J. Phys. Chem. 1981, 85,2965-2970
2965
Differential-Capacitance Studies of Silane-Modified SnOp Electrodes at Low Modulation Frequencies N. R. Armstrong” and V. R. Shepard, Jr. Department of Chemistty, University of Arizona, Tucson, Arizona 85721 (Recelved: July 25, 1980; In Final Form: May 15, 1981)
Differential-capacitancestudies, at low modulation frequencies, were conducted on a variety of silane-modified SnOzelectrodes. Silanes with amine functionality, when added to the SnOzsurface, perturb the ionic population of the gel-like surface layer. This phenomenon and the nonideal capacitance behavior observed for SnOzelectrodes at low modulation frequencies can be explained by the presence of a surface-statecapacitance term in parallel with the normal space-charge capacitance.
Introduction The study of chemically modified electrode surfaces has been undertaken by several laboratories with a variety of goals.l-lS Common to most of these studies is the aim of producing an electrode surface with specific catalytic properties, to facilitate the redox reaction of a solution species which is normally kinetically inhibited.2,3J3-16Our primary modification strategy has been aimed at reactive metal oxide ~urfaces.’-~J*~~ It has been apparent that the surface chemistry of metal oxide electrodes was not fully understood and that attachment of a coupling group such as a silane changed this surface chemistry in ways that were observable by voltammetric,surface-conductance,and differential-capacitance techniques.13 It has also become apparent to us that attachment of various redox couples to the silanized SnOz surface may be enhanced by adsorption processes such as those observed with other modified electrodes, and in some cases these may be the dominant modes of attachment.l6-lg This paper deals with our differential-capacitance studies of SnOz electrode surfaces before and after modification with various silane coupling agents and various (1) N. R. Armstrong, A. W. C. Lin, M. Fujihira, and T. Kuwana, Anal. Chem., 48, 741 (1976). (2) D. Hawn and N. R. Armstrong, J. Phys. Chem., 82, 1288 (1978). (3) V. R. Shepard, Jr., and N. R. Armstrong, J. Phys. Chem., 83,1268 (1979). (4)R. F. Lane and A. T. Hubbard, J.Phys. Chem., 77, 1401 (1973). (5) P. R. Moses, L. Wier, and R. W. Murray, Anal. Chem., 47, 1883 (1975). (6) J. R. Lenhard and R. W. Murray, J. Electroanal. Chem., 78, 195 (1977). (7) P. R. Moses and R. W. Murray, J. Am. Chem. SOC.,98,7435 (1976). (8) P. R. Moses, L. Wier, J. C. Lennox, H. 0. Finklea, J. R. Lenhard, and R. W. Murray, Anal. Chem., 50, 576 (1978). (9) D. G. Davis and R. W. Murray, Anal. Chem., 49, 194 (1977). (10) H. 0. Finklea and R. W. Murray, J. Phys. Chem., 83,353 (1979). (11) M. Fujihira, T. Mataue, and T. Osa, Chem. Lett., 875 (1976). (12) B. F. Watkins, J. R. Behling, E. Kariv, and L. Miller, J. Am. Chem. SOC.,97, 3549 (1975). (13) J. F. Evans, T. Kuwana, M. T. Henne, and G. P. Royer, J. Electroanal. Chem., 80,409 (1977). (14) A. W. C. Lin, P. Yeh, A. M. Yacynych, and T. Kuwana, J.Electroanal. Chem., 84,411 (1977). (15) M. F. Dautartas, 3. F. Evans, and T. Kuwana, Anal. Chem., 51, 104 (1978). (16) 6. A. Koval and F. Anson, Anal. Chem., 50, 223 (1978). (17) H. Oyama, A. P. Brown, and F. C. Anson, J.Electroanal. Chem., 87,435 (1978). (18) A. P. Brown, C. Koval, and F. C. Anson, J.Electroanal. Chem., 72, 379 (1976). (19) N. R. Armstrong and V. R. Shepard, Jr., J. Electroanal. Chem., 115, 253 (1980). (20) V. R. Shepard, Jr., and N. R. Armstrong, submitted for publication in J. Electroanal. Chem. (21) T. M. Mezza, C. L. Linkow, V. R. Shepard, N. R. Armstrong, M. Kenney, and B. Nohr, J. Electroanal. Chem., in press. ’
