J . Phys. Chem. 1993, 97, 3333-3337 3333

Identification of the Mechanism in the Photoelectrochemical Reduction of Oxygen on the Surface of a Molecular Semiconductor

D. Schlettwein' and N. I. Jaeger Institut f i r Angewandte und Physikalische Chemie, FB2, Universitat Bremen, W-2800 Bremen 33, FRG

Received: May 4, I992

The rate-limiting step in the photoelectrochemical reduction of oxygen on a molecular semiconductor is found to be the charge transfer a t the electrode surface. The electrode consisted of a thin film of phthalocyaninatozinc- (11) in a mixture of poly(viny1idene fluoride) on indium-tin oxide. The current transients as the response upon illumination are analyzed under variation of the light intensity and of the oxygen partial pressure. A model is presented that explains the photoelectrochemical behavior on the basis of oxygen adsorption a t the electrode surface according to Langmuir's adsorption isotherm prior to the charge-transfer step. The importance of surface states created by this mechanism for the photoelectrochemical properties of a molecular semiconductor is thereby elucidated.

Introduction The properties of molecular semiconductors such as the

phthalocyanines can be modified in contact with donor or acceptor molecules either in the bulk or at the surface. Their electrical conductivities and photoconductivities as well as surface pho- tovoltages are strongly influenced by different ambient conditions, e.g., if a gas such as ammonia or oxygen has access to the surface. These effects have already stimulated interest in phthalocyanine thin films for chemical sensor purposes.I-l6 Changes in the electrical conductivity and photoconductivity induced by mo- lecular oxygen from the gas phase could be adequately described by assuming simpleadsorption kinetics of 02 at the phthalocyanine thin films followed by an electron transfer to the oxygen m~lecule .~ Direct experimental evidence for adsorbed oxygen species at the surface was obtained by temperature-programmed desorption, 7 ~ 1 8 X-ray photoemission ~pectroscopy,'~ and quartz crystal micro- gravimetry3 studies.

Chemical interaction on the mofecular scale has also to be considered if thin films of these materials are studied in view of their energy conversion ability in either all-solid or photoelec- trochemical cells. Photoelectrochemical activities of phthalo- cyanines or, in general, molecular semiconductors in several reactions have been reported, and the influences of the central metal, the ligand system, polymer matrices, and the redox couple in the electrolyte have been Unsubstituted phth- alocyanines behave like p semiconductors and cathodic photo- currents were observed. From the relative position of the highest possible energy level (the most cathodic equilibrium potential) of redox pairs that still lead to a significant photocurrent, it was suggested that the charge is transferred through s u r f a c e ~ t a t e s . ~ ~ - ~ ~ A capacity in the bandgap region which was calculated from potential-dependent impedance measurements was also ratio- nalized by surface states.32

In a recent paper about photoelectrochemical experiments at porphyrin and phthalocyanine thin-film electrodes,22 it had been reported that electrodes prepared from phthalocyaninatozinc- (11) (PcZn) embedded within a matrix of poly(viny1idene fluoride) (PVDF) on indium-tin oxide (ITO) turned out to be most photoactive in the reduction of molecular oxygen dissolved in an aqueous electrolyte. In this paper we focus on the importance of surface processes for the overall photoelectrochemical reaction. It will be shown that large photocurrents can be explained by charge transfer via surface states. Additional experiments analyzing the photocurrent-time behavior under various oxygen partial pressures and light intensities will be presented, and the experimental data will be analyzed in detail. A simple model

0022-3654 193 /2091-3333304.00/0

based on the adsorption of oxygen at the electrode surface will be derived which describes the dependence of the currents under illumination on the oxygen partial pressure, which explains the current transients in response to the onset of illumination and which simulates the time dependence of the transients under differing light intensities reasonably well. The influence of adsorbed molecules on the photophysical properties of phthalo- cyanines or molecular semiconductor thin films in general via the formation of surface states is confirmed by the results of these photoelectrochemical experiments.

