A precursor-dependent distinctive polymerization process controls the optoelectronic properties of graphitic carbon nitride photocatalyst

Abstract

Due to its metal-free polymeric nature, ease of synthesis using low-cost earth-abundant precursors and tunable optoelectronic properties, graphitic carbon nitride (GCN) is extensively used in solar fuel production. Despite two decades of extensive research, the fundamentals of the thermal polymerization process leading to the formation of GCN are inadequately understood. In this work, we employ cyanamide (CYN) and dicyandiamide (DCDA) precursors and systematically reveal the polymerization mechanism. Though CYN has half the amount of C and N compared with DCDA, it yielded virtually similar structural properties and a similar degree of conjugation that determines the energetic difference for π-to-π* fundamental (optical) transitions and photoexcited lifetimes. Detailed complementary analysis using thermal methods, along with quantifying the amount of NH₃ released using the temperature-programmed desorption technique, offered unique insights into the polymerization process. Unlike previous notions, the results unambiguously demonstrate that GCN formation need not always release NH₃ as a result of a thermal condensation reaction. Rather, it is possible that molecular rearrangement (dimerization and/or cyclization) of intermediate condensates can also play a major role in the formation of melamine, which is found to be an important intermediate. The obtained mechanistic insights into the thermodynamics of the polymerization process and its impact on optoelectronic properties and photoelectrochemical performance will aid the rational design of GCN to enhance the efficiency of solar energy conversion.