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.     

The fluorination of Butane over cobalt trifluoride

The fluorination of butane over cobalt trifluoride has given a complex mixture of partially fluorinated compounds: 51 of these have been identified, comprising over 99% of the products. Most were polyfluorobutanes but 1-2% were polyfluoro-2-methylpropanes. The reaction has no synthetic utility. There was some selectivity in the fluorination: secondary C-H was converted into C-F more easily then primary, and the ease of replacement of a particular H was reduced by geminal and vicinal fluorines. A computer of the fluorination was only partially successful, perhaps because the fluorination proceeded in part by simple F for H replacement and in part via alkenes: the model only allowed for the former.     

Surface structure and stability of MoSx model clusters

Density functional theory (DFT) computations have been carried out to study the structure and stability of MoSx clusters with the change of sulfur coverage at both Mo and S edges. DFT shows that adding sulfur to the Mo edge is always exothermic. However, deleting corner sulfur from the S edge is exothermic for 67 and 50% sulfur coverages, while deleting edge sulfur from the S edge is endothermic for 33 and 0% sulfur coverages. On the basis of the computed free energies along a wide range of H2S/H2 ratios, it is found that there are two stable structures with 33 and 50% sulfur coverages on the Mo edge by having 100% sulfur coverage on the S edge and one stable structure with 67% sulfur coverage on the S edge by having 0% sulfur coverage on the Mo edge. Under fully sulfiding atmosphere or at a very high H2S/H2 ratio, triangle MoSx structures with 100% sulfur coverage on the Mo edge are computed to be more stable than those with 100% sulfur coverage on the S edge, in agreement with the observation of scanning tunneling microscopy. In addition, the effects of cluster sizes on the surface structures are discussed.     

一种经氟化和热处理在双壁碳纳米管上生成缺陷的方法(英文)

Nanoscale defects in the outer tube to preserve the electrical and optical features of the inner tube can be engineered to exploit the intrinsic properties of double walled carbon nanotubes (DWCNTs) for various promising applications. We demonstrated a selective way to make defects in the outer tube by the fluorination of DWCNTs followed by the thermal detachment of the F atoms at 1000 °C in argon. Fluorinated DWCNTs with different amounts of F atoms were prepared by reacting with fluorine gas at 25, 200, and 400 °C that gave the stoichiometry of CF0.20, CF0.30, and CF0.43, respectively. At the three different temperatures used, we observed preservation of the coaxial morphology in the fluorinated DWCNTs. For the DWCNTs fluorinated at 25 and 200 °C, the strong radial breathing modes (RBMs) of the inner tube and weakened RBMs of the outer tube indicated selective fluorine attachment onto the outer tube. However, the disappearance of the RBMs in the Raman spectrum of the DWCNTs fluorinated at 400 °C showed the introduction of F atoms onto both inner and outer tubes. There was no significant change in the morphology and optical properties when the DWCNTs fluorinated at 25 and 200 °C were thermally treated at 1000 °C in argon. However, in the case of the DWCNTs fluorinated at 400 °C, the recovery of strong RBMs from the inner tube and weakened RBMs from the outer tube indicated the selective introduction of substantial defects on the outer tube while preserving the original tubular shape. The thermal detachment of F atoms from fluorinated DWCNTs is an efficient way to make highly defective outer tubes for preserving the electrical conduction and optical activity of the inner tubes.     

Electronic structure of octagonal boron nitride nanotubes

The effect of an octagonal lattice configuration on a boron nitride nanotube is explored using first principle calculations. Calculations show that the formational energy of an octagonal boron nitride nanotube (o‐BNNT) is an exothermic reaction. Boron and nitrogen atoms within an o‐BNNT have an average of 2.88 electrons and 9.09 electrons, respectively, indicating ionic‐like bonding. In addition, the electronic structure of the octagonal boron nitride nanotube shows semiconductive properties, while h‐BNNT is reported to be an insulator. Additional o‐BNNTs with varying diameters are calculated where the results suggest that the diameter has an effect on the binding energy and bandgap of the o‐BNNT. The defect sites of the o‐BNNT are reactive against hydrogen where a boron defect is particularly reactive. Thus, this work suggests that physical and chemical properties of a boron nitride nanotube can be tailored and tuned by controlling the lattice configuration of the nanotube.     

