Electrochemical study of the reduction of aromatic radical cations at a zinc oxide electrode

ELECTROCHEMICAL STUDY OF THE REDUCTION OF AROMATIC RADICAL CATIONS AT A ZINC OXIDE ELECTRODE E. W. GRABNER and C. lnstitut

ZINGEL

fur Physikalische und Theoretische Chemie der Universitat Frankfurt, Robert-Mayer-StraRe D-6UOO Frankfurt am Main 1, Federal Repubhc of Germany (Rewiced

21 April

11.

1982; in ret)isedforrn 11 Ocrober 1982)

Abstract-The reduction of radical cations of rubrene. tri-p-tolylamine, and 9, lO-diphenylanthracene, which exhibit different electronic excited state energies, was studied al a ZnO electrode by voltammetric and capacity measurements. The current-voltage curves obtained for the three systemsdo not differ significantly, indicating that the electrode kineticsare not affected substantially by the involvement of excited states during electron transfer. The kinetics are rathercontrolled by irreversibleelectron transfer ria surfacestates ofwhich the existence has been confirmed by the capacity experiments. The energy of the surface states was estimated to be approx. 0.6 eV below the conduction band, and the rate constant k,,, for the irreversible transitions has of radical been determined to be 10 ’ cm s I. From the change in the flat band potential on the generation cations, it is concluded that the presence of the latter causes a change in the polential drop across the Helmholtz layer.

INTRODUCTION It was shown for the first time by Marcus[l] that electronic excited states can be involved in heterogeneous electron transfer processes leading possibly to the emission of radiation. Some years later Gecischer[2] formulated this idea more specifically and demonstrated that heterogeneous electron transfer to or from excited states should occur only on semiconductor electrodes. For the system rubrene radical cations at an n-type ZnO electrode, proposed for heterogeneous electrogenerated chemiluminescence[3], the formation of excited states following heterogeneous electron transfer has actually been supported by the detection of emitted light[4]. This system is chosen in the present study to investigate whether the kinetics of electron transfer processes as reflected experimentally in current-voltage curves, is influenced by the participation of excited states. Thus, the reduction of radical cations of rubrene, which exhibits low lying excited states, is compared with the reduction of radical cations of tri-ptolylamine (TPTA), of which the excited states are located very high, but the ground state is however

identical to that of rubrene. In addition, the reduction of radical cations of 9, IO-diphenylanthracene (DPA) is included in this study, since the relevant energies of this system are located in between the extremes of rubrene and TPTA (cfTable I and Fig. I). No attempt was undertaken to detect the emitted light, since the comparative study of these systems concerns current-voltage curves irrespective of the emission of light following heterogeneous electron transfer (in the case of rubrene and, possibly, of DPA). The voltammetric experiments are complemented by capacity measurements in order to determine flat band potentials and lo prove the existence of surface states. The voltammetric measurements show that with those substances exhibiting low excited states (rubrene and DPA) current peaks do not occur at potentials where electron transfer to excited states can occur according to the energy correlation scheme. Current peaks are rather observed at potentials where the probability of electron transfer from the conduction band directly to the ground state, as well as to excited states, is very low. These current peaks, however, can be attributed to electron transfer via surface states as the combination of voltammetric and capacity measure-

1. Electrochemical and photochemical data for rubrene, TPTA and DPA for energy correlation with a ZnO electrode (cf Fig. 1).

Table

LJo,,(V rs SW)*

E,(eV)+

-qeW

Rubrene

0.81 [S]

1.2[6J

2.3[3]

Tri-p-tolylamine 9,10-Diphenylanthracene

O.X2[S] 1.22[5]

2.9[7] 2.0[8]

3.5[7] 3.2181

* Redox benzonitrile electrode. ! Energy * Energy

potential of the couple radical cation/neutral with 0.1 M tetrabutylammonium perchlorate of the triplet state. of the lowest excited singlet state. 651

molecule in at a platinum

E. W.

652

GRABNER AND C. ZINGEL

0

energy

‘TPTA* 1DpA*

-* __-__-___-___ ‘TPTA

decomposrilon potential Of

----_-

benzonltrlle

I

T

DPAT

rubrene

tri-ptolyl amane

9,10-diphenylanthracene

Fig. 1. Energy correlation between ZnO surface in flat band position (U, = 0.35V US see in presence of radical cations) and the oxidised and excited states of rubrene. TPTA and DPA (relevant data are given in Table 1).

