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On the kinetics of electron transfer reactions at illuminated InP electrodes
685
J. Electrvanal. Chem., 214 (1986) 685-698 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
ON THE KINETICS OF ELECTRON TRANSFER REACI’IONS ILLUMINATED InP ELECTRODES *
K. TUBBESING, Institut
D. MEISSNER,
R. MEMMING
AT
** and B. KASTENING
ftir Physikalische Chemte der Universitiit, Bundesstr. 45, 2ooO Hamburg 13 (F. R.G.)
(Received
25th July 1985; in revised form 5th June 1986)
ABSTRACT Various reactions at highly doped n- and p-type InP electrodes were investigated. Impedance measurements have shown that the flatband potential is changed upon illumination. The corresponding shift of the energy bands at the surface of InP was interpreted by the trapping of minority carriers in a surface oxide layer. The reduction and oxidation of various redox systems at p-InP were investigated in the dark and under illumination and it is shown that a thin oxide surface layer influences mainly the oxidation process. The results are compared with those obtained with GaAs and various charge transfer models are discussed in detail.
INTRODUCTION
InP is of great interest for electrochemical studies because both n- and p-type materials are available. The positions of the energy bands in aqueous electrolytes are known [1,2] so that it is easy to estimate whether a charge transfer between the semiconductor electrode and a redox system occurs via the conduction or valence band. A number of redox reactions have been investigated which prove the predictions [3]. It has also been shown that InP dissolves under anodic polarization [4] and that the dissolution rate is orientation dependent [5]. In addition, InP dissolves chemically in concentrated HCl solutions [6]. In connection with the possible application of photoelectrochemical cells for solar energy conversion, Cardon et al. [3] have investigated the possibility of preventing the photoelectrochemical decomposition by means of competing anodic oxidation of reducing agents. Other scientists have studied p-type-based liquid
Dedicated to the memory of Professor H.W. Nbmberg. * Permanent address: Philips GmbH Forschungslaboratorium Hamburg 54, F.R.G.
l l
0022-0728/86/$03.50
0 1986 Elsevier Sequoia S.A.
Hamburg,
Vogt-Kolln-Str.
30, 2000
686
junction solar cells [7]. In contrast to n-type electrodes, photocathodes are protected against corrosion because electrons are driven towards the interface. In the case of p-type electrodes, however, a relatively high overvoltage was observed for the onset of the cathodic photocurrent with respect to the flatband potential, which seemed to decrease in the presence of a redox couple such as V2+/V3+ [&lo]. Various successful attempts were made by Heller and co-workers to decrease the overvoltage by depositing a metal catalyst on p-type InP [7,8,10]. The overvoltage for the photocurrent onset is a phenomenon which is not entirely restricted to p-InP. It has been observed with a number of n- and p-type electrodes [ll]. In the case of n- and p-type GaAs [ll-131 and n-WSe, [14] electrodes, the overvoltage could be explained by the trapping of minority carriers in surface states, leading to a change of the potential drop across the Helmholtz layer and by recombination processes. InP is an interesting electrode material for studying the parameters responsible for the overvoltage in more detail, because here various redox processes can compete more easily with the hydrogen evolution than on GaAs. Corresponding results obtained with InP are given in the present paper. EXPERIMENTAL
The InP crystals were obtained from the Philips Laboratory in Eindhoven (The Netherlands). The crystal faces were of [lOO] orientation. The carrier density was of the order of some 1018 cmv3 if not stated otherwise. The electrodes were etched in concentrated HCl solutions. All solutions were prepared with reagent-grade chemicals. The electrochemical measurements were made in a conventional cell using a saturated calomel electrode (SCE) as the reference electrode. The scan rate of the electrode potential was 40 mV s- ‘. The impedance measurements were made by imposing a high-frequency modulation, typically 10 mV at 20 kHz, on the potential applied to the cell. Measurements of the phase and amplitude of the corresponding high-frequency current component through the cell, by applying the potentioscan current output signal to a Heterodyne lock-in amplifier, enabled the capacitance to be calculated. For illumination, a 150 W Xe lamp was used installed in an Amco housing (ALH 215). The light passed through a monochromator before entering the cell. RESULTS
In Figs. la and 2a typical current-potential dependences for n- and p-type InP are presented as measured in darkness and under illumination using 1 A4 HCl as electrolyte. The corresponding Mott-Schottky curves, also measured in darkness and under illumination, are given in Figs. lb and 2b. The capacity values could be evaluated in the whole potential range from impedance measurements using an equivalent circuit with a resistance and a capacitance in series. The same equivalent circuit could be applied for electrodes under illumination for the potential range in
687
-10 I’
Iph Inl.x
10
:
u/v
-05
c/
(0)
(b)
% $
10
dark
0
lllum \
I
5
\ \
\
\ \
‘1
\ \
electrode
potentiol
ECE)/V
I
I
-10
-0.5
I
‘1, \
\
0 electrode potenttot ISCEI/V
,‘q 05
1
UP,
Fig. 1. Normalized photocurrent (a) and capacity (b) vs. electrode potential for n-InP in 1 M HCl. Fig. 2. Normalized photocurrent (a) and capacity (b) vs. electrode potential for p-InP in 1 M HCl.
which the photocurrent is constant. In the other range, i.e. that in which the photocurrent rises strongly with increasing potential, we did not attempt to derive capacity values from the impedance data. Other authors [15], studying GaAs electrodes, evaluated the impedance data in this range also, using a very complex equivalent circuit. Since the latter cannot be interpreted unambiguously, we did not adopt this evaluation. The shift of the Mott-Schottky curves upon illumination depends on the light intensity and saturates at higher intensities. These results are similar to those found with p-GaAs [12]. The excitation spectrum of the saturation photocurrent obtained with highly doped n- and p-InP is given in Fig. 3. The absorption curve in the same figure as given in the literature [16] is shown. According to Figs. 1 and 2, the onset potential of the photocurrent occurs in all cases (besides for the 1016 cme3 n-type sample) at potentials considerably different from that of the flatband determined in darkness. We have investigated this effect in more detail at p-InP in electrolytes containing different redox systems. The corresponding photocurrent-potential dependences are presented in Fig. 4. In one case (Eu3+) we also measured the photocurrent-potential curves at different concentrations of the redox system in the range 5 X lop3 < cn,, < 10-l M. The corresponding results indicated a shift of the curves towards positive potentials with increasing concentration. These experiments were limited to a small concentration range because the light intensity had to be reduced to a very low level for measurements at
electrode -10
wovelength/
-05
potentlol lSCE)/V 0
+0.5
Iph Irn.l
nm
I
Fig. 3. Excitation spectrum of photocurrent at p-InP in 1 M HCl. UE = -0.9 V. Fig. 4. Photocurrent vs. electrode potential at p-InP in 1 M HCl in the presence of various redox systems.
much lower Eu3+ concentrations in order to avoid diffusion limitation. Then, however, the photocurrents were small and side reactions dominated in the photocurrent-potential behavior which were not reproducible. Concerning the capacity measurements performed with solutions containing redox systems, it should be
k-4
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-0~
-a2
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El?+ +OL
+06
Fig. 5. Dark current vs. electrode potential in 1 M HCI + lo-’
M Eu*+. (a) p-InP; (b) n-InP.
