J . Phys. Chem. 1987, 91, 6663-6669 that if we compare the Christiansen effect in different carbons, with one or more probe adsorbates, we can empirically evaluate the similarity/dissimilarity of their internal surfaces. Distorted band shapes in activated carbons have been recorded previously;2 however, probably because of reasons such as low signal-to-noise (e.g. Mattson et al.) and low signal-to-background (Bouwman et al.), they have not been mentioned in the text of the articles. We believe that if our techniques are used to remeasure the samples from earlier studies, then enhanced trans-
6663
mission will be confirmed in many of those experiments as well. Acknowledgment. I am grateful to Dr. Barbara Bolton for sharing her insights into the biphenyl structure and to Dr. Jim Scherer for his interest in the spectral distortions. I also wish to thank Dr. Brian York for making the X-ray measurements and Dr. Leo Volpe for his insights into the carbon structure. Registry No. C, Engelhard CG-5, Norit RB-1,7440-44-0; H20, 7732-18-5; COz, 124-38-9;biphenyl, 92-52-4.
Organized Self-Assembling Monolayers on Electrodes. 2. Monolayer-Based Ultramicroelectrodes for the Study of Very Rapid Electrode Kinetics Eyal Sabatani and Israel Rubinstein* Department of Materials Research, The Weizmann Institute of Science, Rehovot 76100, Israel (Received: May 1 , 1987)
Organized monolayers were constructed on gold electrodes by self-assembly of octadecyl derivatives with trichlorosilane or mercaptan head groups. The monolayers, which are highly oriented and densely packed, provide effective blocking of electrochemical reactivity at coated electrodes. With fractional surface coverages 6 close to unity, the remaining exposed electrode surface 1 - 6 is distributed as an array of extremely small ultramicroelectrodes with an average diameter of 5-10 nm. It is shown that such electrodes provide distinct advantages in various types of fundamental electrochemical studies, including background suppression, electron-transfermediation, and most notably, in the measurement of very large heterogeneous electron-transfer rate constants ko. Several such cases are demonstrated, including convenient determination of ko values as high as 5.0 cm/s. Values of ko measured in the present work are in good agreement with those calculated from known self-exchange rate constants by using the Marcus relationship.
Introduction Chemically modified electrodes have been the subject of considerable interest for more than a decade.' The modification of electrode surfaces involves the attachment of chemical substances to the electrode surface by physical adsorption, chemical bonding, or polymer coating. The modified electrode can thereby exhibit properties related to those of the modifying substance, e.g., electron-transfer mediation; acceleration or inhibition of electrode reactions; chemical, electrostatic, or steric selectivity; photosensitivity; and so on. It is quite clear that in order to perform increasingly complicated and demanding tasks using chemically modified electrodes, complex molecular structures would have to be constructed, with a high degree of control over the design and structure of the systems. This need has been recognized and tackled to some extent in the case of polymer films on electrodes, where it was demonstrated that by constructing polymeric bilayers on electrodes in a rational manner the electrode system may display unique Organized organic monomolecular layers appear highly promising as building blocks for chemical microstructures on electrodes. Such monolayers, comprising amphiphilic molecules which are assembled as compact oriented monomolecular layers on solid supports, provide a means of controlling the structure at the molecular level. Two principal methods are used to prepare such monolayers (and multilayers) on various substrates: the wellknown Langmuir-Blodgett (LB) techniq~e,~.' where an ordered (1) Murray, R. W. In Electroanalytical Chemistry; Bard, A. J., Ed.; Dekker: New York, 1984; Vol. 13, pp 191-368. (2) Pickup, P. G.; Kutner, W.; Leidner, C. R.; Murray, R. W. J . Am. Chem. SOC.1984, 106, 1991. (3) Kittlesen, G. P.; White, H. S . ; Wrighton, M. S . J . Am. Chem. SOC. 1985, 107, 7373. (4) Rubinstein, I. J . Electroanal. Chem. 1985, 195, 431. (5) Rubinstein, I.; Rubinstein, I. J . Phys. Chem. 1987, 91, 235. (6) (a) Langmuir, I. J . Am. Chem. SOC.1917, 39, 1848. (b) Langmuir, I. Trans. Faraday SOC.1920, 15, 62.
