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Anisotropic Copper Nanocrystals Synthesized in a Supersaturated Medium: Nanocrystal Growth C. Salzemann,†,‡ I. Lisiecki,‡ J. Urban,† and M.-P. Pileni*,‡ Fritz-Haber-Institut der MPG, Abt. AC, 4-6 Faradayweg, D-14195 Berlin, Germany, and Laboratoire L.M.2.N., U.M.R. C.N.R.S., 7070 Universite´ Pierre et Marie Curie (Paris VI), B.P.52, 4 place Jussieu, F-752 31 Paris Cedex 05, France Received March 18, 2004. In Final Form: August 5, 2004 In the chemical reduction of copper ions in mixed reverse micelles it is found that a large excess of reducing agent favors the production of a new generation of copper nanocrystals. At low reducing agent concentration, nanocrystals are mostly spherical, while in the supersaturated regime, they have various shapes such as pentagons, squares, triangles, and elongated forms. The nanocrystal structures, characterized by high-resolution transmission electron microscopy, are based on the face-centered cubic structure. A tentative explanation for the growth mechanism of copper nanocrystals having various shapes is proposed.
I. Introduction Controlling the size and shape of nanocrystals is a real challenge in the nanotechnology field.1 It is now quite easy to produce well-defined spherical nanocrystals, and their size can be readily varied by controlling that of reverse micelles in the used nanoreactors.2 However, to get the best spherical nanocrystals, syntheses have to be carried out in the supersaturation regime corresponding to a phase diagram domain in which reverse micelles no longer exist.3 Thus, overall control of nanocrystal shapes is still a real challenge in the nanotechnology field. Over the past decade, several groups succeeded in producing nanocrystals having a given shape but each synthesis is unique, and it turns out that no general rules can be applied.1 Several ways to control the nanocrystal shape have been described in the literature. A partial control of the shape of nanocrystals4 can be obtained using colloids. However, as a result of selective adsorption on various facets of the nanocrystal during the crystal growth, the presence of additives is the most important factor.1 Other approaches to this control, via soft chemistry, are solvothermal processes,5 inorganic synthesis,6 and seed production.7 In most cases, a surfactant is needed to control the particle shape and each set of reported results is specific to the method used for the synthesis. The synthesis of copper nanocrystals has been largely extended in our laboratory in the past decade. We first controlled size8 and shape9 of copper nanocrystals. A quantitative study proved that the presence of a salt is one of the key parameters in the shape control.10 † ‡
Fritz-Haber-Institut der MPG. U.M.R. C.N.R.S.
(1) Pileni, M.-P. Nat. Mater. 2003, 2, 145. (2) Pileni, M.-P. J. Phys. Chem. 1993, 97, 6961-6973; Langmuir 1997, 13, 3266-3276. (3) Lisiecki, I.; Pileni, M.-P. Langmuir 2003, 19, 9486. (4) Pileni, M.-P. Langmuir. 2001, 17, 7476-7486. (5) Li, Y.; Ding, Y.; Wang, Z. Adv. Mater. 1999, 11, 847. (6) Puntes, V. F.; Kristhmann, K. M.; Alivisatos, P. Science 2001, 291, 2115. (7) Jana, N. R.; Gearheart, L.; Murphy, C. Chem. Commun. 2001, 617. (8) Lisiecki, I.; et al. J. Am. Chem. Soc. 1993, 5, 3887; J. Phys. Chem. 1996, 100 (10), 4160. (9) Tanori, J.; Pileni, M.-P. Adv. Mater. 1995, 7, 862; Langmuir 1997, 13, 639. (10) Filankembo, A.; et al. J. Phys. Chem. B 2000, 104, 5867; 2003, 107, 7492.
