766
J. Phys. Chem. 1995, 99, 766-772
Electron-Transfer Kinetics of Self-Assembled Ferrocene Octanethiol Monolayers on Gold and Silver Electrodes from 115 to 170 K John N. Richardson, Stephen R. Peck? Larry S. Curtin,* Leonard M. Tender,$ Roger H. Terrill, Michael T. Carter, and Royce W. Murray* Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290
Gary K. Rowel and Stephen E. Creager Department of Chemistry, Indiana University, Bloomington, Indiana 47405 Received: July 26, 1994; In Final Form: October 17, 1994@
The electron-transfer kinetics of CpFeCp(CH2)sSH and of CpFeCpCOz(CH2)sSH monolayers cochemisorbed with n-octanethiol diluent on gold and silver electrodes have been measured in chlororethanehutyronitrile solvent over temperatures of 115- 170 K by cyclic voltammetry and potential step experiments. Rate constants for CpFeCpC02(CH2)gSH monolayers are smaller than those for CpFeCp(CH2)sSH monolayers by a factor approximately equivalent to insertion of two -CH2- groups. Activation plots of rate constants k" for CpFeCp(CH2)8SH and for CpFeCpCOz(CH2)gSH monolayers on gold give reorganization energies (A) of about 0.95 eV which is -0.2 eV larger than predicted from dielectric continuum theory. Broadened voltammetric peaks and curved potential step ln(i) vs time plots indicate a kinetic inhomogeneity among the surface ferrocene population; the effects of this inhomogeneity appear to cause artificially small values of reorganization energies (0.02-0.34 eV) when cyclic voltammetric wave shapes and log(kApp) vs 17 plots are analyzed for 1. Preliminary results for rate constants k" for CpFeCp(CH2)sSH monolayers on Ag are similar to those on Au but with a smaller barrier energy.
Self-assembled alkanethiol monolayers on Au electrodes have recently gained attention as excellent platforms for investigations of electron-transfer dynamics.1-5 Specifically, the well-ordered alkane chains can be made to act as spacers between an electron donorlacceptor species and the electrode surface, which allows inspection of the electronic coupling and distance dependence of the electron-transfer process. Also, cochemisorption of other alkanethiols with a redox terminated thiol can be used to manipulate and explore microenvironmental effects on the electron-transfer r e a ~ t i o n , ~an. ~ important current theme in electrochemical science.8 This report presents a study of the electron-transfer dynamics of two ferrocene-terminated octanethiol monolayers, chemisorbed on Au and on Ag electrodes, over a 115-170 K range of temperatures in a cryoelectrochemical solvent9 of chloroethane/butyronitrile/O.O75 M Bu4NPF6. The octanethiols, CpFeCp(CH2)sSH (Cp = cyclopentadienyl) and CpFeCpC02(CH2)sSH, are cochemisorbed with a diluent, CH3(CH2)7SH, to minimize ferrocene-ferrocene interactions. lo A previous'l qualitative study showed that CpFeCp(CH2)8SH monolayers exhibit exceptionally stable voltammetry in this cryoelectrochemical solvent. In the present work, heterogeneous electron-transfer rate constants ko, are extracted from cyclic voltammetric h E p ~ data u by comparisons to voltammograms'2 simulated using the Marcus relation13 and accounting for the density of electronic states in the electrode (analogous to theory by Chidsey'). The dependency of these rate constants on Present address: E. I. DuPont de Nemours & Co. (Inc), P. 0. Box 1217. Parkersbure. WV 26102. Resent addre&: Department of Chemistry, Temple University, Philadelphia, PA 19122. I Present address: Department of Chemistry, Stanford University, Stanford, CA 94305. '-Present address: Department of Chemistry, University of North Carolina, Chapel Hill, NC 27599-3290. Abstract published in Advance ACS Abstracts, December 15, 1994.
*
@
0022-365419512099-0766$09.00/0
temperature is used to evaluate the reorganization energies ( I ) to electron transfer for the two monolayers. The reorganization energy is ca. 0.2 eV higher than theory for CpFeCp(CH2)sSH and CpFeCpCO2(CH2)sSHmonolayers on Au and the same as theory for CpFeCpCOz(CH2)sSHmonolayers on Ag electrodes. There are no previous low-temperature kinetic data for redoxtagged alkane thiols on Au and none at all on Ag surfaces. Rate constants for the ferrocene monolayer electrode reactions were also measured by the potential step method described by Chidsey.' The current transients obtained produce curved ln(i) vs time plots indicating a kinetic dispersion in the ferrocene surface population; the kinetics were analyzed by measuring the overpotential dependency of segments of the reacting population. Reorganization energies are in principle evaluable both from the dependency of reaction rate on the potential applied and from the temperature dependency of reaction rate at zero overpotential (Le., k"). The barrier 1 measured from the temperature dependency of ko for CpFeCpCOz(CH2)sSH monolayers on Au agrees with that measured by cyclic voltammetry, whereas the I value obtained from ln(rate)overpotential plots is much smaller. Measuring reorganization energies by the latter procedure appears to give specious results in the present experimental system.
