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Electrolyte Electroreflectance Study of the Oscillatory Hydrogen Peroxide Reduction on n-GaAs M. T. M. Koper,*,† A. M. Chaparro,‡,§ H. Tributsch,‡ and D. Vanmaekelbergh| Laboratory of Inorganic Chemistry and Catalysis, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands, Hahn-Meitner Institut, Abteilung Solare Energetik, Glienicker Strasse 100, D-14109 Berlin, Germany, and Debye Institute, Utrecht University, P.O. Box 80.000, 3508 TA Utrecht, The Netherlands Received January 13, 1998. In Final Form: March 26, 1998 The oscillatory reduction of hydrogen peroxide on an n-type gallium arsenide electrode was studied by means of the electrolyte electroreflectance technique. It is found that during the current oscillations both the reflectivity and electroreflectance oscillate with qualitatively different patterns. A simple model, which attributes the current oscillations to an anomalous dependence of the band bending in the semiconductor depletion layer on the potential drop across the semiconductor/electrolyte interface, is considered for comparison. It is concluded that the absolute reflectivity detects a slow variable of the system, most likely related to the chemical composition of the surface, and the electroreflectance detects a fast variable of the system, most likely related to the interfacial potential distribution.
1. Introduction The occurrence of spontaneous current and potential oscillations in electrochemical systems has attracted the attention of electrochemists for many decades. In recent years, many advances in our understanding of these phenomena have been made,1,2 to a large extent owing to the “revolutionary” developments in nonlinear dynamics and chaos theory and their application to chemically reactive systems.3 Most of the experimental examples of electrochemical oscillations concern reactions at metal electrodes. However, there are a few examples of electrochemical oscillations at semiconductor electrodes.4-11 These oscillations are of interest, since they exhibit phenomena not observed at metal electrodes, such as photocurrent oscillations and oscillatory light emission.9 In addition, the potential distribution across the semiconductor/electrolyte interface is markedly different from that across the metal/electrolyte interface. Typically, for a semiconductor electrode in a concentrated electrolyte, the potential difference across the semiconductor/electrolyte interface is accommodated partly across the depletion layer in the semiconductor and partly across the Helmholtz layer in the electrolyte. In this respect, it was recently suggested that the * To whom correspondence should be addressed. † Eindhoven University of Technology. ‡ Hahn-Meitner Institut. § New address: Instituto de Energias Renovables, CIEMAT, 28040 Madrid, Spain. | Utrecht University. (1) Hudson, J. L.; Tsotsis, T. T. Chem. Eng. Sci. 1994, 49, 1493. (2) Koper, M. T. M. Adv. Chem. Phys. 1996, 92, 161. (3) Scott, S. K. Chemical Chaos; Clarendon Press: Oxford, 1991. (4) Tributsch, H. Ber. Bunsen-Ges. Phys. Chem. 1975, 79, 570. (5) Gerischer, H.; Lu¨bke, M. Ber. Bunsen-Ges. Phys. Chem. 1988, 92, 573. (6) Marcu, V.; Tenne, R. J. Phys. Chem. 1988, 92, 7089. (7) Cattarin, S.; Tributsch, H. J. Electrochem. Soc. 1992, 139, 1328. (8) Chazalviel, J.-N.; Ozanam, F.; Etman, M.; Paolucci, F.; Peter, L. M.; Stumper, J. J. Electroanal. Chem. 1992, 327, 343. (9) Koper, M. T. M.; Meulenkamp, E. A.; Vanmaekelbergh, D. J. Phys. Chem. 1993, 97, 7337. (10) Koper, M. T. M.; Vanmaekelbergh, D. J. Phys. Chem. 1995, 99, 3687. (11) Pohlmann, L.; Neher, G.; Tributsch, H. J. Phys. Chem. 1994, 98, 11007.
