In-situ x-ray specular reflectivity and glancing incident angle x-ray diffraction measurements have been performed in the Au(001) surface in two solutions under potential control in an electrochemical cell. In both the 0.01 M HCl0{sub 4} and 0.01 M KBr solutions a ``(5 {times} 20)`` reconstruction is formed at sufficient negative potentials. The reconstruction is similar to that obtained for the clean surface in vacuum

Ocko, B. M.

Wang, Jia

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BNL--45 045. . DE92 007613..ELECTROCHEMICALLY INDUCED RECONSTRUCTION OF THE Au(001)'," "SURFACE: AN X-KAY SCATTERING STUDYB.M. Ocko and Jia WangPhysicsDepa_¢ment ' ' :'_i ,_,ii_::_!:_Brookhaven National Laboratory 'i __:.,,._!' ':Upton, New York 11973ABSTRACTIn-situx-rayspecularreflectivi,tyand glancingincidentangle x-raydiffractionmeasurements have been performed in the Au(001) sur- _.faceintwo solutionsunder potentialcontrolinan electrochemicalcell.In both the 0.01 M HCIO4 and 0.01 IrlKBr solutionsa "(5 × 20)"reconstructionisformed at sui_cientnegativepotentials.The recon-structionissimilarto thatobtainedforthe cleansurfacein vacuum.The utilization of surface x-ray scattering techniques (SXS), scanning tunnelingmicroscopy (STM), and atomic force microscopy (AFM) techniques during the pastdecade has greatly enhanced our understanding of surfaces in vacuum. Currently,these same tect_tiques are rapidly being utilized to study electrode surfaces on anatomic scale with an increasing level of sophistication [1-6]. Here we report theprinciple results of a SXS study of the Au(001) electrode surface in an acid solution(0.01 M HC104 ) and in a salt solution (0.01 M KBr). In a SXS measurement thediffracted intensity couples directly to the periodicity of the top several layers ofmetal atoms. This facilitates a direct in situ measurement of the phase behaviorversus the applied potential with a high degree of accuracy.In vacuum, the Au(001 )surface exhibits a hexagonal reconstruction ,vhere thereare nearly six surface atoms for every five bulk ato_s along the [110] direction [7-10]. The reconstruction is often referred to 'as "5 × 20" although the actual toplayer is more accurately described as incommensurate [9]. The in-plane surfacediffraction (see Figure la) is described by a hexagonal pattern centered aroundthe origin with a wavevector v/2&la*(/kl = A2) , where the incommensurab;-!ity6 = A1 -- 1 = 0.206 :i: 0.001 [8,11], a* = 2_:/a, and where a = 4.081 /_ is the sizeof the face centered cubic _mit cell for Au. The orientation of the reconstructedlayer is rotated by ±0.8 ° from the [110] axis, with a surface corrugation amp,_udeof <_ 0.50 _ peak-peak [9,11,12], has 25T6 excess mass relative to the underlyingbulk layers [13,14] and a 20% interlayer expansion [14]. The excess mass and theexpansion nearly conserve the bulk packing density.4. The present study has been motivated by extensive electrochemical studies ofthe Au(001) surface [15,16]. From measurements of the potential of zero char_'e(PZC) [15,16] which is related to the work function, optical reflectivity(OR) [1_5]|and second harmonic generation [17] it has been inferred that a potential changecan induce a surface structural trm_sition. F_x-situ low energy electron diffractionmeasurements [18], after emersion from an electrochemical cell, suggest that thestructuraltransition inferred from the PZC and OR measurements corresponds tothe lifting of the hexsgonal reconstruction. More recently, STM studies of theAu(001) surface in HC104 solutions [6] have directly imaged the hexagonal recon-struction with atomic resolution under potential control.In thepresentpaper,we reporttheresultsofan x-rayscatteringstudyfrom theAu(001) electrodeinHCIO4 and KBr electrolytes.The structureofthe goldsurfacehas been determined at fixedpotentials.In