J. Phys. Chem. 1987, 91, 643-648 frequencies, often leading to somewhat larger enhancements in the red region of the spectrum. This effect is especially noteworthy for densely packed spheroids such as are relevant to surfaces produced by electrochemical anodization. Whether this effect is sufficient to account for the missing factor of 101-102 in the enhancement factor is not clear from what has been published since these studies all omit some aspect of the electrodynamic model (such as size-dependent dielectric constants, electrodynamic corrections, or full surface averaging) that we find is important in determining quantitative results. Interparticle interactions are apparently not very important to the microlithographically prepared U p s t "experiments for which the most quantitative comparisons between electromagnetic theory and experiment have been made.40 Note that our calculated relative enhancements for Ag, Cu, and Au do seem to correlate well with measured values, as do the (40) Chu, L. C.; Wang, S.Y . Phys. Rev. 1985, 831, 693.
643
apparently large enhancements noted for Li, Na, and In. A1 remains somewhat of an anomaly since its predicted enhancements are comparable to Ag at 2.6 eV, and yet most measurements on A1 are not even close to those on Ag. Surface oxidation may be playing an important role here. Two metals which have not received much attention to date are Zn and Ga, and our results suggest that Zn is the better candidate of the two, especially near 2.5 eV. Cd has already played an important role in SEP measurements, but our results suggest that electromagnetic effects are small relative to other metals.
Acknowledgment. Helpful discussions with R. P. Van Duyne are gratefully acknowledged. This research was supported by the National Science Foundation-Solid State Chemistry Grant DMR-8405839. Registry No. Ag, 7440-22-4; Au, 7440-57-5; Cu, 7440-50-8; Li, 7439-93-2; N a , 7440-23-5; AI, 7429-90-5; Ga, 7440-55-3; In, 7440-74-6; Zn, 7440-66-6; Cd, 7440-43-9.
Vectorial Electron Transport at Ion-Exchanged Zeolite-Y-Modified Electrodes Zhuyin Li and Thomas E. Mallouk* Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712 (Received: July 7, 1986)
Chemically modified SnOzelectrodes were prepared, using zeolite Y which had been ion-exchanged with a metal tris(bipyridy1) complex, M ( b p ~ ) , ~(M + = Ru, Os), and a metallocene cation, M(CPR)~+or M(Cp)(CpR)+ (M = Co, Fe; Cp = q5cyclopentadienyl; R = -H, -CH3, -NH2, -COOCH3, -CHZN(CH3),). The rate of charge transfer between the electrode and the metallocene contained within the zeolite is enhanced at least tenfold by adsorbing M ( b ~ y ) , ~onto + the zeolite surface. Both oxidation and reduction of the metallocene are facile if the potentials of the M ( b p ~ ) , ~and + M(CPR)~+couples are matched, but only one of these processes occurs if the potentials are dissimilar. This behavior is attributed to a rapid electron-transfer cross reaction between the two complexes. The equilibrium potentials of the zeolite-bound M(CPR)~+/O couples were found to be 300400-mV positive of the corresponding potentials in polar organic solvents. The charge transport diffusion coefficient for Co(CpCH3),+/0 in zeolite Y , from linear sweep voltammetry, was found to be ca. 2 X cm2/s.
Introduction Much of the current interest in modified electrodes is directed toward the design of electrocatalytic surfaces. Forming structured modified electrodes which incorporate inorganic components such as zeolites and in addition to redox-active molecules is a new direction in this research. The ability of these inorganic hosts to absorb molecules of only a certain charge and size can result in high product selectivities for electrosynthetic reactiow6 These same structural features should allow one to create molecular assemblies for unidirectional charge transfer, provided the right molecules are chosen. Elegant examples of such electron unfortransport chains on polymer-modified electrodes (1) Gemborys, H. A.; Shaw, B. R. J . Electroanal. Chem. 1986, 208,95. (2) Murray, C. G.; Nowak, R. J.; Rolison, D. R. J. Electroanal. Chem. 1984, 164, 205. (3) (a) Ghosh, P. K.; Bard, A. J. J . Am. Chem. SOC.1983,105, 569. (b) Ghosh, P. K.; Mau, A. W. H.;Bard, A . J. J . Electroanal. Chem. 1984,169, 315. (c) Ege, D.; Ghosh, P. K.; White, J. R.; Equey, J. F.; Bard, A. J. J . Am. Chem. SOC.1985, 107, 5644. (d) White, J. R.; Bard, A. J. J . Electroanal. Chem. 1986, 197, 233. (e) Itaya, K.; Bard, A. J. J . Phys. Chem. 1985, 89, 5565. Rudzinski, W. E.; Bard, A. J. J . Electroanal. Chem. 1986, 199, 323. (4) Liu, H.-Y.; Anson, F. C. J. Electroanal. Chem. 1985, 184, 421. (5) Yamagishi, A.; Aramata, A. J. Chem. SOC.,Chem. Comm.1984, 452. (6) de Vismes, B.; Bedioui, F.; Devynck, J.; Bied-Charreton, C.; PerreeFauvet, M. N o w . J . Chim. 1986, 10, 81. (7) Leidner, C. R.; Murray, R. W. J . Am. Chem. SOC.1985, 107, 551. (8) (a) Pickup, P. G.; Kutner, W.; Leidner, C. R.; Murray, R. W. J . Am. Chem. SOC.1984, 106, 1991, (b) Chidsey, C. E. D.; Murray, R. W., Science 1986, 231, 25. (9) (a) Kittlesen, G. P.; White, H.S.; Wrighton, M. S.J . Am. Chem. Soc. 1985, 107, 7373. (b) Smith, D. K.; Lane, G. A.; Wrighton, M. S.J . Am. Chem. SOC.1986, 108, 3522.
