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Electrochemical Scanning Tunneling Microscopy Observation of Ordered Surface Layers on an Anionic Clay-Modified Electrode Ken Yao, M. Taniguchi, M. Nakata, M. Takahashi, and Akihiko Yamagishi* Division of Biological Sciences, Graduate School of Science, Hokkaido University, Sapporo 060, Japan Received October 17, 1997. In Final Form: March 3, 1998 Electrochemical scanning tunneling microscopy (STM) observations have been performed on a highly ordered pyrolytic graphite (HOPG) electrode modified with a cast film of hydrotalcite (HT) crystal ([Mg6Al2(OH)16]3/4CO3‚1/2Cl‚2H2O). The electrode was in contact with an aqueous solution of 5 mM Na3[Fe(CN)6] and 0.1 M Na2SO4 at 600 mV (vs Ag/AgCl). The STM image of the electrode showed a two-dimensional lattice with a ) 1.40 ( 0.04 nm, b ) 1.85 ( 0.04 nm, and R ) 63 ( 3°. One bright spot in the image was present at every 3.4 [Mg6Al2(OH)16]2+ units, leading to the conclusion that the spot represented a [Fe(CN)6]3ion adsorbed on a crystal surface. At 81 mV (vs Ag/AgCl), the STM image exhibited a two-dimensional lattice with a ) 1.74 ( 0.04 nm, b ) 1.83 ( 0.04 nm and R ) 60 ( 3°. This was ascribed to the lattice made by a mixture of [Fe(CN)6]3- and [Fe(CN)6]4- on a HT surface. At -200 mV (vs Ag/AgCl), the STM image showed a two-dimensional lattice with a ) 1.94 ( 0.04 nm, b ) 1.90 ( 0.04 nm and R ) 60 ( 3°, which was ascribed to the lattice made by reduced [Fe(CN)6].4- These results confirmed that an ordered molecular adsorption layer changed dynamically during the electrochemical processes.

Introduction Recently the thin film of an inorganic layered compound has been used as a functional material.1 In these attempts, the layers of inorganic materials and organic polymers were deposited artificially in an alternatively way by use of electrostatic interactions.1 A hydrotalcite clay takes a unique position in these attempts. The material has a layered structure of octahedral sheet with anion-exchange capacity.2,3 Because of these properties, an ion-exchanged adduct of hydrotalcite and anionic molecules has been applied as a film to accomplish functions such as electrontransfer, energy conversion, and molecular recognition.4-12 To design and synthesize such a supramolecular system as based on a layered inorganic material, it is of vital importance to establish the adsorption structures of guest molecules at the stage of reactions. The molecular arrangement and orientation of a molecule should be clarified especially in connection with the steric control by a solid surface. So far no such investigation has been reported yet, although some studies by probe microscopes * To whom correspondence should be addressed: tel, 81 11 706 2769; fax, 81 11 706 4992; e-mail, [email protected]. ac.jp. (1) (a) Kleinfeld, E. R.; Ferguson, G. S. Science 1994, 256, 370 and references therein. (b) Lvov, Y.; Ariga, K.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 6117. (2) Miyata, S.; Okada, A. Clay Clay Miner. 1974, 22, 14. (3) Miyata, S.; Hirose, T. Clay Clay Miner. 1978, 26, 441. (4) Gaillon, L.; Bationi, P.; Bedioui, F.; Devynck, J. J. Eletroanal. Chem. 1993, 347, 435. (5) Keita, B.; Belhouri, A.; Nadjo, L. J. Eletroanal. Chem. 1993, 355, 235. (6) Itaya, K.; Change, H.-C.; Uchida, I. Inorg. Chem. 1987, 26, 624. (7) Mousty, C.; Therial, S.; Forano, C.; Besse, J. P. J. Eletroanal. Chem. 1994, 374, 63. (8) Therias, S.; Mousty, C.; Forano, C.; Besse, J. P. Langmuir 1996, 12, 4914. (9) Yun, S. K.; Constantino, V. R. L.; Pinnavaia, T. J. Clay Clay Miner. 1995, 43, 503. (10) Basile, F.; Bsini, L.; Formasari, G.; Gazzano, M.; Trifiro, F.; Vaccari, A. J. Chem. Soc., Chem. Commun. 1996, 2435. (11) Narayanan, S.; Krishna, K. Appl. Catal., A 1996, 147, L253. (12) Qin, J.; Villemure, G. J. Eletroanal. Chem. 1995, 395, 159.