Effects of fluorine substitution on the edge-to-face interaction of the benzene dimer

To gain some insight into the effects of fluorination on the aromatic-aromatic interactions found in protein-ligand complexes, like those observed in the set of N-(4-sulfamylbenzoyl)benzylamine (SBB) inhibitors bound to Human Carbonic Anhydrase II (HCAII), we have produced potential energy curves for the edge-to-face interactions of a set of fluorinated benzene dimer compounds. All calculations were carried out at the MP2/aug-cc-pVDZ level of theory using the counterpoise method of Boys and Bernardi (Boys, S. F.; Bernardi, F. Mol. Phys. 1970, 19, 553) to account for the basis set superposition error. Fluorine substitutions are made onto the face molecule of the edge-to-face benzene dimer. As one might expect, the substitution of additional fluorines into this system generally resulted in a decrease of the binding energy. It was also found that the positioning of the fluorine substituents on isosubstituted compounds has a large effect on the total binding energy of these types of systems. More specifically, complexes with fluorines that are substituted closer to the hydrogen atoms of the edge benzene will tend to be stabilized by an electrostatic interaction between the partially negative fluorine atoms and the partially positive hydrogen atoms. However, our findings do not explain the recent crystallographic findings for the SBB-HCAII protein-ligand complex, where increased fluorination resulted in closer edge-to-face contacts, which suggests that there are factors, other than edge-to-face aromatic interactions, influencing this system's behavior.     

Electronic structures and transport properties of fluorinated boron nitride nanoribbons

By applying the nonequilibrium Green's functions and the density-functional theory, we investigate the electronic structures and transport properties of fluorinated zigzag-edged boron nitride nanoribbons. The results show that the transition between half-metal and semiconductor in zigzag-edged boron nitride nanoribbons can be realized by fluorination at different sites or by the change of the fluorination level. Moreover, the negative differential resistance and varistor-type behaviors can also be observed in such fluorinated zigzag-edged boron nitride nanoribbon devices. Therefore, the fluorination of zigzag-edged boron nitride nanoribbons will provide the possibilities for a multifunctional molecular device design.     

SiC-doped boron nitride nanotubes: computations of 11B and 14N quadrupole coupling constants

Abstract  

Density-functional theory calculations have been performed to investigate the properties of the electronic structures of silicon–carbon-doped boron nitride nanotubes (BNNTs). The geometries of zigzag and armchair BNNTs were initially optimized and the quadrupole coupling constants subsequently calculated. The results indicate that doping of B and N atoms by C and Si atoms has more influence on the electronic structure of the BNNTs than does doping of B and N atoms by Si and C atoms. The changes of the electronic sites of the N atoms are also more significant than those of the B atoms.     

Hydrogen adsorption on zigzag (8,0) boron nitride nanotubes

The chemical adsorption of H atoms on an (8,0) zigzag boron nitride nanotube is studied using the density functional theory with the supercell method. One to four H atoms per 32 B and 32 N are considered. The results show that H atoms prefer to adsorb on the top sites of adjacent B and N atoms to form an armchair chain along the tube axis. An even-odd oscillation behavior of the adsorption energy of H atoms on the tube is found, and the average adsorption energy of even H atoms is obviously bigger than that of odd H atoms. The results can be understood with the frontier orbital theory. Based on this adsorption behavior, several high-symmetric structures of H adsorbed boron nitride nanotubes with 50% and 100% coverages are studied. The pairs of lines' pattern with 50% coverage has the biggest average adsorption energy per H(2) among the chosen configurations, corresponding to approximately 4 wt % hydrogen storage.     

Fluorination of multiwall nitrogen-doped carbon nanotubes

A film of oriented nitrogen-doped multiwall carbon nanotubes was grown on a silicon substrate as a result of the thermolysis of an acetonitrile + ferrocene mixture. The fluorination of the film by BrF₃ vapor at room temperature removed the substrate; however, the vertical orientation of the nanotubes was not destroyed. Analysis of micrographs of a fluorinated sample obtained with a high-resolution transmission electron micro-scope showed that only the surface walls of the nanotubes were fluorinated. The fluorine concentration of the product as determined from X-ray photoelectron spectroscopy was about 16%. A comparison of the N1s spectra of the starting and fluorinated samples showed that the nitrogen atoms of CN_x nanotubes changed their electronic state as a result of fluorination. Matching of the X-ray photoelectron spectroscopic data with the results of quantum-chemical calculations for fragments of fluorinated nitrogen-doped nanotubes showed that fluorine atoms preferred to attach to pyridine-like nitrogen atoms or to carbon atoms in the ortho or meta positions relative to a nitrogen atom.     

