Theoretical and Experimental Study on MIIMIII-Layered Double

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The Journal of Physical Chemistry

Theoretical and Experimental Study on MΙΙΜΙΙΙ-Layered Double Hydroxides as Efficient Photocatalyst toward Oxygen Evolution from Water

Si-Min Xu, Ting Pan, Yi-Bo Dou, Hong Yan,* Shi-Tong Zhang, Fan-Yu Ning, Wen-Ying Shi, Min Wei*

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, 100029, Beijing, P. R. China

*Corresponding authors: [email protected] (H. Yan); [email protected] (M. Wei). Telephone: +86-010-64412131 (M. Wei).

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Abstract Recently, layered double hydroxides (LDHs) have attracted extensive attention in the field of energy storage and conversion, and a deep understanding of their semiconducting properties is rather limited. In this work, the electronic properties (band structure, density of states (DOS), band edge placement) of MΙΙΜΙΙΙ-LDHs (MΙΙ = Mg, Co, Ni and Zn; ΜΙΙΙ = Al and Ga) were studied in detail. The thermodynamic mechanism toward oxygen evolution reaction (OER) was investigated by using the density functional theory plus U (DFT + U) method. The calculation results of band structure indicate that Mg and Zn-based LDHs (band gap energies larger than 3.1 eV) are ultraviolet responsive while Co and Ni-based LDHs are responsive to visible light (band gap energies less than 3.1 eV). The DOS calculations reveal that the photogenerated hole localizes on the surface hydroxyl group of LDHs, facilitating the oxidization of water molecule without a long transportation route. The band edge placements of MΙΙΜΙΙΙ-LDHs show that NiGa-, CoAl-, ZnAl- and NiAl-LDHs have a driving force (0.965 eV, 0.836 eV, 0.667 eV and 0.426 eV, respectively) toward oxygen evolution. However, the thermodynamic mechanism of these four LDHs reveal that only CoAl-LDH can overcome the reaction barrier (0.653 eV) via the driving force of photogenerated hole (0.836 eV). Experimental observations of MgAl-, CoAl- and ZnAl-LDHs further prove that only CoAl-LDH is an efficient oxygen evolution photocatalyst (O2 generation rate: 973 µmol h−1 g−1), agreeing well with the theoretical prediction. Therefore, this work provides an effective theoretical and experimental combined method for screening possible photocatalysts, which can be extended to other semiconductor materials in addition to LDHs.

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1. Introduction Solar energy conversion to clean fuels by splitting water into hydrogen and oxygen has received considerable attention since Fujishima and Honda’s first water-splitting system based on TiO2.1 To achieve efficient hydrogen production by water splitting, much effort has been devoted to the design of highly-efficient, durable and earth-abundant oxygen evolution reaction (OER) photocatalysts with low over-potential. Although metal oxides (TiO2,2−3 ZnO,4 Fe2O3,5 WO3,6 KTaO37 and SrTiO38) have mostly been investigated as semiconductor photoanode candidates for the OER, they normally are UV light-responsive (e.g., TiO2) or require an additional voltage bias for the OER operation (e.g., Fe2O3).9,10 Since the efficiency of a photocatalyst is determined by its electronic band gap energy, electron-hole pair lifetime, and charge mobility,10 introducing other elements (e.g., transition metals4,5,6,8 or nitrogen7) has been intensively performed to adjust the band gap so as to spontaneously enable visible light-responsive OER activity.11 However, a uniform doping into the original semiconductor at the atomic level is still beyond the state of the art; in addition, impurity states are often introduced into the forbidden zone by doping,12 which facilitates the decay of excited state and thus induces an unexpected recombination of hole and electron. Therefore, the development of new materials or methodologies for obtaining high-performance visible-light photocatalysts toward the OER remains a challenging goal. Recently, layered double hydroxides (LDHs) have attracted considerable attention in the field of photocatalysis. LDHs are important layered clays generally expressed by the formula [M2+1−xM3+x(OH)2]x+(An−)x/n·mH2O, where M2+ and M3+ are divalent and trivalent metal cations and An− is the anion compensating for the positive charge of the hydroxide layers.13 In the previous reports, transition metal-containing LDHs materials have shown photocatalytic behavior.

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For instance, ZnCr-,14,15 NiTi-16,17 and CoFe-LDHs18 serve as efficient photocatalysts for oxygen evolution while MgCr- and NiCr-LDHs19 exhibit photocatalytic activity toward decomposition of organic pollutants. Although much progress has been made, to the best of our knowledge, a rational design based on their semiconductor properties is rather limited, which restricts the exploration. The above mentioned LDHs just contain the transition metal; does the LDH including main group elements such as Al and Ga show the photocatalytic activity? In addition, it is highly essential to understand the structure-property correlation of LDHs-based photocatalysts so as to give a rational material design. With recent progress in computational hardware and software, the band gaps of fluorite TiO2,20 graphene derivatives,21 Sc2CO2,22 NaTaO323 and some other semiconductors12,24 have been engineered by density functional theory (DFT). Calculations on band structures of semiconductors for the photocatalytic OER can reveal the light response range (visible light or UV responsive). However, whether an external bias is needed to perform the OER can not be predicted by merely calculating the band structure of semiconductors. In addition, the combination of band gap engineering and the OER mechanism calculations toward the photocatalytic OER is rare,25,26 which limits a deep investigation. Therefore, it is essential to carry out a systematic computational study on semiconductor properties of LDHs-based photocatalysts (including band gap, band edge placement and photocatalytic mechanism), so as to provide a detailed understanding of the OER and offers rational design for material. In this work, MΙΙΜΙΙΙ-LDHs (MΙΙ = Mg, Co, Ni and Zn; ΜΙΙΙ = Al and Ga) are investigated by using the density functional theory plus U (DFT + U) method, so as to give an in-depth understanding of their photocatalytic OER capabilities. In this work, the divalent cations (Co2+,

