Point Defect Effects on Photoelectronic Properties of the Potential

Subscriber access provided by Umea University Library

Article

Point Defect Effects on Photoelectronic Properties of the Potential MetalFree C2N Photocatalysts: Insight from First-Principles Computations. Haijun Zhang, Xiao Zhang, Guang Yang, and Xiaomeng Zhou J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12428 • Publication Date (Web): 26 Feb 2018 Downloaded from http://pubs.acs.org on February 27, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

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

To whom correspondence should be addressed. Email: [email protected] (HZ); [email protected] (XZ)

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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, O@C and H@N dopants, as well as the interstitial O in the benzene ring and big ring, result in more appropriate band structures, 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.


Page 2 of 39

Page 3 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. Introduction Since the first experimental report on the photoelectrochemical water splitting at a TiO2 electrode in 1972,1 a variety of metal-based 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 as well as 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 metal-based photocatalysts,7-9 may cause the 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 non-metal compounds.13-16 Remarkably, the 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 non-toxic and earth-abundant elemental composition, high stability in both acidic and alkaline solution, as well as 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 Due to its graphene-like structure and novel characteristics, 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 electronic28 and adsorption29 properties of the C2N crystal. Remarkably, in view of its visible-light absorption (1.96 eV), low-cost fabrication, as well as ecofriendly 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 intrinsic characteristics and consequently improve the catalytic activity of photocatalysts. For instance, the

V B'''iV O V B'''i vacancy

can effectively promote the solar-driven photocatalytic

activity of unltrathin BiOCl nanosheets.38 Due to the existence of nitrogen vacancies, the recombination of photo-induced 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


Page 4 of 39

Page 5 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 scanning tunneling microscopy (STM) topography image of the C2N-h2D crystal. Considering the real existence and possible influence of these point defects, it’s 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 C2N-h2D 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 photo-induced electron and holes, which are highly recommended



for improving the activities of 2D C2N photocatalysts. 2. Computational Details

Fig. 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.


Page 6 of 39

Page 7 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 Г-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 was 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 was 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 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


Page 8 of 39

Page 9 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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. 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 I (ω ) =

2ω [ ε 1 (ω ) 2 + ε 2 (ω ) 2 − ε 1 (ω )]1 2

As presented in the expression, only if the imaginary part

(1)

ε 2 ( ω ) > 0 , the absorption

coefficient I(ω) will be above zero. Therefore, positive value of ε 2 (ω ) reflects the light absorption at a given frequency ω. The imaginary part is determined by a summation over empty states using follow expression:51

ε αβ (2) (ω ) =

4π 2 e 2 1 lim 2 0 q → q Ω

∑ 2w δ (ε

r c ,v , k

r k

∗ r ck

− ε vkr − ω ) × uckr + e

r αq

uvkr

uc kr + e

β

r q

uv kr

(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 E f (V acancy ) = E T (V acancy ) − E T (P erfect ) + µ R ,

µ R = µ C or µ N

E f (Doped) = ET (Doped) − [ ET (Perfect) + µ dopant − µ R ],

µ R = µ C or µ N ; µ dopant = µ O , µ H , µ C or µ N , E f ( In terstices) = E T (In terstices ) − [ E T ( P erfect ) + µ X ], µ X = µ O , µ H , µ C o r µ N

(3) (4) (5)

where ET(Vacancy), ET(Doped), ET(Interstices) represent the total energies of C2N crystal with point defects of vacancies, dopants and interstitial impurities, respectively,



Page 10 of 39

while E(Perfect) is the total energy of the perfect C2N crystal. The solid graphite (4 atoms in a unit cell), gaseous N2, O2, H2 were used to determine the chemical potentials: µC = µ(graphite)/4, µN = µ(N2)/2, µO = µ(O2)/2 and µH = 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 following equation52-53

m* =h 2a

(6)

where a is the curvature of the bands near the conduction band minimum (CBM) and valence band maximum (VBM), respectively, and h 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, respectively.

