Surface Investigation and Magnetic Behavior of Co Nanoparticles

Mar 10, 2009 - Telephone: +82 2 705 8882. Fax: +82 2 ... The surfactant-capped Co nanoparticles prepared by the modified polyol process show excellent...
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J. Phys. Chem. C 2009, 113, 5081–5086

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Surface Investigation and Magnetic Behavior of Co Nanoparticles Prepared via a Surfactant-Mediated Polyol Process Chang Woo Kim,† Hyun Gil Cha,† Young Hwan Kim,‡ Abhijit P. Jadhav,† Eun Sun Ji,† Dong In Kang,† and Young Soo Kang*,† Department of Chemistry, Sogang UniVersity, Seoul 121-742, South Korea, and Department of Functional Layers, GMBU e.V., P.O. Box 520 165, D-01317 Dresden, Germany ReceiVed: October 15, 2008; ReVised Manuscript ReceiVed: January 28, 2009

This work reports on the structural and magnetic properties of 10 nm-sized Co nanoparticles, along with a surface investigation of chemi-adsorption of fatty acid-like oleic acid on their surfaces. The hexagonal closepacked superparamagnetic Co nanoparticles were prepared by a surfactant-mediated polyol process that consists of refluxing a solution of the metal precursor, using oleic acid in ethylene glycol. The nature of an adsorbed fatty acid on a Co nanoparticle surface was studied, using transmission electron microscopy and Fourier transform infrared spectroscopy techniques. The magnetic properties of the surfactant-capped and uncapped Co nanoparticles are compared. The surfactant-capped Co nanoparticles prepared by the modified polyol process show excellent superparamagnetic behaviors, with high magnetic susceptibility, negligible coercivity, and high saturation magnetization. Introduction In the past decades, transition metal (TM) and metal oxide nanoparticles prepared through wet-chemical routes demonstrated outstanding properties for their use in electronic devices and biomedical applications. To mention a few, they are used in photonics, optoelectronics, ferrofluids, nanosensors, and color imaging because of their brilliant size-dependent properties derived from their very narrow size distribution at the nanometer scale.1-4 Large-scale physical manufacturing techniques such as mechanical alloying and sputtering have been used for synthesizing pure TM and metal oxide nanoparticles since a couple of decades ago.5,6 However, these typical physical manufacturing techniques produce nanostructures with poor morphology control and wide particle size distribution.6 Besides these physical techniques, various chemical synthesis routes such as reduction,7-9 thermal decomposition of organic complexes,10-12 reverse micelles,13 the nonhydrolytic sol-gel process,14 the lowtemperature catalytic method,15 and hydrothermal routes16 have been utilized for manipulating TM and metal oxide nanoparticles with well-defined morphologies. Coprecipitation is one of the preferred routes for the easy synthesis of nanocrystals on a large scale.6,17 Hot injection methods can also be used to synthesize the monodispersed nanoparticles with various morphologies.6 Presently, thermal decomposition of metal-organic complexes seems to be the best method for morphology control of metal nanoparticles.6,17 On the other hand, the hydrothermal or solvothermal synthesis technique is rarely used for the synthesis of magnetic nanocrystals, and the mechanism of synthesis is not often studied, although it allows us to synthesize high-quality crystals.17 We have synthesized TM-based nanoparticles, including Fe and Co nanocrystals, using several methods such as solventless synthesis,18 thermal decomposition,19 laser pyrolysis, γ-irradiation,20 and the coprecipitation method,21,22 although * To whom correspondence should be addressed. E-mail: yskang@ sogang.ac.kr. Telephone: +82 2 705 8882. Fax: +82 2 701 0967. † Sogang University. ‡ GMBU e.V.

