Promising Photocatalysts for Water Splitting in BeN2 and MgN2

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Promising Photocatalysts for Water Splitting in BeN2 and MgN2 Monolayers Yining Wei, Yandong Ma, Wei Wei, Mengmeng Li, Baibiao Huang, and Ying Dai J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01081 • Publication Date (Web): 27 Mar 2018 Downloaded from http://pubs.acs.org on March 27, 2018

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The Journal of Physical Chemistry

Promising Photocatalysts for Water Splitting in BeN2 and MgN2 Monolayers Yining Wei, Yandong Ma*, Wei Wei, Mengmeng Li, Baibiao Huang, Ying Dai* School of physics, State Key Laboratory of Crystal Materials, Shandong University, 250100 Jinan, PR China

Corresponding authors: [email protected] (Y.M.); [email protected] (Y.D.)

ABSTRACT Currently, identifying suitable photocatalysts, especially in two-dimensional materials, for photocatalytic water splitting is still challenging. Here, we report the identification of BeN2 and MgN2 monolayers as efficient photocatalysts for water spitting. Our phonon spectra and ab initio molecular dynamics simulations provide eloquent examinations for the dynamical and thermal stabilities of these two monolayers. We find that BeN2 monolayer exhibits a direct band gap of 2.26 eV, while MgN2 monolayer shows an indirect band gap of 2.33 eV. The carrier mobility for BeN2 monolayer is up to 105 cm2V-1s-1 and 104 cm2V-1s-1 for MgN2 monolayer. More importantly, the BeN2 monolayer has appropriate band edge alignment with respect to the water reduction and oxidation levels in water splitting, while MgN2 monolayer satisfy the reduction levels in water splitting only. These results imply that BeN2 monolayer can be a potential photocatalyst for water splitting, while MgN2 monolayer can be a potential hydrogen production material.

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INTRODUCTION Hydrogen fuel produced from photocatalytic water splitting is one of the promising solutions to address the energy and environmental issues.1,2 To maximize the efficiency of solar energy utilization, i) the material should have a suitable band gap between 1.55 and 3.0 eV, ii) its band edge positions must straddle the reduction and oxidation potentials of water, iii) its conduction band minimum (CBM) should be higher than the hydrogen reduction potential, while the valence band maximum(VBM) should be lower than the water oxidation potential.3-5 Currently, it has been one of the glorious missions of material science to seek for desirable photacatalyst with proper band gap and band alignments. Two-dimensional (2D) materials have aroused the upsurge of research since graphene was first synthesized in 2004.6-15 Many 2D materials have been identified as promising photocatalyst. Compared with traditional 3D photocatalyst, such as TiO2 and Ag-based photocatalyst, 2D materials have its unique superiority: high specific surface areas, good crystallinity, rich options of host–guest species, better charge carrier separation, and abundant surface active sites.16-22 The typical example is C3N4 monolayer, which has been identified as an attractive photacatalyst both theoretically and experimentally.23 Other 2D materials, including GeX (X = S, Se), p-SiC, TiN, and phosphorene also are proven to be suitable photacatalysts in experiment or theory.24-27 Although significant progress has been made in the study of 2D materials for the application in photocatalysis field, new 2D photocatalyst materials are still highly desirable. What’s more, the oxidation reaction of the water splitting is more complicated, and it suffers from a higher overpotential than the reduction reaction.23, 28

So it is of significance to find suitable photocatalyst with lower oxidation potential,

to make sure the water oxidation reaction will happen. Beryllium has been found to be an ideal ligand to stabilize molecules with planar hypercoordinate carbon, such as Cal4Be, CBe5.29, 30 Other stable beryllium-based 2D materials, such as Be2C, Be5C2, BeC and BeN2 monolayer, have been proposed with various properties.31-34 In this work, using first-principles calculations, we identify that BeN2 and MgN2 monolayer are predicted to be stable, and BeN2 can be an ACS Paragon Plus Environment

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efficient photocatalyst with excellent performances, and MgN2 monolayer is a potential materials to generate hydrogen. Our cohesive energy, phonon spectra calculations and ab initio molecular dynamics (AIMD) simulations provide compelling evidences for energy, thermal and dynamical stabilities of these two monolayers. To firmly demonstrate their photocatalytic performance, we provide the redox capabilities, optical absorption and carrier’s mobility. Also, we investigate the related properties of the double-layers of BeN2 and MgN2.

