Langmuir 1998, 14, 6639-6647
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Influence of a Terminal Functionality on the Physical Properties of Surfactant-Stabilized Gold Nanoparticles S. R. Johnson,† S. D. Evans,*,†,§ and R. Brydson‡ Department of Physics and Astronomy, and School of Process, Environment and Materials Engineering, Leeds University, Leeds LS2 9JT, U.K. Received October 16, 1997. In Final Form: September 1, 1998 Small aromatic thiol molecules HS-C6H4-X, where X ) OH, COOH, NH2, and CH3, have been used to stabilize gold nanosized clusters. The physical properties of the surfactant-coated nanoparticles, both in solution and as thin films, were found to be dependent upon the nature of the functional group X. In particular it was found that particles bearing COOH and NH2 functionalities were prone to significant aggregation through hydrogen bond formation.
Introduction The optical and electrical properties of metallic and semiconductor materials are usually considered to be determined by their bulk electronic structure. However, by reducing the dimensions of the material to small enough sizes, one can change these properties significantly. For example, the effect of sample size on the electronic properties starts to become noticeable as the dimensions of the metal, or semiconductor, become comparable with the bulk mean free path of the electrons. At this point electron scattering from the surfaces of the material becomes important and makes a significant contribution to the electronic transport properties (in the case of nanoparticles the electron mean free path is frequently taken to be the same as the particle radius1). If the dimensions of the material are reduced still further, the transport properties may begin to exhibit effects due to quantum confinement, with conduction now occurring through transitions between quantized eigenstates. In semiconducting media, CdS and CdSe for example, the optical and electronic properties can be readily controlled, since the band gap is dependent on the particle size.2-5 In the noble metal nanoparticles, of interest to us, the optical properties have two significant contributions. First, due to their small dimensions in comparison with the wavelength of light these exhibit Rayleigh scattering. In addition they also show a distinct absorption band due to the excitation of surface plasmons.6,7,8 Decreasing the particle size directly affects the damping frequency ωd of the metal (ωd ) vf /R where vf is the Fermi velocity of electrons within the metal and R is the radius of the particle), which is in turn related to the wavelength at which the surface plasmon absorption occurs.9 Therefore, * To whom correspondence should be addressed. † Department of Physics and Astronomy. ‡ School of Process, Environment and Materials Engineering. § Centre for Self-Organising Molecular Systems. (1) Kreibig, U.; Fragstein, C. v. Z. Phys. 1969, 224, 307. (2) Brus, L. E. J. Chem. Phys. 1984, 80, 4403. (3) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226. (4) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 15, 8706. (5) Colvin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc. 1992, 114, 5221. (6) Kreibig, U.; von Fragstein, C. Z. Phys. 1969, 224, 307. (7) Kreibig, U. J. Phys. F 1974, 999. (8) Henglein, A. J. Phys. Chem. 1993, 97, 5467. (9) Mulvaney, P. Langmuir 1996, 12, 788.
it is possible to tune the optical properties of materials by altering the particle dimensions. To utilize the novel electronic properties of such nanoparticles, it is desirable to be able to control the growth of nanoparticle superstructures. It has been shown that it is possible to form well-defined structures using combinations of self-assembled monolayers (SAMs) and metallic colloids, via sequential deposition.10 Extensions of this procedure using conducting organic spacers to form a network linking the individual nanoparticles have also been demonstrated.11,12 Finally, 3-D structures have been formed on various substrates using alternate nanoparticle-polyelectrolyte adsorption.13 These various investigations have demonstrated that it is possible to control the electronic properties of the superstructures formed via manipulation of the size of the metallic colloids and organic monolayers. In order that we may study the interaction of nanoparticles with a wide range of molecular species from simple organic molecules to proteins, a range of bifunctional surfactants have been studied. In particular, we have chosen to look at short ω-functionalized thiols of the form HS-C6H4-X, where X ) OH, COOH, NH2, and CH3. The use of thiol derivatives has significant advantages over other routes for nanoparticle stabilization in that they allow the precipitation and resolubilzation of the nanoparticles without deterioration of the product.14 The use of small aromatic thiols in this study has the advantage of reducing the thickness of the insulating coating and permits the “core” to “core” distance to be reduced significantly. The potential benefit of this is in studying the electronic transport behavior in nanoparticle films.15,16 Terrill et al. have suggested that the conductivity within films of these nanoparticles is best described in terms of (10) Grabar, K. C.; Allison, K. J.; Baker, B. E.; Bright, R. M.; Brown, K. R.; Freeman, R. G.; Fox, A. P.; Keating, C. D.; Musick, M. D.; Natan, M. J. Langmuir 1996, 12, 2353. (11) Bethell, D.; Brust, M.; Schiffrin, D. J.; Kiely, C. J. Electroanal. Chem. 1996, 409, 137. (12) Dorogi, M.; Gomez, J.; Osifchin, R.; Andres, R. P.; Reifenberger, R. Phys. Rev. B 1995, 52, 9071. (13) Kotov, N. A.; Dekany, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 13065. (14) (a) Badia, A.; Demers, L.; Dickinson, L.; Morin, G.; Lennox, R. B.; Reven, L. J. Am. Chem. Soc. 1997, 119, 11104. (b) Alvarez, M. M.; Khoury, J. T.; Schaaff, T. G.; Shafigullin, M. N.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 1997, 101, 3706. (c) Hostetler, M. J.; Wingate, J. E.; Zhong, C.-J.; 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.
