Theoretical and Experimental Studies of the Reactions between Hyperthermal O(3P) and Graphite: Graphene-Based Direct Dynamics and Beam-Surface Scattering Approaches

Synergistic effects of atomic oxygen and thermal cycling in low earth orbit on polymer-matrixed space material

Polymer-matrixed materials are widely used in the spacecrafts' structures. However, crafts located in the LEO（Low Earth Orbit） would suffer from hazardous environment factors when orbiting in the space. It has been reported that the space environment factors' integral effect (which represents the factual detriment in space) is not equivalent to the simple summation of each individual. Hence, atomic oxygen and thermal cycling were selected as the starting point for studying the typical LEO synergistic effects on polymer-matrixed space material. In this work, methods such as surface morphology observation, surface components analyzation and inter-laminar-shear strength test were embraced to gather the basic information for the study of degradation. As a result, focusing on the composites selected in this work, synergistic effects do exist between the two factors (AO&TC, representing for atomic oxygen and thermal cycling combined). Besides, a quantified index was proposed to represent synergistic characteristics，so as to lay the foundation for the scientific evolution of material characterization.

Spin-dependent reactivity and spin-flipping dynamics in oxygen atom scattering from graphite

The formation of two-electron chemical bonds requires the alignment of spins. Hence, it is well established for gas-phase reactions that changing a molecule's electronic spin state can dramatically alter its reactivity. For reactions occurring at surfaces, which are of great interest during, among other processes, heterogeneous catalysis, there is an absence of definitive state-to-state experiments capable of observing spin conservation and therefore the role of electronic spin in surface chemistry remains controversial. Here we use an incoming/outgoing correlation ion imaging technique to perform scattering experiments for O(3P) and O(1D) atoms colliding with a graphite surface, in which the initial spin-state distribution is controlled and the final spin states determined. We demonstrate that O(1D) is more reactive with graphite than O(3P). We also identify electronically nonadiabatic pathways whereby incident O(1D) is quenched to O(3P), which departs from the surface. With the help of molecular dynamics simulations carried out on high-dimensional machine-learning-assisted first-principles potential energy surfaces, we obtain a mechanistic understanding for this system: spin-forbidden transitions do occur, but with low probabilities.

High-Dimensional Atomistic Neural Network Potential to Study the Alignment-Resolved O2 Scattering from Highly Oriented Pyrolytic Graphite

A high dimensional and accurate atomistic neural network potential energy surface (ANN-PES) that describes the interaction between one O₂ molecule and a highly oriented pyrolytic graphite (HOPG) surface has been constructed using the open-source package (aenet). The validation of the PES is performed by paying attention to static characteristics as well as by testing its performance in reproducing previous ab initio molecular dynamics simulation results. Subsequently, the ANN-PES is used to perform quasi-classical molecular dynamics calculations of the alignment-dependent scattering of O₂ from HOPG. The results are obtained for 200 meV O₂ molecules with different initial alignments impinging with a polar incidence angle with respect to the surface normal of 22.5° on a thermalized (110 and 300 K) graphite surface. The choice of these initial conditions in our simulations is made to perform comparisons to recent experimental results on this system. Our results show that the scattering of O₂ from the HOPG surface is a rather direct process, that the angular distributions are alignment dependent, and that the final translational energy of end-on molecules is around 20% lower than that of side-on molecules. Upon collision with the surface, the molecules that are initially aligned perpendicular to the surface become highly rotationally excited, whereas a very small change in the rotational state of the scattered molecules is observed for the initial parallel alignments. The latter confirms the energy transfer dependence on the stereodynamics for the present system. The results of our simulations are in overall agreement with the experimental observations regarding the shape of the angular distributions and the alignment dependence of the in-plane reflected molecules.

Self-assembled graphene-based microfibers with eclectic optical properties

The construction of graphene-based microfibers with reinforced mechanical and electrical properties has been the subject of numerous researches in recent years. However, the fabrication of graphene-based fibers with remarkable optical features still remains a challenge and has not been addressed so far. This paper aims to report a series of flexible self-assembled fibers, synthesized through a few-minute sonication of thermally oxidized graphene oxide nanosheets, so-called Nanoporous Over-Oxidized Graphene (NOG), in an acidic medium. These free-standing glassy fibers were classified into four distinct morphological structures and displayed a collection of intriguing optical properties comprising high transparency, strong birefringence, fixed body colorations (e.g. colorless, blue, green, and red), tunable interference marginal colorations, UV-visible-near IR fluorescence, and upconversion emissions. Moreover, they exhibited high chemical stability in strongly acidic, basic, and oxidizing media. The foregoing notable attributes introduce the NOG fiber as a promising candidate both for the construction of graphene-based photoluminescent textiles and the development of a wide variety of optical applications.

