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Effect of sulfuric acid concentration on the structure of sulfate adlayer on Au(1 1 1) electrode
Electrochemistry Communications 8 (2006) 725–730 www.elsevier.com/locate/elecom
Effect of sulfuric acid concentration on the structure of sulfate adlayer on Au(1 1 1) electrode Kazuhiro Sato a, Soichiro Yoshimoto
a,*,1
, Junji Inukai
a,b,2
, Kingo Itaya
a,c,*
a
Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, 6-6-04 Aoba, Sendai 980-8579, Japan b NICHe, Tohoku University, 6-6-10 Aoba, Sendai 980-8579, Japan c Core Research Evolutional Science and Technology organized by the Japan Science and Technology Agency (CREST-JST), Kawaguchi Center Building, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan Received 11 February 2006; accepted 1 March 2006 Available online 31 March 2006
Abstract The adsorption of sulfate on Au(1 1 1) single-crystal electrode was examined in sulfuric acid solutions of various concentrations (0.5– p p 7.5 M) by cyclic voltammetry and in situ scanning tunneling microscopy (STM). The well-known sulfate adlayer with ( 3 · 7) unit cell was found in 0.5 M sulfuric acid by in situ STM. In 2.5 and 5.0 M sulfuric acid, a new order–disorder phase transition, from the p p ( 3 · 7) adlayer to a disordered structure, was found to take place at ca. 1.1 V vs. AgjAgCljKCl(sat.). In 7.5 M sulfuric acid, no current spikes were seen in the CV profile, and in situ STM showed that the sulfate adlayer was disordered. 2006 Elsevier B.V. All rights reserved. Keywords: Au(1 1 1); Sulfuric acid solution; Concentration; in situ STM; Sulfate adlayer; Phase transition
1. Introduction Single crystal electrodes have been widely used for elucidating fundamental electrochemical processes, notably since the establishment of the flame-annealing–quenching technique by Clavilier et al. [1]. One of the major issues studied by using single crystal electrodes is the adlayer structure of anions, especially sulfate/bisulfate ions (hereafter called sulfate) [2]. In sulfuric acid solution, the cyclic voltammogram (CV) of Pt(1 1 1) showed two characteristic potential regions of adsorption above the hydrogen *
Corresponding authors. Tel./fax: +81 29 861 6167/6177. (S. Yoshimoto); tel./fax: +81 22 795 5868. (K. Itaya). E-mail addresses: [email protected] (S. Yoshimoto), itaya@ atom.che.tohoku.ac.jp (K. Itaya). 1 Present address: National Institute of Advanced Industrial Science and Technology (AIST), Division of Biological Resources and Functions, Central 6, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8566, Japan. 2 Present address: Clean Energy Research Center, University of Yamanashi, 7-32 Miyamae-cho, Kofu 400-0021, Japan. 1388-2481/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.03.001
evolution potential [1,3]. The first region from 0.05 to 0.3 V vs. the reversible hydrogen electrode (RHE) corresponded to the usual range of hydrogen adsorption on polycrystalline Pt electrode. The second region appeared immediately above 0.3 V vs. RHE over a range of 0.2 V. The latter was called anomalous because of its absence at polycrystalline platinum electrode. In the anomalous region, a very sharp, characteristic pair of anodic and cathodic spikes was seen at ca. 0.45 V vs. RHE [1,3]. These sharp spikes were also found at Au(1 1 1) single crystal electrode [4–8]. It is now recognized that these spikes are associated with an order–disorder phase transition of the sulfate adlayer [2]. Detailed investigations on the adsorption of sulfate on Au(1 1 1) surface were carried out by in situ scanning tunneling microscopy (STM) [9–11], chronocoulometry [12], radioactive labeling [12], second harmonic generation [13], electrochemical quartz microbalance [14], in situ infrared reflection absorption spectroscopy (IRAS) [15,16] and surface-enhanced infrared reflection absorption spectroscopy
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(SEIRAS) [17,18]. Among these techniques, in situ STM 2 1 showed the ordered structure of sulfate on 1 2 Au(1 1 1) [2,10,11]. In a unit cell of the adlayer, one bright spot and two weakerspots were seen [9,10]. Interestingly, 2 1 the identical adlayer structure, or the so-called 1 2 p p ( 3 · 7) structure, was also found on the (1 1 1) planes of Pt [19–21], Rh [22], Ir [21,23], Pd [24,25], Cu [26–28] and on Ru(0 0 0 1) [21,29]. So far, dilute sulfuric acid solutions with concentrations equal to or lower than 0.5 M have been used for investigating sulfate adlayers on metal surfaces. However, in the field of fuel cell, for example, highly concentrated sulfuric acid is often used as an electrolyte solution [30,31]. Atomic-scale understanding of the adlayers of sulfate in highly concentrated sulfuric acid solutions by using single crystal electrodes, is expected to contribute not only to the advancement of fundamental electrochemistry but also to that of applied electrochemical fields. In the present study, we carried out cyclic voltammetry and in situ STM measurements on a well-defined Au(1 1 1) single crystal electrode in sulfuric acid solutions more concentrated than 0.5 M. A pair of new spikes was observed in 2.5 and 5.0 M solutions. Disordered adlayers of sulfate were observed at potentials more positive than those of the newly found spikes.
