Langmuir 2007, 23, 8575-8584
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First Synthesis by Liquid-Liquid Phase Transfer of Magnetic CoxPt100-x Nanoalloys A. Demortie`re and C. Petit* Laboratoire des Mate´ riaux Me´ soscopiques et Nanome´ triques, UMR CNRS 7070, UniVersite´ Pierre et Marie Curie, 4 place Jussieu, 75252 Paris Cedex 05, France ReceiVed March 12, 2007. In Final Form: May 5, 2007 Controlled synthesis of magnetic nanoparticles with a well-known size and composition is always a challenge. A soft chemical synthesis was developed to obtain magnetic alloy nanocrystals with a high ability to control composition, size, and polydispersity. Cobalt-platinum alloy nanocrystals were synthesized using a colloidal approach by the liquid-liquid phase transfer method. Structural characterization using HRTEM and XRD was carried out on nanocrystals in the range of 25-75% cobalt composition, which indicated the formation of nanoalloy bimetallic CoxPt100-x. Adjusting the alkylamine capping agent and the kinetics of the reduction process allowed tuning of the size in the range of 1.8-4 nm while keeping an equiatomic composition. The narrow size distribution led to the possibility of inducing nanoparticle self-organization over a long range. The magnetic properties of the Co50Pt50 nanoalloy in the disordered face-centered cubic phase A1 were studied for different nanoparticle sizes.
1. Introduction The emergence of new methods to design nanocrystals has attracted increasing attention in the domain of magnetic materials. Magnetic nanocrystals are of particular interest due to their potential as ultra-high-density recording media1,2 and in ferrofluid technology3 as well as in the biomedical field as magnetic sensors.4 However, several problems remain to be solved before these applications become possible. In fact, devices based on magnetic properties are limited by thermal fluctuation of the magnetization. At room temperature, superparamagnetic behavior5 is one of the most important effects due to the reduction of the nanocrystal size. Moreover, the magnetic dipolar interaction between nanocrystals is an important factor of limitation. A detailed understanding of the magnetic properties of assemblies6,7 of nanocrystals with control of the interparticle distance is therefore essential to the development of magnetic applications. Alloy nanostructures are emerging as the most promising solutions. Indeed, there is an enhancement in specific properties upon alloying, due to a rich diversity of compositions, structures, and properties of the metallic alloys. Thus, alloys such as CoPt (or FePt) have an ordered crystalline phase (L10) around the equiatomic composition,8,9 which is intrinsic to the tetragonal symmetry (fct) of the crystal structure. The ordered L10 phase of the CoPt system is of particular interest because of the high coercivity (10 kOe) and the high magnetocrystalline anisotropy (4.9 × 107 ergs/cm3). In fact, CoPt nanocrystals in the L10 phase have large uniaxial magnetic anisotropy energy and so have the potential to exceed the superparamagnetic limit.10,11 Moreover, (1) Grundy, P. J. J. Phys. D: Appl. Phys. 1998, 31, 2975. (2) Weller, D.; Moser, A. IEEE Trans. Magn. 1999, 35 (6), 4423. (3) Rosensweig, R. E. Chem. Eng. Prog. 1989, 85, 53. (4) Moodera, J. S.; Kinder, L. R.; Nowak, J. J. Appl. Phys. 1997, 81, 5522. (5) Dormann, J. L.; Fiorani, D.; Tronc, E. AdV. Chem. Phys. 1997, 98, 283. (6) Russier, V.; Petit, C.; Legrand, J.; Pileni, M. P. Phys. ReV. B 2000, 62, 3910. (7) Klemmer, T. J.; Liu, C.; Shukla, N.; Wu, X. W.; Weller, D.; Tanase, M.; Laughlin, D. E.; Soffa, W. A. J. Magn. Magn. Mater. 2003, 266, 79. (8) Sun, X.; Jia, Z. Y.; Huang, Y. H.; Harrell, J. W.; Nikles, D. E.; Sun, K.; Wang, L. M. J. Appl. Phys. 2004, 95, 6747. (9) Klemmer, T. J.; Shukla, N.; Liu, C.; Wu, X. W.; Svedberg, E. B.; Mryasov, O.; Chantrell, R. W.; Weller, D. Appl. Phys. Lett. 2002, 81, 2220. (10) Carpenter, E. E.; Sims, J. A.; Wienmann, J. A.; Zhou, W. L.; O’Connor, C. J. J. Appl. Phys. 2000, 87, 5615.
