Band Structure Engineering of Transition-Metal-Based Layered

Article pubs.acs.org/JPCC

Band Structure Engineering of Transition-Metal-Based Layered Double Hydroxides toward Photocatalytic Oxygen Evolution from Water: A Theoretical−Experimental Combination Study Si-Min Xu, Hong Yan,* and Min Wei* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, 100029 Beijing, People’s Republic of China S Supporting Information *

ABSTRACT: Considerable attention has been focused on layered double hydroxides (LDHs) for their applications in solar energy storage and conversion recently, but the in-depth investigation on the semiconducting properties of LDHs is limited. Herein, the electronic properties (band structure, density of states (DOS), surface energy, and band edge placement) of 14 kinds of MIInMIII/IV−A−LDHs (MII = Mg, Co, Ni, Cu, Zn; MIII = Cr, Fe; MIV = Ti; n = 2, 3, 4; A = Cl−, NO3−, CO32−) which contain transition-metal cations as well as their thermodynamic reaction mechanism toward the oxygen evolution reaction (OER) were studied using a density functional theory plus U (DFT + U) method. The calculation results indicate that the (003) plane is the most preferably exposed surface, and all these calculated LDHs are visible light responsive. The OER driving force and overpotential for these LDHs were obtained via their band edge placement and thermodynamic mechanism, and the results show that 10 of the calculated 12 LDHs (Ni2Ti−Cl−, Cu2Ti−Cl−, Zn2Ti−Cl−, Ni2Cr−Cl−, Zn2Cr−Cl−, Co2Fe−Cl−, Ni2Cr−NO3−, Ni2Cr−CO3−, Ni3Cr−Cl−, and Ni4Cr−Cl− LDHs) can overcome the reaction barriers by virtue of their driving force of photogenerated hole. Experimental observations further prove that NinCr−A−LDHs (n = 2, 3, 4; A = Cl−, NO3−, CO32−) are efficient OER photocatalysts, among which Ni2Cr− Cl−LDH shows the most active photocatalytic OER performance (O2 generation rate 1037 μmol h−1 g−1). In the meantime, Mg2Cr−Cl−LDH has no OER activity, agreeing well with the theoretical prediction. This work provides theoretical insight into the photocatalytic OER performance of LDHs materials which contain transition-metal cations with semiconducting property, which would show potential application in optical/optoelectronic field. divalent and trivalent metal cations and An− is the anion compensating for the positive charge of the hydroxide layers.8,9 LDHs which contain transition-metal cations (ZnCr−,10−13 ZnTi−,10 CuTi−,14 NiTi−,15 CoFe−,16 and CoAl−LDHs17) have shown semiconducting properties and intriguing photocatalytic activity to OER.10−17 Besides, the photocatalytic OER performance of the rare-earth cation-doped ZnCr−LDH18 and g-C3N4 combined NiFe−LDH19 have also been reported. However, most of these studies are based on experimental research, instead of an intrinsic reaction mechanism study and theoretical estimation. In recent years, the mechanism studies of TiO2,7 doped TiO2,20,21 and other OER photocatalysts22 have been widely reported. In our previous work, the band structure of MIInMIII−A−LDHs (MII = Mg, Co, Ni, Zn; MIII = Al, Ga; n = 2, 3; A = Cl, OH, NO3) and their corresponding OER mechanism were calculated by using a density functional theory plus U (DFT + U) method,16 and CoAl−LDH was found to be an efficient visible-light-responsive OER photo-

1. INTRODUCTION Photocatalytic water splitting to hydrogen and oxygen has received considerable attention in recent decades since Fujishima and Honda’s seminal work based on TiO2 toward the utilization of solar energy.1,2 Accordingly, the majority of the overpotential in water splitting usually arises from the oxygen evolution reaction (OER).3,4 Therefore, to achieve efficient hydrogen production, the design of highly efficient OER photocatalyst is a necessity. Two criteria are applied to identify an excellent OER photocatalyst:5 (1) visible-light response with a low band-gap energy (Eg < 3.1 eV) is desirable, which facilitates use of the most energy in the solar spectrum;6 (2) no external bias is needed, indicating the photocatalytic OER driving force (Edf) can overcome the overpotential (η).7 However, the exploration and development of promising OER photocatalysts with high performance and cost effectiveness remains to be a challenge. Among the OER photocatalysts that have been studied so far, layered double hydroxides (LDHs) materials have received much attention. LDHs are important layered anionic clays generally expressed by the formula [M2+1−xM3+x(OH)2]x+(An−)x/n·mH2O, where M2+ and M3+ are © XXXX American Chemical Society

Received: October 11, 2016 Revised: January 16, 2017 Published: January 16, 2017 A

DOI: 10.1021/acs.jpcc.6b10159 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C Table 1. Chemical Formula, Referred and Calculated Lattice Parameters, Together with the Calculated Energy of the MIIMIII/IV−LDHs lattice parameters referred sample Ni2Ti−Cl−LDH Cu2Ti−Cl−LDH Zn2Ti−Cl−LDH Mg2Cr−Cl−LDH Co2Cr−Cl−LDH Ni2Cr−Cl−LDH Cu2Cr−Cl−LDH Zn2Cr−Cl−LDH Co2Fe−Cl−LDH Ni2Fe−Cl−LDH Ni2Cr−NO3−LDH Ni2Cr−CO3−LDH Ni3Cr−Cl−LDH Ni4Cr−Cl−LDH

chemical formula Ni18Ti9(OH)54Cl18 Cu18Ti9(OH)54Cl18 Zn18Ti9(OH)54Cl18 Mg18Cr9(OH)54Cl9 Co18Cr9(OH)54Cl9 Ni18Cr9(OH)54Cl9 Cu18Cr9(OH)54Cl9 Zn18Cr9(OH)54Cl9 Co18Fe9(OH)54Cl9 Ni18Fe9(OH)54Cl9 Ni18Cr9(OH)54(NO3)9 Ni36Cr18(OH)108(CO3)9 Ni36Cr12(OH)96Cl12 Ni60Cr15(OH)150Cl15

a (Å)

calculated c (Å)

15

3.03 2.9514 3.0515 3.0124 2.9924 2.9825 3.0325 3.0815 3.0817 3.0426 2.9825 2.9825 2.9825 2.9825

23.58

23

26.3723 22.9523 23.5823

a (Å)

c (Å)

energy (eV)

3.05 3.07 3.06 3.02 3.00 3.01 3.05 3.03 3.04 3.03 3.01 3.01 3.01 3.01

22.01 22.16 22.15 22.09 22.14 22.13 22.23 22.07 22.16 22.18 24.51 23.09 22.10 22.06

−70 604.7977 −72 779.6469 −77 017.8593 −67 937.7894 −69 068.8406 −74 706.0192 −76 861.4044 −81 107.8500 −54 629.8419 −60 255.2731 −85 194.6803 −155 196.1735 −63 353.4395 −192 295.6882