3. Results and discussion 3.1. 0.5 M sulfuric acid Fig. 1(a) shows CVs of a well-defined Au(1 1 1) single crystal electrode in 0.5 M sulfuric acid recorded at a scan rate of 5 mV s1. The gray curve shows a CV obtained in the potential range between hydrogen evolution and surface oxidation, whereas the CV in red color was recorded in the double-layer potential region at an expanded current scale of 50 times. The CVs for the Au(1 1 1) single crystal electrode shown in Fig. 1(a) are identical to those reported previously [5–8,11,33–35], indicating that a well-defined Au(1 1 1) surface was exposed to the solution. The broad anodic peaks observed at 0.32 and 0.50 V are p attributed to the anion-induced reconstruction from ( 3 · 22) to (1 · 1) and the adsorption of sulfate, respectively [5,10,11]. The pair of spikes observed at 0.80 V is due to the order–disorder phase transition of adsorbed sulfate anions [5,10,11]. Fig. 1(b) shows a high-resolution STM image obtained by stepping the potential from 0.9 to 0.78 V. The scanning was carried out in the downward direction. At 0.9 V, bright, large spots and weaker, smaller spots are seen in the upper half of Fig. 1(b). The brighter
2. Experimental Au(1 1 1) single-crystal electrodes were prepared by the Clavilier method [1]. A gold single-crystal bead was cut and successively polished with finer grades of alumina paste, and the electrode was then annealed at ca. 950 C for 12 h in an electric furnace to remove mechanical damages [32]. The Au(1 1 1) single crystal was annealed in hydrogen flame for about 1 min and quenched into ultrapure water (Milli-Q SP-TOC; P18.2 MX cm) saturated with hydrogen [32]. The Au(1 1 1) electrode was carefully and immediately transferred into an electrochemical cell filled with sulfuric acid solution (Cica-Merck, ultrapur grade), which was placed in a clean bench to avoid contamination. Cyclic voltammetry was carried out at 20 C using a potentiostat (HOKUTO HAB-151) with the hanging meniscus method in a three-compartment electrochemical cell under Ar atmosphere. In situ STM measurements with a tungsten tip etched in 1 M KOH were performed using a Nanoscope E (Digital Instruments) in the atmosphere of high purity Ar in a glove box system (VAC). One of the (1 1 1) facets formed on the gold single-crystal bead was used as the substrate for in situ STM measurements. To minimize residual faradaic currents, tips were coated with polyethylene (Aldrich). STM images were recorded in the constant-current mode. All potential values were referred to AgjAgCljKCl (sat.).
Fig. 1. (a) CVs of a well-defined Au(1 1 1) electrode in 0.5 M sulfuric acid recorded at the scan rate of 5 mV s1. (b) Potential-dependent STM images (9 · 9 nm2) of Au(1 1 1) surface in 0.5 M sulfuric acid acquired at 0.9 V (upper half) and 0.78 V (lower half). Tunneling current was 3.0 nA.
K. Sato et al. / Electrochemistry Communications 8 (2006) 725–730
spots and the weaker spots form separate rows, p each running in the Æ1 2 1æ direction, or the so-called 3 direction, rotated by 30 with respect to the atomic rows of the underlying Au(1 1 1) lattice. The rows consisting of weaker spots show a zigzag feature. The distances between the brighter spots in the p directions indicated by arrows A and B are ca. 0.5 ( 3 times Au–Au distance: aAu) and 0.78 nm p ( 7aAu), respectively. The angle between the atomic rows marked by arrows A and B is ca. 75. On the basis of the symmetry of the atomic array, a unit cell issuperimposed 2 1 in Fig. 1(b). The adlattice is assigned to the struc1 2 p p ture, or the so-called ( 3 · 7) structure, which is identical to that found previously [9–11,18,21,33]. The bright spots are thought to be either sulfate or bisulfate ions [2]. The weaker spots are attributed to water-related species [2], namely water molecules (H2O) [17,22,34], hydronium ions (H3O+) [10,11], or hydrated hydronium ions ðH5 Oþ 2Þ [16,18,21]. The incorporation of those cations in the p p ( 3 · 7) adlayer was expected to reduce the coulombic repulsion between adjacent sulfate ions on Au(1 1 1) [10]. By stepping the electrode potential from 0.90 to 0.78 V, small, hexagonally arranged spots appeared immediately as seen in the lower half of Fig. 1(b). Because the distance between the spots is ca. 0.29 nm, these spots are attributed to Au atoms on Au(1 1 1)–(1 · 1). The appearance of the Au(1 1 1)–(1 · 1) substrate was made possible by the mobility of sulfate ions at the lower potential after the order–disorder phase transition [2]. A careful inspection of the composite STM image reveals that the weaker spots in the upper half of Fig. 1(b) are located at the on-top sites of Au atoms. The STM images of sulfate were found to depend strongly on the magnitude of tunneling current. Fig. 2(a) shows a close-up view of a sulfate layer at the tunneling current of 3.0 nA, as was observed in the upper half of Fig. 1(b). When the tunneling current was decreased from 3.0 to 1.0 nA, each spot dramatically changed in appearance. Fig. 2(b) shows an STM image of the sulfate adlayer obtained at 1.0 nA. The brighter spots are circular in shape in Fig. 2(a), while they became triangular in Fig. 2(b). Each triangle consists of three bright spots at the apices, which can be attributed to three O atoms of sulfate adsorbed on Au atoms. Between the triangular features, the weaker spots, resulting from water-related species appeared more p clearly, showing a zigzag configuration in the p 3 direction. p The distances between the bright spots in the 3 and 7 directions are ca. 0.5 and 0.78 nm, respectively; thus, the adlayer structure is identical to that found from Fig. 2(a). The unit cells are superimposed in Fig. 2(a) and (b). On the basis of the STM images shown p inpFigs. 1(b), 2(a) and (b), the structural model for the ( 3 · 7) adlayer illustrated in Fig. 3 was derived. By taking the symmetry of the Au(1 1 1) substrate into consideration, it was assumed that three oxygen atoms in each sulfate ion are located at on-top sites, and that each water-related species is also situated on an on-top site, as reported in our previous papers
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Fig. 2. High-resolution STM images (6 · 6 nm2) of Au(1 1 1) surface in 0.5 M sulfuric acid acquired at 0.9 V. Tunneling current was (a) 3.0 and (b) 1.0 nA, respectively.
