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Synthesis of Magnetic Noble Metal (Nano)Particles Krishna N. K. Kowlgi,† Ger J. M. Koper,*,† Stephen J. Picken,‡ Ugo Lafont,‡,§ Lian Zhang,|| and Ben Norder‡ Self-Assembling Systems, ‡NanoStructured Materials, Department of Chemical Engineering, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands § National Centre for High Resolution Electron Microscopy, Delft University of Technology, Lorentzweg 1, 2628 CJ Delft, The Netherlands Fundamental Aspects of Materials and Energy, Faculty of Applied Sciences, Delft University of Technology, Mekelweg 15, 2628 JB Delft, The Netherlands
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bS Supporting Information ABSTRACT: Noble metal particles can be made strongly ferromagnetic or diamagnetic provided that they are synthesized in a sufficiently strong magnetic field. Here we outline two synthesis methods that are fast, reproducible, and allow broad control over particle sizes ranging from nanometers to millimeters. From magnetometry and light spectroscopy, it appears that the cause of this anomalous magnetism is the surface anisotropy in the noble metal particles induced by the applied magnetic field. This work offers an elegant alternative to composite materials of noble metals and magnetic impurities.
1. INTRODUCTION Magnetic noble metals have the potential to significantly advance nanomaterial-based applications such as catalysis, selfassembly, and spintronics.1 3 They have been predicted to revolutionize medicine in the areas of biomolecule recognition, drug release control, and cancer treatment.3,4 There are several procedures that can be used to synthesize magnetic noble metals that include, but are not limited to, the use of capping agents,1 3,5 laser ablation,4 nanotemplating by a polymer,6 and embedding in zeolites.7 Despite the relatively high magnetic moments achieved, applications as mentioned before will require considerable amounts of magnetic material, hence there is a great demand for simple synthesis routes. Here, we present two methods to produce magnetic noble metals where only basic colloid chemistry is employed except for the application of a strong magnetic field during synthesis. The methods can be carried out at room temperature in a single vessel and are fast, reproducible, and allow particle sizes ranging from nanometers in the case of individual nanoparticles to millimeters in the case of (nano)particle clusters. Previous findings on magnetic noble metals were focused on nanoparticles, and saturation magnetization values of up to 13 A m2 kg 1 8 have been reported, which is about 5% of that of bulk iron.9 Some procedures yield (super)paramagnetic samples6,7 whereas others produce materials having a ferromagnetic behavior.1 5 The basic underlying protocol of all of these procedures is the reduction of a metal precursor such as hexachloroplatinate (H2PtCl6). The precise preparation conditions strongly influence the resulting particle size distribution.10 12 r 2011 American Chemical Society
For instance, once the nanoparticles are synthesized they cluster and sediment unless sufficiently protected. Nevertheless, with the help of a templating microemulsion13 16 or the addition of capping agents,1,3 5 homogeneously dispersed and uniformly sized nanoparticles are produced. It is well established that anomalous magnetic properties are promoted when materials are confined to nanosized structures.1 7 On the nanoscale, the high surface to volume ratio, broken translation symmetry, and reduced coordination of surface atoms have serious repercussions on the electrical and magnetic properties of a material. This is further enhanced by factors such as particle dimensions becoming comparable to those of magnetic domains; interactions with capping agents or the chemical environment; and finally defects, vacancies, inclusions, and imperfections present in the material structure.17 As reported above, nanosized noble metals can also exhibit strong magnetism, which is remarkable because their bulk counterparts are weakly diamagnetic or paramagnetic.1 7 Our preparation methods involve the reduction of noble metal precursors similar to hexachloroplatinate (H2PtCl6) with a reducing agent such as hydrazine (N2H4) under standard conditions1 3,5,6,10 16 (detailed Experimental Section). The complete reaction can proceed in a single vessel at room temperature. The only difference from the standard procedures is the application of a Received: December 21, 2010 Revised: March 31, 2011 Published: May 20, 2011 7783
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magnetic field. We have applied this magnetic field by means of permanent magnets of varying strengths. Stronger fields yield more ferromagnetic particles whereas the absence of a magnetic field yields no magnetic particles (Table S1 in the Supporting Information). In addition to ferromagnetic particles, our synthesis method also yields (strongly) diamagnetic particles. Homogeneously dispersed magnetic nanoparticles are obtained using microemulsion templating, whereas (nano)particle clusters can be grown when no protective agent is used so that the nanoparticles aggregate.
