in situ scanning tunneling microscopy of benzene, naphthalene, and anthracene adsorbed...

In Situ Scanning Tunneling Microscopy of Benzene,Naphthalene, and Anthracene Adsorbed on Cu(111) in

Solution

Li-Jun Wan† and Kingo Itaya*,‡

Itaya Electrochemiscopy Project, ERATO/JST, The Research Institute of Electric andMagnetic Materials, 2-1-1 Yagiyama-minami, Sendai 980, Japan, and Department ofApplied Chemistry, Faculty of Engineering, Tohoku University, Sendai 980, Japan

Received June 30, 1997. In Final Form: October 13, 1997X

In situ scanning tunneling microscopy (STM) was used to probe the adlayer structures of benzene,naphthalene, and anthracene on a well-defined Cu(111) electrode surface in aqueous HClO4 solution.These molecules were found to form highly ordered adlayers on a clean Cu(111) surface with a flat-lyingorientation. The molecular orientation and packing arrangement were also determined. The orderedbenzene adlayer was observed to form a (3 × 3) structure with a characteristic triangular shape for eachbenzene molecule. The naphthalene adlayer was observed with a (4 × 4) symmetry. The naphthalenemoleculeswere preferentially alignedwith their longC2 axes along the atom rows of theCu(111) substrate.A long range ordered structure was also found for the anthracene adlayer. Straight molecular chainsformed along the atomic rows of Cu(111) with a side-by-side configuration.

Introduction

Thebondingand coordination of organicmoleculeswithmetal electrode surfaces are the fundamental issue bothin electrochemistry1 and in catalysis in the gas phase.2Thestructureandcomposition featuresof organicadlayerswere traditionally investigated by ultrahigh vacuum(UHV) methods such as low-energy electron diffraction(LEED), Auger electron spectroscopy (AES), and electronenergy-loss spectroscopy (EELS).2 Hubbard and co-workers have intensively examined various organic ad-layers formed on metal electrodes in solution by usingthese ex situ techniques, the so-called UHV-electro-chemical system (UHV-EC), in order to understandelectrode/electrolyte interfaces.3,4 However, recent effortsusing in situ scanning probe microscopies (SPMs), inparticular in situ scanning tunneling microscopy (STM),have established them as invaluable methods for deter-mining the structures of organic adlayers with atomic ormolecular resolution both inUHV5-11 and in solution.12-21

Our recent paper demonstrated that the adlayerstructures of benzene were visualized with an extra-ordinarily high resolution on Rh(111) and Pt(111) in HFsolution.15 On Rh(111), two potential-dependent struc-tures of benzene adlayers with c(2x3 × 3)rect and (3 ×3) symmetries were identified in solution,15 while similarstructures had previously been found to form in UHVconditions.2,5 The fact that the adlayer structures ofbenzeneonRh(111) are similar inUHVand inHFsolutionsuggests that the adsorbate-substrate interaction ispredominant for hydrophobicmolecules such as benzene,and the role of water molecules is surprisingly minor inthe formation of benzene adlayers in solution. On theother hand, it is known that benzene does not form anorderedstructureonPt(111) inUHV,2whileawell-ordered(x21 × x21)R10.9° structure was found at cathodicpotentials inHFsolution.15 This result indicates that thenature of electrified interfaces can play an important rolein the ordering processes of adsorbed organic molecules.We recently reported that in situ STM allowed us tovisualize the adlayers of larger aromatic molecules suchas naphthalene and anthracene.16 Naphthalene yieldeda long range ordered (3x3 × 3x3)R30° structure onRh(111) inHF,whereasanthraceneproducedadisorderedstructure. On Pt(111), disordered structures were foundfor both naphthalene and anthracene.16 These resultsindicate that aromatic molecules are more stronglyadsorbed onPt(111) than onRh(111), resulting in a lesser

* To whom correspondence should be addressed.† The Research Institute of Electric and Magnetic Materials.‡ Tohoku University.X Abstract published in Advance ACS Abstracts, December 1,

1997.(1) Lipkowski, J., Ross, P. N., Eds. Adsorption of Molecules at Metal

Electrodes; VCH Publisher: New York, 1992.(2) Somorjai, G. A. Introduction to Surface Chemistry andCatalysis;

John Wiley & Sons Inc.: New York, 1994.(3) Hubbard, A. T. Chem. Rev. 1988, 88, 633.(4) Soriaga, M. P. Prog. Surf. Sci. 1992, 39, 525.(5) Ohtani, H.; Wilson, R. J.; Chiang, S.; Mate, C. M.Phys. Rev. Lett.

