Dye-loaded polymers on semiconductor electrodes

Dye-loaded polymers on semiconductor electrodes

J. Electroanal Chem, 159 (1983) 49-62 49 Elsevier Sequoia S.A, Lausanne - Printed in The Netherlands DYE-LOADED POLYMERS ON SEMICONDUCTOR ELECTRODE...

608KB Sizes 2 Downloads 51 Views

J. Electroanal Chem, 159 (1983) 49-62


Elsevier Sequoia S.A, Lausanne - Printed in The Netherlands


P R A S H A N T V. K A M A T and M A R Y E A N N E FOX *

Department of Chemtstry, The Umverstty of Texas at Austin, Austin, Texas 78712 ( U S A ) (Recewed 18th October 1982; m revised form 18th Aprd 1983)

ABSTRACT The effect of surface modification of an n-type sermconductor on the electrochemical behavtor of a soluble ferrocene derivative is reported. The incorporation of an a m o m c dye (croconate violet) into poly(vlnylpyndme)-coated polycrystalhne SnO 2 electrodes produces slgmficant alteration of the kinetics of electroox~datlon of solution phase redox couples A mechanism revolving &ffusion of the electroacave substrate a n d / o r an electron through the dye-loaded polymer film is suggested.


Various techniques for chemical modification of electrodes have been developed recently to produce materials which could be useful in electrocatalysis, in electrochromic devices, or in corrosion prevention. Of particular interest have been efforts at modification to stabdize small bandgap semiconductors used in photoelectrochemlcal cells [1]. The casting of specially designed polymer films on electrodes shows promise as an effective, yet simple, technique useful in achieving these goals. Such polymer-coated electrodes are of two types: those containing electroactive groups on the backbone of the polymer [2] and those in which electroactive ions are held electrostat~cally to polar groups pendant from the non-electroactwe polymer matrix [3]. In the latter type, the electroactive guest has been reported either to mediate electron exchange [4] or to facilitate charge transfer by electronic and iomc conduction [5]. We wished to probe the properties of analogous electrodes in which a species having electroactivity and/or photoactivity was electrostatically bound to the polymer coating, to determine whether such electrodes might be useful in catalytic applications. Croconate violet (I), an anionic oxocarbon derivative, having been reported to exhibit strong absorption in the visible region (?~m~x= 530 nm, ~ = 105 M - I cm-~, reversible electrochemical oxidative behavior, [6] and excellent semiconductor prop0022-0728/83/$03 O0

© 1983 Elsevier Sequota S.A.


Nc :

cN Fe

N C / K o - - ~ OK "CN ( I )


erties [7], was chosen as the electroactwe dye guest. We have reported that the fluorescence yield of I can be increased by nearly an order of magnitude by addition of poly(4-vinylpyridine) (PVP) to ethanohc solutions of I [8]. It was our hope that these observations in solution might translate into useful surface modifications, i.e., that this dye could be effectively loaded onto a polymer-coated n-type semiconductor and that the resulting modified electrode might exhibit novel photochemical properties. Although several approaches to coating electrodes with sensitizing dyes using electrochemical or covalent attachment [9], vapor deposition [ 10], or adsorption [11 ] techniques have been described, little attention has been paid to the incorporation of dyes into polymer films coated on the electrodes. We now report the electrochemical behavior of one such modified electrode: n-type SnO2 coated with poly(vmylpyndine) loaded with I. EXPERIMENTAL

Materials I, a gift of Dr. A.J. FatiadL U.S. National Bureau of Standards, and [(N, N, N-trlmethylammonio)methyl]ferrocenehexafluorophosphate (II) [12] were recrystalhzed before use. All other chemicals were reagent grade and were used without further purification. Unless otherwise specified, the electrolyte composition for electrochemical measurements was 0.1 M CF3COONa in Millipore-filtered water, buffered to pH 3.3. with CF3COOH.

