Influence of the Colloidal Environment on the Magnetic Behavior of

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Langmuir 2007, 23, 11740-11746

Influence of the Colloidal Environment on the Magnetic Behavior of Cobalt Nanoparticles Guangjun Cheng,† Cindi L. Dennis,‡ Robert D. Shull,‡ and A. R. Hight Walker*,† Optical Technology DiVision, Physics Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8443 and Metallurgy DiVision, Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8552 ReceiVed April 13, 2007. In Final Form: July 11, 2007 The magnetic properties of 10 nm diameter surfactant-coated cobalt (Co) nanoparticles in 1,2-dichlorobenzene (DCB) are investigated by a series of sequential magnetic moment (m) vs temperature (T) measurements. A rapid rise in magnetic moment around 250 K during warming and an abrupt drop at 234 K during cooling are observed when a nonsaturating external magnetic field is applied. Differential scanning calorimetry (DSC) measurements demonstrate that the rapid rise and abrupt drop in magnetization are associated with the melting and freezing of the solvent. Magnetic measurements of these Co nanoparticles in DCB are also used to probe their aging over a period of 70 days. The saturation magnetic moment of Co nanoparticles in DCB stored in air at room temperature decreases by nearly 40% over 70 days. Transmission electron microscopy (TEM) characterizations are reported to show the time evolution in the size, shape, and crystalline structures of DCB-immersed nanoparticles.

Introduction Since the 1960s, ferrofluids consisting of single-domain magnetic nanoparticles dispersed in a liquid carrier have found applications in semiconductor manufacturing, as a coolant for loudspeakers, and in magnetic seals.1 Recent advancements in synthesizing magnetic nanoparticles with controllable size and shape2 have improved the quality and quantity of ferrofluids available and expanded their applications into biology.3 In addition, ferrofluids provide an ideal system to investigate magnetic relaxation mechanisms and dipole-dipole interactions between magnetic nanoparticles. Frozen ferrofluids, where the magnetic nanoparticles are embedded in a frozen solvent, have been used to investigate the relaxation mechanism of the magnetic moment.4,5 The dipolar chains formed by the magnetic nanoparticles dispersed in a liquid carrier, with and without an external magnetic field, have been investigated both experimentally6 and theoretically.7 In all these cases, since the magnetic nanoparticles are dispersed and stored in a liquid carrier, a detailed study of how the colloidal environment affects their magnetic behavior would facilitate their potential applications. * To whom correspondence should be addressed. Phone: 301-975-2155. E-mail: [email protected]. † Optical Technology Division, Physics Laboratory. ‡ Metallurgy Division, Materials Science and Engineering Laboratory. (1) (a) Rosensweig, R. E. Ferrohydrodynamics; Cambridge University Press: Cambridge, 1985. (b) Du Tre´molet de Lacheisserie, EÄ .; Gignoux, D.; Schlenker, M. Magnetism; Kluwer Academic Publishers: Norwell, MA, 2002. (2) (a) Murray, C. B.; Sun, S.; Doyle, H.; Betley, T. MRS Bull. 2001, 26, 985. (b) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. (c) Hyeon, T. Chem. Commun. 2003, 927. (d) Leslie-Pelecky, D. L.; Rieke, R. D. Chem. Mater. 1996, 8, 1770. (e) Redl, F. X.; Black, C. T.; Papaefthymiou, G. C.; Sandstrom, R. L.; Yin, M.; Zeng, H.; Murray, C. B.; O’Brien, S. P. J. Am. Chem. Soc. 2004, 126, 14583. (3) (a) Pankhurst, Q. A.; Connolly, J.; Jones, S. K.; Dobson, J. J. Phys. D: Appl. Phys. 2003, 36, R167. (b) Tartaj, P.; Morales, M. P.; Veintemillas-Verdaguer, S.; Gonzalez-Carreno, T.; Serna, C. J. J. Phys. D: Appl. Phys. 2003, 36, R182. (c) Berry, C. C.; Curtis, A. S. G. J. Phys. D: Appl. Phys. 2003, 36, R198. (4) (a) Luo, W. L.; Nagel, S. R.; Rosenbaum, T. F.; Rosensweig, R. E. Phys. ReV. Lett. 1991, 67, 2721. (b) Zhang, J.; Boyd, C.; Luo, W. L. Phys. ReV. Lett. 1996, 77, 390. (5) (a) Erne´, B. H.; Butter, K.; Kuipers, B. W. M.; Vroege, G. J. Langmuir 2003, 19, 8218. (b) Klokkenburg, M.; Erne´, B. H.; Philipse, A. P. Langmuir 2005, 21, 1187. (c) Bu¨scher, K.; Helm, C. A.; Gross, C.; Glo¨ckl, G.; Romanus, E.; Weitschies, W. Langmuir 2004, 20, 2435.

