Electroreflectance Study of Hemin Adsorbed on a HOPG Electrode

incident light on the incident angle was analyzed at the formal potential E°′ of adsorbed hemin. ...... in-Aid from the Ministry of Education, Scie...
0 downloads 0 Views 95KB Size
Download PDF
J. Phys. Chem. B 1998, 102, 521-527

521

Electroreflectance Study of Hemin Adsorbed on a HOPG Electrode: Estimation of Molecular Orientation and Analysis of Nonfaradaic Electroreflectance Signal Due to the Stark Effect Takamasa Sagara,* Masato Fukuda, and Naotoshi Nakashima Department of Applied Chemistry, Nagasaki UniVersity, Bunkyo, Nagasaki 852-8521, Japan ReceiVed: April 24, 1997; In Final Form: October 6, 1997X

The structural and spectroelectrochemical properties of a layer of hemin (iron(III) protoporphyrin IX chloride) adsorbed on the basal plane surface of a highly oriented pyrolytic graphite (HOPG) electrode were studied using electroreflectance (ER) spectroscopy. The intensity dependence of the ER response to p- or s-polarized incident light on the incident angle was analyzed at the formal potential E°′ of adsorbed hemin. Hemin molecules were found to lie almost flat on the HOPG electrode; i.e., the angle between the porphyrin plane and the surface normal of the electrode is 70-80°. At potentials both far positive and negative from E°′, adsorbed hemin exhibited a nonfaradaic ER response to p-polarized and s-polarized incident light attributable to the Stark effect, indicating that the adsorbed hemin orients obliquely. The linear dependence in the shift of the ER spectral band due to the Stark effect indicates that the orientation of hemin molecules is insensitive to changes in electrode potential.

Introduction The orientation of molecules immobilized on an electrode surface is one of the key factors affecting the functional properties of such molecules. The molecular orientation of porphyrins immobilized on an electrode surface has been extensively investigated, since the orientation strongly influences their electrocatalytic and photoelectrochemical properties.1-17 The orientation of an electrocatalytic porphyrin immobilized on an electrode surface is known to govern the catalytic activity in several ways including the interaction of the porphyrin with the electrode surface, the interaction among porphyrin molecules adsorbed on the surface, and the accessibility of substrates in the solution phase to the porphyrin. Attempts have also been made to control the orientation of metalloporphyrins by derivatizing the porphyrin with thiol terminals8-10 or by anchoring the porphyrin covalently on glassy carbon through an amide linkage.11 The molecular orientation of porphyrins adsorbed on an electrode surface has been studied using electrochemical and spectroscopic techniques.12-15 A flat orientation has been observed in most cases including R,β,γ,δ-tetra(4-(trimethylammonium)phenyl)porphyrin on a Ag electrode,12 cobalt(II) hexadecyltetrapyridylporphyrin amphiphile on a Au electrode,13 and chlorophyll on an amalgamated Au electrode,14 with the exception of µ-oxo[meso-tetrakis(methoxyphenyl)porpyrinato]duron, which has been found to assume an edge-on orientation on a Ag electrode.15 Scanning probe microscopy (SPM) using scanning tunneling microscopes (STMs) or atomic force microscopes (AFMs) has recently been used to observe porphyrins or metalloporphyrins on an electrode surface in water.1-7,16 High-resolution STM images of hemin (iron(III) protoporphyrin IX chloride) on highly oriented pyrolytic graphite (HOPG)1-3 and tetrakis(N-methylpyridinium-4-yl)porphine on iodine-modified Au(111), Ag(111), and Pt(111) electrodes4-7 show a flat * Corresponding author. E-mail: [email protected]. X Abstract published in AdVance ACS Abstracts, December 15, 1997.

and two-dimensionally aligned adsorption structure. Although SPM provides a powerful tool for observing the molecular orientation, complementary spectroscopic data are still indispensable. The reason is that SPM is a pseudocontact method, in contrast to the spectroscopic method. The apparent adsorption structure under the SPM probe is sometimes altered by the presence of the probe and differs from the structure in the absence of the probe.18,19 Furthermore, it is uncertain whether the SPM image is representative of all adsorbed molecules or represents only the local structure. On the other hand, spectroscopic methods have the inherent limitation of providing information that has been averaged over the area of the light spot used for irradiation. Unless the electrode surface is known to be homogeneous and defectless, it is quite difficult to evaluate information about each of the distinct adsorption states using spectroscopic data alone. Electroreflectance (ER) spectroscopy is one of the spectroelectrochemical techniques that can be used to estimate the molecular orientation of adsorbed molecules.20-22 An ER study of the molecular orientation of adsorbed hemin was previously conducted on a pyrolytic graphite (PG) electrode.21 The results of ER measurements with polarized incident light suggested that hemin orientates obliquely on the electrode surface. However, the surface of the PG electrode was not flat enough to allow a quantitative conclusion about the orientation. Recent STM studies on the adsorption structure of hemin on the basal plane of a HOPG electrode have reported conflicting results. Snyder and White presented STM images showing that hemin forms aggregates approximately 30 Å in diameter on the basal plane of HOPG in a 0.1 M Na2B4O7 solution containing 0.5 mM of hemin.3 On the other hand, Tao et al. found from STM observation that a hemin (sub)monolayer, in which the hemin molecules are lying flat, is formed in a 0.1 mM hemin solution if the adsorption time is short (approximately 10 min).2 The present study used ER spectroscopy to investigate the structure of hemin adsorbed on the basal plane surface of a

S1089-5647(97)01392-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/15/1998

522 J. Phys. Chem. B, Vol. 102, No. 3, 1998

Sagara et al.

