5072
J . Phys. Chem. 1987, 91, 5072-5076
Determination of Substrate Diffusion in Polymeric Films on Electrode Surfaces Otto Haas* and Brigitte Sandmeier Swiss Federal Institute f o r Reactor Research, CH-5303 Wuerenlingen, Switzerland (Received: November 12, 1986)
A new technique to determine the substrate diffusion in the polymeric films of modified electrodes is described. The technique is based on a double-coated electrode. The first coating serves as an electrocatalyst whereas the second coating, which is placed on top of it, is the material in which one wants to measure the diffusion of the substrate. This technique has been
used to measure the diffusion of in a [ R U ( ~ ~ ~ ) ~ C ~ ( P V[PVP P)]C = Ipoly(4-vinylpyridine)J film, while electrochemically polymerized 1 -hydroxyphenazine has been used as the electrocatalyst.
Introduction To characterize catalytically active polymeric electrode coatings, one needs information about electron transfer and the diffusion of electrochemically active species (substrates) in the coatings. Saviant et al.’ showed that the mediation of electrochemical reactions by redox polymer films may be described with four characteristic currents, i,, iE, is, and i K ,where the lowest characteristic current is most important as a rate-limiting process. A similar theoretical treatment of the electrochemical behavior of modified electrodes has been published by Albery et al.* Instead of describing the system in terms of characteristic currents, Albery et al. express their results in terms of an electrochemical rate constant klMe,which they call the modified electrode rate constant. Both theories distinguish eight different cases of reaction mechanism. The connection between the two sets of notation is given in Table I of Albery’s paper.* In order to evaluate the important parameters, experiments with rotating disk electrodes and potential step experiments on stationary electrodes might be used. The substrate diffusion from the solution to the film ( i A ) and the cross exchange reaction between the redox center and the substrate in the film (iK)may be obtained from Koutecky-Levich p10ts.~ The electron transfer between the redox centers in the film ( i E )is obtained from Cottrell plots.4 The intercept of the Koutecky-Levich plot in some cases gives information about substrate diffusion in the film (is), but in general it will not be possible to separate this kinetic factor from iE and iK by this technique alone. Since the substrate diffusion in the film is a parameter that is very important for an understanding of the reaction mechanism in catalytic coatings,s$12it would be of great advantage if it could be measured by an independent method. The determination of is is relatively simple when the heterogeneous electron exchange rate at the electrode underneath the film is fast and no cross exchange reaction takes place in the film. In this case is can be evaluated by measuring the limiting currents ( i L ) of a covered electrode and an uncovered electrode. This method has been used by Murray et al.’ to determine the permeation of bromide, ferrocene, and certain ruthenium complexes in poly[Ru(vbpy),]*+ films in aprotic solvents. Unfortunately, current-potential curves resulting from substrate electron exchange at the electrode underneath a polymer coating are often very sluggish. For example, even with a Pt electrode underneath the PVP film, no useful current-potential curves can be measured for the reduction of Fe(II1) ions; thus no current plateau is observed, and iL cannot be deduced from this experiment. To overcome this difficulty, an interlayer with catalytic
Experimental Section Materials. [ R U ( ~ ~ ~ ) ~ C I ( P Vwas P ) ]a Cgift ~ obtained from Dr. J. G. Vos. It was prepared according to the previously published procedure.10 The Ru:PVP ratio was 1.5. 1Hydroxyphenazine was prepared as described in ref 6. The substrate solution was prepared with 1 M HCI (Titrisol Merck) and FeC1, (p.a. Merck). Apparatus. The electrochemical investigations were performed with an Amel 551 potentiostat/galvanostat, an Amel 567 function generator, a Linseis LY 1800 recorder, and a Tacussel ED1 rotating disk electrode. The disk speed was controlled with an IKA-TRON DZM 1 from Janke Kunkel, Staufen (BRD). Electrode Preparation. Three-millimeter-diameter glassy carbon electrodes were polished with a 1-pm diamond suspension from Buehler-Met Ltd., Basel, and washed with distilled water and ethanol. The electrodes were first coated with l-hydroxyM phenazine, by exposing them to a solution containing 1-hydroxyphenazine in 1 M H 2 S 0 4and scanning the potential between -0.3 and 1.3 V vs. SSCE until the desired surface concentration of the 1-hydroxyphenazine was reached. On top of this I-hydroxyphenazine layer (film l), the [Ru(bpy)zC1(PVP)]Cl coating (film 2) was applied by a spin-coating technique using a solution of the polymer containing 10 mg of
(1) Andrieux, C. P.; Savbant, J. M. J . Electroanal. Chem. Interfacial Electrochem. 1982, 134, 163. (2) Albery, W. J.; Hillman, A. R. Annu. Rep. Prog. Chem., Sect. C 1981, 78, 377. (3) (a) Koutecky, J.; Levich, V . G . Zh. Fiz. Khim. 1958, 32, 1565. (b) Anson, F. C.; Osaka, T.; Savbant, J. M. J. Am. Chem. SOC.1983, 105,4883. (4) Daum, P.; Lenhard, J. R.; Rolison, D.; Murray, R. W. J . Am. Chem. Sot. 1980, 102, 4649. (5) Ikeda, T.; Schmehl, R.; Denisevich, P.;Willman, K.; Murray, R. W. J . Am. Chem. Soc. 1982, 104, 2683.
