the Sc Transition Metal Doping

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A new Effective Approach to Prevent the Degradation of Black Phosphorus: the Sc Transition Metal Doping Xinbo Wang, Chunmei Tang, Weihua Zhu, Xiaofeng Zhou, Qionghua Zhou, and Chun Cheng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01089 • Publication Date (Web): 19 Feb 2018 Downloaded from http://pubs.acs.org on February 24, 2018

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The Journal of Physical Chemistry

A New Effective Approach to Prevent the Degradation of Black Phosphorus: the Sc Transition Metal Doping Xinbo Wanga Chunmei Tanga, * Weihua Zhua,* Xiaofeng Zhoua

Qionghua Zhouc Chun Chengb,*

a

College of Science, Hohai University, Nanjing, Jiangsu 210098

b)

Department of Materials Science and Engineering，Southern University of Science and Technology,

Wisdom Garden Building I, Room 308;Tangchang Rd.,Xili,Nanshan District,Shenzhen,Guandong, 518055 c)

School of Physics, Southeast University, Nanjing, Jiangsu 211189

ABSTRACT: The density functional calculations are used to investigate the effective approach to prevent the degradation of black phosphorus(BP). For the few-layer BP, it is found that the Sc atom monodoped 3x3 unit cell with the doping ratio of 1.4％ can remain the semiconductor character with the moderate band gap(Egap) of 0.84eV. Obviously, the two layer BP(2MLBP) with one Sc atom binding to three P atoms on the upmost layer has the lowest formation energy (-4.57eV), exploring its most stability. Moreover, the calculated binding energy of Sc above the three P atoms is the largest(4.24eV), which is larger than the experimental cohesive energy of bulk Sc(3.90 eV), indicating the clustering of Sc atoms on the surface of BP can be avoided. Importantly, the Sc atom adsorbed 2MLBP structure (2MLBP-Sc) has the conduction band minimum(CBM) value of -4.18 eV, which is below the redox potential of O2/O2-(-4.11eV). When the O2 molecule is adsorbed on the 2MLBP-Sc, although the Sc-O and O-P bonds are formed based on the broken of the O-O bond, forming the 2MLBP-Sc-2O structure contains the intact 2MLBP. Importantly, the 2MLBP-Sc-2O structure has a direct Egap of 0.32eV with the CBM of -4.75eV which 1

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is far below the redox potential of O2/O2-, therefore, its Egap is not located in the range of visible light, indicating it is a good direct semiconductor and stable in the air. When the structure further interacts with the H2O molecule, the resulting 2MLBP-Sc-2O-H2O structure can maintain its integrity during the dynamics course. Our research can supply a new effective method to protect the BP in the illumination, oxygen, and water environment.

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INTRODUTION More recently, the black phosphorus (BP), a new isomer of phosphorus, has attracted increasing attention as a monolayer or few-layer material with extraordinary electronic and optoelectronic properties. Due to its intrinsic direct energy band gap (Egap), a good compromise between charge-carrier mobility and current on/off ratios, and because of its unusual in-plane anisotropy, BP has tremendous potential in both electronics and optoelectronics.1–6 The bulk BP can be synthesized under 10 kbar pressure and high temperature, and it is the most stable phosphorus allotrope at ambient conditions.7,8 The few-layer BP has a high carrier mobility up to 1000 cm2V-1s-1 at room temperature on the two dimensional form and a moderate Egap of about 1.5 eV.3,9 The BP crystal can be exfoliated to few-layer BP or single-layer BP by scotch tape based on the mechanical exfoliation method.3 The weak Van der Waals interlayer interaction exists between P layers.10-14 The single-layer BP is named as phosphorene. It has been reported that the direct Egap can be tunable from 0.3eV for bulk to 2.2eV for phosphorene,15 which can be directly coupled with the visible light. However, the few-layer BP can easily react with oxygen and water under ambient conditions,16 leading to the fast degeneration within a short time.4,17,18 The degradation in air and other environments is an issue that may limit the future applications of BP. Therefore, how to prevent the degradation of few-layer BP becomes an urgent issue. The degradation mechanism of few-layer BP has been highly debated but still far from clear. For example, in the degradation course of BP, the triplet-to-singlet 3



