J. Phys. Chem. 1993,97, 7337-7341
7337
Oscillatory Behavior of the H202 Reduction at GaAs Semiconductor Electrodes Marc T. M. Koper,’*+Eric A. Meulenkamp,t and Daniel Vanmaekelbergbt Departments of Electrochemistry and Condensed Matter, Debye Research Institute, University of Utrecht, P.O.Box 80.000, 3508 TA Utrecht, The Netherlands Received: March 2, 1993; In Final Form: May 10, 1993
Spontaneous current and potential oscillations during reduction of hydrogen peroxide at GaAs semiconductor electrodes are described and discussed in the context of dynamical systems theory. The oscillations are shown to be accompanied by oscillations in the intensity of the electroluminescence. The oscillatory behavior is discussed on the basis of existing models for the kinetics of the H202 reduction at semiconductor electrodes.
Introduction
Experimental Section
Spontaneous current or potential oscillations accompanying electrochemicalreactionsat semiconductor/electrolyteinterfaces have been reported by a number of authors.’“ Although they are usually bracketedtogetherwith oscillationsat metal electrodes, semiconductor electrodes can give rise to some interesting new phenomena, such as photocurrent ~ s c i l l a t i o nand s ~ ~oscillatory ~~~ light emission. The commonly expressed belief that their physicochemical interpretation is essentially similar to that of oscillations at metal electrodesdoes therefore not seem evidently justified, and so far no detailed explanation has been given for any of the observed oscillations at semiconductor electrodes. In this paper, we report on the oscillatory behavior observed during hydrogen peroxide reduction at GaAs semiconductor electrodes at elevated temperatures (40-50 “C) and H202 concentration 1.0 mol dm-3. This system is interesting as its kinetics has been studied in detail under less severe conditions under which no oscillations are ~bserved.~ At room temperature and lower H202 concentrations, a potential-independentlimiting current due to the reduction of H202 was found, which was much smaller than expected for the case of diffusion limitation. The limiting current was attributed to the potential-independent adsorption of HzO2 onto the GaAs surface. Under the conditions described in this investigation, a negative faradaic impedance and spontaneouscurrent oscillations occur in the potential region (-0.61-1.O V vs SCE) where a potential-independent current is observed under the conditions previously investigated.9 We have studied the H202 reduction at n-GaAs under potentiostatic and galavanostatic conditions and under the condition of a fixed potential across the electrochemical cell and a series resistor. Current or potential oscillations occur in all three operation modes. It will further be shown that the current or potential oscillations are accompanied by oscillations in the intensity of the luminescence. This so-called electroluminescence originates from recombination-in the bulk of the semiconductor-of free holes, injected during the H202 reduction, with the majority carriers, i.e., conduction band electrons. In the case of illuminated p-type GaAs, photocurrent oscillations are observed. The paper concludes with a more detailed discussion of the H202 reduction kinetics at GaAs. It is worth mentioning that the observation of current or potential oscillations during H202 reduction in itself is not new. Other authors have described them for Cu~FeS4,lCuInSe2,6 Hg,lo, P t l l J 2 Ag,” and Au14 electrodes, the first two also being semiconductors.
The experiments were carried out in a thermostatically controlledelectrochemicalcell in a three-electrodeconfiguration. The working electrode consisted of a GaAs single crystal (3.85 mm in diameter) glued onto a copper substrate and embedded in a PVC holder, which was screwed onto a rotating disk unit. Both n- and p-type (100)material, having doping levels of (2,02.2)X 1017cm-’ (Si) and (1.0-3.0) X 1017~m-3(Zn),respectively, were purchased from MCP Electronic Materials Ltd. (UK). The counter electrode was a large platinum sheet, and the reference electrodewas a saturated calomel electrode(SCE). All potentials are given with respect to the SCE. Potentiostatic control was performed by a Wenking ST72 potentiostat, connectedto a Wenking Model VSG83 voltagescan generator. For galvanostatic experiments, the potentiostat was used in the galvanostat mode.15 For impedance measurements a Solartron 1250 frequency analyzer was used in combination with a Solartron 1286 electrochemical interface. In a number of experiments,a silicon photodiodewas installed underneath the cell, about 3 cm from the working electrode surface. For illumination of the p-type electrode, a white light source (Schott KL 1500) was employed in combination with a glass fiber light cable. In all experiments, the electrolyte solution was 1.0 M H202 (Merck, 30% perhydrol) in 1.0 M H2S04 (Merck). From impedance measurements at sufficiently high frequencies (>50 kHz), the ohmic cell resistance was found to be 12 Q. During the experiments, the electrode rotation rate was 1000 rpm and the cell temperature was controlled at 40 OC (unless otherwise stated).