Preparatton of the electrode A modificauon of the method employed by Anson et al. for the incorporation of IrC13- into PVP was employed [4]. NESA-glass plates (SnO2-coated, from PPG Industries) were cleaned by previously described techmques [13]. The cleaned surface was then polymer-coated by applying a solution of 2% PVP (Polysclences, Inc.) in ethanol and allowing the solvent to evaporate. The PVP-coated electrode, referred to as (SnO2/PVP), was then soaked for 15 min in an aqueous solution of I (ca. 1 m M ) buffered to pH 3.3 with 0.1 M CF3COONa and CF3COOH. The electrode was then removed and thoroughly washed with Millipore-filtered water. A pink coloration of the polymer film confirmed the incorporation of the dye. Such an electrode will be referred to as (SnO2/PVP/I). Because of the extremely low pK d of


I [7], the dye exists exclusively as the dianion under the conditions of the experiment. The thickness of the dry film, determined with a Sloan Dektak surface profilometer, was 0.025-0.050 /~M. Typically, a surface coverage of approximately 8 × 10 -9 m o l / c m 2 of I in the polymer film was obtained by this procedure.

Electrochemtstry All cychc voltammetric measurements were conducted with a Princeton Applied Research (PAR) Model 173 potentiostat/galvanostat, a PAR Model 175 universal programmer, and a Houston Instruments x-y recorder. All current-potential measurements were carried out in a standard three compartment cell with a saturated calomel electrode (SCE) used as reference. Rotating disc electrode ( R D E ) / experiments were performed with a Pine Instruments Co. RDE assembly. The surface of a glassy carbon disc, fitted with heat shrinkable Teflon tubing, was polished with Metadi II, a 1 ~tm diamond polishing compound on a microcloth (Buehler Ltd.), and washed with water and ethanol. The coating procedure and the dye incorporation were identical to those used with SnO 2. RESULTS A N D DISCUSSION

Tin oxide electrodes modified by adsorption of PVP containing I have significantly different electrochemical properties than their unmodified counterparts. The specific differences between these dye-loaded polymer electrodes are reflected in both physical and optical properties and in the influence of the surface modification on the kinetics of electrochemical redox reactions of electroactive substances dissolved m the electrolyte.

Absorptton charactertsttcs A typical absorption spectrum observed for a dye-loaded polymer-coated n-SnO 2 electrode is shown in Fig. 1. The absorpUon maximum of (n-SnO2/PVP/I) (556 nm) is considerably red-shifted compared to the absorption maximum of I in ethanol (~ma× = 538 nm). The extinction coefficient of the occluded dye is also slightly decreased from that observed in flmd solution. These observations parallel our previous report of shifts of both absorption and emission maxima of I in the presence of a polymeric microcage structure [8] and lend credence to our suspicion that solution-phase excited state behavior may find parallel behavior at modified surfaces.

Electrochemtcal charactertstws of (Sn 0 e / P VP / I) Typical cyclic voltammograms of I in aqueous solution and as a dye-loaded polymer-coated electrode are shown in Fig. 2. When the potential range of the anodlc scan was limited to a one-electron oxidation of I (0.70-0.75 V vs. SCE. Fig.

52 0"80 0.70 0.80 0.50 0,40 0,30 0.20 D. I0

0"D i









WAVELENGTH/nm Fig. I AbsoFptlon spectra of transparent n-SnO2 electrodes at various stages of modlfiCatlon (against air). (...... ) natwe SnO2; ( . . . . ) (SnOE/PVP). 0025 /Lm thlck); ( ) (SnO2/PVP/I)" 0.025 /~m

thlck; 8 × 10-9 tool of dye/cm2; ( . . . . . ) I as a ddute ethanohc solutlon

2a), the electrode was stable. Repeated scans reproduced an identical cyclic voltammogram characteristic of I. When the potential range was extended to 1.0 V vs. SCE, an irreversible oxidation was attributable to I could be seen (at 0.8 V, Fig. 2b). This observation closely parallels that reported by Fatiadi et al. in a thin layer cell [6]. Repeated scans beyond 0.75 V led to the destruction of the dye in the polymer matrix, an occurrence which could be easily monitored by observing the decreased anodic peak currents at 0.6 V. After the destruction of the dye, the electrode could be restored to its original behavior by redipping it in an aqueous, buffered solution of I. This observation implies that I is more strongly bound to PVP than is its second oxidation product. Protonation of PVP in the film was found to be critical m attaining the electroactivity for (SnO2/PVP/I). At pH greater than 3.5, only a small fraction of I contained in the polymer layer was found to be electroactive. An analogous requirement for nearly complete protonation of the film in attaining full electroactivity of substrates dispersed in PVP-coated electrodes has been discussed by Anson et al. who, as in our observations, found pH dependence on electroactlvity at values much lower than required for polymer protonation or for dye incorporation [4]. The cyclic voltammogram observed for (SnO2/PVP/I), Fig. 2a, allows for characterization of the type of redox reaction involved. For a one-electron Nernstian redox reaction at 25°C, the cyclic voltammogram of a surface confined layer is