p p Fig. 3. Structural model of the ( 3 · 7) adlayer on Au(1 1 1)–(1 · 1).
on Rh(1 1 1) [22], Ir(1 1 1) [23], and Pd(1 1 1) [24,25]. This configuration of sulfate ion on Au(1 1 1) is supported by SEIRAS measurements [18], electrochemical double-layer
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simulation in the vacuum [16], and quantum-chemical calculation [36]. 3.2. More concentrated sulfuric acid solutions CV measurements were carried out in 1.0, 2.5, 5.0, and 7.5 M sulfuric acid solutions at a scan rate of 5 mV s1. Fig. 4 shows the results obtained. The shape of the CVs remained unchanged after potential cycles in these concentrated solutions. With the increase in sulfate concentration, the peak potential for Au oxidation (gray lines in Fig. 4) is
Fig. 4. CVs of a well-defined Au(1 1 1) single-crystal electrode in (a) 1.0, (b) 2.5, (c) 5.0, and (d) 7.5 M sulfuric acid solutions, respectively. The scan rate was 5 mV s1.
seen to shift in the positive direction, from 1.40 V at 0.50 M to 1.66 V at 7.5 M. Furthermore, the Au oxidation peak became broader with the increase in sulfate concentration, while the charge density for the peak was almost independent of concentration (ca. 450 lC cm2). In 1.0 M sulfuric acid, anodic and cathodic currents at ca. 1.2 and 1.0 V, respectively, are seen on the red curve in Fig. 4(a), which could be assigned to the adsorption/desorption of hydroxyl groups. In the 2.5 M solution, the currents for the adsorption/desorption of hydroxyl groups shifted in the positive direction, whereas a pair of new sharp spikes appeared at 1.15 V (Fig. 4(b), red line). The shape of the sharp spikes is indicative of phase transition of the adlayer, which will be discussed in conjunction with STM results. As the concentration was increased further, the peaks for Au(1 1 1) surface reconstruction located at ca. 0.3 V became smaller. In the 5.0 M solution, the pair of new spikes shifted in the negative direction to 1.07 V. The pair of spikes originated from the order–disorder transition of sulfate layer observed at 0.80 V in Fig. 1(a) slowly moved in the negative direction as the concentration of sulfate was increased, and at 5.0 M, the order–disorder transition peaks of sulfate were observed at 0.75 V. In the 7.5 M solution, the peaks observed at the lower concentrations disappeared except those for the adsorption/desorption of sulfate located at ca. 0.5 V. To understand the origin of the new spikes, in situ STM measurements were carried out in 5.0 M sulfuric acid. Fig. 5(a) shows a composite STM image obtained by stepping the potential from 1.03 to 1.08 V. The scanning was done in the upward direction during the data acquisition. At 1.03 V, which is a potential morepnegative p than that for the new spikes, the highly ordered ( 3 · 7) adlayer was found as seen in the lower half of the image even in the highly concentrated 5.0 M solution. Here, sulfate ions and water-related species are clearly seen. After stepping the potential from 1.03 to 1.08 V, which is a potential more positive than that of the new spikes, p pthe STM image dramatically changed, i.e., the ( 3 · 7) adlayer immediately became disordered. Fig. 5(b) shows a high-resolution STM image acquired at 1.08 V in 5.0 M sulfuric acid. Although the terrace was entirely covered with a disordered structure, careful inspection revealed several triangles consisting of three bright spots as indicated by white circles in Fig. 5(b). These triangles are considered to be individual sulfate ions, because the size of each triangle is the same as that observed in Fig. 2(b). At potentials more negative than 0.75 V, the (1 · 1) structure of Au(1 1 1) substrate lattice was observed in the 5.0 M solution as in the 0.5 M solution (Fig. 1(b)). The difference between the disordered sulfate layers at negative and positive potentials is that at negative potentials, sulfate ions are weakly bound and mobile, whereas at positive potentials, sulfate ions are strongly bound and observable by STM. The mechanism of disordering of the sulfate layer at positive potentials, such as 1.1 V, is not clear. One of the
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Fig. 5. (a) Potential-dependent STM image (9 · 9 nm2) of the Au(1 1 1) surface in 5.0 M sulfuric acid acquired at 1.03 V (lower half) and 1.08 V (upper half). Tunneling current was 10.3 nA. (b) High-resolution STM image (6 · 6 nm2) of the Au(1 1 1) surface in 5.0 M sulfuric acid acquired at 1.08 V. Tunneling current was 1.2 nA.
