The synthesis of carbon nanotubes at low temperature via carbon suboxide disproportionation

Carbon nanotubes were synthesized via a novel route using an iron catalyst at the extremely low temperature of 180 °C. In this process, carbon suboxide was used as carbon source, which changed to freshly formed free carbon clusters through disproportionation. The carbon clusters can grow into nanotubes in the presence of Fe catalyst, which was obtained by the decomposition of iron carbonyl Fe₂(CO)₉ at 250 °C under nitrogen atmosphere. The products were characterized with XRD, TEM, HRTEM and Raman spectroscopy.     

Optimisation of the arc-discharge production of multi-walled carbon nanotubes

The effect of varying current density and pressure during arc generation on the yield and purity of multi-walled nanotube-containing carbon soot has been studied in this work. Various soots were produced and characterised qualitatively using transmission electron microscopy and quantitatively using electron paramagnetic resonance and thermogravimetric analysis. It was found that both yield and purity increase as current density and pressure are increased to the limit of our experimental investigations, i.e. 195 A/cm² and 500 Torr of helium. Under these conditions a yield of 24 mg/min soot containing 48% by mass nanotubes was obtained.     

Catalyst deactivation in CVD synthesis of carbon nanotubes

A computational fluid dynamics (CFD) model with multistep chemical reactions was applied to predict the yield of multiwalled carbon nanotubes produced from our xylene-based chemical vapor deposition (CVD) reactor. Two-step xylene decomposition in the gas phase and catalytic decomposition of hydrocarbons to nanotubes on the growth surfaces were adopted based on exhaust-gas composition measurements. Using the experimentally obtained exhaust-gas concentrations, we conducted inverse calculations to determine apparent rate constants of the catalytic surface reactions. During the CVD process, catalyst deactivation was observed probably due to carbon formation on the catalyst surface. Its effect on the apparent rate constant was empirically correlated with a simple exponential equation. Applying the CFD model, we predicted the local yielding rate of nanotubes along the axis of the reactor. The total yield was then computed by integrating the local yielding rate over the growth surfaces and compared favorably (∼95%) with the experimental results. The proposed model is expected to help researchers optimize the process parameters to achieve the maximum nanotube yield.     

Catalytic activity of carbon nanotubes in the oxidative dehydrogenation of ethylbenzene

Multi-walled carbon nanotubes (MWNT), before and after different oxidative treatments and hence possessing different oxygen surface groups, were used as catalysts for the oxidative dehydrogenation of ethylbenzene (ODE), and their performances compared to those of activated carbon and graphite samples. MWNT are active catalysts for ODE and show the highest specific activity per initial surface area amongst all the materials tested in this study. Moreover, the main advantage of using MWNT over activated carbons is their higher stability under oxidative conditions. It was shown that pre-oxidised MWNT are more active for ODE during the initial stages of the reaction, highlighting the importance of oxygenated surface groups.     

Controlling the diameter distribution of carbon nanotubes grown from camphor on a zeolite support

Single-wall and multi-wall carbon nanotubes (SWNTs and MWNTs, respectively) of controlled diameter distribution were selectively grown by thermal decomposition of a botanical hydrocarbon, camphor, on a high-silica zeolite support impregnated with Fe-Co catalyst. Effects of catalyst concentration, growth temperature and camphor vapor pressure were investigated in wide ranges, and diameter distribution statistics of as-grown nanotubes was analyzed. High yields of metal-free MWNTs of fairly uniform diameter (∼10 nm) were grown at 600-700 °C, whereas significant amounts (∼30%) of SWNTs were formed at 850-900 °C within a narrow diameter range of 0.86-1.23 nm. Transmission electron microscopy and micro-Raman spectroscopy reveal that camphor-grown nanotubes are highly graphitized as compared to those grown from conventional CNT precursors used in chemical vapor deposition.     

Multi-walled carbon nanotubes on amorphous carbon films

Nanotubular structures composed of layered graphite sheets or other layered materials have been studied intensely by both scanning and transmission electron microscopy. In this paper, we will show how graphite structures, that are inherent to the production process of the amorphous carbon support films, used for both SEM and TEM studies can be easily mistaken for the actual sample structures. We will further report that these artifacts appear in both commercial as well as homemade holey carbon support films on copper grids, and suggest that to successfully study the “real” nanotubular structures only support films made from materials other than carbon should be used.     

High purity multiwalled carbon nanotubes under high pressure and high temperature

We have studied the structural modifications that multi-walled carbon nanotubes (MWNTs) undergo when treated under high pressure and high temperature (HP-HT). Uniaxial pressures of 5.5-6.5 GPa were applied to samples of purified MWNTs between 850 and 950 °C for 30 min. The resulting material was then analysed by X-ray diffractometry, Raman spectroscopy and transmission electron microscopy. Comparison of the data obtained by such analyses before and after treatment does show changes in the Raman spectra, a slight linear decrease of the d₀₀₂ spacing, and some structural modifications that could be due to MWNT segmentation and interlinking.