Nanoparticle Films as Electrodes - American Chemical Society

May 18, 2007 - Metal electrodes have a continuum of electronic levels, and redox ... in voltammetry.4 The energy gaps of the latter nanoparticles are ...
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Langmuir 2007, 23, 7372-7377

Nanoparticle Films as Electrodes: Voltammetric Sensitivity to the Nanoparticle Energy Gap Srikanth Ranganathan,† Rui Guo,‡ and Royce W. Murray* Kenan Laboratories of Chemistry, UniVersity of North Carolina, Chapel Hill, North Carolina 27599-3290 ReceiVed January 2, 2007. In Final Form: April 10, 2007 Electron-transfer reactions of redox solutes at electrode/solution interfaces are facilitated when their formal potentials match, or are close to, the energy of an electronic state of the electrode. Metal electrodes have a continuum of electronic levels, and redox reactions occur without restraint over a wide span of electrode potentials. This paper shows that reactions on electrodes composed of films of metal nanoparticles do have constraints when the nanoparticles are sufficiently small and molecule-like so as to exhibit energy gaps, and resist electron transfers with redox solutes at potentials within the energy gap. When solute formal potentials are near the electronic states of the nanoparticles in the film, electron-transfer reactions can occur. The electronic states of the nanoparticle film electrodes are reflected in the formal potentials of the electrochemical reactions of the dissolved nanoparticles at naked metal electrodes. These ideas are demonstrated by voltammetry of aqueous solutions of the redox solutes methyl viologen, ruthenium hexammine, and two ferrocene derivatives at films on electrodes of 1.1 nm core diameter Au nanoparticles coated with protecting monolayers of phenylethanethiolate ligands. The methyl viologen solute is unreactive at the nanoparticle film electrode, having a formal potential lying in the nanoparticle’s energy gap. The other solutes exhibit electron transfers, albeit slowed by the electron hopping resistance of the nanoparticle film. The nanoparticles are not linked together, being insoluble in the aqueous medium; a small amount of an organic additive (acetonitrile) facilitates observing the redox solute voltammetry.

Introduction Small metal and semiconductor nanoparticles are active research topics owing, among others, to their potential applications in catalysis and in bioanalytical chemistry, to the scientific importance of the metal-to-molecule transition, and to the technological importance of achieving continued diminution of feature sizes in microelectronics fabrication.1 Our and other laboratories2 have investigated nanoparticles composed of Au nanocrystals stabilized by a firmly affixed monolayer of organothiolate ligands. These monolayer-protected Au clusters (MPCs) provide an attractive vehicle to explore the metal-tomolecule transition. By manipulation of the conditions of MPC synthesis and the choice of the protecting monolayer thiol,3 MPC core sizes can be reduced to dimensions producing quantized, one-electron electrochemical double layer charging and, at smaller dimensions, molecule-like optical band edges and energy gaps in voltammetry.4 The energy gaps of the latter nanoparticles are presumably devoid of electronic states that could support electron* To whom correspondence should be addressed. E-mail: [email protected]. † Present address: Nanosys, Inc., Palo Alto, California, 94304. ‡ Present address: Wilson Greatbatch, Buffalo, New York. (1) (a) Schmid, G., Ed. Clusters and Colloids; VCH: Weinheim, 1994. (b) Schmid, G., Ed. Nanoparticles: From Theory to Applications; Wiley-VCH: Weinheim, 2004. (c) Klabunde, K. J., Ed. Nanoscale Materials in Chemistry; Wiley-Interscience: New York, 2001. (d) Fendler, J. H., Ed. Nanoparticles and Nanostructured Films; Wiley-VCH: Weinheim, 1998. (e) Brus, L. E. J. Phys. Chem. 1986, 90, 2555. (f) Heath, J. R. Science 1995, 270, 1315. (g) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270, 1335. (h) Weller, H. Angew. Chem., Int. Ed. 1993, 32, 41. (i) Alivisatos, A. P. Science 1996, 271, 933. (j) Feldheim, D. L.; Keating, C. D. Chem. Soc. ReV. 1998, 27, 1. (k) Hayat, M. A., Ed. Colloidal Gold: Principles, Methods and Applications; Academic Press: San Diego, CA, 1991. (2) (a) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (b) 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. (c) 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. (d) Templeton, A. C.; Wuelfing, P. W.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27.