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Ni2+ and Zn2+) are expected to serve as the active site, owing to their redox property; while the trivalent cations (Al3+, Ga3+) are inert.27 Although some studies report that the inert trivalent cations may impose influence on the photocatalytic behavior of divalent cations.28 The electronic properties (band structure, DOS, band edge placement) and the thermodynamic mechanism toward the OER were studied, respectively. The calculations indicate that Mg and Zn-based LDHs (band gap energies larger than 3.1 eV) are UV light responsive while Co and Ni-based LDHs respond to visible light (band gap energies less than 3.1 eV). The band edge placements and thermodynamic mechanism study reveal that NiGa-, CoAl-, ZnAl- and NiAl-LDHs may have enough driving force (0.965 eV, 0.836 eV, 0.667 eV and 0.426 eV, respectively) toward the OER, but only CoAl-LDH can overcome the reaction barrier (0.653 eV) via the driving force of photogenerated hole (0.836 eV). Experimental investigations further demonstrate the efficient photocatalytic activity of CoAl-LDH toward the OER (O2 generation rate: 973 mmol h−1 g−1), matching well with the theoretical predictions. This is among the highest values of O2 generation rate reported previously.14−18

2. Computational methods and experimental details 2.1 Computational methods The models of MΙΙΜΙΙΙ-LDHs (MΙΙ = Mg, Co, Ni and Zn; ΜΙΙΙ = Al and Ga): The models of MΙΙΜΙΙΙ-LDHs (MΙΙ = Mg, Co, Ni and Zn; ΜΙΙΙ = Al and Ga) were constructed according to the crystal data of these LDHs reported previously.29−37 The formulae of these models are shown in Table 1. The space groups of all these ten LDHs are r 3 m, with the unit cell parameters α = β = 90°, γ = 120°.38 The other three unit cell parameters, a, b (ranging from 3.05 to 3.11 Å for a and b)

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and c (22.47 to 23.58 Å), are referred to the powder X-ray diffraction data of these LDHs and shown in Table S1 (see Supporting Information Table S1).29−37 Except the model Co3Al-Cl−-LDH, the molar ratio of MΙΙ (MΙΙ = Mg, Co, Ni and Zn) to ΜΙΙΙ (ΜΙΙΙ = Al and Ga) is set to be 2. The molar ratio of Co2+ to Al3+ is 3 in Co3Al-Cl−-LDH. Thus the influence of the molar ratio on electronic properties of LDHs can be quantified by comparing Co2Al-Cl−-LDH and Co3Al-Cl−-LDH. Since the molar ratio of MΙΙ to MΙΙΙ is 2 for all the models except Co3Al-Cl−-LDH, constructing the LDHs supercell by 3 × 3 × 1 in the a-, b- and c- directions is suitable. For Co3Al-Cl−-LDH, it is 4 × 4 × 1 in the a-, b- and c- directions. In Co2Al-OH−-LDH and Co2Al-NO3−-LDH, hydroxyl anion and nitrate anion are intercalated into the interlayer space of CoAl-LDH, respectively. In the other eight models, chloride anion is added into the interlayer space of LDHs to keep the model neutral. By comparing Co2Al-Cl−-LDH, Co2Al-OH−-LDH and Co2Al-NO3−-LDH, the effect of guest anion on the electronic properties of LDHs can be quantified.

Table 1 The formula, band gap energy (Eg), energy difference between the Fermi level and conduction band minimum (x), work function (W), the energy level of conduction band minimum/valence band maximum (ECBM/EVBM) relative to the vacuum level and the driving force (Edf) for each MΙΙMΙΙΙ-LDHs (MΙΙ = Mg, Co, Ni and Zn; MΙΙΙ = Al and Ga) Sample

Formula

MgAl-LDH

Mg18Al9(OH)54Cl9

Band gap

x

Work function

ECBM

EVBM

Edf

energy / eV

/ eV

/ eV

/ eV

/ eV

/ eV

4.631

4.435

5.052

−0.617

−5.248

\

NiAl-LDH

Ni18Al9(OH)54Cl9

2.326

1.305

4.725

−3.420

−5.746

0.426

ZnAl-LDH

Zn18Al9(OH)54Cl9

3.495

2.491

4.983

−2.492

−5.987

0.667

MgGa-LDH

Mg18Ga9(OH)54Cl9

4.040

3.831

4.730

−0.899

−4.939

\

NiGa-LDH

Ni18Ga9(OH)54Cl9

2.275

0.634

4.644

−4.010

−6.285

0.965

ZnGa-LDH

Zn18Ga9(OH)54Cl9

3.225

3.104

4.663

−1.559

−4.784

\

Co18Al9(OH)54Cl9

2.480

1.909

5.585

−3.676

−6.156

0.836

Co18Al9(OH)54(OH)9

2.403

1.813

5.525

−3.712

−6.115

0.795

−

Co2Al-Cl -LDH −

Co2Al-OH -LDH

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Co2Al-NO3−-LDH

Co18Al9(OH)54(NO3)9

2.429

1.889

5.575

−3.686

−6.115

0.795

Co3Al-Cl−-LDH

Co36Al12(OH)96Cl12

2.469

1.839

5.632

−3.793

−6.262

0.942

All the calculations are performed with the CASTEP code in the Materials Studio version 6.1 software package (Accelrys software inc.: San Diego, CA).39 The density functional theory (DFT) calculations are performed using a plane wave implementation40 at the generalized gradient approximation (GGA) Perdew-Burke-Ernzerhof (PBE) level.41 Spin-polarized DFT + U theory is applied to correct the well-known DFT self-interaction errors for the strongly correlated electrons in the first-row transition metal ions (Co2+ and Ni2+ here),42−44 because DFT + U theory can give a more accurate prediction on redox potentials and oxidation energies than standard DFT.45,46 In this work, we choose the values of U - J (Ueff) to be 3.52 eV46−49 for Co2+ and 3.8 eV50−52 for Ni2+, respectively. These values are referred to the unrestricted Hartree-Fock theory calculations over various cobalt oxides and nickel oxides, in which the coordination environment of Ni2+ and Co2+ is similar to that in LDHs layers. Moreover, these U values have been proven to be effective for the Hubbard correction.46-52 The combination of DFT and Hubbard correction makes sense, because Hartree-Fock for the intra-atomic exchange interactions among localized open-shell electrons eliminates self-interaction error while the use of DFT describes the delocalized electrons well.53 For the cations with closed-shell configuration (Mg2+, Zn2+, Al3+ and Ga3+), the value of Ueff is 0 eV. The DFT dispersion correction is dealt with the Tkatchenko-Scheffler method to describe the noncovalent forces, such as hydrogen bonding and van der Waals interaction. The ionic cores are described by ultrasoft pseudopotentials to improve transferability and reduce the number of plane waves required in the expansion of the Kohn-Sham orbitals.48,49 The Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm is used to search the potential energy surface during optimization.54 The structure optimization is based on the following points: (1) an

