In-situ observation of oxygen adlayer formation on Cu(110) electrode surfaces

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Surface Science 326 (1995) L461-L466

Surface Science Letters

In-situ observation of oxygen adlayer formation on Cu(ll0) electrode surfaces John R. LaGraff, Andrew A. Gewirth * Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA Received 28 September 1994; accepted for publication 13 December 1994

Abstract

In-situ atomic force microscopy (AFM) was used to image Cu(110) single crystals in aqueous solutions during the initial stages of oxidation. Images obtained in pH 2.5-2.7 HCIO4 and H 2 S O 4 solutions revealed the growth of oxygen adlayers consisting primarily of [001] oriented chains. A majority of these chains (ca. 70%) were arranged in (2 x 1) and (3 x 1) structures. These chain structures were observed in the thermodynamically forbidden region of the pH-potential phase diagram, which indicates that stable oxygen adlayers develop prior to bulk oxide formation. Keywords: Adatoms; Atomic force microscopy; Copper; Metal-electrolyte interfaces; Solid-liquid interfaces; Surface structure; Surface thermodynamics

A fundamental understanding of adlayer structure on single crystal metal surfaces immersed in liquids is extremely important in developing a structural basis for electrochemical reactivity, including, corrosion and catalysis. While significant progress has been made in imaging electrochemical processes with lattice resolution on Au [1] and Ag [2] surfaces, Cu has received scant attention owing to its tendency to oxidize and for its narrow (ca. 400-600 mV) double-layer (ideally polarizable) region. Copper single crystal surfaces immersed in aqueous solutions have been studied by a variety of techniques [3-5]. However, the results are extremely sensitive to surface preparation, ostensibly due to adventitious oxide species. Mixed metal-oxygen adlayers have been

* Corresponding author. Fax: + 1 [email protected].

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observed on Cu surfaces in the ultra high vacuum (UHV) environment [6-8], but their existence in the acidic electrochemical environment remains problematic. In particular, thermodynamic potential-pH considerations suggest that a bulk oxide is unstable at pH < 3.5 at the rest potential [9]. Consequently, the determination of oxygen adlayer structure evolution on copper single crystal surfaces is invaluable in helping develop an atomic-level understanding of copper oxidation and corrosion in aqueous environments. The Cu(ll0) surface is a particularly attractive candidate on which to examine the initial stages of oxidation as distinctive Cu-O rows are clearly seen in the UHV environment, the growth mechanism of which provides considerable insight into the initial stages of Cu oxidation [6]. It is important to compare oxygen adsorption processes between the UHV and electrochemical environments. In this Letter, we re-

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J.R. LaGraff, A,4. Gewirth / Surface Science 326 (1995) L461-L466

in situ real space images of the C u ( l l 0 ) s o l i d - l i q u i d interface in acidic aqueous solutions as it undergoes the initial stages of oxygen adsorption. The working electrode was formed from C u ( l l 0 ) single crystals (1 cm 0 , 99.999%, Monocrystals Company) which were mechanically polished down to 0.3 /xm with A I 2 0 3, rinsed with Millipore-Qpurified water, dried in flowing air, and then electropolished with an applied current of ~ 1.5 A / c m 2 in a solution containing H3PO4(85%) : H2SO4(96%) : H 2 0 in a 6 : 1 : 3 by volume ratio [3]. The reference electrodes were formed from a H g / H g 2 S O 4 (MSE) cell connected to the A F M cell through a Luggin capillary. In order to facilitate thermodynamic comparisons all potentials are reported relative to the normal hydrogen electrode (NHE) which is - 6 4 0

m V versus MSE. The counter electrode was a 0.5 mm diameter A u wire prepared by flame annealing followed by quenching into a solution containing the appropriate supporting electrolyte. Electrolytes were prepared with Millipore-Q water, H2SO 4 (J.T. Baker, Ultrex), HC10 4 (J.T. Baker, Ultrex), and NaC1 (reagent grade, Fisher). In most experiments the electrolytes were not deoxygenated prior to use, but images were also obtained in an A r purged atmosphere. In situ A F M images were obtained with a Nanoscope II A F M [10] equipped with a glass cell and operating in constant force mode. Tips were microfabricated with integral pyramidal Si3N 4 tips. The instrument was calibrated using the known spacings and structures for mica and C u ( l l 0 ) . Upon

