Point Defect Effects on Photoelectronic Properties of the Potential

Article Cite This: J. Phys. Chem. C 2018, 122, 5291−5302

pubs.acs.org/JPCC

Point Defect Effects on Photoelectronic Properties of the Potential Metal-Free C2N Photocatalysts: Insight from First-Principles Computations Haijun Zhang,*,†,‡ Xiao Zhang,† Guang Yang,∥ and Xiaomeng Zhou*,†,§ †

Center for Aircraft Fire and Emergency, Economics and Management College, Civil Aviation University of China, Tianjin 300300, P. R. China ‡ School of Physics and Materials Science, Anhui University, Hefei 230601, P. R. China § College of Environmental Science and Engineering, Nankai University, Tianjin 300071, P. R. China ∥ College of Energy and Environmental Engineering, Hebei University of Engineering, Handan 056038, Hebei Province, China S Supporting Information *

ABSTRACT: Through first-principles computations on the structural, electronic, and optical properties of perfect and defective two-dimensional C2N crystals, the effects of point defects on photoelectronic characteristics of this potential photocatalysts were investigated. The introduction of point defects, including N vacancies, interstitial C impurities, O@C and H@N dopants, and the interstitial O in the benzene ring and big ring, should result in more appropriate band structures and broadened optical absorptions and generally promoted carrier mobilities of C2N photocatalysts. Remarkably, the defective C2N with N vacancy, interstitial O in benzene/big ring, and interstitial C in benzene ring are highly recommended for the photocatalytic applications due to their broadened optical absorption, spatially separated e−−h+ pairs, excellent redox capacities, and fast carrier migrations. Our theoretical results can provide some guidance for further exploring the utilization of 2D C2N material and some possible strategies for improving its photoactivities.

1. INTRODUCTION Since the first experimental report on the photoelectrochemical water splitting at a TiO2 electrode in 1972,1 a variety of metalbased semiconductors have been developed for photocatalytic applications, such as the metal (oxy)nitrides and modified titanium dioxides for the hydrogen generation from water splitting,2,3 metal oxides for the CO2 reduction into hydrocarbon fuels,4 and perovskite materials and metal−organic frameworks for the degradation of organic pollutants.5,6 However, the probably high cost and environmental toxicity, during the synthesis and utilization process of these metalbased photocatalysts,7−9 may cause a serious restriction of their widespread utilizations. In order to search for the alternatives to the metal-based materials, a class of metal-free semiconductors have emerged as attractive photocatalysts, including the elemental semiconductors10−12 and nonmetal compounds.13−16 Remarkably, the © 2018 American Chemical Society

graphitic carbon nitride (g-C3N4), which is commercially available and can be easily fabricated, was proven a low-cost and high-efficiency photocatalyst for the production of hydrogen from water even in the absence of catalytic metals.17−20 This polymeric carbon nitride has attracted so much research interest and attained great achievements in the past years, owing to its nontoxic and earth-abundant elemental composition, high stability in both acidic and alkaline solution, and independence of noble metal cocatalysts.21 Recently, another two-dimensional (2D) carbon nitride crystal with a C2N stoichiometry, namely the C2N-h2D, was efficiently synthesized via a simple wet-chemical reaction.22 Because of its graphene-like structure and novel characteristics, Received: December 18, 2017 Revised: February 1, 2018 Published: February 26, 2018 5291

DOI: 10.1021/acs.jpcc.7b12428 J. Phys. Chem. C 2018, 122, 5291−5302

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The Journal of Physical Chemistry C

Figure 1. Geometric structures before optimization for (a) the perfect C2N-h2D crystal as well as the crystals with (b) vacancies and substitutional dopants, (c) interstitial impurities in benzene and pyrazine rings, and (d) interstitial atoms in big hole ring. Red dashed line presents the lattice framework.

intrinsic characteristics and consequently improve the catalytic ·· activity of photocatalysts. For instance, the V‴ Bi VOV‴ Bi vacancy can effectively promote the solar-driven photocatalytic activity of unltrathin BiOCl nanosheets.38 Because of the existence of nitrogen vacancies, the recombination of photoinduced e−−h+ pair is effectively restrained, and the population of long-lived charge carriers is greatly increased, which result in the overall enhancement of the photocatalytic activity of the g-C3N4.39 Nitrogen doped into substitutional sites of TiO2 has proven to be indispensable for broadening the optical absorption spectrum and improving the photocatalytic activity.40 According to the synthesis and characterization results of C2N-h2D crystals, there must be varieties of point defects in this 2D material,22 which should play a significant role in affecting its physicochemical properties.38−40 The characterization results in this paper suggest that the elemental composition of C2N-h2D crystal includes H and O impurities. The lattice vacancies can also be observed in the large-area

