Comparative Electrochemical Scanning Tunneling Microscopy Study

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Comparative Electrochemical Scanning Tunneling Microscopy Study of Nonionic Fluorosurfactant Zonyl FSN Self-Assembled Monolayers on Au(111) and Au(100): A Potential-Induced Structural Transition Yongan Tang, Jiawei Yan,* Feng Zhu, Chunfeng Sun, and Bingwei Mao State Key Laboratory for Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, Fujian 361005, PR China Received September 23, 2010. Revised Manuscript Received December 15, 2010 We investigate the structure of nonionic fluorosurfactant zonyl FSN self-assembled monolayers on Au(111) and Au(100) in 0.05 M H2SO4 as a function of the electrode potential by electrochemical scanning tunneling microscopy (ECSTM). On Au(111), a (31/2  31/2)R30° arrangement of the FSN SAMs is observed, which remains unchanged in the potential range where the redox reaction of FSN molecules does not occur. On Au(100), some parallel corrugations of the FSN SAMs are observed, which originate from the smaller distance and the repulsive interaction between FSN molecules to make the FSN molecules deviate from the bridging sites, and ECSTM reveals a potential-induced structural transition of the FSN SAMs. The experimental observations are rationalized by the effect of the intermolecular interaction. The smaller distance between molecules on Au(100) results in the repulsive force, which increases the probability of structural change induced by external factors (i.e., the electrode potential). The appropriate distance and interactions of FSN molecules account for the stable structure of FSN SAMs on Au(111). Surface crystallography may influence the intermolecular interaction through changing the molecular arrangements of the SAMs. The results benefit the molecular-scale understanding of the behavior of the FSN SAMs under electrochemical potential control.

Introduction Surface self-assembly of organic molecules is an attractive way to modify the properties of solid surfaces.1 Molecule-surface and intermolecular interactions are responsible for the formation of self-assembled monolayers (SAMs). Other factors such as electrode potential also play important roles in the structures of SAMs. A variety of SAMs based on chemical or physical molecule-surface interactions have been investigated.2-5 Recently, we reported scanning tunneling microscopy (STM) studies of nonionic fluorosurfactant zonyl FSN self-assembly on Au(111) and Au(100) surfaces under ambient conditions.6,7 Zonyl FSN is a kind of nonionic fluorosurfactant with a molecular formula of F(CF2CF2)3-8CH2CH2O(CH2CH2O)xH in which the hydrophilic part consists of a polyoxyethylene chain and the hydrophobic part consists of a fluorocarbon chain. A schematic illustration of the structure of the FSN molecule is shown in Figure 1. It has been found that FSN SAMs on Au(111) form a (31/2  31/2)R30° adlayer structure. An important characteristic of FSN SAMs on Au(111) is its very large domain with almost no defects, which benefits the quality of FSN SAMs.6 This phenomenon is in contrast to the most extensively studied thiolate SAMs on Au(111), which usually contain several kinds of defects such as vacancies of Au islands with monatomic depth, missing rows, vacancies of molecules, and a considerable number of domain *Corresponding author. E-mail: [email protected]. (1) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (2) Ulman, A. Chem. Rev. 1996, 96, 1533. (3) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103. (4) De Feyter, S.; De Schryver, F. C. Chem. Soc. Rev. 2003, 32, 139. (5) Wan, L. J. Acc. Chem. Res. 2006, 39, 334. (6) Tang, Y. A.; Yan, J. W.; Zhou, X. S.; Fu, Y. C.; Mao, B. W. Langmuir 2008, 24, 13245. (7) Yan, J. W.; Tang, Y. A.; Sun, C. F.; Su, Y. Z.; Mao, B. W. Langmuir 2010, 26, 3829. (8) Yang, G. H.; Liu, G. Y. J. Phys. Chem. B 2003, 107, 8746.

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Figure 1. Schematic illustration of the molecular structure of FSN.

boundaries.8 The physical interaction of FSN with the substrate and the high mobility of the molecules as well as the surface Au atoms during the self-assembly process are responsible for the formation of such uniform SAMs. On the Au(100) surface, the characteristics of FSN SAMs are the result of surface crystallography and the mobility of Au atoms and adsorbed molecules during the self-assembly process. STM images reveal a 3 1

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registration of the SAMs with the Au(100) surface and some parallel corrugations. The distance of the two nearest FSN molecules is 0.41 nm, which is smaller than 0.50 nm on Au(111). The repulsive interaction that results from the smaller distance between FSN molecules makes FSN molecules deviate from bridge sites and form corrugations.7 Being a fluorosurfactant, FSN has high surface activity as well as good chemical and thermal stability for use in acidic or alkaline solution. Moreover, FSN and other nonionic fluorosurfactants have been used for electrochemical studies including electrochemical analysis and electrogenerated chemiluminescence, which show the potential applications of fluorosurfactants

