A surface analytical investigation of the electrochemical double layer on silver electrodes in chloride solutions

ELSEVIER

Journal of Electroanalytical Chemistry 436 (1997) 109-118

A surface analytical investigation of the electrochemical double layer on silver electrodes in chloride solutions D. Hecht *, H.-H. Strehblow Instimt )iIr Physikalisrhe Chemic und Elektrochemie, H¢inrich.tteine Um+,ersitlltDl~sseldmf Unieersitiitsstr, I, 40225 Diisseldo~i Germany

R~+ceived4 March 1997: ~ceived in revised lotto 13 June 1~7

Abstract The el¢cl~xa:hemical double layer on Ag in NaCI solutions was examined with X+ray photoelectron spectroscopy (XPS) and ion scattering Sl~Ctmscopy (ISS). The potential del~ndent surh+ce concentration of Ihe adsor~d s~cies (Na +, CI H ~O) as well as the cationic excess charge was delemfined, The s~cific adsorption of CI +' is accompanied by coadsorption of cations and waler, Angle-resolved XPS measurements and ISS experiments gave more detailed information about the structure of the double layer perpendicular to the silver electrode surface, A layered structure of the specifically adsorbed chloride anions and the coadsorbed cations was determined, in agreement with recently published studies. © 1997 Elsevier Science S.A.

1. Introduction The electrochemical double layer on silver electrodes has been intensively studied in the past. Polycrystalline electrodes as well as single-crystal surfaces have been examined. Reviews and the current statm+ of data are given in Refs. [I,2]. The specifically adsorbing halide ions have attracted great interest (see for instance, Ref. [1]). Among the,~, the electrosorption of C i was intensively studied. Electro°reflectance measurements revealed the role of sur+ face states on the bond tbrmation during the adsorption of chloride on different silver single-crystal surfaces [3]. The electrochemical formation and reduction of thin AgO films on silver electrodes in aqueous chloride solutions has been investigated with electrochemical methods [4]. From these experiments it was deduced that the first step in the growth process involves the two+dimensional growth of either a partial monolayer of a AgO film or the adsorption of chloride ions with a partial charge transfer [4]. At an overpotential of about 0.02-0.04 V, the nucleation and growth of a throe-dimensional AgCI film is observed [4]. In general, the adsorption processes of the halide ions on silver electrodes were studied with electrochemical methods (e.g., Refs. [I.4]). electrode resistance monitoring [5,6], ellipsometry [7] and radiotracer methods [8], These

" Corresponding author. E-mail: [email protected]. 0022-0728/97/$17.00 © 1997 Elsevier Science S.A. All rights re~rved. Pil S0022-0728(97)00340-9

experiments showed that the strength of adsorption in+ creases from CI ++to Br + and I +++[I,5+7]. Ex situ investiga+ tions of electrode surfaces with surface analytical tech+ niques such as Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED) after their con+ trolled emersion from the electrolyte and a transfer into an ultrahigh vacuum system confirmed these conclusions [9]+ In addition, well ordered adlattices of the adsorbed halides were found on Ag(l l l) surfaces with LEED [9] and scanning tunneling microscopy (STM) [!0]. Ahhough surface analytical techniques like AES. X°ray photoelectron spectroscopy (XPS) or UV photoelectron spectroscopy {UPS) have been applied to the investigation of the elcctr~hemical double layer for nearly twenty years (e.g., Refs. [9,1 I-16]). it seemed useful to perform further studies due to the increasing sensitivity of surface analytical instruments during the past decade, i.e,. more detailed information about the electrochemical adsorption phenomena is available, For example, the XPS investigations are no longer restricted to elements with high photoionization cross +sections and the quantification of surface concentra° tions, element binding energies, etc. can be performed with higher accuracy. In the present study, we repro1 on ~ome results obtained for the electrochemical double layer on polycrystalline silver electrodes in chloride ~olutim~+, h~ addition to XPS and angle+resolved XPS experiment+, the application of ion back+,~tt~fing spectroncopy (!S~) will be described. Due to the complete neutralizatio+l of noble gas ions which penetrate into the s~cimen sur|~ceo ig~:

| iO

O. Hi~t'hl. tt. ~H. 5ilr¢.hllh,~w/Journal ~ Eh,,'troanalytical Chemisto' 436 f i ~71 109-118

becomes inherently surface-.~nsitive, i.e., only ions scattered at the outn~st atomic layer of the sample can be detected in the sca|tered beam [17-19]. Therefore the application of iSS ~ems to be u.~ful for ex situ investigations of the electrochemical double layer.

2. IE~peHm~tal surface analytical experiments were performed with a commercial spectrometer (ESCALAB 200X, Fi~ns/VG Sciemific) with three chambers, an analy~r chamber with diffe~m radiation ~3urces aM a spherical .~cmr analy~r (VO Scientific MKII), a preparation chamber aM a thsto entry air k~k ~parated fn~m each other by valves° i1~e elec~hemical ~ c i m e n preparation was ~dbrmed in an additional chamber which i~ attached m the ent~ lock. The H~cimens were mounted o~ ~ c i m e n stubs and a mechano ical transport ~ystem was used ior the specimen transfer ~tween the different chambers. I)emils of the system, the specinren transfer within the closed system tTom the eleco tro|yte to the ultrahigh vacuum (UHV) and its performance were previously ~ f i b e d in Refs. [13.14]. XPS spectra ~ r e taken in the constant analyst energy (CAE) mode with a pass energy of 20 eV. Nonomonochromatized Mg K.o and AI K,,-radiation ( h v ~ 1253.6 eV and 1486.6 eV, respectively) with an input IX~wer of 301) W (15 kV, 20 mA) were u ~ . The calibration of the spcctrOl'neler was routinely checked with sputterocleaned Au, Ag and Cu stx~cimens ~co~ing to Ref. [20]. Due to the low surface concentration of t ~ adsorbed double-layer ctmstituolts, an integration time of I up to 2 h was necessary in order to ~tain a sufficient signal-t~noise ratio of the measured XPS data. The phol~lectron takeooff angle ~ (measu~ against the suri~tee normal) was vari~ ~tween if' and 70'~ for the aagleo~solved XPS ¢x|~fiments. Under routine ~ti~ conditions, the v~uum in the analyser cham~r was ~Ite¢ than 5 × 10 ~~ Pa. He* k ~ a t ~ f i n g experiments we~ ~ f f o r n ~ with a dilTei~ntially l ~ m ~ ion gun (~X05, Fisons/VG Scieno tiff): the kirtle energy of the emitted He" ions was in range between 600 eV and 2 keV. The scattering angle betw~een the impinging and the refl~,~ztexl He* ions was adjusted to 90 °, le~ieg to a simple e~rgy~mass relation [18,21]. The target current was ~t to 30 nA while the area was ate-at O, I cm:. Owing to the low intensb f~s of tl~ ISS signals, t ~ pigs energy of t ~ annlyser wz~s to 200 eV t'o~ the ISS ex~rin~nls. Prio~ to any electrochemical ex~rin~nt, the [~lyc~so talti~ silve¢ e t ~ t ~ (Johnson Malthey Chemicals. ~,~, 13-ram diaa~ier) ~ r e n~hanically ~ l i s ~ to I1 ~m f i l l gradi~ ~ carefully c l e a ~ with triply di~iik~ ~atet. After this treatment, the silver electrodes u:,aia||y r e v ~ water-repelling surfaces, which ~ necess~$ ~ ex situ UHV studies of double-laver constituents. Light-microsc~ical investigations ensured that the Ag S~i~s a~ free of any grieves ~r scratches remaining

