Comparative study of hydrogen adsorption on carbon and BN nanotubes

The physisorption and chemisorption of hydrogen in BN nanotubes, investigated by density functional theory (DFT), were compared with carbon nanotubes. The physisorption of H2 on BN nanotubes is less favorable energetically than on carbon nanotubes; BN nanotubes cannot adsorb hydrogen molecules effectively in this manner. Chemisorption of H2 molecules on pristine BN nanotubes is endothermic. Consequently, perfect BN nanotubes are not good candidates for hydrogen storage by either mechanism. Other strategies must be utilized if BN nanotubes are to be employed as hydrogen storage media such as utilizing them as supporting media for hydrogen-absorbing metal nanoclusters.     

Defects-enhanced dissociation of H2 on boron nitride nanotubes

With the density-functional theory and nudged elastic band method, the adsorption and dissociation of the hydrogen molecule on the boron nitride (BN) nanotubes with and without defects are studied theoretically. Hydrogen molecule physically adsorbs on the surface of the BN layer and nanotubes. The dissociation of the hydrogen molecule on the surface of the perfect BN layer and nanotubes is endothermic, and the energy barrier reduces with the decrease of the diameter of the tubes, while it is still larger than 2.0 eV for the (7,0) BN nanotube. Antisite, carbon substitutional, vacancy, and Stone-Wales 5775 defects on the wall of the tube are considered. With the presence of the defects, the dissociation of the hydrogen molecule becomes exothermic and the dissociation barrier can be reduced to about 0.67 eV.     

Adsorption of hydrogen molecules on the platinum-doped boron nitride nanotubes

Adsorption of hydrogen molecules on platinum-doped single-walled zigzag (8,0) boron nitride (BN) nanotube is investigated using the density-functional theory. The Pt atom tends to occupy the axial bridge site of the BN tube with the highest binding energy of -0.91 eV. Upon Pt doping, several occupied and unoccupied impurity states are induced, which reduces the band gap of the pristine BN nanotube. Upon hydrogen adsorption on Pt-doped BN nanotube, the first hydrogen molecule can be chemically adsorbed on the Pt-doped BN nanotube without crossing any energy barrier, whereas the second hydrogen molecule has to overcome a small energy barrier of 0.019 eV. At least up to two hydrogen molecules can be chemically adsorbed on a single Pt atom supported by the BN nanotube, with the average adsorption energy of -0.365 eV. Upon hydrogen adsorption on a Pt-dimer-doped BN nanotube, the formation of the Pt dimer not only weakens the interaction between the Pt cluster and the BN nanotube but also reduces the average adsorption energy of hydrogen molecules. These calculation results can be useful in the assessment of metal-doped BN nanotubes as potential hydrogen storage media.     

氮化硼纳米管的研究进展

氮化硼纳米管的研究进展;结构;制备;性能;储氢;综述     

True nanocable assemblies with insulating BN nanotube sheaths and conducting Cu nanowire cores

Nanocable models comprised of BN nanotubes filled with close-packed Cu nanowires were investigated by gradient-corrected density functional theory (DFT) computations. The optimal distance between the sidewall of BN nanotubes and the atoms in a copper nanowire is about 0.35 nm, with a weak insertion energy (ca. -0.04 eV per Cu atom). Hence, such nanocables are assembled by weaker van der Waals (vdW) forces, rather than by chemical bonding interactions. The electronic band structures of the BN/Cu hybrid systems are superposition of those of the separate components, the BN nanotubes, and the Cu nanowires. Since charge density analyses show that the conduction electrons are distributed only on the copper atoms, charge transport will occur only in these inner nanowires, which are effectively insulated by the outer BN nanotubes. On the basis of these computational results, BN/Cu hybrid structures should be ideal nanocables.     

H2与C,BN和GaN纳米管的相互作用势能

基于C,B,N和Ga与H原子间的L-J势函数,系统计算了H2处于(n,n)(n=8,10,12)单壁C,BN和GaN纳米管内部及外部不同处的势能.根据势能变化曲线,分析了3种纳米管氢物理吸附能力的差异,给出了H2在3种纳米管外部的势能表达式.研究结果表明:3种纳米管内部的氢吸附力均分别高于管外;随着纳米管直径的增加,各纳米管管内的氢吸附力均略有下降,而管外变化不明显;GaN,BN和C纳米管依次具有更好的储氢能力.     

