In Situ Scanning Tunneling Microscopy Study of the Anodic Oxidation

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J. Phys. Chem. B 2001, 105, 4263-4269

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In Situ Scanning Tunneling Microscopy Study of the Anodic Oxidation of Cu(111) in 0.1 M NaOH Julia Kunze,†,‡ Vincent Maurice,*,† Lorena H. Klein,† Hans-Henning Strehblow,‡ and Philippe Marcus† Laboratoire de Physico-Chimie des Surfaces, CNRS (ESA 7045), UniVersite´ Pierre et Marie Curie, Ecole Nationale Supe´ rieure de Chimie de Paris, F-75231 Paris Cedex 05, France, and Institut fu¨ r Physikalische Chemie und Elektrochemie, Heinrich-Heine-UniVerista¨ t, D-40225 Du¨ sseldorf, Germany. ReceiVed: October 31, 2000; In Final Form: February 7, 2001

In situ electrochemical scanning tunneling microscopy (STM) measurements of the anodic oxidation of Cu(111) in 0.1 M NaOH are reported. Anodic oxidation is preceded, in the underpotential range, by adsorption of an ordered layer assigned to OH species. This ordered adlayer is a precursor of the oxide growing at higher potential with the copper surface reordering to mimic the structural arrangement of a (111) oriented Cu2O oxide. In the potential range of Cu(I) oxidation, a Cu2O(111) oxide film is formed with a faceted, and most likely hydroxylated, surface. The nucleation, growth, and crystallization of this Cu(I) oxide depend on the overpotential of oxidation. At low overpotential, poorly crystallized and one-monolayer-thick islands partially covering the substrate are formed after preferential nucleation at step edges. At higher overpotential, well crystallized and several-monolayer-thick films are formed, and the step edges are not preferential sites of nucleation. In the potential range of Cu(II) oxidation, a crystalline Cu2O/CuO,Cu(OH)2 duplex film is formed. The cathodic reduction of these anodic oxides rebuilds the original extended and flat terraces of the substrate for oxides of monolayer thickness but produces a faceted Cu surface when a thicker oxide film is reduced.

Introduction The passivation of copper in aqueous solutions has been described by several authors in the past.1-16 The formation of protecting anodic oxides in solutions of pH > 5 is in agreement with the thermodynamic predictions of the Pourbaix diagram.1 Potentiodynamic polarization curves show characteristic anodic and cathodic peaks that are related to the formation and reduction of these anodic oxides (Figure 1). The potentials of these peaks show hysteresis of ca. 0.4 V with an average value close to the thermodynamically expected value of oxide formation. In acid solutions, the dissolution of the anodic oxides is too fast to provide passivity. The composition of the layers has been studied qualitatively and quantitatively with electrochemical and surface analytical methods such as X-ray photoelectron spectroscopy (XPS)2-4 and ion scattering spectroscopy (ISS)2 and in situ Raman5-7 and infrared8 spectroscopies. A Cu2O layer is formed at E > 0.58-0.059 pH/V, whereas a Cu2O/CuO,Cu(OH)2 duplex film is found for E > 0.78-0.059 pH/V (all potentials are referred to the standard hydrogen electrode, SHE). The total thickness of the anodic layer increases with the electrode potential but does not exceed 6 nm. Thicker films are formed in strongly alkaline electrolytes in the potential range of the second anodic peak (AII) of the polarization curve, presumably by a dissolution-precipitation mechanism. The investigation of oxide layers formed by anodic pulses followed by immediate and fast reduction scans suggests the intermediate formation of a precursor oxide close to Cu2O9,10 whose prop* Corresponding author: e-mail [email protected]; fax 33 1 46 34 07 53. † Universite ´ Pierre et Marie Curie, Ecole Nationale Supe´rieure de Chimie de Paris. ‡ Heinrich-Heine-Univerista ¨ t.

