Anal. Chem. 1990, 62,2647-2650
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TECHNICAL NOTES I n Situ Ultraviolet-Visible Reflection Spectroscopy of Cobalt Tetrasulfonated Phthalocyanine Irreversibly Adsorbed on the Basal Plane of Highly Oriented Pyrolytic Graphite Sunghyun Kim, X. Xu, I. T. Bae, Z. Wang, and D. A. Scherson* Case Center for Electrochemical Sciences and the Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44016
INTRODUCTION The development of in situ spectroscopic techniques applicable to the study of adsorbed species on electrode surfaces has been the subject of intense investigation in recent years ( I ) . Much of the interest in this area has been prompted by the need of acquiring highly specific information regarding modifications in the electronic, vibrational, and structural properties of atomic and molecular species adsorbed onto various electrode materials induced by changes in the applied potential across the interface. A better understanding of the factors governing these phenomena is expected to provide considerable insight into such areas of fundamental and technological importance as metal electrodeposition and electrocatalysis. Reflectance spectroscopy in the UV-vis region is especially suited for the study of a variety of interfacial phenomena ( I ) . Specifically, (i) the changes in reflectivity attending processes, such as the formation of oxide layers of molecular dimensions and the adsorption of ions and underpotential deposited metals, are large enough to be detected with rather conventional instrumentation, and (ii) aqueous solutions are essentially transparent over a wide energy range, making the use of thin-layer cells unnecessary. From a theoretical viewpoint, the reflectivity of the interface between two phases j and k, Rjk is defined as lrjkI2where rjk is the Fresnel coefficient of that interface. Such absolute reflectivities are very difficult to measure in systems involving conventional electrochemical cells (2). More amenable to experimental determination, however, is the normalized reflectivity defined as AR/R = R(d)/R(O) - 1 at constant geometry for which the various sources of errors are common and therefore cancel. In the case of potential-modulated reflectance spectroscopy it is customary to define a relative reflectivity, i.e. AR/R = R( V(sample))/R(V(reference)) R(V(sample)),where R(V(i)) is proportional to the light intensity collected at the detector with the electrode polarized a t the specified potential, V(i). Applications of UV-vis reflectance spectroscopy to molecular species that exhibit large extinction coefficients in this energy region are particularly important, as detailed information may be gained into modifications in their electronic properties brought about by surface bonding, the field at the interface, and changes in oxidation state. Large relative reflectivity signals have been observed, for example, by employing potential modulation in combination with lock-in detection methods in the m e of p-nitroaniline adsorbed onto Pt (3), cytochrome c ( 4 ) , and a series of macrocycles irreversibly adsorbed on gold and carbon surfaces. These include cobalt and metal-free 5,10,15,20-tetrakis(4-N-methylpyridy1)porphine (5) and Co, Ni, Zn, Fe, and Cu phthalocyanines (6) and some of their water-soluble sulfonated counterparts (7). 0003-2700/90/0362-2647$02.50/0
An alternate experimental approach introduced much earlier by Nikolic et al. (8)involves careful measurements of the reflectivity as a function of wavelength first in the absence and then in the presence of the irreversibly adsorbed species. Unlike the modulation techniques mentioned above, which provide a measure of the changes in the optical properties induced by potential variations, this method makes it possible to obtain normalized reflectivities that are closely related to the actual spectra of the species on the surface. In fact, very similar results were obtained by using this approach for cobalt(I1) and iron(II1) tetrasulfonated phthalocyanine, TsPc, adsorbed onto platinum and onto the basal plane of highly ordered pyrolytic graphite, HOPG(bp), at a single potential, and for the same materials in solution phase. Transition-metal macrocycles of the porphyrin and phthalocyanine type have received much attention