Kinetic study of laser-induced photoelectrochemical processes at a

Publication Date: March 1985. ACS Legacy Archive. Cite this:J. Phys. Chem. 1985, 89, 7, 1285-1290. Note: In lieu of an abstract, this is the article's...
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J. Phys. Chem. 1985,89, 1285-1290 be prepared and that they can be made, through the appropriate choice of reagents, to show respectable energy conversion efficiencies. In terms of long-term stability, these materials, due to the high density of exposed edges, might be inherently more susceptible to degradation than single-crystal materials, especially in aqueous solvents. As such, we are currently exploring the use of these thin film electrodes in nonaqueous solvents. In addition, we believe that the efficiencies are being limited in part by the low quantum efficiencies. W e believe that through the control

1285

of stoichiometry and the addition of controlled levels of impurities, we will be able to improve the photocharacteristics of these materials. These studies are currently in progress and will be reported on in the future.

Acknowledgment. Support of this work by the Materials Science Center of Cornel1 University is gratefully acknowledged. Registry No. OPD, 95-54-5; WSe2, 12067-46-8; Hl,1333-74-0; Pt, 7440-06-4; poly(benzylviologen), 32168- 10-8.

Kinetic Study of Laser-Induced Photoelectrochemical Processes at a Dye Soiution/Semiconductor Interface A. Frippiat and A. Kirsch-De Mesmaeker* Universitt Libre de Bruxelles, Facultt des Sciences, CP 160, Chimie Organique, B- 1050 Bruxelles, Belgium (Received: October 5, 1984)

The photopotentials induced by pulsed laser illumination of a rhodamine B solution in contact with a Sn02 or ZnO electrode are studied from the nanosecond to the microsecond time scale, in open-circuit and short-circuit conditions. From short-circuit measurements, the capacitances and series resistances of an equivalent electrical circuit are obtained, in agreement with those determined by impedance measurements at high frequency. The study of the open-circuit photopotential relaxation with highly doped Sn02 as a function of the prepolarization potential and the hydroquinone concentration gives the kinetic rate constants for the tunneling back electron transfer to the oxidized dye (R’.), for the chemical reaction of R’., and for the trapping of R’. by the reductant responsible of the supersensitization.

Introduction Up to now most photoelectrochemical studies have been carried out under continuous illumination and very little is known about the dynamics of interfacial processes occurring on the submillisecond time scale. Richardson et a1.l and Gottesfeld et al.z have recently developed a technique to detect laser-induced photoelectrochemical transients of semiconductor/electrolytejunctions. A similar study has also been performed with n-Ti02 in contact with a~etonitrile.~We applied this technique to the study of electrode sensitization and supersensitization kinetics at a semiconductor/dye solution i n t e r f a ~ e . ~ The semiconductor sensitization by dyes has already been extensively studied under steady-state irradiation condition^.^ Previous work from this laboratory examined the supersensitization mechanism of different dye plus reductant systems at a very highly doped SnOz electrode and showed that the Stem-Volmer constant Ksv = kQT corresponding to the supersensitization process by the reductant could be deduced from a study of the photooxidation current as a function of the supersensitizer c~ncentration.~*’A Ksv value on the order of 700 M-’ was obtained for the rhodamine B-hydroquinone system.6 It was concluded that this value could not correspond to the supersensitization process called “quenching” where the excited rhodamine is reductively quenched by hydroquinone to generate semiquinone and a reduced rhodamine which is oxidized afterward at the electrode to regenerate the dye. Indeed, introducing the highest value for kQ,a diffusion-controlled rate constant determined from the rhodamine fluorescence quenching by hydroquinone, gives a lifetime 7 on the order of 70 ns. This is much too long for the lifetime of the excited singlet state of the dye which is 1.5 ns;* moreover, as the intersystem crossing quantum yield is zero in solution: 7 could not correspond to the excited triplet. Consequently, the ksv value obtained had to correspond to the supersensitization mechanism called “trapping” where the reductant traps the photoelectrochemically generated oxidized dye before its reduction a t the .electrode by *Research Associate of the National Fund for Scientific Research (Belgium).

the back electron transfer tunneling through the thin SnO, space charge. An indirect indication was thus reached in favor of the “trappingn mechanism but no clear-cut conclusions could be drawn. In this paper, we show that the kinetic study of photoelectrochemical transients induced by laser pulsed irradiation allows an independent determination of kQ and T whereas under continuous illumination only the product k p r can be determined; direct evidence for the “trapping” process is so obtained.

