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Nov 2, 2016 - hybrid Ni−Fe-MOF-74, named as the monopolar magnetic semiconductor (MMS), is a new type of porous polarization material that can be ...
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Monopolar Magnetic MOF-74 with Hybrid Node Ni−Fe Lihong Wei,† Baihai Li,‡ Qiuju Zhang,*,†,§ Liang Chen,*,† and Xiao Cheng Zeng*,§ †

Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, Zhejiang 315201, China School of Energy Science and Engineering, University of Electronic Science and Technology of China, Chengdu 611731, China § Department of Chemistry, University of NebraskaLincoln, Lincoln, Nebraska 68588, United States ‡

S Supporting Information *

ABSTRACT: Effects of metal hybridization on the electronic properties of the metal− organic framework (MOF) Fe-MOF-74 with different Ni content are investigated using the first-principles method with the Heyd−Scuseria−Ernzerhof (HSE06) functional. The hybrid Ni−Fe-MOF-74, named as the monopolar magnetic semiconductor (MMS), is a new type of porous polarization material that can be easily converted to a half-metal. On the basis of our investigation of the effects of Ni content and the hybrid node arrangement on the band gap of the MOF, we found that the interchain Ni−Fe−Fe arrangement with a Ni content of 33−50% is likely the most suitable configuration due to the narrowing of the spin-down band gap to ∼0.62−0.91 eV from 1.38 eV of the pristine Fe-MOF-74. Hybrid nodes can provide an effective way to narrow the electronic band gap of MOFs and to allow easy conversion of MOFs to polarization materials. Our computation also suggests that the pure Ni-MOF-74, with a band gap of 2.10 eV, can serve as a good visible-light photocatalyst.

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INTRODUCTION Spin polarization materials are a special class of half-metals that can be exploited for spintronic applications due to their ability to conduct single-spin current under a gate voltage. For example, bipolar magnetic semiconductors (BMSs) that can generate spin-polarized currents with reversible spin polarization were recently demonstrated in semihydrogenated singlewalled nanotubes by Yang et al.1,2 Two additional types of spintronic materials proposed are spin-gapless semiconductors and half-metals that exhibit antiferromagnetism.3 Until now, most of these spin polarization materials are either metal oxides, or two-dimensional (2D) materials, or nanotubes. Future spintronic application calls for greater efforts in developing new materials that can be easily converted to halfmetals under a gate voltage. Due to their high surface area and good electrical conductivity, metal−organic frameworks (MOFs) can find applications in batteries, supercapacitors, and electronic devices.4−7 One specific MOF, namely, Fe-MOF-74, exhibits high charge mobility due to the loose binding of the Fe β electron. This high mobility offers some important hints on rational design of conductive MOFs.8 The electrical conductivity of Fe-MOF-74 is a million times higher than that of Mn-MOF-749 due to the 1.0 eV narrowed band gap. We recently reported density functional theory (DFT) computation and showed that ferromagnetic (FM) metastable Fe-MOF74 exhibits spin polarization with a smaller band gap in the spin-down channel and a larger gap in the spin-up channel.10 A recent HSE06 DFT computation suggested that the band gap of antiferromagnetic (AFM) Fe-MOF-74 is ∼1.4 eV,8 making its conversion to a single-spin material difficult. However, the © XXXX American Chemical Society

newly synthesized MOF-74 with a hybrid metal node and different magnetic transition metals gives rise to a new type of MOF.11 Motivated by the hybrid node effects, we explore a series of hybrid M−Fe-MOF-74 nodes to study the hybrid node effects on electronic band gap. By varying the content of the transition metals, the conversion to single-spin current materials can be realized via reducing the band gap. In this work, we examine the effects of mixing Mg, Mn, Ni, or Co atoms with Fe atoms to form the hybrid nodes of M−FeMOF-74 because these elements have been shown to form M− Fe hybrid node due to successful synthesis of topological MOF74 structures. Here, however, results of only Ni−Fe hybrid node are reported as the Ni−Fe hybrid node is the most effective to tune the band gap of Fe-MOF-74. The electronic properties, including the density of states (DOS) and band structures, are computed for different Ni content using the HSE06 hybrid functional. The Ni−Fe hybrid node with 33− 50% Ni is found to be the most suitable candidate to ease the conversion of the MOF to half-metal under a low gate voltage as it narrows the band gap for spin-down polarization to ∼0.62−0.91 eV. Contrary to BSMs, the porous material appears to favor the production of spin-down current. Hereafter, we consider this type of MOF as a monopolar magnetic semiconductor (MMS). The substitution of different transition metals to narrow the band gap is also applicable to nanowire metal oxide chains that can exhibit electron mobility along the metal oxide chain. The computed work function indicates that Received: September 10, 2016 Revised: October 25, 2016 Published: November 2, 2016 A

