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Preparation and characterization of Co–Pt bimetallic magnetic nanoparticles
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 299 (2006) 21–28 www.elsevier.com/locate/jmmm
Preparation and characterization of Co–Pt bimetallic magnetic nanoparticles Xueyan Du, Makoto Inokuchi, Naoki Toshima Department of Materials Science and Environmental Engineering, Tokyo University of Science, Yamaguchi, Onoda-shi, Yamaguchi 756-0884, Japan Received 27 September 2004; received in revised form 26 October 2004 Available online 7 April 2005
Abstract CoPt3 nanoparticles are synthesized by a two-stage route using NaBH4 as a reductant. The nanoparticles are characterized by thermogravimetry (TG) and differential thermal analysis (DTA), Fourier transform infrared (FT-IR), transmission electron microscopy (TEM) and X-ray diffraction (XRD). Structural and spectroscopic studies show that the nanoparticles adopt a face-centered-cubic (FCC) crystalline structure with an average particle size of 2.6 nm. SQUID studies reveal that as-synthesized nanoparticles are superparamagnetic at room temperature and ferromagnetic at 1.85 K with coercivity of 980 Oe. Annealing of the samples at 500 1C causes an increase of particle size and a decrease of coercivity. r 2005 Elsevier B.V. All rights reserved. Keywords: Nanoparticles; Cobalt; Platinum; Magnetic properties
1. Introduction Nanoparticle-based materials have achieved much attention due to the novel electrooptic, magnetic and catalytic properties which arise from the quantum size effects and large surface areas that are characteristic of nanosized species [1,2]. In particular, magnetic nanosized materials attract Corresponding author. Tel.: +81836883500;
fax: +81836884567. E-mail address: [email protected] (X. Du).
growing interest because of their potential application to ultrahigh-density magnetic recording systems [3]. The magnetic and chemical properties of the monometallic materials are known to be significantly enhanced by the formation of alloys with additional metals. The magnetic alloys provide many advantages such as high magnetic anisotropy, enhanced magnetic susceptibility, and large coercivities [3–7]. Co–Pt alloy is one of the candidates for ultrahigh-density magnetic recording media because of their high magnetic anisotropy and good
0304-8853/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2005.03.013
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X. Du et al. / Journal of Magnetism and Magnetic Materials 299 (2006) 21–28
chemical stability upon corrosion [8–10]. Most of the magnetic alloy syntheses to date have been focused in the area of thin films by using vacuum deposition techniques [9–11]. However, random nucleation and growth of nanoparticles usually result in relatively large crystalline size and broad size distributions, which adversely affect their magnetic performance. Recent reports suggest that chemical approaches in solution, such as the decomposition of organometallic precursors [4,12–16], utilization of reverse micelles [7,17–19] and application of polyol reduction [20–22] are excellent alternative methods for better size control and self-assembly. For example, Sobal et al. [13] reported that monodispersed, bimetallic Pt–Co nanoparticles with a controllable core–shell structure have been synthesized by a two-stage procedure. The Pt–Co nanoparticles showed a narrow size distribution with a maximum of 7.6 nm. Pt–Co nanoparticles with an average size of 3–4 nm prepared using a water-in-oil reverse microemulsion was reported [17] showing high catalytic activity toward methanol oxidation. Shevchenko and co-wokers [21] prepared CoPt3 nanoparticles by reduction of platinum acetylacetonate and thermal decomposition of cobalt carbonyl in the presence of 1-adamantanecarboxylic acid. As-synthesized CoPt3 nanoparticles with the size of 1.5–7.2 nm distribution range have been assembled into 2D and 3D structures. However, the potential carcinogenicity of chemicals (i.e., cobalt carbonyl) could restrict their applications to production on a large scale. We have prepared various kinds of polymerprotected monometallic and bimetallic nanoparticles by chemical methods [23–25]. We have then tried to extend our approach to Co–Pt system which display interesting magnetic properties. In this paper, we present a two-stage procedure for the preparation of Co–Pt bimetallic nanoparticles just by using NaBH4 as a reductant. Pure Co particles with definite diameter were formed at first. Later, the reduction of platinum ions was performed in the presence of Co seeds. It is worthy of noting that the coercivity of nanoparticles prepared by a coreduction of Co and Pt ions is much lower than that of samples prepared by twostage procedure.
