Chemical Insights from Theoretical Electronic States in Nickel

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Chemical Insights from Theoretical Electronic States in Nickel Hydroxide and Monolayer Surface Model Yuki Sakamoto, Yusuke Noda, Kaoru Ohno, and Shinichiro Nakamura J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07564 • Publication Date (Web): 27 Sep 2017 Downloaded from http://pubs.acs.org on October 4, 2017

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The Journal of Physical Chemistry

Chemical Insights from Theoretical Electronic States in Nickel Hydroxide and Monolayer Surface Model Yuki Sakamoto,†,‡,§ Yusuke Noda,‖ Kaoru Ohno,⏊ Shinichiro Nakamura†,‡* †

Research Cluster for Innovation, Nakamura Laboratory, RIKEN, 2-1, Hirosawa, Wako, Saitama

351-0198, Japan ‡

Computational Chemistry Applications Unit, Advanced Center for Computing and

Communication, RIKEN, 2-1, Hirosawa, Wako, Saitama 351-0198, Japan §

Department of Biological Information, Tokyo Institute of Technology,

4259 Nagatsuta, Midori-ku, Yokohama 226-8501, Japan ‖

Center for Materials research by Information Integration (CMI2),

National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan ⏊

Department of Physics, Graduate School of Engineering, Yokohama National University, 79-5

Tokiwadai, Hodogaya, Yokohama 240-8501, Japan

ACS Paragon Plus Environment

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Abstract

We performed the density functional calculation of bulk β-Ni(OH)2 and monolayer Ni(OH)2 as a surface model to investigate their chemical features. The Nickel hydroxide (Ni(OH)2) and Nickel oxyhydroxide (NiOOH) redox pair is one of the most attractive materials for electrocatalytic oxygen evolving reaction (OER) with low overpotential. However, its reaction mechanism has not yet been fully understood. Their band structures and visualized pseudo charge density contributions indicate strong hydrogen interaction between the layers in the βNi(OH)2. In the experimental electrocatalytic OER reaction, the Fe atom is required; the structure is called NiFe-Layered Double Hydroxide (LDH). We report the Fe effect in the monolayer Ni(OH)2 framework as the surface model of NiFe-LDH. The Fe atom is suitable for receiving electrons from substrate waters. This study provides crucial basic information for the OER mechanism on the surface of Ni(OH)2/NiOOH redox pairs.


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Introduction Natural plants use manganese atoms as a water splitting catalyst in their photosynthesis1. For future artificial photosynthesis, it is important to realize the artificial catalyst of the oxygen evolving reaction (OER), composed of first transition metals such as manganese, iron, cobalt, and nickel. Among these materials, nickel hydroxide Ni(OH)2, and nickel oxyhydroxide NiOOH redox pairs (referred to as NiOxHy hereafter) are currently one of the most attractive materials that may enable OER with low overpotential2-6. NiOxHy is widely used as an alkaline battery electrode. The battery using NiOxHy is known as Ni-MH (M refers to adequate metals) battery and is currently utilized in various applications such as hybrid cars due to its safety and reliability7. Since the 1960’s, the properties of NiOxHy have been studied by a number of researchers. Examples include Bode’s studies on the phase transition of NiOxHy 8-9. The scheme is known as Bode’s scheme (Scheme 1). According to the Bode’s scheme, there are four phases of Ni(OH)2/NiOOH redox pairs: αNi(OH)2, β-Ni(OH)2, γ-NiOOH, and β-NiOOH. Among them, only β-Ni(OH)2 is structurally determined including proton positions through neutron diffraction experiments10. Various physical properties obtained from density functional theory (DFT) calculations were reported by Hermet et al11. The structure of β-NiOOH is ambiguous due to the lack of critical information such as stacking pattern and positions of coordinated protons. Thus, several models of β-NiOOH are proposed by experiments12 and theoretical calculations13-15. Recently, Conesa reported DFT calculations in which the energies of different β-NiOOH configurations have very similar values which cannot be distinguished by experiments16. The α-Ni(OH)2 and γ-NiOOH show large intersheet distances due to the intercalation of water molecules and/or various ions17. Since these


