Biomimetic strategies for the control of size, shape and self-organization of nanoparticles

COLLOIDS AND A ELSEVIER

Colloids and Surfaces A: Physicochemical and Engineering Aspects 123 124 (1997) 561 573

SURFACES

Biomimetic strategies for the control of size, shape and self-organization of nanoparticles M.P. Pileni *, J. Tanori, A. Filankembo Laboratoire SRSI, URA CNRS 1662, Universit~ P. et M. Curie, Bdt. F, 4 Place Jussieu, 75005 Paris, France CEA, DSM-DRECAM Service de Chimie Mol~culaire CE Saclay, 91 191 Gif sur Yvette Cedex, France

Received 14 June 1996; accepted 25 July 1996

Abstract An unusual phase diagram is presented. It is composed of copper(II) bis(2-ethylhexyl) sulfosuccinate Cu(AOT)zisooctan~water. Keeping the concentration of the Cu(AOT)2 isooctane solution constant, increasing in the amount of water induces various phase transitions. At low water content, spherical and cylindrical reverse micelles are formed. By increasing the water content, a bicontinuous system appears. Further addition of water leads to the formation of planar and spherulite type lamellae. As more water is added only spherulites remain in the phase. Still further addition of water leads to a reappearance of an interconnected network and then reverse micelles. Syntheses performed in the various parts of the phase diagram show that the use of colloidal assemblies as templates favors the control of the shape of nanoparticles. Cylindrical metallic copper particles having the same size can be obtained in various parts of the phase diagram when the template is made of interconnected cylinders. A very low amount of cylinders (13%) is formed when the synthesis is performed in cylindrical reverse micelles. When the colloidal self-assembly is a mixture of several phases, various types of shapes can be obtained. In some cases, the polydispersity in size is so low that metallic particles are able to self-assemble in a hexagonal network. Multilayers can be observed and are arranged in a face centered cubic structure. Keywords: Nanoparticles; Colloidal assemblies; Copper metal

1. Introduction N a t u r e has t a k e n a d v a n t a g e of the rich liquidcrystalline b e h a v i o r of a m p h i p h i l i c liquids to create o r d e r e d yet fluid b i o m e m b r a n e structures a n d to m o d u l a t e d y n a m i c processes in cells [1,2]. A key step in the c o n t r o l of m i n e r a l i z a t i o n e m p l o y e d by a l m o s t all o r g a n i s m s is the initial isolation of a space. T h e n u n d e r c o n t r o l l e d conditions, minerals are i n d u c e d to form within the space [ 3 ] . The space is usually delineated by cellular m e m b r a n e s , vesicles o r p r e d e p o s i t e d m a c r o m o l e c u l a r m a t r i x * Corresponding author.

frameworks. Filling up these spaces with a m o r p h o u s minerals would a p p e a r to require a quite different strategy as c o m p a r e d to filling space with crystalline material. The simplest w a y to fill a space with crystals is to create as high a local supersatu r a t i o n as possible, and then induce n u c l e a t i o n or let the system s p o n t a n e o u s l y reach a state of lower energy by crystallization, while at the same time r e m o v i n g the excess solvent. This situation is o b s e r v e d in spherulites of calcium c a r b o n a t e which s p o n t a n e o u s l y form in m e t a s t a b l e s u p e r s a t u r a t e d solutions. " D r o p l e t s " of calcium c a r b o n a t e a n d water s e p a r a t e from solution a n d with time, or u p o n drying, crystallize to lead to spherulites [ 4 ] .

0927-7757/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved PH S0927-7757(97) 03785-5

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Increasing attention is now being focussed on shear-induced transitions in complex fluids [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 upon an increase of surfactant concentration and/or upon addition of salt, alcohols etc. In that case, micelles have been shown to grow to elongated more or less flexible rodlike micelles [ 7-15] in agreement with theoretical prediction on micellization [16,17]. In terms of growth of particles, some analogies between surfactant self-assemblies and natural media can be proposed. In both cases, the growth of particles needs a supersaturated medium where the nucleation takes place. Increasingly chemists are contributing to the synthesis of advanced materials with enhanced or novel properties by using colloidal assemblies as templates. Recently, selforganizations have been used to make calcium carbonate material [ 18,193. In this paper we show that, as in nature, the shape of the particles can be partially controlled by the shape of the template used. It is demonstrated that the shape and the number of metallic copper particles strongly depend on the colloidal structure in which the chemical reduction of Cu(II) takes place. Self-organization of metallic copper particles can be obtained from syntheses in colloidal assemblies.

2. Experimental section

2.1. Compounds Synthesis of copper(II) bis(2-ethylhexyl) sulfosuccinate (Cu(AOY)2) has been described previously [20]. Cu(AOT)2 in isooctane solutions of various water contents are analyzed by Karl Fisher titration using a Mettler automatic titrator. The concentrations of Cu(AOT)2 are determined by adding a sample to a ~0.03 M hydrochloric acid, ~0.3 M ammonium acetate solution and subsequently titrating for copper(II) using a 0.01 M sodium EDTA solution with 4-(2-pyridylazo) resorcinol as indicator. Isooctane was supplied by Fluka (99.5% puriss.), ammonium acetate (98%),

sodium EDTA and 4-(2-pyridylazo) resorcinol (99%) by Prolabo. Single-distilled water was passed through a Millipore "MilliQ" system cartridge until its resistivity reached 18 Mf~ cm. All chemicals were used without further purification.

