(Invited) Diffusion and Depletion Effects in Chronoamperometry of Diazonium based Electrochemical Grafting

Abstract

Chemisorption on electrode surfaces by means of electrochemical “grafting” of diazonium based compounds has demonstrated the ability to introduce robust functional character to electrode surfaces, i.e., selective binding and recognition. Unfortunately, chemical decomposition of the active diazonium compounds frequently plagues the extent of surface functionalization. Impacts of diazonium degradation on the grafting process can be limited by activation (diazotization) from stable aniline precursors moments before the electrochemical treatment. In this work, we investigate how kinetic control of this diazotization activation reaction impacts the resultant electrochemical grafting on highly oriented pyrolytic graphite (HOPG) electrode surfaces. Using simple aniline precursors, diazotization reactions are carried out with four different protocols: ex situ activation, ex situ activation & stirring, in situ activation, and in situ activation & stirring. The resultant HOPG electrode surfaces are analyzed using scanning tunneling microscopy and Raman spectroscopy, to reveal hindered surface functionalization from in situ activation methods. Increases in the already high grafting densities achieved from ex situ diazotization can be enhanced by stirring the solution while applying the electrochemical potential. Stirring is shown to replenish the depletion layer that arises after the electrochemical potential is applied. Overall, this work highlights the role of diffusion, depletion, and electric double layers involved in diazonium based electrochemical grafting processes.