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The Preparation and in Situ Scanning Tunneling Microscopy Study of Fe(110) Surface De-Sheng Kong,†,‡ Shen-Hao Chen,*,† Li-Jun Wan,‡ and Mei-Juan Han‡ Department of Chemistry, Shandong University, Jinan, Shandong 250100, China, and Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China Received September 19, 2002. In Final Form: November 12, 2002 In situ scanning tunneling microscopy was employed to study the pretreatment method of Fe(110) surface and the adlayer structure of o-aminothiophenol (OATP) formed on it in 0.1 M NaClO4. A preparation method consisting of chemical etching and electrochemical annealing steps was primarily determined, which can consistently give atomically flat terraces on Fe(110) surface. This way of preparation could open up new perspectives for the investigations of surface structural properties on Fe that were normally not accessible because of easy oxidation and contamination outside ultrahigh vacuum. For the first time, the atomic resolution Fe(110) surface and the molecular resolution adlayer on it were imaged in solution. OATP molecules adsorbed on the Fe(110) surface and formed well-ordered p(2×2) structures with a coverage of 0.25.
1. Introduction The development of scanning tunneling microscopy (STM) has contributed significantly to understanding of the adlayer properties by yielding detailed structural information. While most of the STM studies of selfassembled monolayers (SAMs) so far were performed in ultrahigh vacuum (UHV) or in air, the number of investigations of SAM-covered metal surfaces in an electrochemical environment is increasing.1 In the early 1990s, atomic resolution images on metals were obtained in solution by STM,2,3 and the intervening years have seen considerable growth in the area.4,5 Preparation of metallic surfaces for work in UHV typically consists of a series of polishing steps outside the vacuum chamber followed by in vacuo steps consisting of ion sputtering and/or high-temperature annealing.4,6,7 For surfaces that are to remain outside UHV, these processing steps are not available. This recognition led to considerable effort on the part of the electrochemical community to develop methods for metal electrode surface preparation. A major breakthrough was achieved when Clavilier et al.8 demonstrated that high-quality single-crystal platinum surfaces could be prepared simply with a Bunsen burner. This so-called flame-annealing quench technique was subsequently shown to work also for gold and silver. At the present time, flame-annealing methods exist for several noble metals such as Au,9-11 Pt,12 Ph,13,14 and Ir.15,16 * Corresponding author. Fax: +86-531-8565167. E-mail: shchen@ sdu.edu.cn. † Department of Chemistry, Shandong University. ‡ Institute of Chemistry, Chinese Academy of Sciences. (1) Kolb, D. M. Angew. Chem., Int. Ed. 2001, 40, 1162. (2) Magnussen, O. M.; Hotlos, J.; Nichols, R. J.; Kolb, D. M.; Behm, R. J. Phys. Rev. Lett. 1990, 64, 2929. (3) Bard, A. J.; Abrun˜a, H. D.; Chidsey, C. E.; Faulkner, L. R.; Eldberg, S. W.; Itaya, K.; Majda, M.; Melroy, O.; Murray, R. W.; Porter, M. D.; Soriaga, F. M. P.; White, H. S. J. Phys. Chem. 1993, 97, 7147. (4) Gewirth, A. A.; Niece, B. K. Chem. Rev. 1997, 97, 1129. (5) Itaya, K. Prog. Surf. Sci. 1998, 58, 121. (6) Lorenzo, M. O.; Haq, S.; Bertrams, T.; Murray, P.; Raval, R.; Baddeley, C. J. J. Phys. Chem. B 1999, 103, 10661. (7) Maruyama, T.; Sakisaka, Y. Surf. Sci. 1991, 253, 147. (8) (a) Clavilier, J.; Faure, R.; Guinet, G.; Durand, R. J. Electroanal. Chem. 1980, 107, 205. (b) Clavilier, J. J. Electroanal. Chem. 1980, 107, 211. (c) Clevilier, J.; Rodes, A.; Achi, K. El; Zamakhchari, M. A. J. Chem. Phys. 1991, 88, 1291. (9) Wan, L.-J.; Terashima, M.; Noda, H.; Osawa, M. J. Phys. Chem. B 2000, 104, 3563.
