Microwave-Assisted Synthesis of Pt Nanocrystals and

Copyright © 2006 American Scientific Publishers All rights reserved Printed in the United States of America

Journal of Nanoscience and Nanotechnology Vol. 6, 175–179, 2006

Microwave-Assisted Synthesis of Pt Nanocrystals and Deposition on Carbon Nanotubes in Ionic Liquids Zhimin Liu,∗ Zhenyu Sun, Buxing Han, Jianling Zhang, Jun Huang, Jimin Du, and Shiding Miao Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, P. R. China

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In this work, a simple, fast and efficient route is presented for the metal (such as Pt, Rh, etc.) nanocrystal synthesis and deposition on carbon nanotubes (CNTs) in ionic liquids (ILs) via microwave heating. In this method, inorganic salts (such as H2 PtCl6 · 4H2 O, RhCl3 · 2H2 O, etc.) dissolved in ILs, 1,1,3,3-tetramethylguanidinium trifluoroacetate or 1,1,3,3-tetramethylguanidinium Delivered by Ingenta to: of microwave heating, and the lactate, were reduced to metal nanoparticles by glycol with the aid College Dublin produced metal nanoparticles could beTrinity decorated on CNTs in the presence of CNTs in ILs. The 134.226.1.229 resulting nanomaterials were characterizedIP by :means of transmission electron microscopy and X-ray diffraction. It was demonstrated that the homogeneously dispersed Pt nanocrystals with the size of Thu, 29 Mar 2007 08:19:17 2 ∼ 3 nm were obtained using H2 PtCl6 · 4H2 O as precursor, and they deposited on CNTs with the similar size when CNTs was present in ILs. This technique also can be extended to fabricate other noble metal nanocrystals (including Rh, Au, etc.) and corresponding CNT composites.

Keywords: Platinum Nanocrystals, Synthesis, Deposition, Carbon Nanotubes.

1. INTRODUCTION Room temperature ionic liquids (RTILs) possess unusual properties, such as low melting temperature and extended temperature range in the liquid state, air and water stability, nonflammability, high ionic conductivity, excellent solvent power for organic and inorganic compounds, and importantly very low vapor pressure.1–5 Therefore, they have been applied as environmentally benign solvents for a number of chemical processes, including separations,6 organic reactions,7 8 electrochemistry,9 10 electrodeposition11 and others.12 13 In contrast to their application in organic chemistry, the use of RTILs in inorganic synthesis is still in its infancy. One of the reasons is that the most RTILs used have weak solvent power for inorganic compounds, while strong solvent power for organic compounds. Thus inorganic metallic compounds were seldom used as precursors for synthesis of metallic compounds in RTIL media. Owing to the presence of large organic cations with a high polarizability, RTILs are good media for absorbing microwaves, which can lead to a very high heating rate. The application of microwave chemistry to material synthesis in conjunction with RTILs hardly has been exploited. By combining the advantages of RTILs and microwave heating, Zhu et al.14 developed ∗

Author to whom correspondence should be addressed.

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a microwave-assisted ionic liquid method for the fast controlled synthesis of tellurium nanorods and nanowires via the reduction reaction of TeO2 with NaBH4 . Although some metal nanoparticles such as iridium,15 platinum,16 metal alloys,17 and nanorods and nanowires14 have been fabricated in RTILs, few reports on the synthesis of nanocomposites (especially carbon based nanocomposites) have been found in a literature survey. Recently, carbon nanotubes (CNTs) have attracted much attention due to their exceptional electrical and mechanical properties. Especially, more attention has been paid on exploring their applications with the success of mass production of CNTs.18–20 The CNT-based composites provide new properties with the original features of CNTs, which widen the potential applications of CNTs. The nanocomposites of CNTs decorated with metal or metallic compounds can be used as catalysts in fuel cells, functional materials, etc., which have stimulated more interests of scientists. Up to date, several techniques, including capillary action,21 chemical vapor deposition or wet chemical processing,22 arc-discharge technique,23 catalyzed hydrocarbon pyrolysis,24 and electroless deposition,25 have been developed to impregnate metals or metal oxides in the cavities or immobilize them on the external walls of CNTs. Nevertheless, new approaches are still desirable to the modification of CNTs.

