Chemical Oxidation of Graphite: Evolution of the Structure and Properties

Graphene oxide is a complex material whose synthesis is still incompletely understood. To study the time evolution of structural and chemical properties of oxidized graphite, samples at different temporal stages of oxidation were selected and characterized through a number of techniques: X-ray photoelectron spectroscopy for the content and bonding of oxygen, X-ray diffraction for the level of intercalation, Raman spectroscopy for the detection of structural changes, electrical resistivity measurements for probing charge localization on the macroscopic scale, and scanning transmission electron microscopy for the atomic structure of the graphene oxide flakes. We found a nonlinear behavior of oxygen uptake with time where two concentration plateaus were identified: Uptake reached 20 at % in the first 15 min, and after 1 h a second uptake started, reaching a highest oxygen concentration of >30 at % after 2 h of oxidation. At the same time, the interlayer distance expanded to more than twice the value of graphite and the electrical resistivity increased by seven orders of magnitude. After 4 days of chemical processing, the expanded structure of graphite oxide became unstable and spontaneously exfoliated; more than 2 weeks resulted in a significant decrease in the oxygen content accompanied by reaggregation of the GO sheets. These correlated measurements allow us to offer a comprehensive view into the complex oxidation process.