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in modifying the properties of electrode surfaces in an electrochemical field.9-15 The structure of a fluorosurfactant film on electrodes is directly relevant to its properties. As mentioned above, besides moleculesurface and intermolecular interactions, the electrode potential also plays an important role in the structures of SAMs. For example, the electrode potential may induce a phase transition in organic monolayers. Kolb’s group reported potential-induced structural transitions of SAMs on a Au electrode in 0.1 M H2SO4, such as ethanethiol SAMs on Au(111)16 and Au(100)17 and propanethiol SAMs on Au(100).18 Structural transitions of other adlayers on gold single-crystal electrodes including 2,20 -bipyridine,19,20 uracil,21 trimesic acid,22 pyridine,23 and hexadecane24 have also been revealed. On Rh(111) and Pt(111) electrodes, structural transition of the benzene adlayer was observed.25 Taniguchi et al. reported a structural change of bis(2-anthraquinyl)disulfide ((2-AQS)2) SAMs on the Au(100) surface induced by the electrochemical redox reaction.26 Therefore, it is important to study the effect of the electrode potential on structures of adlayers on electrode surfaces. For this purpose, electrochemical STM (ECSTM) is a powerful tool that can directly address a wealth of structure information. In this work, we investigate the structures of FSN SAMs on Au(111) and Au(100) in 0.05 M H2SO4 as a function of the electrode potential by ECSTM. On the basis of the comparative study of FSN SAMs on Au(111) and Au(100), we demonstrate that the electrode potential can induce the structural transition of FSN SAMs only if a larger repulsive force exists among FSN molecules.

Experimental Section Electrochemical STM measurements were performed on a Nanoscope IIIa multimode SPM (Digital Instruments). Electrochemically etched and polyethylene-insulated tungsten tips were used. Au single-crystal beads were prepared following the Clavilier method.27 Briefly, one end of a Au wire with a diameter of 0.5 mm was melted in a hydrogen-oxygen flame to form a singlecrystal bead with a diameter of about 3 mm. The bead was then fixed on a Au foil with a (111) facet or a (100) facet facing upward and served as the substrate for in situ STM measurements. Prior to each experiment, the Au surface was subjected to electrochemical polishing followed by flame annealing to obtain a highquality clean surface. A platinum wire was used as reference electrode for in situ STM measurements. However, potentials in this work are quoted with respect to the saturated calomel electrode (SCE). (9) Cha, C. S.; Zu, Y. B. Langmuir 1998, 14, 6280. (10) Chen, Z. F.; Zheng, H. Z.; Lu, C.; Zu, Y. B. Langmuir 2007, 23, 10816. (11) Chen, Z. F.; Zu, Y. B. J. Electroanal. Chem. 2007, 603, 281. (12) Chen, Z. F.; Zu, Y. B. J. Electroanal. Chem. 2008, 624, 9. (13) Li, F.; Zu, Y. B. Anal. Chem. 2004, 76, 1768. (14) Li, M. J.; Chen, Z. F.; Yam, V. W. W.; Zu, Y. B. ACS Nano 2008, 2, 905. (15) Chen, Z. F.; Zu, Y. B. J. Phys. Chem. C 2009, 113, 21877. (16) Hagenstrom, H.; Schneeweiss, M. A.; Kolb, D. M. Langmuir 1999, 15, 2435. (17) Schweizer, M.; Hagenstrom, H.; Kolb, D. M. Surf. Sci. 2001, 490, L627. (18) Schweizer, M.; Manolova, M.; Kolb, D. M. Surf. Sci. 2008, 602, 3303. (19) Cunha, F.; Tao, N. J. Phys. Rev. Lett. 1995, 75, 2376. (20) Dretschkow, Th.; Wandlowski, Th. Electrochim. Acta 1999, 45, 731. (21) Dretschkow, Th.; Dakkouri, A. S.; Wandlowski, Th. Langmuir 1997, 13, 2843. (22) Su, G. J.; Zhang, H. M.; Wan, L. J.; Bai, C. L.; Wandlowski, Th. J. Phys. Chem. B 2004, 108, 1931. (23) Cai, W. B.; Wan, L. J.; Noda, H.; Hibino, Y.; Ataka, K.; Osawa, M. Langmuir 1998, 14, 6992. (24) He, Y. F.; Tao, Y.; Borguet, E. J. Phys. Chem. B 2002, 106, 11264. (25) Yau, S. L.; Kim, Y. G.; Itaya, K. J. Am. Chem. Soc. 1996, 118, 7795. (26) Yoshimoto, S.; Hirakawa, N.; Nishiyama, K.; Taniguchi, I. Langmuir 2000, 16, 4399. (27) Clavilier, J.; Faure, R.; Guinet, G.; Durand, R. J. Electroanal. Chem. 1980, 107, 205.

