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gold bimetallic nanoparticles
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 311 (2007) 31–35 www.elsevier.com/locate/jmmm
Synthesis and characterization of cobalt/gold bimetallic nanoparticles Guangjun Cheng, Angela R. Hight Walker Optical Technology Division, Physics Laboratory, National Institute of Standards and Technology, 100 Bureau Drive, MS 8443, Gaithersburg, MD 20899-8443, USA Available online 8 December 2006
Abstract Cobalt/gold (Co/Au) bimetallic nanoparticles are prepared by chemically reducing gold (III) chloride to gold in the presence of presynthesized Co nanoparticles. Transmission electron microscopy (TEM), ultraviolet-visible (UV-vis) absorption spectrometry, and a superconducting quantum interference device (SQUID) magnetometer have been used to characterize as-prepared bimetallic nanoparticles. Our findings demonstrate Au not only grows onto Co nanoparticles, forming a surface coating, but also diffuses into Co nanoparticles. The introduction of Au alters the crystalline structure of Co nanoparticles and changes their magnetic properties. Dodecanethiols induce a reorganization of as-prepared Co/Au bimetallic nanoparticles. r 2006 Elsevier B.V. All rights reserved. Keywords: Cobalt nanoparticles; Magnetic nanoparticles; Cobalt/gold; Bimetallic nanoparticle; Magnetic properties
Nanoparticles possess unique properties due to their size and surface effects [1]. With recent advancements in the synthesis of nanoparticles with controllable sizes and shapes, the nanoparticles themselves have been used as reaction agents to prepare new nanomaterials. For example, it has been demonstrated that hollow gold (Au) nanostructures can be synthesized by using silver (Ag) nanoparticles as the templates via a replacing reaction between Au3+ salt and Ag [2,3]. In terms of bimetallic nanoparticles, Au nanoparticles and copper (Cu) nanoparticles have been used to prepare atomically ordered intermetallic nanocrystals AuCu and AuCu3 by controlling Cu diffusion into Au [4]. Therefore, in addition to the conventional physical sputter deposition and electrochemical deposition methods, the reactions using nanoparticles can provide a new avenue to fabricate novel bimetallic nanoparticles. In this paper, we report on the synthesis and characterization of Co/Au bimetallic nanoparticles. This system is of great importance for its multiple applications, especially those in medical sensors and biomedicine [5,6]. The Co/Au bimetallic nanoparticles are prepared by chemically reducing gold (III) chloride to gold in the presence of cobalt Corresponding author. Tel.: +1 301 975 2155; fax: +1 301 975 6991.
E-mail address: [email protected] (A.R. Hight Walker). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.11.164
nanoparticles. Of obvious interest is how the introduction of Au influences the magnetic properties of Co nanoparticles. Transmission electron microscopy (TEM) images and ultraviolet-visible (UV-vis) spectra confirm the formation of the resultant Co/Au bimetallic nanoparticles. The results from electron diffraction and superconducting quantum interference device (SQUID) measurements show that the introduction of Au alters the crystalline structure of Co nanoparticles and changes their magnetic properties. Further modifications with alkanethiols on as-prepared bimetallic nanoparticles show that Au components are stripped from the bimetallic nanoparticles, and larger gold nanoparticles with various shapes are produced. Fig. 1 shows the TEM images and electron diffraction patterns of Co nanoparticles synthesized in our lab using a conventional thermo-decomposition method [7,8]. Trioctylphosphine oxide (0.2 g, TOPO, 90%, Alfa Aesar, Ward Hill, MA) and oleic acid (0.1 mL, OA, 99%, Aldrich, Milwaukee, WI) were degassed in argon in a flask for 20 min. Then, 15 mL of 1,2-dichlorobenzene (DCB, 99%, anhydrous, Aldrich, Milwaukee, WI) was introduced into the flask under an argon atmosphere. The solution was heated to the reflux temperature of DCB (182 1C) and 0.54 g of di-cobalt octa-carbonyl (Co2(CO)8, containing 1–5% hexane as a stabilizer, Alfa Aesar, Ward Hill, MA) diluted in 3 mL of DCB was quickly injected into the
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G. Cheng, A.R. Hight Walker / Journal of Magnetism and Magnetic Materials 311 (2007) 31–35
Fig. 1. TEM characterization of Co nanoparticles: (a) image and (b) selected area diffraction patterns.
