2446 J . Phys. Chem. 1984, 88, 2446-2451

thalpy of the surface reactions have been calculated from ex- perimentally determined heats of adsorption and heats of metal oxide formation. These enthalpies indicate the relative thermo- dynamic driving force for each of the reactions. The results suggest that, on all the samples studied except for silver, metallation of the O H bond may be expected and this is observed. The de- composition of methanol to CO rather than H 2 C 0 on the group 8 metals is explained by the greater thermodynamic stability of adsorbed CQ and H than H2C0 on these surfaces.

On more reactive surfaces such as platinum the changes in the reactivity due to the presence of oxygen are more compl5x than on Ag(l10) since the clean surface is already very reactive. The modification of the reactivity can occur by direct-reaction with the oxygen or through the steric or ligand eUds on the substrate. Consequently, it may be difficult to predict the effects of the oxygen on such surfaces. On very reactive metals such as mol- bydenum oxygen tends to passivate the surface, blocking sites required for reaction rather than acting as a strong base as on Ag(l10). Thus while methoxy species form on Mo(100) far fewer form on a partly oxidized Mo( 100) surface.84

ARTICLES

Concluding Remarks An indication of the potential of the modern spectroscopic

methods for elucidating the elementary steps occurring during reaction on a metal surface has been provided by an overview of the surface chemistry of oxygen-containing organic molecules as revealed by these techniques. In many of the reactions discussed stable reaction intermediates have been identified and shown to be important in controlling the rate and selectivity of the reactions occurring. Much remains to be done, however, before critical comparison between the chemistry occurring on surfaces and on clusters can be made in order to discover trends in chemical properties which may be of predictive use.

The elucidation of the role surface oxygen plays in many ox- idation reactions and the surface acid-base reactions provide synthetic routes which may prove useful when trying to form a surface complex of desired structure for surface reactivity studies.

Acknowledgment. Support of the National Science Foundation and the donors of the Petroleum Research Fund, administered by the American Chemical Society, is gratefully acknowledged.

Resonance Raman Spectroelectrochemistry of Semiconductor Electrodes: The Photooxidation of Tetrathiafulvalene at n-GaAs( 100) in Acetonitrile

Richard P. Van Duyne* and Jeanne P. Haushalter

Department of Chemistry, Northwestern University, Evanston, Illinois 60201 (Received: July 1 , 1983)

Resonance Raman spectroelectrochemistry (RRSE) is applied for the first time to the study of a laser-driven photoelectrochemical process at a 111-V semiconductor in nonaqueous solution. The specific system investigated is the one-electron photooxidation of tetrathiafulvalene (TTF) to form the corresponding cation radical (TTF'.) at n-type GaAs( 100) in acetonitrile solution. Resonance Raman spectra (RRS) of TTF'. excited into both electronic excited states accessible to visible lasers as well as millisecond time scale resonance Raman intensity (RRI) vs. time transients are reported. Both the spectra and transients represent the detection of TTF'. in the diffusion layer adjacent to the electrode surface rather than in the bulk of solution.

Introduction Resonance Raman spectroelectrochemistry (RRSE) has been

shown to be a very useful means of monitoring electrochemical reaction products with the excellent molecular specificity inherent in vibrational spectro~copy.'-~ Applications of RRSE include studies of species of photosynthetic i m p ~ r t a n c e ~ , ~ and of organic metal component^.'*^-^ Now, we have found this technique to

(1) R. P. Van Duyne, J . Phys. (Paris), 38, C5-239 (1977). (2) D. L. Jeanmaire and R. P. Van Duyne, J. Electroanal. Chem., 66,235

(3) D. L. Jeanmaire, M. R. Suchanski, and R. P. Van Duyne, J . Am.

(4) T. M. Cotton and R. P. Van Duyne, Biochem. Eiophys. Res. Commun.,

( 5 ) T. M Cotton, K. D. Parks, and R. P. Van Duyne, J. Am. Chem. SOC.,

( 6 ) R. P. Van Duyne, M. R. Suchanski, J. M. Lakovits, A. R. Sielde, K.

