The improvement of photocatalytic activity of monolayer g-C3N4via 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-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.


Introduction
Graphitic carbonic nitride (g-C 3 N 4 ) 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 Therefore g-C 3 N 4 has been found to be important in applications in diverse elds, [1][2][3][4][5][6][7][8][9][10][11] including water splitting, 1,2 CO 2 reduction, 3,4 contaminant degradation, 5,6 and so on. Unfortunately, bulk g-C 3 N 4 is a medium band gap semiconductor with visible light response (up to 450 nm), but low carrier mobility and insufficient sunlight absorption limit its photocatalytic activity, 1,[8][9][10][11] which cannot meet the prerequisites of high activity in photocatalytic reaction (both strong light absorption and suitable redox potential). 12,13 Therefore, there are many reports on the improvement of photocatalytic activity for pristine g-C 3 N 4 . 14-32 For instance, Ma et al. proposed an effective structural doping approach to modify the photoelectrochemical properties of g-C 3 N 4 by doping with nonmetal (sulfur or phosphorus) impurities, which reduced the energy gap to enhance the visible-light absorption of g-C 3 N 4 . 16 Lu et al. reported that the photocatalytic efficiency of g-C 3 N 4 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 Although much effort has been devoted to improving the photocatalytic activity of g-C 3 N 4 , it shows great necessity and urgency to discover new efficient approach to overcome the above mentioned problems and broaden the applications of g-C 3 N 4 in water splitting and environmental pollutants degradation elds.
It has been reported that element doping is an efficient method to tune the unique electronic structure and band gap of g-C 3 N 4 , which considerably broadens the light responsive range and enhance the charge separation. 16,17,20 However, the conventional doping with elemental impurities methods usually can introduce any bulk defects into the semiconductor lattice. In contrast, the surface charge transfer doping (SCTD) approach is nondestructive and does not induce any bulk defects into the semiconductor lattice, thus retaining the high performance of nanostructures by reducing carrier scattering in the bulk. [33][34][35] In this approach, through controlling Fermi level (E F ) misalignment of surface dopants with respect to underlying semiconductor nanostructures, electrons can be extracted from (or injected into) the nanostructures, forming an electron-decient (or electron-rich) surface layer. Carrier concentration and even conduction type of the semiconductor nanostructures can be readily tuned by varying the types as well as densities of surface dopants, leading to effective p-and n-type doping on the nanostructures. [33][34][35] SCTD has been proven to be a simple, nondestructive, and effective method to tune both the electronic and optical properties of low-dimensional semiconductors, [36][37][38][39][40][41][42][43][44] which is of fundamental importance to enable their wide applications in optoelectronic and electronic devices. For example, the electronic properties and carrier density of monolayer MoS 2 monolayer could be effectively modulated with electron acceptor, tetracyanoquinodimethane (TCNQ), and electron donor, tetrathiafulvalene (TTF). 41 Zhang et al. 44 showed that phosphorene could be p-and n-type doped by modifying the surface with TCNQ and TTF, respectively. On the other hand, it was also reported that the optical properties of two dimensional (2D) materials could be modulated by SCTD. Jing et al. 41,43 disclosed that TCNQ and TTF could enhance the optical properties of MoS 2 and phosphorene for effective light harvesting. All these reports suggested that SCTD is of fundamental importance to broaden their applications in optoelectronic and electronic devices. Although the SCTD scheme has been demonstrated to be very effective for MoS 2 and phosphorene, so far there are few works that apply this method on the monolayer g-C 3 N 4 . 28,29 Herein, based on the density functional theory (DFT) with rst-principles calculations, we demonstrated an efficient approach to improve the photocatalytic activity of monolayer g-C 3 N 4 by SCTD using TCNQ and TTF as surface dopants. Our calculations revealed that TCNQ and TTF could act as acceptor and donor to inject holes and electrons into monolayer g-C 3 N 4 , leading to electron-decient and -rich surface layers, respectively. The remarkable surface charge transfer between the adsorbed molecules and the monolayers made TCNQ/TTF an efficient surface dopant to rationally tune the electronic and optical properties of monolayer g-C 3 N 4 . For TCNQ and TTF modied systems, the Fermi levels moves into the valence/ conduction band region, together with the increase/decrease of work functions, thus leading to p-/n-type doping of monolayer g-C 3 N 4 . Moreover, the SCTD with TCNQ and TTF has also been found to be an effective way to enhance the light harvesting capabilities of the monolayer g-C 3 N 4 in the visible-light region, which improves the photocatalytic activity and broadens the applications in splitting water and degrading environmental pollutants under sunlight irradiation.

