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Annealing Process of Anisotropic Copper Nanocrystals. 1. Cylinders† I. Lisiecki,‡ H. Sack-Kongehl,§ K. Weiss,§ J. Urban,§ and M.-P. Pileni*,‡ Laboratoire SRSI, URA CNRS 1662, Universite´ P. et M. Curie (Paris VI), BP 52, 4 Place Jussieu, F-752 31 Paris Cedex 05, France, and Fritz-Haber-Institut der MPG, Abt. AC, Faradayweg 4s6, D-14195, Berlin, Germany Received March 7, 2000. In Final Form: June 16, 2000
The annealing process of truncated decahedral cylindrical copper particles with 5-fold symmetry (7.5 × 20.5 nm) is presented. The melting point of the cylinders lies below the melting point of the bulk phase. During the annealing process, various stages of surface diffusion, premelting, and melting take place. Depending on the particle coating with the surfactant, two behaviors are observed. When the nanocrystals are well-coated, they disappear completely at 520 °C. At the opposite, when they are not well-coated and are placed closely together during the premelting phase (at 380 °C), a percolation process leads to the formation of large aggregates which tend to crystallize again at a melting point far above 520 °C.
I. Introduction Because of its possible demand in technology, the synthesis of nanoparticles has been extensively studied. As to metal nanoparticles,1,2 the electronic properties were shown to depend on the size and shape of the particles.3-6 Additionally, metal nanoparticles very often act as active catalysts.7 Colloids can be used as templates8 to control the size3,4,6,9 and shape1,4,9-12 of the particles. In terms of particle growth, some analogies between surfactant selfassemblies and natural media are proposed.1 In both cases, the particle growth needs a supersaturated medium for the nucleation to take place. Chemists are increasingly contributing to the synthesis of advanced material with enhanced or novel properties by using colloidal assemblies as templates. Recently, it was demonstrated that the shape of copper metal particles strongly depends on the colloidal structure in which the chemical reduction of Cu(AOT)2 takes place.9-12 If the template is made of interconnected cylinders, cylindrical copper particles are obtained. The present paper describes the annealing process of cylindrical copper nanocrystals.
as described elsewhere.6 Residual water present in the initial Cu(AOT)2/isooctane solutions, before addition of water, was analyzed by the Karl Fisher titration method using a Mettler automatic titrator. The Cu(AOT)2 concentration was determined by adding a sample to a solution of 0.03 M hydrochloric acid and 0.3 M ammonium acetate, and subsequently titrating for copper(II) using a 0.01-M sodium EDTA solution with 4-(2-pyridylazo)resorcinol as a color indicator. Isooctane was supplied by Fluka (99.5% purity), and ammonium acetate (98%), sodium EDTA, and 4-(2-pyridylazo)resorcinol (99%) were supplied by Prolabo. II.2. Apparatus. The transmission electron microscopy (TEM) experiments were carried out using a Philips CM200 FEG microscope operating at 200 kV, Cs ) 1.35 mm, with an information limit better than 0.18 nm. The images were digitized with a pixel size of about 0.03 nm, and the corresponding power spectra (PS), i.e., square of the Fourier transform of the image, were calculated. A sample holder constructed by GATAN was used to heat the sample from 20 to 600 °C. To minimize the effect due to the thermal drift, the specimen holder was cooled when it reached a temperature higher than 500 °C. The temperature given here was measured using a thermocouple attached to the furnace.
II. Experimental Section
The copper metal particles were prepared by reducing copper(II) bis(2-ethylhexyl)sulfosuccinate, Cu(AOT)2, in Cu(AOT)2-isooctane-water colloidal self-assemblies. Hydrazine was used as the reducing agent, and the reaction that takes place under N2 atmosphere and at 22 °C started immediately after hydrazine was added to the colloidal solution. In a Pyrex tube, 3 mL of 5 × 10-2 mol dm-3 Cu(AOT)2-isooctane was mixed with 140 µL of water. The Cu(AOT)2 and water concentrations were 4.8 × 10-2 mol dm-3 and 0.80 mol dm-3, respectively. Then, 22 µL of 22 mol dm-3 hydrazine was added to the colloidal solution, which was vigorously stirred. Thus, the ratio of Cu(AOT)2 to hydrazine was one-third, and the corresponding ratio of water to AOT concentration was w:30. The colloidal assembly immediately turned dark because of the reduction of Cu2+ to Cu(0), and the solution became darker with time. The reaction lasted 2 h.
