J. Phys. Chem. 1988,92, 7043-7045
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Wavelength (nm) Figure 1. Transient absorption spectra of cis- and tram-stilbene radical cations obtained 0.5 ps after excitation. identical conditions of secondary electron transfer, as an actinometer. Under the secondary electron-transfer conditions used to generate the stilbene cation radicals (Scheme I), their decay is mainly second order. After 10 ps, when the absorption due to the cis cation has decreased by ca. 1 order of magnitude, the spectrum is essentially unchanged, with only a slight shoulder observable at ca. 470 nm, the absorption maximum of the trans cation. Thus, we conclude that thermally activated unimolecular isomerization of the cis-stilbene cation radical does not occur on the microsecond time scale in solution. At the higher concentrations of cis-stilbene (0.05-0.1 M) used in previous attempts to generate its cation radical by photoinduced electron transfer,2s6secondary electron transfer to trans-stilbene present as an impurity or generated photochemically is most likely responsible for the observation of the trans-stilbene cation radical." Reaction of cis-stilbene cation radical and neutral to form a dimer cation radical may also contribute to the disappearance of the monomer cation radical a t higher cis-stilbene concentrations. The efficiency of reactions of photogenerated radical ions is determined by the quantum yields for separation of the initially Determined by electrochemical generation of the stable cation radical. Lenhard, J. R.,private communication. (1 5 )
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formed radical ion pair to form free radical ions and of their subsequent reactions. Quantum yields for cage escape1*of the stilbenes determined with DCA as the acceptor, 0.05 M stilbene as the primary donor, and 5 X lo-" M 4,4'-dimethoxystilbene as the secondary donor are 0.134 and 0.254 for cis- and trans-stilbene, respectively. Quantum yields for DCA-sensitized isomerization under conditions of steady-state irradiation are 0.32 and 0.002 for 0.05 M cis- and trans-stilbene, respectively.' Thus, isomerization efficiencies of the free radical ions are 2.4 and 0.008 for cis- and trans-stilbene, respectively. The isomerization efficiency for the cis but not the trans isomer increases with stilbene concentration, in accord with a chain mechanism. Quantum yields for benzaldehyde formation from the DCA-sensitized oxygenation of cisand trans-stilbene are 0.12 and 0.18, respectively.'J6 The higher observed quantum yield for photooxygenation of trans- vs cisstilbene is consistent with its higher cage escape efficiency. The smaller cage escape efficiency for the cis- vs trans-stilbene cation/DCA anion pair might not have been expected since cisstilbene has a higher oxidation potential than that of trans-stilbene, and it has been shown that cage escape yields for ion pairs of similar structures are higher for donors of higher oxidation potential due to the energy gap effect.I2 However, we have also shown recently that cage escape is quite sensitive to small changes in molecular structure and that lower escape yields are characteristic of molecules in which electron delocalization is smaller.12b The present results suggest that the cis-stilbene radical cation might adopt a conformation in which the charge is less delocalized due to steric crowding and that this cation is thus more "benzene like" than the less crowded trans cation. In conclusion, we have generated the cis- and trans-stilbene cation radicals in solution under conditions where they do not interconvert and have measured their cage escape efficiencies. This has permitted for the first time the correlation of cation radical structure with both cage escape and product formation efficiency. Acknowledgment. We thank A. G. Davies and T. W. Ebbesen for informing us of their results prior to publication. Work performed at Northwestern University is supported by the National Science Foundation (Grant CHE-8618994). (16) Photooxygenation quantum yields are dependent upon oxygen concentration light intensity and conversion, and these values ( I atm of 0 2 , P = 7.4 x lo8 einstein s-I) are not optimized.
An Application of Optical Waveguldes to Electrochemistry: Construction of Optical Waveguide Electrodes Kiminori Itoh* and Akira Fujishima Department of Synthetic Chemistry, Faculty of Engineering, The University of Tokyo, Hongo, Bunkyo- ku, Tokyo 113, Japan (Received: July 1, 1988; In Final Form: October 13, 1988)
The optical waveguide (OWG) method was applied to electrochemical measurements for the first time. OWG electrodes were fabricated by coating conductive Sn02 onto a glass OWG. Basic OWG characteristics and optical sensitivity of the OWG electrodes were examined. Optical changes associated with redox reactions of a dye (methylene blue) adsorbed onto the electrode surface were sensitively monitored on the OWG electrodes.