(6) Haas, 0.;Zumbrunnen, H . R. Helu. Chim. Acta 1981, 64, 854. (7) Haas, 0.;Vos, J. G. J . Electroanal. Chem. Interfacial Electrochem. 1980. 113. 139. ( 8 ) (a)’Murray, R. W.Philos. Trans. R. Sot. London, A 1981, 302, 253. (b) Haas, 0.Extended Abstracts, 36th ISE Meeting, Salamanca, Spain; ISE: Zurich, 1985; p 05520. (c) Andrieux, C. P.; Haas, 0.;Savbant, J. M. J . Am. Chem. Soc. 1986, 108, 8175. (9) (a) Kittlesen, G. P.; White, H. S.; Wrighton, M. S . J . A m . Chem. Soc. 1985, 107, 7373. (b) Murray, R. W.Electroanal. Chem. 1984, 13, 340. (10) Clear, J. M.; Kelly, J. M.; O’Connell, C. M.; VOS.J. G. J . Chem. Res., Miniprint 1981, 3039.
0022-3654/87/209 1-5072$01.50/0
properties may help. It has been shown that in some cases twin and sandwich electrode arrangements have been useful to get some information about diffusion in electrode coating^.^ In the present work we applied a [ R U ( ~ ~ ~ ) ~ C ~ ( P V film P)]CI (Figure 1) to a glassy carbon electrode already coated with electrochemically polymerized 1-hydroxyphenazine. As a substrate we used ferric ions in acidic solutions. The electrochemical properties of electrodes coated with 1-hydroxyphenazine6 and [Ru(bpy),C1(PVP)]CI7 have been described in previous papers. It was shown that Fe(1I) ions may be oxidized at [Ru(bpy),CI(PVP)]Cl-coated electrodes, whereas Fe(II1) ions can be reduced at 1-hydroxyphenazine-coated electrodes. For thermodynamic reasons the inverse electron transfers are almost completely suppressed in these coatings. When Fe(II1) ion reduction is studied with the double-coated electrodes, the iron ions first must diffuse through the [Ru(bpy)*CI(PVP)]Cl layer before they can be reduced in the polymeric 1-hydroxyphenazine film. By measuring the limiting currents on single- and double-coated electrodes, we should be able to determine is for iron ions in the [Ru(bpy),CI(PVP)]Cl film.
+
0 1987 American Chemical Society
Diffusion in Polymeric Films on Electrode Surfaces
The Journal of Physical Chemistry, Vol. 91, No. 19, I987
5073
Figure 1. [Ru(bpy),CI(PVP)]Cl; Ru/PVP = 1/5.
