Renzo Marazzi

Synthesis of multiwall carbon nanotubes by electric arc discharge in liquid environments

This work reports the experimental results from the production of multiwall carbon nanotubes (MWCN) synthesized by an electric arc discharge performed in liquid environments between pure graphite electrodes. Both liquid nitrogen and deionised water were suitable for a successful synthesis of this form of carbon aggregation. We report a successful synthesis of MWCN by arc discharge submerged in deionised water. Electron microscopy observations of both the reaction products and the surface of the as-synthesized raw material showed the presence of structural degradation of the MWCN, which probably operates after their growth at the cathode. The degradation is tentatively ascribed to a combination of overheating and high current density experienced by the as-synthesized MWNT, which can be caused by the loose structure of the as-deposited material. The damage appeared to be less severe in water environments, probably owing to the better cooling capacity of water relative to liquid nitrogen.

AC arc discharge synthesis of single-walled nanohorns and highly convoluted graphene sheets

In this paper we report the synthesis of single-walled carbon nanohorns (SWNHs) and highly convoluted graphene sheets by AC powered arc discharge in air. The experimental strategy was based on the design of an experimental set-up able to enhance the synthesis of these compounds at the expense of the usually found cathode crust where multi-walled nanotubes are often found. In this set-up the arc discharge is ignited between pure graphite electrodes experiencing a symmetrical situation as far as the heat generation by the arc and the heat dissipation at the heat sinks is concerned. This is achieved by AC powering two electrodes of the same size and length. A cylindrical steel collector able to control the heat exchange with the surroundings completes the experimental set-up. The yield of the device, in terms of synthesis rate and of fraction of sublimated carbon converted to nanostructures, has been evaluated, and it resulted in being particularly high, indicating that AC powering can represent a key factor in the large-scale synthesis of SWNHs and highly convoluted graphene sheets by arc discharge. The structure of the synthesized powder has been determined by transmission electron microscopy.

Arc-discharge synthesis of carbon nanohorns and multiwalled carbon nanotubes

Carbon nanohorns and multiwalled carbon nanotubes have been synthesized by DC arcdischarge carried out at room pressure in air and Ar-enriched environment, by a specially designed experimental device. The resulting nanostructured material, characterized by electron microscopy and X-ray diffraction, shows different structures according to the condensation channels through which the sublimated carbon atoms are re-condensed in the solid state. Multi-Walled Carbon Nano- Tubes are mainly found in the hard crust formed at the cathode, while nano-horned particles can be recovered from a cylindrical collector surrounding the discharge. Further material, rag-like shaped and with an amorphous structure, can be collected in the reaction area. When the discharge occurs under Ar atmosphere, a larger quantity of this latter phase is synthesized. This suggests that the atmospheric oxygen could play an active role by burning the most reactive among the synthesized phases, like amorphous carbon contributing so to an “in situ” purification of the raw material.

Quantitative evaluation of nanotube content produced by arc discharge in a raw material

The amount of carbon nanotube in a raw material grown by electric arc discharge, ignited in different liquid environments, has been evaluated by combined use of X-ray diffraction, electron microscopy and differential thermal analysis (TG-DTA) in a reactive environment. X-ray diffraction was used to discriminate the various carbon microstructures utilizing the difference in lattice parameter between curved and planar graphite structures. The results are repeatable and in good agreement with TG-DTA measurements carried out in a reactive environment, where the relative amount of carbon nanostructures is evaluated on the basis of the difference in the reaction kinetics with atmospheric oxygen. In analyzed specimens, SEM and TEM show the presence of only two allotropic forms of carbon, namely nanotubes and globular particles, the relative amounts of which depend on the nature of the liquid surrounding the discharge and on the voltage applied to the electric arc.

Improvement of DMFC Electrode Kinetics by Using Nanohorns Catalyst Support

One of the factors limiting direct methanol fuel cells (DMFC) performance is the slow kinetics of methanol oxidation at the anode. The importance of the catalyst support for fuel cells has been recognized and different forms of carbon have been suggested. Single wall nanohorns (SWNH) are a new class of carbon with a similar graphitic structure of carbon nanotubes. They are self-assembling materials that produce aggregates of about 100 nm. In the present study, the comparison of the performance of a DMFC equipped with electrocatalysts supported on a commercial carbon black and on SWNH was carried out. The SWNH were synthesized by the arc discharge method in air. The deposition of the Pt and Pt/Ru catalysts on the carbon supports was accomplished by using ethylene glycol as reducing agent. The synthesized catalyst nanoparticles have a very small diameter size (ca. 2.5 nm) and they are uniformly distributed on both carbon supports. The supported electrode catalysts were tested in a DMFC and results indicate that employing SWNH is very promising showing catalytic activities 60 % higher.

Carbon Nanostructures Produced by an AC Arc Discharge

Carbon nanostructures are under deep investigation due their peculiar properties and possible applications. In particular, development of new methods for the synthesis of these materials and their mechanism of formation represent interesting research fields. Arc discharge allows to produce different forms of carbon nanostructures. The parameters involved in the process, voltage, current density, type and pressure of the surrounding gas can be controlled especially for achieving high quantity of material with enhanced characteristics in terms of purity while the use of transition metal-graphite mixtures has been used to produce single wall structures. Moreover direct current (DC) and alternating current (AC) are suitable for producing carbon nano-materials, but different results can be obtained. In this work the effect of the power frequency in an AC arc discharge technique on the synthesis of carbon nanostructures is reported. Pure graphite electrodes have been arched in air in an homemade apparatus where the material can be collected directly on a cylindrical collector fixed near the arc. In order to avoid the formation of deposits under the arc a symmetrical configuration of the electrodes has been set. The production of carbon soot containing Single Wall Nanohorns (SWNH) and highly convoluted graphene sheets is optimized. The range of power frequencies 32-1000Hz has been investigated and the arcs have been ignited fixing the voltage at 28 V. The materials has been analyzed by field emission scanning electron microscope and high resolution transmission electron microscope. The microstructure of the material synthesized by this apparatus is affected by the power frequency, as the experimental results demonstrate. The samples produced at low frequency presented high amounts of single wall structures, SWNH-type. More compact structures, similar to large onion-like structures, have been found in samples synthesized at high frequency values.

Microstructure of Pt particles and aggregates deposited on different carbon materials for fuel cells application

PEM (polymer electrolyte membrane fuel cells) and DMFC (direct methanol fuel cells) have demonstrated to be suitable devices in order to realize a widespread diffusion of electrical-H2 fed vehicles in the near future [1].