1988, 60, 2389.(6) Tanaka, H.; Nakagawa, T.; Kawai, T. Surf. Sci. 1996, 364, L575.(7) Stranick, S. J.; Kamna,M.M.;Weiss, P. S.Science 1994, 266, 99.(8) Junk, A. T.; Schlittler, R. R.; Gimzewski,Nature 1997, 386, 696.(9) Weiss, P. S.; Eigler, D. M. Phys. Rev. Lett. 1993, 71, 3136.(10) Chiang, S. InScanningTunnelingMicroscopy I;Wiesendanger,

R.,Guntherodt,H.-J., Eds.; Springer-Verlag: NewYork, 1991; pp 181-205.

(11) Ikai, A. Surf. Sci. Rep. 1996, 26, 261.(12) Siegenthaler, H. In Scanning Tunneling Microscopy II; Wie-

sendanger, R., Guntherodt, H.-J., Eds.; Springer-Verlag: New York,1992; pp 7-49.

(13) Tao, N. J.; Cardenas, G.; Cunha, F.; Shi, Z. Langmuir 1995, 11,4445.

(14) Cuhan, F.; Tao, N. J. Phys. Rev. Lett. 1995, 75, 2376.(15) Yau, S.-L.; Kim, Y.-G.; Itaya, K. J. Am. Chem. Soc. 1996, 118,

7795.(16) Yau, S.-L.; Kim, Y.-G.; Itaya, K. J. Phys. Chem. 1997, 101, 3547.(17) Kunitake, M.; Batina, M.; Itaya, K. Langmuir 1995, 11, 2337.(18) Batina, N.; Kunitake,M.; Itaya, K. J. Electroanal. Chem. 1996,

405, 245.(19) Kunitake, M.; Akiba, U.; Batina, N.; Itaya, K. Langmuir 1997,

13, 1607.(20) Ogaki, K.; Batina, N.; Kunitake, M.; Itaya, K. J. Phys. Chem.

1996, 100, 7185.(21) Itaya, K.; Batina,N.; Kunitake,M.; Ogaki, K.; Kim, Y.-G.;Wan,

L.-J.;Yamada,T. InSolid-LiquidElectrochemical Interfaces; Jerkiewicz,G., Soriaga, M. P., Uosaki, K., Wieckowski, A., Eds.; ACS SymposiumSeries 656; American Chemical Society: Washington, DC, 1997; p 171.

7173Langmuir 1997, 13, 7173-7179

S0743-7463(97)00693-8 CCC: $14.00 © 1997 American Chemical Society

degreeof surfacediffusionofadsorbedmoleculesandhencemore disordered phases on Pt(111).Wehavealsopreviously revealedan important fact that

relatively large organic molecules such as porphyrinformedanorderedadlayer onan iodine-modifiedAu(111),while they formed a disordered adlayer in the absence ofthe iodinemonolayer.17-21 The iodine-modified Au(111)17-19

and Ag(111)20 electrodes were found to be suitablesubstrates on which to form highly ordered adlayers oflarger molecules, demonstrating that the interactionbetween molecules and substrates is an important factorin the processes of ordering of molecular adsorbates onsubstrates. VanderWaals type interactions between thehydrophobic iodine adlayer and the organic moleculesallowmolecularmotion on the surface to form the orderedphase.19, 21 The adsorption of organic molecules on Cuelectrodes is an interesting subject, because of relativelyweaker interactionsbetweenaromaticmoleculesandCu.1,2It is also well-known that benzotriazole is an effectivecorrosion inhibitor for Cu electrodes. Note that Behmand his co-workers recently reported adlayer structuresof benzotriazole adsorbed on Cu(100).22Here,we describe, for the first time, results of an in situ

STM study of adlayer structures of simple aromaticmolecules such as benzene, naphthalene and anthracenedirectly attached to a well-defined Cu(111) in aqueousHClO4 solution. ACu(111)-(1×1) structurewas observedon an atomically flat Cu(111) surface in a double-layerpotential range in the absence of organic molecules. Themolecules of benzene, naphthalene, and anthracenewerefound to adsorb on Cu(111) to form ordered layers with(3 × 3), (4 × 4), and (4 × 5) structures, respectively. Themolecular orientation and packing arrangement for eachadlayer were derived from high-resolution STM images.