ments shows. From these results, valuable informations about the kinetics of electron transfer via surface states are obtained. EXPERIMENTAL Single crystals of ZnO supplied by Prof. E. Mollwo (Universittit Erlangen-Niirnberg) were grown by using gas transport. The donor density determined by Hall measurements was 2 x lOI4 cm- ’ Ohmiccontact was made by a droplet of indium fused on the back side of the ZnO plates. In all experiments the (0001) surface etched by 85 y0 H, PO4 for about 1 ruin prior to measurements was exposed to the electrolyte. Electrical insulation against the electrolyte was achieved by a silicon adhesive (General Electric RTV 108). The preparation and purification of the samples, solvent (benzonitrile) and supporting electrolyte (tetrabutylammonium perchlorate TBPA) have been described elsewhereC3, 5,9]. The electrochemical cell was of conventional design containin a small surface area platinum electrode (0.035 cm P ) for voltammetric measurements and a larger surface area platinum electrode (4cm”) for electrolysis and capacity measurements besides the ZnO working electrode (0.04cm2). The reference electrode compartment was attached to the cell by a turnable joint so that the Luggin capillary could be located alternatively in front of the ZnO electrode, the platinum microelectrode or the platinum macroelectrode. Thus with one and the same solution filled into the cell the following procedures could be performed[S]: the electrolytic generation of radical cations, capacity measurements, cyclic voltammetry at the platinum microelectrode for concentration control as well as the reduction of radical cations at the ZnO electrode.

Electrochemical measurements were made with a Wenking potentiostat POS 73 and capacity measurements with a PAR lock-in amplifier model 126. For coulometric control a Wenking integrator SSI 70 was used. All manipulations were carried out ina glove box fiHed with dry nitrogen. Oxygen was removed from solutions by repeated freeze-pumpthaw cycles.

RESULTS Capacity measurements The differential capacity of the ZnO-electrode was measured as function of the electrode potential at frequencies of 0.2-10 kHz resulting in a parabolic relation between capacity C and the band bending A+ in the space charge layer according to the MottSchottky relationship, i/C2 = (ZjNneee,) (A& - kT/e),

(1)

where N D = donor density, e = charge of the electron, +, = vacuum permittivity and E = dielectric constant. Thus from a plot of l/C’ us the electrode potential the flat band potential U, could be determined which apparently depends on radical cations being present in the electrolyte or not (Table 2). Prior to capacity the stationary potential 77, was measurements, measured, which also depends on the presence of radical cations (cj_Table 2). The slope of the Mott-Schottky plot representing a measure of the donor density No according to (1) depends on the frequency of measurements becoming smaller with increasing frequency (Fig. 2). For comparison with literature data[ll, 121 capacity measurements were performed in aqueous KC1 solutions resulting in a flat band potential ( - 0.28 V us see) which agrees well with that found by Dewald[l 1)

Reduction

of aromatic

cations

653

Table 2. Flat band potential Ufiand stationary potential U, (in V 11ssce)of 1 mM solutions of rubrene, TPTA and DPA in benzonitrile (0.1 M TBAP) at a ZnOelectrode Degree of

Pure eleclrolyle Rubrene TPTA DPA

0.1 -0.05 -0.1 0.05

0.1 0 u 0

0.35 0.35

0.6 0.6 0.5

30 65 I8

* Radical cations are absent. * Radical cations are present. t When radical cations are generated.

of peak currents Ai, of non-electrolysed and electrolysed solutions and the radical cation concentration c: c = const . Ai,,

IO0

10' -

VlkHz

Fig. 2. Slopes of Mott-Sehottky plots as function frequency for a ZnO electrode in benzonitrile (0.1 M . pure electrolyte; o with rubrem; + with rubrene cations; x with TPTA; D with TPTA radical cations; DPA (concentration of all solutes ~a. 1 mM).

