Permanent Trapping of CO(2) in Single-Walled Carbon Nanotubes Synthesized by the HiPco Process

Infrared spectroscopy is used to study trapped and physisorbed CO2 in single-walled carbon nanotube bundles (SWNTs) synthesized by the HiPco process. CO2 is entrapped within the SWNTs by acid oxidation of the unpurified sample followed by vacuum heating to 700 K. The trapped CO2 has a single nu3 mode at 2327 cm-1, is stable during temperature cycling from 77 to 700 K, and remains after venting to room air. CO2 physisorption studies show a nu3 mode at 2330 cm-1 for the as-received HiPco samples, 2340 cm-1 for the acid-oxidized sample, and 2327 and 2340 cm-1 for the oxidized sample after vacuum heating. The sites responsible for the infrared peaks of the physisorbed and trapped species are discussed.     

Cutting of single-walled carbon nanotubes by ozonolysis

Cutting of single-walled carbon nanotubes (SWNT) has been achieved by extensive ozonolysis at room temperature. Perfluoropolyether (PFPE) was selected as a medium for cutting SWNT due to its high solubility for ozone (O3). A mixture of 9 wt % of O3 in O2 was bubbled through a homogeneous suspension of pristine SWNT in PFPE, at room temperature. The intense disorder mode in the Raman spectra of ozonated SWNT indicates that extensive reaction with the sidewalls of SWNT occurs during ozonolysis. Atomic force microscopy (AFM) images of SWNT, before and after ozonolysis, provided a measure of the extent of the cutting effects. Monitoring of the evolved gases for both pristine and purified SWNT indicates CO2 was produced during the ozonolysis process with a dependence on both system pressure and temperature. During heating, FTIR analysis of gases released indicated that carbon oxygen groups on the sidewalls of SWNT are released as CO2. SWNT was found to be extensively cut after an ozone treatment with a yield of approximately 80% of the original carbon.     

Gas sensor array based on metal-decorated carbon nanotubes

Here we demonstrate design, fabrication, and testing of electronic sensor array based on single-walled carbon nanotubes (SWNTs). Multiple sensor elements consisting of isolated networks of SWNTs were integrated into Si chips by chemical vapor deposition (CVD) and photolithography processes. For chemical selectivity, SWNTs were decorated with metal nanoparticles. The differences in catalytic activity of 18 catalytic metals for detection of H(2), CH(4), CO, and H(2)S gases were observed. Furthermore, a sensor array was fabricated by site-selective electroplating of Pd, Pt, Rh, and Au metals on isolated SWNT networks located on a single chip. The resulting electronic sensor array, which was comprised of several functional SWNT network sensors, was exposed to a randomized series of toxic/combustible gases. Electronic responses of all sensor elements were recorded and the sensor array data was analyzed using pattern-recognition analysis tools. Applications of these small-size, low-power, electronic sensor arrays are in the detection and identification of toxic/combustible gases for personal safety and air pollution monitoring.     

Electrochemical functionalization of single-walled carbon nanotubes in large quantities at a room-temperature ionic liquid supported three-dimensional network electrode

Electrochemical functionalization of single-walled carbon nanotubes (SWNTs) was one of selective, clean, and nondestructive chemical methods. But in previous studies it met difficulties in homogeneous electrografting of SWNTs in large quantities because the reaction was often localized on a very thin film (ca. 2 microm). In this report, a room-temperature ionic liquid (RTIL) supported three-dimensional network SWNT electrode was first utilized to break through this barrier. In this work, large quantities of SWNTs were considerably untangled in RTILs so as to greatly increase the effective area of the electrode. N-succinimidyl acrylate (NSA), as a model monomer, was dissolved in the supporting RTILs and was electrografted onto SWNTs (SWNTs-poly-NSA). As an application example, glucose oxidase was directly covalently anchored on the SWNTs-poly-NSA assembly, and the electrocatalytic oxidation of glucose in this assembly was investigated. RTILs opened a new path in electrochemical functionalization of SWNTs.     