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energy tolerance of 1 × 10−5 eV per atom; (2) a maximum force tolerance of 0.03 eV/Å; (3) a maximum displacement tolerance of 1 × 10−3 Å. A Fermi smearing of 0.1 eV and Pulay mixing are used to ensure the fast convergence of the self-consistent electron density.25 For the calculation of band structure and density of states (DOS) of MΙΙΜΙΙΙ-LDHs, the Γ-point-centered k-point meshes used for Brillouin zone integrations are 3 × 3 × 1 k-points. The number of empty bands is 24 for every LDH. In this work, these parameter settings are referred to the previous report, which can guarantee a high-level calculation precision with acceptable computational cost.23,55,56 The driving force of OER: Figure 1 illustrates the schematic diagram of water splitting into H2 and O2 over photocatalysts. Photocatalysis on semiconductor particles involves three main steps: (a) absorption of photons with energies exceeding the semiconductor band gap, leading to the generation of electron (e−) and hole (h+) pairs in the semiconductor particles; (b) charge separation followed by migration of these photogenerated carriers in the semiconductor particles; (c) surface chemical reactions between these carriers with water. The energetic position of the conduction band minimum (CBM) should be more negative than the reduction potential of water (0 V vs. SHE when pH is 0, 0.41 V vs. SHE when pH is 7) to produce H2, and/or the valence band maximum (VBM) must be more positive than the oxidation potential of water (1.23 V vs. SHE when pH is 0, 0.82 V vs. SHE when pH is 7) to produce O2.57

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Figure 1. The schematic diagram of photocatalytic water splitting to produce H2 and O2 when pH is 7. The driving force (Edf) and work function (W) are defined. Two criteria are applied to identify an excellent semiconductor photocatalyst for oxygen evolution: (a) visible light response reflected by band gap energy (Eg < 3.1 eV); (b) no external bias needed.58,59 The band gap energy (eigenvalue), Eg, can be calculated by equation (1): (1)

Eg = ECBM − EVBM

where ECBM and EVBM represent the energy level of conduction band minimum (CBM) and valence band maximum (VBM), respectively (Figure 1). The eigenvalue band gap is calculated with the Hubbard correction to avoid the underestimation of band gap, as depicted above. For the second criterion, the need for a supplementary bias is due to the smaller driving force (named as Edf)10,25,26,60 compared with the kinetic barrier (mostly composed of the overpotential, η) in the OER. Edf is assumed to be the difference between EVBM of the semiconductor and the O2 evolution redox potential (E(O2/H2O) = 0.82 V vs. SHE when pH = 7), as illustrated in Figure 1. Therefore, Edf can be calculated with equation (2): Edf = 0.82 eV + 4.5 eV − EVBM = 5.32 eV − EVBM

(2)

The work function and band edge placement of MΙΙΜΙΙΙ-LDHs: To obtain the band edge

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placement of a semiconductor, the work function (W) of this semiconductor needs to be studied first. The work function is the minimum thermodynamic work (i.e., energy) needed to remove an electron from a solid to a point in the vacuum immediately outside the solid surface, as shown in Figure 1. The work function is calculated with equation (3): W = −eφ − EF

(3)

where e is the charge of an electron; Φ is the electrostatic potential in the vacuum nearby the surface, and EF is the Fermi level inside the material.61 By calculating the band structure of the semiconductor, the energy difference (x) between the Fermi level (EF) and CBM (ECBM) is obtained by equation (4): (4)

x = ECBM − EF

Subsequently, the band edge placement of this semiconductor can be deduced by equation (5) and (6):12 ECBM = EF + x = −W + x

(5)

EVBM = ECBM − Eg = −W + x − Eg

(6)

Work function is surface dependent.60 For LDH materials, several previous reports have shown that the (003) facet is the most preferably exposed surface.62-64 Therefore, we choose (003) facet as the reaction surface. The (003) slabs of each LDH with different thickness (from one layer to five layers) are constructed. 30 Å of vacuum is incorporated along the direction perpendicular to the surface to separate the periodic images. The geometries of the slabs with five layers of MΙΙΜΙΙΙ-LDHs (MΙΙ = Mg, Co, Ni and Zn; ΜΙΙΙ = Al and Ga) are shown in Figure S1 (see Supporting Information). The work functions with respect to the slab thicknesses are converged within 0.02 eV so that the work function of the slab can represent the bulk material work function,

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as shown in Table S2 and Figure S2 (see Supporting Information). The work functions of slabs with five layers are applied in further calculations. The computational method used in the geometry optimization of slab model is the same as depicted in 2.1. The thermodynamic mechanism of OER over CoAl-LDH, NiAl-LDH, ZnAl-LDH and NiGa-LDH: To calculate the overpotential (η) of the OER, Norskov’s method is referred.25,48,49,65,66,67,68 For the photocatalytic OER over semiconductors, three possible mechanisms have been reported. The overpotential of the mechanism reported by Norskov et al. is the smallest;25,65 therefore, it is widely used as the predominant pathway.25,48,49,65,66,67,68 This reaction mechanism scheme is as follows: H2O + * → *OH + H+ + e−

(A)

*OH → *O + H+ + e−

(B)

*O + H2O → *OOH + H+ + e−

(C)

*OOH → * + O2 + H+ + e−

(D)