redox couples. Voltammetric studies of SnOz electrodes which have been modified with monolayer concentrations of silanes and further modified by the attachment of transition-metal ions or macrocyclic complexes, e.g., phthalocyanines, are published e1se~here.l~ In both of these studies we have attempted to further the understanding of the Sn02/solution interface especially in those instances where the surface has been silanized. The SnOZ surface has been considered as consisting of only a discrete plane of ionizable and reactive oxide groups. The surfaces of most metal oxides, however, exposed to aqueous solutions, consist of a gel-like hydration layer which extends to depths of 10-1000 A.22-33 This layer is the site of ion-exchange activity and of mobile ionic species. The results shown below support models of the electrode surface which include capacitance terms due to bulk electronic phenomena, the ionic population of the gel-like surface layer, and the solution Helmholtz capacitance. The ionic terms are the ones chiefly affected by the addition of the silane coupling agent to the electrode surface. Capacitance studies made at low modulation frequencies are capable of measuring these ionic contributions.
Experimental Section Differential-capacitance studies were carried out by using a potentiostat of conventional design, equipped with positive feedback compensation. Potential modulation was applied to the input of the potentiostat as a 20-mV, 100 Hz-10 000-Hz sine wave signal. The quadrature component of the output from the potentiostat was monitored via a lock-in amplifier. Capacitance data were taken at frequencies where Faradaic distortion was minimized. For most of the experiments reported here, we used modulation (22) P. B. Harrison and E. W. Thornton, J. Chem. SOC.,Faraday Trans. 1. 72. 1310. 1317 (19761: 74. 2703 (1978). (23) S . Kittaka,’S. Kdemoto, &d T. Morimoto, J. Chem. SOC.,Faraday Trans. 1, 74, 676 (1978). (24) S. Ahmed, J.Phys. Chem., 73,3546 (1969); S. Ahmed in “Oxides and Oxide Films”, Vol. 1,J. W. Diggle, Ed., Marcel Dekker, New York, 1972, pp 320-499. (25) P. Kirkov, Electrochim. Acta, 17, 519 (1972). (26) R. P. Buck and I. Krull, J. Electroanal. Chem., 18, 381, 387 (1968); J. R. Sandifer and R. P. Buck, ibid., 385 (1974). (27) H. B. Boehm, Discuss. Faraday SOC.,52, 121 (1971). (28) R. DeGryse, W. P. Gomes, F. Cardon, and J. Vonnik, J.Electrochem. Soc., 122, 711 (1975). (29) E. C. Dutoit, R. L. Van Meirhaeghe,F. Cardon, and W. P. Gomes, Ber. Bunsenges. Phys. Chem., 80, 1206 (1976). (30) F. Mollers and R. Memming, Ber. Bunsenges. Phys. Chem., 76, 469 (1972). (31) D. M. Tensch and E. Yeager, J. Electrochem. Soc., 120, 164 (1973). (32) S. Toshima and I. Uchida, Electrochim. Acta, 15, 1717 (1970). (33) S. R. Morrison, “Electrochemistryat Semiconductorand Oxidized Metal Electrodes”, Plenum Press, New York, 1980, pp 133-7. ~~
0022-3654/81/2085-2965$01.25/00 1981 American Chemical Society
2986
Armstrong and Shepard
The Journal of Physical Chemistty, Vol. 85, No. 20, 1981
