Functionalization of Single‐Walled Carbon Nanotubes

Through chemical functionalization of single‐walled carbon nanotubes, the prerequisites for possible applications of such nanostructures are established. The derivatized tubes differ from the crude materials in their good solubility, which enables both a more extensive characterization and subsequent chemical reactivity. Current derivatization methods include defect and covalent sidewall functionalization, as well as noncovalent exo‐ and endohedral functionalization. In this way, for example, a range of nanotubes can be prepared: with sidewall substituents, wrapped with polymers, or with guest molecules included. The current state of the literature is presented in this Minireview.     

Extraction of (9,8) Single‐Walled Carbon Nanotubes by Fluorene‐Based Polymers

Selective polymer wrapping is a promising approach to obtain high‐chiral‐purity single‐walled carbon nanotubes (SWCNTs) needed in technical applications and scientific studies. We showed that among three fluorene‐based polymers with different side‐chain lengths and backbones, poly[(9,9‐dihexylfluorenyl‐2,7‐diyl)‐co‐(9,10‐anthracene)] (PFH‐A) can selectively extract SWCNTs synthesized from the CoSO₄/SiO₂ catalyst, which results in enrichment of 78.3 % (9,8) and 12.2 % (9,7) nanotubes among all semiconducting species. These high‐chiral‐purity SWCNTs may find potential applications in electronics, optoelectronics, and photovoltaics. Furthermore, molecular dynamics simulations suggest that the extraction selectivity of PFH‐A relates to the bending and alignment of its alkyl chains and the twisting of its two aromatic backbone units (biphenyl and anthracene) relative to SWCNTs. The strong π–π interaction between polymers and SWCNTs would increase the extraction yield, but it is not beneficial for chiral selectivity. Our findings suggest that the matching between the curvature of SWCNTs and the flexibility of the polymer side chains and the aromatic backbone units is essential in designing novel polymers for selective extraction of (n,m) species.     

Selective and Scalable Chemical Removal of Thin Single‐Walled Carbon Nanotubes from their Mixtures with Double‐Walled Carbon Nanotubes

Double‐walled carbon nanotubes (DWCNTs) are materials in high demand due to their superior properties. However, it is very challenging to prepare DWCNTs samples of high purity. In particular, the removal of single‐walled carbon nanotubes (SWCNTs) contaminants is a major problem. Here, a procedure for a selective removal of thin‐diameter SWCNTs from their mixtures with DWCNTs by lithium vapor treatment is investigated. The results are evaluated by Raman spectroscopy and in situ Raman spectroelectrochemistry. It is shown that the amount of SWCNTs was reduced by about 35 % after lithium vapor treatment of the studied SWCNTs–DWCNTs mixture.     

Photoinduced Luminescence Blinking and Bleaching in Individual Single‐Walled Carbon Nanotubes

The temporal evolution of photoluminescence in individual single‐walled carbon nanotubes (SWNT) under strong laser irradiation is studied and pronounced blinking and bleaching is observed, caused by photoinduced oxidation that subsequently quenches mobile excitons. The nanotubes are isolated with sodium cholate and spun onto either a glass or mica surface. Their bleaching behavior is investigated for variable laser intensities in air and argon atmosphere. The decay rate for luminescence bleaching generally increases with higher laser intensity, however saturating on mica substrates, which is attributed to limited availability of oxygen in the vicinity of the nanotubes. Step‐like events in the luminescence time traces corresponding to single oxidation events are analyzed regarding relative step height and suggest an exciton diffusion range of about 105 nm.     

Microwave‐Assisted Synthesis of Crosslinked Poly(vinyl alcohol) Nanocomposites Comprising Single‐Walled Carbon Nanotubes,Multi‐Walled Carbon Nanotubes,and Buckminsterfullerene

We report a facile method to accomplish the crosslinking reaction of PVA with SWNTs, MWNTs, and C‐60 using MW irradiation. Nanocomposites of PVA crosslinked with SWNT, MWNT and C‐60 were prepared expeditiously by reacting the respective carbon nanotubes with 3 wt.‐% PVA under MW irradiation, maintaining a temperature of 100 °C, representing a radical improvement over literature methods to prepare such crosslinked PVA composites. This general preparative procedure is versatile and provides a simple route to manufacture useful SWNT, MWNT and C‐60 nanocomposites.