X-ray absorption near edge structure study of BN nanotubes and nanothorns

Two boron nitride (BN) nanostructures, the bamboo-like nanotubes and nanothorns where the nanosize h-BN layers are randomly stacked looking like thorns, were synthesized selectively via thermal chemical vapor deposition of B/B(2)O(3) under the NH(3) flow at 1200 degrees C. Electron energy-loss spectroscopy reveals the N-rich h-BN layers with a ratio of B/N = 0.75-0.85. Angle-resolved X-ray absorption near edge structure of these two N-rich nanostructures has been compared with that of h-BN microcrystals. The pi transition in the N K-edge shifts to the lower energy by 0.8-1.0 eV from that of h-BN microcrystals, and the second-order signals of N 1s electrons become significant. We suggest that the N enrichment would decrease the band gap of nanostructures from that of h-BN microcrystals. The Raman spectrum shows the peak broadening due to the defects of N-rich h-BN layers.     

Multi-walled BN nanotubes synthesized by carbon-free method

Pure multi-walled BN nanotubes were synthesized via a carbon-free chemical vapor deposition process using boron and gallium oxide mixture as reaction precursor. Transmission electron microscopy was used to investigate their structure, morphology and defects. The wall deformation, dependent on tube diameter, was observed and explained in terms of strain relaxation of bond rotation. Opposed to carbon nanotubes, bending of BN nanotubes typically results in fracture at their concave side. Ring defect-related mechanism was proposed to interpret the fracture. The ring defects also result in the formation of a nanocone with 300° disclination. The nanocones end up with BN nanotubes exhibiting the small innermost shell ∼0.4 nm in diameter.     

Encapsulation of carbon chain molecules in single-walled carbon nanotubes

The vacuum space inside carbon nanotubes offers interesting possibilities for the inclusion, transportation, and functionalization of foreign molecules. Using first-principles density functional calculations, we show that linear carbon-based chain molecules, namely, polyynes (C(m)H(2), m = 4, 6, 10) and the dehydrogenated forms C(10)H and C(10), as well as hexane (C(6)H(14)), can be spontaneously encapsulated in open-ended single-walled carbon nanotubes (SWNTs) with edges that have dangling bonds or that are terminated with hydrogen atoms, as if they were drawn into a vacuum cleaner. The energy gains when C(10)H(2), C(10)H, C(10), C(6)H(2), C(4)H(2), and C(6)H(14) are encapsulated inside a (10,0) zigzag-shaped SWNT are 1.48, 2.04, 2.18, 1.05, 0.55, and 1.48 eV, respectively. When these molecules come inside a much wider (10,10) armchair SWNT along the tube axis, they experience neither an energy gain nor an energy barrier. They experience an energy gain when they approach the tube walls inside. Three hexane molecules can be encapsulated parallel to each other (i.e., nested) inside a (10,10) SWNT, and their energy gain is 1.98 eV. Three hexane molecules can exhibit a rotary motion. One reason for the stability of carbon chain molecules inside SWNTs is the large area of weak wave function overlap. Another reason concerns molecular dependence, that is, the quadrupole-quadrupole interaction in the case of the polyynes and electron charge transfer from the SWNT in the case of the dehydrogenated forms. The very flat potential surface inside an SWNT suggests that friction is quite low, and the space inside SWNTs serves as an ideal environment for the molecular transport of carbon chain molecules. The present theoretical results are certainly consistent with recent experimental results. Moreover, the encapsulation of C(10) makes an SWNT a (purely carbon-made) p-type acceptor. Another interesting possibility associated with the present system is the direction-controlled transport of C(10)H inside an SWNT under an external field. Because C(10)H has an electric dipole moment, it is expected to move under a gradient electric field. Finally, we derive the entropies of linear chain molecules inside and outside an open-ended SWNT to discuss the stability of including linear chain molecules inside an SWNT at finite temperatures.     

Catalyzed collapse and enhanced hydrogen storage of BN nanotubes

The novel morphology of BN nanotubes with a collapsed structure has been discovered by a metal-catalyzed treatment. The collapse causes the dramatic enlargement of a specific surface area of BN nanotubes and remarkably enhances the hydrogen storage capacity of BN nanotubes.     

Theoretical study on interaction of hydrogen with single-walled boron nitride nanotubes. II. Collision, storage, and adsorption

Collision and adsorption of hydrogen with high incident kinetic energies on a single-walled boron nitride (BN) nanotube have been investigated. Molecular-dynamics (MD) simulations indicate that at incident energies below 14 eV hydrogen bounces off the BN nanotube wall. On the other hand, at incident energies between 14 and 22 eV each hydrogen molecule is dissociated at the exterior wall to form two hydrogen atoms, but only one of them goes through the wall. However, at the incident energies between 23 and 26 eV all of the hydrogen atoms dissociated at the exterior wall are found to be capable of going inside the nanotube and then to recombine to form hydrogen molecules inside the nanotube. Consequently, it is determined that hydrogen should have the incident energy >22 eV to go inside the nanotube. On the other hand, we find that the collisions using the incident energies >26 eV could result in damaging the nanotube structures. In addition our MD simulations find that hydrogen atoms dissociated at the wall cannot bind to either boron or nitrogen atoms in the interior wall of the nanotube.     

