Solvent-free derivatization of pristine multi-walled carbon nanotubes with dithiols

We report on direct solvent-free derivatization of pristine multi-walled carbon nanotubes (MWNTs) with aliphatic dithiols (1,4-butanedithiol, 1,6-hexanedithiol and 1,8-octanedithiol), by means of heating at 130–150 °C under reduced pressure. This method requires no additional chemical activation and about 2 h only for completion. Studies by high-resolution transmission electron microscopy and scanning electron microscopy with energy dispersive X-ray spectroscopy showed that dithiol-derivatized MWNTs have a high affinity to ZnCl₂ in solution, which covers the nanotubes with a dense amorphous layer. According to PM3 semi-empirical calculations, employing a closed-cap zigzag (10,0) single-walled carbon nanotube (SWNT) model incorporating a Stone-Wales defect, site-specificity of the addition depends on the mutual position of pentagons. If the nanotube contains pyracylene units or Stone-Wales defect, the addition takes place on their 6,6 or 7,7 bonds, respectively, whereas for isolated pentagons, preferential reaction sites are their C–C bonds. Ideal graphene sheet sidewalls with cylindrical curvature are relatively inert (although one cannot discard the possibility to activate the reaction by heating). Dithiol groups introduced in the way proposed can be used as chemical linkers for anchoring metal complexes and nanoparticles to carbon nanotubes, attaching SWNTs to gold tips for atomic force and scanning tunneling microscopy, and potentially for adsorption and concentration of trace metal ions.     

Solvent-free derivatization of pristine multi-walled carbon nanotubes with amines

We performed direct solvent-free amination of multi-walled carbon nanotubes (MWCNTs) with nonylamine, dodecylamine, octadecylamine, 4-phenylbutylamine and 1,8-ocanediamine at a temperature of 150-170 degrees C and reduced pressure. Thermogravimetric analysis and temperature-programmed desorption-mass spectrometry revealed that a major amine fraction decomposes in a temperature interval of 250-500 degrees C, thus existing on multi-walled carbon nanotubes as chemically bonded species; a minor amine fraction was found in physisorbed form. The new derivatization technique combines simplicity in implementation and attractive features of "green" chemistry. It requires no additional chemical activation, but thermal activation instead; it is relatively fast since it can be completed in about 2 h; the high temperature allows one to spontaneously remove excess amine from the nanotube and minimize the possibility of physical adsorption; there is no need to use an (organic) solvent medium. In the case of diamines (represented in this study by 1,8-ocanediamine), the functional groups introduced can be potentially used as chemical linkers for anchoring metal complexes and nanoparticles to multi-walled carbon nanotubes, for adsorption and concentration of trace metal ions.     

Coating of carbon nanotubes with amorphous carbon nitride thin films and characterization of long-term emission stability

The effects of amorphous carbon nitride (CN) thin films that were coated on carbon nanotubes (CNTs) and their thermal treatment were investigated, in terms of the chemical bonding and morphologies of the CNTs and their field emission properties. CNTs were directly grown on conical tip-type tungsten substrates via the inductively coupled plasma-chemical vapor deposition (ICP-CVD) system, and the CNTs were coated with CN films using the RF magnetron sputtering system. The CN-coated CNTs were thermally treated using the rapid thermal annealing (RTA) system by varying the temperature (300-700 °C). The morphologies, microstructures, and chemical compositions of the CN-coated CNTs were analyzed as a function of the thickness of the CN layers and the RTA temperatures. The field emission properties of the CN/CNT hetero-structured emitters, and the fluctuation and long-term stability of the emission currents were measured and compared with those of the conventional non-coated CNT-emitter. The results showed that the electron emission capability of CNT was noticeably improved by coating a thin CN layer on the surface of the CNT. This was attributed to the low work function and negative electron affinity nature of the CN film. The CN-coated CNT-emitter had a more stable emission characteristic than that of the non-coated one. In addition, the long-term emission stability of the CN-coated emitter was further enhanced by thermal treatment, which was verified by x-ray photoelectron spectroscopy (XPS) analysis.     

