Alkanethiolate Gold Cluster Molecules

Alkanethiolate Gold Cluster Molecules

(Parte 1 de 5)

Alkanethiolate Gold Cluster Molecules with Core

Diameters from 1.5 to 5.2 nm: Core and Monolayer Properties as a Function of Core Size

Michael J. Hostetler,† Julia E. Wingate,† Chuan-Jian Zhong,‡ Jay E. Harris,† Richard W. Vachet,† Michael R. Clark,† J. David Londono,§ Stephen J. Green,†

Jennifer J. Stokes,† George D. Wignall,§ Gary L. Glish,† Marc D. Porter,‡ Neal D. Evans,| and Royce W. Murray*,†

Kenan Laboratories of Chemistry, University of North Carolina,

Chapel Hill, North Carolina 27599-3290, Department of Chemistry, Iowa State University,

Ames, Iowa 50011-3111, Oak Ridge National Laboratory, P.O. Box 2008,

Oak Ridge, Tennessee 37831, and Oak Ridge Institute for Science and Education, P.O. Box 117, Oak Ridge, Tennessee 37831

Received June 5, 1997. In Final Form: October 23, 1997X

Themeansizeofthegold(Au)coreinthesynthesisofdodecanethiolate-stabilizedAuclustercompounds can be finely adjusted by choice of the Au:dodecanethiolate ratio and the temperature and rate at which the reduction is conducted. The Au clusters have been examined with a large number of independent analytical tools, producing a remarkably consistent picture of these materials. Average cluster and core dimensions,asascertainedby1HNMRlinebroadening,high-resolutiontransmissionelectronmicroscopy, small-angle X-ray scattering, and thermogravimetric analysis, vary between diameters of 1.5 and 5.2 nm ( 110-4800 Au atoms/core). The electronic properties of the Au core were examined by UV/vis and X-ray photoelectron spectroscopy; the core appears to remain largely metallic in nature even at the smallest core sizes examined. The alkanethiolate monolayer stabilizing the Au core ranges with core size from 53 to nearly 520 ligands/core, and was probed by Fourier transform infrared spectroscopy, differential scanning calorimetry,contact-anglemeasurements,andthermaldesorptionmassspectrometry. Thedodecanethiolate monolayeronsmallandlargecoreclustersexhibitsdiscernabledifferences;thelinedividing“3-dimensional” monolayersandthoseresemblingself-assembledmonolayersonflatAu(2-dimensionalmonolayers)occurs at clusters with 4.4 nm core diameters.

Theabilitytoselectivelysynthesizemetalnanoparticles of any desired size or shape would generate significant opportunities in chemistry, because catalytic, optical, magnetic, and electronic activities can be dimensionally sensitive.1-3 Verysmallclusters(< 50metalatoms)act like large molecules, whereas large ones (> 300 atoms) exhibit characteristics of a bulk sample of those atoms.

Between these extremes lie materials with intermediate chemical and physical properties that are largely unknown; gaining access to and evidence of such materials is one of the themes of this paper.

Asanexample,theopticalpropertiesofametalcluster, including the intensity and energy of its surface plasmon bands, have been strongly correlated to its size.4 The smallest clusters of some [including gold (Au)], but not all, metals exhibit electronic spectra with molecular transitions;asthenumberofatomsincreases,overarange that depends on the particular metal in the cluster, the plasmon band intensifies until the optical spectrum resembles that of the bulk metal. Clearly, investigations ofadditionalclusterpropertiesoverabroadrangeofsizes would be valuable. What is needed is the ability to selectively synthesize large quantities of these materials with ease.

In this regard, recent reports on the synthesis and characterization of relatively monodisperse Au nanoparticles are noteworthy.5-7 These clusters of Au atoms are stabilized to a remarkable degree by a monolayer of chemisorbed alkanethiolate ligands and are readily

* Author to whom correspondence should be addressed. † KenanLaboratoriesofChemistry,UniversityofNorthCarolina. ‡ Department of Chemistry, Iowa State University. § Oak Ridge National Laboratory. | Oak Ridge Institute for Science and Education. X Abstract published in Advance ACS Abstracts, December 15,

