© Copyright 2000 by the American Chemical Society
VOLUME 104, NUMBER 25, JUNE 29, 2000
LETTERS Is the Template of Self-Colloidal Assemblies the Only Factor That Controls Nanocrystal Shapes? A. Filankembo and M. P. Pileni* Laboratoire SRSI URA CNRS 1662, UniVersite´ P. et M. Curie (Paris VI), B.P. 52, 4 Place Jussieu, F-752 31 Paris Cedex 05, France ReceiVed: January 21, 2000; In Final Form: April 28, 2000
In the present Letter we demonstrate that particle shape can be controlled even if the macroscopic structure of the self-assembly used as a template remains unchanged. We demonstrate that the control of nanocrystal shape is influenced by the addition of salt while the same template is kept.
In these past few years many groups have used colloidal selfassemblies as templates to control the size and shape of nanocrystals. This has been extensively described in two feature articles.1,2 It has been well demonstrated that the shape of the template plays a role on that of the nanocrystals. However, in some cases, it is shown that the nanoparticle shape is not related to that of the template.3 Several factors can be invoked to explain such differences. The most important one is the solubilization of relatively large reactants in the colloidal self-assemblies. This provokes drastic changes in the structure itself, and the chemical reaction does not take place in the expected template. This was clearly demonstrated, several years ago: addition of acrylamine in AOT reverse micelles induces a phase transition from micelles to interconnected cylinders, keeping an optically clear solution.4 Similarly, the phase diagram changes with temperature. Thus, the structure of the template determined at a given temperature and participating in the chemical reaction at a different temperature, as reported in ref 5, cannot be taken into account. One possibility to avoid such changes is to use a surfactant with specific functional groups. The structure of the template can then be determined under the same experimental conditions as those for the chemical reaction.2,6-11 However, in this case the structure of the template perturbs the structure of the selfassembly itself. From our expertise in this domain the templating is valid only if the chemical reaction is not complete. This would induce very small changes in the templating structure. Another
factor in the control of the particle shape is the kinetic rate of the chemical reaction involved. In biological media this process needs to be very slow.12 Taking into account all these considerations, we succeeded in controlling the size1,2,6,7 and shape8-11 of nanocrystals characterized by a very high crystallinity. With copper rods nanocrystals the structure is not fcc as in the bulk phase. It is characterized by a 5-fold symmetry.13 In the present letter we demonstrate that particle shape can be controlled even if the macroscopic structure of the selfassembly used as a template remains unchanged. We demonstrated that the control of nanocrystal shape can be influenced by the addition of salt while keeping the same template. The system used as template is Cu(AOT)2-isooctane-water. Cu(AOT)2 is the double-chained surfactant copper(II) bis(2ethylhexyl)sulfosuccinate. The phase diagram has been characterized by small-angle X-ray scattering (SAXS),14 conductivity,15 and freeze fracture electron microscopy (FFEM).16 The phase diagram has been described in detail in one of our previous papers.17-19 It is prepared as follows: 5 × 10-2 M Cu(AOT)2 is solubilized in isooctane and water added to the solution. [Samples were sealed, vigorously shaken, and centrifuged. They are allowed to stand for several days. In fact, the equilibrium phases are formed in a few minutes. Each sample is checked for reversibility to temperature cycles.] Let w ) [H2O]/[AOT]
10.1021/jp000268c CCC: $19.00 © 2000 American Chemical Society Published on Web 06/03/2000
5866 J. Phys. Chem. B, Vol. 104, No. 25, 2000
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TABLE 1: Relative Volume of the Isotropic Phase (Vis), the Birefringent Phase (Vb), and Isooctane (Vi), the Characteristic Diameter of Cylinders (d), and the Head Polar Group Area (σ) salt Vi Vis Vb d (Å) σ (A2)
none
NaCl
KCl
NaBr
KBr
NaNO3 KNO3 NaHSO3
0.826 0.143 0.031 30 40
0.838 0.142 0.020 32 38
0.838 0.156 0.005 32 38
0.836 0.158 0.006 32 38
0.832 0.163 0.004 32 38
0.841 0.157 0.002 32 38
0.820 0.176 0.004 32 38
0.822 0.175 0.003 32 38
