Langmuir 1996, 12, 4723-4730
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Synthesis and Characterization of Hydrophobic, Organically-Soluble Gold Nanocrystals Functionalized with Primary Amines Daniel V. Leff, Lutz Brandt, and James R. Heath* Department of Chemistry and Biochemistry, University of California, Los Angeles, 405 Hilgard Avenue, Los Angeles, California 90095-1569 Received May 6, 1996. In Final Form: July 11, 1996X We report on the synthesis and characterization of 25-70 Å diameter amine-capped gold nanocrystals. In particular, we show how these particles can be prepared by a simple procedure and confirm the particle composition (including the identity of the amine surface passivant) through several materials characterization techniques that include infrared spectroscopy, nuclear magnetic resonance spectroscopy, ultravioletvisible spectroscopy, mass spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, X-ray diffraction analysis, differential scanning calorimetry, thermogravimetric analysis, and elemental analysis. All physical characterizations are consistent with a charge-neutral amine/gold surface interaction described by a weak covalent bond. The stability of the particles appears to be largely kinetic, rather than thermodynamic, in nature. Comparison of these nanocrystals to amines adsorbed onto bulk Au surfaces indicates that the stability of the nanocrystal/amine system is a finite-size effect.
I. Introduction The synthesis and physical characterization of nanometer-scale metal and semiconductor colloids is currently very active area of research due to the potential applications of these materials in optoelectronics,1 catalysis,2 reprography,3 and other areas.4 Gold colloids represent one of the most widely studied5 of these nanoparticle systems, and they have a rich history dating back to the original work of Faraday.6 The bulk of this work, however, consists of the preparation of gold sols (gold colloids prepared and stabilized in aqueous media).5 Brust and co-workers have recently reported a synthesis for the preparation of gold nanocrystals which are passivated with covalently bound alkanethiols.7 These particles represent a departure from standard gold sols in that they are extremely soluble and resoluble in organic media such as hexane or toluene. In addition, they represent nanocrystal analogs to the prototypical selfassembled monolayer (SAM) systems of alkanethiols on bulk Au surfaces. There do exist critical differences between the nanocrystal and bulk systems, however. For example, it is unlikely that the highly curved surface of a nanocrystal can support ordered domains of the bound organic monolayer, although such order often lends additional stability to the bulk monolayer. In addition, there are indications that the curved particle surface supports a higher density adsorbate coverage relative to the bulk. Thus, the finite size (or high surface curvature) of a metal nanocrystal may introduce unique factors which * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, September 1, 1996. (1) (a) Wang, Y.; Herron, N. J. Phys. Chem. 1991, 95, 525. (b) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (2) (a) Schmid, G. Chem. Rev. 1992, 92, 1709. (b) Hoffman, A. J.; Mills, G.; Yee, H.; Hoffman, M. R. J. Phys. Chem. 1992, 96, 5546. (3) (a) Hamilton, J. F.; Baetzold, R. C. Science 1979, 205, 1213. (b) Hair, M., Croucher, M. D., Eds. Colloids and Surfaces in Reprographic Technology; A.C.S. Symposium Series No. 200; American Chemical Society: Washington, DC, 1982. (4) Dagani, R. Chem. Eng. News 1992, Nov. 23 (Nanostructured Materials Promise to Advance Range of Technologies), 18. (5) Hayat, M. A., Ed. Colloidal Gold: Principles, Methods, and Applications; Academic Press, Inc.: San Diego, CA, 1989; Vol. 1. (6) Faraday, M. Philos. Trans. R. Soc. London 1857, 147, 145. (7) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801.
