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In Situ Scanning Tunneling Microscopy Observation of a Porphyrin Adlayer on an Iodine-Modified Pt(100) Electrode K. Sashikata,† T. Sugata,† M. Sugimasa,† and K. Itaya*,‡ Department of Applied Chemistry, Faculty of Engineering, Tohoku University, Aoba-ku, Sendai 980-77, Japan, and Itaya Electrochemiscopy Project ERATO/JST, Research Institute of Electric and Magnetic Materials, Yagiyama-Minami 2-1-1, Sendai 982, Japan Received December 23, 1997. In Final Form: March 3, 1998 The adlayer structure of 5,10,15,20-tetrakis(N-methylpyridinium-4-yl)-21H,23H-porhine tetrakis(ptoluenesulfonate) (TMPyP) at an iodine-modified Pt(100)(I-Pt(100)) surface was investigated in HClO4 solution by in situ scanning tunneling microscopy (STM). The structure of the iodine adlayer was (x2 × 5x2)R45°. TMPyP molecules were adsorbed with flat-lying orientation at the I-Pt(100)) surface and aligned along the 〈100〉 direction that was parallel to the row of iodine atoms. The van der Waals type interaction between TMPyP and the iodine adlayer seems to be relatively weak because of the characteristic structure of iodine, which allows TMPyP to diffuse on the iodine adlayer on Pt(100) in specific directions to form a highly ordered array. The observed structure of TMPyP was very different from that found previously on I-Au(111), I-Ag(111), and I-Pt(111). A square lattice was found to form on I-Pt(100) with a side-by-side configuration. It is concluded that the structure of the iodine adlayer plays an important role in the formation of the ordered structure.
Introduction The adsorption of organic molecules on electrode surfaces in electrolyte solutions has also long been an important subject in electrochemistry for elucidating the role of molecular properties and the atomic structure of electrode surfaces.1 To understand the adsorption of relatively simple aromatic molecules on various substrates, we have recently used in situ scanning tunneling microscopy (STM) to investigate the adlayer structures of molecules such as benzene, naphthalene, biphenyl, and anthracene on well-defined Rh(111) and Pt(111) electrodes in HF solutions.2-5 Highly ordered adlayers of benzene were observed on Rh(111) and Pt(111), depending on the electrode potential. On the other hand, larger molecules such as anthracene formed disordered adlayers on both the Rh and Pt electrodes, which was attributed to strong adsorbate-substrate interaction.3 More recently, however, it was found to our surprise that these aromatic molecules formed ordered adlayers on Cu(111) in HClO4,6 suggesting that the interaction with the substrate is one of the most important factors determining the adlayer structure of organic molecules. For even larger molecules such as porphyrins, it was found in our previous studies that no ordered adlayer of a water-soluble porphyrin, TMPyP, formed on a welldefined Pt(111) nor on Au(111), whereas iodine-modified * To whom correspondence is addressed. † Tohoku University. ‡ Research Institute of Electric and Magnetic Materials. (1) Lipkowski, J., Ross, P. N., Eds. In Adsorption of Molecules at Metal Electrodes; VCH Publishers: New York, 1992. (2) Yau, S.-L.; Kim, Y.-G.; Itaya, K. J. Am. Chem. Soc. 1996, 118, 7795. (3) Yau, S.-L.; Kim, Y.-G.; Itaya, K. J. Phys. Chem. 1997, 101, 3547. (4) Yau, S.-L.; Kim, Y.-G.; Itaya, K. In Proceedings of the Sixth International Symposium on ELECTRODE PROCESSES VI; Wieckowski, A., Itaya, K., Eds.; The Electrochemical Society: Pennington, 1996; pp 243-256. (5) Yau, S.-L.; Itaya, K. Colloids Surf. A, in press. (6) Wan, L.-J.; Itaya, K. Langmuir, in press.
