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C: Energy Conversion and Storage; Energy and Charge Transport 2

Blue Phosphorus/Mg(OH) van der Waals Heterostructures as Promising Visible-Light Photocatalysts for Water Splitting Bao-Ji Wang, Xiao-Hua Li, Xiao-Lin Cai, Weiyang Yu, Li-Wei Zhang, Ruiqi Zhao, and San-Huang Ke J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12408 • Publication Date (Web): 13 Mar 2018 Downloaded from http://pubs.acs.org on March 16, 2018

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The Journal of Physical Chemistry

Blue Phosphorus/Mg(OH)2 van der Waals Heterostructures as Promising Visible-Light Photocatalysts for Water Splitting Bao-Ji Wang,† Xiao-Hua Li,∗,† Xiao-Lin Cai,† Wei-Yang Yu,† Li-Wei Zhang,† Rui-Qi Zhao,‡ and San-Huang Ke∗,¶ †School of Physics and Electronic Information Engineering, Henan Polytechnic University, Jiaozuo 454000, China. ‡School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo 454000, China. ¶MOE Key Labortoray of Microstructured Materials, School of Physics Science and Engineering, Tongji University, Shanghai, 200092, China. E-mail: [email protected]; [email protected]

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Abstract In order to construct efficient solar-driven devices, considerable efforts have been made to search desirable photocatalyts for water splitting. Motivated by recent successful syntheses of single-layer blue phosphorus and Mg(OH)2 , we systematically investigate the stability, energy band structure, charge transfer and potential photocatalytic properties of blue phosphorus/Mg(OH)2 van der Waals (vdW) heterostructure using the first-principles method. It is found that all the heterostructures considered possess similar electronic and optical properties and exhibit type-II band alignment and indirect band gap characteristic. And the ground-state configuration (β-stacking) of the heterostructure is found to be a potential photocatalyst for water splitting . In particular, its band gap, band edge positions, and optical absorption can be tuned by in-layer biaxial strain to improve the efficiency of the photocatalytic water splitting. Our findings provide a new possibility of designing efficient photocatalysts for water splitting.

Introduction The efficient conversion of solar energy into hydrogen fuels via photocatalytic processes is an attractive technology for producing clean, low-cost and renewable energy, where a photocatalyst involves excited electrons and holes migrating to the semiconductor surface and serving as redox sources which react with the adsorbed water to produce H2 and O2 . 1 There are several criteria for a semiconductor to become a high-efficiency photocatalyst. First, it must have decent band edge positions for the redox potentials, where the conduction band minimum (CBM) energy is higher than the reduction potential (−4.44 eV) of H+ /H2 while the valence band maximum (VBM) energy is lower than the oxidation potential (−5.67 eV) of O2 /H2 O. 2 Consequently, the criteria of bandgap for water splitting correspond to the minimum potential difference, and that is 1.23 eV, which corresponds to the infrared wavelength. However, when energy losses due to over-potential are counted, a much larger band gap is 2


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usually needed for appreciable water splitting reaction. Second, the material should have the ability to capture the visible portion of the solar spectrum, which covers more than 40% of the solar energy compared to the ultraviolet fraction (< 5%). Accordingly, opportune bandgaps for visible-light photocatalytic water splitting should be around 2.0 ∼ 2.2 eV. 3 Two-dimensional (2D) materials for photocatalytic water splitting have big advantages due to their enhanced photocatalytic properties. 3,4 Compared to bulk photocatalysts, 2D photocatalysts are expected to offer intriguing features such as high specific surface areas available for photocatalytic reactions, high photon-harvesting efficiency in the visible light region, low exciton recombination rate. Several experimental and theoretical studies have explored graphene and related 2D materials for photocatalytic water splitting. 5–7 Besides graphene, various other 2D materials such as monolayer transition-metal dichalcogenides (TMDs), 2,8 single-layer group-III (IV) monochalcogenides 9,10 and others, 3,4,11–13 have also shown potential applications as excellent photocatalyst. Interestingly, black phosphorus (BlackP) is found stable in liquid water and can be engineered as a water splitting photocatalyst by means of strain at certain pH values. 11,14 Very recently, a 2D layered allotrope of BlackP, termed as blue phosphorus (BlueP), was also successfully fabricated through molecular beam epitaxial growth on Au(111) substrate by using black phosphorus as precursor. 15 It was reported that BlueP is an indirect semiconductor with a larger band gap than BlackP and its band gap can be modulated in a large range by applying external strains or electric fields. 16,17 In addition, the structural and electronic properties of BlueP-based heterostructures such as BlueP/BlackP, 18 BlueP/TMDs, 19,20 BlueP/graphene, 21,22 and BlueP/AlN 23 heterostructures have also attracted much attention from researchers. In particular, the BlueP/AlN heterostructure exhibits decent band alignments with the redox potential of hydrogen generation from water, and fantastic optical adsorption index. These make it a promising candidate of photocatalyst for water splitting. More recently, Mg(OH)2 monolayer, a member of water-insoluble alkaline metal hydroxides (AMHs), was fabricated successfully from its layered bulk crystals. 24 Accordingly, Mg(OH)2 /2D heterostructures, such as