transfer reactions with other species at energies within the gap; this paper explores this premise in a study of the voltammetry of dissolved redox probes at films of Au MPC nanoparticles coated on Au electrodes. The Au nanoparticles employed here have nearly monodisperse 1.1-1.2 nm diameter cores protected by a monolayer of phenylethanethiolate ligands (PhC2S-).5 While we originally interpreted the analytical data5a,b for this nanoparticle as a Au38 core, new mass spectrometry results5c,d unequivocally identify (3) For example: (a) Hostetler, M. J.; Wingate, J. E.; Zhong, C.-Z.; 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. (b) Woehrle, G. H.; Warner, M. G.; Hutchison, J. E. J. Phys. Chem. B 2002, 106, 9979. (c) Woehrle, G. H.; Brown, L. O.; Hutchison, J. E. J. Am. Chem. Soc. 2005, 127, 2172. (d) Brown, L. O.; Hutchison, J. E. J. Am. Chem. Soc. 1999, 121, 882. (e) Song, Y.; Murray, R. W. J. Am. Chem. Soc. 2002, 124, 7096. (f) Hostetler, M. J.; Templeton, A. C.; Murray, R. W. Langmuir 1999, 15, 3782. (g) Kell, A. J.; Workentin, M. S. Langmuir 2001, 17, 7355. (h) Lee, D.; Donkers, R. L.; DeSimone, J. M.; Murray, R. W. J. Am. Chem. Soc. 2003, 125, 1182. (i) Hostetler, M. J.; Green, S. J.; Stokers, J. J.; Murray, R. W. J. Am. Chem. Soc. 1996, 118, 4212. (j) Guo, R.; Song, Y.; Wang, G.; Murray, R. W. J. Am. Chem. Soc. 2005, 127, 2752. (k) Ionita, P.; Caragheorgheopol, A.; Gilbert, B. C.; Chechik, V. J. Am. Chem. Soc. 2002, 124, 9048. (l) Tsunoyama, H.; Negishi, Y.; Tsukuda, T. J. Am. Chem. Soc. 2006, 128, 6036. (m) Negishi, Y.; Takasugi, Y.; Sato, S.; Yao, H.; Kimura, K.; Tsukuda, T. J. Phys. Chem. B 2006, 110, 12218. (4) (a) 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. (b) Yang, Y.; Chen, S. Nano Lett. 2003, 3, 75. (c) Chen, S.; Murray, R. W.; Feldberg, S. W. J. Phys. Chem. B 1998, 102, 9898. (d) Schaaff, T. G.; Shafigullin, M. N.; Khoury, J. T.; Vezmar, I.; Whetten, R. L.; Cullen, W. G.; First, P. N.; Gutie´rrez-Wing, C.; Ascensio, J.; Jose-Yacama´n, M. J. J. Phys. Chem. B 1997, 101, 7885. (e) Templeton, A. C.; Cliffel, D. E.; Murray, R. W. J. Am. Chem. Soc. 1999, 121, 7081. (f) Quinn, B. M.; Liljeroth, P.; Ruiz, V.; Laaksonen, T.; Kontturi, K. J. Am. Chem. Soc. 2003, 125, 6644. (g) Lee, D.; Donkers, R. L.; Wang, G.; Harper, A. S.; Murray, R. W. J. Am. Chem. Soc. 2004, 126, 6193. (h) Balasubramanian, R.; Guo, R.; Mills, A. J.; Murray, R. W. J. Am. Chem. Soc. 2005, 127, 8126. (5) (a) Donkers, R. L.; Lee, D.; Murray, R. W. Langmuir 2004, 20, 1945. (b) Jimenez, V. L.; Georganopoulou, D. G.; White, R. J.; Harper, A. S.; Mills, A. J.; Lee, D.; Murray, R. W. Langmuir 2004, 20, 6864. (c) While the initial results interpreted as a Au38 composition were sound, a more definitive set of experiments based on electrospray mass spectrometry5d show that our preparations contain larger populations of Au25L18 MPCs. (d) Tracy, J. B.; Kalyuzhny, G.; Crowe, M. C.; Balasubramanian, R.; Choi, J.-P.; Murray, R. W. J. Am. Chem. Soc., published online May 4, http://dx.doi.org/10.1021/ja071042r.

