Electronic, Magnetic and Catalytic Properties of Thermodynamically

Article Cite This: Chem. Mater. 2017, 29, 8892-8900

pubs.acs.org/cm

Electronic, Magnetic, and Catalytic Properties of Thermodynamically Stable Two-Dimensional Transition-Metal Phosphides Yangfan Shao,†,‡ Xingqiang Shi,*,‡ and Hui Pan*,† †

Institute of Applied Physics and Materials Engineering, University of Macau, Macau SAR, China Department of Physics, Southern University of Science and Technology, Shenzhen 518055, China

‡

S Supporting Information *

ABSTRACT: Two-dimensional (2D) nanomaterials have attracted extensive interest due to their unique properties and versatile applications. In the current work, a new family of 2D nanostructures, 2D transition-metal phosphide monolayer (M2P), is designed to find its multifunctional applications on the basis of density functional theory (DFT) calculations. We show that the 2D M2P monolayers are thermodynamically stable by carrying out molecular dynamic simulations, analyzing their phonon dispersions, and calculating their mechanical properties. We find that all of the stable 2D M2P monolayers are metallic, and their magnetic states strongly depend on the transition metals and geometric phases. We further demonstrate that 2D W2P and Fe2P monolayers in the 2H phase show the best catalytic performance in hydrogen evolution reaction (HER) due to the relatively low overpotentials at high hydrogen coverage. Importantly, oxygen functionalization can efficiently improve the HER activities of the 2D M2P monolayers at low hydrogen coverage. Our systematic study predicts that these new thermodynamically stable 2D transition-metal phosphides could be applicable in nanodevices, spintronics, and electrocatalysis.

1. INTRODUCTION Since the discovery of graphene, two-dimensional (2D) materials have triggered increasing interest because of their unusual physical and chemical properties1−4 and wide applications from nanodevices to catalysis.5−10 For example, 2D transition-metal dichalcogenides (TMDs) showed intriguing electronic and optical properties, leading to versatile applications in logic transistors, chemical sensors, Li-ion batteries, catalysts, etc.11−23 Recently, MXenes, 2D transitionmetal carbides, nitrides, and carbonitrides, have attracted great attention because of their specific metallic and magnetic properties.24−28 The MXene monolayer has a chemical formula of MnXn−1 (n = 2−4),27,29 where M is a transition metal and X is carbon or nitrogen. For example, transition metals in M2X are at the top and bottom, and the carbon or nitrogen layer is in between. MXenes have been fabricated by chemical etching30 and chemical vapor deposition (CVD),31 and have found various applications in energy storage,32−37 hydrogen production,38 and electromagnetic interference shield.39 The CVD is viewed as a very promising and scalable fabrication technique, which has recently made tremendous advances in growth of high-quality and large-scale 2D materials.40,41 MXenes showed high catalytic activity in hydrogen evolution reactions (HERs) in a wide range of hydrogen densities, which is comparable to novel metals.42 Most recently, 2D Tl2O was predicted on the basis of DFT calculations, and the family of metal-shrouded 2D materials has expanded to metal oxides.43 © 2017 American Chemical Society

Recently, M. Yoon et al. reported theoretically that layered transition-metal phosphides (TMPs), Sr2P and Ba2P, are thermodynamically stable and could be applied to 2D electrides.44 However, a systematic study on 2D TMPs and their various applications has not been available. In this work, we investigate the structural stability, and electronic, magnetic, and catalytic properties of a new family of 2D TMP monolayers on the basis of DFT calculations. We consider 24 M2P monolayers, including M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, and Ni in both trigonal (T) and hexagonal (H) phases. We find that nine systems are stable by analyzing the dynamic, thermal, and mechanical properties. We show that all of the stable monolayers are metallic, while their magnetic ground states depend on the transition metals and geometrical phases. We further show that the catalytic activity of 2D TMPs depends also on the transition metal and phase, and oxygen functionalization can efficiently improve their HER activities.

