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Preparation of silica-coated Co–Pt alloy nanoparticles
Materials Letters 60 (2006) 2046 – 2049 www.elsevier.com/locate/matlet
Preparation of silica-coated Co–Pt alloy nanoparticles Yoshio Kobayashi a , Mitsuru Horie a , Daisuke Nagao a , Yasuo Ando b , Terunobu Miyazaki b , Mikio Konno a,⁎ a
Department of Chemical Engineering, Graduate School of Engineering, Tohoku University, 6-6-07 Aoba, Aramaki-aza, Aoba-ku, Sendai 980-8579, Japan b Department of Applied Physics, Graduate School of Engineering, Tohoku University, 6-6-05 Aoba, Aramaki-aza, Aoba-ku, Sendai 980-8579, Japan Received 20 February 2005; accepted 22 December 2005 Available online 13 January 2006
Abstract A previously proposed method for silica-coating of Co nanoparticles was extended to silica-coating of Co–Pt alloy nanoparticles. The alloy nanoparticles were prepared from CoCl2 (1.2 × 10− 3 M), H2PtCl6 (1.2 × 10− 3 M), citric acid (4 × 10− 4 M) and NaBH4 (1.2 × 10− 2 M). The silicacoating was performed in aqueous/ethanolic solution with a silane coupling agent, 3-aminopropyltrimethoxysilane (8 × 10− 5 M), and a silica source, tetraethoxyorthosilicate (7.2 × 10− 4 M) in the presence of the alloy nanoparticles. TEM observation and X-ray diffraction measurements revealed that the alloy nanoparticles were coated with silica and consisted of fcc Co–Pt alloy crystallites with crystal sizes of 4.6–10.7 nm. Magnetic properties of the silica-coated particles were strongly dependent on annealing temperature. Maximum values of 1.4 emu/g-sample for saturation magnetization and 450 Oe for coercive field were obtained for the particles annealed at 300 °C. Annealing above 300 °C transformed the crystallite from the alloy to pure Pt probably through oxidation of cobalt in the alloy, resulting in reductions in saturation magnetization and coercive field. © 2005 Elsevier B.V. All rights reserved. Keywords: Composite materials; Magnetic materials; Metals and alloys; Nanocomposites; Nanomaterials; Sol–gel preparation
1. Introduction Magnetic nanoparticles are applied to various materials such as magnetic inks [1], isolating device [2], model materials for interparticle interactions [3], and recording media [4]. Some of the most representative magnetic materials are metal oxides such as various ferrites [5–8]. Apart from the magnetic metal oxides, pure metals such as Fe, Co and Ni are also used in various fields of magnetism [9,10]. However, the main difficulty for the use of pure metal nanoparticles arises from their instability toward oxidation in air, which easily occurs as their size becomes small. On the other hand, metal alloys have high chemical stability [11], strong perpendicular magnetic anisotropy [12] and suitability for magnetooptical storage media [13–17]. Particularly, the Co–Pt alloy is a candidate for ultrahigh-density magnetic recording media because of its high magnetic anisotropy and good chemical stability to oxidation [18–22]. ⁎ Corresponding author. Tel.: +81 22 795 7239; fax: +81 22 795 7241. E-mail address: [email protected] (M. Konno). 0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.12.078
Extensive researches have been performed for the synthesis and characterization of Co–Pt alloy nanoparticles. The Co–Pt particles are often prepared from organometallic compounds such as cobalt carbonyl or metal complexes such as platinum acetylacetonate as a suspension in organic phases [23–25]. However, the use of such organic compounds is harmful and hazardous. Deposition of silica shells on magnetic particles has been employed by several authors [26–32]. This method has the advantages: exceptional stability of aqueous dispersions, easy surface modification that allows the preparation of non-aqueous colloids and easy control of interparticle interactions. We have recently developed [33] a technique for silica-coating of Co nanoparticles prepared through reduction of cobalt inorganic salts in aqueous solution. In the present paper, we propose a silica-coating technique that allows encapsulation of Co–Pt nanoparticles prepared from aqueous solutions of inorganic salts. Ions of Co and Pt were reduced with NaBH4 in the presence of citric acid stabilizer to generate Co–Pt metallic nanoparticles, which were silica-coated by a sol–gel method after surface modification with a silane
Y. Kobayashi et al. / Materials Letters 60 (2006) 2046–2049
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Fig. 1. TEM images of CoPt@SiO2 particles as-prepared (a) and annealed at 500 °C (b).
coupling agent. The silica-coated particles were annealed at various temperatures for measurements of magnetic properties.