10.1021/la070008n CCC: $37.00 © 2007 American Chemical Society Published on Web 05/18/2007

Nanoparticle Films as Electrodes Scheme 1. Formal Potentials of the MPC1+/0 and MPC2+/1+ Couples and of Indicated Redox Probe Couplesa

a The formal potentials of the MPC1+/0 and MPC2+/1+ couples are centered within two pairs of vertical lines that represent potential ranges (see red arrows) within which the concentration ratios of the MPC1+/0 and MPC2+/1+ states change from 100:1 to 1:100. The formal potentials ([) of the two ferrocene redox probe couples lie within the range of the MPC2+/1+ states; that is, they should be oxidized by the MPC2+ states. The formal potential of the Ru complex lies in the MPC1+/0 range, and oxidation by MPC1+ is expected, at least thermodynamically. The formal potential of the MV2+/1+ couple lies outside the MPC potential ranges.

the predominance of MPC cores with core Au atom counts of 25. We will abbreviate the nanoparticles in this paper simply as Au MPCs. These very small Au25 nanoparticles display interesting spectral, electrochemical, and electronic conductivity properties.3h,4g,5a,b,6 Optical band edge data5a,b suggest a highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) energy gap of 1.3 eV, which is consistent with the potential gap in their solution voltammetry,4g after accounting for charging energy. Oxidative voltammetry reveals a pair of one-electron steps spaced by ∼0.3 V that is thought to reflect a doubly occupied HOMO level. We will label these redox steps as MPC0/1+ and MPC2+/1+. Another doublet of electron-transfer steps at more positive potentials defines a second molecular orbital. Voltammetry of this MPC is clearly moleculelike, and on that basis, we expect an energy gap devoid of electronic states that could support electron-transfer reactions with other species within the energy gap. The preceding expectation was tested by inspecting reactions of a set of dissolved redox probes at a MPC/solution interface. The MPCs were used as a film coating on a Au electrode, with the potential of the MPC/solution interface being controlled by the potential applied to the underlying Au electrode, and with the charge being transported to the MPC/solution interface by electron hopping within the MPC film. The potentials of the redox states of the solid MPC film are assumed to be adequately represented by their formal potentials as dissolved electrode reactants. The redox probes were chosen to test their reactivity at potentials where the MPC film has redox reactivity (and thus accessible electronic states) and at potentials within the band gap where the film presumably lacks electronic states and should not show reactivity. The experimental plan is outlined in Scheme 1. The potential ranges encompassed by the two pairs of dashed vertical lines are centered on the formal potentials of the MPC1+/0 and MPC2+/1+ redox couples. The 0.24 V width of the potential (6) Choi, J. P.; Murray, R. W. J. Am. Chem. Soc. 2006, 128, 10496.

