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

The 2D CNs can realize the strong band-to-band visible-light absorption. Such visible-light absorption band can enable a number of photons to harvest a larger portion of solar spectrum and reduces the recombination probability of the charge carriers. [Display omitted] •Ultrathin red 2D CNs with a bandgap of 2.05 eV is realized.•The red 2D CNs can effectively increase a number of photons.•IPCE activity of red CNs/TiO2 photoanode significantly exceeds that of TiO2 photoanode. 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 visible-light 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.