CH基团与金刚石(111)面的碰撞反应及其对碳膜生长的影响

等离子体增强化学气相沉积技术中的碳膜选择性自组装机理是高性能碳膜制备过程中的挑战性基础课题.采用经典分子动力学方法,模拟了不同能量(1.625-65 eV)的CH基团在清洁金刚石和吸氢金刚石(111)面上的轰击行为,获得了吸附、反弹、反应等各类事件的发生概率,并据此探讨了含氢碳膜制备过程中CH基团的贡献.结果表明,随着入射能量的增加,CH基团对薄膜生长的贡献由单纯的吸附、反弹机理向反应、吸附混合机理转变,其中最主要的反应过程是释放一个或两个氢原子的反应,而释放氢分子的反应则很少发生.这些反应不仅使薄膜生长过程更均匀、薄膜表面更平整,还降低了薄膜的氢含量.生长机理的转变导致低能量条件下所成薄膜中的多数碳原子都包含一个氢原子作为配位原子,而高能量条件下的薄膜中的碳原子则很少有氢原子作为配位原子.另外,通过分析sp~3-C和sp~2-C数目的变化,研究了CH基团对金刚石基底的破坏作用.     

无序碳单层的电子结构及氢原子吸附的第一性原理

本研究运用第一性原理计算方法,系统地研究了无序碳单层材料不同位点的电子结构及其析氢性能.计算结果显示无序结构中的C-C键相比于石墨烯中的C-C键在26.7%的范围内有不同程度的拉伸或压缩,使得C原子电荷在-0.17～+0.16个电子范围内变化,导致部分C原子电子局域化.电子的局域化增强了C原子的化学活性,从而表现出了较强的吸附性能.我们发现H原子与C原子的键合及析氢性能与C原子间的键角相关.对于三配位的碳原子,其中三个价电子通过sp~2杂化轨道与最邻近的碳原子结合形成较强的共价键,而余下的一个p_z轨道电子可以与H原子在垂直于原子层的方向形成较弱的化学键.无序结构可以打破三个sp~2杂化轨道的对称性,进而影响p_z轨道与氢的成键.本研究在一定程度上揭示了单层无序碳材料结构-性能的构效关系,为实验上设计特定性能的无序碳功能材料提供理论指导.     

Sites for catalysis and electrochemistry in solid oxide fuel cell (SOFC) anode

Fuel cells represent a challenging overlap of catalysis and electrochemistry. This is illustrated by anode reactions in a solid oxide fuel cell. The sites for catalytic conversion of methane and electrochemical conversion of hydrogen on an SOFC anode appear not to be the same. The fuel (methane, hydrogen, etc.) is activated by chemisorption on the nickel surface of the anode. This is linked to the electrochemical reaction at the interface of the electrolyte and the nickel crystals converting oxygen ions into electrons and water by reactions with adsorbed hydrogen atoms resulting from the activation of the fuel. The sites for these reactions appear not to be the same. This is reflected by different sensitivities of the two steps to sulphur poisoning. The role of different sites on the nickel surface for the steam reforming reaction is well understood in terms of impact on activity for methane activation, carbon formation and sintering. The study is supplemented by an analysis of anodes having been exposed to 13000 of operation using a number of characterisation methods. PACS 82.47.Ed; 82.45.-h; 82.65.-s     

A model of the multiple adsorption of hydrogen atoms on the surface of carbon nanotubes

A model of the multiple adsorption of atomic hydrogen on the surface of single-walled carbon nanotubes of the zigzag and arm-chair types was constructed. The adsorption model is based on the Anderson periodic model. An analytic equation for the band structure of carbon nanotubes with adsorbed hydrogen atoms was obtained, and the special features of this structure were studied. The dependence of the band structure of carbon nanotubes on the concentration of adsorbed hydrogen atoms was analyzed. The model constructed can be used to study adsorption of other univalent atoms on the surface of carbon particles.     

