Theoretical investigation of methane adsorption onto boron nitride and carbon nanotubes

Abstract

Methane adsorption onto single-wall boron nitride nanotubes (BNNTs) and carbon nanotubes (CNTs) was studied using the density functional theory within the generalized gradient approximation. The structural optimization of several bonding configurations for a CH₄ molecule approaching the outer surface of the (8,0) BNNT and (8,0) CNT shows that the CH₄ molecule is preferentially adsorbed onto the CNT with a binding energy of ?2.84 kcal mol^?1. A comparative study of nanotubes with different diameters (curvatures) reveals that the methane adsorptive capability for the exterior surface increases for wider CNTs and decreases for wider BNNTs. The introduction of defects in the BNNT significantly enhances methane adsorption. We also examined the possibility of binding a bilayer or a single layer of methane molecules and found that methane molecules preferentially adsorb as a single layer onto either BNNTs or CNTs. However, bilayer adsorption is feasible for CNTs and defective BNNTs and requires binding energies of ?3.00 and ?1.44 kcal mol^?1 per adsorbed CH₄ molecule, respectively. Our first-principles findings indicate that BNNTs might be an unsuitable material for natural gas storage.     

Comparative experimental study of methane adsorption on multi-walled carbon nanotubes and granular activated carbons

In this study, methane storage capacity of granular activated carbons (GACs) and two types of multi-walled carbon nanotubes (MWCNTs) was investigated and compared. An experimental apparatus consisting a dual adsorption vessel was set up for measurement of equilibrium adsorption of methane on adsorbents using volumetric technique at pressure range of 0–50?bar at different temperatures. The first type of MWCNs has shown lower methane uptake (4.5?mmol?g^?1) compared to GACs (6.5?mmol?g^?1) at the temperature of 283.15?K and the pressure of 50?bar, while 33?mmol?g^?1 of methane storage capacity was achieved using the second type of MWCNTs that is much higher than methane storage on GACs at the same operating conditions. The superior uptake performance for the second type of MWCNTs can be attributed to its specific characteristics such as smaller pore size and higher pore volume. The experimental data of adsorption were almost equally well described by Langmuir, Freundlich and Sips equations to determine the model isotherms. The isosteric heat of methane adsorption on the adsorbent was calculated based on Clausius–Clapeyron and the Sips isotherm model using the experimental data at different temperatures. Results revealed that the isosteric heat of methane adsorption on MWCNTs was lower than the heat of methane adsorption on GACs. Low values obtained for isosteric heats of adsorption indicated dominance of physisorption mechanism for all adsorbents. In general, the obtained data indicated that some well-structured MWCNTs with uniform and narrow size distribution as well as higher pore volume are potential materials for methane storage and deserve further study.     

Synthesis of multiwall boron nitride nanotubes dependent on crystallographic structure of boron

Synthesis and growth of multiwall boron nitride nanotubes (BNNTs) under the B and ZrO₂ seed system in the milling–annealing process were investigated. BNNTs were synthesized by annealing a mechanically activated boron powder under nitrogen environment. We explored the aspects of the mechanical activation energy transferred to milled crystalline boron powder producing structural disorder and borothermal reaction of the ZrO₂ seed particles on the synthesis of BNNTs during annealing. Under these circumstances, the chemical reaction of amorphous boron coated on the seed nanoparticles with nitrogen synthesizing amorphous BN could be enhanced. It was found that amorphous BN was crystallized to the layer structure and then grown to multiwall BNNTs during annealing. Especially, bamboo-type multiwall BNNTs were mostly produced and grown to the tail-side of the nanotube not to the round head-side. Open gaps with ∼0.3 nm of the bamboo side walls of BNNTs were also observed. Based on these understandings, it might be possible to produce bamboo-type multiwall BNNTs by optimization of the structure and shape of boron coat on the seed nanoparticles.     

Self-assembly of carbon nanotubes and boron nitride nanotubes into coaxial structures

Coaxial carbon nanotube/boron nitride nanotube (CNT/BNNT) multi-walled structures are ideal components in nanoelectronic systems. Our molecular dynamics simulations show that separate CNTs and BNNTs can self-assemble into stable coaxial structures in water under appropriate conditions. In case study three types of representative coaxial structures: (5, 5) CNT/(10, 10) BNNT, (5, 5) BNNT/(10, 10) CNT and (5, 5) BNNT/(10, 10) BNNT are obtained. Simulation results also reveal that the self-assembly time between two separate BNNTs is increased remarkably due to the polarization of BNNTs in water. The mechanism of self-assembly among these tubes is demonstrated in detail. Further, coaxial (10, 10) BNNT/(10, 10) CNT/(15, 15) BNNT nanoheterojunctions are achieved for potential application in nanoelectronic systems. The present work shows the feasibility to fabricate the coaxial nanodevices such as insulating high-strength cables, high frequency oscillators and nanojunctions using self-assembly approach.     