mixture. The reaction continued for another 10 min and then the black colloidal solution was extracted using an airtight syringe and stored in a glass vial under argon. TEM samples were prepared by dropping the nanoparticle solution onto a carbon-coated TEM grid (Formvar/ Carbon Cu grids, Ted Pella, Inc. Redding, CA). The solvent was allowed to evaporate in air. TEM images and electron diffraction patterns were obtained on a HITACHI H-600 microscope (100 kV). As we can see from Fig. 1a, Co nanoparticles have a narrow-size distribution. The average size of the nanoparticles is determined to be 10.5 nm with 1.4 nm standard deviation. Selected-area electron-diffraction patterns in Fig. 1b show the characteristic diffraction rings of e-Co phase [7]. One milliliter of as-prepared Co nanoparticles in DCB were concentrated by centrifugation under ambient condition. The resulting black precipitations were re-dispersed into 5 mL toluene (99.5+%, Aldrich, Milwaukee, WI) containing 140 mg OA, and then were used as the starting materials or ‘‘seeds’’ to prepare Co/Au bimetallic nanoparticles. 10.5 nm Co nanoparticles were used because of their relative stability in air compared with the ones with a smaller size [9]. Co/Au bimetallic nanoparticles are synthesized by reducing Au precursor solution in toluene in the presence of pre-synthesized Co nanoparticles under ambient conditions [10]. In detail, the Au precursor solution contains 75 mg gold (III) chloride (AuCl3, 99.99%, Aldrich, Milwaukee, WI) and 235 mg didodecyl-dimethyl-ammonium bromide (DDAB, Research Chemical Ltd., Heysham, Lancs) in 5 mL toluene. The reducing solution contains 50 mg tetra-butyl-ammonium borohydride (TBAB, 98%, Aldrich, Milwaukee, WI) and 94 mg DDAB in 2 mL toluene. During the reaction, we observe that Co nanoparticles can also perform as the reducing agents for the Au precursor solution. The surfactants, OA and TOPO, on Co nanoparticles play an important role in the reactivity of Co nanoparticles. When purified Co nanoparticles are dispersed into toluene without the additional OA and used as the seeds, a clear green solution is obtained right after the addition of Au precursor, even without any addition of
TBAB. The green solution is related to the replacement reaction between Co nanoparticles and Au3+. At this stage Co nanoparticles reduce Au3+ only to Au+ because there is no reddish color indicating the formation of Au nanoparticles. When purified Co nanoparticles are dispersed into toluene containing OA, there is no obvious color change for 2 h with the addition of gold precursor solution. Therefore, the excess of surfactants slows down the reaction between Co nanoparticles and Au3+ by acting as a barrier or capping layer on the Co nanoparticles. In our synthesis of Co/Au bimetallic nanoparticles, 0.2 mL Au precursor solution was added into the flask containing re-dispersed Co nanoparticle colloidal solution with additional OA, and 0.4 mL reducing solution was slowly injected into the flask under stirring to avoid mass production of pure gold nanoparticles. The reaction took approximately 30 min. The solution was removed from the flask, and put into a glass vial. A magnet with 0.05 T field strength was placed outside of the glass vial and was used to attract the magnetic components in the solution to the side of the vial. This magnetic separation is used to separate pure gold nanoparticles from the rest of magnetic nanoparticles. The remaining solution was decanted, and the magnetic portion was rinsed with toluene. The magnetic extraction procedure was repeated three times. The sample was re-dispersed into toluene for TEM, UV-vis and SQUID measurements described below. Fig. 2 shows the TEM image and electron diffraction patterns of as-prepared Co/Au bimetallic nanoparticles after the magnetic separations. TEM measurements were performed on a JOEL 4000 FX microscope (300 kV). As we can see from Fig. 2a, in contrast to the mono-dispersed 10.5 nm Co nanoparticles shown in Fig. 1a, the size of Co/Au nanoparticles is larger and the size distribution is broader. The average size of the nanoparticles is determined to be 14.2 nm with 3.6 nm standard deviation. Obviously, the Co nanoparticles are enlarged. We speculate the enlargement is due to the growth of Au layer onto Co nanoparticles. During the reduction process, Co nanoparticles behave like ‘‘seeds’’ or nucleation sites for the resultant Co/Au bimetallic nanoparticle. In Fig. 2b, the selected area diffraction patterns indicate Co/Au bimetallic
ARTICLE IN PRESS G. Cheng, A.R. Hight Walker / Journal of Magnetism and Magnetic Materials 311 (2007) 31–35
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Fig. 2. TEM characterization of as-prepared Co/Au bimetallic nanoparticles: (a) image and (b) selected area diffraction patterns.
