1596
J. Phys. Chem. 1988, 92, 1596-1601
Adsorption of Acetic Acid at Platinum and Gold Electrodes: A Combined Infrared Spectroscopic and Radotracer Study Dennis S. Corrigan,' Elizabeth K. Krauskopf,2 Lesa M. Rice? Andrzej Wieckowski,** and Michael J. Weaver*] Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, and Department of Chemistry, University of Illinois, Urbana, Illinois 61801 (Received: July 28, 1987)
The potential-dependent specific adsorption of acetic acid and acetate at polycrystallineplatinum- and goldaqueous interfaces has been examined by potentialdifference infrared spectroscopy (PDIRS) in conjunction with quantitative surface concentration measurements using the radiotracer technique. Both the infrared and radiotracer measurements utilize a similar thin-layer solution arrangement. Extensive adsorption of acetic acid occurs at both platinum and gold, which increases at more positive potentials and reaches a maximum at the onset of anodic oxide formation. The PDIR spectra contain several features identified with solution acetic acid in the 1200-1800-~m-~ region, including vC4 and vC4 bands at 1710 and 1280 cm-I, respectively, arising from adsorption-induced changes in the thin-layer solution composition as well as a single band, at 1415 and 1400 cm-' on platinum and gold, respectively, due to adsorbed acetic acid. The intensity of the former bands can be employed to evaluate the extent of adsorption as a function of potential; the results are in semiquantitative agreement with the surface concentration-potential data from radiotracer measurements. Although the radiotracer measurements indicate that acetate is not significantly adsorbed, acetate solutions in unbuffered media also yield intense PDIR spectra at platinum and gold, the latter arising merely from pH-induced changes in the thin-layer solution composition associated with anodic oxide formation. The 1400-cm-' feature for adsorbed acetic acid is consistent with a symmetric carboxylate stretching mode; the absence of the corresponding asymmetric vibration is indicative of a C, geometry with both carboxylate oxygens oriented toward the metal surface. In view of the infrared spectra together with the observed absence of acetate adsorption, the most likely modes of acetic acid adsorption involve hydrogen bonding between the carbonyl oxygen and inner-layer water molecules, or by self-association to form dimers or chain structures.
The recent development and application of vibrational spectroscopic probes, namely infrared and Raman spectroscopies, to the in situ characterization of metal-solution interfaces3v4 is contributing significantly to the increasing emphasis being placed on molecular-level surface characterization in electrochemistry. Nevertheless, in order to aid the spectral interpretation as well as provide a more complete picture of interfacial structure it is clearly desirable to combine such spectroscopic measurements with independent evaluations of surface composition. The importance of such combined in situ spectral-compositional measurements is reinforced by the anticipated sensitivity of surface infrared and Raman intensities not only to surface concentration but also to adsorbate structure and orientation (via selection rules and other factor^^,^). As a consequence, the vibrational band intensities for adsorbates may not display any simple relation to surface composition. Added complications with in situ infrared measurements arise from interferences from solution-phase features and from difficulties in distinguishing these from bands due to adsorbed species. The acquisition of quantitative surface compositional information in conjunction with such spectral measurements should therefore provide important information on these additional factors that influence the latter data as well as aid the structural interpretation of the former. In addition to surface-enhanced Raman spectroscopy (SERS), the Purdue group has recently been applying potential-difference infrared spectroscopy (PDIRS) using a Fourier transform instrument to characterize adsorbates on ~ i l v e r ,gold,6 ~ . ~ and platinum' electrodes. For systems featuring potential-dependent adsorption-desorption equilibria, the alterations in the thin-layer
solution concentration induced by alteration of the electrode potential give rise to additional vibrational bands that can provide an approximate guide to the adsorbate c o ~ e r a g e . It ~ remains important, however, to employ additional more reliable means of assessing adsorbate coverage. One such method involves radiotracer measurements. This technique has recently been utilized in conjunction with infrared measurements to characterize the adsorption of phosphoric acid and related systems on platinum.* The Illinois group has recently developed a variant of the radiotracer method that, by employing a thin-layer arrangement with a highly polished glass scintillator and electrode surface, enables extremely accurate measurements of surface concentration to be a ~ h i e v e d . ~This method is applicable to a miscellany of organic adsorbates by using I4C radiotracers. It provides a valuable method for examining adsorption on a variety of surfaces, including those for which conventional electrochemical methods cannot be utilized. Moreover, the similar 1 cm2 area) thin-layer configurations and electrode geometries (a. employed with the radiotracer and infrared techniques portend well for their broad-based combined application. The study described herein involves the application of this combined approach to examining the potential-dependent adsorption of acetic acid and acetate on platinum and gold electrodes. While the adsorption of formic acid on platinum involves irreversible chemisorption to form carbon monoxide and other dissociation products,1o higher carboxylic acids appear to adsorb reversibly over a wide range of electrode potentials, including regions where surface oxides are formed.11J2The present infrared
(1) Purdue University. (2) University of Illinois. (3) For a review, see Bewick, A.; Pons, S . In Advances in Infrared and Raman Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Wiley Heyden: New York, 1985; Vol. 12, Chapter 1. (4) For example: Chang, R. K.; Laube, B. L. CRC Crft.Rev. Solid State Mater. Sci. 1984, 12, 1. ( 5 ) (a) Corrigan, D. S.; Weaver, M. J. J. Phys. Chem. 1986,90, 5300. (b) Corrigan, D. S . ; Brandt, E. S.; Weaver, M. J. J. Electroanal. Chem. 1987, 235, 327. (6) (a) Corrigan, D. S.; Gao, P.; Leung, L-W. H.; Weaver, M. J. hngmuir 1986, 2, 744. (b) Corrigan, D. S.; Weaver, M. J. J. Electroanal. Chem. 1988, 239, 5 5 . ( c ) Corrigan, D. S.; Weaver, M. J. Langmuir, in press.