2. COMPUTATIONAL METHODS AND EXPERIMENTAL DETAILS 2.1. Computational Methods. Models of Bulk MIInMIII/IV− A−LDHs (MII = Mg, Co, Ni, Cu, Zn; MIII = Cr, Fe; MIV = Ti; n = 2, 3, 4; A = Cl, NO3, CO3). In this work, the divalent cations in LDHs matrix were chosen to be Mg2+, Co2+, Ni2+, Cu2+, and Zn2+, which are the most common divalent cations for LDHs.23 The trivalent or quadrivalent cations in LDHs matrix were chosen to be Cr3+, Fe3+, and Ti4+ because several kinds of LDHs with these cations have been reported to be photocatalytically OER active.10−15,17 The models of these LDHs were constructed according to their crystal data and the powder X-ray diffraction data reported previously14,15,17,23−26 (Table 1). Since the space group of many natural LDHs (hydrotalcite, Mg3Al−CO3−LDH;27 pyroaurite, Mg3Fe−CO3−LDH;28 stichtite, Mg3Cr−CO3−LDH;29 takovite, Ni3Al−CO3−LDH;30 reevesite, Ni3Fe−CO3−LDH;31 desautelsite, Mg3Mn−CO3− LDH32) is R3̅m,33 the space group of R3̅m is used for all LDHs in this work. Thus, the unit cell parameters can be determined to be α = β = 90° and γ = 120°. The other three lattice parameters, a = b (ranging from 2.95 to 3.08 Å) and c (ranging from 22.95 to 26.37 Å), were referred to the powder X-ray diffraction data of these LDHs.14,15,17,23−26 The molar ratio of MII (Mg2+, Co2+, Ni2+, Cu2+, Zn2+) to MIII/IV (Ti4+, Cr3+, Fe3+) is set to be 2, 3, and 4 because the molar ratios of 2 and 3 are the only ones where the MIII/IV cation is only surrounded by divalent cations. Also, all natural LDHs occur in molar ratios of 2 and 3.34 Besides, the optimal molar ratio of MII to MIII is observed to be between 2 and 6;23 thus, the molar ratio of Ni2+ to Cr3+ being 4 is also considered in the model of Ni4Cr−Cl− LDH. Chloride, nitrate, and carbonate anions are chosen to be the interlayer guests of LDHs because these three anions are common in LDHs.23 The calculations were performed with the CASTEP code in the Materials Studio 6.1 software package (Accelrys Software Inc., San Diego, CA).35 The DFT calculations were performed using a plane wave implementation36 at the generalized gradient approximation (GGA) Perdew−Burke−Ernzerhof (PBE) level.37 Spin-polarized DFT + U theory is applied for the first-row transition-metal cations (Co2+, Ni2+, Cu2+, Ti4+, Cr3+, and Fe3+). The values of U − J (Ueff) are 3.52 eV for

catalyst. Nevertheless, these studied LDHs only involve trivalent main group metal cations (Al3+ and Ga3+).16 Since a majority of OER active LDHs consist of trivalent or quadrivalent transition-metal cations (e.g., Cr3+, Fe3+, and Ti4+),10−17 investigations on the OER mechanism for LDHs which contain transition-metal cations are urgently required. On the other hand, in our previous work, the photocatalytic OER active sites are found to be divalent transition-metal cations instead of trivalent main group metal cations in trivalent main group metal cations-based LDHs.16 Therefore, the trivalent main group metal cations are thought to be inert in OER. The photocatalytic OER activities of transition-metalbased LDHs are proposed to be different from trivalent main group metal cations-based LDHs because both divalent metal cations and trivalent metal cations can be the OER active sites. In addition to the species of the metal cations in the LDHs matrix, the interlayer anion, named as A, together with the molar ratio of MII to MIII/IV, defined as n, also influence the structure of LDHs. Therefore, in this work, in total 14 kinds of MIInMIII/IV−A−LDHs (MII = Mg, Co, Ni, Cu, Zn; MIII = Cr, Fe; MIV = Ti; n = 2, 3, and 4; A = Cl, NO3, and CO3) which contain transition-metal cations are constructed. The electronic structures and OER mechanisms of these LDHs are investigated by the DFT + U method. All calculated LDHs are found to be visible-light responsive, and their (003) planes are the most preferably exposed surface. The OER driving force and overpotential of MIInMIII/IV−A−LDHs are thus obtained. By comparing the driving force and overpotential, it is expected that 10 kinds of LDHs (Ni2Ti−Cl−, Cu2Ti−Cl−, Zn2Ti−Cl−, Ni2Cr−Cl−, Zn2Cr−Cl−, Co2Fe−Cl−, Ni2Cr−NO3−, Ni2Cr− CO3−, Ni3Cr−Cl−, Ni4Cr−Cl−LDHs), which hold a larger driving force, can oxidize water spontaneously under visiblelight illumination. Experimental investigations further demonstrate the efficient photocatalytic activities of Ni2Cr−Cl−LDH, Zn2Cr−Cl−LDH, Ni2Cr−NO3−LDH, Ni2Cr−CO3−LDH, Ni3Cr−Cl−LDH, and Ni4Cr−Cl−LDH toward OER (O2 generation rate 1037, 1079, 669, 614, 871, and 714 μmol h−1 g−1), matching well with the theoretical predictions. B


Article

The Journal of Physical Chemistry C Co2+,38,39 3.8 eV for Ni2+,40,41 3.6 eV for Cu2+,42 6.0 eV for Ti4+,43 3.2 eV for Cr3+,44 and 4.3 eV for Fe3+.44 These values are referred to the unrestricted Hartree−Fock theory calculations over corresponding oxides, where the coordination environments of these transition cations are similar to that in LDHs layers.38−44 For cations with closed-shell configuration (Mg2+ and Zn2+), the value of Ueff is 0 eV. Spin-polarized DFT + U theory is applied for correcting the well-known DFT selfinteraction errors for the strongly correlated electrons in the first-row transition-metal ions because DFT + U theory can give a more accurate prediction on the band-gap energies, redox potentials, and oxidation energies than standard DFT.45,46 The effectiveness of the DFT + U method has been confirmed in various materials (α-Fe2O3,47 β-NiOOH,48 β-Ni(OH)2,48 and TiO249). Although the many-body GW method is regarded as the gold standard for calculation of band structure,50 the computational cost of the GW method for LDHs materials is not affordable. DFT + U can provide satisfactory results with affordable computational cost. Since the noncovalent forces, such as hydrogen bonding and van der Waals interactions, are crucial for the formation, stability, and function of LDHs materials, the DFT dispersion correction is dealt with the Tkatchenko−Scheffler method to describe the noncovalent forces. 51 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.38,39 The Broyden−Fletcher−Goldfarb−Shanno (BFGS) algorithm was used to search the potential energy surface during optimization.52 The cutoff energy is determined to be 380 eV (converged to 0.3 meV/atom) because of the cost effectiveness (for detailed discussion, see Supporting Information). Structure optimization is based on the following points: (1) an energy tolerance of 1 × 10−5 eV per atom, (2) a maximum force tolerance of 0.03 eV/Å, and (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-consisitent field iterations.7 For calculation of the band structure and density of states of MIInMIII/IV−A−LDHs, the Γpoint-centered k-point meshes used for the Brillouin zone integrations are chosen to be 6 × 6 × 1 k-points referred to previous work in order to obtain an accurate band-gap energy.53 Surface Energy. For LDHs materials, the natural exposed surfaces can be the (003) or (110) plane. In order to determine the most preferably exposed surface of these LDHs, the surface energy (γ) of each LDH is derived from eq 154 γ=

