Computational study of B- or N-doped single-walled carbon nanotubes as NH3 and NO2 sensors

The adsorption of NH₃ and NO₂ in B- or N-doped (10, 0) single-walled carbon nanotubes (SWCNTs) was investigated by using density functional computations to exploit their potential applications as gas sensors. NH₃ can be chemisorbed only in B-doped SWCNTs with apparent charge transfer, so B-doped SWCNTs can be used as NH₃ sensors. Both B- and N-doping make NO₂ chemisorption feasible in SWCNTs, but the binding of NO₂ with B is too strong, indicating an impractical recovery time as gas sensors. Due to the medium (optimal) adsorption energy and the conductance reduction accompanied with the charge transfer between SWCNTs and gas molecules, N-doped SWCNTs are potentially good NO₂ sensors.     

Hydrogen adsorption studies on single wall carbon nanotubes

Hydrogen adsorption data on as-grown and heat-treated single walled carbon nanotubes (SWNTs) obtained by a volumetric procedure using a Quantachrome Autosorb-1 equipment are presented. The amounts of hydrogen adsorbed at atmospheric pressure reach approximately 0.01 wt.% at 298 K and 1 wt.% at 77 K. The isosteric heat of adsorption has been calculated for both samples from H₂ equilibrium adsorption data at three temperatures, having initial values of 7.42 and 7.75 kJ mol⁻¹. Studies in porous structure by N₂ adsorption and density measurements in helium pycnometer are reported.     

The role of carbon nanotube structure in purification and hydrogen adsorption

Structural properties of carbon nanotubes were studied by using samples from various manufacturers synthesized by different processes. A two-stage purification method was applied to all samples. Relationships between synthesis techniques and carbon nanotube structure are discussed. The role of carbon nanotube structural features such as degree of crystallinity, tube diameter, tube wall structure, and bundling behavior in purification and hydrogen adsorption were investigated by a combination of transmission electron microscopy and magnetic resonance techniques. It is suggested that MWNTs with low crystallinity and SWNTs with large diameters and open tube ends yielded the highest hydrogen uptake capacities. Both MWNTs and SWNTs show low hydrogen storage capacities (less than 1 wt%) at hydrogen pressures up to 1480 kPa.     

Easy method to prepare N-doped carbon nanotubes by ball milling

N-doped carbon materials are promising metal-free catalysts for a number of applications. In this work, a cost effective and easy method to prepare N-doped carbon nanotubes by ball milling was developed, which avoids the use of solvents and production of wastes. Melamine and urea were used as nitrogen precursors. The textural and chemical properties of the N-doped materials were characterized by N₂ adsorption at −196 °C, temperature programmed desorption, elemental analysis, X-ray photoelectron spectroscopy and thermogravimetric analysis. The highest efficiency of N-doping was obtained with melamine. This method leads to the incorporation of large amounts of N-groups, namely quaternary nitrogen, pyridinic, and pyrrolic groups.     

Physisorption of hydrogen in single-walled carbon nanotubes

The interaction of hydrogen with single-walled carbon nanotubes (SWNTs) was analysed. A SWNT sample was exposed to D₂ or H₂ at a pressure of 2 MPa for 1 h at 298 or 873 K. The desorption spectra were measured by thermal desorption spectroscopy (TDS). A main reversible desorption site was observed throughout the range 77 to 320 K. The activation energy of this peak at about 90 K was calculated assuming first-order desorption. This corresponds to physisorption on the surface of the SWNTs (19.2±1.2 kJ/mol). A desorption peak was also found for multi-walled carbon nanotubes (MWNTs), and also for graphite samples. The hydrogen desorption spectrum showed other small shoulders, but only for the SWNT sample. They are assumed to originate from hydrogen physisorbed at sites on the internal surface of the tubes and on various other forms of carbon in the sample. The nanosized metallic particles (Co:Ni) used for nanotube growth did not play any role in the physisorption of molecular hydrogen on the SWNT sample. Therefore, it is concluded that the desorption of hydrogen from nanotubes is related to the specific surface area of the sample.     