2. COMPUTATIONAL METHOD The spin-polarized DFT calculations were performed by using the Vienna ab initio simulation package (VASP).45,46 Projector augmented wave (PAW) potentials were employed to describe the core electrons, and the valence electron orbitals were expanded on the basis of plane Received: September 11, 2017 Revised: September 24, 2017 Published: September 24, 2017 8892

DOI: 10.1021/acs.chemmater.7b03832 Chem. Mater. 2017, 29, 8892−8900

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Chemistry of Materials waves with a kinetic energy cutoff of 500 eV.47 The Perdew−Burke− Ernzerhof generalized gradient approximation (PBE−GGA)48 exchange and correlation functional was used. For structure relaxation, both lattice constants and atom coordinates were optimized until the force is less than 0.02 eV/Å. The Monkhorst−Pack k-point sampling for the 2D cell was 15 × 15. For density of states (DOS) calculations, Gaussian smearing was 0.05 eV, and Monkhorst−Pack k-point sampling was 41 × 41. The vacuum layer in the vertical direction was 20 Å. The phonon dispersion curves were calculated by using the finite displacement method as implemented in the Phonopy program,49 which is interfaced with the density functional perturbation theory as implemented in VASP. The 3 × 3 supercells were adopted to simulate phonon dispersion curves. The kinetic energy cutoff for phonon calculations was 700 eV.

nearly single-atomic-layer structure) with a vertical distance of 0.69 Å after geometry optimization (Table S2). 3.1. Dynamic, Thermal, and Mechanical Stability. The structural stability cannot be ensured, even though the structures can be optimized by minimizing the total energy. To further confirm the structural stability of 2D TMPs, we calculate the phonon dispersions of the energetically stable M2P systems, which is an important indication of structural stability. The phonon modes at some K points in the Brillouin zone will be imaginary when a system is dynamically unstable. There are no significant imaginary vibrational frequencies in the calculated phonon dispersion curves of 2D M2P monolayers (M = Ti, Zr, Nb, Ta, Cr, Mo, W, and Fe) in the 2H phase (M2P−2H), indicating that these systems are dynamically stable (Figure S1a,c−g,i,k). For 2D M2P in the 1T phase (M2P−1T), Zr2P−1T, W2P−1T, and Fe2P−1T are found to be dynamically stable from the calculated phonon dispersion (Figure S1b,h,j). However, other systems (Ti2P, V2P, Nb2P, Ta2P, Cr2P, Mo2P, Mn2P, and Ni2P in 1T phases; and Mn2P and Ni2P in 2H phases) are dynamically unstable because of imaginary frequencies in phonon dispersions (Figure S2). The above dynamically stable structures are obtained at T = 0 K. To investigate their thermal stability at higher temperatures, we carry out ab initio molecular dynamics (AIMD) calculations. After the supercells with 108 atoms are annealed at 300 K for 3 ps with a time step of 1 fs, most of the predicted stable structures, except Cr2P−2H and W2P−1T (Figure S3), have no structure reconstruction. We thus conclude that the predicted M2P−2H (M = Ti, Zr, Nb, Ta, Mo, W, and Fe) and M2P−1T (M = Zr and Fe) can be stable at room temperatures. Although the MD simulation confirms the thermodynamic stability of the systems, it is necessary to assess the effect of lattice distortion on structural stability because the supercells are fixed in the MD simulation, which can be confirmed by calculating elastic constants. Mechanical stabilities of these systems are obtained on the basis of Born criteria.50 For a 2D sheet, the four nonzero 2D elastic constants are c11, c22, c12, and c66 using the standard Voigt notation, 1-xx, 2-yy, and 6-xy.51 The hexagonal structure has c11 = c22 because of the symmetry 1 and the additional relation c66 = 2 (c11 − c12). 51 As an indication of a mechanically stable 2D sheet, the elastic constants must satisfy c11c22 − c212 > 0 and c66 > 0. The in-plane planar Young’s and shear moduli are derived from elastic constants as follows:

3. RESULTS AND DISCUSSION In our calculations, we focus on searching new stable 2D transition-metal phosphide monolayers with a sandwich structure (M2P). The M2P monolayer is a three-atom-thick layer in a sequence of M−P−M and has two possible phases (1T and 2H). Both 1T and 2H phases are considered in our study to find the stable structure (Figure 1). The H structure is

