Electrochemical, in-situ surface EXAFS and CTR studies of Co monolayers irreversibly adsorbed onto Pt(111)

Electrochemical, in-situ surface EXAFS and CTR studiesof Co monolayers irreversibly adsorbed onto Pt(111)

Enrique Herrero, Jun Li, HeÂ ctor D. AbrunÄ a *

Department of Chemistry, Baker Laboratory, Cornell University, Ithaca, NY 14853-1301, USA

Received 11 May 1998; received in revised form 14 August 1998

Abstract

Irreversibly adsorbed Co adlayers with surface coverages of about 0.6 and 1 ML can be deposited onto Pt(111)

from CoSO4 in 0.1 M H2SO4 and CoCl2 in 0.1 M NaOH solutions. Once adsorbed, the adlayers were studied in 0.1M H2SO4 and 0.1 M NaOH solutions respectively, by electrochemical, surface EXAFS (SEXAFS) and crystaltruncation rod (CTR) measurements. In 0.1 M H2SO4, an irreversible oxidation with a peak potential value of+1.00 V (vs. Ag/AgCl) corresponding to the oxidation of the cobalt adlayer as well as the Pt substrate was

observed. There was also a reduction wave present at ÿ0.29 V and which was ascribed to, at least in part, theevolution of hydrogen and which resulted in the displacement of the cobalt adlayer. In 0.1 M NaOH two redoxprocesses were also observed at formal potential values of ÿ0.355 and ÿ1.04 V. The former, which exhibited

signi®cant kinetic limitations, was ascribed to a CoIII(OH)3/CoII(OH)2 redox process. The latter was ascribed to the

generation of Co0. SEXAFS measurements in sulfuric acid (at an applied potential of 0.0 V) and in 0.1 M NaOH(at applied potential values of ÿ0.06 and ÿ0.75 V) were consistent with the presence of an oxidized cobalt layer with

Co, O and Pt near neighbors. CTR data were also consistent with the presence of an oxidized cobalt bi-layerstructure. In 0.1 M NaOH and at a coverage of ca. 1 ML, the deposited cobalt appears to form a 1 � 1 structurewith the cobalt atoms occupying three-fold hollow sites. Relative changes in the number of oxygen near neighborswith applied potential (derived from EXAFS data) were also consistent with electrochemical data. In 0.1 M H2SO4

(at an applied potential of 0.0 V) and at a cobalt coverage of about 0.6 ML, the deposited cobalt appears to forman incommensurate monolayer with an expanded lattice (Co±Co distance of 2.96 AÊ ) likely involving co-adsorbed(bi)sulfate. # 1999 Elsevier Science Ltd. All rights reserved.

Keywords: Co monolayers; Irreversible adsorption; Pt(111); EXAFS; CTR

1. Introduction

In recent years, much attention has been devoted to

the study of Co/Pt multilayers, since they exhibit a per-

pendicular magnetic anisotropy and large Kerr e�ects

at small optical wavelengths [1]. For this reason they

are considered good candidates for high density mag-

neto-optical recording media.

Some of these studies have stressed the necessity of

knowing the growth mode(s) of cobalt on well de®ned

platinum surfaces since platinum and cobalt structures

present large di�erences [2±5]. The stable phase of

cobalt at room temperature is hcp whereas platinum is

always present in an fcc lattice. There is also a large

lattice mismatch with the Co±Co and Pt±Pt distances,

being 2.50 and 2.77 AÊ , respectively. This mismatch

generates an interfacial tension when cobalt is depos-

ited on platinum and this, in turn, makes epitaxial

growth di�cult.

Electrochimica Acta 44 (1999) 2385±2396

0013-4686/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.

PII: S0013-4686(98 )00362-4

PERGAMON

* Corresponding author. Tel.: +1-607-255-4720; e-mail:

[email protected]

Although a variety of deposition methods could, in

principle, be employed, most of the studies of cobalt

deposition onto well-de®ned platinum surfaces have

been carried out in UHV [2±4]. Electrochemical tech-

niques can also be used to deposit cobalt onto metallic

substrates. There are some distinct advantages to elec-

trochemical deposition including a less stringent en-

vironment, easy control of the amount deposited and

growth mode through the potential or current applied

during deposition. However, the potential for cobalt

electrodeposition onto platinum electrodes overlaps

with the hydrogen evolution reaction, which not only

acts as a competing reaction but also can obstruct

cobalt deposition.

There have also been some studies on the oxidation

of bulk Co electrodes, mainly in alkaline media. In one

of the earlier studies, Behl and Toni [6] proposed the

initial formation of a Co(OH)2 ®lm with subsequent

oxidation to Co3O4 and CoOOH. ArvõÂ a et al. have stu-

died the electrochemical behavior of CoO [7] and

Co(OH)2 [8] electrodes formed by surface oxidation of

the bulk metal as well as precipitated layers on Pt

electrodes [9]. In all of these cases the electrochemical

behavior was quite complex due to the multitude of

oxide phases that can be present (and their potential

dependence). In addition, all of these studies have

involved relatively thick cobalt-containing layers. In a

recent study, Itaya and coworkers [10] studied the dis-

solution of Co(0001) by in-situ electrochemical-STM in

pH 3 sulfate media. They found that dissolution

occurred without passivation and that the initial pro-

cesses involved the formation of a 5 � 5 surface struc-

ture corresponding to either CoO(111) or to Co(OH)2(0001).

In the present study we have employed the irrevers-

ible adsorption method [11, 12] to deposit up to mono-

layer amounts of cobalt onto Pt(111) electrodes. In

this method, the electrode is immersed in a cobalt-con-

taining solution from which cobalt species spon-

taneously adsorb onto the electrode surface. Three

di�erent techniques have been used to characterize

these cobalt modi®ed Pt(111) electrodes; cyclic voltam-

metry, surface extended X-ray absorption ®ne struc-

ture/X-ray absorption near edge structure (SEXAFS/

XANES) and specular crystal truncation rod (CTR)

measurements. The voltammetric measurements

allowed the electrochemical characterization of the

modi®ed electrodes and the establishment of the redox

behavior of the cobalt adlayer. As cobalt may be pre-

sent on the electrode surface in di�erent oxidation

states, depending on the applied potential, SEXAFS

and XANES measurements were carried out at di�er-

ent electrode potentials in an e�ort to determine the

cobalt's oxidation state and local structure. These stu-

dies were complemented with CTR measurements,

which provide information on the adlayer structurenormal to the electrode surface.