Experimental Section Cell and Electrodes. When the potentiostatic control is omitted, the photoelectrochemical (PEC) cell is composed of a small semiconducting electrode (from 8 to 20 mmz) and a large surface area Pt counterelectrode (-20 cm2). The use of a large Pt electrode is essential to keep a constant counterelectrode potential (1) J. H. Richardson, S. B. Deutscher, S.P.Perone, J. Rosentha!, and J. N. Ziemer, J . Electrochem. Soc., 127, 2580 (1980); S.B. Deutscher, J. H. Richardson, S. P. Perone, J. Rosenthal, and J. Ziemer, Faraday Discuss. Chem. SOC.,70, 33 (1980); J. H. Richardson, S. P. Perone, and S. B. Deutscher, J . Phys. Chem., 85, 341 (1981); J. H. Richardson, S. P. Perone, L. L. Steinmetz, and S. B. Deutscher, Chem. Phys. Lett., 77, 93 (1981). (2) Z. Harzion, N. Croitoru, and S. Gottesfeld, J. Electrochem. Soc., 128, 551 (1981); S. Gottesfeld and S. W. Feldberg, J . Electroanal. Chem., 146, 47 (1983); Z. Harzion, D. Huppert, S. Gottesfeld, and N. Croitoru, J. Electroanal. Chem., 150, 571 (1983). (3) P. V. Kamat and M. A. Fox,J. Phys. Chem., 87, 59 (1983). (4) A. Frippiat, A. Kirsch-De Mesmaeker, and J. Nasielski, J . Elecrrochem. SOC.,130, 237 (1983). (5) H. Gerischer and F. Willig, Top. Curr. Chem., 61, 31 (1975). (6) A. Kirsch-De Mesmaeker, P. Leempoel, and J. Nasielski, Electrochim. Acta, 23, 605 (1978); A. Kirsch-De Mesmaeker, P. Leempoel, and J. Nasielski, Nouu. J . Chim., 2, 497 (1978). (7) A. Kirsch-De Mesmaeker, J. Kanicki, P.Leempoel, and J. Nasielski, Bull. Soc. Chim.Belg. 87,849 (1978); A. Kirsch-De Mesmaeker, P.Leemp e l , and J. Nasielski, Nouv. J . Chim., 3, 239 (1979); M. Wyart-Remy, A. Kirsch-De Mesmaeker, and J. Nasielski, Nouu. J . Chim., 3, 303 (1979); A. Kirsch-De Mesmaeker, M.Wyart-Remy, and J. Nasielski, Sol. Energy, 25, 117 (1980); A. Kirsch-De Mesmaeker and R. Dewitt, Electrochim. Acta, 26, 297 (1981). (8) V. J. Koester and R. M. Dowben, Reu. Sci. In.“., 49, 1186 (1978). (9) A. K. Chibisov, H. A. Kezle, L. V. Levshin, and T. D. Slavnova, J . Chem. SOC.,Chem. Commun., 1292 (1972).

0022-3654/85/2089-1285$01.50/00 1985 American Chemical Society

1286 The Journal of Physical Chemistry, Vol. 89, No. 7, 1985

Frippiat and Kirsch-De Mesmaeker

TABLE I: Electrode Characteristics

D. U.

n-Sn02 poly: single

thickness exposed area, mm2 N,," cm-' V,, at pH 4.5," V/SCE Cscat +0.3

V/SCE,bcnF Csc at +0.3 V/SCE,bdnF R,,bdQ

crystalline' crystal 320nm -1"

-

-1"

-8 2x

-0.160

-0.350

-0.620

500

80

2.3

160

50

1.6

-200

-1000

-1000

POT.

/

n-ZnO single crystal

20 7 x 10'9

osc.

I 0. I .

D. U.

F.C.

-8 1020

-

PEC

.

