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Tailoring the Electronic Band Gap and Band Edge Positions in CN Monolayer by P and As Substitution for Photocatalytic Water Splitting M.R. Ashwin Kishore, and Ponniah Ravindran J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07776 • Publication Date (Web): 18 Sep 2017 Downloaded from http://pubs.acs.org on September 20, 2017

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The Journal of Physical Chemistry

Tailoring the Electronic Band Gap and Band Edge Positions in C2N Monolayer by P and As Substitution for Photocatalytic Water Splitting† M. R. Ashwin Kishore†,‡ and Ponniah Ravindran∗,†,‡,¶,§ †Department of Physics, Central University of Tamil Nadu, Thiruvarur, Tamil Nadu, 610101, India ‡Simulation Center for Atomic and Nanoscale MATerials, Central University of Tamil Nadu, Thiruvarur, Tamil Nadu, 610101, India ¶Department of Materials Science, Central University of Tamil Nadu, Thiruvarur, Tamil Nadu, 610101, India §Center for Materials Science and Nanotechnology and Department of Chemistry, University of Oslo, Box 1033 Blindern, N-0315 Oslo, Norway. E-mail: [email protected]

Abstract Exploiting earth-abundant and low-cost photocatalyst for high efficient photocatalytic water splitting is of profound significance. Herein, we report an improved photocatalytic water splitting activity by P and As substitution at N-site in C2 N monolayer using state-of-the-art hybrid density functional calculations. Our results show that the band gap can be reduced in C2 N by increasing the concentrations of P and As substitution and at the same time the obtained band gap value is higher than the free energy of water splitting except for As with concentrations of x=0.333. This indicate

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that these new compositions of P/As substituted C2 N monolayers are thermodynamically suitable to drive hydrogen evolution reaction. The calculated effective mass of charge carriers illustrates that charge transfer to the reactive sites would be easier in the substituted system than the pure C2 N and also our results suggest that the recombination rate would be lower in substituted system indicating the enhancement in the efficiencies of photocatalytic water splitting. The band edge position with respect to the redox potentials of water shows that P/As substituted C2 N monolayer are the potential photocatalysts for water splitting than the pristine C2 N monolayer. From the optical absorption spectra, we found that P/As substituted C2 N monolayer show optical absorption extended more into the visible region, indicating enhanced energy harvesting. Our results reflect that P/As substituted C2 N monolayer could be the potential visible-light photocatalysts for overall water splitting.

Introduction Hydrogen production by photocatalytic water splitting using solar energy is an attractive and sustainable solution to solve the global energy problem and environmental issues. 1–3 Nano-sized semiconductors, especially the two-dimensional (2D) materials exhibit enhanced photocatalytic activity than their bulk counterparts due to the presence of more surface active sites, improved electron-hole separation, fast mobility of charge carriers, short diffusion length, and reduced recombination rate. Although, graphene, a single layer of carbon atoms of honeycomb-like layered material possesses intriguing electronic and mechanical properties, but its zero band gap makes it unsuitable to use in optoelectronic devices and photocatalysis. Therefore, great efforts have been made to open an appropriate band gap in graphene and also the band gap shortcoming pave the way to search for other 2D materials with finite band gap and favorable carrier mobility. It has been realized experimentally and predicted theoretically that graphene-analogous materials such as BN, transition metal dichalcogenides, phosporene, MXenes, g-C3 N4 , and so forth can significantly remedy graphene’s shortcoming