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crystals have relatively low crystallinities, their chemical natures which determine the performance of the catalyst are mostly unrevealed. In 1980’s, Corrigan et al. reported their study on the OER occurring on the NiOxHy battery during charging3,

18-19

. In battery usage, this reaction is harmful and thus prevented through

numerous devices. After Corrigan’s report, a number of studies focusing on OER efficiency and mechanism have been published. Trotochaud et al. provided important information for understanding the OER mechanism of NiOxHy redox pairs in 201420. Before this work, the βNi(OH)2/β-NiOOH redox pair was considered as an active phase for OER. The fact that iron impurities enhance OER is known, however, it is not considered to be an essential part for the reaction. Trotochaud et al. proved the following points; (i) iron is indispensable for OER with low overpotential through their experiments with the rigorous control of iron concentration, and (ii) Ni/Fe-layered double hydroxide (LDH), whose distance between the layers is larger than βNi(OH)2, is observed in the OER active phase. It is also reported that exfoliated NiFe-LDH enhances the water splitting catalysis21. There are several experimental studies on its electronic and structural properties22-25. Theoretical studies focusing on electronic structures14, 16, 26-28 and reaction mechanism29-32 have also been published. Recently, Fidelsky et al. discussed the roles of Fe atoms and H vacancies for low overpotential through theoretical calculations31-32. Previously, we investigated the electronic states in various MnO2 crystals33. We recognized the fact that the information obtained from band structure calculations are more useful for chemical insights than those currently accepted34. We here extend work focusing on nickel hydroxides explicitly considering the magnetic structures. In the present study, we performed first principles calculations on the β-Ni(OH)2 and monolayer Ni(OH)2 to take into account large inter-sheet


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distances. This paper discusses its properties by comparing the bulk β-Ni(OH)2 and Fe inserted sheet. Since the degree of deprotonation for NiOxHy during the OER is unknown, we focused on the most simple and pristine structures of NiOxHy, that is Ni(OH)2 without any H-vacancy. The remaining part of this paper is organized as follows. We describe the Computational Details used in this work. In Result and Discussions, first, we discuss the magnetic configuration and band structures of bulk Ni(OH)2, then we compare the band structures with monolayer Ni(OH)2. In the last part, we discuss the effects of iron substitution in the Ni(OH)2 monolayer, and end with a conclusion. We believe that this study provides crucial basic information for clarifying the OER mechanism on the NiOx surface.

Scheme 1. Bode’s scheme describing phase transition of Ni(OH)2 / NiOOH redox pairs.


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Computational Details We performed DFT calculations with ultrasoft pseudopotential35 implemented on Quantum ESPRESSO36

program

package.

The

Perdew-Burke-Ernzerhof

generalized

gradient

approximation (PBE)37 functional and Hubbard U correction38 (5.5 eV for Ni and 3.3 eV for Fe29) are used. The cutoff energies for wave function and charge density are set at 40 Ry and 400 Ry, respectively. The magnetic moment of each Ni atom is initially set at ± 2.00 µB corresponding to the low spin state of Ni(II). The geometry of the bulk β-Ni(OH)2 primitive cell is made referencing the neutron diffraction structure reported by Kazimirov et al10. 1×1×2 super cells are used for the antiferromagnetic A type39 (AAFM: the intralayer coupling is ferromagnetic (FM) while the interlayer coupling has an antiferromagnetic (AFM) configuration). The geometry of the monolayer Ni(OH)2 is made from primitive cells with 15 Å vacuum region. The geometry of Ni0.75Fe0.25(OH)2 monolayer is made through 2×2×1 replicated super cell of monolayer Ni(OH)2. We carried out geometrical optimizations with the unit-cell relaxations. For the k-points, we used a Monkhorst-Pack grid40 with 12×12×8, 12×12×4 and 12×12×1 for the Ni(OH)2 primitive cell, AAFM configuration Ni(OH)2, and monolayer Ni(OH)2 models, respectively. For the monolayer Ni0.75Fe0.25(OH)2, the 6×6×2 Monkhorst-Pack grid was used, since the self-consistent calculations during the geometrical optimizations were unstable with that of 6×6×1 grid. For the density of states (DOS) and partial DOS calculation, three times as many numbers of kpoints along each axis except the vacuum layers were used for sampling. All symmetry k-points and symmetry k-lines in the Brillouin zone are determined by automatic flow (AFLOW)

41-42

.