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 (Fig. 1a). In view of the planarity, the planar structure can be well maintained in the defective C2N with C/N vacancy or substitutional impurities (Figure S2-S3). While the impurity atoms are not at the same plane of the C2N framework with interstitial impurities (Figure 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


Page 11 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


energetically preferable positions to reduce the total energy of the defective system, and thus, to reach a more energetically favorable geometry, as shown in Figure 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 the same geometric structure of the interstitial H in pyrazine ring and big ring (Figure 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 the FigureS3c. 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 atom 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).



Page 12 of 39

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 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 and S6e). 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.

Table 1 The formation energies (Ef in eV) of the defective C2N. Vacancy

C Vacancy

5.21

N Vacancy

2.82

C-Position Substitutional Defects N-position

Interstitial

Benzene Ring

Nitrogen Dopant

0.96

Oxygen Dopant

2.24

Hydrogen Dopant

1.48

Carbon Dopant

1.91

Oxygen Dopant

-1.17

Hydrogen Dopant

0.52

Oxygen Atom

-0.09

Hydrogen Atom

-1.18


Page 13 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Impurities

Pyrazine Ring

Big Ring

Carbon Atom

1.62

Nitrogen Atom

3.31

Oxygen Impurity

-0.05

Hydrogen Impurity

-3.29

Carbon Atom

4.27

Nitrogen Atom

3.73

Oxygen Impurity

-0.05

Hydrogen Impurity

-3.29

Carbon Atom

3.10

Nitrogen Atom

2.78

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, 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, 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 @ C (0.96 eV) is energetically more favorable than the C dopant @ 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 contrary, the positive formation energies of C and N impurities (1.61~4.27 eV) indicate the relatively high difficulty of realizing these interstitial defects in experimental studies. For interstitial C (or N) atom, the benzene-ring (Ef = 1.62 eV) (or 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-shape C-N-C and N-N-C bonds (Figure S6b and S6e). 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 firstly calculated by PBE functional and presented in Figure S7-S10. The PBE computations suggest that the perfect C2N crystal has direct gap of 1.66 eV (Figure S7a). The C2N with N


Page 14 of 39

Page 15 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


vacancy is a p-type 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. In view of the substitutional and interstitial impurities (Fig. 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 attentions should be paid to the deep gap states, which could act as the recombination center of the photo-activated 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 (Figure S11-S12), indicating the severe localization of gap states that can efficiently capture the photo-induced carriers. In other words, the recombination centers spatially located around the defects could easily capture both the photo-activated 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-defect C2N, including the defective C2N with C substituted by N and H, N substituted by C and 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 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 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, C2N-Big-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 CBM located at Γ point, which agree well with the previous hybrid functional computations.30


Page 16 of 39

Page 17 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 2 Band structures (left column) and spatial charge distribution of CBM and VBM (right column) computed by HSE06 method for (a) P-C2N 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.



Fig. 3 Band structures (left column) and spatial charge distribution of CBM and VBM (right column) computed by 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 Fermi level at 0 eV.


Page 18 of 39

Page 19 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 4 Band structures (left column) and spatial charge distribution of CBM and VBM (right column) computed by 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 Fermi level at 0 eV.



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 (Fig. 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 eV, 1.16 eV, 1.12 eV, 2.19 eV, 2.25 eV, 1.78 eV, 1.99 eV and 1.04 eV, respectively, which may result in the broadened optical absorption. Remarkably, the O@C and H@N substitution should lead to the direct-to-indirect-gap transfer of the C2N semiconductor, indicating different position of photo-activated e--h+ in momentum space, and accordingly, the probably decreased rate of e--h+ recombination.54-55 Because most photo-generated 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 photo-induced e--h+ pairs (right column in Figure 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 photo-induced e--h+ pairs within perfect C2N are not spatially separated and may recombine easily. For most of the defective C2N semiconductors, the spatial distribution of CBMs and VBMs are entirely or partially separated, implying the spatial separation of photo-activated e--h+ pairs. The completely overlapped distribution of CBM and VBM within the C2N-N-H (Fig. 3b), C2N-Pyrazine-C (Fig. 4b) may lead to fast recombination of the photo-induced electron and holes. Due to the attractive force between the unlike charges, the photo-induced


Page 20 of 39

Page 21 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 possible capture center of the activated electrons and holes. As shown in Figure 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.