0022-3654/87/2091-6663$01.50/0
monolayer (usually comprising long-chain hydrocarbon molecules with a polar head group) is transferred from the water-air interface onto a solid surface by applying mechanical pressure; and the self-assembly technique,* based upon spontaneous adsorption of similar molecules from appropriate organic solutions onto solid surfaces. In the latter technique the polar head groups are designed to specifically interact with the substrate or chemically bind to it. This, as well as the fact that the self-assembly process is spontaneous, implies that such monolayers are inherently more stable than those obtained by the LB technique. Sagiv and co-workers have thoroughly investigated silane-based self-assembling monolayer^.^ They have shown that monolayers of long-chain amphiphiles of this kind constructed on various solid substrates are highly organized in a densely packed structure, with the molecules arranged in a preferred orientation perpendicular to the substrate surface. The trichlorosilane head groups, which hydrolyze with traces of water during adsorption, may either covalently bind to certain substrates (e.g., glass or silicon), in-plane linearly polymerize, or do both. The merits of organized monolayer techniques for electrode modification have been recognized recently by several groups, using both the LB technique'*I2 and self-assembling monolayers.I3J4 (7) Blodgett, K. B. J . Am. Chem. SOC.1935, 57, 1007. (8) Bigelow, W. C.; Pickett, D. L.; Zisman, W. A. J . Colloid Sci. 1946, 1, 513. (9) (a) Sagiv, J. Isr. J. Chem. 1979, 18, 346. (b) Maoz, R.; Sagiv, J. J . Colloid Interface Sci. 1984, 100, 465. (c) Gun, J.; Iscovici, R.; Sagiv, J. Ibid. 1984, 101, 201. (d) Pomerantz, D. M.; Segmuller, A.; Netzer, L.; Sagiv, J. Thin Solid Films 1985, 132, 153. (e) Maoz, R.; Sagiv, J. Ibid. 1985, 132, 135. (f) Cohen, S . R.; Naaman, R.; Sagiv, J. J. Phys. Chem. 1986,90, 3054. (10) (a) Daifuku, H.; Aoki, K.; Tokuda, K.; Matsuda, H. J. Electroanal. Chem. 1985,183, 1. (b) Park, S. G.; Aoki, K.; Tokuda, K.; Matsuda, H. Ibid. 1985, 195, 157. (1 1) (a) Fujihira, M.; Poosittisak, S . J. Electroanal. Chem. 1986, 199, 481. (b) Fujihira, M.; Araki, T. Ibid. 1986, 205, 329. (12) Facci, J. S . ; Falcigno, P. A,; Gold, J. M. Langmuir 1986, 2, 732. (13) Miller, C. J.; Majda, M. J . Am. Chem. SOC.1986, 108, 3118. (14) Finklea, H.0.;Robinson, L. R.; Blackburn, A,; Richter, B.; Allara, D.; Bright, T. Langmuir 1986, 2, 239.
0 1987 American Chemical Society
6664 The Journal of Physical Chemistry, Vol. 91, No. 27, 1987
The majority of these publication^'^'^ deal with electroactive monolayers on electrodes as means of obtaining organized electroactive microstructures. A basic concern with such monolayer films is the degree of molecular organization. The bulky electroactive groups or counterions included in the monolayers are likely to disturb the compactness and thus the degree of organization of the systems. The mere fact that the monolayers exhibit electroactivity implies free movement of counterions therein, and thus a decreased compactness and organization. A different approach was undertaken by Finklea et al.,14 who studied the organization and blocking properties of octadecyltrichlorosilane on gold electrodes toward electroactive species in solution. They reported good blocking of the surface by the monolayer film and suggested the existence of small electroactive pinholes. A major drawback in the work of Finklea et a l l 4 was the instability of the films; they were reported to deteriorate rapidly upon performing electrochemical measurements, in fact during a single voltage scan. This severely limited the electrochemical characterization of the system and prevented any possible application. Recent publications from this describe the preparation and characterization of organized monolayer films on gold electrodes, prepared from octadecyltrichlorosilane(OTS), octadecyl mercaptan (OM), or both. We have demonstrated that highly organized monolayers of OTS and O M can be prepared on flat gold electrodes (gold deposited on microscope slides). Reflection-absorption FTIR (RA-FTIR) and wettability measurements indicated densely packed, perpendicularly oriented hydrocarbon chains in the all-trans conformation. The monolayers were highly stable in repeated electrochemical experiments for prolonged periods of time and in different environments. We have used the characteristic gold oxide formation/removal peaks in cyclic voltammetry in H2S04for quantitative measurement of the fractional surface coverage 0, assuming that areas covered with the dense monolayer are electrochemically inert. It was found that superior blocking properties, i.e. 0 > 0.99, can be obtained with mixed monolayers (OTS OM), where the monomeric O M is adsorbed onto OTS-covered slides. It was found that residual pinholes in the otherwise blocking monolayers behave as an array of extremely small microelectrodes, possibly in the size range 5-10 nm7.l6 Microelectrodes and microelectrode assemblies are presently the subject of considerable interest.I7 Microelectrode assemblies exhibit unique diffusional behavior depending on the size, spacing, and the time domain of the experiment.I8 It thus appears that the monolayer-based microelectrode assembly, with extremely small size and fine distribution, may provide unique possibilities. It was already shown by Amatore et al.I9 that under certain conditions an electrode reaction at a microelectrode assembly will appear kinetically slower than the same reaction measured at a macroelectrode. We have shown,16 using the Fe(CN),4-/3- couple, that this predicted phenomenon, combined with the extremely small size and fine distribution of the monolayer-based microelectrodes, can be used for the study of rapid electrode kinetics. With the Fe(CN)6e/3- couple, which is only moderately fast, we were able to compare ko values obtained with monolayer-coated electrodes to those obtained with macroelectrodes, and demonstrated excellent agreement for a wide range of coverages.I6 These results provide a strong support for the microelectrode model, indicating that in the range of coverages studied the oxide removal peaks are a true measure of the total pinhole area available for an undisturbed penetration of relatively large ions.
+
(15) (a) Rubinstein, I.; Sabatani, E.; Maoz, R.;Sagiv, J. Ext. Abstr. Electrochem. SOC.1986, Vol. 86-1, Abstr. No. 546; (b) In Elecrrochemical Sensors for Biomedical Applications; Li, C.K.N., Ed., Electrochemical Society: Pennington, 1986; Proc. Vol. 86-4. (16) Sabatani, E.; Rubinstein, I.; Maoz, R.;Sagiv, J. J.Electroanal. Chem. 1987, 219, 365. (17) See for example: Bard, A. J.; Crayston, J. A,; Kittlesen, G. P.; Shea, T. V.; Wrighton, M. S . Anal. Chem. 1986,58, 2321 and references therein. (18) Reller, H.; Kirowa-Eisner, E.; Gileadi, E. J . Electrwnal. Chem. 1982, 138, 65. (19) Amatore, C.; Saveant, J. M.; Tessier, D. J. Electrounal. Chem. 1983, 147, 39.