from the polishing procedure, in some experiments, Ar ion sputtering (Leybold/Specs IQP 63, 3 keV, ~ 30 btA, 5 rain) in the preparation chamber completed the specimen pretreatment. Owing to the large number of specimen preparation~ which is neces~ry for a systematic study of the potential dependency of the electn~so~d s~cies (more than 50 electrochemically prepared ~mples were investigated in this study), we omitted a rather time-consuming sputtering-vacuum annealing procedure to improve the remaining surface roughness of the samples, i.e., the application of such a |reatment should be restricted to singlecrystal surfaces. Subs~iuentiy, Ihe samples were immersed into the solution at sufficiently negative potential ( - 1 . 0 V) and puled to the potential of interest for the desired time. After l~)larisation, the samples were removed from the solution under i~tential conrad and intn~duced to the UHV within some few minutes. The work function of the emersed electrodes was deduced I?oln He (I) UPS investigations (h v ~ 21.22 eV~ The I: i col~lation between the measured work fimcfion a,d the erection potential proved that the double layer stayed intact during the ~moval of the el¢ctnxles l'tx~mthe el~trolyte and the transi~r into the UHV of lhe speclro,r~ter [ 15,221. While the i,a~lential Ur of the working electrode was measured using a HglHg~SOaKO.5 M) H~SOa reference electn'~le (Ur~ ~, ~ +0.68 V vs, standard hydrogen dect , ~ e [SHE]), all polentia!s given in the text are referred to the SHE The N~CI ~dutions were pte~red from analytically pure substances and triply distilled water. The pH of the .,a~lution ~v~s adjusted to pH 3 by the addition of applv~priate amt,tmts of HCI. The solution:~ were purged with purified Ar (oxysorh, Mes~r Griesheim) l~tb,*c the elect|x~hemical p~paration to remove any traces of oxygen. The qummtative evaluation of the deleted XPS signals was ~ d b m ~ d with computer software develol~d in our gamp [23} and i~luded a background sublraction according to Shirley [24] and a I~ak deconvolution. For the calculation of the surface concentration of any adsorbate. the tbllowing procedure was applied. For a silver electrode covered with a thin-surface film of a species A and |hickness d,,,, the intensity ratio of a phot~leetmn signal of the surface film and the Ag substrate la/!a~ is given by F~. (I) (Ta, Ta~: transmission function of the energy analyzer; Aa~, Aa~.a: inelastic mean free path of Ag phot~lec~nms in the substmte and the surface film; An: inelastic mean f~e path of ~he adsotbate species; o"a, (rAg: photoionitation cross ~ctions; NA, Nas: atomic density of A in the surface film and Ag in the bulk metal, respeclively): exp(

da ~Ag .ACOS

(i)

D. He(,iit. !t.-H. Sirehbhnr / Jo~irmd o f EM'mumalytical Chl,iilistry 436 f IVY7} 109+ i I<~

where the molar concentration NA+ of Ag atolns in tile bulk material was estimated to N,x+ = 0.097 mol c m ~ from the atomic concentration of metallic silver (nap = 5.85 × I0 ~' atorns cm-++), in this study, the photoionization cross sections according Io Ref. [25] with the correction described in Ref. [26] were used. XPS peak areas were token as a measure of the intensity. For a very thin fihn or an adsorbate, i.e., dA/AcosO ~ I, Eq. (I) can be developed in a Taylor series and truncated alter the linear tenn. If the prtvJuct of the volume concentration Na and the Ihickness dA is combined to a surfilce concentration n a = Nad a and with Ti C/(Ekum) 1t2 ibr the energy used analyser [27] and A, = Bi Ei/'~+ki,,i[28], the following result was obtained (Eq. (2)): =

& ~{~ 3as Na+~cos 0 aa IIA® &l~ ira IJA

(2)

In general, B~ descuil~;'s tile inlluence of maleriaI imlper o ties on tile iuehtslic electron lUeiiul fl~e i~alll; Ior elenlelltS. i.e+. fiir Ag, It .... BA~ ++ (),()54 nm e+ r. ~ i2rl, Ttie vahle used I'or II A is of no i+eleviince in tile pre.,ienI case. since ii is conipeusaled by AA +° .-an.++i,,.al+'i!++in Ett. (2). However. several liSSUlilpiions have to be made to derive Eqs. ( I ) and (2). The most iinporianl tire (a) thai the overhiyer is holriogeneously composed and hits a sharply defined border to the subsinile; (b) thai the local I'ihTi <++i + + ,,+

+i,

t h i c k n e s s d a is the s a m e at e v e r y point o f tile surfilce; a n d

(c) that this surf'we is flat on the niicroscopic as well as on the macroscopic scale (see Ref. [29]). Since hydrophilic samples were identified by their abnomlaily intensive adsorbate XPS signats and were excluded from the data evaluation procedure, reslrictio;ls (a) and (b) are only of minor importance for XPS investigations of adsorbates, However, thill overlayers on rough surfaces appt:ar much thinner for XPS observations at high take+off angles t9 when colnpared with small or medium values of t'-) [31/=32]. ~ h validity of Eq. (2) was checked with tinderpotential deposited lead nlonolayers on Ag [33]. The surface concen+ tration of the Pb atoms amounts to hi, , + !.77 ± 0.15 nnml cm +~ as determined from ex situ XPS investigatiolls. For comparison, the value calculated for a closeopacked monoo layer of Pb atoms with a radius Ri, , + 0.175 nm is ni,h = 1.65 nmol tin '+: [33]. Thus it can be expected that the application of Eq. (2) yields reasonable values for any adsorbate surface concentration. Eq. (3) is valid for the quantitative evaluation of ISS spectra (e.g., Refs. [18.34]). The ion current density i,~ scattered from atoms of a species A with a surface concentration n A gives rise to a current I,~ measured in the deteetor: =

d~

(u -

(3)

where d o-/d m denotes the differential backscattering cro~.~ section, A oj and T are the angle of acceptance and the

