Adlayer Structure and Electrochemical Reduction of O2 on Self

Adlayer Structure and Electrochemical Reduction of O2 on Self-Organized ... Langmuir , 2003, 19 (3), pp 672–677 ... Adlayers of 5,10,15,20-tetraphen...
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Langmuir 2003, 19, 672-677

Adlayer Structure and Electrochemical Reduction of O2 on Self-Organized Arrays of Cobalt and Copper Tetraphenyl Porphines on a Au(111) Surface Soichiro Yoshimoto, Akinori Tada, Koji Suto, Ryuji Narita, and Kingo Itaya* Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, Aoba-yama 04, Sendai 980-8579, Japan Received August 21, 2002. In Final Form: November 12, 2002 Adlayers of 5,10,15,20-tetraphenyl-21H,23H-porphine cobalt(II) and copper(II) (CoTPP and CuTPP) formed on a Au(111) electrode by immersion into a benzene solution containing either CoTPP or CuTPP were investigated in 0.1 M HClO4 by cyclic voltammetry and in situ scanning tunneling microscopy (STM). Highly ordered arrays of CoTPP and CuTPP molecules were observed on the Au(111) surface. Highresolution STM images revealed the characteristic shape, internal molecular structure, and molecular orientation of individual CoTPP and CuTPP molecules in ordered arrays. The adlayers of CoTPP and CuTPP formed on Au(111) were in an identical packing arrangement with the same orientation. The center cobalt ion in CoTPP appeared as the brightest spot in the STM image, whereas the copper ion in CuTPP was dark. On the adlayers of highly ordered CoTPP and CuTPP molecules, O2 reduction was carried out in 0.1 M HClO4 saturated with O2. Voltammetric results indicated that CuTPP did not catalyze the reaction, whereas the highly ordered CoTPP adlayer on Au(111) enhanced the two-electron reduction of O2 to H2O2.

Introduction Formation and characterization of ordered adlayers of porphyrin molecules at electrolyte-electrode interfaces are important from the fundamental and technological points of view. Porphyrins are of great interest in such diversified fields as biology,1 photosynthesis,1 electrocatalysis,2 and molecular devices.3 In electrochemistry, thin films of porphyrins have been intensively studied because of interest in electrocatalytic reactions, such as the reduction of O2 for developing efficient fuel cells.2,4-6 For electrochemical catalytic reduction of O2, the reactivity has been investigated on graphite electrodes modified with various metalloporphyrins and metallophthalocyanines (MPc’s).2,4-6 However, not much attention has been paid so far to the adlayer structure of those porphyrins. Adlayer structures of porphyrins have been studied in the past mostly in ultrahigh vacuum (UHV) using scanning tunneling microscopy (STM) on various metal surfaces.7-12 Lippel et al. reported the first STM images of a copper phthalocyanine (CuPc) adlayer on a Cu(100) surface.7 Gimzewski and co-workers investigated adlayer structures * To whom correspondence should be addressed. Phone/Fax: +81-22-214-5380. E-mail: [email protected]. (1) Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH: New York, 2001; Vol. 3. (2) Collman, J. P.; Wagenknecht, P. S.; Hutchison, J. E. Angew. Chem., Int. Ed. 1994, 33, 1537 and references therein. (3) Molecular Electronics; Jortner, J., Ratner, M., Eds.; IUPAC: Oxford, 1997. (4) (a) Shigehara, K.; Anson, F. C. J. Phys. Chem. 1982, 86, 2776. (b) Shi, C.; Steiger, B.; Yuasa, M.; Anson, F. C. Inorg. Chem. 1997, 36, 4294. (c) Shi, C.; Anson, F. C. Inorg. Chem. 1998, 37, 1037. (d) Song, E.; Shi, C.; Anson, F. C. Langmuir 1998, 14, 4315. (5) (a) Kuwana, T.; Fujihira, M.; Sunakawa, K.; Osa, T. J. Electroanal. Chem. 1978, 88, 299. (b) Kobayashi, N.; Matsue, T.; Fujihira, M.; Osa, T. J. Electroanal. Chem. 1979, 103, 427. (6) Forshey, P. A.; Kuwana, T. Inorg. Chem. 1983, 22, 699. (7) Lippel, P. H.; Wilson, R. J.; Miller, M. D.; Wo¨ll, C.; Chiang, S. Phys. Rev. Lett. 1989, 62, 171. (8) (a) Jung, T. A.; Schlittler, R. R.; Gimzewski, J. K.; Tang, H.; Joachim, C. Science 1996, 271, 181. (b) Jung, T. A.; Schlittler, R. R.; Gimzewski, J. K. Nature 1997, 386, 696. (9) Yokoyama, T.; Yokoyama, S.; Kamikado, T.; Okuno, Y.; Mashiko, S. Nature 2001, 413, 619.

