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Shape control of copper nanocrystals
Applied Surface Science 164 Ž2000. 260–267 www.elsevier.nlrlocaterapsusc
Shape control of copper nanocrystals A. Filankembo, M.P. Pileni ) Laboratoire SRSI, URA CNRS 1662, UniÕersite´ Pierre et Marie Curie (Paris VI), B.P. 52, 4 Place Jussieu, F-752 31 Paris Cedex 05, France
Abstract In this short paper, it is demonstrated that addition of various salts differing by their counter ions markedly changes the shape of copper metal nanocrystals. These follow the Hofmeister series. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Shape control; Copper nanocrystal; Salt
1. Introduction These last 10 years, several groups used colloidal solution as template. By using reverse micelles Žwater in oil droplets. we demonstrated, for the first time w1x, that it is possible to make spherical nanocrystals. The size of the water droplets controls that of the nanomaterial. A very large variety of nanocrystals has been prepared by using this technique w2–13x. However some groups did not succeed to keep the shape of the nanomaterials w14–16x. This is due to the fact that addition of salt as reactants induces change in the colloidal structure. Then the template does not have the structure expected. Reverse micelles are water in oil droplets w17x. Water is readily solubilized in the polar core, forming as so called ‘‘water pool’’, characterized by w, the water-surfactant molar ratio Ž w s wH 2 OxrwSx.. ) Corresponding author. Laboratoire SRSI, URA CNRS 1662, Universite´ Pierre et Marie Curie ŽParis VI., B.P. 52, 4 Place Jussieu, F-75231 Paris Cedex 05, France. Tel.: q33-1-44-27-2516; fax: q33-1-44-27-25-15. E-mail address: [email protected] ŽM.P. Pileni..
We make no distinction between swollen micelles or microemulsions because there is none. However, some groups call ‘‘reverse micelles’’ aggregates containing a low amount of water Ž w - 15. whereas ‘‘microemulsion’’ for larger water content. For NaŽAOT. –water–isooctane solution, the water pool radius is controlled by the water content w17–20x. One of the challenges in nanomaterial Science is not only to control the size of particles but also the shape. As a matter of fact, these two factors are expected to induce markedly changes in the physical properties. Cylindrical water in oil droplets were used to make nanocrystals and cylindrical particles were produced w7x. This study has been extended to various parts of the diagram differing by the shape of the self-assemblies with formation of either interconnected cylinders, lamellae or spherulites. It has been well demonstrated that the shape of the template plays an important role on the shape of the material produced w21,22x. In this paper, we demonstrate that in one given region of the phase diagram of the system CuŽAOT. 2 –water–isooctane, the presence of slight
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 0 0 . 0 0 3 4 5 - 7
A. Filankembo, M.P. Pileni r Applied Surface Science 164 (2000) 260–267
amount of salt induces change in the shape of the material. The salt anion plays the major role whereas that of the cation is totally minor.
2. Experimental section Copper Ž II . bis Ž 2-ethylhexyl . sulfosuccinate, CuŽAOT. 2 , has been described previously w23x. Single distilled water was passed through a Millipore MilliQ system cartridge until its resistivity reached 18 M V cm. All chemicals were used without further purification. Electron micrographs are obtained with a PHILIPS electron microscope ŽModel EM 430, high tension 300 kV..
3. Colloidal system 3.1. General Õiew In solution, surfactant molecules self assemble to form aggregates w24,25x. Microstructure is set by interfacial curvature which is prescribed by a balance between head polar group forces, electrostatic double layer and opposing hydrocarbon tail interactions. This has to be taken together with geometric constants set by volume ratio of components. At another level, these interactions between interfacial regions, which together with entropy factors determine two or three phase regions, are very subtle. The contribution of the entropy of the folded film is predominant in the free energy of the solution, while the morphology has little influence. When functionalized surfactant is used Žthe reactant is associated to the surfactant as CuŽAOT. 2 the progression of the phase diagram CuŽAOT. 2 – water–isooctane with increasing the water content is extensively described w26–28x. The overall concentration of CuŽAOT. 2 remains constant. However, phase transitions take place and the amount of CuŽAOT. 2 differs from one phase to the other. At low water contents Ž1 - w - 5. a homogeneous reverse micellar solution Žthe L 2 phase. is formed. In this range, the shape of the droplets changes from spheres Žbelow w s 4. to cylinders.