(1) Matijevic, E. Curr. Opin. Coll. Interface Sci. 1996, 1, 176-183. (b) Belloni, J. Curr. Opin. Coll. Interface Sci. 1996, 1, 184-196. (c) Klabunde, K. J.; Stark, J.; Koper, O.; Mohs, C.; Park, D. G.; Decker, S.; Jiang, Y.; Lagadic, I.; Zhang, D. J. Phys. Chem. 1996, 100, 12142- 12153.(d)Haberland,H.,Ed.ClustersofAtomsandMolecules;Springer- Verlag: New York, 1994. (e) Clusters and Colloids. From Theory to Applications; Schmid, G., Ed.; VCH: New York, 1994. (f) Heinglein, A. Ber. Bunsenges Phys. Chem. 195, 9, 903-913. (g) Schmid, G. Chem. Rev. 1992, 92, 1709-1727. (h) Fendler, J. H.; Meldrum, F. C. Adv. Mater. 1995, 7, 607-632. (i) Schmid, G.; Maihack, V.; Lantermann, F.; Peschel, S. J. Chem. Soc., Dalton Trans. 1996, 589-595. (2) (a) Schon, G.; Simon, U. Colloid Polym. Sci. 1995, 273, 101-117. (b) Schon, G.; Simon, U. Colloid Polym. Sci. 1995, 273, 202-218. (3) (a) Weller, H. Angew. Chem., Int. Ed. Engl. 1993, 32,4 1-53. (b)

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Eds. Springer-Verlag: New York, 1995. (b) Underwood, S.; Mulvaney, P. Langmuir 1994, 10, 3427-3430. (c) Mulvaney, P. Langmuir 1996, 12, 788-800. (d) Kreibig, U.; Fauth, K.; Quinten, M.; Schonauer, D. Z. Phys. D 1989, 12, 505. (5) (a) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman,

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S0743-7463(97)0588-X C: $15.0 © 1998 American Chemical Society Published on Web 01/06/1998 prepared in large quantities. Manipulation5a,7a of the preparative reaction conditions can effect changes in cluster dimensions; these monolayer-protected clusters (MPCs)arethusappealingforthestudyofsize-dependent physical and chemical properties.

In this paper, we present the synthesis and characterization of a series of unfractionated (i.e., as-prepared) dodecanthiolate-protectedAu clusterpreparations. Averagecorediametersrangefrom1.5to5.2nm;remarkably, even the smallest core sizes exhibit surface plasmon and electronbindingenergypropertiesthatcanbeconsidered metallic, implying that the transition between molecular and metallic behavior lies at smaller core sizes.8

This paper also describes how the alkanethiolate monolayer changes as a function of Au core size. Alkanethiolate-protected Au clusters can be regarded as 3-dimensional(3D)analogsofself-assembledmonolayers (3D-SAMs) formed by alkanethiolate ligands on flat Au surfaces (2D-SAMs).9 Two structural features inherent in the Au cluster molecules, large populations of core surfaceAuatomsthatonaflatsurfacewouldberegarded asdefectsites,andahighradiusofcorecurvature,promote differencesbetween3D-and2D-SAMs. Fouriertransform infrared (FTIR) spectroscopy has already detected examples of these dimensional distinctions; the conformational order of dropcast films of cluster compounds,6c,7c although strong, is less than that of a 2D-SAM of corresponding chainlength. These studies suggest that the structure (and perhaps reactivity) of the solid-state 3D-SAM should also depend on the size of the Au core, which is confirmed in the present case by FTIR spectroscopy, differential scanning calorimetry (DSC), mass spectrometry(MS),X-rayphotoelectronspectroscopy,and contact-angle observations. Surprisingly, the disorder/ ordertransitionroughlymarkingtheborderlinebetween a 3D- and a 2D-SAM lies at a relatively small core size.

Experimental Section

Chemicals. HAuCl4âxH2O was either purchased (Aldrich, 9.9%)orsynthesized.10 Allotherreagentswereacquiredfrom standard sources and used as received.

