Molecular hydrogen and oxygen interactions with armchair Si nanotubes

Molecular hydrogen and oxygen adsorptions on a (6, 6) armchair silicon nanotube have been studied by optimizing the distances of the admolecules from both inside and outside the tube. Full geometry and spin optimizations have been performed without any symmetry constraints with an all electron 3-21G* basis set and the B3LYP functional. The molecule is originally placed perpendicular or parallel to the tube axis. Hydrogen adsorption with the molecular axis aligned parallel to the surface of the nanotube is less favorable. Hydrogen molecule does not dissociate while oxygen molecule dissociates after optimization. The on-top site is the only preferred site for hydrogen molecule with an adsorption energy of 3.71 eV and an optimized distance of 3.31 for external adsorption whereas the on-top site is the most preferred site with adsorption energy of 3.69 eV for internal adsorption. For oxygen, the molecule dissociates and the most preferred sites are the two bridge sites with an adsorption energy of 9.64 eV, the optimized distance being 1.65/1.68 Å when it is adsorbed from the outside of the tube. When oxygen molecule is originally placed at on-top site it will hold as a molecule after adsorption with a slightly increased bond length. For the internal adsorption of oxygen, the molecules also dissociate in most cases and the zigzag bridge site is the most preferred site. After molecular adsorption for both hydrogen and oxygen, the buckling of the nanotubes increased. Frustration effects in the nanotube due to molecular adsorption are also noted.     

Titanium-decorated carbon nanotubes as a potential high-capacity hydrogen storage medium

We report a first-principles study, which demonstrates that a single Ti atom coated on a single-walled nanotube (SWNT) binds up to four hydrogen molecules. The first H2 adsorption is dissociative with no energy barrier while the other three adsorptions are molecular with significantly elongated H-H bonds. At high Ti coverage we show that a SWNT can strongly adsorb up to 8 wt % hydrogen. These results advance our fundamental understanding of dissociative adsorption of hydrogen in nanostructures and suggest new routes to better storage and catalyst materials.     

Regular chemisorption of hydrogen on achiral single-walled carbon nanotubes

Regular chemisorption of hydrogen on achiral single-walled carbon nanotubes has been investigated with the use of AM1 quantum-chemical semiempirical method. It has been found that regular hydrogen chemisorption deforms nanotubes, in some cases leading to stable prismatic modifications. The dependence of the adsorption energy on the density of hydrogen coverage has been found. A procedure for determining the adsorption energy by the spectra of thermally stimulated desorption has been proposed.     

O在Au(111)表面吸附的密度泛函理论研究

应用密度泛函理论，本文系统地研究了O在Au(111)表面上的吸附能、吸附结构、功函数、电子密度和投影态密度，给出了覆盖度从0.11ML到1.0ML的范围内，O的吸附特性随覆盖度变化的规律.研究发现O的稳定吸附位为3重面心立方(fcc)洞位，O在fcc洞位的吸附能对覆盖度比较敏感，其值随着覆盖度的增加而减小；O诱导Au(111)表面功函数的变化量与覆盖度成近线性关系，原因是Au表面电子向O偏移，形成表面偶极子；O—Au的相互作用形成成键态和反键态，且反键态都被占据，造成O—Au键很弱，O吸附能较小. 关键词： 表面吸附 Au(111)表面 密度泛函理论 电子特性     

Hydrogen adsorption on SiC nanotube under transverse electric field

Using density functional theory, we present a model to illustrate that under a transverse electric field the overall amount of hydrogen storage can be increased on the SiC nanotube. Due to the cylindrical shape of the nanotube, an electric field does not have the similar effects on the different adsorption sites. Although it has the desired effects on some sites, the electric field may lead the binding energy to decrease on some other sites. We demonstrate that the binding energy decreases slightly just on the two small areas and increases significantly on the largest part of the nanotube surface.     

Molecular dynamics simulations of hydrogen adsorption/desorption by palladium decorated single-walled carbon nanotube bundle

Utilising molecular dynamics simulations, the hydrogen molecules adsorption isotherms of the (8,?0) palladium decorated single-walled carbon nanotube (SWNT) were obtained. The hydrogen adsorption was studied on the external, interstial and internal surfaces of the SWNT bundle at several temperatures ranging from 77 to 400?K. The results were compared with the bare single-walled carbon nanotube bundle under the same conditions. The decorated carbon nanotube bundle hydrogen adsorption was significantly higher than that of the bare one. The hydrogen desorption and readsorption were studied using temperature as the readsorption/desorption variable. The rate constants were calculated for the hydrogen desorption at different temperatures. The calculated decorated SWNT bundle hydrogen desorption activation energy was higher than that for the bare SWNT bundle. The calculated activation energies for the hydrogen desorption in both nanotube bundles specified the temperature dependency of hydrogen desorption.     