Experimental Section Chemicals. Ferrocene thiols CpFeCpCOz(CH2)8SH and CpFeCp(CH2)sSHwere prepared using literature p r o ~ e d u r e s ' ~ J ~ a-Octylmercaptan (Aldrich, >97%), butyronitrile (Aldrich, 99%), tetrabutylammonium hexafluorophosphate ( B W F 6 , Fluka, puriss), and absolute ethanol (AAPER Alcohol and Chemical Co.) were used as received. Chloroethane (Linde) was condensed and stored in sealed vacuum transfer pipets at room temperature. Monolayer Preparation. Thiol monolayers were assembled on gold or silver disk electrodes (0.5 mm diameter) which were 0 1995 American Chemical Society
ET Kinetics of Ferrocene Octanethiol Monolayers
J. Phys. Chem., Vol. 99, No. 2, 1995 161
TABLE 1: Heterogeneous Electron-Transfer Rate part of an electrode assembly discussed elsewhere." Electrodes Constants Obtained for a CpFeCp(CH&SH Monolayer' on were first polished with 1pM aqueous alumina (Buehler), then Ag from Cyclic Voltammetric AE~EAK Resultd 1 p M diamond paste (Buehler), followed by extensive rinsing T, K v, mVls A&EAK,mV kocV,C s-' To mol cm-2 x loLo with water and sonication. The electrode assembly (which included a 2 mm diameter Au disk quasi-reference electrode) 120 10 76 0.085 2.11 88 0.12 2.16 20 was then etched in dilute aqua regia (3:1:6 HCl:HN03:H20, by 50 120 0.15 2.21 volume) for 5 min,14rinsed successively with water and ethanol, 159 0.13 2.19 100 and placed in an ethanol coating solution containing 1:3 mole 2.16 200 198 0.11 ratio ferrocene akanethiol to n-akanethiol diluentlO (1 mM total 2.17 500 271 0.063 thiol concentration) for 24 h. For Ag electrodes, the etching 1000 339 0.035 2.16 step was deleted and the electrode exposed to the ethanolic thiol 135 50 61 0.70 2.25 15 75 0.70 2.31 solution for only 30 min to minimize sulfiding side reactions." 100 78 0.90 2.26 Electrodes were thoroughly rinsed with ethanol and water and 200 100 1.10 2.26 dried after removal from the coating solutions. Chemisorbed 500 146 1.05 2.24 thiol was removed from the Au quasi-reference disk electrode lo00 183 1.05 2.23 surface by abrasion with fine-grit sandpaper (Buehler). 150 100 51 2.0 2.38 200 56 3.4 2.38 Electrochemical Measurements. The cylindrical electrode 500 90 3.8 2.29 assembly prepared as above was fitted into a slotted stainless 1000 120 4.1 2.30 steel sleeve whose bottom, serving as the auxiliary electrode, "Prepared from a 1:3 (mo1:mol) ethanol coathg solution of was set at ca. 0.5 mm from the monolayer-coated working 0.5 h. Uncompensated resisCpFeCp(CHz)$3Hand CH~(CHZ)~SH, electrode to minimize cell resistance. This assembly was in tances determinedusing ac impedance at T = 120,125,130,135,140, turn placed into an A1 container with 0.5 mL of dry ice145, 150, 155, 160, and 170 K are R ~ =c 1.7 x lo6, 5.5 x l@, 2.3 prechilled (to avoid thiol desorption) butyronitrile/B&dVPF6 x lo5, 9.3 x 104, 4.1 x 104, 2.7 x 104, 1.6 x 104, 1.1 x lo", 1.0 x (0.075 M final cell concentration), and the cold container (kept lo4,7.2 x lo3 Q, respectively. Measured from experimental A E ~ K values as described in text using a working plot like Figure 2 for 1 = cold using a dry ice/acetone slush bath) sealed and evacuated. 0.69 eV. Chloroethane (1 mL) was vacuum transferred into the cell which was then immediately bolted to the cold finger of a Janis helium gram in Figure lA, experimental AEPE.A.K = 154 mV, RIJNC = refrigerator cryostat controlled by a Lakeshore Cryogenics 320 1.7 x lo6 Q and ~PEAKRIJNC20 mV; by these parameters this Autotuning Temperature Controller. Equilibration at target magnitude of i R ~ cassuming , no positive feedback compensatemperatures (1 15-140 K) was achieved, starting from ca. 273 tion had been applied, would depress the determined rate K, within 4 h. constant by ca. 30%. Given that the positive feedback Staircase potential sweeps and potential steps were generated compensation actually applied in the experiment would reduce digitally and converted to analog excitations with a Tecmar the iRmc and the error in ko considerably below the above Labmaster D/A board. The staircase sweeps were 2.44 mV estimate, and that this is a near-worst case example (Le., RIJNC steps with current sampling at 1-2 kHz, recording the average is largest at 120 K), we belive that resistance effects on data current for each step. This is a good approximation of a linear like that of Table 1 are minimal. potential sweep. Currents were measured using a potentiostat of local design. Limited low-pass filtering was employed for Results and Discussion long time scale experiments. Data were stored and analyzed digitally using locally written software. Electrode Kinetics by Cyclic Voltammetry. Cyclic voltamAll potential sweeps and steps were