Fig. 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.

As shown in Figure S17, the perfect C2N has 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


Page 22 of 39

Page 23 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


eV).56 As 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 eV, 0.83 eV, 0.89 eV, 1.53 eV, 1.39 eV, 1.21 eV, 1.38 eV 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 eV, 1.46 eV, 1.52 eV, 2.16 eV, 2.02 eV, 1.84 eV, 2.01 eV and 1.54 for the C2N-C-V, 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 materials, respectively. Compare 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, as well as 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 photo-induced 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 photo-generated 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 to -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, suggest 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.

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

In consideration of the photo-induced holes, which can capture the electrons and


Page 24 of 39

Page 25 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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-Big-O, 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 abovementioned seven C2N materials are comparable to those of some widely used photocatalysts of bismuth oxyhalides (1.44-1.93 eV)Error! Bookmark not defined.

and g-C3N4 (1.97 eV),20 suggesting the notable oxidation capability of

photo-generated holes in them. For the photocatalytic water splitting, the P-C2N, C2N-N-V, C2N-Benzene-O, C2N-Big-O, C2N-Benzene-C and C2N-Pyrazine-C have both the more negative CBM levels than the Ф(H+/H2) potential and more positive VBM levels than the Ф(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 photo-activated 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

photo-generated

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 photo-induced 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. 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 photo-activated 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 K→Γ direction and the C2N-Benzene-O has the lightest holes (|m*| = 0.09 me) along K→Γ direction. Accordingly, the photo-induced holes should have the greatest mobility in the C2N-Benzene-O and the lowest mobility in the C2N-Pyrazine-C, while the photo-activated electrons should have the highest mobility in the C2N-N-V and the lowest mobility in the C2N-Pyrazine-C.


Page 26 of 39

Page 27 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Notably, the point defects have remarkable impacts on the transport properties of the 2D holey C2N material. The effective mass of electrons along 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 K→Γ direction within the perfect C2N (0.49 me) also decrease 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 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 abovementioned results of effective mass, most of the point defects should reduce the effective mass of photo-induced 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 photo-activated 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 photo-activated carriers in the P-C2N and C2N-Benzene-C. The introduction of interstitial C at pyrazine and big rings lead 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-Big-C. Compared to the C2N-Pyrazine-C and C2N-Big-C, the C2N-Benzene-C has not only the comparably broadened visible-light absorptions and lower formation energy, but also greater redox capability and faster migration of photo-activated electron and holes, which suggest the better performance of this potential C2N-Benzene-C photocatalyst.

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), 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 photo-generated e--h+


Page 28 of 39

Page 29 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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, as well as 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 photo-activated 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.

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

Supporting Information

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 photo-induced electron and holes for C2N crystals; Structural information of the investigated C2N with and without defects.

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

REFERENCES (1) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature, 1972, 238, 37-38. (2) Hisatomi, T.; Kubota, J.; Domen, K. Recent Advances in Semiconductors for Photocatalytic and Photoelectrochemical Water Splitting. Chem. Soc. Rev., 2014, 43, 7520-7535. (3) Ma, Y.; Wang, X. L.; Jia, Y. S.; Chen, X. B.; Han, H. X.; Li, C. Titanium Dioxide-Based Nanomaterials for Photocatalytic Fuel Generations. Chem. Rev.,

2014, 114, 9987-10043. (4) Habisreutinger, S. N.; Schmidt-Mende. L.; Stolarczyk, J. K. Photocatalytic Reduction of CO2 on TiO2 and Other Semiconductors. Angew. Chem. Int. Ed.,

2013, 52, 7372-7408. (5) Wang, W.; Tadè, M. O.; Shao, Z. P. Research Progress of Perovskite Materials in Photocatalysis- and Photovoltaics-Related Energy Conversion and Environmental Treatment. Chem. Soc. Rev., 2015, 44, 5371-5408.