E/V vs SCE










1 btA

Io ¸ E/V vs SCE


Fag. 2. Cychc voltammograms. (a) (SnO2/PVP/I). 0-0 7 V vs. SCE; (b) (SnO2/PVP/I). 0-0 9 V vs SCE, (c) I at nauve SnO2. Electrolyte: 0 1 M aq. CF3COONa, pH 3.3, scan rate 10 mV/s. Cathodic currents are plotted upwards.




At.ply o! ,ll

0 -3


I -2


log i /


Fig 3 Dependence of oxidative and reduct~ve peak separation (AEp) on scan rate (v) for the oxldauon of II at the surface of (SnO2/PVP/I) in 0.1 M aq CF3COONa, pH 3 3

54 •






150 Ip, alAoA





2t/v s

Fig 4 Dependence of anodlc peak current (ip, a) on scan rate ( v ) at the surface of ( S n O z / P V P / I ) m 0 1 M aq. CF3COONa, p H 3 3.

expected to exhibit no separation between anodic and cathodic peak potentials (i.e., AEp-----gp,a - - E p , c = 0) [14]. Very few films, however, exhibit such ideal behavior [2,15,16]. With (SnO2/PVP/I), the observed peak separation was 50 mV at scan rates less that 20 mV/s. As can be seen from Fig. 3, the peak separations were constant at slower scan rates and increased at scan rates greater than 50 mV/s. Similarly, the anodic peak current varied hnearly with scan rate at slow scan rates and deviated from lineanty at higher rates (Fig. 4). This deviation from linearlty is relatively common with polymer-coated electrodes and has been attributed to film resistance or to diffusion-like behavior of the substrate within the film, particularly in thick layers [ 15-17].

Electroehermstry of H at (SnO2/PVP/ I) Cyclic voltammetric scans of dissolved II at differently modified S n O 2 electrodes are shown m Figs. 5-8. Reversible oxidation of II could be seen at unmodified tin

02 E/Vvs


F~g 5. Cychc voltammogram for II at an unmodified SnO 2 electrode Electrolyte" 3 × 10 - 4 M II, 0 1 M Cf3COONa, p H 3 3. Room temperature, scan rate 10 m V / s

55 o x i d e (Ep. a = 0 . 4 1

V, Ep. c = 0.32 V, A E p = 90 m V ;

Fig. 5). W h e n

the electrode

c o a t e d w i t h o n l y P V P w a s e m p l o y e d ( c o m p l e t e l y p r o t o n a t e d , a t p H 3.3 [41), b o t h t h e a n o d i c a n d c a t h o d i c p e a k s c o r r e s p o n d i n g t o t h e r e v e r s i b l e o x i d a t i o n o f II c o u l d still



Fig. 6 Cyclic voltammogram for lI at a modified SnO2 electrode coated with PVP (film thickness = 0A ptm). Electrolyte' 3 x 10 -4 M II, 0 1 M CF3COONa, pH 3 3. Room temperature, scan rate 10 m V / s

Fig 7. Cychc voltammogram for II at a modified SnO2 electrode coated with PVP and loaded with I. Electrolyte 3 × 10 -4 M II, 0 1 M CF3COONa, pH 3 3. Room temperature, scan rate 10 mV/s. Fdm thickness 0 1 #m; F0 = 8× 10 -9 mol/cm 2

Fig 8. Cychc voltammogram for II at a modified SnO2 electrode coated with PVP and loaded with I, after anodlc destruction of the dye. Electrolyte' 3 × 10 -4 M II, 0 1 M CFaCOONa, pH 3 3. Room temperature, scan rate 10 mV/s.