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in optoelectronic devices, photocatalysis, etc. 4–8 In recent years, carbon-nitride based nanostructures have attracted tremendous attention in the field of photocatalytic water splitting due to their earth abundant nature, high thermal as well as chemical stability, tunable electronic structure, and economical affordability. Two dimensional graphitic phase of carbon nitride (g-C3 N4 ) has been synthesized by Wang. et.al. 9 and their results showed a good photocatalytic performance for hydrogen or oxygen production via water splitting. Its unique structure and exotic properties created interest among the researchers to search for another two-dimensional covalent triazine frameworks and are shown great promise for a wide variety of applications. C4 N3 , an another polymorphs of carbon nitride composed of single-ring triazines displays a ferromagnetic ground state and possesses an intrinsic half-metallicity. 10 Srinivasu et. al. 11,12 reported that s-triazine based porous graphitic carbon nitride (g-CN) as a potential photocatalyst for water splitting. Recently 2D C4 N has been identified by first principles calculations in combination with a swarm structure search and it could be a promising material for applications in nanoelectronics. 13 3D mesoporous carbon nitride with C3 N5 stoichiometry has been synthesized very recently which shows superior photocatalytic water splitting performance under visible light. 14 Nitrogenated holey graphene (C2 N), a newly synthesized layered 2D network structure possessing evenly distributed holes. 15 In C2 N structure, the benzene rings are bridged by pyrazine rings, which consists of a six-membered D2h ring with two nitrogen atoms facing each other. This material has a direct band gap and the optically measured band gap value is 1.96 eV. The fabricated field effect transistor based on C2 N multilayer exhibits a high on/off ratio of 107 . It is to be noted that the direct band gap, high on/off ratio, and porous nature of C2 N is expected to exhibit desirable properties for electronics, opto-electronics, energy conversion as well as storage, gas storage applications, etc. It was reported by Sahin 16 that the phonon modes of C2 N monolayer are close to that of graphene, indicating the high structural stability of C2 N monolayer. Guan et al. 17 demonstrated the effect of strain on electronic and optical properties on C2 N monolayer for opto-



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electronic device applications. Theoretical predictions have shown that the band gap and band edge positions of C2 N can be tuned by varying their stacking order, layer number, and external electric field 18,19 for optoelectronic and photocatalytic water splitting applications. An enhanced photocatalytic mechanism has been observed in g-C3 N4 /C2 N 20 and C2 N/graphene 21 heterostructure. C2 N/MoS2 heterostructure has been reported as a potential photovoltaic material. 22 Isovalent element-alloyed C2 N monolayer have been predicted to have promising applications in optoelectronic and photocatalytic water splitting. 23,24 Recently, it has been reported that C2 N can be an ideal candidate for hydrogen storage material. 25 Furthermore, earlier reports revealed its possibility to use for gas separation, 26–28 single-atom catalyst, 29,30 HER catalyst, 31,32 and OER catalyst. 33 Computational simulation and design play a great role to assist experimental efforts in screening or identifying new potential photocatalysts. In many cases, it is necessary to modulate the electronic and optical properties of the material to broaden its widespread applications or improve the efficiency. There are several ways to achieve this, for example, doping, point and line defects, layer-by-layer or heterostructure mapping, chemical adsorption, strain, and external electric field. Herein, we demonstrate using density functional calculations that the isoelectronic substitution at N site in C2 N monolayer improves the photocatalytic water splitting activity. We find that P/As substitutions reduces the band gap of C2 N monolayer make it optimum for water splitting and thus enhance the optical absorption in the visible region of the solar spectrum. The low effective mass of charge carriers indicating an efficient charge separation in the P/As substituted system. More importantly, the band edge positions are even more favorable in the P/As substituted system than the pristine C2 N for water decomposition.


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Figure 1: The schematic structure of C2 N monolayer: a) top view, b) side view. The atoms are denoted as C (Blue) and N (Orange) in this picture, c) the contour map of electron localization function (ELF) is viewed from [0 0 1] direction. The reference bar for ELF value is provided at the left and d) band structure of C2 N monolayer calculated using HSE06 functional.



