Covalent functionalization of single-walled carbon nanotubes by aniline electrochemical polymerization

Electrochemical polymerization of aniline in an HCl solution on a single-walled carbon nanotubes (SWNTs) film has been studied by Raman and FTIR spectroscopy. It is shown that this method leads to a covalent functionalization of SWNTs with polyaniline (PANI). A careful study in Raman scattering shows that the increase in the intensity of the band at 178 cm⁻¹ associated with radial breathing modes of SWNTs bundles suggests an additional nanotubes roping with PANI as a binding agent. A post chemical treatment with the NH₄OH solution of polymer-functionalized SWNTs involves an internal redox reaction between PANI and carbon nanotubes. As a result, the polymer chain undergoes a transition from the semi-oxidized state into a reduced one.     

An effective surfactant-free isolation procedure for single-wall carbon nanotubes

An optimised isolation procedure of single-wall carbon nanotubes (SWNTs) from a SWNT soot without using any surfactant is reported. Amorphous carbon and small graphitic particles were washed away with N-methyl-2-pyrrolidone (NMP) and acetone. A large amount of graphite-coated metal particles were removed with the oxidation of the SWNT material with HNO₃ (6.5 and 4 M) and by washing the oxidised SWNT material with a mixture of methanol (MeOH) and deionised water. The isolated material was investigated with transmission electron microscopy (TEM) and Raman scattering (647.1 and 532 nm). An elemental analysis of the content of Co and Ni in the SWNT samples isolated at different steps of the isolation procedure was performed. On the basis of the TEM images and elemental analysis it was estimated that the purified material contains more than 75 wt.% of SWNTs.     

Synthesis of water-soluble single-walled carbon nanotubes by RAFT polymerization

Water-soluble single-walled carbon nanotubes (SWNTs) were synthesized by grafting poly(acrylamide) (PAM) from the surface of SWNT via reversible addition-fragmentation chain transfer (RAFT) polymerization. The RAFT agents were covalently attached to the SWNTs by functionalizing SWNTs with in situ generated diazonium compounds. The product was characterized by means of FT-IR, Raman, ¹H NMR, TGA and TEM. The results showed that PAM chains had successfully grafted from SWNT by RAFT polymerization. The amount of PAM grown from SWNT increased with the polymerization time. The acrylamide conversion increased linearly with the polymerization time, indicating the “living” characteristics of the RAFT polymerization. TEM was utilized to image PAM-g-SWNT, showing relatively uniform polymer coatings present on the surface of individual, debundled nanotubes.     

Complete elimination of metal catalysts from single wall carbon nanotubes

The complete removal of entrapped metallic impurities (i.e. Ni and Co) incorporated within single wall carbon nanotubes (SWNTs) has been a long-standing issue. A sonication-mediated treatment of as-obtained SWNT soot in a 1:1 mixture of aqueous hydrofluoric and nitric acids resulted in the complete elimination of these impurities as shown by energy dispersive X-ray analysis (EDAX). Contact angle measurements indicated that the wetting of SWNTs is enhanced in the presence of HF. The presence of HNO₃ and surfactant was found essential in removing the catalyst due to SWNT etching of end-caps/defects and providing better dispersion, respectively. Moreover, Raman spectroscopy indicated that the structural purity of the SWNTs is not compromised by the HF/HNO₃ purification treatment.     

Dynamic mechanical and Raman spectroscopy studies on interaction between single‐walled carbon nanotubes and natural rubber

The effects of the incorporation of single‐walled carbon nanotubes (SWNTs) on the physical and mechanical properties of natural rubber (NR) are described. Characterization of these new materials has been performed by dynamic mechanical analysis, differential scanning calorimetry, and Raman spectroscopy to obtain information about of the possible interactions between both materials as well as the dispersion of SWNTs on elastomer matrix. The results are then compared with those obtained for NR–carbon black composites. Dynamic mechanical analysis indicates a stronger filler–matrix interaction in the case of SWNTs incorporation, showing a noticeable decrease of the height of tan δ peak, as well as a marked shift of T_g towards higher temperatures. In particular, the increase of the storage modulus indicates a beneficial effect of SWNTs incorporation with respect to NR filled with carbon black and the pristine polymer matrix. In addition, calorimetric analysis indicates that both fillers accelerate the NR vulcanization reaction, this effect being more evident when SWNTs are added into the matrix. Raman spectroscopy indicates that SWNTs dispersion into the elastomer matrix creates residual strain on the nanotubes bundle. We demonstrate that the Raman microprobe technique provides a means for load transfer effectiveness of SWNTs. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3394–3400, 2004     