Figure 1. Potentiodynamic polarization curve of Cu(111) in 0.1 M NaOH, scan rate 20 mV/s.

erties change upon aging by dissociation into the duplex film for potentials E > EAII.10-13 The structure of these anodic oxide films has been examined more recently.14-16 Small oxide grains, 1.5-3 nm wide and ca. 0.3 nm high, have been found by in situ electrochemical scanning tunneling microscopy (STM) in borate buffer.14 They form preferentially at the step edges of the Cu(111) terraces when the potential approaches the value of Cu(I) oxide formation. At more positive potentials, oxide grains are formed very rapidly all over the surface due to the higher overpotential of oxidation (i.e., larger driving force). STM images get unstable when the oxide thickness exceeds the value of a monolayer. The duplex film formed at higher potential can be imaged with STM; small grains, 2-5 nm wide, are found for this oxide layer in borate buffer.14 No atomic resolution was achieved on the

10.1021/jp004012i CCC: $20.00 © 2001 American Chemical Society Published on Web 04/12/2001

4264 J. Phys. Chem. B, Vol. 105, No. 19, 2001 oxide grains formed in a borate buffer solution, and the STM data indicate an amorphous structure of both the Cu(I) single layer and the Cu(I)/Cu(II) duplex layer.14 In contrast, a crystalline and epitaxial Cu(I) oxide layer has been observed by in situ electrochemical atomic force microscopy (AFM) in 0.1 M NaOH:15 Cu2O(111) is formed on Cu(111), and Cu2O(100) is formed on Cu(100). The difference with the results obtained in the borate buffer solution14 indicates a strong anion and/or pH effect on the crystallinity of the anodic oxide film. Very recent in situ STM investigation performed in 0.1 M NaOH show that the formation of anodic layers is preceded by adsorption at more negative potentials (-0.70 V < E < -0.25 V).16 A small anodic peak and an equally small cathodic peak of ca. 55 µC/cm2 are observed in the polarization curve. The adsorbed species are assigned to OH groups on the basis of the associated charge transfer and structure of the adlayer. Indeed, the charge-transfer yields a density of ∼0.35 × 1015 OH‚cm-2 that is confirmed by the STM measurements. During adsorption, a reconstruction process of the Cu surface is evidenced by the lateral growth of the terraces. This is explained by the structural model proposed for the adsorbed phase in which the topmost Cu plane of the Cu(111) surface reorders from the close-packed structure of (111)-oriented metallic copper into the hexagonal structure of the Cu planes in (111)-oriented Cu2O. This reconstruction corresponds to a decrease of 30% of the atomic density in the topmost Cu plane. The excess Cu atoms are transferred to the step edges of the terraces, which results in the lateral growth of the terraces. The ratio of OH groups to Cu atoms in the reconstructed topmost plane is 1:4 as inferred from the structure of Cu2O. Indeed, in the (111) orientation, Cu2O consists of a face-centered cubic stacking of Cu planes (density of ∼1.28 × 1015 atoms‚cm-2) embedded between two oxygen planes (density of ∼0.32 × 1015 atoms‚cm-2).17,18 The formation of the adsorbed reconstructed phase is therefore thought to correspond to a precursor stage in the formation of the Cu(I) oxide (Cu2O) layer at higher potential. At low overpotential of adsorption, the adsorbed layer nucleates preferentially but not only at the step edges of the terraces. This is not observed at higher overpotential due to the increased density of nucleation centers. The adsorbed OH layer increases the image resolution of the step edges that appear fuzzy in their absence. This is assigned to a lower mobility of the Cu atoms at the step edges. The objective of the experiments reported in the following was to investigate the growth of oxide layers on Cu(111) in the same strongly alkaline solution (0.1 M NaOH). In situ electrochemical STM was used. It was intended to study the structural changes occurring on the copper surface from the initial stages of oxidation to the later stages when 3D crystalline oxide layers are formed. Experimental Section The Cu(111) crystals were oriented by Laue diffraction with a precision of 1° and mechanically polished with diamond spray with a final 0.25 µm grading. They were then electropolished in 60% orthophosphoric acid for 5 min at 1.80 V versus a Cu counterelectrode and subsequently annealed at about 1000 K for 16 h in a flow of ultrapure (6 N) hydrogen at normal pressure to heal out defects and to enlarge the terraces. The single crystals were mounted in a small electrochemical cell of the STM with a surface of 0.16 cm2 exposed to 0.1 M NaOH. The electrolyte was prepared from ultrapure NaOH (Gen-Apex) and Millipore water (resistivity > 18 MΩ‚cm). The electrode surface was exposed to the electrolyte at open circuit potential and then