1533-4880/2006/6/175/005

doi:10.1166/jnn.2006.027

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Microwave-Assisted Synthesis of Pt Nanocrystals and Deposition on Carbon Nanotubes in Ionic Liquids

+

N CH

NH2

– RCOO

N

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Fig. 1.

The chemical structure of RTILs used in this work.

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126 W, which took about 30 seconds. The solution turned opaque, which meant that the precipitates were formed. The solution were maintained at 130 C for 1 min, then microwave heating was terminated and the solution was allowed to cool to room temperature. The precipitates were separated from RTIL solution via centrifugating and washing with ethanol for several times, and they were then dried at 60 C in a vacuum overnight.

In this work, we report a simple method to synthesize 2.3. Synthesis of Pt/CNT Nanocomposites noble metal nanocrystals and deposit them on CNTs from Based on the procedures to prepare Pt nanoparticles, the corresponding inorganic precursors dissolved in RTILs Pt/CNT composites were fabricated as follows. 5 mg of via microwave heating. The RTILs used in this work are H2 PtCl6 · 6H2 O was dissolved in the mixture of 2 mL of newly synthesized in our group,26 which contain a basic RTIL and 0.5 mL of glycol loaded in a 10 mL glass tube, functional group, 1,1,3,3-tetramethylguanidinum (TMG), then about 5 mg of CNTs were dispersed in the solution to and different anions such as trifluoroacetate, lactate, etc. form a stable suspension. Subsequently the suspension was The chemical structure of the RTILs used is shown in microwave-heated to 130 C at a power setting of 126 W, Figure 1. Two RTILs, 1,1,3,3-tetramethylguanidinium tri2 which took about 30 seconds. The suspension was mainfluoroacetate (TMGTFA) with viscosity of 143.7 mm /s −1 Delivered by Ingenta tainedto:at 130 C for 1 min, then microwave heating was and conductivity of 1886 S · cm at 45 C, and 1,1,3,3Trinity College Dublin terminated and the suspension cooled to room temperature tetramethylguanidinium lactate (TMGLA) with viscosity 2 −1 IP : 134.226.1.229 naturally. The products were separated from RTIL soluof 388.2 mm /s at 45 C and conductivity of 101 S · cm Thu, 29 Mar 2007 08:19:17 tion via repeated washing, centrifugating, and followed by at 25 C, were used as reaction media. The melting point vacuum-drying at 60 C overnight. and decomposition temperature of the former are, 42 C and higher than 300 C, while those of the later are lower 2.4. Characterization than −50 C and about 191 C, respectively. Both RTILs have good solvent power for hydrated metallic salts, The morphology and microstructure of the products were such as H2 PtCl6 · 6H2 O, HAuCl4 · 4H2 O, RhCl3 · 2H2 O, examined by means of transmission electron microscopy Zn(CH3 COO)2 · 2H2 O, Fe(NO3 )3 · 6H2 O, etc. Moreover, (TEM) on JEOL JEM-2010 equipped with an energy disCNTs can homogeneously disperse in them to form stable persive X-ray spectrometer under 200 kV operating voltsuspension. In combination with the advantages of these age. The X-ray powder diffraction (XRD) patterns of RTILs and microwave heating, we successfully synthethe Pt/CNTs composites were collected on a D/MAX-RC sized some metal nanocrystals such as Pt, Rh, and their diffractometer operated at 30 kV and 100 mA with Cu K nanocrystal/CNT nanocomposites. The adopted method radiation. is a fast, high-efficiency route for the production of nanomaterials.

3. RESULTS AND DISCUSSION

2. EXPERIMENTAL DETAILS 2.1. Materials All the chemical reagents used in this work were of analytical grade, which were commercially obtained and used as received. The ILs were synthesized following the procedures described previously.23 The CNTs, provided by Shenzhen Nanotech Port Co. Ltd, were prepared by the catalytic decomposition of CH4 .27 The CNTs were sinuous and highly entangled, characterized by open ends and with outer diameters of 40–60 nm and lengths of 1–12 m. 2.2. Synthesis of Pt Nanocrystals Pt nanocrystals were prepared as follows. Around 5 mg of H2 PtCl6 · 4H2 O dissolved in the mixture of 2 mL of RTIL (TMGTFA or TMGLA) and 0.5 mL of glycol was loaded in a 10 mL glass tube at room temperature. The solution was microwave-heated to 130 C at power setting of 176

