Synthesis and characterization of helically coiled carbon nanotubes

Abstract

Although coiled carbon nanofibers can be synthesized on large scale, the selective synthesis of coiled carbon nanotubes is still a challenge for the scientific community. In the present work we aimed to produce helically coiled CNTs (HCNTs) in at least 10% of the product, taking in consideration previous works. Alumina-, sepiolite and silica supported Co-; Fe-; Co-Fe and Co-Pr catalysts were prepared and tested in CNT synthesis reactions applying different conditions. Sepiolite and alumina supported catalysts showed low activity in the synthesis of HCNTs. The helices were maximum 1-2% of all the synthesized CNTs. A more detailed study was carried out with silica supported catalysts. The catalysts were prepared with the ion-adsorption-precipitation (IAP) method. A preliminary study was carried out to observe the behavior of different silica supported catalysts. Co-; Fe-; Co-Fe and Co-Pr catalysts were prepared with different metal loadings. Special attention was dedicated to the Co-Pr catalysts that showed higher activity in the HCNT production. The 2.5%Co-2.5%Pr; 4%Co-1%Pr and the 1%Co-4%Pr catalysts were chosen for a more detailed study. The effect of the temperature, the carbon source flow and the carrier gas flow variation was tested in the synthesis of helically coiled carbon nanotubes. The most favorable reaction conditions for the HCNT synthesis in our conditions are presented in table 1. Catalyst Reaction temperature (°C) C2H2 flow (ml/min) N2 flow %HCNTs 2.5%Co-2.5%Pr 700 30 300 15 4%Co-1%Pr 650 30 300 14-15 4%Co-1%Pr 700 30 600 14-15 1%Co-4%Pr 700 30 300 15-17 1%Co-4%Pr 700 30 600 14-15 Table 1.: Most favorable reaction conditions for HCNT formation over 5%Co-Pr catalysts with different metal ratios The synthesis products contain approximately 15% helically coiled MWCNTs. This gives the possibility to apply the synthesized HCNTs in nanocomposite materials, and exploit the peculiar properties of these structures. However, the purification process of the samples should be optimized. The characteristics, such as coil diameter and coil pitch, of the helices produced on 5%Co- Pr/SiO2 catalysts were analyzed. The morphology of these helices vary from wavy coils, Sshaped tubes, to tight helices and loose telephone-cord-like nanotubes. Their coil diameter varies from 25 nm to 270 nm but the most frequent values are present in the range of 25-50 nm. The coil pitch varies from 20 nm to 300 nm. The most frequent coil pitch values ae between 20-130 nm. Occasionally, helical tubes with coil pitch higher than 300 nm were found in the samples. In most of the samples S-shaped elongated coils are prevalent, however, tight coils are typical for the synthesis using 1%Co-4%Pr and the 4%Co-1%Pr catalysts applying different nitrogen flows. The aim of this work to obtain more than 10% of helically coiled carbon nanotubes in the synthesized product was achieved, however further study is needed to understand the formation of the helices and the role of the different reaction parameters