The symbol “*” represents the (003) surface of MΙΙΜΙΙΙ-LDHs (MΙΙ = Mg, Co, Ni and Zn; ΜΙΙΙ = Al and Ga). The “*OH”, “*O” and “*OOH” denote the surface with the corresponding chemisorbed species residing in the (003) surface of MΙΙΜΙΙΙ-LDHs. This mechanism contains four oxidation steps, each of which results in a proton ejected into the electrolyte that will eventually meet a transferring electron at the cathode. For MΙΙΜΙΙΙ-LDHs, H2O molecule firstly adsorbs onto the (003) surface and then undergoes two subsequent oxidation reactions to form *O; *O then reacts with another water molecule to produce *OOH intermediate. Finally, O2 is released from *OOH. The energy of H+ + e− is replaced implicitly with the energy of half a H2 molecule referred to the standard hydrogen electrode (SHE, 0.5 H2 → H+ + e−, pH = 0, p= 1 atm,

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T = 298 K). Entropic contributions (referred to the standard tables) and the calculated zero point

energy are listed in Table S3 (see Supporting Information).50 Thus, the reaction free energies are calculated as follows:

∆GA = E*OH + 0.5 EH 2 − E* − EH 2O + (∆ZPE − T∆S ) A − eU − kT ln 10pH

(7)

∆GB = E*O − E*OH + 0.5 EH 2 + (∆ZPE − T∆S ) B − eU − kT ln 10pH

(8)

∆GC = E*OOH − E*O − EH 2O + 0.5 EH 2 + (∆ZPE − T∆S )C − eU − kT ln 10pH

(9)

∆GD = E* − E*OOH + EO 2 + 0.5 EH 2 + ( ∆ZPE − T∆S ) D − eU − kT ln 10pH

(10)

EH2O, EH2 and EO2 are the calculated energies for the isolated gaseous molecule H2O, H2 and O2, respectively; U represents the external bias. Since the sum of ∆GA, ∆GB, ∆GC and ∆GD equals to the free-energy change of the total reaction 2H2O(l) → 2H2(g) + O2(g), which has been calculated to be 4.92 eV (Table S3, see Supporting Information), matching well with the experimentally determined value25. The theoretical overpotential (η) is independent of the pH value or the external bias (U). Since the free energies obtained by using equation (7)-(10) vary in the same way with pH and bias, the potential determining step remains unchanged.67 Therefore, the analysis performed for the free energies is at standard conditions (pH = 0, T = 298.15 K) and U = 0. The catalytic performance is estimated by the magnitude of the potential-determining step for the OER, GOER. The potential-determining step is the last step to become downhill in free energy as the potential increases, i.e., the reaction step with the largest ∆G among the four steps (A)−(D), as calculated by the equation (11):67 GOER = max[∆GA , ∆GB , ∆GC , ∆GD ]

(11)

Then the overpotential, η, is obtained by equation (12):67

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η = GOER / e − (∆GA + ∆GB + ∆GC + ∆GD ) / 4e = GOER / e − 1.23 V

(12)

here 1.23 V means one fourth of 4.92 eV/e (the sum of ∆GA, ∆GB, ∆GC and ∆GD). For an ideal OER catalyst, ∆GA = ∆GB = ∆GC = ∆GD = 1.23 eV. The (003) slabs of CoAl-, NiAl-, ZnAl- and NiGa-LDHs are therefore constructed. Each slab model consists of one layer of LDH, with 9 metal atoms. 30 Å of vacuum is incorporated along the direction perpendicular to the surface to separate the periodic images. For each radical (OH, O or OOH) on one slab, there are four possible adsorption positions (top, hollow, bridge and fcc). Each adsorption position of one radical (OH, O or OOH) is calculated (for the detailed energy, see Table S4, Supporting Information). These four models are named as model CoAl-LDH-1, NiAl-LDH-1, ZnAl-LDH-1 and NiGa-LDH-1. To evaluate the interaction between water molecule in electrolyte and intermediates, three water molecules are added in each model, to give four models named as CoAl-LDH-2, NiAl-LDH-2, ZnAl-LDH-2 and NiGa-LDH-2. On the other hand, to evaluate the influence of the different coverage of radicals (OH, O and OOH) on the OER performance, model CoAl-LDH-3, NiAl-LDH-3, ZnAl-LDH-3 and NiGa-LDH-3 were built. The concentrations of radicals (OH, O and OOH) in CoAl-LDH-3 and so on are improved to three times of that in model CoAl-LDH-1, NiAl-LDH-1, ZnAl-LDH-1 and NiGa-LDH-1. The computational method used in the geometry optimization of slab model is the same as depicted in 2.1.

2.2 Experimental details Preparation of nitrate-containing MgAl-LDH, CoAl-LDH and ZnAl-LDH samples: Colloidal LDHs suspension were prepared according to the separate nucleation and aging steps (SNAS) method reported by our group.69-71 Typically, 100 mL of solution A (Al(NO3)3·9H2O:

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0.10 M and Mg(NO3)2·6H2O: 0.20 M) and 100 mL of solution B (urea: 1.0 M) were mixed together. The mixed solution was transferred into a Teflon-lined stainless steel autoclave and hydrothermally treated at 110 °C for 24 h. The resulting LDH slurry was obtained by centrifugation, washed thoroughly and dried in vacuum at 60 °C for 12 h to obtain the final MgAl-LDH. The samples of CoAl- and ZnAl-LDHs were prepared by a similar method via replacing Mg(NO3)2·6H2O by Co(NO3)2·6H2O and Zn(NO3)2·6H2O, respectively.

Characterization: Powder X-ray diffraction patterns of MgAl-, CoAl- and ZnAl-LDHs samples were collected on a Shimadzu XRD-6000 diffractometer using a Cu Kα source, with a scan step of 0.02° and a scan range between 3° and 70°. The morphology of these three samples were investigated using a scanning electron microscope (SEM; Zeiss SUPRA 55) with an accelerating voltage of 20 kV. UV-vis diffuse reflectance spectra of the samples were recorded with a Beijing PGENERAL TU-1901 spectrometer in the 200-800 nm wavelength range.