be the ratio of the water to surfactant concentration and kept to w ) 10. As described previously18 the system consists of three phases. Very flat menisci signifying ultralow tension separate all phases. The upper phase is isooctane. No Cu(AOT)2 molecules are detected. The middle phase is birefringent. By freeze fracture planar and onion phases are observed. This indicates the coexistence of two birefringent phases. The data obtained at higher water content and described in ref 20 permit us to assume that such an onion phase contains in its interior a small amount of microemulsion made of interconnected cylinders. The volume of the birefringent phase is too small to permit any structural study. The lower phase is optically clear. It is a classical microemulsion phase made of interconnected cylinders with a rather large conductivity (1.23 milliSiemens/cm). Various salts at a given overall concentration (10-3 M) solubilized in aqueous solution are added. From the macroscopic point of view, the phase diagram is not drastically changed by salt addition. The three phases remain present. Table 1 gives the relative volume of the various phases (isooctane, birefringent, and isotropic phases). It is observed that the relative volume of isooctane remains constant and is rather large compared to the others. Slight changes in the relative volume of birefringent and isotropic phases are observed. These variations are closed to experimental errors (10%). Table 1 shows no change in the relative volumes when water is replaced by a solution containing NaCl and KCl. The SAXS measurements of the lower phase (microemulsion) remain quite unchanged with the various salts. A slight increase in the characteristic diameter of the cylinders and a decrease in the head polar area are observed (Table 1). Again these differences could be in the experimental error domain. No change in these parameters is observed by changing the salt, and the scattering curves obtained with various salts are all superimposed. A part of the water is replaced by hydrazine, and Cu(AOT)2 is partly reduced. Hence after hydrazine addition to a selfassembly, the system is kept during 3 h. Then a drop of a solution is deposited on a carbon grid and a TEM pattern is obtained. A careful study of the crystal structure of the rods using transmission electron microscopy shows a truncated decahedral 5-fold symmetry13 and not a fcc structure as in the bulk phase. In the following results, the change in the particle shape has been observed several times and it is obtained over all the TEM grid.21 In the absence of salt added to the Cu(AOT)2-isooctanewater-hydrazine solution colloidal assembly, most of the particles are spherical. Very few cylinders can be seen (Figure 1A). When sufficient aqueous solutions of various salts were used to reach w ) 10, drastic changes in the particle shape are observed: (i) With NaCl, very long rods are seen (Figure 1B). Of course, in equilibrium with these rods, particles having various sizes
Figure 1. TEM patterns of particles obtained in the presence of chloride salts: (A) [salt] ) 0, (B) [NaCl] ) 10-3 M, (C) [KCl] ) 10-3 M.
and shapes (square, triangle, etc.) are observed. Similar behaviors are obtained when NaCl is replaced by KCl (Figure 1C). From these data it is concluded that addition of chloride salt to a self-assembly induces drastic changes in the particle shape. Because the number of rods are too small, their sizes cannot be well described statistically. However, the shorter length and width of the rods are 650 and 15 nm whereas the longest ones are 1600 and 25 nm, respectively. When NaCl is replaced by KCl, no drastic changes in size and shape are observed. However, a very slight change in the number of rods formed is obtained (the number increases when sodium is replaced by potassium ions). Such a chloride anion effect has to be related to previous data obtained in mixed [Na(AOT)-Cu(AOT)2 ] reverse micelles,22 where addition of a very small amount of CTAC (10-4 M) induces a drastic change in the particle size dependence on the water content and in the copper reduction yield. As in the present case, no structural changes in the template (water in oil droplets) in shape and size were observed. However, from FTIR spectral data, it was concluded that the water structure changes.23 Data shown in Figure 1 confirm that chloride salts play an important role in the templating. (ii) With NaBr (Figure 2A) and KBr (Figure 2B) the number of rods produced markedly decreases. Various shapes characterize most of the particles. However, concerning the rods the longest length and width are 400 and 56 nm whereas the shortest are 230 and 24 nm, respectively. Hence, all the rods obtained are shorter in length and larger in width than those obtained with chloride derivatives. The major effect is to markedly decrease the number of rods formed. From this it is concluded that the salt anion plays a role in the control of the rods. Very good confirmation of this is given below. (iii) With NaNO3 (Figure 2C) and KNO3 (Figure 2D) most of the particles are not well-defined and form aggregates. Very
Letters
J. Phys. Chem. B, Vol. 104, No. 25, 2000 5867 The anions and cations can be classified into so-called Hofmeister series:
SO42- > CO32- > HPO42- > F- > Cl- > Br- > NO3- > I- > ClO4- > SCNNa+ > K+ > Li+ > Rb+ > Cs+
Figure 2. TEM patterns of particles obtained in the presence of bromide and nitrate salts: (A) [NaBr] ) 10-3 M, (B) [KBr] ) 10-3 M, (C) [NaNO3] ) 10-3 M, (D) [KNO3] ) 10-3 M.
