A novel metal-free two-dimensional material for photocatalytic water splitting – phosphorus nitride (γ-PN)

Abstract

Visible-light-driven water-splitting technology has attracted wide attention because of its sustainable and renewable production of hydrogen. The key to photocatalysis field is to search materials with suitable oxidizing and reducing potential. Herein we investigated the novel 2D γ-phosphorus nitride (γ-PN) monolayer as a candidate photocatalyst for reducing water into hydrogen and oxygen from the first-principles and molecular dynamics calculations. Our results show that the 2D γ-PN monolayer is an indirect semiconductor whose conduction and valence band edges matches well with the chemical potential of H⁺/H₂ and O₂/H₂O. Strain effect is considered to tune the electronic properties of γ-PN. The calculation results reveal that tensile strain decreases the band gap and upshifts the work function of the γ-PN monolayer. The γ-PN monolayer at the 10% tensile strain shows the optimal performance in catalysing the water-splitting due to the low band gap and the better absorption spectrum at the UV-visible light range. Furthermore, the molecular dynamics shows that the γ-PN in the aqueous solution always keep stable after 100 ps simulation. Both the density functional theory (DFT) and molecular dynamics (MD) simulation reflect that the γ-PN monolayer is reliable and promising as a water-splitting photocatalyst.

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Introduction

The consumer demand for renewable and clean energy is in growth steadily nowadays due to the shortage of exhaustible fossil fuels and the accompanying environmental problems. Seeking alternative environment-friendly energy sources is an effective approach to meet this urgent demand. Hydrogen, as a high-density green energy produced from water splitting with the assistance of sunlight, is considered to balance the energy requirement and environmental protection hopefully in the long term.^1–3 The key to this method is to search high-performance photocatalysts whose band positions should fit the water reduction and oxidation potentials strictly in order to provide a driving force for reducing water.

In the last few decades, a variety of novel nanomaterials are designed as homogeneous and heterogeneous catalysts in the field of catalysis by making use of their high specific surface area and rich active sites. Advanced density functional theory (DFT) calculations have been widely applied to guide the experimenters to understand the fundamental nature of the physical and chemical phenomenon, and to design materials with different structures and optimize the catalytic performance.^4,5 Screening and designing materials with suitable band position through the first-principles calculations play a vital role in current water-splitting studies. The traditional benchmark water-splitting catalyst is the bulk TiO₂ inspired by the pioneering work of Fujishima and Honda.^6,7 However, relatively wide band gap (3 eV in the rutile phase) restricts TiO₂ to making full use of solar energy, resulting in the final less than 1% photoenergy conversion efficiency, which hinders its massive commercial application. Tuning band structure of TiO₂ with externally doped elements is an effective approach to enhance the photocatalysis activity. Both theoretical calculation and experimental results show that doping with 3d transition metal ions (Cr³⁺, Co³⁺, Ni²⁺, Mn³⁺)⁸ or non-metal element (B, C, N)⁹ will induce doping energy level and change the properties of light absorption obviously.

In recent years, metal-free water-splitting catalysts, such as graphitic carbon nitride (g-C₃N₄), and graphene(G), have spring up due to their distinctive advantages in easy synthesis and no poison. Typically, the g-C₃N₄ investigated by Wang and co-workers are chemically and thermally stable semiconducting materials with direct bandgaps around 2.1 eV, exhibiting steady 2–4% H₂ production efficiency with a small amount Pt content.¹⁰ Meanwhile, compared with the previous TiO₂, these 2D polymeric carbon nitride materials can absorb more visible light due to the narrower bandgap. Furthermore, the doping and defects of g-C₃N₄ and the coupling with other 2D materials can synergistically promote the proton adsorption and reduction kinetic, which shows better photocatalytic performance than that of traditional metallic catalysts.^11–14 Recently, different 2D porous phosphorus polymorphs and transition metal dichalcogenides are also expected as elemental water-splitting photocatalysts with tunable band gaps.^15–17

To date, the earliest synthesized phosphorus nitride crystal is the α-P₃N₅ and β-α-P₃N₅ made up of the sharing PN₄ tetrahedra, reported by the Wolfgang Schnick et al.¹⁸ A variety of phosphorus nitride compound have been predicted theoretically, such as the PN₂ and PN₃ as superconductor under high pressure.¹⁹ Very recently, we designed novel 2D phosphorus nitride with α, β and γ phases and confirmed that those 2D phosphorus nitride materials are stable and are possible to synthesize at the surface of silver (110), (001) and (111).²⁰ The 2D PN materials with a 1 : 1 mole fraction of P and N with different electronic and magnetic properties have potentials in the application of nano-electronics.