such as an atomic force microscope (AFM) have revealed the static aspects of adsorbed layers on a surface.13 In the present work, we report the in-situ observation of redox process of adsorbed [Fe(CN)6]3- on a hydrotalcite crystal with an electrochemical scanning tunneling microscope (ECSTM). [Fe(CN)6]3- acted as an electroactive species when an electrode substrate was modified with a film of [Fe(CN)6]3- and a hydrotalcite crystal. As a result, the metal complexes were revealed to form a twodimensional lattice on a surface. Most interestingly the size of a unit lattice changed dynamically with the progress of electrochemical reduction. Observation on the same samples by the AFM method proved to be impossible because these molecules were so weakly bound to the surface that they were removed during scanning with the AFM tip. The present results may open a way to apply the STM method to the surface reaction of an inorganic layered compound. Experimental Section Materials. A hydrotalcite clay (denoted by HT) was synthesized according to the literature method.13 An aqueous mixture of 1.9 mmol of MgCl2, 0.63 mmol of AlCl3, and 31 mmol of urea was stored in a sealed bottle at 60 °C for 1 month. After the mixture was filtered, the precipitate was washed with water and freeze-dried for 24 h. The crystal of HT thus obtained had a size of ca. 5 µm in diameter as determined by transmission electron microscopy. According to the X-ray powder diffraction measurements, the crystal had the basal spacing of 0.76 nm.2,3 Results of chemical analyses were consistent with the chemical composition of [Mg6Al2(OH)16]3/4CO3‚1/2Cl‚2H2O. Na3[Fe(CN)6] was used as purchased from Wako Pure Chemical Ind. Co. (Japan). Instruments. AFM measurements were performed with a Nanoscope III Scanning Probe Microscope (Digital Instrument). The cantilever [Si3N4] integral tip with spring constant of 0.06 nm-1 (Park Scientific) was used. The “A” scan head was used for 0.7 µm maximum scan. The image was obtained in the constant force mode with filters off. (13) Cai, H.; Hiller, A. C.; Franklin, K. R.; Nunn, C. C.; Ward, M. D. Science 1994, 266, 1551.

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Figure 1. Plot of the specific resistivity of the thin pellet of a hydrotalcite crystal (surface-to-length ratio ) 0.036 cm) versus the reciprocal of temperature. The resistance was measured under the application of direct voltage of 10 V.

Figure 2. Cyclic voltammogram of a HOPG electrode modified with a hydrotalcite crystal. The electrode was in contact with an aqueous solution of 5 mM Na3[Fe(CN)6] and 0.1 M Na2SO4. The potential sweep rate was 2 mV/s.