1016

'The characteristics of the chemically modified S n 0 2 film are the same as those of the unmodified Sn02. bThese data are obtained by impedance measurements with a Solartron-Schlumberger 1250 frequency response analyzer. The capacitance values depend on the measurement frequency; however, the flat-band potential ( VFB)obtained by extrapolationof the plot 1/C? vs. Vis frequency independent. cMeasurementsat 100 Hz. dMeasurementsat 10000 Hz. during the laser-induced SnOz potential jump and during its relaxation. It was checked that the response of the cell does not depend on the counterelectrode in the time scale 20 ns < t < 10 ms, by testing three different Pt areas at various distances from the working electrode. The counterelectrode capacitance can thus be neglected in the equivalent electrical circuit of the cell. For the measurements under potentiostatic control, the cell used contains four electrodes: a working semiconducting electrode, a platinum counterelectrode, and a dual reference consisting of a saturated calomel electrode (SCE) capacitively coupled (470 nF) to a platinum quasi-reference.ls10 This fourth electrode allows the measurement of cell potential changes even at times shorter than 10 M, Le., in a time domain where any reliable measurement relative to the calomel electrode alone is precluded because of its high impedance. This dual reference also allows the measurement of potentials relative to S C E on a longer time scale. Essentially four different semiconducting electrodes are used for photoelectrochemical studies: a n-SnO, polycrystalline film deposited on a glass plate (Glaverbel), a chemically modied SnO, film, and a n-Sn02 and a n-ZnO single crystal. The electrode characteristics are summarized in Table I. The single crystals are polished with 3 - ~ mdiamond paste and etched in 4 N HCl for 3 min before use. The chemically modified SnOzis obtained by anchoring rhodamine B to the Sn02OH groups via an ester linkage according to the method of Fujihira and Osa;" before this reaction in acetonitrile in the presence of d i c y c l o h e x y l c a r b d e (DCC), the Sn02 was dipped in a basic medium (4 M NaOH) for 15 h, rinsed with distilled water, and dried. The attachment of the dye to the Sn02 has been tested by the following experiments: (i) washing the chemically modified SnO, with ethanol does not produce any change in the laser-induced photopotential signal, although the dye preadsorbed from a rhodamine B aqueous solution is completely desorbed by ethanol; (ii) no kinetic analysis can be performed on an SnO, electrode with rhodamine B preadsorbed from a dye solution and in contact with the electrolyte; indeed this electrode does not give any photopotential signal after one or two pulses, whereas in the same conditions, the chemically modified electrode produces always a photopotential signal, which after many pulses, corresponds to the deethylated rhodamine. The electrical contacts are made with silver paint (Acheson, Dag 1415) on SnOz and with an In/Ga alloy on ZnO electrodes. The connections to the conducting wires are glued with silver paint and the electrodes mounted on a glass support with Dow Corning 730 RTV glue. Reagents. Solutions are prepared with triply distilled water and deaerated by bubbling high-purity N,. Rhodamine B (UCB, (10) C. C. Herrmann, G. G. Perrault, and A. A. Pilla, Anal. Chem., 40, 1173 (1968). (11) M. Fujihira, N. Ohishi, and T. Osa, Nature (London), 268, 226 (1977); M. Fujihira, T. Osa, D. Hursh, and T. Kuwana, J. Electrounal. Chem., 88, 285 (1978).

T \

P.G.

T

Figure 1. Block diagram of the experimental setup with prepolarization:

D.U.,delay unit; OW., oscilloscope; SW.,switch; POT., potentiostat; PEC, photoelectrochemical cell; F.C., Faraday cage; L, lens; D.L.,dye optical isolator; J., joulemeter; P.G., pulse laser; NIL., nitrogen laser; O.I., generator; T, trigger. analytical reagent), sodium acetate (UCB), the supporting electrolyte LiN03 (UCB, lo-' M), and the DCC (Aldrich) are used as such. The hydroquinone (UCB) is purified by three successive sublimations under reduced pressure (0.13 Pa); the acetic acid (Merck) is distilled over KMnO,; the acetonitrile (Merck) is distilled and then dried Over molecular sieves. All the experiments are performed in 5 X lo-* M acetate buffer, pH 4.75. Instrumentation. The excitation source is a Molectron DLII tunable dye laser with an 8-11s pulse width, pumped by a Molectron U.V. 24 nitrogen laser (337 nm). The intensity of the full laser pulse is on the order of 0.5 mJ. In some experiments, this intensity is attenuated by inserting neutral filters in the light path. The electronic optically isolated system for triggering the laser and the oscilloscopes is represented in Figure 1. The laser beam at 565 nm, corresponding to the absorption maximum of the adsorbed rhodamine B, is split by a beam splitter into a transmitted beam irradiating the dye solution/semiconductor interface and a reflected beam illuminating a pyroelectrical joulemeter (Molectron 53-05 DW) connected to an oscilloscope (Philips P M 3234) to measure the relative light intensity of each pulse. The electrochemical cell is mounted in a Faraday cage made of 1 0 " aluminum plates and placed 10 m from the laser source. The electrical connections are made with BNC connectors kept as short as possible to reduce the overall circuit capacitance. For open-circuit photopotential measurements, the cell output is usually directly fed to a 100-MHz dual-beam storage oscilloscope (Philips PM 3266, 1-MSZ input); for some experiments on a millisecond time scale, a high-input-impedance (lo8 0)differential amplifier (Burrbrown 3554) is used between the cell and the oscilloscope to increase the value of the external load. For short-circuit measurements, the potentials across variable external loads (50 SZ Q Re Q 1000 0)are recorded on the oscilloscope. The oscilloscope traces are photographed and analyzed on a Hewlett-Packard HP-85 microcomputer. For open-circuit measurements under potentiostatic control, the experimental procedure involves first an adjustment of the semiconducting electrode potential to the selected value (relative to SCE) for a few seconds, via a potentiostat. A solid-state electronic switch (Analog Devices, AD7512DI) then opens the circuit and after a ~ O - N Sdelay the pulsed laser is triggered and the transient potential difference between the working and reference electrodes is monitored on the oscilloscope in ac mode. It was checked that the slow decay of the electrode potential from the prepolarization value to the open-circuit equilibrium value occurs on the hundreds of milliseconds time scale and therefore does not disturb the measurements. Description of the Method The method consists of irradiating, by a laser pulse, the rhodamine B (R) solution (IO4 M) in contact with the semiconducting electrode, which causes the electron injection from the excited dye (R*) into the semiconductor conduction band. This process nearly instantaneously charges the capacitance Csc associated with the semiconductor space charge by a quantity AQo

The Journal of Physical Chemistry, Vol. 89, No. 7, 1985 1287

Study of Dye/Semiconductor Interface

1

Rs

InAV

Re

0

tie

I Nm.

AV a 460

260

-

Figure 3. Short-circuitrelaxations on polycrystalline Sn02in the absence of hydroquinone at external loads Re of (a) 75, (b) 325, (c) 625, and (d) 1000 0.

-

Figure 2. Equivalent electrical circuit of the cell.

of charge and induces a negative shift AVO= AQo/Csc of the semiconducting electrode potential. At the same time, oxidized rhodamine (R+.) is produced at the electrode. The discharge of this capacitance just after the laser pulse is studied, Le., the photopotential relaxation with time, as a function of different parameters, such as the external load placed in the measurement M) in the dye circuit, the supersensitizer concentration (65 solution, the external bias applied to the working electrode, and the semiconductor doping level. The following reaction scheme was proposed by Gerischerl2 and Memming13 to explain the photosensitization of highly doped semiconductors by adsorbed dyes and the supersensitization by reductants such as hydroquinone: R

-

R*

tlP1

(1,)

where AV(t) represents the semiconductor photoinduced potential shift. After the laser pulse, the capacitance will discharge preferentially through the external circuit (i,) or through the leak resistance (it)depending on the value of & The discharge current i, of the capacitance is given by i, = i, it (11)

+

Taking into account reactions 3-5 and assuming that the species are adsorbed, we have

+ +

dI'R+./dt -(k3 k4 k,[H2Q])r~+. (111) where rR+. is the superficial concentration of the oxidized dye. Assuming that the rate constants are independent of AV, and thus of time, which is reasonable since the initial potential jump AVO is never more than 30 mV (by using neutral filters), we can write rR+, =

rR+.OeWf

(IV)

+ + k5[H2Q].The expression for the tunneling

with a = k3 k4 current is then

where reaction 2 represents electron injection from the adsorbed excited dye into the semiconductor conduction band, reaction 3 represents back-transfer from the electrode to the adsorbed oxidized dye by tunneling through the thin space-charge layer, and process 4 represents the reaction of the dye radical cation to a product P. Reaction 5 corresponds to the supersensitization process called "trapping"; the photoelectrochemically produced oxidized dye is trapped by hydroquinone (H2Q),in competition with its reduction at the electrode (reaction 3), which regenerates the dye and enhances the observed photocurrent. Thus, in the laser experiment with the rhodamine B-hydroquinone system, reactions 1 and 2 take place during the laser pulse whereas reactions 3-5 occur during the relaxation of the photopotential AV. In this experiment, the PEC cell can be represented by the equivalent electrical circuit of Figure 2. In this circuit, Csc represents the capacitance associated with the.semiconductor space charge, R, the resistance of the semiconductor and of the dye solution, Re the external load placed on the measurement circuit, and RI the variable leak resistapce characteristic of the tunneling current corresponding to the back electron transfer through the semiconductor space charge. According to this equivalent circuit, the photopotential measured across the external load is given by

