The Journal of Physical Chemistry, Vol. 82, No. 25, 1978 2719
Electrochemistry of Semiconductor Electrodes in DMF
References and Notes This mechanism is sometimes referred to in the current literature as the Eley-Rideal mechanism, presumably on account of D. D. Eley and E. K. Rideal, Nature(London), 146, 401 (1940). In the present paper and in ref 2 and 6 we have used the designation "Langmuir-Rideal" because of I. Langmuir, Trans. Faraday Soc., 17, 607, 621 (1921),and E. K. Rideai, R o c . Camb. Phil. Soc., 35, 130 (1939). It seems plain to us that Langmuir's name should be associated with this mechanism, whatever else is done. M. E. Shuman and W. Brennen, to be submitted for publication. J. A. Schwartz and R. Madix, Surface Sci., 46, 317 (1974). D. R. Olander and A. Ullman, Int. J . Chem. Kinet., 8, 625 (1976). H . 4 . Chang and W. H. Weinberg, Surface Sci., 65, 153 (1977);72, 617 (1978). W. Brennen and M. E. Shuman, J . fhys. Chem., 79, 741 (1975). R. W. Hamming, "Introduction to Applied Numerical Analysis", McGraw-Hill, New York, 1971, p 203 et seq. This reference describes the basic idea of predictor-corrector methods. We designed and carefully tested a program specifically for solving eq 5 and 6, which included an automatic, variable time-step feature to maintain accuracy during the interval from 19 = 0 to a time beyond the maximum in F and to minimize computer time usage thereafter. The program may be found in M. E. Shuman, Ph.D. Dissertation, University of Pennsylvania, 1978. M. Bodenstein and H. Lutkemeyer, 2.fhys. Chem., 114, 208 (1924). J. R. Bowen, A. Acrivos, and A. K. Oppenheim, Chem. Eng. Sci., 16, 177 (1963). L. A. Farrow and D. Edelson, Int. J . Chem. Kinet., 6, 787 (1974). L. A. Farrow and T. E. Graedel, J. Phys. Chem., 61, 2480 (1977). F. A. Paneth and K. F. Herzfeld, 2.Elektrochem., 37, 577 (1931). I. M. Campbell and B. A. Thrush, R o c . R . SOC.London, Ser. A , 296, 201 (1967). C. Mavrovannis and C. A. Winkler. Can. J. Chem.. 39. 1601 11961). J. T. Herion, J. L. Franklin, P. Bradt, and V. H. Dibeier, J . C h e i . Phys., 30, 879 (1959). R. A. Young, J. Chem. fhys., 34, 1292 (1961).
2 .o
,,. P = 2 ,
0
2
4
R: 10 8
6
1 0 1 2 1 4
6 Figure 8. Graphs of y/yo vs. 6 compujed for R = 1 and various values of p with K = m (solid curves) and K = 2,lO (dotted curves).
finite constant asymptotically at long time. Figure 8 shows the dependence of eq 20 on the choice of p for the case R = 1. Included as dotted curves are examples calculated from eq 17 for the unrestricted mechanism. Cases in which R # 1 are qualitatively similar to those shown. It is clear from this discussion that in the context of the Langmuir-Rideal mechanism it is not generally possible to view the quantity y as a constant attribute of the catalytic surface, since it depends not only on rate constants and active surface site density but also on the instantaneous values of fractional site coverage and atom concentration.
Electrochemistry of n-type CdS, Gap, and GaAs and p-type Ge Semiconductor Electrodes in N,N-Dimethylformamide Solutions Lun-Shu Ray Yeh Allied Chemical Go., Morristown, New Jersey 07960
and Norman Hackerman* Depatiment of Chemistiy, Rice University, Houston, Texas 7700 1 (Received January 9, 1978; Revised Manuscript Received June 5, 1978) fubllcation costs assisted by the Robert A. Welch Foundation
The electrochemical and photoelectrochemical behavior of p-Ge and n-type CdS, Gap, and GaAs single crystal semiconductors as electrodes for the reduction and reoxidation of anthracene, p-benzoquinone, p-nitroaniline, trans-stilbene, diethyl fumarate, and iodine in N,N-dimethylformamide solutions was investigated in the dark and under illumination. The redox potentials of the various compounds at these electrodes were compared with the reversible behavior at a platinum electrode. The redox potentials in solution were also correlated with the flat-band potentials of the semiconductors which were estimated by using photocurrent onset potential under continuous irradiation,chopped light experiments, and Schottky-Mott plots. Good agreement was obtained in determining the flat-band potential for CdS using these three methods. The electrode behavior of n-GaP and n-GaAs depends strongly on electrode pretreatment. The presence of invisible films or surface states after the first reduction or oxidation voltammetric scan also is influential. Redox potentials of various compounds at these semiconductors generally occurred at higher negative potentials than at a platinum electrode. However, trans-stilbene reduced at lower potentials on n-GaP and n-GaAs than at Pt when the semiconductors were exposed to light. At illuminated n-CdS the oxidation of iodide to triiodide occurred at a potential more negative than that at Pt indicating the CdS was stabilized against photodecomposition. Introduction General theory and models of reaction mechanisms a t semiconductor-solution interfaces have been given by Gerischer1g2and Myamlin and Pleakova3 Due to the low free carrier concentration of the semiconductor compared to an electrolyte the potential would drop within the 0022-3654/78/2082-2719$01.00/0
interior of the semiconductor, the space charge region, rather than a t the semiconductor-electrolyte interface. Because of this, polarizing the semiconductor electrode would result in bending of the conduction and valence bands. Even though there is a potential drop in the Helmholtz double layer by changes of the potential a t the 0 1978 American Chemical Society
2720
The Journal Of Physical Chemistry, Vol. 82, No. 25, 1978
L A . R.