nanoparticles adopt face-centered cubic (fcc) structure, which is the typical crystalline structure for Au. The disappearance of the original epsilon crystalline structure of Co nanoparticles and the appearance of a new crystalline structure indicate the possibilities of not only the growth of Au layer onto Co nanoparticles, but also the diffusion of Au into Co nanoparticles. The introduction of Au into the cobalt nanoparticles alters the epsilon crystalline structure of cobalt nanoparticles. Curves a–c in Fig. 3 show the UV-vis absorption spectra for Co, Au, and Co/Au bimetallic nanoparticles. The UVvis spectra were collected using a Lambda 45 UV-vis spectrometer (Perkin Elmer Instruments, Wellesley, MA). Co nanoparticles show no absorption peak in the 300–900 nm range. For Au nanoparticles, there is a sharp absorption peak around 520 nm. For Co/Au bimetallic nanoparticles, the absorption peak broadens, covering a range from 500 to 750 nm, which is commonly observed in other Au bimetallic systems [11]. Alkanethiols have been widely used to modify gold surfaces by forming self-assembled monolayers through a sulfur-Au bond. Here the modification of Co/Au bimetallic nanoparticles with alkanethiol was performed by adding 100 mL 1-dodecanethiol (C12H26S, 98+%, Aldrich, Milwaukee, WI) into 5 mL re-dispersed Co/Au bimetallic nanoparticle solution after magnetic separation. After the addition of 1-dodecanethiol, the Co/Au bimetallic nanoparticle solution turns a reddish color. Curve d in Fig. 3 shows the UV-vis absorption spectrum for the resulting Co/Au bimetallic nanoparticles in toluene after dodecanethiol modification. There is a broadened absorption peak around 620 nm. Fig. 4 shows the TEM images of the resulting nanoparticles after dodecanethiol modification. There are two different contrast imaging regions observed on the TEM grids. In Fig. 4a, we can see 150 nm nanorods and nanoparticles are present in the higher-contrast region and in Fig. 4b, a lower-contrast region, hollow nanoparticles can be observed. By combining the results of UV-vis spectra and TEM image, the absorption peak at 620 nm in Fig. 3d is an indication of the formation of large Au nanoparticles and nanorods, which are shown in Fig. 4a. In the literature, surfactants are reported to re-organize the core-shell structured nanoparticles. Cetyltrimethylammonium bromide (CTAB) was reported to chemically
Fig. 3. UV-vis absorption spectra in toluene for (a) Co nanoparticles, (b) Au nanoparticles, (c) Co/Au bimetallic nanoparticles, and (d) resulting Co/Au colloidal solution after 1-dodecanethiol modification.
Fig. 4. TEM images of resulting Co/Au colloidal solution after 1-dodecanethiol modification in (a) a high-contrast region and (b) a low-contrast region.
reshape silica-gold core-shell nanostructures [12]. After the addition of CTAB, Au nanoshells went through a radical morphological change to large, elongated gold nanoparticles or toroids, and the dissolution of the silica core was observed. It was also reported that alkanethiols can induce the structural rearrangements in silica-gold core-shell nanoparticles [13]. The string-like gold nanoparticle structures on the silica surface were produced when the thiols with short alkane groups were used, while the hexagonally packed arrays of gold nanoparticle structures were produced when the thiols with long alkane groups
ARTICLE IN PRESS G. Cheng, A.R. Hight Walker / Journal of Magnetism and Magnetic Materials 311 (2007) 31–35
a
0.08 0.06 0.04
Moment (mA.m2)
were used. In these two cases, the inner core is inert silica. Here we have cobalt nanoparticles. The results of UV-vis spectra and the TEM image indicate that the 1-dodecanethiol molecules strip the gold components out of Co/Au bimetallic nanoparticles and reorganize them into Au nanoparticles. The hollow nanoparticles shown in Fig. 4b are similar to the reported Co oxides and Co sulfide hollow structures [14]. It was also demonstrated that the anisotropically phase-segregated cobalt/palladium (CoPd) sulfide nanoparticles, consisting of crystalline Co9S8 and amorphous PdSx phases, can be synthesized by reducing the corresponding metal precursors with 1,2-hexadecanediol in the presence of various alkanethiols, and the thiols are requisite to the conversion of Co and Pd to their sulfides [15]. Here in our Co/Au system, after the removal of Au components, the remaining Co components may react with the oxygen in the air/1-dodecanethiol in the solution, and turn into hollow Co oxides/sulfides. Therefore, although the solid Co/Au nanoparticles can be obtained, these bimetallic nanoparticles can be re-organized when the surfactants have strong affinities with the components. We have shown that the introduction of Au alters the crystalline structure of Co nanoparticles, and the resulting bimetallic nanoparticles re-dispersed in toluene containing OA are still responsive to external magnetic field. However, Co/Au bimetallic nanoparticles lose their magnetic response to the external field over time. A SQUID (Quantum Design MPMS) magnetometer was used to measure the magnetic properties of Co nanoparticles and Co/Au bimetallic nanoparticles stored in toluene for 24 h. The sample was prepared by taking out the solvent and drying Co nanoparticles and Co/Au bimetallic nanoparticles in a gel capsule under ambient conditions. The magnetization curves were recorded at 5 K. Fig. 5 shows the magnetization curves of pure Co nanoparticles and Co/Au bimetallic nanoparticles stored in toluene for 24 h. As we can see from Fig. 5a, there is an obvious hysteresis loop and Co nanoparticles show the ferromagnetic behavior at 5 K with a coercivity of 12.0 kA/m (150 Oe) and the remanence ratio (Mr/Ms, the ratio of coercivity of the remanence to the saturation magnetization) of 0.5. For Co/Au nanoparticles stored in toluene for 24 h, the hysteresis loop is almost gone and the magnetic behavior is more like a paramagnetic one. We have shown that the introduction of Au alters the crystalline structure of Co nanoparticles and changes their magnetic properties. In the literature, several studies have been carried out concerning Co/Au bimetallic systems prepared by physical methods [16–18]. Supersaturated Au–Co solid solutions have been obtained by ion-beam mixing of thin deposited Au and Co layers of compositions Au75Co25, Au50Co50, Au25Co75 and Au5Co95. X-ray diffraction measurements showed that the metastable alloys resulting from room-temperature ion implantation had an fcc structure and that the lattice parameters of the alloys varied almost linearly with the composition [16].
