Fine Tuning Of The Sizes Of Fept Nano Particles

Japanese Journal of Applied Physics Vol. 44, No. 2, 2005, pp. 1147–1149 #2005 The Japan Society of Applied Physics

Fine Tuning of the Sizes of FePt Nanoparticles Satoru M OMOSE, Hiroyoshi K ODAMA, Takuya U ZUMAKI and Atsushi T ANAKA Advanced Magnetic Recording Laboratory, Fujitsu Laboratories Ltd., Kanagawa 243-0197, Japan (Received July 7, 2004; accepted November 4, 2004; published February 8, 2005)

Fine tuning of the sizes of FePt nanoparticles has been realized by a simple modification of a solution phase synthesis, which consists of the decomposition of Fe(CO)5 and the reduction of Pt(acac)2 . The key method used to control the particle size is the use of surfactants, and only a change in their amount is required to vary the average sizes of the FePt nanoparticles from 2 to above 4 nm. The resultant nanoparticle volume is proportional to the surfactant amount; this makes the synthesis of FePt nanoparticles of desired sizes possible. [DOI: 10.1143/JJAP.44.1147] KEYWORDS: FePt nanoparticles, nanocrystalline metal particles, monodispersed particles, chemical synthesis, particle size control

1.

Introduction

2.

Recently, nanometer-sized particles of magnetic materials such as FePt,1–4) Co,5–7) CoPt,8–10) and Fe3 O4 11–16) have been fabricated by solution phase synthesis. These chemically synthesized nanoparticles are characterized by uniform size distributions. At the same time, high-anisotropy-density materials such as FePt17) allow a reduction in size to 4 nm or less, because of their high magnetic stability.18,19) Chemically synthesized FePt nanoparticles, with diameters reported to be 4 nm, are therefore very attractive candidates for ultrahigh density magnetic recording media, because high recording densities require a small material grain and a narrow size distribution.18,19) Generally, the magnetic properties of materials highly depend on grain size, and since magnetic recording media require rigorous control of their magnetic properties, the tuning of the sizes of the magnetic nanoparticles used in magnetic recording applications is also required. However, since many factors can affect the particle size during synthesis, and those effects are complex, no general procedure previously existed for the tuning of the sizes of alloy systems, even though a successful case with CoPt3 has been reported.20,21) Here, we will report on the development of a method for FePt nanoparticle synthesis that easily enables the fine tuning of the particle size. The key point of this method is the ability to control the particle growth using a surfactant. Whatever the particle growth mechanism, the separation of nucleation and growth in time is required for the formation of particles with a near-monodisperse size distribution.22) For this kind of system, some procedures are necessary in order to alter the particle size. These include controlling the reaction time or, following nucleation, changing the quantity of materials that contribute to particle growth. However, the former is applicable only to adequately slow reactions, and the latter is difficult to apply except in cases of adding materials. Another technique is the control of the number of nuclei at the first step of particle formation, because an increase in this number can be expected to result in small particles. The tendency of materials to nucleate is most likely related to the affinity between the nucleating material and solvent, which can be controlled using a favorable surfactant. We, therefore, examined a method for FePt nanoparticle synthesis that is based on the reduction of platinum(II) acetylacetonate (Pt(acac)2 ) and the decomposition of iron carbonyl (Fe(CO)5 ),1,2) using various surfactant quantities.

Experimental

2.1 Synthesis The synthesis was carried out using standard airless techniques in an argon atmosphere. The reagents were obtained from commercial sources and used without further purification. A typical synthetic procedure is as follows: Pt(acac)2 (198 mg, 0.5 mmol), 1,2-hexadecanediol (389 mg, 1.5 mmol), dioctylether (20 mL), oleic acid (0.16 mL, 0.5 mmol), oleyl amine (0.17 mL, 0.5 mmol), and Fe(CO)5 (0.13 mL, 1 mmol) were mixed and heated to 230 C, and then the temperature was maintained for 30 min with stirring. The reaction vessel was cooled to room temperature, and the black product was precipitated by the addition of ethanol (40 mL) and centrifugation. The black precipitate was then dispersed in hexane (20 mL) and centrifuged, and the resultant supernatant was added to ethanol (20 mL) and centrifuged to precipitate out FePt nanoparticles. In this study, the only modifications in the reaction conditions were changes in the amounts of surfactants, oleic acid and oleyl amine (the total amounts are 0.13, 0.25, 0.5, 1.0 and 2.0 mmol). The molar ratio of the acid to the amine was fixed at 1. 2.2 Characterization The particle size was determined by transmission electron microscopy (TEM). TEM samples were prepared by depositing a droplet of the diluted nanoparticle dispersion onto a carbon-coated copper grid. TEM observations were carried out at 200 keV using JEOL JEM-2010F. The crystal structure of the nanoparticles was studied by X-ray powder diffraction (XRD) analysis on a Rigaku ATX-G diffractometer under Cu K radiation. Fe and Pt elemental analyses of the nanoparticles were performed on an ICP-atomic emission spectrometer (Perkin Elmer 4300DV). 3.