! 11

tmn:mlission Ihctor of the energy analyzer o n is the neutndization probability of the projectile ions and GA is a geometrical filctor, which corresponds to the partial screen= il;g of one surface sNcies by a second. The straightforward calculation of the iudividual parameter.,, of Eq. (3) is rather complicated. However, the quantification of ISS spectra with experimentally determined sensitivity factors OPtss has proven to give reasonable results [17,35,36]. The sensitivity thctors used in this study are determined from the ISS spectra of Ag elecmntes, which were removed li'om 0. I M NaCI solutions with a hydrophilic surface+ i.e., covered with a thin NaC! film alter the evaporation of coadsorbed water into the UHV. Such a hydrophilic sure face can be prepared, e.g., by prolonged potential cycling i,~,,ween hydrogen evohition and tile b~ginning of AgCI I'orrnalk~a. It it is assumed that the surface concentration of Na' cations and CI anions are equal for the NaCI l'ill~lm prepared as described above, the relnlive sensilivily factors of Na arid CI were obtained froln |he 1+elated peak areas of the ISS spectra which were nle,t+ t red directly after lli¢ enler,,iion ol+tile electrode hlIo tile electrolyll:. In gell~:ral, a linear background was sul tnicled froln tile SS s~clra in order to determine the peak areas of the individual signal,,+. Alter the sputter removal of the NaC! fihn. the peak die+ ...... of the clean Ag electrode was determh+cd l+or identical experimental conditions. Relative sensitivity factors lot Ag, Na and Ci were calcuhited as intensity ratios of the measured Ag, Na and Ci intensities, taking into account that the surface concentrations lor Na and CI elich my about 1114 nnlol c m ~ as determined from tile cryslalloo graphic structure of bulk NaCI, while the Ag atom surlhce concentration calculated for a close packed int;nohly¢l of Ag atoms ix about 2.07 nnlol cm ~ In thi~ COllleXl+ it .,,hould be nlentioned that a sin)ilar i}rocedur+' was success+ fully applied Ior tile determination of ;++|)S sensitivity factors in the past [12], The relative ISS sensitivity thctor~ determined this way agree qualitatively with those calcuo hired by Binghani [37]. However. the corresponding sen~io tivity lactor of oxygen is not accessible this way. Due m tile close fit of the measured +r~,~s values for Ag, Na and CI with the c,'Yculated values, the sensitivity factor for oxygen was estimated by an extnipolation of the calculated values published in Ref, [37], The ~en~itivity factor~ trl,~i used in this study are conlpiled in Table I; they were normalized with respect to the value obtained tbr oxygen, Due to the low mass oi' the +He ' ions u~ed Ii,' the ion scattering experiments described iu this section+ tlic re+ulto ing sputtering effect wits very small. Depending on the

1++d+le I Rehiliv¢ !+aek++'+illerm~cr.,+++r+e+'li+m~l.r ++lie' i,m+ I E + 2 lw+d)dew++ lilhled ¢~l+ril+¢lenlIillyl'++rdifl'c++r¢llIelcllwlil++ ........... Element

Ag

CI

N~J

{}

ot,~.~

2.J42

I,+~3f+

1,3 I~

I

112

D. Hecht. It. -H. Strehbhm' / Y,mmal of Electroanalytwal Chemisto" 430 t 1997) 109-118

area analyzed and the ion current, the sputter rate was estimaicd in ~ about 2 to 3 h for one monolayer, i.e.. = 0.1 to 0.2 nm h-~. Therefore. a slow removal of the double-layer constituents is possible by continuous bombardment with ~He* ions. Though He + ion scattering experiments can be carried out simultaneously, a sputter depth ~ f i t i n g of ~ electrochemical double layer on the emersed Ag e l e c t r ~ s can ea~ily be performed. Similarly to the analysis of the XPS signals, one might expect that any surf~'e roughness might a i ~ have an influence on the ~sulting IS$ s~ctra. However. surface roughness is only of minor importance for the pre~:tlt ~tudy as d i ~ u ~ d in Section 3. ~ the one hand. the influence of ~l~ading effect~ on depth profiling ex~rinlents arc negligible since shMowed area~ &~ not contribute to tM ~attet~d intensity. On the other hand, arty surfiice roughnes~ changes the ~mtering ~ometry Mtween rite incoming and the ~¢attered ion ~ a m and migl~t result in a slightly changed k i r t l e energy of the scattered ions [ 18,21 ]. Therefore, surt~'tce roughness causes broader !SS signals of arty investigated species but leaves the relative intensities u n a f f ~ J , Since a cttanged geometry might also have an influc~e on the actual sputter rate; sputter times tbr the complete removal of the double layer were d i s u s e d only qualitatively,

3,

Results arid

discussion

3, I, XPS #w,¢stig~amms Typical XPS s ~ t r a obtained tbr the different double° layer constituents alter emenion from 0. I M NaCI + I0 '~ M HCI (pH 3) at, shown in Fig. I for mveral erection ~entials, ~ M w ca, ~ 0 3 V, no CI ~ c ~ M dete, ted on

~ecf~r~e~,/eV

the Ag working electrode surfaces, i.e., CI- anions are not incorporated in the electrochemical double layer below a certain potential. In contrast, adsorbed CIO4- anions can still be detected on silver electrodes in the region of the hydrogen evolution at about - 1 . 0 V [38,39]. Above --- 0.5 V, the intensity of the CI ' signal a~ a binding energy of about 198 eV increases continuously with Up. While the intensity of the Na + signal at E~ = 1072 eV decreases with potential between U~,---1.2 V and - 0 . 4 V, it strongly increa~s again above = 0.4 V. The oxygen XPS O Is spectra reveals only o ~ broad l~ak centered at E~ ~ 533 eV which correslxmds to adsorbed water; the intensity of this ixmk inc~ams with the polarization potential, For i~.~tentials positive to +0.33 V. the sample could not be emer.~d with a water=reviling surface. These samples were transferred to the UHV system of dte s|x~ctmmeo ter after cautious rinsing of the surface with triply distilled water. 11~e quantitative XPS analysis revealed the t'omm~ tion of a compact AgCI layer. Tl~e Nemst i~tential of AgO fomtation ( UA~/~c~) according to Ag + CI ~ * AgCi + e = is UA~/A~C~== U~, - 0 . 0 5 9 V log a¢~