possibilities is the desorption of water-related species (H2O, H3O+, H5 Oþ 2 ) at such positive potentials; the desorption of water related species would destroy thepnetwork p of sulfate ions (Fig. 3). It was reported p that the ( 3 · 7) structure p of sulfate changed into a ( 3 · 3)R30 structure on Pt(1 1 1) when the water was lost upon transfer into vacuum [37]. It was also reported that the codeposition of SO3 and p p H2O on Pt(1 1 1) could form the ( 3 · 7) structure on Pt(1 1 1) in vacuum, but p p that an insufficient amount of H2O formed a ( 3 · 3)R30 structure [16]. Apparently, the amount of water-related species strongly affects the structure of sulfate adlayer. Another possible factor responsible for the disordering could be the chemical state of sulfate on the surface. Kolb and co-workers [11] observed the disordered structure of sulfate in 0.1 M Na2SO4. Because a 0.1 M Na2SO4 solution is almost neutral, SO2 ion must be the dominant species in solution. 4 Kolb and co-workers thus proposed that HSO 4 forms a p p ( 3 · 7) adlayer, whereas SO2 4 forms a disordered structure on Au(1 1 1). If their argument holds, the newly found spikes might be attributable to the transformation between 2 HSO 4 and SO4 on the Au(1 1 1) electrode. In 7.5 M sulfuric acid, no ordered structure of sulfate was observed. The concentration of water molecules in 7.5 M sulfuric p acid p is approximately 55.5 M. In order to form the ( 3 · 7) sulfate network on Au(1 1 1) surface, the amount of water-related species available might be too small in this highly concentrated sulfuric acid solution. At 0.2 V, which is a potential more negative than that for the desorption of sulfate (Fig. 4(d)), in situ STM showed a reconstructed structure of Au(1 1 1). Therefore, sulfate ions have little influence on Au(1 1 1) at very negative potentials even in 7.5 M sulfuric acid. 4. Conclusions CV and in situ STM measurements were carried out in sulfuric acid solutions with concentrations ranging from 0.5 to 7.5 M on a well-defined Au(1 1 1) single crystal elec-
trode. In 2.5 and 5.0 M solutions, a pair of new anodic and cathodic spikes was found at ca. 1.1 V. In situ STM revealed that these spikes were associated with a new order–disorder transition of a sulfate adlayer. Sulfate ions were disordered at potentials more positive than that for the new spikes, and the ions were observed consistently by STM. In 7.5 M sulfuric acid, no ordered structure of sulfate was observed by STM. It is proposed that the network of sulfate and water-related species is not formed in such highly concentrated sulfuric acid solutions. At a very negative potential between the potentials of sulfate desorption and that of hydrogen evolution, a reconstructed structure of Au(1 1 1) substrate was observed in the 7.5 M solution. Therefore, sulfate has little influence on the surface of Au(1 1 1) at this negative potential even at the concentration as high as 7.5 M solution. Acknowledgments This work was supported in part by CREST-JST and by the Ministry of Education, Culture, Sports, Science and Technology, a Grant-in-Aid for Young Scientists (B) (No. 16750106) and the Center of Excellence (COE) Project, Giant Molecules and Complex Systems, 2006. The authors acknowledge Dr. T. Sawaguchi of AIST for useful discussion and the assistance provided by Dr Y. Okinaka in writing this manuscript. References [1] J. Clavilier, R. Faure, G. Guinet, R. Durand, J. Electroanal. Chem. 107 (1980) 205. [2] O.M. Magnussen, Chem. Rev. 102 (2002) 679. [3] J. Clavilier, J. Electroanal. Chem. 107 (1980) 211. [4] D.A. Scherson, D.M. Kolb, J. Electroanal. Chem. 176 (1984) 353. [5] A.S. Dakkouri, D.M. Kolb, in: A. Wieckowski (Ed.), Interfacial Electrochemistry, Marcel Dekker, New York, 1999 (Chapter 10). [6] H. Angerstein-Kozlowska, B.E. Conway, A. Hamelin, L. Stoicoviciu, Electrochim. Acta 31 (1986) 1051. [7] H. Angerstein-Kozlowska, B.E. Conway, A. Hamelin, L. Stoicoviciu, J. Electroanal. Chem. 228 (1987) 429.
