J . Phys. Chem. 1990, 94, 4375-4376 reversed only with sweeping the potential back to values where cathodic current flow inverts the surface-state charging. A light intensity sufficient for producing a photocurrent of 1 PA, as used by Salvador, provides enough charges for filling all the surface states within a few seconds and enough to oxidize the whole surface within only 1 min. Therefore, this technique does not allow even in principle the determination of dark properties of an electrode in which surface-state charging occurs. Also, photocorrosion will play an important role within the time scale necessary for the EER experiments. As our experience shows, sulfite does not prevent sulfur formation by photocorrosion, and the subsequent dissolution to form S2032-(aq) is much too slow to keep the surface free of sulfur. The observation that no hysteresis occurs in the potential sweep is simply due to the necessity of changing the potential only very slowly in EER measurements. I n summary, Salvador's arguments cannot be accepted. The evidence given in his paper is that of a MottfSchottky curve obviously obtained with an electrode poisoned not only by sulfur but also by cadmium metal, deposited from the solution. His EER measurements are meaningless as evidence for the flat-band potential in the dark because they are the result of an illumination experiment. It agrees fairly well with the flat-band potential we obtained under illumination, which is at least 0.6 V more positive than that obtained in the dark. Registry No. CdS, 1306-23-6.
Dieter Meissner
Institut f u r Solarenergieforschung GmbH Sokelantstrasse 5 0-3000Hannover 1 , West Germany Received: August 28, 1989
Comments on "Photoelectrochemistry of Cadmium Sulfide. 2. Influence of Surface-State Charging" Sir: In a second paper1 concerning the influence of surface-state charging, Meissner et al. study the transient photocurrent-time behavior of CdS single crystals (clean (0001) Cd face), as well as the cathodic dark currents in the presence of dissolved oxidized species. The authors interpret their experimental results on the basis of flat-band potential (V,) shifts toward anodic potentials due to positive charging of surface states (S2- h+ S*-,or S2XCq) S'- X("+')-,where h+ represents a photogenerated hole and X" is a dissolved, electron acceptor species). This model considers again the controversial, previous result2 that the flat-band potential of CdS in the dark is much more negative (about 1 V) than generally assumed. Although we agree with the authors that V, shifts are involved in their experimental results, we cannot agree on the origin of these shifts. One of the electrochemical reactions taking place at the CdS/electrolyte interface is surface electroreduction:
+
-
CdS
+
+
+ 2H30+ + 2e-cb
-
CdO,,,
-
+ H2S + 2 H 2 0
(1)
Le., two electrons from the conduction band are trapped at a Cd0(,) surface atom and a S2-ion is dissolved. The reversible standard However, with a CdS single potential of (1) is -0.83 V crystal this reaction takes place with an overpotential of near -1 .O V, unless a big density of surface lattice defects are present. For instance, after photocorroding the (0001) face of a CdS single crystal, we have shown4 that (1) starts at -1 .O V (SCE); Le., the overpotential for (1) to occur decreases up to -0.17 V. According to reaction 1, during the cathodic dissolution of CdS the electrode surface becomes charged negatively (Cdzf(,)
-
(1) Meissner, D.; Lauermann, I.; Memming, R.; Kastening, B. J . Phys. Chem. 1988, 92, 3484. (2) Meissner, D.; Memming, R.; Kastening, B. J . Phys. Chem. 1988, 92,
3476. ( 3 ) Kolb. D. M.; Gericher, H. Electrochim. Acta 1973, 18, 987. (4) Ferrer, I. J.; Pujadas, M.: Salvador, P. Chem. Phys. Lett. 1987, 139, 271.
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4315
Figure 1. EER cyclic voltammogram of a CdS single crystal (Cd(0001) face) in 0.5 M N a 2 S 0 3 electrolyte. Note that the voltammogram is practically reversible if during the cathodic sweep starting at 0.0 V (SCE) the potential does not reach a value more negative than -1.1 V (SCE). For more negative potentials (Vdc = -1.5 V) a big hysteresis appears due to a negative shift of V , (AV, N -0.4 V).
