9486
Langmuir 2003, 19, 9486-9489
Synthesis of Well-Defined and Low Size Distribution Cobalt Nanocrystals: The Limited Influence of Reverse Micelles I. Lisiecki and M. P. Pileni* Laboratoire LM2N, UMR CNRS 7070, Universite´ P. et M. Curie Baˆ t F, 4 Place Jussieu, 75005 Paris, France Received April 2, 2003. In Final Form: July 17, 2003 Cobalt nanocrystals are produced in cobalt(II) bis(2-ethylhexyl)sulfosuccinate (Co(AOT)2) reverse micelles. It is found that their size distribution is related to the volume of reducing agent added to the micellar system. At a low volume of reducing agent, the micelles play the role of nanoreactors in which take place the nucleation and growth of cobalt. In the supersaturate regime, that is, at a high volume of reducing agent, micelles are destroyed. In the first case, the nanocrystal size polydispersity is about 29%, while in the latter case, it can decrease to 8%. This low size polydispersity has to be related on one hand to the excess of the reducing agent that modifies the nucleation and growth processes and on the other hand to the structural and chemical evolutions of the micellar system. To obtain well-defined two-dimensional superlattices, the size distribution needs to be less than 13%.
I. Introduction During the past decade, due to the emergence of a new generation of high-technology materials, the number of groups involved in nanomaterials has increased exponentially.1,2 Nanomaterials are implicated in several domains such as chemistry, electronics, high-density magnetic recording media, sensors, and biotechnology. This is, in part, due to their novel material properties, which differ from those of both the isolated atoms and the bulk phase. An ultimate challenge in materials research is now the creation of perfect nanometer-scale crystallites that are identically replicated in unlimited quantities in a state that can be manipulated and that behave as pure macromolecular substances. Thus the ability to systematically manipulate nanomaterials is an important goal in modern materials chemistry. As the electrical, optical, and magnetic properties of inorganic nanomaterials vary widely with their sizes and shapes, optimizing this ability requires an understanding of nanocrystal growth, which turns out to be a complex process. One of the challenges, which has not so far been met, is to order magnetic nanocrystals in 2D and 3D superlattices on a macroscopic scale. These superlattices provide a new horizon in fundamental physics and are used as model systems for considering phenomena related to dipolar interactions within the solid. In the past three years, several groups have concentrated their efforts on producing self-organized, magnetic nanocrystals in 2D compact hexagonal networks. When magnetic nanocrystals are deposited on a surface, collective properties due to long-range dipolar interactions are observed.3-13 * Corresponding author. (1) Gleiter, H. Nanostructured Materials; 1992. (2) Acc. Chem. Res. 1999, 32 (5), special issue on Nanoscale Materials. (3) Petit, C.; Taleb, A.; Pileni, M. P. J. Phys. Chem. B 1999, 103, 1805. (4) Petit, C.; Taleb, A.; Pileni, M. P. Adv. Mater. 1998, 10, 259. (5) Murray, C. B.; Sun, S.; Gaschler, W.; Betley, T. A.; Kagan, C. R. IBM J. Res. Dev. 2001, 45, 47. (6) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989. (7) Russier, V.; Petit, C.; Legrand, J.; Pileni, M. P. Phys. Rev. B 2000, 62, 3910. (8) Ngo, T.; Pileni, M. P. Adv. Mater. 2000, 12, 276.