of 5,10,15,20-tetrakis(3,5-di-tertiarybutylphenyl) porphine copper(II) (CuTBPP) on Cu(100), Au(110), and Ag(110) surfaces in UHV.8 It was found that the packing arrangement of CuTBPP depends on the metal substrates used.8 Yokoyama et al. found that the CN-substituted TBPP molecules adsorbed on Au(111) formed supermolecular assemblies such as monomers, trimers, tetramers, and extended wirelike structures.9 Hipps and co-workers reported various MPc’s (M: Cu,10a,b Co,10a,b Ni,10c Fe,10c VO10d) and metal(II) tetraphenyl-21H,23H-porphines (MTPPs)12 on reconstructed Au(111). The brightness of the center spot of Pc or TPP depended on the active center metal. The difference in contrast between the metal ions in STM images was explained in terms of the occupation of the dz2 orbital.10,12 Recently, STM has been widely accepted as a powerful tool for understanding the structure of adlayers of molecules in solution,13-15 and porphyrin layers have also been observed in solution.13,15-20 We have previously reported that highly ordered arrays of water-soluble (10) (a) Lu, X.; Hipps, K. W.; Wang, X. D.; Mazur, U. J. Am. Chem. Soc. 1996, 118, 7197. (b) Hipps, K. W.; Lu, X.; Wang, X. D.; Mazur, U. J. Phys. Chem. 1996, 100, 11207. (c) Lu, X.; Hipps, K. W. J. Phys. Chem. B 1997, 101, 5391. (d) Barlow, D. E.; Hipps, K. W. J. Phys. Chem. B 2000, 104, 5993. (11) Chizhov, I.; Scoles, G.; Kahn, A. Langmuir 2000, 16, 4358. (12) (a) Scudiero, L.; Barlow, D. E.; Hipps, K. W. J. Phys. Chem. B 2000, 104, 11899. (b) Scudiero, L.; Barlow, D. E.; Mazur, U.; Hipps, K. W. J. Am. Chem. Soc. 2001, 123, 4073. (13) Itaya, K. Prog. Surf. Sci. 1998, 58, 121. (14) Gewirth, A. A.; Niece, B. K. Chem. Rev. 1997, 97, 1129. (15) (a) Tao, N. J.; Cardenas, G.; Cunha, F.; Shi, Z. Langmuir 1995, 11, 4445. (b) Tao, N. J. Phys. Rev. Lett. 1996, 76, 4066. (c) Tao, N. J.; Li, C. Z.; He, H. X. J. Electroanal. Chem. 2000, 492, 81. (16) Batina, N.; Kunitake, M.; Itaya, K. J. Electroanal. Chem. 1996, 405, 245. (17) (a) Kunitake, M.; Batina, N.; Itaya, K. Langmuir 1995, 11, 2337. (b) Kunitake, M.; Akiba, U.; Batina, N.; Itaya, K. Langmuir 1997, 13, 1607. (18) Ogaki, K.; Batina, N.; Kunitake, M.; Itaya, K. J. Phys. Chem. 1996, 100, 7185. (19) Sashikata, K.; Sugata, T.; Sugimasa, M.; Itaya, K. Langmuir 1998, 14, 2896. (20) Wan, L.-J.; Shundo, S.; Inukai, J.; Itaya, K. Langmuir 2000, 16, 2164.

