J. Phys. Chem. C 2007, 111, 8135
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COMMENTS Comment on “The Origin of Surface Stress Induced by Adsorption of Iodine on Gold” Alexandre Tkatchenko* Departamento de Quimica, UniVersidad Autonoma Metropolitana Iztapalapa, AV. San Rafael Atlixco No. 186, Col. Vicentina, Del. Iztapalapa, C.P. 09340, Mexico D.F., Mexico ReceiVed: January 3, 2007; In Final Form: April 23, 2007 In a recent paper published in this journal, Evans and Craig analyzed the surface stress induced by the adsorption of iodine onto a gold(111) surface by an experimental cantilever technique.1 One of the most important conclusions of their work (as judged by the abstract and the concluding section) is an explanation of the experimentally determined surface stress by a simple theoretical model for lateral interaction potential in the adsorbed iodine monolayer. The authors claim to obtain a good (almost perfect) agreement between the experimental and the theoretical surface stress. In my opinion, however, their use of a Lennard-Jones expression for the lateral interaction between adsorbed atoms constitutes a large oversimplification. Namely, the authors seem to overlook a large body of work on lateral interactions in adsorbed monolayers that has been performed over the last few decades (see, e.g., refs 2-3). They do not even cite a paper by Wang et al., which focuses on the lateral potential of the I-Au(111) system, that is highly relevant to their research and that was published in J. Phys. Chem. B several years ago.4 Furthermore, the I-Au(111) system, employed for the surface stress investigation in the commented paper, is a very well studied system using a wealth of different experimental techniques, yet the authors omit references to the majority of these studies (see ref 5 for a recent review). Evans and Craig1 propose that upon adsorption from the gas phase, the iodine molecule is physisorbed onto the gold(111) surface, and it then dissociates to form two chemisorbed iodine atoms. The authors go on to propose that a Lennard-Jones potential can be employed to predict the lateral interaction energy difference between the iodine atoms in the gas phase and those adsorbed onto the gold(111) surface. However, as has been shown in numerous studies,2,3 the interaction potential between atoms that are chemisorbed onto surfaces is largely modified from the corresponding gas-phase potential. This change is especially noticeable for halogens on metal surfaces, as evidenced by the highly expanded monolayer lattice constants.5 Namely, the saturation coverage (the maximum compressibility) of the iodine monolayer on the Au(111) surface is * E-mail:
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Θ ) 0.45, which gives an iodine-iodine bond length of 4.3 Å in the monolayer.6 The corresponding iodine-iodine distance in the gas phase is merely 2.6 Å. The authors comment on this fact in their paper; however, they later neglect it by assuming the same interaction potential for both iodine phases (gas and monolayer). For instance, it has been shown that the effective atom-atom interaction is repulsive for a large variation of iodine monolayer coverage for a closely related I-Pt(111) system, both experimentally7 and theoretically.8 In particular, for the I-Au(111) system at Θ ) 0.45, it means that the total lateral interaction energy is repulsive with the value on the order of 10-15 kJ/mol.8 Furthermore, the iodine monolayer on the Au(111) surface can form a large variety of structures, from the commensurate (x3×x3)R30°, to uniaxially compressed c(p×x3)R30° with variable p, to rotated hexagonal ones. The formation of these monolayers has been attributed to a competition between the complex atom-surface and atom-atom interactions.9 Nevertheless, the authors do not comment on this fact, and consequently, it is not clear in the paper which monolayer symmetry the iodine possesses (i.e., no surface structure analysis was performed). Finally, to obtain the hexagonal monolayer interaction energy, the authors multiply the pairwise interaction potential by 6 instead of the correct value of 3 (to avoid double counting for an atom i interacting with atom j). In summary, I believe that the theoretical method used by Evans and Craig in their paper1 contains several inaccuracies, and it cannot be used for direct comparison with the employed experimental technique, which severely weakens one of their main conclusions. Therefore, I would appreciate if the authors could revise their model in light of my comments and carefully consider the influence of the surface structure and the repulsive lateral interactions on the surface stress in the I-Au(111) system. References and Notes (1) Evans, D. R.; Craig, V. S. J. J. Phys. Chem. B 2006, 110, 1950719514. (2) Feibelman, P. J. Annu. ReV. Phys. Chem. 1989, 40, 261-290. (3) Einstein, T. L. In Physical Structure of Solid Surfaces; Unertl, W. N. Ed.; Elsevier: Amsterdam, 1996; Vol. 1, pp 577-650. (4) Wang, X.; Chen, R.; Wang, Y.; He, T.; Liu, F.-C. J. Phys. Chem. B 1998, 102, 7568-7576. (5) Magnussen, O. M. Chem. ReV. 2002, 102, 679-725. (6) Ocko, B. M.; Watson, G. M.; Wang, J. J. Phys. Chem. 1994, 98, 897-906. (7) Labayen, M.; Furman, S. A.; Harrington, D. A. Surf. Sci. 2003, 525, 149-158. (8) Tkatchenko, A.; Batina, N.; Galva´n, M. Phys. ReV. Lett. 2006, 97, 036102. (9) Tkatchenko, A.; Batina, N. J. Phys. Chem. B 2005, 109, 2171021715.
10.1021/jp070046f CCC: $37.00 © 2007 American Chemical Society Published on Web 05/12/2007