Nickel nanoparticles decoration of ordered mesoporous silica thin films for carbon nanotubes growth

We report in situ successive depositions of nickel nanoparticles and carbon nanotubes (CNTs) on ordered mesoporous silica films used as template for the catalyst particles. The mesoporous films are synthesized by the evaporation-induced self-assembly process from tetraethyl orthosilicate derived oligomers and a di-block copolymer from dip-coating deposition method. The substrates are decorated with Ni nanoparticles through Ion Beam Deposition and posterior annealing to induce metal coalescence in the mesoporous cavities. CNTs were then grown by Chemical Vapor Deposition in the presence of an electric field. These techniques provide a simple control method producing ordered arrangements of catalyst nanoparticles and ordered nanostructures for large area applications.     

Consideration of critical axial properties of pristine and defected carbon nanotubes under compression

Carbon nanotubes are hexagonally configured carbon atoms in cylindrical structures. Exceptionally high mechanical strength, electrical conductivity, surface area, thermal stability and optical transparency of carbon nanotubes outperformed other known materials in numerous advanced applications. However, their mechanical behaviors under practical loading conditions remain to be demonstrated. This study investigates the critical axial properties of pristine and defected single- and multi-walled carbon nanotubes under axial compression. Molecular dynamics simulation method has been employed to consider the destructive effects of Stone-Wales and atom vacancy defects on mechanical properties of armchair and zigzag carbon nanotubes under compressive loading condition. Armchair carbon nanotube shows higher axial stability than zigzag type. Increase in wall number leads to less susceptibility of multi-walled carbon nanotubes to defects and higher stability of them under axial compression. Atom vacancy defect reveals higher destructive effect than Stone-Wales defect on mechanical properties of carbon nanotubes. Critical axial strain of single-walled carbon nanotube declines by 67% and 26% due to atom vacancy and Stone-Wales defects.     

Electrodeposited carbon nanotube thin films

A successful attempt to grow carbon nanotubes (CNTs) by electrodeposition technique for the first time is reported here. Carbon nanotubes were grown on Si (001) substrate using acetonitrile (1% v/v) and water as electrolyte at an applied d.c. potential ∼20 V. The films were characterized by X-ray Diffraction (XRD), Scanning Electron Microscope (SEM), Raman, optical absorbance, Fourier Transform Infra Red spectroscopy (FTIR) and Electron Spin Resonance (ESR) measurements. The effect of magnetic field on the growth of nanotubes was studied critically. It was found that the presence of magnetic field during electro-deposition played a crucial role on the growth of carbon nanotubes and hence the electronic properties. Photoluminescence (PL) studies indicated band edge luminescence ∼0.72-0.83 eV. Field emission studies indicated lower turn-on voltage and higher current density for films deposited with magnetic field.     

Factors controlling the electrodeposition of metal nanoparticles on pristine single walled carbon nanotubes

The electrodeposition of metal (Pd and Pt) nanoparticles on networks of pristine single walled carbon nanotubes (SWNTs) has been investigated using a microelectrochemical cell. A microcapillary containing electrolyte solution and a reference electrode is contacted with a silicon oxide substrate bearing a SWNT network, connected as the working electrode. Electrodeposition is promoted by applying a potential between the SWNT network and the reference electrode. By combination of analysis of the resulting current-time curves with atomic force microscopy and field emission scanning electron microscopy imaging of the network surfaces after electrodeposition, the nature of metal nanoparticle formation on SWNTs has been elucidated. In particular, the parameters controlling nanoparticle number density, distribution, and size have been identified, with short deposition times and high driving forces favoring the formation of ultrasmall particles at high density. Capacitance and network resistance effects are minimized in the microcapillary configuration, making it possible to accurately analyze short time-scale deposition processes (millisecond time scale). Furthermore, it is also possible to make many measurements on a pristine sample, simply by moving the position of the microcapillary to a new location on the substrate.     

Influence of single wall carbon nanotubes and thermal treatment on the morphology of polymer thin films

Homogeneous and stable thin films of poly(butylene terephthalate) PBT and its nanocomposites based on single wall carbon nanotubes (SWCNTs) were prepared by spin coating. PBT thin films show crystalline structures for thicknesses above 40 nm, consisting of submicrometer size 2D-spherulites. In the case of nanocomposites, carbon nanotubes act as nucleating agents and provide a template for the crystallization of PBT. This gives rise to hybrid shish-kebab structures, even in the thinnest films (∼10 nm thick). Melting and recrystallization provoke the crystallization of PBT and its nanocomposites, and can be used to control morphology. For PBT thin films, the orientation of crystalline lamellae undergoes a transformation, changing from a disposition perpendicular to the substrate (“edge-on”) to a parallel arrangement (“flat-on”) after recrystallization. In the case of the nanocomposites, the CNT influence on the polymer crystallization morphology in thin films is less significant than in the bulk due to the effect of the substrate interactions. Using Raman microscopy it is possible to directly observe both, the degree of dispersion and the location of carbon nanotubes in the films. The results reveal that bigger agglomerates act as more effective nucleating points than isolated bundles of SWCNTs during crystallization of the polymer matrix.     