oscillations occurring during the hydrogen peroxide reduction at n-GaAs electrodes are caused by what was termed a “band bending anomaly”.9,10 In a certain region of the voltammogram, a negative shift in the applied potential does not lead to a decrease in the semiconductor band bending, as expected for an n-type semiconductor/ electrolyte interface,12 but to an increase in the band bending. This remarkable phenomenon (which is not to be confused with the more commonly known effect of Fermi level pinning (in which the band bending does not change with applied potential, as all changes in applied potential are accommodated across the Helmholtz layer) leads to a decrease in the reduction current and to negative impedance characteristics of the interface.10 In combination with a sufficiently large Ohmic drop, either internal or external to the cell, this phenomenon may lead to oscillations in the cathodic current. Impedance measurements gave support to the explanation of the oscillations in terms of this band bending anomaly.10 However, independent evidence for this phenomenon would be desirable. The simplest and most suitable technique in this respect is electrolyte electroreflectance (EER), which has been used by many workers in order to gain information on the potential distribution across the semiconductor/ electrolyte interface.13-16 In this paper, we apply the EER technique to investigate the n-GaAs/H2O2 interface and show that the technique can be used to follow electrochemical oscillations at semiconductor electrodes. By measuring the electroreflectance at varying frequency of the potential modulation, we show the existence of (at least) two contributions to the EER signal, one stemming from the semiconductor space charge layer and one stemming from surface compounds. The results will be discussed in relation to the model suggested previously. (12) Morrison, S. R. Electrochemistry at Semiconductor and Oxidized Metal Electrodes; Plenum: New York, 1990. (13) Cardona, M. Modulation Spectroscopy; Academic Press: New York, 1969. (14) Tomkiewicz, M. J. Electrochem. Soc. 1979, 126, 1505. (15) Hamnett, A.; Lane, R. L.; Trevellick, P. R.; Dennison, S. In Comprehensive Chemical Kinetics; Compton, R. G., Hamnett, A., Eds.; Elsevier: Amsterdam, 1989; Vol. 29, p 385. (16) Salvador, P. Electrochim. Acta 1992, 37, 957.
S0743-7463(98)00059-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/10/1998
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2. Experimental Section The experiments were carried in a conventional three-electrode electrochemical cell, controlled thermostatically. The working electrode was an n-type GaAs(100) single crystal (MCP Electronic Materials Ltd. (U.K.)) having a doping level of 2 1017 cm-3. The electrolyte solution was either a 0.2 or 1.0 M H2O2 solution in 1 M H2SO4. The counter electrode was a platinum wire and the reference electrode a mercurous sulfate (0.5 M) electrode (MSE). All potentials are referred to the MSE. The experimental setup for the EER measurements has been described elsewhere.17 The dc potential V of the cell was controlled by a Wenking potentiostat (POS 73). The electrode surface was illuminated under normal incidence with a halogen lamp. A monochromator (ORIEL) was used to obtain EER spectra, and an EG&G PARC 5210 lock-in amplifier was used for signal detection. The alternating voltage ∆V ˜ (either 15, 40, 120, or 200 Hz modulation frequency) superimposed on the dc bias voltage had an amplitude ∆V of 200 mV.