these CoPt magnetic nanocrystals are monodomain magnetic particles for a typical size below 10 nm.12 Beyond this typical size the nanocrystals become polydomain magnetic, and this leads to domain wall formation.13 Therefore, high control of the chemical composition and size of the alloy nanocrystals is essential for optimizing the magnetic nanoscale behavior. In recent years, to synthesize nanocrystals, different methods have been developed in the colloidal approaches.14 In these soft chemistry processes, two main routes have been used for noble metals such as platinum, silver, and gold: the in situ synthesis in the water pools of reverse micelles developed in the 1980s by Pileni et al.15,16 and the phase transfer method developed by Brust et al. in the 1990s.17-19 In the first method, the reverse micelles can be considered as a nanoreactor and the nanoparticle size is often approximately limited by the droplet size. This method has been used for the synthesis of semiconductor materials such as CdS,20,16 of metallic nanoparticles such as Pt,21,22 Cu,23,24 Co,25 and Ag,26 and of nanoalloys such as CoPt,27,28 PtPd,29 and (11) Huang, Y.; Zhang, Y.; Hadjipanayis, G. C.; Simopoulos, A.; Weller, D. IEEE Trans. Magn. 2002, 38, 2604. (12) Spratt, G. W. D.; Bissell, P. R.; Chantrell, R. W.; Wohlfarth, E. P. J. Magn. Magn. Mater. 1998, 75, 309. (13) Zeng, H.; Sun, S.; Vedantam, T. S.; Liu, P.; Dai, Z.; Wang, Z. L. Appl. Phys. Lett. 2002 80, 2583. (14) Pileni, M. P. Ber. Bunsen-Ges. Phys. Chem. 1997, 101, 1578-1587. (15) Pileni, M. P. J. Phys. Chem. 1993, 97, 6961. (16) Petit, C.; Pileni, M. P. J. Phys. Chem. 1988, 92, 2282. (17) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (18) Brust, M.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. AdV. Mater. 1995, 7, 795. (19) Brust, M.; Bethell, D.; Kiely, C. J.; Schiffrin, D. J. Langmuir 1998, 14, 5425. (20) Petit, C.; Lixon, P.; Pileni, M. P. J. Phys. Chem. 1990, 94, 1598. (21) Ha¨relind Ingelsten, H.; Bagwe, R.; Palmqvist, A.; Skoglundh, M.; Svanberg, C.; Holmberg, K.; Shah, D. O. J. Colloid Interface Sci. 2001, 241, 104. (22) Ha¨relind Ingelsten, H.; Beziat, J.-C.; Bergkvist, K.; Palmqvist, A.; Skoglundh, M.; Qiuhong, H.; Falk, L. K. L.; Holmberg, K. Langmuir 2002, 18, 1811. (23) Tanori, J.; Gulik-Krzywicki, T.; Pileni, M. P. Langmuir 1997, 13, 632. (24) Lisiecki, I. J. Phys. Chem. B 2005, 109, 12231. (25) Petit, C.; Taleb, A.; Pileni, M. P. AdV. Mater. 1998, 10, 259. (26) Petit, C.; Lixon, P.; Pileni, M. P. J. Phys. Chem. 1993, 97, 12974. (27) Petit, C.; Rusponi, S.; Brune, H. J. Appl. Phys. 2004, 95, 4251. (28) Carpenter, E. E.; Seip, C. T.; O’Connor, C. J. J. Appl. Phys. 1999, 85, 5184. (29) Yashima, M.; Falk, L. K. L.; Palmqvist, A. E. C.; Holmberg, K. J. Colloid Interface Sci. 2003, 268, 348.
10.1021/la700719h CCC: $37.00 © 2007 American Chemical Society Published on Web 06/29/2007
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FeCu.30 However, in the case of nanoalloys the ability to control the size distribution and shape was not precise enough, and consequently, no self-organization was obtained. The second method, in a two-phase approach, involves the transfer of metallic ions from an aqueous phase to an organic phase. This route has been used for the synthesis of metallic nanoparticles such as Au,17-19,31,32 Ag,33,34 Pt,35,36 or Pd37 but never for nanoalloys. Of course, there are also other physical or chemical methods to synthesize nanoalloys; for example, chemical routes based on organometallic decomposition38,39 have also been used, but such synthesis procedures are more complex than those of the soft chemistry routes presented above. In fact, the difficulty remains in the preparation of samples with a uniform composition around the ratio 50:50 using chemical synthesis. A deviation as small as 5% of this composition can result in a large fraction of soft magnetic CoPt nanoparticles after annealing of the disordered phase of the as-prepared nanocrystals.8 However, if monodisperse 4 nm diameter Co50Pt50 nanocrystals in the L10 phase can be obtained, the superparamagnetic limitation could be overcome. In addition, a narrow size distribution leads to the possibility of forming ordered arrays of CoPt nanoparticles, and consequently, control over the particle size and dispersity is essential. In this paper, we report the first utilization of the liquidliquid phase transfer method to synthesize alloy nanocrystals. The method was adapted to allow a fine control of the composition of the nanoalloy and, by tuning of the reaction parameters, control of the average size. The as-synthesized CoPt nanocrystals have a good crystallinity and are homogeneous in composition. A structural study is carried out to characterize the nanoalloy. Magnetic properties of the CoPt nanocrystals in the disordered phase face-centered cubic (fcc) (A1) are also reported. 2. Experimental Section 2.1. Chemicals. Platinum(IV) chloride (PtCl4) (Aldrich, 99,9%), cobalt(II) chloride hexahydrate (CoCl2‚6H2O) (VWR), cobalt acetate tetrahydrate (Co[CH2COO]2‚4H2O), tetrakis(decyl)ammonium bromide (TDAB), sodium bis(2-ethylhexyl) sulfosuccinate (AOT) (Fluka, 99%), and sodium borohydride (NaBH4) (Aldrich, 99%) were used as purchased without further purification. The alkylamines used in the study were octadecylamine (C18H37NH2), hexadecylamine (C16H33NH2) and octylamine (C8H17NH2) (Fluka, 99%), dodecylamine (C12H25NH2) and decylamine (C10H21NH2) (Fluka, 99.5%), nonylamine (C9H19NH2) (Aldrich, 98%), and heptylamine (C7H15NH2) (Aldrich, 99%). Toluene, ethanol, and hydrochloric acid were purchased from a variety of sources and used without purification. Water was purified with a Millipore water system (18.2 MΩ). After synthesis the nanocrystals were stored under nitrogen in a glovebox. 2.2. Transmission Electron Microscopy Measurements. Transmission electron microscopy (TEM) images were obtained using a JEOL 1011 operated at 100 kV with magnifications of up to 500000×. TEM samples were prepared by the drop deposition method on an (30) Duxin, N.; Brun, N.; Colliex, C.; Pileni, M. P. Langmuir 1998, 14, 1984. (31) Shon, Y.-S.; Mazzitelli, C.; Murray, R. W. Langmuir 2001, 17, 7735. (32) Saunders, A. E.; Sigman, M. B., Jr.; Korgel, B. A. J. Phys. Chem. B 2004, 108, 193. (33) Heath, J. R.; Knobler, C. M.; Leff, D. V. J. Phys. Chem. B 1997, 101, 189. (34) Korgel, B. A.; Fullam, S.; Connolly, S.; Fitzmaurice, D. J. Phys. Chem. B 1998, 102, 8379. (35) Wikander, K.; Petit, C.; Holmberg, K.; Pileni, M. P. Langmuir 2006, 22, 4863. (36) Horswell, S. L.; Kiely, C. J.; O’Neil, I. A.; Schiffrin, D. J. J. Am. Chem. Soc. 1999, 121, 5573. (37) Chen, S.; Huang, K.; Stearns, J. A. Chem. Mater. 2000, 12, 540. (38) Ramirez, E.; Jansat, S.; Philippot, K.; Lecante, P.; Gomez, M.; MasdeuBulto´, A. M.; Chaudret, B. J. Organomet. Chem. 2004, 4601. (39) Zitoun, D.; Amiens, C.; Chaudret, B.; Fromen, M. C.; Lecante, P.; Casanove, M. J.; Respaud, M. J. Phys. Chem. B 2003 107, 6997.
