Electron Paramagnetic Resonance as a Structural Tool to Study Graphene Oxide: Potential Dependence of the EPR Response

Electron paramagnetic resonance (EPR) spectroscopy is reported as a tool to probe the behavior of graphene oxide (GO). The potential-dependent response of GO is reported for the first time and correlated with the observed electrochemical response. The EPR signal, deconvoluted into two constituent parts, was used along with lineshape simulations and the temperature dependence to probe the electrochemical processes. The EPR signal is found to be well described by two components. The narrower one is associated with unpaired electrons on localized functional groups and shows a reversible increase as GO is biased to positive potentials. The Curie behavior of this component suggests that it increases because of the formation of stable radical species, such as semiquinones, derived from quinones and other carbonyl functional groups found on GO. A stronger dependence of the narrow component with potential, and an elevated g value over 2.0034, is found in alkaline conditions compared to neutral electrolytes, reflecting the greater stability of seminquinone-like species at higher pH. By contrast, the second, broader component of the EPR signal was found to be potential-independent. The EPR approach described here offers a solution phase alternative, which can be employed under electrochemical control, to techniques such as X-ray photoelectron spectroscopy and Raman spectroscopy, as a means to probe the structure of GO and related materials.