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Mesostructures of Cobalt Nanocrystals. 2. Mechanism V. Germain and M. P. Pileni* Laboratoire LM2N, UniVersite´ Pierre et Marie Curie (Paris VI), BP 52, 4 place Jussieu, F-752 31 Paris Cedex 05, France ReceiVed: September 27, 2004; In Final Form: December 8, 2004
Mesoscopic patterns of cobalt nanocrystals produced by applying a magnetic field perpendicular to the substrate during the deposition process are presented. These mesoscopic patterns markedly differ with the size distribution of the cobalt nanocrystals. Well-defined columns are produced when the size distribution of cobalt nanocrystals is low; conversely, the coalescence of columns with formations of labyrinths occurs for a large size distribution. A formation mechanism of these structures is proposed.
1. Introduction Several studies have been undertaken to describe the ferrofluid patterns in Hele-Shaw cells with an applied perpendicular magnetic field.1-3 The observed patterns are columns or dots that may be disordered or ordered (in a hexagonal array) and labyrinths. At a given strength of the applied field, a pattern made of ordered columns is obtained, and upon an increase in the time at the same magnetic field intensity, a second structure also consisting of a hexagonal array is formed. This second structure is obtained by a phase transition where each column splits in two.2 The mechanism for the formation of columns and labyrinths is not well understood. We produced similar patterns in our laboratory in 2001 by applying a magnetic field during the evaporation of a fluid containing cobalt nanocrystals dispersed in a nonpolar solvent.4 However, a thick film of cobalt nanocrystals underneath such “supra” structures was observed. In this case, the applied magnetic field is quite high and is in the nonlinear regime. Furthermore, the size distribution of nanocrystals dispersed in the nonmagnetic fluid is rather wide. Very recently, we have been able to obtain good control of the size distribution of cobalt nanocrystals.5 The physical properties of mesoscopic assemblies differ from those of isolated nanocrystals and those of the bulk phase.6,7 Furthermore, the mesoscopic structure itself is also a key parameter in the control of the physical properties.8-11 Hence, over the past 5 years, collective magnetic, optical, and transport properties were found.6 They are mainly due to dipole-dipole interactions. Intrinsic properties due to self-organization also open a new research area.12 This concerns the physical, chemical, and mechanical properties of these assemblies. Here, we propose a mechanism for the formation of mesoscopic structures of cobalt nanocrystals. 2. Experimental Section 2-1. Chemicals. Cobalt acetate, lauric acid, and sodium borohydride were from Aldrich, isooctane and hexane were from Fluka, and di(ethylhexyl)sulfosuccinate was from Sigma. The synthesis of cobalt (2-ethylhexyl)sulfosuccinate [Co(AOT)2] is described elsewhere.13 All of the chemicals were used without further purification. * To whom correspondence should be addressed:
[email protected].
2-2. Equipment. Scanning electron microscopy (SEM) was done with a JSM-5510LV instrument. The applied magnetic field was produced either by a permanent magnet or by an electromagnet (Oxford Instruments N38 and Oxford Instruments PS2-120, respectively). We used different magnetic flux densities, 0.46 and 0.59 T, for the experiments with the electromagnet and 0.25 T for those with the permanent magnet. Videos were made using the video camera of a Parc Scientific Autoprobe AFM connected to a PowerMac through a Formac VideoDV. 2-3. Synthesis. The synthesis of cobalt nanocrystals with a narrow size distribution was described in a previous paper.5 Reverse micelles of 5 × 10-2 M Co(AOT)2 with a water content w ) [H2O]/[AOT] ) 32 are formed in the Co(AOT)2/isooctane/ water system. Co(AOT)2 is reduced by sodium borohydride (NaBH4), with the amount of NaBH4 added to the solution being monitored by the ratio R ) [NaBH4]/[AOT]. The nanocrystals are coated by adding lauric acid (C12H25COOH) to the solution; this induces a covalent attachment to cobalt atoms located at the interface.14 The coated cobalt nanocrystals are then extracted from the solution by theaddition of ethanol. They are washed and centrifuged several times with ethanol to remove all of the surfactant molecules and then dispersed in hexane. The concentration of the cobalt nanocrystal solution is set at 4.8 × 10-4 mol L-1. Control of the nanocrystals’ size and distribution is obtained as already demonstrated; the syntheses, done at various R