Results and Discussion

Figure 1(c) shows a TEM image of FePt nanoparticles created under the aforementioned typical reaction conditions (total surfactant amount is 1.0 mmol). Figures 1(a), 1(b) and 1(d) similarly show nanoparticles created with total surfactant amounts of 0.13, 0.25, and 2.0 mmol, respectively. It is clearly visible that the sizes of the particles increase with an increase in surfactant amount. Both oleic acid and oleyl amine have a double bond in the hydrocarbon chain; accordingly, they are good ligands for metal atoms.23,24) It is also clear that an increase in surfactant amount results in

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a

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FePt (111)

b

FePt (200)

10 nm

10 nm

d

c

Intensity (arb. units)

2.1 (2.2) nm Fe47Pt53 2.6 (2.7) nm Fe46Pt54 3.0 (3.0) nm Fe50Pt50

3.3 (3.2) nm Fe53Pt47 4.3 (4.1) nm Fe48Pt52

35

40

45

50

55

60

2θ (deg)

10 nm

10 nm

Fig. 1. TEM images of (a) 2.1 nm FePt nanoparticles synthesized using a total of 0.13 mmol of surfactants: oleic acid and oleyl amine, (b) 2.6 nm FePt nanoparticles synthesized using 0.25 mmol of the surfactants, (c) 3.3 nm FePt nanoparticles synthesized using 1.0 mmol of the surfactants, and (d) 4.3 nm FePt nanoparticles synthesized using 2.0 mmol of the surfactants.

Particle Volume (nm 3)

Fig. 2. XRD patterns of as-synthesized FePt nanoparticles of various average TEM sizes. In parentheses, XRD sizes calculated using the Debye–Sherrer equation are also indicated.

30

20

10

0 0

Table I. Experimental conditions and resultant nanoparticles. Surfactant amount (mmol)

Precursor mole ratio Fe : Pt

Yield (mg)

Composition Fe : Pt

Ave. diameter (TEM) (nm) (Dispersion, %)

0.13

2.00

121

47 : 53

2.1 (17)

0.25

1.95

89

46 : 54

2.6 (20)

0.5 1.0

2.01 1.99

89 106

50 : 50 53 : 47

3.0 (11) 3.3 (10)

2.0

2.05

124

48 : 52

4.3 (12)

increases in the stability and affinity of metal atoms to the solvent. This effect is expected to prevent the metal atoms from nucleation, and thus, the number of nuclei decreases and each resultant particle volume increases. The correspondence of reaction conditions to resultant nanoparticles is indicated in Table I. Substantially, equiatomic compositions were obtained in all cases. XRD patterns indicated in Fig. 2 show the chemically disordered fcc crystal structure of these nanoparticles and the broadening of the peaks with decreasing particle size. Figure 2 also indicates that the average sizes of the nanoparticles from XRD, calculated using the Debye–Sherrer equation, are similar to those from TEM. Figure 3 shows a plot of the molar ratio of the surfactant to Pt(acac)2 versus particle volume: the average volumes of the particles are calculated from the average sizes, assuming that they are spherical. A linear relationship between these

1

2

3

4

5

Surfactant to Pt(acac)2 molar ratio Fig. 3. Molar ratio of surfactant to Pt(acac)2 vs average volumes of resultant FePt nanoparticles; the solid line represents a least-squares fitting, and error bars are standard deviations.

parameters is observed from the graph. This linear relationship can be explained qualitatively according to a suggestive precedent by Watzky and Finke.22) They evidenced a growth mechanism for Ir nanoparticles that consists of a slow, continuous nucleation step and a fast surface growth step, and that this schematic formation process was thermodynamically reasonable. Namely, the surface growth should proceed irreversibly because of the metal–metal bond stability, while the critical nucleus is the state of balance between the energy gain by metal–metal bond formation and the cost due to surface tension enthalpy increase with entropy loss, thus the nucleation should be a reversible and relatively slow process. Accordingly, metal nanoparticles, perhaps, are generally formed by a similar process under homogeneous reaction conditions. Then monodispersed particles are formed when the growth rate is significantly larger than the nucleation rate, and the growth step is adequately short, because whole particles grow simultaneously and nuclei are formed continuously during the reaction.22) This argument seems to be applicable to our FePt nanoparticles. Under this mechanism, the resultant nanoparticle volume in principle turn proportion-