(7) D. L. Jeanmaire and R. P. Van Duyne, J . Am. Chem. SOC., 98,4029

(8) D. L. Jeanmaire and R. P. Van Duyne, J . Am. Chem. SOC., 98,4034

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107, 6399 (1980).

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(1976).

(1976).

0022-3654/84/2088-2446$01 .SO10

be particularly applicable to the study of semiconductor (SC) electrode based photoelectrochemical (PEC) systems. This is partly a consequence of the dual role of the laser excitation which both drives the photoelectrochemistry and monitors the photo- generated species by resonance Raman spectroscopy (RRS). Furthermore, the molecular specificity of RRSE will be very useful in sorting out the complex processes which can contribute to the photocurrent at an illuminated SC electrode and compete with the desired photoelectrochemical reaction. Consider, for example, that production of an electron-hole (e-hc) pair through absorption of light by an n-type SC electrode can lead to the following competitive processes: (1) the desired photooxidation of a solution species, (2) lattice oxidation or photocorrosion of the SC electrode surface, (3) nonradiative recombination of the electron-hole pair, and (4) radiative recombination of the electron-hole pair with resulting luminescence. The detailed understanding of these reaction channels, especially their kinetic aspects, is very important for the development of efficient and stable solar energy devices

~~~~ ~ ~~

(9) M. R. Suchanski and R. P. Van Duyne, J . Am. Chem. Soc., 98, 250 (1976).

0 1984 American Chemical Society

RRSE of Semiconductor Electrodes

based on the SC-liquid junction.’@19 Various spectroelectrochemical approaches have been previously

applied to the study of PEC systems. Photoresponse spectra have been reported for many of the PEC cells developed, but of par- ticular interest are systems involving sensitized SC electrodes20v21 and SC electrodes with surface states. Heller et al. have shown that surface states can be observed by using a laser pumping technique.22 Electron-hole pair recombination luminescence has been rnonitored to map e-h+ separation in graded SC-electrode material^.^^,^^ These and other spectroelectrochemical techniques have proven to be indispensible to the study and development of PECs.

Both normal Raman spectroscopy (NRS) and RRS have been previously used to study bulk semiconductors as well as semi- conductor electrodes. For example, the NRS vibrational properties of anodically grown, multilayer oxide films on Gap, GaSb, and GaAs have been reported.2s RRS has been used to characterize both multilayer metallophthalocyanine films on glassy carbon electrodes26 as well as an unspecified coverage of a dye (viz., Rose Bengal) adsorbed on a ZnO electrodes2’ In both of these cases the electrodes were operated under potential control in aqueous solution. Quite recently nanosecond pulsed lasers have been used to initiate photochemical reactions on the surface of colloidal TiOz particles and to monitor the time-resolved vibrational spectra of reaction products in the solution layer surrounding the particles.28 Modern Raman spectroscopic techniques have clearly been rec- ognized as potentially powerful tools for probing SC photochemical and photoelectrochemical systems.

In this paper we demonstrate the unique capabilities of RRSE for the study of SC electrodes. RRSE can selectively monitor photoelectrochemical reaction intermediates and products in bulk solution, in the diffusion layer adjacent to the electrode, and even on the electrode surface itself under favorable circumstances. Here we use RRSE to monitor the photooxidation of tetrathiafulvalene (TTF) to its cation radical (TTF+.) a t a small band-gap SC electrode (n-GaAs( 100)) in nonaqueous electrolyte (0.1 M tet- rabutylammonium perchlorate (TBAP) in acetonitrile (ACN)). It should be noted, however, that RRSE is also applicable to aqueous systems as well. To our knowledge, this is the first example of the application of a vibrational spectroscopy to direct monitoring of a dynamic process a t a SC electrode. Our ex- periments were performed on the millisecond time scale but any time regime from picoseconds to seconds is, in principle, accessible by RRSE by use of pulsed laser excitation.