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ranges represents expected Nernstian changes of ratios of ox/red nanoparticle states from 100:1 to 1:100 (from left to right across the range). Potentials at the range edges correspond to those at which, thermodynamically, at least 1% of the nanoparticle population is in either the oxidized or reduced state. It is assumed that the thermodynamics of dilute CH2Cl2 solution voltammetry4g of the MPCs can, using ferrocene as a reference potential,7 be used to represent the aqueous redox potentials of the MPCs in the electrode film. Also indicated in the Scheme are the aqueous formal potentials of four redox probes: methyl viologen (MV2+/1+), ruthenium hexammine (Ru(NH3)63+/2+), ferrocenylmethyltrimethyl ammonium (FcTMA2+/1+), and 1,1′-ferrocenedimethanol (FcMeOH1+/0). The potentials for the two ferrocene couples lie within the range of the MPC2+/1+ redox couple, and that of the Ru(NH3)63+/2+ lies within the range of the MPC1+/0 redox couple. One expects then that those dissolved redox probes would be oxidized by electron transfers from the MPC2+ and MPC1+ electronic states at the MPC/solution interface, respectively. In contrast, the formal potential of the MV2+/1+ couple lies well into the Au MPC energy gap and falls short of accessing the MPC-1 state (not possible in aqueous medium), and it should thereby be unreactive at the MPC/solution interface. The nanoparticles in the films comprising the electrodes are not linked together, with the intent of not disturbing their electronic energy structures with new ligands. The Au MPCs remain as films because they are not soluble in aqueous electrolyte. A small volume percentage of an organic component (acetonitrile) is added to the aqueous phase to facilitate ion permeation into the MPC interphase. Some films are simply cast atop a metal (Au) disk electrode and are contacted by the redox probe solution (these are labeled (redox)/Au MPC film/Au electrodes). Other MPC films are overcoated with a film of Nafion, into which the cationic redox couples are sequestered from solutions, and the electrode is then transferred to a probe-free electrolyte solution (labeled Nafion(redox)/Au MPC film/Au electrodes; without the MPCs, these are labeled just Nafion(redox)/Au electrodes). The results confirm that Au MPCs retain their discrete energy levels even when they are contacted by a nonsolvent, and they also indicate that, in the specific experiments described, the rate of electron hopping through the Au MPC film is probably the current limiting step in the electron-transfer process between the electrode and redox systems. Scheme 1 can be recognized as an extension of older ideas underlying “bilayer electrodes” made from redox polymer films.8 Other explorations of nanoparticles as electrode surfaces have used linked-together, solvent-swollen nanoparticle films on rotated disk electrodes.9a Dynamic contact angles9c have been measured on potential-controlled, unlinked MPC films that are contacted by a sessile droplet of an ionic liquid. Experimental Section Chemicals. Phenylethanethiolate-protected Au MPCs were prepared in a similar manner to those described in previous work.5a Sodium p-toluenesulfonate (NaTos, Aldrich), methyl viologen dichloride hydrate (MV2+, Aldrich), hexammineruthenium(III) (7) Noviandri, I.; Brown, K. N.; Fleming, D. S.; Gulyas, P. T.; Lay, P. A.; Masters, A. F.; Phillips, L. J. Phys. Chem. B 1999, 103, 6713. (8) (a) Abruna, H. D.; Denisevich, P.; Umana, M.; Meyer, T. J.; Murray, R. W. J. Am. Chem. Soc. 1981, 103, 1. (b) Pickup, P. G.; Murray, R. W. J. Electrochem. Soc. 1984, 131, 833. (c) Pickup, P. G.; Kutner, W.; Leidner, C. R.; Murray, R. W. J. Am. Chem. Soc. 1984, 106, 1991. (9) (a) Brennan, J. L.; Branham, M. R.; Hicks, J. F.; Osisek, A. J.; Donkers, R. L.; Georganopoulou, D. G.; Murray, R. W. Anal. Chem. 2004, 76, 5611. (b) The absence of Au oxide formation currents in pure aqueous media shows that the MPC film completely covers the underlying Au electrode. (c) Wang, W.; Murray, R. W. Anal. Chem. 2007, 79, 1213.

7374 Langmuir, Vol. 23, No. 13, 2007 chloride (Ru(NH3)63+, Strem Chemicals), acetonitrile (biotech grade, Sigma-Aldrich), methylene chloride (CH2Cl2, Fisher), and phenylethanethiol (PhC2SH thiol, Aldrich) were used as received. Ferrocenylmethyltrimethyl ammonium trifluoromethanesulfonate (FcTMA1+, prepared from an iodide salt (Strem Chemicals) and sublimed) and 1,1′,-ferrocenedimethanol (FcMeOH0) were from a stock of previous research samples in our laboratory. Electrochemical Measurements. Cyclic voltammetry (CV) was conducted with a CH Instruments electrochemical workstation (CHI660A). The 1.6 mm diameter Au working electrodes (Bioanalytical Systems, BAS, West Lafayette, IN) were hand polished with 1 µm diamond paste (BAS) and then rinsed with water and ethanol in succession, followed by sonication in water for 10 min. The electrodes were then electrochemically cycled in 0.5 M H2SO4 until a sharp oxide reduction peak was seen. Adhesion of a Au nanoparticle film to a bare Au working electrode was poor but was substantially improved by first forming a self-assembled monolayer (SAM) of phenylethanethiolate (PhC2S-) by immersion in a freshly prepared ethanolic solution (∼50 mM) of the thiol for 60 min. The PhC2S-SAM promotes adhesion probably by hydrophobic interactions. The electrodes were rinsed thoroughly in ethanol and air dried prior to the drop-casting of the nanoparticle films. A single compartment, three-electrode cell with a Pt counter and Ag quasi-reference electrodes was employed. Film Preparation. Au MPC films were drop-cast onto Au electrodes from their solutions in CH2Cl2 (∼5 mg/mL). Several droplets (dried serially) were required to thoroughly coat the working electrode surface, as judged visually. Parallel experiments on glass slides showed (surface profilometry) that the films are approximately 4-8 µm thick and are rather rough. Thicker films had poor electrochemical behavior. The electrochemical responses of these films were studied in 5 mM aqueous solutions of the redox probes, in aqueous 0.1 M NaTos containing 4-10% CH3CN. Some of the Au MPC films were overcoated with Nafion films formed by a cast droplet of a 2.5 wt % solution of Nafion in isopropyl alcohol (Nafion perfluorinated ion-exchange resin solution, Aldrich), followed by air drying. Some variations in Nafion film thickness occurred, since the droplets did not always spread uniformly. Visually, the Au MPC films seemed undisturbed by the process of forming the Nafion film. Sequestering of cationic electroactive species into the Nafion film10 was done by soaking the Nafion-coated nanoparticle film electrode for 20 min in aqueous solutions that were 5 mM in a redox probe, 0.1 M NaTos electrolyte, and contained 4-10% CH3CN. The films were thoroughly rinsed with water and then soaked in the same medium (absent the redox species) for another 20 min; subsequent voltammetry was done in fresh, redox probe-free, aqueous 0.1 M NaTos electrolyte containing 4-10% CH3CN. Some analogous experiments were done with Nafion-coated Au electrodes that lacked the Au MPC film. These electrodes are labeled as Nafion(redox)/Au MPC film/Au electrodes (with the Au MPC film) and Nafion(redox)/ Au electrode (without the Au MPC film). Voltammograms were simulated using Digisim software (BAS), setting k°HET at 20 cm/s (assuming fast electron transfer and no accompanying homogeneous chemical reactions). The simulated CVs were iteratively matched to the experimental CVs obtained using Nafion-coated MPC films at different potential scan rates by varying, mainly, RUNC to match the ∆EPEAK values and, secondarily, the redox probe concentration (C/) to match the peak currents of the experimental and simulated voltammograms while maintaining a close match between the simulated and literature D values.