Experimental Section

Thin-film electrodes of approximately 250-nm thickness on IT0 were prepared from a solution of 1 g L-I of PcZn and 1 g L-1 of PVDF in N,N-dimethylacetamide and characterized by visible absorption spectroscopy and surface texture measurements. The photoelectrochemical experiments were performed poten- tiostatically at -60 mV vs NHE in a gas-tight glass cell using 0.5 M aqueous KN03 as the electrolyte. The concentration of oxygen dissolved in the electrolyte was adjusted by its partial pressure in a mixture with argon assuming the validity of Henry's law. The electrolyte was saturated with the appropriate gas mixture prior to each measurement. Rotating disk experiments were performed under air. The cell was illuminated by the white light of a xenon arc lamp at an intensity of 400 mW cm-2. Details of the experiments were reported elsewhere.20-22.40

The Model

The initial current upon theonset of illumination, the transient, and the steady-state current are evaluated under the assumption that only adsorbed oxygen of a molar surface concentration r(t) is irreversibly reduced to a fast desorbing product. The current i is assumed to be proportional to the surface concentration [e*Io of electrons in excited states, either in the conduction band (LUMO) or in states of the bandgap region. If the dark current is neglected, the current density can be written as follows:

i /nF = k,r(t)[e*], (1) where kr is the rate constant for the electrochemical reaction.

In the most simple case [e*]o is taken to be. independent of time, current, and coverage by oxygen and to be a function of only the applied potential E and the incident light intensity a. The adsorption of oxygen is assumed to follow Langmuirs law

0 1993 American Chemical Society

3334 The Journol of Physical Chemistry, Vol. 97, No. 13, 1993 Schlettwein and Jaeger

combined with its irreversible photoelectrochemical reduction:

d r / d t = k(rma, - r ) p o - k ’ r - k,[e*],I’ (2)

where k is the rate constant of adsorption, k’is the rate constant of desorption, po is the oxygen partial pressure (mol L-I) at the electrode surface, and rmax is the maximum surfaceconcentration of 0 2 arising from the occupation of all available adsorption sites.

The change of [e*], is assumed to occur fast compared to a change in r. Therefore, the initial current ii,, the maximum of the transient, arises from the equilibrium value of r in the dark not influenced by any current. The concentration of oxygen in the electrolyte at the electrode surface is equal to the value in the bulk of the electrolyte.

From d r / d t = 0 and [e*Io = 0 it follows from (2) that

and inserting i from (1) under constant E and Q and the assumption that this is leading to a constant [e*Io:

k’iin = kpi,,, - kpi,,

This is reorganized to

;=e+& (1) l i n imax ‘ma,

where i,,, is the hypothetical current arising from rmax. The current in the steady state istcad results from the steady-

state value of r which has to be calculated from (1) and (2) taking into account the last term of (2) and a diffusion dependence ofp,. If Fick’s first law of diffusion is used, the following equation is obtained for islead:

4,g+ k’/k + k,[e*],/k + ”( 1 --) (11) k c a d Imax imax nFD

where d is the Nernst diffusion layer thickness and D is the diffusion coefficient of 0 2 .

To describe the current-time behavior following the start of the illumination, a time-dependent solution of (2) has to be found. Ifpo is assumed to equal p independent of time, which means that diffusion does not limit the current a t any time, and using the boundary conditions arising from (I) and (11)

fort--,= = rmaxp + (k’+ k,[e*],)/k

rmaxPk + r = kp + k’+ kr[e*],

exp [-( k p + k ’ + k , [ e*] o) t ] rmax~kf[e*l o (p + k’/k)(kp + k’+ k,[e*],)

is obtained as a solution. Using (I) and (11) this equation can be transformed to

kf[e*lo’k - k(p + k’/k + k,[e*],/k)t (111) p + k’/k In - = In - jstead

‘stead

to simulate the influence of a change in [e*],, or to

which is an explicit expression for i as a function of time.

-l io

I L 1 Figure 1. Current transient measured in the dark and under illumination [L] at a rotational speed of 2400 rpm in an electrolyte saturated by air.

Experimental Results

The current under illumination depends in many ways on the applied potential since not only the electrochemical rate constant kr is changed according to the Butler-Volmer equation4’ but also the number of excited electrons at the surface [e*],, is influenced through the complex potential dependence of the charge carrier generation, transportation, and recombination processes in the bulk of the film and at its surface.8 All experiments reported in this paper have therefore been performed at a sufficiently cathodic constant potential of -60 mV vs NHE.