be seen. Such behavior is consistent with its diffusion through the polymer film. However, both anodic and cathodic peaks were shifted towards positive and negative potentials respectively (Ep. a = 0.55 V, Ep.c = 0.10 V, ~Ep = 450 mV, Fig. 6). Such a separation in the peak potential has been reported prevxously m the oxidation of Fe 3+ at PVP-coated pyrolytic graphite electrodes [4] and can be explained on the basis of repulsive electrostatic interactions introduced in the polycationic layer. When (SnO2/PVP/I) was employed in the same solution, two separate reversible cyclic voltammetric peaks corresponding to I and II were s e e n ( E p . a and Epx for II were 0.43 V and 0.34 V, respectively, Fig. 7). These waves were almost identxcal to those seen at the bare SnO 2 electrode. The cyclic voltammetric peaks corresponding to I were also retained, indicating the presence of the dye in the film. The cyclic voltammogram of II was of a diffusional type (/p.a varied linearly with v-1/2) but that of I was of a surface type. When the dye in the film was deliberately destroyed by extending the anodic scan to 1.0 V, the reduction peak for II shifted toward more negative potentials, Fig. 8. In successive scans, the cyclic voltammogram reverted in appearance to that obtained at (SnO2/PVP), Fig. 6. This reversible shift, the most interesting feature of this polymer-coated electrode, clearly demonstrates that I incorporated in the PVP film faciliates the reversible oxidation of II. A similar shift in the diffusional wave was observed for the oxidation of II when another anionic dye, erythrosin B, was incorporated into the polymer in place of I.

Rotatmg dlsc electrode (RDE) experiments RDE techniques [ 18] were used to investigate the electrochemical behavior of II on our dye-loaded film-coated electrodes. Cyclic voltammetric studies established that glassy carbon, in native, polymer-coated, or dye-loaded polymer-coated form, behaved almost identically to the analogously treated SnO 2 electrodes. Typical cyclic voltammograms of II at a rotated unmodified glassy carbon electrode and at a glassy

•6 !

.4 I

.2 I


O E/V vs SCI~

Fig. 9. Cychc voltammograms of II ( 3 × 10-'* M ) at a rotated glassy carbon &sc electrode, 0 1 M CF3COONa, p H 3.3. (a) unmo&fied; (b) coated with I-loaded PVP film, (c) coated with PVP film. RotaUon rate = 1000 rpm, electrode area = 0.65 cm 2, room temperature, scan rate = 20 m V / s .




Ihm /J.IA 100





50 f.A~ 1/2/r pm I/2

Fig. 10 Levlch plots for p e r m e a t i o n wave for II o x i d a t i o n at glassy c a r b o n (e); ( C / P V P ) (m); a n d ( C / P V P / I ) (X) electrodes. [ I I ] = 3 × l 0 - 4 M, F0 = 8 × l 0 - 9 mol c m 2, electrode area = 0.65 c m 2 15


"l/mA-1 im








-I12/ rpm-lJ2 F i g 11 R e c i p r o c a l Levlch p l o t s for p e r m e a t i o n wave for II o x i d a t i o n at glassy c a r b o n (e), ( C / P V P (It); a n d ( C / P V P / I ) (X) electrodes [I1] = 3 × 10 - 4 M, F0 = 8 × 10 - 9 m o l / c m 2 ; electrode area = 0.65 c m 2.