Fig. 3. Crystal size of CoPt@SiO2 particles as a function of annealing temperature.
2. Experimental Cobalt chloride hexahydrate (Wako Pure Chem. Ind., 99.5%) and hydrogen hexachloroplatinate (IV) hexahydrate (Wako Pure Chem. Ind., 98.5%) were used as cobalt and platinum precursors, respectively. Sodium borohydride (Wako Pure Chem. Ind., 90%) and citric acid monohydrate (Wako Pure Chem. Ind., 99.5%) were used as reducing and stabilizing reagents for preparation of Co–Pt nanoparticles, respectively. Special grade reagents of 3-aminopropyl-trimethoxysilane (APS) (Aldrich, 97%), tetraethylorthosilicate (TEOS) (Wako Pure Chem. Ind., 95%) and ethanol (Wako Pure Chem. Ind., 99.5%) were used for silica-coating. All the chemicals were used as received. Ultrapure deionized water (resistivity higher than 18 MΩ cm) was used in all the preparations. Preparation of Co–Pt particles was performed with a method similar to the previous paper [33]. An aqueous solution (0.6 ml) containing CoCl2 (0.4 M) and H2PtCl6 (0.4 M) was added to 200 mL of 1.2 × 10− 2 M NaBH4 in a deaerated aqueous solution containing citric acid (1.2 × 10− 2 M), and particle formation
Fig. 2. XRD patterns of CoPt@SiO2 particles annealed in air for 2 h. Annealing temperatures are written above each pattern. Attribution of the peaks is indicated by the symbols: (●) Co–Pt alloy (cubic) and (○) Co3O4 (cubic).
gave rise to a gray coloration of the solution. Nitrogen bubbling was maintained during the reactions. Silica coating was performed by addition of 800 ml of ethanolic solution containing 14.4 μL of APS and 169 μL of TEOS (in which a molar ratio of APS to TEOS was 1 : 9) to 200 mL of the aqueous Co–Pt colloid, 1 min after mixing the CoCl2–H2PtCl6 and NaBH4 solutions. In the water/ethanol mixture, adsorption of APS onto the surface of Co–Pt particles should take place, followed by hydrolysis/condensation of both APS and TEOS that results in the formation of the silica shells. For crystallization, the silica-coated Co–Pt particles were separated from the solution with centrifugation, dried and annealed in air at temperatures between 200 and 600 °C for 2 h. The samples were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and vibrating sample magnetometry (VSM). A Zeiss LEO 912 OMEGA microscope was used for TEM at 100 kV of accelerating voltage. The samples for TEM were prepared by dropping and evaporating the obtained silica-coated Co–Pt colloid on top of a collodion-coated copper grid. XRD measurements were made with a Rigaku RU-200A diffractometer at 40 kV and 30 mA
Fig. 4. d-Spacing of (111) peak of CoPt@SiO2 particles as a function of annealing temperature.
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Fig. 5. Saturation magnetization and coercive field of CoPt@SiO2 particles annealed in air for 2 h at various temperatures.
