Stable Amorphous Cobalt Nanoparticles Formed by an in Situ Rapidly

Jul 14, 2009 - To minimize or eliminate any remnant magnetization in the sample space (i.e., the location where measurement of the sample will take pl...
0 downloads 0 Views 3MB Size
Download PDF
pubs.acs.org/Langmuir © 2009 American Chemical Society

)

Stable Amorphous Cobalt Nanoparticles Formed by an in Situ Rapidly Cooling Microfluidic Process Yujun Song,*,†,‡ Laurence L. Henry,§ and Wantai Yang †

)

Key State Laboratory of Aerospace Materials & Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China, ‡Center for Advanced Micro Structures and Devices (CAMD), Louisiana State University, Baton Rouge, Louisiana 70806, §Department of Physics, Southern University and A & M College, Baton Rouge, Louisiana 70813, and College of Materials and Engineering, Beijing University of Chemical Technology, Beijing 100029, China Received March 20, 2009. Revised Manuscript Received May 24, 2009 The controlled synthesis of nanoparticles (NPs) with stable crystal structures and stable physical and chemical properties is a key issue for commercial applications. The use of a microfluidic reactor (MR) process has proven to be a flexible approach to control the fine crystal structures and the magnetic properties during the ripening and aging of the NPs. We have developed an in situ rapidly cooling microfluidic process (IRCMP) in which Co NPs with stable crystal structures and magnetic properties are synthesized by using elevated reaction temperatures followed by rapid quenching of the colloids to reduced temperatures. The Co NPs that are obtained by this process demonstrate stable crystal structures and stable magnetic properties for a much longer period of time (at least 3 months) than for Co NPs obtained by performing the reaction and the quenching processes at room temperature or under sonication.

1. Introduction Over the past decade, much attention has been given to using microfluidic reactor (MR) processes in the synthesis and preparation of specific materials as a result of easy in situ spatial and temporal control of the reaction kinetics, along with highly efficient mass and heat transfer.1-9 Recently, application of microfluidic reactors techniques have been expanded beyond the improvement of the chemical reaction efficiency to the controlled synthesis of micro- and nanoscale materials.6,10-20 *Corresponding author. E-mail: [email protected].

(1) Watts, P.; Wiles, C. Chem. Commun. 2007, 443–467. (2) Fletcher, P. D. I.; Haswell, S. J.; Paunov, V. N. Analyst 1999, 124, 1273–1282. (3) Sounart, T. L.; Safier, P. A.; Voigt, J. A.; Hoyt, J.; Tallant, D. R.; Matzke, C. M.; Michalske, T. A. Lab Chip 2007, 7, 908–911. (4) Wang, H.; Li, X.; Uehara, M.; Yamaguchi, Y.; Nakamura, H.; Miyazaki, M.; Shimizu, H.; Maeda, H. Chem. Commun. 2004, 48–50. (5) Pennemann, H.; Watts, P.; Haswell, S. J.; Hessel, V.; Lowe, H. Org. Process Res. Dev. 2004, 8, 422–439. (6) Edel, J. B.; Fortt, R.; deMello, J. C.; deMello, A. J. Chem. Commun. 2002, 1136–1137. (7) deMello, A. J. Nature 2006, 442, 394–402. (8) Sahoo, H. R.; Kralj, J. G.; Jensen, K. F. Angew. Chem., Int. Ed. 2007, 46, 5704–5708. (9) Song, Y.; Kumar, C. S. S. R.; Hormes, J. Small 2008, 4, 698–711. (10) Brivio, M.; Oosterbroek, R. E.; Verboom, W.; Goedbloed, M. H.; van den Berg, A.; Reinhoudt, D. N. Chem. Commun. 2003, 1924–1925. (11) He, S.; Kohira, T.; Uehara, M.; Kitamura, T.; Nakamura, H.; Miyazaki, M.; Maeda, H. Chem. Lett. 2005, 34, 748–749. (12) Zhao, B.; Moore, J. S.; Beebe, D. J. Science 2001, 291, 1023–1026. (13) Kenis, P. J. A.; Ismagilov, R. F.; Whitesudes, G. M. Science 1999, 285, 83– 85. (14) Boleininger, J.; Kurz, A.; Reuss, V.; Sonnichsen, C. Phys. Chem. Chem. Phys. 2006, 8, 3824–3827. (15) Song, Y.; Zuo, J.; Zhang, T.; Jin, P.; Han, L.; Yang, W.; Jiang, L.; Zhao, S. J. Nanopart. Res. 2008, under review. (16) Song, Y.; Zhang, T.; Yang, W. T.; Albin, S.; Henry, L. L. Cryst. Growth Des. 2008, 8, 3766–3772. (17) Krishnadasan, S.; Brown, R. J. C.; deMello, A. J.; deMello, J. C. Lab Chip 2007, 1434–1441. (18) Hung, L.-H.; Lee, A. P. J. Med. Biol. Eng. 2007, 27, 1–6. (19) Jahn, A.; Reiner, J. E.; Vreeland, W. N.; DeVoe, D. L.; Locascio, L. E.; Gaitan, M. J. Nanopart. Res. 2008, 10, 925–934. (20) Song, Y.; Kumar, C. S. S. R.; Hormes, J. J. Nanosci. Nanotechnol. 2004, 4, 788–793.

