Efficient surfactant-free and chemical reductant-free solvothermal deoxidation of solution-processable sub-stoichiometric graphene oxide

Abstract

We report an efficient solvothermal process to achieve deep deoxidation of octadecylamine functionalized sub-stoichiometric graphene oxides (sub-GOx) in organic solvents. An initial average carbon oxidation state of ca. 0.6 (i.e., 0.6 OH per basal-plane C) could be reduced to ca. 0.15, while retaining a critical density of the alkyl chains for solvent processability, ca. 0.02 chains per basal-plane C. The products can thus be characterized as single-sheet dispersions of alkyl-functionalized disordered “graphenes”. The oxidation state was characterized by X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy and Raman spectroscopy. We found a strong solvent effect in the relative rates of alkyl chain degrafting and/or chain scission versus deoxidation and/or regraphenization. The desired deoxidation and/or regraphenization are promoted at higher temperatures and by the use of aprotic amide solvents. Furthermore we found that carbon monoxide (CO) was a remarkably efficient fugitive deoxidizer. A key advantage of this solvothermal deoxidation process is that the resultant dispersion of conductive graphenes can be directly used for printing, coating or formulating into nanocomposites, without further purification or heat treatment. Thin films of the deoxidized sub-GOx give dc conductivities of up to 40 S cm⁻¹, which is the highest reported to date for solvent-processable graphene derivatives. These films show temperature independent conductivity, which suggests that its conductivity is largely limited by tunneling between the alkyl chain spacers.