In this paper, we concentrate our efforts on understanding the growth mechanism of copper nanocrystals differing by their shape. A new way to make copper nanocrystals via a supersaturated regime is described. II. Experimental Section II.1. Chemicals. Copper(II) bis(2-ethylhexyl)sulfosuccinate, Cu(AOT)2, is prepared by ion exchange with the sodium salt as described elsewhere.11 Copper(II) sulfate, CuSO4, was from prolabo. Isooctane, sodium (2-ethylhexyl)sulfosuccinate (NaAOT), and hydrazine were from Sigma and Fluka. II.2. Apparatus. A Philips CM200 FEG 200 kV microscope, equipped with a field emission gun and a Gatan imaging filter (GIF 100), was used. The coefficient of spherical aberration was Cs ) 1.35 nm. The images were digitized in 1024 pixels × 1024 pixels with a pixel size of 0.022 65 nm by using a charge-coupled device camera. To determine the structures and the plane distances, the power spectra (PS, square of the Fourier transform of the images) were calculated.12 II.3. Simulation. Cerius version 5 was used for high-resolution transmission electron microscopy (HRTEM) simulations, singlecrystal diffraction calculations, and structure simulations. Image simulations were performed using the multislice algorithm13 to describe the solutions to full N-beam dynamical scattering. To do the calculations, we create a model characteristic of the structure. The atomic positions are known. Then the model is placed in a supercell, and the HRTEM image of this model is calculated. For the symmetry interpretation and measurements of structural data like lattice parameters and the corresponding lattice angles, the PS of the images are also calculated. All the calculations are performed with the data of the microscope described in II.2.
III. Synthesis The synthesis of copper nanocrystals in reverse micelles has been described in detail in ref 8. A mixture of surfactants, 10-2 M Cu(AOT)2 and 8 × 10-2 M NaAOT, is solubilized in isooctane and forms spherical reverse micelles. To produce copper nanocrystals, Cu(AOT)2 is reduced by various concentrations of hydrazine (N2H4). The ratio R, R ) [N2H4]/[Cu(AOT)2], varies from 3 to 15 corresponding to changes in the hydrazine concentration from 3 × 10-2 M to 0.15 M. (Hydrazine is a liquid.) To (11) Petit, C.; Lixon, P.; Pileni, M. P. Langmuir 1991, 7, 2620. (12) Urban, J.; Sack-Kongehl, H.; Weiss, K. Z. Phys. D 1993, 28, 245. (13) Moodie, A. F.; et al. Acta Crystallogr. 1957, 609, 10; Acta Crystallogr., Sect. A 1979, 30, 280.
10.1021/la0492862 CCC: $27.50 © 2004 American Chemical Society Published on Web 11/19/2004
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Figure 1. TEM images of copper nanocrystals obtained at various R values: (A) R ) 3, (B) R ) 5, (C) R ) 10, and (D) R ) 15. The insets show the electron diffraction patterns corresponding to each sample in a collection of nanoparticles.
ensure that the data are as consistent as possible, we assumed that a molecule of water has the same volume as that of a molecule of hydrazine and both contribute similarly to the polar volume fraction. The overall polar volume fraction, f, is kept constant at 2.6%. In the absence of hydrazine, this corresponds to a water content, defined as w ) [H2O]/[AOT], of 10. After hydrazine addition, the solution immediately turns dark, indicating the reduction of Cu2+ to Cu0. Three hours later, a few drops of the colloidal solution are deposited on an amorphous carbon film supported by a copper grid and allowed to evaporate. To prevent oxidation, the reaction takes place in an inert atmosphere. IV. Results and Discussion The copper nanocrystals are synthesized at R ) 3, 5, 10, and 15 (R ) [N2H4]/[Cu(AOT)2]). Figure 1 shows the TEM images obtained at various R values. At low hydrazine concentrations, R ) 3 and 5, most of the nanocrystals are spherical. Very small amounts of elongated nanocrystals and triangles are observed (Figure 1A,B). By increasing R, the proportion of anisotropic shapes and sphere crystallinity markedly increases (Figure 1C,D). Because it is rather difficult to distinguish between all the various forms, we call the pentagons, the hexagons, and the more or less well-defined shapes “spheres”. The histograms of spherical nanocrystals at R ) 3 and 15 (Figure 2) show an increase in the average size (from 12 to 20 nm) and their distribution (from 14 to 21%). At R ) 15, various shapes such as elongated nanocrystals, triangles, spheres, and cubes are observed (Figure 1D). A careful evaluation of the percentage of nanocrystals
Figure 2. Size distributions of spherical copper metallic particles synthesized at various R values: (A) R ) 3 and (B) R ) 15.
differing by their shapes is made using about 500 particles. It is found that 31% spheres, 30% triangles, 9% squares, and 30% elongated particles are produced. The average
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Figure 3. Representation of the stable precursor nuclei for the formation of copper nanocrystals.