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The increase of the water content Ž5 - w - 9., destabilizes the solution, and the L 2 phase separates into a more concentrated reverse micellar solution ŽL)2 . and an almost pure isooctane phase. Structural studies of L)2 phase is characterized by a bicontinuous network of cylinders. The number of connections increases with increasing w. Upon further addition of water Ž9 - w - 10., the coacervate becomes more and more concentrated, and subsequently a turbid, birefringent, lamellar phase ŽL a . starts to coexist with the L)2 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 Ž10 - w - 15.5.. Stable emulsion made of spherulites containing in the interior and external interconnected cylinders is formed. Further addition of water molecules leads to appearance of a L)2 Žinterconnected cylinders. phases in coexistence with the L a and isooctane phases Ž15.5 - w - 26.. The increase in the water content w from 15.5 to 26 induces a progressive disappearance of L a phase. Above w s 26, L)2 is in equilibrium with isooctane. The increase of the water content induces an increase in the volume of the L)2 phase and a decrease in the isooctane volume. This corresponds to a dilution of the interconnected cylinders. At w s 28.5, the upper phase totally disappears and remains an isotropic region attributed to water-in-oil droplets. 3.2. Description of the template Cu Ž A O T . 2 is solubilized in isooctane ŽwCuŽAOT. 2 x s 5 = 10y2 M. and an isotropic solution is formed. Water is added to the solution to reach a water concentration of 5 = 10y1 M. A phase transition appears with formation of three phases. Structures have been studied by small angle scattering ŽSAXS., conductivity, freeze fracture electron microscopy ŽFFEM. w26–28x. Ži. The isotropic phase is made of interconnected cylinders with formation of channel of water ŽFig. 1A.. Žii. The birefringent phase is made by a mixture of planar ŽFig. 1B. and onion Žspherulites. ŽFig. 1C. phases.
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Fig. 1. Freeze fracture electron microscopy and its cartoon corresponding to the self organization: ŽA. interconnected cylinders; ŽB. planar lamellar phase; ŽC. spherulite Žor oignon phase. made of lamellae.
Žiii. The isooctane is repelled by phases described in Ži. and Žii.. Addition of low concentration Ž10y3 M. of various salts totally dissociated in aqueous phase does not macroscopically change the phase diagram. The three phases seem to remain the same. However, it
will be demonstrated below it drastically changes the shape of nanocrystals. 3.3. Chemical reaction Water is replaced by hydrazine ŽwN2 H 4 x s 0.15 . M . Immediately after mixing the solution turns to brown. This is due to the reduction of CuŽII. to
A. Filankembo, M.P. Pileni r Applied Surface Science 164 (2000) 260–267
CuŽ0.. With time the darkness of the solution increases. The overall chemical reaction is 2Cu Ž AOT . 2 q N2 H 4
™ 2CuŽ 0. q 4HŽ AOT. q N . 2
Ž 1.
The chemical reaction takes 3 h. 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 ŽTEM. and electron diffraction ŽED.. The synthesis of copper nanocrystals is made in the absence or presence of various salts. The ED shows concentrical circles characteristic of a face-centered cubic phase with a lattice dimen˚ as it is observed with bulk sion equal to 3.61 A
263
copper metal material and nanoparticles w7,10, 21,22,29x.
4. Hofmeister series The main idea is that the solubility of organic compounds in water can be adjusted by adding inorganic salts. The anions and cations can be classified into so-called Hofmeister series w30x SO42y) CO 32y) HPO42y) Fy) Cly) Bry) NOy 3 ) Iy) ClO4y) SCNy Naq) Kq) Liq) Rbq) Csq. The ‘‘salting out’’ process indicates a decrease in the water solubility of organic solutes whereas ‘‘salt-
Fig. 2. TEM patterns of particles obtained after 2 h in the presence of various salts ŽA. wsaltx s 0; ŽB. wNaClx s 10y3 M, ŽC. wNaClx s 1.8.10y3 M, ŽD. wKClx s 10y3 M, wCuŽAOT. 2 x s 5.10y2 M, w s 11, wN2 H 4 x s 0.15 M.
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ing in’’ is the opposite Žan increase.. The Hofmeister series is universal in sense that the order of the sequence does not depend on the nature of the organic solute and is applied for instance to alcohol, polymers, surfactants, proteins, etc. These ‘‘salting out’’ and ‘‘salting in’’ processes are more pronounced for anions than cations. As an example, Naq and Kq behave quite similarly. Two hypotheses have been proposed to explain Hofmeister series behavior. Ža. The salt affects the water structure. The salts on the left hand-side of the series are believed to be ‘‘structure makers’’ while those on the right handside ‘‘structure breakers’’ w30–34x. Žb. The ‘‘salting out’’ and ‘‘salting in’’ phenomena have an interfacial origin. Salts either desorb or
adsorb at the water-organic solute interface producing an increment in the solute free energy and thereby modifying the phase equilibrium w35–39x. The controversy between these two interpretations is far from being resolved. Hence, two classes of inorganic electrolytes have to be distinguished w40x.