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[8] A. Hamelin, J. Electroanal. Chem. 407 (1996) 1. [9] O.M. Magnussen, J. Hagebo¨ck, J. Hotlos, R.J. Bhem, Faraday Discuss. 94 (1992) 329. [10] G.J. Edens, X. Gao, M.J. Weaver, J. Electroanal. Chem. 375 (1994) 357. [11] A. Cuesta, M. Kleinert, D.M. Kolb, Phys. Chem. Chem. Phys. 2 (2000) 5684. [12] Z. Shi, J. Lipkowski, M. Gamboa, P. Zelenay, A. Wieckowski, J. Electroanal. Chem. 366 (1994) 317. [13] Z. Shi, J. Lipkowski, S. Mirwald, B. Pettinger, J. Electroanal. Chem. 396 (1995) 115. [14] H. Uchida, N. Ikeda, M. Watanabe, J. Electroanal. Chem. 424 (1997) 5. [15] Y. Shingaya, M. Ito, Electrochim. Acta 44 (1998) 745. [16] Y. Shingaya, M. Ito, J. Electroanal. Chem. 467 (1999) 299. [17] K. Ataka, M. Osawa, Langmuir 14 (1998) 951. [18] Th. Wandlowski, K. Ataka, S. Pronkin, D. Diesing, Electrochim. Acta 49 (2004) 1233. [19] A.M. Funtikov, U. Linke, U. Stimming, R. Vogel, Surf. Sci. 324 (1995) L343. [20] A.M. Funtikov, U. Stimming, R. Vogel, J. Electroanal. Chem. 428 (1997) 147. [21] M. Nakamura, Y. Shingaya, M. Ito, Surf. Sci. 502–503 (2002) 474. [22] L.-J. Wan, S.-L. Yau, K. Itaya, J. Phys. Chem. 99 (1995) 9507.
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Effect of sulfuric acid concentration on the structure of sulfate adlayer on Au(1 1 1) electrode Kazuhiro Sato a, Soichiro Yoshimoto
a,*,1
, Junji Inukai
a,b,2
, Kingo Itaya
a,c,*
a
Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, 6-6-04 Aoba, Sendai 980-8579, Japan b NICHe, Tohoku University, 6-6-10 Aoba, Sendai 980-8579, Japan c Core Research Evolutional Science and Technology organized by the Japan Science and Technology Agency (CREST-JST), Kawaguchi Center Building, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan Received 11 February 2006; accepted 1 March 2006 Available online 31 March 2006
Abstract The adsorption of sulfate on Au(1 1 1) single-crystal electrode was examined in sulfuric acid solutions of various concentrations (0.5– p p 7.5 M) by cyclic voltammetry and in situ scanning tunneling microscopy (STM). The well-known sulfate adlayer with ( 3 · 7) unit cell was found in 0.5 M sulfuric acid by in situ STM. In 2.5 and 5.0 M sulfuric acid, a new order–disorder phase transition, from the p p ( 3 · 7) adlayer to a disordered structure, was found to take place at ca. 1.1 V vs. AgjAgCljKCl(sat.). In 7.5 M sulfuric acid, no current spikes were seen in the CV profile, and in situ STM showed that the sulfate adlayer was disordered. 2006 Elsevier B.V. All rights reserved. Keywords: Au(1 1 1); Sulfuric acid solution; Concentration; in situ STM; Sulfate adlayer; Phase transition
1. Introduction Single crystal electrodes have been widely used for elucidating fundamental electrochemical processes, notably since the establishment of the flame-annealing–quenching technique by Clavilier et al. [1]. One of the major issues studied by using single crystal electrodes is the adlayer structure of anions, especially sulfate/bisulfate ions (hereafter called sulfate) [2]. In sulfuric acid solution, the cyclic voltammogram (CV) of Pt(1 1 1) showed two characteristic potential regions of adsorption above the hydrogen *
Corresponding authors. Tel./fax: +81 29 861 6167/6177. (S. Yoshimoto); tel./fax: +81 22 795 5868. (K. Itaya). E-mail addresses: [email protected] (S. Yoshimoto), itaya@ atom.che.tohoku.ac.jp (K. Itaya). 1 Present address: National Institute of Advanced Industrial Science and Technology (AIST), Division of Biological Resources and Functions, Central 6, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8566, Japan. 2 Present address: Clean Energy Research Center, University of Yamanashi, 7-32 Miyamae-cho, Kofu 400-0021, Japan. 1388-2481/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.03.001
evolution potential [1,3]. The first region from 0.05 to 0.3 V vs. the reversible hydrogen electrode (RHE) corresponded to the usual range of hydrogen adsorption on polycrystalline Pt electrode. The second region appeared immediately above 0.3 V vs. RHE over a range of 0.2 V. The latter was called anomalous because of its absence at polycrystalline platinum electrode. In the anomalous region, a very sharp, characteristic pair of anodic and cathodic spikes was seen at ca. 0.45 V vs. RHE [1,3]. These sharp spikes were also found at Au(1 1 1) single crystal electrode [4–8]. It is now recognized that these spikes are associated with an order–disorder phase transition of the sulfate adlayer [2]. Detailed investigations on the adsorption of sulfate on Au(1 1 1) surface were carried out by in situ scanning tunneling microscopy (STM) [9–11], chronocoulometry [12], radioactive labeling [12], second harmonic generation [13], electrochemical quartz microbalance [14], in situ infrared reflection absorption spectroscopy (IRAS) [15,16] and surface-enhanced infrared reflection absorption spectroscopy
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(SEIRAS) [17,18]. Among these techniques, in situ STM 2 1 showed the ordered structure of sulfate on 1 2 Au(1 1 1) [2,10,11]. In a unit cell of the adlayer, one bright spot and two weakerspots were seen [9,10]. Interestingly, 2 1 the identical adlayer structure, or the so-called 1 2 p p ( 3 · 7) structure, was also found on the (1 1 1) planes of Pt [19–21], Rh [22], Ir [21,23], Pd [24,25], Cu [26–28] and on Ru(0 0 0 1) [21,29]. So far, dilute sulfuric acid solutions with concentrations equal to or lower than 0.5 M have been used for investigating sulfate adlayers on metal surfaces. However, in the field of fuel cell, for example, highly concentrated sulfuric acid is often used as an electrolyte solution [30,31]. Atomic-scale understanding of the adlayers of sulfate in highly concentrated sulfuric acid solutions by using single crystal electrodes, is expected to contribute not only to the advancement of fundamental electrochemistry but also to that of applied electrochemical fields. In the present study, we carried out cyclic voltammetry and in situ STM measurements on a well-defined Au(1 1 1) single crystal electrode in sulfuric acid solutions more concentrated than 0.5 M. A pair of new spikes was observed in 2.5 and 5.0 M solutions. Disordered adlayers of sulfate were observed at potentials more positive than those of the newly found spikes.