[Ru(bpy),CI(PVP)Cl] in 10 mL of methanol. With a 10-pL syringe about 1 pL was placed on the surface of the l-hydroxyphenazine-coated electrode. Spinning was then started immediately with the disk facing upward. The speed used was 3000 rpm. By this method films of very uniform thickness could be prepared. Surface Concentration Measurements. To measure the surface concentration (moles per square centimeter) of l-hydroxyphenazine on the electrodes, the potential was scanned between -200 and +500 mV vs. SSCE (the redox peak is at 50 mV) while the charge in the cathodic scan was determined. For the calculation we assumed a two-electron-transfer process. To measure the corresponding surface concentration at the double-coated electrodes, the potential was scanned from +lo00 to -200 mV vs. SSCE after it was held at 1000 mV vs. SSCE for 2 min. All the coulometric experiments were performed in 1 M HCI under argon in the dark and with a scan rate of 25 mV/s. Note that this coulometric method can be used to measure the surface concentration only, when electron transfer within the films is fast as compared with the applied scan rate. One therefore must work in the region where the current density is proportional to the scan rate.
Theoretical Background The electrochemical reaction of substrate A at the double-coated electrode can be characterized by a scheme involving three consecutive steps: A,-
k 'D
A,-
k's
A, + e -
k'Mc
B
Figure 2 shows the above scheme in terms of concentration profiles, where A , is the concentration in the bulk solution, A2 is the concentration at the solution/film interface, A , is the concentration at the film l/film 2 interface, and A , is the concentration at the electrode surface. ktD= D/6 in centimeters per second, and k', = D , / @ 2 in centimeters per second. D is the diffusion coefficient of the substrate in solution, and D, is the diffusion coefficient of the substrate in film. and a2are the thicknesses of the first and second films, 6 is the thickness of the . Levich diffusion layer,I3 where 6 = 0 . 6 4 3 ~ - ' / ~ v ' / ~ Dw~is/ ' the disk speed in hertz, and u is the kinematic viscosity in square centimeters per second. ktMeis the heterogeneous rate constant for the reduction of Fe(II1) ions at the 1-hydroxyphenazine-coated electrode; it can be deduced from the intercept of a KouteckyLevich plot (curves a and c in Figure 5 ) . Following SavCant but reformulated for the double-coated electrode, under steady-state conditions, iL, iA, is, and iKcan be expressed as Levich depletion layer [ R u ( ~ ~ ~ ) ~ C I ( P V Pfilm )]CI
iL = FSkfs(A2- A , ) catalytic film
i , = FSkIMeA,
Figure 2. Assumed stationary concentration profiles at a negatively polarized double-coated rotating disk electrode. The picture shows the simplified situation where D, = D B and K = 1.
The characteristic currents may then be defined. Substrate diffusion from the bulk solution to the rotating electrode: iA = FSk'p4,
(4)
Substrate diffusion through the second film: is = FSk',KA,
(5)
Electron change reaction between the redox centers of the first film and the substrate: iK = FSk'MeKA,
(6)
Here F is Faraday's constant, S is the electrode surface area, P is the concentration of the redox sites in the film, D is the diffusion coefficient of substrate in the solution, D, is the diffusion of substrate in the film, and K is the partition coefficient of the substrate between the film and the solution. Using eq 4-6 to eliminate A , and A 2 in eq 1-3, we obtain
l/iL = l/i,
+ l/is + l/iK
(7)
In this equation, only 1/ i Adepends on disk speed whereas 1/ i s and I / i K are additive components of the intercept. The third term drops out when klMeis much larger than kts and k b . These conditions arise when the second film is thick and the rotation speed of the electrode (w)is low.
