In-Situ Scanning Tunneling Microscopy of CarbonMonoxide Adsorbed on Au(111) Electrode

Chia-Haw Shue,† Liang-Yueh Ou Yang,§ Shueh-Lin Yau,*,‡ and Kingo Itaya*,‡,§

Department of Chemistry, National Central University, Chungli, Taiwan 320, CREST, JST,4-1-8 Kawaguchi, Saitama 332-0012, Japan, and Faculty of Engineering, Tohoku University,

6-6-04 Aoba, Sendai 980-8579, Japan

Received August 30, 2004. In Final Form: November 25, 2004

In-situ scanning tunneling microscopy (STM) coupled with cyclic voltammetry was used to examine theadsorption of carbon monoxide (CO) molecules on an ordered Au(111) electrode in 0.1 M HClO4. Molecularresolution STM revealed the formation of several commensurate CO adlattices, but the (9 × x3) structureeventually prevailed with time. The CO adlayer was completely electrooxidized to CO2 at 0.9 V versus RHEin CO-free 0.1 M HClO4, as indicated by a broad and irreversible anodic peak which appeared at thispotential in a positive potential sweep from 0.05 to 1.6 V. A maximal coverage of 0.3 was estimated forCO admolecules from the amount of charge involved in this feature. Real-time in-situ STM imaging alloweddirect visualization of the adsorption process of CO on Au(111) at 0.1 V, showing the lifting of (x3 × 22)reconstruction of Au(111) and the formation of ordered CO adlattices. The (9 × x3) structure observedin CO-saturated perchloric acid has a coverage of 0.28, which is approximately equal to that determinedfrom coulometry. Switching the potential from 0.1 to -0.1 V restored the reconstructed Au(111) with nochange in the (9 × x3)-CO adlattice. However, the reconstructed Au(111) featured a pairwise corrugationpattern with two nearest pairs separated by 74 ( 1 Å, corresponding to a 14% increase from the ideal valueof 65.6 Å known for the (x3×22) reconstruction. Molecular resolution STM further revealed that protrusionsresulting from CO admolecules in the (9 × x3) structure exhibited distinctly different corrugation heights,suggesting that the CO molecules resided at different sites on Au(111). This ordered structure predominatedin the potential range between 0.1 and 0.7 V; however, it was converted into new structures of (7 × x7)and (x43 × 2x13) on the unreconstructed Au(111) when the potential was held at 0.8 V for ca. 60 min.The coverage of CO adlayer decreased accordingly from 0.28 to 0.13 before it was completely removed fromthe Au(111) surface at more positive potentials.

IntroductionThe study of carbon monoxide (CO) adsorbed on gold

electrodes is of interest from the perspectives of industrialapplications and fundamental research. In particular, thediscovery of catalytic activity of gold nanoparticles towardCO oxidation at low temperature has triggered researchon the interaction of CO molecule with gold metal usingthe traditional surface science approach as well astheoretical modeling.1-4 Although CO molecule is notadsorbed on Au(111) and Au(332) surfaces in a vacuumat room temperature,5,6 raising the pressure of CO to theorder of mbar yields CO adsorption on the Au(111) andAu(110) surfaces, as indicated recently by using infraredspectroscopy and X-ray scattering techniques.7-10 STMhas also been used to gain an insight into the adsorption

of CO on gold surfaces, particularly Au(111) and Au(110).Results show that although CO adspecies produce sub-stantial changes in the structure and surface morphologyof these gold substrates, STM has failed to image COadsorbate directly.8,9 One can only conjecture that this isdue to the high diffusion rate of CO on gold substrates atroom temperature.

Apart from these studies of gas-solid interfaces, theelectrocatalytic property of gold electrodes toward COoxidation has been examined also by using electrochemicalmethods and surface vibrational spectroscopy.11-15 Thesuperior activity of gold over Pt, Cu, and Ag toward COoxidation in alkaline media was noted nearly 20 yearsago.11 This is followed by further electrochemical studiesemploying well-defined gold single-crystal electrodes,which indicated the involvement of water molecules andOH species in the electroxidation of CO in acidic andalkaline solutions, respectively.12-15 IR studies of CO onAu(111), Au(100), and Au(210) show that CO moleculesare mainly linearly-bonded to these electrodes.12-15 Inaddition, in-situ STM has been used to examine theadsorption of CO on gold electrodes.12 However, none ofthose studies has succeeded in imaging CO adsorbate insolution. Again, it is not clear why in-situ STM could not

* To whom correspondence should be addressed. E-mail:yausl@atom.che.tohoku.ac.jp; tel: 81-22-2174177; fax: 81-22-2174177.

† National Central University.‡ CREST.§ Tohoku University.(1) Haruta, M.; Tsubota, S.; Kobayashi, T.; Kageyama, H.; Genet, M.