3. Theoretical Preliminaries In this section, we discuss a few concepts mainly related to the interpretation of the EER spectra that are necessary to understand the results and their discussion to be described in the next section. First, we want to emphasize that the existence of an Ohmic drop either internal or external to the electrochemical cell plays an important role in the generation of electrochemical current oscillations.2 It is therefore important to distinguish, from the outset, between the applied potential V and the actual interfacial potential φ. Under oscillatory conditions, V is fixed but φ oscillates. In the absence of an Ohmic drop, or under stationary conditions, they are equal or differ by a constant. By ∆V or ∆φ, we denote the amplitude of the modulation of the corresponding quantity, and by ∆V ˜ and ∆φ˜ we denote the time-dependent modulation itself. Electrolyte electroreflectance spectra result from the effect of an electric field on the optical properties of a solid or a semiconductor in the surface region. During an EER experiment, the normalized field-induced change in reflectivity ∆R/R is measured by application of a periodic modulation of the interfacial potential ∆φ˜ ) ∆φ sin(ωt) and studied as a function of the wavelength of the light λ ) c/ν, the modulation frequency ω, and the dc electrode potential φ. For a semiconductor, the modulation of the field in the space-charge layer (SCL) yields a relatively simple third-derivative spectrum in which sharp signals appear corresponding to direct transitions. This semiconductor EER signal can be approximated by a formula derived by Aspnes18
(∆R R)
SCL
)-
2e0ND∆φsc Ln(pv) 0sc
(3.1)
where ∆φsc is the amplitude of the band bending modulation ()∆φ if no surface states are present), ND is the donor density, Ln(pν) is the third-derivative line shape function, and the other symbols have their usual meaning. Formulas for the line shape Ln(pν) may be found in Aspnes’ original paper;18 the important point to note here is that the Aspnes equation does not predict the line shape to depend on the dc potential but only on the wavelength of the light. It is important to remark that this formula is derived for low fields in the semiconductor and high lifetime broadening of the electron energy. (17) Chaparro, A. M.; Salvador, P.; Coll, B.; Gonzalez, M. Surf. Sci. 1993, 293, 160. (18) Aspnes, D. E. Surf. Sci. 1973, 37, 418.
Surface states present at the interface may influence the interfacial potential distribution. For the semiconductor/electrolyte interface, Tomkiewicz et al.19 have modified the Aspnes equation in order to account for changes in the field modulation across the Helmholtz layer, due to the potential-dependent emptying and filling of surface states. For sufficiently low frequency, ∆φsc ) ∆φ - ∆φH ) ∆φ - e0∆Nss/CH, so that eq 3.1 becomes
( ) ∆R R
SCL
)-
[
]
2e0ND e0∆Nss Ln(pν) 1 ∆φ (3.2) 0sc CH∆φ
where φH is the Helmholtz layer potential, e0∆Nss is the amplitude of the modulation of the charge stored in the surface states, and CH is the Helmholtz layer capacity. For finite modulation frequencies, ∆Nss and hence ∆R become frequency dependent and ∆R ˜ (ω)SCL will exhibit a frequency-dependent intensity and a phase shift with respect to ∆φ˜ (ω). In the low-field region, (∆R ˜ (ω)/R)SCL is ˜ ss(ω)/CH] and still proportional to ∆φ˜ sc(ω) [)∆φ˜ (ω) - e0∆N the equations will be similar to those derived for the electrical impedance response of a semiconductor/electrolyte interface.20 Tomkiewicz’s equation predicts that the EER signal may depend on the dc potential if ∆Nss depends on the dc potential. In addition to changing the potential distribution at the semiconductor/electrolyte interface, surface states may also more specifically change the optical properties of the surface layer and hence the reflectivity.21 Therefore, the measured EER signal may consist of two contributions, one due to electroreflectance from the semiconductor space-charge layer and the other due to electroreflectance from the surface (“surf”):
∆R ∆R ) R R
( )
SCL
+
(∆R R)
surf
(3.3)
Note that surface states play a dual role in the EER signal. First, they change the potential distribution across the interface and thereby the (∆R/R)SCL signal. Second, they may introduce a new EER surface signal (∆R/R)surf. At sufficiently high modulation frequencies, the charge relaxation in the surface states does not follow the modulation so that ∆N ˜ ss(ω) ) 0. Under these circumstances, we expect ∆φ to tend to ∆φsc and to be independent of the dc applied potential V. Hence, according to eq 3.2, the EER signal should be independent of the applied dc potential V. For the same reason, we expect the (∆R ˜ (ω)/ R)surf contribution to drop out at high modulation frequency. Hence, by varying the modulation frequency, it should be possible to differentiate between the different EER signals. Electroreflectance at variable frequency is a valuable tool for the study of complex semiconductor/ electrolyte interfaces and has already been applied to the study of the RuS2/electrolyte interface.22 We note that Hamnett et al.23-25 have studied the EER spectra of GaAs/electrolyte interfaces more carefully and found that, in general, the Aspnes equation did not (19) Tomkiewicz, M.; Siripala, W.; Tenne, R. J. Electrochem. Soc. 1984, 131, 736. (20) Vanmaekelbergh, D. Electrochim. Acta 1997, 42, 1135. (21) McIntyre, J. D. E.; Aspnes, D. E. Surf. Sci. 1971, 24, 417. (22) Chaparro, A. M.; Alonso-Vante, N.; Salvador, P.; Tributsch, H. J. Electrochem. Soc. 1997, 144, 2991. (23) Abrantes, L. M.; Peat, R.; Peter, L. M.; Hamnett, A. Ber. BunsenGes. Phys. Chem. 1987, 91, 369. (24) Batchelor, R. A.; Brown, A. C.; Hamnett, A. Phys. Rev. B 1990, 41, 1401. (25) Hamnett, A.; Gilman, J.; Batchelor, R. A. Electrochim. Acta 1992, 37, 949.
