5612
Langmuir 1998, 14, 5612-5619
Three-Dimensional Monolayers: Voltammetry of Alkanethiolate-Stabilized Gold Cluster Molecules Stephen J. Green,† Jeremy J. Pietron, Jennifer J. Stokes, Michael J. Hostetler, Han Vu, W. Peter Wuelfing, and Royce W. Murray* Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290 Received April 1, 1998. In Final Form: July 15, 1998 Voltammetry of nonspecifically adsorbed and of freely diffusing monolayer protected clusters (MPCs) is presented. The MPC monolayers are mixtures of n-octanethiolates and ω-functionalized ferrocenyloctanethiolates. MPC adsorption coverages range from a few percent to roughly a full monolayer of cluster molecules. Rotated disk electrode voltammetry of the ferrocenated MPCs has two principal features: the ferrocene oxidation wave and sloping current baselines at prewave and postwave potentials. Each MPC molecule can have multiple ferrocene units; characteristics of the ferrocene wave indicate that the polyelectron-transfer oxidations occur as a rapid sequence of single electron transfers. Comparisons of different modes of polyelectron transfer suggest that the present one involves rotational diffusion. Deviation from ideal Nernstian one-electron-transfer behavior is modeled as a Gaussian distribution of E0′ values. The sloping current baselines are attributed to double layer charging of the cluster cores that is controlled by hydrodynamic mass transport. Further aspects of previously described relations governing the corecharging property are compared to and found to be consistent with experimental behavior. Finally, preliminary experiments for determining MPC diffusion coefficients by the Taylor dispersion method are described.
Following the seminal report by Brust et al.1 on the synthesis of gold clusters stabilized by monolayers of alkanethiolate ligands (monolayer protected clusters, MPCs), we launched an investigation of their synthesis as to core size andsin ω-functionalized-alkanethiolate formssof their chemical, electrochemical, structural, and physical properties.2,3 Pertinent to the present report are recent synthetic and voltammetric results2,3b showing that ω-ferrocenated MPCs not only exhibit ferrocene electrochemistry but also exhibit a double layer charging phenomenon analogous to that of a metal electrode/ solution interface. The presence of the double layer capacitative behavior means, in effect, that the cluster molecules act as tiny electrodes dissolved in the electrolyte solution around the working electrode, i.e., that they are freely diffusing nanoelectrodes.2 A mass transportcontrolled double layer charging model was used to analyze the steady state, rotated disk electrode voltammetric results. MPC core surface area-normalized double layer capacitances thus obtained were similar to those4 of alkanethiolate self-assembled monolayers on flat Au surfaces.2 Diffusion coefficients, and thereby estimations of MPC hydrodynamic radii, were also described. This paper presents further results of voltammetry performed on ω-ferrocenated alkanethiolate Au clusters, using cyclic voltammetry (CV, a transient method) and rotated disk voltammetry (RDE, a steady state method). † Present address: University of Exeter, Department of Chemistry, Exeter, Devon, EX4 4QD, U.K.
(1) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D.J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801-802. (2) Green, S. J.; Stokes, J. J.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W. J. Phys. Chem. B 1997, 101, 2663-2668. (3) 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.; 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.; Wingate, J.; Zhong, C.-J.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. W.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir, 1998, 14, 17-30.
