Application of scanning electrochemical microscopy to studies of

Anal. Chem. 1992, 64, 2021-2028

2021

AC RESEARCH

Application of Scanning Electrochemical Microscopy to Studies of Charge Propagation within Polyelectrolyte Coatings on Electrodes I1 Cheol Jeont and Fred C. Anson’ Division of Chemistry and Chemical Engineering, Arthur Amos Noyes Laboratories, California Institute of Technology, Pasadena, California 91125

The technique of scanning electrochufnicalmicroclcopyk used to examine the properties of polyelectrolyte coatings on electrodes. Coatlngs of protonated poly(bvinytpyrkline) exhlblt time dependences of their redox conductlvles wAh Fe(CN)% or as counierbnsIncorporatedInthe coathgs. By contrast, responses obtainedfrom coatings of Naflonshow no comparable time dependewe. A pordble reallon for the difference k ruggosted. Images of the two types of coatings obtakndwlththescannlngelectrochemlcai~indude apparent topoioglcai features in the poly(4-vlnyipyrldlne) coatings which are shown to ark. from dgniflcant variations in redox conductivliy across the surface of aged coatings rather than from true changes in the coating’s dimendon. I n the caw of Nafkncoatlngs, only smooth surfaces are observed Inscanning eiectrochemkai mkroscopeImageswhether they are recordedunder condltknrof podtlveor negativefeedback. Tlte absence of dgniflcant apatlal varlatlons in redox conductMty k bellovedto be rwpodble for the featurelessImages obtalned wlth Nafion coatings.

Im”

In recent report@ we described the application of microelectrodetechniques such as those proposed by Engat”= and the scanning electrochemical microscope introduced by Bard and co-workers6l2to examinations of ion and electron motions at and within polyelectrolyte coatings on electrodes. The coatings are electronic insulators when they contain only electroinactive counterions and exhibit negative feedback (in Permanent address: Department of Chemistry, Jeonbuk National University, Jeonju, 560-756, Korea. (1) Kwak, J.; Amon, F. C. Anal. Chem. 1992, 64, 250. (2) Lee, C.; Anson, F. C. A d . Chem. 1992,64,528. (3) Engstrom, R. C.; Weber, M.; Wunder, D. J.; Burgess, R.; Winquist, S. A d . Chem. 1986,58, 844. (4) Engstrom, R. C.; Meaney, T.; Tople, R.; Wightman, R. M. Anal. Chem. 1987,59, 2005. (5) Engstrom, R. C.; Wightman, R. M.; Kriatensen, E. Anal. Chem. 1988,60,652. (6) Bard, A. J.;Fan,F.-R.; Kwak, J.; Lev, 0. Anal. Chem. 1989,61,132. (7) Kwak, J.; Bard, A. J. Anal. Chem. 1989, 61, 1221. (8) Kwak, J.; Bard, A. J. Anal. Chem. 1989,61, 1794. (9) Kwak, J.; Lee, C.; Bard, A. J. J. Electrochem. SOC.1990,137,1481. (10) Bard, A. J.; Denault, G.; Lee, C.; Mandler, D.; Wipf, D. 0. Acc. Chem. Res. 1990,23,357. (11) Wipf, D. 0.;Bard, A. J. J. Electrochem. SOC.1991,138, 469. (12) Unwin, P. R.; Bard, A. J. J. Phys. Chem. 1991,95, 7814. 0003-2700/92/0364-202 1$03.00/0