the C2N-h2D was utilized as high-efficiency catalyst for the production of hydrogen.23,24 There are also theoretical achievements on the mechanical,25 thermal conductivity and stability,26 magnetic,27 electronic,28 and adsorption29 properties of the C2N crystal. Remarkably, in view of its visible-light absorption (1.96 eV), low-cost fabrication, and eco-friendly and earth-abundant elemental composition (C and N), this newly discovered C2N-h2D may be another promising metal-free photocatalyst for high-efficiency hydrogen production and pollutant degradation. Many theoretical30−37 studies have explored the possibility of its photocatalytic applications and found that the primary or modified C2N-h2D materials may have extraordinary electronic and optical properties for its probable utilization in visible-light-driven photocatalysis. Remarkably, a large number of experimental and theoretical studies suggest that the point defects, including vacancy, impurity, and interstitial atoms, commonly exist in the semiconductors and can be utilized to effectively modify the 5292


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interstitial impurities (Figure 1c,d) are systematically investigated to evaluate the effect of point defects on the photoactivities of the C2N-h2D crystal. According to the synthesis and characterization results, there are C, N, O, and H elements in the products of this holey 2D carbon nitride.22 Therefore, only the foreign impurities of O and H are considered in the defective C2N-h2D crystal. The perfect and defective C2N-h2D crystals were placed in the x−y plane with the z direction perpendicular to the layer plane, and a vacuum space of 20 Å in the z direction was tested to be large enough to avoid interactions between adjacent layers (Figure S1). To test the equilibrium configurations of the adsorption geometries, namely the C2N systems with point defects of interstitial C, N, O, or H atoms, ab initio molecular dynamic (AIMD) simulations were performed in NVT ensemble lasts for 10 ps with a time step of 2.0 fs. The temperature set at 300 K was controlled by using the Nosé−Hoover method.49 The dipole correction was also considered in the geometry optimizations to obtain the lowest-energy configurations and total energies of the investigated defective C2N systems. Therefore, all the equilibrium configurations are based on the results of AIMD simulations and dipole corrections. The optical absorptions of the perfect and defective C2N unit cell were analyzed by computing the complex dielectric constants (ε) at a given frequency. The 21 × 21 × 1 k-point mesh was used in the computations of dielectric constants. After the obtainment of the real and imaginary part of the dielectric constants ε(ω) = ε1(ω) + iε2(ω), the absorption coefficient I(ω) can be calculated the by following formula:50

scanning tunneling microscopy (STM) topography image of the C2N-h2D crystal. Considering the real existence and possible influence of these point defects, it is indispensable to investigate the point defects effects on physicochemical properties of the C2N-h2D crystal in order to further explore the practical application of this newly emerged 2D material and to propose optimization strategies on improving its photocatalytic activities. Herein, we computed the structural, electronic, optical, and transport properties of C2Nh2D crystals with various point defects, including the C and N vacancies, as well as the substitutional and interstitial impurities of C, N, O, and H atoms. Our computational results suggest that most of the point defects could effectively reduce the band gaps and broaden the optical absorptions of C2N crystals. Some point defects have negative effects on the photocatalytic activities of C2N due to the deep gap states in the band structures of C2N with these types of defects. The defects of N vacancy, interstitial O in benzene/big ring, and interstitial C in benzene ring result in the broadened optical absorption, faster carrier migration, and greater redox capacities of photoinduced electron and holes, which are highly recommended for improving the activities of 2D C2N photocatalysts.

2. COMPUTATIONAL DETAILS Our first-principles computations were carried out by using the projector-augmented plane wave (PAW)41,42 method as implemented in the Vienna ab inito simulation package (VASP).43,44 The electron exchange-correlation functional was treated by the generalized gradient approximation (GGA) in the form proposed by Perdew, Burke, and Ernzerhof (PBE).45 The energy precision of the computations was set to 10−5 eV, and the atomic positions were fully relaxed until the maximum force on each atom was less than 10−3 eV/Å. The cutoff energy of 580 eV was used for all computations. In the geometry relaxation and self-consistent computations, the Brillouin zone was sampled with a 11 × 11 × 1 Γ-centered Monkhorst−Pack k-points grid. Considering the underestimated band gaps by the PBE functional, the Heyd−Scuseria−Ernzerhof (HSE06)46 hybrid functional, which was proven a reliable method for the calculation of electronic and optical properties,47,48 was employed to calculate the band structures of the investigated C2N-h2D crystals. In view of the computational burden of the large systems, namely the defective C2N crystal lattice with more than 70 atoms, only the optical properties of C2N unit cell with 18 atoms was computed by using the HSE06 hybrid functional. All the optical properties of defective C2N systems were computed by the PBE functional. Notably, both the HSE06 and PBE functionals were used to calculate the dielectric constants of C2N unit cell to provide a benchmark for the correction of underestimated optical gap computed by PBE functional. The optimized structures of C2N-h2D are presented in Figure 1a. This holey 2D carbon nitride consists of benzene rings and nitrogen atoms, in which each benzene ring connects to three adjacent benzene rings through bridge connection of nitrogen atoms. The supercell of 2 × 2 × 1 was tested to be large enough for the simulation of the point defects in C2N crystal. Actually, the ratio between the impurity and host is about 1/70 ≈1.4% in number of atoms, which is realizable in experiment. All the carbon (or nitrogen) atoms in C2N-h2D are equivalent in geometry. In our computations, C- or N-position vacancies and substitutional defects (Figure 1b) as well as

