Charge Transport Dynamics and Redox Induced Structural Changes

National Centre for Sensor Research, School of Chemical Sciences, Dublin ... transfer may limit homogeneous charge transport through these solid depos...
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Charge Transport Dynamics and Redox Induced Structural Changes within Solid Deposits of a Ruthenium Dimer Graeme A. Snook, Yvonne Cooney, Tia E. Keyes, and Robert J. Forster* National Centre for Sensor Research, School of Chemical Sciences, Dublin City University, Dublin 9, Ireland Received May 9, 2002. In Final Form: September 11, 2002 Solid particles of a ruthenium dimer, [(Ru(bpy)2)2 bpzt-]3+, have been abrasively attached to macroelectrodes and microelectrodes, where bpy is 2,2′-bipyridine and bpzt- is 3,5-bis(pyrazin-2-yl)-1,2,4triazole. The voltammetry of these solids deposits is unusually ideal in NH4PF6-containing aqueous solutions, and the response is characterized by semi-infinite linear diffusion for scan rates between approximately 50 and 2000 mV s-1. SEM imaging reveals that sparse films of solid particles (1-10 µm in diameter) are efficiently transformed into microcrystals by voltammetric cycling. The charge transport diffusion coefficient, DCT, has been determined by systematically varying the voltammetric scan rate. For reduction of the deposits, DCT increases from 2.4 to 3.6 × 10-10 cm2 s-1 as the NH4PF6 concentration is increased from 0.1 to 2.0 M, while, for oxidation of the deposit, DCT increases from 1.1 to 3.9 × 10-10 cm2 s-1 over the same concentration range. The maximum DCT observed would correspond to an electron self-exchange rate constant of 1.1 × 105 M-1 s-1. Despite the smaller electron-transfer distance expected within the solid, this apparent self-exchange rate constant is more than 2 orders of magnitude smaller than that typically found for ruthenium bis-bpy complexes in solution. This observation suggests that ion rather than electron transfer may limit homogeneous charge transport through these solid deposits.

Introduction Traditionally, electroanalysis has been restricted to dilute solutions in which the redox active species is dissolved in a highly conducting electrolyte solution. However, solid-state voltammetry, in which microparticles or crystals of a solid are abrasively attached to the working electrode, promises to provide significant new insight into charge and mass transport in solids. Switching the oxidation state of a solid deposit involves ion permeation, the movement of charge compensating counterions through the solid and electron transfer at the electrode/solid interface. Therefore, the overall rate at which the redox composition of the solid can be switched depends on the relative dynamics of electron and ion movement. Of particular interest is the impact of the medium, that is, solution versus solid state, on the dynamics of electron transfer either between adjacent oxidized and reduced reactant pairs, that is, electron self-exchange, or between the delocalized electronic states of the electrode and the molecular orbitals of a redox active molecule. Recent studies on solid deposits have focused on forming a relatively sparse distribution of microparticles on a microelectrode surface. This approach has the advantage of efficient mass transport of charge compensating counterions, making it more likely that electron transfer rather than ion transport will represent the rate determining step. Also, because the particles are separated by more than 2-3 particle diameters, the zones around each microparticle that are depleted of electrolyte as the particles are oxidized do not interact with one another. Consequently, the availability of charge compensating counterions is less likely to limit the overall charge transport rate than is the case in single macroscopic crystals. This behavior is similar to that expected for the electrolysis of a solution containing a dissolved redox * To whom correspondence should be addressed.

species at a random array of microelectrodes.1 Moreover, the small size of the particles (typically 1-10 µm) means that radial diffusion of electrolyte to and from these individual particles is possible. Furthermore, the surface area-to-volume ratio of the smaller crystals compared to macroscopic systems means that the extent of the triple interface between solid/electrode/electrolyte is maximized. These advantages lead to the possibility of using more practical scan rates (mV s-1 range) compared to the case of some previous studies on solid electrodes, for example, those used in mineral electrochemistry, which requires extremely slow scan rates (µV s-1) to achieve well-defined voltammetry.2 In this contribution, the solid-state voltammetry of a ruthenium dimer, [(Ru(bpy)2)2 bpzt-]3+ (Chart 1), where bpy is 2,2′-bipyridine and bpzt- is 3,5-bis(pyrazin-2-yl)1,2,4-triazole, is reported. Ruthenium complexes are particularly suitable for studying the voltammetric behavior of solid deposits because they are stable in several oxidation states and have large self-exchange rate constants.3,4 However, while using disperse arrays of microparticles supports efficient transport of charge compensating counterions to the deposit/solution interface, ion transport within the deposit must be facile if the dynamics of electron transfer are to be measured.5 For example, we recently demonstrated that solid deposits of [Os(bpy)2 bpt Cl], where bpy is 2,2′-bipyridyl and bpt is 3,5-bis(pyridin4-yl)-1,2,4,-triazole, exhibit nearly ideal reversible voltammetric responses.6 However, in this system, the rate of charge compensating counterion motion, rather than (1) Fletcher, S.; Horne, M. D. Electrochem. Commun. 1999, 1, 502. (2) Haber, J.; Nowak, P. Langmuir 1995, 11, 1024. (3) Quaranta, F.; Rella, R.; Siciliano, P.; Capone, S.; Epifani, M.; Vasanelli, L.; Licciulli, A.; Zocco, A. Sens. Actuators B 1999, 350. (4) Chan, M. S.; Wahl, A. C. J. Phys. Chem. 1978, 82, 2542. (5) Deuber, R. E.; Bond, A. M.; Dickens, P. G. J. Electrochem. Soc. 1992, 139, 2363. (6) Forster, R. J.; Keyes, T. E. Phys. Chem. Chem. Phys. 2001, 3, 1336.

