J. Phys. Chem. 1988, 92, 4103-41 10 electron scavenger is present. The rest of the deviation could be due to experimental scatter. However, it is interesting to note that, whereas the fit in Figure 7 is not perfect, a plot of l/Yo vs [SI"with n = 1 andfdlrect= 0.08 gives an ideal fit to the data (see Figure 10). The same conclusion was reached by Choi and L i p ~ k y .They ~ measured the effect of perfluoro-n-hexane on the steady-state fluorescence of irradiated cyclohexane and extracted n = 1 andfdlrect= 0.1. As Choi and Lipsky pointed out, the physical meaning of n = 1 in the case of electron scavenging is not known. The interpretation of information available on the efficiency of scavenging of geminate electrons in the radiolysis of cyclohexane is uncertain. For example, at 0.05 M scavenger, our values of n = 0.6 and a = 50 M-I for C 0 2 and n = 0.7 and a = 43 M-' for perfluoro-n-hexane result in 63.4% and 63.7% scavenging, respectively. The values of n = 0.5 and a = 13 M-' for C 0 2 given by Klein and Schuler16 give 44.6% scavenging. The values of n = 1 and a = 50 M-' obtained by Choi et aL4 for perfluoro-nhexane result in 71.4% scavenging. The value of a should be proportional to the rate constant for reaction with the electron.'* The rate constant for CO, is 4.3 X lo', M-I s-I,l9 and the value for perfluoro-n-hexane is 1.6 X 10l2 M-' sTherefore, the similarity of our results for C 0 2 and perfluoro-n-hexane, as well as the disparity between our perfluoro-n-hexane result and the Klein and Schuler C 0 2 result, is unexpected. The difference between our results and those of Choi et al. is not as large and could perhaps be due to experimental uncertainties. The magnitude of the discrepancy between our results for C 0 2 and N,O and those of Klein and Schuler is further illustrated by the lower curves in Figures 8 and 9, which are drawn using eq IV withfdlrect (18) Rzad, S. J.; Infelta, P. P.; Warman, J. M.; Schuler, R. H. J . Chem. Phys. 1970, 52, 3971. (19) Baxendale, J. H.; Keene, J. P.; Rasburn, E. J. J. Chem. Sot., Faraday Trans I 1974, 70, 718. (20) Sauer, Jr., M. C.; Schmidt, K. H., unpublished results.
4103
= 0, Y = 2.9 M-I, and the values of Klein and Schuler for n and a.
Conclusions The present experiments have clearly shown that most or all of the cyclohexane excited state (CH*) can be attributed to ion recombination reactions. This is similar to the results of Choi et al.$5 but is in conflict with the results of Busi et aL3 The present measurements are considerably simpler than any of the previous experiments in that the effect of an electron scavenger on the initial yield of the excited state and its effect on excited-state molecules are directly separated in our measurements. The lifetime of CH* was determined from our experimental measurements to be 0.88 ns in good agreement with previously measured v a 1 ~ e s . l ~ The dependence of the initial yield of CH* on concentration of electron scavenger is not compatible with the results of studies where a product resulting from the capture of the electron was measured.15J6 The latter results, when compared with those presented here (and those of Choi and L i p ~ k y ~on , ~CH*, ) indicate that the efficiency of electron scavenging is appreciably lower than the efficiency of the reduction of the CH* from geminate-ion recombination. This may mean that the implicit assumption that the formation of the excited state depends only on the reaction of an electron and the positive ion is not valid. That is, there could be a change in the positive ion (for example, a change in internal energy, rearrangement of the positive ion, or fragmentation of the positive ion) which would affect the probability of CH* production from the ion recombination reaction. Further work is needed to explore the consequences of such a suggestion and whether it can explain the disparate experimental observations. Acknowledgment. We would like to thank Don Ficht, George Cox, and Ed Kemereit for running the accelerator and making the experiments possible. Registry No. CO,, 124-38-9; N,O, 10024-97-2; cyclohexane, 11082-7; perfluoro-n-hexane, 355-42-0.
Adsorption of Telluride Ions on Cadmium Telluride. Consequences for Photoelectrochemical Cells D. Lincot* and J. Vedel Laboratoire d'Electrochimie Analytique et AppliquPe (UA 216), ENSCP, 1 1 , rue Pierre et Marie Curie, 75231 Paris Cedex 05, France (Received: April 20, 1987; In Final Form: January 4, 1988) The adsorption of telluride ions on cadmium telluride electrodes is studied in a basic solution (1 M NaOH) by means of capacitance measurements. Negative shifts of the flat-band potential are observed (by up to -0.7 V) when telluride ions are introduced in the solution (the upper concentration was about lo-* M) showing the adsorption of negative charges at the surface. This process is dependent upon the polarity of the surface: the adsorption is stronger on Cd-terminated (1 11) surfaces than on Te ones. The results are compared with a model of adsorption based on the Langmuir isotherm. Within this hypothesis, it appears that the process involves more likely the net exchange, at the surface, of an unitary charge, HTealone or Te2-with OH- substitution. When the interface is illuminated,the adsorption leads to a$egative shift of the photocurrent onset as compared with its position in the blank electrolyte. A transition from a one-wave to a two-wave photocurrent/voltage curve appears when the illumination level increases, associated with a large positive shift of the flat-band potential. The first wave is attributed to the oxidation of telluride ions in solution whereas the second one corresponds mainly to the corrosion reaction. These results are interpreted by considering that the band edge position under illumination is fixed by the kinetics of the electrode reactions. A simple phenomenological model is presented to discuss the interfacial behavior.
Introduction One major problem encountered in the field of photoelectrochemical conversion is the photocorrosion of the semiconductor. To decrease it, many efforts are currently made.' They deal with (1) (a) Heller, A. Semiconductor Liquid Junction Solar Cell; Electrochemical Society: Pennington, NJ, 1977. (b) Nozik, A. J. ACS Symp. Ser. 1981, 146. (c) Schiavello, M. Photoelectrochemistry, Photocatalysis and Photoreactions; Reidel: Dordrecht, 1985.
