A handbook for graphitic carbon nitrides: revisiting the thermal synthesis and characterization towards experimental standardization

Abstract

Graphitic carbon nitrides (g-C3N4s) have continued to attract attention as metal-free, low-cost semiconductor catalysts. Herein, a systematic synthesis and characterization of g-C3N4s prepared using four conventional precursors (urea (U), dicyandiamide (DCDA), semicarbazide hydrochloride (SC-HCl), and thiosemicarbazide (TSC)) and an unexplored one (thiosemicarbazide hydrochloride (TSC-HCl)) is presented. Equal synthesis conditions (e.g. heating and cooling rates, temperature, atmosphere, reactor type/volume etc) mitigated the experimental error, offering fair comparability for a library of g-C3N4s. The highest g-C3N4 amount per mole of the precursor was obtained for D-C3N4 (∼37.85 g), while the lowest was for S-C3N4 (∼0.78 g). HCl addition to TSC increased the g-C3N4 production yield (∼5-fold) and the oxygen content (T-C3N4 ∼ 3.17% versus TCl-C3N4 ∼ 3.80%); however, it had a negligible effect on the level of sulphur doping (T-C3N4 ∼ 0.52% versus TCl-C3N4 ∼ 0.45%). S-C3N4 was the darkest in color (reddish brown), and the band gap energies were S-C3N4(2.00 eV) < T-C3N4(2.74 eV) < TCl-C3N4(2.83 eV) ≤ D-C3N4(2.84 eV) < U-C3N4(2.97 eV). The experimentally derived conduction band position of S-C3N4(−0.01 eV) was closer to the Fermi energy level than the others, attributable to high oxygen atom doping (∼5.11%). S-C3N4 displayed the smallest crystallite size (∼3.599 nm by XRD) but the largest interlayer distance (∼0.3269 nm). Furthermore, BET surface areas were 138.52 (U-C3N4), 22.24 (D-C3N4), 18.63 (T-C3N4), 10.51 (TCl-C3N4), and 9.31 m2 g−1 (S-C3N4). For the first time, this comprehensive handbook gives a glimpse of a researcher planning g-C3N4-based research. It also introduces a novel oxygen-sulphur co-doped g-C3N4 (TCl-C3N4) as a new halogen-free catalyst with a relatively high production yield per mole of precursor (∼24.09 g).