Experimental Section

A bulk Cu(111) single-crystal disk with a diameter of 10 mm(from MaTeck) was used as a working electrode for bothelectrochemicalmeasurementand in situSTMobservation.Afterhavingbeenmechanicallypolishedwith successively finer gradesof diamondpastewithparticle sizes down to0.05µm, theCu(111)sample was annealed at ca. 600-700 °C in a quartz tube underhydrogen atmosphere for 2-3 h to remove the damaged layerproduced by themechanical polishing. TheCu(111) surfacewasthen electropolished in a phosphoric acid solution (50mL of 85%H3PO4 and 50 mL water) at 0.8-1.0 A cm-2 for 3-5 s. The Cucrystal was rinsed repeatedly with ultrapure Millipore water(Millipore-Q). Adroplet ofwaterwas left on the electrode surfaceto protect it from contamination during transfer to the electro-chemical cell. The Cu sample was then mounted in theelectrochemical cell for either cyclic voltammogrammeasurementor in situSTMobservation. Awell-defined single-crystal surfacewas prepared in situ by an anodic dissolution. It is shown belowthat the anodic dissolution in pure HClO4 was found to proceedin the layer-by-layermode, exposingatomically flat terraces overlarge areas.A solution of 0.1MHClO4 was prepared by diluting ultrapure

HClO4 (Cica-Merck, Kanto Chemicals) with ultrapureMilliporewater. Reagent grade benzene, naphthalene, and anthracenewere fromKantoChemicalsCo. Ltd. The solubilities of benzeneand naphthalene inwater were reported to be ca. 9 and 0.23mMat room temperature, respectively.23 The solutions containingthese organicmoleculeswerepreparedat a specific concentration(benzene) or at saturated concentrations (naphthalene andanthracene). Although the solubility of anthracenewasexpectedto be much lower than that of benzene or naphthalene, it wasfound in this study that the formation of a monolayer ofanthracenewas easily established in a saturated solutionwithin

a few minutes. The home-made electrochemical cell containeda reversible hydrogen electrode (RHE) in 0.1 M HClO4 and a Ptcounter electrode. All electrode potentials are reported withrespect to the RHE.The in situ STM apparatus used was a Nanoscope III (Digital

Instrument Inc., Santa Barbara, CA). The tunneling tips wereprepared by electrochemically etching a tungstenwire (0.25mmindiameter) in0.6MKOH. Anacvoltage of 12-15Vwasapplieduntil the etching stopped. The W tips were then coated withclear nail polish to minimize the Faradaic current.15 DuringSTMobservation the potential of the tunneling tipwas carefullyadjusted at a valuemore anodic than theCudeposition potentialtoavoidCudepositionon the tip, asdescribed in the literature.24,25Most of the STM images shown here were acquired in theconstant-current mode to evaluate the corrugation heights ofthe Cu(111) substrate and the adsorbed molecules.

Results and Discussion

Cyclic Voltammetry. Steady state cyclic voltammo-grams of Cu(111) were obtained by using the so-calledhanging meniscus method26 in the absence and presenceof benzene, naphthalene, or anthracene in 0.1 M HClO4.The first scan of each CV was made in the negativedirection from the open circuit potential (OCP; ca. 0.23V). Figure 1(a) shows a CV of Cu(111) in the absence oforganic molecules. It can be seen that the double-layerregion extends from -0.35 to 0.15 V. At ca. 0.15 V, theanodic dissolution of Cu started, and an abrupt increasein anodic current commenced at ca. 0.25 V. The cathodicpeak at 0.2 V observed upon reversal of the potential scancorresponds to the electrodeposition of Cumetal from theCu2+ ions formed during the anodic scan. The cathodiccurrent beginning at ca. -0.35 V is ascribed to hydrogenevolution. Note that although a similar voltammogramwas reported in a HCl solution,24 a small broad cathodic

(22) Vogt, M. R.; Polewska, W.; Magnussen, O. M.; Behm, R. J. J.Electrochem. Soc. 1997, 144, L113.