Ži. The lyotropic salts decrease the mutual solubility between water and surfactant as SO42y, CO 32y, Cly. Žii. The hydrotropic salts increase the mutual solubility between water and surfactant as ClO4y or SCNy.
Fig. 3. TEM patterns of particles obtained after 3 h in the presence of various salts ŽA. and ŽB. wNaBrx s 10y3 M, ŽC. wNaHSO 3 x s 10y3 M and ŽD. wNaNO 3 x s 10y3 M.
A. Filankembo, M.P. Pileni r Applied Surface Science 164 (2000) 260–267
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5. Results
6. Discussion
In the absence of salt, very few elongated and cylindrical of copper metal particles are observed ŽFig. 2A.. Most of the particles are spherical. Addition of 1 = 10y3 M of NaCl in the microphases induces a marked change in the particle shape with appearance of very long rods ŽFig. 2B.. The length and width of the longest are about 1 mm and 15 nm, respectively. The structure of these rods have been studied in detail w41x and attributed to troncated decahedron. On increasing the NaCl concentration from 1 = 10y3 to 1.8 = 10y3 M, the length of the rods decreases until 220 nm whereas the width increases until 22 nm ŽFig. 2C.. By replacing 10y3 M of NaCl by same amount of KCl, similar data are obtained ŽFig. 2D.. There is no change in size and shape when NaCl is replaced by KCl. In apposition, the replacement of 10y3 M of NaCl by 10y3 M of NaBr induces a large variety of shape of copper metal particles with formation of very few rods ŽFig. 3A and B.. It has to be noticed that the main particles are not cylindrical ŽFig. 3B.. Similar behavior is observed with KBr instead of NaBr. With 10y3 M of NaHSO 3 , Fig. 3C shows formation of a well dispersed elongated particles with a very low shape distribution. In the present case, the kinetic of growth is very slow. Because of the very low distribution in shape and in size, the particles tend to self assemble. In presence of 10y3 M of NaNO 3 , most of copper nanocrystals are spherical. Very few are elongated ŽFig. 3D.. With KNO 3 , similar behavior at that obtained with the sodium derivative is observed. We have to note that the kinetic of growth drastically varies with salts addition. In the absence of salt and in presence of 10y3 M of NaCl, the kinetic is roughly the same. After 2 h, the size and the shape of the particles remain unchanged. At the opposite with NaHSO 3 , the chemical reduction is very slow. With NaBr the reduction is very fast. This work is too preliminary to give quantitative data. However by yes, it is very easy to see the spide of the brown color appears which is characteristic of copper reduction.
From data presented in Figs. 2 and 3, it is clearly demonstrated that addition of salts drastically perturbs the shape of the copper metal particles. The salts used are totally dissociated and act as strong electrolyte. In colloids addition of salts is well known to induce an increase in the interface rigidity. This cannot be retained to explain the change in the particle shape by using various salts. They are expected to act similarly. To explain these data, we have to take into account the Hofmeister series based on the change in the solubility of organic molecules with salt addition. In Hofmeister series HSOy 3 is not reported. In first approximation, we could assume that it acts as SO42y and HSOy 3 is highly lyotropic. This anion desorbs at the water–oil interface and then decreases the solubility between water and surfactant. The water is more structured. That means that the water molecules are highly bound to the surfactant. This process provokes various phenomena. Ži. The water–surfactant interface is characterized by a higher rigidity compared to the absence of salt. Žii. The high rigidity of the microenvironment modifies the redox potential of CuŽAOT. 2 . Such high rigidity of the microphase and the change in the redox potential can be related to what has been described above at very low water content in reverse micelles. Because the amount of reduced CuŽAOT. 2 is low, the structure of the template will not be strongly perturbed. Taking into account such changes in the local microstructure of the template, we could assume that very few number of copper atoms are reduced. Because of fact that the surfactant is chiral, cylindrical clusters are formed. The copper atoms produced during the chemical reaction are in a very visquous environment. This prevents the atoms to diffuse inside the matrix and the atoms one by one bound to the cylindrical nucleus. This explains why the particles shown on Fig. 3C are characterized by a rather low size and shape distribution. y On replacing HSOy 3 by Cl , the desorption of the salt at the water–surfactant interface decreases. The