The CV results reveal physisorption of clusters on the electrode. The RDE experiments further test our previous analysis of steady state voltammetry for values of MPC capacitances and diffusion coefficients, including the behavior of cluster solutions that are initially mixed valent. A further analysis of the steady state, RDE ω-ferrocenated cluster wave shape is conducted to address deviations from ideal thermodynamic one-electron behavior, using a model like that previously employed5 on flat Au surfaces coated with ω-ferrocene-alkanethiolate monolayers, in which the ferrocenes are assumed to exhibit a Gaussian distribution of formal potentials. The Taylor dispersion method was unsuccessfully explored as an alternate route to diffusion constants. The MPC samples used previously2,3b and here are modestly polydisperse in core size. Accordingly, the reported diffusion coefficient and double layer capacitances are averages over the different cluster sizes present. Since the previous study, we have also reported6 differential pulse voltammetry (DPV) and CV of solutions of MPCs that are monodisperse in core size, in which the uniform double layer capacitances of individual clusters allowed observation of discrete DPV and CV current peaks. This phenomenon, which is an electrochemical analogue of a “Coulomb staircase”, is not seen in the present work, owing to the averaging over the mixture of cluster core sizes and thus capacitances. Experimental Section Chemicals. Tetrabutylammonium perchlorate, Bu4NClO4 (Fluka, >99%), was dried under vacuum at 80 °C, dichlo(4) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568. (5) Rowe, G. K.; Carter, M. T.; Richardson, J. N.; Murray, R. W. Langmuir 1995, 11, 1797-1806. (6) (a) Ingram, R. S.; Hostetler, M. J.; Murray, R. W.; Schaaf, T. G.; Khoury, J. T.; Whetten, R. L.; Bigioni, T. P.; Guthrie, D. K.; First, P. N. J. Am. Chem. Soc. 1997, 119, 9279-9280. (b) Chen, S.; Ingram, R. S.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W.; Schaaff, T. G.; Khoury, J. T.; Alvarez, M. M.; Whetten, R. L. Science 1998, 280, 2098-2101.
S0743-7463(98)00362-X CCC: $15.00 © 1998 American Chemical Society Published on Web 08/26/1998
Three-Dimensional Monolayers romethane (Mallinckrodt, 99.9%) was dried over 4Å molecular sieves, and acetonitrile (Fisher, 99.9%) was redistilled from CaH2. Octanethiolate-stabilized gold MPCs were prepared by two different procedures (previously described2,3a). One involved reaction at room temperature of a 1:1 mole ratio of octanethiol to AuCl4-, which gives a product with an average 2.0 nm diameter core that we have estimated3c to be a mixture of Au225 and Au314 MPCs. Voltammetric results for this preparation are calculated assuming the Au314 core size, which would have ca. 90 alkanethiolate chains. The second MPC preparation involved reaction of a 3:1 mole ratio of thiol to AuCl4- at 0 °C, giving a Au140 product with an average 1.6 nm diameter core with an estimated 46 chain monolayer. All of the MPC compositions are somewhat approximate being averages of somewhat polydisperse (in core size) samples and involving an assumed core shape (truncated octahedral).7 ω-Ferrocenyloctanethiol was place-exchanged3b onto these MPCs producing, by varying the molar feed ratio of reactants (ratio of ω-ferrocenyl octanethiol to octanethiolates on the MPC), clusters bearing mole ratios of ω-ferrocenyloctanethiolate chains to octanethiolate chains (abbreviated C8Fc/C8) of 1:5.5, 1:11.5, and 1:24 for the Au314 MPC, and 1:2.6, 1:3.7, 1:11.5, and 1:32 for the Au140 MPC. The mole ratio analysis was based, as before,3b on the integrals of proton NMR resonances of the MPC ω-ferrocene and methyl protons. The chemical oxidant iron(III) phenanthroline was made via bulk electrolysis of in-house synthesized iron(II) phenanthroline perchlorate. Electrochemical Measurements. Cyclic voltammetry was conducted with a locally constructed potentiostat with PCcontrolled potential waveform and current acquisition; RDE experiments relied on an MSR rotator (Pine Instruments Company). The electrodes were 0.15 cm2 glassy carbon and 0.12 cm2 Pt electrodes. The glassy carbon and Pt electrodes were, respectively, polished between experiments with 0.25 µm diamond and 0.05 µm alumina polishing compounds (Buehler) and rinsed with isopropyl alcohol and water (Barnstead Nanopure, >18 MΩ/cm2) and water, before each use. The cell was described previously.2 RDE experiments were conducted at rotation rates of 400-3600 rpm, and CV experiments, with potential scan