The role of oxygen during thermal reduction of graphene oxide studied by infrared absorption spectroscopy

Understanding the thermal reduction of graphene oxide (GO) is important for graphene exfoliation, and chemical and morphological modifications. In this process, the role of trapped water and the evolution of oxygen during annealing are still not well-understood. To unravel the complex mechanisms leading to the removal of oxygen in reduced GO, we have performed in situ transmission infrared absorption spectroscopy measurements of GO films upon thermal annealing at 60-850°C in vacuum (10^-3-10^-4 Torr). Using cluster-based first-principles calculations, epoxides, ethers (pyrans and furans), hydroxyls, carboxyls, lactols, and various types of ketones and their possible derivatives have been identified from the spectroscopic data. Furthermore, the interactions between randomly arranged nearby oxygen species are found to affect the spectral response (red and blue shifts) and the overall chemistry during annealing. For instance, the initial composition of oxygen species (relative amounts and types of species, such as hydroxyls, carboxyls, and carbonyls) and reduction times determine the final oxygen concentration (% of initial concentrations), varying from ∼46-92% in multilayer GO to ∼3-5% in single-layer GO. In the multilayer case, there is no dependence on the layer thickness. An important indicator of the reduction efficiency is the relative concentration of carbonyls at intermediate annealing temperatures (∼200°C). These observations suggest that thermal annealing can foster the formation of free radicals containing oxygen in the presence of trapped water in GO, which further attack carboxyls, hydroxyls, and carbonyls, preferentially at edges rather than on basal plane defects. These findings impact the fabrication of electronics and energy storage devices.