In this paper, 10 nm diameter surfactant-coated cobalt (Co) nanoparticles synthesized in 1,2-dichlorobenzene (DCB) by thermo-decomposition2b,6d are used to investigate the effect of the colloidal environment on the magnetic behavior of the Co nanoparticles. The magnetic properties are investigated by a series of magnetic moment (m) measurements over a range of temperature between 4.2 and 300 K with and without an external magnetic field. Earlier, we demonstrated the formation of magnetic-field-induced chains of Co nanoparticles in a colloidal solution.6d Dipolar chain formation by 8 nm iron nanoparticles6b and 21 nm magnetite nanoparticles6c in colloidal dispersions in zero applied field have also been observed using cryogenic TEM. In the present study, we are particularly focused toward understanding how the dipolar chains formed by Co nanoparticles in DCB change under the influence of external magnetic fields of different magnitudes by monitoring the changes in magnetic moment of the ferrofluid. Since the solvent goes through a phase transition over the temperature range of interest, which creates a transition between a frozen and a liquid environment for the magnetic nanoparticles,8 differential scanning calorimetry (DSC) is used to investigate the thermal behavior of the ferrofluid. We also studied the shelf life and aging process of Co nanoparticles in DCB by monitoring the change in magnetization over time. The magnetic properties of the DCB-immersed Co nanoparticles stored in air for 70 days are compared with those of a freshly prepared sample. Transmission electron microscopy (TEM) is used to characterize the evolution in the size, shape, and crystalline structure of these nanoparticles over time. (6) (a) Donselaar, L. N.; Frederik, P. M.; Bomans, P. H. H.; Buining, P. A.; Humbel, B. M.; Philipse, A. P. J. Magn. Magn. Mater. 1999, 201, 58. (b) Butter, K.; Bomans, P. H. H.; Frederik, P. M.; Vroege, G. J.; Philipse, A. P. Nat. Mater. 2003, 2, 88. (c) Klokkenburg, M.; Vonk, C.; Claesson, E. M.; Meeldijk, J. D.; Erne´, B. H.; Philipse, A. P. J. Am. Chem. Soc. 2004, 126, 16706. (d) Cheng G.; Romero D.; Fraser G. T.; Hight Walker A. R. Langmuir 2005, 21,12055. (e) Zhang, L.; Manthiram, A. Phys. ReV. B 1996, 54, 3462. (7) (a) Stevens, M. J.; Grest, G. S. Phys. ReV. E 1995, 51, 5962. (b) Camp, M. J.; Shelley, J. C.; Patey, G. N. Phys. ReV. Lett. 2000, 84, 115. (c) Tlusty, T.; Safran, S. A. Science 2000, 290, 1328. (8) (a) Pakhomov, A. B.; Bao, Y.; Krishnan, K. M. J. Appl. Phys. 2005, 97, 10Q305. (b) Miyajima, H.; Inaba, N.; Taketomi, S.; Sakurai, M.; Chikazumi, S. J. Appl. Phys. 1988, 63, 4267. (c) Farrell, D.; Ding, Y.; Majetich, S. A. J. Appl. Phys. 2004, 95, 6636.

10.1021/la7010887 CCC: $37.00 © 2007 American Chemical Society Published on Web 10/09/2007

