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A Colloidal Templating Method To Hollow Bimetallic Nanostructures
Scheme 1. Preparation Sequence of Hollow Au/Pt Nanostructures
Lehui Lu, Guoying Sun, Shiquan Xi, Haishui Wang, and Hongjie Zhang* Laboratory of Physical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China Tiedong Wang and Xiaohuan Zhou The Center Laboratory, University of Quartermaster of PLA, Changchun 130062, China Received September 7, 2002. In Final Form: December 12, 2002
Introduction Recently, considerable effort has been devoted to the design and controlled fabrication of composite nanostructured materials with functional properties.1-9 In particular, composite metallic nanomaterials6a,b,8,9 including Au/ Pt, Au/Pd, and Cu/Pt are of continuing interest due to their fascinating catalytic properties, which are dramatically different from those of their single components. For example, the polymer-stablized Au-Pt bimetallic colloidal sol exhibits more efficient catalytic activity than monometallic Pt colloid for both hydrogenation of olefins8 and visible light induced hydrogen generation from water.9 Moreover, in the construction of metallic nanostructured catalysis, it is very important for practical purposes to control the structure and morphology of metallic nanomaterials. Consequently, effective strategies to build composite metallic nanomaterials with tailored structures are required in order to meet the ever-increasing technical demands. Our ongoing effort is to fabricate the composite metallic nanomaterials with a hollow interior. It is expected that such materials may enhance catalytic activity for special chemical reactions and optical signal sensitivity for analytical application. We demonstrate herein that this can be achieved by a general colloidal templating method. * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: +86-431-5685653. (1) (a) Fendler, J. H. Nanoparticles and Nanostructured Films; WileyVCH: Weinheim, Germany, 1998. (b) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257 and references therein. (c) Schmid, G. Chem. Rev. 1992, 92, 1709. (d) See also all the review articles in the Feb 16, 1996, issue of Science. (2) (a) Caruso, F. Adv. Mater. 2001, 13, 11 and references therein. (b) Zhong, C. J.; Maye, M. M. Adv. Mater. 2001, 13, 1507 and references therein. (3) (a) Henglein, A. J. Phys. Chem. B 2000, 104, 2201. (b) Henglein, A. Langmuir 2001, 17, 2329. (c) Link, S.; Wang, Z. L.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 3529. (4) (a) Hutter, E.; Fendler, J. H. Chem. Commun. 2002, 378. (b) Lu, L. H.; Wang, H. S.; Zhou, Y. H.; Xi, S. Q.; Zhang, H. J.; Hu, J. W.; Zhao, B. Chem. Commun. 2002, 144. (5) (a) Sun, Y.; Mayers. B. T.; Xia, Y. Nano Lett. 2002, 2, 481. (b) Oldfield, G.; Ung, T.; Mulvaney, P. Adv. Mater. 2000, 12, 1519. (6) (a) Mizukoshi, Y.; Fujimoto, T.; Nagata, Y.; Oshima, R.; Maeda, Y. J. Phys. Chem. B 2000, 104, 6028. (b) Toshima, N.; Wang, Y. Langmuir 1994, 10, 4574. (c) Schmid, G.; Lehnert, A.; Malm, J. O.; Bovin, J. O. Angew. Chem., Int. Ed. 1991, 30, 874. (7) (a) Nashner, M.S.; Frenkel, A. I.; Somerville, D.; Hills, C. W.; Shapley, J. R.; Nuzzo, R. G. J. Am. Chem. Soc. 1998, 120, 8093. (b) Hills, C. W.; Nasher, M. S.; Frenkel, A. I.; Shapley, J. R.; Nuzzo, R. G. Langmuir 1999, 15, 690. (8) Chen, C. W.; Akashi, M. Polym. Adv. Technol. 1999, 10, 127. (9) Toshima, N.; Yonezawa, T. Makromol. Chem. Macroml. Symp. 1992, 59, 281.