It has been well demonstrated that reverse micelles are good candidates for use as nanoreactors.14,15 However, some discrepancies were recently pointed out.16 Particularly, when functionalized reverse micelles are mixed with a large amount of reducing agent, this induces the destruction of the nanoreactors and then drastic changes in the nanocrystal size and shape are observed. Hence, silver nanodisks are produced from Ag(AOT) (silver(I) di(ethylhexyl)sulfosuccinate) in the presence of a large amount of hydrazine17 with control of the particle size.18 The nanodisk size is tuned by the relative amount of reducing agent involved in the synthesis, whereas their aspect ratios remain the same order of magnitude. As demonstrated below, to reach the supersaturation regime, reverse micelles have to be destroyed and molecules themselves play a major role in the shape control as already discussed in various papers (see the review in ref 16). We demonstrate here that addition of a high concentration of reducing agent to a micellar solution favors formation of rather large and well-defined cobalt nanocrystals. The low size distribution obtained induces their organization on a substrate. II. Experimental Section Materials. All materials were used without further purification. Cobalt acetate, Co(CH3CO2)2, lauric acid, and sodium borohydride, NaBH4, were from Aldrich, isooctane and hexane were from Fluka, and Na(AOT), sodium di(ethylhexyl) sulfo(9) Black, C. T.; Murray, C. B.; Sandstrom, R. L.; Sun, S. Science 2000, 290, 1131. (10) Russier, V. J. Appl. Phys. 2001, 89, 1287. (11) Ngo, T.; Pileni, M. P. J. Phys. Chem. B 2001, 105, 53. (12) Legrand, J.; Petit, C.; Pileni, M. P. J. Phys. Chem. B 2001, 105, 5643. (13) Ngo, T.; Pileni, M. P. Appl. Phys. 2002, 92, 46. (14) Pileni, M. P. J. Phys. Chem. 1993, 97, 1. (15) Pileni, M. P. Langmuir 1997, 13, 3266. (16) Pileni, M. P. Nat. Mater. 2003, 2, 145. (17) Maillard, M.; Georgio, S.; Pileni, M. P. Adv. Mater. 2002, 14, 1084. (18) Maillard, M.; Georgio, S.; Pileni, M. P. J. Phys. Chem. B 2003, 107, 2466.
10.1021/la0301386 CCC: $25.00 © 2003 American Chemical Society Published on Web 09/25/2003
Synthesis of Co Nanocrystals in Reverse Micelles
Langmuir, Vol. 19, No. 22, 2003 9487
succinate, was from Sigma. The synthesis of cobalt(II) bis(2ethylhexyl)sulfosuccinate (Co(AOT)2) was described previously.19 Instrumentation. Transmission Electron Microscopy (TEM). A JEOL (200 kV) model JEM 200CX was used to obtain micrographs of the cobalt nanocrystals on amorphous carbon coated TEM grids. The magnetic measurements were made with a commercial SQUID magnetometer (Cryogenic S600).
III. Results and Discussion Reverse micelles of 5 × 10-2 M Co(AOT)2 form an isotropic phase. The amount of water added in solution is fixed to reach a water concentration defined as w ) [H2O]/[AOT] ) 32. Sodium borohydride, NaBH4, added to the micellar solution reduces the cobalt ions. The sodium borohydride content changes by varying the volume of a fixed concentration solution ([NaBH4] ) 1 M) added to the micellar solution. R, the NaBH4 content, is defined as R ) [NaBH4]/[Co(AOT)2]. Immediately after NaBH4 addition, the micellar solution color turns from pink to black, indicating the formation of colloidal cobalt particles. Related to the R value, two behaviors can be distinguished: (i) The addition of a low volume of NaBH4 (R < 1) maintains the stability of the reverse micelles which play the role of nanoreactors in which the nucleation and growth of cobalt nanocrystals take place. (ii) In the supersaturate regime, that is, R g 1, micelles are destroyed because of the limiting water concentration, w. In such a case, two nanocrystal populations are produced. Whatever the R values are, the nanocrystals are coated and then extracted from reverse micelles or from the surfactant. The coating process is as follows: In a solution containing nanocrystals, surfactants, water, and isooctane, 0.2 M lauric acid, C12H25COOH, is added inducing a “quasi” covalent attachment with cobalt atoms located at the interface. The coated cobalt crystals are then washed and centrifuged several times with ethanol to remove all the AOT surfactant, and the obtained black powder is dispersed in hexane. To eliminate the larger size crystals formed when synthesis takes place in a supersaturate regime, the solution is centrifuged and only the upper phase containing the smaller sizes is collected. Ten 10-µL drops of a solution with a particle concentration of 3 × 10-7 M are deposited on a carbon grid. The data drastically change with the sodium borohydride content, R. At a low R value (R ) 0.5), the TEM image shows nanocrystals with a local ordering. Over long distances, the nanocrystals are randomly deposited on the substrate (Figure 1A). The average size is 6 nm with a size distribution of the nanocrystals which is rather large (Figure 2A). This is in good agreement with previously published data,12,20 from which it has been clearly demonstrated that cobalt metal particles are produced. At R ) 1, the TEM image shows almost similar behavior as that observed at R ) 0.5 (Figures 1B and 2B) with an increase in the average nanocrystal size to 7 nm (Table 1) while there is decrease in the size distribution. At larger R values (R ) 2, 4, 6), the average nanocrystal diameter remains the same as for R ) 1, that is, 7 nm, while the size distribution decreases and then the nanocrystals self-organize in a hexagonal network (Figures 1C-E and 2C-E). At R ) 8, the average nanocrystal diameter increases again to reach 8 nm while the size distribution further decreases (Figures 1F and 2F and Table 1). The assemblies shown in Figure 1C-F clearly show that to obtain organizations of nano(19) Petit, C.; Lixon, P.; Pileni, M. P. J. Phys. Chem. 1990, 94, 1598. (20) Legrand, J.; Petit, C.; Bazin, D.; Pileni, M. P. Appl. Surf. Sci. 2000, 164, 193.