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Mg(OH)2 /MoS2 , Mg(OH)2 /WS2 , Mg(OH)2 /AlN, et al., have also attracted increasing interest due to their extraordinary electronic and optical properties. 24–26 What is more, Mg(OH)2 and BlueP monolayers share the same hexagonal crystal structure with a small lattice mismatch, 17,24 which is advantageous to the fabrication of BlueP/Mg(OH)2 heterobilayer in laboratory. As a consequence, it is natural to see whether BlueP and Mg(OH)2 can form a Van der Waals (vdW) heterostructure to obtain enhanced optical properties beyond pristine BlueP and Mg(OH)2 sheets. In this work, we focus on the exploration of BlueP/Mg(OH)2 vdW heterostructures as photocatalysts for water splitting. We first determine the relative stabilities of the heterostructures with different stacking configurations, followed by the study of their electronic structures by performing accurate hybrid density functional theory (DFT) calculations. Next, we investigate how biaxial strains can be utilized to modulate the bandgaps, band edge positions, and optical absorptions of the BlueP/Mg(OH)2 vdW heterostructures to increase the potential efficiency of solar energy conversion. Finally, to understand the strain effects on the photocatalytic properties of the heterostructures, we investigate the integrated charge density difference and the charge transfer for different strains.

Computational methods In this work, we use the Vienna Ab initio Simulation Package (VASP) 27 to do the DFT calculations. The electron-ion interactions are treated by the projector-augmented wave (PAW) method. 28 The electron exchange and correlation are described by the generalized gradient approximation (GGA) 29 in the version of Predew-Burke-Ernzerhof (PBE). 30 Besides the PBE functional, the hybrid Heyd-Scuseria-Ernzerhof (HSE06) functional 31 is further adopted to correct the underestimated bandgaps given by DFT/PBE calculations. The vdW interaction between the two monolayers, which is absent in DFT, is added by the DFTD2 correction of Grimme. 32 This method has been shown to be able to reasonably describe

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various vdW heterostructures. 33–37 The single-electron wave function is expanded with plane waves whose cutoff kinetic energy is set to be 500 eV. The convergence criteria for total energy is set to be 10−5 eV per atom. The integration over the Brillouin zone (BZ) is sampled by a 36 × 36 × 1 (15 × 15 × 1) Monkhorst-Pack k-point mesh for the PBE (HSE06) calculations. Periodic boundary conditions in the three dimensions are applied to the systems and the artificial interactions between the periodic images are removed by introducing a vacuum spacing of 25 Å . Test calculations show that the above technical parameters adopted in this work are good enough to give well converged results. The atomic structures of the systems studied, including the size of the supercell and the atomic positions in the supercell are fully optimized by minimizing the internal stresses and the atomic forces acting on each atom (< 0.01 eV/Å ). The binding energy (Eb ) of the BlueP/Mg(OH)2 heterostructure is calculated by Eb = EBlueP/Mg(OH)2 − EBlueP − EMg(OH)2 , where EBlueP/Mg(OH)2 , EBlueP , and EMg(OH)2 represent total energies of the BlueP/Mg(OH)2 heterostructure, BlueP and Mg(OH)2 monolayers, respectively. The applied in-layer biaxial strain (ε) is defined as ε = (L − L0 )/L0 , where L and L0 are the lattice constants of the structures with and without strains, respectively. Positive values correspond to expansion, while negative values refer to compression. The optical absorbance A(ω) of the 2D heterostructure can be calculated by A(ω)=ωLIm(ω)/c, where ω is the frequency of light; L is the vacuum spacing in the normal direction; Im(ω) is the imaginary permittivity calculated within the independent particle approximation; c is the speed of light in vacuum. 38 Moreover, Im(ω) is accurately calculated using the HSE06 functional. The averaged electron density difference of the heterostructure along the z direction is deR R R fined as ∆ρ(z) = ρBlueP/Mg(OH)2 (x, y, z) dxdy− ρBlueP (x, y, z) dxdy− ρMg(OH)2 (x, y, z) dxdy, where ρBlueP/Mg(OH)2 (x, y, z), ρBlueP (x, y, z) and ρMg(OH)2 (x, y, z) are the charge density at (x, y, z) points in the BlueP/Mg(OH)2 bilayer, BlueP and Mg(OH)2 monolayers, respective-

