Abstract:

Photocatalytic hydrogen production is a promising strategy for utilizing inexhaustible solar energy as a source of clean energy. Graphitic carbon nitride (g-C₃N₄) is a widely used photocatalytic material in photocatalytic hydrogen production because of its simple preparation process, suitable band structure, and high stability. However, the low charge carrier separation efficiency and small specific surface area of pristine g-C₃N₄ restrict its photocatalytic activity. It has been demonstrated that the construction of intramolecular donor-acceptor (D-A) systems and ultra-thin nanosheet structures are effective strategies for enhancing the photocatalytic activity of g-C₃N₄. Herein, an intramolecular D-A structured g-C₃N₄ nanosheet photocatalyst is synthesized through the thermal copolymerization of dicyandiamide and methylene blue (MB), followed by thermal exfoliation. X-ray diffraction, Fourier transform infrared spectrometry, solid-state ¹³C nuclear magnetic resonance, and X-ray photoelectron spectroscopy analyses reveal that MB is successfully incorporated into the g-C₃N₄ framework and well retained after thermal exfoliation. The resulting D-A system induces intramolecular charge transfer from the donor units (MB segment) to the acceptor units (tri-s-triazine rings) and extends the absorption edge to approximately 500 nm. The ultra-thin nanosheet structure produced by thermal exfoliation shortens the charge transfer distance from the interior to the surface of g-C₃N₄ and reduces the charge transfer resistance, which increases the charge carrier separation efficiency. Furthermore, the introduction of MB generates a flaky structure during copolymerization, which promotes thermal exfoliation and results in a remarkably increased specific surface area. The transient photocurrent response, electrochemical impedance spectra, and time-resolved photoluminescence decay spectra reveal that the charge transfer and separation of g-C₃N₄ are further promoted by integrating the intramolecular D-A system and ultra-thin nanosheet structure. Density functional theory calculations further demonstrate that MB donates electrons to tri-s-triazine rings (electron acceptor). Moreover, the highest occupied molecular orbit of D-A structured g-C₃N₄ is mostly distributed around the MB segment, while the lowest unoccupied molecular orbit is distributed around tri-s-triazine rings, resulting in spatially separated photogenerated electron-hole pairs. Through integrating the intramolecular D-A system and ultra-thin nanosheet structure, the obtained photocatalyst exhibits enhanced charge carrier separation, an extended absorption edge, and enlarged specific surface area. As a consequence, the D-A structured g-C₃N₄ nanosheet shows a considerably improved photocatalytic hydrogen production activity (2275.6 μmol·h⁻¹·g⁻¹), which is 5.30, 2.60, and 1.30 times that of bulk g-C₃N₄, D-A structured bulk g-C₃N₄, and g-C₃N₄ nanosheet, respectively. This work offers a valuable strategy for developing D-A-modified photocatalytic materials for solar energy conversion.

Key words: Donor-acceptor, Methylene blue, g-C₃N₄ nanosheet, Photocatalytic hydrogen production