(23) Dean, J. A., Ed. Lange’s Handbook of Chemistry; McGraw-Hill:New York, 1985.

(24) Suggs, D. W.; Bard, A. J. J. Am. Chem. Soc. 1994, 116, 10725.(25) Suggs, D. W.; Bard, A. J. J. Phys. Chem. 1995, 99, 8349.(26) Yamada, T.; Batina,N.; Itaya, K.J. Phys. Chem. 1995, 99, 8817.

Figure1. Cyclic voltammogramsofCu(111) in (a) 0.1MHClO4,(b) 0.1 M HClO4 + 1 mM benzene, (c) 0.1 M HClO4 +naphthalene, and (d) 0.1MHClO4 + anthracene. The potentialscan rate was 50 mV s-1.

7174 Langmuir, Vol. 13, No. 26, 1997 Wan and Itaya

peak previously observed in HCl before the hydrogenevolution reaction did not appear in a HClO4 solution inthe absence of chloride ions.The CVs obtained in the presence of benzene, naph-

thalene, and anthracene are shown in Figure 1b,c, and d.The overall shape of each CV is almost the same as thatin Figure 1a obtained in the absence of organicmolecules.Only the electric charge involved in the double-layerpotential range becomes smaller due to adsorption of themolecules. Ourprevious resultshaveshownthatbenzene,naphthalene, and anthracene were chemisorbed onRh(111) and Pt(111) electrode surfaces, resulting inblockage of the hydrogen adsorption and desorptionreactions.15,16 The voltammograms shown in Figure 1appear to suggest that the organic molecules are moreweakly adsorbed onCu(111). In general, it is known thatthe adsorption of small aromatic molecules such asbenzene on metals of group IB elements, i.e., Cu, Ag, andAu,has the character ofphysisorption.1,2 Arecent infraredspectroscopic study on the adsorption of benzene on PtandCu inUHVclearly indicates that benzene is adsorbedon Cu more weakly than on Pt.27 The results mentionedabove strongly encouraged us to use a Cu electrode toinvestigate the adsorption of the aromatic molecules,including relatively large molecules such as anthracene.We expected the formation of highly ordered adlayers ofthemolecules onCu(111) in solutionbasedonourpreviousresults obtained on iodine-modified electrodes,15-21 be-cause of weaker interactions between Cu and the mol-ecules.In Situ STM. (1) Cu(111)-(1 × 1). To produce an

atomically flat Cu(111) surface for STM observation, afreshlypolishedsurfacewas first immersed in0.1MHClO4at the OCP in the STM electrochemical cell, and theelectrode potential was swept negative at the scan rate50 mV/s to the hydrogen evolution region and then backto theCudissolutionregion. After threeor five suchcycles,the electrodewas finally held at a potential near the onsetof the anodic dissolution. Although atomically roughsurfaces were consistently observed soon after the elec-tropolishing,well-defined terrace-step featureswere foundtodevelopgradually, indicating that theanodicdissolutionproceededbya layer-by-layermode inHClO4. The similarlayer-by-layer dissolution was recently found for Cu inHCl,24,25 S-modifiedNi,28,29 I-modifiedPd,30 and I-modifiedAg.31 The terrace width further increased when theelectrode potentialwasheld in the double-layer region forat least30-60min. Thisobservationsuggests thatsurfaceannealing also takes place on Cu(111).Figure 2a is a typical large scale STM image of a so-

preparedCu(111) surface acquired at-0.1V. Monatomicsteps (0.22 nm) are seen to run nearly parallel or at anangle of ca. 60° to each other, as expected for surfaceswith a threefold symmetry. However, it is clear that thesteps includemanydefects such as kinks because the steplineswerenot atomically straight, as canbe seen inFigure2a. The terraces are atomically flat with a low defectdensity, suggesting that the Cu(111) surface has a well-defined structure in HClO4.High-resolution STM imaging made it possible to