2. Experimental 2.1. Materials All chemicals were purchased from Wako Pure Chemical Industries Ltd. and used without further purification. Distilled and deionized water was used throughout. 2.2. Preparation of nanoparticles In nitrogen atmosphere, 100 mL of NaBH4 ethanol solution (0.066 M) was dropped into 100 mL CoCl2 ethanol solution (3.6 mM) containing 0.5000 g PVP under vigorous stirring at 0 1C. The color of the solution changed from blue to brown, indicating the formation of the suspension of cobalt nanoparticles. Then, 100 mL H2PtCl6 ethanol solution (10.8 mM) was dropped into the above suspension. After keeping vigorous stirring for 30 min, 100 mL of NaBH4 solution (0.066 M) was slowly added. The mixtures were kept stirring at 0 1C for 3 h. Afterwards, the colloid dispersions were filtered by an ultra-fine membrane filter in nitrogen atmosphere and washed by dehydrated ethanol for three times. The residual ethanol of Co/Pt colloid was removed by using of a rotary evaporator. Co/Pt powders were finally obtained by vacuum drying at 40 1C for 48 h. 2.3. Characterization X-ray diffraction (XRD) patterns of the samples were recorded on a Rigaku Rint 2400 diffractometer with Cu Ka radiation. The thermal decomposition behavior of assynthesized nanoparticles was examined by means of thermogravimetry (TG) and differential thermal analysis (DTA) using a Mac Science WS200-TGDTA instrument with the simultaneous recording of weight losses and temperature variations. These measurements were carried out at a heating rate of 10 1C min1 with a nitrogen gas flow. Transmission electron microscopy (TEM) and HRTEM measurement were performed on a Hitachi H-7000 microscope operated at 80 kV and a Hitachi H-9000 NAR microscope operated at 300 kV, respectively. Samples for these
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3. Results Fig. 1 shows the TG-DTA curves which display the thermal decomposition behavior of as-synthesized nanoparticles. The full line corresponds to the gravimetric thermal curve and the dashed line represents the differential scanning calorimetry curve. Two strong endothermic peaks appear from the room temperature to 800 1C. One endothermic peak around 85 1C is attributed to the departure of the remaining water molecule. The other one corresponds to the decomposition of organic stabilizer. The total mass loss is up to 48% during the entire heating cycle, implying the PVP is burned out since the initial content of PVP is about 48%. Thus, a temperature of 500 1C is selected as annealing temperature for ensuring the organic decomposition. XRD patterns of CoPt3 nanoparicles before and after heat treatment are given in Fig. 2. The samples are assigned to face-centered-cubic (FCC) CoPt3 crystal phase from the diffraction lines of
2.0 0 -10 1.0
-20
0.5
-30
DTA (µV/mg)
Weight loss (%)
1.5
0.0
-40 -50
415°C
85°C 0
-0.5
100 200 300 400 500 600 700 800 Temperature (°C )
Fig. 1. TG-DTA curves of the as-synthesized Co–Pt nanoparticles. These measurements were carried out at a heating rate of 10 1C min1 with nitrogen flow. Full line corresponds to the gravimetric thermal curve and dashed line represents the differential scanning calorimetry curve.
(111)
250
150
(220)
(200)
200 Intensity (a.u.)
measurements were prepared by depositing a drop of the dispersion in the mixed solvent of hexane and ethanol onto carbon-coated copper grids. The excess of solvent was wicked away with a filter paper, and the grids were dried in air. The average diameter and standard deviation of nanoparticles were measured from TEM images with at least 200 particles. Infrared spectra were recorded on a JIRWINSPEC 50 Fourier transform infrared (FTIR) spectrophotometer. The samples were prepared as follows: In a Fischer–Porter bottle, a dispersion of 100 mg of the colloidal product in 20 mL ethanol is submitted to 1 atm of carbon monoxide. After 20 h, the solution was used directly to measure as a liquid film in air. Magnetic measurements were undertaken with a Quantum Design MPMS SQUID magnetometer. The temperature was varied between 1.85 and 300 K according to a classical zero field cooling/ field cooling (ZFC/FC) procedure in the presence of a very weak applied magnetic field (100 Oe). Hysteresis measurements were performed in a magnetic field varying from +9000 to –9000 Oe.
23
(b) 100
50 (a) 0 10
20
30
40 50 60 2 Theta (degree)
70
80
Fig. 2. XRD pattern of CoPt3 nanoparticles (a) in the asprepared state and (b) after heat treatment at 500 1C for 2 h.
(1 1 1), (2 0 0) and (2 2 0) planes in the XRD profile. The width of peaks of CoPt3 nanoparticles after heat treatment becomes narrower than that of asprepared nanoparticles, indicating an increase in crystalline size after heat treatment. No peak appears near 521 and 611 which are attributed to cubic Co, indicating that as-synthesized nanoparticles are not mixture of Pt and Co metallic nanoparticles. The transition from the disordered FCC to the ordered FCT (L10) phase during heat
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treatment does not take place since the reflection near 331 cannot be observed in XRD profile, which is attributed to (1 1 0) reflection of L10 superlattice [22]. Fig. 3 shows the TEM micrographs of CoPt3 nanoparticles before and after heat treatment at 500 1C for 2 h, where spherical nanoparticles with average sizes of 2.6 and 3.3 nm, respectively, can be seen. More specifically, Fig. 3(b) shows polydispersity of larger size nanoparticles compared with Fig. 3(a), as is also evident from the corresponding histograms. This indicates that the annealing results in the increase of particle size, which is in good agreement with the results of XRD. The insetting HRTEM image clearly shows the lattice planes with the periodicity of 0.22 nm typical for a cubic crystallograghic CoPt3 structure
and platinum structure. In order to distinguish between CoPt3 and Pt structure, energy-dispersive X-ray (EDX) spectrometric analysis was performed. It is evident that the presence of both Pt and Co metal is qualitatively confirmed, implying pure Pt structure is ruled out in as-synthesized nanoparticles. EDX results show the ratio between Co and Pt was close to 1:3, which coincided well with the results of ICP AES analysis. Although the XRD pattern and HRTEM results are in good agreement with the formation of CoPt3 nanoparticles, core–shell structure cannot completely be excluded from these data. To get more evidence for bimetallic phase formation, we decided to use carbon monoxide as a probe of the surface state. Thus, carbon monoxide has been adsorbed on the surface of the samples. In the case
Distribution (%)
30
dav = 2.6 nm = 1.3 nm
20
10
0 0
2
(a)
4 6 8 Diameter (nm)
10
12
Distribution (%)
50 dav = 3.3 nm = 2.7 nm
40 30 20 10 0 0
(b)
2
4 6 8 Diameter (nm)
10
12
Fig. 3. TEM micrograph and size distribution of (a) the as-synthesized CoPt3 nanoparticles and (b) the samples annealed at 500 1C for 2 h. Inset shows HRTEM image.