with CuKα radiation. VSM measurements of silica-coated Co– Pt powder were made at room temperature with a Toei VSM-515 magnetometer. 3. Results and discussion Fig. 1(a) shows TEM image of as-prepared silica-coated Co–Pt particles. Darker and lighter parts of particles were Co–Pt and silica, respectively. Multiple cores were observed in silica-coated particles formed. Fig. 2 shows XRD pattern of silica-coated Co–Pt particles annealed at various temperatures. The XRD pattern of the as-prepared silicacoated Co–Pt particles showed several peaks at 39.96°, 46.41° and 67.86°, which corresponded to d-values of 0.2254, 0.1955 and 0.1380 nm, respectively. These d-values were between those of Pt (0.2265 (111), 0.1962 (200) and 0.1387 (220) nm) (JCPDS card No. 4802) and fcc CoPt3 (0.2224 (111), 0.1927 (200) and 0.13627 (220) nm) (JCPDS card No. 29-499), which indicated that the core particles were fcc Co–Pt alloy or pure Pt metal. The XRD patterns of the particles annealed at 200–600 °C showed profiles similar to the as-prepared particles, though peak positions and peak widths slightly changed with annealing. Fig. 3 shows the average crystal sizes estimated from the broadening of (111) peak as a function of annealing temperature. The as-prepared silica-coated Co–Pt particles had an average crystal size of 4.6 nm. The average crystal size increased from 5.5 to 10.7 nm with an increase in the annealing temperature from 200 to 600 °C, which indicated crystal growth due to the annealing. Fig. 4 shows d-value for (111) peak position versus annealing temperature. The d-value increased with an increase in the annealing temperature. For the annealing at 600 °C, the d-value was almost the same as 0.2265 nm that is the d-value of pure Pt (111). These results indicated that cobalt atoms of the Co–Pt alloy was probably oxidized in air that diffused through the silica shell and/or by oxygen atoms that composed the silica. Consequently the Co–Pt alloy was transformed to pure Pt metal. For the particles annealed at 500 and 600 °C, peaks attributed to Co3O4 (cubic) were also appeared around 37.3°, 59.1° and 65.1° (Fig. 2). The transformation from Co–Pt alloy to Pt might have been accelerated at high temperature. In our previous work for silica-coated cobalt particles [33], formation of Co3O4 by annealing in air was strongly related to their synthesis conditions. In the present work, it was speculated that the Pt formation provided acceleration of the Co3O4 formation interdependently, though precise reason for the acceleration
mechanism is still unclear. TEM micrograph of particles annealed in air at 500 °C is shown in Fig. 1(b). The annealing led to spread of a slightly darker area. Taking the XRD results into account, the TEM observation implied that cobalt oxide diffused toward the silica shell and consequently Pt metal stayed at the center of particles. Fig. 5 shows the room temperature saturation magnetization and coercive field of the silica-coated Co–Pt particles as a function of annealing temperature. The magnetization was expressed in units of emu per gram of powder. The saturation magnetization and the coercive field increased with an increase in the annealing temperature up to 300 °C, because of high crystallinity provided with the annealing. Maximum values of the saturation magnetization (1.4 emu/g-sample) and the coercive field (450 Oe) were obtained for the 300 °C-annealed particles. Decreases in the saturation magnetization and the coercive field were observed above an annealing temperature of 300 °C. This temperature was almost in good agreement with the annealing temperature above which the (111) peak position remarkably shifted to lower degree. The annealing provided transformation from the Co– Pt alloy to the Pt metal that did not show any magnetic properties, which led to the decreases in the saturation magnetization and the coercive field.
4. Summary In the present work, we could prepare colloidally stabilized magnetic cobalt–platinum alloy nanoparticles surrounded by uniform silica shells. Measurement of magnetic properties showed that the silica-coated particles annealed at 300 °C had a saturation magnetization of 1.4 emu/g-sample and a coercive field of 450 Oe. Annealing above 300 °C provided the transformation from cobalt–platinum alloy to pure metal due to oxidation of cobalt in the alloy, which led to reductions in saturation magnetization and coercive field. Acknowledgements This research was partially supported by a Grant-in-Aid for Science Research (No. 15656163) and by the COE project, Giant Molecules and Complex Systems from the Ministry of Education, Culture, Sports, Science and Technology of Japan. References [1] R. Chandrasekhar, S.W. Charles, K. O'Grady, J. Imag. Technol. 3 (1987) 55. [2] K. Nakatsuka, H. Yokoyama, J. Shimoiizaka, T. Funaki, J. Magn. Magn. Mater 65 (1987) 359. [3] L.N. Donselaar, A.P. Philipse, J. Colloid Interface Sci. 212 (1999) 14. [4] A. Yoshizawa, Y. Nakane, N. Arisawa, H. Imaizumi, K. Sugino, S. Harada, IEEE Trans. Magn. MAG-23 (1987) 2874. [5] R. Massart, IEEE Trans. Magn. MAG-17 (1981) 1247. [6] S. Neveu, A. Bee, M. Robineau, D. Talbot, J. Colloid Interface Sci. 255 (2002) 293. [7] F. Grasset, N. Labhsetwar, D. Li, D.C. Park, N. Saito, H. Haneda, O. Cador, T. Roisnel, S. Mornet, E. Duguet, J. Portier, J. Etourneau, Langmuir 18 (2002) 8209. [8] S. Sun, H. Zeng, J. Am. Chem. Soc. 124 (2002) 8204. [9] M. Giersig, M. Hilgendorff, J. Phys. D: Appl. Phys. 32 (1999) L111. [10] S. Sun, C.D. Murray, D. Weller, L. Folks, A. Moser, Science 287 (2000) 1989. [11] T.A. Tyson, S.D. Conradson, R.F.C. Farrow, B.A. Jones, Phys. Rev., B 54 (1996) R3702.