2.2. Apparatus and treatments Electrical conductivity measurements were made with a Tacussel CD 810 instrument using a TD 100 (platinum) electrode from the same manufacturer. The measurements were made at 22°C once a stable reading had been established. Small angle X-ray scattering (SAXS) experiments were performed on the High Resolution X-ray Scattering Apparatus from S.C.M-C.E.A. Saclay (France). The wave vector range was: 0.02A 1 < q < 0 . 3 5 ~ - 1 where q=4nsin(O)/2, 20 is the scattering angle, and 2 is the radiation wavelength. The scattered X-ray intensity I(q) is I(q) = P(q) S(q) where q is the wave vector, equal to 4n(sin 0)/2 (20 is the diffusion angle), P(q) is the form factor and S(q) the structure factor. This describes the interactions between the aggregates [213. Determination of the self-assembly structures has been obtained by using the classical treatments described for spheres [21], interconnected cylinders [22] and sponge phase [23]. Electron micrographs are obtained with a Jeol electron microscope (model Jem.100 CX.2). Histograms are obtained by measuring the diameter Di of all the particles from different parts of the grid (magnification 160000) for an average number of particles close to 500. The standard deviation, ~r, is calculated from the following equation: o- = {Y [ni(D i - D)z]/(N - 1 )} 1/2 where D and N are the average diameter and the number of particles respectively.

2.3. Syntheses and characterization of metallic copper particles The copper particles are prepared by reduction of the Cu(AOT)2 in isooctane solution containing

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various amounts of water. Hydrazine is used as the reducing agent. The reaction takes place under N2 atmosphere and starts immediately after hydrazine addition to Cu(AOT)2 in isooctane. The reaction takes 3 h. The ratio of Cu(AOT)2 to hydrazine is kept equal to ½. The water content, w, is defined as the ratio [ H 2 0 ] / [ A O T ] . The concentration of Cu(AOT)2 is kept constant and equal to 5 x 10 -2 M. At the end of the reaction a drop of the solution is placed on a carbon film supported by a copper grid and the sample is examined by transmission electron microscopy, (TEM) and electron diffraction (ED). The electron diffraction shows concentric circles characteristic of a face centred cubic phase with a lattice dimension equal to 3.61 A as observed with bulk metallic copper material.

3. Results

content, water in oil spherical droplets are formed whereas at higher water content cylindrical droplets are present. These behaviors are similar to those previously observed for similar bimetallic surfactants [20]. Such changes in the droplet shape have been observed for many systems [24] and are attributed to changes in the average curvature [,16,17,25] of the interfacial film. As a matter of fact, the increase in the water content favors the hydration of the copper ions and the polar head groups which induces a decrease in the electrostatic interactions between the polar head group and copper ions and favors the increase in the curvature. Syntheses performed in this region of the phase diagram show formation of a relatively small number of metallic copper particles. Most of the particles are spherical (87%) with a very low percentage (13%) of cylinders (Fig. 1A). The average size of spherical particles is characterized by a

The 5 x 10 2 M Cu(AOT)2-isooctane solution is an isotropic phase. The water content is defined as w = [ H 2 0 ] / [ A O T ] . By water addition, the phase diagram evolves progressively.

O

3.1. Below w = 5.5

The solution is isotropic. The conductivity of C u ( A O T ) 2 - H 2 0 isooctane solution is very low. It varies from 10 to 230 nS when increasing the water content to w = 5.5. At low water content, w = 2, the behavior of the intensity, I(q), observed in the Cu(AOT)2-isooctane-water solution is characteristic of a spherical structure: the radius of the microaggregate determined from the slope of the Guinier plot, ln[-I(q)] vs. q2 is found equal to 1 nm. By increasing the water content above w = 4, the scattering spectrum, at very small wave vectors, is no longer characteristic of a spherical structure. Cylindrical water in oil droplets are observed. The radius determined from the slope of the linear relationship obtained by plotting ln[-q I(q)] vs. q2 is also in good agreement with that deduced from Porod's plot. The persistence length of the cylinder, h, is found equal to 7 nm. From the conductivity and from SAXS measurements, it can be concluded that, at low water

m

21nm B

C

xl0

25 20 15 10 5 Ih

b I

5

10

5 20

Spheres diameter (nm)

-1

I I [

1

3

5

7

9

Cylindrical axial ratio

Fig. 1. (A) TEM pattern obtained after synthesis at w=4, [Cu(AOT)2]=5 x 10 2 M; (B) histogram of diameter of spheres; (C) ratio of the length over the width of cylinders.