Au(111) and Ag(111) electrodes were found to be ideal substrates to form ordered adlayers of various large organic molecules.7-11 Although highly ordered pyrolytic graphite (HOPG) and similar layered crystals such as MoS2 have been popular choices as the substrate for the study of adsorbed molecules such as liquid crystals,11 biological molecules,12 and porphrins,13 we believe that the use of iodine-modified electrodes is more appropriate for investigating the role of the substrate in ordering processes of organic molecules. It is well-known that iodine atoms are strongly attached to various metal surfaces and form specific structures depending on the kind of metals such as Pt, Au, Ag, Pd, and Rh.14-16 It was particularly emphasized that the iodine layer on Au(111) played a crucial role in the formation of highly ordered TMPyP arrays.7-11 The relatively weak van der Waals type interaction on the iodine adlayer seemed to be a key factor in the formation of ordered molecular arrays of large molecules. However, the relation between the TMPyP and iodine adlayer structures was not fully understood in our previous work,7-10 because the iodine (7) Kunitake, M.; Batina, N.; Itaya, K. Langmuir 1995, 11, 2337. (8) Batina, N.; Kunitake, M.; Itaya, K. J. Electroanal. Chem. 1996, 405, 245. (9) Ogaki, K.; Batina, N.; Kunitake, M.; Itaya, K. J. Phys. Chem. 1996, 100, 7185. (10) Kunitake, M.; Akiba, U.; Batina, N.; Itaya, K. Langmuir 1997, 13, 1607 and references therein. (11) Itaya, K.; Batina, N.; Kunitake, M.; Ogaki, K.; Kim, Y.-G.; Wan, L.-J.; Yamada, T. In Solid-Liquid Electrochemical Interfaces; Jerkiewicz, G., Wieckowski, A., Uosaki, K., Soriaga, M. P., Eds.; Americal Chemical Society: Washigton, DC, 1996; pp 171-188. (12) Ikai, A. Surf. Sci. Rep. 1996, 26, 261. (13) Tao, N. J.; Cardenas, G.; Cunha, F.; Shi, Z. Langmuir 1995, 11, 4445. (14) Hubbard, A. T. Chem. Rev. 1988, 88, 633. (15) Soriaga, M. P. Prog. Surf. Sci. 1992, 39, 325. (16) Yamada, T.; Batina, N.; Ogaki, K. Okubo, S.; Itaya, K. In Proceedings of the Sixth International Symposium on ELECTRODE PROCESSES VI; Wieckowski, A., Itaya, K., Eds.; The Electrochemical Society: Pennington, 1996; pp 43-57.
S0743-7463(97)01410-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/21/1998
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adlayer structures on Au(111) and Ag(111) are complicated by a potential dependent compression in the adlayers.16-18 In evaluating the structural relationship, we were particularly interested in the iodine adlayer on Pt(100), which is known to form a (x2 × 5x2)R45° structure, because of its characteristic double-row structure.18-24 The iodine atoms in two nearest bright rows along the direction of 〈100〉 are located near on-top sites of Pt(100), and in every third row of iodine atoms, which appears as a dark row, are located on 4-fold hollow sites.21 We expected in this study that such a unique structure might affect the ordering processes and adlayer structures of TMPyP. In this paper, we first discribe a well-defined Pt(100)(1 × 1) structure and the iodine adlayer with the (x2 × 5x2)R45° structure, and then the adsorption of TMPyP on the I-Pt(100). Experimental Methods A single-crystal Pt bead, 3 mm in diameter, was prepared at one end of a Pt wire using a hydrogen-oxygen flame.2 Wellprepared single-crystal beads showed not only eight (111) facets in octahedral configuration as described previously25 but also six (100) facets in hexagonal configuration. Although the (100) facets were substantially smaller than the (111) facets, they could be used directly for the in situ STM study without using a mechanical polishing procedure. Similar to the (111) facet, the (100) facet showed atomically flat terrace-step features. For electrochemical measurements, the Pt(100) surface was exposed by metallographically polishing a Pt bead parallel to the (100) facet, and then it was annealed in a hydrogen-oxygen flame at 1000 °C for several hours to remove surface defects produced during the mechanical polishing. The iodine adlayer was prepared by cooling an annealed sample in iodine vapor. Relatively large domains with the (x2 × 5x2)R45° structure similar to those reported previously could be seen.20,22 The domain size strongly depended on cooling conditions in the iodine vapor. STM measurements were carried out with a Nanoscope E (Digital Instruments) equipped with a custom-made electrochemical cell. STM tips used were made from an electrochemically etched tungsten wire. To minimize residual background currents, the side wall of tips was coated with nail polish. STM observations were performed in 0.1 M HClO4 in the absence and presence of 1 µM TMPyP. All electrode potentials are referred to as a reversible hydrogen electrode (RHE) in 0.1 M HClO4. All chemicals were prepared with Merck ultrapure grade reagents and Milli-Q ultrapure water.