We also show that RRSE can provide information about changes in the surface of a SC photoelectrode due to photocor- rosion and passivation. Spectra obtained in a RRSE experiment can be affected by these SC electrode surface changes in a number

(10) A. J . Nozik, Annu. Rev. Phys. Chem. 29, 189 (1978). (11) M. A. Wrighton, Acc. Chem. Res., 12, 303 (1979). (12) R. Memming, Electrochim. Acta, 25, 77 (1980). (13) K. Rajeshwar, P. Singh, and J. DuBow, Electrochim. Acta, 23, 117

(14) A. Heller and B. Miller, Electrochim. Acta, 25, 29 (1980). (15) A. Heller, Acc. Chem. Res., 14, 154 (1981). (16) A. J. Bard, Science, 207, 139 (1980). (17) H . Gerischer, Top. Appl. Phys., 31, 115-72 (1979). (18) M. A. Wrighton, ACS Symp. Ser., No. 192, 99-132 (1982). (19) A. J. Nozik, Ed., ACS Symp. Ser., No. 146 (1981). (20) H. Tributsch and H. Gerischer, Ber. Bunsenges. Phys. Chem., 73,251

(21) R. Memming, Discuss. Faraday SOC., 58, 261 (1974). (22) A. Heller, K. C. Chang, and B. Miller, J . Am. Chem. Soc., 100,684

(23) M. K. Carpenter, H. H. Strecker, and A. B. Ellis, J . Solid State

(24) H. H. Streckert and A. B. Ellis, J . Phys. Chem., 86, 4921 (1982). (25) G. P. Schwartz, G. J. Gultieri, J. E. Griffiths, C. D. Thurmond, and

(26) C. A. Melendres and F. A. Cafasso, J . Electrochem. Soc., 128,755

(27) H. Yamada, T. Amamiya, and H. Isubomura, Chem. Phys. Lett., 56,

(28) R. Rossetti, S. M. Beck, and L. E. Brus, J. Am. Chem. Soc., 104,

(1978).

(1969).

(1978).

Chem., 45, 51 (1982).

B. Schwartz, J . Electrochem. Soc., 127, 2488 (1980).

(1981).

591 (1978).

7322 (1982).

The Journal of Physical Chemistry, Vol. 88, No. 12, 1984 2441

of ways. The overall intensity can decrease due to altered re- flectivity, background changes (e.g., due to a film on the surface) can occur, and scatering from bulk phonons of the SC electrode can decrease or disappear. By observing these changes in time, it is possible to follow photocorrosion and passivation processes.

it should be noted here that it is also possible to observe e-h+ pair recombination luminescence by using the same experimental arrangement used to monitor Raman scattering from photoe- lectrochemical reaction products. Thus, RRSE can simultaneously provide information about many if not all the important com- petitive processes which occur a t an illuminated SC electrode.

Experimental Section Materials and Methods. Tetrathiafulvalene (TTF) was ob-

tained from Aldrich Chemical Co. It was sublimed and sealed under vacuum or argon until use. Tetrabutylammonium per- chlorate (TBAP) was obtained from Southwestern Analytical Chemicals, Inc. I t was recrystallized twice from reagent grade ethyl acetate, dried under vacuum, and desiccated until use. Acetonitrile was spectroscopic grade (Burdik & Jackson Labo- ratories, Inc.) and dried by stirring over activated alumina.

Si-doped n-type GaAs( 100) was obtained from Laser Diode Laboratories, Inc. (dopant density was 0.95 X 1018-1.6 X 10**/cm3). Electrodes were fabricated by attaching ca. 0.5-cm2 wafers, which had been first abraded on the back by fine sandpaper and wetted with GaIn eutectic (Alfa Products), to Cu wire with Ag epoxy (EPO-TEK 417, Epoxy Technology). The Cu wire was encased in 6-mm Pyrex tubing and portions of the Cu wire and the n-GaAs (including the contact) were masked with solvent resistant epoxy resin (Varian Associates Torr Seal). The electrodes were etched in an unstirred solution of H2S04/30% H202 (1:l by volume mixture) immediately before use.

Cyclic Voltammetry. Cyclic voltammetry was performed in a “sandwich-type” Raman spectroelectrochemical cell of the kind previously described in the l i t e r a t ~ r e . ~ ~ The potentiostat and triangular wave generator were of conventional operational am- plifier design and are described in detail e l ~ e w h e r e . ~ ~ Cyclic voltammograms were recorded on a Hewlett-Packard 7015B X-Y recorder. Illumination of the n-GaAs SC electrodes was provided by the same laser (with the same power density) as used in the Raman experiments. A Ag/Ag+ (0.01 M) reference electrode was used in all experiments although all potentials are reported vs. SCE.

