Langmuir 2004, 20, 5145-5148
Formation and Characterization of Water-Soluble Platinum Nanoparticles Using a Unique Approach Based on the Hydrosilylation Reaction
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Scheme 1. Hydrosilylation of Q8M8H and SA-PEG
Junchao Huang, Chaobin He,* Xueming Liu, Yang Xiao, Khine Yi Mya, and Jianwei Chai Institute of Materials Research and Engineering (IMRE), 3 Research Link, Singapore 117602 Received November 13, 2003. In Final Form: March 6, 2004
Introduction Metal and semiconductor nanoparticles in the colloidal solution have size-dependent optical, optoelectronic, and material properties.1,2 The ability of colloidal nanoparticles to form ordered arrays is expected to lead to nanostructures with a range of practical applications.3,4 Discrete nanoparticles with controlled chemical composition and size distribution can be readily synthesized using technologies in confined reaction media, such as reverse micelles and microemulsions.5-7 These nanoparticles exhibit several unique characteristics, such as a small average diameter, narrow size distribution, and similar shapes. However, poor dispersibility of alkyl capping nanoparticles in water hinders their potential application in biological systems, such as biolabeling, molecular markers, biosensors based on nanoparticles with receptor or reporter sites, and materials in the field of biolistics.8,9 Progress in designing water-soluble monolayer-protected clusters (MPCs) has been demonstrated over the last several years. The solubility of MPCs is dominated by the monolayer and particularly their peripheral functionalities. The polar monolayers have been used to promote the water-soluble or polar organic-soluble Au, Pd, Pt, and alloy clusters synthesized using capping agents such as tiopronin,8,10 poly(ethylene glycol) (PEG),11 glutathione,12 4-hydroxythiophenol,13,14 mercaptobenoic acid,15 mercaptosuccinic acid,16 lysine,17 sulfonic acid,18 and ammonium ions.19,20 * To whom correspondence should be addressed. Tel: 6568748145. Fax: 65-68727528. E-mail:
[email protected]. (1) Li, M.; Schnablegger, H.; Mann, S. Nature 1999, 402, 393. (2) He, R.; Qian, X. F.; Yin, J.; Zhu, Z. K. Chem. Phys. Lett. 2003, 369, 454. (3) Kumar, A.; Mandal, S.; Mathew, S. P.; Selvakannan, P. R.; Mandale, A. B.; Chaudhari, R. V.; Sastry, M. Langmuir 2002, 18, 6478. (4) Ma, X. D.; Qian, X. F.; Yin, J.; Zhu, Z. K. J. Mater. Chem. 2002, 12, 663. (5) Taleb, A.; Petit, C.; Pileni, M. P. Chem. Mater. 1997, 9, 950. (6) Motte, L.; Billoudet, F.; Lacaze, E.; Peleni, M. Adv. Mater. 1996, 8, 1018. (7) Qian, X. F.; Yin, J.; Feng, S.; Liu, S. H.; Zhu, Z. K. J. Mater. Chem. 2001, 11, 2504. (8) Templeton, A. C.; Chen, S.; Gross, S. M.; Murray, R. W. Langmuir 1999, 15, 66. (9) Christou, P. Plant J. 1992, 2, 275. (10) Templeton, A. C.; Cliffel, D. E.; Murray, R. W. J. Am. Chem. Soc. 1999, 121, 7081. (11) Wuelfing, W. P.; Gross, S. M.; Miles, D. T.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 12696. (12) Schaff, T. G.; Knight, G.; Shaffigulin, M. N.; Borkman, R. F.; Whetten, R. L. J. Phys. Chem. B 1998, 102, 10643. (13) Brust, M.; Fink, J.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. J. Chem. Soc., Chem. Commun. 1995, 1655. (14) Chen, S. Langmuir 1999, 15, 7551. (15) Johnson, S. R.; Evans, S. D.; Brydson, R. Langmuir 1998, 14, 6639. (16) Chen, S.; Kimura, K. Langmuir 1999, 15, 1075. (17) Selvakannan, P. R.; Mandal, S.; Phadtare, S.; Pasricha, R.; Sastry, M. Langmuir 2003, 19, 3545.
