The improvement of photocatalytic activity of monolayer g-C3N4 via surface charge transfer doping.

Graphite-like carbon nitride (g-C3N4) has attracted much attention due to its peculiar photocatalytic performance as a visible-light-responsive photocatalyst. However, its insufficient sunlight absorption is not conducive to the photocatalytic activity of the g-C3N4. Herein, by using first-principles density functional theory (DFT) calculations, we demonstrated a simple yet efficient way to achieve improvement of photocatalytic activity of monolayer g-C3N4 via surface charge transfer doping (SCTD) using the electron-drawing tetracyanoquinodimethane (TCNQ) and electron-donating tetrathiafulvalene (TTF) as surface dopants. Our calculations revealed that the electronic properties of monolayer g-C3N4 can be affected by surface modification with TCNQ and TTF. These dopants are capable of drawing/donating electrons from/to monolayer g-C3N4, leading to the accumulation of holes/electrons injected into the monolayer g-C3N4. Correspondingly, the Fermi levels of monolayer g-C3N4 were shifted towards the valence/conduction band regions after surface modifications with TCNQ and TTF, along with the increase/decrease of work functions. Moreover, the optical property calculations demonstrated that the TCNQ and TTF modifications could significantly broaden the optical absorption of monolayer g-C3N4 in the visible-light regions, yielding an improvement in the photocatalytic activity of monolayer g-C3N4. Our results unveil that SCTD is an effective way to tune the electronic and optical properties of monolayer g-C3N4, thus improving its photocatalytic activity and broadening its applications in splitting water and degrading environmental pollutants under sunlight irradiation.