Density Functional Studies on Layered Perovskite Oxyhalide

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Density Functional Studies on Layered Perovskite Oxyhalide BiMOX Photocatalysts (M=Nb and Ta, X=Cl, Br, and I) 4

8

Xin Zhou, and Hao Dong J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06576 • Publication Date (Web): 06 Sep 2017 Downloaded from http://pubs.acs.org on September 12, 2017

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The Journal of Physical Chemistry

Density Functional Studies on Layered Perovskite Oxyhalide Bi4MO8X Photocatalysts (M=Nb and Ta, X=Cl, Br, and I) Xin Zhou a,* and Hao Dong b,* a

College of Environment and Chemical Engineering, Dalian University, Dalian

116622, China. b

Institute of Chemistry for Functionalized Materials, School of Chemistry and

Chemical Engineering, Liaoning Normal University, Dalian, Liaoning 116029, China

* Corresponding author. E-mail addresses: [email protected] (X. Zhou), [email protected] (H. Dong)

ACS Paragon Plus Environment


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Abstract Layered perovskite oxyhalide Bi4MO8X (M=Nb and Ta, X=Cl, Br and I) have recently emerged as suitable photocatalysts for the photocatalytic water-splitting reaction and degradation of organics. Here, we present a comparative study on the crystal structure, electronic structure, water adsorption, and oxygen evolution reaction of these systems. The calculated band gaps using hybrid density functional method HSE06 are smaller than 2.75 eV and increase with the increase of X atomic number, which are in excellent agreement with experimental data. All Bi4MO8X systems possess indirect band gaps, which benefits the separation of photogenerated electron-hole pairs. The density of states reveal that for all the Bi4MO8X cases, the valence band maximum is mostly compose of O 2p states rather than X np states, which can explain the observed stability of these materials against photocorrosion. It is found that the molecular adsorption of water is energetically favorable on Bi4MO8X (001) surfaces. As a result, the computed free energy change for every step in oxygen evolution reaction show that the rate-determining step is the first step of generating OH* species for all the cases. The computed overpotentials (0.69  0.77 V) of Bi4MO8X for oxygen evolution reaction are comparable to and even lower than those of widely used photocatalysts for water oxidation, such as TiO2, WO3, BiVO4 and α-Fe2O3. The calculations suggest that Bi4MO8X (M=Ta and Nb, X=Cl, Br and I) are potential photocatalysts for overall water splitting in the visible light region and we hope that the results reported in this work will stimulate experimental tests of our predictions.

Keywords: photocatalyst, perovskite, oxyhalide, density functional theory


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1. Introduction Photocatalytic technology has attracted increasing attention as a potential strategy to address energy shortages and environmental crises, resulting in the rapid development of efficient and practical photocatalysts. Under solar light irradiation, semiconductor-based photocatalysts can split water to produce hydrogen, convert carbon dioxide into hydrocarbon fuel, and degrade pollutants to harmless inorganics. However, the critical issues of conventional photocatalysts are narrow range of photo-absorption, mainly under UV irradiation, low photocatalytic activity and poor photocatalytic stability. During the recent years, continuing breakthroughs have been made in the improvement of various properties of semiconductor-based photocatalysts by

doping

ions,1-3

controlling

crystal

morphologies,4-7

synthesizing

novel

photocatalysts,8-12 constructing heterostructures from different semiconductors,13-15 loading suitable cocatalysts.16-18 Among these strategies, developing novel photocatalysts has been extensively investigated. A number of oxynitrides and nitrides have recently synthesized and exhibited narrow band gaps, and superior photoelectrocatalytic properties.10, 19-21 However, they are mostly unstable upon light irradiation and prone to self-oxidation during the photocatalytic reaction. Other materials such as oxysulfides and oxyhalides are considered as potential photocatalytsts under visible light irradiation, because the p orbitals of S and X (X=Cl, Br and I) are located at higher energies than O 2p orbitals, leading to a reduction of the band gap compared with the corresponding oxides.22-24 Bismuth oxyhalides Bi4MO8X (M=Nb, Ta and X=Cl, Br, I) are layered semiconductors and belong to the Sillén-Aurivillius family. The structure of Bi4MO8X is composed of fluorite-related [Bi2O2]2+ slabs with strong intralayer interactions alternating separated by either MO4 perovskitic or halid layers.25-27 Bi4MO8X has advantages of unique crystal structures, excellent electronic and optical properties, and thus exhibits many promising applications. In 2007, Lin and co-workers found that Bi4NbO8Cl has higher photocatalytic activity for degrading methyl orange than anatase TiO2 under visible-light illumination. They proposed that polarizing fields in NbO6 and BiO8 local structures and the internal static electrical fields between [Bi2O2]



