The structure of bromide and chloride adlayers on Au(100) electrodes: an in situ STM study

Surface Science 465 (2000) 310–316 www.elsevier.nl/locate/susc

The structure of bromide and chloride adlayers on Au(100) electrodes: an in situ STM study A. Cuesta, D.M. Kolb * Department of Electrochemistry, University of Ulm, 89069 Ulm, Germany Received 5 May 2000; accepted for publication 29 June 2000

Abstract Bromide adsorption on Au(100) electrodes was investigated using in situ scanning tunneling microscopy (STM ). In perfect agreement with published surface X-ray scattering data, two quasi-hexagonal structures were found: a c(E2×2E2)R45° commensurate structure at potentials between the first and second spike in the cycling voltammogram of Au(100) in Br− containing solutions and a uniaxially incommensurate c(E2×p)R45° structure (2E2≥p≥2.5), at potentials positive of the second spike. The incommensurate character of the latter bromide structure provokes the appearance of stripes which run parallel to one of the E2 directions of the substrate. The electrocompression of this adlattice is reflected in the change of shape and corrugation length of the stripes upon changing the potential. In the potential region of the ordered structures, the Au(100) surface is covered by gold islands which are nearly hexagonal in shape and by steps forming angles of ca. 120°, indicating that, in the presence of the ordered Br− adlayer, the gold steps run parallel to the close-packed rows of the bromide structure. Chloride adsorption on Au(100) electrodes was also investigated by in situ STM. A uniaxially incommensurate structure was found at potentials positive of the spike in the cyclic voltammogram of Au(100) in Cl− containing solutions. It shows a quasi-hexagonal geometry and can be described as a c(E2×p)R45° superstructure (2E2≥p≥2.3). As in the case of the uniaxially incommensurate bromide superstructure, stripes can be seen in the STM images of the chloride adlattice, the corrugation length of which depends on the electrode potential. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Chemisorption; Gold; Halides; Low index single crystal surfaces; Metal–electrolyte interfaces; Scanning tunneling microscopy

1. Introduction Anion adsorption plays a key role in determining the properties of the metal/electrolyte interface. For this reason, adsorption processes at the electrode/electrolyte interface have been extensively studied using conventional electrochemical techniques [1,2], radiochemical methods [3] or spectroelectrochemical techniques [4,5]. More * Corresponding author. Fax: +49-731-5025409. E-mail address: [email protected] (D.M. Kolb)

recently, the use of scanning tunneling microscopy (STM ) [6 ] and surface X-ray scattering (SXS ) [7] has allowed one to obtain in situ information about the structure of adsorbate covered electrodes with atomic or molecular resolution. Halides are by far the most exhaustively studied class of specifically adsorbing anions. Early STM studies of adsorbates on electrode surfaces already focused on halide adlattices on single crystal electrodes [8]. In situ STM measurements of ordered halide adlayers on gold single crystals have been reported for iodide on Au(111) [9–15],

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Au(100) [16 ] and Au(110) [17] and bromide on Au(111) [10]. The structure of bromide adsorbed on Au(100) has been determined in situ using SXS [18]. Our STM measurements, reported here, agree perfectly with this data, but provide in addition new information regarding the effect of the bromide adlayer on the step orientation, which can only be obtained by real space imaging. Regarding chloride on gold single crystal electrodes, only one SXS study for Au(111) was found in the literature [19]. Here, we report for the first time the in situ observation of an ordered chloride adlayer on Au(100).

2. Experimental The Au(100) electrode 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 ca. 6 min and cooled down to room temperature in a stream of nitrogen. A gold wire served as counter electrode, a platinum wire was used as reference electrode (+0.55 V versus SCE ). However, all potentials are quoted against the saturated calomel electrode (SCE ). A Topometrix TMX 2010 Discoverer STM was used. Experiments were performed with tungsten tips, etched from a polycrystalline wire in aqueous NaOH [20]. The tips were then coated with an electrophoretic paint [21] in order to reduce the faradaic current at the tip/electrolyte interface. All images were recorded in the constant-current mode. The electrolytes were prepared from Merck suprapure chemicals and Milli-Q water (18.2 MV cm and 3 ppb of TOC ).