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M.P. Pileni et al. / Colloids Surfaces A." Physicochem. Eng. As'pects 123 124 (1997) 561 573

diameter equal to 12 nm (Fig. 1B) with a polydispersity in size equal to 14%. Histograms of the cylinders show that the average ratio of the length over the width of the particle is equal to 2.3. The length and width of the cylinders are 18.5 _+ 2.2 nm and 8.2 _+ 0.6 nm. Spherical particles characterized by the same size self-assemble on the T E M grid as shown on Fig. IA. 3.2. 5.5 < w < 1 1

Above w = 5.5, the L 2 phase is destabilized and is separated into a more concentrated reverse micellar solution (L~') and an almost pure isooctane phase. By increasing the water content we observe the following. (i) An increase in the isooctane volume. This induces an increase in the Cu(AOT)2 concentration in the lower phase. (ii) An increase in the conductivity of the lower phase from 0.12 to 1.5 mS. (iii) A progressive increase in the viscosity with increase of w and then a decrease. The maximum in viscosity is reached for a w value close to 7. (iv) The variation of the scattered intensity with the wave vector, q, is always characteristic of cylinders. A linear relationship is obtained plotting In [q I(q)] vs. q2, indicating the scatter of cylinders. The persistence length of the cylinders is deduced, at various water contents. Table 1 shows drastic changes in neither gyration radius nor in the persistence length with increase in the water content. From this it can be deduced that the average size of the cylinders remains constant when the water content increases. (v) Freeze fracture image shows a homogeneous system made up from only very small objects. The increase in conductivity and viscosity with the increase of the water content can be related to the increase in Cu(AOT)2 concentration. At low water content, the L* phase is rather dilute. The increase in the water content induces an increase in the Cu(AOT)2 concentration of the L~' phase. This favors the increase in the number of connections between cylinders to form a bicontinuous network. Similar behavior has already been observed with other self-assembling surfactants and is attributed to formation of a disordered

Table 1 Structural parameters obtained in various parts of the phase diagram w

Structure

RgInm)

Rp(nm)

Linm)

6.5 7.5 8.5 9.5 10 11 20 22 24 26 28 30 32 35

L2

1 0.9 l 1 1 1.1 2.4 2.4 2.4 2.5 2.6 3.9 4.3 4.8

1.3 1.3 1.5 1.5 1.5 1.5

3 3 3.2 3.1 3 3.1 6.4 6.8 6.8 7.4 7.4 8 11

L~" + L s

L~ L2

--

-5.9

o-(nm)

..... -4.9 4.9 5 5.3 5.5 ....

The various phases are: reverse micelles (L2), interconnected cylinders (L*) and sponge phase (L~). From simulation of the scattered intensity obtained by SAXS, the gyration radius (Rg), the average radius obtained from the maximum and the minimum of the Porod plot (Rp), the persistence length obtained from the plot of ln[l(q} q] vs. qZ(L) and the thickness of the bilayer obtained from the plot of ln[/(q) q2] vs. q2(cr) are derived.

open-connected microemulsion [ 7 - 1 0 ] . At high water content w > 7, the decrease in the viscosity can be explained in terms of branching of one cylinder onto another with a local saddle structure. Similar behavior has been observed in oil in water micelles [26]. Syntheses performed at w = 6 show (Fig. 2) formation of a relatively large amount of metallic copper cylinders (32%) in coexistence with 68% of spheres. The average diameter of the spherical particles is 9.5 nm and a rather large polydispersity (27%) is obtained. The histogram shows formation of large cylinders (Fig. 2C). The average ratio of the length over width of the cylinder is found equal to 3.5 with 40% polydispersity. The average 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 performed at water content higher than w = 6 (w= 8,10) induce formation of an excessive amount of material which makes their observation by T E M impossible.

M.P. Pileni et al. ,/Colloids Suff~tces A: Physicochem. Eng. Aspects 123 124 (1997) 561 573

A

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10

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20

Spheres d i a m e t e r (nm)

L .....

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. . . . . . . . .

-1

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7

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Cylindrical axial ratio

565

and spherulites. In the first case, the conductivity is expected to be higher than in the latter. As shown below, the planar lamellae disappear and spherulites remain. Hence the decrease in the conductivity and the non-periodic variation of the characteristic distance with increasing water content can be attributed to an increase in the number of spherulites and a decrease in the planar lamellae. Syntheses performed in this region of the phase diagram show formation of a very large number of cylinders in coexistence with spheres. Fig. 3A shows the TEM pattern obtained at w = 12. In this region of the phase diagram a slight increase in the number of cylinders is observed (38% of the particles are cylindrical whereas 62% are spherical). The average diameter of the spherical particles is equal to 10.9 nm with polydispersity 17% in size distribution. The length and width of the cylinders are 25 _+ 4 nm and 7.3 _+ 1.4 nm respectively. From the histogram given on Fig. 3C, a slight increase

A

Fig. 2. (A) TEM pattern obtained after synthesis at w=6, [ C u ( A O T ) 2 ] = 5 x 10 2 M; (B) histogram of diameter of spheres; (C) ratio of the length over the width of cylinders.