Resonance Raman Spectroelectrochemistry. RR spectra and RRI vs. time transients were obtained by use of the 457.9-nm line of an argon ion laser (Coherent, Inc. Model CR-3). The double monochromator, photon-counting electronics, photomultiplier tube, and computerized data acquistion system have all been previously d e s ~ r i b e d . ’ * ~ , ~ ~ Laser power and monochromator band-pass were typically 100 mW and 4 crn-’, respectively.

Potential modulation was accomplished by applying square wave pulses of an appropriate amplitude, pulse width, and duty cycle to the second input of the potentiostat. The first input was set a t the negative potential limit for the modulation. The pulse generator used as a Continental Specialties Corp. Model 4001. In order to obtain steady-state spectra of TTF’. in the diffusion layer of the electrode, we used a 50% duty cycle and 50-ms pulse width. For the transients, a 5% duty cycle and aperiod of 1 s was used. The amplitude of the square wave pulse varied depending on the particular electrode used.

Transient data were collected with a EG&G-PAR Model 4203 signal averager. The data was then transmitted to and stored on an Apple 11+ microcomputer equipped with dual floppy disks and a SSM AS10 Apple serial 1/0 interface. The data were then

Although we will reserve the specific details for a future

(29) J. P. Haushalter, B. K. Johnson, and R. P. Van Duyne, unpublished work.

(30) R. P. Van Duyne in “Chemical and Biochemical Applications of Lasers”, Vol. 4, C. B. Moore, Ed., Academic Press, New York, 1979, pp

(3 1) J. M. Lakovits, Ph.D. Dissertation, 198 1 Northwestern University, Evanston, IL.

101-83.

2448 The Journal of Physical Chemistry, Vol. 88, No. 12, 1984 Van Duyne and Haushalter

A P t

~ , , , , , , , " " " " ' , , " " " 0.0 1.0

VOLTS VS SCE

B n - G a A s

I i g h t

HT , t t , ! V I , I , , , 1 ,

0 . 0 0.5 1.0 1.5 VOLTS VS SCE

Figure 1. Cycic voltammograms of 2.5 mM MlTF/O.l M TBAP/ACN on Pt and n-GaAs (light and dark). The arrows indicate the limits of the potential modulation for the RRSE experiment.

transferred to a NOVA 2/10 minicomputer with 65K bytes of memory, dual floppy disks, CRT terminal, printer, and incremental plotter for plotting of the transients. Plots of RRI vs. t 1 / 2 and

were obtained with a commercial curve-fitting software package (Curfit by Interactive Microwave) with the Apple II+.

The spectrum in Figure 4 was taken with an argon ion (Co- herent, Inc. Model CR-8) pumped CW dye laser (Coherent, Inc. Model 590). Rhodamine 6G was the dye used. The dye laser was tuned until maximum laser power was transmitted through a 589.6-nm Na line filter. The precise excitation wavelength was determined by using the Jobin-Yvon Ramanor HG-2 double monochromator. Multichannel detection was accomplished with a Princeton Applied Research OMA with Model 1420 intensified diode array cooled to 0 OC in conjunction with Model 1218 de- tector controller and Model 1215 console.

Results and Discussion Figure 1 shows cyclic voltammograms for TTF on both Pt and

n-GaAs. In the dark, essentially no oxidation takes place on n-GaAs. In the light, however, the oxidation potential is shifted negative of its value on Pt by ca. 0.4 V. The photooxidation of TTF at illuminated n-GaAs can be expressed as follows:

TTFO + h+ + TTF+* E, = -0.04 VS. SCE (1) (photogenerated hole)

The first anodic peak occurs on Pt at +0.34 V vs. SCE. Following a procedure outlined by W r i g h t ~ n , ~ ~ the anodic peak potentials on n-GaAs and Pt are compared to give an observed photovoltage of 0.38 v. Theoretically, the photovoltage ( vphoto) is a function of the redox potential as follows:

I/photo = Ivfb - Vredonl (2) where V, is the flat-band potential and Vrdox is the electrochemical potential of the electrolyte in contact with the SC electrode. Bard reports V, for n-GaAs in 0.1 M TBAP/ACN as -1 .O f 0.2 V vs. SCE.33 If we take Vredox = E, = +0.34 V vs. SCE for the TTI?/TTF+. oxidation at Pt in the same medium, the calculated