of the TBAP). radical I with

( - 0.3 V US see) and Lohmann[l Z] ( - 0.33 V US see) after correction for the different donor concentrations. In this case the slope of the Mott-Schottky plot increases somewhat with increasing frequency. Above 6 kHz the slope is constant and from this value a donor concentration of about 4 x 101* cmmJ is derived, in good agreement with the result of the Hall measurements. Vffltammerric measurements Cyclic voltammetric measurements at the platinum microelectrode was performed to determine the half wave potentials for the oxidation of the substances to radical cations (cfTable 1). In addition, the electrolytic generation of radical cations was controlled by cyclic voltammetry. Since the oxidised, as well as the reduced, form of the aromatics are present in the solution simultaneously the voltammograms are different than those predicted by theory[lO]. However, an empirical linear relationship was obtained between the difference

where const = 0.47 mM PA-’ at a scan rate of 0.1 Vs ’ (cf Fig. 3). This method of concentration control could only be applied to solutions of TPTA. The solutions of rubrene and DPA revealed decomposition of radical cations at prolonged electrolysis times (more than 50% electrolysis of 1 mM solutions). The decomposition products were detected by the occurrence of additional current peaks in the voltammograms at potentials different from the redox potentials of radical cations. Thus, with the solutions of rubrene and DPA the concentration of radical cations was determined solely by coulometry with an uncertainty of about 10% due to unavoidable background currents and radical cation decomposition. The cyclic voltammograms of the reduction of rubrene, TPTA and DPA at a ZnO electrode are shown in Figs 4 and 5. In all three cases an irreversible current peak occurs at ~0. 0.2 V US see despite the fact that the redox potential of the formation of DPA radical cations isabout 0.4 V more positive than that of the formation of rubrene and TPTA radical cations (d Table 1). In Fig. 4 the voltammogram of the reduction of neutral rubrene is added showing a current peak at - 1.6 V which must be ascribed to the reduction of rubrene to radical anions. This peak is also observed when radical cations are present (Fig. 4, curve 2). At prolonged electrolysis of rubrene and at moderate electrolysis of DPA, additional current peaks at CCI. - 1.2 V are obtained, which correspond to additional current peaks observed at the platinum electrode and which must be attributed to radical cation decomposition products (Fig. 5). The current peak at 0.2 V occurring whenever radical cations are present, was studied in detail. Thus, voltammograms were recorded as functions of the scan rate and the concentration as shown for TPTA in Figs (u = scan rate) are in6 and 7. The i,/v 1’2c-values dependent of the scan rate u except at small u where natural convection causes an increase in the current. do not give a single The fact that the i ulJ2c-values straight line paralla f/to the abscissa results from errors in the determination of the true radical cation concentration The linear dependence of the peak current on

654

E. W.

GRABNER

AND

C. ZINGEL

Fig. 3. Cyclic voltammograms of 0.8 mM TPTA m benzonittile (0.1 M TBAP) at a platinum elatrode (0.035rm2) for dilkent concentratmns of electrolytically generated radical cations: OmM (curve l), 0.17mM (curve 2), 0.3SmM (curve 3), 0.59mM (curve 41, 0.79mM (curve 5). V&age scan 0.1 Vs-I.

itp

I

Fig. 4. Cyclic voltammograths of 1 mM solutions of rubrene anti TPTA,respectively’, in benzonitrilc (0.1 M TBAP) at a ZnO electrode with OmM rubrene radial calions (wrve I), ea. O.SmM rubrene radical cations (ra. 50% ekctrolysis, curve 2), 0.47 mM TPTA radical cations (47 2 electrolysis, curve 3). Voltage scan 0.1 vs-1.

I

0

- 0,5

I

- I.5

-2

Fig. 5. Cyclic voltammograms of 1 mM solutions of rubrene and DPA in benzomlnle (0.1 M TBAP) at a ZnO electrodewith ~a. 0.98 mM rubrcne radical cations 198 ‘x electrofyss, curve I) and ca. 0.2mM DPA radlcai cations (20% electrolysrs, curve 2). Voltage scan 0.1 V s-l.