689
mentioned that the shift of the Mott-Schottky curves upon illumination is reduced. Practically no shift was detectable any longer in the case of Fe’+. In addition, we also investigated the current-potential behavior of n- and p-type InP in the dark in the presence of the reduced species of various redox couples. A typical result, obtained with Eu*+, is shown in Fig. 5a and b. At the p-type electrode (Fig. 5a), no additional anodic current was found in the presence of Eu*+, whereas in the case of n-type an oxidation current was detectable in the whole potential range (Fig. 5b). Almost identical results were obtained with V*+. DISCUSSION
It is well known from the literature that InP is electrochemically dissolved during positive polarization [1,4,6], whereas during negative polarization mainly hydrogen is produced [l]. Since the anodic dissolution is a valence-band process, the dark current at n-type InP is limited to a very low value. On the other hand the hydrogen evolution proceeds via the conduction band. Accordingly, the latter process occurs at p-InP only during light excitation. The flatband potentials for n- and p-type electrodes in darkness are Ufb”= -0.62 V and +0.65 V vs. SCE, respectively. The difference between the two flatband potentials amounts to 1.27 V. This difference is practically equal to the energy gap (1.3 ev), which is reasonable for highly doped material. It can be concluded from this result that the position of energy bands at the surface is equal for the n- and p-type electrodes, as found for many other semiconductor electrodes [17]. Before discussing the electron transfer reactions, the spectral distribution of the photocurrent should be analyzed further. Analysis of the excitation spectrum
According to Reichman [18], the current carried by the minority carriers during light excitation is given by: I
i
0
P=
i”P
eU
0)
ii + i, exp - kT ) (
in which (Y represents the absorption coefficient, L the diffusion length of the minority carriers, i, the dark saturation current, ‘z the exchange current at equilibrium, U the overpotential with respect to equilibrium and 1, the light intensity. The thickness of the space charge layer d, is given by [17]
(2) in which U, is the potential drop across the space charge region and no the carrier density in the bulk.
690
Assuming (U x=- F),
that i, is sufficiently eqn. (1) is reduced
small and selecting
to the so-called
a large bias toward
G&rtner equation
depletion
[17,19]
Accordingly, the photocurrent depends on the thickness of the space charge layer d, and on the absorption coefficient and consequently on the penetration depth of light (L, = l/a). The thickness of the space charge layer can be varied by changing the band bending U,,, but it is also determined essentially by the bulk majority density no. Since we used rather heavily doped material, d, should be relatively small. For instance in the case of no = 3 X lOI cmp3, d, = lop6 cm for U,, = 1 V. Using light of high penetration depth, (small a) so that ad,, e 1, the exponential term in eqn. (4) can be extended into a series; eqn. (4) can be approximated by [20] L+a-’ eI0 -= (4) L+d, ‘ph As already pointed out by Geiger et al. [20] a plot of eI,/i,, vs. LY-’ should then yield a straight line and the intercept with the c--~ axis should give the diffusion length of the minority carriers, i.e. eZ0 y---t()+L=
--(y-l
(5)
‘ph
The same authors tried to apply eqn. (4) to their photocurrent results. Since they had data available only for 3 or 4 wavelengths, the applicability of eqn. (4) could not be tested definitively. We analyzed the excitation spectra of the photocurrent over a much wider range by using the absorption data given in ref. 16. Replotting the data from Fig. 3 according to eqn. (4) one indeed obtains straight lines, as shown in Figs. 6 and 7. From the intercept we obtained: L,=2x10-6cm
for p. = 6 X 1018 cmp3 (p-type)
L, = 3.5 X lop6 cm
for p. = 3 X 1018 cmp3 (p-type)
Lp=1.4x10-5cm
for no = 2 X 1018 cmm3 (n-type)
\
>.
EL-
‘.
s
‘1
b 2-
‘\\ IIIII,I,II -10 penetrotm
-5
,,,,,,,IIJ,
0 depth of light ,iOOL,/pm
5
I_& 10
1L
Fig. 6. Reciprocal values of photocurrent vs. penetration depth of light for n-InP in 1 M HCl.
691
-15 -
-10 penetrollon
-5 0 depth of llghl ,100
2 L L, /em
Fig. 7. Reciprocal values of photocurrent vs. penetration depth of light for p-InP in 1 M HCI. (a) P0 =6XlO’“cm3; (b) PO= 3 x lo’* cmm3.
Very similar values were obtained from typical solid-state measurements [21]. These results show that the G&tner model describes the long-wavelength region of the excitation spectrum rather well. The decrease of the photocurrent below 450 nm cannot be explained by this model, however. At present we have no reasonable interpretation for this phenomenon. Potential
distribution
during right excitation
The Mott-Schottky curves are shifted during illumination toward negative potentials for p-type and toward positive potentials for n-type electrodes (Figs. 1 and 2). Similar results have been obtained with GaAs [12], GaP [ll], CdS [ll], RuS, [22], and with some layer compounds such as MoSe, and WSe, [14]. In earlier publications we interpreted the shift and the flatband potential through a corresponding change of the potential distribution. Accordingly, the potential drop across the Helmholtz layer 17, must be different during light excitation from that in darkness, at least in the range where the photocurrent is constant. Such a change in U, can only be caused by an accumulation of minority carriers below or at the surface. Their density can be estimated by using the equation [12]
(6) and N, =
Q/e
(7)
in which C, is the Hehnholtz capacity, AU,,,,, the change in the potential drop across the Helmholtz layer and AQ the resulting charge. Assuming C, - lop5 F cm-* and taking from Figs. 1 and 2 the experimental values of AUuu,,, (0.18 V for n-type and 0.3 V for p-type) one obtains N, = 1.8 X 1013 cm-* for p-InP and
692
1 x 1013crne2 for n-InP. As already discussed in earlier papers, the accumulation of the minority carriers can be interpreted by assuming trapping in surface states. The shift of the Mott-Schottky curves and the corresponding change in the flatband potential indicates an upward shift of the energy bands at the surface of the p-type electrode and a downward shift at n-type InP. We have investigated the model in detail for the case of p-GaAs [12]. It was shown by quantitative analysis that the rate of electron transfer from the conduction band to protons in the solution must be small, so that electrons excited by light are easily trapped in surface states. The quantitative evaluation of the intensity dependence-of the flatband shift and of the photocurrent indicated that finally electron transfer from surface states to protons occurs which limits the surface-state charging and the upward shift of the bands [12]. Since the results obtained with p-InP are qualitatively identical to those found with p-GaAs, we assume that the same model is valid here. Charge transfer processes
According to Figs. 1 and 2, the onset of the photocurrent occurs at considerable overvoltages with respect to the flatband potential in the dark. Especially large overvoltages were found with p-type InP and the difference between the onset potential U,, of the photocurrent and the flatband potential U,, is also greater than the shift of the flatband potential upon illumination. The large overpotential may be due partly to problems in the kinetics of hydrogen evolution but must also be due to strong surface recombination [lo]. The recombination rate seems to be even stronger than for p-GaAs [12] because the difference (U,, - U,) is larger for p-InP than for p-GaAs. On the other hand it is remarkable that at p-InP the onset of photocurrent occurs at much less negative potentials if a redox system with a standard potential negative to that of H,/H+ is added to the electrolyte, examples being Cr 3+, Eu3+, V3 + and MV2+ (Fig. 4). This res ult was not found with p-GaAs [12]. In the latter case a smaller overvoltage was observed only with redox systems having a rather positive standard potential, such as Fe3+ or [Fe(CN),13-. We interpreted the latter result by an electron transfer via surface states [12]. From the energy scheme illustrated in Fig. 8 one would expect that the reduction of the other redox couples should occur by an electron transfer directly from the conduction band. From the thermodynamic point of view one should even expect that a direct electron transfer from the conduction band to the redox system is more favorable for GaAs than for InP. As already mentioned above, however, the opposite was found. The energy situation can be illustrated even more clearly by plotting the complete distribution of energy states, as shown for Eu2+13+ in Fig. 9. Here a reorientation energy of about A = 1 eV has been assumed, a value similar to that found for Fe2+/Fe3+ (1.25 eV) [17]. Since the position of the conduction bands of GaAs and InP differ by 0.24 eV, the density of the empty energy states of Eu’+/Eu’+ should be higher by about two orders of magnitude at the lower edge of the conduction band of GaAs than that of
693
EISCEl/eV
InP v-’ f&&C -0 7
0
--_[FelCN1,1’~“~
t +05
GoAs
--FeYFeZ+ zz
+1.0
/EV +lO
redor couple 1o-6 lo-‘
+1.5
Fig. 8. Energy
levels at the interface
Fig. 9. Energy
scheme at the interface
of InP and redox couples
10-Z
1
in 1 M HCl.