TABLE I : Silanes Used f o r Surface Modification silane (3-aminopropyl)trie thoxysilane [ 3-[(2-aminoethy1)aminol p r o p y l ] trimethoxysilane ( trimethoxysily1)propyldie thylenetriamine (2-cyanoethy1)triethoxysilane phenyltrimethoxysilane dimethyldiethoxysilane
(3-mercaptopropyl)trimethoxysilane
frequencies of 100-1000 Hz. Frequency dispersion of the capacitive response is a problem in the study of semiconductor electrodes.28~29Capacitance measurements were taken at more than one frequency, and variations with frequency were confirmed to exist for the SnOzelectrodes. The capacitance numbers reported here are compared at the same modulation frequency, so that the numbers have significance in their relative magnitudes. A discussion of this frequency dependence is given below. Calculation of no values assumed a surface roughness factor of 2. A major problem in past studies of differential capacitance of semiconductor electrodes has been the variability of response from one electrode to another. Even for SnOz electrodes obtained commercially, two electrodes from the same synthetic preparation can differ in surface composition in ways that preclude comparison of small changes in their capacitive behavior. To circumvent this problem, we have studied the capacitive and voltammetric behavior of between 100 and 200 electrodes before and after each surface modification step. The electrodes were carefully handled and catalogued so that the same area of each electrode was exposed to the electrochemical environment for each investigation. The large number of electrodes were examined so that any changes in behavior caused by chemical modification, pH changes, etc., could be observed over a statistically meaningful population. All capacitance data, unless otherwise indicated, were obtained after 5-10 min of exposure to the electrolyte. The silanes which have been attached to the SnOz electrodes reported here are shown in Table I along with the abbreviations for their use in the text. The methods of attachment for these coupling agents have been reported previ~usly.'-~~'-~~ Tetrasulfonated phthalocyanines of cobalt (CoTSPc) or copper (CuTSPc) were adsorbed to the silanized electrode surfaces as discussed previo~sly.'~ All of the SnOzelectrodes used were obtained from PPG Industries as the fluoride-doped, 500-700-nm thin film, chemically vapor deposited on glass. Surface-analysis studies were conducted on either a Physical Electronics Industries 548-XPS/AES system (Ol), a Physical Electronics TFA-Auger analysis system (02), or a GCA-McPherson ESCA-36 system (GCA). Base pressures were between lo-' torr (GCA) and torr (01 and 02). X-ray photoelectron spectra were run by using a Mg Ka source; all spectra were charge-shift corrected to the C(ls), 284.6 eV line. Auger electron spectroscopy (AES) was conducted by using a 2-kV electron beam with 5-20-pA target current. Ion milling was conducted by using a 2-kV argon ion gun (01 and 02). Results and Discussion Previous studies of the solution interface of metal oxide semiconductor electrodes have involved electrical equivalents of the double-layer region which vary in complexity.1,10,11,24-26,28,29,31-33 Morrison has recently reviewed pertinent models of semiconductor electrolyte interface^.^^ The simplest model assumes a series of space-charge and Helmholtz capacitances with negligible contribution from