addition,the potentialdependence ofthe scatteringhas alsobeen measured. The x-ray measurements in HCIO4 havebeen previouslyreported[4].Two typesof x-rayscatteringmeasurement were carriedout. In-planesurfacex-ray di_T'ractionmeasurements, with the x-raysincidentat a very shallow(--_)angleto the surface,has been used to determinethe crystallographicstructureof2-D surfaceoverlayers.X-ray specularreflectivitymeasurements have been usedto determine the electrondensityprofilein the directionnormal to the surq_ace.The reflectivitydata providesinformationon the density,rouglmessand interlayerspacingofato_--_iclayersparallelto the crystalsurface.These two independentandcomplementary measurements can be used togetherto give a detailedpictureofsurfacestn2cture.Fig. l(a)shows the electrochemicalcell.A 6_m polypropylenewindow coversand sealsthe cellwith a thincapillaryfilm(< 20_m) between the metal surfaceand the polypropylenefilm.A detaileddescriptionof the apparatus ispresentedelsewhere[4,19].Loss of signaldue to absorptionof the x-ray'sby thesolutionandthe polymer filmissmallsincethe absorptionlengthof x-raysin thesematerialsisseveralmillimetersforx-raysof 10 keV.The in-planediffractionpatternforthe Au(001)surface,isshown inFig.l(b).Fig. 2(a) shows a scan actors the hexagonal diffraction point (Al, Al, 0) for 0.01 MHC104 . This rocking curve exhibits two distinct peaks for the approximate hexag-c:a2 reconstruction at low potentials (open circles). The rotation of the hexagonalreconstructed layer with respect to the [110] direction agrees with the 0.8 ° rotation- angleobtainedunder vacuum conditions[9,10].Above 0.5V, the surfacescatteringvanishesasshown inFig.2(a)(closedcircles)..Atthesepotentials,thereconstructediayerhas liftedand onlysurfacereflectionsconresponding_o the bulk (squaresym-111 metry, see Fig. (lb)) axe observable. The presence of the reconstructed phase hasbeen determined by integrating the rocking curves at the hexagonal positions shownin Fig. 2(a). Fig. 2(b) shows this integrated intensity as a function o'_potential acrossthe transition region between 0 and 0.6 V at an effective scan rate of 0.004 mV/sec..The integrated inteusity starts to decrease at about 0.20 V, falls to half at 0.35 V,and nearly vanishes by 0.50 V. The transition occurs at somewhat higher potentialswhen the scan rate is increa_sed.At negative potentials, the in-pla,_e diffraction features a near hexagonal pattern- with A i = 1.205:t=0.002 and _2 = 1.200"t-0.005 where _I and A2 are the ma,_nitudeof the hexagonal wavevector along the [110] and rotated directions as shown inFig. 3. The incommensurability, is in near perfect agreement with vacuum studiesof the Au(001) surface [9-10]. There is a snmLt change in A1 and virtually no changein _2 with potential. This is in contrast to the Au(111) surface where the stripeseparation changes continuously from p=23 at the most negative potentials to p=30before the reconstruction lifts [19]. We do not observe a corresponding continuoustransition at the Au(001) surfaces.The surface norton:!: distribution of the electron density profile can be directlyrelated to the specular reflectivity profile. In a specular reflectivity measurementthe scattered intensity, along the surface normal wavevector, is measured on an• absolute scale [19]. Fig. 3(a) shows the specular reflectivity for the reconstructed" (-0.4 V, open circles) and lifted (+1.0 V, closed circles) surface structures in 0.01M HCIO4 . There are distinct differences between these two curves which axe mostapparent at L, = 1 where the reflectivity differs by a factor of 5. The specularreflectivity R(Q,) where Qz = 2_rL/a can be modeled as a sum over atomic layers^ o-Q_q_/2.iQ.