单壁纳米碳管的纯化及其储氢特性

针对半连续氢电弧法制备的单壁纳米碳管提出了一种纯化方法。采用HNO3和H2O2回流水煮的方法对单壁纳米碳管进行了纯化处理，透射电镜观察及热重分析表明样品中的无定形炭、纳米碳颗粒及金属催化剂颗粒等杂质可被有效去除，提纯后单壁纳米碳管的收率约为35％，纯度在95％以上；研究发现该纯化方法对单壁纳米碳管的孔径分布和比表面积有较大影响。采用体积法测定了纯化前后单壁纳米碳管样品的储氢容量，结果表明纯化样品的储氢量为1．65％，比未提纯样品（0．56％）有较大提高。     

N-doped anodic titania nanotube arrays for hydrogen production

Titanium dioxide (TiO₂) nanotube arrays are grown in a mixed electrolyte by anodizing process. The anodic nanotubes for N-doping were calcinated at 773 K in a tube furnace with a mixture of NH₃ and Ar gas. The photocatalytic activity of N-doped TiO₂ nanotubes was carried out in a water-splitting reaction under UV and visible light irradiation. Various characterization techniques (Scanning electron microscopy, X-ray diffractometry, X-ray photo-electron spectroscopy, etc.) are used to study the surface morphology, phase of structure, and binding energy.     

Selective interaction of single-walled carbon nanotubes with conducting dendrimer

We report on the transport properties of a system composed of single-wall carbon nanotubes (SWNTs) noncovalently linked to a new electrically conducting dendrimer poly(amidoamine) modified with a substituted naphthalenediimide (PAMAMC). SEM images show how the adsorption of the conducting dendrimer on SWNTs leads to the unroping of the bundles. The adsorption of PAMAMC molecules on SWNTs has been also investigated by electrical transport measurements. The electrical conductance of SWNTs drastically increases upon adsorption of conducting dendrimer. UV–Vis spectroscopy indicates that there was a modification in the electronic structure of the dendrimer as consequence of nanotube introduction while the appearance of new bands on the Raman spectra may suggest that metallic nanotubes are selectively functionalized.     

Adsorption of hydrogen on single-walled carbon nanotubes with defects

We present molecular dynamics (MD) simulations and density functional theory (DFT) calculations of hydrogen adsorption on single-walled carbon nanotubes (SWCNT) with various kinds of defects. The nature of defects, which is characterized here by the number of carbon atoms present in a ring on the surface of nanotube, plays a significant role in determining the hydrogen adsorption capacity of the SWCNT. Nanotubes containing the Stone–Wales defect with 5 and 8-member rings were found to have the largest hydrogen adsorption ability that increases further with the number of rings with such defects. Whereas, the presence of defects with 5, 3-5-8-member rings and the Stone–Wales defect with 5 and 7-member rings decreases the adsorption ability of the defective SWCNT significantly with respect to defect-free nanotubes. Our results indicate that the huge discrepancies in hydrogen storage capacities of SWCNT reported in the literature could be attributed to the nature of defects present in nanotubes. DFT calculations also reveal that the adsorption energy depends not only on the nature and number of defects present on the surface of nanotube but also on the equilibrium structure of rings.     

Adsorption of methane and ethane into single-walled carbon nanotubes and slit-shaped carbonaceous pores

The adsorption equilibria of methane, ethane and their binary mixture in single-walled carbon nanotubes (SWNTs) and slit-shaped carbonaceous pores were studied by using a Grand Canonical Monte Carlo (GCMC) method. We used a slit-shaped pore for microporous structure of activated carbons and an armchair type of cylindrical pore for SWNTs. Methane was modeled as a spherical Lennard-Jones (LJ) model and ethane as two LJ sites with the unified methyl group. The isotherms of both components in micropore region displayed Type I adsorption by Brunauer et al., which corresponds to unimolecular adsorption. At low pressure the storage capacity of SWNTs for pure components of methane and ethane was higher than that for slit-shaped pore geometries of the same size, and the selectivities of equimolar bulk gas mixture were much higher. GCMC was shown to give good qualitative agreement with Ideal Adsorbed Solution Theory (IAST).     