Demortie` re and Petit
Figure 1. EDX analysis of CoPt nanocrystals on a silicon wafer showing the characteristic peaks of KR, Kβ cobalt and MR, LR platinum. Inset: SEM image of a thin film of nanocrystals with a thickness of about 10 µm. amorphous carbon coated TEM grid. To determine the average diameter, 〈D〉, and the size distribution, σ, of CoPt nanocrystals, around 400 nanocrystals were measured for each sample and presented in a histogram. A log-normal distribution has been used to describe the size distribution: f(D) )
(
ln(D/〈D〉) 1 exp 1/2 (2π) σD 2σ2
)
2
(1)
The polydispersity index was defined as the ratio σ/〈D〉. For highresolution transmission electron microscopy (HRTEM) measurements of the crystallinity of the as-prepared CoPt nanoparticles, a JEOL 2010 UHR instrument operated at 200 kV (LaB6) was used. 2.3. Scanning Electron Microscopy. To determine the average composition of the CoPt nanocrystals with a good accuracy, the EDX analyzer of an SEM JEOL 5510LV was used. A thin film of nanocrystals was made by slow evaporation of a concentrated solution of nanocrystals at room temperature. The resulting film (inset of Figure 1) was thicker than 10 µm and presented characteristic cracks due to the drying process.40 Figure 1 shows a typical energy-dipersive X-ray analysis (EDX) obtained for Co50Pt50 nanoparticles. 2.4. X-ray Diffraction Measurements. The crystalline structure of the as-synthesized CoPt nanoparticles was investigated by X-ray diffraction (XRD) measurements in transmittance mode using a Siemens Kristalloflex diffractometer with a STOE goniometer (Co KR, λ ) 1.7902 Å) setup. Samples were prepared by repetitive dropwise addition of a concentrated nanoparticle solution onto an X-ray transparent tape until a sufficient sample film had been established. The intensity of the diffracted beams was detected in the interval 30-120° (2θ) with a step size of 0.05° and an integration time of 10 s. The X-ray diffractograms were fitted by a least-squares fit assuming a Gaussian peak shape. In this form, the Scherrer equation has been used to calculate the diameter of the nearly spherical CoPt nanocrystals: (40) Pileni, M. P.; Lalatonne, Y.; Ingert, D.; Lisiecki, I.; Courty, A. Faraday Discuss. 2004, 125, 251. (41) Cheng, Y.; Schiffrin, D. J. J Chem. Soc., Faraday Trans. 1996, 92, 3865. (42) Birdi, K. S. Handbook of surface and colloid chemistry; CRC Press: New York, 1994. (43) Pileni, M. P. Nat. Mater. 2003, 2, 145. (44) Petit, C.; Lixon, P.; Pileni, M. P. Langmuir 1991, 7, 2620. (45) Darling, A. S. Platinum Met. ReV. 1963, 7, 96. (46) Shevchenko, E. V.; Talapin, D. V.; Rogach, A. L.; Kornowski, A.; Haase, M.; Weller, H. J. Am. Chem. Soc. 2002, 124, 11480. (47) Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J.; Gou, L.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857. (48) Prasad, B. L. V.; Stoeva, S. I.; Sorensen, C. M.; Klabunde, K. J. Langmuir 2002, 18, 7515. (49) Shimizu, T.; Teranishi, T.; Hasegawa, S.; Mayake, M. J. Phys. Chem. B 2003, 107, 2719. (50) Batlle, X.; Labarta, A. J. Phys. D 2002, 35, R15. (51) LaMer, V. K.; Dinegar, R. H. J. Am. Chem. Soc. 1950, 72, 4487. (52) Pich, J.; Friedlander, S. K.; Lai, F. S. J. Aerosol Sci. 1970, 1, 115. (53) Motte, L.; Billoudet F.; Pileni, M. P. J. Phys. Chem. 1995, 99, 16425. (54) Tronc, E. NuoVo Cimento 1996, 18D, 163. (55) Park, J. I.; Jeon, J. J. Am. Chem. Soc. 2001, 123, 5743.
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Figure 2. Size histogram (A-D) and TEM images (E-H) of CoPt nanocrystals obtained by methods (A, E) 1A, (B, F) 1B, (C, G) 2A, and (D, H) 2B.