instrumentally corrected mograms of monolayers of the ferrocene-terminated ester for solution resistance effects with positive feedback compensaCpFeCpC02(CHz)gSHand allcanethiol CpFeCp(CH2)gSH on Au tion.15 We have found that this older technique is well-suited electrodes are quite stable and very well defined at 120 K to the moderately long time scales of low-temperature electro(Figure 1, panel A) and 150 K (Figure 1, panel B). Monolayer chemical measurements. A feedback potential proportional to voltammetry on Ag electrodes is similarly behaved (Figure lA, the cell current is added to the sweep or step potential applied - - -). Integration of charges under the reduction branches to the working electrode to compensate for the "ohmic" potential gives surface coverages (rc) corresponding to roughly 20-30% drop (iRmc) resulting from the electrolyte ionic resistance of a full monolayer of ferrocene sites.1° between working and reference electrodes. Feedback potentials The increases in voltammetric peak potential separations that just equal iRmc produce an unstable cell circuit (current AEPEAK at lower temperature (Figure 1A vs B) or with large oscillation); stability is reestablished by decrease of feedback potential scan rates (Table 1) reflect control of the voltammetry compensation by 5- 10% by the rate of heterogeneous electron transfer reactions of the Heterogeneous electron-transfer rate constants measured from ferrocene monolayers. Ohmic losses (I'RIJNc)appear to be cyclic voltammetry using positive feedback compensation negligible when positive feedback compensation is employed exhibit little or no dependence on the potential sweep rate (a (Table 1). (This potential problem has been extensively criterion for absence of i R ~ distortion c from the results). examined; see Experimental Section.) Values of ~EPEAK are Example data are shown in Table 1 for a CpFeCp(CH2)sSH larger for monolayers of CpFeCpCO2(CHz)gSH than for CpFeCp monolayer chemisorbed on silver. Except for the fastest sweep (CH&SH, indicating slower kinetics for the former films. rate at 120 K, the measured (vide infra) ko values vary by less Extraction of electron-transfer rate constants ko from AEPEAK than a factor of 2 over 1-2 decades of sweep rate variation. results is done by comparison to AEPEAKparameters of voltammograms digitally simulated for various rate constants Additionally, the product of voltammetric peak current (which ko, reorganization energies A, potential sweep rates v, and are quite small) and R ~ (measured c directly with ac impedance; temperatures, plotted as working curves of AEPEAKvs log(v/ values given in Table 1 footnote) gives a simple estimate of the possible magnitude of iRmc relative to experimental A E ~ E A K ko). This procedure, described in full elsewhere,12ais analogous to the well-known use of cyclic voltammetry in determining values. For the CpFeCp(CH2)gSWAu monolayer voltammo-
Richardson et al.
768 J. Phys. Chem., Vol. 99,No. 2, 1995
2E-08
1
I
i A
1200 1000
i--
- 2 - 1 0
5E-08 CI
3
4
5
various ratios of potential sweep rate and rate constant, log(v/
o
kO) and different values of 1. As we foundlZaat 298 K, AEPEAK is mildly insensitive to 1(especially at larger A), and so ko can
5
0
-5E-08 -1E-07 0.6
2
Figure 2. Simulated curves of AEPEAK vs log(v/ko, units of volts) at 120Kfor1= l.OeV(0),0.8eV(V),0.6eV(O),0.4eV(A),and0.2 eV (+). Note mild dependence of AEPEAK on 1 below 9 = 300 mV.
h
9 =g!
1
log (v/ka)
1E-07
0.4
0.2 0 -0.2 Overpotential (V)
-0.4
-0.6
Figure 1. Panel A: cyclic voltammograms at 120 K (v = 50 mV/s) of (solid curve) CpFeCpCOz(CH2)sSH monolayer on Au (r,= 1.1 x mol/cm2, AEPEAK = 298 mV); (bold curve) CpFeCp(CH2)sSH monolayer on Au (r,= 1.2 x 1O-Io mol/cm*, AEPWK= 154 mV); (dashed curve) CpFeCp(CH&SH monolayer on Ag (r,= 2.2 x mol/cm2, = 120 mV). Panel B: cyclic voltammograms at 150 K of (solid curve) CpFeCpCOz(CH2)sSH monolayer on Au (v = 100 mV/s, rc= 1.3 x mol/cm2,A E p =~96 mV) and (bold curve) CpFeCp(CH2)sSH monolayer on Au (v = 300 mV/s, r, = 1.4 x mol/cm2, AEPEM = 41 mV). Monolayers prepared from 1:3 (mol: mol) ethanol coating solutions of ferrocene thiol and CHs(CH&SH diluent. the electrode kinetics of diffusingl58l6 and electrode-boundI7 redox species but relies on Marcus theory13 rather than the classical Butler-Volmer theory15J7to connect a reaction rate constant kv to electrode overpotential 7 ( = D p E M / 2 ) . Marcus theory is more appropriate than Butler-Volmer theory when 7 is not negligible in relation to the reorganization energy 1 for the reaction and has been applied in several recent investigations of electrode kinetics of d i f f u ~ i n g ~and , ~ ~electrode-bound species.1-3~6~12*19 The specific relation combines the Marcus relation with an integration of the continuum of electronic states in the metal electrode'
