Oxidation of a two-dimensional hexagonal boron nitride monolayer: a first-principles study

Two-dimensional (2D) hexagonal boron-nitride oxide (h-BNO) is a structural analogue of graphene oxide. Motivated by recent experimental studies of graphene oxide, we have investigated the chemical oxidation of 2D h-BN sheet and the associated electronic properties of h-BNO. Particular emphasis has been placed on the most favorable site(s) for chemisorption of atomic oxygen, and on the migration barrier for an oxygen atom hopping to the top, bridge, or hollow site on the h-BN surface, as well as the most likely pathway for the dissociation of an oxygen molecule on the h-BN surface. We find that when an oxygen atom migrates on the h-BN surface, it is most likely to be over an N atom, but confined by three neighbor B atoms (forming a triangle ring). In general, chemisorption of an oxygen atom will stretch the B-N bond, and under certain conditions may even break the B-N bond. Depending on the initial location of the first chemisorbed O atom, subsequent oxidation tends to form an O domain or O chain on the h-BN sheet. The latter may lead to a synthetic strategy for the unzipping of the h-BN sheet along a zigzag direction. A better understanding of the oxidation of h-BN sheet has important implications for tailoring the properties of the h-BN sheet for applications.     

Pt(3) and Pt(4) clusters on graphene monolayers supported on a Ni(111) substrate: Relativistic density-functional calculations

Density-functional theory including spin-orbit coupling and corrections for dispersion forces has been used to investigate the structural and magnetic properties of Pt(3) and Pt(4) clusters deposited on a graphene layer supported on a Ni(111) substrate. It is shown that the strong interaction of the Pt atoms with the Ni-supported graphene stabilizes a flat triangular and a slightly bent rhombic structure of the clusters. Pt atoms are located nearly on top of the C atoms of the graphene layer, slightly shifted towards the bridge positions because the Pt-Pt distances are larger than the C-C distances of the graphene sheet lattice-matched to the Ni support. The strong interaction with the substrate leads to a substantial reduction of both the spin and orbital moments of the Pt atoms, not only compared to the clusters in the gas-phase, but also compared to those adsorbed on a freestanding graphene layer. The trends in the magnetic moments and in the magnetic anisotropy of the cluster/substrate complex have been analyzed and it is demonstrated that the anisotropy is dominated by the Ni support.     

Attractive interaction between transition-metal atom impurities and vacancies in graphene: a first-principles study

We present a density functional theory study of transition metal adatoms on a graphene sheet with vacancy-type defects. We calculate the strain fields near the defects and demonstrate that the strain fields around these defects reach far into the unperturbed hexagonal network and that metal atoms have a high affinity to the non-perfect and strained regions of graphene. Metal atoms are therefore attracted by the reconstructed defects. The increased reactivity of the strained graphene makes it possible to attach metal atoms much more firmly than to pristine graphene and supplies a tool for tailoring the electronic structure of graphene. Finally, we analyze the electronic band structure of graphene with defects and show that some defects open a semiconductor gap in graphene, which may be important for carbon-based nanoelectronics.     

Preparation of phosphorus-doped carbon nanospheres and their electrocatalytic performance for O2 reduction

Phosphorus-doped carbon nanospheres without any metal residues were synthesized and characterized.The results revealed that the doping phosphorus atoms could significantly improve the electrocatalytic activity of graphitic carbon for the oxygen-reduction reaction(ORR) both in acidic and alkaline media,and the materials exhibited high electrocatalytic activity,long-term stability,and excellent tolerance to crossover effects especially in alkaline media.Quantum mechanics calculations with the density functional theory demonstrated that the changes in charge density and energetic characteristics of frontier orbitals for the P-doped graphene sheet could facilitate the ORR.     