10.1021/la026449i CCC: $25.00 © 2003 American Chemical Society Published on Web 01/09/2003

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Chart 1. Chemical Formula of 5,10,15,20-Tetraphenyl-21H,23-H-porphine Metal(II) (MTPP, M ) Co or Cu)

Figure 1. Cyclic voltammograms of bare (dashed line) and CoTPP-adsorbed (solid line) Au(111) electrodes in 0.1 M HClO4 under a N2 atmosphere. The potential scan rate was 50 mV s-1.

5,10,15,20-tetrakis(N-methylpyridinium-4-yl)-21H,23Hporphine (H2TMPyP) molecules were formed on iodinemodified (I-) Au(111),16,17 I-Ag(111),18 I-Pt(100),19 and sulfur-modified (S-) Au(111) electrodes.20 Besides the information concerning the internal molecular structure, our STM images revealed the symmetry and the structure of the H2TMPyP array. However, it has been difficult to investigate adlayer structures of water-insoluble molecules such as TPP and Pc. There are some recent efforts to prepare adlayers of water-insoluble porphyrins having thiol moieties chemisorbed onto Au surfaces21,22 and those of coordinated 4-pyridinethiol monolayers on Au.23,24 However, the adlayer structures of those molecules were not characterized at the molecular level. We have very recently succeeded in the preparation of highly ordered adlayers of waterinsoluble organic molecules such as coronene,25 fullerene monomer (C60) and [2 + 2] C60 dimer (C120)26 by dissolving those molecules in pure benzene. Since benzene does not adsorb on the Au(111) surface, adlayers of water-insoluble aromatic molecules were formed without interference of the adsorption of benzene.25b This method for the preparation of ordered adlayers using benzene solutions is also expected to be applicable to the investigation of adlayers of porphyrin molecules on Au(111). In the present paper, we describe highly ordered adlayers of CoTPP and CuTPP (Chart 1) directly attached to Au(111) in 0.1 M HClO4. The CoTPP- or CuTPPadsorbed Au(111) was prepared by immersion into a benzene solution containing either CoTPP or CuTPP molecules. The internal structure and packing arrangement of CoTPP and CuTPP on Au(111) were determined in 0.1 M HClO4 by high-resolution STM. It was also found that the CoTPP-adsorbed Au(111) electrode exhibited electrocatalytic activity for the reduction of O2 to H2O2. Experimental Section CoTPP and CuTPP were purchased from Aldrich and used without further purification. Benzene was obtained from Kanto (21) Zak, J.; Yuan, H.; Ho, M.; Woo, L. K.; Porter, M. D. Langmuir 1993, 9, 2772. (22) (a) Hutchison, J. E.; Postlethwaite, T. A.; Murray, R. W. Langmuir 1993, 9, 3277. (b) Postlethwaite, T. A.; Hutchison, J. E.; Hathcock, K. W.; Murray, R. W. Langmuir 1995, 11, 4109. (c) Hutchison, J. E.; Postlethwaite, T. A.; Chen, C.-H.; Hathcock, K. W.; Ingram, R. S.; Ou, W.; Linton, R. W.; Murray, R. W.; Tyvoll, D. A.; Chng, L. L.; Collman, J. P. Langmuir 1997, 13, 2143. (23) Zhang, Z.; Hou, S.; Zhu, Z.; Liu, Z. Langmuir 2000, 16, 537. (24) Kanayama, N.; Kanbara, T.; Kitano, H. J. Phys. Chem. B 2000, 104, 271. (25) (a) Yoshimoto, S.; Narita, R.; Itaya, K. Chem. Lett. 2002, 356. (b) Yoshimoto, S.; Narita, R.; Wakisaka, M.; Itaya, K. J. Electroanal. Chem. 2002, 532, 331. (26) Yoshimoto, S.; Narita, R.; Tsutsumi, E.; Matsumoto, M.; Itaya, K.; Ito, O.; Fujiwara, K.; Murata, Y.; Komatsu, K. Langmuir 2002, 18, 8518.

