Attenuated Total Reflection Surface-Enhanced InfraredAbsorption Spectroscopy at a Cobalt Electrode

SHENG-JUAN HUO, JIN-YI WANG, DA-LIN SUN, and WEN-BIN CAI*Department of Material Science, Fudan University, Shanghai 200433, China (S.-J.H. D.-L.S.); and Shanghai Key Laboratory for Molecular

Catalysis and Innovative Materials and Department of Chemistry, Fudan University, Shanghai 200433, China (S.-J.H., J.-Y.W., W.-B.C.)

In situ surface-enhanced infrared absorption spectroscopy (SEIRAS) in

attenuated total reflection (ATR) configuration has been extended to a Co

electrode fabricated by potentiostatic deposition of a 50-nm-thick Co

overlayer onto a Au underlayer chemically preformed on the reflecting

plane of an ATR Si hemi-cylindrical prism. The as-prepared Co-on-Au

film was characterized with atomic force microscopy (AFM) and X-ray

photoelectron spectroscopy (XPS). The AFM images of the films before

and after Co coating revealed island structures facilitating the SEIRA

effect with Co nanoparticles much smaller than the underlying Au ones.

The XPS spectrum did not contain any characteristic peaks related to Au,

suggestive of a virtually pinhole-free nature of the Co overlayer. The

voltammetric response of the as-prepared films in phosphate buffer

solution (PBS, pH 6.9) was characteristic of a polycrystalline bulk Co

electrode. Normally directed unipolar bands were found for surface probe

CO molecules on Co surfaces in the PBS with their major band (COL)

intensity being one order of magnitude higher than that obtained with

conventional IR reflection–absorption spectroscopy (IRRAS). By taking

advantage of the higher detection sensitivity, the bands for linearly

bonded CO (COL) at 1965–2005 cm�1 and the multi-bonded (COM) band

at 1845–1875 cm�1 were clearly detected with their Stark tuning rates

being 59 and 63 cm�1�V�1, respectively, which would be otherwise

unobtainable with the conventional IRRAS in the neutral solution.

Index Headings: Surface-enhanced infrared absorption spectroscopy;

SEIRAS; Attenuated total reflection; ATR; Cobalt electrode; Carbon

monoxide; electrodeposition.

INTRODUCTION

Cobalt is an important additive of metallic functionalmaterials in electrocatalysis and anti-corrosion. Specifically,Pt–Co alloys are found to be CO-tolerant anode catalysts andmethanol-tolerant or oxygen-reduction active cathode catalystsin proton exchange membrane fuel cells.1–3 It is thusinteresting to exploit high-sensitivity surface-enhanced infraredspectroscopy (SEIRAS) to elucidate the adsorption andreaction on Co electrodes, before more complex alloyelectrodes can be tackled. External IR reflection–absorptionspectroscopy (IRRAS) on Co electrodes was reported byCuesta’s and Sun’s groups,4,5 in which the infrared beam has topass through 103–104 layers of electrolytes before striking theelectrode surfaces. Cuesta et al.4 pioneered the study of the pHeffect on the CO adsorption at the Co electrode; the rather weakbipolar bands (around 10�3 in relative reflectance units)complicated the interpretation of IR spectroscopic data.Specifically, in a neutral solution high Co electro-oxidationseverely deformed the spectra, leading to difficulty in reliablyobtaining the Stark tuning rate. Sun et al.5 reported abnormalIR effects (AIREs) for CO at Co films electrodeposited onglassy carbon in basic solution, with a maximum absolute band

intensity of approximately 2 3 10�3. Despite the improvedsensitivity due to the Co nanostructured film, the explanation ofthe spectral results was still controversial. In addition, they didnot provide spectra of CO adsorption on Co electrodes in aneutral solution.

Surface-enhanced infrared absorption spectroscopy (SEI-RAS) in attenuated total reflection (ATR) configuration isregarded as a powerful analytical tool for characterizing surfaceadsorption and reactions at film electrodes owing to its highsurface sensitivity, simple surface selection rules, and unre-stricted mass transport.6,7 A special advantage of ATR-SEIRAS over IRRAS is that the former can provide thespectral features around 3600 cm�1 due to interfacial free waterwhile the latter can not.8 To our best knowledge, extension ofATR-SEIRAS to Co electrodes has not been attempted.