Reduction

1.5

~As”~

of aromatic

x

-

655

cations

0.47mM

0.24.mM p-

mV”*

mM

O.l*mM

0,026~~ 0.7

I

0 o-

mM

I

50

Fig. 6. Dependence

100

200

’ on the scan rate v for the reduction of TPTA electrode (current peak at 0.2 V USsee).

of i,v-. l”c

the concentration up to about 50 7; electrolysis (Fig. 7) indicates that the electron transfer at 0.2 V is diffusion controlled. According to theory[ IO], the peak potential U, Of an irreversible electron transfer should be proportional to the logarithm of the scan rate. This is shown for TPTA in Fig. 8.

radical

cations

at a 2110

space charge layer and the band bending A@ = U - U,[ll, 133: C, = (eZN,&E,/2kn”2 exp (eA&/kT) - 1 X [exp (eAd/kT) - eAd/kT - 11~” ’

With the data related to ZnO (E = 8.5 and No = 4 x lOI cmT3) C, was calculated as function of U -7J,. The resulting curve is compared with values obtained from the capacity measurements in Fig. 9. The measured capacity C is greater than C, by about one order of magnitude indicating that an additional capacity C,, exists,

DISCUSSION Surjhce

states

For a semiconductor with fully ionised donor impurities the solution of the Poisson equation combined with the Boltzmann distribution law leads to the following relationship between capacity C, of the

c z c,+c,.

250

./ 1 /

ip/pA

mv 5 -1

/

I

0

10

1OOmV

x ‘/

5

/ /

x_/4’:

s-l

50mv s-1 25mv*-’

7

[/

1OmV

l-’

2,5mvs-’

,

,

,

.

0.3

0,I

0,5

0.6

*&c:~8”

0.1

0,2

ClrnM

Fig. 7. Dependence

of the peak current

reduction of TPTA radical

(2)

i, yt 0.2V us see on the radical cation concentration c for the catIons at a ZnO electrode at different scan rates.

(3)

656

E. W. GRABNER AND C. ZINGEL

of Zn atoms on the surface is lOI5 cme2, the surface states cover only 0.01 “/b of the ZnO surface. There is an apparent paradox phenomenon which has to be explained: the decrease of the slope of Mott-Schottky plots (cf Fig. 2) with increasing frequency, involving the enhancement of capacity with increasing frequency. Following the argumentation by Gobrecht and Meinhardt[lS], this behaviour can be understood simply by assuming the capacity C,, of the surface states to be negative. Thus with increasing frequency w the second frequency dependent term of the following capacity relationship[lB] will become 2

4

smaller,

6 In(vlmV

5-9

c = c,+c,,/(l

+R&C;p2),

(51

where R, = ohmic resistance of surface states, ie the Fig. 8. Dependence of the peak potential of the reduction of overall capacity C increases with increasing frequency TPTA radical cations at a ZnO electrode on the logarithm of for a negative C,,. The concept of a negative capacity the scan rate for different TPTA radical cation concenaccounts for the fact that if acceptor surface states trations: x 0.2mM; o 0.4mM; l ClS5mM; + 0.7 mM. exist, the change of voltage in a negative direction (from the blocking to the passing region of an n-type This capacity C,, must beascribed to surface states[ 1 l] semiconductor) causes an accumulation of negative and can be related to the density of surface states r a more SS charge in the surface states and consequently WC141 po~iritdy charged space charge layer, ie C = dQ/dU -z 0 (Q = charge in the space charge layer). 9 - l exp (E, - E, - eA#)/kT C,= (e’/W)r From the concept of a negative capacity for the ~[l+g~‘e~p(E,,-_~-eA~)/kT]~ surface states, it must hence be concluded that the (4) surface states occurring on the ZnO electrode are where g = degeneration factor,E,, = energy of surface acceptor states. Their energy E,can be deduced from states and E, = Fermi energy. C, has a maximum the fact that the stationary potential U,is shifted from value of TSSe2/4kT when the band bending is so large cu. OV us see of the electrolyte without any redox that the Fermi energy E, just equals the energy of the couple to ca. 0.6 V vs see when radical cations are surface states E,. Thus an upper value for TSS of ca. present (cJ‘ Table 2). Since this shift seems to be 10” cm- ‘can beestimated from theexperimental and independent of the redox potential of the different calculated capacity [cf (3)] (C = 60 nF cm-’ and C,, radical cations used, electron transfer for attainment of = 10 nF cm-’ at U,- U, = 0.25 V). Since the density the steady state will take place from occupied surface

