Few-layer P4O2: A Promising Photocatalyst for Water Splitting - ACS

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Few-layer PO: A Promising Photocatalyst for Water Splitting Baichuan Lu, Xiaoyan Zheng, and Zesheng Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21001 • Publication Date (Web): 20 Feb 2019 Downloaded from http://pubs.acs.org on February 21, 2019

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ACS Applied Materials & Interfaces

Few-layer P4O2: A Promising Photocatalyst for Water Splitting Baichuan Lu, Xiaoyan Zheng*, Zesheng Li* Key Laboratory of Cluster Science of Ministry of Education, Beijing Key laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing, 100081, China. KEYWORDS：photocatalytic water splitting, two-dimensional materials, carrier mobility, phosphorus oxide, firstprinciple calculations

ABSTRACT: Photocatalytic water splitting by two-dimensional (2D) material is a promising technology for producing clean and renewable energy. Development of this field requires candidate materials with desirable optoelectronic properties. Here we present a detailed theoretical investigation of the atomic and electronic structure of few-layer P4O2 systems to predict their optoelectronic properties. We predict that the three-layer P4O2 with normal packing (α-3), ingeniously combining all desired optoelectronic features, is an ideal candidate for photocatalytic water splitting. It fascinatingly bears nearly direct band gap (1.40 eV), appropriate band edge position, high solar-to-hydrogen (STH) efficiency (17.15%), high sunlight absorption efficiency, and ultrahigh carrier mobility (21460 cm2V-1s-1) at room temperature. These results make three-layer P4O2 a promising candidate for photocatalytic water splitting.

INTRODUCTION Photocatalytic water splitting is a promising technology that use photocatalyst in an electrochemical reaction to initiate the splitting of water into its elemental components (H2 and O2) by sunlight without additional energy, providing a fascinating route toward producing clean and renewable energy1-5. In recent decade, twodimensional (2D) materials have drawn tremendous attention as a potential candidate to be used in the photocatalytic water splitting, such as semiconductors molybdenum disulfide (MoS2)6-8 and graphitic carbon nitride (g-C3N4)9-11. Since the 2D materials exhibit two desirable advantages, the photocatalytic efficiency of water splitting can be improved12. First it has large contact surface area for sunlight absorption and photocatalytic water splitting reactions. Second it has the short distance for photogenerated electron and hole to mobilize onto the water interface. However, the disadvantages of g-C3N4 and MoS2 limit their practical application of photocatalytic water splitting, such as the large bandgap and low yield of sunlight absorption13-14. For 2D materials, to be a potential candidate of semiconductor photocatalyst in water-splitting reaction, some key condition must be satisfied12, 15, for example, the appropriate band gap (1.3-1.8 eV) for efficient absorption of sunlight in visible light region, and the carrier mobility for electron and hole must be high enough to enhance carrier transport efficiency onto the water interface. The last but not the least, the band edge positions of 2D materials must straddle over the water redox potentials in water splitting

reaction. Therefore, the exploration of novel and promising 2D semiconductors as high-efficiency photocatalyst for water splitting is quite meaningful, but extremely challenge. Black phosphorus (BP) is a fascinating 2D material with thickness-tunable bandgap from 0.3 eV (bulk) to 1.58 eV (monolayer) and high free-carrier mobility16-19. But it can only be used as the photocathode for hydrogen evolution reaction (HER) in solar water splitting cells with poor efficiency because of its too high conduction band minimum (CBM) position20. To modulate the band edge position of a semiconductor photocatalyst, scientists proposed many ways, such as mechanical strain, electrical bias, pH and defect or dopant doping, but the desired performance for photocatalytic water splitting are impacted by the complex factors in the experimental process21-25. Recently Hu et al.20 proposed that the pseudohalogen edge-modified phosphorene nanoribbons could be used as efficient photocatalysts for water splitting and the intrinsic optoelectronic properties of phosphorene were preserved. However, the phosphorene derivatives are sensitive to oxygen and are easy to degrade in ambient condition. Wang et al.8, 26 reported the environmental degradation mechanism of BP and proposed that a fully oxidized BP layer as a native capping can be taken as a feasible protection layer for BP. Lu et al.27 demonstrated that phosphorus oxides have tunable band gaps experimentally. Xiang et al.28 found that P4O4 was suitable for photochemical water splitting and P2O3 possesses stable ferroelectric structures. Edmonds et al.29 reported that the prolonged air exposure of BP (about 2 days) leads

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to its surface oxidation and generates a certain amount of layered P4O2 with air stability, and this phosphorus oxide only forms at the top layer of bulk black phosphorus. Therefore, P4O2 may be a potential 2D candidate for water splitting photocatalyst with high air stability. However, the detailed structural and optoelectronic properties are still need to uncover.

finite system. To minimize the edge effect, the 2 × 2 supercell was chosen for all the P4O2 systems. The vacuum space with at least 20 Å was constructed perpendicular to the layer plane of each few-layer P4O2 system. Moreover, the values of the CBM and VBM energy levels are relative to the vacuum level, they are the difference between the CBM/VBM energy level and vacuum level.