Page 30 of 39

Page 31 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(6) Wang, C. C.; Li, J. R.; Lv, X. L.; Zhang, Y. Q.; Guo, G. S. Photocatalytic Organic Pollutants Degradation in Metal-Organic Frameworks. Energy Environ. Sci., 2014, 7, 2831-2867. (7) Ran, J. R.; Zhang, J.; Yu, J. G.; Jaroniec, M.; Qiao, S. Z. Earth-Abundant Cocatalysts for Semiconductor-Based Photocatalytic Water Splitting. Chem. Soc. Rev., 2014, 43, 7787-7812. (8) Zou, X. X.; Zhang, Y. Noble Metal-Free Hydrogen Evolution Catalysts for Water Splitting. Chem. Soc. Rev., 2015, 44, 5148-5180. (9) Rahman, M. Z.; Kwong, C. W.; Davey, K.; Qiao, S. Z. 2D Phosphorene as a Water Splitting Photocatalyst: Fundamentals to Applications. Energy Environ. Sci., 2016, 9, 709-728. (10) Liu, G.; Niu, P.; Yin, L. C.; Cheng, H. M. α-Sulfur Crystals as a Visible-Light-Active Photocatalyst. J. Am. Chem. Soc., 2012, 134, 9070-9073. (11) Wang, F.; Ng, W. H. K.; Yu, J. C.; Zhu, H.; Li, C.; Zhang, L.; Liu, Z.; Li, Q. Red Phosphorus: An Elemental Photocatalyst for Hydrogen Formation from Water. Appl. Catal. B, 2012, 111-112, 409-414. (12) Liu, G.; Yin, L.; Niu, P.; Jiao, W.; Cheng, H. Visible-Light-Responsive β-Rhombohedral Boron Photocatalysts. Angew. Chem. Int. Ed., 2013, 52, 6242-6245. (13) Huang, C. J.; Chen, C.; Zhang, M. W.; Liu, L. H.; Ye, X. X.; Lin, S.; Antonietti M.; Wang, X. C. Carbon-Doped BN Nanosheets for Metal-Free Photoredox Catalysis. Nat. Commun., 2015, 6, 7698.



(14) Liu, J. K.; Wen, S. H.; Hou, Y.; Zou, F.; Beran, G. J. O.; Feng, P. Y. Boron Carbides as Efficient, Metal-Free, Visible-Light-Responsive Photocatalysts. Angew. Chem. Int. Ed., 2013, 52, 3241-3245. (15) Luo, J.; Zhang, X.; Zhang, J. Carbazolic Porous Organic Framework as an Efficient, Metal-Free Visible-Light Photocatalyst for Organic Synthesis. ACS Catal., 2015, 5, 2250-2254. (16) Wang, X. C.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.; Domen, K.; Antonietti, M. A Metal-Free Polymeric Photocatalyst for Hydrogen Production from Water under Visible Light. Nat. Mater., 2009, 8, 76-80. (17) Liu, J.; Liu, Y.; Liu, N. Y.; Han, Y. Z.; Zhang, X.; Huang, H.; Lifshitz, Y.; Lee, S. -T.; Zhong, J.; Kang, Z. H. Metal-Free Efficient Photocatalyst for Stable Visible Water Splitting via a Two-Electron Pathway. Science, 2015, 347, 970-974. (18) Wang, Y.; Wang, X. C.; Antonietti, M. Polymeric Graphitic Carbon Nitride as a Heterogeneous Organocatalyst: from Photochemistry to Multipurpose Catalysis to Sustainable Chemistry. Angew. Chem. Int. Ed., 2012, 51, 68-89. (19) Zhang, J. S.; Chen, Y.; Wang, X. C. Two-Dimensional Covalent Carbon Nitride Nanosheets: Synthesis, Functionalization, and Applications. Energy Environ. Sci.,

2015, 8, 3092-3108. (20) Zhang, H. J.; Zuo, X. Q.; Tang, H. B.; Li, G.; Zhou, Z. Origin of Photoactivity in Graphitic Carbon Nitride and Strategies for Enhancement of Photocatalytic Efficiency: Insights from First-Principles Computations. Phys. Chem. Chem. Phys., 2015, 17, 6280-6288.


