Awakening n-π* electron transition in structurally distorted g-C3N4 nanosheets via hexamethylenetetramine-involved supercritical CO2 treatment towards efficient photocatalytic H2 production

Abstract

Graphitic carbon nitride (g-C₃N₄) has been regarded as highly potential photocatalyst for solar energy utilization. However, the restricted absorption of visible light for pristine g-C₃N₄ significantly limits the solar-light-driven chemical reaction efficiency. Herein, structurally distorted g-C₃N₄ nanosheets with awakened n-π* electron transition were successfully synthesized through hexamethylenetetramine (HMTA)-involved supercritical CO₂ (scCO₂) treatment and following pyrolysis of melamine precursor. ScCO₂ treatment was conductive to homogeneously dissoving melamine precursor and HMTA, and then the modification by HMTA with three-dimensional structure changed the g-C₃N₄ photocatalyst from a symmetrical planar structure to an asymmetrical non-planar structure. The resulting awakened n-π* electron transition in structurally distorted g-C₃N₄ nanosheets greatly extended the photoresponse range of g-C₃N₄ and increased the amount of catalytically active π electrons. Moreover, the unique distorted structure of g-C₃N₄ enhanced photogenerated charge carriers separation and provided sufficient reactive sites for photocatalytic H₂ production. Consequently, the structurally distorted g-C₃N₄ nanosheets exhibited enhanced photocatalytic H₂ production performance, which was up to 6.4 times that of pristine g-C₃N₄. This work presents a promising scCO₂ strategy towards precursor treatment to regulate the microstructure of g-C₃N₄, and provides valuable guidance to obtain efficient g-C₃N₄ photocatalyst by microstructure engineering.

Keywords: Graphitic carbon nitride; Hydrogen; Microstructure engineering; Photocatalysis; Solar energy; Supercritical fluid.