Reductive annealing methods have also developed for Ag17 and Ni.18,19 Cleaning and annealing procedures for all other metals remain somewhat less well developed. The other metals, such as Fe, W, Cr, or Cu, readily form an oxide film in the presence of oxygen, the absence of which is hard to ensure outside of UHV.4,20 Recently, on the basis of Siegenthaler and Juttntr’s work,21 researchers22-26 gradually developed a pretreatment method for singlecrystal copper to obtained a clean, oxide-free surface, which utilizes electrochemical etching in a phosphoric acid solution. But to the best of our knowledge, both surface absorption and surface reaction studies reported in the literature by now on single-crystal iron have been all in UHV condition.7,27-36 After being mechanically polished outside (10) Jiang, P.; Liu, Z.-F.; Cai, S.-M. Langmuir 2002, 18, 4495. (11) Xu, Q.-M.; Wan, L.-J.; Wang, C.; Bai, C.-L.; Wang, Z.-Y.; Nozawa, T. Langmuir 2001, 17, 6203. (12) Nagatani, Y.; Hayashi, T.; Yamada, T.; Itaya, K. Jpn. J. Appl. Phys. 1996, 35, (Part 1, No. 2A), 720. (13) Wan, L.-J.; Yau, S.-L.; Itaya, K. J. Solid State Electrochem. 1997, 1, 45. (14) Yau, S.-L.; Itaya, K. J. Phys. Chem. 1995, 99, 9507. (15) Zou, S.; Weaver, M. J. Surf. Sci. 2000, 446, L95. (16) Carabineiro, S. A. C.; Nieuwenhuys, B. E. Surf. Sci. 2002, 505, 163. (17) Teshima, T.; Ogaki, K.; Itaya, K. J. Phys. Chem. B 1997, 101, 2046. (18) Ando, S.; Suzuki, T.; Itaya, K. J. Electroanal. Chem. 1996, 412, 139. (19) Suzuki, T.; Yamada, T. J. Phys. Chem. 1996, 100, 8954. (20) Cuesta, A.; Kibler, L. A.; Kolb, D. M. J. Electroanal. Chem. 1999, 466, 165. (21) Siegenthaler, H.; Juttntr, K. J. Electroanal. Chem. 1984, 163, 327. (22) Suggs, D. W.; Bard, A. J. J. Phys. Chem. 1995, 99, 8349. (23) Vogt, M. R.; Polewska, W.; Magnussen, O. M.; Behm, R. J. J. Electrochem. Soc. 1997, 144, L113. (24) Wan, L.-J.; Itaya, K. Langmuir 1997, 13, 7173. (25) Wan, L.-J.; Itaya, K. J. Electroanal. Chem. 1999, 473, 10. (26) Wang, D.; Xu, Q.-M.; Wan, L.-J.; Wang, C.; Bai, C.-L. Langmuir 2002, 18, 5133. (27) Lu, J.-P.; Albert, M. P.; Bernasek, S. L. Surf. Sci. 1991, 258, 269. (28) Hodgson, A.; Wight, A.; Worthy, G. Surf. Sci. 1994, 319, 119. (29) Cheng, L.; Bocarsly, A. B.; Brenasek, S. L.; Ramanarayanan, T. A. Langmuir 1994, 10, 4542. (30) Wight, A.; Condon, N. G.; Leibsle, F. M.; Worthy, G.; Hodgson, A. Surf. Sci. 1995, 333, 133. (31) Cheng, L.; Bernasek, S. L.; Bocarsly, A. B. Chem. Mater. 1995, 7, 1807. (32) Rufael, T. S.; Batteas, J. D.; Friend, C. M. Surf. Sci. 1997, 384, 156.