Sabatani and Rubinstein The first part of the present paper describes in detail the characterization of monolayer-based microelectrode assemblies. Some applications in signal-to-noise improvement in electrochemical experiments are then exemplified. The remainder of the paper describes the determination of very high heterogeneous electron-transfer rate constants ko using monolayer-coated gold electrodes and the ac-impedance technique, in aqueous as well as organic solvents. A convenient determination of ko of up to 5.0 cm sdl is demonstrated. Experimental Section Chemicals. Octadecyltrichlorosilane(OTS) (Merck), octadecyl mercaptan (OM) (Fluka), K,Fe(CN),, (Merck, GR), K,Fe(CN), (BDH), and FeC1, (Merck, GR) (for the preparation of Prussian blue), ascorbic acid (Merck, Extra Pure), R u ( N H ~ ) ~ C (Strem ~, Chemicals), and tetrabutylammonium fluoroborate (TBAF) (Southwestern Chemicals) were used as received. Tris(2,2’-bipyridine)ruthenium (Ru(bpy),*+) perchlorate was prepared by metathesis of the corresponding chloride salt (Strem Chemicals). Ferrocene (Fc) (Aldrich Chemcal Co.) was recrystallized from petroleum ether. Trimethylaminomethylferrocene (Me,N+MeFc) perchlorate was prepared by metathesis of the corresponding iodide salt (Strem Chemicals). Acetonitrile (Bio-Lab, H.P.L.C. Grade), CHC1, (Frutarom, AR), CCl, (Frutarom, AR), bicyclohexyl (Aldrich), and “Electrostatic Dispersant” (Hunt Chemical Corp.) were all purified by passing through basic alumina (ICN Biomedicals). Aqueous solutions were prepared with triply distilled water. All solutions were deaerated by passing Ar. Cell and Instrumentation. A conventional three-electrode cell was used. For quantitative electrolysis an auxiliary electrode, separated by a porous glass plug, was added. A mercurous sulfate reference electrode ( M S E +0.400 V vs SCE) was used in sulfate solutions;a saturated calomel electrode (SCE) was used in all other aqueous solutions. (Potentials in the figures are plotted with respect to a SCE.) A Ag/Ag+ reference electrode separated by a porous glass plug was used in acetonitrile solutions. All dc and ac electrochemical measurements were performed using a Solartron 1286 potentiostat and a Solartron 1250 frequency response analyzer controlled by a Zenith microcomputer. Ac measurements were done at 5 mV rms around E” of the studied couple, and readings were taken at seven discrete frequencies per decade. Procedure. Flat gold electrodes were prepared as described previously16by sputter-deposition of 100-nm gold on precleaned glass microscope slides. OTS and O M were adsorbed by generally following the published p r o c e d ~ r e .OTS ~ ~ ~was ~ adsorbed from a solution of 2.0 X lo-, M OTS in 80% bicyclohexyl 12% CCl, 8% CHC1,; O M was adsorbed from a solution of 1.0 X low2 M O M in 90% bicyclohexyl 10% CHC1,. Alternatively, OTS and OM can be adsorbed from solutions of similar compositions in which “Electrostatic Dispersant” (a branched hydrocarbon mixture) replaced the bicyclohexyl. Au/(OTS OM) electrodes were prepared by adsorption of O M onto OTS-coated electrodes.I6 Adsorption times were 15 min for OTS and 5-15 min for OM, depending on the desired final surface coverage. Monolayer-coated electrodes were routinely tested (before and after electrochemical experiments) by contact angle measurements and RA-FTIR,I6 to assure a high degree of order and directionality as well as monolayer stability. Solutions used for ac-impedance measurements always contained equal concentrations of the oxidized and reduced forms of the redox couple. They were prepared by dissolving one form ( R u ( N H ~ ) ~Me,N+MeFc, ~+, or Fc) and performing quantitative electrolysis at a constant current until the potential of a Pt wire in the solution reached the standard redox potential of the couple (taken as E I l 2determined from cyclic voltammetry). It was verified in all cases that the potential of the Pt wire remained constant during the ac measurements.
-
+
+
+
+
Results Electrochemical Characterization of GoldlMonolayer Electrodes. The total fraction of pinhole area in Au/monolayer electrodes was determined quantitatively by performing cyclic
The Journal of Physical Chemistry, Vol. 91, No. 27, 1987 6665
Organized Self-Assembling Monolayers on Electrodes A
0 2I m A I
m
A
B
1
C
D
T
W
uuuu
00 0 2 04 GO 0 2 0 4 00 02
04 00 0 2
04
E (VOLTS v s SCE ) I
04
I
I
I
I
C
00 I2 E (VOLTS vs SCE)
I
16
Figure 1. Steady-statecyclic voltammograms in 0.1 M H2S04(scan rate, 0.10 V/s) for bare Au (A), Au/OTS of 1 - B = 0.17 (B), and Au/(OTS OM) of 1 - 8 = 0.0024 (C). Geometric area, 1.60 cm2.
+
Figure 3. Cyclic voltammmograms (first scan) for 3 mM Fe(CN)$- in 0.5 M KCI (scan rates, 0.025, 0.05, 0.10 V/s) for bare Au (A) and Au/(OTS OM) of 1 - 0 = 0.038 (B), 0.021 (C), 0.0055 (D). Geometric area, 0.25 cm2.