nanoparticles with different particle size, shape, and composition are produced depending on their reaction conditions. Over the various physical and wet-chemical routes commonly used for the fabrication of metallic nanocrystals, the Fie´vet polyol synthesis presents added advantages for the production of monodispersed metallic nanocrystals with a controlled particle size at nanometer scale, while using polyvinylpyrrolidone (PVP) as the surfactant.23-28 This traditional polyol method, utilizing ethylene glycol and trimethylene glycol, is an energy-efficient and environmentally friendly process as the reaction is carried out under closed system conditions compared with that of thermal decomposition synthesis using toxic organic solvents.29 The polyol process allows the nanoparticles to be nucleated at the high boiling point of polyol and is also used to manipulate the morphology of the particles at the nanoscale as the organic liquid alcohols act not only as the reducing agents but also as their dissolving media.30,31 Although the polyol synthesis process has been explored widely for the synthesis of metal nanoparticles since work by the Fie´vet group,24-29 until the recent works reported by Y. Xia and co-workers,32-34 most of the reported works have been limited to the synthesis of noble metals because of insufficient understanding of their synthesis mechanism.35 Many reports have treated either the manipulation of synthesis conditions to manipulate physical properties such as optical, magnetic, and electrical properties of metal nanoparticles or simply morphology control in the view of their self-assembly without enhancing physical properties. As a consequence, the focus of many research groups is confined only to the surface stability of nanocrystals for their applications or simply their morphology search in the synthesis of various nanocrystals.6,36 Moreover, very few studies have been carried out on the comparison of magnetic properties of surfactant-capped and uncapped magnetic nanocrystals, although fatty acids are widely used as surfactants to prepare chemically stable magnetic nanocrystals.37,38 Our purpose in this work is to investigate the magnetic behavior and the surface properties of hexagonal close-packed (hcp) Co nanoparticles synthesized via a surfactant-mediated polyol

10.1021/jp809113h CCC: $40.75  2009 American Chemical Society Published on Web 03/10/2009

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Figure 1. Schematic presentation of the different stages of the reaction process in polyol synthesis.

process and to compare them with uncapped particles. We report on the synthesis of superparamagnetic Co nanoparticles with hexagonal close-packed structures and of saturation magnetization (Ms) close to its bulk value, along with chemical interaction with the surfactant oleic acid. Experimental Section Reagents and Apparatus. All chemicals and solvents were used directly as received without further purification. Cobalt acetate [Co(CH3CO2)2, 95%], sodium hydroxide (NaOH, 97+%), ethylene glycol (HOCH2CH2OH, 99+%), and oleic acid [CH3(CH2)7CHdCH3(CH2)7COOH] were purchased from Aldrich Chemical Co. N2 gas of 99.999+% purity was used during synthesis of Co nanoparticles to prevent oxidation and drying. Morphology of the as-synthesized nanoparticles was studied using a field emission scanning electron microscope (FE-SEM) (JEOL LTD JSM 890). The size, shape, and fine structure of the particles were investigated using low-resolution HITACHI H-7500 and JEOL JEM2010 high-resolution transmission electron microscopes (TEM). For TEM observations, Co nanoparticles were first dispersed in hexane. Then a drop of the colloidal sample was placed on a 300 mesh carbon-coated copper grid and dried in vacuum. Size distribution of the particles was performed by measuring their diameters from the enlarged TEM images. Elemental analyses of the samples were performed by using energy dispersive X-ray microanalysis (EDX) in the JEOL JEM2010 TEM operated under an acceleration voltage of 200 kV. The surface investigation of the fatty acid (oleic acid) over the Co nanoparticles was carried out using TEM and Fourier transform infrared (FT-IR) spectroscopy (Spectrum GX, PerkinElmer). The thermal stabilities of the oleic acid and Co nanoparticles were analyzed by thermogravimetric analysis (TGA), using a heating rate of 10 °C/min under highpurity argon. The crystal structure of the as-synthesized nanoparticles was identified by X-ray diffraction (XRD) in a powder diffraction mode, using the Cu KR radiation source (λ ) 0.154056 nm) of a Philips X’pert-MPD diffractometer. Magnetic susceptibilities and hysteresis loops of the samples were obtained with a super conducting quantum interference device (SQUID) magnetometer. The magnetic behavior of the nanoparticles was

studied in zero-field cooling (ZFC) and field cooling (FC) modes in the temperature range of 5.0-300.0 K under an applied field of 200 oersted (Oe). Magnetization curves were obtained under the fields from 0 to 7 T, after the powder samples were set with wax to prevent physical rotation of the particles. Synthesis of Oleic Acid-Capped Co Nanoparticles by a Polyol Process. In a typical synthesis of Co nanoparticles, 20 mmol of cobalt acetate, Co(CH3CO2)2, and 20 mmol of NaOH were mixed and stirred in 100 mL of deoxygenated ethylene glycol inside a 250 mL round-bottomed flask under N2 atmosphere. The solution was heated to 150 °C at a rate of 3 °C/min. At this temperature, 20 mmol of oleic acid was added to the solution. The mixture was heated to 200 °C at a rate of 3 °C/min and refluxed for 2 h. After the reaction, the solution was cooled to room temperature, and then ethanol was added to this solution, containing a Co nanoparticle. The precipitate was separated and washed repeatedly with excess ethanol and dried with nitrogen gas. For comparison, Co nanoparticles were also synthesized without using an oleic acid surfactant under the same conditions. Results and Discussion In a typical polyol synthesis strategy, the concentration of the metal precursor solution can be kept constant with the precipitation of the bivalent metal cation in the solid phase. The nucleation step can be separated from the crystal growth step according to eq 1 as shown in Figure 1.39 nucleation