(_/2)(,,,-_)>R ( Q z) cx < _,,,z_ ,.m_--.Owhere the proportionality factor can be calculated from fundamental constants.Each term in the sum corresponds to an atomic layer m where pm is the electrondensity relative to a bulk (001) layer, ern is tb.e increase in atomic interlayer spac-ing relative to a bulk (001) layer and am is the root-mean-square (r.m.s.) atomicdisplacement.For a perfectly terminated crystal (square .symmetry), the calculated specularreflectivity is shown in Fig. 4(a) (broken line). It does not agree with data fromeither the reconstructed or the lifted surfaces. At -0.4 V (reconstructed surface), anexcellent fit to the data (Fig. 4(a), solid line) is obtained for a top layer density of1.21 with a 20% outward expansion of the top gold layer, relative to the bulk gold-gold layer spacing of 2.04/1_. The excess density and expansion of the top layer arein close agreement with the close-packed hexagonal layer found in vacuum [9,11].At +1.0 V, the best fit is obtained with a top layer density of 0.22 and no expansionof the interlayer spacing for the top layer of gold. Constraining the top layer densityto unity causes a dramatic increase in the error of the fit. At the Au(111) electrodesurface, adsorbed electrolyte species modify the observed reflectivity [19]. Thiseffect has not been included in the analysis of the Au(O01)/electrolyte interface.• The transition between the reconstructed and the lifted surfaces can be repeatedon cycling the potential. Fig. 4(b) shows the voltage dependence of the reflectedintensity at (0,0,2.3) for two different potential scan rates in the HC104 solution.The lower scan rate shows a smaller hysteresis for the two scan directions. Wenote that in the forward sweep direction, the intensity at the wavevector (0, 0, 2.3)increases when the reconstructed layer lifts. This is because the asymmetry in thereflectivity, in the wings of the (002) Bragg peak (see Fig. 4(a)), vanish when theslc'face transforms from the reconstructed to a (lxl) surface layer. The changein intensity showr: "n Fig. 4(b) is primarily due to changes in the density of thegold layers and not due to aniola adsorption. The latter has been observed at theAu(lll) electrode surface in salt solutions [19].We have also carried out x-ray di_r'raction studies of the Au(001) surface incontact with 0.01 M KBr. In salt solutions_ the double layer region extends to -0.8V versus an Ag/ AgC/ reference whereas in 0.01 M HC104 , hydrogen evolutionstm-ts at-0.4 V. The in-plane x-ray difraction pattern is identical to tha_ observedin the HC104 _olution. In Fig. 5 we show the potential dependence of the scatteringat three wavevectors; (1.205, 1.205,0.3), (1, 1, 0.3), and (0,0,2.3) at a scan rate of2 mV/sec, in the 0.01 M KBr electrolyte. The first two wavevectors are in thegazing incident geometry and are sensitive to the in-plane atomic structure. Thewavevector (0, 0, 2.3) corresponds to a specular position which is sensitive to theelectron density distribution along the surface normal direction. In all three panelsof Fig. 5 the diffuse background has been subtracted and the maximum intensityhas been normalized to unity.The scattered imensity at a wavevector (1.205, 1.205,0.3), rotated by 0.8 °, issensitive to the order of the hexagonal reconstruction. Above 0.0 V the scatteredintensity, after background subtraction, is zero as sho_wn in Fig. 5(a). This is becausethere is no longer a reconstructed surface layer above this potential. In the negativepotential sweep, the reconstruction does not start to form until -0.45 V. Concurrentwith the formation of the reconstruction there is a decrease in the scattered intensityat a wavevector (1, 1, 0.3) as shown in Fig. 5(b). This reciprocal space positioncorresponds to the in-plane periodicity of the underlying gold layers. In the positivedirection, the scattered intensity exhibits a dip before the reconstruction is lifted.This reflects a loss of surface order during