where p is the distance-dependent coupling between the redox species and the electrode, e the density of states of the metal electrode, kb the Boltzmann constant, and 7 the applied overpotential relative to the formal potential E"'. Equation 1 gives ko when q = 0. Using eq 1 to calculate a cyclic voltammogram requires a value of reorganization energy 1as well as a trial value of k" and v. We have shown,la on the other hand, that at small ratios of vlko, it is not required that il be known accurately to obtain meaningful results for ko. Figure 2 shows MPEM parameters at 120 K for voltammograms digitally simulated with eq 1 for
be estimated by comparing experimental values of AEPEAK to Figure 2 with only a crude specification of 1. For the present purposes, we have used "outer-sphere" reorganization energies calculated from the dielectric continuum mode11J3~20
where e is the electronic charge, N Avogadro's number, €0 the permitivity of free space, a the molecular radius of the redox probe, d the distance of the redox probe from the electrode surface, topthe optical dielectric constant, and ts the static dielectric constant.21 Equation 2 gives221 = 0.68 eV and 0.71 eV for CpFeCp(CH2)gSH and CpFeCpCOz(CHz)gSH, respectively; working curves like those in Figure 2 were constructed for these values of 1 and used to determine k" from experimental AEPEAK values. Rate constants thus obtained as a function of v and temperature (averaging results at different v) are given in Tables 1 and 2 (indicated as kocv), respectively, for the two ferrocenethiols chemisorbed on Au surfaces and for (discussed later) CpFeCp(CH2)gSH chemisorbed on Ag. Table 2 shows that, on average, ko at any given temperature is ca. 1l-fold larger for CpFeCp(CH2)gSH monolayers relative to those of CpFeCpC02(CH2)gSH. This factor may reflect the role of the ester linkage in the electronic coupling between metal electrode and ferrocene site. If the ester linkage is assumed to approximate two methylene spacers, using a coupling term @) of l . l l / m e t h ~ l e n e ,a~ ~rate difference of about !&fold is predicted. Given the similarity to the observed result, the ester linkage has a kinetic effect equivalent to extension of the alkane chain by two methylene groups. Reorganization energies were determined from the temperature dependencies of the rate constants ko in Table 2 based on the Arrhenius equation: ko = ,u@kbTexp(-AG*/kBT)
(3)
where A the usual preexponential frequency factor becomes the ,u@kbTprefactor of eq 1, AG* the free energy of activation is assumed equal to the enthalpy of activation and is related13 to the reorganization energy by 1 = 4AG*, and Tis temperature. We assume a negligible entropy of activation based on previous dataN for the ferrocene+'0 couple. Plots25of h(ko/kbT'") vs 1/T for the cyclic voltammetric results for CpFeCp(CH&SH and
J. Phys. Chem., Vol. 99,No. 2, 1995 769
ET Kinetics of Ferrocene Octanethiol Monolayers
TABLE 2: Heterogeneous Electron-Transfer Rate Constants for CpFeCpCOz(CH2)sSH and CpFeCp(CH2)sSH Monolayers on Au and Ag electrodes gold silver CpFeCpCO2(CHz)sSH CpFeCp(CHt)8SH CpFeCp(CH2)sSH kocv,LI k"T.w,b kocv,F k"T.w,b k"cv,d k0Tm: T, K s-l S-1 S-1 S-1 S-1 S-1 115 0.012 0.099 120 0.0056 0.069 0.02 0.19 125 0.017 0.22 0.03 0.43 0.56 130 0.043 0.097 0.05 0.96 0.92 135 0.21 0.10 2.0 2.0 1.6 1.5 140 0.40 0.30 4.1 2.1 145 0.67 8.2 3.3 150 1.4 155 160 2.4 3.9 165 170 6.6 "Determined from experimental A E ~ values ~ K with theoretical working plots like Figure 2, for A = 0.71 eV, rate constants are averages of results over a range of potential sweep rates; extrapolation in a plot of in(k0,&~")vs 1/T gives k0273 K = 4.1 x lo3 s-I and p@= 1.4 x lo9 eV-' s-l (see text). Determined from analysis of oxidation branch of a log(kMp) vs composite plot for fust 10-20, 20-30, and 3040% segments of reacting ferrocene population. As in (a) but with working plot for 1= 0.68 eV, extrapolation in a plot of ln(ko/kJ'ln) ' eV-I s-l. vs 1/T gives k0273 K = 4.5 x 104 s-l and p@ = 1.7 x 1OO As in (a) but with working plot for 1= 0.68 eV, extrapolation in a plot of h(ko/kbT"2)vs 1/T gives k0273 K = 2.2 x lo3 s-' and p@= 5.9 x lo7 eV-' SKI. e As in (b) but from the reduction branch.
1
$: *L - 3
Tafel ana&\
\
1 0.0055
0.0075
0.0065
0.0085
l/T (K-') Figure 4. Activation (Arrhenius) plots of data in Table 2, for kocv of CpFeCpCOz(CH2)gSH on Au (0,bG* = 0.24 eV, I = 0.95 eV, j q = 1.4 x lo9 eV-I s-l, k0273 = 4.1 x lo3 s-l) and ~OT.A.F of CpFeCpCOz(CHz)sSH on Au (0,bG* = 0.24 eV, I = 0.95 eV, p@= 1.0 x lo9 eV-l s-I, Po273 = 3.1 x lo3 s-l).