Yeh and N. Hackerman
semiconductor-electrolyte interface, it is not modulated with conducting epo-tek (silver base) epoxy cement (Epoxy as opposed to the metal-electrolyte interface where the Technology, Inc., Watertown, Mass.). The semiconducting potential drops mainly across the Helmholtz double layer. crystal was then mounted on a glass disk connected to a The polarization of the semiconductor electrode would also glass tube with silicone adhesive (Dow Corning Co., populate or depopulate the free carrier concentration at Midland, Mich.). The glass tube provided an internal path the semiconductor surface where the redox reactions occur. for the Cu wire and only the semiconductor surface was S t u d i e ~ of ~ -the ~ charge transfer processes taking place exposed to solution. Electrode areas of n-GaP and n-GaAs through the conduction band or valence band of the were 0.5 cm2, and p-Ge and n-CdS had areas of 1and 0.2 semiconductor electrodes with the redox couples in cm2, respectively. n-GaAs was doped with tellurium at a aqueous solutions are available, but are limited by the 1.23 doping level of 5 X 101'/cm3, and n-GaP was doped with V of water stability range. Some large band gap semiS also at a doping level of about 5 X 1017/cm3. The (100) conductors such as Ti02,10J1Sn02,12J3SrTi03,14,15WO 3, and (111) faces were used for n-GaP and n-GaAs, reand KTa0317 with E, close to or greater than 3 eV are spectively. p-Ge contained about 5 X 1014/cm3Hg. The inherently stable to photoanodic dissolution in aqueous dopant and doping level of n-CdS were unknown. Consolutions. Others, however, are complicated by filming and centrated HCl was used to etch n-CdS for 30 s, and n-GaP, photoanodic dissolution in aqueous solutions. n-GaAs, and p-Ge for about 1 min before each measureNonaqueous solutions provide extended stable potential ment. ranges and the availability of numerous redox reactions The DMF (Matheson Coleman and Bell) was treated without kinetic complications. The stability and ease of with molecular sieves (Linde, Type 4A) and anhydrous preparation of high purity solutions permits the study of cupric sulfate and then vacuum distilled.28 Only the small band gap semiconductors. Still relatively little such middle portion of the distillate was used. Tetra-n-buwork has been reported. Among these acetonitrile is tylammonium perchlorate (TBAP), polarographic grade probably the most widely used solvent, e.g., with large band (Southwestern Analytical Chemicals, Inc.), was dried at gap electrodes such as SnOa transparent electrodesI8 and ca. 120 "C for 30 h under vacuum and stored over Drierite. Ti02,19Zn0;20with medium band gap electrodes such as TBAP was used as the supporting electrolyte in DMF and n-type GaP20-23and CdS;20 and with small band gap the solution purity was ascertained by recording cyclic electrodes such as n- and p-type Si electrode^.^^ N,Nvoltammograms at a platinum electrode before each exDimethylformamide (DMF) solutions were used to study periment. Diethyl fumarate and trans-stilbene (Aldrich capacitance at Ge and for spectrophotometric measureChemical) were used as received. p-Nitroaniline (Eastman ments at a Ge internal reflection plate during reductive Organic Chemical Co.) was recrystallized several times electrolysis of 8-quinolin01.~~ Benzonitrile was used to before use. Anthracene and p-benzoquinone (Matheson study the direct heterogeneous generation of chemiluColeman and Bell) were recrystallized at least three times minescence of rubrene at ZnO electrode.26 before use. Resublimed iodine (Allied Chemical Co.) was In acetonitrile solutions, Bard and c o - w o r k e r ~ ~ ~ used , ~ ~ as ~ ~received. ~ showed that species with standard potentials above the The cell was a conventional three-compartment design. flat-band potential (V,) reduced via the conduction band. The auxiliary electrode and reference electrode comIn the dark, no reoxidation peaks were observed after the partments were separated from the main working comreduction of species by cyclic voltammetry when the partment by medium porosity fritted-glass disks to prevent standard potentials were located below the flat-band mixing of the solutions. The reference compartment was potential; oxidation peaks appeared on exposure to light. a removable glass tube slanted toward the working elecThe work reported here deals with studies of n-type trode to minimize any uncompensated potential drop CdS, Gap, GaAs, and p-type Ge electrodes in DMF soacross the working and reference electrodes. A Pt wire lutions. The comparison of the behavior of several organic working electrode (0.25 cm2) was also used in conjunction redox systems at these electrodes in the presence and with the semiconductor electrode. The auxiliary electrode absence of illumination helps determine the flat-band was Pt foil and the reference electrode was Ag wire. A potentials and locations of the bands of the semiconductor large area Pt foil was used for