,o,,

3.3. 11 < w < 1 5

At w = 11, a birefringent phase appears in equilibrium with the L~' phase and isooctane. By increasing the water content to w=15, the L* phase disappears progressively. The SAXS pattern of the birefringent phase is rather surprising. The Bragg peak is very broad and no second order is observed at the various water contents. This indicates disorder in the lamellar structure. This is confirmed by freeze fracture images which show the coexistence of a planar lamellar phase with spherulites. The characteristic distance is equal to 68 A, 55 A and 57 A for w = 12, 13 and 14 respectively. It does not follow a regular behavior with the increase of the water content. The conductivity of the lamellar phase decreases with increasing the water content (from w = 13 to 14). This is consistent with the fact that the freeze fracture images reveal the presence of two types of lamellar phase, namely planar lamellae

t n

21nm

B 6 ~xl0

C

~. 25 20

•

15 I0

© 5 . . . .

.,l 5

IL i.,

10

15

' 20

Spheres d i a m e t e r (nm)

-I

I|,,,,, 1

3

5

7

9

Cylindrical axial ratio

Fig. 3. (A) TEM pattern obtained after synthesis at w = 12, [-Cu(AOT)2]=5x 10 -2 M; (B) histogram of diameter of spheres; (C) ratio of the length over the width of cylinders.

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M.P. Pileni et al. ' Colloids Surfaces A: Physicochem. Eng. Aspects 123-124 (1997) 561-573

Fig. 4. High resolutionimage of self-assemblymade of cylindrical metallic copper particles (insert self-assemblies);w= 12.

in the size distribution is observed. The average length over width ratio of the cylinders is equal to 3.7 with 44% in polydispersity. In some regions of the carbon grid, particles having similar sizes selfassemble (insert Fig. 4). The average distance between particles is kept constant and equal to 1 nm. A high resolution image of such a selfassembly (Fig. 4) shows that the interlattice distance is similar to that obtained for the 111 phase in bulk material (3.6 A). This indicates the high crystallinity of the material synthesized in a selforganized assembly. 3.4. 15 < w < 2 0

At w = 15 the birefringent phase remains in equilibrium with pure isooctane. A drop in the conductivity is observed with an average conductivity equal to 12.8 itS. This indicates a decrease in the degree of interconnectedness. The birefringence and the decrease in conductivity strongly support

the formation of a lamellar phase which consists of thin aqueous and thick isooctane lamellae. Over this entire range, the freeze fracture replicas show only the presence of spherulites. The spherulites size distribution is large with sizes varying from 100 to 8000 rim. They are formed here apparently without any external forces, in contrast to those usually observed after shearing of lamellar phases [27,28]. These data are confirmed from SAXS measurements. As at lower water content, one Bragg peak is observed without second order and a strong increase in the scatter at low angle. The characteristic distance, D*, linearly increases with increase in the water content. This can be explained as follows: the equilibrium with almost pure isooctane forces the isooctane lamellae to be maximally swelled. Addition of water causes swelling of the aqueous lamellae and increases the characteristic distance, D*. However D* is smaller than the value expected from the measured isooctane content. This may be due to onion-like lamellar structures containing the excess isooctane in between the spherulites and in their centers. Syntheses performed in this region of the phase diagram (15 < w < 2 0 ) show the formation of particles having a higher polydispersity in size and in shape (Fig. 5A), than observed at low water content. As a matter of fact, Fig. 5 shows formation of triangles, squares, cylinders and spheres. However in some regions of the carbon grid selfassemblies made of spherical particles are observed (insert Fig. 5A) (24% of particles are characterized by a spherical shape). The average diameter of the spherical particles is equal to 10.4 nm with a size distribution equal to 31%. Because of the marked change in the shape of the particles it is difficult to produce histograms. The size distribution has been measured as follows: triangular and tetrahedral particles have been assimilated to spheres. When the difference in length and width of particles was higher than 3 nm, it has been assumed that the particles were cylinders. Table 2 confirms the very high polydispersity in size and shape. 3.5. A t w = 20

At w = 2 0 a new isotropic phase appears in equilibrium with spherulites and isooctane solutions.

M.P. Pileni et al. / Colloids' SurJaces A. Physicochem. Eng. Aspects 123-124 (1997) 561 573

,,A~x 10

B

~- 2o

Syntheses show very surprising data. We observe formation of spherical particles having an average diameter equal to 7.8 nm and characterized by a very low polydispersity in size (11%). Because of that, the particles self-arrange with a very high organization. In all the various parts of TEM patterns, monolayers and multilayers made of spherical particles are observed (Fig. 6). Fig. 6A shows monolayers made of 7.8 nm spherical particles. They are arranged in a hexagonal network with an average distance between particles of 11.00 nm. In some regions of the TEM pattern several layers can be observed. Fig. 6B shows addition of second and third layers. This of course drastically changes the contrast. This is attributed to packing of hexagonal layers in a face centered cubic structure. At such water content (w=20), only 3% of cylinders are formed. They are characterized by a length and width equal to 10.9 _+ 1.7 nm and 6.8 _+ 1.0 nm respectively. This result is rather surprising. As a matter of fact, below and above w = 20, a large polydispersity in size and shape is observed (Table 2).