The hydrosilylation reaction, an addition of a hydrosilane unit (Si-H) to a double bond (CdC) to form an alkylsilane (Scheme 1), is widely utilized in the production of silicon polymers, liquid injection molding products, paper release coatings, and pressure-sensitive adhesives.21 The hydrosilylation reaction can be initiated in numerous ways, and one of the most common used platinum-based catalysts is the Karstedt catalyst [Pt(dvs) complex].22,23 During the course of the Pt-catalyzed hydrosilylation reaction, the formation of colloidal Pt species was previously regarded as an undesired side reaction which resulted in coloration of the final reaction solution.24 In contrast, this “side reaction” can be exploited to synthesize Pt nanoparticles. In this paper, we report the synthesis of Pt nanoparticles, which is stabilized in situ by branched PEG during the hydrosilylation reaction and investigate the self-assembly behaviors of Pt nanoparticles in toluene and water solutions, which are two typical organic and inorganic phases, respectively. Experimental Section General Methods. Q8M8H as shown in Scheme 1 was provided by Hybrid plastics. Single-allyl-terminated PEGs (SA-PEG) were synthesized from methoxy-poly(ethylene glycol) following the literature procedures.25,26 Toluene was distilled over Na/ben(18) Shon, Y. S.; Wuelfing, W. P.; Murray, R. W. Langmuir 2000, 17, 1255. (19) Cliffel, D. E.; Zamborini, F. P.; Gross, S. M.; Murray, R. W. Langmuir 2000, 16, 9699. (20) Yonezawa, T.; Onoue, S.; Kimizuka, N. Langmuir 2000, 16, 5218. (21) Lewis, L. N.; Stein, J.; Gao, Y.; Colborn, R. E.; Hutchins, G. Platinum Met. Rev. 1997, 41, 66. (22) Hichcock, P. B.; Lappert, M. F.; Warhurst, N. J. W. Angew. Chem., Int. Ed. Engl. 1991, 30, 438. (23) Zhang, C.; Laine, R. M. J. Am. Chem. Soc. 2000, 122, 6979. (24) Marko, I. E.; Sterin, S.; Buisine, L.; Mignani, G.; Branlard, P.; Tinant, B.; Declercq, J. Science 2002, 298, 204. (25) Huang, J. C.; Li, X.; Lin, T.; He, C.; Mya, K. Y.; Xiao, Y.; Li, J. J. Polym. Sci., Part B: Polym. Phys. 2004, 42, 1173. (26) Maitra, P.; Wunder, S. L. Chem. Mater. 2002, 14, 4494.
10.1021/la036135a CCC: $27.50 © 2004 American Chemical Society Published on Web 05/11/2004
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zophenone under nitrogen immediately prior to use. The platinum divinyltetramethyldisiloxane complex [Pt(dvs)] was obtained from Aldrich and diluted to a 2 mM solution in anhydrous toluene before use. Other chemicals were used as received without further purification. X-ray diffraction (XRD) measurements were carried out on a Bruker gadds axs system (KR ) 1.542 Å, 40 kV, 40 mA). Fourier transform infrared (FTIR) spectra were measured with a BioRad 165 FTIR spectrophotometer. UV-vis spectra were collected using a SHIMADZU UV-2501PC UV-vis recording spectrophotometer. Transmission electron microscope (TEM) images were acquired on a Philip CM300 TEM operating at an acceleration voltage of 300 kV. TEM samples were prepared by casting several drops of diluted colloidal solution onto standard carbon-coated copper grids and drying in air for 1 h. X-ray photoelectron spectroscopy (XPS) spectra were obtained using a VG Scientific EscaLab 220 IXL with a monochromator Al KR X-ray source (hν ) 1486.6 eV). Synthesis of Pt Nanoparticles. In a typical synthesis, the stoichiometric ratio of PEG repeat unit to Pt(dvs) was 100, the concentration of Pt(dvs) in the reaction solution was 1 mM, and the feed ratio of vinyl of PEG to hydrosilane was 0.8. Q8M8H (18.6 mg, 0.0183 mmol, 0.146 mmol hydrosilane) and SA-PEG (Mn ) 750; 88 mg, 2 mmol PEG repeat unit, 0.117 mmol vinyl) were placed in a 50 mL Schlenk flask with a magnetic stirrer. The reaction flask was charged by anhydrous toluene (10 mL), evacuated, and refilled with nitrogen three times. After that, Pt(dvs) (2 mM solution, 10 mL) was added by a syringe, then the reaction was stirred under nitrogen at 60 °C. The color of the reaction solution became light brown about 0.5 h later and deep brown 1 h later, indicating the formation of Pt nanoparticles. The resulting dark brown solution could be stable over several months. Transfer of Pt Nanoparticles. Typically, 2 mL of Pt colloidal solution in toluene was mixed with 6 mL of deionized water. The mixture was sonicated for 5 min and dried under a mild nitrogen stream. Once all toluene and an aliquot of water were evaporated, a clear and dark brown colloidal solution was obtained.