0022-365418812092-4103$01.50/0
the optimization of the solution and with semiconductor surface treatments in order to increase the kinetics of the stabilization reaction toward the decomposition one. A redox species that adsorbs on the electrode surface appears to be very promising to suppress photocorrosion.24 This is shown, for example, by the (2) Wrighton, M. S.; Bolts, J. M.; Bocarsly, A. C.; Palazzotto, M. C.; Walton, E. G.J . Vac. Sci. Technol. 1978, 15, 1429.
0 1988 American Chemical Society
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The Journal of Physical Chemistry, Vol. 92, No. 14, 1988
stabilization of CdX (X = S, Se, Te) photoanodes in the presence of chalcogenide ions (X2-),5-7awhich are adsorbed at the semiconductor surface.7b12 Similar results have been obtained with organic thiols at CdS and CdSe electrodes.3 However, adsorption studies at semiconductor electrodes are still little developed. In addition to the previous cases, they concern, for instance, the adsorption of OH- ions on various electrodes,13cations on Sn02,14 chloride ions on GaAs in molten salt^,'^^'^ and a great variety of species on silicon-based electrodes (inorganic cations and even biological molecules) in the field of ISFET devices.17 In this paper we present new experimental results concerning the adsorption of Te2- ions on CdTe in darkness as compared with the previous studies of Ellis et al.,8 and its consequences on the behavior under illumination. Energetic modifications occurring at the interface are obtained from capacitance measurements during the potential scannings under illumination. A model is then proposed to discuss the experimental results. The interest of CdTe/telluride photoelectrochemical cells has been recently illustrated by Lyons et al.7a They show that efficiencies close to 15% are achievable by using concentrated telluride solutions (0.1 M) in 5 M KOH.
Experimental Section The samples of CdTe are cut from ingots prepared by a Bridgman technic at the Laboratoire de Physique des Solides (CNRS, Meudon, France). They are indium-doped n-type sam. encapsulation in an epoxy ples in the range of I O l 7 ~ m - ~Before resin, except the front surface (a few mm2), a back Ohmic indium contact is taken on the rear surface. Before the experiments the surface is mechanically polished down to 0.2-pm grade and etched in a bromine in methanol solution (0.5%) for at least 30 s. Electrolytic N a O H solutions are prepared with reagent grade chemicals. Telluride ions are introduced in situ by an electrochemical method: A solution of ditelluride ions, Te22-, is first prepared by the cathodic reduction of an elemental tellurium electrode at -1.2 V vs SCE. The introduced concentration is calculated from the integration of the current. Then the Te electrode is taken out of the solution, and the Te2- ions are further reduced in Te2- ions on a mercury pool electrode at -1.6 V vs SCE. In both steps the counter electrode is placed in separate compartment. These operations are repeated for each required concentration of telluride ions. The experimental arrangement is described elsewhere.18 It allows to record simultaneously the current-voltage and capa-
(3) Natan, M. J.; Thackeray, J. W.; Wrighton, M. S. J. Phys. Chem. 1986, 90, 4089.
(4) Evenor, M.; Huppert, D.; Gottesfeld, S. J. Electrochem. SOC.1986, 133, 296. (5) (a) Ellis, A. B.; Kaiser, S. W.; Wrighton, M. S. J. A m . Chem. SOC. 1976,98, 1635. (b) Ellis, A. B.; Kaiser, S. W.; Wrighton, M. S. J . Am. Chem. SOC.1976, 98, 6418. (6) Hodes, G.; Manassen, J.; Cahen, D. Nature (London)1976, 261,403. (7) (a) Lyons, L. E.; Morris, G . C.; Raftery, M. A.; Young, T. L. Aust. J. Chem. 1987,40,655. (b) Lyons, L. E.; Young, T. L. Ausr. J . Chem. 1987, 40. 723. (8) Ellis, A. B.; Kaiser, S. W.; &Its, J. M.; Wrighton, M. S. J. Am. Chem. SOC.1977, 99, 2839. (9) Minoura, H.; Watanabe, T.; Oki, T.; Tsuiki, M . Jpn. J . Appl. Phys. 1977, 16, 865. (10) Ginley, G. S.;Butler, M. A. Electrochem. SOC.1978, 125, 1968. (1 11 Inoue, T.: Watanabe. T.: Fuiishima, A,: Honda. K. Bull. Chem. SOC. Jpn. 1979, 52, 1243. (12) Frese, K. W.; Canfield, D. J . Electrochem. SOC.1984, 131, 2614. (13) (a) Gerischer, H. Physical Chemistry-An Advanced Treatise; Eyring, H.,Ed.; Academic: New York, 1970; Vol. IXA. (b) Morrison, S. R. Electrochemistry at Semiconductor and Oxidized Metal Electrodes; Plenum: New York, 1980. (14) Uchida, I.; Akahoschi, H.; Toshima, S. J. Electroanal. Chem. Interfacial Electrochem. 1978, 88, 79. (15) Singh, P.; Singh, R.; Gale, R.; Rajeshwar, K.; Dubow, J. J. Appl. Phys. 1980, SI,6286. (16) Rajeshwar, K.; Mraz, T. J. Phys. Chem. 1983, 87, 742. (17) Martelet, C.; Jaffrezic-Renault, N. Spectra 2000 1986, 14, 21. (18) Lincot, D.; Vedel, J. J . Electroanal, Chem. Interfacial Electrochem. 1987, 220, 179.