Y. Kobayashi et al. / Materials Letters 60 (2006) 2046–2049 [12] D. Weller, H. Brändle, C. Chappert, J. Magn. Magn. Mater. 121 (1993) 461. [13] W. Grange, M. Maret, J.-P. Kappler, J. Vogel, A. Fontaine, F. Petroff, G. Krill, A. Rogalev, J. Coulon, M. Finazzi, N. Brookes, Phys. Rev., B 58 (1998) 6298. [14] D. Weller, H. Brändle, G. Gorman, C.-J. Lin, H. Notarys, Appl. Phys. Lett. 61 (1992) 2726. [15] C.-J. Lin, G.L. Gorman, Appl. Phys. Lett. 61 (1992) 1600. [16] A.L. Shapiro, P.W. Rooney, M.Q. Tran, F. Hellman, K.M. Ring, K.L. Kavanagh, B. Rellinghaus, D. Weller, Phys. Rev., B 60 (1999) 12826. [17] G. Chang, Y. Lee, J. Rhee, J. Lee, K. Jeong, C. Whang, Phys. Rev. Lett. 87 (2001) 067208. [18] O. 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. [19] E.E. Carpenter, C.T. Seip, C.J. O’Connor, J. Appl. Phys. 85 (1999) 5184. [20] S.H. Liou, S. Huang, E. Klimek, R.D. Kirby, Y.D. Yao, J. Appl. Phys. 85 (1999) 4334. [21] M. Thielen, S. Kirsch, A. Weinforth, A. Carl, E.F. Wassermann, IEEE Trans. Magn. 34 (1998) 1009.
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[22] Y. Yamada, T. Suzuki, E.N. Abarra, IEEE Trans. Magn. 34 (1998) 343. [23] E.V. Shevchenko, D.V. Talapin, A.L. Rogach, A. Kornowski, M. Haase, H. Weller, J. Am. Chem. Soc. 124 (2002) 11480. [24] J.-I. Park, J. Cheon, J. Am. Chem. Soc. 123 (2001) 5743. [25] Z. Zhang, D.A. Blom, Z. Gai, J.R. Thompson, J. Shen, S. Dai, J. Am. Chem. Soc. 125 (2003) 7528. [26] M. Ohmori, E. MatijeviÇ, J. Colloid Interface Sci. 150 (1992) 594. [27] M. Ohmori, E. MatijeviÇ, J. Colloid Interface Sci. 160 (1993) 288. [28] A.P. Philipse, M.P.B. van Bruggen, C. Pathmamanoharan, Langmuir 10 (1994) 92. [29] M.A. Correa-Duarte, M. Giersig, N.A. Kotov, L.M. Liz-Marzán, Langmuir 14 (1998) 6430. [30] Q. Liu, Z. Xu, J.A. Finch, R. Egerton, Chem. Mater. 10 (1998) 3938. [31] T. Tago, T. Hatsuta, K. Miyajima, M. Kishida, S. Tashiro, K. Wakabayashi, J. Am. Ceram. Soc. 85 (2002) 2188. [32] Y. Lu, Y. Yin, B.T. Mayers, Y. Xia, Nano Lett. 1 (2002) 183. [33] Y. Kobayashi, M. Horie, M. Konno, B. Rodríguez-González, L.M. LizMarzán, J. Phys. Chem., B 107 (2003) 7420.
