Langmuir 1995,11, 2337-2340 2337

Self-organized Porphyrin Array on Iodine-Modified Au( 111) in Electrolyte Solutions: In Situ Scanning

Tunneling Microscopy Study

Masashi Kunitake,+ Nikola Batina,' and Kingo Itaya*>t

Itaya Electrochemiscopy Project, ERATO, JRDC, 2-1 -1 Yagiyama-Minami, Taihaku-ku, Sendai 982, Japan, and Department of Engineering Science, Faculty of Engineering, Tohoku

Uniuersity, Aoba-ku, Sendai 980, Japan

Received March 22, 1995. In Final Form: May 8, 1995@

The visualization of highly ordered adlayers of 5,10,15,2O-tetrakis(N-methylpyridinium-4-y1)- 2VI,23H-porphine tetrakis(ptoluenesu1fonate) (TMPyP) on the iodine-modified Au( 111) in 0.1 M perchloric acid solutions was achieved by in situ scanning tunneling microscopy (STM). TMPyP was found to form highly ordered molecular layers on the iodine-modified Au( 11 1) substrate. High-resolution images of individual TMPyP molecules showed a typical square porphyrin ring with additional four bright spots around the ring. These bright spots are attributed to the pyridinium units.

Formation and characterization of the ordered adlayers of organic molecules a t solid-electrolyte interfaces are important from the fundamental and technological points of view. The recently developed in situ scanning tunneling microscopy (STM) technique revealed exciting possibilities of the direct observation and characterization of electrode surfaces in so1utions.l This new technique also triggered considerable interest in studying organic molecules, particularly biologically important macromolecules a t native conditions, Le., in electrolyte solutions.2-5

Here we report, for the first time, a successful in situ imaging of a highly ordered adlayer of 5,10,15,20-tetrakis- (N-methylpyridinium-4-yl)-21H,23H-porphine tetrakis- (p-toluenesulfonate) (TMPyP) in electrolyte solutions. It

TMPyP

was surprisingly found in this study that well-ordered TMPyP arrays were formed on the iodine-modified Au- (111) surface in perchloric acid solution. In order to form stable and well-ordered adlayers of TMPyP on electrode

* To whom correspondence should be addressed. ' Itaya Electrochemiscopy Project. 0 Tohoku University. @ Abstract published in Advance ACS Abstracts, J u n e 15, 1995. (1) Siegenthaler, H. In Scanning Tunneling Microscopy II; Wiesen-

danger, R., Guntherodt, H. J., Eds.; Springer-Verlang: New York, 1992; Vol. 28; Chapter 2.

(2) Srinivasan, R.; Murphy, J. C.; Fainchtein, R.; Pattabiraman, N. J. Electroanal. Chem. 1991, 312, 293.

surfaces in electrolyte solutions, we have attempted to use a variety of well-defined substrates such as HOPG and flame-annealed Pt and Au single crystal^.^ In our preliminary studies, we did not succeed in resolving TMPyP layers on HOPG. Only the atomic structure of HOPG was observable in all experiments. It was also found that STM imaging showed evidence for adsorption of TMPyP on an atomically ordered Au(ll1) surface in perchloric acid solution but not for the formation of the ordered adlayer.6 On the other hand, an ordered TMPyP adlayer was exclusively formed in the process of spon- taneous adsorption from the solution containing TMPyP on the iodine-modified Au( 111) substrate. This paper is the first report to describe the formation ofhighly ordered molecular adlayers a t iodine-modified metal surfaces.