C

-8 -6

t5

-4 © 5

-2

[J~-- : : ',: ', ',',: ',',

I I~ll

5

10

15

-1

20

Spheres diameter (nm)

3

5

7

567

9

Cylindrical axial ratio

Fig. 5. (A) T E M pattern obtained after synthesis at w = 1 8 , [ C u ( A O T ) z ] = 5 x 10 Z M; (B) histogram of diameter of spheres; (C) ratio of the length over the width of cylinders.

3.6. 20 < w < 30

By increasing the water content from w = 20 to 30, the lamellar phase disappears with an increase Table 2 Variation with the water content, w, of the average diameter of sphere, (ds); polydispersity of spheres, or; the percentage of spheres, %s; the percentage of cylinders, %~; the percentage expressed in weight of metallic copper cylindrical particles, %c(weight); average length, (L¢), and width, (l¢), of the cylinders; polydispersity in the average length, aLe(%), and width, ~ ( % ) , of the cylinders; the average ratio of the cylinder axis, (Lolls), and polydispersity in this ratio, Cre~,~¢ Parameter

w 4

6

12

18

20

24

26

34

38

40

44

(ds)(nm) ~s(%) %s %c %c(weight) (Lc)(nm) ~Lc(%) (/c)(nm) ak(%)

12.3 13 87 13 13 18.5 23.5 8.2

9.5 27 68 32 45.5 22.6 24 6.7

10.9 17 62 38 48.6 25 32 7.3

10.4 31 86 14 44.7 20.7 34 13.4

7.8 11 97 3 4.7 10.9 10.9 6.8

21

29

57

16

17

3.5 40

3.7 44

1.9 51

1.7 31

8.6 22 86 14 28.2 16.8 27 7.8 29 2.4 39

17

2.3 43

9.5 19 58 42 51.4 19.8 27 6.5 24 3.2 33

7.4 19 86 14 28 12.8 22 7.1

16

9.2 22 91 9 13.8 17.1 25 7.0 22 2.5 26

7.5 16 93 7 12.3 12.0 21 6.6

(Lc/lo)

9.4 26 87 13 19.9 19.3 29.8 6.9 34 2.9 35

1.9 32

1.9 37

~'Lc/lc(%)

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M.P. Pileni et al. / Colloids' Surfaces A: Physicochem. Eng. Aspects 123-124 (1997) 561 573

water contents. From these data it is difficult to draw conclusions. The freeze fracture images show the presence of two types of objects: large interconnected spherulites and densely packed, much smaller objects. The interconnected spherulites may not represent the original three-dimensional structure. They could be induced, instead, by insufficiently rapid quenching of an original sponge phase containing a large amount of isooctane, since this solvent is extremely difficult to vitrify. The densely packed, much smaller objects may well correspond to an interconnected cylinder structure. Syntheses, performed at various water content from w=21 to 29, induce formation of roughly 10% of cylinders and 90% of spheres. The percentage of cylinders decreases with increasing the water content from 13 to 7% whereas the length and the width of the cylinders remain unchanged. The length and width of the cylinders are found equal to 19.0 _+ 2.5 nm and 6.7 _+ 8 nm respectively. The diameter of the spheres slightly decreases and the polydispersity increases with increasing water content, from 9.7 _+ 0.7 nm to 8.1 ± 1.2 nm at w = 22 and w = 28 respectively. 3.7. 30 < w < 3 5 Fig. 6. TEM pattern obtained [ C u ( A O T h ] = 5 x 10 2 M.

after synthesis at

w=20,

in the volume of the isotropic phase. The conductivity of the isotropic phase is very high and is nearly constant. The structure of the isotropic phase is not well defined. From SAXS measurements, it is difficult to differentiate between a sponge phase and an interconnected cylinder structure. As a matter of fact, a linear relationship is obtained by plotting either in [I (q) q2 ] or In [7 (q) q ] VS. q2. The correlation factors of these plots are similar (0.99). Assuming the scatter is due to interconnected cylinders, the persistence length can be deduced from the plot of l n [ I ( q ) q ] vs. q2. Similarly assuming a sponge phase the thickness of the bilayer can be deduced by plotting ln[l(q) q2] vs. q2. Table 1 shows good agreement between these two parameters obtained at various

At w = 30, the lamellar phase totally disappears and the isotropic phase remains in equilibrium with isooctane. From w = 30-35, the volume of the upper phase decreases with an increase in that of the lower phase. This phenomenon is followed by a decrease in the conductivity. SAXS patterns show the scatter characteristic of cylinders. These data indicate a progressive dilution of the L* phase with a decrease in the number of connections between the cylinders. Syntheses, overall this water content range, show formation of spherical and cylindrical metallic copper particles. Fig. 7 shows a TEM pattern obtained at w=34. As at lower water content, cylindrical (42%) and spherical (58%) nanoparticles are observed. The average size of spherical particles remains the same as that observed in most of the cases, with a diameter equal to 9.5 + 0.9 rim. The length and width of the cylinders are equal to 19.8 _+ 2.7 nm and 6.5 _+ 0.8 nm respectively.

