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Langmuir 2000, 16, 9368-9373
Highly Ordered p-Xylene Adlayer Formed on Rh(111) in HF Solution: In Situ STM and Ex Situ LEED K. Suto, M. Wakisaka, M. Yamagishi, L.-J. Wan, J. Inukai, and K. Itaya* Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, Aoba-yama 04, Sendai 980-8579, Japan Received June 19, 2000 Structure of the adlayer of p-xylene on Rh(111) in HF solution was investigated by the complementary use of in situ scanning tunneling microscopy (STM) and ex situ low-energy electron diffraction (LEED). In situ STM images revealed that p-xylene molecules lie flat on the Rh(111) surface, forming a highly ordered adlayer with a c(2x3 × 4)rect structure. High-resolution imaging revealed not only the packing arrangement but also the internal structure of each molecule. Two methyl functional groups at the para position of benzene were clearly discerned. The adlayer structure obtained by LEED was consistent with that determined by in situ STM.
Introduction Understanding the structure of organic adsorbates on metal surfaces in solution is a major subject of modern interfacial electrochemistry.1-3 The adsorption of organic molecules on various metal surfaces is also important for evaluating gas-phase catalytic reactions such as hydrogenation and dehydrogenation.4 Several recent investigations have demonstrated scanning tunneling microscopy (STM) to be a powerful technique for in situ characterization of surfaces both in solution5,6 and in ultrahigh vacuum (UHV).7 We have previously investigated the adsorption of simple aromatic molecules such as benzene, naphthalene, and anthracene on well-defined Rh(111), Pt(111),8,9 and Cu(111)10 to elucidate the role of interaction between molecules and substrates in ordering processes on the bare electrodes. For example, the structure of benzene on Rh(111), Pt(111), and Cu(111) unveiled the role played by substrates in the process of ordering of molecular adlayers. In general, highly ordered structures of benzene were formed on Rh(111)8 and Cu(111),10 whereas only local ordering was found on Pt(111).8 More importantly, it was discovered that the adlayer structures of benzene and naphthalene on Rh(111) observed in solutions were similar to those found at solid-gas interfaces, indicating that adsorbate-substrate interaction plays a crucial role in the formation of ordered adlayer structures of those hydrophobic molecules.8,9 On the other hand, we have recently found that intermolecular interactions also play an important role in the formation of the ordered adlayer of benzene derivatives.11 To elucidate the role of intermolecular interactions, * Author for correspondence. (1) Adsorption of Molecules at Metal Electrodes; Lipkowski, J., Ross, P. N., Eds.; VCH Publishers: New York, 1992. (2) Interfacial ElectrochemistrysTheory, Experiment, and Applications; Wieckowski, A., Ed.; Marcel Dekker: New York, 1999. (3) Hubbard, A. T. Chem. Rev. 1988, 88, 633. (4) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; John Wiley & Sons: New York, 1994. (5) Itaya, K. Prog. Surf. Sci. 1998, 58, 121. (6) Gewirth, A. A.; Niece, B. K. Chem. Rev. 1997, 97, 1129. (7) Chiang, S. Chem. Rev. 1997, 97, 1083. (8) Yau, S.-L.; Kim, Y.-G.; Itaya, K. J. Am. Chem. Soc. 1996, 118, 7795. (9) Yau, S.-L.; Kim, Y.-G.; Itaya, K. J. Phys. Chem. B 1997, 101, 3547. (10) Wan, L.-J.; Itaya, K. Langmuir 1997, 13, 7173.
the adsorption of benzoic acid and terephthalic acid was investigated on Pt(111) in 0.1 M HClO4. It was found that benzoic acid formed disordered structures on Pt(111), whereas terephthalic acid formed a highly ordered adlayer with a c(2x3 × 4)rect structure, which was thought to be stabilized by hydrogen bonds between adjacent -COOH groups.11 These results prompted us to investigate adlayer structures of para-substituted benzene derivatives to understand further the effect of intermolecular interactions on the formation of ordered adlayers. Somorjai and co-workers reported results of an STM study of the adsorption of p-xylene on Rh(111) at room temperature in UHV.12 They observed that there were many areas of the monolayer where the p-xylene molecules were disordered and that the ordered domain size was about 7 nm. Their results strongly encouraged us to investigate the adsorption of p-xylene on Rh(111) in aqueous solution. It was also of our great interest to compare the adlayers of p-xylene formed on Rh(111) in solution and in UHV. It is shown in this paper that p-xylene molecules formed long-range ordered adlattices on Rh(111) in HF solution. Ordered domains with three orientations differing by 60° or 120° were clearly discerned. The complementary use of in situ STM and ex situ low-energy electron diffraction (LEED) revealed that the ordered structure of p-xylene formed on Rh(111) in solution can be designated as c(2x3 31 × 4)rect or 1 3 .
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Experimental Method Single-crystal beads of Rh, 3 mm in diameter, were made by the crystallization of a molten ball formed at the end of a Rh wire in a hydrogen-oxygen flame.8,9,13 The (111) facets formed on the single-crystal bead were used for STM measurements. For electrochemical and LEED measurements, a laser beam deflection method was used to determine the orientation of the single crystal bead to expose the (111) plane, which was then mechanically polished with successively finer-grade diamond pastes down to 0.05 µm with an accuracy in the angle of