HOPG electrode in Na2B4O7 solution. Both the faradaic and nonfaradaic ER responses to polarized light were analyzed, typically under the same conditions as in Tao’s work.2 The effect of surface defects was also investigated by using an intentionally roughened basal plane HOPG electrode prepared by electrochemical pretreatment. The HOPG electrode provides a flat surface at the atomic scale over the area of the light spot used for ER measurement. This flatness is necessary when attempting to determine the molecular orientation using the ER method with polarized incident light.22 The faradaic ER signal represents the interaction between the electric dipole moment for absorption and the electric field of the standing waves of incident light as a function of the oxidation state of the adsorbed molecules, whereas the nonfaradaic ER signal represents the interaction among the electric dipole moment, the static electric field at the electrode interface, and the electric field of the standing waves of polarized incident light. Both faradaic and nonfaradaic ER responses to polarized light are sensitive to the molecular orientation. Experimental Section Hemin, purchased from Acros, was used as received. Water was purified through a Milli-Q Plus Ultra-pure water system (Millipore Co.). Resistivity of the purified water was greater than 18 MΩ cm. All other chemicals were of reagent grade. For all measurements, a 0.1M Na2B4O7 solution (pH 9.2) was used as the base solution. The highly oriented pyrolytic graphite (HOPG, Panasonic graphite PGC-X-05 grade; size, 13 mm × 13 mm × 3 mm-thickness) was purchased from Matsushita Electric Co. The electrode was cleaved using Scotch tape to expose a fresh basal plane (graphite (0001) surface) with a surface area of 0.79 cm2. The electrode was mounted in a homemade Kel-F holder with a fluorinated rubber O-ring and immediately immersed in deaerated base solution. The capacitance of the HOPG electrode was then measured in the base solution by cyclic voltammetry in the scan range between -0.5 and 0.3 V to evaluate the quality of the surface prior to starting the adsorption procedure. Only HOPG electrodes with a capacitance lower than 2.5 µF cm-2 at 0.0 V were used. Voltammetric and ER measurements were made using a quartz cell with an optically flat window. The reference electrode was a Ag/AgCl electrode in saturated KCl solution. The counter electrode was a coiled gold wire. All measurements were made in ambient argon (99.998% purity) at 22 ( 3 °C. The instrumentation used for the ER measurements is described elsewhere.23 A 300-700 nm polarizer (Sigma Koki, extinction ratio 1/10000) was used for irradiating p- or spolarized light. The waveform used to modulate the electrode potential is described as

E ) Edc +

∆Eac

x2

exp(jωt)

(1)

where E is the electrode potential, Edc is the dc potential, ∆Eac is the rms amplitude, j ) (-1)1/2, ω ) 2πf, which is the angular frequency (f is the frequency of the potential modulation), and t is the time. In the ER spectral measurements, both the real part of the ER signal (in-phase component of ∆R/R with respect to the potential modulation) and the imaginary part (90° outof-phase component) were monitored simultaneously during the stepwise wavelength scan, where ∆R/R is the ac reflectance divided by the time-averaged reflectance. A lock-in-amplifier (EG&G model 5210) was used for the phase-sensitive detection of the ER signal. Optical constants of the bare HOPG electrode

Figure 1. (A) Cyclic voltammogram in 0.1 M Na2B4O7 solution for hemin-adsorbed HOPG electrode prepared by 10 min adsorption from 0.03 mM hemin solution. Sweep rate in mV s-1 are the following: (a) 20; (b) 40; (c) 60; (d) 80; (e) 100; (f) 150. (B) Peak current (ip) of cyclic voltammogram versus sweep rate (V): (b), cathodic peak current; (O) anodic peak current.

were obtained using a spectroscopic ellipsometer (M-150, JASCO Co.). Electrochemically pretreated HOPG electrodes were prepared according to the procedure reported by Bowling et al.24 by polarizing the electrode potential at 1.95 V in 0.1 M HNO3 for 2 min. The resulting HOPG electrode exhibited a peak separation of 70 mV for the cyclic voltammetric response of [Fe(CN)6]3-/4- at 100 mV s-1, while that for the untreated HOPG was approximately 780 mV. The capacitance of the electrochemically pretreated HOPG electrode at 0.0 V was approximately 21 µF cm-2. The surface morphology before and after pretreatment was imaged using SEM (Hitachi, S-2250N) in the secondary electron mode. The SEM image after pretreatment was quite similar to the image reported by Bowling et al.24 Several line-defects were clearly visible within the 100 × 100 µm2 area. When the pretreated HOPG surface was illuminated by visible laser light, a nondiffusive reflection spot was seen, indicating that the area surrounding the defects was still optically flat. Results Voltammetric Measurements. Hemin was adsorbed by exposing a freshly prepared HOPG basal plane surface to a 0.1M Na2B4O7 solution (pH 9.2) containing 0.03 mM of hemin for a given adsorption time. After the adsorption procedure, the HOPG electrode was rinsed well with the base solution and placed in deaerated base solution in the cell. Voltammetric measurements were started after Ar gas was thoroughly bubbled in the solution for longer than 30 min. Figure 1A shows a typical cyclic voltammogram (CV) for a hemin-adsorbed HOPG electrode. The adsorption time was 10 min. The redox wave of hemin was observed at a formal potential E°′ of -0.460 V. Figure 1B shows a plot of peak

