钇对石墨烯储氢性能的影响

应用基于密度泛函理论的第一性原理方法研究过渡金属钇（Y）修饰对石墨烯储氢性能的影响。考虑Y原子在石墨烯上易形成团簇,采用B原子掺杂有效阻止了团簇形成。通过模拟计算得到的改性体系稳定、储氢性能优异,可吸附6个H₂分子,平均吸附能范围为-0.539到-0.655 eV （per H₂）,理论上满足理想的氢吸附能范围。经Bader电荷初步计算和基于Y/B/graphene （G）体系吸附H₂分子的电子态密度及电荷差分密度图分析得,Y原子与石墨烯间通过电荷转移产生结合,与H₂分子则发生典型的Kubas型相互作用。Y原子改变了H₂分子与石墨烯基的电荷分布,成为连接两者电子云的桥梁,从而增强了H₂分子的吸附能。改性石墨烯体系吸附的均为氢分子,有利于在环境温度和压力条件下进行循环控制,是具有良好发展前景的储氢材料之一。     

Adsorption, diffusion, and recombination of hydrogen on pure and boron-doped graphite surfaces

Boron inserted as impurity by substitution of carbon atoms in graphite is known to modify the reactivity of the surface in interaction with hydrogen. Boron induces a better H retention capability in graphite while it makes easier the recombination into molecular hydrogen under heating in thermal-desorption experimental conditions. It has already been calculated that boron modifies the electronic structure of the surface, which results in an increase of the adsorption energy for H. This result seems in good agreement with the better retention for H in doped graphite, but contradictory with the easier recombination observed. The aim of this work is to dismiss this contradiction by elucidating the modifications induced by boron in the recombination mechanism. We studied the diffusion of H on pure and boron-doped graphite in the density functional theory framework. We determined a diffusionlike mechanism leading to molecular hydrogen formation. Finally, we have shown the fundamental modifications induced by boron on the [0001] graphite surface reactivity. From these calculations it stands out that recombination is the result of desorption on pure graphite and diffusion on B-doped surfaces, while the activation energy for the rate limiting step is half reduced by boron. The results are compared to experimental observations. The connection between the cluster and periodic quantum modes for graphite is also discussed.     

Insights into hydrogen atom adsorption on and the electrochemical properties of nitrogen-substituted carbon materials

The nitrogen substitution in carbon materials is investigated theoretically using the density functional theory method. Our calculations show that nitrogen substitution decreases the hydrogen adsorption energy if hydrogen atoms are adsorbed on both nitrogen atoms and the neighboring carbon atoms. On the contrary, the hydrogen adsorption energy can be increased if hydrogen atoms are adsorbed only on the neighboring carbon atoms. The reason can be explained by the electronic structures analysis of N-substituted graphene sheets. Nitrogen substitution reduces the pi electron conjugation and increases the HOMO energy of a graphene sheet, and the nitrogen atom is not stable due to its 3-valent character. This raises an interesting research topic on the optimization of the N-substitution degree, and is important to many applications such as hydrogen storage and the tokamaks device. The electronic structure studies also explain well why nitrogen substitution increases the capacitance but decreases the electron conductivity of carbon electrodes as was experimentally observed in our experiments on the supercapacitor.     

空位和B掺杂对Si在石墨烯上吸附的影响

利用基于密度泛函理论的第一性原理计算了空位和B替位掺杂对Si在石墨烯上吸附的影响. 结果表明: 对完整的石墨烯结构, Si吸附在桥位最稳定, Si吸附改变了石墨烯中C原子的自旋性质; 空位和B替位掺杂均加强了Si在缺陷处的吸附, 空位对Si在石墨烯上吸附的影响相对较大; B掺杂改变了Si的稳定吸附位置(由桥位移到顶位); Si在空位和B掺杂石墨烯上吸附, 体系不具有磁性; B掺杂提高了石墨烯体系的导电性能; 单空位缺陷不易形成, 结构不稳定, B掺杂结构相对较稳定.     