S0743-7463(96)00445-3 CCC: $12.00
govern the chemical stability of covalently bound monolayers. In this paper, we present work on a metal nanocrystal/organic passivant system in which the chemical stability of the system must be derived from the finite size of the nanocrystal, since the bulk analog is chemically unstable. We report here on organically-soluble gold nanocrystals passivated with alkylamines, which have chemical and physical characteristics that are consistent with charge-neutral amine/gold surface interactions described by weak covalent bonds. Amines form only weakly bound and chemically unstable monolayers on bulk Au surfaces. In contrast, the amine-capped nanocrystals discussed here are nearly as stable as their thiol-capped counterparts. Physical characterization of the nanocrystals indicates that the stability of these particles originates from a kinetic rather than a thermodynamic effect. Monolayers on Bulk and Nanocrystal Gold Surfaces. The preparation and characterization of supported, self-assembled monolayers (SAMs) of oriented long-chain organic molecules has generated model systems for the study of a variety of interfacial phenomena.8 Examples include catalysis,9 lubrication,10 corrosion,11 adhesion,12 adsorption of proteins at surfaces,13 electrochemistry,14 and electrical conduction,15 among others. The most widely studied system has been that of self-assembled, well-ordered, close-packed monolayers of long-chain organothiols on zero-valent gold (Au0[111]) surfaces.16-18 The thiol linkage apparently provides a nearly unique path toward imparting stable organic functionality to an Au0 surface. Some reports, however, indicate that a phosphine (8) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (9) (a) Richard, M. A.; Deutsch, J.; Whitesides, G. M. J. Am. Chem. Soc. 1979, 100, 6613. (b) Somorjai, G. A. Chemistry in Two Dimensions: Surfaces; Cornell University Press: Ithaca, NY, 1981. (10) (a) Bowden, F. P.; Tabor, D. The Friction and Lubrication of Solids; Oxford University Press: London, 1968. (b) Adamson, A. W. Physical Chemistry of Surfaces, 3rd ed.; Wiley: New York, 1976; Chapter 10. (11) Notoya, T.; Poling, G. W. Corrosion 1979, 35, 193. (12) Kaelble, D. H. Physical Chemistry of Adhesion; Wiley-Interscience: New York, 1971. (13) Prime, K. L.; Whitesides, G. M. Science 1991, 252, 1164. (14) (a) Murray, R. W. Acc. Chem. Res. 1980, 13, 135. (b) Soriaga, M. P.; Hubbard, A. T. J. Am. Chem. Soc. 1982, 104, 3937, and references therein. (c) Hubbard, A. T. Acc. Chem. Res. 1980, 13, 177. (15) (a) Polymeropoulos, E. E.; Sagiv, J. J. Chem. Phys. 1978, 69, 1836. (b) Sugi, M.; Fukui, T.; Lizima, S. Phys. Rev. B 1978, 18, 725. (c) Furtlehner, J. P.; Messier, J. Thin Solid Films 1980, 68, 233.
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linkage can provide a second route,17-19 but competitive adsorption experiments between long-chain alkanethiols and trialkylphosphines attest that there is preferential adsorption of the thiol.17,18 Alcohols, the isolectronic second row analogs to thiols, exhibit no appreciable interaction with the Au0 surface. As for amines (the phosphine analogs), it has been shown (from the standard adsorption methodology from the solution phase) that they do not form stable, close-packed, ordered monolayers on the Au0 (111) surface.17,18 However, Xu et al.20 have recently reported on the use of Fourier transform infrared (FTIR) external reflectance spectroscopy to characterize vapor-phase ambients adsorbed onto naked Au substrates. Their work indicates that vapor-phase deposition of CH3(CH2)9NH 2 does apparently result in a selfassembled, ordered monolayersalthough these amine monolayers only form and remain stable under certain conditions. For example, they report that amineterminated n-alkanes do not spontaneously adsorb onto Au surfaces from high-polarity solvents such as ethanol, because the Au0/amine interaction is insufficient to compete with solvent interactions. In addition, they find that a preformed n-alkylamine SAM on an Au surface is only stable for hours in vapor-phase media such as N2 but is actually unstable when exposed to a polar solvent. They argue that vapor-phase stability of the SAM arises from a combination of Au-N and interchain van der Waals interactions. Despite the extensive work on SAMs for bulk surfaces, little of what has been learned has been extended to the study of finite-size systems. In this paper, we make such analogies and interpret the unique behavior of our chemical system in the context of a finite-size effect. In particular, we describe the synthesis and characterization of 25-70 Å gold