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and [Cl] slabs induce efficient separation of photogenerated electron-hole pairs.28 Then, Zhang et al. found that the Bi4TaO8X (X=Cl, Br and I) show a high degradation efficiency of methyl orange in aqueous solution and azo dye.29-30 Under visible light irradiation, the photocatalytic activity decreased in the order Bi4TaO8I  Bi4TaO8Br  Bi4TaO8Cl.29

He and co-workers synthesized Bi4NbO8Br by a solid state reaction

and found it has an indirect-transition optical band-gap of 2.34 eV and exhibited photocatalytic activity of decomposing rhodamine-B under visible-light region.31 Bhat and Sundaram reported hierarachical nanostructure of Bi4NbO8Cl shows comparable photocatalytic performance in sunlight and reduced recombination rate based on bulk structure, which can be attributed to the distinct morphology of nanostructure.32 In 2016, Fujito et al. found that Bi4NbO8Cl is an efficient O2-evolving photocatalyst under visible light and enable to overall split water by coupling with a H2-evolving photocatalyst in a Z-scheme system. They investigated electronic structures of Bi4NbO8Cl and Bi4NbO8Br by density functional theory (DFT) calculations, and found that valence band maximum (VBM) of these photocatalysts is predominately composed of O 2p states.33 Recently, Nawaz and co-workers prepared Bi4TaO8Cl with flower-like hierarchical structure, which exhibited higher photocatalytic activity of hydrogen production than commercial Ta2O5.34 Since the synthesis and photocatalytic performance of Bi4MO8X (M=Nb, Ta and X=Cl, Br, I) have been investigated by independent research groups, their photocatalytic capacities could not be directly compared due to different experimental conditions. Furthermore, the origins of photocatalytic activity and varying trends in the properties with different halides have not been studied in detail. Therefore, a comprehensive theoretical investigation is valuable to further understand their essential photocatalytic mechanisms and to predict their photocatalytic ability. In the present work, DFT calculations were performed on the crystal and electronic structures, water adsorption modes and the mechanism of water oxidation reaction of Bi4MO8X. Based on the calculated results, a reasonable explanation for previous experimental observations is revealed, which may benefit designing potentially efficient photocatalysts based on Bi4MO8X.


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2. Computational details All calculations were performed using the projected augmented wave (PAW) method with a plane-wave basis set, implemented in the Vienna Ab initio simulation package (VASP).35-37 The exchange correlation potential was described through the Perdew-Burke-Ernzerhof

(PBE)

functional

within

the

generalized

gradient

approximation (GGA) formalism.38 In the basis we treated explicitly 15 valence electrons for Bi (5d106s26p3), 11 for Ta (5p66s25d3) and Nb (4p65s14d4), 6 for O (2s22p4), and 7 for Cl (3s23p5), Br (4s24p5) and I (5s25p5). The kinetic cut-off energy for the plane-wave expansion was set to 400 eV. Full optimization of the cell parameters for the bulk Bi4MO8X including 56 atoms was carried out by using the 551 Monkhorst-Pack type k-point sampling. On top of the optimized geometries obtained at the GGA-PBE level, a more accurate screened Coulomb hybrid functional HSE06 with 25% HF exchange was adopted to compute band structure and density of states.39-41 The q-space sampled for the hybrid functional calculations is the default value of 1. The slab models of Bi4MO8X surfaces were created by cleaving the crystal structure based on the optimized lattice parameter. In this work, we focused on the (001) surface of Bi4MO8X by use of GGA-PBE calculations. Each 1  1 supercell of the stoichiometric Bi4MO8X unit cell contains 56 atoms in total. For all the surface calculations, a vacuum layer of 15 Å in the perpendicular direction was used in order to avoid the interaction between periodic slabs. During the optimization of the surfaces covered with adsorbates, a Monkhorst-Pack set of 551 k-points was applied. The upper half of the slab and the adsorbates were allowed to relax, while the bottom half of the slab was held fixed at its optimized bulk position. For both bulk and surface calculations, convergence criterions were 10-5 eV for the self-consistent electronic energy minimization and 0.01eV/Å for ionic relaxation. 3. Results and discussion 3.1 Crystal structures Bi4MO8X possesses a layered structure with Sellén-Aurivillius intergrowths, which is described by the orthorhombic space group P21cn.26-27 This structure consists of