Boron‐Double‐Ring Sheet,Fullerene, and Nanotubes: Potential Hydrogen Storage Materials

Similar to carbon‐based graphene, fullerenes and carbon nanotubes, boron atoms can form sheets, fullerenes, and nanotubes. Here we investigate several of these novel boron structures all based on the boron double ring within the framework of density functional theory. The boron sheet is found to be metallic and flat in its ground state. The spherical boron cage containing 180 atoms is also stable and has I symmetry. Stable nanotubes are obtained by rolling up the boron sheet, and all are metallic. The hydrogen storage capacity of boron nanostructures is also explored, and it is found that Li‐decorated boron sheets and nanotubes are potential candidates for hydrogen storage. For Li‐decorated boron sheets, each Li atom can adsorb a maximum of 4 H₂ molecules with g_d=7.892 wt %. The hydrogen gravimetric density increases to g_d=12.309 wt % for the Li‐decorated (0,6) boron nanotube.     

Electronic and magnetic properties of armchair and zigzag graphene nanoribbons

The electronic properties, band gap, and ionization potential of zigzag and armchair graphene nanoribbons are calculated as a function of the number of carbon atoms in the ribbon employing density functional theory at the B3LYP6-31G* level. In armchair ribbons, the ionization potential and band gap show a gradual decrease with length. For zigzag ribbons, the dependence of the band gap and ionization potential on ribbon length is different depending on whether the ribbon has an unpaired electron or not. It is also found that boron and nitrogen zigzag and armchair doped graphene nanoribbons have a triplet ground state and could be ferromagnetic.     

Linear augmented cylindrical wave Green’s function method for perfect nanotubes and nanotubes with regular and point impurities

Nanotubes are giant cage molecules looking like closed hollow cylindrical shells. This review deals with basic principles of the linear augmented cylindrical Green’s function method and its applications to calculation of the electronic structure of perfect nanotubes and those containing substitutional impurities. A major argument for using cylindrical waves to describe nanotubes is that such a choice of the basis set makes it possible to explicitly consider the actual cylindrical geometry of nanotubes, which, in particular, ensures rapid convergence of iterative procedures. A computation technique has been described and the results of calculations of the band structure and densities of states of carbon and boron nitride nanotubes have been reported. Special attention has been paid to the changes in the electronic properties of nanotubes induced by the substitution of nitrogen, boron, or oxygen for C atoms in the carbon nanotubes, as well as to the isoelectronic substitution of P, Sb, or As for the nitrogen and of Al, In, or Ga for the boron in boron nitride nanotubes.     

NH3在硼纳米管表面吸附的密度泛函理论研究

利用密度泛函理论研究了NH₃在完整和含有缺陷的硼纳米管上的吸附行为以及相关电子性质. 计算结果表明, 对于α硼纳米管, 在不同的直径和手性条件下, NH₃均倾向于吸附在配位数为6的顶位上. 电子结构计算结果表明, NH₃能够吸附在纳米管表面主要是由于N和B原子产生了较强的相互作用. 表明硼纳米管是一种潜在的NH₃气气敏材料.     

First-principles study of electronic and magnetic properties of transition metal adsorbed h-BNC2 sheets

Adsorption of Fe, Co and Ni atoms on a hybrid hexagonal sheet of graphene and boron nitride is studied using density functional methods. Most favorable adsorption sites for these adatoms are identified for different widths of the graphene and boron nitride regions. Electronic structure and magnetic properties of the TM-adsorbed sheets are then studied in detail. The TM atoms change the electronic structure of the sheet significantly, and the resulting system can be a magnetic semiconductor, semi-metal, or a non-magnetic semiconductor depending on the TM chosen. This gives tunability of properties which can be useful in novel electronics applications. Finally, barriers for diffusion of the adatoms on the sheet are calculated, and their tendency to agglomerate on the sheet is estimated.     