Langmuir 2009, 25(17), 10209–10217

An important goal for magnetic NPs is controlling the size, shape, and crystal structure for stable crystal structures and stable magnetic properties.16,21 The magnetic properties depend not only on the interaction between atoms but also on the constructed compounds, which are in turn dependent on the combination of particle size, shape, and crystal structure.22 Hence, developing a method to control these parameters during the synthesis process should provide a very effective technique to produce the final magnetic properties of the NPs. It is not a trivial thing to achieve the above goals by routine methods. The main reason is difficulty in preventing aggregation and coarsening of the NPs, caused by Ostwald ripening (OR), the oriented attachment (OA) process, and concurrent phase transformations, which often occur in the simple batch process. These effects also appear in microfluidic reactor processes if the growth of the NPs is not sufficiently controlled. Hence, process optimization needs to be performed to suppress these conditions, even in MR processes. With appropriate optimization, microfluidic processes can provide excellent kinetic control of nanoparticle formation at different stages. Hence, it can be used to obtain the desired morphology, crystal structure, and magnetic property of the nanoparticles. In this article, the evolution of the magnetic property with the growth of nanoparticles is first investigated to elucidate the effect of the OR and OA processes on the magnetic properties of NPs. Then we present results of investigations on cobalt NPs that were fabricated by the developed in situ rapidly cooling microfluidic process (IRCMP), which involves performing the reaction at elevated temperatures followed by rapid quenching of the colloids to reduced temperatures. (21) Song, Y.; Henry, L. L. Nanoscale Res. Lett. 2009, available online, DOI 10.1007/s1671-009-9369-8. (22) Cullity, B. D. Introduction to Magnetic Materials; Addison-Wesley: Menlo Park, CA, 1972; Chapter 1, pp 7 and 17; Chapter 7, p 357.

Published on Web 07/14/2009

DOI: 10.1021/la9009866

10209

Article

Song et al. Scheme 1. In Situ Rapid Cooling Microfluidic Process (IRCMP) for the Synthesis of Stable Amorphous Cobalt Nanoparticles

Table 1. Magnetic Property Change for Nanoparticles under Different Reaction and Quenching Conditionsa experimental no.

R-1

R-2

R-3 Fresh

R-4 Aged

Fresh

R-5 Aged

Fresh

R-6 Aged

Fresh

Aged

reaction temperature (°C) 20 20 20 20 20 50 ripening time (h) 0 3 >8 >8 0 0 quenching temperature (°C) 20 20 20 20 20 4 flow rate (mL/min) 0.9 0.9 0.9 0.08 0.9 0.0 0.9 sonication or not no no no no yes no diameter (nm) 3.5 3.6 3.8 8.2 4.7 3.7 3.7 3.9 10 K 63 77 74 19 136 76 16 16 144 154 Ms (emu/g) 300 K 63 85 86 21 118 76 5.6 1.5 152 155 26 34 44 8 69 35 1.6 0.4 94 56 Mr at 10 K (emu/g) 0.41 0.44 0.51 0.42 0.51 0.46 0.1 0.03 0.65 0.36 Mr/Ms at 10 K 10 K 330 2800 3500 5350 1400 4450 1550 350 1350 1050 Hc (Oe) 300 K 20 16 10 18 30 36 18 15 7 35 30 35 35 35 13 35 40 40 13 14 Hs At 10 K (kOe) 125 180 300 ∼330 >300 >330 170 300 310 ∼340 Tb (K) 7 3 2.0 2.5 3.5 ∼0.4 >1.8 2.1 3.8 ∼3.7 K  10 (erg/cm ) hysteresis loop shape at 10 K W W NW NW NW NW NW NW NW NW a Fresh: the nanoparticles as synthesized. Aged: the powder nanoparticles aged for 3 months. Ms: saturation magnetism, Mr: remnant magnetism. Hc: coercivity. Hs: saturation magnetic field. Tb: blocking temperature. W: wasp waist. NW: no wasp waist. K: the anisotropy constant, K = 25kBTb/V. kB and V are the Boltzmann constant and volume of nanoparticles, respectively.

The Co NPs obtained by this process demonstrate more stable crystal structures and magnetic properties for a much longer period of time (at least 3 months) than for Co NPs obtained by performing the reaction and quenching processes at room temperature or under sonication.

2. Experiments 2.1. Synthesis of Co Nanoparticles at Different Reaction Conditions. Cobalt nanoparticles were synthesized in a microfluidic reactor by the reduction of CoCl2 in tetrahydrofuran (THF) solution using lithium hydrotriethylborate (Li[B(C2H5)3H]) as a reducing agent and 3-(N,N-dimethyldodecylammonia)-propanesulfonate (SB12) as a stabilizer according to the following chemical (23) Song, Y.; Kumar, C. S. S. R.; Hormes, J. J. Micromech. Microeng 2004, 14, 932–940. (24) Song, Y.; Modrow, H.; L., H. L.; K., S. C.; Doomes, E. E.; Palshin, V.; J., H.; R., K. C. S. S. Chem. Mater. 2006, 18, 2817–2827. (25) Bonnemann, H.; Braun, G.; Brijoux, W.; Brinkmann, R.; Schulze, A. T.; Seevogel, K.; Siepen, K. J. Organomet. Chem. 1996, 520, 143–162. (26) Glavee, G. N.; Klabunde, K. J.; Sorensen, C. M.; Hadjipanayis, G. C. Inorg. Chem. 1993, 32, 474–477.