* To whom correspondence should be addressed. t
Department of Electrochemistry. Department of Condensed Matter.
Results n-TypeGaAs: Potentiostatic Operation. In Figure 1 a typical current-voltage scan is given for n-type GaAs in the dark. The light emitted by the electrode, detected as a photodiode current, is also shown. It should be noted that, in the potential range between -0.7 and -0.9 V, light emission also occurs, but with a much smaller intensity than in the potential range negative with respect to -0.9 V. The flatband potential is determined in the potential region where H202 is not reduced (-0.3/ 1.OV). Linear MottSchottky plots are observed at all frequencies(10-1 05 Hz), which converge to a common x-axis intercept from which the flatband potential is extrapolated to be -1 V, as indicated in Figure 1. Since the currents are not appreciably affected by the rotation rate of the working electrode,the process is under kinetic control.16 In the forward scan, in the negative potential direction, oscillations are usually not observed until the potential reaches -1 .O V. The sudden increase in current at this potential is due to hydrogen evolution and enhanced H202 reduction. In the
0022-3654f 9312097-1337%04.00/0 0 1993 American Chemical Society
Koper et al.
7338 The Journal of Physical Chemistry, Vol. 97, No. 28, 1993 -1 .o
V I V (SCE)
0
-0.5
I/mA
I
","
Figure 1. Cathodic current-potential scan for an n-GaAs electrode in 1.0 M H202, 1 M H2SO4 solution at 40 "C. Scan rate 2 mV/s. Also shown is the light emission of the electrode detected as a photodiode
current.
X
I
I,,,,(N
-0.77V
Figure 3. (a) Oscillatorycurrent time series at the indicated potentials illustrating the transition from monoperiodic( V i-0.90 V), to biperiodic (V= -0.89V), to chaotic (V = -0.88 V) oscillations. (b) Corresponding
time evolution of the luminescenceintensity. (c) A next-amplitudemap obtainedby plotting the (absolute)currentmaximaat V =-0.88 V against the previous maximum; the dashed curve is the one-dimensional map expected in the case of deterministicchaos (n-GaAs, 1 M H202 + 1 M H2SO4, 40 "c).
V / V (SCE)
-0.95
-0.90
-0.85
-0.80
-0.75
-0.70 -2
-3
Figure 2. Some typical oscillatorytime series observed at the indicated potentials in the region where the negative impedance is observed (nGaAs, 1 M H202 + 1 M H2SO4,40 "C).
reverse scan, going from -1.1 to 0.0 V, a negative differential resistance is observed, and strong current oscillations appear in the potential range -0.81-0.7 V. At lower scan rates and higher temperatures, these oscillations also appear in the forward scan, suggesting that the hysteresis in Figure 1 should be ascribed to the relatively high scan rate. When the potential is arrested at values around -1 .OV, the oscillations usually die out slowly. The oscillations in this region have a strongly harmonic character, and their relation with oscillations occurring a t more positive potentials remained unclear. Our interest in this paper will be exclusively devoted to oscillations occurring around V = -0.75 V.
A typical sequence of oscillatory states observed on stepping the cell potential in the negative direction is illustrated in Figures 2 and 3 and summarized in the schematic bifurcation diagram of Figure 4. At about V = -0.74 V, the current changes from a steady state to a small-amplitude oscillation, presumably via
Figure4. Schematic bifurcationdiagram giving the minima and maxima of the oscillatory or stationary current as a function of the applied cell potential V (n-GaAs, 1 M H202 + 1 M HzS04, 40 "C).
a Hopf bif~rcation.1~ The amplitude increases with more negative potentials, until around V = 4 . 7 7 V a relaxation spike appears on the scene. The small oscillations persist during the slow part of the relaxation oscillation: this type of behavior is often referred to as mixed-mode oscillation (MMO).I7 In the potential region close to this transition, it was observed that the system sometimes returned to small-amplitude oscillations, which in turn could be restored to MMO by what we assume to be a fluctuation or drift. This indicates that the behaviors observed at -0.76 and -0.77 V actually are bistable over a certain potential region.