II.1. Products. Copper(II) bis(2-ethylhexyl)sulfosuccinate (Cu(AOT)2) was prepared by ion exchange with the sodium salt * To whom all correspondence should be addressed. † Part of the Special Issue “Colloid Science Matured, Four Colloid Scientists Turn 60 at the Millennium”. ‡ Universite ´ P. et M. Curie (Paris VI). § Fritz-Haber-Institut der MPG. (1) Pileni, M.-P. Langmuir 1997, 13, 3266. (2) Glossman, M. D.; Iniguez, M. P.; Alfonso, J. A. Z. Phys. D 1992, 22, 541. (3) Lisiecki, I.; Pileni, M.-P. J. Am. Chem. Soc. 1993, 115, 3887. (4) Lisiecki, I.; Billoudet, F.; Pileni, M.-P. J. Phys. Chem. 1996, 100, 4160. (5) Petit, C.; Lixon, P.; Pileni, M.-P. J. Phys. Chem. 1993, 97, 12974. (6) Lisiecki, I.; Pileni, M. P. J. Phys. Chem. 1995, 99, 5077. (7) Bommannavan, A. S.; Montano, P. A.; Yacaman, M. J. Surf. Sci. 1985, 156, 426. (8) Pileni, M.-P. J. Phys. Chem. 1993, 97, 6961. (9) Pileni, M.-P.; Ninham, B. W.; Gulik, T.; Tanori, J.; Lisiecki, I.; Filankembo, A. Adv. Mater. 1999, 11 (16). (10) Tanori, J.; Pileni, M.-P. Adv. Mater. 1995, 7, 862. (11) Tanori, J.; Pileni, M.-P. Langmuir 1997, 13, 639. (12) Pileni, M.-P.; Gulik-Krzywicki, T.; Tanori, J.; Filankembo, A.; Dedieu, J. C. Langmuir 1998, 22, 7359.
III. Synthesis of Copper Cylinders
IV. Results At the end of the synthesis described above, a drop of a solution was deposited on an amorphous carbon film
10.1021/la0003443 CCC: $19.00 © 2000 American Chemical Society Published on Web 09/22/2000
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Figure 1. Influence of temperature on the melting of a collection of copper particles. The annealing experiment is performed in situ in the electronic microscope, where temperature is measured using a thermocouple attached to the furnace.
Figure 2. Influence of temperature on the melting of a collection of copper particles. The annealing experiment is performed in situ in the electronic microscope, where temperature is measured using a thermocouple attached to the furnace.
supported by a nickel grid. The solvent evaporated after a few minutes. Some drops of isooctane were deposited on the grid to remove parts of the surfactant. However, a rather large amount of surfactant remained on the grid and coated the particles. The AOT layers were observed by high resolution around the elongated copper particles. Related to the synthesis and the washing stages, the number of layers is variable. Surfactant must be present to prevent oxidation. The TEM patterns obtained after deposition showed formation of cylinders made of copper metal nanocrystals (Figures 1 and 2). Spheres were also produced. In this paper we concentrate on the cylinders. In a previous paper,13 we demonstrated formation of long rods by using a procedure similar to that described above. A careful structural study of these rods indicates formation of truncated decahedral 5-fold symmetry, and not a face-centered cubic (fcc) structure as in the bulk phase. In the following paragraphs some data indicating a similar structure for cylindrical nanocrystals as obtained with rods are reported. Images obtained by tilting the sample around its long rod axis are presented in Figure 3. Together with the images, the corresponding power spectra are calculated. The TEM patterns of a sample tilted at various angles are presented in Figure 3. The tilt angles are 0°, (18°, and (36°. After tilting the cylindrical
particle, a periodic sequence can be observed, i.e., every 36° a complete repetition of the high-resolution image, clearly displayed in the power spectrum, is achieved. A second different configuration is obtained by tilting around + and -18°. From the real image, it can be seen that for tilt angles 0° and (36°, the central part of the image shows atomic resolution with strong contrast. However, the particle tilted by (18° shows only lattice planes on the left or the right part. This result is related not to the fcc structure but rather to the tilt series of Cu nanorods observed in the previous study.13 Indeed, the reflections obtained under the different tilt angles correspond to the same lattice parameters, and the angles between planes are also in good agreement. Therefore, in analogy to the structural study previously investigated on Cu nanorods,13 the structure of copper cylinders can be explained as truncated decahedra with 5-fold symmetry. Hence, the particles are not single-crystalline of fcc structure but are composed of a set of deformed fcc tetrahedra bounded by (111) faces with additional intermediate planes (110). It must be noted that copper oxide has been depicted neither in the power spectrum nor during electron diffractions made directly on the particles. This finding is in agreement with the electron energy loss microscopy (ELLS) made on Cu nanorods prepared in the same conditions,14 which indicates formation of pure metal copper nanocrystals. The structural investigation performed on a collection of particles shows the presence of distortions and defects.
