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Formation of Uniform Fluorinated Gold Nanoparticles and Their Highly Ordered Hexagonally Packed Monolayer Tetsu Yonezawa,*,†,‡ Shin-ya Onoue,† and Nobuo Kimizuka*,† Department of Applied Chemistry, Faculty of Engineering, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan, and “Structural Ordering and Physical Properties”, Precursory Research for Embryonic Science and Technology, Japan Science and Technology Corporation, Hakozaki, Fukuoka 812-8581, Japan Received October 10, 2000. In Final Form: January 29, 2001 Fluorocarbon-stabilized gold nanoparticles were prepared for the first time by reduction of AuCl4- in the presence of fluorinated alkane thiols as stabilizers. Transmission electron microscopy revealed the formation of uniform gold nanoparticles. These fluoro-nanoparticles are soluble only in fluorocarbon media, and casting of these dispersions provided highly ordered, hexagonally packed monolayers on solid substrates.
Metal nanoparticles display specific catalytic, magnetic, electronic, and photophysical properties, depending on their size (quantum size effect).1 Many research studies have been exploited toward the realization of future nanomaterials. Especially, the synthesis, surface modification, and organization of gold nanoparticles have been attracting much attention.2 The choice of stabilizer molecules is of paramount importance, because they determine the size and size distributions, dispersion media, and functions of the obtained nanoparticles.3 Alkylthiols have been widely used to prepare lipophilic gold nanoparticles, and water-soluble nanoparticles were synthesized by the use of citric acid, glutathione,3h and * To whom correspondence should be addressed: Dr. Tetsu Yonezawa, Department of Applied Chemistry, Faculty of Engineering, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan. Tel: +81-92-642-3598. Fax: +81-92-642-2011. E-mail: tetsutcm@ mbox.nc.kyushu-u.ac.jp. † Kyushu University. ‡ Japan Science and Technology Corporation. (1) For example: (a) Toshima, N.; Yonezawa, T. New J. Chem. 1998, 1179. (b) Bigioni, T. P.; Whetten, R. L.; Dag, O ¨ . J. Phys. Chem. B 2000, 104, 6983. (c) Hicks, J. F.; Templeton, A. C.; Chen, S.; Sheran, K. M.; Jasti, R.; Murray, R. W.; Debord, J.; Schaaff, T. G.; Whetten, R. L. Anal. Chem. 1999, 71, 3703. (d) Green, S. J.; Pietron, J. J.; Stokes, J. J.; Hostetler, M. J.; Vu, H.; Wuelfing, W. P.; Murray, R. W. Langmuir 1998, 14, 5612. (e) Andres, R. P.; Bein, T.; Dorogi, M.; Fneg, S.; Henderson, J. I.; Kubiak, C. P.; Mahoney, W.; Osifchin, R. G.; Reifenberger, R. Science 1996, 272, 1323. (f) Markovich, G.; Collier, C. P.; Henrichs, S. E.; Remacle, F.; Levine, R. D.; Heath, J. R. Acc. Chem. Res. 1999, 32, 415. (g) Pileni, M.-P. Langmuir 1997, 13, 3266. (2) (a) Schmid, G.; Beyer, N. Eur. J. Inorg. Chem. 2000, 835. (b) Schmid, G.; Ba¨umle, M.; Beyer, N. Angew. Chem., Int. Ed. 2000, 39, 181. (c) Korgel, B. A.; Fullam, S.; Connolly, S.; Fitzmaurice, D. J. Phys. Chem. B 1998, 102, 8379. (d) Heath, J. R.; Knobler, C. M.; Leff, D. V. J. Phys. Chem. 1997, 101, 189. (e) Kiely, C. J.; Fink, J.; Brust, M.; Bethell, D.; Schiffrin, D. J. Nature 1998, 396, 444. (f) Schmid, G.; MeyerZaika, W.; Pugin, R.; Sawitowski, T.; Majoral, J.-P.; Caminade, A.-M.; Turrin, C.-O. Chem.sEur. J. 2000, 6, 1693. (g) Harfenist, S. A.; Wang, Z. L.; Alvarez, M. M.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. 1996, 100, 13904. (h) Yonezawa, T.; Onoue, S.; Kunitake, T. Adv. Mater. 1998, 10, 414. (i) Yonezawa, T.; Onoue, S.; Kunitake, T. Chem. Lett. 1998, 689. (j) Whetten, R. L.; Shafigullin, M. N.; Khoury, J. T.; Schaaff, T. G.; Vezmar, I.; Alvarez, M. M.; Wilkinson, A. Acc. Chem. Res. 1999, 32, 397. (3) (a) Yonezawa, T.; Matsune, H.; Kunitake, T. Chem. Mater. 1999, 11, 33. (b) Yonezawa, T.; Onoue, S.; Kunitake, T. Chem. Lett. 1999, 1061. (c) Yonezawa, T.; Onoue, S.; Kunitake, T. Kobunshi Ronbunshu 1999, 56, 855. (d) Yonezawa, T.; Onoue, S.; Kimizuka, N. Langmuir 2000, 16, 5218. (e) Yonezawa, T.; Yasui, K.; Kimizuka, N. Langmuir 2001, 17, 271. (f) Schmid, G.; Pfeil, R.; Bo¨se, R.; Bandermann, F.; Meyer, S.; Calis, G. H. M.; van der Velden, J. W. A. Chem. Ber. 1981, 114, 3634. (g) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (h) Schaaff, T. G.; Whetten, R. L. J. Phys. Chem. B 2000, 104, 2630.