0022-3654/87/2091-0643$01.50/0
tunately, the preparation of these polymer structures is necessarily equally elegant. In this paper we describe the preparation and electrochemical behavior of simple, self-assembling vectorial electron transport chains on zeolite-Y-modified electrodes. Zeolites are crystalline aluminosilicates with cavities and window openings of molecular dimensions.10 The cavities are occupied by exchangeable molecules and cations. Equations which describe ion-exchange equilibria in other inorganic and organic ion exchangers usually cannot be applied to zeolites, because in the latter structural space-filling effects are important." Many neutral, monocationic, and dicationic molecules are freely mobile in hydrated zeolites and so are accessible to electrochemical experiments. Rolison and co-workers2 were able to demonstrate size-exclusion effects for oxygen reduction at zeolite-A-modified electrodes. Cyclic voltammograms for Cd2+and Ag+ in zeolites have also been obtained.I2J3 Larger electroactive molecules may be exchanged into the bulk, or onto the outer surface, of particles of the faujasitic zeolites (X or Y), and the electrochemistry of metall~porphyrins~,'~ and methylviologenl so adsorbed has been examined. We have examined the cyclic voltammetry of zeol(10) Barrer, R. M. Hydrothermal Chemistry of Zeolites; Academic: London, 1982. (1 1) Breck, D. W. Zeolite Molecular Sieves; Wiley: New York, 1984; p 529. (b) Barrer, R. M. Zeolites and Clay Minerals as Sorbents and Molecular Sieues; Academic: New York, 1978; p 103. (12) Susic-Milenko, V. Electrochim. Acta 1979, 24, 535. (13) Periera-Ramos, J.; Messina, R.; Perichon, J. J . Electroanal. Chem. 1983, 146, 157. (14) de Vismes, B.; Bedioui, F.; Devynck, J.; Bied-Charreton, C. J . Electroanal. Chem. 1985, 187, 197.
0 1987 American Chemical Society
644
The Journal of Physical Chemistry, Vol. 91, No. 3, 1987
A
B
Figure 1. Scanning electron micrographs of a freshly clcaved 7eolite Y electrode. Sn02/glass is on the left side in (A), and on the right in (B) and (C).
ite-Y-modified electrodes in which a metal polypyridyl complex was adsorbed onto the zeolite surface, and a metallocene cation was exchanged into the bulk. An electrocatalytic effect was observed; that is, electron transfer to and from bulk species was found to occur almost entirely via the inteimediacy of the metal polypyridyl ions. Experimental Section Maferials. Sodium zeolite Y(Na-Y), ideal formula Na56A1S6Si,360384=250H20, was obtained from Union Carbide, Linde Division, and used as received. This powdered zeolite had an average particle size of ca. 1 pm, determined by SEM (Figure 1). From the crystal ~ t r u c t u r e '(cubic, ~ a. = 24.7 A, eight supercages per unit cell), the ratio of bulk-to-surface supercages in this material is in the range (1-5) X lo2. Cobaltocenium hexafluorophosphate, (ZV,N'-dimethylaminomethy1)ferrocene methiodide, and tris( 2,2'-bipyridyl)ruthenium( I I) chloride hexahydrate were obtained from Strem Chemicals and used without further purification. 1 ,l'-Dimethylcobaltocenium hexafluorophosphate (Co(CpCH,),+), 1,l'-diaminocobaltocenium hexafluorophosphate,and tris(2,2'-bipyridyl)osmium(11) choride were prepared and purified as described in the literat~re.'~.'' 1,l'( 1 5) Meier, W. M.;Olson, D. H. Atlas ofzeolite Structure Types: Juris Druck + Verlag AG: Zurich, 1978;p 37.
