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2D Self-Organization of Core/Shell Cohcp/Co Nanocrystals I. Lisiecki,† M. Walls,‡ D. Parker,† and M. P. Pileni*,† Laboratoire des Mate´ riaux Me´ soscopiques et Nanome´ triques, UMR CNRS 7070, UniVersite´ P. et M. Curie, Baˆ t. F, 4 Place Jussieu, 75005 Paris, and Laboratoire de Physique des Solides, UMR 8502, UniVersite´ Paris-Sud, Baˆ t. 510, 91405 Orsay Cedex, France ReceiVed October 16, 2007. In Final Form: NoVember 30, 2007 In this paper we report the preparation of ordered hexagonal 2D arrays of core/shell Cohcp/CoO nanocrystals. A full structural investigation has been carried out using high-resolution transmission electron microscopy, electron diffraction, and electron energy-loss spectroscopy.
Introduction Nanoparticles with a sufficiently low size distribution selfassemble in hexagonal 2D arrays.1-3 We showed in a previous paper that these arrays can be annealed at 350 °C to give rise to an atomic structural transition from poorly crystallized fcc (native nanoparticles) to monocrystalline hcp nanocrystals.4 This annealing process did not induce a coalescence of the hexagonal arrays. After the sample is annealed, the hcp nanocrystals are exposed to air, inducing oxidation of the nanocrystals with formation of a Cohcp/Co core/shell structure. A careful structural study of these core/shell nanocrystals is presented. Previous studies on Co/CoO particles have shown that they exhibit interesting magnetic properties arising from the ferromagnetic/ antiferromagnetic core/shell interface.5-8 However, in all these cases, the cobalt core has an fcc structure. We show here the first example of 2D arrays consisting of Co/CoO nanocrystals with an hcp Co core. This new core structure is expected to lead to a significant change in the exchange anisotropy compared to that of the existing fcc systems. Experimental Section Products. All materials were used without further purification. Cobalt acetate and dodecanoic acid are from Aldrich, isooctane, hexane, and sodium bis(ethylhexyl) sulfosuccinate (Na(AOT)) are from Fluka, sodium borohydride is from Acros. The synthesis of cobalt(II) bis(2-ethylhexyl) sulfosuccinate, (Co(AOT)2) has been described previously.9 Apparatus. Transmission electron microscopy (TEM) and electron diffraction were performed using a Jeol JEM-1011 microscope. Highresolution transmission electron microscopy (HRTEM) was performed using a Jeol JEM-2010 microscope. High-angle annular dark field (HAADF) and electron energy-loss spectroscopy (EELS) * To whom correspondence should be addressed. E-mail: pileni@sri. jussieu.fr. † Universite ´ P. et M. Curie. ‡ Universite ´ Paris-Sud. (1) Pileni, M. P. J. Phys. Chem. 2001, 105, 105. (2) Wang, Z. L. Mater. Charact. 1999, 42, 101. (3) Sun, S.; Murray, C. B. J. Appl. Phys. 1999, 85, 4325. (4) (a) Lisiecki, I.; Salzemann, C.; Parker, D.; Albouy, P. A.; Pileni, M. P. J. Phys. Chem. C 2007, 111, 12625. (b) Parker, D.; Lisiecki, I.; Salzemann, C.; Pileni, M. P. J. Phys. Chem. C 2007, 111, 12632. (5) Gangopadhyay, S.; Hadjipanayis, G. C.; Sorensen, C. M.; Klabunde, K. J. J. Appl. Phys. 1993, 73, 6964. (6) Nogue´s, J.; Skumryev, V.; Sort, J.; Stoyanov, S.; Givord, D. Phys. ReV. Lett. 2006, 97, 157203. (7) Tracy, J. B.; Weiss, D. N.; Dinega, D. P.; Bawendi, M. G. Phys. ReV. B 2005, 72, 64404. (8) Tracy, J. B.; Bawendi, M. G. Phys. ReV. B 2006, 74, 184434. (9) Petit, C.; Lixon, P.; Pileni, M. P. J. Phys. Chem. 1993, 97, 12974.