conversion of O2 greatly lowers the oxygen dissociation barrier, then the dangling O atoms increases the hydrophilicity of BP.19 Castellanos et al.5 have suggested that visible light, oxygen, and water three factors can cause the degradation of BP. Favron et al.20 have proved that visible light can promote the oxidation of BP. However, Huang et al.16 and Wang et al.21 have presented that the degradation of BP caused by the oxygen excluding the water initially. Recently，Zhou et al.22 have calculated that the valence band maximum(VBM) and the conduction band minimum(CBM) of phosphorene are -5.46 eV and -3.95 eV respectively. Therefore, the direct Egap of 1.51eV is within the visible light range and can induce the electron excitation. Moreover, the redox potential of O2/O2-(-4.11eV) is just below the CBM and located within the energy gap, causing O2 to accept an electron, generating a large amount of O2-, which will further react with phosphorene and start the degradation course. When the O2- adsorbed BP is reacting with the H2O molecule, because the H-O bond between the H atom in the water molecule and the O atom on the surface is stronger than the P-P bond, so the outmost P layer of few-layer BP has been destroyed, causing the inner P atoms to be exposed to the air. This can be circulated until it is completely degraded.16,22,23 Researchers have tried many methods to prevent the degradation of few-layer BP. For example, to use the two-dimensional nanomaterials such as graphene, hexagonal boron nitride(h-BN), aluminum oxide(Al2O3) layers to coordinate with outermost P layer. However, how to prevent BP from the contamination and damage during the deposition of exotic layers is still need to be solved. Another method is to oxidize the 4


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P atoms on the outmost BP surface, which can prevent the degradation of BP under the illumination, oxygen and water condition. However, it also looks forward to an intentional water-free oxidization.4,17,22,23-27 Thus, to efficiently protect BP from the air and water is an enormous challenge at present. Recently, Kim et al.28 have revealed that the direct Egap of few-layer BP can be tuned between 0 and 0.6 eV by the dopant K atom. Yang et al.29 have used Te atom to decorate few-layer BP, which can obviously reduce the CBM of the structure. The aim of this paper is to prevent the degradation of few-layer BP by the method of metal doping, which can reduce the Egap to below the visible light range and the CBM to under the redox potential of O2/O2- respectively. Therefore, the few-layer BP can be effectively protected. In what follows, we will first describe the computational details in Section 2, then present our results and discussion in Section 3, and end with our final conclusions in Section 4. COMPUTATIONAL METHODS All calculations are carried by the density functional theory(DFT) implemented in the Vienna ab initio Simulation Package(VASP).30,31 The exchange-correlation interactions are Perdew-Burke-Ernzerhof type(PBE) within the generalized gradient approximation(GGA).32 In the geometry optimization course, the Van der Waals interactions are considered by the vdW-DF level with the optB88 exchange functional(optB88-vdW).33,34 For the pure BP, the plane-wave cutoff energy and the k-mesh is chosen as 400eV and 8×10×1 respectively. The geometric structures are 5



fully relaxed until the force on each atom is smaller than 0.001 eV/Å. For the doped structure, the cutoff energy of 600 eV and the k-meshes of 3×4×1 are used. All the structures are fully relaxed until the force on each atom is less than 0.01 eV/Å. The vacuum layer of 15Å is introduced to minimize the interaction between the adjacent layers. The electronic structures are calculated by hybrid functional (HSE06) method. 35,36

Since it yields reasonably accurate predictions for energy band gaps,37-39 thus, the