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M.P. Pileni et al. / Colloids" Surfaces A: Physicochem. Eng. Aspects 123 124 (1997) 561 573

A

21 n m

xlO ~- 20

8

0 15 10 O

.41j [IL 6

5

.I 5

-'2rl *

......

l0

15

20

Spheres diameter (nm)

-1

1

3

~- 40

8

o 30

-6

20 © ~. 10

.4

~

. . . . . . . . .

5 7

9

Cylindrical axial ratio

Fig. 7. (A) TEM pattern obtained after synthesis at w=34, [Cu(AOT)2]=5x 10-2 M; (B) histogram of diameter of spheres; (C) ratio of the length over the width of cylinders.

2

ill ........ 5

10 15 20

Spheres diameter (nm)

'IIII -1

LI, ............ 3

5 7

9

Cylindrical axial ratio

Fig. 8. (A) TEM pattern obtained after synthesis at w=40, [Cu(AOT)2]=5 x 10-2 M; (B) histogram of diameter of spheres; (C) ratio of the length over the width of cylinders. 4. D i s c u s s i o n

3.8. 3 5 < w < 4 0

At w = 35 one isotropic phase is observed and the conductivity is very low. The increase in the water content to w = 4 0 does not induce macroscopic changes and the conductivity of the solution remains very low (nanosiemen level). SAXS experiments show the scatter of spheres characterized by radii given in Table 2. The low conductivity and the scatter typical of spheres allow us to conclude that water in oil droplets are formed. Syntheses have been performed at w = 3 5 and above. Most of the particles formed are spherical. The percentage of cylinders remains small (Table 1). N o drastic changes in the particle sizes and in the percentage of cylinders are observed by increasing the water content from w = 3 5 to 40. Fig. 8 shows the T E M pattern obtained at w = 40. The particle size is found equal to 7.5 _+ 0.6 nm.

The phase diagram described shows various colloidal structures. By increasing the water content, the colloidal system evolves from reverse micelles to bicontinuous systems formed first by interconnected cylinders and then by a lamellar phase. Such behavior has already been observed in m a n y systems [-29-35]. However, just the o p p o site to what is well known, the increase in water content does not induce an increase in the average curvature to form oil in water aggregates. In the present case, the increase in the water content induces formation of multiphases having spherulites, sponge structure, interconnected cylinders and then reverse micelles. Hence this phase diagram shows presence of reverse micelles in two different water content domains (below w = 5.5 and above w = 3 5 ) . Similarly, interconnected cylinders can be observed in two different water content

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domains (5.5 < w < 1 1 and 3 0 < w<35). At other w values several phases in equilibrium can be obtained. In order to try to find some correlation between the colloidal structure used as a template and the size and shape of the metallic copper particles we will discuss the data obtained in the various colloidal structures.

4.1. Reverse micellar solution Reverse micelles are formed below w = 5.5 and above w = 35. From the data described above and given in Table 2, most of the metallic copper particles obtained in this region of the phase diagram are spherical. The percentage of spheres varies from 86% to 93%. At low water content (w=4), the diameter of the particles is larger (12.3 nm) than that observed at higher water content (w > 35). A slight decrease in the particle diameter with increasing water content is observed (Table 2). The length and width of the cylinders are larger at low water content and decrease with increase in w. Furthermore, the number of particles formed strongly increases with increasing w (Figs. 1 and 8). These differences could be mainly attributed to hydration phenomena. At low water content the number of water molecules is too small to hydrate the polar head groups of the surfactant and the counter ions. This induces a relatively high rigidity of the water content with low value of the intermicellar exchange constant. The chirality of the surfactant molecules could favor formation of nuclei having a boat configuration and induce cylindrical particles. The local rigidity, due to the low w value, could favor this process. At higher water content, "free" water molecules are present in the microphase and the dynamic process between micelles increases. This could favor the growth of the particles in various directions, resulting in a decrease in the size of the cylinders with increasing water content. Comparison between Figs. 1 and 8 shows a strong increase in the number of particles formed when the water content increases. At low water content (w=4), the local environment cannot be regarded as an aqueous solution. The local confinement of water molecules in the internal micro-

phase creates a medium of lower dielectric constant, as in the presence of "free water". This could strongly change the redox potential of Cu(AOT)2. The increase in the water content induces a better solvation of Cu(AOT)2 and then an increase in the amount of material formed. It can be noted that the size of spherical metallic particles is not controlled by the water content, as has been demonstrated previously in N a ( A O T ) Cu(AOT)z-isooctane water reverse micelles [36-38]. This can be explained as follows. In the previous study, the concentration of Cu(AOT)2 was very small (10 .3 M) compared to that used in the present study (5 x 10 .2 M). At a given water content, the number of nuclei formed by chemical reduction of Cu(AOT)2 is much higher in the present study than in mixed micelles. This increase in the number of nuclei induces an increase in the particle size. Hence at w = 4 , in pure Cu(AOT)2 reverse micelles, the average size of metallic copper particles is equal to 12.3 nm whereas in mixed micelles it is 7.3nm [36]. Furthermore mixed micelles form spherical water in oil droplets with an average radius equal to 0.6 nm whereas pure Cu(AOT)2 reverse micelles are cylindrical with a gyration radius equal to 4 nm.