3.1. Synthesis of Pt Nanocrystals We first utilized H2 PtCl6 · 6H2 O as metallic salt precusor, TMGTFA as RTIL medium, to prepare Pt nanoparticles via microwave heating. Figure 2 shows the TEM images of the as-prepared product. A large number of homogeneously dispersed nanoparticles with uniform size can be clearly observed in Figure 2a, and these particles are crystals with visible lattice (shown in Fig. 2b). The mean particle size was about 2 ∼ 3 nm estimated from the TEM images. These particles display an irregular shape, but show a monomodal particle size distribution from evaluation of their characteristic diameter. The energy dispersive X-ray spectroscopy (EDS) analysis during the TEM observation illustrates that the particles only contain element Pt (presented in Fig. 2c), which means that metallic Pt was formed. Based on the TEM observation, Pt nanoparticles with size of 2 ∼ 3 nm were also obtained when TMGLA was J. Nanosci. Nanotechnol. 6, 175–179, 2006

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(b)

3.2. Synthesis of CNT Composites

(c)

Based on the experiments to prepare metal nanocrystals in RTILs (TMGTFA or TMGLA), we dispersed CNTs in RTIL containing metal precursor to form a stable suspension, followed by heating with microwave, resulting in CNT nanocomposites. Figure 3 shows the XRD pattern of the Pt/CNT composites prepared using H2 PtCl6 as precursor and TMGTFA as reaction medium. From this

CPS

(002)

6.9 5.5

6.00

8.00

10.00

Energy (KeV)

Fig. 2. The TEM images of Pt nanoparticles synthesized using H2 PtCl6 as precursor and TMGTFA as reaction medium: (a) low magnification of particles; (b) HRTEM images of some particles; (c) EDS analysis of the product shown in (a).

J. Nanosci. Nanotechnol. 6, 175–179, 2006

10

20

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(311)

4.00

(020)

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Pt

Cu

(101)

Pt

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C 1.4

(100)

Cu

2.7

Intensity (a.u.)

4.1

80

90

2θ (degree) Fig. 3. The XRD patterns of the as-synthesized Pt/CNT composites. ♣ stands for Pt and for CNT.

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used as reaction medium under the same other conditions. This means that these two ILs, TMGTFA, and TMGLA, have the same effect on the morphology of the Pt nanoparticles. Similarly, with HAuCl4 , RhCl3 as metal precursors, respectively, Au particles of around 50 nm and Rh nanocrystals of about 2.0 nm were also obtained. The discrepancy in the particle size maybe result from the different reaction rate of different precursors in the ILs. The possible mechanism to form monodispersed metal nanoparticles via microwave heating in RTILs can be discussed as follows. On heating with microwave, the temperature of RTIL solution increased rapidly to the reaction temperature for metal salt reduction by glycol; thus, metal nanoparticles were formed. The intrinsic high charge plus the steric bulk of IL can create an electrostatic and steric colloid-type stabilization for the metal nanoparticles. Thus, the newly formed Pt nanoparticles were stabilized by IL. This result is similar to that reported by Dupont et al.,12 whoIngenta easily synthesized stable and redispersible transitionDelivered by to: metal nanoparticles by simple reduction, with molecuTrinity College Dublin lar hydrogen, of transition metal compounds dissolved IP : 134.226.1.229 in 1-n-butyl-3-methylimidazolium hexafluorophosphate. In Thu, 29 Mar 2007 08:19:17 this work, the used ILs, TMGTFA, and TMGLA, also acted as stabilizing agents for the metal nanoparticles. Therefore, the formed metal nanoparticles can be homogeneously dispersed in this kind of ILs.

