4576
J. Phys. Chem. C 2010, 114, 4576–4579
Electrodeposition of Sb on Au(111) from an Acidic Chloroaluminate Ionic Liquid: An in Situ STM Study Feng-Xia Wang, Ge-Bo Pan,* Ying-Dan Liu, and Yan Xiao Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215125, People’s Republic of China ReceiVed: NoVember 18, 2009; ReVised Manuscript ReceiVed: February 6, 2010
The electrodeposition of Sb on Au(111) in an acidic chloroaluminate ionic liquid has been investigated by cyclic voltammetry and scanning tunneling microscopy (STM). In situ STM imaging has revealed an ordered structure of the adsorbed SbCl2+ at 1.1 V vs Al/Al(III), which has further compressed into a Moire´-like pattern at 1.05 V. In the UPD range of Sb, a sudden smoothing of surface has been observed at 1.0 V, which is similar to those in BMIBF4. In the later stage, the nucleation process begins to occur and is followed by the formation of 2D-nanostripe architectures. Instead of width, the number and the length of Sb nanostripes increase with time. When the potential goes more negative, clusters are formed on the top of close-packed Sb nanostripes. These results are different from the Sb UPD in the neutral melt, implying that the acidicity of ionic liquid might play an important role in the deposition of Sb. 1. Introduction Electrochemical modification of solid surfaces with foreign metals is an important research field in both fundamental and technological areas.1-4 Among various elements, antimony is a semimetal with sp3 hybridization and has a rhombohedral crystal structure, showing a large mismatch with the Au(111) substrate. This is different from most metal heteroepitaxy systems wherein the substrate and overgrown metal possess similar crystal structure and bonding characteristics. Therefore, heteroepitaxy is possible for this system, but a simple epitaxial relationship would not be expected. More recently, in situ and ex situ studies that focus primarily on the superlattice structure of Sb monolayers formed by UPD have been reported in aqueous solutions.5,6 Ionic liquids (ILs) exhibit a wide electrochemical window, high conductivity, and low vapor pressure, enabling the electrodeposition of metals and semiconductors.7-11 Being composed of substituted imidazolium cations and weakly coordinating anions, ILs have a completely different environment from aqueous solution.12,13 Recently, Freyland et al. have reported on the nanoscale electrodeposition of the compound semiconductor AlSb from the neutral MBIC-AlCl3 (1:1).14 Mao et al. have studied the UPD of Sb on Au(111) and Au(100) in a BMIBF4 ionic liquid.15 The UPD behavior of Sb in BMIBF4 is found to be dramatically different from those observed in both aqueous solutions and neutral chloroaluminated ionic liquids. Herein, we report a detailed study on the electrodeposition of Sb on Au(111) in a Lewis acidic ionic liquid, 1-methyl-3butylimidazolium chloride-aluminum chloride (MBIC-AlCl3, 42:58 mol %) containing 5 mM SbCl3. In this melt, the predominant anions are reported to be Al2Cl7- + AlCl4-. Our interest has been focused on the initial stages of phase formation of Sb in both under- (UPD) and overpotential deposition (OPD) ranges. For this aim, in situ STM has been employed to monitor the Sb electrodeposition. A different UPD of Sb has been observed, which has further induced the nanostripe growth mode * To whom correspondence should be addressed. Fax: +86-51262872663. E-mail:
[email protected].
