electron exposure (A) and after electron beam exposure (B).
These data clearly show that the water content and the intermicellar exchange rate constant are not the only parameters which play a role in controlling particle size. II.1.4.2. Silver Iodide. Syntheses in reverse micelles to produce silver iodide nanoparticles show, as previously, an increase in the particle size with the water content. Conversely to what has been observed previously, the polydispersity in size is huge (100%). Hence, reverse micelles do not strongly control the size of silver iodide. Furthermore, all the particles are not spherical, as has been observed in the cases described above. Figure 7A shows a nonspherical particle. The large increase in the polydispersity in size and shape could be due to its high affinity for water molecules.110 The electron diffraction pattern indicates formation of AgI nanosized particles having two phases (R and β phases). On electron beam irradiation, the R phase disappears and only the β phases remain (Figure 7B). In the bulk phase, several AgI phases111 are observed. At room temperature, the β and γ phases are stable. At 146 °C, a phase transition takes place and the β phase is transformed to a R phase. At 558 °C the R phase disappears. (110) Tatsumisago, M.; Shinkuma, Y.; Saito, T.; Minami, T. Solid State Ionics 1992, 50, 273. (111) Landolt-Bornstein. Hellwege, K. H., Ed.; Springer-Verlag: Berlin, Heidelberg, New York, 1982; Vol. 17, subvol. b.
Langmuir, Vol. 13, No. 13, 1997 3271
Hence, conversely to what is obtained in the bulk phase, in reverse micelles, the R phase is formed at room temperature and is less stable than the β phase. This indicates a change in the phase diagram of AgI nanoparticles compared to the bulk. Hence, a phase unstable in the bulk material is obtained in reverse micelles. Similar behavior has been observed in the quantum dot semiconductors (II-VI): In the bulk phase these semiconductors are characterized by a hexagonal structure (Wurtzite). A decrease in the particle size induces a change in the crystalline structure from hexagonal to face-centered cubic (zinc blende). This has been observed for CdS, CdSe, CdyZn1-yS, and CdyMn1-yS, having diameters in the 2-4 nm range. Furthermore, in the bulk phase CdyZn1-yS is not stable at room temperature. II.2. Control of the Particle Size by Using Oil in Water Micelles. Surfactants having a positive curvature, above a given concentration, usually called the critical micellar concentration (cmc), self assemble to form oil in water aggregates called normal micelles. The most often used surfactant is sodium dodecyl sulfate (Na(DS)). To make particles, the counterion of the surfactant is replaced by ions which participate in the chemical reaction. These are called functionalized surfactants. Hence to make copper metal particles,22 copper dodecyl sulfate (Cu(DS)2) is used. In the case of magnetic fluids,23 mixed micelles made of cobalt and iron dodecyl sulfate (Fe(DS)2 and Co(DS)2) are needed. The critical micellar concentrations of Cu(DS)2, Fe(DS)2, and Co(DS)2 are 1.3 × 10-3, 1.42 × 10-3, and 1.2 × 10-3 M, respectively. II.2.1. Magnetic Fluid of Cobalt Ferrite Nanosized Particles. In this section, we will show, by using oil in water micelles, the formation of cobalt ferrite magnetic fluid and how the size changes compared to that of those in aqueous solution. Syntheses of CoFe2O4 particles have been extensively studied in homogeneous solutions.112 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) low reactant concentrations, (ii) the absence of Fe(III) ions, and (iii) room temperature. 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.). This induces large changes in the magnetic properties of the particles. It is rather difficult to derive a 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 room temperature, addition of base to the Co(DS)2 and Fe(DS)2 mixed micelles induces formation of a precipitate made of stoichiometric CoFe2O4 nanoparticles.113,114 The magnetic fluid is formed by washing this precipitate. Its characterization indicates formation of CoFe2O4 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)2 and Fe(DS)2 concentrations in the 10-3 M range whereas, in aqueous solution, the reaction does not take place at such low concentrations. This could be attributed (112) Charles, S. W. J. Magn. Magn. Mater. 1987, 65, 350. (113) Moumen, N.; Pileni, M. P. J. Phys. Chem. 1996, 100, 1867. (114) Moumen, N.; Pileni, M. P. Chem. Mater. 1996, 8, 1128.
3272 Langmuir, Vol. 13, No. 13, 1997
Pileni
Figure 9. TEM patterns of colloidal copper dispersions prepared in the presence of various sodium dodecyl sulfate concentrations: [Cu(DS)2] ) 5 × 10-4 M; [NaBH4] ) 10-3 M; [NaDS] ) 10-3 M (A); [NaDS] )10-3 M (B).