Kunze et al. scanned from E ) -0.25 to -1.20 V to reduce the oxide film formed in air. After this pretreatment, the electrode was stepped to the potential of interest. The small electrochemical STM cell was made of Teflon and contained a Pt counterelectrode and a Pt pseudo-reference electrode. The reference electrode was calibrated by the welldefined cathodic peaks of the polarization curve of Cu in 0.1 M NaOH (Figure 1) and proved to be sufficiently stable. Prior to each measurement, the electrochemical STM cell with the two Pt electrodes mounted was cleaned first in a 2:1 mixture of concentrated H2SO4 and H2O2 and second in concentrated HNO3 and then thoroughly rinsed with Millipore water. The STM was a Molecular Imaging system with a Nanoscope E controller (Digital Equipment). The tungsten tips were prepared from a 0.25 mm diameter wire by electrochemical etching in 3 M NaOH and covered by Apiezon wax. All images were obtained in the constant current mode. Results The detailed examination of the anodic oxidation of Cu(111) in 0.1 M NaOH was performed at the characteristic potentials indicated in the polarization curve of Figure 1. The terrace topography of the Cu surface prior to oxidation was systematically checked at E ) -0.75 V. The OH adsorption was confirmed at -0.60 V; its details have been previously reported.16 The growth of the Cu(I) oxide film was followed at -0.25 and -0.2 V. The duplex Cu(I)/Cu(II) oxide film was investigated at 0.75 V. Stepping the potential from -0.75 V, where the terrace topography of the bare Cu(111) surface is observed (Figure 2a), to -0.25 V leads to the immediate (on the time scale of the STM measurements) adsorption of the OH-. This is due to the applied high overpotential for this process. The kinetics of the adsorption process could be investigated at lower potentials and has been reported previously.16 Figure 2b shows the immediate formation after the potential step of the characteristic dark islands in the center of the terraces. At higher magnification, a hexagonal lattice with a parameter of 0.6 ( 0.05 nm is measured (Figure 3a), in very good agreement with the lattice parameter of the O planes in the (111)-oriented cuprite structure. This superstructure is identical to that previously reported and assigned to the formation of the OH adlayer with the related reconstruction of the Cu topmost plane.16 The dark appearance is an electronic effect observed by STM for O species adsorbed on metals19 and does not correspond to the true topography of the Cu surface with the OH adsorbate. At -0.25 V, the growth of the Cu(I) oxide is also observed and is confirmed by subsequent cathodic reduction. It is characterized by the formation of brighter islands that cover the surface only partially. They form preferentially but not exclusively at step edges (Figure 2b). At higher magnification, one observes an ordered hexagonal structure, but only in some parts of the oxide islands, indicating an incomplete crystallization of the oxide film (Figure 3b). This evolves slowly with time with more crystallized areas being observed. The period of the measured hexagonal lattice is 0.3 ( 0.05 nm, which is in good agreement with the period of the Cu plane in the (111)oriented Cu2O (0.3 nm). This indicates that the Cu(I) oxide formed at this potential is a Cu2O(111)-like film. The measured height difference between the oxide and adsorbed islands formed on the initially same Cu terrace is ∼0.25 nm. This suggests that the oxide islands contain one more layer of Cu than the adsorbed islands and therefore that the oxide islands are one

In Situ STM Study of Anodic Oxidation of Cu(111)

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Figure 2. Sequence of topographic in situ STM images of Cu(111) in 0.1 M NaOH showing the growth of monolayer-thick islands of Cu(I) oxide; It ) 1 nA, Etip ) -0.60 V, z range ) 3 nm. Panel a was recorded at E ) -0.75 V, and panel b was recorded after a potential step at E ) -0.25 V. The potential step is marked by the horizontal arrow (b). Scan direction, time evolution, and potential are indicated. The distortion of the image between scans a and b is a scanner artifact. Adsorption and oxide islands are marked in panel b.