experiments were performed in a VG HB5 scanning transmission electron microscope (STEM) operating at 100 kV. The HAADF inner collection semiangle was 45 mrad. The probe convergence and EELS collection semiangles R and β were 7.5 and 22 mrad, respectively. The EELS spectrum-imaging mode was used, in which an entire spectrum is recorded for each probe position in the image,11 starting at the top-left position and scanning over a square sample area. The acquisition time per pixel in the 64 × 64 spectrum images was 100 ms, leading to a total acquisition time of 7 min for an entire spectrum image. Synthesis. The synthesis of the Co nanoparticles was performed as described in ref 10. The synthesis takes place in pure reverse micelles of Co(AOT)2, the size and form of which are controlled by the water content, defined here as w ) [H2O]/[AOT] ) 32. The cobalt ions are reduced by addition of sodium borohydride, NaBH4, to the micellar solution; the optimal concentration of reducing agent to reduce the size dispersion of the nanoparticles is R ) [NaBH4]/ [Co(AOT)2] ) 6. After synthesis, the Co nanoparticles are extracted from the AOT surfactant by adding dodecanoic acid molecules that covalently bond to the metallic surface. After being washed with alcohol, the particles are dispersed in hexane. The system is then centrifuged to precipitate bulk Co and larger particles, and we recover only the smaller nanoparticles with a low size dispersion. The entire synthesis was carried out in a nitrogen glovebox.
Results and Discussion 2D arrays of Co nanoparticles are prepared by dropwise deposition of approximately 150 µL of a colloidal solution (5.5 × 10-7 M) onto a copper grid coated with amorphous carbon. Such a thin film is required to do structural investigations based on electronic techniques. The cobalt nanoparticles are characterized by an average size and size distribution of 7.5 ( 0.4 nm and 9.5%, respectively. As already described4,10 the native nanoparticles are characterized by very low crystallinity, are composed of a few small fcc domains, and are highly stable to an exposure to air over several days. The monolayers are annealed in a furnace at 350 °C for 20 min under a flow of nitrogen and cooled in the nitrogen glovebox. The sample is removed from the glovebox for characterization. As previously observed on highly orientated pyrolytic graphite (HOPG), Figure 1 shows that the average diameter remains the same after annealing (within the accuracy limit of the measurements), as is observed for native nanoparticles. There is no coalescence of the nanocrystals.4a Because we use an amorphous carbon instead of an HOPG substrate, the nanoparticles self-organize on a smaller scale than (10) Lisiecki, I.; Pileni, M.-P. Langmuir 2003, 19, 9486. (11) Jeanguillaume, C.; Colliex, C. Ultramicroscopy 1989, 28, 252.
10.1021/la703215g CCC: $40.75 © 2008 American Chemical Society Published on Web 03/12/2008
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Figure 2. TEM image of a 2D array of Co nanoparticles annealed at 350 °C and exposed to air for 2.5 h.
Figure 1. (A) TEM image of a 2D array of 7.5 nm Co nanoparticles after being annealed at 350 °C. Inset: higher magnification image of the same nanoparticles. (B) HRTEM image of one of the Co nanoparticles in (A). (C) Electron diffraction pattern of the nanoparticles in (A).
described previously.12,13 The high-resolution image (Figure 1B) of a single particle shows no core/shell contrast, confirming previous observations.4 The high crystallinity is illustrated by the regular 2.00 Å spacing of the lattice planes (the characteristic distance corresponding to the 002 plane of hcp Co). The electron diffraction pattern taken from a population of several hundred (12) Courty, A.; Albouy, P. A.; Mermet, A.; Duval, E.; Pileni, M. P. J. Phys. Chem. B 2005, 109, 21159. (13) Lisiecki, I.; Albouy, P.-A.; Pileni, M.-P. AdV. Mater. 2003, 15, 712.