HSE06 method is used to calculate the electronic band structures of few-layer BP and metal doped structures in this paper. The optical absorption spectra are calculated by the HSE06 method. To reproduce the experimental conditions more correctly, the calculated optical absorption spectra are broadened for the wavelength range 0-1500nm. The real part of the dielectric function is then calculated using the Kramers–Kronig transformation.40 The climbing-image nudged elastic band(CI-NEB) method41 incorporated with the spin-polarized DFT are used in the transition state(TS) calculation. The ab-initio molecular dynamics(ABMD) simulations are performed at 300 K and the total simulation time is 5ps with a time step of 1.0 fs, the 5ps time has been proved sufficient to simulate the whole dynamics course.22,42,43 The accuracy of our computational method is firstly tested. The optimized average length of the P-P bond is 2.243Å, which is in good agreement with the value of 2.248 Å gotten by other researchers.22,44 The average lengths of the P-P bond calculated by the PBE, PBEsol, B3LYP are 2.243Å, 2.236Å, 2.235Å respectively, which are similar to the experimental values (2.240 Å) obtained by X-ray diffraction (XRD).45 The Egap of phosphorene calculated by the HSE06 method is 1.51eV, which 6


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is in good agreement with 1.51eV obtained by Zhou et al.22 Therefore, these methods are more suitable to study the structural and electronic properties of BP. Above all, the computational scheme used in this paper is reliable. RESULTS AND DISCUSSIONS For the few-layer BP, the stacking pattern between layers should be taken into consideration. There are three different stacking patterns between the layers of BP as shown in Figure 1. For the AA-stacking, the top layer is stacked directly above the bottom. For the AB-stacking, the edge of the puckered hexagon in the top layer is located at the center of the puckered hexagon in the bottom layer, which can be gotten by shifting the top layer half a cell in the Y direction. For the AC-stacking, the top and bottom layers are mirror images of each other. We calculate that the total energies are -77.05eV, -80.79eV, and -77.63 eV for AA-, AB-, and AC-stacking structures respectively. Therefore, the AB-stacking BP should be the most stable, which is the same as that of Dai et al.44 The 3x3 unit cell of phosphorene are usually considered for the following study. 22,29

We calculate that the direct Egap for the 3x3 unit cell of phosphorene is 1.51eV,

which is similar to the value explored by Zhou et al.22 Obviously, the Egap is in the energy range of the visible excitation, indicating that the phosphorene will produce excitations under ambient light. Importantly, the CBM of phosphorene (-3.95 eV) is above the redox potential of O2/O2-(-4.11 eV),20,22,46 thus, it can cause the electron excitation from the conduction band to the O2 molecule, generating the O2- ions and 7



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inducing the degradation of few-layer BP. Very recently, Yang et al.29 and Kim et al.28 have explored that the Te and alkali metal atom doped BP can effectively reduce the CBM and adjust the Egap to locate in the 0~0.6 eV energy range respectively. However, the research on the BP doping of other metals are absent. Therefore, we choose many different kinds of metals such as alkaline-earth elements, transition elements concluding Mg, Ca, Sr, Sc, Y to decorate the few-layer BP. For the sake of exploring interlayer interactions clearly, the bilayer BP (2MLBP) is taken into consideration. In order to search out the most stable metal atom (M) decorated 2MLBP structure, the formation energy(Ef) of the decorated structure with the metal atom located at four different positions is calculated as follows:22,47 Ef= EP×NP-EM+E2MLBP-M

(1)

Where, EP, EM, and E2MLBP-M are the total energy of the isolated P, metal, and metal decorated 2MLBP respectively. NP is the total number of P atoms in metal doped 2MLBP. The Ef can reflect the stability of a structure. The positive value means an exothermic reaction, a more stable structure should have a lower Ef.22,47 Based on the research proved by Yang et al.,29 four different doping positions for the metal atom have been considered, as shown in Figure 2. They are respectively: (a) replacing one P atom on the surface. (b) replacing two P atoms on the surface. (c) in the middle of up and down layers of 2MLBP. (d) bonding to three dangling P atoms 8