structure (EtO),SiCH,CH,CII, NH, (MeO),SiCH, CH,CH,NHCH,CH,NH, (MeO),Si(CH, ),NH(CH, ),NH(CH, ),NH, (EtO),SiCH, CH,CN PhSi(OEt), (EtO),Si(CH, )z (MeO),SiCH,CH,CH, SH
symbol Pr en ta CY
Ph me sh
Faradaic t e r m ~ . ~Since B ~ most studies are carried out in high ionic strength electrolytes, and with semiconductors with effective carrier densities of 1019/cm3or less, the contribution of the Helmholtz-capacitance term, CH,to the measured capacitance, CT,is negligible over a wide potential range. Mott-Schottky behavior is obeyed according to 1/C,,2 = 1/cT2= [ 2 / ( t ~ ~ n ~-) EFB ] ( E- k T / e ) (1) where C, = space-charge capacitance, E = dielectric constant for SnOz (12.7), eo = permitivity of free space, no = ionizable donor population in the space-charge region, Em = flat-band potential, E = electrode potential, k T / e has its usual significance. Several workers have shown that, where the effective carrier density, no, of the semiconductor electrode is high, as in the case for the SnOz electrodes discussed here, Helmholtz-capacitance terms (CH) are no longer negligible and the Mott-Schottky relationship takes on the form of eq 2. The importance of the Helmholtz capacitance be1/cT2= [2/(etotno)](E- EFB - k T / e ) + 1 / c H 2 (2) comes apparent when the deviation of the E - EFB term is computed at 1/cT2= 0. Errors of 10 mV to 1V can be obtained in the apparent flat-band potential (Em)when the Helmholtz-capacitance term is significant. We show here that the capacitance data obtained on clean, highly doped (1019 5 no 5 lOZ1/cm3)SnOz electrodes, in various aqueous electrolytes, before and after modification, is subject to the changes imposed by two nonnegligible solution capacitance terms. For all of the capacitance data discussed here, plots of electrode capacitance (CT)vs. potentid (E)and 1/cT2vs. potential are shown. Several studies have indicated that the l/cTz= 0 potential intercept varies from the flat-band potential calculated from photoelectrochemicald a h a The apparent carrier density (no)calculated from the slope of the 1/C2 vs. potential plots may also vary from no measurements made by other physical techniques.~a1~B~34 The 1/C2vs. potential plots are nevertheless quite sensitive to small but significant changes in electrode surface composition, and the changes in no and EFB values computed from these data are directly related to the changes in the true values. Capacitance Behavior of Clean SnOz Electrodes. Figure 1 shows the capacitance/potential response for a clean, unmodified SnOz electrode in pH 1 and 7 solutions. As shown in previous accounts, the capacitive response in the anodic potential region is dictated by the charging and discharging of the depletion-layer region of the semiconductor.1~5~11~25~28~29 In both 0.1 N HC1 or 0.1 N HzS04 and pH 7 phosphate and KHP buffers, negligible Faradaic activity was observed in the anodic potential region, at the modulation frequencies employed. In the potential region from 0.0 to -0.4 V vs. Ag/AgCl, a Faradaic response was (34) M. A. Butler and (1978).
D.S. Ginley, J. Electrochem. SOC.,126, 228
The Journal of Physical Chemistry, Vol. 85, No. 20, 198 1 2967
Stlane-Modified SnO, Electrodes
I
.064/y
056
Scheme I
I\
ti 12
1
;
I
k
\
0 0 I
I 0
A
I
I
-4
1
-8
1
.
1
-12
APPLIED POTENTIAL (volts vs Ag/Ag C I )
Flgure 1. 1/C: and CTvs. potential for an unmodified SnO, electrode in (0)pH 7, KHP/NaOH buffer and (0)pH 1, 0.1 N HCI or H,SO,. Modulation frequency = 200 Hz.