3. Results and discussion 3.1. 0.5 M sulfuric acid Fig. 1(a) shows CVs of a well-defined Au(1 1 1) single crystal electrode in 0.5 M sulfuric acid recorded at a scan rate of 5 mV s1. The gray curve shows a CV obtained in the potential range between hydrogen evolution and surface oxidation, whereas the CV in red color was recorded in the double-layer potential region at an expanded current scale of 50 times. The CVs for the Au(1 1 1) single crystal electrode shown in Fig. 1(a) are identical to those reported previously [5–8,11,33–35], indicating that a well-defined Au(1 1 1) surface was exposed to the solution. The broad anodic peaks observed at 0.32 and 0.50 V are p attributed to the anion-induced reconstruction from ( 3 · 22) to (1 · 1) and the adsorption of sulfate, respectively [5,10,11]. The pair of spikes observed at 0.80 V is due to the order–disorder phase transition of adsorbed sulfate anions [5,10,11]. Fig. 1(b) shows a high-resolution STM image obtained by stepping the potential from 0.9 to 0.78 V. The scanning was carried out in the downward direction. At 0.9 V, bright, large spots and weaker, smaller spots are seen in the upper half of Fig. 1(b). The brighter
2. Experimental Au(1 1 1) single-crystal electrodes were prepared by the Clavilier method [1]. A gold single-crystal bead was cut and successively polished with finer grades of alumina paste, and the electrode was then annealed at ca. 950 C for 12 h in an electric furnace to remove mechanical damages [32]. The Au(1 1 1) single crystal was annealed in hydrogen flame for about 1 min and quenched into ultrapure water (Milli-Q SP-TOC; P18.2 MX cm) saturated with hydrogen [32]. The Au(1 1 1) electrode was carefully and immediately transferred into an electrochemical cell filled with sulfuric acid solution (Cica-Merck, ultrapur grade), which was placed in a clean bench to avoid contamination. Cyclic voltammetry was carried out at 20 C using a potentiostat (HOKUTO HAB-151) with the hanging meniscus method in a three-compartment electrochemical cell under Ar atmosphere. In situ STM measurements with a tungsten tip etched in 1 M KOH were performed using a Nanoscope E (Digital Instruments) in the atmosphere of high purity Ar in a glove box system (VAC). One of the (1 1 1) facets formed on the gold single-crystal bead was used as the substrate for in situ STM measurements. To minimize residual faradaic currents, tips were coated with polyethylene (Aldrich). STM images were recorded in the constant-current mode. All potential values were referred to AgjAgCljKCl (sat.).
Fig. 1. (a) CVs of a well-defined Au(1 1 1) electrode in 0.5 M sulfuric acid recorded at the scan rate of 5 mV s1. (b) Potential-dependent STM images (9 · 9 nm2) of Au(1 1 1) surface in 0.5 M sulfuric acid acquired at 0.9 V (upper half) and 0.78 V (lower half). Tunneling current was 3.0 nA.