(12) B. Pettinger, H. R. Sch6ppe1, and H. &&her, Eer. Bunsenges. Phys. Chem., 77, 960 (1973). (13) R. Memming and G. Karsten, Eer. Bunsenges. Phys. Chem., 76, 4 (1972).

it = -Sm~+?k$-"' (VI where S is the electrode area, F the Faraday constant, and rR+! the superficial concentration of R+-just after the laser pulse. Expressions I1 and V lead to

which, after integration and provided that Csc is constant, gives

AV(t) = CY -k3 l / T e AV0e-"'

(

+ AVO

1-

- e-'/'c

(VII)

Ck!-k;/Te)

+

with AVO= smRt.O/cscand T , = (Re R,)Csc. For a 1-MQ external load, thus in open-circuit (OC), T, is large and expression VI1 simplifies to

k3 AVm,,(t) = AV(t) = AVo-eat a

+ AVO

The capacitance discharges then by the back electron transfer tunneling, through the leak resistance of the equivalent electrical circuit. In short circuit, T , is small and the capacitance discharges exponentially with time through the external circuit

Experimental Results and Discussion Short-circuit Experiments. The short-circuit measurements are performed without potentiostatic control at the semiconductor dark equilibrium potential. With highly doped polycrystalline Sn02and sufficiently small external resistance values corresponding to short-circuit conditions

1288 The Journal of Physical Chemistry, Vol. 89, No. 7, 1985

Frippiat and Kirsch-De Mesmaeker

i

80

I

200

400

Re(n)

600

Figure 4. Time constant re of the short-circuit exponential decay vs. the external load Re obtained on polycrystalline SnO, in the absence of

hydroquinone.

Figure 5. Rise of the open-circuit photopotential at the rhodamine Bpolycrystalline Sn02 interface: [H2Q] = O M;E = +0.3 V/SCE.

TABLE II: Short-circuit ( R e = 100 Q) Time Constants T, and Dark Equilibrium Potentials E , as a Function of the Hydroquinone Concentration

W2Q1,

M

0 10-4

5 x 10-4 lo-'

T ~ PS ,

37 55 58 68

E,, mV/SCE 300 240 210 190

Csc:

nF

150 220 230 270

301

1 1500

(50 R d Re d 550 Q ) , it turns out that all the relaxations are exponential (Figure 3). From the arguments of the exponentials, the time constants re = ( R , + R,)Csc are obtained (eq IX) and are plotted vs. Re (Figure 4). The slope of the straight line in Figure 4 gives Csc = 150 nF and from the intercept R, is found to be 150 R. For Re > 550 52, the relaxation is no more exponential (Figure 3) because Csc also discharges by the back electron tunneling and, as calculated previously (eq VJI), the discharge is then the sum of two exponentials. These results are obtained without hydroquinone in the dye solution. Table I1 gives the values of re found with Re = 100 Cl and increasing hydroquinone concentrations. By adding hydroquinone to the solution, the dark equilibrium potential is shifted to less positive values, which increases the capacitance Cscof the electrode and hence 7,. The re vs. R, (R, C 600 a), in absence of hydroquinone, have also been measured with the SnOzsingle crystal; it is found that, at a dark equilibrium potential of +0.25 V/SCE, Csc = 60 n F and R, = 900 R. For the poorly doped ZnO sample the electron back tunneling is slower and the relaxations remain exponential up to 1000 R, leading to Csc = 2 n F and R, = 700 Q, without hydroquinone, at a dark equilibrium potential of +0.25 V/SCE. The short-circuit experiments thus allow the determination of R, and Csc at high frequency; the values obtained are in good agreement with those found by impedance measurements at lo5 Hz (Table 1).

