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In-situ observation of an ordered sulfate adlayer on Au(100) electrodes
Surface Science 430 (1999) L521–L526
www.elsevier.nl/locate/susc
Surface Science Letters
In-situ observation of an ordered sulfate adlayer on Au(100) electrodes M. Kleinert, A. Cuesta, L.A. Kibler, D.M. Kolb * Department of Electrochemistry, University of Ulm, 89069 Ulm, Germany Received 14 December 1998; accepted for publication 17 February 1999
Abstract An ordered adlayer of sulfate or bisulfate has been found on the unreconstructed Au(100) surface in 0.1 M H SO by in-situ STM. The ordered adlayer is formed concomitantly with lifting of the (hex)-reconstruction. The 2 4 new structure consists of sulfate/bisulfate rows that run parallel to the edges of the islands that were created during ˚ , and that between rows the (hex)(1×1) transition. The distance between the adions within the rows is 4.0±0.3 A 1.4±0.1 0 ˚ 10.4±0.7 A, corresponding to a ( 0 3.6±0.2) superstructure with a surface coverage of about 0.2. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Au(100) electrodes; Metal–electrolyte interfaces; Ordered adlayers; STM; Sulfate
1. Introduction One of the most intensively pursued goals in today’s electrochemistry is to understand the structure and composition of the electrochemical double layer, in general, and to understand adsorption processes at the electrode/electrolyte interface, in particular (see, for example, [1,2]). Much work has been carried out for the mercury/electrolyte interface [3], since the use of the dropping mercury electrode provides an easy way to expose a clean surface to the solution. Similarly well-defined conditions have been achieved in recent years for single crystal surfaces of noble metals [4]. The * Corresponding author. Fax: +49 731-502-5409. E-mail address: [email protected] (D.M. Kolb)
existence of a large double-layer region, besides the easy preparation and handling, has been the main reason for gold electrodes being employed as a model system for the study of interfacial properties of solid electrodes [5]. The report by Clavilier et al. [6,7] of an anomalous adsorption state in the voltammograms of a well-defined Pt(111) electrode in sulfuric and perchloric acid solutions, which was attributed by these authors to hydrogen adsorption, but later shown by Al Jaaf et al. [8] to be due to specific adsorption of anions, has stimulated the study of sulfate adsorption on a number of single crystal metal electrodes [9–15]. From the very beginning of their observation, current spikes have been attributed to structural transitions on the surface and in ionic adlayers [8,16 ].
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Magnussen et al. [11] were the first to provide direct evidence by in-situ STM for an ordered adlayer with a (앀3×앀7) superstructure on Au(111) in 0.1 M H SO . This structure can be 2 4 readily seen at potentials positive of the sharp spikes in the cyclic voltammogram at 0.78 V vs. SCE, which arises from a disorder–order phase transition of the sulfate adlayer. They proposed a model for the (앀3×앀7) structure in which the adsorbed species was bisulfate with a coverage of 0.4. Weaver and co-workers have also studied this system by a combination of IRRAS and STM and found the same structure [13]. They reached the conclusion that the adlayer was formed by sulfate, rather than bisulfate, with a maximum coverage of 0.2, and coadsorbed hydronium cations. Chronocoulometric measurements by Shi et al. [12] also yielded a coverage of 0.2. Similar STM images have later been reported for sulfate adsorbed on Pt(111) [15] and Rh(111) [14]. In the latter case, a model was proposed according to which the images would correspond to an ordered sulfate adlayer and hydrogen-bonded water chains along the 앀3 direction between neighboring rows of adsorbed sulfate [14]. Very recently, two groups have reported on the in-situ observation of an ordered sulfate adlayer on Cu(111) [17,18]. The structure is similar to that found on Au(111) [11,13], Pt(111) [15] and Rh(111) [14], with the only difference being that on Cu(111), a Moire´ pattern was observed [17,18]. The appearance of a Moire´ pattern was explained by Li et al. [17] as being due to the incommensurate character of the adlayer, whereas Wilms et al. [18] ascribed it to the adsorption of sulfate on a reconstructed Cu surface. Sulfuric acid adlayers on emersed Au(111), Au(100) and Au(110) electrodes have been studied ex situ by Zei et al. [19,20] using LEED, RHEED and AES. They found a (2×앀3) rectangular structure on the reconstructed Au(100)-(hex) surface and a p(2×2) structure on the unreconstructed Au(100)-(1×1) surface. For Au(111), they reported a (앀7×앀7) structure for the sulfuric acid adlayer. More recently, Moraes et al. [21] have studied the adsorption of sulfate and nitrate anions on Au(100) electrodes using IRRAS. They found
that in 0.5 M KF+0.69 M HF+10−2 M Na SO 2 4 (pH 2.8), only sulfate was present on the surface, the adsorption starting at +0.23 V vs. SCE. In a 7.3 M HF solution (pH 0.23) containing 10−2 M Na SO , both sulfate and bisulfate were found on 2 4 the surface, the ratio sulfate/bisulfate increasing with potential. In such a solution, sulfate adsorption started at +0.49 V vs. SCE. Here, we report for the first time the observation by in-situ STM of an ordered sulfate or bisulfate adlayer on Au(100) in 0.1 M H SO . For the sake 2 4 of brevity, we shall use in the following only the term ‘sulfate’ for describing the adlayer.