Thionine-mediated chemistry of carbon nanotubes

Thionine can be employed as a kind of useful functional molecule for the non-covalent functionalization of carbon nanotubes, as it shows a strong interaction with either SWNTs or MWNTs. Attachment of thionine molecules onto the sidewalls of carbon nanotubes would improve the solubility and lower the thermal stability of original carbon nanotubes. More importantly, it may functionalize the surface of carbon nanotubes with rich NH₂ groups and therefore open up more opportunities for the surface chemistry of carbon nanotubes. It has been proved that through the modification of small thionine molecules, other kinds of species such as cytochrome C and TiO₂ nanoparticles could be easily and selectively introduced onto the surface of carbon nanotubes. With this approach, SWNTs or MWNTs can be tailored with desired functional structures and properties.     

Raman spectroscopy and nitrogen vapour adsorption for the study of structural changes during purification of single-wall carbon nanotubes

Raman spectroscopy and nitrogen adsorption measurements were combined to study the surface features of semi-conducting and metallic single-wall nanotubes (SWNTs). The nanotubes were treated chemically and with heat under moderate conditions that more than doubled the mesopore volume of the tested samples, which consistently led to a significant rise in the total surface area of up to 1550 m²/g. The large increase in the number of micropores of less than 1 nm in diameter was associated with the loosening of nanotube bundles as well as the creation of structural flaws on the surface of individual SWNTs due to chemical treatment. Micropores in the 1.0-1.8 nm range were associated with the holes created on the surface of individual tubes. Heating at 1000 °C was shown to restore nanotube diameter to their initial pre-chemical treatment levels with the change in the chirality of SWNTs and diminish the porosity by closing small holes. It was assumed that the intermediate frequency range (500-1100 cm⁻¹) was associated with the degree of imperfection of HiPco SWNTs crystalline structures, and therefore provided information about the degree of tube surface damage due to the presence of functional groups. A hypothesis explaining the transformation of SWNT porous structure during heat treatment is proposed.     

Single-wall carbon nanotubes with diameters approaching 6 nm obtained by laser vaporization

Single-wall carbon nanotubes (SWNTs) with large diameters from 2 to 5.6 nm were prepared by pulsed laser vaporization of carbon rods doped with Co, Ni and FeS in an atmosphere of Ar:H₂. The SWNT material was characterized by SEM, HRTEM, Raman, IR, UV-VIS-NIR absorption spectroscopy and thermogravimetric analysis.     

Coalescence of single-walled carbon nanotubes and formation of multi-walled carbon nanotubes under high-temperature treatments

Electric arc-discharge single-wall carbon nanotubes are annealed between 1600 and 2800 °C under argon flow. Their stability and evolution are studied by coupling TEM, X-ray diffraction and Raman spectroscopy. The first modifications appear at 1800 °C with a significant decrease of the crystalline order. It is due to SWNTs coalescence leading to smaller bundles but with an increase of the tube diameters from 2 to 4 nm. From 2200 °C, SWNTs progressively disappear to the benefit of MWNTs having at first two to three carbon layers then reaching 7 nm external diameter. The possible mechanisms responsible for the SWNTs coalescence and instability and their transformation in MWNTs are discussed.     

Single-walled carbon nanotubes grown on natural minerals

Single-walled carbon nanotubes (SWNTs) are successfully grown on magnesite crystal by pyrolysis of methane gas under moderate conditions, demonstrating the possibility of naturally occurring SWNTs.The obtained SWNTs were analyzed by Raman scattering, transmission electron microscope (TEM), and thermogravimetric (TG) measurements. These measurements revealed that high purity SWNTs having diameters of about 1-1.8 nm occur on the surface of natural magnesite sample by the pyrolysis of methane gas at 1073-1173 K. Structural properties and formation mechanism of the obtained SWNTs were discussed.     