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Computational details Total energy calculations were done using projector augmented wave (PAW) method which is implemented in Vienna Ab initio Simulation Package (VASP). 34 Geometry optimization is carried out by employing the generalized gradient approximation proposed by Perdew, Burke and Ernzrhof (GGA-PBE) 35 with van der Waals(vdW) correction proposed by Grimme. 36 Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional 37 was used to calculate electronic structures owing to the band gap underestimation of semiconductors by PBE functional. The Brillouin zone was sampled using Monkhorst-Pack scheme 38 and employs a 9×9×1 k-mesh for geometry optimization and a 5×5×1 k-mesh for the more expensive HSE06 calculations. A plane-wave basis set with a cutoff energy of 520 eV was used. A large vacuum of ∼ 15 ˚ A is used to avoid interlayer interaction due to the periodic boundary condition used in the present calculations. The convergence criterion for energy was set to be 10−5 eV between two consecutive SCF cycle and the force is get converged when the Helman Feyman force acting on each atom was less than 0.01 eV/˚ A upon ionic relaxation. The postprocessing of VASP calculated data was done by using VASPKIT code. 39 The positions of VBM and CBM of pristine and P/As substituted C2 N monolayers are determined by the calculated band gap center energy (EBGC ) and the band gaps (Eg ), using the following relation ECBM/V BM = EBGC ±

1 2

EgHSE06

where EBGC is the band gap center energy calculated using PBE functional. As EBGC is quite insensitive to the choice of exchange correlation functional, we have used the less expensive PBE functional. A more accurate HSE06 is used to calculate the band gaps. To align the energy levels for pristine and P/As substituted C2 N monolayers, their vacuum levels are set to zero. In order to study the optical absorption property of pristine and P/As substituted C2 N monolayers, the frequency-dependent dielectric function (ω)=1 (ω)+i 2 (ω) was calculated. 6 ACS Paragon Plus Environment

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The interband contribution to the imaginary part of the dielectric functions 2 is calculated by summing transitions from occupied to unoccupied states with fixed k vector over the Brillouin zone, weighted with the appropriate matrix element giving the probability for the transition. To be specific, the components of 2 are given by. 40 αβ 2 (ω) =

1 X 4π 2 e2 lim 2 2ωk δ(ck − vk − ω)× Ω q→0 q c,v,k

(1)

hµck+qeα |µvk ihµvk |µck+qeβ i∗ where the indices c and v refer to conduction and valence band states, respectively. µck represents the cell periodic part of the wavefunctions at the k-point. The real part of the components of the dielectric tensor 1 (ω) are then calculated using the Kramer-Kronig transformation. The absorption coefficient α(ω) was evaluated according to the following equation, 41 q 21 √ 2 2 α(ω) = 2ω 1 (ω) + 2 (ω) − 1 (ω)

(2)

where 1 (ω) and 2 (ω) are the real and imaginary parts of the frequency dependent complex dielectric function (ω), respectively. In order to account the tensor nature of the dielectric function, 1 (ω) and 2 (ω) are averaged over three polarization vectors (along x,y, and z directions).

Results and discussion Bonding character and structural stability The schematic structure of C2 N monolayer is shown as top view and side view in Fig. 1a and b, respectively. The visualization of the electron localization function (ELF) and band structure calculated using HSE06 functional gives us insight into the character of the bonding and nature of the band gap. The relaxed lattice constant obtained from present GGA-PBE calculation (a=8.325 ˚ A) is consistent with that reported from experimental and theoretical 7 ACS Paragon Plus Environment


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studies. 15,17,19 The in-plane bond lengths were found to be 1.429 ˚ A , 1.468 ˚ A , and 1.336 ˚ A for the C−C(1), C−C(2), and C−N bond, respectively. The C−N−C angle is 117.5o , slightly deviated from 120o , which is almost the same as the value of 117.4o by Liang et. al. 42 from similar calculation. The nature of the chemical bond or the electron density distribution between atoms can be described accurately by ELF. 43–45 The ELF refers to the jellium-like homogeneous electron gas and renormalized to the values between 0.00 and 1.00. The region with an ELF value of 1.00, 0.50, and 0.00 correspond to fully localized electrons, fully delocalized electrons, and very low charge density, respectively. It can be seen from the Figure. 1c that the C and N sites are characterized by low charge densities. However the ELF value close to 1.00 can be seen between carbon atoms and also between C and N resembling that the electrons are fully localized between these atoms confirms the presence of covalent bonding in C2 N monolayer. From this figure, it is also evident that the geometrical structure of C2 N has pyrazine rings contain nitrogen atoms having six lone-pair electrons. In order to tune the electronic band gap and band edge positions, we have done isoelectronic substitution at N site with three different substitution concentrations such as x = 0.083, 0.167, and 0.333 to simulate C2 N1−x Mx (M = P and As) monolayers. We have used the supercell approach to simulate different substitution concentrations. For C2 N1−x Mx (M = P and As) monolayers, substitution concentrations x = 0.083 and 0.167 was obtained by replacing one N atom with one P/As atom in the 2x1 (36 atoms) and 1x1 (18 atoms) C2 N cell, respectively. The substitution concentration x = 0.333 was obtained by replacing two N atoms with P(As) atoms in the C2 N unit cell. The cohesive energy per atom in a primitive unit cell was calculated (see Table 1) to evaluate the relative stability of C2 N1−x Mx monolayers using the formula;