2. Experimental

SEXAFS and XANES measurements were con-

ducted at the B-2 station of the Cornell High EnergySynchrotron Source (CHESS) using a Si(111) doublecrystal monochromator. The incident beam intensity

was monitored with an ionization chamber ®lled withnitrogen. The Co (Ka) ¯uorescence was monitoredwith a solid state Si(Li) detector (Princeton GammaTech) coupled to an EG&G 673 spectroscopy ampli-

®er. A single channel analyzer (Tennelec) was used todiscriminate against the background and other undesir-able contributions to the signal. The incident beam

was detuned 50% to eliminate higher order harmonicphotons.All measurements were carried out in an in-plane

(s-polarization) geometry at grazing incidence (angleof incidence was typically 1.5±2.08). The incidenceangle on the sample was optimized to give the best sig-nal-to-noise ratio over the energy range from 7600 to

8600 eV. SEXAFS data were collected using SPECsoftware. A typical SEXAFS scan took 20±30 min. 20scans were averaged in order to obtain a good signal

to noise ratio. In order to ensure the integrity of theadsorbed cobalt layer, cyclic voltammograms wererecorded at regular intervals during the acquisition of

spectra.Data were analyzed using well-established

procedures [13] using BAN and MFIT programs

(Tolmar Instruments). In the analysis, the EXAFSfunction w(k) was k 1 weighted. No signi®cant di�er-ences were found when using k 2 or k 3 weighing. Thek�w(k) function was Fourier ®ltered using only the con-

tributions from the ®rst and second shells. Phase-shiftsand amplitudes used to ®t the SEXAFS data for theCo±Co and Co±Pt correlations were calculated with

the theoretical EXAFS code FEFF version 5.04 [14, 15].A hexagonal cobalt layer with a Co±Co distance of2.77 AÊ (which represents the lattice constant for Pt)

was used as input for the program for the Co±Co cor-relations. A platinum (111) substrate on top of whicha cobalt monolayer was deposited on three-fold hollowsites was built for the Co±Pt correlations. Both the Pt±

Pt and Co±Pt distances were set at 2.77 AÊ . The Co±Ophase-shift and amplitude were obtained from theEXAFS spectra of CoO.

Specular crystal truncation rod (CTR) measurementswere carried out at the Exxon X10-B beamline at theNational Synchrotron Light Source using a bent

Si(111) crystal. X-ray photons of 1.1282 AÊ wereemployed. CTR's were measured across the (00l)Bragg di�raction points. A complete CTR measure-

E. Herrero et al. / Electrochimica Acta 44 (1999) 2385±23962386

ment consisted of a series of surface rocking curves atQz ranging from 0 to 2 rlu (reciprocal lattice units).

Each rocking curve was ®tted to a Lorentzian line-shape to derive the integrated intensity, which was sub-sequently used for ®tting with a model consisting of

atomic layers along the surface normal. A full descrip-tion of the ®tting model has been givenelsewhere [16, 17].

For the X-ray measurements a Pt(111) electrode ofca. 1 cm diameter was used. The crystal was grownfrom the melt at the Materials Science Center at

Cornell University. It was cut and polished with a mis-cut of less than 0.38. The Pt(111) electrode used in theelectrochemical measurements was oriented, cut andpolished using Clavilier's technique [18]. Cobalt depo-

sition was carried out by the irreversible adsorptiontechnique [11, 12]. Prior to any measurement, electro-des were ¯ame annealed, quenched with ultrapure

water [18], dipped in the cobalt containing solutionuntil the desired cobalt coverage was attained and thentransferred to the cell for electrochemical or X-ray

measurements. It should be noted that cobalt is notpresent in the electrolyte solution in the cell, so there isno interference neither in the electrochemical nor in

the X-ray measurements from cobalt species in sol-ution. The cell, procedures and apparatus used in theX-ray [19±21] and in the electrochemical [22] exper-iments have been described elsewhere.

All potentials are reported versus a Ag/AgCl elec-trode in 3 M NaCl without regard for the liquid junc-tion potential. A large area coiled platinum wire was

used as a counter electrode both in the electrochemicaland X-ray cells. All experiments were carried out atroom temperature.

Solutions were prepared using ultrapure water (18MO; Millipore Milli-Q1 water). Aqueous solutionswere prepared from high-purity sulfuric acid (Baker,Ultrex) and NaOH (Aldrich). Irreversible adsorption

of cobalt was carried out from solutions preparedusing 0.01 M CoSO4�H2O (Aldrich) in 0.1 M H2SO4

and 0.01 M CoCl2 (Aldrich) in 0.1 M NaOH. All cov-

erage values are de®ned as the ratio between the num-ber of adatoms in the adlayer and the number ofsurface platinum atoms.

3. Results

3.1. Electrochemical results

3.1.1. 0.1 M H2SO4 electrolyteAs mentioned in Section 2, cobalt modi®ed Pt(111)

electrodes were prepared by immersing the ¯ame

annealed electrode in a relatively concentrated (10ÿ2

M) CoSO4 solution. A fully covered electrode couldnormally be obtained after a deposition time of ca. 2

min. The solution concentration of cobalt required for

its deposition is much higher than that of Bi or

As [11, 12], indicating that cobalt deposition is less

favored than the deposition of these other adatoms.

The use of more dilute cobalt solutions (e.g. 1 mM)

gave irreproducible results in terms of coverage and

required very extensive adsorption times. The irrevers-

ible adsorption of cobalt in acid media only takes

place when CoSO4 is used. No deposition was

observed when the electrode was immersed in solutions

of chloride or perchlorate salts of cobalt. These results

suggest that the cobalt adlayer likely interacts strongly

with sulfate and/or bisulfate anions.

