J . Phys. Chem. 1989, 93, 3269-3275 in the case of Os3+,longer term stability runs at the maximum power point indicated no change in power output for >3000 C/cm2. We have described the I-Vproperties of the most effective metal ion treatment, Os3+,in detail in a separate paper.28 The spectral response behavior of the n-GaAs(0sS+)/KOH-Se-/” cell is reported in Figure 8, and these data reflect the outstanding junction properties of the interface. The spectral response data in Figure 8 were collected with a longer optical path length through the KOH-Se-/2- solution than is typically used in our photoelectrochemical efficiency measurements, and the short-wavelength quantum yields in Figure 8 are reduced accordingly. The measured J , for matte-textured surfaces of 25.0 mA/cm2 under 88 mW/cmz of illumination scales to 28.4 mA/cmZ under 1 Sun AM1 .O (100 mW/cm2) conditions; thus, the observed J , represents 86% of the theoretical photocurrent density attainable from the incident photons above 1.42 eV in energy (33 mA/cm2 at AMl.0).3a Typical cell efficiencies in this system are 15.5-16.5%, but we have observed efficiencies as high as 17.2% with optimized values of J , and fill factor. Also of note is the beneficial effect of Co(I1) chemisorbed from base, which contrasts with the null effect observed in previous studies of Co(I1) chemisorption from acidic media! The n-GaAs I-V data from a Co(I1) pretreatment and from a basic Co(NH3)6Br3solution were extremely similar, and such behavior is expected from the hypothesis that Co(II1) complexes bind to n-GaAs by reduction to the Co(I1) state.26 In general, the close correlation between ions that are effective electrocatalysts for SeZ-oxidation and ions that yield improved fill factors at n-GaAs photoanodes confirms the usefulness of this hypothesis in designing improved photoelectrochemical cells in the KOH-Se-/2- electrolyte. These cells have been observed to yield outstanding photovoltaic performance, providing efficiencies that are superior to those reported to date for conventional solid-state surface barrier devices from GaAs or from other 111-V These efficiencies underscore the ability of properly constructed photoelectrochemical cells to provide superior photovoltaic performance as compared to their Schottky barrier counterparts. The bulk diffusion/recombination limited value of V , for these n-GaAs samples is 1.05 V at 300 K and J,, = 20 mA/cm2;3-29thus, improved metal ion chemisorption procedures (28) Tufts, B. J.; Abrahams, I. L.; Santangelo, P. G.; Ryba, G. N.; Casagrande, L. G.; Lewis, N. S. Nature (London) 1987, 326, 861.
3269
might be expected to yield even further performance enhancements. Such treatments are currently being explored in our laboratory, as are chemical methods to control and passivate surface recombination processes. Conclusions The open circuit voltage of the n-GaAs/KOH-Se-/2- junction exhibits only a small dependence on the value of the GaAs dopant density. This behavior is in accord with the junction properties of n-GaAs/nonaqueous electrolyte and n-GaAs/metal interfaces and is in contrast to the predictions of digital simulations of interfacial transport at the n-GaAs/KOH-Se-/” junction. Matte texturing of photoelectrode surfaces can yield increased effective minority carrier collection lengths, due to the interplay between the optical penetration depth and the distance that excited carriers must traverse in order to reach the junction depletion region. Chemisorption of metal ions has been shown to effect decreases in kinetic overpotentials for Se2-oxidation at Inz03 electrodes. Chemisorption of metal ions also has been found to yield increased anodic and cathodic currents at p-GaAs electrodes in the KOHSe-/2- electrolyte. The behavior of the p-type GaAs surfaces is consistent with the metal ions introducing electrocatalytic states near the valence band in energy but is inconsistent with the passivation of hole surface recombination processes as the dominant I-Veffect in KOH-Se-/2- electrolyte. Degenerately doped n+-GaAs electrodes exhibit high kinetic overpotentials for SeZoxidation, but metal ion chemisorption yields improved anodic currents and verifies the notion that chemisorbed metal ions can act as electrocatalysts for Se” oxidation a t GaAs surfaces. Acknowledgment. We thank the Department of Energy, Office of Basic Energy Sciences, for support of this work. We thank Dr. C. L. R. Lewis of Varian Associates, Palo Alto, CA, for a generous supply of GaAs samples, R. Dominguez and C. Gronet for assistance in some of the experiments, and Dr. Adam Heller of AT&T Bell Laboratories for helpful discussions. N.S.L. also acknowledges support as an A. P. Sloan Fellow and as a Dreyfus Teacher-Scholar. Registry No. GaAs, 1303-00-0; KOH, 1310-58-3; Se?-, 25778-65-8; Se2-, 22541-48-6; Os”, 22542-06-9; Co2+, 22541-53-3; Sn,7440-31-5; In203, 1312-43-2; Ru3+, 22541-88-4; Pb2+, 14280-50-3; Rh3+, 1606589-7; Ir3+, 22555-00-6; Co(NH3):+, 14695-95-5. (29) Lewis, N. S. Annu. Rev. Mater. Sei. 1984, 14, 95.