Insights into adsorption, diffusion, and reactions of atomic nitrogen on a highly oriented pyrolytic graphite surface

To gain insight into the nitrogen-related gas-surface reaction dynamics on carbon-based thermal protection systems of hypersonic vehicles, we have investigated the adsorption, diffusion, and reactions of atomic nitrogen, N(⁴S), on the (0001) face of graphite using periodic density functional theory with a dispersion corrected functional. The atomic nitrogen is found to bind with pristine graphite at a bridge site, with a barrier of 0.88 eV for diffusing to an adjacent bridge site. Its adsorption energy at defect sites is significantly higher, while that between graphene layers is lower. The formation of N₂ via Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) mechanisms was also investigated. In the LH pathway, the recombinative desorption of N₂ proceeds via a transition state with a relatively low barrier (0.53 eV). In addition, there is a metastable surface species, which is capable of trapping the nascent N₂ at low surface temperatures as a result of the large energy disposal into the N-N vibration. The desorbed N₂ is highly excited in both of its translational and vibrational degrees of freedom. The ER reaction is direct and fast, and it also leads to translationally and internally excited N₂. Finally, the formation of CN from a defect site is calculated to be endoergic by 2.75 eV. These results are used to rationalize the results of recent molecular beam experiments.

Reaction Mechanism of the Reduction of Ozone on Graphite

The kinetics of the ozonation of graphite with different particle sizes (106 μm, G₁₀₆; 6.20 μm, G_6.2) was studied at several temperatures under a flow of O₃ diluted in O₂. The reaction was first-order with respect to graphite and to the consumption of ozone. X-ray photoelectron spectrum (XPS) showed that the reactions occurring in the solid under steady-state conditions maintain the original stoichiometry, as predicted by the postulated mechanism for SO₂. The deoxygenation reaction occurred along with the ozonation reaction at 100 °C. The rate of oxygen elimination in the flow system has the same rate-determining kinetic barrier as ozone insertion. Ozonation and deoxygenation reactions are sequentially related. Ozonation occurs with the insertion of O₃, forming a 1,2,3-trioxolane followed by an oxygen transfer that produces a peroxide valence tautomer in equilibrium with 1,3-dicarbonyl, [peroxide ↔ dicarbonyl], and an oxirene that eliminates atomic oxygen. The decarboxylation reaction was studied at 600 °C from the ozonated G₁₀₆ (ΔG^≠ = 83.60 ± 0.08 kcal·mol^-1). Total decarboxylation at 600 °C matched the number of moles of CO₂ removed and the oxygen content after ozonation, showing that the reduction of ozone on graphite was essentially a clean reduction with no secondary oxidations. When ozonized graphite was heated to 600 °C, only [peroxide ↔ dicarbonyl] species remained in the matrix. The peroxide tautomer isomerized to dioxirane and eliminated CO₂ as a dioxicarbene. Total deoxygenation of decarboxylated graphite G₁₀₆ was obtained by pyrolysis. There was residual oxygen that arose from the atomic oxygen eliminated from the oxirene, intercalated in graphite layers, and formed basal epoxy groups. Also, incoming O atoms reacted with the intercalated O atoms to produce O₂ molecules. Thermal annealing deintercalated molecular oxygen (600-900 °C).

Multiscale modeling of damaged surface topology in a hypersonic boundary

In this work, we used molecular dynamics (MD) to perform trajectory simulations of ice-like argon and amorphous silica aggregates on atomically smooth highly ordered pyrolytic graphite (HOPG) and a comparatively rougher quartz surface. It was found that at all incidence velocities, the quartz surface was stickier than the HOPG surface. The sticking probabilities and elastic moduli obtained from MD were then used to model surface evolution at a micron length scale using kinetic Monte Carlo (kMC) simulations. Rules were derived to control the number of sites available for the process execution in kMC to accurately model erosion of HOPG by atomic oxygen (AO) attack and ice-nucleation on surfaces. It was observed that the effect of defects was to increase the material erosion rate, while that of aggregate nucleation was to lower it. Similarly, simulations were performed to study the effects of AO attack and N₂ adsorption-desorption on surface evolution and it was found that N₂ adsorption-desorption limits the surface available for erosion by AO attack.