Eslab − E bulk 2A

on the surface energy, the slab models without water monolayer were also considered. Driving Force of OER. Scheme 1 illustrates the schematic diagram of water splitting into H2 and O2 over the Scheme 1. Schematic Diagram of Photocatalytic Water Splitting To Produce H2 and O2 when pH is 7a

a

Photocatalysis on semiconductor particles involves three main steps: (a) adsorption of photons, leading to generation of electron (e−) and hole (h+) pairs; (b) charge separation followed by the migration of these photogenerated carriers; (c) surface chemical reactions between these carriers with water.

photocatalyst. The electronic band-gap energy (eigenvalue), Eg, can be calculated by eq 216 Eg = ECBM − E VBM

(2)

where ECBM and EVBM represent the energy of the conduction band minimum (CBM) and valence band maximum (VBM), respectively. The optical band-gap energy is the difference between the electronic band-gap energy (eigenvalue) and the exciton binding energy. Most semiconductors exhibit very small exciton binding energies, and therefore, the distinction between electronic and optical band gaps is frequently ignored.56 The driving force is assumed to be the difference between the O2 evolution redox potential (E(O2/H2O) = 0.82 V vs SHE when pH = 7) and the EVBM of the semiconductor.7,16,57 Therefore, Edf can be calculated with eq 3 Edf = (0.82V − (− 4.5V))e − E VBM = 5.32eV − E VBM (3)

where −4.5 V is the vacuum level. Work Function and Band Edge Placement. The work function is calculated with eq 4

(1)

where Eslab is the total energy of the optimized slab containing the same number of formula units as the bulk LDH (E003 and E110 here), Ebulk is the energy of the bulk LDH, and A is the surface area of one side of the slab. Thus, the slabs of the (003) and (110) planes for MIInMIII/IV−A−LDHs were constructed. Each slab contains three layers of LDH, three layers of intercalated guest, and 15 Å of vacuum (see Figures S4 and S5, Supporting Information). The higher index planes, (012) and (015) planes, of Ni2Cr−Cl−LDH were also considered for comparison.53,55 Since the photocatalytic OER occurs in aqueous solution, the solvent effect of water was taken into consideration. One monolayer of water molecule was added in each model. Also, to evaluate the influence of the solvent effect

W = −eϕ − E F

(4)

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.58 The values of ϕ and EF are directly obtained from the Accelrys Materials Studio software. For an undoped material, the Fermi level lies in the middle of the forbidden zone.59 Subsequently, the band edge placement of this semiconductor can be deduced by eqs 5 and 659,60

C

ECBM = −W + 0.5Eg

(5)

E VBM = −W − 0.5Eg

(6) DOI: 10.1021/acs.jpcc.6b10159 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C The work function is surface dependent.60 In this work, the (003) plane has been calculated to be the most preferably exposed surface in these LDHs (see section 3.1, surface energy part), matching well with the previous reports.61−63 Therefore, the (003) plane is chosen as the reaction surface. Thermodynamic Mechanism of OER over MIInMIII/IV−A− LDHs (MII = Mg, Co, Ni, Cu, Zn; MIII = Cr, Fe; MIV = Ti; n = 2, 3, 4; A = Cl, NO3, CO3). In previous work, several kinds of photocatalytic OER mechanism schemes have been proposed. The OER mechanism scheme depends on the pH value of solvent environment.6 In this work, the photocatalytic OER happens in neutral solvent (pH = 7). Therefore, the mechanism proposed by Lyons and Brandon,64 which happens in alkaline solvent, is not considered here. In neutral solvent, two kinds of photocatalytic OER mechanism schemes (in neutral solvent) have been proposed. Mechanism 1, proposed by Norskov and co-workers,7,38,39,53,65−68 is as follows H 2O + * → *OH + H+ + e−

(A.1)

*OH → *O + H+ + e−

(B.1)

*O + H 2O → *OOH + H+ + e−

(C.1)

*OOH → * + O2 + H+ + e−

(D.1)

η = GOER /e − (ΔGA + ΔG B + ΔGC + ΔG D)/4e = GOER /e − 1.23 V

where 1.23 V means one-fourth of 4.92 eV/e (the sum of ΔGA, ΔGB, ΔGC, and ΔGD, which equals to the Gibbs free energy change of the reaction, 2H2O(l) → 2H2(g) + O2(g)).16 For an ideal OER catalyst, ΔGA = ΔGB = ΔGC = ΔGD = 1.23 eV. The (003) slabs of MIInMIII/IV−A−LDHs were constructed. Each slab model consists of two layers of LDH. H2O molecule and the radicals (OH, O, and OOH) were thus put on the surface of each LDH slab. Normally, the slab with four layers of atoms is used for metal, metal oxide, and other materials.7 However, for LDHs, the interaction between the matrix and the guest anion is mainly composed of the Coulomb force, hydrogen bonding, and van der Waals force, which is weaker than the covalent bond. Therefore, the slab with two layers of LDH matrix and two layers of guest anion is able to describe the property of LDHs while saving computational cost at the same time. Thus, the slab with two layers is applied in this work. The second mechanism, proposed by Doyle and coworkers,70 is as follows

The asterisk (*) represents the (003) surface of these calculated LDHs. “*OH”, “*O”, and “*OOH” denote the surface with the corresponding chemisorbed species residing in the (003) surface of these LDHs. The energy of H+ + e− is replaced implicitly with the energy of one-half a H2 molecule with respect to the standard hydrogen electrode (SHE, 0.5 H2 → H+ + e−, pH = 0, p = 1 atm, T = 298 K).7 Thus, the reaction Gibbs free energies are calculated as follows7 ΔGA = G OH + 0.5G H2 − G − G H2O − eU * * − kT ln 10·pH

(7)

(8)

(A.2)

*OH + H 2O → *H 2O2 + H+ + e−

(B.2)

*H 2O2 → *OOH + H+ + e−

(C.2)

*OOH → * + O2 + H+ + e−

(D.2)

ΔGA = G OH + 0.5G H2 − G − G H2O − eU * * − kT ln 10·pH

(13)

ΔG B = G H2O2 + 0.5G H2 − G H2O − G OH − eU * * − kT ln 10·pH

(14)

ΔGC = G OOH + 0.5G H2 − G H2O2 − eU − kT ln 10·pH * *

(9)

(15)

