Kinetically driven graphite-like to diamond-like carbon transformation in low temperature laminar diffusion flames

Abstract

Historically, the synthesis of diamond and graphite via combustion flames stands out as a simplified, scalable and inexpensive approach. Unfortunately, this method is not beneficial for industrial applications in coatings due to limitations related with the high flame and substrate temperatures. Here, we report novel findings about the formation mechanism of graphite-like and diamond-like supported nanostructures in low temperature laminar diffusion flames. Both materials are formed upon controllable combustion at atmospheric pressure of a cylindrical paper wick immersed in rapeseed oil. An accurate adjustment of the incident air flow and the amount of available fuel allow deposition of carbon soot or diamond-like carbon (DLC). The DLC formation is favorable in a narrow stoichiometric range at flame temperatures within ~210–260°C and beyond this range the particles precipitate as soot. The comparative structural analysis using scanning electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and transmission electron microscopy, along with the full thermal and stoichiometric profiles for the chosen combustion conditions, suggest a kinetically driven graphite-to-diamond transformation rather than a thermodynamically induced phase transition. Our results reveal a new direction in the principles of graphite and diamond formation in flames that could be applied to surmount the existing shortcomings in flame synthesis.