observe individual Cu atoms on the terraces. To ourknowledge, atomic resolution STM images have not yet

been achieved on bare Cu(111) in solution,24 althoughatomic force microscopy (AFM) showed a Cu(111)-(1× 1)structure.32 Our first atomic STM image of a Cu(111)surface is shown inFigure2b. Thehexagonal close-packedstructure can be seen with an interatomic distance of ca.0.26nm(thediameter of aCuatomis0.256nm), indicatingthat the Cu(111) surface has a (1 × 1) structure. Thearrows point out the directions of close-packed Cu(111)determined by the crystallographic orientation of thecrystal. The corrugation amplitude was too low to bemeasured accurately, i.e., ca. 0.01-0.015 nm, which issignificantly smaller than that observed on Au, Pt, Rh, orAg, typically 0.02-0.03 nm.12,15,31 The identical (1 × 1)structurewas consistently observed in thepotential rangebetween-0.35 and 0.15V. No evidencewas found for theoxidation of Cu(111) in HClO4, in contrast to the reportbasedon in situAFMexperiments that theCu(100) surfacemight be covered with adsorbed O or OH-, exhibiting a(x2 × x2)R45° structure, even in HClO4.32 The imageshown in Figure 2b demonstrates that, under the presentconditions, the Cu(111) surface retains a clean andunreconstructed (1 × 1) structure.(2) BenzeneAdlayer. After the atomic resolution image

of the Cu(111)-(1 × 1) structure shown in Figure 2b was

(27) Haq, S.; King, D. A. J. Phys. Chem. 1996, 100, 16957.(28) Suzuki, T.; Yamada, T.; Itaya, K. J. Phys. Chem. 1996, 100,

8954.(29) Ando, S.; Suzuki, T.; Itaya, K. J. Electroanal. Chem. 1996, 412,

139.(30) Sashikata, K.; Matsui, Y.; Itaya, K.; Soriaga, M. P. J. Phys.

Chem. 1996, 100, 20027.(31) Teshima,T.;Ogaki,K.; Itaya,K.J.Phys.Chem.1997,101, 2046.

(32) Cruickshank, B. J.; Sneddon, D.; Gewirth, A. A. Surf. Sci.Lett.1993, 281, L308.

Figure 2. STM top views of Cu(111) in 0.1MHClO4. (a) Largescale STM image showing an atomically flat Cu(111) surface.(b) Cu(111)-(1 × 1) structure. The set of arrows point out theclose-packed directions of Cu(111). Scanning rates were (a) 8.1and (b) 21.7 Hz, respectively. Tunneling currents were (a) 2and (b) 20 nA.

Aromatics Adsorbed on Cu(III) in Solution Langmuir, Vol. 13, No. 26, 1997 7175

observed, a small amount of benzene solutionwasdirectlyinjected into the electrochemical cell at -0.3 V. Theaverage concentration of benzene was ca. 1 mM. Figure3a is a large scale STM image revealing the molecularfeature of the benzene adlayer. This image was acquiredat -0.3 V. Although several single molecular defects areapparent, it can be seen that a well-ordered adlayer ofmolecules with a characteristic molecular shape andpacking arrangement is extended over wide atomicallyflat terraces. The molecular rows cross each other at anangle of either 60° or 120° within an experimental errorof (2°. From a comparison with the crystal orientation

of ⟨110⟩ determined by the Cu(111)-(1 × 1) atomic imageas indicated by the arrows, it can be easily concluded thatall molecular rows are almost perfectly parallel to theunderlying Cu(111) atomic rows. All benzene moleculesexhibit a corrugation height of ca. 0.02-0.025 nm.STM images acquired with even higher resolution