Results and Discussion Electrodes with Single Film. In Figure 3 the electrochemical behavior of single-coated stationary and rotating glassy carbon (GC) electrodes is summarized. Curve d shows the cyclic voltammogram (CV) of a 1-hydroxyphenazine-coated electrode and curve e the CV of a [Ru(bpy),C1(PVP)]C1-~oatedelectrode in 1 M HC1. The upper part of Figure 3 shows the current-potential curves obtained with rotating disk electrodes in 1 M HCI containing M FeCI, and M FeCI, as a substrate. The current-potential curve a was obtained with an uncoated, polished G C electrode. There is almost no reaction occurring on this electrode. A [R~(bpy)~Cl(PVP)]Cl-coated electrode shows an anodic wave at the formal potential of the [ R u ( ~ ~ ~ ) ~ C I ( P V P ) ] C I polymer (curve b), whereas, with a 1-hydroxyphenazine-coated electrode, a cathodic wave at the formal potential of the 1hydroxyphenazine polymer is obtained (curve c). The thickness of the catalytic 1-hydroxyphenazine film was optimized in order to get the highest klMevalue possible. This maximum was attcrined for r = 0.5 X mol/cm2. For this r-value at low and intermediate disk speeds, the limiting current for the Fe(1II) reduction at the 1-hydroxyphenazine-coated electrode is very close to the expected Levich current controlled
5074
0.7 -
F.-.-
i
0.5-
i
0.4-
i
0.30.2
b
i
0.6-
i
-
N
,E
Haas and Sandmeier
The Journal of Physical Chemistry, Vol. 91, No. 19, 1987
01-
\
a
E
\
O- b - a
I 4 -0.1
1 - 1 0.5
-02 -
- 0 3-
-04I
-05-
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-0 6 -07 -
\
06
4
0.4
-
\
I
I
'
1000
Figure 4. (Upper panel) Current-potential curves of rotating disk electrodes in a solution containing M FeCI, in 1 M HCI. Curve a, mol/" 1-hydroxyphenazine. Curve electrode coated with 0.8 X b, electrode coated with 0.9 X mol/cm2 [ R u ( ~ ~ ~ ) ~ C I ( P V Pon) ] C I top of 0.8 X mol/cm2 I-hydroxyphenazine. Disk speed = 1000 rpm. (Lower panel) Cyclic voltammograms of the double-coated stationary electrode. Curve c, potential cycled between -500 and +500 mV vs. SSCE. Curve d, potential scanned from +lo00 to -500 mV vs. SSCE
C
A
/ I
0.2 -
after waiting for 2 min at +lo00 mV vs. SSCE.
a E
,
0 500 E/mV vs. SSCE
O-
-0.2 -0.4 -
- 500
0
500
1000
E/mV vs. SSCE Figure 3. (Upper panel) Current-voltage curves obtained with a rotating M FeCI2and IO-, disk electrode (2000 rpm) in solution containing M FeCI, in 1 M HCI. Curve a, with a polished rotating GC electrode, diameter = 3 mm. Curve b, with the electrode spin-coated with 7 X lo4 mol/cm2 [Ru(bpy),CI(PVP)]CI. Curve c, with the electrode of curve a coated with 5.2 X mol/cm2 electrochemically polymerized 1hydroxyphenazine. (Lower panel) Curve d, cyclic voltammogram of a stationary I-hydroxyphenazine-coated electrode in 1 M HCI (scan rate = 100 mV/s). Curve e, cyclic voltammogram of a stationary Ru(bpy),CI(PVP)]Cl-coated electrode in 1 M HCI (scan rate = 100 mV/s).
by diffusion of the substrate from the solution to the film/solution interface. Double-Coated Electrodes. From curve a of Figure 4 we find iA = 0.445 mA/cm2 at 1000 rpm, which is in good agreement with measurements using the same solution at active platinum electrodes. With an additional layer of [Ru(bpy),CI(PVP)]Cl (rRu =9X mol/cm2) on top of the 1-hydroxyphenazine layer, we obtain curve b of Figure 4. Here the limiting current is 25% lower than i A of curve a: iL = 0.33 mA/cm2. Putting i A and i, into eq 7 and ignoring the third term ( l/iK),we find is = 1.3 mA/cm2. is should be inversely proportional to the surface concentration of the film. The surface concentration of the [Ru(bpy),Cl(PVP)]Cl film can be found from the results shown in Figure 4 (lower panel). Curve c shows the cyclic voltammogram obtained for the double-coated electrode in 1 M HCI when the potential was cycled between -500 and +500mV vs. SSCE, while curve d was obtained when the potential was first held at 1000 mV vs. SSCE for 2 min and then scanned from +lo00 to -500 mV vs. SSCE. There is almost no signal from the ruthenium redox couple at its formal potential (700 mV vs. SSCE); however, curve d shows an increased wave at +50 mV vs. SSCE in the cathodic scan. which is due to the reduction of the ruthenium sites by means of a mediated electron transfer via the partially reduced 1-
15
20: 10
/Y 1/SORT(w)
Figure 5. Koutecky-Levich plot of single- and double-coated GC disk