Characterization of the Silver/Aqueous Electrolyte Interface by Optical Second Harmonic Generation and Differential Capacitance H. M. Rojhantalab and G. L. Richmond* Chemical Physics Institute, University of Oregon, Eugene, Oregon 97403 (Received: August 2, 1988)
The optical second harmonic response from the silver/aqueous electrolyte interface is analyzed in terms of the simultaneously measured differential capacitance. This is the first study in which a direct comparison between these two methods has been made. The results provide important insight into the various relative contributions of the components in the interfacial region to the overall polarizability as the electrode is biased within the ideally polarizable region. Both smooth and electrochemically roughened polycrystalline silver surfaces have been examined in a series of simple electrolytes.
Introduction Optical studies of metal/electrolyte interfaces are increasingly recognized to yield valuable information about adsorption and reactivity at the solid/liquid interface. When these optical studies are combined with more conventional electrochemical methods, one has the advantage of controlling many of the adsorption and reactive processes by the application of a dc electric field. One 0022-3654/89/2093-3269$01.50/0
shortcoming to these optical studies, however, is a lack of discrimination between bulk and surface properties. It is also desirable that the optical method does not rely on surface roughness features such as is necessary in surface-enhanced Raman spectroscopy (SERS) and that it is applicable to a variety of materials. Optical second harmonic generation (SHG) meets many of these stringent requirements for an interfacial probe. Within the electric 0 1989 American Chemical Society
3270 The Journal of Physical Chemistry, Vol. 93, No. 8, 1989 dipole approximation, this nonlinear process is forbidden in centrosymmetric media and is thus restricted to regions where the inversion symmetry is lifted, namely, at the interface. Numerous studies have appeared that demonstrate the versatility of this nonlinear optical method.'-3 SHG has been shown to be sensitive to adsorption c~nverage?~ molecular orientation,bs adsorption binding site^,^,^ surface structure,lO.lland electrochemical reaction^.^-'^-^^ SHG in reflection from smooth silver surfaces that are reversibly polarized within the ideally polarizable region (IPR) in aqueous electrolytes shows a strong potential and concentration dependen~e.~.I~-'* Whereas earlier results with simple electrolytes have suggested that this effect is due to a variation in the excess charge density on the metal electrode that occurs upon ionic a d ~ o r p t i o n , ~ *the ~ ~contribution -'~ of the adsorbate to the nonlinear optical properties of the interface has not been explored in detail. To distinguish the contribution of the metal side of the junction from that of the solution side and to test the validity of the simple electrostatic model, correlations between the charging of the double layer and the S H signal must be examined. In previous S H experiments of this laboratory and others, the correlation between the S H response and the surface charge density has been made by comparison with published differential capacitance measurements of similar electrochemical systems. However, because surface morphology and consequently the interfacial optical properties are very sensitive to surface preparation and even minute contaminants in the solution will render data incompatible, it is essential to make such a comparison directly. In this paper the correlation between the S H response and the electrostatics of the silver/aqueous electrolyte interface is established by simultaneous measurements of SHG and differential capacitance. Both nonspecifically adsorbing ions (fluoride and perchlorate) and specifically adsorbing ions (chloride, bromide, and iodide) on smooth polycrystalline silver will be examined. In addition, the effect of surface roughness on the S H response and the electrostatics of the double layer will also be examined.