InP/Eu2*/3+.
InP. Accordingly, the question arises as to whether strong recombination is really responsible for the large overvoltage of the photocurrent. In connection with this problem another, different electrode behavior for p-InP and p-GaAs should be mentioned. From the energy scheme in Fig. 9 one would expect that the reverse process, i.e. the oxidation of the redox couple, occurs preferably via the valence band because the density of occupied states is much larger at the upper edge of the valence band than at the lower edge of the conduction band. A valence band process, however, requires holes which are not available in n-type semiconductors. Accordingly, the small additional anodic dark current found with n-type InP in the presence of Eu2+ or V*+ (Fig. 5b) must be due to electron injection into the conduction band. This process is expected to be independent of potential, which is confirmed experimentally. In the case of n-GaAs such an oxidation current was not detected. This result is reasonable because the conduction band is located at higher energies, which makes electron injection more difficult, The experiments with p-type InP electrodes did not yield any additional oxidation current either. At first sight, this result seems to be reasonable because the hole density in the valence band at the interface may be too low for a large downward band bending. Corresponding measurements with p-GaAs have shown, however, that large anodic currents do occur in the presence of Eu*+ even at relatively strong downward band bending (e.g. 10 mA cm-* at US, = -0.45 V), as illustrated for comparison in Fig. 10a [23]. The difference in behaviour can certainly not be interpreted on the basis of the energy scheme in Fig. 9. These considerations show quite clearly that the electron transfer from Eu2+ or Vzt into the valence band of InP must be inhibited by a different process. The
694
inhibition may be related to a thin oxide film which is easily formed on InP [&lo]. Thicknesses of about 1 nm are reported in the literature [lo]. Assuming that the empty states of the oxide are located close to the conduction band, the oxide would not block the electron transfer from the conduction band to Eu3+. The reverse process, i.e. electron transfer from Eu2+ into the valence band, however, can occur only at a considerably reduced rate because the electrons must tunnel through the oxide, as shown in Fig. 11. It should once more be emphasized that reduction and oxidation of the europium redox couple can occur via different energy bands at p-GaAs. Large oxidation currents were observed in the potential range where no cathodic photocurrent was detectable. Accordingly, if an electrolyte is used which contains only the oxidized species of the redox system, e.g. Eu3+, then the reduced species (Eu2’) is formed during illumination by electron transfer via the conduction band. The latter (Eu2’) would then be reoxidized by hole transfer via the valence band. In this case the cathodic photocurrent would be compensated by an anodic dark current, provided that the Eu2+ produced in the first step does diffuse away from surface. Accordingly, in this model the large overvoltage for the onset of photocurrent found with most redox systems at p-GaAs could be interpreted by a recombination shunt in the
-
Eu3’
hv (
Eu ‘+
E”
-+ -06 (bl : 6 02
06 -0L -02
Eu2’
0 +Q2 +OL U(SCE)V
n-GoAs dark ELI” p-InP
Fig. 10. Dark current vs. electrode potential p-GaAs: (b) n-GaAs (from ref. 23).
for GaAs in 1 M HCI+10-2
Fig. 11. Energy levels of p-InP covered with a thin oxide layer.
M Eu”
at 5000 rpm. (a)
695
electrolyte [23]. This model would also explain the lower overvoltage at p-type InP because the photocurrent cannot be compensated due to the blocking of the valence band process by the oxide layer. However, it is difficult to prove this model. A detailed estimate of the kinetics, especially of the diffusion of the Eu*+ away from the surface, has shown that a recombination shunt in the electrolyte is possible in principle [23]. The diffusion of Eu*+ away from the electrode surface can be influenced by using different speeds for a corresponding rotating disc electrode. Accordingly, one should expect that at high rotation speeds Eu*+ moves sufficiently fast into the electrolyte so that the reoxidation probability is reduced, i.e. the photocurrent onset should occur at less negative potentials. However, we failed to see any shift of the photocurrent onset upon changing the rotation speed. The model would still work if the Eu*+ remains adsorbed for a sufficient time. Since this assumption seems to be rather unlikely we have to rule out this model. Accordingly, overvoltages in the photocurrent onset must be explained by surface recombination as also suggested for p-Gap electrodes [25]. Further, it has to be concluded that the surface recombination rate must be smaller for InP, possibly due to the presence of the oxide [24]. The onset of the photocurrent should depend on the concentration of the oxidized species of the redox system, as confirmed experimentally with Eu3+ within a limited concentration range. It is interesting to note that Heller et al. [8,10] have found relatively high photovoltages in regenerative cells containing both Eu*+ and Eu3+ or V*+ and V3+. This must be due to the fact that the oxidation of Eu*+ or V *+ is blocked at p-InP by the oxide layer. So far, charge transfer processes with redox systems of rather negative standard potentials were discussed extensively. On the other hand, very small over-potentials were found with redox systems exhibiting a relatively positive standard potential, such as for instance Fe3+ (Fig. 4). This was also found with p-GaAs. Since in this case no shift of the flatband potential occurred upon illumination, we assumed an electron transfer via surface states at the GaAs-electrode [12]. The same interpretation could be applied in principle for InP. The question arises, however, as to whether the surface states are located at the InP/oxide or at the oxide/electrolyte interface. In the first case the electrons trapped in surface states have to tunnel through the oxide, which should lead to a rather low rate. In addition, one may ask whether an assumption of electron transfer via surface states is really conclusive. With respect to this problem the relative position of energy states on both sides of the interface InP/oxide/Fe *+13+ should be inspected more closely (Fig. 12). For simplicity and better comparison with the case for europium (Fig. 7), we used the same reorientation energy of X = 1 eV. Assuming further that the empty energy states of the oxide are located only slightly below the conduction band of InP, the maximum of the distribution of the empty energy states in the electrolyte occurs close to the conduction band. The density of empty states of the Fe3+/Fe2+ couple exceeds that of the Eu3+/Eu2+ couple by orders of magnitude at E = E,” (compare Figs. 12 and 9). Accordingly, the rate of electron transfer from the oxide to Fe3+ should be much higher than in the case of Eu3+, as was found experimentally. As regards the large overpotential for the photocurrent onset in solution free
696 E ISCEl/eV eV
log D
Fig. 12. Energy scheme at
the interface InP/Fe3+j4+.
