(Invited) Synthesis of Helical-CNT and Stacked Graphene-CNF Using a Copper Foil As Catalyst in a Chemical Vapor Deposition System

Abstract

Helical carbon nanotubes (H-CNT) and stacked graphene carbon nanofibers (SG-CNF) were synthesized in an aerosol assisted chemical vapor deposition system (AACVD) using a copper foil as catalyst and substrate and ethanol vaporized as precursor. The copper foils were cleaned and pre-treated with ethanol, isopropanol and acetone in sonication bath during one hour each to obtain a slight surface modification. The AACVD was carried in one tubular furnace at 980 °C where several copper foils of 0.25 mm thickness and 4 cm x 1 cm area were placed inside the reactor from the highest to the coolest temperature zone. This carbon nanostructures were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction, Raman spectroscopy, atomic force microscopy (AFM) and cyclic voltammetry using a Calomel electrode in a KCl 0.5 M solution as electrolyte. A variety of carbon morphologies were found passing from the growing of graphite at the highest temperature to the formation of the H-CNT and SG-CNF at the temperature dropping zone (figure 1a-1f). Migration of sharp copper nanoparticles (10 nm – 200 nm) was observed as copper nanoparticles were found at the surface covered by carbon materials, while as we go out the furnace, faceted copper nanoparticles are partially covered of graphitic materials. A diminution of the Cu(200) crystalline plane of the copper foil as the temperature decreases is visible and the formation of copper oxide at the farthest position play a significant role in the growing of the carbon nanostructures. The H-CNT are helical structures with no practically gap between each coil with a pitch of ~27 nm by a tubular structure with ~20 graphitic layer in each wall (figure 1g-1i), in the case of the SG-CNF the graphitic layers are almost perpendicular to the CNT axis similar to a stacked short graphene layers directed outwards the structure that leds to the edge formation. Raman characterization illustrates differences between the graphitic carbon structures found at early stages from those at the dropping temperature zone where at the higher temperature mainly graphite the relation of the ID/IG bands is 0.88, while for the H-CNT and SG-CNF the ID/IG relation is 1.79 indicative of high defective graphitic structure which is related with the “herringbone structure” of the SG-CNF, in the case of the coolest zone the Raman spectra is characteristic of low sp3 amorphous carbon (ID/IG of 0.67). The cyclic voltammetry results indicates that the sample with the H-CNT and SG-CNF structures becomes more capacitive in a cuasi-reversible process with practically no modification of the standard oxide-reduction potential for Cu, while for the other sections this oxide-reduction potential increases. Another aspect of the sample with the H-CNT and SG-CNF is its hydrophobic surface with a contact angle of 122 °. With this work we demonstrate that a light surface modification of the copper foil and the temperature profile inside the furnace has a high influence in the formation of the H-CNT and the SG-CNF, also the shape of the copper nanoparticle influence the type of carbon nanostructure catalyzed over the copper substrate. This material could be a good for oxide-reduction reactions. Figure 1