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Zonyl FSN-100 (F(CF2CF2)3-8CH2CH2O(CH2CH2O)xH) was purchased from Aldrich and used as received. All solutions were prepared with Milli-Q water (18.2 MΩ 3 cm, Millipore). FSN aqueous solutions (0.25%) were freshly prepared prior to selfassembly experiments. The FSN SAMs were prepared by immersing an electrochemically polished, flame-annealed Au singlecrystal bead into an FSN-containing aqueous solution for 3 h; the bead was then thoroughly rinsed with Milli-Q water.

Results and Discussion FSN SAMs on Au(111). ECSTM images of FSN SAMs on Au(111) in 0.05 M H2SO4 are shown in Figure 2; these images were recorded at 0.60 V. Over an area of 200 nm  200 nm as shown in Figure 2A, no Au islands can be observed, but this is expected because the Au(111) substrate was flame annealed before FSN self-assembly. The absence of Au islands supports the conclusion in our previous paper6 that the expelled Au atoms that originated from the lifting of reconstruction should emerge in step edges because of the FSN adsorbate-enhanced mobility of Au atoms during the self-assembly process. Similar to the FSN SAM-covered Au(111) surface under ambient conditions,6 FSN SAMs on Au(111) in 0.05 M H2SO4 under potential control also present a uniform structure without any domain boundaries within the image area. The molecular-resolution image of FSN SAMs is given in Figure 2B, which appears in a hexagonal arrangement of bright spots with each spot corresponding to an FSN molecule. The shortest distance between two bright dots is 0.50 ( 0.04 nm, and a unit cell is highlighted on the image, which corresponds to a (31/2  31/2)R30° arrangement of the FSN SAMs on Au(111) showing the same characteristics as the FSN SAMs under ambient conditions. The FSN molecules are reduced at -0.10 V and oxidized at þ1.10 V. When the electrode potential is more positive than þ0.90 V, the Au electrode tends to be unstable. Therefore, we choose the potential region of 0.00-0.80 V to guarantee the stability of FSN molecules and the Au substrate and demonstrate the influence of the electrode potential on the structure of FSN SAMs on Au(111). ECSTM results indicate that the (31/2  31/2)R30° structure of the FSN SAMs is stable in the above potential range where the redox reaction of FSN molecules does not occur. The shortest distance between two FSN molecules is 31/2a (a = 0.288 nm, Au(111) lattice constant), which results in the appropriate interaction of FSN molecules and thus the stable structure. Therefore, the change in electrode potential cannot influence the structure of FSN SAMs on Au(111). FSN SAMs on Au(100). In the potential range from þ0.20 to þ0.80 V, the structure of FSN SAMs on Au(100) in 0.05 M H2SO4 remains unchanged. Typical ECSTM images are shown in Figure 3, which are recorded at þ0.25 V. In Figure 3A, some parallel corrugations of the FSN SAMs are observed, with the white lines being placed to aid the eye. This kind of corrugation could be observed in two perpendicular directions, which is consistent with the 4-fold symmetry of the Au(100) substrate. Figure 3B is a molecular-resolution image of FSN SAMs on Au(100), which shows that an ordered structure of FSN molecules is formed between two corrugation stripes. The structures are the same as those of the FSN SAMs on Au(100) under ambient conditions, which are analyzed in detail including the structural arrangement, the adsorption sites, and the possible orientation of FSN molecules in our previous paper.7 The FSN SAM-covered Au(111) surface under potential control (0.00-0.80 V) shows the same characteristics as the FSN SAMs under ambient conditions, and the FSN SAM-covered Au(100) surface under potential control (0.20-0.80 V) also shows the same characteristics as the Langmuir 2011, 27(3), 943–947

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Figure 2. ECSTM images of an FSN-covered Au(111) at 0.60 V in 0.05 M H2SO4. (A) Large-scale (200  200 nm2) and (B) high-resolution (10  10 nm2) ECSTM images. Schematic illustration of the (31/2  31/2)R30° adlayer structure on the Au(111) surface is shown in the inset. (Gray circles) FSN molecules; (black circles) Au atoms.