nanocrystals whose surfaces are functionalized with n-alkylamines. The chemical and physical characteristics concerning the bonding mode of the alkylamine surface passivants are consistent with a chargeneutral amine/gold surface interaction described by a weak covalent bond. In terms of the specific Au0/amine interaction, this system should be similar to the case of n-alkylamine SAMs on a bulk Au surface. However, in other ways, it is quite different. For example, high surface curvature of the nanocrystal implies that adsorbed monolayers are unlikely to be orderedsan effect which should decrease the stability of the monolayer. Conversely, a high surface curvature also implies a radially increasing volume above the particle surface (relative to a surface with low curvature) that is available to the adsorbate molecules, potentially leading to an increase in the number of adsorbates (per metal surface atom) bound to the surface. Indeed, recent work by us21 on Au nanocrystals capped with long-chain alkanethiols indicates that the number of thiol molecules per metal surface atom gradually decreases with increasing particle size, from roughly one thiol per two Au surface atoms for a small (D ≈ 20 Å) particle, to the bulk value of one thiol per three Au surface atoms. Murray’s group has reported a slightly higher
Leff et al.
number for 20 Å dodecanethiol-capped Au nanocrystals (two thiols for every three surface Au atoms),22 although the trend is the same. This effect should increase the relative stability of a monolayer on the nanocrystals, especially for the smallest particles. The amine-capped Au nanocrystals reported here do exhibit certain properties which are reminiscent of the amine monolayers on bulk Au surfaces reported by Xu et al. For example, DSC measurements on various Au nanocrystals indicate that the Au0/N interaction is much weaker than the Au0/S interaction. In addition, certain of these particles are unstable under certain conditions (with respect to ligand desorption and particle agglomeration). However, this decomposition correlates with the presence of trace amounts of Br- associated with the particles. Particles which do not contain this impurity exhibit remarkable long term stability in a variety of chemical environments and may even be stored under ambient conditions for days or weeks without effect. Indeed, in many important respects, amine-capped Au nanocrystals behave very similar to their thiol-capped counterparts. In light of what is known about the chemistry of bulk Au surfaces, this result is surprising. Our data indicate, however, that the stability of aminecapped Au nanocrystals is a finite-size effect which is largely kinetic, rather than thermodynamic, in origin. II. Experimental Section A. Materials. The following chemicals were all used as obtained: from Sigma Chemical Co., dodecylamine (laurylamine, C12H25NH2); from Aldrich Chemical Co., oleylamine (C18H35NH2; tech., 80%), hydrogen tetrachloroaurate trihydrate (chloroauric acid, HAuCl4‚3H2O), sodium borohydride (sodium tetrahydridoborate, NaBH4; 99%), and tetraoctylammonium bromide (N(C8H17)4Br; 98%); from Fisher Scientific Co., toluene (certified A.C.S. reagent) and hexane (certified A.C.S. reagent, spectranalyzed); from the UCLA Department of Chemistry and Biochemistry, 95% ethanol. B. Particle Preparation. Amine-capped gold nanocrystals, ranging in average diameter from ≈25 to 70 Å, were synthesized. Two different reaction schemes were employed (Scheme 1 and Scheme 2), and all reactions were carried out at room temperature
Scheme 1 (a) mAuCl4-(aq.) + mN(C8H17)4+(toluene) f m[N(C8H17)4+AuCl4-](toluene) (b) m[N(C8H17)4+AuCl4-](toluene) + n(alkylamine)(toluene) + 3me-(aq.) f 4mCl-(aq.) + (Aum)(alkylamine)(n-x)(toluene) + x(alkylamine)(toluene) + m[N(C8H17)4+] (toluene; aq.) Scheme 2 mAuCl4-(aq) + n(alkylamine)(toluene) + 3me-(aq.) f 4mCl-(aq.) + (Au(m-y))(alkylamine)(n-x) (toluene) + yAu(s) + x(alkylamine)(toluene)
(16) See, for example: (a) Reference 8, above. (b) Reference 13, above. (c) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (d) Bain, C. D.; Whitesides, G. M. Science 1988, 240, 62. (e) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (17) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (18) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155. (19) (a) Uvdal, K.; Persson, I.; Liedberg, B. Langmuir 1995, 11, 1252. (b) Steiner, U. B.; Neuenschwander, P.; Caseri, W. R.; Suter, U. W. Langmuir 1992, 8, 90. (20) Xu, C.; Sun, L.; Kepley, L. J.; Crooks, R. M. Anal. Chem. 1993, 65, 2102. (21) Leff, D. V.; Ohara, P. C.; Heath, J. R.; Gelbart, W. M. J. Phys. Chem. 1995, 99, 7036.