Hz02 Reduction at GaAs Semiconductor Electrodes
The Journal of Physical Chemistry, Vol. 97, No. 28, 1993 7339 T
P
eas
Figures. Complex mixed-modedationobscrvedat a n-GaAS electrode in 1 M H202 + 1 M H2S04 at V = -0.82 V and T = 50 O C .
The MMOs prevailed for over a wide range of potential values; their period and amplitude increased with more negative cell potential, as illustrated in Figure 2. This type of oscillations could survive for several hours. For potentials more negative than -0.88 V, small-amplitude oscillations were again observed. On severaloccasions, a significant period doubling was observed, as in Figure 3a at V = -0.89 V. The irregular time series at V = -0.88 V was analyzed for the occurrenceof deterministicchaos. In Figure 3c, a next maximum map is shown in which the (absolute) current maximum of the oscillation is plotted against the previous maximum. For a chaotic time series arising from a series of period doublings, a single-humped set of points is expected." Although the points show some scatter, they indicate a deterministic chaotic component in the time series at V = -0.88 V. From Figures 3b and 6, it can be seen that the oscillations in the luminescence intensity have the same period as the current oscillations, and both show a related fine structure. Consider for example the MMO in Figure 6b. At the beginning of the period the current shows a spike and then drops drastically; in this time interval the luminescenceintensity decreases drastically, too. In the long time interval in which the current increases slowly, the luminescence intensity decreases slowly. A schematicoverview of the dynamicbehavior is given in Figure 4. One peculiar feature of this diagram is that the MMOs occur in the entire potential region between the transitions to smallamplitudeoscillation. The more usual behavior is that the MMOs take place only in a small parameter region close to these transitions.'* It is interesting to note that a hypotheticalchemical reaction model of Hudson, Rhsler, and Killory shows a similar type of bifurcation diagram.19 Sometimes, usually at more elevated temperatures, a more complex MMOs than those depicted in Figure 2 occurred. A typical example is given in Figure 5. It looks though the smallamplitude oscillations undergo chaotic transients which seem to be the remainings of a chaotic attractor of the kind illustrated in Figure 3a. Although the role of noise cannot be ruled out, an argument in favor of the deterministic character of this complex oscillation is the observation that the large spike always occurs after a series of harmonic oscillations of small or growing amplitude (see also Figure 5 , R,= 300 a). Similar behavior was Observed by Krischer et al. for the oxidation of hydrogen at platinum.20 They described the oscillation in terms of an "interior crisis", in which a chaotic attractor collides with a coexisting unstable orbit, resulting in a sudden enlargement of the attractor. Although few examples of this type of bifuraction exist in differential equation~,l~*~1 they do not quite give the behavior observed in Figure 5 . n-Type GOAS: Fixed PotentialDifference acrossCelland Series Resistor. A type of cell operation often employed in the study of oscillatory behavior is that of a fixed potential differenceacross the cell and an external resistor that is connected in series with
T
Bo8
Figure 6. Some typical oscillatory current time series observed at V = -0.90 V for the indicated values of the external series resistance. Both the electric current and the electrode's light emission are shown (n-GaAs, 1 M H202 + 1 M &Sod, 40 "C). V I V (SCE)
-0.8 -0.7 -0.6
-0.5 -0.4
-0.3 -0.2 -0.1
,
i
/i---
- -3 - -4
-5
I
Rgure 7. Current-potential Scan at a n-GaAs electrode in 1 M H!02 1 M H2!3O4 solution at 40 OC, with the current as the scanning parameter. Scan rate 0.02 mA/s.
+
the working electrode. This kind of operation is a sort of intermediate form between truly potentiostatic and truly galvanostatic conditions. The reason for studying an oscillating electrochemical cell with an ohmic resistor in series stems from the fact that ohmic drop plays a crucial role in destabilization of a negative faradaic i m p e d a n ~ e . ~ ~A - * ~straightforward and systematic way to study the role of the ohmic drop is to connect a resistor in series with the cell. In Figure 6 four typical time series are shown at V = -0.90 V, with increasing external resistance value. Both the oscillating electrical current and light emission are shown. n-Type CaAs: Calvanostatic Operation. Figure 7 shows a current-potential plot obtained with the current as the scanning parameter. A clear hysteresis is observed, due to the region of negative impedance that was already visible in Figure 1. When the current isslowly increased(in thenegativedirection), stable steady-state behavior prevails until I = -3.5 mA. There, an oscillatory escape occurs toward the opposite potential branch.