I(ω) =

2 ω[ ε1(ω)2 + ε2(ω)2 − ε1(ω)]1/2

(1)

As presented in the expression, only if the imaginary part ε2(ω) > 0 will the absorption coefficient I(ω) be above zero. Therefore, a positive value of ε2(ω) reflects the light absorption at a given frequency ω. The imaginary part is determined by a summation over empty states using the expression51 (2) (ω) = εαβ

4π 2e 2 1 lim ∑ 2wk ⃗δ(εck ⃗ − εvk ⃗ − ω) Ω q → 0 q2 c , v , k ⃗

× ⟨uck ⃗ + eαq ⃗|uvk ⃗⟩⟨uck ⃗ + eβq ⃗|uvk ⃗⟩*

(2)

The formation energies of defective C2N with C/N vacancies, C/N/O/H dopant, and interstitial C/N/O/H atoms were calculated according to the following formulas:20 Ef (vacancy) = E T(vacancy) − E T(perfect) + μR ,

μR = μC or μ N

(3)

Ef (doped) = E T(doped) − [E T(perfect) + μdopant − μR ], μR = μC or μ N ;

μdopant = μO , μH , μC , or μ N

(4)

Ef (interstices) = E T(interstices) − [E T(perfect) + μX ], μX = μO , μH , μC , or μ N

(5)

where ET(vacancy), ET(doped), and ET(interstices) represent the total energies of C2N crystal with point defects of vacancies, dopants and interstitial impurities, respectively, while E(perfect) is the total energy of the perfect C2N crystal. The solid graphite (four atoms in a unit cell) and gaseous N2, O2, and H2 were used to determine the chemical potentials: μC = μ(graphite)/4, μN = μ(N2)/2, μO = μ(O2)/2, and μH = 5293


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The Journal of Physical Chemistry C μ(H2)/2. In all the computations of chemical potentials and geometry optimizations, electronic spin was considered. The effective mass m* of electrons and hole in perfect and defective C2N was calculated by the equation52,53

m* = ℏ/2a

C4N3 rings in the C2N crystal (Figure S6d). The interstitial C and N atom added into the pyrazine ring should result in the C−N−C and N−N−C bridge bond, respectively, which are perpendicular the plane of C2N framework (Figure S6b,e). In view of the interstitial atoms in the big ring, the interstitial carbon atom should be bonded with two N atoms at adjacent pyrazine ring (Figure S6c), while the interstitial nitrogen atom should be connected with one N atom at pyrazine ring (Figure S6f). The above-mentioned reconstructed geometries and redistributed electrons of C2N crystal may lead to unique characteristics of these defective C2N materials. The formation energies of defective C2N crystal were also calculated to evaluate the relative stability and experimental feasibility of them (Table 1). In view of the one-atom vacancies,

(6)

where a is the curvature of the bands near the conduction band minimum (CBM) and valence band maximum (VBM), and ℏ is the reduced Planck constant. The parameter a can be obtained by the second-order term in a quadratic fit of E(k) approach to the CBM and VBM.

