铜族金属与完整及氮掺杂石墨烯的相互作用

基于广义梯度密度泛函理论和周期平板模型,研究了铜族金属单原子和双原子簇与完整及氮掺杂石墨烯的结合情况.结果表明,氮掺杂后石墨烯的电子结构特性由半金属性变为金属性;铜族金属在完整及石墨型氮掺杂石墨烯上的吸附较弱,结合能约为0.5eV,而在吡啶型氮掺杂和吡咯型氮掺杂石墨烯上有较强的化学吸附,结合能一般大于1eV;吡咯型氮掺杂后的构型不稳定,金属原子及簇与包含该结构的石墨烯衬底作用时会使其向吡啶型氮掺杂转变,并最终得到基于吡啶型氮掺杂的稳定吸附构型.Mulliken电荷布居分析显示,吸附在吡啶型氮掺杂石墨烯上的金属单原子与金属双原子簇带电性质相反.态密度及轨道分析表明,Cu与吡啶型氮掺杂石墨烯空位处留有悬挂键的三个原子成键,而Au与其中两个C原子成键.     

Atomic-scale deformation in N-doped carbon nanotubes

We present the N-doping induced atomic-scale structural deformation in N-doped carbon nanotubes by using density functional theory calculations. For substitutional N-doped nanotube clusters, the N dopant with an excess electron lone pair exhibits the high negative charge, and the homogeneously distributed dopants enlarge the tube diameter in both zigzag and armchair cases. On the other hand, in pyridine-like N-doped ones, the concentrated N atoms result in a positively curved graphene layer and, thus, can be responsible for tube wall roughness and the formation of interlinked structures.     

Study of the coupling reaction of pyridine in the gas phase

The coupling reaction of pyridine in the gas phase to form bipyridyl and terpyridyl has been studied by electron ionization using an ion trap mass spectrometer. In contrast to the difficulty in carrying out electrophilic substitutions at carbon atoms in the pyridine ring under highly acidic solvent conditions, reactions in the gas phase overcame the conjugate acidification of pyridine in the solvent phase, thus decreasing the hardness of this electrophilic coupling. Through product ion mass spectra of the ion at m/z 157, we have shown that this ion was protonated bipyridyl rather than the ion/molecule adduct. A computational study of the heat of formation surface also supported the formation of polypyridyls through the electrophilic substitution of pyridine. We have confirmed the reaction through a study of pyridine-d(5) coupling in the gas phase.     

Low temperature growth of highly nitrogen-doped single crystal graphene arrays by chemical vapor deposition

The ability to dope graphene is highly important for modulating electrical properties of graphene. However, the current route for the synthesis of N-doped graphene by chemical vapor deposition (CVD) method mainly involves high growth temperature using ammonia gas or solid reagent melamine as nitrogen sources, leading to graphene with low doping level, polycrystalline nature, high defect density and low carrier mobility. Here, we demonstrate a self-assembly approach that allows the synthesis of single-layer, single crystal and highly nitrogen-doped graphene domain arrays by self-organization of pyridine molecules on Cu surface at temperature as low as 300 °C. These N-doped graphene domains have a dominated geometric structure of tetragonal-shape, reflecting the single crystal nature confirmed by electron-diffraction measurements. The electrical measurements of these graphene domains showed their high carrier mobility, high doping level, and reliable N-doped behavior in both air and vacuum.     

The electric field as a novel switch for uptake/release of hydrogen for storage in nitrogen doped graphene

Nitrogen-doped graphene was recently synthesized and was reported to be a catalyst for hydrogen dissociative adsorption under a perpendicular applied electric field (F). In this work, the diffusion of H atoms on N-doped graphene, in the presence and absence of an applied perpendicular electric field, is studied using density functional theory. We demonstrate that the applied field can significantly facilitate the binding of hydrogen molecules on N-doped graphene through dissociative adsorption and diffusion on the surface. By removing the applied field the absorbed H atoms can be released efficiently. Our theoretical calculation indicates that N-doped graphene is a promising hydrogen storage material with reversible hydrogen adsorption/desorption where the applied electric field can act as a switch for the uptake/release processes.     

