Tuning Vertical Electron Transfer on Graphene Bilayer Electrochemical Devices

Pristine graphene electrodes exhibit slow vertical (off-plane) heterogeneous electron transfer (HET) rate that limits their application in electrochemical devices. This can be minimized by generating defects in the graphene structure. However, these defects adversely affect the horizontal (in-plane) electrical conductivity characteristic to graphene. Here, the fabrication of graphene bilayer devices with modulated vertical HET is reported. Strategically, the upper sheet is used as a sacrificial layer for the introduction of extrinsic defects via electrochemical oxidation while preserving the structure of the graphene underlying layer. For [Fe(CN)(6)](4-)/[Fe(CN)(6)](3-) vertical HET in solution-phase, oxidized electrodes present a very low charge transfer resistance. For vertical HET in surface adsorbed ferrocene on oxidized electrodes, the vertical HET rate constant is about five times higher than on pristine electrodes. Based on data from scattering-type scanning near-field optical microscopy (s-SNOM) and Raman spectroscopy, the improvement on the electrochemical properties is attributed to the defects that are incorporated in the upper layer graphene lattice. This fundamental study on the atomic behavior of defects and stacking layers of graphene provides a new strategic design of graphene-based devices with superior electrochemical performance.