10.1021/la9711342 CCC: $15.00 © 1998 American Chemical Society Published on Web 10/16/1998
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an activated tunneling/hopping process, in which the tunneling term decays exponentially with increasing corecore separation and is thus highly sensitive to ligand length (approximately 1 order of magnitude per angstrom increase in separation). The activation energy term is dependent on the core size and the dielectric medium separating the cores (i.e., the ligand shell, adsorbed solvent, and vacancies).17 Changing the ω-functionality of the surfactant stabilizing the nanoparticles not only affects how the particles interact with other molecular species but also how they interact with each other. To date there have been a variety of investigations into ω-functionalized nanoparticles; these have been concerned with long chain surfactants and a small number of terminal functionalities for specific investigations.18 The emphasis of the work presented here however is on how the nature of the terminal group affects the particle-particle interactions both in solution and in thin films. Experimental Section Synthesis of Thiol-Stabilized Gold Nanoparticles. The synthesis of the gold nanoparticles was performed using the following materials: hydrogen tetrachloroaurate, 99.999% (chloroauric acid); sodium borohydride, 98%; methanol, 98% (HPLC grade); HS-C6H4-OH, 90% (sample I); HS- C6H4-NH2, 90% (sample III); and HS-C6H4-CH3, 98% (sample IV), which were obtained from Aldrich. The HS-C6H4-COOH, 97% (sample II) derivative was purchased from Toronto Research Chemicals. Solutions of all reagents employed were formed using standard volumetric techniques, and all glassware was cleaned using Piranha solution.19 The nanoparticle synthesis was similar to that outlined by Brust et al.20 and was carried out as follows: Step 1: 20 mL of a bright yellow HAuCl4‚3H2O (methanol) (0.053 M) solution was added to a flask at 20 °C. Step 2: 20 mL of a clear colorless thiol solution (methanol) (0.095 M) was added; on addition there was no change in the color of the solution. Step 3: 10 mL of acetic acid was added to the mixture. (There was no color change.) Step 4: 10 mL of a NaBH4 (0.891 M) solution was added, while the mixture was being vigorously stirred. There was an immediate color change of the solution from yellow to black in the flask. The solution was then left, for 3 h to achieve thermodynamic equilibrium, at 20 °C, while being continually stirred. Step 5: The solvent was evaporated off under a steady stream of N2. Step 6: The product was washed three times with diethyl ether and water and dried under a stream of N2, before being finally dried on a vacuum line. N.B. Sample IV was synthesized using a route described previously.21 The nanoparticles formed as described above had the gold/sulfur ratios in solution 0.55:1, 0.74:1, 0.73:1, and 0.85:1 (15) Terrill, R. H.; Postlethwaite, T. A.; Chen, C.; Poon, C.; Terzis, A.; Chen, A.; Hutchison, J. E.; Clark, M. R.; Wignall, G.; Londono, J. D.; Superfine, R.; Falvo, M.; Johnson, C. S.; Samulski, E. T.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 12537. (16) Musick, M. D.; Keating, C. D.; Keefe, M. H.; Natan, M. J. Chem. Mater. 1997, 9, 1499. (17) Sheng, P.; Abeles, B.; Arie, Y. Phys. Rev. Lett. 1973, 31, 44. (18) (a) Ingram, R. S.; Hostetler, M. J.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 9175. (b) Sastry, M.; Patil, V.; Mayya, K. S. Langmuir 1997, 13, 4490. (c) Templeton, A. C.; Hostetler, M. J.; Warmoth, E. K.; Chen, S.; Hartshorn, C. M.; Krishnamurthy, V. J.; Forbes, M. D. E.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 4845. (d) Templton, A. C.; Hostetler, M. J.; Kraft, C. T.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 1906. (19) Caution: A solution of H2O2 in H2SO4 is a very strong oxidant and reacts violently with many organic materials. It should be handled with extreme care. (20) Brust, M.; Fink, J.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. J. Chem. Soc. Chem Commun. 1995, 1655. (21) Johnson, S. R.; Evans, S. D.; Mahon, S. W.; Ulman, A. Langmuir 1997, 13, 51.
Johnson et al.
Figure 1. Schematic representation of the surface of a gold nanoparticle, indicating the range of short aromatic thiol molecules used in this study. for samples I-IV, respectively. The nanoparticles formed were soluble in a variety of solvents, dependent on the nature of the ω-functionality. Samples I, II, and III, for example, were soluble in polar solvents such as methanol, while sample IV was only soluble in nonpolar solvents. Transmission Electron Microscopy. Bright field images were obtained using a Philips CM20 TEM operating at 200 kV. The samples were prepared by placing a dilute solution containing the nanoparticles on a carbon-coated copper grid (400 Mesh) and allowing the solvent to evaporate. The nanoparticle size distributions were determined via a Kontron Electronik Image Analyzer using IBAS software version 2.5. Atomic Force Microscopy. Tapping-mode atomic force microscopy (AFM) was performed using a Nanoscope III (Digital Instruments, Santa Barbara, CA) using a silicon nitride etched tip (force constant ) 100 N/m, radius of curvature of tip ) 5-10 nm). The samples were prepared by placing one or two droplets of the appropriate solution onto a freshly cleaved mica surface and allowing the solvent to evaporate. To minimize drift in the signal, the apparatus was allowed to equilibrate for 5 h before any scans were recorded. X-ray Photoelectron Spectroscopy. XPS measurements were made using the Scienta ESCA-300 instrument at Daresbury Laboratories, Warrington, U.K. A monochromated Al KR X-ray source at 1487.6 eV was used. C 1s, O 1s, Au 4f, and S 2p levels were recorded, at an electron takeoff angle of 90°. The base pressure in the sample chamber was