     

Spectroscopic Characteristics of Differently Produced Single‐Walled Carbon Nanotubes

Single‐walled carbon nanotubes (SWNTs) synthesized with different methods are investigated by using multiple characterization techniques, including Raman scattering, optical absorption, and X‐ray absorption near edge structure, along with X‐ray photoemission by following the total valence bands and C 1s core‐level spectra. Four different SWNT materials (produced by arc discharge, HiPco, laser ablation, and CoMoCat methods) contain nanotubes with diameters ranging from 0.7 to 2.8 nm. The diameter distribution and the composition of metallic and semiconducting tubes of the SWNT materials are strongly affected by the synthesis method. Similar sp² hybridization of carbon in the oxygenated SWNT structure can be found, but different surface functionalities are introduced while the tubes are processed. All the SWNTs demonstrate stronger plasmon resonance excitations and lower electron binding energy than graphite and multiwalled carbon nanotubes. These SWNT materials also exhibit different valence‐band X‐ray photoemission features, which are considerably affected by the nanotube diameter distribution and metallic/semiconducting composition.     

DFT Studies of Ag‐Loading Intrinsic and Functionalized Single‐Walled Carbon Nanotubes

The first principles study was performed on the stability of Ag adsorbed on the internal walls of single‐walled carbon nanotube (SWCNT) and loaded on acid modified SWCNT. The calculation results show that Ag can be adsorbed stably on the internal walls of SWCNT. With the increase of SWCNT diameter, the adsorption energy increases in a certain range. Ag can also be loaded on the modified SWCNT surface in the form of COOAg and OAg groups, and COOAg group is more stable than OAg group. For either the adsorption on the inner SWCNT or the load on the modified SWCNT surface, only a small proportion of the Ag ions can be stably bonded to the walls of SWCNT.     

Covalent Attachment of Anderson‐Type Polyoxometalates to Single‐Walled Carbon Nanotubes Gives Enhanced Performance Electrodes for Lithium Ion Batteries

Single‐walled carbon nanotubes (SWNTs) covalently functionalized with redox‐active organo‐modified polyoxometalate (POM) clusters have been synthesized and employed as electrode materials in lithium ion batteries. The Anderson cluster [MnMo₆O₂₄]^9? is functionalized with Tris (NH₂C(CH₂OH)₃) moieties, giving the new organic–inorganic hybrid [N(nC₄H₉)₄]₃[MnMo₆O₁₈{(OCH₂)₃CNH₂}₂]. The compound is then covalently attached to carboxylic acid‐functionalized SWNTs by amide bond formation and the stability of this nanocomposite is confirmed by various spectroscopic methods. Electrochemical analyses show that the nanocomposite displays improved performance as an anode material in lithium ion batteries compared with the individual components, that is, SWNTs and/or Anderson clusters. High discharge capacities of up to 932 mAh g^?1 at a current density of 0.5 mA cm^?2 can be observed, together with high long‐term cycling stability and decreased electrochemical impedance. Chemisorption of the POM cluster on the SWNTs is shown to give better electrode performance than the purely physisorbed analogues.     

Influence of Polymer Electronics on Selective Dispersion of Single‐Walled Carbon Nanotubes

The separation and isolation of semiconducting and metallic single‐walled carbon nanotubes (SWNTs) on a large scale remains a barrier to many commercial applications. Selective extraction of semiconducting SWNTs by wrapping and dispersion with conjugated polymers has been demonstrated to be effective, but the structural parameters of conjugated polymers that dictate selectivity are poorly understood. Here, we report nanotube dispersions with a poly(fluorene‐co‐pyridine) copolymer and its cationic methylated derivative, and show that electron‐deficient conjugated π‐systems bias the dispersion selectivity toward metallic SWNTs. Differentiation of semiconducting and metallic SWNT populations was carried out by a combination of UV/Vis‐NIR absorption spectroscopy, Raman spectroscopy, fluorescence spectroscopy, and electrical conductivity measurements. These results provide new insight into the rational design of conjugated polymers for the selective dispersion of metallic SWNTs.     

Photochemical Behavior of Single‐Walled Carbon Nanotubes in the Presence of Propylamine

This report describes the photochemical behavior of single‐walled carbon nanotubes (SWNTs) in the presence of propylamine. The SWNTs are characterized by absorption and Raman spectroscopy. The spectral changes due to photoirradiation indicate that reactions occur predominantly with the metallic SWNTs and small‐diameter SWNTs. The detection of amine radicalcation species by ESR spectroscopy reveals photoinduced electron transfer from the amine to the excited SWNTs. After exposure of the photoirradiated SWNTs to air, the characteristic spectra were recovered, except for that of the small‐diameter SWNTs. The results suggest that, after photoreduction of the SWNTs, subsequent selective sidewall functionalization of the small‐diameter SWNTs occurs.     