In this work, we theoretically investigated 2D γ-phosphorus nitride (γ-PN) material as a novel metal-free water-splitting catalyst. This γ-PN nanomaterial is an indirect semiconductor with a bandgap of 2.85 eV based on the HSE06 functional. The conduction band margin (CBM) and valence band margin (VBM) of γ-PN locate energetically around the potential of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), indicating their great potentials in metal-free photocatalytic water splitting. Strain effect is considered to enhance the photocatalysis performance. In addition, this γ-PN keep stable after 100 ps molecular dynamics simulation at 300 K in the aqueous environment.

Computational details

All the DFT calculations are performed by the CASTEP module²¹ in the Materials Studio (version 8.0). The generalized gradient approximation (GGA) realized by Perdew–Burke–Ernzerhof (PBE)²² functional with norm-conserving pseudopotentials²³ was used to optimize all the structures. Grimme correction method²⁴ was used to describe the van der Waals interaction. The valence electron for N and P was 2s²p³ and 3s²3p³. The cut-off energy was set to 500 eV and the convergence criteria are 5.0 × 10⁻⁶ eV per atom for energy and 0.01 eV Å⁻¹ for force, which are enough to ensure reasonable simulation results. Over 20 Å vacuum slab is used to avoid the interaction between neighboring layers. The 13 × 13 × 1 K-points were used in the geometry optimization and self-consistent calculations for the density of states (DOS) and band structure. Considering traditional GGA-PBE may underestimate the band gap of 2Dmaterials, advanced hybrid functional Heyd–Scuseria–Ernzerhof (HSE06)²⁵ was used to correct the band gap with a 7 × 7 × 1 K-points.

The absorption spectrum is calculated by the frequency dependent dielectric matrix, where the real part ε₁(ω) is obtained by the Kramers–Kronig transformation and the imaginary part ε₂(ω) is determined by a summation over empty states using the following equation:^26,27

where the indices c and v refer to conduction and valence band states, respectively. φ^c_k is the cell periodic part of the orbitals at the k point in the Brillouin zone and the u is the vector defining the polarization of the incident electric field. The absorption coefficient A(ω) is defined as following:

Molecular dynamics of 4 × 4 γ-PN supercells surrounded with water molecules at 300 K under the canonical ensemble (NVE) was run to estimate the thermodynamic stability and the total simulation time is set as 100 ps. The COMPASS²⁸ force field is chosen, which is compatible with our first-principles calculations. The electrostatic interaction and van der Waals interaction used the atom based method.

Results and discussion

Based on our previous work,²⁰ there are three types of stable PN monolayers, individually α-, β-, and γ-PN. All of them show highly structural and dynamic stability. The formation enthalpy of γ-PN is calculated to be negative (−0.08 eV), suggesting its existence in reality is possible. As shown in Fig. 1, the γ-PN forms a graphene-like hexagonal lattice where the bond length between N and P is uniformly 1.79 Å. The valence electrons in the outer layer of N (2p³) and P (3p³) atom result in covalent bonding between N and P according to the bond order of 0.84. The larger electronegativity of N atom than P atom indicates the transfer of electrons from P atoms to N atoms during the formation of γ-PN. Similar to black phosphorus and varieties of phosphorus allotropes, the P atoms are sp² hybridized with three neighboring N atoms by the covalent bond, which stabilized the whole framework in principles. Then advanced hybrid density functional HSE06 is adopted to calculate the band structure and partial density of states because traditional GGA and LDA functionals will underestimate about ∼1.0 eV in band gap for 2D materials. The band gap of the γ-PN monolayer is 2.85 eV from the advanced hybrid density functional HSE06 compared with 1.72 eV from GGA-PBE calculation. The PDOS shows a hybrid character of p orbitals from N and P atoms in the γ-PN configuration, which contributes to 3-fold coordination typically. While for α-PN and β-PN monolayer, they are also indirect semiconductors with a corresponding band gap of 2.61 eV and 2.29 eV shown in Fig. S1 of ESI.† Fig. S1c† shows that the valance band maximum of α-PN and β-PN is −5.29 eV and −5.09 eV respectively, both of which are higher than the oxidation potential of water (E_H2O/O2 = −5.67 eV) and have little potential in the water splitting.

Fig. 1 (a) The top view and side view of 2D γ-PN monolayer, as well as the corresponding path in the Brillouin zones and (b) the band structure and partial density of states of γ-PN based on the HSE06 functional (pink and blue balls represent the phosphorus and nitrogen atoms respectively).