rates of 5-25mV/s, in degassed CH2Cl2 solutions that were micromolar in cluster and 0.1 M in Bu4NClO4. Chemical Oxidation of MPCs. The ferrocene sites in CH2Cl2/CH3CN (95:5 v:v) solutions of ferrocenated MPCs were halfoxidized by incrementally adding microliter volumes of iron(III) phenanthroline solution (mM concentrations in 0.1 M Bu4NClO4/ CH3CN), until the E1/2 of the resulting RDE voltammetric wave was approximately centered on the zero current axis of the voltammogram. The dilution of the initial MPC concentration was less than 2%. The acetonitrile solvent component was needed for adequate solubility of the metal complex, but caused some changes in MPC adsorption, as noted later. Taylor Dispersion Experiments. Peak widths of plugs of MPC solution injected into an open tubular column were measured with the aim of measuring MPC diffusion coefficients and, thereby, hydrodynamic radii. The experiments were done with a Waters Model 510 HPLC pump, U6K injector, and Model 410 differential refractometer detector, with data plotted using a Waters 745B Data Module (Millipore Corporation, Milford, MA). The capillary column was 1/16 in. i.d. × 0.030 in. o.d. × 50 ft length PEEK tubing (Alltech, Deerfield, IL). Curve Fitting and Digital Simulation. Experimental RDE wave shapes were matched to simulated RDE voltammograms that were calculated from the hydrodynamic wave equation8 assuming a Gaussian distribution of formal potentials, using a commercial spreadsheet program (Sigmaplot for Windows, version 2.1, Jandel Scientific, San Rafeal, CA). Simulated cyclic voltammograms, used in estimations of cluster adsorption, were generated by Digisim (version 2.1, Bioanalytical Systems, West Lafayette, IN). (7) (a) 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. (b) Luedtke, W. D.; Landman, U. J. Phys. Chem. 1996, 100, 13323-13329. (8) Bard, A. J., Faulker, L. R. Electrochemical Methods; Wiley: New York, 1980, Ch. 8.
Langmuir, Vol. 14, No. 19, 1998 5613
Figure 1. Cyclic voltammetric parameters for 1 µM Au314 MPC with 1:5.5 C8Fc:C8 composition, (corresponds to ca. 15 Fc/MPC), in 0.1 M Bu4NClO4/CH2Cl2 at 0.12 cm2 Pt disk. Top: CV at 50 mV/s. Bottom: (b) ∆EPEAK (mV); (2) 10 × peak current (µA); (9) 1011 × adsorbed ferrocene coverage (mol/cm2).
Results and Discussion Ferrocenated MPC Adsorption on Electrodes. Alkanethiolate and ω-ferrocene-alkanethiolate MPCs are nonpolar and poorly soluble in most polar solvents. That ferrocenated MPCs adsorb on electrodes from CH2Cl2 was evident3b in our initial cyclic voltammetric observations. The adsorption is weak and reversible, being readily rinsed away by transfer of the electrode to fresh solvent. The tendency toward MPC adsorption has directed our electrochemical studies of MPCs to methods such as RDE voltammetry, since reactions of adsorbed species do not contribute to steady state currents at sufficiently slow potential sweep rates. It is nonetheless important to inspect the extent and characteristics of the adsorption process, and an brief analysis of ferrocenated MPC cyclic voltammetry is presented next. Figure 1(top) shows an illustrative cyclic voltammogram (50 mV/s) at a Pt electrode in a micromolar CH2Cl2 solution of a ferrocenated MPC bearing ca. 7.7 Fc/cluster. The voltammogram’s oxidation peak current (2) is roughly proportional to the potential scan rate (v), and the wave shape is roughly symmetrical with a peak potential separation (∆EPEAK, b) at 50 mV/s that is only 12 mV. These factors9 are all consistent with most of the current arising from reaction of an adsorbed layer of ferrocenated MPC. (The possibility that the small ∆EPEAK arises from a multielectron transfer from diffusing, multiply ferrocenated MPC is eliminated by an analysis of steady-state RDE results that is discussed later.) At smaller potential scan rates, ∆EPEAK grows since on longer time scales more of the current arises from diffusing MPCs. At larger scan rates, the increase (b) in ∆EPEAK can be attributed to uncompensated resistance and/or kinetic effects. The latter ∆EPEAK changes are most likely uncompensated resistance effects, but the detailed studies required for a certain assignment have not been carried out. Figure 1(9) shows the adsorbed ferrocene coverage Γ calculated from the charge under the CV wave at different potential scan rates. The increase in apparent coverage seen at slow scan rates arises from the increasing relative (9) Murray, R. W. in Molecular Design of Electrode Surfaces; R. W. Murray, Ed. Wiley: New York, 1992, Chapter 1.
