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One-Phase Synthesis of Thiol-Functionalized Platinum Nanoparticles Chanel Yee,†,§ Michael Scotti,†,§ Abraham Ulman,*,†,§ Henry White,‡,§ Miriam Rafailovich,‡,§ and Jonathan Sokolov‡,§ Department of Chemical Engineering, Chemistry and Materials Science, Polytechnic University, Six Metrotech Center, Brooklyn, New York 11201, Department of Materials Sciences, SUNY at Stony Brook, Stony Brook, New York 11794-2275, and The NSF MRSEC for Polymers at Engineered Interfaces Received October 13, 1998. In Final Form: January 7, 1999 A new one-phase synthesis of thiol-functionalized platinum nanoparticles is presented. Using tetrahydrofuran as the solvent and lithium triethylborohydride (“superhydride”) as the reducing agent, platinum nanoparticles functionalized by octadecanethiol were prepared. Fourier transform infrared spectroscopy (FTIR), tunneling electron microscopy (TEM), and powder X-ray diffraction were used to analyze the nanoparticles. The results show that the nanoparticles are single crystals with fcc structure, that their average size is ∼3 nm, and that the octadecyl chains are close packed in a solid-like assembly.
Nanoparticles have become the focus of academic and industrial interest, because of their size-dependent physical properties and because using functionalized nanoparticles as building blocks for microassemblies and nanoassemblies may provide access to novel materials unattainable in any other way.1 One of the important contributions to this area was the development of a facile method for the preparation of 1-3 nm diameter alkanethiol-functionalized gold nanoparticles by Schiffrin and co-workers.2 They used sodium borohydride as the reducing agent and either water/toluene or methanol for * Send correspondence to Abraham Ulman. Phone: (718) 2603119. Fax: (718) 260-3125. E-mail:
[email protected]. † Polytechnic University. ‡ SUNY at Stony Brook. § NSF MRSEC for Polymers at Engineered Interfaces. (1) (a) Ozin, G. A. Adv. Mater. 1992, 4, 612. (b) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226. (c) Belloni, J. Curr. Opin. Colloid Interface Sci. 1996, 2, 184. (d) Brus, L. Curr. Opin. Colloid Interface Sci. 1996, 2, 197. (e) Fendler, J. H.; Meldrum, F. C. Adv. Mater. 1995, 7, 607. (f) Matijevic´, E. Curr. Opin. Colloid Interface Sci. 1996, 1, 176. (g) Haberland, H., Ed. Clusters of atoms and molecules; Springer-Verlag: New York, 1994. (h) Clusters and Colloids. From Theory to Applications; Schmid, G., Ed.; VCH: New York, 1994. (2) (a) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (b) Brust, M.; Fink, J.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. J. Chem. Soc., Chem. Commun. 1995, 1655. (c) Brust, M.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. J. Adv. Mater. 1995, 7, 795. (d) Bethell, D.; Brust, M.; Schiffrin, D. J.; Kiely, C. J. J. Electroanal. Chem. 1996, 409, 137. (e) Johnson, S. R.; Evans, S. D.; Mahon, S. W.; Ulman, A. Langmuir 1997, 13, 51. (f) Badia, A.; Cuccia, L.; Demers, L.; Morin, F.; Lennox, R. B. J. Am. Chem. Soc. 1997, 119, 2682. (g) Hostetler, M. J.; Wingate, J. E.; Zhong, C.-J.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wingall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17. (3) Yee, C. K.; Jordan, R.; Ulman, A.; White, H.; King, A.; Rafailovich, M.; Sokolov, J. J. Am. Chem. Soc. 1999, 121, 1016. (4) (a) Petroski, J. M.; Wang, Z. L.; El-Sayed, M. A. J. Phys. Chem. 1998, 102, 3316. (b) Marzan, L. L.; Philipse, A. P. Colloids Surf. 1994, 90, 95. (c) Kyotani, T.; Tsai, L. F.; Tomita, A. J. Chem. Soc., Chem. Commun. 1997, 701. (d) Ahmadi, T. S.; Wang, Z. L.; El-Sayed, M. A. Science 1996, 272, 1924. (e) Rodriguez, A.; Amiens, C.; Bradley, J. S. Chem. Mater. 1996, 8, 1978. (f) El-Sayed, M. A.; Ahmadi, T. S.; Wang, Z. L. Chem. Mater. 1996, 8, 1161. (g) Wang, Z. L.; Ahmad, T. S.; ElSayed, M. A. Surf. Sci. 1997, 380, 302. (h) Chen, C.-W.; Akashi, M. Langmuir 1997, 13, 6465. (i) Sarathy, K. V.; Kulkarni, G. U.; Rao, C. N. R. J. Chem. Soc., Chem. Commun. 1997, 537. (5) (a) Hepel, M. J. Electrochem. Soc. 1998, 145, 124. (b) Marzan, L. L.; Philipse, A. P. Colloids Surf. 1994, 90, 95. (c) Gamez, A.; Richard, D.; Durand, R. Electrochim. Acta 1996, 41, 307. (d) Gloaguen, F.; Leger, J.-M.; Lamy, C. J. Appl. Electrochem. 1997, 27, 1052. (e) Somorjai, G. A. Appl. Surf. Sci. 1997, 121/122, 1.