2. EXPERIMENTAL SECTION 2.1. Materials. Surfactant sodium bis(2-ethyhexyl)sulphosuccinate or AOT (C20H37NaO7S, 98%), oil n-heptane (C7H14, g99.9%), noble metal precursors chloroplatinic acid hydrate (H2PtCl6 3 6H2O, g99.9%), gold chloride hydrate (HAuCl4 3 3H2O, g99.9%), and copper chloride (CuCl2, g99.999%), and reducing agent sodium borohydride (NaBH4, g99%) were all purchased from Sigma-Aldrich BV. Silver metal precursor silver nitrate (AgNO3, g99.8%) was obtained from Merck KGaA. Reducing agent hydrazine hydrate (N2H4 3 H2O, 100%) was purchased from Fischer Emergo BV. Reagent-grade water produced by a Milli-Q ultrapure water purification system from Millipore BV was used in all sample formulations. All materials were used as received without further purification. 2.2. Methods. Most of the reactions (for nanoparticles and their clusters as detailed below) were carried out on an IKA Labortechnik RCT or RET Basic magnetic stirrer set at rotational speeds from 600 to 1000 rpm. These stirrers were ordered from IKA -Werke GmbH & Co. KG. No magnetic stir bars or beans were introduced during the preparation procedure. The stirrer plate, which produces 20 mT at the point of contact, was the only source of magnetic induction. Magnetic clusters were identified and separated by looking at the ones that spin with the rotation of the stirrer’s magnetic field. A video is available for reference (see Supporting Information). To obtain higher yields of ferromagnetic clusters, either a neodymium iron boron permanent magnet of strength 1.8 T or a sintered neodymium iron boron magnet coated with nickel of strength 0.5 T was used. The stronger magnet was supplied by the Baotou Research Institute of Rare Earths, China, and the weaker magnet was ordered from Webcraft GmbH. 2.3. Nanoparticles. Preparation follows the standard microemulsion nanoreactor-based synthesis procedure,13 16 where a microemulsion is obtained after adding a mixture of surfactant and oil to aqueous solutions in corresponding proportions. Then, two microemulsions (one containing a metal precursor in its aqueous solution and the other containing a reducing agent) are mixed together in equal volumes to form metal nanoparticles in the microemulsion. The approach used in this work is detailed as follows. A microemulsion composition of AOT/n-heptane/aqueous solution (15:82.5:2.5 by mass %) was employed. The metal precursor (H2PtCl6 3 6H2O, HAuCl4 3 3H2O, CuCl2, or AgNO3, 1 mmol) was solubilized in water, and the reducing agent (N2H4 3 H2O or NaBH4) solution was set at 10 times the molar concentration of the precursor in order to obtain complete reduction. 2.4. (Nano)Particle Clusters. Aqueous solutions were prepared by mixing the noble metal precursor or the reducing agent with water in the required concentrations. Clusters were prepared by mixing equal volumes of aqueous solutions of the metal precursors with the reducing agents in a 1:10 molar ratio and keeping the precursor solution concentration constant (1 mmol). This method predominantly produces diamagnetic particles and just a fraction of ferromagnetic particles. 2.5. Instrumentation. Powder X-ray diffraction (XRD) was performed using a Bruker AXS D8 Discover instrument from Bruker AG equipped with a general area detector diffraction system (1024