Our strategies to fabricate such materials are shown in Scheme 1. First, the surface of the silica nanoparticles is functionalized with (3-aminopropyl)trimethoxysilane (APTMS) (step 1). This is followed by adsorption of negatively charged Au colloidal nanoparticles onto the APTMSmodified silica nanoparticles via the amine group (step 2). These gold-coated silica nanoparticles as “seeds” are then added to the H2PtCl6 and ascorbic acid solution (growth solution) to obtain Pt/Au-coated silica nanoparticles (step 3). The coverage of Pt coating on the “seed” can be tuned by controlling the “seeds” concentration in growth solution. Finally, the silica template is removed by treating the Pt/Au-coated silica nanoparticles with HF solution to obtain Au/Pt nanostructures with hollow interior (step 4). Similar procedure is also applicable to prepare hollow Au/Pd and Au/Ag nanostructures. To the best of our knowledge, this is first reported fabricate of hollow bimetallic Au/Pt and Au/Pd nanostructures. Experimental Section Materials. (3-Aminopropyl)trimethoxysilane (APTMS), tetraethyl orthosilicate (TEOS), H2PtCl6, and HAuCl4 were obtained from Aldrich. NaBH4 and sodium citrate were purchased from Sigma. K2CO3, NH4OH, HF, and absolute ethanol were purchased from Beijing Chemical Reagents Industry. All chemicals were used as supplied. Throughout the experiment, Milli-Q water was used. Preparation of SiO2@Au Seed. Monodispersed silica nanoparticles with a diameter of 78 nm were prepared as described by Sto¨ber et al.10 The concentration of the resultant silica nanoparticles was ∼ 4 × 1013 particles/mL. An excess of APTMS (∼300 µL, 1.6 mmol) was added to a 100 mL portion of the silica nanoparticles colloid while vigorously stirring and allowed to react for 2 h. After stirring ceased, the APTMS-functionalized silica nanoparticles were observed to precipitate to the bottom. The APTMS-functionalized silica nanoparticles were purified by centrifuging and redispersing in ethanol. Colloidal gold (2.6 nm) was prepared according to ref 11. SiO2@Au nanoparticles were synthesized by the general method described by Halas et al.12 Mainly, the APTMS-functionalized silica nanoparticles dispersed in ethanol were placed in a 50 mL tube and 25 mL of the above gold colloid was added. The tube was shaken gently for several minutes and then allowed to sit for 2 h. The redcolored SiO2@Au nanoparticles were obtained after the excess gold colloid was removed by at least four repeated centrifugation cycles (2000 rpm). The purified SiO2@Au naoparticles were redispersed and sonicated in water until use. A subsequent (10) Sto¨ber, W.; Fink, A. J. Colloid Interface Sci. 1968, 26, 62. (11) Grabar, K. C.; Allison, K. J.; Baker, B. E.; Bright, R. M.; Brown, K. R.; Freeman, R. G.; Fox, A. P.; Keating, C. D.; Musick, M. D.; Natan, M. J. Langmuir 1996, 12, 2353. (12) Oldenburg, S. J.; Averitt, R. D.; Westcott, S. L. Halas, N. J. Chem. Phys. Lett. 1998, 288, 243.
10.1021/la026521c CCC: $25.00 © 2003 American Chemical Society Published on Web 02/20/2003
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Figure 1. TEM images of (a) APTMS-functionalized SiO2 nanoparticles, (b) SiO2@Au nanoparticles, (c) SiO2@Au/Pt nanoparticles, and (d) hollow Au/Pt nanostructures. Scale bar ) 100 nm. (e, f) HR-TEM images taken from the edge and center of the hollow Au/Pt nanostructures, respectively. Scale bar ) 10 nm. procedure by Halas et al.12 was also exploited to obtain the SiO2@Au naoparticles with high Au coverage by a reaction of HAuCl4 and NaBH4 on the surface of gold nanoparticles. Procedures for Hollow Au/Pt Nanostructures. A typical preparation procedure was as follows. First, a solution containing 1.5 mL of 100 mM freshly prepared ascorbic acid and 10 mL of 0.8 mM H2PtCl6 was prepared. Next, 5 mL of solution containing SiO2@Au nanopaticles as “seeds” was added while stirring. Within 1 min, the red color of SiO2@Au changed to the dark brown of Pt colloid. After five repeated centrifugation cycles, the SiO2@Au/ Pt nanoparticles were redispersed and sonicated in water. Finally, hollow Au/Pt nanostructures were produced by treating the SiO2@Au/Pt nanoparticles with 10 M HF solution (Extreme Caution! Great care and appropriate protective clothing must be employed when performing this procedure.) The thickness of hollow Au/Pt nanoshell increased with decreasing SiO2@Au/Pt “seeds” concentration. Procedures for Hollow Au/Pd Nanostructures. Similarly, 10 mL of 0.75 mM H2PdCl4 solution (0.013 g PdCl2 and 1 mL of 0.15 M HCl in 100 mL of H2O) was mixed with 1.5 mL of 100 mM ascorbic acid. Afterward, 5 mL of solution containing SiO2@Au/ Pt nanoparticles was added while stirring. After 3 h, an isolation procedure was carried out as described above. The product consisting of dark brown SiO2@Au/Pd nanoparticles was redis-
persed and sonicated in water. The silica template was removed with 10 M HF to obtain hollow Au/Pd nanostructures. Characterization. The structure of the hollow Au/Pt and Au/Pd nanostructures was investigated by power X-ray diffraction (XRD, D/max 2000, Rigaku, Cu KR radiation) and transmission electron microscopy (TEM, JEOL 2000-FX). The surface composition of hollow Au/Pt and Au/Pd nanostructures was analyzed by X-ray photoelectron spectroscopy (XPS, VG ESCA MK). All TEM samples were prepared by adding a drop of solution separated by centrifugation onto a carbon-coated copper grid and allowing evaporation of the solvent. Samples for XPS and XRD measurement were prepared by adding several drops of condensed solution to a glass substrate and left to dry in air. The sample etchings with a argon ion sputtering system were performed similar to the method by Borchert et al.13
Results and Discussion The uniform nanosized silica spheres with an average diameter of 78 nm were prepared using the stepwise seeded growth process. Functionalization of these nano(13) Borchert, H.; Haubold, S.; Haase, M.; Weller, H.; McGinley, C.; Riedler, M.; Moller, T. Nano. Lett. 2002, 2, 151.
