J. Phys. Chem. 1996, 100, 6393-6396
6393
Surface Anisotropic Electroreflectance Response at an Edge-Plane Pyrolytic Graphite Electrode Takamasa Sagara,* Hiroshi Nomaguchi, and Naotoshi Nakashima Department of Applied Chemistry, Nagasaki UniVersity, Bunkyo, Nagasaki 852, Japan ReceiVed: January 22, 1996X
We found that the potential-modulated UV-vis reflectance (electroreflectance, ER) spectrum measured with linearly polarized incident light at an edge-plane graphite (EPG) electrode with adsorbed redox species shows surface anisotropy. At a hemin or methylene blue-adsorbed EPG electrode, s-polarized incident light gave rise to greater ER response than p-polarized when the c axis of the graphite electrode is parallel to the plane of incidence, while when the c axis is perpendicular to the plane of incidence p-polarized light gave rise to greater response. The anisotropy was also observed for an EPG electrode coated with a Nafion film in which adsorptive species was incorporated. In contrast, methylviologen-incorporated Nafion films did not produce the anisotropy. The possible origin of the anisotropy was discussed in light of the surface morphology of the EPG as observed by the SEM.
Introduction
Experimental Section
Pyrolytic graphite has been widely used as a material of the solid electrode in the field of electrochemistry.1 A basal-plane pyrolytic graphite (BPG) is prepared by cleaving the ab planes of well-oriented graphite. The exposed surface of BPG is preferentially the ab plane. The graphite cut parallel to the c axis is referred to as an edge-plane pyrolytic graphite (EPG). The surface of the EPG in contact with an aqueous solution has been regarded as a hydrophilic surface with carbonyl, carboxyl, and quinone functionalities. However, we suspect that unless the surface of EPG is microscopically flat, the basal graphite planes which are perpendicular to the macroscopic surface are also in contact with the solution, because the EPG surface may possess both microscopic basal and edge planes. To see as to whether an adsorbate prefers a basal or an edge plane, a microscopic view of the adsorption site is quite important. As the methods by which one can discriminate between basal and edge planes as the real adsorption site, the in situ surface spectroscopic techniques with linearly polarized light would be useful. Recently, Zhao and McCreery reported that the anisotropic polarized-Raman spectroscopy reveals the oriented adsorption of metallophthalocyanine on carbon surfaces.2 Among the spectroelectrochemical methods, potentialmodulated UV-visible reflectance spectroscopy (electroreflectance spectroscopy, ER) enables us to readily analyze the adsorption states of electroactive species on not only metal but also graphite electrode surfaces.3,4 Since the UV-vis reflectance of polarized light is sensitive to the orientation of adsorbed chromophores,5-7 the adsorption site on the EPG electrode is expected to be explored. In the present work, we examined the anisotropy of the ER response at the EPG electrode, on which various redox dyes were immobilized. We found that ER spectrum measured with linearly polarized incident light shows surface anisotropy. The possible origin of the observed anisotropy was briefly discussed in light of the SEM image of EPG, which displayed the existence of large area of basal plane.