DOI: 10.1021/acs.jpcc.6b09175 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 1. (a) Unit cell of the Fe-MOF-74 with 54 atoms. (b,c) Inter- and intra-chain hybrid modes for Ni hybridization into Fe-MOF-74 at the hybridized amount (N = 2). Ni1 and Ni2 correspond to Fe−Fe−Ni and Fe−Ni−Ni chains due to their different arrangements. The blue, green, red, gray, and white balls represent Fe, Ni, O, C, and H atoms, respectively.

Table 1. Exchange Energy (Eex = EFM − EAFM) between the FM and AFM Sites in Fe-MOF-74 and Ni−Fe-MOF-74 along with the Mtotal and Mlocal for Each Ground State and the Metastable States Mlocal (μB) Fe-MOF-74 Ni1 N = 2 interchain N = 3 interchain

FM AFM FM AFM FM AFM FM AFM

Eex (eV)

Fe1

Fe2

Fe3

Fe4

0.03

3.66 3.66 3.67 3.66 3.65 3.69 3.67 3.66

3.66 3.66 3.66 3.73 3.66 3.67 3.75 3.65

3.66 −3.66 3.66 −3.66 3.68 −3.75 3.65 −3.67

3.66 −3.66 3.66 −3.66 3.67 −3.68 1.71 (Ni) −1.67 (Ni)

−0.015 0.098 0.032

Fe5 3.66 −3.66 3.66 −3.66 1.64 −1.57 −0.02 1.68

(Ni) (Ni) (Ni) (Ni)

Fe6 3.66 3.66 −0.07 1.62 0.02 1.64 1.59 −0.03

(Ni) (Ni) (Ni) (Ni) (Ni) (Ni)

Mtotal(μB) 23.19 0.00 19.32 −1.93 17.37 −0.04 15.47 3.84

contents and relative Ni atom locations. Here, we put two Ni atoms (hybridization N = 2) in one unit cell as an example to illustrate the difference of two hybrid nodes, as presented in Figure 1. The hybridization node with Ni atoms distributed in differential metal oxide chains (M−O−M) is called interchain hybridization (Figure 1b), whereas the other hybridization node with Ni atoms hybridized into the same metal oxide chain is called intrachain hybridization (Figure 1c). Different initial guesses for the local magnetic moments of the FM and AFM states were examined to determine the most stable state and spin alignments for the M-MOF-74 systems. An illustration of inter- and intrachain formation at hybridized amounts of N = 1 ∼ 4 is shown in Figure S1 in the Supporting Information (SI).

the Ni-MOF-74 likely gives good photocatalytic activity under visible light.