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water molecules are less bound to the interface. As in reverse micelles, on increasing the water content the reduction yield increases. Hence more CuŽAOT. 2 are reduced. However the yield remains rather low and the template is not deeply perturbed. Concerning Bry and NOy 3 , these anions are not lyotropic. From Hofmeister series we know that Bry and NOy 3 tend to induce less and less ‘‘salting out’’. These anions are at the broad between lyotropic and hydrotropic salts. It can be expected that the water structure containing Bry and NOy 3 is close to that of bulk water. The salt tends to adsorb at the interface and replaces the water molecules, which are expelled inside the microphase. This screens the interactions between CuŽII. and AOTy. CuŽAOT. 2 tends to behave as CuŽII. ions solubilized in the aqueous phase of the microphase. The role of the interface is strongly reduced and the template loses its efficiency. The surfactant behaves now as a polymer to protect the particle growth but not as a template. From Hofmeister series Bry is on the left-hand side compared to NOy 3 . Hence, if it plays a role, we would expect less y rods with NOy 3 than Br . This is observed. As y matter of fact with Br , very few rods are observed whereas with NOy 3 only spheres are formed. The change in the water structure with salt addition is consistent with the kinetic of growth. If the water is highly structured Žlyotropic salt., the kinetic is very slow whereas it is very fast when the water structure is like bulk phase. We know that the Hofmeister series is slightly sensitive to the cations. This is clearly observed in the particle shape. Similar behavior is obtained with KCl and NaCl, with NaNO 3 and KNO 3 and with NaBr and KBr, respectively. These similarities in the behaviors strongly support the important role of Hofmeister series in the control of the particle shape.
7. Conclusion In this paper, we have demonstrated that the growth of the rods of copper metal particles depend drastically on the counter ions of the added salt. The nanocrystal shape seems to be governed by the anions Hofmeister series.
Acknowledgements All TEM pictures were obtained at SRMA, CEASaclay. We are indebted to Dr. Ph. Dubuisson for giving us facilities.
References w1x C. Petit, M.P. Pileni, J. Phys. Chem. 92 Ž1988. 2282. w2x P.J. Atkinson, M.J. Grimson, R.K. Heenan, A.M. Howe, B.H. Robinson, J. Chem. Soc., Chem. Commun Ž1989. 1807. w3x C. Petit, P. Lixon, M.P. Pileni, J. Phys. Chem. 94 Ž1990. 1598. w4x B.H. Robinson, A.N. Khan-Lodhi, T.F. Towey, J. Chem. Soc. Faraday Trans. 86 Ž1990. 3757. w5x L. Motte, C. Petit, P. Lixon, L. Boulange, M.P. Pileni, Langmuir 8 Ž1992. 1049. w6x P. Kumar, V. Pillai, S.R. Bates, D.O. Shah, Mat. Lett. 16 Ž1993. 68. w7x I. Lisiecki, M.P. Pileni, J. Am. Chem. Soc. 115 Ž1993. 3887. w8x T. Hirai, H. Sato, I. Komasawa, Ind. Eng. Chem. Res. 33 Ž1994. 3262. w9x J. Cizeron, M.P. Pileni, J. Phys. Chem. 99 Ž1995. 17410. w10x I. Lisiecki, M.P. Pileni, J. Phys. Chem. 99 Ž1995. 5077. w11x M. Kishida, T. Fujita, K. Umaxoshi, J. Ishiyama, H.J. Nagata, Chem. Soc. Chem. Commun Ž1995. 763. w12x K. Suriki, M. Harada, A. Shioi, J. Chem. Eng. Jpn. 109 Ž1996. 245. w13x C. Sangregorio, M. Galeotti, U. Bardi, P. Baglioni, Langmuir 12 Ž1996. 5800. w14x J.D. Hopwood, S. Mann, Chem. Mater. 9 Ž1997. 1819. w15x D. Walsh, S. Mann, Adv. Mater. 9 Ž1997. 658. w16x D. Walsh, S. Mann, Chem. Mater. 8 Ž1996. 1953. w17x M.P. Pileni ŽEd.., Reactivity in Reverse Micelles, Elsevier, 1989. w18x M. Zulauf, H.F. Eicke, J. Phys. Chem. 83 Ž1979. 480. w19x M.P. Pileni, T. Zemb, C. Petit, Chem. Phys. Lett. 118 Ž1985. 414. w20x M.P. Pileni, P. Brochette, B. Hickel, B. Lerebours, J. Colloid Interface Sci. 98 Ž1984. 549. w21x J. Tanori, M.P. Pileni, Adv. Mater. 7 Ž1995. 862. w22x J. Tanori, M.P. Pileni, Langmuir 13 Ž1997. 639. w23x C. Petit, P. Lixon, M.P. Pileni, Langmuir 7 Ž1991. 2620. w24x M.P. Pileni, J. Phys. Chem. 97 Ž1993. 6961. w25x M.P. Pileni, Langmuir 13 Ž1997. 3266. w26x J. Tanori, T. Gulik, M.P. Pileni, Langmuir 13 Ž1997. 632. w27x I. Lisiecki, P. Andre, ´ A. Filankembo, C. Petit, J. Tanori, T. Gulik-Krzywicki, M.P. Pileni, J. Phys. Chem. 103 Ž1999. 9168. w28x I. Lisiecki, P. Andre, ´ A. Filankembo, C. Petit, J. Tanori, T. Gulik-Krzywicki, M.P. Pileni, J. Phys. Chem. 103 Ž1999. 9176.
