Mapping concentration profiles within the diffusion layer of an electrode; Part I. Confocal resonance Raman microscopy

Confocal microspectroscopy is known to be a very efficient means for probing composition of spatially resolved micrometric volumes inside a macroscopic sample. In this paper, the applicability of confocal Raman microspectroscopy for imaging molecular diffusion at microelectrodes with a micrometric resolution is established. The efficiency and versatility of the method have been tested by probing the composition of the two different diffusion layers which build up in the vicinity of an ultramicroelectrode during reduction of tetracyanoquinodimethane (TCNQ) on its first or second electrochemical wave. This is performed by mapping the concentration profiles of the TCNQ.- anion radical under each condition using its resonance Raman spectrum. As a correlation, this provides the first direct experimental proof of a conproportionation reaction taking place when the electrode potential is poised on the second wave of a two-wave EE electrochemical system. In both cases, the concentration profiles of the radical anion TCNQ.- agree extremely well with theoretical predictions.