on Au(111). The results offer possibilities for the better understanding of medium effects on metal UPD. 2. Experimental Section Anhydrous SbCl3 (Aldrich, 99.999%) was used as received. MBIC was prepared as described in the literature.16 Anhydrous AlCl3 (Fluka, 99.99%) was sublimed three times before the meltpreparation. The as-prepared MBIC and AlCl3 were then slowly mixed in a molar ratio of 42:58. All the above preparation processes were carried out in an argon-filled glovebox (O2 and H2O < 1 ppm). A single-crystal Au(111) (10 × 10 mm2) was used as the working electrode. Aluminum and platinum wires were used as quasi-reference and counter electrodes, respectively. Before each experiment, the gold sample was annealed in a H2 flame, cooled slowly in the Ar flow, and directly used for cyclic voltammetry and in situ STM measurements. All the electrode potentials are reported with respect to an Al quasi-reference electrode. In situ STM measurements were carried out with a NanoScope E (Digital Instrument) microscope. The tunneling tips were prepared by electrochemical etching of Pt/Ir (90/10) in 3.5 M NaCN. The surface of the tip was sealed with electrophoretic paint to minimize Faradaic currents. All the experiments were performed in a vacuum-tight STM chamber filled with Argon. 3. Results and Discussion 3.1. Cyclic Voltammetry. Figure 1 shows cyclic voltammograms (CVs) of a Au(111) electrode in the acidic ionic liquid MBIC-AlCl3 (42:58) + 5 mM SbCl3. The redox couple at 1.65/ 1.68 V, which has also been observed in pure MBIC-AlCl3, can be assigned to Au step-edge oxidation. Bulk Au oxidation only occurs at potentials more positive than 1.85 V. These results indicate that the oxidation of Au(111) is independent of SbCl3. On the other hand, the addition of SbCl3 has induced four current peaks, which were not observed in the case of pure MBIC-AlCl3. It has been shown that SbCl2+ was the dominant
10.1021/jp910973c 2010 American Chemical Society Published on Web 02/24/2010
Electrodeposition of Sb on Au(111)
J. Phys. Chem. C, Vol. 114, No. 10, 2010 4577
Figure 1. CVs of an Au(111) electrode in MBIC-AlCl3 + 5 mM SbCl3. The potential window was extended negatively by an interval of 50 mV, and two cycles up to each new potential were performed. The second cycles of several typical sets are shown here. The scan rate was 50 mV/s. (a) UPD of Sb. (b) OPD of Sb and Al. The inset is an overall CV of Al OPD.
Figure 2. STM images of the SbCl2+ adsorption on Au(111) in MBIC-AlCl3 + 5 mM SbCl3. (a) E is scanned from 1.2 to 1.10 V and (b) E ) 1.10 V. (c) Large-scale and (d) high-resolution STM images acquired at E ) 1.05 V.
species and no oxidation of Sb(III) was observed in the acidic BuPyC-AlCl3,17 and this may also be true in the MBIC-AlCl3. Moreover, the CV analyses have indicated that the adsorbed SbO+ is involved in the reduction of Sb(III) on Au(111).18 Therefore, the cathodic peaks at 1.1 (C1) and 1.05 (C2) are possibly related to the adsorption of SbCl2+, whereas the other one at 0.85 V (C3) is related to the Sb UPD. The details of surface structural changes are revealed by in situ STM and shown in a latter section. In the subsequent scan, a steep current increase appears at 0.6 V, which is ascribed to the Sb OPD and is well separated from the Al bulk deposition at -0.1 V. 3.2. In Situ STM. Prior to the Sb UPD, the Au surface was examined by in situ STM. As described in the literature,19,20 ordered adlayers of AlCl4- were recorded at potentials positive of 1.3 V. Upon decreasing the potential, desorption of AlCl4starts, the STM image becomes fuzzy, and a new domain appears gradually (as shown in Figure 2a). Figure 2b shows a typical STM image acquired at 1.10 V. An interesting feature in the image is a well-ordered structure that extends all over
the Au(111) surface. The lattice constants extracted from STM image are a ) 1.0 ( 0.1 nm, b ) 1.0 ( 0.1 nm, and R ) 82 ( 2°, indicating that this structure is different from previous reported results.14,15 Although the physical origin of this ordered superstructure is still unclear at the moment, a possible explanation is attributed to a less closely packed Sb adlayer or the adsorption of SbCl2+ on Au(111). The latter seems more reasonable since the SbCl2+ cation is the main Sb(III) species in the acidic ionic liquid.17 The SbCl2+ adsorption in MBIC-AlCl3 is comparable to the SbCl3 adsorption in BMIBF4 and the SbO+ adsorption in aqueous solution.15,18 The difference is that the distance between neighboring SbCl2+ complexes is larger than those of SbCl3. This is reasonable because the strong electrostatic repulsion would prevent the clustering of such highly charged species. Upon the potential going more negative, structural transition occurs. As shown in Figure 2c, a Moire´-like pattern is finally formed at 1.05 V. The image shows a larger repeat spacing of ∼2.5 nm. The larger spacing is attributed to the lattice mismatch between the adsorbate and substrate. Figure 2d shows a higher resolution STM image, revealing the structural details. Obviously, each cluster consists of a set of small bright pots, which are indicated by white circles and arranged in hexagonal lattice. To determine the content of the unit cell is not straightforward because the structural details are missing from the STM images. However, careful inspection has revealed a large separation of ∼0.7 nm between two neighboring spots. This value is much bigger than those of iodine and PF6- on Au(111),21 indicating that the bright spots of the clusters cannot be from elemental Sb(0), instead of SbCl2+. It is noted that the (3 × 3) and (7 × 7) structures reported previously could not be observed in the present study.14,15,22 Since the ionic melts in the separate experiments were not identical, but differed in composition and acidity, this can have a strong impact on the reduction potentials and on anion adsorption and thus on the UPD process. As a matter of fact, the CVs of a Au(111) electrode in neutral and acidic MBIC-AlCl3 exhibit a clear distinction in the UPD region. Similar to the Sb UPD in BMIBF4,15 the sudden smoothing of the surface leads to the formation of domains of an ordered but complex structure as shown in Figure 3. In each domain, the atomic structure is characterized by periodic rows (indicated by arrows) with a repeat distance of 0.9 ( 0.1 nm. The rows are composed of two pairs (see the inset) of aligned short bars with a periodicity of 0.5 ( 0.1 nm and aligned along the 〈110〉
4578
J. Phys. Chem. C, Vol. 114, No. 10, 2010
Figure 3. STM images of Sb UPD on Au(111) in MBIC-AlCl3 + 5 mM SbCl3: (a) from 1.05 to 1.0 V and (b) E ) 1.0 V. The inset is a close-up STM image.