Preparation of silica-coated Co–Pt alloy nanoparticles Yoshio Kobayashi a , Mitsuru Horie a , Daisuke Nagao a , Yasuo Ando b , Terunobu Miyazaki b , Mikio Konno a,⁎ a
Department of Chemical Engineering, Graduate School of Engineering, Tohoku University, 6-6-07 Aoba, Aramaki-aza, Aoba-ku, Sendai 980-8579, Japan b Department of Applied Physics, Graduate School of Engineering, Tohoku University, 6-6-05 Aoba, Aramaki-aza, Aoba-ku, Sendai 980-8579, Japan Received 20 February 2005; accepted 22 December 2005 Available online 13 January 2006
Abstract A previously proposed method for silica-coating of Co nanoparticles was extended to silica-coating of Co–Pt alloy nanoparticles. The alloy nanoparticles were prepared from CoCl2 (1.2 × 10− 3 M), H2PtCl6 (1.2 × 10− 3 M), citric acid (4 × 10− 4 M) and NaBH4 (1.2 × 10− 2 M). The silicacoating was performed in aqueous/ethanolic solution with a silane coupling agent, 3-aminopropyltrimethoxysilane (8 × 10− 5 M), and a silica source, tetraethoxyorthosilicate (7.2 × 10− 4 M) in the presence of the alloy nanoparticles. TEM observation and X-ray diffraction measurements revealed that the alloy nanoparticles were coated with silica and consisted of fcc Co–Pt alloy crystallites with crystal sizes of 4.6–10.7 nm. Magnetic properties of the silica-coated particles were strongly dependent on annealing temperature. Maximum values of 1.4 emu/g-sample for saturation magnetization and 450 Oe for coercive field were obtained for the particles annealed at 300 °C. Annealing above 300 °C transformed the crystallite from the alloy to pure Pt probably through oxidation of cobalt in the alloy, resulting in reductions in saturation magnetization and coercive field. © 2005 Elsevier B.V. All rights reserved. Keywords: Composite materials; Magnetic materials; Metals and alloys; Nanocomposites; Nanomaterials; Sol–gel preparation
1. Introduction Magnetic nanoparticles are applied to various materials such as magnetic inks [1], isolating device [2], model materials for interparticle interactions [3], and recording media [4]. Some of the most representative magnetic materials are metal oxides such as various ferrites [5–8]. Apart from the magnetic metal oxides, pure metals such as Fe, Co and Ni are also used in various fields of magnetism [9,10]. However, the main difficulty for the use of pure metal nanoparticles arises from their instability toward oxidation in air, which easily occurs as their size becomes small. On the other hand, metal alloys have high chemical stability [11], strong perpendicular magnetic anisotropy [12] and suitability for magnetooptical storage media [13–17]. Particularly, the Co–Pt alloy is a candidate for ultrahigh-density magnetic recording media because of its high magnetic anisotropy and good chemical stability to oxidation [18–22]. ⁎ Corresponding author. Tel.: +81 22 795 7239; fax: +81 22 795 7241. E-mail address: [email protected] (M. Konno). 0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.12.078
Extensive researches have been performed for the synthesis and characterization of Co–Pt alloy nanoparticles. The Co–Pt particles are often prepared from organometallic compounds such as cobalt carbonyl or metal complexes such as platinum acetylacetonate as a suspension in organic phases [23–25]. However, the use of such organic compounds is harmful and hazardous. Deposition of silica shells on magnetic particles has been employed by several authors [26–32]. This method has the advantages: exceptional stability of aqueous dispersions, easy surface modification that allows the preparation of non-aqueous colloids and easy control of interparticle interactions. We have recently developed [33] a technique for silica-coating of Co nanoparticles prepared through reduction of cobalt inorganic salts in aqueous solution. In the present paper, we propose a silica-coating technique that allows encapsulation of Co–Pt nanoparticles prepared from aqueous solutions of inorganic salts. Ions of Co and Pt were reduced with NaBH4 in the presence of citric acid stabilizer to generate Co–Pt metallic nanoparticles, which were silica-coated by a sol–gel method after surface modification with a silane
Y. Kobayashi et al. / Materials Letters 60 (2006) 2046–2049
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Fig. 1. TEM images of CoPt@SiO2 particles as-prepared (a) and annealed at 500 °C (b).
coupling agent. The silica-coated particles were annealed at various temperatures for measurements of magnetic properties.
Fig. 3. Crystal size of CoPt@SiO2 particles as a function of annealing temperature.