THEORETICAL METHODS First-principles calculations were performed by using the density functional theory (DFT) calculations in conjunction with the projector augmented wave (PAW) scheme, as implemented in the plane-wave basis code VASP (Vienna ab initio simulation package).35,36 The generalized gradient approximation (GGA) as formulated by Perdew–Burke–Ernzerhof

(PBE)

was

used

for

exchange

and

correlation

contributions.37 Band structures are also examined by the Heyd–Scuseria–Ernzerhof (HSE06) hybrid functional to give more accurate results.38 A kinetic energy cutoff of 630 eV was chosen for the plane-wave expansion of wave functions and the Monkhorst–Pack scheme of k-point sampling was adopted for the integration over the first Brillouin zone.39 A 10 × 10 × 1 grid for k-point sampling was used for geometry optimization, while 15 × 15 × 1 for the static total energy calculations. A vacuum space of 20 Å is applied as periodic boundary conditions along the z direction, normal to the interface, which is large enough to avoid the interactions between the adjacent layers. The convergence criteria of energy and force were set to 10-5 eV and 0.01 eV/Å, respectively. The DFT-D2 semi-empirical dispersion correction approach was employed to correct the van der Waals (vdW) interactions between the layers when considering the double layers.40 Besides, ab initio MD simulations (AIMD) calculations were carried out to examine thermal stability by using 3 × 3 supercells at 800K within each time step of 1 fs.

3. RESULTS AND DISCUSSION ACS Paragon Plus Environment


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As shown in Figure. 1a, the optimized structures of BeN2 and MgN2 monolayers adopt a flat but distorted hexagonal structure. Both monolayers possess a P-62m symmetry, with two metal atoms (Be, Mg) and four N atoms in one primitive cell. The optimized lattice parameters and bond lengths are shown in Table 1. The lattice constants of BeN2 and MgN2 monolayers are 4.54Å and 5.26Å, respectively. The bond length between Be atom and N atom is 1.62 Å, while the bond length between Mg atom and N atom is 2.0 Å for MgN2 monolayer. The bond length between N atoms is ~1.33 Å for both systems. We then investigate the chemical bonding features by analyzing their electron localized functions (ELF). From Figure. 1b, we can see that substantial concentration of electrons are located between Be (Mg) and N atoms, indicating strong covalent bonding between metal atoms and N atoms.

Table 1. The optimized lattice parameters and bond lengths for BeN2 and MgN2 monolayers. Lattice parameter (Å) BeN2 MgN2

4.54 5.26

Be-N bond length(Å) 1.62 2.0

N-N bond length(Å) 1.32 1.33


cohesive energy(eV/atom) 4.98 3.89

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Figure 1. (a) Crystal structures and (b) electron localization functions (ELFs) for BeN2 (left) and MgN2 (right) monolayers.

The cohesive energies for BeN2 and MgN2 monolayers are 4.98eV/atom and 3.89eV/atom, respectively. Both value is larger than that for black phosphorus (3.48eV/atom),34 implying that they both are more stable than black Phosphorus. Since few-layer black phosphorus has been synthesized experimentally, BeN2 and MgN2 monolayers are expected to be synthesized in experiment in the near feature.41 To confirm their dynamical and thermal stabilities, we perform the phonon spectrum and AIMD simulations. The AIMD simulation is carried out after running 5000 steps at the temperature of 800K, there are neither structure reconstruction nor bond broken, suggesting that the structures of BeN2 and MgN2 monolayers are stable even at a temperature of up to 800 K. From the phonon spectrum, we can see that there are no imaginary vibration modes in the entire Brillouin zones for both BeN2 and MgN2 monolayers, which verifies the dynamical stabilities of BeN2 and MgN2 monolayers. The evolution of free energy for BeN2 and MgN2 monolayers during AIMD simulation at 800 K and the phonon spectrum are shown in the Supporting Information (Figure S1). After checking the dynamical and thermal stabilities, we further examine the mechanical properties of BeN2 and MgN2 monolayers. The in-plane elastic stiffness coefficients for 2D materials can be written as: C11 =