values, induce the formation of nanocrystals with different size distributions with a slight change in the average size.5 2-4. Mesoscopic Structure: Sample Preparation. The mesostructures are produced as described in ref 4: The magnetic fluid (200 µL) is deposited in a beaker containing a highly oriented pyrolitic graphite (HOPG) substrate. The beaker is then covered with Parafilm to reduce the evaporation rate and is subjected, or not, to an external magnetic field perpendicular to the substrate. The solvent slowly evaporates, and the magnetic field is applied until the evaporation is complete. For video experiments, a microscope glass plate, kept in place and sealed by the Parafilm, is used. At the end of the evaporation, the substrate is removed and the mesoscopic structures are recorded by SEM.15 Similar data are obtained upon replacing HOPG with silicon, and the type of magnet used does not affect the mesostructures.16
10.1021/jp0456180 CCC: $30.25 © 2005 American Chemical Society Published on Web 03/05/2005
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Figure 2. SEM patterns of mesostructures of cobalt nanocrystals. A field strength of 0.25 T is applied during the evaporation; silicon is used as the substrate. The mesostructures in parts A, D, and G are obtained with nanocrystals A, B, and C, respectively. Parts B and C, E and F, and H and I are magnifications of the structures in parts A, D, and G, respectively
Figure 1. SEM patterns of mesostructures of cobalt nanocrystals. A field strength of 0.25 T is applied during the evaporation; silicon is used as the substrate. The mesostructures in parts A, B, and C are obtained with nanocrystals A, B, and C, respectively.
3. Results and Discussion The mesostructures described below are formed of nanocrystals with the same average size but different size distributions. To easily identify the various systems used, we denote nanocrystals having average diameters of 5.7, 5.9, and 5.9 nm and respective size distributions of 13%, 16.8%, and 18% by A, B, and C, respectively. The applied magnetic field perpendicular to the substrate is 0.25 T. Figure 1 compares the SEM patterns of A-C. Obviously, they markedly differ; A has a welldefined and very compact structure (Figure 1A), the B nanocrystals’ patterns are less regular (Figure 1B), and there are a large number of flowerlike patterns for C (Figure 1C). For all of the samples, the patterns are rather homogeneous on a very large scale. Figure 2 shows magnifications of the SEM images in Figure 1, and these confirm the changes in the structural
patterns upon increasing the nanocrystal size distribution. Hence, A nanocrystals induce the formation of well-defined patterns (Figure 2A) with columns or dots and very few labyrinths. Magnifications of the patterns show that some columns are upright (Figure 2B), whereas others have fallen (Figure 2C). With B nanocrystals, cylinders (Figure 2D), either isolated or associated with each other, are produced. Figure 2E shows that the cylinder borders are not well-defined and include numerous defects. Furthermore, in a few parts of the substrate, coalesced cylinders are observed (Figure 2F). With C nanocrystals, as shown in Figure 2G, there is a formation of flowerlike and labyrinthine (see arrows) patterns. Magnification of these structures shows a coalescence of either upright (Figure 2H) or fallen columns (Figure 2I). The latter form wormlike or labyrinthine structures (see arrows). From these data, it is concluded that the mesoscopic patterns of cobalt nanocrystals markedly differ with a slight change in the size distribution. At narrow size distributions, columns or dots and well-defined cylinders are formed, whereas upon an increase in the size distribution, columns tend to fuse to form wormlike and labyrinthine patterns. The number of isolated columns or cylinders is very high when the size distribution is low (A), as observed in Figures 1A and 2A, whereas it markedly decreases upon increasing the size distribution (B and C). Hence, very few isolated columns (or cylinders) are produced with C nanocrystals. The lengths and widths of the isolated cylinders vary: with A, B, and C nanocrystals, the lengths are 3.05 µm ( 9%, 1.36 µm ( 7.5%, and 3.8 µm, respectively, whereas the widths are 1.7 µm ( 7%, 0.2 µm ( 16%, and 0.7 µm, respectively. For C nanocrystals, the number of isolated cylinders is too low to determine a size distribution. To explain this change in the behavior, let us consider the transmission electron microscopy (TEM) patterns obtained with A nanocrystals. While the experimental conditions are kept similar, the HOPG substrate