capacitance measurements. electrodes. Due to the different solvation energies of redox The potential of the Ag electrode was determined in each couples in different solvents, some redox systems, which experiment by obtaining a cyclic voltammogram of a would generally cause dissolution of the semiconductor known redox couple at Pt. However, all the potentials electrode materials in one solution, may be used to stabilize reported in this work are referred to the aqueous saturated the same materials in other solutions. This is because in calomel electrode (SCE). The semiconductor electrodes aprotic solutions, such as DMF, the photodissolution were placed close to the reference electrode to reduce IR current of the semiconductor electrodes occurs at more drop and close to the wall of the cell to reduce the abpositive potentials and the redox potentials of depolarizers sorption of the irradiating light during the photoexcitation occur at more negative potentials than those in water. We experiments, A glass thermometer adaptor was used to were, therefore, interested in studying the electrochemistry adapt the semiconductor for use with the cell. Nitrogen of various electrodes in DMF. gas was bubbled through the DMF solutions for at least 30 min before the experiments, and wm kept flowing above Experimental Section the solution during the experiments. Single crystal semiconductors p-Ge and n-type CdS, The potentiostat and ramp generator used were conGap, and GaAs were cut to about 1 mm thickness and structed in this laboratory using Teledyne-Philbrick oppolished to a mirror finish on the side exposed to the erational amplifiers and the design was similar to that solutions. Ohmic contacts to the n-type semiconductors already published.29 Data were recorded on a Hewlettwere made by electroplating In on the unpolished side from Packard, Model 7044A, X-Y recorder. For' photoexcitation 0.1 M InCI, solution, The contacts to n-GaP were heated under H2 at about 400 "C for 2 h. For ~ - G ~ Au A was s ~ ~ experiments, a 500-W General Electric Quartzline lamp with very little focusing was used. Cyclic voltammetry was plated on the In and heated at about 400 "C under H2. performed by manually switching the direction of scan and Electroplating Au on p-Ge directly provided good ohmic the scan rate was 100 mV/s unless otherwise stated. contacts. A Cu wire was connected to the ohmic contacts
The Journal of Physical Chemistry, Vol. 82,No. 25, 1978 2721
Electrochemistry of Semiconductor Electrodes in DMF
I
I
'
3c
! 't o
,
1
-02
-04
I
-as
-08
-10
,
-12
a -14V
Figure 1. Schottky-Mott plot of the space charge capacity (C2) vs. potentials for (a) n-CdS and (b) n-GaP electrodes using single current pulse of 5-ps duration in 0.1 M TBAP-DMF solution.
Positive feedback was employed to compensate for the solution resistance and the internal resistance of the semiconductor. The single pulse capacitance m e a s u r e r n e n t ~were ~~~~~ done with a Tektronix wave form generator (Type 162) to trigger a Tektronix pulse generator (Type 161). The electrode potential was controlled by a Wenking potentiostat (Model 61RS) while a constant current pulse of a few microseconds duration was superimposed on it for these measurements. The voltage-time curves were detected using a Tektronix oscilloscope (Model 7603) and recorded by a Tektronix oscilloscope camera (Model (2-59).
Results and Discussion The capacitance measured across the semiconductorelectrolyte system is mainly the response of the capacitance in the space charge region of the semiconductor (C,& Information about the doping level (the donor or acceptor density) and the flat-band potential, V,, of a semiconductor can be obtained according to the Schottky-Mott equation1S2
C,;'
= 2(V - V, - hT/e)/ccoeN
-
A plot of C,L~ vs. V yields a straight line and V , at C,c2 0 (with a small correction for k T / e ) . The measurements of C,, were made in solutions free of electroactive species. A square current pulse of ca. 2 mA/cm2 and 5-ys duration was superimposed on a constant potential. The output potential was observed as a function of time:
C,, = i(dt/dV) where i is the value of the constant current pulse. The Schottky-Mott plots for n-CdS and n-GaP are shown in Figures l a and lb. Dielectric relaxation, surface states, and surface roughness are factors in the deviation of Schottky-Mott plots from linearity as discussed by Gomes and ~ o - w o r k e r s In . ~ view ~ ~ ~ of ~ these difficulties, our results do not seem to be exceptional. A straight line for n-CdS was not obtained but the flat-band potential was
1
1
$/ 1
1
0
I - 1
I -2 V
YS
SCE
Figure 2. Cyclic voltammogram of the background photooxidation and reduction under illumination of (a) n-CdS, (b) n-Gap, (c) n-GaAs, and (d) p-Ge electrodes. Dashed line in (d) indicates that the electrode is in the dark. The numbers on the curves indicate the sequence of the scans. The scan rate was 100 mV/s and supporting electrolyte was 0.1 M TBAP.