CdO,,)); consequently, a negative shift of V, is to be expected. Recently, we were able to directly measure such a band unpinning by means of the electrolyte electroreflectance (EER) t e c h n i q ~ e . ~ In fact, Figure 1 shows the EER voltammogram at the direct fundamental transition of a CdS single crystal kindly provided by Prof. Memming. The EER experiment was performed in 02-saturated 1 M N a 2 S 0 3under a photon flux producing small at 0.0 V (SCE)). It is interesting photocurrents ( 1 pA to note that the EER voltammogram is reversible and a V, = -0.9 V (SCE), independent of the sweep sense, is obtained if the cathodic sweep does not exceed a potential more negative than -1.1 V (SCE). However, when the voltammogram is extended to more negative potentials (V, < -1.1 V (SCE)) where reaction 1 starts, a clear hysteresis appears in the EER voltammogram. Thus, when a potential as negative as -1.5 V is reached during the cathodic potential sweep, the V, obtained during the further anodic sweep is not -0.9 V but -1.3 V (SCE). This cathodic shift of V, is even greater with polycrystalline electrodes, which have a greater density of lattice defects and a lower overpotential for reaction I . 4 In light of these results, the interpretation of the photocurrent transients observed by Meissner et a1.l (see their Figure 2) must be different than that they assume. The anodic phototransient at small band bending is mainly due to surface recombination of photogenerated holes trapped at S*-(,) surface states (S2-(,) + h+vb So-(,))with conduction band electrons (So-(,) e-cb S2-(,)). The rate of this reaction, in a first approximation, can be expressed by N
-
+
qVR = qKRns[S'-(s)l
-
(2)
KR = uu is the recombination rate constant ( u is the cross section for electron capture of S'-(,) species, u is the thermal speed of electrons), and n, is the surface concentration of electrons at the conduction band (n, = n b exp(-eUsc/KT), where nb is the bulk electron density). Using the same notation as Meissner et al.,' at the steady state the anodic photocurrent can be written Iph
= I p - qVR
(3)
Simultaneously, the concentration of So-(,)surface species reaches a value [S'-(s)]s,. When the light is turned off, the anodic photocurrent disappears, but the cathodic current (qVR) remains until all the So-(,)surface species are neutralized with electrons. Therefore, initially the cathodic current will be (qvR)in = qKRns[S'-(s)lst
(4)
and will decrease with time until [S'-(,)] = 0; consequently a cathodic transient is observed in the dark. The transient behavior can be detected when (qV& k I p f l O , i.e., at sufficiently low band bending where n, is high. In Figure 2 of ref 1 this happens in the range of potential 5-0.5 V (SCE), for Usc 5 400 mV. Between -0.5 and -1 .O V (SCE) (qVR)in diminishes very little because the increase of n, is practically compensated by a decrease of [S'-(,)],, as Ipdecreases. However, between -1 . I and -1.5 V (SCE) both 0 1990 American Chemical Society
J . Phys. Chem. 1990, 94, 4316-4311
4376
I , and qVR increase again. This evidences an increase of Us, and nE,associated with a shift of Vb toward negative values as the CdS surface begins to be electroreduced (Le., becomes charged negatively). At -1.5 V (SCE) the negative shift of V , stops and the 0; photocurrent practically disappears, which means that Us, therefore, it must be V , N -1.4 V (SCE). This interpretation is consistent with our EER results of Figure 1, according to which the CdS flat-band potential is near -0.9 V (SCE) and shifts by 0.5 V toward negative values as the electrode is polarized cathodically and its surface electroreduced. Finally, Meissner et a1.l also find that the cathodic dark currents corresponding to the electroreduction of various oxidized species occur in two different potential regions. This is also consistent with a true flat-band potential at about -0.9 V (SCE) that shifts toward negative values when the electrode is polarized at potentials more negative than V,. In fact, the reversible redox potentials of AI/AI3+. Mn/Mn2+, and Zn/Zn2+ couples are more negative than -0.9 V (SCE). This means that AI3+,Mn2+,and Zn2+ions can on14 be electroreduced at potentials where the CdS surface becomes electroreduced and V, shifts cathodically by an amount hV,. Consequently, the onset potential of the cathodic dark current, which should be close to V,, shifts toward negative potentials by the same amount AV,. The behavior is different for couples like Fe2+/Fe3+,Fe(CN)4-/3-, Fe(EDTA)2+/3+,etc., which have redox potentials more positive than V,, and whose reduction starts before Cdu is electrodeposited and the flat-band potential shifts negatively. Finally, we would like to mention that shifts are in fact expected under illumination. However, such shifts only would be observed under high photocurrents when hole trapping at surface states is important, but never under conditions of low illumination intensity and small band bending producing very small photocurrent^.^ This kind of effect has been directly shown for the first time with the help of the EER technique at T i 0 2 electrodes.h Registry No. CdS, 1306-23-6. (5) Salvador, P. J . Phys. Chem. 1985, 89, 3863. (6) Pujadas, M.; Tafalla, D.; Salvador. P. Surf. Sci. 1989, 215, 190.