3. RESULTS AND DISCUSSION 3.1. Geometries and Relative Stabilities of Defective C2N. The planar C−N framework of perfect C2N is constructed by benzene rings and pyrazine rings, which surround the big holes within the holey structure (Figure 1a). In view of the planarity, the planar structure can be well maintained in the defective C2N with C/N vacancy or substitutional impurities (Figures S2 and S3), while the impurity atoms are not at the same plane of the C2N framework with interstitial impurities (Figures S4−S6), except for defective C2N with a hydrogen atom in the pyrazine ring and big ring (Figure S5b,c) as well as the C and N atoms in the pyrazine ring. According to the AIMD simulations, all the interstitial atoms should transfer to energetically preferable positions to reduce the total energy of the defective system and, thus, to reach a more energetically favorable geometry, as shown in Figures S4−S6. For instance, the H/O atom will transfer from the center of pyrazine ring into the big ring to reduce the total energy of system, resulting in the same geometric structure of the interstitial H in pyrazine ring and big ring (Figures S4b,c and S5b,c). For the defective C2N with vacancy (Figure S2), the disappearance of C leads to the unsaturated carbon and nitrogen atoms, while the absence of nitrogen atom produces a five-member C−N ring, which could result in the redistribution of electrons and, accordingly, the possible active sites for the catalytic reaction. To a certain degree, there are local reconstructions around the dopants of the substitutional C2N (Figure S3). For example, the pyrazine and benzene rings are severely deformed after the carbon was substituted by hydrogen atom, as shown in Figure S3c. Considering the C2N with interstitial impurities, the O impurity in the benzene ring should result in the C−O−C bridge bonds connecting the adjacent carbon atoms within the six-member carbon rings (Figure S4a), while the interstitial O added into the pyrazine ring should transfer to the big ring (Figure S4b). After the O atom was added into the big hole ring, one of the edge N atoms will be saturated by the O atom and N−O bonds will not stay in the plane of C−N framework. The addition of H atom into the pyrazine ring and big hole ring will lead to the same geometry, which has an absolutely planar structure with one N atom passivated by H atom (Figure S5b,c). When the H atom was added into the benzene ring, one carbon atom was saturated by the H and the C−H bond perpendicular to C2N plane was formed (Figure S5a). The interstitial C and N atoms at different positions were also computed and the equilibrium configurations are represented in Figure S6. The foreign C atom added into the benzene ring should replace one N atom at the neighbor pyrazine ring and a C3N2 five-member ring is formed (Figure S6a), while the interstitial N atom at benzene ring should replace one C atom of the benzene ring, leading to a C5N and

Table 1. Formation Energies (Ef in eV) of the Defective C2N vacancy substitutional defects

C vacancy N vacancy C position

N position

interstitial impurities

benzene ring

pyrazine ring

big ring

nitrogen dopant oxygen dopant hydrogen dopant carbon dopant oxygen dopant hydrogen dopant oxygen atom hydrogen atom carbon atom nitrogen atom oxygen impurity hydrogen impurity carbon atom nitrogen atom oxygen impurity hydrogen impurity carbon atom nitrogen atom

5.21 2.82 0.96 2.24 1.48 1.91 −1.17 0.52 −0.09 −1.18 1.62 3.31 −0.05 −3.29 4.27 3.73 −0.05 −3.29 3.10 2.78

the positive formation energies of C and N vacancies suggest that the vacancy defects in 2D holey C2N are not that favorable in energy. Relatively, the N vacancy (2.83 eV) is energetically more preferable than the C vacancy (5.21 eV). Considering the excessively high formation energy of C vacancy, which indicates that the C vacancy in 2D C2N is energetically unfavorable and the extremely small realizability in experiment, the computations on electronic and optical properties of C2N with C vacancy will not be performed and discussed in the following sections of this paper. For substitutional defects, only the oxygen dopant at the N position is energetically favorable, which is indicated by its negative Ef (−1.17 eV). For the same dopant at different position, the hydrogen dopant at the N position (0.52 eV) is also energetically more preferable than that at C position (1.48 eV). The O dopant can replace the N atom more easily than the replacement of C atom in C2N. For the C2N structure with self-element substitutions of N or C atom, the N dopant at C (0.96 eV) is energetically more favorable than the C dopant at N (1.91 eV). Generally, the negative formation energies of C2N crystal with interstitial impurities of H and O suggest the promising stability and energetic preferability of these interstitial defects. On the contrary, the positive formation energies of C and N impurities (1.61−4.27 eV) indicate the relatively high difficulty 5294


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Figure 2. Band structures (left column) and spatial charge distribution of CBM and VBM (right column) computed by the HSE06 method for (a) PC2N and (b) C2N-N-V. The isosurface value of the charge distribution is 0.002 au. The green dashed lines represent Fermi level at 0 eV.