Figure 3. TEM patterns of particles obtained in the presence of [NaHSO3] ) 10-3 M, with (insert) a large aggregate of particles.
few cylinders are formed. This confirms the influence of the anion in the salt addition. (iv) With NaHSO3 most of the particles are slightly elongated (Figure 3). The mean size is comparatively small and the particles tend to be aggregate in 2D and 3D lattices. All the salts used (except NaHSO3) are usually called “spectator”. This means that they are known not to react with any compound. In most cases, these salts are used to change the ionic strength of the solution. Obviously, in the present case others factors play a role in changing the shape of the particles. We have to remember that the overall amount of salt added is very low (10-3 M). From the phase diagram no drastic changes are observed. It can thus be concluded that microscopic properties inside the self-assembly play an important role in the produced particle shape. Hence, the shape of the template is not the only factor, as has been established.1-2,6-11 To propose an explanation for the change in the shape with salt addition, we have to take into account the Hofmeister series24 discovered a century ago. It is based on the change in the solubility of organic molecules dissolved in aqueous solution. The solubility of various molecules such as alcohol, polymer, surfactant, protein, etc.25-27 can be adjusted by salt addition.
This series is universal in that the order of the sequence does not depend on the nature of the organic molecules. This solubility effect is more pronounced for anions than cations. A low solubility of organic molecules (salt out) is obtained by using salts on the left side of the series. On changing the salt from left to right, the solubility increases to reach a zone in which it is higher than that obtained in pure aqueous solution (salting in). HSO3- is not reported in the Hofmeister series. To a first approximation we could assume that HSO3- acts as SO42and 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. This was very recently confirmed by surface-specific vibration spectroscopy.28 This means the water molecules are highly bound to the surfactant. This induces a high rigidity of the water-surfactant interface, as observed in reverse micelles, at very low water content. Because the fraction of reduced Cu(AOT)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, as in reverse micelles, that very few copper atoms are reduced. The copper atoms produced during the chemical reaction are in an environment characterized by a large viscosity. This prevents the atoms from diffusing inside the matrix and the atoms one by one binding to the cylindrical nucleus. This explains why the particles shown in Figure 3 are characterized by a rather low size and shape distribution. On replacing HSO3- by Cl-, the desorption of the salt at the watersurfactant interface decreases. The water molecules are less bound to the interface. This induces changes in the Cu(AOT)2 redox potential. As in reverse micelles,22 the reduction yield increases in the presence of chloride ions. Hence more Cu(AOT)2 are reduced. However, the yield remains rather low and the template is not highly perturbed. As with HSO3-, cylindrical clusters are formed. However, the yield of copper atoms increases and the template is less rigid. The copper atoms can thus diffuse inside the template, and the growth of nanocrystals is favored. In our previous paper10 we demonstrated that the kinetic growth of copper rods drastically changes with and without added salt. In fact, the growth of small cylinders was observed in a preferential direction whereas in the presence of NaCl the length and the width increase simultaneously. The Brand NO3- anions are not lyotropic. From the Hofmeister series we know that nitrate salts out more than the bromide (which tends to be salting in). In this Letter we demonstrate that salt addition (with a rather low overall concentration) in the template induces drastic changes in the particle shape. This can be related to the Hofmeister series. Change in the water structure and the rigidity of the template induces changes in the length and in the number of rods produced. This proves the influence of the colloidal structure on the microscopic scale on the nanocrystal shape. Acknowledgment. The authors would like to thank Pascal Andre´ and Dr. C. Petit from our laboratory for useful discussions and providing the SAXS results. Thanks are also due to Dr.