sizes are 23, 19, and 20 nm for the triangles, the squares, and the “spheres”, respectively. The elongated particles have an average length, width, and aspect ratio of 22 nm, 13 nm, and 1.8, respectively. Note that for triangular nanocrystals, the contrast is very weak indicating formation of very thin nanocrystals. The electron diffraction patterns of a collection of nanocrystals are recorded for the various syntheses and are shown in the Figure 1 insets. For any R, the diffraction patterns show the same rings corresponding to the 111 (d111 ) 0.2087 nm), 200 (d200 ) 0.1807 nm), 220 (d220 ) 0.1278 nm), and 311 (d311 ) 0.1089 nm) reflections. These distances are characteristic of the face-centered cubic (fcc) structure of copper (metal). It must be noted that the rings are constituted of a collection of spots typical of the different nanocrystal orientations. Hence, it can be concluded that there is formation of (metal) copper nanocrystals without any detectable traces of copper oxide. The increase in R induces an increase in the reduction yield, that is, in the number of copper atoms, as observed previously.14 As the polar volume fraction remains unchanged (φ ) 2.6%), the nuclei concentration increases, leading to an increase in the nanocrystal size with R. For copper, the stable nuclei are decahedral, cuboctahedral, and tetrahedral15 (Figure 3). The increase in the nanoparticle crystallinity with R is explained by the increase in the number of nuclei. Similar behavior has been observed previously with silver nanocrystals: spheres are produced at low R values16 whereas monocrystal spheres with nanodisks17 form at higher R. The syntheses described above show a large variety of morphologies depending on many factors. The crystallographic structure of the nuclei and seeds during the nucleation process and their subsequent growth stage are key parameters. Furthermore, the bis(2-ethylhexyl)sulfosuccinate (AOT) surfactant adsorbs on certain facets inducing truncation and preferential growth of the corresponding facets. For the present study, we have to keep in mind that copper nanocrystals are synthesized in mixed reverse micelles in which only 20% of the surfactant (NaAOT) is functionalized with the counterions Cu2+. This means that the amount of surfactant for any production of copper nanocrystals is high, and it is assumed that the capping involves the AOT molecules and, of course, other compounds such as those resulting from the chemical reaction. In the following we study the structure of these various nanocrystals differing by their shapes. (i) Spherical Nanoparticles. Spherical nanoparticles are observed at various R values. However, their percentage and the number of polycrystals are higher at low R values (14) Lisiecki, I.; Bjo¨rling, M.; Motte, L.; Ninham, B.; Pileni, M.-P. Langmuir 1995, 11, 2385. (15) Kirkland, A. I.; Jefferson, D. A.; Duff, D. G.; Edwards, P. P.; Gameson, I.; Johnson, B. F. G.; Smith, D. J. Proc. R. Soc. London, Ser. A 1993, 440, 589. (16) Courty, A.; Lisiecki, I.; Pileni, M.-P. J. Chem. Phys. 2002, 116, 8074. (17) Maillard, M.; Giorgio, S.; Pileni, M.-P. Adv. Mater. 2002, 14, 1084.
Figure 4. Copper decahedron oriented along the fivefold axis nanocrystals obtained at R ) 3: (A) HRTEM image, (B) calculated PS, and (C, D) computer simulations.
Figure 5. Spherical fcc copper nanocrystals in the [110] orientation produced at R ) 15: (A) HRTEM image, (B) calculated PS, and (C, D) computer simulations.
(R ) 3). The nanoparticles are then characterized by many defects, and no structure can be determined. This is explained by a random coalescence of nuclei differing or not by their structures. On increasing R, the number of well-crystallized particles increases. Regular decahedral (Figure 4) and cuboctahedral (Figure 5) nanocrystals are produced. The HRTEM images of a nanocrystal (Figures 4A and 5A) are in good agreement with the simulated images of a decahedron in the fivefold orientation (Figure 4B) and a cuboctahedron in the [110] orientation (Figure 5B). For the decahedron, the calculated PS is characterized by 10 pairs of reflections: five pairs, labeled 1 (d ) 0.208 nm), and five others, labeled 2 (d ) 0.180 nm), corresponding respectively to (111) and (200) planes of the five subunits.18 Figure 4C is in good agreement with the simulation (Figure 4D). For the cuboctahedron, the calculated PS (Figure 5C) shows four pairs of reflections, labeled 1-4, that correspond to the lattice parameters d1 ) d2 ) 0.208 nm, d3 ) 0.180 nm, and d4 ) 0.128 nm related to the (111), (200), and (220) planes, respectively, of a fcc structure. These data provide evidence that spherical nanocrystals are characterized either by decahedral or by cuboctahedral (18) Urban, J. Cryst. Res. Technol. 1998, 33 (7), 1009.
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Figure 7. Schematic of the growth mode of cubic nanocrystals.