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ture-dependent suscepitibility of the as-synthesized nanoparticles before and after heat treatment are shown in Fig. 5a. It can be found that there is a cusp in the ZFC suscepitibility at the blocking temeperature TB. Above TB, in the superparamagnetic regime, no coercivity or remanence is observed because the particles align freely with the field during the measuring time. Thermal energy at room temperature is sufficient to overcome the coercive field of the particles and thus the orientation of magnetization of isolated particles fluctuates in time. Cooling at zero magnetic field freezes the moments of individual nanoparticles into random orientations. When external field is applied, it energetically favors the moments of the individual particles to reorient with the field at low temperature, but there is no enough thermal 0.009 0.008 χ (emu g-1)
of CoPt3 colloid, three adsorption bands can be observed in the 2000–2060 cm1 range, as shown in Fig. 4b. The band at 2000 cm1 is attributed to CO coordinated on cobalt, as reported by OuldEly and co-workers [4]. CO coordinated on platinum causes the appearance of the band at 2059 cm1, as shown in the case of platinum colloid (see Fig. 4c). The band at 2022 cm1 is probably attributed to CO coordinated simultaneously on cobalt and platinum. In the case of cobalt colloid, however, there is no band appearance at 2000 cm1 (see Fig. 4a), which is probably the result of the oxidation of cobalt since cobalt nanoparticles are very sensitive to oxygen. Indeed, despite the careful preparation of the sample, oxygen contamination cannot be totally avoided during IR studies because the liquid film remain in air for measurements. It appears that both Pt and Co metal existed on the surface of as-synthesized Co–Pt bimetallic nanoparticles, which excludes the possibility of Co-core/Pt-shell structure formation. It could be presumed that Pt atoms deposited on the Co seed nanoparticles can easily penetrate into Co crystals to form the alloy phase. The results of magnetic measurements performed on powdered samples reveal that the assynthesized nanoparticles have a single domain structure and are superparamagnetic at room temperature. Representative plots of the tempera-
25
0.007 0.006
Tb
0.005 0.004 0.003
1.4 (a)
0.2 2300
2200
2100
χ (emu g-1)
1924
2059
2135
0.4
2000
1900
100 150 200 250 Temperature (K)
300
350
0
50
100 150 200 250 Temperature (K)
300
350
0.024
(b)
(c)
0.6
50
0.028
1923 1926
2059
0.8
2022 2000
1.0
2173
Transmittance (a.u.)
1.2
0 (a)
0.020 0.016 0.012 0.008
1800
Wavenumbers (cm-1) Fig. 4. Infrared spectra of carbonylated samples of (a) PVPprotected Co colloid, (b) PVP-protected CoPt3 colloid and (c) PVP-protected Pt colloid.
(b)
Fig. 5. ZFC/FC curves corresponding to (a) as-synthesized CoPt3 nanoparticles and (b) those after heat treatment at 500 1C for 2 h.
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energy to overcome the coercive field pinning the moments. As the sample is allowed to warm, the particles gain some thermal energy and begin to reorient their magnetic polarization with the external field and the total magnetization increases. At the blocking temperature, the magnetization reaches a maximum as thermal energy becomes comparable to the energy gained by aligning the particle magnetic vectors in the weak field and the magnetization drops with further heating. The annealed nanoparticles appear as a broad curve compared with as-synthesized sample, as shown in Fig. 5b. It possibly relates to the size distribution of the nanoparticles [4]. Fig. 6 shows 1.85 K hysteresis measurements from as-synthesized CoPt3 nanoparticles before and after annealing. The as-synthesized nanoparticles show a coercive field of 980 Oe, which is 8 HC =980 Oe
M (emu/g)
-10000
0
10000
20000
M (emu/g)
¼ 2Pt0 ðmetalÞ þ Naþ þ BðOC2 H5 Þ3 þ 7Hþ ; (3)
15
Pt4þ þ 2H2 ¼ Pt0 ðmetalÞ þ 4Hþ :
10
Produced Pt metal atoms penetrate into Co metals and form CoPt3 alloy,
5
3Pt þ Co ¼ CoPt3 : 0 -20000
(2)
2Pt4þ þ NaBH4 þ 3C2 H5 OH
-8 H (Oe)
-10000
0
10000
20000
-5 -10
(b)
NaBH4 þ 4C2 H5 OH ¼ NaOC2 H5 þ BðOC2 H5 Þ3 þ 4H2 ðgasÞ:
Then, platinum ions are reduced by both the excess of NaBH4 and hydrogen gas by-produced from Reaction (2):
-4
HC =920 Oe
It is first important to notice that the synthesis procedure consists of two stages: the first is the reduction of cobalt ion, and the second is the reduction of platinum ions. This process can be outlined by chemical equations as follows: Firstly, cobalt ions are reduced by NaBH4 according to Eq. (1) and hydrogen gas is byproduced according to Eq. (2): ¼ 4Co0 ðmetalÞ þ Naþ þ BðOC2 H5 Þ3 þ 7 Hþ ;(1)
0
(a)
4. Discussion
4Co2þ þ NaBH4 þ 3C2 H5 OH 4
-20000
much higher than that of CoPt3/Au (415 Oe at 10 K) prepared via microemulsion technique [18]. After annealing at 500 1C under argon for 2 h, the sample shows a decrease in coercivity to 920 Oe. This could be explained that the annealing will cause a reduction of the interparticle separation, a growth of gains and a release of interparticle strains [16].
-15 H (Oe)
Fig. 6. Hysteresis loops at 1.85 K of (a) as-synthesized CoPt3 nanoparticles and (b) after heat treatment at 500 1C for 2 h.