In view of the substitutional and interstitial impurities (Figures S8−S10), most of these point defects should result in deep gap states around the Fermi level, except for the defective C2N with C substituted by O (Figure S8c), N substituted by H (Figure S8d), interstitial O in benzene ring (Figure S9c) or big ring (Figure S9d), and interstitial C (Figure S10a−c). These deep gap states may result from the redistributed electrons around the defects within above-mentioned defective C2N. Serious attention should be paid to the deep gap states, which could act as the recombination center of the photoactivated electron and holes within these point-defect C2N.54,55 Especially, the spatial charge distribution of these deep gap states, which are mainly located around the substituted and interstitial impurities (Figures S11 and S12), indicates the severe localization of gap states that can efficiently capture the photoinduced carriers. In other words, the recombination centers spatially located around the defects could easily capture both the photoactivated electrons and holes within C2N with these types of defects. To test the reliability of PBE computations on these gap states, the HSE06 method was also used to computed the band structure of defective C2N with C substituted by N atom. The HSE06 and PBE computations result in the same band lines across the Fermi level of the defective C2N, as represented by Figure S13, indicating the reliability of our PBE results on the above-mentioned deep gap states. Accordingly, these point-

of realizing these interstitial defects in experimental studies. For interstitial C (or N) atom, the C@benzene ring (Ef = 1.62 eV) (or N@big-ring (Ef = 2.78 eV)) configuration is the most favorable in energy, while the interstitial C (or N) at pyrazine ring with the formation energy of 4.27 eV (or 3.73 eV) should be most difficult to realize in experiments. The relatively high Ef of defective C2N with interstitial C and N atoms in pyrazine ring may result from their severely reconstruction of geometry, which have bridge-shaped C−N−C and N−N−C bonds (Figure S6b,e). 3.2. Electronic and Optical Properties of the C2N with and without Point Defects. The band structures and optical absorptions of the perfect and defective C2N crystals were also computed to investigate the point-defect effects on these 2D systems in order to select appropriately defective C2N materials with promising electronic and optical properties for photocatalytic applications. All the band structures of C2N crystal with and without point defects were first calculated by PBE functional and are presented in Figures S7−S10. The PBE computations suggest that the perfect C2N crystal has direct gap of 1.66 eV (Figure S7a). The C2N with N vacancy is a ptype semiconductor with an indirect gap of 1.37 eV, which is a little smaller than that of perfect C2N. Accordingly, the N vacancy will reduce the band gap of C2N material and possibly expand the optical absorption spectrum. 5295


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Figure 3. Band structures (left column) and spatial charge distribution of CBM and VBM (right column) computed by the HSE06 method for defective C2N crystal of (a) C2N-C-O, (b) C2N-N-H, (c) C2N-Benzene-O, and (d) C2N-Big-O. The isosurface value of the charge distribution is 0.002 au. The green dashed lines represent the Fermi level at 0 eV.

of our study, owing to the existence of e−−h+ recombination center in these point-defect C2N crystals. For the reliability of our computations, the HSE06 method was used to calculated the band structures of perfect and

defect C2N, including the defective C2N with C substituted by N/H and N substituted by C/O, as well as interstitial H and interstitial N, should be prohibited in the photocatalytic application and will not be investigated in the following part 5296


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Figure 4. Band structures (left column) and spatial charge distribution of CBM and VBM (right column) computed by the HSE06 method for defective C2N with interstitial C atoms at (a) benzene, (b) pyrazine, and (c) big rings. The isosurface value of the charge distribution is 0.002 au. The green dashed lines represent the Fermi level at 0 eV.

CBM located at Γ point, which agree well with the previous hybrid functional computations.30 Similar to the PBE results, the HSE06 computations also suggest that these semiconducting C2N with point defects have smaller band gaps than perfect C2N, to some degree (Figures 2−4). The C2N-N-V, C2N-C-O, C2N-N-H, C2N-Benzene-O, C2N-Big-O, C2N-Benzene-C, C2N-Pyrazine-C, and C2N-Big-C have band gaps of 1.90, 1.16, 1.12, 2.19, 2.25, 1.78, 1.99, and 1.04 eV, respectively, which may result in the broadened optical absorption. Remarkably, the O@C and H@N substitution

defective C2N with appropriate band structures for further investigation. These appropriate band-structure C2N crystals include the perfect C2N crystal as well as the defective C2N with C substituted by O, N substituted by H, one-atom N vacancy, interstitial O in benzene and big ring, and interstitial C in benzene, pyrazine, and big ring, which were simply labeled as P-C2N, C2N-C-O, C2N-N-H, C2N-N-V, C2N-Benzene-O, C2NBig-O, C2N-Benzene-C, C2N-Pyrazine-C, and C2N-Big-C, respectively. The HSE06 computations suggest that the perfect C2N crystal has direct band gap of 2.46 eV and the VBM and 5297


