Precursor-dependent distinctive polymerization process controls optoelectronic properties in 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 for solar fuel production. Despite two decades of extensive research, the fundamental understanding of the thermal polymerization process leading to the formation of GCN is inadequately understood. In this work, we employ cyanamide (CYN) and dicyandiamide (DCDA) precursors and systematically reveal polymerization mechanism. Though CYN has half the number of C and N content than DCDA, it yielded virtually similar structural properties, degree of conjugation that determines the energetic difference between π to π* fundamental (optical) transition and photoexcited lifetimes. Detailed complementary analysis using thermal methods along with quantifying the amount of NH₃ released using temperature programmed desorption technique offered unique insights into the polymerization process. Unlike previous notion, results unambiguously demonstrate that GCN formation need not always release NH₃ as a result of thermal condensation reaction. Rather, it is amenable that molecular rearrangement (dimerization and/or cyclization) of the intermediate condensates would also play a major role in the formation of melamine, which is found to be an important intermediate. Obtained mechanistic insights into thermodynamics of polymerization process and its impact on optoelectronic properties and photoelectrochemical performance will aid in rational design of GCN to enhance the efficiency of solar energy conversion.