Linear scaling calculation of band edge states and doped semiconductors

Linear scaling methods provide total energy, but no energy levels and canonical wave functions. From the density matrix computed through the density matrix purification methods, we propose an order-N [O(N)] method for calculating both the energies and wave functions of band edge states, which are important for optical properties and chemical reactions. In addition, we also develop an O(N) algorithm to deal with doped semiconductors based on the O(N) method for band edge states calculation. We illustrate the O(N) behavior of the new method by applying it to boron nitride (BN) nanotubes and BN nanotubes with an adsorbed hydrogen atom. The band gap of various BN nanotubes are investigated systematically and the acceptor levels of BN nanotubes with an isolated adsorbed H atom are computed. Our methods are simple, robust, and especially suited for the application in self-consistent field electronic structure theory.     

First-principle study of adsorption of hydrogen on Ti-doped Mg(0001) surface

Ab initio density functional theory (DFT) calculations are performed to study the adsorption of H2 molecules on a Ti-doped Mg(0001) surface. We find that two hydrogen molecules are able to dissociate on top of the Ti atom with very small activation barriers (0.103 and 0.145 eV for the first and second H2 molecules, respectively). Additionally, a molecular adsorption state of H2 above the Ti atom is observed for the first time and is attributed to the polarization of the H2 molecule by the Ti cation. Our results parallel recent findings for H2 adsorption on Ti-doped carbon nanotubes or fullerenes. They provide new insight into the preliminary stages of hydrogen adsorption onto Ti-incorporated Mg surfaces.     

A study of the reactions of molecular hydrogen with small gold clusters

This work presents a study of reactions between neutral and negatively charged Au(n) clusters (n=2,3) and molecular hydrogen. The binding energies of the first and second hydrogen molecule to the gold clusters were determined using density functional theory (DFT), second order perturbation theory (MP2) and coupled cluster (CCSD(T)) methods. It is found that molecular hydrogen easily binds to neutral Au(2) and Au(3) clusters with binding energies of 0.55 eV and 0.71 eV, respectively. The barriers to H(2) dissociation on these clusters with respect to Au(n)H(2) complexes are 1.10 eV and 0.59 eV for n=2 and 3. Although negatively charged Au(n) (-) clusters do not bind molecular hydrogen, H(2) dissociation can occur with energy barriers of 0.93 eV for Au(2) (-) and 1.39 eV for Au(3) (-). The energies of the Au(2)H(2) (-) and Au(3)H(2) (-) complexes with dissociated hydrogen molecules are lower than the energies of Au(2) (-)+H(2) and Au(3) (-)+H(2) by 0.49 eV and 0.96 eV, respectively. There is satisfactory agreement between the DFT and CCSD(T) results for binding energies, but the agreement is not as good for barrier heights.     

Origins of dihydrogen binding to metal-inserted porphyrins: electric polarization and Kubas interaction

Using density functional theory calculations, we have investigated the interactions between hydrogen molecules and metalloporphyrins. A metal atom, such as Ca or Ti, is introduced for incorporation in the central N(4) cavity. Within local density approximation (generalized gradient approximation), we find that the average binding energy of H(2) to the Ca atom is about 0.25 (0.1) eV/H(2) up to four H(2) molecules, whereas that to the Ti atom is about 0.6 (0.3) eV per H(2) up to two H(2) molecules. Our analysis of orbital hybridization between the inserted metal atom and molecular hydrogen shows that H(2) binds weakly to Ca-porphyrin through a weak electric polarization in dihydrogen, but is strongly hybridized with Ti-porphyrin through the Kubas interaction. The presence of d orbitals in Ti may explain the difference in the interaction types.     

Different surface chemistries of water on Ru[0001]: from monomer adsorption to partially dissociated bilayers

Density functional theory has been used to perform a comparative theoretical study of the adsorption and dissociation of H(2)O monomers and icelike bilayers on Ru[0001]. H(2)O monomers bind preferentially at atop sites with an adsorption energy of approximately 0.4 eV/H(2)O. The main bonding interaction is through the H(2)O 1b(1) molecular orbital which mixes with Ru d(z)2 states. The lower-lying set of H(2)O molecules in an intact H(2)O bilayer bond in a similar fashion; the high-lying H(2)O molecules, however, do not bond directly with the surface, rather they are held in place through H bonding. The H(2)O adsorption energy in intact bilayers is approximately 0.6 eV/H(2)O and we estimate that H bonding accounts for approximately 70% of this. In agreement with Feibelman (Science 2002, 295, 99) we find that a partially dissociated OH + H(2)O overlayer is energetically favored over pure intact H(2)O bilayers on the surface. The barrier for the dissociation of a chemisorbed H(2)O monomer is 0.8 eV, whereas the barrier to dissociate a H(2)O incorporated in a bilayer is just 0.5 eV.     