0.02 0 -0.02 -0.04 -0.06 -0.08 -20
b
-10 0 10 Applied Field (1/4π (MA/m))
20
0.002 0.0015 0.001
Moment (mA.m2)
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0.0005 0 -0.0005 -0.001 -0.0015 -0.002
-60
-40 -20 0 20 40 Applied Field (1/4π (MA/m))
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Fig. 5. Magnetization curves measured at 5 K of (a) Co nanoparticles and (b) Co/Au bimetallic nanoparticles stored in toluene for 24 h.
It was also reported that Au–Co nanoparticles electrodeposited on an amorphous carbon electrode from aqueous electrolytes were homogeneously alloyed and the composition of the alloyed nanoparticles was dependent on the electrodepositing potential [17]. Multiply twinned particles (MTPs) in these alloyed particles were observed. It is proposed that due to the smaller radii of the deposited Co atoms mixed in the Au top-layer of the growing particles there is a stronger tendency for the alloyed particles to grow into larger MTPs that are more stable than Au MTPs of the same size at high negative potentials. Deki et al. reported the preparation of Co/Au binary alloy nanoparticles via a vacuum co-evaporation method [18]. As-deposited Au/Co nano-alloys, with the crystal structure close to that of Au, were in a thermodynamically metastable state, and subsequent heat-treatment induced structural relaxation to release their internal stress, which resulted in the formation of a disordered fcc solid solution in thermodynamic equilibrium. The authors attributed the alloy-phase stability of Au/Co nano-alloys to their high specific surface energy. The Co/Au bimetallic nanoparticles we present here were synthesized under ambient conditions at room temperature.
ARTICLE IN PRESS G. Cheng, A.R. Hight Walker / Journal of Magnetism and Magnetic Materials 311 (2007) 31–35
The results show that Au not only grows onto Co nanoparticles, but also diffuses into Co nanoparticles. The introduction of Au alters the crystalline structure of Co nanoparticles and changes their magnetic properties. Dodecanethiols induce the reorganization of as-prepared Co/Au bimetallic nanoparticles. Therefore, we believe that these as-prepared Co/Au bimetallic nanoparticles have Co/ Au cores with Au surface coating, and Co/Au cores are in a thermodynamically metastable state [18]. A high-temperature annealing process may help reduce disorder of Co/Au bimetallic nanoparticles and improve their magnetic performance, which could lead to their biological applications in magnetic resonance imaging and hyperthermia treatment. Acknowledgment We acknowledge the help of Dr. Tiejun Zhang at the University of Maryland, College Park for TEM measurements. We thank Ms. Rosetta V. Drew and Dr. Robert D. Shull (Materials Science and Engineering Laboratory, NIST) for their help with SQUID measurements. Disclaimer: We identify certain commercial equipment, instruments, or materials in this article to specify adequately the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Institute of Standards and Technology, nor
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does it imply that the materials or equipment identified are necessarily the best available for the purpose.
References [1] Z.L. Wang, Characterization of Nanophase Materials, Wiley-VCH, Weinheim, Germany, 2000. [2] Y. Sun, Y. Xia, Nano Lett 3 (2003) 1569. [3] Y. Sun, Y. Xia, Anal. Chem. 74 (2002) 5297. [4] A.K. Sra, R.E. Schaak, J. Am. Chem. Soc. 126 (2004) 6667. [5] Y. Bao, K.M. Krishnan, J. Magn. Magn. Mater. 293 (2005) 15. [6] M. Chen, S. Yamamuro, D. Farrell, S.A. Majetich, J. Appl. Phys. 93 (2003) 7551. [7] V.F. Puntes, K.M. Krishnan, A.P. Alivisatos, Science 291 (2001) 2115. [8] G. Cheng, D. Romero, G.T. Fraser, A.R. Hight Walker, Langmuir 21 (2005) 12055. [9] G. Cheng, J.D. Carter, T. Guo, Chem. Phys. Lett. 400 (2004) 122. [10] N.R. Jana, X. Peng, J. Am. Chem. Soc. 125 (2003) 14280. [11] J.L. Lyon, D.A. Fleming, M.B. Stone, et al., Nano Lett 4 (2004) 719. [12] C.M. Aguirre, T.R. Kaspar, C. Radloff, N.J. Halas, Nano Lett 3 (2003) 1707. [13] F. Osterloh, H. Hiramatsu, R. Porter, T. Guo, Langmuir 20 (2004) 5553. [14] Y. Yin, R.M. Rioux, C.K. Erdonmez, et al., Science 304 (2004) 711. [15] T. Teranishi, Y. Inoue, M. Nakaya, et al., J. Am. Chem. Soc. 126 (2004) 9914. [16] B.Y. Tsaur, S.S. Lau, J.W. Mayer, Philos. Mag. B 44 (1981) 95. [17] D. Lu, K. Domen, K. Tanaka, Langmuir 18 (2002) 3226. [18] H. Nabika, M. Mizuhata, A. Kajinami, et al., J. Electroanal. Chem. 559 (2003) 99.