Spectroscopic study on the centrifugal fractionation of soluble single-walled carbon nanotubes

Optical absorption and resonant Raman spectra are proven to be convenient and effective to monitor the centrifugal fractionation and to evaluate the quality of soluble single-walled carbon nanotubes (SWNTs) achieved by inorganic oxidation and organic functionalization. Through a systemic study of a series of centrifuged solutions, we confirmed that heavily functionalized amorphous carbon was fractionated into the early centrifuged solutions, whereas lightly functionalized graphite fragments as well as polyhedral carbon and metal catalysts particles were fractionated into the late centrifuged solutions and centrifuged residue, and then highly pure and well dispersed SWNTs were collected from the middle centrifuged solutions. It is proposed that the purity, dispersibility, and aggregation state of SWNTs can be qualitatively estimated by the relative intensity of their absorption features, the fine structure and slope of their absorption curves. The Raman features of centrifuged SWNTs are found systematically up-shifted except the disorder-induced D band in comparison with those of as-prepared material, indicating that the SWNTs in centrifuged solutions are individual or in thin bundles. Two new features were identified at 1428 and 941 cm⁻¹ in the Raman spectrum of thermally annealed centrifuged SWNTs, which were assigned to achiral nanotubes and combined mode, respectively.     

Hierarchical morphology of carbon single-walled nanotubes during sonication in an aliphatic diamine

Dispersion of single-walled carbon nanotubes (SWNTs) by sonication into diamine curing agents is studied as a means to improve the dispersion of SWNTs in cured epoxy. Cured and uncured specimens are analyzed by light microscopy, electron microscopy, light scattering (LS), ultra small-angle X-ray scattering (USAXS), electrical conductivity and Raman spectroscopy. A flexible diamine (D2000) forms a stable SWNT suspension leading to good homogeneity in both the diamine and the cured epoxy. High resolution transmission electron microscopy (TEM) shows that small ropes of SWNTs (mostly under 15 nm) are present despite the sample's visual homogeneity. Further morphological investigation of cured and uncured D2000 resins using light and small-angle X-ray scattering indicates that the SWNTs are networked into fractal clusters that electrically percolate at low SWNTs loadings (0.05 wt%).     

Field emission properties and electronic structures of ultra small diameter carbon nanotubes

The field emission mechanisms of the ultra small armchair and zigzag single-walled carbon nanotubes with finite and infinite length are studied individually by using the first principle theory. For carbon nanotubes with finite length, the field emission is affected not only by the electronic behaviors related to the magnitude of diameter, but also by the geometrical conditions at the edge of finite nanotubes. On the other hand, for the ultra small carbon nanotube with infinite length, the work functions are enhanced significantly due to their remarkable curvature effects compared to the large one. For the open-ended carbon nanotubes, the no-bonding valence electrons induced at the mouth layer after geometrical relaxations are corresponding to the variation of work functions. The localized states at the mouth layer of the open-ended (3,0) and (5,0) exhibit the stable sp³-like and sp² structures, which will influence the occupied and the unoccupied states near the Fermi level and improve the field emission properties of carbon nanotubes.     

Low cost growth route for single-walled carbon nanotubes from decomposition of acetylene over magnesia supported Fe-Mo catalyst

A large amount of single wall carbon nanotubes (SWNTs) was successfully produced by thermal decomposition of C₂H, at 800 °C over magnesia supported Fe-Mo bimetallic catalysts in a tubular flow reactor under an atmosphere of hydrogen flow. The growth density of SWNTs increased with increasing the weight percent of the catalyst metals (wt% ratio of two metals: 50 : 50) supported on magnesia (MgO) from 5 to 30 wt%. The yield of SWNTs reached 144.3% over 30 wt% metal-loaded catalyst. Raman measurements showed the growth of bundle type SWNTs with diameters ranging from 0.81 to 1.96 nm. The growth of SWNTs was also identified by thermal gravimetric analysis (TGA) and Raman spectroscopy.     

Persistent currents in finite zigzag carbon nanotubes

Magnetic properties of finite zigzag carbon nanotubes are studied within the tight-binding model. The spin-B interaction (Zeeman splitting) causes the metal-semiconductor transition and thus produces a large persistent current (J) with special jump structures. This effect makes all zigzag carbon nanotubes exhibit a gigantic paramagnetism. It also destroys the periodicity of magnetic properties. The dependence on the magnetic flux, the length (w), the radius (r), the temperature (T), and the chirality (zigzag or armchair) is strong. The amplitude of J quickly decreases with increasing of (w, r, T). Zigzag carbon nanotubes differ significantly from armchair carbon nanotubes (or infinite zigzag carbon nanotubes) in features such as magnetic susceptibility and in special structures in J.     