Uo ~" + 0.22 V

i.c., U~/A~to~ ~ +0.28 V in the p~seut ca~. Therelb~ the oveqmienttal ~ fi~r the formation of bulk AgCl can M detemfined to be ~ ~ 0.05 ~ 0.06 V, in agreement with ~cently publislk.-d values obtained from electrochemical investigations [4]. In Fig. 2, the surface concentrations of anions n c l , cations ~%~., calculated with Eq. (2) and the surface excess charge nci ~ ns, ,, on Ag electrodes which were hydrophobically extracted fn~m 0.1 M NaCI + I0~ *~ M HCI are p r e ~ n t ~ as a function of the emersion potential.

~ene~/ev

~'~ingeneoy/eV

~g. |.('I2p. Na is and O Is XPS spectra~'tected on Ag ¢l~trodes which were emersed hydrophobically from 0.I M NaCI + I0 --~ M HCI for different

D. Hecht. H.-H. Slrehhlow / J o , m a l ¢~f Electr()analylical Chemisto' 436 f i ~ 7 ) I { ~ - 118

U / V($HE) -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0

0.2 0.4

% °'r

".r/

°

-°"o,,t o.o r 0°~

l

11

J

00=,4 ~_._ ~__~__~__~_~., I

!

"

I

'

I

"

O, ~

~

1

I

.

i

.

~

,

, ..... = ...... ,

"

i

'

I

'

I

o o ]i~p

"

I

'

.

-

O,r*

~:~ o0,4 '

~ ..~

oO, f;

o0,8~ o 1.0

,

.

,

.

~

...

_ ...... ~

I

~ ,

L

i

, J ~ = _ _ L

.--L----.--

•

,

.I,9 .t.O .0,8 .0,6 .0o4 .0,2 0.0

/

II

0.~

O.a

up/ V(SHE) Fig. 2. Potenti~ll dependence of tile surl~ce concentration of anions no.I . cations nr~,,, and exces~ charge "rl - " ~ , , ' on Ag electrodes which were hydrophobically exit°acted from 0.1 M NaCl+ IO "~ HCI. The literature value of the potential of zero charge Up,, is indicated [40].

The potential of zero-charge Ups, (Up, ¢ = - 0 . 5 4 V for polycrystailine silver [40]) is indicated wire an arrow. In this graph, the results of two series of experiments arc compiled. While the specimen preparation involved an electrochemical reduction at = 1.0 V and a potential change to the desired emersion potential for the first series, the samples were additionally cleaned by argon ion sputtering prior to their introduction into the electrolyte for the second series. As can be seen from Fig. 2, the scatter between the two sets of data points is negligibly small, In contrast to previously published results (e,g., Refs. [I 3,4 I]). the sputter.cleaned samples obviously conserved their hydrophobic behaviour although any surface contaminations of carbon or oxygen species were removed by the argonsputter treatment. In addition, a slight roughening of the electrode surface will be induced by the sputtering [;,r~,ct. dure. In general, the same trends which could already be deduced from the raw XPS data are observed, While t?o adsorbed chloride was detected for U ~ - 0 . 6 V, the CI ~surface concentration net- increases approximately lind early with potential for Ut, >_ - 0 . 6 V, reaching a maximum value of about 1.4 nmol cm -2 at Ut, ~ +0.33 V. From the ionic radius of the chloride ion (Rcn - = 0.181 nm), the coverage of a close-packed hard-sphere mend-

! i3

layer was calculated to be Fc~- = 1.46 nmol cm- 2 i.e.. the maximum surface concentration of CI- detected on the silver electrodes is slightly lower than monolayer coverage. Similar values were delermined from an in ~;itu eilipsometric study [7]. While the quantification of the XPS spectra is straightforward (see Section 2), however, several assumptions have to be made for the evaluation of the ellipsometric data [7]. For example, the influence of coadsorbed cations and water on the refractive index of the anions was not included in the theoretical description of the optical properties of the adsorbate-covered surfaces [7]. On the other hand, the XPS experiments clearly show the incocporation of all these species in the double layer ell silver (Fig. I); therefore sucl~ an influence is very likely. I,~adiotracer studies in mixed HCi + H e l d 4 solutions also show similar resuhs lbr the potentially dependent C I surface concen|ration [8], in this case, however, condo sorbed CI "q O ,~ anions will probably influence the actual alriount of ~ldsorbed C I , The Na" surfi~ce concentration decreases co||tinuously from about 0.4 nmol cm =° tbr UI, ,~ =~ !.2 V to about O, I nmol cm ~": tbr Urn,- - 0 . 5 V; this is the potential region where no C! = was detected on the Ag electrodes. Parallel to the incre~lsing incorporation of CI anions into the double layer above - 0 . 5 V, aN,,. increases up to a value of -~ 0.4 nmoi c m - : for Ut, = +0.33 V, The observed behaviour of the coadsorL~ed cations is characteristic for a specific adsorption of the anions: Na + ions are incorpoo rated in the double layer with increasing amounts for charge compensation as predicted by the classical double° layer theory [42]. Similar trends were observed for the coadsorption of Cs and C! .... on gold electrodes [ I I,I 2,43]. The cationic excess charge "N,' ~ "(1 decreases, ape i~roximately linearly, wid)in the whole potential region studied. For U~, ~ =0.6 V, the excess charge changes its sign, i.e,, the determined potential of zero charge is about 0,6 V which is slightly lower than the value given in the literature (U0t~,=~ = 0.54 V, [40]). "l'his slight shift of Ut,~ tow~rds more negative potentials can also be understood in terms of the specific adsorption of the chloride ion.,; (Esin-Markov effect, [44]). In Fig. 3, the surface concentration of adsorbed water i,~ depicted as a function of the emersion potential, Again, results related to both specimen pretreatments are shown, As can be seen in Fig. 3, the amount of adsorbed water increases with potential from about I nmol cm : for U p = - 1 . 2 V to ~2.5 nmol cm ~ tbr Ua,~ +0,33 V, ic., the average value of tlH~ O amounts to ~bout one mor,olayer. This value i~. predicted by c!a~ical double-layer ,,~;dc|s [45], Again, no significant de~nael~ce on the specimen pretreatment was found, For potentials Ut, ~0,4 V, the increase of n,~o wi;h Uo is ~lightly ~teel~.r than below ~(),4 V. Therefore the adsorbed C! ..... anion~ seem to bind larger amounts of water i, the double layer. Results of angle-resolved XPS experiments are pre° sented in Fig. 4 where the intensity ratios of the XPS peok~

D. Hecht. H.-H. Strehblow / Jounml t$'lATectroanalytical Chemislrj' 436 ( I~71 109- 118

I 14 3,0

~ ' "A~ ' " I "0.1M " ' " " NaCl " " " "+' 10:~'M"HC# ~ ' " " " ' " " """ ~"-i • elctrachemtca/~reduced o sl~uttercleaned .