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Figure 2. EER voltammograms at λ ) 600 nm and various modulation frequencies (15, 40, 120, 200 Hz). n-type (100) GaAs in contact with 1 M H2O2 in 1 M H2SO4, T ) 25 °C: scan rate, 10 mV s-1. The inset shows the simultaneously recorded current.
Figure 1. (a) EER spectra at Vdc ) -0.99 V for the modulation frequencies 15, 40, 120, and 200 Hz. (b) EER spectra at Vdc ) -0.50 V for the modulation frequencies 15 and 200 Hz. All other experimental parameters as in part a.
adequately describe all features of the system. Although for the n-GaAs/0.1 M KOH interface they found the EER signal to be proportional to the modulation amplitude, the signal was found to depend on the dc potential under conditions where this is not expected according to the Aspnes equation.23 One may attribute this effect to a potential-dependent line shape function. Clearly this fact will further complicate an unambiguous interpretation of the results, as there are two possible reasons for a dependence of the EER signal on the dc potential: surface states and deviations from low-field conditions. A third possible cause would of course be a potential-dependent (∆R/R)surf signal. 4. Results and Discussion EER spectra in the energy range 1.6-2.8 eV, for four different frequencies and for two different potentials, are shown in Figure 1. The corresponding voltammograms for λ ) 600 nm (2.07 eV) are shown in Figure 2; the current-potential scan is shown in the inset. For V ) -0.5 V, where no cathodic current is flowing, it is seen that the EER spectra (Figure 1b) do not depend on the modulation frequency, which is clear evidence for the absence of charge relaxation in surface states at this potential. For V ) -0.990 V, the EER spectra depend strongly on the modulation frequency (Figure 1a). For 120 and 200 Hz the spectra are almost the same, indicating that the surface states responsible for the strong EER signal at this potential cannot follow the 200 Hz modulation frequency. This implies that at 25 °C for ω > 200 Hz modulation occurs primarily in the SCL. To study the potential dependence of the surface signal, the EER at
600 nm, where the semiconductor signal at 200 Hz is not very strong, was measured at the four different frequencies. In the potential region from -0.3 to -0.6 V, Figure 2 shows that ∆R/R does not change with V, in agreement with impedance measurements which show Mott-Schottky behavior in this potential region.10 In the potential region negative with respect to -0.6 V, we observe a pronounced influence of the modulation frequency on the electroreflectance. At 15 and 40 Hz a significant surface contribution (∆R/R)surf shifts the EER signal abruptly to negative values. At 120 and 200 Hz the surface component is small. It seems reasonable to assume that at 200 Hz the signal is almost entirely due to electroreflectance from the SCL. From the results at 200 Hz it can be clearly seen that the potential distribution across the depletion and Helmholtz layers changes drastically at around -0.85 V (negative-going scan) and -0.65 V (positive-going scan). These are precisely the potentials at which maxima are observed in the current-potential scan, suggesting that a redistribution of the interfacial potential triggers the negative slope (negative impedance) in the currentpotential curve. Next we studied the current oscillations occurring in the region of negative slope of the current-potential curve at more elevated temperature (40 °C). At this temperature, the Ohmic potential drop in the electrochemical cell can no longer be neglected and may destabilize the negative slope, resulting in spontaneous