58 carbon electrode coated with PVP and loaded with I ( C / P V P / I ) are shown in Fig. 9. A diffusion controlled limiting current attributable to II oxidation was seen at approximately + 0.4 V. This was accompanied by a surface wave at about + 0.6 V for the oxidation of I. Limiting currents for the first wave at a bare electrode exhibited a linear Levich plot, but deviations from linearity were seen both at ( C / P V P ) and at ( C / P V P / I ) electrodes at higher rotation rates (Fig. 10). Reciprocal Levich plots were linear, however, and displayed nonzero intercepts (Fig. 11). Mechamsm Four different electrochemical reaction mechanisms can be proposed for electrochemical reactions of redox couples at polymer-coated electrodes [16]: (1) diffusion of the electroactive species through channels or pinholes in the film, (2) mediated electron transfer through the electroactive sites in the polymer, (3) electron (or hole) conduction through the film material itself, or (4) diffusion of the electroactive species through the film, a process characterized by a different diffusion coefficient than observed in solution. Diffusion of the electroactive species to the bare tin oxide surface through surface defects seems unlikely. First, no cracks or pinholes could be seen in the film of ( S n O 2 / P V P / I ) when examined by a scanning electron microscope (SEM) with a magnification of 20,000 times. Second, AEp for II shifted from 90 mV to 450 mV when I which was loaded into the polymer is destroyed. If the electrochemical redox reaction were governed by diffusion through pinholes, no shift in the cyclic voltammetric peaks should have been seen. Although mediated electron transfer, the second mechanism, has been reported for many polymer-coated electrodes [4,17], this may not be a viable process in our case. No firm evidence could be found to assign these observed peaks as exclusively having been caused by mediated electron transfer through the polymer. In fact, the following observations seem to suggest the importance of non-mediated electron transfer: (i) Two clean superimposed cyclic voltammograms corresponding to those observed separately for I and II with no enhanced peak currents are seen at S n O 2 / P V P / I . In a simple mediated process, enhanced oxidation should be seen at the potential at which I is oxidized and no peak would be seen which corresponds to the reduction of oxidized I on scan reversal. (ii) The anodic peak current corresponding to the oxidation of II was found to be independent of the concentration of I in the polymer film over the range of concentrations of 5 × 10-10 m o l / c m 2 to 10 -8 m o l / c m 2. (iii) That reduction of oxidized II occurs not at 0.10 V vs. SCE (as observed at SnO2/PVP, Fig. 6) but rather at 0.34 V is more consistent with the possibility that the diffusion of the oxidized form of II is faclhtated within the dye-loaded polymer film. (iv) Qualitatively analogous shifts in redox peak potentials are seen with other redox couples in this dye-loaded film, e.g., if Ru(NH3)36 ÷ replaces II or if erythrosin B replaces I in the above experiment. These arguments are compelling only if the effective diffusion coefficient of I in the film is less than or equal to that of II. Since these diffusion coefficients are unknown and since they

59 m a y d e p e n d on the film structure, we c a n n o t u n a m b i g u o u s l y exclude m e d i a t e d electron transfer. T h e b u l k of the evidence, however, weighs against the m e d i a t i o n m e c h a n i s m as the sole route for electrochemistry within the film. The possible i n v o l v e m e n t o f electronic conduction, the third m e c h a n i s m , has sometimes been e v a l u a t e d b y the m e a s u r e m e n t of resistance across p o l y m e r films c o a t e d with a c o n d u c t i v e metal b y v a c u u m deposition. Such m e a s u r e m e n t s , unfortunately, are often misleading a n d unreliable [ 16]. A n a l t e r n a t e technique involves an e x a m i n a t i o n of the characteristics of the electrochemical reaction as a f u n c t i o n of the p o l y m e r thickness. If the d y e - l o a d e d film were conductive, one w o u l d expect the cychc v o l t a m m e t r l c b e h a v i o r to be i n d e p e n d e n t of the film thickness. W i t h our d y e - l o a d e d p o l y m e r coatings, however, b o t h the o b s e r v e d p e a k currents a n d p o t e n tials for the o x i d a t i o n of II d e p e n d e d on the thickness of the film, thicker films l e a d i n g to larger s e p a r a t i o n of a n o d i c a n d c a t h o d i c p e a k p o t e n t i a l s ( T a b l e 1). T h e d e p e n d e n c e of the rate of heterogeneous electron transfer on the film thickness suggests that electronic c o n d u c t i o n t h r o u g h the film is n o t a m a j o r process for electrochemical o x i d a t i o n s of r e d o x couples p r e s e n t in the electrolyte. T h e last m e c h a n i s m seems m o s t promising. Diffusion of the electroactive species I I t h r o u g h the p o l y m e r film can readily explain the o b s e r v e d behavior. If the p o l y m e r layer is c o n s i d e r e d as a highly viscous m e d i u m swelled with solvent, s u p p o r t e d electrolyte, a n d a n y o t h e r ions present, a r e a c t w e species might r e a d i l y diffuse through to reach the electrode surface if i n t r o d u c t i o n of the d y e does not

TABLE 1 Effect of film thickness on the cychc voltammetric behavior of II Electrodes

Scan rate/ mV s- i

Bare SnO2

10 100

90 ll0

SnO2/PVP/I 0.025-0.05 ~m thick 0 8 × 10-8 mol of I/cm 2

10 100

90 110

SnO2/PVP/I 0.1-0.2 t~m thick 1.2 x 10 -8 tool of I/cm z

10 100

100 120

SnOz/PVP/I > 0.5 t~m thick 5.2× 10-s mol of I/cm 2

10 100

440 --

SnO//PVP 0.025-0 05 #rn thick

10 100

450 - 600



SnO2/PVP/I 0.025-0.05 t~rn thick, 0.8 x 10-8 mol of I/cinz, but after the destruction of I