J.; Delmon, B. J. Catal. 1993, 144, 175.(2) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 1647.(3) Hammer, B.; Morikawa, Y.; Nørskov, J. K. Phys. Rev. Lett. 1996,

76, 2141.(4) Liu, Z. P.; Hu, P.; Alavi, A. J. Am. Chem. Soc. 2002, 124, 14770.(5) Outka, D. A.; Madix, R. J. Surf. Sci. 1987, 179, 351.(6) Ruggiero, C.; Hollins, P. Surf. Sci. 1997, 377, 583.(7) Peters, K. F.; Steadman, P.; Isern, H.; Alvarez, J.; Ferrer, S. Surf.

Sci. 2000, 467, 10.(8) Jugnet, Y.; Cadete Santos Aires, F. J.; Deranlot, C.; Piccolo, L.;

Bertolini, J. C. Surf. Sci. 2002, 521, L639.(9) Piccolo, L.; Loffreda, D.; Cadete Santos Aires, F. J.; Deranlot, C.;

Jugnet, Y.; Sautet, P.; Bertolini, J. C. Surf. Sci. 2004, 566-568, 995.(10) Gottfried, J. M.; Christmann, K. Surf. Sci. 2004, 566-568, 1112.

(11) Kita, H.; Nakajima, H.; Hayashi, K. J. Electroanal. Chem. 1985,190, 141.

(12) Chang, S.-C.; Hamelin, A.; Weaver, M. J. Surf. Sci. Lett. 1990,239, L543.

(13) Chang, S.-C.; Hamelin, A.; Weaver, M. J. J. Phys. Chem. 1991,95, 5560.

(14) Edens, G. J.; Hamelin, A.; Weaver, M. J. J. Phys. Chem. 1996,100, 2322.

(15) Sun, S.-G.; Cai, W.-B.; Wan, L.-J.; Osawa, M. J. Phys. Chem. B1999, 103, 2460.

1942 Langmuir 2005, 21, 1942-1948

10.1021/la047832l CCC: $30.25 © 2005 American Chemical SocietyPublished on Web 01/28/2005

image CO on gold electrodes in the electrochemicalenvironment.

In this paper, we present high-quality STM molecularimages of CO admolecules on Au(111) recorded underpotential control in CO-saturated 0.1 M HClO4, whichshow that several commensurate CO adlattices at 0.1 Vare formed, but the (9 × x3) structure (coverage θ ) 0.28)prevailed with time. Adjusting the potential to 0.8 V andwaiting for ca. 60 min yielded (7 × x7) and (x43 × 2x13)structures with coverages of roughly 0.13 before the COadlayer was completely eliminated at 0.85 V.

Experimental Section

The Au(111) single-crystal electrode used for voltammetricmeasurements and for STM experiments were prepared from agold wire, as devised originally by Hamelin.16 The Au(111)electrode was first annealed in a hydrogen flame and then cooledin a quenching tube containing hydrogen-saturated Milliporetriple-distilled water. The as-prepared Au(111) electrode wastransferred to the electrochemical cell or mounted in the STM.The electrochemical cell had a three-electrode configurationincluding a reversible hydrogen electrode (RHE) as the referenceelectrode and a Pt wire as the counter electrode. The voltammetricexperiments for studying electrocatalytic activity of Au(111)toward CO oxidation were conducted by flushing the electro-chemical cell with Ultrapure CO for 5 min while the potentialof Au(111) was held at 0.1 V. For the voltammetry of COmonolayer on Au(111), the CO dissolved in the solution wasremoved by purging with Ultrapure nitrogen gas for 3 min. Detailsof STM and linear sweep voltammetry experiments are describedelsewhere.16-18 The STM used was a Nanoscope-E (SantaBarbara, CA), and the constant-current imaging mode was usedthroughout this study. The tips were made of tungsten (diameter0.3 mm) prepared by electrochemical etching in 2 M KOH. Atypical in-situ STM experiment involved imaging the Au(111)substrate prior to the introduction of CO. Confirming the exposureof a clean and ordered Au(111) surface was followed by imagingin a CO-saturated perchloric acid solution.

A Pt wire served as the quasi-reference electrode in the in-situSTM experiment. However, all potentials reported here refer toa reversible hydrogen electrode (RHE). Carbon monoxide (highpurity 99.5%) was purchased from Matheson Tri-Gas (Dallas,Texas). The electrolyte was always 0.1 M HClO4 prepared fromMillipore water (18.2 MΩ) and superpure perchloric acid (Darm-stadt, Germany). Before each measurement, the solution wasdeaerated by bubbling pure nitrogen. All experiments werecarried out at room temperature (ca. 25 °C).