Computational methods
All the rst-principles calculations were performed using DFT based methods as implemented in the Cambridge Sequential Total Energy Package (CASTEP) program in Materials Studio 6.1 package of Accelrys Ltd. [45][46][47] The Generalized Gradient Approximation (GGA) 48 with the Perdew-Burke-Ernzerhof functional (PBE) 49,50 was adopted to describe the correction of the electronic exchange and correlation effects. Meanwhile, 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. A slab of vacuum of 15Å in thickness in the Z direction was applied to avoid the interaction with the image atoms. All of the structure models were fully relaxed, and the convergence criteria for geometric optimization and energy calculation were set to 2.0 Â 10 À5 eV per atom, 0.02 eVÅ À1 , 0.005Å and 2.0 Â 10 À6 eV per atom for the tolerance of energy, maximum force, maximum ionic displacement, and self-consistent eld (SCF), respectively.
In addition, the available adsorption sites of TCNQ and TTF on the surface of monolayer g-C 3 N 4 were explored by comparing the adsorption energy (DE). And the DE was calculated according to the following denition: N 4 , and E dopant are the total energy of the surface modied system, intrinsic monolayer g-C 3 N 4 , and isolated dopant, respectively.

Results and discussion
To investigate the surface charge transfer doping effects on the monolayer g-C 3 N 4 , two typical p-and n-type organic surface dopants, including one electron-withdrawing molecule (TCNQ) and one electron-donating molecule (TTF), were chosen in this paper, as shown in Fig. 1a and b. And Fig. 1c also presents the top views of the energetically most favorable conguration of monolayer g-C 3 N 4 . For each molecule, we considered several possible adsorption sites and the energetically favorable conguration for these two molecules on the monolayer g-C 3 N 4 are presented in Fig. 2. And the possible adsorption sites of TCNQ and TTF on the surface of monolayer g-C 3 N 4 were explored by comparing the adsorption energy (DE), which was dened in the computational method section and a more negative DE indicated a more favorable conguration.
The electron-withdrawing/donating molecules, TCNQ and TTF, prefer adsorption on the basal surface of monolayer g-C 3 N 4 along the z direction with a vertical distance. And the adsorption energy of the three different congurations of TCNQ and TTF modied systems is À0.12, À0.11, À0.12, À0.12, À0.12 and À0.12 eV (Table 1), respectively, showing a non-covalent interaction between the surface dopants and monolayer g-C 3 N 4 . It can be noted that the structure of monolayer g-C 3 N 4 was obviously bending aer TCNQ and TTF surface modication due to the non-covalent interaction between the surface dopants and monolayer g-C 3 N 4 . In addition, 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-C 3 N 4 to TCNQ, revealing that the TCNQ molecule can act as a strong acceptor on the surface of monolayer g-C 3 N 4 . In contrast, for the TTF modied monolayer g-C 3 N 4 systems, TTF acts as a donor and injects electrons into the monolayers (as shown in Table 1). As a result, the adsorption of TCNQ and TTF lead to positively and negatively charged monolayer g-C 3 N 4 , manifesting that surface modication by TCNQ and TTF molecules could be an effective approach to modulate the carrier concentrations in g-C 3 N 4 monolayers.