2. Experimental The Au(100) electrode used for STM measurements was a single crystal disc with a 12-mm diameter (MaTecK, Ju¨lich), oriented to better than 1° and polished down to 0.03 mm. Before each experiment, the crystal was annealed in a hydrogen flame for about 6 min and cooled down to room temperature in a stream of nitrogen. Two platinum wires were used as reference and counter electrodes in the STM cell, but all potentials are quoted against SCE. A Topometrix TMX 2010 Discoverer STM was used. Experiments were performed with tungsten tips etched from a polycrystalline wire in aqueous NaOH. The tips were then coated with an electrophoretic paint [22] in order to reduce the faradaic current at the tip/electrolyte interface. All images were recorded in the constant-current mode. The electrolyte (0.1 M H SO ) was prepared 2 4 from Merck suprapure chemicals and Milli-Q water (18.2 MV · cm and 3 ppb of TOC ).
3. Results and discussion Contrary to what is found for Au(111), the cyclic voltammogram for Au(100) in 0.1 M H SO (Fig. 1) shows no spikes, which would 2 4 indicate an order–disorder transition within the sulfate adlayer at positive potentials. Only one peak at +0.34 V vs. SCE is seen in the first positive-trending sweep, which corresponds to the
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Fig. 1. Cyclic voltammogram for a freshly prepared, reconstructed Au(100) electrode in 0.1 M H SO , starting at −0.2 V 2 4 in negative direction, with the (hex)(1×1) transition occurring at +0.34 V on the positive scan. Sweep rate: 10 mV s−1.
lifting of the Au(100)-(hex) reconstruction [23,24]. It is well-known that the (hex)(1×1) transition causes a shift of the pzc by 220 mV to negative values, and hence, additional charging of the electrode surface is required to maintain during the structural transition the applied potential on the positive side of the pzc [24]. However, the absence of a spike does not mean that sulfate ions are not specifically adsorbed on the electrode surface. In fact, it has been shown that specific adsorption of anions plays a crucial role in lifting of the reconstruction: the stronger the interaction between specifically adsorbed anions and the gold surface, the more negative is the potential at which lifting occurs [24]. For sulfuric acid solutions, a critical anion coverage for the (hex)(1×1) transition of only about 5% of a monolayer has been estimated [23]. It has also been shown [23,24] that the charge under the transition peak (e.g. at 0.34 V ) depends strongly on the anion in solution, indicating that both double-layer charging due to lifting of the reconstruction and the specific adsorption of anions contribute to this peak. Fig. 2 shows a 65×65 nm2 image of a Au(100) surface in 0.1 M H SO at +0.35 V vs. SCE, i.e. 2 4 at a potential just positive of the current peak in the cyclic voltammogram. The monoatomic high gold islands on the surface were formed by the extra amount of gold atoms required for the more densely packed reconstructed surface and expelled during the structural transition (hex)(1×1). The
Fig. 2. STM image (65×65 nm2) of a Au(100) electrode in 0.1 M H SO , obtained at +0.35 V vs. SCE with a tungsten 2 4 tip. I = 2 nA; U =−0.35 V (tip negative). T T
