Three-dimensional graphene with the decoration of elemental sulfur-based sulfur-nitrogen dots for CO₂ capture

Using a mild hydrothermal process, elemental sulfur was directly used to develop uniformly decorated sulfur-nitrogen (NS) dots on three-dimensional (3D) graphene. UV-Visible spectroscopy confirmed that hydrophobic elemental sulfur complexed with the primary amine in polyethylenimine (PEI) solution to form a hydrophilic sulfur-amine complex, and the resulting solution served as a nitrogen (N) and sulfur (S) source while improving dispersibility in GO solution. Hydrothermal treatment of aqueous GO and PEI-sulfur solution produces NS co-doped 3D graphene foam, while x-ray diffraction (XRD) and Raman spectroscopy results reveal GO reduction to graphene, and thermal properties are determined using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). X-ray photoelectron spectroscopy (XPS) confirms NS doping on graphene sheets in the foam structure, while scanning and transmission electron microscopy (SEM and TEM) revealed homogeneous dispersion of NS dots with an average size of 5 nm. The high dispersibility and compatibility of amine-sulfur in aqueous GO results in the formation of ultra-small, uniformly distributed NS dots on graphene layers, whereas the amount of S and N in PEI solution is easily controlled by adjusting the stoichiometric ratio. Furthermore, the textural properties of NSG-1 were determined, and the obtained BET surface area was 245 m²/g, while the pore volume and average pore size were 1.91 cm³/g and 31.1 nm, respectively. The developed NS-co-doped graphene exhibits a room-temperature adsorption capacity of 2.77 mmol/g at 1 bar, with faster kinetics and CO₂/N₂ capture selectivity of 16.45 and 9.19 at 0.1 and 1 bar, respectively.