Hemin on HOPG Electrode

J. Phys. Chem. B, Vol. 102, No. 3, 1998 523

Figure 3. ER spectra with p- or s-polarized incident light at five different incident angles (θ). Edc ) -460 mV; ∆Eac ) 50 mV; f ) 14 Hz. Solid line is for p-polarized incident light; dotted line is for s-polarized incident light. Figure 2. (A) ER spectrum of hemin-adsorbed (0.70 ML) HOPG electrode in 0.1 M Na2B4O7 solution measured with nonpolarized incident light. Edc ) -460 mV; ∆Eac ) 30 mV; f ) 14 Hz. Solid line represents the real part; dotted line represents the imaginary part. (B) ER voltammogram of hemin-adsorbed HOPG electrode measured with nonpolarized incident light. Wavelength is 440 nm; ∆Eac ) 50 mV, f ) 14 Hz; sweep rate is -3 mV s-1. Solid line represents the real part; dotted line represents the imaginary part.

current ip versus sweep rate V. The peak current is proportional to V, indicating that the voltammetric response is due to the redox reaction of surface-confined hemin. Under deaerated conditions, the voltammetric response of adsorbed hemin remained stable for hours. The change in ip was less than 5% over a 12-h period. An additional small peak was observed at -0.27 V at low sweep rates. A small, constant cathodic current was seen in the double-layer charging region negative to E°′. The cathodic peak heights were slightly greater than the anodic peak heights (Figure 1B). All these additional cathodic voltammetric responses became more pronounced when the Ar bubbling time decreased. Therefore, the additional cathodic current is attributable to the O2 reduction catalyzed by the adsorbed hemin. It should be noted that reduction of residual O2 at the HOPG electrode surface is extremely enhanced by the presence of adsorbed hemin. The peak separation in the V range of 20-150 mV s-1 was almost zero, indicating that a reversible redox reaction occurs in this V range. The wave was symmetrical in shape with respect to the peak potential. The half-height peak width was 130 mV, which is greater than the ideal value of the reversible CV wave for the one-electron-transfer reaction of a noninteracting adsorbed species. The amount of adsorbed hemin was calculated from the anodic peak charge at V ) 40 mV s-1 as being 7.7 × 10-11 mol cm-2. If hemin molecules assume a flat orientation with the alignment suggested by the reported STM image2 on a perfectly flat HOPG surface, the monolayer coverage is 1.1 × 10-10 mol cm-2. The coverage obtained in the present experiment corresponds to 0.70 ML (ML ) monolayer). The experiment was repeated seven times in total under the same adsorption condition. The voltammetric characteristics were the same as described above in all seven experiments when using HOPG electrodes with a capacitance in the range 1.82.5 µF cm-2. The coverage was on average 0.40 ( 0.20 ML with a maximum of 0.70 ML and a minimum of 0.10 ML. Electroreflectance Measurements of Redox Reaction. Figure 2A shows a plot of the ER signal as a function of the wavelength of incident light λ. The electroreflectance (ER) spectrum was measured at Edc ) E°′ for the same heminadsorbed HOPG electrode as that used in the voltammetric measurements. The background ER signal of the HOPG

electrode was negligibly small. Thus, the ER signal in Figure 2A can be considered to originate totally from the adsorbed hemin. The ER spectral bands around 400-500 nm correspond to the Soret bands of hemin. The ER spectral bands at the longer wavelengths correspond to the Q-bands of hemin. All the cross points between the real and imaginary spectra fall on the zero line. The peak wavelengths of the real and imaginary parts are the same for all four peaks. The ER spectral profile was independent of the modulation frequency. These facts indicate that the ER signal consists of a single component. Figure 2B shows a plot of the ER response at 440 nm, which is the wavelength of the maximal ER signal in Figure 2A, as a function of Edc throughout the linear sweep of the potential. This ER voltammogram shows a symmetrical peak with respect to the peak potential at -0.464 V, which is consistent with the E°′ obtained from CV. This confirms that the ER spectrum in Figure 2A is due to the redox reaction of adsorbed hemin. It is important to note that a nonzero real part of the ER signal is seen in the double-layer potential region both positive and negative relative to E°′. This signal is not due to the background ER signal at the HOPG electrode but is a nonfaradaic ER response from the adsorbed hemin. This nonfaradaic ER response will be discussed later in detail. Polarization Dependence of Faradaic ER signal. Figure 3 shows a series of ER spectra at E°′ in response to polarized incident light. The spectral profile and peak wavelengths are independent of the polarization type and the incident angle θ at the air/cell-wall interface. The magnitude of the ER response in the Soret band region to s-polarized light decreases relative to p-polarized light as the incident angle increases. The dependence of the ER signal on the incident angle of polarized light was analyzed using a method described in detail elsewhere.22 The assumptions used in the present calculations are: (1) The molecular orientation of adsorbed hemin is independent of the oxidation states; (2) The orientation angle φ, which is the angle between the surface normal of the HOPG substrate and the porphyrin plane of hemin, is the same for all hemin molecules; (3) The electric dipole moment of the hemin molecule for the transition in the Soret band region is circularly polarized on the iron-porphyrin plane.25 This means that the Soret band transition moment for a hemin molecule can be divided into two orthogonal dipoles of identical magnitude on the porphyrin plane. This is equivalent to assuming that hemin molecules are freely rotating about a director, which is inclined to the surface normal at an angle φ. First, we used a two-phase model as an approximation. The electrode interface was modeled as a simple two-phase system consisting of water and HOPG. To calculate the ratio of the ER response to p-polarized and s-polarized light ([p/s] ratio) as a function of θ, the mean-square electric field of the surface