Herein we perform the theoretical exploration of fewlayer P4O2, by combing the first-principle density functional theory (DFT) calculation and ab-initio molecular dynamics (AIMD) simulations, to discover the promising semiconductor candidate for photocatalytic water splitting. We demonstrate that few-layer P4O2 systems with different packing manners, from the monolayer up to five-layer structure, show tunable band gaps as a function of thickness. Their charge carrier mobility at room temperature are high and exhibit obviously asymmetric between electrons and holes. In particular, the three-layer P4O2 with normal packing (α-3), ingeniously combining all desired features, is an ideal candidate for photocatalytic water splitting. It fascinatingly bears small band gap (1.40 eV), appropriate band edge position, high solar-to-hydrogen (STH) efficiency (17.15%), high efficiency of sunlight absorption, small photogenerated carrier recombination possibilities and ultrahigh carrier mobility. We are quite optimistic about the experimental realization of few-layer P4O2 in the near future29.

Carrier Mobility Calculation. The carrier mobility (μ) was calculated based on the deformation-potential (DP) theory39, which has been successfully used in many 2D materials40-41. In 2D system, the carrier mobility is given by the expression:

COMPUTATIONAL METHODS Geometric and Electronic Structures. All geometric and electronic structures were performed using DFT calculations by the Vienna Ab initio Simulation Package (VASP)30-31. We choose the generalized gradient approximation of Perdew, Burke, and Ernzerhof (GGAPBE)32 as the exchange-correlation functional with the plane wave basis set to calculate the geometric structures of few-layer P4O2. The DFT-D3 method with Becke–Jonson damping was adopted to accurately account for the weak van der Waals interactions in all few-layer P4O2 systems3334. The cut-off energy was set to 500 eV. The Brillouin zone was regulated with 7 × 7 × 1 and 5 × 5 × 1 k-points for monolayer P4O2 and other few-layer P4O2 systems, with layer number from 2 to 5. The PBE-D3 functional is known to underestimate the bandgap because of their missing derivative discontinuity in the exchange-correlation energy across the gap35. While the Heyd-Scuseria-Ernzerh of hybrid functional (HSE06) functional, with its fraction of screened short-ranged Hartree–Fock exchange, yields reasonably accurate predictions for energy bandgaps in semiconductors, giving band gaps in better agreement with experiment 36-37. To correct the underestimated band gaps, the electronic band structures were calculated again by HSE06 on the basis of the geometric structures obtained from PBE-D338. All the geometry structures were fully relaxed until the convergence criteria of energy (10-5 eV) and force (0.01 eV/Å) are satisfied. The periodic boundary condition was applied to minimize the edge effect in a

𝜇2𝐷 =

𝑒ħ3𝐶2𝐷 𝑘𝐵𝑇𝑚𝑒∗ 𝑚𝑑(𝐸𝐷𝑃)2

(1)

where T is the temperature, e is the electron charge, and ħ is the reduced Planck constant. 𝑚𝑒∗ is the effective mass in the transport direction and 𝑚𝑑 is the average effective mass ∗ ∗ determined by 𝑚𝑑 = 𝑚𝑥 𝑚𝑦 . 𝐸𝐷𝑃 is the deformationpotential constant, defined as 𝛥𝐸 = 𝐸𝐷𝑃(𝛥𝑙/𝑙0), where 𝛥𝐸 is the shift of the band edge positions with respect to the lattice dilation 𝛥𝑙/𝑙0 along the specific direction of the orthogonal cell. 𝐶2𝐷 is the elastic modulus of X and Y uniformly deformed crystal for emulating the lattice distortion activated by the strain, defined as 𝐶2𝐷 = [∂2𝐸/∂𝛿2]/𝑆0, here 𝐸 is the total energy of the supercell, 𝛿 is the applied uniaxial strain, and 𝑆0 is the area of the equilibrium supercell. The Ab Initio Molecular Dynamics Simulation. To evaluate the stability of few-layer P4O2 systems themselves and their aqueous solution, we performed AIMD simulations using VASP for monolayer P4O2 and threelayer P4O2 (α-3) both in vacuum condition and in aqueous solution. The AIMD simulations were carried out under the NVT ensemble (T = 300 K). The temperature was controlled by the weak coupling of Nose-Hoover thermostat42. The simulation time for each system was 10 ps with a time step of 1 fs. The periodic boundary condition was applied in three dimensions to minimize the edge effect in the finite system. The snapshots were saved by a time interval of 1 fs to collect data for analysis. The STH efficiency. The STH efficiency was obtained following the methodology of Yang et al43. 𝜂𝑆𝑇𝐻 is estimated by 𝜂𝑆𝑇𝐻 = 𝜂𝑎𝑏𝑠 × 𝜂𝑐𝑢, where 𝜂𝑎𝑏𝑠 is the efficiency of light absorption and 𝜂𝑐𝑢 is carrier utilization. The 𝜂𝑎𝑏𝑠 is defined as ∞

