ADVANCES IN IN-SITU SPECTROELECTROCHEMICAL FOURIER TRANSFORM INFRARED SPECTROSCOPY

Abstract

The level of information provided by electrochemical measurements can be substantial as evident by the use of electrochemistry in varied disciplines spanning from materials research to cellular biochemistry. However, electrochemistry on its own does not provide direct information concerning redox induced changes in molecular structure. This information can only be elucidated by coupling spectroscopic and/or separation techniques with traditional electrochemical methodologies. In principle, infrared (IR) spectroelectrochemistry (SEC) is ideal for such studies but in practice coupling IR spectroscopy and electrochemistry are often experimentally incompatible. Since the inception of in-situ IR SEC techniques in the 1980’s, two competing methodologies (using either external- or internal- IR reflection geometries), were developed to deal with the two major challenges associated with IR SEC (strong infrared absorption of the electrolytes and weak analytical signals). The primary focus of this thesis is the successful advancement of IR SEC techniques through the implementation of synchrotron infrared radiation with ultramicroelectrodes (UMEs; electrode diameters < 25 µm) to study spectroelectrochemical processes on the microsecond time scale. Several examples using Surface Enhanced Infrared Absorption Spectroscopy (SEIRAS) are presented including the adsorption of dimethylaminopyridine (DMAP) on gold substrates and the proton-coupled electron-transfer (PCET) kinetics of electrochemically-active 1,4-benzoquinone terminated self-assembled monolayers (SAMs). These studies highlight the benefits of coupling electrochemistry and infrared spectroscopy. For instance, in-situ spectroscopic evidence shows that small amounts of DMAP’s conjugate acid (DMAPH+) adsorb on gold electrodes in acidic electrolytes and at negative potentials. This result was not forthcoming from previous electrochemical measurements and was only realized through in-situ SEIRAS. Finally, the largest contribution in advancing in-situ IR SEC methodologies was through the development of utilizing synchrotron infrared radiation on UMEs to study fast electrochemical processes. This work was technically very challenging and emphasized the interfacing of an electrochemical cell containing an UME with fast infrared data acquisition techniques (i.e. rapid scan and step-scan interferometry). The use of a prototypical electrochemical system, i.e. the mass-transport controlled reduction of ferricyanide, indicate that at short times the spectroscopic signal closely matches the electrochemical signal but at long time scales it deviates due to edge effects associated with the diffusion environment of the UME.