square shape of the islands reflects the twofold symmetry of the Au(100)-(1×1) surface. A noval feature in the image, however, are lines that cover the whole surface and are aligned parallel to the edges of the monoatomic high islands. These lines are not interrupted by the islands, but run over them. It can also be seen in Fig. 2 that these lines form domains rotated by 90°, as one would expect for a surface with twofold symmetry. Although the overall appearance of the new structure in Fig. 2 resembles the (hex)-reconstruction, the line ˚ is far too small to assign separation of 10.4 A these lines to areas where the reconstruction had not been lifted. We assign the new structure to an ordered sulfate adlayer on the unreconstructed Au(100)-(1×1) surface. Fig. 3 shows a molecularly resolved image of the adlayer, and Fig. 4 shows an atomically resolved image of the Au(100)-(1×1) surface proper for comparison. The image in Fig. 3 reveals rows of main maxima separated by rows of secondary maxima, and hence is very similar to those for the ordered sulfate adlayer on Au(111) [11,13], Rh(111) [14] and Pt(111) [15], the only difference
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Fig. 4. STM image (6×6 nm2) of a Au(100)-(1×1) surface in 0.1 M H SO , obtained at +0.35 V vs. SCE with a tungsten 2 4 tip, showing the bare gold surface. I =50 nA; U = −0.35 V. T T
Fig. 3. STM images of a 7×7 nm2 area (top view and 3-D plot) of a Au(100) electrode in 0.1 M H SO , obtained at +0.35 V 2 4 vs. SCE with a tungsten tip, showing a close-up of the adlayer structure. Same conditions as in Fig. 2.
being that in the present case, the main maxima are arranged in a rectangular way (the small deviation from an exactly rectangular unit cell is due to drift). The distance between the main ˚ maxima in the close-packed rows is 4.0±0.3 A ˚ and that between rows is 10.4±0.7 A, which corresponds to a (1.4±0.1×3.6±0.2) superstructure. The main axes of its unit cell must be parallel to the crystallographic [011] and [01: 1] directions since in Fig. 2, the lines are seen to run parallel to the gold island edges. According to this, the struc1.4±0.1 0 ture can be better described as ( 0 3.6±0.2) with
respect to the substrate’s surface unit cell and would correspond to an incommensurate ordered sulfate adlayer on Au(100). The lack of an order–disorder phase transition for the sulfate adlayer independent of the (hex)(1×1) transition makes it very difficult to obtain an image composed of both the substrate (1×1) and the sulfate structure, as in the case of Au(111), which would have allowed us to determine the parameters of the adlayer unequivocally. However, changing the experimental conditions, namely increasing the tunneling current, enabled us to obtain atomically resolved images of the Au(100)-(1×1) surface only a few minutes after having measured the sulfate structure, which provides the data for an internal calibration of the adlayer structure parameters. This procedure confirmed our conclusions that the sulfate rows run parallel to the substrate’s crystallographic main axes and that the adlayer forms an incommensurate (1.4±0.1×3.6±0.2) structure. Further support of the incommensurate character of the adlayer structure is provided in Fig. 5, where a molecularly resolved image of a
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Fig. 5. STM image (27×27 nm2) of a Au(100) electrode in 0.1 M H SO , obtained at +0.35 V vs. SCE with a tungsten 2 4 tip, showing the adlayer structure at domain boundaries, under the same conditions as in Fig. 2.