from any redox system, it should be mentioned finally that Heller et al. found a pronounced displacement of the photocurrent onset towards positive potentials after having deposited a surface film of platinum, ruthenium or silver on p-InP [7,8,10]. In a very recent paper by this group, it was shown further that a Pt film of 2 nm thickness was sufficient to create such a shift [26]. The authors concluded from this observation that platinum catalyzes the hydrogen evolution. Accordingly, the electron transfer becomes faster than the recombination process. In corresponding experiments we made similar observations; our results were not very reproducible, however, which may be due to the deposition technique. It must be emphasized here that a positive displacement of the photocurrent onset does not necessarily mean a decrease of the overvoltage with respect to the flatband potential. Since a metal was deposited on InP, the potential distribution, and consequently the flatband potential, may be different from that of metal-free surfaces. For instance, Aspnes and Heller [27] have shown that contact between a p-InP electrode and a thin layer of Pt saturated with hydrogen (without any electrolyte) leads to a large Schottky barrier, i.e. the energy bands are bent downwards at equilibrium. According to these data, the Fermi level is located about 0.4 eV below the conduction band at equilibrium. If such a p-InP electrode with its Pt surface is brought into contact with an electrolyte, it is difficult to predict the position of the energy bands. Accordingly, one cannot predict the flatband potential nor the overvoltage of the photocurrent onset with respect to the flatband potential. Corresponding Mott-Schottky measurements have not been published yet. These may be difficult because for very thin Pt-layers the electrolyte not only makes contact with the platinum but also with the bare InP surface. In this case one may have different positions of the energy bands in areas
697
where the electrolyte is in direct contact with the InP and those where it is in contact with the Pt, as discussed recently by Nakato and Tsubomura [28]. CONCLUSION
In the present paper we have described various electron transfer processes at nand p-type InP. Especially the different onset potentials of cathodic photocurrents at p-InP with and without redox systems were analyzed and compared with corresponding results obtained with GaAs. According to the relative position of the energy bands on both sides of the interface, the reduction of the redox systems considered here occurs via the conduction band, whereas the oxidation is expected to proceed via the valence band, as concluded from comparable measurements with p-GaAs. This valence band process did not occur at p-InP because the transfer of holes seems to be blocked by the oxide layer. The result that the oxidation and reduction of a redox system occur via different energy bands is caused by the relatively large reorientation energy [23]. In order to avoid charge transfer processes via both bands, investigations with redox systems whose molecules contain large ligands or are embedded in rigid organic cages (small h values), may be more favorable. ACKNOWLEDGEMENTS
The authors are very much indebted to Dr. J.J. Kelly, Philips Research Laboratories, Eindhoven (The Netherlands) for the generous supply of InP single crystals as well as for many valuable discussions. Financial support by the Deutsche Forschungsgemeinschaft and by the Fonds der chemischen Industrie is gratefully acknowledged. REFERENCES 1 A.M. van Wezemael, W.H. Laflere, F. Cardon and W.P. Comes, J. Electroanal. Chem., 87 (1978) 105. 2 M.P. Dare-Edwards, A. Hamnett and J.B. Goodenough, J. Electroanal. Chem.. 119 (1981) 109. 3 F. Cardon, W.P. Comes, F. van de Kerchove, D. van Mackelbergh and F. van Overmeire, Faraday Discuss. Chem. Sot., 70 (1980) 153. 4 A.B. Ellis, J.M. Bolts and MS. Wrighton, J. Electrochem. Sot., 124 (1977) 1603. 5 P.A. Kohl, C. Wolowodink and F.W. Ostermayer, J. Electrochem. Sot., 130 (1983) 2288. 6 P.H.L. Notten, J. Electrochem. Sot., 131 (1984) 2641. 7 A. Heller in J. Rabani (Ed.), Photochemical Conversion and Storage of Solar Energy, The Weizmann Science Press, Jerusalem, 1982, p. 63, and literature cited therein. 8 A. Heller and R.G. Vadimsky, Phys. Rev. Lett., 46 (1981) 1153. 9 K. Kosaki and H. Kita, Sol. Energy Mater., 8 (1983) 421. 10 A. Heller, H.J. Leamy. B. Miller and W.D. Johnston, J. Phys. Chem., 87 (1983) 3239. 11 R. Memming and J.J. Kelly in J.S. Connally (Ed.), Photochemical Conversion and Storage of Solar Energy, Academic press. New York, 1980, p. 243. 12 J.J. Kelly and R. Memming, J. Electrochem. Sot., 129 (1982) 730. 13 K. Schrader and R. Memming. Ber. Bunsenges. Phys. Chem., 89 (1985) 385.
698 14 A.J. McEvoy, M. Etman and R. Memming, J. Electroanal. Chem.. 190 (1985) 225. 15 P. Allongue, H. Cachet and G. Horowitz, J. Electrochem. Sot., 130 (1983) 2352. 16 B.O. Seraphin and H.E. Bennett in R.K. Willardson and A.C. Beer @is.), Semiconductors and Semimetals, Vol. 3, Academic Press, New York, 1967, p. 499. 17 R. Memming in B.E. Conway, J.O’M. Bockris, E. Yeager, S.U.M. Khan and R.E. White (Eds.), Comprehensive Treatise of Electrochemistry, Vol. 7, Plenum Press, New York, 1983, p. 529. 18 J. Reichman, Appl. Phys. Lett., 35 (1980) 574. 19 W. Gtitner, Phys. Rev., 116 (1953) 84. 20 T. Geiger, R. Nottenberg, M.L. Pelaprat and M. GrBtzel, Helv. Chim. Acta, 65 (1982) 2507. 21 J.J. Kelly, private communication. 22 S. Piazza, H.M. Kbhne and H. Tributsch, J. Electroanal. Chem. 1% (1985) 53. 23 D. Meissner, C. Sinn, R. Memming, P.H.L. Notten and J.J. Kelly in E. Pelizetti (Ed.), Homogeneous and Heterogeneous Photocatalysis, Reidel, Dordrecht, 1986, p. 317. 24 A. Heller, B. Miller and F.A. Thiel, Appl. Phys. Lett.. 38 (1981) 282. 25 W.J. Albery and P.N. Bartlett, J. Electrochem. Sot., 129 (1982) 2254. 26 A. Heller, D.E. Aspnes, J.D. Porter, T.T. Sheng and R.G. Vadimsky, J. Phys. Chem., 89 (1985) 4444. 27 D.E. Aspnes and H. Heller, J. Phys. Chem., 87 (1983) 4919. 28 J.Y. Nakato and H. Tsubomura, J. Photochem., 29 (1985) 257.