Figure 4. STM images of the nanostripe formation on Au(111) in MBIC-AlCl3 + 5 mM SbCl3: (a) large-scale and (b) high-resolution STM images. E ) 0.87 V.
directions of the Au(111) surface. According to the literature,15 a (4 × 9) structure can also be concluded, regardless of the constituents of the unit cell. Moreover, the slight difference still exists in two different systems. Unlike in BMIBF4 where hole structure is formed, only smooth surface has been observed in the Sb UPD in MBIC-AlCl3 when the potential goes positive. This is possibly due to the existence of chloride in MBIC-AlCl3, which can improve the diffusion of surface atoms.23 Reducing the potential to 0.87 V, the deposition of Sb becomes obvious and a few nuclei are formed both on the terraces and along the step edges of Au(111). As the deposition process is continued, the nuclei develop into anisotropic nanostripes as shown in Figure 4a and b. Interestingly, the nanostripes are confined to a width of ∼2.0 nm and to a height of ∼1.0 nm and randomly lie on the Au(111) surface. Atomic resolution images are not achieved on these nanostripes though efforts such as using different STM tips and varying the current set point and the bias voltage have been made. This observation is comparable to the Sb deposition on Cu(100).24 However, the width and orientations of nanostripes are different in the two systems. This is possibly attributed to different lattice constant for the two substrates, Au(111) and Cu(100). Although the growth behavior is still not clear, we believe that the covalent bonding tendency and the layered structure of the rhombohedral R-Sb with a lattice constant of 4.501 Å play a major role in forming the 2D nanostripe architectures.24
Wang et al.
Figure 5. STM images of the Sb OPD on Au(111) in MBIC-AlCl3 + 5 mM SbCl3: (a) E ) 0.87 V and (b) E ) 0.8 V.
These growth characteristics have been maintained until the whole Au(111) surface is covered by nanostripe architectures (Figure 5). As the deposition process is continued at 0.8 V, the number of deposited nanostripes increases rapidly and they spread all over the Au(111) surface. The width of nanostripes remains constant during the deposition of Sb, similar to the Sb deposition on Cu(100).24 Nevertheless, the difference is that once the surface is thoroughly covered by the nanostripes, 3D clusters begin to grow on top of the nanostripes, whereas the vertical alignment of the nanostripes is achieved following the layerby-layer epitaxy of nanostripes on Cu(100). Evidently, the electrochemical behavior of Sb on Au(111) in acidic MBIC-AlCl3 is different from that in neutral MBIC-AlCl3 as well as in BMIBF4. This is because the predominant anions in the three electrolytes are different from each other. It has been realized for a long time that the presence of specifically adsorbed ions can significantly affect the chemical reactivity of a metal electrode.18,25,26 The predominant anions in acidic MBIC-AlCl3, in neutral MBIC-AlCl3, and in BMIBF4 electrolyte are AlCl4- + Al2Cl7-, AlCl4-, and BF4-, respectively. The acidicity of ionic liquid might play an important role in the deposition of Sb. 4. Conclusion The electrodeposition of Sb on Au(111) in an acidic chloroaluminate ionic liquid has been investigated by cyclic voltammetry and STM. In situ STM imaging has revealed an ordered structure of the adsorbed SbCl2+ at 1.1 V vs Al/Al(III), which has further compressed into a Moire´-like pattern at 1.05 V. In the UPD range of Sb, a sudden smoothing of surface has been observed at 1.0 V, which is similar to those in BMIBF4. In the later stage, the nucleation process begins to occur and is followed by the formation of 2D-nanostripe architectures. Instead of width, the number and the length of Sb increase with time. When the potential goes more negative, the clusters are formed on the top of close packed Sb nanostripes. These results are different from those of the Sb UPD in the neutral melt, implying that the acidicity of ionic liquid might play an important role in the deposition of Sb. Acknowledgment. This work was supported by the National Natural Science Foundation of China (No. 20873160), the National Basic Research Program of China (No. 2010CB934100), and the Chinese Academy of Sciences. We thank Prof. W. Freyland for providing us with a home-built scanner, which is designed for ILs/Solid surface study. References and Notes (1) Budevski, E.; Staikov, G.; Lorenz, W. J., Eds. Electrochemical Phase Formation and Growth; VCH: Weinheim, Germany, 1996.