Results and Discussion

Cyclic Voltammetry. Linear sweep voltammetry wasperformed with an as-prepared Au(111) electrode in 0.1M HClO4 to ensure the degree of ordering of its surfaceand the cleanliness of the electrochemical environment.The resultant steady-state CV (the dotted line in Figure1) recorded at a scan rate of 50 mV/s shows an extendeddouble-layer charging region spanning between 0 and 1.1V, two anodic features, A1 and A2, at 1.35 and 1.55 V inthe positive sweep, and peak C1 at 1.16 V in the negativesweep. These features are typical for a well-orderedAu(111) electrode, as reported previously.16 The prominentfeatures of A1, A2, and C1 are ascribed to the formationand the reduction of an oxygen adlayer. The CV shown bysolid line in Figure 1 was obtained with a Au(111) electrodeimmersed in 0.1 M HClO4 saturated with carbon monoxideafter 20 mL of the electrolyte was flushed with CO for 3min at a fixed potential of 0 V. The potential was scannedrepeatedly from 0 to 1.6 V, producing a broad oxidation

wave with three peaks at 0.9, 1.3, and 1.5 V, labeled A0,A1, and A2, respectively. Oxidation of CO started at 0.6V, reaching a maximum current density of 180 µA/cm2 at0.9 V (A0), which was followed by a gradual decrease toa minimum of 50 µA/cm2 at 1.6 V. The features of A1 andA2 still persisted as was observed with the bare Au(111),suggesting that the oxidation of surface-bound CO mol-ecules and that of the gold electrode proceeded simulta-neously at these potentials. As is known for mosttransition-metal electrodes, the oxygen adlayer acted asa passivating layer on Au(111), inactivating the goldelectrode toward CO oxidation. The CO oxidation wascompletely blocked till the oxygen adlayer was removed,as indicated by the sharp rise of anodic current at 1.25 Vin the negative potential sweep. The peak current densityof A1, 180 µA/ cm2, is comparable to that observed witha gold film electrode.15 However, it is substantially lowerthan the values found with Au(100), Au(210), and Au(110)electrodes.12-14 This sensitivity to the crystallographicorientation can be understood on the basis of the fact thatthe current densities for these anodic features are directlyproportional to the kinetics of the electrode processesinvolved.

Figure 2 shows the voltammogram of a Au(111) electrodemodified with a monolayer of CO produced by bubblingCO directly into the electrochemical cell for 10 min at 0.1V. A positive-going potential sweep was initiated at a scanrate of 50 mV/s after dissolved CO was removed by purgingwith N2 for several minutes. Because CO adsorbs revers-ibly, both the pressure of nitrogen and the time of purgingcan influence the amount of CO adsorbed on the Au(111).Extensive experimental work indicated that for a volumeof 20 mL in the electrochemical cell, a 3-min purging withN2 at the pressure set at 0.3 psi removed nearly alldissolved CO with a minimal loss of surface-bound COmolecules. The resultant CV profile is shown in the solid

(16) Hamelin, A. J. Electroanal. Chem. 1996, 407, 1.(17) Itaya, K. Prog. Surf. Sci. 1998, 58, 121.(18) Chen, J.-H.; Yau, S.-L.; Chang, S.-C. J. Phys. Chem. B 2002,

106, 9079.

Figure 1. Cyclic voltammograms of Au(111) with (solid line)and without (dotted line) saturated carbon monoxide in 0.1 MHClO4. The potential of Au(111) was held at 0.1 V while COwas bubbling into the cell, and the potential was scannedpositively from 0.1 to 1.6 V at 50 mV/s. Only the first potentialsweep is shown. The subsequent sweeps showed a continuousdecrease in current density with continued potential cycles.

Figure 2. Voltammogram of Au(111) modified with a mono-layer of CO deposited potentiostatically at 0.1 V, followed bypotential sweep from 0.1 to 1.6 V at a scan rate of 50 mV/s. Thebroad anodic wave with a current peak at 0.9 V is due to theelectroxidation of CO adlayer. The profile in dotted trace is dueto Au(111) after the CO adlayer was removed electrochemically.

In-Situ Scanning Tunneling Microscopy Langmuir, Vol. 21, No. 5, 2005 1943

trace in Figure 2, which contains a broad oxidation wavelabeled A0 with a 60 µA/cm2 peak current at 0.9 V.Furthermore, typical features of the formation andsubsequent reduction of a surface oxide appeared im-mediately after the CO adlayer was removed, indicatingthat the broad oxidation feature at 0.9 V involved theoxidation of surface-bound CO, rather than dissolved CO.After correction for the double-layer charging, the amountof charge involved in peak A0 is estimated to be 130 ( 10µC/cm2, which corresponds to a coverage by CO of 0.30,if one assumes the corresponding oxidation process to beCO + H2O f CO2 + 2H+ + 2e-. This CO stripping profilecontrasts markedly with that at the Pt(111) electrode,where a sharp current spike with a full width at half-maximum of 25 mV is observed at 0.8 V. Furthermore,the broad shape of this CV reflects the sluggish kineticsof CO oxidation at Au(111), in strong contrast to thesituation at Pt(111).19,20 However, the kinetics of thisreaction could vary with the surface atomic structure ofgold electrode. Indeed, this structural effect was notedpreviously for the oxidation of surface-bound CO at Rhelectrodes.21 This issueatgoldelectrodes is currentlyunderinvestigation.

