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.     

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     

Preparation and electrochemical hydrogen storage of boron nitride nanotubes

Boron nitride (BN) nanotubes were synthesized through chemical vapor deposition over a wafer made by a LaNi5/B mixture and nickel powder at 1473 K. Scanning electron microscopy, transmission electron microscopy, energy-dispersive spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy were performed to characterize the microstructure and composition of BN nanotubes. It was found that the obtained BN nanotubes were straight with a diameter of 30-50 nm and a length of up to several microns. We first verify that the BN nanotubes can storage hydrogen by means of an electrochemical method, though its capacity is low at present. The hydrogen desorption of nonelectrochemical recombination in cyclic voltammograms, which is considered as the slow reaction at BN nanotubes, suggests the possible existence of strong chemisorption of hydrogen, and it may lead to the lower discharge capacity of BN nanotubes. It is tentatively concluded that the improvement of the electrocatalytic activity by surface modification with metal or alloy would enhance the electrochemical hydrogen storage capacity of BN nanotubes.     

毛刺状竹节结构氮化硼纳米管的合成与生长机理

以非晶硼和氧化镍纳米颗粒为原料,在氨气中1100℃下合成了毛刺状竹节结构的氮化硼纳米管. 利用X射线衍射和透射电镜研究了氮化硼纳米管的结构和形貌. 竹节结构纳米管表面的毛刺是六方氮化硼的纳米薄片. 提出了一种基于固态硼和气态二氧化硼扩散的毛刺形貌生长机理.     

Fluorination and electrical conductivity of BN nanotubes

Fluorination of BN nanotubes has been performed using a catalytic growth method, which leads to the appearance of markedly curved fluorine-doped BN sheets and converts originally insulating BN nanotubes to semiconductors, as confirmed by the comparative electron transport four-probe measurements on doped and undoped individual BN nanotubes.     

Thin boron nitride nanotubes with unusual large inner diameters

BN nanotubes, displaying the characteristics of few concentric layers (2–6 layers) but unusual large inner diameters (ranging from 8 to more than 10 nm), are synthesized by a chemical vapor deposition (CVD) method on -Al₂O₃ micrometer-range particles. The inner diameters are at least 5 nm larger than the previously reported BN nanotubes of similar layers. Some BN nanotubes are observed to be filled with B–N–O-based amorphous materials. Crystalline core fillings (in the form of boron carbide nanorods) were also discovered. The discussions suggested that the CVD growth behavior of BN nanotubes may be closely dependent on the underlying substrates, which may be helpful to the possible rational synthesis of BN nanotubes.     

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.     

First-principles study of interaction between H2 molecules and BN nanotubes with BN divacancies

The interaction between H(2) molecules and boron nitride (BN) single-walled nanotubes with BN divacancies is investigated with density-functional theory. Our calculations reveal that H(2) molecules adsorb physically outside defective BN nanotubes, and cannot enter into BN nanotubes through bare BN divacancies because the energy barrier is as high as 4.62 eV. After the defects are saturated by hydrogen atoms, the physisorption behavior of H(2) molecules is not changed, but the energy barrier of H(2) molecules entering into BN nanotubes through the defects is reduced to 0.58 eV. This phenomenon is ascribed to hydrogen saturation induced reduction of electrostatic potential around the defects.     

Covalent attachments of boron nitride nanotubes through a carboxylic linker: Density functional studies

Properties of attached boron nitride (BN) nanotubes based on linking two zigzag nanotubes through a carboxylic (–(CO)O–) linker were investigated by performing density functional theory (DFT) calculations. The linking boron and nitrogen atoms at the edges of two zigzag BN nanotubes were linked to the –(C]O)O– linker to make possible the attachments of two BN nanotubes together. Total energies, energy gaps, dipole moments, linking bond lengths and angles, and quadrupole coupling constants were obtained for the optimized structures to determine the properties of the attached BN nanotubes. The results indicated that different properties could be seen for the investigated models based on their linking status. For quadrupole coupling constants, the most significant changes of parameters were observed for the linking atoms among the investigated models of attached BN nanotubes.     