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spots and the weaker spots form separate rows, p each running in the Æ1 2 1æ direction, or the so-called 3 direction, rotated by 30 with respect to the atomic rows of the underlying Au(1 1 1) lattice. The rows consisting of weaker spots show a zigzag feature. The distances between the brighter spots in the p directions indicated by arrows A and B are ca. 0.5 ( 3 times Au–Au distance: aAu) and 0.78 nm p ( 7aAu), respectively. The angle between the atomic rows marked by arrows A and B is ca. 75. On the basis of the symmetry of the atomic array, a unit cell issuperimposed 2 1 in Fig. 1(b). The adlattice is assigned to the struc1 2 p p ture, or the so-called ( 3 · 7) structure, which is identical to that found previously [9–11,18,21,33]. The bright spots are thought to be either sulfate or bisulfate ions [2]. The weaker spots are attributed to water-related species [2], namely water molecules (H2O) [17,22,34], hydronium ions (H3O+) [10,11], or hydrated hydronium ions ðH5 Oþ 2Þ [16,18,21]. The incorporation of those cations in the p p ( 3 · 7) adlayer was expected to reduce the coulombic repulsion between adjacent sulfate ions on Au(1 1 1) [10]. By stepping the electrode potential from 0.90 to 0.78 V, small, hexagonally arranged spots appeared immediately as seen in the lower half of Fig. 1(b). Because the distance between the spots is ca. 0.29 nm, these spots are attributed to Au atoms on Au(1 1 1)–(1 · 1). The appearance of the Au(1 1 1)–(1 · 1) substrate was made possible by the mobility of sulfate ions at the lower potential after the order–disorder phase transition [2]. A careful inspection of the composite STM image reveals that the weaker spots in the upper half of Fig. 1(b) are located at the on-top sites of Au atoms. The STM images of sulfate were found to depend strongly on the magnitude of tunneling current. Fig. 2(a) shows a close-up view of a sulfate layer at the tunneling current of 3.0 nA, as was observed in the upper half of Fig. 1(b). When the tunneling current was decreased from 3.0 to 1.0 nA, each spot dramatically changed in appearance. Fig. 2(b) shows an STM image of the sulfate adlayer obtained at 1.0 nA. The brighter spots are circular in shape in Fig. 2(a), while they became triangular in Fig. 2(b). Each triangle consists of three bright spots at the apices, which can be attributed to three O atoms of sulfate adsorbed on Au atoms. Between the triangular features, the weaker spots, resulting from water-related species appeared more p clearly, showing a zigzag configuration in the p 3 direction. p The distances between the bright spots in the 3 and 7 directions are ca. 0.5 and 0.78 nm, respectively; thus, the adlayer structure is identical to that found from Fig. 2(a). The unit cells are superimposed in Fig. 2(a) and (b). On the basis of the STM images shown p inpFigs. 1(b), 2(a) and (b), the structural model for the ( 3 · 7) adlayer illustrated in Fig. 3 was derived. By taking the symmetry of the Au(1 1 1) substrate into consideration, it was assumed that three oxygen atoms in each sulfate ion are located at on-top sites, and that each water-related species is also situated on an on-top site, as reported in our previous papers
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Fig. 2. High-resolution STM images (6 · 6 nm2) of Au(1 1 1) surface in 0.5 M sulfuric acid acquired at 0.9 V. Tunneling current was (a) 3.0 and (b) 1.0 nA, respectively.
p p Fig. 3. Structural model of the ( 3 · 7) adlayer on Au(1 1 1)–(1 · 1).
on Rh(1 1 1) [22], Ir(1 1 1) [23], and Pd(1 1 1) [24,25]. This configuration of sulfate ion on Au(1 1 1) is supported by SEIRAS measurements [18], electrochemical double-layer
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simulation in the vacuum [16], and quantum-chemical calculation [36]. 3.2. More concentrated sulfuric acid solutions CV measurements were carried out in 1.0, 2.5, 5.0, and 7.5 M sulfuric acid solutions at a scan rate of 5 mV s1. Fig. 4 shows the results obtained. The shape of the CVs remained unchanged after potential cycles in these concentrated solutions. With the increase in sulfate concentration, the peak potential for Au oxidation (gray lines in Fig. 4) is
Fig. 4. CVs of a well-defined Au(1 1 1) single-crystal electrode in (a) 1.0, (b) 2.5, (c) 5.0, and (d) 7.5 M sulfuric acid solutions, respectively. The scan rate was 5 mV s1.
seen to shift in the positive direction, from 1.40 V at 0.50 M to 1.66 V at 7.5 M. Furthermore, the Au oxidation peak became broader with the increase in sulfate concentration, while the charge density for the peak was almost independent of concentration (ca. 450 lC cm2). In 1.0 M sulfuric acid, anodic and cathodic currents at ca. 1.2 and 1.0 V, respectively, are seen on the red curve in Fig. 4(a), which could be assigned to the adsorption/desorption of hydroxyl groups. In the 2.5 M solution, the currents for the adsorption/desorption of hydroxyl groups shifted in the positive direction, whereas a pair of new sharp spikes appeared at 1.15 V (Fig. 4(b), red line). The shape of the sharp spikes is indicative of phase transition of the adlayer, which will be discussed in conjunction with STM results. As the concentration was increased further, the peaks for Au(1 1 1) surface reconstruction located at ca. 0.3 V became smaller. In the 5.0 M solution, the pair of new spikes shifted in the negative direction to 1.07 V. The pair of spikes originated from the order–disorder transition of sulfate layer observed at 0.80 V in Fig. 1(a) slowly moved in the negative direction as the concentration of sulfate was increased, and at 5.0 M, the order–disorder transition peaks of sulfate were observed at 0.75 V. In the 7.5 M solution, the peaks observed at the lower concentrations disappeared except those for the adsorption/desorption of sulfate located at ca. 0.5 V. To understand the origin of the new spikes, in situ STM measurements were carried out in 5.0 M sulfuric acid. Fig. 5(a) shows a composite STM image obtained by stepping the potential from 1.03 to 1.08 V. The scanning was done in the upward direction during the data acquisition. At 1.03 V, which is a potential morepnegative p than that for the new spikes, the highly ordered ( 3 · 7) adlayer was found as seen in the lower half of the image even in the highly concentrated 5.0 M solution. Here, sulfate ions and water-related species are clearly seen. After stepping the potential from 1.03 to 1.08 V, which is a potential more positive than that of the new spikes, p pthe STM image dramatically changed, i.e., the ( 3 · 7) adlayer immediately became disordered. Fig. 5(b) shows a high-resolution STM image acquired at 1.08 V in 5.0 M sulfuric acid. Although the terrace was entirely covered with a disordered structure, careful inspection revealed several triangles consisting of three bright spots as indicated by white circles in Fig. 5(b). These triangles are considered to be individual sulfate ions, because the size of each triangle is the same as that observed in Fig. 2(b). At potentials more negative than 0.75 V, the (1 · 1) structure of Au(1 1 1) substrate lattice was observed in the 5.0 M solution as in the 0.5 M solution (Fig. 1(b)). The difference between the disordered sulfate layers at negative and positive potentials is that at negative potentials, sulfate ions are weakly bound and mobile, whereas at positive potentials, sulfate ions are strongly bound and observable by STM. The mechanism of disordering of the sulfate layer at positive potentials, such as 1.1 V, is not clear. One of the
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Fig. 5. (a) Potential-dependent STM image (9 · 9 nm2) of the Au(1 1 1) surface in 5.0 M sulfuric acid acquired at 1.03 V (lower half) and 1.08 V (upper half). Tunneling current was 10.3 nA. (b) High-resolution STM image (6 · 6 nm2) of the Au(1 1 1) surface in 5.0 M sulfuric acid acquired at 1.08 V. Tunneling current was 1.2 nA.