Figure 1. Top and side views of 2D M2P in 1T (a) and 2H (b) phases. The purple and blue spheres denote phosphorus and transition metals, respectively.

honeycomb with D3h point-group symmetry, while the T structure is centered honeycomb with C3v symmetry. Here, a new family of 2D M2P monolayers (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, and Ni) is designed. To find their structural stability, we first calculate the total energies of M2P monolayers in both T and H phases, respectively. The lowest total energy can be found by optimizing the supercell size and geometry as well as the atomic coordinates in the supercell. Hf2P monolayers in 1T and 2H phases are unstable because their total energies cannot converge to a local minimum, and will not be considered in the following discussion. The 1T phases of Ti2P, Zr2P, and Mn2P monolayers are more energetically stable than their 2H counterparts because their energies in the 1T phase are lower than those in the 2H phase by 0.210, 0.362, and 0.350 eV per unit cell, respectively. For other 2D M2P structures (M = V, Nb, Ta, Cr, Mo, W, Fe, and Ni), their 2H phases are more energetically stable than the 1T phases (Table S1). The optimized lattice constants of M2P (M = Ti, Zr, V, Nb, Ta, Cr, Mo, W, Mn, Fe, and Ni) are listed in Table S2. We notice that only Fe2P−1T becomes nearly flat (a

Ex2D =

2D 2D 2D 2D c11 c 22 − c12 c 21 2D c 22

,

Ey2D =

2D 2D 2D 2D c11 c 22 − c12 c 21 2D c11

,

2D 2D Gxy = c66 2D 2D 2D 2D where c2D 11 , c12 , c21 , c22 , and c66 are components of the 2D elastic constant. The calculated elastic constants (Table 1) 2D show that the Young’s moduli (E2D x and Ey ), and in-plane 2D planar shear moduli (Gxy ), are positive, indicating that the structures are mechanically stable. We conclude, therefore, that the 2D M2P−1T (M = Zr, W, and Fe) and M2P−2H (M = Ti, Zr, Nb, Ta, Cr, Mo, W, and Fe) are mechanically stable. By calculating the total energies and the dynamical, thermal, and mechanical properties of 2D M2P systems (Figure 2, Figures S1−S4, Table 1, and Table S1), we find: (1) Hf2P in both 1T and 2H phases is unstable; (2) there are 11 M2P monolayers, M2P−1T (M = Zr, W, and Fe) and M2P−2H (M = Ti, Zr, Nb, Ta, Cr, Mo, W, and Fe), that are dynamically and

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Chemistry of Materials Table 1. In-Plane Planar Elastic Constants c2D 11 , Young’s 2D 2D Moduli E2D x and Ey , and In-Plane Planar Shear Moduli Gxy of 2D M2P−1T (M = Zr, W, and Fe) and M2P−2H (M = Ti, Zr, Nb, Ta, Cr, Mo, W, and Fe) Systems Ti2P−2H Zr2P−1T Zr2P−2H Nb2P−2H Ta2P−2H Cr2P−2H Mo2P−2H W2P−1T W2P−2H Fe2P−1T Fe2P−2H

c2D 11 (N/m)

E2D x (N/m)

E2D y (N/m)

G2D xy (N/m)

51 78 59 129 156 27 114 228 96 48 44

51 55 59 87 112 27 22 48 96 23 41

51 55 59 87 112 27 22 44 21 23 40

27 18 30 28 36 16 6 17 6 7 28

= Zr and Fe) and M2P−2H (M = Ti, Zr, Nb, Ta, Mo, W, and Fe), are stable. Accordingly, their electronic and magnetic properties are investigated. We calculate their spin-polarized density of states (DOS) and band structures to investigate the electronic properties of the stable 2D TMP monolayers (Figures S7−S12). The electronic structures show that all of the stable 2D M2P monolayers are metallic.