27×27 nm2 area of the sulfate adlayer is shown. Some of the sulfate rows form bows at the domain boundary, which implies that these anions are not aligned along any of the substrate’s main axes, but are in intermediate positions, although the distance between adions within the rows seems to be kept constant. From the (1.4±0.1×3.6±0.2) superstructure, an approximate sulfate coverage of 0.2 is derived, if only the main maxima are considered to correspond to adsorbed sulfate. Magnussen et al. [11] have assigned both the main and secondary maxima in the STM images of the Au(111)− (앀3×앀7) superstructure to adsorbed bisulfate, which results in a coverage of 0.4. However, this value is at variance with chronocoulometric data, which suggest a coverage of 0.2 [12]. According to previous suggestions for Au(111) [13] and Rh(111) [14], the secondary maxima in the Au(100)-(1.4±0.1×3.6±0.2) superstructure (Fig. 3) should correspond either to hydrogenbonded hydronium cations [13] or to water molecules [14]. Support for the latter has recently been
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provided by Ataka and Osawa [25] by means of in-situ ATR-SEIRAS. These authors propose a model in which all secondary maxima in the STM images of sulfate on Au(111) are assigned to water molecules bridging through hydrogen bonding sulfate molecules in neighboring rows. Bridging hydrogen bonds would diminish the Coulomb repulsions between adjacent sulfate anions, as had already been pointed out by Edens et al. [13] for the case of coadsorbed hydronium ions. Regarding this, it should be noted that Ataka and Osawa [25] also provide evidence that hydronium ions are not incorporated in the sulfate adlayer. Normally, the highest coverages for adlayers are achieved on the densely packed (111) surface. This means that the estimated coverage for the (1.4±0.1×3.6±0.2) sulfate structure is very close to saturation or even is the saturation coverage. This is supported by the fact that the current in the cyclic voltammogram ( Fig. 1) remains constant after the peak at +0.34 V, indicating a negligible increase in specific adsorption. This also explains the absence of spikes for order–disorder phase transitions at more positive potentials, since a high coverage is already reached immediately after lifting the surface reconstruction. The peak at +0.34 V in the cyclic voltammogram ( Fig. 1) should therefore be interpreted as being due to the transition from a reconstructed Au(100)-hex surface to an unreconstructed Au(100) surface covered by an ordered (1.4±0.1×3.6±0.2) sulfate adlayer. A last point that should be discussed is why this structure has not been previously reported, despite the fact that gold single crystals are the most intensively studied surfaces in electrochemical STM investigations. We have found the structure to be very labile: after some time of measuring (about 30–40 min), the structure was destroyed by the tip. We have noted that the best conditions for imaging the structure were low tunnel currents and high tunnel voltages, i.e. with the tip far away from the surface. The use of higher tunnel currents or smaller tunnel voltages (i.e. approaching the tip to the surface) resulted in a faster destruction of the adlayer structure. The time during which the structure persisted on the surface was also found to be dependent on the cleanliness of both the
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surface and the solution: the presence of contaminations on the surface reduced the life time of the structure.
4. Conclusions We have found that upon lifting the hex reconstruction, an ordered sulfate or bisulfate adlayer is formed on the unreconstructed Au(100) surface. The adlayer has an incommensurate (1.4±0.1× 3.6±0.2) structure with an approximate coverage of 0.2. In-situ STM reveals that the structure is composed of main and secondary maxima, similar to the STM images of the (앀3×앀7) sulfate structure observed on Au(111) [11,13], Pt(111) [15] and Rh(111) [14]. In analogy to Au(111) [13], only the main maxima are assumed to correspond to sulfate molecules, the secondary maxima probably representing coadsorbed water molecules [14,25]. The stability range of the ordered adlayer on the unreconstructed Au (100) surface, particularly for the negative potential scan, is currently under investigation.
Acknowledgement A.C. acknowledges a Marie Curie fellowship within the TMR program of the European Commission.
References [1] J. Lipkowski, P.N. Ross ( Eds.), Adsorption of Molecules at Metal Electrodes, VCH, New York, 1992.
