Langmuir 2004, 20, 3431-3434
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Microwave Synthesis of Core-Shell Gold/Palladium Bimetallic Nanoparticles Riki Harpeness and Aharon Gedanken* Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel Received October 23, 2003. In Final Form: February 2, 2004 The microwave-assisted polyol reduction method was applied to the synthesis of core-shell gold/palladium bimetallic nanoparticles by the simultaneous reduction of the AuIII and PdII ions. The thickness of the palladium shell was calculated to be ∼3 nm, and the gold core diameter is 9 nm. The structure and composition of the bimetallic particles were characterized by high-resolution transmission electron microscopy equipped with a nanoarea energy-dispersive X-ray spectroscopy attachment, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy.
Introduction In recent years, many studies have been carried out on nanoparticles of noble metals because of their extremely small size and large specific surface area.1-3 They have many potential applications in optoelectronics, semiconductors, catalysis, photocatalysts, magnetic materials, drug delivery, and other areas.4-6 The bimetallic particles are also an attractive subject for study because their catalytic properties7 are superior to those of single metallic nanoparticles. They are also of importance because of the change in their surface plasma band energy8 relative to that of the separate metals. Various methods have been reported so far for their preparation, for example, alcohol citrate reduction,8b,10 alcohol reduction,7,9 polyol process,11 solvent extraction reduction,8a,12 sonochemical method,13 photolytic reduction,14 decomposition of organometallic precursors,15 and electrolysis of a bulk metal.16 * Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) (a) Ozin, G. A. Adv. Mater. 1992, 4, 612. (b) Chan, R. W. Nature 1992, 359, 591. (c) Ozin, G. A. Science 1996, 271, 920. (2) Fendler, J. H. Nanoparticles and nanostructured films: preparation, characterization and applications; Wiley-VCH: Weinhein, Germany, 1998. (3) Schimid, G. Clusters and Colloids: Form Theory to Application; VCH: Weinhein, Germany, 1994. (4) Toshima, N.; Yonezawa, T. New J. Chem. 1998, 1179. (5) (a) Kamat, P. V. Chem. Rev. 1993, 93, 267. (b) Lewis, L. N. Chem. Rev. 1993, 93, 2693. (c) Gates, B. C. Chem. Rev. 1995, 95, 511. (6) (a) Hoffman, A. J.; Mils, G.; Yee, H.; Hoffman, M. R. J. Phys. Chem. 1995, 99, 4414. (b) Lee, A. F.; Baddeley, C. J.; Hardacre, C.; Ormerod, R. M.; Lambert, R. M.; Schmid, G.; West, H. J. Phys. Chem. 1995, 99, 6096. (7) (a) Toshima, N.; Yonezawa, T.; Kushihashi, K. J. Chem. Soc., Faraday Trans. 1993, 89, 2537. (b) Toshima, N.; Harada, M.; Yamazaki, Y.; Asakura, K. J. Phys. Chem. 1992, 96, 9927. (c) Wang, Y.; Toshima, N. J. Phys. Chem. B 1997, 101, 5301. (8) (a) Han, S. W.; Kim, Y.; Kim, K. J. Colloid Interface Sci. 1998, 208, 272. (b) Link, S.; Wang, Z. L.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 3529. (9) (a) Toshima, N.; Wang, Y. Langmuir 1994, 10, 4574. (b) Yonezawa, T.; Toshima, N. J. Mol. Catal. 1993, 83, 167. (c) Harada, M.; Asakura, K.; Ueki, Y.; Toshima, N. J. Phys. Chem. 1993, 97, 10742. (d) Yonezawa, T.; Toshima, N. J. Chem. Soc., Faraday Trans. 1995, 91, 4111. (10) Miner, R. S.; Namba, S.; Turkevich, J. Proceedings of the International Congress on Catalysis, 7th; Tokyo, Japan, 1981. (11) Silvert, P.-Y.; Vijayakrishnan, V.; Vibert, P.; Herrera-Urbina, R.; Elhsissen, K. T. Nanostruct. Mater. 1996, 7, 611. (12) Esumi, K.; Shiratori, M.; Ishizuka, H.; Tano, T.; Torigoe, K.; Meguro, K. Langmuir 1991, 7, 457. (13) (a) Mizukoshi, Y.; Fujimoto, T.; Nagata, Y.; Oshima, R.; Maeda, Y. J. Phys. Chem. B 2000, 104, 6028. (b) Mizukoshi, Y.; Okitsu, K.; Maeda, Y.; Yamamoto, T. A.; Oshima, R.; Nagata, Y. J. Phys. Chem. B 1997, 101, 7033.
