Klára Hernádi

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.

CVD synthesis of high-purity multiwalled carbon nanotubes using CaCO3 catalyst support for large-scale production

Abstract Multiwalled carbon nanotubes (MWCNTs) were synthesized in a catalytic reaction using CaCO3 as catalyst support. Impregnated with conventional catalysts CaCO3 enabled the production of MWCNTs in a fixed-bed flow reactor at relatively low reaction temperature. The purification was performed in one step: both metallic particles and catalyst support were dissolved in a diluted acid. Hence, disadvantages, namely multi-step processes and hazardous chemicals, were avoided. A further advantage of CaCO3 support is efficient and stable growth of MWCNTs. First results obtained in our new rotary-tube oven indicated high quality and pure MWCNTs with a yield of 100 g/day by this method.

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.

PATTERNED FILMS OF NANOTUBES USING MICROCONTACT PRINTING OF CATALYSTS

change in the vibrational structure of the PL spectrum were effected by MWNTs. The reduction of the PL efficiency can be a result of energy transfer and partial hole transfer from PPV chains to MWNTs, together with scattering and absorption by MWNTs. Using the composite, photovoltaic devices have been fabricated by employing MWNT as a hole-collecting electrode. We obtained good quantum efficiency (1.8 % at 2.9±3.2 eV), about twice that of the standard ITO device. It is considered that the high efficiency arises from a complex interpenetrating network of PPV chains with MWNTs and the relatively high work function of the MWNT film. The present results suggest the possible application of carbon nanotubes as a new interesting electrode material in macroscale devices.

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.

Synthesis of MWNT-based composite materials with inorganic coating

Abstract Multiwalled carbon nanotube (MWNT) based metal oxide composites were prepared by an impregnation method using organometallic compounds as precursor. Aluminium isopropoxide (AlIP), tetraethyl orthosilicate (TEOS), and tetraethyl orthotitanate (TEOTi) were used as inorganic sources and decomposed by hydrolysis on the surface of carbon nanotubes. The composites were subsequently investigated by transmission electron microscopy and their coverage was compared. A direct, solvent-free impregnation technique turned out to be the most successful for all organometallic compounds and provided homogeneous inorganic cover layer on the surface of purified MWNTs.

Reactivity of different kinds of carbon during oxidative purification of catalytically prepared carbon nanotubes

In the purification procedure of catalytically prepared carbon nanotube samples, the oxidative removal of amorphous carbon was investigated. Oxidation was carried out by KMnO4, H2O2, O3 or HClO4. The selectivity of the amorphous carbon burning reaction was studied as a function of the reaction temperature by transmission electron microscopy. The advantages of the different methods are as follows: although oxidation by KMnO4 in an acidic suspension provides nanotubes free of amorphous carbon, and can easily be controlled, it must be followed by filtration and cc HCl treatment in order to dissolve the MnO2 formed during the reaction. Oxidation with hydrogen peroxide does not give undissolved residue on the surface of the tubes. Ozone treatment produces gaseous by-products, but requires higher reaction temperature. This method is more difficult to control. During oxidation, competitive oxidation always takes place: carbon nanotubes and amorphous carbon react simultaneously. While amorphous carbon can be attacked from any direction, carbon nanotubes can be oxidized only from the ends.

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.