10.1021/la025928e CCC: $22.00 © 2002 American Chemical Society Published on Web 11/12/2002

Solid Deposits of a Ruthenium Dimer Chart 1

electron hopping between the redox centers, limits the overall rate of charge propagation. In contrast, complexes incorporating bulkier side groups, such as [Os(OMe-bpy)3](PF6)2, where OMe-bpy is 4,4′-dimethoxy-2,2′-dipyridyl, methoxybipyridyl, appear to have a higher internal free volume that facilitates fast ion motion leading to electron self-exchange being rate determining.7 For the dimer considered here, one might anticipate that the molecular shape would disfavor close packing making the solids rapid ion conductors. To probe the redox switching dynamics, cyclic voltammetry has been performed under semi-infinite linear diffusion conditions to measure the rate of homogeneous charge transport through the deposits.8 By systematically varying the concentration of the supporting electrolyte, we have obtained an insight into whether electron transfer or counterion transport limits the rate at which the redox composition can be switched. This information is essential for the construction of molecular devices, since it determines the overall response time of any device.9-11 For technological applications ranging from molecular electronics to sensors, it is important to understand those factors that control the rate of electron transfer across the interface between a metal substrate and a molecular material. Therefore, we have probed the rate of heterogeneous electron transfer for this complex both as a solid deposit and as a solution phase reactant. These investigations into homogeneous charge transport and heterogeneous electron transfer for solid deposits will underpin developments ranging from electrocatalysis to redox switchable nonlinear optical materials. Experimental Section Chemicals. Lithium perchlorate (95+%), ammonium hexafluorophosphate (95+%), sodium tetrafluoroborate (98%), and sodium persulfate (98+%) were all obtained from Aldrich. The synthesis of the ruthenium dimer, [(Ru(bpy)2)2 bpzt-]3+, where bpy is 2,2′-bipyridine and bpzt- is 3,5-bis(pyrazin-2-yl)-1,2,4triazole, was described previously.12 Acetonitrile (HPLC grade) (99.9%) was obtained from Lab Scan. Milli-Q water was used to prepare all aqueous solutions. Instrumentation. Cyclic voltammetry was performed using a CH Instrument model 660 electrochemical workstation and a conventional three electrode cell placed within a Faraday cage. UV-visible spectra for the solid-state deposits were recorded (7) Keane, L.; Hogan, C.; Forster, R. J. Langmuir, in press. (8) Wightman, R. M.; Wipf, D. O. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1989; Vol. 15. (9) Yang, C.; He, G.; Wang, R.; Li, Y. J. Electroanal. Chem. 1999, 471, 32. (10) Nazar, L. F.; Goward, G.; Leroux, M.; Duncan, H.; Kerr, T.; Gaubicher, J. Int. J. Inorg. Mater. 2001, 3, 191. (11) Wrighton, M. S. Science 1986, 231, 32. (12) Coates, C. G.; Keyes, T. E.; Hughes, H. P.; Jayaweera, P. M.; McGarvey, J. J.; Vos, J. G. J. Phys. Chem. 1998, 102, 5013.

Langmuir, Vol. 18, No. 25, 2002 9875 using a Nikon Eclipse ME600 microscope equipped with both a 100 W halogen lamp and a 100 W mercury arc source. The films were deposited on conducting glass slides (ITO) and were positioned against a water-immersion objective (10× magnification) and observed through the microscope. An Ag/AgCl reference electrode and a large area platinum wire completed the three electrode electrochemical cell. Spectra were recorded using an Andor Technology gated intensified CCD coupled to an Oriel model MS125 spectrograph fitted with a 600 lines/inch grating. Typically the gate width was 2 ms, and for surface coverages less than 1 × 10-7 mol cm-2, signal averaging was necessary. The potential of the working ITO electrode was controlled using a CH Instruments model 660 electrochemical workstation. A Dell Dimension Pentium PC was used for data acquisition and analysis. For the surface images of the macro- and microelectrodes, a scanning electron microscope (Hitachi S3000N) was used. Electrodes. The electrodes used were a glassy carbon macroelectrode (1.5 mm diameter), a platinum macroelectrode (1.7 mm diameter), and a gold macroelectrode (1.7 mm diameter). For analysis using microelectrodes, 50 µm diameter platinum microelectrodes were used. A basal plane pyrolytic graphite stub (6.5 mm in diameter) was used for SEM imaging of voltammetric cycling. For all the cyclic voltammetry experiments, the reference electrode used was an Ag/AgCl electrode filled with 3 M KCl (aq) (CH Instruments). For both solution phase and solid-state experiments, all solutions were deoxygenated for at least 15 min using nitrogen gas. During the experiments, a blanket of nitrogen gas was kept over the solution. The working electrode was polished using 12.5, 5.0, and then 0.3 µm alumina for the macroelectrodes with an additional polish with 0.05 µm alumina for the microelectrodes. For solid-state measurements, the complex was attached to the electrode surface by rubbing the tip of the electrode onto the filter paper containing this solid. SEM Imaging. A basal plane pyrolytic graphite cylindrical stub was used as a working electrode for SEM imaging before and after cyclic voltammetry. The graphite stub was cleaned by lightly scraping the top layers off using emery paper. The solid compound was then attached to the surface of the electrode using a combination of the filter paper and a cotton bud. Poorly adhering solid was then removed using distilled water. The modified stub was then dried under vacuum before SEM imaging. Following imaging, the solid deposit was voltammetrically cycled. The electrode was then soaked in distilled water for 15 min to remove any remaining electrolyte. Finally, the stub was again thoroughly rinsed with distilled water, and this process was repeated three times. The graphite stub was dried under vacuum before imaging using SEM.