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Experimental Section

The average diameter of the nanoparticles is determined to be 10.0 nm with a standard deviation of 1.4 nm. The selected-area electron diffraction pattern in Figure 1b shows the characteristic diffraction rings of the -Co phase.10 Figure 1c shows the hysteresis loops of freshly prepared Co nanoparticles in DCB (the total weight of Co nanoparticles and DCB is 90 mg) at 298 and 5 K. At 298 K, the magnetic moment reaches its maximum value at 79.6 kA/m (1000 Oe) and is nearly constant for the strength of the external magnetic field values between 79.6 kA/m (1000 Oe) and 3.98 MA/m (50 000 Oe). In contrast, at 5 K, for fields above 79.6 kA/m (1000 Oe), the magnetic moment increases gradually with increasing field, and the magnetic moment is nearly saturated at 3.18 MA/m (40 000 Oe). An expanded plot is shown in the insert for fields between -79.6 kA/m (-1000 Oe) and +79.6 kA/m (+1000 Oe). At 5 K, the Co nanoparticles in DCB have a coercivity of 44.6 kA/m (561 Oe) and a remanence ratio (the ratio of magnetic moment at zero applied field, mr, to the saturation magnetic moment, ms) of 0.5. At 298 K the coercivity and remanence ratio are both near zero. In our previous work6d we showed that at room temperature Co nanoparticles in a colloidal solution formed linear chains along the field lines of an external magnetic field. The chains remained after removal of the magnetic field, but they lost their linearity. The Co nanoparticles bound in the chains could be redispersed into the colloidal solution by vigorous shaking or sonication. Thus, although the coercivity and remanence ratio are near zero at 298 K in our present sample, there are still interparticle interactions between the Co nanoparticles. In the absence of an external magnetic field, magnetic nanoparticles in a colloidal dispersion can form either isotropic aggregation by van der Waals attractions or dipolar chains by dipolar coupling.6a As shown experimentally by the results on the dipolar chains formed by 8 nm iron nanoparticles6b or 21 nm magnetite nanoparticles6c in a colloidal dispersion in zero applied magnetic field, in order to form the dipolar chains, the dipolar potential needs to exceed thermal fluctuation energies. Therefore, the dipolar coupling constant λ needs to be larger than 2, where λ ) µoµ2/(4πkBTσ3), µo) 4π × 10-7 J/(A2 m), µ is the magnetic moment per nanoparticle (estimated by the magnetic volume of a nanoparticle times the bulk magnetization), kB is the Boltzmann constant, T is the absolute temperature, and σ is the hard-sphere diameter, which is the diameter of the magnetic nanoparticle plus the thickness of surfactant layer. Here, we estimate that the dipolar coupling constant is 4.6 when the bulk magnetization (from hexagonal close-packed (hcp) Co) of 1.37 × 106 A/m1b is used for 10 nm diameter Co nanoparticles coated with a 2 nm organic surfactant layer at 298 K (σ ) 14 nm). Given the size distribution of 1.4 nm on the diameter of these nanoparticles, λ varies between 2.5 and 7.5. Therefore, we hypothesize that the dipolar chains of Co nanoparticles exist in DCB even in zero applied field. To further investigate the chains formed by Co nanoparticles in DCB, with and without an external magnetic field, we characterized these nanoparticles using a series of m vs T measurements as shown in Figure 2. Prior to the m vs T measurements, the colloidal sample was exposed to an applied magnetic field during the measurements for the hysteresis loops. Therefore, presumably the field-induced dipolar chains already exist in the solution at the beginning of the m vs T data measurement. The sample was then cooled to 4.2 K in zero applied field. The magnetization measurements shown in Figure 2a were made in zero applied field with a small starting magnetic moment (-1.50 × 10-4 mA m2) at 4.2 K. During warming, the