Figure 8. TEM patterns of a magnetic fluid obtained at various surfactant concentrations keeping [Co(DS)2]/[Fe(DS)2] ) 0.325 and [Co(DS)2]/[NH2CH3] ) 1.3 × 10-2: [Fe(DS)2] ) 6.5 × 10-3 M (A); [Fe(DS)2] ) 1.3 × 10-2 M (B); [Fe(DS)2] ) 2.6 × 10-2 M (C).
to the fact that formation of micelles induces high local concentration of the reactant at the interface (Co2+ and Fe2+) and then favors the chemical reaction. No obvious explanation can be given for the formation of CoFe2O4 in micelles at room temperature 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 size23,113,114 is controlled by keeping the ratio of reactants ([Fe(DS)2]/[CH3NH3OH] and [Fe(DS)2]/[Co(DS)2]) constant and by increasing the Fe(DS)2 concentration by a factor of 4 (6.5 × 10-3 M < [Fe(DS)2] < 1.2 × 10-3 M). Figure 8 shows 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)2 concentration
increases115 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. Similar behavior has been observed with Fe3O4 and γ-Fe2O3 nanosized particles. The chemical reaction takes place at low reactant concentration, at room temperature, and in the absence of Fe(III) ions.116 In homogeneous solutions, syntheses in the absence of Fe(III) ions induce formation of very well defined particles in the micrometer range.117 Controlling the particle size does not require large changes in the experimental conditions. This allows, to a first approximation, us 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 CoFe2O4 nanosized particles shows, from magnetization curves113 and Mo¨ssbauer118 spectroscopy, an increase in the anisotropy with a decrease in the particle size. For particles having an average size of 5 nm, 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. II.2.2. Copper Metal Particles. Copper metal particles are obtained from reduction, in the absence of air, of Cu(DS)2 micellar solution by sodium borohydrate.22 The formation of copper metal particles in aqueous solution is rather surprising. The reaction is instantaneous, showing a strong micellization effect on the chemical reduction of Cu(II). As a matter of fact, by replacing Cu(DS)2 by its salt derivative, Cu(Cl)2, a copper oxide precipitate appears instantaneously. To control the size of the copper metal particle, syntheses are performed at fixed Cu(DS)2 concentration and various sodium dodecyl sulfate (Na(DS)) concentrations. Figure 9 shows a decrease in the particle size by increasing Na(DS) concentration. This could be due to a decrease in the average number of Cu(DS)2 molecules per Na(DS) micelle. (115) Moumen, N.; Lisiecki, I.; Briois, V.; Pileni, M. P. Supramol. Sci. 1995, 2, 161. (116) Feltin, N.; Pileni, M. P. Langmuir, in press. (117) Matijevic, E. Chem. Mater. 1993, 5, 412. (118) Moumen, N.; Bonville, P.; Pileni, M. P. J. Phys. Chem. 1996, 100, 14410.
Nanosized Particles Made in Colloidal Assemblies
Figure 10. TEM patterns of metallic copper particles obtained in Cu(AOT)2-isooctane solution at various water contents: w ) 4, (A); w ) 6 (B); w ) 12, (C); w ) 18 (D); w ) 34 (E); w ) 44 (F); [Cu(AOT)2] ) 5 × 10-2 M.
This would induce a decrease in the number of nuclei per micelle and then smaller particle sizes. III. 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. III.1. Controlling the Shape by Using Water in Oil Self Assemblies. Copper ions have been reduced in colloidal assemblies differing by their structures.119 In all cases, copper metal particles are obtained. Figure 10 shows syntheses performed at various water contents. No oxide is detectable. Furthermore, the particles are a single crystal. High-resolution measurements show that the copper metal particles formed are in a single crystalline phase. The colloidal system consists of 5 × 10-2 M Cu(AOT)2-isooctane-water. The colloidal structure is changed by increasing the amount of water in Cu(AOT)2isooctane solution.120 Syntheses are performed in various regions of the phase diagram.121 (i) At low water content from w ) 2 to 5.5, a homogeneous reverse micellar solution (the L2 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 formation of a relatively small amount of copper metallic particles. Most of the particles are spherical (87%) with a very low percentage (13%) of cylinders (Figure 10A). The average size of spherical particles is characterized by (119) Petit, C.; Lixon, P.; Pileni, M. P. Langmuir. 1991, 7, 2620. (120) Tanori, J.; Gulik, T.; Pileni, M. P. Langmuir 1997, 13, 632. (121) Tanori, J.; Pileni, M. P. Langmuir 1997, 13, 639.