Synthesis. The syntheses followed a standard procedure,6c withinwhichthreereactionconditionsweresystematicallyvaried

(Table 1): (1) the temperature of the NaBH4 reduction step; (2) the mole ratio of dodecanethiol:HAuCl4âxH2O; and (3) the rate of addition of NaBH4. Toavigorouslystirredsolutionof1.5goftetraoctylammonium bromide (2.5 equiv) in 80 mL of toluene was added 0.31 g of

HAuCl4âxH2O(1equiv)in25mLofdeionizedwater. Theyellow HAuCl4âxH2O aqueous solution quickly cleared and the toluene phase became orange-brown as the AuCl4- was transferred into it. The organic phase was isolated, the desired amount of dodecanethiol was added, and the resulting solution was stirred for 10 min at room temperature. (For dodecanethiol:AuCl4- g 2, the orange-brown solution became either very pale yellow or colorless within 5 min.) Adjusting the solution temperature as desired for the reduction step, the reaction solution was then vigorously stirred and NaBH4 (0.38 g, 10 equiv) in 25 mL of deionized water was added11 over periods of either 10 s, 2 min, or 15 min (referred to as fd, md, and sd for fast, medium, and slow delivery, respectively). The now very dark organic phase was further stirred at the reduction temperature for 30 min and atroomtemperatureforatleastanother3h. Theorganicphase wascollected,andthesolventwasremovedonarotaryevaporator (forthelargerclusters,thisstepshouldnotexceed50°Ctoprevent partialproductdecomposition). Theblackproductwassuspended in 30 mL of ethanol, briefly sonicated to ensure complete dissolution of byproducts, collected on a glass filtration frit, and washed with at least 80 mL of ethanol and 150 mL of acetone. Upon air drying (or drying at room temperature in vacuo), materials were found to be spectroscopically (NMR) clean. The reactions have been successfully scaled up fivefold. These reactions were also carried out in oxygen-saturated/aerated solutions, but no evidence of oxygen-containing thiol or cluster products was ever seen (by 1H NMR or MS).

Cluster Synthesis at -78 °C. This low temperature preparation (coded -78°,2X,sd) was identical to the aforemen- tionedprocedureexceptthattheNaBH4reductantwassuspended in20mLofabsoluteethanolandaddedtoarapidlystirredAuCl4-/ dodecanethiol toluene solution held at -78 °C (dry ice, acetone).

Following addition of the reductant, the solution was stirred for 1ha t -78 °C, warmed slowly to room temperature, stirred an additional 3 h, and then worked up.

Spectroscopy (see Supporting Information for further details). Infrared absorbance spectra of clusters (either as dropcast thin films on KBr plates or pressed into a KBr pellet) were acquired using a Bio-Rad 6000 spectrometer. The 1H and

13C NMR spectra of C6D6 solutions were collected at 200 MHz and50MHz,respectively,onaBrukerAC200spectrometer,and

UV/vis spectra of hexane solutions with a ATI UNICAM spectrometer. X-ray photoelectron spectra for the gold clusters (dropcast from hexane onto a molybdenum substrate) were

(6) (a) Terrill, R. H.; Postlethwaite, T. A.; Chen, C.-H.; Poon, C.-D.;

Terzis,A.;Chen,A.;Hutchison,J.E.;Clark,M.R.;Wignall,G.;Londono, J. D.; Superfine, R.; Falvo, M.; Johnson, C. S., Jr.; Samulski, E. T.; Murray,R.W.J.Am.Chem.Soc.1995,117,12537-12548.(b)Hostetler, M. J.; Green, S. J.; Stokes, J. J; Murray, R. W. J. Am. Chem. Soc. 1996, 118, 4212-4213. (c) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Langmuir1996,12,3604-3612.(d)Green,S.J.;Stokes,J.J.;Hostetler, M. J.; Pietron, J. J.; Murray, R. W. J. Phys. Chem. B 1997, 101, 2663. (e) Hostetler, M. J.; Murray, R. W. Curr. Opin. Colloid Interface Sci. 1997,2,42-50.(f)Ingram,R.S.;Hostetler,M.J.;Murray,R.W.J.Am. Chem.Soc.1997,119,9175.(g)Ingram,R.S.;Hostetler,M.J.;Murray, R. W.; Schaaff, T. G.; Khoury, J. T.; Whetten, R. L.; Bigioni, T. P.; Guthrie, D. K.; First, P. N. J. Am. Chem. Soc. 1997, 119, 9279. (h) Templeton,A.C.;Hostetler,M.J.;Kraft,C.T.;Murray,R.W.,submitted for publication. (7) (a) Leff, D. V.; Ohara, P. C.; Heath, J. R.; Gelbart, W. M. J. Phys.