Direct Observation of Atomic Dynamics and Silicon Doping at a Topological Defect in Graphene

Chemical decoration of defects is an effective way to functionalize graphene and to study mechanisms of their interaction with environment. We monitored dynamic atomic processes during the formation of a rotary Si trimer in monolayer graphene using an aberration‐corrected scanning‐transmission electron microscope. An incoming Si atom competed with and replaced a metastable C dimer next to a pair of Si substitutional atoms at a topological defect in graphene, producing a Si trimer. Other atomic events including removal of single C atoms, incorporation and relocation of a C dimer, reversible C? C bond rotation, and vibration of Si atoms occurred before the final formation of the Si trimer. Theoretical calculations indicate that it requires 2.0 eV to rotate the Si trimer. Our real‐time results provide insight with atomic precision for reaction dynamics during chemical doping at defects in graphene, which have implications for defect nanoengineering of graphene.     

Can Li Atoms Anchored on Boron- and Nitrogen-Doped Graphene Catalyze Dinitrogen Molecules to Ammonia? A DFT Study

The most successful electrochemical conversion of ammonia from dinitrogen molecule reported to date is through a Li mediated mechanism. In the framework of the above fact and that Li anchored graphene is an experimentally feasible system, the present work is a computational experiment to identify the potential of Li anchored graphene as a catalyst for N₂ to NH₃ conversion as a function of (a) minimum number of Li atoms needed for anchoring on graphene sheets and (b) the role of chemical modification of graphene surfaces. The studies bring forth an understanding that Li anchored graphene sheets are potential catalysts for ammonia conversion with preferential adsorption of N₂ through end-on configuration on Li atoms anchored on doped and pristine graphene surfaces. This mode of adsorption being characteristic of Nitrogen Reduction Reaction (NRR) through enzymatic pathway, examination of the same followed by analysis of electronic properties demonstrates that tri-Li atoms (Tri Atom Catalysts, TACs) are more efficient as catalysts for NRR as compared to two Li atoms (Di Atom Catalysts, DACs). Either way, the rate determining step was found to be *NH₂→*NH₃ step (mixed pathway) with ΔG_max=1.02 eV and *NH₂−*NH₃→*NH₂ step (enzymatic pathway) with ΔG_max=1.11 eV for 1B doped TAC and DAC on graphene sheet, respectively. Consequently, this work identifies the viability of Li anchored graphene based 2-D sheets as hetero-atom catalyst for NRR.     

Labeling Graphene Oxygen Groups with Europium

Graphene‐related materials contain chemically bonded oxygen atoms in the form of epoxy, hydroxy, carboxy, and carbonyl groups. It is important to determine the quantity of oxygen atoms and to understand their position on the graphene sheet. However, visualization of these groups by standard methods is a challenge. Here, we utilize europium(III) as a selective label for oxygen‐containing groups. We studied three different graphene‐related materials: 1) graphene oxide, 2) chemically reduced graphene oxide, and 3) thermally reduced graphene oxide (the number of oxygen containing groups decreases from material 1 to 3). We show that it is possible to efficiently use Eu as a label of oxygen‐containing groups. This Eu label could be applied to determine the precise location of oxygen‐containing groups on graphene sheets and also induce novel optical, electrochemical, and catalytic properties.     

Density functional study of hydrogen adsorption and diffusion on Ni‐loaded graphene and graphene oxide

In this work, we performed density functional calculations to investigate the adsorption and diffusion of hydrogen on Ni‐loaded graphene and single layer graphene oxide (SLGO). We evaluated the feasibility of hydrogen spillover in the presence of Ni₄ cluster and the role of oxygen‐containing groups. Our calculations indicate that the hydrogen diffusion is difficult to take place on the Ni/graphene interface due to the stronger Ni? H bond strength. Further, the chemisorbed H atoms are also hard to diffuse freely on the graphene surface. For the SLGO surface, both hydroxyl and epoxide groups may not facilitate the hydrogen diffusion. Instead, they are readily attracted by the nearby Ni catalyst and hydrogenated to water molecules. © 2014 Wiley Periodicals, Inc.     