Figure 1. TEM micrographs of a nanoparticle (made from K2PtCl4) monolayer. The FFT analysis of interparticle spacing (inset) shows a pronounced ring at 0.8 nm-1). Electron diffraction of the platinum nanoparticles shows an fcc packing arrangement.
two-phase2a or one-phase reaction,2b respectively. Recently, we have developed a one-phase synthesis of thiol-functionalized gold nanoparticles using tetrahydrofuran as the solvent and lithium triethylborohydride as the reducing agent.3 High-resolution transmission electron microscopy (HRTEM) revealed the formation of truncated spherical particles 4 nm in average diameter. Electron diffraction shows that the nanoparticles are single crystals with fcc structure. Infrared spectra reveal closely packed
10.1021/la9814283 CCC: $18.00 © 1999 American Chemical Society Published on Web 05/26/1999
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Figure 2. Histogram of the TEM in Figure 1 showing the size distribution with a peak at 2.75-3.25 nm.
n-alkyl chains, mainly in the all-trans conformation. In this communication we report that the new synthesis can be used for the preparation of thiol-functionalized platinum nanoparticles. The preparation of platinum nanoparticles4 has been motivated by their catalytic properties.5 Our motivation is the preparation of functionalized nanoparticles both as self-assembled complex moieties6 and as for biomedical application. We have examined two platinum sources, dihydrogen hexachloroplatinate(IV) (H2PtCl6‚6H2O) and potassium platinum(II) tetrachloride (K2PtCl4). Both gave similar nanoparticles and thus will be discussed together. For the synthesis starting from chloroplatinic acid, 0.146 g (0.5 mmol) of octadecanethiol (ODT, C18H37SH) was added under vigorous stirring to a solution of 0.263 g (0.5 mmol) of H2PtCl6‚6H2O in 10 mL of distilled, anhydrous THF. The reaction mixture was stirred for approximately 40 min at room temperature, before a 1.0 M solution of lithium triethylborohydride in THF was added dropwise (10 mL). H2 gas bubbled immediately upon addition, and the mixture turned reddish brown. For the synthesis starting from the potassium salt, 0.146 g (0.5 mmol) of ODT was added under vigorous stirring to a suspension of 0.208 g (0.5 mmol) of K2PtCl4 in 10 mL of distilled, anhydrous THF. After 18 h of stirring, the reaction mixture was homogeneous, and 6 mL of “superhydride” was added dropwise. There was no immediate change of color, and the reaction mixture was heated to ∼35 °C. After a few minutes, the mixture became darker, and finally, after 2 h, it became reddish brown. Dry ethanol (40 mL) was added to the resulting solution mixtures, and they were centrifuged. The supernatant liquid was removed, and the precipitate was sonicated in dry ethanol and then centrifuged. The supernatant was
Figure 4. TGA spectra of octadecanethiol-functionalized platinum nanoparticles. The heating rate was 5 °C/min.