To explore the work function variations of monolayer g-C 3 N 4 before and aer surface modication, the electrostatic potential calculations were further performed to study the changes of work functions (DF ¼ F dopant/g-C 3 N 4 À F g-C 3 N 4 ) for monolayer g-C 3 N 4 aer surface modications. As shown in Table 1, there is an obvious increase of work functions for monolayer g-C 3 N 4 by 1.21, 1.20, and 1.19 eV, respectively, aer the adsorption of TCNQ, attributing to the injection of holes from TCNQ into the monolayers. In contrast, due to the injection of electrons from TTF molecule, the adsorption of TTF results in a decrease of work function by 0.30, 0.17, and 0.13 eV, respectively. It can be also noted that the work function of TCNQ modied monolayer g-C 3 N 4 is higher than that of intrinsic monolayers, while the work function of TTF modied monolayer g-C 3 N 4 is lower than that of intrinsic monolayers (shown in Fig. 3), in accordance with the literature reports. 40,56 These phenomena indicate that the monolayer g-C 3 N 4 may be tuned into p/n-type materials by doping with the electron-withdrawing/donating TCNQ and TTF molecules.
To examine whether the surface dopants (TCNQ and TTF) have an effect on electronic properties of the monolayer g-C 3 N 4 due to charge transfer between surface dopants (TCNQ and TTF) and monolayer g-C 3 N 4 , the electronic band structures of monolayer g-C 3 N 4 before and aer surface modication were computed (Fig. 4). Fig. 4a-c presents the electronic band structures of the intrinsic and surface modied monolayer g-C 3 N 4 , respectively. Remarkably, the intrinsic monolayer g-C 3 N 4 is an indirect-gap semiconductor with a bandgap of 1.15 eV at GGA/PBE level, while with a bandgap of 2.70 eV at  a DF is dened as DF ¼ F dopant/g-C 3 N 4 À F g-C 3 N 4 , where F dopant/g-C 3 N 4 and F g-C 3 N 4 are the work functions of the surface modied system and the intrinsic monolayer g-C 3 N 4 , respectively. HSE06 level, which agrees well with previous reports. 24,57 For TCNQ modied system, it can be noted that the Fermi level (E F ) moves into the valence band region that probably attributed to the adsorption of TCNQ molecule. And the shi of original E F in monolayer g-C 3 N 4 to the lower energy regions leads to the increase of work function. These phenomena reveal that monolayer g-C 3 N 4 can be p-type doped with TCNQ surface modication. In contrast, for TTF modied system, there appears a new at energy level below the Fermi level that mainly attributed to the TTF molecule. The appearance of the new empty at band in the bandgaps shis the original E F in the monolayer g-C 3 N 4 to the higher energy region, thus decreases the work functions. The new at level can act as a donor state in TTF-modied system, which is in favor of the charge transfer from TTF to monolayer g-C 3 N 4 . In the meantime, the new at band below the Fermi level is close to the conduction band minimum (CBM) in the TTF modied system (Fig. 4c), verifying that it acts as a donor state for n-type doping. The density of states (DOS) of the molecule-modied systems and the projected density of states (PDOS) for both the adsorbed molecules and monolayer g-C 3 N 4 in these adsorption systems were also computed (Fig. 5), which validate the results from the band structures and conrm that the new at energy levels were generated by the absorbed molecule. The above results collectively demonstrate that TCNQ and TTF can p-and n-type doping of monolayer g-C 3 N 4 , respectively. It can be noted that the charge transfer between the surface dopants (TCNQ and TTF) and monolayer g-C 3 N 4 have an obvious effects on the electronic properties of monolayer g-C 3 N 4 . Therefore, the electron density difference (Dr) of monolayer g-C 3 N 4 before and aer TCNQ and TTF modication was calculated to visualize the charge transfer between the surface dopants (TCNQ and TTF) and monolayer g-C 3 N 4 (Fig. 6). Dr illustrates how the electron density changes during the adsorption process and is dened as Dr ¼ r dopant/g-C 3 N 4 À r dopant À r g-C 3 N 4 , in which r dopant/g-C 3 N 4 , r dopant , and r g-C 3 N 4 denote the electron density of the surface modied system, the isolated molecule and the isolated monolayer g-C 3 N 4 , respectively. Fig. 6a and b show the change of electron density in the monolayer g-C 3 N 4 aer TCNQ and TTF surface modication, where the gain and loss of electrons is presented in red and blue color, respectively. For TCNQ modied system, there are obvious electrons depletion on the surface of monolayer g-C 3 N 4 , while strong electrons accumulation around TCNQ molecule. In contrast, for TTF modied system, the adsorption of TTF leads to the electrons enrichment on the surface of monolayer g-C 3 N 4 and the electrons depletion around TTF molecule. These phenomena intuitively reveal that TCNQ and TTF can draw and donate electrons from/to monolayer g-C 3 N 4 as acceptor and donor, respectively, and these results are consistent with the Mulliken charge transfer analysis in Table 1. In addition, both electrons accumulation and depletion appear on the surface of monolayer g-C 3 N 4 , suggesting there are charge transfers both between intramolecules and intermolecules.