(4)
(5)
The coercivity of Co–Pt alloy nanoparticles prepared by this two-stage process is much higher than that of Co–Pt nanoparticles prepared by a coreduction method, implying that the sequence of reduction of metal ions plays a crucial role on the magnetic properties of nanoparticles in Co–Pt system. It should be mentioned that the atomic contents of platinum and cobalt in the products prepared by the two-stage process are in very close
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ratios to those initially used in the synthesis procedure, as indicated ICP AES measurements. The elaboration of bimetallic system of controlled average composition is thus possible according to this procedure. The first direct evidence for the presence of bimetallic nanoparticles results from the HRTEM study. Pure cobalt nanoparticles or pure platinum particles cannot be found by HRTEM. Another evidence is the structure studies. In all previous studies, platinum particles have been found nearly systematically FCC, whereas cobalt particles were seen to adopt a non-periodic structure. A typical FCC structure has been found for as-synthesized samples. An indirect evidence for the presence of bimetallic nanoparticles results from the study of carbon monoxide adsorbed on the surface of asprepared nanoparticles. Three adsorption bands in 2000–2060 cm1 range imply that cobalt and platinum atoms coexist on the surface of the nanoparticles. Another means to detect the presence of bimetallic nanoparticles is the study on the magnetic behavior of the materials. The coercivity of pure cobalt nanoparticles prepared by the same synthesis procedure can be found scarcely. The mechanism of alloy formation can be elaborated from crystal structure. As we know, there are only two stable crystal phases known for element cobalt at ambient pressure. The bulk hexagonal-close-packed (HCP) form is stable at temperature below 425 1C, while the bulk of FCC form is the stable structure at higher temperature. The crystal lattice constants (CLC) of HCP cobalt, a and c, are 0.2505 and 0.4060 nm, repectively (See JCPDS card, No. 5-727). Generally, only FCC structure is found for element platinum and its CLC is 0.3923 nm (See JCPDS card, No. 4-802). The value of c for element cobalt is larger than that of CLC for element platinum, which favors platinum atoms entering into cobalt bulk, whereas, it is hardly the case that cobalt atoms penetrate into platinum bulk. In our case, since cobalt is produced firstly as a bulk, platinum produced subsequently could enter into cobalt bulk and form an alloy phase. However, in the case of coreduction, where cobalt ions and platinum ions are reduced simultaneously, platinum ions are
27
reduced firstly and cobalt ions later by comparison of redox potentials of platinum and cobalt ions, indicating that the CoPt3 phase cannot form due to lack of penetration of cobalt atoms into platinum bulk. In this case, the easier oxidation of cobalt leads to much lower coercivity. The ratio of cobalt and platinum was selected 1:3 in our experiments because of the magnetic behavior of a series of Co–Pt nanoparticles. A series of Co–Pt nanoparticles with different metal ratios have been prepared with two-stage synthesis procedure. The results are not shown in this paper. The coercivities of these nanoparticles increase with increasing Pt concentration and get the maximum at Co:Pt ¼ 1:3. This is in good agreement with that of literatures reported [4,7]. It is obvious that the structure inside these bimetallic nanoparticles causes the different values of coercivities. However, it is clear that the investigation of the structure inside these bimetallic nanoparticles is a difficult task, especially in the case of element-specific methods such as EXAFS and WAXS.
5. Conclusions A two-stage route to synthesize CoPt3 nanoparticles by using NaBH4 has been presented. CoPt3 nanoparticles with average particle size of 2.6 nm formed a FCC alloy phase. SQUID studies revealed that as-synthesized nanoparticles were superparamagnetic at room temperature and ferromagnetic at 1.85 K with a coercivity of 980 Oe. Annealing at 500 1C caused an increase of particle size and a decrease of coercivity.
Acknowledgments This work is supported by Japan Society for the Promotion of Science (JSPS, ID: P02385) and Ministry of Education, Culture Sports, Science and Technology, Japan (MEXT to N.T. No. 15310678). The authors would like to thank Mr. Toru Mastushita for FT-IR measurement and Mr. Norihisa Watanabe for XRD and TEM measurements.
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References [1] [a] G. Schmid, Nanoparticles: From Theory to Application, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2004; [b] L.M. Liz-marzan, P.V. Kamat, Nanoscale Materials, Kluwer Academic Publishers, London, 2003; [c] K.J. Klabunde, Free Atoms, Clusters, and Nanoscale Particles, Academic Press, California, 1994. [2] [a] A.P. Alivisatos, Science 271 (1996) 933; [b] A.P. Alivisatos, J. Phys. Chem. 100 (1996) 13226. [3] S. Sun, C.B. Murray, D. Weller, L. Folks, A. Moser, Science 287 (2000) 1989. [4] T. Ould-Ely, C. Pan, C. Amiens, B. Chaudret, F. Dassenoy, P. Lecante, M.J. Casanove, A. Mosset, M. Respaud, J.M. Broto, J. Phys. Chem. B 104 (2000) 695. [5] S.H. Liou, S. Huang, E. Klimek, R.D. Kirby, Y.D. Yao, J. Appl. Phys. 85 (1999) 4334. [6] E.E. Carpenter, J.A. Sims, J.A. Wienmann, W.L. Zhou, C.J. O’Connor, J. Appl. Phys. 87 (2000) 5615. [7] E.E. Carpenter, C.T. Seip, C.J. O’Connor, J. Appl. Phys. 85 (1999) 5184. [8] T.A. Tyson, S.D. Conradson, R.F.C. Farrow, B.A. Jones, Phys. Rev. B 54 (1996) R3702. [9] M. Thielen, S. Kirsch, A. Weinforth, A. Carl, E.F. Wassermann, IEEE Trans. Magn. 34 (1998) 1009. [10] Y. Yamada, T. Suzuki, E.N. Abarra, IEEE Trans. Magn. 34 (1998) 343.
