Borax-assisted hydrothermal carbonization to fabricate monodisperse carbon spheres with high thermostability

A modified hydrothermal carbonization (HTC) process in the presence of borax was developed to produce uniform and monodispersed micro-carbon spheres. Corn starch was used as carbon precursor and borax acts as a structure directing agent. The introduction of borax could catalytically transform the starch into hydroxymethylfurfural (HMF) and favoring the rapid nucleation and slow growth of particle owing to the buffer function of borax additive. The inclusion of borax thus allowed for the controlling of particles morphology as well as tailoring the particle diameters. Uniform solid carbon spheres with a narrow particle size distribution, ranging from 0.6 to 2 μm, was demonstrated by turning the borax concentration and the reaction time in a single-step hydrothermal process. The retained boron content and chemical structure of the carbonaceous products could be adjusted as revealed by inductively coupled plasma (ICP) and Fourier transformed infrared (FTIR) analysis. Furthermore, the boron-dopant increased the final product's thermostability from 536 °C to 629 °C at 0.70 wt% boron content. This method therefore also provides a facile and novel approach to prepare boron-doped carbon materials.