Ranganathan et al.

Figure 1. Differential pulse voltammetry (DPV) of Au MPC films drop-cast on a Au electrode (solid lines) in aqueous 0.1 M NaTos solution containing 4% CH3CN. The dashed curve represents a dilute solution DPV voltammogram of Au MPCs in CH2Cl2 with 0.1 M tetrabutylammonium perchlorate on a bare Pt electrode. The MPC film voltammetry was not very reproducible; the potential of the MPC2+/1+ wave in particular varied from 0.3 to 0.5 V more positive than the MPC1+/0 wave, whereas the MPC waves in solution are separated by 0.3 V.

Voltammetry of Au Films in Electrolyte Only. Voltammetry of films of these small Au MPCs cast on Au metal electrodes is featureless when contacted by aqueous electrolyte9b containing no organic dopant or, as reported elsewhere,9c when contacting

an imidazolium hexafluorophosphate ionic liquid. The addition of a small volume percentage of an organic solvent (acetonitrile in combination with NaTos was chosen after tests of several solvents and electrolytes) was successful in provoking electroactivity of the Au films, although it is quite distorted in comparison to that of dilute solutions of Au MPCs. Figure 1 illustrates differential pulse (DPV) voltammetry of a Au MPC film coating on a Au electrode in aqueous 0.1 M NaTos containing 4% CH3CN. The dashed curve shows the characteristic doublet of oxidation peaks (MPC1+/0 and MPC2+/1+) seen in dilute solutions of Au MPCs in CH2Cl2/0.1M Bu4NClO4 electrolyte at a bare Pt electrode.4g The film voltammetry only crudely resembles the dilute solution voltammetry, and it is poorly reproducible in regards to peak definition and spacing between the two redox couples. The potential at which the MPC1+/0 reaction occurs is more reproducible than that of the MPC2+/1+ couple, which at the solid state/solution interface is variable and lies at between 0.3 and 0.5 V more positive potential. The cyclic voltammetric scans are also poorly reproducible, sometimes showing features (see the Supporting Information, Figure S-1) suggestive of Au oxide formation and reduction, which in aqueous media can occur on the surfaces of Au MPCs.11 The voltammetric appearance also varies with different synthetic preparations of the Au nanoparticles and with the organic solvent component and electrolyte employed. Some electrolytes (including LiClO4, Bu4NPF6, and Bu4NClO4) produce no discernible voltammetry; the somewhat hydrophobic tosylate anion produced the most consistent voltammetry. Too much CH3CN (in which Au MPCs are soluble) in the water medium causes slow dissolution of the film. These observations suggest a general importance of solvent swelling and electrolyte anion penetration of the film to electrochemically form cationic MPC charge states, an internal mixed valency, and a more electronically conductive film. CH3CN and the tosylate anion are thought to promote charge transport and electrolytic conversion of some portion of the film to cationic MPC states in the manner of well-known redox polymer metal complexes.8 Voltammetry of Dissolved Redox Species at Au Film Electrodes: (Redox)/Au MPC Film/Au Electrodes. As noted above and in Scheme 1, the redox species were selected based

(10) (a) Martin, C. R.; Dollard, K. A. J. Electroanal. Chem. 1983, 159, 127. (b) Szentirmay, M. N.; Martin, C. R. Anal. Chem. 1984, 56, 1898.