monolayer thick. The charge transfer measured in subsequent cathodic reduction (see below) indicates that a fraction of a monolayer of oxide has been formed and supports this interpretation. Figure 3c reveals the simultaneous presence of both structures with the oxide islands decorating the step edges. The effect of aging on the crystallization of the anodic oxide islands should be seen as an Ostwald ripening effect that assumes first the formation of a metastable nonordered structure, which rearranges with time into a stable crystalline structure. The two types of structure, adsorbed and oxide islands, could be observed concomitantly for a prolonged period of time at -0.25 V, indicating that this potential value is near the equilibrium value for the formation of the Cu(I) oxide. The terrace topography of the substrate is still preserved at this potential, which is a second indication that the oxide film is very thin. The partial coverage and monolayer thickness of the oxide layer are confirmed by the potentiodynamic reduction curve of the electrode, which gave a CI peak corresponding to a charge transfer of 94 µC/cm2. This amounts to about half (47%) a monolayer of Cu2O(111) (one monolayer corresponds to 202 µC/cm2), in very good agreement with the partial coverage observed by STM. After reduction, large and atomically smooth terraces similar to those observed prior to partial oxidation were recovered. Stepping the potential from -0.75 to -0.2 V causes faster oxide growth. Figure 4 presents a sequence of images starting with an adsorbate-free Cu surface (Figure 4a) stepped at -0.2 V during the STM scan of Figure 4b. Oxide (bright) islands

Figure 3. Topographic in situ STM images recorded at E ) -0.25 V (Etip ) -0.60 V). Panel a shows the ordered adsorbed OH layer with the lattice marked (It ) 1.5 nA, z range ) 1 nm). Panel b shows a crystallized Cu(I) oxide domain with the lattice marked (It ) 3 nA, z range ) 0.4 nm). Panel c shows adjacent domains of OH adsorption and oxide (It ) 3 nA, z range ) 1.2 nm).

nucleate all over the surface with no preferential sites (Figure 4b). Darker islands corresponding to the OH adsorbed layer separate them. The height difference between oxide and adsorbed islands in these initial stages of oxidation is measured to range from 0.2 to 0.6 nm. This indicates, by comparison with the monolayer islands formed at -0.25 V, that several (possibly up to three) additional layers of Cu are contained in the oxide islands formed at -0.2 V. About 40 s. after the potential step, the surface is nearly fully covered by the oxide islands. After about 100 s, triangular facets characteristic of a crystalline film are observed. The topography then evolves more slowly to reveal a crystalline Cu(I) oxide layer terminated by a faceted surface (Figure 4c). The average width of terraces is ∼2.7 nm and the separating steps have a height of 0.2-0.25 nm. They correspond to monatomic steps of the oxide since a (111)oriented monolayer of Cu2O has a height of 0.246 nm from bulk values. At higher magnification, a hexagonal structure is measured on the terraces (inset of Figure 4c). The lattice period is ∼0.3

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Kunze et al.

Figure 5. Topographic in situ STM images of Cu(111) in 0.1 M NaOH after reduction of 7.4 monolayers of Cu(I) oxide formed at E ) -0.2 V, Itip ) 1 nA, Etip ) -0.60 V, z range ) 4 nm (a) or z range ) 3 nm (b).

Figure 4. Sequence of topographic in situ STM images of Cu(111) in 0.1 M NaOH showing the growth of a multilayer-thick crystalline Cu(I) oxide; Etip ) -0.60 V, z range ) 2 nm, Itip ) 1.0 nA (a, b) or Itip ) 2 nA (c). Panel a was recorded at E ) -0.75 V; panels b and c were recorded after a step at E ) -0.2 V marked by the horizontal arrow in panel b. Scan direction, time evolution, and potential are indicated. The inset in panel c shows the measured atomic lattice of the crystallized oxide. The distortion of the image between scans a and b is a scanner artifact. Adsorption (dark areas) and oxide (bright areas) islands are marked in panel b.