The structures and surfaces are visualized with VESTA43.


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Results and discussions Band structure of β-Ni(OH)2 and interaction between layers We calculated the band structures of β-Ni(OH)2. We assumed the FM and AAFM configuration, since there are experimental reports on the presence of FM couplings in each layer in the β-Ni(OH)244. On the other hand, Ni(OH)2 also have metamagnetic properties in which the degree of magnetization changes depending on the temperatures45. The comparison of these magnetic configurations is presented in section S1 of the supporting information (SI). Band structure calculations were performed with the magnetic primitive cells (1×1×1 primitive cells for the FM configuration and 1×1×2 replicated super cells for the AAFM configuration). The relaxed lattice constants from our geometrical optimizations were a = 3.183 Å, c = 4.616 Å for FM configuration, and a = 3.182 Å, c = 9.225 Å for AAFM configuration. The results are shown in Figure 1 and Figure 2 for FM and AAFM configurations, respectively. According to the results of PBE+U calculation, the band structures in the FM calculations were split into the up and down spins. On the other hand, these of the AAFM configuration were degenerated. The energy of FM in the 2×2×2 super cells is 0.01 eV lower than that of AAFM configuration (see S1 in detail). This small energy difference indicates that the magnetic interaction between the Ni(OH)2 layers is weak. Thus, it is natural that β- Ni(OH)2 shows metamagnetic property. In the FM configuration, the valence band maximum (VBM) of the up-spin band is located on the M point, while the conduction band minimum (CBM) is located on the Γ point in the reciprocal space. The O-2px and O-2py orbitals are degenerated and involved in the valence bands near the Fermi level. On the other hand, the O-2pz orbitals lie at the lower energy level than O-2px and O-2py orbitals (shown in Figure 1 (a)). As for the two conduction bands with small energy dispersion (Figure 1 (b)), they are clearly consisting of the down spin 3d bands


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localized on Ni (Figure 1 (a)). These 3d bands consist of eg orbital of Ni(II) atom, to be discussed later in Spin Distribution. The calculated band gap is 2.91 eV. As for the AAFM configuration, VBM is located on the area from the Γ to A points, which is almost degenerated with the M point. While CBM is on the point between the Γ and M points with the band gap of 3.02 eV. Although there are some differences such as band gap between FM and AAFM configuration, the overall band structures are similar. From these observation, in β-Ni(OH)2, the magnetic and electrostatic interaction between the layers are weak. Therefore, in order to understand the chemical features relating to the OER activity, it is relevant to analyze just one layer of Ni(OH)2.

Figure 1. (a) Partial DOS and (b) band structures of β-Ni(OH)2 with FM configuration. The Fermi energy is set to VBM. Note that the localized DOS of 2p orbitals of oxygen in (a) are decomposed into 2pz and (2px + 2py). Up and down-spin bands colored in blue and red respectively are shown in (b).


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Figure 2. (a) DOS and (b) band structure of β-Ni(OH)2 with AAFM configuration. In (a), note that Ni1, O1 and O2 are located in the same layer, while Ni2, O3, and O4 are located in the other layer in the 1×1×2 replicated super cells. Up and down-spin bands colored in blue and red respectively are shown in (b).

Band structure of monolayer Ni(OH)2 A clue to understanding the mechanism of OER may be found in the peculiar geometry of the Layered Double Hydroxide (LDH) structure in which the Ni(OH)2 surfaces are exposed effectively. In order to investigate the chemical nature of the Ni(OH)2 surface, we then calculated the band structure of monolayer Ni(OH)2 (two-dimensional sheet) as a surface model. To compare the band structures with that of bulk β-Ni(OH)2 fairly, we assumed that Ni atoms coupled ferromagnetically in the sheet. Previously, the details of the magnetic configuration in the monolayer were reported theoretically by Tang et al46. Their focus is on the relation between biaxial strain and band gap or magnetic couplings in the ground state. We study chemical and catalytic properties. The DOS


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and band structures produced by our PBE+U calculations are shown in Figure 3. The results show that both the VBM and CBM are on the Γ point, having the band gap of 1.98 eV. Since this band gap is narrower than that of FM (Figure 1(b)) and AAFM (Figure 2 (b)) by 1 eV, the electrical conductivity of monolayer Ni(OH)2 may be higher than that of β-Ni(OH)2. On the other hand, the band structure on the Γ-M-K-Γ path is similar to the FM (Figure 1 (b)) and AAFM (Figure 2 (b)). In real space, the path corresponds to intra-sheet area, the similarity suggest that the inter-sheet interaction does not much influence on the electronic structures in the Ni(OH)2 layers. As a result, the magnetic interaction as well as electrostatic interaction between the layers of β-Ni(OH)2 are weak.