Figure 3. (a) Calculated energy difference of the M2P monolayer between ferromagnetic and antiferromagnetic states. Alignment configurations of magnetic moments: ferromagnetic ground state (b) and antiferromagnetic ground state (c).

mechanically stable; (3) Cr2P−2H and W2P−1T are thermally unstable; and (4) nine 2D M2P systems (Ti2P, Nb2P, Ta2P, Mo2P, and W2P in the 2H phase; Zr2P and Fe2P in both 1T and 2H) are thermodynamically stable. The 1T phase of the Zr2P monolayer is more energetically stable than its 2H phase, while the 2H phase of Fe2P is more energetically stable than its 1T counterpart. Our calculations show that the stability of 2D M2P strongly depends on its composition and structure. As reported in literatures, 2D transition-metal dichalcogenides (TMDs) can be stable in 2H or 1T phases. For example, MoS2 is stable in 2H,19 while TiS2 is stable in 1T.52 Thus, we believe that the stability of 2D M2P is determined by the hybridization between d and p electrons from M and P atoms, respectively, and the symmetry. In addition, we calculated the formation energies of the stable 2D M2P monolayer to evaluate the formation possibility in experiments by comparing with stable MxPy compounds that experimentally exist. The negative (positive) formation energy indicates that the process of fabrication is exothermic (endothermic). According to the convex hull, most new 2D M2P monolayers, except Ta2P−2H and W2P−2H, can be synthesized from bulk metal and phosphorus easily (Figure S4). Although the fabrication of Ta2P−2H and W2P−2H is endothermic, the low formation energies indicate the possibility of experimental realization. 3.2. Electronic and Magnetic Properties. On the basis of the analysis of the dynamical, mechanical, and thermal stability, we find that nine 2D M2P monolayers, including M2P−1T (M

The stable M2P monolayers show a rich diversity of magnetic properties. Depending on the transition metal and phase, 2D M2P monolayers can be ferromagnetic (FM), antiferromagnetic (AFM), or nonmagnetic (NM) metals. There are several possible AFM orderings in a supercell with 2 × 2 × 1 unit cells, named as AFM1, AFM2, AFM3, AFM4, and AFM5 (Figure S5). For AFM1, the metal atoms in the same layer have the same magnetic ordering, while the atoms in a different metal layer have opposite magnetic ordering. For AFM2 and AFM3, the two atoms in the same metal layer of the supercell have opposite magnetic directions. For AFM4 and AFM5, the spin of one atom is opposite to those of another three atoms in the same metal layer of a supercell. In the AFM3 and AFM5 configurations, the spins in the different metal layers are antiparallel to each other. In contrast, for AFM2 and AFM4, the spins in the different metal layers are parallel. All of the possible configurations are relaxed to find the stable magnetic ground state. Our calculations show that AFM2, AFM3, AFM4, and AFM5 are destroyed and automatically convert to the FM or AFM1 state after relaxation. The calculated exchange energies (EAFM − EFM) show that the ground magnetic states of Ti2P− 2H, Zr2P−1T, Fe2P−1T, and Fe2P−2H are ferromagnetic

Figure 2. Summary of dynamical, mechanical, and thermal stability analysis, and magnetic properties of 2D M2Ps. M is a transition metal. FM, AFM, and NM refer to ferromagnetic, antiferromagnetic, and nonmagnetic ordering, respectively. 8894


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Chemistry of Materials (Figure 3a,b) because the exchange energies are positive; Zr2P−2H is antiferromagnetic (Figure 3a,c) because of the negative exchange energy, and the other 2D M2P monolayers (M = Nb, Ta, Mo, and W) are nonmagnetic because of the zero exchange energies. The magnetic moments of P atoms in the ferromagnetic M2P are antiparallel to those of metallic atoms (Table 2 and Figure 3b). For a better understanding of the