allowed us to determine the internal structure andorientationof eachbenzenemoleculeadsorbedonCu(111).The image shown in Figure 3b is an example acquired inan area including molecular defects. It is clear that eachbenzene molecule appears as a set of three spots withalmost the same corrugation height. The intermoleculardistance along the close-packed directions of Cu(111) ismeasured to be ca. 0.77 nm, which corresponds to threetimes the lattice distance ofCu(111). Themolecular rowscross each other at an angle of either 60° or 120° withinexperimental error. A clear dip exists at the center ofeach triangle with three lobes. The difference in cor-rugation height between the spot and the dip is ca. 0.006nm.Therefore,weconclude that thebenzeneadlayer formsa (3 × 3)-C6H6 structure with a surface coverage of 0.11,as illustrated in Figure 3c. Each benzene moleculeappeared with three isolated spots in a triangular con-figuration. Similar three spots have been found forbenzene onRh(111) inUHV5 and in solution,15 suggestingthat each benzene molecule is also located on the three-fold hollow site on Cu(111).It is rather surprising that all features observed for

benzeneadsorbed onCu(111) are almost identical to thosefound on Rh(111), as described in our previous paper.15However, there is a difference in detail between thebenzene adlayer on Cu(111) and that on Rh(111). Eachunit cell of the benzene adlayer on the Rh(111) surfacewas always accompanied by a weaker additional spot,which was attributed either to coadsored CO in UHV5,10

or to a water molecule or a hydronium cation in aqueoussolution.15 On the other hand, no such additional spotwas found on Cu(111), as can be seen in Figure 3b.Although many factors should be taken into account forthe weak additional spots,5,15 the interatomic distance ofCu onCu(111), 0.256nm, is smaller than that ofRh, 0.268nm. It is reasonably expected that the benzene adlayeris more closely packed on Cu(111) than on Rh(111),resulting in the disappearance of hydroniumcations fromthe unit cell.It is also important to note that the adsorbed benzene

monolayer on Cu(111) is desorbed in UHV in the tem-perature range between 150 and 240 K.33 At roomtemperature, all benzenemolecules except thoseadsorbedat defect sites undergo complete desorption from theCu(111) surface.33 This result obtained in UHV is incontrast to the present in situ STM result. The benzeneadlayer on Cu(111) is stable at room temperature insolution, suggesting that water molecules play an im-portant role in stabilizing theadlayer. On the otherhand,the nature of electrode/electrolyte interfaces is alsoexpected to be one of the most important factors. Theordered (3× 3) structurewas consistently observed in thepotential range from-0.35 to-0.2 V. At potentialsmorepositive than-0.2V, theorderedstructurebecameunclearand produced noisy images, suggesting that either de-sorption or reorientation took place at the positivepotentials. No clear peak corresponding to the phasetransition between the ordered and disordered phasesappears inCV,as shown inFigure1, indicating that chargetransferprocessesarenot involved in thephase transition.Note that no peak was also observed for benzene on

(33) Xi, M.; Yang, M. X.; Jo, S. K.; Bent, B. E. J. Chem. Phys. 1994,101, 9122.

Figure 3. (a and b)High-resolution STM images of a Cu(111)-(3 × 3)-C6H6 adlattice acquired at -0.3 V. Tunneling currentwas 20nA. Scanning ratewas 9.04Hz. (c) Real space structuralmodel. All molecules are assigned to threefold hollow sites.


Rh(111), as described previously.15 The potential of zerocharge (pzc) was reported to be ca. -0.25 V vs. SCE forCu electrodes.34 Recently, more negative values wereobtained on Cu(111).35 The interaction energy betweenaromaticmoleculesandCuelectrodes shouldbedependenton the charge on theelectrode. Weexpect that thepositivecharge on theCu(111) electrode decreases the interactionbetween benzene molecules and Cu to form an orderedadlayer in solution. However, further studies usingdifferent techniques such as capacitance measurementand in situ IR are needed to understand the nature of thebenzene adlayer at potentials more positive than -0.2 V.A similar potential dependence was also found fornaphthalene but not for anthracene, as described below.(3) Naphthalene Adlayer. Figure 4 shows high-resolu-