Figure 6. Cubic copper cuboctahedron nanocrystals in the [001] orientation produced at R ) 15: (A) HRTEM image, (B) calculated PS, and (C, D) computer simulations.
structures. Both of these structures are directly related to the crystallographic phase of the nuclei. Indeed, the homogeneous growth of cuboctahedral nuclei makes possible the formation of spherical nanocrystals. The decahedron consists of five deformed tetrahedral subunits twinned by their {111} planes, and then it is characterized by a fivefold symmetry. The formation of such an unusual structure results from the simultaneous and regular growth of the five {111} facets. No transformation such as facet truncation or preferential growth of facets occurs during the growth process. The AOT surfactant does not play a role in such growths. On the other hand, it must be noted that the produced decahedra have an average size of 10-15 nm. For many years, several authors used to claim that the maximal size for such fivefold symmetry was around 5 nm.19 Above this size, this structure was not considered to be stable. Conversely, and in good agreement with our work, some other papers20,21 have shown large decahedra. Their stability is explained by either the deformation of the tetrahedral subunits or the accommodation of the angle between the units. Moreover, a gain of stability due to the AOT adsorption on the {111} facets cannot be excluded. (ii) Cubic Nanocrystals. The HRTEM image (Figure 6A) and simulation (Figure 6B) show a square with two characteristic lattice planes perpendicular to each other corresponding to a pure fcc structure oriented in the [001] direction. The calculated (Figure 6C) and simulated (Figure 6D) PS confirm this structure. Reflections 1 and 2 correspond to the (200) (d1 ) d2 ) d200 ) d020 ) 0.18 nm) planes and reflections 3 and 4 correspond to the (220) (d3 ) d4 ) d220 ) d22h 0 ) 0.127 nm) planes. The planes (200) are perpendicular to each other just as for the (220) planes. This nanocrystal corresponds to a cuboctahedron in the [001] direction. Squares of copper10 and platinium22 nanocrystals are characterized by {100} facets whereas the cuboctahedral precursors have a surface made with {100} and {111} facets (Figure 7). Therefore, it is assumed that square particles are formed by the specific growth of {111} facets of the cuboctahedral nucleus. Such a com(19) Reinhard, D.; Hall, B. D.; Berthoud, P.; Valkealahti, S.; Monot, R.; Phys. Rev. B 1998, 58, 4917. (20) Gardea-Torresday, J. L.; Tiemann, K. J.; Gamez, G.; Dokken, K.; Tehuacanero, S.; Yacaman, M. J. J. Nanopart. Res. 1999, 1, 397. (21) Ijima, S. Jpn. J. Appl. Phys., Part 1 1987, 26, 357. (22) Ahmadi, T. S.; Wang, Z. L.; Henglein, A.; El-sayed, M. A. Chem. Mater. 1996, 8 (6), 1161.
Figure 8. Elongated copper nanocrystals in the [001] orientation produced at R ) 15: (A) HRTEM image of a truncated decahedron, (B) PS in the [001] orientation, and (C, D) computer simulation.
petitive growth indicates that the {111} surface typically has a higher surface energy than that of the {100} surface. Indeed, at equilibrium, a crystal has to be bounded by facets with a minimum total energy.23 This behavior is explained by the selective adsorption of AOT surfactant on {100} facets. In fact, this capping prevents or retards the growth of {100} facets and this slow growing facet becomes the dominant surface of the Cu nanocrystal. (iii) Elongated Particles. The elongated nanocrystals are characterized by an aspect ratio close to 1.7. Comparison of the HRTEM (Figure 8A) and simulation (Figure 8B) images indicates formation of fivefold symmetry with lattice planes better defined in the center of the particle than at the border. The calculated PS (Figure 8C) shows three pairs of equatorial reflections labeled 1-3 together with sideband reflections labeled 4-9. These reflections correspond to the lattice parameters d1 ) 0.128 nm, d2 ) 0.210 nm, d3 ) 0. 338 nm, d4 ) d9 ) 0.181 nm, d5 ) d8 ) 0.217 nm, and d6 ) d7 ) 0.250 nm. Reflections 1, 2, 4, and 9 correspond to the Cu (220), (111), (200), and (020) plane spacings, respectively (0.1278, 0.2087, 0.1808, and 0.1808 nm). The angle between (200) and (020) is 90°. The appearance of the (200) and (020) reflections together with the (111) reflection proves that the particle consists of more than one subunit that must be twinned together. Reflections 3 and 5-8 are caused by multiple scattering effects.24 Such a PS clearly indicates a truncated decahedron, that is, a decahedron with additional intermediate (100) planes, and is in good agreement with the simulated PS (Figure 8D). It is interesting to note that elongated copper particles or nanorods have been already produced (23) Wulff, G. Z. Kristallog. 1901, 34, 449. (24) Lisiecki, I.; et al. Phys. Rev. B 2000, 61, 4968; Langmuir 2000, 16, 8802; 8807.