because of their ability to promote the rates of dioxygen reduction when adsorbed onto a variety of electrode surfaces (9). In the course of an investigation aimed at monitoring the spectral properties of irreversibly adsorbed CoTsPc on HOPG(bp) in its various oxidation states, it was found that polarization of the electrode for long periods of time in acid electrolytes at potentials positive enough to oxidize the adsorbed macrocycle lead to the desorption of the material from the surface. Such a conclusion was made on the basis of the gradual decrease in the cyclic voltammetric peaks associated with the Col*/ComTsPcredox couple. This phenomenon was not observed, however, either upon polarizing the electrode for the same period of time with the adsorbed species in the fully reduced Co(I1) state or upon cycling the electrode repetitively in a linear fashion to sufficiently positive potentials to bring about the oxidation of a significant fraction of the adsorbed species. This note will present results of UV-vis reflectance spectroscopy measurements obtained during continuous potential cycling with a combination of light intensity modulation and signal averaging techniques. This method shares some commonalities with that described by Crouigneau et al. (IO) in connection with their studies of viologen adsorption of Pt electrodes in that the potential is scanned (as opposed to stepped) between two potential limits using a signal averager to improve the signal to noise ratio. The high quality of the data obtained in this work has made it possible to show that the changes in relative reflectivity at a constant wavelength, as evaluated from the spectroscopic data, are directly proportional to the amount of oxidized (or reduced) species, as determined from the cyclic voltammetry. In addition, the relative reflectivity as a function of potential has been found to increase or decrease depending on the energy of the incident radiation, indicating that the normalized reflectivity associated with the oxidized species may be, as expected, larger or smaller than that of the reduced species. 0 1990 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 23, DECEMBER 1, 1990 10.0
*
Y \ , I -
TEFLON FRAME
.-'
-i
OUTLET
E
J
I
INLET
=- ~-
I
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& \\ / STAINLESS STEEL PLATE
+ p r c
,
--
y
/
-
-
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__ a - _ ~ ~ ~ b_- - _ _-._ ---d
Figure 1. Schemetic diagram of the electrochemical cell for in situ reflectance measurements in the UV-vis range: (a)21 mm; (b) 41 mm; (c) 15 mm.
EXPERIMENTAL SECTION All experiments were conducted with a near-IR-vis spectrometer designed originally for circular dichroism measurements (11). It consists of a 250-W halogen-tungsten lamp, a mechanical chopper (PAR Model 125A) set at a frequency of 535 Hz, a Czerny-Turner type monochromator (Spex, Model 1700-11),a photomultiplier detector (Hamamatsu, Model R406),and various sets of mirrors used to optimize the overall optical path. A schematic diagram of the electrochemical cell for in situ reflectance spectroscopy is shown in Figure 1. The main component of the cell is a Teflon piece in the form of a frame interposed between a 45' quartz prism (and the HOPG(bp) electrode, forming a cavity (15 x 21 x 3.5 mm) that is filled with electrolyte. The Teflon piece has an inlet and an outlet that are used to exchange solutions without disturbing the relative position of the cell with respect to the optical path. A thin flexible Teflon tubing is used to connect the inlet to the external solution reservoir, which is in turn separated from the reference electrode compartment by a glass stopcock. The counter electrode is a Pt wire in the form of a loop placed along the frame so as to avoid blocking the light beam. The assembly of the cell is effected by compressing all the components with a set of screws in an aluminum frame. The latter fits in a translation-rotation stage that enables fine adjustments to be made in order to optimize the detector response. All measurements reported in this work were performed at an incident angle of 50°. The light was polarized parallel to the plane of incidence by using an uncoated Glan-Taylor calcite polarizer (Karl Lambrecht Co., Model MGTYB15). Cyclic voltammetry measurements