ΔG D = G − G OOH + GO2 + 0.5G H2 − G OOH − eU * * * − kT ln 10·pH (10)

ΔG D = G − G OOH + GO2 + 0.5G H2 − G OOH − eU * * * − kT ln 10·pH (16)

G is the Gibbs free energy; U represents the external bias; the terms “eU” and “kT ln 10·pH” are 0 because the Gibbs free energy analysis performed is at standard conditions (pH = 0, T = 298.15 K) and U = 0. The Gibbs free energies of all reactants and products were obtained by calculating the vibrational frequencies of that molecule or intermediate.69 The detailed method for calculating the vibrational frequency is illustrated in the Supporting Information. The catalytic performance is estimated by the magnitude of the Gibbs free energy of the potential-determining step for the OER, GOER. The potential-determining step is the reaction step with the largest ΔG among the four steps A.1−D.1, as calculated by eq 1167 GOER = max[ΔGA , ΔG B , ΔGC , ΔG D]

H 2O + * → *OH + H+ + e−

The overpotential, η, of mechanism 2 is calculated using a similar method with that of mechanism 1. The reaction Gibbs free energies are calculated as follows

ΔG B = G O + 0.5G H2 − G OH − eU − kT ln 10·pH * * ΔGC = G OOH + 0.5G H2 − G O − G H2O − eU * * − kT ln 10·pH

(12)

2.2. Experimental Details. Preparation of MIInMIII/IV−A− LDHs Samples. Solution A: MgCl2·6H2O and CrCl3·6H2O with a Mg2+/Cr3+ ratio of 2 were dissolved in deionized water (45 mL) to give a solution with Mg2+ concentration of 1.0 M. Solution B: NaOH (3.0 M) was dissolved in deionized water (45 mL) to form the base solution. Solution B was added drop by drop in solution A until the pH of the total solution is 10 ± 0.1. The mixture was aged 24 h at 120 °C. The samples of Co2Cr−Cl−LDH, Ni2Cr−Cl−LDH, Cu2Cr−Cl−LDH, and Zn2Cr−Cl−LDH were prepared by a similar method via replacing MgCl2·6H2O by CoCl2·6H2O, NiCl2·6H2O, CuCl2· 6H2O, and ZnCl2·6H2O. The samples of Co2Fe−Cl−LDH and Ni2Fe−Cl−LDH were prepared using a similar method by replacing MgCl2·6H2O and CrCl3·6H2O with CoCl2·6H2O (or NiCl2·6H2O) and FeCl3·9H2O. The samples of Ni2Cr−NO3−

(11)

Then the overpotential, η, is obtained by eq 12

67

D


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The Journal of Physical Chemistry C LDH and Ni2Cr−CO3−LDH were also prepared by a similar method via replacing MgCl2·6H2O and CrCl3·6H2O by corresponding nitrates and carbonates. The samples of Ni3Cr−Cl−LDH and Ni4Cr−Cl−LDH were synthesized using a similar method to that of Ni2Cr−Cl−LDH by changing the added amount of NiCl2·6H2O and CrCl3·6H2O. The sample of Ni2Ti−Cl−LDH was prepared by referring to Garcia’s synthesis protocol.10 NiCl2·6H2O and TiCl4 with a Ni2+/Ti4+ molar ratio of 2 were dissolved in a solution of urea (1 M). This solution was heated at 120 °C for 48 h under stirring. The samples of Cu2Ti−Cl−LDH and Zn2Ti−Cl−LDH were prepared by applying a similar method via replacing NiCl2·6H2O with CuCl2·6H2O and ZnCl2·6H2O. All these solids were recovered by filtration and washed copiously with distilled water. The samples were dried at 60 °C and then stored at room temperature. Characterization. Powder X-ray diffraction patterns of MIInMIII/IV−A−LDHs (MII = Mg, Co, Ni, Zn; MIII = Cr, Fe; MIV = Ti; n = 2, 3, 4; A = Cl, NO3, CO3) 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 morphologies of these 12 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 (Mg2Cr−Cl−, Ni2 Cr−Cl−, Zn2 Cr−Cl−, Ni 2Cr−NO 3 −, Ni2 Cr−CO 3−, Ni3Cr−Cl−, Ni4Cr−Cl−LDHs) 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 30 °C, connected with a closed gas circulation system. A 0.02 g amount of each sample (Mg2Cr− Cl−, Ni2Cr−Cl−, Zn2Cr−Cl−, Ni2Cr−NO3−, Ni2Cr−CO3−, Ni3Cr−Cl−, Ni4Cr−Cl−LDHs) 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.).

Figure 1. Optimized geometry of Ni2Cr−Cl−LDH. The color of each element is listed. For the optimized geometries of the other 13 LDHs, see Figure S2 (Supporting Information).

MII2MIII/IV−Cl−LDHs, which indicates a decreasing energy barrier of the d−d electronic transition in the sequence of Co2+-, Ni2+-, and Cu2+-containing LDHs with the same trivalent or quadrivalent cations. The Eg of Zn-based LDHs is the largest among these LDHs with the same MIII/IV. This can be well understood since Zn2+ (d10) has a fully filled d-orbital, implying that the d−d electronic transition is hardly allowed. Mg2Cr− Cl−LDH has an even larger Eg than Zn2Cr−Cl−LDH because Mg2+ has no d-electrons. The band-gap energy of a semiconductor plays a decisive role in light absorption. An Eg value less than 3.1 eV enables visible light absorption. The calculated band-gap energy of each LDH is less than 3.1 eV; thus, all of them are visible-light responsive. Nevertheless, a rather small Eg may lead to a poor oxidation ability of the photogenerated hole; the Eg should be no less than 1.23 eV.5 The influence of the interlayer guest to the band-gap energy of Ni2Cr−A−LDH (A = Cl, NO3, CO3) is smaller than 0.1 eV (2.310−2.393 eV). With the increasing Cr content in NinCr−Cl−LDH, the bandgap energy of NinCr−Cl−LDH (n = 2, 3, 4) decreases. On the other hand, it is found that the momentum (k-vector, see Figure S3 and Table 2) of the VBM and CBM are the same for Ni 2 Ti−Cl−LDH only. For the other 13 LDHs, the momentums of their excited electrons are different from that of the photogenerated holes. Therefore, Ni2Ti−Cl−LDH is a direct band-gap semiconductor, while the other LDHs are indirect band-gap semiconductors. For indirect band-gap semiconductor, a photon cannot be absorbed before the electron passes through an intermediate state and transfer momentum to the crystal lattice (i.e., phonon).75 Therefore, direct band-gap semiconductor, like Ni2Ti−Cl−LDH, is thought to be more suitable for photocatalytic OER than the other LDHs. Density of States. The total density of state (TDOS) of each LDH is shown in Figure 2 with a 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 S1. It is found that for all calculated LDHs, the O-2p