Li and Mallouk
Bis(carb0methoxy)cobaltocenium hexafluorophosphate was prepared by dissolving 1,I '-bis(chlorocarbony1)cobaltocenium hexafluorophosphate and triethylamine in anhydrous methanol and was recrystallized from methanol/acetone. (Carb0methoxy)coba 1tocen i u m hexa fl uorop hospha te, CoC p( C pCOOC H 3)4, was prepared by a modification of the procedure of El Murr and Laviron;16 we found that the two-electron reduction and in situ carbonylation of cobaltocenium hexafluorophosphate could be conveniently effected with sodium amalgam. The amalgam (0.65 g of Na dissolved in 25 mL of Hg) was placed in a 500-mL flask together with 350 mL of dimethylformamide. The solution was purged with a vigorous stream of C 0 2 and stirred magnetically while 1.5 g of solid cobaltocenium hexafluorophosphate was added. After 1 h the C 0 2 purge was stopped and the deep red suspension was decanted away from the amalgam. Subsequent reaction with methyl iodide and triphenylcarbenium tetrafluoroborate, and isolation of the product, followed the literature method.16 All other chemicals were reagent grade (Aldrich) and used as received. Aqueous solutions were 1 mM KH2P04 in 18.3 Mohm cm deionized water and were deaerated with nitrogen prior to the electrochemical experiments. Apparatus. Electrochemical experiments were carried out in a standard two-compartment, three-electrode cell. The reference electrode was saturated calomel (SCE). Sweep voltammetry was performed with a Pine Instruments RDE 4 bipotentiostat. UVvisible spectra of solutions and electrodes were obtained with an H P 845 1A (Hewlett-Packard) diode array spectrophotometer. Ion Exchange. The ion-exchanged zeolite-Y samples were prepared by stirring a suspension of 0.5 g of Na-Y in 10 mL of water containing the appropriate electroactive compounds at approximately 0.1 M concentration. After 24 h the zeolite was filtered, washed several times with distilled water, and air-dried at 40 "C.The degree of ion exchange was determined by comparing UV-visible spectra of the aqueous solutions before and after the exchange reaction. The concentrations of cobaltocenium and ferrocene species in the zeolite were found to be typically (1-2) X mol/g of zeolite, while R ~ ( b p y ) , ~and + O ~ ( b p y ) , ~were + in the range (2-3) X lod mol/g of zeolite. These observations are consistent with exchange of the smaller metallocene cations into the bulk, and the larger, substitution-inert M(bpy),*+ complexes onto only the outer surface of the zeolite particles. Electrode Preparation. Conductive Sn02-coated glass (NESA glass, PPG Industries) was cut into small squares and washed with absolute ethanol. Contact to a copper wire was made with colloidal silver paint (Ted Pella, Inc.). The copper wire was sealed into a glass tube and the silver contact was covered with 5-min epoxy (Dow Corning), and then with an overlayer of RTV silicone rubber (Devcon Corp.). The remaining exposed area of S n 0 2 was about 0.25 cm2. Current densities reported herein refer to this apparent surface area and do not take into account surface roughness. A suspension containing 0.1 g of the ion-exchanged zeolite powder and 0.01 g of polystyrene per milliliter of TH F was prepared,14 the electrode was dipped once into this suspension and allowed to dry in air. In all the experiments discussed herein, very similar results were obtained if the electrode was prepared by using Na-Y containing no electroactive ions, and the ion-exchange reaction was carried out after the electrode was fabricated. Fabrication of electrodes using preexchanged zeolite Y was preferred only because of the ease with which the degree of ion exchange could be quantified. Results and Discussion Electrode Film Structure. The structure of a zeolite/polystyrene film on Sn02glass is shown in scanning electron micrographs, Figure 1. The film thickness is about 60 pm, with most of the polystyrene forming an apparently porous layer at the outer surface of the film. These edge-on micrographs were obtained from a freshly cleaved electrode prepared with cobaltocenium(16) (a) Sheats, J. E.; Rausch, M. D. J. Org.Chem. 1970.35, 3245. (b) El Murr, N.; Laviron, E. Can. J. Chem. 1976,54, 3350. ( 1 7) Gaudiello, J. G.; Bradley, P. G.; Norton, K. A.; Woodruff, W. H.; Bard, A. J. Inorg. Chem. 1984, 23, 3.