were performed with a RDE-3 (Pine Instruments) potentiostat. A modification was introduced to the built-in signal generator to obtain an output signal at the end of each sweep reversal, which was used to trigger a Nicolet 1170 signal averager and thus to synchronize all the scans. The photomultiplier current output was fed directly into a lock-in amplifier (Stanford Instruments, Model 510), using the frequency of the chopper as the reference frequency, and the lock-in output was then fed to the y channel of the signal averager. About thirty voltammetric scans were found to be sufficient to yield signals with adequate S/N values. After each spectra were completed, the data were transferred to a computer (IBM XT) and stored for further processing. Prior to each run, the HOPG(bp) surface was cleaved with adhesive tape until no defects could be found by visual inspection. The cell was then assembled, mounted in the holder at the desired incidence angles, and filled with neat 0.05 M H2S04,which had been previously deaerated in the extemal solution reservoir. Minor adjustments of the off-axis elliptical mirror were always made in order to achieve maximum light intensity at the detector. Subsequently, the overall electrochemical response of the cell was examined by cyclic voltammetry and a series of reflectance vs potential curves then acquired at different wavelengths in the region between 550 and 750 nm, while the potential was scanned at 250 mV s-l between the desired limits. The cell was then emptied and filled with a solution 10 pM CoTsPc in 0.05 M HaOI. A series of cyclic voltammograms were recorded until curves
I
I
I
\
-10 0 0.00
0.20
0 40
0 60
0.80
100
'.20
V v s SCE Figure 2. Cyclic voltammogram of HOPG(bp)before (curve A) and after (curve 6)adsorption of a single monolayer of CoTsPc in 0.05 M H,SO,. Scan rate 100 mV s-'; electrode area ca.3 om2. The dotted line WSIS used as a background for estimating the coverage as a function of potential.
believed to be characteristic of a single monolayer of CoTsPc adsorbed onto HOPG(bp) were obtained. The CoTsPc solution was removed, and the cell was rinsed repeatedly with 0.05 M H2S04and finally filled with the same electrolyte. At this time a new set of reflectance versus potential curves was acquired at the same wavelengths at which the data had been collected for the bare HOPG(bp) surface. Spectroelectrochemicalmeasurements involving solution-phase CoTsPc (0.2 mM in 0.05 M H2S04) were conducted with a modified version of a cell described elsewhere (12) in a Cary-2300 (Varian) instrument.
RESULTS AND DISCUSSION The cyclic voltammetry of HOPG(bp) in 0.05 M H2S04 including the value of the interfacial capacity (ca. 3 WFcm-2) (see curve A, Figure 2) was found to be practically identical with those reported by other workers (13,14). Within the limit of sensitivity of the experimental setup employed, reflectance versus potential curves recorded for bare HOPG(bp), R(HOPG(bp)) vs V , were featureless and essentially independent of the wavelength in the range examined (550-800 nm). A small increase in the signal was observed, however, upon reversal of the potential sweep, an effect that is most probably due to some type of electronic coupling. These curves were used to normalize the relative reflectivity curves for the surface in the presence of the adsorbate and thus remove the asymmetry observed in the negative- and positive-going potential sweeps (vide infra). Characteristic peaks associated with the CO"/CO~~~TSPC redox couple were observed after adsorption of the material onto the HOPG(bp) surface in the absence of macrocycle in solution (see curve B, Figure 2). These curves are in agreement with those obtained earlier by Zagal et al. (15). The number of electroactive species in the adsorbed layer was estimated by integration of the voltammetric peak using as a background the current observed a t a potential just between the two redox peaks (see the dotted line in curve B, Figure 2), yielding a value of about (2.9 f 0.1) X lo-" mol cm-2. Although this procedure may be regarded as approximate, the charge obtained was found to be in good agreement with that determined by fitting the voltammetric peak with a nonideal isotherm of the type reported by Brown and Anson (16) and later by Gerischer and Scherson (17). The same procedure was used to evaluate the coverage of the oxidized species as a function of potential. The smaller redox peak a t 0.45 V could only be observed after adsorption of the macrocycle. The nature of the process responsible for this feature, however, still remains to be elucidated. Also noteworthy is the change in the background capacity