Electrochemical STM Imaging of Au(111). Au(111)Surface in 0.1 M HClO4 under Potential Control. Figure3 shows in-situ STM images of Au(111) surface obtainedat 0.1 V in 0.1 M HClO4 under typical imaging conditionsof 100-200 mV in bias voltage and 2-5 nA in feedbackcurrent. The topography scan of 1000 × 1000 Å in Figure

3a is intended to show typical surface characteristics foundon terraces. This image reveals distinctly protruded stripesrunning in the ⟨121⟩ directions, or 30° rotated from theclose-packed atomic rows of the Au(111) substrate. Thesefeatures are the well-known signature of the Au(111)-(x3 × 22) reconstruction. They can be aligned along three⟨121⟩ directions, yielding three rotational domains, labeledI, II, and III in Figure 3a. The corrugated stripes canextend more than 500 Å in length, as seen in domain I,or merely 100 Å in length in domain III. The weight of adomain depends on the step structure or details of theelectrode pretreatment, as described previously.22 Thehigh-resolution STM scan in Figure 3b reveals the atomicstructure of this reconstructed Au(111) surface. Thisatomic array is “pseudo” hexagonal, resulting from auniaxial compression of the uppermost plane of Au(111)with surface gold atoms located on face-center-cubic (fcc),hexagonal-close-pack (hcp), and asymmetric sites, ascompared to fcc sites only for unreconstructed Au(111).The energetically more favorable fcc domains are alwaysbroader than hcp ones.22 The corrugated stripes of theherringbone structure are due to gold atoms located atasymmetric sites. Two neighboring pairs of the corrugatedrows are separated by a characteristic length D of 65.6 Å(22 × 2.99 Å), as can be seen in Figure 3b. Details ofreconstructed Au(111) have been reported.22 It is generallybelieved that the potential of a gold surface determinesits real-space structure. For a negatively charged Au(111),the lateral interatomic interactions between gold atomsin the uppermost layer is enhanced, leading to theaforementioned uniaxial compression of the reconstructedAu(111). However, reconstruction snaps back to form the(1 × 1) lattice at potentials positive of the pzc (ca. 0.34 Vvs SCE).23,24 The lattice constant along the compressedatomic rows of the reconstructed Au(111) is 4.3% shorterthan that of the (1 × 1) structure.

In-Situ STM Imaging of the Adsorption Process of COon Au(111). A series of time-dependent in-situ STM imagesare shown in Figure 4 to reveal the dynamics of COadsorption on a Au(111) electrode at 0.1 V in 0.1 M HClO4.These images were consecutively acquired at intervals of1 min, which reveal changes that took place at the surface5 min after the electrolyte was completely replaced withCO-saturated 0.1 M HClO4 containing 1.57 mM of CO.25

Because of the weak interaction between CO and Au(111),it was critical to optimize imaging conditions to achievehigh-quality STM resolution. A bias voltage of 200 mVand a setpoint current of 1 nA or less are suitable to achievehigh-quality STM molecular resolution. Atomically flatterraces delineated by monatomic steps (∆z ) 2.5 Å) areapparent in these images. Meanwhile, the herringbonecharacteristics on the terrace clearly indicate that theAu(111) surface was reconstructed, as indicated by arrow1 in Figure 4a. The introduction of CO into the STM cellyielded no immediate change in the surface image for about5 min, when some locally ordered arrays began to appear.It is likely that weakly adsorbed CO molecules diffusedrapidly after they landed on the Au(111) surface. Theyprobably underwent intermolecular collisions before ag-gregating into larger domains that could be imaged bySTM. Our in-situ STM did not disclose the processes ofmolecular diffusion and collision, but it clearly showed

(19) Yau, S. L. Ph.D. Thesis, Purdue University, 1990.(20) Villegas, I.; Weaver, M. J. Chem. Phys. 1994, 101, 1648.(21) Gomez, R.; Orts, J. M.; Feliu, J. M.; Clavilier, J.; Klein, L. H. J.

Electroanal. Chem. 1997, 432, 1.

(22) Barth, J. V.; Brune, H.; Ertl, G.; Behm, R. J. Phys. Rev. B 1990,42, 9307.

(23) Wang, J.; Davenport, A. J.; Isaacs, H. S.; Ocko, B. M. Science1992, 255, 1416.

(24) Kolb, D. M. In Structures of Electrified Interfaces; Lipkowski, J.,Ross, P. N., Eds.; VCH Publishers: NewYork, 1993.

(25) Silberger, M. S. Chemsitry: The Molecular Nature of Matterand change; MacGraw Hill: New York, 2000; p 487.

Figure 3. In-situ STM images of the (x3 ×22) reconstructedAu(111) at 0.1 V in 0.1 M HClO4. The gross scan in (a) revealsthe general herringbone features seen on the surface, while thehigh-resolution scan in (b) shows the atomic arrangement ofthe herringbone pattern, whose periodicity is 65.6 Å as indicatedin the figure.

