Oxidation of graphene with variable defects: alternately symmetrical escape and self-restructuring of carbon rings

Effect of Hyperthermal Hybrid Gas Composition on the Interfacial Oxidation and Nitridation Mechanisms of Graphene Sheet

Graphene is one of the most promising thermal protection materials for high-speed aircraft due to its lightweight and excellent thermophysical properties. At high Mach numbers, the extremely high postshock temperature would dissociate the surrounding air into a mixture of atomic and molecular components in a highly thermochemical nonequilibrium state, which greatly affects the subsequent thermal chemical reactions of the graphene interface. Through establishing a reactive gas-solid interface model, the reactive molecular dynamics method is employed in this study to reveal the influences of the thermochemical nonequilibrium gas mixture on the thermal oxidation and nitridation mechanisms of graphene sheet. The results show that three distinctive stages can be recognized during bombardment of various nonequilibrium gas components toward the graphene sheet: (i) collision and surface adsorption stage, (ii) gas-solid heterogeneous reaction stage, and (iii) gas phase homogeneous reaction stage. The surface catalysis effect is found to be dominant during the first two stages, which can influence the following ablation behavior of graphene significantly at high-temperature conditions. Moreover, surface catalysis, oxidation, nitridation, and ablation mechanisms between nonequilibrium gas and graphene interface are revealed, which is of high relevance for future interfacial design and application of graphene as a thermal protection material.

The Adsorption Mechanism of Hydrogen on FeO Crystal Surfaces: A Density Functional Theory Study

The hydrogen-based direct reduction of iron ores is a disruptive routine used to mitigate the large amount of CO₂ emissions produced by the steel industry. The reduction of iron oxides by H₂ involves a variety of physicochemical phenomena from macroscopic to atomistic scales. Particularly at the atomistic scale, the underlying mechanisms of the interaction of hydrogen and iron oxides is not yet fully understood. In this study, density functional theory (DFT) was employed to investigate the adsorption behavior of hydrogen atoms and H₂ on different crystal FeO surfaces to gain a fundamental understanding of the associated interfacial adsorption mechanisms. It was found that H₂ molecules tend to be physically adsorbed on the top site of Fe atoms, while Fe atoms on the FeO surface act as active sites to catalyze H₂ dissociation. The dissociated H atoms were found to prefer to be chemically bonded with surface O atoms. These results provide a new insight into the catalytic effect of the studied FeO surfaces, by showing that both Fe (catalytic site) and O (binding site) atoms contribute to the interaction between H₂ and FeO surfaces.

Ab Initio Investigation of the Adsorption of CO2 Molecules on Defect Sites of Graphene Surfaces: Role of Local Vacancy Structures

An in-depth investigation into the adsorption of CO₂ on graphene vacancies is essential for the understanding of their applications in various industries. Herein, we report an investigation of the effects of vacancy defects on CO₂ gas adsorption behavior on graphene surfaces using the density functional theory. The results show that the formation of vacancies leads to various deformations of local carbon structures, resulting in different adsorption capabilities. Even though most carbon atoms studied can only trigger physisorption, there are also carbon sites that are energetically favored for chemisorption. The general order of the adsorption capabilities of the local carbon atoms is as follows: carbon atoms with dangling bonds > carbon atoms shared by five- and six-membered rings and a vacancy > carbon atoms shared by two six-membered rings and a vacancy. A stronger interaction in the adsorption process generally corresponds to more obvious changes in the partial density of states and a larger amount of transferred charge.

Parallel-Self-Assembling Stack, Center-Capture Effect, and Reactivity-Enhancing Effect of N-Layer (N = 1, 2, 3) Cyclo[18]carbon

Cyclo[18]carbon (C₁₈) is an captivating allotrope of carbon synthesized recently, which has drawn the attention among scientists. There are still few studies on the dynamic behaviors of C₁₈. To gain knowledge in this area, we systematically explored the stacking behaviors and the oxidation kinetics of C₁₈, as well the electronic transport behaviors of C₁₈ oxides, by density functional theory and nonequilibrium Green's function calculations combined with reactive force field molecular dynamics simulations. The parallel-self-assembling behaviors were observed in the stack of two- or three-layer C₁₈. During the oxidation process of C₁₈, we found an evident center-capture effect in which the hollow rings would preferentially attract an O₂ molecule into their centers. Moreover, the adsorption of O₂ on the O₂-doped rings was dramatically enhanced by the O₂ at the center of the ring, showing the reactivity-enhancing effect. The excellent electron transport property of central-O₂-doped C₁₈ among 13 types of C₁₈ oxides demonstrates the potential of C₁₈ oxides as promising molecular devices for various applications. This study reveals the dynamic behaviors of C₁₈ and provides theoretical guidance for use of C₁₈ and C₁₈ oxides in molecular devices.