estimated to be -0.9 V vs. SCE. The curve for n-GaP was not linear but the results indicate the V, would fall in the range of -1.1 to -1.5 V. Cyclic voltammograms of n-CdS, n-Gap, n-GaAs, and p-Ge electrodes under illumination and in the dark for p-Ge in DMF solutions containing 0.1 M TBAP are shown in Figure 2. Except for p-Ge the oxidation current was slightly smaller in the dark when the electrode potential was more positive than 0 V. All other electrodes when illuminated showed strong oxidation currents at potentials where no oxidation currents were detectable in the dark. A small oxidation current was observed for n-CdS polarized at potentials more positive than -0.9 V with the electrode illuminated. Two reduction peaks at -0.45 and -1.0 V were observed on scan reversal after the electrode was scanned up to 0 V,. These peaks grew after the potential was held at 0 V for a moment as shown in scan 2. Similar behavior at an n-CdS electrode has been seen in acetonitrile solutionz0 and the reduction peaks can be attributed to the reduction of lattice photooxidation products. n-GaP and n-GaAs showed similar behavior (Figures 2b and 2c) and the lattice photooxidation products did not seem to reduce at the electrodes even at rather negative potentials. These lattice photooxidation products formed insulating films blocking the electrode surfaces and the photooxidation currents were less pronounced during the second potential scan. To a lesser degree similar behavior was observed at the p-Ge electrode. The insulating films were removed effectively by etching the used electrodes in concentrated HC1 to restore the original film-free conditions. The peak potentials and the shapes of the curves of these irreversible film-forming processes were strongly dependent upon the scan rate. A number of compounds which show reversible redox potentials a t Pt become irreversible at semiconductor electrodes and render the peak potentials (E,) dependent upon the scan rates.
The Journal of Physical Chemistry, Vol. 82, No. 25, 1978
2722
Pt
n.Cds DARK LIGHT
Epa Epc Epa
-21
n4aP DARK LIGHT
Epc Epo Epc
Epa Epc Epa Epc
L A . R. Yeh and N. Hackerman
n.GaAs pGe DARK LIGHT DARK LIGHT Epn Evc E i a EPC Epa Epc EPC
-2.0 U YI
'1 -1.5n >"
j-1
5
.o -
z + -0.5n.
ot Figure 3. The reduction and reoxidation peak potentials of various compounds at a platinum electrode and at the four semiconductor electrodes in the dark and illuminated with white light. The solution was 0.1 M TBAP-DMF and the scan rate was 100 mV/s.
.^
c .13 0
il i 5
I
1
0
-5
, -1
-15
I -2VviSCE
Figure 4. Cyclic voltammogram of a 2.0 mM p-benzoquinone solution at an n-CdS electrode (a) in the dark, (b) illuminated with white light, and (c) at a Pt electrode. The scan rate was 100 mV/s.
Cyclic voltammetry was carried out for a number of compounds at these semiconductor electrodes in the dark and under illumination and the results were compared to those obtained at a platinum electrode immersed in the same solutions. The cathodic peak potential, Epc,and anodic peak potential, Epa,at n-CdS, n-Gap, n-GaAs, p-Ge, and platinum are summarized in Table I and illustrated along with V, in Figure 3, The results for each individual semiconductor are described in detail. n-CdS. The i-V curve for CdS in 0.1 M TBAP-DMF solution under illumination is shown in Figure 2a. A small photoanodic current is observed at potential of about -0.9 V; no anodic current is observed there in the dark. This potential could be used as V, and compared to 4 . 9 V from the Schottky-Mott plot in Figure la. At potentials more positive than -0.25 V the photoanodic current increased rapidly due to lattice photooxidation. Therefore, all potential scans up to this potential were avoided, especially under illumination. The electrochemistry of a number of compounds which have redox potentials varying from -2.2 to +0.6 V on Pt were investigated at CdS. p-Benzoquinone (BQ) has a redox potential El value (Ell2is taken here as (Epc+ E )/2) of -0.52 for the first reduction at platinum ($;gure 4c). It showed an E,, of -0.85 V, just positive of V , for the reduction with no oxidation peak at CdS in the
v'
z
+'8 w
0
The Journal of Physical Chemistry, Vol. 82, No. 25, 1978 2723
Electrochemistry of Semiconductor. Electrodes in DMF
"'7
0
I
- 2
-.4
-.a
-6 Poten1
01.
v,
"I
-1ov
SCE
Figure 5. Slow potential scan toward positive potentials at 12.5 mV/s in a 2.0 mM p-benzoquinone solution at an n-CdS electrode illuminated with periodic white light.