Open-circuit Experiments. I . Initial Photopotential. Figure 5 shows the rise of the open-circuit photopotential induced at the rhodamine B-polycrystalline SnO, interface. The rise time depends on the RC constant of the electrical device, i.e., on the cell resistance R, (see Table I) and on the combined values of cable capacitances and input capacitance of the oscilloscope (typically less than 100 pF). This leads to a rise time on the order of 20 ns for the polycrystalline sample and 100 ns for the single crystals. The photopotential AVOreached after the rise and normalized to the same incident light intensity, as a function of the exciting wavelength, shows a maximum at 562 nm and looks very much like the action and absorption spectra of rhodamine B adsorbed on Sn0; (Figure 6), indicating that the signal originates from the adsorbed dye. It has to be noted that without dye or by

6 6 0 A(nm)

5 50

Figure 6. Spectrum of the initial open-circuit photopotential AVOvs. the exciting wavelength: polycrystalline SnO,; [H2Q] = 0 M;E = +0.3

V/SCE. TABLE III: AV,,, Cw, and AQo (= C,AVo) Values Obtained without Hydroquinone on Polycrystalline SnO, as a Function of the Applied Potential

E , mV/SCE -- 100 0 100

300 500 750

AVO,mV

CSC,nF

AQo, nC

20.5 25.0 28.0 34.0 40.0

365 210 185 180 175

7.5 5.3 5.2 6.1 7.0

illuminating the cell at a wavelength where rhodamine B does not absorb, no photopotential is observed. From the AVO value and the knowledge of Csc from the short-circuit experiments, it is possible to estimate the amount of electrons injected from the excited rhodamine to the SnO, conduction band: ne-,inj= AQo/q = CscAVo/q, where q is the elementary charge. Moreover, the knowledge of the light pulse intensity Io determined with the joulemeter and the evaluation of the percentage of light absorbed by the adsorbed dye allow us to calculate the amount of photons absorbed: flhu,abs. The percentage of light absorbed by the dye (20% Io) was measured at the rhodamine B-Sn0, interface by attenuated total refle~tion.'~The ratio h-bj/n,,uak gives an estimate of the electron injection quantum yield &,j into the semiconductor; it is found that is of the order of 5 X lo4 for polycrystalline SnO, and 5 X 10'7 for single crystals. These values are comparable to those found by Spitler et al.14 for the photooxidation of rose bengal on poorly doped TiOz and ZnO. These very low quantum yields could be attributed to an important deactivation of the excited dye by ground-state molecules or aggregates of the adsorbed 1 a ~ e r . l ~ '

~

~~~

~

~~~

(14) The attenuated total reflection experiments have been carried out in

Professor M. Spitler's laboratory (Caw Laboratory of Mount Holyoke College, South Hadley, MA; M. Spitler, M. Liibcke, and H. Gerischer, Chem. Phys. Let?., 56, 577 (1978); M. Spitler, M. Liibcke, and H. Gerischer, Eer. Eunsenges. Phys. Chem., 83, 663 (1979)

The Journal of Physical Chemistry, Vol. 89, No. 7, 1985 1289

Study of Dye/Semiconductor Interface TABLE I V AVOValues Measured on Polycrystalline SnOl at +300 mV/SCE as a Function of the Hydroquinone Concentration AVO,mV [HzQI, M 0 27.8 29.6 10-4 s x 104 30.2 10-3 M 30.4 28.2 5 x 10-3

Av(mvr 40. 0

TABLE V Dependence of the Rate Constants k3, k, (Obtained without Hydroquinone) and kdon the Electrode Potential Ep 10-'k3, lO-'k4, 104k5, (k5)TR+., E, mV/SCE s-I S-' M-1 s-I M-l 0 2.8 3.6 5.4 645 3.5 5.8 740 50 2.0 100 1.9 3.6 3.3 755 200 1.7 3.6 3.1 785 85 1 300 1.3 3.4 3.3 920 500 0.95 1050 760 0.4 3.1 'rR+.= (k,

+ kJ';

(k5)is the average of the values of column 4.

AV(mW 30.0

10.0

[ l 15. 0

0.0 0.0

I. 0

.5

1.5

t(ms)

Figure 7. Relaxations of the open-circuit photopotential observed on polycrystalline SnOl at different prepolarizationpotentials: [HzQ] = 0 M. Prepolarizations (mV/SCE): (a) = 0; (b) = 100; (c) = 200; (d) = 300; (e) = 500 and (f) = 760.