observed, which several workers have hypothesized to re1. We have previously accounted for some of this nonisult from a partial reduction of the metal oxide surdeality in capacitive behavior by assuming that the efface.1*25,22 This process is reversible and has a reduction fective carrier density of the SnOz electrodes was large and potential which shifts cathodically as a function of inthat solution contributions to the measured capacitance creasing solution pH.'J5 Extensive voltammetric inveswould not be ignored.' tigation of this response has prompted Lin35to speculate The simplest capacitance model and a more complete that the voltammetric process actually involves the remodel which comes from the previous studies of metal duction of a hydroxylated tin species, e.g., Sn(OH)4. oxxide i n t e r f a c e ~ ~are ~ ~shown ~ J - ~in~ Scheme I. We feel Surface-analysis data tend to confirm the hypothesis of that the SnOz interfacial capacitance results from (a) a metal oxide r e d u ~ t i o n . 'Coincident ~ ~ ~ ~ ~ ~with this surface depletion-layer capacitance, C, (1-5 pF/cm2), whose value reaction is a marked increase in the measured capacitance is affected mainly by the number of ionizable donors in response in the vicinity of the reduction potential as seen the semiconductor, and the potential gradient in the dein Figure 1 (see also Figure 17 of ref 35 and ref 19). pletion-layer region, (b) a large Helmholtz-capacitance Plots of 1/c2 vs. potential yield linear plots in the anodic term (20-40 pF/cm2), in series with C,,, whose value is potential region, according to the Mott-Schottky relaaffected by electrolyte composition and applied potential, tionship.' The flat-band potential (Em) for the ideal metal and (c) a capacitance term C,, in parallel with C,, arising oxide electrode should show a shift of -59.0 mV per pH from mobile, ionic species in the gel-like surface layer of unit increase, resulting from a change in the potential drop the SnOz electrode. From the model in Scheme lB, the across the ionizable layer at the solution interface, with total capacitance should be changing hydrogen ion a c t i ~ i t y . 'When ~ ~ ~ a~ wide ~ ~ range CT = c H ( c s c + c , s ) / ( c H + c s c + cw) (3) of pH solutions is investigated, where the solution buffer composition must be altered (e.g., HzSO4,KHP, etc.), the When CHis larger than C,, C, and the variation of C, pH dependence of the flat-band potential exceeds 100with potential has the same general form as that for C,, 115-mV change per pH ~ n i t . ~In paddition ~ ~ ~to ~this ~ ~ ~(eq ~ l),the 1/C2vs. potential plots remain linear but with shift in flat-band potential, a change in the slope of the an increase or decrease in slope proportional to the change l/Cz vs. potential plots is also observed for widely varying in the ionic population (co*) of the gel-like surface layer pH solutions, indicating an unexpected relationship be(eq 4). The larger than expected shifts in the intercept tween the apparent carrier density, no,of the SnOz electrode and the solution composition (Figure 1). The average d(l/CT2)/dE a no* (4) carrier density, no,computed for 30 electrodes in pH 7 of the 1/C2 vs. potential plots (EFB) with pH can be ramedia was (5.5 f 0.1) X 1020/cm3,and this value changed tionalized by assuming a nonnegligible contribution of CH to (3.7 f 0.1) X 10m/cm3when these same electrodes were (C, and C, are not negligible on the measured capacitance exposed to a pH 1,HCl or H2S04solution. The apparent CH). Changes in electrolyte composition, compared to flat-band potential shifted from ca. -1.2 V vs. Ag/AgCl to necessary to change solution pH, lead to changes in CH, -0.2 V when the solution was changed from pH 7 to pH which can be detected for semiconductor electrodes with no I 1019/~m3.33 (35)A. W. C. Lin, Ph.D. Dissertation, The Ohio State University, Changes in the slope of the 1/c2 vs. potential plots result Columbus,OH,1978. from a potential-dependent C,, term in parallel with C,,, (36) H. A. Laitinen, C.A. Vincent, and T. M. Bednarski, J. Electrochern. Soc., 115, 1024 (1968). as has been suggested by Boddy and others for germani-
+
2968
The Journal of Physical Chemistry, Vol. 85, No. 20, 1981