Reversible dispersion of single-walled carbon nanotubes based on a CO2-responsive dispersant

A CO(2)-responsive dispersant, N,N-dimethyl-N'-(pyren-1-ylmethyl) acetimidamidinium (PyAH(+)), which bears both a pyrene moiety and an amidinium cation, has been successfully synthesized. Through strong π-π interaction between the pyrene moiety and single-walled carbon nanotubes (SWNTs), we have demonstrated that PyAH(+) can be modified onto SWNT surfaces to promote the dispersion of SWNTs in water. Furthermore, taking advantage of gas triggered interconversions between the amidinium cation and amidine, reversible control on the solubility of SWNTs has been achieved simply through alternated bubbling of CO(2) and Ar. This work has demonstrated a new method for controlled dispersion and aggregation of SWNTs, and it may contribute to the development of gas responsive carbon materials.     

Adsorption of CF4 on the internal and external surfaces of opened single-walled carbon nanotubes: a vibrational spectroscopy study

Infrared spectroscopy has been used to make the first experimental discrimination between molecules bound by physisorption on the exterior surface of carbon single-walled nanotubes (SWNTs) and molecules bound in the interior. In addition, the selective displacement of the internally bound molecules has been observed as a second adsorbate is added. SWNTs were opened by oxidative treatment with O(3) at room temperature, followed by heating in a vacuum to 873 K. It was found that, at 133 K and 0.033 Torr, CF(4) adsorbs on closed SWNTs, exhibiting its nu(3) asymmetric stretching mode at 1267 cm(-1) (red shift relative to the gas phase, 15 cm(-1)). Adsorption on the nanotube exterior is accompanied by adsorption in the interior in the case of opened SWNTs. Internally bound CF(4) exhibits its nu(3) mode at 1247 cm(-1) (red shift relative to the gas phase, 35 cm(-1)). It was shown that, at 133 K, Xe preferentially displaces internally bound CF(4) species, and this counterintuitive observation was confirmed by molecular simulations. The confinement of CF(4) inside (10,10) single-walled carbon nanotubes does not result in the production of lattice modes that are observed in large 3D ensembles of CF(4).     

Sensitivity of ammonia interaction with single-walled carbon nanotube bundles to the presence of defect sites and functionalities

Ammonia adsorption on single-walled carbon nanotubes (SWNTs) was studied by means of infrared spectroscopy at both cryogenic (approximately 94 K) and room (approximately 300 K) temperatures. At 94 K, vacuum-annealed SWNTs showed no detectable ammonia uptake. However, the ammonia adsorption was found to be sensitive to the functionalities and defects on the nanotube surfaces. NH3 adsorption was detected on HNO3-treated nanotubes, characterized by significant functionalities and defects, prior to vacuum annealing. NH3 desorbed from those nanotubes above 140 K, indicating a weak adsorbate-nanotube interaction (approximately 30 kJ/mol). Exposure of annealed samples to ambient air, which possibly regenerated functionalities and defects on nanotube surfaces, restored partially the ammonia uptake capacity. No ammonia adsorption on SWNTs was observed by infrared spectroscopy at room temperature with up to 80 Torr dosing pressure. This work suggests the influence of functionalities and/or defect densities on the sensitivity of SWNT chemical gas sensors. Our theoretical studies on NH3 adsorption on pristine and defective tubes, as well as oxidized tubes, corroborate these findings.     