1944 Langmuir, Vol. 21, No. 5, 2005 Shue et al.

the formation of molecular aggregates near step ledges,as indicated by arrow 2 in Figure 4a. Along with theappearance and the growth of CO molecule aggregates atthe step, two patches of CO molecules appeared 1 minlater, which are labeled P1 and P2 in Figure 4b near thestep and on the terrace, respectively. The difference inlocation influenced profoundly their subsequent stabilityon the surface, as P1 continued to expand with the timeof STM imaging, whereas P2 is no longer visible in Figure4c. These results suggest that CO molecules prefer toadsorb at step sites, as was observed also for the adsorptionof CO on Au(111) and Au(110) in a vacuum.8,9 It is shownthat CO adsorption yields rugged steps with a significantincrease in the number of step and kink sites.8,9 This isconsistentwithour finding thatstepsbecamesubstantiallyrougher after CO adsorption.

Close examination of these STM images reveals thatthe areas occupied by CO molecules appear dimmer thanthe gold surface by ca. 2.0 ( 0.2 Å, a surprising findingbecause the CO molecules are located on top of the goldsubstrate. (The 2.0 Å difference in corrugation wasmeasured between the tip of protrusions and the terraceof Au(111) in the same scan line.) The same phenomenonwas observed in replicate experiments conducted withdifferent tips. There are some STM reports on theadsorption of CO on metal surfaces of Pt, Ag, Rh, and soforth in a vacuum and electrochemical environment.20,26-30

However, none of these studies shows depressed imagesof CO molecules. On Pt(111), the corrugation heights ofCO molecules at all adsorption sites are less than 0.5 Å.26-28

To our knowledge, only oxygen and nitrogen adatomsproduce consistent depressive appearances on metalsurfaces such as Pd, Ni, and Ru.31-33 These results havebeen explained by the adsorbate-induced shift of electronicstates at a metal substrate away from the Fermi level,which reduces the electronic resonance between thesubstrate and the STM tip.34,35 Meanwhile, the atomic

orbitals of oxygen and nitrogen adatoms are eithermismatched in energy with those of the tip or contractedto such an extent that they make no contribution toelectron tunneling. This brings about the depressiveappearances in STM images. If this model is applied toaccount for the present result observed with the CO/Au(111) system, it can be conjectured that adsorbed COmolecules reduce the electronic states at the Au(111)surface. This contention would have to be confirmed byfurther theoretical analysis. Another unique aspect of COadsorbed on gold and other coinage metal surfaces is thedecrease of work function by nearly 1 eV when a saturatedCO monolayer is adsorbed.36,37 This is in strong contrastto most transition metals, suggesting that CO is phys-isorbed on Au(111), but chemisorbed on Pt and other nobletransition metals.20

Furthermore, the herringbone pattern disappeared orthe (x3 × 22) reconstruction was lifted to produce the (1× 1) structure upon CO adsorption. This phase transitionwas coupled with an injection of 4.3% gold atoms to thesurface, which coincides with the appearance of a fewmounds (indicated by arrows in Figure 4c) in the vicinityof CO adlayer. Since the Au(111) electrode was poten-tiostated at 0.1 V, which expectedly encouraged theformation of reconstructed Au(111), it is surprising thata weak adsorbate such as CO can cause lifting of thereconstruction of Au(111). This result implies that COwas adsorbed more strongly at the unreconstructed thanat the reconstructed Au(111), and the difference inadsorption energy was large enough to compensate forthe loss of surface energy as the structure of Au(111)changed. However, this issue cannot be further substan-tiated without the knowledge of the difference in surfaceenergy of the two structures of Au(111). According to theprevious vacuum studies, the maximal heat of adsorptionfor CO on Au(111) amounts to 42 kJ/mol,7 which lies atthe borderline of physiosoprtion and chemisorption. It isspeculated that applying a negative potential to theAu(111) electrode could increase the heat of adsorption ofCO and eventually yield lifting of reconstruction. Thisview is born out from previous theoretical calculations,which show changes in electronic density of the frontieratomic orbitals, 5σ and 2π*, for CO molecule adsorbed ona Pt(111) electrode.36

Our STM results indicate that the adsorption processof CO was slow, as it took roughly 30 min to complete thedeposition of a full monolayer of CO on Au(111). This

(26) Stroscio, J. A.; Eigler, D. M. Science 1991, 254, 1319.(27) Vestergaard, E. K.; Thostrup, P.; An, T.; Lægsgaard, E.;

Stensgaard, I.; Hammer, B.; Besenbacher F. Phys. Rev. Lett. 2002, 88,259601.

(28) Jensen, J. A.; Rider, K. B.; Salmeron, M.; Somorjai, G. A. Phys.Rev. Lett. 1998, 80, 1228.

(29) Yau, S.-L.; Gao, X.; Chang, S.-C.; Schardt, B. C.; Weaver, M. J.J. Am. Chem. Soc. 1991, 113, 6049.