524 J. Phys. Chem. B, Vol. 102, No. 3, 1998

Sagara et al.

standing wave of light at the HOPG surface was calculated using the two-phase model. Then the interaction of the electric field with the electric dipole moment of hemin was calculated. The optical constants used were the refractive index of water, n1 ) 1.333, and the complex dielectric constant of the HOPG substrate at 440 nm, ˆ ) ′ + j′′ ) 5.902 + j10.52. The optical constant of the HOPG was measured in this study by ellipsometry. The values reported here are slightly different from the values reported previously.26,27 The rationale behind the use of the two-phase model is as follows. Since the ER response magnitude was less than 4 × 10-3, the interaction between the electric field of the light and the transition moment can be assumed to be so small as to have a negligible effect on the electric field of the light. In other words, the presence of the chromophore-containing layer is thought to introduce a negligibly small perturbation to the electric field. The p/s ratio for a chromophore with a circularly polarized transition moment can be expressed as22,28

p/s )

2Ep⊥2 sin2 φ + Ep|2(1 + cos2 φ) Es2(1 + cos2 φ)

(2)

where Ep⊥2 and Ep|2 are the perpendicular-to-surface and parallel-to-surface components, respectively, of the mean-square electric field of the surface standing wave (〈E2〉) of p-polarized incident light at the position of the chromophore, and Es2 is the parallel-to-surface component of 〈E2〉 of s-polarized incident light at the position of the chromophore. Figure 4A shows working curves of the p/s ratio calculated based on the two-phase model for various φ at 440 nm. The p/s ratio is plotted as a function of incident angle θ. The incident angle at the electrode surface with respect to the surface normal is equal to arcsin(n0 sin θ/n1), where n0 ) 1 is the refractive index of air. The values of 〈E2〉 at the position of hemin molecules were calculated using Hansen’s formula29 based on the two-phase model shown in the inset. The p/s ratio data obtained by ER measurements at 440 nm are also shown in the figure. When the coverage is 0.70 ML (open circles), the data points for φ fall between 80 and 83° except for the point at θ ) 45°. When the point at θ ) 45° is excluded, the average for φ is 81.0 ( 1.1°. This indicates that hemin molecules orientate nearly, but not perfectly, flat. The experiment was repeated for a hemin-adsorbed HOPG electrode with a coverage of 0.30 ML (closed circles). At this lower coverage, the average orientation angle was 77.5 ( 2.9°. Working curves were also calculated based on a three-phase model. In this model, a layer of hemin is sandwiched between the water and HOPG phases. Figure 4B shows the set of working curves calculated with the three-phase model. The layer of hemin was regarded as an optically homogeneous continuum, and the optical anisotropy of the layer was not taken into consideration. A complex refractive index of 1.4 + j1.0 was assumed as the optical constant of the adsorption layer. The thickness of the adsorption layer was set as being 0.20 nm. The values of 〈E2〉 at the position of hemin molecules were calculated using Hansen’s formula29 based on the three-phase model shown in the inset. When the working curves in Figure 4B are compared to those in Figure 4A, the position of all the curves is shifted downward. It is noteworthy that changes in the assumed optical constant of the adsorption layer do not affect the curves at higher angles of φ in Figure 4B. The orientation angles obtained by plotting the measured p/s ratios are 75 ( 1.6° for a 0.70 ML hemin film (open circles) and 70 ( 4.4° for a 0.30 ML hemin film (closed circles).

Figure 4. (A) Plot of the p/s ratio of ER response versus incident angle (θ) for various orientation angles (φ). Working curves are calculated using a two-phase model. Optical parameters are shown in the inset. Open circles are for hemin-adsorbed HOPG electrode with 0.70 ML; closed circles are for hemin-adsorbed HOPG electrode with 0.30 ML. (B) Plot of p/s ratio of ER response versus θ for various φ. Working curves are calculated using a three-phase model. Details are same as in part A above.