The Journal of Physical Chemistry, Vol. 91, No. 3, 1987 645
Electron Transport at Zeolite-Y-Modified Electrodes SCHEME I 2+/+ eCH2NMe3
R
e C O C H 3
e%
-CY
&r
Fe
c o +IO
c o +IO
3i
+1 .o
+OS
0.0
1
Os(bpy)t+'2+
E,,2 (V vs. SCE) in 0.1 M CH$N/(n-C4Hd4NBF4 exchanged zeolite Y. Energy-dispersive X-ray emission spectra (not shown) clearly indicate the presence of Co-containing species in the (zeolite-containing) part of the film near the S n 0 2surface, but not in the polystyrene overlayer. The highest magnification view (Figure IC) of the zeolite/Sn02 interface shows that the film is quite porous and that a compact layer of ca. 1-pmdiameter particles sticks fast to the SnOz substrate. Electrochemistry. R ~ ( b p y ) , ~ + - Z e o l i Y. t e A typical cyclic voltammetric curve for R~(bpy),~+-zeoliteY/Sn02 electrodes is shown in Figure 2. The reduction and oxidation peaks at -0.76 and -0.55 V vs. S C E are ascribed to the zeolite surface-bound R ~ ( b p y ) , ~ + couple. /+ It is interesting to note that the half-wave potential of this couple (Ed N -0.66 V vs. SCE) is shifted about 600 mV positive of its value18 in polar organic solvents (-1.24 V vs. SCE). This shift indicates that the relatively nonpolar Ru( b ~ y ) ~cation + is greatly stabilized, relative to R ~ ( b p y ) , ~ +by, adsorption onto the zeolite. The quantity of charge passed in the C/cmZ) suggests that only Ru(bpy)?+/+ reduction wave (1 X the R ~ ( b p y ) , ~adsorbed + onto zeolite particles directly contacting the Sn02 electrode surface is electrochemically accessible. C~(CpCH,)~+-Zeolite Y. Figure 2b shows a cyclic voltammogram obtained for a C~(CpCH,)~+-zeoliteY/Sn02 electrode. In spite of the fact that the C O ( C ~ C H ~loading ) ~ + onto the zeolite is about ten times that of R ~ ( b p y ) ~in~the + R~(bpy),~+-zeolite Y/Sn02 electrodes (Figure 2b), the charge passed in the cathodic sweep is still only 1 X C/cm2. Hence only a small fraction of the C O ( C ~ C H ~contained )~+ in the zeolite particles at the S n 0 2 surface is reduced and oxidized. The equilibrium potential of the C O ( C ~ C H ~ wave ) ~ + /is~also shifted to a more positive value (-0.66 V vs. SCE), compared to the corresponding solution value,19 -1.10 V. Zeolite Electrodes Containing both Surface and Bulk Ions. When electrodes were prepared from zeolite Y exchanged in a solution containing both R ~ ( b p y ) , ~and + CO(CPCH,)~+,or when the C ~ ( C p C H ~ ) ~ + - z e o lY i t eelectrodes were soaked briefly in a 10 mM R ~ ( b p y ) , ~solution + and rinsed with water, the cyclic voltammogram shown in Figure 2a was obtained. Approximately a tenfold increase in the height of the cathodic wave, compared to Figure 2b, is seen. The quantity of charge passed in this wave (1 X loq4C/cm2) corresponds to the quantity of zeolite-bound cobaltocenium and Ru(bpy)?+ ions contained within the ca.1-pm compact layer of particles at the SnO, surface. Similar current enhancements were found (vide infra) for a number of substituted cobaltocenium (Co(CpR),+) species, provided the C O ( C ~ R ) ~ + / O wave was positive of, or coincident with, the R ~ ( b p y ) , ~ +wave. /+ The half-wave potentials, in acetonitrile solution, of the compounds used in this study18J9 are shown in Scheme I. We rationalize
-
(18) Saji, W.; Aoyagui, S. J . Electroanal. Chem. 1975, 63, 31. (19) Sheats, J. E. In Organometallic Chemistry Reviews, Vol. I, Seyferth, D., Ed.; Elsevier: Amsterdam, 1979; p 461.