The adsorption of hydrogen and the reaction of hydrogen with oxygen on Pt (100)

The adsorption of hydrogen on Pt (100) was investigated by utilizing LEED, Auger electron spectroscopy and flash desorption mass spectrometry. No new LEED structures were found during the adsorption of hydrogen. One desorption peak was detected by flash desorption with a desorption maximum at 160 °C. Quantitative evaluation of the flash desorption spectra yields a saturation coverage of 4.6 × 10¹⁴ atoms/cm² at room temperature with an initial sticking probability of 0.17. Second order desorption kinetics was observed and a desorption energy of 15–16 kcal/mole has been deduced. The shapes of the flash desorption spectra are discussed in terms of lateral interactions in the adsorbate and of the existence of two substates at the surface. The reaction between hydrogen and oxygen on Pt (100) has been investigated by monitoring the reaction product H₂O in a mass spectrometer. The temperature dependence of the reaction proved to be complex and different reaction mechanisms might be dominant at different temperatures. Oxygen excess in the gas phase inhibits the reaction by blocking reactive surface sites. At least two adsorption states of H₂O have to be considered on Pt (100). Desorption from the prevailing low energy state occurs below room temperature. Flash desorption spectra of strongly bound H₂O coadsorbed with hydrogen and oxygen have been obtained with desorption maxima at 190 °C and 340 °C.     

A theoretical study of hydrogen adsorption and diffusion on a W(1 1 0) surface

We have used density functional theory to investigate hydrogen adsorption and diffusion on a W(1 1 0) surface. Hydrogen adsorption structures were examined from low coverage to one monolayer, and a threefold hollow site was found to be the most stable site at all coverages. In contrast to previous assertions, the work function decrease is not due to electron transfer from the hydrogen atoms to the W surface, but due to electron depletion at the vacuum region above the hydrogen atoms. Hydrogen atoms can diffuse via short-bridge sites and long-bridge sites at a coverage of θ = 1.0. Although the calculated activation energy for hydrogen diffusion via a short-bridge site is as small as 0.05 eV, field emission microscope experiments have shown that the activation energy for hydrogen diffusion is about 0.20 eV, which agrees fairly well with our calculated value of the activation energy via a long-bridge site. This discrepancy can be related to hydrogen delocalization on the W(1 1 0) surface, which has been suggested by electron energy loss spectroscopy experiments.     

First-principle study of interaction of molecular hydrogen with BC<Subscript>3</Subscript> composite single-walled nanotube

The physisorption of molecular hydrogen in BC₃ composite single-walled nanotube, investigated using density functional theory, was compared with single-walled carbon nanotube. Both external and internal adsorption sites of these two nanotubes have been studied with the hydrogen molecular axis oriented parallel to the nanotube wall. The calculated results show that: ([see full textsee full text]) the physisorption energies of a H₂ molecule are larger for BC₃(8,0) composite nanotube than for C(8,0) nanotube at all adsorption sites examined. ([see full textsee full text]) For these two nanotubes, the physisorption energies are larger for hydrogen bound inside the nanotubes than for adsorption outside the nanotubes. The different behavior between these two nanotubes is explained by the contour plots of electron density and charge-density difference of them. The present computations suggest that BC₃ nanotube may be a better candidate for hydrogen storage than carbon nanotube.     

Coverage effects on phenol adsorption on Si(1 0 0)2 × 1 as: A first principle calculation

We investigated the adsorption of a 6-dimers Si(1 0 0)2 × 1 surface as a function of coverage and adsorption type (molecular/dissociative) by first principle calculations. In particular, we performed calculations on models with 2, 3, 4 and 6 phenol molecules, corresponding to coverage Θ = 0.34, 0.5, 0.67 and 1. We found that total adsorption energy, when at least one phenol is in a molecular state is lower than the sum of the corresponding singly adsorbed molecules. The dissociative adsorption of multiple molecules, both in parallel and switched configuration is most favoured for a coverage Θ = 0.34 (2.6 eV per adsorbed molecule). This values decreases to 2.0 eV and remains constant till the coverage 1 is reached.The energy barrier for the molecular-to-dissociated transition of a phenol molecule, in presence of another dissociatively adsorbed molecule is ∼0.008 eV and it is similar to the value in case of single adsorption. Possible hydrogen displacements were also considered.     