The SEIRA effect originates from the interactions of IRphotons with metal nanoparticles and adsorbed molecules.6,7

Growing thin metal films with the SEIRA effect and reasonableelectrochemical response is essential for the successfulapplication of electrochemical ATR-SEIRAS. Chemical depo-sition has been successfully applied for fabricating Au,9,10

Ag,11,12 Cu,13 Pt,14,15 Pd,16 and Ni–P17 film electrodes on Si.But not all the metals can be deposited simply with theelectroless method. Therefore, we proposed a more generaltwo-step wet process, which incorporates initial chemicaldeposition of a Au underfilm followed by electrodeposition ofa second metal overlayer,18 considering the facts that Aupossesses a very wide inert potential window in aqueoussolution and the thick overlayer yields a sufficient SEIRAeffect, while maintaining its pinhole-free nature. With thisstrategy, broader ATR-SEIRAS applications can be expectedon metals such as Pt, Pd, Ru, Rh,18 Ni,19 and Cd electrodes.20

In the present work, we aim to extend this strategy to explore insitu ATR-SEIRAS at Co electrodes by using CO as the probemolecule in a neutral electrolyte solution. At least one order ofmagnitude higher band intensity was enabled for CO on Cofilm electrodes with ATR-SEIRAS as compared to previousdata with IRRAS or AIREs in the literature.

EXPERIMENTAL

A non-doped Si hemicylindrical prism (PASTEC, Japan,Osaka) was used as the ATR-IR window. A Au underlayer waschemically deposited on the reflecting plane of the Si prismaccording to previous reports,9,18,19 followed by electro-deposition of the Co overlayer in a bath containing 0.01 MCoSO4þ0.1 mM H2SO4þ0.01 M H3BO3 at�1.0 V (vs. SCE)for 600 s in a custom-made spectro-electrochemical cell.Afterwards, the entire cell was thoroughly rinsed with copiousamounts of Milli-Q water (.18 MX�cm), followed byimmediate injection of 0.1 M K2HPO4 þ 0.1 M KH2PO4

(PBS, pH 6.9). The potential was held at�1.0 V for 10 min in

Received 26 May 2009; accepted 16 July 2009.* Author to whom correspondence should be sent. E-mail: wbcai@fudan.edu.cn.

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� 2009 Society for Applied Spectroscopy

order to reduce the native oxides on the Co surfaces. Thegeometrical surface area of the Co working electrode was 1.33cm2. All electrolytes were deaerated with high-purity Ar gasprior to spectroelectrochemical measurements. A CHI 660Belectrochemistry workstation (CH Instruments, Shanghai) wasemployed for potential/current control and to record cyclicvoltammograms. All electrode potentials are cited withreference to the saturated calomel electrode (SCE).

Inductively coupled plasma atomic emission spectroscopy(Varian Inc.) was used to determine the concentration ofdissolved Au and Co ions from the films on Si with hot aqua-regia so that the mass-equivalent thicknesses for Au and Cocan be estimated by assuming the same densities as their bulkmaterials.

Atomic force microscopy (AFM) images of Au nanofilmchemically deposited on a Si wafer before and afterelectrodeposition of a Co overlayer were acquired with thetapping mode under ambient conditions with a Pico-SPM (MI).Si cantilevers having spring constants between 1.2 and 5.5 Nm�1 were used at resonance frequencies between 60 and 90kHz.

The X-ray photoelectron (XPS) spectra of the as-depositedsamples were recorded on a Perkin Elmer PHI-5000C ESCAsystem equipped with a dual X-ray source, of which the Al Karadiation (hm ¼ 1486.6 eV) anode and a hemispherical energyanalyzer were used. The background pressure during dataacquisition was maintained at 10�9 Pa. Measurements wereperformed at a pass energy of 93.90 eV. The survey XPSspectrum (0;1200 eV) and the narrow spectra of Au withmuch higher resolution were both recorded.

The spectroelectrochemical cell with internal reflectionconfiguration (Kretschmann ATR) was described previous-ly.6,18 Briefly, unpolarized infrared radiation from a ceramicsource was focused at the electrode/electrolyte interface bypassing it through the silicon prism. The incident angle was 708

from the surface normal. A Varian 3100 Fourier transform

infrared (FT-IR) spectrometer equipped with a liquid-nitrogen-cooled mercury cadmium telluride (MCT) detector was usedfor the spectral acquisition at a resolution of 4 cm�1. Sixty-four(64) interferograms were co-added to each single-beamspectrum in the multi-step mode with CO used as the probemolecule for ATR-SEIRAS at Co electrodes. The absorbance isdefined as –log(R/R0), where R and R0 represent the reflectedIR intensities (single-beam spectra) corresponding to thesample and reference single-beam spectra, respectively. COpre-dosing was carried out by keeping the electrode potential at�0.8 V for 30 min in a CO-saturated PBS solution, followed bypurging the solution with high-purity Ar for 1 h to remove thedissolved CO.