Figure 3. ECSTM images of an FSN-covered Au(100) at 0.25 V in 0.05 M H2SO4. (A) Large-scale (100 100 nm2) and (B) high-resolution (10  10 nm2) ECSTM images. Schematic illustration of the   3 -1 1 1 adlayer structure and corrugation on the Au(100) surface are shown in the inset. (Red and blue circles) FSN molecules and (white circles) Au atoms. Three nearest-neighbor directions for the arrangement are indicated by arrows.

FSN SAMs under ambient conditions. Therefore, according to our previous results under ambient conditions,6,7 we infer that FSN molecules in an electrochemical environment are also in a standing-up configuration with a tilt angle and that fluorocarbon chains are exposed on the outer surface of the FSN SAMs. As shown in the inset of Figure 3B, the shortest distance between two FSN molecules is 21/2a, which is much smaller than 31/2a on Au(111). We infer that because of the smaller distance there exists a repulsive interaction between FSN molecules. The accumulation of such repulsive interactions makes FSN molecules deviate from the bridge sites in the corrugation region (i.e., FSN molecules move gradually to the near-top sites). However, even by forming the corrugations, the strain can only be partially relieved and the adsorptive sites in the corrugation region, deviating from the bridge sites, are not stable ones. Therefore, the structure of FSN SAMs on Au(100) is not stable enough and could be influenced easily by the other factors such as the electrode potential. Interestingly, ECSTM does reveal a potential-induced structural transition of FSN SAMs on Au(100). When the electrode potential was moved negatively to þ0.05 V, the structure of FSN SAMs on Au(100) in 0.05 M H2SO4 changed markedly. The ECSTM images are shown in Figure 4. From the large-scale image Langmuir 2011, 27(3), 943–947

in Figure 4A, the corrugations of the FSN SAMs, which were observed in two perpendicular directions in Figure 3A, are absent. The absence of corrugations indicates a potential-induced structural transition of FSN SAMs on Au(100). A close examination of the ECSTM image in Figure 4B shows that some pits form on the surface. It seems that the pits appear at places where corrugation exists. A section analysis of some pits has been performed. A typical result is shown in the upper right corner of Figure 4B. The depth of the pit is about 0.10 nm, which indicates that the pit does not originate from a gold vacancy. We infer that the disappearance of corrugations is accompanied by the rearrangement of FSN molecules, which results in the formation of vacancies of FSN molecules because the SAMs after the structural transition have a denser structure than does the previous one. Because the size of the pits is about 0.70 nm, the missing of FSN molecule accounts for the appearance of a pit. A molecular-resolution image of FSN SAMs on Au(100) after the structural transition is given in Figure 5, which shows an ordered arrangement of FSN molecules without the existence of corrugations. Three nearest-neighbor directions for the arrangement are indicated by arrows. aB0 is parallel to the direction DOI: 10.1021/la103812v

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Figure 4. ECSTM images of FSN-covered Au(100) at 0.05 V in 0.05 M H2SO4. (A) Large-scale (100  100 nm2) image showing the structural

transition. (B) High-resolution (20  20 nm2) image showing the formation of pits. (Inset) Section analysis of a pit.

Figure 5. Molecular-resolution ECSTM image of an FSN-covered Au(100) at 0.05 V in 0.05 M H2SO4. Scan size: 8  8 nm2. Three nearest-neighbor directions for the arrangement are indicated by arrows.

of aB (inset of Figure 3), along which the shortest distance between two bright dots is 0.41 ( 0.03 nm, which should originate from the adsorbed molecules in the 21/2 direction. However, bB0 and c00B are rotated by 12 and 9° against the directions of bB and B c , and the neighbor distances along the directions of bB and B c are 0.43 ( 0.03 and 0.46 ( 0.03 nm, respectively. Although STM resolves the distorted hexagonal arrangement of FSN molecules, we cannot propose an adlayer model because the change in the directions of bB and B c results in the complexity of the adsorptive sites for FSN molecules. The problem is similar to that of propanethiol on Au(100), for which offering an adlayer model for the observed structures of propanethiol is also difficult.18 Kolb’s group reported potential-induced structural transitions of SAMs on the Au electrode, including those of propanethiol SAMs on Au(100) and ethanethiol SAMs on Au(111) and Au(100).16-18 They pointed out that such observations are rather surprising because the gold thiolate moiety is not an ionic species and hence the influence of the electrode potential seems not to be obvious. They also pointed out that the chain length (i.e., intermolecular interaction) is related to the stability of the SAMs and determines whether the structure of the SAMs will be affected by potential changes.18 Taniguchi et al. observed a structural change in SAMs of bis(2-anthraquinyl)disulfide ((2-AQS)2) on the Au(100) surface, which was induced by a redox reaction.26 Though the driving force for the structural change with the redox reaction is not clear, they suggested that it may be due to the 946 DOI: 10.1021/la103812v