[2] J. Lipkowski, P.N. Ross (Eds.), Structure of Electrified Interfaces, VCH, New York, 1993. [3] R. Payne, in: D.A. Cadenhead, J.F. Danielli, M.P. Rosenberg ( Eds.), Progress in Surface and Membrane Science Vol. 6, Academic Press, New York, 1973, p. 51. [4] A. Hamelin, in: B.E. Conway, R.E. White, J.O’M. Bockris (Eds.), Modern Aspects of Electrochemistry, Vol. 16, Plenum Press, New York, 1985, p. 1. [5] A. Hamelin in: Nanoscale Probes of the Solid/liquid Interface, A.A. Gewirth, H. Siegenthaler (Eds.), Nato ASI Series E, Vol. 288, Kluwer, Dordrecht, 1993, p. 285. [6 ] J. Clavilier, R. Faure, G. Guinet, R. Durand, J. Electroanal. Chem. 107 (1980) 205. [7] J. Clavilier, R. Faure, G. Guinet, R. Durand, J. Electroanal. Chem. 107 (1980) 211. [8] K. Al Jaaf-Golze, D.M. Kolb, D. Scherson, J. Electroanal. Chem. 200 (1986) 353. [9] H. Angerstein-Kozlowska, B.E. Conway, A. Hamelin, L. Stoicoviciu, Electrochim. Acta 31 (1986) 1051. [10] H. Angerstein-Kozlowska, B.E. Conway, A. Hamelin, L. Stoicoviciu, J. Electroanal. Chem. 228 (1987) 429. [11] O.M. Magnussen, J. Hagebo¨ck, J. Hotlos, R.J. Behm, Faraday Discuss. Chem. Soc. 94 (1992) 329. [12] Z. Shi, J. Lipkowski, M. Gamboa, P. Zelenay, A. Wieckowski, J. Electroanal. Chem. 366 (1994) 317. [13] G.J. Edens, X. Gao, M.J. Weaver, J. Electroanal. Chem. 375 (1994) 357. [14] L.-J. Wan, S.-L. Yau, K. Itaya, J. Phys. Chem. 99 (1995) 9507. [15] A.M. Funtikov, U. Stimming, R. Vogel, J. Electroanal. Chem. 482 (1997) 147. [16 ] D.A. Scherson, D.M. Kolb, J. Electroanal. Chem. 176 (1984) 353. [17] W.-H. Li, R.J. Nichols, J. Electroanal. Chem. 456 (1998) 153. [18] M. Wilms, P. Broekmann, C. Stuhlmann, K. Wandelt, Surf. Sci. 416 (1998) 121. [19] M.S. Zei, D. Scherson, G. Lehmpfuhl, D.M. Kolb, J. Electroanal. Chem. 29 (1987) 99. [20] M.S. Zei, D. Weick, Surf. Sci. 208 (1989) 425. [21] I.R. Moraes, M.C.P.M. da Cunha, F. Nart, J. Braz. Chem. Soc. 7 (1996) 453. [22] C.E. Bach, R.J. Nichols, W. Beckmann, H. Meyer, A. Schulte, J.O. Besenhard, P.D. Janakoudakis, J. Electrochem. Soc. 140 (1993) 1281. [23] D.M. Kolb, J. Schneider, Electrochim. Acta 31 (1986) 929. [24] D.M. Kolb, Prog. Surf. Sci. 51 (1996) 109. [25] K. Ataka, M. Osawa, Langmuir 14 (1998) 951.
www.elsevier.nl/locate/susc
Surface Science Letters
In-situ observation of an ordered sulfate adlayer on Au(100) electrodes M. Kleinert, A. Cuesta, L.A. Kibler, D.M. Kolb * Department of Electrochemistry, University of Ulm, 89069 Ulm, Germany Received 14 December 1998; accepted for publication 17 February 1999
Abstract An ordered adlayer of sulfate or bisulfate has been found on the unreconstructed Au(100) surface in 0.1 M H SO by in-situ STM. The ordered adlayer is formed concomitantly with lifting of the (hex)-reconstruction. The 2 4 new structure consists of sulfate/bisulfate rows that run parallel to the edges of the islands that were created during ˚ , and that between rows the (hex)(1×1) transition. The distance between the adions within the rows is 4.0±0.3 A 1.4±0.1 0 ˚ 10.4±0.7 A, corresponding to a ( 0 3.6±0.2) superstructure with a surface coverage of about 0.2. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Au(100) electrodes; Metal–electrolyte interfaces; Ordered adlayers; STM; Sulfate
1. Introduction One of the most intensively pursued goals in today’s electrochemistry is to understand the structure and composition of the electrochemical double layer, in general, and to understand adsorption processes at the electrode/electrolyte interface, in particular (see, for example, [1,2]). Much work has been carried out for the mercury/electrolyte interface [3], since the use of the dropping mercury electrode provides an easy way to expose a clean surface to the solution. Similarly well-defined conditions have been achieved in recent years for single crystal surfaces of noble metals [4]. The * Corresponding author. Fax: +49 731-502-5409. E-mail address: [email protected] (D.M. Kolb)
existence of a large double-layer region, besides the easy preparation and handling, has been the main reason for gold electrodes being employed as a model system for the study of interfacial properties of solid electrodes [5]. The report by Clavilier et al. [6,7] of an anomalous adsorption state in the voltammograms of a well-defined Pt(111) electrode in sulfuric and perchloric acid solutions, which was attributed by these authors to hydrogen adsorption, but later shown by Al Jaaf et al. [8] to be due to specific adsorption of anions, has stimulated the study of sulfate adsorption on a number of single crystal metal electrodes [9–15]. From the very beginning of their observation, current spikes have been attributed to structural transitions on the surface and in ionic adlayers [8,16 ].