Nonfaradaic ER Signal in Double-Layer Potential Region. As described in the previous section, a nonfaradaic ER signal was observed in the double-layer potential region (see Figure 2B). With the exception of line d, the ER spectra in Figure 5A are the nonfaradaic ER signals measured with nonpolarized incident light at Edc ) 0.100 V. The potential is so positive with respect to E°′ that all the hemin molecules are of the oxidized form. The spectra of the nonfaradaic ER signal represent a bipolar structure in the Soret band region. The magnitude of the nonfaradaic ER signal (IER) was proportional to ∆Eac up to 150 mV (Figure 5B). For the faradaic ER response at E°′, the magnitude of the imaginary part of the ER signal was comparable to the real part (Figure 2A). In contrast, the imaginary part of the nonfaradaic response (line e in Figure 5A) is much smaller than the real part. This suggests that the nonfaradaic response is due to a kinetically much faster process than the redox reaction.30 The magnitude of the nonfaradaic ER response was approximately 6% of the faradaic ER response at E°′ given the same ∆Eac and f. The wavelength of the zero-ER response λ0 is defined as the wavelength at which the bipolar-shaped nonfaradaic ER spectrum in the Soret band region intercepts the zero line. Figure 5C shows a plot of  ) hc/λ0 as a function of Edc at a constant ∆Eac, where  is the energy corresponding to λ0, h is the Planck

Hemin on HOPG Electrode

Figure 5. (A) ER spectra of hemin-adsorbed (0.70 ML) HOPG electrode in base solution measured with nonpolarized incident light at f ) 14 Hz. Lines a-c are real part spectra at Edc ) 100 mV with ∆Eac in mV: (a) 150; (b) 100; (c) 50. Line d represents a real part spectrum of faradaic ER response at Edc ) -460 mV. Lines e are imaginary part spectra at Edc ) 100 mV corresponding to lines a-c. (B) Plot of ER response intensity (IER) at λ ) 419 nm, Edc ) 100 mV, as a function of ∆Eac. (C) Plot of  as a function of Edc.

constant, and c is the velocity of light in a vacuum. The data points can be fit to a straight line. The first derivative in the negative-going band of the real part of the faradaic ER spectrum (line d in Figure 5A) with respect to the hc/λ gives rise to a bipolar-shaped curve. When this curve is shifted by 0.07 eV toward higher energy, it can be superposed on the nonfaradaic ER spectrum at Edc ) 0.100 V. The negative-going band in the Soret region of the faradaic ER spectrum corresponds to the apparent absorption band of the oxidized form of adsorbed hemin. Therefore, the obtained nonfaradaic ER response exhibits the same spectral structure as the first derivative of the apparent absorption band of the oxidized form of adsorbed hemin. When the kinetically faster process, proportionality of ER intensity to ∆Eac, linear band energy shift in response to Edc, and shape of the spectral curve are all taken into account, the origin of the nonfaradaic ER response can be assigned to electrochromism resulting from the potential-dependent interaction of the static electric dipole responsible for the light absorption of the chromophore in the ground and excited states with the static electric field at the electrode surface. This phenomenon is known as the Stark effect (vide infra).20,21,31-35 Figure 6A shows the nonfaradaic ER response measured with polarized incident light at a positive Edc relative to E°′. At this potential, all the adsorbed hemin molecules are of the oxidized form. The ER response due to the Stark effect occurred in response to both p-polarized and s-polarized incident light. Figure 6B shows nonfaradaic ER spectra measured with nonpolarized incident light at a negative Edc relative to E°′. At this potential, nearly all the adsorbed hemin molecules are of the reduced form. A nonfaradaic ER response was observed regardless of the oxidation state of the adsorbed hemin. At a negative potential relative to E°′, the nonfaradaic ER spectrum of a bipolar structure exhibited the same curve shape as the first derivative of the positive-going peak of the real part of the faradaic ER spectrum in the Soret region shifted to a shorter wavelength by 20 nm. This suggests that the nonfaradaic ER spectrum at negative potentials is also due to the Stark effect

J. Phys. Chem. B, Vol. 102, No. 3, 1998 525

Figure 6. (A) ER spectrum (real part) of nonfaradaic response with p- or s-polarized incident light: Edc ) 100 mV; ∆Eac ) 100 mV; f ) 14 Hz. Solid line is for p-polarized light; broken line is for s-polarized light. (B) ER spectrum of nonfaradaic response with nonpolarized incident light: Edc ) -800 mV; ∆Eac ) 100 mV; f ) 14 Hz. Line a is the real part; line b is the imaginary part. Line c is same curve as line d in Figure 5A, showing faradaic ER spectrum.

