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.

Enhanced visible light absorption performance of SnS2 and SnSe2 via surface charge transfer doping.

The layered two-dimensional (2D) SnS2 and SnSe2 have received intensive attention due to their sizable band gaps and potential properties. However, it has been shown that the visible light absorption of SnS2 and SnSe2 are restricted as photocatalysts and light-harvesting material absorbers for water splitting and high-performance optoelectronic devices. Herein, to enhance the visible light absorption performance of SnS2 and SnSe2, we performed a systematic investigation on tuning the electronic and optical properties of monolayers SnS2 and SnSe2 via surface charge transfer doping (SCTD) with the adsorption of molybdenum trioxide (MoO3) and potassium (K) as surface dopants based on density functional theory. Our calculations reveal that MoO3 molecules and K atoms can draw/donate electrons from/to SnS2 and SnSe2 as acceptors and donors, respectively. The adsorption of MoO3 molecules introduces a new flat impurity state in the gap of the monolayers SnS2/SnSe2, and the Fermi level moves correspondingly to the top of valence band, resulting in a p-type doping of the monolayer SnS2/SnSe2. With the adsorption of K atoms, the electrons can transfer from K atoms to the monolayer of SnS2 and SnSe2, making K an effective electron-donating dopant. Meanwhile, the bandgaps of monolayers SnS2 and SnSe2 decrease after the MoO3 and K doping, which leads to the appearance of appreciable new absorption peaks at around ∼650/480 and ∼600/680 nm, respectively, and yielding an enhanced visible light absorption of SnS2 and SnSe2. Our results unveil that SCTD is an effective way to improve the photocatalytic and light-harvesting performance of SnS2 and SnSe2, broadening their applications in splitting water and degrading environmental pollutants under sunlight irradiation.