Figure 1. TEM images of cobalt nanocrystals made at various sodium tetrahydroboride concentrations, R: (A) R ) 0.5, (B) R ) 1, (C) R ) 2, (D) R ) 4, (E) R ) 6, and (F) R ) 8. The inserts show TEM images obtained at a lower magnification.
crystals in compact hexagonal networks, the size distribution must not exceed about 13%. Below this value, no change in the self-organization is observed. After addition of a large amount of reducing agent, the reverse micelles are destroyed. The reduction of Co2+ (from Co(AOT)2) with NaBH4 results in the formation of atomic cobalt and various species such as Na+, BO2-, H+, OH-, H2, and AOT-. Surfactant ions can either recombine with Na+ to form Na(AOT) or can also hydrolyze; the latter process is amplified by the presence of certain ions in the solution. Nevertheless, it is not possible to obtain information on the transient behavior of the system because the cobalt ion is a part of the surfactant and the chemical reaction takes place immediately after adding the reactant. It is likely that the presence of surfactant in its hydrolyzed state and anions such as BO2- play a role during the growth of magnetic nanocrystals through specific adsorption processes on crystallographic faces. This is supported by the fact that at high reducing agent concentration, R, cobalt nanocrystals appear to be very well facetted. Other convincing experiments on the influence of molecules in the control of nanocrystal shape are the production of ZnTe21 and CdTe22 nanowires and PbS nanorods.23 In addition, Glave et al.24 showed that the reduction of cobalt ions in an aqueous phase is not complete if the R value is lower than 2. Hence, in the present case, the (21) Li, Y.; Ding, Y.; Wang, Z. Adv. Mater. 1999, 11, 847. (22) Li, Y.; Liao, H.; Ding, Y.; Fan, Y.; Zhang, Y.; Qian, Y. Inorg. Chem. 1999, 38, 1382. (23) Wang, S.; Yang, S. Langmuir 2000, 16, 389. (24) Glave, G. N.; Klabunde, K. J.; Sorensen, C. M.; Hadjipanayis, G. C. Langmuir 1993, 9, 162.
9488
Langmuir, Vol. 19, No. 22, 2003
Lisiecki and Pileni
Figure 3. High-resolution TEM images of cobalt nanocrystals at R ) 6. Table 2. Magnetic Characterizations Recorded at 3 K of Cobalt Nanocrystals Dispersed in Hexane at Various R Values. R Ms (emu/g) Mr/Ms Hc(T)
Figure 2. Size distribution of cobalt nanocrystals made at various sodium tetrahydroboride concentrations, R: (A) R ) 0.5, (B) R ) 1, (C) R ) 2, (D) R ) 4, (E) R ) 6, and (F) R ) 8. (The average number of particles counted is 500.) Table 1. Average Diameter of Nanocrystals, D, and Size Distribution, σ, at Various R Values R D (nm) σ (%)
0.5
1
2
4
6
8
6 30
7 18
7 13
7 12
7 12
8 8
smaller magnetic particles obtained at low R values (which can be considered as seeds) grow with increasing R values, that is, with increasing the yield of the reduction reaction. On the other hand, as concerns the size selection, which occurred at the end of the particle preparation, we collect only the smaller nanocrystals. Therefore, the increase in the amount of NaBH4 favors a decrease in the particle size distribution. In our case, that is, in the presence of surfactant, the yield of the reaction probably continues to change up to R ) 8, which can explain the change in size and then the change in size distribution up to this R value. The electron microscope high-resolution image (Figure 3) clearly shows that nanocrystals are organized in a very regular hexagonal network. Furthermore, from the lattice fringes of these nanocrystals, formation of twin particles is observed. The nanocrystal structure is found to be the face-centered-cubic structure.25 One question arises concerning these nanocrystals: Do we still produce cobalt nanocrystals? In our previous papers, we demonstrated by the magnetic study and by EXAFS (extended X-ray absorption fine structure) the formation of cobalt metal nanocrystals when the particles (25) Lisiecki, I.; Ding, Y.; Wang, Z. L.; Pileni, M. P. Private communication.
