Langmuir 1994,10, 920-922
920
Complementarity in Radiochemical and Infrared Spectroscopic Characterization of Electrode Adsorption Andrzej Wieckowski Department of Chemistry, University of Illinois, Urbana, Illinois, 61801 Received September 7, 1993. In Final Form: November 1 6 , 1 9 9 9
Radioactive labeling and infrared spectroscopy are frequently used as direct, in situ probes into the structure of the electrochemiacl solidlliquid interface. These techniques are compared, in a polemical fashion,in the context of a recent publication by Parry et al. (Langmuir 1993,9,1878)where the research potential of the former technique was not adequately depicted. It is shown below that radiotracers can clearly differentiate between the surface and solution species, both neutrals and anions. In addition to the surface specificity, the radiotracers offer a quantitative determination of adsorbate surface concentrations, a feature not yet demonstrated with surface infrared spectroscopy in electrochemistry. Therefore, these two techniques are complementary. Examples of the combined radiochemical and spectroscopic measurements of adsorption with equivalent (smooth) electrode surfaces are quoted. Due to the progress in the development of surfacesensitive radiochemical, spectroscopic, and X-ray instrumentation, high precision and accuracy data on electrode adsorption have been obtained.lP2 The very essence of the application of a wide variety of methods is the premise of complementarity: namely, an assumption that some specific aspects of adsorbate characteristics are obtained better by one rather than by another method, and that scientificknowledge results from asynthesis of all possible types of information obtained. However, the complementarity approach cannot be fully appreciated unless methodological features of a given method are properly presented, and its contribution to science assimilated. Recently, for instance, in the context of reporting on infrared measurements of sulfate adsorption on polycrystalline gold, and on copper deposited on gold, the following statementswere made. “Radiotracerexperiments?~~ which do indeed provide data on electrolyte behavior in a thinlayer cell, cannot differentiate between the surface and solution species. Furthermore, the radiotracer techniques do not allow for the determination of the orientation of adsorbed anions and do not provide behavior on surface waterq6Such a perception of the radiochemical technique prevents an interpretative comparison of surface radiochemical data with those obtained by other methods, including infrared spectroelectrochemistry.6 It will be shown below that none of the statements of ref 5 quoted above is strictlycorrect and that the f i t assertion is simply incorrect. The evidence will be provided separately for neutral species and anions, since the anionic diffuse electrode layer may be perceived as an ultrathin solution film adjacent to the electrode surface. With neutral species, radiochemical methods differentiate easily between surface and solution radioactive components through a straightforward subtraction of the solution count rates from the total count rates, the difference being directly proportional to the coverage. (Howlow must the bulk concentration be to permit a 0 Abstractpubliehedin Advance ACSAbstracts,
February 1,1994.
(1) Electrochemical Znterfacea: Modern Technique8 for In-Situ
Surface Characterization; Ab-, H., Ed.;VCH Publishers: New York, 1991. (2) Frontier8 of Electrochemietry; ROES,P. N., Lipkowski, J., Eds.; VCH Publishers: New York, 1992. (3) Zelenay, P.; Wieckowski, A. In ref. 1, pp 479-527. (4) Horanyi, G.;Rizmayer,E.;Joo, P. J. Electroanal.Chem. 1986,162, 211. (5) Parry, D. B.; Samant, M. G.;Seki, H.; Philpott, M. R.; Ashley, K. Langmuir 1998,9,1878.
satisfactorysubstractiondependson a surfaceroughnees.6) An advantage of radiochemistry is that is permits study
of electroactive and nonelectroactive, as well as spectroscopically active and inactive species on all possible electrodesubstrates, includingpowders and singlecrystals. Perhaps, because of these featUies,the radiochemicalwork can provide directionsfor more spectroscopicallyadvanced surface techniques in electrochemistry. For instance, radiochemical and infrared data on species adsorbed on equivalent electrode surfaces have already been compared,7f*radiochemical and NMR results with rough platinum Surfaces are being jointly analyzed,9 and radiodata have been used in concert with ultrahigh vacuum (UHV) surface analysis and Monte Carlo modeling of adsorption.1° There are several recent review articles where the experimental and mathetmatical details of this technique, and its limitations, have been explained.s+3J1 Radiochemical measurements cannot, indeed, provide information on surface water in the systems reported in ref 5. However, successful measurements of water adsorptionon platinum electrodesin nonaqueousmedia have been reported.1214 Neither can the technique provide spectroscopic information on the molecular (ionic) orientation in a direct way, but, in the same manner as the thin layer electrochemistrymethodology does (TLE),ls it can provide indirect evidence as to such an reorientation.l8 More importantly, the surface sensitivity of the radiochemical method can be evidenced by comparing surface concentrations obtained from radiochemistry with those (6) Wieckowski, A. In Modern Aspect8 of Electrochemietry; Bockris, J. O’M., Conway, B. E., White, R. E., Edq Plenum Preae: New York, 1990; VOl. 21.