(M2+)solid phase 98 M2+solution 98 M0 (M ) OH-

growth

transition metal)

(1)

Through the polyol synthesis process, it is possible to synthesize single and binary alloy particles with different structures and morphologies, depending on the reaction conditions such as reaction temperature and the reducing medium.39,40 The polyol synthesis procedure proceeds through oxidation of the polyol by reducing the metallic precursor to its zero-valent

Investigation and Behavior of Co Nanoparticles

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Figure 2. FE-SEM images (a and b) and low-resolution TEM image (c) of Co nanoparticles synthesized by a polyol process without using oleic acid. Note the small nanoparticles aggregated to form larger particles.

metallic state according to eqs 1 and 2 and Figure 1.40 Equation 2 shows the reaction involved in the reduction of Co2+ to Co0. When the zero-valent metal is nucleated from the bivalent metal ion at the high boiling point of ethylene glycol, metal particles are surrounded with a lone pair electron of oxygen of 2,3butanedione derived from ethylene glycol.35 When 1,2-propanediol is used instead of ethylene glycol as a solvent, 3,4hexanedione is formed. The presence of a lone pair electron keeps the formed metal particles separated, and therefore, the particle size is regulated and controlled without surfactants such as PVP, oleic acid, and triphenylphosphine.41

(2)

Figure 2 shows the FE-SEM and TEM images of the Co nanoparticles synthesized without using oleic acid. As shown in Figure 2, using ethylene glycol as the solvent and reducing medium, Co nanoparticles of about 200 nm are formed through the aggregation of smaller particles. FE-SEM, TEM images, EDX spectrum, and the size distribution of Co nanoparticles prepared using oleic acid as surfactants are shown in Figure 3. We can see that the addition of a surfactant such as oleic acid to the reaction mixture produces Co nanoparticles of about 10 nm in average size, in contrast to the larger particles (∼200 nm) produced without using the surfactant. To improve our understanding of the chemisorption process of fatty acids on the surface of Co nanoparticles, FT-IR measurements were performed on the pure oleic acid (liquid) and Co nanoparticle capped with oleic acid (spectra shown in Figure 4). In a typical FT-IR spectrum of oleic acid (Figure 4a), because there is no other functional group having broad and intense bands between 3500 and 2500 cm-1, the broadband appearing in this region is associated with the O-H stretching band of carboxylic acid.16 The CH2 groups produce two characteristic bands at about 2922 and 2853 cm-1, corresponding to the asymmetric (υasCH2) and symmetric (υsCH2) stretching modes, respectively. The intense bands appearing at about 1710 and 1286 cm-1 reveal the existence of CdO stretching and C-O stretching, respectively.16 The appearance of asymmetric (υasCH2) and symmetric (υsCH2)

stretching bands at about 1710 and 1286 cm-1, respectively, in the FT-IR spectrum of the as-synthesized Co nanoparticle coated with oleic acid (Figure 4b) indicates the presence of fatty acids in the adsorbed state on the surface of the Co nanoparticle.42 Two new characteristic bands appeared at about 1563 and 1408 cm-1 in the FT-IR spectrum of the oleic acid-capped Co particles (Figure 4b), which correspond to asymmetric (υasCOO-) and symmetric (υsCOO-) stretching bands, respectively.16,42 This demonstrates that the fatty acid was chemisorbed as carboxylate onto the Co nanoparticle surface, coordinating their two oxygen atoms symmetrically to the surface of the Co nanoparticle. The carboxyl functional group (-COO-) of oleic acid can be coordinated to Co atoms forming Co carboxylate, and the tails of oleic acid have hydrophobic and van der Waals interactions between hydrocarbon alkyl chains, producing a gap between the nanoparticles (Figure 5). As shown in the TEM image of Figure 5, the Co nanoparticles are separated by a distance of about 2.2 nm. If the alkyl chains in oleic acid were fully extended, the chain length from the first carbon atom to the last carbon atom in the terminal methyl group could be calculated as 24.507 Å, from 20.048 Å (1.253 Å per C-C × 16), plus 1.113 Å (bond length of C-H), 2.009 Å (distance between C-H in COOH), and 1.337 Å (bond length of CdC). The C-C bond length in -OOC-CH2- is the same as that in -CH2-CH2- and -CH2-CH3.16 Considering that the oleic acid chain is in cis form, we found the total chain length should be shorter than the calculated chain length. The measured interparticle distance for all of the particles was about 2.2 nm, which is the interpenetration distance of the oleic acid chains (schematic drawing in Figure 4). The high-resolution transmission electron microscope (HRTEM) lattice image of a typical Co particle shown in Figure 5 indicates its high crystalline structure. The measured lattice spacing of about 2.05 Å was consistent with the lattice spacing of the (111) plane of the Co crystal. The mass loss occurred from the thermal decomposition of oleic acid in the TGA spectrum of the oleic acid-capped Co nanoparticles (Supporting Information), indicating that the alkyl chain on the surface of Co nanoparticles dissociated at about 300 °C and decomposed by a CO2 elimination pathway after oleic acid was chemisorbed on the surface of a Co nanoparticle.43 Figure 6 shows the XRD patterns of Co nanoparticles under various conditions. The crystal structures of the Co nanoparticles have been confirmed as hexagonal close-packed (hcp) structures. The diffraction peaks in Figure 6c can be indexed to (100),