Comparison of analytical techniques for purity evaluation of single-walled carbon nanotubes

We compare popular analytical techniques, including scanning and transmission electron microscopy (SEM and TEM), thermogravimetric analysis (TGA), and Raman and near-infrared (NIR) spectroscopy, for the evaluation of the purity of bulk quantities of single-walled carbon nanotubes (SWNTs). Despite their importance as imaging techniques, SEM and TEM are not capable of quantitatively evaluating the purity of typical inhomogeneous bulk SWNT samples because the image frame visualizes less than 1 pg of SWNT material; furthermore, there is no published algorithm to convert such images into numerical data. The TGA technique is capable of measuring the amount of metal catalyst in an SWNT sample, but does not provide an unambiguous separation between the content of SWNTs and carbonaceous impurities. We discuss the utilization of solution-phase near-infrared spectroscopy and solution-phase Raman spectroscopy to quantitatively compare arbitrary samples of bulk SWNT materials of different purities. The primary goal of this paper is to provide the chemical community with a realistic evaluation of current analytical tools for the purity evaluation of a bulk sample of SWNTs. The secondary goal is to draw attention to the growing crisis in the SWNT industry as a result of the lack of quality control and the misleading advertising by suppliers of this material.     

Electrochemical templating of metal nanoparticles and nanowires on single-walled carbon nanotube networks

The use of single-walled carbon nanotube (SWNT) networks as templates for the electrodeposition of metal (Ag and Pt) nanostructures is described. Pristine SWNTs, grown on insulating SiO2 surfaces using catalyzed chemical vapor deposition, served as the working electrode. In the simplest case, electrical contact was made by depositing a gold strip on the SWNT substrate (device 1). Deposition of Ag and Pt over extensive periods (30 s) resulted in a high density of particles on the SWNTs, with almost contiguous nanowire formation from the Au/SWNT boundary moving to isolated nanoparticles at further distances from the contact. For direct electrochemical studies of Ag and Pt nucleation, the assembly was coated in a resist layer and a small window opened up to expose only the electrically connected SWNTs to solution (device 2). In this case, the electrochemical signature in voltammetric and amperometric studies of metal deposition was due solely to processes at the SWNTs. Coupled with high-resolution microscopy measurements (atomic force microscopy and field emission scanning electron microscopy), this approach provided detail on the nucleation and growth mechanisms of Ag and Pt on SWNTs under electrochemical control. In particular, Ag growth was found to be rapid and progressive with an increasing nanoparticle density with time, whereas Pt deposition was characterized by lower nucleation densities and slower growth rates with a tendency for larger particles to be produced over long times.     

New Multifunctional Porous Materials Based on Inorganic–Organic Hybrid Single‐Walled Carbon Nanotubes: Gas Storage and High‐Sensitive Detection of Pesticides

Single‐walled carbon nanotubes (SWNTs) that are covalently functionalized with benzoic acid (SWNT‐PhCOOH) can be integrated with transition‐metal ions to form 3D porous inorganic–organic hybrid frameworks (SWNT‐Zn). In particular, N₂‐adsorption analysis shows that the BET surface area increases notably from 645.3 to 1209.9 m² g^?1 for SWNTs and SWNT‐Zn, respectively. This remarkable enhancement in the surface area of SWNT‐Zn is presumably due to the microporous motifs from benzoates coordinated to intercalated zinc ions between the functionalized SWNTs; this assignment was also corroborated by NLDFT pore‐size distributions. In addition, the excess‐H₂‐uptake maximum of SWNT‐Zn reaches about 3.1 wt. % (12 bar, 77 K), which is almost three times that of the original SWNTs (1.2 wt. % at 12 bar, 77 K). Owing to its inherent conductivity and pore structure, as well as good dispersibility, SWNT‐Zn is an effective candidate as a sensitive electrochemical stripping voltammetric sensor for organophosphate pesticides (OPs): By using solid‐phase extraction (SPE) with SWNT‐Zn‐modified glassy carbon electrode, the detection limit of methyl parathion (MP) is 2.3 ng mL^?1.     