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Figure 9. Schematic of the growth mode of elongated copper nanocrystals.
Figure 10. Triangular particle of copper oriented in the [111] direction: (A) HRTEM image, (B) calculated PS, and (C, D) computer simulation.
by changing the experimental conditions.9 Whatever the aspect ratio is, it was found that the structure with a truncated decahedron remains the same.9 The elongated nanocrystal structure (Figure 9), already observed,24 is the result of additional intermediate (110) planes in a decahedral nucleus. In other words, their formation is induced by the truncation of the five subunit edges of the decahedral nucleus. Thus, they are limited at the edges by {100} facets and at each extremity by {111} facets. This clearly indicates that the nanocrystal growth is hindered and takes place in the [110] direction, that is, the fivefold symmetry resulting in elongated nanocrystals.
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Such a formation is determined from the preferential growth of the {111} facets compared to the growth of {100} facets. This process is attributed to selective adsorption of AOT surfactant on {100} facets. (iv) Flat Triangular Nanocrystals. The HRTEM image (Figure 10A) shows a triangular nanocrystal, which exhibits three lattice planes, and the calculated PS (Figure 10B) with four pairs of reflections labeled 1-4. The lattice parameters are d1 ) d2 ) d3 ) 0.221 nm and d4 ) 0.128 nm. Reflections 1-3 correspond to the forbidden 1/3{422} reflections related to the fcc structure. The angle between these planes is 60°. It has to be mentioned that these reflections have mostly been observed in flat, noble metal samples.15 This behavior is related to the presence of stacking faults inside the nanocrystal. Reflection 4 corresponds to the (220) planes of a fcc copper single crystal oriented in the [111] direction. The PS of such a regular fcc nanocrystal in the [111] orientation consists of three reflections with a threefold symmetry corresponding to the {220} spacing. The lattice parameter d220 is 0.128 nm. As a result of the limited resolution of the microscope and the size limitation of the nanocrystals taken as models in the simulations (∼3 nm), these reflections are not visible in the simulated PS. However, for a crystal, as a result of its greater size (∼20 nm) the intensity of these reflections is enhanced and, hence, they become observable. From these considerations, it can be concluded that a fcc nanocrystal with stacking faults oriented in the [111] direction is formed. These objects are also called “trigonal lamellar particles” or “trigonal platelets”. The initial precursor nucleus for these nanocrystals must contain a unique threefold axis, which is expressed in the final shape of the nanocrystal.15 Among the stable precursor nuclei, the decahedron, the cuboctahedron, and the tetrahedron, only the latter can be modified simply to give a trigonal lamellar particle. Indeed, if a regular fcc tetrahedron is truncated on a {111} surface and then twinned by reflection at this surface then a suitable nucleus for the trigonal lamellar particles can be obtained (Figure 11). Such a bitetrahedral precursor nucleus possesses the required threefold symmetry and, more importantly, contains three active sites for growth to maintain the overall threefold symmetry in the final nanocrystal. The truncation can be attributed to the capping of surfactant on one of the edges (as in the previous case related to the truncation of decahedra) and then on the {111} facets of the tetrahedral units, which induces the formation of new {111} facets that will twin together. V. Conclusion We show in this paper that the synthesis of copper nanocrystals in a micellar system and under supersaturated conditions favors the formation of various isotropic and anisotropic shapes. Cubes, flat triangles, pentagons, and elongated forms are, thus, observed. Through HRTEM, the various structures have been identified and explanations related to their growth processes are proposed involving the nucleus structure and the capping conditions. These data clearly show that syntheses have to be carried out in a “super” saturated region to produce
Figure 11. Schematic of the growth mode of triangular copper nanocrystals.
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well-defined nanocrystals. The structure of the precursor nucleus and the surfactant adsorption on specific facets of Cu nanocrystals are the dominating factors for the growth and then the determination of the final shapes. However, it is difficult to predict which specific interactions take place. For this we need to know what driving force favors the specific adsorption of surfactant on a given
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facet. This is still unclear and, thus, opens a new research area. If this problem can be solved, it will probably be possible to control the growth of anisotropic nanocrystals. LA0492862