Lincot and Vedel
2
-1 5
1
-0 5
0
APPLIED POTEMIAL VSCE)
Figure 1. Capacitancevoltage curves obtained on n-CdTe (oriented near ( I l l ) a ) for various concentration of telluride ions in solution (1 M NaOH): [Te*-] = 0 ( l ) , 10” (2), (3), lo4 (4), and lo-’ M ( 5 ) . Insert: Corresponding values of flat-band potentials and calculated adsorbed charges for C, = Fan-*.
citance-voltage curves at various frequencies between a few Hz and 200 kHz. The variation of the capacitance is presented here as (capacitance)-2/voltage curves measured at 50 kHz. Under these conditions the capacitance is almost that of the space-charge layer of the semiconductor.ls In the depletion range it is given by the relation 1/cSC2
= (2/qeedv,)(vSC - k T / q )
(1)
where t is the relative dielectric constant of the semiconductor (=lo for CdTe), ND its donor concentration, and V, the potential drop across the space-charge layer. When the band edges are fvted at the interface, V ~ isClinearly related to the applied potential V by Vs, = V - VFB (2) VFB being the flat-band potential (Vsc = 0). Replacing (2) in (1) leads to the well-known Mott-Schottky relation:
CX-* = (2/qetdVD)(V- VFB
- kT/q)
(3)
Relation 2 is often not observed in the whole range of applied potential but only in limited ranges. In those regions the value of VFBobtained from relation 3 by extrapolation is not the true flat-band potential. However, this extrapolated value gives the band edge position in the region of extrapolation, by taking into account the difference between the conduction band and the Fermi level in the bulk, VBc= V , - 0.1 V. This will be the case in the present study.
Results and Discussion 1 . Adsorption Studies in Darkness. a. Results. In Figure
1 are reported the Cz- Vcurves obtained on an n-type electrode, for different concentrations of Te2- ions in sooriented (1 1l)a, lution, between 0 and about M. At each concentration the curve is a straight line according to relation 3. As expected from this relation, the slope is independent of the concentration of Te” ions since it depends only on the doping level of the electrode, ND. The effect of Te2- addition is to shift the extrapolated flat-band potential toward more negative values. Taking into account one basic relation at semiconductor electrodes between V F B and the potential drop across the Helmholtz layer, VH V F B - V, = constant (4)
the change of VFB is associated with an equal change of VH. As a consequence, it reflects any change of the charge or dipole configuration in the inner Helmholtz layer. In the presence of adsorption of ionic species, the change of VH,and thus of VpB, is given by the relation AVH =
(5) = a,q(ANads/CH) taking the hypothesis that the Helmholtz layer behaves like a plane AQab/cH
Adsorption of Telluride Ions on Cadmium Telluride
The Journal of Physical Chemistry, Vol. 92, No. 14, I988
A
4105
'
0
-4
A -5
-2
LOG, 0 [ Te2-] log [X']
.D
Figure 2. Calculated curves showing influence of the total superficial density of adsorption sites on the adsorption isotherm for a monovalent species: C, = lo-' Fa-?, AGO = -6.8 kcal/mol, PH= 0.
capacitor with a constant capacitance C H . AQ,* is the net change of adsorbed charge, ANadsthe corresponding change of the superficial density of adsorbed ions, and cyi their number of charges. From the values determined from Figure 1 it appears that V,, (Le., VH) varies closely linearly with the logarithm of the concentration of Te2- ions, as reported in the insert of Figure 1 , with a slope of about -65 mV. This indicates, from relation 5, the net adsorption of negative species. This behavior can be compared with that reported for the adsorption of sulfide ions on CdS giving slopes between -50 and -75 mV.9-11 Values of about -60 mV have been recently measured in the CdTe/selenide system for freshly prepared electrode^.'^ If we consider a Helmholtz capacitance value of F-cm-2, the total charge of VH corresponds to 5 X lOI3 cm-2 adsorbed charges. This value is associated with a coverage of about 10% of the total density of atomic sites at the surface ("3 X loi4cm-2 for a (1 11) face of CdTe). b. Discussion. These results can be discussed by considering the general adsorption equilibrium X"-(sol) + S s S-X"-(ads) (6) where X"-(sol) is the adsorbate in solution and S an adsorption site at the electrode surface. The equilibrium situation corresponds to a zero change of the energy of the system: AGO+ R T log [a(SX"-)]/[a(X"-)U(S)]
- nFVH = 0 (7)
with AGOthe standard variation of the energy of reaction, (6), and a, the activities of the different species which will be taken here proportional to their concentrations (Langmuir-type adsorption). This assumes that the interaction between adsorbed species is weak, which is justified by the upper value of filled adsorption sites (10%). Different relations have to be considered in other cases [see ref 191. The Helmholtz potential is then written as
PH= I/oH + A v H with A v H = -nqNse/CH
(8)
where B is the fraction of occupied sites, Ns the total concentration of sites, and PH the value of VH at zero coverage, that is, in the blank electrolyte. Relation 7 can be rewritten, giving with decimal logarithm log [x"-]
-3
= log (e/(l - e)) - ( ~ F / ~ . ~ R T )-kAD~ H (9)
with D = AGo/(2.3RT) - ( n F / 2 . 3 R T ) p ~
(10)
which is a constant. Replacing 0 by its value as a function of AvdH from (8) gives the relation between the shift of band edges ( A v H ) and the concentration of X"- ions in solution. On Figure 2 are reported the curves calculated for different values of Ns and a set of reasonable parameters: n = 1 , vDH = 0 (arbitrary choice), and AGO= -6.8 kcal/mol (this supposes a
Figure 3. Dependence of flat-band potential with concentration of Te" ions and polarity of the exposed surface for two different holders (square and triangular symbols): filled symbols (1 11)Cd; open symbols (1 1 l)Te. Dashed line: theoretical slope of 60 mV/decade.