Chem. 195, 9, 7036-7041. (b) Badia, A.; Gao, W.; Singh, S.; Demers, L.; Cuccia, L.; Reven, L. Langmuir 1996, 12, 1262-1269. (c) Badia, A.; Singh, S.; Demers, L.; Cuccia, L.; Brown, G. R.; Lennox, R. B. Chem. Eur. J. 1996, 2, 359-363. (d) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vezmar, I.; Wang, Z. L.; Stephens, P. W.; Cleveland, C. L.; Luedtke, W. D.; Landman, U. Adv. Mater. 1996, 8, 428-433. (e) Ohara, P. C.; Leff, D. V.; Heath, J. R.; Gelbart, W. M. Phys. Rev. Lett. 1995, 75, 3466-3469. (f) Luedtke, W. D.; Landman, U. J. Phys. Chem. 1996, 100, 13323-13329. (g) Weisbecker, C. S.; Merritt, M. V.; Whitesides,G.M.Langmuir1996,12,3763-3772.(h)Leff,D.V.;Brandt, L.; Heath, J. R. Langmuir 1996, 12, 4723-4730. (i) Alvarez, M. M.; Khoury, J. T.; Schaaff, T. G.; Shafigullin, M. N.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 1997, 101, 3706. (j) Cleveland, C. L.; Landman, U.;Shafigullin,M.;Stephens,P.W.;Whetten,R.L.Z.Phys.D1997,40, 503. (k) Vezmar, I.; Alvarez, M. M.; Khoury, J. T.; Salisbury, B. E.; Whetten, R. L. Z. Phys. D 1997, 40, 147. (l) Johnson, S. R.; Evans, S. D.;Mahon,S.W.;Ulman,A.Langmuir1997,13,51-57.(m)Badia,A.; Cuccia, L.; Demers, L.; Morin, F.; Lennox, R. B. J. Am. Chem. Soc. 1997, 119, 2682-2692. (n) Yonezawa, T.; Tominaga, T.; Richard, D. J. Chem. Soc., Dalton Trans. 1996, 783-789. (o) Alvarez, M. M.; Khoury, J. T.; Schaaf, T. G.; Shafigullen, M.; Vezmar, I.; Whetten, R. L. Chem. Phys. Lett. 1997, 266, 91. (p) Yonezawa, T.; Sutoh, M.; Kunitake, T. Chem. Lett. 1997, 619. (q) Buining, P. A.; Humbel, B. M.; Philipse, A. P.;Verkleij,A.J.Langmuir1997,13,3921.(r)Whetten,R.L.,personal communication. (s) Andrews, R. P.; Bielefeld, J. D.; Henderson, J. I.; Janes,D.B.;Kolagunta,V.R.;Kubiak,C.P.;Mahoney,W.J.;Osifchin, R.G.Science1996,273,1690-1693.(t)Andrews,R.P.;Bein,T.;Dorogi, M.; Feng, S.; Henderson, J. I.; Kubiak, C. P.; Mahoney, W.; Osifchin, R. G.; Reifenberger, R. Science 1996, 272, 1323-1325. (8) Recent work in this lab indicates that the transition between metallicandmolecularbehavioroccursatagoldcoresizeof 40atoms; see ref 6g. (9) (a) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437-463. (b) Ulman, A. An Introduction to Ultrathin Organic Films, Academic: New York, 1991. (c) Bain, C. D.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1989, 28, 506-512. (d) Ulman, A. Chem. Rev. 1996, 96, 1533-1554.

(10) (a) Handbook of Preparative Inorganic Chemistry; Brauer, G.,

Ed.; Academic: New York, 1965, p 1054-1059. (b) Block, B. P. Inorg. Syn. 1953, 4,1 4-17. (1) The quantity of NaBH reductant was also varied in the 0,2x,fd clusterpreparation.Judgingby HNMRlinewidths,increasedamounts of NaBH had no apparent effect; decreased amounts (NaBH :AuCl mole ratios of 5:1 and 2:1) slightly increased average particle size (FWHM, 19 and 21 Hz, respectively). A mole ratio of 1:1 significantly increased particle size (FWHM ) 32 Hz) with 50% toluene-insoluble product; a mole ratio of 1:2 gave only insoluble product.

18 Langmuir, Vol. 14, No. 1, 1998 Hostetler et al.

collected on a Physical Electronics Industries 50 surface analysis system with Al KR source, hemispherical analyzer, torroidal monochromater, and multichannel detector (the pass energy was 187.9 eV, with a resolution of 0.3 eV). The peak positions of S 2p3/2 and S 2p1/2 were determined by spectral deconvolution with a Gaussian-profile curve fitting with the constraint of a doublet of peaks with a FWHM of 0.90 ( 0.05 eV.12 PeakpositionswereinternallyreferencedtotheC1speak (from the methylene chain of the ligand) at 284.9 eV.