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(b)

(a)

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(c)

2.256Å

2.260Å

2.257Å

α-stacking

β-stacking

γ-stacking

Figure 1: (color online) Top view (upper panel) and side view (down panel) of the BlueP/Mg(OH)2 heterostructures in different possible stacking configurations: (a) αstacking, (b) β-stacking, and (c) γ-stacking. The optimized interlayer distances are indicated. ly. Also, the amount of transferred electrons up to z is given by ∆Q(z) =

Rz −∞

0

0

∆ρ(z ) dz .

Consequently, the total number of electrons transferred between BlueP and Mg(OH)2 layers is determined by the value of ∆Q(z) at the BlueP/Mg(OH)2 interface.

Results and discussion Before studying the vdW hybrid structures of BlueP and Mg(OH)2 , we first calculate the structural parameters of the isolated BlueP and Mg(OH)2 monolayers using DFT/PBE. After the total energy relaxation, the optimized lattice constants of BlueP and Mg(OH)2 monolayers are 3.27 and 3.13 Å respectively, which are in good agreement with previous results. 16,17,22,24 Therefore, the lattice mismatch between the two monolayers is about 4%, which is in an acceptable range and accessible in experimental synthesis since both BlueP and Mg(OH)2 monolayers have good flexibility due to their puckered structures. 19,39 The optimized lattice constants of the heterostructures are about 3.21Å for all the stacking configurations discussed below. With different atomic stacking configurations, the BlueP and Mg(OH)2 monolayers can

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form three high-symmetry heterobilayers named as α-, β-, and γ-stacking, which are separately depicted in Fig. 1(a-c). For the α-stacking, P atoms in the BlueP sublayer reside on top of OH groups. As for the β (γ)-stacking, one P sublattice is located on top of Mg atoms and the other is on top of upper (lower) OH groups. The optimized structural parameters and the binding energies are given in Table S1. The results show that the binding energies per unit cell of BlueP/Mg(OH)2 heterostructures in the α-, β-, and γ-stacking configurations and the corresponding interlayer distances are −42.5 meV, −44.4 meV, −43.9 meV and 2.256 Å, 2.257 Å, 2.260 Å, respectively. The very close negative binding energies mean that these three BlueP/Mg(OH)2 heterostructures are all stable and possess similar stabilities. Furthermore, the ground-state structural properties of the heterostructures are nearly identical, leading to similar electronic and optical properties regardless of the stacking order (see Supplemental Material for details: Table S1 and Fig. S1). Similar behaviors are also found in germanene/BeO heterostructures 40 and Arsenene/Ca(OH)2 heterostructures. 13 Thus, in the following sections, we take the β-stacking BlueP/Mg(OH)2 heterostructure as a representative to show our results. The band structures of the two monolayers and the projected band structure of the heterostructure given by the DFT/HSE06 calculations are shown in Fig. 2(a-c), respectively. The bandgap of BlueP is indirect with the value of 2.74 eV. The CBM locates between the Γ and M points while the VBM appears between the K and Γ points. Mg(OH)2 monolayer exhibits a direct bandgap of 4.74 eV with both the VBM and CBM located at the Γ point. These results are in good agreement with previous theoretical calculations or experimental measurements 16,17,24,25 and thereby justifying our approach (see Supplemental Material for further discussion: Discussion S1). In the band structure of the BlueP/Mg(OH)2 heterostructure (Fig. 2(c)), the bands plotted with cyan and magenta circles denote the contributions from the BlueP and Mg(OH)2 layers, respectively. We can clearly see that the system is a semiconductor with an indirect band gap of 2.21 eV, which is larger than the required 1.23 eV minimum band gap for the photocatalysis reactions, showing the potential appli-

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(a)

(c)

(b)

(e)

(d)

CBM

VBM

Figure 2: (color online) The band structures for (a) BlueP, (b) Mg(OH)2 monolayer, and (c) BlueP/Mg(OH)2 heterostructure. The solid arrows indicate the lowest energy transition. (d) The band decomposed charge densities of the VBM and CBM of the heterostructure, respectively. (e) The total and partial density of states of the BlueP/Mg(OH)2 heterostructure. All the results are from the HSE06 calculations. cation of BlueP/Mg(OH)2 heterostructure as a visible light photocatalyst. Its VBM and CBM are localized on the Mg(OH)2 and BlueP layer, respectively. Therefore, a type-II band alignment is formed at the BlueP/Mg(OH)2 interface, which will spontaneously separate the free electrons and holes, enabling high-efficiency solar energy conversion. Further support can be obtained from the decomposed charge densities of the VBM and CBM in Fig. 2(d), in which P and O atoms contribute to the CBM and VBM, respectively. To show more details, the total and partial density of states (PDOS) are calculated, and the results are depicted in Fig. 2(e). It is clear that the states around VBM are mainly occupied by O p(x,y) states, while the CBM mainly consists of P 3s and 3p orbitals. Here, we would like to mention that 8


