Low-temperature growth of carbon nanofiber using a vapor–facet–solid process

Abstract

Most carbon nanofibers (CNFs) are grown at temperatures higher than 700°C with a chemical vapor deposition (CVD) process and their growths are explained using the vapor–liquid–solid (VLS) mechanism. Herein we report the realization of low temperature growth of CNFs and the interpretation of their growth with a vapor–facet–solid (VFS) mechanism. CNFs were synthesized via a thermal CVD process at the temperature as low as 350°C and characterized using elemental analysis, gas chromatography–mass spectrometry, X-ray photoelectron spectroscopy, and Raman spectroscopy. They feature unique structures of partly ordered discontinuous and hydrogen rich polymer sheets with a diameters of 0.5–1.5nm. Based on a trimerization reaction occurring on the Fe catalyst surface, their initial growth step is the formation of six-membered rings from the source gas (i.e. C2H2). Subsequently, these rings act as structural unit and construct various larger planar molecules. Due to catalytic difference of the crystalline faces for a given Fe catalyst particle, a concentration gradient of hydrocarbon molecule introduces simultaneously. This gradient drives the diffusion of hydrocarbon molecule from the Fe(110) to the Fe(100) face, leading to the formation of disordered hydrogen-rich polymer structures. Highly graphitic CNFs can be obtained simply by annealing those polymer structures at higher temperatures. This growth mode proposed is workable whenever transition metal catalyzed nanostructures are synthesized by a thermal CVD process at low temperatures.