As shown in Table I11 for polycrystalline SnOz, AVOincreases at more positive applied potentials. At first sight, this AVOincrease could be related to the evolution of Cscor to the variation of the amount of charge AQo injected into the semiconductor with the applied bias. Table I11 also gives the Cscvalues obtained by impedance measurements at high frequency (los Hz) and the corresponding calculated AQo = CscAVo values. It turns out that the AVO increase with the potential is due only to the evolution of C, and not to an increased amount of charge AQo injected during the light pulse, indicating thus that &j does not depend on the applied is given by k J k 2 k , which also means that the potential. 4inj. excited-state lifetime (k2 k J 1 is not influenced by the applied potential. With the ZnO single crystal, the potential dependence of AVOis not clear, probably because of a poor experimental evaluation of AVOdue to the too long R C time constant with ZnO, which increases the AVrise time. On the other hand, for the same applied bias, the initial photopotential AVO,and hence &j, is reasonably independent of the hydroquinone concentration (Table IV), showing that the reductant does not quench the excited dye efficiently before its oxidation at the electrode. 2. Photopotential Relaxation on Highly Doped Sn02. As mentioned before, in open-circuit conditions, the charge injected into the electrode has to decay by back electron transfer to the oxidized dye acrw the thin space charge layer (reaction 3). Figure 7 shows the open-circuit photopotential relaxations observed on polycrystalline Sn02 without hydroquinone and for different prepolarization values varying from 0 to 760 mV/SCE. It has to be noted that for electrode potentials more positive than 800 mV/SCE, the adsorbed rhodamine is oxidized in the dark by a tunneling effect,6 introducing thus a dark component and decreasing the amount of nonoxidid dye adsorbed on the electrode, and hence the initial photopotential AVO. All curves of Figure 7 are normalized to the same incident light intensity. It is seen that the photopotential does not relax entirely to its initial dark value; the final constant potential can be explained by reaction 4 where the oxidized dye generates P, a long-lived species, and so escapes the back-reduction. When increasing applied potential, the space charge becomes thickerI6 which slows the photopotential

+

+

(15) N. Nakashima, K. Yoshihara, and F. Willig, J. Chem. Phys., 73,3553

(1980).

0. 0

I

1.0

.5

1.5

t (ms)

Figure 8. Relaxations of the open-circuit photopotential observed on polycrystalline SnOl at different hydroquinone concentrations: (a) = 0 M; (b) = lo4 M; (c) = 5 X IO4 M; and (d) = M. E = +0.2 V/SCE.

relaxation by electron transfer tunneling and increases the final AVvalue. By analyzing curves a to f of Figure 7 it turns out that they are all exponential on the 100-ps time scale, in agreement with eq VIII. It should be noted, however, that without hydroquinone AVcontinues to decrease slowly after this first relaxation, for a few milliseconds. This slower decay could be attributed to the reduction at the electrode of a long-lived oxidized species generated by reaction of R+.: R+*

-+

R,,

%,

-

(i

P'

P)

(4) (7)

where P' represents a nonreducible species. The amplitude of the slower relaxation is, however, much too small to allow a thorough analysis. Furthermore, it does not disturb the exponential fast decay up to 1 ms and is not observed in the presence of hydroquinone which efficiently traps the long-lived R,, species. Identical relaxation curves are obtained when the experiments are performed with rhodamine B chemically bound to the SnOz surface by an ester linkage" (see also the Experimental Section), indicating that a kinetic treatment with the adsorbed species only was indeed reasonable. The three experimental values, the initial photopotential AVO, the potential reached after the first decay A & = AVo[l - ( k 3 / a ) ] , and the argument a = k3 k4 of the exponentials of Figure 7, allow us to determine the kinetic rate constants k3 and k4 given in Table V. The fact that k4 is potential independent strongly indicates that it represents a purely chemical reaction consuming the oxidized dye R+.. On the other hand, the decrease of k3 at more positive potentials is explained by the increase of the electrode space-charge thickness, making electron tunneling less probable. Results similar to those given in Table V are obtained with the

+

(16)

(1975).