Behavior of pionic hydrogen atoms in liquid organic compounds

Negative pion transfer from pionic hydrogen to other heavier atoms was studied in some liquid organic compounds. The pion capture probability on hydrogen at a particular site in the molecules was obtained using deuterated compounds for alcohols and carboxylic acids. The transfer process to carbon atoms in different chemical states was also studied in the mixtures of C₆ H₁₂ or C₆ H₆+ CCl₄. These methods revealed that the pionic hydrogen atoms originating from hydrogen in different chemical environments showed different behavior. The transfer process was discussed using a large mesomolecular model combined with an external transfer process. This revised version was published online in August 2006 with corrections to the Cover Date.     

Unambiguous Assignment of the 13C NMR Resonances of the Side-Chain Carbon Atoms of Dipropylglyoxal Bis(Amidinohydrazone) by DEPT and Selective Heteronuclear Proton Decoupling Techniques

The previously unassigned carbon-13 NMR resonances of the side-chain carbon atoms of the enzyme inhibitor dipropylglyoxal bis(amidinohydrazone) (DPGBG) have been unambiguously assigned with the aid of DEPT measurements and experiments involving the selective decoupling of the protons of one of the methylene groups. The chemical shifts of the side-chain carbon atoms of DPGBG decrease in a nearly linear fashion as a function of the position of the atom in the side chain, the terminal methyl groups having the lowest shift value. The carbon-13 shifts are positively correlated with the chemical shifts of the corresponding hydrogen atoms.     

Accumulation of hydrogen by silicon powders in HF inductive discharge plasma

The possibility of saturation with hydrogen of micro- and nanostructured silicon powders in electrodeless plasma is demonstrated. It is established that the hydrogen concentration in the bulk of the powder is determined by the size of its particles, the hydrogen pressure in the reaction chamber, and the HF power. The mechanism of plasma saturation with hydrogen is based on the chemical interaction of hydrogen atoms and active radicals with the surface of particles of the silicon powder, which is a surface with a distorted order of stacking of silicon atoms.     

Encapsulation of methane molecules into carbon nanotubes

Methane gas (CH₄) is a chemical compound comprising a carbon atom surrounded by four hydrogen atoms, and carbon nanotubes have been proposed as possible molecular containers for the storage of such gases. In this paper, we investigate the interaction energy between a CH₄ molecule and a carbon nanotube using two different models for the CH₄ molecule, the first discrete and the second continuous. In the first model, we consider the total interaction as the sum of the individual interactions between each atom of the molecule and the nanotube. We first determine the interaction energy by assuming that the carbon atom and one of the hydrogen atoms lie on the axis of the tube with the other three hydrogen atoms offset from the axis. Symmetry is assumed with regard to the arrangement of the three hydrogen atoms surrounding the carbon atom on the axis. We then rotate the atomic position into 100 discrete orientations and determine the average interaction energy from all orientations. In the second model, we approximate the CH₄ molecule by assuming that the four hydrogen atoms are smeared over a spherical surface of a certain radius with the carbon atom located at the center of the sphere. The total interaction energy between the CH₄ molecule and the carbon nanotube for this model is calculated as the sum of the individual interaction energies between both the carbon atom and the spherical surface and the carbon nanotube. These models are analyzed to determine the dimensions of the particular nanotubes which will readily suck-up CH₄ molecules. Our results determine the minimum and maximum interaction energies required for CH₄ encapsulation in different tube sizes, and establish the second model of the CH₄ molecule as a simple and elegant model which might be exploited for other problems.     

Hydrogen incorporation in diamond: the vacancy-hydrogen complex

We report the identification of the vacancy-hydrogen complex in single crystal diamond synthesized by chemical vapor deposition. The S=1 defect is observed by electron paramagnetic resonance in the negative charge state. The hydrogen atom is bonded to one of the carbon atoms neighboring the vacancy. Unlike the analogous defect in silicon, no symmetry lowering reconstruction occurs between the three remaining carbon dangling orbitals. The very small measured hydrogen hyperfine interaction is explained by dipolar coupling between the hydrogen and the unpaired electron probability density delocalized on the three equivalent carbon neighbors.     