The combined gold and palladium bimetallic nanoparticles have received significant attention because of their special catalytic properties. Turkevich et al.17 have synthesized the gold/palladium bimetallic particles and described their morphologies. Toshima et al.7b have described the catalytic activity and analyzed the structure of the poly(N-vinyl-2-pyrolidone)-protected gold/palladium bimetallic clusters prepared by the simultaneous reduction of HAuCl4 and PdCl2 in the presence of poly(N-vinyl-2pyrolidone). Other groups have reported the formation of the gold/palladium bimetallic particles with a palladiumrich shell by the simultaneous alcoholic reduction method.18 In contrast, successive alcoholic reduction did not give the core-shell-structured products but, instead, “clusterin-cluster” structured products or mixtures of the monometallic particles.20 Mizukoshi et al.13 reported the preparation and structure of gold/palladium bimetallic nanoparticles by sonochemical reduction of the gold(III) and palladium(II) ions. Wu et al.19 described the synthesis of Au/Pd bimetallic nanoparticles in the reverse micelles of water/sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/isooctane by the coreduction of HAuCl4 and H2PdCl4 with hydrazine at 25 °C. Schmid et al.21 reported the synthesis of the ligandstabilized core-shell clusters, which were prepared in successive chemical reductions using hydroxylamine hydrochloride. Davis22 and co-workers immobilized coreshell-structured gold/palladium bimetallic nanoparticles on a silica support. In this paper, we report the synthesis of core-shell gold/ palladium bimetallic nanoparticles, by the microwaveassisted polyol method. The polyol method has been developed over the last two decades and applied to the preparation of submicrometer (14) Ramita, S.; Mostafavi, M.; Delcourt, M. O. Radiat. Phys. Chem. 1996, 47, 275. (15) (a) Baradley, J. S.; Hill, E. W.; Klein, C.; Chaudret, B.; Duteil, A. J. Chem. Mater. 1993, 5, 254. (b) Pan, C.; Dassenoy, F.; Casanove, M. J.; Philippot, K.; Amiens, C.; Lecante, P.; Mosset, A.; Chaudret, B. J. Phys. Chem. B 1999, 103, 10098. (16) (a) Reetz, M. T.; Helbig, W.; Quaiser, S. A. J. Chem. Mater. 1995, 7, 2227. (b) Reetz, M. T.; Quaiser, S. A. Angew. Chem., Int. Ed. Engl. 1995, 34, 2240. (17) Turkevich, J.; Kim, G. Science 1970, 169, 873. (18) Liu, H.; Mao, G.; Meng, M. J. Mol. Catal. 1992, 74, 275. (19) Wu, M. L.; Chen, D. H.; Huang, T. C. Langmuir 2001, 17, 3877. (20) Harada, M.; Asakura, K.; Toshima, N. J. Phys. Chem. 1993, 97, 5103. (21) Schmid, G.; Lehnert, A.; Malm, J.-O.; Bovin, J.-O. Angw. Chem., Int. Ed. Engl. 1991, 30, 874. (22) Davis, R. J.; Boudart, M. J. Phys. Chem. 1994, 98, 5471.
