6054
J. Phys. Chem. 1993, 97, 6054-6059
Time-Resolved Resonance Raman and Surface-Enhanced Resonance Raman Scattering Study on Monocation Radical Formation Processes of Heptylviologen at Silver Electrode Surfaces Yasuhito Misono, Koichi Shibasaki, Naoaki Yamasawa, Yasushi Mineo, and Koichi Itoh’ Department of Chemistry, School of Science and Engineering, Waseda University, Shinjuku-ku, Tokyo 169, Japan Receiued: November 3, 1992; In Final Form: February 5, 1993
Time-resolved resonance Raman and surface-enhanced resonance Raman scattering ( R R S and SERRS) spectra were measured for the monocation radical of heptylviologen (HV’+) formed on a silver electrode surface as a function of time ( t ) after switching the electrode potential from -0.2 V (vs Ag/AgCl) to -0.65 V (the monocation radical formation potential). The intensity of the R R S band at 1530 cm-I due to HV’+ increases as a linear function of t 3 / 2in the first step (0 to ca. 4 ms) and as a linear function of t l / * in the second (4-10 ms) and third steps (10-30 ms). The t312-dependence indicates an instantaneous nucleation mechanism, which consists of instantaneous formation of a nucleus and subsequent three-dimensional growth of the nuclei, and the t’i2-dependence indicates one-dimensional diffusion-controlled film growth processes. Measurement of transient currents confirmed the existence of the two kinds of one-dimensional diffusion-controlled film growth processes. Time-resolved SERRS spectral measurements proved that the monocation radical formation process in a few monolayers of the HV2+molecules adsorbed at a roughened silver electrode surface proceeds following a mechanism similar to that of the first step observed for the smooth silver electrode surface. The time-resolved R R S and S E R R S spectra indicated that at the beginning of the reduction process the radicals exist as a dimeric state at both the smooth and roughened silver electrode surfaces and that the conversion from the dimer to a radical salt begins when the radicals form a film of a thickness of at least 5 monolayers.
Introduction Viologen homologs (l,l’-dialkyL4,4’-bipyridiniumdications) have been used as electron mediators in biological systems1and phot~electrolysis.~~~ One of the homologs, 1,l’-diphenyl-4,4’bipyridinium dication (or heptylviologen, HV2+(X-)2, X- = a counter anion), in an aqueous solution, performs a one-electron redox reaction (see Scheme I) with a half-wave potential near -0.55 V (vs Ag/AgC1),4 and on stepping the electrode potential to a value more negative than -0.55 V, a monocation radical (HV’+.X-) is formed at an electrode surface, giving an insoluble red-purplefilm. On reversing the potential to a value more positive than -0.55 V, the colored film disappears. This property may be used for an electrochromic display d e v i ~ e . ~In~ sview of these electrochemical properties, a number of studies have been performed to elucidate the mechanism of the redox reactions of HV*+. Bruinink and Kregting6and Cieslinski and Armstrong7.s have performed chronoamperometric measurements on the monocation radical film formation processes of HV2+on optical transparent electrodes (OTE) such as a tin oxide coated glass. These studies indicated that (i) at an initial stage of the film formation, transient current increases as a linear function of t l i 2 , which conforms to an instantaneous nucleation and threedimensionalgrowthof thenuclei, and (ii) the current subsequently decreases as a linear function of t l / z which , corresponds to a diffusion-controlled film growth process. In order to get more insight into the monocation radical film formation processes, it is of crucial importance to know what kinds of species are present in the processes and to determine the interaction mode of the species with the electrode surface. Surface-enhanced Raman and resonance Raman scattering (SERS and SERRS) spectroscopies have been applied for this p u r p o ~ e . ~ -Researchers, l~ especially the group of Cotton,I3-15 have made systematic measurements of the SERS and SERRS spectra of viologens and studied adsorption structures at silver electrodes of each redox intermediate (dication, monocation radical, and neutral species). Time-resolvedabsorption spectroscopicstudies16 have also been performed to elucidate what kind of chemical species is formed 0022-3654/93/2097-6054%04.00/0
SCHEME I
-e
if
+e
at the beginning of the reduction process of HVZ+. The results, however, gave little information with regard to these points. In addition, the monocation radical formation process at metal electrodes such as platinum and silver proceeds much faster than that at the OTE surfaces. Consequently, there have been no electrospectrochemical study on the process at metal electrodes except for studies done by Osawa et al.I7JS Osawa et al.lS applied time-resolved resonance Raman scattering (RRS) spectroscopy to study the initial stage of the monocation radical formation of HV2+at a platinum electrode surface. They reported a linear diffusion-controlledfilm growth process in which the intensity of a RRS band due to the monocation radical increases as a function of t l / * and suggested the existence of three kinds of species, a radical monomer, a radical dimer, and a radical salt. The time-resolution of their measurement system, however, is about 5 ms. The initial steps (the nucleation and subsequent growth of a few monolayers of the radical film) at metal electrodesurfacessuch as platinum and silver proceed within a few milliseconds, which means that in order to perform a timeresolved RRS study on these processes we need much higher time-resolution. In the present paper we constructed a time-resolved Raman scattering electrospectroscopy apparatus, which allows us to observe spectra of electrochemicallygenerated species near and/ or at electrode surfaces with a time-resolutionof submilliseconds, and applied the apparatus to study the initial processes of the heptylviologen monocation radical formation at silver electrode surfaces. Further, we measured the time-resolved RRS spectra of the monocation radical formed within a Nafion-coated silver 0 1993 American Chemical Society
Heptylviologen at Silver Electrode Surfaces -0.20V.-
The Journal of Physical Chemistry, Vol. 97, No. 22, 1993 6055
--lTul
POTENT I A L
-0.65 V
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u I Atzllnsec
I
LASER PULSE
Figure 1. Step potentials employed by time-resolved RRS and SERRS measurements.
electrodeI9and compared the kinetic features of the process with those observed for the silver electrode surfaces.
Experimental Section Apparatus and Methods. The time-resolved Raman electrospectroscopic apparatus consists of (i) a usual three-electrode electrochemical cell containing a silver working electrode, an Ag/AgCl reference electrode, and a platinum auxiliary electrode,2OI2' (ii) an electrochemical apparatus containing a programmable digital function generator (Hokuto Denko Co. Ltd., Model HB105) modified to operate by an outer TTL trigger and a potentiostat (Hokuto Denko Co., Ltd., Model HA301), (iii) a Raman spectrometer (SPEX Industries, Inc., Triplemate 1877) equipped with a multichannel detector (Princeton Instruments Inc., Model ICCD-576), and (iv) a Q-switched Nd:YAG laser (Spectron Laser Systems, Model SL404) and a dye laser excited by the Nd:YAG laser (Spectron Laser Systems, Model SL4000G). Either a 532-nm line from the Nd:YAG laser or a dye (Rhodamine 640) laser line in the 570-615-nm range, which has a pulse width of about 15 ns and a maximum repetition rate of about 15 Hz, was used as an excitation source. The application of step potentials to the electrode, the firing of the excitation laser pulse, and the opening of the detector gate were synchronously coupled with each other by a sequence of TTL pulses from a pulse generator (Stanford System Inc., Model DG535) (see Figure 1). In time-resolved RRS and SERRS measurements, we applied step potentials as shown in Figure 1; tl and t 2 were adjusted so that HV*+films formed during the 21 (E < -0.65 V) step were completely reduced to HVZ+during the t 2 (-0.20 V) step. The spectra were recorded as a function of time ( t ) after the application of the step potential from -0.2 to E (