the sense defined by Kwak and Bard’), as microtip electrodes are moved closer and closer to the coating surface in solutions containing electroactive redox couples which are not incorporated by the coatings. By contrast, when the electroactive redox couples employed can also serve as counterions for the polyelectrolyte coatings, the currents at the microtip electrodes increase as the tip is moved closer to the surface of the coatings. This positive feedback’ matches that obtained with uncoated, conducting electrodes but only with relatively low concentrations of the redox couple.112 As the concentration of the redox couple in solution is increased, the extent of positive feedback at coated electrodes diminishes and ultimately gives way to negative feedback.’v2 This behavior was attributed to the limit on the rate of electron propagation across the coatings that is set by the rate of diffusion of the electroactive counterions (by physical motion or electron hopping). The electronic (redox) conductivity of the coatings is determined by this diffusional rate. At concentrations of the counterions in solution which produce currents at the tip electrode that exceed the maximum electron propagation rate acrossthe coatings, the positive feedback currents are smaller than those obtained at uncoated electrodes under the same conditions. At even higher concentrations, the current at the tip electrode can decrease as it approaches the substrate. The present experiments were initiated in order to investigate this concentration-dependent feedback phenomenon in more detail by examining redox couples with differing intrinsic electron self-exchange rates, by extending the measurements to additional types of polyelectrolyte coatings, and by exploring the effects of variation in the position of the tip across the coating surface.

EXPERIMENTAL SECTION Materials. Commercialsamplesof K$e(CN)s (Mallinckrodt) and NaJrCL.6HzO (Alfa) were used as received. Os(bpy)&l* (bpy = 2,2’-bipyridine)was prepared as described in ref 13. Poly(4-vinylpyridine)(MW = 7.4 X 106) (Polysciences)and a solution of Ndion (equiv wt = 1100) (Aldrich) were used as received. Laboratory-distilled water was used after passage through a purification train (Barnsted Nanopure). Apparatus and Procedures. The electrochemical cell, glassy-carbonsubstrate electrodes, and 11-pm-diametercarbon(13) Creutz, C.; Chan, M.; Netzell, T. L.; Okumura, M.; Sutin, N. J. Am. Chem. SOC.1982,104,1309.

0 1992 American Chemical Society

2022

ANALYTICAL CHEMISTRY, VOL. 64, NO. 18, SEPTEMBER 15, 1992 cu

-

m LD -7

cu

8.-

. ..,c b

I

m 0

Lo 0

=x 0

S i , ,I

, I

, , I , ,

/,,,,,, ;

, , I , ,

' O B 0 7 0 6 0 5 0 4 0 3 0 2 0 1

I

0

, , , ,I

~

,,!

-01-02-03-04

2 I 0

0

fiber microtip electrodehoused in a conicallytapered glass sheath have been described: as has the procedure for preparingcoatings of cross-linkedpoly(4-vinylpyridme)on the substrate e1ectrode.l Nafion coatings were applied to a glassy-carbon substrate electrode (diameter = 3.1 mm)by transferring 0.9 p L of a 0.6 w t % Nafion solution in 2-propanol to the electrode surface and allowing the solvent to evaporate at room temperature. The thicknesses of coatings, estimated with a Dektak profilometer, were 0.5 and 0.2 pm for the poly(4-vinylpyridine)and Nafion, respectively. Electroactive complexes were introduced into coatings by immersing the coated electrodes in solutions of the incorporating complex and appropriate supporting electrolyte. Potentials were measured with respect to a saturated calomel electrode. Ekperimenta were conducted at the ambientlaboratory temperature, 22 f 2 OC. The scanning electrochemicalmicroscope employed has been previously described.lJ4 Tip currentaas a function of the distance between the tip and substrate electrodes were usually recorded while the tip was advanced toward the substrate at 0.5 pm s-'. There was no significant difference in the curves obtained at an approach rate of 0.05 pm s-l. The distance separating the tip and substrate was determinedfrom the magnitudeof the increases in tip current as the separation became comparableto the radius of the tip using the curves calculated by Kwak and Bard.' With coated electrodes,low concentrationsof redox couples in the test solution were employed to assure that the measured currents were not affected by the charge propagation rates within the coatings. To obtain images of coated electrodes, the tip electrode, maintained at a constantverticalposition 20pm above the coating, was scanned in the directions parallel to the coating at 26.5 pm s-1. Scan rates of 3 and 0.5 pm s-1 produced images which showed only minor differences from those obtained at the higher scan rate.