current oscillations.9,10 For this purpose we used two different types of modulation: modulation of the applied potential (i.e., electroreflectance) and modulation of the incoming light intensity by employing a chopper. This was done under four different conditions of illumination: white-light illumination, monochromatic subbandgap illumination (875 nm), bandgap illumination (840 nm), and suprabandgap illumination (688 nm). At these wavelengths the semiconductor EER (∆R/R)SCL signal exhibits a peak. The electroreflectance measurements were carried out at 200 Hz. Our measurements allowed us to follow three different signals: the absolute reflectivity R, the electroreflectance ∆R, and the photocurrent due to electronhole pair excitations caused by the illumination. Results are shown in Figures 3-6. In Figure 3 spontaneous oscillations of current, photocurrent, and reflectivity are shown for two different potentials during light modulation. The light modulation itself has hardly any influence on the current or the current oscillations but allows us to follow accurately the (small)
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Figure 5. As in Figure 4 but at λ ) 840 nm.
Figure 6. As in Figure 4 but at λ ) 875 nm. Figure 3. Simultaneous oscillations of the photocurrent Iph, total current I, and reflectivity R at (a) Vdc ) -1.08 V and (b) Vdc ) -1.06 V: n-GaAs electrode in contact with 1 M H2O2 in 1 M H2SO4, T ) 40 °C.
Figure 4. Simultaneous oscillations of the total reflectivity R, the electroreflectance ∆R/R, and the total current I at Vdc ) -0.9 V: n-GaAs electrode in contact with 1 M H2O2 in 1 M H2SO4, T ) 40 °C. Illumination with λ ) 688 nm, ∆Vdc ) 300 mV, and a 200 Hz modulation frequency.
photocurrent and reflectivity by the lock-in technique. The photocurrent oscillation is due to the creation of electronhole pairs, which are separated by the field in the semiconductor and give rise to a small positive current.
In principle, they should contain information about the strength of the field in the semiconductor (i.e. band bending), but the signal probably reflects quite a complicated interplay of various effects. In this respect, it is to be noted that the photocurrent oscillations have very different waveforms in Figure 3a and b, and also their relations to the current oscillations are quite different for the two potentials. Figures 4-6 show ∆R/R and R measured during the spontaneous current oscillations. Not surprisingly, both reflectivity and electroreflectance also exhibit oscillations. The ∆R/R signal oscillates in phase with the current and, more importantly, is observed to display sharp transitions when the current also shows sharp transitions. This implies that interfacial potential distribution and the current are directly coupled and evolve on the same time scale. Recall, however, that at 200 Hz the EER signal is mainly from the SCL, and therefore the ∆R/R oscillations mainly reflect oscillations in the band bending modulation not in the band bending itself. However, at 40 °C we also expect an enhanced contribution of the surface EER signal. The situation for ∆R/R should be contrasted with the oscillations in the absolute reflectivity R, which shows a qualitatively different oscillation pattern. It exhibits only one relatively fast change followed by a long and slow transient. Note that this R oscillation has a waveform completely similar to that of the R oscillation in Figure 3b. The R oscillation becomes less intense with progressively negative potential. By visual inspection under the optical microscope, the absolute reflectivity is observed to oscillate essentially synchronously all over the surface.