rather strong adsorption, of the order of the value given by Frese and CanfieldI2 for the adsorption of S2- on CdSe electrodes at pH 14). In each case IAVHl increases with the concentration till it reaches a limiting value corresponding to the complete coverage. However, there is a range especially for large values of Ns, where a quasilinear portion appears with a slope close to 60 mV ( ~ 2 . 3 R T / F ) . This extends for any value of n leading to slopes close to 60/n mV. As a consequence, the experimental results are associated with a value of unity for n. This implies that the observed behavior is not due to Te2- ions only. This result is in agreement with the conclusions of Ginley and Butler" in a work on the CdS/S2system, who observed a 60-mV slope and proposed that HS-, and no S" ions as expected, are the adsorbing species. Following their conclusions, HTe- ions should be in this case the adsorbed species. Lyons et al.7b also propose an adsorption of HSe- ions in the CdTe/selenide system but through an exchange reaction between Se- and adsorbed selenium. Another possibility is that the adsorption occurs via Te" ions by replacement of a monovalent anion already adsorbed on the surface, such as the hydroxyl ion OH-, leading also to a net exchange of one charge per site. This supposes that OH- ions are adsorbed on the surface in the blank electrolyte. Frese and Canfield12proposed such a mechanism for the CdSe/S2system, for which they found a pH dependence of VFBin basic solutions contrary to the independence observed at lower pH values.20 The same behavior is found for CdS electrodes,21 meaning that this explanation could be also valid in that case. The presence of OH- ions at the surface of CdTe is also expected in basic solutions: flat-band-potential shifts with pH have been observed by us either in darkness22or under i l l ~ m i n a t i o n . ' ~ * ~ ~ Moreover, it can be argued that, in basic media, the corrosion of CdTe involves species containing the OH- ligand via reaction (1 1) further in the text. As a consequence, intermediates of decomposition containing OH- ligands are presumably present at the surface under anodic polarization. Effect of Surface Orientation. Only a few studies report on the effect of surface orientation on adsorption [for example ref 7b, 9, and 171. In the case of sulfide ions on CdS, Minoura et a1.9 reported a strong anisotropy of the adsorption with the polarity of CdS( 1 1 1) surfaces: S2- ions (or HS-IO) adsorb only on cadmium-terminated surfaces and Cd2+ ions preferentially on sulfur-terminated surfaces. This behavior is also observed, but with a weaker extent for CdTe in presence of Se2- ions.7b This behavior could be the same for Te" ions on CdTe. In order to check this point, we determined the band edge position for both (1 11) polarities. To be sure that the experimental conditions remain and (1 1l)Tcsamples were mounted in the same identical, (1 1l)cd (20) Frese, K. W., Jr. J . Appl. Phys. 1982, 53, 1571. (21) Dewitt, R.; Kirsch de Mesmaeker, A. J . Electrochem. SOC.1983, 130, 1995. (22) Lincot, D.; Vedel, J. J . Electroanal. Chem. Interfacial Electrochem.
1984, 175, 207.
(19) Gileady, E. Electrosorpfion; Plenum: New York, 1967.
(23) Lincot, D.; Vedel, J. J . Cryst. Growth 1985, 7 2 , 426.
4106 The Journal of Physical Chemistry, Vol. 92, No. 14, 1988
L
I
-1-5
I
-1
I
- 0.5
I
Lincot and Vedel
-’5
0
-05
1 APPLIED
POTENTIAL
(VSCE)
APPLIED POTENTIAL (VECE) A
Figure 4. Current-voltage curves of n-CdTe under illumination for different electrolyte compositions: 1 M NaOH (l), +Te?- ions only (2), +Te2- ions only (3). Potential scan rate 20 mV/s. Vi are the flat-band potentials measured in darkness. epoxy holder. The results obtained with two different holders (square and triangle symbols) are presented in Figure 3. They demonstrate clearly the influence of the polarity on the adsorption process on CdTe surfaces. For (1 11)Cd surfaces the variation is similar to that described previously, with no saturation of the shift of VFB.The slope of 60 mV (indicated in the figure) is not far from that deduced from the experimental points. For (1 1l)Te surfaces the band edge position levels off, indicating that the densities of available absorption sites are much lower than for the (1 11)Cd surfaces. The difference in the heights of saturation obtained between the two (1 1l)Tesurfaces probably arises from small changes in the surface preparation. Experimental results can be compared with the calculated curves of Figure 3 for Ns = 2.5 X lo3 cm-2 and 5 X 1014 cm-2, for example. It can be mentioned that the potential translation between the linear portions of the calculated curves (about 100 mV) seems to be also verified experimentally. 2. Behavior under Illumination. a. Results. Influence of Electrolyte Composition. In Figure 4 are reported I-V curves recorded under illumination in 1 M N a O H only (curve l ) , with either ditelluride ions (curve 2) or with telluride ions (curve 3). In the latter cases the photocurrent onset is shifted negatively by more than 0.6 V, showing that both kinds of ions are photoxidized on n-CdTe. These shifts are related with those of the flat-band potentials which are indicated in the figure. The lower shift of the photocurrent onset observed with ditelluride ions is due to the appearance of a large reduction current of ditelluride ions in telluride ions. General Behavior in Telluride Solutions. In Figure 5 is shown the influence of the illumination intensity on both I-Vcurves and C2-Vcurves in a solution containing only telluride ions. When the illumination intensity increases, a remarkable effect appears around a photocurrent threshold value of 0.2 mA cm-z under these experimental conditions (Figure 5a): For values lower than the threshold the I-Vcurves present only one wave corresponding to curve 3 in Figure 4, the height of which varies linearly with the light intensity. The corresponding Cz-V curves shift slowly toward positive potentials when the illumination increases (Figure 5b). The value of the flat-band potential is kept at about -2 V as a consequence of the adsorbed charge. For illumination levels higher than the threshold a second wave appears on the I-V curve, its onset being at the same position as that of the unique wave observed in the pure NaOH electrolyte (Figure 4, curve 1). On the Cz-V curve (Figure 5b) the appearance of this second wave is associated with a large change of the flat-band potential toward more positive values (Figure 5b). The shift of band edges occurs mainly in the A-B range as demonstrated by the almost constant value of the capacitance in this range; that means that the space-charge potential is also constant, any change of the applied potential appearing across
2
N
w
! a +
2 4
2
‘ 5
05
1
APPL’ED POTENTIAL
IV
SCE,