Electrodeposition of Sb on Au(111) (2) Freyland, W.; Aravinda, C. L.; Borissov, D. In Electrocrystallization in Nanotechnology; Staikov, G., Ed.; Wiley-VCH: Weinheim, Germany, 2007. (3) Budevski, E.; Staikov, G.; Lorenz, W. J. Electrochim. Acta 2000, 45, 2559. (4) Dogel, J.; Tsekov, R.; Freyland, W. J. Chem. Phys. 2005, 122, 094703. (5) Ward, L. C.; Stickney, J. L. Phys. Chem. Chem. Phys. 2001, 3, 3364. (6) Wu, Q.; Shang, W. H.; Yan, J. W.; Mao, B. W. J. Mol. Catal. A: Chem. 2003, 199, 49. (7) Welton, T. Chem. ReV. 1999, 99, 2071. (8) Wasserscheid, P.; Welton, T., Eds. In Ionic Liquids in Synthesis, 2nd ed.; Wiley VCH: Weinheim, Germany, 2008. (9) Sheldon, R. Chem. Commun. 2001, 2399. (10) MacFarlane, D. R.; Golding, J.; Forsyth, S.; Forsyth, M.; Deacon, G. B. Chem. Commun. 2001, 1430. (11) Freyland, W.; Zell, C. A.; Abedin, S. Z. E.; Endres, F. Electrochim. Acta 2003, 48, 3053. (12) Mamantov, G.; Mamantov, C., Eds. AdVances in Molten Salt Chemistry; Elsevier: New York, 1983; Vols. 5 and 6. (13) Hussey, C. L. Pure Appl. Chem. 1988, 60, 1763.
J. Phys. Chem. C, Vol. 114, No. 10, 2010 4579 (14) Mann, O.; Aravinda, C. L.; Freyland, W. J. Phys. Chem. B 2006, 110, 21521. (15) Fu, Y. C.; Yan, J. W.; Wang, Y.; Tian, J. H.; Zhang, H. M.; Xie, Z. X.; Mao, B. W. J. Phys. Chem. C 2007, 111, 10467. (16) Zell, C. A.; Freyland, W. Langmuir 2003, 19, 7445. (17) Habboush, D. A.; Osteryoung, R. A. Inorg. Chem. 1984, 23, 1726. (18) Li, F.-H.; Wang, W.; Gao, J.-P.; Wang, S.-Y. J. Electrochem. Soc. 2009, 156, D84. (19) Pan, G. B.; Freyland, W. Electrochim. Acta 2007, 52, 7254. (20) Borissov, D.; Aravinda, C. L.; Freyland, W. J. Phys. Chem. B 2005, 109, 11606. (21) Pan, G. B.; Freyland, W. Chem. Phys. Lett. 2006, 427, 96. (22) Aravinda, C. L.; Freyland, W. Chem. Commun. 2006, 1703. (23) Honbo, H.; Sugawara, S.; Itaya, K. Anal. Chem. 1990, 62, 2424. (24) Wu, J. H.; Yan, J. W.; Xie, Z. X.; Xue, Q. K.; Mao, B. W. J. Phys. Chem. B 2004, 108, 2773. (25) Magnussen, O. M. Chem. ReV. 2002, 102, 679. (26) Herrero, E.; Buller, L. J.; Abrun˜a, H. D. Chem. ReV. 2001, 101, 1897.
JP910973C