4.2. Interconnected cylinders Interconnected cylinders having similar persistance lengths are observed in Cu(AOT)z-iSooctane-water solution in two different water content domains (at 5.5 < w < l l and 30 < w<35). Syntheses performed in these two domains show very strong correlation and similar data. Spherical and cylindrical particles are formed in both cases. No other particle shapes have been observed. In both regions, the average diameter of spherical particles is the same (9.5 nm). Similarly, the size of the cylinders remains identical with an average length and width equal to 21.2 nm and 6.6 nm respectively. The polydispersity is a little higher at low water content (Table 2). The number of cylinders increases with increasing the water content. In percentage by weight, more than 50% of metallic copper particles are cylinders with a rather low polydispersity in size. Hence a very small increase in the water content (from w = 4 to w = 6 ) induces

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a strong change in size and shape of the nanoparticles. Such an increase cannot be attributed to changes in the hydration of the polar head group. In fact, in reverse micelles (see above), a strong increase in the water content does not induce a drastic change in the particle morphology. Furthermore a very high similitude in the size and shape of nanoparticles is observed in the two parts of the phase diagram differing by their water content and formed by interconnected cylinders. Hence the same average diameter and same ratio of cylinder axes (~3.3) are observed at low (5.5
571

4.3. Multiphase regions As described in the phase diagram, the increase in the water content induces formation of several colloidal structures in equilibrium.

4.3.1. Lamellae and interconnected cylinders At w = 11 a birefringent phase made of regular lamellae and spherulites in equilibrium with interconnected cylinders appears. By increasing the water content from 11 to 13, a progressive disappearance of interconnected cylinders takes place. Syntheses at w = 12 show formation of spheres and cylinders. A very few particles (less than 1%) have a triangular shape. The average diameter of spheres (10.9 nm) is similar to that observed in the other cases and their percentage is 62%. 38% of cylinders are formed and are characterized by a relatively high polydispersity (Table 2). Hence the presence of various colloidal assemblies used as templates induces an increase in the polydispersity. However, an increase in the number of cylindrical particles can be noted. A relatively high weight of copper material exists as cylinders.

4.3.2. Spherulites aggregates In the water content range (13 < w < 19) spherulites are in equilibrium with isooctane. Syntheses performed in this region show formation of particles characterized by various shapes. Strong variations in the contrast can be observed on Fig. 5. As has already been mentioned this is due to changes in the electron density under the electron beam. This again indicates the presence of twins. It can be noted that the twins are not aligned, as observed in cylindrical particles (Fig. 4), and they are characterized by various orientations. This phenomenon could explain formation of various shapes: because of the onion structures, nuclei could be formed in various orientations. The progressive growth of the particles could evolve in various directions. In some cases, the growth of the particles in two directions could cost too much energy which would prevent such growth. The shape of the particles would depend on the number of nuclei formed and on the number of directions.

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5. Conclusion From the data described in the present paper, it seems reasonable to claim that the size and the shape of metallic copper particles can be partially controlled by the shape of the colloidal assemblies used as template. In cylindrical reverse micelles, most of the metallic copper particles remain spherical and 13% are cylinders. Because of the large number of nuclei formed in each droplet, the size of copper particles is rather large and does not vary much with the water content. However, the number of particles synthesized strongly increases with the hydration of the polar head group of the surfactant. By using interconnected cylinders as template, spherical and cylindrical copper metallic particles are formed. The size of the cylinders and their polydispersity remain unchanged over the whole range studied. A rather large increase in the water content does not change the size of the cylinders. As in reverse micelles the number of metallic copper cylinders increases with the water content. This has to be related to hydration of the polar head group and the changes in the Cu(AOT)2 redox potential by increasing the water content. Of course, the system used is dynamic and this prevents formation of 100% of cylinders. However, the consistency of the data obtained in the two regions of the phase diagram (at low and high water content) in which interconnected cylinders are formed strongly supports the fact that as in nature, the shape of the template controls the shape of the particles. When several phases are in equilibrium a strong change in shape and polydispersity of the particles can be observed. In some cases, the polydispersity in size is small enough to induce self-assembly of nanoparticles. The particles arrange in a hexagonal network. Formation of multilayers made of particles can be observed. They are arranged in centered cubic face superlattices.

References [ 1] L. Addadi and S. Weiner, Angew. Chem. Int. Ed. Engl., 31 (1992), 153.