Figure 3. (a) DOS and (b) band structure of Ni(OH)2 monolayer with FM configuration. Note that the localized DOS of 2p orbitals of oxygen in (a) are decomposed into 2pz and (2px + 2py). Up and down spin bands respectively colored in blue and red are shown in (b).


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Frontier orbitals of β-Ni(OH)2 and monolayer Ni(OH)2 In an attempt to locate the critical area of OER, we focus on the frontier orbitals. Here, let us compare the electronic structures in the bulk and monolayer systems. We discuss common and different points qualitatively through the molecular orbital (crystal orbital) theory, putting on the attention of topological features of orbitals. In both models, the two valence bands exist with degeneration at the Γ-point. These pseudo wave functions at the Γ-point are shown in Figure 4. According to the pseudo wave functions (shown in Figure 4 (a) and (b)) and partial DOS (shown in Figure 1 (a) for bulk and Figure 3 (a) for monolayer system), valence bands in both models mainly consist of the O-2px, 2py and Ni-3d bands.

Figure 4. Absolute square of pseudo wave function of the valence band with the highest energy at Γ-point in (a) β-Ni(OH)2 (FM configuration) and (b) Ni(OH)2 monolayer. The iso-values of these surface are set to ±0.003 (a.u.)-3. The color of the surface shows the phase of the pseudo wave function.

It is also crucial to discuss the conduction bands, since they must play a role in accepting electrons from water or hydroxide ions during the OER. In both models, there are two narrow


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down spin bands above the band gap. As shown in the DOS (Figure 1 (b) for bulk and Figure 3 (b) for monolayer system), they are assigned to the Ni-3d and O-2p orbitals. Other bands which are not assigned to the Ni-3d orbitals and have large dispersion along with k-points, are almost degenerated of the up and down spins as shown in Figure 1 (b). These pseudo wave functions (β-Ni(OH)2, up spin bands) are shown in Figure 5. The corresponding wave functions of monolayer Ni(OH)2 are shown in Figure 6. These unoccupied orbitals of monolayer Ni(OH)2 are clearly more localized than that of β-Ni(OH)2. This difference of localization reflects the difference of the conduction band dispersion between the bulk and monolayer β-Ni(OH)2.

Figure 5. Absolute square of pseudo wave function of the lowest two up spin conduction bands of β-Ni(OH)2 at the Γ-point. The energy levels at the Γ-point are 2.840 eV in (a) and 7.073 eV in (b). The isovalues are set at 0.003 (a.u.)-3.


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Figure 6. Absolute square of pseudo wave function of lowest two up spin conduction bands of monolayer Ni(OH)2 at Γ-point. The energy levels at the Γ-point are 1.895 eV in (a) and 2.462 eV in (b). The isovalues are set at 0.003 (a.u.)-3.

Orbital interaction between layers As we discussed earlier, although the interaction between layers is weak, there are some orbital interactions. In the hexagonal system, the orbital interactions between layers are reflected on the Γ to A point in the band structures, such as the conduction bands of β-Ni(OH)2. We notice that there are four valence bands changing their relative energy to a large extent from the Γ to A point in the FM β-Ni(OH)2 band structure (Figure 1 (b)). Two of them are up spin bands and the others are down spin bands. Similar bands which show a remarkable energy change can also be observed in the AAFM configuration (Figure 2 (b)). The wave functions of these bands at the Γ point are shown in Figure 7. The DOS (Figure 1 (a)) and the visualized pseudo wave functions (Figure 7 (a) and (b)) indicate that these bands are mainly contributed by the O-2pz and H-1s orbitals. These visualized wave functions, in-phase as well as out-of-phase interactions of the OH groups, indicate that there is only weak covalent interaction between neighboring Ni(OH)2 layers. On the other hands, for the monolayer Ni(OH)2, the corresponding orbitals are also found, although there are no neighboring layers to interact