Upon illumination of a resting electrode of PcZn in PVDF in contact with an aqueous electrolyte containing dissolved oxygen the current rises from a negligible dark current to a maximum initial value ii, and then relaxes to a steady-state value during further illumination. The photocurrent rises with the incident light intensity at an expected power law i - as reported earlier.22 The shape of the transient already indicates that the current under illumination is not limited by charge carrier generation and transport witbin the film alone.” The nonlinear dependence of the oxygen partial pressure and the time scale of the decay from the initial current ii, to the steady-state value islead

further are not compatible with relaxation processes in the solid. To find out whether or not diffusion of oxygen from the electrolyte to the surface of the electrode has a marked influence on the transient, rotating disk experiments have been performed. The shape of the transient is preserved in principle under different rotational speeds up to 3700 rpm (Figure 1). Mechanical instability of the electrode, however, leads to a permanent decline of its absolute height. The stationary current rises with the rotational speed but far less than expected according to the square- root law of diffusion-limited currents.

The dependence of the observed currents on the oxygen partial pressure (Figure 2) further excludes diffusion limitation for the initial as well as for the steady-state currents as the values do not rise linearly with the oxygen partial pressure. The dependence of the currents under illumination upon oxygen partial pressure and time is much better reproduced by the model derived in the previous chapter assuming the reduction of adsorbed oxygen. Figure 3 shows a plot of the ratio of the oxygen partial pressure p over the initial currents i,, againstp. The evaluation according to (I) yields i,,, = (1 32 f 9) MA cm-2 from the slope of the least-squares fit and, together with its intercept a value for k’/k = (2.4 f 0.8) X lo4 mol L-I. In Figure 4 the ratios of p over the steady-state currents are plotted against p . A fairly good straight line is obtained. From (11) it is seen that this isonly possible if either islead << i,,,, istcad imax or d/nFD << p/iStcad. As iatead varies between 20 and 100 FA, both of the first cases fail. I t is therefore concluded that the diffusion layer thickness is

Surface of a Molecular Semiconductor The Journal of Physical Chemistry, Vol. 97, No. 13, 1993 3335

i / pA cm'?

1 135 1

105 1 i

75 4

i 45 v-

I ' I ,f'

IO3 p / mol I-]

Figure 2. Currents in the dark [+] and under illumination: initially [ *] and in the steady state [ # I . Functions according to Langmuir's adsorption isotherm [- - -1.

0

0 3 0 / + .

. 0

0 0

0

0 0

10' p / mol I-'

Figure 3. Initial currents under illumination [+I evaluated according to ( I ) including the least-squares linear fit [- - -1.

0 *

,li- .le4 , , .ea , . ,750 , , 1.0s , , t.s , ,

10' p / mol I-'

Figure 4. Steady-state currents under illumination [ +] evaluated according to (11) including the least-squares linear fit [- - -1.

comparably small, Le., that diffusion of oxygen is not rate limiting and hence, that po = p . From the slope a value of i,,, = (1 37 f 9) pA cm-2 is calculated and from the intercept kr[e'lo/k = (4.0 f 1.5) X lo4 mol L-I.

As the resultsobtained according to (I) and (11) clearly indicate that diffusion does not limit the current, eqs 111 and IV which have been derived for this cpse are suitable for the analysis of the time dependence of the photocurrent. In Figure 5 the In ( ( i - irtcad)/&tcad) is plotted against time for transients recorded at constant oxygen partial pressure under different light intensities.

i - i,M In -

'dad

1

a : b :

d : e : f :

c :

40 mW cm" 80 mW cm-' 120 mW cm-' 200 mW cm-' 320 mW cm-2 400 mW cm-2

7 /,,I -!

20 0 60 0 100 t aa 1W

t / s

Figure 5. Current transients under different light intensities evaluated according to (111).

i / pA cm-'

a: 1.3 x)" mol I-' b: 9.1 lo4 mol I-' c: 3.9 io-' mol I-' d: 1.3 IO-' mol I-'

70.