(≈22 °C) and ambient pressure over a period of ≈12 h. In both of these schemes, AuCl4- is reduced in a two-phase water/toluene system in the presence of a linear alkylamine. The difference between these two reactions is that, in Scheme 1, the AuCl4- is quantitatively transferred from the aqueous into the organic phase by complexing it with a phase transfer reagent [N(C8H17)4Br] prior to the reduction step. In Scheme 2 the phase transfer reagent is omitted. In both cases, the gold nanocrystals of interest are in the organic phase at the end of the reaction period. In both of these schemes, the e-’s are supplied in the form of BH4- (from NaBH4), and the possible reaction products include (22) Terrill, R. H.; et al. J. Am. Chem. Soc. 1995, 117, 12537.
Gold Nanocrystals Functionalized with Primary Amines H2 (g), sodium borate, and NaCl, among others. The nanocrystal products formed by these two reaction schemes do differ somewhat, and this will be discussed in more detail below. Schemes 1 and 2 were carried out using varying amounts of one of two different amines as surface passivants, dodecylamine and oleylamine (C12H25NH2 and C18H35NH2, respectively). Fresh solutions and appropriate volumetric glassware were used for all reactions. A description of a Scheme 1 synthesis is given here. A yellow solution of 112 mg (0.284 mmol) of HAuCl4.3H2O(s) in 25 mL of deionized water was prepared. Next, 0.365 g (0.667 mmol) of N(C8H17)4Br(s) was dissolved into 25 mL of toluene and added to the aqueous solution of the Au salt while rapidly-stirring. An immediate two-layer separation resulted, with an orange/red organic phase on top and an orange-tinted aqueous phase on the bottom. This mixture was vigorously stirred until all color was removed from the aqueous phase, indicating quantitative transfer of the AuCl4- moiety into the organic phase. Next, 0.574 g (3.10 mmol) of C12H25NH2(s) in 25 mL of toluene was added to the rapidly stirring two-phase mixture. Upon the addition of this solution, the aqueous layer immmediately became beige/murky white (presumably due to the formation of a precipitate complex between the protonated amine and the gold anion, C12H25NH3+AuCl4-(s)).23 Finally, 0.165 g (4.36 mmol) of NaBH4 in 25 mL of deionized water was added to the rapidly stirring mixture. There was an instant color change of the organic phase to black/ brown and then quickly to dark purple. The white precipitate and cloudiness in the aqueous phase dissipated as the reaction proceeded. After 10 min, the aqueous layer was clear and colorless. The reaction was continued (open to ambient atmosphere) for ≈12 h while rapidly stirring. Once the reaction time was finished, the aqueous phase was separated and discarded, and the organic phase was reduced in volume to ≈5 mL by rotary evaporation. The particles were precipitated from the toluene by adding 350 mL of 95% ethanol and cooling to -60 °C for 12 h. The dark purple precipitate was then filtered with 0.65 µm nylon filter paper, washed with an excess of ethanol, and dried on a vacuum line to give ≈60 mg of dry product. The product was analyzed by X-ray diffraction (XRD) to be face-centered cubic (fcc) Au nanocrystals with an avearge domain size of ≈26 Å. Variation of the amine capping agent from dodecylamine to oleylamine did not affect the reaction in any way, nor did it alter the final particle domain size. Scheme 2 reactions were