Carbon nanotubes: dynamics of synthesis processes

Heat and mass transport are obtained in a solar reactor using ‘in situ’ measurements linked to numerical simulation and allow the interpretation of the vaporization process as well as the determination of the cooling regime. Comparison with other processes (laser ablation or electric arc) point out some common behavior like the great influence of the cooling rate of vapors on the structure and yield of nanostructured carbon material. We also investigate the growth mechanisms of single wall carbon nanotubes (SWNTs) produced by the solar method as a function of the nature of catalysts and the temperature variation in the condensing area. The Raman spectra clearly show that the change of catalyst induces differences in the diameter of SWNT whereas TEM pictures enhance the change of both length and diameter of the bundles. All these results are explained considering that the key parameter is the temperature at which the SWNTs are formed. This temperature range can be related to the sublimation temperature of the target and to the eutectic temperature of the binary phase diagram. Finally we propose a new mechanism to explain the nucleation process and segregation rate which seems to depend on the capacity of catalyst to form carbide.     

Synthesis of Y-junction single-wall carbon nanotubes

Y-junction single-wall carbon nanotubes (SWNTs) are synthesized using controlled catalysts by chemical vapor deposition. Mo-doped Fe nanoparticles supported by aluminum oxide particles are used as catalysts for growing Y-junction single-wall carbon nanotubes. Distribution of Mo-doped Fe particles plays an important role in Y-junction formation. Transmission electron microscopy confirmed the formation of single-walled structures of Y-junctions with diameters of 2-5 nm. Radial breathing mode peaks in Raman spectra show that our sample has both metallic and semiconducting nanotubes, indicating the possible formation of Y-branching with different electrical properties. The different electrical properties of branch and stem can be utilized in nanoscale three terminal electronic devices. The growth mechanism of Y-junction SWNTs is proposed.     

Prediction of elastic properties for single-walled carbon nanotubes

Based on a link between molecular and solid mechanics, an analytical method was developed for modeling the elastic properties of single-walled carbon nanotubes (SWNTs). A SWNT is regarded as a continuum-shell model which is composed of the discrete molecular structures linked by the carbon-to-carbon bonds. The elastic properties were investigated for the SWNTs as a function of the nanotube size in terms of the chiral vector integers (n,m). The theoretical prediction on elastic properties agreed reasonably with the existing experiment and theoretical results. The present formulas are able to serve as a good approximation of the elastic properties for SWNTs.     

High growth of SWNTs and MWNTs from C2H2 decomposition over Co-Mo/MgO catalysts

A series of MgO supported catalysts having Co and Mo metals 5-40 wt.% in a ratio of 1:1 was prepared by impregnation method. Carbon nanotubes (CNTs) were grown over the catalysts by decomposition of C₂H₂ at 800 °C for 30 min. It was found that 5 and 10 wt.% Co-Mo/MgO catalysts produced single-wall nanotubes (SWNTs), whereas 20, 30 and 40 wt.% Co-Mo/MgO catalysts produced multi-wall nanotubes (MWNTs). The catalyst Mo/MgO was inactive in growing CNTs. In Co-Mo/MgO catalysts, however Mo generated a favorable environment to grow SWNTs. The growth of SWNTs was strongly dependent on the formation of small clusters of cobalt, which may generate from the decomposition of CoMoO₄ species during the nanotube growth. MWNTs were produced over comparatively larger cobalt clusters generated from Co₃O₄ phase during the nanotube growth stage. The yields of SWNTs were about 6% and 27% over 5 and 10 wt.% Co-Mo/MgO catalysts, respectively. MWNTs yield (576%) was observed over 40 wt.% Co-Mo/MgO catalyst. Carbon yield (%) highly varied with acetylene concentration.     

The synthesis of carbon nanotubes at low temperature via carbon suboxide disproportionation

Carbon nanotubes were synthesized via a novel route using an iron catalyst at the extremely low temperature of 180 °C. In this process, carbon suboxide was used as carbon source, which changed to freshly formed free carbon clusters through disproportionation. The carbon clusters can grow into nanotubes in the presence of Fe catalyst, which was obtained by the decomposition of iron carbonyl Fe₂(CO)₉ at 250 °C under nitrogen atmosphere. The products were characterized with XRD, TEM, HRTEM and Raman spectroscopy.