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Figure 3. TEM patterns obtained by using a magnetic field of 0.25 T and a copper TEM grid covered with amorphous carbon. The patterns in parts A-D are obtained with A nanocrystals. Parts A and B are low and high magnifications of the structures obtained without an applied magnetic field, respectively; part B is a FFT of the area denoted by a dashed line. The scale bar represents 86 nm-1. Parts C and D show low and high magnifications of the structures obtained with a field strength of 0.25 T applied perpendicularly. The patterns in parts E-H are obtained with nanocrystals having 6.9 nm average diameter and 18% size distribution. Parts E and F are low and high magnifications of the structure obtained without an applied magnetic field, respectively, whereas parts G and H (low and high magnifications, respectively) are obtained with 0.25 T applied perpendicularly.
is replaced by a TEM grid covered with amorphous carbon. Parts A and B of Figure 3 show the TEM pattern obtained by the deposition of A nanocrystals with no magnetic field, and parts C and D of Figure 3 exhibit the pattern obtained with an applied magnetic field. Large aggregates are formed with sizes between 2.5 and 9 µm (Figure 3A). When the magnification of these aggregates is increased, well-defined superlattices of nanocrystals (Figure 3B) are observed. The fast Fourier transform (FFT) of a given area of the superlattice (inset, Figure 3B) exhibits a 6-fold symmetry, indicating an ordering of the nanocrystal in a face-centered cubic (fcc) structure. This agrees with previous data from which “supra” crystals with fcc structures are produced17 when the size distribution of nanocrystals is narrow enough.18 In a 0.25 T applied magnetic field (perpendicular to the substrate), well-defined cylinders with a rather narrow size distribution are produced (Figure 3C). Figure 3D shows that their extremities are made of well-ordered nanocrystals. The structures’ thicknesses prevent any TEM observation of the nanocrystal ordering inside the cylinders, whereas the lack of material rules out any small-angle X-ray scattering
Germain and Pileni
Figure 4. SEM pattern of the mesostructure obtained on a HOPG substrate with nanocrystals of 7.2 nm diameter and 20% size distribution. For parts A and B, the strengths of the applied magnetic fields are 0.17 and 0.46 T, respectively.
experiments. From data already obtained with “supra” crystals17 and from Figure 3D, it is concluded that the cylinders are made of nanocrystals ordered in a fcc structure. Experiments similar to those above are carried out using nanocrystals with 6.9 nm average diameter and 18% size distribution. From a previous study,16 we know that, in the range of 5-8 nm (for the nanocrystals’ diameters), the structures are not affected by the nanocrystals’ sizes. With no applied field, when the nanocrystals’ size distribution is increased (18%), the aggregates formed are not well-defined and their sizes decrease (Figure 3E). Furthermore, the nanocrystal ordering disappears (Figure 3F). By using the same nanocrystals with an applied magnetic field, elongated aggregate (Figure 3G) and fingerlike (Figure 3H) structures without organized nanocrystals appear. From these results, it is concluded that the narrow size distribution of nanocrystals, favoring their self-organization in a fcc structure, produces very well defined cylinders and dots (columns) with large widths that are isolated from each other. The loss of the nanocrystal ordering decreases the columns’ cohesion and favors coalescence. At around a 20% size distribution, as can be clearly seen in Figure 4, there is a fusion of columns to form either wormlike or labyrinthine structures at various strengths of the applied field. Hence, in the 0.17 T (Figure 4A) to 0.59 T (Figure 4B) applied field range, the fusion of columns is observed upon using cobalt nanocrystals having a rather wide nanocrystal size distribution, whereas for a narrow size distribution at any applied field strength, only isolated columns are produced (Figures 1A and 2A) and very few fusions of columns are observed. In a previous study, we demonstrated, from a theoretical model based on the minimization of the total free energy and
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Figure 5. Video microscopy images obtained during the evaporation of a cobalt nanocrystal solution in a 0.25 T magnetic field at the moment when the columns appear. A silicon substrate is used. The arrows in B-E indicate a column; the dashed arrows (D) indicate the direction of column diffusion.