Figure 5. Influence of the illumination level on the characteristics of n-CdTe in a telluride ion solution ([Te2-]= lo-) M), magnetic stirring. (a) Current-voltage curves (20 mV/s). (b) Simultaneous (capacitance)-2/voltagecurves. Insert: shift of band edges with the photocurrent value. Dashed line, extrapolated behavior. the Helmholtz layer. When the illumination level still increases, the height of the second wave remains constant whereas that of the first still increases. The flat-band potential comes back close to that obtained in the blank electrolyte with few further changes under increasing illumination levels. The appearance of the second current wave leads to an enhanced hysteresis on the I-Vcurves. This indicates that the characteristics are not stationary under these conditions. Figure 6 shows the influence of the rotation rate w of the electrode on the I-Vcurves. The change of the height of the first wave demonstrates that it is controlled by diffusion of ions from the solution. This behavior has already been observed on the CdS/S*- system by Inoue et al.” This is confirmed by the linear relation observed between the current and w l / * . From the value at 1000 rpm, taking the concentration in solution of 6.34 X 10’’ cm-3 M) and assuming the thickness of the Nernst layer to be 1.5 X cm, the diffusion coefficient is evaluated as D =2 X cm2 s-l, which is a reasonable value. Band Edge Shifts in Telluride Solutions. In order to study more precisely the band edge shifts, experiments have been performed by recording the capacitance at a fixed potential value, in the photocurrent plateau region (here -0.57 V/SCE), as a function of slowly varying illumination level. In these conditions the use of relation 3 gives the corresponding values of the flat-band potential. Figure 7 gives the results obtained for different rotation rates of the electrode. For a given value two different behaviors can be observed. At lower illumination levels (region A-B for w = 1000 rpm) the variation is closely linear and weakly dependent on the rotation rate. When the illumination level variation is reversed in this region, the hysteresis remains small. This indicates that the interface is in a stationary condition which corresponds to the one wave situation. For higher levels (region B-C for w = 1000 rpm) the shift of band edges becomes steeper. This is
The Journal of Physical Chemistry, Vol. 92, No. 14. 1988 4107
Adsorption of Telluride Ions on Cadmium Telluride
i
a NE 0
4
E 1
I
r /
0
,
,
,
I
,
,
,
,
-05
-1
POTENTIAL ( V / SCE )
b
~
SEMICONDUCTOR
SOLUTION
F g v e 8. Correlation between the shift of band edges under illumination and the redox potentials of the charge-transfer reactions (pH 14): (-) band diagram in the stabilization regime ( [Te2-] = M); (- -) band diagram in the corrosion regime.
-
/+ 0
I
I
I
I
10
20
30
40
[ROTATION RATE ( rpm ) ] I M
Figure 6. (a) Influence of rotation rate of the electrode on the I-Vcurves under illumination: 100 (l), 200 (2), 300 (3), 400 (4), 500 (5), 650 (6), 800 (7). 1000 (8), 1250 (9), and 1500 rpm (10). Sweep rate 25 mV/s, 1 M NaOH, [Te2-] = lV3M. (b) Height of the first wave as a function of the rotation rate of the electrode at -1 V.
H w
0
z
m a
8
-
tI cn
0.2
~
0
0.5
1.5
1
PHOTOCURRENT ( mA.cnY2 )
Figure 7. Shift of flat-band potential as a function of photocurrent value, for different rotation rates: 250 (l), 500 (2), 1000 (3), and 1500 rpm (4). 1 M N a O H . [Tel-] = lo-' M. Applied potential -0.57 V. The arrows indicate the values of the plateau current, obtained, for the same rotation rate, from Figure 6 .
associated to the entrance in the two-wave situation. The onset of this behavior is related to the value of the limiting diffusion current under the given conditions. This is shown by reporting in Figure 7 (arrows) the values obtained independently from Figure 6. When the illumination level variation is reversed in this region, a hysteresis appears, as for I-V curves. b. Discussion. The most negative wave in Figure 5a is attributed to the oxidation of telluride ions to ditelluride ions. They arrive at the surface by diffusion. The second wave position corresponds to the photocorrosion of the material itself, which is the main oxidation process in indifferent aqueous solutions," following the reaction CdTe
with Eo = -0.58 V vs SCE at pH 14. However, in pure NaOH, the photocurrent does not decrease with time as observed here showing that another process occurs at the interface in this case. In Figure 8 are represented the band edge positions for illumination levels lower and higher than the threshold value, toward the standard redox potentials of the oxidation reactions which can m u r at the interface, calculated from thermochemical data" with AG = -25.5 kJ mol-' for the formation energy of CdTe? For the Te2-/Te2- value an uncertainty exists since a more positive value of -1.05 V is also given,8-26which seems more consistent with our own observations (the reduction to telluride ions can be achieved at -1.3 V). By considering the thermodynamic criteria of oxidation at semiconductor electrcdes?'J* that is a valence band edge potential more positive than the redox potential, the behavior of the system can be explained as follows: For the lower illumination levels the valence band edge position only allows reactions A and B. As ditelluride ions are not present in the solution, reaction A (2Te2- - 2e Te?-) is the only possible charge-transfer reaction. However, an influence of reaction B (Te?- - 2e 2Te) in interfacial processes is not to exclude due to the formation of ditelluride ions by reaction A. Under illumination photoholes reach the surface; they increase the net surface charge, explaining the positive shift of the band edges in this situation (relation 5). The process involves probably the oxidation of adsorbed telluride ions to ditelluride ions and further to elemental tellurium. In this process when the diffusion of telluride ions from the solution is not the limiting step, the shift of band edges reflects the kinetics of the charge-transfer mechanism. When the photocurrent still increases, the diffusion becomes limiting. The corresponding current is no more sufficient to compensate the generated photocurrent. This leads to a rapid increase of the surface charge and thus to a large shift of the band edge position explaining the observed threshold. This shift brings the valence band edge in a position allowing new transfer reactions (C, D). When these reactions become sufficiently fast to evacuate all the photogenerated holes, the surface charge remains almost
-
+ 6h+ + 90H-
HCd02- + Te0,2-
+ 4H20
(1 1)
-
(24) Pourbaix, H. Atlas d'Equilibres Electrochimiques;Gauthier-Villars: Paris, 1963. (25) Kubaschewski, 0.; Evans, E. L. Metallurgical Thermochemistry; Pergamon: London, 1955; Table A. (26) Latimer, W. M. Oxidation Potentials, 2nd ed.; Prentice Hall: New York, '1952. (27) Gerischer, H. Surf. Sci. 1980, 101, 518. (28) Bard, A. J.; Wrighton, M. S . J . Electrochem. SOC.1977, 124, 1706.