1 ∂ 2U 1 ∂ 2U 1 ∂ 2U = C = C 22 12 2 A0 ∂ε 112 A0 ∂ε 22 A0 ∂ε 11∂ε 22 ,

where A0 is the equilibrium unit cell area, U is the strain energy, ε11 and ε22 are the strain along the x and y directions. ε is set from −0.02 to 0.02 for each direction, with an increment of 0.01. C11 and C22 represent the elastic stiffness along x and y directions, respectively. The coefficients C11, C22 and C12 are calculated using fully relaxed atomic configurations. The in-plane elastic stiffness coefficients can be obtained by fitting the strain energy U to the in-plane strain states (ε11, ε22):



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1 1 U = C11ε112 + C22ε 222 + C12ε11ε 22 . 2 2 What is more, the Young’s modulus Ex (Ey) is defined as:

Ex = C11 −

C122 C2 E y = C22 − 12 C22 C11 ,

these results related to the C11, C22, C12 and Young’s modulus are listed in Table 2. C11 is almost the same with C22 for both monolayers, resulting from the isotropy geometric structures along the two directions. C11 for BeN2 monolayer is larger than that for MgN2 monolayer, which means the strain can be applied on MgN2 monolayer more easily than BeN2 monolayer. The Young’s modulus of BeN2 and MgN2 are 166 N/m and 92.1 N/m with isotropy, respectively. From the above analyzation we can get that MgN2 monolayer is more flexible than BeN2 monolayer, and both BeN2 and MgN2 monolayers are more flexible than graphene.42, 43

Table 2. In-plane elastic stiffness C11, C22 and C12 and in-plane Young modulus of BeN2 and MgN2 monolayers. The Young modulus of MoS2 monolayer and graphene are also listed for reference. Materials

C11（N/m）

C22(N/m)

C12(N/m)

BeN2 MgN2 graphene MoS2

180.7 112.5

179.6 112.5

51.0 47.9

in-plane Young modulus(N/m) 166.0 92.1 340 123

Next, we investigate the electronic properties of BeN2 and MgN2 monolayers, the corresponding band structures and density of states are plotted in Figure 2. BeN2 is a direct semiconductor with a band gap of 2.26eV, with both the valence band maximum (VBM) and the conduction band minimum (CBM) locating at the Γ point. Different from BeN2, MgN2 presents an indirect-gap band structure, and its VBM located at the K point and CBM lies on the Γ point. From the PDOS and partial charge densities shown in Figure. 2a, we can find that for BeN2 monolayer, the


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electronic states of the valence band near the Fermi level are mainly dominated by the edge N atoms, while conduction band close to the Fermi level mainly originates from the N center atoms. And for MgN2 monolayer, the VBM is contributed by both N edge atoms and Mg atoms, the CBM originates from the N center atoms. These results agree well with the partial charge density analyze plotted in inset of Figure. 2a and

Figure. 2c. Analysis of the partial DOS, as shown in Figure. 2b and Figure. 2d reveals that the VBM mainly originates from N-pz states for BeN2 monolayer and N-px states for MgN2, while N-pz orbits contribute to the CBM for bot h BeN2 and MgN2 monolayers. We also investigate the double layer structures of BeN2 and MgN2, the geometry and electrical properties are shown in the Supporting Information (Figure S2).

Figure 2. Band structures and projected density of states (PDOS) of (a) BeN2 and (c) MgN2 monolayers, the CBM and VBM of BeN2 and MgN2 monolayers are presented as insets in the PDOS. (b) (d) The orbit contribution PDOS.