新型掺杂多孔芳香骨架材料的储氢性能模拟

基于PAF-301分子模型通过Li掺杂或B取代等模式设计了几种新型多孔芳香骨架(PAFs)材料,采用量子力学和分子力学方法对新材料的储氢性能进行研究.由量子力学计算得到了不同分子片段与H2之间的结合能,并结合DDEC方法计算了各分子片段的原子电荷分布.利用巨正则蒙特卡洛(GCMC)模拟方法计算了77和298 K下H2在不同PAFs材料中的吸附平衡性质.结果表明,H2直接与苯环的结合能较低,但掺杂Li原子能够提高H2与六元环的结合能,同时Li原子体现出较高的正电性质,B原子取代苯环中的两个C原子后,使得原有C原子电负性增强;77 K下PAF-301Li具有最高的储氢性能,而PAF-C4B2H4-Li2-Si和PAF-C4B2H4-Li2-Ge体现出较好的常温储氢性能,各种材料的常温储氢性能远低于其低温储氢性能.通过77 K下H2在PAFs材料中的等位能面分布和吸附平衡质心密度分布对H2在PAFs材料中的优先吸附位置进行分析,发现在PAF-301和PAF-301Li骨架中,由于中心能量较低的等位能区域范围较宽,H2在其中存在四个明显的吸附高密度分布区域,而其它三种PAFs晶胞中心能量较低的等位能区域范围较窄,使得H2在其中只存在两个明显的吸附高密度分布区域.     

Substitutional doping of symmetrical small fullerene dimers

Magnetic carbon nano-structures have potential applications in the field of spintronics as they exhibit valuable magnetic properties. Symmetrically sized small fullerene dimers are substitutional doped with nitrogen (electron rich) and boron (electron deficient) atoms to visualize the effect on their magnetic properties. Interaction energies suggests that the resultant dimer structures are energetically favorable and hence can be formed experimentally. There is significant change in the total magnetic moment of dimers of the order of 0.5 μ_B after the substitution of C atoms with N and B, which can also be seen in the change of density of states. The HOMO-LUMO gaps of spin up and spin down electronic states have finite energy difference which confirm their magnetic behaviour, whereas for non-magnetic doped dimers, the HOMO-LUMO gaps for spin up and down states are degenerate. The optical properties show that the dimers behave as optical semiconductors and are useful in optoelectronic devices. The induced magnetism in these dimers makes them fascinating nanocarbon magnetic materials.     

缺陷石墨烯吸附Au、Ag、Cu的第一性原理计算

采用基于密度泛函理论的投影缀加波方法研究了Au、Ag、Cu吸附在缺陷石墨烯单侧和双侧的体系,对吸附体系的吸附能、磁性、电荷转移和电子结构进行了计算和分析.缺陷石墨烯吸附Au、Ag、Cu体系的吸附能比本征石墨烯增加2 eV以上,说明三种金属原子更容易吸附在缺陷位置;吸附体系的电荷密度差分和电子结构的结果表明,Au、Ag、Cu与缺陷石墨烯之间均为化学吸附.计算吸附体系的磁性发现,单侧吸附时三种吸附体系均有磁性,磁矩大约为1μB;双侧吸附时,三种吸附体系磁矩大约为2μB.     

Energetic stability of boron nitride nanostructures doped with one carbon atom

We have investigated, using first‐principles calculations, the role of a substitutional carbon atom on the geometric stability of boron nitride monolayers, nanotubes, and nanocones. It is shown that the formation of energy depends on the number of atoms for the monolayers and on the diameter for the tubes. It is also found, for the carbon‐doped boron nitride nanotubes, that the value for the strain energy approaches the one obtained for nondoped tubes with increasing diameter. For the structural stability, we have verified that the doping, which introduces an excess of nitrogen or boron, makes each structure more favorable in its reverse atmosphere, i.e., excess of nitrogen is more stable in a boron‐rich growth environment, whereas excess of boron is preferred in a nitrogen‐rich condition. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

缺陷石墨烯吸附Au、Ag、Cu的第一性原理计算

采用基于密度泛函理论的投影缀加波方法研究了Au、Ag、Cu吸附在缺陷石墨烯单侧和双侧的体系,对吸附体系的吸附能、磁性、电荷转移和电子结构进行了计算和分析. 缺陷石墨烯吸附Au、Ag、Cu体系的吸附能比本征石墨烯增加2 eV以上,说明三种金属原子更容易吸附在缺陷位置;吸附体系的电荷密度差分和电子结构的结果表明,Au、Ag、Cu与缺陷石墨烯之间均为化学吸附. 计算吸附体系的磁性发现,单侧吸附时三种吸附体系均有磁性,磁矩大约为1μ_B;双侧吸附时,三种吸附体系磁矩大约为2μ_B.     