Ecoh = [NC EC + NM EM + NN EN − Etot ]/[NC + NM + NN ]

(3)

where Etot is the total energy of the model systems, NC , NM and NN are the number of C, M(P and As), and N atoms per system, and EC , EM and EN are the energies of the 8 ACS Paragon Plus Environment

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Table 1: The calculated lattice constant, cohesive energy, and the band gap values of pristine C2 N, C2 N1−x Px and C2 N1−x Asx monolayers. The calculated PBE direct band gap values for C2 N0.917 P0.083 and C2 N0.917 As0.083 are given in parentheses. System

C2 N C2 N0.917 P0.083 C2 N0.833 P0.167 C2 N0.667 P0.333 C2 N0.917 As0.083 C2 N0.833 As0.167 C2 N0.667 As0.333

Lattice constant (˚ A)

Cohesive energy per atom (eV)

8.325 8.426 8.515 8.771 8.454 8.570 8.921

8.64 8.63 8.44 8.22 8.50 8.37 8.09

Band gap (eV) PBE HSE 1.66 2.46 1.33 (1.39) 2.22 1.36 2.06 0.94 1.47 1.31 (1.35) 2.08 1.30 1.89 0.60 1.09

Table 2: Calculated bond strength from Integrated COHP 46–48 for C2 N, C2 N1−x Px and C2 N1−x Asx monolayers. Composition C2 N C2 N0.917 P0.083 C2 N0.833 P0.167 C2 N0.667 P0.333 C2 N0.917 As0.083 C2 N0.833 As0.167 C2 N0.667 As0.333

C-C(1) C-C(1) C-C(1) C-C(1) C-C(1) C-C(1) C-C(1)

−6.76 −6.14 −6.37 −6.04 −6.07 −6.38 −6.14

Bond strength (eV) C-C(2) −5.75 C-N −6.91 C-C(2) −5.58 C-N −6.67 C-C(2) −5.90 C-N −6.67 C-C(2) −5.61 C-N −6.79 C-C(2) −5.65 C-N −6.60 C-C(2) −5.64 C-N −6.61 C-C(2) −5.82 C-N −6.71


C-P −5.03 C-P −5.03 C-P −5.00 C-As −4.51 C-As −4.52 C-As −4.39


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isolated C, M(P and As), and N atoms, respectively. From our cohesive energy calculations we have found that the cohesive energies of P/As substituted C2 N monolayers are slightly smaller than that of pristine C2 N indicating that stability will be reduced by substitution, but yet the higher values indicate that they can be stable. The cohesive energy of C2 N1−x Mx monolayers show a decreasing tendency with the increasing concentration of x. The decrease in cohesive energy is due to weakening of bond strength and hence a decrease in optical band gap 49 as seen from our band structure analysis. In order to substantiate this observation, we have calculated the bond strength between various constituents of C2 N and C2 N1−x Mx monolayers using the Integrated Crystal Orbital Hamiltonian Population (ICOHP) as we have done earlier. 50 The calculated ICOHP values are listed in Table. 2. It is evident from the table that the bond strength of various constituents of C2 N1−x Mx monolayers are decreasing when P/As substitution concentrations are increasing and thus the cohesive energy.