The preparation of the TMPyP array involved the following procedures. Au( 111) facets formed on the single crystal bead, which usually showed atomically flat and wide terraces, were used as the substrate after annealing in a hydrogen-oxygen flame and quenching in pure water according to the well-established pr~cedure .~ The modi- fication by iodine was started with immersion into 1 mM KI solution for 3 min.8s9 After the immersion, the sample was thoroughly rinsed with 0.1 M HC104 solution. Contact of the iodine-modified Au( 111) with a 0.1 M HC104 solution containing 5 x M TMPyP (Doujin Co., Ltd., Japan, used without further purification) was carried out under potential control in an STM electrochemical cell. The electrode potential was usually kept a t approximately 0.8 Vvs RHE, which corresponds to the open circuit potential (OCP) for the I/Au(ll l) sample. At this potential, the iodine adlayer possesses a 42.5 x d3R-30") s t r u c t ~ r e . ~ J ~ The surface of the iodine-modified electrode was perfectly flat at the atomic level, uniformly covered by neutral iodine atoms, and strongly hydrophobic. In situ STM images were acquired in the electrolyte solutions, both in the absence and in the presence of TMPyP at potentials near OCP. A Nanoscope I11 (Digital Instruments, USA) was used. Two Pt wires were used as reference and counter

(3) Oden, P. I.; Thundet, T.; Nagahara, L. A.; Lindsay, S. M.; Adams,

(4) Tao, N. J.; DeRose, J. A,; Lindsay, S. M. J. Phys. Chem. 1993,97, G. B.; Sankey, 0. F. Surf Sei. Lett. 1991, 254, L454.

910. ( 5 ) Tao, N. J.; Shi, Z. J. Phys. Chem. 1994,98, 1464. ( 6 ) Kunitake, M.; Batina, N.; Itaya, K. In preparation. (7) Honbou, H.; Sugawara, S.; Itaya, K.Anal. Chem. 1990,62,2424. (8 ) Sugita, S.; Abe, T.; Itaya, K. J. Phys. Chem. 1993, 97, 8780. (9) Yamada, T.; Batina N.; Itaya, K. Sur f Sei., in press. (10) Ocko, B. M.; Watson, G. M.; Wang, J . J. Phys. Chem. 1994,98,

897.

0743-746319512411-2337$09.00/0 0 1995 American Chemical Society

2338 Langmuir, Vol. 11, No. 7, 1995 Letters

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100 nm 0 50 Figure 1. STMimages (top view) of the fine TMPyP molecular array on I/Au( 111). Image A was recorded in 0.1 M He104 with 5 x lop7 M TMbP at the sample potential of 0.80 V vs RHE, tip potential of 0.34 V vs RHE, and tunneling current of 4 nA. Image B was recorded in pure 0.1 M HClO4 after rinsing at 0.83 V vs RHE. The tip potential was 0.34 V vs RHE, and the tunneling current was 0.66 nA.

electrodes, respectively. The tunneling tip was made from a W wire electrochemically etched in 1 M KOH. The end of the W wire, 1 mm in length, was submerged into the KOH solution and etched by applying 15 V ac. The etched W wire was sealed with a clear nail polish to minimize the faradaic current. All images shown in this paper were taken in the constant current mode.

Figure 1A shows a typical high-resolution STM image of the highly ordered TMPyP array acquired in 0.1 M HC104 solution containing TMPyP. It is clear that the molecular array is extended over the wide atomically flat terrace of the iodine-modified Au(ll1) substrate. It is seen that the iodine-modified surface is almost completely covered with TMPyP molecules. Despite the relatively large area of the image, each TMPyP molecule can be recognized as a bright square within a skewed array of similar motifs. Figure 1B shows an STM image of a 100 x 100 nm2 area obtained after rinsing the sample thoroughly with pure 0.1 M HC104. Note that the intact

LO nm

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l’0 nm Figure 2. High-resolution STM images (A, top view; B, surface plot) of the TMPyP array in 0.1 M HC104 with 5 x lop7 M TMPyP. Image was recorded at 0.82 Vvs RHE, with tip potential of 0.29 V vs RHE and tunneling current of 2 nA.

TMPyP array can still clearly be seen. The highly ordered TMPyP array with almost the same structure was observed in a TMPyP-free perchloric acid solution, indi- cating that the adsorption of TMPyP occurs strongly on the iodine-modified Au surface.