3. Results and discussion 3.1. Bromide on Au(100) Fig. 1 shows the well-known cyclic voltammogram of Au(100) in 0.1 M H SO +1 mM KBr. 2 4 Four different potential regions can be discerned in the current-potential curve, labelled I–IV in Fig. 1. Region I corresponds to the potential

Fig. 1. Cyclic voltammogram for a freshly prepared, thermally reconstructed Au(100) electrode in 0.1 M H SO +1 mM KBr, 2 4 starting at −0.25 V in the negative direction. Sweep rate: 10 mV s−1.

interval where the reconstructed surface is stable. Here, only the Au(100)-hex structure can be seen by in situ STM, and specifically adsorbed anions are absent, or present in very small amounts [22]. The peak at −0.12 V corresponds to the lifting of the hex-reconstruction. In region II, that is, at potentials between the peak at −0.12 V and the spike at 0.10 V, no structure other than the Au(100)-(1×1) substrate can be seen in the in situ STM images, suggesting that, within this potential region, bromide adsorption is disordered. The same conclusion was reached in a SXS study [18], where no ordered bromide adlayer could be detected in this region. Sharp spikes like those at 0.10 and 0.48 V in the cyclic voltammogram ( Fig. 1) are usually associated with phase transitions within adsorbed adlayers. The spike at 0.10 V marks the structural transition from region II to region III. Fig. 2A shows a typical STM image within the latter potential region. Both the geometrical arrangement of the maxima (distorted hexagons instead of squares) and the distances between them (with

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Fig. 2. (A) STM image (19×19 nm2) of a Au(100) electrode in 0.1 M H SO +1 mM KBr, obtained at 0.40 V. 2 4 I =10 nA; U =0.40 V (tip negative). (B) STM image (7×7 nm2) of a Au(100) electrode in 0.1 M H SO +1 mM KBr, showing T T 2 4 both the bromide adlayer, as observed within potential region III (0.15 V ), and the underlying Au(100)-(1× 1) substrate (−0.05 V ). E2 directions of the substrate and the unit cell of the bromide adlattice are indicated in the image. The I =20 nA; U =0.25/0.05/0.25 V (tip negative). T T

˚ clearly bigger than those expected 4.08 and 4.56 A for gold atoms) prevent us from assigning this image to the Au(100)-(1×1) substrate, and, hence, it must be assigned to an ordered bromide structure. Accordingly, the spike at 0.10 V in the cyclic voltammogram of Fig. 1 corresponds to an order– disorder transition within the bromide adlayer. This phase transition allows us to very easily obtain STM images where both the substrate and the adsorbate are present, by simply switching the potential from a point within region II to another within region III, while recording the image. Such an image can be seen in Fig. 2B, where the upper and lower part correspond to the bromide superstructure and the middle part to the Au(100)(1×1) substrate (the deviation from a perfect square geometry, as expected for a Au(100)-(1×1) surface, is due to thermal drift). Images like that in Fig. 2B are ideal to accurately determine the relation between the adlayer and the substrate structure and the distances between ions in the ordered adlayer, even in the presence of drift, since

the substrate’s lattice parameters are well known. It can be clearly seen in Fig. 2B that one of the close-packed rows in the bromide superstructure runs parallel to one of the diagonals of the substrate’s unit cell (often called the E2 direction). The distance between bromide ions along this row ˚ as calculated from Fig. 2B is 4.03±0.07 A ˚ ). Along the other diagonal, (a M E2=4.08 A Au Au ˚. the distance between bromide ions is 8.16±0.1 A These data allow us to describe the bromide adlayer in region III as a c(E2×2E2)R45° superstructure, in perfect agreement with published SXS data [18]. The nearest neighbour spacing within ˚ , and the next nearest this ordered adlayer is 4.08 A ˚ . It corresponds to a neighbour spacing 4.56 A surface concentration C=6.0×1014 cm−2 and a surface coverage of h=0.5. The transition from region III to region IV is indicated by the spike at 0.48 V in the cyclic voltammogram ( Fig. 1). As indicated above, such spikes usually correspond to phase transitions within adsorbed adlayers, and this is also the case

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Fig. 3. (A) STM image (16 × 16 nm2) of a Au(100) electrode in 0.1 M H SO +1 mM KBr showing the phase transition asso2 4 ciated with the spike at 0.48 V. I =10 nA; U =0.45/0.55 V (tip T T negative). (B) STM image (16×16 nm2) of a Au(100) electrode in 0.1 M H SO +1 mM KBr showing the electrocompression 2 4 of the uniaxially incommensurate c(E2×p)R45° bromide structure. I =10 nA; U =0.55/0.60 V (tip negative). T T

for the spike at 0.48 V. The upper part of Fig. 3A corresponds to the structure of region III described above as c(E2×2E2)R45°. When the electrode potential was switched from 0.45 V, in region III, to 0.55 V, in region IV (see arrow), the structural