A. Filankembo, M.P. Pileni r Applied Surface Science 164 (2000) 260–267 w29x M.P. Pileni, T. Gulik-Krzywicki, J. Tanori, A. Filankembo, J.C. Dedieu, Langmuir 14 Ž1998. 7359. w30x F. Hofmeister, Arch. Exp. Pathol. Pharthol. 24 Ž1888. 247. w31x K.D. Collins, M. Washabaugh, Quarterly Rev. Biophys. 18 Ž1985. 323. w32x P.H. Von Hippel, K.Y. Wong, Science 145 Ž1964. 577. w33x K. Franks, in: K. Franks ŽEd.., Water-A. Comprehensive Treatise vol. 2 Plenum, New York, 1973, p. 1. w34x D. Eagland, in: K. Franks ŽEd.., Water-A. Comprehensive Treatise vol. 4 Plenum, New York, 1973, p. 305. w35x R. Aveyard, Can. JK. Chem. 60 Ž1981. 1317.
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w36x J. Schellman, Biopolymers 26 Ž1987. 549. w37x L. Piculell, S. Nilsson, J. Phys. Chem. 93 Ž1989. 5598. w38x L. Piculell, S. Nilsson, Prog. Colloid Polym. Sci. 82 Ž1990. 198. w39x L. Piculell, B. Lindman, Adv. Colloid Interface Sci. 94 Ž1992. 5149. w40x M. Kahlweit, E. Lessner, R. Strey, J. Phys. Chem. 88 Ž1984. 1937. w41x I. Lisiecki, A. Filankembo, H. Sack-Kongehl, K. Weiss, M.P. Pileni, J. Urban, Phys. Rev. B, in press.
Shape control of copper nanocrystals A. Filankembo, M.P. Pileni ) Laboratoire SRSI, URA CNRS 1662, UniÕersite´ Pierre et Marie Curie (Paris VI), B.P. 52, 4 Place Jussieu, F-752 31 Paris Cedex 05, France
Abstract In this short paper, it is demonstrated that addition of various salts differing by their counter ions markedly changes the shape of copper metal nanocrystals. These follow the Hofmeister series. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Shape control; Copper nanocrystal; Salt
1. Introduction These last 10 years, several groups used colloidal solution as template. By using reverse micelles Žwater in oil droplets. we demonstrated, for the first time w1x, that it is possible to make spherical nanocrystals. The size of the water droplets controls that of the nanomaterial. A very large variety of nanocrystals has been prepared by using this technique w2–13x. However some groups did not succeed to keep the shape of the nanomaterials w14–16x. This is due to the fact that addition of salt as reactants induces change in the colloidal structure. Then the template does not have the structure expected. Reverse micelles are water in oil droplets w17x. Water is readily solubilized in the polar core, forming as so called ‘‘water pool’’, characterized by w, the water-surfactant molar ratio Ž w s wH 2 OxrwSx.. ) Corresponding author. Laboratoire SRSI, URA CNRS 1662, Universite´ Pierre et Marie Curie ŽParis VI., B.P. 52, 4 Place Jussieu, F-75231 Paris Cedex 05, France. Tel.: q33-1-44-27-2516; fax: q33-1-44-27-25-15. E-mail address: [email protected] ŽM.P. Pileni..
