Selective oxidation of single-walled carbon nanotubes using carbon dioxide

A simple process for selective removal of carbon from single-walled carbon nanotube samples was developed based on a mild oxidation by carbon dioxide. The reactivity profiles of as prepared and purified nanotube samples were determined using both TG and a related analytical technique, controlled atmosphere programmed temperature oxidation (CAPTO). The complex differential rate curves for weight loss (DTG) or carbon dioxide evolution (CAPTO) could be resolved by a series of Gaussian peaks each associated with carbonaceous species of different reactivity. Comparisons were made between samples as received after preparation by the laser ablation method, after purification by nitric acid oxidation, and both of these after reaction with CO₂. The DTG of as prepared tubes had a broad major peak centered about 410 °C. Mild oxidation of as prepared nanotubes under flowing carbon dioxide at 600 °C preferentially removed more reactive carbon species leaving behind a narrower distribution about the major peak in DTG. In contrast to the as prepared material, the sample that had been purified using nitric acid had a more distinct separation of the major DTG peaks between more and less readily oxidized material. Oxidation of this sample with CO₂ selectively removed the peak associated with the most readily oxidized material. The original CO₂ oxidation experiments performed on the analytical scale were successfully scaled up to a small preparative scale.     

Bio-nano interaction of proteins adsorbed on single-walled carbon nanotubes

We applied X-ray absorption near edge structure (XANES) spectroscopy to investigate the adsorption of proteins onto single-walled carbon nanotubes (SWCNTs). Specific XANES spectral features such as peptide CO bonds in proteins were recognized and found to be affected by the corresponding aromatic structure of SWCNTs. Experimental data combined with first-principle calculation of the investigated nano-complex allow the understanding of adsorption mechanism and reveal that an interface interaction occurs leading to precise structural distortions of proteins. The study also demonstrates that XANES is a powerful tool to characterize structural details of proteins at the interface of complex systems.     

Increasing the semiconducting component in transparent conducting films of single-walled carbon nanotubes

A simple process was developed for increasing the semiconducting component of single-walled carbon nanotubes (SWCNTs) with narrowed diameter distribution through directly eliminating metallic SWCNTs and semiconducting SWCNTs (s-SWCNTs) with smaller diameters in transparent conducting films of SWCNTs. The process is based on the oxidation with diazonium reagents and air. The transparent films of s-SWCNTs obtained had high-purity and retained the original structure. The possible mechanism of the process is discussed.     

Spectroscopic characterization of contaminants and interaction with gases in single-walled carbon nanotubes

Several spectroscopic techniques have been used to investigate the presence of contaminants in a commercial purified single-walled carbon nanotube (SWCNT) bucky paper, to determine their cleaning procedure in ultra-high-vacuum conditions and to study how impurities influence the interaction between SWCNTs and gas phase molecules. Nickel catalyst particles and sodium-containing species, likely a residual of the surfactant bath, were fully removed only after prolonged (>2 h) annealing at 1270 ± 30 K. Other impurity elements (S and Si) remain in the material as localised clusters that do not interact with the SWCNTs and do not interfere with their properties.A dramatic difference was observed when the Na-contaminated or the Na-free nanotubes interacted with molecular oxygen. O₂ adsorption was strongly altered by the Na traces, which simulated an intense sample oxidation causing a modification of the tube electronic properties. On the contrary, for the Na-free sample the lack of adsorbed oxygen and the stability of the C1s core level after large O₂ doses demonstrated the absence of any chemical bond between SWCNTs and O₂. Similarly, exposures to N₂, H₂O and CO do not have influence on the electronic properties of SWCNTs. Instead, a sizeable effect on the electronic spectra was observed for SO₂, NO and NO₂ adsorption. The sensitivity of the SWCNT electronic spectra to ppb quantities of nitrogen oxides and sulfur oxide undoubtedly foresees applications in the field of toxic gas sensing.     

Ferrocene-filled single-walled carbon nanotubes

Ferrocene molecules are successfully introduced into the inner hollow space of Single-walled carbon nanotubes (SWNTs) to get ferrocene-filled SWNTs (Fc@SWNTs). This nanohybrid material was carefully characterized by high resolution microscopy, FTIR spectrum, and Cyclic voltammetry (CV). This new material may not only act as air stable n-type field-effect transistors based on nanotubes, but it may also be employed as building blocks for various devices based on the redox activity of ferrocene. What’s more, upon high temperature annealing, the encapsulated ferrocene molecules will decompose and change into interior tubes, forming double-walled carbon nanotubes (DWNTs). This provides convincing evidence that ferrocene molecules are inserted into the hollow cavities SWNTs. This result also presented a controllable way to synthesize DWNTs.     

Chemistry of single-walled carbon nanotubes

In this Account we highlight the experimental evidence in favor of our view that carbon nanotubes should be considered as a new macromolecular form of carbon with unique properties and with great potential for practical applications. We show that carbon nanotubes may take on properties that are normally associated with molecular species, such as solubility in organic solvents, solution-based chemical transformations, chromatography, and spectroscopy. It is already clear that the nascent field of nanotube chemistry will rival that of the fullerenes.     