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Figure 5. Imaginary parts of dielectric constants computed by PBE functionals for the defective C2N crystal of (a) C2N-N-V, (b) C2N-C-O, (c) C2N-N-H, (d) C2N-Benzene-O, (e) C2N-Big-O, (f) C2N-Benzene-C, (g) C2N-Pyrazine-C, and (h) C2N-Big-C. Green dashed lines tangent to curves are used to determine the optical gaps.

possible capture center of the activated electrons and holes. As shown in Figures S14−S16, the ground-state electrons are completely delocalized at the C−N network within semiconducting C2N with or without point defects, which suggests that there is no positively or negatively charged regions on the framework of these investigated 2D carbon nitrides. Therefore, the carrier migration will not be affected by this factor. The reduced band gaps of defective C2N imply the possibly broadened optical absorption of these materials. Accordingly, we further computed the dielectric constants of perfect and defective C2N. In view of the severely computational burden of the dielectric constants calculation by using the HSE06 methods, the PBE functionals was used for the computation of optical properties for the huge defective C2N models with about 72 atoms (Figure 5). For the benchmark of scissors value, both the HSE06 and PBE functionals were used to calculate the dielectric constants of perfect C2N unit cell with 18 atoms. As shown in Figure S17, the scissors value of +0.63 eV should be added to correct the PBE results of optical absorptions. As shown in Figure S17, the perfect C2N has an optical gap of 2.20 eV (HSE06 result) and, thus, pronounced visible-light absorption, which is in good agreement with experimental results (1.96 eV).22 The optical absorption of C2N increases with photon energy over the range of visible light and reaches the first absorption peak at about 3.03 eV, which agree well with the previously computational results (3.20 eV).56 As

should lead to the direct-to-indirect-gap transfer of the C2N semiconductor, indicating different position of photoactivated e−−h+ in momentum space and, accordingly, the probable decreased rate of e−−h+ recombination.54,55 Because most photogenerated electrons and holes are mainly located at CBM and VBM, respectively,47,48 we also computed the spatial charge distribution of CBM and VBM for the semiconducting C2N crystals to evaluate the spatial separation of photoinduced e−−h+ pairs (right column in Figures 2−4). For P-C2N crystal, the CBM and VBM at Γ point, which are mainly composed of p orbitals of C and N atoms, respectively, are uniformly distributed at the entire C−N framework of it. Therefore, the photoinduced e−−h+ pairs within perfect C2N are not spatially separated and may recombine easily. For most of the defective C2N semiconductors, the spatial distributions of CBMs and VBMs are entirely or partially separated, implying the spatial separation of photoactivated e−−h+ pairs. The completely overlapped distribution of CBM and VBM within the C2N-N-H (Figure 3b) and C2N-Pyrazine-C (Figure 4b) may lead to fast recombination of the photoinduced electron and holes. Because of the attractive force between the unlike charges, the photoinduced electrons and holes tend to accumulate around the positively and negatively charged areas, respectively. Accordingly, the electron localization functions (ELFs) of the C2N semiconductors were also computed to analyze the 5298


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Figure 6. Band edge positions of perfect and defective C2N semiconductors. The redox potentials of H+/H2 (Φ(H+/H2) = 0 vs ENHE) and H2O/O2 (Φ(H2O/O2) = 1.23 vs ENHE) at pH = 0 are also given as a reference.

In consideration of the photoinduced holes, which can capture the electrons and directly oxidize the adsorbate, all the perfect and defective C2N materials have the capability to serve as the oxidative photocatalyst. Relatively, the P-C2N, C2N-BigO, C2N-Benzene-O, C2N-N-V, C2N-N-H, C2N-Benzene-C, and C2N-Pyrazine-C photocatalysts may have better performance in oxidation photocatalysis, thanks to their VBM levels located at 2.05, 1.93, 1.89, 1.54, 1.33, 1.50, and 1.95 eV, respectively. The VBM levels of above-mentioned seven C2N materials are comparable to those of some widely used photocatalysts of bismuth oxyhalides (1.44−1.93 eV)54 and gC3N4 (1.97 eV),20 suggesting the notable oxidation capability of photogenerated holes in them. For the photocatalytic water splitting, the P-C2N, C2N-N-V, C2N-Benzene-O, C2N-Big-O, C2N-Benzene-C, and C2NPyrazine-C have both more negative CBM levels than the Φ(H+/H2) potential and more positive VBM levels than the Φ(H2O/O2) potential, which suggest that they can simultaneously reduce H+ and oxidize OH− to release H2 and O2, respectively, during the photocatalysis. Therefore, the P-C2N, C2N-N-V, C2N-Benzene-O, C2N-Big-O, C2N-Benzene-C, and C2N-Pyrazine-C are promising photocatalysts for hydrogen generation from water. However, the C2N-C-O, C2N-N-H, and C2N-Big-C are unsatisfactory for the photocatalytic water splitting and other photocatalytic reactions, owing to the relatively poor reduction and oxidization capacities of photoactivated electrons and holes, respectively. Moreover, the band gaps of C2N-C-O, C2N-N-H, and C2N-Big-C are too small to restrain the transition of activated electrons from the CBM to the VBM and recombination of e−−h+ pairs.58 After photoactivation, the photogenerated electron and holes within photocatalysts must quickly migrate to the active sites to participate the photocatalytic reactions. Accordingly, the migration efficiency of the perfect and defective C2N should also be computed to assess the transport characteristics of them. Considering the extremely high cost of the HSE06 computations on carrier mobilities, only the effective mass was computed for these huge C2N systems with more than 70 atoms. The effective mass was also often used to reliably evaluate the migration efficiencies of photoinduced electrons and holes in the previous studies.54,56 Generally, the carrier with lower effective mass should have higher mobility and, thus, faster migration to the reactive sites.

