J. Phys. Chem. B 2001, 105, 1829-1833
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In Situ STM Study of Chloride Adsorption on Cu(110) Electrode in Hydrochloric Acid Aqueous Solution W. H. Li,† Y. Wang,†,‡ J. H. Ye,*,† and S. F. Y. Li‡ Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, Republic of Singapore, and Department of Chemistry, National UniVersity of Singapore, Singapore 119260, Republic of Singapore ReceiVed: September 28, 2000; In Final Form: December 22, 2000
The adsorption of chloride on Cu(110) single-crystal surface has been investigated in hydrochloric acid aqueous solution by using in situ scanning tunneling microscopy (STM). A (1 × 1) structure of Cu(110) substrate lattice was visualized at the potentials negative than -450 mV vs SCE, where chloride anions were desorbed. In the potential range from -400 mV to -150 mV vs SCE, two main complex adlayers with 3-fold and 4-fold periodicity along [11h0] direction of the substrate were observed. These structures with long-range periodicity are attributed to ordered chloride adlayers on unreconstructed Cu(110)-(1 × 1) structure. The different corrugation heights and periodic modulations along [11h0] direction observed in STM images indicate that chloride anions are located at nonequivalent binding sites. The average measured lattice constants for the two adlayers have the same size as the substrate lattice along [001] direction and have 4 and 5 times the lattice spacing of the underlying copper substrate along [11h0] direction. Models are proposed to interpret these two structures to (4 × 1) and (5 × 1) chloride adlayers containing three and four chloride anions, respectively.
Introduction Fundamental studies of the interaction of chloride with copper is of great interest from both scientific and technological viewpoints since it is related to the copper electroplating and also the halogen etching processes in the fabrication of metal interconnections in microelectronic devices.1,2 There are a lot of investigations of copper polycrystalline and single crystalline surfaces, which have been carried out by employing traditional electrochemical methods and ex situ techniques.2-4 The development of in situ electrochemical STM provides a powerful technique to in situ characterize electrode surface on an atomic scale in various electrochemical fields.5 They include specific adsorption of anions, adsorption of organic molecules, underpotential deposition, overpotential deposition, reconstructions, electrochemical dissolution, electrochemical etching, and corrosion of metals and semiconductors in electrolytes.6,7 Recently, in situ STM has been used to study the adsorption of anions, such as halides8-14 and (bi)sulfate,15-18 on copper single-crystal electrodes. The reordering and dissolution processes of copper single crystals in acidic solutions have also been studied by in situ STM.8-10,18 For the Cu(111) surface, Suggs and Bard have reported a (6x3×6x3)R30° chloride adlattice on Cu(111) surface over the whole potential region in HCl solution.8 Kruft et al. have observed a (x3×x3)R30° structure for chloride on Cu(111) surface,11 which is different from the observation of the incommensurate structure reported in ref 8. Recently, Itaya and co-workers reported a different structure with a c(p ×x3R30°) (p ) 2.53-2.48) chloride adlayer on the Cu(111) surface by using in situ STM and lowenergy electron diffraction (LEED) in ultrahigh vacuum,12 in * Corresponding author. Tel: (065) 874 8378. Fax: (065) 872 0785. E-mail:
[email protected]. † Institute of Materials Research and Engineering. ‡ Department of Chemistry.