Effective electron–hole separation and high charge mobility are two crucial aspects for the application of 2D materials. Herein, we study the effective mass and carrier mobility to identify the carriers transfer properties along specific directions. By fitting



parabolic functions to the VBM and CBM of the band structure, we investigate the effective masses of electrons and holes based on the following formula:

d 2 Ek −1 m = ±h ( 2 ) dk , *

2

where k is the wave vector, and Ek is the energy corresponding to the wave vector k. As for mh*, there are two fold degenerate states: the heavier holes correspond to the less dispersive band, the lighter holes are related to the more dispersive band, which means there are two effective masses for holes. We only consider the heavy hole during the

photocatalysis process.44 The carrier mobility in 2D materials is

dominated by acoustic phonon scattering via intra- and inter-valley deformation potential (DP) couplings at room temperature, proposed by Bardeen and Shockley:45, 46

µ=

2eh3C 2

3 K BT m* Ed 2

,

where e, KB, and T represent the electron charge, Boltzmann constant, and temperature, respectively. Ed is the DP constant defined as the shift of the band edges (Eedge) (CBM for electrons and VBM for holes) induced by strain ( ε ): dEedge / d ε . And C =  ∂ 2 E / ∂ε 2  / S 0 is the elastic modulus of 2D system activated by strain, where E is the total energy and S0 is the area of the system. The isotropic strain is defined as ε = ∆c / c0 , where c0 is the lattice constants of the freestanding monolayer. The calculated relative energy and the CBM and VBM positions as a function of the uniaxial strain ε along x and y directions for BeN2 monolayer are shown in the

Supporting Information (Figure S3). It is shown that the linear fittings of relative position of CBM and VBM and the parabolic fitting of total energies fit well with theoretical calculations. The carrier effective mass and mobility of BeN2 and MgN2 along x and y directions are summarized in Table 3. The electron mobility at the CBM along x direction is about 3.24 × 105 cm2V-1s-1, while the holes mobility at the VBM along x direction is about 1.14 × 104 cm2V-1s-1 and 4.48 × 105 cm2V-1s-1 for two


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different degenerate edges. However, for the photocatalysis process, we consider the heavy hole only. The value is higher than most of the 2D materials, including MoS2 (200 cm2V-1s-1), phosphorene (104 cm2V-1s-1) and GeS (2500 cm2V-1s-1).24, 47, 48 For MgN2 monolayer, the mobility along x direction is 1.41 × 104 cm2V-1s-1 for electrons and 172 cm2V-1s-1 for holes. The ultrahigh carriers mobilities suggest fast migration of photo-generated electrons and holes to the surface of the materials to participate in the redox reaction of water splitting. Moreover, the tremendous difference between electron mobility and holes mobility is excellent for the separation of carriers, and could further reduce the probability of the recombination of photo-generated carriers, significantly improving the photocatalytic activity. Furthermore, we investigate the effect of isotropic strains on their electronic structures and band gaps, as shown in

Figure 3. For both BeN2 and MgN2, by exerting isotropic strains ranging from -5% to 5%, there is no change of the position of the CBM and VBM, and the band gap exhibit very little change, which means the electronic properties are not sensitive to the strain.

Table 3. Carrier effective masses (m*) and carrier mobilities of BeN2 and MgN2 monolayers along x and y directions. BeN2

MgN2

Carrier type Electrons (x) Heavy holes (x) Lighter holes(x) Electrons (y) Heavy holes (y) Light holes(y)

m∗/m0 0.27 1.87 0.30 0.26 1.82 0.29

µ(cm2V-1s-1) 3.24 × 105 1.14 × 104 4.48 × 105 3.23 × 105 1.49 × 104 5.88 × 105

Electrons (x) Holes (x) Electrons (y) Holes (y)

0.52 1.78 0.51 1.61

1.41 × 104 1.72 × 102 3.00 × 104 2.62 × 102



Figure 3. Band structures of BeN2 monolayer under (a) −5%, (b) -3%, (c) 3% and (d) 5% biaxial strain. And band structures of MgN2 monolayer under (d) −5%, (e) -3%, (f) 3% and (h) 5% biaxial strain. The Fermi level is set to 0 eV.