A pyrolytic graphite plate of 6 mm thickness, the product of Union Carbide, was purchased from Tomoe Industrial, Tokyo. X-ray diffraction analysis of the graphite revealed that d002 ) 0.345 nm, the content of the impurity planes < 1%, and the full width at half-height of (002) diffraction peak ) 0.56 degree on the 2θ scale. A piece of cubic graphite cut from the plate was connected to a copper wire with silver paste and sheathed with epoxy cement resin (Torr Seal, Varian) so that the basal or edge plane is exposed as the electrode surface. Apparent (viz. geometrical) electrode area was ∼0.3 cm2. The surface of the EPG electrode was polished with a wetted 1500 grit SiC emery paper in a way that the direction of movement of the emery paper with respect to the EPG electrode is in parallel to the direction of the basal graphite plane (i.e., perpendicular to the c axis). Then the electrode was sonicated in water. The surface of BPG electrode was peeled off at least five times by using Scotch tape. Water was purified to >18 MΩ cm resistivity. Methylene blue (3,7-bis(dimethylamino)phenothiazin-5-ium chloride, MB), purchased from Junsei Chemical, was recrystallized three time from ethanol. Nafion 5 wt % solution purchased from Aldrich was further diluted to be 0.5% Nafion content with ethanol-water (9:1 in volume) mixture. All other chemicals were of analytical or reagent grade and used as received. The procedures of modification of electrode surfaces with dyes are reported in the captions of Figures 2 and 3. The instruments for ER measurements were described elsewhere.8,9 The detector of reflected light intensity was a photomultiplier (PM R928 combined with C1556 preamplifier, Hamamatsu Photonics). The incident monochromatic light was p-polarized (parallel to the plane of incidence) or s-polarized (perpendicular to the plane of incidence). The ER response, ∆R/R, was defined as being the ac component of reflectance in response to the sinewave potential modulation divided by the time-averaged reflectance. Both the real part (in-phase component) and the imaginary part (out-of-phase component) were simultaneously monitored as a function of wavelength of the incident light at the formal potential of the redox reaction of interest. As reference and counter electrodes, a Ag/AgCl electrode in a saturated KCl solution and a Pt wire were used, respectively. In the ER measurements, the angle of c axis with respect to the
* To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, April 1, 1996.
0022-3654/96/20100-6393$12.00/0
© 1996 American Chemical Society
6394 J. Phys. Chem., Vol. 100, No. 16, 1996
Figure 1. SEM images of EPG surfaces as polished as described in the experimental section. Magnifications were ×2000 (A) and ×10000 (B).
plane of incidence of light, φ, was set by rotating the EPG electrode. All the measurements were conducted in nitrogen atmosphere at 23 ( 2 °C. A scanning electron microscope (JEOL T100) was employed to obtain the SEM images. Because the graphite is intrinsically a good conductor, the metal overlayer was not deposited for SEM observation. Results Surface Morphology of EPG As Observed by SEM. The SEM images of the EPG surface are shown in Figure 1. In Figure 1A, unidirectional alignment of the graphite ab plane is obviously seen. In Figure 1B, mica-like leafy forms reflecting the cleavability are observed. The facets composed of basal planes are almost perpendicular to the macroscopic surface. The maximal distance between two adjacent facets exceeds 0.3 µm. The depth of the deepest ditch walled by the two adjacent facets is deeper than 2.5 µm. The SEM images connote that large area of ab plane is exposed at the EPG surface. If the effective diameter of a solution species is much smaller than the width of the ditch, it may recognize the presence of the larger area of the ab plane as well as the edge-plane of the EPG electrode. ER Measurements at Dye-Adsorbed Graphite Electrodes. Figure 2 shows the ER spectra of a hemin-adsorbed EPG electrode. It was confirmed by cyclc voltammogram (CV) that the electrode reaction was due only to surface-confined hemin
Letters
Figure 2. ER spectra of a hemin-adsorbed EPG electrode in 0.1 M Na2B4O7 solution (pH 10.1) free of hemin at -450 mV. Adsorption of hemin was made by dipping the EPG electrode in 0.1 mM hemin solution for 5 min. This adsorption procedure assumes nearly monolayer adsorption of hemin.10 The potential modulation was 100 mVrms and 14 Hz. Since both spectral structure and p/s ratio were almost the same for real and imaginary parts, the imaginary parts were shown as representatives. Incident light was p-polarized (solid line) or s-polarized (broken line). A, φ ) 0°; B, φ ) 90°.