Fig. 1 shows the voltammetric pro®les for partially

(dashed line) and fully (solid line) cobalt covered

Pt(111) electrodes. For the partially covered electrode,

both hydrogen as well as sulfate/bisulfate adsorption/

desorption processes on Pt(111) electrodes are still

clearly visible in the voltammetric pro®le, but the

charge associated with them has clearly diminished

relative to the unmodi®ed Pt(111) electrode whose vol-

tammetric pro®le is also presented in the ®gure (dotted

line). For the fully cobalt-covered electrode, the vol-

tammetric pro®le shows only a ¯at line corresponding

to double layer charging/discharging processes. The

voltammetric pro®les for the partially and fully cobalt-

covered electrodes did not change upon cycling

between ÿ0.23 and +0.70 V, indicating that the depos-

ited cobalt layer is stable over this potential range.

Unlike other irreversibly adsorbed adatoms [11, 12],

there appears to be no redox process corresponding to

the adsorbed adatoms in sulfuric acid electrolyte over

Fig. 1. Cyclic voltammetric pro®les, at 50 mV/s for a clean

(dotted line), partially (dashed line) and fully (full line) cobalt

covered Pt(111) electrode in 0.1 M H2SO4.

E. Herrero et al. / Electrochimica Acta 44 (1999) 2385±2396 2387

this potential range. Thus, no direct information

regarding the cobalt coverage (as the number of cobalt

atoms per surface platinum atom) could be obtained

from voltammetric measurements under these con-

ditions. However, the fraction of the platinum surface

covered by the cobalt adatoms could be evaluated (by

di�erence) by measuring the charge remaining in the

hydrogen and sulfate/bisulfate adsorption/desorption

processes.

Although no redox response ascribable to the cobalt

adlayer could be observed in the range from ÿ0.23 to

+0.70 V, if the upper limit in the voltammetric scan

was extended, a sharp peak was observed at +1.00 V

(Fig. 2A). This peak likely corresponds to the oxi-

dation of the cobalt adatoms present on the electrode

surface together with the oxidation of the platinum

surface itself, as the presence of a double reduction

wave on the subsequent cathodic scan suggests. The

onset of surface oxidation for a clean Pt(111) electrode

in this medium is around +0.90 V, however the pre-

sence of adsorbed cobalt appears to hinder the oxi-

dation of the platinum surface. The oxidation of

cobalt adatoms also appears to involve their partial

dissolution, as the reappearance, on the subsequent

cathodic sweep, of the voltammetric peaks associated

with hydrogen and sulfate/bisulfate adsorption/de-

sorption between 0.00 and ÿ0.23 V indicates.

However, oxidation/reduction cycles of the platinum

surface to potentials higher than +0.90 V disrupt the

surface order and this, in turn, a�ects the voltammetricresponse associated with hydrogen and sulfate/bisulfateadsorption/desorption.

At potentials below ÿ0.23 V an additional redoxprocess for the cobalt adlayer can be observed.Although the onset of hydrogen evolution on a plati-

num electrode in this medium (0.1 M H2SO4) is ÿ0.23V, the presence of cobalt (a metal with a much higherhydrogen evolution overpotential) displaces the onsettowards more negative values. Between ÿ0.23 V and

the onset of hydrogen evolution on the adlayer, a newvoltammetric peak can be seen (Fig. 2B) at ÿ0.29 Vcorresponding to the desorption of the cobalt adlayer

(likely via displacement by H2), since after severalcycles up to ÿ0.30, the electrode recovers the typicalvoltammetric pro®le of a clean Pt(111) electrode. It

should be mentioned that the recovery of the charac-teristic Pt(111) voltammetry described above at nega-tive potentials was only observed for electrodes whose

potential had not been previously scanned to potentialsabove +0.90 which, as mentioned before, results inthe disruption of the surface's long range order.

3.1.2. 0.1 M NaOH electrolyteIrreversible adsorption of cobalt in basic electrolytes

was carried out from 0.01 M CoCl2 in 0.1 M NaOH

solutions. The cyclic voltammogram of a fully coveredelectrode in 0.1 M NaOH is shown in Fig. 3 (solidline) and compared to that for an unmodi®ed Pt(111)

electrode (dashed line) in the same medium. In thiscase, a new pair of voltammetric peaks centered atÿ0.355 V and associated with the cobalt adlayer

Fig. 2. Cyclic voltammetric pro®les at 50 mV/s in 0.1 M

H2SO4 for a Pt(111) electrode covered with a cobalt mono-

layer in (A) the platinum oxide region and (B) the hydrogen

evolution region.

Fig. 3. Cyclic voltammetric pro®les at 50 mV/s in 0.1 M

NaOH for a clean (dashed line) and cobalt covered (full line)

Pt(111) electrode.


appears in the voltammetric pro®le. The peak separ-ation (DEp) of 140 mV at a sweep rate of 50 mV sÿ1

suggests that the redox process is kinetically slow. Noadditional voltammetric features were observed in thevoltammogram when the upper potential limit was

increased up to +0.30 V. When the voltammetric scanwas extended to more negative potentials (Fig. 4), anadditional voltammetric feature, which overlapped

with the onset of hydrogen evolution, could beobserved with cathodic and anodic peak potentialvalues of ÿ1.04 V and ÿ0.97, respectively. Unlike thepeaks appearing in sulfuric acid solution, these peaks

did not result in the dissolution of the cobalt adlayer,so that oxidation±reduction cycles did not alter thecyclic voltammogram.