Emerging transparent electrodes based on thin films of carbon nanotubes, graphene, and metallic nanostructures

Transparent electrodes are a necessary component in many modern devices such as touch screens, LCDs, OLEDs, and solar cells, all of which are growing in demand. Traditionally, this role has been well served by doped metal oxides, the most common of which is indium tin oxide, or ITO. Recently, advances in nano-materials research have opened the door for other transparent conductive materials, each with unique properties. These include CNTs, graphene, metal nanowires, and printable metal grids. This review will explore the materials properties of transparent conductors, covering traditional metal oxides and conductive polymers initially, but with a focus on current developments in nano-material coatings. Electronic, optical, and mechanical properties of each material will be discussed, as well as suitability for various applications.     

Growth and characterization of bamboo-shaped carbon nanotubes using nanocluster-assembled ZnO:Co thin films as catalyst

Bamboo-shaped carbon nanotubes (CNTs) had been successfully fabricated by a plasma enhanced chemical vapor deposition method, in which nanocluster-assembled ZnO:Co thin film was used as catalyst. It was found that bamboo-shaped CNTs were generally grown in a direction perpendicularly to the substrate surface with the tops of CNTs dominated by the droplet-like catalyst covered by the carbon layer. The diameter of CNTs was ranged from 20-50 nm. High resolution of TEM image showed that the typical CNT had a multi-walled structure with an inner core presented. The ordered graphite layers were inclined to an axis of CNT about 18 degrees and the interlayer space of a CNT was about 0.35 nm. Two peaks in Raman spectrum at 1586 cm(-1) and 1372 cm(-1) were identified as G-band and D-band for graphite, respectively. The results showed that catalyst based on ZnO:Co thin films could be used for the growth of CNTs with bamboo-shaped structure.     

Functionalization and solubilization of thin multiwalled carbon nanotubes

The reaction of thin multiwalled carbon nanotubes with a mixture of concentrated HNO₃ and H₂SO₄ has been studied by IR absorption and x-ray photoelectron spectroscopies. The results indicate the attachment of-C(O)OH groups to nanotubes and subsequent conversion of these groups to-C(O)Cl groups via reaction with SOCl₂ and then to-C(O)NR₂ via reaction with didodecylamine. The yield of the carboxylated nanotubes is 53%, and that of the amidated nanotubes is 28%. The O:C atomic ratio in the carboxylated tubes is 1.0:9.0. The solubility of the carboxylated tubes in water is 3.13 g/l, and that of the amidated tubes in chloroform is 1.30 g/l.     

Macrodispersion of multi-walled carbon nanotubes for conductive films

Understanding of the effect of the multi-walled carbon nanotube (MWCNT) dispersion process on physical properties of MWCNT film is crucial in process optimization of MWCNT film-based products. In the present work, the electrical conduction property of MWCNT films according to various conditions in MWCNT dispersion is investigated. Spectroscopic analysis of dispersed MWCNTs show that the electrical resistance of the MWCNT conductive film is affected by an increase in the electrical contacts between adjacent CNTs due to CNT debundling and physical damage caused by ultrasonic processing. Based on the two conflicting parameters, dispersion guidelines for highly conductive MWCNT film are presented.     

X-ray images obtained from cold cathodes using carbon nanotubes coated with gallium-doped zinc oxide thin films

X-ray imaging data obtained from cold cathodes using gallium-doped zinc oxide (GZO)-coated CNT emitters are presented. Multi-walled CNTs were directly grown on conical-type (250 μm-diameter) tungsten-tip substrates at 700 °C via inductively coupled plasma-chemical vapor deposition (ICP-CVD). GZO films were deposited on the grown CNTs at room temperature using a pulsed laser deposition (PLD) technique. Field-emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM) were used to monitor the variations in the morphology and microstructure of the CNTs before and after GZO coating. The formation of the GZO layers on the CNTs was confirmed using energy-dispersive X-ray spectroscopy (EDX). The CNT-emitter that was coated with a 10-nm-thick GZO film displayed an excellent performance, such as a maximum emission current of 258 μA (at an applied field of 4 V/μm) and a threshold field of 2.20 V/μm (at an emission current of 1.0 μA). The electric-field emission characteristics of the GZO-coated CNT emitter and of the pristine (i.e., non-coated) CNT emitter were compared, and the images from an X-ray system were obtained by using the GZO-coated CNT emitter as the cold cathode for X-ray generation.     