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Magnussen et al. [11] were the first to provide direct evidence by in-situ STM for an ordered adlayer with a (앀3×앀7) superstructure on Au(111) in 0.1 M H SO . This structure can be 2 4 readily seen at potentials positive of the sharp spikes in the cyclic voltammogram at 0.78 V vs. SCE, which arises from a disorder–order phase transition of the sulfate adlayer. They proposed a model for the (앀3×앀7) structure in which the adsorbed species was bisulfate with a coverage of 0.4. Weaver and co-workers have also studied this system by a combination of IRRAS and STM and found the same structure [13]. They reached the conclusion that the adlayer was formed by sulfate, rather than bisulfate, with a maximum coverage of 0.2, and coadsorbed hydronium cations. Chronocoulometric measurements by Shi et al. [12] also yielded a coverage of 0.2. Similar STM images have later been reported for sulfate adsorbed on Pt(111) [15] and Rh(111) [14]. In the latter case, a model was proposed according to which the images would correspond to an ordered sulfate adlayer and hydrogen-bonded water chains along the 앀3 direction between neighboring rows of adsorbed sulfate [14]. Very recently, two groups have reported on the in-situ observation of an ordered sulfate adlayer on Cu(111) [17,18]. The structure is similar to that found on Au(111) [11,13], Pt(111) [15] and Rh(111) [14], with the only difference being that on Cu(111), a Moire´ pattern was observed [17,18]. The appearance of a Moire´ pattern was explained by Li et al. [17] as being due to the incommensurate character of the adlayer, whereas Wilms et al. [18] ascribed it to the adsorption of sulfate on a reconstructed Cu surface. Sulfuric acid adlayers on emersed Au(111), Au(100) and Au(110) electrodes have been studied ex situ by Zei et al. [19,20] using LEED, RHEED and AES. They found a (2×앀3) rectangular structure on the reconstructed Au(100)-(hex) surface and a p(2×2) structure on the unreconstructed Au(100)-(1×1) surface. For Au(111), they reported a (앀7×앀7) structure for the sulfuric acid adlayer. More recently, Moraes et al. [21] have studied the adsorption of sulfate and nitrate anions on Au(100) electrodes using IRRAS. They found
that in 0.5 M KF+0.69 M HF+10−2 M Na SO 2 4 (pH 2.8), only sulfate was present on the surface, the adsorption starting at +0.23 V vs. SCE. In a 7.3 M HF solution (pH 0.23) containing 10−2 M Na SO , both sulfate and bisulfate were found on 2 4 the surface, the ratio sulfate/bisulfate increasing with potential. In such a solution, sulfate adsorption started at +0.49 V vs. SCE. Here, we report for the first time the observation by in-situ STM of an ordered sulfate or bisulfate adlayer on Au(100) in 0.1 M H SO . For the sake 2 4 of brevity, we shall use in the following only the term ‘sulfate’ for describing the adlayer.
2. Experimental The Au(100) electrode used for STM measurements was a single crystal disc with a 12-mm diameter (MaTecK, Ju¨lich), oriented to better than 1° and polished down to 0.03 mm. Before each experiment, the crystal was annealed in a hydrogen flame for about 6 min and cooled down to room temperature in a stream of nitrogen. Two platinum wires were used as reference and counter electrodes in the STM cell, but all potentials are quoted against SCE. A Topometrix TMX 2010 Discoverer STM was used. Experiments were performed with tungsten tips etched from a polycrystalline wire in aqueous NaOH. The tips were then coated with an electrophoretic paint [22] in order to reduce the faradaic current at the tip/electrolyte interface. All images were recorded in the constant-current mode. The electrolyte (0.1 M H SO ) was prepared 2 4 from Merck suprapure chemicals and Milli-Q water (18.2 MV · cm and 3 ppb of TOC ).
3. Results and discussion Contrary to what is found for Au(111), the cyclic voltammogram for Au(100) in 0.1 M H SO (Fig. 1) shows no spikes, which would 2 4 indicate an order–disorder transition within the sulfate adlayer at positive potentials. Only one peak at +0.34 V vs. SCE is seen in the first positive-trending sweep, which corresponds to the
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Fig. 1. Cyclic voltammogram for a freshly prepared, reconstructed Au(100) electrode in 0.1 M H SO , starting at −0.2 V 2 4 in negative direction, with the (hex)(1×1) transition occurring at +0.34 V on the positive scan. Sweep rate: 10 mV s−1.
