Langmuir 1994,10, 640-642
640
Real-Time Monitoring of Electrochemical Dynamics by Submillisecond Time-Resolved Surface-Enhanced Infrared Attenuated-Total-ReflectionSpectroscopy Masatoshi Osawa,' Katsumasa Yoshii, Ken-ichi Ataka, and Takao Yotsuyanagi Department of Molecular Chemistry and Engineering, Faculty of Engineering, Tohoku University, Aoba, Aramaki, Aoba-Ku, Sendai 980, Japan Received December 14, 199P We have succeeded for the first time in submilliiecondtime-resolvedinfrared spectroscopicobservations of the electrochemical interfaces. The measurementa were performed with a combination of step-scan Fourier transform interferometry and the so-called Kretschmann attenuated-total-reflection (ATR) technique. Thinsilver films evaporatedon the ATR prism were used as the working electrode. Enhancement of the infrared absorption of molecules on the metal surface (surface-enhanced infrared absorption phenomenon) enables the dynamic probing of electrochemicalreactions. Introduction plication of FT-IRonlyto relativelyslow processes.6Wery recently, however, the development of modern step-scan In situ infrared spectroscopy provides molecular level interferometry instrumentation allowed FT-IR to be information at the electrochemicalinterfaces that cannot applicable to the study of timedependent phenomena with be obtained by conventional electrochemicaltechniques.'$ time-resolution from tens of nanoseconds to tens of However, most of the infrared measurements reported so milliseconds.8 far have been performed under static conditions (at static In the present paper, we report the first submillisecond potentials) and, therefore, electrochemicaldynamics could time-resolved infrared measurements of electrochemical not be studied. Although spectra obtained at potential dynamics using a step-scan FT-IRspectrometer. A study modulated conditions contain dynamic information, the of the one-electron reduction of l,lJ-diheptyl-4,4'-dipyelectrochemical modulation and demodulation with lockridine (better known as heptylviologen; HV2+)to monoin detection technique has been used only for background subtraction1*2except by Chazalviel and his c o - w ~ r k e r s . ~ ~ ~cation radical H V + (HV2++ e- H V + ) is reported here as an example. The radical precipitates on the electrode If the time-resolved infrared spectroscopy technique is surface and forms a violet film in aqueous solutions available,the infrared data can be directly correlated with containing the proper anion^.^ This electrochemical simultaneously obtained electrochemical data (cyclic system was chosen as a test sample because the mechanism voltammetry, chronoamperometry, etc.). The ultimate of the film deposition in the initial stage of the reduction time-resolution desired for spectroelectrochemistry is of has been receiving considerable attention and has been the order of 10-7-106 s because monitoring of fast investigated with time-resolved resonance Raman specelectrochemical transient phenomena is restricted to this troscopy.loJ1 time region due to the response of electrochemical equipment, especially of the potentiostat (0.5-10 ps). In the present investigation we employed the so-called The first millisecond time-resolved electrochemical Kretschmann attenuated-total-reflection (ATR) techinfrared spectra were reported by Ito et almsThe spectra nique12(the ATR measurement with a prismtmetal film/ were obtained with a dispersive spectrometer by repeating solution configuration) instead of reflection-absorption measurements of transients of surface infrared signals at spectroscopy (RAS)technique that has been used most fixed wavelengths. The data stored in a computer were widely for in situ studies at electrochemical interfaces.192 rearranged to time-resolvedspectra. The time-resolution A thin metal film vacuum-evaporated on the ATR prism was limited rearranged to 10 ms by the AID converter is used as the working electrode. Since infrared absorption they used. Dispersive spectrometers with the point-byof molecules on or very near the metal surface is greatly point data collections are easily applicableto timeresolved enhanced due to surface-enhanced infrared absorption measurements, but at the expense of very long data (SEIRA) phenomenon,1g16 only the molecules near the collection times, and seriously limited throughput and electrode surface are detected selectively at a higher spectral resolution. sensitivity and less interference from strong electrolyte Fourier transform infrared (FT-IR) interferometry absorption compared with RAS.lS The advantage of the offers the combination of broad spectral range with relatively high spectral resolution and fast data acquisition (6) h u n g , L.-W. H.; Weaver, M. J. J. Electroanal. Chem. 1988,240, time (due to multiplexand throughput advantages). Until 341. (7) Xing, X.;Bae, I. T.; Shao, M.; Liu, C.-C. J. Electroanal. Chem. recently, equipment limitations have restricted the ap-
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* Author to whom correspondence should be addreesed.