H.Gerischer, J. Electroonal. Chem. Interfacial Electrochem., 58,263

1290 The Journal of Physical Chemistry, Vol. 89, No. 7, 1985

2 80

I

AV (mV)

+350

16-

-40.-

-*15c

50

are observed (see short-circuit experiments). Indeed, for R, values such that T~ is of the same order of magnitude as T ~ + . ,the relaxation of AV, given by eq VII, takes place simultaneously through the external circuit and by the faradic reaction 3. With hydroquinone concentrations 2 lo-, M, sometimes the photopotential, instead of relaxing, increases slowly for a few milliseconds after the laser pulse! The origin of this irreproducible slow increase will be discussed elsewhere. It has been checked that the photopotential relaxations presented here are not disturbed by this phenomenon. 3. Photopotential Evolution with Time on Poorly Doped ZnO. Figure 9 shows the open-circuit photopotential evolution with time observed with a ZnO single crystal for prepolarization values varying from -0.5 to +0.65 V/SCE, in the absence of hydroquinone. There is no significant photopotential relaxation in the microsecond time scale, except for electrode potentials more negative than -0.3 V/SCE. As expected for a poorly doped semiconductor having a thick space-charge layer, electron tunneling is rather slow, except near the electrode flat-band potential. The charge injected during the laser pulse has then to decay exponentially through the external circuit, Le., through the 1-MQ resistance of the oscilloscope with a time constant 7, = (Re &)Cx i= R,Cx = 2 ms, as observed experimentally. Introducing a high-input impedance (1 O8 a) differential amplifier between the cell and the oscilloscope increases the experimental time constant of the exponential decay to 0.2 s. Hydroquinone does not influence the photopotential evolution down to -0.3 V/SCE. This confirms that the photopotential relaxation observed on highly doped SnO, originates essentially from the back electron tunneling through the thin space-charge layer.

I ‘

120:

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Frippiat and Kirsch-De Mesmaeker

I

t

Figure 9. Time evolution of the open-circuit photopotential observed on poorly doped ZnO at different applied potentials (mV/SCE).

highly doped SnOz single crystal. Figure 8 gives a series of curves obtained a t +200 mV/SCE with polycrystalline SnO,; for different hydroquinone concentrations, the final AVvalue becomes more important and is reached more rapidly meaning that more photoelectrochemically produced oxidized dye is trapped by the reductant to regenerate the dye instead of being reduced at the electrode. The same series of curves were recorded for different applied potentials. For each prepolarization value, a plot of the argument a of the exponential vs. the hydroquinone concentration gives a straight line; the slope k5 does not vary much with the applied potential (Table V). It should be stressed that the values of k5 are rather inaccurate since they come from two successive curve fittings. Furthermore, the addition of hydroquinone for electrode potentials more positive than 200 mV/SCE leads to decay curves which are too flat to warrant any quantitative analysis. In conclusion, these results show that the measurement of open-circuit photopotentials allows us to determine kinetic rate constants associated with photoelectrochemical reactions of ad(Table V), where sorbed species. Moreover, the product k5~R+. T ~ += . (k3+ k4)-l represents the lifetime of the adsorbed oxidized dye R+., agrees pretty well with the value of the Stern-Volmer constant Ksv found previously under continuous illumination6 for the supersensitization process. This c ~ n f ithat i in the rhodamine B-hydroquinone system the main supersensitization mechanism corresponds to “trapping”. It should also be mentioned that the values of T ~ + found . (-200 p s ) are in agreement with the results obtained with external loads Re > 550 R, Le., 7, > 105 ps, for which no exponential decays

+

Acknowledgment. We thank Professor J. Nasielski for his stimulating discussions and Professor M. Spitler for his helpful contribution to the attenuated total reflection experiments, carried out at the Carr Laboratory of Mount Holyoke College, South Hadley, MA, thanks to a NATO grant for collaborative research. We are grateful to Professor R. Helbig from the ErlangenNiimberg University and to Professor G. Heiland from the Aachen Technische Hochschule for gifts of the SnOa and ZnO single crystals. We thank also Professor J. Vereecken and Dr.J. Hubrecht (Metallurgy and Electrochemistry Department, V.U.B., Belgium) for allowing us to perform the impedance measurements with the Solartron apparatus. A.F. thanks the Institut pour 1’Encouragement de la Recherche Scientifique dans 1’Industrie et 1’Agriculture (I.R.S.I.A.) for a fellowship. Registry No. Rt.,67627-95-6;SnO,, 18282-10-5; ZnO, 1314-13-2; rhodamine B, 81-88-9;hydroquinone, 123-31-9.