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on the surface of 2MLBP. Table 1 lists the Egap and Ef of one Mg(Ca, Sr, Sc, Y) atom decorated 2MLBP. Obviously, the Ef of Sc atom above three dangling P atoms on the surface is -4.57eV, which is the lowest, indicating this structure should be the most stable. Based on the PBE calculation considering the calculation cost, the Mg(Ca, Sr, Sc, Y) doped structure can obviously reduce the Egap. Among of all, the Sc doped black phosphorus reduces the CBM value most. Furthermore, we continue to use HSE06 to calculation the electronic band structures of Sc doped 2MLBP(d style), which has a Egap of 0.84 eV based on HSE06 (as shown in Figure 2e). Figure 2(e) presents the partial density of states (PDOS) of Sc and BP in the 2MLBP-Sc structure. The PDOS are obtained by the Lorentzian extension of discrete energy levels, with weights given by the orbital populations of the levels, and a summation over them. The fermi energy (Ef) is at 0 eV, shown by the black dotted line in the figure. The CBM of -4.18 eV is lower than the redox potential of O2/O2-. Meanwhile, we can find from the PDOS hat there is obvious hybridization between the d orbitals of Sc and BP in the range of energy from 0.6eV to 1.3eV. This suggests that there is a strong chemical interaction between Sc and BP. Therefore, the Sc doped BP perhaps can avoid the degradation in the illumination, oxygen, and water environment. We define Nmetal∕NP as doping ratio, where NP is the number of P atoms, Nmetal is the number of metal atoms. Obviously, with the same number of metal atoms, the structure with the larger lattice size will have the smaller metal ratio. Which should 9



be the most suitable doping ratio for the metal doped BP with the lowest CBM? In order to answer this question, different unit cell of BP are tested in the following. As can be seen from Table 2, for the Mg(Ca, Sr, Sc, Y) doped 2MLBP structure, the 2x2 unit cell with the doping ratio of 3.1％ are gapless or has a higher CBM than the redox potential of O2/O2-. Meanwhile, the CBM of the 4x4 unit cell with the ratio of 0.78％and the 5x5 unit cell with the ratio of 0.50％ are all higher than the redox potential of O2/O2-. Thus, the CBM of the 3x3 unit cell with the ratio of 1.4％should be the lowest. It is known from figure 3a that the Sc doped 3x3 super cell of BP can effectively reduce the CBM to under the redox potential of O2/O2- and has a Egap of 0.84eV. In order to find out the most suitable doping ratio of Sc doped 2MLBP, doping of multiple Sc atoms is considered at the following. Figure 3b presents the structure with two and three Sc atoms doped in different positions respectively. Because the calculated lattice constants are 13.53Å and 9.99Å in the x and y direction for the 3x3 unit cell of BP, therefore, the distance between a and b (3.33Å) is not equal to the distance between c and d (4.51Å). Therefore, totally five different isomers for the two Sc atoms doped 2MLBP are considered, as shown in Figure 3. They are respectively: (1) two Sc atoms are adjacent in the same column on the surface of BP; (2) two Sc atoms are interphase by one P atom in the same column on the surface of BP; (3)two Sc atoms are adjacent in the same row on the surface of BP; (4)two Sc atoms are interphase by one P atom in the same row on the surface of BP; (5) two Sc atoms are in the diagonal position separated by three P atoms on the surface of BP. 10


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When two Sc atoms are in the same column, the CBM are reduced to -4.02 eV, and they are still higher than the redox potential of O2/O2-. If two Sc atoms are in the same row, the structure has no Egap, moreover, the structure with three Sc atoms adjacent in the same column is still gapless. Therefore, only the doping of one Sc atom in the 3x3 unit cell of BP perhaps can protect the BP from degradation. In order to certify the binding strengthen between the Sc atom and the black phosphorus, as well as to search out the most stable position of the metal atom, the binding energy(Eb) of the Sc atom to 2MLBP is calculated as follows: Eb= (E2MLBP+ESc)-E2MLBP-Sc

(2)