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METHODS All energy and atom position optimizations were based on firstprinciples calculations using DFT methods implemented in VASP 5.4.12−14 The projector-augmented plane wave (PAW) potentials were used to describe the electron−ion interactions. The Perdew−Burke−Ernzerhof (PBE) exchange−correlation functional was selected for first-stage computation.15−18 All atoms and lattice parameters were fully relaxed until the Hellmann−Feynman forces on each atom and total energy were converged to less than 0.02 eV/Å and 10−4 eV, respectively. The cutoff energy of the plane wave was set to 480 eV, and K points in the Brillouin zone were sampled as a 2 × 2 × 2 mesh. The main electronic properties, including the electron DOS and band gap, were computed using a hybrid exchange−correlation functional (HSE06) with 25% of the short-range semilocal exchange replaced by the exact nonlocal Hartree−Fock exchange.19 To evaluate the photocatalytic activity of M-MOF-74, we compute the work function using the method proposed by Butler et al.20 They assumed that the vacuum potential of a MOF, whose pore radius is larger than 8 Å, reaches a constant with an error range of 0.1 eV. The spherical average of the electrostatic potential was calculated using the formula · 1 φav = V ∫ φ(r′) d3(r′), where V and r are the volume and V radius of the sphere, respectively. To model the dehydrated hybrid M−Fe-MOF-74 node, we considered a unit-cell model with 54 atoms including six metal atoms, as shown in Figure 1. The metal hybridization and locations are two key factors that can affect the structure and inherent electronic properties of the M−Fe-MOF-74 nodes. Two hybridized nodes were produced with different Ni

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RESULTS AND DISCUSSION Magnetic Structures of Ni−Fe-MOF-74. The structures and magnetic properties are computed to evaluate the hybrid node effect. Our previous study showed that the lattice parameters of Ni-MOF-74, a, b, and c, are nearly 0.25 Å smaller than those of pure Fe-MOF-74, while the lattice angle remains nearly the same.10 Compared to pure Fe-MOF-74, the hybrid Ni−Fe-MOF-74 exhibits no obvious variation in the lattice parameters or angles and maintains the triclinic crystal structure, as summarized in the SI Table S1. Our computed lattice parameters are in good agreement with the experimental values of MOF-74 (a = b = c = 15.14 Å, α = β = γ = 117.8°).21 The apical bond length of Ni−O in MO5 coordination is enlarged to 2.7−2.8 Å, much longer than the four planar Ni−O bonds of ∼2.0−2.1 Å, when the Ni hybridization is increased from N = 1 to 3. The neighboring Fe−O bonds in the FeO5 fragment remain the original values of 2.0−2.1 Å, similar to those of the pure Fe-MOF-74. To determine the magnetic ground state, both FM and AFM order were studied by setting different initial magnetic directions of the hybrid metal nodes. The corresponding energies, together with the total magnetic B

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moment directions on different Fe and Ni atoms ranging from ±|5.79| to ±|1.93| μB, as plotted in SI Figure S2. Monopolar Magnetic Semiconductor (MMS) of the Ni−Fe Hybrid Node of MOF-74. Next, we study the effects of Ni−Fe hybridization on the electronic properties of the MOFs considered. Our previous study indicates a significant spin gap difference between spin-up and -down channels in metastable FM state of Fe-MOF-7422 and Co-MOF-74. We reexamine the electronic properties of pure Fe-MOF-74 using the HSE06 hybrid functional. The spin-down and -up band gaps are increased from previous estimates (0.30 and 1.75 eV) to 1.38 and 2.44 eV, respectively, due to the underestimation of the band gap by the PBE computation (reported in a previous study). Although there is still a large difference in the band gap for up and down spin channels, larger spin-down gap would imply higher voltage to generate single-spin current. Therefore, we aim to further reduce the spin-down gap by selecting suitable hybrid metal atoms. After attempting Mn, Mg, Zn, Co, and Ni atoms, we found that the Ni−Fe hybrid node is the best candidate to reduce the spin-down gap by utilizing different Ni contents, as shown in Figure 3. It is known that both the hybrid modes and degree of hybridization are key factors that can influence the band gap. Although both the spin-up and spin-down gaps are narrowed by increasing the hybrid Ni content from N = 1 to 3, N = 2 in one unit cell can give rise to the smallest spin-down gap of 0.62 eV, compared to the original 1.38 eV gap for Fe-MOF-74 based on the HSE06 computation. From the DOS, it can be seen that the hybridized Ni can shift the conductive band minimum (CBM) to lower energy and hence reduce the band gap. The corresponding PDOS indicates that the lowered spin-down CBM is mainly contributed from the Ni-3d at a peak of about 0.50 eV, while the spin-down valence band maximum (VBM) stems mainly from the Fe-3d band. The spin-up CBM consists of Ni-3d and organic linkers O-2p and C-2p from Ni−O and C−O bonds. At the Ni hybridization level of N = 3, the rise of the spin-down Ni-3d band near the Fermi level reduces the band gap of the spin-down channel to 0.91 eV. The spin-up gaps are simultaneously reduced to 1.70 and 2.20 eV for N = 2 and 3, respectively. Ni−Fe hybrid modes, including intrachain and interchain hybridization, will result in different effects on changing the band gap. Taking N = 2 as an example, interchain and intrachain Ni hybridization could cause the spin-down gap to be reduced to 0.62 and 1.05 eV, respectively. For interchain hybridization at N = 2 (Figure 1b), the two hybrid Ni atoms can be distributed in two different metal oxide chains to yield the Fe−Fe−Ni chain. From projection of the DOS at the Ni1 and Ni2 atoms in SI Figure S3, such a hybrid pattern creates a lower-energy peak of the Ni1−3d band at 0.50 eV. Hence, it largely narrows the spin-down gap to about 0.62 eV. However, in intrachain hybridization (Figure 1c), the formed Fe−Fe−Fe and Fe−Ni−Ni chains coexist, while neither of them can cause a significant energy shift of the VBM. The hybrid Ni2−3d band at the Ni−Ni−Fe chain is located at ∼0.90 eV and hence causes little change to the spin-down gap. Such a difference can be further verified for the Ni hybridization content at N = 3. In this case, the interchain mode can narrow the spin-down gap to 0.91 eV (Figure 3c,d), while the intrachain mode can only reduce the gap to 1.09 eV. The interchain hybrid Ni atoms at N = 3 yield Fe−Ni−Ni and Fe−Fe−Ni chains. The lower CBM also stems from the Ni-3d band in the Fe−Fe−Ni chain, and higher-energy Ni-3d stems from Ni−Ni−Fe. Similarly, the