(32) S. Tanaka, J. A. Bruce, and M. S. Wrighton, J . Phys. Chem., 85,

(33) P. A. Kohl, and A. J. Bard, J. Electrochem. Soc., 126, 59 (1979). 3778 (1981).

2 0 t

tl I I I I I I I 250 350 450 550 650

WAVELENGTH inmi Figure 2. Absorption spectra of TTI? and TTFt..

photovoltage is 1.3 f 0.2 V vs. SCE. The discrepancy between the observed and calculated values may be due to Fermi-level ~innning.3~93~

The RRSE experiment consists of two parts: (1) steady-state RRS of photoelectrogenerated species in the diffusion layer and (2) transient RRS. TTF+. was detected in the diffusion layer adjacent to the n-GaAs electrode surface by modulating the ap- plied potential between the limits indicated by the vertical arrows on Figure 1B. When a 50% duty cycle is used, a steady-state diffusion layer of TTF'. is produced, the thickness ( d ) being determined by the diffusion coefficient (0) for TTP. and the pulse width ( 7 ) of the modulation approximately expressed by'

d - (07 ) ' / 2 (3) If D is taken to be 1 X cm2 s-' and T is 50 ms, the diffusion layer thickness is calculated to be ca. 7 pm. TTF+. in this very thin layer can be selectively detected by exciting resonance Raman scattering with either 457.9- or 580-590-nm laser radiation, as these wavelengths correspond to the two visible absorption bands on the cation radical. The neutral species (TTFO) in the large volume of bulk solution will not be detected since it is resonant only at laser excitation wavelengths shorter than 375 nm as seen in Figure 2.36

Figure 3 shows resonance Raman spectra of electrogenerated TTF+. in bulk solution at two excitation wavelengths corresponding to the visible absorption bands of TTF'.. With 457.9-nm exci- tation, a band at 1427 cm-' dwarfs all other spectral features. This was selected as the TTF'. specific band to monitor in the second part of the RRSE experiment. Excitation at 580.0 nm, however, results in resonance enhancement of additional modes and the resonance Raman spectrum of TTF'. is no longer essentially a single-band spectrum.36 For comparison, a spectrum of TTFO solid excited at 600.0 nm is included in Figure 3. Because the spectra of TTF+. and TTFO are so different, these species could be se- lectively detected in a mixture even if they did not exhibit very different resonance enhancement behavior.

Figure 4 shows results from the first part of the RRSE ex- periment. When the applied potential was fixed at -0.8 V vs SCE, only bands due to the solvent are observed (Figure 4B). However, with potential modulation, the 1427-cm-' band due to the central C=C totally symmetric stretching mode of TTF+. was observed (Figure 4A). When the potential modulation was stopped, TTF'. diffused away from the electrode surface and the 1427-cm-' band was no longer observed (spectrum 4A was obtained before spectrum 4B).

(34) A. J. Bard, A. B. Bocarsly, F. R. Fan, E. G . Walton, and M. S.

(35) G . Nagasubramanian, B. L. Wheeler, F.-R. Fan, and A. J. Bard in

(36) Mary R. Suchanski, Ph.D. Dissertation, 1977, Northwestern Univ-

Wrighton, J . Am. Chem. SOC., 102, 3671 (1980).

Proceedings of the 160th Meeting of the Electrochemistry Society.

ersity, Evanston, IL.

RRSE of Semiconductor Electrodes The Journal of Physical Chemistry, Vol. 88, No. 12, 1984 2449

r 1 4 2 7

T T F t 4579 A

T T F ' 6000 A

I ' I ( I I 1 ' I 2200 1800 1400 1000 600 200

Figure 3. Resonance Raman spectra of electrogenerated TTF+- and T T P solid. The TTF+- (1 .O mM) was electrogenerated by controlled potential coulometry at +0.5 V vs. SCE in 0.1 M TBAP/ACN at a Pt electrode. Top: laser power = 70 mW, monochromator band-pass = 2 cm-', scan rate = 0.333 A/s. Center: laser power = 90 mW, mono- chromator band-pass = 3 cm-', scan rate = 0.333 A/s. Bottom: laser power = 30 mW, monochromater band-pass = 1.2 cm-', scan rate = 0.333 A/s.

CM-I

tj s

A

A I \

, / / l l ~ l I , I l I l l l I l l , , , , , I , , I

1500 1400 1300