on the reduced form of adsorbed hemin. At negative potentials, the nonfaradaic ER response to both p- and s-polarized light at nearly the same magnitude was observed. Measurements Using Electrochemically Pretreated HOPG Electrode. All the above results are for the freshly cleaved HOPG basal plane (graphite (0001) surface). It should be noted that the HOPG surface produced by careful peeling could still contain defect sites. At these defects, the edges of the basal planes are also exposed to the solution. Since the spot size of the light used for ER measurement was much larger than the size of a (0001) face terrace, the ER response could involve signals from hemin molecules at such defect sites. To determine whether the dominant contributor to the ER response was hemin adsorbed on defect-free (0001) planes or hemin at defect sites, we measured ER on a HOPG electrode surface that was pretreated electrochemically in order to intentionally introduce defects. Hemin was adsorbed on the pretreated electrode using the same procedure described in the previous sections. Hemin concentration was 0.03 mM and adsorption time was 10 min. A reversible voltammetric response for the adsorbed hemin was observed in the V range of 20-150 mV s-1. The differences from the data in Figure 1 were the following: (1) E°′ was -0.450 V, which was 10 mV more positive for the pretreated electrode than E°′ for the untreated electrode; (2) The coverage of hemin calculated using the surface area of 0.79 cm2 was 6.2 × 10-11 mol cm-2, which was slightly less than the coverage in Figure 1; (3) The half-height peak width was 170 mV, which was greater than that in Figure 1. These results suggest either that the defect sites are at least not the preferential adsorption sites for hemin or that hemin molecules adsorbed at the defect sites are electrochemically inactive. The ER spectrum at E°′ showed a spectral profile identical to the spectrum in Figure 2A. The electroactive hemin adsorbed on the pretreated HOPG electrode exhibited the same ER spectral features for the faradaic and nonfaradaic response as those on the untreated HOPG electrode. ER spectral measurements were carried out with por s-polarized light at various angles of incidence. By use of the two-phase model and the optical parameters of pretreated HOPG obtained by ellipsometry (ˆ ) 4.038 + j8.580), the orientation angle of adsorbed hemin molecules was calculated to be approximately 80° with respect to the surface normal.

526 J. Phys. Chem. B, Vol. 102, No. 3, 1998

Sagara et al.

Discussion Considering the surface properties of the graphite electrode, the present ER study provides a significantly more quantitative insight into the orientation of adsorbed hemin than the previous work.21 Although a basal plane produced by a peeling procedure was used in the previous work,21 the PG electrode surface properties were largely different from those of the (0001) surface of HOPG. The surface in the previous work did not present a mirrorlike appearance, and the interfacial capacitance was as high as 20 µF cm-2, indicating a high surface roughness and the presence of impurities such as edge faces. The (0001) surface of HOPG used in the present study is flat at the atomic scale over the area of the light spot used for ER measurement. It is likely that the adsorption procedure used in the present study yielded a submonolayer of hemin with a deeply oblique orientation. In light of the report by Tao et al.,2 adsorption time was short enough to create an aggregationless submonolayer. The coverage was less than the monolayer coverage with a flatly lying orientation. The half-height peak width as well as the ER spectral profile were independent of the coverage. This indicates that increased coverage does not induce changes in the electric interaction between hemin molecules, which might become pronounced if a multilayer had been formed. Even at prolonged adsorption times, we were unable to obtain a heminadsorbed electrode with more than a monolayer coverage judging from the CV peak charge. Since Tao et al. found that a relatively long adsorption time results in islands of aggregated hemin 8.5 Å high and that the aggregates are electrochemically inactive,2 we cannot be certain that the hemin layers formed in the present study were aggregationless submonolayers and that all adsorbed hemin molecules remained electroactive. This should be confirmed using in situ STM imaging. Nevertheless, we can conclude that electroactive hemin molecules, which are likely formed into a submonolayer, assume a deeply oblique orientation (φ ≈ 70-80°) on average, since the faradaic ER response arises only from electrochemically active hemin. It should be noted that the nonfaradaic ER signal may partially involve a contribution from electroinactive hemin molecules, in contrast to the faradaic ER signal. The Stark effect brings about a shift in the absorption bands.34,35 As for the linear Stark effect, the shift in band energy can be expressed as

∆ ) hc/∆λ ) k∆µF cos R

(3)

where ∆λ is the wavelength shift, k is a constant, ∆µ is the change in the static dipole moments in the ground and excited states of the chromophore (∆µ b)b µexcited - b µground), F is the static electric field at the position of the chromophore, and R is the angle between the direction of charge displacement and F.31,32 In the case of Soret band absorption for hemin, R is equal to the angle between the porphyrin plane and F. Since the direction of field F is perpendicular to the flat electrode surface, φ can be substituted for R. The ER spectrum can be regarded as representing the difference between the apparent absorption spectra at positive and negative potentials relative to Edc. Therefore, when a peakshaped apparent absorption band shifts with Edc according to eq 3, the resulting ER spectral shape constitutes the first derivative of the spectral band with respect to hc/λ and presents a bipolar feature. All the experimentally obtained spectral profiles of the nonfaradaic ER response agree well with this prediction. We can conclude that the nonfaradaic response is due to the Stark effect.