Journal of Magnetism and Magnetic Materials 311 (2007) 31–35 www.elsevier.com/locate/jmmm
Synthesis and characterization of cobalt/gold bimetallic nanoparticles Guangjun Cheng, Angela R. Hight Walker Optical Technology Division, Physics Laboratory, National Institute of Standards and Technology, 100 Bureau Drive, MS 8443, Gaithersburg, MD 20899-8443, USA Available online 8 December 2006
Abstract Cobalt/gold (Co/Au) bimetallic nanoparticles are prepared by chemically reducing gold (III) chloride to gold in the presence of presynthesized Co nanoparticles. Transmission electron microscopy (TEM), ultraviolet-visible (UV-vis) absorption spectrometry, and a superconducting quantum interference device (SQUID) magnetometer have been used to characterize as-prepared bimetallic nanoparticles. Our findings demonstrate Au not only grows onto Co nanoparticles, forming a surface coating, but also diffuses into Co nanoparticles. The introduction of Au alters the crystalline structure of Co nanoparticles and changes their magnetic properties. Dodecanethiols induce a reorganization of as-prepared Co/Au bimetallic nanoparticles. r 2006 Elsevier B.V. All rights reserved. Keywords: Cobalt nanoparticles; Magnetic nanoparticles; Cobalt/gold; Bimetallic nanoparticle; Magnetic properties
Nanoparticles possess unique properties due to their size and surface effects [1]. With recent advancements in the synthesis of nanoparticles with controllable sizes and shapes, the nanoparticles themselves have been used as reaction agents to prepare new nanomaterials. For example, it has been demonstrated that hollow gold (Au) nanostructures can be synthesized by using silver (Ag) nanoparticles as the templates via a replacing reaction between Au3+ salt and Ag [2,3]. In terms of bimetallic nanoparticles, Au nanoparticles and copper (Cu) nanoparticles have been used to prepare atomically ordered intermetallic nanocrystals AuCu and AuCu3 by controlling Cu diffusion into Au [4]. Therefore, in addition to the conventional physical sputter deposition and electrochemical deposition methods, the reactions using nanoparticles can provide a new avenue to fabricate novel bimetallic nanoparticles. In this paper, we report on the synthesis and characterization of Co/Au bimetallic nanoparticles. This system is of great importance for its multiple applications, especially those in medical sensors and biomedicine [5,6]. The Co/Au bimetallic nanoparticles are prepared by chemically reducing gold (III) chloride to gold in the presence of cobalt Corresponding author. Tel.: +1 301 975 2155; fax: +1 301 975 6991.
E-mail address: [email protected] (A.R. Hight Walker). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.11.164
nanoparticles. Of obvious interest is how the introduction of Au influences the magnetic properties of Co nanoparticles. Transmission electron microscopy (TEM) images and ultraviolet-visible (UV-vis) spectra confirm the formation of the resultant Co/Au bimetallic nanoparticles. The results from electron diffraction and superconducting quantum interference device (SQUID) measurements show that the introduction of Au alters the crystalline structure of Co nanoparticles and changes their magnetic properties. Further modifications with alkanethiols on as-prepared bimetallic nanoparticles show that Au components are stripped from the bimetallic nanoparticles, and larger gold nanoparticles with various shapes are produced. Fig. 1 shows the TEM images and electron diffraction patterns of Co nanoparticles synthesized in our lab using a conventional thermo-decomposition method [7,8]. Trioctylphosphine oxide (0.2 g, TOPO, 90%, Alfa Aesar, Ward Hill, MA) and oleic acid (0.1 mL, OA, 99%, Aldrich, Milwaukee, WI) were degassed in argon in a flask for 20 min. Then, 15 mL of 1,2-dichlorobenzene (DCB, 99%, anhydrous, Aldrich, Milwaukee, WI) was introduced into the flask under an argon atmosphere. The solution was heated to the reflux temperature of DCB (182 1C) and 0.54 g of di-cobalt octa-carbonyl (Co2(CO)8, containing 1–5% hexane as a stabilizer, Alfa Aesar, Ward Hill, MA) diluted in 3 mL of DCB was quickly injected into the
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G. Cheng, A.R. Hight Walker / Journal of Magnetism and Magnetic Materials 311 (2007) 31–35
Fig. 1. TEM characterization of Co nanoparticles: (a) image and (b) selected area diffraction patterns.