0.5
1
2
6
68 0.38 0.20
70 0.37 0.22
72 0.37 0.28
72 0.40 0.10
are synthesized at R ) 0.512,20 and at R ) 1.3 Nevertheless, we could argue that the increase of the reducing agent concentration leads to the formation of Co2B. To reject the boron derivative formation, the magnetic properties of nanocrystals dispersed in hexane are recorded at 3 K with a superconducting quantum interference device (SQUID). Table 2 gives the major parameters for magnetic nanocrystals. The saturation magnetization, reduced remanence, and coercive field remain almost unchanged at various R values. The slight differences given in Table 2 are in the experimental error range. Except for some cases,26 we know that the saturation magnetization of nanocrystals is often smaller than that of the bulk phase,27 and here its value is rather low compared to that of the cobalt bulk phase (166 emu/g).28 However, it is higher than that corresponding to the Co2B bulk phase (60 emu/ g)29 so that formation of Co2B nanocrystals can be excluded. In this case, the saturation magnetization would markedly drop compared to that shown in Table 2. In the present data, the fact that no change in the magnetic properties is observed above R ) 1 compared to R ) 0.5 and R ) 1 clearly shows that whatever the reducing agent concentration, cobalt metal nanocrystals are formed. On the other hand, chemical reduction of iron salt by NaBH4 in aqueous solutions has been studied by Linderoth et al.30 They demonstrate by Mo¨ssbauer spectroscopy that for the highest borohydride molarity, primarily R-Fe is produced (26) Respaud, M; Broto, J. M.; Rakoto, H.; Fert, A. R.; Thomas, L.; Barbara, B.; Vereslst, M.; Snoeck, E.; Lecante, P.; Mosset, A.; Osuna, J.; Ould Ely, T.; Amiens, C.; Chaudret, B. Phys. Rev. B 1998, 57, 2925. (27) Pileni, M. P. Adv. Funct. Mater. 2001, 11, 323. (28) Nishikawa, M.; Kita, E.; Erata, T.; Tasaki, A. J. Magn. Magn. Mater. 1993, 126, 303. (29) Yiping, Y.; Hadjipanayis, G. C.; Sorensen, C. M.; Klabunde, K. J. J. Magn. Magn. Mater. 1989, 79, 321. (30) Linderoth, S.; Morup, S. J. Appl. Phys. 1991, 69, 5256.
Synthesis of Co Nanocrystals in Reverse Micelles
while with decreasing borohydride molarity, the amount of the crystalline component decreases to give rise finally to an amorphous Fe-B alloy. To a first approximation, it is reasonable to conclude that cobalt nanocrystals are formed. However, the presence of boron inclusions cannot be excluded. X-ray photoelectron spectroscopy (XPS) experiments were performed to evaluate the amount of boron in the nanocrystals, but the coating and presence of surfactant prevent any detection. Nevertheless, the various magnetic parameters do not depend on R, indicating no changes in the nanocrystal composition. Formation of Co2B would induce an amorphous material that would exhibit melting under the high-energy electron beam used for obtaining TEM images.26 This is not the case, and high-resolution electron microscopy patterns are obtained as shown in Figure 3. In addition, the fact that particles appear well facetted at high R values is in favor of a nonamorphous material.
Langmuir, Vol. 19, No. 22, 2003 9489
IV. Conclusion In this paper, we demonstrate that the amount of reducing agent is one of the key parameters in controlling the size distribution of nanocrystals. Furthermore, a slight difference in average particle size is observed. Because it has been possible to make the same nanocrystals having various size distributions, we demonstrate that for selforganization of cobalt nanocrystals the size distribution has to be less than 13%. A lower size distribution does not change the efficiency of the formation of 2D superlattices organized in hexagonal networks. Acknowledgment. We are grateful to Dr. Eric Vincent, Dr. G. Lebras, and L. Lepape of DSM-DRECAMSPEC for provision of the SQUID equipment. We thank Dr. A. Loiseau of ONERA (Chaˆtillon) for providing us with facilities for using the transmission electron microscope. LA0301386