of the amount of 1.1 × 10-9 mol cm-2. When the c axis of the graphite was parallel to the plane of incidence (φ ) 0°), s-polarized light gave rise to greater ER response than the p-polarized light. In contrast, when the c axis was perpendicular to the plane of incidence (φ ) 90°), the ER response with p-polarized light was greater. The anisotropy of the ER response, viz. the dependence of the p/s ratio of the ER response upon φ, is apparent. This anisotropy of the ER response was observed regardless of the amount of adsorbed hemin up to 2.6 × 10-9 mol cm-2. Identical anisotropy was observed also for MB or hemin(1-(3-aminopropyl)imidazole)2 complex adsorbed on the EPG electrodes. It is important to note that (1) the anisotropy was not observed at a BPG electrode with adsorbed hemin at all and (2) when the incident light was nonpolarized, the magnitude of the ER response was independent of φ. When the EPG electrode surface was highly polished with 0.05 µm silica slurry, the anisotropy of ER response of hemin was no longer observed. That is, hemin adsorbed on the edge plane at a flat EPG electrode does not contribute to the anisotropy of the ER response. ER Measurements at Nafion-Coated Graphite Electrodes. The anisotropy of the ER response was also observed for the
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J. Phys. Chem., Vol. 100, No. 16, 1996 6395
Figure 4. Schematic cross-sectional view of the EPG electrode and a model proposed for the interaction of the linearly polarized light with transition dipoles of hemin adsorbed on the microscopic basal planes.
Figure 3. ER spectra of an EPG electrode coated with a Nafion film, in which methylviologen dichlorides (MV2+‚Cl2) was incorporated, measured in 0.1 M phosphate buffer solution, pH 7.0. 50 µL of the diluted Nafion solution containing 0.1 mM MV2+‚Cl2 was coated on the EPG electrode and dried in air. The estimated thickness of the film was 35 µm. Measurements was made at -645 mV, where the electrode reaction of MV2+ + e- a MV+• takes place, with the potential modulation of 50 mVrms and 3 Hz. Since both spectral structure and p/s ratio were almost the same for real and imaginary parts, the imaginary parts were shown as representatives. A, φ ) 0°; B, φ ) 90°.
electrode reaction of MB incorporated in Nafion film (Nafion/ MB film) coated on the EPG electrode surface. It is known that MB adsorbs on the graphite electrode surface at the BPG/ Nafion interface.11 The same ER spectral structure at the Nafion/MB-film coated electrode as that at a MB-adsorbed EPG electrode, sweep rate dependence of CV, and frequency dependence of the ER response, all combined together confirm that the anisotropic ER response arises from adsorbed MB. Figure 3 shows the ER spectra at a Nafion/methylviologen (MV2+) film coated on the EPG electrode surface. The ER response with p-polarized light is greater than that with s-polarized light at the transition wavelengths, regardless of φ, indicating that the anisotropy is insignificant. The sweep rate dependence of CV represented a thin-layer electrochemistry of MV2+/MV•+ in the interior of the Nafion film. In a separate experiment, we measured the electrochemistry of MV2+/MV•+ couple at a bare EPG electrode. It was confirmed that MV2+/ MV•+ couple does not adsorb on the EPG electrode surface. These results indicate that the direct adsorption of the redox species on the electrode surface is a necessary condition for the appearance of surface anisotropic ER response.
Anisotropy of the Reflectivity of the EPG Substrate. To see as to whether or not the anisotropy of the ER response is related to the intrinsic anisotropy of the reflectivity of the EPG substrate itself, the spectrum of reflectivity was examined. In the practical experiment, instead of the reflectivity, the PM highvoltage value necessary to maintain 1.00 ( 0.05 V PM output was monitored as a function of wavelength. As the result, at both φ ) 0° and 90°, a higher voltage was needed for p-polarized light in the wavelength range 350-700 nm. This means that the reflectivity of s-polarized light is greater than that of p-polarized light at both φ ) 0° and 90° in the wavelength range 350-700 nm. This fact reveals that the effect of reflectivity anisotropy of the substrate on the anisotropy of ER response is unnecessary to be considered. Discussion In light of the SEM images of the EPG, the cross-sectional picture of the EPG electrode surface was depicted schematically in Figure 4, together with the illustration of the relationship between the basal plane assembly of a facet and the electric field of the incident light. Since the ER