Infrared Irradiation-Lattice Vibration Coupling-Initiated N Electronic Transition in Carbon Nitride Nanosheets for Increased Photocatalysis

Enabling n ->pi* electronic transition in graphitic carbon nitride (g-CN) can significantly enhance its photocatalytic performance by extending visible light absorption. However, achieving this transition remains a grand challenge due to the selection rule that restricts it to planar tri-s-triazine units of g-CN. Here, an effective strategy to induce the n ->pi* transition in g-CN nanosheets by coupling infrared irradiation with lattice vibrations is presented. Theoretical simulations reveal two prominent vibration modes perpendicular to tri-s-triazine units at 219 and 737 cm-1 can promote the corrugation of tri-s-triazine units and facilitate the n ->pi* electronic transition when coupled with infrared irradiation. Additionally, these vibrational modes enable the efficient exfoliation of bulk g-CN into 2D nanosheets by overcoming the interplanar binding energy. Through femtosecond transient absorption spectra (fs-TAS), the n ->pi* transition prolongs the lifetime of photocatalytically active shallow electron trapping states and charge separation states in g-CN is demonstrated. Accordingly, the activated n ->pi* electronic transition and prolonged lifetime of photocatalytically active charges lead to an H2 generation rate of 2485 mu mol h-1 gcat-1, approximate to 40 times higher than that of the bulk counterpart. This work highlights the effectiveness of coupling lattice vibrations with infrared irradiation to drive the n ->pi* transition for enhanced visible-light photocatalysis. The lattice vibration modes perpendicular to heptazine units in g-CN, which induce both bulk exfoliation into nanosheets and the formation of corrugated heptazine building blocks, are identified as the origin of n ->pi* electronic transition. This transition enables extended visible light absorption and a prolonged lifetime of photocatalytically active charges, thereby resulting in a remarkable 40-fold enhancement of photocatalytic H2 generation.image