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It is of interest to compare the experimental signals with the predictions of a simple model suggested previously.10 Although the model does not explain all the different features of the experimental system, it is good enough for our illustrative purposes. Kinetic measurements by Minks et al.26,27 have suggested that H2O2 is being reduced in its adsorbed state. The adsorption is a relatively slow process. The adsorbed H2O2 takes up an electron from the conduction band, and the resulting surface radical rapidly injects a hole in the valence band. The last fact explains the experimentally observed current doubling and electroluminescence. We write the following differential equation for the surface coverage of H2O2, θHP:
dθHP Γm ) kads(1 - θHP) - keθHPns(φsc) dt
(4.1)
where Γm is the maximum surface coverage (∼1015 cm-2), kads is the adsorption rate constant, ke is the electron capture rate constant, and ns ) ND exp(-e0φsc/kBT) the surface concentration of conduction band electrons. The total interfacial potential φ is the sum of the band bending and the Helmholtz layer potential φ ) φsc + φH. In the limit of Csc/CH , 1, its time evolution is given by ref 10:
dφ V - φ + 2FkeφHPns(φsc) Csc ) dt ARΩ
(4.2)
where Csc is the space charge capacity, A is the electrode surface area, RΩ is the Ohmic resistance of the cell, F is the Faraday constant, and V is the applied potential, equal to φ only if RΩ ) 0. The Faradaic current is JF ) -2FkeφHPns(φsc), the factor of two stemming from the current doubling. As mentioned, in the potential region of the H2O2 reduction, other surface states are formed which alter the potential distribution across the interface. We associated them with the formation of a hydride layer. Most probably, protons are already adsorbed at the interface and are being reduced into adsorbed hydrogen.28,29 This charges the surface negatively and therefore increases the band bending, which is conveniently described by the phenomenological parameter γH10
φsc ) φ + γHφH
(4.3)
(which means that φH ) -γHθH). In a previous paper10 it was assumed that the reduction of adsorbed protons is a reversible process, which follows the changes in the interfacial potential essentially instantaneously. Recent impedance measurements by Uhlendrof et al.30 support this assumption that the Volmer reaction on n-GaAs is very fast. We can therefore write the hydride coverage as a parametric function of φ:10
1 θH(φ) ) 1 + (K3/ND) exp(e0φ/kBT)
(4.4)
where K3 (in cm-3) is an equilibrium constant. Equations 4.1-4.4 constitute our simple model for the H2O2 reduction (26) Minks, B. P.; Oskam, G.; Vanmaekelbergh, D.; Kelly, J. J. J. Electroanal. Chem. 1989, 273, 119. (27) Minks, B. P.; Vanmaekelbergh, D.; Kelly, J. J. J. Electroanal. Chem. 1989, 273, 133. (28) Gerischer, H.; Mu¨ller, N.; Haas, O. J. Electroanal. Chem. 1981, 119, 41. (29) Lafle`re, W. H.; Cardon, F.; Gomes, W. P. Surf. Sci. 1974, 44, 541. (30) Uhlendorf, I.; Reineke-Koch, R.; Memming, R. Ber. Bunsen-Ges. Phys. Chem. 1995, 99, 1082.
Figure 7. Steady-state current-voltage curve of the model: kads ) 2 × 10-9 cm-2 s-1 mol-1, ke ) 10-20 cm s-1, K3 ) 6 × 1013 cm-3, γH ) 0.25 V, ND ) 1018 cm-3, T ) 300 K. The bottom part shows the dependence of θH, φsc, and ∆φsc on φ for the same parameter values.