Chemicals. Dicobalt octacarbonyl (Co2(CO)8) containing 1-5% hexane as a stabilizer, oleic acid (OA, 99%), and 1,2-dichlorobenzene (DCB, 99%, anhydrous) were purchased from Aldrich (Milwaukee, WI).9 Trioctylphosphine oxide (TOPO, 90%) was purchased from Alfa Aesar (Ward Hill, MA).9 All chemicals were used without further treatment. Synthesis of Co Nanoparticles. Co nanoparticles were synthesized by a standard procedure.2b,6d First, 0.25 g of TOPO and 0.1 mL of OA were degassed in Ar in a flask for 20 min. Then 12 mL of DCB was introduced into the flask under an Ar atmosphere. The solution was heated to the reflux temperature of DCB (∼455 K), and ∼0.5 g of Co2(CO)8 dissolved in 3 mL of DCB was quickly injected into the 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. (The weight percentage of Co in DCB for the as-prepared colloidal solution is approximately 1%.) TEM Characterization. Images and electron diffraction patterns of Co nanoparticles were obtained on a HITACHI H-600 transmission electron microscope (100 kV).9 TEM samples were prepared by dropping the solution onto a carbon-coated TEM grid (Formvar/ Carbon Cu grids, purchased from Ted Pella, Inc., Redding, CA).9 The solvent was allowed to evaporate in air. Magnetic Characterization. A superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS)9 was used to measure the magnetic properties of the Co nanoparticles in DCB. The fresh colloidal sample was taken immediately after synthesis and loaded into a screw-sealed Kel-F sample holder (purchased from Lake Shore Cryotronics, Inc., Westerville, OH)9 for immediate magnetic measurements. (The total mass of the sample, including DCB, is 90 mg.) Afterward, the colloidal sample was stored in air and used as an aged sample for magnetic measurements. For comparison, a dried sample was prepared by placing the colloidal Co nanoparticles in a gelatin capsule, attracting the Co nanoparticles to the bottom using a hand-held magnet, extracting the solvent, and blowing the sample dry with argon. All of the samples were measured in the same manner. First, the hysteresis loop at 298 K was measured, starting at an applied magnetic field of 3.98 MA/m (50,000 Oe). Then, the sample was cooled to 5 K in zero applied field, and the hysteresis loop was measured at 5 K. For the m vs T measurements, the sample was first cooled from 300 to 4.2 K in zero applied field, followed by measuring the magnetic moment in zero applied field during warming the sample from 4.2 to 300 K and during the subsequent cooling from 300 to 4.2 K. Immediately afterward, an external magnetic field of 15.9 kA/m (200 Oe) was applied, and the magnetic moment was measured during warming from 4.2 to 300 K and while subsequently cooling from 300 to 4.2 K. Then, an external magnetic field of 39.8 kA/m (500 Oe) was applied, and the measurements were repeated during warming and cooling. Finally, the measurements were performed again with an applied field of 3.98 MA/m (50 000 Oe) during warming and cooling. DSC Characterization. DSC scans were performed with a model 2910 TA Instruments differential scanning calorimeter.9 Two samples, one of pure DCB solvent and the second of Co nanoparticles in DCB, were put into the stainless steel DSC cells and sealed with rubber O-rings. These two samples were used for the measurements with an empty cell as a reference. Measurements were made for five complete heating and cooling cycles between 218 and 298 K at a constant rate of 1 K/min.

Results and Discussion Figure 1a shows a TEM image of freshly prepared surfactantcoated Co nanoparticles. The Co nanoparticles form a twodimensional hexagonal array during drying on the TEM grid. (9) 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 does it imply that the materials or equipment identified are necessarily the best available for the purpose.

(10) Cheng, G.; Carter, J. D.; Guo, T. Chem. Phys. Lett. 2004, 400, 122.

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Figure 1. Characterization of freshly prepared Co nanoparticles: (a) TEM image, (b) selected area diffraction pattern, and (c) hysteresis loops measured at 5 and 298 K.

magnetic moment gradually increases an order of magnitude to 1.95 × 10-3 mA m2 as the temperature increases and then starts to decrease at 250 K, reaching a minimum of 1.37 × 10-3 mA m2 at 265 K. Afterward, the magnetic moment stays nearly constant as the temperature increases from 265 to 300 K. During cooling, the magnetic moment remains almost constant as the temperature decreases from 300 to 239 K. However, there is an abrupt drop from 1.43 × 10-3 mA m2 at 239 K to 1.20 × 10-3 mA m2 at 234 K. Below 234 K, the magnetic moment remains nearly constant as the temperature decreases to 4.2 K. The total change in the magnetic moment over the whole temperature range is relatively small at zero applied field. Since the field-induced chains exist in the solution prior to freezing, the initial magnetic moment in the absence of the external field is dependent on the state in which the intertwined chains are trapped during freezing. The field magnitude, prior to zero field cooling, may be as high as 3.98 MA/m (50 000 Oe) or as low as 79.6 kA/m (1000 Oe). However, the magnet is reset immediately prior to cooling, so the sample is “zero field cooled” in a field of magnitude less than 159 A/m (2 Oe). In our repeat measurements, different initial magnetization values and different magnetization changes around 250 K are observed during warming in zero applied field because a different state of intertwined chains is captured and embedded in frozen DCB.