Recent progress in chemical detection with single-walled carbon nanotube networks

Single-walled carbon nanotubes (SWNTs) have had significant impact on the development of gas sensors in the last decade. However, useful applications of SWNTs are limited by the lack of manufacturable routes to device formation. This Highlight article chronicles recent progress in this area and demonstrates the great promise of a new room temperature deposition method for SWNT networks in gas sensing applications. This liquid deposition technique allows the deposition of pre-treated, highly aligned SWNT networks on a wide variety of substrates. A significant advantage of SWNT-network sensors is that fluctuations in the electrical response of individual SWNTs become less important as the size of the network increases. Therefore, device properties can be controlled by the overall density of the network rather than the physical properties of any individual SWNT. At densities where semiconducting pathways dominate, highly sensitive thin-film chemoresistive sensors can be fabricated. Such devices also have higher signal-to-noise ratios and are easier to fabricate than devices based on a single SWNT.     

Large-scale fabrication of aligned single-walled carbon nanotube array and hierarchical single-walled carbon nanotube assembly

Aligned single-walled carbon nanotubes (SWNTs) and hierarchical SWNT assembly were fabricated by electrospinning. The high fiber elongation and high DC electric field applied during the electrospinning process result in the orientation of the SWNTs along the axial direction of the fiber. The alignment of the electropsun composite fiber transfers this local SWNT orientation to macroscopically aligned SWNTs. After removing the polymer component from the aligned composite fiber, we produced large area aligned SWNTs. The results show that the directional control of SWNT alignment and debundling of SWNTs into individual tubes can be simultaneously realized.     

Vibrational behavior of adsorbed CO2 on single-walled carbon nanotubes

We present theoretical and experimental evidence for CO(2) adsorption on different sites of single walled carbon nanotube (SWNT) bundles. We use local density approximation density functional theory (LDA-DFT) calculations to compute the adsorption energies and vibrational frequencies for CO(2) adsorbed on SWNT bundles. The LDA-DFT calculations give a range of shifts for the asymmetric stretching mode from about -6 to -20 cm(-1) for internally bound CO(2), and a range from -4 to -16 cm(-1) for externally bound CO(2) at low densities. The magnitude of the shift is larger for CO(2) adsorbed parallel to the SWNT surface; various perpendicular configurations yield much smaller theoretical shifts. The asymmetric stretching mode for CO(2) adsorbed in groove sites and interstitial sites exhibits calculated shifts of -22.2 and -23.8 cm(-1), respectively. The calculations show that vibrational mode softening is due to three effects: (1) dynamic image charges in the nanotube; (2) the confining effect of the adsorption potential; (3) dynamic dipole coupling with other adsorbate molecules. Infrared measurements indicate that two families of CO(2) adsorption sites are present. One family, exhibiting a shift of about -20 cm(-1) is assigned to internally bound CO(2) molecules in a parallel configuration. This type of CO(2) is readily displaced by Xe, a test for densely populated adsorbed species, which are expected to be present on the highest adsorption energy sites in the interior of the nanotubes. The second family exhibits a shift of about -7 cm(-1) and the site location and configuration for these species is ambiguous, based on comparison with the theoretical shifts. The population of the internally bound CO(2) may be enhanced by established etching procedures that open the entry ports for adsorption, namely, ozone oxidation followed by annealing in vacuum at 873 K. Xenon displacement experiments indicate that internally bound CO(2) is preferentially displaced relative to the -7 cm(-1) shifted species. The -7 cm(-1) shifted species is assigned to CO(2) adsorbed on the external surface based on results from etching and Xe displacement experiments.     