3. RESULTS AND DISCUSSION 3.1. Electronic Properties of MIInMIII/IV−A−LDHs (MII = Mg, Co, Ni, Cu, Zn; MIII = Cr, Fe; MIV = Ti; n = 2, 3, 4; A = Cl, NO3, CO3). The optimized geometries of bulk MIInMIII/IV− A−LDHs are displayed in Figures 1 and S2 (Supporting Information). The lattice parameters of these optimized geometries agree well with the experimental observations (Table 1). The slight distortions of the matrix for Cu2Ti−Cl− LDH and Cu2Cr−Cl−LDH are observed (Figure S2), which is derived from the “Jahn−Teller effect” of Cu.23 Band Structure. The band structures of these LDHs are shown in Figure S3 (Supporting Information). As shown in Table 2, the band-gap energy of Ni2Fe−Cl−LDH is the lowest (1.340 eV) and that of Zn2Ti−Cl−LDH is the largest (3.002 eV) among these LDHs. In previous experimental work, the band-gap energies of NiTi−LDH, ZnTi−LDH, CoCr−LDH, NiCr−LDH, CuCr−LDH, ZnCr−LDH, and CoFe−LDH are determined to be 2.20,71 3.06,72 2.48,73 2.05,74 1.73,73 2.40,74 and 2.25 eV,17 respectively, which are close to the theoretical prediction in this work. The increasing d-electrons of MII (d7 for Co, d8 for Ni, and d9 for Cu) leads to a decreasing Eg of E


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The Journal of Physical Chemistry C

Table 2. Band Gap Energy (Eg), k-Vector of VBM and CBM, Work Function (W), Energy Level of Conduction Band Minimum/ Valence Band Maximum (ECBM/EVBM) Relative to the Vacuum Level, Driving Force (Edf), and Overpotential (η) for MIInMIII/IV−A−LDHsa

a

sample

Eg (eV)

k-vector of VBM

k-vector of CBM

W (eV)

ECBM (eV)

EVBM (eV)

Edf (eV)

η (V)

Ni2Ti−Cl−LDH Cu2Ti−Cl−LDH Zn2Ti−Cl−LDH Mg2Cr−Cl−LDH Co2Cr−Cl−LDH Ni2Cr−Cl−LDH Cu2Cr−Cl−LDH Zn2Cr−Cl−LDH Co2Fe−Cl−LDH Ni2Fe−Cl−LDH Ni2Cr−NO3−LDH Ni2Cr−CO3−LDH Ni3Cr−Cl−LDH Ni4Cr−Cl−LDH

2.249 1.874 3.002 2.721 2.364 2.310 1.641 2.634 2.001 1.340 2.393 2.384 2.294 2.192

0.000 0.000 0.000 0.320 0.745 0.000 0.000 0.863 0.000 0.933 0.941 0.245 0.905 0.683

0.000 0.863 0.366 0.500 0.500 0.866 0.866 0.500 0.866 0.000 0.382 0.531 0.000 0.000

5.526 6.168 4.978 4.874 6.749 4.759 5.163 5.054 4.968 4.283 4.869 4.805 4.908 4.782

−4.402 −5.231 −3.477 −3.514 −5.567 −3.604 −4.343 −3.737 −3.968 −3.613 −3.673 −3.613 −3.761 −3.686

−6.651 −7.105 −6.479 −6.234 −7.931 −5.914 −5.984 −6.371 −5.969 −4.953 −6.066 −5.997 −6.055 −5.878

1.331 1.785 1.159 0.914 2.611 0.594 0.664 1.051 0.649

0.972 0.954 0.904 1.351 0.581 0.810 0.910 0.496

0.746 0.677 0.735 0.558

0.650 0.668 0.632 0.525

MII = Mg, Co, Ni, Cu, Zn; MIII = Cr, Fe; MIV = Ti; n = 2, 3, 4; A = Cl, NO3, CO3.

is calculated to be 1.0491 and 0.9471 J·m−2, respectively (Figure S6, Supporting Information), which are also higher than that of the (003) plane. Therefore, the mostly preferably exposed surface is the (003) plane for LDHs materials, matching well with the previous experimental observations.61−63 This phenomenon can be well understood: the (003) plane is constructed by cleaving the nonbonding interaction between LDH matrix and guest anion, while the (110), (012), and (015) planes are built with cleavage of the M−O bond in the LDH matrix. This nonbonding interaction is mainly consisting of the Coulomb force, hydrogen bond, and van der Waals interaction, which is much weaker than the covalent M−O bond. The surface energies of the (003) and (110) slabs for these LDHs without considering the solvent effect were also calculated and are shown in Table S3. It is found that the surface energies are obviously overestimated if the solvent effect was not considered. Therefore, it is necessary to take the solvent effect into consideration when calculating the surface energy. Band Edge Placement. Table 2 lists the work function (W) obtained by eq 4 based on the electrostatic potential EF and ϕ in Figure S7 (Supporting Information). The work function ranges from 4.283 (Ni2Fe−Cl−LDH) to 6.749 eV (Co2Cr− Cl−LDH) for these calculated LDHs. Subsequently, the energy of CBM/VBM (ECBM/EVBM) relative to the vacuum level of each LDH was calculated with eqs 5 and 6 and transferred to the electric potential as shown in Figure 3; the driving force was further calculated with eq 3. With the obtained driving force, the photocatalytic OER ability of each LDH can be evaluated. For oxygen evolution from water splitting at pH 7, the driving force should be positive. According to Table 2 and Figure 3, all calculated LDHs except Co2Cr−Cl−LDH and Ni2Fe−Cl− LDH have possibilities toward the OER because they provide a positive driving force to oxidize water. Ni2Fe−Cl−LDH does not possess the driving force toward OER because the VBM of Ni2Fe−Cl−LDH lies higher in energy than the oxidation potential of oxygen. For Co2Cr−Cl−LDH, its CBM is under the O2 redox potential, E(O2/H2O). Therefore, it is more favorable for the photogenerated hole to recombine with the photogenerated electron in the CBM than to oxidize water. In order to judge whether the above 12 LDHs can undergo the OER without external bias, the driving force needs to be