(7) Conigm, D. 5.; Krauakopf, E. K.; Rice, L. M.; Wieckowski, A.; Weaver, M. J. J. Phys. Chem. 1988,92,1596. (8) Rice Jackson, L. M.; Zelenay, P.; Wieckowski, A.; b u n g , L.-W.; Weaver, M. J.ExtendedAbrtract, 175thMeetingofTheElectrochemical Society, Loa Angeles, CA. (9) Franaezczuk, K.; Wu, J.; Wieckowski, A.; Montez, B.; Oldfield E. In preparation. (10) Gamboa-Aldeco, M.; Mr-k, P.; Rhw, C. K.; Wieckowski, A.; Rikvold, P.; A.; Wang, Q. Surf. Scr. Lett., in press. (11) Kauakopf, E. K.; Wieckowski, A. In ref 2, pp 119-169. (12) (a)Zelenay,P.;Winnicka-Meurin,M.;Sobkoweki,J.J.Electroonal. Chem. 1990,278,361. (b) Zelenay, P.; Szklarczyk, M.; Winnicka-Maurin, M.; Sobkowski, J. J. Electroanal. Chem. lSSl,308, 269. 1976,122, 252. (13) Wieckowski, A. J. EZectrochem. SOC. (14) Wieckowski, A.: Szklarmk, M.: Sobkowski, J. J. Electroanal. Chem. 1980,13,79. (15) Soriaga, M. P.; Hubbard, A. T. J. Am. Chem. SOC.,1982, 104, 2735.
Q743-7463/94/241Q-Q92Q$Q4.5Q/Q(8 1994 American Chemical Society
Langmuir, Vol. 10, No. 3, 1994 921
Complementary in Techniques
of the same adsorbate obtained by other techniques. For instance, the work on hydroquinone adsorption on platinum by TLE and radiochemistry, as well as by Auger electron spectroscopy and radiochemistry, gave surface concentrationscomparablewithin 34% and 4% accuracy, respectively (a better coincidence of the TLE and radiochemical data was expected if the surface treatment was identical).’8 Notably, in this comparison,the radiochemical surface concentration was lower than the TLE results showing that if any factor accounts for the discrepancy, it is not due to the contribution from the thin film radioa~tivity.~ A joint paper between the Guelph and Urbana groups on pyridine adsorption on gold provided practically the same surface data with experimental accuracyof ca. 20%.17 Furthermore, the most recent work on urea adsorption by radiochemistry and Auger electron spectroscopy gave coverages 0.26 f 0.04 and 0.24 f 0.03 ML, respectively.’” As far as surface concentrationelectrode potential relationships are concerned, joint studies by radiochemistry and surface infrared spectroscopy with equivalent platinum surfaces gave identical trend~.~lB Study of anions’ surface chemistry, being of a clear concern to ref 5, has been a particularly attractive and broadly explored area of the radiochemical research. Investigators who have initiated work with radiolabeled anions adsorbed on rough surfaces,1g2Oand those who have studied anions on single crystal electrodes:J1*21 have recognized that the anions are present both in the inner and in the diffuse part of the interface.22 Being interested in the inner part of the interface only, the work has been conducted in solutions containing an excess of a weakly adsorbing supporting electrolyte with respect to the studied solute. It has been assumed that the diffuse part of the double layer is significantlyreduced of the adsorbing anions (minority anions) due to the presence of the electrolyte anions (majority anions). However, in view of the doubts in ref 5, a more direct evidence, preferably, a quantitative proof, is needed to demonstrate that the amount of anions in the diffuse part of the interface in the earlier radiochemical studies could indeed be ignored. A very relevant study to the case raised in ref 5 has already been completed and is being published.2s Namely, using several sulfate and/or perchloate solutions, chronocoulometry and radiochemistry techniques have independently been applied for measurements of the Gibbs excessesof SO$ anions at the Au(ll1) electrode. Stating that the surface concentration measured by the radiochemical technique is the sum of S O P and HSO4- ions, a thermodynamic analysis was carried out to discriminate between the S04” and HS04- surface components, and a conclusionwas reached that the SO4%anion is the majority surfacespecies. More importantly,evidencewas presented that the amount