Results and Discussion Well-Defined Pt(100). When a flame-annealed Pt(100) was quenched in water or hydrogen-saturated water, voltammetric features were different from those of a welldefined Pt(100). A set of double peaks was found at ca. 0.2 and 0.3 V as shown previously.26,27 However, a dominant peak was found only at 0.35 V with a small (17) Yamada, T.; Batina, N.; Itaya, K. J. Phys. Chem. 1995, 99, 8817. (18) Yamada, T.; Ogaki, K.; Okubo, S.; Itaya, K. Surf. Sci. 1996, 369, 321. (19) Wieckowski, A.; Rosasco, S. D.; Schardt, B. C.; Stickney, J. L.; Hubbard, A. T. Inorg. Chem. 1984, 23, 565. (20) Vogel, R.; Baltruscat, H. Surf. Sci. Lett. 1991, 259, L739. (21) Vogel, R.; Kamphausen, I.; Baltruschat, H. Ber. Bunsen-Ges. Phys. Chem. 1992, 96, 525. (22) Baltruschat, H.; Bringemeier, U.; Vogel, R. Faraday Discuss. 1992, 94, 317. (23) Albers, J.; Baltruschat, H.; Kamphausen, I. J. Electroanal. Chem. 1995, 395, 99. (24) Vitus, C. M.; Chang, S.-C.; Schardt, B. C.; Weaver, M. J. J. Phys. Chem. 1991, 95, 7559. (25) Itaya, K.; Sugawara, S.; Sashikata, K.; Furuya, N. J. Vac. Sci. Technol. 1990, A8, 515. (26) Clavilier, J.; Armand, D.; Wu, B. L. J. Electroanal. Chem. 1982, 135, 159. (27) Clavilier, J.; Rodes, A.; Achi, K. El.; Zamakhchari, J. Chim. Phys. 1991, 88, 1291.
Figure 1. Cyclic voltammograms of well-ordered Pt(100) in 0.5 M H2SO4 (a) and iodine-modified Pt(100) in 0.1 M HClO4 in the presence (solid line) and the absence (dashed line) of 10 µM TMPyP (b). Scan rate was 10 mV/s.
shoulder at 0.28 V in 0.5 M H2SO4 as shown in Figure 1a when the flame-annealed electrode was cooled in a hydrogen stream and then immersed in a hydrogensaturated pure water. The oxidation of the Pt(100) surface commenced at ca. 0.8 V. The cyclic voltammogram (CV) shown in this figure is consistent with that previously reported by Clavilie et al.27 Figure 1b shows cyclic voltammograms of an I-Pt(100) electrode in 0.1 M HClO4 with (solid line) and without (dashed line) 10 µM TMPyP. As shown in Figure 1a, the formation of the Pt oxide layer at Pt(100) surfaces started from 0.8 V, while the doublelayer region extended beyond 1.2 V in the anodic direction, indicating that the oxide formation was inhibited by the iodine adlayer. The oxidative desorption of the iodine adlayer and Pt oxide formation occurred at potentials more positive than 1.2 V. The cathodic current observed at potentials less positive than 0.2 V should be due to hydrogen adsorption followed by the hydrogen evolution reaction accompanying partial desorption of the iodine adlayer. After the electrode was immersed into 0.1 M HClO4 + 10 µM TMPyP, new reversible peaks appeared at 0.35 V and the peak height increased gradually for 15 min, depending on the concentration of TMPyP. Similar peaks were found for adsorbed TMPyP on I-Au(111) as described previously.10 These peaks can be assigned to the two-electron redox reaction of metal-free TMPyP adsorbed on I-Pt(100).10,28 Note that the reversible peaks were consistently observed even after the electrode was transferred to a 0.1 M HClO4 solution containing no TMPyP, suggesting that TMPyP molecules were irreversibly adsorbed on I-Pt(100) and stable in the pure HClO4 solution. After correction for the background-charging current shown in Figure 1b, the total amount of charge consumed for the surface redox reaction was found to be about 9 µC/cm2, which corresponds to a surface concentration of ca. 2.8 × 1013 molecules/cm2 if the two-electron process is assumed. Similar values of 2.5 × 1013 and 3.7 (28) Forshey, P. A.; Kuwana, T. Inorg. Chem. 1981, 20, 693.
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Figure 2. (a) STM image of well-ordered Pt(100) in an area of 200 nm × 200 nm obtained in 0.1 M HClO4. The sample potential was 0.75 V vs RHE, and the tunneling current 1 nA. (b) High-resolution STM image of Pt(100) in an area of 6 nm × 6 nm. The sample potential was 0.15 V vs RHE, and the tunneling current 26 nA.