ANALYTICAL CHEMISTRY, VOL. 62, NO. 23,DECEMBER 1, 1990 2849
-.- , 0.00
0.40
0.20
0.60
0.80
1.00
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Figure 4. Plots of ARIR in Figure 3 versus 8, the coverage of the oxidized species, determined by integration of the voltammetric curve in Figure 28 for three different wavelengths: (A) 750 nm (slope (S) = -1.30 X lo-? intercept (I)= 9.15 X 10"; correlation factor, CF = 0.999); (B) 700 nm (S = -1.71 X I = -9.67 X CF = I = -6.45 X 0.989); (C) 655 nm (S= -2.52 X CF =
0,999). 0.3
0.5 0.7
0.9 0.7
0.5 0.3
V vs SCE
Flgure 3. Relative reflectance versus potential plots for CoTsPc ad-
sorbed onto HOPG(bp) recorded at 655 (A), 700 (B),and 750 nm (C) and normalized by the corresponding curves obtained for bare HOPG(bp)at the same wavelengths. Curve D shows the current versus potential curves obtained during the spectroscopic measurements (see text for other details). of the HOPG(bp) induced by the presence of the macrocycle, a phenomenon that may be ascribed to modifications in the effective dielectric constant of the media immediately adjacent to the HOPG(bp) surface. A series of reflectance versus potential curves were then recorded at the same wavelengths as those for bare HOPG(bp), denoted as R(CoTsPc/HOPG(bp)). The results obtained a t 655,700,and 750 nm, normalized by the corresponding data for the macrocyclic free HOPG(bp), R(HOPG(bp)) are displayed in Figure 3. As noted above, this normalization was found to be necessary in order to remove instrumental artifacts. Also shown in this figure is the essentially time-invariant current-potential profile for the positive- and negative-going voltammetric scans recorded during the actual spectroscopic measurements. As can be seen, the reflectivity curves are highly symmetric about the potential Iimit, providing strong evidence that the signals are directly related to the redox process on the surface. Calculations were performed to estimate possible solution-phase contributions to the signal due to an improper rinsing of the cell following the adsorption step, under the conditions used in the experiments. On the basis of the values for the extinction coefficients at 700 nm extracted from the spectroelectrochemical measurements = 6.5 X 103 M-' cm-' and cox = 1.8 X 10" M-' cm-') and a common value for the diffusion coefficient of the oxidized and reduced forms of the macrocycle (4.9 X lo4 cm2 s-l), it was found that concentrations of CoTsPc in solution as high as lo-' M would give rise to (asymmetric) reflectivity changes 2 orders of magnitude smaller than those observed experimentally. If it is assumed that (i) the total relative reflectivity is simply the sum of the relative reflectivities due to areas of the surface covered by the adsorbate in the oxidized and reduced states and (ii) the optical properties of the adsorbed species are not a function of the coverage and also are not affected by the presence of species in the layer in the other oxidation state, it may be shown that a R / R = fl[(aR/R)ox- ( M / R ) r e d I / [ ( m / R ) r e d + 11
0.20
I
400
450
500
550
600
650
700
750
Wavelength (nm) Flgure 5. Transmission spectra of solution-phase CoTsPc in 0.05 M H,SO,, in the fully reduced (solld line)and fully oxidized (dashed line) states obtained by spectroelectrochemicl techniques (see text). where (AR/R)i = (Ri - Rbare)/Rbareare the normalized reflectivities of i = red and ox, and AR/R = (R(V) - Rrd)/Rr, are the relative reflectivities using Vrd as the reference potential. Support for this simple model is provided by the linear character of the plots of AR/R observed spectroscopically versus the coverage of the oxidized species, Box, evaluated by a direct integration of the voltammetric peak and by the zero intercept of the lines (see Figure 4) (a similar, although more restricted, model has been proposed; see ref 18). According to the equation above, the slopes of these curves a t a given wavelength are