𝜂𝑎𝑏𝑠 =

∫𝐸 𝑃(ℎ𝜔)𝑑(ℎ𝜔) 𝑔

∞ ∫0 𝑃(ℎ𝜔)𝑑(ℎ𝜔)

(2)

where 𝐸𝑔 is the band gap and 𝑃(ℎ𝜔) is the AM1.5G solar energy flux at the photon energy ℎ𝜔. The 𝜂𝑐𝑢 is defined as


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𝜂𝑐𝑢 =

∞ 𝑃(ℎ𝜔) ℎ𝜔 𝑑(ℎ𝜔)

Δ𝐺∫𝐸

(3)

∞ ∫𝐸 𝑃(ℎ𝜔)𝑑(ℎ𝜔) 𝑔

where Δ𝐺 is the potential difference for water splitting (1.23 eV) and 𝐸 is the energy of photons that can actually be used for water splitting. 𝐸 is determined by 𝐸𝑔,(𝜒(𝐻2) ≥ 0.2, 𝜒(𝑂2) ≥ 0.6) 𝐸𝑔 + 0.2 ― 𝜒(𝐻2), (𝜒(𝐻2) < 0.2, 𝜒(𝑂2) ≥ 0.6) 𝐸= 𝐸𝑔 + 0.6 ― 𝜒(𝑂2), (𝜒(𝐻2) ≥ 0.2, 𝜒(𝑂2) < 0.6) 𝐸𝑔 + 0.8 ― 𝜒(𝐻2) ― 𝜒(𝑂2),(𝜒(𝐻2) < 0.2, 𝜒(𝑂2) < 0.6)

{

where χ(H2) is over potential for HER, and χ(O2) is over potential for oxygen evolution reaction (OER). RESULTS AND DISCUSSIONS Geometry Structure and Stability of Few-layer P4O2 systems. The P4O2 is an oxide formation on 2D BP surface in the air. Because of no available atomic structure and electronic properties in experiments, we begin our work on setting up few-layer P4O2 models with different packing manners based on the framework of BP to provide a comprehensive understanding of the potential application

Figure 1. Geometric structure of monolayer P4O2 in 2 × 2 supercell marked with coordinate axes. (a, b) Top views of atomic structure of monolayer P4O2 on oxygen-rich surface and phosphorus-rich surface, respectively. The orientation of each crystal structure of few-layer P4O2 systems is lattice constant a along X direction and lattice constant b along Y direction. (c) The definition of the Brillouin zone. (d, e) Side views of atomic structure of monolayer P4O2 along X and Y directions. The pink and red balls stand for P and O atoms, respectively. (f) Evolution of total energy per atom of monolayer P4O2 in 4 × 4 supercell obtained from 10 ps AIMD simulations. The initial conformation at t = 0 ps (Str.@ 0 ps) and final configuration at t = 10 ps (Str.@ 10 ps) are shown in the inset for comparison.



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Figure 2. The geometric structure of few-layer P4O2 systems in 2 × 2 supercell marked with coordinate axes. (a, b) Side views of α-3 (three-layer P4O2 in α packing manner). (c, d) Side views of β-3 (three-layer P4O2 in β packing manner). The pink and red balls stand for P and O atoms, respectively. Each single layer and interlayer spacing among adjacent P4O2 layers are defined and labeled in atomic structure of β-3 as an example.