um/electrolyte i n t e r f a ~ e s ~and ~ dby ~ Tensch and Yeager for nickel oxide electrolyte interfaces.3l As the modulation frequency in the capacitance measurement was increased from 100 to 1000 Hz or greater, the slope of the 1/C2vs. potential plots converged to a constant value. This observation is consistent with the hypothesis that ionic surface states are the predominant cause of the C, term. RC time constants for the perturbation of ionic populations a t the electrode surface are larger than for electrbnic states.31-33*37 The C, term has a smaller contribution to CT at higher modulation frequencies. Dutoit et al. have considered in some detail the problem of nonideal capacitive behavior of semiconductor electrodes.29They chose to represent the frequency variation of electrode capacitance as belonging to one of two classes: (A) the case where there is a large dependence of the 1/C2 = 0 intercept on frequency but little frequency dependence upon the slope of the 1/C2 vs. potential plots upon frequency but little frequency dependence on the 1/C2 = 0 intercept. It appears that Ti02 and the Sn02thin-film electrodes belong to this last class. The frequency dependence of the measured capacitance was assumed by Dutoit et al. to be due to the presence of a “disturbed layer” which has the same approximate dimensions as the depletion-layer region of the solid. We assume that in these experiments the capacitance (C,) is due to the surface gel layer of the metal oxide and that the size of the gel layer at the Sn02surface extends to near the thickness of the depletion region. Evidence to support the fact that the Sn02surface is fully hydrated and possesses gel-like properties comes from electrochemical studies as well as surface-analysis data. Fully dehydrated Sn02 surfaces were produced by extracting in several nonaqueous solvents and then drying at 150-200 “C, or by vacuum drying at those temperatures directly. These fully dehydrated surfaces were confirmed to have less than one molecular layer of water present by analysis using X-ray photoelectron spectroscopy (XPS). The O(1s) signal in the XPS spectrum on such Sn02 surfaces showed only one peak, at a binding energy of 532.0 eV (vs. C(ls), 284.6 eV), indicative of oxygen in the Sn02 lattice. Upon immersion of such electrodes into the electrolyte, immediate observation of the capacitance measured at low modulation frequencies (100 Hz or less) showed CT increasing steadily by as much as 20% over the first several minutes of exposure. This effect was much less pronounced at modulation frequencies of 1000 Hz of greater. Steady-state capacitance measurements were achieved after exposure times to solution in excess of 15 min. Examination of these Sn02 surfaces by XPS following exposure to such electrolytes showed an O(1s) spectrum that indicated a fully hydrated surface; the major O(1s) transition occurred at 532.0 eV consistent with OHtype surface oxygen. In those cases where ions detectable by photoelectron spectroscopy were used in electrolyte buffers, their presence could be seen in the SnOz surface spectra as well. In these studies, as well as in previous r e p ~ r t s ,we ~ ! have ~ ~ observed the presence of potassium below the surface of the hydrated Sn02 electrodes to depths in excess of 100 A, through the use of ion-milling techniques in combination with the surface spectroscopies to reveal subsurface composition. The extent of the hydration layer detected by XPS or Auger electron spectroscopy (as indicated by an oxygen-to-tinatomic ratio that exceeded 2.0) was at least 100-200 A on those surfaces that had been allowed to reach steady state in the electrochemical capacitance measurements. The actual hydration layer may extend beyond this distance, because both the (37) P. J. Boddy, Surf. Sci., 13, 52 (1969).
Armstrong and Shepard
AES and XPS techniques require examination of the sample in ultrahigh-vacuum environments. Recently, Bard et al. have characterized several semiconductor electrodes which have demonstrated high concentrations of surface states.