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1 DISTANCE
Figure 9. Simplified energetic diagram and current mechanisms used for the modelization of the illuminated n-type semiconductor/electrolyte junction. The effect of a band edges shift is illustrated for two different applied potentials, V' > V, with Qs' > Qs.
constant, preventing further shift of the band edges.ls This behavior is observed in the blank electrolyte under stationary conditions, the transfer reaction being in that case the decomposition reaction alone (D). In the present case it occurs simultaneously with other ones which must lead to the deposition of tellurium at the surface. As tellurium is a low band gap semiconductor (EG i= 0.3 eV), it strongly absorbs the incident light. This explains the decrease of the photocurrent under these conditions and the related hysteresis effect. This behavior is not observed for the CdS/S2- system by Inoue et al.," who demonstrate by ring-disk experiments that the second wave is due to the corrosion reaction. This can be explained by the fact that elemental sulfur is transparent in the considered wavelength range. The mechanism, based on the shift of band edges, is probably also valid to discuss their results. The authors, who have not performed simultaneous capacitance measurements, give another interpretation than ours; they suppose implicitly that the band edges remain fixed at the interface. R. H. Wilson in ref 29 and 30 reconsiders their interpretation and also proposes a shift of band edges. An interesting aspect of the above mechanism is that the adsorption modifies the relative positions of the band edges and the decomposition potential, which is brought more positive than the valence band edge, decreasing the rate of the corrosion reaction. These results also show the importance of taking into account the possible band edge shifts under illumination as recently recognized for various semiconductors especially since the results and modelization of Kelly et al.31332 They also point out the role of kinetics in fixing band edge positions at illuminated semiconductor/electrolyte interface^.'^^^^ c. Modelization. In order to discuss in more detail the above mechanism, we have considered the behavior predicted by a simple phenomenological model. The associated energetic diagrams are represented in Figure 9. Three main current mechanisms are involved: JLis the hole photocurrent reaching the interface, JT the transfer current across the interface, and J R the recombination current. The surface charge and thus the Helmholtz potential (relation 5) is supposed to be determined by the balance between these different currents: dQs/dt = J L - JT - J R (12) this relation will also govern the band edge position (relation 4). (29) Wilson, R. H. J . Electrochem. SOC.1979, 126, 1187. (30) Wilson, R. H., reference lb, p 104. (31) Kelly, J. J.; Memming, R. J . Electrochem. SOC.1982, 129, 730. (32) Kelly, J. J.; Notten, P. H. L. J . Electrochem. SOC.1983, 130, 2452. (33) Kiihne, H. M.; Tributch, H. J . Electroanal. Chem. Interfacial Electrochem. 1986, 201, 263.
This is illustrated in Figure 9 for two different values of the applied potential ( V > Vl). In this treatment, according to the experimental results, we will consider that the transfer current is the sum of two contributions, J I and J2, representing the stabilization current and the corrosion current. The stabilization current can be decomposed in several steps, some of them involving the adsorbed s p e ~ i e s . ~ ,The ~ ~ corrosion -~~ current also results from a multistep mechanism involving successive breaking of surface bonds.37 A complete modelization implies a combination of both mechanism^,^^^^^ involving under the present conditions many unknown parameters. Moreover, the lack of stationarity in the two-wave situation hinders the study of the competition between corrosion and stabilization processes in this system. As a consequence, we have chosen a more qualitative approach to describe the system, especially for the corrosion reaction, based on the following simplifying assumptions: (i) The generated photocurrent is independent of the applied potential in the depletion range. (ii) The charge-transfer reactions involve only one electronic step. Diffusion from the solution is considered for J , . (iii) The photoholes reaching the surface are rapidly transferred to surface states. They can be of two kinds, oxidized adsorbed specie^^^^^ or corrosion sites at the surface.40 They will be assumed to be all of the same type. (iv) The recombination comes only from surface recombination. Following these assumptions, it results that the different reactions at the interface are mainly controlled by the amount of oxidized surface states. Their total number is directly related to the shift of band edges under illumination by Qs = q AT/,," = CH( ,Hi' - VHd) (13)
VHdis the value of VHin darkness in the presence of adsorption, given by relations 8 and 9. Within the experimental range (Figure l ) , we take the relation VHd = -0.06 log IX-1 + C (14) If we consider that V, = 0 in darkness in the blank electrolyte, the constant C is here -0.8 V. Derivation of the Various Contributing Currents. Recombination Current. Jr = krQ~ns (15) where ns = N D exp(-aVsc) is the equilibrium superficial concentration of electrons in the conduction band and a = q / k T (38.6 V-1). Transfer Current JI. Two steps are considered, the diffusion of the reducing species in solution toward the surface Red(so1) Red(surf) (16) associated with the diffusion current
-
Jl = JLo(1 - C / C o )
(17)
where JLo is the limiting diffusion current C and Co are the concentrations of the reducing species at the surface and in the bulk of the solution; and the charge transfer with the oxidized surface states with a rate constant K , J I = KiCQs (18) This leads, after equating both currents, to the expression of the current, J l : J I = KIQsCO/(l + ( K , Q S C " / J D ~ ) ) Transfer Current J2. J 2 = K2Qs
(19)
(20)
(34) Tenne, R.; Mulier, N.; Mirovsky, Y . ;Lando, D. J . Electrochem. SOC. 1983, 130, 852.