tion STM images of a naphthalene adlayer at -0.3 V. Along range ordered adlayer can be clearly seen over thelarge area (20 × 20 nm), as shown in Figure 4a.Naphthalene molecules uniformly cover the Cu(111)surface with a small number of defects. Each spot has anelongated feature corresponding to an individual naph-thalene molecule. The molecular rows were found to beparallel to the ⟨110⟩ direction of the underlying Cu(111)lattice. This ordered adlayer was consistently observedin the potential range from -0.35 to -0.15 V. At morepositive potentials, the ordered adlayer disappeared,resulting in noisy STM images, which was observed alsofor the adsorption of benzene as described above.The STM image shown in Figure 4b showsmore details

of the packing arrangement and the internal structure ofthe naphthalene adlayer. In this image the two-ringstructureof thenaphthalenemoleculeappearsas twomainspots with additional details of the internal structure.The distance between the two main spots is ca. 0.56 nm,as expected from the molecular model of naphthalene. Itis clear that each molecule obviously bonds to Cu(111)with a flat-lying orientation. Thenaphthalenemoleculesare alignedwith the longermolecular axis (C2) in the samedirection along the Cu rows. The observed distancebetween the nearest neighbormolecules is 1.0( 0.02 nm,which is nearly equal to four times theCu lattice distanceof 0.256 nm. All features in the STM images indicatethat the structure of the naphthalene adlayer can bedefined as Cu(111)-(4 × 4)-C10H8. A unit cell is outlinedin Figure 4a and b. This (4 × 4) structure results in asurface coverage of 0.0625.It was reported in our recent paper that naphthalene

formed a highly ordered structure with (3x3× 3x3)R30°symmetry on Rh(111) in HF solution.16 The internalstructure of naphthalene was more clearly discerned onRh(111) than on Cu(111), which allowed us to determinea micro-orientation of naphthalene molecules along themolecular rows.16 Although the image shown in Figure4b can fairly accurately determine the symmetry of theadlayer, (4×4), thedeterminationof themicro-orientationwas not as easy as that on Rh(111) even though optimumimagingconditionsweresearchedbyvarying the tunnelingcurrent (1-50nA) and the bias voltage (20-300mV). The(3x3 × 3x3)R30° structure found on Rh(111) results ina surface coverage of 0.11, which is nearly twice as largeas that found on Cu(111). This difference suggests thatnaphthalene is more strongly attached on Rh(111) thanon Cu(111). The repulsive interaction between adjacentnaphthalene molecules seems to be a predominant factorfor the formation of the (4 × 4) structure. A tentativelyproposed ball model for the (4 × 4)-C10H8 structure is

shown in Figure 4c. In this model each naphthalenemolecule is placed onto twoneighboring twofold siteswithits C2 axis aligned along ⟨110⟩ of the Cu(111) substrate.A similar binding site was assumed for naphthalene onRh(111), as described previously.16 In this open adlayerstructure, however, one can expect thatmolecularmotionisnot completelyprohibited in theunit cell. Weanticipate

(34) Hamelin, A.; Sevastyanov, E.; Popov, P. J. Electroanal. Chem.1983, 145, 225.

(35) Hartinger, S.; Doblhofer, K. J. Electroanal. Chem. 1995, 380,185.

Figure 4. (a and b) High resolution STM images of a Cu(111)-(4 × 4)-C10H8 adlayer acquired at -0.3 V. Tunneling currentwas 20 nA. Scanning rate was 10.85 Hz. The close-packeddirections of theCu(111) lattice are indicated bya set of arrows.(c) Schematic representation for the (4 × 4) structure.


that this type of motion decreased the resolution in STMimaging on Cu(111).(4) Anthracene Adlayer. It was surprising to find an

extraordinarily ordered adlayer of anthracene onCu(111)becauseanthracene forms completelydisorderedadlayersonRh(111) andPt(111), as described in our recent paper.16Even in a large area of 30 × 30 nm, the highly orderedmolecular array can be seen withmolecular resolution asshown in Figure 5a. Each molecule appeared as an

elongated spot. The imagewas acquired at-0.25 V in ananthracene-saturated 0.1 M HClO4 solution.Figure 5b shows a high-resolution STM image showing

