Langmuir 2008, 24, 375-378
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Morphological Control of Synthesis and Anomalous Magnetic Properties of 3-D Branched Pt Nanoparticles Hai-Tao Zhang,* Jun Ding, and Gan-Moog Chow Department of Materials Science and Engineering, Faculty of Engineering, National UniVersity of Singapore, Singapore 117574 ReceiVed October 15, 2007. In Final Form: NoVember 26, 2007 Morphology-controllable platinum nanostructures could be obtained by modulating the growth kinetics in oleylamine. The nanostructures evolve from spherical particles to branched networks with decreasing reaction temperature, and the complexity of the branched-network nanostructures increases with the extended reaction period. Size-dependent magnetic properties and enhanced ferromagnetism in dodecanethiol-capped Pt branched nanostructures indicate that the permanent magnetic moments are probably introduced by broken symmetry and charge transfer because charge transfers more effectively from dodecanethiol than from oleylamine.
Introduction Noble metal nanoparticles (NPs) have exhibited many exotic electronic, optical, and magnetic properties that are highly affected by their morphology, including size, shape, and geometry.1 Metal NPs can be used not only as “artificial atoms” to form nanocrystal superlattices but also as “artificial molecules” after they have been regioselectively attached with functional molecules.2 Pt NPs have been intensively investigated because of their promising technological applications in catalysts, sensors, and nanoscale devices.3 Owing to their potential applications, spherical, cubic, 1-D and multipod nanostructures have been obtained.4 Despite the synthesis progresses made in the past, one simple route for monodisperse Pt nanocrystals with controlled morphologies is greatly desired. As is well known, the form (shape, size, and morphology) and properties are inextricably linked.5 Therefore, the prerequisite for obtaining NPs with controllable physical and chemical properties is the development of a synthesis technique to generate * To whom correspondence should be addressed. E-mail: msezh@ nus.edu.cn. (1) (a) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem 2000, 1, 18. (b) He, L.; Musick, M. D.; Nicewarner, S. R.; Salinas, F. G.; Benkovic, S. J.; Natan, M. J.; Keating, C. D. J. Am. Chem. Soc. 2000, 122, 9071. (c) Shipway, A. N.; Lahav, M.; Willner, I. AdV. Mater. 2000, 12, 993. (2) (a) Shevchenko, E. V.; Talapin, D. V.; Kotov, N. A.; O’Brien, S.; Murray, C. B. Nature 2006, 439, 55. (b) Redl, F. X.; Cho, K. S.; Murray, C. B.; O’Brien, S. Nature 2003, 423, 968. (c) Perepichka, D. F.; Rosei, F. Angew. Chem., Int. Ed. 2007, 46, 2. (d) Kalsin, A. M.; Fialkowski, M.; Paszewski, M.; Smoukov, S. K.; Bishop, K. J. M.; Grzybowski, B. A. Science 2006, 312, 420. (3) (a) Tseng, R. J.; Tsai, C. L.; Ma, L. P.; Ouyang, J. Y. Nat. Nanotech. 2006, 1, 72. (b) Tian, N.; Zhou, Z. Y.; Sun, S. G.; Ding, Y.; Wang, Z. L. Science 2007, 316, 732. (c) Joo, S. H.; Choi, S. J.; Oh, I.; Kwak, J.; Liu, Z.; Terasaki, O.; Ryoo, R. Nature 2001, 412, 169. (d) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; ElSayed, M. A. Science 1996, 272, 1924. (e) Zhao, M. Q.; Crooks, R. M. AdV. Mater. 1999, 11, 217. (f) Brugger, P. A.; Cuender, P.; Gratzel, M. J. Am. Chem. Soc. 1981, 103, 2923. (g) Chen, C. W.; Serizawa, T.; Akashi, M. Chem. Mater. 1999, 11, 1381. (h) Polsky, R.; Gill, R.; Kaganovsky, L.; Willner, I. Anal. Chem. 2006, 78, 2268. (i) Gill, R.; Polsky, R.; Willner, T. Small 2006, 2, 1037. (4) (a) Wang, Z. L.; Petroski, J. M.; Green, T. C.; El-Sayed, M. A. J. Phys. Chem. B 1998, 102, 6145. (b) Wang, C.; Daimon, H.; Lee, Y.; Kim, J.; Sun, S. H. J. Am. Chem. Soc. 2007, 129, 6974. (c) Niesz, K.; Grass, M.; Somorjai, G. A. Nano Lett. 2005, 5, 2238. (d) Lee, H.; Habas, S. E.; Kweskin, S.; Butcher, D.; Somorjai, G. A.; Yang, P. Angew. Chem., Int. Ed. 2006, 45, 7824. (e) Mayers, B.; Jiang, X.; Sunderland, D.; Cattle, B.; Xia, Y. J. Am. Chem. Soc. 2003, 125, 13364. (f) Chen, J.; Herricks, T.; Geissler, M.; Xia, Y. J. Am. Chem. Soc. 2004, 126, 10854. (g) Chen, J.; Xiong, Y.; Yin, Y.; Xia, Y. Small 2006, 2, 1340. (h) Chen, J.; Herricks, T.; Xia, Y. Angew. Chem., Int. Ed. 2005, 44, 2589. (i) Teng, X.; Yang, H. Nano Lett. 2005, 5, 885. (5) (a) Kim, S. H.; Mokari, T.; Rothenberg, E.; Nanin, U. Nat. Mater. 2003, 2, 155. (b) Manna, L.; Milliron, D. J.; Meisel, A.; Scheer, E. C.; Alivisatos, A. P. Nat. Mater. 2003, 2, 382. (c) Wang, D. L.; Lieber, C. M. Nat. Mater. 2003, 2, 355.