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with. These orbitals are shown in Figure 8. There are possibilities that these orbitals participate in the interaction with reactant water molecules at the surface. There exists the same hexagonal crystal system in Ca(OH)2, while there are no d-electrons or spin polarization47. There are no reports of spin related OER catalytic activity as far as we know. For comparison, we carried out the band structure calculation for Ca(OH)2 as shown in Figure S2. The result also contains similar bands that spread in the range of -6 to -4 eV along the Γ-A path. It is reported that there are strong hydrogen bonds (for which no d orbitals are necessary) between the Ca(OH)2 sheets48. Thus, we can infer that there is a similar hydrogen-bond interaction as in the β-Ni(OH)2. This is a characteristic of β-Ni(OH)2 as well as Ca(OH)2. Since this hydrogen-bond interaction does not exist in the monolayer Ni(OH)2, α-Ni(OH)2, and NiFeLDH, due to the large distance between the sheets, this hydrogen-bond interaction may mainly maintain crystal stability and be less related with the catalytic reactivity.

Figure 7. Absolute square of pseudo wave function of FM Ni(OH)2 up-spin valence bands consisted of O-2pz orbitals at the Γ point. The energy levels at the Γ point are -8.924 eV in (a) and -4.361 eV in (b). The isovalues are set to ±0.003 (a.u.)-3.


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Figure 8. Absolute square of pseudo wave function of monolayer Ni(OH)2 up-spin valence bands consisted of O-2pz orbitals at the Γ point. The energy levels at the Γ point are -8.489 eV in (a) and -5.344 eV in (b).

Spin distribution The spatial distribution of the spin polarization, the difference between the number of up-spin and down-spin electrons (ρ(α)-ρ(β)), for bulk β-Ni(OH)2 and monolayer Ni(OH)2 are shown in Figures 9 (a), (b), and (c). In all the calculations, the spin densities are distributed along the Ni-O bonds centered on the Ni atoms. In order to understand this feature, we employed the Löwdin charge analysis. The results on Ni atoms are summarized in Table 1. The Löwdin charge analysis revealed that the amount of spin polarization of Nickel atoms are 1.71 in these three models. Thus, the Ni atoms in these models can be considered as the Ni(II) oxidation state. Moreover, these polarizations are contributed by d orbitals. Therefore, these spin distributions are attributed to the eg orbitals of Ni atoms in the octahedral crystal field splitting (Scheme 2).


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Figure 9. Spin distribution of (a) β-Ni(OH)2 (FM), (b) β-Ni(OH)2 (AAFM), and (c) monolayer Ni(OH)2. The isovalues of the spin densities are set at 0.01 (a.u.)-3. The isosurfaces plotted in yellow and blue mean the up and down spins, respectively.

Table 1. Löwdin charge analysis of Ni in each calculated model.

4s (up/dn)

FM

AAFM1

monolayer

0.25/0.25

0.25/0.25

0.25/0.25

0.25/0.25 4p (up/dn)

0.00/0.00

0.00/0.00

0.00/0.00

0.00/0.00 3d (up/dn)

4.97/3.27

4.97/3.27

4.97/3.26

3.27/4.97 Total (up+dn)

8.74

8.74

8.73

8.74 Poralization (up-dn)

1.71

1.71

1.71

-1.71 1

The β-Ni(OH)2 with AAFM configuration contains two Ni atoms in a cell.


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Scheme 2. Formal electronic configurations of Ni(II), Fe(III), Fe(II), Mn(III) and Mn(IV) in the octahedral crystal field.

Effect of iron atoms for spin distribution Finally, we discuss the catalytically important effect of Fe atoms in the Ni(OH)2 sheet, since experimentally it is reported that the actual water oxidation reaction occurs on the surface of the NiFe-LDH. We assumed that the ratio of the amount of Fe to Ni is 1:3 (Ni0.75Fe0.25(OH)2) in our calculation models, as shown in Figure 10 (a). For the sake of simplicity, we assumed that all the metals coupled ferromagnetically. We examined both models of the sheet to be neutral and positive (Fe(II) in Figure 10 and Fe(III) in Figure 11). The calculated DOS in the neutral model is shown in Figure 10(b). The pseudo wave functions at the Γ point near the Fermi level are described in S4.