Fig. 1. AFM images of Cu(ll0) in 0.003M HCIO4 (pH 2.5). The [001] direction is indicated in each figure. (a) 6 X 6 nm bare Cu(ll0) region observed at -340 5:10 mV. (b) 9 X 9 nm image of two domain region with Cu(ll0) (upper right) and (2 X 1) rows (lower left) observed at E0 after a potential of -550 + 10 mV was applied. Lines indicate the registry between the overlayer and the substrate. (c) 8 X 8 nm image of periodic (001) chain structures at Eo corresponding to (1 X 1), (2 X 1), and (3 x 1) rows and (d) the corresponding Fourier transform with multiple periodicities along [li0]: a = 0.24 ± 0.02 nm, b = 0.31 ± 0.03 nm, c = 0.54 + 0.05 nm, d = 0.74 + 0.07 nm; and in the [001] direction: e = 0.34 ± 0.03 rim.

J.R. LaGraff, A.A. Gewirth / Surface Science 326 (1995) L461-L466

completion of the experiments, the (110) in-plane crystal orientations - [110] and [001] - determined from the images were confirmed by using Laue backscattering. Fig. la shows an image of the Cu(ll0) surface in dilute (0.003M, pH = 2.5) HCIO 4 obtained at a potential of 140 mV which is 200 mV negative of the potential of Cu dissolution [Cu2++ 2 e - , ~ Cu°; E ° = 340 mV]. This surface exhibits a rectangular lattice with periodicities of 0 . 3 6 _ 0.03 in the [001] direction and 0.25 + 0.02 nm in the [110] direction. This was the dominant lattice structure seen in this potential range. The spacing and structure of this lattice correspond well with that expected for the bare, unreconstructed Cu(ll0) surface. Sweeping the potential positive to 300 mV caused additional structures to form on the Cu(ll0) surface. Fig. lb shows a region of the surface which developed 10-20 min after stepping from 90 to 300 mV. In the upper right there are atomic scale features exhibiting the same spacing and structure as the Cu(ll0) surface seen in Fig. la. However, the lower left of Fig. lb reveals atomic scale chains with a 0.34 + 0.02 nm spacing in the [001] direction and a 0.49 + 0.05 nm spacing in the [110] direction. The height of the chains above the Cu(ll0) surface was 0.15 + 0.08 nm; these correspond to atomic scale dimensions. This is equivalent to a (2 X 1) overlayer and the image shows this overlayer growing in on top of the bare Cu(ll0) lattice. Fig. lc shows an AFM image of the Cu(ll0) surface obtained at 300 mV. The corresponding two-dimensional Fourier transform of this image is shown in Fig. ld. Equivalent images were found either after allowing the surface shown in Fig. lb to develop for ca. 30-120 min after an excursion to negative potentials or after immersion of the Cu(110) surface into the electrolyte at E 0 following electropolishing. The image shows a series of atomic scale chains on the surface all aligned along the [001] direction. The lattice periodicity in the [001] direction, along the chains, is 0 . 3 4 _ 0.03 nm, the expected Cu-Cu spacing (spot e, Fig. ld). There are, however, multiple spacings evident along the [110] direction in Fig. lc with lengths of 0.24 _ 0.02 nm, 0.31 _ 0.03 nm, 0.54 _ 0.05 nm, and 0.74 _ 0.07 nm. These correspond to the spots labeled a, b, c, and d in Fig. ld. The lattices represented by these

(2xl) on Cu(L,,z

[001]

T [li0] Fig. 2. Schematic of Cu(110) surface with proposed (2 x 1) chain structure with adatoms located in two-fold bridging sites along the [110] close packed rows. Copper and the adlayer are represented by light and dark circles, respectively. While the AFM image in Fig. lb unambiguously places the chain atoms on the close-packed rows, the registry [110] along cannot be determined from the present data set.

spacings are approximately (1 X 1), unknown, (2 X 1), and (3 X 1) structures, respectively. These chain features coexist with both disordered regions and bare Cu(ll0). The 0.31 nm separation (spot b, Fig. lc) may be a result of lateral chain interactions in particular regions of the surface which would change individual chain-chain separations. Fig. 2 is a schematic of the Cu(ll0) surface with [001] chains of atoms arranged in one possible (2 x 1) structure. The AFM imaging mechanism and the ultimate resolution available with this technique remains the subject of considerable controversy [11]. In particular, there are strong indications that the AFM provides an average structure rather than a true single atom image. Except under very low force conditions [12], not used here, step edges are found to be either indistinct or to evince a low resolution region arising from the finite AFM tip radius. However, by comparison with the results of scanning tunneling microscopy and surface X-ray scattering measurements, we showed that the images obtained by AFM are truely representative of the lattice structure present on the surface for both commensurate and incommensurate overlayers [13]. The images in Fig. 1 show that an ordered overlayer structure grows in