Page 32 of 39

Page 33 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(21) Cao, S. W.; Low, J. X.; Yu, J. G.; Jaroniec, M. Polymeric Photocatalysts Based on Graphitic Carbon Nitride. Adv. Mater., 2015, 27, 2150-2176. (22) Mahmood, J.; Lee, E. K.; Jung, M.; Shin, D.; Jeon, I. -Y.; Jung, S. -M.; Choi, H. J.; Seo, J. -M.; Bae, S. -Y.; Sohn, S. -D.; Park, N.; Oh, J. H.; Shin, H. -J.; Baek, J. -B. Nitrogenated Holey Two-Dimensional Structures. Nat. Commun., 2015, 6, 7486. (23) Mahmood, J.; Jung, S.-M.; Kim, S.-J.; Park, J.; Yoo, J.-W.; Baek, J.-B. Cobalt Oxide Encapsulated in C2N-h2D Network Polymer as a Catalyst for Hydrogen Evolution. Chem. Mater. 2015, 27, 4860–4864. (24) Mahmood, J.; Li, F.; Jung, S. -M.; Okyay, M. S.; Ahmad, I.; Kim, S. -J.; Park, N.; Jeong, H. Y.; Baek, J.-B. An Efficient and pH-Universal Ruthenium-Based Catalyst for the Hydrogen Evolution Reaction. Nat. Nanotechnol., 2017, 12, 441-446. (25) (a) Mortazavi, B.; Rahaman, O.; Rabczuk, T.; Felipe, L.; Pereira, C. Thermal Conductivity and Mechanical Properties of Nitrogenated Holey Graphene. Carbon, 2016, 106, 1-8. (b) Yagmurcukardes, M.; Horzum, S.; Torun, E.; Peeters, F. M.; Senger, R. T. Nitrogenated, Phosphorated and Arsenicated Monolayer Holey Graphenes. Phys. Chem. Chem. Phys., 2016, 18, 3144-3150. (26) (a) Wang, X. Y.; Hong, Y.; Ma, D. W.; Zhang, J. C. Molecular Dynamics Study of Thermal Transport in a Nitrogenated Holey Graphene Bilayer. J. Mater. Chem. C, 2017, 5, 5119-5127. (b) Zhang, T. T.; Zhu, L. Y. Giant Reduction of Thermal Conductivity in Two-Dimensional Nitrogenated Holey C2N nanosheet. Phys.



Page 34 of 39

Chem. Chem. Phys., 2017, 19, 1757-1761. (27) (a) Gong, S.; Wan, W. H.; Guan, S.; Tai, B.; Liu, C.; Fu, B. T.; Yang, S. A.; Yan, Y.

G.

Tunable

Half-Metallic

Magnetism

in

an

Atom-Thin

Holey

Tow-Dimensional C2N Monolayer. J. Mater. Chem. C, 2017, 5, 8424-8430. (b) Choudhuri, I.; Pathak. B. Ferromagnetism and Half-Metallicity in Atomically Thin Holey Nitrogenated Graphene Based Systems. CHEMPHYSCHEM, 2017, 18, 2336-2346. (28) (a) Zheng, Z. D.; Wang, X. C.; Mi, W. B. Tunable Electronic Structure of Monolayer Semiconductor g-C2N by Adsorbing Transition Metals: A First-Principles Study. Carbon, 2016, 109, 764-770. (b) Sun, J. Y.; Zhang, R. Q.; Li, X. X.; Yang, J. L. A Many-Body GW+BSE Investigation of Electronic and Optical Properties of C2N. Appl. Phys. Lett., 2016, 109, 133108. (29) (a) Liu, Y. Z.; Meng, Z. S.; Guo, X. J.; Xu, G. J.; Rao, D. W.; Wang, Y. H.; Deng, K. M.; Lu, R. F. Ca-Embedded C2N: An Efficient adsorbent for CO2 Capture. Phys. Chem. Chem. Phys., 2017, 19, 28323-28329. (b) Tromer, R. M.; Luz, M. G. E.; Ferreira, M. S.; Pereira, L. F. C. Atomic Adsorption on Nitrogenated Holey Graphene. J. Phys. Chem. C, 2017, 121, 3055-3061. (30) Zhang, R. Q.; Li, B.; Yang, J. L. Effects of Stacking Order, Layer Number and External Electric Field on Electronic Structures of Few-Layer C2N-h2D. Nanoscale, 2015, 7, 14062-14070. (31) Zhang, R. Q.; Yang, J. L. Few-Layer C2N: A Promising Metal-Free Photocatalyst for Water Splitting. arXiv:1505.02768, 2015