Encapsulation of cellulose chain into carbon nanotubes and boron nitride nanotubes

Encapsulation of cellulose chain into carbon nanotubes and boron nitride nanotubes was investigated to find out the possibility of band gap engineering in these nanotubes. The structural stability and the electronic properties of the zigzag carbon nanotubes and boron nitride nanotubes filled with cellulose chain were studied using density functional theory. It was found that encapsulation of cellulose chain into nanotubes was an exothermic process. The metallic properties of the carbon nanotubes did not change by cellulose encapsulation. The semiconductor and insulator nanotubes filled with cellulose were shown semiconducting properties. The energy band gap of these tubes was decreased by cellulose encapsulation. The results demonstrated the ability of band gap engineering through the encapsulation of cellulose chain into carbon nanotubes and boron nitride nanotubes.     

Theoretical study of the interactions of carbon monoxide with Rh-decorated (8,0) single-walled carbon nanotubes

Recent laboratory studies have shown that metal nanoparticles-decorated single-walled carbon nanotubes (SWCNTs) can be used to detect carbon monoxide (CO) gases at room temperature, which is known not able to be adsorbed on pure SWCNTs. In this paper, we investigated the Rh-decorated (8,0) SWCNT and its interaction with CO gases by using density functional theory (DFT) methods. Upon Rh atom adsorption, the conductivity of the (8,0) SWCNT and the atomic charges of some carbon atoms around Rh atom are enhanced dramatically. The Rh-adsorption may be thought of as providing “activated” carbon-sites of adsorbing foreign species. Both the Rh-site and the “activated” C-sites are considered as reactivity sites for the adsorption of CO gases. The binding energy is larger for CO-adsorption on the Rh-site than on the “activated” C-sites. Since the interaction between CO gases and the Rh-site is very strong, the Rh-decorated SWCNT is not reusable for CO gases detecting due to the large binding energy. On the other hand, the CO gases can also be adsorbed on the “activated” C-site with the binding energy of about −0.80 eV and 0.12 electrons transfer. The electronic properties have changed dramatically upon the CO gases. These calculation results are useful not only to explain the sensing mechanisms but also to evaluate the potential for SWCNTs-based sensing materials at room temperature.     

Facile synthesis of boron nitride coating on carbon nanotubes

Boron nitride (BN) coating on the surface of carbon nanotubes (CNTs) was synthesized by the direct reaction of NaBH₄ and NH₄Cl in the temperature range of 500–600 °C. X-ray diffraction, field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) confirm the formation of BN coating. It is revealed that the BN coating follows the shape of CNTS without damaging the surface of CNTs, and the elements B and N distribute homogenously along the whole CNTs without chemical bonds between carbon and BN layers. Besides, the oxidation resistance of the CNTs improved a lot after being coated with BN.     

Estimating Bounds on Collisional Relaxation Rates of Spin-Polarized 87Rb Atoms at Ultracold Temperatures

We present quantum scattering calculations for the collisional relaxation rate coefficient of spin-polarized ⁸⁷Rb(f = 2,m = 2) atoms, which determines the loss rate of cold Rb atoms from a magnetic trap. Unlike the lighter alkali atoms, spin-polarized ⁸⁷Rb atoms can undergo dipolar relaxation due to both the normal spin-spin dipole interaction and a second-order spin-orbit interaction with distant electronic states of the dimer. We present ab initio calculations for the second-order spin-orbit terms for both Rb₂ and Cs₂. The corrections lead to a reduction in the relaxation rate for ⁸⁷Rb. Our primary concern is to analyze the sensitivity of the ⁸⁷Rb trap loss to the uncertainties in the ground state molecular potentials. Since the scattering length for the a³Σ⁺_u state is already known, the major uncertainties are associated with the X¹Σ⁺_g potential. After testing the effect of systematically modifying the short-range form of the molecular potentials over a reasonable range, and introducing our best estimate of the second-order spin-orbit interaction, we estimate that in the low temperature limit the rate coefficient for loss of Rb atoms from the f = 2,m = 2 state is between 0.4 × 10⁻¹⁵ cm³/s and 2.4 × 10⁻¹⁵ cm³/s (where this number counts two atoms lost per collision). In a pure condensate the rate coefficient would be reduced by 1/2.     

Stability,electrochemical behaviors and electronic structures of iron hydroxyl-phosphate

Iron hydroxyl-phosphate with a uniform spherical particle size of around 1 μm, a compound of the type Fe_2−y□_y(PO₄)(OH)_3−3y(H₂O)_3y−2 (where □ represents a vacancy), has been synthesized by hydrothermal methods. The particles are composed of spheres of diameter <100 nm. The compound exhibits good electrochemical performance, with reversible capacities of around 150 mAh g⁻¹ and 120 mAh g⁻¹ at current densities of 170 mA g⁻¹ and 680 mA g⁻¹, respectively. The stability of crystal structure of this material was studied by TGA and XRD which show that the material remains stable at least up to the temperature 200 °C. Investigation of the electronic structure of the iron hydroxyl-phosphate by GGA + U calculation has indicated that it has a better electronic conductivity than LiFePO₄.     