Results and discussion A coordination olefin is used to synthesize the in situ capping agents; it is generally accepted that the coordination capping agents, such as poly(vinyl pyrrolidone), poly(vinyl alcohol), and PEG, are able to be adsorbed on the surface of metal nanoparticles and consequently prevent them from aggregation. Moreover, Pt nanoparticles formed from the hydrosilylation reaction of hydrosilane and olefins are expected to be stabilized by the product of the hydrosilylation reaction, because the formation of platinum-silicon or platinum-carbon bonds in the reaction27 links the in situ stabilizer to the surface of Pt nanoparticles. In this study, SA-PEG was selected to react with Q8M8H using a highly reactive Pt(0) catalyst [Pt(dvs)]. At the excess hydrosilane concentration, all PEG molecules could react with Q8M8H and Pt-Si bonds were formed on the surface of Pt nanoparticles.27 XPS analyses (Figures 1 and 2) confirm the formation of Pt-Si bonds in Pt nanoparticles. XPS results of the Pt nanoparticles from the toluene phase exhibit peaks at 73.0 eV [Pt(4f7/2)] and 76.5 eV [Pt(4f5/2)], which agree well with the Pt(4f7/2) binding energy of Pt-Si bonds in the literature.27,28 As reported previously,27,29 the Karstedt’s catalyst [Pt(dvs)] is a low-valent, platinum olefin complex and formation of Pt-Si bonds in the final platinum product leads to a shift (27) Stein, J.; Lewis, L. N.; Gao, Y.; Scott, R. A. J. Am. Chem. Soc. 1999, 121, 3693. (28) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Physical Electronics, Inc.: Eden Prairie, MN, 1995; p 181. (29) Lewis, L. N.; Stein, J.; Smith, K. A.; Messmer, R. P.; LeGrand, D. G.; Scott, R. A. In Progress in Organosilicon Chemistry; Marciniec, B., Chojnowski, J., Eds.; Gordon and Breach: Amsterdam, 1995; p 263.
Notes
Figure 1. Pt(4f) core-level spectra of a 10 wt % Pt/charcoal mixture (a), Pt(dvs) (b), Pt nanoparticles from the toluene phase at a PEG/Pt ratio of 100 (c), and Pt nanoparticles from the water phase (d).
Figure 2. Si(2p) core-level spectra of Q8M8H (a), Pt nanoparticles from the toluene phase at a PEG/Pt ratio of 100 (b), and Pt nanoparticles from the water phase (c).