A Mössbauer and X-ray study of Fe2P1−xBx compounds (x < 0.15)

Boron/phosphorus substitution in Fe₂P has been studied by ⁵⁷Fe Mössbauer spectroscopy. The magnetic ordering temperature increases rapidly with increasing boron content. Replacement of a phosphorus atom by boron in the immediate environment of an iron atom results in a substantial increase of the magnetic hyperfine field, while the centroid shift and the quadrupole splitting are almost unchanged. The hyperfine parameters for iron atoms at larger distances from the boron atom remain unaffected. Boron substitutes preferentially for phosphorus at the singlefold P(2) position in the Fe₂P structure.     

铁脂质体/水分配系数测定及影响因素考察

基于PAF-301分子模型通过Li 掺杂或B取代等模式设计了几种新型多孔芳香骨架(PAFs)材料, 采用量子力学和分子力学方法对新材料的储氢性能进行研究. 由量子力学计算得到了不同分子片段与H₂之间的结合能, 并结合DDEC方法计算了各分子片段的原子电荷分布. 利用巨正则蒙特卡洛(GCMC)模拟方法计算了77和298 K下H₂在不同PAFs材料中的吸附平衡性质. 结果表明, H₂直接与苯环的结合能较低, 但掺杂Li 原子能够提高H₂与六元环的结合能, 同时Li 原子体现出较高的正电性质, B原子取代苯环中的两个C原子后, 使得原有C原子电负性增强; 77 K下PAF-301Li 具有最高的储氢性能, 而PAF-C₄B₂H₄-Li₂-Si 和PAF-C₄B₂H₄-Li₂-Ge体现出较好的常温储氢性能, 各种材料的常温储氢性能远低于其低温储氢性能. 通过77 K下H₂在PAFs材料中的等位能面分布和吸附平衡质心密度分布对H₂在PAFs 材料中的优先吸附位置进行分析, 发现在PAF-301 和PAF-301Li 骨架中, 由于中心能量较低的等位能区域范围较宽, H₂在其中存在四个明显的吸附高密度分布区域, 而其它三种PAFs晶胞中心能量较低的等位能区域范围较窄, 使得H₂在其中只存在两个明显的吸附高密度分布区域.     

Boron substituted graphene: energy landscape for hydrogen adsorption

We have analyzed the modifications of interaction energy between a molecule of hydrogen and graphene layers partially substituted by boron. We show that the presence of boron modifies the symmetry of the energy landscape. It is due to both the larger boron size (with respect to carbon) and its stronger interaction with hydrogen molecules. The changes of energy surface are not confined to the neighborhood of substituted sites but extend over several graphene carbon sites, making the surface more heterogeneous. We show that the average increase of adsorption energy could meet DOE targets for hydrogen storage if a partial charge transfer between boron and hydrogen occurs during adsorption.     

Why h atom prefers the on-top site and alkali metals favor the middle hollow site on the Basal plane of graphite

In this work, the different adsorption properties of H and alkali metal atoms on the basal plane of graphite are studied and compared using a density functional method on the same model chemistry level. The results show that H prefers the "on-top site" while alkali metals favor the "middle hollow site" of graphite basal plane due to the unique electronic structures of H, alkali metals, and graphite. H has a higher electronegativity than carbon, preferring to form a covalent bond with C atoms, whereas alkaline metals have lower electronegativity, tending to adsorb on the highest electrostatic potential sites. During adsorption, there are more charges transferred from alkali metal to graphite than from H to graphite.     

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.     

Predicting two-dimensional boron-carbon compounds by the global optimization method

We adopt a global optimization method to predict two-dimensional (2D) nanostructures through the particle-swarm optimization (PSO) algorithm. By performing PSO simulations, we predict new stable structures of 2D boron-carbon (B-C) compounds for a wide range of boron concentrations. Our calculations show that: (1) All 2D B-C compounds are metallic except for BC(3) which is a magic case where the isolation of carbon six-membered ring by boron atoms results in a semi-conducting behavior. (2) For C-rich B-C compounds, the most stable 2D structures can be viewed as boron doped graphene structures, where boron atoms typically form 1D zigzag chains except for BC(3) in which boron atoms are uniformly distributed. (3) The most stable 2D structure of BC has alternative carbon and boron ribbons with strong in-between B-C bonds, which possesses a high thermal stability above 2000 K. (4) For B-rich 2D B-C compounds, there is a novel planar-tetracoordinate carbon motif with an approximate C(2)(v) symmetry.