Langmuir, Vol. 13, No. 13, 1997 3273
a diameter of 12 nm with a size polydispersity of 14%. Plotting the cylinders data as a histogram shows that the average ratio of the length to the width of the particles 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 the same size self assemble on the TEM grid as shown in Figure 10A. (ii) The increase in 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 studies120 indicate that L2* is characterized by a bicontinuous network 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 (〈l〉 ) 3 nm) with increasing w. Syntheses121 at w ) 6 show (Figure 10B) 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 polydispersity 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 nm and 6.7 ( 1.4 nm, respectively. Because the number of connections and then the local concentration increases by increasing the water content, syntheses at water content above w ) 6 (w ) 8 and 10) induce formation of too much material, which makes this impossible to be observed by TEM. (iii) At the phase transition (w ) 11), a LR phase made up of a mixture of planar lamellae and spherulites appears. Syntheses performed in this region of the phase diagram show formation of a very large amount of cylinders in coexistence with spheres. Very few large rods can be observed (Figure 10C). Their sizes vary from 0.1 to 1 µm.122 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% polydispersity in size distribution. The length and width of the cylinders are 25 ( 4 nm and 7.3 ( 1.4, nm respectively. A slight increase in the size 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 10D) than those observed at low water content. As a matter of fact, Figure 10D shows formation of triangles, squares, cylinders, and spheres. (v)At w ) 20, an isotropic phase appears. By increasing the water content from w ) 20 to w ) 29, the lamellar phase progressively disappears, giving rise to 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 from w ) 21 to w ) 29 induce formation of roughly 10% cylinders and 90% spheres. The percentage of cylinders decreases with increasing 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 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. It is attributed to the L2* phase, which (122) Tanori, J.; Pileni, M. P. Adv. Mater. 1995, 7, 862.
3274 Langmuir, Vol. 13, No. 13, 1997
Pileni
Table 1. Variation with the Water Content, w, of the Average Diameter of the Sphere, 〈ds〉, the 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), the Average length, 〈Lc〉, and Width, 〈l) w
〈ds〉 (nm)
σs (%)
%s
%c
%c(weight)
〈Lc〉 (nm)
6 34
9.5 9.5
27 19
68 58
32 42
45.5 51.4
22.6 1.8
w
σLc (%)
〈lc〉 (nm)
σlc (%)
〈Lc/lc〉
σLc/lc (%)
6 34
24 27
6.7 6.5
21 24
3.5 3.2
40 33
is similar to that obtained at lower water content (in the range w ) 5.6-11). By increasing the water content from w ) 30 to w ) 35, the interconnected cylinder network is diluted with a decrease in the number of connections. Over all this water content range, formation of spherical and cylindrical copper metallic particles is observed. Figure 10E 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 from w ) 35 to w ) 40, no drastic changes in the particle sizes and in the percentage of cylinders are observed. Figure 10F 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 polydispersity 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 10A 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 (Table 1). 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 (L2*) 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 cylinder axes (*3.3) are observed at low (5.5 < w < 11) and high (30 < w < 35) water content. Because of this large similarity under various experimental conditions, this phenomenom is attributed to the structure of the colloid used as a template. This control of the particle shape could be explained as in nature,1,2 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
Figure 11. TEM patterns of colloidal copper dispersions prepared in a pure copper dodecyl sulfate solution: [Cu(DS)2] ) 5 × 10-4 M (A); 7.5 × 10-4 M (B); 1.2 × 10-3 M (C); [NaBH4] ) 2‚[Cu(DS)2].