shown in Figure 5, the optical gaps of C2N-N-V, C2N-C-O, C2N-N-H, C2N-Benzene-O, C2N-Big-O, C2N-Benzene-C, C2N-Pyrazine-C, and C2N-Big-C are 1.52, 0.83, 0.89, 1.53, 1.39, 1.21, 1.38, and 0.91 eV, respectively, which are computed by PBE functionals and should be corrected by the scissors value of +0.63 eV. Therefore, the optical gaps should be corrected to 2.15, 1.46, 1.52, 2.16, 2.02, 1.84, 2.01, and 1.54 eV for the C2N-C-V, C2N-N-V, C2N-C-O, C2N-N-H, C2NBenzene-O, C2N-Big-O, C2N-Benzene-C, C2N-Pyrazine-C, and C2N-Big-C materials, respectively. Compared to the optical gap of perfect C2N (2.20 eV), all the optical absorptions are further broadened by introducing the points defects of N vacancies, O@C substitution, H@N substitution, interstitial O at benzene and big ring, and the interstitial C atom at benzene, pyrazine, and big ring, which have positive effects on the photocatalytic activities of 2D C2N. 3.3. Redox Capability and Effective Mass of the Photoinduced Carriers. As another predominant factor that has a significant impact on the photoactivities, the band edge positions should be computed to assess the reduction and oxidation capacities of photogenerated electrons and holes, respectively. The band edge alignments were determined by the CBM/VBM energies relative to the vacuum levels of the corresponding C2N with or without defects, which were computed by the VASP code and were set at 0 eV. Similar to other computational studies, the potential of a normal hydrogen electrode (ENHE), which equals −4.5 eV with respect to the absolute vacuum scale (EAVS = 0 eV), was utilized as a reference to compare the redox potential of the band edges within the investigated C2N crystals. In semiconductors, the valence-band holes that have chemical potential of +1.0 to +3.5 V (vs ENHE) are powerful oxidants, while the conduction-band electrons with chemical potential of +0.5 to −1.5 V (vs ENHE) are good reductants.57 The band alignment, as represented in Figure 6 and Table S1, suggests that the most perfect or defective C2N have negative CBM levels (vs ENHE) and VBM levels larger than +1.0 V, which indicates the powerful reductants and oxidants within these C2N semiconductors, respectively. The computed redox potential for perfect C2N crystal are in good consistence with the previous computations,31 implying the reliability of our computations. 5299


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4. CONCLUSION By employing the density functional theory (DFT) computations, the structural, electronic, optical, and transport characteristics of perfect and defective 2D C2N crystals were investigated to uncover the effects of point defects on the photoactivities of these potential metal-free photocatalysts. In general, the results of electronic structures and optical absorptions suggest that the 2D holey C2N material could be a promising candidate for the photocatalytic application. The point defects of vacancies, substitutions, and interstitial impurities should obviously reduce the band gaps and broaden the optical absorptions of C2N crystals. However, there is deep gap states in the band structures of defective C2N with C substituted by N (or H) and N substituted by C (or O) as well as interstitial H (or N) in pyrazine, benzene, and big rings. Because the deep gap states could act as the recombination centers of photogenerated e−− h+ pairs, these types of point defects should be avoided if the 2D C2N was utilized in photocatalysis. Significantly, the point defects of N vacancies, O@C and H@ N substitutions, interstitial O in the benzene ring and big ring, and interstitial C in benzene, pyrazine, and ring lead to more appropriate band structures, broadened optical absorptions, and generally promoted carrier mobilities of C2N photocatalysts. Among these point defects with positive effects, the N vacancy, interstitial O in benzene/big ring, and interstitial C in benzene ring result in not only the widened optical absorptions and spatially separated e−−h+ pairs but also the superior photoactivated carriers with higher redox capacities and faster migrations, which are strongly recommended as the promising strategies for further improving the photoactivities of potential metal-free C2N photocatalysts.