+
A 8 ' I
JOTS
1 OM
f
8
B
4 LC
F i p e 2. Schematicdrawing of the model for OTS and OM adsorption. OTS polymerizes laterally upon adsorption, while OM is adsorbed as
monomeric units. TABLE I: Voltammetric Results Summarized for the Electrodes in Figure 3" at 0.025 V/s at 0.05 V/s at 0.10 V/s 1-8 ,i AE, i, hE, i, AE, 1.oo 1.00 60 1.00 60 1.00 60 100 75 1.00 85 0.038 1.00 0.94 95 0.77 110 0.74 130 0.021 0.81 0.0055
0.81
155
0.74
180
0.72
230
"i, is the anodic peak current normalized to the corresponding one for the bare electrode (3A). AE, is the peak separation in mV.
voltammetry in 0.1 M H2S04.'53'6Typical voltammograms for different electrodes are shown in Figure 1. Note that the voltammograms in Figure 1B,C were perfectly stable upon repetitive cycling. The integrated area under the oxide removal peak, indicating the degree of water penetration to the gold surface, is taken as a measure of the total fractional pinhole area 1 - 0, 0 being the fraction of monolayer coverage. In Figure 1, 1 - 0 is 0.17 and 0.0024 for 1B and lC, respectively, compared to the integrated area for the bare electrode (1A). This behavior is typical and indicates the improved barrier efficiency obtained with Au/(OTS + OM) compared to Au/OTS. The possibility of having more than a single monolayer with OTS O M can be ruled out on the basis of contact angle values and RA-FTIR results such as those given in ref 16. Thus, we believe that the improved blocking is due to the different modes of binding of the two monolayer-forming molecules to the gold substrate.I6 As shown schematically in Figure 2, the polymeric OTS bridges over certain surface irregularities and thus provides a monolayer network with superior stiffness and directionality;" the monomeric OM blocks the remaining sites which are not compatible with the polymeric OTS due to steric hindrance. Note that measurements of 1 - 0 as described above were always performed before and after each of the electrochemical experiments described in the following sections, to ensure the stability of the monolayer films as well as the invariance of 1 - 0. In order to gain a better insight into the detailed distribution of pinholes in monolayer films on electrodes, linear scan voltammetric experiments with Fe(CN)," were performed, with electrodes of different 1 - 0 and at various scan rates. A set of results of this kind is shown in Figure 3 and summarized in Table I. As seen in Figure 3 and Table I, it can be generally said that with fractional surface coverages, 0 I 0.98, very small if any decrease
+
Figure 4. Schematic drawing of the model for the diffusion to microe-
lectrode assemblies at different times (details in the text). in peak currents is observed at intermediate scan rates. With increased coverage a moderate peak current diminution is observed, but even with 1 - 0 = 0.0055 (Le., only 0.55% of the electrode area is active) the decrease in ,i is only about 30%. At the same time, a substantial increase in peak-to-peak separation AE, is observed with increased surface coverage, indicating an apparent decrease in the rate of the heterogeneous electron-transfer step. The theoretical model for diffusional behavior of microelectrode a s s e m b l i e ~ , schematically ~~J~ drawn in Figure 4, predicts three types of behavior, depending on the microelectrode size and distribution and the time scale of the experiment: At relatively short times, when the diffusion layer thickness 6 is small compared to the microelectrode diameter 4 and the distance d between the centers of two adjacent microelectrodes, a macroelectrode-type behavior (semiinfinite linear diffusion) is expected, corresponding to the cumulative areas of the individual microelectrodes (4A). At longer times when 6 becomes large compared to 4 but still small compared to d, a microelectrode-type behavior is expected with characteristic spherical diffusion to individual microelectrodes (4B). At even longer times when 6 becomes large relative to d , the individual spherical diffusion layers overlap, and a semiinfinite linear diffusion is again expected, this time corresponding to the entire geometric area (4C). Under the latter conditions one expects diffusion-controlled currents comparable to those obtained with a bare electrode. The fact that only a fraction (and possibily, as in our case, only a very small fraction) of the total area is active will manifest itself under these conditions as an apparent kinetic 1imitati0n.l~Thus, a linear scan voltammogram taken under the conditions of Figure 4C is expected to display nearly &normaln peak currents and increased peak separation. Evidently, the voltammetric behavior of monolayer-based microelectrode assemblies conforms quite well to the theoretical predictions. It deviates, however, from ideal behavior in that the decrease in peak currents in Figure 3C,D should in principle be accompanied by a gradual change to a polarogram-type shape,lg indicative of the transition from the situation of Figure 4C to that of 4B. This is not clearly observed in our case. (A transition to a polarogram shape is observed only at even higher coverages). We attribute this nonideal behavior to a certain distribution in the size and spacing of microelectrodes around an average value,
Sabatani and Rubinstein
6666 The Journal of Physical Chemistry, Vol. 91, No. 27, 1987
A
TABLE 11: Voltammetric Results at High Scan Rates for Au/(OTS + OM) Electrodes (Y,,, d , 4 Are Defined in the Text) 1- a 0.010 0.005
VIS 4-10 2-4
vmnx,
d, nm
4, nm
94-60 94-6
9.4-6.0 6.6-4.6