WAVENUMBERS IN C M - l Figure 4. Resonance Raman spectra taken at a n-GaAs surface in 2.5 mM TTF/O.l M TBAP/ACN. A,, = 457.9 nm, laser power = 100 mW, monochromator band-pass = 4 cm-', scan rate = 1 cm-'/s. A. Potential was modulated from -0.8 to +0.1 V vs. SCE. The duty cycle was 50%, the period was 100 ms. B. Potential was held constant at -0.8 V vs. SCE.

When an excitation wavelength corresponding to the lower energy absorption band of TTF+. was used in the RRSE exper-

I ' I , , , , , , 1 1 1 , , , 1600 1400 1200 1000 800 60c' 4[

AAVENUMBERS I N C M - l Figure 5. Resonance Raman difference spectrum taken at a n-GaAs surface in 2.5 nM TTF/O.l M TBAP/ACN by using multichannel de- tection. A,,, = 592.8 nnm, laser power = 75 mW, resolution = 0.09 nm/channel, accumulation time = 60 s. A spectrum taken at -0.8 V, vs. SCE was subtracted from a spectrum taken while modulation the potential from 1.2 to +0.1 V vs. SCE (duty cycle = 5055, period = 100 ms) .

P t

B

0 100 200 300

TIME (rnSEC1

n-GaAs

0 100 200 300

TIME CrnSEC) Figure 6. Resonance Raman transients observed in the diffusion layer of (A) n-GaAs electrode and (B) Pt electrode. A,,, = 457.9 nm, laser power = 100 mW, monochromator band-pass = 4 cm-', 600 scans av- eraged. iment, a new set of resonance Raman bands was observed which arise from the C-S stretching mode. Figure shows a RRSE spectrum of TTF . excited at 592.8 nm. This spectrum is actually a difference spectrum obtained by subtracting a spectrum taken at -0.8 V vs. SCE from one obtained while modulating the po- tential between -0.8 and +0.05 V vs. SCE. These spectra were obtained in 1 min by using multichannel detection. In addition to the 1427-cm-I band, a new fundamental mode was observed at 5 11 em-' along with overtones at 1020 and 153 1 cm-'. Also, the band at 757 cm-' increases in magnitude with 592.8-nm excitation. The negative going solvent peak at 920 cm-' is an artifact of the subtraction and due to the fact that, a t -0.8 V vs SCE, Raman scattering from acetonitrile is more intense than when TRF'. is present in the diffusion layer because of the lack of absorption by TTF'..

The transient part of the RRSE experiment involved monitoring the intensity of the 1427-cm-' band with time during and im-

2450 The Journal of Physical Chemistry, Vol. 88, No. 12, 1984 Van Duyne and Haushalter

0

m 1

n G a A s

* /

z .;/ .. / * * . '.

/ / * * ..

P t

Figure 7. Plots of (rn..e'/P)

RRI vs. t ' / * and RRI vs. O1/z. = ( t - T ) I / ~ for

mediately following a double potential step between the same potential limits used in the steady-state RRS experiment. These potential limits correspond to diffusion-controlled conditions. It is possible to perform this experiment under conditions in which the electrode reaction rate is controlled by the heterogeneous electron transfer rate; however, for preliminary studies, the dif- fusion control regime was used to check the validity of several assumptions (which have been enumerated in ref 2). It was also necessary to ensure that the absorption of laser radiation by n-GaAs does not cause thermal convection problems.

Results for the second part of the RRSE experiment are shown in Figure 6 . Resonance Raman intensity (RRI) transients are shown for both Pt and n-GaAs. These RRI transients were obtained in 10 min and represent an average of 600 single-shot measurements. The signal-to-noise ratio can be improved by simply averaging more measurements. The duty cycle and fre- quency used for these experiments were 5% and 1 Hz, respectively.

It is clear from both the cyclic voltammograms and the RRI transients that the reverse reaction (rereduction of TTF'. on the reverse potential step or scan) occurs more slowly on n-GaAs than on Pt. When the RRI data for the forward step is plotted vs. t1l2, both the Pt and n-GaAs system give a linear plot indicating diffusion-controlled behavior (see Figure 7). For the reverse step, however, only Pt shows diffusion-controlled behavior (Le., gives a linear plot for RRI vs. = t1 I2 - ( t - 7) ' i2 . For the n-GaAs system, the reverse reaction is kinetically slow and the rate of the heterogeneous electron transfer rather than the rate of diffusion of molecules to the surface is the rate-limiting step in the electrode reaction. Figure 6 shows the plots of RRI vs. t1I2 and 81/2 for both Pt and n-GaAs.