Koper et al.
7340 The Journal of Physical Chemistry, Vol. 97, No. 28, I993 V / V (SCE)
.3.7mA
-9.4
-9.2
-9.3
-9.1
0;:
- pj-l,5
dark
-3.5 mA
-3.4 mA
.1-1
-0.05 V
I
-3.3 mA
.3.1 mA
-2.9 mA
-2.0 mA L ,
/
Figure 8. Some typical oscillatory potential time series observed at the indicated values of the externallycontrolled current (n-GaAs, 1 M HzOz + 1 M HzS04,40 "C). V I V (SCE)
-9.8
-01.6
-0.07V
.0.06V
I
Figure 10. Current-potential scan for p-GaAs in 1 M H202 + 1 M
60s
-1;O
:1
illumination
-01.4
-O>
/
il L-l
L -5 Figure 9. Schematic bifurcation diagram giving the minimum and maximum of the oscillatory or stationary current as a function of the applied current I. Broken lines representunstablestationarystateswhich should exist but cannot be reached in this operation mode (n-GaAs,1 M H202 1 M &So.$, 40 "C).
+
This branch consists of fairly long-period oscillations (Figure 8). On this branch, the transition from steady-stateto large-amplitude periodic behavior occurs at I z -3.8 mA, giving a relaxation oscillation with two distinctly different periods. The oscillations become faster when the current is made less negative, until they finally become extinct at Z = -2.8 mA, where a transition to the other potential branch is made. Thus, between Z = -2.8 and -3.5 mA, there is a significant bistability of steady-state behavior at relatively positive potentials and periodicbehavior on the relatively negative potential branch. The substantial period shortening found when making the current less negative excludes the possibility of a homoclinic bifurcation'' at I P -2.8 mA; this would seem the most logical thing to happen, in view of the proximity of an unstable steady state, Le., the negative impedance branch. In Figure 9, a schematic bifurcation diagram of the system under galvanostatic control is given. p-Type GaAs: Potentiostatic Operation. In Figure 10 the current-potential scans are given for p-type GaAs in the dark and under illumination. Photocurrent oscillations are observed if the incident light intensity is so high that the cathodic
HzSO4 solution in the dark and under illumination (maximum incident light intensity). Scan rate 1 mV/s, T = 40 "C. In the inset, some typical oscillatory photocurrent time series are shown for the indicated values of the applied cell potential. photocurrent equals the (limiting) cathodic current observed at n-type electrodes, at the same temperature and H202 concentration. Photocurrent oscillations occur over a large potential range in both the forward and reverse scans. In the inset, some typical time series are given for the case in which the potential is arrested at the indicated values. In general, the photocurrent oscillations show a much less regular behavior than theoscillations at n-type GaAs in the dark, which seriously hampered a clear identification of bifurcation scenarios.
Discussion It seems fairly well established that the reduction of H202 at both metaP and semicond~ctor~~ electrodes occurs via two distinguishable electron-transfer reactions, with an OH' radical as intermediate. From the observation that optical excitation of the p-type semiconductor leads to an increase of the cathodic current, M e m m i ~ (who ~ g ~ studied ~ the system at Gap) concluded that at least one step of the mechanism involves a photoexcited conduction band electron:
H,O, + eCB-- HO' + OH-
This also explains the experimentallyobserved 'current doubling" at p-type materials, since for every photon absorbed two electrons are transferred across the semiconductor/electrolyte interface. Since H202 is a strongoxidizing agent, there is a process parallel to (l), being the chemical etching (dissolution) of the semiconductor:
-
3H,O, + GaAs + 6H+ 6H20 + Ga" + As3+ (2) Experiments by Minks et al.9.16have shown processes 1 and 2 to be correlated, and Memming's mechanism was revised9 to the extent that reactions 1 and 2 were assumed to have a common precursor, postulatedto be some chemisorbed H2Oz intermediate. This model was shown to account for most of the results obtained at n- and p-type GaAs electrodes in H202 solutions of moderate concentration and temperature. The main features can be summarized in the following reaction scheme?