for few-layer P4O2 on photocatalytic water splitting. We validate our computational method by reproducing the equilibrium lattice constants, band gap and band edge position of general investigated monolayer BP16, 20. And the geometry optimization for monolayer BP and monolayer P4O2 were performed by using both PBE and PBE-D3 correction methods to gauge which method provide best fit to previous computational results. To correct the band edge position of the valence band maximum (VBM) and CBM, we performed band structure calculation again by HSE06 based on the geometric structures of PBE-D3. The obtained band gap (1.58 eV) and band edge position (-5.49 eV and -3.91 eV, relative to vacuum level) can well reproduce previous computational results (more details in Figure S1 and Table S1 in SI)16, 20. For monolayer P4O2, after considering dispersion correction, the lattice constant a decreases by 0.19 Å and the bandgap increases by 0.12 eV (Table 1), indicating the importance of dispersion correction on weak van der Waals interactions driving fewlayer P4O2 systems. We find that predicting atomic structures of few-layer P4O2 by PBE-D3 functional and combining with HSE06 functional to obtain the band gap and band edge positions can give the reasonable prediction of few-layer P4O2 systems. The fully relaxed crystal structure of monolayer P4O2 is shown in Figure 1. We find that the backbone of monolayer P4O2 exists in the folded form due to its intrinsic nature of sp3-hybridized P atom, which is similar to the backbone of BP, in addition, it includes exceptional bridge oxygen P-OP motif and dangling P=O motif. The involvement of oxygen atoms in P4O2 endows the monolayer bearing two asymmetric surfaces: one is oxygen-rich surface with dangling P=O motif (Figure 1a, d-e), and the other is

phosphorus-rich surface near P-O-P motif (Figure 1b, d-e). To systematically investigate the structure-property relationship of few-layer P4O2 systems, three representative packing manners are considered. First, all the oxygen-rich surface oriented in the same direction (normal packing), referred as α-N (Figure 2a-b and Figure S2 in SI). Here N represents the layer number. Therefore, α-N bears oxygen-rich and phosphorus-rich surfaces simultaneously. Second, by overturning the top monolayer P4O2 of α-N, we get few-layer P4O2 with only phosphorusrich surface, termed as β-N (Figure 2c-d and Figure S2 in SI). Third, by overturning the bottom monolayer P4O2 of αN, we get few-layer P4O2 with only oxygen-rich surface, named as γ-N (see Figure S5 in SI). Table 1 and Table S2-4 in SI summarize the change of the geometric properties as a function of the layer number from 1 to 5 of all few-layer P4O2 systems including α-N, βN and γ-N. From monolayer to α-5, as the layer number increasing, the lattice constants a increases by 0.05 Å and b grows only 0.03 Å. For β-N systems the lattice constants a and b increase by 0.07 Å and 0.03 Å, respectively. These changes reflect the existing interlayer van de Waals interactions. Taken α-2 as an example, from the results of Bader Charge Analysis (BCA) 44-45(Figure S3 in SI), it is found that most P atoms are positive charged while O atoms are negative charged, that is to say, there is repulsive force between interlayer pairwise P…P atoms or O…O atoms, while attractive force exists between interlayer pairwise P…O atoms. To balance the interlayer interactions, the initial aligned adjacent P4O2 monolayers staggered away and shifted apart along X and Y directions to reduce the electronic repulsion and to keep stability, this causes the regular increase of the lattice constant a in few-layer α-


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N and β-N systems. This is also supported by increased bond length: O2-P2, P1-P3, P3-P4 and P2-P4 shown in Figure S4 and Table S3 in SI. It should be noted that the interlayer spacing d1 (1.94 to 1.95 Å) of β-N is always smaller than that of α-N (2.41 to 2.43 Å) with the same layer number, while the other interlayer spacing d2, d3 and d4 between α-N and β-N (with the same layer number) are almost the same (Table 1 and Table S2). The different interlayer spacing d1 between α-N and β-N should attribute to the different packing manners between the top twolayers. In α-2, there are mainly two pairs of atoms with attractive interactions: O1(L2)…P2(L1), O1(L2)…P4(L1), the high electronegative dangling oxygen O1 in L2 withdraws electrons from P2 and P4 in L1. Thus the total charge of L2 decreases while that of L1 increases (Figure S3b). While in β-2 the interlayer attractive interactions of pairs of atoms in L1 and L2 are interactive, the dangling oxygen O1 of L1 withdraws electrons from the P1 and P3 of L2, at the same time, the dangling oxygen O1 of L2 has the same attractive interactions with P1 and P3 of L1. Thus L1 and L2 are equivalent and the net charge of L1 and L2 are all equal to zero (Figure S3c). Thus the interlayer attractive interaction of β-N is more favorable than α-N, leading to their smaller interlayer spacing d1. The intrinsic stability of 2D P4O2 is crucial for experimental fabrication and practical applications in promising optoelectronics. Taking monolayer and α-3 as examples, the stability of P4O2 systems are confirmed by performing the AIMD simulations at room temperature. As shown in Figure 1f and Figure S6, we performed 10 ps AIMD Table 1 The structural information for few-layer P4O2 in α-N, β-N and monolayer BP obtained by PBE and PBE-D3 functional. The lattice constants a, b and c (h: the height of P4O2 in each atomic structure). The interlayer spacing between two adjacent monolayer of few-layer P4O2 systems. The atomic number of O/P atoms in unit cell in monolayer and each few-layer P4O2 of α-N and βN. System

a (Å)

b (Å)

h (Å)

d1 (Å)