More details about the symmetry and orientation of the TMPyP molecules in the layer were revealed by STM images acquired with even higher resolution (Figure 2). The high resolution achieved in our in situ STM study is comparable to the atomic resolution obtained in STM studies carried out in ultrahigh vacuum (UHV).11-13 The top view and height-shaded surface plot of a high- resolution STM image acquired in an area of 15 x 15 nm2 are presented in Figure 2A and Figure 2B, respectively. Flat-lying TMPyP molecules can be recognized in the image as a square with four additional bright spots. The shape of the observed features in the image clearly corresponds to the known chemical structure of TMPyP molecule. The characteristic four bright spots (0.4-0.5

(ll)Ohtani, H.; Wilson, R. J.; Chiang, S.; Mate, C. M. Phys. Rev.

(12) Lippel, P. H.; Wilson, R. J.; Miller, M. D.; Woll, C.; Chiang, S.

(13) Weiss, P. S.; Eigler, D. M. Phys. Rev. Lett. 1993, 71, 3139.

Lett. 1988, 60, 2398.

Phys. Rev. Lett. 1989, 62, 171.

Letters Langmuir, Vol. 11, No. 7, 1995 2339

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.e 3. Schematic representation of TMPyP molecular array with outlined unit cell.

nm in diameter) placed at the four corners of a square have been found regularly and should be associated with the internal molecular structure. The bright spots are ca. 0.04 nm higher than the substrate and ca. 0.02 nm higher than the central part of the TMPyP molecule which appear as the square. The center to center distance between the bright spots is 1.3 f 0.1 nm (measured diagonally), which corresponds to the distance between two diagonally located pyridinium units in the molecular model presented in Figure 3. The molecular model for TMPyP was conducted using a CAChe software package (CAChe Scientific Co., Ltd., USA). The size of the observed bright spots fits the size of the pyridinium units. Note that the pyridinium units are more easily visible than the rest of TMPyP molecule. This may be due to a specific rotational conformation of the pyridinium units. Those units seem to be slightly elevated above the molecular plane due to tilting with respect to the porphyrin ring. Since it is well-known that STM reveals topographic and electronic features, we cannot rule out a possibility that

the pyridinium unit appears as a bright spot due to specific electronic properties. l2

Besides the information concerning the internal mo- lecular structure, our STM images revealed a great deal of information concerning the symmetry and the struc- ture of the TMPyP array. Even a superficial look at any of the presented STM images reveals that the molecular array is two-dimensionally well-organized. Two different molecular rows are marked by arrows I and I1 in Figures lA, 2A, and 3. These rows cross each other at ca. 60". We also found that the molecular rows of TMPyP consist of flat-lying molecules with two different rotational orienta- tions in the surface plane. In the row marked I, every TMPyP molecule possesses the same molecular orientation with the pyridinium units rotated by ca 23" or 113" with respect to the direction of I. On the other hand, every second molecule along the row marked I1 shows the same molecular orientation. The rotation angle of ca. 45" can be easily recognized between two neighboring molecules in row 11. Even under such relatively complex symmetry,

2340 Langmuir, Vol. 11, No. 7, 1995 Letters

an oblique unit cell lattice can be outlined as shown in Figure 2A and Figure 3 with the lattice parameters of a = 3.4 f 0.2 nm and b = 1.8 f 0.1 nm, and with an angle of ca. 60". Each unit cell includes two TMPyP molecules, which corresponds to a surface concentration of about 3.7 x 1013 molecules/cm2. According to the model shown in Figure 3, the array seems to be almost a closed-packed molecular adlayer.