9 Abstract published in Advance ACS Abstracts, February 15, 1994. (1) Bewick, A.; Pons, S. In Advances in Infrared and Raman Spectroscopy; Clark, R. H. J., Heater, R. E., Eds.; Wiley Hyden: Chichestar, 1985, Vol. 12, Chapter 1. (2) Beden, B.; Lamy, C. In Spectroelectrochemietry: Theory and Practice; Gale, R. J., Ed.;Plenum Preee: New York, 1988, Chapter 5. (3) Ozanam, F.; Chazalviel, J.-N. Rev. Sci. Instrum. 1988,59, 242. (4) Chazalviel, J.-N.; Dubin, V. M.; Mandal, K. C.; Ozanam, F. Appl. SpeCtrO8C. 1993, 47, 1411. ( 5 ) Nakamura, M.; Ogaeawara, H.; Inukai, J.; Ito, M. Surf. Sci. 1993, 283,248.
1993,346,309. (8) Palmer, R.;Chao, J. L.; Dittmar, R. M.; Gregoriou,V. G.; Plunkett, S . E.Appl. Spectrosc. 1993,47, 1297 and the references therein. (9) Bird, C. L.; Kuhn, A. T. Chem. SOC.Rev. 1981,101,49. (10) Osawa, M.; SuBtaka, W. J. Electroonal. Chem. 1989, 270, 261. (11) Misono, Y.; Shibaeaki, K.; Yamasawn, N.; Mineo, Y.; Ito, K. J. Phys. Chem. 1993,97,6045. (12) Kretachmaun, E.;Raether, H. 2.Naturforsch. 1968, A23,2135. (13) Oeawa, M.; Ikeda, M. J. Phys. Chem. 1991,96,691. (14) Matauda, N.; Y d i , K.;Ataka, K.; Osaw4M.; Matsue,T.;Uchida, I. Chem. Lett. 1992,1385. (15) osawa, M.; Ataka, K.; Yoehii, KO; Nishikawa, Y. Appl. Spectrosc. 1993,47, 1497. (16) Oeawa, M.; Ataka, K.; Yoshii, K.; Yotauyanagi, T. J. Electron Spectrosc. Relut. Phew" 1993,64/65,371.
0743-7~63/94/24~0-0S40$04.50/0 0 1994 American Chemical Society
Letters
Langmuir, Vol. 10, No.3, 1994 641
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ATR-SEIRA spectroscopy technique in time-resolved measurements is discussed. Experimental Section Heptylviologen dibromide (HVBrd purchased from Eaatman Kodak Co. was used as received without further purification. Other chemicals were reagent grade. Solutions were prepared with triply distilledwater and were deaerated with nitrogen prior to experiments. A Bio-Rad FTS-60&896 FT-IR spectrometer equipped with a fast-response TGS detector was used for the infrared measurements. The minimum time resolution of the spectrometer is 2.6 w. The spectrometer was operated at a 8-cm-' resolution. p-polarized infrared radiation was used at an incident angle of 60° and the spectra were obtained with single reflection. The spectra are shown in the absorbance unit defined aa -log(R/Ro), where R and RO are the single beam spectra at sample and reference potentiale, respectively. The spectroelectrochemicalcell has been described previously.14J8A thin metal film (the working electrode)was evaporated on the base plane of a silicone hemicylinder prism in a vacuum at a base pressure of 2 X 10-8 Torr. The thickness of the metal film waa controlled to 20 nm with a quartz microbalance. The metal-coated prism was attached to the cell body made with Kel-F by sandwiching a silicone rubber sheet and a copper foil contact. A platinum wire and a AgIAgC1electrode were used aa the counter and referenceelectrodes,respectively. The potential quoted in the present study is against the AgIAgC1 electrode. Electrode potential was controlled and modulated with a potentiatat (HokutoDenko, HAB-161)and a function generator (Biomation, 2202A) synchronized with the spectrometer, respectively.
