In Situ STM Observation of Stable Dislocation Networks during the

pubs.acs.org/JPCL

In Situ STM Observation of Stable Dislocation Networks during the Initial Stages of the Lifting of the Reconstruction on Au(111) Electrodes Cristina Vaz-Domínguez,† Asier Aranz abal,‡ and Angel Cuesta* Instituto de Química Física “Rocasolano”, CSIC, C. Serrano 119, E-28006 Madrid, Spain ABSTRACT In a surface charge density range between -5 and 28 μC cm-2, and independent of the support electrolyte, the surface of a Au(111) electrode shows a morphology that differs from both the well-known surface reconstruction and the unreconstructed state and is apparently composed only of fcc regions and dislocations wider than those observed on the reconstructed surface and arranged in rotational domains that follow the three main crystallographic axes of the substrate. The size of the dislocations increases with the positive charge up to 28 μC cm-2. At higher charge densities, the unreconstructed surface, covered by small triangular islands accommodating the 4-5% excess atoms present in the reconstructed surface, is formed. Our results demonstrate that lifting of the reconstruction is induced by the positive charge on the surface and not by specifically adsorbed anions. SECTION Surfaces, Interfaces, Catalysis

S

region where lifting of the reconstruction takes place. Despite the large amount of STM studies on this system, both in UHVand at the electrode-electrolyte interface, we have observed new dislocation networks, √ different from that characteristic of the well-known (22 3) reconstruction, which are stable in a narrow potential range preceding the formation of the unreconstructed (1 1) surface. As shown by parallel electrochemical measurements, the charge density on the electrode surface √ dictates the potential at which the well-known (22 3) reconstruction transforms into the new dislocation networks and the potential at which the (1 1) surface is formed. Figure 1 shows a typical cyclic voltammogram (CV, Figure 1a) of a Au(111) electrode in 0.1 M H2SO4, and in situ STM images (Figure 1b-f) showing the surface morphology in different potential regions. At potentials more negative than 0.24 V and more positive than √ 0.49 V, the well-known, electrochemically induced4 (22 3) reconstruction (Figure 1b) and the unreconstructed (1 1) surface (Figure 1f) can be observed. Clear changes start to be observed at around 0.24 V, at the onset of the peak corresponding to the lifting of the reconstruction. In this potential region, the dislocation lines separating the narrow hcp domains from the wider fcc ones appear to merge, causing some of the hcp domains to disappear and creating new, wider dislocations ∼30 pm high, clearly larger than the √ 10 pm z corrugation typically measured by us for the (22 3) reconstruction. The fraction of surface covered by these new dislocations increases with potential, and at around

urface reconstruction, particularly on gold singlecrystal surfaces, is arguably the most studied surface phenomenon, both in ultrahigh vacuum (UHV) and at the electrode-electrolyte interface. The driving force for surface reconstruction is the large surface tensile stress of the unreconstructed surface, which is reduced by increasing the number of atoms on the surface1 and accommodating them in a dislocation network.2 In the case of Au(111) in UHV, the reconstructed state, consisting of a ∼4% compression of the top layer in one of the three (110) directions, is the thermodynamically stable form at room temperature,3 this being also the case for negatively charged Au(111) electrodes.4 However, depleting electrons from the surface results in an induced compressive stress that stabilizes the unreconstructed state,5 so that at positive enough potentials, the surface reconstruction is lifted.4 It has been observed that the potential at which the lifting of the reconstruction occurs shifts in the negative direction in the presence of specifically adsorbing anions, the shift being the larger the stronger the anion adsorbs. It has been discussed whether this lifting is caused by the adsorption bond6 or by the redistribution of charges accompanying adsorption,7 a question that can only be studied at the electrode-electrolyte interface, where the surface charge density can be extensively varied with a high precision. A careful thermodynamic analysis of interfacial capacity measurements on Au(111) and Au(100) electrodes, using the surface tension, γ, as the correct thermodynamic potential describing equilibrium conditions, reached the conclusion that lifting of the reconstruction at positive potentials can be induced by charging the surface.8 We have investigated, using in situ scanning tunneling microscopy (STM), the changes occurring on the surface of Au(111) electrodes in the narrow potential

r 2010 American Chemical Society

Received Date: May 21, 2010 Accepted Date: June 15, 2010 Published on Web Date: June 18, 2010

2059

DOI: 10.1021/jz1006803 |J. Phys. Chem. Lett. 2010, 1, 2059–2062

pubs.acs.org/JPCL

Figure 1. Cyclic voltammogram at 10 mV s-1 (a) and surface morphologies observed using in situ STM on a Au(111) electrode at -0.01 (b), 0.29 (c), 0.34 (d), 0.44 (e), and 0.49 V (f) in 0.1 M H2SO4. The tip was kept at -0.105 V.