Electrochemical performance of diamond-like carbon thin films

Diamond-like carbon (DLC) thin films used in this study were intended for their electrochemical properties. The DLC films were deposited by a filtered cathodic vacuum arc (FCVA) process on p-type silicon (100) substrates biased at different pulse voltages (0-2000 V). The chemical bonding structures of the DLC films were characterized with micro-Raman spectroscopy and the electrochemical properties were evaluated by means of electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization. The DLC films showed high impedance, high polarization resistance and high breakdown potential in a 0.5 M H₂SO₄ aqueous solution, which were attributed to the high sp³ content and uniformity of the films. The excellent chemical inertness of the DLC films made them promising corrosion resistant coating materials.     

Implantation of carbon into thin iron films

Polycrystalline iron films, 1000 Å thick, were implanted with carbon ions at 38 kV to an average concentration of 6 at.%. After implantation and subsequent annealing, lattice parameter and microstructural determinations were made with transmitted electrons. Immediately after implantation, the carbon is retained in supersaturated solid solution and the lattice type is transformed from the normal body-centered cubic of alpha iron to body-centered tetragonal with a c/a ratio of 1.6. After annealing above 330 °C for several hours, the carbon precipitates from solid solution as a carbide phase and the c/a ratio is reduced. These results can be interpreted in terms of the formation and annealing of the metastable martensite constituent of the iron-carbon system.     

Comparison of cytotoxicity of pristine and covalently functionalized multi-walled carbon nanotubes in RAW 264.7 macrophages

Carbon nanotubes may be applied in different fields including biomedicine and mechanical engineering. It is important to understand the potential hazards of carbon nanotubes. In the present study, the toxicological effects of the pristine multi-walled carbon nanotubes (p-MWCNTs) and taurine functionalized multi-walled carbon nanotubes (tau-MWCNTs) were assessed on RAW 264.7 macrophages. We tested cell viability, GSH/GSSG ratio, apoptosis, intracellular calcium concentration, ultrastructural changes of cell morphology, and the release of IL-8. We observed the loss of cell viability, decline in the cellular GSH/GSSG ratio, increase of IL-8, and the increase of intracellular calcium concentration in RAW 264.7 macrophages when exposed to p-MWCNTs at high dosage. Additionally, exposure to p-MWCNTs resulted in ultrastructural and morphological changes in RAW 264.7 macrophages. In contrast, the RAW 264.7 macrophages exposed to the tau-MWCNTs did not exhibit altered morphology. Our results conclude that the tau-MWCNTs show lower toxicity than that of p-MWCNTs.     

Spectroscopic investigation confirms retaining the pristine nature of single-walled carbon nanotubes on dissolution in aniline

Carbon nanotubes in all forms are very much insoluble in both organic and inorganic solvents due to its high agglomeration and entangled morphology. General methods for dissolution of single-walled carbon nanotubes (SWNTs) are mostly associated with complexation or polymerization or addition of macromolecules which change the physical or chemical properties of SWNTs and the pristine nature of SWNTs is lost. Dissolution of SWNTs in a solvent like aniline is practiced here which is a very simple reaction method. Here aniline is capable to form a SWNT-aniline charge transfer complex without attachment of macromolecules or polymer which is also soluble in other organic solvents. Solvation of SWNTs by this method is also capable of maintaining the similarity between the structure of SWNTs before and after the dissolution, which means that the pristine nature of SWNTs is preserved. Formation of charge transfer complex in this reaction has been proven by UV-Vis/NIR absorption and photoluminescence spectroscopy. Raman spectroscopy and electron microscopy (FESEM and TEM) are the evidences for protection of the pristine nature of SWNTs even after high-temperature complexation reaction with aniline and also after solubilization in organic solvents.