system reach a lower energy state. In such a local supersaturation regime, the chiral molecules used to form the colloidal template (Cu(AOT)2) impose an orientation of each reactant. The control of the crystal morphology could be due to the presence of a high concentration of surfactant (or compounds resulting from the chemical reaction), which specifically adsorbs on certain crystal faces. A similar approach has been recently proposed.123,124 Self organizing media have been used in an elegant method for producing 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.125,126 III.2. Controlling the Shape by Using Oil in Water Micelles. As above, copper dodecyl sulfate (Cu(DS)2) forms micelles above the cmc. Copper particles have been synthetized below and above the cmc.22 Syntheses of Cu(II) salt in aqueous solution, by sodium borohydrate addition, induces flocculation of pure copper oxide materials. When Cu(II) salt is replaced by Cu(DS)2 at a concentration below the cmc, a brown and isotropic solution is obtained and no flocculation appears. At the end of the chemical reaction, a drop of the solution is left on a carbon grid. The TEM pattern shows (Figure 11A) large rods made of copper oxide. A synthesis at a higher (123) Walsh, D.; Hopwood, J. D.; Mann, S. Science 1994, 264, 1576. (124) Walsh, D.; Mann, S. Nature 1995, 377, 320. (125) Weissbuch, I.; Addadi, L.; Berkovitch-Yellin, Z.; Gati, E.; Lahav, M.; Leiserowitz, L. Nature 1984, 310, 161. (126) Weissbuch, I.; Frolow, F.; Addadi, L.; Lahav, M.; Leiserowitz, L. J. Am. Chem. Soc. 1990, 112, 7718.
Nanosized Particles Made in Colloidal Assemblies
Langmuir, Vol. 13, No. 13, 1997 3275
Figure 13. Monolayer made of coated Ag2S nanosized particles having an average diameter of 5.8 nm.
Figure 12. TEM patterns (A) and expanded picture (B) of the copper network prepared in a pure copper dodecyl sulfate solution: [Cu(DS)2] ) 1.2 × 10-3 M; [NaBH4] ) 2.4 × 10-3 M.
Cu(DS)2 concentration shows formation of rods which tend to be more elongated (Figure 11B). The electron micrograph pattern indicates formation of a mixture of copper metal and oxide material. At the Cu(DS)2 cmc, synthesis results in the formation of an interconnected network made of pure copper metallic particles (Figure 11C). The network is obtained over a very long range (Figure 12). Enhancement of the network shows a change in the shape of the material. Such behavior is rather surprising. However, by small X-ray grazing diffraction of sodium dodecyl sulfate, formation of crystals at the water-air interface of sodium dodecyl sulfate has been observed,127 below the cmc. The formation of a large network of (Cu0)n could be explained as follows: below the cmc, Cu(DS)2 molecules interact at the vessel air-water interface and form an oriented self assembly. Addition of sodium borohydride induces a reduction of copper ions at this air-water interface, favoring formation of a large rod network. Changes in optical properties with size17,62 and shape22 of copper metallic particles have been observed. IV. Self Assemblies of Particles Colloidal self organization with nanocrystallites requires a hard sphere repulsion, a controlled size distribution, and the inherent Van der Waals attraction between particles. IV.1. Three-Dimensional Crystal Made of Different-Sized Silver Sulfide Particles. As shown in Figure 6, the size of silver sulfide increases with increasing water content.41 By thiododecane addition, the Ag2S nanocrystallites are coated and extracted from reverse micelles. The coated nanocrystallites are dispersed in heptane, forming an optically clear solution. The average size of coated particles increases from 3 to 5.8 nm. The extraction induces a size selection with a strong decrease in the size distribution, which drops from 30% to 14%. (127) Weissbuch, I. Personal communication.
Figure 14. TEM of an island of Ag2S in a {100} plane of a close-packed structure made of 3 nm (A and B), 4 nm (C and D), and 5.8 nm (E and F) particles.
By leaving a drop of a dilute solution of coated particles on a carbon grid, the particles arrange themselves in a hexagonal network (Figure 13) and form a monolayer over a large domain.42,128 In some cases, the monolayer area reaches 100 µm2. Instead of leaving a drop on the carbon grid, the support is left inside a concentrated solution of colloidal particles. Very large aggregates are formed, and they are surrounded (128) Motte, L.; Billoudet, F.; Lacaze, E.; Douin, J.; Pileni, M. P. J. Phys. Chem. B 1997, 101, 138.
3276 Langmuir, Vol. 13, No. 13, 1997
Figure 15. TEM of copper metal particles: [Cu(AOT)2] ) 5 × 10-2 M; w ) 20.