0.36-0.37 V (coinciding with the peak potential of the peak attributed to the lifting of the reconstruction, Figure 1d), the surface is completely covered by a network of these new dislocations (z corrugation of ∼50√pm), the ones separating hcp and fcc domains in the (22 3) reconstruction having completely disappeared. The dislocation lines arrange themselves in domains rotated 120°, and some of them have merged, forming wider lines and geometrical figures like triangles, chevrons, and trigonal stars, all of these motifs maintaining the trigonal symmetry of the surface. The width of the new dislocations continues growing with potential (see Figure 1e, where the z corrugation has decreased again to ∼30 pm), and at a potential of 0.49 V, clearly after the peak attributed to the lifting of the reconstruction, the dislocations disappear and are substituted by a flat surface covered by some small, ∼0.25 nm high islands. This morphology is typical of the Au(111)-(1 1) surface, where the small, one-atom-high islands accommodate the 4-5% √ excess of atoms initially present in the Au(111)-(22 3) reconstructed surface.4,9,10 We have obtained similar results in HClO4 and in chloridecontaining solutions (see Supporting Information), the presence or absence of a specifically adsorbing anion and its chemical nature affecting only the onset potential and the potential window over which the reported changes can be observed. As was to be expected, the more strongly the anion adsorbs, the more negative the potential at which we start observing changes in the surface morphology, and the narrower the potential window in which the reported dislocation networks are stable. Figure 2 shows charge density versus potential plots in 0.1 M KClO4, 0.1 M H2SO4, and 0.1 M KClO4 þ 1 mM KCl, obtained by integration of the corresponding CVs, taking 0.365 V as the potential of zero charge (pzc) of reconstructed Au(111) and assuming that, at the negative limit of integration, where all of the CVs superpose, the charge density is independent of the anion present in the solution. The potential at which the new dislocations start to be observed in each solution and the potential at which one-atom-high islands are first observed on


Figure 2. Surface charge density versus potential plot of Au(111) in 0.1 M KClO4 (black curve), 0.1 M H2SO4 (red curve), and 0.1 M KClO4 þ 1 mM KCl (green curve). The vertical solid lines indicate the potential at which the first changes in the surface morphology are detected, and the vertical dashed lines are the potential at which lifting of the reconstruction is complete, as determined using in situ STM in 0.1 M KClO4 (black), 0.1 M H2SO4 (red), and 0.1 M KClO4 þ 1 mM KCl (green). The horizontal solid and dashed lines indicate the surface charge density when the first changes in the surface morphology are detected and when the lifting of the reconstruction has been completed, respectively.

the surface, and therefore the Au(111)-(1 1) surface has been formed, are indicated with vertical lines. In all three cases, the formation of the new dislocation starts at a surface charge density of ∼-5 μC cm-2, and formation of the (1 1) surface occurs at a surface charge density of ∼28 μC cm-2. Santos and Schmickler8 found that around the pzc, the surface tensions for the reconstructed and unreconstructed surfaces are very similar, and only at a potential clearly above the pzc does the unreconstructed surface become more stable, in good agreement with the results reported here. In addition, our results

2060


pubs.acs.org/JPCL

detected the onset of the lifting of the reconstruction (0.07 e/atom = -15.6 μC cm-2) and that at which lifting of the reconstruction has been completed√(taken as the √ point at which the scattering intensity at (0.038/ 3, 1 þ 0.038/ 3, and 0.2) becomes zero in Figure 4 of ref 7, -0.1 e/atom = 22.3 μC cm-2) is ∼38 μC cm-2, in very good agreement with the 33 μC cm-2 determined from our results. Similarly, at a scan rate of 1 mV s-1, the second harmonic intensity for Au(111) in 0.01 M HClO √4 started to decrease from that characteristic of the (22 3) reconstruction to that characteristic of the unreconstructed surface at -0.4 V versus SCE, but the transition was completed only at E > 0.7 V.11 We would also like to note that in one of the earliest reports of electrochemical STM, Tao and Lindsay10 very briefly reported the observation in a small fraction of the Au(111) surface of a structure whose description closely resembles the dislocation networks reported here by us; they observed regions of less densely packed stripes with the average distance √ between two adjacent stripes ∼30% greater than 22 3 stripes, where the hcp regions had disappeared. In conclusion, our results clearly demonstrate that lifting of the reconstruction is induced by positively charging the surface, specifically adsorbing anions simply shifting negatively the potential at which the critical surface charge density is reached by retrieving electrons from the surface when the adsorption bond is formed. We have also shown for the first time that, independent of the anion present in the support electrolyte, at a surface charge density of -5 μC cm-2, the hcp domains present on the reconstructed Au(111) surface start to disappear, causing the dislocations separating them from the fcc domains to merge and form wider dislocations with a larger z corrugation (30-50 √ pm as compared to 10 pm typical of the Au(111)-(22 3) surface). Between -5 and 28 μC cm-2, a series of stable dislocation networks can be observed on the surface of Au(111) electrodes, the width of the dislocations increasing with increasing positive charge density.