Probing the Chemical Functionalization of Single‐Walled Carbon Nanotubes with Multiple Carbon Ad‐Dimer Defects

Drying‐tube‐shaped single‐walled carbon nanotubes (SWCNTs) with multiple carbon ad‐dimer (CD) defects are obtained from armchair (n,n,m) SWCNTs (n=4, 5, 6, 7, 8; m=7, 13). According to the isolated‐pentagon rule (IPR) the drying‐tube‐shaped SWCNTs are unstable non‐IPR species, and their hydrogenated, fluorinated, and chlorinated derivatives are investigated. Interestingly, chemisorptions of hydrogen, fluorine, and chlorine atoms on the drying tube‐shaped SWCNTs are exothermic processes. Compared to the reaction energies for binding of H, F, and Cl atoms to perfect and Stone–Wales‐defective armchair (5,5) nanotubes, binding of F with the multiply CD defective SWCNTs is stronger than with perfect and Stone–Wales‐defective nanotubes. The reaction energy for per F₂ addition is between 85 and 88 kcal mol^?1 more negative than that per H₂ addition. Electronic structure analysis of their energy gaps shows that the CD defects have a tendency to decrease the energy gap from 1.98–2.52 to 0.80–1.17 eV. After hydrogenation, fluorination, and chlorination, the energy gaps of the drying‐tube‐shaped SWCNTs with multiple CD defects are substantially increased to 1.65–3.85 eV. Furthermore, analyses of thermodynamic stability and nucleus‐independent chemical shifts (NICS) are performed to analyze the stability of these molecules.     

Intrinsic defects and dopants in LiNH2: a first-principles study

The lithium amide (LiNH(2)) + lithium hydride (LiH) system is one of the most attractive light-weight materials options for hydrogen storage. Its dehydrogenation involves mass transport in the bulk (amide) crystal through lattice defects. We present a first-principles study of native point defects and dopants in LiNH(2) using density functional theory. We find that both Li-related defects (the positive interstitial Li(i)(+) and the negative vacancy V(Li)(-)) and H-related defects (H(i)(+) and V(H)(-)) are charged. Li-related defects are most abundant. Having diffusion barriers of 0.3-0.5 eV, they diffuse rapidly at moderate temperatures. V(H)(-) corresponds to the [NH](2-) ion. It is the dominant species available for proton transport with a diffusion barrier of ～0.7 eV. The equilibrium concentration of H(i)(+), which corresponds to the NH(3) molecule, is negligible in bulk LiNH(2). Dopants such as Ti and Sc do not affect the concentration of intrinsic defects, whereas Mg and Ca can alter it by a moderate amount. Ti and Mg are easily incorporated into the LiNH(2) lattice, which may affect the crystal morphology on the nano-scale.     

Transformation from chemisorption to physisorption with tube diameter and gas concentration: computational studies on NH3 adsorption in BN nanotubes

Using first-principles computations, we studied NH3 adsorption on a series of zigzag (n,0) single-walled BN nanotubes (BNNTs) and the effect of gas coverage. Tube diameter and NH3 coverage play important roles on the tube-NH3 interaction. Chemisorption of a single NH3 molecule on top of B site is energetically preferable for all the tubes studied, but the adsorption energy decreases sharply with increasing tube diameter, and then gradually approaches the value for NH3 physisorption on BN graphene layer. On the sidewall of (10,0) BNNT, NH3 molecules prefer to pair arrangement on top of B and N atoms opposite in the same hexagon. At low coverages, NH3 molecules are partly chemically bound to BNNTs. With the increase of NH3 coverage, hydrogen bonds form between the adsorbed NH3 molecules or between the NH3 molecules and N atoms in BNNTs. When the coverage reaches 25%, the chemisorption of NH3 transforms to physisorption completely. NH3 adsorption does not modify the overall band structures of BNNTs, irrespective of NH3 coverage, but the band gap is narrowed due to the NH3-tube coupling and tube deformation.     

铂掺杂氮化硼纳米管及CO在其表面的吸附行为的理论研究

采用基于密度泛函理论的PBEPBE方法对铂(Pt)掺杂的氮化硼(BN)纳米管进行了理论研究. 计算结果表明, Pt原子突出BN纳米管表面, Pt的d轨道暴露到外面, 使它更容易和外来分子发生相互作用, 提高了纳米管的反应活性. Pt取代掺杂缩小了纳米管的能隙, 从而提高BN纳米管的导电性. 一氧化碳(CO)在Pt掺杂BN纳米管上的吸附行为表明, 2个CO能化学吸附到纳米管表面, 更多的CO分子吸附是物理吸附.