of Pt(4f7/2) binding energy to a high energy field. Figure 1 shows that the Pt(4f7/2) binding energy shifts from 72.2 eV for the starting catalyst to 73.0 eV for the final Pt nanoparticles, indicating the existence of Pt-Si bonds in the final Pt nanoparticles. In addition, a higher Pt(4f) binding energy of the Pt nanoparticles was obtained as compared with that of the Pt/charcoal mixture; a lower Si(2p) binding energy of the Pt nanoparticles (Figure 2) was obtained as compared with that of Q8M8H as a result of the formation of Pt-Si bonds. Similarly, the XPS results (Figures 1 and 2) also confirm that the Pt nanoparticles from the aqueous phase contain the Pt-Si bonds. The effect of different feed ratio of PEG repeat unit to Pt(dvs) (PEG/Pt) was investigated. The PEG/Pt ratio was varied from 100 to 5, while the concentration of Pt(dvs) and the ratio of vinyl of PEG to hydrosilane were kept constant at 1 mM and 0.8, respectively, and the molecular weight of SA-PEG was 750. When the PEG/Pt ratio decreased to 10 or 5, the hydrosilylation reaction solutions were almost colorless even though the reaction proceeded for 1 day at 60 °C. The reaction solution with a PEG/Pt ratio of 50 became a bit brown when the reaction was conducted for about 5 h. However, the reaction solution with a PEG/Pt ratio of 100 became dark brown quickly. UV-vis spectroscopy was used to monitor the progress of Pt nanoparticle formation and, hence, hydrosilylation. From the 550-nm absorbance change30 as shown in Figures 3, S1, and S2 (Supporting Information), it can be observed that the formation of Pt nanoparticles in the system of (30) Chen S.; Kimura, K. J. Phys. Chem. B 2001, 105, 5397.
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Figure 3. Hydrosilylation reaction monitored by UV-vis spectroscopy. Plot of the change of the absorbance at 550 nm of the reaction solutions versus the reaction time. (100) PEG/ Pt ) 100; (50) PEG/Pt ) 50; (Without Q8M8H) PEG/Pt ) 100, no Q8M8H was added.
PEG/Pt(100) was very fast during the initial 4 h and then slowed significantly. For the system of PEG/Pt(50), the formation of Pt nanoparticles was slow and slowed gradually with the increase of the reaction time. On the other hand, without additional of Q8M8H, there is no formation of Pt nanoparticles detected. These observations suggest that the concentration of the PEG vinyl and the hydrosilane play an important role in the rate of Pt nanoparticles formation. It was also observed that the PEG/Pt ratio plays a significant role on the stabilization of Pt nanoparticles in the reaction solution. When the reaction solutions of PEG/ Pt (5, 10, and 50) were stirred at 60 °C for 1 day, a bit of Pt precipitate on the wall of the reaction bottle appeared, while in the solution of PEG/Pt ratio (100) no Pt precipitate was observed. The stability of Pt nanoparticles has also been studied by the turbidity measurements (Figure S4, Supporting Information). In this measurement, the reaction solutions were sonicated for 5 min before the turbidity monitoring. The change of absorbances at 550 nm was recorded. No absorbance change was observed for the PEG/ Pt (100) system, while the PEG/Pt (50) system showed a remarkable absorbance decrease with the increase of stand time due mainly to the aggregation and precipitation of the Pt nanoparticles. To confirm that the formation of Pt nanoparticles is due to the hydrosilylation reaction, a reaction without addition of Q8M8H was carried out. Pt(dvs) and linear SAPEG with a molecular weight of 750 were respectively dissolved in toluene and stirred at 60 °C without Q8M8H, while the PEG/Pt ratio was kept constant at 100. The solution did not become brown at all, even if it was heated at 60 °C for 2 days. As shown in Figures 3 and S3 (Supporting Information), the absorbance at 550 nm hardly increases, suggesting that the Pt nanoparticles could not be formed without the hydrosilylation reaction. Moreover, a very small amount of Pt nanoparticles formed by thermodecomposition of Pt(dvs) could not be stabilized by the linear SA-PEG; the Pt nanoparticles precipitated or adhered to the wall of the reaction bottle. The influence of the SA-PEG molecular weight was also investigated. In the same way, the SA-PEGs of molecular weight 2000 and 5000 were used instead of SA-PEG (Mn ) 750), while the PEG/Pt ratio, the concentration of Pt(dvs), and the ratio of vinyl to hydrosilane were kept at 100, 1 mM, and 0.8, respectively. In contrast to those of PEG (Mn ) 750), the color of the resulting solution was lighter because of the low concentration of the vinyl and
Figure 4. TEM and HRTEM images of Pt nanoparticles from the toluene phase (a-c) and ED pattern of Pt nanoparticles from the toluene phase (d). TEM images of Pt nanoparticles from water phase (e, f). For Pt nanoparticles in all images, the PEG/Pt ratio is 100.