Figure 1. Transmission electron microscopy (TEM) micrographs of magnetic (nano)particle clusters of platinum in (A) zoomed-out and (B) close-up views. channels 1024 channels). Samples were placed on a 0.1-mm-thick plastic substrate (background contribution was subtracted from results) and set 13 cm away from the detector, which was positioned at 40° (Ω = 20°) with respect to the X-ray source. The X-ray source was a tube operated at 40 kV and 40 mA and produced predominantly Cu KR radiation of wavelength 1.54 Å. The radiation emanating from the source was filtered and replete with KR2 radiation by cross-coupled G€obbel mirrors. It was also sent through a 0.5 mm collimator before reaching the sample. Resultant data from the experiments were integrated along the diffraction angle to obtain the intensity as a function of the scattering angle (2θ). Transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) were conducted using a FEI Tecnai TF20 electron microscope operated at 200 kV. Samples were mounted on Quantifoil microgrid carbon polymer supported on a copper grid by dropping a sample suspension on the grid. EDS was performed using the nanoprobe (high-resolution) and microprobe (low-resolution) modes. Magnetometry was carried out on a superconducting quantum interference device (SQUID) MPMS XL magnetometer from Quantum Design. Magnetic (nano)particle clusters were washed at least twice with ethanol and water before drying and loading them into the standard gelatin capsules (where the background contribution was subtracted from the results) for measurement. Inductively coupled plasma atomic emission spectroscopy (ICPAES) was executed on the Optima 5300dv instrument from PerkinElmer Incorporated. All samples were acidified with 0.6 M hydrochloric acid. Calibration solution concentrations were in the range of 0 to 1 mg L 1. Dynamic light scattering was done on the Zetasizer Nano ZS from Malvern Instruments Limited using the 173° noninvasive back-scatter mode and the M3 phase analysis light scattering mode. The instrument used a 4.0 mW, 633 nm He Ne laser. The multiple-peak highresolution fitting procedure was utilized to obtain the particle size distribution from the autocorrelation function. The UV/visible spectrum was obtained with a UV-1800 spectrophotometer from Shimadzu Corporation.
3. RESULTS AND DISCUSSION The particles synthesized by our methods are pure noble metals as revealed by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The quantities of the most probable ferromagnetic impurities (cobalt, iron, neodymium, and nickel) are 3 orders of magnitude lower than the yields of ferromagnetic noble metals produced on synthesis (Table S2 in the Supporting Information). Therefore, the magnetism observed in the noble metal particles is not due to the ferromagnetic impurities included by the action of the magnetic field generator. Besides, ferromagnetic impurities would not explain the diamagnetic noble metal particles that are also produced with our synthesis method. 7784
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Figure 2. Hysteresis loops of magnetic (nano)particle clusters of platinum at (A) 300 K, (B) 300 K showing coercivity and remanence, (C) 5 K, and (D) 5 K showing coercivity and remanence.