Kinetics of CO oxidation on high-concentration phases of atomic oxygen on Pt(111)

Temperature-programmed reaction spectroscopy (TPRS) and direct, isothermal reaction-rate measurements were employed to investigate the oxidation of CO on Pt(111) covered with high concentrations of atomic oxygen. The TPRS results show that oxygen atoms chemisorbed on Pt(111) at coverages just above 0.25 ML (monolayers) are reactive toward coadsorbed CO, producing CO(2) at about 295 K. The uptake of CO on Pt(111) is found to decrease with increasing oxygen coverage beyond 0.25 ML and becomes immeasurable at a surface temperature of 100 K when Pt(111) is partially covered with Pt oxide domains at oxygen coverages above 1.5 ML. The rate of CO oxidation measured as a function of CO beam exposure to the surface exhibits a nearly linear increase toward a maximum for initial oxygen coverages between 0.25 and 0.50 ML and constant surface temperatures between 300 and 500 K. At a fixed CO incident flux, the time required to reach the maximum reaction rate increases as the initial oxygen coverage is increased to 0.50 ML. A time lag prior to the reaction-rate maximum is also observed when Pt oxide domains are present on the surface, but the reaction rate increases more slowly with CO exposure and much longer time lags are observed, indicating that the oxide phase is less reactive toward CO than are chemisorbed oxygen atoms on Pt(111). On the partially oxidized surface, the CO exposure needed to reach the rate maximum increases significantly with increases in both the initial oxygen coverage and the surface temperature. A kinetic model is developed that reproduces the qualitative dependence of the CO oxidation rate on the atomic oxygen coverage and the surface temperature. The model assumes that CO chemisorption and reaction occur only on regions of the surface covered by chemisorbed oxygen atoms and describes the CO chemisorption probability as a decreasing function of the atomic oxygen coverage in the chemisorbed phase. The model also takes into account the migration of oxygen atoms from oxide domains to domains with chemisorbed oxygen atoms. According to the model, the reaction rate initially increases with the CO exposure because the rate of CO chemisorption is enhanced as the coverage of chemisorbed oxygen atoms decreases during reaction. Longer rate delays are predicted for the partially oxidized surface because oxygen migration from the oxide phase maintains high oxygen coverages in the coexisting chemisorbed oxygen phase that hinder CO chemisorption. It is shown that the time evolution of the CO oxidation rate is determined by the relative rates of CO chemisorption and oxygen migration, R(ad) and R(m), respectively, with an increase in the relative rate of oxygen migration acting to inhibit the reaction. We find that the time lag in the reaction rate increases nearly exponentially with the initial oxygen coverage [O](i) (tot) when [O](i) (tot) exceeds a critical value, which is defined as the coverage above which R(ad)R(m) is less than unity at fixed CO incident flux and surface temperature. These results demonstrate that the kinetics for CO oxidation on oxidized Pt(111) is governed by the sensitivity of CO binding and chemisorption on the atomic oxygen coverage and the distribution of surface oxygen phases.     

Adsorption of transition-metal atoms on boron nitride nanotube: a density-functional study

Adsorption of transition atoms on a (8,0) zigzag single-walled boron nitride (BN) nanotube has been investigated using density-functional theory methods. Main focuses have been placed on configurations corresponding to the located minima of the adsorbates, the corresponding binding energies, and the modified electronic properties of the BN nanotubes due to the adsorbates. We have systemically studied a series of metal adsorbates including all 3d transition-metal elements (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) and two group-VIIIA transition-metal elements (Pd and Pt). We found that many transition-metal atoms can be chemically adsorbed on the outer surface of the BN nanotubes and that the adsorption process is typically exothermic. Upon adsorption, the binding energies of the Sc, Ti, Ni, Pd, and Pt atoms are relatively high (>1.0 eV), while those of V, Fe, and Co atoms are modest, ranging from 0.62 to 0.92 eV. Mn atom forms a weak bond with the BN nanotube, while Zn atom cannot be chemically adsorbed on the BN nanotube. In most cases, the adsorption of transition-metal atoms can induce certain impurity states within the band gap of the pristine BN nanotube, thereby reducing the band gap. Most metal-adsorbed BN nanotubes exhibit nonzero magnetic moments, contributed largely by the transition-metal atoms.     