RESULTS AND DISCUSSION Temporal Changes in PWH+ Coatings. The voltammetric responses obtained from Fe(CN)& anionsincorporated in cross-linked coatings of protonated poly(4-vinylpyridine) (PVPH+) change slowly if the coatings are allowed to stand in supporting electrolyte solutions. For example, the cyclic voltammograms in Figure 1 were recorded with a coating immediately after Fe(CN)& counterions were incorporated in the coating (curve A) and after the Fe(CN)&-PVPH+ coating had soaked in the supporting electrolyte (0.1 M KCl0.01 M HC1) for 24 h (curveB) or 48 h (curve C). The decrease (14)Lee, C.; Kwak, J.; Anson, F. C. A w l . Chem. 1991,63, 1501.

10

15

20

25

30

SEPARATION / pin

E vs S C E (VOLT)

Cydicvotta"ogramsof the Fe(CN)e3-'C coupleincorporated mol cm-2 of PVPH+ on the giassy-carbon substrate electrode. After incorporation, the coated electrode was transferred to a supporting electrolyte of 0.1 M KCI-0.01 M HCi to record the vottammograms with a scan rate of 5.8 mV s-l. The vottammograms were recorded immediately (A) and 24 h (B) and 48 h (C) after the incorporation. Figure 1. in 2.8 X

5

Approach curves recorded as the carbon microtip electrode was moved at 0.5 pm s-' toward a glassy-carbon substrate electrode coated with 2.8 X mol cm-2of PVPH+. The ordinate Is the ratio of the tip current, h, to its value when the tip is moved to a posiUon far from the substrate, h,-. The tlp and substrate electrodes were maintained at -0.2 and +0.8 V, respecttvely. The four curves were obtalned with (fromtop to bottom)0.26,0.88,2.9, and 9.8 mM dutlons of Fe(CN)e" in a supporting electrolyte of 0.1 M KCI-0.01 M HCi. Flguro 2.

in the magnitudes of the peak current with time results from the loss of a portion of the Fe(CN)s3-anions by reverse ion exchange, as indicated by the decreasing areas encompassed by the voltammograms. Comparisonsof the areas of the three cathodic waves in Figure 1 show that 11%of the original Fe(CN)e3-was lost during the first 24 h and an additional 4 % during the next 24 h. However, aging of the coating involves more than the loss of Fe(CN)e3- counterions: The redox conductivity of the coating also changes. Approach Curves for Coatings of PWH+. Approach curves are plots of tip current vs the distance between the tip and substrate electrode as the tip is advanced toward the substrate in a solution containing a reactant which can be reduced (or oxidized) at the tip and substrate electrodes.7 To examine P W H +coatings on the substrate electrode, approach curves were recorded in acidic solutions of Fe(CN)6*. With fresh coatings, the approach curves did not differ greatly from those obtained with uncoated substrate electrodes; i.e., the tip current (normalizedwith respect to its value when the tip was located far from the substrate, i ~ ,increased ~ ) as the tip approached the substrate because of the positive feedback produced when the Fe(CN)64-anions generated at the tip are reoxidized at the substrate electrode and diffuse back to the tip where they reenter the reduction-reoxidation cycle.7 In Figure 2 is shown a set of approach curves for four solutions of Fe(CN)&. The separation distances for the abscissa in Figure 2 were calibrated by using the lowest concentration of Fe(CN)a3-and the calculated curves of Kwak and Bard.7 At an uncoated substrate electrode the four curves similar to those in Figure 2 were superimposed at all separation distances. The differences among the curves in Figure 2 as the separation diminishes reflect the difference between the magnitudes of the positive feedback currents that could have flowed and the smaller, redox conductivity-limited current that did flow across the coating.' As the coatings age, the disparity between the diffusionlimited tip currents and the redox conductivity-limited tip currents increases and exhibits a greater dependence on the point on the coating where the approach curve is measured. For example, in Figure 3 are shown approach curve8 recorded with a coating similar to that used to record Figure 2 but after it had been aged for 48 h. The sensitivity of the tip currents to the tipsubstrate separation is clearly much lower. The tip was moved to various positions above the coating to record the series of curves in Figure 3. At some sites the redox