Photocatalytic tests: The photocatalytic reaction was carried out for solar light induced oxygen generation. The photocatalytic reaction was performed in a Pyrex glass cell with a stationary temperature at 50°C, connected with a closed gas circulation system. Each sample (MgAl-, CoAland ZnAl-LDH, respectively) (0.02 g) was suspended in an aqueous solution (100 ml) containing AgNO3 (0.01 g) as a sacrificial reagent. The suspension was then thoroughly degassed and irradiated using a Xe lamp (300 W). The amount of oxygen generation was analyzed at the given time intervals using an online gas chromatograph (GC-7890II; Techcomp. Co., Ltd). The activities of MgAl-, CoAl- and ZnAl-LDHs were determined on the basis of the average rate of O2 generation at least 3 cycles.

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3. Results and Discussion 3.1 The electronic properties of MΙΙΜΙΙΙ-LDHs (MΙΙ = Mg, Co, Ni and Zn; ΜΙΙΙ = Al and Ga) Band structure: The optimized geometries of MΙΙMΙΙΙ-LDHs (MΙΙ = Mg, Co, Ni and Zn; MΙΙΙ = Al and Ga) are displayed in Figure 2 and Figure S3. The lattice parameters of these optimized geometries agree well with the experimental observations (Table S1). Obviously, the matrix of each LDH is smooth and the guest anions are well distributed in the LDH gallery. The band structures of these LDHs are shown in Figure 3. All the calculated LDHs are found to be semiconductors because their Fermi levels are within the forbidden zone. As can be seen in Figure 3 and Table 1, the band gap energy of these LDHs increases in the following sequence: NiGa-LDH (2.275 eV) < NiAl-LDH (2.326 eV) < CoAl-LDH (2.403 - 2.480 eV) < ZnGa-LDH (3.225 eV) < ZnAl-LDH (3.495 eV) < MgGa-LDH (4.040 eV) < MgAl-LDH (4.631 eV). On the one hand, Ga-based LDHs have smaller band gap energies than Al-based LDHs with the same

ΜΙΙ. This can be explained by that Ga has the 3d-orbitals while Al does not. The Ga-3d orbital gives a significant contribution to the VBM and CBM of Ga-based LDHs according to the analysis of density of states (Table 2 and Figure 4). Therefore, the CBM and VBM in Ga-based LDHs are rather close in energy, compared with those in Al-based LDHs. On the other hand, for LDHs with the same ΜΙΙΙ, Ni-based LDHs shows the smallest band gap energy (Eg), followed by Co-based LDHs, Zn-based LDHs and Mg-based LDHs. This is derived from their different electronic structures. Mg2+ has no d-orbital and Zn2+ (d10) has a fully-filled d-orbital, implying that d-d electronic transition is hardly allowed. Both Co2+ (d7) and Ni2+ (d8) have unoccupied d-orbitals, so Co and Ni-based LDHs show smaller band gap energies compared with Mg and Zn-based LDHs. Band gap energy of a semiconductor plays a decisive role on the light

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absorption. An Eg value less than 3.1 eV enables visible light absorption, but a rather small Eg may lead to a poor oxidation ability of photogenerated hole. According to the above Eg values, Co and Ni-based LDHs are estimated to be visible light responsive while Mg and Zn-based LDHs are only UV light responsive. In addition, by comparing the energy difference between EF and

ECBM (defined as x) of these LDHs, it is found that the Fermi level is close to the CBM for NiGa-LDH while near to the VBM for the other six LDHs. The results indicate NiGa-LDH is an n-type semiconductor while the other six LDHs are p-type ones. The n and p nature of LDHs determines the main charge carrier for electric conduction. The main charge carrier for electric conduction for p-type and n-type semiconductors are hole and electron, respectively. The band gap energies of Co2Al-Cl−-LDH and Co3Al-Cl−-LDH are 2.480 eV and 2.469 eV, respectively (Figure 3). The composition of density of states (DOS) for these two samples are the same (Figure 4); the other electronic properties (work function and band edge placement) of these two samples are also close (Table 1). Therefore, the influence of molar ratio on the electronic properties of LDHs is not significant. Thus the electronic properties based on the models with MΙΙ/MΙΙΙ of 2 are mainly discussed. On the other hand, the band gap energies of Co2Al-Cl−-LDH, Co2Al-OH−-LDH and Co2Al-NO3−-LDH are 2.480 eV, 2.403 eV and 2.429 eV, respectively (Table 1). The DOS composition of these three samples are the same (Figure 4); the other electronic properties (work function and band edge placement) of Co2Al-Cl−-LDH, Co2Al-OH−-LDH and Co2Al-NO3−-LDH are also close (Table 1). Therefore, the influence of guest anions on the photocatalytic OER properties of LDHs is not significant, the electronic properties of LDHs are mainly determined by the host matrices. We thus discuss the electronic properties mostly from the models with Cl− ainon.

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Figure 2. The optimized geometries of Co2Al-Cl−-LDH and Co2Al-NO3−-LDH. The color of element is set as follows: blue for Co, pink for Al, red for O, white for H, green for Cl and dark blue for N, respectively.

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Figure 3. The band structure of MΙΙΜΙΙΙ-LDHs (MΙΙ = Mg, Co, Ni and Zn; ΜΙΙΙ = Al and Ga). The band gap energy of each MΙΙΜΙΙΙ-LDHs is listed in the bracket and the dashed blue line is the Fermi level.