From an analysis of the Pourbaix diagram for cobaltspecies [23], there are only four stable species, all ofthem insoluble, at this pH over the potential range stu-

died: Co, Co(OH)2, Co3O4 and Co(OH)3. Since thestability region for Co3O4 is extremely narrow, thisspecies was neglected. The potentials for the possiblereactions calculated at pH = 13 are

Co�OH �2 � 2H � � 2eÿ $ Co� 2H2O,

E � ÿ0:897 vs: Ag=AgCl,�1�

Co�OH �3 �H � � eÿ $ Co�OH �2 �H2O,

E � ÿ0:001 vs: Ag=AgCl,�2�

Whereas the potential for Eq. (1) is quite close to theone observed experimentally in 0.1 M NaOH, that forEq. (2) is not. The di�erences between these values de-

rived from thermodynamic data and the valuesobtained experimentally are likely due, at least in part,

to the e�ects of the platinum substrate on the elec-tronic structure of the adlayer and their e�ects on theredox potential, as is often found for metal UPD

monolayers on foreign metal substrates. Moreover, itwould be extremely unlikely that the redox process atabout ÿ0.35 V would correspond to the generation of

Co0. Thus, although for Eq. (2) there is some discre-pancy with the experimentally obtained value, we ten-tatively assign the peaks around ÿ1.0 and ÿ0.355 V in

0.1 M NaOH to Eqs. (1) and (2), respectively. Thisimplies that there are three di�erent cobalt species pre-sent in the adlayer; Co0 at potentials below ÿ1.0 V,Co(OH)2 between ÿ1.0 and ÿ0.355 V and Co(OH)3 at

potentials above ÿ0.355 V.The coulometric charge (after background subtrac-

tion) associated with the redox process at ÿ0.355 V is

250 mC cmÿ2. In order to determine the surface cover-age from this value, the number of electrons trans-ferred per cobalt adatom has to be known. That is, the

initial and ®nal oxidation states of the cobalt adlayerhave to be determined. The coverage can then be cal-culated as

YCo � QCo

n � 243 , �3�

where n is the number of electrons exchanged in theredox process and 243 mC cmÿ2 is the charge corre-sponding to a process that exchanges 1 electron per

surface platinum atom at a Pt(111) surface. Since themeasured charge is very close, within the experimentalerror, to 243 mC cmÿ2, the cobalt coverage is approxi-

mately 1, assuming that each cobalt exchanges oneelectron as suggested by Eq. (2).

3.2. EXAFS and XANES results

In order to gain some insight as to the surface struc-ture, composition and properties of the irreversibly

deposited cobalt adlayer, EXAFS spectra wereacquired in the two electrolytes previously mentioned;namely 0.1 M H2SO4 and 0.1 M NaOH. In sulfuric

acid, the electrochemical results indicated that therewas only one stable cobalt species. Although twodi�erent redox processes were observed in this electro-lyte, the one appearing at potentials close to the onset

of hydrogen evolution resulted in the total dissolutionof the cobalt adlayer whereas the one at high poten-tials involved partial dissolution of the adlayer and dis-

ordering of the platinum surface. Due to the instabilityof the adlayer in these potential ranges, neither wasdeemed appropriate for X-ray measurements. As a

result, a potential of 0.00 V, which is in the middle ofthe stability region, was chosen for EXAFS exper-iments in sulfuric acid.

Fig. 4. Cyclic voltammetric pro®le at 50 mV/s in 0.1 M

NaOH in the hydrogen evolution region for a Pt(111) elec-

trode covered with a cobalt monolayer.


In NaOH, there are three di�erent stable species, as

the two di�erent redox processes would indicate.

However, the stability range for the ostensibly most

reduced species, tentatively assigned to Co0, over-

lapped with the hydrogen evolution reaction prevent-

ing the acquisition of a reliable EXAFS spectrum in

this region. Therefore, spectra were obtained at two

potentials, ÿ0.75 and ÿ0.06 V, where the two remain-

ing cobalt species are stable.

Fig. 5 shows the normalized EXAFS spectra, after

background subtraction, for three reference com-

pounds (Co, CoO and CoSO4) as well as for cobalt

adlayers on Pt(111) electrodes in 0.1 M H2SO4 at 0.00

V and in 0.1 M NaOH at ÿ0.75 and ÿ0.06 V, respect-

ively. From a qualitative inspection, it is evident that

the spectra for the cobalt adlayers on Pt(111) electro-

des most closely resemble the spectrum obtained for

CoO. Information about the oxidation state of the

metal adlayer can be obtained from the near edge spec-

tra (XANES). Fig. 6 shows the near edge spectra for

the reference compounds and the Co adlayers.

Although the spectral features for Co0 were quite

di�erent from those of CoO and Co2O3 (not shown),

the di�erences between the latter two were small so

that qualitatively we could only distinguish between

oxidized (Co(II) or Co(III)) and reduced (Co0) cobalt

species. A more precise assignment of the oxidation

state of cobalt (II or III) in the adlayer (at the di�erent

potentials) would require a more extended data acqui-

sition and detailed analysis of the near edge spectra,

which is beyond the scope of this work. Nonetheless,

some qualitative analysis can be made. As Fig. 6

demonstrates, in all cases studied the XANES spectral

features for the cobalt adlayers on Pt(111) correspond

to those of an oxidized cobalt species and, as was the

case for the EXAFS data, most closely resemble those

of CoO. All the spectra for the deposited layers exhib-

ited a small bump prior to the edge (ascribed to a tran-

sition to a localized state), a well de®ned white line,

characteristic of oxidized cobalt and an additional

peak at around 7.78 keV.

Fig. 5. Normalized EXAFS spectra after background subtrac-

tion for (A) Co, (B) CoO and (C) CoSO4 and for cobalt

adlayers on Pt(111) electrodes in (D) 0.1 M H2SO4 at 0.00 V,

(E) 0.1 M NaOH at ÿ0.75 and (F) 0.1 M NaOH at ÿ0.06 V.

Fig. 6. Normalized XANES spectra after background subtrac-

tion. (A), (B), (C), (D), (E) and (F) as in Fig. 5.