0) SnOZ/Zeolite Nay/
elect rode
v = 100 mV/sec
T
67pA/cm2 I
t
0
I
I
I b) SnO2/Zeolite NoY/
2+ SnOz /ZeoliteNaY/ Ru[bpy)3 electrode
d -
+oo
-I 0
-05
POTENTIAL, V vs. S C E
Figure 2. Cyclic voltammetry of ion-exchanged zeolite Y electrodes in 1 mM aqueous KHzPOl solution. Scan rate = 100 mV/s. a. Ru(bpy)32+,C~(CpCH~)~+-zeolite Y electrode. b. C~(CpCH,)~+-zeolite Y and Ru(bpy),*+-zeolite Y electrodes.
the observation of current enhancement when the potentials for Ru(bpy)$/+ (surf.) and C O ( C ~ R ) ~(bulk) + / ~ are nearly coincident (as is the case for R = CH3) according to the following mechanism: cathodic process: Ru(bpy)32+(surf 1 t e-(electrode) Ru(bpy):(wrf.)
-
t
-
Ru(bpy)32+(surf.)t Co(CpCb):(bulk)
+ Co(CpCH3);(bulkl
The large cathodic wave represents the reduction of bulk Co(CPCH,)~', mediated by the surface-adsorbed R ~ ( b p y ) ~ * We +. also observe an enhanced anodic wave, attributable to the reverse electrocatalytic reaction: anodic process:
-
R u( bpy)s2+(s u r f.) t Co(CpCH3)z(buIk)
t R u ( b ~ y ) 3 ~ + ( s u r ff. ) e-
(electrode)
C O ( C ~ C H ~ )bu ~ ' I(k)
t R u ( bpy)3+(surf.)
646 The Journal of Physical Chemistry, Vol. 91, No. 3, 1987
0.2
Li and Mallouk
CH3
-
-Sn02 /Zeolite N o Y / R u ( b p y ~ 2 3 t C ~ ( ~ ) * e l e c t r 3 3 e --- Sn02/Zeolite
2+ NaY/Ru(bpy) electrode
3
CH3
w
0.1
C;(bj2 electrode
. . . Sn02 /Zeolite/
-
c)
z
2a
0 v)
m U
0
a
. . , . . . . . . , . . . . .. . . . . . . . . . . .
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: - -
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/
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i ' 2 L d
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/
\
/
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:
~
c
\
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.-
. . , . . . .~ _.
I
1
I
1
I
I
I
Figure 3. Difference absorption spectra for ion-exchanged zeolite-Y electrodes, recorded at -0.90 V vs. SCE. Reference spectra are the same electrodes held at 0.00 V vs. SCE. 0)
SnOe /Zeolife N o Y /
A
electrode 100 mV/ sec
f
7 0 p A / cm2
1 v = 100 mV/sec
0.0
-0.5
- I .o
POTENTIAL, V vs. SCE
Figure 4. CoCp,+, Ru(bpy),2+-zeoliteY electrode. Scan rate = 100
mV/s. The large (210 mV) value of A E peak and the unusual shape of the anodic wave (Figure 2a) are reminiscent of nonideal waves for polyvinylferrocene, and other nonpolar polymers, in contact with aqueous media;20 this nonideality is thought to arise from the fQrmation of a constant-activity solid phase upon reduction and concomitant dehydration of the polymer film. A similar mechanism may be operative in the bulk of the zeolite when neutral cobaltocene species are formed. Visible spectra for the C O ( C ~ C H , ) ~ +R/ ~ (, b p y ) , ~ + / +and , C~(CpCH,)~+/~-Ru(bpy)~~+/+/zeolite Y electrodes are shown in Figure 3. These are difference spectra, obtained by recording the absorbance 5 s after a potential step from 0.0 to -0.9 V vs. SCE, and subtracting the absorbance recorded before the potential step. For the C O ( C ~ C H , ) ~electrodes, +/~ practically no absorbance change is observed, consistent with the small amount of Co(CpCH,)? produced. The R ~ ( b p y ) , ~ + /electrode + shows a slight decrease in absorbance near 450 nm, where the oxidized form of the complex absorbs strongly;21the Ru(bpy),+ absorbance, with (20) (a) Willman, K. W.; Rocklin, R. D.; Nowak, R.; Kuo, K.; Schultz, F. A.; Murray, R. W. J . Am. Chem. Soc. 1980, 102,1629. (b) Daum, P.; Murray, R. W. J . Phys. Chem. 1981,85, 389. (21) Melsel, D.; Matheson, M. S.; Rabani, J. J . Phys. Chem. 1977, 81, 1449.
0
-0 5 POTENTIAL, V v s SCE
-
____
-I 0
1st SCAN 2nd SCAN 3rd SCAN
- 4ih
SCAN, AFTER HOLDING ONE M I N U T E AT O O V
Figure 5. Co(CpCOOCH3),+,Ru(bpy),2+-zeolite Y electrode. Successive cyclic scans at 100 mV/s. Electrode was held 1 min at 0.0 V
between third and fourth scans.