J. Electrvanal. Chem., 214 (1986) 685-698 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
ON THE KINETICS OF ELECTRON TRANSFER REACI’IONS ILLUMINATED InP ELECTRODES *
K. TUBBESING, Institut
D. MEISSNER,
R. MEMMING
AT
** and B. KASTENING
ftir Physikalische Chemte der Universitiit, Bundesstr. 45, 2ooO Hamburg 13 (F. R.G.)
(Received
25th July 1985; in revised form 5th June 1986)
ABSTRACT Various reactions at highly doped n- and p-type InP electrodes were investigated. Impedance measurements have shown that the flatband potential is changed upon illumination. The corresponding shift of the energy bands at the surface of InP was interpreted by the trapping of minority carriers in a surface oxide layer. The reduction and oxidation of various redox systems at p-InP were investigated in the dark and under illumination and it is shown that a thin oxide surface layer influences mainly the oxidation process. The results are compared with those obtained with GaAs and various charge transfer models are discussed in detail.
INTRODUCTION
InP is of great interest for electrochemical studies because both n- and p-type materials are available. The positions of the energy bands in aqueous electrolytes are known [1,2] so that it is easy to estimate whether a charge transfer between the semiconductor electrode and a redox system occurs via the conduction or valence band. A number of redox reactions have been investigated which prove the predictions [3]. It has also been shown that InP dissolves under anodic polarization [4] and that the dissolution rate is orientation dependent [5]. In addition, InP dissolves chemically in concentrated HCl solutions [6]. In connection with the possible application of photoelectrochemical cells for solar energy conversion, Cardon et al. [3] have investigated the possibility of preventing the photoelectrochemical decomposition by means of competing anodic oxidation of reducing agents. Other scientists have studied p-type-based liquid
Dedicated to the memory of Professor H.W. Nbmberg. * Permanent address: Philips GmbH Forschungslaboratorium Hamburg 54, F.R.G.
l l
0022-0728/86/$03.50
0 1986 Elsevier Sequoia S.A.
Hamburg,
Vogt-Kolln-Str.
30, 2000
686
junction solar cells [7]. In contrast to n-type electrodes, photocathodes are protected against corrosion because electrons are driven towards the interface. In the case of p-type electrodes, however, a relatively high overvoltage was observed for the onset of the cathodic photocurrent with respect to the flatband potential, which seemed to decrease in the presence of a redox couple such as V2+/V3+ [&lo]. Various successful attempts were made by Heller and co-workers to decrease the overvoltage by depositing a metal catalyst on p-type InP [7,8,10]. The overvoltage for the photocurrent onset is a phenomenon which is not entirely restricted to p-InP. It has been observed with a number of n- and p-type electrodes [ll]. In the case of n- and p-type GaAs [ll-131 and n-WSe, [14] electrodes, the overvoltage could be explained by the trapping of minority carriers in surface states, leading to a change of the potential drop across the Helmholtz layer and by recombination processes. InP is an interesting electrode material for studying the parameters responsible for the overvoltage in more detail, because here various redox processes can compete more easily with the hydrogen evolution than on GaAs. Corresponding results obtained with InP are given in the present paper. EXPERIMENTAL
The InP crystals were obtained from the Philips Laboratory in Eindhoven (The Netherlands). The crystal faces were of [lOO] orientation. The carrier density was of the order of some 1018 cmv3 if not stated otherwise. The electrodes were etched in concentrated HCl solutions. All solutions were prepared with reagent-grade chemicals. The electrochemical measurements were made in a conventional cell using a saturated calomel electrode (SCE) as the reference electrode. The scan rate of the electrode potential was 40 mV s- ‘. The impedance measurements were made by imposing a high-frequency modulation, typically 10 mV at 20 kHz, on the potential applied to the cell. Measurements of the phase and amplitude of the corresponding high-frequency current component through the cell, by applying the potentioscan current output signal to a Heterodyne lock-in amplifier, enabled the capacitance to be calculated. For illumination, a 150 W Xe lamp was used installed in an Amco housing (ALH 215). The light passed through a monochromator before entering the cell. RESULTS
In Figs. la and 2a typical current-potential dependences for n- and p-type InP are presented as measured in darkness and under illumination using 1 A4 HCl as electrolyte. The corresponding Mott-Schottky curves, also measured in darkness and under illumination, are given in Figs. lb and 2b. The capacity values could be evaluated in the whole potential range from impedance measurements using an equivalent circuit with a resistance and a capacitance in series. The same equivalent circuit could be applied for electrodes under illumination for the potential range in
687
-10 I’
Iph Inl.x
10
:
u/v
-05
c/
(0)
(b)
% $
10
dark
0
lllum \
I
5
\ \
\
\ \
‘1
\ \
electrode
potentiol
ECE)/V
I
I
-10
-0.5
I
‘1, \
\
0 electrode potenttot ISCEI/V
,‘q 05
1
UP,
Fig. 1. Normalized photocurrent (a) and capacity (b) vs. electrode potential for n-InP in 1 M HCl. Fig. 2. Normalized photocurrent (a) and capacity (b) vs. electrode potential for p-InP in 1 M HCl.
which the photocurrent is constant. In the other range, i.e. that in which the photocurrent rises strongly with increasing potential, we did not attempt to derive capacity values from the impedance data. Other authors [15], studying GaAs electrodes, evaluated the impedance data in this range also, using a very complex equivalent circuit. Since the latter cannot be interpreted unambiguously, we did not adopt this evaluation. The shift of the Mott-Schottky curves upon illumination depends on the light intensity and saturates at higher intensities. These results are similar to those found with p-GaAs [12]. The excitation spectrum of the saturation photocurrent obtained with highly doped n- and p-InP is given in Fig. 3. The absorption curve in the same figure as given in the literature [16] is shown. According to Figs. 1 and 2, the onset potential of the photocurrent occurs in all cases (besides for the 1016 cme3 n-type sample) at potentials considerably different from that of the flatband determined in darkness. We have investigated this effect in more detail at p-InP in electrolytes containing different redox systems. The corresponding photocurrent-potential dependences are presented in Fig. 4. In one case (Eu3+) we also measured the photocurrent-potential curves at different concentrations of the redox system in the range 5 X lop3 < cn,, < 10-l M. The corresponding results indicated a shift of the curves towards positive potentials with increasing concentration. These experiments were limited to a small concentration range because the light intensity had to be reduced to a very low level for measurements at
electrode -10
wovelength/
-05
potentlol lSCE)/V 0
+0.5
Iph Irn.l
nm
I
Fig. 3. Excitation spectrum of photocurrent at p-InP in 1 M HCl. UE = -0.9 V. Fig. 4. Photocurrent vs. electrode potential at p-InP in 1 M HCl in the presence of various redox systems.
much lower Eu3+ concentrations in order to avoid diffusion limitation. Then, however, the photocurrents were small and side reactions dominated in the photocurrent-potential behavior which were not reproducible. Concerning the capacity measurements performed with solutions containing redox systems, it should be
k-4
3 $ 62 E b’
pdb’,;p
I;
wlthout redox syst -?
-0~
-a2
II”/;”
El?+ +OL
+06
Fig. 5. Dark current vs. electrode potential in 1 M HCI + lo-’
M Eu*+. (a) p-InP; (b) n-InP.