by smaller aggregates. Magnification of one part of the aggregate shows that it consists of nanosized particles (insets of Figure 14) which appear not to be placed at random but to be collectively organized. It is clear from the TEM observations that Ag2S layers are usually overlapping, indicating a three-dimensional structure. By tilting the sample, it has been repeatedly demonstrated that it is always possible to find an orientation for which the stacking of monolayers appears to be periodic. Monolayers of Ag2S particles are made up of close-packed particles mimicking the stacking of atoms in a {111} plane of a close-packed structure. It is thus believed that, when overlapping, layers conserve this close-packed arrangement, the particles adopt a three-dimensional close-packed structure, face-centered cubic (fcc) or hexagonal closepacked (hcp). To discriminate between the compact fcc and hcp stackings, tilt experiments have been performed by using a TEM double-tilt holder. The projected positions of Ag2S particles show a pseudohexagonal structure. The “hexagons” are in fact elongated along the vertical direction, but the ratio of the distances between particle positions in the horizontal and vertical directions is close to x2/2. This may correspond to the stacking of a {110} plane of the fcc structure (Figure 14) and there is no direction in a perfect hcp structure for which the projected positions of the particles would take this configuration. Tilting of the sample by 45° while keeping the horizontal direction constant leads to the observation of a fourfold symmetry. This symmetry is again characteristic of the stacking of {001} planes of the cubic structure but cannot be found in the hexagonal structure. From these observations, we can undoubtly conclude that multilayers of Ag2S particles are composed by stacking monolayers in a face-centered cubic structure. Measurements of the distance between projected positions of particle as well as angles between directions joining particles positions are consistent with this conclusion. The distance between two particles core to core in the [010] direction of the superlattice of particles is about 11 nm, which corresponds to the cell parameter (a0) of the pseudo-fcc crystal formed
Pileni
by Ag2S particles. The average particle diameter is about 5.8 nm, while the shortest distance between particles is a0x2/2 ) 7.8 nm. This gives a calculated spacing between two particles of 2 nm. This is in good agreement with the average distance between particles (2 nm) obtained from the observations of monolayers. It is possible to make different-sized particles. Using the same procedure as that described above, we have produced 3D superlattices for particles with an average size of 2 nm (inset of Figure 14A) and 3 nm (inset of Figure 14B). These aggregates are very similar to those formed with particles with an average size of 5.8 nm (inset of Figure 14C). Higher resolution observations of these aggregates (Figure 14) show a fourfold symmetry typical of the fcc stacking. Furthermore, the inset of Figure 14C shows a stacking defect in the structure as in a crystal.128 By AFMTM and GAXS129,130 techniques, it is observed that the produced islands are truncated bipyramids. The three-dimensional organization does not depend on the support used. It has been observed by using silicon wafers, graphite, gold, or mica as a support. Formation of 2D and 3D superlattices of silver metal particles have been obtained by the same technique.131 IV.2. Self Assemblies of Copper Metallic Particles. Reduction of 5 × 10-2 M Cu(AOT)2 isooctanen-water solution characterized by a water content of 20 induces the formation of a large number of particles with a low polydispersity. A drop of the solution is left on a carbon grid, and from the TEM pattern, formation of several layers of copper metallic particles can be observed (Figure 15). Acknowledgment. I would like to thank my coworkers who participated in this work: T. Abdelhafed, F. Billoudet, J. Cizeron, S. Didierjean, N. Duxin, N. Feltin, A. Filankemboi, L. Francois, J. F. Hochepied, D. Ingert, L. Levy, J. L. Lindor, Dr. I Lisieki, Dr. L. Motte, Dr. N. Moumen, P. Millard, A. Ngo, Dr. C. Petit, and J. Tanori. Glossary S ) surfactant Surfactants Used Na(AOT) ) sodium bis(2-ethylhexyl)sulfosuccinate Ag(AOT) ) silver bis(2-ethylhexyl)sulfosuccinate Cu(AOT)2 ) copper bis(2-ethylhexyl)sulfosuccinate Na(DS) ) sodium dodecyl sulfate Cu(DS)2 ) copper dodecyl sulfate Co(DS)2 ) cobalt dodecyl sulfate Fe(DS)2 ) iron dodecyl sulfate CTAC ) Cetyltrimethylammonium chloride w ) [H2O]/[S] Techniques FTIR ) Fourier transformed infrared SAXS ) small angle X-ray scattering GSAXS ) grazing incidence small angle X-ray scattering TEM ) transmission electron microscopy AFMTM ) atomic force microscopy in tapping mode XANES ) X-ray absorption near edge spectroscopy LA960319Q (129) Motte, L.; Billoudet, F.; Lacaze, E.; Pileni, M. P. Adv. Mater. 1996, 8, 1018. (130) Motte, L.; Billoudet, F.; Thiaudie`re, D.; Naudon, A.; Pileni, M. P. J. Phys. III 1997, 7, 517. (131) Taleb, A.; Petit, C.; Pileni, M. P. Chem. Mater. 1997, 9, 950.