Adsorption of hydrogen on normal and pentaheptite single wall carbon nanotubes

Density functional calculations of the physisorption of molecular hydrogen and the dissociative atomic chemisorption on the external surface of hexagonal and pentaheptite carbon nanotubes, have been carried out. Physisorption binding energies are near 100 meV/molecule and are similar on metallic and semiconducting nanotubes. Full coverage of the nanotube with one molecule per graphitic hexagon decreases the binding energy per molecule. Chemisorption binding energies per H atom are larger on pentaheptites than on hexagonal carbon nanotubes. The molecular physisorption and dissociative chemisorption states on pentaheptites have very similar total energies (some chemisorbed states are even slightly more stable than the physisorbed states), while on hexagonal carbon nanotubes molecular physisorption is more stable than dissociative chemisorption. However, a substantial energy barrier has to be overcome to go from physisorption to dissociative chemisorption in both types of nanotubes.     

Reactor scale modeling of multi-walled carbon nanotube growth

As the mechanisms of carbon nanotube (CNT) growth becomes known, it becomes important to understand how to implement this knowledge into reactor scale models to optimize CNT growth. In past work, we have reported fundamental mechanisms and competing deposition regimes that dictate single wall carbon nanotube growth. In this study, we will further explore the growth of carbon nanotubes with multiple walls. A tube flow chemical vapor deposition reactor is simulated using the commercial software package COMSOL, and considered the growth of single- and multi-walled carbon nanotubes. It was found that the limiting reaction processes for multi-walled carbon nanotubes change at different temperatures than the single walled carbon nanotubes and it was shown that the reactions directly governing CNT growth are a limiting process over certain parameters. This work shows that the optimum conditions for CNT growth are dependent on temperature, chemical concentration, and the number of nanotube walls. Optimal reactor conditions have been identified as defined by (1) a critical inlet methane concentration that results in hydrogen abstraction limited versus hydrocarbon adsorption limited reaction kinetic regime, and (2) activation energy of reaction for a given reactor temperature and inlet methane concentration. Successful optimization of a CNT growth processes requires taking all of those variables into account.     

Hydrogen adsorption on graphene: a first principles study

We present a systematic ab initio study of atomic hydrogen adsorption on graphene. The characteristics of the adsorption process are discussed in relation with the hydrogenation coverage. For systems with high coverage, the resultant strain due to substrate relaxation strongly affects H atom chemisorption. This leads to local structural changes that have not been pointed out to date, namely localized surface curvature. We demonstrate that the hydrogen chemisorption energy barrier is independent of the optimization technique and system size, being associated with the relaxation and rehybridization of the sole adsorbent carbon atom. On the other hand, the H desorption barrier is very sensitive to a correct structural relaxation and is also dependent on the degree of system hydrogenation.     

H2S在Fe(100)面吸附的第一性原理研究

基于密度泛函理论第一性原理, 在广义梯度近似下, 研究了表面覆盖度为0.25 ML (monolayer)时硫化氢分子在Fe(100)面吸附的结构和电子性质, 并与单个硫原子吸附结果进行了对比. 结果表明: 硫化氢分子吸附在B2位吸附能最小为-1.23 eV, 最稳定, B1位吸附能最大为-0.01 eV, 最不稳定; 并对硫化氢分子在B1位和B2位吸附后的电子态密度进行了分析, 也表明了吸附在B2位稳定, 且吸附在B2位后硫化氢分子几何结构变化不大; 将硫化氢中硫原子吸附与单个硫原子吸附的电子性质进行了比较, 发现前者吸附作用非常微弱; 同时对吸附后的Fe(100)面进行了对比, 单个硫原子吸附的Fe(100)面电子态密度出现了一系列峰值且离散分布, 生成了硫化亚铁, 表明在硫化氢环境下, 主要是硫化氢析出的硫原子发生了吸附. 关键词： 第一性原理 Fe(100)表面 吸附能 硫化氢     

A Theoretical Study of a Single-Walled ZnO Nanotube as a Sensor for H_2 O Molecules

We have studied the property of single-walled ZnO nanotubes with adsorbed water molecules, and theoretically designed a new sensor for detecting water molecules using single-walled ZnO nanotubes using a combination of density functional theory and the non-equilibrium Green's function method. Details of the geometric structures and adsorption energies of the H 2 O molecules on the ZnO nanotube surface have been investigated. Our computational results demonstrate that the formation of hydrogen bonding between the H 2 O molecules and the ZnO nanotube, and adsorption energies of the H 2 O molecules on the ZnO nanotube are larger than the adsorption energies of other gas molecules present in the atmospheric environment. Moreover, the current-voltage curves of the ZnO nanotube with and without H 2 O molecules adsorbed on its surface are calculated, the results of which showed that the H 2 O molecules form stable adsorption configurations that could lead to the decrease in current. These results suggest that the single-walled ZnO nanotubes are able to detect and monitor the presence of H 2 O molecules by applying bias voltages.     