dark (Figure 4a). This reduction peak potential shifted slightly to -0.80 V under illumination and a well-defined reoxidation peak at -0.70 V was observed on scan reversal. A small reoxidation peak was also observed for the second BQ reduction under illumination, comparable to that at Pt. Iodine reduces on Pt at potentials well positive of V , on CdS and leads to two reduction peaks corresponding to the formation of triiodide and iodide. The reduction of triiodide to iodide is irreversible and a large current peak separation is observed on potential scan reversal.34 In acetonitrile solutions, the iodide/triiodide redox system was reported to stabilize an n-CdS anode against phot o d e c o m p ~ s i t i o n . ~Iodine ~ reduction on CdS in DMF solutions was observed at -0.55 V to form triiodide but no reoxidation peak was observed in the dark. The reduction peak of iodine under illumination apparently shifted to a positive potential where photooxidation of CdS lattice occurred and no well-defined peak was observed. However, the reduction of triiodide to iodide and the oxidation of iodide to triiodide were observed to be rather reversible and reproducible under the light. It is interesting to note that under illumination the oxidation of iodide to triiodide occurred at a potential more negative than the potential at Pt. This indicates that n-CdS is ~ t a b i l i z e d . ~ ~ Anthracene (A), trans-stilbene (ST),diethyl fumarate (DEF), and p-nitroaniline (NA) all reduce reversibly on platinum at potentials well above the V , of CdS. These compounds also showed reversibility at the CdS electrode both under illumination and in the dark. The separation of the reduction peaks and reoxidation peaks were rather large, up to 100-200 mV, compared to 60 mV at Pt. When the semiconductor electrode is polarized at a high negative potential, an accumulation layer',' is formed and produces metallike behavior. Therefore, similar cyclic voltammetric behavior is observed for Pt and CdS electrodes. It is unlikely that the irradiation of an n-type electrode with a moderate doping level would increase the electron density at the electrode surface appreciably, especially at a potential well negative of V,. No change in i-V curves was expected for those compounds. Irradiation of the CdS electrode, however, did increase the reduction peak current slightly and decrease the redox peak potential separations slightly. This could be interpreted as due to heating the semiconductor thus reducing the internal resistance and increasing convection in the solution leading to a higher mass transfer rate.20 An experiment a t CdS in a BQ solution with periodic illumination was performed while the electrode potential was scanned linearly from negative to positive potentials at a scan rate of 12.5 mV/s (Figure 5). At potentials positive of V , each light pulse causes the oxidation of BQ-. resulting in an anodic current spike decaying during the
light-on period. The same phenomenon was observed by Kohl and Bard20with oxazine-1in acetonitrile at CdS. The anodic current decay is due to the decrease in BQ-. concentration around the electrode surface. This technique is similar to pulse polargraphy where the electrochemical reaction stops during the potential-off light-off period to restore a somewhat fresh condition while waiting for a new potential or light pulse. Therefore, when the electrode is polarized to a potential which is capable of oxidizing BQ-. only under illumination and when a light pulse comes in, after a dark period, a photooxidation current spike is observed. When the light pulse is turned off a cathodic current spike is observed to reduce generated oxidation product BQ which is thermodynamically reducible at the electrode potential. This cathodic current also decays quite rapidly due to the decrease in BQ concentration at the electrode surface. As the electrode was scanned to a little more positive potential higher anodic and cathodic current spikes were observed. This higher quantum efficiency for the photoprocess is due to a sufficient thickness of the space charge region after band bending such that the photogenerated excess of holes suffer very little recombination and are delivered rapidly to the surface.37 On the other hand, the low photoanodic current at potentials just positivaof V , is due to the thin space charge region compared to the light penetration depth, and provides less efficient photoabsorption. As the potential was scanned toward a very positive potential (-0.3 V, at the foot of the reduction curve, Figure 4a) only the anodic current was seen. This is due to the CdS lattice photooxidation and when the light was turned off the current immediately dropped back to the background current level, i.e., no reaction. For an n-type semiconductor at a potential more negative than V , the bands bend downward to form an electron accumulation layer at the electrode surface which would repel photogenerated holes, so no photosensitized anodic current should occur. When the electrode potential is more positive than V , the bands bend upward to form an electron depletion layer. This stabilizes the photogenerated holes, and hole injection into solution becomes possible, so a photoanodic current is observed. Therefore, the onset potential of the anodic current spike could be used to determine the flat-band potential. Vfi determined by this method is -0.88 V compared to -0.90 V by photocurrent onset potential of continuous illumination (Figure 2a) and -0.9 V from the Schottky-Mott plot. The low availability of electroactive species in solution having a redox potential near V , and the limited sensitivity of the recorder means that the periodic illumination method could be more sensitive in determining V , than the continuous illuminatioh method.26 n-Gap. n-GaP behaved somewhat differently than CdS in DMF solutions. The i-V curves of n-GaP in background electrolyte under illumination is shown in Figure 2b. The rapid increase in anodic current of an illuminated electrode at potentials more positive than -1.0 V is apparently due to lattice photooxidation. The insulating products cover the electrode surface to prevent further increase in photocurrent and a rather flat wave is seen at potentials more positive than +0.25 V. The insulating film did not seem to be reducible at -3.0 V even by holding at that potential for a few minutes, and the second potential scan showed less photoanodic current due to this film formation. The first voltammetric scan of an unetched electrode also showed curves similar to that of a photooxidized film electrode. Both unetched electrode and the photooxidized film electrode showed very smeared and irreproducible