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particles with APTMS and the centrifugation/redispersion steps had no effect on their size and morphology as determined by TEM images (Figure 1a). When the APTMS-modified silica nanoparticles were mixed with a solution of very small gold colloid, these small gold nanoparticles were immobilized onto the surface of the silica nanoparticles (Figure 1b). Evident from Figure 1b, individual small gold nanoparticles were well separated from each other on the surface. But gold nanoparticles coverage was limited by repulsive interparticle interaction that inhibited additional particles immobilization on the surface of silica nanoparticles. In this case, the selfassembly of monodispersed gold nanoparticles on the APTMS-modified silica nanoparticles yielded the gold monolayer of approximately 25% coverage that was evaluated from the enlarged TEM images. This result was consistent with previous reported coverage of gold nanoparticles onto planar amine-functionalized surface of solid substrates.14-16 However, by using silica nanoparticles as the substrates, the available surface area upon which small gold nanoparticles may be assembled can obviously be increased compared to the limited surface area of planar substrates. The SiO2@Au nanoparticles with high Au coverage can be obtained by a subsequent reduction of an aged mixture of HAuCl4 and K2CO3 by a solution of NaBH4.12 The seeding reaction of H2PtCl6 and ascorbic acid resulted in increasing coverage of Pt on the surface of SiO2@Au nanoparticles (>90%), where the SiO2@Au nanoparticles as seeds can catalyze the reduction of PtCl62- ions by ascorbic acid due to particle-mediated electron transfer from ascorbic acid to PtCl62- ions.17 This seeding growth was completed in 1 min. Importantly, the Au/Pt molar ratio can be controlled solely by the added amount of SiO2@Au seeds. As seen in Figure 1c, the average size of particles increased from 78 nm (silica naoparticles) to 98 nm, and these SiO2@Au/Pt nanoparticles were uniformly spheres in morphology. Such SiO2@Au/Pt nanoparticles are also potential candidates for active and recyclable catalysis. During this step, it is crucial to prepare the SiO2@Au seeds because the quality of SiO2@Au seeds directly influences the resultant SiO2@Au/Pt nanoparticles. We found that the SiO2@Au/ Pt nanoparticles with poor quality were obtained when the SiO2@Au nanoparticles with low gold coverage or poor distribution of gold on the surface were used as seeds. By treatment with dilute HF solution, the silica template was removed since the Au/Pt shell was still permeable to the acid and the dissolved silicon. As observed from the Figure 1d, these hollow nanostructures possess an incompletely closed shell after treatment with HF solution. The wall thickness around the shell is estimated to be ca. 11 nm from the TEM images. The strong contrast between the dark edges and plate centers provides evidence of their hollow nature.18 The existence of a hollow structure can be confirmed by the direct observation of the broken spheres as shown in Figure 1d. High-resolution TEM (HRTEM) images taken from the edge and center of such (14) Freeman, R. G.; Grabar, K. C.; Allison, K. J.; Bright, R. M.; Davis, J. A.; Guthrie, A. P.; Hommer, M. B.; Jackson, M. A.; Smith, P. C.; Walter, D. G.; Natan, J. Science 1995, 267, 1629 and references therein. (15) Doron, A.; Kata, E.; Willner, I. Langmuir 1995, 11, 1313. (16) Li, W.; Huo, L. H.; Wang, D. M.; Zeng, G. F.; Xi, S. Q.; Zhao, B.; Zhu, J.; Wang, J.; Shen, Y.; Lu, Z. Colloids Surf., A 2000, 175, 217. (17) Jana, N. R.; Rearheart, L.; Murphy, C. J. Chem. Mater. 2001, 13, 2313. (18) (a) Fowler, C. E.; Khshalani, O.; Mann, S. J. Mater. Chem. 2001, 11, 1968. (b) Caruso, F.; Caruso, R. A.; Mohwald, H. Science 1998, 282, 1111. (c) Wang, C. R.; Tang, K. B.; Yang, Q.; Hu, J. Q.; Qian, Y. T. J. Mater. Chem. 2002, 12, 2426.