Nonlinear Optical Properties of the Interface The general theory of surface SHG, which was developed by Bloembergen and Pershan? has been applied to numerous systems including the solid/vacuum! liquid/air,lg alkali/glass,20 and (1)For a review, see: Richmond, G. L.; Robinson, J. M.; Shannon, V. L. Prog. Surf. Sci. 1988,I , 1. (2)Bloembergen, N.; Pershan, P. S.Phys. Rev. 1962,128, 606. (3)Lee, C. H.; Chang, R. K.; Bloembergen, N . Phys. Reu. Lett. 1967,18, 167. (4)Chen, C. K.; Heinz, T. F.; Ricard, D.; Shen, Y. R. Phys. Rev. Letf. 1981,46, 1010 Chem. Phys. Lett. 1981,83, 180;Ibid. 1981,83, 455. (5)Richmond, G. L.Chem. Phys. Left. 1984,110,571. (6)Heinz, T. F.; Chen, C. K.; Ricard, D.; Shen, Y. R. Phys. Reu. Lett. 1982,48,478.Heinz, T. F.; Tom, H. W. K.; Shen, Y. R. Phys. Reu. 1983, A28, 1883. (7) Voss, D. F.; Nagumo, N. M.; Goldberg, L. S.; Bunding, K. A. J . Phys. Chem. 1986. 90. 1834. (8)Kemnitz,'K.; Bhattacharyya, K.; Hicks, J. M.; Pinto, G. R.; Eisenthal, K. B.; Heinz, T. F. Chem. Phys. Lett. 1986,131, 285. (9)Tom, H . W. K.; Mate, C. M.; Zhu, X. R.; Crowell, J. E.; Heinz, T. F.;Somarjai, G. A,; Shen, Y. R. Phys. Rev. Left. 1984,52, 348. (10) Tom. H . W. K.: Heinz. T. F.: Shen. Y. R. Phvs. Rev. Lett. 1983.51. 1983. 'Tom, H.W. K.; Aumiller, G. D. Phys. Rev. i986,8 3 3 , 8818. (1 I ) Shannon, V. L.; Koos, D. A.; Richmond, G. L. J . Chem. Phys. 1987, 87, 1440;J . Phys. Chem. 1987,91, 5548. (12)Richmond, G. L. Chem. Phys. Lett. 1984,106,26; Surf. Sci. 1984, 147, 115. (13)Chen, C. K.;Heinz, T. F.; Ricard, D.; Shen, Y. R. Phys. Rev. 1983, 8 2 7 , 1965. (14)Berkovic, G.;Rasing, Th.; Shen, Y. R. J . Chem. Phys. 1986,85,7374 (15)Richmond, G. L. Langmuir 1986,2, 132. ( 1 6) Richmond, G.L.; Rojhantalab, H. M.; Robinson, J. M.; Shannon, V. L. J . Pure Appl. Chem. 1987,59, 1265. Robinson, J. M.; Rojhantalab, H . M.; Shannon, V. L.; Koos, D. A,; Richmond, G. L. J . Opt. Soc. A m . 8 1987, 4 , 228. (17) Rojhantalab, H. M.; Richmond, G. L. Aduances in Laser Science; Lapp, M., Kenney-Wallace, G. A,, Eds.; American Institute of Physics: New York, 1987;Vo1:II. (18) Corn, R. M.; Romagnoli, M.; Levenson, M.D.; Philpott, M. R. Chem. Phys. Lett. 1984,106, 30;J . Chem. Phys. 1984,81,4127.
Rojhantalab and Richmond solid/liquid interface^.^ The expression for the nonlinear polarizability, PNLS(2u),of an interface between two centrosymmetric, isotropic bulk media has been described in detail elsewhere.20*2' To summarize, under the electric dipole approximation and in the presence of the dc electric field, PNLS(2u)consists of two parts:
PNLS(2u)a x(~):E(w).E(w)S(Z) + x(~):E(w).E(w).E~,(1) where x ( ~and ) x ( ~are ) the second- and third-order nonlinear surface susceptibility tensors for the interface. These tensors contain the electric dipole contribution from both sides of the interface such that x(2)
=
xs(2)
+ Xm(2)
(2)
x(3)
=
xs(3)
+ Xm(3)
(3)
xS(*)is the second-order susceptibility of the solution (solute plus
solvent), and x ~ (is ~the) nonlinear susceptibility of the metal. In this formalism, x ( ~is) assumed to be potential independent, with the polarizability from this term labeled as PONLS.The intensity of the SH signal is then proportional to the square of the nonlinear polarizability: ISH a lP"l2 = lPoNLS PlNJ-S12 (4)
+
where the third-order hyperpolarizability P I N Lcontains S the potential-dependent term of eq l . Experiments have shown that adsorption of ions and molecules at the interface can induce a dramatic change in the observed SHG.I5 This sensitivity has been exploited in ionic and molecular adsorption studies on polished metal surfaces in an electrochemical e n ~ i r o n m e n t . ~ JSeveral ~ , ' ~ conclusions have been drawn from these studies concerning the relative contributions of the second-order and potential-dependent third-order terms to the overall response. In general, x ( ~is) several orders of magnitude larger than x ( ~ ) . However, considering the enormous dc electric field strength (=lo7 V/cm) present at the electrochemical interface, it is expected that should make a significant the field-dependent part of eq 1, PINLS, contribution to the overall PNLsrelative to the second-order term. Previous studies for single-crystal and polycrystalline silver electrodes polarized within the IPR have suggested that the observed potential dependence is from the metallic side of the int e r f a ~ e . ~ J ~ -In' ~that - ' ~ case, the second harmonic response was found to originate from the third-order term, x , ( ~ ) ,as a consequence of Edcat the metal surface. With use of Gauss's law, a simple model has been proposed that relates the S H signal to the square of the excess surface charge This expression takes the form density on the metal, 12w(u,) 0: a + b2((4m,/tl(o)) - c ) ~ (5) where the constants a, b, and c are given by 1Im Pol2, (y + y')lE(u)12,and Re Po/b. If the overall polarizability is dominated by the x ( ~term, ) then the equation can be simplified to the form Earlier studies from this laboratory showed a good correlation between Pwand the estimated um2for simple electrolyte^.'^^'^ In the analysis below, this correlation is explored explicitly as the f- and um2values are determined simultaneously. This is followed by a further examination of the S H response in terms of eq 5. Relative values of the a, b, and c coefficients obtained from the best fit to the data are then discussed.