a situation which is not accounted for in the theoretical models. According to Amatore et al.,I9 one can estimate the characteristic parameters C#I and d for a given microelectrode assembly by gradually increasing the scan rate until a transition is observed from situation 4C to 4B. If v,,, is the scan rate corresponding to the transition, thenI9
where, for disk-shaped microelectrodes
f(i - e) = o . q i - e)-l/2
(2)
4 = d(l - B)’/*
(3)
With our system, the partially nonideal behavior complicates the choice of v,,,; however, to obtain a rough estimate of C#I and d we used for the calculation the scan rate at which a deviation of more than 5% in the linearity of i,, vs ul/* is observed. e was calculated from the gold oxide peaks (as in Figure 1). Experimental results are given in Table I1 for two electrodes. Values in the range d 50-100 nm and 4 5-10 nm are typical for electrodes of high surface coverage. It thus appears that the residual natural pinholes in the monolayers on gold substrates provide a fine array of extremely small ultramicroelectrodes, which can be reasonably well characterized electrochemically. Improved Electrochemical Signal io Noise. An outstanding property of the monolayer-based ultramicroelectrodes is the voltammetric behavior exhibited at moderate scan rates for diffusion-controlled reactions (reflecting the geometric surface area) compared to surface reactions at the same electrodes (reflecting the real surface area). Thus, with an electrode of, say, 1 - 0 = 0.02, voltammetric currents originating from surface reactions (such as oxide formation/removal) will decrease by a factor -50 (as in Figure 1) relative to the naked electrode, whereas peak currents corresponding to diffusion-controlled reactions will be little affected (as in Figure 3). This can be used to achieve substantial signal-to-noise improvement in certain electrochemical experiments. Two such examples are demonstrated below. In Figure 5 the voltammetric behavior of Ru(bpy);+ at a bare Au and at a Au/(OTS OM) electrode of 1 - e = 0.015 is shown. If the concentration of Ru complex in solution is chosen to be small as in Figure 5, the voltammetric peaks for the redox couple appear as barely distinguishable, small shoulders, superimposed on the much larger background peaks of the gold oxide. With the Au/(OTS OM), on the other hand, the situation is completely different. The oxide peaks are suppressed proportionally to 1 8, while the diffusion-controlled peaks for the Ru complex remain practically unaffected and thus well resolved at the higher sensitivity used. The second example is presented in Figure 6 , showing the catalytic oxidation of ascorbic acid (AA) with Prussian blue (PB, Fe,(Fe(CN)&). The electrochemical oxidation of AA is irreversible at a gold electrode, with a rather large overpotential (at pH 6 , Eo is more negative than -0.10 V vs SCE,*Owhile E,, at Au/(OTS OM) = +0.40 V). The PB, deposited on the electrode according to the published procedure*’ (the amount of PB deposited was proportional to 1 - e), serves as an electron transfer mediator-catalyst for the oxidation of AA. Due to the low concentration of AA chosen in Figure 6 , at a bare Au electrode the catalytic increase in the PB peak current is very small, about an order of magnitude smaller than the background PB peak. On
-
-
+
+
+
~~~~
~
(20) Facci, J.; Murray, R. W. Anal. Chem. 1982, 54, 112. (21) Itaya, K.; Akahoshi, H.; Toshima, S. J . Electrochem. Soc. 1982, 129, 1498.
L
06
I
I
I
I
1.0 I.4 E (VOLTS vs. SCE)
Figure 5. Cyclic voltammograms (first scan) for 0.5 mM Ru(bpy),2+ in 0.1 M H 2 S 0 4 (scan rate, 0.05 V/s) for bare Au (A) and Au/(OTS OM) of 1 - = 0.015. Geometric area, 1.55 cm2. Arrows indicate the Ru(bpy),*+ peaks.
+
u 00
04 00 E (VOLTS vs SCE)
04
Figure 6. Dashed lines: Cyclic voltammograms (first scan) in 0.1 M KCI (pH 6.0) for Prussian blue deposited on bare Au (A) and on Au/(OTS OM) of 1 - 0 = 0.02 (B) (scan rate, 0.05 V/s). Solid lines: Same, with 1.0 mM ascorbic acid added. Geometric area, 0.5 cm2.
+
+
the other hand, with a Au/(OTS OM) electrode the PB background surface waves are greatly suppressed, while the catalytic wave for AA is almost twice the size of the background, with only a small residual cathodic peak. Thus, with the Au/(OTS OM) electrode a greatly improved sensitivity for AA is observed, indicating that AA catalytic oxidation by adsorbed PB is partly controlled by AA diffusion to the electrode. This example, therefore, emphasizes both the considerably increased sensitivity of the measurement as well as the usefulness of Au/(OTS OM) electrodes in estimating the relative importance of surface and diffusion processes in an overall electrode reaction.22 Determination of Very Large Heterogeneous Rate Constants. As discussed above (see Figure 3 and Table I), an important phenomenon expected for microelectrode assemblies under appropriate conditions is an apparent decrease in the rate of the heterogeneous electron transfer. This provides, in principle, the possibility of “slowing down” a rapid electrode reaction to the extent that it can be conveniently studied, provided that the “real” rate constant can be then calculated from the apparent one. As will be shown below, this decrease is directly proportional to the
+
+
(22) Porat, 2.; Zinger, B.; Tricot, Y.-M.; Rubinstein, I., to be published.
Organized Self-Assembling Monolayers on Electrodes
The Journal of Physical Chemistry, Vol. 91, No. 27, 1987 6667 300
IO
x
p
1
200
0
"
"
~
100
~
~
~
~
~
200 300 Z' (OHM CM2)
'
"
~
400
"
'
500
"
~
'
"
cm s3
+ OM) Electrodes
Me,N+MeFc+/O 0.46 4.06 0.67 3.35
7.3 3.7
0.0040 66.5 0.0020 158 0.0015 288
7.8 3.3 1.8
1.95 1.65 1.20
0.0007
3.5
5.0
,
,
, ,,
i
.