mol/cm2 electrodes. Curves a and c, electrode coated with 0.5 X 1-hydroxyphenazine. Curves b and d, double-coated electrode with 0.7 X mol/cm2 [ R u ( ~ ~ ~ ) ~ C ~ ( P V onPtop ) ] of C Ithe 1-hydroxyphenazine coating. Curves a and b, M FeCI, in 1 M HCI. Curves c and d, 2 X M FeCI, in 1 M HCI. hydroxyphenazine layer. The reoxidation of the [Ru(bpy),Cl(PVP)]Cl through the I-hydroxyphenazine film is very slow, as expected; it can only be achieved by holding the electrode for some time at a potential more positive than the formal potential of the Ru-polymer. By measuring the charge (coulombs) under curve c, we obtain the surface concentration of the I-hydroxyphenazine mol/cm2, while curve d yields a good layer, rPh= 8 X approximation for the sum of both surface concentrations, from
The Journal of Physical Chemistry, Vol. 91, No. 19. 1987 5075
Diffusion in Polymeric Films on Electrode Surfaces
TABLE I: Substrate Diffusion Current (is) for Iron Ions in [Ru(bpy),CI(PVP)KI Films” I o9rph. I 09sR,, mol/cm2 mol/cm2
I o~A,, mol/L
1 03k’Me, is, cm/s mA/cm2
is939
mA/cm2
10.3
26
0.2 1 .o 5.0
38
0.107 0.4 1.9
13.9 10.4 10.3
4.7
7
0.2 1 .o 5.0
46
0.28 1.6 6.2
10.4 11.5 9.1
5.6
6
0.2 1 .o 5.0
37
0.30 1.35 7.1
9.4 8.6 9.0
“is is the value measured for the conditions indicated. is93 is arithM Fe(II1). metically reduced to r R u = IO4 mol/cm2 and A, =
which we deduce rRu= 9*10-9 mole/cm2. When the electrocatalyst is not able to generate a diffusioncontrolled current (Levich current), the system with double-coated electrodes should be analyzed in terms of Koutecky-Levich plots. As shown in Figure 5, for the system investigated these are parallel straight lines for the single- (curves a and c) and double- (curves b and d) coated electrodes. The slopes of the curves are inversely proportional to the concentration of the substrate (CFe)in solution and equal to the expected Levich slope for a bare electrode with a fast heterogeneous reaction rate. The intercepts of curves a and c may be used to evaluate the ) Table I) for 1-hydroxelectrochemical rate constant ( I c ’ ~ ~(see yphenazine-modified electrodes.2*11This rate constant relates the flux of electrons to the concentration of the substrate ( A , ) at the film/solution interface of the 1-hydroxyphenazine-coated electrode. k’Meis directly proportional to A,; this follows from the ratio of the intercepts of curves a and c in Figure 5. It is probably limited by the cross exchange reaction or the substrate is not altered noticeably diffusion in this film. We assume that libMc by the introduction of the second coating. In eq 7 only 1 / i A is rotation-speed-dependent whereas 1/ i s and I / i K are additive components of the intercept in the KouteckyLevich plot. Therefore 1 / i s may be determined from the difference between the intercepts in the Koutecky-Levich plots of singleand double-coated electrodes. The results obtained from Koutecky-Levich plots of doublecoated rotating electrodes in acidic (1 M HCI) solutions with Fe(II1) substrate are listed in Table I. is should be proportional to the substrate concentration (A,) and inversely proportional to the surface concentration r R u . Over the range covered, there is only one measurement that deviates more than 15% from this relation. For the purpose of comparison, we find it useful to normalize is to arbitrary standard surface concentration rRu= IO4 mol/cm2 and substrate concentration A, = M Fe(II1); we call this quantity iS93,and from the results of Table I, is93 = 10.3 f 1.6 mA/cm2. A general expression may be used to cal)]CI culate is for iron ions in the [ R u ( ~ ~ ~ ) ~ C I ( P V P polymer: where C = (1.03 f 0.16) X IO-’ mA/cm when is, A,, and rRu are given in milliamperes per square centimeter, moles per liter, and moles per square centimeter, or C = (1.03 f 0.13) X IO” A/m when amperes per square meter, moles per cubic meter, and moles per square meter are used. When eq 8 is used for the experiment depicted in Figure 4 ( r R u = 9 X mol/cm2; A, = M) we find is = 1 .I4mA/cm2, which is in good agreement with the value (1.3 mA/cm2) found from the limiting currents in this figure. Estimate of the Diffusion Coefficient in the Film. Since 92 is unknown, Ds can only been estimated. Assuming that the polymer density (polymer bulging occurs in the electrolyte) is about 0.5 g/cm3 and the molecular weight per redox center is 1000, we cm for rRu = get a film thickness a2of about 2 X ( 1 I ) Albery, W. J. Electrode Kinetics; Clarendon: Oxford, 1975.