D)R
Kλ w cos θ
(2)
where D is the particle diameter, λ is the wavelength of the radiation, θ is the angle of the considered Bragg reflection, w is the width on a 2θ scale, and K is a constant with a value of 0.9. The broadening of the Bragg reflections is determined by the number of unit cells along columns perpendicular to the diffraction planes. For spherical particles, the length of such columns of unit cells varies within a
given particle. Consequently, to take into account this aspect, we have taken an effective diameter Deff ) D/R with R ) 1.33.56 Assuming an fcc structure for CoPt nanocrystals, the lattice parameter alattice for different lattice planes is given by dhkl )
alattice (h + k2 + l2)1/2 2
(3)
2.5. Magnetic Properties. The magnetic measurements were made with a commercial SQUID magnetometer (Cryogenic S600) at the
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Table 1. Average Diameter and Composition of the CoPt Nanocrystals Coated by Hexadecylamine Obtained for the Different Methods Presented method
platinum salt
cobalt salt
av diam (nm)
polydispersity (%)
Co composition (%)
std dev of the composition
A1 A2 B1 B2
PtCl6(TDA)2 PtCl6(TDA)2 PtCl6(TDA)2 PtCl6(TDA)2
cobalt acetate CoCl2 Co(AOT)2 CoCl4(TDA)2
1.8 2.0 2.1 2.0
17 13 15 12
35 38 45 50
15 11 7 2
SPEC (CEA-Saclay, France). A magnetic field of 75 G was applied during the ZFC/FC measurements. Hysteresis loops were recorded at 3 K for a film of CoPt nanocrystals deposited on silicon substrates with the magnetic field applied parallel to the film.
3. Nanocrystal Synthesis Cobalt-platinum nanocrystals were synthesized using a modification of a two-phase liquid-liquid system, which was developed by Brust and Schiffrin.17-19 Two methods of synthesis were developed by adjusting the initial cobalt salt either in the aqueous phase (method 1) or in the organic phase (method 2). The quaternary ammonium TDAB was used as a phase transfer agent to transfer the metallic salts into the toluene. 3.1. Method 1. A 21.05 mg sample of PtCl4 was dissolved in 5 mL of water and 5 mL of hydrochloric acid to produce the platinum complex PtCl62-. This aqueous solution was mixed with 40 mL of toluene containing 0.25 g of TDAB. To ensure maximum transfer of platinum ions from the water phase to the organic phase, the transferring agent (TDAB) was present in large excess. The twophase mixture was vigorously stirred until all the PtCl62- was transferred into the organic phase (30 min), which became yellow while the aqueous phase became colorless. Then, the aqueous phase was separated and discarded, and 15 mL of an aqueous solution of cobalt salt was added. We have used two different salts of cobalt: Sample 1A was made with 15.56 mg of cobalt acetate tetrahydrate. Sample 1B was made with 14.86 mg of cobalt(II) chloride hexahydrate. With an aim of obtaining Co50Pt50, the platinum precursor in toluene and the cobalt precursor in water were in equimolar concentration. A 1 mL sample of hexadecylamine (C16H33NH2) was then added to the organic phase. A freshly prepared aqueous solution of sodium borohydride (378 mg, 10 mL of H2O) was slowly added with vigorous stirring. Both the reducing agent and the stabilizing agent were added in large stoichiometric excess relative to the salts. The mixture changed relatively quickly from yellow to dark brown, indicating that the reduction had taken place. After further stirring for 16 h, the organic phase was separated and the water phase containing the byproduct of the reaction was discarded. 3.2. Method 2. In this case, both salts of platinum and cobalt were transferred in the organic phase. A 168.45 mg sample of PtCl4 was dissolved in 5 mL of water and 10 mL of HCl to obtain PtCl62-. An 80 mL volume of toluene containing 2 g of TDAB was mixed with this platinum complex solution. This mixing was performed in four steps (4 × 20 mL) to optimize the platinum salt extraction. After the phase transfer, the aqueous phase was separated and discarded. For sample 2A, a functionalized surfactant, Co(AOT)2 (6.25 × 10-3 mol‚L-1, 80 mL of toluene), was used as the cobalt salt precursor. This surfactant is highly soluble in organic phases such as toluene. Co(AOT)2 was prepared as described in ref 20. For sample 2B, as for the platinum salt, 118.9 mg of CoCl2 was added to 5 mL of water and 10 mL of HCl to produce the cobalt complex CoCl42- with a typical blue color (6.25 × 10-3 mol‚L-1). A 2 g sample of TDAB was used to induce the phase transfer, in the same split process. Then the aqueous phase was discarded. For the reaction, as previously, the total volume of the organic phase was maintained at 40 mL. Thus, 10 mL of platinum organic phase and 10 mL of cobalt organic phase (2A, Co(AOT)2; 2B, CoCl4(TDA)2) were added to 20 mL of toluene. Hence, the volume of the solution and the concentration were similar to those of method 1. (56) Bodker, F.; Morup, S.; Linderoth, S. Phys. ReV. Lett. 1994, 72, 282.
Table 2. Average Diameter Measured and Calculated from XRD Patterns, Blocking Temperature, and Anisotropy Constant of the CoPt Nanocrystals Obtained for Method 2B injection time (s) av diam 〈D〉 (nm) Debye-Scherrer 〈D〉 (nm) polydispersity (%) blocking temp (K) Ka (105 ergs/cm3)
0 2.0 1.94 17 3.3 30.7
5 2.5 2.66 15 4.4 20.9
10 3.4 3.62 13 6.2 11.7
20 4.0 4.04 9 9.0 10.5
After addition of hexadecylamine (1 mL), an aqueous solution of NaBH4 (378 mg, 10 mL of H2O) was subsequently introduced into the mixture with rapid stirring. The organic phase was separated after 16 h of stirring. 3.3. Extraction of the Nanocrystals. For both methods, the organic phase containing the CoPt nanocrystals was evaporated (until 1 mL) using a rotary evaporator. Ethanol was added in excess (40 mL) to the mixture, and the nanocrystals were precipitated by centrifugation (5000 rpm, 10 mn). This step allowed us to discard the supernatant containing excess phase transfer, alkylamine, and reaction byproducts. This procedure was repeated twice, and finally, the nanocrystals were dispersed in 4 mL of toluene. In addition, when using heptylamine (C7H15NH2), a volume of 20 µL of alkylamine was required to obtain dispersion. A final centrifugation was performed to eliminate unstable nanocrystals or aggregates in the solution.