Chemical Co. (spectroscopy grade). The electrolyte solution was prepared with HClO4 (Cica-Merck) and ultrapure water (Milli-Q SP-TOC; g18.2 MΩ cm). The Au(111) single-crystal electrode was prepared by the Clavilier method.27 The CoTPP adlayer was formed by immersing the Au(111) electrode into ca. 50-100 µM CoTPP-benzene solution for 10-20 s, after annealing in a hydrogen flame and quenching into ultrapure water saturated with hydrogen.28 The CoTPP-adsorbed Au(111) electrode was then rinsed with ultrapure water and transferred into an electrochemical or STM cell for voltammetric and STM measurements. The same preparation procedure was used to prepare CuTPP adlayers on well-defined Au(111) electrodes. Cyclic voltammetry was carried out in 0.1 M HClO4 under N2 at 20 °C using a potentiostat (HOKUTO HAB-151, Tokyo) and the hanging meniscus method in a three-compartment electrochemical cell. The catalytic activity of the CoTPP- and CuTPPmodified Au(111) electrodes for the reduction of O2 was examined in 0.1 M HClO4 saturated with O2 using the hanging meniscus rotating disk method29 with a model 636 ring-disk system (EG&G). Electrochemical STM measurements were performed by using a Nanoscope E (Digital Instruments, Santa Barbara, CA) with a tungsten tip etched in 1 M KOH. To minimize residual faradic currents, the tip was coated with nail polish. STM images were recorded in the constant-current mode. All potential values are referred to the reversible hydrogen electrode (RHE).

Results and Discussion Voltammetry. Figure 1 shows cyclic voltammograms (CVs) of bare and CoTPP-adsorbed Au(111) electrodes in 0.1 M HClO4 recorded at a scan rate of 50 mV s-1. The voltammogram for the bare Au(111) (dashed line) in the double-layer potential region is identical to that reported previously,25b which shows that a well-defined Au(111) surface was exposed to the HClO4 solution. The CV profile of a Au(111) electrode immersed in pure benzene for 1060 s and rinsed by ultrapure water was essentially the same as that observed on a clean Au(111) electrode.25b This result indicates that benzene did not adsorb on the Au(111) surface in the present experimental conditions. The solid line in Figure 1 is the CV obtained in the first scan of a CoTPP-adsorbed Au(111) electrode. The open circuit potential (OCP) of the CoTPP-adsorbed Au(111) electrode was around 0.7-0.85 V, and the potential scan was started in the negative direction from the OCP. The effect of the adsorption of CoTPP was clearly observed in the double-layer charging current. The decrease in the double-layer charging current in Figure 1 suggests that the Au(111) surface is covered with hydrophobic CoTPP molecules. The repetitive potential cycling between 0.05 (27) Clavilier, J.; Faure, R.; Guinet, G.; Durand, R. J. Electroanal. Chem. 1980, 107, 205. (28) Honbo, H.; Sugawara, S.; Itaya, K. Anal. Chem. 1990, 62, 2424. (29) Abe, T.; Swain, G. M.; Sashikata, K.; Itaya, K. J. Electroanal. Chem. 1995, 382, 73.