Effect of surface fluorination on the electrochemical behavior of petroleum cokes for lithium ion battery

Surface structure change and electrochemical behavior of fluorinated petroleum coke samples (petroleum cokes: petroleum coke and those heat-treated at 1860 °C, 2300 °C and 2800 °C, abbreviated to PC, PC1860, PC2300 and PC2800, respectively) have been investigated. Surface oxygen of petroleum coke was decreased by the fluorination using elemental fluorine. Raman and EPR spectroscopies revealed that surface fluorination increased surface disorder and lattice defects. ¹⁹F NMR spectrum suggests that distribution of fluorine atoms in PC fluorinated 300 °C was similar to that in graphite fluoride with covalent CF bonds. Surface areas of fluorinated petroleum cokes were nearly the same as those of non-fluorinated ones or only slightly increased by fluorination, except PC fluorinated at 300 °C. It is noted that first coulombic efficiencies of PC2300 and PC2800 were highly increased to 80-84% by the fluorination at 300 °C. These values of 80-84% were 12-18% higher than those of non-fluorinated PC2300 and PC2800.     

A computational study of tetrafluoro-[2.2]cyclophanes

A computational study of the isomers of tetrafluorinated [2.2]cyclophanes persubstituted in one ring, namely F4-[2.2]paracyclophane (4), F4-anti-[2.2]metacyclophane (5a), F4-syn-[2.2]metacyclophane (5b), and F4-[2.2]metaparacyclophane (6a and 6b), was carried out. The effects of fluorination on the geometries, relative energies, local and global aromaticity, and strain energies of the bridges and rings were investigated. An analysis of the electron density by B3PW91/6-31+G(d,p), B3LYP/6-31+G(d,p), and MP2/6-31+G(d,p) was carried out using the natural bond orbitals (NBO), natural steric analysis (NSA), and atoms in molecules (AIM) methods. The analysis of frontier molecular orbitals (MOs) was also employed. The results indicated that the molecular structure of [2.2]paracyclophane is the most affected by the fluorination. Isodesmic reactions showed that the fluorinated rings are more strained than the nonfluorinated ones. The NICS, HOMA, and PDI criteria evidenced that the fluorination affects the aromaticity of both the fluorinated and the nonfluorinated rings. The NBO and NSA analyses gave an indication that the fluorination increases not only the number of through-space interactions but also their magnitude. The AIM analysis suggested that the through-space interactions are restricted to the F4-[2.2]metacyclophanes. In addition, the atomic properties, computed over the atomic basins, gave evidence that not only the substitution, but also the position of the bridges could affect the atomic charges, the first atomic moments, and the atomic volumes.     

Plasma fluorination of organosilicon polymeric films for gas separation applications

SF₆ plasma treatment using an RF discharge was carried out for the surface fluorination of polytrimethylsilylpropyne (PTMSP) and polyvinyltrimethylsilane (PVTMS) films. Gas permeation of the fluorinated and untreated films for O₂, N₂, He, H₂, CH₄ and CO₂ gases has been measured. Plasma fluorination increases the ideal selectivities of the PTMSP films decreasing their permeances for all the gases measured, and does not affect the permeances and selectivities of the PVTMS films. The composition and chemical structure of the fluorinated polymer surface were investigated using X-ray photoelectron spectroscopy (XPS) and ¹⁹F nuclear magnetic resonance (NMR) spectroscopy. Within the range of the treatment parameters studied, permselectivity and surface composition of the fluorinated PTMSP films depend slightly on the treatment time and the density of the fluorine atom flux on the modified surface. The trimethylsilyl substituents are detached and carbon atoms are partially fluorinated during modification. The structure of the fluorinated layer contains crosslinks and unsaturated bonds.     

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.     