J.R. LaGraff, A~4. Gewirth / Surface Science 326 (1995) L461-L466

- - - r . . . . . . . . . 0) lattice. This overlayer is representative of the real structure present on the surface. Single atom resolution is not necessary for the argument presented here. In order to ascertain the origin of the chains found on the C u ( l l 0 ) lattice, we examined the surface in H2SO 4 electrolyte in the same pH range. Slight differences were observed between the interaction of C u ( l l 0 ) with the two anions, however, the same (2 X 1) and (3 X 1) chain structures were also observed in the H2SO4-containing solutions [14]. In addition, measurements made utilizing a Pt wire quasi-reference electrode gave identical results to those made with an MSE, ruling out contamination from this source. Another possible origin of the chain features is C I - introduced as a contaminant in the sulfate or perchlorate electrolyte [15]. C I - strongly adsorbs on and reacts with copper surfaces, especially in acidic solutions or upon exposure to HC1 gas [16,17]. Fig. 3a shows the overlayer which was observed on C u ( l l 0 ) in a solution containing 0.001M NaCI and 0.003M HCIO 4 (pH = 2.5) at a potential of 310 ___30 mV. The bright rows in Fig. 3a are separated by 0.75 + 0.05 nm (three times the [ l i 0 ] C u - C u spacing), while in the [001] direction the atoms possess the substrate periodicity. In Fig. 3b, the Fourier transform of Fig. 3a reveals spacings along [110] corresponding to a = 0.24 ___0.02, b = 0.37 + 0.04, and c = 0.75 + 0.07 nm while the other spacings are d = 0.27 + 0.03 nm, e = 0.32 + 0.03 nm, f = 0.33 + 0.03 nm, g = 0.26 + 0.03 nm. We can ascribe the structure to a pseudo-hexagonal basis with angles of 60 + 10 degrees and spacings of 0.30 + 0.03 nm (spots a, d, e, f, and g, Fig. 3b). However, there is an obvious three-fold periodicity along [110]. This structure is clearly different from the [001] chains shown in Figs. lb and lc; the latter show a range of (n × 1) overlayers (n = 1, 2, 3) and also do not exhibit a hexagonal unit cell. The chloride overlayer could be disordered with a sufficiently negative potential. However bare C u ( l l 0 ) was never observed which suggests a strong adsorption of chloride a n d / o r multiple chloride layers (e.g., bulk CuCI) [18]. Cations and an intrinsic surface reconstruction can be excluded as origins of the chain structures. We observed the overlayer growing in on C u ( l l 0 ) at

Fig. 3. (a) 6x6 nm image of CI- overlayer on Cu(ll0) at E o observed in 0.001M NaCI/0.003M HCIO4 (pH =-2.5) solutions and, (b), corresponding Fourier transform; a = 0.24-1-0.02 nm, b= 0.37+0.04 nm, c = 0.75+0.07 nm, d= 0.27+0.03 nm, e = 0.32 + 0.03 nm, f = 0.33 + 0.03 nm, g = 0.26 + 0.03 nm. The [001] direction is indicated.

positive potentials. Cations would be expected to exhibit facile adsorption at negative potentials, consequently, positively charged species such as H ÷ are excluded as possible adsorbates. Au single crystals reconstruct without the aid of adsorbates in the electrochemical environment [19]. However, this reconstruction is driven by excess charge on the Au surface and occurs as the potential is swept negative. The opposite potential dependence here suggests that an intrinsic (non-adsorbate induced) reconstruction is not responsible for the observed adlattice on Cu(110).