Effects of hyperthermal atomic oxygen on a cyanate ester and its carbon fiber-reinforced composite

The durability of cyanate ester (CE) to hyperthermal atomic oxygen (AO) attack in low Earth orbit may be enhanced by the addition of carbon fiber to form a carbon fiber-reinforced cyanate ester composite (CFCE). To investigate the durability of CFCE relative to CE, samples were exposed to a pulsed hyperthermal AO beam in two distinct types of experiments. In one type of experiment, samples were exposed to the beam, with pre- and post-characterization of mass (microbalance), surface topography (scanning electron microscopy (SEM)), and surface chemistry (X-ray photoelectron spectroscopy (XPS)). In the second type of experiment, the beam was directed at a sample surface, and volatile products that scattered from the surface were detected in situ with the use of a rotatable mass spectrometer detector. CFCE exhibited less mass loss than pure CE with a given AO fluence, confirming that the incorporation of carbon fiber adds AO resistance to CE. Erosion yields of CE and CFCE were 2.63 ± 0.16 × 10−24 and 1.46 ± 0.08 × 10−24 cm3 O-atom−1, respectively. The reduced reactivity of CFCE in comparison to CE was manifested in less oxidation of the CFCE surface in XPS measurements and reduced CO, CO2, and OH reaction products in beam-surface scattering experiments. The surface topographical images collected by SEM implied different surface deterioration processes for CE and CFCE. A change of surface topography with increasing AO fluence for CE indicated a threshold AO fluence, above which the erosion mechanism changed qualitatively. CFCE showed almost intact carbon fibers after relatively low AO fluences, and while the fibers eventually eroded, they did not erode as rapidly as the CE component of the composite.

Reactive Molecular Simulation of the Damage Mitigation Efficacy of POSS-, Graphene-, and Carbon Nanotube-Loaded Polyimide Coatings Exposed to Atomic Oxygen Bombardment

Reactive molecular dynamics simulation was employed to compare the damage mitigation efficacy of pristine and polyimide (PI)-grafted polyoctahedral silsesquioxane (POSS), graphene (Gr), and carbon nanotubes (CNTs) in a PI matrix exposed to atomic oxygen (AO) bombardment. The concentration of POSS and the orientation of Gr and CNT nanoparticles were further investigated. Overall, the mass loss, erosion yield, surface damage, AO penetration depth, and temperature evolution are lower for the PI systems with randomly oriented CNTs and Gr or PI-grafted POSS compared to those of the pristine POSS or aligned CNT and Gr systems at the same nanoparticle concentration. On the basis of experimental early degradation data (before the onset of nanoparticle damage), the amount of exposed PI, which has the highest erosion yield of all material components, on the material surface is the most important parameter affecting the erosion yield of the hybrid material. Our data indicate that the PI systems with randomly oriented Gr and CNT nanoparticles have the lowest amount of exposed PI on the material surface; therefore, a lower erosion yield is obtained for these systems compared to that of the PI systems with aligned Gr and CNT nanoparticles. However, the PI/grafted-POSS system has a significantly lower erosion yield than that of the PI systems with aligned Gr and CNT nanoparticles, again due to a lower amount of exposed PI on the surface. When comparing the PI systems loaded with PI-grafted POSS versus pristine POSS at low and high nanoparticle concentrations, our data indicate that grafting the POSS and increasing the POSS concentration lower the erosion yield by a factor of about 4 and 1.5, respectively. The former is attributed to a better dispersion of PI-grafted POSS versus that of the pristine POSS in the PI matrix, as determined by the radial distribution function.

Current and future directions in electron transfer chemistry of graphene

The participation of graphene in electron transfer chemistry, where an electron is transferred between graphene and other species, encompasses many important processes that have shown versatility and potential for use in important applications.