1
1 4
1
'-IOK
0
" , , ' , , ~ " ' ' ' l . , ~ , l l . , ,
0.56 0.40
100
200
300
400
500
2' (OHM CM2)
Figure 8. Complex impedance plots at -0.640 V vs MSE for 0.5 mM
Ru(NH3):' + 0.5 mM Ru(NH3):+ in 0.2M Na2S04,for bare Au (A) and Au/(OTS + OM) of 1 - B = 0.0040 (B),0.0020(C), 0.0015(D). Geometric area, 0.14cm2.
1060, ct, c, cm s-I cm2s-I SF cm-2 SF cm-2
P,
0.016 142 0.0055 284
,
IOK
TABLE III: Ac-Impedance Results for the Au/(OTS in Figures 7 to 9 D cm2
,,cd,i,,
1
0
-o
1
tu
'
Figure 7. Complex impedance plots at 0.396V vs MSE for 0.25 mM Me3NtMeFc + 0.25 mM Me3NtMeFct in 0.1 M H2S04, for bare Au (A) and Au/(OTS + OM) of 1 - 8 = 0.016 (B),0.0055 (C).Geometric area, 0.25cm2.
103k, ,
I
I
IlOK I
L:-O lK
I
I
iI
"
I.
R,,
I
2001
N^ I 2001
0
I
A
200
200
I
I
1.4
1-
0.29 0.31
0.75 0.80
0.79 0.73 0.55
0.73 0.69 0.53
1.9 1.8 1.4
0.79
0.78
2.0
RU(NH&,~+/~+ 2.43 1.97 3.53
FctIo 75
11.0
fraction of active surface. Due to the unique distribution of monolayer-based microelectrodes, effecting high current densities with a minute fraction of active surface, combined with a facile measurement of 8, such electrodes appear particularly promising for the determination of very large heterogeneous electron-transfer rate constants. We have previously demonstrated the feasibility of such measurements with the Fe(CN),&/'- couple,16 where excellent self-consistency in the values of ko, D (the diffusion coefficient), and C,,, (the monolayer capacitance) was shown for bare Au and Au/monolayer of various surface coverages. In what follows, the determination of some very large heterogeneous electron-transfer rate constants will be demonstrated. ko in all cases is large enough to prevent observation of kinetic limitations with a bare electrode, but the kinetics are easily extracted with Au/(OTS OM) electrodes. The ac-impedance technique was chosen for the measurements for the following reasons (i) The small voltage perturbations used (in our case, 5mV rms) minimize mass-transport limitations. (ii) The technique may provide an efficient separation in frequency domains between diffusion and kinetic control. Thus, once the kinetic parameters are evaluated free of any mass-transport control, a certain statistical distribution of pinhole sizes and spacings, which is likely to occur in the present system, need not be considered. As discussed in a previous section, such a distribution may have a substantial effect with other techniques, e.g., linear scan voltammetry. (iii) The ac technique enables at the same time a convenient determination of diffusion coefficients and the capacitance of the system. Figures 7-9 and Table I11 present complex impedance plots and calculated parameters for three very rapid electrode couples
+
0
120 180 240 Z' (OHM CM2)
60
330
Figure 9. Complex impedance plots at 0.075V vs Ag/Agt for 1 .O mM Fc + 1.O mM Fct in acetonitrilecontaining 0.1M TBAF, for Au/(OTS + OM) of 1 - B = 0.0007. Geometric area, 1.75 cm2.
studied in the present work, measured with a bare Au electrode and Au/(OTS OM) electrodes of various 1 - 8 values. The charge-transfer resistance R , is measured as the diameter of the semicircle in the higher frequency domain of the impedance plots.23 The diffusion coefficient D is calculated from the 4 5 O Warburg line at lower frequencies, where the reaction is controlled by semiinfinite linear diffusion to the entire geometric area (as in Figure 4C).16 Within the time domain where a kinetic semicircle is observed in the complex impedance plot, the electrode reaction is totally controlled by the electron-transfer kinetics. Thus, the following simple relationships hold for a 1-electron, first-order reaction with Cox= Crd = C (io is the exchange current per unit geometric area)
+
R = -1- RT '' io F
(4)
io = (1 - 8)FkOC
(5)
or io = Fk,,,C (23) Rubinstein, I.; Rishpon, J.; Gottesfeld, S. J . Electrochem. SOC.1986, 133, 729 and references therein.