(30) Barth, J. V.; Zambelli, T. Surf. Sci. 2002, 513, 359.(31) Rose, M. K.; Borg, A.; Dunphy, J. C.; Mitsui, T.; Ogletree, D. F.;

Salmeron M. Surf. Sci. 2004, 561, 69.(32) Kopatzki, E.; Behm, R. J. Surf. Sci. 1991, 245, 255.(33) Calleja, F.; Arnau, A.; Hinarejos, J. J.; Va’zquez de Parga, A. L.;

Hofer, W. A.; Echenique, P. M.; Miranda, R. Phys. Rev. Lett. 2004, 92,206101.

(34) Sautet, P. Surf. Sci. 1997, 374, 406.(35) Bocquet, M.-L.; Sautet, P. Surf. Sci. 1998, 415, 148.

(36) Illas, F.; Mele, F.; Curulla, D.; Clotet, A.; Ricart, J. M.Electrochim.Acta 1998, 44, 1213.

(37) Gottfried, J. M.; Schmidt, K. J.; Schroeder, S. L. M.; Christmann,K. Surf. Sci. 2003, 536, 206.

Figure 4. Time-dependent STM images showing the continuous adsorption of CO admolecules at 0.1 V in 0.1 M HClO4. The firstimage a was obtained 5 min after the injection of CO, whereas b and c were acquired consecutively at intervals of 1 min in thesame scan area. Arrows 1 and 2 in a denote the herringbone pattern and the domain of CO adlattices. P1 and P2 indicate twonew patches of CO molecules. The former expanded but the later disappeared in c. Arrows in c indicate mounds of gold atomsinjected from the uppermost layer of gold surface, which was produced by structural transition. The bias voltage and setpointcurrent were ca. 400 mV and 4 nA, respectively. Scan sizes are 750 × 750 Å for all three images.

In-Situ Scanning Tunneling Microscopy Langmuir, Vol. 21, No. 5, 2005 1945

sluggish adsorption kinetics can be correlated with theweak adsorption of CO on Au(111). However, the repulsionbetween CO admolecules, which was proposed to accountfor the weak adsorption of CO on gold in a vacuum,37

probably was not operative in the electrochemical envi-ronment. In contrast, CO molecules on Au(111) appearedto interactattractively in theelectrochemicalenvironment,as they aggregated into ordered adlattices from the verybeginning of adsorption to the end of formation of amonolayer. Again, the electrochemical potential is thoughtto be an important factor in altering the intermolecularinteraction at liquid/solid interfaces.

Molecular Resolution STM of CO Adlattices on Au(111).To study whether the charge on Au(111) or CO adsorbatepredominated in determining the surface structure ofAu(111), the number of excess charges was increased byswitching the potential from 0.1 to -0.1 V. The STMimages shown in Figure 5a-c were acquired subsequentlyto elucidate the ordered structure of CO molecules at -0.1V in CO-saturated 0.1 M HClO4. In addition to the orderedpattern, four corrugated stripes indicated by arrows wereobserved in Figure 5a. These stripes are aligned along theherringbone features observed prior to CO adsorption.These features indicate that this negative potential steprestored the reconstructed Au(111) initially lifted by theadsorption of CO. The ordered CO adlayer was barelyaffected by this process. This restructuring event wascompleted in 10 min. The STM image in Figure 5b showsthe coexistence of two rotational domains on the same

terrace, and the image in Figure 5c reveals the internalarrangement of the domain in the upper right portion ofFigure 5b. The unit cell of this ordered array outlined asa dotted rectangle in Figure 5c is determined to be (9 ×x3) with five CO molecules covering 18 Au atoms, whichcorresponds to a coverage of 0.28. This agrees reasonablywell with the coverage determined by the aforementionedelectrochemicalmeasurement.The intermolecularspacingbetween nearest neighbors is measured to be 5.0 Å, whichis 56% larger than the spacings observed at Pt and Irelectrodes.20,39

Another interesting feature of this CO-covered Au(111)surface is the periodicity D of the corrugated rows. In thepresent case, D is averaged as 75 ( 1 Å, which is ca. 15%larger than the ideal value of 65.6 Å for the (x3 × 22)reconstruction. This result indicates that there was a clearrelaxation of the reconstructed Au(111)-(x3 × 22) toward(1 × 1) upon CO adsorption. A similar phenomenon wasobserved previously with X-ray scattering, showing thatdosing Au(111) with 50 mbar CO at room temperaturecauses stretching of the herringbone pattern by 15%, orthe spacing between two neighboring pairs of protrusionsincreases from 65.6 to 75 Å.8 The saturated CO concen-tration used in this study corresponds to a pressure of 34mbar,25 which is close to that (50 mbar) used in vacuumstudies.7 However, two recent STM studies of CO onAu(111) and Au(110) in a vacuum did not identify theadsorbate under high pressure at room temperature.9,10

In contrast, this study, also conducted at room temper-ature, clearly discerned ordered structures of CO mol-ecules. Again, the electrochemical potential could beresponsible for the enhanced interaction between CO andAu(111).