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.     

A Two‐Step Method for Transferring Single‐Walled Carbon Nanotubes onto a Hydrogel Substrate

Carbon nanotube (CNT)‐hydrogel nanocomposites are beneficial for various biomedical applications, such as nerve regeneration, tissue engineering, sensing, or implant coatings. Still, there are impediments to developing nanocomposites, including attaining a homogeneous CNT‐polymer dispersion or patterning CNTs on hydrogels. While few approaches have been reported for patterning CNTs on polymeric substrates, these methods include high temperature, high vacuum or utilize a sacrificial layer and, hence, are incompatible with hydrogels as they lead to irreversible collapse in hydrogel structure. In this study, a novel two‐step method is designed to transfer CNTs onto hydrogels. First, dense CNTs are grown on quartz substrates. Subsequently, hydrogel solutions are deposited on the quartz‐grown CNTs. Upon gelation, the hydrogel with transferred CNTs is peeled from the quartz. Successful transfer is confirmed by scanning electron microscopy and indirectly by cell attachment. The efficient transfer is attributed to π‐interactions pregelation between the polymers in solution and the CNTs.

     

Supramolecular Surface Modification and Solubilization of Single‐Walled Carbon Nanotubes with Cyclodextrin Complexation

A novel approach to solubilize single‐walled carbon nanotubes (SWCNTs) in the aqueous phase is described by employing supramolecular surface modification. We use cyclodextrin complexes of synthetic molecules that contain a planar pyrene moiety or a bent, shape‐fitted triptycene moiety as a binding group connected through a spacer to an adamantane moiety that is accommodated in the cyclodextrin cavity. The binding groups attach to the sidewalls of SWCNTs through a π–π stacking interaction to yield a supramolecular system that allows the SWCNTs to dissolve in the aqueous phase through the formed hydrophilic cyclodextrin shell. The black aqueous SWCNT solutions obtained are stable over a period of months. They are characterized through absorbance, static, and time‐resolved fluorescence spectroscopy as well as Raman spectroscopy, TEM, and fluorescence‐decay measurements. Furthermore, the shape‐fitted triptycene‐based system shows a pronounced selectivity for SWCNTs with a diameter of 1.0 nm.     

Surfactant‐Dependent Charge Transfer between Polyoxometalates and Single‐Walled Carbon Nanotubes: A Fluorescence Spectroscopic Study

Hybridizations of redox‐active polyoxometalates (POMs) with single‐walled carbon nanotubes (SWNTs) have been widely investigated for their diverse applications. For the purpose of constructing high‐quality electronic devices, controlling charge transfer within POM/SWNT hybrids is an inevitable issue. As determined by means of fluorescence spectroscopy, electron transfer between SWNTs and a common POM dopant, phosphomolybdic acid (PMo₁₂), can be tuned simply by an alteration of nanotube surfactant type from anionic to nonionic. The mechanism is attributed to the influence of surfactant type on the stabilization of the electron donor–acceptor hybrid and effect of surfactant–nanotube interactions. These results will be important to control charge‐transport behavior in nanohybrids consisting of carbon nanotubes.     

Separation of Semiconducting Single‐Walled Carbon Nanotubes by Using a Long‐Alkyl‐Chain Benzenediazonium Compound

We designed and synthesized 4‐dodecyloxybenzenediazonium tetrafluoroborate ( 1 ), which preferentially reacts with metallic single‐walled carbon nanotubes (SWNTs) by kinetic control. We first determined the suitable experimental conditions for the preferential reaction of 1 with individually dissolved SWNTs by monitoring the decrease in absorbance for the metallic SWNT in the range of 400–650 nm in the absorption spectrum of the SWNTs. The reacted SWNTs were thoroughly rinsed with THF to obtain THF‐insoluble SWNTs. The Raman spectrum of the THF‐insoluble SWNTs showed a strong peak near 180 cm^?1, which corresponds to a semiconducting breathing band. The metallic breathing bands (≈220 cm^?1) and Breit–Wingner–Fano (BWF) modes (1520 cm^?1) corresponding to the metallic SWNTs were much weaker than those of the pristine SWNTs. We also confirmed that metallic peaks in the range of 400–650 nm in the absorption spectrum of THF‐insoluble SWNTs that were individually dissolved in an aqueous micelle of sodium cholate were almost nondetectable. All the results indicate that the THF‐insoluble SWNTs are semiconducting.