Results and Discussion Effect of Electrolyte Anion on the Solid-State Voltammetry. Figure 1 shows the voltammetric response for a 2 mM solution of the dimer dissolved in acetonitrile containing 0.1 M LiClO4 as supporting electrolyte. The working electrode is a glassy carbon macroelectrode, and the scan rate is 0.1 V s-1. This figure also illustrates the response obtained for a solid deposit where the supporting electrolyte is aqueous 0.1 M NH4PF6. When the complex is dissolved in solution, two well-defined redox processes are observed with formal potentials, E°′, of 0.900 and 1.205 V. These redox processes correspond to the stepwise oxidation of the two Ru2+ centers within the dimer and their subsequent re-reduction. Both processes are electrochemically reversible; that is, ∆Ep, the difference between the anodic and cathodic peak potentials, Epa and Epc, respectively, is 57 ( 6 mV and the ratio of the anodic, ipa, and cathodic, ipc, peak currents is 1.05 ( 0.05, for scan rates, υ, between 10 and 5000 mV s-1. The significant separation between these two metal based redox processes is attributed to the asymmetry of the triazolate bridge and, in particular, to strong electronic communication between the two metal centers.

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Figure 1. Cyclic voltammograms for a 2 mM solution of [(Ru(bpy)2)2bpzt-]3+ dissolved in acetonitrile (thin line) and as a solid deposit (thick line). For the solution phase experiment, the supporting electrolyte is 0.1 M LiClO4 while, for the solid deposit, it is aqueous 0.1 M NH4PF6. In both cases, the scan rate is 0.1 V s-1 and the working electrode is a 1.5 mm radius glassy carbon electrode.

Figure 1 shows that solid deposits of the dimer give well-defined voltammetric responses where the supporting electrolyte is aqueous 0.1 M NH4PF6. While it is usual to take the average of the anodic and cathodic peak potentials as E°′ for electrochemically reversible redox reactions involving a solution phase reactant, this may not be appropriate for solid deposits because of differences in the energetics and dynamics of ion ingress and egress. While this issue is addressed in more detail in a later section, it is apparent from Figure 1 that the redox processes in the solid state occur at similar potentials to those found for the complex dissolved in acetonitrile. The ∆Ep values are 200 and 170 mV, respectively, for the first and second metal centers, respectively, which is significantly larger than the 57 mV expected for an electrochemically reversible redox reaction involving the transfer of one electron. The ∆Ep does not decrease with decreasing scan rate, suggests that slow heterogeneous electron transfer across the electrode/solid interface is not responsible for the observed behavior. Moreover, the total cell resistance, as measured using potential step chronoamperometry in a potential region where the deposit is not electroactive, is 390 Ω, leading to an ohmic drop of approximately 12 mV, which is negligible compared to the experimentally observed ∆Ep values. The ∆Ep values depend markedly on the identity of the supporting electrolyte anion. For example, while the average of Epa and Epc is approximately 120 mV more positive in 0.1 M LiClO4 than in 0.1 M NH4PF6, the ∆Ep values are smaller in hexafluorophosphate-containing media, that is, 130 and 110 mV for the first and second metal centers. Surprisingly, the difference in potential between the two metal oxidations changes little between solution and solid, at approximately 300 mV. This consistency between solid and solution phase reactants suggests either that the local dielectric constant within the deposit is similar to that of acetonitrile or, more likely, that the potential of the second oxidation process is relatively insensitive to electrostatic effects. The relatively large difference in formal potentials for the first and second redox processes confirms that electronic coupling across the bpzt bridge is strong in both solution and solid phase systems. The sensitivity of the solid-state voltammetry to the identity of the electrolyte anion reflects differences in the properties of the charge compensating counterions. The ionic radii of both anions are similar, that is, approximately 236 pm for perchlorate and 246 pm for hexafluorophos-