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transition could be clearly seen: stripes running parallel to each other appear now on the otherwise apparently unchanged structure. These stripes suggest the formation of an incommensurate structure, which leads to a height variation due to nonregistry between adlayer and substrate. Ocko et al. [18] demonstrated, using SXS, that the spike at 0.48 V corresponds to a commensurate–incommensurate phase transition, where the incommensurate structure, described as c(E2×p)R45° (with p≤2E2), compresses continuously and uniaxially as the potential is made more positive and the bromide coverage increases. Images like that shown in Fig. 3A can be used to determine the structure of the incommensurate adlattice. A close inspection of these images reveals that the stripes run parallel to the close-packed direction of the bromide structure which is parallel to one of the E2 directions of the substrate. The distance between maxima along this direction ˚ , like in the remains constant and equal to 4.08 A commensurate structure. Along the other E2 direction and at potentials more positive than 0.48 V, ˚ the distance between maxima is always <8.16 A and decreases with increasing potential. These data confirm that the incommensurate structure observed in the lower part of Fig. 3A and in Fig. 3B corresponds to the c(E2×p)R45° adlattice described by Ocko et al. [18]. Although STM cannot measure distances with the accuracy achieved by SXS, the electrocompression of the incommensurate adlayer can also be observed, since a change in the parameter p provokes a change in the corrugation frequency of the stripes. This can be clearly seen in Fig. 3B, where the potential was switched from 0.55 to 0.60 V (see arrow). The values of the parameter p as deter˚ ) at mined from the STM images are 2.6 (7.50 A ˚ 0.5 V and 2.5 (7.21 A) at 0.6 V. At potentials more positive than 0.6 V the onset of gold dissolution makes it very difficult to obtain good STM images. According to the data shown above, and again in perfect agreement with published SXS data [18], it must be concluded that, in region IV, the bromide adlayer forms a uniaxially incommensurate ˚) c(E2×p)R45° structure, where 2E2 (8.16 A ˚ ). The nearest neighbour spacing ≥p≥2.5 (7.21 A

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˚ , but the next nearest within this structure is 4.08 A neighbour distance decreases continuously from ˚ at the point at which the transition occurs 4.56 A ˚ at 0.6 V. As a consequence of the comto 4.14 A pression of the adlayer, the surface concentration increases from C=6.0×1014 to C=6.8×1014 cm−2 and the surface coverage from h=0.5 to h=0.57. A very interesting effect with regard to the influence of the bromide adlayer on the morphology of the gold surface was observed at potentials above 0.10 V, that is, in potential regions III and IV, where the surface is covered by a quasihexagonal ordered bromide adlayer. This effect is shown in Fig. 4, where it can be clearly seen that, despite the square geometry of the Au(100) substrate, the surface is covered by approximately hexagonally shaped islands and the step edges form angles close to 120° with each other. This means that the steps are forced to run parallel to the close-packed rows of the strongly adsorbed quasi-hexagonal bromide adlayer. A similar effect has been found for Cu(100) in the presence of the c(2×2) chloride adlayer [23–26 ], where the steps

Fig. 4. STM image (822×822 nm2) of a Au(100) electrode in 0.1 M H SO +1 mM KBr, obtained at 0.40 V. I =2 nA; 2 4 T U =0.40 V (tip negative). T

are perfectly straight and oriented along the closepacked directions of the Cl− adlattice, which in this case has a square symmetry rotated 45° with respect to the Cu(100) substrate. A novel aspect in the present case is the fact that distorted hexagonal islands and nearly 120° angles (Calculated values for the c(E2×2E2)R45° structure: 116.53 and 126.94°. The deviation from these values in Fig. 4 is due to thermal drift.) are stabilised on a surface with square symmetry by an adlattice with a quasi-hexagonal structure. At potentials within region II (i.e., where the surface is covered with a disordered and mobile bromide adlayer) square gold islands and steps forming angles of 90° are found on the surface. 3.2. Chloride on Au(100) Fig. 5 shows the cyclic voltammogram of Au(100) in 0.1 M H SO +1 mM HCl. Region I 2 4 corresponds to the potential interval where the (hex)-reconstruction is stable [22]. In region II only the Au(100)-(1×1) surface can be imaged by in situ STM, suggesting that the surface is covered by a disordered layer of adsorbed chloride ions. Region III is separated from region II by the spike at 0.71 V, indicating a phase transition within the adlayer. Fig. 6 shows the STM image of Au(100) in 0.1 M H SO +1 mM HCl at 0.8 V, 2 4 that is, a potential within region III. As can be seen in Fig. 6, the chloride adions form a distorted