There are several reasons for obtaining RRI transients. First, all Faradaic processes including preceding and following chemical steps can be selectively monitored without interference from a capacitive contribution. The information available from RRI transients is analogous to that of chronocoulometry; however, a much higher level of specificity is afforded by the Raman detection technique. The signal vs. time format of the data allows for easy identification of processes which are not mass transport controlled. And finally, it is conceivable that a RRI transient for an un- characterized system could be compared to the current vs. time

where

both

nGaAs

4 *. . jY

1 , i , , , , , , , , , , , , , , , , , , , , 3 4 5 6 7

91/e (rn.ecl/?

500

2 a:

Pt

2 4 6 g1/2 ( m r . c l / * )

Pt and n-GaAs. T = 50 ms.

2500 1 '1 I

I I I I I 1 1 , I , I

1500 1400 1300

AAVENUMBERS IN CM-l Figure 8. Raman spectrum taken at the surface of a partially passivated n-GaAs electrode in 2.5 mM TTF/O.l M TBAP/ACN. The potential was modulated from -0.8 to +0.1 V vs. SCE (duty cycle = 50%, period = 100 ms). A,, = 457.9 nm, laser powder = 100 mW, monochromator band-pass = 4 cm-', scan rate = 1 cm-'/s.

behavior of well-characterized electrochemical systems thereby providing some insight into what processes may be occurring.

It should be noted that, for n-GaAs, the noise in the RRI transient signal increased for the reverse potential step. This may be due to photocorrosion of the electrode over the 10-min data collection period. If photocorrosion were to affect the rereduction more than the photooxidation, the spread in the data points could

J. Phys. Chem. 1984, 88, 2451-2455 245 1

be due to slightly different rates of reaction at the beginning and end of the data collection period.

Diffusion-controlled behavior for Pt and for the forward reaction on n-GaAs indicate that the assumptions made in ref 2 are valid for these systems. It is also safe to assume that thermal convection is not a problem. The fact that the reverse reaction on n-GaAs is not diffusion controlled is a consequence of the electrode kinetics and not thermal convection.

Figure 8 shows the spectrum for a n-GaAs electrode which has become passivated (at the spot where the laser is focussed). That is, the rate a t which TTF'. is being generated photoelectro- chemically is greatly diminished. This is clear from the relative intensities of the 1427-cm-' band due to TTP. and the acetonitrile band at 1370 cm-'. The overall quality of the spectrum is inferior to that of an unpassivated electrode suggesting a change in the optical properties (i.e., reflectivity) of the electrode surface. Such a change is often apparent by simply visually noting the intensity of the Raleigh scatter from the surface of the SC electrode. Also, a large background has grown into the spectrum upon passivation. This background does not change with excitation wavelength and is therefore not fluorescence. Moving the laser beam to a new spot on the electrode surface cause the background (and passi- vation) to disappear temporarily. This implies that the background is due to a passivating film on the electrode. Often, Raman scattering from bulk phonons of the SC electrode will decrease or completely disappear upon passivation or photocorrosion of the SC electrode.2g Thus, RRSE can supply information about passivation and photocorrosion processes. Following spectral changes described above as a function of time by use of multi- channel detection should provide important information about photocorrosion which could conceivably aid in development of more stable PECs. Conclusion

It has been shown that RRSE can be successfully applied to