mixture. The reaction continued for another 10 min and then the black colloidal solution was extracted using an airtight syringe and stored in a glass vial under argon. TEM samples were prepared by dropping the nanoparticle solution onto a carbon-coated TEM grid (Formvar/ Carbon Cu grids, Ted Pella, Inc. Redding, CA). The solvent was allowed to evaporate in air. TEM images and electron diffraction patterns were obtained on a HITACHI H-600 microscope (100 kV). As we can see from Fig. 1a, Co nanoparticles have a narrow-size distribution. The average size of the nanoparticles is determined to be 10.5 nm with 1.4 nm standard deviation. Selected-area electron-diffraction patterns in Fig. 1b show the characteristic diffraction rings of e-Co phase [7]. One milliliter of as-prepared Co nanoparticles in DCB were concentrated by centrifugation under ambient condition. The resulting black precipitations were re-dispersed into 5 mL toluene (99.5+%, Aldrich, Milwaukee, WI) containing 140 mg OA, and then were used as the starting materials or ‘‘seeds’’ to prepare Co/Au bimetallic nanoparticles. 10.5 nm Co nanoparticles were used because of their relative stability in air compared with the ones with a smaller size [9]. Co/Au bimetallic nanoparticles are synthesized by reducing Au precursor solution in toluene in the presence of pre-synthesized Co nanoparticles under ambient conditions [10]. In detail, the Au precursor solution contains 75 mg gold (III) chloride (AuCl3, 99.99%, Aldrich, Milwaukee, WI) and 235 mg didodecyl-dimethyl-ammonium bromide (DDAB, Research Chemical Ltd., Heysham, Lancs) in 5 mL toluene. The reducing solution contains 50 mg tetra-butyl-ammonium borohydride (TBAB, 98%, Aldrich, Milwaukee, WI) and 94 mg DDAB in 2 mL toluene. During the reaction, we observe that Co nanoparticles can also perform as the reducing agents for the Au precursor solution. The surfactants, OA and TOPO, on Co nanoparticles play an important role in the reactivity of Co nanoparticles. When purified Co nanoparticles are dispersed into toluene without the additional OA and used as the seeds, a clear green solution is obtained right after the addition of Au precursor, even without any addition of
TBAB. The green solution is related to the replacement reaction between Co nanoparticles and Au3+. At this stage Co nanoparticles reduce Au3+ only to Au+ because there is no reddish color indicating the formation of Au nanoparticles. When purified Co nanoparticles are dispersed into toluene containing OA, there is no obvious color change for 2 h with the addition of gold precursor solution. Therefore, the excess of surfactants slows down the reaction between Co nanoparticles and Au3+ by acting as a barrier or capping layer on the Co nanoparticles. In our synthesis of Co/Au bimetallic nanoparticles, 0.2 mL Au precursor solution was added into the flask containing re-dispersed Co nanoparticle colloidal solution with additional OA, and 0.4 mL reducing solution was slowly injected into the flask under stirring to avoid mass production of pure gold nanoparticles. The reaction took approximately 30 min. The solution was removed from the flask, and put into a glass vial. A magnet with 0.05 T field strength was placed outside of the glass vial and was used to attract the magnetic components in the solution to the side of the vial. This magnetic separation is used to separate pure gold nanoparticles from the rest of magnetic nanoparticles. The remaining solution was decanted, and the magnetic portion was rinsed with toluene. The magnetic extraction procedure was repeated three times. The sample was re-dispersed into toluene for TEM, UV-vis and SQUID measurements described below. Fig. 2 shows the TEM image and electron diffraction patterns of as-prepared Co/Au bimetallic nanoparticles after the magnetic separations. TEM measurements were performed on a JOEL 4000 FX microscope (300 kV). As we can see from Fig. 2a, in contrast to the mono-dispersed 10.5 nm Co nanoparticles shown in Fig. 1a, the size of Co/Au nanoparticles is larger and the size distribution is broader. The average size of the nanoparticles is determined to be 14.2 nm with 3.6 nm standard deviation. Obviously, the Co nanoparticles are enlarged. We speculate the enlargement is due to the growth of Au layer onto Co nanoparticles. During the reduction process, Co nanoparticles behave like ‘‘seeds’’ or nucleation sites for the resultant Co/Au bimetallic nanoparticle. In Fig. 2b, the selected area diffraction patterns indicate Co/Au bimetallic
ARTICLE IN PRESS G. Cheng, A.R. Hight Walker / Journal of Magnetism and Magnetic Materials 311 (2007) 31–35
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Fig. 2. TEM characterization of as-prepared Co/Au bimetallic nanoparticles: (a) image and (b) selected area diffraction patterns.