2. Experimental Cobalt chloride hexahydrate (Wako Pure Chem. Ind., 99.5%) and hydrogen hexachloroplatinate (IV) hexahydrate (Wako Pure Chem. Ind., 98.5%) were used as cobalt and platinum precursors, respectively. Sodium borohydride (Wako Pure Chem. Ind., 90%) and citric acid monohydrate (Wako Pure Chem. Ind., 99.5%) were used as reducing and stabilizing reagents for preparation of Co–Pt nanoparticles, respectively. Special grade reagents of 3-aminopropyl-trimethoxysilane (APS) (Aldrich, 97%), tetraethylorthosilicate (TEOS) (Wako Pure Chem. Ind., 95%) and ethanol (Wako Pure Chem. Ind., 99.5%) were used for silica-coating. All the chemicals were used as received. Ultrapure deionized water (resistivity higher than 18 MΩ cm) was used in all the preparations. Preparation of Co–Pt particles was performed with a method similar to the previous paper [33]. An aqueous solution (0.6 ml) containing CoCl2 (0.4 M) and H2PtCl6 (0.4 M) was added to 200 mL of 1.2 × 10− 2 M NaBH4 in a deaerated aqueous solution containing citric acid (1.2 × 10− 2 M), and particle formation
Fig. 2. XRD patterns of CoPt@SiO2 particles annealed in air for 2 h. Annealing temperatures are written above each pattern. Attribution of the peaks is indicated by the symbols: (●) Co–Pt alloy (cubic) and (○) Co3O4 (cubic).
gave rise to a gray coloration of the solution. Nitrogen bubbling was maintained during the reactions. Silica coating was performed by addition of 800 ml of ethanolic solution containing 14.4 μL of APS and 169 μL of TEOS (in which a molar ratio of APS to TEOS was 1 : 9) to 200 mL of the aqueous Co–Pt colloid, 1 min after mixing the CoCl2–H2PtCl6 and NaBH4 solutions. In the water/ethanol mixture, adsorption of APS onto the surface of Co–Pt particles should take place, followed by hydrolysis/condensation of both APS and TEOS that results in the formation of the silica shells. For crystallization, the silica-coated Co–Pt particles were separated from the solution with centrifugation, dried and annealed in air at temperatures between 200 and 600 °C for 2 h. The samples were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and vibrating sample magnetometry (VSM). A Zeiss LEO 912 OMEGA microscope was used for TEM at 100 kV of accelerating voltage. The samples for TEM were prepared by dropping and evaporating the obtained silica-coated Co–Pt colloid on top of a collodion-coated copper grid. XRD measurements were made with a Rigaku RU-200A diffractometer at 40 kV and 30 mA
Fig. 4. d-Spacing of (111) peak of CoPt@SiO2 particles as a function of annealing temperature.
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Fig. 5. Saturation magnetization and coercive field of CoPt@SiO2 particles annealed in air for 2 h at various temperatures.
with CuKα radiation. VSM measurements of silica-coated Co– Pt powder were made at room temperature with a Toei VSM-515 magnetometer. 3. Results and discussion Fig. 1(a) shows TEM image of as-prepared silica-coated Co–Pt particles. Darker and lighter parts of particles were Co–Pt and silica, respectively. Multiple cores were observed in silica-coated particles formed. Fig. 2 shows XRD pattern of silica-coated Co–Pt particles annealed at various temperatures. The XRD pattern of the as-prepared silicacoated Co–Pt particles showed several peaks at 39.96°, 46.41° and 67.86°, which corresponded to d-values of 0.2254, 0.1955 and 0.1380 nm, respectively. These d-values were between those of Pt (0.2265 (111), 0.1962 (200) and 0.1387 (220) nm) (JCPDS card No. 4802) and fcc CoPt3 (0.2224 (111), 0.1927 (200) and 0.13627 (220) nm) (JCPDS card No. 29-499), which indicated that the core particles were fcc Co–Pt alloy or pure Pt metal. The XRD patterns of the particles annealed at 200–600 °C showed profiles similar to the as-prepared particles, though peak positions and peak widths slightly changed with annealing. Fig. 3 shows the average crystal sizes estimated from the broadening of (111) peak as a function of annealing temperature. The as-prepared silica-coated Co–Pt particles had an average crystal size of 4.6 nm. The average crystal size increased from 5.5 to 10.7 nm with an increase in the annealing temperature from 200 to 600 °C, which indicated crystal growth due to the