Experimental Method Electrochemical Cell. The electrochemical cell consisted of two compartments made of Kel-F, one containing the working and counter electrode and the other holding the reference electrode. (19) Berkovic, G.;Shen, Y. R. In Nonlinear Optical and Elecfroactive Polymers; Proceedings of the 193rd Meeting of National American Chemical Society; Plenum Press: New York, 1987. (20)Wang, C. C. Phys. Rev. 1969,178,1457. Wang, C. C.; Duminski, A. N. Phys. Rev. Lett. 1968,20, 668. (21)Bloembergen, N.; Chang, R. K.; Jha, S. S . ; Lee, C. H. Phys. Rev. 1968,74, 813. (22)Valette, G.; Hamelin, A. J . Electroanal. Chem. 1973,45, 301;Opt. Commun. 1973,9, 132.
The Journal of Physical Chemistry, Vol. 93, No. 8. 1989 3271
Ag/Aqueous Electrolyte Interface electronic detections
,
electronic detections
I
monochromator
bandpass ( 2 w ) filter(2wl
f i l t e r (2w I
telescope
B
V I I
polarization analyzer(w1
n n r / /
L'2 plate Electrochemical Cell
Figure 1. Experimental arrangement for SHG and differential capacitance measurements.
The working electrode chamber was fashioned into a barrel shape to incorporate an air-tight plunger. The counter electrode was a platinum wire loop concentric with the working electrode. The Ag/AgCI reference electrode was connected to the main cell via a low-flow ceramic junction that was placed as close as possible to the working electrode. The working electrode was a polycrystalline silver disk (5 X 10 mm, 99.999% purity) which was press fit into a '/2-in. Kel-F rod. The latter acted as a plunger and could be moved back and forth with a micrometer to adjust the light path. Electrical contact was made through a silver wire in a contact with the back of the silver disk. The working electrode was mechanically polished to a mirror finish by using successively finer alumina grits (1,0.3,0.05 wm) and electrochemically polished in a cyanide bath.22,23 The electrode was then transferred to the cell and biased at the potential of zero charge (PZC). The cell was purged for 0.5 h prior to this transfer to minimize O2contamination. Clean and reproducible surfaces were obtained by this method as judged by the featureless cyclic voltammograms. Optical Second Harmonic Generation (SHG). The experimental arrangement is shown in Figure 1. For the S H G experiment, p-polarized 1064-nm radiation from a Q-switched Nd:YAG laser (10-11s pulses at 10 Hz) was directed to the surface of the electrode at about 45O. The incident beam was collimated to -0.5 cm2 and passed through a polarizing beam splitter and color filters before impinging on the electrode surface. Throughout the experiment, the laser power was kept at 4-5 mJ/pulse to avoid ~ SH signal surface damage or laser-induced d e s o r p t i ~ n . ~The was collected near the angle of specular reflection and, after passing through IR filters, was focused onto the entrance slit of a Spex 0.5-m double monochromator. The light was detected with a cooled photomultiplier tube. Gated integration techniques were used to capture the amplified detector signal. The monochromaticity and the quadratic power dependence of the signal were checked throughout the IPR. Any noise due to laser power fluctuations was minimized by normalizing the signal with respect to the reference SHG response obtained by passing a small fraction of the incident laser through a KDP crystal. Differential Capacitance and Cyclic Voltammetry. Cyclic voltammograms were obtained by using a PAR 173 potentiostat/galvanostat, a PAR 179 digital coulometer, and an X-Y recorder. The differential capacitance was measured in the range 50-125 Hz and a peak-to-peak amplitude of 4-7 mV by using a PAR 128A lock-in amplifier equipped with an internal oscillator and tuned amplifier. The data were collected in both positive and negative sweep ramps at 2-5 mV/s. The uncertainty in the (23) Larkin, D.; Guyer, K. L.; Hupp, J. T.;Weaver, M. J. J . Electroanal. Chem. Interfacial Electrochem. 1982, 138, 401. (24) Chen, T. T.; vonRaben, K. U.; Murphy, D. V.;Chang, R. K.; Laube, B. L. Surf. Sei. 1983, 124, 529.
5
5
3
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v
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>
35
Y.