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Bi2O2 layers alternately separated by deformed MO6 octahedra and X groups, shown in Fig. 1(a). Based on the experimental characterizations, we built the crystal structures of Bi4MO8X (M=Nb and Ta, X=Cl and Br). Since there is no report on the Bi4MO8I structures, we replaced Br by I to optimize the crystals. In Table 1, we collected the calculated and experimental crystal parameters and selected bond lengths around M, Bi1 and Bi2 atoms displayed in Fig. 1(b). The calculated lattice constants are a=5.4960 Å, b=5.5390 Å, and c=28.5383 Å for Bi4NbO8Cl, a=5.5211 Å, b=5.5676 Å, and c=28.9225 Å for Bi4NbO8Br, a=5.4770 Å, b=5.5166 Å, and c=28.5658 Å for Bi4TaO8Cl, a=5.4942 Å, b=5.5356 Å, and c=29.0375 Å for Bi4TaO8Br. As can be seen, the difference between computed and experimental crystal parameters (a, b, and c) of Bi4MO8Cl and Bi4MO8Br is less than 0.05 Å,26-27 which shows a good consistence and suggests that the computational method in the present work is reasonable. Table 1 shows the lattice parameters a, b and c gradually expand with the increase of ionic size of X- for both Bi4NbO8X and Bi4TaO8X. It is found that there is a slight difference between lattice parameters of Bi4NbO8X and corresponding Bi4TaO8X owing to the similar cationic radius of Nb5+ (0.69 Å) and Ta5+ (0.68 Å). The computed M-O, Bi-O and Bi-X bond lengths are in reasonable agreement with experimental data with a difference smaller than 0.16 Å. The Bi-X bonds are very long ( 3 Å), so there are no bonds between Bi atoms and halogen atoms shown in Figure 1. 3.2 Electronic structures The band structures of Bi4MO8X (M=Nb and Ta, X=Cl, Br and I) using HSE06 functional are plotted in Figure 2, which are calculated along the paths connecting the following high-symmetry point: U(0,0.5,0.5)  X(0,0.5,0)  S(-0.5,0.5,0)  Y (-0.5,0,0)  (0,0,0)  Z(0,0,0.5)  T(-0.5,0,0.5)  R(-0.5,0.5,0.5)  U(0,0.5,0.5) in the k-space. The energy zero (i.e., the Fermi energy level) is set at the VBM. In the cases of Bi4MO8Cl and Bi4MO8Br, the VBM is located on the k-point line of Y and very close to  point. While as to Bi4MO8I, the VBM is located in the region of S Y and close to S point. In all the cases, the conduction band minimum (CBM) is