nanoparticles adopt face-centered cubic (fcc) structure, which is the typical crystalline structure for Au. The disappearance of the original epsilon crystalline structure of Co nanoparticles and the appearance of a new crystalline structure indicate the possibilities of not only the growth of Au layer onto Co nanoparticles, but also the diffusion of Au into Co nanoparticles. The introduction of Au into the cobalt nanoparticles alters the epsilon crystalline structure of cobalt nanoparticles. Curves a–c in Fig. 3 show the UV-vis absorption spectra for Co, Au, and Co/Au bimetallic nanoparticles. The UVvis spectra were collected using a Lambda 45 UV-vis spectrometer (Perkin Elmer Instruments, Wellesley, MA). Co nanoparticles show no absorption peak in the 300–900 nm range. For Au nanoparticles, there is a sharp absorption peak around 520 nm. For Co/Au bimetallic nanoparticles, the absorption peak broadens, covering a range from 500 to 750 nm, which is commonly observed in other Au bimetallic systems [11]. Alkanethiols have been widely used to modify gold surfaces by forming self-assembled monolayers through a sulfur-Au bond. Here the modification of Co/Au bimetallic nanoparticles with alkanethiol was performed by adding 100 mL 1-dodecanethiol (C12H26S, 98+%, Aldrich, Milwaukee, WI) into 5 mL re-dispersed Co/Au bimetallic nanoparticle solution after magnetic separation. After the addition of 1-dodecanethiol, the Co/Au bimetallic nanoparticle solution turns a reddish color. Curve d in Fig. 3 shows the UV-vis absorption spectrum for the resulting Co/Au bimetallic nanoparticles in toluene after dodecanethiol modification. There is a broadened absorption peak around 620 nm. Fig. 4 shows the TEM images of the resulting nanoparticles after dodecanethiol modification. There are two different contrast imaging regions observed on the TEM grids. In Fig. 4a, we can see 150 nm nanorods and nanoparticles are present in the higher-contrast region and in Fig. 4b, a lower-contrast region, hollow nanoparticles can be observed. By combining the results of UV-vis spectra and TEM image, the absorption peak at 620 nm in Fig. 3d is an indication of the formation of large Au nanoparticles and nanorods, which are shown in Fig. 4a. In the literature, surfactants are reported to re-organize the core-shell structured nanoparticles. Cetyltrimethylammonium bromide (CTAB) was reported to chemically
Fig. 3. UV-vis absorption spectra in toluene for (a) Co nanoparticles, (b) Au nanoparticles, (c) Co/Au bimetallic nanoparticles, and (d) resulting Co/Au colloidal solution after 1-dodecanethiol modification.
Fig. 4. TEM images of resulting Co/Au colloidal solution after 1-dodecanethiol modification in (a) a high-contrast region and (b) a low-contrast region.
reshape silica-gold core-shell nanostructures [12]. After the addition of CTAB, Au nanoshells went through a radical morphological change to large, elongated gold nanoparticles or toroids, and the dissolution of the silica core was observed. It was also reported that alkanethiols can induce the structural rearrangements in silica-gold core-shell nanoparticles [13]. The string-like gold nanoparticle structures on the silica surface were produced when the thiols with short alkane groups were used, while the hexagonally packed arrays of gold nanoparticle structures were produced when the thiols with long alkane groups
ARTICLE IN PRESS G. Cheng, A.R. Hight Walker / Journal of Magnetism and Magnetic Materials 311 (2007) 31–35
a
0.08 0.06 0.04
Moment (mA.m2)
were used. In these two cases, the inner core is inert silica. Here we have cobalt nanoparticles. The results of UV-vis spectra and the TEM image indicate that the 1-dodecanethiol molecules strip the gold components out of Co/Au bimetallic nanoparticles and reorganize them into Au nanoparticles. The hollow nanoparticles shown in Fig. 4b are similar to the reported Co oxides and Co sulfide hollow structures [14]. It was also demonstrated that the anisotropically phase-segregated cobalt/palladium (CoPd) sulfide nanoparticles, consisting of crystalline Co9S8 and amorphous PdSx phases, can be synthesized by reducing the corresponding metal precursors with 1,2-hexadecanediol in the presence of various alkanethiols, and the thiols are requisite to the conversion of Co and Pd to their sulfides [15]. Here in our Co/Au system, after the removal of Au components, the remaining Co components may react with the oxygen in the air/1-dodecanethiol in the solution, and turn into hollow Co oxides/sulfides. Therefore, although the solid Co/Au nanoparticles can be obtained, these bimetallic nanoparticles can be re-organized when the surfactants have strong affinities with the components. We have shown that the introduction of Au alters the crystalline structure of Co nanoparticles, and the resulting bimetallic nanoparticles re-dispersed in toluene containing OA are still responsive