2724
The Journal of Physical Chemistry, Vol. 82, No. 25, 1978
L.-S. R. Yeh and N. Hackerman
0
-05
-10
,
I
-15v
0
1
I
-05
-10
-15VVSSCE
Figure 7. Slow potential scan toward positive potentials at 12.5 mVls in a 2.0 mM trans-stilbene solution at (a) n-GaP and (b) n-GaAs I ’ electrodes illuminated with periodic white light. o - 5 -1 -15 -2 -25VrrSCE
solutions and trans-stilbene would then reduce at a more negative potential (-2.22 V) than at Pt. However, due to the poor reproducibility of peak current at the GaP a good peak current-scan rate relationship was not obtained. reduction peaks at very negative potentials for all the Since trans-stilbene failed to produce the reduction peak compounds studied in this work. Therefore, the electrode at -2.22 V when the underpotential reduction was observed, the early reduction of it due to the possibility of was etched and dried immediately before each experiment and potential scans more positive than -1.0 V under ilstrong absorption causing a prewave was rather unlikely. lumination were avoided. The reason for the unusual interaction between transFor the various compounds studied only reduction peaks stilbene and n-GaP and the cause for the underpotential even under dim room light are not clear. and no reoxidation peaks on scan reversal were observed n-GaAs. The behavior of n-GaAs in DMF solutions for potentials even well negative of V , both in the dark and under illumination. Similar results are r e p ~ r t e d ~ @ ~resembles ~ that of n-GaP as shown in Figure 2c. The for the reduction of several compounds at n-GaP in reduction peak potentials for a number of compounds a t acetonitrile solutions. This is due to the formation of an an unetched electrode also occurred at rather negative insulating film on the GaP surface during r e d u c t i ~ n ~ l - ~ potentials ~ with poor reproducibility. Again, the photowhich blocks the oxidation on scan reversal. This behavior oxidized electrode showed similar behavior to that of the unetched electrode even though they were visually inis different from that of CdS, Zn0,2O and Ti029 electrodes. distinguishable from the one which was just etched. Only anthracene and p-nitroaniline (Figure 6) showed Therefore, the electrode was etched immediately before reversible behavior with oxidation peaks on scan reversal each experiment and the potential scan up to -0.75 V or when the electrode was illuminated. more positive potentials under illumination was avoided. The reduction peak height of nitroaniline under illuThe electrochemical behavior of various substances at mination (Figure 6b) is slightly higher than that in the dark (Figure 6a). This is due to the heating effect thus reducing n-GaAs also resembles those at an n-GaP electrode. No reoxidation peaks were observed in the dark on scan rethe internal resistance of the semiconductor and increasing convection in the solution leading to a higher mass transfer versal after the reduction peaks occurred at potentials well negative of V , (Figure 3). This might be due to film rate as explained before. The chopped light experiments formation on the GaAs during reduction as proposed for also produced anodic and cathodic current spikes for n-Gap.21-23The film appeared to be photooxidizable and various compounds. This indicates that the film produced anthracene, p-nitroaniline, and p-benzoquinone showed in the reduction process could be at least partially phoreoxidation peaks under illumination on scan reversal. tooxidized thus allowing some current to flow. Compounds Cyclic voltammograms of p-nitroaniline on the GaAs such as trans-stilbene do not show a reoxidation peak electrode in the dark as well as under illumination are under illumination on scan reversal but diagnostic anodic shown in Figure 8. The chopped light experiment with and cathodic current spikes are observed (Figure 7a) by trans-stilbene placed V , at -1.6 V (Figure 7b). The rethe chopped light experiment. This places V, at -1.40 V duction potentials for various compounds also shifted to for the GaP electrode. This value coincidently falls into more negative potentials compared to those on Pt. the tail point of the Schottky-Mott plot (Figure lb). trans-Stilbene also showed very high underpotential of 0.48 The reduction potentials of various compounds at n-GaP V in dim room light and 0.55 V under illumination. Again, shifted to rather negative potentials compared to those at like n-Gap, no underpotential was detected in this system platinum. trans-Stilbene, on the other hand, showed a when n-GaAs was in complete darkness. We also noted positive potential shift for reduction, Le., the reduction that the underpotential could easily disappear due to peak occurred at a more positive potential on GaP than standing in the DMF solutions and trans-stilbene would Pt when the GaP was exposed to light even to the dim then reduce at -2.50 V, which is a more negative potential room light. This “underpotential” or “negative than at Pt. overpotential” showed 0.23 V in the dim room light and p-Ge. The mercury doped germanium electrode was the 0.33 V under illumination. No underpotential in this only p-type semiconductor used in this study. The i-V system was detected when the electrode was in complete curves for p-Ge in 0.1 M TBAP-DMF solutions under darkness. For other redox systems studied at n-Gap, illumination (solid line) and in the dark (dashed line) are identical results were obtained both in the dark and under shown in Figure 2d. At positive potentials, oxidation by dim room light, and the dim room light failed to produce charge transfer via its majority carrier at the valence band any positive potential shifts compared to those in the dark. is expected and essentially no difference in anodic currents We also noted that this underpotential could disappear in the dark or under illumination should be observed. easily due to potential scans or even standing in the DMF
Figure 8. Cyclic voltammogram of a 2.0 rnM p-nitroaniline solution at an n-GaP electrode (a) in the dark, (b) illuminated with white light, and (c) at a Pt electrode. The scan rate was 100 mV/s.
Electrochemistry of Semiconductor Electrodes in DMF
The Journal of Physical Chemistry, Vol. 82, No. 25, 1978 2725
0-
,
v
1
I
o
-5
.I
-15
I
- 2
-25V
I "1
SCE
Figure 8. Cyclic voltammogram of a 2.0 mM p-nitroaniline solution at (a) Pt electrode, and an n-GaAs electrode (b) in the dark and (c) illuminated with white light. The scan rate was 100 mV/s.