Hydrogen desorption from Ni sites on (110) CuNi surfaces

Hydrogen adsorption on Ni-rich (110) CuNi alloy surfaces has been studied by means of thermal desorption spectroscopy. After adsorption near room temperature the hydrogen desorption spectra exhibit a coverage dependence similar to that known from pure (110)Ni. Besides a slightly composition dependent desorption energy the alloy surfaces behave like a (110)Ni surface diluted by practically inert Cu. These results are compared to those reported by Yu Ling and Spicer.     

Theoretical study of hydrogen adsorption in Ti-decorated capped carbon nanotube

We present ab initio study using dispersion-corrected density functional theory calculations to investigate the hydrogen interaction with Ti-coated, one end closed, single-walled carbon nanotube (SWCNT). Our results demonstrate that a single Ti atom binds up to five hydrogen molecules on SWCNT cap top, whereas adsorption of four hydrogen molecules is energetically more favourable. The analyses from adsorption energy profile, highest occupied molecular orbital–lowest unoccupied molecular orbital gap and Mulliken charge distribution show contrast in first hydrogen molecule adsorption compared with the rest of four configurations. This is clearly due to the strongly different bonding nature of first hydrogen adsorption among others, between hydrogen molecules and Ti-coated SWCNT. These results not only support our understanding of adsorption nature of hydrogen in Ti-coated SWCNTs but also suggest new directions for smart storage techniques.     

Hydrogen chemisorption on <Emphasis Type="BoldItalic">sp</Emphasis><Superscript><Emphasis Type="Bold">2</Emphasis></Superscript>-bonded carbon: Influence of the local curvature and local electronic effects

We report on the interaction of hydrogen with sp²-bonded carbon which has been investigated on graphite (0001), single-walled carbon nanotubes and C₆₀ multilayer films. These substrates have been chosen to represent a large range of curvature in the carbon network. The photoelectron spectroscopy study of the samples treated with atomic hydrogen and low-energy hydrogen ions reveals that hydrogen is chemisorbed on the basal plane of the sp²-bonded carbon networks, as evidenced by the lowered emission from -derived states and a lowering of the electron work function of up to 1.3 eV. The hydrogen adsorption energy barrier is found to strongly depend on the local curvature of the carbon network whereby the barrier is lowered with increasing curvatures. Whereas in the case of C₆₀ and single-walled carbon nanotubes, hydrogen chemisorption can be achieved by exposure to atomic hydrogen, the chemisorption on graphite (0001) requires hydrogen ions of low kinetic energy (1 eV). Furthermore, the adsorption energy barrier is found to increase with hydrogen coverage.The scanning tunnelling microscopy study of individual adsorption sites on the graphite (0001) surface reveals long-ranged (5 nm) electronic effects observed as a (sqrt(3)×sqrt(3))R30° superstructure in the local density of states. It is shown that this superstructure is due to the scattering of delocalized electron wavefunctions at the point defects. The resulting standing waves induce a redistribution of the local density of states which is directly related to the point-like Fermi surface of graphite. PACS 68.43.-h; 71.20.Tx; 68.37.Ef     

A Theoretical Study of a Single-Walled ZnO Nanotube as a Sensor for H2O Molecules

We have studied the property of single-walled ZnO nanotubes with adsorbed water molecules, and theoretically designed a new sensor for detecting water molecules using single-walled ZnO nanotubes using a combination of density functional theory and the non-equilibrium Green's function method. Details of the geometric structures and adsorption energies of the H₂O molecules on the ZnO nanotube surface have been investigated. Our computational results demonstrate that the formation of hydrogen bonding between the H₂O molecules and the ZnO nanotube, and adsorption energies of the H₂O molecules on the ZnO nanotube are larger than the adsorption energies of other gas molecules present in the atmospheric environment. Moreover, the current-voltage curves of the ZnO nanotube with and without H₂O molecules adsorbed on its surface are calculated, the results of which showed that the H₂O molecules form stable adsorption configurations that could lead to the decrease in current. These results suggest that the single-walled ZnO nanotubes are able to detect and monitor the presence of H₂O molecules by applying bias voltages.