Density of states (DOS): The total density of state (TDOS) of each MΙΙΜΙΙΙ-LDH is shown in

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Figure 4 with black line, and the partial density of state (PDOS) of each orbital is analyzed. The main components of the frontier orbitals (VBM and CBM) are listed in Table 2. It is found that for all the calculated LDHs, O-2p orbital is a main component of the VBM, which indicates the photogenerated hole tends to be localized in the O atom of hydroxyl group. As the hydroxyl group of LDHs interacts with water molecules via hydrogen bonding, the hole localized in the O atom facilitates the oxidation of water molecules. The Cl-2p orbital also plays an important role in the VBM of Mg, Ni and Zn-based LDHs. The photogenerated hole localized in chloride anion tends to recombine with electron instead of participation in water oxidization. However, CoAl-LDH is an exception because its VBM is composed of Co-3d and O-2p orbitals only. In the cases of Co and Ni-based LDHs, Co-3d and Ni-3d orbital give important contribution to the VBM. On the other hand, the CBM of each MΙΙΜΙΙΙ-LDH is mainly composed of the orbitals of metal cations. It should be noted that Ga-4s orbital shows a big proportion in the CBM of Ga-based LDHs while Al-2p orbital does not contribute to the CBM of any Al-based LDHs. The DOS suggests that Al cation is not the active site. However, the existence of Al cation in LDH host matrix may give a contribution on the photocatalytic OER performance, as confirmed in the electrocatalytic OER system.72-74 Co-3d, Ni-3d and Zn-4s orbital also consist of the CBM of the corresponding Co, Ni and Zn-based LDHs. Interestingly, Mg-2p orbital contributes to the CBM of MgAl-LDH while in the CBM of MgGa-LDH, Mg-2p orbital is insignificant. Moreover, O-2p orbital is one part of the CBM of Ni-based LDHs, but this phenomenon is not found in other LDHs. In general, the density of states (especially the frontier orbitals) of LDHs is predominantly influenced by the metal cations in the LDHs matrix.

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Figure 4. The total density of states (TDOS) and partial density of states (PDOS) for MΙΙΜΙΙΙ-LDHs (MΙΙ = Mg, Co, Ni and Zn; MΙΙΙ = Al and Ga). The Fermi level is displayed with a dashed blue line.

Table 2. The composition of valence band maximum (VBM) and conduction band minimum (CBM) of each MΙΙMΙΙΙ-LDHs (MΙΙ = Mg, Co, Ni and Zn; MΙΙΙ = Al and Ga) 20


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Sample

Valence band maximum (VBM)

Conduction band minimum (CBM)

MgAl-LDH NiAl-LDH ZnAl-LDH MgGa-LDH NiGa-LDH ZnGa-LDH Co2Al-Cl−-LDH Co2Al-OH−-LDH Co2Al-NO3−-LDH Co3Al-Cl−-LDH

O-2p, Cl-2p O-2p, Cl-2p, Ni-3d O-2p, Cl-2p O-2p, Cl-2p O-2p, Cl-2p, Ni-3d O-2p, Cl-2p, Zn-4s O-2p, Co-3d O-2p, Co-3d O-2p, Co-3d O-2p, Co-3d

H-1s, Mg-2p O-2p, Ni-3d Zn-4s Ga-4s O-2p, Ni-3d, Ga-4s Zn-4s, Ga-4s Co-3d Co-3d Co-3d Co-3d

Band edge placements: The optimized geometries of the slabs of MΙΙMΙΙΙ-LDHs (MΙΙ = Mg, Co, Ni and Zn; MΙΙΙ = Al and Ga) are shown in Figure S1. According to these geometries, the electrostatic potential (EF and Φ, identified in equation (3)) of each LDH is calculated and shown in Figure S2. Table 1 lists the work function (W) obtained by using equation (3) based on EF and

Φ in Figure S2. For MΙΙMΙΙΙ-LDHs (MΙΙ = Mg, Co, Ni and Zn; MΙΙΙ = Al and Ga), the work function (W) ranges from 4.644 eV (NiGa-LDH) to 5.632 eV (Co3Al-Cl−-LDH). The location of Fermi level relative to the CBM (x) and the band gap energy (Eg) were calculated with equation (4) and (1) and shown in Table 1. Subsequently, the energy level of CBM/VBM (ECBM/EVBM) relative to the vacuum level of each LDH was calculated with equation (5) and (6); the driving force (Edf) was further calculated with equation (2).

ECBM and EVBM relative to the vacuum level can be transferred to electric potential as shown in Figure 5. For oxygen evolution of water splitting at pH = 7, the driving force (Edf) should be positive. According to Table 1 and Figure 5, photogenerated hole of ZnGa-LDH, MgGa-LDH and MgAl-LDH can not proceed with water oxidization because their VBM are not more positive than the oxidation potential of water to O2. In contrast, CoAl-LDH, NiAl-LDH, ZnAl-LDH and

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NiGa-LDH may have the possibility toward the OER because they provide a positive Edf to oxidize water. In order to judge whether these four LDHs can undergo the OER without external bias, the driving force (Edf) needs to be compared with the overpotential (η).

Figure 5. The band edge placements of MΙΙMΙΙΙ-LDHs (MΙΙ = Mg, Co, Ni and Zn; MΙΙΙ = Al and Ga). The two dashed red lines (−0.41 V and 0.82 V vs. SHE) represent the reduction potential of H2 and oxidation potential of O2 at pH = 7.

3.2 The thermodynamic mechanism of OER over CoAl-LDH, NiAl-LDH, ZnAl-LDH and NiGa-LDH After geometry optimization of radical OH, O and OOH adsorbed at various positions of model CoAl-LDH-1, NiAl-LDH-1, ZnAl-LDH-1 and NiGa-LDH-1, it is found that for all the four LDHs, OH adsorbs at fcc site, O at the bridge site and OOH at the hollow site (for detailed energy, see Table S4 in Supporting Information). This finding can be well understood as follows: *O has two unbonded electrons which needs two bonds connecting with LDHs surface; while *OH or *OOH has only one unbonded electron, which is more suitable for fcc site and hollow site. The optimized geometries of *, *OH, *O and *OOH of these four LDHs are displayed in Figure 6 and Figure S4 (see Supporting Information). The free energies of reactions A−D (∆G298, in eV) of oxygen evolution over model MΙΙMΙΙΙ-LDH-(1-3) (MΙΙMΙΙΙ-LDH = CoAl-LDH, NiAl-LDH, ZnAl-LDH and NiGa-LDH) were calculated with equation (7)−(10) and displayed in Figure 7. The zero point energy and entropic contribution of each intermediate (*, *OH, *O and 22


