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Hexanethiolate Monolayer Protected 38 Gold Atom Cluster Victoria L. Jimenez, Dimitra G. Georganopoulou,† Ryan J. White,‡ Amanda S. Harper, Allan J. Mills,§ Dongil Lee,| and Royce W. Murray* Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290 Received March 19, 2004. In Final Form: May 11, 2004 The nucleation-growth-passivation Brust reaction has been modified so as to enrich the product in useful quantities of a 38-atom gold nanoparticle coated with a hexanethiolate monolayer. Two modifications are described, using -78 °C reduction temperature and a hyperexcess of thiol. Compositional evidence is presented that establishes the product as a Au38(C6)24 hexanethiolate monolayer protected cluster (MPC), based on transmission electron microscopy, laser ionization-desorption mass spectrometry, thermogravimetric analysis, and elemental analysis. Reverse phase HPLC confirms the relatively good monodispersity of the MPC products, but high-resolution double-column HPLC reveals that the MPCs are a mixture of closely related but chromatographically distinct products. Voltammetry, low energy spectrophotometry, and spectroelectrochemistry reveal, respectively, a 1.6 eV electrochemical energy gap between the first oxidation and the first reduction, an optical HOMO-LUMO energy absorbance edge at 1.3 eV, and a bleaching of optical absorbance near the 1.3 eV band edge that accompanies electrochemical oxidation of the nanoparticle.
Introduction Research on nanometer-sized semiconductor and metal particles, where quantum confinement and molecule-like behavior appear, has been active in recent years because of the technological appeal of such materials1-3 and, more importantly, the need for an improved scientific understanding of nanoscopic materials of all kinds. The synthesis of nanoscopic materials is always of course a prerequisite to study and discovery of any interesting properties. Except for preparation and study of nanoparticles in the gas phase,4 it is essentially universal that nanoparticle synthesis is accompanied by coating the nanoparticles with a stabilizing (protective, capping) layer to prevent nanoparticle aggregation. A large literature exists on using different kinds5-8 of aqueous, nonaqueous, micellar, and two-phase media for the reductive generation of nanoparticle cores of different compositions and to provide the required stabilizing chemistry. Stabilizers range from adsorbed polymer coatings8 to adsorbed small ion layers6 to explicit monolayers of organic ligands.9,10 The focus in this paper is on Au nanoparticles. Au colloids have been known since antiquity, and aqueous †
Present address: Nanotechnology Institute, Evanston, IL. Present address: Department of Chemistry, University of Utah, Salt Lake City, UT. § Department of Chemistry, University Liverpool, Liverpool, U.K. | Present address: Department of Chemistry, Western Michigan University, Kalamazoo, MI. ‡
(1) Simon, U. Adv. Mater. 1998, 10, 1487. (2) Optical Properties of Metal Clusters; Kreibig, U., Vollmer, M., Eds.; Springer Series in Material Science, Vol. 25; Springer-Verlag: Berlin, 1995. (3) Small Wonders, Endless Frontiers: A Review of the National Nanotechnology Initiative; Committee for the Review of the National Nanotechnology Initiative, National Academy Press: Washington, DC, 2002. (4) Koropchak, J. A.; Sadain, S.; Yang, X.; Magnusson, L.; Heybroek, M.; Anisimov, M.; Kaufman, S. L. Anal. Chem. 1999, 71, 386A-394A. (5) Selvakannan, P. R.; Mandal, S.; Pasricha, R.; Adyanthaya, S. D.; Sastry, M. Chem. Commun. 2002, 1334-1335. (6) Henglein, A. J. Phys. Chem. B 2000, 104, 2201-2203. (7) Fendler, J. H. Chem. Mater. 2001, 13, 3196-3210. (8) Pileni, M. P. Langmuir 1997, 13, 3266-3276. (9) Brust, M.; Walker, M.; Bethell, D.; Schriffin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801-802.