However, the cooling curves with the abrupt drop at 234 K are repeatable for all measurements in zero applied field. When an external field of 15.9 kA/m (200 Oe) was applied at 4.2 K as shown in Figure 2b, the magnetic moment increases from 1.07 × 10-3 to 5.18 × 10-3 mA m2 due to the increased alignment of magnetic moments along the applied field direction. As the temperature increases to 210 K, the magnetic moment gradually increases to 2.23 × 10-2 mA m2, then it increases rapidly around 250 K, reaching 3.21 × 10-2 mA m2 at 260 K, and then gradually increases again to 3.57 × 10-2 mA m2 at 300 K. During the subsequent cooling, the magnetic moment remains nearly constant as the temperature decreases from 300 to 239 K. The magnetic moment has an abrupt drop from 3.64 × 10-2 mA m2 at 239 K to 3.40 × 10-2 mA m2 at 234 K and then remains nearly constant as the temperature decreases from 234 to 4.2 K. The measurements with an applied field of 39.8 kA/m (500 Oe) in Figure 2c show a similar trend to the ones with 15.9 kA/m (200 Oe): a rapid rise of magnetic moment around 250 K during warming and an abrupt drop at 234 K during cooling. When an external field of 3.98 MA/m (50 000 Oe) is applied, there is an increase in magnetic moment around 260 K during warming, but the abrupt drop at 234 K during cooling is negligible. Furthermore, an increase in magnetic moment is observed with decreasing temperature, which is in agreement with the difference in magnetic

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Figure 3. DSC measurements of (a) pure DCB and (b) colloidal Co nanoparticles in DCB. Figure 2. Sequential m vs T measurements (from a to d) of freshly prepared Co nanoparticles in DCB with an applied magnetic field of (a) 0 kA/m, (b) 15.9 kA/m (200 Oe), (c) 39.8 kA/m (500 Oe), and (d) 3.98 MA/m (50 000 Oe). The red curves were measured during warming and the blue curves during cooling.

moment values from the hysteresis measurements at 3.98 MA/m (50 000 Oe) between 5 and 298 K in Figure 1c. The deviation from Bloch law at low temperatures in a saturating field, attributable to the surface spin disorder, has been reported in other magnetic nanoparticle systems.11 Since the Co nanoparticles are dispersed in DCB, the thermal behavior of DCB in the colloidal solution was measured using DSC. Figure 3 shows the DSC measurements on (a) pure DCB and (b) Co nanoparticles in DCB. Figure 3a shows that for pure DCB all five heating curves nearly overlap and there is a symmetric endothermic peak centered at 256.8 K, close to the melting point of DCB (255 to 256 K) as provided by the supplier. Freezing of DCB is clearly demonstrated by the sharp exothermic peak near 233 K, indicating that crystallization of DCB experiences supercooling. The mean value of the onset freezing temperature was 233.0 K with a standard deviation of 1.3 K for five measurements. The thermal behavior of the Co nanoparticles in DCB in Figure 3b is similar to that of pure DCB in Figure 3a. The symmetric endothermic peak centers at 257.2 K, and the average of five onset freezing points is 232.4 K with a standard deviation of 4.5 K. In the presence of Co nanoparticles, the melting point and onset freezing point of DCB do not change (11) (a) Kodama, R. H.; Berkowitz, A. E.; McNiff, E. J.; Foner, S. Phys. ReV. Lett. 1996, 77, 394. (b) Kodama, R. H.; Berkowitz, A. E.; McNiff, E. J.; Foner, S. J. Appl. Phys. 1997, 81, 5552. (c) De Biasi, E.; Ramos, C. A.; Zysler, R. D. Phys. ReV. B 2002, 65, 144416. (d) Aquino, R.; Depeyrot, J.; Sousa, M. H.; Tourinho, F. A.; Dubois, E.; Perzynski, R. Phys. ReV. B 2005, 72, 184435. (e) Caizer, C. Appl. Phys. A 2005, 80, 1745.

Figure 4. m vs T measurements of dried Co nanoparticles with an applied magnetic field of 15.9 kA/m (200 Oe). The red curve was measured during warming and the blue curve during cooling.

significantly, suggesting that the Co nanoparticles in DCB do not appreciably alter the nucleation of the DCB. The rapid rise in magnetic moment around 250 K during warming and abrupt drop at 234 K during cooling correspond well to the melting point (257.2 K) and onset freezing point (232.4 K) of the DCB from the DSC analysis. The thermal behavior associated with the freezing and melting of DCB also provides supporting evidence. The symmetry of the endothermic peak in the DSC measurement is reflected by the smooth continuous rise in the magnetization around 250 K, while the asymmetry of the exothermic DSC peak is reflected by the abrupt drop in the magnetization at 234 K. For comparison with the magnetic measurements for Co nanoparticles in DCB, Figure 4 shows the m vs T measurements for a 10 mg dried Co nanoparticle sample using an external magnetic field strength of 15.9 kA/m (200 Oe). A gradual increase