However, at a positive potential the anodic current under illumination was slightly greater than that in the dark. This is the enhanced oxidation of the Ge electrode itself via the decrease of the electrode resistance due to illumination and heating. At potentials more negative than -2 V, a photoreduction current deviating from the dark current was seen. Continuous potential scans under illumination also showed the behavior of lattice oxidation products blocking the electrode surface to render a decrease in oxidation current in the following scan. The lattice oxidation products were more reducible than those on the n-GaP and n-GaAs electrodes, and the decrease in oxidation current intensity during the second scan was less pronounced than that on n-GaP or n-GaAs. The possibility of photoreduction of the lattice oxidation products could also be explained using a number of compounds as discussed below. Knowing Vfi for p-Ge is imperative to interpret and discuss the experimental results. Unfortunately, conclusive results were not obtained using the three methods used for n-CdS. However, in aqueous solutions P l e ~ k o vand ~~ Gerisher2 have shown that the reduction current of Fe3+ ions at a p-Ge electrode is not electron limited and that the reaction is controlled only by mass transfer in solution. This leads to the conclusion that Fe3+reduction is via the valence band and the conduction band edge for this small band gap electrode is slightly positive or near 0 V vs. hydrogen electrode. In DMF solutions for the various organic compounds studied, very little difference in reduction currents in the dark and under illumination were found. Reductions which proceed via the conduction band for p-Ge would be expected to be limited by the minority carrier supply a t the electrode surface and very small cathodic currents might be expected in the dark. This reduction current via the minority carrier could be increased significantly by light with energy equal to or greater than the band gap of the semiconductor. Therefore, the reduction process of these organic compounds was believed to occur not through the conduction band by the minority carriers but through the valence band after extreme band bending due to strong negative polarization. It is conceivable that for a small band gap (0.7 eV) semiconductor, under an enormous band bending, the edge
-5
-I
-15
-2
-25VvrSCE
Figure 9. Cyclic voltammogram of a 2.0 mM anthracene solution at a p-Ge electrode, (a) illuminated with white light, (b) in the dark, and (c) at a Pt electrode. The scan rate was 100 mV/s.
of the conduction band at the surface could be the same or even below the energy of the valence band in the bulk semiconductor. Then a direct electron transfer across the space charge region from the valence band to the edge of the conduction band or to a surface intermediate level in the energy gap is possible. Similar process has been proposed by Laser and Bardz4in the reduction of some organic compounds on a slightly larger band gap p-Si electrode. The redox potentials of various compounds in the dark and under illumination on p-Ge is shown in Table I and Figure 3. The reduction and oxidation peak separations of the cyclic voltammograms under illumination are in general smaller than those in the dark. The cyclic voltammogram for anthracene, for example, is shown in Figure 9a for an electrode under light, and Figure 9b in the dark. This should be compared with that at Pt in Figure 9c. The decrease in peak potential separation of an illuminated electrode implies a decrease in insulating film thickness and a decrease in resistance for charge transfer across the semiconductor-electrolyte interface. Acknowledgment. The authors thank the Robert A. Welch Foundation for supporting this work. The authors express their appreciation to Texas Instruments, Inc., Dallas, Texas, via Richard Chapman and George Cronin for the generous donation of the semiconductor materials. They tfiank Professor Allen J. Bard of the University of Texas a t Austin for helpful discussions.
References and Notes (1) H. Gerischer, Adv. Electrochem. Electrochem. Eng., 1 (1961). (2) H. Gerischer in "Physical Chemistry", Vol. I X A, H. Eyring, D. Henderson, and W. Jost, Ed., Academic Press, New York, 1970. (3) V. A. Myamlin and Yu V. Pleskov, "Electrochemistry of Semiconductors", Plenum Press, New York, 1967. (4) K. H. Beckmann and R. Memming, J . Nectrochem. Soc., 116, 368 (1969). (5) H. Kiess, J . Phys. Chem. Solids, 31, 2379 (1970). (6) H. Gerischer, Surface Sci., 18, 97 (1969). (7) R. A. L. Vannen Berghe, F. Cardon, and W. P. Gomes, Ber. Bunsenges. Phys. Chem., 77, 290 (1973). (8) H. Kokado, T. Nakayama, and E. Inoue, J. Phys. Chem. Sollds, 35, 1169 (1974). (9) 8. Petthger,H. R. Schoppel, and H. Gerischer, Ber. Bunsenges. phys. Chem., 80, 849 (1976). (10) A. Fujishima and K. Honda, Bull. Chem. SOC.Jpn., 44, 1148 (1971); Nature (London), 238, 37 (1972).
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K. L. Hardee and A. J. Bard, J . Electrochem. SOC.,122, 739 (1975), and references cited therein. H. Kim and H. A. Laitinen, J. Electrochem. SOC., 122, 52 (1975). F. Moilers and R. Memming, Ber. Bunsenges. Phys. Chem., 76, 469 (1972). M. S. Wrighton, D. S. Ginley, P. T. Wolczanski, A. 8. Ellis, D. L. Morse, and A. Linz, Proc. Natl. Acad. Sci. U.S.A., 72, 1518 (1975). T.Watanabe, A. Fujishima, and K. Honda, Bull. Chem. SOC.Jpn., 40, 355 (1976). G.Hodes, D. Cahen, and J. Manass& Nature(London), 260, 312 (1976). A. B. Ellis, S. W. Kaiser, and M. S.Wrighton, J. Phys. Chem., 80, 1325 (1976). T. Osa and T. Kuwana, J . Nectroanal. Chem., 22, 389 (1969). S. N. Frank and A. J. Bard, J. Am. Chem. SOC.,97, 7427 (1975). P, A. Kohl and A. J. Bard. J . Am. Chem. Soc.. 99. 7531 (1977). R. Landsberg, P. Janietz, and R. Dehmlow, 2. Cheh., i 4 , 363 i1974j; 15, 38, 106 (1975). R. Landsberg, P. Janietz, and R. Dehmlow, 2.Phys. Chem. (leipzig), 257, 657 (1976). R. Landsberg, P. Janietz, and R. Dehmlow, J . Electroanal. Chem., 65, 115 (1975). D. Laser and A. J. Bard, J. Phys. Chem., 80, 459 (1976).