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phate.13 Therefore, it is unlikely that differences in the rates of ion transport through the solid are responsible for the observed differences in the voltammetric behavior. Rather it appears that it is energetically more difficult to insert the ClO4- anion into the solid for charge neutralization, causing the oxidation potential to be more positive for ClO4- relative to PF6-. However, once incorporated within the solid, it appears that the ClO4- anion is relatively more mobile, causing smaller ∆Ep values to be observed. Behavior of this kind has previously been observed for the solid-state voltammetry14,15 of compounds such as TCNQ and is related to the energy barrier for insertion and expulsion of counterions. When the solid deposits are repeatedly cycled in aqueous 0.1 M NH4PF6 supporting electrolyte, the peak current decreases by less than 5% over a 5 h period. This result indicates that the ruthenium dimer is essentially insoluble in both oxidation states in this electrolyte. In contrast, where the supporting electrolyte is aqueous 0.1 M LiClO4, the peak current decreases significantly over a 20 min period of repeated cycling. Dissolution of the solid deposit occurs even more rapidly when the dimer is in the fully oxidized (5+) state. This behavior is common and reflects the relative thermodynamic stabilities of the oxidized compound in the solid and solution phases.16 Morphological Effects of Voltammetric Cycling. We have previously demonstrated that monomeric bipyridyl based complexes can be efficiently electrocrystallized by voltammetric cycling.6,7 Here, scanning electron microscopy, SEM, has been used to probe the physical character of the deposits and to determine if voltammetric cycling results in any morphological changes in the deposit. Figure 2A and B illustrates SEM images obtained prior to voltammetry. The solid is well spread over the surface with the ruthenium dimer existing as an array of amorphous particles ranging in size from approximately 1 to 20 µm in diameter. Figure 2C and D reveals that voltammetric cycling between +0.400 and +1.300 V in aqueous 0.1 M LiClO4 triggers significant changes in the nature of the deposit. Following cycling, a mixture of amorphous particles and microcrystals is observed. While Figure 2C suggests that the majority of the particles electrocrystallize under these conditions without growing significantly, Figure 2D shows that some very large crystals are also formed. The large crystals that were observed are approximately 100 µm in diameter, suggesting that Oswald ripening may have occurred.17,18 Oswald ripening involves growth of large particles at the expense of the dissolution of small particles in order to minimize the total free energy of the system. Voltammetric cycling of the deposits accelerates electrocrystallization because the oxidized product is more soluble than the reduced form. The larger volume-to-surface area for these bigger particles results in them being thermodynamically more stable. However, they may not be as suitable for solid-state voltammetry, since the triple interface where solid/solution/electrode meet and the particle surface where solid and electrolyte meet have been decreased relative to the initial array of smaller particles. (13) Marcus, Y. Ion Properties; Marcel Dekker: New York, 1997. (14) Bond, A. M.; Fletcher, S.; Marken, F.; Shaw, S. J.; Symons, P. G. J. Chem. Soc., Faraday Trans. 1996, 92, 3925. (15) Bond, A. M.; Fletcher, S.; Symons, P. G. Analyst 1998, 123, 1891. (16) Bond, A. M.; Colton, R.; Mahon, P. J.; Snook, G. A.; Tan, W. T. J. Phys Chem. B 1998, 102, 1229. (17) Weber, R.; Skorupa, W. Nucl. Instrum. Methods Phys. Res. B 1999, 149, 99. (18) Kukushkin, S. A.; Slyozov, V. V. J. Phys. Chem. Solids 1996, 57, 195.

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Figure 2. Scanning electron microscopy images of [(Ru(bpy)2)2bpzt-]3+ solid deposits. (A and B) as deposited, before electrochemical cycling; (C and D) images obtained after 100 voltammetric cycles between +0.400 and +1.300 V in 0.1 M LiClO4 at a scan rate of 0.1 V s-1. In part A the magnification is 100× while in parts B and C it is 500× and in part D it is 250×.

In attempting to probe the dynamics of charge transport through solid deposits of this kind, vide infra, it is important to have an estimate of the area of the electrode that is covered with the solid. SEM images of microelectrode surfaces following immobilization of the solid were used to estimate the area covered by the microparticles of the dimer. Using the mechanical abrasion approach, approximately 30% of the electrode is coated, and this percentage covered is reproducible to within (5%. Indeed, when a higher percentage of the electrode is coated with solid, the voltammetry becomes ill-defined. Potential Dependent UV-Visible Spectroscopy. Since these films exist as discrete particles, it is important to determine what percentage of the film is electrochemically active. In this way, an insight can be obtained into the extent to which individual particles are interconnected. Potential dependent UV-visible spectroscopy represents a convenient approach to probing this issue. Figure 3 illustrates the changes in the UV-visible spectrum that occur during electrolysis of a solid-state film in 1.0 M NH4PF6 at +1.400 V, where the surface coverage as determined voltammetrically is approximately 6 × 10-8 mol cm-2. The film exhibits absorption maxima at approximately 250 and 290 nm that are attributed to ligand based π-π* transitions.19 Prior to oxidation, the film also shows strong absorbances between 375 and 530 nm that are attributed to Ru (dπ) to bpy and bpt (π*) MLCT transitions. The spectroscopic transitions and relative peak intensities of the solid-state films are generally consistent with those (19) Lever, A. B. P. Inorganic Electronic Spectroscopy; Elsevier Science Publishers: New York, 1984.

