Carbon Oxidation at the Atomic Level: A Computational Study on Oxidative Graphene Etching and Pitting of Graphitic Carbon Surfaces

In order to understand the oxidation of solid carbon materials by oxygen-containing gases, carbon oxidation has to be studied on the atomic level where the surface reactions occur. Graphene and graphite are etched by oxygen to form characteristic pits that are scattered across the material surface, and pitting in turn leads to microstructural changes that determine the macroscopic oxidation behavior. While this is a well-documented phenomenon, it is heretofore poorly understood due to the notorious difficulty of experiments and a lack of comprehensive computational studies. The main objective of the present work is the development of a computational framework from first principles to study carbon oxidation at the atomic level. First, the large body of literature on carbon oxidation is examined with regards to experimental observations of the pitting phenomenon as well as relevant theoretical studies on different aspects of the mechanistic details of carbon oxidation. Next, a comprehensive, atomic-scale kinetic mechanism for carbon oxidation is developed, which comprises only elementary surface reactions with reaction rates derived from first principles. The mechanism is then implemented using the Kinetic Monte Carlo (KMC) method. This framework for the first time allows the simulation of oxidative graphene etching at the atomic scale to relevant time- and lengthscales (up to seconds and hundreds of nanometers), and in a wide range of conditions (temperatures up to 2000 Kelvin, pressures ranging from vacuum to atmospheric pressure). The numerical results reveal information about the pitting process in heretofore unattained detail: Pit growth rates (and therefore intrinsic oxidation rates) are calculated and validated against a set of different experimental data at a wide range of conditions. Such information is crucial for modelling of material behavior on meso- and macroscales. The dependence of the pit geometry (hexagonal vs. circular) on temperature and gas pressure is assessed. This is important for utilizing oxidative etching as a manufacturing technique for graphene-based nanodevices. More subtle phenomena like pit inhibition at low pressures and temperatures are also discussed. Moreover, all these findings are examined with respect to the underlying reaction mechanism. This unveils the fundamental reasons for the observed reaction behavior, in particular different activation energies and reaction orders at low and high temperatures, as well as the transition of the pit geometry. The present work is a first step in an ongoing effort to develop predictive models for carbon oxidation in Thermal Protection Systems (TPS), with the ultimate goal of improved safety for hypersonic flight vehicles