1164
Anal. Chem. 1989, 61, 1164-1167
tribution to band broadening by the nebulizer is almost unalthough a little peak is Observed' Regu1ar oscillations in the ion intensity, which occur with a period of about 6 s, are due to flow fluctuation introduced by the HPLC pump.
(9) Handa, S.; Kushi, Y. Satellite Symposium of the ISS-ASN Joint Meeting, Pueruto La Cruz. Venezuela, 1987. (10) Kozuka, M,; Hashimoto, K,; Takasaki, K,; Konoshjma, K,; Kat&, y,; Amano, T. Poster Abstr. J p n . - U S . Congr. Pharm. Sci. 1987, 205. (11) Sakairi, M.; Kambara, H. Anal. Sci. 1988, 4 , 141. (12) Blakley. C. R.; Vestal, M. L. Anal. Chem. 1983, 5 5 , 750. (13) . . Liberato. J. D.: Fenselau. C. C.: Vestal. M. L.: Yeraev. A. L. Anal. Chem. 1983. 55, 1741. (14) Vestal, M. L.; Fergusson, G. J. Anal. Chem. 1985. 5 7 , 2373. (15) Dodd, E. E. J . Appl. Phys. 1953, 224, 73. (16) Loeb, L. B. Science 1945, 102, 1363. (17) Hsu, F. F.; Edmonds. C. G.; McCloskey, J. A. Anal. Left 1986, 19, 1259. (18) Butfering, L.; Schmelzeisen-Redeker.G.; Rollgen, F. W. J . Chromatog. 1987, 394, 109. (19) Robins, R. H.; Crow, F. W. Presented at the 34th ASMS Conference on Mass Spectrometry and Allied Topics, Cincinnati, OH, 1986. 570. (20) Kim. H. Y.; Salem, N., Jr. Anal. Chem. 1986. 58, 9. (21) Kim, H. Y.; Salem, N., Jr. Anal. Chem. 1987, 5 9 , 722. 1
LITERATURE CITED Covey, T. R.; Lee, E. D.; Bruins, A. P.; Henion, J. D. Anal. Chem. 1988, 58, 1451A. Bruins, A. P.; Covey, T. R.; Henion, J. D. Anal. Chem. 1987, 5 9 , 2642. Covey, T. R.; Bruins, A. P.; Henion, J. D. Org. Mass. Spectrom. 1988,
23, 178.
Whitehouse, c, M,; D ~R , N,;~ yamashita, ~ ~M.; F ~ ~ J, ,B, ~ Anal, ~ , Chem. 1985, 5 7 , 675. Thomson, B. A.; Iribarne, J. V.; Dziedzic, P. J. Anal. Chem. 1982, 5 4 , 2219. Sakairi. M.; Kambara, H. Mass Spectrosc. 1983, 31,87. Sakairi, M.; Kambara. H. Presented at the 35th ASMS Conference on Mass Spectrometry and Allied Topics, Denver, CO, 1987. 407. Sakairi, M.; Kambara, H. Anal. Chem. 1988, 6 0 , 774.
.
RECEIVED for review October 4,1988. Accepted February 13, 1989.
CORRESPONDENCE
Absorption Spectroscopy Sir: Considerable progress has been made over the last decade toward the development of infrared spectroscopy as a probe of the vibrational and structural properties of species adsorbed on electrode surfaces. Fourier transform infrared reflection absorption spectroscopy (FTIRRAS) has become perhaps the most versatile technique for acquiring in situ data, as evidenced by the growing number of applications reported in the literature (1,2). Particular attention has been focused on the use of this methodology for the study of interactions involving simple ions and metal surfaces. In the case of Pt in sulfuric acid solutions, spectral features associated with both sulfate and bisulfate were observed in the p-polarized spectra (3). The relative intensities and frequencies of these bands were found to be a function of the applied potential, the latter providing strong evidence that the signals are indeed associated with species adsorbed on the electrode surface. The occurrence of electrolyte bands in potential difference FTIR spectra, however, may not always be ascribed to species adsorbed on the electrode surface. This is due to the fact that the thickness of the electrolyte layer between the electrode and the window is most often smaller than the wavelength of infrared radiation ( 4 ) . Hence, all IR-active species present in the thin layer can be detected with radiation polarized parallel to the plane of incidence, a factor that must be taken into account in the interpretation of spectral data. This communication will present in situ FTIRRAS spectra for Au and Pt electrodes in nitric and perchloric acid solutions and electrolytes containing both perchlorate and nitrate ions. The results obtained have afforded rather conclusive evidence that the absorption bands observed for potentials in the oxide formation region can be attributed to the relatively large amount of charge passed between the working electrode in the thin-layer cell and the externally located counter electrode, resulting in the migration of anions into the gap of the thinlayer cell. Specifically, the charge of the protons produced during anodic film formation must be compensated by the entrance of anions into the gap. 0003-2700/89/036 1-1164$01.50/0
