Adsorption of atomic hydrogen on the surface of the boron-carbon nanotubes

Results of theoretic studies of geometrical, electronic, and energy composition of monolayer boron-carbon BC₃ nanotubes of the zig-zag type (n, 0) and the cylindrical symmetry are presented. The mechanism of adsorption of atomic hydrogen on the outer surface of the boron-containing BC₃-nanotubes of the (6, 0) type is studied. The calculations are carried out on the basis of models of the ion-incorporated covalent-cyclic cluster using the semiempiric MNDO scheme and density functional (DFT) methods.     

Evaluating the hydrogen chemisorption and physisorption energies for nitrogen-containing single-walled carbon nanotubes with different chiralities: a density functional theory study

The hydrogen adsorption energies for nitrogen-containing carbon nanotubes (N-CNTs) and for bare carbon nanotubes were calculated using the density functional theory methods at the B3LYP/6–31-G(d) level, including dispersion force corrections. The N-CNTs were finite saturated and non-saturated single-walled carbon nanotubes that contained one or more pyrimidine units, the relative positions of which defined the different configurations of the nanotube. The chemisorption of atomic hydrogen to a full exocyclic monolayer of zigzag, armchair, and chiral N-CNTs was studied as a function of the structural parameters. Zigzag N-CNTs of any configuration, with a larger number of nitrogen atoms, a small diameter and a small length, are more reactive compared to chiral and armchair N-CNTs. The presence of nitrogen in the carbon nanotubes enhances their reactivity to chemisorb atomic hydrogen, showing exothermic energy values. In contrast, the physisorption of molecular hydrogen was endothermic for most of the studied saturated N-CNTs, even when including corrections for van der Waals interactions. The endothermicity was greatest for zigzag nanotubes, then decreased for chiral nanotubes and decreased again for armchair nanotubes. In general, the endothermicity decreased for longer nanotubes, which have larger diameters, and a small number of nitrogen atoms. The results of this study suggest that, with saturated bare carbon nanotubes, saturated, and unsaturated N-CNTs could potentially have a higher capacity as hydrogen-storage media than the corresponding unsaturated carbon nanotubes.     

Studies on the encapsulation of F- in single walled nanotubes of different chiralities using density functional theory calculations and Car-Parrinello molecular dynamics simulations

In this study, the encapsulation of F(-) in different nanotubes (NTs) has been investigated using electronic structure calculations and Car-Parrinello molecular dynamics simulations. The carbon atoms in the single walled carbon nanotube (CNT) are systematically doped with B and N atoms. The effect of the encapsulation of F(-) in the boron nitride nanotube (BNNT) has also been investigated. Electronic structure calculations show that the (7,0) chirality nanotube forms a more stable endohedral complex (with F(-)) than the other nanotubes. Evidence obtained from the band structure of CNT calculations reveals that the band gap of the CNT is marginally affected by the encapsulation. However, the same encapsulation significantly changes the band gap of the BNNT. The density of states (DOS) derived from the calculations shows significant changes near the Fermi level. The snapshots obtained from the CPMD simulation highlight the fluctuation of the anion inside the tube and there is more fluctuation in BNNT than in CNT.     

Adsorption of atomic hydrogen on single-walled carbon nanotubes

We have investigated atomic and electronic structures of hydrogen-chemisorbed single-walled carbon nanotubes (SWCNTs) by density functional calculations. We have searched for relative stability of various hydrogen adsorption geometries with coverage. The hydrogenated SWCNTs are stable with coverage of H/C, theta >/= 0.3. The circular cross sections of nanotubes are transformed to polygonal shapes with different symmetries upon hydrogen adsorption. We find that the band gap in carbon nanotubes can be engineered by varying hydrogen coverage, independent of the metallicity of carbon nanotubes. This is explained by the degree of sp(3) hybridization.     

Adsorption and diffusion on a stepped surface: atomic hydrogen on Pt(211)

We present density functional theory calculations for atomic hydrogen interacting with a stepped surface, the Pt(211) surface. The calculations have been performed at the generalized gradient approximation level, using a slab representation of the surface. This is the state-of-the-art method for calculating the interaction of atoms or molecules with metal surfaces, nevertheless only few studies have used it to study atoms or molecules interacting with stepped surfaces, and none, to the best of our knowledge, have considered hydrogen interacting with stepped platinum surfaces. Our goal has been to initiate a systematic study of this topic. We have calculated the full three-dimensional potential energy surface (PES) for the H/Pt(211) system together with the vibrational band structure and vibrational eigenfunctions of H. A deep global minimum of the PES is found for bridge-bonded hydrogen on the step edge, in agreement with experimental results for the similar H/Pt(533) system. All the local vibrational excitations at the global minimum have been identified, and this will serve as a helpful guide to the interpretation of future experiments on this (or similar) system(s). Furthermore, from the calculated PES and vibrational band structure, we identify a number of consequences for the interpretation or modelling of diffusion experiments studying the coverage and directional dependence of atomic hydrogen diffusion on stepped platinum surfaces.     