Fig. 7 shows the radial distribution function (RDF)

(uncorrected for phase shifts) of the EXAFS spectra

for Co, CoO and the cobalt adlayers on Pt(111) after

background subtraction, truncation and k= 1 weigh-

ing and Fourier transformation. As anticipated, only

one main peak (for r < 3 AÊ ) can be observed for the

Co sample (Fig. 7A), corresponding to the backscatter-ing from the ®rst cobalt shell. The RDF for CoO(Fig. 7B), exhibits two main peaks (for r < 3 AÊ ) with

the one at lower r corresponding to the ®rst oxygenshell and the other to the ®rst cobalt shell. For thecobalt adlayers on Pt(111) two main peaks (for r< 3

AÊ ) are also found. Given the nature of the adlayer, thesubstrate and electrolyte composition, only three di�er-

ent chemical species could contribute as backscatterersfor the cobalt adlayers on Pt(111): cobalt, oxygen andplatinum. Owing to the fact that the cobalt adlayers

are in an oxidized state, and the similarities in thespectra for the adlayers with the CoO sample, we ten-

tatively assign the peak at lower distances to the ®rstoxygen shell and the second peak to the ®rst cobaltshell, respectively. However, other possible interpret-

ations cannot be ruled out a priori. No other signi®-cant features were found at distances of up to 6 AÊ

(where EXAFS can provide information) in the RDF

of the cobalt adlayers.Given that the ®rst peak (coordination shell) in the

RDF for all cobalt adlayers was at distances (uncor-rected for phase shift) of about 2 AÊ , the only reason-able backscatterer was oxygen. However, the second

shell could have contributions from both cobalt andplatinum as backscatterers. These e�ects were explored

initially for the spectrum of the cobalt adlayer onPt(111) in 0.1 M NaOH at an applied potential ofÿ0.75 V since it exhibited the best resolution between

the ®rst two shells in the RDF (Fig. 7D). In this case,the contribution from the second shell was isolatedand inverse Fourier transformed to obtain its contri-

bution to the EXAFS function. As a ®rst approxi-mation, a ®t was calculated using only Co±Co phase-

shifts and amplitudes. As can be seen in Fig. 8 (dottedline), this model provides a good ®t up to 9 AÊ ÿ1.Although the peak at around 2.5 AÊ in the RDF would

appear to be a single peak, it may contain contri-butions from two di�erent shells, provided that the dis-tances of these shells are quite similar, which could

likely be the case in this study. When an adatom (es-pecially if it is smaller than platinum as in the present

case) is deposited onto a Pt(111) substrate, it tends tooccupy the three-fold hollow sites since they representthe most energetically favorable position, as is the case

for Cu UPD on Pt(111) [22] and Au(111)electrodes [24]. Owing to the similar distances expected

for Co±Co and Co±Pt, their contributions to the RDFmay appear unresolved. Using Co±Co and Co Pt con-tributions a second ®t was obtained. This second

model (Fig. 8, dashed line) provides a better ®t to theoriginal function over the entire range giving Co±Coand Co±Pt distances of 2.86 and 2.84 AÊ , respectively.

However, based on our data, we are clearly unable toresolve such a di�erence.

Fig. 7. Radial distribution function (RDF) (uncorrected for

phase shifts) for (A) Co and (B) CoO and for cobalt adlayers

on Pt(111) electrodes in (C) 0.1 M H2SO4 at 0.00 V, (D) 0.1

M NaOH at ÿ0.75 and (E) 0.1 M NaOH at ÿ0.06 V.


In these studies, we employed a s-polarization geo-

metry, where the plane of polarization is parallel to the

electrode surface, so that it is relatively insensitive to

bonds perpendicular to the surface. However, there

will still be some interactions with the platinum sub-

strate since in the three-fold hollow sites there is still

an in-plane projection that would give rise to Co±Pt

scattering.

Since both peaks appearing in the RDF at distances

lower than 3 AÊ overlap, a complete isolation of the

contributions giving rise to them was not possible. To

overcome this problem, the following scheme was used

to obtain the distances (R), number of nearest neigh-

bors (NNN), variations in Debye±Waller factors (Ds 2)

and di�erences in energy origin (DE0):

1. The contribution from the second shell was Fourier

back ®ltered and the EXAFS function was ®tted

using Co±Co and Co±Pt correlations as described

above.

2. The ®rst and second shells were then back ®ltered

together. In the ®tting of the EXAFS function with

Co, Pt and O near neighbors, the values obtained

for the Co±Co and Co±Pt shells obtained in step 1

were used as initial input values. In the ®tting, the

NNN and DE0 for Co±Co and Co±Pt were ®xed,

and R and Ds 2 were allowed to vary.

Figs. 9±11 show the raw, Fourier ®ltered and ®tted

EXAFS functions obtained for the cobalt adlayers on

Pt(111) electrodes. In 0.1 M NaOH (Figs. 9 and 10),

the model provides a very good ®t to the EXAFS func-

Fig. 8. EXAFS function generated by inverse Fourier trans-

forming of the second peak in the RDF for a cobalt adlayer

on a Pt(111) electrode in 0.1 M NaOH at ÿ0.75 V (solid line)

compared with the ®ts obtained using only Co (dotted line),

and Co and Pt contributions (dashed line).Fig. 9. (A) Raw EXAFS function (dashed line) compared

with the inverse transform of the RDF with a Fourier ®lter

window between 1 and 3 AÊ (full line) for a cobalt adlayer on

a Pt(111) electrode in 0.1 M NaOH at ÿ0.75 V. (B)

Comparison between the Fourier ®ltered function (full line)

and the ®t to the EXAFS function obtained using Co, Pt and

O contributions (dashed line).

Fig. 10. Same as Fig. 9 for a cobalt adlayer on a Pt(111) elec-

trode in 0.1 M NaOH at ÿ0.06 V.


tion obtained. However, in the case of the cobalt

adlayers in 0.1 M H2SO4 (Fig. 11) the ®t was clearly

not as good. In 0.1 M H2SO4 the X-ray signals

obtained were typically less intense and noisier than

that in 0.1 M NaOH. The weighing scheme ampli®ed

the noise at high k values, making the EXAFS func-

tion less reliable in this region.