TABLE 3: Reorganization Energies for Electrode Reactions of CpFeCpCOt(CHz)sSHand CpFeCp(CH&SH Monolayers on Au and Ag, from Activation Plot and log(kApp) vs q Analyses gold silver CpFeCpCOz(CHt)&H CpFeCp(CH2)SSH CpFeCp(CH&SH Itheory?'
eV 0.71
AARRH? IT-:
eV eV 0.95b 0.34 0.95'
ltheory?
eV 0.68
I-?
eV 0.96
ATAF:
eV 0.15
&.RH?
eV 0.70
ATAFld
eV 0.20
10 9 - 0
'Equation 2. From Cyclic voltammetric data, plot of ln(kOcv/kbPn) vs 1/T. From Oxidation branch of 140 K log(kApp) vs 7 plot, best fit using eq 1. As in ( c ) but reduction branch, 140 K. e As in ( b ) but using koT@ data.
F 7 9 6
K for comparison with those results. Extrapolating his resultsz9 on the alkane chain-length dependence of ko to N = 8 gives ko273 % 3 x lo3 s-l and a corresponding (via his result for A, 0.85 eV, and eq 1) value of ,u@= 3.7 x lo8 eV-' s-l. Extrapolation of Figure 4 (0)gives e 2 7 3 % 4.1 x lo3 s-l and ,u@= 1.4 x lo9 eV-' s-l for the K N / E t C l solvent. Similarly, good agreement results in analogous comparisons for the 12 and 16 carbon ferrocene alkanethiols, described e l s e ~ h e r e . ~ ~ , ~ ~ Considering the extrapolations and the differences in solvent dielectric properties, these results are in excellent agreement and lend confidence to the reorganization energies derived Erom the low-temperature results in Figure 3 and 4. While the voltammetric waves of CpFeCp(CH2)gSH monolayers are somewhat asymmetrical?2those of CpFeCpCOz(CHz)8SH are well-formed and symmetrical. They are, however, much broader than quantitatively expected; a reversible surface wave would have a FWHh4 of ca. 45 mV at 150 K, which is small compared to the results in Figure 1B. The voltammetric peak broadening is thoughtlZato arise from a dispersion in the rates at which the individual electroactive ferrocene sites react (due to a dispersion in E"' andlor in the actual ko)" and unfortunately invalidates use of voltammetric wave shapes to measure the reorganization energy A. Although AEPEAK values are rather insensitive to the value of A, the actual wave shapes are shown from theoretical calculations to be very sensitive12 to A. Decreasing values of 1 act to broaden the wave shape, especially when overpotential is comparable to A. Figure 5 shows an attempt at fitting a theoretical voltammogram to the voltammetric wave shape of a CpFeCpCOz(CH2)sSH monolayer; a fair fit can be obtained but only with unrealistically small and temperature-dependent reorganization energies (A = 0.15 -0.02 eV). The proposed kinetic dispersion-related peak broadening
Q
0'
255 E - 4
32 1t 0.006
1
I
0.007
0.008
0.009
1/T (K-') Figure 3. Activation (Arrhenius) plots of data in Table 2, for PCV of CpFeCp(CH2)gSH on Au (0,G* = 0.24 eV, I = 0.96 eV, p@= 1.7 x 1O1O eV-' sw1, P273 = 4.5 x 104 s-') and Ag (0,AG* = 0.174 eV, I = 0.70 eV, p@ = 5.9 x lo7 eV-' s-l, P273= 2.2 x lo3 s-l).
CpFeCpCOz(CH2)8SH monolayers are shown in Figures 3 (0) and 4 (O), respectively; values of A thus obtained are given in Table 3 (as A m ) . The results for the ester and alkane ferrocene thiols are essentially identical and about 0.2 eV larger than predicted by eq 2. (Use of a Figure 2 working plot that assumed A = 0.9 eV would produce no significant change in the results.) It is not presently possible to assign an origin to the ca. 0.2 eV difference between the predicted and experimental reorganization energies. An unforeseen inner-sphere barrier contribution, the approximationz6 of eq 2 and of our averaging of the binary solvent dielectric properties, a nonnegligible entropy of activation, interaction between the hydrophobic monolayer and the organic solvent are all potential factors. We believe the kinetic dispersion evident in potential step results (vide infra) is on the other hand not a likely f a ~ t o r . ~ ~ , ~ ~ Since Chidsey has reported 273 K data29for CpFeCpCOz(CH~)NSWCH~(CH~)N-~SH monolayers on gold (in aqueous perchloric acid), it is instructive to extrapolate Figure 4 to 273
770 J. Phys. Chem., Vol. 99,No. 2, 1995
5 5E-09 h
f
Richardson et al. -12 1 -13 4 --14
I
CI
c
a
5 0
0
3