which the structures are compressed almost linearly with electrode potential. For the Cu(100) surface, Suggs and Bard have reported a (x2×x2)R45° chloride structure on Cu(100) surface in the potential region where no dissolution occurred in HCl solution.9 However, Vogt et al. have observed a (1 × 1) substrate at potentials negative than -400 mV and a c(2 × 2) Cl adlattice only at potentials above -400 mV in HCl solutions.10 These STM results for both Cu(111) and Cu(110) surfaces, together with the STM results we present in this work, are summarized in Table 1. It is noticed that most of previous STM investigations of anion adsorption on copper single crystals in electrolytes have been reported for Cu(111) and Cu(100) surfaces. There are only two STM/AFM studies of anion adsorption on Cu(110) surface in electrolytes13,14 and a few STM studies of adsorption on Cu(110) in ultrahigh vacuum system.19-21 It is known that missing (or added) row reconstruction is a major feature of the surface structural chemistry of (110) surface of fcc transition metals. The (110) surfaces of the metals, such as copper,22 nickel,23 silver,24 palladium,25 and rhodium26 undergo reconstructions upon oxygen adsorption. A (2 × 1) reconstruction of the Cu(110) surface occurring upon oxygen adsorption at room temperature has been known for a long time.22 However, only in the 1990s, LEED27 and STM28 experiments have established that the structure of the (2 × 1) phase is of the added row type. The STM studies by Coulman et al. indicated that the (2 × 1) reconstruction is formed by the condensation of chemisorbed O-adatoms and mobile Cu atoms, which have been evaporated from steps.28 This model for the growth of the reconstruction has been confirmed by many other further experimental works.29,30 There are also some theoretical calculations on the oxygen adsorption and the reconstruction of Cu(110) surface.31 This infers that a rather careful exploration is desirable for the study of Cu(110) surface. However, copper oxidation is unlikely to occur at potentials negative than about
10.1021/jp003542b CCC: $20.00 © 2001 American Chemical Society Published on Web 02/10/2001
1830 J. Phys. Chem. B, Vol. 105, No. 9, 2001
Li et al.
TABLE 1: Comparison of STM Results of Ordered Adlayer Structures for Chloride on Cu(111), Cu(100), and Cu(110) Surfaces electrode
structure
Cu(111)
(6x3× 6x3)R30° (x3×x3)R30° c (p ×x3R30°) (p ) 2.53-2.48) (x2×x2)R45° (1 × 1) c(2 × 2) (1 × 1) (4 × 1) on reconstructed Cu surface (1 × 1) (4 × 1)
Cu(100)
Cu(110)
(5 × 1)
potential range in which the ordered structure observed
coverage
ref
-600 mV ∼ -100 mV vs SCE -950 mV ∼ -500 mV vs Hg/HgSO4 -600 mV ∼ -50 mV vs SCE
0.45 0.33 0.395-0.403
8 11 12
Potential region where no dissolution occurred. -600 mV ∼ -400 mV vs Ag/AgCl -400 mV ∼ -100 mV vs Ag/AgCl -650 mV ∼ -360 mV vs SCE -330 mV ∼ -150 mV vs SCE -700 mV ∼ -400 mV vs SCE both structures coexist at the potential range of -400 mV ∼ -100 mV vs SCE
0.50 0.50
9 10
-160 mV in acidic solutions and between about -160 and -860 mV depending on the pH of electrolyte.32 Gewirth and co-workers have studied the initial stages of oxidation of Cu(110) electrode in aqueous HClO4 and H2SO4 solutions by using AFM.13 Oxide monolayers grown on Cu(110) substrate consisted of [001]-oriented chains arranged mostly in (2 × 1) and (3 × 1) structures. Addition of chloride anions yielded a completely different structure, which is pseudohexagonal with an obvious 3-fold periodicity along [11h0] direction.13 Most recently, Itaya and co-workers have investigated Cu(110) electrodes in HClO4 solutions with addition of trace amount of halide ions (I-, Br- and Cl-) by using in situ STM.14 A Cu(110)-(1 × 1) structure was observed consistently in pure HClO4 solution. The STM image observed after addition of chloride was attributed to adsorbed chloride anions on a reconstructed Cu(110) surface where a (4 × 1) reconstructed Cu structured was formed by adding two copper atomic rows to the underlying (1 × 1) surface in the [001] direction with every two rows absent. Chloride adsorbed on the reconstructed Cu(110) surface with the same (4 × 1) registry.14 In the present work, in situ STM has been used to investigate the adsorption of chloride anions on Cu(110) surface in hydrochloric acid aqueous solution. A (1 × 1) structure of Cu(110) substrate was observed at the electrode potential negative than -450 mV. High-resolution STM images revealed that chloride adsorbed on the Cu(110) surface produce coexisting (4 × 1) and (5 × 1) adlayer structures in the potential range from -400 mV to -150 mV. Models have been proposed for the adlayer structures for the chloride adsorption on Cu(110) surface. A key point is that these periodicities are interpreted in terms of chloride adlayer arrangements on a Cu(110)(1 × 1) substrate, i.e., without involving any substrate reconstruction. Experimental Section A Cu(110) single crystal disk electrode (99.999%, diameter of 10 mm) was commercially available from Goodfellow, UK. The crystal was mechanically polished down to 0.05 µm Al2O3 powder and then electrochemically polished in a phosphoric acid/sulfuric acid solution (130 mL of 85% H3PO4, 20 mL of 97% H2SO4, and 60 mL of H2O) according to ref 16. This process produced a mirrorlike Cu surface. For the electropolishing, a DC voltage of 30 V was applied between the Cu single crystal (anode) and a Pt counter electrode (cathode) for a few seconds. Following the electropolishing, the crystal was rinsed thoroughly with Milli-Q water and immediately transferred to an STM cell or an electrochemical cell and covered with 0.1 M HCl solution.