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The Journal of Physical Chemistry, Vol. 91, No. 27, 1987
where
kapp= kO(1
-
e)
(7)
Equations 4 and 6 yield RT 1 kapp= -F2 RCtC Thus, kappis calculated from R,, in Figures 7-9, and ko is then extracted by using eq 7 and the known value of 1 - 0 (measured as in Figure 1). The results are listed in Table 111. It is clearly observed in Figures 7 and 8 (and the same holds for Fc as well) that no kinetic information can be obtained with a bare electrode under these conditions (enlargement of the high-frequency region in Figures 7A and 8A still does not provide any kinetic information). It is also clear that kinetic control is observed with Au/(OTS OM) electrodes in all cases, and that RCtis quite large and increases with increasing 8, as expected. Average ko values for the three couples (including additional measurements not presented) are 0.56 cm/s for Me,N+MeFc+/O in HZSO,; 1.8 cm/s for R u ( N H ~ ) ~ ~in+ NazS04; /~+ and 5.0 cm/s for Fc in CH3CN. The electrical capacitance Cexpfor each system can be evaluated from the frequency at the upper point of the semi-circle in the according toz3 complex impedance plot w,,,
+
Sabatani and Rubinstein reported in the literature for the same electrode couples. If ko for Fc in aqueous LiCl can be taken as a resonable comparison to Me,N+MeFc+/O in aqueous HZSO4, the value found in the literature is 0.7 cm/s24 measured with the galvanostatic doublepulsed technique. This value is in very good agreement with our + / ~measured + by the measured ko of 0.56 cm/s. R u ( N H ~ ) ~ ~was staircase voltammetry technique, with ko estimated to be > 1 c m / ~ .This ~ ~ lower limit also agrees with our present result of 1.8 cm/s. ko was measured for Fc in CH3CN by using cyclic voltmmetry, with ko reported as 0.22 cm/s.26 This value is considerably lower than 5.0 cm/s measured in the present work. Evidently, the discrepancy is attributable to the cyclic voltammetry technique, which may be too slow for the study of such a rapid couple. The results presented in Figures 7-9 suggest that with the present Au/(OTS + OM) electrodes one should be able to measure ko values of 2 1 0 cm/s. This may be of substantial interest as regards existing theories of electron-transfer reactions. Many attempts have been madez7to test the relationship between the electrochemical outer-sphere heterogeneous rate constant ko and the respective homogeneous self-exchange rate constant k,,, as derived by M a r c u ~ ~ ~ ~ ~ ~
where Z,, and Zsoln are the collision frequencies for the electrode and solution, respectively. In some cases of relatively slow electron-transfer reactions a reasonable agreement is obtained It is reasonable to assume as a simple approximation that CeFp (usually up to an order of magnitude differen~e).~'With rapid is a parallel combination of the double-layer capacitance Cdlin electron-transfer reactions, a major obstacle is the experimental the pinholes and the monolayer capacitance C,, thus providing difficulties involved in kinetic measurements of very fast electrode the following equation for the evaluation of Cm16(all capacitances reactions, where mass-transport limitations become overwhelming. are per unit area) One may suspect that in cases where the measured ko is considerably lower than the value calculated from k,,, the reason may well be experimental limitations in the determination of ko which and the monolayer dielectric constant t can be then calculated may lead to erroneous values. This emphasizes the potential of by using the monoalyer thickness of 2.3 nm.9d The results are Au/monolayer electrodes, which enable a determination of very given in Table 111. C, (and e) values obtained with R u ( N H ~ ) ~ ~ + / ' + large ko values with minimal experimental difficulties. It should and with Fc are in reasonable agreement with those calculated be noted, though, that eq 11 is valid only in cases where ko and for Fe(CN)b4-/,-.I6 C, values obtained with Me,N+MeFc+/O ke, are measured in the same medium, and thus deviations may appear too low, which may indicate that in certain cases the simple be expected if the size of pinholes becomes small enough so that parallel model for the capacitive contributions may not be comthe surrounding monolayer chains effect a reduced dielectric pletely satisfactory. constant. This does not seem to be the case in the present work, where the pinholes are quite large in molecular terms. Discussion Regarding the three electrode couples studies here, values of We have shown that Au/(OTS OM) electrodes of high k,, for two are found in the literature: 8.5 X loz and 4 X lo3 monolayer coverage (0 approaching unity) provide arrays of + aqueous solution,30and 5.9 X lo6 M-' s-l for R u ( N H , ) ~ ~ + / 'in microelectrodes of extremely small dimensions and unique for Fc in CH3CN.,* The expected ko values calculated from eq properties, which can be quite well characterized electrochemically ~ ~ 77 + , cm/s for 11 are 0.92 and 2.0 cm/s for R U ( N H , ) ~ ~ +and by using microelectrode theories. Such electrodes may display Fc. The former is in excellent agreement with our measured value superior performance in various electrochemical situations, inof 1.8 cm/s for R u ( N H ~ ) ~ ~ +ko/ ~measured +. for Fc in CH,CN cluding background suppression, electron-transfer mediation/ (5.0 cm/s) is roughly an order of magnitude lower than the catalysis, complex electrode reactions involving surface and difcalculated value. However, as was shown by Kojima and Bard,32 fusion steps, and the study of exceedingly rapid electrode reactions, a diffuse double-layer correction in organic solvent of similar thus providing a potent tool for fundamental electrochemical dielectric constant (DMF) will increase the measured ko by about studies. The electrodes are durable, need no special care, and can an order-of-magnitude (in their case the use of mercury electrodes be used repeatedly for long periods of time in aqueous and nonenabled such a correction.) Thus, the uncorrected ko value of 5.0 aqueous solutions. (The only case where slow aging was observed cm/s for Fc in CH3CN may as well be considered in general is Ru(NH,):+/*+ in Na2S04. The reason is unclear at this point.) Of the above-mentioned applications, perhaps the most interesting is the possibility to determine very large heterogeneous (24) Saji, T.; Maruyama, Y . ;Aoyagui, S . J . Electroanal. Chem. 1978,86, 219. electron-transfer rate constants. ko values measured in the present (25) Endicott, J. F.; Schroeder, R. R.; Chidester, D.H.; Ferrier, D.R. J . work are among the largest reported in the electrochemical litPhys. Chem. 1973, 77, 2579. erature, and moreover, the measurements involve no particular (26) Sharp, M.; Petersson, M.; Edstrom, K. J . Electroanal. Chem. 1980, 109, 271. experimental difficulties once the appropriate Au/monolayer (27) Southampton Electrochemistry Group; Inrrrumenral Merhods i n electrodes are prepared. This results from the fact that we do €lecrrochemisrry; Ellis Horwood Series in Physical Chemistry; Wiley: S e n not attempt to measure directly a very rapid electrode reaction, York, 1985; pp-102-104. but rather "slow down" the reaction by using the Au/(OTS + (28) Marcus, R. A. J. Phys. Chem. 1965, 43, 679. (29) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New OM) electrodes, measure the apparent rate constant conveniently, York, 1980; p 620. and then convert the experimental value to the "real" rate constant (30) Meyer, T. J.; Taube, H. Inorg. Chem. 1968, 7 , 2369. by using a simple relationship (eq 7). (31) Yang, E. S.;Chan, M.-S.; Wahl, A . C. J . Phys. Chem. 1980, 84, It is interesting to compare the values of ko measured with the 7094 _ "_ Au/(OTS + OM) electrodes in the present work with values (32) Kojima, H.; Bard, A . J. J . Am. Chem. SOC.1975, 97, 6317.