Figure 3. STM images (176 176 nm2) of a Au(111) electrode in 0.1 M H2SO4 at 0.39 V. The image on the left was acquired immediately after stepping the potential to 0.39 V, and the image on the right corresponds to the same surface area 30 min later. UT = 0.5 V (tip negative).

suggest that lifting of the reconstruction is induced by positively charging the surface and that specific adsorption of anions simply shifts negatively the potential at which the critical charge density is reached, in good agreement with previous reports.7,8 The dislocation networks observed for surface charge densities between -5 and 28 μC cm-2 are stable and do not disappear or change their general aspect even after 30 min, as shown in Figure 3. However, the surface atoms are very mobile in this potential region, giving the images a frizzy aspect and precluding us from obtaining atomically resolved images. The mobility of the surface atoms is also evident in Figure 3: although after 30 min at 0.39 V the general aspect of the surface has not changed and the fraction of the surface covered by the dislocations remains the same, the individual features have modified their shape. Our observation of new surface morphologies different both from the reconstructed and from the bulk-terminated Au(111) surface agrees with earlier findings obtained with SXS7 and SHG;11 Wang et al.7 found that at an effective scan rate of 0.5 mV s-1 in 0.01 M NaCl, the X-ray scattering profile along the qr axis at L=0.2 changed gradually between -0.4 and 0.1 V versus Ag/AgCl (KClsat) from that typical of the reconstructed surface to that characteristic of the unreconstructed Au(111)-(1 1) surface. They also found that in all of the cases studied (0.1 M NaF, 0.1 M NaCl, and 0.1 M NaBr), lifting of the reconstruction started at the same surface charge density of 0.07 ( 0.02 e/atom (-15.6 ( 4.5 μC cm-2). Although this is clearly more negative than the charge density of -5 μC cm-2 at which we start to observe changes in the surface morphology, it must be noted that the capacitance curve of Au(111) in 0.1 M NaF used by Wang et al. to calculate their charge density versus potential plots, taking 0.37 V as the pzc of reconstructed Au(111), was obtained by averaging the capacitance data in 0.05 and 0.5 M NaF reported by Lecoeur and Hamelin12 and by Hamelin,13 respectively. This most likely introduced an error in the corresponding charge density versus potential plot that would be dragged to those determined for 0.1 M NaCl and 0.1 M NaBr, which were calculated assuming that the surface charge density at the negative limit of integration is the same in all three solutions. However, the difference between the surface charge density at which Wang et al.


EXPERIMENTAL SECTION The working electrode used was a single-crystal disk (10 mm diameter) from MaTeck (J€ ulich, Germany). Before each experiment, the crystal was annealed in the flame of a Bunsen burner. The auxiliary electrode was a platinum wire. A Ag/AgCl(KClsat) electrode (CV experiments) and a platinum wire (STM experiments) were used as the reference and quasi-reference electrodes, respectively. All of the potentials in the text are referenced to the Ag/AgCl(KClsat) electrode, unless otherwise stated. The STM used was a PicoLE Molecular Imaging with a PicoScan 2100 Controller. Experiments were performed with tungsten tips, etched from a polycrystalline wire in 2 M NaOH and coated with nail polish (measurements in 0.1 M H2SO4) or polyethylene glue (ethyl-vinylacetate copolymer) from a polyethylene glue gun (measurements in 0.1 M HClO4 and 0.1 M HClO4 þ 1 mM KCl) in order to reduce the faradaic current at the tip/electrolyte interface. All images were recorded in the constant-current mode (IT = 2 nA). Raw STM images were treated using WSxM software.14

2061


pubs.acs.org/JPCL

SUPPORTING INFORMATION AVAILABLE Cyclic Voltammograms and in situ STM images in 0.1 M HClO4 and 0.1 M H2SO4 þ 1 mM HCl. This material is available free of charge via the Internet at http://pubs.acs.org.