The values of R, NNN, Ds 2 and DE0 obtained from

the ®ts are given in Table 1. The values obtained for

0.1 M NaOH are in agreement with an oxidized cobalt

layer occupying three-fold hollow sites on the Pt(111)

electrode with the cobalt adatoms bonded to oxygen

atoms, which likely correspond to OH groups. The

change in electrode potential from ÿ0.75 to ÿ0.06 V

resulted in an increase in the number of OH groups

from 6 to 8.5. According to the electrochemical results,Eq. (2) is expected to take place upon changing the po-tential form ÿ0.75 to ÿ0.06 V. The ratio between the

OH groups for both species is 2:3, very close to thevalue obtained with from EXAFS data (2:2.8). Itshould be mentioned that whereas the determination

by EXAFS of absolute values of NNN typically hasan error of 220%, relative measurements (as donehere) are more reliable.

3.3. CTR measurements

Specular CTR measurements can provide infor-mation about the adlayer structure in a direction nor-mal to the Pt(111) surface. The parameters obtained

Fig. 11. Same as Fig. 9 for a cobalt adlayer on a Pt(111) elec-

trode in 0.1 M H2SO4 at 0.00 V.

Table 1

Bond lengths (R), e�ective coordination number (NNN), variations in Debye±Waller factor (Ds 2) and energy variation (DE0)

obtained from EXAFS. R and NNN values have a 0.03 AÊ and a 20% error, respectively

Solution Bond R (AÊ ) NNN Ds 2 (AÊ 2) DE0 (eV)

0.1 M NaOH, E=ÿ 0.75 V Co±Co 2.85 5.2 ÿ1.29 � 10ÿ4 ÿ5.3Co±Pt 2.83 2.5 9.26 � 10ÿ4 ÿ4.4Co±O 1.93 6.0 1.70 � 10ÿ3 ÿ4.1

0.1 M NaOH, E=ÿ 0.06 V Co±Co 2.83 5.0 ÿ8.44 � 10ÿ4 ÿ8.0Co±Pt 2.77 2.9 ÿ1.23 � 10ÿ3 ÿ6.5Co±O 1.92 8.5 5.84 � 10ÿ3 ÿ4.9

0.1 M H2SO4, E= 0.00 V Co±Co 2.96 4.3 6.91 � 10ÿ3 ÿ2.5Co±Pt 2.83 2.1 ÿ4.98 � 10ÿ3 6.9

Co±O 1.95 7.2 3.32 � 10ÿ3 ÿ6.5

Fig. 12. CTR for a cobalt adlayer on a Pt(111) electrode in

0.1 M H2SO4 at 0.00 V (solid circles). The dashed line rep-

resents the calculated pro®le for an ideally terminated Pt(111)

surface and the full line corresponds to the best ®t obtained

(see text for details).


from ®ts to the CTR pro®le are the coverage or den-sity rm (de®ned as the atomic ratio of the species in

the mth layer to that of Pt atoms in a bulk Pt(111)layer), the atomic root-mean-square (rms) displacementsm, and the interlayer distance dm (from the mth layer

to the (mÿ 1)th layer). Fig. 12 shows the CTRmeasurement for a cobalt adlayer on a Pt(111) elec-trode in 0.1 M H2SO4 where the solid circles are the

experimental data and the solid line is the best ®t. Thedashed line represents the calculated pro®le for an ide-ally terminated Pt(111) surface. As can be seen, the ex-

perimental data deviates signi®cantly from the idealcurve, showing deeper valleys which shift asymmetri-cally. This is a clear indication of the presence of over-layers on the electrode surface since it cannot be

attributed solely to the root-mean-square displace-ments of the surface atoms. Taking into account theresults obtained from the EXAFS measurements, a

two-layer model was used for ®tting the experimentaldata. It consisted of a cobalt layer in contact with theplatinum surface with a bisulfate overlayer on top of

the cobalt (see Section 4). Based on this model, a dis-tance of 2.220.1 AÊ between the cobalt layer and thesurface was obtained. In addition, a coverage of the

cobalt adlayer of 0.4020.06 was also determined.

4. Discussion

4.1. Co adlayers on Pt(111) electrodes in 0.1 M NAOH

In general, for experiments carried out in 0.1 MNaOH, the EXAFS results are in good agreement withthe electrochemical ones. From the electrochemicalmeasurements, three di�erent cobalt species were pro-

posed over three di�erent potential ranges: Co0 at po-tentials below ÿ1.0 V, Co(II) between ÿ1.0 and ÿ0.355V and Co(III) above ÿ0.355 V. Although the XANES

data did not allow for an unambiguous distinctionbetween Co(II) and Co(III), EXAFS data were consist-ent with these assignments. In essence, the normalized

ratio of the number of oxygen nearest neighbors perCo for the adlayers at the two di�erent potentials (seeTable 1) was 2:3, consistent with the change in the oxi-dation state as well as with the electrochemical data.

Due to the insolubility of Co(II) and Co(III) speciesin this medium [23], the adlayer is likely composed ofhydroxide type species. However, unlike the adsorption

of Co(II) species onto ZnO, and ZnS [25, 26], no evi-dence of the formation of a multilayer Co(OH)2 phasewas observed. The presence of a cobalt hydroxide mul-

tilayer would have given rise to an additional Co±Cocontribution to the EXFAS spectra at ca. 3.1 AÊ , a

typical distance value for Co±Co in Co(OH)2.However, no contribution was found in this region,ruling out the presence of cobalt multilayers.

Moreover, the CTR pro®le would be signi®cantlychanged over that observed. In addition, due to thenature of the irreversible adsorption process, depo-

sition is restricted to the adsorption of only a singlecobalt monolayer.The Co±Co distances obtained are slightly longer

than the Pt±Pt distance in a Pt(111) surface (2.78 AÊ ).However, the cobalt adlayer can be considered as a(1 � 1) layer deposited on the Pt(111) surface. The(1 � 1) structure is in good agreement with the cobalt

coverage (YCo11) determined from the coulometriccharge associated with the voltammetric wave atÿ0.355 V in 0.1 M NaOH. These results are also in

agreement with the growth mode of vapor depositedcobalt ®lms on Pt(111) surfaces [5], which show aquasi-epitaxial growth up to coverages of 3 ML.