10.1021/la035978z CCC: $27.50 © 2004 American Chemical Society Published on Web 03/04/2004
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metallic nanoparticles of the easily reducible transition metals.23 This method is based on alcohols (such as ethylene glycol (EG) and other glycols) acting as reducing agents of metallic cations to form the corresponding metals. In almost all of the reactions, the polyol plays the role of the solvent for the same reaction. Generally, the solution reactions for the formation of binary chalcogenides are relatively slow24 and are conducted under hydro- or solvothermal conditions in special high temperature and pressure equipment in order to accelerate their rate. Even under these conditions the reaction takes many hours or even days.25 Recently, we have found that the application of microwave radiation or ultrasound greatly facilitates the use of the polyol method for the preparation of binary chalcogenides (selenides and tellurides).26 In most experiments, the microwave reaction is completed within a few minutes, the maximum duration being 1 h. The first stage in the synthesis of these chalcogenides is the synthesis of the metallic particles. The microwave-assisted polyol reaction yielding metallic nanoparticles was first carried out by Komarneni.27 He emphasized the advantages of using the microwave-polyol process for the preparation of nanophase metallic powders. In this paper, we extend this method to the case of a bimetallic compound and report the first microwave synthesis of core-shell gold/palladium bimetallic nanoparticles by the simultaneous reduction of the AuIII and PdII ions. The structure and composition of the bimetallic particles were characterized by high-resolution transmission electron microscopy (HRTEM) equipped with a nanoarea energy-dispersive X-ray spectroscopy (EDAX) attachment, transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Experimental Section 1. Materials and Characterization. All of the reagents were of the highest commercially available purity. Palladium(II) chloride, hydrogen tetrachloroaurate(III) hydrate, and EG were purchased from Aldrich and used without further purification. The XRD patterns of the products were recorded with a Bruker AXS D8 advance powder X-ray diffractometer (using Cu KR λ ) 1.5418 Å radiation). Peak fitting and lattice parameter refinement were computed using the Topas and Metric programs (Bruker Analytical X-ray Systems). The transmission electron micrographs were imaged on a JEOL-JEM 100SX microscope, using a 100 kV accelerating voltage. Samples for TEM were prepared by placing a drop of the sample suspension on a copper grid (400 mesh, Electron Microscopy Sciences) coated with a carbon film. The grid was then air-dried. HRTEM images were taken using a JEOL 3010 with a 300 kV accelerating voltage and, for EDAX, by using the attached function to a scanning electron microscopy (SEM). A conventional monochrome CCD camera, with resolution of 768 × 512 pixels, was used to digitize the images. The digital images were processed with the Digital Micrograph software (23) (a) Chow, G. M.; Kurihara, K. L.; Ma, D.; Feng, C. R.; Schoen, P. E.; Martinez-Miranda, L. J. Appl. Phys. Lett. 1997, 70, 2315. (b) Fie’vet, F.; Lagier, J. P.; Figlarz, M. MRS Bull. 1989, 29. (c) Hegde, M. S.; Larcher, D.; Dupont, L.; Beaudoin, B.; Tekaia-Elhsissen, K.; Tarascon, J. M. Solid State Ionics 1997, 93, 33. (24) (a) Li, J.; Chen, Z.; Wang, R. J.; Prospero, D. M. Coord. Chem. Rev. 1999, 192, 707. (b) Yang, J.; Yu, S. H.; Yang, X. L.; Qian, Y. T. Chem. Lett. 1999, 839. (25) Soliman, H. S.; Ali, N. A.; El-Shazly, A. A. Appl. Phys. A 1995, 61, 87. (26) (a) Kerner, R.; Palchik, O.; Gedanken, A. Chem. Mater. 2001, 13, 1413. (b) Palchik, O.; Kerner, R.; Zhu, J.; Gedanken, A. J. Solid State Chem. 2000, 154, 530. (c) Zhu, J. J.; Palchik, O.; Chen, S.; Gedanken, A. J. Phys. Chem. B 2000, 104, 7344. (d) Palchik, O.; Kerner, R.; Gedanken, A. Chem. Mater. 2002, 14, 2094. (27) Komarneni, S.; Pidugu, R.; Li, Q. H.; Roy, R. J. Mater. Res. 1995, 10, 1687.