.- ---------- 20.0 60.0 100. 140. 110.

"'1 L , ,

t / s Figure 6. Current transients under different oxygen partial pressures [-] compared to functions according to (IV) [- - -1.

Aside from some experimental noise and strong deviations as i approaches &dr straight lines are obtained as required by (111).

In Figure 6 exponential functions according to (IV) are compared to experimental transients recorded at constant light intensity but under different oxygen partial pressures. Different values of k, varying from 33 to 380 L mol-' s-1 have to be used in order to obtain reasonable agreement between the experimental and the simulated curves.

Discussion The assumption of oxygen adsorption prior to the rate-limiting

charge transfer at the electrode surface appears reasonable already in view of the dependence of the currents under illumination in the steady state on the oxygen partial p r e s ~ u r e . ~ * . ~ ~ It has been reported that the overall reaction rate of oxygen photoreduction catalyzed by a dispersion of phthalocyanine particles in an aqueous solution can be explained by rate-limiting adsorption of oxygen.43 New results in the theory of semiconductor electrochemistry further indicate that thecharge transfer through a relatively small number of surface states might dominate that through the well- populatedvalenceor conduction bandseven in thecaseof inorganic semiconductors if a strong electronic coupling exists between the surface and the molecule or ion in solution.44 Adsorption phenomena are of general importance in electrochemistry4s48 and in heterogenous ~a ta lys i s .~3 .~~-5~ Rate equations have been derived from various models to explain the dependence of the current on potential, time and concentrations in the electro- l~te.~5.~6.51 In the case of the photoelectrochemical reduction of oxygen on a molecular semiconductor, however, the simple model presented in this paper proves to be adequate for the simulation of the experimental results.

Langmuir's adsorption isotherm is assumed to be valid in the case of oxygen adsorption at the surface of PcZn. Although

3336 The Journal of Physical Chemistry, Vol. 97, No, 13, 1993 Schlettwein and Jaeger

there are examples reported where the empirical Freundlich adsorption isotherm appeared to be appropriate,4111 there are also reports in which the results of conductivity changes of phtha- locyanine thin films in dependence on the oxygen partial pressure in the gas phase could be well explained by assuming Langmuir's law.5.6 The latter turns out to be appropriate also in the studied photoelectrochemical reaction even if r N rmax. This is reasonable as the central metal can be looked at as the atom to which oxygen binds as indicated by the stronger bond to phthalocyanines of those central metals that are known to have a strong affinity toward additional axial ligand^.^,^^-^^ The metal atom at the surfaceof a phthalocyaninecrystal is located in an isolated position with respect to the other surface metal atoms as it is surrounded by the comparatively large organic The assumption of an adsorption step under Langmuir's conditions prior to the rate-limiting charge-transfer step turns out to describe the observed transients reasonably well and especially accounts for the observed initial current in contrast to a model based on the time dependence of charge carrier generation and transport according to which the current should continually rise upon the onset of illumination until its steady-state value is reached.37

From the ratio k'/k = (2.4 f 0.8) X IO4 mol L-I as a result of (I) (Figure 3), it is indicated that oxygen adsorbs much faster to the surface than it desorbs again. Nevertheless, in none of the experiments the total number of available adsorption sites of the surface was occupied by oxygen (I' < I",,) as its concentration in the electrolyte was quite low. From (11) (Figure 4) oxygen diffusion in the electrolyte can be ruled out to be rate-limiting. Therefore, the oxygen partial pressure at the surface po equals its valuep in the electrolyte bulk. If the presented model should be valid to describe the dependence of the currents on p , the slopes of Figures 3 and 4 should be equal as both should give the maximum current under illumination i,,, corresponding to the maximum surfaceconcentration of oxygen rmax. From the results it can be seen that this is the case and that further the intercept of Figure 4 is larger than that of Figure 3 as also expected from the additional term kr[e*]o/kimax in (11) compared to (I). The same term would demand the ratio istead/iin to be smaller than 1 and rise withp. By increasingp the following ratios arecalculated from the experimental values: 0.33; 0.48; 0.53; 0.69; 0.79; 0.80; 0.75;0.74; 0.77;0.81. The predicted trend isobservedqualitatively at least in the experiments at lower oxygen partial pressures.

