Alkali-assisted hydrothermal route to control submicron-sized nanoporous carbon spheres with uniform distribution

Abstract

Carbon microspheres with nanoporous structures were synthesized based on hydrothermal reaction of saccharide solutions in the presence of potassium hydroxide. The influence of synthesis conditions, including types of saccharide, content of alkali, concentration and reaction time of saccharide solution, on the morphology of the microspheres was systematically investigated, with emphasis on controlling the microsphere size and understanding the role of alkali in hydrothermal reaction. The results show that the microspheres with regular spherical shapes and high surface areas around 500m2g−1 are obtained. The morphology of the microspheres can be tailored by changing the synthesis conditions. The mean diameter of the microspheres varies from 0.16 to 5.61μm. At different stages of the hydrothermal reaction of starch solution, alkali exhibits entirely different effects: (i) to slow the initial hydrolysis of starch, and (ii) to accelerate the subsequent polymerization of dehydrated and fragmented products and the growth of the microspheres. As a result, the competition between the hydrolysis kinetics and the polymerization and growth kinetics plays a critical role in the changes of the morphology and yield of the microspheres. The alkali in dilute starch solution (0.073–0.219M) promotes the increase of the size and yield of the microspheres. In contrast, the alkali in concentrated starch solution (0.292M) results in the reduction and the more uniform distribution of the size of the microspheres with almost constant yield. Note that this alkali assisted hydrothermal route can endow the microspheres with more surface functional groups, including hydroxyl and carboxyl groups, compared with the typical hydrothermal route without alkali. In addition, the formation of the stacks of graphene sheets and the decrease of functional groups occur in the microspheres during pyrolysis, which are accompanied by a significant weight loss of about 50% and a slight contraction in scale.