Polyvinyl pyrrolidone-assisted synthesis of size-tunable polymer spheres at elevated temperature and their conversion to nitrogen-containing carbon spheres.

Polyvinyl pyrrolidone (PVP)-stabilized polymer spheres synthesized at elevated temperature by Stӧber-like method are carbonized to obtain carbon spheres. Their size is tunable in the range from 875 nm to 60 nm by adjusting the amount of PVP during synthesis at elevated temperature. These spheres are obtained by using resorcinol and formaldehyde as carbon precursors, ethylene diamine (EDA) as nitrogen source and basic catalyst, and PVP as stabilizer and an additional source of nitrogen dopant. A synergistic effect is shown between (i) elevated temperature that facilitates the formation of a large number of small polymeric nuclei and (ii) varying amount of PVP that controls the growth of formed nuclei by hindering polymerization of resorcinol and formaldehyde. The elevated temperature synthesis not only produces monodispersed carbon spheres but also eliminates the need for hydrothermal treatment that generally requires high-pressure autoclave vessel. The nitrogen content increases from 3.0 wt% to 5.0% with increasing PVP amount. Besides nitrogen originating from ethylene diamine, an additional amount of doped nitrogen in carbon spheres is supplied by PVP during its decomposition. The resulting carbon spheres were subjected to the post-synthesis CO2 activation in order to improve their structural properties. The surface area of activated carbon spheres increased almost twice from (516 m2/g-581 m2/g) to (1010 m2/g-1060 m2/g). A significant enhancement of microporosity as well as presence of nitrogen species in activated carbon spheres resulted in high CO2 uptake at ambient temperatures. The tunable size, high microporosity, nitrogen doping and cost-effective synthesis make these carbon spheres attractive for a variety of uses ranging from adsorption, catalysis, electrode materials to biomedical applications.