(13) Lisiecki, I.; Filankembo, A.; Sack-Kongehl, H.; Weiss, K.; Pileni, M. P.; Urban, J. Phys. Rev B 2000, 61. In press.
(14) Sauer, H. Private communcation.
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Figure 4. Variation of the average number of elongated nanocrystals with temperature.
Figure 3. Tilt series of a Cu cylinder: from top to bottom, +36, +18, 0, -18, and -36°. HRTEM images and corresponding PS.
The copper cylinders shown at 20 °C in Figures 1 and 2 are then heated. The experiments presented below have been repeated several times. A collection of cylinders is selected and the TEM grid is heated to various temperatures (20, 250, 380, 450, and 520 °C). The following two melting behaviors are observed: (1) The TEM patterns (Figure 1) show no significant change with temperature below 380 °C. At and above 320 °C, the length of the cylinders drastically decreases. Prints of the cylinders can be observed because of the formation of a carbon shell induced by the burning of the surfactant. At 520 °C, all cylinders are evaporated. Despite the fact that some of the cylinders are highly close together, they do not coalesce. To describe such behavior more precisely, we made histograms by counting almost 300 of the dispersed particles. The number of elongated nanocrystals (irrespective of their length) is counted at various temperatures (Figure 4). As already mentioned, when the temperature is increased, a slight decrease in the number of elongated nanocrystals is observed. At 380 °C, more than 80% of cylinders are observed. Above 380 °C, the number markedly decreases to reach 18% at 520 °C. At 600 °C all the cylinders are evaporated and only spheres remain. To make data more quantitative, histograms of the cylinders’ width and length are shown in Figures 5
Figure 5. Histograms of the cylinder width with annealing temperature.
and 6, respectively. The mean value of the width and the length of the cylinders is given in Table 1. For temperatures below 320 °C, Figure 5 clearly shows no drastic change in the cylinder width (7.6 nm) apart from a slight increase in the size polydispersity. In contrast, the length of the cylinders remains in the same order of magnitude below 320 °C as compared to width. This finding is well demonstrated in Figure 7, where the variation of the length-over-width ratio is plotted at various temperatures. It decreases from 2.6 to 1.8 when the temperature is increased from 250 to 450 °C. Figure 7 shows that almost 20% of the nanocrystals are spheres at 520 °C. (2) Another behavior is observed on the TEM. At low temperature (below 250 °C), no drastic change in size and shape is observed (the change is similar to that observed previously). At 380 °C, conversely to what is observed in Figure 1, Figure 2 clearly shows the formation of a small “island” of connected copper cylinders starting in the center
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Figure 6. Histograms of the cylinder length with annealing temperature. Table 1. Mean Width, Length, and Corresponding Ratio, R ) Length/Width, of Copper Cylinders, Recorded at Different Temperaturesa temperature (°C)
mean width (nm)
mean length (nm)
mean R
20 250 320 380 450 520
7.5 7.6 7.8 8 8 7.9
20.5 19.5 17.7 16.8 14.2 15.6
2.7 2.6 2.3 2.1 1.8 2
a
The error on the width and length is 0.3 nm.
of the cylinder assembly. At 450 °C, the “island” size increases and all the surrounding cylinders disappear. This large copper aggregate is still present at 600 °C. It must be noted that prints left by the surrounding cylinders appear thinner compared with the ones observed in Figure 1. This must be related to a better carbon coating for these last particles. Discussion The above-described results clearly show that the Cu cylinders change drastically when submitted to annealing. The melting point of cylindrical copper nanocrystals is far below the melting point of the bulk phase. At 600 °C, all particles evaporate, whereas the melting point is 1084 °C in the bulk phase. The major difference is found in the structure of the two materials. Cylinders are characterized by a 5-fold symmetry with a truncated decahedral structure, whereas the bulk phase is fcc. The cylinders are characterized by high crystallinity. However, the structural investigation conducted on different particles shows some distortions and defects. These defects induce formation of an initial liquid nucleus and provide good nucleation sites with the formation of a partial or complete
Figure 7. Histograms of the length-over-width ratio of the cylinders with annealing temperature.