In addition to the band gap values, another criterion to facilitate the water splitting reaction is that the position of CBM must be higher (more negative) than the hydrogen reduction potential of H+/H2 and the VBM must be lower (more positive) than the water oxidation potential of H2O/O2. We then study the band edges by the HSE functional with respect to the vacuum level. As shown in Figure 4, for BeN2


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monolayer, CBM and VBM satisfy the reduction and oxidation potential of water splitting, respectively. Remarkably, the position of the VBM is -6.284eV, which is much lower than the water oxidation potential of H2O/O2 (-5.67eV). It has been reported that the oxidation reaction has a much higher overpotential, so the lower VBM is essential to the oxidation reaction.28 For MgN2 monolayer, only the CBM position is higher than the reduction level of H+/H2O, which means MgN2 monolayers can sever as potential photoacatalyst to generate hydrogen. The double layers of BeN2 can meet the oxidation potential of water splitting, for MgN2, the reduction potential is satisfied.

Figure 4. The locations of VBM and CBM of BeN2 and MgN2 with respect to the vacuum level which is labeled as 0 eV. The positions of the reduction level of H+ to H2 and the oxidation potential of H2O to O2 are indicated by the dashed lines.

For efficient and feasible photocatalysts in practical applications, the efficient utilization of solar energy is of great importance. The optical absorption properties are analyzed based on the imaginary part of the dielectric function. From Figure 5, the ACS Paragon Plus Environment


absorption edges for both BeN2 monolayer and MgN2 monolayer lie in the visible light region, and the substantial absorption is at ultraviolet area. Since the visible light accounts for more than 50% of solar energy, the optical absorption of BeN2 and MgN2 monolayers would guarantee high efficiency in the utilization of solar energy. Besides, we find that, for both BeN2 and MgN2 double layers the absorption of light have red shift, which is due to the smaller band gap for the double layers.

Figure 5. Imaginary part of the dielectric function of BeN2, MgN2 monolayers and double layers.

CONCLUSION In summary, we theoretically investigate the structural, electronic and optical properties of 2D BeN2 and MgN2 monolayers based on the first-principles calculations. The dynamical stabilities of the two monolayers are confirmed by phonon spectra with no imaginary frequencies. AIMD simulations provide further evidence for their thermal stability, which can withstand temperature even at 800 K ACS Paragon Plus Environment

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with tiny lattice distortion and bond broken. We reveal that BeN2 and MgN2 monolayers are semiconductor with band gap of 2.26eV and 2.33eV, respectively. Attractively, both monolayers have extremely high carrier mobility: for BeN2 it reaches 105 cm2V-1s-1 and for MgN2, 104 cm2V-1s-1. Remarkably, the band edges of BeN2 monolayers can meet the requirement of the reduction and oxidation levels in water splitting, while MgN2 monolayers can satisfy the reduction levels. Meanwhile, BeN2 and MgN2 monolayers also exhibit good optical absorption in the visible-light wavelengths. Our study shows that the BeN2 monolayer is a promising candidate as a visible-light water splitting photocatalyst, while MgN2 monolayer is a potential hydrogen production material. However, the insight reaction between BeN2, MgN2 with water, and the stability of the two monolayers in aqueous solution are still unknown, which is needed to be solved in the future.

Supporting Information AIMD simulation and Phonon dispersion curves for BeN2 and MgN2 monolayers, optimized configure and band structure for BeN2 and MgN2 double layers, rectangle unit cell, the variation of band edge and energy as a function of the uniaxial strain along x directions for BeN2

ACKNOWLEDGMENTS This work is supported by the National Basic Research Program of China (973 program, under Grant No. 2013CB632401), the National Natural Science foundation of China (under Grant No.21333006, 11404187and 11374190), the Taishan Scholar Program of Shandong Province, the 111 Project B13029, and Qilu Young Scholar Program of Shandong University.

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Promising Photocatalysts for Water Splitting in BeN2 and MgN2

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