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in the magnetic moment is observed as the temperature increases from 4.2 to 300 K. Also, the magnetic moment changes little as the temperature decreases from 300 to 4.2 K. Most notably, without the DCB solvent, the rapid rise in magnetic moment around 250 K during warming and abrupt drop at 234 K during cooling disappear. Just as in the relaxation of magnetic moments upon removal of an external magnetic field,4,5 two mechanisms are involved in the rotation of magnetic moments of isolated magnetic nanoparticles in the presence of an external field: Ne´el rotation where the magnetic moment rotates with respect to the crystalline axes in the particle and Brownian rotation where the nanoparticle itself physically rotates to align its magnetic moment with the external magnetic field. In this system, these basic mechanisms are complicated by the interparticle interactions which cause cooperative alignment between the neighboring nanoparticles.6e In the dried sample the chains are in close contact with the sample holder and each other and lack a colloidal environment to allow physical movement. Here, during warming, the thermal energy is provided gradually and a Ne´el rotation is possible to align the magnetic moments of Co nanoparticles in the chains with the field. Once the maximum alignment with the applied field is reached, the magnetic moment changes little, as shown in Figure 4 as a plateau. During cooling the dipole-dipole couplings maintain the alignment of these magnetic dipole moments and the measured values of the magnetic moment remain essentially constant. In frozen ferrofluids magnetic nanoparticles and their dipolar chains are embedded in frozen solvent. Studies have shown that the size, size distribution, and aggregation of magnetic nanoparticles affect their magnetic relaxation behavior.5c It was long held that in a frozen ferrofluid Brownian rotation by individual magnetic nanoparticles and the rotation of the whole dipolar chain were prohibited and only Ne´el rotation was possible.4,5 However, recent results from measurements on the dynamic magnetic susceptibility of magnetic nanoparticles in a frozen solvent (10 K below the onset freezing point of the solvent) provide evidence for the existence of local Brownian rotation of individual nanoparticles within a chain.5b As shown in Figure 2, each time the field magnitude is increased at 4.2 K, the magnetic moment increases, despite the DCB being completely frozen at that temperature. For example, after cooling to 4.2 K in an applied field of 15.9 kA/m (200 Oe), when the field magnitude increases to 39.8 kA/m (500 Oe), the magnetic moment increases from 3.04 × 10-2 to 3.52 × 10-2 mA m2. However, the induced chains formed in an applied field of 15.9 kA/m (200 Oe) are trapped in the frozen DCB and cannot physically move to form the favorable structures in the new applied field of 39.8 kA/m (500 Oe) until the temperature increases to near the melting point of the solvent. Though these trapped chains cannot move physically at 4.2 K, Co nanoparticles in the chains can align their magnetic moments with the field by a cooperative Ne´el rotation when a new field with a higher magnitude is applied. As the temperature gradually increases from 4.2 K during warming in Figure 2, but still below the melting temperature of the solvent, the thermal energy gained by the Co nanoparticles enables them to further align their magnetic dipole moments by cooperative Ne´el rotation and, near the DCB melting point, by local Brownian rotation within the chains, resulting in the gradual increase in magnetic moment. Once the solvent melts the chains can physically rotate as a whole and move to form the favorable structure under the new applied magnetic field (in agreement with the results from our previous studies6d). Investigations on the field-dependent optical