synthetic quaternary ammonium-bearing thiol molecules.3b-c However, there have been no reports on the synthesis of gold nanoparticles that are dispersible in fluorocarbon media. The use of fluorocarbon media should provide a unique opportunity to the organization of nanoparticles, because of their extremely low surface tension and immiscibility to the common organic solvents. In this paper, we describe the synthesis, structural analysis, and solubility of fluorocarbon-stabilized gold nanoparticles and the formation of highly ordered monolayers on solid surfaces. As the stabilizer, we have employed 1H,1H,2H,2Hperfluorodecanethiol and 1H,1H,2H,2H-perfluorooctanethiol (F8 and F6, respectively, Adzmax Co., Japan). Fluorocarbon-stabilized gold nanoparticles were obtained by reduction of HAuCl4 (Nacalai Tesque, GR) in ethanol (5.0 mM, 30 mL) in the presence of the stabilizer (S/Au ) 1.0). Aqueous NaBH4 was added dropwise to the above solution, and the mixture was vigorously stirred for 3 h. The yellow AuCl4- solution immediately turned brown by the addition of NaBH4, indicating the progress of reduction. After the stirring was stopped, black precipitates separated from colorless supernatants were collected by filtration over a poly(tetrafluoroethylene) membrane filter (pore size ) 0.2 µm). The black color of the precipitates indicates the formation of very small gold nanoparticles. To completely remove excess noncoordinating stabilizer molecules, the filtrate was washed several times by water, hot CHCl3, and hot ethanol under ultrasonication. The absence of the unbound stabilizer molecules was confirmed by repeated elemental analyses, which reached constant values after these ultrasonic purification steps. As F8- and F6-stabilized gold nanoparticles showed similar structural properties, those of F8-stabilized gold nanoparticles are described here in detail. The surface occupied area of one F8 molecule on the purified fluoroAu nanoparticle was calculated to be 19.7 Å2, based on the elemental analysis data.4 This value is slightly smaller (4) A fluoro-gold nanoparticle with the diameter of 2.6 nm () 2r) is comprised of 545 gold atoms () FAu(4/3(πr3)/Ar(Au)), where FAu ) 19.3 g mL-1 and Ar(Au) (atomic mass) ) 196.96). From the carbon content in elemental analysis (C, 8.17; H, 0.32), a 2.6 nm Au nanoparticle contains 32.6 wt % of F8 (MW ) 480.2; C, 25.07; H, 0.84). Therefore, one 2.6 nm Au nanoparticle is surrounded by ca. 108 molecules of F8. The occupied area of one molecule of F8 on the nanoparticle surface can be calculated as 19.7 Å2. Assuming that extended F8 molecules are radially oriented from the nanoparticle surface, the occupied area of one F8 molecule at the -CH2CF2- moiety comes to ca. 35.3 Å2, by considering the length of the HSCH2CH2- unit (4.6 Å).
10.1021/la001427m CCC: $20.00 © 2001 American Chemical Society Published on Web 03/23/2001
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Figure 1. (a) Schematic illustration of F8-stabilized gold nanoparticles. Occupied areas were estimated with the average particle size and the elemental analysis data. (b) TEM image of the ordered hexagonal assembly of F8-stabilized Au nanoparticles obtained by casting the dilute HCFC-225 dispersion on a carbon-coated copper grid. Inset: Fourier transform image of the TEM image. The TEM image was taken with a JEOL 200CX TEM at 200 kV (HVEM Lab., Kyushu Univ.). (c) Interpenetration model of F8 molecules in the hexagonal-packed monolayer of gold nanoparticles. The fluorocarbon chain has an all-trans conformation.