Figure 6. Video microscopy images obtained at the end of the evaporation of a cobalt nanocrystal solution in a 0.25 T magnetic field. A silicon substrate is used. The arrows indicate the direction of the wave created by the capillary forces.
from experiments, that the mechanism of formation of these mesostructures involves a liquid-gas phase transition with an average calculated interfacial surface tension of 5 × 10-5 N m-1. From this, it is concluded that cylinders are formed during the evaporation process.15 To prove this claim, a video recording is made during the evaporation process (around 12 h). During the first hours of the evaporation, as seen in Figure 5A,B, there are defects due to dust (no particular care was taken to ensure clean conditions). After 7 h, a number of dots appear (see the arrows in Figure 5C). The number of columns increases progressively (Figure 5D), and these then migrate in the solution (Figure 5E,F) to form hexagonal arrays. This confirms that the columns form during the evaporation process. The same video is obtained for the various nanocrystals used (A-C). At the end of the evaporation, as the video in Figure 6 clearly indicates, there is a progressive collapse of the columns. From these data, it is concluded that, during the evaporation process, the applied magnetic field perpendicular to the substrate induces the formation of columns. Some of them fall because of the waves induced by capillary forces. The results of these two opposite waves are also observed at the end of the evaporation (Figure
7), giving rise to fernlike structures made of columns. This explains the formation of cylinders and dots (columns) observed at the end of the evaporation process (Figures 1 and 2). Similar behaviors are obtained upon replacing HOPG by silicon. For experimental reasons, it has not been possible to record the videos at various applied fields. However, the SEM patterns observed at the end of the evaporation for applied fields of 0.17 T (Figure 8A), 0.33 T (Figure 8B), 0.46 T (Figure 8C), and 0.59 T (Figure 8D) clearly show the formation of a fernlike structure with a disorder in the organization. Magnifications of such SEM patterns (Figure 9) show that these fernlike structures are made of columns and that some of them tend to fuse to form wormlike structures or labyrinths. From these results, a mechanism of pattern formation in a perpendicular applied field is proposed: During the evaporation process, a liquid-gas phase transition occurs with the formation of a concentrated solution of nanocrystals in equilibrium with a diluted one. In the concentrated phase, columns (dots) are progressively formed and tend to migrate. The ordering in a fcc structure of the nanocrystals, having a narrow size distribution, favors the formation of well-defined and compact columns.
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Figure 7. Video microscopy image obtained at the end of the evaporation process of a cobalt nanocrystal solution in a 0.25 T magnetic field. A HOPG substrate is used. The arrows indicate the direction of the wave created by the capillary forces.
assemble via van der Waals interactions, forming wormlike and labyrinthine structures, as can be readily seen in Figure 3H. This explains the change in the height of the labyrinths. The patterns show fallen columns, called cylinders above, because of the waves induced by capillary forces during the evaporation time. Because little solvent is still present when the columns are subjected to the waves, they easily collapse. At the end of the evaporation, traces of such waves can be observed. The calculations made in our laboratory19 and those made by others3 found no difference in the energy to produce columns and labyrinths. 4. Conclusion
Figure 8. SEM pattern in a fernlike structure obtained at various field strengths: 0.17, 0.33, 0.46, and 0.59 T for A, B, C, and D, respectively. A HOPG substrate is used.