Formylation of single-walled carbon nanotubes

Formyl or aldehyde groups are transferred to the surface of single-walled carbon nanotubes (SWCNTs) by reaction of reduced carbon nanotubes with N-formylpiperidine. This could open the way for more versatile chemical modification reactions of carbon nanotubes than is currently possible using functionalization methods reported to date. The formylated SWCNTs were characterized by thermogravimetric analysis-mass spectrometry and Raman, UV-vis-NIR and FTIR spectroscopy. The location and distribution of the functional groups was determined by AFM using electrostatic interactions with gold nanoparticles. The formylated SWCNTs were further derivatized with a fluorescent dye and studied using fluorescence spectroscopy.     

Electron-induced cutting of single-walled carbon nanotubes

Electron beam irradiation with moderate fluences of approximately 10¹⁶-10¹⁷ electrons per cm² is used for controllable, bulk-scale cutting of single-walled carbon nanotubes (SWCNTs). The effectiveness of high energy electron irradiation in cutting SWCNTs is dependent on the nature of the sidewall. While pristine nanotubes are very stable under irradiation conditions, ozonated SWCNTs combined with a moderate fluence of electrons resulted in bulk-scale cutting of nanotubes. The length distribution of the cut SWCNTs could be controlled by adjusting the irradiation fluence. The average length of the cut nanotubes was 65 nm with 85% of the nanotubes shorter than 100 nm.     

Photophysics of individual single-walled carbon nanotubes

Single-walled carbon nanotubes (SWNTs) are cylindrical graphitic molecules that have remained at the forefront of nanomaterials research since 1991, largely due to their exceptional and unusual mechanical, electrical, and optical properties. The motivation for understanding how nanotubes interact with light (i.e., SWNT photophysics) is both fundamental and applied. Individual nanotubes may someday be used as superior near-infrared fluorophores, biological tags and sensors, and components for ultrahigh-speed optical communications systems. Establishing an understanding of basic nanotube photophysics is intrinsically significant and should enable the rapid development of such innovations. Unlike conventional molecules, carbon nanotubes are synthesized as heterogeneous samples, composed of molecules with different diameters, chiralities, and lengths. Because a nanotube can be either metallic or semiconducting depending on its particular molecular structure, SWNT samples are also mixtures of conductors and semiconductors. Early progress in understanding the optical characteristics of SWNTs was limited because nanotubes aggregate when synthesized, causing a mixing of the energy states of different nanotube structures. Recently, significant improvements in sample preparation have made it possible to isolate individual nanotubes, enabling many advances in characterizing their optical properties. In this Account, single-molecule confocal microscopy and spectroscopy were implemented to study the fluorescence from individual nanotubes. Single-molecule measurements naturally circumvent the difficulties associated with SWNT sample inhomogeneities. Intrinsic SWNT photoluminescence has a simple narrow Lorentzian line shape and a polarization dependence, as expected for a one-dimensional system. Although the local environment heavily influences the optical transition wavelength and intensity, single nanotubes are exceptionally photostable. In fact, they have the unique characteristic that their single molecule fluorescence intensity remains constant over time; SWNTs do not "blink" or photobleach under ambient conditions. In addition, transient absorption spectroscopy was used to examine the relaxation dynamics of photoexcited nanotubes and to elucidate the nature of the SWNT excited state. For metallic SWNTs, very fast initial recovery times (300-500 fs) corresponded to excited-state relaxation. For semiconducting SWNTs, an additional slower decay component was observed (50-100 ps) that corresponded to electron-hole recombination. As the excitation intensity was increased, multiple electron-hole pairs were generated in the SWNT; however, these e-h pairs annihilated each other completely in under 3 ps. Studying the dynamics of this annihilation process revealed the lifetimes for one, two, and three e-h pairs, which further confirmed that the photoexcitation of SWNTs produces not free electrons but rather one-dimensional bound electron-hole pairs (i.e., excitons). In summary, nanotube photophysics is a rapidly developing area of nanomaterials research. Individual SWNTs exhibit robust and unexpectedly unwavering single-molecule fluorescence in the near-infrared, show fast relaxation dynamics, and generate excitons as their optical excited states. These fundamental discoveries should enable the development of novel devices based on the impressive photophysical properties of carbon nanotubes, especially in areas like biological imaging. Many facets of nanotube photophysics still need to be better understood, but SWNTs have already proven to be an excellent starting material for future nanophotonics applications.     

Torsional behavior of single-walled carbon nanotubes

Atomistic simulations were performed to investigate the deformation behavior of single-walled carbon nano-tubes (SWCNTs) under torsional loading. The evolutions of the potential energy and stresses were presented. Radial distribution functions (RDFs) were calculated to analyze structural evolution during torsional deformation. The results show that during torsion, the tensile stress component along the tube axis is most significant and other stress components are almost negligible. The tensile stress stretched the C–C bonds until they reached the bond length of 0.18 nm. The torsional strength of the SWCNTs is about 30% of the tensile strength. Buckling took place at a few degrees of torsional angle and propagated along the tube as the torsional angle increased, and collapse of the tube wall followed buckling. These structural evolutions can be well described with the RDFs. Two new peaks appeared at 0.21 nm and 0.18 nm in the RDFs, corresponding to the minimum spacing between the atoms in the collapsed layers, and the maximum bond length that can be reached in stretching before rupture.     

