Langmuir 2000, 16, 8807-8808
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Annealing Process of Anisotropic Copper Nanocrystals. 2. Rods† 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 well-defined long truncated decahedral Cu rods of a diameter in the nanometersize range and a length of the order of 1 micron was studied. For isolated rods on the grid, two melting processes that initiated a drastic change in the melting temperature were observed. These variations are explained in terms of the crystallinity of the nanorods. In the absence of defects, the nanorods are highly stable with a melting temperature close to the bulk phase. In contrast, in the presence of defects, the melting temperature drastically decreases. In this case, nanocrystals can undergo a shape transformation before melting from rod to cylinder.
I. Introduction A few years ago, the formation of copper nanorods by the use of a colloidal template1-4 was demonstrated. This method opens new paths of fundamental research and its technological applications. As a matter of fact, a shape anisotropy is expected to create drastic changes in optical and/or charge transport properties5 and in catalysis.6 To enable a close study of the physical properties of such nanorods, we needed to develop careful structural experiments. In the first step, the structure of such nanocrystals was investigated.4 The particles are not single-crystalline of face-centered cubic (fcc) structure, but are composed of a set of deformed tetrahedra bounded by (111) faces with additional intermediate planes (110). It must be noted that copper oxide is not depicted on the power spectrum, nor is it shown during the electron diffractions made directly on the particles. This observation is confirmed by electron energy loss spectroscopy (EELS).7 However, the structural investigation conducted on a collection of rods shows the presence of defects as stacking faults and/or dislocations. The present paper describes the annealing processes of these rods. II. Experimental Section The products and the technique used in these experiments are described in a previous paper.8 Concerning the synthesis of the nanorods, all experimental conditions * 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) Tanori, J.; Pileni, M.-P. Adv. Mater. 1995, 7, 862. (2) Tanori, J.; Pileni, M.-P. Langmuir 1997, 13, 639. (3) Pileni, M.-P.; Gulik-Krzywicki, T.; Tanori, J.; Filankembo, A.; Dedieu, J. C. Langmuir 1998, 22, 7359. (4) Lisiecki, I.; Filankembo, A.; Sack-Kongehl, H.; Weiss, K.; Pileni, M.-P.; Urban, J. Phys. Rev. B 2000, 61; in press. (5) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Toennies, J. P., Ed.; Springer Series in Material Sciences, Vol. 25; Springer: New York, 1993. (6) Ahmadi, T. S.; Wang, Z. L.; Heiglein, A.; El-Sayed, M. A. Chem. Mater. 1996, 8 (6), 1161. (7) Sauer, H. Private communication. (8) Lisiecki, I.; Sack-Kongehl, H.; Weiss, K.; Urban, J.; Pileni, M.-P. Langmuir, in press.
remain the same, with two exceptions: (1) The water content, defined as the ratio of water over AOT concentration, is equal to 9.5 rather than 30. (2) A mixture of sodium chloride solution replaces the water added to the micellar solution. The overall NaCl concentration is 10-3 mol dm-3. III. Results and Discussion At the end of the chemical reaction described in the preceding paper in the Experimental Section, a drop of the solution was deposited on a carbon grid. Large rods were observed (Figures 1-3) at 20 °C. In this paper we concentrate on the isolated rods. After the solution was deposited on a transmission electron microscopy (TEM) grid, a given collection of particles was chosen and the sample was heated from 20 to 925 °C. Above this temperature, the system is unstable. The TEM patterns were recorded at various temperatures. The experiments described below were reproduced several times. When the rods are isolated on the TEM grid, the following two behaviors are observed: (1) The copper rods keep the same shape and electron diffraction pattern with increasing temperature (Figure 1). Below 925 °C, the rods are characterized by truncated decahedra, regardless of temperature. Above 925 °C, the copper rod totally evaporates and leaves its print on the TEM grid because of the surfactant remaining in carbon form. This behavior clearly indicates that the premelting process does not take place.9-11 The melting temperature is close to the bulk phase temperature (1083 °C), which leads us to conclude that rods of an average length of 1 µm and width of 20 nm behave like the bulk material, i.e., no premelting temperature and an immediate evaporation process. The slight difference in the melting temperature can be attributed to the structural change observed between the nanorods and the bulk material.12,13 Furthermore, the substrate could play a role, as has already been observed with nanoparticles.8 (9) Garrigos, R.; Cheyssac, P.; Kofman, R. Z. Phys. D: At., Mol. Clusters 1989, 12, 497. (10) Buffat, Ph.; Borel, J. P. Phys. Rev. A 1976, 13, 2287. (11) Sambles, J. R. Proc. R. Soc. London, Ser. A 1971, 324, 339. (12) Reinhard, D.; Hall, B. D.; Berthoud, P.; Valkealahti, S.; Monot, R. Phys. Rev. B 1998, 58 (8). (13) Valkealahti, S.; Manninen, M. Phys. Rev. B 1991, 45 (16).
10.1021/la000345v 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 copper nanoparticles. 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 copper nanorod. The annealing experiment is performed in situ in the electronic microscope, where temperature is measured using a thermocouple attached to the furnace.
Lisiecki et al.
Figure 4. Diffraction patterns of copper nanocrystal shown in Figure 2: (A) rod at 20 °C and (B) cylinder at 650 °C.
observed at 20 °C, and also to the instability at high temperatures. The explanation of such behaviors can be found in the annealing data observed on materials having defects.14 The imperfections in the nanorods favor the formation of the liquid nucleus produced by the atoms located on the surface. These atoms are weakly bound to the material. This induces an increase in the size of the liquid nucleus. Above a critical size formed by statistical fluctuation, the liquid nucleus turns into a crystal state. When the temperature is increased, a collection of atoms is activated and migrates to the cylinder. This activity induces a progressive decrease in the rod length and an increase in the cylinder size. At the end of the process, a cylindrical nanocrystal is formed. No obvious explanation is presented at hand to explain why the cylinders reconstruct with the same structure as the rod and not that of the bulk phase. Therefore, we concluded that the defects are responsible for inducing the transition from rods to cylinders. Conclusion
Figure 3. Influence of temperature on the melting of a copper nanorod. The annealing experiment is performed in situ in the electronic microscope, where temperature is measured using a thermocouple attached to the furnace.
(2) The shape and structure of copper rods remain unchanged below 450 °C. At and above 450 °C, Figures 2 and 3 clearly show appearances of an aggregate located either on the tip (Figure 2) or in the middle of the rod (Figure 3). When the temperature is increased, the size of the aggregate increases; simultaneously, the length of the rod decreases to reach a rather large cylinder. Electron diffraction at 650 °C (Figure 4B) causes the material to self-arrange in the same structure (5-fold symmetry) as long rods (Figure 4A). However, the pattern shown in Figure 4B is not well-resolved. This is due to the orientation, which is slightly tilted with respect to the rod (14) Peppiatt, S. J. Proc. R. Soc. London, Ser. A 1975, 345, 401.
The rods studied here are not single-crystalline of structure but are truncated decahedra. They are composed of a set of deformed tetrahedra bounded by (111) faces with additional intermediate planes (110). Two melting behaviors are observed: (1) The rods are highly stable and melt at a temperature close to that of the bulk phase. (2) The melting temperature drops drastically with shape transformation from rod to cylinder with no structural change. This process is attributed to defects which could be either dislocations or stacking faults. Acknowledgment. This 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. LA000345V
