Potassium assisted reduction and doping of graphene oxides: towards faster electron transfer kinetics

Abstract

The current scientific interest and fervour in graphene research is fuelled by their potential uses in a variety of fields such as energy storage, electrochemical and biomolecule sensing applications. In particular, many of these require the bulk production of graphene materials, using approaches such as the oxidation of graphite to graphite oxide with chlorate or manganate oxidants in the presence of strong acids. This is followed by its reduction via thermal exfoliation, or through chemical and electrochemical reduction methods. Elemental potassium is a very strong reducing agent with a chemical potential of ca. E = −2.93 V versus the standard hydrogen electrode. Therefore, in this paper we evaluate the effects of the presence of elemental potassium as a strong reductant during the thermal exfoliation of graphite oxides. We employ both the Hofmann (concentrated sulfuric and nitric acids with KClO₄) and Hummers (concentrated sulfuric acid with NaNO₃ and KMnO₄) methods of graphite oxidation, followed by their thermal reduction at 500 °C both in the absence and presence of elemental potassium. Collectively, these treatments invariably result in differences in morphologies, defect densities and the quantities of oxygen-containing groups within these materials, thus affecting their heterogeneous electron transfer rates and electrochemistry. Extensive analyses with scanning electron microscopy (SEM), Raman spectroscopy, high resolution X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDS), powder X-ray diffraction (XRD), BET surface area analysis, combustible elemental analysis and cyclic voltammetry (CV) are performed to elucidate the different properties and characteristics of our materials. We observed that the elemental potassium-assisted thermal reduction of graphene oxide led to a product with increased heterogeneous electron transfer rates towards the Fe(CN)₆^4−/3− redox probe, despite having more oxygen-containing groups. This is in contrast to prior reports where larger amounts of oxygen groups corresponded to poorer electrochemistry. There were also no significant differences in the density of sp³ defects between the graphene materials. Hence, this improved electrochemistry may be attributed to the formation of stable complexes between potassium and oxygen groups on the graphene surface in a manner we propose to be similar to carboxylic acid salts. These results would therefore have important implications in our understanding of oxygen-containing groups in graphene materials and their influence on their inherent electrochemistry.