the study of a photoelectrochemical system. It is possible to not only gain kinetic information about a SC-electrode reaction but also to monitor changes in the SC electrode surface. The potential of RRSE for the study of photoelectrochemical systems has been clearly demonstrated. The superior specificity of the technique is indispensible for sorting out the various processes which can occur simultaneously at a photoelectrode. Although RRSE with visible lasers requires colored reaction products which may be undersirable for a truly viable PEC, RRSE is still quite applicable to the study of certain model PECs. Future work to establish UV or near-IR RRSE may overcome this limitation.

As will be reported in a future paper,29 RRSE can be applied to the study of SC electrodes which have been pretreated in an attempt to improve reaction kinetics, prevent photocorrosion, and decrease e-h+ pair recombination at the surface. In addition, experiments in which reaction products are monitored at the surface of silver modified GaAs electrodes (Ag/GaAs) will be described. Ag overlayers on n-GaAs have only a minimal effect on photoelectrochemistry while allowing detection and molecular characterization of adsorbed species by surface-enhanced reso- nance Raman spec t ro~copy .~~

Acknowledgment. We thank Brian K. Johnson for computer programs enabling the acquisition of RRI transients and for general assistance. The support of this research by the Office of Naval Research (Contract No. N00014-79-C-0369) is gratefully acknowledged. We also thank Brian E. Miller and John Roper for their assistance in obtaining the OMA data.

Registry No. GaAs, 1303-00-0; Pt, 7440-06-4; TTF, 31366-25-3; TTF'., 35079-56-2.

(37) R. P. Van Duyne and J. P. Haushalter, J . Phys. Chem., 87, 2999 (1983).

Fluorescence of Rhodamine B on Semiconductor and Insulator Surfaces: Dependence of the Quantum Yield on Surface Coverage

Y. Liang, P. F. Moy, J. A. Poole, and A. M. Ponte Goncalves*

Department of Chemistry, Temple University, Philadelphia, Pennsylvania 191 22 (Received: October 24, 1983; In Final Form: February 24, 1984)

The fluorescence quantum yield of rhodamine B deposited on semiconductor (indium oxide) and insulator (glass) surfaces was measured as a function of surface coverage. At high coverage the quantum yield is low on both types of surface, but somewhat lower on the semiconductor. As the surface coverage approaches zero the quantum yield increases by more than one order of magnitude on glass; the increase is much smaller on the semiconductor. The results are discussed in terms of injection and energy transfer quenching.

Introduction Adsorbed dyes may be used to sensitize large bandgap n-type

semiconductors to visible and thus make them potentially interesting as electrodes in photoelectrochemical cells for solar energy conversion.3 Sensitization may occur if the dye excited state is above the conduction band of the semiconductor, in which case the dye injects an electron into the semiconductor and be-

(1) H. Gerischer and F. Willig, Top. Current Chem., 61, 31 (1976). (2) H. Gerischer, M. T. Spitler, and F. Willig in 'Proceedings of the Third

Symposium on Electrode Processes 1979", S . Bruckenstein, Ed., The Elec- trochemical Society, Princeton, NJ, 1980, p 1 1 5.

(3) A. Heller, Acc. Chem. Res., 14, 154 (1981).

comes oxidized. The overall conversion efficiency, q5 = [(number of electrons detected)/(number of photons absorbed)], is governed by the following: (1) competition between the desired injection of an electron into the semiconductor and any other processes which also shorten the lifetime of the dye excited state; (2) com- petition between the successful escape of the electron into the bulk of the semiconductor and undesirable processes such as trapping at surface states and recombination with the oxidized dye. Thus: 4 = where 4i is the injection efficiency and & is the escape efficiency.* In an attempt to examine the processes which de- termine q5i, we have recently measured the fluorescence lifetime, 7, of dyes deposited on semiconductor and insulator surfaces. For rhodamine B, 7 = 55 ps on indium oxide and 7 = 46 ps on glass,4

0022-3654/84/2088-2451$01.50/0 0 1984 American Chemical Society