Figure 3. Time dependent changes in the UV-vis spectrum of a solid-state [(Ru(bpy)2)2bpzt-]3+ deposit attached to an ITO electrode when electrolyzed at +1.400 V in 1.0 M NH4PF6. From top to bottom, the spectra represent electrolysis times of 0, 10, 50, 100, 500, and 2000 ms. The inset shows a plot of the absorbance at 480 nm vs t-1/2.

observed for the complex dissolved in 90:10 H2O/CH3CN. However, the absorption maxima are typically shifted to higher energy by approximately 10 nm for the solid films. This behavior most likely arises because of a lower dielectric constant within the film; for example, we found previously that the apparent pKa for the pyridine group within [Os(bpy)2 3,6-bis(4-pyridyl)-1,2,4,5-tetrazine Cl]+ films was 3.5 ( 0.1 compared to 2.7 ( 0.2 in essentially aqueous solution.6 Consistent with oxidation of the Ru2+ centers, the intensities of the MLCT bands decrease systematically with increasing electrolysis time at +1.150

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V. Significantly, the spectral changes are complete within a rather short period of time, that is, 2 s compared to the 40 s observed previously for solid deposits of the dimeric complex [Os(bpy)2 Cl 4-bpt Os(bpy)2 Cl]PF6, where bpy is 2,2′-bipyridyl and bpt is 3,5-bis(pyridin-4-yl)-1,2,4,-triazole.20 This observation suggests that solid-state charge transport is relatively faster in this system. While we address this issue in a later section, the inset of Figure 3 shows that the absorbance at 480 nm decreases linearly as t-1/2, indicating that semi-infinite linear diffusion controls the electrolysis rate. Perhaps the most important result of these experiments is that the intensity of the MLCT transition collapses to less than 7% of its initial value after the films have been oxidized for approximately 2 s. Given that this potential is sufficient to oxidize both of the two Ru2+ centers, this result indicates that the vast majority of the individual particles are electrically connected to the electrode surface and that close to 100% of the deposit is electrochemically active. Resistance and Interfacial Capacitance. When attempting to extract quantitative data from voltammetric data, for example, formal potential, charge transport diffusion coefficients, or heterogeneous electron-transfer rate constants, it is important to consider both the effects of the electrode response time and ohmic effects. Also, by examining the resistance as a function of the supporting electrolyte concentration, it ought to be possible to obtain a limited insight into the permeability of the deposit toward electrolyte ions. When a polycationic deposit, in this case the dimer initially in the 3+ state, is placed in a dilute solution of a strong electrolyte, the concentration of counterions (PF6 in this case) within the deposit is typically considerably larger than that found in the contacting solution. For the deposits considered here, the anion concentration initially present in the deposit is expected to be up to 4.5 M. Thus, under the influence of the concentration gradient, counterions may diffuse from the deposit into the solution until the concentrations become equal in the two phases. However, if diffusion of charged counterions occurs, then electroneutrality within the deposit would be violated, and an electrical potential would develop at the interface. This “Donnan potential” would then increase until an equilibrium was reached in which it completely opposes the tendency of the counterions to move down the concentration gradient. Under these equilibrium conditions the net diffusion of counterions across the interface would be zero, and co-ions would be excluded from the solid deposit.21 The existence of such a permselective response for these solid deposits has been probed by determining the contribution of the deposit resistance to the total cell resistance as the supporting electrolyte concentration is changed. In the case of an ideally permselective response, ions would be effectively excluded from the membrane, and the deposit resistance would be independent of the supporting electrolyte concentration. To determine the total cell resistance, we have performed short time scale, small amplitude, potential step chronoamperometry, in a potential region where no Faradaic response is observed. In a typical experiment, the potential was stepped from -50 to 0 mV at both bare and modified microelectrodes, and the resulting current was recorded over the following (20) Walsh, D. A.; Keyes, T. E.; Forster, R. J. J. Electroanal. Chem., in press. (21) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; Wiley: New York, 1980.

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Figure 4. Current-time transients for a 25 µm radius platinum microelectrode modified with an [(Ru(bpy)2)2bpzt-]3+ deposit following potential steps from -0.050 to 0.000 V. From top to bottom, the data correspond to 0.1, 0.5, and 1.1 NH4PF6. The inset illustrates the corresponding semilog current vs time plots.