EXPERIMENTAL SECTION The electrochemical cell involved in these studies is shown in Figure 1A. The Au electrode, a cylinder of 19-mm diameter and 2 mm-height, was cast in a Kel-F block exposing a circular area of about 2.8 cm2. It was then polished with a series of alumina powders of successively smaller size down to 0.05 pm, washed with nitric acid, and finally cleaned under ultrasonic agitation in ultrapure water. The electrode holder consisted of a threaded Teflon cap that could be screwed onto the back of the Kel-F piece so as to trap loosely a Teflon rod ending in a hemispherical head of larger diameter. After the cell was assembled, the Teflon rod, which was inserted in the back of the cell body, could be pushed against a hemispherical cavity machined in the back of the Kel-F block, forcing the electrode surface parallel to the CaF, window. Prior to the optical measurements, the electrode was moved away from the window. It was then subjected to a series of oxidation-reduction cycles between potentials close to hydrogen evolution and oxygen generation until voltammetry curves characteristic of clean Au were obtained. The electrolyte was then exchanged and the electrode pushed against the window for a complete series of spectroscopic measurements. Infrared spectra were obtained with a Michelson-Genzel type FTIR instrument (IR/98, IBM Instruments, Inc.) equipped with a liquid nitrogen cooled HgCdTe detector. The optical attachment involved in the in situ measurements is shown schematically in Figure 1B. Reflection spectra at a given potential were obtained by adding 500 interferometric scans. The data are presented in the form of -A.R/R vs wavenumber, where AR = (RmPh - R ) and R is the reflection spectrum at an arbitrary reference potential. Ultrahigh-purityHNO, and HCIOI were purchased from Baker and the solutions made with ultrapurified water obtained from a modified Gilmont distillation system. All potentials were measured versus a standard calomel electrodeplaced in an external compartment connected to the cell through a long Teflon capillary. A Au wire was used as a counter electrode. All measurements were conducted at room temperature. The experiments with Pt electrodes (0.79 cm2jwere carried out with a commercially available spectroelectrochemicalcell designed specifically for in situ IR measurements (Chemical Electronics Associates, Hickory, NCj. 1989 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 10, MAY 15, 1989
A
ELECTROCHEMICAL CELL
Figure 1. Schematic diagram of (A) in situ electrochemical cell for FTIRRAS measurements involving gold electrodes and (B) retroreflective absorption optics.
RESULTS AND DISCUSSION A series of FTIRRAS spectra for a Au electrode in 0.1 M HC104,using the reflection spectrum at 0.0 V as the reference, is shown in Figure 2A. The curves obtained at 0.8 V, as well as at potentials negative to this value (not shown in the figure), were not found to provide evidence for peaks associated with the electrochemical system. A clearly defined absorption peak could be observed, however, at potentials of 1.00 V and higher,
1165
a region in which Au undergoes surface oxidation. The intensity of the band in this potential region was found to increase monotonically with the extent of surface oxidation, without any appreciable shift in its absorption energy in the whole potential range examined. The position of this peak, i.e. 1110 cm-', is in excellent agreement with that associated with the triply degenerate u3 mode for C104- ( 5 ) . Additional measurements were performed in which NaN03 was dissolved in the 0.1 M HC104 solution in an amount sufficient to achieve equal concentration of the two anions. As shown in Figure 2B, the spectra in this case were found to exhibit in addition to the perchlorate feature a peak at 1370 cm-'. Both the position and the shape of the peak are in accordance with the doubly degenerate u3 mode of nitrate ion in aqueous media.5 It is interesting to note that despite the differences in the oscillator strengths of the two normal modes involved, the intensities of the nitrate and perchlorate peaks were very similar. The splitting of the doubly degenerate u3 mode, ca. 40 cm-', is due to a reduction in the symmetry of the species from D3h to C2",due to hydrogen bonding (6). A band splitting may also be caused by a field asymmetry effect induced by ion pairing (7,8). This does not appear to play a role in the systems examined, however, as all the species involved are believed to be solvated in aqueous media. In fact the degree of the splitting was found to be identical upon comparing spectra of KNOB,NaN03, and HN03. Data were also acquired for Au in 0.1 M H N 0 3 solutions and are shown without vertical offset of Figure 3. Included in this figure (see upper curve A) is the transmission spectrum of 0.1 M NaN03 obtained in a liquid IR cell with CaF2 windows. As expected, the spectral features are identical with those obtained in the electrochemical experiments. I t may be noted that the intensity of the nitrate peak in this perchlorate-free solution in the thin-layer electrochemical reflection cell is about twice that obtained when the two ionic species were mixed in equal proportions (see Figure 2B). Similar observations were made after examining the results for Pt obtained in the same electrolytes (see Figure 4). The onset of oxide formation in this case is about 0.5 V, which corresponds to the potential at which the bands begin to increase. The poorer signal to noise ratio in these spectra is due to the smaller size of the Pt electrode surface compared to that of the gold.