(a)

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figure, it can be observed that the pattern shows the char(a) acteristic diffraction peaks at 26.1 , 43.2 , 44.6 , 53.9 , corresponding to (002), (100), (101), (004) reflections of graphite, respectively.25 Besides those peaks of graphite, the typical diffraction peaks at 39.8 , 46.1 , 67.0 , 81.4 can be well indexed as (111), (200), (220), (311) reflections of crystalline metal Pt,26 which further confirms that H2 PtCl6 has converted into metal Pt via microwave heating in TMGTFA, and the formed Pt nanoparticles have a fcc lattice structure. The diffraction peaks of Pt nanoparticles were relatively broad, suggesting the small sizes of the nanoparticles. Transmission electron microscopy equipped with an EDS provided detailed insight into the morphology and composition of the as-prepared Pt/CNT composites. From the TEM observation, it was demonstrated that most of the produced Pt (0) nanoparticles deposited on the surfaces of CNTs, and almost every carbon nanotube was decorated (b) with monodispersed Pt nanoparticles uniformly. In this Delivered by Ingenta to: study, CNTs can be dispersed homogeneously in RTILs to Trinity College Dublin form a stable suspension due to the interactions between IP : 134.226.1.229 the used RTILs and CNTs, which offers the opportunity Thu, 29 Mar 2007 08:19:17 for the precursor molecules dissolved in TMGTFA to contact every CNT. Thus, upon heating with microwave, the precursor was reduced and the newly formed metal nanoparticles deposited on the nearest CNT surfaces. From the TEM observation, it is estimated that the diameter of the Pt nanoparticles deposited on CNT surface is about 2 ∼ 3 nm, which is consistent with that of Pt nanoparticles synthesized in IL under the same conditions in the absence of CNTs. This suggests that the used RTIL medium maybe play an important role to prevent the produced metal nanoparticles from assembling, and at the same time, immobi1 nm lize Pt nanoparticles on CNT surfaces. The HRTEM image of the Pt/CNTs composites reveals the high crystallinity of the Pt nanoparticles with the (c) resolved lattice spacing of 0.227 nm, corresponding to (111) plane of Pt, as illustrated in Figure 4b. SAED pattern given in Figure 4c exhibits typical Pt diffraction circles, attributing to (111), (200), (220), (311) planes, respectively. The diffraction does not exhibit as clear spots, but as concentric rings, each of which consists of a large number of very small spots, implying that the nanoparticles are composed of many fine crystallites. The diffraction spots corresponding to (002) plane of CNTs also can be clearly observed in Figure 4c. In this work, other metal nanocrystal/CNT composites were also prepared in RTILs (TMGTFA or TMGLA) with corresponding inorganic salts as precursors via microwave heating. For example, similar Pt/CNT nanocomposites with the Pt nanoparticle size of about 2 ∼ 3 nm were obtained in TMGLA via microwave heating. And Rh/CNT nanocomposites were also fabricated with the similar techFig. 4. (a) TEM, (b) HRTEM images, and (c) electron diffraction of nique. The diameter of Rh nanoparticles is less than Pt/CNT composites. 2 nm, which is smaller than that of Pt nanoparticles prepared under the same experimental conditions. From these 178


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experiments, conclusions can be drawn that the method described in this work may be extended to prepare other nanocomposites. It is known that the transition metal nanocrystals with size less than 5 nm can act as catalysts with high activity during chemical reactions. The as-prepared metal nanocrystals (Pt, Rh, etc.) in this work possess small size of 2 ∼ 3 nm, which may be potentially used as catalysts. Meanwhile, the stable IL system containing metal nanocrystals and/or CNT decorated with metal nanocrystals probably also can serve as catalyst system for some reactions. Further work is under way.

Received: 12 April 2005. Revised/Accepted: 29 July 2005.


