Physicochemical properties of nitrogen-doped carbon nanotubes from metallocenes and ferrocenyl imidazolium compounds

Abstract

Shaped carbon nanomaterials (SCNMs) were synthesized via the chemical vapour deposition (CVD) technique by using typical metallocenes (ferrocene, nickelocene, cobaltocene, and ruthenocene), and more interestingly, by use of novel ferrocenyl imidazolium derivatives, containing -Cl (FcImCl), -NO2 (FcImNO2) and -CH3 (FcImCH3) substituents as catalysts. Acetonitrile was applied both as a carbon and nitrogen source at temperatures 800–900 °C. The SCNMs, namely, carbon nanotubes (CNTs), carbon spheres (CS), carbon fibres (CF) and amorphous carbons (ACs) were obtained in varying ratios depending on the catalyst and carbon sources. The ferrocenyl imidazolium catalysts produced nitrogen-doped CNTs (N-CNTs) with bamboo-like structures. The yields of various reactions were temperature-dependent, with the highest amount of N-CNTs obtained at 850 °C. In all samples, the composition was mainly of CS and N-CNTs except for nickelocene at 800 °C that gave CFs as a “minor” product. Ferrocene and nickelocene in acetonitrile produced well-aligned N-CNTs while cobaltocene and ruthenocene gave ‘spaghetti-like’ structures. In the case of ferrocenyl imidazolium catalyst, a coiled N-CNTs morphology was produced from FcImCl catalyst. Also, higher percentage of N-CNTs with traces of CS were obtained from the FcImCl and FcImCH3 catalysts in acetonitrile at 850 °C, while higher percentage of CS and AC were obtained for FcImNO2 catalyst. In all the catalysts, the use of acetonitrile promoted nitrogen-doping (samples with more disordered and with smaller outer-diameters). Thus, this study demonstrates that the synthesis of N-CNTs from nitrogen-containing ferrocenyl imidazolium compounds as catalyst sources, provided higher percentage of N-CNTs which can be suitable for various application.