Enhanced directional transfer of charge carriers and optimized electronic structure in fluorine doped polymeric carbon nitride nanosheets for efficient photocatalytic water splitting

The high photogenerated charge carrier recombination and sluggish oxygen evolution reaction (OER) kinetics of polymeric carbon nitride (PCN) photocatalysts limit their application in photocatalytic water splitting. Herein, fluorine (F) doped PCN (PCNF-x) nanosheets with high crystallinity were prepared using dicyandiamide (C₂H₄N₄) and ammonium hydrogen fluoride (NH₄HF₂) through high temperature thermal polymerization. This process not only resulted in PCNF-x nanosheets with a large number of pores, but also improved the crystallinity of PCNF-x nanosheets. Under illumination, the PCNF-0.5 nanosheets exhibited an excellent photocatalytic water splitting activity with a comparable H₂ evolution rate of 135.30 μmol h^-1 g^-1 and O₂ evolution rate of 63.75 μmol h^-1 g^-1, which were 2.3-fold, 3.3-fold, and 25-fold as compared to those of PCNF-1, PCNF-0.2, and pristine PCN nanosheets, respectively. Photoluminescence (PL) spectra and density functional theory (DFT) calculations indicate that F doping of PCN nanosheets brings two changes in PCNF-x nanosheets, one is the increase in crystallinity after F doping effectively weakens the bulk defects of PCNF nanosheets, which is conducive to the directional transfer of charge carriers; the other is the modulation of the electronic structure after F doping, which optimizes the reaction mechanism of the OER in PCNF-x nanosheets. Both the enhancement in charge carrier transfer and the optimization of the reaction mechanism significantly contribute to the improved photocatalytic performance of water splitting in the fluorine doped PCN nanosheets.