H202 Reduction at GaAs Semiconductor Electrodes In the first step (rate constant ka),H202 is adsorbed onto a free surface site XO.In the following step an electron from a surface bond is transferred to the adsorbedspecies and a dipolelike surface complex is formed, consisting of an electron-deficient surface bond XI, an electrostatically attached OH- ion, and an adsorbed OH* radical. This surface intermediate is assumed to be the precursor for further reduction and for chemical etching (Le., reaction 2). In the next step of the H202 reduction, a conduction band electron recombines with the surface complex (rate constant Itr), leading to a resotred surface bond, and an OH- ion in the solution. In a subsequent step, the adsorbed OH' injects a hole into the valence band; this process has a large rate constant khj. The hole is consumed by recombination with a conduction band electron at the surface or in the bulk of the semiconductor. The luminescence is due to bulk recombination. The above scheme provides a basis for the discussion of the results obtained at relatively high H202 concentration and temperature. In the potential range positive with respect to the current maximum ( V > 4 . 7 V), recombination (reaction 4) limits the cathodic current. In this potential range, oscillations do not occur. In the potential range between 4 . 7 and -0.9 V, sufficient conduction band electrons are available at the surface, and the H202 adsorption (reaction 3) becomes rate limiting; this is expected to result in a potential-independentcurrent. However, a decrease of the cathodic current with more negative potential is observed, and spontaneous current oscillations occur in this potential region. A similar passivation was also observed during H202 reduction at Pt** and Ge.29 According to Gerischer and Mindt,29 this negative impedance is due to inhibition of the H202 reduction at Ge' radical surface sites by a competing reaction of H* radicals with these surface sites. The negative impedance observed during H202 reduction at GaAs electrodes presumably also results from an inhibitionof the H202 adsorption. The exact nature of this inhibition process is as yet not clear, but it is probably also related to competitiveadsorption of H' radicals at the GaAs surface.30 At n-GaAs, oscillations occur when adsorption (reaction 3) is rate limiting. This is in accordance with the observation that oscillations at p-GaAs only occur if the incident light intensity is so high that the supply of photogenerated conduction band electrons does not limit the cathodic photocurrent. It is remarkable that at p-GaAS electrodes oscillations occur if the photocurrent is as high as the maximum of the cathodic dark current at n-GaAs electrodes. Attention is paid now to the oscillations in the intensity of the luminescence accompanying the long period oscillations in the cathodic current (see Figure 6). The dramatic drop in luminescence intensity at the beginning of the period corresponds to the fast current drop and can be ascribed to a considerable decrease of the rate of injection of holes (reaction 5 ) in this time interval. During the long time interval in which the cathodic current slowly increases, the luminescence intensity decreasesslowly. This result is as yet not understood. It is however worthwhile remarking that not only the rate of hole injection is of importance for the luminescenceintensity but also the competition between surface and bulk recombination. Surface recombination is believed to occur nonradiatively,31whereas bulk recombinationis responsible for the luminescence. The competition between these two recombinationmechanismsis determined by two factors. Firstly, when the band bending changes, according to the Boltzmann equilibrium the bulk concentration of free holes changes too and, as a consequence, the bulk recombination rate. Secondly, the chemical nature of the interface determines the surface recom-
The Journal of Physical Chemistry, Vol. 97, No. 28, 1993 7341
bination rate. The transition from a hydroxide- to hydride-covered surface, for example, has been used to explain the complex potential dependence of the electroluminescenceof n-GaAs in the presenceof Fe(CN)63-.32It is likely that during theoscillations both the chemical nature of the GaAs surface and the band bending change. These two effects, together with the variation of the hole injection rate, have to be taken into account for a detailed explanation of the oscillations in the luminescence intensity. Finally, it is worthwhile commenting on the difference in the potential distributions at the semiconductor/electrolyteand the metal/electrolyte interface and how this may affect a possible theoretical explanation of the oscillatory behavior. In contrast to the metal/electrolyte case, the interfacial potential across a semiconductor/electrolyte interface consists of, at least, two contributions;one is due to the depletion layer in the semiconductor material, and the second is due to the Helmholtz layer in the electrolyte phase. This additional degree of freedom, together with the role of the ohmic electrolyte resistance, should be taken into account in any model approach to oscillatory behavior at semiconductor electrodes.