N-doped carbon nanotubes for liquid-phase CC bond hydrogenation

The introduction of foreign elements inside the channel of carbon nanotubes could lead to a significant modification of the intrinsic properties of these nanomaterials. Nitrogen atoms entering in the graphene sheets as substitute of carbon could modify in a large extend the acido-basic properties and also adsorption of the nanotube itself. Depending on the synthesis conditions, i.e. nature of the N-source, temperature and C-to-N atomic ratio, various N-doped carbon nanotubes can be synthesized with different surface properties. The aim of the present work is to report the synthesis of N-doped CNTs using a common nitrogen source precursor namely ammonia (NH₃) with C₂H₆ as carbon source. The as-synthesized N-CNTs were subsequently employed as catalyst support in the liquid-phase hydrogenation of cinnamaldehyde using palladium as an active phase.     

Fundamental electronic properties and applications of single-walled carbon nanotubes

Recent scanning tunneling microscopy studies of the intrinsic electronic properties of single-walled carbon nanotubes (SWNTs) are overviewed in this Account. A brief theoretical treatment of the electronic properties of SWNTs is developed, and then the effects of finite curvature and broken symmetry on electronic properties, the unique one-dimensional energy dispersion in nanotubes, the interaction between local spins and carriers in metallic nanotubes systems, and the atomic structure and electronic properties of intramolecular junctions are described. The implications of these studies for understanding fundamental one-dimensional physics and future nanotube device applications are also discussed.     

A simple method to synthesize continuous large area nitrogen-doped graphene

Large area nitrogen (N)-doped graphene films were grown on copper foil by chemical vapor deposition. The as-grown films consisted of a single atomic layer that was continuous across the copper surface steps and grain boundaries, and could be easily transferred to a variety of substrates. N-doping was confirmed by X-ray photoelectron spectroscopy, Raman spectroscopy, and elemental mapping. N atoms were suggested to mainly form a “pyrrolic” nitrogen structure, and the doping level of N reached up to 3.4 at.%. The N-doped graphene exhibited an n-type behavior, and nitrogen doping would open a band gap in the graphene. This study presents use of a new liquid precursor to obtain large area, continuous and mostly single atom layer N-doped graphene films.     

Adsorption and properties of aromatic amino acids on single-walled carbon nanotubes

We investigated the adsorption of three aromatic amino acids-phenylalanine, tyrosine, and tryptophan-on the sidewalls of a number of representative single-walled carbon nanotubes (SWNTs) using density-functional tight-binding calculations, complemented by an empirical dispersion correction. The armchair (n, n) SWNTs (n = 3-12) and zigzag (n, 0) SWNTs (n = 4-12) were thoroughly examined. We found that the most stable amino acid/SWNT complexes for different SWNTs have similar local structures, and that the distance between the amino acid and SWNT is about 3 ?. Owing to the π-π and H-π stacking interactions, the benzene and indole rings are not exactly parallel to the SWNTs but instead lie at a small angle. We also investigated the diameter and chirality dependences of binding energies and found that SWNT (5, 0) has an especially large binding energy that can be used for SWNT identification or selection.     

Intrinsic linear scaling of hydrogen storage capacity of carbon nanotubes with the specific surface area

The linear scaling of the gravimetric hydrogen storage capacity of single- and multi-walled carbon nanotubes (SWNTs and MWNTs) with the specific surface area is investigated at ambient temperature (298 K) and technically relevant pressures (0.9–1.6 MPa). All samples are found to adsorb hydrogen reversibly and their adsorption exhibits type-II BET isotherms according to the IUPAC classification. While there is strong sample-dependency on their pressure–composition isotherms, all of them follow the Henry's Law in the pressure range under consideration. A comparison of the observed slope of specific surface area versus gravimetric storage capacity with that of a theoretically predicted one using a hypothetical condensation model and that of chemically modified carbon nanotubes revealed that the hydrogen storage capacity depends on the accessibility of internal surfaces of nanostructured carbon. The linear scaling of hydrogen storage capacity with the respective specific surface area suggests that the hydrogen adsorption in carbon nanotubes depends on the specific surface area and is irrespective of the type of the nanotubes that is used.     