4. Results and Discussion 4.1. Comparison of the Different Modes of Synthesis: Route To Obtain Precise Control of the Composition. As mentioned above, it is difficult to obtain a precise control of the composition of CoPt nanocrystals by chemical synthesis. Here, we compare the different ways to achieve alloy nanocrystals with the desired equiatomic composition. Figure 2 shows TEM images and the corresponding size distribution of the CoPt nanocrystals coated with hexadecylamine obtained by the four methods described above. The nanocrystals look quite similar with an average diameter of 2 nm (Table 1). However, the size polydispersity is higher if the cobalt acetate salt is used (sample 1A). If we compare the electronic contrast of the TEM pictures, all the samples are similar, indicating a homogeneous composition. Nevertheless, average compositions and standard deviations obtained experimentally by EDX analysis (see the Experimental Section) on a thin film made of uncoalesced CoPt nanocrystals (Figure 1) indicate a strong influence of the method and the cobalt salt used (Table 1). For method 1, with the cobalt salt in the aqueous phase and platinum salt in the organic phase, a low amount of cobalt is obtained in the final alloy nanocrystals. Composition measurements are of 35% and 38% cobalt (samples 1A and 1B, respectively), with an initial salt ratio of 50:50. Moreover, the homogeneity in composition of the sample is very low. The standard deviations are 15% (1A) and 11% (1B). This implies a large heterogeneity of composition from one point to another in the nanocrystal film. Conversely, if both the cobalt and platinum salts are transferred in the organic phase prior to the addition of the reducing agent, the composition is closer to the initial ratio with a great homogeneity in composition. For sample 2B, the standard deviation is only 2%. This composition variation could be explained by considering the mechanism of the reaction in the phase transfer method. In
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Figure 5. Cobalt content of the precursor salt in solution (%) versus the cobalt content in alloy nanocrystals.
Figure 3. TEM images of CoPt nanocrystals of (A) 2 nm diameter (sample 2B) and (B) 4 nm diameter obtained by kinetic control of the growth process (see the text). In this case self-organization in a local hexagonal network is observed. Insets A and B: electronic diffraction pattern of CoPt nanocrystals.
Figure 4. HRTEM images of CoPt nanocrystals of 2 nm (A-C) and 4 nm (D-F). Both nanocrystal sizes prepared with method 2B: (A-C) as-prepared and (D-F) with the kinetic control process.
colloidal chemistry, to obtain stable nanocrystals homogeneous in size, it is necessary to use confined media or interfaces where the nanocrystals could be synthesized and stabilized prior to addition of a capping agent to stop the precipitation. In Brust’s method, this is obtained by transferring the salt in the organic phase by way of the TDAB surfactant. The interest of this method is that the kinetics of nanocrystal growth is controlled by the surface coverage and thus the cluster size is controlled by the reaction conditions at the interface and not by the metal ion reduction kinetics in the homogeneous aqueous phase.41 The large difference in the redox potential of platinum and cobalt can
induce a variation in the reduction kinetics. In the first method (samples 1A and 1B) where platinum is in the organic phase and cobalt in the water phase, the reduction takes place during the emulsification of the solution by stirring when the reducing agent is added. If the reduction of platinum is dominated by the interface, the reduction of cobalt is dominated by the reduction kinetics of the salt in the aqueous phase. Thus, the coreduction is carried out in two distinct ways. Therefore, a strong discrepancy occurs in the average content of the cobalt in the nanocrystal compared to the expected ratio, and due to the change in the characteristics of the emulsion droplets from one to another, the homogeneity in composition is low. In the second method, where both cobalt and platinum salts are in the organic phase interacting with an interface (samples 2A and 2B), only the reaction conditions are predominant and the difference in redox potentials is no longer problem for this interfacial reaction.41 However, the best results are obtained when cobalt is in the same form as the platinum: CoCl2(TDA)2 and PtCl4(TDA)2. These two molecules could be considered as functionalized surfactants and could form reverse micelles or micellar aggregates in toluene:42 there are probably mixed reverse micelles with an average correct 50:50 composition ((2%). Hence, the reduction in the confined media yields a precise control of the composition. When Co(AOT)2 is used as the cobalt salt, the mixing between the two surfactants is probably not random or a segregation may occur in the toluene phase because of the high efficiency of Co(AOT)2 to form reverse micelles43 compared to CoCl2(TDA)2. In fact, it is known that the mixing of two different surfactants can yield the formation of domains in the interface especially for mixtures of cationic and anionic surfactants.44 This has been observed, for example, by mixing Co(AOT)2 and Na(AOT), which yields elongated micelles.43 Thus, the dispersity in composition is higher with a lower cobalt content as the diffusion of the Co0 and Pt0 nuclei could be prevented by the segregation of the precursor in the organic phase. In addition, the interfacial tension could change depending on the interfacial composition. This modification of the parameter can bring an alteration of the interfacial reaction conditions. This can explain the lower quality of the control of composition in sample 2A. 4.2. Characterization of the CoPt Nanoalloys. Let us now consider only the nanocrystals obtained with method 2B. Figure 3A shows a low magnification of a monolayer made of 2 nm
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Figure 6. (A) XRD pattern of CoPt nanocrystals varying by their composition (full line, 100% Pt; dashed line, 25% Co; dotted line, 50% Co). A clear shift of the 111 band is observed by increasing the Co content. Inset: Gaussian fit of the XRD pattern. (B) Variation of the lattice parameter with the alloy composition in platinum (dashed line, bulk data for CoPt; circles, calculated with (111); triangles, calculated with (220)). Inset: characteristic XRD patterns showing the fcc structure of CoPt.