Dynamics of energy transfer and soft-landing in collisions of protonated dialanine with perfluorinated self-assembled monolayer surfaces

Chemical dynamics simulations are reported which provide atomistic details of collisions of protonated dialanine, ala2-H(+), with a perfluorinated octanethiolate self-assembled monolayer (F-SAM) surface. The simulations are performed at collision energies Ei of 5.0, 13.5, 22.5, 30.00, and 70 eV, and incident angles 0° (normal) and 45° (grazing). Excellent agreement with experiment (J. Am. Chem. Soc., 2000, 122, 9703-9714) is found for both the average fraction and distribution of the collision energy transferred to the ala2-H(+) internal degrees of freedom. The dominant pathway for this energy transfer is to ala2-H(+) vibration, but for Ei = 5.0 eV ∼20% of the energy transfer is to ala2-H(+) rotation. Energy transfer to ala2-H(+) rotation decreases with increase in Ei and becomes negligible at high Ei. Three types of collisions are observed in the simulations: i.e. those for which ala2-H(+) (1) directly scatters off the F-SAM surface; (2) sticks/physisorbs on/in the surface, but desorbs within the 10 ps numerical integration of the simulations; and (3) remains trapped (i.e. soft-landed) on/in the surface when the simulations are terminated. Penetration of the F-SAM by ala2-H(+) is important for the latter two types of events. The trapped trajectories are expected to have relatively long residence times on the surface, since a previous molecular dynamics simulation (J. Phys. Chem. B, 2014, 118, 5577-5588) shows that thermally accommodated ala2-H(+) ions have an binding energy with the F-SAM surface of at least ∼15 kcal mol(-1).

Eley-rideal reactions with N atoms at Ru(0001): formation of NO and N(2)

Forward-directed NO molecules with large translational energies are formed upon exposure of an O-covered Ru(0001) surface to a nitrogen (N+N_{2}) beam. This is an unequivocal experimental demonstration of the Eley-Rideal reaction for a "heavy" (i.e., nonhydrogenated) neutral system. The time dependence of prompt NO formation exhibits an exceptionally fast decay as a consequence of shifting reaction pathways and probabilities over the course of the exposure. Prompt production shuts down as the O coverage decreases due to competition from more favorable Eley-Rideal production of N_{2}.

Nonadiabatic reaction mechanisms of the O((3)P) with cyclopentene

The reaction mechanism of the ground state oxygen atom O((3)P) with cyclopentene is investigated theoretically. The triplet and singlet potential energy surfaces are calculated at the CCSD(T)//MP2/6-311G(d,p) level and the minimum energy crossing points (MECPs) between the two surfaces are located by means of the Newton-Lagrange method, from which the complex nonadiabatic reaction pathways are revealed. Based on the theoretical results, the most probable reaction mechanism of O((3)P) with c-C5H8 is described, which agrees with the experimental results nicely, including the condensed phase experiment. At the same time, the newly revealed reaction mechanism clarifies the previous controversial product distribution, and predicts the possible existence of the new enol product, cyclopentenol.

Large scale computational chemistry modeling of the oxidation of highly oriented pyrolytic graphite

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.

Hyperthermal oxidation of graphite and diamond

Carbon materials have mechanical, electrical, optical, and tribological properties that make them attractive for use in a wide range of applications. Two properties that make them attractive, their hardness and inertness in many chemical environments, also make them difficult to process into useful forms. The use of atomic oxygen and other forms of oxidation has become a popular option for processing of these materials (etching, erosion, chemical functionalization, etc.). This Account provides an overview of the use of theory to describe the mechanisms of oxidation of diamond and graphite using hyperthermal (few electronvolts) oxygen atoms. The theoretical studies involve the use of Born-Oppenheimer molecular dynamics calculations in which on-the-fly electronic structure calculations have been performed using either density functional theory or density-functional-tight-binding semiempirical methods to simulate collisions of atomic oxygen with diamond or graphite. Comparisons with molecular-beam scattering on surfaces provide indirect verification of the results. Graphite surfaces become oxidized when exposed to hyperthermal atomic oxygen, and the calculations have revealed the mechanisms for formation of both CO and CO(2). These species arise when epoxide groups form and diffuse to holes on the surface where carbonyls are already present. CO and CO(2) form when these carbonyl groups dissociate from the surface, resulting in larger holes. We also discuss mechanisms for forming holes in graphite surfaces that were previously hole-free. For diamond, the (111) and (100) surfaces are oxidized by the oxygen atoms, forming mostly oxy radicals and ketones on the respective surfaces. The oxy-covered (111) surface can then react with hyperthermal oxygen to give gaseous CO(2), or it can become graphitized leading to carbon removal as with graphite. The (100) surface is largely unreactive to hyperthermal atomic oxygen, undergoing large amounts of inelastic scattering and supporting reactions that create O(2) or peroxy radicals. We did not observe a mechanism for the removal of carbon for this surface. These results are consistent with experimental studies that show formation of CO and CO(2) in graphite oxidation and preferential etching on (111) CVD diamond surfaces in comparison with (100) surfaces.