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*OOH) are shown in Table S5 and S6 (see Supporting Information). For all the twelve LDHs, the proceeding of reaction A (H2O + * → *OH + H+ + e−) is not difficult. The hydrophilic hydroxyl group is localized on the surface of LDHs, which facilitates the adsorption of H2O on the LDHs surface and the subsequent reaction A. In general, the reaction B and C are more difficult to occur, compared with reaction A and D. The reaction B is the removal of H from the adsorbed hydroxyl (*OH → *O + H+ + e−) and reaction C is the generation of radical *OOH (*O + H2O → *OOH + H+ + e−). For all the four LDHs models, ∆GC is the largest one among ∆GA, ∆GB, ∆GC and ∆GD (as shown in Figrue 7), which indicates that reaction C with the highest barrier is the rate-determining step in oxygen evolution. The overpotential (η) values of water oxidation over model CoAl-LDH-1, NiAl-LDH-1, ZnAl-LDH-1 and NiGa-LDH-1 (calculated by equation (12)) are 0.653 V, 0.596 V, 1.103 V and 0.936 V, respectively (Figure 7). Futhermore, the OER overpotential (η) over model CoAl-LDH-2, NiAl-LDH-2, ZnAl-LDH-2 and NiGa-LDH-2 (with water molecule on LDHs surface) are 0.661 V, 0.702 V, 1.091 V and 0.867 V, respectively (Figure 7). The results show the influence of water molecule on the OER performance is not obvious (less than 0.1 eV in overpotential). Moreover, the OER overpotential (η) over model CoAl-LDH-3, NiAl-LDH-3, ZnAl-LDH-3 and NiGa-LDH-3 (with three radicals in *OH, *O and *OOH) are 0.630 V, 0.559 V, 1.097 V and 0.789 V, respectively (Figure 7). Therefore, the influence of the concentration of radical on the OER performance is not significant (less than 0.15 V in overpotential). This finding matches well with the previous report that the OER is fairly localized, exhibiting limited coverage dependence.50

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Figure 6. Optimized structures of the OER intermediates: the first, second, third and fourth column represent reaction site (*), intermediate *OH, *O and *OOH, respectively; while the first, second, third, fourth, fifth and sixth row denote model CoAl-LDH-1, CoAl-LDH-2, CoAl-LDH-3, NiAl-LDH-1, NiAl-LDH-2 and NiAl-LDH-3, respectively.

Figure 7. Standard free energy diagrams for the OER, the first, second, third and fourth row are CoAl-LDH, NiAl-LDH, ZnAl-LDH and NiGa-LDH, respectively. The overpotential (η) is calculated with equation (12). 24


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3.3 Photocatalytic activity of MgAl-LDH, ZnAl-LDH and CoAl-LDH by experimental study According to our calculation results, these LDHs can be divided into three types: Type 1 (MgAl-, MgGa- and ZnGa-LDH) possesses no driving force; Type 2 (NiAl-, ZnAl-, and NiGa-LDH) owns a less driving force than its overpotential; Type 3 (CoAl-LDH) can overcome its overpotential (0.653 V) via its driving force (0.836 eV). The photocatalytic activities of MgAl-LDH (Type 1), ZnAl-LDH (Type 2) and CoAl-LDH (Type 3) toward the OER were further studied by experiments, so as to validate the computational results. The nitrate-containing MgAl-LDH, ZnAl-LDH and CoAl-LDH were prepared by the separate nucleation and aging steps (SNAS) method. Figure 8a shows the PXRD patterns of MgAl-LDH, ZnAl-LDH and CoAl-LDH samples, from which the reflections can be indexed to a hexagonal lattice with r 3 m

rhombohedral symmetry, commonly used for the description of LDHs structure.75 All these three PXRD patterns display the reflections of (003), (006), (009) and (110). The basal spacing of MgAl-LDH, ZnAl-LDH and CoAl-LDH is 8.66 Å (2θ = 10.20°), 8.76 Å (2θ = 10.08°) and 8.75 Å (2θ = 10.10°), respectively. The lattice parameter a of MgAl-LDH, ZnAl-LDH and CoAl-LDH is 3.03 Å (2θ = 61.06°), 3.06 Å (2θ =60.48°) and 3.06 Å (2θ =60.48°), respectively. The basal spacings of MgAl-LDH, ZnAl-LDH and CoAl-LDH match well with other nitrate-containing LDHs.69-71 SEM images of MgAl-LDH, ZnAl-LDH and CoAl-LDH (Figure 8b−d) reveal that the as-synthesized LDHs are platelike crystals with the particle size of 60−70 nm. UV-vis diffuse reflectance spectra of these three samples are shown in Figure 8e, which indicates that only CoAl-LDH is visible light responsive while MgAl-LDH and ZnAl-LDH are ultraviolet responsive. The plots of (αhν)2 vs. (hν)2 (α is the absorbance) of MgAl-, ZnAl- and CoAl-LDH are obtained by further analyzing their UV-vis diffuse reflectance spectra (shown in Figure S5).

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Thus, the band gap energies of MgAl-LDH, ZnAl-LDH and CoAl-LDH are estimated to be 4.18 eV, 3.22 eV and 2.23 eV, respectively. These experimental values of band gap energy are about ten percent smaller than the DFT calculation results (4.631 eV, 3.495 eV and 2.480 eV, respectively).

Figure 8. The powder X-ray diffraction patterns (a), SEM images (b d), UV-vis diffuse reflectance spectra (e) and the volume of O2 generation as function of irradiation time (f) for MgAl-LDH, ZnAl-LDH and CoAl-LDH, respectively. The photocatalytic activity toward water splitting was evaluated by monitoring the time-dependent production of O2 in solar light-illuminated catalyst suspension. AgNO3 has been widely used as sacrificial electron acceptor in the photocatalytic OER.14-18 Although Ag deposition on the LDHs surface occurs, this would not result in significant decrease of the photocatalytic OER activity. As shown in Figure 8f, MgAl-LDH and ZnAl-LDH do not show the photocatalytic OER activities; while the sample of CoAl-LDH displays an O2 generation rate of 973 µmol h−1 g−1, which is among the highest values of previously reported visible-light responsive OER photocatalysts.14−18 Moreover, the lifetime and reproducibility of CoAl-LDH were evaluated, showing a constant photocatalytic activity (952 µmol h−1 g−1) over 3 consecutive cycles (Figure 8f). Therefore, the photocatalytic activity and stability of CoAl-LDH for toward 26


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the OER are demonstrated, which matches well with the above theoretical prediction.