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and 0.9 V caused no change in the CV profile, suggesting that the CoTPP adlayer on Au(111) is stable. In Figure 1 (solid line), a reductive peak is seen at 0.32 V during the cathodic scan. The increase in cathodic current commencing at -0.05 V is due to the H2 evolution reaction. An oxidation peak broader than that of the cathodic peak is seen in the potential range between 0.3 and 0.7 V during the anodic scan. Murray and co-workers investigated a self-assembled monolayer of a CoTPP derivative having four thiol moieties, 5,10,15,20-tetrakis(o-(2-mercaptoethoxy)phenyl) porphyrin (Co(o-TMEPP)), on evaporated Au films.22b The CV profile obtained in the present study was similar to that of the previous study.22b According to the reports by Murray’s group, the Co(o-TMEPP)-modified Au electrode exhibited the redox wave of the Co(III/II) couple at ca. 0.15 V versus a saturated calomel electrode (SCE) in 1 M HClO4 at a scan rate of 50 mV s-1. Even in a nominally degassed acid solution, Murray and coworkers found that residual oxygen fully oxidized the CoII(o-TMEPP) monolayer formed on Au within 10 s.22b In our case, the oxidation to Co(III) probably took place on the CoTPP-adsorbed Au(111) electrode at the OCP in 0.1 M HClO4, because the prepared electrode was transferred into the electrochemical cell after washing by undegassed ultrapure water. The cathodic peak observed at 0.32 V, therefore, is attributed to the reduction of Co(III) to Co(II). The total cathodic charge density consumed for the peak at 0.32 V was calculated to be 8.7 ( 0.3 µC cm-2, which is equivalent to (9.0 ( 0.3) × 10-11 mol cm-2. This value and the size of the CoTPP molecule indicate that CoTPP should form a monolayer on the Au(111) surface. The value obtained in this work is close to that estimated from the Co(o-TMEPP) chemisorbed on Au.22b In Situ STM. (1) CoTPP Array. Figure 2 shows typical STM images of a CoTPP adlayer on Au(111) acquired at 0.85 V (near the OCP) in 0.1 M HClO4. In the image for the relatively large area of 50 × 50 nm2 in Figure 2a, it is clear that atomically flat terraces are extended with steps of monatomic height. It is seen that the atomically flat terraces of the Au(111) surface are completely covered with highly ordered CoTPP molecules. Each CoTPP molecule can be clearly recognized as a bright spot not only on the atomically flat terraces but also near the steps. Some defects missing molecules are seen as dark spots in the ordered domain. Furthermore, reconstruction of the Au(111) surface can be seen over a large area. Careful inspection of the Au(111) surface reveals rows with spacings ranging from ca. 6.3 to 9.1 nm on the terrace, as indicated by arrows in Figure 2a. The difference in height is only ca. 0.05-0.07 nm, which is much smaller than that of the atomic steps. These results reveal that the Au(111) surface was reconstructed from (1 × 1) upon adsorption of CoTPP. We also carried out the adsorption of CoTPP onto a thermally reconstructed Au(111) surface. The image obtained was essentially the same as that shown in Figure 2a. In general, the Au(111) surface shows a (1 × 1) structure after adsorption of an organic compound such as coronene25 and fullerenes26 near the OCP. It was surprising that the reconstruction of the Au(111) surface was induced by the adsorption of CoTPP during immersion in the benzene solution and that the reconstruction was retained even in the aqueous solution at potentials near OCP. Kolb and co-workers reported that the adsorption of coumarin stabilizes the reconstructed Au(100) surface in aqueous solution.30 Like coumarin, highly ordered CoTPP arrays might have protected the specific adsorption (30) (a) Skoluda, P.; Hamm, U. W.; Kolb, D. M. J. Electroanal. Chem. 1993, 354, 289. (b) Ho¨lzle, M. H.; Kolb, D. M. Ber. Bunsen-Ges. Phys. Chem. 1994, 98, 330.

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Figure 2. Large-scale (50 × 50 nm2) (a) and high-resolution (20 × 20 nm2) (b) STM images of the CoTPP adlayer on the Au(111) surface in 0.1 M HClO4, acquired at 0.85 V versus RHE. The potential of the tip and the tunneling currents were 0.35 V and 2.0 nA (a) and 1.25 nA (b), respectively. The set of three arrows in (a) indicates the close-packed directions of the Au(111) substrate.

of anions. The reason for the reconstruction being induced by the adsorption of CoTPP is not clear, but the following mechanism may be proposed: the adsorbed CoTPP induces a negative charge on the Au(111) surface so that the potential of zero charge (Epzc) of the CoTPP-adsorbed Au(111) shifts to a value more positive than that of bare Au(111) (0.24 V vs SCE) in 0.01 M HClO4.31 Figure 2b shows an STM image acquired on an atomically flat terrace in an area of 20 × 20 nm2, revealing internal molecular structures and orientations in the ordered adlayer. It is clear that all molecules are oriented in the same direction on Au(111). In the ordered domains, some CoTPP molecules are seen to be missing as indicated by the white arrow in Figure 2b. To clarify structural details of the CoTPP adlayer formed on the Au(111) surface, a high-resolution STM image shown in Figure 3 was acquired at 0.85 V. An individual CoTPP molecule can be recognized in Figure 3a as a propeller-shaped image with the brightest spot at the center and four additional bright spots at the corners of (31) Kolb, D. M.; Schneider, J. Electrochim. Acta 1986, 31, 929.

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Figure 4. Structural model for the CoTPP adlayer formed on the Au(111) surface.