Optical waveguides (OWGs) are essential components of integrated optics. An interesting feature of the OWGs is that a light wave propagating through the OWG layer is very sensitive
to optical constants of the OWGs and of the circumstance surrounding them. For instance, temperature sensors can be made with the OWGs,' and molecules a t the surfaces and/or at the
0022-3654/88/2092-7043$01 .50/0 0 1988 American Chemical Society
7044 The Journal of Physical Chemistry, Vol. 92, No. 25, 1988 laser beam
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Figure 1. Structure of the multilayered conductive optical waveguide
(OWG electrode). The incident angle of the laser beam (e)was changed by using a micrometer-driventurntable.
interfaces of the OWGs significantly absorb2 or scatter3 the guided light waves. This leads us to an idea that small optical changes taking place at electrode surfaces in contact with solution will be monitored with great sensitivity if the electrodes can be fabricated into the OWGs. Such OWGs will be very useful in various fields including electrochemistry and photoelectrochemistry. In connection with this, we recently carried out flash photolysis measurements on the OWGs to detect reaction products with high ~ensitivity.~ We report here the construction of electrically conductive OWGs, Le., OWG electrodes,s and preliminary results on a model reaction. The OWGs for electrochemical use should satisfy the following conditions. First, the surface of the OWG should be conductive enough (e.g., C1 kQ/cm2). Second, attenuation of the guided light in the OWG should be minimized. Third, the guided light should have sufficiently large electric field at the surface of the OWG; this condition is needed to make optical measurement of the electrode surface sensitive. Thus, it is concluded that the multilayered OWG shown in Figure l matches our purpose. The OWG layer consists of two parts. One layer is nonconductive and has a relatively small refractive index (n) and large thickness. The thin conductive layer at the top of the OWG has a large n so that the guided light can penetrate well into the conductive layer. We made a sample of such OWGs by coating a glass OWG with conductive S n 0 2 ( n = 2.0). The glass OWG layer was made by doping a glass plate ( n = 1.51) with K+ ions in molten KN03 (370 O C , 4 h). The specimen was then coated with an S n 0 2 film (ca. 1000 A in thickness, determined with a reflection interference method) by the spray pyrolysis technique.6 SbC13 was added into the spray solution as a dopant to make the SnOz film conductive. Two diffraction gratings made of a photoresist polymer (spacing 0.51 pm) were attached onto the surface of the OWG by using holographic patterning. A beam of a He-Ne laser (wavelength 633 nm) for monitoring was introduced into the OWG layer through one grating and was taken out from the other grating. The intensity of the guided light (IowG) was monitored with a photomultiplier. Figure 2 shows typical OWG characteristics for a glass OWG (A) and that for an OWG having an S n 0 2 film on the top (B). The latter OWG is considered to have the same K+-doped layer as the former OWG except for possible extra diffusion of K+ during the spray pyrolysis process (350-400 O C , 15 min). These (1) Johnson, L. M.; Leonberger, F. J.; Pratt, G. W., Jr. Appl. Phys. Lett. 1982, 41, 134.
(2) Swalen, J. D.; Tacke, M.; Santo, R.; Rieckhoff, K. E.; Fischer, J. Helu. Chim. Acta 1978, 61, 960. (3) Ho, Z. Z.; Wijekoon, W. M. K. P.; Koenig, E. W.; Hetherington, W. M., 111. J . Phys. Chem. 1987, 91, 757. (4) Itoh, K.; Fujishima, A. J . Am. Chem. SOC.1988, 110, 6267. ( 5 ) A part of this work was presented at the 195th American Chemical Society Meeting at Toronto, Canada, 1988. (6) Chopra, K. L.; Kainthla, R. C.; Pandya, K. K.; Thakoor, A. P. In Physics of Thin Films; Hass, G., Francombe, M. H., Vossen, J. L., Eds.; Academic Press: New York, 1982; Vol. 12, p 167.
A.
0
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Figure 2. Examples of OWG characteristics: intensity of the guided light as a function of the incident angle of the laser beam. (A)A glass OWG (K+-dopedglass). (B) A glass OWG coated with SnOz.
05
Figure 3. Dependence of the intensity of the guided light (lowo)as a function of the electrode potential, observed at an OWG electrode (500 S2/cm2). IO-) mol/dm3 aqueous solution of methylene blue; 0.1 mol/dm3 KC1; scan rate, 0.1 V/s. A cyclic voltammogram was recorded at the same time.
OWGs had four main propagation modes ( m = 0-3) as the figures show. The thickness of the OWG layer was estimated to be ca. 8 pm by using a known relation between the number of modes and the OWG layer thickness. The increase in n due to the K+ doping for the present type of the glass OWG is reported to be ca. 0.01 at the OWG surface.' The degree of attenuation for waveguide B was ca. 10 dB/cm for the highest mode, m = 3. This attenuation is due very likely to free electrons in SnOzand to the rather rough surface of the sprayed SnOz. Figure 3 shows an example of electrochemical measurements on the OWG electrode. Electrochemical reduction of a blue thiazine dye, methylene blue (MB), was used as a model reaction because MB is easily adsorbed onto S n 0 2 electrodes and gives a colorless leuco form when reduced. IoWG was monitored as a function of the electrode potential, and a cyclic voltammogram (7) Neuman, V.; Parriaux, 0.;Walpita,
704.