a-17 c -18 -19 -I1 I -20 L '
-
-5E-09 -1E-08 -1.5E-08 0.4
0.2 0 -0.2 Overpotential (V) Figure 5. Oxidation wave shape fits for monolayer prepared from a 1:3 (molmol) coating solution of CpFeCpCOz(CH2)sSHand CH3(CH2)7SH on Au. Experimental voltammogramsacquired at 50 mV/s at 120K (0,r, = 1.1 x lo-'' mol/cmz,leftmost curve), 130 K (v, r, = 1.2 x mol/cm2),and 140 K (0,rC= 1.3 x moYcm*);respective simulated voltammograms (solid curves) using best-fit parameters ko = 0.008 s-l, 1 = 0.15 eV at 120 K ko = 0.06 s-l, 1 = 0.06 eV at 130 K and ko = 0.21 s-l, 1 = 0.02 eV at 140 K. The unrealistically small 1 values are thought to be caused by kinetic dispersion (see text). mimics a small reorganization energy, producing, we believe, erroneously small results for 1. Only when kinetic inhomogeneities are absent, as evidenced by linear In(i) vs time plots in potential step experiments,' can the stimulated-voltammetric wave shapes be employed to evaluate kl*In the present case, In(i) vs time plots are not linear (vide infra). Electrode Kinetics Using Potential Step Experiments. In an experiment in which the applied electrode potential is suddenly changed from E"' to an oxidizing or reducing value, the current for the first-order reaction of the ferrocene monolayer ideally follows the equation'*2
where kapp,ais the sum of the forward and reverse electrontransfer rate constants at the overpotential q,and Q is the charge passed during the electron-transfer reaction. Equation 4 predicts a linear plot of In(i) vs time with slope of kappaThe linearity of such a plot indicates that all the sites in the redox population react at the same rate, i.e., the monolayer is kinetically homogeneous. From study of a large number of potential step experiments for ferrocene thiol monolayers in the low-temperature PrCN/ EtCl solvent, we have concluded that improperly compensated i R ~ can c seriously compromise quantitative kinetic determinations. Figure 6 shows representative examples of current transients (as ln(i) vs time plots) for a 17 = 150 mV potential step at a CpFeCp(CHz)&H monolayer-coated Au electrode at 140 K in which positive feedback compensation is (-) and is not (bold -) applied. The differences are dramatic. The distortions in the absence (bold -) of positive feedback compensation cause a deceiving linearity, but resistance compensation reveals a decidedly more curved decay plot (-). The changing slope indicates a substantial kinetic dispersion (about a factor of 10-fold in rate) in the ferrocene monolayer population. It is unclear at the present time whether this dispersion reflects real differences in the rate constants of individual ferrocene sites or only apparent ones owing to a dispersion in the sites' formal potential^,^^ or both, or double layer effects.33 In either case, it seems for the purpose of comparing electrode kinetics at varied overpotentials sufficient to analyze the data
*
0.0
I
0.05 Time(s)
0.1
Figure 6. ln(i) vs time plots at 140 K for a 150 mV potential step initiated from E"' of a CpFeCp(CH2)7SH monolayer on Au (prepared and from a 1:3 (mo1:mol) coating solution of cpFecp(cH~)~SH CHs(CH&SH diluent), without (bold) and with (solid)positive feedback compensation employed. The vertical lines delineate the first 2040% of reacting ferrocene sites, by charge passed. Currents corrected for background using 150 mV potential steps in double layer potential region.
\
0.5
-0.1 0 0.1 0.2 0.3 Overpotential(V) Figure 7. Plot of experimental log(kApp,s)vs 7 at 140 K for (0,top to bottom at each 7 ) fust 10-20,20-30, and 30-40% of ferrocene sites reacting (by charge segments) in a CpFeCp(CH2)7SH monolayer on Au. Solid curves are best-fit from eq 1, for (oxidation branch) ko~m = 2.0 S - ' , = 0.15 eV, and pug = 1.16 x lo3 eV-' s-I , and (reduction branch) k"~m= 3.0 s-I, l ~ = m 0.17 eV, and pug = 2.6 x lo3 eV-' s-l. Bold curves are calculated from eq 1 for k" = 2.0 s-' and 1 = 0.68 eV (oxidation branch) and ko = 3.0 s-' and 1 = 0.68 (reduction branch). The fitted LTAF and p@ values are thought to be artifacts.