orbital is a main component of the VBM, which indicates that the photogenerated hole tends to be localized in the O atom of the 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 molecule. The d-orbitals of Co, Ni, Cu, Zn, Cr, and Fe also contribute to the VBM of the corresponding LDHs, while Ti-3d does not show a strong influence to the VBM of Ti-based LDHs. Therefore, Ni3d, Cu-3d, and Zn-3d orbitals also contribute to the photocatalytic OER activities of Ti-based LDHs. The CBM of the calculated LDHs are mainly composed of the d-orbitals of the transition metals. According to the obtained composition of the VBM and CBM for the calculated LDHs, the electronic structure of LDHs can be modified by changing the metal cations in the LDHs matrix, which is facile for the synthesis of LDHs. In our previous work, the main components of the VBM and CBM of MIInMIII−A−LDHs (MII = Mg, Co, Ni, Zn; MIII = Al, Ga; n = 2, 3; A = Cl, OH, NO3) were calculated.16 It was found that the orbitals of trivalent main group metal cations (Al and Ga) were not the main components of the VBM and CBM. Therefore, the trivalent main group metal cations (Al and Ga) were thought to be inert in photocatalytic OER. However, in this work, the trivalent/quadrivalent transition-metal cation (Cr, Fe, Ti) also contributes to the frontier orbitals (VBM and CBM) of LDHs. Therefore, trivalent/quadrivalent transitionmetal cations-based LDHs may have larger variety in photocatalytic OER behavior than trivalent main group metal cations-based LDHs because both divalent metal cation and trivalent/quadirvalent metal cation can be the OER active sites. Surface Energy. The optimized geometries of the (003) and (110) slabs for MIInMIII/IV−A−LDHs are shown in Figures S4 and S5 (Supporting Information), respectively. As is shown, the two-dimensional layered structures of these LDHs are well kept during geometry optimization and the guest anions are uniformly distributed in the LDHs gallery. The (110) slabs of these 14 LDHs are higher in energy than (003) slabs (Table S2), which indicates that the (003) slabs are more stable than the (110) slabs for LDHs materials. The surface energies of the (003) and (110) slabs for the 14 LDHs are calculated by eq 1 and shown in Table S2. The surface energy of the (003) plane is smaller than that of the (110) plane for each LDH. The surface energy of (012) and (015) planes for Ni2Cr−Cl−LDH F


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Figure 2. Total density of states (TDOS) and partial density of states (PDOS) for the MIInMIII/IV−A−LDHs (MII = Mg, Co, Ni, Cu, Zn; MIII = Cr, Fe; MIV = Ti; n = 2, 3, 4; A = Cl, NO3, CO3). The Fermi level is displayed with a dashed red line.

compared with the overpotential. The calculations of the overpotentials for these LDHs are demonstrated in section 3.2. Besides, the species of interlayer guests and the molar ratio of Ni to Cr also influence the band edge placements of NinCr−A− LDH (n = 2, 3, 4; A = Cl, NO3, CO3). The influences of interlayer guest species and molar ratio on the band edge placements are both smaller than 0.15 eV. 3.2. Thermodynamic Mechanism of OER over MIInMIII/IV−LDHs (MII = Mg, Co, Ni, Cu, Zn; MIII = Cr, Fe; MIV = Ti; n = 2, 3, 4; A = Cl, NO3, CO3). The optimized geometries of 12 LDHs, which are calculated to be able to undergo the OER without external bias, with H2O, radical OH, O, OOH, and H2O2 adsorbed on their surfaces are displayed in Figures 4, S8, S9, and S10. The energies of the radicals adsorbed on various sites (top, bridge, and fcc) for MIInMIII/IV− A−LDHs are listed in Table S4. It is found that for all of the 12 LDHs, radical OH and OOH adsorb at the fcc site and radical O at the bridge site, which is also observed in our previous

Figure 3. Band edge placements of Ni2Ti−Cl−LDH (a), Cu2Ti−Cl− LDH (b), Zn2Ti−Cl−LDH (c), Mg2Cr−Cl−LDH (d), Co2Cr−Cl− LDH (e), Ni2Cr−Cl−LDH (f), Cu2Cr−Cl−LDH (g), Zn2Cr−Cl− LDH (h), Co2Fe−Cl−LDH (i), Ni2Fe−Cl−LDH (j), Ni2Cr−NO3− LDH (k), Ni2Cr−CO3−LDH (l), Ni3Cr−Cl−LDH (m), and Ni4Cr− Cl−LDH (n). Detailed values of the band edge placements are listed in Table 2. Ttwo dashed red lines (−0.41 and 0.82 V vs SHE) represent the reduction potential of H2 and the oxidation potential of O2 at pH 7.

G


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Zn2Cr−Cl−LDH, Ni2Cr−NO3−LDH, Ni2Cr−CO3−LDH, Ni3Cr−Cl−LDH, and Ni4Cr−Cl−LDH, ΔGB is the largest among the four steps, while ΔGC is the largest one for Zn2Ti− Cl−LDH, Cu2Ti−Cl−LDH, Cu2Cr−Cl−LDH, and Co2Fe− Cl−LDH. The reaction with the highest barrier is the potentialdetermining step in OER. Therefore, the potential-determining step is reaction B.1 for the former eight LDHs and reaction C.1 for the latter four LDHs. In other words, the combination of *O and a water molecule to form *OOH is the most difficult step to occur for Zn2Ti−Cl−LDH, Cu2Ti−Cl−LDH, Cu2Cr− Cl−LDH, and Co2Fe−Cl−LDH, while generation of *O is the most difficult step for the other eight LDHs. The overpotential values of water oxidation over these LDHs listed in Table 2 show that the overpotentials of Fe-based LDHs are the smallest, followed by Cr-based LDHs. Also, the overpotentials of Ti-based LDHs are the largest. The influences of interlayer guest species and molar ratio of Ni to Cr on the OER overpotentials of NinCr−A−LDHs are both smaller than 0.1 eV. Whether the LDH can undergo the OER without external bias is influenced by both its overpotential and driving force. In previous work, the scaling relation between the binding energies of the intermediates *OH and *OOH has been widely reported.38,39,53,65−68 They showed that the adsorption energies of *OOH and *OH scale according to the relation ΔE*OOH = ΔE*OH + 3.2 eV, with 95% of the points within ±0.4 eV. In this work, it has been found that radicals OH and OOH adsorb at the same fcc site on LDHs surface. Therefore, the scaling relation between the binding energies of *OH and *OOH is also suitable for these LDHs materials. Thus, the sum of ΔGB and ΔGC is deduced to be 3.2 ± 0.4 eV, which matches with the calculated LDHs here (3.135−3.813 eV). Therefore, the proceeding of step B and step C is usually more difficult than that of step A and step D. The Gibbs free energy of intermediate *O influences the value of ΔGB, and ΔGC directly (eqs 8 and 9). Therefore, the Gibbs free energy of *O determines the potential-determining step to be reaction B.1 or C.1. According to Table S5, the Gibbs free energies of *O for Zn2Ti−Cl−LDH, Cu2Ti−Cl−LDH, Cu2Cr−Cl−LDH, and Co2Fe−Cl−LDH are smaller than the other eight LDHs. This can explain why the potential-determining step is reaction C.1 for Zn2Ti−Cl−LDH, Cu2Ti−Cl−LDH, Cu2Cr−Cl−LDH, and Co2Fe−Cl−LDH and is reaction B.1 for the other eight LDHs. 3.3. Experimental Study of the Structures and Photocatalytic OER Activities of MIInMIII/IV−LDHs. Texture and Crystallinity. In order to test the stability and crystallinity of these 14 MIInMIII/IV−A−LDHs, these 14 samples were prepared by the hydrothermal method (demonstrated in section 2.2). Nevertheless, samples of Cu2Ti−Cl−LDH and Cu2Cr−Cl−LDH were not synthesized successfully. In previous work, Cu2Cr−CO32−−LDH,73 Cu2Cr−LDH with 4phenol carboxylate dihydrate intercalated,76 and Cu2Ti− CO32−−LDH14 have been prepared. The reason why Cu2Ti− Cl−LDH and Cu2Cr−Cl−LDH were not obtained by us is supposed to be the difference in the molar ratio of metal cations, interlayer anion species, and synthesis protocol. In this work, it is found that Cu is difficult to be inserted into LDHs matrix, which is derived from the Jahn−Teller effect of Cu.23 This finding agrees with the theoretical calculation that the distortions of the matrix are observed for Cu2Ti−Cl−LDH and Cu2Cr−Cl−LDH (section 3.1 in this work). Figure 6 shows the PXRD patterns of MIInMIII/IV−A−LDHs samples, from which the reflections can be indexed to a hexagonal lattice with R3m ̅