[11] J.A. Christodoulides, Y. Huang, Y. Zhang, G.C. Hadjipanayis, I. Panagiotopoulos, D. Niarchos, J. Appl. Phys. 87 (2000) 6938. [12] J.-I. Park, J. Cheon, J. Am. Chem. Soc. 123 (2001) 5743. [13] N.S. Sobal, U. Ebels, H. Mo¨hwald, M. Giersig, J. Phys. Chem. B 107 (2003) 7351. [14] L.V. Govor, G.H. Bauer, G. Reiter, E. Shevchenko, H. Weller, J. Parisi, Langmuir 19 (2003) 9573. [15] F. Wiekhorst, E. Shevchenko, H. Weller, J. Ko¨tzler, Phys. Rev. B 67 (2003) 224416. [16] C. Frommen, H. Ro¨sner, D. Fenske, J. Nanosci. Nanotechnol. 2 (2002) 509. [17] X. Zhang, K.Y. Chan, J. Mater. Chem. 12 (2002) 1203. [18] A. Kumbhar, L. Spinu, F. Agnoli, K.Y. Wang, W. Zhou, C.J. O’Connor, IEEE Trans. Magn. 37 (2001) 2216. [19] A.C.C. Yu, M. Mizuno, Y. Sasaki, H. Kondo, K. Hiraga, Appl. Phys. Lett. 81 (2002) 3768. [20] C.N. Chinnasamy, B. Jeyadevan, K. Shinoda, K. Tohji, J. Appl. Phys. 93 (2003) 7583. [21] E.V. Shevchenko, D.V. Talapin, A.L. Rogach, A. Kornowski, M. Haase, H. Weller, J. Am. Chem. Soc. 124 (2002) 11480. [22] C. Frommen, H. Ro¨sner, Mater. Lett. 58 (2003) 123. [23] Y. Wang, N. Toshima, J. Phys. Chem. B 101 (1997) 5301. [24] P. Lu, T. Teranishi, K. Asakura, M. Miyake, N. Toshima, J. Phys. Chem. B 103 (1999) 9673. [25] K. Hirakawa, N. Toshima, Chem. Lett. 32 (2003) 78.
Journal of Magnetism and Magnetic Materials 299 (2006) 21–28 www.elsevier.com/locate/jmmm
Preparation and characterization of Co–Pt bimetallic magnetic nanoparticles Xueyan Du, Makoto Inokuchi, Naoki Toshima Department of Materials Science and Environmental Engineering, Tokyo University of Science, Yamaguchi, Onoda-shi, Yamaguchi 756-0884, Japan Received 27 September 2004; received in revised form 26 October 2004 Available online 7 April 2005
Abstract CoPt3 nanoparticles are synthesized by a two-stage route using NaBH4 as a reductant. The nanoparticles are characterized by thermogravimetry (TG) and differential thermal analysis (DTA), Fourier transform infrared (FT-IR), transmission electron microscopy (TEM) and X-ray diffraction (XRD). Structural and spectroscopic studies show that the nanoparticles adopt a face-centered-cubic (FCC) crystalline structure with an average particle size of 2.6 nm. SQUID studies reveal that as-synthesized nanoparticles are superparamagnetic at room temperature and ferromagnetic at 1.85 K with coercivity of 980 Oe. Annealing of the samples at 500 1C causes an increase of particle size and a decrease of coercivity. r 2005 Elsevier B.V. All rights reserved. Keywords: Nanoparticles; Cobalt; Platinum; Magnetic properties
1. Introduction Nanoparticle-based materials have achieved much attention due to the novel electrooptic, magnetic and catalytic properties which arise from the quantum size effects and large surface areas that are characteristic of nanosized species [1,2]. In particular, magnetic nanosized materials attract Corresponding author. Tel.: +81836883500;
fax: +81836884567. E-mail address: [email protected] (X. Du).
growing interest because of their potential application to ultrahigh-density magnetic recording systems [3]. The magnetic and chemical properties of the monometallic materials are known to be significantly enhanced by the formation of alloys with additional metals. The magnetic alloys provide many advantages such as high magnetic anisotropy, enhanced magnetic susceptibility, and large coercivities [3–7]. Co–Pt alloy is one of the candidates for ultrahigh-density magnetic recording media because of their high magnetic anisotropy and good
0304-8853/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2005.03.013
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chemical stability upon corrosion [8–10]. Most of the magnetic alloy syntheses to date have been focused in the area of thin films by using vacuum deposition techniques [9–11]. However, random nucleation and growth of nanoparticles usually result in relatively large crystalline size and broad size distributions, which adversely affect their magnetic performance. Recent reports suggest that chemical approaches in solution, such as the decomposition of organometallic precursors [4,12–16], utilization of reverse micelles [7,17–19] and application of polyol reduction [20–22] are excellent alternative methods for better size control and self-assembly. For example, Sobal et al. [13] reported that monodispersed, bimetallic Pt–Co nanoparticles with a controllable core–shell structure have been synthesized by a two-stage procedure. The Pt–Co nanoparticles showed a narrow size distribution with a maximum of 7.6 nm. Pt–Co nanoparticles with an average size of 3–4 nm prepared using a water-in-oil reverse microemulsion was reported [17] showing high catalytic activity toward methanol oxidation. Shevchenko and co-wokers [21] prepared CoPt3 nanoparticles by reduction of platinum acetylacetonate and thermal decomposition of cobalt carbonyl in the presence of 1-adamantanecarboxylic acid. As-synthesized CoPt3 nanoparticles with the size of 1.5–7.2 nm distribution range have been assembled into 2D and 3D structures. However, the potential carcinogenicity of chemicals (i.e., cobalt carbonyl) could restrict their applications to production on a large scale. We have prepared various kinds of polymerprotected monometallic and bimetallic nanoparticles by chemical methods [23–25]. We have then tried to extend our approach to Co–Pt system which display interesting magnetic properties. In this paper, we present a two-stage procedure for the preparation of Co–Pt bimetallic nanoparticles just by using NaBH4 as a reductant. Pure Co particles with definite diameter were formed at first. Later, the reduction of platinum ions was performed in the presence of Co seeds. It is worthy of noting that the coercivity of nanoparticles prepared by a coreduction of Co and Pt ions is much lower than that of samples prepared by twostage procedure.