nanostructures with controllable size, shape, and structural complexity.6 Usually, oleylamine can be used not only as a stabilizer and solvent but also as a reductant because of its bivalent carbon unit. Until now, monodisperse metals, metal oxides, and metal sulfides have been formed in oleylamine owing to its multiple advantage.7 Bipod and tripod Pt nanoparticles could be formed in oleylamine with silver acetylacetonate as a trigger and 1,2-hexadecanothiol as a reductant.4i In addition, cubic Pt nanoparticles could be formed in oleylamine in the presence of oleic acid and a trace amount of Fe(CO)5.4b Here we report the novel and simple colloidal chemical synthesis of monodisperse Pt branched network (BN) NPs with controlled complexity and their anomalous magnetic properties. Materials and Methods Chemical Materials. Absolute ethanol and hexane were used without purification. Oleylamine (OA, >70%) and platinum(II) acetylacetonate (Pt(acac)2) were purchased from Sigma-Aldrich. Controlled Synthesis of Pt Nanostructures. The synthesis was carried out using commercial chemical regents in an inert blanket. Pt nanoparticles were formed by the decomposition of Pt(acac)2 in oleylamine. In a typical synthesis process of spherical Pt nanoparticles, 0.25 mmol of Pt(acac)2 was dissolved in 3 mL of oleylamine at 70 °C overnight. Then the orange solution was injected into 10 mL of oleylamine at 250 °C under flowing pure nitrogen gas and maintained for 1 h. The black solution was cooled to room temperature by removing the heating source. Ethanol (20 mL) was added to the reaction medium, and samples were precipitated by centrifugation (8000 rpm, 10 min). The precipitate was dispersed into hexane and precipitated by adding ethanol and using centrifugation; this washing process was repeated twice. In the end, the products were kept in hexane for further characterizations. Elongated nanocrystals and tetrapod nanoparticles were synthesized by annealing the solution at 180 and 150 °C for 1 h after the precursor solution was injected into oleylamine at 250 °C. Hierarchically branched Pt nanoparticles were synthesized in a direct heating process. In a typical synthesis, 0.25 mmol of Pt(acac)2 was dissolved in 10 mL of oleylamine under flowing pure nitrogen gas and kept at 110 °C for 2 h. Then the solution was carefully (6) (a) Mokari, T.; Sztrum, C. G.; Salant, A.; Rabani, E.; Banin, U. Nature 2005, 4, 855. (b) Habas, S. E.; Lee, H.; Radmilovic, V.; Somorjai, G. A.; Yang, P. D. Nat. Mater. 2007, 6, 692. (7) (a) Zhang, H. T.; Wu, G.; Chen, X. H. Langmuir 2005, 21, 4281. (b) Zhang, H. T.; Chen, X. H. Nanotechnology 2005, 16, 2288. (c) Zhang, H. T.; Wu, G.; Chen, X. H.; Qiu, X. G. Mater. Res. Bull. 2006, 41, 495.