the lifting process. Similar behavior hasbeen observed at the Au(lll) electrode surface in contact with salt solutions [19].During the reconstruction lifting process, the reflectivity at (0, 0, 2.3) increases,as shown in Fig. 5(c), since the top layer relaxes inward. This occurs at the samepotential where the reconstruction is Iifted (see Fig. 5(a)). At the most positivepotentials, where bromide ions are adsorbed at the surface, the reflectivity is loweredrelative to a (lxl) Au surface with no adsorbed bromide ions. This is becausethe bromide ions interfere destructively with the scattering from the gold layersat (0, 0, 2.3). As the potential is scanned negatively, the reflectivity increases at apotential starting at about -0.1 V. At -0.45 V there is a sharp drop in the reflectivitybecause the top layer expands outward when the surface reconstructs. Before thisdrop, the reflectiivity continues to increase due to the desorption of bromide ions.In contrast, a plateau (before the drop at -0.45 V) in the reflectivity would haveindicated a completion of the bromide desorption process. These findings indicatethat bromide ions are still adsorbed at the surface when the reconstruction forms.The reconstruction lifting/formation transition is much sharper in the KBr so-lution than in the HC104 solution (see Fig. 5(c) and Fig. 4(b)). In addition, thepotential dii_eren.ce between the reconstruction Lifting and formation transitions{i.e. hysteresis) i:s 0.4 V whereas the corresponding difl'erence in the acid solutionis nearly a full w_lt. _,n part, we can attribute these differences to kinet, ic effectssince the surface mobility of gold atoms is enhanced by the adsorption of bromide.Furthermore, the surface charge as a function of applied potential changes fasterin the bromide solution than in HC104 solution. This potential difference is about0.20 V and 0.10 V for the Au(111) and Au(ll0) surfaces, respectively, in KBr elec-trolytes. For Au(001) there is a shift in the PZC (0.12-0.22 V) between the (lxi)and the reconstructed phases on acco_mt of the change in symmetry between thesetwo phases [15,16]. This shift explains, in part, why there is the greatest potentialhysteresis for the Au(001) surface relative to the (111) and (110) surfaces.These measurements of the potential induced reconstruction of the Au(001)electrode in contact with HC104 and KBr solutions demonstrate that importantstructural information on electrode surfaces can be obtained from in situ surface x-ray scattering experiments. The hexagonal structure of the Au(001)reconstructedelectrode in both the acid and salt solution appears nearly identical to room tem-perature vacuum results. The phase behavior depends very much on the solutionspecies.This work is supported by the Division of Materials Research, U.S. Departmentof Energy, under Contract No. DE-AC02-76CH00016.REFERENCESi. M,F. Toney and O.R. Melroy,in "In-SituStudiesof ElectrochemicalInter-faces,"H.D. Abrufia,Editor,VCH VerlagChemical, Berlin(1991).2. J. Wiechers, T. Twomey, D.M. Kolb, and B.J. Behm, ,1. E_ectroanal Chem.,248, 451 (1988).: 3. S.L. Yau, X. Gao, S.C. Chang, B.C. Schardt, and M. Weaver, J. Am. Chem.Soc., 113, 6049 (1991).: 4. B.M. Ocko, J. Wang, A. Davenport, and H. Isaacs, Phys. Rev. Left., 65, 1466(1990).5. S. Manne, P.K. Hansma, J. Massie, V.B. Elings, and A.A. Gewirth, ScieTzce,25, 183 (1991).6. X. Gao, A. Hamelin, M.J. Weaver, Phys. Rev. Left., 67, 618, (1991).7. P.W. Palmberg and T.N. l:Lhodin, Phys. Rev., 161,586 (1967).