× 1013 molecules/cm2 were found on an I-Au(111) by CV and STM, respectively,10 suggesting that the I-Pt(100) was almost close-packed by flat-lying TMPyP molecules. In situ STM revealed that the Pt(100) surfaces prepared by flame annealing followed by slow cooling in a H2 stream as described above exhibit atomically flat terrace-step features. Figure 2a shows a typical STM image acquired in a relatively large area of 200 × 200 nm2 of the Pt(100) surface in 0.1 M HClO4 solution. It is seen that atomically flat terraces extend over several hundred nanometers in width, and straight monatomic height steps are positioned
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mostly along the 〈110〉 direction. Monatomic step lines with zigzag features parallel to the 〈100〉 direction, rotated by 45° from the 〈110〉 direction can also be seen in the image. Atomically flat terrace-step structures were successfully observed in the potential region between 0 and 0.8 V. The atomic image of the Pt(100)-(1 × 1) structure shown in Figure 2b was relatively easily obtained on an atomically flat terrace at a potential near the hydrogen evolution reaction in H2SO4 and also in HClO4. Almost perfectly aligned square lattices can be seen with an interatomic distance of 0.27 nm. Although the Pt(100)-(1 × 1) structure was previously observed in air on a surface in the presence of iodine,24 the result shown in Figure 2b indicates directly, for the first time, that the unreconstructed Pt(100) surface can be exposed by the flameannealing and quenching method. The observation of the (1 × 1) structure in the solutions is consistent with the recent result of X-ray reflectivitiy measurements that no reconstructed structure exists in solution.29 Note that Kolb et al. previously reported reconstruction rows of the Pt(100)-(5 × 20) for a flame-annealed Pt(100) electrode cooled in air which was immersed into 0.1 M H2SO4 at -0.1 V vs SCE.30,31 We also briefly examined a Pt(111) surface cooled in air, but the surface was always atomically rough because of the surface oxidation of Pt in air. This observation suggests that the previously reported reconstruction might not be due to the formation of the (5 × 20) structure. On the other hand, Pt(100) surfaces prepared by flameannealing followed by rapid quenching in water showed many square-shaped islands measuring 10-20 nm, depending on cooling conditions. Step lines of the islands were predominantly parallel to the direction of 〈110〉. However, we have also succeeded in revaling the (1 × 1) structure on these surfaces. No evidence was found for the existence of reconstructed surface in our experiments. Iodine Adlayer Structure. Figure 3 shows a largescale STM image of an I-Pt(100) surface obtained in 0.1 M HClO4. Atomically flat terraces and monatomic height steps are clearly seen. It is interesting to note that almost all steps are now parallel to the 〈100〉 direction rotated by 45° with respect to the atomic rows of Pt(100). This result strongly suggests that surface Pt atoms were rearranged during the iodine vapor treatment. Wide atomically flat terraces with monatomic steps are seen to intersect one another at 90° because of the 4-fold symmetry of the (100) surface. Figure 4a shows a typical example of the high-resolution STM images obtained on the terraces. It is more clearly seen that iodine atoms are aligned along the 〈100〉 direction to form the characteristic double-row structure. Each bright spot corresponds to an iodine atom adsorbed nearly at the on-top position of the substrate Pt atom, while neighboring dark stripes consist of iodine atoms located on 4-fold hollow sites, which is consistent with the finding reported previously.20,24 The nearest distance between iodine atoms along the bright rows was 0.4 nm, and the periodicity perpendicular to the bright iodine rows was 2.0 ( 0.1 nm. These results allowed us to define the unit cell with (x2 × 5x2)R45° symmetry, which is shown in Figure 4a. A hard ball model illustrated in Figure 4b is essenstially the same as that proposed previously.20 The dark circles in Figure 4b correspond to the iodine atoms (29) Tidswell, I. M.; Markovic, N. M.; Ross, P. N. Phys. Rev. Lett. 1993, 71, 1601. (30) Zei, M. S.; Batina, N.; Kolb, D. M. Surf. Sci. Lett. 1994, 306, L519. (31) Kolb, D. M. Prog. Surf. Sci. 1996, 51, 109.