X-ray powder diffraction (XRD) analysis of the particles showed that they are single crystals organized in a face-centered cubic structure (FCC) having Fm3m (group 225) symmetry (Figure S1 and Table S3 in the Supporting Information). The diffraction peak positions and the corresponding lattice parameters correspond very well to the standard values obtained from the Joint Committee on Powder Diffraction Standards (JCPDS), confirming that the samples are pure noble metals. The crystallite sizes as determined by the Scherrer formula for platinum, gold, and silver are 4.6, 12.4, and 15.8 nm, respectively (Table S4). Direct imaging by transmission electron microscopy (TEM) was employed to study the morphology and microstructure of the magnetic noble metal particles. The electron micrographs of the platinum (nano)particle clusters shown in Figure 1 corroborate well with the X-ray powder diffraction (XRD) results. Clusters of single-crystalline nanoparticles having diameters of around 5 ( 2 nm and d spacings of 0.19 and 0.22 nm can be seen. Gold, silver, and copper particles also exhibited crystalline structures as substantiated by XRD and TEM analysis (Figures S1 and S2 and Tables S3, S4, and S5 in the Supporting Information). The particles were roughly spherical, and we observed neither a particular size or shape nor a preferred facet growth with the ferromagnetic nanoparticles obtained from different noble metals. The magnetic behavior of the noble metal (nano)particle clusters was studied by superconducting quantum interference device (SQUID) magnetometry (Figure S3 in the Supporting Information). As shown in Figure 2, the ferromagnetic platinum sample shows the typical characteristics of a nanoparticulate ferromagnetic material:1,4 there is a saturation magnetization (by mass) of Msat = 4.1 A m2 kg 1, a remanence of MR = 320 mA m2 kg 1, and
a coercivity of BC = 7 mT at 300 K. When subjected to lower temperatures, the same sample exhibits a strong paramagnetic response, which is pronounced at higher magnetic fields as can be observed in Figure 2 (Figure S4 in the Supporting Information). The response of the diamagnetic noble metal particles is displayed in Figure 3 (and in Figure S5 in the Supporting Information). The platinum sample typically showed the largest magnetization, followed by silver, copper, and gold (Table S4 in the Supporting Information). We did not observe a particular optimal size for the magnetization of the noble metal nanoparticles within the size range produced. Let us first summarize the results with respect to the composition of our magnetic noble metal particles. The three techniques used, ICP-AES, XRD, and EDS (Figure S6 in the Supporting Information), clearly demonstrate the absence of magnetic impurities. Moreover, an important experimental fact is that the particles are not magnetic when they are prepared by exactly the same technique in the absence of a magnetic field, as seen in Figure 3 (Figure S7 in the Supporting Information). To illustrate this further, let us calculate the required impurity levels to produce the results obtained for the platinum sample corresponding to Figure 2. The sample had a mass of 6 mg and showed a saturation magnetization of M = 24.6 μA m2 (0.0246 emu), which results in a saturation magnetization (by mass) of Msat = 4.1 A m2 kg 1 or a magnetic moment of 0.143μB /atom, assuming every atom contributed to the magnetization at 300 K. The amount of iron (saturation magnetization (by mass) of Msat = 222 A m2 kg 19) needed to get the same magnetization as that of the platinum sample is approximately 0.1 mg. This is about 2% of the platinum sample mass, which is 4 orders of magnitude larger than the measured impurity level, whereas the 7785
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Figure 3. Hysteresis loops of (A) diamagnetic and (B) nonmagnetic (nano)particle clusters of platinum.
Figure 4. (A) UV/visible absorption is different for magnetic nanoparticles than for the nonmagnetic nanoparticles of platinum as a result of stronger surface plasmon resonance. (B) Magnetization vs the temperature of the ferromagnetic (nano)particle clusters of platinum and silver.
analytical techniques that we have employed have a detection limit of 10 6 % by sample mass. Therefore, if the cause of the magnetism in the noble metal particles were impurities, then it would have been detected. Magnetic noble nanoparticles have been synthesized before,1 7 albeit not on the scale of the present method. Also, there is ample experimental evidence that noble metal nanoparticles can exhibit a significant magnetic moment.1 7 Theoretically, these magnetic moments are predicted for nanoparticles with a particular crystal structure consisting of a magic number of atoms.18 For very small nanoparticles, most of the crystal structures have a significant magnetic moment, but for larger nanoparticles, only a few remain. Zheng et al. have followed the growth of platinum nanocrystals by means of in situ TEM11 and suggest that the growth of colloidal nanocrystals from solution takes different pathways, either by monomer attachment or by coalescence, depending on the freeenergy gain associated with that pathway. Cheong et al. further ratify this by demonstrating that growth is thermodynamically controlled for low precursor concentrated solutions such as those employed in our work.12 This might lead to the conclusion that the presence of a magnetic field favors the formation of those nanocrystals that have a significant magnetic moment. In addition, the energy gain associated with the coalescence of nanocrystals into a larger one might be more favorable when magnetic moments are aligned. Because a significant portion of the initial nuclei formed after bringing a solution to supersaturation is magnetic, these might grow further into larger and larger magnetic nanoparticles and (nano)particle clusters until particle growth is stopped either by the depletion of material or by other means.