O/P

3.42

/

2/4

3.55

/

2/4

PBE monolayer

5.38

3.33 PBE+D3

monolayer

5.19

3.32

α-2

5.20

3.32

9.51

2.42

4/8

α-3

5.23

3.33

15.36

2.42

6/12

α-4

5.24

3.33

22.36

2.42

8/16

α-5

5.24

3.35

27.31

2.41

10/20

β-2

5.25

3.32

8.93

1.95

4/8

β-3

5.25

3.33

14.76

1.95

6/12

β-4

5.25

3.33

20.67

1.94

8/16

β-5

5.26

3.35

26.63

1.94

10/20

simulations for the monolayer and α-3 in a 4 × 4 supercell and 3 × 3 supercell respectively, to check their stability. The framework of monolayer and α-3 are well preserved during the 10 ps AIMD simulations. The total energies of both monolayer and α-3 oscillate in a narrow range (within 0.05 eV) as a function of time. The other important thing is the stability of P4O2 systems in aqueous solution, which is closely related to their applications in the photocatalytic water splitting. As shown in Figure S7 and Figure S8, the structures of both monolayer and α-3 are stable and their atoms only slightly vibrate around their equilibrium positions during the 10 ps AIMD simulations. Therefore, P4O2 systems have high stability in water environment. The preparation of P4O2 may introduces defects, such as atomic vacancies (including phosphorus vacancy VP and oxygen vacancy VO). These defects may lead to instability of P4O2, especially degradation in aerobic and watery environment. The stability of the α-3 with defects can be determined by the AIMD simulations. The structures of four kinds of atomic vacancies, including VP and VO, are taken into consideration in our work (in Figure S9b). As shown in Figure S9: (1) VO1, where the oxygen vacancy locating at the O-rich surface; (2) VP1, where the phosphorus vacancy locating at the O-rich surface; (3) VO2, where the oxygen vacancy locating at the P-rich surface; and (4) VP2, where the phosphorus vacancy locating at the P-rich surface (see Figure S9b). The optimized structures and AIMD simulations of α-3 with vacancies are presented in the Supporting Information (Figures S9 - S13). First, we run 7 ps of the AIMD simulations to check the stability of perfect structure of α-3 in aerobic and watery environment at 300 K (in Figure S9a). We find that the α-3 structure is always intact during the 7 ps simulations, and α-3 is stable in aerobic and watery environment. Secondly, we performed four independent 7 ps AIMD simulations of α-3 with different atomic defects: VO1, VP1, VO2 and VP2 respectively in aerobic and watery environment at 300 K in Figures S10 – S13. We find that (1) for VO1 system, the defected two oxygen atoms can be restored by O2 automatically, and the whole process takes place within 100 fs (see Figure S10); (2) in the case of VP1 system, one of the dangling oxygen in P=O motif (P1-O3) is bonded to its adjacent P atom to form a stable P-O-P (P1-O3-P2) motif (see details in Figure S11); (3) as for VP2 system, the defect itself is stable during the AIMD simulation, while the surroundings can interact partially with O2, we find two dangling P=O motifs formed on atoms P1 and P2, and there is additional single bond (P3-P4) formed between P3 and P4 atoms (see details in Figure S12); (4) the VO2 system is always very stable in aerobic and watery environment (see Figure S13). These results indicate that the defective α-3 structure can be repaired by external O2 or self-healed to achieve a configuration that is stable in aerobic and watery environment. Therefore, in aerobic and watery environment, both the perfect and defective α-3 structures are stable. Electronic Properties of Few-layer P4O2 systems. The bandgap plays a crucial role for 2D semiconductor



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Figure 3. Electronic band structures of few-layer P4O2 systems: (a) α-3 (b) β-3 and (c) monolayer, respectively. The highest valence band (VB) and the lowest conduction band (CB) obtained from PBE and HSE06 functional were highlighted by green and red lines, respectively. The corresponding VBM and CBM are labeled by filled purple circles. (d) Evolution of the band gaps as a function of number of layers for α-N and β-N systems in both PBE and HSE06 methods. (e) The Partial density of states (PDOS) of different types orbitals of P (upper panel) and O (lower panel) atoms in few-layer P4O2. All energy levels of the electronic band structures are referred to the corresponding Femi level, which is set to zero.