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Figure 3: (color online) Band edge alignments of BlueP, Mg(OH)2 monolayer, and BlueP/Mg(OH)2 vdW heterostructure with respect to the water oxidation (O2 /H2 O) and reduction (H+ /H2 ) potentials at pH 0 given by DFT/HSE06. the electronic band structure is calculated with and without a dipole correction applied. It is found that the dipole correction has a very small effect on the band dispersion of the heterostructure no matter if a strain is applied or not. Therefore, in the following discussions we will use the results from the calculations without the dipole correction applied. Besides the requirement of a proper band gap, an efficient photocatalyst also requires appropriate band edge positions for the reduction and oxidation potentials. The redox ability could be assessed by aligning the CBM and VBM with respect to the water redox potential levels, as shown in Fig. 3. As one can see, monolayer BlueP has favorable band positions for water splitting, which is in accordance with other works. 23 But its oversize band gap (> 2.20 eV) is adverse to absorb a significant fraction of the solar spectrum, reducing its potential conversion efficiency. 3,9,41 On the other hand, the VBM of Mg(OH)2 monolayer is more positive than the water oxidation potential, being not suitable for water splitting. The BlueP/Mg(OH)2 vdW heterostructure has a decent CBM position for water reduction reaction, but its VBM over-potential is only 0.23 eV, which may not be sufficient for O2 production. 42 As we know that strain engineering is a feasible avenue to tune the band

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(a)

(b)

Figure 4: (color online) Strain effects on (a) the band gap and strain energy, and (b) the band-edge positions of BlueP/Mg(OH)2 vdW heterostructure. The results are from the vdWDFT/HSE06 calculations. The redox potentials of water splitting at pH 0 (black dashed line) and pH 7 (blue dashed line) are shown for comparison. gap and band-edge positions, which has been exemplified by many studies for different materials. 11,13,14,42 The evolutions of the band gap and band-edge position of the BlueP/Mg(OH)2 vdW heterostructure with strain are displayed in Fig. 4(a) and (b), respectively, together with the strain energy. It is found that the band gap slowly decreases with the tensile strain increasing (up to 10%) and then drops sharply for strains beyond 10% while it quickly decreases with the increasing compressive strain, as shown in Fig. 4(a). Such band-gap evolution is mainly due to the strain-induced band-energy shifts. For example, when ε > 10%, the conduction band bottom at Γ-point has a lower energy than the original CBM and becomes the new CBM. Nevertheless, when an increasing compressive strain is applied, the conduction band bottom at K point drops faster than the original CBM and eventually becomes the new CBM. The variations of the highest valence band and the lowest conduction band of the BlueP/Mg(OH)2

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heterostructure with the varying strain are shown in Supporting Information (Fig. S2). Here, it should be emphasized that under a moderate tensile strain in the range of 2% ∼ 10%, the band gaps are slightly decreased to around 2.00 ∼ 2.20 eV, which are the most suitable ones for visible light harvesting. 3 More interestingly, as shown in Fig. 4(b), the corresponding band edges are shifted to appropriate positions, perfectly straddling the redox potential energies of water at pH 0. Meanwhile the CBM and VBM overpotentials are all close to 0.5 eV, which are adequate for H2 /O2 production from water. Larger tensile or compressive strains show no benefit to the photocatalysis properties since the band gap is further reduced far from the visible light range. At the same time, we find that the overlarge compressive strains shift the VBM above the O2 /H2 O oxidation potential, while the oversized tensile strains reduce the CBM below the reduction potential of H+ /H2 , making the system unsuitable for water splitting under such strains. Besides the strain engineering, changing the pH value also provides another means to tune the water-splitting properties of the heterostructure, which is based on the dependence of the redox potentials upon the pH value. 8,14 An increase in pH value shifts both the energy levels of H+ /H2 and O2 /H2 O upward, as illustrated in Fig. 4(b), making the heterostructure an effective photocatalyst with appropriate band edge alignments. In Fig. 4(a)(right y-axis), we give the calculated strain energy per atom as a function of the biaxial strain, Es = (Estrained − Eunstrained )/n, with n being the number of atoms in the unit cell. The result shows that the biaxial strain is in perfect quadratic function with the energy per atom, indicating that the system is flexible and all the strains considered are within the elastic limit and, therefore, are fully reversible. As mentioned above, an outstanding optical absorption in visible light region is needed for an efficient catalyst. In Fig. 5 we compare the optical absorptions of the BlueP/Mg(OH)2 heterostructure under different tensile strains. The results show that biaxial tensile strain red-shifts the optical spectra at uniform incremental intervals in the range of visible light, leading to significantly enhanced optical absorption in the region of [2.05 eV, 3.11 eV]. Specif-