nm, which again fits very well the lattice parameter of the Cu planes in (111)-oriented Cu2O. The orientation of the oxide lattice with respect to the substrate crystallographic direction indicates a parallel (or antiparallel) epitaxy: Cu2O(111)[11h0]// Cu(111)[11h0] (or Cu2O(111)[11h0]//Cu(111)[1h10]). The reduction charge of this Cu2O layer to Cu metal required a charge density of 1488 µC/cm2, which corresponds to 7.4 equivalent monolayers of Cu2O(111). Thus a 3D crystalline oxide layer is formed on Cu(111) at -0.2 V in 0.1 M NaOH. Its structure is Cu2O(111) in parallel or antiparallel epitaxy and its surface is faceted in agreement with previous findings.15 After reduction, the Cu surface also shows a faceted topography characterized by narrow (111) terraces (Figure 5).). This is assigned to an

Figure 6. Topographic in situ STM image of Cu(111) in 0.1 M NaOH at E ) 0.75 V after formation of a duplex Cu(I)/Cu(II)oxide layer; Itip ) 4 nA and Etip ) -0.55 V.

insufficient mobility of the Cu atoms on the reduced surface to heal out the structural modifications related to the formation of several-monolayer-thick oxide films. Thus, the irreversible faceting of the topography of the copper electrode caused by the reduction treatment depends on the thickness of the prior oxide film. At high potential in the passive range (E > 0.5 V), a duplex film is formed with an inner Cu(I) oxide layer and an outer mixed layer of Cu(II) oxide and hydroxide. The STM measurements revealed that a potential step to E ) 0.75 V causes the formation of a crystalline film. Figure 6 shows its topography with terraces having a width ranging from a few nanometers to tens of nanometers and a step height of ∼0.3 nm. The step edges have a linear and kinked morphology different from the curved morphology of the step edges observed at the surface of the Cu(I) oxide layer. No atomic lattice could be imaged on the terraces of this crystalline duplex oxide film.

In Situ STM Study of Anodic Oxidation of Cu(111)

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The potentiodynamic reduction of this duplex oxide occurs in two steps (Figure 1), which permits the determination of the amounts of oxide formed in each layer based on eqs 1, 1′, and 2:

2CuO + 2H+ + 2e- f Cu2O + H2O

(1)

2Cu(OH)2 + 2H+ + 2e- f Cu2O + 3H2O

(1′)

+

-

Cu2O + 2H + 2e f 2Cu + H2O

(2)

The thickness of the layers (δ) is obtained from the charge density (Q) of the reduction peaks according to

δ ) QV ˜ /zF

(3)

with V ˜ being the molar volume of Cu2O, Cu(OH)2, and CuO (23.9, 29, and 12.4 cm3/mol, respectively), F being the Faraday constant, and z being the number of exchanged electrons. The measured reduction charge of the peak CII, QII ) 1.287 mC/ cm2, amounts to a thickness δII of 1.65 nm, assuming that only CuO is formed, or 3.87 nm, assuming that only Cu(OH)2 is formed. The amount of oxide in the inner part of the duplex film, i.e., its Cu2O part, is given by the difference of charge between the two cathodic peaks CI and CII, i.e., QI ) 3.481 1.287 ) 2.194 mC/cm2. This amounts to a thickness of δ1 ) 2.72 nm. The total thickness of the duplex film is then δ ) δI + δII ) 4.37 or 6.59 nm if the outer layer is assumed to be oxide or hydroxide, respectively. Discussion The STM results reported here evidence the influence of the applied potential on the growth and structure of the anodic Cu(I) oxide formed on copper. If the applied potential is close to the equilibrium potential of formation of the oxide layer (-0.25 V), one observes the nucleation of the oxide at preferential sites that are the step edges of the Cu(111) terraces. If a higher potential is applied (-0.2 V), a more random nucleation with no marked preferential growth at step edges is observed. This influence of the overpotential was also observed on the nucleation and growth rate of the OH adsorbed layer.16 The preferential role of step edges is controversial in the gaseous oxidation of copper, with some authors reporting a preferential role in the formation of copper oxides20 not observed by others.21 The present anodic oxidation experiments show that the preferential role of the step edges is strongly dependent on the overpotential of oxidation since an increase of 50 mV causes a change of mechanisms from preferential nucleation at step edges to nucleation on both step edges and terraces. In the underpotential range of oxidation (-0.7 V < E < -0.25 V), the formation of an adsorbed layer of oxygen species (assigned to OH groups), reported previously,16 has been confirmed in this series of measurements. The STM measurements evidence the formation of a hexagonal structure with a period of 0.6 nm. This period corresponds to the interatomic distance between oxygen species in the (111) planes of Cu2O. The STM measurements also evidence a change of morphology of the Cu terraces indicating the reconstruction of the topmost Cu layer, presumably according to the hexagonal lattice of the Cu planes in (111)-oriented Cu2O. Thus the Cu surface adopts already the appropriate structure for the growth of Cu2O(111)like layer in this preoxidation stage where the adsorbed oxygen species are OH groups. This adsorbed structure can be viewed as a precursor for the formation of the crystalline Cu2O(111)