"

Ha+

I

Cf ,

•

ood 7

ClIII}_/

q

A,

!................... . ..................... ~.al ........................ *..:~""-.*-.'t

1.5

:

.'o"

!

l

o'."

0~1. 0 O,P 0,0 .f,,i .l,~I .'1,0 .0,8 .0,6 °0,4 .0,# 0,0

0,2 0,4

u~ / V($Ue) ~i I, ~, Aiilount of a d ~ d wttt¢r ori All ele¢lrode~ enier~ed frotll O,I M N a C I . I0 =:~ tt~1 G)f diff, l~nt i~tl¢,ii~il~, The d~hed line repr¢~,t~ a nloclola~¢r olvet'ag¢ of It ~0,

i

~,~- __0_0

C)

ac,:~

c,

tl

~,

C)

Fig. 5. Schematic repre.~ntation of the electrical double layer on Ag electr(xles in 0.1 M NaCI + IO '~ M HCI derived I~m augle-resolved XPS measurements. Sit~e the CI ~ coverage nearly i~aches a monolayer coverage ai high positive potentials, t!-.¢~2 ions eail ~ top,seined by a clo~>paeked layer of thickness d ( 1 , (a) Owing to the I o ~ r Na* concetltrati~m, tl~ thiekt~ess of a rela|cd layer"has no physically m~:aniugo tttl value, (b) ll~r~fi~e, tl~e ¢~lsothed Na* ions were ttvated as statistically adm~bed particles,

intensity ratio inerea~s witi) O, indicating tile existo)ce of

a concentration gradient of ~tl~i sl~cies in the double layer of the detected doubleolayer constituents (Na +. C I . H ~O) and the Ag 3d~/~o~ak of the Ag working elecmxle are plott~ as a function of the photoelectron take-off angle O. The o l ~ t r ~ e was emersed at Up ~ + 0.33 V. As expected from ~ . (2). the intensity ratios of CI ° :Ag. Na + :Ag and H~O;Ag increase with increasing angle. The intensity ratios of the measured data (circles) are smaller than the calculated data (full line. Eq. (2)) for high take-off angles due to surPace roughness eff.~ts as discussed in detail in Refs. [30-32]. The H:O:Na* and H , O : C ! ratios do not show any angle-dependent trend. However. the Na +:CI-

f i S d

--

ri

°1 J •

t~ndicular

to the surface [46], It se~ms as if th~ ado

sorbed CI: anions are a d s o ~ d in c o n t a c t with the A g metal surface, while the c o a d s o ~ d Na ~ ions are k~atcd near the electrolyte side of the double layer. A simple onedayer m ~ e l is the~fore not suited to describe, the double-layer properties completely. The surface concentralion of the adsorbed CI~ ions is a~mt 1.3 nmol cm- ~ for U0 ~ +0.33 V ( s ~ Fig. 2), i.e., nearly a complete monolayer of CI- is a d s o ~ d Ibr this potential as already mentioned, From the angular de~ndency of the Na + :Ci ~ intensity ratio, the twodayer model presented in Fig. 5a and b can thereiore be deduced. The adsorbed CI ~ anions are represented by a compact- layer with a thickness dcl which is located on the Ag electrode surface, A thin film of adsorbed Na* species is present on top of this C! ..... layer (see Fig, 5a), Due Io the high surface ~mcentration of CI ~, dc~ ~ 0,3 nm is clo~ to the cort¢o spending value of a CI monolayer (d = 2 R o- ~ 0.36 nm), The intensity ratio In~./Ica for this twodayer model is ~presented hy Eq. (4)"

4

e,~

4

~0

Ici

(r¢.i No,

~ , ~

exp ,~c~COS0 ~.~

,,0[.

•

I - exp

. d 0 ~0 ~0 ,~0 ~0 ~0 ~ ~ ~ ,,~,oe~,"

~ ~ "

Fig. 4. &#gk~t~-~t~vcd XPS hlvc.~i.~lioa ,ff an Ag ~ i m e n exu'acted from 0.1 M N a ~ + l0 -~ M HCl at Up= +0.33 V: Dependence of the ~ ~l" ~ detected .species on the take-off angle O. The ~t~es wc~e itot ~ . ~ - t e d with the nelated photoionization cross

~s

f~ th¢se~

[ cos it ~c~ do Actcos 0

')I

AN,

.

(4) However, the comparison of the Na + surface concentration with that of a Na + monolayer ( F n a , = 5 . 3 1 n m o l cm ° ' ) results in a physically meaningless thickness of this sublayer (ds~,+= 0.01 nm). Therefore the Na + ions were treated as isolated particles similar to the derivation of Eq. (2). i.e.. the corresponding exponential functions in Eq. (4) were linearized in a Taylor series, which was truncated after the first term. This modified two-layer model is

D, He(,ht. H. -H. Strehhlow/ Journal ~ Ele~'troanalytical Chemist~3,436 f ! ~7) 109- i 18 AO I o. ~M~ c t + W'*M HCi 1.e4 ~..__.~,, . ~ . . . m " , ~ , , , m " , , ' #

~..~

O~= +020 V

/

/

1.78 .

