Amand Lucas

The study of carbon nanotubules produced by catalytic method

Abstract Catalytic methods for the production of carbon nanotubules have been developed based on the decomposition of acetylene on well-dispersed metal particles strongly adsorbed on a support. Cobalt on silica was found to be the best catalyst—support combination for the production of graphitic tubules. The method for the catalyst preparation and the reaction conditions were optimized. Straight and coiled carbon tubules were obtained with inner and outer diameter of 3–7 and 15–20 nm, respectively, and up to 300 μm in length. These nanotubules were not coated by amorphous carbon. Traces of amorphous carbon could be removed by hydrogen. High resolution electron microscopy images and electron diffraction patterns of the straight nanotubules were similar to those obtained by the arc-discharge method. Coiled nanotubules were revealed by TEM to be regular polygonized helices where the bends are caused by pairs of pentagon-heptagon carbon rings among the hexagonal network.

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.

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.

The Texture of Catalytically Grown Coil-Shaped Carbon Nanotubules

The growth of micron-size carbon fibres from thermal decomposition of hydrocarbons catalyzed by a metal has been widely studied. Coil-shaped fibres often grow among straight or twisted filaments. Their internal structure has not been studied in detail as yet. In the present work, the thermal cracking of acetylene on Co nanoparticles dispersed on porous silica has produced relatively well graphitized hollow nanotubules, including straight filaments and regular helices. The small diameter of the coiled tubules and the absence of an amorphous coating allowed a determination of their texture by transmission electron microscopy (TEM). The coiled tubules consist of regularly polygonized, coaxial graphene tubes whose angular bends are aligned. The bends are probably caused by the occurrence along the helix of pairs of pentagon-heptagon carbon rings in the hexagonal network. Such a structure was recently predicted to be a thermodynamically stable topology for helical, single-sheet carbon tubes. A molecular model, consistent with theoretical predictions on how to connect cylindrical tubule segments, is provided.

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.

Electron diffraction and microscopy of nanotubes

Carbon nanotubes were discovered by electron microscopy in the carbon soot produced in an electric arc between graphite electrodes, as used in the production of fullerenes. Details of this microstructure have been studied mainly by the combined use of electron microscopic imaging and electron diffraction. Due to the small size of the tubes, diffraction patterns of single tubes, which are the most informative ones, can only be obtained by electron diffraction. For a complete interpretation of the observed diffraction effects a detailed theory is required. Successively more refined approximations of the theory allow us to understand the origin of the different features of the diffraction patterns. The most complete kinematical theory for the diffraction by single shell chiral straight tubes is obtained by the direct summation of the complex amplitudes of the waves scattered by the carbon atoms arranged on a helically wound graphene network. The closed form analytical expressions deduced in this way make it possible to compute the geometry and the intensity distribution of diffraction space. Diffraction patterns are computed as planar sections of this diffraction space. High-resolution electron microscopic images reveal the geometry of individual graphene sheets and their defects in multishell tubes. As well as the characteristic features of straight nanotubes those of helix shaped tubes are also discussed. It is shown how the combined use of electron diffraction and electron microscopy makes it possible to completely characterize the geometry of carbon nanotubes.

Catalytic synthesis and purification of carbon nanotubes

Carbon deposition on a catalyst surface during decomposition of different carbon-containing compounds can be used for the synthesis of carbon nanotubes of graphitic structure. Different supported transition metal oxides were found to be active in the production of these nanotubes. The selectivity of the catalytic method is significantly higher than that of either the arc discharge or the flame method. The experiments were carried out in a flow system, at 700 °C, with various acetylene/nitrogen feeds. Deposited carbon was investigated by transition electron microscopy. The best samples were selected for further investigation and for the synthesis of carbon nanotubes in large amounts. For the purification, different methods were utilized. Separation of nanotubes from the catalyst (support and metal particles) and from other carbon products (soot, fibres) can be carried out only by the combination of ultrasound and various chemical treatments.

Optimization of catalytic production and purification of buckytubes

Carbon nanotubes were produced in large amounts by catalytic decomposition of acetylene in the presence of supported Co and Fe catalysts. The influence of various parameters such as the way of catalyst preparation, the nature of the support, the size of active metal particles, and the reaction conditions on the buckytube formation was studied. The process was optimized towards the large-scale production of buckytubes having the same diameters as the fullerene nanotubes obtained from the arc-discharge method. The separation of the buckytubes from the catalyst was also achieved.

Model structure of perfectly graphitizable coiled carbon nanotubes

Abstract The connection of two straight chiral or achiral cylindrical carbon nanotube sections of approximately the same diameters connecting at a “knee” angle of π 5 is described. Such knees are based on the insertion in the plane of the knee of diametrically opposed pentagonal and heptagonal rings in the hexagonal network. Relationships are also established between the nanotubes and their concentric graphitic layers. A growth mechanism leading to perfect carbon tubules and tubule connections on a catalyst particle at a molecular level is described. The mechanism suggested explains the formation of curved nanotubes, tori or coils involving the heptagon-pentagon construction of Dunlap.