10210 DOI: 10.1021/la9009866

reactions:20,23-27 C12 H25 NðCH3 Þ2 ðC3 H6 -SO3 Þ THF

þ LiBEt3 H sf C12 H25 NðCH3 Þ2 ðC3 H6 -SO3 Þ 3 LiBEt3 H C12 H25 NðCH3 Þ2 ðC3 H6 -SO3 Þ 3 LiBEt3 H THF

þ CoCl2 sf Cocolloid 3 C12 H25 NðCH3 Þ2 ðC3 H6 -SO3 Þ 3 LiCl 1 þ BEt3 þ H2 2 The design and fabrication of the microfluidic reactor has been discussed extensively in our previous publications.9,20,23,24,27 Scheme 1 shows a schematic of the MR processes. Two preheating devices are used to control the reactant temperatures, one thermostatic chamber is used to maintain the microfluidic reactor at a constant temperature, and one chiller is used to adjust the temperature of the quenching solution. Table 1 presents the (27) Song, Y.; Domes, E. E.; Prindle, J.; Tittsworth, R.; Hormes, J.; Kumar, C. S. S. R. J. Phys. Chem. B 2005, 109, 9330–9338.

Langmuir 2009, 25(17), 10209–10217

Song et al. magnetic property data for nanoparticles under different reaction and quenching conditions. The differing reaction and quenching conditions represented by “experimental no.” are indicated as R-1 through R-6. R-1, R-2, and R-3 represent the reaction and quenching process carried out at room temperature (RTMP). The processes are briefly described below, and additional details can be found elsewhere.20,23-27 The saturated CoCl2/THF solution (4 g/L) and the reducing agent THF solution (complex of Li[B(Et)3H] with SB12 in THF and 200 mL of SB12 (3.4 g/L) and Li[B(Et)3H] (9.0 g/L)) are delivered to the microfluidic reactor at room temperature using self-priming pumps (120SPI-30, BioChem Valve Inc.) at a flow rate of 0.9 mL/min, and the reaction is performed at room temperature (20 ( 2 °C). Three different types of nanoparticles were collected in the receiver flask: NPs from R-1 were used as synthesized with no Ostwald ripening, NPs from R-2 were Ostwald ripened for 3 h, and NPs from R-3 were Ostwald ripened for more than 8 h. All three sets of NPs were quenched in a mixture of ethanol and THF (1:3 v/v) directly in the flask. NPs from R-4 underwent the same reaction condition as those in R-3 with the exception that the flow rate was decreased to 0.08 mL/min. NPs from R-5 experienced the same reaction conditions as those in R-1 but were collected under sonication when the nanoparticle solution entered the receiver flask with the quenching solution. In R-6, the reaction was carried out at 50 °C, and the nanocolloids were quenched immediately in 100 mL of the quenching solution under rapid stirring at a temperature of approximately 4 °C that is controlled by a chiller (IRCMP). All processes, R-1 through R-6, were performed under an inert atmosphere of N2. Following the precipitation of the Co nanoparticles, the supernatant solution was decanted and the particles were washed three times using a mixture of ethanol and THF (20 V% ethanol) to remove the reducing agent and most of the surfactants. Finally, the particles were dried under vacuum to obtain a fine black powder with a metal content of 60-75%, as determined by TGA. One of the TGA curves (R-5) (i.e., heating the particle powder to 1300 °C under an N2 atmosphere) is shown in Supporting Information (Figure S1). It suggests about a 35% impurity in the NPs. In addition, the powders from R-3, R-5, and R-6 were aged for 3 months at room temperature under an inert atmosphere (N2 gas). These will be used for comparison with the as-prepared samples. 2.2. Characterization of Cobalt Nanoparticles. Magnetization data were collected using a Quantum Design MPMS-5S superconducting quantum interference device (SQUID) magnetometer system.22 The measurements were carried out on powder samples. To carry out the measurements, the powder samples were placed in gelatin capsules under inert gas conditions (N2) inside a drybox. A wad of cotton was stuffed inside the capsule to keep the powder stationary. Care was also taken to minimize air exposure to the sample as the capsule containing the powder sample was transferred into the magnetometer interlock. To minimize or eliminate any remnant magnetization in the sample space (i.e., the location where measurement of the sample will take place) in the magnetometer, the magnetic field at the location was cycled from þ5 T to 0 by alternating between decreasing positive and negative values before the sample was placed there. Because it is important to minimize the possibility of air having leaked into the sample chamber during the transfer of the sample and during the measurements, resulting in contamination of the sample data, the sample chamber temperature was periodically raised to 300 K and the chamber was purged before starting the measurements and in between sets of measurements. Both magnetization M versus temperature T (i.e., M vs T) and magnetization versus applied magnetic field H (i.e., M vs H) measurements were performed. The measurement sequence was set up so that for a sample the M versus T set of measurements was performed first, and the M versus H set of measurements followed. For the M versus T data, both zero-field-cooled (ZFC) and fieldcooled (FC) measurements were made. For the ZFC measurements, the sample was first cooled from room temperature to 10 K in zero Langmuir 2009, 25(17), 10209–10217