moment (Mtotal) and local magnetic moment (Mlocal) on each metal atom, are summarized in Table 1. Following previous studies, the positive value of Eex (Eex = EFM − EAFM) indicates that the hybrid MOFs favor AFM ordering with the exception of one Ni hybridization. However, the small energy differences (Eex) for these Ni−Fe hybrid MOFs indicate that easy transformation to the half-metal state is possible for these MOFs. To estimate the magnetic anisotropy along different directions and the length scale of magnetic ordering, the spin density corresponding to the FM and AFM states of Ni−Fe MOF-74 is plotted (Figure 2 and SI Figure S2). As listed in

Figure 2. Computed spin density of Ni atoms hybridized into an interchain node at N = 2 for one pore structure. The upper panel corresponds to the Ni−Fe hybridized structure at N = 2. The middle panel shows the FM (left) and AFM (right) coupled spin densities for one pore. The bottom panel is a side view of Ni−Fe−Fe chain spin densities.

Table 1, most of the magnetic moments are due to the central NiO5 or FeO5 fragment, as seen from the spin density distribution. In the FM states of hybrid Ni−Fe MOF, Mtotal decreases from 23.19 to 15.47 μB with increasing Ni concentration from N = 0 to 3. This is reasonable due to the smaller local magnetic moment on Ni (Mlocal(Ni) ≈ 1.60 μB) than that on Fe (Mlocal(Fe) ≈ 3.66 μB). The AFM state of the Fe-MOF-74 exhibits a zero Mtotal due to equal and opposite local magnetic moments on the six Fe atomic sites in each unit cell. When Ni and Fe atoms with opposite magnetic moment orientations are mixed, a residual Mtotal in Ni−Fe MOF arises due to uneven opposite local moments on Ni and Fe atomic sites (Figure 2). In the case of N = 1 at the AFM state, the residual Mtotal is computed to be ±|1.93| μB. As far as the N = 2 interchain hybridization is concerned, possible residual Mtotal is ±|0.04| μB or ±|3.86| μB with different initial magnetic moment directions. When N is increased to 3, a more complicated residual Mtotal value is obtained by setting different magnetic C

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Figure 3. Spin-resolved total DOS (TDOS), electronic band structures, and local DOS (LDOS) of the FM hybrid node Ni−Fe-MOF-74 at N = 2 and 3. (a,c) Computed TDOS and the corresponding band structures, illustrating large differences in the band gaps between the spin-up and spindown channels. (b,d) LDOS of C-2p, O-2p, and M-3d (M = Fe and Ni), showing main contributions of the VBM and CBM at the Fermi level.