However, it should also be mentioned that in vapordeposition, the cobalt is deposited as Co0 whereas inthe present case and at the potentials studied, the

cobalt exists in oxidized forms.With regards to the cobalt adsorption site, we

believe that the cobalt species are deposited on thethree-fold hollow sites of the Pt(111) surface. First of

all, adsorption on a-top sites can be ruled out. Due tothe fact that a s-polarization geometry was employedin the EXAFS measurements we are essentially insensi-

tive to scattering contributions perpendicular to theelectrode surface. However, the fact that Co±Pt contri-butions are clearly detected indicates that the Co±Pt

bond cannot be perpendicular to the electrode surfacethus ruling out the possibility of adsorption on a-topsites. Of the remaining possibilities (bridge and three-fold sites), the CTR measurements were consistent

with deposition on the three-fold hollow sites, but noton bridge sites of the Pt(111) surface (vide infra).

4.2. Co adlayers on Pt(111) electrodes in 0.1 M H2SO4

The EXAFS results suggest that the oxidation stateof the cobalt adlayer at an applied potential of 0.0 Vin this medium is Co(II). In addition, there appear tobe some oxygen containing species (co)adsorbed in the

adlayer. In the supporting electrolyte solution, fourdi�erent oxygen-containing species can be found:water, hydroxyl anions and sulfate and bisulfate

anions1. The electrochemical results suggest that(bi)sulfate anions are required for the irreversibleadsorption to take place, since it only occurs when

CoSO4 solutions are employed. Therefore, (bi)sulfateanions clearly play a key role in the adsorption processand one would presume to be present in the cobalt

1 Since the techniques employed cannot distinguish between

sulfate and bisulfate anions, the term (bi)sulfate is used to

refer to both anions indistinctly.


adlayer. However, the presence of other oxygen con-

taining species, particularly water, cannot be ruled out.In-situ studies of Pt(111) and Au(111) electrodes havedemonstrated that water always coexists with (bi)sul-

fate adlayers [27±30]. On the other hand, the Co±Odistance found (1.95 AÊ ) agrees well with the distancefound for the adsorption of Co(II) species onto

ZnO [25, 26].The presence of (bi)sulfate in the cobalt adlayer has

an in¯uence in its structure. As mentioned in Section3, the normalized edge intensity for the cobalt adlayersin sulfuric acid solutions was signi®cantly lower (ap-

proximately 50%) than that obtained in NaOH sol-utions. Since the normalized edge intensity is

proportional to the number of absorbing species, thecoverage in sulfuric acid must be about 50% of thatobtained in NaOH. In the adlayer, this lower coverage

is re¯ected in a longer Co±Co distance and in a lowernumber of cobalt nearest neighbors. Using the R andNNN values for the Co±Co contribution derived from

an analysis of the EXAFS data, an approximate cover-age of 0.6 can be calculated. This represents 60% of

the coverage obtained in NaOH (0.95), in agreementwith the lower intensity of the edge. The di�erence incoverage and boding distances might re¯ect the di�er-

ence in size of (bi)sulfate relative to hydroxide.The Co±Co distance of 2.96 AÊ obtained indicates

that the cobalt adlayer is likely incommensurate with

the Pt(111) substrate, so that the all the cobalt ada-toms are not deposited in equivalent positions on the

Pt(111) surface, i.e., they may be deposited on three-fold hollow, bridged-bonded or at intermediate pos-itions. In the model used for analyzing the EXAFS

data, we have assumed that all cobalt adatoms aredeposited on three-fold hollow sites and this likely con-tributes to a worse ®t of the experimental data as was

pointed out earlier. However, a calculation of thetheoretical EXAFS function for such a case would

require knowledge of the exact positions of the cobaltadatoms on the Pt(111) surface.From the EXAFS data a Co±Pt distance of 2.83 AÊ

was obtained. The distance between the Pt(111) surfaceand the Co layer clearly depends on the adsorption

site. Assuming that the cobalt adatoms are depositedon three-fold hollow sites, forming a tetrahedron withthe three platinum atoms acting as the base, a Co±Pt

distance of 2.83 AÊ would correspond to a vertical dis-tance between the cobalt layer and the platinum sur-face of 2.31 AÊ . This value is within the experimental

error of the distance found in the CTR measurements(2.220.1 AÊ ). Thus, one can deduce that cobalt ada-

toms must be deposited on or near three-fold hollowsites. Although neither of the cobalt coveragesobtained by EXAFS and CTR can be considered accu-

rate, there is a clear indication that it must be around0.5.

As in the case of the cobalt adlayers in NaOH sol-utions, cobalt adsorption is restricted to a single layer,

as revealed by the absence of any contribution in theRDF around 3 AÊ , the typical distance for Co±Co incobalt hydroxides.

5. Conclusions

Co adlayers at coverages of about 0.6 and 1 ML canbe deposited by irreversible adsorption from 0.1 M

H2SO4/CoSO4 and 0.1 M NaOH/CoCl2, respectively.In 0.1 M H2SO4, an irreversible oxidation with a peakpotential value of +1.00 V (vs. Ag/AgCl) correspond-

ing to the oxidation of the cobalt adlayer as well asthe Pt substrate was observed. There was also a re-duction wave at ÿ0.29 V which corresponded to the

dissolution of the cobalt adlayer.In 0.1 M NaOH two redox processes were observed

at formal potential values of ÿ0.355 and ÿ1.00 V. The®rst, at ÿ0.355 V, exhibited signi®cant kinetic limi-

tations and was ascribed to a Co(OH)3/Co(OH)2 redoxprocess. The one at ÿ1.00 V was ascribed to the gener-ation of Co0 which, unlike in sulfuric acid, did not des-

orb. SEXAFS measurements in sulfuric acid (at anapplied potential of 0.0 V) and in 0.1 M NaOH (atapplied potential values of ÿ0.06 and ÿ0.75 V) were

consistent with the presence of an oxidized cobalt layerwith Co, O and Pt near neighbors. CTR data werealso consistent with the presence of an oxidized cobaltbi-layer structure.