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Figure 1. XRD pattern of core-shell Au/Pd (the asterisk indicates Pd).
package (Gatan, Inc., Pleasanton, CA). Samples for TEM were prepared by placing a drop of the sample suspension on a copper grid (400 mesh, Electron Microscopy Sciences) coated with a carbon film. The grid was then air-dried. XPS spectra were recorded using a Kratos Analytical AXIS, HIS, 165, ULTRA (Kratos Analytical). The microwave-assisted reaction was carried out in a Spectra900 W domestic microwave oven, with a 2.45 GHz working frequency. The oven was modified to include a refluxing system. In all of the experiments, the microwave oven was cycled as follows: on for 21 s, off for 9 s, with the total power always at 900 W. This cycling mode was chosen in order to reduce the risk of superheating the solvent. All of the reactions were conducted under nitrogen flow. 2. The Preparation of the Core-Shell. The preparation of the core-shell bimetallic mixture was achieved by introducing equal molar amounts of palladium(II) chloride and hydrogen tetrachloroaurate(III) in EG. The system was purged for a few minutes with nitrogen prior to switching on the microwave reactor. The reactions were conducted for 1 h under nitrogen. In the postreaction treatment, the product was centrifuged once with the mother liquid and a few times with ethanol, at 20 °C and 9000 rpm. The product was then dried overnight under vacuum.
Results and Discussion The color of the solution containing AuIII and PdII in EG was pale yellow, and after the reaction, we obtained a clear solution with a black precipitate. 1. XRD. The products obtained in the polyol reaction described above are nanocrystalline as evidenced from the XRD patterns. The XRD results also indicate whether the gold/palladium structure is that of an alloy or a twosolids mixture that would fit a core-shell structure. If the product was a gold/palladium alloy, an XRD pattern of gold would be detected. The gold peaks are somewhat shifted when compared with the diffraction angles of pristine gold. Figure 1 shows the XRD patterns obtained for the product of the microwave reaction. This figure depicts peaks that fit very well with both the gold and palladium diffraction peaks published in the literature (PDF 04-0784 and 46-1043, respectively). The asterisk indicates Pd. When these results are combined with the TEM, HRTEM, and XPS data, we claim that the XRD data are supportive of a core-shell structure for the Au/ Pd mixture. One advantage of our XRD results over those of a previous study,13b where an Au/Pd core-shell structure was prepared, is that the Pd diffraction peaks, which were missing in that study, are clearly observed in the current investigation. The reason for the absence of the Pd diffraction peaks was due to the 0.8 nm thickness of the shell. This thin shell corresponds to merely a few layers of palladium atoms, resulting in a very weak diffraction. The calculated widths (Scherrer equation) are 17 nm for the Au diffraction peaks and 24 nm for the Pd. These
Core-Shell Gold/Palladium Bimetallic Nanoparticles
Langmuir, Vol. 20, No. 8, 2004 3433 Table 1. Results of the EDAX and XPS Analyses of Au/Pd Bimetallic Nanoparticles EDAX
Figure 2. Transmission electron micrograph of the as-prepared sample of Au/Pd bimetallic nanoparticles.
Figure 3. HRTEM image of core-shell Au/Pd.
results deviate from the thickness calculated from the HRTEM. The Pd peaks are observed in our study because of the much thicker layer observed in the HRTEM pictures. 2. Electron Microscopy Studies (TEM and HRTEM). The TEM image of the as-prepared sample is illustrated in Figure 2. This picture shows highly aggregated spherical particles, with an average diameter of ∼8 nm for the individual particles. To examine whether a core-shell structure was obtained in the microwave reaction, HRTEM of the asprepared material was measured. HRTEM is depicted in Figure 3. The image clearly shows that microwaveprepared gold/palladium bimetallic nanoparticles are composed of a large gold core and a thin palladium shell. The thickness of the palladium layer as determined by the HRTEM observation was calculated to be ∼3 nm, and the gold core diameter is 9 nm. The lattice fringes are shown in the micrograph of Figure 3. The distance between the adjacent fringes in the particle’s core is ∼0.23 nm, which fits well with the distance between the (111) planes of the face-centered cubic gold. We observe the fringes separated by ∼0.24 nm in the shell as well. They are due to the (111) of palladium. This assignment can be questioned because of the similarity of the distances between the fringes of Pd and Au. However, we rely on our XPS and EDAX results in assigning the shell fringes to Pd.