Article applied magnetic field. The applied magnetic field was then set to 100 Oe, and the ZFC data was collected as the sample temperature was increased to the maximum. The FC data was collected as the temperature was cycled back to 10 K. The particle size and morphology were characterized using transmission electron microscopy (TEM 2100F, 200 kV, JEOL) by placing a droplet of oxygen-free water or ethanol containing Co nanoparticles on a carbon-coated copper TEM grid at room temperature and then allowing the liquid to evaporate before data collection. The crystal structures were characterized by powder X-ray diffraction (XRD) measurements with monochromatized Co KR radiation (λ =1.7091 A˚) using a Rigaku high resolution powder diffractometer. Extended X-ray absorption fine structure (EXAFS) spectroscopy characterization was carried out at the X-ray microprobe double-crystal monochromator beamline, port 5A, synchrotron radiation source at the Center for Advanced Microstructures and Devices (CAMD) operated by Louisiana State University. The storage ring was operated at an electron beam energy of 1.3 GeV. For these Co K-edge EXAFS measurements, the beamline monochromator was calibrated with a 7.5 μm hcp cobalt foil. Ge(220) crystals provided monochromatic X-rays in the region of interest. The energy bandwidth for the excitation radiation was less than 2 eV over the range of energies examined. The collected data were background subtracted and normalized using standard procedures.

3. Results and Discussion The fine crystal structure transitions of Co nanoparticles at different OR stages have been observed using the RTMP synthesis technique in a previous study.15,16,24 3.1. Evolution of Magnetic Properties of Co Nanoparticles with OR. Figure 1 presents the M versus H and M versus T data. The data clearly shows the evolution of the magnetic properties of the Co NPs during OR. Figures 1A,C,E presents the overall M versus H data showing the saturation magnetization and symmetry of the hysteresis loop about H = 0 Oe. These plots also include insets showing the M versus T data from which the blocking temperature Tb is obtained. Figures 1B,D,F show the low applied magnetic field M versus H data from Figures 1A,C,E, respectively. Figures 1A,B show data from the as-synthesized samples (i.e., no OR), Figures 1C,D show data for the samples that are Ostwald ripened for 3 h, and Figures 1E,F, show data for the samples that are Ostwald ripened for more than 8 h. The data clearly show changes in the hysteresis loop to indicate that OR significantly affects the magnetic properties of the NPs. An inspection of the low-field M versus H data at 10 K for the as-synthesized nanoparticles, Figure 1B, shows a distinct dualterraced (“wasp-waist”) feature and coercivity (Hc) of 330 Oe for the as-synthesized NPs. This terraced feature suggests two magnetic phases in the nanoparticles, one that can be saturated at low magnetic fields and the other that saturates at high magnetic fields.28 The crystal structure analysis of the NPs, based on X-ray absorption near the K-edge structure (XANES), indicates the presence of two anisotropic crystal structures, mainly mixed fcc structure with low coercivity and hcp structure with a large amount of coercivity.16,24,28 The wasp-waist feature in the data suggesting different magnetic phases supports this mixture of crystal structures.28 Figures 1C,D show a significant reduction in the wasp-waist feature after OR for 3 h, and after 8 h (Figures 1D,E), the feature has completely disappeared with the data showing a more square shape instead. With the disappearance of the wasp-waist feature in the magnetization hysteresis loop, there is a significant increase in the coercivity (Hc): 2800 and 3500 Oe at 10 K for NPs with (28) Sun, S.; Murray, C. B. J. Appl. Phys. 1999, 85, 4325–4330.

DOI: 10.1021/la9009866

10211

Article

Song et al.

Figure 1. Ostwald ripening time effects on magnetic properties of Co nanoparticles synthesized at high flow rate. (A) The as-synthesized nanoparticles show a wasp-waist hysteresis loop at 10 K and a blocking temperature Tb of 125 K (inset). (B) The magnified region of the waspwaist hysteresis loop suggests a two-terrace feature and a coercivity (Hc) of 330 Oe at 10 K. (C) The nanoparticles after 3 h of OR show a reduced wasp-waist hysteresis loop at 10 K and a Tb of 190 K (inset). (D) The magnified region of the wasp-waist hysteresis loop suggests an increased Hc of 2800 Oe at 10 K. (E) The nanoparticles after more than 8 h of OR show a hysteresis loop at 10 K in which the wasp waist has disappeared and the blocking temperature is 270 K (inset). (F) The magnified region of the wasp-waist hysteresis loop suggests an increased Hc of 3500 Oe at 10 K.

Ostwald ripening for 3 more hours and more than 8 more hours, respectively (Table 1). The monotonically decreasing (increasing) nature of the hysteresis curves of all three sets of samples during the magnetic field excursions from positive to negative values (negative to positive values) suggests that the NPs are single domains. Hence, it appears that the evolution of the hysteresis loop is caused by the combination of a crystal structure transition,16 nanoparticle size changes (Table 1), and the result of a gradual relaxation of the internal crystal structure stresses during OR. The reduction in the anisotropy constant suggests that the crystal structure becomes more isotropic. We believe that the ripening process gives the atoms in the nanoparticles an opportunity to diffuse into their thermodynamically favored positions.16 This atom-reforming process alleviates the inner stress forced on the crystal structure of the nanoparticles by the chip effect.16,20 A more random arrangement of atoms and the innerstress release during OR reduces the crystal anisotropy and permits more magnetic domain wall alignment parallel to the easy axis. This is consistent with the enhanced coercivities and magnetic anisotropy constants (Table 1). Clearly, OR significantly affects the magnetization, and after OR for more than 8 h, the samples are clearly in the single magnetic phase. Fine crystal structural changes by the OR process also affect other magnetic parameters. As shown in Table 1, the saturation magnetization (Ms) is increased from 63 emu/g for 10212 DOI: 10.1021/la9009866