单壁碳纳米管分离研究

根据导电性能不同,单壁碳纳米管可以分为金属型和半导体型。目前所有方法制备的单壁碳纳米管,其产物为金属型和半导体型单壁碳纳米管的混合物,且很难将它们分开,这极大地阻碍了单壁碳纳米管在很多领域的应用。本文介绍了单壁碳纳米管的结构与导电性能的关系,着重综述了最新金属型和半导体型单壁碳纳米管的分离方法。     

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.     

Chemical modification of single-walled carbon nanotubes with sulfur-containing functionalities

Photolysis of cyclic disulfides with single-walled carbon nanotubes led to a sidewall modification of the surface to introduce sulfur-containing functional groups, confirming by means of Raman, XPS, FT-IR, mass and UV–vis–NIR measurements. Subsequent treatment of sulfur-containing substituents modified SWNTs with gold nanoparticles gave an attachment of gold on the surface of SWNTs through thioalkylthiol linkage.     

Modification of fluorinated single-walled carbon nanotubes with aminosilane molecules

Single-walled carbon nanotubes (SWCNTs) modified with amino groups were prepared via chemical addition of fluorine on the carbon nanotube surface by plasma treatment. The amino termination makes possible to realize hybrid nanostructures made out of SWCNTs and alkoxy-silane (3′-(aminopropyl)tri-ethoxysilane (APTES)) molecules. The functionalization of the SWCNTs was evidenced by transmission electron microscopy, infrared spectroscopy and thermogravimetric measurements. It was found that the application of a dc electric field enhances the assembly of APTES modified SWCNTs into ordered films with rectifying diode behavior.     

Chemical modification of single-walled carbon nanotubes with peroxytrifluoroacetic acid

A new and simple method for chemical modification of single-walled carbon nanotubes (SWNTs) is presented. Purified SWNTs ropes prepared by CVD growth were reacted with peroxytrifluoroacetic acid (PTFAA) under ultrasonication. Samples before and after treatment were characterized using Raman, FTIR, UV/Vis/NIR, XPS, and AFM. Data from these experiments conclusively showed that, in addition to oxygen-based functional groups, trifluoroacetic groups were covalently attached to the SWNTs. Moreover, these modified SWNTs were shortened into ca. 300 nm in length in the same step of functionalization, resulting in exfoliation of nanotube ropes to yield small bundles and individual nanotubes. The resultant SWNTs were easily dispersed in polar solvents such as dimethylformamide, water and ethanol. The PTFAA treatment described herein should be useful to tailor SWNTs’ chemical and physical properties and to broaden their chemical processibility and reactivity.     

Functionalization of single-walled carbon nanotubes with optically switchable spiropyrans

The design, synthesis and complete characterization of a smart material composed of single-walled nanotubes functionalized with spiropyran-based photo switchable molecules are reported. The chemical complexity of the system requires the use of a range of complementary techniques in order to provide a complete picture of the composition and performance of the nanomaterial. Thermal gravimetric analysis ensured both the high degree of chemical functionalization and the presence of molecular switches in the nanotube sample; micro-Raman indirectly confirmed the successful oxidation of the tubes, FT-IR proved the nature of the functional groups introduced based on their characteristic stretching vibrations and atomic force microscopy demonstrated nanotube lengths of approximately 600 nm. UV-Vis-NIR absorption spectroscopy was used to evaluate the photoresponsive behaviour of the nanomaterial, and the intensity of the absorption band at 556 nm could be modulated by light-induced reversible conversion of the spiropyran molecules attached to the SWCNT from the spiro close conformation to the merocyanine open form. We were also able to provide the first example of a continuous solution based on-off switching in a spiropyran-nanotube material.     

Theoretical study of work function of conducting single-walled carbon nanotubes by a non-relativistic field theory approach

We study a non-relativistic many-fermion system on a small cylindrical surface. The interaction between the fermions is modeled as an attractive two-body contact effective potential, which allows binding of fermions on the cylinder surface. The N-fermion model is solved in a self-consistent Hartree-Fock (HF) approximation, and is applied to study the electronic properties of metallic single-walled carbon nanotubes (SWCNT). The many-fermion HF approach with contact interactions is known to mimic a first order density functional theory. We derive an analytic form of the SWCNT work function (WF), that is parameterized by the Fermi kinetic energy, and reproduces the graphene work function for large radius (R). This model allows us to understand theoretically the small experimental WF fluctuations and the non-trivial left-right asymmetry of the graphene WF distribution found experimentally in a large set of SWCNT with radius above 5 Å. By extending our model for SWCNT with very small radius, we found a WF that increases linearly with 1/R.     

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.