Various defects in graphene: a review

Pristine graphene has been considered one of the most promising materials because of its excellent physical and chemical properties. However, various defects in graphene produced during synthesis or fabrication hinder its performance for applications such as electronic devices, transparent electrodes, and spintronic devices. Due to its intrinsic bandgap and nonmagnetic nature, it cannot be used in nanoelectronics or spintronics. Intrinsic and extrinsic defects are ultimately introduced to tailor electronic and magnetic properties and take advantage of their hidden potential. This article emphasizes the current advancement of intrinsic and extrinsic defects in graphene for potential applications. We also discuss the limitations and outlook for such defects in graphene.

CO oxidation over defective and nonmetal doped MoS2monolayers

Defective (missing S atoms) and nonmetal (C- and N-) doped MoS₂monolayers in the 2H and 1T' phases have been evaluated for catalyzing CO oxidation based on first-principles calculations. For the reaction 2CO + O₂→ 2CO₂, the oxidization of the first CO molecule is fairly easy and sometimes is even spontaneous, as the O₂ molecule is highly activated or dissociates upon adsorption. However, for the defective (2H-), C-doped (1T'-), and N-doped (2H- and 1T'-) MoS₂monolayers, the remaining O^*adatom often refuses to react with other CO molecules and is hard to be removed (barrier > 1.20 eV). Only when over the C-doped 2H- and defective 1T'-MoS₂monolayers, the removal of the second O^*adatom requires to overcome moderate barriers (0.74 and 0.88 eV, respectively) by reacting with another CO molecule via the Eley-Rideal mechanism and the catalysts are recovered. The barriers can be further reduced by applying either tensile or compressive strain to the MoS₂nanosheet. In contrast, the Langmuir-Hinshelwood mechanism is followed over the metal-containing MoS₂nanosheets, as the bigger size of metal dopants allow the co-adsorption of CO and O₂. Therefore, the C-doped 2H- and defective 1T'-MoS₂monolayers are promising nonmetal-doped catalysts for CO oxidation.

Theoretical insights into the thermal reduction of N2 to NH3 over a single metal atom incorporated nitrogen-doped graphene

Density functional theory calculations have been performed to study the reaction mechanism of N₂ thermal reduction (N₂TR) over a single metal atom incorporated nitrogen-doped graphene. Our results reveal that the type of metal atoms and their coordination environments have a significant effect on the catalytic activity of N₂TR. Regarding CoN₄- and FeN₄-embedded graphene sheets that the metal atom is fourfold coordinated, they are inactive for N₂TR owing to the poor stability of the adsorbed H₂ and N₂ molecules. In contrast, if the monodisperse metal atom is surrounded by three N atoms, namely, CoN₃/G and FeN₃/G show activity toward N₂TR, and catalytic conversion of N₂ into ammonia is achieved through the associative mechanism rather than the dissociative mechanism. Further investigations show that the synthesis of NH₃ over the two surfaces is mainly through the formation of an NHNH^* intermediate; however, the detailed reaction mechanisms are sensitive to the type of metal atom introduced into N-doped graphene. Based on the calculated kinetic barriers, FeN₃/G exhibits a better catalytic activity for N₂TR. The superior performance of FeN₃/G can be attributed to the fact that this surface prefers a high spin-polarized state during the whole process of N₂TR, while the non-spin polarized state is predicted as the ground state for most of the elementary steps of N₂-fixation over CoN₃/G. The present study provides theoretical insights into developing graphene-based single atom catalysts with a high activity toward ammonia synthesis through N₂TR.

Catalytic role of graphitic nitrogen atoms in the CO oxidation reaction over N-containing graphene: a first-principles mechanistic evaluation

The catalytic role of graphitic nitrogen atoms of a series of nitrogen-doped graphene surfaces is explored for low-temperature oxidation of CO using periodic DFT calculations.