(35) Flaisher, H.; Tenne, R. J . Appl. Phys. 1984, 56, 2930. (36) Frese, K. W.; Canfield, D. J . Elecrrochem. SOC.1985, 132, 1649. (37) Gerischer, H.; Mindt, W. Eleclrochim. Acra 1968, 13, 1329. (38) Cardon, F.; Gomes, W. P.; Van den Kerchove, F.; Vanmaekelbergh, D.; Van Overmeire, F. Faraday Discuss. 1980, 70, 153. (39) Frese, K. W., Jr.; Madou, M. J.; Morrison, S.R. J. Phys. Chem. 1980,84, 3172. (40) Vammaekelbergh, D.; Gomes, W. P.; Cardon, F. J . Electrochem. SOC. 1982, 129, 546.
The Journal of Physical Chemistry, Vol. 92, No. 14, 1988 4109
Adsorption of Telluride Ions on Cadmium Telluride 1
>
x
1
0
05
1
0.2
15
Figure 10. Calculated curves for various illumination levels accounting for the experimental behavior of Figure 5 . Parameters: ko2 = lo-' s, b2 = 38 V-l, kl = cm3 s-l, bl = 20 V I , k, = lo4 cm3 s-l, CQ = 10-3 M,JD1 = 3 X lo4 A.cm-2, JL = 5 X lo4 A.cm-2, C, = 10" F.cm-2, N D = 1.5 X lo1' cm-), C = -0.8 V. Solid lines: current-voltage curves. Dotted lines: Vsc-voltage curves (Le., CZ-Vcurves). (Insert) Shift of band edges versus generated photocurrent (dots: experimental values).
This supposes that diffusion in solution is not a limiting step. It is generally the case for corrosion reactions in concentrated acidic or basic electrolytes since the ligands involved in reaction 11 are available in large amounts. The rate constants are further supposed to depend on the electrode potential through an exponential dependence with VH."I
K , = kol exp(blVH) and K2 = ko2 exp(b2VH) (21) The system can now be solved by adding the continuity relation for the currents, under stationary conditions:
JL = J,
+ J1 + J2
2 0.5 0
NORMALIZEDAPPLIEDPOTENTIAL ( V )
(22)
1
1.5
PHOTOCURRENT( mA.cm-* )
Figure 11. Influence of the rate of the stabilization reaction only on the total shift of band edges as a function of the generated photocurrent and for different values of the diffusion current (indicated by the arrows in the figure). Solid lines: experimental results of Figure 7. Broken lines: cm3 s-l and bl = 0 (l), 20 (2), 25 calculated curves for k l = 3.2 X (3), and 40 V-' (4).
are almost fixed; they can be associated respectively with the "stabilization" and the "corrosion" ranges. As a consequence, the band edge position reflects the dominant transfer process at the interface. It can give a simple criterion for determining if a photoelectrochemical cell works under stabilization conditions or not, as already shown by other r e ~ e a r c h e r s . ~ ~ Several consequences are suggested by using this model: The influence of the stabilization rate constant on the shift of band edges can be isolated by considering that the corrosion rate is 0. The corresponding shift of band edges (i.e., AVHi') as a function of the generated photocurrent is then given by relation 19 by replacing Jl by JL and Qsfrom (13):
and for the electrostatic potential drop across the interface: V = VsC
+ VH
(23)
The faradaic current is given by
J = J1
+ J2
In Figure 10 are presented calculated curves with a choice of parameters values allowing accounting for the experimental curves. The applied potential is normalized to zero at V = Vmd, obtained by replacing VH by AVHilin relation 23. The value of k, can be evaluated from the magnitude of the space-charge potential when the shift of band edges occurs (Le., in Figure 5b in the A-B range) by means of relation 15 with J , = JL - JID. Values of the order of lo-* cm3 s-l are found by this method, indicating that the adsorbed ions act as efficient recombination centers. This has also been pointed out in the case of sulfide ions.4 The value of K2 is supposed to be the same as that used by us in pure NaOH:I8 ko2 = 0.1 6' and b2 = 38 V-'. For K1we used a fitting procedure, which will be presented below. For a better convenience we have referenced the rate at the position of band edges in darkness with adsorbed species (VHa): K l = k l exp(blAVH"). Values of k l of cm3 s-' and bl of 20 V-' have been evaluated. The agreement between the two sets of curves is good, showing the validity of the simplified model to describe the most striking features of the experimental behavior: On I-V curves (Figure 10, solid lines) the transition between the one-wave and the two-wave curve when the photocurrent becomes higher than the diffusion current is well described. On the calculated capacitance curves (broken lines), presented as Vsc-V curves, the transition is also associated with a large positive shift of the band edges. The band edge shift is more precisely analyzed in the insert (solid line; dots are the experimental values) as a function of the photocurrent value. Two limiting regions appears for which the band edges (41) Vetter, K. J. EIectrochemical Kinetics; Academic: New York, 1967.