RESULTS AND DISCUSSION

Film Characterization. The Au underlayer on Si exhibiteda golden color as usual, and a shiny silver appearance wasfound after the electrodeposition of a thick Co overlayer. Basedon the inductively coupled plasmas-atomic emission spectros-copy (ICP-AES) measurements, the equivalent mass thick-nesses for Co and Au films are located at 45–50 and 60–70 nm,respectively. Shown in Fig. 1 are AFM images of the film on aSi wafer before and after Co coating. The presumablyellipsoidal Au nanoparticles are interconnected and partlyaggregated with an average lateral size of approximately 120nm (Fig. 1a). Quite different from the Au underfilm, the Cooverlayer consists of densely packed nanoparticles with sizesdown to approximately 50 nm (Fig. 1b). The nanostructuredfilms facilitate the interaction of IR radiation with the metal andadsorbed molecules, resulting in enhanced absorption for theallowed vibrations of the adsorbate. In addition, the denselypacked Co nanoparticles ensure a virtually pinhole-free natureof the Co overlayer, as further confirmed by the followingmeasurements (vide infra).

Shown in Fig. 2 are XPS spectra for the Co-coated Au filmon a Si wafer substrate. The inserted Co 2p3/2 bands with

FIG. 1. AFM images for Au film (a) before and (b) after coating with Co on a Si wafer. Refer to the text for the details.

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binding energies of 785.8 eV and 781.6 eV can be ascribed totwo chemical states of Co species, a metallic state and anoxidized state (due to partial oxidation without potentialcontrol and sample transfer), respectively. Peaks located at60.5, 101.7, 482.0, 605.4, 796.9, and 928.6 eV correspond toCo 3p, Co 3s, Co KLL, Co KLL, Co 2p1/2, and Co 2s states,respectively. Except for the regular impurities C (C1s ¼284.6eV) and O (O1s ¼ 532.3 eV), no other elements weredetected on the survey spectra. Most importantly, no Au peakswere observed in the left inset, indicating that the Co-coatingoverlayer was virtually pinhole free. This feature is essential forextending electrochemical ATR-SEIRAS to Co electrodes.

Electrochemical Measurement. The cyclic voltammogramfor the as-deposited Co electrode in an Ar-saturated PBSsolution showed an anodic peak at �0.26 V with a charge of4.2 mC cm�2 (dotted trace), the onset potential of which wasapproximately�0.7 V. In contrast, in a CO-saturated PBS, theonset potential shifted positively to approximately�0.3 V andthe anodic peak to 0.13 V with a charge of 7.7 mC cm�2 (solidtrace). The suppression of hydrogen evolution and Co electro-oxidation as seen in Fig. 3 suggests the adsorption of CO at theCo electrode. The increase in the electro-oxidation peak chargemay be attributed to the oxidation of both adsorbed anddissolved CO superposed with the oxidation of the Coelectrode itself. Degassing CO with Ar bubbling hardlychanged the voltammetric feature in the forward scan of thefirst cycle, indicative of strong CO binding to Co surfaces. Itshould be pointed out that the voltammetric responses for Cofilm electrodes are rather close to those for bulk Co electrodes,4

rendering the following SEIRAS measurement meaningful.In situ Attenuated Total Reflection Surface-Enhanced

Infrared Absorption Spectroscopy. For the whole potentialrange under investigation, we are unable to find a potential atwhich CO can be totally oxidized from Co electrodes. Figure 4shows the difference ATR-SEIRAS spectrum of CO/Co fortwo adsorption potentials, by defining DR/R as R2 � R1/R1,where R1 and R2 represent the reflected single-beam spectraacquired at�0.2 V and�0.8 V, respectively. The bipolar bandsare simply due to the potential induced frequency shift for COadsorbed on the Co electrode, rather than to the nanostructuredinduced ‘‘Fano-like’’ effect.5 By comparing the above spectrumwith that reported previously (Fig. 7 in Ref. 5), we can see notonly greatly enhanced CO band intensities (at approximately2000 and 1870 cm�1), but also previously unseen bands atapproximately 3600 cm�1 due to co-adsorbed isolated water.

