Electroreduction of Viologen Phenyl Diazonium Salts as a Strategy To Control Viologen Coverage on Electrodes

A majority of the reported electrografting of aryldiazonium salts result in the formation of covalently attached films with a limited surface coverage of below 5 nmol·cm-2. Herein, we report the preparation of higher-thickness redox-active viologen-grafted electrodes from the electroreduction of viologen phenyl diazonium salts, by either cyclic voltammetric (CV) sweeps or electrolysis using a fixed potential. Both of the methodologies were successfully applied for various conductive surfaces, including glassy carbon (GC), gold disc, indium tin oxide glass, mesoporous TiO2 electrodes, and 3D compacted carbon fibers. A robust maximal viologen coverage, Γviologen = 9.5 nmol·cm-2, was achieved on a GC electrode by CV electroreduction. Electroreduction held at a fixed potential at Eappl. = -0.3 V can fabricate viologen-grafted electrodes with Γviologen in the range of 0-37 nmol·cm-2 in a controllable way, by simply adjusting the electrodeposition time tappl.. Time-dependent Γviologen were found to be 10 nmol·cm-2@2 min, 20 nmol·cm-2@4.2 min, and 30 nmol·cm-2@7 min. Furthermore, a TiO2 electrode coupled with Γviologen of 140 nmol·cm-2 exhibited electrochromic performance, with the color changing from pale yellow to blue and red brown.