(7) (a) Leung, L-W. H.; Weaver, M. J. J . Am. Chem. SOC.1987, 109, 51 13. (b) Corrigan, D. S.; hung, L-W. H.; Weaver, M.J. Anal. Chem. 1987, 59, 2252. (c) Corrigan, D. S.; Weaver, M. J. J. Electroanal. Chem. 1988, 241, 143. (d) Leung, L-W. H.; Weaver, M. J. J . Electroanal. Chem. 1988, 240, 341. (8) (a) Zelenay, P.; Habib, M. A.; Bockris, J. O'M. Langmuir 1986, 2, 393. (b) Habib, M. A.; Bockris, J. O'M. J . Electrochem. SOC.1983, 130, 25 10. (9) Krauskopf, E. K.; Chan, K.; Wieckowski,A. J . Phys. Chem. 1987, 91, 2321. (10) (a) Beden, B.;Bewick, A.; Lamy, C. J . Electroanal. Chem. 1983,148, 147. (b) Beden, B.; Bewick, A,; Lamy, C. J. Electroanal. Chem. 1983, 150, 505.
0022-365418812092-1596$01SO10 0 1988 American Chemical Society
Adsorption of Acetic Acid at Pt and Au Electrodes
The Journal of Physical Chemistry, Vol. 92, No. 6,1988 1597
results shed some light on the nature of the surface coordination for adsorbed acetic acid, discussed earlier on the basis of radiotracer measurements by one of us.12The results also demonstrate some virtues of radiotracer measurements in aiding the interpretation of potential-difference infrared data. In particular, PDIR spectra obtained for acetate solutions a t platinum and gold electrodes are identified as arising entirely from pH changes within the thin-layer cavity rather than from acetate adsorption.
Experimental Section Most experimental details of the surface infrared measurements are given in ref 5a. The infrared spectrometer was a Bruker-IBM IR 98-4A Fourier transform instrument, equipped with a liquid nitrogen cooled MCT narrow-band detector (Infrared Associates Model HCT-18A). The spectral resolution was 8 cm-'. The incident light was p-polarized by means of a KRS-5 wire-grid polarizer (Harrick). The polycrystalline platinum and gold electrodes for both the infrared and radiotracer measurements were ca. 1 cm diameter disks mounted on a glass plunger (PDIR) or a platinum rod (radiotracer). The platinum and gold surfaces were pretreated immediately prior to use by mechanical polishing successively with 1, 0.3, and 0.05 pm almina (or diamond paste down to 0.25 pm for the radiotracer measurements), dipping in hot chromic acid, followed by rinsing and potential cycling for 10 min at 50 mV s-l between -0.25 and 1.4 V vs S C E in 0.1 M HC104. An electrochemical thin-layer configuration was formed in the PDIR experiments by pressing the electrode up against the CaFz optical window. The spectrometer was maintained under vacuum and the electrochemical cell was placed in an external compartment purged with nitrogen. The spectral acquisition was synchronized with the potential alterations by means of trigger pulses from the spectrometer to the potentiostat (PAR Model 173) as outlined in ref 13. Spectra were obtained by altering the potential between the base and sample values after every 32 interferometer scans, a total of 1024 scans being acquired. Integrated molar absorptivities in bulk aqueous solution for infrared modes of interest here were measured by using a transmission IR cell with CaF2windows and a Teflon spacer. The path length was 0.031 mm as determined from the interference fringes in the mid-IR region. The solute concentrations were comparable to those employed in the surface infrared measurements. The radioelectrochemical method has been described in detail in ref 9. Carbon-14 labeled acetic acid or sodium acetate having a specific activity of 5 mCi/mmol was used. As noted above, the thin-layer configuration of the radiotracer measurements is similar to that in the PDIR experiment; the electrode surface is pressed against a glass scintillator surface polished to optical flatness. The use of a thin-layer configuration arises in both cases from the need to minimize the contribution to the observed (infrared or radiotracer) signal from the bulk solution. The radiotracer methodology involves equilibrating the electrode with the solution at a given electrode potential prior to forming the thin layer. By minimizing the thin-layer thickness (1 pm can now be achieved) and correcting for the solution radioactivity component by calibrating with NaHI4CO3,precise surface concentration values may be obtained with solution concentrations up to several millimole^.^ The glass scintillator detector responds both to the radioactive adsorbate and to the species trapped in the ca. 1 pm thick solution thin layer. The surface infrared and radiotracer measurements were performed at Purdue and Illinois, respectively. The cyclic voltammetric responses of the platinum and gold electrodes employed for these measurements were, however, identical. (1 1) (a) Kazarinov, V. E.; Ginina, G. P. Elektrokhimiya 1967,3, 107. (b) Zusman, R.I.; Vasil'ev, Yu.B. Elektrokhimiya 1976,12,935. (c) Arkharona, G. L.;Bobdanovskii, G. A.; Vasil'ev, Yu.B. Elektrokhimiya 1918.14, 1515. (d) Horanyi, G. J. Electroanal. Chem. 1974, 51, 163. (12) Wieckowski, A.; Sobkowski, J.; Zelenay, P.; Franaszezuk, K. Elecrrochim. Acta 1981, 26, 11 11. (13) Corrigan, D. S.; Milner, D. F.; Weaver, M. J. Rev. Sci. Instrum. 1985, 56, 1965.