Figure 3. High-resolution STM image (9 × 9 nm2) (a) and height-shaded plot (b) of the CoTPP adlayer formed on the Au(111) surface in 0.1 M HClO4, acquired at 0.85 V versus RHE. The potential of the tip and the tunneling current were 0.35 V and 1.25 nA, respectively.

each CoTPP molecule. The brightest spot at the center and the four additional spots of each CoTPP molecule can be attributed, respectively, to the cobalt ion and the phenyl moieties in the CoTPP molecule with a flat-lying orientation. Similar bright central spots have been reported also for CoPc10a,b and CoTPP12 molecules in UHV. A heightshaded view of the high-resolution STM image is shown in Figure 3b. The shape of the features in the image is reminiscent of the structure of the CoTPP molecule shown in Chart 1. The high resolution achieved in solution is comparable to or even better than that of the molecular images of CoTPP obtained in UHV.12 As can be seen in Figure 3a,b, the nearly close-packed molecular array is two-dimensionally well organized. The molecular rows consist of flat-lying CoTPP molecules on the surface. The intermolecular distance was found to be 1.41 ( 0.03 nm. Molecular rows, which are marked by arrows I and II in Figure 3a, cross each other at an angle of approximately 85°. It is clear that all CoTPP molecules possess the same orientation in both rows. The packing arrangement of CoTPP on Au(111) in Figure 3 was similar to that

of H2TMPyP adlayers formed on I-Ag(111)18 and S-Au(111)20 surfaces, although lattice parameters were different.18,20 The unit cell is superimposed in Figure 3a. Each unit cell includes one CoTPP molecule, which leads to a surface concentration of ca. 8.4 × 10-11 mol cm-2. The surface concentration estimated from the STM image is in fair agreement with that obtained by the voltammetric experiment in Figure 1. The observed packing arrangement of CoTPP was also consistent with that obtained in UHV by Scudiero et al.12 A structural model for the CoTPP lattice is shown in Figure 4. The exact relation between the CoTPP adlayer and the underlying Au(111) lattice could not be determined in the present study. No potential-dependent structural change was observed in the potential range between 0 and 0.9 V, and an identical structure was consistently observed. When the potential was held at a value more negative than -0.1 V, the wellordered molecular array of CoTPP gradually disappeared and only a reconstructed Au(111) surface was observed. This result suggests that CoTPP molecules are either highly mobile on the Au(111) surface or desorbed from the Au(111) surface at negative potentials. (2) CuTPP Array. Investigations were carried out for CuTPP on the Au(111) surface using the same technique as that described above. Figure 5 shows typical STM images of a CuTPP adlayer on Au(111) acquired at 0.8 V in 0.1 M HClO4. A highly ordered molecular array of CuTPP was observed consistently over the wide terraces as shown in Figure 5a. It is clearly seen that the Au(111) surface is reconstructed as discussed in the previous section. In this STM image, molecular features of each CuTPP can be recognized on atomically flat terraces even in the relatively large area of 50 × 50 nm2. Figure 5b shows a typical high-resolution STM image acquired in an area of 10 × 10 nm2. The molecular rows consisted of flat-lying CuTPP molecules with the same orientation, crossing each other at an angle of approximately 85° as was described for CoTPP in the previous section. The intermolecular spacings between CuTPP molecules were found to be ca. 1.41 nm, indicating that the adlayer structure of CuTPP is identical to that of CoTPP. We can therefore conclude that the adlayer structure of CuTPP on Au(111) is essentially the same as that of CoTPP. The highly ordered square arrangement of CuTPP molecules

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Figure 6. Cyclic voltammograms for the O2 reduction of bare (a) and CoTPP-adsorbed (b) Au(111) electrodes in 0.1 M HClO4 saturated with O2. The potential scan rate was 50 mV s-1.