Thermal properties of BeX (X = S, Se and Te) compounds from ab initio quasi-harmonic method

We report ab initio calculations of the structure, elastic constants, lattice dynamics and thermodynamic properties of BeS, BeSe and BeTe compounds. The fully minimized structure parameters and elastic constants of BeS, BeSe and BeTe compounds are in good agreement with previous theoretical and experimental data. The density functional perturbations theory with quasi-harmonic approximation QHA methods are applied to determine the phonon dispersion relations, phonon density of states, phonon decomposition density of states, and thermal quantities. The computed thermodynamic properties such as Debye temperature is in agreement with the previous work. The vibrational entropy and constant-volume specific heat are shown for the first time.     

A comparative study of the reported performance of ab initio protein structure prediction algorithms

Protein structure prediction is one of the major challenges in bioinformatics today. Throughout the past five decades, many different algorithmic approaches have been attempted, and although progress has been made the problem remains unsolvable even for many small proteins. While the general objective is to predict the three-dimensional structure from primary sequence, our current knowledge and computational power are simply insufficient to solve a problem of such high complexity.Some prediction algorithms do, however, appear to perform better than others, although it is not always obvious which ones they are and it is perhaps even less obvious why that is. In this review, the reported performance results from 18 different recently published prediction algorithms are compared. Furthermore, the general algorithmic settings most likely responsible for the difference in the reported performance are identified, and the specific settings of each of the 18 prediction algorithms are also compared.The average normalized r.m.s.d. scores reported range from 11.17 to 3.48. With a performance measure including both r.m.s.d. scores and CPU time, the currently best-performing prediction algorithm is identified to be the I-TASSER algorithm. Two of the algorithmic settings--protein representation and fragment assembly--were found to have definite positive influence on the running time and the predicted structures, respectively. There thus appears to be a clear benefit from incorporating this knowledge in the design of new prediction algorithms.     

Stability and electronic properties of armchair boron nitride/carbon nanotubes

In this work are studied the electronic and structural properties of armchair boron nitride/carbon nanotubes using first principles calculations. The density functional within the generalized gradient approximation (HSEh1PBE-GGA) is used. For each composition, different bonding schemes for the construction of the hybrid systems were employed. Among them, structural stability with neutral charge was determined for the following compositions: T1: B₄₀N₃₅C₇₅H₂₀, T2: B₃₅N₄₀C₇₅H₂₀, T3: B₃₇N₃₈C₇₅H₂₀, T4 : B₃₇N₃₇C₇₆H₂₀, and T7: B₃₅N₃₅C₈₀H₂₀. All these hybrid nanotubes have high polarity; the T3, T4 and T7 are semiconductors: whereas T1 and T2 are conductor in character. The formers also have magnetic behavior. These properties together with a low-chemical potential suggest applications as nano-vehicle for drug delivery. These mixed nanotubes also have potential applications in the electronic devices based on the small work function.     

Can a Pressure Standard be Based on Capacitance Measurements?

We consider the feasibility of basing a pressure standard on measurements of the dielectric constant ϵ and the thermodynamic temperature T of helium near 0 °C. The pressure p of the helium would be calculated from fundamental constants, quantum mechanics, and statistical mechanics. At present, the relative standard uncertainty of the pressure u_r(p) would exceed 20 × 10⁻⁶, the relative uncertainty of the value of the molar polarizability of helium A_ϵ calculated ab initio. If the relativistic corrections to A_ϵ were calculated as accurately as the classical value is now known, a capacitance-based pressure standard might attain u_r(p) < 6 × 10⁻⁶ for pressures near 1 MPa, a result of considerable interest for pressure metrology. One obtains p by eliminating the density from the virial expansions for p and ϵ − 1. If ϵ − 1 were measured with a very stable, 0.5 pF toroidal cross capacitor, the small capacitance and the small values of ϵ − 1 would require state-of-the-art capacitance measurements to achieve a useful pressure standard.     