of sulfate in the diffuse layer is negligible. This evidence goes along the following lines. The total Gibbs excess of SO4% ions present in the interfacial region is equal to the sum of the Gibbs excess (16) Krauekopf, E. K.; Wieckoweki, A. J. Electroanal. Chem. 1990, 296,159. (17) Stolberg, L.; Lipkoweki, J.; Morin, S.; Irish, D. E.; Zelenay, P.; Gamboa-Aldeco, M.; Wieckoweki, A. J. Electroaml. Chem. 1993, 366, 147. (18) Horanyi, G.; Solt, J.; Nagy, F. J. Electroanal. Chem., 1971,31,95. (19) Horanyi, G. Electrochim. Acta 1980,25,43. (20) Bockrie, J. O’M.; Gamboa-Aldeco, M.; Szklarczyk, M. J. Electroanat. Chem. 1992.339.355. (21) Krauekopf, E. K.;Rice, L. M.; Wieckoweki, A. J. Electroanal. Chem. . .. . 1988.244.347. -(22) Schmickler, W.; Henderson, D. B o g . Surf. Sci. 1986, 22, 323. (23) Shi, Z.; Lipkoweki,Gamboa-Aldeco, M.; Zelenay, P.; Wieckoweki, A. J. Electroanal. Chem., in prese.
----.---.
of Sod2- in the diffuse and in the inner part of the double layer. While the radiochemical measurements do not provide informationabout the location of the Gibbs excess in the interfacial region, the chronoamperometric data give both the charge density at the metal surface and the total Gibbs excess, Figure 1. One can therefore employ the diffuse layer theory to verify the diffuse layer component of the total Gibbs excess. In Figure 1, the charge density at the metal surface and the absolute value of the charge corresponding to the measured Gibbs excess (Iz&”so,~, where = 2) are plotted against the electrode potential for a 5 X le M KzSO4 + 0.05 M KC104 + 0.02 M HClO4 solution. These data show that the absolute value of the charge of adsorbed SO4” is much larger than the charge on the metal. Therefore, adsorption of sod” has a superequivalent character, meaning that the electric field in the diffuse part of the double layer has a negative sign even at the positively charged metal surface. Moreover, the measured Gibbs excess corresponds to specifically adsorbed sulfate anions, namely, is equal to the surface concentration of the adsorbed anions. This is shown by the calculationsof the charges due to the Gibbs excesses of cations (z+ where z+ = l), perchlorate ion ({zyITclo4--2”,where 12-1 = l), and sulfate ion ((Z&”QO,*, where 1z-J= 2) in the diffusepart of the double layer using the formula for mixed electrolytes derived by Joshi and Parsons”
ri%=
* (&)‘I2 J*:
C:
[exp(-z@/RT)
{c
- 11
d@ (1) c?[exp(-ziF@/RT) - 11)’/2
I
Where the lower integration constant, 49, can be determined from
In eqs 1and 2, = r+%+ rso42-20+ rcIo4-2J,@ and are the inner potentials at a point within the diffuse layer and at the location of the outer Helmholtz plane, respectively, Cib is the bulk concentration of ion i, zj is the ion valency, and e is the dielectric permittivity of the solvent. In Figure 1,the calculatedchargescorresponding to the Gibbs excess of the ionic species in the diffuse layer are plotted against the electrode potential. Since the absolute values of ionic valency lzil were taken to calculate the ionic charges, the sign of the charge is determined by the sign of the corresponding Gibbs excess. The results show that the charge of S O P present in the diffuselayer is negligible in comparison with the charge corresponding to the measured Gibbs excess. Likewise, it shows that the diffusion layer is, in effect, not depleted of the anions. Instead, the negative (superequivalent) charge is compensated for in the diffuse layer by the positive ions. The charge density plots taken in the double layer regionof the gold electrode have been integrated to give a relative value of the interfacial tension.24Subsequently, the Gibbs excesses of the adsorbates have been calculated by differentiation of the interfacial tension with respect to In C K Z ~ O , .Figure 2 showsplots of the Gibbs excessversus the electrode potential determined in a 5 X 10-4 M K2S04 (24) Joshi, K. M.; Parsons, R. Electrochim. Acta 1961,4, 129.