First-principles calculations of AlN nanowires and nanotubes: atomic structures, energetics, and surface states

We explore the atomic and electronic structures of single-crystalline aluminum nitride nanowires (AlNNWs) and thick-walled aluminum nitride nanotubes (AlNNTs) with the diameters ranging from 0.7 to 2.2 nm by using first-principles calculations and molecular dynamics simulations based on density functional theory (DFT). We find that the preferable lateral facets of AlNNWs and thick-walled AlNNTs are {1010} surfaces, giving rise to hexagonal cross sections. Quite different from the cylindrical network of hexagons revealed in single-walled AlNNTs, the wall of thick-walled AlNNTs displays a wurtzite structure. The strain energies per atom in AlNNWs are proportional to the inverse of the wire diameter, whereas those in thick-walled AlNNTs are independent of tube diameter but proportional to the inverse of the wall thickness. Thick-walled AlNNTs are energetically comparable to AlNNWs of similar diameter, and both of them are energetically more favorable than single-walled AlNNTs. Both AlNNWs and AlNNTs are wide band gap semiconductors accompanied with surface states located in the band gap of bulk wurtzite AlN.     

Adsorption model of atomic hydrogen on the surfaces of carbon nanotubes

A model for the adsorption of atomic hydrogen on the surfaces of single-walled zig-zag and armchair carbon nanotubes is constructed on the basis of the single-impurity periodic Anderson model. Features of the bands caused by the adsorption of hydrogen atoms in the structure of carbon nanotubes are studied. A reduction in the forbidden gap as a result of adsorption is revealed, and its dependence on the diameter of the semiconducting nanotubes is established. It is concluded that the model can be used to study the adsorption of other monovalent atoms on the surfaces of carbon particles.     

The effect of atomic hydrogen adsorption on single-walled carbon nanotubes properties

We investigated the adsorption of hydrogen atoms on metallic single-walled carbon nanotubes using ab initio molecular dynamics method. It was found that the geometric structures and the electronic properties of hydrogenated SWNTs can be strongly changed by varying hydrogen coverage. The circular cross sections of the CNTs were changed with different hydrogen coverage. When hydrogen is chemisorbed on the surface of the carbon nanotube, the energy gap will be appeared. This is due to the degree of the sp³ hybridization, and the hydrogen coverage can control the band gap of the carbon nanotube.     

Interaction of atomic hydrogen with single-walled carbon nanotubes: a density functional theory study

We have studied the interaction of atomic hydrogen with (5,5) and (10,0) single-walled carbon nanotubes (SWNT) using density functional theory. These calculations use Gaussian orbitals and periodic boundary conditions. We compare results from the local spin density approximation, generalized gradient approximation (GGA), and hybrid density functionals. We have first kept the SWNT geometric structure fixed while a single H atom approaches the tube on top of a carbon atom. In that case, a weakly bound state with binding energies from -0.8 to -0.4 eV was found. Full geometry relaxation leads to a strong SWNT deformation, weakening the nearest C-C bonds and increasing the binding energy by about 1 eV. Full hydrogen coverage of the (5,5) SWNT converts this metallic nanotube into an insulator with a band gap of 3.4 eV for the GGA functional and 4.8 eV for the hybrid functional. Hybrid functionals perform similar to pure density functional theory functionals for the calculation of binding energies while band gaps critically depend on the functional choice.     

Determination of surface diffusion coefficients of the hydrogen adatom and surface oh radical on platinum electrode

A localized Pt/Nafion interface was established on a planar polycrystalline platinum electrode and the diffusion flux of adsorbed H atoms and OH radical toward the boundary of the interface was measured as the transient current with the cyclovoltammetric technique. Then the surface diffusion coefficients of adsorbed H-atom and OH radical on platinum surface were estimated by solving the differential equation of surface diffusion This article was submitted by the authors in English.     

DFT and MO calculations of atomic and molecular chemisorption energies on surface cluster models

Summary Density functional theory (DFT) (including gradient corrections) and MCPF calculations have been performed for atomic (H, C, N, O) and molecular CH _x (x = 1–3) chemisorption on cluster models of different sites of the Cu(100) surface. The DFT and MCPF results are in good agreement once the important effects of core-valence correlation have been accounted for in the MCPF calculations by including contributions from a core polarization potential (CPP); in the DFT approach the core-valence correlation is obtained directly from the total density using the functional. Very large effects on the four-fold hollow site binding energy from core-valence correlation are found for C, N and CH. Several different DFT functionals were employed and compared in the calculations.     

First-principles calculations of hydrogen diffusion on rutile TiO2(110) surfaces

Density functional calculations are performed to study the H-atom diffusion on titanium dioxide (110) surface in the cases of water-molecule dissociation and splitting of the adjacent hydroxyl OH pair. It is shown that, when a water molecule is adsorbed at a surface oxygen-vacancy site, a fragment H atom of the water molecule tends to diffuse toward the nearest-neighboring bridging-oxygen sites by using a straight-line or relay-point path. As the result, a pair of surface hydroxyl OH is formed on the same oxygen row. In a thermal process, on the other hand, such OH pair favorably splits only by using a relay-point path, i.e., by transferring one H atom from a bridging-oxygen site to a next-neighboring one along the same oxygen row by way of another in-plane oxygen site. We found that the latter splitting reaction is activated around room temperature.     

