Additive-assisted hydrothermal route to synthesize nanoporous carbon microspheres with controlled particle size

Carbon microspheres were synthesized by the hydrothermal treatment of saccharides (fructose and starch) in the presence of additives (HCl and KCl) and the subsequent pyrolysis, and the role of the additives in the hydrothermal reaction was also investigated. The results show that carbon microspheres with well-developed nanoporous structures and surface oxygen functional groups are obtained. Both of the additives in the saccharide solutions induce the enlargement of the size of the microspheres. The mean diameter of the microspheres can be tailored in a range from 0.53 mu m to 6.67 mu m. The morphology of the microspheres depends on the reaction kinetics during hydrothermal treatment. During pyrolysis, the microstructure of the microspheres is transformed from polymer to glassy carbon, which is accompanied by a mass loss of about 50 % and a contraction of over 20% in scale. Both of the additives display distinct effects on the stages of the hydrothermal reaction of the saccharide solutions. The HCl mainly accelerates the hydrolysis kinetic of polysaccharide and the dehydration and fragmentation of monosaccharide, while the KCl chiefly enhances the growth kinetic of the microspheres. The growth of the microspheres is performed through two main ways: the reactive oxygen functionalities present in both the surfaces of the microspheres and the building units and the merged fine microspheres.