Notes
Figure 2. XPS spectra for SiO2@Au/Pt nanoparticles without HF treatment. The samples were etched for 1 min.
Figure 3. XPS spectra for hollow Au/Pt nanostructures: (a) Pt4f orbital; (b) Au4f orbital.
hollow nanostructures provide further insight into their structures (Figure 1e,f). As observed in Figure 1e,f, these images show well-resolved fringes indicative of the formation of crystalline structures for the metallic wall. It is evident that these hollow nanostructures are not completely closed, which is evidenced by the existence of
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Figure 5. TEM image of hollow Au/Pd nanostructures. Figure 4. XRD patterns of SiO2@Au/Pt nanoparticles (a) without and (b) with HF treatment.
many gaps. It is noteworthy that these hollow structures are robust, which can be confirmed by the fact that the centrifugation, redispersion and sonication steps did not destroy such structures. XPS was employed to investigate the composition of SiO2@Au/Pt nanoparticles without and with HF treatment. As for the sample without HF treatment, XPS analysis showed Si2p peaks corresponding to silica nanoparticles after etching for 1 min similar to the method by Borchert et al.13 (Figure 2). In contrast, for the sample with HF treatment, no signal characteristic of silica nanoparticles was observed even after further etching for 3 min, suggesting that silica templates were completely removed and the hollow nanostructures were produced. Moreover, XPS patterns of these hollow nanostructures showed significant Pt4f signal corresponding to the binding energy of metallic Pt (Figure 3a) and Au4f signal characteristic of metallic Au were also observed after etching for 1 min (Figure 3b). It is evident that hollow Au/Pt nanostructures can be obtained via the colloidal templating method. The crystal structure of SiO2@Au/Pt nanoparticles without and with HF treatment was investigated by power X-ray diffraction (XRD). The resulting XRD patterns are shown in Figure 4. As seen in Figure 4, three diffraction lines were observed in the XRD pattern of SiO2@Au/Pt nanoparticles without HF treatment at 2θ ∼ 39.6, 46.3, and 67.5° (Figure 4a). These diffraction lines correspond to the (111), (200), and (220) reflections, respectively, for the face-centered cubic structure of metallic Pt with space group Fm3m (JCPDS, card no. 4-802). As for SiO2@Au/Pt nanoparticles with HF treatment, the same XRD pattern was observed (Figure 4b), indicating that HF treatment of SiO2@Au/Pt nanoparticles did not influence the crystal structure of Au/Pt nanoshell. The nanometer size of samples was evidenced by the broad X-ray diffraction lines. No detectable Au diffraction lines were observed in the XRD patterns of SiO2@Au/Pt nanoparticles without and
with HF treatment. We explain the result as the lower Au molar fraction in samples, which was also confirmed by the XPS analysis. Moreover, the present method has also been extended to fabricate hollow nanosrtuctures from other metals such as Au/Pd and Au/Ag. Figure 5 indicates a representative TEM image of hollow bimetallic Au/Pd nanostructures prepared by the above-mentioned procedure. As is evident from Figure 5, these uniform hollow Au/Pd nanostructures obtained using this procedure also exhibit the morphology similar to that of the hollow bimetallic Au/Pt nanostructures. Conclusion The aim of the present work has been to explore an effective strategy for the fabrication of hollow composite metallic nanostructures. It has been demonstrated that the colloidal templating method provides a successful pathway for fabricating such materials. This approach to hollow bimetallic nanostructures is attractive for four reasons. First, the whole experiment can be carried out at room temperature. Second, the bimetallic molar ratio can be controlled solely by varying the added “seeds” amount. Third, the shape and hollow size of the resultant nanostructures are determined by the dimensions of the employed template. Finally, the method is applicable to many other hollow composite metallic nanostructures. Such materials will find use in various applications in catalysis, plasmonic devices, sensors, and many other fields. Acknowledgment. We thank Dr. Xiaoming Sun and Professor Yadong Li from Tsing University for TEM measurement. This work is supported by the National Natural Science Key Foundation of China (No. 20171043) and the National Key Project for Fundamental Research of Rare Earth Functional Materials. LA026521C