Electrochemical STM (ECSTM) measurements were performed with a Nanoscope III electrochemical scanning tunneling microscope (Digital Instrument). Pt wires were used as reference and auxiliary electrodes. Observation was made in the constant current mode. The tunneling tip was an electrochemically etched tungsten wire. Prior to the measurements on samples, the surface of freshly cleaved mica or highly orientated pyrolytic graphite (HOPG) was imaged to standardize scale calibration for AFM or STM measurements, respectively. The UV-visible spectrum due to [Fe(CN)6]3- ions incorporated in HT was monitored during the reduction of [Fe(CN)6]3-. For that measurement, an indium-tin oxide coated (ITO) glass electrode was modified with a cast film of the ion-exchange adduct of HT and [Fe(CN)6]3- (denoted by HT-[Fe(CN)6]3-). A potentiostat 2020A (Toho Technical, Japan) was used for cyclic voltammogram measurements. Preparation of an Electrode for ECSTM Measurements. An aqueous suspension of HT (ca. 65 mg mL-1) was deposited onto a freshly cleaved HOPG substrate. After the suspension was removed, the sample was dried at 100 °C for 1 h under air. The average thickness of a sample is examined by AFM to be 50-70 nm. The ECSTM measurements were performed in contact with an aqueous solution of 5 mM Na3[Fe(CN)6] and 0.1 M Na2SO4. Before the ECSTM observations, the potential of the electrode was maintained at 600 mV (vs Ag/AgCl) for 2 h in order to stabilize [Fe(CN)6]3- on a HT surface. The scanning rate of potential was 2 mV/s during ECSTM observations. The size of a unit lattice was determined by averaging the results of three independent measurements. Electric Conductivity Measurements of Hydrotalcite. The electric conductivity measurement was performed for a thin rectangular pellet of hydrotalcite (HT) crystals by the twoterminal method. The surface-to-length ratio (f) of the sample was 0.036 cm. The specific conductivity was obtained to be 107108 Ω cm at 20 °C under the direct voltage of 10 V. This value is of the same order as those of Fe2O3 and Cu2O on which the STM images have been reported.14 log F (specific resistance) decreased linearly with 1/T as shown in Figure 1, showing that the material was a semiconductor. Thus the STM measurements have been possible because the present materials are not an insulator but a semiconductor.

and reduction peaks of [Fe(CN)6]3-/4- couple as below:5

Results Figure 2 shows the cyclic voltammogram on a glassy carbon electrode modified with a cast film of HT. The electrode was in contact with an aqueous solution of 5 mM Na3[Fe(CN)6] and 0.1 M Na2SO4. The peaks at 345 and 64 mV (vs Ag/AgCl) corresponded to the oxidation (14) Scanning Probe Microscopy of Clay Minerals; Nagy, K. L., Blum, A. E., Eds.; CMS Workshop Lectures; The Clay Minerals Society: 1994; Vol. 7.

[Fe(CN)6]3- + e S [Fe(CN)6]4-

(1)

Figure 3A shows the 9 nm × 9 nm AFM image of a HT crystal deposited on HOPG as described in the Experimental Section. The sample was in contact with an aqueous solution of 0.1 M Na2SO4. The lower half of the figure represented the periodic image, while the upper portion did not show any periodicity, indicating that the region was poorly crystallized. Figure 3B shows the 3 nm × 3 nm AFM image of the same sample. The periodicity of bright spots confirmed the presence of a two-dimensional lattice with a unit lattice of a ) 0.31 ( 0.02 nm, b ) 0.31 ( 0.02 nm, and R ) 58 ( 3°. When the image is compared with the ideal (0001) surface of a HT crystal,13 it is deduced that the bright spots corresponded to the external hydroxyl groups. When the AFM measurements were performed on the same HT crystal in contact with an aqueous solution of 5 mM Na3[Fe(CN)6] and 0.1 M Na2SO4, they gave the identical image as in Figure 3B. The results implied that an AFM tip removed the adsorbed metal complexes during scanning and always contacted the bare surface of a HT crystal. In other words, the present metal complexes were adsorbed so weakly that they were not imaged by the AFM method. Figure 4A is the 10 nm × 10 nm STM image of a HT crystal deposited on HOPG. The sample was in contact with an aqueous solution of 0.1 M Na2SO4. In this case, the whole surface showed the same periodic lattice except for the partly dark portions. These portions were probably the deficient part of a layer. Figure 4B is the 5.8 nm × 5.8 nm STM image of the same sample. From the periodicity of bright spots, the presence of a twodimensional lattice was deduced with the unit lattice of a ) 0.64 ( 0.03 nm, b ) 0.64 ( 0.03 nm, and R ) 60 ( 3°. The size of a unit lattice size did not change under the potential sweep from 600 to -200 mV (vs Ag/AgCl). The lengths of a unit lattice were nearly twice as large as those of a unit lattice in Figure 3B in both a and b directions. The results indicated that the bright spot in the image represented an aluminum atom at an octahedral site inside a layer (as schematically shown in Figure 4C).15 (15) Yao, Ken; Taniguchi, M.; Nakata, M.; Takahashi, M.; Yamagishi, Akihiko Langmuir 1998, 14, 2410.

