W. K. Hsu

Controlled production of aligned-nanotube bundles

Carbon nanotubes might be usefully employed in nanometre-scale engineering and electronics. Electrical conductivity measurements on the bulk material, on individual multi-walled and single-walled nanotubes and on bundles of single-walled nanotubes have revealed that they may behave as metallic, insulating or semiconducting nanowires, depending on the method of production—which controls the degree of graphitization, the helicity and the diameter. Measurements of Youngs modulus show that single nanotubes are stiffer than commercial carbon fibres. Methods commonly used to generate nanotubes—carbon-arc discharge techniques, catalytic pyrolysis of hydrocarbons and condensed-phase electrolysis—generally suffer from the drawbacks that polyhedral particles are also formed and that the dimensions of the nanotubes are highly variable. Here we describe a method for generating aligned carbon nanotubes by pyrolysis of 2-amino-4,6-dichloro-s-triazine over thin films of a cobalt catalyst patterned on a silica substrate by laser etching. The use of a patterned catalyst apparently encourages the formation of aligned nanotubes. The method offers control over length (up to about 50 μm) and fairly uniform diameters (30–50 nm), as well as producing nanotubes in high yield, uncontaminated by polyhedral particles.

Enhanced magnetic coercivities in Fe nanowires

We describe a way of generating films (<2 mm2; <40 μm thick) of aligned Fe-filled carbon nanotubes. These Fe nanowires are usually composed of single Fe crystals, and have dimensions from 5–40 nm outer diameter and <10 μm in length. The carbon tubes, which coat the wires, have external diameters of ∼20–70 nm and are <40 μm in length. High-resolution electron energy loss spectroscopy, x-ray powder diffraction, and elemental mapping of the tubular structures reveal only characteristic metallic signals and the effective absence of oxygen (or any other nonmetallic element) within the wires. The material exhibits coercivities in the 430–1070 Oe range, i.e., greater than those reported for Ni and Co nanowires.

Metal particle catalysed production of nanoscale BN structures

Abstract Graphite-like nanostructures including nanotubes and encapsulated polyhedral particles have been obtained by arcing hexagonal boron nitride (h-BN) and tantalum in a nitrogen atmosphere. High resolution electron microscopy (HRTEM), electron energy loss spectroscopy (EELS) and X-ray diffraction studies have been used to study these new materials, which contain B:N ratios of ≈ 1:1. The observations reveal interesting new information on the dynamics of metal cluster catalysed nanostructure formation. The structures provide strong circumstantial evidence for the presence of B 2 N 2 squares at the tips, in addition to B 3 N 3 hexagons in the main body of the tubes.

Efficient route to large arrays of CNx nanofibers by pyrolysis of ferrocene/melamine mixtures

We report a high-yield route to two-dimensional arrays (<400×400 μm2) of aligned C49Nx (x ⩽ 1) nanofibers (<100 nm o.d.; <60 μm length), by pyrolyzing mixtures of ferrocene and melamine at 950–1050 °C under an Ar flow. The fibers exhibit unusual interlinked stacked-cone morphologies, ascribed to the presence of nitrogen. High-resolution electron energy-loss spectroscopy of the individual fibers reveals a 2% nitrogen content with ionization energies mainly at ∼400.9 eV, corresponding to N bonded to three C atoms within a hexagonal framework. The nanofibers may be useful for the economic fabrication of field emission sources and robust composites.

Electrolytic formation of carbon nanostructures

Abstract Carbon nanotubes (with and without encapsulated material) as well as nanoparticles and onion-like structures have been generated by electrolysis in molten alkali halide salts using carbon electrodes under an argon atmosphere. The nature of the products depends upon several factors including the electrolysis voltage and current, depth of electrode immersion in the electrolyte, the length of time the current is maintained and the electrolyte.

Carbon nanotubes as polymer antioxidants

The oxidation of polystyrene, polyethylene, polypropylene and poly(vinylidene fluoride) is retarded by carbon nanotubes. Incorporation of boron into the nanotubes enhances the electron affinity of the tubes and leads to a small increase in antioxidant efficiency.

Pyrolytically grown BxCyNz nanomaterials: nanofibres and nanotubes

Pyrolysis of CH3CN · BCl3 at ca. 900–1000°C over Co powder generates novel graphitic BxCyNz nanofibres and nanotubes possessing a range of morphologies (e.g. curled, branched and bent). In these experiments the metal particles play an important role in the growth since nanotube formation appears to occur at the metal surface. High resolution electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS) studies suggest that the stoichiometry of the filaments is ca. [BC2Nz]n (z = 0.3−0.6). Higher relative concentrations of carbon occur at the tips where the metal particle is found. A possible growth mechanism for these new materials is proposed.

Selective Co-catalysed growth of novel MgO fishbone fractal nanostructures

Abstract Novel MgO fishbone- or fern-like nanostructures, and Al 2 O 3 fibre networks, have been generated for the first time from an Al 2 O 3 matrix containing MgO and SiO 2 . The significance of the selective Co-catalysed growth of the elegant three-dimensional fractal fishbones is discussed. We have adopted the vapour–liquid–solid (V–L–S) and nucleation–aggregation (N–A) mechanisms, which take effect equally during the heating process, to account for the formation of these uniquely stacked fractal structures.

Compartmentalized CNx nanotubes: Chemistry, morphology, and growth

A systematic study of the effect of different synthesis parameters on N incorporation into C nanotubes is presented. CNx nanotubes prepared by catalyzed pyrolysis of melamine exhibit a highly compartmentalized morphology with a remarkable periodicity structure along the nanotube axis. Spatially resolved electron energy loss spectroscopy (spectrum-imaging mode) indicates that the nanotubes are made of carbon and nitrogen, inhomogeneously distributed with an enrichment of carbon on the external surfaces. The evolution of the C-K-edge shape across the nanotube reveals a transition from a graphitic stacking on the outside to a disorganized-mixed type in the core of the nanotube. For the N-K edge, the situation is more complex. The fine structure of the N-K edge differs depending on the used catalyst, which indicates differences in the bonding configuration. When Ni is used as a catalyst, N replaces C in the graphitic structure whereas C–N pyridinic-like bonds are formed when the catalyst is Fe. The compartmen...

Preparation of aligned carbon nanotubes catalysed by laser-etched cobalt thin films

Abstract Pyrolysis of 2-amino-4,6-dichloro-s-triazine over laser-etched thin films of cobalt deposited on an inverted silica substrate generates aligned bundles and films of carbon nanotubes of uniform dimensions. Scanning electron microscopy, high resolution transmission electron microscopy, energy dispersive X-ray analyses, electron energy loss spectroscopy and electron diffraction studies reveal that the aligned tubes, which usually grow perpendicular to the substrate surface, are mainly straight (length ⩽100 μm; 30–50 nm OD), contain traces of nitrogen (