on n-GaAs. Note that we do not take into account that absorbed H2O2 and hydride may compete for the same sites. This is mainly for reasons of convenience; all that follows will remain valid if competitive adsorption is accounted for, though with somewhat different parameter values. At any rate, the surface concentration of H2O2 is probably low in the region of oscillations, and also the fact that adsorbed H2O2 is a precursor to the chemical dissolution of GaAs will keep its concentration low.26,27 As can be seen from eq 3.1, (∆R/R)SCL is proportional to ∆φsc. If we assume that the external modulation frequency is always much higher than the frequency of the intrinsic oscillations, from eq 4.3 the amplitude of modulation of the band bending ∆φsc is given by
∆φsc ) ∆φ + γH∆θH
(4.5)
(this is of course simply Tomkiewicz’ eq 3.2). Since θH(φ) is a parametric function of φ as given by eq 4.4, θH follows φ without any time delay and ∆θH is given by
∆θH(φ) ) θH(φ + ∆φ) - θH(φ)
(4.6)
Hence we can calculate the oscillations in ∆φsc from (the oscillations in) the interfacial potential φ by making use of eqs 4.4-4.6. Note that ∆φ ) ∆V. The above differential equations can be solved numerically by standard procedures. Figure 7 shows the typical current-voltage curve JF-φ and the corresponding dependence of θH, φsc, and ∆φsc on φ; Figure 8 shows a typical oscillation that can be obtained in the potential region of negative polarization slope for ARΩ ) 290 Ω cm2. It is seen that the current oscillation is similar in shape to the experimental current oscillation (the total current density is J ) (V - φ)/ARΩ). This fact is not surprising nor particularly meaningful, as it is the typical shape of a relaxation oscillation. In a relaxation oscillation, the two variables necessary to describe spontaneous oscillations
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slow variable in a relaxation oscillation. Its behavior is more reminiscent of that of the experimental reflectivity oscillation. From the above, we derive that the absolute reflectivity detects a slow variable, related to the chemical composition of the surface, whereas oscillations in current, photocurrent, and the electroreflectance (at 200 Hz) are typical for a fast variable, related to the interfacial potential or the band bending. In terms of our model, the absolute reflectivity would be high if the surface concentration of H2O2 is low. This seems reasonable, since adsorbed H2O2 is a precursor to a chemical dissolution product and therefore roughening the surface.
Figure 8. Model oscillation of J, φsc, θHP, and -∆φsc for V ) 0.18 V, ARΩ ) 290 Ω cm2, and ∆φ ) 0.2 V.
evolve on very disparate time scales.3 One variable is said to be fast, and the other is said to be slow. The fast variable in our model is the interfacial potential φ. Since both the current and the band bending are directly related to φ, their model profiles also display sharp transitions in which strong changes occur on a short time scale. Note that the oscillation in φsc shows additional maxima and minima, since it depends on φ in a more complicated manner. The oscillation in ∆φsc ∝ (∆R/R)SCL has a shape similar to that of the current oscillation, a behavior qualitatively comparable to that observed in the experimental oscillation profiles of the electroreflectance ∆R/R. The remaining difference in the experimental and theoretical oscillation profiles of respectively ∆R/R and ∆φsc can be ascribed to our model being too simplistic and to the fact that we neglected the surface contribution to ∆R/R in the model calculation. The oscillation in θHP shows a qualitatively different wave form, which is typical for the
5. Conclusion In this paper we applied the electrolyte electroreflectance (EER) technique to study the reduction of hydrogen peroxide on n-GaAs from an aqueous sulfuric acid solution. As was shown in previous papers,9,10 this system exhibits a negative impedance for certain dc potentials, which in conjunction with a sufficiently large Ohmic drop may give rise to spontaneous current oscillations. By varying the frequency of the potential modulation, we were able to discriminate between two different EER signals, one due to the semiconductor and the other due to the surface. The signal due to the surface reflects the relaxation of surface states and is suppressed at lower temperatures and higher modulation frequencies. A comparison of a simple model calculation with EER measurements at 200 Hz during the current oscillations suggests that the electroreflectance detects a fast variable of the system, related to the variations in the interfacial potential distribution due to the reversible formation of surface states which charge the surface negatively. These fast surface states are most likely due to hydride formation. The absolute reflectivity, however, detects a slow variable of the system. It has been suggested previously that the slow variable is a H2O2 surface intermediate. Acknowledgment. M.T.M.K. acknowledges financial support from the European Commission (TMR fellowship) and the Royal Netherlands Academy of Arts and Sciences (KNAW). A.M.C. acknowledges a grant from the “Ministerio de Educacı´on y Cultura” of Spain. LA9800598