20 µs. This capacitive current versus time transient can be described by eq 1:8

ic(t) ) (∆E/R) exp(-t/RCdl)

(1)

where ∆E is the pulse amplitude, R is the total cell resistance, and Cdl is the integral double layer capacitance. For both modified and bare electrodes, the current decays in time according to a single exponential, which is consistent with double layer charging alone.21 Figure 4 illustrates ic(t) versus t and semilog current versus time plots for the solid deposits as the NH4PF6 concentration is changed from 0.1 to 0.5 to 1.1 M. The absolute slope of the semilog plots represents the reciprocal of the cell time constant RCdl. Table 1 presents RCdl values for an electrode before and after modification with the dimer as a function of the perchlorate concentration. This table shows that both the bare and the modified electrode cell time constants decrease with increasing electrolyte concentration, as expected.21 However, the response time is considerably more sensitive to the supporting electrolyte concentration for the microelectrode coated with the solid deposit. It is apparent from eq 1 that R can be extracted from the intercepts of the semilog plots shown in Figure 4. Figure 5 shows the total cell resistance for a bare and a coated electrode as the PF6- concentration is changed from 0.1 to 1.1 M. It is apparent that in both circumstances R is reduced at high electrolyte concentrations, reflecting a reduced solution resistance. Significantly, for hexafluorophosphate concentrations greater than about 0.7 M, the total cell resistances with and without the deposit are very similar. This result suggests that, for relatively high electrolyte concentrations, the deposit resistance is low, probably because electrolyte can permeate the individual particles that exist on the microelectrode surface, vide supra. Ion Pairing Effects. The formal potential of the immobilized electroactive group is sensitive to both the solvation shell of the redox center and the extent of ion pairing. It is therefore a sensitive probe of the local microenvironment within the solid deposit. Shifts in formal potential with changes in electrolyte concentration reflect differences in the relative stability between the two redox

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Table 1. Resistance, R, Double Layer Capacitance, Cdl, and Electrode Response Times, RC, for 25 µm Radius Platinum Microelectrodes before and after Modification with a Solid Deposit of [Ru-L-Ru]3+ as the Concentration of NH4PF6 Is Systematically Varieda bare

a

modified

[NH4PF6]/M

R/Ω

Cdl/µF

RCdl/µs

R/Ω

Cdl/µF

RCdl/µs

0.0 0.1 0.2 0.4 0.6 0.8 1.0

2410 (172) 2221 (185) 2096 (123) 1911 (1370) 1720 (122) 1510 (17) 1395 (650)

5.99 (0.56) 6.44 (0.49) 6.47 (0.48) 7.12 (0.16) 7.08 (0.32) 7.42 (0.76) 8.33 (0.51)

1.44 (0.27) 1.43 (0.24) 1.36 (0.22) 1.36 (0.14) 1.21 (0.12) 1.12 (0.13) 1.16 (0.13)

3660 (233) 3541 (211) 3294 (189) 2968 (245) 2720 (203) 1880 (145) 1672 (33)

4.10 (0.26) 4.48 (0.28) 4.85 (0.37) 5.22 (0.39) 5.04 (0.19) 5.60 (0.73) 5.97 (0.46)

1.50 (0.19) 1.59 (0.20) 1.60 (0.22) 1.55 (0.25) 1.37 (0.16) 1.05 (0.22) 0.99 (0.10)

The numbers in parentheses represent errors obtained from at least three independent experiments.

Figure 5. Dependence of the total cell resistance, R, on the concentration of NH4PF6 as supporting electrolyte. Data for a bare 25 µm radius platinum microelectrode are shown on the lower curve (b) while the upper curves are for the same microelectrode modified with a thick (≈1 µm) [(Ru(bpy)2)2bpzt-]3+ solid deposit (9).

states and can be used to probe ionic interactions. Therefore, by examining the electrolyte concentration dependence of E°′, information about the extent of ion pairing can be obtained. For the solid deposits considered here, as the concentration of supporting electrolyte was increased, the oxidation and reduction waves became more ideally reversible. However, as discussed above, differences in the energetics of ion ingress and egress, as well as electrochemical irreversibility and ohmic effects, may make it invalid to take the average of Epa and Epc as a measure of E°′. To address this issue, potential step chronocoulometry has been used to determine the redox composition of the deposit as a function of the applied potential. To eliminate the influence of any slow kinetic processes such as slow heterogeneous electron transfer, ion transport, or redox induced structural changes, the deposits were exhaustively electrolyzed by recording the charge passed for periods up to several hundred seconds. It is important to discriminate between the charges associated with double layer charging, Qdl, and with the Faradaic process of interest, QF. The charge associated with the double layer was determined from the short time scale response, and this charge was subtracted from the total charge passed. In all cases, Qdl was less that 10% of the total charge passed. Figure 6 illustrates a Nernst plot of E versus ln [Ru3+/Ru2+] using data obtained in this way. Consistent with the predictions of the Nernst equation, these plots are linear for both redox processes within the dimer irrespective of whether the deposit is initially fully reduced or oxidized. Moreover, the slopes observed for the first and second reduction processes, as

Figure 6. Dependence of the redox composition as determined using exhaustive potential step chronocoulometry of a [(Ru(bpy)2)2bpzt-]3+ solid deposit. The supporting electrolyte is aqueous 0.1 M NH4PF6, and the working electrode is a 1.5 mm radius glassy carbon electrode.