In 0.1 M NaOH the deposited cobalt at a coverageof ca. 1 ML appears to form a 1 � 1 structure with thecobalt atoms occupying three-fold hollow sites.

Changes in the number of oxygen near neighbors withapplied potential were consistent with electrochemicaldata. In 0.1 M H2SO4 (at an applied potential of 0.0

V) and at a cobalt coverage of about 0.6 ML, thedeposited cobalt appears to form an incommensuratemonolayer with an expanded lattice (Co±Co distance

of 2.96 AÊ ) likely involving co-adsorbed (bi)sulfate.

Acknowledgements

This work was supported by the O�ce of NavalResearch and the National Science Foundation.

EXAFS/XANES measurements were performed at theCornell High Energy Synchrotron Source, which issupported by the National Science Foundation. CTR

experiments were performed at Exxon beamline X10-Bat the National Synchrotron Light Source,Brookhaven National Laboratory, which is supported

by the U.S. Department of Energy, Division ofMaterials Science and Division of Chemical Sciences(DOE contract No. DE-AC02-76CH0016). E.H.


acknowledges support by a fellowship from theMinistry of Education and Science of Spain.

References

[1] P.F. Garcia, A.D. Mainhaldt, A. Suna, Appl. Phys. Lett.

47 (1985) 178.

[2] C. Boeglin, A. Barbier, B. CarrieÁ re, R. Cousandier, J.P.

Deville, C. Speisser, Surf. Sci. 251/252 (1991) 602.

[3] C. Boeglin, A. Barbier, F. Scheurer, B. CarrieÁ re, J.P.

Deville, J. Magn. Magn. Mater. 93 (1991) 31.

[4] C. Boeglin, B. CarrieÁ re, J.P. Deville, F. Scheurer, C.

Guillot, N. Barret, Phys. Rev. B 45 (1992) 3834.

[5] P. GruÈ tter, U.T. DuÈ rig, Phys. Rev. B 49 (1994) 2021.

[6] W.K. Behl, J.E. Toni, J. Electroanal. Chem. 31 (1971)

63.

[7] H. GoÂ mez Meier, J.R. Vilche, A.J. ArvõÂ a, J. Electroanal.

Chem. 134 (1982) 251.

[8] H. GoÂ mez Meier, J.R. Vilche, A.J. ArvõÂ a, J. Electroanal.

Chem. 138 (1982) 367.

[9] M.E. Folquer, J.R. Vilche, A.J. ArvõÂ a, J. Electroanal.

Chem. 172 (1982) 235.

[10] S. Ando, T. Suzuki, K. Itaya, J. Electroanal. Chem. 431

(1997) 277.

[11] J. Clavilier, J.M. Feliu, A. Aldaz, J. Electroanal. Chem.

243 (1988) 419.

[12] J.M. Feliu, A. FernaÂ ndez-Vega, A. Aldaz, J. Clavilier, J.

Electroanal. Chem. 256 (1988) 149.

[13] B.K. Teo, EXAFS: Basic Principles and Analysis,

Springer-Verlag, Berlin, 1986.

[14] J.J. Rehr, J. Mustre de Leon, S.I. Zabisnky, R.C. Albers,

J. Am. Chem. Soc. 113 (1991) 5135.

[15] J. Mustre de Leon, J.J. Rehr, S.I. Zabisnky, R.C. Albers,

Phys. Rev. B 44 (1991) 4146.

[16] J. Wang, B.M. Ocko, A.J. Davenport, H.S. Isaacs, Phys.

Rev. B 46 (1992) 10321.

[17] J. Li, E. Herrero, H.D. AbrunÄ a, Colloids Surf. A 134

(1998) 113.

[18] J. Clavilier, D. Armand, S.G. Sun, M. Petit, J.

Electroanal. Chem. 205 (1986) 267.

[19] H.S. Yee, H.D. AbrunÄ a, Langmuir 9 (1993) 2460.

[20] H.S. Yee, H.D. AbrunÄ a, J. Phys. Chem. 97 (1993) 6278.

[21] J. Li, H.D. AbrunÄ a, J. Phys. Chem. B 101 (1997) 244.

[22] J.H. White, H.D. AbrunÄ a, J. Electroanal. Chem. 330

(1991) 521.

[23] M. Pourbaix, Atlas of Electrochemical Equilibrium in

Aqueous Solution, National Association of Corrosion

Engineers, Houston, TX, 1974, pp. 322±329.

[24] O.R. Melroy, G.L. Borges, L. Blum, H.D. AbrunÄ a, M.J.

Albarelli, M.G. Samant, M. McMillan, J.H. White, J.G.

Gordon, Langmuir 4 (1988) 728.

[25] P. Persson, G.A. Parks, G.E. Brown, Physica B 208

(1995) 453.

[26] P. Persson, G.A. Parks, G.E. Brown, Langmuir 11

(1995) 3782.

[27] O.M. Magnussen, J. HageboÈ ck, J. Hotlos, R.J. Behm,

Faraday Discuss. Chem. Soc. 94 (1992) 329.

[28] G.J. Edens, X. Gao, M.J. Weaver, J. Electroanal. Chem.

375 (1994) 357.

[29] T. Nishizawa, T. Nakada, Y. Kinoshita, S. Miyashita,

G. Sazaki, Surf. Sci. 367 (1996) L73.

[30] P.W. Faguy, N.S. Marinkovic, R.R. Adzic, Langmuir 12

(1996) 243.


Documents

Electrochemical, in-situ surface EXAFS and CTR studies of Co monolayers irreversibly adsorbed onto Pt(111)