(11) Cliffel, D. E. University of North Carolina, Chapel Hill, NC. Unpublished results, 1999.

Results and Discussion

Nanoparticle Films as Electrodes

Figure 2. Cyclic voltammograms of 5 mM solutions of indicated redox probes in aqueous 0.1 M NaTos solution (containing 4% CH3CN) on bare (dashed lines) and Au MPC coated (solid lines) Au electrodes. Potential scan rate ) 100 mV/s.

on their electrochemical formal potentials12 relative to those of the Au MPCs. The latter, for the MPC+1/0 and MPC+2/+1 couples, are -0.54 and -0.24 V, respectively, versus Fc/Fc+ in CH2Cl2,13 and upon conversion7 to the aqueous medium and to a Ag/AgCl reference scale, they are E°′ ) -0.03 and 0.27 V, respectively, versus Ag/AgCl. The potentials of the redox probes in the aqueous/acetonitrile medium, at naked Au electrodes versus Ag/AgCl, are (Scheme 1) E°′ ) +0.36 V (FcTMA2+/1+), +0.28 V (FcMeOH1+/0), -0.10 V (Ru(NH3)63+/2+), and -0.60 V (MV2+/1+). According to Scheme 1, the first three of these redox species should (thermodynamically) be capable of serving as electron donor acceptors at the Au MPC film electrode, whereas in the case of MV2+/1+, electron transfers are unlikely. That is, the Boltzman distribution of electronic states at the Au MPC film surface at the MV2+/1+ potential will be very small. The electrochemical responses of the four dissolved redox species at a Au MPC film/Au electrode are shown in Figure 2. The dashed curves are their CV responses on a naked Au electrode (same concentration) after the Au MPC film was removed by washing with acetone. The CV responses could be measured on fresh (redox)/Au MPC film/Au electrodes, or they could all be measured on a single film specimen (rinsing the Au MPC film electrode thoroughly with water between different redox probe solutions). The data shown in Figure 2 were taken on a single Au MPC film. As qualitatively anticipated from Scheme 1, currents for the FcTMA2+/1+, FcMeOH1+/0, and Ru(NH3)63+/2+ electrode reactions are seen at the Au MPC film, and none are observed in the MV2+/1+ solutions. The potentials of the ferrocene reactions are shifted slightly positive from their values on naked electrodes, which may be accounted for by the tendency of the MPC2+/1+ couple on the Au MPC film to appear at more positive potentials (e.g., Figure 1) than Scheme 1 would suggest. As noted above, it seems likely that the two ferrocenes become oxidized through the formation of MPC2+ states on the electrode film, while that of Ru(NH3)62+ occurs through the MPC1+ states at the MPC film/solution interface. The Figure 2 currents for the FcTMA2+/1+, FcMeOH1+/0, and Ru(NH3)63+/2+ electrode reactions on (redox)/Au MPC film/Au (12) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001. (13) Guo, R.; Murray, R. W. J. Am. Chem. Soc. 2005, 127, 12140.

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Figure 3. Cyclic voltammograms of cationic redox systems that had been exchanged into thin Nafion films in aqueous 0.1 M NaTos solution (containing 4% CH3CN) on bare Au electrodes (dashed lines) and on Nafion(redox)/Au MPC film/Au electrodes (solid lines). Potential scan rate ) 10 mV/s.