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Figure 3. STM image obtained at iodine-modified Pt(100) in 0.1 M HClO4. The sample potential was 0.6 V vs RHE, the tip potential 0.1 V vs RHE, and the tunneling current 1.0 nA.
located on 4-fold hollow sites of the Pt(100) surface. Note that the (x2 × 5x2)R45° structure was consistently obtained at least in the potential range between 0.3 and 0.7 V in a pure HClO4 solution, while Baltruschat et al. reported that the iodine adlayer on Pt(100) was potentialdependent in a solution containing iodide ions.22 Although we did not observe other structures such as a (x2 × x2)R45°22 under our experimental conditions, we did find some defects such as irregularly formed missing rows of iodine and antiphase boundaries, similar to those found by Vogel and Baltruschat.20 The defects seemed to be formed mainly under iodine-dosing conditions with insufficient supply of iodine vapor. Adsorption of TMPyP. After the atomic resolution shown in Figure 4a was achieved, a TMPyP solution was directly injected into the STM cell, typically at 0.6 V. After the TMPyP solution was injected, the atomic image of the iodine adlayer blurred and then disappeared, and several minutes later, a small ordered domain of adsorbed TMPyP appeared on the surface. The blurred STM image was caused by adsorbed TMPyP molecules having surface mobility. A similar situation was encountered in the TMPyP adsorption on I-Au(111)10 and I-Ag(111)9 at an initial stage as described previously. Small domains of ordered TMPyP adlayers expanded over the entire area of atomically flat terraces, incorporating randomly adsorbed molecules. It was interestingly observed that the ordered domain of the TMPyP adlayer increased in area in the particular direction of 〈100〉, which was parallel to the iodine atomic rows. Figure 5 shows a typical example of STM images acquired after completion of the formation of an ordered adlayer. Each bright spot now corresponds to a flat-lying TMPyP molecule, and they make straight molecular rows along the 〈100〉 direction. Periodically aligned TMPyP molecules were observed not only in atomically flat terraces but also near the step edges of the substrate. It is also recognized that TMPyP molecules form almostsquare lattices with the same molecular orientation. Figure 6 shows a higher resolution STM image of the
Figure 4. High-resolution STM image obtained at iodinemodified Pt(100) in 0.1 M HClO4 (a) and pictorial representation of iodine adlayer (b). The unit cell of the iodine adlayer is shown in (a) as a white rectangule. The sample potential was 0.6 V vs RHE, the tip potential 0.4 V vs RHE, and the tunneling current 6.8 nA.
TMPyP array. The square shape of the bright spots reflect the molecular structure of TMPyP. It is more clearly seen that all TMPyP molecules are aligned with the same orientation and with the side-by-side configuration along
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Figure 5. STM image of TMPyP adlayer on iodine-modified Pt(100) in 0.1 M HClO4 + 1µM TMPyP. The sample potential was 0.6 V vs RHE, tip potential was 0.13 V vs RHE, and tunneling current was 1 nA.
Figure 6. High-resolution STM image of the TMPyP adlayer on iodine-modified Pt(100) in 0.1 M HClO4 + 1 µM TMPyP. The sample potential was 0.6 V vs RHE, the tip potential 0.13 V vs RHE, and the tunneling current 1 nA.
the molecular rows in the direction of 〈100〉. It is also important to note that a similar side-by-side configuration can be seen along the other 〈100〉 direction perpendicular to the direction shown by the arrow sign in Figure 6. The nearest-neighbor distance between TMPyP molecules along the 〈100〉 direction was ca. 1.9 ((0.05) nm, which is almost identical to that found for an intermediate structure of TMPyP with the side-by-side configuration formed on I-Au(111).10 It is slightly larger than 0.17 nm,
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Figure 7. Composite STM image of iodine and TMPyP adlayers on Pt(100) surface in 0.1 M HClO4 + 1 µM TMPyP. The upper part of the composite image was obtained at 0.6 V vs RHE with the tip potential of 0.2 V vs RHE, and the tunneling current of 1 nA. The lower part of the image was recorded after increasing the tunneling current from 1 to 40 nA in the middle of the scan.