well as for oxidation and reduction, are experimentally distinguishable. Significantly, the slope observed, 0.032 ( 0.003, is similar to that expected on the basis of the Nernst equation, 0.315, for a one-electron-transfer process. The formal potentials, E°′, as determined from the intercepts of these plots, are 1.007 ( 0.005 and 1.200 ( 0.004 V for the first and second oxidation processes, respectively. These values are significantly more positive, by approximately 40 and 35 mV for the first and second processes, respectively, than the average of the cyclic voltammetry Epa and Epc values. This observation indicates that only at a time scale of several hundred seconds does the redox and ion composition, as well as perhaps the physical structure, reach a global equilibrium, making it meaningful to apply the Nernst equation. Figure 7 shows that, for both redox processes, increasing the concentration of supporting electrolyte causes the E°′, as determined using long time scale potential step chronoamperometry, to shift in a negative potential direction. This negative shift indicates that oxidation of the redox center becomes thermodynamically more facile at high electrolyte concentrations and is consistent with ion pairing between the electrolyte anion and the redox centers. This situation is summarized in the following Nernstian reaction:

[RuLRu]3+(X-)ζ - e- + p(X-) T [RuLRu]4+(X-)ζ+p [RuLRu]4+(X-)ζ+p - e- + q(X-) T [RuLRu]5+(X-)ζ+p+q where both redox forms are considered to participate in the ion pairing equilibria.

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Figure 7. Dependence of the formal potential on log[NH4PF6] as supporting electrolyte for the first (b) and second (9) metal base redox processes within [(Ru(bpy)2)2bpzt-]3+ solid deposits. Figure 9. Dependence of the homogeneous charge transport diffusion coefficient on the concentration of NH4PF6 as supporting electrolyte for solid deposits of [(Ru(bpy)2)2bpzt-]3+: (9) oxidation; (O) reduction.

the supporting electrolyte is 0.1 M NH4PF6. For scan rates up to 2000 mV s-1, as illustrated in the figure inset, the voltammetric peak currents increase as υ1/2. This behavior is consistent with semi-infinite linear diffusion control in which the deposit is not exhaustively electrolyzed and the depletion zones remain within individual microparticles. Under these circumstances, the peak current, ip, can be described in terms of the Randles-Sevc¸ ik equation:

ip ) (2.69 × 105)n3/2ADCT1/2Ceffυ1/2

Figure 8. Scan rate dependence of the voltammetric response for a deposit of [(Ru(bpy)2)2bpzt-]3+ formed on a 25 µm radius platinum microelectrode. The supporting electrolyte is 0.1 M NH4PF6. From top to bottom, the scan rates are 10 000, 3500, 1000, and 200 mV s-1. Inset: ip vs υ1/2 for films under these conditions.

Figure 7 illustrates a plot of E°′ versus the logarithm of the PF6- concentration for both redox processes. Since the theoretical slopes for these semilog plots are (59/p) and (59/q) mV/decade, the difference in the numbers of ions that are paired with the redox center in its reduced and oxidized states can be determined for both metal based redox processes. Taking into account activity effects, the slopes of 74 ( 5 and 65 ( 5 mV/decade for the charge states 3+/4+ and 4+/5+, respectively, are close to the 59 mV/decade response expected for the situation where a single extra anion becomes paired with the redox center as each ruthenium center is oxidized. Homogeneous Charge Transport Rates. The welldefined metal based oxidations observed for this solid deposit make it an attractive system for investigating the dynamics of charge transport in the solid state. To achieve this objective, cyclic voltammetry has been used to determine apparent diffusion coefficients as the concentration of supporting electrolyte is systematically varied. For macroelectrodes, the ohmic loss becomes significant at the higher scan rates causing large ∆Ep values and distorted voltammograms to be observed. For this reason, solid deposits have been formed on microelectrodes and used to probe the dynamics of charge transport through the solids. Figure 8 shows how the voltammetric responses obtained for solid deposits change as the scan rate is systematically varied from 200 to 10 000 mV s-1, where

(2)

where n is the number of electrons transferred, A is the area of the working electrode, DCT is the apparent diffusion coefficient, and Ceff is the effective fixed site concentration of the redox center. Measurement of the density of the solid, steady-state voltammetric studies on structurally related systems, and crystallographic data all suggest that the fixed site concentration is 1.5 ( 0.1 M.22 Analyzing the data presented in Figure 8 using this approach yields DCT values of (1.0 ( 0.26) × 10-10 and (2.3 ( 1.0) × 10-10 cm2 s-1 for oxidation and reduction of the solid deposit in 0.1 M NH4PF6, respectively. While these diffusion coefficients are large for solid deposits, they are still many orders of magnitude smaller than those found for the complex dissolved in solution, (6.2 ( 1.1) × 10-6 cm2 s-1. These relatively small diffusion coefficients are likely to significantly limit the technological exploitation of materials of this kind. For example, under semi-infinite linear diffusion conditions, it would take approximately 15 s to switch a monolithic 1 µm thick film from one oxidation state to another. Effect of Changing Electrolyte Concentration on DCT. Where the overall rate of charge transport is limited by the movement of charge compensating counterions, DCT is likely to increase significantly with increasing electrolyte concentration. Randles-Sevcik plots of ip versus υ1/2 are linear for 50 e υ e 2000 mV s-1 where 0.1 < [NH4PF6] < 2.0 M for both oxidation and reduction processes and for both redox couples. The slopes of these plots have been used in conjunction with eq 2 to probe the effect of the electrolyte concentration on DCT. Figure 9 illustrates a plot of DCT versus electrolyte concentration for the first oxidation process and the first reduction process. It is seen that the diffusion coefficient increases (22) Juris, F.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; von Zelewsky, A. Coord. Chem. Rev. 1988, 82, 85.