[A -0.IV
t----dl
!SO0
I
IB -0.IV I
lv(cm-') , 1000I 2500 I
v(cm
)
1000
Figure 1. Potential-difference infrared spectra obtained in the 10002500-cm-' region at a smooth platinum electrode. Electrolyte: (A) 0.01 M CH,COOH + 0.1 M HC104 and (B) 0.1 M HC10,. In each case, the potential was stepped from a base value of -0.2 V vs SCE to the sample values indicated. Spectra are an average of 1024 interferometer scans at each potential.
Sodium hydroxide (ultrapure), acetic acid (Gold label), and sodium acetate-I3C were obtained from Aldrich; sodium perchlorate (G. F. Smith) was recrystallized twice from water. The source of (2-14 labeled acetic acid as well as sodium acetate was CH3I4COONa (ICN Radiochemicals), having 9.6 mCi/mmol specific activity. Other chemicals were reagent grade and used as supplied. Water was purified by means of a "Milli-Q" system (Millipore Corp.). All potentials are quoted versus the saturated calomel electrode (SCE) and all measurements were made at room temperature, 23 f 1 O C .
Results and Discussion Acetic Acid. A representative series of PDIR spectra is shown in Figure 1A for 10 mM CH3COOH 0.1 M HC104at platinum in the frequency region 1000-2500 cm-'. The strongly acidic condition was selected to ensure that acetic acid (pK, 'v 4.75)14 did not undergo significant dissociation in solution. These spectra were obtained by altering the potential to the indicated series of more positive sample values from a common base value of -0.2 V. (This potential was selected since it corresponds to only negligible acetic acid adsorption, vide infra.) As is con~entional,~ the spectra are reported as relative changes in the reflected infrared intensities, AR/R.The corresponding series of spectra obtained with the same electrode in 0.1 M HC104alone is pictured in Figure 1B. It is seen in Figure 1A that, for sample potentials between 0.30 and 0.70 V, three positive-going bands, at 1710, 1380, and 1280 cm-I, and two negative-going bands, at 1410-1415 and 1110 cm-', are clearly observed. The last band is obtained in 0.1 M HC104 alone (Figure 1B) as well as in the acetic acid containing electrolyte (Figure 1A). On the basis of the frequency and bandwidth, this feature is identified with the major infrared-active C1-0 stretch, Y ~ for~ perchlorate ~ . anions. Its presence arises from the migration of perchlorate anions into the thin-layer cavity in order to maintain electroneutrality upon stepping the potential from the base to sample values.6b The three positive-going bands are identical in form with that observed for solution acetic acid, with the 1710-, 1380-, and
+
~~
(14) Serjeant, E. P.; Dempsey, B. Ionization Constants of Orgonic Acids in Aqueous Solution;IUPAC Chemical Data Series No. 23; Pergamon: Oxford, U.K., 1979.
Corrigan et al.
1598 The Journal of Physical Chemistry, Vol. 92, No. 6, 1988
0-
I 0
04 E/
0%
V
I2
vs SCE
Figure 2. Plots of acetic acid surface concentration,I', against electrode potential for platinum (circles) and gold (squares) in aqueous 0.1 M HC104. The open (right-hand y axis) and closed (left-hand y axis) symbols refer to data extracted from infrared and radiotracer measurements, respectively. The acetic acid concentration was 10 and 1 mM, respectively, in these two cases. (See text for details.)