Where, E2MLBP-Sc, E2MLBP, and ESc is the total energy of Sc doped 2MLBP, 2MLBP and the isolated Sc atom respectively. The Eb can reflect the binding strength between the atom and the substrate. The larger Eb indicates a stronger binding strength between the Sc atom and the surface. We calculate that the Eb of Sc atom above three dangling P atoms on the surface is 4.24eV, which is larger than the experimental cohesive energy of bulk Sc(3.90 eV),48 thus, the clustering of Sc atoms can be avoided when Sc is bound to two P atoms. In addition, it is found that the second layer P atoms of 2MLBP-Sc are stable without any shift. Therefore, the Sc atom only affects the first layer of BP. Moreover, Yang et al.29 have obtained the similar structure of Te doped black phosphorus in their experiment, so we believe that the Sc doped black phosphorus can be synthesized in experiment.

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Figure 4 presents the Egaps of MLBP, MLBP-Sc, 2MLBP, 2MLBP-Sc, 3MLBP, and 3MLBP-Sc respectively. On one hand, the Egap changes from 1.51eV for MLBP to 0.8 eV for 3MLBP. Thus, the thicker BP has a lower Egap. On the other hand, the Egap of Sc decorated few-layer BP is lower than that of BP. Moreover, the CBM will shift downward to below the redox potential of O2/O2- starting from the 2MLBP-Sc. Therefore, the 2MLBP- Sc is inert to generate O2- under ambient light. The average adsorption energy(Ead) of a molecule on the substrate is defined in the following: 43 Ead=E2MLBP-Sc+Eadsorbate-E2MLBP-Sc-adsorbate

(3)

Where, EBP-Sc, Eadsorbate, and E2MLBP-Sc-adsorbate are the total energy of 2MLBP-Sc, adsorbate, and 2MLBP-Sc-adsorbate respectively. A small or negative Ead means that the spontaneous adsorption and desorption of gas molecules is impossible at room temperature. It has been reported that the Ead of O2 and O2- above the 2MLBP-Sc is 2.26eV and -0.23eV respectively, therefore, the adsorption of O2- by 2MLBP-Sc is endothermic, indicating the adsorption of O2- on the 2MLBP-Sc surface is difficult. Moreover, the CBM of 2MLBP-Sc has been dropped below the redox potential of O2/O2-, indicating the continuous generation of O2- becomes very difficult, while the O2 is easier to be adsorbed by 2MLBP-Sc. The reaction process between O2 and 2MLBP-Sc are discussed by the transition state (TS) calculation shown in Figure 5. As shown in Figure 5, the O-O bond length 12


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of the O2 molecule in the TS structure, final structure are 2.15Å, and 3.28Å, which are much larger than that of the free O2 molecule (1.24 Å), indicating that chemical adsorption occurs between O2 and 2MLBP-Sc. Obviously, the O2 molecule in the final structure has been decomposed into two O atoms, while the Sc-O and P-O bonds are formed. The energy barrier is 0.92eV, a little larger than that of 2MLBP-Sc-O2, however the final structure is 13.2eV lower in the energy than the initial structure for 2MLBP-Sc-2O, much larger than that of 2MLBP-Sc. The bond length of Sc-O and P-O is 1.94Å and 1.54Å respectively, which are much shorter than the sum of the covalent radius of Sc, O, and P respectively. Moreover, it is known from the Bader charge of Sc, O, and P in the initial and final structure shown in Figure 5 that the electrons should be transferred from P and Sc to the O atom, indicating the Sc-O and P-O bonds should be stronger than the Sc-P bond. Furthermore, Sc is bonded to 4 atoms around itself without lone pair electrons and empty orbit, indicating that the Sc atom is in a stable state. As can be seen from Figure 6a that the calculated direct Egap of the final structure calculated by HSE06 is 0.32eV with the CBM of -4.75eV far below the redox potential of O2/O2-, therefore, the Egap of BP-Sc-O2 is not located in the range of visible light. It can be further found from the optical absorption spectrum shown in figure 6b that there is no light absorption peak in the wavelength range of visible light(390nm-760nm), indicating the 2MLBP-Sc-2O structure is stable in the air.6,22,44 Figure 6c shows the ab initio molecular dynamics (ABMD) simulations of 2MLBP-Sc-2O with the total simulation time of 5ps and a time step of 1.0 fs. The 5ps 13