As we know, most photoelectric devices made by 2D materials should deposit on different substrates. On the one hand, the substrates are propitious to protect and stabilize the 2D materials. On the other hand, the lattice mismatch between substrates and 2D materials is also an effective way to tune the photoelectric properties of 2D materials,^29,30 which arises the importance of strain effect. Considering the successful synthesis of silicene on Ag substrates,³¹ it is predicted that three monolayer PN nanosheets deposited on the Ag substrate by CVD with cyclic phosphazenes in our previous work with a roughly 2%–13% lattice mismatch where the Ag substrate can stabilize the growth of monolayer PN.²⁰ Here, the strain effect is considered by changing the lattice parameter in proportion, and the corresponding electronic properties are tailored simultaneously. The tensile/compressive strain is defined as ε = (a − a₀)/a₀, where a and a₀ are the lattice constants of the relaxed and strained structure, respectively. Firstly, the ideal strength of 2D PN allotropes are determined by the stress–strain relationship and the maximum stress corresponds to the instability point.^32,33 As shown in Fig. 2, the ideal strength of γ-PN along uniaxial direction is 12% strain, corresponding to a maximum stress of 4.33 GPa.

Fig. 2 Stress–strain curves of 2D γ-PN monolayer.

Secondly, the strain effect on electronic properties of 2D γ-PN monolayer is taken into consideration. Introducing strain into 2D materials will not only change the structure, but also cause the electronic redistribution and further alter the band gap of 2D semiconductor materials. Here strains ranging from −10% (compressive) to +10% (tensile) are considered in our calculations because that the γ-PN shows a maximum strain higher than 10%, so that strains in this range could assure the stability of γ-PN. Fig. 3 shows the varying tendency of the bandgap of γ-PN monolayer based on the HSE06 functional. It should be noted that the chosen functional (HSE06 or PBE) do not influence the common tendency of band gap against the applied strain. A band gap range of 1.2–3.2 eV is tuned by strain with the type of band structure keeps indirect. At the strain range from −5% to 10%, the band gap will linearly decrease. Band gap at the −5% compressive strain and +10% tensile strain corresponds to the maximum (3.1 eV) and the minimum (2.27 eV). Furthermore, the energy position of VBM and CBM are considered. The VBM is viewed as the work function of 2D materials, calculated from the difference between the vacuum level (set as 0) and Fermi level of 2D materials determined by the GGA-PBE functional. The band gap obtained by HSE06 functional is then used to locate the position of CBM.³⁴ According to Fig. 3b, the VBM and CBM level of the original γ-PN monolayer are −6.82 eV and −3.98 eV respectively. The work function of γ-PN shows different varying tendency from the band gap and will have a little upshift under the tensile strain. Both VBM level and band gap lead to the decrease of CBM of the γ-PN. It should be noted that the standard reduction potential of H⁺/H₂ and oxidation potential of O₂/H₂O is −4.44 eV and −5.67 eV relative to the vacuum level, which is a determining factor of water-splitting materials driven by visible light.³⁵ With the increased tensile strain, band gap of the γ-PN monolayer linearly decreases and the work function of γ-PN monolayer increase gradually, which is useful to enhance the performance of the γ-PN.

Fig. 3 (a) The band gap of γ-PN monolayer as a function of the uniaxial strain and (b) the located position of VBM and CBM.

Fig. 4a plots the band edge alignment of the γ-PN monolayer with respect to the Standard Hydrogen Electrode (SHE). The redox potential of H⁺/H₂ changes to 0 eV and the redox potential of O₂/H₂O is set to 1.23 eV relative to the SHE. At ambient condition without strain, the VBM and CBM of γ-PN locate around the redox potential of H⁺/H₂ and O₂/H₂O, which can provide a driven force for the oxidation and reduction reaction. With the introduced tensile strain, the decrease of the band gap and the upshift of VBM will decrease the difference between the VBM of γ-PN and the redox potential of H₂O, which helps to keep the balance of water-splitting reaction. Especially, the γ-PN with +10% tensile strain shows a narrower band gap (2.27 eV) and suitable band alignment, which indicates its better performance in photocatalysis of water splitting. The wave function distribution of VBM and CBM of γ-PN are given in Fig. 4b. It is found that the CBM and VBM at the situation of no strain are mainly contributed by the nitrogen atoms. The CBM and VBM distribution of γ-PN have a little overlapping, which is unfavourable for carriers to diffuse. However, the CBM distribution at the +10% tensile strain localizes to the phosphorus atoms, which helps to decrease the orbital overlapping efficiently. The less overlapping between CBM and VBM as well as narrower band gap caused by strain is beneficial to the separation and diffusion of charge carriers and could make full use of them, thus enhancing the catalysing activity. Our calculations indicate that strain engineering of the γ-PN monolayer is a useful way to improve the performance of photocatalysis.