2. Experimental 2.1. Materials All chemicals were purchased from Wako Pure Chemical Industries Ltd. and used without further purification. Distilled and deionized water was used throughout. 2.2. Preparation of nanoparticles In nitrogen atmosphere, 100 mL of NaBH4 ethanol solution (0.066 M) was dropped into 100 mL CoCl2 ethanol solution (3.6 mM) containing 0.5000 g PVP under vigorous stirring at 0 1C. The color of the solution changed from blue to brown, indicating the formation of the suspension of cobalt nanoparticles. Then, 100 mL H2PtCl6 ethanol solution (10.8 mM) was dropped into the above suspension. After keeping vigorous stirring for 30 min, 100 mL of NaBH4 solution (0.066 M) was slowly added. The mixtures were kept stirring at 0 1C for 3 h. Afterwards, the colloid dispersions were filtered by an ultra-fine membrane filter in nitrogen atmosphere and washed by dehydrated ethanol for three times. The residual ethanol of Co/Pt colloid was removed by using of a rotary evaporator. Co/Pt powders were finally obtained by vacuum drying at 40 1C for 48 h. 2.3. Characterization X-ray diffraction (XRD) patterns of the samples were recorded on a Rigaku Rint 2400 diffractometer with Cu Ka radiation. The thermal decomposition behavior of assynthesized nanoparticles was examined by means of thermogravimetry (TG) and differential thermal analysis (DTA) using a Mac Science WS200-TGDTA instrument with the simultaneous recording of weight losses and temperature variations. These measurements were carried out at a heating rate of 10 1C min1 with a nitrogen gas flow. Transmission electron microscopy (TEM) and HRTEM measurement were performed on a Hitachi H-7000 microscope operated at 80 kV and a Hitachi H-9000 NAR microscope operated at 300 kV, respectively. Samples for these
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3. Results Fig. 1 shows the TG-DTA curves which display the thermal decomposition behavior of as-synthesized nanoparticles. The full line corresponds to the gravimetric thermal curve and the dashed line represents the differential scanning calorimetry curve. Two strong endothermic peaks appear from the room temperature to 800 1C. One endothermic peak around 85 1C is attributed to the departure of the remaining water molecule. The other one corresponds to the decomposition of organic stabilizer. The total mass loss is up to 48% during the entire heating cycle, implying the PVP is burned out since the initial content of PVP is about 48%. Thus, a temperature of 500 1C is selected as annealing temperature for ensuring the organic decomposition. XRD patterns of CoPt3 nanoparicles before and after heat treatment are given in Fig. 2. The samples are assigned to face-centered-cubic (FCC) CoPt3 crystal phase from the diffraction lines of
2.0 0 -10 1.0
-20
0.5
-30
DTA (µV/mg)
Weight loss (%)
1.5
0.0
-40 -50
415°C
85°C 0
-0.5
100 200 300 400 500 600 700 800 Temperature (°C )
Fig. 1. TG-DTA curves of the as-synthesized Co–Pt nanoparticles. These measurements were carried out at a heating rate of 10 1C min1 with nitrogen flow. Full line corresponds to the gravimetric thermal curve and dashed line represents the differential scanning calorimetry curve.
(111)
250
150
(220)
(200)
200 Intensity (a.u.)
measurements were prepared by depositing a drop of the dispersion in the mixed solvent of hexane and ethanol onto carbon-coated copper grids. The excess of solvent was wicked away with a filter paper, and the grids were dried in air. The average diameter and standard deviation of nanoparticles were measured from TEM images with at least 200 particles. Infrared spectra were recorded on a JIRWINSPEC 50 Fourier transform infrared (FTIR) spectrophotometer. The samples were prepared as follows: In a Fischer–Porter bottle, a dispersion of 100 mg of the colloidal product in 20 mL ethanol is submitted to 1 atm of carbon monoxide. After 20 h, the solution was used directly to measure as a liquid film in air. Magnetic measurements were undertaken with a Quantum Design MPMS SQUID magnetometer. The temperature was varied between 1.85 and 300 K according to a classical zero field cooling/ field cooling (ZFC/FC) procedure in the presence of a very weak applied magnetic field (100 Oe). Hysteresis measurements were performed in a magnetic field varying from +9000 to –9000 Oe.
23
(b) 100
50 (a) 0 10
20
30
40 50 60 2 Theta (degree)
70
80
Fig. 2. XRD pattern of CoPt3 nanoparticles (a) in the asprepared state and (b) after heat treatment at 500 1C for 2 h.