ANALYTICAL CHEMISTRY, VOL. 64, NO. 18, SEPTEMBER

csd rr-l

I

I

I

I

I

F

0

5

10

15

20

25

30

SEPARATION I p n

Flgurr 8. Approach curves recorded wlth an aged (48 h) PVPH+ coating wlth a Q.8mM solutlon of F~I(CN)&. other condttlons were asinFigve2. Eachcurvewasobtained wlththetlpelectrodepositbned above a different region of the PVPH+ coating. These regions were separated by 10-20 pm and are marked in the image shown in Figure 4.

conductivity of the coating is evidently so low that, with the relativelyhigh concentrationof Fe(CN)6gemployed, negative feedback is observed at approach distances less than -10 pm. This is the behavior to be expected if the substrate were an insulator instead of a conductor.' This spatial variation in the redox conductivities of aged coatings can give rise to false features in images of P W H + coated substrate electrodes obtained with the scanning electrochemicalmicroscope. For example, in Figure 4is shown an image of a 100-rm X 300-pm area of the aged coating used to obtain the approach curves in Figure 3. The apparent valley in the center of the image is actually the result of diminished positive feedback in this region resulting from the smaller redox conductivity of this part of the coating. If a true topographical depression were present in the coating (which retained its redox conductivity), one would expect to obtain individual approach curves exhibiting positive feedback. Instead, approach curves recorded with the tip positioned over the center of the apparent "depressionwin the coating exhibited increasingly negative feedback for separations of ca. 20 pm or less (Figure 3). At the same time, approach curves recorded with the tip centered over the featureless area on the left of Figure 4 exhibited positive feedback (Figure 3). Thus, the apparent depression in the image of Figure 4 represents depressed redox conductivity, not a topographical depression. The spatial variation in redox conductivity observed with PVPH+ coatings in solutions of Fe(CN)& is also present in solutions of IrCl&. The left-hand side of Figure 5 contains an image of an aged coating obtained in a 10 mM solution of IrCl&. Individual approach curvesobtained at the two pointa identified on the image are shown on the right-hand side of Figure 5. The correlation between negative feedback and apparent (false) topological features is clear. It follows from our interpretation of the origin of the false topological features in images such as the one in Figure 5 that the features should diminish if the concentration of the redox couple used to obtain the image is decreased. At concentrations low enough to assure that only positive feedback is obtained, the features should disappear. This prediction was tested by imaging the same portion of a PVPH+ coating using four different concentrations of IrCl&. The results, shown in Figure 6, match the expected behavior. The apparent valley which appears in the rear of the images obtained with 10mM and 3.1 mM IrCl$- solutions (C and D in Figure 6) are absent when the same region is imaged using 1.0 or 0.31 mM IrC&2-