proportional to the difference in the values of (AR/R),d and (AU/R),, (which in turn will depend among other factors on the angle of incidence and the polarization state of the light). In fact, both positive- and negative-going reflectivity curves were observed in this work. As is well-known, the differential spectra of surface films on absorbing substrates are strongly modified by the optical properties of the substrate and therefore may not exhibit transmission-like characteristics (2). Therefore, the positive and negative changes in the relative reflectivity as a function of potential cannot be expected to be consistent with the relative extinction coefficients (at the prescribed energies) obtained from solution-phase transmission-type experiments. This is illustrated in Figure 5, in which the UV-vis spectra
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Anal. Chem. 1990. 62.2650-2654
of CoIITsPc is compared with that of Co"'TsPc obtained by spectroelectrochemical techniques. These results are in agreement with those reported earlier by Nevin et al. (19) and indicate that t for the reduced species is lower than that for the oxidized species a t the wavelengths examined. Further complications may arise in comparing surface versus solution-phase spectra as the concentration (and most likely the coverage) may affect the extent of molecular aggregation and thus the spectral features. It may be noted that preliminary three-layer type calculations in which account has been taken of the anisotropic (uniaxial) character of HOPG(bp) have indicated that the overall reflectivity of the substrate can actually increase upon adsorption of similar macrocycles. This effect is due to the rather poor reflectivity of HOPG(bp) and does not appear to occur for metals such as Au, Pt, and Ag. Efforts are now in progress in this laboratory to improve the stability of the instrumental array so as to enable quantitative data to be acquired in an automatic fashion over a wide energy range. It is expected that the results obtained with such arrangement will make it possible to compare in a more precise fashion the optical properties of solution- and adsorbed-phase species and thus gain insight into the effects of surface bonding on the electronic properties of these species.
LITERATURE CITED (1) Spectr~lectrochemistry: Theory and Practice; Gale, R. J. Ed.; Plenum Press: New York, 1988. (2) McIntyre, J. D. E. I n Advances in Eiechochemistry and Electrochemi-
cal Engineering; Delahay, P., Tobias, C. W., Eds.; John Wiley d Sons: New York, 1973;VoI. 9. (3) Schmidt, P. H.; Pliith, W. J. J. Elechoanal. Chem. Interfackri€lectrochem. 1888, 207, 163. (4) Hinnen, C.; Parsons, R.; Niki, K. J. Necfroanal. Chem. Interfacial flectrochem. 1883, 747, 329. (5) Bedioui, F.; Devynck, J.; Hinnen, C.; Rouseau, A,; Bied-Charreton. C.; Gaudemer, A. J. Electrochem. SOC. 1885, 732, 2121. (6) van den Ham, D.; Hinnen, C.; Magner, G.; Savy, M. J. fhys. Chem. 1887, 97, 4743. (7) Hinnen, C.; Coowar, F.; Savy, M. J. Elechoanal. Chem. Interfacial Electrochem. 1888, 264, 167. (8) Nikoiic, B.1 Adzic, R. R.; Yeager, E. 8. J. Elechoanal. Chem. Interfacial Electrochem. 1878, 703,281. (9) (a) Jahnke, H.; Schoenborn, M.; Zimmerman, G. Top. Curr. Chem. 1878, 67, 133. (b) van der Brink, F.; Barendrecht, E.; Visscher. W. Reci.: J. R . Neth. Chem. SOC. 1880, 99, 253. (c) Tarasevich, M. R.; Radyushkina, K. A. Russ. Chem. Rev. (Engl. Trans/.)1880, 49, 718. (d) Yeager, E. J. Mol. Catal. 1888, 38, 5. (10) Crouigneau, P.; Enea, 0.; Beden, E. J. Elechoanal. Chem. Interfackrl Nechochem. 1887, 218, 307. (11) Koehier, M. E.; Urbach, F. L. Appl. Spectrosc. 1878, 33, 563. (12) Scherson, D.; Sarangapani, S.; Urbach, F. L. Anal. Chem. 1885, 5 7 ,
1501. (13) Zagal, J.; Sen, R . K.; Yeager, E. J. Electroanal. Chem. Interfackrl Electrochem. 1877, 8 4 , 207. (14) Randin. J. P.; Yeager, E. J. Electrochem. SOC. 1871, 778, 711. (15) Zagal, J.; Bindra, P.; Yeager, E. J . Electrochem. SOC. 1880, 127, 1506. (16) Brown, A. P.; Anson, F. C. Anal. Chem. 1877, 49, 1569. (17) Gerischer. H.; Scherson, D.A. J. Elechoanal. Chem. InterfaClalElechochem. 1885, 788, 33. (18) Plieth, W. J. Ber. Bunsen-as. Phys. Chem. 1873, 77, 871. (19) Nevin, W. A.; Liu, W.; Melnik, M.; Lever, A. 8. P. J. fiectroanai. Chem. InterfacialElechochem. 1888, 213, 217.