A first principle study of adsorption of two proximate nitrogen atoms on graphene

Different possible configurations of two nitrogen‐adatoms on graphene are studied using density functional theory. Adsorption of single nitrogen atom on the bridge site of graphene is accompanied by distortion of the sheet. Electronically, this case amounts to p‐type doping. Two N atoms adsorbed on the graphene sheet can share a bond in two ways. They acquire positions either just above two adjacent carbon atoms or they form a bridge across opposite bonds of a hexagon in the sheet. Both these configurations also induce structural distortion of the sheet. Another stable configuration consists of two N atoms bonded as an N₂ molecule physisorbed on the graphene sheet. It is also possible to adsorb two N atoms on opposite sides of the graphene sheet, bonded to the same two C atoms. Moreover, two N atoms can be individually adsorbed on alternate bridge sites of neighboring hexagons experiencing a repulsion, the energy for which arises from the additional distortion of the graphene sheet. The densities of states near the Fermi level are found to be dependent on the adsorption configurations of two nitrogen atoms on graphene. Thus the electronic properties of graphene can be controlled by the selective adsorption of two nitrogen atoms. © 2014 Wiley Periodicals, Inc.     

Monte Carlo molecular simulation of the hydration of Na-montmorillonite at reservoir conditions

The hydration of Na-saturated Wyoming-type montmorillonite is investigated by Monte Carlo simulations at constant stress in the NP(zz)T ensemble and at constant chemical potential in the microVT ensemble, at the sedimentary basin temperature of 353 K and pressure of 625 bar, equivalent to 2-4 km depth. The simulations use procedures established in Chavez-Paez et al. [J. Chem. Phys. 114, 1405 (2001)]. At these conditions, simulations predict a single stable form of 1,2-water layer Na-montmorillonite, containing 164.38 mg/g or 53.37 molecules/layer of adsorbed water and having a spacing of 12.72 A. The corresponding density is 0.32 g/ml. Sodium ions are coordinated with six molecules of water separated 2.30-2.33 A. Water molecules are closer to the central interlayer plane and the spacing is larger than that at 300 K and 1 bar. The interlayer configuration consists of two symmetrical layers of oriented water molecules 1.038 A from the central plane, with the hydrogen atoms in two outermost layers, 3.826 A apart, and the sodium ions on the central plane located between the water layers. The interlayer configuration can be considered to be a stable two-layer intermediate between the one- and two-layer hydrates. Our simulations do not predict formation of other hydrates of Na-montmorillonite at 353 K and 615 bar.     

Regeneration of a Conjugated sp2 Graphene System through Selective Defunctionalization of Epoxides by Using a Proven Synthetic Chemistry Mechanism

Graphene is a promising material capable of driving technological advancement. It is, however, a challenge to obtain pristine graphene in large quantities given the limitation of current synthetic methods. Among the numerous methods available, the chemical approach provides an optimistic outlook and has garnered much interest within the graphene community as a potential alternative. One of the most crucial steps of the chemical approach is the chemical reduction of graphene oxide as this dictates the final quality of the graphene sheets. In recent years, much of the focus has shifted to the usage of established reducing agents or oxygen removal reagents, frequently applied in organic chemistry, onto a graphene oxide platform. Herein, the selective removal of epoxide groups and subsequent regeneration of disrupted conjugated sp² system is highlighted, based on the synergistic effect of indium and indium(I) chloride. The morphological, structural, and electrical properties of the resulting graphene were fully characterized with X‐ray photoelectron, Fourier transform IR, solid‐state ¹³C NMR, and Raman spectroscopy; thermogravimetric analysis; scanning electron microscopy; and conductivity measurements. The as‐prepared graphene showed a tenfold increase in conductivity against conventional graphene treated with hydrazine reducing agent and demonstrated a high dispersion stability in ethanol. Moreover, the selective defunctionalization of the epoxide groups provides opportunities for potential tailoring of graphene properties for prospective applications.     

Unique chemical reactivity of a graphene nanoribbon's zigzag edge

The zigzag edge of a graphene nanoribbon possesses a unique electronic state that is near the Fermi level and localized at the edge carbon atoms. The authors investigate the chemical reactivity of these zigzag edge sites by examining their reaction energetics with common radicals from first principles. A "partial radical" concept for the edge carbon atoms is introduced to characterize their chemical reactivity, and the validity of this concept is verified by comparing the dissociation energies of edge-radical bonds with similar bonds in molecules. In addition, the uniqueness of the zigzag-edged graphene nanoribbon is further demonstrated by comparing it with other forms of sp2 carbons, including a graphene sheet, nanotubes, and an armchair-edged graphene nanoribbon.     