Page 35 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(32) Sun, J. Y.; Zhang, R. Q.; Li, X. X.; Yang, J. L. A Many-Body GW+BSE Investigation of Electronic and Optical Properties of C2N. arXiv:1602.03987v1,

2015 (33) Guan, S.; Cheng, Y. C.; Liu, G.; Han, J. F.; Lu, Y. H.; Yang, A. S.; Yao, Y. G. Effects of Strain on Electronic and Optic Properties of Holey Two-Dimensional C2N Crystals. Appl. Phys. Lett., 2015, 107, 231904. (34) Wang, H.; Li, X.; Yang, J. The g-C3N4/C2N Nanocomposite: A g-C3N4-Based Water-Splitting Photocatalyst with Enhanced Energy Efficiency. ChemPhysChem,

2016, 17, 2100-2104. (35) Luo, X.; Wang, G.; Huang, Y.; Wang, B.; Yuan, H.; Chen, H. A Two-Dimensional Layered CdS/C2N Heterostructure for Visible-Light-Driven Photocatalysis. Phys. Chem. Chem. Phys., 2017, 19, 28216-28224. (36) Kishore, A.; Pavindran, P. Enhanced Photocatalytic Water Splitting in a C2N Monolayer by C-Site Isoelectronic Substitution. ChemPhysChem, 2017, 18, 1526-1532. (37) Yu, S.; Rao, Y.; Duan, M. Modulating the Properties of Monolayer C2N: A Promising Metal-Free Photocatalyst for Water Splitting. Chin. Phys. B, 2017, 26, 087301. (38) Guan, M. L.; Xiao, C.; Zhang, J.; Fan, S. J.; An, R.; Cheng, Q. M.; Xie, J. F.; Zhou, M.; Ye, B. J.; Xie, Y. Vacancy Associates Promoting Solar-Driven Photocatalytic Activity of Ultrathin Bismuth Oxychloride Nanosheets. J. Am. Chem. Soc., 2013, 135, 10411-10417.



(39) Niu, P.; Liu, G.; Cheng, H. M. Nitrogen Vacancy-Promoted Photocatalytic Activity of Graphitic Carbon Nitride. J. Phys. Chem. C, 2012, 116, 11013-11018. (40) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Visible-Light Photocatalysis in Nitrogen-Doped Titanium Oxides. Science, 2001, 293, 269-271. (41) Blöchl P. E. Projector Augmented-Wave Method. Phys. Rev. B, 1994, 50, 17953-17979. (42) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-wave method. Phys. Rev. B, 1999, 59, 1758-1775. (43) Kresse, G.; Hafner, J. Ab initio molecular dynamics for open‐shell transition metals. Phys. Rev. B, 1993, 48, 13115-13118. (44) Kresse, G.; Furthmuller, J. Efficient iterative schemes for ab initio total‐energy calculations using a plane‐wave basis set. Phys. Rev. B, 1996, 54, 11169‐11186. (45) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett., 1996, 77, 3865‐3868. (46) Heyd, J.; Scuseria, G. E.; Ernzerhof, M. Hybrid functionals based on a screened Coulomb potential. J. Chem. Phys., 2003, 118, 8207‐8215. (47) Zhang, H.; Wu, D.; Tang, Q.; Liu. L.; Zhou, Z. ZnO-GaN Heterostructured Nanosheets for Solar Energy Harvesting: Computational Studies Based on Hybrid Density Functional Theory. J. Mater. Chem. A, 2013, 1, 2231-2237. (48) Zhang, H.; Yang, G.; Zuo, X.; Tang, H.; Yang, Q.; Li, G. Computational Studies on the Structural, Electronic and Optical Properties of Graphene-Like MXenes