15, 1892

2028

(A and B in Figure 6). The small hill in the coating that appears near y = 30 pm in part A of Figure 6 is believed to be a true topographic feature. That it becomes less prominent as the concentration of IrC&2- is increased probably reflects a decreasein poeitivefeedbackwhen the tip is over this portion of the coating which apparently has sufficient redox conductivity to support some positive feedback. Behavior of Nafion Coatings. The time-dependent behavior of P W H + coatings is not exhibited by Ndion coatings under the same conditions. The cyclic voltammetry of the Os(bpy)33+/2+couple in a freshly loaded Nafion coating changes very little when the coating is soaked in pure supporting electrolyte for 48 h (left-hand side of Figure 7). Two approach curves recorded for a Nafion coating in solutions of O~(bpy)~2+ are shown on the right of Figure 7. Positive feedback is obtained with the lower concentrations of O~(bpy)~2+ but negative feedback occurs at the higher concentration. The cause of the negative feedback with Nafion coatings is believed to be the same as with P W H + coatings, namely, limited redox conductivity of the coating. However, in contrast with P W H + coatings, the behavior of the Ndion coatings was independent of the region of the coating examined; the behavior shown on the right of Figure 7 was obtained wherever the tip was positioned above the coating. Images of Nafion coatings obtained with the scanning electrochemical microscope were essentially featureless and independent of whether they were obtained with concentrations of Os(bpy)g2+ which produced positive or negative feedback (Figure 8). Thus,the (false)features obtainedwith P W H + coatings in regions where the coating produced a negative feedback effect on the tip current did not appear with Nafion coatings under conditions of negative feedback because there were no nearby regions of the coating where positive feedback prevailed to produce the contrasting responses characteristic of aged coatings of PWH+. As mentioned in the introduction, approach curves for substrate electrodes coated with polyelectrolytesexhibit only negative feedback when the components of the redox couple employed to produce current at the tip are not counterions for the polyelectrolyte so that the substrate coating contains no electroactive counterions. Such behavior is shown on the left side of Figure 9 (curves B)where solutions of IrCl& were employed for approaches to a Nafion-coated substrate electrode. The coating behaves as an insulator and, as expected, the extent of negative feedback is not affected by the concentration of IrCl$ employed. Half of the glassycarbon substrate WBB coated with Nafion and used to record the two curves labeled B. When the tip was moved to a position over the uncoated electrode, the curves labeled A in Figure 9 were obtained. The concentration-independent positive feedback is also as expected. Shown on the right side of Figure 9 is an image of the half-coated substrate electrode in the vicinity of the boundary between its coated and uncoated halves. The positive feedback when the tip is over the bare electrode surface gives way to negative feedback as the tip is moved a c r w the boundary. The resulting image shows that the transition between the coated and uncoated electrode occurs over a range of ca. 30 pm. A similar image of the boundary between an insulating polypyrrole coating and a conducting substrate was obtained by Kwak et al.9 In principle, data such as those in Figure 9 might be used to obtain estimates of the thickness of the insulating coating. However, the Ndion coating employed in Figure 9 had an estimated thickness of ca. 0.5 pm which is beyond the resolution of the scanning electrochemical microscope that was utilized to obtain the approach curves and image.

2024

ANALYTICAL CHEMISTRY, VOL. 64, NO. 18, SEPTEMBER 15, 1992

1

f ie

Flgure 4. Image of an aged PVPH+ coating obtained with the scanning electrochemicalmicroscope with a 9.8 mM solution of Fe(CN)63-. The tip electrode was held 20 pm above the coating as it was rastered across the surface. Other conditions were as in Figure 2. The scale shown on the Zaxis in this and other images is labeled nonlinearly because of rounding by the computer.

8,

b

e111

4

% 4

8 b

bCI

P 0;

0

SEPARATION / pm Flgure 5. (Left) image of an aged PVPH' coating obtained with the scanning electrochemicalmicroscope in a 10.8 mM solution of IrC16*-. The potentials of the tip and substrateelectrodeswere maintained at 0.2 and 0.9 V, respectively. (Right)approachcurves recordedwith the tip electrode positioned above the two regions identified on the image. The sudden increase in current at ca. 4 pm is an experimental artifact.

Images of the boundary region between the coated and uncoated electrode surface were also obtained using a range of concentrations of O ~ ( b p y )as ~~ the + imaging redox reactant (Figure 10). Since Os(bpy)s2+is incorporated as a counterion by the Nafion coating where the O~(bpy)3~+/~+ couple provides redox conductivity, the responses from the coated and uncoated surface are not inherently different as was true with IrCb2-as the imaging reactant. At the lowest concentrations of Os(bpy)32+the image conveys almost no indication of the presence of the coated/uncoated boundary (Figure 10A). However, as the concentration of O~(bpy)3~+ is increased, the presence of the boundary becomes increasingly clear (Figure 10B-D) because the Nafion coating is unable to sustain feedback currents as large as those that flow at the uncoated surface.