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situated at  point, which indicates that they are indirect band gap semiconductors. The calculated minimum band gap, the distance between VBM and CBM, is 2.40 eV for Bi4NbO8Cl, 2.45 eV for Bi4NbO8Br, 2.51 eV for Bi4NbO8I, 2.49 eV for Bi4TaO8Cl, 2.53 eV for Bi4TaO8Br and 2.74 eV for Bi4TaO8I. We have also calculated the band gap of these materials using the GGA-PBE method, the obtained band gaps are as follows: 1.42 eV for Bi4NbO8Cl, 1.47 eV for Bi4NbO8Br, 1.74 eV for Bi4NbO8I, 1.48 eV for Bi4TaO8Cl, 1.55 eV for Bi4TaO8Br and 1.86 eV for Bi4TaO8I. The results obtained by HSE06 method shows a better agreement than those by GGA-PBE method compared with experimental measurements (2.38-2.6 eV for Bi4NbO8Cl, 2.34 eV for Bi4NbO8Br, 2.25-2.47 eV for Bi4TaO8Cl, 2.54 eV for Bi4TaO8Br and 2.34 eV for Bi4TaO8I)28-34 The results confirms the crucial need for using the HSE06 functional for obtaining accurate band gap values. Our calculations show that the band gap of Bi4MO8X is wider and wider along with the increase of X atomic number (ClBrI), which supports the experimental observations,29, 33 but opposite to the trend found in BiOX.42-43 During the process of photocatalytic reactions, a photocatalytst absorbs light from sunlight or an illuminated light source. The electrons in the valence band of the photocatalyst are excited to the conduction band, while the holes are left in the valence band. Then, some of separated photo-generated electrons and holes recombine in bulk or at surface, and others migrate to the surface of photocatalyst to act as reducing agent and oxidizing agent to produce H2 and O2, respectively. Concerning the photocatalytic activity, the value and type of band gap are important: the value of band gap determines the response range for solar light. For Bi4MO8X (M=Nb and Ta, X=Cl, Br and I), the band gap is calculated to be less than 2.75 eV, which means that these materials have the ability of absorbing visible light. A direct band gap is favorable for the absorption of photon energy, while an indirect band gap is favorable for hindering the recombination of photogenerated electron-hole pairs.43-44 By means of HSE06 method, the calculated total density of states (TDOS) and partial density of states (PDOS) of Bi4MO8X (M=Nb and Ta, X=Cl, Br and I) are



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illustrated in Figure 3. The Fermi level is set to zero and marked by a vertical dashed line. The CBM is mainly composed of a mixture of Bi 6p and Nb 4d states for Bi4NbO8X and consists of Bi 6p states hybridized with Ta 5d states for Bi4TaO8X. While the VBM is predominately contributed by O 2p states for all the six systems, which is unexpectedly different from the compositions of TDOS in BiOX.43 In BiOBr and BiOI, X-np states importantly contribute for the formation of the VBM, leading to the narrowing of the band gap compared with BiOCl. As shown in Figure 3, for Bi4MO8X, np states of halogen elements are located in the energy level no higher than O 2p states. As the increase of X atomic number (ClBrI), the X-np related PDOS peak is energetically up-shifting. These behaviors are further confirmed by the charge density distribution of the highest valence bands and lowest conduction bands, shown in Figure 4. Due to the similar spatial distribution of charge density distribution for all cases, we only take Bi4NbO8Cl and Bi4NbO8I as examples to perform the analysis. As can be seen, VBM, VBM-1 and VBM-2 have the characteristics of the O 2p states from different O atoms for both Bi4NbO8Cl and Bi4NbO8I. While the CBM and CBM+1 display the similar pattern with most densities from Bi 6p states. The CBM+2 is mainly composed of Nb 4d states for Bi4NbO8Cl and consists of the mixture of Bi 6p and Nb 4d states for Bi4NbO8I. The unique electronic structure of Bi4MO8X may be one of reasons for the stability during photocatalytic activity.33 Photocorrosion is an oxidation or reduction process of photocatalysts, which roots in the interaction among photocatalysts, photogenerated electrons/holes and the surrounding media such as O2 and H2O, or concurrent occurrence of both events. As a result of photocorrosion, the lifetime and performance of photocatalysts are significantly decreased. Generally speaking, for mixed anion materials (oxynitrides, oxysulfides, and oxyhalides), the DOS in VBM are mainly composed of p orbitals of nonoxide anion (S2-, N3-, X-) with a smaller electronegativity. Photogenerated holes in VBM preferentially oxidize these anions instead of water molecules (e.g., S2- + 2h+  S, 2N3- + 6h+  N2, 2X- +2 h+  X2), resulting in an inactive surface.45-46 On the contrary, the VBM of Bi4MO8X is predominately composed of O 2p states so that the holes populated on the stable