time has been proved sufficient to simulate the whole dynamics course.22,42,43 The geometric structures are intact after the dynamics course, exploring that the 2MLBP-Sc-2O structure should be stable at room temperature. In general, we can summarize the properties of 2MLBP-Sc-2O as follows: (a) the structure of 2MLBP is intact when reacting with O2. (b) The direct Egap of 0.32eV is not located in the visible light range. (c) The CBM is far below the redox potential of O2/ O2-. We continue to investigate the effect of water in the degradation course of BP. For the BP, the stability of the surface is challenged by the interaction between the dangling O of the P-O bond and the H atom in the H2O molecule, and the initial collapse begins with the P-P bond breaking.16,22 In order to confirm whether the H2O molecule will react with the BP-Sc-O2 structure, the ABMD simulation with the total simulation time of 5ps for MLBP-Sc-2O-H2O and 2MLBP-Sc-2O-H2O is taken into consideration. In order to guarantee the accuracy and rationality of the experimental results, the H2O molecule is initially located above the dangling O atom of doped structure. The initial distance between H2O and the dangling O atom is set as 3.0Å. Figure 7a shows the geometric structure of MLBP-Sc-2O-H2O after 1ps, 3ps, and 5ps dynamics course. It is clearly that the MLBP-Sc-2O-H2O structure can maintain the structure integrity during the dynamic course. The distance between H2O and MLBP-Sc-2O changes from 3.03Å(1ps) to 2.29Å(2ps). Due to the hydrogen bonding effect, the H2O will move closer to MLBP-Sc-2O before 3ps, so it 14


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is eventually stabilized at about 1.89 Å(3ps). Because of the stronger Sc-O chemical bonds and the limit hydrogen bonding force between H and O, the distance between H2O and the dangling O atom is almost unchanged from 3ps to 5ps. In order to test if the Sc atom affects the second layer of BP, the dynamics structure for 2MLBP-Sc-2O-H2O after 1ps, 3ps, and 5ps is also shown in Figure 7b. Obviously, the second layer is almost unchanged in the dynamics course. The distance between H2O and MLBP-Sc-2O changes from 3.04Å(1ps) to 1.90Å(3ps) to 1.89Å(5ps). Therefore, the Sc atoms doped few-layer BP can maintain the good structure integrity in the illumination, oxygen and water environment. CONCLUSION The Sc atom monodoped 3x3 unit cell of bilayer BP with the doping ratio of 1.4％ can remain the semiconductor character with the moderate energy gap of 0.84eV. Obviously, the calculated formation energy of Sc above three P atoms on the surface of BP is the lowest of -4.57eV, exploring the best doping site of Sc atom. Obviously, the calculated binding energy of Sc above the three P atoms is the largest(4.24eV), which is larger than the experimental cohesive energy of bulk Sc(3.90 eV), indicating the clustering of Sc atoms on the surface can be avoided. When the O2 is adsorbed, the Sc-O and O-P bond are formed. The energy gap of 2MLBP-Sc-2O(0.32eV) is below visible light range. Importantly, the conduction band minimum of 2MLBP-Sc(-4.75 eV) is lower than that of the redox potential of O2/O2-(-4.11eV), therefore, the Sc doped BP can avoid the light-induced degradation 15



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in the air moisture environment. By the dynamic course simulation, the 2MLBP-Sc-2O and 2MLBP-Sc-2O-H2O structure can maintain the structure integrity during the dynamic course. Our research can supply a new effective method to protect the black phosphorus in the illumination, oxygen, and water environment. AUTHOR INFORMATION Corresponding author Chunmei Tang, E-mail: [email protected]; Weihua Zhu, E-mail:[email protected]; Chun Cheng, E-mail: [email protected]