Electronic properties The calculated band structures using GGA-PBE functional for C2 N and C2 N1−x Mx are displayed in Supplementary Information Fig.S1 and S2, respectively. It can be seen from the Fig.S1 that the pristine C2 N monolayer is a direct band gap semiconductor as the valence band maximum (VBM) and the conduction band minimum (CBM) both lie at the Γ point with the band gap value of 1.66 eV which is consistent with earlier reports. 19 The obtained band gap value is much smaller than the value measured from optical measurements (1.96 eV) 15 and the value calculated theoretically 19 using Heyd-Scuseria-Ernzerhof (HSE06) functional. The P/As substitutions also preserve the direct band gap nature except for x = 0.083 concentrations, where indirect band gap behavior is observed with the VBM lie at M point and the CBM lie at the Γ point. It is well known that the PBE functional underestimate the band gaps of semiconductors 51 and also it may wrongly predict the nature of the band dispersion. Therefore, we carried out band structure calculations by HSE06 hybrid functional which has been demonstrated to be more accurate in describing 10 ACS Paragon Plus Environment

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Figure 2: The calculated band structure of (top panel) C2 N1−x Px and (bottom panel) C2 N1−x Asx monolayers with different substitution concentrations (x = 0.083, 0.167, 0.333) obtained from HSE06 functional. The Fermi level is set to zero. the exchange-correlation energy of electrons in solids. The band structures of pristine and C2 N1−x Mx (M = P and As) monolayers calculated with HSE06 functional are depicted in Fig. 1d and Fig. 2, respectively. It can be clearly seen from the band structures of C2 N and C2 N1−x Mx monolayers that all these systems are belong to direct band gap semiconductors. As we mentioned earlier, the PBE functional may wrongly predict the nature of band dispersion, the band structure calculated using PBE functional has two degenerate flat bands in the VBM (see Fig.S1) whereas in the band structure computed using HSE06 functional, these two bands moved to lower energy and the bands at −1 and −2.5 eV moved upwards and forms the VBM. Also, the band structures obtained using HSE06 functionals show that P/As substitution for x = 0.083 11 ACS Paragon Plus Environment


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Figure 3: The orbital projected band structure of C2 N, C2 N1−x Px , and C2 N1−x Asx monolayers calculated using HSE06 functional. The size of the circles illustrates the projected weight of the orbitals. The Fermi level is set to zero.


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also having direct band gap nature which was wrongly predicted as an indirect band gap material by PBE functional. It may be noted that the isovalent impurities do not induce deep defect energy levels in the band gap in semiconductors and hence no localized states are present in the P/As substituted C2 N monolayers as evident from Fig. 2. As a result, bands originating from substituents such as P/As in C2 N will not act as recombination centers for the photogenerated electron hole pairs. Also, from the figure, it is clear that the bands in the valence and conduction band edges of these materials are well dispersed. Hence, the lifetime of the photogenerated charge carriers in these systems is expected to be longer, which will result in better photocatalytic efficiency. The calculated band gap values and the corresponding lattice constants are listed in Table. 1 for C2 N and C2 N1−x Mx monolayers. From this Table, we can see that the increasing lattice constant due to the substitution of bigger size atoms may lead to a decrease of the band gap which is in agreement with previous report. 23 It is to be noted that the optimum band gap value for a semiconductor to be employed for water splitting application using solar radiation is between 1.8 eV and 2.2 eV. 52 Interestingly, the band gap values for P/As substituted C2 N fall exactly in this optimum range (see Table. 1) except for As substitution with x = 0.333 concentrations. The calculated band gap value for x = 0.333 is lower (1.09 eV) than the water redox potential of 1.23 eV and hence this composition is not suitable to use for photocatalytic water splitting. Therefore, the photocatalytic properties of this composition are not analyzed further. The substitution of isoelectronic elements will preserve the electron per atom ratio which may stabilize the structure and at the same time it distort the structure due to strain introduced by variation in the atomic radii of the substituents as a result the band gap as well as band edge positions may alter. The great advantage in substituting isoelectronic elements in C2 N is that it preserves direct band gap behavior and more importantly, it allows to tune the band gap from 0.24 eV to 0.99 eV and from 0.38 eV to 1.37 eV upon varying the concentration (x = 0.083 to 0.333) of P and As, respectively. This significant reduction in the band gap due to substitutions display better light absorption