Interaction of hydrogen with ZnO: surface adsorption versus bulk diffusion

The interaction of hydrogen (H) and a ZnO(0001)-O surface has been investigated using a temperature programmed desorption (TPD) technique. When the surface is exposed to atomic hydrogen below 400 K, hydrogen is adsorbed on the surface. As the hydrogen exposure increases, bulk diffusion of hydrogen takes place. The existence of surface and bulk hydrogen has been confirmed using X-ray photoelectron spectroscopy (XPS). When the ZnO surface dosed with hydrogen is heated, surface hydrogen is desorbed at 432 K and bulk hydrogen is evolved at ～539 K. Diffusion of hydrogen into the ZnO bulk is an activated process, and the activation energy is estimated to be 0.19 eV.     

Reactions of atomic nitrogen and atomic hydrogen with methylacetylene

The reaction of nitrogen atoms with methylacetylene has been studied using a fast-flow low-pressure reactor coupled to a mass spctrometer by a nozzle-beam sampling system. Hydrogen atom concentrations were measured by ESR analysis. Experimental second-order rate constants for the consumption of N atoms, of C₃H₄, and for the formation of N₂ were determined in the temperature range of 283° to 485°K. Product profiles of all stable species and of hydrogen atoms and methyl radicals were obtained for different initial concentrations of the reactants. Two different reaction pathways can be distinguished: one provides for recombination of N atoms, and the second leads to the formation of cyano compounds and other hydrocarbons. Only the latter process is influenced by the addition of hydrogen atoms. Mechanisms for the two pathways are discussed.     

氢原子在镍台阶面上吸附、振动和表面扩散行为的理论研究

本文用对势方法研究了氢原子在Ni(510)台阶面上的吸附和振动, 计算结果与实验符合得很好。并考察了氢原子在Ni(997)台阶面上的吸附和扩散, 结果表明,台阶对下台面上扩散的氢原子开成捕获势阱, 对上台面扩散的氢原子形成反射势,这也很好地支持了实验结果。     

Interaction of atomic hydrogen with the beta-SiC(100) 3 x 2 surface and subsurface

We investigate clean and atomic hydrogen exposed beta-SiC(100) 3 x 2 surfaces by synchrotron radiation-based Si 2p core-level photoemission spectroscopy. The clean 3 x 2 surface reconstruction exhibits three surface and subsurface components. Upon hydrogen exposures, those surface and subsurface components are shifted to lower binding energies by large values, indicating significant charge transfer to the surface and subsurface regions, in excellent agreement with the recently discovered H-induced beta-SiC(100) 3 x 2 surface metallization. In addition, the interaction of hydrogen results in a large reactive component at Si 2p supporting an asymmetric charge transfer in the third plane below the surface, in agreement with previous experimental investigations. However, the results are inconsistent with recent ab initio theoretical "frozen" calculations predicting H atom to be in a bridge-bond position.     

Water diffusion inside carbon nanotubes: mutual effects of surface and confinement

The mutual effects of two crucial features of carbon nanotubes (CNTs) (surface and confinement) on the temperature-dependent water diffusion are studied through molecular dynamics simulations. A two-stage diffusion mechanism is detected in the CNTs of diameter smaller than 12.2 ?, which becomes obscure as the temperature increases. This peculiar phenomenon can be ascribed to the cooperation of the small confinement and the periodic surface. The diffusion coefficient of the confined water exhibits a nonmonotonic dependence on the confinement size and an unexpected increase inside the large CNTs (compared to that of bulk water). These anomalous behaviors can be attributed to the competition of the smooth surface and the small confinement. Considering the mutual effects, an empirical formula is proposed on the basis of two groups of numerical examples, whose results indicate that the confinement effect will dominate over the surface effect until the CNT diameter increases up to ~16 ?, whereas thereafter the surface effect becomes dominant and finally both of them vanish gradually.     

DFT calculations on the cyclic ethers hydrogen-bonded complexes: molecular parameters and the non-linearity of the hydrogen bond

B3LYP/6-311+G(d,p) calculations were used to explore the geometry, intermolecular energy and the vibrational harmonic spectrum of heterocyclic complexes formed between 2,5-dihydrofuran and thiophene cyclic ethers and the HCl and HF acids. The simulated structures of these hydrogen complexes are discussed in terms of the linearity deviation of the n...HX hydrogen bond. Theoretical results are satisfactory as compared to the experimental equilibrium structure. The energies of the hydrogen bonds were determinate through the difference between the complex and its correspondent isolated monomers. Moreover, to obtain the correct energies of the hydrogen bonds, it was included the values of the zero point vibrational energy and the basis set superposition error. The infrared spectra reveal the direct relationship between the distance of the hydrogen bond and its stretching frequencies, as well as a good interpretation of the bathochromic effect of the HCl and HF stretching modes from intermolecular charge transfer.