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Figure 10. (a) Calculated model of Ni0.75Fe0.25(OH)2 sheet. Atoms colored in silver, gold, red, and white are Ni, Fe, O and H, respectively. (b) DOS of Ni0.75Fe0.25(OH)2 sheet.

The DOS shows that there are down spin bands below the Fermi energy level which mainly consist of Fe-3d orbitals. According to the Löwdin charge analysis, the number of electrons assigned to the 4s, 4p and 3d orbitals of the Fe atom are 0.45, 0.00 and 6.25, respectively (summarized in Table 2). Since the Fe(II) atom contains about six electrons in the 3d orbital, it should contain down spin electrons as well. We then calculated the positively charged model of the Ni0.75Fe0.25(OH)2 sheet ([Ni0.75Fe0.25(OH)2 ]+). Since in the actual NiFe-LDH, there are number of various intercalated


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anions such as CO3-, OH-, this model must be also considered (initial geometry is the same as Figure 10 (a)). The DOS is shown in Figure 11. The pseudo wave functions at the Γ point near the Fermi level are described in S5. The DOS indicates that the Fe-3d bands with down spin electrons are above the band gap (as shown by a red arrow in the DOS (Figure 11(a))). The Löwdin charge analysis of the Fe atom is shown in Table 2. Compared with the neutral model, the number of electrons with down spin in the Fe-3d orbital decreased. The change of the frontier orbitals also reflects these observations. The highest down-spin valence band in neutral Ni0.75Fe0.25(OH)2 and the lowest down-spin conduction band in positively charged model at the Γ point are visualized in Figure 12 (a) and (b), respectively. These orbitals clearly consist of the same Fe-3d orbitals. From these observations, the Fe-3d bands in Ni0.75Fe0.25(OH)2 behave like impurity energy levels in the semiconductors bandgap. The unoccupied Fe-3d bands in [Ni0.75Fe0.25(OH)2]+ indicate that the Fe sites can accept electrons which are necessary for H2O oxidation reaction. From another point of view, we noticed that the NiFe-LDH can be considered as the combined structure of Ni(OH)2 and Fe(OH)2, because the crystal system of Fe(OH)2 is the same as Ni(OH)2. We calculated the electronic structures of Fe(OH)2 with the same methods. The calculated DOS and the band structure are shown in Figure S8. These results indicate that two down spin bands, consist of Fe-3d orbitals, exist around the Fermi energy. Therefore, in the Ni0.75Fe0.25(OH)2, the Fe-3d bands can be considered to behave as impurity levels in the Ni(OH)2 band framework. These observations indicate that the Fe atom rather than Ni may be reaction center during the water splitting reaction. DOS of neutral system, Figure 10, shows 3d occupied impurity level at the valence band top and that of charged system, Figure 11, shows 3d unoccupied impurity level. The existence of this


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level is very important to favor the initial reaction to split water molecule into OH- and H+ and emit proton, because the neutral Fe(OH)2 i.e. the Fe(II), has a tendency to at least weakly attract another OH- to become Fe(OH)3, i.e. the Fe(III). This reaction cannot occur for pure Ni system because these is no such impurity level. The latter state Fe(III) is obviously energetically favorable than the former state Fe(II), which can be seen in the Scheme II.

Figure 11. The DOS of [Ni0.75Fe0.25(OH)2]+ sheet.

Table 2. Löwdin charge analysis of Fe atoms. Fe in [Ni0.75Fe0.25(OH)2]

Fe in [Ni0.75Fe0.25(OH)2]+

4s (up/dn)

0.23/0.21

0.24/0.23

4p (up/dn)

0.00/0.00

0.00/0.00

3d (up/dn)

4.96/1.29

4.94/0.87

Total (up+dn)

6.70

6.28

Polarization (up-dn)

3.68

4.08


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Figure 12. (a) Absolute square of pseudo wave function of (a) the valence band with the highest energy at Γ-point in Ni0.75Fe0.25(OH)2 and (b) the conduction band with the lowest energy at Γpoint in [Ni0.75Fe0.25(OH)2]+. The iso-values of these surface are set to ±0.003 (a.u.)-3. The color of the surface shows the phase of the pseudo wave function.