Nitrogen doping effects on the structure behavior and the field emission performance of double-walled carbon nanotubes

The nitrogen (N) doping effect and field emission properties of double-walled carbon nanotubes (DWCNTs) were investigated. Diameter transformation and defect generation in the N-doped DWCNTs mainly depend on the amount of nitrogen employed. By applying N-doping into DWCNTs (1.5 N at.%), the average diameters of the DWCNTs were increased from 1.7 to 2.4 nm, and the crystallinity (I_G/I_D) was decreased from 13.5 to 5. Field emission properties were enhanced by the N doping into DWCNTs. The turn-on field, corresponding to a current density of 0.1 μA/cm², was about 0.9 V/μm for the N-doped DWCNTs (1.5 N at.%). The field enhancement factor of the N-doped DWCNTs was higher than that of the undoped DWCNTs. It was found that the field emission properties were controlled by pyridine-like N in the graphite due to N-doping.     

Simultaneous reduction and N-doping of graphene oxides by low-energy N2+ ion sputtering

An easy and catalyst-free method was used to obtain N-doped reduced graphene oxides (N-RGO) through low-energy N₂⁺ ion sputtering of graphene oxides (GO). The simultaneous reduction and N-doping of GO during the sputtering were systematically investigated by X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure and Raman spectroscopy. The N-doping and reduction levels, which are determined by the N/C and O/C atomic ratios from the quantitative XPS analysis, respectively, can be easily controlled by varying the N₂⁺ ion sputtering time. In addition, three different N species, namely, nitrile-like N, graphitic N and pyridinic N, can be distinguished in N-RGO.     

Nanotubes as polymers

In this review, we show that the structure and behavior of single-walled nanotubes (SWNTs) are essentially polymeric; in fact, many have referred to SWNTs as “the ultimate polymer”. The classification of SWNTS as polymers is explored by comparing the structure, properties, phase behavior, rheology, processing, and applications of SWNTs with those of rigid-rod polymers. Special attention is given to research efforts focusing on the use of SWNTs as molecular composites (also termed nanocomposites) with SWNTs as the filler and flexible polymer chains as the host. This perspective of “SWNTs as polymers” allows the methods, applications, and theoretical framework of polymer science to be appropriated and applied to nanotubes.     

Diameter control of single-walled carbon nanotubes by plasma rotating electrode process

We investigated plasma rotating electrode process (PREP) as a method for diameter control of single-walled carbon nanotubes (SWNTs) with a Ni–Y catalyst in He ambient. Compared with the diameters of SWNTs produced by the conventional arc discharge (0 rpm), those of SWNTs, which were synthesized by PREP (5000 and 10 000 rpm), were decreased. This result indicated that the centrifugal force by the rotation of anode could control the diameter distributions of SWNTs by controlling kinetic energies of carbon and/or metal species or formation region of SWNTs.     

碳纳米管对活性污泥胞外聚合物的吸附特性研究

以单壁碳纳米管(SWNTs)为吸附剂,系统地研究了其对活性污泥胞外聚合物(EPS)的吸附特性.结果表明,SWNTs能够快速吸附EPS,当吸附剂投加量为0.9 g/L时,吸附在20 min即可达到平衡,吸附动力学符合准二级动力学模型.吸附等温线能较好地用Langmuir 吸附等温模型来描述,最大单分子吸附量为123.577 mg/g.pH值对吸附有较大影响,最佳吸附pH范围为5～7.