Abstract
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-C3N4via 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.
Highlights
Graphitic carbonic nitride (g-C3N4) has attracted much attention since it was rst developed to be a visible-light-driven photocatalyst by Wang and co-workers in 2009 due to its abundance, high stability, and excellent capacity for solar utilization.[1]
It can be noted that the structure of monolayer g-C3N4 was obviously bending a er TCNQ and TTF surface modi cation due to the non-covalent interaction between the surface dopants and monolayer g-C3N4
Mulliken charge analysis of bond population[54,55] for the TCNQ modi ed systems show that there are about À0.11 charge transfers from the monolayer g-C3N4 to TCNQ, revealing that the TCNQ molecule can act as a strong acceptor on the surface of monolayer g-C3N4
Summary
Effective structural doping approach to modify the photoelectrochemical properties of g-C3N4 by doping with nonmetal (sulfur or phosphorus) impurities, which reduced the energy gap to enhance the visible-light absorption of g-C3N4.16 Lu et al reported that the photocatalytic efficiency of g-C3N4 can be enhanced by H ions plus B, N, Si, O, P and As ions with high coverage rates plus halogen ions with low coverage rates.[17]. The van der Waals interactions between the monolayers and surface dopants (TCNQ and TTF) were described by the DFT-D2 method of Grimme.[51] The interactions between valence electrons and ionic core were described by the Vanderbilt ultraso pseudopotential.[52] The cutoff energy was set as 550 eV, and 8 Â 8 Â 1 kpoints with the Monkhorst–Pack[53] scheme in the rst Brillouin zone was employed in the present work. Both the cutoff energy and k grid were tested to be converged in the total energy. The DE was calculated according to the following de nition: DE 1⁄4 Edopant/g-C3N4 À Eg-C3N4 À Edopant where Edopant/g-C3N4, Eg-C3N4, and Edopant are the total energy of the surface modi ed system, intrinsic monolayer g-C3N4, and isolated dopant, respectively
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