(1 1 1), (2 0 0) and (2 2 0) planes in the XRD profile. The width of peaks of CoPt3 nanoparticles after heat treatment becomes narrower than that of asprepared nanoparticles, indicating an increase in crystalline size after heat treatment. No peak appears near 521 and 611 which are attributed to cubic Co, indicating that as-synthesized nanoparticles are not mixture of Pt and Co metallic nanoparticles. The transition from the disordered FCC to the ordered FCT (L10) phase during heat
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treatment does not take place since the reflection near 331 cannot be observed in XRD profile, which is attributed to (1 1 0) reflection of L10 superlattice [22]. Fig. 3 shows the TEM micrographs of CoPt3 nanoparticles before and after heat treatment at 500 1C for 2 h, where spherical nanoparticles with average sizes of 2.6 and 3.3 nm, respectively, can be seen. More specifically, Fig. 3(b) shows polydispersity of larger size nanoparticles compared with Fig. 3(a), as is also evident from the corresponding histograms. This indicates that the annealing results in the increase of particle size, which is in good agreement with the results of XRD. The insetting HRTEM image clearly shows the lattice planes with the periodicity of 0.22 nm typical for a cubic crystallograghic CoPt3 structure
and platinum structure. In order to distinguish between CoPt3 and Pt structure, energy-dispersive X-ray (EDX) spectrometric analysis was performed. It is evident that the presence of both Pt and Co metal is qualitatively confirmed, implying pure Pt structure is ruled out in as-synthesized nanoparticles. EDX results show the ratio between Co and Pt was close to 1:3, which coincided well with the results of ICP AES analysis. Although the XRD pattern and HRTEM results are in good agreement with the formation of CoPt3 nanoparticles, core–shell structure cannot completely be excluded from these data. To get more evidence for bimetallic phase formation, we decided to use carbon monoxide as a probe of the surface state. Thus, carbon monoxide has been adsorbed on the surface of the samples. In the case
Distribution (%)
30
dav = 2.6 nm = 1.3 nm
20
10
0 0
2
(a)
4 6 8 Diameter (nm)
10
12
Distribution (%)
50 dav = 3.3 nm = 2.7 nm
40 30 20 10 0 0
(b)
2
4 6 8 Diameter (nm)
10
12
Fig. 3. TEM micrograph and size distribution of (a) the as-synthesized CoPt3 nanoparticles and (b) the samples annealed at 500 1C for 2 h. Inset shows HRTEM image.
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ture-dependent suscepitibility of the as-synthesized nanoparticles before and after heat treatment are shown in Fig. 5a. It can be found that there is a cusp in the ZFC suscepitibility at the blocking temeperature TB. Above TB, in the superparamagnetic regime, no coercivity or remanence is observed because the particles align freely with the field during the measuring time. Thermal energy at room temperature is sufficient to overcome the coercive field of the particles and thus the orientation of magnetization of isolated particles fluctuates in time. Cooling at zero magnetic field freezes the moments of individual nanoparticles into random orientations. When external field is applied, it energetically favors the moments of the individual particles to reorient with the field at low temperature, but there is no enough thermal 0.009 0.008 χ (emu g-1)
of CoPt3 colloid, three adsorption bands can be observed in the 2000–2060 cm1 range, as shown in Fig. 4b. The band at 2000 cm1 is attributed to CO coordinated on cobalt, as reported by OuldEly and co-workers [4]. CO coordinated on platinum causes the appearance of the band at 2059 cm1, as shown in the case of platinum colloid (see Fig. 4c). The band at 2022 cm1 is probably attributed to CO coordinated simultaneously on cobalt and platinum. In the case of cobalt colloid, however, there is no band appearance at 2000 cm1 (see Fig. 4a), which is probably the result of the oxidation of cobalt since cobalt nanoparticles are very sensitive to oxygen. Indeed, despite the careful preparation of the sample, oxygen contamination cannot be totally avoided during IR studies because the liquid film remain in air for measurements. It appears that both Pt and Co metal existed on the surface of as-synthesized Co–Pt bimetallic nanoparticles, which excludes the possibility of Co-core/Pt-shell structure formation. It could be presumed that Pt atoms deposited on the Co seed nanoparticles can easily penetrate into Co crystals to form the alloy phase. The results of magnetic measurements performed on powdered samples reveal that the assynthesized nanoparticles have a single domain structure and are superparamagnetic at room temperature. Representative plots of the tempera-
25
0.007 0.006
Tb
0.005 0.004 0.003
1.4 (a)
0.2 2300
2200
2100
χ (emu g-1)
1924
2059
2135
0.4
2000
1900
100 150 200 250 Temperature (K)
300
350
0
50
100 150 200 250 Temperature (K)
300
350
0.024
(b)
(c)
0.6
50
0.028
1923 1926
2059
0.8
2022 2000
1.0
2173
Transmittance (a.u.)
1.2
0 (a)
0.020 0.016 0.012 0.008
1800
Wavenumbers (cm-1) Fig. 4. Infrared spectra of carbonylated samples of (a) PVPprotected Co colloid, (b) PVP-protected CoPt3 colloid and (c) PVP-protected Pt colloid.
(b)
Fig. 5. ZFC/FC curves corresponding to (a) as-synthesized CoPt3 nanoparticles and (b) those after heat treatment at 500 1C for 2 h.