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A

B

Figure 3. (A) The 9 nm × 9 nm AFM image of an HOPG electrode modified with a hydrotalcite crystal in contact with an aqueous solution of 0.1 M Na2SO4.. (B) The 3 nm × 3 nm AFM image of the same sample. The unit lattice of a ) 0.31 ( 0.02 nm, b ) 0.31 ( 0.02 nm, and R ) 58 ( 3° is indicated in the figure.

Figure 5A shows the 57 nm × 57 nm STM image of a HT crystal on the HOPG electrode in contact with an aqueous solution of 5 mM Na3[Fe(CN)6] and 0.1 M Na2SO4. The potential of the electrode was maintained at 600 mV (vs Ag/AgCl) (position A in Figure 2). There were several dark domains observed in the image. When such a portion was magnified to the smaller scale, the image gave a two-dimensional lattice identical to either that of Figure 4B or the image of a HOPG surface. Thus the dark domains were ascribed to either a bare hydrotalcite surface or the deficient part of a layer. The other part of the image consisted of a two-dimensional lattice whose axes took nearly unique directions over the whole surface as indicated by the arrows. The results suggest that the observed lattice represented the surface layer of adsorbed molecules and that the orientation of the lattice was determined by the interaction with the lattice of a HT crystal.

C

Figure 4. (A) The 10 nm × 10 nm STM image of the same HT crystal in contact with the aqueous solution of 0.1 M Na2SO4. (B) The 5.8 nm × 5.8 nm STM image of the same sample. The unit lattice of a ) 0.64 ( 0.03 nm, b ) 0.64 ( 0.03 nm, and R ) 60 ( 3° is indicated in the figure. The scanning parameters were -275 mV and 2.28 nA. (C) Schematic representation of the [Mg6Al2(OH)16]2+ hydrotalcite crystal surface.

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Figure 5. (A) The 57 nm × 57 nm STM images of a hydrotalcite crystal in contact with an aqueous solution of 5 mM Na3[Fe(CN)6] and 0.1 M Na2SO4. The potential was maintained at 600 mV (vs Ag/AgCl) which corresponded to position A in the cyclic voltammogram curve in Figure 1. (B) The 12 nm × 12 nm STM image under the same condition as (A). The unit lattice of a ) 1.40 ( 0.04 nm, b )1.85 ( 0.04 nm, and R ) 63 ( 3° is indicated in the figure. (C) The 12.5 nm × 12.5 nm STM images of the same electrode. The potential was maintained at 81 mV (vs Ag/AgCl) which corresponded to position B in the cyclic voltammogram curve in Figure 1. The unit lattice of a ) 1.74 ( 0.04 nm, b )1.83 ( 0.04 nm, and R ) 60 ( 3° is indicated in the figure. (D) The 15 nm × 15 nm STM images of the same electrode. The potential was maintained at -200 mV (vs Ag/AgCl) which corresponded to position C in the cyclic voltammogram curve in Figure 1. The unit lattice of a ) 1.94 ( 0.04 nm, b )1.90 ( 0.04 nm, and R ) 60 ( 3° is indicated in the figure. (E) The 15 nm × 15 nm STM images of the same electrode. The potential was maintained at 405 mV (vs Ag/AgCl) which corresponded to position D in the cyclic voltammogram curve in Figure 2. The unit lattice of a ) 1.65 ( 0.04 nm, b )1.71 ( 0.04 nm, and R ) 60 ( 3° is indicated in the figure. To compare the size of a unit lattice under the identical condition, the scanning parameters were kept at -218 mV and 3.96 nA for all observation processes..