Figure 7. Section view of a model proposed for the Cu(111) surface terminated by the precursor phase of adsorbed OH and by one monolayer of (111)-oriented Cu2O with a hydroxylated surface.

film that grows at higher potential. Figure 7 shows the proposed stacking sequence of the surface planes in this precursor adsorbed phase. The unreconstructed metallic Cu planes have a hexagonal close-packed structure with a parameter of 0.25 nm. The topmost Cu plane is reconstructed: the symmetry remains hexagonal but the lattice parameter is 0.3 nm. The coincidence cell between these two structures of the Cu planes has been discussed previously.16 An adsorbed layer of OH groups terminates the surface. It forms a hexagonal structure with a (2 × 2) periodicity with respect to the topmost reconstructed Cu plane; the lattice parameter is 0.6 nm. At E ) -0.25 V, one monolayer of Cu2O(111)-like oxide partially covering the substrate is formed concomitantly with the adsorbed precursor phase. According to the measured height difference of ∼0.25 nm with respect to the OH adsorption islands, the islands of Cu(I) oxide contains one more plane of Cu atoms. This implies a partial coverage by oxide islands of monolayer thickness confirmed by the reduction charge measurements. The surface model is shown in Figure 7. The Cu atoms ejected from the topmost substrate layer under the adsorption-induced reconstruction likely form the additional Cu layer. The fact that only a fraction of a Cu2O(111)-like monolayer is formed at this stage supports this possibility. Indeed, the reconstruction of the topmost substrate layer decreases the atomic density by 31% of a Cu(111) plane, making the ejected Cu atoms available to form 45% of a monolayer of Cu2O(111). Hence, it can be concluded that at -0.25 V the anodic oxidation of Cu(111) is essentially a 2D process leading to the formation of Cu2O(111)-like monolayer islands partially covering the substrate. At higher potential (-0.2 V), the height of the oxide islands observed in the initial stages up to complete coverage of the substrate ranges from 1 to 3 monolayer (ML) equivalent of Cu2O(111). These measurements of height between the oxide and adsorbate islands are influenced by the tunneling conductivity that can differ on oxide and adsorbate islands. However, the monolayer thickness of the oxide islands formed at -0.25 V deduced from such height measurements is confirmed by the reduction charge measurements, which indicates that the uncertainty related to the differences in tunneling conductivity between the oxide and adsorbate islands is negligible. We assume this is also the case for the oxide islands formed at -0.2 V and therefore that their thickness can be estimated as 1-3 ML as indicated by the height measurements. This implies that, at -0.2 V, preferential 2D growth operates in the initial stages until complete coverage of the substrate by the growing Cu(I) oxide film. Subsequent thickening up to about 7 equivalent monolayers as measured from the cathodic reduction is then thought to proceed by cation transport through the growing oxide film according to the high-field mechanism described by Fehlner, Mott, and Cabrera.22,23 The thicker oxide film formed at -0.2 V has a faceted surface characterized by nanometric (111) facets (2.7 nm wide in

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Figure 8. Section view of a model proposed for the 7 ML thick and (111)-oriented Cu2O anodic film grown epitaxially on Cu(111) at -0.2 V in 0.1 M NaOH. The stacking sequence of the O2- and Cu+ planes in the oxide is illustrated by the gray lines. The oxide surface is thought to be terminated by a monolayer of hydroxyl/hydroxide groups. The faceted surface corresponds to a tilt of 5° between the two lattices.