Acknowledgment. We thank Dr. Bart Minks, who "discovered" the oscillations in this reaction and did some preliminary investigations. References and Notes (1) Tributsch, H. Be?. Bunsen-Ges. Phys. Chem. 1975, 79, 570. (2) van Meirhaghe, R. L.; Cardon, F.;Gomes, W. P. Electrochim. Acra 1979.24, 1047. (3) Gerischer, H.; Liibke, M. Ber. Bunsen-Ges. Phys. Chem. 1988, 92, 573. (4) Marcu, V.; Tenne, R. J. Phys. Chem. 1988, 92, 7089. (5) Marcu, V.; Strehblow, H.-H. Electrochim. Acra. 1991, 36, 869. (6) Cattarin, S.;Tributsch, H. J . Electrochem. SOC.1992, 139, 1328. (7) Chazalviel, J.-N.; Ozanam, F.;Etman, M.; Paolucci, F.; Peter, L. M.; Stumper, J. J. Electroanal. Chem. 1992, 327, 343. (8) Josseaux,P.; Michaeu, J. C.; Kirsch-De Mesmaeker, A. Electrochim. Acra 1985, 30, 1093. (9) Minks, B.P.; Vanmaekelbergh, D.; Kelly, J. J. J. Electroanal. Chem.
1989, 273, 133. (10) von Antropoff, A. Z . Phys. Chem. (Munich) 1908,62, 513. (1 1) Lingane, J. J.; Lingane, P. J. J. Electroanal. Chem. 1963, 5, 41 1. (12) Fetner, N.; Hudson, J. L. J. Phys. Chem. 1990,94, 6506. (13) Honda, M.; Kodera, T.; Kita, H. Electrochim. Acra 1986, 31, 377. (14) Strbac, S.;Adzic, R. K. J. Electroanal. Chem. 1992, 337, 355. (15) Techniques in Electrochemistry, Corrosion, and Metal Finishing: A Handbook; Kuhn, A. T., Ed.; J. Wiley: New York, 1989; p 33. (16) Minks, B. P.; Oskam, G.; Vanmaekelbergh, D.; Kelly, J. J. J . Electroanal. Chem. 1989, 273, 119. (17) Scott, S. K. Chemical Chaos; Clarendon Press: Oxford, 1991. (18) Koper, M. T. M.; Gaspard, P.; Sluyters, J. H. J. Phys. Chem. 1992, 96, 5674. (19) Hudson, J. L.; Rbssler, 0. E.; Killory, H. Chem. Eng. Commun. 1986, 46, 159. (20) Krischer,K.;Liibke,M.; Wolf, W.;Eiswirth,M.;Ertl,G.Ber.BunsenGes. Phys. Chem. 1991, 95, 820. (21) Richetti, P.; DeKepper,P.;Roux, J. C.; Swinney, H. L. J. Srat. Phys. 1987, 48, 977. (22) Degn, H. Trans. Faraday Soc. 1968,64, 1348. (23) de Levie, R. J . Electroanal. Chem. 1970, 25, 257. (24) Epelboin, I.; Gabrielli, C.; Keddam, M.; Lestrade, J.-C.; Takenouti, H. J. Electrochem. SOC.1972,119, 1632. (25) Koper, M . T. M. Electrochim. Acra 1992, 37, 1771. (26) Winkelmann, D. Z . Elektrochem. 19!%,60, 731. (27) Memming, R. J. Electrochem. Soc. 1969, 116, 785. (28) Gerischer, R.; Gerischer, H. Z . Phys. Chem. (Munich) 19!%,6,178. (29) Gerischer, H.; Mindt, W. Surf. Sci. 1966, 4, 440. (30) Gerischer, H.; MOller, N.; Haas, 0. J. EIectroanaL Chem. 1981, 119, 41. (31) Smandek, B.; Gerisher, H. Electrochim. Acra 1985, 30, 1101. (32) Decker, F.; Pettinger, B.;Gerischer, H. J . Electrochem. SOC.1983, 130. 1335.