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25
0 15 J
c
d
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-1200
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0
E Ag/agCl(mv) Figure 2. (a) Cyclic voltammogram for smooth polycrystalline silver electrode in 200 mM NaC104 polarized within the limits of the IPR at pH = 6.7. (b) Differential capacitance for polycrystalline Ag in NaCIO4 of concentrations of 1 (0),10 (0).100 (A),200 (0),and 500 m M (0).
potential measurement is estimated at 20 mV, whereas a 5% uncertainty exists for the differential capacitance measurements. Solution changes were made under potentiostatic control at, or near, the PZC. Materials. Most salts were recrystallized from water three times before use. Sodium fluoride was calcined at 750 OC for 12 h to oxidize any organic impurities. All solutions were prepared by using nanopure water, and their pH was adjusted to 6.5-7.0. Before admission to the cell, solutions were purged with nitrogen that was deoxygenated by passage through a dual column of BASF R3-11 catalyst. Calculation of Surface Charge Density. Changes in the surface charge density of the electrode due to ionic adsorption were calculated from the capacitance-potential, C,(E), data for NaC104, NaF, and mixtures of NaCIO4-NaX (X = C1-, Br-, I-) at constant ionic strength. Surface charge density in each case was obtained by back integration of the C,(E) data from a negative potential where the capacitance of the mixture, regardless of the X- concentration, was the same as that of the pure electrolyte. By back integrating the C,(E) curves relative to this value, a series of surface excess charge versus potential, a,(E), curves were calculated for each mixture.
3272 The Journal of Physical Chemistry, Vol. 93, No. 8, 1989 1600
1
a
N
Rojhantalab and Richmond 1200,
0
.
0 '
b'
400
0 .
1
0
.*
..ee
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,
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-
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ib
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/
,
,
,
1
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-1200
1
.
-900
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-600
-300
,
,I 0
(mv)
Figure 3. Comparison of the um2(E)and Is#) from a polished polycrystalline silver electrode in NaCIO4polarized in the IPR at pH = 6.7. (a) om2vs bias potential in NaCIO, concentrations of 100 (0)and 500 m M (e). (b) SH response as a function of potential for the solutions in (a).
Results Nonspecijkally Adsorbing Anions. Sodium Perchlorate. The cyclic voltammogram shown in Figure 2a is typical of the current-voltage pattern observed for the IPR in these experiments. Extreme care is always taken to avoid any faradaic processes that might alter the surface morphology or solution composition. The limit of the negative potential depends on the composition of the electrolyte and must be approached carefully to avoid any hydrogen evolution. The presence of hydroxide ion is minimized by working in slightly acidic solutions so as to avoid its adsorptive contribution to the S H response and to attempt to minimize silver oxide formation, which can drastically reduce the S H signal from the bare silver surface.I2 Figure 2b shows the differential capacitance results for various concentrations of NaClO,. The C,,,(E) curves indicate a sharp minimum at -880 mV for the 1.0 and 10 mM concentrations, which becomes less pronounced as the perchlorate concentration is increased. The potential of this minimum agrees well with the reported PZC values of -950 f 50 mV vs Ag/AgCl for polycrystalline silver.22,23-25For comparison with the S H response, the potential dependence of u,,, was calculated. Since PZC is not well defined for 100 and 500 mM NaC104 electrolyte, integrations were carried out from potentials negative to PZC where C,,,(E) has its minimum value. In Figure 3a, the square of the surface charge density, am2,versus potential is plotted for the concentration range of interest in these mixed electrolytes studies. The potential dependence of u,,,, and therefore am2,is found to be virtually independent of concentration. Such behavior is expected for adsorbates that do not specifically adsorb. The absence of any sharp changes in the profile of these curves with concentration implies a very weak, if any, specific adsorption of Clod- on Ag. The interaction between the perchlorate ion and the metal surface involves primarily long-range electrostatic forces. These observations agree with previous differential capacitance work for ClO, on siIver.22,2s The results of the SHG experiments which were recorded simultaneously with the C,,,(E)curves are shown in Figure 3b. The two profiles of Figure 3a,b have qualitatively the same behavior, a coincident rise in signal at potentials positive to the PZC. Furthermore, the S H response is independent of electrolyte (25) Leikis, D. 1.; Ribalka, K. V.; Sevastyanov, E. S.;Frumkin, A. N. J . Electroanal. Chem. Interfacial Electrochem. 1973, 46, 161.
5 r
Ib
0 ' -1500
'
'
"
-1200
"
-900
I
'
-600
-300
0
E A9 ,Ag C ,Pv) Figure 4. Comparison of the om2(E)and ZsH(E)for a smooth polycrystalline silver electrode in NaF at pH = 6.7. (a) um2(E)for NaF concentrations of 100 (0) and 500 mM (e). (b) SH response as a function of potential for the solutions in (a).