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7. R. A. Brown, P. Pollet, E. McKoon, C. A. Eckert, C. L. Liotta, and P. G. Jessop, J. Am. Chem. Soc. 123, 1254 (2001). 8. A. C. Cole, J. L. Jensen, I. Ntai, K. L. T. Tran, K. J. Weaver, D. C. Forbes, and J. H. Davis, J. Am. Chem. Soc. 124, 5962 (2002). 9. M. C. Buzzeo, R. G. Evans, and R. G. Compton, Chem. Phys. Chem 5, 1107 (2004). 10. F. Endres, Z. Phys. Chem 218, 255 (2004). 11. F. Endres, Chem. Phys. Chem. 3, 144 (2002). 12. W. Wu, B. X. Han, H. X. Gao, Z. M. Liu, T. Jiang, and J. Huang, Angew Chem. Int. Ed. 43, 2415 (2004). 13. J. Huang, T. Jiang, H. X. Gao, B. X. Han, Z. M. Liu, W. Z. Wu, Y. H. Chang, and G. Y. Zhao, Angew. Chem. Int. Ed. 43, 1397 (2004). 14. Y. J. Zhu, W. W. Wang, R. J. Qi, and X. L. Hu, Angew. Chem. Int. Ed. 43, 1410 (2004). 15. J. Dupont, G. S. Fonseca, A. P. Umpierre, P. F. P. Fichtner, and 4. CONCLUSION S. R. Twixeira, J. Am. Chem. Soc. 124, 4228 (2002). 16. C. W. Scheeren, G. Machado, J. Dupont, P. F. P. Fichtner, and S. R. In summary, we have shown a simple and highly efficient Twixeira, Inorg. Chem. 42, 4738 (2003). route in which nanocrystalline metals and their CNT com17. F. Endres, M. Bukowski, R. Hempelmann, and H. Natter, Angew. Chem. 115, 3550 (2003); Angew Chem. Int. Ed. 42, 3428 (2003). posites can be made from their inorganic salts in ionic 18. S. J. Tans, A. R. M. Verschueren, and C. Dekker, Nature 393, 49 liquids via microwave heating. The resulting nanocrystals Delivered by Ingenta to: (1998). possess small size less than 5 nm, which and their CNT19. E. W. Wong, P. E. Sheehan, and C. M. Lieber, Science 277, 1971 Trinity College Dublin based nanocomposites may find potential applications as (1997). IP : 134.226.1.229 catalysts for chemical reactions. 20. P. Poncharal, Z. L.Wang, D. Ugarte, and W. A. D. Heer, Science Thu, 29 Mar 2007 08:19:17 283, 1513 (1999). 21. P. M. Ajayan, O. Stephan, P. Redlich, and C. Colliex, Nature 375, Acknowledgments: This work is financially supported 564 (1995). by National Natural Science Foundation of China 22. Y. L. Hsin, K. C. Hwang, F.-R. Chen, and J.-J. Kai, Adv. Mater. 13, (No. 20374057, 50472096) and opening research fund of 830 (2001). state key laboratory of heavy oil processing (No. 2004-10). 23. S. Subramoney, Adv. Mater. 10, 1157 (1998). 24. B. K. Pradhan, T. Kyotani, and A. Tomita, Chem. Commun. 1317 (1999). References and Notes 25. Z. L. Liu, X. H. Lin, J. Y. Lee, W. D. Zhang, M. Han, and L. M. Gan, Langmuir 18, 4054 (2002). 1. P. Wasserscheid and W. Keim, Angew Chem. Int. Ed. 39, 3772 26. H. X. Gao, B. X. Han, J. C. Li, T. Jiang, Z. M. Liu, W. Z. Wu, (2000). Y. H. Chang, and J. M. Zhang, Synth. Commun. 34, 3083 (2004). 2. J. Dupont, R. F. de Souza, and P. A. Z. Suarez, Chem. Rev. 102, 27. B. C. Liang, S. H. Liu, Z. J. Tang, Q. Li, L. Z. Li, B. L. Gao, and 3667 (2002). Z. L. Yu, Acta Chim. Sin. (Chin. Ed.) 58, 1336 (2000). 3. T. Welton, Chem. Rev. 99, 2071 (1999). 28. O. Zhou, R. M. Fleming, D. W. Murphy, C. H. Chen, R. C. 4. R. Schldon, Chem. Commun. 2399 (2001). Haddon, A. P. Ramirez, and S. H. Glarum, Science 263, 1744 5. H. Olivier-Bourbigou and L. Magna, J. Mol. Catal. A 182–183, 419 (1994). (2002). 29. C. W. Scheeren, G. Machado, J. Dupont, P. F. P. Fichtner, and S. R. 6. E. D. Bates, R. D. Mayton, I. Ntai, and J. H. Davis, Jr., J. Am. Chem. Twixeira, Inorg. Chem. 42, 4738 (2003). Soc. 124, 926 (2002).