Metal-catalyst-free growth of carbon nanotubes/carbon nanofibers on carbon blacks using chemical vapor deposition

Abstract

Metal-catalyst-free growth of carbon nanostructures, e.g. carbon nanotubes (CNTs) or carbon nanofibers (CNFs), on typical materials is a promising route to achieve further applications without interference of unwanted metal catalysts. Here, we report that carbon black (CB) can act as a catalyst to controllably grow CNTs or CNFs yielding bi-functional hybrid structure-CNTs/CB or CNFs/CB through a metal-catalyst-free chemical vapour deposition (MCF-CVD) in a reaction temperature range of 800–1000 °C. We find that the decomposition steps of the carbon source are a crucial factor to determine the formation of CNTs (ethylene), CNFs (acetylene) or amorphous carbons (ethanol and cyclohexane) evidenced by the gas-composition analysis. The growth yields of CNTs or CNFs catalysed by various CBs display a linear trend corresponding with the surface areas of CBs, indicating that structural morphologies of CB dominate the growth of the carbon nanostructures. The formation of CNTs/CB or CNFs/CB is proposed to obey the vapour–solid–surface–solid model with an activation energy of 59.0 kJ mol⁻¹. A growth mechanism of CNTs/CB or CNFs/CB is suggested involving a direct dehydrogenation route of carbon sources (ethylene and acetylene) occurring on the CB surfaces through self-assembly. The multi-step decomposition route of the carbon sources (ethanol and cyclohexane) would generate amorphous carbons only. These findings open a new route to prepare unique carbon structures through a metal-catalyst-free CVD.