Figure 4. Spin-resolved TDOS, electronic band structures, and LDOS of the AFM hybrid node Ni−Fe-MOF-74 at hybridized amounts of N = 1 and 2. (a,c) Computed TDOS and the corresponding band structures for showing loss of symmetry between spin-up and spin-down components upon Ni hybridization. The corresponding LDOS (b,d) shows Ni-3d bands emerging at energies near the Fermi level.

D

DOI: 10.1021/acs.jpcc.6b09175 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 5. Comparison of the DOS of a nanowire model containing (a) an Fe−Fe−Ni chain at N = 2 and (b) an Fe−Ni−Ni chain at N = 3 to illustrate the band gap variation induced by different hybridization amounts along the metal oxide direction.

intrachain hybrid causes two separated Fe−Fe−Fe and Ni− Ni−Ni chains, it results in little shift of the VBM, and therefore it has little effect on the spin-down gap. The small band gap for the spin-down channel suggests electrical control of the spin-polarized current by adding a small gate voltage. The easily induced single-spin current renders the Ni−Fe hybrid node MOFs possible as potential electrodes in suitable sandwich devices. This is why we name these materials with a single-spin band gap as MMS. The AFM state exhibits symmetric electronic bands for pure Fe-MOF-74 (SI Figure S4), while the hybrid Fe-MOF-74 containing Ni atoms would obviously destroy such symmetric electron densities, as shown in Figure 4. The hybrid spin-down Ni-3d band at N = 1 is above the Fermi level at 0.95 eV, causing conversion of the total spin density from symmetric to single polarization with all electrons being spin-down. In this case, the residual total magnetization is ±|1.93| μB rather than 0, and the electronic band structure is similar to that in the FM state. The same situation is also seen when increasing the Ni hybrid content to N = 2. In this case, the Ni-3d band appears above the Fermi level at 0.80 eV for both spin orientations, which causes both band gaps to narrow to 0.80 and 0.90 eV for the spin-up and spin-down orientations, respectively. When the Ni hybrid content is increased further to N = 3, the spin-up Ni-3d band shifts to a lower energy and results in a smaller band gap of 1.17 eV, compared to that of spin-down (1.40 eV). Such a spin polarization variation caused by difference in the

hybridization content is expected to allow tuning of the spin gap to alter spin currents. MMS in Nanowire Ni−Fe-MOF-74. In 3D porous MOFs, the magnetic ordering and charge mobility exhibit significant direction anisotropy along the metal oxide chain. To evaluate whether the electronic properties along the metal oxide chain of Fe−Ni−O exhibit similar MMS properties upon hybridization with differing Ni content, we constructed a 1D nanowire Ni− O−Fe at hybridization ratios of N = 2 and 3 to study their electronic properties near the Fermi level (Figure 5). Each nanowire model consists of a metal oxide chain truncated with benzene so that the nanowire is a metal oxide−metal (M−O− M) chain. Similar to the 3D porous model, the 1D nanowire of the metal oxide chain in Fe-MOF-74 could reduce the band gaps to achieve MMS by the Ni hybridization method. The total DOS of the two nanowires at N = 2 and 3 indicates that Fe−Fe−Ni hybrid chain ordering still plays an important role in narrowing the spin-down gap. As shown in the LDOS (Figure 5a), the Ni1-3d band (CBM) at about 0.50 eV contributes to the reduced band gap for the spin-down channel, while the VBM still depends on the Fe-3d band. As far as the Fe−Ni−Ni chain is concerned, although the CBM of the Fe− Ni−Ni chain consists of a Ni-3d band, its location is shifted toward higher energy, increasing the band gap to 1.12 eV for the spin-down channel. The different Ni-3d band location illustrates that the existence of the Fe−Fe−Ni chain mode is a key factor to the decrease of the spin-down gap in Ni−FeMOF-74, suggesting that the Ni hybridization content should E