withdrawn and the process was repeated four times to ensure removal of all starting materials. After the last purification cycle, the supernatant was free of ODT, as verified by thin-layer chromatography (TLC). Finally, the nanoparticles were dried in a vacuum desiccator overnight. Thin films of functionalized platinum nanoparticle solution in chloroform were drop cast onto a 300 mesh carbon-supported film copper grid. Bright-field images and normal incidence selected area electron diffraction (SAED) patterns were obtained using the Phillips CM-12 transmission electron microscope (100 keV) to determine the particle size, distribution, and structure. Figure 1 shows bright-field images of platinum nanoparticles. NIH Image (release Beta 3) was used to determine the average particle size and distribution (Figure 2). SAED patterns (Figure 1) of a collection of nanoparticles resulted in a typical cubic Debye-Scherrer ring pattern for polycrystalline materials with random crystal orientation. To investigate the n-octadecyl layer, Fourier transform infrared spectroscopy was carried out.7 FTIR is one of the most utilized characterization techniques for identification of 2D and 3D SAMs with respect to their degree of order and relative orientation. Figure 3 shows the 2750-3050
Figure 3. As-recorded transmission FTIR spectrum of a drop-cast film (chloroform) of Pt/ODT nanoparticles on a KBr window.
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as well as the 700-1600 cm-1 region. It is clear that the obtained nanoparticles are Pt/ODT composites, and taken together with the TEM and electron diffraction results, a platinum core-ODT shell can be proposed. Hence, the one-phase synthesis using a strong reducing agent is probably a general route for the preparation of thiolfunctionalized nanoparticles. The position of the ν(CHx) band maximum indicates crystalline packing of alkyl chains in the ODT-functionalized platinum nanoparticles. The methyl symmetric and asymmetric stretching modes at 2955 and 2872 cm-1 for the ODT/nanoparticle composites are less pronounced due to the random orientation of the octadecyl chains, then, for example in a SAM on a flat metallic surface. The appearance of the methylene stretching modes of Pt/ODT particles at νas(CH2) ) 2918 and νs(CH2) ) 2850 cm-1 indicates highly ordered all-trans alkyl chains surrounding a platinum core. This is similar to the ordered octadecyl chains others and we have observed for thiol-functionalized nanoparticles of gold. The thermal stability of octadecanethiol-functionalized platinum nanoparticles was studied using thermal gravimetric analysis (TGA). Figure 4 shows that weight loss from nanoparticles prepared from H2(PtCl6) is significantly larger than that from the corresponding particles prepared from K2(PtCl4). This could be justified if there was a difference in the average size of the two different nanoparticles. The two histograms are too similar to provide a clear indication that nanoparticles made of H2PtCl6 are smaller, and further studies are needed to clarify this point. In both cases there are two peaks in the derivative, (6) “It is important to emphasize, however, that self-assembly is not limited to the molecular level, and one can imagine self-assembly of molecular clusters, organic, and inorganic crystals, etc.” Kuhn, H.; Ulman, A. In Organic Thin Films and Surfaces: Directions for the Nineties; Ulman, A., Ed.; Academic Press: Boston, 1995. (7) Infrared spectra were recorded on a Nicolet 760 spectrometer with 2 cm-1 resolution in transmission mode (500 scans). A solid sample on a KBr infrared window was prepared by evaporating a solution of the nanoparticles in chloroform. The ODT bulk spectrum was recorded under the same conditions. (8) Chen, C.-W.; Akashi, M. Langmuir 1998, 13, 66465.
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Figure 5. UV spectrum of the thiol-functionalized platinum nanoparticles shown in Figure 1. The concentration was 10 mg of particles in 10 mL of chloroform.
one at ∼214 °C and the other at ∼280 °C. The origin of these peaks is under study. The UV spectrum of the ODT-functionalized platinum nanoparticles is presented in Figure 5. The plasmon band is observed at 266 nm, in agreement with the spectra published in the literature for other platinum nanoparticles.8 In conclusion, we described a new one-phase synthesis of thiol-functionalized platinum nanoparticles. Using tetrahydrofuran as the solvent and lithium triethylborohydride (“superhydride”) as the reducing agent, platinum nanoparticles functionalized by octadecanethiol were prepared. Fourier transform infrared spectroscopy (FTIR), tunneling electron microscopy (TEM), and powder X-ray diffraction were used to analyze the nanoparticles. The results show that the nanoparticles are single crystals with fcc structure, that their average size is ∼3 nm, and that the octadecyl chains are close packed in a solid-like assembly. LA9814283