% In the case of these narrow band gap semiconductors (nonmetal oxides), the concentration of surface states can be so high as to dictate a condition of “Fermi level pinning”. This condition occurs when the total charge of the surface state (q,), at a given energy, exceeds the charge normally assumed in the space-charge region, qsc. From the results presented here we conclude that the charge due to the ionizable surface states is nonnegligible fraction of the charge due to intrinsic carriers but is not high enough to create the condition of pinning of the Fermi level. Changes in potential in the electrode surface are stil expressed principally across the surface layer and not across the Helmholtz-solution region. Capacitance Behavior of Modified SnOz Electrodes. When a silane is attached to the Sn02surface, the ionic composition and the dielectric constant of the gel-like surface region is affected. This compositional change is reflected chiefly in changes to C,. Figure 2, A and B, shows the differential-capacitance behavior of Sn02electrodes which have been modified with pr-silane. Similar results were obtained for all of the amine silane modified electrodes. The 1/C2 = 0 intercept for apparent flat-band potential changes by less than an average of 50 mV for all electrodes examined, before and after modification as reported previously.’JO At the same modulation frequency the addition of the silane to the Sn02 electrode surface changes the slope of the 1/C2 vs. potential plots. In the pH 7 medium (Figure 2B) above the pKa for the protonated primary amine, the addition of the silane causes the slope of these plots to increase (apparent no decreases),and at pH 1(Figure 2A), below the pKa for the amine, the slope decreases (apparent no increases compared to the response observed for the unmodified electrodes). When the capacitance behavior of an electrode was compared before and after its modification with the amine silane, no decreased by an average of 27% in pH 7 or increased by an average of 30% in pH 1solutions. Addition of the silane to the Sn02 electrode causes an increase in capacitive current (C, increase) in acidic media, and a decrease in capacitive current (CeSdecreases) in neutral media. This behavior should be explored for other electroanalytical applications of these electrodes. In terms of the model stated above, these results are consistent with an increase in the effective ionic population-of the gel layer in neutral media and with a decrease of this population in acidic media. The lack of significant change in the flat-band potential after silanization of the electrode surface is similar to the results of Finklea and Murray on Ti02 electrodes.1° It is apparent that only a fraction of the ionizable surface sites on the Sn02 electrode participate in the modification reactions, thereby leaving the energy levels of the semiconductor surface largely unchanged. Previous voltammetric and capacitance studies of amine silane modified Sn02electrodes had shown that the capacitance current increased in acidic media over that of the unmodified electrodes5 or that the capacitance current decreased in neutral media.’V2 No complete rationalization for these phenomena has been given before this study. Comparison of the changes caused by the addition of amine silanes of increasing molecular weight and number of secondary amine groups, the en-silane and the ta-silane, showed that these silanes affected the capacitance of the (38) A. J. Bard, A. B. Bocarsly, F.-R. F. Fan, E. G. Walton, and M. S. Wrighton, J.Am. Chem. SOC.,102, 367 (1980).
The Journal of Physical Chemistry, Vol. 85, No. 20, 198 1 2969
Sllane-Modified SnO, Electrodes
L
I2
8
0
4
-.4
APPLIED POTENTiAL (volts vs. Ag/Ag CI)
APPLIED POTENTIAL (volts vs Ag/AgCI) B
056
\
048
040 N
E
s
3
032
024
016
1.2
.4
0
-.4
-.a
-I
APPLIED POTENTIAL (wlts vs. Ag/AgCI)
Figure 2. (A) 1/cT2and CT vs. potential for the SnOp electrode in Figure 1 , pH 1, 0.1 N HCi: (0)before surface modification and (0) after modification with py-silane. (B) 1/CT2and CT vs. potential for a SnOp electrode in pH 7, KHP/NaOH buffer: (0)before surface after modification with py-silane. modification and (0)
Sn02by nearly the same magnitude and in the same direction as the pr-silane. The addition of one or two more protonatable groups did not have a significant effect on electrode capacitance. In order to ascertain whether the acidlbase chemistry of the amine silanes was important to the observed capacitance changes, several other silanes with different chemical functionalities were also attached to Sn02electrodes and their electrochemical behavior was compared before and after modification. For all of these materials
Flgure 3. 1/CT2and CT vs. potential for the SnO, electrode from following adsorption Figure 2A: (0)modlfied with py-silane and (0) of CoTSPc.
