A. Fonseca

Structural Defects Play a Major Role in the Acute Lung Toxicity of Multiwall Carbon Nanotubes: Physicochemical Aspects.

Carbon nanotubes (CNT) have been reported to elicit toxic responses in vitro and in vivo, ascribed so far to metal contamination, CNT length, degree of oxidation, or extent of hydrophilicity. To examine how structural properties may modulate the toxicity of CNT, one preparation of multiwall CNT has been modified (i) by grinding (introducing structural defects) and subsequently heating either in a vacuum at 600 degrees C (causing reduction of oxygenated carbon functionalities and reduction of metallic oxides) or in an inert atmosphere at 2400 degrees C (causing elimination of metals and annealing of defects) and (ii) by heating at 2400 degrees C in an inert atmosphere and subsequently grinding the thermally treated CNT (introducing defects in a metal-deprived carbon framework). The presence of framework and surface defects, metals, and oxygenated functionalities was monitored by means of a set of techniques, including micro-Raman spectroscopy, adsorption calorimetry, X-ray photoelectron spectroscopy, inductively coupled plasma mass spectrometry, and atomic emission spectroscopy. Contrary to traditional toxicants, such as asbestos, CNT may quench rather than generate oxygenated free radicals. The potential of the modified CNT to scavenge hydroxyl radicals was thus evaluated by means of electron spin resonance spectroscopy (spin trapping). The original ground material exhibited a scavenging activity toward hydroxyl radicals, which was eliminated by heating at 2400 degrees C but restored upon grinding. This scavenging activity, related to the presence of defects, appears to go paired with the genotoxic and inflammatory potential of CNT reported in the companion paper. Thus, defects may be one of the major factors governing the toxic potential of CNT.

Fe-catalyzed carbon nanotube formation

Abstract The catalytic production of carbon nanotubes was investigated using various iron catalysts. Catalyst samples were made by different preparation methods in order to improve both the quality and the quantity of as-prepared carbon nanotubes. The catalysts were tested in the decomposition of different hydrocarbons in the temperature range 650–800 °C using either fixed bed flow or fluidized bed reactor. The quality of the products was characterized by means of transmission electron microscopy. By using Fe/silica, the highest activity ever observed in catalytic nanotube formation can be reached.

Production of short carbon nanotubes with open tips by ball milling

Short multi-wall carbon nanotubes can be obtained by ball milling. The average length of the ball milled carbon nanotubes, synthesised by decomposition of acetylene on different types of supported metal catalysts, is ca. 0.8 μm. The cleavage was caused by the collision between one agate ball and the nanotube powder contained in an agate mortar.

CATALYTIC PRODUCTION AND PURIFICATION OF NANOTUBULES HAVING FULLERENE-SCALE DIAMETERS

Carbon nanotubules were produced in a large amount by catalytic decomposition of acetylene in the presence of various supported transition metal catalysts. The influence of different parameters such as the nature of the support, the size of active metal particles and the reaction conditions on the formation of nanotubules was studied. The process was optimized towards the production of nanotubules having the same diameters as the fullerene tubules obtained from the arc-discharge method. The separation of tubules from the substrate, their purification and opening were also investigated.

Catalytic synthesis of carbon nanotubes using zeolite support

Catalytic synthesis of carbon nanotubes having fullerene-like structure applying supported transition metal/zeolite catalysts is introduced in this work. Decomposition of unsaturated hydrocarbons was carried out under relatively mild conditions in a fixed bed flow reactor. The quality of the carbon deposit was characterized by means of transmission electron microscopy. For the separation of carbon nanotubes and catalyst particles, chemical methods were applied.

Control of the outer diameter of thin carbon nanotubes synthesized by catalytic decomposition of hydrocarbons

Abstract Multi-wall carbon nanotubes have been produced by the catalytic decomposition of acetylene. Co–Mo, Co–V and Co–Fe mixtures supported either on zeolite or corundum alumina were used as catalysts. When Fe or V is added to Co, the carbon deposit increases. The nanotubes were characterized by both low and high resolution TEM. From histograms representing the outer diameter distributions, it is clear that the outer diameter of the nanotubes can be controlled by choosing the appropriate catalyst.

Large scale production of short functionalized carbon nanotubes

A simple mechano-chemical modification of multiwall carbon nanotubes is described. The use of ball-milling in specific atmosphere allows us to introduce functional groups like thiol, amine, amide, carbonyl, chlorine, etc. onto carbon nanotubes. The resulted functional groups are characterized using infrared spectroscopy and X-ray photoelectron spectroscopy.

Production of nanotubes by the catalytic decomposition of different carbon-containing compounds

Carbon nanotubes were prepared in the catalytic decomposition of different carbon containing compounds over supported transition metal catalysts. Besides acetylene, ethylene, propylene, acetone, n-pentane, methanol, toluene, and methane were tested and each resulted in carbon nanotube formation. The quality of as-made nanotubes was investigated by TEM and was found to be at least as good as obtained in acetylene decomposition. Ethylene and propylene showed somewhat lower reactivity in the buckytube formation with respect to acetylene, simultaneously suppressed formation of amorphous carbon on the outer surface was found.

Structural and electronic properties of bent carbon nanotubes

Atomic models of two single joints connecting (a) (9, 0) to (5, 5) and (b) (10, 0) to (6, 6) nanotubes have been constructed and relaxed on the computer using a molecular-mechanics model. Each connection is based on a pair of diametrically opposed pentagon and heptagon which bend the structure. The electronic properties of these metal-metal and semiconductor-metal junctions are explored within a tight-binding description of the π bands of the carbon sp2 network.

Different purification methods of carbon nanotubes produced by catalytic synthesis

Different purification methods of carbon nanotubes produced by catalytic process are described, based on the reactivity differences between carbon nanotubes and impurities (principally amorphous carbon). The quality, but also the major modifications of the nanotubes caused by these treatments are investigated by transmission electron microscopy.