electrodes are smaller than those at naked Au electrodes, and the ∆EPEAK values are much larger. The smaller currents and larger ∆EPEAK values may arise from the internal resistance of the Au film (i.e., an ohmic loss caused by rate determining electron hopping in the film) or from slow heterogeneous electron transfers of the redox species at the Au MPC film/solution interface. We believe the former to be more likely from comparisons of experimental and simulated voltammograms as discussed later. Currents from the MPC film are not prominent in Figure 2; they are smaller than those of the redox probe and indeed seem to be suppressed. The FcTMA panel in Figure 2 shows a hint of the film reduction at the ∼ -0.1 V wave. Voltammetry of Redox Species in Nafion Films on Au and Au MPC Film Electrodes. The three cationic redox probes (Ru(NH3)63+/2+, FcTMA2+/1+, and MV2+/1+) were ion-exchanged into Nafion films resting either on a naked Au working electrode or on a Au MPC film. The latter circumstance mimics previously studied8 redox polymer film bilayer assemblies. In the present bilayer assembly, the Au MPC film acts as one electroactive layer and the redox species in the Nafion film acts as a second electroactive layer. The focus in the bilayer assembly is to exploit the defined energy levels of the molecule-like Au MPCs, analogous to the earlier work. Cyclic voltammetry curves of the three redox systems are shown in Figure 3 for the Nafion(redox)/Au MPC film/Au (solid lines) and Nafion(redox)/Au electrodes (dashed lines). In both cases, the redox probes were first sequestered into the Nafion film and the experiment was then done in redox-free electrolyte solution. The CV responses of the redox probes in the Nafion(redox)/Au electrode (dashed lines) cases are qualitatively similar to those for the redox species freely diffusing in solution (Figure 2), except that the wave for the Ru(NH3)63+/2+ couple is shifted to more negative potentials (as expected from previous work10b). The Nafion(redox)/Au MPC film/Au (solid lines) results in Figure 3 are fully consistent with those in Figure 2; electrode reactions can be observed for Ru(NH3)63+/2+ and FcTMA2+/1+ but not for MV2+/1+. For the former two, the ∆EPEAK values are again larger in the presence of the Au MPC film. For MV2+/1+, no current response is seen in the presence of the Au MPC film, irrespective of the potential scan rate. (We are unable to observe charge trapping and untrapping reactions of the redox layer by accessing

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Table 1. Results from Comparisons of Experimental and Simulated CVs for Nafion(Redox)/Au and Nafion(Redox)/ Au MPC Film/Au Electrodes DNAF lit valuesa

Nafion(redox)/Au electrodes

C//Mb DNAF/(×10-9 cm2 s-1) RUNC/kΩc

2.3

0.2 2.4 3

0.2 2.5 13

C//Mb DNAF/(×10-9 cm2 s-1) RUNC/kΩc

0.4

2.0 0.6 2.5

2.0 0.4 6.5

C//Mb DNAF/(×10-9 cm2 s-1) RUNC/kΩc

1.9 2.5 0.8

1.9

1.8

redox couple

parameters

Ru(NH3)63+/2+ FcTMA2+/1+

MV2+/1+

Nafion(redox)/Au MPC film/Au electrodes

>1 × 107

a From ref 10a. b From the i versus ν1/2 plots for the Ru(NH3)63+/2+, FcTMA2+/1+, and MV2+/1+ electrode reactions. C/ was adjusted upward ∼2-fold to achieve satisfactory current comparison. c Adjusted so that the simulation fits the experimental ∆EPEAK values.

the MPC1- state, which thermodynamically would be able to reduce MV2+, because the MPC0/1- reaction occurs at negative potentials that are not accessible in the aqueous medium.) Different films had to be used for each redox species in the Figure 3 experiments, so a comparison of currents is less exact in Figure 3 than in Figure 2; it is clear nonetheless that they are similar for electrodes with and without the Au MPC film. Finally, we should note the possibility that the electrochemical activity of the Au MPC films is additionally promoted by minor amounts of dissolution of oxidatively charged Au MPCs into the contacting electrolyte and/or Nafion medium, in the manner demonstrated by Bond et al.14 in a variety of particle-coated electrode experiments. We have, however, no direct evidence that this occurs here. CV Simulations. The currents seen at the Nafion(redox)/Au MPC film/Au electrodes vary linearly with (potential scan rate)1/2 (over a range of 0.01-0.10 V/s), showing that the Nafion film is sufficiently thick (and that charge transport is sufficiently slow) that semi-infinite diffusion conditions12 are maintained (Supporting Information, Figure S-2). This offered the opportunity for a rough simulation analysis of the Figure 3 CV data to ascertain the possible current-limiting factor(s) by comparing the observed combinations of CV peak currents and ∆EPEAK values with digitally simulated values. The approach taken was to assign the main sources of ∆EPEAK enhancement to ohmic potential loss due to Nafion film ionic conductivity and the internal resistance (slow electronic hopping) of the Au MPC film, computing CVs to match the experimental ones by taking the ∆EPEAK values as the unknown parameter. The simulations required knowledge of the diffusion coefficients (DNAF) and concentrations (C/) of the Ru(NH3)63+/2+ and FcTMA2+/1+ redox species within the Nafion film. Literature reports10 (Table 1) supplied the DNAF values (ignoring any effect of the CH3CN content of the aqueous medium). The slopes of the Figure S-2 plots (see the Supporting Information) of peak current versus (potential scan rate)1/2 were analyzed using the relation for a reversible reaction in a linear sweep voltammogram,12 giving the product DNAF1/2C/. C/ was then estimated using the literature DNAF values (Table 1). Using these data, values of iRUNC were then manipulated to produce a best fit between the simulated and experimental ∆EPEAK results. The simulation was performed first for the Nafion(redox)/Au electrode experiments, where comparing experimental and simulated CVs for a good match and a ∆EPEAK value output was (14) (a) Zhang, J.; Bond, A. M. Anal. Chem. 2003, 75, 2694. (b) Hultgren, V. M.; Mariotti, A .W. A.; Bond, A. M.; Wedd, A. G. Anal. Chem. 2002, 74, 3151. (c) Zhang, J.; Bond, A. M.; Belcher, W. J.; Wallace, K. J.; Steed, J. W. J. Phys. Chem. B 2003, 107, 5777.