CoPt nanocrystals. It can be noticed that the nanocrystals are homogeneous in size, shape, and electronic contrast. The selected area electron diffraction (SAED) pattern (inset in Figure 3A) is similar to that of platinum, which indicates the formation of the chemically disordered (A1) fcc CoPt structure. This is characteristic of an alloy by substitution. The as-prepared CoPt nanoparticles have high crystallinity as evidenced by clearly resolved lattice fringes in high-resolution transmission electron microscopy (HRTEM; Figure 4A-C). It is known that CoPt bulk alloys form a complete solid solution.45 Therefore, the interlattice distance has a quasi-linear dependency between Co(111) fcc and Pt(111) fcc. From the HRTEM pictures, the interlattice distance (111) is estimated to be 2.2 ( 0.1 Å compared to 2.27 Å for pure platinum and 2.04 Å for pure cobalt. Assuming a linear dependence of the structural phase A1 of the CoPt bulk, we expect a value of 2.16 Å for Co50Pt50 in agreement with the experimental value. The accuracy of the interlattice estimation is low especially for nanocrystals showing only 10 fringes. Nanophase segregation cannot be totally excluded from these data. However, we have further evidence excluding segregation. Domains of pure Co have a lower contrast in TEM than those of Pt or CoPt;46 thus, Co domains should be distinguishable from domains of Pt and CoPt. However, we observe homogeneous contrast in the TEM and HRTEM images and no core-shell structure by HRTEM (Figures 3 and 4). Other evidence of the real efficiency of our synthesis is the control of the composition of the nanocrystals. Figure 5 shows the variation of the final composition of the CoxPt100-x nanocrystals after extraction depending on the concentration ratio of salt in the initial solution. A linear variation can be observed with a slope equal to 1, indicating a perfect control of the composition by the concentration of salt. This reinforces the hypothesis of the formation of nanoalloys. This is confirmed by X-ray diffraction (Figure 6A), which shows with better accuracy than HRTEM the change in the crystalline structure due to the formation of the nanoalloys. It is clear, from the diffractogram, that the crystalline structure of the nanoalloys is fcc (inset in Figure 6B). A shift is observed for the (111) band of the fcc structure compared with the pure platinum nanocrystals. This is consistent with the variation of the interlattice parameter observed by HRTEM. Plotting the variation of the lattice parameter of the fcc unit with the average composition of the nanocrystals (Figure 6B) shows a clearly linear variation. Indeed, the variation of the lattice parameter of the nanocrystals obeys Vegard’s law. This is characteristic of alloy formation. In addition, the shoulder corresponding to the
(200) band is drastically reduced (inset in Figure 6A). The effect is due to the random substitution of the platinum in the fcc lattices by cobalt, which has a smaller atomic radius. As a result, the chemical disordering of the crystalline structure increases. This confirms the formation of Co50Pt50 and suggests negligible Pt or Co surface segregation. The constant stoichiometry observed for each composition, as well as the TEM and diffraction results, rules out the possibility of nanophase segregation or core-shell formation. Indeed, our procedure allows a complete and precise control of the composition over a large range. The structural investigation is in agreement with the formation of CoxPt100-x nanoalloys. 4.3. Size Control of the Nanoalloys. 4.3.1. Effect of the Alkylamine Chain Lengths. The phase transfer method does not allow precise control of the size of the nanocrystals. A posttreatment in the form of a germination process,47 digestive ripening,48 or heat treatment49 is often used to increase the size of the nanocrystals. Thus, it is not trivial to increase the size while maintaining the spherical shape of the nanoparticles. Here, it is also important to keep the composition and low size dispersity. It has been shown recently that there is an effect of the length of the alkylamine chain on the average size of platinum nanocrystals synthesized by the Brust method.35 A systematic increase of the average size was obtained by using shorter alkylamines. Figure 7 shows the TEM images and the corresponding histograms of the nanocrystal diameter obtained by systematically varying the number of carbon atoms in the alkylamine chain of the stabilizer between 7 and 18. The calculated average diameter and the polydispersity are plotted in Figure 8. The data show an inverse relationship between the size of the nanoparticles and the length of the alkylamine. Furthermore, the polydispersity decreases when the size increases. This is characteristic of a diffusion-controlled growth process.32 As demonstrated in the case of pure platinum nanocrystals,35 this evolution with the length of the alkyl chains is due to differences in their solubility in toluene. An alkylamine with a short hydrocarbon tail is much more soluble in toluene (which is slightly polar) than a long-chain alkylamine. Thus, the former will diffuse more into the bulk toluene phase than the latter. To a higher degree than their shorter homologues, the longer alkylamines will be located at the interface, where the nanoparticles are being generated. This location is a prerequisite for a good stabilizing effect. Only when the alkylamine is situated at the interface will the amino group be able to coordinate to the metallic surface. To stabilize the nanocrystals when short chains are used, it is
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Figure 7. Size histograms (A-D) and TEM images (E-H) of CoPt nanocrystals obtained using different alkylamines as the capping agent: (A, E) C12H25NH2, (B, F) C10H21NH2, (C, G) C8H17NH2, (D, H) C7H15NH2.