Facilely synthesized N-doped graphene sheets and its ferromagnetic origin

Inducing ferromagnetism into graphene is vital today because it has a wide range of applications such as spintronics devices and magnetic memory devices. In this paper, we will report a new method to synthesize ferromagnetic graphene by nitrogen doping. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy were utilized to testify the N-doped material and further discuss the N-doped process. The superconducting quantum interference device (SQUID) was put in and used to analyze the magnetic properties of the N-doped graphene sheets. It shows that the material exhibits ferromagnetism at both 3 K and 300 K and the ferromagnetic saturation moment is 0.412 emu/g and 0.051 emu/g, respectively. The mechanism of the origin of the ferromagnetism in N-doped graphene sheets will also be discussed in this paper. It shows that, when the amount graphitic N reached the threshold, the origin of the ferromagnetism will change from defects induced by nitrogen atoms to the transition in energy band caused by graphitic N.     

Nitrogen-doped multiwalled carbon nanotubes and their electrocatalysis towards oxidation of NO

Nitrogen-doped multiwalled carbon nanotubes (N-MWCNTs) were synthesized by thermal decomposition of pyridine and iron phthalocyanine over an iron catalyst in an atmosphere of ammonia. The N-MWCNTs thus obtained were analyzed by X-ray photoelectron spectroscopy. They were found to contain three types of nitrogen (N) atoms, namely pyridine-like, graphite-like, and molecular N. The effect of the pyridine-like N and the graphite-like N was investigated. The pyridine-like N is absorbing nitric oxide (NO) more easily than the graphite-like N. The N-MWCNTs with higher N content (especially the pyridine-like N) have higher catalytic activity (in terms of electrooxidation of NO) than those containing less N. The N-MWCNTs with high levels of pyridine N were incorporated into an electrode which suitable for sensing NO and for removal of NO due to its excellent electrocatalytic activity.     

Distribution and structure of N atoms in multiwalled carbon nanotubes using variable-energy X-ray photoelectron spectroscopy

We investigated the inhomogeneous distribution of concentration and electronic structure of the nitrogen (N) atoms doped in the multiwalled carbon nanotubes (CNTs) by variable-energy X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge structure, and electron energy-loss spectroscopy. The vertically aligned N-doped CNTs on the substrates were grown via pyrolysis of iron phthalocyanine (FePc), cobalt phthalocyanine (CoPc), and nickel phthalocyanine (NiPc) in the temperature range 750-1000 degrees C. They usually have a bamboo-like structure, and the diameter is in the range of 15-80 nm. As the photon energy of XPS increases from 475 to 1265 eV, the N content increases up to 8 atomic %, indicating a higher N concentration at the inside of nanotubes. We identified three typed N structures: graphite-like, pyridine-like, and molecular N(2). The pyridine-like N structure becomes significant at the inner walls. Molecular N(2) would exist as intercalated forms in the vicinity of hollow inside. The XPS valence band analysis reveals that the pyridine-like N structure induces the metallic behaviors. The CNTs grown using NiPc contain the higher content of pyridine-like structure compared to those grown using FePc and CoPc, so they exhibit more metallic properties.     

Binding characteristics of pyridine on Ag(110)

A combination of low-temperature scanning tunneling microscopy and density functional theory calculations was used to determine the binding characteristics of single pyridine molecules at a low coverage on a silver surface. The results indicated that pyridine binds to silver through the nitrogen atom in either a perpendicular or a parallel configuration with the latter structure being more prevalent. Both configurations are produced predominantly through electrostatic interaction between nitrogen and silver atoms. This is induced by charge redistribution in the pyridine molecule and nearby silver atoms upon pyridine adsorption.     

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.     

Proton Transfer Influence on Geometry and Electron Density in Benzoic Acid–Pyridine Complexes

The influence of the proton transfer on the geometry of donor and acceptor molecule in benzoic acid–pyridine complexes is investigated by theoretical calculations at the B3LYP/6‐311++G** level of theory. Systematic shifts of the H‐atom in the H‐bond are reflected in the geometry of the COOH group and the lengths of aromatic ring bond lengths of the proton acceptor. Changes in electron densities have been studied by atoms in molecules analysis. A systematic natural bond orbital analysis has been performed to study the proton transfer mechanism. Two donor orbitals are engaged in the proton transfer process which is accompanied by a change in orbital delocalization of H‐atom that can switch between two donor orbitals so the path of proton transfer in intermolecular H‐bond is not determined by the orbital shape. Theoretical results have been confirmed by experimental results published previously.     