[2] S. Mann, in D.W. Bruce and D. O'Hare (Eds.), Inorganic Materials, Vol. J. Wiley & Sons Ltd, 1992, p. 238. [3] Simkiss in B.S.C. Leadbeat and R. Riding (Eds.), Biomineralization in Lower Plants and Animals, pp 19. [4] E.V. Lengyel, Z. Kristallogr., 97 (1937), 67. [5] R.G. Larson, Rheol. Acta, 31 (1992), 497. [6] C. Tanford, The Hydrophobic Effect, Wiley, New York, 1973. [7] S.J. Chen, D.F. Evans, B.W. Ninham, D.J. Mitchell, F.D. Blum and S. Pickup, J. Phys. Chem., 90 (1986), 842. [8] D.F. Evans, D.J. Mitchell and B.W. Ninham, J. Phys. Chem., 90 (1986), 2817. [9] I.S. Barnes, S.T. Hyde, B.W. Ninham, P.J. Deerian, M. Drifford and T.N. Zemb, J. Phys. Chem., 92 (1988), 2286. [10] B.W. Ninham, I.S. Barnes, S.T. Hyde, P.J. Derian and T.N. Zemb, Europhy. Lett., 4 (1987), 561. [11 ] N. Mazerr, G. Benedek and M.C. CareT, J. Phys. Chem., 80 (1976), 1075. [12] D. Blankschtein, G.M. Thurston and M.C. Carey, Phys. Rev. Lett., 85 (1986), 7268. [13] G. Porte, J. Appell and Y. Poggi, J. Phys. Chem., 84 (1980), 3105. [14] H. Hoffmann, G. Platz and W. Ulbricht, J. Phys. Chem., 85 (1981), 3160. [15] J.R. Mishic and M.R. Fisch, J. Chem. Phys., 92 (19901, 3222. [16] D.J. Mitchell and B.W. Ninham, J. Chem. Soc., Faraday Trans. 2, 77 (1981), 601. [17] S.A. Safran, L.A. Turkevich and P.A. Pincus, J. Phys. Lett., 45 (1984), L69. [18] D. Waish, J.D. Hopwood and S. Mann, Science, 264 (1994), 1576. [19] D. Waish and S. Mann, Nature, 377 (1995), 320. [20] C. Petit, P. Lixon and M.P. Pileni, J. Phys. Chem., 94 (1990), 1598. [21 ] A. Guinier and G. Fournet (Eds.), Small Angle Scanerings of X-Rays, Wiley, New York, 1955. [22] B. Cabane, in R. Zana (Ed.), Surfactant in Solution, New Methods of Investigation, 1985. [23] M. Filali, J. Appell and G. Porte, J. Phys. II, Fr., 5 (1995), 657. [24] J.M. Seddon and R.H. Templer, Philos. Trans. R. Soc. London, Ser. A, 344 (1993), 377. [25] J.N. Israelachvili, D.J. Mitchell and B.W. Ninham, J. Chem. Soc., Faraday Trans. 2, 72 (1976), 1525. [26] J. Appell, G. Porte, J.F. Berret and D.C. Roux, Prog. Colloid Polym. Sci., 97 (1994), 233. [27] O. Diat and D. Roux, J. Phys. II, 3 (1993), 9. [28] J.F. Berret, D.C. Roux, G. Porte and P. Lindner, Europhys. Lett., 25 (1994), 521. [29] I.S. Barnes, S.T. Hyde, B.W. Ninham, P.J. Deerian, M. Drifford and T.N. Zemb, J. Phys. Chem., 92 (1988) 2286. [301 N. Mazerr, G. Benedek and M.C. Carey, J. Phys. Chem., 80 (1976), 1075.

M.P. Pileni et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 123-124 (1997) 561 573 [-31] D. Blankschtein, G.M. Thurston and M.C. Carey, Phys. Rev. Lett., 85 (1986) 7268. [32] G. Porte, J. Appell and Y. Poggi, J. Phys. Chem., 84 (1980) 3105. [33] H. Hoffmann, G. Platz and W. Ulbricht, J. Phys. Chem., 85 (1981), 3160. 1-34] J.R. Mishic and M.R. Fisch, J. Chem. Phys., 92 (1990), 3222. 1-35] D.J. Mitchell and B.W. Ninham, J. Chem. Soc., Faraday Trans, 2, 77 (1981) 601.

573

[36] I. Lisieki and M.P. Pileni, J. Am. Chem. Soc., 115 (1993), 3887. [37] I. Lisieki and M.P. Pileni, J. Phys. Chem., 99 (1995), 5077. [38] I. Lisieki, M. Borjling, L. Motte, B.W. Ninham and M.P. Pileni, Langmuir, 11 (1995), 2385. [39] I. Weissbuch, L. Addadi, Z. Berkovitch-Yellin, E. Gati, M. Lahav and L Leiserowitz, Nature, 310 (1984), 161. [40] I. Weissbuch, F. Frolow, L. Addadi, M. Lahav and L. Leiserowitz, J. Am. Chem. Soc., 112 (1990) 7718.