单壁碳纳米管上毒性苯气体净化的分子模拟(英文)

采用巨正则系综蒙特卡罗(GCMC)方法研究了空气中微量苯组分在单臂碳纳米管(SWNTs)上的吸附净化.模拟表明,具有较大孔径的(20,20)纳米管比较适合吸附纯苯蒸汽,而对于移除空气中的毒性苯物质,苯的吸附选择性分别在(12,12)纳米管及4.0 MPa时和(18,18)纳米管及0.1 MPa时出现最小值和最大值.为了解释这一异常行为,我们进一步分析了N2-O2-C6H6混合物的局部密度分布、吸附分子构型和概率密度分布,发现(18,18)纳米管内外完全被苯分子占据,而对于(12,12)纳米管,由于存在更强的吸附质-吸附剂相互作用,空气分子更倾向于吸附在管与管之间的间隙.此外,吸附分子的空间有序参数表明大多数苯分子采取"平躺"在纳米管表面的定位,而线性的N2和O2分子则多数平行于孔轴方向.最后研究了温度和苯分子主体相浓度对分离效果的影响.我们发现较大孔中的选择性随着温度的增加比小孔下降更加明显.与此对比,主体相苯浓度对小孔中的选择性起到更加重要的作用.     

Separation of single-walled carbon nanotubes on silica gel. Materials morphology and Raman excitation wavelength affect data interpretation

In this report, procedures are discussed for the enrichment of single-walled carbon nanotube (SWNT) types by simple filtration of the functionalized SWNTs through silica gel. This separation uses nanotube sidewall functionalization employing two different strategies. In the first approach, a crude mixture of metallic and semiconducting SWNTs was heavily functionalized with 4-tert-butylphenyl addends to impart solubility to the entire sample of SWNTs. Two major polarity fractions were rapidly filtered through silica gel, with the solvent being removed in vacuo, heated to 700 degrees C to remove the addends, and analyzed spectroscopically. The second approach uses two different aryldiazonium salts (one with a polar grafting group and one nonpolar), appended selectively onto the different SWNTs by means of titration and monitoring by UV analysis throughout the functionalization process. The different addends accentuate the polarity differences between the band-gap-based types permitting their partial separation on silica gel. Thermal treatment regenerated pristine SWNTs in enriched fractions. The processed samples were analyzed and characterized by Raman spectroscopy. A controlled functionalization method using 4-fluorophenyl and 4-iodophenyl addends was performed, and XPS analyses yielded data on the degree of functionalization needed to affect the van Hove singularities in the UV/vis/NIR spectra. Finally, we demonstrate that relative peak intensity changes in Raman spectra can be caused by morphological changes in SWNT bundling based on differing flocculation or deposition methods. Therefore a misleading impression of separations can result, underscoring the care needed in assessing efficacies in SWNT enrichment and the prerequisite use of multiple excitation wavelengths and similar flocculation or deposition methods in comparative analyses.     

Thermal physics in carbon nanotube growth kinetics

The growth of single wall carbon nanotubes (SWNTs) mediated by metal nanoparticles is considered within (i) the surface diffusion growth kinetics model coupled with (ii) a thermal model taking into account heat release of carbon adsorption-desorption on nanotube surface and carbon incorporation into the nanotube wall and (iii) carbon nanotube-inert gas collisional heat exchange. Numerical simulations performed together with analytical estimates reveal various temperature regimes occurring during SWNT growth. During the initial stage, which is characterized by SWNT lengths that are shorter than the surface diffusion length of carbon atoms adsorbed on the SWNT wall, the SWNT temperature remains constant and is significantly higher than that of the ambient gas. After this stage the SWNT temperature decreases towards that of gas and becomes nonuniformly distributed over the length of the SWNT. The rate of SWNT cooling depends on the SWNT-gas collisional energy transfer that, from molecular dynamics simulations, is seen to be efficient only in the SWNT radial direction. The decreasing SWNT temperature may lead to solidification of the catalytic metal nanoparticle terminating SWNT growth or triggering nucleation of a new carbon layer and growth of multiwall carbon nanotubes.     