It is known that the differential capacitance at the (0001) HOPG electrode/solution interface is totally dominated by the space-charge layer capacitance of HOPG,36-40 since HOPG is a semimetal. Therefore, the value of F in eq 3 cannot be obtained directly by traditional capacitance measurements. Gerischer et al. estimated the potential drop across the Helmholtz layer (∆φH).37 According to their results (Table 1 in ref 37), approximately 20% of the total potential difference (E - Epzc) is applied across the Helmholtz layer and φH can be regarded approximately as a linear function of E - Epzc in the region of |E - Epzc| e 0.5 V, where Epzc is the potential of zero charge. Therefore, if R is independent of ∆φH, the Stark shift is a linear function of Edc according to eq 3. We have obtained two significant results: IER is proportional to ∆Eac (Figure 5B) and ∆ is a linear function with respect to Edc (Figure 5C). These results confirm that R is independent of ∆φH at least in the Edc range from -0.05 to 0.40 V. We define n as the energy corresponding to the peak wavelength of the Soret band negative-going ER peak of the real part of the faradaic ER response measured at E°′. Since Epzc is midway between E°′ and 0.10 V (the reported Epzc value for HOPG is -0.20 V vs EAg/AgCl36,37,40), it is reasonable to assume that n does not lie on the extrapolated straight line of the plot of  versus Edc. Actually, it does not, as shown in Figure 5C. The magnitude of the change in F in response to potential modulation decays steeply as the distance from the electrode surface increases.37 Therefore, even if hemin molecules are partially formed into a multilayer, the molecules in direct contact with the electrode surface are the major contributor to the ER response to the Stark effect. If all the adsorbed hemin molecules lie completely flat on the HOPG surface, the Stark effect should not be observed because the dipole responsible for the transition cannot interact with the surface field as expressed by eq 3. If the porphyrin ring of all the adsorbed hemin molecules assumed an upright orientation, the p-polarized light would give rise to a significantly greater ER signal than the s-polarized light. Therefore, the appearance of the Stark effect with both p- and s-polarization at nearly the same intensity provides strong evidence for the oblique orientation of adsorbed hemin. The intensities of the ER response to the Stark effect with p-and s-polarized incident light are nearly the same also at negative potentials below E°′. The intensity of the nonfaradaic ER voltammetric signal at potentials more negative than E°′ in Figure 2B appears to be independent of Edc. These facts suggest that hemin molecules orientate obliquely also in the range from -0.80 to 0.40 V. Measurements using an electrochemically pretreated HOPG electrode have demonstrated that defects induced on the HOPG surface modify the electrochemistry and average molecular orientation only slightly. If the results measured from the untreated HOPG electrode were dominated by the behavior of hemin molecules at the defects, pretreatment would have enhanced the voltammetric and electroreflectance response compared to the response from the untreated HOPG electrode. Consequently, the majority of hemin molecules analyzed by the CV and ER methods are thought to be adsorbates at the basal plane rather than at the defects, on both untreated and pretreated HOPG electrodes. Our electroreflectance studies in the vicinity of the formal potential and in the double-layer potential region allow us to conclude that hemin orients obliquely. The potential-insensitive orientation suggests that the oblique orientation is not due to the electrostatic interaction of hemin with HOPG. Since the basal plane of HOPG possesses no functional group or dangling

Hemin on HOPG Electrode bonds, strong chemical interaction can be ruled out. The fact that the orientation is independent of the coverage implies that the interaction between adsorbed hemin molecules may not be an important factor affecting the orientation. The presence of an OH- ligand to central iron of hemin between the hemin molecule and the graphite surface or steric hindrance of the propionate group for flat orientation can be speculated as the reason, though further discussion is unwarranted in light of present results. Since the ligand to central iron and the proton association of the propionate group depend on pH, we are currently investigating the pH dependence of the orientation of adsorbed hemin. The Stark effect in the UV-vis wavelength region at the solid/liquid interfaces of metal complexes has been reported.41,42 However, to the best of our knowledge, this report may be the first one confirming the Stark effect in metalloporphyrins and at the HOPG electrode surface. It is interesting to note that the Stark effect was clearly observed even though the change in φH, and consequently in F too, in response to potential modulation is much smaller than the change in E - Epzc. Conclusion The structural and spectroelectrochemical properties of a layer of hemin adsorbed on the (0001) surface of a HOPG electrode were studied using CV and ER methods in a Na2B4O7 solution. The dependence of the intensity of the faradaic ER response at the formal potential of adsorbed hemin measured with p- or s-polarized incident light on the incident angle was analyzed. Electroactive hemin adsorbed on the HOPG electrode surface, presumably formed into a submonolayer, was found to orientate nearly, but not perfectly, flat on the surface. The angle between the porphyrin plane and surface normal was estimated to be 70-80°. At potentials both far positive and negative to the formal potential, a nonfaradaic ER response due to the Stark effect was observed. The ER response due to the Stark effect appeared with both p- and s-polarized incident light. The band shift due to the Stark effect was found to be linear with respect to electrode potential. The results indicate that adsorbed hemin assumes an oblique orientation on the electrode surface and that the orientation is insensitive to change in electrode potential. Nevertheless, it remains unclear why the evaluated molecular orientation reported in the present study differs from that reported by Tao et al.2 Further studies using complementary SPM and spectroelectrochemical methods are needed in order to shed more light on the adsorption structure of the electrode surface. Acknowledgment. This work was supported by a Grantin-Aid from the Ministry of Education, Science, Sports, and Culture of Japan to T.S. Financial support from Toray Science Foundation to N.N. and from Sumitomo Science Foundation to T.S. is also gratefully acknowledged. The authors also thank Professor McCreery (The Ohio State University) for his kind advice on the method for cleaving HOPG to expose the basal plane and Dr. T. Fukazawa (JASCO Corporation, Hachioji,