In order to obtain ‘‘absolute’’ spectra of adsorbed CO speciesand their Stark tuning rates, the single-beam spectrum collectedat �0.8 V in the neat PBS solution before CO bubbling was

FIG. 2. Survey XPS spectrum of an electrodeposited Co overlayer (50 nm) on a Au nanofilm (70 nm) chemically deposited on a Si substrate and (insets) the narrowspectra of Au 4f and Co 2p3/2.

FIG. 3. Cyclic voltammograms at a scan rate of 50 mV s�1 for a Co filmelectrode in PBS (pH 6.9) without (dotted curve) and with (solid curve)dissolved CO and for a CO-pre-dosed Co film electrode in PBS after purgingCO with Ar for 1 h (dashed curve).

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used as the reference spectrum, and the single-beam spectra atall the potentials after pre-dosing CO served as the samplespectra. As a result, Fig. 5a shows the potential-dependentATR-SEIRA spectra for a CO-pre-dosed Co electrode in PBS(pH 6.9). Unlike the pervious report, normally directed andunipolar CO absorption bands with significant enhancementwere observed without the emergence of bipolar or abnormallyinverted ones. The major band at 1965–2005 cm�1 can beassigned to the linearly adsorbed CO (COL) and the minor bandat 1845–1875 cm�1 to the multi-bonded CO (COM) on Cosurfaces.4,21 As compared to the case of CO adsorption on Ni

electrode in the same PBS, the ratio of COL versus COM ismuch higher on the Co electrode;19 this may be explained bystronger back donation of 3d electrons from Ni to CO 2p*orbitals, considering that the valence electron structures of Coand Ni are 4s23d7 and 4s23d8, respectively. It is well knownthat for a given metal, a more negative potential favors the backdonation of electrons and also benefits the multi-bonded COadsorption.22,23 Shown in Fig. 5b are the corresponding plotsof the center frequencies and band intensities as a function ofpotential. The potential-dependent shifts in frequency arecaused by the weakening of the back-donation of Co d-electrons into the CO 2p* orbital (a change of C–O bondstrength). The linear parts from �1.0 to �0.4 V yield Starktuning rates of 59 and 63 cm�1�V�1 for COL and COM,respectively. It should be mentioned that these data were notavailable for CO at Co electrodes in a neutral electrolyte, due tothe lower signal-to-noise ratio and bipolar band shape in theprevious IRRAS measurement. The integrated intensities ofCOL and COM peaks were largely unchanged at potentialsranging from �1.0 V to �0.6 V, and the COM band intensitydecreased significantly accompanied by the increase of theCOL band at potentials from �0.6 to �0.3 V, suggesting thepartial conversion of COM to COL, similar to the case of Nielectrodes.19 At higher potentials coincident with the emer-gence of the anodic current in voltammetry, both the intensitiesand frequencies for these two bands are nearly invariant. Thiscan be attributed to the adsorption of CO on passivated Cosurfaces.4 The passivation of the Co electrode at higherpotentials in a neutral solution caused a substantial ohmic dropacross the interface, and thus surface CO moieties are notsensitive to the increased potential. For this reason, the bipolarbands, as shown in Fig. 4, can be understood.

The ATR-SEIRAS spectra of the CO adsorption on Co-coated and bare Au electrode surfaces in PBS solution at�0.3 V are shown in Fig. 6. It is necessary to point out that the

FIG. 5. (a) Potential-dependent SEIRA spectra for a CO-pre-dosed Co electrode in PBS (pH 6.9). The reference spectra were taken at the corresponding potentialsindicated in neat PBS before introducing CO (the reference spectra can be referred to the text for details). (b) Plots of the stretching frequencies (upper panel) andintegrated intensities (lower panel) of COL and COM bands versus the electrode potential.

FIG. 4. Difference ATR-SEIRAS spectrum by taking the single-beam spectraacquired at �0.2 and �0.8 V for the CO-pre-dosed Co electrode in PBS (pH6.9) as the sample and reference spectra, respectively.