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Figure 5: (color online) The optical absorbance spectrum A(ω) of the BlueP/Mg(OH)2 heterostructure under biaxial tensile strains calculated using the HSE06 functional. ically, the optical absorption of the heterostructure at +4% strain reaches 1.8% at 3.0 eV, which is nearly four times larger than that at zero strain and is far greater than that of monolayer GaSe in a similar condition. 9 This behavior is related to the aforementioned results that strains cause bandgap reductions. Finally, to reveal the possible charge transfer between the two monolayers in the heterostructure and how it is affected by strain, we calculate the integrated charge density difference ∆ρ(z) to visualize the charge redistribution along z-direction normal to the heterostructure. The results are given in Fig. 6. It can be seen that the charge redistribution is mainly around the interface of the two monolayers: The Mg(OH)2 layer donates electrons to the BlueP layer no matter if the strain is applied, implying an electrostatic field across the interface. This electric field may significantly influence the carrier dynamics and make the excitonic behavior in the heterostructure quite different from that in the isolated BlueP layer as it may facilitate the separation and transport of the photogenerated charges. 43 As a result, during the photocatalytic water splitting the hydrogen and oxygen production processes will occur separately in the BlueP layer and Mg(OH)2 layer, respectively, 44 which protects BlueP from property degradation arising from water and oxygen molecules. 45,46 Meanwhile, we find

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Figure 6: (color online) Plane-averaged differential charge density ∆ρ(z) (black or green) and the amounts of transferred charge ∆Q(z) (red or blue) along the normal direction of the heterostructure under the strains of 0% and 8%, respectively. The vertical dashed line denotes the BlueP/Mg(OH)2 interface. that the mount of transferred electrons from the Mg(OH)2 to BlueP layer (determined by the value of ∆Q at the BlueP/Mg(OH)2 interface) increases with the tensile strain increasing, as shown in Fig. 6 (right y-axis), which suggests an enhanced interlayer interaction, leading to the shifts of band edges.

Conclusions In summary, the geometric, electronic and optical properties of BlueP/Mg(OH)2 heterostructure are investigated systematically by performing first-principles calculations with vdW corrections to exploit its potential applications in water splitting. The similar negative binding energies mean that the three BlueP/Mg(OH)2 heterostructures considered are all stable. The calculation of band structure reveals that the BlueP/Mg(OH)2 heterostructure with the ground-state configuration is a type-II semiconductor with an indirect band gap of 2.21 eV. Comparing the band edge positions with the redox potentials of water reveals that the BlueP/Mg(OH)2 heterostructure is a potential photocatalyst for water splitting though 13



the small VBM over-potential may not be sufficient for O2 production. What is more, we show that mechanical strain can be utilized to modulate the bandgap, band edge positions and optical absorption for a better match with the redox potentials of water and the solar spectrum in order to improve the efficiency of the photocatalytic water splitting, which arises from the enhancement of the interlayer interaction induced by applied strain. In short, our calculations predict that the BlueP/Mg(OH)2 heterostructure is a promising candidate for enhanced photocatalytic water splitting in the visible light region.

Supporting Information Available The structural, electronic, and optical properties of BlueP, Mg(OH)2 monolayers and their heterostructures. This material is available free of charge via the Internet at http://pubs. acs.org.

Acknowledgement This research was supported by the National Natural Science Foundation of China (No. 11374226 and 11174220), the Innovation Scientists and Technicians Troop Construction Projects of Henan Province (No. CXTD2017089), the Key Scientific Research Project of the Henan Institutions of Higher Learning (No. 16A140009 and 18A140018), the Natural Science Foundation of Henan Province of China (No. 162300410116), the Program for Innovative Research Team of Henan Polytechnic University (No. T2016-2), the Doctoral Foundation of Henan Polytechnic University (No. B2015-46), and by the Open Project of Key Laboratory of Radio Frequency and Micro-Nano Electronics of Jiangsu Province (LRME201601). This research used computational resources of the High-performance Grid Computing Platform of Henan Polytechnic University.

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