Ti2P−2H, Fe2P−1T, and Fe2P−2H monolayers are ferromagnetic, and the d orbitals of metal atoms dominate their magnetic properties. In addition, there is a strong spin polarization around the Fermi level of Ti2P−2H, Fe2P−1T, and Fe2P−2H, and the out-of-plane components Ti_dπ and Fe_dπ make a key contribution (Figure S6c−e). For the Ti2P− 2H monolayer, the spin-up bands are partially occupied, and the spin-down bands are nearly empty, leading to a magnetic moment of 0.77 μB per Ti atom (Figure S6c). For Fe2P−1T and Fe2P−2H, the spin-up bands are fully occupied, while the spin-down bands are partially occupied, resulting in a magnetic moment of 2.13 μB in the 1T phase and 1.49 μB in 2H phase per Fe atom (Figure S6d,e). The magnetic moment is mainly contributed by the Fe_d orbitals. The p electrons of P atoms in ferromagnetic Ti2P−2H, Fe2P−1T, and Fe2P−2H monolayers are also weakly spin-polarized, leading to small magnetic moments of 0.045, 0.032, and 0.081 μB per P atom, respectively, which are antiparallel to those of metal atoms (Figure S6c−e). For magnetic systems, the spin-polarization ratios (SPRs) are calculated to quantify the effect on spin polarization of M2P with ferromagnetic ordering. The spinpolarization ratio (SPR) is defined as SPR(E) = [DOS↑(E) − DOS↓(E)]/[DOS↑(E) + DOS↓(E)], where DOS↑(E) and DOS↓(E) are the DOS for the majority-spin and minority-spin, respectively. There is a strong spin polarization of 50−85% around the Fermi level of Ti2P−2H (Figure 4b). For the Fe2P− 2H structure (Figure S10b), there is also strong spin polarization around the Fermi level, which could be useful to spintronics. 3.3. HER Activity of Pure M2P. Transition-metal phosphides have been reported to be promising catalysts for hydrogen evolution reactions (HERs).58−68 The highly conductive 2D M2P monolayers may lead to high catalytic activity in the HER. To study the catalytic activity of stable 2D TMP monolayers, including M2P−2H (M = Ti, Zr, Nb, Ta, Mo, W, and Fe) and M2P−1T (M = Zr and Fe), we first investigate the adsorption of the hydrogen atom on the monolayers. Three possible sites with high symmetry are

Table 2. Magnetic Momenta (in μB) of M2P−1T (M = Zr and Fe) and M2P−2H (M = Ti, Zr, Cr, and Fe) Structures magnetic moment (μB) M P a

Ti2P−2H

Zr2P−1T

Zr2P−2H

Fe2P−1T

Fe2P−2H

0.77 −0.045

0.20 −0.013

0.32 0

2.13 −0.032

1.49 −0.081

The magnetic moment is mainly from M atoms.

physical origin of the magnetic properties of 2D M2P monolayers, the spin-polarized partial density of states (PDOS) of TM and P atoms are calculated (Figures S6a−e). The TM d orbitals are decomposed to in-plane component dσ and out-of-plane component dπ (Figure S6). The Zr2P monolayer in the 1T phase is ferromagnetic. Zr_dπ dominates the spin polarization around the Fermi level, leading to a moment of 0.20 μB per Zr (Figure S6a). The magnetic moment of the P atom is about 0.013 μB in Zr2P−1T and antiparallel to those of metal atoms, indicating that the carrier-mediated double exchange is the dominant mechanism of ferromagnetism.3,53−55 However, its 2H phase is antiferromagnetic. The PDOS of the Zr2P−2H monolayer show that the electrons of the P atom are spin-unpolarized because of the asymmetric magnetic structures (the two magnetic values are equal in the opposite direction) between two Zr atoms. Both Zr_dσ and Zr_dπ make the main contribution to the spin polarization near the Fermi level and the p−d hybridization between P and Zr atoms also played a significant role (Figure S6b). From the antiparallel magnetic moments between two Zr layers, we see superexchange among Zr atoms via nonmagnetic P atoms.3,56,57

Figure 4. Spin-polarized density of states (DOS) (a), and spin-polarization ratio (SPR; b) of Ti2P−2H. Spin-up (c) and spin-down (d) band structures of Ti2P−2H. 8895


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Figure 5. Side and top views of the structure of M2P−2H with hydrogen adsorption at HC (M2P−HC; a) and M2P−2H with hydrogen adsorption at TP (M2P−TP; b) for a 3 × 3 × 1 supercell. (c) Calculated H-adsorption energies at different sites. (d) Calculated Gibbs free energies of W2P− HC and Fe2P−TP as a function of H coverage on pure M2P monolayers. d-ΔG: differential Gibbs free energy; a-ΔG: average Gibbs free energy.