Figure 4. Optimized structures of the OER intermediates: first, second, third, fourth, and fifth rows represent the reaction site (*), intermediates *H2O, *OH, *O, and *OOH, respectively; while the first, second, and third columns denote Ni2Ti−Cl−LDH (a), Cu2Ti− Cl−LDH (b), and Zn2Ti−Cl−LDH (c), respectively. The color of each element is the same as in Figure S2. Optimized structures of the OER intermediates for the other nine LDHs are shown in Figures S8 and S9, Supporting Information.

work.16 This finding can be well understood as follows: *O has two unbonded electrons which need two bonds connecting with the LDHs surface, while *OH or *OOH has only one unbonded electron, which is more suitable for the fcc site. In our previous work, the coverage effect of OER radicals (OH, O, and OOH) has been calculated. It is found that the influence of the coverage effect on the OER overpotential of LDHs is minor (smaller than 0.15 eV).16 The Gibbs free energies of the calculated LDHs are listed in Table S5. According to mechanism 1, the free energy changes of reactions A.1−D.1 (ΔG298, in eV) of oxygen evolution over LDHs were calculated with eqs 7−10 and are displayed in Figure 5. The free energy changes under mechanism 2 were also calculated with eqs 13−16 and are displayed in Figure S11. By comparing the OER overpotentials calculated with mechanism 1 and mechanism 2 for each LDH, it is found that the overpotentials of mechanism 1 are smaller than that of mechanism 2 for all of the calculated LDHs. Since a mechanism with a smaller overpotential is preferred, all calculated LDHs apply mechanism 1 (with intermediate *O instead of *H2O2). For the calculated LDHs, the proceeding of reaction A.1 (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 subsequent reaction A.1. In general, it is more difficult for reactions B.1 and C.1 to occur, compared with reactions A.1 and D.1. Reaction B.1 is the removal of H from the adsorbed hydroxyl (*OH → *O + H+ + e−), and the reaction C.1 is the generation of radical *OOH (*O + H2O → *OOH + H+ + e−). For Ni2Ti−Cl−LDH, Mg2Cr−Cl−LDH, Ni2Cr−Cl−LDH, H


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Figure 5. Standard free energy diagrams for the OER on MIInMIII/IV−A−LDHs (MII = Mg, Co, Ni, Cu, Zn; MIII = Cr, Fe; MIV = Ti; n = 2, 3, 4; A = Cl, NO3, CO3). Blue numbers (in eV) represent the Gibbs free energy changes of the corresponding OER elementary steps.

Figure 6. Powder X-ray diffraction patterns of MIInMIII/IV−A−LDHs (MII = Mg, Co, Ni, Zn; MIII = Cr, Fe; MIV = Ti; n = 2, 3, 4; A = Cl, NO3, CO3).

MIInMIII/IV−A−LDHs (Table 1). According to Figure 6, seven kinds of LDHs (Ni2Ti−Cl−, Mg2Cr−Cl−, Ni2Cr−Cl−, Zn2Cr−Cl−, Co2Fe−Cl−, Ni2Fe−Cl−, and Ni2Cr−CO3− LDHs) reveal high and sharp LDHs characteristic peaks. According to the Debye−Scherrer equation,78 the thickness of crystal is inversely proportional to the half-peak width of its

rhombohedral symmetry, commonly used for the description of LDHs structure.77 All of these 12 PXRD patterns display the reflections of (003), (006), (009), and (110), which are labeled in Figure 6. The lattice parameters (a = b and c) of MIInMIII/IV− A−LDHs are calculated with the Bragg equation23 and listed in Table S6 (Supporting Information). The obtained lattice parameters match with the calculated lattice parameters of I


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The Journal of Physical Chemistry C characteristic peaks. Therefore, it is speculated that these seven LDHs possess larger thicknesses than the other five LDHs. Furthermore, SEM images of MIInMIII/IV−A−LDHs are displayed in Figure 7, which indicate that the as-synthesized

Figure 7. Scanning electron microscope images of MIInMIII/IV−A− LDHs (MII = Mg, Co, Ni, Zn; MIII = Cr, Fe; MIV = Ti; n = 2, 3, 4; A = Cl, NO3, CO3).

Figure 8. UV−vis reflectance diffuse spectroscopy for Mg2Cr−Cl− LDH, Ni 2Cr−Cl−LDH, Zn2Cr−Cl−LDH, Ni2 Cr−NO3 −LDH, Ni2Cr−CO3−LDH, Ni3Cr−Cl−LDH, and Ni4Cr−Cl−LDH.

LDHs are platelike crystals. The particle sizes of these MIInMIII/IV−A−LDHs are thus obtained and listed in Table S6 (Supporting Information). The particle sizes of these 12 LDHs range from 0.11 to 1.86 μm. Mg2Cr−Cl−, Ni2Cr−Cl−, Zn2Cr−Cl−, Co2Fe−Cl−, Ni2Fe−Cl−, Ni2Cr−NO3−, and Ni2Cr−CO3−LDHs show larger particle sizes than the other five LDHs, which indicate the better crystallinity of these seven LDHs. Photocatalytic Activity. According to the calculated driving forces and overpotentials of these 12 LDHs they can be divided into two types. Type 1 (Mg2Cr−Cl−LDH, Co2Cr−Cl−LDH, and Ni2Fe−Cl−LDH) has a larger overpotential than its driving force, while type 2 (Ni2Ti−Cl−LDH, Zn2Ti−Cl−LDH, Ni 2 Cr−Cl−LDH, Zn 2 Cr−Cl−LDH, Co 2 Fe−Cl−LDH, Ni2Cr−NO3−LDH, Ni2Cr−CO3−LDH, Ni3Cr−Cl−LDH, and Ni4Cr−Cl−LDH) possesses a larger driving force compared to its overpotential. Therefore, type 1 needs an external bias to proceed the OER, while type 2 can overcome its OER overpotential with its OER driving force. Among type 2, the photocatalytic OER activities of NiTi−, ZnTi−, ZnCr−, and CoFe−LDHs have been reported.10−17 Therefore, the photocatalytic OER activity of NinCr−A−LDH (n = 2, 3, 4; A = Cl, NO3, CO3) (type 2) needs to be further studied by experiments so as to validate the computational results. Mg2Cr−Cl−LDH (type 1) and Zn2Cr−Cl−LDH (type 2) are also chosen to be studied as the check sample. In total, seven samples, Mg2Cr−Cl−LDH (type 1), Ni2Cr−Cl−LDH, Zn2Cr−Cl−LDH, Ni2Cr−NO3−LDH, Ni2Cr−CO3−LDH, Ni3Cr−Cl−LDH, and Ni4Cr−Cl−LDH (type 2), are further studied with experiments. UV−vis diffuse reflectance spectra of these seven samples are shown in Figure 8, which indicates that all these LDHs are visible-light responsive. The plots of (αhν)2 vs hν (α is the

