A new approach to electrode kinetics and dynamics by potential modulated Fourier transform infrared spectroscopy

In order to study very fast dynamic processes at the electrochemical interface that cannot be readily accessible by conventional electrochemical techniques, we have developed a new dynamic infrared spectroscopy in which step-scan Fourier transform infrared (FT-IR) interferometry, surface-enhanced infrared absorption spectroscopy (SEIRAS), and attenuated-total-reflection (ATR) techniques were coupled. The principle of this technique is similar to that of ac voltammetry or impedance spectroscopy. The electrode potential is sinusoidally modulated at a certain frequency, and in-phase and quadrature spectra (real and imaginary components, respectively, of the infrared response with respect to the potential modulation) are. measured with a lock-in amplifier. From the analysis of the infrared data collected at several modulation frequencies, kinetic and dynamic information is obtained. The cross talk between Fourier frequencies and the potential modulation was removed by using step-scan FT-IR interferometry. The response of the spectroelectrochemical cell with respect to the externally applied potential modulation was improved by the use of the ATR technique instead of the so-called reflection-absorption spectroscopy technique, which allows us to change the modulation frequency up to 100 kHz, Furthermore, the high sensitivity of SEIRAS shortens the spectral acquisition time significantly. A self-assembled monolayer of 4-mercaptopyridine on an Au electrode was used as a model sample. The spectrum of the monolayer changed with potential due to the charge-transfer between the molecule and the electrode. The charge-transfer rate was estimated to be about 5 x 10(5) s(-1). (C) 1999 Elsevier Science S.A. All rights reserved.