where E(M2P + nH) and E(M2P) are the total energies of the M2P monolayer with and without hydrogen atoms, respectively. n is the number of H atoms adsorbed on single-layer M2P. ΔSH is the difference in entropy and T is room temperature. ΔEZPE is the difference in zero-point energies of hydrogen between the adsorbed and the gas phases. ΔEZPE − TΔSH is about 0.24 eV. The Gibbs free energies are defined as differential d-ΔGH and average a-ΔG H . The individual process describes the production of the hydrogen molecule one by one, which can be assessed by d-ΔGH. The collective process shows that all of the hydrogen atoms on the surface are simultaneously converted to molecules, which can be express by a-ΔGH. To investigate the effect of hydrogen coverage on HER performance, we constructed a 3 × 3 × 1 supercell of pure M2P monolayer for the adsorption of n hydrogen atoms with n changed from 1 to 9. We consider the magnetic properties in the study of HER performance for the magnetic system, including Ti2P−2H, Zr2P−1T, Zr2P−2H, and Fe2P−2H. We see that the ΔGH strongly depends on the H coverage and adsorption site (Figures S14−S17). For the hydrogen adsorption on HC (Figure 5d), the 2D W2P−2H monolayer shows the best HER performance due to the relatively lower overpotentials (absolute value of the calculated Gibbs free energy). Specifically, the d-ΔGH of W2P−2H is 0.051, −0.048, 0.115, and −0.114 eV at n = 3, 6, 8, and 9, respectively, indicating high catalytic activity in the individual process. However, the overpotential in the average process is far away from zero, leading to a poor collective process. For hydrogen adsorbed on the TP site, the calculated Gibbs free energies show that Mo2P−2H and Fe2P−2H are better than other predicted 2D TMPs in HER performance because of the relatively lower overpotentials (Figures S16 and S17). Specifically, the d-ΔGH values of Mo2P−2H and Fe2P−2H are −0.065 and −0.080 eV, approximately equal to 0, at full H coverage (n = 9), indicating good HER activity at high H density in the individual process. Similarly, the average Gibbs

considered for hydrogen atoms adsorbed on the M2P monolayer, including the top of the hexagonal center (HC), top of P atoms (TP), and top of M atoms (TM) (Figure 5a,b, and Figure S13). The adsorption energies (Ead) of different H adsorption sites are calculated to find the stable adsorption configuration, as expressed by Ead = E(M2P + H) − E(M2P) − (1/2)E(H2), where E(M2P + H) and E(M2P) are the total energies of M2P monolayers with and without hydrogen adsorption. The calculated energies show that both HC and TP sites on most of the 2D M2P monolayers are stable to hold hydrogen atoms because of the negative adsorption energy, except Fe2P−1T, for which it is difficult to hold hydrogen atoms. The H atom on the TM site moves away to other sites after relaxation, indicating that the top of the metal is unstable to hold the hydrogen atom. HC is the most stable site for hydrogen adsorption because of the relatively lower adsorption energy (Figure 5c). However, hydrogen can also adsorb on TP because of the small adsorption energy difference between the adsorptions on HC and TP sites. The HER performance of the 2D M2P monolayer can be quantified by the reaction Gibbs free energy of hydrogen adsorption (ΔGH).69,70 In principle, an advanced high-HERactivity catalyst should have a ΔGH near 0 eV, which can be obtained from the following equation: ΔG H = ΔE H + ΔEZPE − T ΔSH

(1)

where ΔEH is the hydrogen chemisorption energy. This can be the differential chemisorption energy, defined as ΔE H = E(M 2P + nH) − E(M 2P + (n − 1)H) −

1 E(H 2) 2 (2)

or the average chemisorption energy, defined as ⎛ ⎞ n ΔE H = ⎜E(M 2P + nH) − E(M 2P) − E(H 2)⎟ /n ⎝ ⎠ 2

(3) 8896


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Figure 6. Side and top views of the structure of M2PO2−HC (a) and M2PO2-TP (b) for a 3 × 3 × 1 supercell. Calculated differential Gibbs free energies of oxidized M2P monolayers as a function of H coverage: individual (c) and collective (d) processes.