successful (Supporting Information, Figure S-3A) for Nafion films containing the Ru(NH3)63+/2+ couple but required adjusting the estimated C/ values upward by ∼2-fold for the FcTMA2+/1+ (Supporting Information, Figure S-3B) and MV2+/1+ results to match the current responses. The values employed are listed in Table 1, along with the resulting estimates of RUNC, which varied from 0.8 to 3.0 kohms, with an average of 1.8 kohms. Here, RUNC presumably measures the Nafion film ionic conductivity. Adopting the same values of C/, CV experiment/simulation comparisons were made for the Nafion(redox)/Au MPC film/Au electrodes (the bilayer electrode assemblies). The comparisons were done for data at different potential scan rates, in this case adjusting DNAF slightly (see Table 1) for the best match of the experimental and simulated ∆EPEAK values (see the Supporting Information, Figure S-3). The results for ∆EPEAK are given in Table 1 as RUNC. For MV2+/1+, the data could not be fit with any reasonable DNAF value and RUNC in Digisim had to be set at g1 × 107 ohms to obtain a base line similar to the flat one in Figure 3. The key result in Table 1 is that, in each case, the value of RUNC is larger for the Nafion(redox)/Au MPC film/Au electrodes than for the Nafion(redox)/Au electrodes. The difference is a factor of 3-4-fold for the Ru(NH3)63+/2+ and FcTMA2+/1+ couples and is much larger for the MV2+/1+ data. The latter result is consistent with the preceding discussion that the Au MPC film lacks electronic states suitable for the MV2+/1+ reaction, so that it acts as an insulator. The RUNC value for the Ru(NH3)63+/2+ and FcTMA2+/1+ couples is quantitatively consistent with other measurements6 of the electronic conductivity of similarly prepared, dry Au MPC films. For example, assigning a RUNC value of ∼6.5 kohms to the Au MPC film and assuming a 4 µm Au MPC film thickness gives an estimated film electronic conductivity of ∼3 × 10-6 ohm-1 cm-1. Results6 on electronic conductivities of dry Au MPC films give 3 × 10-8 ohm-1 cm-1 for 1:1 mixed valent MPC1+/0 films. Considering that the present Au MPC films have a certain solvent content and ionic transport level, which the dry films6 did not, these conductivities are reasonably close. The CV experiment/simulation comparisons (Supporting Information) are admittedly rough but are consistent with the primary current-limiting factor in the experiments of Figure 3 (and most likely Figure 2 as well) being the ohmic potential loss within the Au MPC film itself. In the case of the Au MPC film at the negative potentials of the MV2+/1+ couple, the large ohmic potential loss actually represents an absence of electronic states that allows charge transport through the Au MPC film at the appropriate energies. At more positive potentials, electron

Nanoparticle Films as Electrodes

transport is possible at the appropriate energies, but its rate is limited by electron hopping between the Au MPC cores. Acknowledgment. This research was supported by grants from the National Science Foundation and the Office of Naval Research.

Langmuir, Vol. 23, No. 13, 2007 7377

Supporting Information Available: Cyclic voltammetry, current versus (scan rate)1/2 plots, and comparisons of experimental and simulated voltammograms. This material is available free of charge via the Internet at http://pubs.acs.org. LA070008N