Hydrogen adsorption on N-decorated single wall carbon nanotubes

Using density functional theory and molecular dynamics we found that N-decorated single walled (8,0) carbon nanotubes are potential high capacity hydrogen storage media. This system could store up to 6.0 wt% hydrogen at 300 K and ambient pressure, with average adsorption energy of −80 meV/(H₂). Nitrogen coverage was C₈N.     

Thermal conductance for single wall carbon nanotubes

We report a theoretical analysis of the phonon thermal conductance, κ(T), for single wall carbon nanotubes (SWCN). In a range of low temperatues up to 100 K, κ(T) of perfect SWCN is found to increase with temperature, approximately, in a parabolic fashion. This is qualitatively consistent with recent experimental measurements where the tube-tube interactions are negligibly weak. When the carbon-carbon bond length is slightly varied, κ(T) is found to be qualitatively unaltered which implies that the anharmonic effect does not change the qualitative behavior of κ(T). Received 12 June 2001     

Exciton-phonon coupling strength in single wall carbon nanotubes

The coupling strength of the exciton and different phonons, including the radial breathing mode (RBM), longitudinal (LO) and transverse (TO) optical phonons, are calculated for different diameter single wall carbon nanotubes (SWNTs) in the framework of tight-binding model. It is found that the exciton-phonon coupling strength with the LO mode or RBM shows a clear (2n+m)-family behavior due to the trigonal warping effect, but it with the TO mode remains to be zero. In the same SWNT, the E₂₂ exciton-phonon coupling strength is found to be slightly smaller than that of E₁₁ exciton. Finally, the exciton-RBM-phonon coupling strengths for several SWNTs are found to be in good agreement with the recent experimental data [Phys. Rev. Lett. 98, 037405 (2007)].     

Nitrogen ion implantation in single wall carbon nanotubes

We report an X-ray photoemission and electron energy loss study on 3 keV ion implantation in single wall carbon nanotubes. Our results show that nitrogen atoms can bind to carbon in tetrahedral sp³, defects related pyridine-like, and triangular sp² configurations and such bondings are stable for annealing up to 650 K. Heating at higher temperatures results in preferential substitutional nitrogen doping. This technique opens a new channel for controlled doping in carbon nanotubes for device applications.     

Hydrogen adsorption on Pt-decorated closed-end armchair (3,3), (4,4) and (5,5) single-walled carbon nanotubes

ABSTRACT

Structures of small lengths of capped (3,3), (4,4) and (5,5) single-walled carbon nanotubes (SWCNTs) and their structures decorated by Pt atom and Pt_n clusters (n = 2–4) were obtained using density functional theory calculations. Binding abilities of Pt atom and Pt_n clusters on the outer surface of SWCNTs at various adsorption sites were explored. Adsorptions of H₂ onto Pt atom of the Pt-decorated (3,3), (4,4) and (5,5) SWCNTs were studied and their adsorption energies are reported. The thermodynamic properties and equilibrium constants for H₂ adsorptions on the Pt₄-decorated (3,3), (4,4) and (5,5) SWCNTs were obtained. The adsorption of H₂ on the Pt atom of the Pt₄/(3,3) SWCNT was found to be the most preferred reaction of which enthalpy and free energy changes at room temperature are ?46.61 and ?23.99 kcal/mol, respectively.     

Hydrogen adsorption on activated carbon nanotubes with an atomic-sized vanadium catalyst investigated by electrical resistance measurements

Activated multi-walled carbon nanotubes were prepared with appended vanadium as a hydrogen storage medium. The pore structure was significantly improved by an activation process that was studied using Raman spectroscopy, field emission transmission electron microscopy and pore analysis techniques. X-ray photoelectron spectroscopy and X-ray diffraction results reveal that the vanadium catalyst was introduced into the carbon nanotubes in controlled proportions, forming V₈C₇. The improved pore structure functioned as a path through the carbon nanotubes that encouraged hydrogen molecule adsorption, and the introduced vanadium catalyst led to high levels of hydrogen storage through the dissociation of hydrogen molecules via the spill-over phenomenon. The hydrogen storage behavior was investigated by electrical resistance measurements for the hydrogen adsorbed on a prepared sample. The proposed mechanism of hydrogen storage suggests that the vanadium catalyst increases not only the amount of hydrogen that is stored but also the speed at which it is stored. A hydrogen storage capacity of 2.26 wt.% was achieved with the activation effects and the vanadium catalyst at 30 °C and 10 MPa.     