Studies of adsorption of 2,2′-bipyridyl on the surface of highly regulated silica matrix of the MCM-41 type by means of <Superscript>15</Superscript>N NMR spectroscopy

The study of 2,2′-bipyridyl adsorption on the surface of highly regular MCM-41 silica at 300 and 130 K was carried out by the ¹⁵N NMR spectroscopy. It was shown that at 300 K the adsorbed molecules were involved in the processes of isotropic reorientation accompanied by the formation and rupture of hydrogen bonds with the surface-located hydroxy groups. Each molecule of 2,2′-bipyridyl forms no more than one hydrogen bond at a time, and their surface density is about one molecule per 1 nm² of the surface. At 130 K 2,2′-bipyridyl forms a monolayer on the surface of silica including about 1.6 molecule per 1 nm². In this monolayer each molecule forms a hydrogen bond with one hydroxy group and prevents the interaction of the other bipyridyl molecules with one more hydroxy group.     

Theoretical analysis of pyridine protonation in water clusters of increasing size.

The protonation of pyridine in water clusters as a function of the number of water molecules was theoretically analyzed as a prototypical case for the protonation of organic bases. We determined the variation of structural, bonding, and energetic properties on protonation, as well as the stabilization of the ionic species formed. Thus, we used supermolecular models in which pyridine interacts with clusters of up to five water molecules. For each complex, we determined the most stable unprotonated and protonated structures from a simulated annealing at the semi ab initio level. The structures were optimized at the B3LYP/cc-pVDZ level. We found that the hydroxyl group formed on protonation of pyridine abstracts a proton from the ortho-carbon atom of the pyridine ring. The "atoms in molecules" theory showed that this C-H group loses its covalent character. However, starting with clusters of four water molecules, the C-H bond recovers its covalent nature. This effect is associated with the presence of more than one ring between the water molecules and pyridine. These rings stabilize, by delocalization, the negative charge on the hydroxyl oxygen atom. Considering the protonation energy, we find that the protonated forms are increasingly stabilized with increasing size of the water cluster. When zero-point energy is included, the variation follows closely an exponential decrease with increasing number of water molecules. Analysis of the vibrational modes for the strongest bands in the IR spectra of the complexes suggests that the protonation of pyridine occurs by concerted proton transfers among the different water rings in the structure. Symmetric water stretching was found to be responsible for hydrogen transfer from the water molecule to the pyridine nitrogen atom.     

On the possibility of the existence of molecular nitrogen allotropes

The possibility of the existence of nitrogen molecules with an even number of atoms of composition N₄, N₆, N₈, and N₁₀ has been discussed with the use of the QCISD and G3 quantum-chemical methods. From these data, a conclusion has been made that three new nitrogen allotropes with an even number of atoms in a molecule can exist, namely, N₄ shaped as a rectangle and regular tetrahedron and N₆ with a shape remotely resembling an “open book.” The bond lengths and bond and torsion angles in each of these molecules have been reported.     

DFT Study on the Effect of Hydrogen-bond Formation on the Adsorption of Aminotriazines on Graphene

DFT calculations have been performed to explore the aminotriazine adsorption on graphene surfaces.Relative energies,equilibrium geometries and electronic structures of monomer and dimer of aminotriazine molecules adsorbed at the surface were investigated and analyzed in details.It was found that the hydrogen atoms in the NH2 group of aminotriazine molecules are directed toward the graphene surface,and the adsorption energy increases as the NH2 group is added.The adsorbed aminotriazine molecules facilely form a dimer through the hydrogen bonding interactions,and the two aromatic rings of optimized structure of 2-amino-1,3,5-triazine(B) dimmer(denoted by B2) and melamine(D) dimmer(denoted by D2) are parallel to the graphene sheet.The large deviation of the averaged adsorption energy of B2 and D2 compared to monor adsorption may reflect the increase of π-π repulsion and the effect of hydrogen bond formation.The electronic structure analyses reveal that the formation of hydrogen bonds in melamine dimer has great influence on the adsorption mode at the graphene surface.     