Thermal Analysis (see Supporting Information for further details). The DSC was performed with a Seiko SSC 5200 thermal analysis system and thermogravimetric analysis (TGA) with a Seiko RTG 220 robotic TGA.

ContactAngles(seeSupportingInformationforfurther details). Advancingcontactanglesofinterfacesofsessilewater droplets and cluster films on smooth Au films were measured withacontactanglegoniometer(Rame-Hart,Inc.);datareported are averages from six different droplets.

Small-Angle X-ray Scattering (SAXS). Experiments on hexane cluster solutions in an airtight stainless steel cell with Mylar windows were performed on the Oak Ridge National Laboratory 10 m SAXS instrument13ab with sample-detector distances of 1.119 and 5.119 m using Cu KR radiation (ì ) 1.54 Å) and a 20 20 cm2 area detector with cell size 3 m. The data were worked up as described previously.6a,13c-h

High-Resolution Transmission Electron Microscopy

(TEM). Samples for TEM were prepared by dropcasting one drop of a 1 mg/mL cluster solution in toluene onto standard carbon-coated (200-300 Å) Formvar films on copper grids (600 mesh)anddryinginairforatleast45min. Phase-contrastimages of the particles were obtained with a side-entry Phillips CM12 electronmicroscopeoperatingat120keV. Threetypicalregions of each sample were obtained at 580 000X. Size distributions of the Au cores were measured from enlarged TEM image photographs for at least 80-150 individual cluster core images.

Mass Spectrometry (see Supporting Information for further details). Thermal desorption experiments were performedwiththedirectinsertionprobeofaforward(EB)geometry Finnigan MAT 900 instrument using 70 eV electron ionization. Theprobetemperaturewasrampedto200°Cin1minandthen from200to300°Cin20minatarateof5°C/min,whilescanning the spectrometer over a mass range of 40 to 481 Da at a rate of 1.1 scan/degree.

Results and Discussion

Synthesis. SmallAuclustersprotectedbyPR3ligands, suchasthosefirstreportedbySchmid’sgroup,havebeen knownforsometime.1e,g However,thereliableformation of larger and more stable MPCs remained for Brust et al.,5a who demonstrated that an organic-phase reduction of HAuCl4 by dodecanethiol and NaBH4 leads to stable, modestly polydisperse, dodecanethiolate-protected Au clusterswithacentralpopulationofcoredimensions2.0- 2.5 nm. Subsequent reports have shown that this basic protocol can give access to clusters protected by a range ofalkanethiolatechainlengths(C3-C24)6candfunctional groups,5b,7b and that variation7a of the RSH:AuCl4- ratio (2:1 to 1:6) changes the cluster core size produced.

WehavehereextendedtheRSH:AuCl4-moleratioused in the Brust et al.5a reaction from 4:1 to 1:12, and have varied the temperature and rate of reductant addition as shown in Table 1 (see footnote a for coding of reaction conditions). The color and physical consistency of the product clusters vary substantially from the top to the bottom of the table, which is arranged (vide infra) from smallest to largest core sizes. For the smallest cores, the cluster material is black and waxy. With increasing core size(suchasRT,1/2X,fd),thetextureoftheclustermaterial becomes “clumpy” and less waxy and then powdery but stillblack(suchasRT,1/6X,fd). Forthelargestsizes(such as RT,1/10X,fd), the dark, free-flowing powder displays a distinct golden hue.

In reactions with RSH:AuCl4- mole ratios of g2:1, the orange-brown R4N+AuCl4-/toluene solution fades within 5 min after adding the alkanethiol, which may signify an alkanethiol-induced reduction to Au(I). Consistent with this hypothesis are (i) the formation of oxidized thiol (the corresponding disulfide, seen by NMR) as a major byproduct of the reaction, and (i) the stoichiometric requirement of a threefold excess of thiol to completely reduce the metal center to Au(I).7o Also, literature syntheses for Au(I) alkanethiolate complexes, which are thought to be [Au-S(R)-]n polymers, follow a similar procedure.14 In principle, forming a Au(0) cluster should require only a single further equivalent of reductant, but the Brust et al., procedure5a recommends a 10:1 BH4-:

(Parte 1 de 5)

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