absorbance) of Mg2 Cr−Cl−, Ni2 Cr−Cl−, Zn2 Cr−Cl−, Ni2Cr−NO3−, Ni2Cr−CO3−, Ni3Cr−Cl−, and Ni4Cr−Cl− LDHs are obtained by further analyzing their UV−vis diffuse reflectance spectra (Figure S12, Supporting Information). Thus, the band-gap energies of Mg2Cr−Cl−, Ni2Cr−Cl−, Zn2Cr−Cl−, Ni2Cr−NO3−, Ni2Cr−CO3−, Ni3Cr−Cl−, and Ni4Cr−Cl−LDHs are estimated to be 2.70, 2.35, 2.58, 2.38, 2.38, 2.48, and 2.50 eV, respectively. These experimental values of band-gap energy match well with the DFT calculation results (2.72, 2.31, 2.63, 2.39, 2.38, 2.29, and 2.19 eV, respectively). The photocatalytic activity toward water splitting was evaluated by monitoring the time-dependent production of O2 in a solar light-illuminated catalyst suspension. AgNO3 has been widely used as a sacrificial electron acceptor in the photocatalytic OER.10−17 As shown in Figures 9 and S13, Mg2Cr−Cl−LDH does not show the photocatalytic OER activities, while the samples of Ni2Cr−Cl−LDH, Zn2Cr−Cl− LDH, Ni2Cr−NO3−LDH, Ni2Cr−CO3−LDH, Ni3Cr−Cl− LDH, and Ni4Cr−Cl−LDH display good OER activities with an O2 generation rate of 1037, 1079, 669, 614, 871, and 714 μmol h−1 g−1, respectively. The O2 generation rate of Zn2Cr− Cl−LDH agrees with the previous experimental observation (1073 μmol h−1 g−1).10 On the other hand, the O2 generation rate of Ni2Cr−Cl−LDH (1037 μmol h−1 g−1) is the highest among NinCr−A−LDHs (n = 2, 3, 4; A = Cl, NO3, CO3). This value is close to that of Zn2Cr−Cl−LDH (1079 μmol h−1 g−1), which is regarded as an efficient OER photocatalyst.10−12 Thus, the photocatalytic activities of NinCr−A−LDHs (n = 2, 3, 4; A = Cl, NO3, CO3) toward the OER are demonstrated, which matches well with the above theoretical predictions. The photocatalytic activity of Ni2Cr−Cl−LDH is better than that of J


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optimized structures, calculated band structures, optimized geometries of (003) and (110) planes, work functions, optimized geometries of the OER intermediates, OER mechanism, plots of (αhν)2 vs hν, oxygen generation rate; composition of frontier orbitals, energies, and surface energies of the (003), (110), (012), and (015) planes, Gibbs free energies of OER intermediates, lattice parameters, and particle sizes for MIInMIII/IV−A−LDHs (MII = Mg, Co, Ni, Cu, Zn; MIII = Cr, Fe; MIV = Ti; n = 2, 3, 4; A = Cl, NO3, CO3) (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *Phone: +86-010-64412131. E-mail: [email protected].

Figure 9. Temporal profile of the evolved O2 volume during the irradiation for Mg2Cr−Cl−LDH, Ni2Cr−Cl−LDH, Zn2Cr−Cl−LDH, and Ni2Cr−NO3−LDH using 0.01 M AgNO3 as sacrificial acceptor.

Co2Al−NO3−LDH (O2 generation rate 973 μmol h which was found in our previous work.16

−1

ORCID

Hong Yan: 0000-0003-0285-3704 Min Wei: 0000-0001-7540-0824

−1

g ),

Notes

The authors declare no competing financial interest.

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4. CONCLUSION The electronic properties and the thermodynamic mechanisms of OER of 14 MIInMIII/IV−A−LDHs (MII = Mg, Co, Ni, Cu, Zn; MIII = Cr, Fe; MIV = Ti; n = 2, 3, 4; A = Cl, NO3, CO3) which contain transition-metal cations are calculated by using the DFT + U method. The results of the band structure and DOS indicate that all 14 LDHs can absorb visible light. The (003) planes are proved to be the most preferably exposed surfaces in LDHs by the values of the surface energies. The band edge placements illustrate the OER driving force of the LDHs and show that except Co2Cr−Cl−LDH and Ni2Fe−Cl− LDH the other 12 calculated LDHs have the possibility toward the OER. The OER overpotentials are calculated by the thermodynamic mechanism studies, and the value of the overpotential is in the sequence of Fe-based LDHs < Cr-based LDHs < Ti-based LDHs. By comparing the driving force and overpotential, it is found that 10 LDHs (Ni2Ti−Cl−, Cu2Ti− Cl−, Zn2Ti−Cl−, Ni2Cr−Cl−, Zn2Cr−Cl−, Co2Fe−Cl−, Ni2Cr−NO3−, Ni2Cr−CO3−, Ni3Cr−Cl−, and Ni4Cr−Cl− LDH) can overcome the overpotential of water splitting via their driving force provided by the photogenerated hole, while the other LDHs need an external bias to proceed the OER. This is further verified by experimental investigations: O2 generation rates ranging from 614 to 1037 μmol h−1 g−1 were achieved for the samples of NinCr−A−LDHs (n = 2, 3, 4; A = Cl, NO3, CO3), while Mg2Cr−Cl−LDH does not show photocatalytic OER activity, validating the accuracy of theoretical predictions. This work not only provides theoretical insight into the photocatalytic OER performance of LDHs materials which contain transition-metal cations with semiconducting property but also applies a theoretical and experimental combined approach to screen and design LDH photocatalysts toward OER, which would show potential applications in optical/optoelectronic field.

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ACKNOWLEDGMENTS This work was supported by the 973 Program (Grant No. 2014CB932101), the National Natural Science Foundation of China (NSFC), and the Specialized Research Fund for the Doctoral Program of Higher Education (20130010110013). M.W. particularly appreciates the financial aid from the China National Funds for Distinguished Young Scientists of the NSFC. We acknowledge the National Supercomputing Center in Shenzhen for providing the computational resources and the materials studio (version 6.1, CASTEP).

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b10159. Detailed information for the setting of the cutoff energy and the calculating method of Gibbs free energy; K


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