,,A
-
510 nm,21 is not resolved. The CO(C~CH,),+/~-RU(bpy)32+/+electrode shows a large absorbance change upon reduction, with a broad band increasing toward the shortest wavelength (350 nm) observable through SnOz glass. This spectrum is consistent with the production of a relatively large amount of C O ( C ~ C H , )& ~ ~ , = 330 nm?2 The difference spectra confirm that most of the current passed in the cathodic wave can be attributed to C O ( C ~ C H ~reduction. )~+ In order to test our hypothesis that electron transfers to and from the cobaltocenium ions were occurring only via the intermediacy of surface-adsorbed R ~ ( b p y ) , ~ we + , prepared electrodes (22) Borrell, P.; Henderson, E. Inorg. Chim. Acta 1975, 1.2, 215
Electron Transport a t Zeolite-Y-Modified Electrodes
The Journal of Physical Chemistry, Vol. 91, No.3, 1987 641
1 " " 1 1 1 " '
- Sn02 /Zeolite NaY/Os(bpy) 2+/ FcCH2 N Me; v.100 m V / s e c
----Sn02 /ZeoliteNaY /Os(bpy);+
""SnO2/ZeoIiteNaY/
A
T
40pA/cm2
1
I
0
n
1
I O 0 mV/sec
FcCH2NMe;
I
1
10
,
3
.
I
a
0 5
0
I
L 00
P0TENTIAL.V vs. SCE
.-W" 0 c
- 0.5
Figure 7. Cyclic voltammetry of FcCH2NMe,+,Os(bpy),2+-zeolite Y electrodes. Scan rate = 100 mV/s.
- 1.0
P O T E N T I A L , Vvs.SCE
reduction and oxidation of bulk ions is suppressed. Successive scans recorded at 1-min intervals show that the cathodic prepeak current is largely independent of scan rate. In the region where - Some electrode a f t e r equilibration the reduction waves are nearly coincident (-0.45 to -0.65 V vs. w i t h Ru(bpy):+ SCE), the current is determined by the electron-transfer rate constant, k ( E ) ,at the electrode/zeolite interface. This waveshape Figure 6. CoCp(CpCOOCH3)-zeolite Y electrode before and after 5is characteristic of totally irreversible (Le., unidirectional) inmin equilibrationwith 10 mM R~(bpy),~+. Electrode held 1 min at 0.0 terfacial electron t r a n ~ f e r . ~ ~ - ~ ~ V between scans. When electrodes were prepared with 1,1'-diaminmbaltocenium hexafluorophosphate ( C O ( C ~ N H ~ ) in ~ ' combination ) with Ruusing several different substituted cobaltocenes. The unsubstituted ( b ~ y ) , ~ the + , electrochemical response was identical with that cobaltocenium hexafluorophosphate, exchanged into zeolite Y in obtained with R ~ ( b p y ) , ~alone. + The potential of the Cocombination with R ~ ( b p y ) , ~ +gave , results (Figure 4) almost (CpNH2)+l0couple is ca. 400 mV negative of the C O ( C ~ C H , ) ~ + / ~ identical with those obtained with C O ( C ~ C H , ) ~ + - R ~ ( ~ ~ ~ ) , ~ + . couple in s o l ~ t i o n , 'so ~ Ru(bpy),+ (surf.) cannot reduce CoThis behavior is expected since the cobaltocene/cobaltocenium (CpNH2)+ (bulk). couple has about the same equilibrium potential as the CoThe system O~(bpy),~+/~+-(N,N'-dimethylaminomethyl)( C P C H , ) ~ +c0up1e.l~ /~ When electrodes were prepared by using ferrocene methiodide (FcCH~N(CH,),~+/+) was investigated in 1,l'-bis(carbomethoxy)cobaltocenium hexafluorophosphate (Coorder to test the generality of our conclusions concerning electron (CpCOOCH3)z+)and R ~ ( b p y ) , ~ only + , a broad reduction wave, transfer through surface-adsorbed species on zeolite Y. Figure and no oxidation wave, is seen (Figure 5 ) . The reduction wave 7 shows cyclic voltammograms obtained from electrodes prepared is markedly irreversible in shape and is peaked at a potential (-0.80 by using O ~ ( b p y ) , ~alone, + FcCH2N(CH3),+ alone, and both V) near E peak (-0.76 V) for R~(bpy),~+/+(surf.).While the together, exchanged into zeolite Y. In the first two cases, no peaks determination of half-wave potentials for irreversible systems is are observed. When both ions are present, oxidation and reduction complicated by kinetic effects, the near-coincidence of these powaves are observed, with E,' about +0.75 V vs. SCE. We conclude tentials suggests that R ~ ( b p y ) , ~ + and , not C O ( C ~ C O O C H , ) ~ + , that the Os(bpy)?