average) separated by monatomic oxide steps. Surface faceting has been previously observed for this system15 and for wellcrystallized anodic oxide films formed on Ni(111)24-27 and Cr(110).28 The faceting corresponds to a tilt between the closepacked planes of the oxide and the close-packed planes of the substrate. This is illustrated for copper in Figure 8, for which, from the average width of the facets and the monatomic step height, the tilt value is calculated to be about 5°. The cause of this tilt is attributed to the large misfit between the oxide and substrate lattices. The lattice misfit causes epitaxial stress that can be released by a tilt between the two lattices when extended oxide crystals are formed as observed in the present case (Figure 4c). For copper, the misfit value calculated from the bulk parameters (0.427 and 0.361 nm for Cu2O and Cu, respectively) is 18.3%. In the [11h0] direction, which is the alignment direction of the two lattices observed experimentally, a perfect fit between oxide and substrate lattices is obtained over a coincidence length of 1.787 nm with a tilt angle of 5° and a contraction of 1% of the oxide lattice or an expansion of 1% of the substrate lattice with respect to the bulk values. This coincidence length of 1.787 nm along [11h0] corresponds to 7 periods of the Cu(111) planes and to 6 periods of the Cu2O(111) planes. In the case of nickel, for which the misfit value between NiO and Ni is nearly equal (18.8%), surface faceting corresponding to a tilt ranging from 3°26,27 to 8°24 has been reported, in good agreement with that observed on Cu(111) in the present study. It should be emphasized that on both systems the perfect fit between oxide and substrate lattices may be obtained with varying tilt values, provided that contractions and/or expansions limited to a few percent of the lattices are allowed. Hence, it can be concluded that the surface faceting of the extended crystals of the anodic Cu2O(111) film formed on Cu(111) at -0.2 V corresponds to the relaxation of the epitaxial stress at the interface with the metallic substrate. It is not observed for the oxide islands formed at -0.25 V due to a lower extent of crystallization. The oxide islands observed prior to complete coverage at -0.2 V are not tilted, probably because of their limited lateral extension, i.e., the stress is likely relaxed at the islands’ boundaries. Also, the smaller thickness of the oxide islands in this early stage probably allows larger lattice distortions due to a lower cohesive energy than after thickening of the oxide film. The chemical termination of the Cu2O(111)-like anodic oxide film formed on Cu(111) has not been directly determined in these measurements. However, a surface termination by OH and/ or OH- groups is very likely in the aqueous electrolyte and is supported by recent angle-resolved XPS measurements.29 This chemical termination by OH groups, illustrated in Figures 7 and 8, allows us to propose an explanation for the fact that the STM images reveal the periodic lattice of the Cu plane in the case of the Cu(I) oxide film (independently of its thickness) and the