14 0.75
this work
0.80
Figure 1. Cyclic voltammogram of a Cu(110) electrode in a solution of 0.1 M HCl at a scan rate of 150 mV‚s-1.
The in situ STM measurements were performed with a Nanoscope E in a constant current mode at room temperature. The STM tips were made of a polycrystalline W wire, which was electrochemically etched in 1.0 M NaOH by applying an AC voltage of about 17 V between the W wire and a Pt wire. The STM tips were then coated with nail polish to reduce the faradaic current at the tip. A Pt wire was used as a quasireference electrode during the in situ STM measurements. However, all potentials cited in this paper have been converted to SCE unless stated otherwise. The electrochemical cell for cyclic voltammetry measurements consists of the Cu(110) single crystal working electrode, a SCE reference electrode, and a Pt wire counter electrode. An EG&G model 273A potentiostat was employed for potential control. The solutions were prepared with HCl (70%, redistilled, Aldrich), H2SO4 (97%, Merck), H3PO4 (85%, Aldrich), and Milli-Q water (18.2 MΩ). Results and Discussion Cyclic Voltammetry. Figure 1 shows a cyclic voltammogram of a Cu(110) electrode in a solution of 0.1 M HCl in the potential region between -850 and -100 mV. An anodic peak at -400 mV and a cathodic peak at -440 mV are due to the adsorption and desorption of chloride adlayer on the Cu(110) surface. Similar peaks were also observed in 0.1 M HClO4 + 1 mM HCl solution reported previously14 except the peak potentials were located at more positive potential. There is a low current potential region between -350 mV and -200 mV, which is due to the charging of electrochemical double layer. The increase of current at negative potentials is due to hydrogen evolution. The exponential increase of current at positive potentials is assigned to copper dissolution. A cathodic peak at -113 mV is attributed to the redeposition of the dissolved copper species. STM Imaging of Cu(110)-(1 × 1) Substrate. Figure 2a shows a STM image of the Cu(110) surface at the potential of
Chloride Adsorption on Cu(110) Electrode
J. Phys. Chem. B, Vol. 105, No. 9, 2001 1831
Figure 3. (a) STM image of (4 × 1) chloride adlayer on Cu(110) surface, and (b) proposed model for Adlayer A of chloride adsorbed on Cu(110) surface.
Figure 2. High-resolution in situ STM images of Cu(110) surface in a solution of 0.1 M HCl. (a) E ) -700 mV, Etip ) -300 mV, It ) 2.2 nA (5.5 nm × 5.5 nm) (Substrate); (b) E ) -140 mV, Etip ) 160 mV, It ) 6.1 nA (18 nm × 18 nm); (c) E ) -350 mV, Etip ) 120 mV, It ) 8.5 nA (8 nm × 8 nm) (Adlayer A); (d) E ) -240 mV, Etip ) 160 mV, It ) 1.9 nA (8 nm × 8 nm) (Adlayer B); (e) E ) -330 mV, Etip ) 170 mV, It ) 9.4 nA (6.8 nm × 6.8 nm) (Adlayers A and B).