+/*+(surf.) and F C C H ~ N ( C H ~ ) , ~ + / +waves (~U~~) is being reduced at the electrode surface. Reduction of zeolitemust be nearly coincident, and that for the latter E,,' is ca. +0.75 bound C O ( C ~ C O O C H , ) ~by+ Ru(bpy),+ is rapid, because the V. The oxidation wave is not sharply peaked, as it was for the electron-transfer reaction is thermodynamically favorable: the RU(~~~),~+/+-CO(C~CH~)~+/~ oxidation, suggesting that a deC O ( C ~ C O O C H ~ )solution ~ + / ~ redox potential is about 500 mV hydrated or constant activity phase is not formed in the case of ~ , it is likely that the positive of C O ( C ~ C H , ) ~or + /C~O C ~ , + / so a bulk 2 + / + couple. A systematic spectroscopic and electroC O ( C ~ C O O C H , ) ~(bulk) + / ~ potential is several hundred millivolts chemical study of substituted ferrocenes sorbed into zeolite Y is positive of the R ~ ( b p y ) , ~ + l(surf.) + couple. However, such a currently being undertaken in order to clarify the reasons for these separation is sufficient to make the surface concentration of effects. Ru(bpy)+ practically nil a t the CO(C~COOCH,)~,+/O (bulk) Measurement of Charge- Transport Diffusion Rates. The equilibrium potential, so that mediation of the oxidation by Rusweep-rate (v) dependence of the cathodic peak current (i,) for ( b ~ y ) , ~ + (surf.) /+ is unobservably slow. In Figure 5 it is shown Ru(bpy):+, C~(CpCH,)~+-zeoliteY electrodes is shown in Figure that the concentration of C O ( C ~ C O O C H , ) is ~ +depleted in each 8. For slow scans ( 5 5 0 mV/s) the peak current varies linearly successive scan, but that it is reoxidized or replaced by diffusional with v, indicating that nearly all of the electrochemically accessible processes if the electrode is held positive of the Coions are reduced. The nonzero intercept of the i, vs. v plot arises (CpCOOCH3)2+/0potential for 1 min. The slowness of this from steady-state background (water reduction) current at the reoxidation is again consistent with our assertion that electron peak potential. Correcting for background and integrating the transfer goes only via the Ru(bpy),*+/+ couple. C is passed (electrode area cathodic wave we find that 3.5 X The irreversibility of electron transfer to electropositive sub= 0.28 cmz) at 20 mV/s. From eq 1 we calculate the surface stituted cobaltocenium ions is clearly revealed in the sweep-rate coverage of electroactive material, r, to be dependence of the current response. Figure 6 shows single cyclic scans for a CoCp(CpCOOCH3)+electrode before and after exposure to a solution containing R ~ ( b p y ) , ~ions. + Before equilibration with R ~ ( b p y ) , ~ +broad , reduction and oxidation waves (23) Reinmuth, W. H. Anal. Chem. 1960, 32, 1831. attributable to CoCp(CpCOOCH,)+(bulk), Ed = -0.40 V, are (24) Nicholson, R. S.; Shain, I. Anal. Chem. 1964, 36,706. (25) Klingler, R. J.; Kochi, J. K. J . Phys. Chem. 1981, 85, 1731 seen. With surface-adsorbed R ~ ( b p y ) , ~ions + present, the direct 4
1
0
---- C o C p ( C p CII O C H 3 1+/ Z e o l i t e
Y electrode
J. Phys. Chem. 1987, 91, 648-654
648
corresponds well to the average particle dimension. At higher scan rates, the cathodic wave is diffusional and i, varies linearly with d12. From the Randles-Sevcik equation (eq 2 ) , taking C
I
i , = 2.69 X IO5 n312Av112D'12C
0.8
(2)
= 1.4 X equiv/cm3, we find that the charge transport diffusion cm2/s. This value is comparable coefficient, D, is about 2 X to that found for charge transport in redox polymer and ion-exchanged polyelectrolyte filmsz6 and underscores the potential viability of incorporating zeolite-modified electrodes into electrocatalytic systems.