L. M. Electronics Lett. 1979,15,
J. Phys. Chem. 1988, 92, 7045-7052 was taken at the same time. The OWG electrode had sheet resistance of ca. 500 O/cmZ and donor density of ca. 1020/cm3 ( [SbC13]in the spray solution = 2.5 wt %); the optical path length was 1 cm. It is apparent from Figure 3 that IowGincreased when MB was reduced to the colorless form, and reoxidation of reduced MB caused a decrease in IoWG.It should be noted that almost the same result was obtained when the dye solution was replaced with a 0.1 mol/dm3 KC1 solution to leave the adsorbed dye on the electrode surface. This shows that the change in ZoWG shown in Figure 3 is mainly due to the redox reaction of the adsorbed dye molecules. In addition, Oz evolution at higher potential regions and H2evolution at lower potential regions did not affect ZOWG. This is because these reactions do not give color changes at the electrode surface. The sensitivity factorZ of this system was estimated by comparing absorption spectra of adsorbed MB taken with a spectrophotometer and attenuation of the guided light due to adsorbed MB; typical values of sensitivity factor were 20-40 depending on the mode number. These values compare well with those for glass OWGs. This shows that the guided light has sufficiently large electric field in the SnOZlayer as well as a t the surface of SnOz as expected. The sensitivity values obtained here is similar to those for a carefully designed internal multireflection measurement.*
7045
We can, however, increase the sensitivity of the OWG electrodes further by increasing the optical path length and by changing the OWG parameters, e.g., refractive indexes of the OWG materials. In order to make accurate and fast electrochemical measurements using the OWG electrodes, the conductivity of the conductive film should be as high as possible. However, an increase in the conductivity of the film causes attenuation of the guided light. We therefore will have a practical limit of the conductivity. However, we can avoid this problem by optimizing the design of the OWG electrodes. For instance, we can coat the surface of the SnOz film with metal except for the OWG region to reduce the total resistance of the electrode; the metal part should be covered with an inert material in order to avoid direct contact with the electrolyte solution. It is rather easy to make such an OWG electrode having an OWG region of 1 mm width and of 1 cm length. This electrode will give a resistance of 50 0 when the resistivity of the SnOz film is 500 0 cm.
Acknowledgment. We thank Mr. 0. Nakamura of Toa Nenryo Kogyo K. K. for helpful discussions. (8) Bard, A. J.; Faulkner, L. R. Electrochemical Method; Wiley: New York, 1980; p 592.
FEATURE ARTICLE Is There Any Beam Yet? Uses of Synchrotron Radiation in the in Situ Study of Electrochemlcal Interfaces H. D. Abruiia,* J. H. White, M. J. Albarelli, G. M. Bommarito, M. J. Bedzyk,+ and M. McMillan Department of Chemistry and Cornell High Energy Synchrotron Source and School of Applied and Engineering Physics, Cornell University, Ithaca, New York I4853 (Received: March 3, 1988; In Final Form: July I , 1988)
The advantages of employing synchrotron radiation for the in situ study of electrochemical interfaces are discussed with emphasis on the techniques of surface EXAFS (extended X-ray absorption fine structure) and X-ray standing waves. The principles behind the techniques are briefly considered followed by a discussion of recent experimental results. Examples include the study of underpotentially deposited metallic monolayers, polymer films on electrodes, and in situ measurement of adsorption isotherms. We conclude with an assessment of future directions.
Introduction The central goal of electrochemical research is to understand and control electrochemical reactivity at the atomic and molecular levels. Such control and understanding, if achieved, would profoundly affect many areas of scientific, technological, and economic importance. The establishment of understanding of structure/ reactivity correlations are critical to the achievement of these goals. The interfacial nature of electrochemical reactions makes their study particularly difficult since they involve the coupling of a multitude of processes including transport rates and the nature and form of the reactant(s) and product(s) as well as the electron-transfer event itself. I q addition, the nature of the electrode surface, including its crystallographic orientation, can profoundly affect reactivity. Furthermore, the very high electric fields present
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Cornell High Energy Synchrotron Source and School of Applied and Engineering Physics.
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will distort the electronic clouds of species in the vicinity of the electrode and will give rise to gradients in potential and ionic distributions.' Finally, since only the region proximal to the electrode/solution interface itself will be affected, we need to develop techniques that will allow us to probe only this region. All of these aspects have, thus far, presented some formidable obstacles to the structural characterization of electrode/solution interfaces. In recent years,* however, there has been a renewed interest in the study of the electrode/solution interface due in part to the (1) (a) Sparnaay, M. J. In The International Encyclopedia of Physical Chemistry and Chemical Physics; Pergamon: Glasgow, 1972; Vol. 14. (b) Bockris, J. OM.; Conway, B. E.; Yeager, E. Comprehensive Treatise of Electrochemistry; Plenum: New York, 1980; Vol. 1. (2) (a) Furtak, T. E.; Kliewar, K. L.; Lynch, D. W . , Eds. Proceedings on the International Conference on Non-Traditional Approaches to the Study of the Solid Electrolyte Interface; Surf. Sci. 1980, 101. (b) See also: J. Electroanal. Chem. 1983, 150.
0 1988 American Chemical Society