Figure 5. Large-scale (50 × 50 nm2) (a) and high-resolution (10 × 10 nm2) (b) STM images of the CuTPP adlayer on the Au(111) surface in 0.1 M HClO4, acquired at 0.8 V vs RHE. The potential of the tip and the tunneling currents were 0.35 V and 10 nA (a) and 15 nA (b), respectively.

was consistently observed in the potential range between 0 and 0.9 V. An individual CuTPP molecule can be recognized as a propeller-shaped image with four additional bright spots at the corners. These bright spots can be attributed to the phenyl moieties in the CuTPP molecule. In contrast, the center of each CuTPP molecule was observed as a dark spot. This is quite different from the image of CoTPP, in which the center spot of each CoTPP appeared as the brightest spot. In the UHV environment, Hipps and co-workers reported that the difference in brightness between the center metals of CoTPP and CuTPP was due to the difference in the mode of occupation of d orbitals.10,12 The d orbital configuration of CoTPP is dxz2, dyz2, dxy2, dz21, whereas that of CuTPP is dxz2, dyz2, dxy2 , dz22, that is, CoTPP has a half-filled dz2 orbital, while CuTPP has a filled dz2 orbital.12b In the case of CuTPP, Hipps et al. reported that the center spot appears dark because the d orbital is filled, whereas the bright spot in the CoTPP appears bright because of the tunneling being mediated by the half-filled dz2 orbital between the Au surface and the tip.12 Electrocatalytic Activity for O2 Reduction. On the highly ordered CoTPP arrays on the Au(111) surface, the reduction of molecular oxygen was carried out in 0.1 M

HClO4 saturated with O2. Figure 6 shows the CVs obtained in 0.1 M HClO4 saturated with O2 for bare (a) and CoTPPmodified (b) Au(111) surfaces. As can be seen in Figure 6, there is a clear difference in electrocatalytic activity between the bare and CoTPP-modified Au(111) electrodes for the reduction of O2. On the bare Au(111) electrode, cathodic current due to the reduction of O2 commenced at ca. 0.55 V, and it gradually increased in the potential range between 0.5 and -0.1 V. On the CoTPP-adsorbed Au(111) electrode, the catalytic current of O2 commenced at ca. 0.55 V during the cathodic scan and gave a clear electrocatalytic reduction peak for O2 at 0.32 V. At potentials more negative than 0.3 V, the reductive current remained almost constant because the process was limited by diffusion of O2. The enhancement of the reductive current for O2 reduction at 0.32 V for the CoTPP-modified Au(111) electrode compared to that at the bare Au(111) electrode indicates that the CoTPP adlayer catalyzes the reduction of O2. The CV profile of the CoTPP-modified Au(111) electrode was stable for repetitive cycles between -0.1 and 0.85 V, indicating that the CoTPP thin film formed on Au(111) is not desorbed in this potential range. As was reported for the reduction of O2 on an evaporated Au film electrode coated with CoTPP,22a it can be roughly estimated from the current density (ca. 0.4 mA cm-2) that the two-electron reduction for O2 to H2O2 occurred on the CoTPP-modified Au(111) electrodes. Note that the behavior of the CuTPP-modified Au(111) electrode was also examined for the reduction of O2 (not shown). In the case of the CuTPP-modified Au(111), the reduction current of O2 was depressed, and the current started to flow only at potentials more negative than ca. 0.2 V. According to previous papers on the effect of center metals on electrocatalytic activity of O2 reduction,32-35 a MTPP derivative, metal(II)-R,β,γ,δ-tetra(p-methoxyphenyl)porphyrin,32 and MPc33-35 decrease electrocatalytic activity in the order of Co(II) > Ni(II) > Cu(II). The difference in electrocatalytic activity for the reduction of O2 can be explained by backbonding, that is, interaction of the filled oxygen orbitals with the vacant dz2 orbital of the center metal.32 (32) Alt, H.; Binder, H.; Sandstede, G. J. Catal. 1973, 28, 8. (33) Savy, M.; Andro, P.; Bernard, C.; Magner, G. Electrochim. Acta 1973, 18, 191. (34) Randin, J.-P. Electrochim. Acta 1974, 19, 83. (35) (a) Zagal, J.; Pa´ez, M.; Tanaka, A. A.; dos Santos, J. R., Jr.; Linkous, C. A. J. Electroanal. Chem. 1992, 339, 13. (b) Ca´rdenas-Jiro´n, G. I.; Gulppi, M. A.; Caro, C. A.; del Rı´o, R.; Pa´ez, M.; Zagal, J. H. Electrochim. Acta 2001, 46, 3227.