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UHV, the single-crystal Fe surfaces were cleaned by repeated Ar+ ion sputtering followed by annealing (ranging from 500 to 800 K) under vacuum, as suggested by Musket et al.,37 until no contamination was detected with Auger electron spectroscopy (AES). So the investigations of Fe single crystals outside UHV (particularly in solutions), including determining a simple and effective pretreatment method, are of significant importance for surface science and corrosion science.20,38 In this paper, we reported our recent in situ studies of the preparation of a clean Fe(110) surface, and the adsorption of OATP, which is frequently used as a corrosion inhibiter,39,40 on Fe(110). It is for the first time that both the atomic resolution images of an Fe(110) surface and the molecular resolution images of an ordered adlayer on Fe(110) were observed in solution by in situ STM. 2. Experimental Section A commercial bulk Fe(110) single-crystal disk (from MaTack, Germany) of 10 mm diameter and 1 mm thickness was used as a working electrode for in situ STM observation. Solutions used were prepared with HClO4 (Kanto Chemical Co., Japan), NaClO4 and o-aminothiophenol (OATP) (Fluca), and ultrapure Millipore water. The in situ STM apparatus used was a Nanoscope E instrument (Digital Instruments Inc., Santa Barbara, CA) equipped with a bipotentiostat. The electrochemical cell used for in situ STM experiments was a standard commercial Teflon cell. Two platinum wires served as the reference and the counter electrodes. The potentials quoted in this paper were versus the Pt reference electrode in 0.1 M NaClO4. To obtain a clean, oxide-free, and atomically flat surface, the preparation of the Fe(110) single crystal was performed in the following way. First, after being polished mechanically with diamond paste of various grain sizes down to 0.03 µm, the Fe(110) electrode was mounted in the electrochemical cell with a droplet of water left on the electrode surface to protect it from contamination. A solution of 0.1 M HClO4 was injected into the cell to etch the freshly polished surface for about 2 min. Second, after being rinsed thoroughly with ultrapure water, the cell was filled with 0.1 M NaClO4 and then transferred to the STM base under potential control, at potentials negative to the open circuit potential (ocp, about -1.4 V in 0.1 M NaClO4), to avoid further oxidation of the Fe(110) surface in the STM cell. Finally, before the STM observation, the so-called electrochemical annealing treatment was carried out by cycling the electrode potential between an anodic dissolution potential of Fe(110) (-1.0 V) and a hydrogen evolution potential (-1.8 V) to remove the residual surface oxide and ensure an Fe(110) surface.
3. Results and Discussion 3.1. Preparation of Fe(110) Single-Crystal Surface. Both the so-called flame-annealing quenching method (as mentioned above) and the electrochemical etching method (frequently applied to copper24-26) could not be suitable for the preparation of single-crystal Fe because of the easy oxidation of Fe, with a thick black oxide film on its surface. After performing many probing experiments, we have tentatively determined a chemical etching plus electro(33) Cheng, L.; Bocarsly, A. B.; Brenasek, S. L.; Ramanarayanan, T. A. Langmuir 1996, 12, 392. (34) Roosendaal, S. J.; Asselen, B. van; Elsenaar, J. W.; Vredenberg, A. M.; Habraken, F. H. P. M. Surf. Sci. 1999, 442, 329. (35) Batteas, J. D.; Rufael, T. S.; Friend, C. M. Langmuir 1999, 15, 2391. (36) Hofer, W. A.; Redinger, J.; Biedermann, A.; Varga, P. Surf. Sci. 2000, 466, L795. (37) Musket, R. G.; McLean, W.; Colmenares, C. A.; Makowiecki, D. M.; Siekhaus, W. J. Appl. Surf. Sci. 1982, 10, 143. (38) Kolb, D. M. Surf. Sci. 2002, 500, 722. (39) Srhiri, A.; Derbali, Y.; Picaud, T. Corrosion 1995, 51, 788. (40) Uehara, J.; Aramaki, K. J. Electrochem. Soc. 1991, 138, 3245.