The effective mass of carriers with the perfect and defective C2N varies from 0.09 to 4.58 me (Table S2), which are comparable to those of 2D materials with high carrier mobilities including the MnPSe3 (0.55−1.22 me),59 α-phosphorene (0.16−6.40 me),60 and Ti2CO2 MXene (0.14−4.53 me).52 These appropriate values of effective mass suggest the efficient migration of photoactivated electrons and holes in the investigated 2D C2N materials. Comparatively, the defective C2N-Pyrazine-C has the heaviest electrons (|m*| = 4.16 me) and holes (|m*| = 4.58 me) along the Γ → M direction. Meanwhile the C2N-N-V has the lightest electrons (|m*| = 0.14 me) along the K → Γ direction, and the C2N-Benzene-O has the lightest holes (|m*| = 0.09 me) along the K → Γ direction. Accordingly, the photoinduced holes should have the greatest mobility in the C2N-Benzene-O and the lowest mobility in the C2N-Pyrazine-C, while the photoactivated electrons should have the highest mobility in the C2N−N−V and the lowest mobility in the C2N-Pyrazine-C. Notably, the point defects have remarkable impacts on the transport properties of the 2D holey C2N material. The effective mass of electrons along the Γ → M direction within the perfect C2N (2.94 me) is obviously reduced by the point defects of N vacancies (0.93 me), substitutional dopants of O (0.28 me), the interstitial impurity of O in the benzene ring (1.82 me), and big ring (0.60 me) as well as the interstitial C at the benzene (2.82 me) and big ring (0.60 me). Also, the effective mass of electrons along the K → Γ direction within the perfect C2N (0.49 me) also decreases with the introduction of N vacancies (0.14 me) and the interstitial O in the benzene ring (0.17 me) and big ring (0.16 me). The defective C2N with O@ C and H@N substitutions as well as the interstitial C impurities have larger effective mass of electron along the K → Γ direction than that of perfect C2N. For holes along both the Γ → M and K → Γ direction, only the interstitial O could reduce their effective mass. According to the above-mentioned results of effective mass, most of the point defects should reduce the effective mass of photoinduced electrons, except for the substitutional defects of O@C and H@N and the interstitial C at the pyrazine and big ring. The effective mass of the photoactivated holes could only be reduced by the interstitial defects of O. Therefore, the carrier mobilities of electrons in C2N can be efficiently enhanced by introducing the point defects of N vacancies as well as interstitial O in benzene ring and big ring. Nevertheless, the interstitial O could also promote the migration of holes in the C2N crystals. The N vacancies and interstitial O are beneficial to the enhancement of carrier mobilities of C2N crystals. Generally, the effective mass of carriers within C2N will not be obviously changed by introducing the interstitial C impurity at benzene ring, indicating the comparable migration efficiency of photoactivated carriers in the P-C2N and C2N-Benzene-C. The introduction of interstitial C at pyrazine and big rings leads to increased effective mass of electrons (0.51−4.16 me) and holes (0.79−4.58 me), implying the reduced efficiency of carrier migrations within the defective C2N-Pyrazine-C and C2N-BigC. Compared to the C2N-Pyrazine-C and C2N-Big-C, the C2NBenzene-C has not only the comparably broadened visible-light absorptions and lower formation energy but also greater redox capability and faster migration of photoactivated electron and holes, which suggest the better performance of this potential C2N-Benzene-C photocatalyst.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b12428. Geometric structures of the completely relaxed defective C2N crystals; band structures computed by PBE functionals for both the perfect and defective C2N; spatial charge distribution of the deep gap states around the Fermi level for the defective C2N; band structures of C2N with N@C substitution computed by both the PBE and HSE06 functionals; ELF results of all C2N crystals; dielectric constants of perfect C2N unit cell computed by PBE and HSE06 functionals; computational details and results of the redox potential; effective mass of the photoinduced electron and holes for C2N crystals; structural information on the investigated C2N with and without defects (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.Z.). *E-mail: [email protected] (X.Z.). ORCID

Haijun Zhang: 0000-0002-9082-3662 Xiaomeng Zhou: 0000-0003-1258-9935 Notes

The authors declare no competing financial interest. 5300


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ACKNOWLEDGMENTS This work was supported by Natural Science Foundation of China (No. 21403001 and 51776219).

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