From the ratio kr[e*]o/k = (4.0 f 1.5) X IO4 mol L-I, it is seen that through the photoelectrochemical reaction a t theapplied potential and light intensity a similar amount of oxygen is consumed from the surface as through the normal desorption process. From thiscomparison it becomes clear that thecoverage of oxygen is lowered by the photoelectrochemical reaction which in turn lowers the photocurrent until the steady state is reached. An explicit expression for this process is given by (IV) and Figure 6 indicates that the equation in principle resembles the exper- imental curve; however, a lowered oxygen partial pressure does not lead to a slower decrease of the initial current to the steady state value as predicted by (IV). Strongly differing fitting parameters are obtained for the transients although not only the initial but also the steady-state values are presented reasonably well. The reason for this failure of the model has to be looked for in the very restrictive assumptions made in the derivation of the simple model to describe the complex situation. Relaxation phenomena in the organic semiconductor influencing both generation and transportation of charge carriers have been neglected. As these processes have to be expected to occur on a time scale comparable to the decay of excited states in the solid and as their decay even in solution is known to occur much faster58 when compared to the time scale of the measurements discussed here, their neglection seems to be justified and the model still leads to a good description of most observations. However, the observed tendency of faster decay rates with lower oxygen partial

CB / LUMO

e' -

VB / HOMO

aqueous KNO,

electrolyte

I T 0

bock electro, photoelectrode PcZn I Figure 7. Energy level diagram including a scheme of the proposed mechanism. Electron energies are plotted upward.

pressures would be explained by a temporarily higher concen- tration of excited electrons at the surface ([e*Io) in the case of lower coverages of oxygen. Enhanced exciton dissociation at the s u r f a ~ e ~ ~ ~ ~ ~ or suppressed recombination reactions at a smaller concentration of surface states would lead to such a situation. This hypothesis is supported by the fact that, at a nearly constant coverage of oxygen, the influence of a change in the light intensity on the time dependence of the photocurrent is well predicted by (111) (Figure 5): the intercept rises and the slope gets steeper with rising light intensity. This is exactly the tendency predicted by (IIJ), if it is taken into account that [e*Io rises with the light intensity. Its dependence is given by a power law of about [e*Io - as reported earlier.8.22

A schematic representation of the presented model describing the illuminated electrode a t a sufficiently cathodic potential in contact with an aqueous electrolyte containing dissolved oxygen is given in Figure 7. The adsorption of oxygen which leads to surface states at the phthalocyanine electrode and the processes leading to excited electrons are included as well as the transfer of excited electrons to adsorbed oxygen molecules which turned out to be rate limiting.

Conclusions

It has been reported22 that at thin-film electrodes of PcZn in PVDF, oxygen dissolved in the electrolyte yields a significantly higher photoelectrochemical efficiency when compared to other reactions. Processes at the electrode surface were found to limit the current under illumination. In this paper a model has been presented which is based on the assumption that the current depends linearly both on the amount of adsorbed oxygen and on the amount of excited electrons present at the surface. The latter is assumed to depend only on the incident light intensity and on the applied potential. The current's dependence on time, oxygen partial pressure, and light intensity are predicted reasonably well. From this coincidence it is therefore concluded that the charge transfer occurs through surface states which are generated by adsorbed oxygen. At constant potential and light intensity this process limits the photocurrent. To yield high photocurrents, especially those redox couples with a strong chemical interaction of the educt with the electrode surface and a small interaction of the product have to be studied. A comparatively fast transfer of excited electrons to the educt further is a valuable tool to suppress photocorrosion of any electrode. Surface chemistry therefore must play a decisive role in the discussion of the photoelectrochemistry of phthalocyanines and of charge transfer from a molecular semiconductor surface to a neighboring phase in general.

Surface of a Molecular Semiconductor The Journal of Physical Chemistry, Vol. 97, No. 13, 1993 3337

Acknowledgment. Financial support by the Universitlt Bremen (FNK NFSPZ/90) and assistance by the Petroleum Research Fund in meeting the publication costs is gratefully acknowledged.

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