Accelerated photocatalytic hydrogen evolution over donor–acceptor type graphitic carbon nitride (g-CN) with simultaneous modification of pyrimidine and thiophene rings

Abstract

Graphitic carbon nitride (g-CN) has attracted interest due to its cost-effectiveness, ease of synthesis, and suitable band structure for hydrogen evolution. However, its application is limited by high charge recombination rates and restricted visible light absorption, which lower photocatalytic performance. This study presents a modified g-CN catalyst, termed UPDB, incorporating π-conjugated and donor–acceptor (DA) structures using urea, 2,4,6-triaminopyrimidine (P), and dibenzothiophene-2-carboxaldehyde (DB). DRS and PL measurements revealed that alongside the π–π* transitions originating from pristine g-CN, UPDB exhibits n–π* transitions influenced by the lone pair electrons and unpaired electrons present in P and DB. This interaction creates a new absorption band (midgap) that broadens visible-light absorption. FT-IR analysis confirmed that the electron donor DB binds to the end of the g-CN backbone, while DFT calculations suggested that DB induces a spatial separation between the HOMO and LUMO, significantly decreasing charge recombination. At the optimal dosage, the hydrogen evolution rate of UPDB-10 (U (10 g), P (10 mg), and DB (1 mg)) reached 1000 μmol g⁻¹ h⁻¹, which was approximately 10 times higher than that of the original carbon nitride (U) calcined from urea alone. Furthermore, the apparent quantum yield (AQY) was 13.7% at 400 nm, 15.5% at 420 nm, and 6.3% at 450 nm in the presence of K₂HPO₄ (KPH), demonstrating high visible-light responsivity. The one-pot calcination method used in this study to introduce π-conjugation and a DA structure provides a novel approach to overcome the limitations of g-CN, paving the way for the advancement of solar energy conversion technology.