Realization of ultrathin red 2D carbon nitride sheets to significantly boost the photoelectrochemical water splitting performance of TiO2 photoanodes

Ultrathin 2D carbon nitride sheets (CNs) are potential materials for solar energy conversion. However, these ultrathin sheets usually suffer from a wider bandgap of 3.06 eV, and thus nothing but ultraviolet range absorption. Here, we realized that red CNs has a narrower bandgap of 2.05 eV with a strong band-to-band visiblelight absorption band by fluorination of atomic CNs followed by thermal defluorination (F-DF). Theoretical calculation results indicate that (i) a defect band arises near the conduction band minimum (CBM) after F-DF, resulting in a downward shift of the CBM, and (ii) the band structures are highly associated with the spatial distribution of defects in ultrathin 2D CNs. Furthermore, we confirm that such visible-light absorption band of red CNs can enable the number of photons to harvest a larger portion of solar spectrum, and suppress the rapid recombination of the photogenerated carriers. Consequently, the red CNs-sensitized TiO2 nanorod-based photoanode exhibits an outstanding photoelectrochemical water splitting activity in the visible light range. This work presents a new strategy for using 2D wide-bandgap material of bandgap narrowed to achieve a strong band-to-band visible-light absorption band.