1280-cm-l bands attributed to the C = O stretch (u-), CH3 bend [S(CH,)], and C-0 stretch (uc4), respe~tive1y.l~ These assignments were confirmed from the frequency downshifts of the first and third bands upon substituting CH312COOH by CH313COOH. This isotopic substitution procedure also aids the distinction between the uM bands and the broad bipolar feature around 1650-1700 cm-', due to water absorption, which also appears in the blank electrolyte (Figure 1B). The positive sign of these three bands is indicative of the loss of solution acetic acid upon stepping from the base to sample potentials, resulting from increased specific adsorption under these conditions. The corresponding negative-going feature at 1410-1 41 5 cm-' is associated with the corresponding adsorbed species that is formed upon stepping to the sample potentials (vide infra). The peak frequency increases slightly with increasing positive potential. Since an essentially fixed amount of adsorbing species is trapped in the thin-layer cavity on the time scale of the PDIR measurements, the intensity of these positive-going bands can be used as a rough measure of the extent of potential-induced adsorption of acetic acid between the base and sample value^.^ As detailed elsewhere,5 this procedure involves integrating the area under one or more of these bands (plotted in absorbance units) and converting into surface concentration, r (mol cm-*), from a knowledge of the molar absorptivity, Ai, of the bands in question. The uc4 band was employed for this purpose; A , was determined to be 5.8 X lo3 M-' cm-2 (base 10 logarithm scale) from transmittance cell (1710 cm-') band was not used for this measurements. The purpose since interference from overlapping water bands made , ~ r values were the determination of A , difficult. As b e f ~ r ethe obtained by assuming that the light effectively made only a single, rather than reflected double, passage through the thin-layer cavity. An inevitable complication is that the effective A , value in the absorption-reflection arrangement can differ somewhat from that obtained in the transmittance cell due to the strong spatial and angular variation of the p-polarized light vector close to the reflecting surface (vide infra).16 Figure 2 contains a plot of the acetic acid surface concentration against the electrode potential so obtained for 10 mM CH3COOH + 0.1 M HC104 on platinum (open circles, r scale on right-hand y axis). Plotted for comparison in Figure 2 are corresponding r-E data for 1 mM CH3COOH + 0.1 M HC104on platinum obtained by the radiotracer method (filled circles; note that these refer to -JL
(15) Ito, K.; Bernstein, H. J. Cun.J. Chem. 1956, 34, 170. ( ! 6 ) (a) Greenler, R. G. J . Chem. Phys. 1966, 44, 310. (b) Seki, H.; Kunimatsu, K.; Golden, W. G. Appl. Spectrosc. 1985, 39, 437.
I
,
,I
0
04 E / V v s SCE
0.8
I .2
Figure 3. Anodic-cathodiccyclic voltammograms obtained at platinum in 0.1 M HC104(solid trace) and after the addition of 0.01 M CH3COOH (dashed trace). The sweep rate was 50 mV s-' and the solution was purged previously with nitrogen. Anodic current is plotted upwards; the electrode area is 0.8 cm2.
different r scale, on left-hand y axis). This plot resembles that obtained previously by using radiotracer measurements at electrodeposited platinum,12although the r values here are somewhat higher. The use of different acetic acid concentrations in these plots requires comment. Data were also obtained by the radiotracer method for 3 and 10 mM acetic acid. Both these plots are similar to that shown for 1 mM acetic acid, only with slightly larger r values, so that the maximum surface concentration is mol cm-2, respectively (i.e., increased to about 5.5 and 8 X up to about 1.8 times larger than for 1 mM acetic acid). The precision of these data, however, is inferior to that for the 1 mM data shown in Figure 2, due to the larger background radioactivity from the solution acetic acid in the thin-layer solution. In contrast to the relative insensitivity of I? to the acetic acid concentration in the 1-10 mM region, the intensities of the PDIR bands decreased markedly for concentrations below 10 mM, even though they were insensitive to concentrations above this value. The origin of this apparent disparity almost undoubtedly lies in the need to maintain a sufficient quantity of acetic acid in the thin-layer cavity so that the solution concentration is not perturbed substantially upon altering the potential between the base and sample values. This circumstance differs from that encountered with the radiotracer technique, where the adsorbate-solution equilibrium is established prior to forming the thin layer at each potential. The observation of the solution PDIR bands, on the other hand, arises from the alteration in the adsorbate-solution equilibrium between the base and sample potentials. These methodology differences notwithstanding, it is of interest to note that while the corresponding infrared and radiotracer r - E plots for acetic acid adsorption at platinum have a similar shape, the adsorbate I? values obtained by the former are about 4-5-fold smaller (Figure 2). While this difference is due in part to the higher acetic acid concentration required for the former measurements, a residual disparity of about 2-3-fold remains. Given that the radiotracer method provides a reliable direct measurement, the larger estimates obtained from the infrared probe presumably arise from a systematic error in the estimation of the effective A , value of the uc-o band (vide supra). Indeed, the somewhat larger A , values that are required to bring the infrared data into quantitative agreement with the radiotracer measurements are consistent with the attenuation of the p-polarized light intensity anticipated within the thin-layer cavity.16 The roughness factors of the platinum electrodes used for the infrared and radiotracer measurements were determined to be within ca. 10% from the charge contained under the hydrogen adsorption/desorption peaks (Figure 3). Most importantly, however, the similar shapes of the two r-E plots attest to the qualitative validity of the infrared
Adsorption of Acetic Acid at Pt and Au Electrodes
The Journal of Physical Chemistry, Vol. 92, No. 6,1988 1599 A OV
02 C
04
1550
04
0
08
E/VvsSCE
I 2
Figure 4. As for Figure 3, but for gold.