5868 J. Phys. Chem. B, Vol. 104, No. 25, 2000 Ph. Dubuisson of DTA/DECM/SRMA,CEA-Saclay, for providing us with facilities for using the transmission electron microscope. References and Notes (1) Pileni, M. P. J. Phys. Chem. 1993, 97, 6961. (2) Pileni, M. P. Langmuir 1997, 13, 3266. (3) Hopwood, J. D.; Mann, S. Chem. Mater. 1997, 9, 1819. (4) Candau, F.; Zekhnini, Z.; Heatley, F.; Franta, E. Colloid Polym. Sci. 1986, 264, 676. (5) Waish, D.; Mann, S. Nature 1995, 377, 320. (6) Lisiecki, I.; Pileni, M. P. J. Am. Chem. Soc. 1993, 115, 3887. (7) Lisiecki, I.; Pileni. J. Phys. Chem. 1995, 99, 5077. (8) Tanori, J.; Pileni, M. P. AdV. Mater. 1995, 7, 862. (9) Tanori, J.; Pileni, M. P. Langmuir 1997, 13, 639. (10) Pileni, M. P.; Gulik-Krzywicki, T.; Tanori, J.; Filankembo, A.; Dedieu, J. C. Langmuir 1998, 22, 7359. (11) Pileni, M. P.; Ninham, B. W.; Gulik-Krzywicki, T.; Tanori, J.; Lisiecki, I.; Filankembo, A. AdV. Mater. 1999, 11, 1358. (12) Addadi, L.; Weiner, S. Angew. Chem., Int. Ed. Engl. 1992, 31, 153. (13) Lisiecki, I.; Filankembo, A.; Sack-Kongehl, H.; Weiss, K.; Pileni, M. P.; Urban, J. Phys. ReV B, in press. (14) SAXS experiments were performed at the D22 diffractometers of the LURE, (CNRS-CEA-Paris XI) in Orsay, France, with high spatial resolution; the wavelength is 1.46 Å and the scattering vectors (q) range from 0.007 to 0.4 Å-1. (15) The measurements were made with a Tacussel CD 810 instrument using a TD 100 (platinum) electrode from the same manufacturer. (16) A thin layer of a sample (20-30 µm thick) was placed on a thin copper holder and then rapidly quenched in liquid propane. The frozen
Letters sample was fractured at liquid nitrogen temperature in a vacuum close to 10-7 Torr, with the liquid nitrogen cooled knife in a Balzers 301 freezeetching unit. The replication was done using unidirectional shadowing, at an angle of 35°, with platinum-carbon 1-1.5 nm the size of mean metal deposit. The replicas were washed with organic solvents and distilled water and were observed in a Philips 301 electron microscope. (17) Filankembo, A.; Andre´, P.; Lisiecki, I.; Petit, C.; Gulik-Krzywicki, T.; Ninham, B. W.; Pileni, M. P. Colloids and Surfaces A: Physicochemical and Engineering Aspects (special issue on “Imaging of Colloidal Structures), in press. (18) Lisiecki, I.; Andre´, P.; Filankembo, A.; Petit, C.; Tanori, J.; GulikKrzywicki, T.; Ninham, B. W.; Pileni, M. P. J. Phys. Chem 1999, 103, 9168. (19) Lisiecki, I.; Andre´, P.; Filankembo, A.; Petit, C.; Tanori, J.; GulikKrzywicki, T.; Ninham, B. W.; Pileni, M. P. J. Phys. Chem. 1999, 103, 9176. (20) Andre´, P.; Lisiecki, I.; Filankembo, A.; Ninham, B. W.; Pileni, M. P. AdV. Mater. 2000, 12, 119. (21) PHILIPS EM 430,300kV. (22) Lisiecki, I.; Borjling, M.; Motte, L.; Ninham, B. W.; Pileni, M. P. Langmuir 1995, 11, 2385. (23) Motte, L., Lisiecki, I., Pileni, M. P., Dore, J., Bellisan, M. C., Eds. Hydrogen Bond Networks; NATO/Kluwer: Dordrecht, The Netherlands, 1994; p 447. (24) Hofmeister, F. Arch. Exp. Pathol. Pharthol. 1888, 24, 247. (25) Collins, K. D.; Washabaugh, M. Q. ReV. Biophys. 1985, 18, 323. (26) Franks, K. In WatersA ComprehensiVe Treatise; Franks, K., Ed.; Plenum: New York, 1973; Vol. 2, p 1. (27) Eagland, D. In WatersA ComprehensiVe Treatise; Franks, K., Ed.; Plenum: New York, 1973; Vol. 4, p 305. (28) Baldelli, S.; Schnitzer, C.; Campbell, D. J.; Shultz M. J. J. Phys. Chem. 1999, 103, 2789.