It predicts a quasilinear (linear when bl = 0) variation at small JL values, allowing estimation of the value of kl from experimental AVH = f(JL) curves. The value of bl is derived by a fitting procedure. In Figure 11 is shown the treatment of experimental curves (solid lines) of Figure 7 by this method. A value of 3.2 X cm3 s-' for k , is obtained, which is not far from that determined in the previous experiment (1 X cm3 s-I), the difference being attributed to small changes in the initial Helmholtz potential from an experiment to another since kl is referred to this position. The curves calculated with different values of bl and by taking the experimental values of the limiting diffusion current determined in separate rotating disk experiments (Figure 6) are reported in the figure in broken lines. The best fit corresponds to a value of bl of about 20 V-' with a satisfactory agreement with the experimental curves. The effect of the potential dependence of the stabilization rate is much more sensitive for high values of the limiting diffusion current than for low values. It can be noticed that the experimental curves tend to present a change of the concavity which is not accounted for by the calculated curves. This behavior is, however, predicted by the exponential dependence of the rate of transfer but it should occur at much higher photocurrent values. This shows that the real mechanism of the stabilization reaction is probably more complex than that assumed in the present paper. Figure 12a illustrates the influence of stabilization rate constant on the I-V and VSc-Vcurves for photocurrent values lower than the limiting diffusion current: The influence of k l results in a change of the photocurrent onset position (curves 1 and 3). When b, varies, the photocurrent onset position is not modified but the fill factor is better for larger values (curves 1 and 2). The cor(42) Allongue, P.; Cachet, H. Abstracts of Papers, 6th International Conference on Photochemistry, Paris; 1986; Extended Abstract D35.
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Lincot and Vedel
NORMALISEDAPPLIED POTENTiAL
NORMALISED APPLIED POTENTIAL
Figure 13. Internal behavior of the model: influence of the recombinacm3s-’, tion kinetics. Same parameters as in Figure 12 with kl = b, = 0 V-I, and (1) k , = lo4 cm3s-l, (2) k, = cm3s d , (3) k, =
cm3S-I.
0
I00
200
300
CURRENT(@^^^)
Figure 12. Internal behavior of the model: influence of the kinetics of the stabilization reaction. (a) I-Vcurves for k o 2 = 10-I s, b2 = 38 V-I, k, = 10” cm3s-l, CO = M, JD1= 3 X lo4 A a r 2 , JL = 2.5 X lo4 C, = loe5 Fan-2, N,, = 1.5 X 10’’ C = -0.8 V, and ( 1 ) kl = cm3s-I, b, = 38 V-I; (2) k , = lo-” cm3s-I, b I = 0 V-I; (3) kl = cm3s-I, b, = 0 V I . (b) Calculated total shift of band edges
as a function of the generated photocurrent for the same parameters.
responding shifts of band edges with the photocurrent value are shown in Figure 12b. In Figure 13 is presented the influence of the recombination rate. For fixed values of the other parameters the total shift of band edges remains unchanged as k, varies. This comes from the fact that in the photocurrent plateau the recombination current is 0. On the other hand, the photocurrent onset is shifted negatively when k, decreases, which is beneficial to photoelectrochemical application. In the present experiments the value of k, is high ( k , = lo-* cm3 s-’). This indicates that the oxidized adsorbed species are probably efficient recombination centers which is a negative point to optimize photoelectrochemical efficiencies. However, as shown in ref 4, this is balanced by their ability to act also as very efficient charge-transfer intermediates. The problem is to find adsorbed species leading to a low recombination rates.
Conclusions In this paper the adsorption of telluride ions on CdTe has been characterized for the first time by means of band edge position measurements with different concentrations of telluride ions in solution. The influence of the polarity of the exposed surface has been clearly demonstrated by using (1 11) oriented electrodes: as observed on CdS electrodes for sulfide ions,9 the adsorption is stronger on cadmium-terminated surfaces than on tellurium ones. This could be explained by the partial ionicity of the Cd-Te bond. The analysis of the isotherms obtained on cadmium surfaces by assuming a Langmuir-like adsorption shows that the process occurs
via a net exchange of a single negative charge (direct adsorption of HTe- ions or more likely adsorption of Te2- with OH- substitution). Under illumination the simultaneous measurement of I-V and C2-V curves shows that the band edges shift toward positive potential values when the photocurrent crosses the interface, the total extent of the shift increasing with the illumination level. This behavior has been interpreted by considering that the position of the band edges under illumination is kinetically fixed by the rate of the different reactions allowing the transfer of the generated photocurrent across the interface, Le., the stabilizing reaction (with telluride ions here) and the photocorrosion reaction. An interesting consequence is that the adsorption shifts cathodically the band edges and brings the valence band at an higher energy level than that corresponding to the decomposition potential, which theoretically strongly reduces the rate of the corrosion reaction. This effect can be used to produce more stable photoelixtrochemical cells. Another beneficial consequence is that adsorption shifts the conduction band relative to the redox potential, increasing the available open circuit potential. Moreover, since large shifts of band edges are observed when the corrosion becomes dominant, the determination of the band edge position can be a method to control the dominant process (stabilization/corrosion) occurring at the interface. This could also be true for photoelectrochemical cells where no adsorption occurs. In this paper we have shown the interest to measure precisely the band edge shift as a function of the photocurrent crossing the interface to study the chargetransfer reactions on semiconductor electrodes. The fact that the measurements are performed in the photocurrent plateau region, where the recombination and forward currents are minimal, allows us to study the transfer reaction alone. This is not the case when the study is made by the analysis of the I-V curves in the photocurrent onset range, where recombination and foward currents have a significant influence. As a consequence, precise measurements of band edge shifts could provide a complementary way to study the kinetics of charge-transfer reactions at semiconductor electrodes.
Acknowledgment. This work was supported by the GRECO “Photoelectrodes Semiconductrices” of the CNRS. Dr. R. Triboulet and G. Didier are acknowledged for the growth of bulk CdTe and Dr. G. Cohen Sola1 is acknowledged for giving us the oriented samples. Registry No. CdTe, 1306-25-8;Te2-, 22541-49-7;NaOH, 1310-73-2; OH-, 14280-30-9.