to external magnetic field. However, Co/Au bimetallic nanoparticles lose their magnetic response to the external field over time. A SQUID (Quantum Design MPMS) magnetometer was used to measure the magnetic properties of Co nanoparticles and Co/Au bimetallic nanoparticles stored in toluene for 24 h. The sample was prepared by taking out the solvent and drying Co nanoparticles and Co/Au bimetallic nanoparticles in a gel capsule under ambient conditions. The magnetization curves were recorded at 5 K. Fig. 5 shows the magnetization curves of pure Co nanoparticles and Co/Au bimetallic nanoparticles stored in toluene for 24 h. As we can see from Fig. 5a, there is an obvious hysteresis loop and Co nanoparticles show the ferromagnetic behavior at 5 K with a coercivity of 12.0 kA/m (150 Oe) and the remanence ratio (Mr/Ms, the ratio of coercivity of the remanence to the saturation magnetization) of 0.5. For Co/Au nanoparticles stored in toluene for 24 h, the hysteresis loop is almost gone and the magnetic behavior is more like a paramagnetic one. We have shown that the introduction of Au alters the crystalline structure of Co nanoparticles and changes their magnetic properties. In the literature, several studies have been carried out concerning Co/Au bimetallic systems prepared by physical methods [16–18]. Supersaturated Au–Co solid solutions have been obtained by ion-beam mixing of thin deposited Au and Co layers of compositions Au75Co25, Au50Co50, Au25Co75 and Au5Co95. X-ray diffraction measurements showed that the metastable alloys resulting from room-temperature ion implantation had an fcc structure and that the lattice parameters of the alloys varied almost linearly with the composition [16].
0.02 0 -0.02 -0.04 -0.06 -0.08 -20
b
-10 0 10 Applied Field (1/4π (MA/m))
20
0.002 0.0015 0.001
Moment (mA.m2)
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0.0005 0 -0.0005 -0.001 -0.0015 -0.002
-60
-40 -20 0 20 40 Applied Field (1/4π (MA/m))
60
Fig. 5. Magnetization curves measured at 5 K of (a) Co nanoparticles and (b) Co/Au bimetallic nanoparticles stored in toluene for 24 h.
It was also reported that Au–Co nanoparticles electrodeposited on an amorphous carbon electrode from aqueous electrolytes were homogeneously alloyed and the composition of the alloyed nanoparticles was dependent on the electrodepositing potential [17]. Multiply twinned particles (MTPs) in these alloyed particles were observed. It is proposed that due to the smaller radii of the deposited Co atoms mixed in the Au top-layer of the growing particles there is a stronger tendency for the alloyed particles to grow into larger MTPs that are more stable than Au MTPs of the same size at high negative potentials. Deki et al. reported the preparation of Co/Au binary alloy nanoparticles via a vacuum co-evaporation method [18]. As-deposited Au/Co nano-alloys, with the crystal structure close to that of Au, were in a thermodynamically metastable state, and subsequent heat-treatment induced structural relaxation to release their internal stress, which resulted in the formation of a disordered fcc solid solution in thermodynamic equilibrium. The authors attributed the alloy-phase stability of Au/Co nano-alloys to their high specific surface energy. The Co/Au bimetallic nanoparticles we present here were synthesized under ambient conditions at room temperature.
ARTICLE IN PRESS G. Cheng, A.R. Hight Walker / Journal of Magnetism and Magnetic Materials 311 (2007) 31–35
The results show that Au not only grows onto Co nanoparticles, but also diffuses into Co nanoparticles. The introduction of Au alters the crystalline structure of Co nanoparticles and changes their magnetic properties. Dodecanethiols induce the reorganization of as-prepared Co/Au bimetallic nanoparticles. Therefore, we believe that these as-prepared Co/Au bimetallic nanoparticles have Co/ Au cores with Au surface coating, and Co/Au cores are in a thermodynamically metastable state [18]. A high-temperature annealing process may help reduce disorder of Co/Au bimetallic nanoparticles and improve their magnetic performance, which could lead to their biological applications in magnetic resonance imaging and hyperthermia treatment. Acknowledgment We acknowledge the help of Dr. Tiejun Zhang at the University of Maryland, College Park for TEM measurements. We thank Ms. Rosetta V. Drew and Dr. Robert D. Shull (Materials Science and Engineering Laboratory, NIST) for their help with SQUID measurements. Disclaimer: We identify certain commercial equipment, instruments, or materials in this article to specify adequately the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Institute of Standards and Technology, nor
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does it imply that the materials or equipment identified are necessarily the best available for the purpose.
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