Figure 13. The spin distribution of (a) Ni0.75Fe0.25(OH)2 and (b) [Ni0.75Fe0.25(OH)2]+. The isovalues of the spin densities are set at 0.01 (a.u.)-3.

As for the spin, its distribution around the Fe atom is distinct when compared with that of Ni atom (Figure 9 (c)), as shown in Figure 13(a) or (b); the spin densities distribute spherically around the Fe nuclear. This observation is consistent with the following arguments. Since Ni(II) usually takes the d8 state, the total magnetic moments are due to the single occupied eg orbitals. On the other hand, Fe(II) or Fe(III) usually prefers the high spin d6 or d5 state (see Scheme 2).


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Thus, the t2g orbitals, in addition to the eg orbitals, are also half occupied to form the open shell state. The elevated population of open shell sites most probably play an important role for reactions from singlet reactant (H2O) to triplet product (O2). In the natural photosynthesis, four manganese atoms are utilized in the oxygen evolving center (OEC). According to a number of experimental and theoretical calculations, these manganese atoms prefer either the Mn(III) or Mn(IV) oxidation state1, 49. Since the Mn(III) and Mn(IV) take the high-spin d4 (three single occupied t2g orbitals and one single occupied eg orbital) state and the d3 (three single occupied t2g orbitals) state, the spatial spin densities also distribute spherically around the manganese nuclear (Scheme 2). This feature is somehow common in Fe atoms in the Ni(OH)2 framework. Summarizing above discussions, the Ni0.75Fe0.25(OH)2 systems have following remarkable chemical features. (i)

The Ni(OH)2 framework has semiconductor properties.

(ii)

The electronic structures of Ni(OH)2 is similar in the β-phase and monolayer, except the results of weak covalent interaction. The experimental results, that LDH is active phase for OER, may be closely related to the accessibility of waters.

(iii)

The Fe doping generates the impurity energy level into the Ni(OH)2 bandgap. This energy level may have a very important role of attracting OH- and deprotonation.

(iv)

The spin distribution around Fe is spherical, but that of Ni is not. These spatial features due to the half occupied t2g orbitals of Fe may be relevant for the OER activity.

Recently, the OER activities of Ni(OH)2 doped with other metals50 is reported. The effect of the bandgap, impurity levels and spin distributions of systems to OER activities will be an important future subject.


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Conclusion We report the study on the electronic properties of the bulk β-Ni(OH)2 and monolayer Ni(OH)2 using first principles calculations. The comparison of the two systems is presented from the point of view of the molecular orbital; the pseudo charge density contribution from both valence and conduction bands, and the orbital interaction between the Ni(OH)2 sheets. In the experimental electrocatalytic water splitting catalyst, the Fe atoms are necessary in the Ni(OH)2 framework. The electronic structure calculation of Ni0.75Fe0.25(OH)2 sheets show that the Fe may play the role of receiving electrons from reactants waters. High spin may be needed for efficient reaction from H2O (singlet) to O2 (triplet). Study on the details of the mechanism is ongoing. We believe that the current results provide a clue to understanding the detailed mechanism of OER.


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ASSOCIATED CONTENT Supporting Information. Comparison of magnetic configuration of β-Ni(OH)2, frontier orbitals of Ni0.75Fe0.25(OH)2, and the band structure of Ca(OH)2 and Fe(OH)2. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Phone: +81-048-467-9477. Fax: +81-048-467-8503. Email: [email protected] (S.N.) ACKNOWLEDGMENT We thank HOKUSAI Greatwave of RIKEN for computational resources.

ABBREVIATIONS OER, oxygen evolving reaction; DOS, density of states; LDH, Layered Double Hydroxide. REFERENCES 1.

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(Summary) The electronic structure of β-Ni(OH)2, monolayer Ni(OH)2 and monolayer Ni0.75Fe0.25(OH)2 were investigated through the DFT calculation with the planewave basis set. Table of Contents Graphic


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Chemical Insights from Theoretical Electronic States in Nickel

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