AE v/ mV

60 induce extensive cross-linking which would adversely influence diffusion. With unloaded polymer (SnO2/PVP), the electrostatic repulsion between the protonated PVP layer and the positively charged ferrocenium derivative retards the diffusion of the latter through the polymer film. Such a repulsive electrostatic interaction will shift the oxidation potential of II to more positwe potentials and its reduction to more negative potentials. In the case involving the dye-loaded polymer (SnO2/PVP/I), however, because the anionic dye is electrostatically bound to the cationic PVP layer, one might reasonably expect reduced cation--cation repulsion between the protonated polymer and the cationic substrate. This, in turn, would facilitate the diffusion of a positively charged reactant through the polymer layer. The R D E experiments indicate that diffusion of the ferrocenium derivative through the dye-loaded film is a major process in the electrooxidation. As seen in Fig. 11, a non-zero intercept for the reciprocal Levich plots exists for polymer-coated ( C / P V P ) and dye-loaded polymer-coated ( C / P V P / I ) electrodes. On these two electrodes, therefore, II diffuses through the film to be oxidized at the electrode surface. The intercept of the Levich plot allows determination of KDp/d, where K is the partition coefficient, Dp the diffusion constant for the ferrocenium derivative in the film, and d the thickness of the film [17,18]. With both coated electrodes, the intercept was found to vary inversely with the concentration of II. The value of KDp/d, which remained almost constant as the concentration of II increases, was an order of magnitude higher when dye was incorporated into the film (Table 2). Although the intercept of the Levich plot may be used as a measure of the rate of the mediation reaction, this clear dependence on the concentration of II requires that at least some additional pathway (perhaps diffusion) be responsible for the enhanced currents. These results clearly support a facilitated diffusional oxidation for II when dye is incorporated into the polymer film. Apart from simple diffusion of the electroactive species, there also exists the possibility of electron hopping or electron transfer between the oxidized and reduced forms of the same reactant within the polymer layer [19]. This type of "electron transfer diffusion," discussed by Dahms and by Ruff [20] allows for self-exchange electron transfer reactions for redox couples in solution. Such exchange reactions could play a significant role in electrochemical charge transport within the swelled

TABLE 2 Comparison of permeaUon rates of II through PVP and PVP/I fdms Q Conc of II/M


Intercept/A- i 1.5 30 6.0

5500 3100 1500

In 0.1 M CF3COONa (pH 3.3)


DpK d- ~/ cms -1

Intercept/A- l

1.93x 10- s 1.71 × 10-5 1.77× 10-5

900 400 200

DpKd- 1// cms -1 1 18× 10- 4 1 33× 10 - 4 1.33× 10-4


polymer phase where physical diffusion constants are lower than in solution [12.21]. Considermg the low concentrations of substrate employed here, however, such a process is probably less important than the diffusion &scussed above. Thus, although we do not completely exclude possible catalytic sites in the polymer film, especially within the first monolayer of the dye-coated polymer, our results are more consistent with diffusion through the PVP film as the major process governing the electrooxidat~on of II. The experiments described here represent a s~mple technique for the kinetic modification of diffusion rates through non-electroacttve polymer coatings. Since this process can be sigmficantly affected by the presence of occluded anionic dyes, important consequences on electrocatalytic processes occurring at such polymercoated electrodes are anticipated. The fact that the &ffusion me&ators are also highly absorptive opens the possibility that such arrays may also find use as photoresponsive catalysts. Indeed, photoelectrochemical effects observed with these dye-loaded systems will be &scussed in future work from our laboratory. ACKNOWLEDGEMENT