ClpFcm-2 10’

I

lDO

10-l

10-i

Fig. 9. Capacity ofa ZnO eIcctrode in contact with solutions of radical cations of TPTA (0.8 mM,e ) and of rubrene (ca. 0.3 mM, o ) in benzonitrile (0.1 M TBPA) at a frequency of 0.23 kHz. Drawn curve: calculated according to (2) with E = 8.5 and N, = 4 x 10’4cm”.

657

Reduction of aromatic cations states to radical cations, thus depleting the space charge layer until the Fermi level is lowered to the surface state energy E,,. At the energy E, the Fermi level is pinned, and this Fermi energy pinning[ 161 manifests itself by a steady state potential being identical with the energy of the surface states. Hence the energy of the surface states can be positioned at about 0.6 V below the conduction band. A further point which has to be considered is the shift of the flat band potential from about 0 V vs see to 0.35V us see upon generation of radical cations (cf Table 2). This shift indicates that the potential drop across the Helmholtz layer increases by about 0.35 V when radical cations are in contact with the ZnO surface, and parallels the observation of the shift of the stationary potential being explained by Fermi energy pinning. The latter involves a relatively strong interaction between surface states and radical cations, probably causing a change in the dipole moment of the ZnO surface and hence a change in the potential drop across the Helmholtz layer. This dependence of the flat band potential on the presence of reactive species in the electrolyte, should be accounted for whenever charge transfer between semiconductor electrodes and reactive species is studied (cfeg[4]).

Kinetics

ofelectron

transfer

via s&ace

states

From the energy correlation scheme (Fig. I), which is related to the situation when radical cations are present (U, = 0.35V us see), it can be seen that electron transfer from the conduction band to unoccupied ground states at potentials near the flat band potential should be very unlikely, although it cannot be excluded entirely[19]. Thus the electron transfer actually observed at these potentials (cfcurrent peaks at ca. 0.2 V in Figs 4 and 5) is probably due to the surface states evidenced by the capacity measurements. From the standpoint of surface physics the irreversible electron transfer via surface states can be described by the following reactions (cf Fig. 10):

EC ,+ 02aev

electrolyte

Fig. 10. Energy correlation scheme of a ZnO surface in contact with a redox electrolyte (radical cations R’ of rubrene or TPTA) in flat band situation (Ur = 0.35 V us see) depicting electron transfer via surface states (U, = stationary potential, U& = redox potential of radical cation generation, k&, k:s, k,, = rate constants).

i, = 0.227 nFAc ki,,exp [ - cm,F (Up-

U’)/RT], (10)