=8. G.E. Rhead, Y. Phys. F, 3, L53 (1973).9. S.G.J. Mochrie, D.M. Zehner, D. Gibbs, and B.M. Ocko, Phys. Rev. Lett.,64, 2925 (1990).10. D. Gibbs, B.M. Ocko, D.M. Zehner, and S.G.J: Mochrie, Phys. Rev. B, 42,_ 7330 (1990).11. B.M. Ocko, D. Gibbs, K.G. Huang, D.M. Zehner, and S.G.J. Mochrie, Phys._ Rev. B, 44, 6429 (1991).12. K.H. Rieder, T. Engel, R.H. Swendsen, and M. Manninen, Surf. Sci., 127,223 (1983).13. D.M. Zehner, B.R. Appleton, T.S. Noggle, J.W. Miller, J.H. Barret, L.H. Jenk-ins, and O.E. Schow, III, J. Vac. Sci. Technol., 12, 454 (1975).14. D. Gibbs, B.M. Ocko, D.M. Zehner, and S.G.J. Mochrie, Phys Rev. B, 38,7303 (1988).15. A. Hamelin, J. Electroana.l. Chem., 142, 299 (1982); A. Hamelin, J. Elec-troanal. Chem., 255, 281 (1988).16. D.M. Kolb and J. Schneider, Surf. Sci., 162, 764 (1985).17. A. Friedrich, B. Pe'thager, D.M. Kolb, G. Lupke, R. Steinhoff, and G. Maxowsky,Chem. Phys. Let_., 163, 123 (1989).18. M.S. Zei, G. Lehmpfuhl, and D._v._.Kolb, Surf. Sci., 221, 23 (1989).19. J. Wang, B.M. Ocko, A.J. Davenport, and H.S. Isaac.s, Submitted to Phys.Rev. B., (1991).,o_(&_, A_,0)PolypropyleneWindow L, _--• ._. (2_) [lOOl/ O Spmmmr/ • sc_eReference • Hexagon=electrocleGoldCrystal Electrolyte B,(a) _)Fig.I (a)Electrochemicalx-rayscatteringKeI-Fcell.(b) In-plane dil_a_tionpattern forthe Au(001) surface.The squares show thepattern from the bulk which has square symmetry while the trianglesshowthe pattern from the hexagona_ reconstructionwhich forms at low applied= potentiaJs.Each point representsa rod of scatteringnormal to the surface..... 1.2.... ! .... Is00_ ( CD) ._, .+0.sv I °°°-°= o -0.4V ) o =CO-%_ooF / \ _ - o&-i °300 o 0.30 o200 _ ,,, , , , . I , , - _ 0• , I , I i I , L-1.6 -0.8 0 0.8 1.6 -().5 0 0.5 1.0 ,_,(d,,g.,.,,) E_IA_Ci(,,oi=)Fig. 9 Ca) Glamcing {nc{dent angle x-ray di_raction rocking] curves for -0.4 V a_Id 0 5- - ._0o, 0 3V in 0.01 M HCIO4 at the primary hexagonal wavevector (1.20,5, i 9 _, . ).(b) PotentiaJ dependence of the inte,g'rated intensity (back_ound subtracted)of the rocking curves shown in (a) upon increasing the potential.1.208 ,Z_ Z_1.206 "_ '_ _z_ AJ_Oo"" 1.204o" _ __ 1.202 "ot,._.w _ _ _ _ _ _ _ _1.198 ' ' ' ' , ,_-0.3 0 0.3E.=m/._jC=(_=)Fig. 3 Potential dependence of the wavevector magnitudes _l and _? in 0.01 MHC104 .10..40.04_$m1°"6, 0.______ ..-=___r.o 10-'_ 3'" -- kl==t 0.02oo.104 ° 0.01• | ,,,, I I I I •o I _ 3 -o_ o o_ .oL(uNmsof:') _'_/_cI (_o_t,)Fig. 4 X-ray reflectivity in from the At1(001) electrode in 0.01 M HC104 .(a) Absolute reflectivity data for the (0,0, L) rod at -0.4V (open circles) and1.0V (closed circles) where L = aQ,../27r.(b) Potential dependence of the specular reflectivity at (0, 0, 2.3) at differentpotential scanning rates.• _ ' ,, n n m n,n I ' n n n I II1.20.8 1 (1.205,1.205,0.3).4-N 1.2_E Cb)o=0.8 o.4C::(I,)--=1.2@)0.80.4.0-I.0-0.5 0 0.5 1.0EAg/AgCl (volts)Fig.5 Potentialdependenceofthescatteredx-raysat theAu(001) surfaceincontactwith 0.01M KBr at (a)(1.205,1.205,0.3)(b)(I,I,0.3)and (c)(0,0,2.3}.DIF_--_AINIERThis report was prepared as an account of work sponsored by an agency of the United StatesGovernment. Neither the United States Government nor any agency thereof, nor any of their- employees, makes any warranty, express or implied, or assumes any legal liability or responsi-bility for the accuracy, completeness, or usefulness of any information, apparatus, product, orprocess disclosed, or represents that its use would not infringe privately owned rights. Refer-All ence herein to any specific commercial product, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom-- mendation, or favoring by the United States Government or any agency thereof. The viewsand opinions of authors expressed herein do not necessarily state or reflect those of theUnited States Government or any agency thereof._I IIII III 111 I .... "" i I iii I I II I I __ir

Electrochemically induced reconstruction of the Au(001) surface: An x-ray scattering study

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Electrochemically induced reconstruction of the Au(001) surface: An x-ray scattering study

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