Effect of hydrogen bond formation on the NMR properties of microhydrated ortho-aminobenzoic acid

The in?uence of the hydrogen bond formation on the nuclear magnetic resonance parameters has been investigated in the case of microhydrated ortho-aminobenzoic acid (o-Abz) in the gas-phase. DFT-B3LYP/aug-cc-pVDZ predicted ¹H and ¹³C isotropic chemical shifts with respect to TMS of the isolated o-Abz are in reasonable agreement with available experimental data. The isotropic and anisotropic chemical shifts for all atoms of o-Abz within the o-Abz?···?(H₂O)_1-3 complexes have been calculated at the Hartree–Fock, and density functional (B3LYP) theoretical levels using the 6-31++G(2d,2p) and aug-cc-pVDZ basis sets and considering the counterpoise corrections for the basis set superposition errors. The chemical shift values of the carboxyl group atoms of microhydrated o-Abz relative to isolated o-abz do not show significant basis set dependence. Both the hydrogen and carbon atoms constituting the carboxyl group of o-Abz suffer downfield shift due to formation of hydrogen bond with water. The length of hydrogen bond formed between o-Abz and water is found to vary with the number of water molecules present around o-Abz. A direct correlation between the hydrogen bond length and isotropic chemical shift of the bridging hydrogen is observed for both C?=?O?···?H-O and O-H?···?O interactions.     

Ti-doped nano-porous graphene: A material for hydrogen storage and sensor

Clustering of Ti on carbon nanostructures has proved to be an obstacle in their use as hydrogen storagematerials. Using density functional theory we show that Ti atoms will not cluster at moderate concentrations when doped into nanoporous graphene. Since each Ti atom can bind up to three hydrogen molecules with an average binding energy of 0.54 eV/H₂, this material can be ideal for storing hydrogen under ambient thermodynamic conditions. In addition, nanoporous graphene is magnetic with or without Ti doping, but when it is fully saturated with hydrogen, the magnetism disappears. This novel feature suggests that nanoporous graphene cannot only be used for storing hydrogen, but also as a hydrogen sensor.     

相对化学位移值：获得CH基团中氢原子精确化学位移值的一种简单方法

为了解决CH基团中氢原子的精确化学位移值的问题,引入相对化学位移值概念．内标法和外标法用于测量全浓度范围的N-甲基乙酰胺水溶液．对于同一个分子来说,选择分子内某个基团的氢原子化 学位移作为一个标准值,得到的其它基团氢原子相对化学位移值随温度和浓度的变化是和测试方法无关的．     

功能化单壁碳纳米管的热导率

采用基于BrennerⅡ势的非平衡态分子动力学方法，模拟研究了300K温度下经氢化学修饰的(10,0)单壁碳纳米管的热导率.研究显示功能化后碳管的热导率有明显减小，当有一列碳原子被氢化后（功能化程度为5%），碳管的热导率减小了大约1/3，为了进一步解释这种功能化对碳纳米管热导率的影响，计算了不同功能化程度下碳纳米管的声子谱.     

功能化单壁碳纳米管的热导率

采用基于BrennerⅡ势的非平衡态分子动力学方法,模拟研究了300K温度下经氢化学修饰的(10,0)单壁碳纳米管的热导率.研究显示功能化后碳管的热导率有明显减小,当有一列碳原子被氢化后（功能化程度为5%）,碳管的热导率减小了大约1/3,为了进一步解释这种功能化对碳纳米管热导率的影响,计算了不同功能化程度下碳纳米管的声子谱.     

Influence of Mechanical Stretching on Adsorption Properties of Nitrogen-Doped Graphene

This paper presents the results of quantum chemical modeling of chemisorption of atomic hydrogen and epoxy, carboxyl, and hydroxyl functional groups on nitrogen-doped graphene. It is shown that the substitutional nitrogen atom does not bind to adsorbing groups directly, but significantly increases the adsorption activity of neighboring carbon atoms. Mechanical stretching of doped graphene reduces the adsorption energy of all the aforementioned radicals. This reduction is significantly greater for the epoxy group than for the other functional groups. The results obtained confirm that, upon a sufficient stretching of a nitrogen-doped graphene sheet, the dissociation of molecular hydrogen and oxygen with subsequent precipitation of the resulting radicals onto graphene can be energetically favorable.

     

Hydrogenation of single-walled carbon nanotubes

Towards the development of a useful mechanism for hydrogen storage, we have studied the hydrogenation of single-walled carbon nanotubes with atomic hydrogen using core-level photoelectron spectroscopy and x-ray absorption spectroscopy. We find that atomic hydrogen creates C-H bonds with the carbon atoms in the nanotube walls, and such C-H bonds can be completely broken by heating to 600 degrees C. We demonstrate approximately 65 +/- 15 at % hydrogenation of carbon atoms in the single-walled carbon nanotubes, which is equivalent to 5.1 +/- 1.2 wt % hydrogen capacity. We also show that the hydrogenation is a reversible process.     

