A graphitic carbon nitride photocatalyst with a benzene-ring-modified isotype heterojunction for visible-light-driven hydrogen production

Abstract

Semiconductor-based photocatalysts show promise for generating H₂ through water splitting. In particular, graphitic carbon nitride (g-C₃N₄) has drawn attention owing to its narrow bandgap and noteworthy optical, structural, and physicochemical attributes. However, pristine g-C₃N₄ allows rapid charge-carrier recombination, which impedes its photocatalytic performance. Therefore, isotype heterojunction formation and aromatic ring incorporation—strategies that have thus far been implemented separately—were combined in this study to enhance the charge-carrier separation and photocatalytic hydrogen-generating activity of g-C₃N₄. Essentially, g-C₃N₄-based photocatalysts with various urea/thiourea/N-phenylthiourea ratios (denoted as UTPh) were synthesized and incorporated with benzene rings and characterized using various techniques. The isotype heterojunction formation and aromatic ring insertion were found to significantly improve the morphology, bandgap, charge-carrier separating tendency, and ultimately, the hydrogen production rate of g-C₃N₄ under visible-light irradiation. The optimal UTPh specimen exhibited a photocatalytic hydrogen production rate that was 6.1 and 20 times higher than those of urea- and thiourea-based g-C₃N₄, respectively, and the hydrogen generation rate almost doubled in the presence of K₂HPO₄. The efficiency and durability of the modified photocatalyst were validated by stability and reusability tests. Density functional theory calculations provided insights into the electronic structural changes and corroborated the experimental findings. The modifications to the energy band structure induced by the isotype heterojunction formation and aromatic ring substitution likely increased the light absorption, charge-carrier separating behavior, and overall photocatalytic activity of g-C₃N₄. This study on modified g-C₃N₄ photocatalysts offers valuable insights that advance sustainable hydrogen production and mitigate energy-related and environmental challenges.