4. Conclusions The band structure and the density of states (DOS) of MΙΙMΙΙΙ-LDHs (MΙΙ = Mg, Co, Ni and Zn; MΙΙΙ = Al and Ga) were calculated by using DFT + U method, and the results indicate that Co and Ni-based LDHs can absorb visible light and may serve as photocatalysts toward the OER from water. The DOS of these LDHs illustrates that the photogenerated hole localizes at the O atom of the hydroxyl group, which benefits the oxidation of water molecule. The band edge placements of all calculated LDHs show that CoAl-LDH, NiAl-LDH, ZnAl-LDH and NiGa-LDH possess driving force (Edf = 0.836 eV, 0.426 eV, 0.667 eV and 0.965 eV, respectively) toward oxidizing water. The thermodynamic mechanism study on oxygen evolution over CoAl-LDH, NiAl-LDH, ZnAl-LDH and NiGa-LDH reveals that only CoAl-LDH can overcome the overpotential of water splitting (0.653 eV) via its driving force provided by the photogenerated hole. This is further verified by experimental investigations: an O2 generation rate of 973 µmol h−1 g−1 was achieved for the sample of CoAl-LDH while none of other LDHs samples shows photocatalytic OER activity, validating the accuracy of theoretical predictions. Therefore, this work provides an effective approach to screen and design LDHs photocatalysts for OER based on the combination of calculation and experiment, which can be extended to other semiconductor materials.

Acknowledgements This work was supported by the 973 Program (Grant No. 2014CB932103), the National Natural

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Science Foundation of China (NSFC), the Beijing Natural Science Foundation (2132043) and the Specialized Research Fund for the Doctoral Program of Higher Education (20130010110013). M. Wei particularly appreciates the financial aid from the China National Funds for Distinguished Young Scientists of the NSFC. We acknowledge National Supercomputing Center in Shenzhen for providing the computational resources and the materials studio (version 6.1, CASTEP).

Supporting Information Available The referred and calculated lattice parameters (a, b and c), and the calculated energy of MΙΙMΙΙΙ-LDHs (MΙΙ = Mg, Co, Ni and Zn, MΙΙΙ = Al and Ga) (Table S1); the work function (W) with respect to the slab thickness of MΙΙMΙΙΙ-LDHs (Table S2); entropic energy contributions (T = 298.15 K) and zero point energy (ZPE) corrections for gaseous molecules (Table S3); the energies of radical OH, O and OOH adsorbed on the different sites (top, bridge, fcc and hollow) for model CoAl-LDH-1, NiAl-LDH-1, ZnAl-LDH-1 and NiGa-LDH-1 (Table S4); the zero point energy and entropic contribution of each OER intermediate (*, *OH, *O and *OOH) in each MΙΙMΙΙΙ-LDHs (Table S5 and Table S6); the optimized geometries for the slabs of MΙΙMΙΙΙ-LDHs (MΙΙ = Mg, Co, Ni and Zn; MΙΙΙ = Al and Ga) (Figure S1); the work function of the (003) facet of MΙΙMΙΙΙ-LDHs (Figure S2); the optimized geometries of MΙΙMΙΙΙ-LDHs (Figure S3); optimized structures of the OER intermediates (Figure S4); the plots of (αhν)2 vs. (hν)2 of MgAl-LDH, ZnAl-LDH and CoAl-LDH according to their UV-vis diffuse reflectance spectra (Figure S5). This information is available free of charge via the internet at http://pubs.acs.org.

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51x50mm (300 x 300 DPI)



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Figure 1. The schematic diagram of photocatalytic water splitting to produce H2 and O2 when pH is 7. The driving force (Edf) and work function (W) are defined. 40x19mm (300 x 300 DPI)


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Figure 2. The optimized geometries of Co2Al-Cl-−-LDH and Co2Al-NO3-−-LDH. The color of element is set as follows: blue for Co, pink for Al, red for O, white for H, green for Cl and dark blue for N, respectively. 79x76mm (300 x 300 DPI)



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Figure 3. The band structure of M(II)-M(III)ΙΜΙΙΙ-LDHs (M(II)= Mg, Co, Ni and Zn;M(III)= Al and Ga). The band gap energy of each M(II)-M(III)-LDHs is listed in the bracket and the dashed blue line is the fermi level. 196x225mm (300 x 300 DPI)


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Figure 4. The total density of states (TDOS) and partial density of states (PDOS) for M(II)M(III)-LDHs ( M(II)= Mg, Co, Ni and Zn; M(III)= Al and Ga). The fermi level is displayed with a dashed blue line. 171x202mm (300 x 300 DPI)



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Figure 5. The band edge placements of M(II)M(III)-LDHs ( M(II) = Mg, Co, Ni and Zn; M(III) = Al and Ga). The two dashed red lines (−0.41 V and 0.82 V vs. SHE) represent the reduction potential of H2 and oxidation potential of O2 at pH = 7. 37x8mm (300 x 300 DPI)


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Figure 6. Optimized structures of the OER intermediates: the first, second, third and fourth column represent reaction site (*), intermediate *OH, *O and *OOH, respectively; while the first, second, third, fourth, fifth and sixth row denote model CoAl-LDH-1, CoAl-LDH-2, CoAl-LDH-3, NiAl-LDH-1, NiAl-LDH-2 and NiAl-LDH-3, respectively. 91x100mm (300 x 300 DPI)



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Figure 7. Standard free energy diagrams for OER, the first, second, third and fourth row are CoAl-LDH, NiAlLDH, ZnAl-LDH and NiGa-LDH, respectively. The overpotential (η) is calculated with equation (12). 91x49mm (300 x 300 DPI)


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Figure 8. The powder X-ray diffraction patterns (a), SEM images (b d), UV-vis diffuse reflectance spectra (e) and the volume of O2 generation as function of irradiation time (f) for MgAl-LDH, ZnAl-LDH and CoAl-LDH, respectively. 61x21mm (300 x 300 DPI)


Theoretical and Experimental Study on MIIMIII-Layered Double

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