10.1021/la7032065 CCC: $40.75 © 2008 American Chemical Society Published on Web 12/11/2007
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Figure 1. TEM images of monodisperse spherical (a), elongated (b), and tetrapod (c) Pt NPs. (I) Scheme of three different ways to control the reduction kinetics and the corresponding shape evolution for the Pt nanostructure. heated to 135 °C at a heating rate of 2 °C/min and held for different periods to form hierarchical Pt nanostructures with different complexities. Samples for TEM characterization were removed with a syringe from the same batch. Products were precipitated and washed using the same process as for spherical nanoparticles. Ligand Exchange of Pt Branched Nanostructures. Nanoparticles (20 mg) and 0.3 mL of 1-dodecanethiol were dissolved in 3 mL of hexane. Then they were mixed by shaking and ultrasonication and maintained overnight. The black powders were separated through centrifugation by adding ethanol and were dried in flowing nitrogen gas. This process was repeated three times. Characterization of Pt Nanoparticles. The microstructural analysis of nanoparticles was conducted using transmission electron microscopy (TEM) imaging and diffraction techniques with a JEOL JEM 3010 TEM operated at 300 kV and a JEOL JEM 2010 TEM operated at 200 kV. The magnetic properties were measured using a superconducting quantum interference device (SQUID, Quantum Design), and the crystal structure was analyzed by X-ray diffraction spectrometry (XRD, Bruker D8 Advance). The corresponding infrared spectra of the samples were recorded at room temperature in the range of 4000-400 cm-1 using a Bio-Rad FTIR model QS-300 spectrometer, which has a resolution of (8 cm-1.
Results and Discussion Three different morphologies could be obtained, namely, monodisperse 7.9 nm spheres, 4 × 8 nm2 elongated NPs, and 9 nm tetrapods with a pod diameter of 3.5 nm (branched NPs) at 250, 180, and 150 °C, respectively, during the annealing of Pt(acac)2 in OA. Figure 1a shows a transmission electron microscope (TEM) image of spherical NPs. The size distribution analysis, as shown in the histogram in Figure 2, reveals that the spherical NPs are monodisperse and have an average size of 7.9 ( 1 nm. Elongated and tetrapod NPs are also uniform, as shown in Figure 1b,c. A more complex branched network (BN) can be formed in a direct heating process at 135 °C. Figures 2 and S1 show typical TEM images of secondary (less than ten pods) and more complex (tens of pods) BNs. They are monodisperse in particle size, as confirmed by the size distribution histogram shown in Figure 2f, and have morphology with an average branch diameter of 3.6 nm. All products are of the fcc Pt phase, as confirmed by X-ray diffraction (XRD) and electron diffraction (Figure S2).8 A high-resolution (HR) TEM image of a single secondary BN (Figure S1) shows that the Pt branches grow mainly (8) For more details, see Supporting Information.
Figure 2. (II) Morphology evolution scheme of BNs with time. TEM images of Pt BNs formed in a direct heating process at 135 °C for 0.5 h (a), 4 h (b), 24 h (c), and 70 h (d). Size distribution histograms of nanostructures formed at 250 °C for 1 h (e) and at 135 °C for 70 h (f).
along the 〈111〉 direction with a lattice spacing of 2.3 Å, which is expected for the {111} planes of the fcc Pt phase. The growth mechanism is illustrated in Figure 1. At low temperatures (e150 °C), Pt tends to grow anisotropically along the 〈111〉 direction, resulting in the formation of network. For fcc metals, the surface energies for different crystallographic planes follow: γ(111) < γ(100) < γ(110).9 The growth along the 〈111〉 directions may result in a relatively high surface energy. Therefore, these tetrapods have a limited length in terms of branches. With the increase in annealing temperature, the anisotropic growth was destroyed, leading to spherical NPs at 250 °C. It is interesting to study the resultant nanostructure if the anisotropic growth is allowed to take place at a low temperature with a low growth rate, as shown in Figure 2c,d, where BN nanoparticles resulted after growing for a prolonged period. The results indicate that the complexity of the network increases with increasing annealing period while the morphology and monodispersity remain the same. The possible growth mechanism is illustrated in Figure 2. The growth along the 〈111〉 directions is terminated, probably because of the relatively higher surface energy. The terminal of the branch serves as a new nucleation side. New branches will grow from the terminal through the (9) Wang, Z. L. J. Phys. Chem. B 2000, 104, 1153.
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Figure 3. (a) Magnetization (M) vs magnetic field (H) loops for spherical nanoparticles (I) and BN nanoparticles (II) at 300 K. (b) FC magnetization dependence of temperature curves for BNs (as shown Figure 2c) coated with OA (amine-Pt) and DDT (thiol-Pt) and M-H loops of amine-Pt and thiol-Pt measured at 300 K (c) and 2 K (d).