the as-synthesized NPs to 77 emu/g for NPs with OR for 3 h and to 74 emu/g with OR for more than 8 h. The remnant magnetization (Mr) of these NPs increases from 30 to 34 and then to 44 emu/g after Ostwald ripening for 0, 3, and >8 h, respectively. The effect of OR on Mr is more significant than that on Ms, as seen by the increased Mr/Ms (i.e., the squareness factor) ratio for the hysteresis loop. The blocking temperatures Tb are also increased from 125 K for R-1 to 180 K for R-2 and to 300 K for R-3 with increasing OR time. In general, the changes in the magnetic properties of the NPs are directly related to Ostwald ripening times, as seen by the enhanced saturation magnetization and remnant magnetization, larger coercivities, and increased blocking temperatures. We have demonstrated that the magnetic properties of Co NPs can be tailored for specific applications of Ms, Mr, Hc, K, and hysteresis loop shape by flexibly tuning the fine crystal structure of NPs via the OR times. In addition, the dualterrace hysteresis feature in the as-synthesized NPs may be very useful for the design of magnetic materials with multiorder magnetic switching applications, as may be required by highdensity storage devices. 3.2. Magnetic Stability of Nanoparticles with Aging. In engineering nanomaterials, one of the key issues is to fabricate NPs with stable crystal structures and physical and chemical properties, as desired for specific applications. Hence, we have Langmuir 2009, 25(17), 10209–10217

Song et al.

Article

Figure 2. Nanoparticles synthesized at high flow rate show a significantly reduced saturation magnetization with an Hc of 5500 Oe (A) and an increased Tb of >300 K (B) after aging for 3 months. The nanoparticles synthesized at low flow rate also show a reduced saturation magnetization and an increased Hc of 4400 Oe (C) and an increased Tb of more than 350 K (D) after aging for 3 months. The as-synthesized cobalt nanoparticles collected under ultrasonication show a hysteresis loop having a paramagnetic nature with a very low remnant magnetization (Mr) of 1.6 emu/g Co, a coercivity (Hc) of 1560 Oe at 10 K (black curve in E), and a blocking temperature of 170 K (black curve in F). The cobalt nanoparticles collected under ultrasonication also suggest unstable magnetic properties with a reduced magnetization (pink curves in E and F), a smaller Hc of 350 Oe (pink curve in E), and an increased blocking temperature (pink curve in F) higher than 320 K.

also investigated the magnetic stabilities of the nanoparticles synthesized by the RTMP method. Figures 2A-F show M versus H and M versus T data for the NPs synthesized at high and low flow rates. The effect of aging on the magnetization (measured at 10 and 300 K) for samples from differing reaction and quenching conditions represented in Table 1 by experimental no. R-3 through R-5 is presented. Figures 2A,B present data for the R-3 sample, Figures 2C,D present data for R-4, and Figures 2E,F present data for the R-5 sample. Figures 2A-D give typical results of stability investigations of the magnetic properties of nanoparticles formed by the RTMP at high flow rate (R-3) and at low flow rate (R-4). Both the Ms and Mr at 10 K are reduced, and the Hc at 10 K is increased significantly after aging for 3 months. The blocking temperatures, Tb, also shift to higher values. These Tb shifts in the magnetic properties suggest that a significant phase change occurs during aging. This phenomenon, which is sometimes referred to as disaccommodation, is very harmful to the long-term application of NPs.29 From a theoretical consideration, the phase stability of NPs can be evaluated by their free energy (i.e., G(p, T, γ)). If (29) Morrish, A. H. The Physical Principles of Magnetism; IEEE Press, The Institute of Electrical and Electronics Engineering, Inc.: New York, 2001; pp 407431.

Langmuir 2009, 25(17), 10209–10217

the nanoparticles are kinetically stable against coarsening or dissolution, then the expression for G(p, T, γ) can be written as30 GðE, S, p, T, γ, A, VÞ ¼ E - TS þ ðp þ p0 ÞV þ γA þ other terms where, E, S, T, and p are the internal energy of NPs, the entropy of NPs, the ambient temperature, and the pressure, respectively. In addition, p0 is an excess internal pressure associated with strain at the NP surface, γ is the interfacial free energy of the NPs, and A is the surface area of the NPs. On the basis of this equation and a previous investigation of the stability of NPs,15,16,24,28,30,31 we deduce that for NPs fabricated by the RTMP method having excess atoms at their highenergy positions (related to the E and S components of the equation) and some unanticipated defects and surface strains (related to S and p0 ), the active surface atoms (related to γ and p0 ) will contribute to their unstable crystal structures and subsequently to their properties. We have thus tried several methods to reduce these crystal structure parameters by reducing surface (30) Gilbert, B.; Zhang, H.; Huang, F.; Finnegan, M. P.; Waychunas, G. A.; Banfield, J. F. Geochem. Trans. 2003, 4, 20–27. (31) Zhang, J.; Lan, Y.-Z.; Chen, D.-G.; Ren, G.-Q. Chin. J. Struct. Chem. 2007, 26, 1145–1152.