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ally decreases as the number of nuclei at the end of the nucleation step increases. Since the critical nucleus volume likely depends on the material substance and is constant irrespective of the reaction conditions, the nanoparticle volume may turn proportionally decrease as the nucleation rate increases. On the other hand, Watzky and Finke also revealed an effect of the initial concentration of a ligand on the nucleation rate. In their reaction system, Ir cation, which is dissociated from the polyoxoanion complex, is reduced and then the resultant Ir atom contributes to the particle formation. The rate-determining step is the dissociation of the complex. Since the dissociation rate is determined from an equilibrium and is turn proportional to the initial concentration of the ligand polyoxoanion, the nucleation rate is also turn proportional to the initial concentration of the polyoxoanion.22) For our reaction system, metal atoms are formed from both the thermal decomposition of Fe(CO)5 and reduction of Pt cations; thus, the metal atoms should complex oleic acid and oleylamine immediately, because these surfactants are good ligands for zero-valent metals as mentioned above. The metals then may be liberated from the complex by dissociation equilibrium and may contribute to the nanoparticle formation, indicating that the nucleation rate turn proportionally decreases as the initial concentration of the surfactant increases. Our result that the nanoparticle volume proportionally increases as the surfactant amount increases is therefore inferred to be due to a turn proportional decrease in nucleation rate caused by an increase in surfactant amount. Since it is considered that transition metal nanoparticle sizes are determined kinetically in solution phase syntheses, the reaction temperature likely significantly affects the particle sizes always. For alloy systems, however, there is a restriction that the temperature must be in a range where all metal precursors can react and produce zero-valent metals. Ligand amount control therefore may be one of the most powerful tools for the tuning of the sizes of alloy nanoparticles. 4.

Conclusion

We have described a simple fine size tuning technique for FePt nanoparticles, which requires only one parameter change, that is, a change in surfactant amount. The resulting size change of the nanoparticles is thought to be the result of the change in the number of nuclei at the first step of particle formation. The resultant nanoparticle volume proportionally

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increases with the surfactant amount, thus allowing the simple fabrication of nanoparticles of preferred sizes. This relationship may imply that the surfactant amount determines the number of nuclei via nucleation rate control, which results from the coordination of the surfactants to metal atoms. Acknowledgement This work was supported in part by the IT program (RR2002) of the Ministry of Education, Culture, Sports, Science and Technology. 1) S. Sun, C. B. Murray, D. Weller, L. Folks and A. Moser: Science 287 (2000) 1989. 2) S. Sun, E. E. Fullerton, D. Weller and C. B. Murray: IEEE Trans. Magn. 37 (2001) 1239. 3) B. Jeyadevan, A. Hobo, K. Urakawa, C. N. Chinnasamy, K. Shinoda and K. Tohji: J. Appl. Phys. 93 (2003) 7574. 4) B. Jeyadevan, K. Urakawa, A. Hobo, N. Chinnasamy, K. Shinoda, K. Tohji, D. D. J. Djayaprawira, M. Tsunoda and M. Takahashi: Jpn. J. Appl. Phys. 42 (2003) L350. 5) S. Sun and C. B. Murray: J. Appl. Phys. 85 (1999) 4325. 6) C. Petit, A. Taleb and P. Pileni: J. Phys. Chem. B 103 (1999) 1805. 7) V. F. Puntes, K. M. Krishnan and A. P. Alivisatos: Science 291 (2001) 2115. 8) J.-I. Park and J. Cheon: J. Am. Chem. Soc. 123 (2001) 5743. 9) M. Chen and D. E. Nikles: J. Appl. Phys. 91 (2002) 8477. 10) C. N. Chinnasamy, B. Jeyadevan, K. Shinoda and K. Tohji: J. Appl. Phys. 93 (2003) 7583. 11) R. S. Sapieszko and E. Matijevic: J. Colloid Interface Sci. 74 (1980) 405. 12) Y. S. Kang, S. Risbud, J. F. Rabolt and P. Stroeve: Chem. Mater. 8 (1996) 2209. 13) C.-Y. Hong, I. J. Jang, H. E. Horng, C. J. Hsu, Y. D. Yao and H. C. Yang: J. Appl. Phys. 81 (1997) 4275. 14) R. Vijayakumar, Y. Koltypin, I. Felner and A. Gedanken: Mater. Sci. Eng. A 286 (2000) 101. 15) T. Fried, G. Shemer and G. Markovich: Adv. Mater. 13 (2001) 1158. 16) S. Sun and H. Zeng: J. Am. Chem. Soc. 124 (2002) 8204. 17) O. A. Ivanov, L. V. Solina, V. A. Demshina and L. M. Magat: Phys. Met. Metallogr. 35 (1973) 81. 18) D. Weller and A. Moser: IEEE Trans. Magn. 35 (1999) 4423. 19) D. Weller, A. Moser, L. Folks, M. E. Best, W. Lee, M. F. Toney, M. Schwickert, J.-U. Thiele and M. F. Doerner: IEEE Trans. Magn. 36 (2000) 10. 20) E. V. Shevchenko, D. V. Talapin, A. L. Rogach, A. Kornowski, M. Haase and H. Weller: J. Am. Chem. Soc. 124 (2002) 11480. 21) E. V. Shevchenko, D. V. Talapin, H. Schnablegger, A. Kornowski, O. Festin, P. Svedlindh, M. Haase and H. Weller: J. Am. Chem. Soc. 125 (2003) 9090. 22) M. A. Watzky and R. G. Finke: J. Am. Chem. Soc. 119 (1997) 10382. 23) S. Ahrland, J. Chatt and N. R. Davies: Quart. Rev. 12 (1958) 265. 24) R. G. Pearson: J. Am. Chem. Soc. 85 (1963) 3533.