水在HfO2(111)和(110)表面的吸附与解离

运用广义梯度近似密度泛函理论方法(GGA-PW91)结合周期平板模型, 研究水分子在二氧化铪(111)和(110)表面不同吸附位置在不同覆盖度下的吸附行为. 通过比较不同吸附位的吸附能和几何构型参数发现:(111)和(110)表面铪原子(top 位)是活性吸附位. 水分子与表面的吸附能值随覆盖度的变化影响较小. 在(111)和(110)表面, 水分子都倾向以氧端与表面铪原子相互作用. 同时也计算了羟基、氧和氢在表面的吸附, Mulliken 电荷布居, 态密度及部分频率. 结果表明, 在两种表面羟基以氧端与表面铪相互作用, 氧原子与表面铪和氧原子同时成键, 而氢原子直接与表面氧原子相互作用形成羟基. 通过过渡态搜索, 水分子在(111)和(110)表面发生解离, 反应能垒分别为9.7和17.3 kJ·mol^-1, 且放热为59.9和47.6 kJ·mol^-1.     

Why the hydrothermal fluorinated method can improve photocatalytic activity of carbon nitride

Carbon nitride(CN) photocatalysts have attracted much attention due to their excellent photocatalytic properties.And hydrothermal fluorination is a common method to improve the photocatalytic effect of CN photocatalyst.Here,the influence of the band gap was first revealed of fluorination and hydroxylation of CN photocatalyst based on the first theoretical principle.Here,the effect of fluorination and hydroxylation on the CN band gap was discussed for the first time using the first theoretical principle.With F atoms and OH doping,the band gap of CN was significantly improved,conduction band and valence band moved up.Then,F-CN photocatalyst with F atoms and OH was successfully synthesized by a hydrothermal fluorinated method.Next,the reasons why F-CN photocatalyst was more effective than that of traditional CN photocatalyst were fully discussed.From the photocatalytic effect of photocatalyst(12,593.2 μmolg^-1 h^-1to the morphology(super-small nanosheets),structure(homojunctions),composition(metal-free),specific surface area(54.1 m²/g),visible light absorption response(AQE is10.9% at 420 nm) and photo-induced carrier life(14.13 ns).Therefore,this work has a great guiding effect on the development of CN photocatalyst.     

Effects of coverage on the structures, energetics, and electronics of oxygen adsorption on RuO2(110)

Plane-wave supercell DFT calculations within the PW91 generalized gradient approximation are used to examine the influence of oxygen coverage on the structure, energetics, and electronics of the RuO2(110) surface. Filling of O(br) and O(cus) sites is exothermic with respect to molecular O2 at all coverages and causes changes in local Ru electronic structure consistent with the changing metal coordination. By fitting the surface energies of a large number of surface configurations to a two-body interaction model, an O atom is calculated to be bound by 2.55 eV within a filled O(br) row and by 0.98 eV along an otherwise vacant O(cus) row. Lateral interactions modify these binding energies by up to 20%. O(cus)-O(cus) interactions are repulsive and diminish binding energy with increasing O(cus) filling. Due to the favorable relief of local strain, O(br)-O(br) interactions are attractive and favor filling of neighbor br sites. These interaction effects are relatively modest in absolute magnitude but are large enough to influence the ability of the RuO2(110) surface to promote oxidation of relatively weak reductants, such as NO and C2H4.     

Oxygen adsorption on Al (111) surface interstitial site calculated by density functional theory

The adsorption possibilities of oxygen atoms at Al (111) surface for different oxygen atom coverages (Θ) from 0.25 to 1 ml have been studied using first principles based on density functional theory with generalized gradient approximation. The results show that the interstitial sites on Al (111) surface are relatively stable, in which binding energies are 0.6 ～ 1 eV/atom lower than those on surface face centered cubic (fcc) sites for the different coverages. The binding energy and work function of the oxygen‐adsorbed surface increase with the oxygen atom coverage. Moreover, the oxygen atom at one tetrahedral site of Al (111) subsurface becomes more and more unstable with the decrease of the coverage, and it moves up to the Al (111) surface hexagonal close packed (hcp) site at Θ = 0.25. All the octahedral absorption sites are also unstable in relatively lower coverages (Θ = 0.5 and 0.25). The bond length and overlap population between Al and O, including the relaxation effects on the oxygen atom coverage are discussed. Copyright © 2010 John Wiley & Sons, Ltd.