Page 36 of 39

Page 37 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(M2CT2, M = Ti, Zr, Hf; T = O, F, H) and Their Potential Applications as Visible-Light Driven Photocatalysts. J, Mater. Chem. A, 2016, 4, 12913-12920. (49) Martyna, G. J.; Klein, M. L.; Tuckerman, M. Nosé-Hoover Chains: The Canonical Ensemble via Continuous Dynamics. J. Chem. Phys., 1992, 97, 2635-2643. (50) Saha, S.; Sinha, T. P.; Mookerjee, A. Electronic Structure, Chemical Bonding, and Optical Properties of Paraelectric BaTiO3. Phys. Rev. B, 2000, 62, 8828. (51) Gajdoš, M.; Hummer, K.; Kresse, G.; Furthmüller, J.; Bechstedt, F. Linear Optical Properties in the Projector-Augmented Wave Methodology. Phys. Rev. B,

2006, 73, 045112. (52) Zhang, X.; Zhao, X.; Wu, D.; Jing, Y.; Zhou, Z. High and Anisotropic Carrier Mobility in Experimentally Possible Ti2CO2 (MXene) Monolayers and Nanoribbons. Nanoscale, 2015, 7, 16020-16025. (53) Jing, Y.; Zhang, X.; Wu, D.; Zhao, X.; Zhou, Z. High Carrier Mobility and Pronounced Light Absorption in Methyl-Terminated Germanene: Insights from First-Principles Computations. J. Phys. Chem. Lett., 2015, 6, 4252-4258. (54) Zhang, H.; Liu, L.; Zhou, Z. Towards Better Photocatalysts: First-Principles Studies of the Alloying Effects on the Photocatalytic Activities of Bismuth Oxyhalides under Visible Light. Phys. Chem. Chem. Phys., 2012, 14, 1286-1292. (55) Zhang, H.; Liu, L.; Zhou, Z. First-Principles Studies on Facet-Dependent Photocatalytic Properties of Bismuth Oxyhalides (BiOXs). RSC Adv., 2012, 2, 9224-9229.



(56) Kishore, M. R. A.; Ravindran, P. Tailoring the Electronic Band Gap and Band Edge Positions in the C2N Monolayer by P and As Substitution for Photocatalytic Water Splitting. J. Phys. Chem. C, 2017, 121, 22216-22224. (57) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Environmental Applications of Semiconductor Photocatalysis. Chem. Rev., 1995, 95, 69-96. (58) Tong, H.; Ouyang, S. X.; Bi, Y. P.; Umezawa, N.; Oshikiri, M.; Ye, J. H. Nano-photocatalytic Materials: Possibilities and Challenges. Adv. Mater., 2012, 24, 229-251. (59) Zhang, X.; Zhao, X.; Wu, D.; Jing, Y.; Zhou, Z. MnPSe3 Monolayer: A Promising 2D Visible-Light Photohydrolytic Catalyst with High Carrier Mobility. Adv. Sci., 2016, 3, 1600062. (60) Zhang, S.; Xie, M.; Li, F.; Yan, Z.; Li, Y.; Kan, E.; Chen, Z.; Zeng, H. Semiconducting Group 15 Monolayers: A Broad Range of Band Gaps and High Carrier Mobilities. Angew. Chem. Int. Ed., 2016, 55, 1666-1669.


Page 38 of 39

Page 39 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Graphic


Point Defect Effects on Photoelectronic Properties of the Potential

Recommend Documents