hydrogen bonding between the hydroquinone groups of 2-AQH2S (reduced form) (i.e., an intermolecular interaction). The above interpretation demonstrates that the intermolecular interaction and the electrode potential are two important factors in inducing the structural transition. In addition, the molecule-surface interaction can influence the structure of SAMs and the intermolecular interaction. Liu et al. reported that benzenethiol molecules adopt the (21/2  21/2)R45° adlayer structure on Pt(100) in 0.1 M HClO4 under potential control because the Pt-S bond is strong enough to compensate for the repulsive interaction between the admolecules.28 Because the interaction between FSN molecules and the Au substrate is weaker than the Pt-S bond, in the model of the adlayer as shown in the inset of Figure 3B, though the distance of the two nearest FSN molecules in the 21/2 direction is 21/2a, FSN molecules prefer to adjust their adsorptive sites to form corrugations and relieve strain, and in the other direction of the unit cell, the distance between two FSN molecules is (10/2)1/2a, which is larger than 21/2a but smaller than 31/2a. The intermolecular interaction mainly involves the van der Waals force, which ensures the efficient formation of the FSN SAMs and plays an important role in its stability. In the electrochemical environment, the intermolecular interaction will be influenced not only by the chain length and the distance but also by the electrolyte. At the electrode/electrolyte interface, the composition of the electrolyte is related to the electrode potential. When the electrode potential is more negative or positive than the potential of zero charge (PZC), the electrochemical environment at the interface will be significantly different. Interestingly, the structural transition of FSN SAMs on Au(100) occurs at 0.05 V, which is slightly more negative than the PZC for the Au(100) electrode (about 0.08 V).29 Therefore, we infer that when the electrode potential moves across the PZC, the electrochemical environment (ion composition) at the interface will change from a bisulfate-dominant composition to a hydronium-dominant composition. The alteration will influence the intermolecular interaction and thus induce the structural transition. In the potential range of þ0.80-0.00 V where the redox reaction of FSN molecules does not occur, the (31/2  31/2)R30° structure of the FSN SAMs on Au(111) remains unchanged and a structural transition of the FSN SAMs on Au(100) is induced by (28) Liu, G. Z.; Ou Yang, L. Y.; Shue, C. H.; Ma, H. I.; Yau, S. L.; Chen, S. H. Surf. Sci. 2007, 601, 247. (29) Dakkouri, A.; Kolb, D. Reconstruction of Gold Surfaces. In Interfacial Electrochemistry: Theory, Experiment, and Applications; Wieckowski, A., Ed.; Marcel Dekker: New York, 1999.

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the electrode potential. By comparing the structure of FSN SAMs on Au(111) with that on Au(100), particularly the distance of the two nearest FSN molecules, we believe that the intermolecular interaction plays an important role in the stability of the FSN SAMs. The smaller distance between molecules on Au(100) results in the repulsive force, which increases the probability of structural change induced by external factors such as the electrode potential. In addition, the surface crystallography may influence intermolecular interactions through changing the molecular arrangements of the SAMs.

Conclusions The structures of FSN SAMs on Au(111) and Au(100) in 0.05 M H2SO4 as a function of electrode potential have been investigated by ECSTM. On Au(111), a (31/2  31/2)R30° arrangement of the FSN SAMs is observed, which remains unchanged in the potential range where the redox reaction of FSN molecules does not occur. On Au(100), some parallel corrugations of the FSN SAMs are observed, which originate from the smaller distance and the repulsive interaction between FSN molecules to make the

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FSN molecules deviate from the bridging site, and ECSTM reveals the potential-induced structural transition of FSN SAMs. By comparing the structural behavior of FSN SAMs on Au(111) with that on Au(100), we infer that the structural transition originates as a result of both intrinsic and extrinsic factors (i.e., the intermolecular interaction and the electrode potential), where the intrinsic factor plays a decisive role. On Au(111), the appropriate distance and intermolecular interaction of FSN molecules account for the stable structure of FSN SAMs, which is not influenced by the electrode potential. On Au(100), external causes become operative through internal causes because of the smaller intermolecular distance and repulsive force; therefore, the electrode potential may induce the structural transition. Acknowledgment. This work was supported by the Natural Science Foundation of China (NSFC nos. 20973144, 21033007, and 21021002) and the Special Funds for Major State Basic Research Project of China (973 project no. 2007CB935603). We sincerely thank Dr. Yanbing Zu for his suggestion to perform STM studies on FSN self-assembly.

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