We make no distinction between swollen micelles or microemulsions because there is none. However, some groups call ‘‘reverse micelles’’ aggregates containing a low amount of water Ž w - 15. whereas ‘‘microemulsion’’ for larger water content. For NaŽAOT. –water–isooctane solution, the water pool radius is controlled by the water content w17–20x. One of the challenges in nanomaterial Science is not only to control the size of particles but also the shape. As a matter of fact, these two factors are expected to induce markedly changes in the physical properties. Cylindrical water in oil droplets were used to make nanocrystals and cylindrical particles were produced w7x. This study has been extended to various parts of the diagram differing by the shape of the self-assemblies with formation of either interconnected cylinders, lamellae or spherulites. It has been well demonstrated that the shape of the template plays an important role on the shape of the material produced w21,22x. In this paper, we demonstrate that in one given region of the phase diagram of the system CuŽAOT. 2 –water–isooctane, the presence of slight
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 0 0 . 0 0 3 4 5 - 7
A. Filankembo, M.P. Pileni r Applied Surface Science 164 (2000) 260–267
amount of salt induces change in the shape of the material. The salt anion plays the major role whereas that of the cation is totally minor.
2. Experimental section Copper Ž II . bis Ž 2-ethylhexyl . sulfosuccinate, CuŽAOT. 2 , has been described previously w23x. Single distilled water was passed through a Millipore MilliQ system cartridge until its resistivity reached 18 M V cm. All chemicals were used without further purification. Electron micrographs are obtained with a PHILIPS electron microscope ŽModel EM 430, high tension 300 kV..
3. Colloidal system 3.1. General Õiew In solution, surfactant molecules self assemble to form aggregates w24,25x. Microstructure is set by interfacial curvature which is prescribed by a balance between head polar group forces, electrostatic double layer and opposing hydrocarbon tail interactions. This has to be taken together with geometric constants set by volume ratio of components. At another level, these interactions between interfacial regions, which together with entropy factors determine two or three phase regions, are very subtle. The contribution of the entropy of the folded film is predominant in the free energy of the solution, while the morphology has little influence. When functionalized surfactant is used Žthe reactant is associated to the surfactant as CuŽAOT. 2 the progression of the phase diagram CuŽAOT. 2 – water–isooctane with increasing the water content is extensively described w26–28x. The overall concentration of CuŽAOT. 2 remains constant. However, phase transitions take place and the amount of CuŽAOT. 2 differs from one phase to the other. At low water contents Ž1 - w - 5. a homogeneous reverse micellar solution Žthe L 2 phase. is formed. In this range, the shape of the droplets changes from spheres Žbelow w s 4. to cylinders.
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The increase of the water content Ž5 - w - 9., destabilizes the solution, and the L 2 phase separates into a more concentrated reverse micellar solution ŽL)2 . and an almost pure isooctane phase. Structural studies of L)2 phase is characterized by a bicontinuous network of cylinders. The number of connections increases with increasing w. Upon further addition of water Ž9 - w - 10., the coacervate becomes more and more concentrated, and subsequently a turbid, birefringent, lamellar phase ŽL a . starts to coexist with the L)2 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 Ž10 - w - 15.5.. Stable emulsion made of spherulites containing in the interior and external interconnected cylinders is formed. Further addition of water molecules leads to appearance of a L)2 Žinterconnected cylinders. phases in coexistence with the L a and isooctane phases Ž15.5 - w - 26.. The increase in the water content w from 15.5 to 26 induces a progressive disappearance of L a phase. Above w s 26, L)2 is in equilibrium with isooctane. The increase of the water content induces an increase in the volume of the L)2 phase and a decrease in the isooctane volume. This corresponds to a dilution of the interconnected cylinders. At w s 28.5, the upper phase totally disappears and remains an isotropic region attributed to water-in-oil droplets. 3.2. Description of the template Cu Ž A O T . 2 is solubilized in isooctane ŽwCuŽAOT. 2 x s 5 = 10y2 M. and an isotropic solution is formed. Water is added to the solution to reach a water concentration of 5 = 10y1 M. A phase transition appears with formation of three phases. Structures have been studied by small angle scattering ŽSAXS., conductivity, freeze fracture electron microscopy ŽFFEM. w26–28x. Ži. The isotropic phase is made of interconnected cylinders with formation of channel of water ŽFig. 1A.. Žii. The birefringent phase is made by a mixture of planar ŽFig. 1B. and onion Žspherulites. ŽFig. 1C. phases.
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A. Filankembo, M.P. Pileni r Applied Surface Science 164 (2000) 260–267
Fig. 1. Freeze fracture electron microscopy and its cartoon corresponding to the self organization: ŽA. interconnected cylinders; ŽB. planar lamellar phase; ŽC. spherulite Žor oignon phase. made of lamellae.