nanocrystals shows rings corresponding to pure hcp Co (see Table 1). Note that the two rings at 1.27 and 1.52 Å are also close to values predicted for the (311) and (220) planes of CoO. However, as no other reflections corresponding to this material are observed, we believe that there is no CoO present. In the same way, the rings at 1.08 and 1.27 Å could also correspond to the (311) and (220) reflections of fcc Co, but as for CoO, the other rings are absent. Magnetization measurements further confirm the absence of oxidation.4b To oxidize the Co nanocrystals, the sample is exposed to air for 2.5 h after being annealed and cooled in the glovebox. The TEM image of a 2D array (Figure 2) shows a fairly well contrasted core/shell structure. To explain the difference in the resistance to oxidation before and after annealing, we have to take into account the coating agent. It is reasonable to assume that during annealing at high temperatures the dodecanoic acid chains are partly desorbed or destroyed, leading to an easier oxidation of the metal nanoparticles. However, as no coalescence occurs after annealing (Figures 1A, 2, and 3A) we also conclude that some of the coating must remain intact. In the TEM image shown in Figure 3, the core/shell structure is clearly observed with an average shell thickness of ∼2 nm and an average core diameter of ∼5 nm. This gives a total average particle diameter (∼9 nm) larger than that observed for nonoxidized Co hcp nanocrystals (7.5 nm). To explain this difference, let us consider the corresponding electronic diffraction pattern (Figure 3B) which shows both the signature of hcp Co and cubic CoO (see Table 1). The strongest evidence of the presence of CoO is the ring at 2.46 Å typical of the (111) planes. Other rings characteristic of CoO could be present, but as they are located at distances close to the hcp Co rings, they cannot be clearly distinguished (see Table 1). The HRTEM image (Figure 3C) shows a single core/ shell particle in a hexagonal network. The core shows regular lattice planes with a typical distance of 2.00 Å, corresponding to the (002) planes of hcp Co. In some cases the orientation of the particle is such that lattice planes can also be observed in part of the CoO shell (Figure 3D) where the characteristic distance of the lattice planes is 2.46 Å, corresponding to the (111) planes of CoO. These data clearly demonstrate the formation of core/ shell nanocrystals comprising CoO as the shell and hcp Co as the core. The increase in the average diameter of the core/shell nanocrystals (1.5 nm), observed above, is attributed to the lower
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Figure 3. Detailed study of the nanoparticles in Figure 2: (A) TEM image showing the core/shell structure; (B) electron diffraction pattern of the nanoparticles in (A); (C) HRTEM image of one of the nanoparticles in (A) showing core Co hcp lattice planes; (D) HRTEM image of one of the nanoparticles in (A) showing core Co hcp lattice planes and shell CoO lattice planes.
Table 1: Comparison between Experimental and Theoretical d Spacings of Co fcc, Co hcp, and CoO Structures of Diffraction Ringsa Co fcc [hkl] indices
Co hcp dhkl (Å)
[hkl] indices
CoO dhkl (Å)
[hkl] indices
111 100
101
dhkl (Å) 2.43
2.19
2.20
2.00
2.05
1.86
1.89
2.1310 400
2.021
422 511
1.6505 1.5559
1.5068
1.50
1.4800
1.52 440
1.4293
620
1.2788
533
1.2330
622 444 711
1.2191 1.1671 1.1321
1.2850
1.26
1.2532
1.27 110
1.2520 222
a
2.438 2.333
1.9100
311
222
311 222
dhkl (Å)
2.4606
2.0230
220
311
4.669 2.860
S2
1.7723
102
220
dhkl (Å)
111 220
S1
2.0467 002
200
dhkl (Å)
2.1650 200
111
Co3O4 [hkl] indices
103 200
1.1490 1.0830
112
1.0660
1.2303
1.15 1.08 642
1.0803
731
1.0524
800 822 751 662
1.0105 0.9529 0.9335 0.9275
1.0688
1.08 201
1.0470
004
1.0150
1.0233
Key: S1, annealed sample not exposed to air; S2, annealed sample after exposure to air.