1
1
V vs. SCE
-AR/R
1.40
1.30
T
1.20
4x10-3 1.10
1
1.00
i-I 1600
0.80 1400
wavenumber
1200
(
cm-1
1000
)
1600
1400
wavenumber
1200
(
cm-1
1000
)
Flgure 2. I n situ FTIRRAS spectra for a Au electrode in solutions of (A) 0.1 M HCIO, and (B) 0.1 M NaNO, plus 0.1 M HCIO, at the indicated potentials. Conditions: electrode area A = 2.9 cm2; number of interferometric scans, 500; resolution, 4 cm-'; Emf = 0.0 V vs SCE. Measurements were conducted at increasingly positive potentials.
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 10, MAY 15, 1989
I
I
1.40 1.30 1.20 1.10 1.00 0.80
74x10-3
i. 1400
1600
1200
1000
wavenumber ( cm-1 ) Flgure 3. (A) FTIR transmission spectrum of 0.1 M NaNO,. (9)I n situ FTIRRAS spectra for a Au electrode in 0.1 M HNO, plotted without vertical offset. Ere,= 0.0 V vs SCE. Other conditions are listed in the caption of Figure 2. On the basis of the results obtained, it may be concluded that neither nitrate nor perchlorate appears to undergo specific adsorption on the substrate surface and, hence, that the increase in intensity must be related to a different type of phenomenon. The proposed explanation for this effect is an increase in the concentration of anions in the electrolyte of the thin-layer cell resulting from the transport of ions in and out of the thin-layer cell induced by the passage of charge between the working and counter electrodes, particularly during oxide formation. It should be stressed that ionic migration provides the predominant form of mass transport into and out of the gap in the thin-layer configurations of the type involved in this work (9, 10). Evidence in support of a charge compensation effect is provided by the linear correlation observed between the intensity of the spectral peak and the charge associated with oxide formation, shown in Figure 5 . The charge associated with the formation of 1 monolayer of Au oxide (400 pC.cm-2) (11) is about 13 times larger than that involved in a shift in potential of 1 V within the double-layer region, assuming that the double-layer capacity of Au is about 30 pF.cm-2. A simple calculation shows that the total number of moles of oxyanions in the thin electrolyte layer for a 1-V change in the double-layer region is 4 X lo-'' mol. This calculation uses transport numbers evaluated from the equivalent conductivities for hydronium ion (t+= 0.84) and the anions (t- = 0.16). Therefore, it is not surprising that no signal could be detected in the normalized reflectance spectra a t 0.8 V.
Another effect that plays a role in determining the actual size of the observed peaks is that associated with window reflections (12). As shown in the Appendix, however, this effect not only is small but would yield peaks with relative intensities of opposite sign to those observed experimentally. Although the charge compensation argument seems consistent with the experimental results, the possibility of nonspecific electrostatic interactions between the anions and the oxide surface cannot be completely discarded. In fact, it has been inferred from other measurements that nitrate undergoes adsorption on iron oxide particles from aqueous solutions (13). In summary, electrochemical reactions carried out in very thin layers of electrolyte (ca. 1 pm) trapped between the electrode and a window, which generate or consume ions, can give rise to modifications in the local concentration of supporting electrolyte ions due to charge compensation. In the case of IR-active species such a phenomenon brings about changes in the relative amplitudes of the spectral features associated with such species, which are not related directly with surface adsorption.
APPENDIX The light intensity reaching the detector, denoted hereafter as I , may be regarded as arising from reflections at the airwindow I,, window-electrolyte I b , and electrode-electrolyte interfaces I,. It will be assumed in what follows that: (i) Contributions due to multiple reflection are vanishingly small. (ii) The internal reflection component at the windowelectrolyte interface is very small. This condition is fulfilled in the experiments described in this work, for which the refractive indexes of the window and the electrolyte are very similar and the angle of incidence is well below the critical angle. (iii) The window is completely transparent to the IR radiation. (iv) The Beer-Lambert law is valid. Within this model, the light intensity reaching the detector will be given by I = I , 1, I , (1)
+ +
or equivalently by
I =
(pa
+ Pb + P e exp(-cexe)lIo
(2)
where I , is the original light intensity entering the window and pi are the effective reflectivities associated with each of the interfaces. In particular pa = ra, &, = (1 - ra),and pc =
Es
I
B
R
I
-AR/R
70.60
4x10-3
b I 1 -
1 1600
1400
1200
1000
1600
1400
1200
1000
wavenumber ( cm-1 ) wavenumber ( cm-1 ) Flgure 4. I n situ FTIRRAS spectra for a t'F electrode in solutions of (A) 0.1 M HCIOI and (B) 0.1 M KNO, plus 0.1 M HCIO,. A = 0.79 cm2; E,,, = -0.2 V vs SCE. Other conditions are specified in the caption of Figure 2. Curves have been arbitrarily displaced along the y axis.