element
whole
center
shell
XPS
Au (atomic %) Pd (atomic %)
47.79 52.21
94.44 5.56
0.33 96.80
27.64 72.36
3. EDAX. HRTEM observations together with the SAEDX (selected-area EDX) measurements, conducted with an electron beam whose size is 25 nm, are presented in Table 1. The results clearly show that the composition of the individual particle is that of a core that is heavily populated with gold, with a shell that is predominantly composed of palladium. The EDAX results lead us to conclude that the microwave-prepared gold/palladium bimetallic nanoparticles are composed of a gold core and palladium shell. 4. XPS. XPS as a surface-monitoring technique was employed to further support the EDAX results. The penetration depth of the XPS is 1-2 nm. It clearly demonstrates that the shell is composed primarily of palladium. The binding energies of Pd3d3/2 and Pd3d5/2 measured for these two peaks are 340.5 and 335.2 eV, respectively. The binding energies of the Au4f5/2 and Au4f7/2 were detected at approximately 88.5 and 83.93 eV, respectively. The energies for Au and Pd are in good agreement with the literature values for the binding energies of Au and Pd.28 According to the intensities of the XPS peaks, the elemental ratios of Au/Pd bimetallic nanoparticles are summarized in Table 1. The enrichment of the surface layer with palladium is clearly observed from the measured data. 5. Proposed Mechanism. In previous polyol reactions, which have been carried out under microwave radiation, we, like others,29 have found that the solvents can undergo profound overheating.23,29-31 As a result, the metal ion can be reduced to its zero oxidation state by the following reactions:23b
2CH2OH-CH2OH f 2CH3CHO + 2H2O
(1)
2CH3CHO + M(OH)2 f CH3-CO-CO-CH3 + 2H2O + M (2) To succeed in obtaining the metallic particles, the reagents must be soluble in EG and independently reducible by the solvent.32 Ions, such as Ag+, Cu2+, Ni2+, Pb2+, Bi3+, etc., were all reduced to their metallic nanoparticles by EG under microwave heating. Gold and palladium can be easily reduced to their metallic state under molecular weight radiation because of their positive reduction potentials. Thermodynamically, it is easier to reduce the gold ions because the reduction potential of gold is more positive than that of palladium. However, the question of whether a core-shell structure is obtained is a kinetic one. It was not known from early studies whether the reduction of the AuCl4- ions was faster than that of the PdCl42- ions. Our results answer this dilemma and show that gold is reduced first forming the particle’s core followed by the reduction of palladium. The (28) Chastain, J.; King, R. C. Handbook of X-ray Photoelectron Spectroscopy; Physical Electronics: Eden Prairie, MN, 1995. (29) Mingos, D. M. P. Res. Chem. Intermed. 1994, 20, 85. (30) Gedye, R. N.; Wei, J. B. Can. J. Chem. 1998, 76, 525. (31) Sridhar, V. Curr. Sci. 1998, 74, 446. (32) Stuerga, D.; Gaillard, P. Tetrahedron 1996, 52, 5505.
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Au0 is acting as a nucleic center for the growth of the Pd layer, which covers Au and grows to its final size. Conclusions The paper reports on a new preparation method for the fabrication of bimetallic particles. The advantages of the described process are its simplicity, the short reaction time, and the easy and cheap preparation of the coreshell Au/Pd structure. The HRTEM and XRD analyses all suggest the formation of a core-shell Au/Pd structure. As
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far as we know, this is the first microwave synthesis of bimetallic nanoparticles with a core-shell structure. Acknowledgment. A.G. and R.H. thank Dr. Y. Grinblat, Dr. T. Tamari, and Dr. G. Salitra for helping with the characterization measurements. A.G. is also thankful for the financial help of the DIP (Deautsche-Israeli Projects) organization. LA035978Z