Computational studies for reduced graphene oxide in hydrogen-rich environment

We employ molecular dynamic simulations to study the reduction process of graphene oxide (GO) in a chemically active environment enriched with hydrogen. We examine the concentration and pressure of hydrogen gas as a function of temperature in which abstraction of oxygen is possible with minimum damage to C-sp(2) bonds, hence preserving the integrity of the graphene sheet. Through these studies we find chemical pathways that demonstrate beneficiary mechanisms for the quality of graphene including formation of water as well as suppression of carbonyl pair holes in favor of hydroxyl and epoxide formation facilitated by hydrogen gas in the environment.     

The Kernel energy method: Application to graphene and extended aromatics

The quantum chemistry of finite aperiodic graphene flakes is a matter of considerable interest because of the anticipated technological importance of such objects. Since real aperiodic graphene flakes will in general be composed of many thousands of carbon atoms, theoretical methods appropriate to such large molecules would need to be used for the ab initio quantum calculation of their properties. The Kernel energy method is discussed here, and it is shown to be accurately applicable to graphenes and analogous extended aromatic molecules. It is necessary to define the kernels of a graphene molecule in a new way because of the extensive aromaticity, which defines its electronic structure. The kernels used in the reconstruction of the full graphene sheet preserve the total number of π‐electrons, Clar sextets, and the approximate overall aromaticity. Sivaramakrishnan et al. [J Phys Chem A, 2005, 109, 1621] define similar “ring conserved isodesmic reactions (RCIR).” The principal innovation of this article is the suggestion that kernels may be mathematically extracted from an extended aromatic molecule such as graphene by a fissioning of aromatic bonds. Hartree Fock (HF) and Møller‐Plesset (MP2) chemical models using a Gaussian basis of 3‐21G orbitals are used to calculate the total energy of a graphene flake. This demonstration calculation is performed on a graphene flake in which dangling bonds are saturated with hydrogens (C₇₈H₂₆) composed of a total of 104 atoms arranged in 27 benzenoid rings. The KEM with both types of chemical model are shown to be accurate to nearly 1 kcal/mol, of a total energy, which is nearly 3000 atomic units, that is, with an absolute error within “chemical accuracy” and a relative error of the order of 5 × 10⁵% of the total energy. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Graphene substrate-mediated catalytic performance enhancement of Ru nanoparticles: a first-principles study

The structural, energetic and magnetic properties of Ru nanoparticles deposited on pristine and defective graphene have been thoroughly studied by first-principles based calculations. The calculated binding energy of a Ru(13) nanoparticle on a single vacancy graphene is as high as -7.41 eV, owing to the hybridization between the dsp states of the Ru particles with the sp(2) dangling bonds at the defect sites. Doping the defective graphene with boron would further increase the binding energy to -7.52 eV. The strong interaction results in the averaged d-band center of the deposited Ru nanoparticle being upshifted toward the Fermi level from -1.41 eV to -1.10 eV. Further study reveals that the performance of the nanocomposites against hydrogen, oxygen and carbon monoxide adsorption is correlated to the shift of the d-band center of the nanoparticle. Thus, Ru nanoparticles deposited on defective graphene are expected to exhibit both high stability against sintering and superior catalytic performance in hydrogenation, oxygen reduction reaction and hydrogen evolution reaction.     