The crystal structures that we have obtained for the noble metal nanoparticles are, however, not discernible from those of bulk noble metals. If the magnetic nanoparticles are to be different, this most likely is a surface effect. Surface anisotropy has been claimed to be responsible for magnetism in otherwise nonmagnetic materials.3,19 21 For instance, the localization of surface electrons by strongly (e.g., thiols, alcohols) or weakly (e.g., amines) bonded capping agents and the presence of crystal defects has been attributed20 to the magnetism found in noble metal nanoparticles.1 4,6 Here, the magnetic field directs the assembly of the nanoparticles such that it enhances the surface anisotropy, leading to magnetism. This is not the same effect as when nanoparticles are contained in a magnetic field after their synthesis, where the spin polarization is believed to be very small as argued by Skomski.19 Experimental evidence for deviating surface behavior has been provided for the case of capped gold nanoparticles where the surface plasmon resonance that is normally present in nonmagnetic particles was suppressed for the thiol-capped magnetic nanoparticles.22 Interestingly, we find the opposite behavior for nanoparticles rendered by our method. Figure 4 shows that the surface plasmon resonance of the magnetic nanoparticles is different compared to the nonmagnetic ones. Because the resonance largely depends on the presence of conduction electrons, we conclude that the surface electronic structure of magnetic nanoparticles significantly differs from that of the nonmagnetic ones. This would also explain why, simultaneously with the ferromagnetic particles, we can produce diamagnetic particles. Finally, the temperature dependence of the magnetization also shows anomalous behavior as observed in Figure 4 (Figure S7 in Supporting Information). In fact, within 7786
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Langmuir our experimental range we have not been able to find a Curie temperature beyond which the magnetization vanishes, albeit the magnetization decreases toward lower temperatures. This anomalous temperature dependence indicates that the behavior is not (super)paramagnetic and provides another argument that it could be the result of surface anisotropy.19 It may be concluded here that the nanoparticles do not show (super)paramagnetic behavior.
4. CONCLUSIONS We have described two methods of producing magnetic noble metal nanoparticles and (nano)particle clusters. These methods rely on the presence of a magnetic field during synthesis. From the measured particle properties, in particular, the enhanced surface plasmon resonance, we conclude that the magnetism of the nanoparticles is due to surface anisotropy induced by means of the magnetic field during the growth of the particles. In addition, the magnetization of the nanoparticles exhibits an anomalous temperature dependence. Finally, the synthesis routes mentioned here are expected to have a siginificant impact on applications involving noble metals,23 especially those that have resorted to combining noble with magnetic materials in order to be affected by a magnetic field.24 ’ ASSOCIATED CONTENT
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Supporting Information. Yields of magnetic materials, quantities of impurities, XRD crystallographic data, additional TEM/optical microscopy micrographs, field/temperature dependence of magnetization for ferromagnetic/diamagnetic samples, particle size distributions, and absorption spectra for different noble metals. This material is available free of charge via the Internet at http://pubs.acs.org.