./"

~ •"

1.76

;,' "/'~"

1.74 ~ ' ° " ° l.TL~

1,86 ~

~a~L~LtJL~.~daa~,_t,o,.d.......t,.,.,~

8,1

o

f,~

~o ~o ~o ~o 5o 60 ~o

i

~_,.LL,J.,,J n. . . . , . . . , . . . m . . . H

o

take off angle/"

m

~0 ~o 40 5o 60 ;'o take eftangle/"

Fig, 6, Fit tff the angle-dependemXPS intensily ratios I~,,,/Ic~ for t~o diffe~nl polenllals, The dashed lines con~espond to a fit with an exact twoJayer m~vdel(see Fig, 5a), while the solid line |~presenlsIhe m~lilied |wooh~yernu~;lel(Fig, 510,

depicted in Fig. 5b, The resulting equation describing the angular dependence of IN,, , / I c ~ is given by Eq. (5):

,.o ic~

~rc~ AN,,cos O Nc~

I

-

exp

(.,c t] Ac~COS0

'

(5)

115

shifted with respect to each other for a better orientation. The peak positions of Ag, CI, Na and O calculated from the mass of the target atoms and the projectile ions are indicated by dotted lines. Directly after the removal of the electrode from the solution, mainly contributions of oxygen (water) at about 1200 eV kinetic energy are visible in the spectra. After approximately 200 s spuuer time, signals at = 1380 eV and --- 1540 eV corresponding to Na and C! are detected on a large and broad background intensity. The latter is typical for low-energy He ion scattering experiments from adsorbate-covered surfaces (for example, see figure 9 in Ref. [47] and figures 2 and 3 in Ref. [48]). Further sputtering of the electrode causes a peak at about 1850 eV which is related to the Ag electrode. The intensity of this peak increases strongly with sputter time, while the re,ative intensity of the background signal decreases con~ tinuously, For the Na and CI signals, a linear background signal is marked in Fig. 7 for a better orientation. The quantification of the individual ISS signal intensities i~ depicted in Fig. 8 in the tbrm of a sputter depth profile~ The relative backscattering cross sections given in Table ! were used for the evaluation of the atomic concentrations. Obviously, the oxygen content decreases drastically during the first few minutes of sputtering. Alter about 10 t i n , no oxygen could be detected on the electrode surface. The CI

If the remaining exponential function in Eq. (5) was also developed in a Taylor series and truncated after the linear term, the ratio IN,./lc~- would not show any take-off angle dependence as can be expected for two statistically arranged adsorbate species. In Fig. 6, the fit of the experimental data using the modified two-layer model is presented for two potentials. For Up ~ + 0.20 V, the related surface concentrations arc nN~, ~ 0.30 nmol cm ~" and nc~ ~ 1.05 nmol cm ~'. resulting in a thickness of the CI ~ layer of dc~ ~ ~ 0.27 nm.

1850o

In both graphs, the solid line represents the fit with the modified two-layer model, while the dashed line gives the best fit with the exact twoolayer model. Although the scatter of the data points is relatively large, the trend, i.e.,

~

~.J oess

the increasing INa,/i(, ~ intensity ratio is clearly visible for both potentials.

~, 620 o 615~

3.2. ISS int,estigatimzs

Due to the small differences in the positions of the adsorbed cations and anions, the variations of the XPS intensity ratio Na + :Ci~ are only small. In addition, only low-signal intensities could be detected. Therefore it is useful to check the significance of the results with an independent method, ht this context, ISS investigations are well suited due to their inherent surface sensitivity [17-19]. 4 He +-ISS spectra (2 keV, 30 nA) obtained from an Ag electrode emersed hydrophobically from 0.1 M NaCI + 10 -3 M HCI for Up = +0.33 V are presented in Fig. 7 for different sputter times; the spectra are normalized and

1o3o6

t03 1000

1~.00

1400

1800

1800

kinetic onorgy/ oV

hydrophobically from 0,1 M NaCI4oI0 ':'~ M HC! at Up ~ +0,,~3 V |~Jl: different sputlcr Umes, The H~¢lra are normalized and ~hifled wilh respect to each other, The peak ~sition~ of Ag, CI, Na and O calculaled from the mass of the target atoms and the proJectile ions are indicatedby dotted lines, For orientation, a linear background signal is indicated t~r Na and CI.

IIb

D. Hecht. H..H. Slrehblow/ Journal vf Eh,ctr,,analytical Chemisto' 436 (1997) 109- 118 100

80

•

~""-'

~'1

'

'

'i'

!

'

'

He . 1$S (2 IceV, 3 0 hA)

u,,,+oJ3v

'

I

'

'

"

i

~

'

- -

p~ ®0

~0

~

'

-

i

i

00

'

O0

I

.

50

• pt~tor tlroo / m~, Fi~, B, I$S H~tler @l~h profile of an Ag ¢1¢¢!i*~t~eol~rm~d hydrophohio c~lly th~an ILl M N~CI+ |0 'l' ~ M HCI a,t Uo ~ +0,33 v,

and Na ~ignals decrea~ more slowly with time. In contrast the XPS investigations, no CI enrichment in compari~m to Na can be found within the double layer. However. the to~l a ~ n t of adsorbed CI (nc~ ~ 1.3 nmoi cm ~' ) should be much larger than that of Na (nn.. ~ 0,M nmol cm ~ ) according to the XPS data analysis, while the ISS depth profile reveals approxima~ly equal amounts of both species. The observed behaviour can be explained by a ~reening of the CI ~ anions by the Na* cations, Acco~o ing to F~, (4) this effect should attenuate the chlorine sign~! in. the !SS spectra, In principle, this ~reening effect is adequate to the twoolayer model postulated from the angleo~solved XPS ~ a s u r e ~ n t s , Several studies in the lite~tu~ have shown that the ISS method is ~nsitive e ~ h to prove such a sc~ning effect, I~'~rexample, ISS has shown that CO is ~ n d on molybdenum via the ¢~: the C atotu is ~ ~ efficionOy by t ~ top-bound o~g~n and can thereft~ give only a we~k ~ k ~ a t t e r i n g

a single Na + ion is approximately identical to that of a Na-CI bond, while the energy related to take off a CI- ion is equal to that of an Ag-CI bond. The detachment of Na+-C! - ion pairs is therefore energetically favoured compared to the removal of single Na + and CI- ions. The rapid decrease of the oxygen signals during the sputter depth profiling can also be explained by the lower binding energy of oxygen (water,) to Ag, Na and CI. The proposed structure of the double layer on Ag electrodes with a chloride layer in direct contact with the working electrode and coad:~rbed Na ÷ species on the electrolyte side of this layer is in agreement with a ~ e n t l y published study with Xoray abso~tion fine structure spectro~opy [~] and X-ray diffraction data [55], In both studies, the structure of underpotentially deposited Cu mooolayers on gold substrates was studied, These experio ments revealed that i~reasing amounts of anions (CI ~ and SO~ °*, respectively) we~ coadsorbed with deceasing potential during the deposition of the Cu monolayer [54], Apparently. Cu ~* cations a~ not completely di~harged during underpotential deposition (upd), but remain ads o ~ on the surface in a Cu ~* state (sac for instance Refs. [56,57]), The quantitative evaluation of the Xoray absoq~|ion data and lhe X-ray diffraction peaks shows that the Cu ions are directly bound to the Au surface, The coadsorbed anions, which are neces~ry for a charge compensation, similarly In the coadsorbed Ha* cations during the speoific adsorption of chloride, are attached to the Cu i~ns, i,e,, a layered swucture Au electrode-Cu ||~onolayer~co~d~rhed CI '~ and SO~ ~ ions is pre~nt during the Cu upd on Au [54,55], Simil~ results have been oblained very recently for the upd of Hg on Au [58] and the ct~d~rption of Br ~ during the Cu upd on Pl [59].