(25) M. D. Krotova and Yu V. Pleskov, pflys. Status Soldi, 3, 2119 (1963). (26) 1.-S. R. Yeh and A. J. Bard, Chem. Phys. Lett., 44, 339 (1976). (27) R. K. Wlllardson and A. C. Beer, Ed., “Semiconductors and Semimetals”, Vol. 7A, Academic Press, New York, 1971, p 65. (28) L. R. Faulkner and A. J. Bard, J. Am. Chem. SOC., 90, 6284 (1968). (29) E. R. Brown, T.G. McCord, D. E. Smith, and D. D. DeFord, Anal. Chem., 38, 1179 (7966). (30) J. S . Riney, G. M. Schmid, and N. Hackerman, Rev. Sci. Instrum., 32, 588 (1961). (31) P. J. Boddy, J . Electroanal. Chem., 10, 199 (1965). (32) W. H. Laflere, R. L. Van Meirhaeghe, F. Cardon, and W. P. Gomes, Surface Sci., 5g, 401 (1976). (33) E. C. Dutoit, R. L. Van Meirhaeghe, F. Cardon, and W. P. Gomes, Ber. Bunsenues. Phys. Chem., 79, 1206 (1976). (34) M. S. Shuman, G. D. Fultz, M. J. Hazelrlgg, and M: K. Reedy, Anal. Chem., 42, 483 (1970). (35) K. Nakatani, S. Matsudaira, and H. Tsubomuna, J. Electrochem. Soc., 125, 406 (1978). (36) A. J. Bard and M. S.Wrighton, J . Necfrochem. SOC.,124, 1706
(1977). (37) D. Laser and A. J. Bard, J . Electrochem. Soc., 123, 1837 (1976). (38) Yu V. Pleskov, Zh.Flz. Khim., 35, 2540 (1961).
Modified Ellipsometry Applied to Organic Films M. S. Tomar Departamento F k x , Universidad Simdn Bolivar, Caracas- 108, Venezuela (Received May 3 1, 1978) Publication costs assisted by Universidad Simon Bolvar
Conventional ellipsometric parameters (A,$), coupled with the measurement of relative change in the reflected intensity, were used to calculate the optical constants and thickness of uniaxial organic films. The method was found convenient and superior to earlier ones, since the three film unknowns were calculated from the three measured values on the same sample using exact theoretical relations. Thus auxiliary measurements as used earlier were avoided.
Introduction It is well known that “built-up” films of organic compounds prepared by the Blodgett-Langmuir (LB) technique1 are transparent uniaxial crystals2 with optic axes normal to the film surface. Measurements of refractive indices and monolayer thickness of LB layers were carried in the past using conventional ellipsometry, where the phase difference (A) and the amplitude ratio (tan #) were measured on films containing several monolayers. In these studies, any two sets of A and $ equations (i.e., out of these four equations any three of them) were used to calculate the three film parameters. This method was successfully used assuming that the monolayers were deposited uniformly. The quality of LB layers mostly depends on the precautions taken to avoid dust particles, mechanical viberations, and impurities in the chemicals used. Among the ellipsometric A and 4, the measured A is more sensitive4 to the surface than $ at an angle of incidence of 60-75’. Therefore the selection of the third equation in earlier work was made using the A equation, to avoid the error in calculated film parameters. This problem can be avoided if all the measurements necessary to calculate the film parameters are made on the same sample so that one does not depend on the measurement of auxiliary A. Therefore it is desirable to use some method where the third measurement could also be made without disturbing the ellipsometric setup. Paik and Bockris5 measured the relative change in the reflected intensity of a beam from the sample along with the A and ) I measurements. They calculated the optical 0022-3654/78/2082-2726$0 1.0010
constants and thickness of a light absorbing surface layer on cobalt in the state of electrochemically induced passivity. T o the best of author’s knowledge this modified ellipsometric technique had not been applied to optical studies of LB layers. In this work we present our data on calcium stearate and stearic acid films using modified ellipsometry. Some of the necessary experimental details follow.
Experimental Section Films were deposited by the celebrated LB technique1p6 described earlier. The trough was filled with triply distilled water. A few drops of stearic acid dissolved in benzene were placed on the water surface, forming a monomolecular layer of stearic acid. For calcium stearate films, a subsolution of calcium carbonate (5 X M) at p H 7 was prepared and placed in the trough. The stearic acid reacts with calcium carbonate and a monolayer of calcium stearate spreads on the surface of the subsolution. These monolayers were deposited by a dipping and withdrawal process on stainless steel polished slides. It was observed that the first layers adhere well on these slides, therefore the films were deposited one over the other uniformly. The films we obtained here were of the Y-type in both cases. Ellipsometric theory suited to LB layers has been given e l s e ~ h e r e . ~The + ~ ellipsometric equation is3 tan J/ exp(iA) = lrlp + rstZp exp(-2iPp)Ill + r 1 ~ exp(-2iflS)l 2 ~