photocatalytic water splitting because it can directly affect the efficiency of sunlight absorption. We next investigate the electron properties of few-layer P4O2 systems. The electronic band structures of monolayer, α-3 and β-3 computed with PBE-D3 are shown in Figure 3a-c. For clarity, the highest valence band (HVB) and the lowest conduction band (LCB) obtained from PBE-D3 functional are highlighted by green lines, while the corresponding HVB and LCB calculated from HSE06 functional are marked by red lines for comparison. The electronic band structures of α-2 and β-2 are shown in Figure S9 in SI. For monolayer, α-2 and α-3 systems, the CBMs of PBE-D3 are located at the high-symmetry Gamma (G) point, while the VBM are located along the G-X direction with k close to G point (see Figure 3a-c). Thus monolayer, α-2 and α-3 are indirect band gap semiconductors. However, the energy difference between VBM and G point of each system (monolayer, α-2 and α-3) is within 0.0114 eV for PBE-D3 (see zoom-in plot in Figure 3c). For HSE06, the corresponding energy difference is 0.0055 eV, which is much smaller than that of PBE-D3. Thus they can be regarded as the nearly direct band gap semiconductors. With respect to β-N systems, both β-2 and β-3 with PBED3 functional possess the direct band gap since their VBM and CBM are both located at G point in the Brillouin zone (see Figure 3b and Figure S14b in SI). It is amazing that the shape of band structures from HSE06 are almost the same

as PBE-D3, thus their VBM and CBM share the same kpoints of the Brillouin zone respectively, while their band edge positions are much different. The band gap of monolayer from HSE06 (2.92 eV) is 0.96 eV higher than that of PBE-D3 (1.96 eV). As the layer number increases from 1 to 5, the energy gap with HSE06 for α-N decrease from 2.92 to 0.39 eV, while that of β-N reduces from 2.92 to 0.33 eV, respectively (Figure 3d). Compared to BP (from 0.3 eV of bulk to 1.58 eV of the monolayer16-19), the tunable band gap range for few-layer P4O2 is much larger, which endows the few-layer P4O2 more potential applications in fabricating the optoelectronic device. The same trend of energy gap change as a function of layer number is also observed in γ-N system (Figure S5 in SI). The energy gap decrease for α-N, β-N and γ-N systems imply that there are interlayer interactions between adjacent P4O2 monolayers. The thicker the few-layer P4O2 is, the stronger the interlayer interactions are, leading to decreasing of the band gap. In addition, the energy gap difference between β-N and the corresponding α-N with the same layer number decrease regularly. To further understand the compositions of VBM and CBM for few-layer P4O2 systems, the partial density of states (PDOS) of different orbitals of elements (phosphorus and oxygen) and each monolayer of few-layer P4O2 systems are shown in Figure 3e and Figure S15 in SI. It


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Figure 4. The band edge positions and band gaps obtained by HSE06 functional for few-layer P4O2 and monolayer BP. The reduction potential of H+/H2 and the oxidation potential of O2/H2O at pH = 0 are marked by the dotted red and blue lines, respectively. The band edge positions (CBM and VBM) for each few-layer P4O2 systems were labeled by the solid red and blue lines, respectively. The relative energy difference to redox potential of water splitting are shown near the corresponding band edge positions. The bandgap values are marked aside the vertical black arrows. All the energy levels are relative to the vacuum level for each system, which is set to zero. The corresponding values of BP are also shown for comparison.

is clear that both the CBM and VBM of all P4O2 systems in Figure 3e are mainly contributed by the 3p states of phosphorus, while the 2p states of oxygen only play a minor role. Interestingly, for α-2 and α-3, the VBMs are mainly originate by the top layer (L1), while the CBMs are dominated by the bottom layer (Figure S15b, d, Figure S17ab and Figure S18a-b). This is also supported by the BCA, in which the amount of charge transfer from L1 to L2 of α-2 is 0.180 e, resulting in the PDOS of L2 in α-2 moves to low energy state relative to the monolayer (see arrows in Figure S15b). The induced potential generated by the interlayer interactions between the oxygen-rich surface and the phosphorus-rich surface lead to the wave function localization of VBM on L1 and CBM states on L2 in α-2 system (Figure S17a-b in SI). For β-2, the two layers contribute equally (Figure S15c in SI), and the wave function of both VBM and CBM of β-2 distributes on the whole system (Figure S17c-d in SI). This is consistent with the results of BCA, that there is no charge transfer between L1 to L2 of β-2 and the two layers of β-2 bears the equivalent contributions. Thus the wave function of both VBM and CBM of β-2 distributes on the whole system (Figure S17cd). This may also be the reason why the bandgap decreases as the number of layer increases. Photocatalytic Water Splitting, STH Efficiency and Optical Property. Photocatalytic water splitting is a