binding e ~ i e s A ~ between ABACI, As=As ~nd Na:-CI bonds tui~h! explain the :~bsonc¢ of a chlorie~ endchmen| in the sputter depth profile (Fig, 8), Val~s for E ~ summarized in Table 2, C o m ~ to the Ag~CI bindip,~ energy, tl~ Na=CI bond is about 20% stronger, It can ~ assumed that the e ~ required for the removal of

~ ~ l ~..

.

~ 4 e l~y¢~"o~ A~ i~ O,I M I',l~C~l+ IO=~ M HCI Io00

1200

1400

1600

1800

kinetic energy / e V ~--O

2s't,4

[531

Fig, 9, '~He+-ISS spectra (2 keY, 30 hA) of an Ag electrode emersed hydrophobically from 0.1 M NaCI+ 10 -3 M HCI at Up -- - 0 . 6 V (i.e.. in the vicinity of the potential of zero charge) for different sputter times. The spectra are normalized and shifted with respect to each other.

D. Hecht, H.-H. Strehblow / Jounml qf Eleclsoanalytical Chen;isto, 436 ¢1997) 1{)9-118 100 ~

~"

"'

¢

'I'

'

"'

~

'1

'

''

""J

'"

'

''

I'

"

'

!17

the sputter time required for the complete removal of the double layer is larger compared to Up = -0~6 V.

"

80

~

HCI

60

V

E 40

4. Conclusions

u:.osv

A

ONa aAg

I ~ 2O¢

~ O~ 0

_

_

5

. 10

&

__J~

15

2O

25

sputter time /min. Fig, I0. ISS sputter depth profile of an Ag ¢lcctm(l¢~ mnersed hydrol~hohio tally fi~m O, I M NaCI + I0 '~~ M HCI at UI, ~ ~ 0.6 V.

Furthm" ISS investigations weir perlhrmcd I'or Ag electrodes emerged from 0, I M NaCI + I0 =~~ M HCI l'or more negative potentials. In Fig. 9, 4He+~|SS spectra are pre~ sented for a potential in the vicinity of the potential o1"zero charge, i,e., Ul~ ~ = 0.6 V; the related sputter depth profile is depicted in Fig. I0. in agreement with the XPS investigations, no CI- can be detected on the electrode for this potential. In addition, only very small amounts of Na + were found. However, oxygen (water) signals are clearly visible in the spectra at about 1200 eV. Therelbre the oxygen concentration in the sputter depth profile are more intense. Compared to Up = +0.33 V, the Ag signals can clearly be seen after a sputter time of about 150 s, there= fore the total amount of adsorbed species is significantly lower for Up = - 0 . 6 V, in agreement with the XPS investigations. A sputter depth profile for an Ag electrode emersed at ,~ I.I V is shown in Fig. l I. Only Na, O and Ag contributions are visible in the ISS spectra. Correo spending to the larger amount of adsorbed Na + (see Fig. 3), the concentration of Na is larger than that of water and

100

.......................

=

,o

N

60

/

"i

40

^

o

0

~He~- ISS(2keY, 30 hA)

":'" '

10

20

40

Acknowledgements We would like to thank J.-M. Abels for help with the appropriate computer software for the XPS peak deconvolution. Stimulating discussions concerning the interpreta° tion of the ISS experiments with D. Schaepers are grate° fully acknowledged.

'

References

o n.

30

The combination of XPS, angle-resolved XPS and ISS was used for the ex situ investigation of the electrochemical double layer on Ag in NaCi solutions. The quantitative evaluation of the XPS spectra shows that approximately one monolayer of CI= is adsorbed prior to the electrochemical formation of bulk AgCI, the overpotential required for bulk AgC! formation is about 0.05 V. The results show that the specifically adsorbed CI ~ anions tend to bind Na + cations in the double layer for charge come pensation. In addition, appl~ciable amounts of adsorbed water (ca. one monolayer) were found over the whole potential region studied, in agreement with classical done ble=layer models. AngleoreSolved XPS data and !SS inveso tigations showed the existence of a concentration gradient of the double,.layer constituents pe~ndicular to the stir° face, i.e., the coadsorbed cations are bound to the adsorbed CI =, which itself is contact=adsorbed on the silver eleco trode. With regard to recently published results of metal upd systems obtained from X-ray absorption fine structure data [54] and X-ray diffraction studies [55,58,59], this structure is probably caused by the attractive potential between the anions which are bound to the positively charged silver surface and cations to the negative charge of the anions. For future work, further investigations, espeo cially with ISS, seem to be promising to obtain more detailed insight in the double°layer structure by ex situ examinations of electrode surfaces, in addition, the investio gation of well=defined and smootll single°crystal sut'face.~ is very desirable.

50

sputter time /min. Fig. I 1. ISS sputter depth profile of an Ag electrode emersed hydrophobically from O. I M NaCI + I 0 - ; M HCI at Up = - I. I V.

[I] G. Valetle. A. Hamelin. R. Par~o,~. Z. Phyla. Chem. NF 113 (1078) 71. [2] A. Hamelin. T, Vil~,ov. E, Seva~lymlov. A, Popov. J, l~l~¢{ro~!n~d, Chem. 145 (1983) 225. [3] C, |:ranke. G, Piazza. D,M. Kolb. El~clr~him. A¢|~l 3~, {I(;1~9) ~'~1 [4] V.I. Birs,. C,K. $milh. Eleclr~him, Acla 32 (1987) 2~9, [5] R.I. Tucceri. D. Po~adas. J. Elecm):mal, Chem, 283 (1990) l~(), [6] D. K0rwer. D. Schumacher. A. Ollo. Ber, Bun~ng~, Phy,~, Chem 95 (1991) 1484.