details of an anthracene adlayer. The image of eachmolecule shows details of the internal structure ofanthracene. It is also clear that theanthracenemoleculesare preferentially aligned with their long C2 axes alongone of the close-packed directions of the Cu(111) lattice,as indicated by the set of arrows. The side-by-sideconfiguration is more clearly seen than that for naph-thalene shown inFigure 4b. The intermolecular distancealong the straight molecular rows is ca. 1.25 ( 0.05 nm,which corresponds to five times the lattice parameter ofCu. A unit cell and a model of the packing arrangementare shown in Figure 5b and c, respectively. The distanceon the shorter side is ca. 1.0(0.05nm. The twodirectionsin the unit cell are parallel to the Cu(111) lattice for allmolecules observed. These features strongly suggest thatthe structure of the observed molecular adlayer is (4 ×5)-C14H10 with a coverage of 0.05, as shown in Figure 5c.We tentatively propose that each anthracene moleculeoccupies nearly three two-fold bridge sites with its longC2 axis along the Cu(111) lattice. The center ring ofanthracenemight be exactly located in the center positionof a two-fold bridge site, whereas two other rings areslightly shifted from the center position. A detailedinspection of the STM image shown in Figure 5b revealedthe nonequivalent appearance of the three rings ofanthracene.It is noteworthy that the anthracene adlayer could be

seen in the potential range between -0.35 and 0 V. Asdescribed above for benzene and napthalene adlayers,those molecules could not be seen at potentials morepositive than ca. -0.15 V. On the other hand, a clearimage similar to that shown in Figure 5b was observedwith anthracene even at 0 V, which is more positive thanthe pzc. This indicates that the interaction betweenanthracene and the Cu(111) surface is stronger than thatof benzene or naphthalene with Cu(111), because of thelarger cross sectional area of anthracene.Finally, the results shown here strongly encouraged us

to explore the potential applications of in situ STM toinvestigate various organic molecules. Behm and his co-workers recently reported adlayer structures of benzo-triazole adsorbed on Cu(100).22 Benzotriazole is knownto be an effective corrosion inhibitor for Cu electrodes.

Conclusion

An atomically flat Cu(111) single-crystal electrodesurface could be exposed by in situ anodic dissolution inHClO4. The Cu(111)-(1 × 1) structure was consistentlyobservedonthewell-definedCu(111) surface in thedouble-layer potential range. High-resolution STM imageswerepresented of molecules of benzene, naphthalene, andanthraceneadsorbedonaCu(111) single-crystal electrodesurface in aqueous HClO4 solution. The packing ar-rangement and the internal molecular structure of themolecules were determined by in situ STM. All of thethree molecules formed long range ordered adlayers onCu(111)with flat-lying orientation. In situSTMrevealeda (3 × 3)-C6H6 structure with a coverage of 0.11. Eachbenzene molecule appeared as a triangular shape, imply-ing its three-fold registry. A (4 × 4)-C10H8 adlayerstructure with a coverage of 0.0625 was observed in a 0.1M HClO4 + naphthalene solution. The naphthalenemolecule was found to adsorb on the electrode surfacewith its longC2 axisalong the ⟨110⟩directionof theCu(111)lattice. The spots corresponding to the two-ring structurewere clearly discerned in high-resolution STM images.

Figure 5. (a and b)High-resolution STM images of a Cu(111)-(4 × 5)-C14H10 adlayer acquired at -0.3 V. Tunneling currentwas 10 nA. Scanning rate was 10.85 Hz. (c) Schematicrepresentation for the (4 × 5) structure.


Eachnaphthalenemoleculewas assumed to be located ona row of Cu atoms across two neighboring twofold sites.Anthracene molecules also formed a long range orderedadlayer on a Cu(111) surface. A (4 × 5)-C14H10 structurewas revealed. The present results show that high-resolution in situ STM images can supply direct informa-tion on the adsorption of organic molecules on activemetallic electrode surfaces. The internal structures of

benzene, naphthalene, and anthracene adlayers on Cu(111)were successfully probed.

Acknowledgment. We thank the ERATO project forfinancial support and Dr. Y. Okinaka for his help in thewriting of this manuscript.

LA9706936


in situ scanning tunneling microscopy of benzene, naphthalene, and anthracene adsorbed...

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