lifting of the Au(100)-(hex) reconstruction [23,24]. It is well-known that the (hex)(1×1) transition causes a shift of the pzc by 220 mV to negative values, and hence, additional charging of the electrode surface is required to maintain during the structural transition the applied potential on the positive side of the pzc [24]. However, the absence of a spike does not mean that sulfate ions are not specifically adsorbed on the electrode surface. In fact, it has been shown that specific adsorption of anions plays a crucial role in lifting of the reconstruction: the stronger the interaction between specifically adsorbed anions and the gold surface, the more negative is the potential at which lifting occurs [24]. For sulfuric acid solutions, a critical anion coverage for the (hex)(1×1) transition of only about 5% of a monolayer has been estimated [23]. It has also been shown [23,24] that the charge under the transition peak (e.g. at 0.34 V ) depends strongly on the anion in solution, indicating that both double-layer charging due to lifting of the reconstruction and the specific adsorption of anions contribute to this peak. Fig. 2 shows a 65×65 nm2 image of a Au(100) surface in 0.1 M H SO at +0.35 V vs. SCE, i.e. 2 4 at a potential just positive of the current peak in the cyclic voltammogram. The monoatomic high gold islands on the surface were formed by the extra amount of gold atoms required for the more densely packed reconstructed surface and expelled during the structural transition (hex)(1×1). The
Fig. 2. STM image (65×65 nm2) of a Au(100) electrode in 0.1 M H SO , obtained at +0.35 V vs. SCE with a tungsten 2 4 tip. I = 2 nA; U =−0.35 V (tip negative). T T
square shape of the islands reflects the twofold symmetry of the Au(100)-(1×1) surface. A noval feature in the image, however, are lines that cover the whole surface and are aligned parallel to the edges of the monoatomic high islands. These lines are not interrupted by the islands, but run over them. It can also be seen in Fig. 2 that these lines form domains rotated by 90°, as one would expect for a surface with twofold symmetry. Although the overall appearance of the new structure in Fig. 2 resembles the (hex)-reconstruction, the line ˚ is far too small to assign separation of 10.4 A these lines to areas where the reconstruction had not been lifted. We assign the new structure to an ordered sulfate adlayer on the unreconstructed Au(100)-(1×1) surface. Fig. 3 shows a molecularly resolved image of the adlayer, and Fig. 4 shows an atomically resolved image of the Au(100)-(1×1) surface proper for comparison. The image in Fig. 3 reveals rows of main maxima separated by rows of secondary maxima, and hence is very similar to those for the ordered sulfate adlayer on Au(111) [11,13], Rh(111) [14] and Pt(111) [15], the only difference
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Fig. 4. STM image (6×6 nm2) of a Au(100)-(1×1) surface in 0.1 M H SO , obtained at +0.35 V vs. SCE with a tungsten 2 4 tip, showing the bare gold surface. I =50 nA; U = −0.35 V. T T
Fig. 3. STM images of a 7×7 nm2 area (top view and 3-D plot) of a Au(100) electrode in 0.1 M H SO , obtained at +0.35 V 2 4 vs. SCE with a tungsten tip, showing a close-up of the adlayer structure. Same conditions as in Fig. 2.
being that in the present case, the main maxima are arranged in a rectangular way (the small deviation from an exactly rectangular unit cell is due to drift). The distance between the main ˚ maxima in the close-packed rows is 4.0±0.3 A ˚ and that between rows is 10.4±0.7 A, which corresponds to a (1.4±0.1×3.6±0.2) superstructure. The main axes of its unit cell must be parallel to the crystallographic [011] and [01: 1] directions since in Fig. 2, the lines are seen to run parallel to the gold island edges. According to this, the struc1.4±0.1 0 ture can be better described as ( 0 3.6±0.2) with
respect to the substrate’s surface unit cell and would correspond to an incommensurate ordered sulfate adlayer on Au(100). The lack of an order–disorder phase transition for the sulfate adlayer independent of the (hex)(1×1) transition makes it very difficult to obtain an image composed of both the substrate (1×1) and the sulfate structure, as in the case of Au(111), which would have allowed us to determine the parameters of the adlayer unequivocally. However, changing the experimental conditions, namely increasing the tunneling current, enabled us to obtain atomically resolved images of the Au(100)-(1×1) surface only a few minutes after having measured the sulfate structure, which provides the data for an internal calibration of the adlayer structure parameters. This procedure confirmed our conclusions that the sulfate rows run parallel to the substrate’s crystallographic main axes and that the adlayer forms an incommensurate (1.4±0.1×3.6±0.2) structure. Further support of the incommensurate character of the adlayer structure is provided in Fig. 5, where a molecularly resolved image of a
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Fig. 5. STM image (27×27 nm2) of a Au(100) electrode in 0.1 M H SO , obtained at +0.35 V vs. SCE with a tungsten 2 4 tip, showing the adlayer structure at domain boundaries, under the same conditions as in Fig. 2.