than that of a dodecanethiol monolayer formed on an Au(111) surface (21.4 Å2),5 indicating that the surface of fluoro-Au nanoparticles is densely covered by radially oriented F8 molecules as shown in Figure 1a. According to this surface organization model, the occupied area of the -CH2CF2- moiety is estimated as 35.3 Å2, which is slightly larger than the cross-sectional area of a CF2 chain (ca. 30 Å2).4 Elemental analysis of F6-stabilized gold nanoparticles similarly indicated a densely covered surface organization of thiol molecules.6 The purified gold nanoparticles are not soluble in common organic solvents, including ethanol, acetone, chloroform, and dimethyl sulfoxide. On the other hand, they are readily dispersed into fluorocarbon media, such as hexafluorobenzene (C6F6, Showa Denko), perfluorobutylalkyl ethers C4F9OCH3 and C4F9OC2H5 (Novec HFE7100 and 7200, respectively; 3M), and especially HCFC225 (AK-225, Asahi Glass; mixture of CF3CF2CCl2H and CF2ClCF2CClFH). The observed solubility is consistent with the fluorinated surface structure depicted in Figure 1a. Electron microscopy was conducted for the sample, which was prepared by dropping the HCFC dispersion onto a carbon-coated copper grid. Figure 1b displays a part of the regularly packed monolayer, which consists of monodisperse nanoparticles. Interestingly, these monolayers were extended over a wide area on the transmission electron microscopy (TEM) grid. This particle size is quite similar to that of the dodecanethiol-stabilized gold nanoparticles prepared at the S/Au ratio of 1.0, in a toluene/ water biphasic system.3e,3g In addition, UV-vis spectra of F8-stabilized gold nanoparticles in HCFC showed a plasmon peak at ca. 510 nm, which is similar to that of dodecanethiol-stabilized gold nanoparticles in toluene. Moreover, their particle size distributions were quite similar, as shown in Figure 2. Therefore, it is apparent that the fluorinated thiol molecules effectively stabilized small gold nanoparticles and maintained the monodisperse size distribution seen with alkylthiols. During the preparation of this manuscript, Korgel et al. reported preparation of F8-stabilized silver nanoparticles.7 Their particles were obtained as precipitates in an acetone/water mixture and were reported to be redis(5) Ullman, A. Chem. Rev. 1996, 96, 1533. (6) F6-stabilized gold nanoparticles possessed an average diamter of 2.4 nm. The occupied area of an F6 molecule on the surface is calculated as 18.6 Å2, and that at the -CH2CF2- moiety is 35.5 Å2. (7) Shah, P. S.; Holmes, J. D.; Doty, R. C.; Johnson, K. P.; Korgel, B. A. J. Am. Chem. Soc., 2000, 122, 4245.
Figure 2. Particle size distributions of dodecanethiol-stabilized (solid bars) and F8-stabilized gold nanoparticles (open bars). Three hundred particles in each enlarged TEM image were measured to obtain these distributions.
persible in acetone. Such an unusual solubility in organic media would be ascribed to insufficient purification of fluoro-nanoparticles, because the presence of physically adsorbed residual F8 molecules and/or the phase transfer reagent employed in the preparation enhances their solubility in organic media. In our case, fluoro-gold nanoparticles were purified under ultrasonic conditions, which was indispensable to thoroughly remove such uncoordinated F8 molecules. Interestingly, the monolayer formed by F8-stabilized gold nanoparticles is in hexagonal packing (Figure 1b). Though several defects are visible in this image, they were generated by the electron beam irradiation. The Fourier transformation of this image shown in the inset of Figure 1b displays sharp spots, and this clearly indicates that the monolayer consists of regularly aligned fluoro-nanoparticles with uniform interparticle distances, in a wide area. The formation of such a uniform monolayer is ascribed to the remarkably low surface tension of HCFC (16.2 mN/m), which can spread over the wide area of the carbon grid with a uniform thickness. The constant interparticle distance in the hexagonalpacked monolayer is attributable to the rigid fluorocarbon chains, which are radially organized around the nanoparticles. From an enlarged TEM image, the edge-to-edge distance between gold cores was determined as 3.0 nm, which is slightly smaller than the bimolecular length of F8 (3.32 nm ) 1.66 nm × 2). It is likely that the fluorocarbon stabilizers are interpenetrated at the chain ends, giving rise to the shortened interparticle distance
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(Figure 1c). Hexagonally packed monolayers were also obtained from F6-stabilized gold nanoparticles. In this case, the observed interparticle distance was 2.4 nm, which is also smaller than the bimolecular length of an F6 molecule (1.40 nm). It is noteworthy that interparticle distances are readily controllable by tuning the length of rigid fluorocarbon stabilizers. In conclusion, monodisperse and small gold nanoparticles with average diameters of 2.4-2.6 nm were synthesized by the use of perfluoroalkanethiol stabilizers. The particle size and their plasmon absorption spectra are comparable to those of the dodecanethiol-stabilized gold nanoparticles, consistent with the formation of stable fluorocarbon monolayers. As expected, purified fluoro-
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nanoparticles are insoluble to common hydrocarbon media, but they are well dispersed in fluorinated solvents. Casting of these dispersions on solid substrates afforded hexagonally aligned, ordered monolayer assemblies in a wide area. These unique features allow us to design novel nanometallic architectures. Acknowledgment. This work was partially sponsored by a Grant-in-Aid for COE Research “Design and Control of Advanced Molecular Assembly Systems” from the Ministry of Education, Culture, Sports, Science and Technology, Japan (08CE2005). LA001427M