The electronic properties of monolayer g-C 3 N 4 can be tuned by the TCNQ and TTF surface modication, and to explore surface modication with TCNQ and TTF whether can improve the optical properties of monolayer g-C 3 N 4 , the optical spectra of the intrinsic and surface-modied monolayer g-C 3 N 4 were calculated (Fig. 7). The imaginary part of the dielectric functions (3 2 ) is an effective parameter to measure the optical absorption ability of materials, and the peaks in 3 2 are caused by the absorption of incident photons and the interband transition of   electrons. 58,59 As shown in Fig. 7, there is an appreciable largest absorption peak around 460 nm for intrinsic monolayer g-C 3 N 4 (blue curve in Fig. 7), which is in good agreement with previous report 16 and denotes the monolayer g-C 3 N 4 is a visible-light semiconductor material. However, the absorption intensity of monolayer g-C 3 N 4 is weak in the visible-light region, which is not enough to contribute to the highly photocatalytic activity of g-C 3 N 4 . For TCNQ modied system, new absorption peaks appear in the region of 530-880 nm, and the absorption intensity of peak around 880 nm is increased compared to that of intrinsic monolayer g-C 3 N 4 peak around 460 nm. These new peaks round 530-880 nm could contribute to improve the photocatalytic activity of monolayer g-C 3 N 4 . Similar to TTF modied system, it can be noted that a new visible-light absorption peak appears around 586 nm, which can also contribute to improve the photocatalytic activity of monolayer g-C 3 N 4 . All these phenomena suggest that both TCNQ and TTF surface modication can induce an increase of optical absorption range and the absorption intensity in the visible-light region of monolayer g-C 3 N 4 . This will be of great importance to improve the photocatalytic activity of monolayer g-C 3 N 4 and broaden its applications in splitting water and degrading environmental pollutants under sunlight irradiation.

Conclusions
In conclusion, we have proposed an efficient approach to improve the photocatalytic activity of monolayer g-C 3 N 4 via surface charge transfer doping with TCNQ and TTF molecules. The electronic and optical properties of intrinsic and surface modied monolayer g-C 3 N 4 were systematically investigated by means of DFT computations. It was found that TCNQ could act as an acceptor to draw electrons from the monolayer g-C 3 N 4 , leading to pronounced holes accumulation in the monolayers and increased work functions. While TTF could act as a donor to inject electrons into the monolayer g-C 3 N 4 , resulting in the accumulation of electrons on the monolayers and decreased work functions. The remarkable surface charge transfer between the adsorbed molecules and the monolayers made TCNQ/TTF an efficient surface dopant to rationally tune the electronic properties of monolayer g-C 3 N 4 . Moreover, the adsorptions of TCNQ and TTF on monolayer g-C 3 N 4 could induce an increase of optical absorption range and the absorption intensity in the visible-light region, improving the photocatalytic activity of monolayer g-C 3 N 4 . Our work demonstrates the great potential of SCTD method on the rational tuning of the electronic and optical properties of monolayer g-C 3 N 4 , opening up the opportunities to improve the photocatalytic activity of monolayer g-C 3 N 4 via SCTD.

Conflicts of interest
There are no conicts to declare. Fig. 7 Computed imaginary dielectric functions versus wavelength for intrinsic (blue lines), TCNQ (red lines) and TTF (green lines) modified monolayer g-C 3 N 4 .