DOI: 10.1021/la9009866

10213

Article

Song et al.

Figure 3. Co nanoparticles obtained by performing the reaction at 50 °C and quenching at 2 °C suggest much more stable magnetic properties than those synthesized at low temperatures. (A) Co nanoparticles after aging for 3 months show a slightly increased saturation magnetization from 144 to 153 emu/g, a slightly reduced Hc from 1350 to 1050 Oe at 10 K (A), almost no change in the saturation magnetization at 300 K (B), and a slightly increased blocking temperature (inset in B).

strains and crystal defects in the Co NPs and passivating their surface atoms to minimize the interfacial free energy. First, the flow rate is considered. Comparing data for R-3 and R-4 from Figures 2A-D and Table 1, Ms and Mr for R-3 are significantly more decreased than those for R-4. In addition, the Hc for R-3 is increased even more than that for R-4. One reason that R-4 is more stable than R-3 may be due to the larger size (R-4 = 4.7 nm; R-3 = 3.8 nm) of the NPs because larger NPs usually have lower general surface energy. Also, fewer defects, probably introduced at a low flow rate compared to high number of defects at a high flow rate, may be another reason for the stability of the R-4 NPs.32 We also attempted to stabilize the magnetic property of the nanoparticles by introducing additional energy through sonication to prompt the atoms to diffuse into their stable positions and alleviate the inner stress. The results that were obtained, however, suggest that sonication does not work very well. Interestingly, the as-synthesized cobalt nanoparticles collected under sonication (R-5) have a potbelly hysteresis loop shape (black curve in Figure 2E and Figure 2F) and reduced Hc compared with those with OR at room temperature (R-3).33 The nature of the hysteresis loop (the magnetization now does not approach saturation) in Figure 2E suggests that the magnetic behavior of both the fresh and aged samples becomes superparamagnetic. These NPs have a very low residual magnetization (Mr) of 1.6 emu/g, a small coercivity (Hc) of 1560 Oe at 10 K, and a blocking temperature at 170 K. Summarized from Figures 2E,F, the data for R-5 in Table 1 suggests a change in the magnetic properties from ferromagnetic to superparamagnetic with a significantly reduced remnant magnetization of 0.4 emu/g, a smaller Hc of 350 Oe, and an increased blocking temperature of more than 320 K for the cobalt nanoparticles collected under sonication and aged for 3 months. Clearly, the nanoparticles collected under sonication still exhibit potbelly behavior after aging, which can be explained by the combination of single domain formation with a smaller Hc value and superparamagnetic behavior with a shallower slope.33 To stabilize the nanoparticles, we considered an alternative method. Previous investigations indicate that high temperatures can reduce both the number of crystal defects and the amount of surface strain by providing the atoms with more energy to reach their minimum-energy local positions.32,34 According to a molecular dynamics simulation on the free-energy change during the (32) Krishnadasan, S.; Tovilla, J.; Vilar, R.; deMello, A. J.; deMello, J. C. J. Mater. Chem. 2004, 14, 2655–2660. (33) Tauxe, L.; Mullender, T. A. T.; Pick, T. J. Geophys. Res., B 1996, 101, 571– 583. (34) Hyeon, T. Chem. Commun. 2003, 8, 927–934.

10214 DOI: 10.1021/la9009866

attachment, it is found that the general energy barrier to aggregation can be enhanced greatly for NPs approaching along different crystal faces.35 The chances for this kind of approach with the higher energy barriers can be easily improved for Co nanoparticles by using slightly elevated formation temperatures according to the surface-energy level in different crystal planes of Co.16 It is also found that strong bonding between the ligands and the nanoparticles will have a stabilizing effect by passivating the surface.34,36,37 We should expect that the binding between the stabilizer (SB12) and the surfaces of nanoparticles can be improved at elevated reaction temperatures. In the microfluidic process, the sample collection time can take about 3.5 h to pump off the 200 mL of reactants at the highest rate of 0.9 mL/min. To avoid OR and OA, it is better to cold quench the reaction solution as fast as possible. This rapid cold quenching may function to deactivate the surface by changing the binding status between the surfactants and the surfaces of the nanoparticles. On the basis of these considerations, we have developed the in situ rapidly cooling microfluidic process (IRCMP) shown in Scheme 1, which has produced very good results. In IRCMP, cobalt nanoparticles are synthesized by performing the formation reaction at an elevated temperature (e.g., 50 °C) and then cold quenching rapidly in a cold pool (e.g., a 4 °C THF and ethanol mixture) that is cooled by a chiller (Scheme 1). The result produces NPs with enhanced, stable magnetization. In this way, both the fine crystal structure with possibly fewer defects formed at 50 °C can be sufficiently retained, and defects introduced by OR and OA can probably be minimized. 3.3. Stable Cobalt NPs Fabricated by IRCMP. Magnetization data (M vs H and M vs T) for Co NPs synthesized using the IRCMP method is given in Figure 3. The data suggest that NPs fabricated by IRCMP have enhanced and more stable magnetic properties than those fabricated at RTMP (Figure 2). The Ms values for the fresh samples produced by IRCMP approach 144 emu/g at a saturation field of 13 kOe (Table 1, R-6), which is less than that for the samples at RTMP, indicating that highpermeability NPs can be obtained by the IRCMP. At 10 K, the coercivity of the IRCMP NPs is 1330 Oe, which is much less than the coercivity of the nanoparticles synthesized by RTMP and experiencing the OR process. After aging for 3 months, the saturation magnetization of Co nanoparticles by the IRCMP method is slightly increased from 144 to 153 emu/g, and their (35) Spagnoli, D.; Barnfield, J. F.; Parker, S. C. J. Phys. Chem. C 2008, 112, 14731–14736. (36) Murray, C. B.; Sun, S.; Gaschler, W.; Doyle, H.; Betley, T. A.; Kagan, C. R. IBM J. Res. Devel. 2001, 45, 47–56. (37) Lu, A.-h.; Salabas, E. L.; Schuth, F. Angew. Chem., Int. Ed. 2007, 46, 1222– 1244.