ACKNOWLEDGMENTS This work is financially sponsored by the Natural Science Foundation of Jiangsu Province (Grant No.BK20161501), the Fundamental Research Funds for the Central Universities (Grant No. 2015B19314), Six talent peaks project in Jiangsu Province (Grant No. 2015-XCL-010). The National Natural Science Foundation of China (Grant No. 51776094 and51406075), the Guangdong Natural Science Funds for Distinguished

Young

Scholars

(Grant

No.

2015A030306044)

and

the

Guangdong-Hong Kong joint innovation project (Grant No. 2016A050503012). REFERENCE (1)

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Page 22 of 39

Table1：The Egap and Ef of one Mg(Ca, Sr, Sc, Y) decorated 3x3 unit cell of 2MLBP(Unit: eV). Position ( the unit size is 3x3 )

metal a

b

c

d

(eV)

CBM

VBM

Egap

CBM

VBM

Egap

CBM

VBM

Egap

CBM

VBM

Egap

Mg

-4.01

-4.38

0.37

-3.97

-4.28

0.31

-3.90

-4.16

0.26

-4.06

-4.47

0.41

-1.12

Ef Ca

-3.90

-3.86

-4.17

Ef

-3.96

-4.57

-4.05

-4.37 -3.34

-4.02

-0.77 0.15

-3.84

-1.43 0.10

-3.85

-3.92

0.40

-4.05

-4.38

0.07

-4.03

-4.31

0.11

-3.92

-3.88

-0.96 0.33

-4.01

-4.28

-2.57

-3.97

-4.22 -1.73

22


0.18

-4.00

0.12

-2.43 0.27

-4.18

-1.86 0.28

-4.10 -2.67

gap = 0

-2.76 0.32

-3.95

-1.44

-1.05

-1.36

-3.42

Ef Y

-3.87

-1.88

Ef Sc

0.16

-1.97

Ef Sr

-4.06

-0.90

-5.02

0.84

-4.57 0.25

-4.08

-4.42 -4.12

0.34

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


Table2: The CBM of different unit cell of 2MLBP-M with M on the surface(Unit: eV) . Unit cell

CBM Mg

Ca

Sr

Sc

Y

2x2

-4.08

-4.01

-3.95

3x3

-4.06

-3.92

-3.88

-4.18

-4.08

4x4

-4.01

-3.88

-3.86

-4.09

-4.05

5x5

-3.93

-3.84

-3.83

-4.06

-3.97

23



Figure captions: Figure 1: Three stacking types of bilayer BP（1x1）. a, b, c are the AA-, AB-, AC-stacking structures respectively. Both Top view and Side view are given in the figure. Figure 2：(a~d) Four isomers of the geometric structures of 2MLBP-Sc; (e) The band structure of 2MLBP-Sc and PDOS of Sc in 2MLBP-Sc. Fermi level is set to zero. Figure 3: (a) The CBM of 2MLBP-Sc with different metal ratio; (b) the CBM of 2MLBP when doped with 2 and 3 Sc atoms respectively. Figure 4:

The CBM and VBM of ML, 2ML, and 3ML respectively. The dashed line

is the redox potential of O2/O2-. Figure 5：(a) The reaction process between 2MLBP-Sc and O2. IS, TS, and FS are the initial (reactant), transitional, and final structure (product) respectively. Figure 6: (a) the band structure of 2MLBP-Sc-2O. (b) Absorption spectra of the 2MLBP-Sc-2O. (c) the ABMD simulation of the 2MLBP-Sc-2O. Figure 7: (a) the ABMD simulation of the 2MLBP-Sc-2O. (b) the ABMD simulations course of 2MLBP-Sc-2O-H2O.

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Figure 1, Wang et al.

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the Sc Transition Metal Doping

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