A portion of Figure 5c was treated with the Fouriertransform method to remove noise. The resultant STMimage shown in Figure 5d reveals the internal moleculararrangement of this structure. A corresponding ball modelis shown in Figure 5e. Because CO molecules appearedas depressions in the STM images (see Figure 4, forexample), the image in Figure 5d is presented in theinverted mode. Evidently, not all protrusions in the cellexhibit the same corrugation height. This structureconsisted of rows of protrusions labeled A-E aligned inthe ⟨121⟩ direction. The spots in row A appear to be higherthan those in row C by ca. 0.4 Å, while those in rows B,D, and E are lower by an additional 0.2 Å. If all brightspots in the image are associated with CO molecules, thisresult implies that these molecules might reside at threedifferent adsorption sites. This view is consistent withthe conclusions drawn from the study of CO/Pt(111),19,20

where thecorrugationheightofamolecular featurereflectsits binding configuration. Specifically, molecular resolu-tion STM shows that the Pt(111)-(2 × 2)-3CO adlatticecontains one CO molecule at on-top site and two COmolecules at three-fold hollow sites with the formerappearing 0.5 Å higher than the latter. Subsequently, themost likely model for CO/Au(111) system would place thosebrightest spots at the corners of (9 × x3) to on-top sites,whereas weaker spots inside the unit cell to high sym-metric sites, including two-fold bridge and three-foldhollow sites. Accordingly, the population ratio of COadmolecules residing at on-top, two-fold, and three-foldsites is equal to 2:1:2. Unfortunately, this viewpoint differssubstantially from that derived from IR measurements,which indicates the atop registry to be predominant forCO admolecules on a gold film supported by silicon wafer.15

(38) McElhiney, G.; Pritchard, J. Surf. Sci. 1976, 60, 397.(39) Yang, L. M.; Yau, S. L. J. Phys. Chem. B 2000, 104, 1769.

Figure 5. a-c. In-situ STM images obtained for Au(111) at0.1 V in 0.1 M HClO4 containing 1.57 mM CO. Image in a showsthe ordered CO adlattice above a herringbone pattern with aperiodicity of 75. Image in b shows the presence of two rotationaldomains, whereas image in c highlights the internal moleculararrangement of the ordered structure. The bias voltage andsetpoint current were ca. 200 mV and 0.5 nA, respectively.Figure 5d-e: Filtered STM molecular images (d) of Au(111)-(9 × x3)-5CO adlattice on Au(111) at 0.1 V in 0.1 M HClO4.A corresponding model (e) depicts the real-space arrangementof this structure. CO molecules adsorbed at atop, two-fold, andthree-fold sites are drawn in different circles.

1946 Langmuir, Vol. 21, No. 5, 2005 Shue et al.

The origin of this discrepancy is not clear at present.However, the intensity of IR absorption may not reflectfaithfully the population of CO molecules at different sites.For example, despite the fact that several atop COmolecules are only a half of that residing at three-foldsites, the intensity of IR bands because of C-O stretchingof CO molecules at atop sites is 3 times greater than thatof the molecules at three-fold sites. This issue is extensivelydiscussed by Weaver.20 On the other hand, it is not clearfrom our STM results whether adsorbed CO admoleculescould affect the structure of the Au(111) surface. This couldbe another important factor that influences the appear-ances of STM molecular resolution.

Potential Effect on Structure of CO Adlattices. Effectsof potential on the structure and oxidation of CO adlayeron Au(111) were examined by stepping the potentialprogressively from 0.1 to 0.8 V. The (9 × x3) structureremained unchanged up to 0.8 V, where new structuresemerged. The STM images shown in Figure 6 illustratethe changes in structure. The resemblance in surfacemorphology between these images ensures that the samearea was examined. The image in Figure 6a obtained at0.2 V shows the prominent (9 × x3) structure along withthe herringbone feature. However, stepping the potentialto 0.8 V and holding it at this value for 67 min caused adistinct change in the CO adlayer, as the STM image inFigure 6b shows. Notably, the (9 × x3)-CO adlattice andcorrugated stripes vanished, and they were replaced bynew ordered arrays with some disordered domains. Thetwo mounds (indicated by arrows) within this scan area

could be due to the gold atoms released by the lift ofreconstruction. The newly formed structures in Figure 6bagain disappeared as the potential was stepped morepositively from 0.8 to 0.825 V. Presumably, the orderedadlattices seen in Figure 6b are due to CO molecules orintermediates produced in the oxidation process beforeCO molecules were desorbed or eventually electrooxidizedoff the Au(111) surface. The chemical natures of theadspecies within the disordered domains are not clear.They could be water or OH molecules and coadsorbedClO4

- anions. The main supplier of oxygen atoms used forthe CO oxidation process in acidic solution can be H2Omolecules or OH adspecies.12-14 Nevertheless, this seriesof STM images show that the electrooxidation of COoccurred preferentially at the perimeters of ordereddomains. This viewpoint is in line with the Langmuir-Hinshelwood mechanism proposed to explain the processof CO oxidation,10,30,40 that is, the reactants of CO andH2O/OH were both adsorbed prior to the oxidationreaction.