promising application of 2D semiconductors. As a candidate of photocatalyst, its band gap should exceed the free energy of water splitting (1.23 eV). As shown in Figure 3d, the band gap of α-4, α-5 and β-4, β-5 are all smaller than 1.23 eV, thus they are not feasible for water splitting applications. Except for the magnitude of the band gap, the band edge positions must straddle the redox potentials of water splitting. The CBM energy should be higher than the reduction potential of H+/H2 (-4.44 eV) and the VBM energy is lower than the oxidation potential of O2/H2O (5.67 eV) respectively (at PH=0). It should be noted that, all the CBM edge positions of γ-N systems are located below the reduction potential of H+/H2 (-4.44 eV), while the corresponding VBM edge positions are much lower than the O2/H2O oxidation potential (-5.67 eV), thus all γ-N systems are not proper to be used as candidate for photocatalytic water splitting (see Figure S5 in SI). After checking the band gap and band edge positions, we focus on the rest five systems: monolayer, α-2, α-3 and β-2, β-3, because their band edges straddle the redox potential of water splitting. Thus they should be potential candidates for water splitting photocatalysts without an external potential. As shown in schematic diagram (Figure 4), all five systems (monolayer, α-2, α-3 and β-2, β-3) demonstrated desired VBM and CBM band edge positions for both HER and OER in photocatalytic water splitting. More fascinatingly, the VBM energy level of α-3 is 0.09 eV



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Figure 5. Optical absorption spectrum of few-layer P4O2 calculated by GW+BSE approach. The Gaussian broadening is adopted 0.05 eV.

The optical absorption property of 2D semiconductor photocatalyst is another key factor in determining photocatalytic water splitting efficiency. The GW plus Bethe−Salpeter equation approach46 is employed to predict the accurate optical absorption. As shown in Figure 5, all five systems have significant optical absorption in both visible and ultraviolet spectrum ranges. Especially the absorption coefficient of α-3 and β-3 are much higher than that of rest three systems, because of the existing interlayer interactions. Moreover, the charge transfer among different layers of α-3 could generate new transition modes and enhance the optical absorption intensity of the sunlight. Therefore, α-3 exhibits better performance on

lower than the oxidation potential of O2/H2O, at the same time, the CBM energy level of α-3 is 0.08 eV higher than the reduction potential of H+/H2. Moreover, STH efficiency is one of most vital indexes for evaluating the performance of catalysts. The calculated STH efficiencies of the monolayer, α-2, α-3, β-2 and β-3 are 2.62%, 12.84%, 17.15%, 3.12% and 9.95%, respectively (details in Table S5). As the quite small band gap (1.40 eV), the STH efficiency of α-3 reach up to 17.15%, which is in close proximity to the conventional theoretical limit of ∼18%43. Therefore, α-3 could be regarded as the highly efficient photocatalysts for full water splitting reactions.

Table 2 The predicted charge carrier mobility of few-layer P4O2 calculated by PBE functional. e and h represent the electron and hole respectively. 𝒎𝒙∗ and 𝒎𝒚∗ are the carrier effective mass along X and Y directions. 𝑪𝒙 ― 𝟐𝑫 and 𝑪𝒚 ― 𝟐𝑫 denote the elastic modulus along X and Y directions, respectively. 𝑬𝒙 and 𝑬𝒚 represent the deformation potential, respectively. 𝝁𝒙 and 𝝁𝒚 are charge carrier mobility of few-layer P4O2 in X and Y directions, respectively. e and h represent electron and hole, respectively. system

monolayer α-2 α-3 β-2 β-3

𝑚𝑥∗ /𝑚0

𝑚𝑦∗ /𝑚0

G-X

G-Y

e

0.24

0.42

5.01

1.21

128.83

102.42

1.39

10.83

h

1.37

1.15

0.87

0.67

128.83

102.42

2.05

3.28

carrier type

𝐸𝑥

𝐸𝑦

𝐶𝑥 ― 2𝐷

𝐶𝑦 ― 2𝐷

(N m-1)

(eV)