Study on the electronic structure and hydrogen adsorption by transition metal decorated single wall carbon nanotubes

The ground state geometry and electronic structure of various 4d transition metal (TM) atom (Y, Zr, Nb and Mo) decorated single wall carbon nanotubes (SWCNTs) are obtained using density functional theory and the projector augmented wave (PAW) method. We found a systematic change in the adsorption site of the transition metal atom with increasing number of d electrons. We also predicted that Y and Zr decorated SWCNTs are metallic whereas Nb and Mo decorated SWCNTs are semiconducting. From detailed electronic structure and Bader charge analysis we found that the systematic variation of the adsorption site with the number of d electrons is related to the decreasing amount of charge transfer from the TM atom to the SWCNT along the 4d series. We have also studied the hydrogen adsorption capabilities of these decorated SWCNTs to understand the role of transition metal d electrons in binding the hydrogen molecules to the system. We found that metallic SWCNT + TM systems are better hydrogen adsorbers. We showed that the hydrogen adsorption by a TM decorated SWCNT will be maximum when all the adsorptions are physisorption and that the retention of magnetism by the system is crucial for physisorption.     

单壁碳纳米管环离散谱和连续谱间的转变

考虑卷曲效应，构造了扶手形单壁碳纳米管环的单Π轨道紧束缚模型.利用波函数分解方法导出了原子间相互作用矩阵元，由此研究了扶手形碳纳米管环的电子性质.随环半径改变，观察到电子结构发生从离散谱到连续谱之间的转变.计算也表明随管半径改变，其能谱也有类似的变化. 关键词： 碳纳米管环 卷曲效应 电子结构     

Nucleation of single wall carbon nanotubes of various chiralities

A simple model for the nucleation and growth of single wall carbon nanotubes from a graphene sheet at the surface of a metallic catalyst saturated in carbon is developed. It enables to predict the geometry and energy of tube embryos of all possible chiralities, as well as the way that they can grow. It is shown that armchair-like chiralities are energy preferred for geometrical reasons. This result is discussed and compared to experimental literature.     

Optical properties of single wall carbon nanotubes dispersed in biopolymers

In this paper we report the optical studies of single wall carbon nanotubes dispersed in biomaterials. We have obtained very stable suspensions of SWNTs, which allowed us to get good photoluminescence signal from the individually dispersed nanotubes. These new hybrid systems may find some applications in bionanocomposites with photoluminescence properties and in biosensors. Furthermore, the dispersion of carbon nanotubes in these biocompatible materials is important for evaluating the toxicity of either isolated or lightly bundled single wall carbon nanotubes.     

Phonon and thermal properties of achiral single wall carbon nanotubes

A detailed theoretical study of the phonon and thermal properties of achiral single wall carbon nanotubes has been carried out using force constant model considering up to third nearest-neighbor interactions. We have calculated the phonon dispersions, density of states, radial breathing modes (RBM) and the specific heats for various zigzag and armchair nanotubes, with radii ranging from 2.8 Å to 11.0 Å. A comparative study of phonon spectrum with measured Raman data reveals that the number of Raman active modes for a tube does not depend on the number of atoms present in the unit cell but on its chirality. Calculated phonon modes at the zone center more or less accurately predicted the Raman active modes. The radial breathing mode is of particular interest as for a specific radius of a nanotube it is found to be independent of its chirality. We have also calculated the variation of RBM and G-band modes for tubes of different radii. RBM shows an inverse dependence on the radius of the tube. Finally, the values of specific heat are calculated for various nanotubes at room temperature and it was found that the specific heat shows an exponential dependence on the diameter of the tube.     

Effects of textural and surface characteristics of microporous activated carbons on the methane adsorption capacity at high pressures

The objective of this study is to relate textural and surface characteristics of selected microporous activated carbons to their methane storage capacity. In this work, a magnetic suspension balance (Rubotherm, Germany) was used to measure methane adsorption isotherms of several activated carbon samples. Textural characteristics were assessed by nitrogen adsorption on a regular surface area analyzer (Autosorb-MP, by Quantachrome, USA). N₂ adsorption was analysed by conventional models (BET, DR, HK) and by Monte Carlo molecular simulations. Elemental and surface analyses were performed by X-ray photoelectronic spectroscopy (XPS) for the selected samples. A comparative analysis was then carried out with the purpose of defining some correlation among the variables under study. For the system under study, pore size distribution and micropore volume seem to be a determining factor as long as the solid surface is perfectly hydrophobic. It was concluded that the textural parameters per se do not unequivocally determine natural gas storage capacities. Surface chemistry and methane adsorption equilibria must be taken into account in the decision-making process of choosing an adsorbent for gas storage.     