Selective synthesis of boron nitride nanotubes by self-propagation high-temperature synthesis and annealing process

Four types of BN nanotubes are selectively synthesized by annealing porous precursor in flowing NH₃ and NH₃/H₂ atmosphere at temperature ranging from 1000 to 1200 °C in a vertical furnace. The as-synthesized BN nanotubes, including cylinder, wave-like, bamboo-like and bubble-chain, are characterized by XRD, FTIR, Raman, SEM, TEM and HRTEM. Three phenomenological growth models are proposed to interpret growth scenario and structure features of the four types of BN nanotubes. Selectivity of nanotubes formation is estimated as approximately 80-95%. The precursor containing B, Mg, Fe and O prepared by self-propagation high-temperature synthesis (SHS) method plays a key role in selective synthesis of the as-synthesized BN nanotubes. Chemical reactions are also discussed.     

Thermal conductivity of nanostructured boron nitride materials

We have measured the thermal conductivity of bulky pellets made of various boron nitride (BN)-based nanomaterials, including spherical nanoparticles, perfectly structured, bamboo-like nanotubes, and collapsed nanotubes. The thermal conductivity strongly depends on the morphology of the BN nanomaterials, especially on the surface structure. Spherical BN particles have the lowest thermal conductivity while the collapsed BN nanotubes possess the best thermoconductive properties. A model was proposed to explain the experimental observations based on the heat percolation passage considerations.     

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.     

Hydrogen uptake in boron nitride nanotubes at room temperature

Hydrogen uptake capacities of 1.8 and 2.6 wt % were obtained on BN multiwalled nanotubes and bamboo-like nanotubes, respectively, under approximately 10 MPa at room temperature. The preliminary results show that chemical interactions mainly take place between hydrogen and BN. Taking into consideration the uniform semiconducting electronic property and ultimate stable chemical and thermal stability, BN nanotubes may exhibit some interesting possibilities in storing hydrogen. This will also provide some viewpoints on designing the carbon-based hydrogen storage system.     

氮化硼纳米管的研究进展

本文较全面地综述了氮化硼(BN)纳米管研究的最新进展,详细讨论了BN纳米管的合成、结构、表征、形成机理及应用等方面的研究现状,并对今后的研究方向进行了展望。     

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.     

Functionalized boron nitride nanotubes with a stannic oxide coating: a novel chemical route to full coverage

BN nanotubes are coated with SnO(2) by a simple chemical reaction in solution. BN nanotubes are stirred inside a SnCl(2) solution at room temperature for 1 h. The coating is uniform with complete coverage. The coating thickness is typically 1-5 nm. The coating layer contains SnO(2) nanoparticles with sizes ranging from 1 to 5 nm.     

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.     

(BN)n富勒烯和单层BN纳米管的图形理论方法研究

应用图形理论方法对(BN)12等富勒烯和单层BN纳米管的能级分布及其稳定性进行研究,发现(BN)n比同构型的(C2)n稳定,且与用DFT方法计算的结果一致.计算结果表明,采用图形理论方法是一种很有意义的解释和预测BN纳米材料的结构和性质的定性研究方法.     

Family behaviour of Raman-active phonon frequencies of single-wall nanotubes of C, BN and BC3

Using a symmetry-based force-constant model of the lattice dynamics, the Raman-active phonon frequencies are calculated for almost 200 single-wall nanotubes of C, BN and BC(3). The n+m=constant family behaviour is found in most branches and these three kinds of nanotubes display different diameter and chirality dependence in different branches. In these branches, vibration modes that C, BN and BC(3) nanotubes have in common are presented in detail. For a particular family, the phonon frequency at Gamma point changes regularly with the chiral angle. Therefore, we may distinguish among single-wall nanotubes with similar diameter and different chiral angle.     

Catalytic synthesis of bamboo-like multiwall BN nanotubes via SHS-annealing process

Bamboo-like multiwall boron nitride (BN) nanotubes were synthesized via annealing porous precursor prepared by self-propagation high temperature synthesis (SHS) method. The as-synthesized BN nanotubes were characterized by the field emission scanning electron microscopy (FE-SEM), transmission electron microscope (TEM), high-resolution TEM (HRTEM), X-ray diffraction (XRD), Raman and Fourier transform infrared (FTIR) spectroscopy. These nanotubes have uniform diameters of about 60 nm and an average length of about 10 μm. Four growth models, including tip, base, based tip and base-tip growth models, are proposed based on the catalytic vapor-liquid-solid (VLS) growth mechanism for explaining the formation of the as-synthesized bamboo-like BN nanotubes. Chemical reactions and annealing mechanism are also discussed.