hydrosilane groups; no spherical aggregations of Pt nanoparticles in the toluene phase were observed by the TEM. Figure 4 presents TEM and high-resolution TEM (HRTEM) images of spherical assemblies of Pt nanoparticles in toluene, together with the selected-area electron diffraction (ED) pattern. In the TEM image (Figure 4a), the spherical aggregates with a diameter of 100 ( 30 nm are observed. HRTEM images (Figure 4b,c) show that these spherical aggregates clearly consisted of small dark spots, which are individual Pt nanoparticles. Each Pt nanoparticle appears as a discrete nanosphere with a size of 3.0 ( 0.6 nm. As shown in the HRTEM image, the majority of the Pt nanoparticles could be discernible as single crystals of a face-centered cubic (fcc) lattice, because clear {111} lattice planes are observed to cover all of the particles if the particles are viewed in a proper direction (e.g., the {110} direction). The lattice spacing, 0.23 nm, is consistent with that of bulk Pt31 and the XRD results (Figure S5, Supporting Information). The selected-area ED shows four Debye-Scherrer rings assigned to {111}, {200}, {220}, and {311} planes, which have been confirmed by the XRD results. The Pt nanoparticles could be directly transferred from the toluene phase into the water phase without the assistance of a phase transfer agent; this is due to the hydrophilic nature of the capping polymer (PEG). Figure 5 shows a photo of the Pt nanoparticles in the toluene phase and in the water phase, respectively. After phase transfer, a clear and brown Pt colloidal (31) The International Centre for Diffraction Data JCPDS-ICDD, PDF-2 Data Base. http://www.icdd.com.
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Notes
Conclusions
Figure 5. Picture showing Pt nanoparticles in the toluene phase (a) and Pt nanoparticles after phase transfer into the aqueous phase (b).
aqueous solution was obtained, suggesting that the Pt nanoparticles were soluble in the aqueous phase. Figure 4e,f presents TEM images of the Pt nanoparticles from the water phase. The shape of the spherical aggregates was damaged to some extent, and the high-magnification TEM image shows that the mean distance between the Pt nanoparticles in the water is larger than that in toluene. However, the size of the Pt nanoparticles remained unchanged. The TEM images also confirm that the Pt nanoparticles in the present study have a good dispersibility in water. The resulting colloidal aqueous solution was stable over a period of several months.
We have demonstrated a novel approach to synthesize Pt nanoparticles by virtue of the hydrosilylation of Q8M8H and SA-PEG in anhydrous toluene. As a result of the formation of platinum-silicon bonds with the in situ capping agent (branched PEG), the Pt nanoparticles have a good stability in the resulting solution. The Pt nanoparticles could be transferred from the toluene phase to the water phase in a quite simple way. TEM results showed that the Pt nanoparticles could be well dispersed in toluene and water, and in toluene a spherical assembly of Pt nanoparticles was observed. According to UV-vis spectra at a PEG(Mn ) 750)/Pt ratio of 100, the formation rate of the Pt nanoparticles in hydrosilylation was high and the resulting Pt nanoparticles could be stabilized in toluene solution; on the contrary, at the low PEG/Pt ratios the formation rate of the Pt nanoparticles was slow and the resulting Pt nanoparticles weren’t stable. The XRD spectra and the selected-area ED pattern reveal that the nanometer-scale Pt particles were formed with a fcc lattice. Acknowledgment. We are grateful to Chow Shue Yin and Lim Poh Chong for their assistance in TEM characterization and XRD measurement. This research was supported by DSO national lab of Singapore and Institute of Materials Research and Engineering, Singapore. Supporting Information Available: UV-vis spectra, plot of the absorbance as a function of the standing time, XRD patterns, FTIR spectra, and a TGA curve for the Pt nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org. LA036135A