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CO bands (Figs. 6a and 6b) can not be ascribed to COadsorption on (if any) exposed sites of the underlying Au. Thedominant COL band on bare Au electrode is located at 2090–2100 cm�1 at potentials�0.5 to 0 V in the neutral solution (Fig.6c) and can be readily removed by Ar purging due to the weakadsorption of CO on Au (Fig. 6d). The strongest band intensityappeared at �0.3 V for CO at bare Au electrode (Fig. 6c).Furthermore, the interaction of CO with Au is weak: CO cannot be detected when it is purged from solution (Fig. 6d). Theabove IR result is in agreement with XPS measurements.

Estimation of the enhancement factor of the Co electrode isessential for its future ATR-SEIRAS applications. Themaximum peak intensity of the COL band is approximately0.013 in absorbance units (or 0.03 in relative reflectivity) in thepresent study, which is at least 30 times higher than thatobtained on a corresponding polycrystalline bulk electrode inexternal reflection mode by using polarized IR radiation (theorder of magnitude is about 10�3).4 According to our previousreport,12,19 the enhancement factor of the Co nanofilm is atleast 20 by taking appropriate calibration of the effects of thesurface roughness factor, surface coverage, incident angles, andpolarization states of IR radiation.

Further Optical Considerations. We have comparedexperimentally the single-beam spectra obtained on coatedand uncoated Si prisms. As shown in Fig. 7, the IR beamintensity reflected from the Si prism coated with 70 nm Au isapproximately 70% of the one from the bare Si prism. With a40-nm-thick Co film deposited on Au, no significant reduction

in the intensity of the single spectrum was observed, and theratio remains at approximately 70% as compared to the one ona bare Si prism. This result is rather close to that obtained by anumerical simulation, which will be reported elsewhere.

The origin of SEIRA is primarily ascribed to the perturbationof the polarizability of metal particles by the adsorbedmolecules, i.e., the local electric fields enhanced via theexcitation of localized plasmon of small metal particles.6 It istrue that the evanescent field is very much reduced by the metallayers, but the vibrations of adsorbed molecules can still beobserved through the metal film, and hence the evanescent fieldis not necessary to penetrate deeply into the bulk solution.Actually, sufficient enhancement was also found on a 60-nm-thick Au plus 40-nm-thick Ni film,19 as well as on a 100-nm-thick Pt film.14

The band shape depends on various factors, such as thethickness of the film, the incident angle of the IR beam, thefilling factor in effective dielectric constant of the film, andthe substrate.24 Different band shapes and directions (thenormal, the Fano-type, and the abnormal) observed experi-mentally on nanoparticle films in external reflection modewere explained by proper application of a well-establishedeffective-medium theory. Such a relevant model may needdeveloping for the ATR system, which is beyond the scope ofthe current work.

CONCLUSION

The ATR-SEIRAS method has been successfully extendedto a Co electrode as prepared by electrodepositing a pinhole-free Co overlayer onto a Au underfilm chemically plated on Si.The voltammetric response for the as-obtained Co filmelectrode is very close to that of a bulk Co electrode. Thesurface IR enhancement factor for CO adsorbed was evaluatedto be above 20, with reference to the band intensity obtainedwith conventional IRRAS measurements. No emergence ofbipolar or abnormally inverted absorption bands due to the filmnanostructure was found. In addition, the Stark tuning effectsfor COL and COM on Co electrode in a neutral solution wereinitially determined to be 59 and 63 cm�1�V�1, respectively.

FIG. 6. ATR-SEIRAS spectra of CO adsorbed on Co-coated and -uncoated Auelectrodes in PBS solution (pH 6.9) at �0.3 V. (a) The Co electrode in CO-saturated PBS and (b) then with Ar purging; the single-beam spectrumcollected at�0.80 V in the neat PBS solution was used as the reference for COon Co. (c) The bare Au electrode in the CO-saturated PBS and (d) then with Arpurging. Single-beam spectrum taken at 0.4 V serves as the reference for CO onAu.

FIG. 7. Single-beam spectra recorded on a Si ATR prism (a) uncoated and (b)coated with a 70-nm-thick Au film, and (c) coated with 70-nm-thick Au and 50-nm-thick Co films in contact with the same PBS solution. The IR incident angleis set at 708.

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This work may promote more ATR-SEIRAS investigation ofthe adsorption and reaction on Co-based electrodes.

ACKNOWLEDGMENTS

This work is supported by NSFC (Nos. 20673027, 20833005, and20873031) and STCSM (No. 08JC1402000 and 08DZ2270500). Valuablecomments from Prof. Lei Xu of Fudan University and Prof. Masatoshi Osawaof Hokkaido University are greatly appreciated.

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