carried out in a manner similar to those of Scheme 1, except that no phase transfer reagent [N(C8H17)4Br(s)] was included. In the course of these reactions, a small amount of black, solid particulate material was produced and separated from the organically soluble material by filtration through 0.65 µm nylon filter paper. The insoluble black material was shown by XRD to be fcc gold with an average particle diameter near 160 Å. The organically-soluble material was treated in an analagous fashion to those reaction products of Scheme 1. The resultant particles were shown by XRD to have an average particle diameter near 55 Å. The fractions of soluble particles prepared in this manner are the Scheme 2 particles referred to throughout the rest of this article. Variation of the initial reactant molar ratios (e.g., Au, N(C8H17)4Br; Au, C12H25NH2; etc.) for both Scheme 1 and Scheme 2 syntheses can produce nanocrystals with a range of average domain sizes. Thus, the various characterization techniques utilized in the present work were necessarily carried out on particles which possess different average domain sizes. A discussion of this “size-altering” chemistry is not included here. Both the Scheme 1 and Scheme 2 particles are extremely soluble (up to 20 mg/mL) in nonpolar organic solvents such as toluene, hexane, and pentane and are insoluble in polar solvents such as water, methanol, and acetone. C. Physical Measurements. 1. Powder X-ray Diffraction. X-ray powder diffraction (XRD) data were taken with Cu KR radiation (λ ) 1.5418 Å), on the UCLA crystal logic powder diffractometer operated in the θ:2θ mode primarily in the 3090° (2θ) range and step-scan of ∆2θ ) 0.1°. Samples from Scheme 1 and 2 were prepared as uniform thin films supported on 0.65 µm pore size nylon filter paper. For all patterns, the observed (23) See, for example: (a) Torigoe, K.; Esumi, K. Langmuir 1992, 8, 59. (b) Ishizuka, H.; Tano, T.; Torigoe, K.; Esumi, K.; Meguro, K. Colloids Surf. 1992, 63, 337. (c) Scherbakova, E. S.; Buslaeva, T. M.; Koteneva, N. A.; Sinitsyn, N. M. Russ. J. Inorg. Chem. 1990, 35 (9), 1366.
Langmuir, Vol. 12, No. 20, 1996 4725 diffraction peaks were those expected for fcc gold based on the bulk lattice constants. The Au nanocrystals were sized by X-ray diffraction peak line-width broadening using the Debye formula for small crystalline spheres.24 All X-ray data were fit to Lorentzian line shapes with Peakfit nonlinear curve-fitting software (Jandel Scientific Co.). All fits gave an r-squared correlation of at least 0.993. The ability of XRD to distinguish different average size samples is well within 10 Å (for particle diameters Scheme 2 amine-capped particles > Scheme 1 aminecapped particles.
Figure 5. Differential scanning calorimetry data for dodecanethiol-capped gold nanocrystals synthesized by Scheme 1 (top) and for dodecylamine-capped particles synthesized by Scheme 1 (middle) and Scheme 2 (bottom). The TOAB label stands for tetraoctylammonium bromide. The dodecanethiolcapped nanocrystals show an endothermic transition beginning at ≈95 °C and peaking at 120 °C (18 J/g). The dodecylaminecapped nanocrystals show a host of features. Both show an exothermic feature beginning near 50 °C. The Scheme 1 particles show an endothermic feature which peaks near 90 °C (7 J/g). The Scheme 2 particles show a sharp, relatively endothermic feature that peaks near 110 °C (4 J/g).