(9)
+
~~
.I
J. Phys. Chem. 1987,91,6669-6673 agreement with the value calculated from kexo Conclusions The application of the mixed monolayer approach to gold electrodes coated with highly organized long-chain hydrocarbon amphiphiles enables us at this point to obtain complete blocking of the electrode, or, by controlling the adsorption conditions, to leave a certain amount of natural pinholes. Both cases are of considerable interest from an electrochemical point of view. In the present work we have exploited the latter possibility, Le., the electroactive pinholes in Au/monolayer electrodes with 0 close to (but less than) unity. Such electrodes provide unique arrays of ultramicroelectrodes with an average diameter of 5-10 nm, probably the smallest microelectrodes which have so far been characterized and used. It was shown that Au/monolayer electrodes can effectively facilitate various electrochemical measurements, including kinetic studies of very rapid electrode reactions. We have used such electrodes for the determination of
6669
several very large heterogeneous rate constants ko, the highest being 5.0 cm/s. However, ko values greater than 10 cm/s can be measured quite conveniently with the present system. A complete blocking of electrodes with a single organized monolayer will be important in electrochemical experiments involving electron transfer via electron tunneling, as well as the future implementation of selective ionic or electronic conductivity in organized monomolecular systems. These directions are presently pursued in our laboratory. Acknowledgment. We are pleased to acknowledge numerous helpful discussions with Drs. J. Sagiv and R. Maoz. I.R. is the Incumbent of the Victor L. Erlich Career Development Chair. Registry No. OTS, 112-04-9; OM, 2885-00-9; AA, 50-81-7; PB, 14038-43-8;Fc’, 12125-80-3;Fco, 102-54-5;Me3N+MeFct,51 150-57-3; Me3N+MeFco,33039-48-4; Au, 7440-57-5; Fe(CN):-, 13408-63-4; Ru(bpy)3’+, 15158-62-0; R u ( N H ~ ) ~ * 19052-44-9; +, Ru(NH~)~’+, 18943-33-4.
CO Methanation and Ethane Hydrogenolysis over Ni Thin Films Supported on W(110) and W(100) C. Michael Greenlief,? Paul J. Berlowitz,* D. Wayne Goodman,* Surface Science Division, Sandia National Laboratories, Albuquerque, New Mexico 871 85
and John M. White Department of Chemistry, University of Texas, Austin, Texas 78712 (Received: June 2, 1987)
The kinetics of methanation and ethane hydrogenolysis have been used to probe the surface chemistry of submonolayeramounts of Ni deposited on W( 110) and W( 100). Ni adsorption up to 1 monolayer is pseudomorphic, and submonolayer films are strained to conform to the W substrate lattice dimensions. Methanation rates per Ni atom on both W substrates are equal to rates found on Ni single crystals and on supported Ni catalysts. From these results we conclude that CO methanation is structure insensitive and that the rate-limiting step likely requires no more than one Ni atom. For ethane hydrogenolysis over Ni/W(110), the activation energy was 21.9 f 1 kcal mol-’ and independent of Ni coverage, but the overall reaction rate (per Ni atom) decreased with increasing Ni coverage. Over Ni/W(100) the activation energy was 22.3 & 1 kcal-’ and was independent of Ni coverage. The results are consistent with the strained (more open) Ni overlayers being more active toward C-C bond breaking and with only a single, unhindered Ni atom being required for the ratedetermining step in ethane hydrogenolysis.
Introduction There has been considerable interest in bimetallic systems over the years because of the commercial success of these materials in altering the catalytic selectivity and/or activity in desirable ways.’” Many fundamental studies have centered on trying to define the roles of “ligand” and “ensemble” effects in the catalysts.+12 Ensemble effects are usually defined in terms of the number and geometry of atoms needed for a catalytic process to occur, for example, C-C bond cleavage. Ligand effects refer to those involving electronic interactions. In an effort to evaluate the relative importance of ensemble and ligand effects, we have undertaken a series of studies involving the addition of an active metal to a relatively inactive metal with the intent of systematically varying the geometry of the local ensemble and its electronic character. The prepared surfaces are then probed by a variety of methods, ranging from ultrahighvacuum (UHV) surface investigations to elevated pressure reaction kinetics. In this paper we present results for two different reactions over strained Ni overlayers (