For the first time, to the best of our knowledge, an overview of the mechanism to explain the pattern formation when a magnetic field is applied perpendicular to the substrate is proposed. Well-defined and isolated columns are formed when the size distribution of nanocrystals is sufficiently narrow. When the size distribution is increased, the columns are no longer ordered in a fcc structure and they lack rigidity. Defects appears along the columns. Because of the low stability of these columns, the entities having similar heights tend to self-organize in solution via van der Waals forces and form wormlike or labyrinthine structures. Furthermore, the disordering of the nanocrystals induces lateral defects on the columns, favoring their coalescence. Hence, labyrinths are made of columns. This agrees with calculations made in our laboratory19 and those made by other groups3 in which columns and labyrinths are produced even if there is no difference in the energy. Further development of this work should tend to avoid the destruction of the arrangement of the mesoscopic structures by the capillary forces; thus, the pattern will present upright structures, giving rise to new magnetic properties. Acknowledgment. The authors thank Dr. D. Ingert and Dr. J. Richardi for fruitful discussions.
Figure 9. SEM pattern columns forming fernlike structures at various field strengths: 0.17, 0.33, 0.46, and 0.59 T for A, B, C, and D, respectively. A HOPG substrate is used.
Because they are formed in solution, they tend to diffuse and self-organize in a hexagonal network. When the size distribution of the nanocrystals is increased, the interactions between the particles markedly decrease and the columns are formed with disordered entities. This creates defects, and the cohesive forces between columns are not large enough to keep them ordered. Columns having more or less the same sizes tend to self-
Supporting Information Available: Three movies in .mov format are available. One movie shows the formation of the pattern on a silica substrate; the two others show the destruction of the arrangement by the capillary forces at the end of evaporation on silica and HOPG substrates. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Cowley, M. D.; Rosensweig, R. E. J. Fluid Mech. 1967, 30, 671. (2) Hong, C. Y.; Jang, I. J.; Horng, H. E.; Hsu, C. J.; Yao, Y. D.; Yang, H. C. J. Magn. Magn. Mater. 1999, 201, 317.
Mesostructures of Cobalt Nanocrystals. 2 (3) Dickstein, A. L.; Erramilli, S.; Goldstein, R. E.; Jackson, D. P.; Langer, S. A. Science 1993, 261, 1012. (4) Legrand, J.; Ngo, A. T.; Petit, C.; Pileni, M. P. AdV. Mater. (Weinheim, Ger.) 2001, 13, 58. (5) Lisiecki, I.; Pileni, M. P. Langmuir 2003, 19, 9486. (6) Pileni, M. P. J. Phys. Chem. B 2001, 105, 3358. (7) Pileni, M. P. AdV. Funct. Mater. 2001, 11, 323. (8) Ngo, A. T.; Pileni, M. P. J. Phys. Chem. 2001, 105, 53. (9) Petit, C.; Russier, V.; Pileni, M. P. J. Phys. Chem. B 2003, 107, 10333. (10) Russier, V.; Petit, C.; Pileni, M. P. J. Appl. Phys. 2003, 93, 10001. (11) Lalatonne, Y.; Motte, L.; Russier, V.; Ngo, A. T.; Bonville, P.; Pileni, M. P. J. Phys. Chem. 2004, 108, 1848.
J. Phys. Chem. B, Vol. 109, No. 12, 2005 5553 (12) Courty, A.; Merme, J.; Duval, E.; Pileni, M. P. Submitted for publication. (13) Petit, C.; Lixon, P.; Pileni, M. P. J. Phys. Chem. 1993, 97, 12974. (14) Wu, N.; Fu, L.; Su, M.; Aslam, M.; Wong, K. C.; Dravid, V. P. Nano Lett. 2004, 4, 383. (15) Germain, V.; Richardi, J.; Ingert, D.; Pileni, M. P. J. Phys. Chem B, submitted for publication. (16) Germain, V.; Pileni, M. P. Submitted for publication. (17) Lisiecki, I.; Albouy, P. A.; Pileni, M. P. AdV. Mater. (Weinheim, Ger.) 2003, 15, 712. (18) Pileni, M. P. J. Phys. Chem. B 2001, 105, 3351. (19) Richardi, J.; Pileni, M. P. Phys. ReV. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 2004, 69, 16304.