preparations of ca. 10 nm diameter colloids retain their importance, in for example current bioanalytical methodology.11-14 The evolution of size-dependent properties starts, however, only at much smaller Au dimensions, of the order of 100-150 atoms. Schmid et al.15 in 1981 reported an early example of a molecule-like Au nanoparticle, formulated as Au55[PPh3]12Cl6. Subsequent investigations15 of this interesting material were hampered by its general instability. A new synthetic opening into making small Au nanoparticles was provided in 1994 by the Brust two-phase protocol,9 in which the Au nanoparticle was formed in the presence of alkanethiols, which formed a passivating and protective coating of thiolate ligands. We10 and others16-18 were attracted to this synthesis, and considerable subsequent work ensued, including use of alkanethiols of different chain lengths,10 functionalized alkanethiols,19 and dialkyl disulfides.20 We label these materials “monolayer-protected clusters” (MPCs). There has also been related use of one-phase synthetic protocols and of more polar thiols, yielding a range of water-soluble MPCs.21,22 (10) Hostetler, M. J.; Wingate, J. E.; Zhong, C.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17-30. (11) Mirkin, C. A. Inorg. Chem. 2000, 39, 2258-2272. (12) Willner, A. N.; Willner, I. Acc. Chem. Res. 2001, 34, 421-432. (13) Shenhar, R.; Rotello, V. M. Acc. Chem. Res. 2003, 36, 549-561. (14) Colloidal Gold: Principles, Methods, and Applications; Hayat, M. A., Ed.; Academic Press: San Diego, 1989; Vols. 1-3. (15) Schmid, G.; Pfeil, R.; Boese, R.; Bandermann, F.; Meyer, S.; Calis, G. H. M.; van der Velden, J. W. A. Chem. Ber. 1981, 114, 3634. (16) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vezmar, I.; Wang, Z. L.; Stephen, P. W.; Cleveland, C. L.; Luedtke, W. D.; Landman, U. Adv. Mater. 1996, 5, 428. (17) (a) Alvarez, M. M.; Khoury, J. T.; Schaaff, T. G.; Shafigullin, M.; Vezmar, I.; Whetten, R. L. Chem. Phys. Lett. 1997, 266, 91-98. (b) Schaaff, T. G.; Shafigullin, M. N.; Khoury, J. T.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 2001, 105, 8785. (18) Schaaff, T. G.; Shafigullin, M. N.; Khoury, J. T.; Vezmar, I.; Whetten, R. L.; Cullen, W. G.; First, P. N. J. Phys. Chem. B 1997, 101, 7885-7891. (19) Johnson, S. R.; Evans, S. D.; Brydson, R. Langmuir 1998, 14, 6639-6647. (20) Porter, L. A., Jr.; Ji, D.; Westcott, S. L.; Graupe, M.; Czernuszewicz, R. S.; Halas, N. J.; Lee, T. R. Langmuir 1998, 14, 73787386.
10.1021/la049274g CCC: $27.50 © 2004 American Chemical Society Published on Web 06/25/2004
Monolayer-Protected Gold Atom Cluster
The Brust strategy of forming the Au nanoparticle in the presence of thiols has been accompanied by considerable effort in purification of the typically polydisperse MPC products (solubility fractionation,16 etching,23 heating,24 vapor treatments,25,26 and annealing27) and in analytically characterizing the polydispersity (transmission electron microscopy (TEM),10 reverse phase HPLC,27 liquid chromatography with electrochemical detection,37 capillary electrophoresis,29 and mass spectrophotometry17). Hutchison et al.30 introduced an additional strategy, of exchanging the triphenylphosphine ligands of Au11 and Au55 nanoparticles with thiolate ligands. These collective efforts have led to thiolate-stabilized MPCs reported as containing 11,31 28,21 38,32,33 55,30 75,17a,34 116,17a and 140-14517,10,27 Au atoms. These various atom numbers correspond to closed shell core structures and lie in the interesting nanoscale in which variations of properties with size are expected. We recently described35 a Brust type synthesis that led in good yield to a stable, rather monodisperse MPC with the analytically established formula Au38(PhC2)24, where PhC2 is phenylethanethiolate. The focus of the present work is to extend our synthetic capacity to gold MPCs of this same interesting core dimension, but stabilized with monolayers of alkanethiolate ligands. The isolation procedure used for the Au38(PhC2)24 nanoparticle fails for alkanethiolates.35 An additional question asked is whether