analysis. Comparison of the data with a corresponding anodiccathodic cyclic voltammogram obtained with and without 1 mM acetic acid at platinum (solid, dashed curves, respectively, in Figure 3) shows that the peak in the r-E plots, a t about 0.5 V, corresponds closely to the onset of oxide formation. Similar, although somewhat less intense, PDIR spectra were also obtained from corresponding measurements at gold, again using 10 m M CH,COOH + 0.1 M HC104. Besides the three positive-going bands at 1710, 1380, and 1280 cm-I due to loss of solution acetic acid upon stepping from the base potential (0.2 V) to the sample potential, a corresponding negative-going band is obtained at a slightly lower frequency, ca. 1395-1405 cm-l, than that on platinum (Figure 1A). The frequency of this latter feature increases slightly as the potential becomes more positive (cf. platinum). The corresponding r-E plot, obtained as before from the integrated absorbance of the positive-going vC4 band, is shown as the open square points in Figure 2. The closed squares refer to data extracted from the radiotracer method, for 1 mM CH3COOH + 0.1 M HC104. Similarly to adsorption on platinum, while the r values extracted from the infrared data are systematically larger than those measured by the radiotracer method, the plot shapes are very similar. In comparison with the corresponding r - E plots on platinum, the r values for acetic acid adsorption on gold are smaller and the r-E peak is shifted to more positive potentials. Interestingly, the potential of such maximum adsorption, 1.0 V, again corresponds to the onset of oxide formation as evaluated from the cyclic voltammograms (Figure 4). Infrared spectra were also obtained at gold as well as platinum in 0.1 M HC104 containing a range of acetic acid concentrations from 1 to 100 mM. The forms of the spectra are generally very similar to those in Figure 1A and the intensities, as noted above for platinum, are only mildly concentration dependent above 10 mM. Acetate. An extensive series of measurements were also made at higher pH values; above ca. pH 5.0 the predominant solution species will be acetate.14 Parts A and B of Figure 5 show representative sets of PDIR spectra obtained under these conditions at platinum and gold, respectively, the electrolyte being 10 mM CH3COONa 0.1 M NaC104 (ca. pH 9.5). The potential was stepped to the sample potentials indicated from a base value of -0.35 and -0.6 V for platinum and gold, respectively. The spectral features are markedly different to those in acidic media. Three major positive-going bands are obtained at 1550, 1420, and 1345 cm-I. These are identified from their frequencies and relative intensities as arising from the asymmetric COz stretch [v,(CO;)], the symmetric COT stretch [vs(C02-)],and the CH3 deformation mode [6(CH3)],respectively, for aqueous acetate anions.17 As
+
(17) Nakamoto, K. Infrared and Roman Spectra of Inorganic and Coordination Compounds, 4th ed.; Wiley: New York, 1986; p 231.
u 1500
v(cm-')
1000
2500
v(cm-l)
1000
Figure 5. Potential-differenceinfrared spectra obtained in 1000-2500cm-' region at smooth platinum and gold (A and B, respectively) in 0.01 M CH,COONa + 0.1 M NaC104. The potential was stepped from a base value of -0.35 V at platinum and -0.6 V vs SCE at gold to the sample values indicated. Spectra are an average of 1024 interferometer
scans at each potential. for acetic acid, this is consistent With a decrease in the acetate concentration in the thin-layer solution as the potential is altered from the base to the more positive sample values. These positive-going bands do not appear until the potential is stepped to a value corresponding to the commencement of oxide formation; indeed the potential at which a maximum intensity is reached is well into the oxide formation region. Note that the ion migration between the thin-layer cavity and the solution reservoir necessary for the maintenance of electroneutrality will be provided predominantly by the perchlorate and the sodium ions which are in large excess.6b Again, the negative-going band at 1110 cm-I in Figure 5A,B is due to C104- migration into the thin layer so to retain electroneutrality upon stepping from the base to the sample potential. Interestingly, the two other negative-going bands observed in Figure 5A,B at 1720 and 1280 cm-I, are notably different not only to the solution features but also from the negative-going band (around 1400-1420 cm-I) obtained by using acetic acid electrolytes (Figure 1A). Corresponding infrared spectra obtained from acetic acid-acetate electrolytes having pH values from 1 to 9 indicate that the 1400-1420-cm-' feature disappeared above about pH 2. Although the intensities of the 1720- and 1280-cm-I bands vary with the sample potential in rough correspondence with those of the positive-going band intensities, their frequencies are independent of potential. In order to interpret the PDIR spectra in Figure 5, it is desirable to evaluate the extent of adsorption of acetate at these electrode surfaces from corresponding radiotracer measurements. Significantly, no acetate adsorption was detected throughout the potential region -0.2 to +1.2 V for both platinum and gold. The maximum surface concentration of acetate must, therefore, be less than the lower limit of detection for this technique, around 3 X IO-" mol cm-2. Consequently, it is very unlikely that the features in the PDIR spectra in Figure 5 arise from acetate adsorption. One way of distinguishing between vibrational modes of adsorbed and solution phase species is to compare spectra obtained by using s- and ppolarized incident radiation. Because the electric vector associated with the s component of the infrared radiation undergoes a 180' phase shift upon reflection at the metal surface, the electric field strength at that surface is zero so that the light