𝜇𝑥

𝜇𝑦

(103 cm2V−1s−1)

e

0.20

0.49

4.54

1.29

244.75

196.78

3.99

16.20

h

1.12

1.11

1.71

0.75

244.75

196.78

1.40

5.91

e

0.23

0.47

4.99

1.38

363.00

304.57

4.00

21.46

h

1.52

1.30

2.81

0.73

363.00

304.57

0.45

6.49

e

0.21

0.38

4.41

1.33

243.57

232.38

4.43

25.69

h

1.56

1.47

1.91

0.38

243.57

232.38

0.59

15.09

e

0.21

0.37

5.00

1.42

359.08

300.26

5.08

29.91

h

1.64

1.50

2.18

0.32

359.08

300.26

0.61

25.91


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absorption of sunlight, which is key for the efficiency of photocatalytic water splitting reaction. The Carrier Mobility of Monolayer and Few-layer P4O2 systems. To further investigate the performance of few-layer P4O2 systems as photocatalysts, the carrier mobility were calculated for quantitative evaluation of the ability of the 2D semiconductors to transfer electron (e) or hole (h) along X and Y directions by applying a standard 2D model. The carrier mobilities of e and h were obtained by combining the deformation potential (DP) theory and the effective mass approximation method at the room temperature, by using Eq. (1). The carrier mobility electronically depend on the effective mass (𝑚 ∗ ) of fewlayer P4O2 systems, at the same time, it is also determined by the other two properties which related to the crystal lattice, namely deformation potential (EDP) and elastic modulus (C2D). These parameters were calculated by PBED3 functional and the data are collected in Table 2. Except for carrier mobility of monolayer along X direction, the predicted electron mobilities for all few-layer P4O2 systems are larger than that of its hole mobility, because of much smaller effective masses of electron than that of hole, supporting by the more dispersive CBM band near the G point than that of VBM (see Table 2). For all P4O2 system, it is clear that there is anisotropy between electrons and holes and the electrons are more mobile in the Y direction than that in X direction, because the deformation potential in Y direction (Ey) is smaller than that of is smaller than that in X direction (Ex). For both αN and β-N systems, the electrons mobility along Y direction increases gradually, as the layer number increasing, due to the C2D increasing. In addition, the charge carrier mobility for both e and h of β-N are much larger than the corresponding value of α-N, because of their more delocalized wave functions, which can enhance the charge carrier transport mobility. Thus α-3 outperforms other α-N systems in both e and h transport ability along Y direction, while β-3 gives the largest carrier mobilities along Y direction for all demonstrated few-layer P4O2 systems. It is worth to note that the electron mobility is an order of magnitude higher than the hole mobility along Y direction in α-3, while those are very close in β-3. The huge difference in carrier mobility between e and h in α-3 facilitates the effective separation of electron-hole pairs and reduces the probability of recombination of electron and hole in photocatalytic water splitting reaction8. Above all, the charge carrier transport of α-3 is promising, it not only bears high carrier mobility (as high as 21460 cm2V−1s−1), but also has highly electron and hole separation efficiency. Moreover, the few-layer P4O2 systems also has high stability in aqueous solution. Therefore, α-3 is an idea candidate for photocatalytic water splitting reaction. CONCLUSIONS In conclusion, we have theoretically proposed a category of novel 2D materials: few-layer P4O2 systems by performing density functional theory calculations and AIMD

simulations. The few-layer P4O2 systems possess perfect band edge positions and high charge carrier mobility, and all these parameters are tunable by change their thickness. Especially, we proposed that the three-layer P4O2 with normal packing (α-3) ingeniously combines all desired features for photocatalytic water splitting reactions, including the small band gap (1.40 eV), perfect band edge position (relative to the redox optional of water splitting reaction), STH efficiency as high as 17.15%, high efficiency of sunlight absorption in both visible and ultraviolet ranges, small photogenerated carrier recombination possibilities and ultrahigh carrier mobility (21460 cm2V−1s−1). In addition, α-3 also has high stability in aerobic and watery environment. It should be an appropriate and promising candidate for photocatalytic water splitting reaction. We are quite optimistic about the experimental realization of few-layer P4O2 in the near future.

ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge on the ACS Publications website at DOI: Geometric structures of few-layer P4O2 systems, band gap, band edge positions, PDOS figures, the distributions of wave functions of VBM and CBMs, STH efficiency and AIMD simulations of monolayer P4O2 and α-3.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank the financially support by the National Natural Science Foundation of China (Grants 21803007, 21473010 ， 91544223) and Beijing Institute of Technology Research Fund Program for Young Scholars. We also thank the Shenzhen Supercomputer Center of China for providing us the excellent platform for all computations in this work.

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