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Figure 4. (A) High-magnification HAADF image of a hexagonally ordered array of Co/CoO core/shell nanoparticles. (B) EELS spectrum of the area shown in (A) after power-law background subtraction. (C) Elemental map for oxygen, using the O K-edge. (D) Elemental map for cobalt, using the Co L-edge. (E) Low-magnification HAADF image of a hexagonally ordered array of hcp Co nanoparticles. (F) EELS spectrum corresponding to the area shown in (E).
density of the CoO material compared to the hcp metallic Co. To confirm this claim, the ordered arrays are analyzed by using HAADF and EELS. Figure 4A shows a high-magnification HAADF view of an ordered zone. The contrast in this technique, which captures mostly the thermal diffuse scattering, is dominated by the mass thickness of the sample (although a small contribution from high-angle Bragg beams is possible). The outer, oxidized part of the nanocrystals, being less dense and thinner in the beam direction, therefore appears darker. The global EEL spectrum from the whole area is shown after power-law background subtraction in Figure 4B. The strong oxygen signal clearly confirms the oxidized state of the cobalt nanocrystals. The zone in Figure 4A is used to record a spectrum image and elemental maps for oxygen, using the O K-edge (Figure 4C), and cobalt, using the Co L-edge (Figure 4D). They are generated using the Egerton method14 with 150 eV background fitting windows before the edges and 75 eV signal integration windows. The oxygen
signal is seen to originate mainly from the “shell” region of the nanocrystals (Figure 4C), while the cobalt is present throughout the nanocrystals but particularly in their centers (Figure 4D). The result clearly indicates a surface oxidation layer whose thickness is in the range 2-3 nm. This is slightly larger than the thickness deduced from the TEM images, probably as a result of the inferior spatial resolution in the STEM (the probe size used was approximately 1 nm). The signal intensity near the bottom of the images is lower because the current from the field emission gun decreases significantly during the 7 min needed to acquire the data. Figure 5A shows a detail of the O K-edge compared with those of reference oxides CoO and Co3O415 (Figure 5B) as measured by X-ray absorption spectroscopy (XAS). (The edge shapes observed using XAS and EELS on the same material should be almost identical). There is a clear lack of fine structure in the O K-edge from the nanocrystals. These experiments are not optimized for fine structure observations, but rather to
(14) Egerton, R. F. Electron Energy-Loss Spectroscopy in the Electron Microscope, 2nd ed.; Plenum Press: New York, 1996.
(15) Soriano, L.; Abbate, M.; Ferna´ndez, A.; Gonza´les-Elipe, A. R.; Sirotte, F.; Sanz, J. M. J. Phys. Chem. B 1999, 103, 6676.
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crystalline Co3O4 present should have contributed a clear peak at 532 eV, as in the reference. The results thus imply that the oxide is more likely to have the CoO stoichiometry and to be mostly nanocrystalline. For a final confirmation of the origin of the oxygen signal, we investigate ordered arrays made with magnetic nanocrystals annealed at 350 °C but not exposed to air. Figure 4F shows the global spectrum from these particles, an HAADF image of which is shown in Figure 4E. No oxygen signal can be detected, clearly indicating that the oxygen signal seen in Figure 4B,C does not originate from the dodecanoic acid molecules.
Conclusion
Figure 5. (A) EELS spectrum of Figure 4B showing a detail of the O K-edge. (B) Reference oxides CoO (top) and Co3O4 (bottom) as measured by XAS.15
maximize the available signal. However, the resolution is easily sufficient to give sharp L2,3 peaks for cobalt (Figure 4), so any
We have shown that by annealing Co nanoparticles at 350 °C and subsequently exposing them to air for a few hours we can form core/shell Cohcp/Co nanocrystals in 2D organizations. Detailed characterization shows that the core is monocrystalline hcp Co and the shell is predominantly polycrystalline cubic CoO. We can expect an increase in the exchange anisotropy compared to that of similar systems reported by other groups which consist of Cofcc/CoO nanocrystals. LA703215G