0.6
0.4
Conclusions
We have prepared zeolite-modified electrodes in which a M ( b ~ y ) ~ ,(+M = Ru, Os) complex is adsorbed at the zeolite surface, and a metallocene is sorbed into the bulk. The metallocene ions appear to diffuse freely in the bulk of zeolite Y, with diffusion coefficients in the cmz/s range. The key finding of this study is that rapid electron transfer from the electrode to the bulk material occurs almost entirely through the M(bpy),,+ complex. The vectorial nature of these electron transfers suggests possible extensions of this work to shape-selective electrocatalysis and artificial photosynthesis. These possibilities will be explored in future work.
'pc (PA'
10.0
I
I
00 10
20
1
30
* ..
v (mb/sec)
40
Figure 8. Scan rate dependence of the cathodic peak current for the CO(C~CH,)~+, Ru(bpy)?+-zeolite Y electrode. a. ipakvs. scan rate plot. b. iFakvs. (scan rate)'I2 plot.
1.3 X IO4 equiv/cm2. Taking into account the bulk concentration of Co(CpCH&+ in the zeolite (ca. 2 X mol/cm3, or one cobaltocenium ion for every -40 supercages), and assuming a packing density of about 0.7 for zeolite particles in contact with the SnO, surface, we calculate a thickness of about 1 pm for the layer containing electrochemically accessible ions. This figure
Acknowledgment. We are grateful to Professor B. Shaw for communicating results to us prior to publication. SEM's were taken by Dr. Michael Schmerling, Department of Engineering, The University of Texas at Austin. This research was supported by a grant from Research Corporation, and by the Robert A. Welch Foundation. Registry No. Ru(bpy)32+,15158-62-0;Os(bpy)?, 23648-06-8; Co(CpCHj)*+, 40759-60-2; CoCp,+, 12241-42-8; Co(CpCOOCH3)2+, 40698-36-0; FcCH2NMe3+,33039-48-4; CoCp(CpCOOCH,)+,4069833-7; C O ( C ~ N H ~40759-59-9; )~+, Sn02, 18282-10-5. (26) Murray, R. W. In Electroamlytical Chemistry, Vol. 13; Bard, A. J., Ed.; Marcel Dekker: New York, 1984; p 191.
pH-Sensitive WO,-Based Microeiectrochemicai Transistors Michael J. Natan, Thomas E. Mallouk, and Mark S. Wighton* Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 021 39 (Received: July 8, 1986)
-
Electrochemical properties of an array of closely spaced (1.2 wm) Au or Pt microelectrodes (- 2 Fm wide X 50 wm long X 0.1 gm high) coated by a 0.15-wm-thicklayer of polycrystalline WO, are reported. The W 0 3 is deposited on the electrodes by radio frequency (rf) sputtering of a WO, target. The cyclic voltammetry of these microelectrodes indicates that W 0 3 connects individual microelectrodes, since the voltammogram of a pair of microelectrodes driven together is indistinguishable from that of an individual microelectrode. WO, becomes a good conductor upon electrochemicalreduction in aqueous solutions. The change in resistance of W 0 3 connecting two microelectrodes as a function of electrochemical potential spans 4 orders of magnitude, from lo6 to lo2 ohm. A pair of W03-connected microelectrodes functions as a microelectrochemical transistor that is sensitive to pH. The cyclic voltammetry is pH-dependent and consistent with pH-dependent transistor characteristics, which indicate that the device is turned on at more positive electrochemical potentials in acidic media. In basic solutions, more negative potentials are needed to turn on WO,-based transistors. The maximum slope of the drain current, ID,vs. gate voltage, VG,plot at fned drain voltage, VD,gives a transconductance of 12 mS/mm of gate width. Potential step and potential sweep measurements indicate that the W03-based transistor can be reversibly turned off and on in seconds; furthermore, the gate current, ZG,and ZD can be measured simultaneously,allowing demonstration of power gain for a sinusoidal variation of VG at fixed VD. Operating at a frequency of 1 Hz, the power amplification by the WO,-based transistor is 200, at pH 1. The power amplification decreases at both higher pH and higher frequency. The properties of the W0,-based microelectrochemical transistor allow its use as a real-time pH sensor: a reproducible change in ID, at fixed VG and VD, is obtained rapidly as the pH of a stream flowed continuously past the electrode is repetitively changed from pH 3.9 to pH
- -
1.2.
In this article we report the properties of W03-based microelectrochemical transistors prepared by derivatization of microelectrode arrays with polycrystalline W 0 3 , which undergoes the 0022-3654/87/2091-0648$01.50/0
following reversible, proton-dependent redox reaction:' WO,
+ nH+ + ne-
0 1987 American Chemical Society
F?
H,WO,
(1)