Langmuir 2009, 25(17), 10209–10217

Song et al.

Article

Figure 5. XRD data for Co nanoparticles synthesized at room temperature and high flow rate after OR for more than 8 h show a peak shift from 52.95° for the as-synthesized nanoparticles (a) to 51.88° for those after aging for 3 months (b). Their XRD spectra also suggest a significant peak-narrowing effect after aging for 3 months. The XRD data for Co nanoparticles synthesized at high temperatures and quenched at low temperatures shows a little peak shift at 52.59° for the nanoparticles as synthesized (c) and for those after aging for 3 months (d), although there is still a slight peak-narrowing effect.

Figure 4. TEM images for samples synthesized by the RTMP method (sample R-3): (A) as-synthesized and aged for 3 months. TEM images for samples synthesized by the IRCMP method (sample R-6): (C) as-synthesized and (D) aged for 3 months. (E) HR-TEM image for single Co NPs as-synthesized by RTMP; (F) HR-TEM image for single Co NPs as-synthesized by IRCMP; and (G) HR-TEM image for single Co NPs after aging for 3 months by IRCMP.

Hc value is reduced from 1350 to 1050 Oe at 10 K (Table 1). The Ms at 300 K changes little (Figure 3B), and the Tb increases only slightly. However, the Mr/Ms ratio is reduced from 0.65 for the fresh sample to 0.36 after aging for 3 months. This suggests that as the crystal structure changes slightly during the aging process the magnetic coupling within the material could be reduced significantly, hence the decrease in the squareness factor. To elucidate the enhanced long-term stability of the magnetic properties of these nanoparticles, their sizes, shapes, morphology, and crystal structure are compared with those for nanoparticles obtained at RTMP. Figure 4 presents HR-TEM images of the samples produced by the RTMP method (Figures 4A,B) and the IRCMP method (Figures 4C,D). As shown in Figure 4A,B, the data for nanoparticles synthesized at RTMP suggests that significant aggregation and shape change occur during the aging period of 3 months. The average diameter increases from 3.8 ( 0.5 to 8.2 ( 3.4 nm after aging for 3 months, respectively, with a slight deviation from the calculated values based on the XRD spectra (Figure 5). The HR-TEM images for the as-synthesized NPs by RTMP (Figure 4E) suggest that these nanoparticles are not in a stable crystalline phase. Figures 4A,B also suggest that the samples do not remain stable for the long term. Figures 4C,D show that the nanoparticles synthesized by IRCMP exhibit little aggregation and retain well-separated spherical shapes after aging for 3 months. The statistical diameters based on the measurement of approximately 100 nanoparticles before and after aging are 3.7 ( 0.4 and 3.9 ( 0.4 nm. The crystal structures for the assynthesized NPs indicate that they are not in a crystalline phase (38) Inoue, A.; Zhang, T.; Takeuchi, A. Appl. Phys. Lett. 1997, 71, 464–466.

Langmuir 2009, 25(17), 10209–10217

but instead appear to be in an amorphous phase (Figure 4F) with only short-range ordering.38 By comparing the TEM images in Figures 4E-G, it can be seen that the atomic arrangement in the NPs fabricated by IRCMP are more uniform, with almost the same electron transmission density between the surface and interior regions as that for the NPs fabricated by RTMP. This suggests that the uniform atomic arrangement formed at elevated temperatures can be frozen by the rapid cooling process. This nonuniform feature of atomic arrangement in the RTMP-produced samples may be one of the reasons that those NPs are not as stable as NPs produced by IRCMP that have a uniform atomic arrangement throughout (Figure 4G). X-ray diffraction (Figure 5) was carried out on four samples produced by the IRCMP method to investigate the crystal structure difference and amorphous crystal feature. Results shown in Figure 5 show only a single broad peak in the XRD spectra, reminiscent of the XRD feature for the amorphous Co metal, hence giving additional evidence for the amorphous phase in these NPs.38 It appears that Co nanoparticles formed by the atom-by-atom growth arrangement at a low reaction temperature will result in the amorphous phase. However, high temperatures usually favor the formation of the highly crystalline phase by increasing the crystallization rate and may not be the only reason for the stable amorphous phase formed by IRCMP. Considering the rapid cooling process for the amorphous metal formation, we hypothesize that our rapid cold quenching process may also have a similar effect because the hot nanoparticle solution can be cold quenched on a very small scale (microlilters or even nanoliters) rapidly by IRCMP. Indeed, on the basis of a hot ball model calculation and assuming that 50 °C droplets of the nanoparticle solution disperse into the 4 °C quenching solution instantaneously, the average cooling rate is estimated to be about 7  104 K/s for 3.6 nm Co NPs, which is close to the necessary cooling rate for conventional amorphous metal (∼105 K/s) formation.39,40 Results from the TEM images show more uniform compact densities in different areas of NPs that are produced by IRCMP than NPs produced by RTMP (Figure 4E-G). We believe that this indicates the development of the stability of the NPs. (39) Klement, W.; Willens, R. H.; Duwez, P. Nature 1960, 187, 869–871. (40) Makino, A.; Kubota, T.; Makabe, M.; Chang, C. T.; Inoue, A. Mater. Sci. Eng. B 2008, 148, 166–170.