-0.3
-0.2
as different segments of reacting populations. The vertical lines in Figure 6 encompass the fist 20-40% of reacting sites (based on charges passed). Figure 7 shows a typical Tafel plot, at 140 K, of the overpotential dependency of rate constants derived from the average slopes of plots like Figure 6 for the 10-20, 20-30, and 30-40% segments of the surface population. It is apparent that the rate constants of the different segments exhibit similar dependencies on applied overpotential and that there is a mild asymmetry between the oxidation and reduction branches, the latter giving somewhat larger reaction rates. Analogous to the cyclic voltammetric wave shapes, Figure 1A (-), asymmetry was always present in data for CpFeCp(CH2)gSH chemisorbed on Au, but never.in data for CpFeCpC02(CHz)gSH monolayers. Plots like those in Figure 7 were examined for mixed CpFeCp(CH2)gSWCH3(CH2)7SH and CpFeCpCO2(CH2)sSW CH3(CH&SH monolayers on Au and Ag, following Chidsey' and FinkIea2, to select values of ko and 1 (and thereby pe) in eq 1 that best fit the experimental data. Figure 7 (-) represents such a best fit to the (average) experimental results in the three 10-40% reacting population segments of a CpFeCp(CH2)gSH
J. Phys. Chem., Vol. 99, No. 2, 1995 771
ET Kinetics of Ferrocene Octanethiol Monolayers monolayer; kApp = 2kO at q = 0 in these curves. Values of rate constants derived from similar fits are given in Table 2 (as ~OTAF). These data agree, within a factor of 2, with the results kocv from analysis of cyclic voltammetric data. Also, Arrhenius plots of the kTm0 data for CpFeCpC02(CH2)8SH monolayers (Figure 4 (0)and Table 3) yield a reorganization energy -A = 0.95 eV, which agrees with that derived from ~OCV. Table 3 also shows values of reorganization energies ATAF derived in overpotential-rate constant fitting l i e that in Figure 7. These results are much smaller than those derived from the temperature dependence ( A m ) of rate constants. The fitting of dTAF in Figure 7 is quite sensitive to the curvature in the log(kApp)-overpotential plots; the bold curves in Figure 7 are drawn for a value of A = 0.68 eV and clearly are far from a satisfactory fit to the data. We believe that the ATAF values obtained (0.15-0.35 eV) are, like those derived from cyclic voltammetric waveshape fits (Figure 5), unrealistically small. Calculations strongly indicate27 that the shapes of log(kApp) vs q plots will be affected by any kinetic dispersion that is present. Calculations that if a dispersion in formal potential of magnitude similar to that observed (Figure 1B) is assumed, the log(kApp) vs q plot is changed in a way such that when analyzed with eq 1 as in Figure 7, erroneously small values of A are produced. The presence of inadequately compensated cell resistance, as a secondary influence on the results, can also have the effect of artificially depressing the observed A. We regard the use of potential stepderived log(kMp) vs 7 plots for measurement of reorganization energies as requiring caution when either kinetic dispersion or resistance effects are present. Kinetics at Silver Electrodes. There are no previous electrode kinetic data for self-assembled monolayers on Ag surfaces. We have observed8 that low-temperature cyclic voltammetry of monolayers of CpFeCp(CH2)gSH cochemisorbed with CH3(CH2)7SH on silver is well-defined (Figure 1A) and stable for long periods of time. Rate constants kO for these monolayers obtained by analysis of the voltammetric AEPEAK values (Table 1) are in good agreement with results on Au electrodes over the 120-150 K temperature range (Table 2). The monolayers on Ag also are kinetically inhomogeneous, giving curved ln(i) vs time plots in reductive potential step experiments at 140 K. (Oxidative potential steps were not collected out of concern of initiating Ag oxidation.) The PTAF derived from the average 10-40% segment of the reacting ferrocene population, 1.5 s-*, is in good agreement with that derived at 140 K from cyclic voltammetry 1.6 s-l (Table 2), but as was seen on Au, the barrier 1 derived from the fit of eq 1 to the rate-overpotential data again seems unrealistically small (0.2 eV, Table 3). While comparing kOcv results at specific temperatures indicates similar kinetic behavior of CpFeCp(CH2)gSHmonolayers on Au and Ag surfaces (Table 2), the temperature dependency of the Ag monolayer kinetics seems to be somewhat different. An Arrhenius plot of the Ag results (Figure 3 (0))is not very linear and produces 1 = 0.70 eV, in good agreement with the dielectric continuum results but somewhat smaller than the result on Au (Table 3). Since our data on Ag surfaces are less extensive than those on Au, the silver result should be taken as a preliminary value.
Acknowledgment. This research was funded in part by grants from the Office of Naval Research and the National Science Foundation.
References and Notes (1) (2) (3) 9, 223. (4)
Chidsey, C. E. D. Science 1991,251, 919. Hanshew, D. D. J. Am. Chem.SOC.1992,114,3173. Finklea, H. 0.; Ravenscroft, M. S.; Snider, D. A. Langmuir 1993, Finklea, H. 0.;
Becka, A. M.; Miller, C. J. J. Phys. Chem. 1992, 96, 2657. ( 5 ) Miller, C.; Cuendet, P. 0.; Gratzel, M. J. Phys. Chem. 1991, 95,