the HER activity of a few 2D TMPs, including Ti2P−2H, Ta2P−2H, and Nb2P−2H, at low H coverage. However, the HER performances of Mo2P−2H and Fe2P−2H are greatly reduced by oxidation because of large overpotentials (Figures S21−S24). In addition, we investigate the effect of oxygen coverage on HER performance. We find that low O coverage leads to reduction of HER performance, and high/full O coverage shows better catalytic ability (Figure S26).

free energies of Mo2P−2H and Fe2P−2H for hydrogen at the TP site are far away from 0 eV, indicating that it is more difficult for the collective processes to take place than the individual process. By comparing the Gibbs free energies at different adsorption sites, we see that Mo2P−2H shows generally better HER performance when hydrogen atoms are at HC sites than when at TP sites, except n = 9, where the situation is reversed. However, the HER performance of Fe2P− 2H is much better for the hydrogen atom at TP sites than for the one at HC sites. Furthermore, the effect of defects in Fe2P− 2H and W2P−2H on HER performance is investigated. We find that their HER performances are slightly reduced by introducing defects (Figure S18). 3.4. HER Activity of Oxidized 2D M2P. It is well-known that MXenes can be easily oxidized because the out-layers are transition metal.71−73 To give a comprehensive understanding on catalytic properties of the 2D TMPs, we investigate the HER performance of oxidized 2D TMPs (M2PO2), where oxygen atoms can adsorb on HC or TP sites (Figure 6a,b) because of the strong binding energy. A 2 × 2 × 1 supercell of the M2PO2 monolayer is used to investigate the effect of hydrogen coverage on the HER performance of the oxidized TMPs. From calculated differential Gibbs free energies, the ΔGH of the M2PO2 monolayer strongly depend also on the H coverage and ΔGH increase with the H coverage in both individual and collective processes. For hydrogen adsorption at the HC site, the d-ΔGH of Nb2PO2−2H is −0.046 eV at the lowest H coverage (n = 1), indicating high HER performance in the individual process at low hydrogen coverage. For hydrogen adsorption at the TP site, the d-ΔGH of Ti2PO2−2H for n = 1, Ta2PO2−2H for n = 1, and Nb2PO2−2H for n = 2 are 0.077, −0.079, and 0.08 eV, respectively, indicating good HER activity at low H density in the individual process. Their HER activities are comparable with those of MXenes and MX2 monolayers when considering the H-coverage density (Table S3).16,42,74−76 Therefore, the oxygen functionalization can efficiently improve

4. CONCLUSIONS In summary, a new family of 2D transition-metal phosphides is designed on the basis of DFT calculations. We predict that these 2D M2P monolayers (M = Ti, Zr, Nb, Ta, Mo, W, and Fe) are thermodynamically stable by the analysis of phonon dispersions, molecular dynamic simulations, and investigation of mechanical properties. We find that all of the stable 2D M2P monolayers are metallic, and the magnetic states strongly depend on the transition metal and phase. We further demonstrate that pure W2P−2H and Fe2P−2H monolayers show the best catalytic performance in the hydrogen evolution reaction at high hydrogen coverage because of the relatively lower overpotentials, and oxygen functionalization can efficiently improve the HER activities of Nb2PO2, Ti2PO2, and Ta2PO2 at low H coverage. It is expected that these 2D M2P monolayers can be applied in spintronics and electrocatalysis.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b03832. Optimized lattice constants, phonon dispersion curves, snapshots of atomic configurations at the end of AIMD simulations, spin-polarized density of states, and Gibbs free energies (PDF) 8897


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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Hui Pan: 0000-0002-6515-4970 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work is supported by the Science and Technology Development Fund from Macau SAR (FDCT-068/2014/A2, FDCT-132/2014/A3, and FDCT-110/2014/SB), the MultiYear Research Grant (MYRG2017-00027-FST and MYRG2015-00017-FST) from the Research & Development Office at University of Macau, the NSF of China (Grants 11474145, and 11334003), the Nanshan Key Lab on Nonvolatile Memory Grant (KC2015ZDYF0003A), and the special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase) under Grant U1501501. The DFT calculations were partially performed at High Performance Computing Cluster (HPCC) of Information and Communication Technology Office (ICTO) at University of Macau.

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