Figure 1. STM topography of a single-crystal Fe(110) surface without pretreatment by chemical etching and electrochemical annealing. The image was obtained in 0.1 M NaClO4 at -1.62 V with a scan rate of 8 Hz and tunneling current of 20 nA.
chemical etching method to obtain flat clean surfaces, which could consistently yield STM images of atomically rough surface. Figure 1 shows a typical large scale STM image of the Fe(110) surface without pretreatment by chemical etching and electrochemical etching. It can be seen that the surface is very rough with the features of pits and clusters of islands, which were produced initially by mechanical polishing. It is impossible to obtain atomic or molecular resolution images on this type of surface. Although electrochemical cycling can function as both a cleaning and an annealing process,41-43 it is not enough to remove the islands on the Fe(110) surface seen in Figure 1. So we attempted the chemical etching step and found it is effective for flattening the surface. After the surface was etched in 1.0 M HClO4 for about 2 min, the damaged surface layer can be removed by dissolution, exposing atomically flat terraces over large areas as shown in Figure 2. Figure 2 shows two examples of STM images of the Fe(110) surface, which were obtained in 0.1 M NaClO4 after chemical etching in 1.0 M HClO4 and electrochemical annealing treatment. The single-crystal surface is characterized by a terrace-and-step topography. The steps are mostly monatomic with a measured height of 0.20 ( 0.1 nm (Figure 2c) and the wide terraces extending over 30 nm were usually observed on a well-prepared Fe(110) electrode as shown in Figure 2. The atomic flatness of the terraces is indicative of both the removal of the damaged layer and the complete reduction of the natural oxide. Different from the situations found on some face-centered cubic metals such as Au(110),10,44 Ag(100),17 Pt(111),45 Cu(110),25 and Ni(100),18 no preferential orientation of the steps along the main crystallographic directions of the substrate was observed. The flat Fe surface terraces as shown in Figure 2 could be imaged within the doublelayer potential range about from -1.80 to -1.30 V, which corresponds to the double-layer potential region of Fe(41) Ho¨lzle, M. H.; Wandlowski, Th.; Kolb, D. M. J. Electroanal. Chem. 1995, 394, 271. (42) Finklea, H. O. In Encyclopedia of Analytical Chemistry/Electroanalytical Method; Meyers, R. A., Ed.; John Wiley & Sons Ltd.: Chichester, 2000; p 1. (43) Suggs, D. W.; Bard, A. J. J. Am. Chem. Soc. 1994, 116, 10725. (44) Honbo, H.; Sugaware, S.; Itaya, K. Anal. Chem. 1990, 62, 2424. (45) Kim, Y.-G.; Yau, S.-L.; Itaya, K. J. Am. Chem. Soc. 1996, 118, 393.
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Figure 3. (a) Atomically resolved STM image of the Fe(110) surface showing a (1×1) structure. The image was obtained in 0.1 M NaClO4 at -1.62 V with the scan rate of 12 Hz and tunneling current of 28 nA. (b) Schematic top views of Fe(110)(1×1) structure. A rhombus indicates the two-dimensional unit cell of the surface lattice, where a is the lattice constant (a ) 2.87 Å).
Figure 2. (a and b) Topographic STM images of an Fe(110) surface after treatment by chemical etching in 1.0 M HClO4 for ∼2 min and by electrochemical annealing in 0.1 M NaClO4. The images were obtained in 0.1 M NaClO4 at -1.44 V (a) and -1.62 V (b), with the scan rates of 12 Hz both for (a) and for (b), and with tunneling currents of 10 nA for (a) and 15 nA for (b), respectively. The cross section profile in (c) corresponds to the line in (b), revealing the monatomic steps of 0.20 ( 0.1 nm in height.