877. (6) Ravenscroft, M. S.; Finklea, H. 0.J. Phys. Chem. 1994,98,3843. (7) Rowe, G. K.; Creager, S. E. Langmuir 1991, 7, 2307. (8) Bard, A. J.; Abruna, H. D.; Chidsey, C. E. D.; Faulkner, L. R.; Feldberg, S.; Itaya, K.; Majda, M. M.; Melroy, 0.; Murray, R. W.; Porter, M.; Soriaga, M.; White, H. S. J . Phys. Chem. 1993, 97, 7147. (9) Ching, S. McDevitt, J. T.; Peck, S. R.; Murray, R. W. J. Electrochem. SOC. 1991, 138, 2308. (10) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Musjsce, A. M. J. Am. Chem. SOC.1990, 112,4301. (11) Curtin, L. S.; Peck, S. R.; Tender, L. M.; Murray, R. W.; Rowe, G. K.; Creager, S. E. Anal. Chem.1993,65, 386. (12) (a) Tender, L. M.; Carter, M. T.; Murray, R. W. Anal. Chem. 1994, 66, 3173. (b) Weber, K.; Creager, S. E. Anal. Chem. 1994, 66, 3164. (13) Marcus, R. A. J . Chem. Phys. 1%5,43, 679. (14) Creager, S. E.; Hockett, L. A.; Rowe, G. K. Langmuir 1992, 8, 854. (15) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1980. (16) Nicholson, R. S. Anal. Chem. 1%5, 37, 1351. (17) Laviron, E. J. Electroanal. Chem. lnterfacial Electrochem. 1979, 101, 19. (18) Richardson, J. N.; Harvey, J.; Murray, R. W. J . Phys. Chem., submitted for publication. (19) Nahir, T. M.; Clark, R. A.; Bowden, E. F. Anal. Chem. in press. (20) Equation 2 is approximate for the ferrocene monolayers, in which the ferrocene site lies at the boundary of a low dielectric (alkane chain) monolayer of constrained mobility and the fluid R C N E C l electrolyte phase. This may alter both its average dielectric environment and the nature of the image term in eq 2. The approximation of eq 2 was also noted by Chidsey.' (21) copand cs display slight but linear temperature dependencies; their values were extrapolated to 175 K where cs = 36.51, cop= 2.05 for RCN and cS = 18.08, cop= 2.05 for EtCl. The change of these constants with temperature is so gradual that these values were judged sufficient for the lower temperatures employed as well. Assuming a mole fraction-weighted averaging of the dielectric properties of the mixed solvent gives cop= 2.05 and cs = 23.42. (22) In eq 2, a = 3.8 x m, d = 9.6 x m (CpFeCp(CH2)sSH) and 12 x m (CpFeCpCO2(CHz)sSH); the values of d assume a 30' tilt angle from the surface normal and the ester linkage was approximated as two additional methylene units. (23) The rate constant is expected to be proportional to exp[-(electronic coupling parameter)(length of alkane chain)]. A coupling parameter of 1.11/ methylene was reportedz9 from data on a series of ferrocene thiols, CpFeCpCOt(CHz)KH3. (24) Genett, T.; Milner, D. F.; Weaver, M. J. J. Phys. Chem. 1985, 89, 2787. (25) (a) In(ko/kb?"n),rather than the h(ko/kbr) indicated by eq 3, is used to include the temperature dependency of p (=(HAB2/h)(d&T)'"). The difference is minor; a plot of h(koCV/kbn vs 1/Tgives 1 = 0.93 eV for the CpFeCpCOz(CH2)12SH data in Table 2. (b) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265. (26) (a) Analysis26b of metal field penetration and solvent spatial correlation effects produces predictions of AOS somewhat different from eq 2. For the specific case of a = 3.8 A, d = 12 A, ECP= 2, and Es = 20. Figure 2 of ref 256 predicts los values smaller than those of eq 2, whereas the experimental 1 found here are larger. (b) Phelps, D. K.; Kornyshev, A. A.; Weaver, M. J. J. Phys. Chem. 1990, 94, 1454. (27) Rowe, G. K.; Carter, M. T.; Richardson, J. N.; Murray, R. W., submitted for publication. (28) The effects of kinetic heterogeneity on cyclic voltammetry and potential step experiments have been modelled as a Gaussian distribution of E"' and of Ip and are more fully discussed elsewhere?' Pertinent to the present experiments are simulations of cyclic voltammograms based on a Gaussian distribution of E"' with u = 41 mV, and with l = 0.8 eV, pe = 1 x lo6 eV-I s-l, and r] = 0 mV, which gives at 125 K, cyclic voltammograms which when analyzed by the method used here give a potential scan rate-independent (5 mV/s to lo00 V/s) apparent rate constant k" (1.15 x s-l), whereas the average ko was 5.7 x s-l. A similar comparison at 150 K gave an apparent ko of 2.6 x s-' relative to an s-'. Thus, the presence of kinetic dispersion average one of 1.5 x causes a slight elevation (2.03- and 1.71-fold, respectively) of the measured ko (relative to the average ko), Le., the method slightly favors the "faster" reacting sites. This bias is nearly temperature independent, producing in
772 J. Phys. Chem., Vol. 99,No. 2, 1995 this example only a -5% error in 1 and a -13% error in re. These errors lie within experimental uncertainty, and we conclude that an ,!?-based kinetic dispersion has little consequence on ko, 1,and determinationsby cyclic voltammetry. Consistent with this conclusion are the nearly identical results in an actual measurement of ko for ferrocene C( 16) alkanethiol monolayers, which were121and were not1 kinetically disperse. (29) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992,43, 437. (30) Carter, M. T.; Rowe, G. K.; Richardson, J. N.; Tender, L. T.; Tenill, R. H.; Murray, R. W., submitted for publication.
Richardson et al. (31) Richardson, J. N.; Rowe, G. K.; Carter, M. T.; Murray, R. W., submitted for publication. (32) Oxidative peak broadening, accompanied by increasing A E m with no change in double-layercapacitance, occurs as the coverage of CpFeCp(CH2)sSH is increased relative to diluent. This behavior is attributable to femene-ferrocene interactions in the crowded monolayer. Voltammetric asymmetry was not observed for the ferrocene-tagged ester thiol. A similar pattern of behavior was reported by Chidsey.’o (33) Creager, S.E.; Weber, K. Langmuir 1993,9, 844.
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