Large scale molecular dynamics (MD) simulations are performed to study the oxidation of highly oriented pyrolytic graphite (HOPG) by hyperthermal atomic oxygen beam (5 eV). Simulations are performed using the ReaxFF classical reactive force field. We present here additional evidence that this method accurately reproduces ab initio derived energies relevant to HOPG oxidation. HOPG is modeled as multilayer graphene and etch-pit formation and evolution is directly simulated through a large number of sequential atomic oxygen collisions. The simulations predict that an oxygen coverage is first established that acts as a precursor to carbon-removal reactions, which ultimately etch wide but shallow pits, as observed in experiments. In quantitative agreement with experiment, the simulations predict the most abundant product species to be O2 (via recombination reactions), followed by CO2, with CO as the least abundant product species. Although recombination occurs all over the graphene sheet, the carbon-removal reactions occur only about the edges of the etch pit. Through isolated defect analysis on small graphene models as well as trajectory analysis performed directly on the predicted etch pit, the activation energies for the dominant reaction mechanisms leading to O2, CO2, and CO product species are determined to be 0.3, 0.52, and 0.67 eV, respectively. Overall, the qualitative and quantitative agreement between MD simulation and experiment is very promising. Thus, the MD simulation approach and C/H/O ReaxFF parametrization may be useful for simulating high-temperature gas interactions with graphitic materials where the microstructure is more complex than HOPG.

No abstract

No abstract

The gas−surface scattering dynamics of nitromethane (CH 3 NO 2 ) and methyl formate (HCOOCH 3 ) on a highly oriented pyrolytic graphite (HOPG) surface have been investigated as part of a broader effort to evaluate the efficacy of a funnel-like neutral gas concentrator that has been proposed as a mass spectrometer inlet for the characterization of tenuous planetary atmospheres or plumes. Molecular beams of CH 3 NO 2 and HCOOCH 3 with incidence energies, E i , of 106.5 and 98.8 kJ mol −1 , respectively, were directed at the surface with incidence angles, θ i , of 70, 45, and 30°. A rotatable mass spectrometer, employing electron-impact ionization, was used to collect angle-resolved time-of-flight (TOF) distributions of the molecules that scattered inelastically from the surface, allowing angular distributions of the scattered product flux and translational energy distributions at a given final angle, θ f , to be obtained. The TOF distributions of the scattered products detected the parent ion mass-to-charge ratios and their respective dominant ion fragments were identical, indicating that CH 3 NO 2 and HCOOCH 3 fragmented in the ionizer of the detector and not while colliding with the surface. The scattering dynamics suggested that the parallel momentum of the molecules was conserved during impact with the surface. The translational energy and angular distributions of CH 3 NO 2 and HCOOCH 3 were identical when θ i = 70°. For θ i = 45 and 30°, the HCOOCH 3 angular distributions were shifted to a slightly larger θ f than the CH 3 NO 2 distributions. The molecules scattered from the surface through impulsive scattering (IS) and quasitrapping (QT) pathways. The IS molecules retained a large fraction of their incidence translational energy when colliding with the surface. The QT molecules transferred more energy, but they did not come completely into thermal equilibrium with the surface before scattering into the vacuum. The QT molecules had a lobular angular distribution with a maximum flux far from the surface normal, indicating that they retained some memory of their incident conditions despite losing a significant amount of energy at the surface. The results presented in this article demonstrate that for E i near 100 kJ mol −1 , these molecules would not dissociate upon impact with the surfaces of a gas concentrator constructed of HOPG. Although the observed scattering dynamics suggest that such a concentrator could perform well for a variety of molecular species, accurate concentration factors are ultimately molecule-specific and determined by the details of the molecule−surface interaction potential.

No abstract