Rectifying diodes from asymmetrically functionalized single-wall carbon nanotubes

Asymmetrically functionalized single-wall carbon nanotubes (SWNTs) have been prepared by a covalent reaction of an 11-mercaptoundecanol-modified Au surface with oxidized SWNT cylinders. While one end of the tubes is attached to gold substrate via ester groups, the free carboxylic substituents on the other end can be either ionized (CO2-) or esterified (CO2Et), creating a donor-acceptor asymmetric and acceptor-acceptor symmetric SWNT, respectively. Study of the SWNT monolayer conductance in Hg drop junction experiments reveals a pronounced diode-like behavior for donor-SWNT-acceptor junctions, while acceptor-SWNT-acceptor junctions are electrically symmetric.     

单壁碳纳米管中甲烷吸附的分子模拟

采用巨正则系统MonteCarlo方法研究了甲烷在单壁碳纳米管(Singlewallcarbonnanotube,SWNT)中于低温74.05K下的吸附等温线及吸附机理,发现在两个较小的孔径(1.225nm和1.632nm)下单壁碳纳米管中甲烷的吸附有着明显的微孔所独有的“填充效应”,而在2.04nm以上的孔的吸附中会出现毛细凝聚现象。通过模拟知道发生毛细凝聚的必要条件是孔内能至少容纳下两层粒子,此外还导出在恒定温度下毛细凝聚吸附量与SWNT孔径关系。本文还模拟了常温300K下甲烷在SWNT内的吸附,对比了2.04nm和4.077nm两种孔径的SWNT吸附甲烷的等温线,推荐在4.077nm孔中的适宜吸附存储压力为5.0～6.0MPa,吸附质量分数可达16%～19%.     

Rational chemical strategies for carbon nanotube functionalization

Whereas the chemistry of fullerenes is well-established, the chemistry of single-walled carbon nanotubes (SWNTs) is a relatively unexplored field of research. Investigations into the bonding of moieties onto SWNTs are important because they provide fundamental structural insight into how nanoscale interactions occur. Hence, understanding SWNT chemistry becomes critical to rational, predictive manipulation of their properties. Among the strategies discussed include molecular metal complexation with SWNTs to control site-selective chemistry in these systems. In particular, work has been performed with Vaska's and Wilkinson's complexes to create functionalized adducts. Functionalization should offer a relatively simple means of tube solubilization and bundle exfoliation, and also allows for tubes to be utilized as recoverable catalyst supports. Solubilization of oxidized SWNTs has also been achieved through derivatization by using a functionalized organic crown ether. The resultant adduct yielded concentrations of dissolved nanotubes on the order of 1 g L(-1) in water and at elevated concentrations in a range of organic solvents, traditionally poor for SWNT manipulation. To further demonstrate chemical processability of SWNTs, we have subjected them to ozonolysis, followed by treatment with various independent reagents, to rationally generate a higher proportion of oxygenated functional groups on the nanotube surface. This protocol has been found to purify nanotubes. More importantly, the reaction sequence has been found to ozonize the sidewalls of these nanotubes. Finally, SWNTs have also been chemically modified with quantum dots and oxide nanocrystals. A composite heterostructure consisting of nanotubes joined to nanocrystals offers a unique opportunity to obtain desired physical, electronic, and chemical properties by adjusting synthetic conditions to tailor the size and structure of the individual sub-components, with implications for self-assembly.     

Separated metallic and semiconducting single-walled carbon nanotubes: opportunities in transparent electrodes and beyond

Ever since the discovery of single-walled carbon nanotubes (SWNTs), there have been many reports and predictions on their superior properties for use in a wide variety of potential applications. However, an SWNT is either metallic or semiconducting; these properties are distinctively different in electrical conductivity and many other aspects. The available bulk-production methods generally yield mixtures of metallic and semiconducting SWNTs, despite continuing efforts in metallicity-selective nanotube growth. Presented here are significant advances and major achievements in the development of postproduction separation methods, which are now capable of harvesting separated metallic and semiconducting SWNTs from different production sources with sufficiently high enrichment and quantities for satisfying at least the needs in research and technological explorations. Opportunities and some available examples for the use of metallic SWNTs in transparent electrodes and semiconducting SWNTs in various device nanotechnologies are highlighted and discussed.