the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) band gap observed is sensitive to the protecting ligand employed. Two synthetic tactics are employed: (A) a -78 °C reaction temperature and (B) a hyperexcess of thiols relative to Au in the reaction. The former tactic is predicated on seeking a slowed growth of the metal core, relative to that of thiolate passivation of the core, and the second on seeking a very rapid passivation reaction. Both tactics produced some success. We additionally describe the analytical formulation of the MPCs as Au38 cores, their ca. 1.3 eV HOMO-LUMO band gap, and an unprecedented level of HPLC resolution in nanoparticle separations of what may be near-isomers of the Au38 nanoparticles. (21) (a) Link, S.; Beeby, A.; Fitzgerald, S.; El-Sayed, M. A.; Schaaff, T. G.; Whetten, R. L. J. Phys. Chem. B 2002, 106, 3410-3415. (b) Link, S.; El-Sayed, M. A.; Schaaff, T. G.; Whetten, R. L. Chem. Phys. Lett. 2002, 356, 240-246. (c) Schaaff, T. G.; Knight, G.; Shafigullin, M. N.; Borkman, R. F.; Whetten, R. L. J. Phys. Chem. B 1998, 102, 1064310646. (22) Huang, T.; Murray, R. W. J. Phys. Chem. B 2001, 105, 1249812502. (23) Schaaff, T. G.; Whetten, R. L. J. Phys. Chem. B 1999, 103, 9394. (24) Devenish, R. W.; Goulding, T.; Heaton, B. T.; Whyman, R. J. Chem. Soc., Dalton Trans. 1996, 673-679. (25) Maye, M. M.; Zheng, W.; Leibowitz, F. L.; Ly, N. K.; Zhong, C. J. Langmuir 2000, 16, 490-497. (26) Zhong, C. J.; Zhang, W. X.; Leibowitz, F. L.; Eichelberger, H. H. Chem. Commun. 1999, 1211-1212. (27) Hicks, J. F.; Miles, D. T.; Murray, R. W. J. Am. Chem. Soc. 2002, 124, 13322-13328. (28) Jimenez, V. L.; Leopold, M. C.; Mazzitelli, C.; Jorgenson, J. W.; Murray, R. W. Anal. Chem. 2003, 75, 199-206. (29) Templeton, A. C.; Chen, S.; Gross, S. M.; Murray, R. W. Langmuir 1999, 15, 66. (30) Brown, L. O.; Hutchison, J. E. J. Am. Chem. Soc. 1997, 119, 12384-12385. (31) Woehrle, G. H.; Warner, M. G.; Hutchison, J. E. J. Phys. Chem. B 2002, 106, 9979. (32) Lee, D.; Donkers, R. L.; DeSimone, J. M.; Murray, R. W. J. Am. Chem. Soc. 2003, 125, 1182-1183. (33) Ha¨kkinen, H.; Barnett, R. N.; Landman, U. Phys. Rev. Lett. 1999, 82, 3264-3267. (34) Gutie´rrez, E.; Powell, R. D.; Furuya, F. R.; Hainfeld, J. F.; Schaaff, T. G.; Shafigullin, M. N.; Stephens, P. W.; Whetten, R. L. Eur. Phys. J. D 1999, 9, 647. (35) Donkers, R. L.; Lee, D.; Murray, R. W. Langmuir 2004, 20, 19451952.
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Experimental Section Chemicals. HAuCl4‚xH2O was synthesized using an established procedure.28 Tetrabutylammonium perchlorate (Bu4NClO4), hexanethiol (CH3(CH2)5SH, abbreviated C6SH), dodecanethiol (C12SH), tetraoctylammonium bromide (Oct4NBr), sodium borohydride (NaBH4), and HPLC-grade toluene, methanol, acetonitrile, dichloromethane, and acetone (Aldrich) were used as received. House distilled water was purified with a Barnstead Nanopure system (18.2 MΩ). Silica gel, 230-400 mesh, was from Fisher Scientific. MPC Synthesis. Two different modified Brust type3 syntheses produced useful yields of small-core MPCs. The first involved the use of low reaction temperatures (the “-78 °C protocol”) and the second (“XXS-thiol protocol”) a very high thiol/gold ratio (300: 1). In the -78 °C protocol, 1.1 g of Oct4NBr in 100 mL of toluene was stirred with 0.31 g of HAuCl4 in 20 mL of Nanopure water for 30 min. After the deep red organic layer was separated, C6SH was added in a 3:1 mole ratio (relative to Au). After 30 min of stirring, the resulting clear and colorless solution was cooled to -78 °C in a dry ice/acetone bath, and a solution of 0.38 g of NaBH4 (10:1 mol, relative to Au) in 20 mL of Nanopure water was added using a fast delivery/stirring technique. This involved using a top stirrer set at a relatively high speed. The added aqueous reductant solution immediately froze upon addition; the fast stirring produces a frozen, coffee-colored slush, which was stirred for 30 min in the dry ice/acetone bath and then poured into a separatory funnel. The icy aqueous layer was discarded, the dark brown organic layer was separated, and the solvent was removed at a temperature of