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Figure 5B shows the 12 nm × 12 nm STM image of the same simple. The bright spots confirmed the presence of a two-dimensional periodicity with the unit lattice of a ) 1.40 ( 0.04 nm, b )1.85 ( 0.04 nm, and R ) 63 ( 3°. If the bright spots represented [Fe(CN)6]3- ions adsorbed on a HT surface, one metal complex was located at every 3.4 [Mg6Al2(OH)16]2+ units. Theoretically two [Fe(CN)6]3ions are located at every 3 [Mg6Al2(OH)16]2+ units for the sake of electrical neutrality. If both sides of a HT layer adsorb the same amount of anions, one [Fe(CN)6]3- covers 3 [Mg6Al2(OH)16]2+ units. On the ideal surface of a HT crystal, the lattice point corresponding to 3 [Mg6Al2(OH)16]2+ units forms a superlattice of a ) 1.24 nm, b )1.86 nm, and R ) 60°. Since the observed density of the spots was close to the theoretical one, it was concluded that the STM image represented [Fe(CN)6]3- ions on a basal surface. Actually the observed surface density of [Fe(CN)6]3- was slightly lower than the density calculated for electrical neutrality. Therefore the surface of a HT crystal still carried the residual charge of +0.13e per [Mg6Al2(OH)16]2+ unit. Probably this residual positive charge was compensated by the adsorption of SO42- ions from a solution. ECSTM observation was performed in scanning the potential toward the negative direction at a rate of 2 mV/s in order to reduce [Fe(CN)6]3- ions. Figure 5C shows the 12.5 nm × 12.5 nm STM image at 81 mV (vs Ag/AgCl) (position B in Figure 2). At this point, [Fe(CN)6]3- was partially reduced to [Fe(CN)6]4-. In the figure, there existed a unit lattice of a ) 1.74 ( 0.04 nm, b ) 1.83 ( 0.04 nm, and R ) 60 ( 3°. In other words, the unit lattice area expanded about 23% compared with that in Figure 5B. As a result, one metal complex was estimated to be located at every 4.1 [Mg6Al2(OH)16]2+ units. Figure 5D shows the 15 nm × 15 nm ECSTM image at -200 mV (vs Ag/AgCl) (position C in Figure 1). At this point, the entire of [Fe(CN)6]3- was reduced to [Fe(CN)6]4-. Under the condition, a unit lattice was obtained to be a ) 1.94 ( 0.04 nm, b ) 1.90 ( 0.04 nm, and R ) 60 ( 3°. As compared with the unit lattice in Figure 5B, the unit lattice area was expanded about 42%. One metal complex was estimated to be located at every 4.8 [Mg6Al2(OH)16]2+ units. For the sake of electrical neutrality, one [Fe(CN)6]4is calculated to cover 4.0 [Mg6Al2(OH)16]2+ units. On the ideal surface of a HT crystal, the lattice point corresponding to 4.0 [Mg6Al2(OH)16]2+ units forms the superlattice of a ) 1.76 ( 0.04 nm, b ) 1.76 ( 0.04 nm, and R ) 60°. The difference between the observed and theoretical surface densities of [Fe(CN)6]4- implied that the surface of a HT crystal still carried the residual charge of +0.2e per [Mg6Al2(OH)16]2+. When the potential sweep was inverted toward the positive direction, the size of a unit lattice was observed to decrease. This was apparently caused by the oxidation of [Fe(CN)6]4- to [Fe(CN)6]3-. Figure 5E shows the 15 nm × 15 nm STM image at 405 mV (vs Ag/AgCl) (position D in Figure 2). The unit lattice of a ) 1.65 ( 0.04 nm, b )1.71 ( 0.04 nm, and R ) 60 ( 3° was observed. As compared with the lattice of Figure 4D, the unit lattice area was compressed about 24%. One metal complex was estimated to be located at 3.7 [Mg6Al2(OH)16]2+ units. When the potential was maintained at 600 mV (vs Ag/AgCl) (position A in Figure 2), the STM image returned to the one as observed in Figure 4B. To examine the possibility of electronic interactions among the metal complexes, the UV-visible spectrum of an ITO glass electrode was monitored during the electrochemical processes. In the experiments, the electrode coated with a cast film of HT-[Fe(CN)6]3- was in contact