possibilities is the desorption of water-related species (H2O, H3O+, H5 Oþ 2 ) at such positive potentials; the desorption of water related species would destroy thepnetwork p of sulfate ions (Fig. 3). It was reported p that the ( 3 · 7) structure p of sulfate changed into a ( 3 · 3)R30 structure on Pt(1 1 1) when the water was lost upon transfer into vacuum [37]. It was also reported that the codeposition of SO3 and p p H2O on Pt(1 1 1) could form the ( 3 · 7) structure on Pt(1 1 1) in vacuum, but p p that an insufficient amount of H2O formed a ( 3 · 3)R30 structure [16]. Apparently, the amount of water-related species strongly affects the structure of sulfate adlayer. Another possible factor responsible for the disordering could be the chemical state of sulfate on the surface. Kolb and co-workers [11] observed the disordered structure of sulfate in 0.1 M Na2SO4. Because a 0.1 M Na2SO4 solution is almost neutral, SO2 ion must be the dominant species in solution. 4 Kolb and co-workers thus proposed that HSO 4 forms a p p ( 3 · 7) adlayer, whereas SO2 4 forms a disordered structure on Au(1 1 1). If their argument holds, the newly found spikes might be attributable to the transformation between 2 HSO 4 and SO4 on the Au(1 1 1) electrode. In 7.5 M sulfuric acid, no ordered structure of sulfate was observed. The concentration of water molecules in 7.5 M sulfuric p acid p is approximately 55.5 M. In order to form the ( 3 · 7) sulfate network on Au(1 1 1) surface, the amount of water-related species available might be too small in this highly concentrated sulfuric acid solution. At 0.2 V, which is a potential more negative than that for the desorption of sulfate (Fig. 4(d)), in situ STM showed a reconstructed structure of Au(1 1 1). Therefore, sulfate ions have little influence on Au(1 1 1) at very negative potentials even in 7.5 M sulfuric acid. 4. Conclusions CV and in situ STM measurements were carried out in sulfuric acid solutions with concentrations ranging from 0.5 to 7.5 M on a well-defined Au(1 1 1) single crystal elec-
trode. In 2.5 and 5.0 M solutions, a pair of new anodic and cathodic spikes was found at ca. 1.1 V. In situ STM revealed that these spikes were associated with a new order–disorder transition of a sulfate adlayer. Sulfate ions were disordered at potentials more positive than that for the new spikes, and the ions were observed consistently by STM. In 7.5 M sulfuric acid, no ordered structure of sulfate was observed by STM. It is proposed that the network of sulfate and water-related species is not formed in such highly concentrated sulfuric acid solutions. At a very negative potential between the potentials of sulfate desorption and that of hydrogen evolution, a reconstructed structure of Au(1 1 1) substrate was observed in the 7.5 M solution. Therefore, sulfate has little influence on the surface of Au(1 1 1) at this negative potential even at the concentration as high as 7.5 M solution. Acknowledgments This work was supported in part by CREST-JST and by the Ministry of Education, Culture, Sports, Science and Technology, a Grant-in-Aid for Young Scientists (B) (No. 16750106) and the Center of Excellence (COE) Project, Giant Molecules and Complex Systems, 2006. The authors acknowledge Dr. T. Sawaguchi of AIST for useful discussion and the assistance provided by Dr Y. Okinaka in writing this manuscript. References [1] J. Clavilier, R. Faure, G. Guinet, R. Durand, J. Electroanal. Chem. 107 (1980) 205. [2] O.M. Magnussen, Chem. Rev. 102 (2002) 679. [3] J. Clavilier, J. Electroanal. Chem. 107 (1980) 211. [4] D.A. Scherson, D.M. Kolb, J. Electroanal. Chem. 176 (1984) 353. [5] A.S. Dakkouri, D.M. Kolb, in: A. Wieckowski (Ed.), Interfacial Electrochemistry, Marcel Dekker, New York, 1999 (Chapter 10). [6] H. Angerstein-Kozlowska, B.E. Conway, A. Hamelin, L. Stoicoviciu, Electrochim. Acta 31 (1986) 1051. [7] H. Angerstein-Kozlowska, B.E. Conway, A. Hamelin, L. Stoicoviciu, J. Electroanal. Chem. 228 (1987) 429.
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