Possible Role of Heterogeneous Electron-Transfer Kinetics. A reviewer suggested that feedback currents at coated substrate electrodes might fall below those obtained at uncoated substrate electrodes because of slow electrontransfer between the redox reactant confined in the substrate coating and the coreactant in solution. This is a reasonable possibility because of the high local rates of electron transfer at the coating/solution interface that are required to sustain diffusion-controlled feedback currents. The behavior to be expected in such cases would be similar to that obtained with an uncoated substrate held at a potential where the current at the substrate was limited by the rate of heterogeneous electron transfer. Such cases have been analyzed by Wipf and Bard.Il The normalized feedback currents are predicted to be smaller than those obtained under diffusion-controlled


2025

Figure 6. Images of an aged PVPH+ coating obtained with different concentrations of IrCls2-in the imaging solution. After each image was recorded the tip was retracted and the imaging solution carefully removed by aspiration and replaced with a more concentrated imaging solution. The tip was then positioned20 pm above the same region of the coatingand another image recorded. The imagingsolutions were 0.1 M NaCI-0.01 M HCI containing (A) 0.31,(B) 1.0, (C)3.1,and (D) 10 mM IrCls2-.

1.2 1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

E vs SCE (VOLT)

0

5

1 0 1 5 a i 3 SEPARATION / pm

3

Figure 7. (Left) cyclic vottammograms of the O~(bpy),~+/~+ couple incorporated in 4.5 X lo-" mol cm-2 of Nafion on the glassy-carbon substrate electrode (supportingelectrolyte; 0.05 M H2S04;scan rate, 5.8 mV SI): (outer curve) immediatelyafter incorporationof O~(bpy),~+ into a freshly deposited coating; (inner curve) 48 h later. (Right) approach curves recorded for a Nafion coating with a low or a high concentration of Os(bpy)32+. The potentials of the tip and substrate electrodes were maintained at 0.9 and 0.2 V, respectively. Concentrations of Os(bpy),*+: (A) 8.4 mM; (B) 0.33 mM.

conditions but the normalized currents are expected to be independent of the concentration of reactant in solution. We verified this expectation by carrying out experiments similar

to those in ref 11 in which Fe3+ was reduced at the tip and the resulting Fe2+was oxidized at a bare substrate electrode. At substrate potentials where the kinetics of the oxidation

2026


?! 8

E *-

8 :

2

a

6i

o

. a

\

E-cd

*-

f?

C

pi

n

L

CISi

?I

On

B

I!

O

L

Flgure 8. Images of a Nafion coating obtained with the' scanning electrochemical microscope in solutions of O~(bpy),~+. (A) [O~(bpy),~+]= 0.33 mM; positive feedback. (B) [Os(bpy),*+] = 8.4 mM; negative feedback. Other conditions were as in Figure 7.

k

8

0

5

1

0

i

5

2

0

~

SEPARATION / pm Flgure 9. (Right) approach curves recorded with the half-coated substrate electrode imaged on the left: (A) tip locatedabove the uncoatedportion

of the substrate; (B) tip located above the coated portion of the substrate. The imaging solution contained 10.1 mM (solid line) or 0.31 mM IrClS2(dotted line) in 0.1 M NaCI-0.01 M HCI. (Left) image of a glassy-carbon substrate electrode, half of which was coated with 4.5 X lo-" mol cm-2 of Naflon. The other half was uncoated. The image was obtained near the border between the coated and uncoatedareas. The imaging solution was 0.1 M NaCI-0.01 M HCi containing 10.1 mM IrClS2-. The potentials of the tip and substrate electrodes were maintained at 0.9 and 0.3 V, respectively.