-700 mV in 0.1 M HCl solution. The rectangular unit cell observed has average interatomic distances of 0.25 ( 0.02 nm in [11h0] direction and 0.36 ( 0.02 nm in [001] direction, which is consistent well with the dimensions of Cu(110) substrate lattice. This (1 × 1) structure is attributed to the Cu(110) substrate, which was observed only at the potentials negative than -450 mV where desorption of chloride anions from Cu(110) occurred. The observation is consistent with the cyclic voltammetry shown in Figure 1, where the desorption peak of chloride is located at -440 mV. STM Imaging of (4 × 1) and (5 × 1) Chloride Adlayers. Figure 2b shows a typical STM image of Cu(110) surface with well-ordered terraces obtained in the potential region from -400 to -150 mV where chloride is adsorbed. The average measured height of steps in this image is 0.21 ( 0.02 nm, which is consistent with a monolayer copper step. Compared with Figure 2a, this image depicts that the straight long steps running from bottom to top-right of the image lie along the [001] direction of substrate lattice. The terrace observed usually has a length of more than 50 nm along [001] direction but relatively narrow width from 2 to 25 nm along [11h0] direction. It is clear that there are some parallel lines on the terraces. These parallel lines lie along the straight steps running along [001] direction. Figures 2c and 2e are high-resolution STM images, which were recorded by zooming in the terraces as shown in Figure 2b. These images exhibit parallel rows running along the [001] direction of substrate lattice. On the terraces, the structures with
the periodicity of 3-fold (called “Adlayer A”) and 4-fold (called “Adlayer B”) are discernible along [11h0] direction, shown in Figures 2c,d, respectively. Similar 3-fold structure was only observed in the potential region from -330 to -150 mV and was proposed to be the ordered chloride anion adlayer adsorbed on (4 × 1) reconstructed Cu(110) substrate.14 In our experiments, both Adlayers A and B are routinely observed in the same potential region where chloride is adsorbed. However, it is difficult to determinate which adlayer dominates the surface by controlling the potential in this region. Figure 2e shows an interesting STM image in which both Adlayers A and B coexist at the instance. There is a marked arrow along [001] direction separating the two structures. The two different adlayers can be clearly distinguished in this figure: one on the top left (Adlayer A) and one on the right (Adlayer B). By analyzing the structures of these two adlayers in details, we found that, for both two adlayers, the average separation between the gray-scale maxima along the [001] direction of substrate lattice is the periodicity of substrate lattice along this direction. The distances between the maxima along [11h0] direction vary and show 3- and 4-fold periodicity for Adlayers A and B, respectively (Figures 2c and 2d). These adlayers observed here are clearly different from the structure of Cu(110)-(1 × 1) substrate observed at potential negative than -450 mV in the same solution (Figure 2a). It is also quite different from the (2 × 1) and (3 × 1) structures observed in the solution of HClO413 and the (2 × 1) reconstruction of the Cu(110) surface upon oxygen adsorption reported previously.28-30 It is worthy noticing that, for copper surface in acidic solution, copper oxides are not stable at the potential negative than -160 mV.32 Therefore, the structures of the two adlayers observed in the present work are attributed to chloride adlayers instead of copper oxides. It is interesting to note that the adsorbed chloride anions observed in the STM images appeared with the different corrugation heights and periodical modulation along [11h0] direction (Figures 2c-2e). This straightforwardly indicates that chloride anions are located at physically nonequivalent binding sites. We assume that the individual bright spots observed in the image stem from adsorbed chloride anions. We consider simple geometrical models33 to explain the changes of corrugation height of the STM patterns, which, of course, are related to the interference of the wave functions of substrate, adlattice and the STM tip. Unit cells can be constructed for the adlayers, shown in both STM images and corresponding proposed models in Figures 3 and 4, respectively. The measured lattice constants for Adlayer A are a ) 0.98 ( 0.06 nm, b ) 0.36 ( 0.03 nm, and R ) 91 ( 4°. The size of vector b is the same as the size of substrate lattice along
1832 J. Phys. Chem. B, Vol. 105, No. 9, 2001
Figure 4. (a) STM image of (5 × 1) chloride adlayer on Cu(110) surface, and (b) proposed model for Adlayer B of chloride adsorbed on Cu(110) surface.