27×27 nm2 area of the sulfate adlayer is shown. Some of the sulfate rows form bows at the domain boundary, which implies that these anions are not aligned along any of the substrate’s main axes, but are in intermediate positions, although the distance between adions within the rows seems to be kept constant. From the (1.4±0.1×3.6±0.2) superstructure, an approximate sulfate coverage of 0.2 is derived, if only the main maxima are considered to correspond to adsorbed sulfate. Magnussen et al. [11] have assigned both the main and secondary maxima in the STM images of the Au(111)− (앀3×앀7) superstructure to adsorbed bisulfate, which results in a coverage of 0.4. However, this value is at variance with chronocoulometric data, which suggest a coverage of 0.2 [12]. According to previous suggestions for Au(111) [13] and Rh(111) [14], the secondary maxima in the Au(100)-(1.4±0.1×3.6±0.2) superstructure (Fig. 3) should correspond either to hydrogenbonded hydronium cations [13] or to water molecules [14]. Support for the latter has recently been
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provided by Ataka and Osawa [25] by means of in-situ ATR-SEIRAS. These authors propose a model in which all secondary maxima in the STM images of sulfate on Au(111) are assigned to water molecules bridging through hydrogen bonding sulfate molecules in neighboring rows. Bridging hydrogen bonds would diminish the Coulomb repulsions between adjacent sulfate anions, as had already been pointed out by Edens et al. [13] for the case of coadsorbed hydronium ions. Regarding this, it should be noted that Ataka and Osawa [25] also provide evidence that hydronium ions are not incorporated in the sulfate adlayer. Normally, the highest coverages for adlayers are achieved on the densely packed (111) surface. This means that the estimated coverage for the (1.4±0.1×3.6±0.2) sulfate structure is very close to saturation or even is the saturation coverage. This is supported by the fact that the current in the cyclic voltammogram ( Fig. 1) remains constant after the peak at +0.34 V, indicating a negligible increase in specific adsorption. This also explains the absence of spikes for order–disorder phase transitions at more positive potentials, since a high coverage is already reached immediately after lifting the surface reconstruction. The peak at +0.34 V in the cyclic voltammogram ( Fig. 1) should therefore be interpreted as being due to the transition from a reconstructed Au(100)-hex surface to an unreconstructed Au(100) surface covered by an ordered (1.4±0.1×3.6±0.2) sulfate adlayer. A last point that should be discussed is why this structure has not been previously reported, despite the fact that gold single crystals are the most intensively studied surfaces in electrochemical STM investigations. We have found the structure to be very labile: after some time of measuring (about 30–40 min), the structure was destroyed by the tip. We have noted that the best conditions for imaging the structure were low tunnel currents and high tunnel voltages, i.e. with the tip far away from the surface. The use of higher tunnel currents or smaller tunnel voltages (i.e. approaching the tip to the surface) resulted in a faster destruction of the adlayer structure. The time during which the structure persisted on the surface was also found to be dependent on the cleanliness of both the
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surface and the solution: the presence of contaminations on the surface reduced the life time of the structure.
4. Conclusions We have found that upon lifting the hex reconstruction, an ordered sulfate or bisulfate adlayer is formed on the unreconstructed Au(100) surface. The adlayer has an incommensurate (1.4±0.1× 3.6±0.2) structure with an approximate coverage of 0.2. In-situ STM reveals that the structure is composed of main and secondary maxima, similar to the STM images of the (앀3×앀7) sulfate structure observed on Au(111) [11,13], Pt(111) [15] and Rh(111) [14]. In analogy to Au(111) [13], only the main maxima are assumed to correspond to sulfate molecules, the secondary maxima probably representing coadsorbed water molecules [14,25]. The stability range of the ordered adlayer on the unreconstructed Au (100) surface, particularly for the negative potential scan, is currently under investigation.
Acknowledgement A.C. acknowledges a Marie Curie fellowship within the TMR program of the European Commission.
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