reaction limited the feedback currents, the normalized currents were independent of the concentration of Fe3+ in solution in the range between 0.3 and 10 mM. In all of the cases examined in this study where the normalized feedback currents were smaller than the corresponding values obtained at bare substrate electrodes, the normalized currents became smaller as the concentration of the redox couple in solution was increased. In addition, the feedback currents decreased as the coating thickness increased, while currents limited only by rates of electron transfer at the coating/solution interface should be independent of the coating thickness. It was these two features of the results which led us to suggestthat the feedback currents were limited by the redox conductivity of the substrate coatings.' However, if redox conductivitywere the only factor limiting the feedback currents, the approach curves should

be independent of the identity of the redox couple in solution. In fact, the approach curves do exhibit some dependence on the identity of the redox couple in the direction to be expected if the kinetics of the electrontransfer between the immobilized reactant in the substrate coating and the redox couple in solution were affecting the current. For example, in Figure 11 are shown the approach curves obtained at substrate electrodes coated with two different thicknesses of NafionO~(bpy)~3+. In Figure 11A the coatings were employed in a solution of O~(bpy)3~+, while in Figure 11B the same coatings were employed with a solution of Fe(CN)&. For the Os( b ~ y ) ~ 3 + / 0 s ( b p ycase, ) ~ ~ +the equilibrium constant for the electron-transfer reaction is near unity and the normalized currents with the thinner coating (upper curve) correspond to a small negative feedback at all of the separation distances shown. When the coating thickness was doubled (lower

~


2027

8 F(

.LI

8

8

Fc

Fc

CI

. I

.LI

\

\

Fc

F( .R

.R

- 0

\

3 e

0

Flguro 10. Images of the halfcoated substrate electrode of Figwe 9 obtained with os(bpyh2+ instead of IrCb2- as the Imaging redox reactant. Supporting electrolyte: 0.05 M H2S04. The concentrationsof Os(bpyh2+were (A) 0.26, (B) 0.87, (C) 2.9, and (D) 8.4 mM. Other conditions were as in Figure 9.

B

A a! r(

'9 r(

4 r(

'v

!-

.?

r(

\

.5 '9 0

'9 4 'v 0

0

5

1

0

5

a

3

3

SEPARATION / pm

I

0

5

!

O

f

l

a

3

3

l

SEPARATION / pm

Flguro 11. Approach curves recorded as the carbon micratip electrode was moved at 0.05 pm s-l toward a glassy-carbon substrate electrode coated wlth Nafkm which was saturated wlth 0@bpyh3+. The solutions contained 10.1 mM 0s(bpyh2+ (A) or Fe(CNh> (B) in 0.05 M HS04. The thicknessesof the coatings were 0.1 pm (upper curves) and 0.2 pm (lower curves). The potentials of the tip and substrate electrodes, respecthrety, were maintained at (A) 0.9 and 0.2 V and (B) -0.2 and 4-0.6V.

curve), the currents decreased a t all separations, as expected contrast, when the O~(bpy)3~+/~+ couple in solution was for currents limited by the redox conductivityof the coatings replaced by the Fe(CN)63-/4-couple,positive feedback currents but not expected if the kinetics of the O~(bpy)3~+/0s(bpy)3~+were obtained a t all separations with the thinner coating (Figure 11B). Even with the thicker coating (lower curve in electron-transfer reaction were limiting the current. By