DOI: 10.1021/la9009866

10215

Article

Figure 6. EXAFS analysis for the Co nanoparticles of three standard crystal structures and Co nanoparticles as synthesized by the microfluidic reactor and aged species. (a-c) R spacing for the theoretical fcc-phase, epsilon (ε)-phase, and hcp-phase Co nanoparticles, respectively. (d) As-synthesized and quenched at room temperature (sample R-4, fresh). (e) Co nanoparticles synthesized and quenched at room temperature and aged for 3 months (sample R-4, aged). (f) Co nanoparticles synthesized at high temperatures and quenched at low temperatures (sample R-6, fresh). (g) Co nanoparticles synthesized at high temperatures and quenched at low temperatures and aged for 3 months (sample R-6, aged). (h) R spacing for the standard hcp-phase Co foil.

All of the XRD spectra have a single broad peak feature. Careful consideration of the location and fwhm for each peak can yield differences between the NPs fabricated by the two methods (RTMP and IRCMP). XRD data from Co nanoparticles synthesized by RTMP, after Ostwald ripening for more than 8 h, show a distinct peak shift from 52.99° for the nanoparticles as synthesized (Figure 5a) to 51.88° for those aged for 3 months (Figure 5b). XRD data also suggest a significant peak-narrowing effect after aging for 3 months, with the fwhm reduced from 5.8 to 3.4°. Calculation of the d spacing suggests a lattice expansion from 2.0065 to 2.0804 A˚, and the calculated crystal size based on the fwhm also increased from 3.6 to 8.6 nm.38,41,42 However, the XRD data for the nanoparticles formed by the IRCMP show little change after aging for 3 months, with a little-changed broad peak at 52.59° and very slight line-width narrowing from 5.0 to 4.5° (Figures 5c,d). Calculation of the d spacing suggests very little expansion with a lattice constant of 2.0191 A˚, and the calculated crystal size based on the fwhm is slightly increased from 4.1 to 4.6 nm, consistent with the data obtained by TEM.43 The XRD data indicate that the NPs formed by RTMP experience a more significant size, shape, or crystal structure change than NPs obtained by IRCMP after 3 months of aging. To further investigate the stability of NPs fabricated by the IRCMP process, EXAFS analysis (Figure 6) was performed on the samples to examine the interatomic spacing R(A˚). The results clearly show that the R spacing in the nanoparticles synthesized by RTMP (Figure 6, plot d: as-synthesized; plot e: aged for 3 months) changes more significantly than for those synthesized (41) Carslaw, H. S.; Jaeger, J. C. Conduction of Heat in Solids ; Clarendon Press: Oxford, England, 1956; p 232. (42) Lienhard, J. H. I.; Lienhard, J. H. V. A Heat Transfer Textbook , 3rd ed.; Phkigiston Press: Lexington, MA, 2003; pp 203-223. (43) Cullity, B. D.; Stock, S. R. Elements of X-ray Diffraction, 3rd ed.; Prentice Hall: Upper Saddle River, NJ, 2001; Appendix 5.

10216 DOI: 10.1021/la9009866

Song et al.

by IRCMP (Figure 6, plot f: as-synthesized; plot g: aged for 3 months). The dual-peak R spacing becomes a monopeak for NPs formed by RTMP, approaching that for the theoretical fcc crystal structure (Figure 5a), which is the stable crystal structure in this size range.43 Although the dual-peak feature is retained for nanoparticles formed by IRCMP that undergo 3 months of aging, the obvious change in the dual-peak shape suggests some change in atomic arrangements in IRCMP NPs. Clearly, EXAFS can be used to detect the fine crystal structure change (e.g., nearestneighbor atom arrangement) more precisely than XRD. Although the XRD data does not indicate a significant crystal structure difference between the nanoparticles formed by IRCMP before and after aging, EXAFS analysis for NPs before and after aging for 3 months still indicates a definite change in atomic arrangements in NPs synthesized by IRCMP. For comparison, we also performed the reaction under the same conditions as in IRCMP but collected the product at room temperature without rapid cold quenching. It was found that some good crystalline particles were formed but clearly the size and shape were dispersed more widely and, more aggregation among the NPs and some crystalline lattice defects appeared (Supporting Information Figure S2). Again, rapid cold quenching favors the formation of NPs with uniform size and shape and stable crystal structures. According to the temperature effect on nanoparticle formation, we believe that more-stable NPs can be synthesized by IRCMP at much higher reaction temperatures than at the current operating temperature. However, limited by the solvent boiling point (56 °C for THF) and the thermal stability of the polymeric microfluidic reactor (