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Figure 6. The change of the UV-visible spectrum of an ITO glass electrode coated with a cast film of hydrotalcite-[Fe(CN)6]3-. The initial spectrum (a) was measured when the potential of the electrode was maintained at 600 mV (vs Ag/ AgCl). Thereafter the spectra b, c and d were measured at 60, 120, and 180 s after the potential was changed to 100 mV (vs Ag/AgCl), respectively.

with an aqueous solution of 0.1 M Na2SO4. As shown in Figure 6, the peak at 425 nm due to the incorporated [Fe(CN)6]3- decreased monotonically with the progress of the reduction. During the process, no shift of the peak position was observed, excluding the possibility that an electronic interaction between [Fe(CN)6]3- and [Fe(CN)6]4led to the intervalent state of Fe(II) and Fe(III). Discussion In the present work, the ECSTM method has been applied to a hydrotalcite clay (HT)-modified electrode with a purpose of obtaining evidence for the formation of a surface layer for the [Fe(CN)6]3-/4- couple. The STM images at various reduction stages indicated that there existed a two-dimensional periodicity that completely differed from the bare surface of a HT crystal. Notably the size of a unit lattice depended on the average charge of an adsorbed molecule remarkably. The results confirmed that we observed the dynamic formation of an ordered molecular layer on a HT crystal during the electrochemical reactions. One conclusion derived by the present results is that even a spherical molecule like [Fe(CN)6]3- or [Fe(CN)6]4constitutes a stable two-dimensional lattice at the surface density lower than the one satisfying charge neutrality. The two-dimensional arrangement of these metal complexes is considered to be realized by the combination of the following effects: (i) the electrostatic attraction by a positively charged HT layer, (ii) the repulsive interaction among the adsorbed molecules, and (iii) the steric control by the network structure of a basal surface of HT. Another interesting conclusion is that the observed STM images represented the dynamic development of the twodimensional lattice with the progress of the redox reaction. In other words, the surface lattice expanded or compressed its size with the increase or decrease of fraction of reduced [Fe(CN)6]4-, respectively. Such a change of the lattice size took place uniformly so that no boundary of different domains was observed at any stage of the redox process. These results indicated that [Fe(CN)6]3- and [Fe(CN)6]4interacted cooperatively on the surface to form a single lattice as a whole. It may be due to the fact that the present metal complexes were adsorbed so weakly as to (16) Sato, H.; Yamagishi, A.; Kato, S. J. Am. Chem. Soc. 1992, 100, 10933.

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Figure 7. Schematic representation of the molecular adsorption of anionic metal complexes, [Fe(CN)6]3- and [Fe(CN)6]4-, on the surface of [Mg6Al2(OH)16]2+ hydrotalcite crystal surface.

be mobile on a HT surface. They always tended to form a negatively charged layer as a whole. The formed lattice was commensurate to an original HT layer as shown schematically in Figure 7. The distance between the metal complexes was estimated to be 1.4-2.0 nm. It was too large to allow them to interact electronically. The measurements of the UVvisible spectrum during the electrochemical reduction (Figure 6) suggested that the Fe(II) complexes did not interact electronically with the Fe(III) complexes in such a way to form the intervalent state. Thus the origin of cooperatively among the adsorbed metal complexes is ascribed to the electrostatic and steric interactions i-iii as stated above.

At present it is not clear whether such interactions may really realize the formation of a uniform two-dimensional lattice for the cases of these mixtures of [Fe(CN)6]3- and [Fe(CN)6]4- ions. We are now planning to perform the simulation calculation of such a system on the basis of Monte Carlo simulations.16 Acknowledgment. Thanks are due to Professor Y. Inabe (Hokkaido University) for the measurements of electric conductivity. This work was partially supported by Grant-in-Aid for Scientific Research on Priority Area of “Electrochemistry of Ordered Interfaces” (No. 282) from the Ministry of Education, Science, Sports and Culture of Japan. LA971137E