Atomic Layer Deposition of Hafnium(IV) Oxide on Graphene Oxide: Probing Interfacial Chemistry and Nucleation by using X‐ray Absorption and Photoelectron Spectroscopies

Interfacing graphene with metal oxides is of considerable technological importance for modulating carrier density through electrostatic gating as well as for the design of earth‐abundant electrocatalysts. Herein, we probe the early stages of the atomic layer deposition (ALD) of HfO₂ on graphene oxide using a combination of C and O K‐edge near‐edge X‐ray absorption fine structure spectroscopies and X‐ray photoelectron spectroscopy. Dosing with water is observed to promote defunctionalization of graphene oxide as a result of the reaction between water and hydroxyl/epoxide species, which yields carbonyl groups that further react with migratory epoxide species to release CO₂. The carboxylates formed by the reaction of carbonyl and epoxide species facilitate binding of Hf precursors to graphene oxide surfaces. The ALD process is accompanied by recovery of the π‐conjugated framework of graphene. The delineation of binding modes provides a means to rationally assemble 2D heterostructures.     

Pt on graphene monolayers supported on a Ni(111) substrate: relativistic density-functional calculations

The structural, energetic, and magnetic properties of Pt atoms and dimers adsorbed on a Ni-supported graphene layer have been investigated using density-functional calculations, including the influence of dispersion forces and of spin-orbit coupling. Dispersion forces are found to be essential to stabilize a chemisorbed graphene layer on the Ni(111) surface. The presence of the Ni-substrate leads not only to a stronger interaction of Pt atoms and dimers with graphene but also to a locally increased binding between graphene and the substrate and a complex reconstruction of the adlayer. The stronger binding of the dimer also stabilizes a flat adsorption geometry in contrast to the upright geometry on a free-standing graphene layer. These effects are further enhanced by dispersion corrections. Isolated Pt adatoms and flat dimers are found to be non-magnetic, while an upright Pt dimer has strongly anisotropic spin and orbital moments. For the clean C/Ni(111) system, we calculate an in-plane magnetic anisotropy, which is also conserved in the presence of isolated Pt adatoms. Surprisingly, upright Pt-dimers induce a re-orientation of the easy magnetic axis to a direction perpendicular to the surface, in analogy to Pt(2) on a free-standing graphene layer and to the axial anisotropy of a gas-phase Pt(2) dimer.     

Fluorination of Isotopically Labeled Turbostratic and Bernal Stacked Bilayer Graphene

Fluorination of graphene opens up a bandgap, which creates opportunities for optoelectronics, and also paves the way for the creation of extremely thin insulating layers, which can be important for applications in devices. However, in spite of many interesting features offered by, for example, unequally doped layers in multilayered systems, most of the work has concerned the fluorination of graphene monolayers. Here, the fluorination process of graphene bilayers is investigated through high‐resolution Raman mapping followed by analysis of more than 10 000 spectra of bilayer graphene. Isotopically labeled bilayers are used, allowing each individual layer in bilayer graphene to be addressed unambiguously. The fluorinated graphene is prepared through exposure to XeF₂. Monolayer graphene is found to be significantly more sensitive to fluorination than bilayer graphene. Through comparison of the D/G area ratio and the position of the G band for turbostratic and Bernal stacked (AB) bilayers, it is found that the fluorination process is more effective for turbostratic than for AB‐stacked bilayer graphene. The fluorination changes the electronic structure similarly for the top and bottom layers in turbostratic bilayers. However, the top layer is more sensitive than the bottom layer in AB‐stacked bilayers.     

Density-functional calculations of HCN adsorption on the pristine and Si-doped graphynes

Graphyne, a lattice of benzene rings connected by acetylene bonds, is one-atom-thick planar sheet of sp- and sp²-bonded carbons differing from the hybridization of graphene (considered as pure sp²). Here, HCN adsorption on the pristine and Si-doped graphynes was studied using density-functional calculations in terms of geometric, energetic, and electronic properties. It was found that HCN molecule is weakly adsorbed on the pristine graphyne and slightly affects its electronic properties. While, Si-doped graphyne shows high reactivity toward HCN, and, in the most favorable state, the calculated adsorption energy is about ?10.1 kcal/mol. The graphyne, in which sp-carbon was substituted by Si atom, is more favorable for HCN adsorption in comparison with sp²-carbon. It was shown that the electronic properties of Si-doped graphyne are strongly sensitive to the presence of HCN molecule and therefore it may be used in sensor devices.