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nanowire of a Ni−Fe chain to evaluate the electronic features along the metal oxide chain. By computing the work function for pure Ni-MOF-74, we find that pure Ni-MOF-74 can be a good visible light photocatalyst with a band gap of 2.10 eV. These results are useful for seeking new applications of porous MOFs in the field of spintronics and photocatalysis.

be no more than 50% (N = 3) in one unit cell. Here, we should notice that short-range 1D magnetic order at the nanometer scale is possible, based on the substrate or kinetics proposed by Yang, although the long-range magnetic order is prohibited at a finite temperature on the spin-lattice.1 Photocatalytic Activity. In addition to possible applications in spintronics, we also predict good photocatalytic activity of M-MOF-74.23 Note that it is a demanding task to compute the work function of porous materials using traditional methods. Recently, Butler et al. developed a new methodology for evaluation of the work function of MOFs with pore radii larger than 5 Å.20 In general, the pore size of MOFs ranges from 2 to 50 Å.24 In the series M-MOF-74, the pore size is near 10 Å, similar to those of MOF-5 and HKUST.25,26 As shown in Figure 6, the work function is computed for the Ni−Fe hybrid

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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.jpcc.6b09175. Structures of the Ni−Fe hybrid node of MOF-74 at N = 1−4, lattice parameters for the FM and AFM Ni−FeMOF-74, spin density of Ni ions hybridized into an interchain node at N = 3, PDOS for Ni1 atoms in an Fe− Fe−Ni chain and Ni2 atoms in an Fe−Ni−Ni chain, and PDOS and TDOS for AFM Fe-MOF-74 and FM NiMOF-74 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Q.Z.). *E-mail: [email protected] (L.C.). *E-mail: [email protected] (X.C.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support of the National Key Basic Research Program of China (Grant No. 2013CB934800), the National Science Foundation of China (Grant No. 11274323, 51472254), the NSF of Zhejiang Province (LR14E020004), and the program for Ningbo Municipal Science and Technology Innovative Research Team (Grants 2015B11002 and 2016B10005). We also thank computation support by the University of Nebraska Holland Computing Center.

Figure 6. Computed work function of MOFs for evaluation of possible photocatalysis of H2O. The vertical blue dashed lines refer to the computed work functions, that is, 5.44, 4.63, 5.05, and 6.35 eV for Fe-, 2Ni−Fe-, 3Ni−Fe-, and Ni-MOF-74, respectively.

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node of MOF-74 and pure Fe- and Ni-MOF-74, utilizing the methodology of Butler et al. Unexpectedly, we found that pure Ni-MOF-74 is predicted to exhibit photocatalytic properties as its CBM is higher than the H+/H2 level and its VBM is lower than the O2/H2O energy level. The band gap of 2.10 eV for NiMOF-74 is also suitable for some visible light photoactivity as the photogenerated electrons could be transferred to H+/H2 to reduce H+ into H2 and oxidize OH− into O2.

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CONCLUSIONS Electronic structures of different M−Fe-MOF-74 hybrids (M = Ni or Co) are computed using the HSE06 hybrid functional method. We name the hybrid node Ni−Fe-MOF-74 a MMS in view of its potential as a new kind of porous material due to the ease with which it can be converted to a half-metal through a small gate voltage. On the basis of a systematic study of the effects of Ni content and the hybrid node arrangement, we find that the Ni−Fe hybrid node is a suitable candidate to narrow the band gap of spin-down polarization to ∼0.62−0.91 eV when Ni−Fe−Fe arrangement is employed in the same chain with a Ni content of 33−50% (N = 2 and 3 in one unit cell). The hybrid node MOFs are expected to provide an effective way of narrowing the band gap to promote easy conversion of MOFs to polarization materials. We also constructed the 1D F

DOI: 10.1021/acs.jpcc.6b09175 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.6b09175 J. Phys. Chem. C XXXX, XXX, XXX−XXX