Žiii. The isooctane is repelled by phases described in Ži. and Žii.. Addition of low concentration Ž10y3 M. of various salts totally dissociated in aqueous phase does not macroscopically change the phase diagram. The three phases seem to remain the same. However, it
will be demonstrated below it drastically changes the shape of nanocrystals. 3.3. Chemical reaction Water is replaced by hydrazine ŽwN2 H 4 x s 0.15 . M . Immediately after mixing the solution turns to brown. This is due to the reduction of CuŽII. to
A. Filankembo, M.P. Pileni r Applied Surface Science 164 (2000) 260–267
CuŽ0.. With time the darkness of the solution increases. The overall chemical reaction is 2Cu Ž AOT . 2 q N2 H 4
™ 2CuŽ 0. q 4HŽ AOT. q N . 2
Ž 1.
The chemical reaction takes 3 h. 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 ŽTEM. and electron diffraction ŽED.. The synthesis of copper nanocrystals is made in the absence or presence of various salts. The ED shows concentrical circles characteristic of a face-centered cubic phase with a lattice dimen˚ as it is observed with bulk sion equal to 3.61 A
263
copper metal material and nanoparticles w7,10, 21,22,29x.
4. Hofmeister series The main idea is that the solubility of organic compounds in water can be adjusted by adding inorganic salts. The anions and cations can be classified into so-called Hofmeister series w30x SO42y) CO 32y) HPO42y) Fy) Cly) Bry) NOy 3 ) Iy) ClO4y) SCNy Naq) Kq) Liq) Rbq) Csq. The ‘‘salting out’’ process indicates a decrease in the water solubility of organic solutes whereas ‘‘salt-
Fig. 2. TEM patterns of particles obtained after 2 h in the presence of various salts ŽA. wsaltx s 0; ŽB. wNaClx s 10y3 M, ŽC. wNaClx s 1.8.10y3 M, ŽD. wKClx s 10y3 M, wCuŽAOT. 2 x s 5.10y2 M, w s 11, wN2 H 4 x s 0.15 M.
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ing in’’ is the opposite Žan increase.. The Hofmeister series is universal in sense that the order of the sequence does not depend on the nature of the organic solute and is applied for instance to alcohol, polymers, surfactants, proteins, etc. These ‘‘salting out’’ and ‘‘salting in’’ processes are more pronounced for anions than cations. As an example, Naq and Kq behave quite similarly. Two hypotheses have been proposed to explain Hofmeister series behavior. Ža. The salt affects the water structure. The salts on the left hand-side of the series are believed to be ‘‘structure makers’’ while those on the right handside ‘‘structure breakers’’ w30–34x. Žb. The ‘‘salting out’’ and ‘‘salting in’’ phenomena have an interfacial origin. Salts either desorb or
adsorb at the water-organic solute interface producing an increment in the solute free energy and thereby modifying the phase equilibrium w35–39x. The controversy between these two interpretations is far from being resolved. Hence, two classes of inorganic electrolytes have to be distinguished w40x.
Ži. The lyotropic salts decrease the mutual solubility between water and surfactant as SO42y, CO 32y, Cly. Žii. The hydrotropic salts increase the mutual solubility between water and surfactant as ClO4y or SCNy.
Fig. 3. TEM patterns of particles obtained after 3 h in the presence of various salts ŽA. and ŽB. wNaBrx s 10y3 M, ŽC. wNaHSO 3 x s 10y3 M and ŽD. wNaNO 3 x s 10y3 M.
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5. Results
6. Discussion
In the absence of salt, very few elongated and cylindrical of copper metal particles are observed ŽFig. 2A.. Most of the particles are spherical. Addition of 1 = 10y3 M of NaCl in the microphases induces a marked change in the particle shape with appearance of very long rods ŽFig. 2B.. The length and width of the longest are about 1 mm and 15 nm, respectively. The structure of these rods have been studied in detail w41x and attributed to troncated decahedron. On increasing the NaCl concentration from 1 = 10y3 to 1.8 = 10y3 M, the length of the rods decreases until 220 nm whereas the width increases until 22 nm ŽFig. 2C.. By replacing 10y3 M of NaCl by same amount of KCl, similar data are obtained ŽFig. 2D.. There is no change in size and shape when NaCl is replaced by KCl. In apposition, the replacement of 10y3 M of NaCl by 10y3 M of NaBr induces a large variety of shape of copper metal particles with formation of very few rods ŽFig. 3A and B.. It has to be noticed that the main particles are not cylindrical ŽFig. 3B.. Similar behavior is observed with KBr instead of NaBr. With 10y3 M of NaHSO 3 , Fig. 3C shows formation of a well dispersed elongated particles with a very low shape distribution. In the present case, the kinetic of growth is very slow. Because of the very low distribution in shape and in size, the particles tend to self assemble. In presence of 10y3 M of NaNO 3 , most of copper nanocrystals are spherical. Very few are elongated ŽFig. 3D.. With KNO 3 , similar behavior at that obtained with the sodium derivative is observed. We have to note that the kinetic of growth drastically varies with salts addition. In the absence of salt and in presence of 10y3 M of NaCl, the kinetic is roughly the same. After 2 h, the size and the shape of the particles remain unchanged. At the opposite with NaHSO 3 , the chemical reduction is very slow. With NaBr the reduction is very fast. This work is too preliminary to give quantitative data. However by yes, it is very easy to see the spide of the brown color appears which is characteristic of copper reduction.