Theoretical Understandings of Graphene-based Metal Single-Atom Catalysts: Stability and Catalytic Performance

Research on heterogeneous single-atom catalysts (SACs) has become an emerging frontier in catalysis science because of their advantages in high utilization of noble metals, precisely identified active sites, high selectivity, and tunable activity. Graphene, as a one-atom-thick two-dimensional carbon material with unique structural and electronic properties, has been reported to be a superb support for SACs. Herein, we provide an overview of recent progress in investigations of graphene-based SACs. Among the large number of publications, we will selectively focus on the stability of metal single-atoms (SAs) anchored on different sites of graphene support and the catalytic performances of graphene-based SACs for different chemical reactions, including thermocatalysis and electrocatalysis. We will summarize the fundamental understandings on the electronic structures and their intrinsic connection with catalytic properties of graphene-based SACs, and also provide a brief perspective on the future design of efficient SACs with graphene and graphene-like materials.

Theoretical Calculation of Different Reaction Mechanisms for CO Oxidation on MnN3-Doped Graphene

In recent decades, great expectation has always been placed on catalysts that can convert toxic CO into CO₂ under mild conditions. The catalytic mechanism of CO oxidation by Mn-coordinated N-doped graphene with a single vacancy (MnN₃-SV) and a double vacancy (MnN₃-DV) was studied by density functional theory (DFT) calculations. Molecular dynamics simulations showed that CO₂ on MnN₃-SV could not be desorbed from the substrate and MnN₃-SV was not suitable for use as a CO oxidation catalyst. MnN₃-DV was more suitable for CO oxidation (COOR) and from the electronic structure it was found that the Mn atom was the main active site, which was the reaction site for CO oxidation. At temperatures of 0 and 298.15 K, CO oxidation on MnN₃-DV via the Langmuir-Hinshelwood (LH) mechanism was the best reaction pathway. The rate-determining step using MnN₃-DV as the catalyst for CO oxidation through the LH mechanism was O₂ + CO → OOCO, and the energy barrier was 0.861 eV at 298.15 K. MnN₃-DV was suitable as a catalyst for CO oxidation in terms of both thermodynamics and kinetics. This study provides a comprehensive understanding of the various reaction mechanisms of CO oxidation on MnN₃-DV, which is conducive to guiding the development and design of efficient catalysts for CO oxidation.

The effect of coordination environment on the kinetic and thermodynamic stability of single-atom iron catalysts

The stability of a single-atom catalyst is directly related to its preparation and applications, especially for high-loading single-atom catalysts. Here, the effect of a coordination environment induced by nitrogen (N) atoms coordinated with iron on the kinetic and thermodynamic stabilities of single-atom iron catalysts supported with carbon-based substrates (Fe_SA/CS) was investigated by density functional theory (DFT) calculations. Five Fe_SA/CS with different numbers of N atoms were modelled. The kinetic stability was evaluated by analyzing the migration paths of iron atoms and energy barriers. The thermodynamic stability was studied by calculating the adsorption and formation energies. Our results indicated that the coordination environment induced by N can promote the kinetic and thermodynamic stability of Fe_SA/CS. N atoms on the substrate promote the kinetic stability by raising the energy barrier for iron migration and not only increase the thermodynamic stability, but also contribute to catalyst synthesis. Doping N on the substrate enhances charge transfer between the iron atoms and substrates simultaneously improving the kinetic and thermodynamic stabilities. This theoretical research provides guidance for synthesizing stable and high loading single-atom catalysts by tuning the coordination environment of single-atom elements.

A comparative DFT study on single-atom catalysis of CO oxidation over Al- and P-embedded hexagonal boron-nitride nanosheets

Density functional theory calculations are performed to compare catalytic oxidation of CO molecule over Al- and P-embedded hexagonal boron nitride nanosheet (h-BN). It is found that the Al and P adatom can be stably anchored on the boron-vacancy site of h-BN, as evidenced by a relatively large adsorption energy and charge-transfer value. According to our findings, the oxidation of CO over these surfaces proceeds via the Langmuir-Hinshelwood mechanism, followed by the elimination of the remaining atomic O by another CO molecule. Meanwhile, the stronger adsorption of O₂ than CO avoids poisoning of the active site of both surfaces. The results of the present study indicate that Al-doped h-BN exhibits higher catalytic activity for CO oxidation than P-doped one, which may provide a valuable guidance on design metal-free catalysts to remove toxic CO molecules.

Insights into the electronic properties and reactivity of graphene-like BC3 supported metal catalysts

Graphene-like BC₃ monolayer is a new two-dimensional nanomaterial with many unique properties, but is still largely unknown.