of weight loss for the dodecylamine-capped nanocrystal samples occurred at about 250 °C, which agrees well with the boiling point of neat dodecylamine (bp 247-249 °C). The total weight loss is consistent with the total amount of bonded ligands found by elemental analysis. The total weight loss for the thiol-capped nanoparticles is about 25% and about 8 and 10% for the Scheme 1 and Scheme 2 amine-capped nanoparticles, respectively. These TGA results have particular significance in that they strongly indicate that the gold surfaces of these organically-capped nanocrystals are indeed almost exclusively covered by neutral amine or neutral thiol ligands. 9. Differential Scanning Calorimetry. Figure 5 shows the DSC spectra for dodecanethiol-capped nanocrystals synthesized by Scheme 1 (top) and for aminecapped nanocrystals synthesized by Scheme 1 (middle) and Scheme 2 (bottom). The dodecanethiol-capped nanocrystals (made in the presence of (octyl)4N+Br-) show an endothermic transition beginning at ≈95 °C and peaking at 120 °C (18 J/g). This is in agreement with the desorption threshold of ≈100 °C for dodecanethiol molecules from dodecanethiol single monolayers self-assembled on Au(111).32 For the dodecylamine-capped gold nanocrystals made with (octyl)4N+Br-, we find a broad exothermic transition (or transitions) extending from 50 to 130 °C, which includes a relatively sharp endothermic feature centered at 90 °C (7 J/g). And for the dodecylamine-capped gold nanocrystals made without (octyl)4N+Br- (Scheme 2), we find a much stronger broad exothermic transition beginning at ≈50 °C with a relatively sharp, and relatively endothermic feature peaking near 110 °C (4 J/g). A subsequent rerun of the samples under the same conditions showed that the desorption of the thiol and amine ligands was irreversible. Unfortunately, it is not possible to extract ligand binding energies from the DSC data, since all observed features correspond to a complicated process of ligand desorption and particle aggregation. Nevertheless, our findings do reveal major differences between the thiol-capped particles and the amine-capped particles. Below 150 °C, the (32) Schonenberger, C.; Jorritsma, J.; Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. J. Phys. Chem. 1995, 99, 3259.
A striking result from Figure 5 is the net calorimetric sums of the DSC features over the 170° scan range for each particle system. These features sum to net endothermic for the thiol-capped particles, weakly exothermic for the Scheme 1 amine-capped particles, and strongly exothermic for the Scheme 2 amine-capped particles. This indicates that the Au/alkanethiol system is thermodynamically stable at finite size, consistent with our previous results.21 However, neither Au/alkylamine nanocrystal system is thermodynamically stable with respect to the formation of bulk Au and free ligand, although there does exist a thermal barrier to such decomposition. This suggests that the stability of these particles is kinetic in origin. On the other hand, the presence of (octyl)4N+Brin the Scheme 1 particles does lend some relative thermodynamic stability to the particles, but it also lowers the kinetic barrier corresponding to particle decomposition. IV. Conclusions In this paper we have reported on the unique system consisting of hydrophobic, organically-soluble gold nanocrystals functionalized with primary amines. Specifically, we have reported on the synthesis, characterization, and stability of 25-70 Å average diameter amine-capped gold nanocrystals. We have shown how these particles can be prepared by simple procedures and confirmed the particle composition (including the identity of the amine surface passivant) through a battery of materials characterization techniques that include IR, NMR, UV-vis, MS, XPS, TEM, XRD, DSC, TGA, and elemental analysis. This extensive characterization leads to the following conclusions: (1) the amine/gold surface interaction is charge-neutral and is best described by a weak covalent bond; (2) particle stability is predominantly kinetic, rather than thermodynamic, in nature. This contrasts with the system of thiol-capped Au nanocrystals, which are shown to possess thermodynamic stability with respect to ligand desorption and subsequent particle agglomeration; (3) from a comparison of the amine-capped gold nanocrystal system to amines adsorbed onto bulk Au surfaces, we have interpreted the stability of the nanocrystals to be a finitesize effect. Acknowledgment. We would like to thank Professor Bill Gelbart, Professor Robin Garrell, Professor David Myles, and Ms. Pam Ohara for helpful discussions. We also acknowledge Kibbey Stovall for performing the XPS measurements. This work was supported by a grant from the National Science Foundation-NYI program and by a David and Lucille Packard Fellowship. LA960445U