Figure 6. Scanning electron microscopc picture from a typical surface covered with [ R u ( ~ ~ ~ ) ~ C I ( P V P ) ] C I .
mol/cm2 and hence Ds = 2 X IW7 cm2/s, when K = 1. We believe that the rather high diffusion rate found for iron ions in the [ R u ( ~ ~ ~ ) ~ C I ( P V Pfilm ) ] CisI a consequence of the uncoordinated pyridines (Figure 1) which are protonated in acidic solutions. The electrostatic repulsion between these protonated pyridines opens the structure of the polymer and facilitates the diffusion of the substrate.14 In the strongly acidic solution ( 1 M HCI) complexation of Fe ions by pyridine may be neglected; however, complexation with CI- leads to monochloro- and dichloroiron complexes. From literature datal7 we can conclude that for the experimental conditions FeC12+,F e W , and FeC12 are the dominating species and FeCI2+is in equilibrium with about 10% of FeCI3. This means that significant amounts of neutral species are present that are not repelled by the pyridinium ions. But even for cationic substrates in cationic polymers the measured Dsis not entirely u n u s ~ a I . ~ J ~ Large diffusion coefficients are expected also when the film structure includes elements of porosity. Figure 6 suggests a certain extent of surface roughness but no through porcs. Figure 6, however, could of course only be obtained after extracting the electrode from the electrolyte solution into vacuum. It should be mentioned that is slightly decreased with increasing age of our electrode coating, probably owing to some change in the structure of the polymer. Furthermore, wc found that is depended on the supporting electrolyte and on the nature of the substrate.IS Strong dependencies of the substrate diffusion coefficient on the supporting electrolyte were also found in methylated poly(viny1pyridine) films.I6
Conclusions The method described is a valuable tool to measure the substrate diffusion current (is) in electrochemically inactive films. It can also be used for electrochemically active films, provided one is (12) Rocklin, R. D.; Murray, R. W. J . Phys. Chem. 1981, 85, 2104. ( 1 3) Levich, V. G. Physicochemical Hydrodynamics; Prentice-Hall: En-
glewood Cliffs, NJ, 1962; p 68. (14) Leddy, J.; Bard, A. J.; Maloy, J. T.;SavCant, J. M.J. Elecrroanal. Chem. Interfacial Electrochem. 1985, 187, 105. ( 1 5 ) Haas, 0.; Vos, J. G. Extended Abstracts, 37th ISE Meeting, Vilnius, USSR; ISE: Zurich, 1986; Vol. 2, p 350. ( 1 6) Niwa, K.; Doblhofer, K. Elecrrochim. Acta 1986, 31, 549. ( 1 7 ) (a) Davies, C. W. Electrochemistry, Ncwncs: London. 1967; p 76. (b) Gill, N. S.J . Chem. SOC.1961, 3512.
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working in the potential region where the film is not active toward the substrate employed. When the electrocatalyst employed (first coating) is very efficient, is can be. evaluated by using eq 7 ( i K is large and the third term can be neglected). Then quantities iA and iL are obtained from the limiting currents of the current-potential curves recorded with single- and double-coated electrodes (Figure 4). In the case where the electrocatalyst is not able to generate a diffusioncontrolled current (Levich current), the system should be analyzed in terms of Koutecky-Levich plots (Figure 5), where the difference of the intercepts of the parallel plots is a direct measure in I/is. The high diffusion current found for iron ions in the [Ru(bpy)zCl(PVP)]C1 fim is of interest. We believe that the unco-
ordinated protonated pyridine groups at the polymer backbone play an important role for the polymer to achieve this property. However, we can not exclude contributions from porosity.