The internal molecular arrangements of the newstructures emerging at 0.8 V are elucidated by the STMimages in Figure 7a and b, which show the unfiltered andfiltered high-resolution scans of the ordered structure inthe upper portion of the terrace (Figure 6b). This adlatticeis characterized as (7 × x7) with three CO molecules percell, or a coverage of 3/21 (0.14). The model depicted inFigure 7c accounts for this structure. In contrast to the(9 × x3) structure, all protrusions within the (7 × x7)structure exhibited similar corrugation heights, althoughthere was some irregularity of intensity as seen in Figure7b. It is assumed that all CO adsorbates occupy similarsites on the Au(111) surface, although the exact registryin this case could not be determined. We arbitrarily assignall CO molecules to on-top sites, as illustrated by the modelin Figure 7c.

Another ordered array seen in Figure 6b is highlightedin Figure 8a and a portion of this structure is filtered andshown in Figure 8b. This structure is determined to be(x43 × 2x13) with six COs per unit cell or a coverage of6/48 (0.125). The similarity in coverage may be the reasonthese two structures were produced at the same time.The relative weights of these two structures have not beendetermined precisely. The phase transition from (9 × x3)to (7×x7) and (x43×2x13) could correspond to processesoccurring at the prewave of the main peak at 0.9 V observedin the positive-going potential sweep from 0 to 1.5 V (Figure2a). Evidently, these CO adlattices differ greatly fromthose observed on the (111) planes of all transition metalssuch as Pt, Ir, and Ru. This contrast again reflects thedifference in the nature of bonding of CO to these metalsurfaces.

Figure 6. Time-dependent in-situ STM images showed therestructuring of an adlayer on Au(111) in CO-saturated 0.1 MHClO4. These image were collected at 0.2 V (a), 67 min at 0.8V (b), 75 min at 0.825 V (c), and 78 min at 0.6 V (d). The scansizes for all images were 67 × 67 nm. The Itip and Et were0.7∼1.3 nA and 250 mV, respectively.

Figure 7. Unfiltered (a) and filtered (b) in-situ STM images of Au(111)-(7 × x7)-4CO, showing the real-space arrangement ofCO molecules at 0.8 V. The bias voltage and setpoint current were -550 mV and 0.6 nA, respectively. A corresponding model isdepicted in (c), where all CO admolecules are arbitrarily assigned to on-top sites.

In-Situ Scanning Tunneling Microscopy Langmuir, Vol. 21, No. 5, 2005 1947

Conclusions

Immersion of a Au(111) electrode in a CO-saturatedperchloric acid solution at 0.1 V formed several orderedstructures, but they were gradually transformed into amore stable (9 × x3) structure, which persisted when thepotential was altered between 0.1 and 0.8 V. DosingAu(111) with a CO-saturated HClO4 at 0.1 V caused thelifting of Au(111)-(x3 × 22) reconstruction, but adjustingthe potential from 0.1 to -0.1 V rendered the reconstructedAu(111) to reform, although the periodicity of the her-ringbone feature increased from 65.5 to 75 Å. Molecularresolution STM revealed CO molecules with unlikecorrugation heights, implying CO molecules are adsorbedon different symmetric sites. Raising the potential from0.7 to 0.8 V resulted in a slow phase transition from (9 ×x3) to (7 × x7) and (x43 × 2x13), as molecules of COwere partially desorbed or electrooxidized with the cover-ages of these adlattices decreasing from 0.28 to 0.14. The

(7 × x7) and (x43 × 2x13) structures appeared assegregated domains amid disordered areas ascribable toadsorbed water molecules or OH adspecies. Potential-dependent STM imaging revealed displacement of orderedCO domains by disordered water domains. These resultssuggest that the electrooxidation of CO proceeds via theLangmuir-Hinshelwood mechanism on Au(111).

Acknowledgment. S.L.Y. acknowledges the financialsupport from the National Science Council, Taiwan (NSC93-2113-M-008-009). This work was partially supportedby the Ministry of Education, Culture, Sport, Science, andTechnology, a Grant-in-Aid for the COE project, GiantMolecules and Complex Systems, 2004. The authors thankDr. Y. Okinaka for his help in writing this manuscript.

LA047832L

(40) Blizanac, B. B.; Lucas, C. A.; Gallagher, M. E.; Arenz, M.; Ross,P. N.; Markovic, N. M. J. Phys. Chem. B 2004, 108, 625.

Figure 8. Original (a) and filtered (b) in-situ STM atomic images of Au(111)-(x43 × 2x13)-6CO, θ ) 0.125 and a correspondingreal-space model (c). The image was obtained 67 min after the potential was stepped from 0.2 to 0.8 V in 0.1 M HClO4.

1948 Langmuir, Vol. 21, No. 5, 2005 Shue et al.