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Figure 3. Typical TEM images (a and b), EDX spectrum (c), and particle size distribution (d) of Co nanoparticles synthesized with oleic acid.

Figure 4. FT-IR spectra of pure oleic acid (a) and oleic acid-capped Co nanoparticles (b). Appearance of carboxylate-related 1563 and 1408 cm-1 bands in spectrum b indicates the presence of chemiadsorbed oleic acid over the Co nanoparticles.

(002), (101), (110), and (103) planes of a hexagonal unit cell of a hcp cobalt structure (JCPDS 05-0727). Magnetic behavior of the Co nanoparticles was characterized with ZFC and FC magnetization curves and hysteresis loops. Figure 7 shows the temperature dependence of the magnetization for ZFC and FC of the oleic acid-capped (Figure 7a) and uncapped (Figure 7b) Co nanoparticles under an applied magnetic field of 200 Oe. The ZFC and FC magnetization curves

Figure 5. Typical TEM and HRTEM images of the hcp-structured Co nanoparticles prepared with an oleic acid surfactant. Note the interparticle distance is close to the interpenetration distance of oleic acid chains (2.2 nm).

are split below the blocking temperature (TB), which is the transformation temperature from ferromagnetic to superparamagnetic states, and overlap with each other above TB as remanence and coercivity vanish.44,45 The ZFC curve of 10 nmsized Co nanoparticles in Figure 7a shows a maximum at about 15 K and decreases rapidly to zero at lower temperature, while the magnetization in the FC curve decreases slightly as the

Investigation and Behavior of Co Nanoparticles

Figure 6. XRD patterns of Co nanoparticles prepared with (a) 1 h of reaction time without oleic acid, (b) 1 h of reaction time with oleic acid, and (c) 2 h of reaction time with oleic acid. Vertical bars indicate the standard peak positions for the hcp Co crystals (JCPDS 05-0727).

Figure 7. Temperature dependence of magnetization measured with an applied field of 200 Oe for (a) surfactant-capped and (b) uncapped Co nanoparticles under the FC and ZFC processes. Note a cusp at TB ) 15 K, indicating the superparamagnetic nature of the Co nanoparticles.

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Figure 8. Hysteresis loops of oleic acid-capped Co nanoparticles measured at (a) 300 K and (b) 5 K.

particles have larger coercivities as the magnetization change cannot occur through a domain wall motion. As the magnetic spin moments are more and more affected by thermal fluctuations, this system shows superparamagnetic behavior due to the reduction of particle size below the size of a single domain.45 Our Co nanoparticles prepared by a surfactant-mediated polyol process are expected to be of single domains at this size value. Compared with that of the capped Co nanoparticles, the ZFC and FC curves of the submicron size uncapped Co nanopartricles (prepared without a surfactant) behave in a different way. The ZFC curve in Figure 7b shows a rise in magnetization at a low temperature for the uncapped Co nanoparticles, indicating the presence of defects like Co oxide in the shell with antiferromagnetic behavior.46 The magnetic moments of the Co nanoparticles come into the line of the applied field at a low temperature. A broad maximum in their ZFC curves indicates the magnetic behavior of agglomerated Co particles. The saturation magnetization (Ms) was measured at room temperature in a maximum applied field of 7 T by SQUID (Supporting Information). Figure 8 shows the hysteresis loops of the oleic acid-capped Co nanoparticles measured at room temperature (Figure 8a) and at 5 K (Figure 8b). As shown, the hysteresis loops are quite different in shape. The coercivity (Hc) at 5 K is as high as 870 Oe in comparison to its coercivity (Hc) at 300 K. Together with the ZFC curve in Figure 7a, Figure 8 shows the capped Co nanoparticles are superparamagnetic at 300 K. The Ms value of the Co nanocrystallites prepared by a polyol process is about 159 emu/g, which is very close to the saturation magnetization of bulk Co (Mbulk ) 162 emu/g) at room temperature. The nominal decrease of the Ms value for our capped Co nanoprticles in comparison to the bulk value might be due to the chemisorption of the surfactant as a carboxylate onto their surfaces. Magnetic susceptibility of the 10 nm sizecapped Co nanoparticles is higher than that of the submicrometer size-uncapped Co particles, as can be seen in their hysteresis loops taken at 7 T (Supporting Information). Conclusions