The influence of hydrogen impurities on the atomic and electronic structures of carbyne crystals

An ab initio DFT study of atomic and electronic structure of carbyne crystals was carried out. The influence of hydrogen impurities on carbyne structure was investigated. Calculations with atomic relaxations showed that carbon chains in the carbyne crystal structure are bow-like curved; free-energy calculations showed that the most probable lengths of those chains are four and six atoms, which is in a good agreement with experiments. Carbyne-crystal electronic-structure analysis showed that there is a small gap of 0.09 eV near the Fermi level in four-atomic carbyne, while there is no such gap in six-atomic carbyne. In studying of the hydrogen impurity influence on the atomic and electronic structure of carbyne crystals, hydrogen atoms were embedded in two directions: across and along carbon chains in the crystal. As a result we found that the crystal structure is not distorted in the case of hydrogen embedded across the chains, while the type of bonding between carbon atoms in carbon chains in the carbyne crystal structure depended on the impurity concentration. The crystal structure was distorted when hydrogen was embedded along the chains. The concentration of impurities influences the conductivity of a carbyne crystal.     

Possibilities of C 1s XPS and N(E) C KVV Auger spectroscopy for identification of inherent peculiarities of diamond growth

The interaction of C-atoms and CH_n-radicals with uncleaned and argon cleaned silicon substrate and with diamond surface after H-treatment have been studied in situ by XPS and Auger spectroscopy. It was found the formation of a new chemical surface state of carbon atoms in the case of carbon atoms and radicals interaction with cleaned silicon. The same chemical state was revealed on the H-treated diamond surface. Graphite-like structure of carbon atoms was observed on the surface of unlearned silicon and H-treated diamond after interaction with carbon atoms and radicals. N(E) C KVV Auger spectrum for the new chemical state of carbon atoms significantly differs from typical spectra for sp²- and sp³-bonded carbon materials. The high energy part of this spectrum was interpreted under the hypothesis of sp³-bonded carbon atoms but with shifted fermi level position.     

Nitrogen doped carbon nano-onions as efficient and robust electrocatalysts for oxygen reduction reactions

We investigated synthesis and electrocatalytic performance of metal-free, nitrogen-doped carbon nano-onions (N-CNOs) for oxygen reduction reactions in alkaline electrolyte. N-CNOs were prepared by chemical oxidation of nanodiamond-derived carbon nano-onions (ox-CNOs), followed by thermal annealing with urea under the flow of argon gas. The chemical oxidation step was critical to successfully internalize nitrogen atoms into carbon network. Morphology, microstructure, and chemical states of carbon nano-onions (CNOs), ox-CNOs, and N-CNOs were characterized by transmission electron microscopy (TEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). Electrocatalytic activity of pristine and modified CNOs was characterized by a series of electrochemical measurements. Electrochemical characterizations were done with thin film electrodes of CNOs mounted on a glassy carbon disk. Compared to CNOs and ox-CNOs, N-CNOs showed remarkably enhanced electron-transfer kinetics with the 4-electron transfer as a dominant reaction pathway. Overall, N-CNOs exhibited electrochemical characteristics comparable to commercial Pt/C catalysts.     

The effect of hydrogen and oxygen atoms on the adsorbed gallium inside defective carbon nanotubes

The adsorption of single gallium atoms on the inner walls of single-walled carbon nanotubes with hydrogen/oxygen-saturated monovacancies are studied by using the density functional theory method. When the monovacancy is saturated by the hydrogen or oxygen atom, the gallium atom prefers to adsorb on the top of the center of a pentagon ring, and the binding energy between the gallium atom and carbon nanotube is significantly lower as compared to the case with a pure monovacancy. In addition, the results of the density of states show that the states originating from the adsorbed gallium atoms shift toward lower energy when the carbon atoms with dangling bonds are saturated by hydrogen or oxygen atoms. Meanwhile, these states have no contribution to the states near the Fermi levels.