Kunze et al. periodic lattice of the OH species in the case of the adsorption layer. The possibility of a difference due to tunneling bias effects can be ruled out since both structures have been observed on the same surface at -0.25 V with the same tip potential. Our explanation is that the surface OH groups are imaged on both structures (oxide and adsorbed layer). This implies that the Cu(I) oxide film is terminated by a (1 × 1) layer of OH groups, i.e., a layer having the same hexagonal symmetry, periodicity (0.3 nm), and density (∼1.28 × 1015 atoms‚cm-2) as the underlying Cu(I) layer. In this case, the electroneutrality of the surface Cu(I) oxide slab (O2--Cu+-OH-) can be achieved if the surface layer is composed of the proper balance of hydroxide ions (OH-) and neutral hydroxyl groups (OH). This is obtained, for example, with densities of 0.32 × 1015 atoms‚cm-2 (O2-), 1.28 × 1015 atoms‚cm-2 (Cu+), 0.64 × 1015 atoms‚cm-2 (OH-), and 0.64 × 1015 atoms‚cm-2 (OH). In this case, the ratio of hydroxide to hydroxyl groups in the topmost plane would be 1. Conclusion The anodic oxidation of Cu(111) in 0.1 M NaOH has been studied in situ by electrochemical STM. In the underpotential range of oxidation (-0.7 V e E < -0.25 V), an ordered adlayer is formed, the structure of which is a precursor for the formation of the Cu2O(111) anodic layer at higher potential. The adsorbed species, assigned to OH groups, form a hexagonal structure with a parameter of ∼0.6 nm and a density of ∼0.32 × 1015 OH‚cm-2. The topmost Cu layer is reconstructed according to the structure of the copper planes in Cu2O(111): it has a hexagonal lattice having a parameter of ∼0.3 nm and its density is ∼1.28 × 1015 atoms‚cm-2. At E g -0.25 V, a Cu(I) oxide layer grows as 2D islands, which develop with time to form a crystalline film having the Cu2O(111) structure: the higher the overpotential of oxidation the faster the growth, the thicker the film, and the better the crystallinity. The potential influences also the nucleation sites, which are preferentially at the step edges of the Cu terraces at -0.25 V but not at -0.2 V. At -0.25 V, monolayer-thick oxide islands partially covering the substrate are obtained. The crystallization is not complete. The growth of the oxide islands is thought to result from the aggregation of the Cu atoms rejected from the topmost reconstructed plane of the substrate in the 2D phase of OH preceding the oxide growth. At -0.2 V, oxide islands are observed in the initial stages of growth. They are estimated to be 1-3 monolayers thick. The Cu(I) oxide layer grows thicker (up to ∼7 monolayers), very likely by a high-field mechanism of transport through the oxide film, after complete coverage of the substrate by the oxide islands. The Cu(I) oxide layer is terminated by small terraces having an average width of 2.7 nm and separated by monatomic oxide steps (faceted surface). The surface faceting results from a tilt estimated at 5° between the oxide and metal lattices, which allows the relaxation of the epitaxial stress at the interface with the substrate. STM images with atomic resolution show that the terraces have a hexagonal periodic lattice (0.3 nm) consistent with the structure of the Cu+ planes in Cu2O(111). The epitaxy with the substrate is parallel or antiparallel. The oxide surface is very likely hydroxylated and terminated by OH- and/or OH groups in (1 × 1) registry with the Cu+ plane (density of ∼1.28 × 1015 atoms‚cm-2). The Cu2O/CuO,Cu(OH)2 duplex oxide film formed at 0.75 V is thicker with an inner layer of Cu(I) oxide 1.6 nm thick and a mixed outer layer of Cu(II) oxide/hydroxide 2-3 nm thick. This duplex film is also crystalline. It is terminated by larger terraces of up to 20 nm width.

In Situ STM Study of Anodic Oxidation of Cu(111) After cathodic reduction of these oxide layers, large substrate terraces can be reobtained for oxides of monolayer thickness but not for thicker oxide layers, which produce a faceted Cu surface after reduction. This is assigned to an insufficient surface mobility of the Cu atom to heal out the structural modifications related to the formation of several-monolayer-thick oxide films. References and Notes (1) Pourbaix, M. Atlas of Electrochemical Equilibria in Aqueous Solutions, 2nd U.S. edition; NACE: Houston, TX, 1974. (2) Strehblow, H.-H.; Titze, B. Electrochim. Acta 1980, 25, 839. (3) Kautek, W.; Gordon, J. G. J. Electrochem. Soc. 1990, 137, 2672. (4) Feng, Y.; Siow, K.-S.; Teo, W.-K.; Tan, K.-L.; Hsieh, A.-K. Corrosion 1997, 53, 389. (5) Hamilton, J. C.; Farmer, J. C.; Anderson, R. J. J. Electrochem. Soc. 1986, 133, 739. (6) Mayer, S. T.; Muller, R. H. J. Electrochem. Soc. 1992, 139, 426. (7) Chan, H. Y. H.; Takoudis, C. G.; Weaver, M. J. J. Phys. Chem. B 1999, 103, 357. (8) Melendres, C. A.; Bowmaker, G. A.; Leger, J. M.; Beden, B. J. J. Electroanal. Chem. 1998, 449, 215. (9) Lohrengel, M. M.; Schultze, J. W.; Speckmann, H. D.; Strehblow, H.-H. Electrochim. Acta 1987, 32, 733. (10) Strehblow, H.-H. Proceedings of the International Symposium on Control of Copper and Copper Alloys Oxidation; Revue de Me´tallurgie: Rouen, France, July 6-9, 1992; p. 33. (11) Strehblow, H.-H.; Borthen, P.; Druska, P. Synchrotron Techniques in International Electrochemistry; Melendres, C. A., Tahdjeddine, A., Ed.;

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