Results and Discussion Figure 1shows a cyclic voltammogram at 20 mV/s for 1 mM HV2+in 0.3 M KBr aqueous solution. A pair of peaks is observed. The reduction peak at -0.51 V corresponds to the reduction of HV2+ to Hv'+.9 The radical is reversibly oxidized to HV2+at -0.45 V in the reverse potential sweep to positive direction. The peak current of the reduction wave was proportional to the square root of the sweep rate, indicating the reduction to be diffusion controlled. Time-resolved infrared measurements were performed for a double-potential step from -0.2 to -0.55 V (80 ms) and back to -0.2 V (see the inset in Figure 3a), which were repeated at 5 Hz. Figure 2 shows a series of infrared spectra of the silver electrode/solution interface. The spectrum obtained at a constant potential of -0.2 V before the timeresolved measurement was used as the reference spectrum. Each spectrum was obtained with 100-ps acquisition time. However, only the spectra of every l-ms interval from t = -6 to 24 me are shown in the figure for clarity. The
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Figure 2. Time-resolved infrared spectra for the reduction of H P to HV'+ for a potential step from -0.2 to -0.66 V. Each spectnun was obtained with 100pa acquisition time, but only the spectra of every 1me interval from t = -6 to 24 me are shown in the f i e for clarity, where t = 0 is taken when potential was stepped from -0.2 to -0.56 V.
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initial time, t = 0, is taken when the potential is stepped from -0.2 to -0.55 V. It is seen that the bands at 1635, 1593,1505,1330,1185,and 116Ocm-l assignedtoHV+ 1 7 3 grow with t. This is seen more clearly in Figure 3a, where the integrated intensity of the 1505-cm-l band is plotted as a function of t. The band intensity decreases rapidly when the potential was stepped back to 4.2 V (at t = 80 ms). We note that the spectra shown in Figure 2 were obtained by only one interferometer scan (it took about 10 min), and no smoothingor averaging of the spectra was performed. If a liquid-Nz-cooled MCT detector is available, signal-to-noise ratios of the spectra will be improved. We show in Figure 3b the intensity of the 1505-cm-1 band as a function of the square root of t, which indicates that the reduction process can be divided into two regions, ~
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(17)Brienue, S.H.R.;Cooney,R.P.;Bowmaker, G. A. J. Chem. SOC., Faraday Tram. 1991,87,1356. (18)Kitamura,F.;Ohsaka,T.;Tokuda,K. J.Electroaml. Chem. 1993, 363,323.
Letters
642 Langmuir, Vol. 10,No. 3,1994 as denoted by I and 11. In region I ( t = 0-12 ms) the intensity can be fitted to a function of t3J2as shown by the dashed curve in the f i i e . On the other hand, the intensity increases as a linear function of t 1 / 2 in region I1 ( t > 12 ms). Quite similar results have been observed in timeresolved Raman measurements of the HV2+reduction.lOJ1 It has been well established by chronoamperometry that the radical film is formed through the initial nucleation process and the subsequent film growth process.lOJ1 In the nucleation process, a fixed number of nuclei are instantaneously born followed by the growth in size (threedimensional growth). After the nucleation, the film grows under linear (one-dimensionl) diffusion conditions. Therefore, the charge consumed by the reduction increases as a function of t3J2in the nucleation process and as a function of t 1 / 2in the subsequent film growth process.lOJ1 The band intensity versus t1l2curve shown in Figure 3b is in good agreement with the chronoamperometric measurements. The successof the present submillisecond time-resolved infrared monitoring of the electrochemical dynamics is largely due to the use of the Kretschmann ATR technique. Although the RAS technique has been used most widely in infrared spectroelectrochemistry,we note here that this technique is not suitable for dynamic probing of electrode surface reactions because of the following reasons. First, intense electrolyte absorption of infrared radiation makes it necessary to use a thin layer optical cell, where the electrolyte is confined to a layer of a few pm thickness between the electrode and a suitable infrared transparent w i n d o ~ . ~ lSince ~ J ~ there exists a large IR drop between the working and reference electrodes in the thin-layer cell, the electrode potential does not respond very quickly with a potential modulation. In addition, diffusion of electroactive speciesfrom bulk solution to the electrode surface (or vice versa) is greatly restricted and only species in the thin layer can be reacted. Furthermore, current distribution over the working electrode surface is not uniform. For the HV2+reduction, the redox peaks were observed by cyclic voltammetry only with very slow potential sweep with the working electrode in thin-layer contact with the window. It was also observed that only the edge of the disk electrode surface turned to purple due to the precipitation of HV'+. Second, the sensitivity of RAS is not good enough to detect monolayer adsorbates. The reflection change of the electrode surface caused by the absorption of one monolayer species is typically of the order of 103in A = -log(R/Ro) units except for strong infrared absorbers such as carbon monoxide ( A Since several hundred to several thousand coadditions of interferograms are required to enhancethe signal-to-noiseratio, time-resolved measurements are very difficult. Third, strong background signals of electrolytes prevent the detection of weak surface signals. Since the background signals are several orders of magnitude larger than the surface signals even with thin-layer contact conditions (A 0.5 for the H-0-H bending mode of waterle), complete subtraction of the background signals is quite difficult. All these problems of the RAS technique are removed with the Kretschmann ATR technique. Since infrared
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(19) Seki, H.; Kunimatau, K.; Golden, W. G . Appl. Spectrosc. 1985,39, 437.