From data presented in Figs. 2 and 3, it is clearly demonstrated that addition of salts drastically perturbs the shape of the copper metal particles. The salts used are totally dissociated and act as strong electrolyte. In colloids addition of salts is well known to induce an increase in the interface rigidity. This cannot be retained to explain the change in the particle shape by using various salts. They are expected to act similarly. To explain these data, we have to take into account the Hofmeister series based on the change in the solubility of organic molecules with salt addition. In Hofmeister series HSOy 3 is not reported. In first approximation, we could assume that it acts as SO42y and HSOy 3 is highly lyotropic. This anion desorbs at the water–oil interface and then decreases the solubility between water and surfactant. The water is more structured. That means that the water molecules are highly bound to the surfactant. This process provokes various phenomena. Ži. The water–surfactant interface is characterized by a higher rigidity compared to the absence of salt. Žii. The high rigidity of the microenvironment modifies the redox potential of CuŽAOT. 2 . Such high rigidity of the microphase and the change in the redox potential can be related to what has been described above at very low water content in reverse micelles. Because the amount of reduced CuŽAOT. 2 is low, the structure of the template will not be strongly perturbed. Taking into account such changes in the local microstructure of the template, we could assume that very few number of copper atoms are reduced. Because of fact that the surfactant is chiral, cylindrical clusters are formed. The copper atoms produced during the chemical reaction are in a very visquous environment. This prevents the atoms to diffuse inside the matrix and the atoms one by one bound to the cylindrical nucleus. This explains why the particles shown on Fig. 3C are characterized by a rather low size and shape distribution. y On replacing HSOy 3 by Cl , the desorption of the salt at the water–surfactant interface decreases. The
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water molecules are less bound to the interface. As in reverse micelles, on increasing the water content the reduction yield increases. Hence more CuŽAOT. 2 are reduced. However the yield remains rather low and the template is not deeply perturbed. Concerning Bry and NOy 3 , these anions are not lyotropic. From Hofmeister series we know that Bry and NOy 3 tend to induce less and less ‘‘salting out’’. These anions are at the broad between lyotropic and hydrotropic salts. It can be expected that the water structure containing Bry and NOy 3 is close to that of bulk water. The salt tends to adsorb at the interface and replaces the water molecules, which are expelled inside the microphase. This screens the interactions between CuŽII. and AOTy. CuŽAOT. 2 tends to behave as CuŽII. ions solubilized in the aqueous phase of the microphase. The role of the interface is strongly reduced and the template loses its efficiency. The surfactant behaves now as a polymer to protect the particle growth but not as a template. From Hofmeister series Bry is on the left-hand side compared to NOy 3 . Hence, if it plays a role, we would expect less y rods with NOy 3 than Br . This is observed. As y matter of fact with Br , very few rods are observed whereas with NOy 3 only spheres are formed. The change in the water structure with salt addition is consistent with the kinetic of growth. If the water is highly structured Žlyotropic salt., the kinetic is very slow whereas it is very fast when the water structure is like bulk phase. We know that the Hofmeister series is slightly sensitive to the cations. This is clearly observed in the particle shape. Similar behavior is obtained with KCl and NaCl, with NaNO 3 and KNO 3 and with NaBr and KBr, respectively. These similarities in the behaviors strongly support the important role of Hofmeister series in the control of the particle shape.
7. Conclusion In this paper, we have demonstrated that the growth of the rods of copper metal particles depend drastically on the counter ions of the added salt. The nanocrystal shape seems to be governed by the anions Hofmeister series.
Acknowledgements All TEM pictures were obtained at SRMA, CEASaclay. We are indebted to Dr. Ph. Dubuisson for giving us facilities.
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