ANALYTICAL CHEMISTRY, VOL. 61, NO. 10, MAY 15, 1989
W,) /I(&) 1 If Pa
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=
APc exP(-ceXe)/(Pa
+ Pb
Pc(Er) exP(-ceXe)I (5)
+ Pb pc(E,), the Pb)/Pcl
It thus follows that for Apc background will shift in the positive direction with the electrolyte bands appearing in the negative direction. As a means of illustration, the experimentally determined parameters observed in the case of Au in 0.1 M HNOB for E, = 0.0 V and E, = 1.4 V vs SCE were found to be [pc(E,)pa + Pb z 0.05 pc(E,),and exp(-%Xe) pc(EJ]/pc(Er) 3 X = 0.44/0.45 = 0.98. Hence, (-AR/R)Amu- (-iVi/R)bkgd = -3 x lo4, a very small negative band.
LITERATURE CITED
0.0
2.0
4.0
Chorga ( mcoul )
Figure 5. (A) Plots of normalized charge for gold oxide formation, as calculated from the cyclic voltammogram, and intensity of the absorption peak vs electrode potential. (B) Plot of spectral intensities vs charge for gold oxide formation.
(1 - rJ(1 - rb)rc. ra and rb may be calculated on the basis of the magnitude of the angle of incidence and the optical properties of the window and electrolyte, whereas pc, as specified, is a potential-dependent quantity that accounts for modifications in the light intensity due to reflection on the electrode. Finally, the term exp(-ceXe) is proportional to the attenuation due to electrolyte absorption, which is assumed to be potential independent. ce in this expression is the effective extinction coefficient of the electrolyte and therefore a function of wavelength, and xe is the actual optical path length associated with the specific configuration, a parameter that will depend on the angle of incidence. Hence, a t the reference potential E,
I(&) = (Pa + Pb + prc(Er) exp(-eexe)llo
Bewick, A.; Pons, S. I n Advances in Infraredand Raman Spectroscopy; Clark, R. J. H., Hester. R. E., Eds.; Heyden: London, 1985; Vol. 12. Ashley, K.; Pons, S. Chem. Rev. 1988, 8 8 , 673. Kunimatsu, K.; Samant, M. G.; Philpott, M. R. J. Electroanal. Chem. Interfacial Electrochem. 1988, 243, 203. Seki, H.; Kunimatsu, K.; Golden, W. G. Appl. Spectrosc. 1985, 39, 437. Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds; Wiley: New York, 1966. Irish, D. H.: Brooker, M. H. I n Advances in Infrared and Raman Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Heyden: London, 1976; Vol. 2. Gibbons, C. S.; Trotter, J. J. Chem. SOC.A 1971, 2058. Devlin. J. P.; Ritzhaupt, G.; Hudson, T. J. Chem. Fhys. 1973, 58, 817. Corrigan, D. S.; Weaver, M. J. J. Nectroanai. Chem. InterfacielElectrochem. 1988, 239, 55. Pons, S.;Davidson, D.; Bewick. A. J. Electroanal. Chem. Interfacial Nectrochem . 1982, 140,21 1. Woods, R. I n Nectroanalytical Chemistry; Bard, A,, Ed.; Marcel Dekker: New York, 1976; Vol. 9. Neugebauer, H. Extended Abstracts ; 39th International Society of Electrochemistry Meeting, Glasgow, Scotland, 1988. Ardizzone, S. J. Electroanal. Chem , Interfacial Nectrochem. 1988. 239, 419.
In Tae Bae Xuekun Xing Ernest B. Yeager Daniel Scherson* Case Center for Electrochemical Sciences and the Department of Chemistry Case Western Reserve University Cleveland, Ohio 44106
(3)
RECEIVED for review December 7 , 1988. Accepted February
where Apc = pc(E,)- pc(E,). The intensity ratio can therefore be expressed as
24, 1989. This work was supported by the Gas Research Institute. Additional funding was provided by the Department of Energy through a subcontract from Lawrence Berkeley Laboratory. The purchase of the FTIR instrument was made possible by a grant from the Department of Defense.
