Preparation of submicron monodisperse melamine resin microspheres and nitrogen-doped carbon microspheres derived from them

Melamine-formaldehyde (MF) microspheres were prepared using acetic acid as the catalyst without aid of any surfactant, and were carbonized to obtain nitrogen-doped carbon microspheres. The effects on the microstructure of stirring time, acid amount and thickness of the layer of MF spheres spread in the crucible during carbonization, and the particle size and distribution of the MF and carbon microspheres were investigated by SEM, TEM, FTIR, TGA, XPS, nitrogen adsorption and elemental analysis. Results indicated that by increasing the stirring time from 1 min to 48 h, the median particle size of the MF microspheres increased rapidly in the first 10 min from 0.6 to 1.6 μm, levelled off from 1.6 to 1.7 μm between 10 and 120 min, then decreased slowly to 1.055 μm from 120 min to 24 h and then levelled off again from 1.055 to 1.047 μm between 24 and 48 h. The particle size became narrower and the surface smoother with increasing stirring time. Increasing the amount of acetic acid reduced the particle size, but broadened the size distribution and led to particle aggregation. The thermal stability and carbonization yield increased and the particle shrinkage during carbonization decreased with stirring time since the molecular weight of the polymers that formed the MF microspheres increased with stirring time. The carbonized samples produced using a thin spread layer have better sphericity and monodispersion than those from a thick and dense spread layer. Severe particle aggregation was found regardless of the carbonization temperature in the latter case. The carbon microspheres had lower nitrogen and oxygen, but higher carbon, contents with a longer stirring time, and nitrogen atoms were in the form of pyridinic N(N-6), pyrrolic-N(N-5) and quaternary-N/graphitic-N(N-4) nitrogen. The carbon microspheres had an inverse core-shell structure with a mesoporous shell and a dense carbon core.