RECEIVED for review March 2,1990. Accepted August 23,1990. This work was supported by the Gas Research Institute.
Design Concepts for Strip-Line Microwave Spectrochemical Sources Ramon M. Barnes* and Edward E. Reszke' DeDartment of Chemistrv. Lederle Graduate Research Center, University of Massachusetts, A ;hers t , Ma.&achuset ts "01003- 0035 In 1976, Beenakker described a TMolo microwave cavity operating in atmospheric-pressure helium as a spectroscopic source ( I ) and gas chromatographic detector (2). This stimulated a long-term development effort with microwave cavities or structures employed especially as spectrochemical detectors for gas chromatography (3-8) that has only recently culminated in the successful development of a commercial chromatographic microwave emission detector (9, 10). Various atmospheric-pressure microwave plasma devices have been developed and are currently being used for different purposes in sciences and technology (11-15), but only two of them, the Beenakker microwave-induced plasma (MIP) (1, 2) and the Surfatron discharges (16-33), are now regularly applied for spectrochemical analysis. A t microwave generator frequencies (e.g., 2.45 GHz), atmospheric-pressure discharges in rare gases are not in local thermodynamic equilibrium (LTE), and the thermal energy (or excitation temperature) is significantly lower than the electron energy (or electron temperature). This makes efficient heat transfer between the discharge and the solvent and/or solute for decomposition, vaporization, and atomization less practical compared to other spectrochemical plasma sources near LTE. Consequently, the low power (e100 W) commonly coupled to the MIP cavity in argon or helium does not provide sufficient plasma enthalpy to desolvate and vaporize aerosols adequately from directly nebulized solutions. Present address: Enterprise for Implementation of Scientific and Technological Progress, Plazmatronika Ltd., ul. Gizycka 54, PL-51163 Wroclaw, Poland.
Although generally excellent as a gas or dry aerosol sample excitation source, both the Beenakker and the Surfatron arrangements are less effective a t low power when aerosol or sample density and/or solvent loading increase substantially. Several design modifications have been made to these MIP sources (34-54) to improve power transfer from the generator to the plasma, so as to provide additional heat exchange to the sample but hopefully not the coupling hardware. For the Beenakker cavity high-efficiency power transfer to the discharge or high operating microwave power input to the cavity have been the primary approach. The common problem arising with the Beenakker cavity is impedance matching in order to obtain a low reflected power and high efficiency with a plasma. In most cases auxiliary matching devices must be used. For example, the Beenakker cavity requires additional tuners in order to minimize the reflected power. Their proper design and implementation reduce power loss as heat. A practical limitation of moderate- to high-power MIP operation is the requirement for an efficient water-cooled cavity and tuner. To avoid the difficulties resulting from high-power operation, such as power cable and coupling loop overheating and discharge tube cracking, Mohamed et al. (55-57) pulsed a magnetron to obtain modulated power (400 W peak) for a Beenakker MIP discharge. Excitation temperatures in the discharge were found to increase with microwave modulation pulse amplitude, width, and frequency corresponding to an increase in signal-to-noise ratio. Some recent efforts to introduce aerosols and liquids with modified, often moderate to high operating power (e500 W), versions of the TMolocavity have been successful (34-40,46,
0003-2700/90/0362-2650$02.50/00 1990 American Chemical Society
