In-situ FTIR spectroscopy at the electrochemical interface: The interest of a modulation technique

Infrared vibrational spectroscopy is increasingly being used as a tool for the in-situ study of the electrochemical interface. The geometry may be that of external reflection with a thin-layer-electrolyte cell, or that of internal reflection in the case of infrared-transparent (e.g., semiconducting) electrodes. The small infrared absorption from the interfacial species can be separated out from the large electrolyte absorption background by making use of its specific dependence upon electrode potential. We describe an original technique combining audio-frequency modulation of the electrode potential, together with Fourier-transform infrared spectroscopy. This technique provides an unprecedented sensitivity of ?I/I~10^-6 under typical operating conditions. It further allows a better identification of the species under study, by using the capability of probing the dynamics, inherent to a modulation technique. This allows one to tell between the different mechanisms that may lead to potential dependent changes at an electrochemical interface. The technique is illustrated by several applications: study of the ionic layer, interfacial species and their relation to electronic surface states at the n-Si/acetonitrile-electrolyte interface, study of the reversible reduction of solute species at a Ge/aqueous electrolyte interface, study of the catalytic mechanism of hydrogen evolution after this interface has been irreversibly modified by reduction of heteropolyanions.