1600 The Journal of Physical Chemistry, Vol. 92, No. 6,1988
cannot interact with adsorbed specie^.^ Comparisons between the PDIR spectra obtained with s- and p-polarized radiation revealed that they both possess negative-going bands at 1720 and 1280 cm-' as well as the positive-going features noted above, implying that all these bands are indeed associated with solution phase rather than adsorbed species. The most likely explanation for the appearance of the negative-going bands in these spectra involves pH changes in the thin-layer cavity. As the potential is stepped from the base to sample potentials in the anodic oxide region, protons are necessarily released into the thin-layer solution. Although this will have a negligible effect upon the solution composition for the strongly acidic electrolytes considered above, in the unbuffered acetate solutions the protons will form an equivalent quantity of acetic acid. Consequently, this effect should yield bipolar PDIR spectra with the positive- and negative-going bands arising from equivalent quantities of acetate and acetic acid formed by this reversible protonation process. The form of the PDIR spectra in Figure 5 are indeed in accordance with this explanation. Thus the negative-going bands at 1280 and 1720 cm-' and the weaker feature at 1380 cm-' are very similar to those obtained for solution acetic acid (cf. Figure 1). Moreover, the ratios of the absorbances for the positive-going 1550-cm-' band and the negative-going 1280-cm-' feature are in good agreement with the ratio of corresponding Ai values for these bands (3.75 X lo4, 5.8 X lo3 M-' cm2) in the solution-phase transmittance spectra for acetate and acetic acid, respectively. Although the acetic acid formed at the sample potential may be significantly adsorbed from the above results, this is expected to be hindered by the presence of extensive surface oxide. The size of the bipolar bands also correlates roughly with the extent of oxide film formation as obtained from cyclic voltammograms. The approximate independence of the band intensities on platinum and gold for potentials beyond 0.6 and 1.0 V probably results from the complete protonation of the acetate thin-layer solution. The addition of hydroxide concentration equal to or larger than the acetate concentration removed entirely the bipolar spectral features, as expected since the solution pH will then be effectively buffered so as to eliminate significant acetic acid formation. The examination of PDIR spectra for trifluoroacetate at platinum, using 10 mM CF3COONa 0.1 M NaC104, provided a further check oh the role of carboxylate protonation since CF3COO- is a sufficiently weak base (pK, = 0.5 for CF3COOH)I4to remain unprotonated under most conditions. Indeed, no significant spectral features in the 1200-2000-cm-' were obtained with this system. Structure of Adsorbed Acetic Acid. The frequency of the single vibrational feature observed for acetic acid adsorbed on platinum, 1410-1415 cm-' (Figure lA), and gold, ca. 1400 cm-I, is shifted markedly from those for the vc-o and vc-0 bands observed for the solution species, a t 1710 and 1280 cm-', respectively (vide supra). The frequency of this singular band is consistent with a symmetric C02- stretching mode, v,(C02-), for an acetate-like The absence of the higher-frequency v,(COc) band partner, expected around 1550-1600 cm-I, is unlikely to be due to interference from a positive-going solution band since none appears within this frequency region (Figure 1A). More likely, the presence of v,(C02-) together with the absence of its asymmetric partner arises from an interfacial binding geometry having C, symmetry, with both carboxylate oxygens oriented toward the metal surface. Thus in this case the latter mode will be infrared inactive on the basis of the "dipole surface selection rule", which deems that only vibrations having a net dipolar component normal to the surface can interact with the p-polarized light.lg Such a bridging coordination geometry has been identified on the basis of similar evidence for the adsorption of gas-phase acetic acid onto platinum, obtained by using electron energy loss spectroscopy for which the same selection rule will apply.20 In this
+
(18) Deacon, G. B.; Phillips, R. J. Coord. Chem. Reo. 1980, 33, 227. (19) (a) Pearce, H. A.; Sheppard, H. Surf. Sci. 1976, 59, 205. (b) Moskovits, M.; Huse, J. E. Surf. Sci. 1978, 78, 397.
Corrigan et al. P3 HO/CnO H\O/Hs-
wvi);:v A CH3
dL \ 0-
-H- 0
h\&
~
H ....Q
H\@H\&
~
NC\ 0-H
~
/
/
~
/
E!