[001] direction (0.362 nm). The vector a for Adlayer A is slightly smaller than four times the lattice spacing of the underlying copper substrate along [11h0] direction (1.02 nm). As shown in both STM image and proposed model of Figure 3, there is a marked angle (β) between the row of chloride anions along [001] direction and the row of chloride anions along the A-A′ direction. The measured value of this angle β is 72 ( 6°. The proposed model is shown in Figure 3b. In this model, chloride anions closely pack along the rows running along [001] substrate direction. Chloride anions in the first row of the unit cell along [001] direction are located at the 4-fold hollows where the chloride atom sits in a hole surrounded by four surface copper atoms. Since the size of chloride anion is larger than that of Cu(110) substrate lattice along [11h0] direction (2.56 Å), the chloride anions in the second and third rows of the unit cell sit on asymmetric sites, which are slightly higher than the 4-fold hollows. The chloride anions of the fourth row are at the similar sites as those of the first row. According to this model, the chloride anions of the first/fourth rows, which correspond to the less bright spots along [001] direction in the STM image, are located at slightly lower level. While the chloride anions of the second and third rows, which correspond to the bright spots along [001] direction in the image, are located at slightly higher level. Thus, there is a 3-fold periodicity along [001] direction observed in the STM images. The calculated value of β between the row of chloride anions along [001] direction and the one along the A-A′ direction is 70.5°. This is consistent with the STM measurements. This model gives a (4 × 1) structure containing three chloride anions. The surface excess for this structure (Adlayer A) is 1.35 × 10-9 mol cm-2 and has a coverage of 0.75. It is interesting that Weaver and co-workers have also observed several ordered bromine adlayers on Au(110) substrate recently,34,35 all featuring close-packed Br rows along the [110] direction. These include (3 × 1) and (4 × 1) unit cells, corresponding to coverage of 0.66 and 0.75, respectively. Our model presented here is similar to the one reported by Zou et al. for Br/Au(110), in which a (4 × 1) Br-adlayer structure on unreconstructed Au(110)-(1 × 1) substrate was deduced.35 It should be emphasized that the top layer of Cu atoms on Cu(110) substrate is not reconstructed in our model. A different model have been purposed previously to explain the similar STM image for Cl/Cu(110),14 in which two copper atomic rows added to the (1 × 1) surface in the [001] direction with every two rows absent, resulting in a (4 × 1) reconstructed structure. Chloride is proposed to adsorb on the reconstructed Cu(110) surface with the same (4 × 1) registry.14
Li et al. Similarly, for Adlayer B, the modulation in the intensity of bright spots in STM images along [11h0] direction infers that the adjacent chloride anions are located at different lattice sites. A unit cell is drawn in both the STM image and the corresponding proposed model, shown in Figures 4a and 4b, respectively. The measured lattice constants for this adlayer are a ) 1.27 ( 0.05 nm, b ) 0.37 ( 0.03 nm and R ) 91 ( 4°. Vector b, has the same dimension for Adlayer A, namely, the size of the substrate lattice along [001] direction. The other vector a for Adlayer B, is 5 times the lattice spacing of the underlying copper substrate along [11h0] direction (1.28 nm) and as such is larger than the a vector for Adlayer A. The measured value of the angle (β) between the rows along [001] and B-B′ in Figure 4a is 54 ( 7°. The proposed model for this adlayer in Figure 4b shows (5 × 1) structure with chloride anions closely packed along [001] direction. Chloride anions are proposed to locate at the 4-fold hollows in the first and fifth rows of the unit cell along [001] direction. The adsorbed chloride anions in these two rows correspond to the less bright spots in the STM image, shown in both STM image and the model in Figure 4. There are three rows between the first and fifth rows. According to this model, the chloride anions of the third row sit at the highest sites (the top position site) of the copper surface atoms and thus correspond to brightest spots in the STM image along [11h0] direction. The relative heights of the adsorbed chloride anions in the second and fourth rows are between those of chloride anions in the third and first or fifth rows. Thus, they correspond to the bright spots in the STM image along [11h0] direction. This