annealing. Fig. 4 shows d-value for (111) peak position versus annealing temperature. The d-value increased with an increase in the annealing temperature. For the annealing at 600 °C, the d-value was almost the same as 0.2265 nm that is the d-value of pure Pt (111). These results indicated that cobalt atoms of the Co–Pt alloy was probably oxidized in air that diffused through the silica shell and/or by oxygen atoms that composed the silica. Consequently the Co–Pt alloy was transformed to pure Pt metal. For the particles annealed at 500 and 600 °C, peaks attributed to Co3O4 (cubic) were also appeared around 37.3°, 59.1° and 65.1° (Fig. 2). The transformation from Co–Pt alloy to Pt might have been accelerated at high temperature. In our previous work for silica-coated cobalt particles [33], formation of Co3O4 by annealing in air was strongly related to their synthesis conditions. In the present work, it was speculated that the Pt formation provided acceleration of the Co3O4 formation interdependently, though precise reason for the acceleration
mechanism is still unclear. TEM micrograph of particles annealed in air at 500 °C is shown in Fig. 1(b). The annealing led to spread of a slightly darker area. Taking the XRD results into account, the TEM observation implied that cobalt oxide diffused toward the silica shell and consequently Pt metal stayed at the center of particles. Fig. 5 shows the room temperature saturation magnetization and coercive field of the silica-coated Co–Pt particles as a function of annealing temperature. The magnetization was expressed in units of emu per gram of powder. The saturation magnetization and the coercive field increased with an increase in the annealing temperature up to 300 °C, because of high crystallinity provided with the annealing. Maximum values of the saturation magnetization (1.4 emu/g-sample) and the coercive field (450 Oe) were obtained for the 300 °C-annealed particles. Decreases in the saturation magnetization and the coercive field were observed above an annealing temperature of 300 °C. This temperature was almost in good agreement with the annealing temperature above which the (111) peak position remarkably shifted to lower degree. The annealing provided transformation from the Co– Pt alloy to the Pt metal that did not show any magnetic properties, which led to the decreases in the saturation magnetization and the coercive field.
4. Summary In the present work, we could prepare colloidally stabilized magnetic cobalt–platinum alloy nanoparticles surrounded by uniform silica shells. Measurement of magnetic properties showed that the silica-coated particles annealed at 300 °C had a saturation magnetization of 1.4 emu/g-sample and a coercive field of 450 Oe. Annealing above 300 °C provided the transformation from cobalt–platinum alloy to pure metal due to oxidation of cobalt in the alloy, which led to reductions in saturation magnetization and coercive field. Acknowledgements This research was partially supported by a Grant-in-Aid for Science Research (No. 15656163) and by the COE project, Giant Molecules and Complex Systems from the Ministry of Education, Culture, Sports, Science and Technology of Japan. References [1] R. Chandrasekhar, S.W. Charles, K. O'Grady, J. Imag. Technol. 3 (1987) 55. [2] K. Nakatsuka, H. Yokoyama, J. Shimoiizaka, T. Funaki, J. Magn. Magn. Mater 65 (1987) 359. [3] L.N. Donselaar, A.P. Philipse, J. Colloid Interface Sci. 212 (1999) 14. [4] A. Yoshizawa, Y. Nakane, N. Arisawa, H. Imaizumi, K. Sugino, S. Harada, IEEE Trans. Magn. MAG-23 (1987) 2874. [5] R. Massart, IEEE Trans. Magn. MAG-17 (1981) 1247. [6] S. Neveu, A. Bee, M. Robineau, D. Talbot, J. Colloid Interface Sci. 255 (2002) 293. [7] F. Grasset, N. Labhsetwar, D. Li, D.C. Park, N. Saito, H. Haneda, O. Cador, T. Roisnel, S. Mornet, E. Duguet, J. Portier, J. Etourneau, Langmuir 18 (2002) 8209. [8] S. Sun, H. Zeng, J. Am. Chem. Soc. 124 (2002) 8204. [9] M. Giersig, M. Hilgendorff, J. Phys. D: Appl. Phys. 32 (1999) L111. [10] S. Sun, C.D. Murray, D. Weller, L. Folks, A. Moser, Science 287 (2000) 1989. [11] T.A. Tyson, S.D. Conradson, R.F.C. Farrow, B.A. Jones, Phys. Rev., B 54 (1996) R3702.
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