Solid Deposits of a Ruthenium Dimer

Langmuir, Vol. 18, No. 25, 2002 9881

with increasing electrolyte concentration. However, DCT is not very sensitive to the supporting electrolyte concentration and increases by a factor of less than 4 on going from 0.1 to 2.0 M NH4PF6. Assuming that electron hopping represents the rate determining step, the Dahms-Ruff equation23,24 can be used to calculate the electron self-exchange rate constant from the maximum DCT value. This equation is given by

DCT ) Dphys + 1/6kSEδ2C

(3)

where Dphys describes physical diffusion in the absence of electron hopping, C is the concentration of ruthenium centers within the solid, and δ is the intersite separation between adjacent ruthenium centers. Electron selfexchange may occur across the bridging ligand (throughbond electron transfer) or between adjacent dimers within the solid (through-space electron transfer). However, to fully oxidize the deposit, electrons must be transferred between adjacent complexes, and on the basis of crystallographic data25 for structurally related bipyridyl complexes, the electron-transfer distance is taken as the separation between adjacent complexes, 9.6 Å. Given that we are dealing with crystalline solid deposits, Dphys is assumed to be zero. The maximum DCT value yields self-exchange rate constants ranging from approximately 0.2 to 1.1 × 105 M-1 s-1. This value is approximately 2 orders of magnitude smaller than the values typically reported for osmium and ruthenium polypyridyl complexes in solution4 or within monolayers.26-28 Given the assumptions made in the calculation of kSE, the value obtained does not allow an unambiguous determination of whether counterion motion or electron transfer limits the overall rate of charge transport. However, the data indicate that the rate of charge transport is significantly higher than that found in structurally related systems, for example, for solid deposits of [Os(bpy)2 3,5-bis(pyridin-4-yl)-1,2,4,-triazole (23) Dahms, H. J. Phys. Chem. 1968, 72, 362. (24) Ruff, I.; Friedrich, V. J.; Demeter, K.; Csillag, K. J. Phys. Chem. 1971, 75, 3303. (25) Juris, F.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; von Zelewsky, A. Coord. Chem. Rev. 1988, 82, 85. (26) Charych, D. H.; Majda, M. Thin Solid Films 1992, 210, 348. (27) Charych, D. H.; Anvar, D. J.; Majda, M. Thin Solid Films 1994, 242, 1. (28) Lee, W.-Y.; Majda, M.; Brezesinski, G.; Wittek, M.; Mo¨bius, D. J. Phys. Chem. B 1999, 103, 6950.

Cl], DCT is 6.3 × 10-12 cm2 s-1 in 1.0 M HClO4, that is, more than 2 orders of magnitude smaller than those found here.6 Conclusions Solid deposits of [Ru(bpy)2 bpt Ru(bpy)2]3+ formed on macro- and microelectrodes exhibit well-defined oneelectron-transfer processes for both metal centers where the supporting electrolyte is aqueous NH4PF6. SEM images reveal that before cycling the solid particles are amorphous in nature and approximately 1-20 µm in diameter. After cycling, a mixture of crystal and amorphous particles is observed. Most of the crystals were of similar size to the original amorphous particles, but some larger crystals were formed (approximately 100 µm in diameter), which suggests that Oswald ripening may have occurred. This electrocrystallization process appears to be facilitated by the higher solubility of the oxidized (5+) compared to the reduced (3+) form. The charge transport diffusion coefficient for the solid-state process is approximately 4 × 10-10 cm2 s-1 for both oxidation and reduction of the deposit in 2.0 M NH4PF6. This small diffusion coefficient prevents steady-state behavior from being observed even at micron dimensioned electrodes within accessible time scales. The observations that the peak-to-peak separation decreases with increasing PF6- concentration and that DCT increases with increasing supporting electrolyte concentration suggest that ion- rather than electrontransfer limits the overall rate of charge propagation through the solid. This conclusion is supported by calculations of the apparent rate of electron self-exchange using the Dahms-Ruff equation, which gives an electron self-exchange rate constant of the order of 104 M-1 s-1, that is, approximately 2 orders of magnitude smaller than that typically found for ruthenium bis-bypridyl complexes in solution. We expect that these investigations of redox active solid deposits will support their application as mediators in the development of reagentless biosensors. Acknowledgment. The financial support from the Higher Education Authority under the Program for Research in Third Level Institutions is gratefully acknowledged. The generous loan of potassium hexachloroosmate(IV) by Johnson Matthey under the loan scheme is deeply appreciated. LA025928E