Microwave absorbing properties of hydrogen plasma in single wall carbon nanotubes

The complex permittivity of hydrogen plasma in single wall carbon nanotubes (SWNTs), which were grown by iron-catalyzed high-pressure disproportionation (HiPco), is deduced using double-fluid theory and phenomenological model. We calculate the absorptional attenuations of microwave in HiPco SWNTs in the range of 300 MHz–30 GHz under different conditions. The experimental phenomenon of strong microwave absorption (around 2.45 GHz) by hydrogen plasma in HiPco SWNTs can be explained numerically. The influences of the liberal electron density and electronic effective collision frequency on microwave absorption are presented theoretically.     

Water vapor adsorption onto activated carbons prepared from cattle manure compost (CMC)

Activated carbons were prepared from cattle manure compost (CMC) using zinc chloride activation. The structural and surface chemical characteristics of CMC-based activated carbons were determined by N₂ adsorption-desorption and Boehm titration, respectively. The water vapor adsorption properties of the prepared activated carbons with various pore structure and surface nature were examined, and the mechanism of water adsorbed onto activated carbon was also discussed. The results show that the adsorption of water vapor on carbons begins at specific active sites at low relative humidity (RH), followed by micropore filling at medium RH through the formation of pentamer cluster of water molecules in the narrow micropores. The water vapor adsorption capacity of activated carbon is predominantly dependent on its pore volume and surface area. Although capillary condensation is not the mechanism for water adsorption onto activated carbon, water can adsorb on narrow mesopore to some extent.     

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.     

微孔对单壁纳米碳管储氢性能的影响

用巨正则蒙特卡罗分子模拟方法研究了单壁纳米碳管中的微孔即单壁纳米碳管基本孔-内管腔和管间孔对单壁纳米碳管储氢性能的影响.与低温下氮气吸附实验结果的比较发现单壁纳米碳管的内管腔是吸附的主要位置.分析单壁纳米碳管内管腔中吸附势的叠加和利用效率，发现管径为2nm左右时单壁纳米碳管内管腔的储氢容量最高.当单壁纳米碳管阵列的管间距增加时，单壁纳米碳管的管间孔也会成为有效的氢吸附位. 关键词： Monte Carlo方法 单壁纳米碳管 储氢 微孔     

Pressure variation of Luttinger liquid parameters in single wall carbon nanotubes networks

We measure electrical transport on networks of single wall nanotube of different origin as a function of temperature T, voltage V and pressure P. We observe Luttinger liquid (LL) behavior, a conductance ∝T^α and a dynamic conductance ∝V^α. We observe a sample dependent P variation of the α parameters, interpreted as Fermi level changes due to pressure induced charge transfer. We show how, through standard four-leads and crossed configuration methods, it is possible to determine α_bulk and α_end, respectively. We study and discuss the pressure and doping level dependences of the number of channels N, the LL parameter g and the intra-rope tube-tube coupling constant U within a phenomenological model.     

Production and characterization of polymer nanocomposite with aligned single wall carbon nanotubes

We reported a simple method to fabricate polymer nanocomposites with single-walled carbon nanotubes (SWNTs) having exceptional alignment and improved mechanical properties. The composite films were fabricated by casting a suspension of single walled carbon nanotubes in a solution of thermoplastic polyurethane and tetrahydrofuran. The orientation as well as dispersion of nanotubes was determined by scanning electron microscopy, transmission electron microscopy and polarized Raman spectroscopy. The macroscopic alignment probably results from solvent-polymer interaction induced orientation of soft segment chain during swelling and moisture curing. The tensile behavior of the aligned nanotube composite film was also studied. At a 0.5 wt.% nanotube loading, a 1.9-fold increase in Young's modulus was achieved.     

First principles study of work functions of single wall carbon nanotubes

We perform first principles calculations on work functions of single wall carbon nanotubes, which can be divided into two classes according to tube diameter (D). For class I tubes (D > 1 nm), work functions lie within a narrow distribution (approximately 0.1 eV) and show no significant chirality or diameter dependence. For class II tubes (D < 1 nm), work functions show substantial changes, with armchair tubes decreasing monotonically with diameter, while zigzag tubes show the opposite trend. Surface dipoles and hybridization effects are shown to be responsible for the observed work function change.