(me-, cy-, sh-, and ph-silanes, Table l),similar changes in capacitance behavior were noted as for the amine silanes although with smaller magnitude. Effective carrier density increases were observed in pH 1 solutions, and decreases were noted in pH 7 solutions after addition of the silanes to the electrode surface. These effects have been previously documented for the sh-silane on SnOz in pH media.' The differences in magnitude of the capacitance variation may reflect small differences in surface coverage of these materials or real chemical differences between the silanes. When CT was measured on the modified electrodes at higher modulation frequencies, (up to 2 kHz), it was noted that the anomolous shift in no as a function of surface modification (figure 2, A and B) was diminished. Simultaneous with these observed changes in capacitance behavior, voltammetric analysis showed that the surface reduction process described above (between -0.1 and 4.5 V) had decreased in intensity or was almost completely suppressed. This fact has been noted previously for silane-modified e1e~trodes.l~~~ Capacitance behavior was also explored for electrodes which had been silanized and then exposed to conditions which would cause adsorption or covalent attachment of CoTSPc, CuTSPc, Co2+,Fe2+,or Cu2+(Figure 3). In all cases, the capacitance behavior was affected only slightly by the addition of the redox center to the surface, except in the vicinity of the potential for its redox reaction. (See also ref 19.) Capacitance Data i n Nonaqueous Solvents. The electrochemical behavior of the modified SnOz electrodes in nonaqueous media is also of interest; the above-described experiments were conducted in nonaqueous solvents. Despite the lower dielectric constant for nonaqueous solvents, the Helmholtz capacitance is large enough that linear 1/C2 vs. potential behavior has been observed for n-type semiconductor electrodes at potentials well positive of the flat-band potential.39 We have observed this to be ~~
(39)S. N. Frank and A. J. Bard, J.Am. Chem. Soc., 97,7427 (1975).
2970
The Journal of Physicai Chemistry, Vol. 85, No. 20, 198 1
Armstrong and Shepard
In nonaqueous media, the metal oxide surface is largely dehydrated, and the contributions of a gel-like layer to the measured capacitance are lessened. The silane layer added to the surface is more readily solvated in nonaqueous media so that a larger perturbation to the Helmholtz layer is expected. Referring to eq 3, C, becomes unimportant in the nonaqueous solvent and CH is lowered to ca. 10-15 vs. 20-40 pF/cm2 in concentrated aqueous electrolytes. C, is not negligible with respect to CH. Addition of the silane with a low effective dielectric constant causes an even further decrease in CH, and the 1/C2 = 0 intercept shifts negatively by 300-500 mV. Further addition of the tetrasulfonated phthalocyanine to the silane layer causes the CH term to increase, above that of the unmodified electrode. This is likely due to the attachment of a partially dissociated tetraanion (CoTSPc"-..(Na+)J to the otherwide un-ionized surface region. The silane and the macrocycle appear to be held away from the electrode surface, in a low-polarity "solvent" sheath. The voltammetric behavior of the attached phthalocyanines in nonaqueous solvents supports this hypothesi~.'~ B
I -
I
I
I
I
I
I
I
I
I
I
Conclusions Differential-capacitance studies of electrode/solution interfaces give sensitive indications of small chemical changes occurring to the electrode surface. In the case of the n-type metal oxides, such as Sn02,measurements of CT or 1/CT2 vs. potential confirm the importance of the hydrated surface layer and the ionic contributions to the capacitance current. Addition of a silane attached to a strong base such as primary or secondary amines further increases the population of ionizable species in the gel-like surface region. Some previous accounts have attributed the contribution of the silane to capacitance changes in CH.lJl Changes in CH by silanization (provided CH is independent of potential) should cause changes in Em which are not observed. It is clear that the data presented above can be rationalized by assuming (a) that CH is not independent of potential or (b) that the ionic population of the surface layer (including the silane) contributes a capacitance term C, which is potential dependent. At a pH less than the zero point charge (pH 5.5 for SnOJ and at positive potentials of EFB, anion adsorption is referr red.^^^^' Raising the pH or adding an amine functionality which is protonated clearly perturbs the ionic population of the surface layer and is measurable only at low modulation frequencies (below 1 kHz).1092a33 This study confirms previous reports that show that the anomalous capacitance behavior reported for several metal oxide semiconductor electrodes is less likely due to experimental artifacts than to surface chemical effects. Acknowledgment. Many helpful discussions with A. W, C. Lin and T. Kuwana are gratefully acknowledged. This
(40) K. W. Frese, Jr., and S. W. Morrison, J. Electrochem. SOC.,126, 1235 (1979).
research was supported by a grant from the National Science Foundation (CHE 7819959).