Flexible Full-Surface Conformal Encapsulation for Each Fiber in Graphene Glass Fiber Fabric against Thermal Oxidation

Encapsulation for carbon-based electronic devices against oxidation can enhance their long-term working stability. Graphene glass fiber fabric (GGFF), as an advanced flexible electrothermal material, also struggles with graphene oxidation. The flexible, full-surface, conformal encapsulation for each fiber in the large-area fabric puts forward high requirements for encapsulating materials and techniques. Herein, the nanometer-thick h-BN layer was in situ grown on cambered surfaces of each fiber in GGFF with the chemical vapor deposition method. Stable heating duration (500 °C) of h-BN-encapsulated GGFF (h-BN/GGFF) was increased by 1 order of magnitude without compromising the electrothermal performances and flexibility. Theoretical simulations revealed that the enhanced oxidation resistance of h-BN/GGFF was attributed to the decreased interaction and adsorption life of oxygen. The proposed flexible, full-surface, conformal encapsulation technique targeting the fiber-shaped graphene electrothermal device is scalable and can be extended to the other carbon materials, even devices with intricate shapes, which will promote the development of flexible electronics.

A density functional theory study on the adsorption reaction mechanism of double CO2 on the surface of graphene defects

Much research has been done on reactions of a single CO₂ molecule with a graphene surface. In this paper, density functional theory calculations are used to investigate the adsorption and reaction of double CO₂ on the surface of single vacancy (SV) and divacancy (DV) defect graphene. The study found that due to the mutual repulsion between CO₂ and the size of the SV defect, it is difficult for two CO₂ molecular to be adsorbed directly above the SV defect at the same time. Regardless of SV or DV, the adsorption of the first CO₂ in the defect center will have a beneficial effect on the adsorption of the second CO₂. In addition, the transition state calculation of the CO₂ reaction on the DV plane was carried out, and the adsorption behavior was analyzed and studied. This in-depth study is helpful to the understanding of the reaction behavior of CO₂ on graphene, and further exploration in the direction of the effective application of graphene to the reaction and adsorption of CO₂. Our work explores the adsorption behavior of CO₂ on graphene surfaces, the physical and chemical adsorption of double CO₂ at the defect was studied and analyzed.

Established Model on Polycrystalline Graphene Oxide and Analysis of Mechanical Characteristic

It may cause more novel physical effects that the combination with in-plane defects induced by grain boundaries (GBs) and quasi three-dimensional system induced by oxidation functional group. Different from those in blocks, these new physical effects play a significant role in the mechanical properties and transport behavior. Based on the configuration design, we investigate the in-plane and out-plane geometric deformation caused by the coupling of GBs and oxygen-containing functional groups and establish a mechanical model for the optimal design of the target spatial structure. The results show that the strain rate remarkably affect the tensile properties of polycrystalline graphene oxide (PGO). Under high oxygen content (R = 50%), with the increasing strain rate, the PGO is much closer to ductile fracture, and the ultimate strain and stress will correspondingly grow. The growth of temperature reduces the ultimate stress of PGO, but the ultimate strain remains constant. When the functional groups are distributed at the edge of the GBs, the overall strength decreases the most, followed by the distribution on the GBs. Meanwhile, the strength of PGO reaches the greatest value when the functional groups are distributed away from the GBs.

A theoretical study of wrinkle propagation in graphene with flower-like grain boundaries

This study investigated dynamic surface wrinkle propagation across a series of flower-like rotational grain boundaries (GBs) in graphene using theoretical solutions and atomistic simulations. It was found that there was significantly less out-of-plane displacement of dynamic wrinkles when curvature of rotational GBs was reduced, which can be explained by a defect shielding effect of flower-like GBs. Potential energy evolved via different modes for pristine graphene and graphene with various GBs. With external excitation, the distinctly different patterns of wrinkle propagation in graphene with various GBs demonstrated how dynamic wrinkling can reveal defects. These results can provide a theoretical basis for guiding the design and implementation of graphene-based nano-mechanical devices such as protectors and detectors.