Electrochemically exfoliating graphite into N-doped graphene and its use as a high efficient electrocatalyst for oxygen reduction reaction

N-doped graphene has been extensively explored because of their intriguing properties. However, most of the conventional heat-processed N-doped graphene (HNG) suffer from the poor hydrophilic property and low electric conductivity when using electrode materials. Herein, we present a facile solution-processed strategy to fabricate N-doped graphene through electrochemical exfoliation of graphite in inorganic electrolyte solution. The resulting electrochemically exfoliated N-doped graphene (ENG) has high level of nitrogen (7.9 at.%) and oxygen (16.5 at.%), moreover, excellent electric conductivity (19 s cm^?1). As a binder-free electrode material for oxygen reduction reaction (ORR), ENG exhibits much better electroactivity than HNG and electrochemically exfoliated graphene (EG), moreover, much better methanol tolerance and long-term durability than that commercial Pt/C catalyst. The results provide new sights into scalable production of noble metal-free catalyst towards ORR.     

Density functional study of super cell N-doped (10,0) zigzag single-walled carbon nanotubes as CO sensor

N-doped SWCNT with different concentration of doped nitrogen atoms were investigated through density functional theory (DFT) calculations for detecting CO molecule. The CO molecule was adsorbed to different sites of the modified nanotubes and their geometric structures and electronic properties were investigated after full optimization. A significant change can be observed in adsorption energies and electronic properties of N-doped SWCNT after CO adsorption. By increasing the number of nitrogen atoms in each unit cell, these properties change more obviously. So these modified nanotubes can be used as CO sensors.     

Well-dispersed high-loading pt nanoparticles supported by shell-core nanostructured carbon for methanol electrooxidation

Shell-core nanostructured carbon materials with a nitrogen-doped graphitic layer as a shell and pristine carbon black particle as a core were synthesized by carbonizing the hybrid materials containing in situ polymerized aniline onto carbon black. In an N-doped carbon layer, the nitrogen atoms substitute carbon atoms at the edge and interior of the graphene structure to form pyridinic N and quaternary N structures, respectively. As a result, the carbon structure becomes more compact, showing curvatures and disorder in the graphene stacking. In comparison with nondoped carbon, the N-doped one was proved to be a suitable supporting material to synthesize high-loading Pt catalysts (up to 60 wt %) with a more uniform size distribution and stronger metal-support interactions due to its high electrochemically accessible surface area, richness of disorder and defects, and high electron density. Moreover, the more rapid charge-transfer rates over the N-doped carbon material are evidenced by the high crystallinity of the graphitic shell layer with nitrogen doping as well as the low charge-transfer resistance at the electrolyte/electrode interface. Beneficial roles of nitrogen doping can be found to enhance the CO tolerance of Pt catalysts. Accordingly, an improved performance in methanol oxidation was achieved on a high-loading Pt catalyst supported by N-doped carbon. The enhanced catalytic properties were extensively discussed based on mass activity (Pt utilization) and intrinsic activity (charge-transfer rate). Therefore, N-doped carbon layers present many advantages over nondoped ones and would emerge as an interesting supporting carbon material for fuel cell electrocatalysts.     

Theoretical study of nitrogen‐doped graphene nanoflakes: Stability and spectroscopy depending on dopant types and flake sizes

As nitrogen‐doped graphene has been widely applied in optoelectronic devices and catalytic reactions, in this work we have investigated where the nitrogen atoms tend to reside in the material and how they affect the electron density and spectroscopic properties from a theoretical point of view. DFT calculations on N‐doped hexagonal and rectangular graphene nanoflakes (GNFs) showed that nitrogen atoms locating on zigzag edges are obviously more stable than those on armchair edges or inside flakes, and interestingly, the N‐hydrogenated pyridine moiety could be preferable to pure pyridine moiety in large models. The UV–vis absorption spectra of these nitrogen‐doped GNFs display strong dependence on flake sizes, where the larger flakes have their major peaks in lower energy ranges. Moreover, the spectra exhibit different connections to various dopant types and positions: the graphitic‐type dopant species present large variety in absorption profiles, while the pyridinic‐type ones show extraordinary uniform stability and spectra independent of dopant positions/numbers and hence are hardly distinguishable from each other. © 2018 Wiley Periodicals, Inc.