concentration. These SH results are in agreement with the previous observations for these systems;lb18 however, these are the first studies in which a direct comparison with the electrostatics has been made. Sodium Fluoride. Similar experiments were performed with sodium fluoride at different pH values. The IPR in N a F shows a stronger dependence on the pH of the solution and the concentration of the electrolyte than in NaC104. At pH = 7, the accessible potential window changed from 0 1 E I -1200 mV for the 1 mM fluoride concentration to 0 1 E 1 -900 mV for the 500 mM fluoride concentration. The C,,,(E)curve for the 10 mM NaF shows a sharp minimum at =-880 mV. In Figure 4a,b the potential dependence of umZand the S H signal are plotted for the concentration range of interest. Again the S H signal and the surface charge density model show a good correlation although with the NaF, the overall rise in signal is somewhat less than that observed in the NaC104 solution. For the 500 mM N a F solution, a small amount of cathodic current was passed (-350 gA/cm2) during the SH scan to -1 100 V. At this low level of cathodic current, the SH response did not seem to be altered relative to that obtained when the scan was terminated prior to any evidence of solvent reduction. However, as noted previously, this is not true when more extensive reduction occurs.1s The correlation between the S H response and am2remains. Nevertheless, since the widest possible IPR is desired, the remainder of the experiments were performed in a supporting sodium perchlorate electrolyte at a pH of 6.6 f 0.2. Specifically Adsorbing Anions. Previous linear26*27 and nonlinear3,4,15-17 optical studies of the silver electrodehalide electrolyte junction have shown that the interfacial optical properties are sensitive to both the adsorption and reactivity of halides. In all of the studies the observed optical effects have been attributed to the properties of the substrate. For the nonreactive cases, the potential dependence has been described in terms of either alterations in the free-electron concentration at the surface or perturbations in surface state^.^^,^^ Minimal or no direct optical contribution from the ionic or molecular adsorbate or from the adjacent water molecules has been indicated. (26) Korshin, G. V.; Shapnik, M. S . Elekrrokhimiya 1984, 21, 1650. (27) McIntyre, J. D. E. In Aduances in Electrochemistry and Electrochemical Engineering; Muller, R. H., Ed.; Wiley Interscience: New York, 1973; Vol. 9,-Chapter 2. (28) Furtak, T.; Lynch, D. W. J . Electroanal, Chem. 1917, 79, 1. (29) Tadjeddine, A.; Kolb, D. M.; Kotz, R. Surf. Sci. 1980, 101, 122.
Ag/Aqueous Electrolyte Interface
-
200
-
160
-
120
-
N
E
NaCI04 NaCl
0
The Journal of Physical Chemistry, Vol. 93, No. 8, 1989 3273
A
c
1
2!W
6-
z
4 --
t-
= i80
v
I
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0
8 --
v)
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'
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"
-7200
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'
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'
-600
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-300
2 --
" 0
(mv)
Aq/A9 CI
Figure 5. Differential capacitance, C,,,(E),vs E for a smooth polycrystalline silver electrode in 10 mM NaX (X = CI-, Br-, I-) in 200 mM NaC104supporting electrolyteat constant ionic strength and pH = 6.7: NaBr ( 0 ) ;NaI (A). The dotted line is an NaCIO, (A);NaCl (0); extrapolation of the experimental points for NaI to the potential at which all capacitance values become congruent. 5 ,
1
c
12 --
9
9.-
z
6 _-
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I v)
04 0
20
40
60
80
1 0
0
20
40
60
80
100
I
01
-1400
3 --
-1100
-800
-500
-200
100
E Ag/Agci(mv) Figure 6. (a) SH response for solutions in Figure 5. (b) Square of the surface charge density vs potential derived from the experiments in Figure 5 for NaCIO, (A), NaCl (0),NaBr ( O ) , NaI (A).
Chloride, bromide, and iodide adsorb strongly on silver in the IPR.23,30*31To examine in detail the effect of the halide ions present in the inner Helmholtz plane, the differential capacitance and the second harmonic response of the interface were measured for NaX (X = C1-, Br-, I-) at constant ionic strengths in the range 0.245. NaC104 was used as a supporting electrolyte. The results obtained in 0.01 mM NaX + 0.19 mM NaC104 mixtures are shown in Figure 5. As the adsorbate is changed from C1- to Brto I-, the maximum in the C,(E) curves increases and moves to more negative potentials. This indicates an increase in the degree of adsorption from CI- to Br- to I-, respectively. By increasing the concentration of CI- and Br-, the C,(E) values increase while the maximum in the curve shifts further to negative potentials. However, for I-, 10 mM seems to be the upper limit that can be safely studied. Beyond that point I< is formed, which can interfere with the adsorption studies. The S H results for these halide containing systems are shown in Figure 6a. As with the nonspecifically adsorbing ions, the S H response increases with positive bias. These results can be compared with the um2values in Figure 6b for this potential region as derived from the data in Figure 5. It is clear that at any given (30) Weaver, M . J.; Barz, F.; Gordon 11, J. G.; Philpott, M. R.Surf. Sci. 1983, 125, 409.
(31) Hupp, J . T.; Larkin, D.; Weaver, M. J. Surf. Sci. 1983, 125, 429.