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To analyze the reduction process in further detail, a rotating CoTPP-modified Au(111) disk electrode was used. The equation expressing the Koutecky-Levich plot is as follows:

1/ilim ) 1/ik + 1/0.62nFD2/3ω1/2v-1/6C*

(1)

where ilim is the limiting current. The first term on the right-hand side is the inverse of the kinetic current density, ik, as expressed by

ik ) nFkΓCoTPPC*

(2)

where n is the number of electrons transferred in the overall electrode reaction, k is the rate constant for the catalyzed reaction (M-1 s-1), ΓCoTPP is the surface concentration of the CoTPP adlayer on the Au(111) surface (mol cm-2), F is the Faraday constant, and C* is the concentration of O2 (mol cm-3). The second term in eq 1 is the inverse of the Levich mass transport limited current density for the reduction of O2. D, v, and ω are the diffusion coefficient, the kinematic viscosity of the solution, and the angular rotation speed in hertz, respectively. For the calculation of the slope, the following values were used: v ) 0.01 cm2 s-1,36 D ) 2.0 × 10-5 cm2 s-1,6 and C* ) 1.2 × 10-3 M.37 Figure 7 shows the current-potential curves for the reduction of O2 at the CoTPP-modified Au(111) electrode. The cathodic scan was started from 0.85 V (near the OCP) for various rotation speeds at the scan rate of 10 mV s-1. The catalytic currents commenced at ca. 0.5 V. The profile observed at the CoTPP-adsorbed Au(111) electrode was similar to that reported for the edged plane graphite (EPG) electrode coated with CoTPP,4d suggesting that the reaction is the two-electron reduction from O2 to H2O2. The Levich plot for the limiting current (ilim) at 0.1 V versus the square root of rotation speed in an O2 saturated solution gave an expected linear relationship (not shown), indicating that the limiting currents observed at the CoTPP-modified Au(111) electrodes are controlled by the rate of diffusion of O2. The corresponding KouteckyLevich plot derived from the limiting current density values at 0.1 V in an O2 saturated solution is shown as the inset in Figure 7. The slope of the plot is close to the calculated value not for the four-electron (n ) 4) reduction but for the two-electron (n ) 2) reduction of O2 (dashed lines). From the slope, the number of electrons, n, is calculated to be 2.0 ( 0.1, as expected from the CV in Figure 6b. (36) Zagal, J.; Bindra, P.; Yeager, E. J. Electrochem. Soc. 1980, 127, 1506. (37) Markovic, N. M.; Adzic, R. R.; Vesovic, V. B. J. Electroanal. Chem. 1984, 165, 121.

Figure 7. Current-potential curves for O2 reduction at a rotating CoTPP-modified Au(111) disk electrode in 0.1 M HClO4 saturated with O2. The inset shows Koutecky-Levich plots. The rotation speeds were 100 (a), 400 (b), 900 (c), 1600 (d), and 2500 (e) rpm. The potential scan rate was 10 mV s-1.

Conclusions By immersing a Au(111) electrode into CoTPP and CuTPP benzene solutions, we succeeded in preparing welldefined adlayers of CoTPP and CuTPP onto the Au(111) surface and in revealing the packing arrangement and even the internal structure of each CoTPP and CuTPP molecule on the Au(111) electrode in aqueous HClO4 solution by high-resolution STM images. Both CoTPP and CuTPP molecules formed ordered adlayers with an identical configuration on the reconstructed Au(111) surface with lattice parameters of approximately 1.41 nm. The adlayers of highly ordered CoTPP and CuTPP formed on the Au(111) electrode surfaces were stable in a wide scanning potential range. It was also found that the CoTPP-modified Au(111) electrode enhanced the cathodic current for electrocatalytic reaction of O2 to H2O2. Acknowledgment. This work was supported in part by a Grant-in-Aid for Science Research (A) (No. 12305055) from the Ministry of Education, Science, Sports and Culture, Japan. The authors acknowledge Dr. Y. Okinaka for his assistance in writing this manuscript and Dr. J. Inukai of Tohoku University for his useful discussion. LA026449I