Graphene oxide (GO) has received tremendous attention over the past decade, in part because it enables a route which allows graphene to be produced from aqueous solutions, which potentially enables large-scale production of graphene-based devices. The structure of GO is a two-dimensional network of carbon atoms, similar to graphene. However, whereas pristine graphene is an uninterrupted network of sp² carbon atoms, GO contains significant interruptions to the sp² lattice caused by the introduction of oxygen species. It is this functionalization with oxygen groups which enables GO to be soluble in water and several other organic solvents. However, breaking the sp²lattice along graphene’s basal plane diminishes the material’s electronic properties. GO can be reduced to a graphene-like material using thermal, chemical, or electrochemical means to restore the electronic properties.

The electron transfer properties of GO are thus of great interest and have been investigated either by studying the heterogeneous electron transfer between a GO electrode and a redox molecule in solution, by measuring the direct reduction of GO films on various electrode substrates, or measuring the electrochemical behavior of GO in solution. With the first technique, information about the electron transfer between a species in solution and the graphene material is obtained. This can provide heterogeneous rate constants, insight into the surface chemistry and reactivity, and general electroanalytical characteristics. These experiments are useful for understanding how the GO-based material will behave in applications such as sensing or fuel cells. Such detailed characterization of the electrochemical processes provides fundamental information about the direct electron transfer processes involved with the material itself and allows a deeper understanding of GO, thus maximizing its potential for future applications.

Using the second strategy, GO films are prepared (e.g., by drop casting) and subsequently reduced electrochemically. With GO fixed to the electrode surface, the current−voltage response is governed by thin-film behavior and arises from the reduction of oxygen-containing functional groups such as epoxides, hydroxides, or peroxides. With this methodology, GO is not able to diffuse to and from the electrode surface, so information about its behavior in solution is lost. To date, only two communications show the electrochemical behavior of native graphene oxide colloids. Chen et al. showed that GO can be electrodeposited onto the surface of glassy carbon electrodes directly from colloidal solutions.¹ Eng and Pumera showed that the voltammetry of GO colloids depends primarily on particle size and pH.²Both of these previous investigations were conducted using carbon electrode materials, which effects the observed voltammetry due to relatively inert nature of the carbon electrode surface.

Here we report a voltammetric analysis of native GO colloids in water using Pt electrodes.³Although much is known about the physical structure of GO, there is still debate concerning GO’s exact chemical properties, especially its acidity in aqueous solutions. The electrochemistry of graphene oxide in solution is shown to be significantly different from the previously reported electrochemistry of surface-bound GO films. We show that suspensions of graphene oxide exhibit a reversible oxidation and reduction that is present under a variety of conditions. We investigate the origin of this response and provide a plausible mechanism for the electrochemical behavior of GO in solution.

References

(1) Chen, L.; Tang, Y.; Wang, K.; Liu, C.; Luo, S. Direct Electrodeposition of Reduced Graphene Oxide on Glassy Carbon Electrode and Its Electrochemical Application. Electrochem. Commun. 2011, 13(2), 133–137.

(2) Eng, A. Y. S.; Pumera, M. Direct Voltammetry of Colloidal Graphene Oxides. Electrochem. Commun. 2014, 43, 87–90.

(3) O’Neil, G. D.; Weber, A. W.; Buiculescu, R.; Chaniotakis, N. A.; Kounaves, S. P. Electrochemistry of Aqueous Colloidal Graphene Oxide on Pt Electrodes. Langmuir 2014, 30, 9599–9606.