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performance and thus photo conversion efficiency than the pristine C2 N. To gain a better insight into the orbital contributions to electronic bands, we present the orbital-projected band structure - so called fat bands calculated using HSE06 in Figure. 3 It is clear that the VBM and CBM of pristine C2 N are mainly occupied by C pz and N pz electrons, respectively. The bands present in the valence band at − 1 eV and − 2 eV are occupied by N s as well as px +py electrons of C and N s as well as px +py electrons of N, respectively. The whole CB is mainly occupied by pz electrons of N with little contributions from pz electrons of C. In the case of P/As substituted C2 N monolayers, the VBM and CBM are occupied by C pz and N pz electrons, respectively for x=0.083 and 0.167. On the other hand, for higher concentration of P (C2 N0.667 P0.333 ), the VBM is occupied by P pz electrons and the CBM is occupied by pz electrons of both C and N. It is interesting to note that the VBM of all compositions distinctly comprise the pz states of C/P, whereas, the CBM is mainly contributed by pz states of N/C atoms. This implies that upon photoexcitation, the holes will reside on the C/P site and the electrons will reside on the N/C site. As a result, the electrons and holes will be well separated and this extends the lifetime of the charge carriers. Hence, the probability of recombination rate would be lower and this will further improves the photocatalytic water splitting efficiency. In order to understand the bonding characteristics and the origin of the band gap reduction of P/As substitution in C2 N monolayers, we have analyzed the total and projected density of states (DOS) as shown in Fig. 4. Here, we discuss the DOS for C2 N0.833 P0.167 and C2 N0.833 As0.167 only as the trend will be same for the other compositions. DOS analysis shows that the VBM of pristine C2 N monolayer is dominated by C-2p states with a noticeable contribution from N-2p states. Similarly the CBM is originating from hybridization of both C-2p and N-2p states. In the case of C2 N0.833 P0.167 monolayer, the VBM is mainly dominated by C-2p states with a noticeable contribution from N-2p as well as P-3p states, while the CBM is mainly originating from p states of C, N, and P. For C2 N0.833 As0.167 monolayer, the VBM is dominated by C-2p states with a noticeable contribution from As-4p and


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Figure 4: The calculated total and partial density of states for C2 N, C2 N0.833 P0.167 , and C2 N0.833 As0.167 monolayers obtained from HSE06 functional are shown in (a), (b), and (c), respectively. The Fermi level is set to zero.



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N-2p states, while the CBM is mainly originating from N-2p, C-2p, and As-4p states. The energetically degenerate nature of DOS distribution coming from C-2p and N-2p along with its broad dispersion in the whole valence band indicating a strong covalent character, which is typical for layered 2D materials. From the ELF analysis discussed above, there is strong covalent bond present between C and N which is also evident from the degenerate nature of the band dispersion from C and N p states in the whole valance band range. Due to this strong covalent bonding, the broad band features are observed at the band edges that can reduce the effective mass of the charge carriers and thus increase their mobility that will be discussed in detail later. The observed band gap narrowing is attributed mainly by p orbitals of P and As in the unoccupied states which broaden the band edges and significantly narrowed down the band gap suitable for photocatalytic water splitting.

Figure 5: The calculated band edge positions of C2 N and C2 N1−x Mx monolayers with respect to vacuum potential. The dashed and solid lines are water redox potentials at pH=0 and pH=7, respectively.