Acknowledgment. This project was supported by the Swiss Nationalscience Foundation (Grant 2.91 5-0.85). We thank Drs. J. F. Equey and K. Mueller from our institute and Dr. C. P. Andrieux, CNRS, Paris, for helpful discussions, Dr. J. G. Vos, NIHE, Dublin, for providing us with the [ R U ( ~ ~ ~ ) ~ C ~ ( P V P ) ] C I polymer, and Maria Mohos for taking the electron microscope picture. Registry No. Fe, 7439-89-6; HCI, 7647-01-0; 1-hydroxyphenazine, polymerized, 105103-78-4; carbon, 7440-44-0.
Photochemlstry of Colloidal Semiconducting Iron Oxlde Polymorphs Jonathan K. Leland and Allen J. Bard* Department of Chemistry, University of Texas, Austin, Texas 78712 (Received: November 14, 1986; In Final Form: May 19, 1987)
Electrochemical charge collection experiments were carried out with different irradiated iron oxide polymorphs (a-FezO3, a-FeOOH, @-FeOOH,6-FeOOH, y-Fe203,and y-FeOOH). A model for direct electron transfer from a particle to an electrode is developed to describe the experimental current-potential behavior. The quasi-Fermi level of electrons for the different species, measured by mediated charge collection experiments, was different for each polymorph. The rate of oxalate and sulfite photooxidation, which was used to probe the relative efficiency of photogenerated charge production and transfer, varied by 2 orders of magnitude among the iron oxides. This efficiency did not correlate with particle size (hydrodynamic radius) or band gap.
Introduction We report photoelectrochemical studies of colloids and particles of different forms of iron oxide. Iron oxides exist in many crystal structures and stoichiometries.' Most of these iron oxides have semiconducting properties, but a comparison of them as photocatalysts has not been made. This polymorphism provides a unique opportunity to study the effects of crystal structure on the semiconducting properties. The most common iron oxide, a-FezO3, has been studied extensively.z a-FeZO3is an n-type semiconductor (1) (a) Bernal, J. D.; Dasgupta, D. R.; MacKay, A. L. Clay Miner. Bull. 1959, 4, 15. (b) Murray, J. W . In Marine Minerals; Burns, R. G., Ed.; Mineralogical Society of American: Washington, DC, 1979; Chapter 2, p 47. (c) Lindsley, D. H . Rec. Mineral. 1976, 3, 1. (d) Gallagher, K. J. Nature (London) 1970,226, 1225. (e) Chukhrov, F. V. In?. Geol. Reo. 1973.19, 873. (f) Okamoto, S. J . A m . Ceram. SOC.1968, 51, 594. (2) (a) Morin, F. J. Phys. Rev. 1954, 93, 1195. (b) Gardner, R. F. G.; Sweett, F.; Turner, D. W. J . Phys. Chem. Solids 1963, 24, 1175. (c) Lewis, D. C.; Westwood, W. D. Can. J . Phys. 1964, 42, 2367. (d) Bailey, P. C. J . Appl. Phys. 1960, 31, 39s. (e) Wickersheim, K. A.; Lefever, R. A. J. Chem. Phys. 1962, 36, 844. (f) Quinn, R. K.; Nasby, R. D.; Baughman, R. J. Mater. Res. Bull. 1976, I ! , 1011. (g) Kennedy, J. H.; Frese, K. W., Jr. J. Electrochem. SOC.1978, 125, 709. (h) Turner, J. E.; Hendewerk, M.; Parameter, J.; Neiman, D.; Somorjai, G. A. J . Electrochem. SOC.1984, 131, 1777. (i) Hardee, K. L.; Bard, A. J. J . Electrochem. SOC.1976, 123, 1024. G ) DareEdwards, M. P.; Goodenough, J. B.; Hamnett, A,; Trevellick, P. R. J . Chem. SOC.,Faraday Trans. 