689
mentioned that the shift of the Mott-Schottky curves upon illumination is reduced. Practically no shift was detectable any longer in the case of Fe’+. In addition, we also investigated the current-potential behavior of n- and p-type InP in the dark in the presence of the reduced species of various redox couples. A typical result, obtained with Eu*+, is shown in Fig. 5a and b. At the p-type electrode (Fig. 5a), no additional anodic current was found in the presence of Eu*+, whereas in the case of n-type an oxidation current was detectable in the whole potential range (Fig. 5b). Almost identical results were obtained with V*+. DISCUSSION
It is well known from the literature that InP is electrochemically dissolved during positive polarization [1,4,6], whereas during negative polarization mainly hydrogen is produced [l]. Since the anodic dissolution is a valence-band process, the dark current at n-type InP is limited to a very low value. On the other hand the hydrogen evolution proceeds via the conduction band. Accordingly, the latter process occurs at p-InP only during light excitation. The flatband potentials for n- and p-type electrodes in darkness are Ufb”= -0.62 V and +0.65 V vs. SCE, respectively. The difference between the two flatband potentials amounts to 1.27 V. This difference is practically equal to the energy gap (1.3 ev), which is reasonable for highly doped material. It can be concluded from this result that the position of energy bands at the surface is equal for the n- and p-type electrodes, as found for many other semiconductor electrodes [17]. Before discussing the electron transfer reactions, the spectral distribution of the photocurrent should be analyzed further. Analysis of the excitation spectrum
According to Reichman [18], the current carried by the minority carriers during light excitation is given by: I
i
0
P=
i”P
eU
0)
ii + i, exp - kT ) (
in which (Y represents the absorption coefficient, L the diffusion length of the minority carriers, i, the dark saturation current, ‘z the exchange current at equilibrium, U the overpotential with respect to equilibrium and 1, the light intensity. The thickness of the space charge layer d, is given by [17]
(2) in which U, is the potential drop across the space charge region and no the carrier density in the bulk.
690
Assuming (U x=- F),
that i, is sufficiently eqn. (1) is reduced
small and selecting
to the so-called
a large bias toward
G&rtner equation
depletion
[17,19]
Accordingly, the photocurrent depends on the thickness of the space charge layer d, and on the absorption coefficient and consequently on the penetration depth of light (L, = l/a). The thickness of the space charge layer can be varied by changing the band bending U,,, but it is also determined essentially by the bulk majority density no. Since we used rather heavily doped material, d, should be relatively small. For instance in the case of no = 3 X lOI cmp3, d, = lop6 cm for U,, = 1 V. Using light of high penetration depth, (small a) so that ad,, e 1, the exponential term in eqn. (4) can be extended into a series; eqn. (4) can be approximated by [20] L+a-’ eI0 -= (4) L+d, ‘ph As already pointed out by Geiger et al. [20] a plot of eI,/i,, vs. LY-’ should then yield a straight line and the intercept with the c--~ axis should give the diffusion length of the minority carriers, i.e. eZ0 y---t()+L=
--(y-l
(5)
‘ph
The same authors tried to apply eqn. (4) to their photocurrent results. Since they had data available only for 3 or 4 wavelengths, the applicability of eqn. (4) could not be tested definitively. We analyzed the excitation spectra of the photocurrent over a much wider range by using the absorption data given in ref. 16. Replotting the data from Fig. 3 according to eqn. (4) one indeed obtains straight lines, as shown in Figs. 6 and 7. From the intercept we obtained: L,=2x10-6cm
for p. = 6 X 1018 cmp3 (p-type)
L, = 3.5 X lop6 cm
for p. = 3 X 1018 cmp3 (p-type)
Lp=1.4x10-5cm
for no = 2 X 1018 cmm3 (n-type)
\
>.
EL-
‘.
s
‘1
b 2-
‘\\ IIIII,I,II -10 penetrotm
-5
,,,,,,,IIJ,
0 depth of light ,iOOL,/pm
5
I_& 10
1L
Fig. 6. Reciprocal values of photocurrent vs. penetration depth of light for n-InP in 1 M HCl.
691
-15 -
-10 penetrollon
-5 0 depth of llghl ,100
2 L L, /em
Fig. 7. Reciprocal values of photocurrent vs. penetration depth of light for p-InP in 1 M HCI. (a) P0 =6XlO’“cm3; (b) PO= 3 x lo’* cmm3.
Very similar values were obtained from typical solid-state measurements [21]. These results show that the G&tner model describes the long-wavelength region of the excitation spectrum rather well. The decrease of the photocurrent below 450 nm cannot be explained by this model, however. At present we have no reasonable interpretation for this phenomenon. Potential
distribution
during right excitation
The Mott-Schottky curves are shifted during illumination toward negative potentials for p-type and toward positive potentials for n-type electrodes (Figs. 1 and 2). Similar results have been obtained with GaAs [12], GaP [ll], CdS [ll], RuS, [22], and with some layer compounds such as MoSe, and WSe, [14]. In earlier publications we interpreted the shift and the flatband potential through a corresponding change of the potential distribution. Accordingly, the potential drop across the Helmholtz layer 17, must be different during light excitation from that in darkness, at least in the range where the photocurrent is constant. Such a change in U, can only be caused by an accumulation of minority carriers below or at the surface. Their density can be estimated by using the equation [12]
(6) and N, =
Q/e
(7)
in which C, is the Hehnholtz capacity, AU,,,,, the change in the potential drop across the Helmholtz layer and AQ the resulting charge. Assuming C, - lop5 F cm-* and taking from Figs. 1 and 2 the experimental values of AUuu,,, (0.18 V for n-type and 0.3 V for p-type) one obtains N, = 1.8 X 1013 cm-* for p-InP and
692
1 x 1013crne2 for n-InP. As already discussed in earlier papers, the accumulation of the minority carriers can be interpreted by assuming trapping in surface states. The shift of the Mott-Schottky curves and the corresponding change in the flatband potential indicates an upward shift of the energy bands at the surface of the p-type electrode and a downward shift at n-type InP. We have investigated the model in detail for the case of p-GaAs [12]. It was shown by quantitative analysis that the rate of electron transfer from the conduction band to protons in the solution must be small, so that electrons excited by light are easily trapped in surface states. The quantitative evaluation of the intensity dependence-of the flatband shift and of the photocurrent indicated that finally electron transfer from surface states to protons occurs which limits the surface-state charging and the upward shift of the bands [12]. Since the results obtained with p-InP are qualitatively identical to those found with p-GaAs, we assume that the same model is valid here. Charge transfer processes
According to Figs. 1 and 2, the onset of the photocurrent occurs at considerable overvoltages with respect to the flatband potential in the dark. Especially large overvoltages were found with p-type InP and the difference between the onset potential U,, of the photocurrent and the flatband potential U,, is also greater than the shift of the flatband potential upon illumination. The large overpotential may be due partly to problems in the kinetics of hydrogen evolution but must also be due to strong surface recombination [lo]. The recombination rate seems to be even stronger than for p-GaAs [12] because the difference (U,, - U,) is larger for p-InP than for p-GaAs. On the other hand it is remarkable that at p-InP the onset of photocurrent occurs at much less negative potentials if a redox system with a standard potential negative to that of H,/H+ is added to the electrolyte, examples being Cr 3+, Eu3+, V3 + and MV2+ (Fig. 4). This res ult was not found with p-GaAs [12]. In the latter case a smaller overvoltage was observed only with redox systems having a rather positive standard potential, such as Fe3+ or [Fe(CN),13-. We interpreted the latter result by an electron transfer via surface states [12]. From the energy scheme illustrated in Fig. 8 one would expect that the reduction of the other redox couples should occur by an electron transfer directly from the conduction band. From the thermodynamic point of view one should even expect that a direct electron transfer from the conduction band to the redox system is more favorable for GaAs than for InP. As already mentioned above, however, the opposite was found. The energy situation can be illustrated even more clearly by plotting the complete distribution of energy states, as shown for Eu2+13+ in Fig. 9. Here a reorientation energy of about A = 1 eV has been assumed, a value similar to that found for Fe2+/Fe3+ (1.25 eV) [17]. Since the position of the conduction bands of GaAs and InP differ by 0.24 eV, the density of the empty energy states of Eu’+/Eu’+ should be higher by about two orders of magnitude at the lower edge of the conduction band of GaAs than that of
693
EISCEl/eV
InP v-’ f&&C -0 7
0
--_[FelCN1,1’~“~
t +05
GoAs
--FeYFeZ+ zz
+1.0
/EV +lO
redor couple 1o-6 lo-‘
+1.5
Fig. 8. Energy
levels at the interface
Fig. 9. Energy
scheme at the interface
of InP and redox couples
10-Z
1
in 1 M HCl.