Fig. 4 (a) Energy alignment of the γ-PN monolayer with no strain and under 10% tensile strain when pH = 0 and (b) the wave function distribution of VBM and CBM of γ-PN monolayer with no strain and 10% tensile strain.

In addition, we compared the optical properties of γ-PN and previously studied graphitic carbon nitride (g-C₃N₄) monolayer based on the HSE06 functional. Fig. 5 displays the absorption spectra as the function of photon energy in the vertical direction of the plane. It is found that the main absorption peaks of the γ-PN and g-C₃N₄ monolayer widely locate at the ultraviolet area because of their relatively large band gap. Although the band gap of the γ-PN and g-C₃N₄ monolayer is about 2.85 eV, the γ-PN shows wider and higher absorption intensity than that of g-C₃N₄ at the range between 1.55 eV and 3.1 eV, corresponding to the visible light area (about 400 nm–800 nm in wavelength). Moreover, the absorption peaks of γ-PN at the +10% tensile strain shows more overlapping with visible light area and stronger absorption for visible light compare with g-C₃N₄ monolayer, equipping the better absorption coefficients with the low band gap of 2.27 eV. Specifically, for γ-PN at the +10% tensile strain there is an enhanced peak located at about 3.5 eV, showing its superior absorption on near-ultraviolet light. The above results reveal that the tensile strain will further enhance the photocatalysis performance of water splitting for γ-PN, which provides a versatile way to tune its optoelectronic properties.

Fig. 5 The absorption coefficient of γ-PN and buckle g-C₃N₄ along the vertical direction of 2D materials.

Furthermore, we simulated the interfacial water decomposition on the γ-PN monolayer in order to unfold the microscopic mechanism shown in Fig. S2 of ESI.† The adsorption of water molecule is weak interaction −0.27 eV while the following water decomposition is an endothermic reaction with a reaction energy of 1.24 eV (from −0.27 eV to 0.97 eV), which is comparable to the water splitting reaction on the 2D Zr₂CO₃ and TiO₂ surface. Moreover, the ultra-high specific surface area of PN monolayer with a board visible light absorption is available to enhance the performance of photocatalytic reactions compared to the bulk TiO₂ and 2D g-C₃N₄.

Last but not least, we run 100 ps molecular dynamics to investigate the structural stability of γ-PN monolayer configurations under the surrounding of water. The constructed periodical box ensures the 1 g cm⁻² density in the whole system. It is clearly found that the γ-PN monolayer is still stable in the environment of H₂O after 100 ps molecular dynamics simulation at 300 K, as shown in Fig. 6. Furthermore, the radius distribution function g(r) of P–N and H₂O are investigated to characterize how the densities of water molecules and N–P varies as a function of space distance and the discrete peaks in Fig. 6 show well crystalline γ-PN and the distance between water molecules are about 1 Å, which is the intrinsic distance of liquid water in the solution and agrees well with the previous XRD measurements.³⁶

Fig. 6 Snapshot of phosphorus nitride in liquid water and the radius distribution function g(r) after 100 ps molecular dynamics at 300 K (the pink, blue, red and white balls present the P, N, O, and H atoms, respectively).

Conclusions

In this work, novel γ-PN monolayer is investigated as a visible-light-driven water-splitting photocatalyst based on the first-principles calculations. We verify that the γ-PN monolayer is an indirect semiconductor with a bandgap of 2.85 eV. The VBM and CBM of γ-PN monolayer are −6.82 eV and −3.98 eV respectively and are located around the reduction and oxidation potentials of H₂O energetically, enabling the γ-PN monolayers as effective and promising metal-free visible-light-driven water-splitting photocatalysts. Meanwhile, the band gap and energy levels of γ-PN can be effectively tuned by the external applied strain effect. The γ-PN monolayer at the 10% tensile strain shows the optimal water-splitting performance due to the low band gap and the superior absorption on UV-visible light. In addition, the stability of γ-PN monolayer in aqueous solution is considered by the molecular dynamics and confirms the reliability of γ-PN monolayer in splitting water. The novel γ-PN nanomaterial has shown good potential in catalysing visible-light-driven water-splitting and is promising as a metal-free photocatalyst if be synthesized.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (Grants 2017YFA0204800 and 2017YFB0701600), the National Natural Science Foundation of China (Grants 51761145013, 21673149), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). Additional support comes from the Fund for Innovative Research Teams of Jiangsu Higher Education Institutions, Jiangsu Key Laboratory for Carbon Based Functional Materials and Devices, Collaborative Innovation Center of Suzhou Nano Science and Technology.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra10305k

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