which is the corresponding value for TMPyP molecules adsorbed on I-Ag(111).9 It can be seen that the distance between neighboring molecular rows is ca. 1.9 nm, which seems to be the same periodicity as that for the long side of the (x2 × 5x2)R45° unit cell shown in Figure 4b, indicating that there is a strong correlation between the adlayers of iodine and that of TMPyP. TMPyP molecules are thought to be adsorbed on specific sites on the iodine adlayer. All TMPyP molecules appear with the same corrugation height of ca. 0.13 nm, indicating that they are located on equivalent binding sites. The TMPyP molecules were usually observable with the tunneling current of 1-5 nA. Imaging with a tunneling current greater than 30 nA usually revealed the underlying iodine adlayer, as demonstrated previously for TMPyP on I-Ag(111).9 When the tunneling current was changed from 1 to 40 nA during the imaging in the same area, a composite image of the underlying iodine and TMPyP adlayers was obtained as shown in Figure 7. The arrow sign on the right-hand side of the STM image indicates the position where the tunneling current was abruptly changed. The upper part of Figure 7 shows the TMPyP adlayer, while the lower part reveals the iodine adlattice with the (x2 × 5x2)R45° structure underneath the TMPyP adlayer. The increase in the tunneling current in the lower part of the image did not disturb the arrangement of the adsorbed TMPyP molecules, as was confirmed by the fact that only molecular images with the same configuration were obtained in the image subsequently acquired using a lower tunneling current of ca. 1 nA. It can be seen from the composite image that N-methylpyridinium moieties are located over the bright iodine atomic rows. The dark stripes seen in the lower part of the STM image in Figure 7, resulting from the iodine row located on the 4-fold hollow sites, are seen to go through the center of each molecule. A few irregularly arranged dark stripes can also be seen because of defects
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Figure 8. Illustrative depiction of the structure of the TMPyP adlayer on iodine modified Pt(100).
in the iodine adlayer. The result shown in Figure 7 strongly suggests that the center of each porphyrin moiety should be located above the dark row of iodine atoms. Such a highly ordered molecular arrangement in both directions seems to be due to the specific structure of the iodine adlayer on Pt(100). On the basis of the results described above, a model structure of TMPyP on the (x2 × 5x2)R45° iodine adlayer was constructed as shown in Figure 8. In this model, the center of each porphyrin ring is located on top of an iodine atom in the dark row. All pyridinium rings are located on the iodine atoms in the bright iodine rows. The length of the square unit cell (a ) b) is calculated to be 1.97 nm, which is very close to the value obtained experimentally as described above. Note that the side-by-side configuration causes a stronger repulsive interaction between the nearest positively charged N-pyridinium groups. A similar side-by-side configuration was observed for TMPyP adsorbed on I-Au(111) at an intermediate stage. This configuration was converted to other structures presumably to minimize the repulsive interaction.10 In the present case, the interaction between TMPyP molecules and the iodine adlayer might be weaker than that on I-Au(111) or I-Ag(111), because a part of the porphyrin ring is not
directly attached to iodine atoms in the dark row. Under these circumstances, we believe that the ion pairing with perchlorate anions plays a crucial role in determining the adlayer structure. Particulary, the side-by-side configuration in the direction b in Figure 8 can only be explained by the coordination of perchlorate anions. The surface coverage and the surface concentration of TMPyP can be calculated to be 0.02 and 2.77 × 1013 molecules/cm2, respectively, based on the model shown in Figure 8. The latter value is surprisingly consistent with that obtained from the CV shown in Figure 1b as described above. A larger value of 3.7 × 1013 molecules/cm2 was obtained for TMPyP on I-Au(111) as described in our previous paper,10 indicating that the TMPyP adlayer has an open-spaced structure on I-Pt(100). Finally, it should be mentioned that we have also investigated the adsorption of TMPyP on an iodinemodified Pt(111). The surface of I-Pt(111) was almost completely covered by flat-lying TMPyP molecules. Each individual TMPyP molecule could be recognized as the characteristic square shape. However, the adsorbed TMPyP molecules did not form an ordered array on the I-Pt(111).11 The surface diffusion of adsorbed TMPyP molecules was rather slow on I-Pt(111). These results
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further support that the structure of the iodine layer on Pt(100) plays an important role in the formation process of the highly ordered TMPyP adlayer. Conclusions In situ STM observation was carried out to determine adlayer structures of TMPyP on the I-Pt(100) surface in HClO4 solution under electrochemical conditions. It was revealed that TMPyP formed an ordered structure with alignment along the iodine atomic row on I-Pt(100). This result clearly indicates that the adlayer structure of TMPyP is controlled by the interaction between the iodine adlayer and TMPyP. The adlayer structure of TMPyP on
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I-Pt(100) is totally different from that observed on I-Au(111) or I-Ag(111). This difference is primarily due to different atomic arrangements of iodine atoms. TMPyP molecules align not only in the direction parallel to the iodine atomic rows but also in the neighboring molecular rows with the same side-by-side-configuration. Such a highly ordered molecular arrangement in both directions seems to result from the stabilization of the repulsive force between methylpyridinium groups by perchlorate anions in solution. The effect of anions is of special interest to us. LA971410C