0
(PC/cm*)
Figure 7. Comparison of the theoretical expression in eq 5 with the I(u) data for (a) 200 mM NaC104with a = 0.26, b = 4.27 X 1O-Io, and c = 6.14 X lo8; (b) 200 mM NaCl with a = 0.656, b = 3.09 X and c = 8.27 X lo8; (c) 200 mM NaBr with a = 0.331, b = 3.07 X and c = 7.28 X lo8. A value of 8.85 X F/m was used for e,(O) as calculated from Gauss's law and assuming Edc = lo7 V/cm.
potential prior to saturation coverage at the most positive potentials, both the um2and the SH intensity follow the order of CI< Br- < I-. It is important to note, however, that the am2results for I- are not very reliable due to the uncertainty in the PZC for this system. A good qualitative agreement is observed between the um2values and IsHfor all ions. However, for a more accurate comparison, the potential-independent contribution to the polarizability must also be included as described below. Comparison with Theory. The results examining the correspondence between the SH response and the excess surface charge density can be analyzed further in terms of the theoretical expression in eq 5 used to describe the S H response from the electrified interface. Figure 7 compares the SH intensity as a function of u, and the best fits to the data by using eq 5. As a starting point, the value of parameter c was determined from the value of u, at the minimum of the S H curves where c = 4m~,/q(O). Values for a and b are then obtained for the best fit by using least-squares analysis. Figure 7a shows the best fit to ISH(um)with the appropriate calculated coefficients for silver in NaC104 at 200 mM. For an arbitrary value of um = 3 1.5 pC/cm2, the second-order contribution is approximately 7% of the overall signal, indicating a
3274
The Journal of Physical Chemistry, Vol. 93, No. 8, 1989
dominance by the third-order hyperpolarizability due to the dc electric field. The data for halide-containing solutions was also fit to eq 5 . For both CI- and Br- a good fit could be obtained at u, values corresponding to potentials negative to the capacitance humps in the differential capacitance scans. However, the difficulty in fitting the curves always occurred at and beyond the capacitance maximum. Figure 7b,c shows the fits to the curves for the u, values obtained at potentials negative to the C, peak. The resulting values for b and c that were determined by the fits in this negative potential region were found to correspond closely to those found for the nonspecifically adsorbing systems. The I- data were not analyzed here due to the uncertainties in calculating n,. The similarity in the a, b, and c values at the low adsorbate coverages for the different anions supports the contention that the surface properties dominate the S H response through the x ( ~ ) term. For the halide ions the fit is not adequate at higher coverages. There are several possible explanations for the deviation. One of the most likely explanations is the possible inadequacy of differential capacitance to yield accurate measurements of ,u at high coverages for specifically adsorbing ions. Weaver and cow o r k e r ~have ~ ~ found this to be true for Br--containing solutions as well as for other adsorbates in regions positive of the C, peak. Such inconsistencies have been attributed to various effects such as the sluggish restructuring of the adsorbate layer which causes a deviation from the usual representation of the interface as an RC circuit. It is conceivable that the deviation is due to a growing ) ~the ) ions next to the contribution from the susceptibility ( x ( ~ of surface at higher coverages. However, that the deviation is abrupt and coincident with the C , peak for all of the ions makes this possibility somewhat suspect. A third possible explanation is that a phase transition within the overlayer is occurring and results in a variation in the surface dipole.23 In all of these studies, the SH response at potentials between the PZC and the more negative potentials up to the point of solvent reduction has been neglected. On the basis of the surface charge density model alone, one might expect the SH response to increase as cations adsorb with negative potential sweep beyond the PZC. However, this effect should be small due to the weak interaction of the cations with the surface relative to the anions. Although a small increase in S H intensity at these relatively negative potentials is observed, such changes are within the experimental uncertainty. Electroreflectance measurements of silver electrodes have also noted negligible changes in response at potentials negative to the PZC, suggesting that the electron density at the surface is not significantly altered by potential in this region.32 Effects of Surface Roughening. Electrochemical roughening of a polished silver electrode surface leads to a dramatic change in the potential-dependent S H response within the IPR.I5,l6 A strong signal appears at potentials negative to the PZC, resulting in a somewhat parabolic response with a minimum occurring approximately 300 mV positive relative to the PZC. This parabolic potential dependence is similar to that observed by using an attenuated total reflection geometry which accesses surface polaritons.'* To explore this behavior in more detail, experiments similar to those described above have been performed on surfaces that have been electrochemically roughened by passage of approximately I O mA of anodic current. Electrochemically roughened surfaces are prepared by stepping the potential of the electrode into the anodic region and deliberately allowing the passage of several milliamps of current. Silver structures are created on the electrode that range from 10 to 1000 A in diameter,33depending upon the degree of roughening. The results of C,(E), am2(,!?),and ZsH(E) are compared for electrochemically polished (Figure 8) and subsequently roughened (Figure 9) polycrystalline silver electrodes in contact with different concentrations of NaBr/NaCIO, a t a constant ionic strength of 0.5 M. As in other previous ~ t u d i e s ,the ~ ~capacitance ,~~ values for each concentration are slightly larger on the roughened surface (32) Kotz, R.; Kolb, D. M . Z.Phys. Chem. N e d Folge 1978, 112, 69. (33) Tuschel, D. D.; Pemberton, J. E.; Cook, J. E.Langmuir 1986, 2, 380.