response of interest arises from the electronic transition (for example, Soret band transition of hemin is explicitly seen in Figure 2 around 440 nm), the magnitude of ER response is determined by the strength and direction of the electric field of the light at the adsorption site (E) and the interaction of E with transition dipoles of the adsorbate. If the basal plane facets act as if they are the conductive wires of a wire grid polarizer, the electric field component in parallel to the basal plane is significantly weakened. However, this may not actually be true in the present electrode surface, because (1) the gap of the facets look much longer than the wavelength (Figure 1) and (2) the reflectivity of the substrate does not represent the considerable anisotropy. Another possible origin of the anisotropy of ER response is the orientation of chromophores of the adsorbed species. In Figure 4, the transition dipole moment of hemin, which is planepolarized in the porphyrin ring, was added assuming the flat orientation of hemin on the microscopic basal planes, as an example. When s-polarized light is irradiated at φ ) 90°, the transition dipole moment is orthogonal to E and thus no
6396 J. Phys. Chem., Vol. 100, No. 16, 1996 interaction is possible, whereas when p-polarized, the electric field can interact with the dipole moment. At φ ) 0°, the electric field of both s- and p-polarized light can interact with the dipole moment, but the former can interact more strongly. Therefore, the interaction between E and dipole moment decreases as in the order of (s-polarized, φ ) 0°) ) (p-polarized, φ ) 90°) > (p-polarized, φ ) 0°) > (s-polarized, φ ) 90°), provided that the amplitude of E at the adsorption site is identical for the four cases as well as that the adsorption orientation of hemin on the microscopic basal plane is two-dimensionally isotropic. The above-mentioned order of the magnitude of the interaction is consistent with the experimental results: (spolarized, φ ) 0°) > (s-polarized, φ ) 90°) and (p-polarized, φ ) 90°) > (p-polarized, φ ) 0°). When the molecular transition dipole is linearly polarized as in the case of MB, the order of the magnitude of ER response is the same according to the above-described model. Since MV2+/MV•+ is incorporated in Nafion film with random orientation, the anisotropy was not observed in Figure 3. To rationalize the model above, we are now undergoing the calculation of the electric field of the light at the EPG surface as well as the search of the adsorbate which selects microscopic edge planes as the adsorption site. These points should be reported in our next publication. Conclusion The anisotropy of the ER response due to the redox reaction of adsorbed species was found at an EPG electrode. The ER response was enhanced when the component of the electric field of the incident light parallel to the basal plane is greater. We proposed a model which attributes the anisotropy to the oriented
Letters adsorption of the redox species on the microscopic basal planes. Although the model is not yet established with a quantitative basis, we may be able to point out that the existence of the microscopic basal planes, which are perpendicular to the macroscopic electrode surface, should not be ignored in use of EPG as an electrode. Acknowledgment. This work was financially supported by the grant-in-aids form the Ministry of Education, Science, Sports and Culture of Japan and the Sumitomo Foundation. References and Notes (1) McCreery, R. L. Carbon Electrodes: Structural Effects on Electron Transfer Kinetics. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1991; Vol. 17, p 221 and references therein. (2) Zhao, J.; McCreery, L. Langmuir 1995, 11, 4036. (3) Sagara, T.; Niki, K. Langmuir 1993, 9, 831. (4) Sagara, T.; Takagi, S.; Niki, K. J. Electroanal. Chem. 1993, 349, 159. (5) Bewick, A.; Cunningham, D. W.; Lowe, A. C. Makromol. Chem., Makromol. Symp. 1987, 8, 355. (6) Henglein, F.; Lipkowski, J.; Kolb, D. M. J. Electroanal. Chem. 1991, 303, 245. (7) Plieth, F.; Kozlowski, W.; Twomey, T. Reflectance Spectroscopy and Ellipsometry of Organic Monolayers. In Adsorption of Molecules at Metal Electrode; Lipkowski, J., Ross, P. N., Eds.; VCH Publishers: New York, 1992; p 239. (8) Sagara, T.; Takeuchi, S.; Kumazaki, K.-I.; Nakashima, N. J. Electroanal. Chem. 1995, 396, 525. (9) Sagara, T.; Sato, H.; Niki, K. Bunseki Kagaku 1991, 40, 641. (10) Tao, N. J.; Cardenac, G.; Cunha, F.; Shi, Z. Langmuir 1995, 11, 4445. (11) Imabayashi, S.; Nakamura, T.; Sagara, T.; Niki, K. Denki Kagaku 1994, 62, 526.
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