Charge mobility and separation The ability of electron/hole transfer along a specific direction is important to have efficient charge separation and thus reduced recombination rate which will improve the efficiency of water splitting activity. It is well known that the carrier mobility is inversely proportional 16 ACS Paragon Plus Environment

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Figure 6: Optical absorption spectrum calculated using the PBE functional followed by a rigid energy shift to take into account the bandgap underestimation of the PBE functional. The area between the red and the violet lines represents the visible range. to the effective mass and hence low effective mass of charge carriers results in high carrier mobility. Therefore, we pay attention to the carrier effective masses. The calculated effective masses of electrons and holes for C2 N and C2 N1−x Mx monolayer are summarized in Table 3. P/As substituted C2 N monolayer manifest low electron effective mass than the pristine and this can be understood from the band structure. In C2 N, the CBM has three bands that are degenerate at the Γ point and among these three bands, the lowest energy band show a flat band nature along Γ-M. This originates from the π state contributed by the localized p orbitals of the nitrogen atoms as evident from our fat band analysis in Electronic properties section. 15 However, the P and As substitutions makes the flat band along Γ-M more dispersive due to increase in overlap interaction by the bigger size of the substituents and thus results in low electron effective mass than the pristine C2 N. Our results show that the electron and hole effective masses along Γ-M direction are much heavier than those along Γ-K direction. At least in one crystallographic direction, the effective mass of charge carrier should be lower than 0.5 me to have an excellent mobility. 52 In our case the effective mass of electrons and holes is lower than 0.5 me along Γ-K direction for C2 N0.833 P0.167 , 17 ACS Paragon Plus Environment


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C2 N0.667 P0.333 , and C2 N0.833 As0.167 . In the case of C2 N0.917 P0.083 and C2 N0.917 As0.083 , the calculated effective masses are relatively smaller and are comparable with that of other known photocatalysts. 53,54 Our effective mass analysis indicates that the charge carriers move faster to the reactive sites and this results in efficient charge separation in these materials and hence the photoconversion efficiency. Our orbital projected band structure analysis in Electronic properties section indicate that the recombination rate will be lower in these systems due to different origin of CBM and VBM. We can also access the recombination rate of electron-hole pairs by evaluating the effective masses of charge carriers. Generally, in semiconductors the effective mass of electrons will be lower than that of holes. But, in these systems the effective mass of holes is lower than the electrons due to the well dispersed bands in the VBM and relatively less dispersed bands in CBM. Therefore, the mobility of holes will be much higher than that of the electrons which results in noticeable difference in electron-hole mobility.

D=

m∗e m∗h

(4)

A higher value of relative ratio of the effective masses (D), indicates a large difference in the electron-hole mobility and thus reduces the recombination rate of electron-hole pairs. C2 N1−x Mx monolayers manifest higher D values along Γ-M direction (ranging between 0.80 and 5.65) than that of Γ-K direction (ranging between 0.66 and 2.35) and the values are comparable to other known photocatalysts. 55,56 Hence, the iso-electronic substituted C2 N monolayers have lower electron-hole recombination rates which caters to enhance the photocatalytic water splitting efficiency.

Reduction and oxidation capabilities An appropriate position of band edges relative to the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) potentials is necessary to identify the potential activity


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Table 3: Calculated effective mass of electron/hole for C2 N1−x Mx (M = P and As) monolayers System

Effective mass

Effective mass

C2 N C2 N0.917 P0.083 C2 N0.833 P0.167 C2 N0.667 P0.333 C2 N0.917 As0.083 C2 N0.833 As0.167

of electron (m0 ) Γ−M Γ−K 12.78 1.20 3.51 0.90 4.30 0.27 2.53 0.24 1.47 1.74 3.10 0.33

of hole (m0 ) Γ−M Γ−K 0.86 0.19 0.91 0.79 0.76 0.23 3.15 0.36 0.30 0.74 2.17 0.21

of a material for photocatalytic water splitting applications. The CBM has to lie above the reduction reaction potential and the VBM has to lie below the oxidation reaction potential to drive the redox reactions. It may be noted that the redox potential depends on pH value 54,57 and hence, the standard redox potential with respect to the vacuum level for H+ /H2 and O2 /H2 O are calculated by using the following equations:

red EH + /H = −4.44 eV + pH × 0.059 eV 2

(5)

EOox2 /H2 O = −5.67 eV + pH × 0.059 eV

(6)

The band edge positions of C2 N, C2 N1−x Px , and C2 N1−x Asx monolayers with respect to the vacuum potential and to the HER and OER potentials at pH=0 and pH=7 are shown in Fig. 5. From this figure one can see that the band edge position of P/As substituted C2 N monolayers has more favorable band edge position than the pure C2 N except for C2 N0.667 P0.333 whose VBM is higher than the oxidation potential of O2 /H2 O at pH = 0. Hence, this composition cannot split water into hydrogen and oxygen in acidic conditions. However, the band edge positions of all the other compositions straddling the HER and OER redox potentials at both pH levels, indicating they are thermodynamically favorable for water splitting reactions to occur without any external bias.