Such an accurate description of the symmetry of molecular array also allowed us to understand the formation of domain boundaries. Several types of domain boundaries were frequently observed in the STM images acquired in the present work. More often, however, the observed domain boundaries run parallel to the molecular row I. Examples of such phase boundaries are shown in Figure 2A. The domain boundaries are marked by the dashed lines. As can be seen, the molecules are aligned with the same orientation in the two nearest neighboring rows along the domain boundary as indicated by the dashed lines. Note that domain boundaries parallel to the direction of the row I1 were not frequently observed. This strongly suggests that the interactions between the molecules aligned in row I, which are identically oriented molecules, are stronger than those between the molecules along the direction 11, alternately oriented molecules. Another type of boundary can be seen in FigurelA, as marked by the dashed line. In this case, the same type (row I) of molecular rows are connected with each other at the boundary with an angle of approximately 140". The observed value is slightly larger than 120" expected for a surface with 3-fold symmetry. It suggests that an important role is played by the structure of the iodine adlayer.

The TMPyP molecular images were usually observed at the conditions with relatively small tunneling currents (0.5-4 nA) and high bias voltages (0.30-0.6OV). Imaging with higher tunneling currents (greater than 40 nA) always revealed the underlying iodine adlayer. A com- posite image of the underlying iodine adlayer (upper part of the image) and the overlaid TMPyP adlayer (lower part of the image) is presented in Figure 4. During the recording of this particular image, the tunneling current was abruptly changed from 80 to 0.5 nA. Similar composite STM images were obtained when the tunneling current was switched in the opposite direction from 0.5 to 80 nA. The STM image in the upper part indicates a relatively flat surface with interatomic distances of 0.45-0.47 nm between the observed atoms. In the lower part of the image, the typical surface features for TMPyP adlayer can be recognized. This experiment clearly demonstrates that the TMPyP array exists on top of the iodine adlayer. Interestingly, during the recording of the image of an underlying iodine adlayer, the TMPyP layer was not disturbed by the tip.14J5 Although the structural commensuration between the TMPyP array, the iodine adlayer, and even the Au(ll1) substrate is of our special interest, the observed result shown in Figure 4 was not very conclusive, because the structure of the iodine adlayer

(14) Gao, X.; Edens, G. J.; Weaver, M. J. J. Phys. Chem. 1994,98,

(15) Gao, X.; Edens, G. J.; Liu, F.-C.; Hamelin, A.; Weaver, M. J. J. 8074.

Phys. Chem. 1994,98, 8086.

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0 0 5 ' Onm Figure 4. A composite STM image of the iodine and the TMPyP layer obtained in 0.1 M HC104. Upper part of the image was recorded at 0.84 V, with the tip potential of 0.44 V and the tunneling current of 80 nA. Lower part ofthe image was recorded after the tunneling current was switched to 0.5 nA in the middle of the scan.

did not appear as clearly as was observed in our previous w ~ r k . ~ ? ~ We believe that the structure of the TMPyP layer is controlled by the underlying iodine adlayer. However, the relation between the two adlayers may not be so simple because of the incommensuration of the two adlayers. Nevertheless, it must be emphasized that the iodine layer on Au(ll1) plays a crucial role in the formation of the well-ordered TMPyP array. Relatively weak interactions between the hydrophobic iodine adlayer and the organic could be the key to produce self-organized molecular arrays. As described before, TMPyP does not form a well- ordered array on bare Au(ll1) surface. This might be most probably due to strong interactions between the gold substrate and organic molecules, which prevent self- ordering processes in the molecular layer. The real mechanism of the formation of the TMPyP array on the iodine adlayer is still not clear. The driving force for 2D self-ordering of the highly charged TMPyP molecules could be intermolecular interactions such as a sharing of counterion in the array, etc. Finally, the results shown here strongly encouraged us to explore the potential applications of the iodine-modified electrodes to investi- gate various organic molecules. Detailed studies are in progress.

Acknowledgment. The authors acknowledge Profes- sor K. Niki (Yokohama University) for helpful discussions and Dr. Y. Okinaka for help in the writing of this manuscript. This work was supported by the ERATO- Itaya Electrochemiscopy Project, JRDC.

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