temperature increases to the 5-300 K range. The ZFC curve shows a cusp at 15 K for superparamagnetic Co nanoparticles. The Co nanoparticles can therefore be considered as magnetic single domains, with a blocking temperature TB around 15 K. Above 15 K, the hysteresis feature vanishes, and the nanoparticles manifest a superparamagnetic behavior. As the size of the particles decreases close to their critical diameter, formation of domain walls becomes energetically unfavorable.45 The

In this study, Co nanoparticles of a 10 nm average size with hcp structures were prepared by surfactant-mediated polyol synthesis. The chemical interaction between a fatty acid surfactant and the Co nanoparticles was studied using FT-IR spectroscopy, and the nature of the chemiadsorbed species on the particle’s surface was identified. FT-IR results revealed the existence of two new bands at about 1563 and 1408 cm-1, corresponding to the asymmetric (υasCOO-) and symmetric

5086 J. Phys. Chem. C, Vol. 113, No. 13, 2009 (υsCOO-) stretching bands, respectively, and two oxygen atoms in the carboxyl functional groups were symmetrically coordinated with the surface of the Co nanoparticle. The magnetic behavior of the Co nanoparticles was studied from their ZFC and FC magnetization curves and hysteresis loops. It was observed that the fatty acid-capped Co nanoparticles of 10 nm average size behave as single magnetic domains, with superparamagnetic behavior and high saturation magnetization at room temperature. Apart from the synthesis of the superparamagnetic Co nanoparticles, our results are useful for understanding the chemical interaction between fatty acids and metal particles of any kind. Acknowledgment. This research was supported by the Korea Science and Engineering Foundation through the Nano R&D Program (Grant 2007-02628) and the Brain Korea 21 project in 2008. Supporting Information Available: Comparisons of the TGA results between a Co nanoparticle and oleic acid, of a hysteresis loop between the surfactant-capped and uncapped Co nanoparticle, and of the magnetic susceptibility of Co particles and oleic acid-capped Co nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Chen, M.; Nikles, D. E. Nano Lett. 2002, 2 (3), 211. (2) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989. (3) Dakhlaoui, A.; Smiri, L. S.; Babadjian, G.; Schoenstein, F.; Molinie, P.; Jouini, N. J. Phys. Chem. C 2008, 112, 14348. (4) Shemer, G.; Tirosh, E.; Livneh, T.; Markovich, G. J. Phys. Chem. C 2007, 111, 14334. (5) Hadjipanayis, G. C. J. Magn. Magn. Mater. 1999, 200, 373. (6) Park, J.; Joo, J.; Kwon, S. G.; Jang, Y.; Hyeon, T. Angew. Chem., Int. Ed. 2007, 46, 4630. (7) Ngo, A. T.; Pileni, M. P. J. Phys. Chem. B 2001, 105, 53. (8) Rondinone, A. J.; Samia, A. C. S.; Zhang, Z. J. J. Phys. Chem. B 2000, 104, 7919. (9) Kang, Y. S.; Risbud, S.; Rabolt, J. F.; Stroeve, P. Chem. Mater. 1996, 8, 2209. (10) Puntes, V. F.; Krishan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. (11) Cordente, N.; Respaud, M.; Senocq, F.; Casanove, M. J.; Amiens, C.; Chaudret, B. Nano Lett. 2001, 1, 565. (12) Yao, C.; Zeng, Q.; Goya, G. F.; Torres, T.; Liu, J.; Wu, H.; Ge, M.; Zeng, Y.; Wang, Y.; Jiang, J. Z. J. Phys. Chem. C 2007, 111, 12274. (13) Chen, J. P.; Lee, K. M.; Sorensen, C. M.; Klabunde, K. J.; Hadjipanayis, G. C. J. Appl. Phys. 1994, 75, 5876. (14) Joo, J.; Yu, T.; Kim, Y. W.; Park, H. M.; Wu, F.; Zhang, J. Z.; Hyeon, T. J. Am. Chem. Soc. 2003, 125, 6553.

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