Residual Metal Catalyst Research Articles

Purification of Single-Wall Carbon Nanotubes by Selective Microwave Heating of Catalyst Particles

A new scalable method for purification of single-wall carbon nanotubes (SWNTs) by using microwave heating in air is developed. The microwaves couple to the residual metal catalyst, raising significantly the local temperature leading to both the oxidation and rupturing of the carbon passivation layer over the metal catalyst particles. With this protective carbon coating weakened or removed, a mild acid treatment is then sufficient to remove most of the metal in the sample, leaving the nanotubes in tact. Using microwave processing and 4 M HCl acid reflux for 6 h we were able to remove residual metal (Ni, Y) in arc-discharge SWNTs to a level lower than 0.2 wt % (∼0.04 at. %). Results from transmission and scanning electron microscopy, Raman spectroscopy, and thermogravimetric studies were discussed.

Synthesis of SiC nanorods from sheets of single-walled carbon nanotubes

We report the unexpected synthesis of SiC nanorods during thermally annealing single-walled carbon nanotube sheets (SWNTs) at 1000 °C between two silicon wafers. The exterior layers of the carbon nanotube sheets were converted into a network of SiC nanorods, while the carbon nanotubes interior to the sheet remain unchanged. The nanotube sheets that underwent this reaction were comprised of small diameter nanotubes made by a high-pressure carbon monoxide process (HiPco), which contain iron catalyst. Using the same reaction conditions, SiC formation was not observed for sheets of purified, larger diameter nanotubes made by laser ablation, which contained a small amount of residual metal catalysts.

Purification and structural annealing of multiwalled carbon nanotubes at graphitization temperatures

In this work, we present a systematic study of the effects of graphitization on the structural perfection of multiwalled carbon nanotubes. High purity nanotubes were produced by a low temperature CVD method and subsequently annealed at temperatures between 1600 and 3000°C. The nanotubes were characterized for chemical purity, interlayer spacing, and defect healing. The graphitization procedure was found to remove residual metal catalyst in the nanotubes and reduce the wall defects as reflected in a reduced interlayer spacing between the graphene shells. Graphitization presents a low-cost, commercially viable method of purifying and ordering multiwall carbon nanotubes.

Purification of SWNTs Using Microwave Heating

AbstractA new method for purifying single wall carbon nanotubes (SWNTs) using microwave heating is developed. The microwaves couple to the residual metal catalyst, raising significantly the local temperature leading to both the oxidation and rupturing of the carbon passivation layer over the metal catalyst particles and sintering. With this protective carbon coating weakened or removed, a mild acid treatment in HCl is then sufficient to remove most of the metal in the sample, leaving the nanotubes in tact. Results from transmission and scanning electron microscopy (TEM & SEM), Raman spectroscopy and thermo-gravimetric studies are discussed.

The use of thermal analysis in the characterization of a polymer surface

Fuel cells are electrochemical systems that convert hydrogen into electricity without combustion. Proton exchange membrane (PEM) fuel cell power systems consist of a polymer membrane, finely dispersed catalyst and a gas humidifying system. Thermogravimetry (TGA) and differential scanning calorimetry (DSC) can be used to characterize the physical properties of a polymer membrane. Ion chromatography can monitor the presence of contaminating ions in the aqueous condensate. TGA can measure decomposition of the polymer material and the residual metal catalyst that remains, which can be correlated to the distribution of metal catalysts on a polymer surface. DSC can be used to measure the concentration of Teflon in a polymer blend by measuring the melting enthalpy.

Anomalous temperature dependence of the resistivity of single-wall carbon nanotubes

The anomalous behavior of conductivity observed in experiment of single-wall carbon nanotubes is shown to be due to the effects of residual transition metal catalysts residing in close contact with the nanotube walls. A combination of analysis of the density of states based on the tight-binding molecular dynamics and a simple model calculation provides a good description of the resistivity behavior observed in experiment.