Figure 6. Possible structures of adsorbed acetic acid at the electrodesolution interface.
gas-metal surface case the acetic acid appears to be coordinated via both oxygens directly to the platinum (Le., as acetate). Such a coordination geometry for adsorbed acetic acid in the present electrochemical environment also appears to be reasonable. However, it is difficult to account on this basis for the observed lack of significant acetate adsorption under higher pH conditions, where acetate rather than acetic acid is present in the bulk solution. One possible rationalization of these results is that acetate surface binding is largely prevented by competitive hydroxide adsorption at higher pH values. There is some evidence for OH- adsorption on platinum under these conditions.2' Alternatively, the acetic acid can be envisaged as adsorbing in undissociated form on top of inner-layer water molecules, involving hydrogen-bonding interaction with the carbonyl oxygen. Such an adsorption geometry has indeed been proposed previously to account for the increase in the extent of acetic acid adsorption as the electrode charge becomes more positive,12since the greater polarization of the adsorbed water molecules under these conditions should aid such hydrogen bonding. This proposed geometry for adsorbed acetic acid is shown schematically in Figure 6A. A further possibility involves the adsorbed acetic acid molecules as forming dimers or possibly oriented polymeric chains, as are pictured in Figure 6B. Evidence for such association in aqueous solution is persuasive;22although vibrational spectroscopic studies of this phenomenon for acetic acid are surprisingly fragmented in the literature, a v,(COJ band around 141C-1430 cm-' is indeed observed for associated species in both solution23 and the gas phase.24 A driving force for the formation of such associated adsorbate species, and indeed for the presence of such high surface concentrations, may well be the relatively low dielectric constant, as well as high field, expected in the vicinity of the inner layer of oriented water molecules. Concluding Remarks. The foregoing provides an illustrative example of how the availability of direct quantitative information on adsorbate surface concentration as provided by radiotracer measurements can aid the interpretation of corresponding infrared spectra obtained by using potential-difference technique. In particular, the PDIR spectra for acetate, identified as arising merely from pH-induced changes in the thin-layer solution composition, could easily have been misinterpreted in the absence of the radiotracer data as being due instead to potential-dependent acetate adsorption. In the case of acetic acid adsorption, the radiotracer data again provides a valuable check on the interpretation of the bipolar bands seen in the PDIR spectra. In the latter case, the PDIR spectra provides information at least on the relative extent of adsorption as well as the nature of the surface coordination and orientation of the adsorbed species. (20) Avery, N. R. J . Vac, Sci. Technol. 1982, 20, 592. (21) Jaaf-Golze, K. A.; Kolb, D. M.; Scherson, D. J . Electroanul. Chem. 1986,200, 353.
(22) For example: Schrier, E. E.; Pottle, M.; Scheraga, H. A. J . Am. Chem. Soc. 1964,86, 3444. (23) Zarakhani, N. G.; Vinnik, M. I. Rum. J . Phys. Chem. 1964,38, 332. (24) Weltner, W., Jr. J . Am. Chem. SOC.1955, 77, 3941. Kishida, S.; Nakamoto, K. J . Chem. Phys. 1964.41, 1558.
/
~
~
J. Phys. Chem. 1988, 92, 1601-1603 Although the approach does require reversible potential-dependent adsorption, it is applicable to systems where the surface composition-potential relationships display considerable hysteresis, as will be the case for the present systems where surface oxidation occurs. The substantial differences in the form of the carboxylate vibrational bands seen between the interfacial and bulk-phase species for each of the systems described here implicates directly the role of carboxyl coordination in the adsorption process. The nature of the surface coordination cannot be completely elucidated on the basis of such frequency shifts alone. It nevertheless dem-
1601
onstrates the heretofore unexploited virtues of surface infrared spectroscopy for examining the structure of electrochemical adsorbates for which the point-group symmetry of coordinating functional groups can be altered upon adsorption.
Acknowledgment. The radiochemicals were purchased from a grant (to A.W.) from Dow Chemical Corp. This work is also supported by the National Science Foundation (via Grant No. CHE-85-13732 to M.J.W.). Registry No. CH3COOH, 64-19-7; CH3COO-, 71-50-1; Pt, 744006-4; Au,7440-57-5; CF3COO-, 14477-72-6.
Surface-State Properties of n-CdSe in Sulfide Solutions Bob L. Wheeler* and Norman Hackerman Department of Chemistry, Rice University, Houston, Texas 77251 (Received: July 30, 1987; In Final Form: October 8, 1987)
The equivalent parallel conductance technique was used to study surfacestate behavior at the n-CdSe single crystal/aqueous sulfide interface. Surface states were found at -1.1 V versus SCE (-0.6 eV below the conduction band minimum) in solutions containing hydroxide and sulfide. The phase-sensitive photocurrent was affected by these states. Addition of dissolved sulfur to the solution removed the hump in the conductance versus voltage curves caused by these states and the photocurrent behavior is also improved.