5614 Langmuir, Vol. 14, No. 19, 1998
Green et al.
Table 1. Summary of Results for Adsorption of Ferrocenated MPCs on Glassy Carbon Electrodes fractional MPC covgc expt
C8Fc:C8a
θFc
no. 1 Au314 no. 2 Au314 no. 3 Au314 no. 4 Au314 no. 5 Au314 no. 6 Au314 no. 7 Au314 no. 8 Au140 no. 9 Au140 no. 10 Au140 no. 11 Au140 50% oxidn no. 12 Au140 50% oxidn no. 13 Au140 no. 14 Au140 50% oxidn no. 15 Au140 no. 16 Au140 50% oxidn no. 17 Au140 no. 18 Au140 50% oxidn
1:24 1:5.5 1:5.5 1:5.5 1:5.5 1:9.5 1:9.5 1:11.5 1:11.5 1:11.5 1:3.7 1:3.7 1:2.6 1:2.6 1:32 1:32 1:32 1:32
4.0 14.6 14.6 14.6 14.6 9.0 9.0 3.7 3.7 3.7 9.9 9.9 13 13 1.4 1.4 1.4 1.4
b
Fc coveraged 10-11 mol/cm2
MPC concn, µM
oxidn
redn
oxidn
redn
1.4 1.1 0.6 2.5 2.0 1.5 1.4 26 13 7.8 11 17 18 18 43 43 63 63
0.08 0.06 0.015 0.05 0.1 0.05 0.04 0.02 0.08 0.01 0.4 1.3 0.8 0.7 0.6 0.9 0.9 1.0
0.1 0.08 0.02 0.1 0.2 0.08 0.06 0.08 0.2 0.017 0.5 1.5 1.0 1.1 0.4 1.1 0.7 1.1
0.75 2.2 0.54 2.0 5.3 1.2 1.0 0.17 0.8 1.3 12 34 30 26 2.6 3.7 3.4 3.9
1.2 2.8 0.8 3.7 9 1.8 1.4 0.8 1.6 1.7 14 40 36 38 1.7 4.4 2.8 4.1
a Relative number of ω-ferrocene-hexanethiolate and hexanethiolate chains on MPC, by proton NMR. b Estimated number of Fc per MPC, based on 90 chains/MPC for Au314 and 46 chains for Au140. c Based on full coverage of MPC as 2.4 × 10-11 mol/cm2. d From integration of CV wave at v g 200 mV/s with correction for diffusing MPC charge.
contribution of diffusing MPCs there. At v g 50mV/s, Γ is ca. 1.4 × 10-11 mole/cm2 of Fc, or at ca. 7.7 Fc/MPC, ca. 1.8 × 10-12 mole/cm2 of adsorbed MPCs. On the basis of hexagonal close-packing of MPCs with 1.5 nm radius, a full MPC monolayer would be ca. 2.4 × 10-11 mol/cm2, so the adsorption in Figure 1 amounts to only a small fraction of a monolayer (ca. 8%). Reaction of this small amount of adsorbed ferrocenated MPC dominates the Figure 1 voltammetry only because the diffusion flux from the 1 µM MPC solution is itself so small. Table 1 shows a collection of adsorption results from CV at glassy carbon electrodes in CH2Cl2 and CH2Cl2/ CH3CN solvents, for MPCs with various ferrocene loadings. The adsorption coverages were taken by integrating the charge under the ferrocene oxidation CV waves (at g200 mV/s). This analysis was refined by subtracting the charge expected to arise from diffusing MPC (based on integrating simulated CVs calculated for RDEmeasured diffusion coefficients; the correction amounted to factors of 1.4-1.8). While this procedure is approximate compared to the well-known double potential step chronocoulometry approach10 to measuring adsorption, it suffices for the present assessment. The Table 1 results for adsorbed ferrocene coverage range from 0.5 × 10-11 to 34 × 10-11 mol/cm2, or when expressed as a fraction of a full MPC monolayer, from
