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 probin g composition of spatially resolved micrometric volumes inside a macroscopi c sample. In this paper, the applicability of confocal Raman microspectrosc opy for imaging molecular diffusion at microelectrodes with a micrometric r esolution is established. The efficiency and versatility of the method have been tested by probing the composition of the two different diffusion laye rs which build up in the vicinity of an ultramicroelectrode during reductio n 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. A s a correlation, this provides the first direct experimental proof of a con proportionation reaction taking place when the electrode potential is poise d 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. (C) 2000 Elsevier Science S.A. All right s reserved.