(110) in 0.1 M NaClO4. At potentials more positive than -1.30 V or more negative than -1.8 V, the terrace-andstep topography became unclear and noisy images were observed, resulting from either the oxidization of the Fe(110) surface or the hydrogen evolution from the Fe(110) surface. 3.2. High-Resolution Atomic Image of Fe(110). The flatness of the Fe(110) terrace, as shown in Figure 2, strongly encouraged us to try to reveal individual iron atoms. Figure 3a, with corrugation heights of 0.3-0.6 Å,
shows a high-resolution STM image of the Fe(110) surface. A rhombic lattice is outlined in this image. The observed nearest and next-nearest neighbor atomic distances are 0.23 ( 0.05 nm and 0.29 ( 0.05 nm, which are in excellent agreement with theoretic values of (x3/2)a and a, respectively, where a is the lattice constant for Fe single crystal (a ) 0.287 nm).46 From the crystallographic features, this adlattice is proposed to have an Fe(110)(1×1) structure. Figure 3b shows a schematic illustration of the body-centered cubic Fe(110) surface atom structure. The 〈001〉 and 〈11 h 0〉 directions of the surface can be determined from the orientation of the (110) facet on the microscope stage. 3.3. In Situ STM of OATP Adlayer on Fe(110). After the atomic resolution image as shown in Figure 3 was achieved, a small drop of about 1 × 10-5 M OATP solution was directly injected into 0.1 M NaClO4 in the electrochemical cell. OATP molecules adsorbed on the Fe(110) (46) Shih, H. D.; Jona, F.; Jepsen, D. W.; Marcus, P. M. Phys. Rev. Lett. 1981, 46, 731.
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Figure 4. High-resolution STM image of an OATP adlayer on a Fe(110) surface obtained in 0.1 M NaClO4. Scan rate was 20 Hz, and tunneling current was 10 nA.
surface formed a long-range ordered self-assembled monolayer which is completely different from the Fe(110) structure. Figure 4 shows a typical molecular resolution STM image acquired on an OATP SAM formed on the Fe(110) surface in 0.1 M NaClO4. This unfiltered STM image was observed at -1.50 V, which was within the double-layer potential region. It can be seen that the adlayer is highly ordered with the features of parallel rows of blobs. Every blob was proposed to represent one OATP molecule, since the size of that measured in the image is 0.31 ( 0.1 nm
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in diameter, which is in accordance with the size of an aromatic ring. The intermolecular distance in the 〈001〉 direction gives a periodic spacing (between point a and point c as indicated in the image) of 0.52 ( 0.1 nm, corresponding to twice the Fe-Fe distance. In the 〈11 h 0〉 direction, the atomic distance (between point b and point d) is 0.81 ( 0.1 nm, which is equal to twice the Fe-Fe distance along the 〈11 h 0〉 direction. The angle between points a and b and a and d is 112 ( 4°. All features observed in this image suggest that the structure of the OATP adlayer on the Fe(110) has a p(2×2) symmetry, which yields a surface coverage of 0.25. The STM images of OATP adlayer on Fe(110) in 0.1 M NaClO4 revealed the presence of long range order over extended areas. This is in contrast with the situation of OATP on Au(111) in 0.1 M H2SO4 solution,47 where the molecular resolution images revealed the coexistence of the areas of ordered adsorption and areas where the OATP seems randomly adsorbed. In summary, we have attempted a simple but effective treatment method for the Fe single crystal to obtain a clean, oxide-free, atomically flat surface in aqueous solution. The atomically resolved images of the Fe(110) surface and of the OATP on it were obtained by using in situ STM. This work would contribute to the investigations of anticorrosion of reactive metals and of the molecularly ordered thin films. Acknowledgment. This work was supported by the Chinese National Science Fund (No. 20173033) and the Special Funds for the Major State Basic Research Projects, G19990650. LA0265777 (47) Kolb, D. M. Electrochim. Acta 2000, 45, 2387.