liquid layer.15 The distortions and defects observed could explain the slight decrease in the number of cylinders observed at 20 and 250 °C (Figure 1). However, it is not reasonable to assume that they explain the markedly decreased melting point of most of the cylinders (below 600 °C) compared to the bulk phase. The annealing process can be explained as follows. At rather low temperatures (below 300 °C), a slight increase in the average width and a slight decrease in the length on the cylinders (Figures 5, 6; Table 1) are observed. The changes are attributed to surface diffusion. The atoms from the particle tip characterized by a rather high curvature move along the length of the cylinders. This interpretation is supported by the following observations: (1) The surface atoms are weakly bound and the constraints are fewer, which favors the motion of the atoms from the tip to the border of the cylinder length. (2) The presence of surfactant surrounding the cylinders prevents the atom motion from the nanocrystal to the outside. Above 300 °C, the premelting takes place with the formation of a layer of liquid on the surface of the particle. At such temperatures the surfactant molecules are burnt and decomposed into carbon atoms and other derivatives. At this point, the behaviors differ. It is difficult to explain such changes. The most probable explanation is the change of the particle coating. We must keep in mind that after the particles are deposited on the carbon grid, drops of isooctane are added to remove the surfactant. In the first approximation, it can be reasonably assumed that the coatings differ. It has been noted that some of the cylinders presented in Figure 1, which are characterized by the same high vicinity as the cylinders presented in Figure 2, do not coalesce. It can therefore be assumed that the surfactant coating, for which thickness is more or less (15) Peppiat, S. J. Proc. R. Soc. London, Ser. A 1975, 345, 401.
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important, can either prevent (Figure 1) or favor (Figure 2) a percolation process of the copper particles. When the cylinders are well-coated (see Figure 1), the particles slowly evaporate and only the carbon shell remains on the grid because of the burning of the surfactant. The particles evaporate from their tips. This occurs because the melting point decreases when the particle curvature is increased. This behavior has been well-established for spherical particles where a decrease in the melting point is observed in conjuction with an increase in the particle curvature, i.e., with a decrease in its radius. This process is usually observed for particles of a diameter smaller than 20 nm.15-29 In the present experiments, the average diameter of the cylinders, at 20 °C, is 7.5 nm. On the tip of the cylinders, the curvature is rather large and the melting point at this edge is likely to be very low. When the amount of surfactant remaining after the grid is washed immediately after particle deposition (see above) is rather large, the particles are well-coated by carbon atoms which retain the liquid formed by premelting. With increasing temperatures, the overall particles turn into a liquid phase. At 520 °C, the liquid evaporates and the skeleton of the cylinders remains because of the carbon coating of the particle. The increase in the size polydispersity observed during the annealing can also be attributed to the production of cylindrical particles. As shown in Figures 5 and 6, at 20 °C the distribution in width and length of the cylinders is rather large. This produces a change in the temperature related to the melting process. As a matter of fact, the melting temperature changes markedly with the size of the cylinders (see the second paper in this series, published in this issue). Besides, the presence of distortions and defects in the nanocrystals cannot be excluded. The annealing behavior shows drastic changes with the high vicinity of the nanocrystals on the carbon grid. At 520 °C, instead of evaporating, cylinders fuse and form large islands. This behavior can be explained as follows: when the particles are very close and not highly coated with surfactant, the liquid formed at the surface of the nanocrystal by premelting migrates from one nanocrystal to the other. With increasing temperature, a percolation process with a cooperative behavior takes place. Formation of large islands induces a change in the melting properties of the aggregates. As a matter of fact, we know that the increase in particle size induces an increase in the melting temperature. Hence, the percolation