2028


Figure llB), positive feedback prevailed until the separation distance was made smaller than 10 pm. The equilibrium constant governing the reaction between Fe(CN)& and Os(bpy)33+is much greater than that for the reaction between O~(bpy)~Z+ and Os(bpy)s3+,and the larger feedback currents obtained with the former pair of reactants suggest that the kinetics of the electron-transfer reaction do affect the magnitude of the feedbackcurrents. These contrasting results indicate that, with some combinations of coating thicknesses and reactant concentrations, both redox conductivity of the substrate coating and kinetics of the electron-transfer cross reactions at the coating/solution interface may play a role in controllingthe currents observed as the tip is advancedtoward coated substrate electrodes. A puzzling aspect of Figure 11B is the broad maximum followed by a sharp decrease in the current in the approach curve for the thinner coating. The positive feedback current obtained at a separation of, e.g., 10 pm demonstrates that both the redox conductivity of the coating and the kinetics of the reaction between Fe(CN)64-and Os(bpyh3+ at the coating/solution interface are adequate to sustain the measured current. Even larger currentsflow at smallerseparation, but the current decreases to values well below that obtained at 10 pm as the separation is decreased below 4 pm. Similar behavior has been observed in approach curves recorded at bare substrate electrodes under conditions where the kinetics of heterogeneous charge transfer at the substrate limited the currents.11 The current maxima were qualitatively attributed to a distance-dependent combination of the responses expected from conductor and insulator substrates.ll The abruptness of the current decrease in Figure 11B compared with that in Figure 11A, which was recorded with the same pair of substrate and tip electrodes, makes it difficult to understand the behavior in terms of a simple combination of insulator and conductor responses. Quantitative simulations of the types carried out by Bard and co-workers+12will be needed to clarify the origin of the unusual shape of approach curves such as the upper curve in Figure 11B.

CONCLUSIONS The data we have obtained make clear the differences between the charge propagation properties of PVPH+ and Ndion coatings containing electroactive counterions. The properties of Ndion coatings appear more homogeneous and less time-dependent than do those of PVPH+ coatings. The (15) Jiang, R.; Anaon, F. C. J. Phys. Chem. 1992,96452. (16)Montgomery, D. D.; Anson, F. C. J. Am. Chem. SOC.1985,107, 3431.

(17)Whiteley, L. D.; Martin, C. R. J. Phys. Chem. 1989,93, 4650. (18) Yeager, H. L.; Steck, A. J. Electrochem. SOC.1984, 128, 1880.

origin of the heterogeneous properties exhibited by PVPH+ coatings remains to be fully delineated. The gradual decrease in redox conductivity which is responsible for the changes in peak currents in Figure 1may be the result of slow changes in electrostatic cross-linking as the freshly incorporated Fe(CN)63-anionsmove within the coating to sites where they can form more, or more stable, electrostatic bonds with the fixed pyridinium sites. Such increases in the extent or strength of electrostatic, cross-linking bonds would be expected to diminish the diffusion coefficientsof the Fe(CN)s3counterions within the coatings and, therefore, the redox conductivity of the coatings. However, the presence of significant differences in redox conductivity in different regions of the same, aged PVPH+ coatings, which appears necessary to explain the apparent features in images such as those in Figures 4 and 5, probably results from additional structural changes within the coatings. In aqueous solution, poly(4-vinylpyridine) is not fully protonated at pH 2,15 and in coatings, the polyelectrolyte is likely to spontaneously segregate into domains of unprotonated pyridine groups separated by larger domains of protonated pyridines.16 Such self-segregationinto domains within the lightly cross-linked coatings (which would be coupled to a corresponding segregation of the Fe(CN)e3- counterions) could be the process which proceeds during the aging of the PVPH+ coatings. The resulting domains would consist of less conducting, partially unprotonated regions that could exhibit negative feedback behavior surrounded by largely protonated regions where the incorporated Fe(cN)~~-counterionS provide the conductivity which produces either positive, or less negative, feedback. The lack of similar behavior with Ndion coatings is consistent with their known properties. The coatings deposited out of evaporating alcoholic solvents adopt structures which swell somewhat when immersed in aqueous solutions but do not undergo significant additional alterations with more extended aging." Although the coatings do contain hydrophilic channels surrounded by hydrophobic polymer,'* the former are distributed uniformly throughout the latter and the distances between them are much too small to be sensed by scanning electrochemical microscopy.

ACKNOWLEDGMENT This work was supported by the National Science Foundationand the US.Army Research Office. I.C.J. was grateful for partial support provided by the Korean Science and Engineering Foundation. Numerous enlightening discussions with Dr. Chongmok Lee are a pleasure to acknowledge. RECEIVED for review February 10, 1992. Accepted June 18, 1992.

Application of scanning electrochemical microscopy to studies of

Recommend Documents