Instituto de Catdlisis y Petroleoquhica (CSIC) Serrano, I 19, 28006 Madrid, Spain
P. Salvador
Receiced: May 8. 1989; In Final Form: December 4, 1989
Reply to Comments on “Photoelectrochemistry of Cadmium Sulfide. 2. Influence of Surface-State Charging” Sir: Besides various kinetic aspects, Salvador’s comments1 are centered around two points. One is the nature of the CdS surface at potentials negative of -1.1 V (SCE). The other is the value of the flat-band potential for both surface charges. The Nature ofthe Charged CdS Surface. We have developed a model for electron-transfer processes based on two cases of surface charging of CdS due to either Cd2+S2-at the surface as in the bulk lattice (uncharged surface), or due to Cd2+S’- (positively charged surface). The latter is formed by trapping holes in the surface (as an intermediate of the corrosion process Sz2h+ So) either formed by oxidation via oxidizing agents in the electrolyte or by illumination. The uncharged surface is obtained only in the presence of reducing agents in the electrolyte or by
+
applying a sufficiently negative potential. Both types of surfaces have their specific flat-band potential due to different potential drops across the Helmholtz double layer as known from metal electrodes. Salvador seems to agree with this view with the exception that he assumes the electrode to be either uncharged (Cd2+S2-), negatively charged (CdoS2-), or positively charged (Cd2+S’-). Whether he also considers a CdZ+Sosurface to be a double positively charged state remains open. Concerning the formation of Cdo at CdS electrodes, Kolb and Gerischer4 have already shown in 1973 that one has to distinguish between two types of reaction mechanisms, the reduction of dissolved Cd2+ and the reduction of lattice Cd2+ at potentials negative of -1.4 V (SCE). These authors even used electrolyte electroreflection (EER) measurements for the direct determination of the Cdo metal formed on the surface. Their results have been confirmed in 1980 by Masuda, Fujishima, and H ~ n d awho , ~ used photoacoustic spectroscopy for the metal detection and found an onset of reduction at -1.5 V (SCE). That the same is true also for small and very small CdS particles was shown by Henglein and co-workers? In various experiments they found, e.g., complete stabilization of colloids in the absence of heavy metals: cathodic decomposition is not easy to achieve whereas excess Cd2+is easily reduced also at CdS suspensions.’ Accordingly, CdS single crystals, suspensions, and colloidal particles behave the same. Therefore, an increase of the specific surface area and thereby of the number of surface states does not change the behavior. Salvador’s understanding of what happens in the CdS surface at negative potentials is based on investigations of single-crystalline CdS after photocorrosionZ and of “polycrystalline” electrodes prepared by 600 “C annealing of CdS powder.3 In the latter case it is probable and in the former obvious that Cd2+ from CdS decomposition is present in the solution. What Salvador has attributed to the reduction of lattice Cd2+really should be due to the reduction of CdZ+ions, the latter being either dissolved in the solution or adsorbed at the surface and encapsulated in his baked powder electrodes. I n principle sulfur and cadmium formation on a CdS surface should not be interpreted as “surface charging”. First of all, a deposition of sulfur on the surface leads to the formation of a new insulator material between the semiconductor and the electrolyte. This forms its specific Helmholtz double layer with corresponding band edge positions (“flat-band potential”) a t the electrolyte interface, a specific space charge, and a respective band bending at the semiconductor interface. Cadmium, on the other hand, provides a metallic contact to the electrolyte and-as found by us-an Ohmic contact to the semiconductor. Therefore, such layers lead to specific and complicated interface energetics and their own specific surface charging effects. The Flat-Band Potentials. The reliability of flat-band potential determination by EER experiments is only given if the reflectivity of the surface is changing with the potential, Le., with the electric field at the surface. As shown by Kolb and Gerischer, the formation of Cdo leads to a strong increase of the A R / R starting at -0.9 V (SCE) in the presence of Cd2+ions and at -1.5 V (SCE) in the case of lattice reduction. This effect certainly includes also surface electric field effects due to the change in the potential drop across the space-charge region. Therefore, no determination of the flat-band potential will be possible. As mentioned in our reply to the comments on our part 1 paper,8 a detailed analysis is easy to achieve by using careful I / E measurements. This was shown already very nicely by Kolb and Gerischer. The observed onset of photocurrent at -1.5 V (SCE) is also explained by Salvador as being due to a shift of the flat-band
+
( I ) Salvador, P. J . Phys. Chem., accompanying paper in this issue. (2) Ferrer. 1. J : Pujadas, M ; Salvador, P. Chem. Phys. Leu. 1987, 139, 27 I ( 3 ) Ferrer. I J . . Salvador, P.; Velasco, J. G. J . Efectroanul. Chem 1985, 189, 363
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(4) Kolb, D. M.; Gerischer, H. Electrochim. Acta 1973, 18, 987. (5) Masuda, H.; Fujishima, A,; Honda, K. Chem. Lett. 1980, 1153; Bull. Chem. SOC.Jpn. 1982, 55, 672. (6) Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1982.86, 301; Top. Curr. Chem. 1988, 141, 113. (7) Gutierrez, M.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1983, 87,
474.
(8) Meissner, D. J . Phys. Chem., preceding paper in this issue.
1990 American Chemical Society