Introduction Cadmium chalcogenides have been extensively studied for use in photoelectrochemical cells because of the many desirable qualities which these compounds possess.' Primary among these are the stability under illumination, especially in polychalcogenide solutions? and the direct optical transition3 as well as the ability to modify the band gap energy for a good match with the solar spectrum by compound formation, such as CdSexTel, and Cd,Znl-xTe.e6 Like the oxide semiconductors in acidic and basic solutions, cadmium chalcogenides have been found to interact strongly with species present in polychalcogenide solutions? For example, CdSe has been shown to exchange selenium for sulfur when immersed in sulfide solutions.8 The flat-band potential of CdS has been shown to depend on sulfide concentration.%" Recent work has shown that the cations in solution also play an important role in the maximization of the energy conversion efficiency, as does the ratio of chalcogen to chalcogenide to hydroxide.'-'*J3 In addition to effects by solution species, properties of the particular semiconductor crystal can have a great influence on the energy conversion efficiency. By variation of the doping and crystal growth procedures, the efficiency can be changed because (1) Licht, S.J. Phys. Chem. 1986, 90, 1096 and references therein. (2) Ellis, A. B.; Kaiser, S. W.; Bolts, J. M.; Wrighton, M. S. J. Am. Chem. Soc. 1977, 99, 2839. (3) Sze, S.M. Physics of Semiconductor Devices, 2nd ed.;Wiley: New York, 1981; p 849. (4) Noufi, R. N.; Kohl, P. A.; Bard, A. J. J . Electrochem. SOC.1978,125, 375. (5) Licht, S.;Tenne, R.; Dagan, G.; Hcdes,G.; Manassen, J.; Cahen, D.; Triboulet, R.; Rioux, J.; Levy-Clement, C. Appl. Phys. Lett. 1985, 46, 608. (6) Lemasson, P.; Forveille-Boutry, A.; Triboulet, R. J. Appl. Phys. 1984, 55, 592. (7) Yoneyama, H.; Hoflund, G. B. h o g . Surf. Sci. 1986, 21, 5. (8) Gerischer, H.;Gobrecht, J. Ber. Bunsen-Ges. Phys. Chem. 1978, 82, 520. (9) Minoura, H.;Watanabe, T.; Oki,T.; Tsuiki, M. Jpn. J . Appl. Phys. 1977, 16, 865. (10) Ginley, D. S.; Butler, M. A. J . Efecrrochem.SOC.1978, 125, 1968. (1 1) Wilson, R. H.J . Elecrrochem. SOC.1979, 126, 1187. (12) Licht, S.;Tenne, R.; Flaisher, H.; Manassen, J. J . Electrochem. SOC. 1984, 131, 950. (13) Licht, S.; Manassen, J.; Hodes, G. J. Elecrrochem. SOC.1986, 133, 272.
0022-3654/88/2092-1601.$01.50/0
of variations in the carrier density, bulk traps, and surface ~ t a t e s . ' ~ , Unlike '~ the former two, surface states can be removed or blocked by various surface treatments before insertion of the semiconductor electrode into the ~ e l l ' ~ as 3 ' ~well as by solution species in the electrolyte it~e1f.I~ Photocurrent-potential curves can give an indication of how surface treatments and solution species affect the energy amversion efficiency. However, this measurement alone cannot determine whether the surface state, and thereby surface recombination, has been affected, or whether the heterogeneous charge-transfer energetics or kinetics have been ~ h a n g e d . ' ~ J *On the other hand, plots of equivalent parallel conductance (G,) versus potential at low frequencies can detect both surface states as well as bulk traps in the space charge region by monitoring the filling and emptying of these states d i r e ~ t l y . ' ~ ' ~ The 7 ' ~ relevant ~ part of the equivalent circuit of the semiconductor/electrolyte interface is shown in Figure 1. Experimental conditions have been adjusted so that other parameters can be neglected. For example, a large counand,,,,C ,, to be insignificant, terelectrode allows both Rcounter the first small and the second large. Addition of supporting electrolyte to the solution enables both the diffuse double layer capacitance and the bulk solution resistance to be neglected. The ac measurements are conducted in the dark and in a potential regime where no faradaic current flows. Therefore, the faradaic impedance is extremely large. The Helmholtz layer capacitance is usually much larger than the space charge capacitance and is unimportant in this ~ a s e . ' ~ -R,, l ~ and C, represent any leakage (14) Wilson, R. H. J . Appl. Phys. 1977, 48, 4292. (15) Finlayson, M. F.; Wheeler, B. L.; Kakuta, N.; Park, K.-H.;Bard, A. J.; Fox, M. A.; Webber, S. E.; White, J. M. J . Phys. Chem. 1985,89, 5676. (16) Wheeler, B. L.; Nagasubramanian, G.; Bard, A. J. J . Electrochem. SOC.1984, 131, 2289. (17) Nagasubramanian, G.; Wheeler, B. L.; Hope, G. A.; Bard, A. J . J . Electrochem. SOC.1983, 130, 385. (18) Reichman, J.; Russak, M. A. J . Appf. Phys. 1982, 53, 708. (19) Nagasubramanian, G.; Wheeler, B. L.; Bard, A. J. J . Efecfrochem. SOC.1983, 130, 1680. (20) Nicollian, E. H.; Goetzberger, A. Bell Syst. Tech. J. 1967, 46, 1055. (21) Barabash, P. A.; Cobbold, R. S.C. IEEE Trans. Electron Deuices 1982, Ed-29, 102. (22) Dubow, J.; Rajeshwar, K. Final Report submitted to SERI Div., Golden, CO 80401, Oct. 1981. (23) Abe, M.; Morisaki, H.; Yazawa, K. Jpn. J . Appf. Phys. 1980, 19, 1421.
0 1988 American Chemical Society