InP/Eu2*/3+.
InP. Accordingly, the question arises as to whether strong recombination is really responsible for the large overvoltage of the photocurrent. In connection with this problem another, different electrode behavior for p-InP and p-GaAs should be mentioned. From the energy scheme in Fig. 9 one would expect that the reverse process, i.e. the oxidation of the redox couple, occurs preferably via the valence band because the density of occupied states is much larger at the upper edge of the valence band than at the lower edge of the conduction band. A valence band process, however, requires holes which are not available in n-type semiconductors. Accordingly, the small additional anodic dark current found with n-type InP in the presence of Eu2+ or V*+ (Fig. 5b) must be due to electron injection into the conduction band. This process is expected to be independent of potential, which is confirmed experimentally. In the case of n-GaAs such an oxidation current was not detected. This result is reasonable because the conduction band is located at higher energies, which makes electron injection more difficult, The experiments with p-type InP electrodes did not yield any additional oxidation current either. At first sight, this result seems to be reasonable because the hole density in the valence band at the interface may be too low for a large downward band bending. Corresponding measurements with p-GaAs have shown, however, that large anodic currents do occur in the presence of Eu*+ even at relatively strong downward band bending (e.g. 10 mA cm-* at US, = -0.45 V), as illustrated for comparison in Fig. 10a [23]. The difference in behaviour can certainly not be interpreted on the basis of the energy scheme in Fig. 9. These considerations show quite clearly that the electron transfer from Eu2+ or Vzt into the valence band of InP must be inhibited by a different process. The
694
inhibition may be related to a thin oxide film which is easily formed on InP [&lo]. Thicknesses of about 1 nm are reported in the literature [lo]. Assuming that the empty states of the oxide are located close to the conduction band, the oxide would not block the electron transfer from the conduction band to Eu3+. The reverse process, i.e. electron transfer from Eu2+ into the valence band, however, can occur only at a considerably reduced rate because the electrons must tunnel through the oxide, as shown in Fig. 11. It should once more be emphasized that reduction and oxidation of the europium redox couple can occur via different energy bands at p-GaAs. Large oxidation currents were observed in the potential range where no cathodic photocurrent was detectable. Accordingly, if an electrolyte is used which contains only the oxidized species of the redox system, e.g. Eu3+, then the reduced species (Eu2’) is formed during illumination by electron transfer via the conduction band. The latter (Eu2’) would then be reoxidized by hole transfer via the valence band. In this case the cathodic photocurrent would be compensated by an anodic dark current, provided that the Eu2+ produced in the first step does diffuse away from surface. Accordingly, in this model the large overvoltage for the onset of photocurrent found with most redox systems at p-GaAs could be interpreted by a recombination shunt in the
-
Eu3’
hv (
Eu ‘+
E”
-+ -06 (bl : 6 02
06 -0L -02
Eu2’
0 +Q2 +OL U(SCE)V
n-GoAs dark ELI” p-InP
Fig. 10. Dark current vs. electrode potential p-GaAs: (b) n-GaAs (from ref. 23).
for GaAs in 1 M HCI+10-2
Fig. 11. Energy levels of p-InP covered with a thin oxide layer.
M Eu”
at 5000 rpm. (a)
695
electrolyte [23]. This model would also explain the lower overvoltage at p-type InP because the photocurrent cannot be compensated due to the blocking of the valence band process by the oxide layer. However, it is difficult to prove this model. A detailed estimate of the kinetics, especially of the diffusion of the Eu*+ away from the surface, has shown that a recombination shunt in the electrolyte is possible in principle [23]. The diffusion of Eu*+ away from the electrode surface can be influenced by using different speeds for a corresponding rotating disc electrode. Accordingly, one should expect that at high rotation speeds Eu*+ moves sufficiently fast into the electrolyte so that the reoxidation probability is reduced, i.e. the photocurrent onset should occur at less negative potentials. However, we failed to see any shift of the photocurrent onset upon changing the rotation speed. The model would still work if the Eu*+ remains adsorbed for a sufficient time. Since this assumption seems to be rather unlikely we have to rule out this model. Accordingly, overvoltages in the photocurrent onset must be explained by surface recombination as also suggested for p-Gap electrodes [25]. Further, it has to be concluded that the surface recombination rate must be smaller for InP, possibly due to the presence of the oxide [24]. The onset of the photocurrent should depend on the concentration of the oxidized species of the redox system, as confirmed experimentally with Eu3+ within a limited concentration range. It is interesting to note that Heller et al. [8,10] have found relatively high photovoltages in regenerative cells containing both Eu*+ and Eu3+ or V*+ and V3+. This must be due to the fact that the oxidation of Eu*+ or V *+ is blocked at p-InP by the oxide layer. So far, charge transfer processes with redox systems of rather negative standard potentials were discussed extensively. On the other hand, very small over-potentials were found with redox systems exhibiting a relatively positive standard potential, such as for instance Fe3+ (Fig. 4). This was also found with p-GaAs. Since in this case no shift of the flatband potential occurred upon illumination, we assumed an electron transfer via surface states at the GaAs-electrode [12]. The same interpretation could be applied in principle for InP. The question arises, however, as to whether the surface states are located at the InP/oxide or at the oxide/electrolyte interface. In the first case the electrons trapped in surface states have to tunnel through the oxide, which should lead to a rather low rate. In addition, one may ask whether an assumption of electron transfer via surface states is really conclusive. With respect to this problem the relative position of energy states on both sides of the interface InP/oxide/Fe *+13+ should be inspected more closely (Fig. 12). For simplicity and better comparison with the case for europium (Fig. 7), we used the same reorientation energy of X = 1 eV. Assuming further that the empty energy states of the oxide are located only slightly below the conduction band of InP, the maximum of the distribution of the empty energy states in the electrolyte occurs close to the conduction band. The density of empty states of the Fe3+/Fe2+ couple exceeds that of the Eu3+/Eu2+ couple by orders of magnitude at E = E,” (compare Figs. 12 and 9). Accordingly, the rate of electron transfer from the oxide to Fe3+ should be much higher than in the case of Eu3+, as was found experimentally. As regards the large overpotential for the photocurrent onset in solution free
696 E ISCEl/eV eV
log D
Fig. 12. Energy scheme at
the interface InP/Fe3+j4+.