process induces a change in the size and shape of the aggregates and then (16) Sambles, J. R. Proc. R. Soc. London, Ser. A 1971, 324, 339. (17) Buffat, Ph.; Borel, J. P. Phys. Rev. A 1976, 13, 2287. (18) Buffat, Ph. Thin Solid Films 1976, 32, 283. (19) Castro, T.; Reifenberger, R.; Choi, E.; Andres, P. Phys. Rev. B 1990, 42, 13. (20) Borel, J. P. Surf. Sci. 1981, 106, 1. (21) Ercolesi, F.; Andreoni, W.; Tosattie, E. Phys. Rev. Lett. 1991, 66, 911. (22) Lewis, J. L.; Jensen, P.; Barrat, J. L. Phys. Rev. B 1997, 4 (56), 2248. (23) Coombes, C. J. J. Phys. F: Met. Phys. 1972, 2, 441. (24) Berman, R. P.; Curzon, A. E. Can. J. Phys. 1974, 11 (52), 923. (25) Couchman, P. R.; Jesser, W. A. Nature 1977, 269, 481. (26) Lai, S. L.; Carlsson, J. R. A.; Allen, L. H. Appl. Phys. Lett. 1998, 9 (72), 1098. (27) Garrigos, R.; Cheyssac, P.; Kofman, R. Z. Phys. D: At., Mol. Clusters 1989, 12, 497. (28) Reiss, H.; Mirabel, P.; Whetten, R. H. J. Phys. Chem. 1988, 92, 7241. (29) Cleveland, Ch. L.; Landman, U.; Luedtke, W. D. J. Phys. Chem. 1994, 98, 6272. (30) Reimer, L. Elektronenm KrusKopische Untersuchungs und Pra¨ parations Methoden. Springer-Verlag: Berlin Heidelberg New York, 1967; 223-231.
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a change in the melting point, thus producing a new crystalization of the aggregates. As a consequence, recrystallization of copper before the “second melting temperature” (higher than 600 °C) is reached is likely. Unfortunately, it has been impossible to obtain a diffraction pattern at such temperatures. However, similar behaviors are observed with large rods (see the second paper in this series, in this issue), and diffraction patterns indicate that the crystal structure of the rods remains unchanged. These two behaviors explain the annealing mechanism of cylindrical particles. However, it remains unclear what process induces a decrease in the melting point of cylindrical particles compared to the bulk phase. It must be noted that the cylinders do not have the same crystalline structure as the bulk phase. In fact, it has been demonstrated that the cylinders are characterized by a 5-fold symmetry. This structure probably is energetically less stable than an fcc structure. The change in the solidliquid surface tension could induce a change in the premelting process related to the wetting of the surface. It has been reported that the melting temperature of silver and gold nanoparticles of the same average size can differ drastically.19 Buffat and Borel17 determined the ratio of the melting temperature (T) of a spherical nanoparticle with a radius r to that of bulk (T0) as
[ ]
2 γSL T )1T0 Fs‚λ r - δ
where Fs is the density of the bulk solid, λ is the heat fusion, and γSL is the solid-liquid surface tension. The liquid layer thickness, δ, can be estimated to be approximately 0.25 r. If we consider a copper spherical nanocrystal having an average diameter of 3.7 nm, Fs ) 8900 kg‚m-3, and λ ) 205 kJ‚kg-1, T ) 400 °C, the surface tension is evaluated to 1.5 N‚m-1. This value is of the order of magnitude of those found for silver (γSL) 0.73 N‚m-1) and gold (γSL) 0.37 N.m-1) nanoparticles. We have to keep in mind that the particles are coated with carbon and the change in the surface tension is probably more important that the one obtained when particles are deposited on a carbon substrate. Conclusion In the present paper we have demonstrated that cylindrical copper particles are not single-crystalline of fcc structure like the copper bulk phase. Their structure is truncated decahedral with 5-fold symmetry. These nanocrystals are composed of a set of deformed fcc tetrahedra bounded by (111) faces with additional intermediate planes (110). During the annealing process, various stages take place. At low temperature the surface diffusion process is observed. When the temperature is increased, premelting phenomena take place with the formation of a liquid layer on the surface. When the particles are well-coated, the melting takes place and the liquid is evaporated at 600 °C. When the particles are not well-coated and located very close together, the liquid formed during the premelting runs from one particle to another, creating a very large domain of liquid which crystallizes again because of its large size. Acknowledgment. This work was supported by the program PROCOPE 1999, “Structural Investigations of Copper Nanoparticles of Different Size and Morphology”. The authors thank the French Foreign Ministry (Ministe`re des Affaires E Ä trange`res [MAE]) and the DAAD (Deutscher Akademischer Austauschdienst) for financial support. LA0003443