Wieckowski
922 Langmuir, Vol. 10, No.3, 1994
I
I
N
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N
5
M
c
.-0
%
\
L
\
b
f 0 F
-20’
”
-200
0
I
200
’
I
400
”
600
‘
I
’
800
E / mV vs SCE Figure 1. Charge density versus electrode potential plots for the Au(ll1) electrode in 0.05 M KC104+ 0.02 M HC104+ 5 X lo-’M solution:29( 0 )charge density at the metal, UM;(a) charge corresponding to the measured Gibbs excess of SO? ion (Iz&”so,a- where kJ = 2); (A)charge corresponding to the Gibbs excess of cations (H+and K+)in the diffuse layer (z+FT+”); ( 0 ) charge corresponding to the Gibbs excess of clod- in the diffuse with 1z-I = 1); (*) charge corresponding to the layer (IzJrc10 Gibbs excess of SO? in the diffuse layer (IzJFrsop” where IzJ = 2); (0)absolutevalues of net charge density in solutionIz&”s~,s
-”
+ 1z-I Frso,s” + 1z-I FI’Clo4-” - Iz+ll;T+&.
solutionfor two supporting electrolytes. For comparison, the Gibbs excesses determined from the radiochemical experiments are also included in this diagram. The data show a very good agreement between the Gibbs excesses determined from the thermodynamicanalysis of the charge density data and the Gibbs excesses measured using the radioactive label anions. It may be mentioned that the surfaceconcentrationdata in the oxide range have been obtained only by radiochemistry, Figure 2. The companion chronocoulometry data are not available since the surface oxidation process makes the purely electrochemical determination of the anion adsorption more difficult than is the case with equivalent measurements in the double layer range. In summary, previous data show that for neutral adsorbates, the surface and solution radioactive species can be easily differentiated,and the thin film radioactivity, present between the electrode surface and the detector,ll can be factored out from the measurements. In the case of charged adsorbates, chronocoulometricdata show that the contribution of anions present in the diffuse layer to
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
E / V v s SCE Figure 2. Gibbs excess-electrode potential curves for S O P adsorption at the Au(ll1) electrode determined from (0) chronocoulometric experiments in a 0.05 M KC104 + 0.02 M HCIOr 5 X lo-‘ M KzSO, solution,29(A)chronocoulometric experiments in 0.1 M HC104 + 5 X lo-‘ M K2S04, ( 0 ) radiochemical experiments with the electrode potential progressively changed in the positive direction, and (m) radiochemical experiment&potentid changed in the negative direction in a 0.1 M HC104 5 X lo-‘ M HzSO4 solution.
+
+
the measured Gibbs excess is negligible.% The favorable comparison between the radiochemical and coulometric results,Figure 2, is an important verification of the capacity of the radiochemicaltechniqueto measure surface coverage of anions in the inner part of the interface. That is, the solutionthin film radioactivitycan be ignored,or corrected for, as in the case of neutral adsorbates. We conclude that the radiochemical technique provides quantitative information about the Gibbs excesses (surface concentrations). It is also known that infrared spectroelectrochemistry cannot, at present, give a quantitative measure of amounts adsorbed. Therefore, the infrared and radiochemical techniques are complementary and a collective interpretation of the spectre and radiodata, if done with morphologically equivalent surfaces,brings an important contribution to the area of electrochemical surface science.798 Acknowledgment. Studies of metal and anion overlayers on single crystal gold electrodes are supported by the Department of Energy, Grant DE-AC02-76ER01198, administered by the Materials Research Laboratory at the University of Illinois.