where U,, U,,, = potential at the current peak isand at the half current peak, respectively, n,+= number of electrons transferred in the rate-determming step, ALJp change in peak potential upon tenfold change m kr v%tage scan rate, A = electrode surface area, c e,+ S zs-, (6) = radical cation concentration and U” = standard k: potential. With na = 1 and U” = U, = 0.6V the transfer coefS-+R. +k’edS+R, (7) ficient c(and the rateconstant k;,,have been determined (Table 3). band, S, Swhere e;s - electron in the conduction In view of errors involved in the voltammetric = unoccupied and occupied surface states respectmeasurements and especially in the determination of ively, R’, R = radical cation and neutral molecule the radical cation concentration, only an approximate respectively, kh, k&, k, = rate constants. value of 0.5 kO.1 can be given for a. Likewise only Reaction (7) represents formally an irreversible coarse values for k,are obtained (ca. 5 x lo-’ cm s-r electro chemical reaction, for TPTA and ca. lo- ’ cm s- ’ for rubrene) which are by an order of magnitude larger than the rate constant kox + e- “‘red, (7a) of reduction of radical cations at n-type GaAs electrodes[ 171. the voltammetric behavior of which can be treated From the electrochemical rate constant kin the rate according to the theory of Nicholson and Shain[ 10-J. constant k, (cfEq. 7) is obtained by division through Thus from voltammograms the transfer coefficient 01, the density of occupied surface states r, which as well as the irreversible rate constant k,, can be depends on the electrode potential. At the stationary derived by application of the following relationships: potential, however, where riz= l/2 r B(cfbelow), an cm a s -’ for krdcan be approximate value of ca. lous- LTp,2= - 1.857 RT/orn,F, (8) given by using the upper value of rIls = 10” cm-’ AU, = (30/mV)an,, obtained from the capacity experiments (cfabove). (9)

E. W.

658

GRABNER

AND

C. ZINGEL

Table 3. Transfer coefficient o and irreversible rate constant k for the reduction ol‘ TPTA and rubrene radical cations on a ZnO electrode% ca 0.2 v IS see -.-

Concentration @M) TPTA

Rubrene

Transfer coemcient o! a b C

k&m

--

s ‘)

0.3 0.7 0.85

0.55 0.43 0.43

0.58 0.58 0.6

0.37 0.36 0.45

5.7 x lo-’ 6.6 x IO-’ 1.4 x 10-Z

ca. 0.2

0.43 0.51 0.48

0.43 0.54 0.55

0.26 0.31 0.31

3.3 x 10- ’ 9 x10-t 7 XlO~Z

ca.0.65 E(I. 0.97

(a) According to (8). (b) According to (9). (c) According to (10).

The rate constants kzsand k,b,can be derived from the rate equation governing electron transfer between the conduction band and the surface states [cf (6)], dl-.Jdt

= k;sn,(T,s-

r,)

-k,b$Cr;-krJs;N;+,

(111

where rgs and I-, = overall density of surface states and density of occupied surface states respectively, nr = electron concentration at the ZnO surface and Nk j = radical cation concentration at the interphase. The occupation of the surface states obeys the Fermi distribution law[14]. Tss=~,,[l+y-‘exp(E,-eA~-EE,)/kT]-’.

(12)

For the special case of steady state at the stationary potential U,, the following simplifications hold: dTJdl = 0, the last term of (II) equals zero since no current Bows, and r, = l-,%/2due to the fact that at the stationary potential the Fermi energy is pinned to the surface state energy, ie ES,-kTln g = E, + eA4. Thus

(1 I) simplifies to k$,=

kL.

(13)

nS is obtained from the band bending at the statidnary potential [A+ = e(U -U,) = 0.25eV] and from the

donor density in the ZnO bulk (Nn = 2 x 1014cm-3), and amounts to 10’*cm-3. Since the timeconstant r,, of the surface states is given by T$*= l/(&n,+

kts),

(14)

the rate constants k&and k&can be estimated from the frequency dependence of capacity combined with (13). Capacity measurements were performed up to frequencies of 10 kHz where the slopes of the Mott-Schottky plots of solutions with different composition reveal approximately the same value (cf Fig. 2). It can therefore be assumed that at a frequency not far from 10 kHz the surface states cannot follow the rapid change of the applied alternating field and consequently do not contribute any more to the overall capacity. Hence an upper limit of the time constant can be given, ie

From this and (13) the following values are obtained: kfSI -~5x10~‘cm’s-‘andk~~~Sx103s~‘.

