Chemical vapour deposition syntheses and characterization of boron-doped hollow carbon spheres

Abstract

We report a novel and simple method for the growth of boron-doped hollow carbon spheres (B-HCSs) synthesized via chemical vapour deposition (CVD) at 900°C using acetylene as the carbon precursor and trimethyl borate as a carbon and boron source. The B-HCSs as well as BOx filled carbon spheres (CSs) were synthesized using a vertical (or horizontal) furnace and the properties of the produced materials were compared to undoped CSs synthesized in a vertical furnace. The morphology and size, as well as the shell thickness of the B-HCSs was not substantially affected by varying the ratio of Ar to H2 carrier gas ratio and the type of furnace used (horizontal vs. vertical furnace); the yield was affected by the Ar/H2 ratio. Thermogravimetric analysis confirmed the presence of B2O3 (25–35%) in the core of the unpurified product and this B2O3 could be removed by boiling water. High-angle annular dark-field scanning transmission electron microscopy analysis confirmed the presence of B2O3 in the centre of the carbon spheres. B-HCSs synthesized with 100% Ar and also 100% H2 were analysed by X-ray photoelectron spectroscopy (XPS). The level of boron content in these samples was found by XPS to be 0.19wt% B when made under H2 while the product made under Ar contained 3.92wt% (mainly B2O3, most of which could readily be removed by water). The final residual B found in all samples is associated with B doping of the HCS. The B2O3 is proposed to act as a template for the formation of the B-HCS structures and a mechanism for the B-HCS formation is proposed. The use of the vertical reactor provides for the continuous synthesis and large scale production of B-HCSs and is preferred over the horizontal furnace.