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transmission of magnetic fluid films demonstrate that magnetic nanoparticles form magnetic columns under the influence of a magnetic field and as the field strength increases more columns are formed.12 Thus, the rapid rise in magnetic moment we observe around 250 K during warming is due to the increased alignment with the field resulting from the physical movement and rearrangement processes of these dipolar chains in the applied field. At 300 K, the Co nanoparticles are in a liquid environment, and they are in a chain structure favored at that applied magnetic field. During cooling, the magnetic moment should be nearly constant since the dipole-dipole couplings are the predominant interaction as the dried sample data show in Figure 4. This is the case before and after the freezing of the solvent. Certainly the freezing and crystallization of DCB account for the abrupt drop in magnetic moment at 234 K during cooling. The freezing of DCB experiences a supercooling process. Once freezing starts, solidification of DCB will proceed immediately because the solution is already 25 degrees below the equilibrium freezing temperature of DCB. During crystallization of DCB the sharp peaks in the DSC data show that the solidification process is fast, meaning that there is quick motion in random directions of many solid/liquid interfaces. A stress could be exerted on the chains due to the sudden crystallization of DCB, causing distortions in the shape of the chains. Also, the burst of the heat of freezing can be conducted to and absorbed by the Co nanoparticles, as demonstrated in the industrial application of ferrofluids as coolants.1 For the field-induced chains there are dipole-dipole interactions among the nanoparticles and magnetic field-chain interactions. A competition exists between retaining the alignment of the chains with the applied field and the disrupting effect of heat and stress. At fields lower than the saturating field, which is approximately 79.6 kA/m (1000 Oe) at 298 K, the stress and heat could induce the misalignment of magnetic moments of the Co nanoparticles in the chains away from the external field during freezing, resulting in the abrupt drop in the magnetization (Figure 2b and 2c). When a saturating external magnetic field of 3.98 MA/m (50 000 Oe) is applied, the magnetic moments of the Co nanoparticles in the chains are perfectly aligned with the field and the interactions between the external field and both the chains and individual nanoparticles are sufficiently strong to overcome the effect of stress and heat burst during freezing (Figure 2d). The differences in magnetic moment between cooling and warming in each applied field (b, c, and d in Figure 2) show the differences in physical and magnetic alignments between the induced chains formed in the new applied field and the trapped chains formed in the previous applied field. Figure 5 shows the plots for the corresponding difference in magnetic moment between cooling and warming as a function of temperature for each applied field. As the magnitude of applied field increases, from b to d in Figure 5, the difference in magnetic moment decreases. When a nonsaturating field is applied (curves b and c in Figure 5), the difference decreases gradually as the temperature increases from 4.2 to 232.4 K, consistent with the analysis above that the thermal energy provides the gradual alignment of magnetic dipole moments of Co nanoparticles in the trapped chains with the direction of the applied field by a cooperative Ne´el rotation and, near DCB melting point, by local Brownian rotation within the chains. However, when a saturating (12) (a) Hong, C. Y.; Chen, C. A.; Chen, C. H.; Horng, H. E.; Yang, S. Y.; Yang, H. C. Appl. Phys. Lett. 2001, 79, 2360. (b) Yang, S. Y.; Chiu, Y. P.; Jeang, B. Y.; Horng, H. E.; Yang, H. C. Appl. Phys. Lett. 2001, 79, 2372.

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Figure 5. Corresponding difference in m between cooling and warming for (b), (c), and (d) in Figure 2 as a function of temperature.

field is applied (curve d in Figure 5), the differences in magnetic moment between cooling and warming are small and remain almost constant as the temperature increases from 4.2 to 232.4 K, suggesting that the applied field is strong enough to align magnetic dipole moments of Co nanoparticles in the trapped chains with the direction of the applied field and that the thermal energy does not significantly change the magnetic alignment of the chains with the field. Concerns associated with the application of ferrofluids include lack of knowledge about their shelf life and aging mechanism. After storage in air for 70 days, a purplish color appears in the original black colloidal solution. The color is indicative of the presence of solvated Co atoms or ions (the exact chemical composition has not been identified), suggesting that, over time, the nanoparticles leach Co into the DCB. The aging process of these colloidal nanoparticles is also exhibited by changes in the magnetic behavior. In comparison with the initial m vs T measurements in Figure 2, Figure 6 shows the m vs T measurements for the same sample of Co nanoparticles in DCB after aging at room temperature for 70 days in air. First, the rapid rise in the magnetic moment around 250 K during warming and the abrupt drop at 234 K during cooling are still present for the nonsaturating external magnetic fields (Figure 6b and 6c). Second, at the same temperature and field the magnetic moment has decreased significantly for the aged sample. For example, in a field of 15.9 kA/m (200 Oe), the magnetic moment for the freshly prepared sample at 300 K was 3.59 × 10-2 mA m2, but it has decreased by 50% to 1.69 × 10-2 mA m2 for the 70-day-old sample. Third, an increase in magnetic moment (which follows a paramagnetic 1/T behavior), not present for the freshly prepared sample in Figure 2, is observed when the temperature decreases from 15 to 4.2 K during cooling in a 15.9 kA/m (200 Oe) field (Figure 6b). The same paramagnetic behavior in the lowtemperature region (between 4.2 and 15 K) appears in the warming and cooling curves for the 39.8 kA/m (500 Oe) data in Figure 6c but with a greater magnitude, consistent with the positive susceptibility of the paramagnetic component. This result is in agreement with the presence of solvated Co atoms or ions in the solution, as deduced from the observed color change of the colloidal solution. Figure 7a shows the hysteresis loops at 5 and 298 K for the 70-day-old sample of Co nanoparticles in DCB. Compared with the freshly prepared sample at 5 K, the aged sample has a saturation magnetic moment of 3.55 × 10-2 mA m2, a decrease of nearly