Electron

tranhfer to excited states

As the energy correlation scheme (Fig. 1) shows, electron transfer from the conduction band to excited states (the lowest triplet state of rubrene and DPA, respectively) cannot occur at the flat band potential and at more positive potentials. When the potential is shifted in negative direction, however, the ZnO surface becomes degenerated which is accompanied with a change in the potential drop across the Helmhottz layer. Since U, = 0.35 V and EC- E, = 0.28eV, degeneration sets in at about 0.05 V us see. Thus the energy of the conduction band electrons is raised successively with increasing negative potential until it equals the energy of excited statesC3,4]_ At these potentials (cu. - 0.4 V USsee for rubrene triplets) electron transfer to excited states takes place, as has been demonstrated for rubrene by detection of emitted light[4]. The question whether this electron transfer can be “seen” eleo ttochemically by current-voltage curves must be denied on the basis of the experimental results (@Fig. 4). The expIanation is straightforward. At about - 0.4 V us see the rubrene radical cations are reduced to triplets[4]. At more positive potentials, however, reduction of radical cations occurs too, leading presumably to ground state molecules and being mediated al least partly by surface states. Both, the electron transfer to tripIets, as well as electron transfer to the ground state directly and/or via surface states, are oneelectron steps. Since the current at potentials more positive than - 0.4 V is diffusion controlled (cfFig. 7), any enhancement of the current at - 0.4 V, which is associated with the transfer to triplets, cannot be expected. Only at more negative potentiaIs, where the energy states of rubrene radical anions and excited singlets are accessible (u, - 1.6V vs see, cfFigs 1 and 4), two-electron reduction of radical cations and additional reduction of neutral rubrene molecules to radical anions take place resulting in an additional current peak. Thus the current due to electron transfer to excited states is masked by the diffusion controlled current at more positive potentials, and the former should be “observable” only if semiconductor surfaces are obtainable without any surface states and if a proper energy correlation exists between semiconductor electrodes and excited states of a redox electrolyte. Acknowledgement-The authors grateFullyacknowledge the helpful cooperation of Prof. E. Brauer and financial support

Reduction from the Deutsche Forschungsgemeinschaft der Chemischen Industrie.

of aromatic

and the Fonds

9. 10.

REFERENCES 1. R. A. Marcus. .I. them. Phvs. 43. 2654 (19651. of 2. H. Gerischer, Plenary lect& helh at the‘22ndmeeting ISE, Dubrovnik (Yugoslavia) 1971. Ber. Bunsenges. phys. Chem. 77. 771 11973j. Acra 20, 7 (1975). 3. E. W. Grabner,.Elec;rochim. 4. I. D. Luttmer and A. J. Bard, J. eiecrrochem. Sot. 125, 1423 (1978). (1979). 5. C. Zingel, Thesis, Frankfurt 6. H. D. Brauer and H. Wagner, Ber. Bunsenges. phys. Chem. 79, 597 (1975); W. G. Herkstroeter and P. B. Merkel, J. Phorochcm. 16. 331 il9Rli. Thesis,‘Amsterdam (1972). 7. K. A. Zachariaise, 8. I. B. Birks, Photophysics of AromaticsMolecules. Wiley, New York (1970). _

11. 12. 13. 14.

15. 16. 17. 18.

19.

cations E. W.

659

Grabner and E. Brauer, Ber. Bunsenges. phys. Chem. 76, 106 (1972). R. S. Nicholson and 1. Shain. Anolyr. Chem. 36, 706 (1964). J. F. Dewald, Bell System Techn. J. 39, 615 (1960). F. Lohmann, Ser. Bunsenges. phys. Chem. 70,428 (1966). M. Green,in Modern Aspects of Eiecrrochemistry (Ediled by I. O’M. Bockris) Vol. II, p. 343. London (1959). H. Gerlscher, in Phys. Chem. (Edited by H. Eyring, D. Henderson and W. Jost) Vol. 1X A, p. 463. New York (1970). H. Gobrecht and 0. Meinhardt. Ber. Eunsenges. phys. Chem. 67, 151 (1963). S. R. Morrison, The Chemical PhyGr,.s qf Surfaces, p. 3 1. Plenum Press, New York (1977). R. Landsbere. P. Janietr and R. Dehmlow, Z. Chem. 3, 106 (1975). S. R. Morrison. Electrochemistry at Semiconductor and Oxidized Metal Electrodes, p, 125. Plenum Press, New York (1980). ibid. p. 106.