Deformation behavior of complex carbon nitride and metal nitride based bi-layer coatings

A bi-layer coating consisting of a TiAlCrN inner layer and complex carbon nitride (CN_x) plus CrCN outer layer was deposited on to a high speed steel (M2) substrate via physical vapour deposition (PVD). Detailed microstructural analysis of the coating has been performed via transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). XPS analysis indicated that the outer layer contained both C-C bonds, associated with an amorphous phase, and the presence of CrCN, which was confirmed by TEM analysis. Localized deformation of the coating was performed using nanoindentation with a spherical indenter. Force-displacement curves of the indentation tests exhibited a number of ‘pop-ins’ during the loading cycle, indicative of cracking and other deformation events. Cross-sectional analysis of the indents revealed extensive cracking in the TiAlCrN layer and shear steps at the steel-nitride interface, consistent with the events observed in the force-displacement curves. On the other hand minimal cracking was observed in the CN_x + CrCN layer. It is believed that the relatively ductile outer layer inhibits the propagation of cracks from the inner, brittle layer.     

Density functional theory investigation of the mechanical properties of single-walled carbon nanotubes

We detail the results of our first-principles study based on density functional theory on the elastic properties of (6, 6) single-walled carbon nanotubes (SWCNTs) in both periodic and non-periodic systems. The Young’s modulus and the shear modulus of nanotubes were evaluated through applying axial and torsion strains on periodic, H-, and C-capped nanotubes. Based on our first-principles calculations, the Young’s modulus of the periodic nanotube tens to increase as the nanotube’s length increases, and finally approaches a constant value at long tube lengths. It was found that the Young’s modulus characteristic of H- and C-capped nanotubes exhibit contradictory behaviors during compression with the periodic nanotube. Our calculations also predict that the Young’s and Shear moduli of C-capped nanotubes are larger than those of other types of nanotubes.     

DFT study of the coverage effects for Al adsorption on Si(1 1 1) surfaces

Density functional theory (DFT) calculations have been carried out to investigate the interactions between the Si(1 1 1) surface and the Al adatoms. Different adsorption sites and coverage effects have been considered. For low Al coverage, the threefold-filled adsorption site is the most energy favored site. With the increase of Al coverage, adatom-adatom interactions become increasingly important and Al atomic chains or clusters are formed. With the clean Si(1 1 1) surface of metallic feature, we found that 1/3 ML Al adsorption leads to a semiconducting surface. The results for the electronic behavior suggest the formation of the polarized covalent bonding between the Al adatom and the Si(1 1 1) surface.     

Relativity and the nobility of gold

Relativistic effects are known to play a fundamental role in determining the properties of gold, even imparting it with a yellow color. Here we use ab initio calculations and an evolving theory that allows the electron charge density to be partitioned into chemically meaningful structures to show that these effects also give rise to gold's well-known resistance to oxidation by making oxygen insoluble in gold. We go on to show that silver, despite having the same lattice constant and oxygen binding energy as gold, is extremely susceptible to oxidation because the relativistic effects in silver are smaller than in gold.     

流化床CVD法制备单壁碳纳米管:反应温度与时间的影响

基于负载法Fe—MgO体系催化剂研究了化学气相沉积（CVD）法在流化床反应器中反应温度和时间对单壁碳纳米管（SWCNTs）生长的影响。通过气相色谱对尾气进行实时在线分析，获得了CH4在反应过程中的转化率随时间的变化规律。并对不同反应温度和反应时间所获产品进行了TEM、Raman和TGA等表征。结果表明，900℃是最佳的反应温度，反应温度过低会降低催化剂活性，反应温度过高则容易使催化剂过快失活。在合适的反应温度下，反应前10min催化剂的平均活性较高，能够得到较高质量的SWCNTs，10min后催化剂基本失活。     

Experimental and theoretical analyses of ZnO nanoparticles deposited onto single-wall carbon nanotubes

Experimental and theoretical studies were performed on Single Wall Carbon Nanotubes (SWCN) decorated with ZnO nanoparticles located on the surface of the SWCN. Measurements of High Resolution Transmission Electron Microscopy (HRTEM) and Photoluminescence were performed, and theoretical calculations were carried out under the Extended Hückel Tight Binding Approximation. HRTEM results provide information about the existence of SWCN with ZnO nanoparticles on top. A considerably enhancement in the Photoluminescence (PL) behavior of the hybrid system was obtained as compared to pristine SWCN. Calculated energy bands of the pristine SWCN provide an energy gap of 0.06 eV. When the ZnO cluster is located onto the SWCN the new system behaves as a metal. Analysis of total and Partial Density of States (PDOS) provides information about each atom orbital contribution to the total DOS. It is shown that carbon contributes with p-orbitals while Zn contributes with p- and a very small contribution of d-orbitals. The overlap of all these contributions produces hybridized orbitals, which may likely be responsible for the enhancement of Photoluminescence on the new hybrid system.     

Electronic properties of magnetically doped nanotubes

Effect of doping of carbon nanotubes by magnetic transition metal atoms has been considered in this paper. In the case of semiconducting tubes, it was found that the system has zero magnetization, whereas in metallic tubes the valence electrons of the tube screen the magnetization of the dopants: the coupling to the tube is usually antiferromagnetic (except for Cr).