necessary to increase the amount of capping agent added (see the Experimental Section). The situation at the interface bears a close resemblance to that of a water-in-oil microemulsion, formulated with one very hydrophobic cationic surfactant and one cosurfactant, in this case represented by TDAB and the alkylamine, respectively. The tendency of the long alkylamine chain to stay at the interface is likely to make this palisade layer static as compared to the palisade layer formed with the short alkylamine chain: a more dynamic interface will favor transport
of new platinum or cobalt complexes from the bulk toluene phase, where they exist in the form of ion pairs, into the water droplets, where they release the platinum to the growing nanoparticles. This difference in permeability of the palisade layer explains why short alkylamine chains give larger particles. Furthermore, as expected, if the growth process takes place in micellar aggregates, this growth mechanism does not change the average composition. Consequently, it is possible to control the size of the nanoparticles formed by the phase transfer method simply
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Demortie` re and Petit Table 3. Moments of the Nanoparticle Size Distribution Calculated from TEM Images
Figure 8. Average diameter (squares) and polydispersity (circles) of the CoPt nanocrystals as a function of the chain length of the alkylamine used as the capping agent.
by adjusting the solubility of the stabilizing agent in the bulk organic medium. 4.3.2. Kinetic Control of the Growth Process. The method presented above only allows increases in the average diameter from 1.8 to 2.6 nm. To reach ferromagnetism at room temperature in the hard phase (L10) of Co50Pt50, it is necessary to obtain a minimum diameter of 4 nm. Increasing the van der Waals interaction between the nanocrystals will in turn favor the selfassembly process, which is also a key point for the development of magnetic devices based on organizations of nanocrystals.50 Taking the larger nanocrystals obtained previously (method 2B, coated by hexadecylamine), we have tried to increase the average size by controlling the kinetics of growth of the CoPt nanocrystals. Considering the theoretical framework of LaMer,51 the chemical process of reduction is modified. LaMer assumes that monodisperse colloids could be prepared by supersaturating a solution with a monomer. Homogeneous nucleation occurs when the concentration exceeds “a critical limiting supersaturation”, which decreases the monomer concentration below the supersaturation point. Then if no aggregation occurs, which is prevented in our case by the surfactant, the remaining monomer deposits uniformly onto existing nuclei to give monodisperse colloids. It has been demonstrated experimentally that separation of nucleation and growth could be realized by controlled addition of a reducing agent in the liquid-liquid phase transfer method for gold nanoparticles.32 Thus, instead of adding the reducing agent in one step, we add the reducing agent (10 mL) in drops of 10 µL at regular intervals of time between 5 and 30 s. For the sample with an injection time of 0 s, the reducing agent was introduced in one step. This allows the formation of numerous monomers and nuclei at the initial stage of the reduction (a light coloration appears in the solution, indicating that the reaction takes place), and then further injection of reducing agent converts the residual metallic salt to Pt0 or Co0, which condenses onto the existing nuclei. This process is energetically favorable, and as a result the size should increase and the polydispersity should decrease. The TEM pictures (Figure 9) clearly demonstrate this behavior. Furthermore, increasing the interval between drops of solution of reducing agent allows an increase of the average size and a decrease in the polydispersity. Again HRTEM shows a homogeneous electronic contrast of the particles and no core-shell structure (Figure 4D-F). The SAED pattern is unchanged, and
injection time (s)
Ra
Rc
Rh
µ1
µ2
0 10 20 30
2.084 3.397 4.005 4.579
2.125 3.416 4.019 5.279
2.041 3.378 3.992 4.090
1.041 1.011 1.006 1.290
0.980 0.994 0.996 0.867
the contrast is homogeneous (Figure 3B). The average composition as determined by EDX analysis is maintained at 50:50 ( 2%. Figure 10 summarizes the increase of the average size and the decrease of the polydispersity of the nanocrystals by systematically increasing the interval of time between drops of solution containing the reducing agent. The longer the waiting time, the larger the diameter and the lower the polydispersity. Thus, the optimal time is 20 s, yielding CoPt nanocrystals with a diameter of 4 nm and a polydispersity of 9% (Table 2 and Figure 9 and 10). The size distribution can be analyzed to study the growth process. Indeed, nanoparticles can grow either by condensation of monomer onto the nuclei or by coagulation of the nuclei. The moments of size distribution function, µ1 ) Rc/Rh and µ2 ) Ra/Rc, show the relative influence of condensation and coagulation on nanocrystal growth.32,52
Ra ) Rc ) Rh )
∑Ri (a) N∞
( )
∑Ri3 1/3 (b) N∞
N∞
∑(1/Ri)
(c)
(4)
where Ra (eq 4a) is the arithmetic mean radius, Rc (eq 4b) is the cube-mean radius, and Rh (eq 4c) is the harmonic mean radius. N∞ is the total number of nanoparticles. Considering the theoretical framework of Friedlander et al.,52 for monodisperse nanocrystals µ1 ) µ2 ) 1, and if µ1 < 1.25 and µ2 > 0.905, then a condensation mechanism controls growth. The nanoparticle size distribution functions were determined from TEM images of three samples with injection times of 0, 10, and 20 s. A total of 500 particles were measured for each sample. For these three samples, the average moments were close to 1 as shown in Table 3. These values indicate that the growth process occurs by condensation and not by coagulation. However, beyond 30 s between drops, a drastic increase of the size polydispersity is observed due to evolution of the average size and shape. This limitation effect occurs when the growth process changes from a condensation mode to a coagulation mode. The average moments were µ1 ) 1.290 and µ2 ) 0.867 (Table 3), indicating that the nanocrystals grow primarily by coagulation. For a long time step, the concentration of nuclei becomes high and the monomer concentration decreases. In this case, the coagulation process is supported and the size polydispersity becomes broad. The increase of size is confirmed by the X-ray diffractogram showing a narrowing of the (111) line when the size is increased without the position of the band being changed, indicating that the composition is unchanged (Figure 11 and Table 2). This is also observed on the SAED pattern (Figure 3). As a result of the increase of the van der Waals interaction,53 the 4 nm diameter CoPt nanocrystals self-assemble in a hexagonal network monolayer (Figures 3B and 9C).