anisotropic growth along the 〈111〉 directions as described above. The continuous growth/termination processes result in a nanostructured BN until the depletion of the precursor, resulting in monodisperse BN particles as shown in Figure 2. Recent research has shown that anomalous magnetic properties appear not only in nanoscale magnetic materials but also in nonmagnetic noble metal clusters and carbon-coated twined Pt nanoparticles.10 Here we mainly study the surfactant effect on the anomalous magnetic properties of the ligand-molecule-coated Pt nanoparticles. For magnetic characterization, a superconducting quantum interference device (SQUID) was used. No magnetic signal was found for the sample holder after a careful cleaning. For the magnetic measurements, a relatively large amount of Pt powder (20-30 mg) was used to ensure the magnetic signal at the level of 10-5 emu, which is well above the instrumental sensitivity of 10-8 emu. We also examined very carefully the composition using energy-dispersive X-ray spectroscopy (EDX) and chemical analysis before and after the chemical reaction. No 3d impurity or other elements (except for the elements of the surfactant) were observed in the starting precursor and resultant Pt nanoparticles and branched networks. The magnetic properties of the resultant Pt NPs are summarized in Figure 3. Our magnetic study was concentrated on the two samples: spherical and branched NPs (Figures 1a and 2c). Ferromagnetism has been reported for carbon-coated Pt nanoparticles with a saturation magnetization of around 10 × 10-3 emu/g.10c As shown in Figure 3, our amine-coated spherical Pt nanoparticles possessed a very low magnetization (around 1 × 10-3 emu/g). The spherical Pt NPs show negligible magnetization at 300 K, which is expected for Pt to be paramagnetic. However, ferromagnetism appears in spherical nanoparticles at low (10) (a) Sampedro, B.; Crespo, P.; Hernando, A.; Litran, R.; Lopez, J. C. S.; Cartes, C. L.; Fernandez, A.; Ramirez, J.; Calbet, J. G.; Vallet, M. Phys. ReV. Lett. 2003, 91, 237203. (b) 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, 087204. (c) Garcia, M. A.; Ruiz-Gonzalez, M. L.; de la Fuente, G. F.; Crespo, P.; Gonzalez, J. M.; Llopis, J.; GonzalezCalbet, J. M.; Vallet-Regi, M.; Hernando, A. Chem. Mater. 2007, 19, 889. (d) Liu, X.; Bauer, M.; Bertagnolli, H.; Roduner, E.; Slageren, J.; Phillipp, F. Phys. ReV. Lett. 2006, 97, 253401. (e) Zhang, J.; Soon, J. M.; Loh, K. P.; Jianhua Yin, J. H.; Ding, J.; Sullivian, M. B.; Wu, P. Nano Lett. 2007, 7, 2370.
temperature, as shown in Figure S6. The branched Pt networks had a much higher magnetization (10 × 10-3 emu/g at room temperature). The branched NPs are ferromagnetic at room temperature, with a coercivity of 106 Oe. The disappearance of ferromagnetism in spherical NPs (Figure 3aI) at room temperature suggests that ferromagnetism appears only in NPs with larger surface to volume ratios. The mechanism of anomalous ferromagnetism in BN particles needs to be investigated further in the near future. Such behavior indicates that the permanent magnetic moment may be introduced by the locally enhanced density of state at the broken symmetry around the surface.10a In addition, charge transfer in surfactant-Pt could plausibly induce a localized moment at the surface such as that appearing in thiol-capped Au NPs.10b After washing several times with a mixture of dichloromethane and ethanol and/or annealing to remove most of the surfactant (OA), the magnetization has been significantly reduced. The result suggests charge transfer from the surfactant molecules. We have studied the use of a stronger surfactantsdodecanethiol (DDT). The replacement of OA by DDT was confirmed by an infrared spectra study.8,11 After ligand exchange, magnetization is enhanced by a factor of 8 at room temperature and by a factor of ∼10 at 2 K because DDT binds more tightly with noble metal atoms than does oleylamine.11 The field cooling curves of the two samples indicate a high Curie temperature of well above 360 K.
Conclusions Monodisperse Pt nanostructures with controllable morphologies could be obtained by modulating the growth kinetics of Pt nanoparticles in OA. The nanostructures evolve from spheres to branched particles with decreasing reaction temperature. In addition, BNs can be achieved in a prolonged annealing process at a reduced annealing temperature while the diameter of the branches remains the same. Size-dependent magnetic properties and enhanced ferromagnetism in Pt-thiol NPs indicate that the permanent magnetic moments are probably introduced by broken (11) Bagaria, H. G.; Ada, E. T.; Shamsuzzoha, M.; Nikles, D. E.; Johnson, D. T. Langmuir 2006, 22, 7732.
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symmetry and charge transfer because charge transfers more effectively from DDT than from OA. The ferromagnetism of Pt nanostructures may be of interest in practical applications and fundamental research. Morphology-dependent catalysis studies are being conducted.
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Supporting Information Available: More TEM, XRD, FTIR, and magnetization curves and further discussion. This material is available free of charge via the Internet at http://pubs.acs.org. LA7032065