This research was supported by the U.S. Dept. of Energy, Fundamental Interactions Branch, Office of Basic Energy Sciences. MAF is also grateful for support as an Alfred P. Sloan Research Fellow and as a Camille and Henry Dreyfus TeacherScholar. We sincerely thank Dr. A.J. Fatiadi for samples of croconate violet used here and Henry S. White and Johnna Leddy for helpful discussions. REFERENCES 1 (a) M A. Fox, J.R. Hohman and P V. Kamat, Can, J Chem., 61 (1983) 888, (b) R M Murray, Acc. Chem Res, 13 (1980) 135 and references cited therein, (c) R. Noufi, AJ. Nozak, J. White and L.F. Warren, J Electrochem Soc, 129 (1982) 2261 2 D R. Rollson, M. Umana, P Burgmayer and R.W. Murray, Inorg Chem., 20 (1981) 2996. 3 (a) N. Oyama and F.C. Anson, J Electrochem. Soc., 127 (1980) 247; N Oyama, T Stumomura, K. Shlgehara and F.C. Anson, J Electroanal. Chem, 112 (1980) 271, (b) I Rubmsteln and A.J. Bard, J. Am. Chem. Soc, 102 (1980) 6641; 103 (1981) 5007. 4 N Oyama and F.C Anson, Anal. Chem., 52 (1980) 1192; (b) K. Shlgehara, N. Oyama and FC. Anson, J. Am. Chem Soc., 103 (1981) 2552 5 T.P. Hennlng, H.S. Wbate and A.J Bard, J. Am Chem Soc., 103 (1981) 3937. 6 L M Doane and A.J. Fatla&, J. Electroanal. Chem., 135 (1982) 193 7 A J. Fatla&, in R West (Ed.), Oxocarbons, Acadermc Press, New York, 1980, Ch. 4 8 P.V Kamat and M.A. Fox, Chem. Phys Lett., 92 (1982) 595 9 (a) K. Itaya, T Ataka and S. Tosluma, J Am Chem. Soc., 104 (1982) 4767; (b) WJ. Albery, A.W, Foulds, K J. Hall and A.R Hdlman, J. Electrochem. Soc., 127 (1980) 654; (c) W.J. Albery, A W. Foulds, K J. Hall, A R. Hdlman, R.G. Egdell and A.F. Orchard, Nature, 282 (1979) 793; (d) P K. Ghosh, and T.G Splro, J. Am Chem Soc., 102 (1980) 5543; (e) J.R Hohman, and M.A. Fox, ibid., 104 (1982) 401 10 (a) V R. Shepard, Jr. and N.R. Armstrong, J Phys Chem., 83 (1979) 1269; (b) A. Glraudeau, F.R F. Fan and A.J Bard, J Am. Chem. Soc., 102 (1980) 5137. 11 (a) Y. Monstnma, M. Isono, Y Itoh and S. Nozakura, Chem Lett., (1981) 1149, (b) M A Fox, F.J. Nobs and T.A. Voymck, J. Am Chem. Soc., 102 (1980) 166

62 12 13 14 15 16 17 18 19



H S. White, J Leddy and A.J. Bard, J Am. Chem. Soc., 104 (1982) 4811. JR. Hohman and M A Fox, J. Am Chem. Soc Soc., 104 (1982) 401 A.J Bard and L.R Faulkner, Electrochemical Methods, Wdey, New York, 1980 TP. Hennmg, H.S White and AJ. Bard, J Am Chem. Soc, in press P.J Peerce and A.J Bard, J. Electroanal Chem, 112 (1980)97 T. Ikeda, R. Schmehl, P Denlsevlch, K. Wdlman and R W Murray, J Am Chem Soc, 104 (1982) 2683. C. Andrleux, J.M. Dumas-Boucl~at and J M Sav6ant, J Electroanal Chem, 131 (1982) 1 (a) C R. Martin, I Rubmsteln and AJ. Bard, J. Am Chem. Soc., 104 (1982) 4817; (b) E Lavlron, J Electroanal. Chem., 112 (1980) 1, (c) F B. Kaufman and E M. Engler, J Am, Chem Soc, 101 (1979) 547. (a) H Dahms, J Phys. Chem., 72 (1968) 362, (b) I. Ruff, Electrochlm Acta, 15 (1970) 1059, (c) I Ruff and I. Korosl-Odor, Inorg Chem, 9 (1970) 186; (d) I. Ruff and V, Fnedrlch, J Phys Chem, 75 (1971) 3297, 3303. (a) J. Facc~ and R Murray, J Phys. Chem, 85 (1981) 2870; (b) DA. Buttry and F C Anson, J Electroanal. Chem, 130 (1982) 333.