Fig. 5. Cyclic voltammogram for a freshly prepared, initially reconstructed Au(100) electrode in 0.1 M H SO +1 mM HCl, 2 4 starting at −0.2 V in the negative direction. Sweep rate: 10 mV s−1.

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tance between chloride ions along this direction ˚ . Along the other diagonal, the being 4.09±0.04 A distance between maxima depends on the electrode potential. Accordingly, we describe the adlattice as a uniaxially incommensurate c(E2×p)R45° structure, which compresses with increasing potential. The compression of the adlattice can be observed by in situ STM due to the change in corrugation frequency of the stripes, as shown in Fig. 7, where the potential was switched from 0.80 to 0.75 V at the point marked by an arrow. ˚) The values for the parameter p are 2.5 (7.21 A ˚ at 0.75 V and 2.3 (6.63 A) at 0.85 V. It was not possible to obtain good quality STM images of the chloride superstructure at potentials between the spikes and 0.75 V or more positive than 0.85, where the proximity of the phase transition and the onset of surface dissolution, respectively, made the images very unstable. By analogy to Br−

Fig. 6. STM image (top view and three-dimensional plot) of a 13×13 nm2 area of a Au(100) electrode in 0.1 M H SO +1 mM HCl, obtained at 0.8 V, showing the ordered 2 4 chloride adlayer. I =20 nA; U =0.80 V (tip negative). T T

hexagonal structure. The presence of stripes in the STM image again suggests a non-equivalence of the adsorption sites and is an indication of the incommensurate character of the adlattice. It is possible to obtain STM images of the Au(100) surface with both the substrate and the chloride structures just by switching the potential from region II to region III while recording the image. As discussed above, these kind of images are ideal for determining the adlattice parameters. From a set of images of this type (not shown), we could determine that the stripes shown in Fig. 6 run diagonal to the substrate’s unit cell, the dis-

Fig. 7. STM image (top view and three-dimensional plot) of a 11×11 nm2 area of a Au(100) electrode in 0.1 M H SO +1 mM HCl showing the compression of the uniaxially 2 4 incommensurate c(E2×p)R45° chloride structure with potential. I =20 nA; U =0.80/0.75 V (tip negative). T T

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adsorbed on Au(100) (see above and Ref. [18]), it seems reasonable to assume that at the spike ˚ ) and, therefore, the potential p=2E2 (8.16 A structure should be described as c(E2×p)R45° with 2E2≥p≥2.3. At some point between 0.75 ˚ ) and the structure is and 0.85 V, p=2.45 (7.07 A perfectly hexagonal. At potentials more negative than this point the nearest neighbour spacing is ˚ and the next nearest neighbour distance 4.08 A ˚ . At potentials more varies between 4.56 and 4.08 A positive than this point the nearest neighbour ˚ and the spacing varies between 4.08 and 3.90 A ˚ . The next nearest neighbour distance is 4.08 A surface coverage increases from h=0.5 at the spike potential to h=0.62 at 0.85 V, corresponding to a surface concentration between 6.0×1014 and 7.4×1014 cm−2.

4. Conclusions Two ordered adlayers were found for bromide adsorbed on Au(100) electrodes: a commensurate c(E2×2E2)R45° structure in potential region III and a uniaxially incommensurate c(E2×p)R45° structure in potential region IV. The incommensurate character of the latter provokes the appearance in the STM images of stripes that run parallel to one of the E2 directions of the substrate. In regions III and IV, the presence of the strongly adsorbed adlayer forces the steps to run along the close-packed bromide rows, and quasi-hexagonal islands and steps forming angles of 120° with each other appear on the surface. Only the uniaxially incommensurate c(E2×p) R45° structure was found for chloride adsorbed on Au(100) electrodes. A height corrugation like that found for the equivalent bromide structure can also be observed in the STM images of this chloride adlattice.

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