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Figure 9. TEM images and size histograms of CoPt nanocrystals obtained at various injection times of (A, D) 0 s, (B, E) 10 s, and (C, F) 20 s.
In our conditions of high supersaturation, the simultaneous increase of the average size and decrease of the polydispersity with the time of reaction indicates that the nanocrystal growth is a condensation mechanism growth.32 This reinforces the micellar hypothesis in the reaction process as exposed above, which implies an isotropic growth. Furthermore, it should be noticed that this effect is more efficient when shorter alkylamine chains are used. As explained above, this is probably due to a dynamic interface allowing the growth process to take place efficiently. 4.4. Magnetic Properties of Co50Pt50 Nanoalloys. We have shown above that liquid-liquid phase transfer allows synthesis of cobalt-platinum nanocrystals with a controlled size and composition. The magnetic properties of the solution containing the nanocrystals in the A1 crystalline phase are studied by SQUID measurements. The susceptibility versus temperature behavior is measured by a zero-field-cooled (ZFC)/field-cooled (FC) experiment. In the ZFC measurement, the sample is cooled to
2.5 K without an applied field starting from a high temperature where all the particles are in the superparamagnetic state. Afterward, the magnetization is measured as a function of the increasing temperature in an applied field of 75 Gauss. In the case of an ideal system of perfectly monodisperse particles, the magnetization measured in the ZFC curve drops upon cooling from a maximum to zero in a few degrees. The temperature TB at which the susceptibility peak occurs represents the particle blocking temperature which is related to the particle magnetic anisotropy energy KV by the relation KaV ) kbTB ln(τ/τ0) ≈ 28 kbTB, where V is the particle volume, Ka is the magnetic anisotropy energy per volume unit, and τ0 ≈ 10-9 to 10-11 s.54 In Figure 12 the ZFC curves of Co50Pt50 are reported for nanocrystals differing by their size. As expected for disordered Co50Pt50 nanocrystals (A1 phase) the blocking temperature is very low (Table 2) and is shifted toward high temperature with increasing size. This is consistent with the structural data presented above and confirms the formation of a nanoalloy (platinum has a
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Demortie` re and Petit
Figure 13. Hysteresis curves: magnetization of CoPt nanocrystals of 2 and 4 nm size (dashed line, 4 nm at 60 K; triangles, 4 nm at 3 K; squares, 2 nm at 3 K).
Figure 10. Average diameter (squares) and polydispersity (circles) of the CoPt nanocrystals as a function of the injection time of the reducing agent.
Figure 11. XRD pattern showing the widening of the (111) and (200) peaks as a function of the nanocrystal size (2.0, 2.5, 3.4, and 4.0 nm). Inset: Gaussian fit of the XRD pattern.
nanocrystals. The magnetic anisotropy energy Ka is surprisingly high especially for the 2 nm nanocrystals (Ka ) 3 × 106 ergs/ cm3). This is due to surface effects.56 It is known that the surface can considerably increase the total magnetic anisotropy energy (MAE) especially for nanocrystals, where 50% of the atoms could be located at the surface. The MAE increases considerably on decreasing the average size of the CoPt nanocrystals (Table 2). The hysteresis loops at 3 K of 2 and 4 nm diameter CoPt nanocrystals are reported in Figure 13. For all nanocrystals, saturation is not observed even at high magnetic fields. Again this could be due to a surface effect (spin canting) but also to a contribution of superparamagnetic particles. The smallest nanocrystals of the size distribution are always in the superparamagnetic state. This is due to the fact that the hysteresismeasuring temperature is very close to the blocking temperature. However, the nanocrystals are clearly ferromagnetic at low temperature with a surprisingly high coercivity field (0.20 T for 4 nm diameter nanocrystals), but this is similar to that reported for 2.7 nm CoPt nanocrystals obtained in reverse micelles.27 This could be due to the disordered state of the nanocrystals and surface defects, which may prevent the coherent rotation of the spin.
5. Conclusion
Figure 12. ZFC curves: magnetic susceptiblility of CoPt nanocrystals of different sizes (triangles, 2.0 nm; circles, 2.5 nm; full squares, 3.4 nm; squares, 4.0 nm).
diamagnetic response, and the blocking temperature for 4 nm cobalt nanocrystals is close to 40 K). It has been shown that for a CoPt core-shell structure the blocking temperature should be close to that observed for similarly sized Co nanoparticles.55 Here, we should expect a value of TB around 20 K assuming a shell of 1 nm of platinum and a core of cobalt 2 nm in diameter. The observed value is 9 K for the 4 nm in diameter CoPt
For the first time, the liquid-liquid phase transfer method has been successfully used to synthesize nanoalloys of CoxPt100-x in the disordered phase (A1) with perfect control of the composition. This is only possible if the reaction is totally interfacial, implying a complexation of the initial salt in a similar structure to form homogeneous aggregates in the organic phase. As observed generally for nanocrystals synthesized by this method, the size is strongly dependent on the control of the growth process either by a change of the dynamics of the interface or by kinetic control of the growing process. This allows us to easily obtain 4 nm diameter nanocrystals with a Co:Pt composition of 50:50. These nanocrystals are highly crystalline, homogeneous in composition, and present the expected magnetic behavior for the disordered phase. Further investigations are in progress to induce the transition toward the L10 magnetic phase. Acknowledgment. The authors would like to express their gratitude to Dr. P. Bonville and Dr. E. Vincent, DRECAM/ SPEC CEA, Saclay, France, for the use of their SQUID equipment. LA700719H