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(6) Yamamoto, Y.; Miura, T.; Nakae, Y.; Teranishi, T.; Miyake, M.; Hori, H. Physica B 2003, 329 333, 1183. (7) Liu, X.; Bauer, M.; Bertagnolli, H.; Roduner, E.; van Slageren, J.; Phillipp, F. Phys. Rev. Lett. 2006, 97, 253401. (8) Liu, X.; Bauer, M.; Bertagnolli, H.; Roduner, E.; van Slageren, J.; Phillipp, F. Phys. Rev. Lett. 2009, 102, 049902. (9) Cullity, B. D.; Graham, C. D. Introduction to Magnetic Materials, 2nd ed.; IEEE Press: Piscataway, NJ, 2009. (10) Turkevich, J.; Miner, R. S.; Babenkova, L. J. Phys. Chem. 1986, 90, 4765. (11) Zheng, H.; Smith, R. K.; Jun, Y.; Kisielowski, C.; Dahmen, U.; Alivisatos, A. P. Science 2009, 324, 1309. (12) Cheong, S.; Watt, J.; Ingham, B.; Toney, M. F.; Tilley, R. D. J. Am. Chem. Soc. 2009, 131, 14590. (13) Pileni, M. Nat. Mater. 2003, 2, 145. (14) Sine, G.; Comninellis, Ch. Electrochim. Acta 2005, 50, 2249. (15) Liveri, V. T. Controlled Synthesis of Nanoparticles in Microheterogeneous Systems; Springer-Verlag: Berlin, 2006. (16) Nagarajan, R.; Hatton, T. A. Nanoparticles Synthesis, Stabilization, Passivation and Functionalization; American Chemical Society: Washington, DC, 2008. (17) Guimar~aes, A. Principles of Nanomagnetism; NanoScience and Technology; Springer: Heidelberg, Germany, 2009. (18) Kumar, V.; Kawazoe, Y. Phys. Rev. B 2008, 77, 205418. (19) Skomski, R. J. Phys.: Condens. Matter 2003, 15, R841. (20) Himpsel, J. F.; Ortega, J. E.; Mankey, G. J.; Willis, R. F. Adv. Phys. 1998, 47, 511–597. (21) Fernandez-Seivane, L.; Ferrer, J. Phys. Rev. Lett. 2007, 99, 183401. (22) Crespo, P.; Litran, R.; Rojas, T. C.; Multigner, M.; de la Fuente, J. M.; Sanchez-Lopez, J. C.; Garcia, M. A.; Hernando, A.; Penades, S.; Fernandez, A. Phys. Rev. Lett. 2004, 93, 87204. (23) Sanchez-Gaytan, B. L.; Park, S. J. Langmuir 2010, 26, 19170–19174. (24) Bardhan, R.; Chen, W.; Bartels, M.; Perez-Torres, C.; Botero, M. F.; McAninch, R. W.; Contreras, A.; Schiff, R.; Pautler, R. G.; Halas, N. J.; Joshi, A. Nano Lett. 2010, 10, 4920–4928.
’ AUTHOR INFORMATION Corresponding Author
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’ ACKNOWLEDGMENT We thank The Ministry of Economic Affairs in The Netherlands for subsidizing this project via the EOS LT framework (EOS LT 02025). Partial support from COST Action D43 is acknowledged. We are also grateful to Wim G. Haije for useful discussions and suggestions. The assistance of Marjolein van Ruijven with microscopy, Joop Padmos with spectrometry, and Aditya Surjosantoso with synthesis is appreciated. ’ REFERENCES (1) Teng, X.; Han, W.; Ku, W.; Hucker, M. Angew. Chem., Int. Ed. 2008, 47, 2055. (2) Zhang, H.; Ding, J.; Chow, G. Langmuir 2008, 24, 375. (3) Teng, X.; Feygenson, M.; Wang, Q.; He, J.; Du, W.; Frenkel, A. I.; Han, W.; Aronson, M. Nano Lett. 2009, 9, 3177. (4) Garcia, M. A.; Ruiz-Gonzalez, M. L.; de la Fuente, G. F.; Crespo, P.; Gonzalez, J. M.; Llopis, J.; Gonzalez-Calbet, J. M.; Vallet-Regí, M.; Hernando, A. Chem. Mater. 2007, 19, 889. (5) Garitaonandia, J. S.; Insausti, M.; Goikolea, E.; Suzuki, M.; Cashion, J. D.; Kawamura, N.; Ohsawa, H.; Gil de Muro, I.; Suzuki, K.; Plazaola, F.; Rojo, T. Nano Lett. 2008, 8, 661. 7787
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