1 1983, 79, 2027. (k) Kennedy, J. H.; Frese, K. W., Jr. J. Electrochem. SOC.1978, 125, 723. (I) Hardee, K. L.; Bard, A. J. J . Electrochem. SOC.1977, 124, 215. (m) Shinar, R.; Kennedy, J. H . J . Electrochem. Sac. 1983, 130, 860. (n) Kennedy, J. H.; Shinar, R.; Ziegler, J. P. J . Electrochev. SOC.1980, 127, 2307. (0)Sammells, A. F.; Ang, P. G. P. J . Electrochem. SOC.1979, 126, 1831. (p) Yeh, L . 3 . R.; Hackerman, N. J . Electrochem. SOC.1977, 124, 833. (9) Wilhelm, S. M.; Yun, K. S.; Ballenger, L. W.; Hackerman, N. J . Electrochem. SOC.1979, 126, 419. (r) Curran, J. S.; Gissler, W. G. J. Electrochem. SOC.1979, 126, 56. (s) Iwanski, P.; Curran, J. S.; Gissler, W.; Memming, R. J. Electrochem. SOC.1981, 128, 2128. (t) Frese, K. W.; Kennedy, J. H. Extended Abstracts, 153rd Meeting of the Electrochemical Society, Seattle, WA; Electrochemical Society: Pennington, NJ, 1983.
0022-3654/87/2091-5076$01.50/0
with a narrow band gap (2.2 eV). The material has a somewhat positive flat-band potential, -0.58 V vs. N H E at pH 12.02' and very low electron and hole mobility, about IO-* cm2/(V.s).za ala-FezO3 can carry out the photooxidation of HzO to 02,2f-i though it suffers from instability in acidic solutions and generally poor efficiencies. The only reports on the photochemistry or photoelectrochemistry of iron oxides colloids have been on aFez03.3 The photoelectrochemical properties of the other iron oxides remain uncharacterized. There are many naturally occurring and synthetic iron oxides, but not all (e.g., the green rusts and dehydrated gels) have well-defined crystal structures. Transformation of one structure to another is possible by hydration-dehydration and/or partial oxidation-reduction.Ia Since the most stable form is a-Fe2O3, any elevated temperature technique used to prepare solid electrodes (like growth of single crystals from melts or sintering of pressed pellets) by starting with the other iron oxides results in transformation to a-Fe203. However, colloidal solutions and particulate dispersions of the different iron oxides can be prepared and were used in this study. Numerous experimental techniques have been used to study the photoelectrochemistry of colloids and compare their behavior to that of solid semiconductor electrodes of the same ~ n a t e r i a l . ~ ~ . ~ . ~ (3) (a) Moser, J.; Gratzel, M. Helu. Chim. Acta 1982, 65, 1436. (b) Gratzel, M.; Kiwi, J.; Morrison, C. L.; Davidson, R. S.; Tseung, A. C. C. J . Chem. SOC.,Faraday Trans. 1 1985,81, 1883. (c) Haupt, J.; Peretti, J.; Van Steenwikel, R. Nouo. J . Chim. 1984, 8, 633. (d) Herrmann, J. M.; Mozzanega, M. N.; Pichat, P. J . Photochem. 1983, 22, 333. (e) Dimitrijevif, N. M.; Savif, D.; MiEiE, 0. I.; Nozik, a. J. J . Phys. Chem. 1984, 88, 4278. (4) (a) White, J. R.; Bard, A. J. J. Phys. Chem. 1985,89, 1947. (b) Ward, M. D.; White, J. R.; Bard, A. J. J. Am. Chem. SOC.1983, 105, 27. (c) Bard, A. J.; Pruiksma, R.; White, J. R.; Dum, W.; Ward, M. D. Proc. Electrochem. SOC.1982, 82-3, 381. (d) Ward, M. D.; Bard, A. J. J . Phys. Chem. 1982, 86, 3599. (e) Dunn, W. W.; Aikawa, Y.; Bard, A. J. J . Am. Chem. SOC.1981, 103, 3456. (f) D u m , W. W.; Aikawa, Y.; Bard, A. J. J . Electrochem. SOC. 1981, 128, 222.
b 1987 American Chemical Societv