Figure 6. Sequential m vs T measurements (from a to d) of 70day-old Co nanoparticles in DCB with an applied magnetic field of (a) 0 kA/m, (b) 15.9 kA/m (200 Oe), (c) 39.8 kA/m (500 Oe), and (d) 3.98 MA/m (50 000 Oe). The red curves were measured during warming and the blue curves during cooling.

40%. At 5 K the sample has a coercivity of 21.1 kA/m (265 Oe), while at 298 K the coercivity is near zero. TEM characterization was also performed on the aged sample. In contrast to the observed initial monodisperse spherical Co nanoparticles and their two-dimensional hexagonal arrays, the TEM image in Figure 7b shows, after aging in DCB for 70 days, that the Co nanoparticles no longer have a uniform distribution on the TEM grid and tend to aggregate. Furthermore, some aged Co nanoparticles are no longer spherical but rather more irregular in shape. The average diameter of the Co nanoparticles increased to 10.8 nm with an accompanying increase in the standard deviation to 1.7 nm. This indicates that Ostwald ripening,13 the growth of larger particles at the expense of the smaller particles, may play an important role in the aging process of these nanoparticles. During this process the smaller Co nanoparticles slowly dissolve, leaching Co (atoms or ions) into the solution, which then either remain in the solution or grow onto a larger nanoparticle. The observed color change, from black to purple, in the aged solution supports this conclusion. This spontaneous process occurs because larger particles are more thermodynamically favored than smaller particles. Finally, the electron (13) (a) Kolthoff, I. M.; Bowers, R. C. J. Am. Chem. Soc. 1954, 76, 1503. (b) Narayanan, R.; El-Sayed, M. A. J. Am. Chem. Soc. 2003, 125, 8340. (c) Hu, J.; Liu, Y. Langmuir 2005, 21, 2121. (d) Stahl, M.; Aslund, B.; Rasmuson, A. C. Ind. Eng. Chem. Res. 2004, 43, 6694.

11746 Langmuir, Vol. 23, No. 23, 2007

Cheng et al.

Figure 7. Characterization of the 70-day-old Co nanoparticles in DCB: (a) Hysteresis loops measured at 5 and 298 K, (b) TEM image, and (c) selected area diffraction pattern.

diffraction ring patterns in Figure 7c, though not as distinct as the ones for the freshly prepared sample shown in Figure 2b, still show the existence of -phase Co nanoparticles. This indicates that there is no significant amount of cobalt oxide forming on the surface (at least within the resolution of the TEM), despite the sample being stored in air.10 However, this does not eliminate the possibility that oxygen or water in the colloidal environment may affect the surfactant shell or perhaps cause further chemical activity.

Conclusion A series of m vs T measurements is systematically performed to study the magnetic properties of 10 nm diameter Co nanoparticles in DCB. A rapid rise in magnetic moment around 250 K during warming and an abrupt drop in magnetic moment at 234 K during cooling are observed when a nonsaturating magnetic field is applied. DSC measurements correlate that the rapid rise and abrupt drop in magnetic moment with the melting and freezing of the DCB. Further analysis shows that the rapid rise in magnetization is due to the ability of the physical movement

and rearrangement of the induced nanoparticle chains. The stress exerted on the chains and heat released due to the sudden crystallization of DCB could contribute to the abrupt drop in magnetic moment at 234 K. Finally, the aging process of these Co nanoparticles in DCB is probed by magnetic measurements over a period of 70 days. The saturation magnetic moment of these nanoparticles has been found to decrease with time due in part to the leaching of Co atoms or ions into the DCB. In addition, a paramagnetic behavior in the low-temperature region (between 4.2 and 15 K) is observed in the m vs T measurements in a nonsaturating field. TEM characterizations show that these nanoparticles maintain their -Co crystalline structure, although their average diameter increases over time. Acknowledgment. We acknowledge the help of Mr. Tiejun Zhang at the University of Maryland, College Park, for TEM measurements. We thank Mr. Dale P. Bentz (Materials and Construction Research Division, NIST) for his help with DSC measurements. LA7010887