J. Phys. Chem. B, Vol. 102, No. 3, 1998 527 Tokyo) for measuring the optical constants of the HOPG substrate. References and Notes (1) Tao, N. J. Phys. ReV. Lett. 1996, 76, 4066. (2) Tao, N. J.; Cardenas, G.; Cunha, F.; Shi, Z. Langmuir 1995, 11, 4445. (3) Snyder, S. R.; White, H. S. J. Phys. Chem. 1995, 99, 5626. (4) Itaya, K.; Batina, N.; Kunitake, M.; Ogaki, K.; Kim, Y.-G.; Wan, L.-J.; Yamada, T. In Solid-Liquid Electrochemical Interface; Jerkiezicz, G., Soriaga, M., Uosaki, K., Wieckowski, A., Eds.; ACS Monograph, ACS Symposium Series 656; American Chemical Society: Washington, DC, 1996; p 171. (5) Ogaki, K.; Batina, N.; Kunitake, M.; Itaya, K. J. Phys. Chem. 1996, 100, 7185. (6) Batina, N.; Kunitake, M.; Itaya, K. J. Electroanal. Chem. 1996, 405, 245. (7) Kunitake, M.; Batina, N.; Itaya, K. Langmuir 1995, 11, 2337. (8) Postlethwaite, T. A.; Hutchison, J. E.; Hathcock, K. W.; Murray, R. W. Langmuir 1995, 11, 4109. (9) Zak, J.; Yuan, H.; Ho, M.; Woo, L. K.; Porter, M. D. Langmuir 1993, 9, 2772. (10) Hutchison, J. E.; Postlethwaite, T. A.; Murray, R. W. Langmuir 1993, 9, 3277. (11) Nagao, K.; Fujihira, M.; Osa, T.; Dong, S. Chem. Lett. 1982, 575. (12) Chen, J.; Hu, J.; Xu, Z.; Sheng, R. Appl. Spectrosc. 1993, 47, 292. (13) Van Galen, D. A.; Majda, M. Anal. Chem. 1988, 60, 1549. (14) Khanova, L. A.; Tarasevish, M. R. J. Electroanal. Chem. Interfacial Electrochem. 1987, 227, 99. (15) Holze, R. Electrochim. Acta 1988, 33, 1619. (16) Wang, E. Anal. Sci. 1994, 10, 155. (17) Basu, J.; Rohatgi-Mukherjee, K. K. Sol. Energy Mater. 1991, 21, 317. (18) Samerson, M.; Neubauer, G.; Folch, A.; Tomotori, M.; Ogletree, D. F.; Sautet, P. Langmuir 1993, 9, 3600. (19) Carpick, R. W.; Salmeron, M. Chem. ReV. 1997, 97, 1163. (20) Plieth, W.; Kozlowski, W.; Twomey, T. In Adsorption of Molecules at Metal Electrodes; Lipkowski, J., Ross, P. N., Eds.; VCH Publishers: New York, 1992; p 239. (21) Sagara, T.; Takagi, S.; Niki, K. J. Electroanal. Chem. 1993, 349, 159. (22) Sagara, T.; Kaba, N.; Komatsu, M.; Uchida, M.; Nakashima, N. Electrochim. Acta, in press. (23) Sagara, T.; Takeuchi, S.; Kumazaki, K.; Nakashima, N. J. Electroanal. Chem. 1995, 396, 525. (24) Bowling, R. J.; Packard, R. T.; McCreery, R. L. J. Am. Chem. Soc. 1989, 111, 1217. (25) Fromherz, P. FEBS Lett. 1970, 11, 205. (26) Wada, N.; Splin, S. A. Physica 1981, 105B, 353. (27) Taft, E. A.; Philipp, H. R. Phys. ReV. 1965, 138, A197. (28) Sperline, R. P.; Song, Y.; Freiser, H. Langmuir 1994, 10, 37. (29) Hansen, W. N. J. Opt. Soc. Am. 1968, 58, 380. (30) Feng, Z.-Q.; Sagara, T.; Niki, K. Anal. Chem. 1995, 67, 3564. (31) Grewer, G.; Lo¨sche, M. Makromol. Chem., Makromol. Symp. 1991, 46, 79. (32) Plieth, W. J.; Schmidt, P.; Keller, P. Electrochim. Acta 1986, 31, 1001. (33) Leow, L. M.; Simpson, L. L. Biophys. J. 1981, 34, 353. (34) Plieth, W. J.; Gruschinske, P.; Hensel, H.-J. Ber. Bunsen-Ges. Phys. Chem. 1978, 82, 615. (35) Platt, J. R. J. Chem. Phys. 1961, 34, 862. (36) Kneten Cline, K.; McDermott, M. T.; McCreery, R. L. J. Phys. Chem. 1994, 98, 5314. (37) Gerischer, H.; McIntyre, R.; Scherson, D.; Storck, W. J. Phys. Chem. 1987, 91, 1930. (38) Gerischer, H. J. Phys. Chem. 1985, 89, 4249. (39) Gerischer, H.; McIntyre, R. J. Chem. Phys. 1985, 83, 1363. (40) Randin, J.-P.; Yeager, E. J. Electroanal. Chem. 1972, 36, 257. (41) Hug, S. J.; Boxer, S. G. Inorg. Chim. Acta 1996, 242, 323. (42) Oh, D. H.; Sano, M.; Boxer, S. G. J. Am. Chem. Soc. 1991, 113, 6880.