X.B. Zhang

A Formation Mechanism for Catalytically Grown Helix-Shaped Graphite Nanotubes

The concept of a spatial-velocity hodograph is introduced to describe quantitatively the extrusion of a carbon tubule from a catalytic particle. The conditions under which a continuous tubular surface can be generated are discussed in terms of this hodograph, the shape of which determines the geometry of the initial nanotube. The model is consistent with all observed tubular shapes and explains why the formation process induces stresses that may lead to spontaneous plastic deformation of the tubule. This result is due to the violation of the continuity condition, that is, to the mismatch between the extrusion velocity by the catalytic particle, required to generate a continuous tubular surface, and the rate of carbon deposition.

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.

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.

Carbon nano-tubes; their formation process and observation by electron microscopy

Abstract Carbon nano-tubes have been investigated thoroughly by transmission electron microscopy. Non-helical tubes are not exceptional and mixing with helical tubes with various helicities is often observed within the same tubule. The helical nature of the tubes is thought to be actually required by lattice continuity during the formation of successive seamless concentric tubes. Besides, introducing dislocations is also a possible accommodation mechanism for the strains produced between the successive tube surfaces. Reciprocal space constructions have been accomplished for explaining the diffraction effects for non-helical tubes as well as for helical ones. Well-resolved high-resolution images are obtained in particular for the non-helical tubes and the image features could be confirmed by computer simulations. A possible growth process for the successive concentric tubes is proposed.

A Structure Model and Growth Mechanism for Multishell Carbon Nanotubes

A model that postulates a mixture of scroll-shaped and concentric, cylindrical graphene sheets is proposed to explain the microstructure of graphite multishell nanotubes grown by arc discharge. The model is consistent with the observed occurrence of a relatively small number of different chiral angles within the same tubule. The model explains clustering in a natural way and is consistent with the observation of asymmetric (0002) lattice fringe patterns and with the occurrence of singular fringe spacings larger than c/2 (c is the c parameter of graphite) in such patterns. Anisotropic thermal contraction accounts for the 2 to 3 percent increase in the c parameter of nanotubes, compared with bulk graphite, but is too small to explain the singular fringe spacings. The model also explains the formation of multishell closure domes. Nucleation is attributed to the initial formation of a fullerene dome.

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.

Electron microscopy study of coiled carbon tubules

Abstract Carbon fibres prepared by the catalytic decomposition of acetylene over finely dispersed cobalt, supported on amorphous silica, are often helix shaped. The structure of such carbon fibres was studied by means of various electron diffraction techniques and by electron microscopy. It was shown that the tubules are hollow and consist of concentric cylindrical graphene sheets. The diffraction patterns are analysed in detail and compared with theoretically predicted patterns. Good semi-quantitative agreement is found. It is shown that the helices are polygonized; their atomic structure is consistent with models predicted on the basis of molecular dynamics simulations.

The reciprocal space of carbon tubes : a detailed interpretation of the electron diffraction effects

Abstract Carbon tubules have been studied by means of transmission electron microscopy and in particular by electron diffraction. It has been found that streaks perpendicular to the axis of the tubule occur at most spots in the diffraction patterns. The streaks often exhibit a fine structure. All diffraction phenomena can be well interpreted in terms of a reciprocal space model. Tilting experiments about an axis perpendicular to the needle axis allowing the exploration of reciprocal space and a quantitative analysis of the geometry of the electron diffraction patterns in terms of the tilting angle and reciprocal vectors are shown to confirm the model.

Catalyst traces and other impurities in chemically purified carbon nanotubes grown by CVD

Multiwall carbon nanotubes grown by the catalytic decomposition of acetylene over supported Co catalyst were subjected to wet and dry oxidation in order to remove the unwanted products and the catalyst traces. The effects of the purification treatment on the Co content Ž. Ž . was monitored by physical methods: Rutherford Backscattering Spectrometry RBS , Particle Induced X-Ray Emission PIXE and Ž. X-Ray Fluorescence XRF . The purified products were investigated by microscopic methods: TEM, Scanning Electron Microscopy Ž. Ž . SEM , Energy Dispersive Spectroscopy EDS and STM. The KMnOrH SO aqueous oxidation procedure was found to be effective in 42 4 reducing the Co content while damaging only moderately the outer wall of the nanotubes. Treatment in HNO rH SO yields a 32 4 bucky-paper like product and produces the increase of the Si and S content of the sample. q 2002 Elsevier Science B.V. All rights reserved.

EM, XPS and LEED study of deposition of Ag on hydrogenated Si substrate prepared by wet chemical treatments

The deposition of Ag on a hydrogenated Si(111) substrate, prepared by wet chemical treatment, was carried out at room temperature (RT), 250°C and above 350°C. The samples were examined in situ by LEED, XPS and then by SEM and TEM. Our results show that flat Ag crystallite domains are growing on the HSi(111) surface at all three substrate temperatures. The 〈hw〉 ratio of the islands increases but the density decreases with increasing substrate temperature which is attributed to the differences between hydrogenated and clean Si(111) surface. The so-called A-type and B-type domains resulting from analyses of TOF ICISS are attributed to twinning in the deposited layer.