from any redox system, it should be mentioned finally that Heller et al. found a pronounced displacement of the photocurrent onset towards positive potentials after having deposited a surface film of platinum, ruthenium or silver on p-InP [7,8,10]. In a very recent paper by this group, it was shown further that a Pt film of 2 nm thickness was sufficient to create such a shift [26]. The authors concluded from this observation that platinum catalyzes the hydrogen evolution. Accordingly, the electron transfer becomes faster than the recombination process. In corresponding experiments we made similar observations; our results were not very reproducible, however, which may be due to the deposition technique. It must be emphasized here that a positive displacement of the photocurrent onset does not necessarily mean a decrease of the overvoltage with respect to the flatband potential. Since a metal was deposited on InP, the potential distribution, and consequently the flatband potential, may be different from that of metal-free surfaces. For instance, Aspnes and Heller [27] have shown that contact between a p-InP electrode and a thin layer of Pt saturated with hydrogen (without any electrolyte) leads to a large Schottky barrier, i.e. the energy bands are bent downwards at equilibrium. According to these data, the Fermi level is located about 0.4 eV below the conduction band at equilibrium. If such a p-InP electrode with its Pt surface is brought into contact with an electrolyte, it is difficult to predict the position of the energy bands. Accordingly, one cannot predict the flatband potential nor the overvoltage of the photocurrent onset with respect to the flatband potential. Corresponding Mott-Schottky measurements have not been published yet. These may be difficult because for very thin Pt-layers the electrolyte not only makes contact with the platinum but also with the bare InP surface. In this case one may have different positions of the energy bands in areas
697
where the electrolyte is in direct contact with the InP and those where it is in contact with the Pt, as discussed recently by Nakato and Tsubomura [28]. CONCLUSION
In the present paper we have described various electron transfer processes at nand p-type InP. Especially the different onset potentials of cathodic photocurrents at p-InP with and without redox systems were analyzed and compared with corresponding results obtained with GaAs. According to the relative position of the energy bands on both sides of the interface, the reduction of the redox systems considered here occurs via the conduction band, whereas the oxidation is expected to proceed via the valence band, as concluded from comparable measurements with p-GaAs. This valence band process did not occur at p-InP because the transfer of holes seems to be blocked by the oxide layer. The result that the oxidation and reduction of a redox system occur via different energy bands is caused by the relatively large reorientation energy [23]. In order to avoid charge transfer processes via both bands, investigations with redox systems whose molecules contain large ligands or are embedded in rigid organic cages (small h values), may be more favorable. ACKNOWLEDGEMENTS
The authors are very much indebted to Dr. J.J. Kelly, Philips Research Laboratories, Eindhoven (The Netherlands) for the generous supply of InP single crystals as well as for many valuable discussions. Financial support by the Deutsche Forschungsgemeinschaft and by the Fonds der chemischen Industrie is gratefully acknowledged. REFERENCES 1 A.M. van Wezemael, W.H. Laflere, F. Cardon and W.P. Comes, J. Electroanal. Chem., 87 (1978) 105. 2 M.P. Dare-Edwards, A. Hamnett and J.B. Goodenough, J. Electroanal. Chem.. 119 (1981) 109. 3 F. Cardon, W.P. Comes, F. van de Kerchove, D. van Mackelbergh and F. van Overmeire, Faraday Discuss. Chem. Sot., 70 (1980) 153. 4 A.B. Ellis, J.M. Bolts and MS. Wrighton, J. Electrochem. Sot., 124 (1977) 1603. 5 P.A. Kohl, C. Wolowodink and F.W. Ostermayer, J. Electrochem. Sot., 130 (1983) 2288. 6 P.H.L. Notten, J. Electrochem. Sot., 131 (1984) 2641. 7 A. Heller in J. Rabani (Ed.), Photochemical Conversion and Storage of Solar Energy, The Weizmann Science Press, Jerusalem, 1982, p. 63, and literature cited therein. 8 A. Heller and R.G. Vadimsky, Phys. Rev. Lett., 46 (1981) 1153. 9 K. Kosaki and H. Kita, Sol. Energy Mater., 8 (1983) 421. 10 A. Heller, H.J. Leamy. B. Miller and W.D. Johnston, J. Phys. Chem., 87 (1983) 3239. 11 R. Memming and J.J. Kelly in J.S. Connally (Ed.), Photochemical Conversion and Storage of Solar Energy, Academic press. New York, 1980, p. 243. 12 J.J. Kelly and R. Memming, J. Electrochem. Sot., 129 (1982) 730. 13 K. Schrader and R. Memming. Ber. Bunsenges. Phys. Chem., 89 (1985) 385.
698 14 A.J. McEvoy, M. Etman and R. Memming, J. Electroanal. Chem.. 190 (1985) 225. 15 P. Allongue, H. Cachet and G. Horowitz, J. Electrochem. Sot., 130 (1983) 2352. 16 B.O. Seraphin and H.E. Bennett in R.K. Willardson and A.C. Beer @is.), Semiconductors and Semimetals, Vol. 3, Academic Press, New York, 1967, p. 499. 17 R. Memming in B.E. Conway, J.O’M. Bockris, E. Yeager, S.U.M. Khan and R.E. White (Eds.), Comprehensive Treatise of Electrochemistry, Vol. 7, Plenum Press, New York, 1983, p. 529. 18 J. Reichman, Appl. Phys. Lett., 35 (1980) 574. 19 W. Gtitner, Phys. Rev., 116 (1953) 84. 20 T. Geiger, R. Nottenberg, M.L. Pelaprat and M. GrBtzel, Helv. Chim. Acta, 65 (1982) 2507. 21 J.J. Kelly, private communication. 22 S. Piazza, H.M. Kbhne and H. Tributsch, J. Electroanal. Chem. 1% (1985) 53. 23 D. Meissner, C. Sinn, R. Memming, P.H.L. Notten and J.J. Kelly in E. Pelizetti (Ed.), Homogeneous and Heterogeneous Photocatalysis, Reidel, Dordrecht, 1986, p. 317. 24 A. Heller, B. Miller and F.A. Thiel, Appl. Phys. Lett.. 38 (1981) 282. 25 W.J. Albery and P.N. Bartlett, J. Electrochem. Sot., 129 (1982) 2254. 26 A. Heller, D.E. Aspnes, J.D. Porter, T.T. Sheng and R.G. Vadimsky, J. Phys. Chem., 89 (1985) 4444. 27 D.E. Aspnes and H. Heller, J. Phys. Chem., 87 (1983) 4919. 28 J.Y. Nakato and H. Tsubomura, J. Photochem., 29 (1985) 257.