Rojhantalab and Richmond
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Figure 8. Comparison of C,(E), un2(E), and IsH(E) for a smooth
polycrystalline silver electrode at different concentrationsof NaBr/NaCIO, at pH = 6.6 and constant ionic strength of 0.5: (a) differential capacitance, ,C, vs potential for NaBr at concentrations of 0 (0),1 (A), 10 (El), and 100 mM (0); (b) um2vs potential; (c) SH response for 10 and 100 mM. 130 w -
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Figure 9. Comparison of C,(E), um2(E),and I s H ( E )for electrochemically roughened polycrystalline silver electrode in different concentrations of NaBr/NaCIO,, Rf= 1.2, pH = 6.6, ionic strength = 0.5: (a) differential capacitancevs potential for NaBr at concentrations of 0.2 ( O ) , 1 (A), 100 m M (0); (b) um2vs potential from data in (a); (c) SH response for 0.2, 10, and 100 mM.
than the corresponding values on the smooth surface, indicating a larger surface area of the electrode upon roughening. This change can be seen by comparison of Figure 8a and Figure 9a. The increase in capacitance shows a roughening factor of Rf= 1.2. Increased adsorption of the ions on the roughened surface is also manifested in the am2(E)curves. However, except for the
Ag/Aqueous Electrolyte Interface difference in magnitude, the am2(E)profile for the roughened surface resembles the um2(E)profile for the smooth surface. Other double-layer properties such as the PZC are unchanged. With knowlege of the surface adsorbate concentration of the bromide ion and the Rffactor, the adsorption values on the roughened surface can be deter~nined.~) The S H profiles for the two different surface preparations are depicted in Figures 8c and 9c. Whereas the S H profile for the smooth surface shows a good correlation with the am2(@curves for different concentrations, the corresponding results on the roughened surface are quite different. Figure 9c shows a parabolic feature for l s H ( E )that is not observed for the smooth surfaces in contact with any of the electrolytes studied. The minimum in S H signal is not at the PZC (=-lo30 mV for 100 mM NaBr/ NaC104) but is shifted anodically by approximately 300 mV. Similar results were observed for other electrolytes. It is interesting to note the different effect that Br- adsorption has on the potential-dependent S H signal on the roughened surface relative to the smooth surface. While increasing the concentration of the bromide with the smooth surface causes an overall signal increase between the PZC and -0.300 V, little effect at the potentials cathodic to the PZC values is observed. In contrast, on the roughened surface the most dramatic increase in intensity occurs at the most negative potentials cathodic to the smooth surface PZC values. Comparison of the S H and differential capacitance measurements for the two different surface preparations indicates that the S H response from the roughened surface at the most cathodic potentials arises from a mechanism different from that of the smooth surface. The S H response no longer follows the electrostatic behavior of the metal surface. The most likely additional contribution to the surface polarizability is the excitation of surface plasmons which are inaccessible on the smooth surface. Surface plasmons create localized electromagnetic waves which are known to enhance surface optical effects.34 In fact, surface plasmon enhanced SHG of smooth silver films'* studied in similar systems (34) Shen, Y . R. The Principles of Nonlinear Optics; Wiley: New York, 1984.
The Journal of Physical Chemistry, Vol. 93, No. 8, 1989 3275 showed a nearly identical response in the IPR as the roughened electrode presented in Figure 9c. Photoacoustic measurements of the linear optical properties of these smooth and roughened surfaces also suggest a contribution from surface plasmon^.^^ The surface plasmon excitation is expected to be potential dependent, but these studies provide evidence that quantitative measurement of adsorption using surface plasmon enhancement techniques for silver is not appropriate.
Summary and Conclusions These comparative studies of SHG and differential capacitance measurements provide the first detailed analysis of the correlation between the potential dependence of the S H response and the electrostatics of the metal surface polarized within the IPR. The potential-dependent S H signal is related to the charge density on the metal, as was shown conclusively for the nonspecifically adsorbed ions. The S H signal is also sensitive to different specifically adsorbing electrolytes and their variable concentrations, again in a manner that is consistent with surface electrostatics. The correlation is also good for halide ions at potentials negative to the capacitance maximum. The effect of electrochemical roughening on the SH response in the IPR has also been investigated in detail. The results provide conclusive evidence that the S H response on roughened surfaces does not follow the doublelayer electrostatics but is more likely associated with surface plasmon optical effects. Acknowledgment. The assistance of E. K. L. Wong in the curve fitting of the data is appreciated. We are grateful for financial support provided by the National Science Foundation (CHE8722798 and CHE-8451346) and the donors of the Petroleum Research Fund, administered by the American Chemical Society. G.L.R. expresses her appreciation to the Alfred P. Sloan Foundation and the Camille and Henry Dreyfus Foundation for awards used to support this research. Registry No. Ag, 7440-22-4; NaC104, 7601-89-0; NaF, 768 1-49-4; NaC1, 7647-14-5; NaBr, 7647-15-6; NaI, 7681-82-5. (35) Chu, P.; Richmond, G. L., to be published.