I 18

D. Het'hL H,,H, S~rehblnw/ Journal ofEleclroanalytical Chemistr3.'436 (1997) 109- i 18

[7] Wo Paik+ MA. ~ s h a w . J.O'M. Bockris. J. Phys. Chem. 74 (1970) 4266, [8] G. Horanyi. E+M. Rizmayer. J. Konya. J. Electroanal. Chem. 176 (1984) 339. [9] G.N. Salaila.F. Lu. L Laguren-Davidson. A.T. Hubbard, J ElecImaaal. Chem. 229 (1987) I. [101 T. Yamada. K+ Ogaki, S+ Oku~+. K. haya, Surf. Sci. 369 (1996) 321. [11] WeN, Han~n+ D.M+ Koib. D.L. Rath, R. Wille, J. Elcctroanal. ~ m . I I0 (1980) 369. [12] D.M+ Kolb, D,L Rath+ R, Wille. W.N. Han~n. Ber. Bun~nges. Phys, ~ m + 87 (1983) 1108. [13] S, Haup¢+ U, Cellist. H.D. Speckmann. H,-H. Strehblow, J. Elect~mnal, Chem, 11)4(1985) 170. [14] S. Haupt, C. Calin~kL U. Call|+|. H.W. HopW. HD. Swekmann. H:H, St~rehblow, Surf. Interf, Anal+ 9 (1986) 357. [i~] |LR. g t~, H. Neff. g. M~ll~r, J, El+clroa~al. Chem. 215 (19S6) 331, [10] D,M, gotb, Z, Phy++ (:hem, NI: 1~4 (1~7) 179, [17] ~, Ta+lauer, W,IL Heihnd. Apple I~ym+ 9 (1976) 261 [1~] W+ H+tlmnd, E, Ta+lau+r, Surf, Set, 6~ (1~77)90 [I~] H+ N+hu+, R, Spil+l, Surf, h)!¢rf, Aaml, 17 (1991) 287, [~)] M,T. A,It~wly. M,P+ S¢f~tl, Surf, Inlet'f. Allal+ 6 (1984) ')~ [21] D,P. Smi+h+ Appt, lSy~, 38 (I~7) 340, [2:2]W,N H:m~,~n,D,M, Kolh. J, l+l¢c|roanal, Chelii~ I(X)(1979) 493. [23] J,oM, At~h, Ph,D, d i ~ a t i o n , HeinrichoiteineoUniverm~iit DUssel+ d~m\ in prepai+ation, [~4] D+A, Sh~dey, Phy~> Rev, B 6 41972) 4709+ [28] J,H, St.~field, J, Elecmm, Spectm~, Related Phe, om, 8 (1976) 12q. [20] R.F. R¢ilma,. A. M~za~, S.T, Man~m, J, Electrum, Sl~ctro~c. Related ~enom, S (1976) 389, [27] A.E. Hughes. C.C. Phillips, Surf. Interf. Anal. 4 (1982) 220. [28] M,P. Scab, W,A, Death, Surf, In|err, Anal, 1 (197~)) 2. [29] C,S, Fmlley, Pang+ Solid Stale Chem. 11 (1976) 265, [,~] M,F+ El
[37] [38] [39] [40]

F.W. Bingham. Sandia report SC-RR-66-506 (1966), D. Heeht, PhoD. thesis, Heinrich-Heine Univer~it:,it Diissehlorf, Iq97, D. Hecht, H.-H. Strehblow, unpublished data, D.D. Bed6 Jr., T.N. Andersen, H, Eyring, J. Phys, Chem, 71 [1967) 792+ [41] O. Hofmann. K. Doblhofer, H. Gerischer, J, Electroanal, Chem, 161 (1984) 337. [42] M.A.V. Devanathan, P. Peries, Trans. Faraday So<:, 50 (1954) 1236, [43] A.T. D'Agoslino, W.N. Hansen, J. Elcc|rou, Slx,-clrosc, Related Phenom. 46 (1988) 155, [44] O.A. F,sin. B,F. Marker, Acla Physicochem, USSR IO (1939) 353. [45] J.O'M, Bockris. M,A. Devanalhan, K. MUller, Prec, R. Soc London. Set. A 247 (1963) 55, [46] C.S, Fadley, R J, Baird. W. Siekhaus, T+ Novakov, S,A,I,o, Bergstt~hn, J, Eleclron, Spot|rose. Related Phenom, 4 (1974) 93, [47] HI|+ Broagersma, TM, Buck, Nucl, Instr. Moth, (in Phys, Res,) 149 (1978) 5(~9. [48] H,F, Helhig+ PJ., Adelmann, A,C. Miller, A,W, Czandenm, Nucl, Insa'+ Me|he (in Phy~, Res..) 149 (1971~)5811 [49] B0 d¢ B. i)m~vcm, B~md Dtsmsciaiion t~n¢tgics iu Situpie Mol¢cules, NSRDSoNBSo310 National Bureau o[ Slandm~ls. Washington IX?, t970, [~0] P. Gra~l~t. KG, Well, Bet, Bun~mbt¢~, Phy,~,Chem, 7h (lU?~)4!?, [~1] J,B, Pedley. ILM, Marshall J+ phys, Chem, Ref, Data 12 {19841 967. [,~3] D,R+ Reddy, T+V,R, Rao, A+S,R, Reddy, ImL 3, Pure AppL Phys+ 27 (191~0) 243, [54] T, T~lis~cl,ak, A,P, Httchcock, S, Wu, J, LipkowskL Ih~meedings of the Ninth Imemati,mml C~mi~ren¢¢ on X-ra~ Absoq~|ion F+oe S|ru¢o lure+ Greaobl,e, I~M++ [SS] M.F. T+mey, +.N. Howai~, +. Riehler, G.I+. Bodes+ +.(L Gordon, O.R. Meh~y, D. Y~+, L.B. So¢¢I~n, Phys. Rev. Ioelt. ?5 (1995) J4721 15¢+] A+ Tadj¢&iln¢, G, T+mriihm, D, Guay, Eh++¢|roehht+,Ac|a 3f~ ( I*F)I] 185~), (57] A~ Tadj~|te~, A I+~hr~¢hi, G, Tourilloa. J, Electroanal. Chem. 3~1 <11~3) 2tM. ~5t~] ,|, Li, H,D+ Ahtu~a, J, Phys.+Cl~+m+ B IOI { 1~')7) 244, 1~9] N,M, Ma~kovic, C.A, I+ucas, H,A Gasleiger, P.N, Ross Jr+. Surf.