model gives a (5 × 1) structure containing four chloride anions. The surface excess for Adlayer B is 1.44 × 10-9 mol‚cm-2 with a coverage of 0.80 for chloride anion on the Cu(110) substrate, which is slightly higher than that for Adlayer A. However, as shown in Figures 3 and 4, both chloride adlayer patterns on Cu(110) involve close-packed chloride rows strictly normal to the (11h0) direction and show distorted hexagonal packing of the adsorbed anions. The hexagonal adsorbate packing is usually observed for halides and other simple chemisorbates on single crystals, reflecting the efficiency of hexagonal packing in minimizing adsorbate-adsorbate repulsions.35 The packing behavior of chloride on Cu(110) observed here indicates that both packing efficiency and the substrate geometry, driven by the intrinsic preference of the atomic chemisorbate for multifold binding sites, make contribution to these interesting structural patterns. STM studies of chloride adsorption on Cu(111), Cu(100), and Cu(110) surfaces were summarized in Table 1. Compared with the coverage of chloride adsorbed on Cu(111) and Cu(100) surfaces, higher coverage of chloride adlayer for Cu(110) surface was proposed in our models. This is due to the open structure of Cu(110) plane, which favors the adsorption of anions, such as chloride and sulfate. It was determined by radiometric method that the order of the sulfate coverage on the three low index mono-crystalline copper electrodes is Cu(110) > Cu(111) > Cu(100).36 It should be also pointed out that the possibility (or absence) of reconstruction/restructuring of metal surface is very much related to the adsorbate and the potential region studied. Zou et al. explored the structure of Au(110) in the presence of bromide electrolytes where close-packed atomic bromine adlayers are formed.34,35 However, markedly different behavior was observed for Au(110) in the presence of iodine electrolytes.37,38 It was also reported that the reconstruction of the Cu(111) in the presence of (bi)sulfate is very different from the reconstruction of the Au(hkl).16-18 The reconstruction of the Au surfaces may be induced by charge or surface stress.39
Chloride Adsorption on Cu(110) Electrode The Cu(111) reconstruction is very much driven by the adsorption of the anion.16,17 Further investigations including both electrochemical studies and ex-situ work, such as surface excess, LEED and surface X-ray scattering of this system will enable us to fully understand the nature of chloride adsorption on Cu(110) surface. Conclusions In situ STM has been employed to study chloride adsorption on Cu(110) electrode in hydrochloride acid aqueous solution. A (1 × 1) structure for Cu(110) substrate was observed at the potential negative than -450 mV, in which the chloride anions are desorbed. The atomic resolution images of chloride adsorbed on Cu(110) surface have been obtained over the potential range from -400 to -100 mV. At the potential positive than -400 mV, the chloride anions are very strongly adsorbed on the Cu(110) surface. The images taken over a large region show parallel rows on the terraces and monolayer straight steps, which run along the [001] direction of the substrate lattice. The parallel row structures observed here are attributed to the chloride adlayers. The different corrugation height and periodical modulation in the height along [11h0] direction observed indicates that chloride anions are located at physically nonequivalent binding sites. Models are proposed to interpret the structures with 3-fold periodicity (Adlayer A) and 4-fold periodicity (Adlayer B) along [11h 0] direction. They are attributed to a (4 × 1) structure containing three chloride anions and a (5 × 1) structure containing four chloride anions, respectively. Acknowledgment. We are grateful to Professor Michael J. Weaver for his criticising reading of this manuscript and helpful discussions. References and Notes (1) Gerischer, H.; Tobias, C. W. AdVances in Electrochemical Science and Engineering; VCH: New York, 1994; Vol. 3, p 55. (2) Bockris, J. O.; Conway, B. E.; Yeager, E.; White, R. E. Electrochemical Materials Science; Plenum: New York, 1981; Vol. 4; p 97. (3) Ehlers, C. B.; Villegas, I.; Stickney, J. L. J. Electroanal. Chem. 1990, 284, 403. (4) Stickney, J. L.; Ehlers, C. B.; Gregory, B. W. Langmuir 1988, 4, 1368. (5) Itaya, K. Progress in Surf. Sci. 1998, 58, 121. (6) Gewirth, A. A.; Siegenthaler, H. Nanoscale Probes of the Solid/ Liquid Interface; Kluwer Academic Publishers: Dordrecht, 1995.
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