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energy to overcome the coercive field pinning the moments. As the sample is allowed to warm, the particles gain some thermal energy and begin to reorient their magnetic polarization with the external field and the total magnetization increases. At the blocking temperature, the magnetization reaches a maximum as thermal energy becomes comparable to the energy gained by aligning the particle magnetic vectors in the weak field and the magnetization drops with further heating. The annealed nanoparticles appear as a broad curve compared with as-synthesized sample, as shown in Fig. 5b. It possibly relates to the size distribution of the nanoparticles [4]. Fig. 6 shows 1.85 K hysteresis measurements from as-synthesized CoPt3 nanoparticles before and after annealing. The as-synthesized nanoparticles show a coercive field of 980 Oe, which is 8 HC =980 Oe
M (emu/g)
-10000
0
10000
20000
M (emu/g)
¼ 2Pt0 ðmetalÞ þ Naþ þ BðOC2 H5 Þ3 þ 7Hþ ; (3)
15
Pt4þ þ 2H2 ¼ Pt0 ðmetalÞ þ 4Hþ :
10
Produced Pt metal atoms penetrate into Co metals and form CoPt3 alloy,
5
3Pt þ Co ¼ CoPt3 : 0 -20000
(2)
2Pt4þ þ NaBH4 þ 3C2 H5 OH
-8 H (Oe)
-10000
0
10000
20000
-5 -10
(b)
NaBH4 þ 4C2 H5 OH ¼ NaOC2 H5 þ BðOC2 H5 Þ3 þ 4H2 ðgasÞ:
Then, platinum ions are reduced by both the excess of NaBH4 and hydrogen gas by-produced from Reaction (2):
-4
HC =920 Oe
It is first important to notice that the synthesis procedure consists of two stages: the first is the reduction of cobalt ion, and the second is the reduction of platinum ions. This process can be outlined by chemical equations as follows: Firstly, cobalt ions are reduced by NaBH4 according to Eq. (1) and hydrogen gas is byproduced according to Eq. (2): ¼ 4Co0 ðmetalÞ þ Naþ þ BðOC2 H5 Þ3 þ 7 Hþ ;(1)
0
(a)
4. Discussion
4Co2þ þ NaBH4 þ 3C2 H5 OH 4
-20000
much higher than that of CoPt3/Au (415 Oe at 10 K) prepared via microemulsion technique [18]. After annealing at 500 1C under argon for 2 h, the sample shows a decrease in coercivity to 920 Oe. This could be explained that the annealing will cause a reduction of the interparticle separation, a growth of gains and a release of interparticle strains [16].
-15 H (Oe)
Fig. 6. Hysteresis loops at 1.85 K of (a) as-synthesized CoPt3 nanoparticles and (b) after heat treatment at 500 1C for 2 h.
(4)
(5)
The coercivity of Co–Pt alloy nanoparticles prepared by this two-stage process is much higher than that of Co–Pt nanoparticles prepared by a coreduction method, implying that the sequence of reduction of metal ions plays a crucial role on the magnetic properties of nanoparticles in Co–Pt system. It should be mentioned that the atomic contents of platinum and cobalt in the products prepared by the two-stage process are in very close
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ratios to those initially used in the synthesis procedure, as indicated ICP AES measurements. The elaboration of bimetallic system of controlled average composition is thus possible according to this procedure. The first direct evidence for the presence of bimetallic nanoparticles results from the HRTEM study. Pure cobalt nanoparticles or pure platinum particles cannot be found by HRTEM. Another evidence is the structure studies. In all previous studies, platinum particles have been found nearly systematically FCC, whereas cobalt particles were seen to adopt a non-periodic structure. A typical FCC structure has been found for as-synthesized samples. An indirect evidence for the presence of bimetallic nanoparticles results from the study of carbon monoxide adsorbed on the surface of asprepared nanoparticles. Three adsorption bands in 2000–2060 cm1 range imply that cobalt and platinum atoms coexist on the surface of the nanoparticles. Another means to detect the presence of bimetallic nanoparticles is the study on the magnetic behavior of the materials. The coercivity of pure cobalt nanoparticles prepared by the same synthesis procedure can be found scarcely. The mechanism of alloy formation can be elaborated from crystal structure. As we know, there are only two stable crystal phases known for element cobalt at ambient pressure. The bulk hexagonal-close-packed (HCP) form is stable at temperature below 425 1C, while the bulk of FCC form is the stable structure at higher temperature. The crystal lattice constants (CLC) of HCP cobalt, a and c, are 0.2505 and 0.4060 nm, repectively (See JCPDS card, No. 5-727). Generally, only FCC structure is found for element platinum and its CLC is 0.3923 nm (See JCPDS card, No. 4-802). The value of c for element cobalt is larger than that of CLC for element platinum, which favors platinum atoms entering into cobalt bulk, whereas, it is hardly the case that cobalt atoms penetrate into platinum bulk. In our case, since cobalt is produced firstly as a bulk, platinum produced subsequently could enter into cobalt bulk and form an alloy phase. However, in the case of coreduction, where cobalt ions and platinum ions are reduced simultaneously, platinum ions are
27
reduced firstly and cobalt ions later by comparison of redox potentials of platinum and cobalt ions, indicating that the CoPt3 phase cannot form due to lack of penetration of cobalt atoms into platinum bulk. In this case, the easier oxidation of cobalt leads to much lower coercivity. The ratio of cobalt and platinum was selected 1:3 in our experiments because of the magnetic behavior of a series of Co–Pt nanoparticles. A series of Co–Pt nanoparticles with different metal ratios have been prepared with two-stage synthesis procedure. The results are not shown in this paper. The coercivities of these nanoparticles increase with increasing Pt concentration and get the maximum at Co:Pt ¼ 1:3. This is in good agreement with that of literatures reported [4,7]. It is obvious that the structure inside these bimetallic nanoparticles causes the different values of coercivities. However, it is clear that the investigation of the structure inside these bimetallic nanoparticles is a difficult task, especially in the case of element-specific methods such as EXAFS and WAXS.
5. Conclusions A two-stage route to synthesize CoPt3 nanoparticles by using NaBH4 has been presented. CoPt3 nanoparticles with average particle size of 2.6 nm formed a FCC alloy phase. SQUID studies revealed that as-synthesized nanoparticles were superparamagnetic at room temperature and ferromagnetic at 1.85 K with a coercivity of 980 Oe. Annealing at 500 1C caused an increase of particle size and a decrease of coercivity.
Acknowledgments This work is supported by Japan Society for the Promotion of Science (JSPS, ID: P02385) and Ministry of Education, Culture Sports, Science and Technology, Japan (MEXT to N.T. No. 15310678). The authors would like to thank Mr. Toru Mastushita for FT-IR measurement and Mr. Norihisa Watanabe for XRD and TEM measurements.
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