radiation is reflected at the electrodelelectrolyteinterface by passing through the ATR element, the solution phase need not be thin. Although the penetration depth of the infrared radiation is of the order of a micrometer, the solution signals are very weak ( A 0.05 for the H-0-H bending mode of waterle) because the penetration of the electric field into the solution phase is greatly damped by the presence of the metal film. Fortunately, however, the infrared absorption of molecules on or near the thin metal electrode is greatly enhanced due to the SEIRA phenomenon.l3-'6 For examle, the peak intensity of the 1505cm-l band of HV'+ is about 0.01 in the ATR spectrum at t = 24 ms in Figure 2. The amount of HV'+ deposited in the initial 24 ms of the reduction is about a 5 monolayer equivalent.ll On the other hand, the peak intensityof the 1505-cm-1 band of HV'+ is about 5 X 1o-L in the RAS spectra for a several monolayer equivalent deposition.lB Therefore, spectra of good signal-to-noise ratios can be obtained by the ATR-SEIRA technique even with the low sensitivity TGS detector and by only one interferogram scan, as shown in Figure 2. In addition, since the intensity of the surface molecule is comparable to that of bulk water, the background absorption is subtracted very successfully.
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Conclusions and Remarks In summary, we succeeded for the first time in monitoring with submillisecond time resolution the electrochemical dynamics by the combined use of the step-scan FT-IRinterferometry and the ATR-SEIRA technique. The initial stage of the one-electron reduction of HV2+ at a silver electrode surface was used as an example. The use of the ATR-SEIRA spectroscopy enabled the detection of a several monolayer speciesat the electrochemicalinterface within the short acquisition time. Since ultimate time resolutions of present FT-IR spectrometers are in the nanosecond range (which are restricted by the response of detectors), the time-resolved ATR-SEIRA spectroscopy technique will become a very powerful tool for investigations of fast transient molecular processes a t electrode surfaces, including electron transfer processes,orientation changes of adsorbed molecules, reaction pathways, unstable reaction intermediates, periodic oscillation phenomena, etc. Finally, we note that step-scan interferometry is applicable only to reversible or repeatable reaction systems in principle.8 This is also the case in the point-by-point measurements with dispersive spectrometers. This limitation is removed to a certain degree by using "rapidscan" FT-IR spectrometers. For the FT-IR spectrometer used in the present study, rapid-scan at 65 scans/s (timeresolution of 15 ms) is possible at a spectral resolution of 16 cm-'. We succeeded in real-time monitoring of irreversible reactions with the rapid-scan mode, which were performed simultaneously with cyclic voltammetry measurements. The results will be reported elsewhere together with a more detailed analysis of the results reported in the present paper. Acknowledgment. We gratefully acknowledge the ALPS Electric Corporation for permission to use the stepscan FT-IR spectrometer. This work is partly supported by a Grant-in-Aid for Scientific Research from The Ministry of Education, Science, and Culture of Japan and by the Shimadzu Science Foundation.