(13)

AUTHOR INFORMATION

(14)

Corresponding Author: *To whom correspondence should be addressed. Tel: þ34915619400. Fax: þ34-915642431. E-mail: [email protected].

Monocrystalline Gold (111) Electrode in Contact with Aqueous Sodium-Fluoride Solutions. C. R. Acad. Sci. C 1974, 279, 1081–1084. Hamelin, A. Coadsorption of Sulphate Ions and Pyridine on the (111), (110) and (100) Faces of Gold. J. Electroanal. Chem. 1983, 144, 365–372. Horcas, I.; Fernandez, R.; Gomez-Rodriguez, J. M.; Colchero, J.; Gomez-Herrero, J.; Baro, A. M.; Wsxm, A Software for Scanning Probe Microscopy and a Tool for Nanotechnology. Rev. Sci. Instrum. 2007, 78, 01370/5–01370/8.

Notes † ‡

E-mail: [email protected]. E-mail: [email protected].

ACKNOWLEDGMENT Financial support from the DGI (Ministerio de Ciencia e Innovaci on) under Project CTQ2009-07017 is gratefully acknowledged. C.V-D. acknowledges a JAE-Doc fellowship from the CSIC. We thank Prof. Claudio Guti errez for a critical reading of the manuscript.

REFERENCES (1)

Fiorentini, V.; Methfessel, M.; Scheffler, M. Reconstruction Mechanism of fcc Transition Metal (001) Surfaces. Phys. Rev. Lett. 1993, 71, 1051–1054. (2) Barth, J. V.; Brune, H.; Ertl, G.; Behm, R. J. Scanning Tunneling Microscopy Observations on the Reconstructed Au(111) Surface: Atomic Structure, Long-Range Superstructure, Rotational Domains, and Surface Defects. Phys. Rev. B 1990, 42, 9307–9318. (3) Perdereau, J.; Biberian, J. P.; Rhead, G. E. Adsorption and Surface Alloying of Lead Monolayers on (111) and (110) Faces of Gold. J. Phys. F 1974, 4, 798. (4) Kolb, D. M. Reconstruction Phenomena at Metal-Electrolyte Interfaces. Prog. Surf. Sci. 1996, 51, 109–173. (5) Ibach, H.; Bach, C. E.; Giesen, M.; Grossmann, A. PotentialInduced Stress in the Solid-Liquid Interface: Au(111) and Au(100) in an HClO4 Electrolyte. Surf. Sci. 1997, 375, 107–119. (6) Bohnen, K. P.; Kolb, D. M. Charge- Versus Adsorbate-Induced Lifting of the Au(100)-(Hex) Reconstruction in an Electrochemical Environment. Surf. Sci. 1998, 407, L629–L632. (7) Wang, J.; Alison, J. D.; Isaacs, H. S.; Ocko, B. M. Surface Charge-Induced Ordering of the Au(111) Surface. Science 1992, 255, 1416–1418. (8) Santos, E.; Schmickler, W. Changes in the Surface Energy During the Reconstruction of Au(100) and Au(111) Electrodes. Chem. Phys. Lett. 2004, 400, 26–29. (9) Gao, X.; Hamelin, A.; Weaver, M. J. Atomic Relaxation at Ordered Electrode Surfaces Probed by Scanning Tunneling Microscopy: Au(111) in Aqueous Solution Compared with Ultrahigh-Vacuum Environments. J. Chem. Phys. 1991, 95, 6993–6996. √ (10) Tao, N. J.; Lindsay, S. M. Observations of the 22 3 Reconstruction of Au(111) under Aqueous Solutions Using Scanning Tunneling Microscopy. J. Appl. Phys. 1991, 70, 5141–5143. (11) Friedrich, A.; Pettinger, B.; Kolb, D. M.; L€ upke, G.; Steinhoff, R.; Marowsky, G. An in Situ Study of Reconstructed Gold Electrode Surfaces by Second Harmonic Generation. Chem. Phys. Lett. 1989, 163, 123–128. (12) Lecoeur, J.; Hamelin, A. Influence of Polarization Range Boundaries on Differential-Potential Capacity Curves of


2062


In Situ STM Observation of Stable Dislocation Networks during the

Recommend Documents