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Optical absorption Although there are other factors like appropriate band gap and suitable band edge positions which determine the efficiency of a photocatalyst, it is also important that a photocatalytic material should able to absorb a significant fraction of incoming sunlight. Fig. 6 shows the optical absorption coefficient of C2 N, C2 N1−x Px and C2 N1−x Asx monolayers as a function of energy as well as wavelength. It may be noted that the PBE functional underestimate the calculated band gap values and hence we have compensated them by using a so called scissor operation with the correct band gap values obtained from HSE06 functional that shift the absorption spectrum rigidly to higher energy. 58 From the estimated optical absorption spectra (see Fig. 6), we have found that the optical absorption of C2 N monolayer increases with photon energy over the range of visible light and reaches a maximum absorption at 3.2 eV with a maximum value as large as 105 cm−1 . This finding agrees well with previous results obtained from similar DFT calculation. 17 This absorption peak at 3.2 eV is arising from the optical interband transition coming from a doubly degenerate flat bands at −1 eV, which originates predominantly from the non-bonding (n) lone pair electronic states of N atoms to the localized pz orbitals of N in the CBM. In other words, the optical absorption is ascribed from the n → π ∗ electron transition involving lone pairs of the nitrogen atoms. However, C2 N1−x Px and C2 N1−x Asx monolayers possess absorption edges that extended more into the visible region as evident from Fig. 6. The observed notable red-shift of the absorption edge under P/As substitutions in C2 N is attributed by the band gap reduction. Hence, it is indeed that C2 N1−x Mx monolayers could efficiently utilize the maximum portion of the solar spectrum which improves the efficiency of photocatalytic water decomposition.

Conclusions In conclusion, we have demonstrated using first principles calculation that P/As substitution enhances the photocatalytic water splitting activity in C2 N monolayer. Our results show


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that P/As substitutions decreases the bandgap of the C2 N with the increase in substitution concentrations and at the same time it is still higher than the free energy of water splitting. Hence, these materials are thermodynamically suitable to drive hydrogen evolution reaction. Our effective mass calculations illustrate that charge transfer to the reactive sites would be easier and also the rate of recombination would be lower which indicates that their photocatalytic efficiencies may be enhanced. The VBM and CBM of P/As substituted C2 N distinctly comprise of different orbitals with well dispersed valence and conduction bands and hence a longer carrier lifetime are consequently expected. The band edge positions with respect to the redox potentials of water at pH=0 and pH=7 shows that P/As substituted C2 N monolayers are potential photocatalysts for water splitting than the pristine C2 N monolayer. From the optical absorption spectra, we could find that P/As substituted C2 N monolayer show optical absorption extended more into the visible region, indicating that it could enhance the photoelectron conversion in the solar spectra. Optimum band edges to the redox potentials, enhanced optical absorption, low effective masses, and lower recombination rate reveals that P/As substituted C2 N monolayers are the promising visible light photocatalysts for overall water splitting.

Supporting Information Available Band structure of pristine as well as P/As substituted C2 N monolayers using GGA-PBE functional. This material is available free of charge via the Internet at http://pubs.acs.org/.

Acknowledgments The authors are grateful to the Research Council of Norway for computing time on the Norwegian supercomputer facilities. This research was supported by the Indo-Norwegian Cooperative Program (INCP) via UGC Grant No. F.No.58-12/2014(IC) and Department of 21 ACS Paragon Plus Environment


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Science and Technology, India via Grant No. SR/NM/NS-1123/2013. The authors thank Helmer Fjellv˚ ag and Anja O. Sj˚ astad for useful discussion.

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