J.R. LaGraff, A.A. Gewirth/ Surface Science 326 (1995) L461-L466

Based on the insensitivity to anion and the different structure observed with a C1- contaminant, we assign the (2 X 1) and (3 × 1) chain structures observed in Figs. lb and lc to an adsorbed oxygen or hydroxide species growing on the bare Cu(ll0) surface. These chains may represent the initial stages of bulk oxide formation on Cu(ll0). The images in Fig. 1 were obtained in the pH-potential region of the Pourbaix diagram where bulk Cu20 oxide formation is thermodynamically forbidden. According to this diagram, derived from bulk thermodynamic considerations, Cu20 should not be found at pH values less than 3.5 for any potential. Consequently, the observation of oxygen chain structures at pH values between 2.5 and 2.7 indicates that prior to bulk oxide formation, a partial oxygen adlayer develops. This is equivalent to saying that the adlayer forms at underpotentials. The oxygen monolayer can be thought of thermodynamically in the same manner as are monolayers of metal adatoms which form at potentials more positive of the bulk deposition potential - a process known as underpotential deposition (UPD) [20]. Applying more negative potentials removes the chain structures enabling resolution of the Cu(ll0) surface. This is in qualitative - but not quantitative agreement with the Pourbaix diagram [9]. There are three possible sources of the oxygen adlayer: (a) the electropolishing procedure, (b) atmospheric oxygen adsorbed during transport to the AFM cell, and (c) hydroxide [21] or oxygen in the aqueous solutions. It would be premature to propose a definitive source of the oxygen species. Existence of a coadsorbed oxygen species on Cu surfaces in acidic solutions (i.e., in the thermodynamically disfavored region of the potential-pH phase diagram) has been inferred from electrochemical [3,4], and second harmonic generation [5] studies; however, the results in this Letter represent the first direct observation of oxygen adlayers on Cu(ll0). A recent AFM study on Cu(100) crystals reported a c(2 X 2) structure attributed to adsorbed oxygen species at a pH near 1 [22]; however, the CI- adlattice on this surface also exhibited a c(2 x 2) structure making it impossible to definitively distinguish between chlorine and oxygen. Structures similar to those observed in Fig. 1, have been reported for both oxygen and water adsorbed on Cu(ll0) surfaces in UHV [6-8]. In this -

environment, the introduction of oxyb . . . . . . . . . . . onto Cu(ll0) surfaces leads to adsorbate-induced reconstructions consisting of [001] Cu-O chains arranged in a (2 X 1) adlayer. We note as well that mixed metal-oxygen chains form on Ni(ll0) surfaces oriented in both the [110] and [001] directions, while on Ag(ll0) a series of n X 1 (n = 2-7) overlayers have been observed [23]. In UHV, the adlayer consists of Cu-O chains which grow when mobile Cu atoms - originating from step edges - bind with oxygen adsorbed on the surface [6-8]. Cu-O chains develop, which eventually become immobile as they increase in length forming a (2 x 1) structure. The UHV and electrochemical chain structures are similar, which suggests that the (2 x 1) chains observed in solution also consist of Cu and O. However, we cannot discern the individual constituents of the chains. The UHV-derived model requires a source of mobile Cu. Possible sources of mobile Cu adatoms on the immersed surface include intrinsic surface mobility as in the UHV case or solution Cu 2÷ arising from exchange between surface and the electrolyte. There is no good measure of surface diffusion in any Cu electrode system. Alternatively, the exchange current density, i0, for Cu(ll0) is between 2 and 2.7 m A / c m 2 [24]. This corresponds to the exchange of between 6 and 9 full monolayers of Cu every second. We note that i 0 is much greater for Cu(ll0) than for the other two low Miller index Cu faces [24]. We have never observed chain growth on these other faces. In summary, [001] oriented chains with (2 X 1) and (3 x 1) structures were observed for the first time on Cu(ll0) interfaces in aqueous HC104 and H2SO 4 solutions at a pH near 2.5-2.7. These structures exists in the thermodynamically forbidden regime of the Pourbaix diagram for bulk Cu20 development indicating that these are likely precursor structures to eventual bulk oxide formation. The different adlayer structures observed with added chloride further supports this conclusion. These oxygen adlayers are thermodynamically analogous to monolayers of metal adatoms formed on other metals by underpotential deposition [20]. The monolayers formed at the solid/liquid interface exhibit structures similar to those observed in the UHV environment and this similarity suggests that they may also be composed of Cu-O chains. An understanding of

J.R. LaGraff, A.A. Gewirth / Surface Science 326 (1995) L461-L466

. . . . . . . . . j . . . . . ygen structures on copper raises possibilities in controlling either this structure or inhibiting its d e v e l o p m e n t in order to halt copper oxidation and corrosion. [8] [9]

Acknowledgments J.R.L. acknowledges a National Science Foundation Postdoctoral Fellowship (CHE-9302406). A.A.G. acknowledges a Presidential Young Investigator Award (CHE-9027593) with matching funds provided by Digital Instruments, Inc and is an A.P. Sloan Foundation Fellow. This work was funded by the Department of Energy (DE-FG02-91ER45349) through the Materials Research Laboratory at the University of Illinois.

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