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oxygen anions near VBM can efficiently oxidize water, without suffering from deactivation by self-oxidation of X-. 3.3 Band edge position Along with a sufficient band gap, the semiconductor also needs to possess suitable band edge alignment for the oxidation and reduction potentials. To drive the overall water splitting reaction, the CBM of semiconductor has to be more negative than the reduction potential of H+/H2 (0 V vs. NHE), whereas the VBM has to be more positive than the oxidation potential of O2/H2O (1.23 V). Therefore, in order to evaluate photocatalytic activities of Bi4MO8X (M=Nb and Ta, X=Cl, Br and I), we computed band edge positions of their valence bands and conduction bands. It has been reported that for Bi4NbO8Cl, the CBM is 0.14 eV above the H+/H2 level and the VBM is 1.00 eV below the H2O/O2 level at pH=0, making it active for an overall water-splitting process.33 In this work, the VBM and CBM values of Bi4NbO8Cl with respect to the NHE potential are taken from experimental values. We have adopted the method proposed by Bulter and Ginley to determine relative band edge positions of other Bi4MO8X compared to the values of Bi4NbO8Cl.47 In this method, band edge positions are estimated from the absolute electronegativity of atoms and the band gap of the semiconductors by the following equations:48 ECB= – χ + E0 + 1/2Eg, and EVB= Eg + ECB, where ECB and EVB are the potentials of the CB and VB edges with respect to the normal hydrogen electrode (NHE), respectively. χ is the electronegativity of the semiconductor and equals the geometric mean of the absolute electronegativity of the constituent atoms; E0 is the scale factor relating the reference electrode redox level to the absolute vacuum scale (E0 = -4.5 eV for a NHE), and Eg is the calculated band gap of the semiconductor. The absolute electronegativity of an atom is given by the Mülliken definition, that is, the arithmetic mean of the atomic electron affinity (A) and the first ionization energy (I). Here, the A, I and χ of the constituent atoms of Bi4MO8X (M=Nb and Ta, X=Cl, Br and I) are obtained by a paper by Pearson.49 As shown in Figure 5, compared with the positions of VBM and CBM of Bi4NbO8Cl, the CBM is raised by 0.06 eV for Bi4NbO8Br, 0.14 eV for Bi4NbO8I, 0.03 eV for Bi4TaO8Cl, 0.09 eV for Bi4TaO8Br, and 0.25 eV for Bi4TaO8I;



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the VBM is raised by 0.01 eV Bi4NbO8Br and 0.03 eV for Bi4NbO8I; the VBM is lowered 0.06 eV for Bi4TaO8Cl, 0.04 eV for Bi4TaO8Br, and 0.09 eV for Bi4TaO8I. The results indicate that with the increase of atomic number of X, the ability of reduction slightly reduces for Bi4MO8X with the same M. The capability of reduction for Bi4TaO8X is a little weaker than that for the corresponding Bi4NbO8X. The difference of positions of VBM is less than 0.03 eV in Bi4NbO8X and less than 0.05 eV in Bi4TaO8X, which reveals that the ability of oxidizing water is almost independent of X. Bi4TaO8X have slightly lower ability of water oxidation than Bi4NbO8X. Figure 5 shows that all the Bi4MO8X (M=Nb and Ta, X=Cl, Br and I) would have the ability of overall water splitting due to the suitable positions of CBM and VBM with respect to the H+/H2 and H2O/O2 redox levels, respectively. 3.4 Water adsorption In photocatalytic applications, most reactions are carried out in aqueous solution or, at least, need the participation of water with surface hydroxyl radicals derived from water decomposition being an important reaction intermediate.50-51 It is important to understand the interaction between water and semiconductor surfaces. Therefore, we have theoretically investigated the water adsorption on Bi4MO8X (001) surface. We have constructed the stoichiometric (001) surface with different terminations and take Bi4TaO8Br as an example to display the terminations and their relative energies in Figure S1. The results show that the symmetric Br-exposed surface has the lowest energy. Therefore, we perform following calculations based on this stoichiometric symmetric (001) surface. Both molecular and dissociative water adsorptions at different sites were considered. For every case, all the dissociatively adsorbed water ends up molecularly adsorbed on the (001) surface. The adsorption structures of H2O on Bi4MO8X (001) surface are shown in Figure 6. The results show that H2O molecule is attached to the surface by the interaction between Bi and O in water (Ow), which is similar with the adsorbing configuration of water molecules in other bismuth oxides, like BiVO4.52-53 The top views in Figure 6 show that H atoms are located above the surface O atoms.


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The calculated Bi-O distance is 2.883 Å for Bi4NbO8Cl, 2.908 Å for Bi4NbO8Br, 2.955 Å for Bi4NbO8I, 2.902 Å for Bi4TaO8Cl, 2.935 Å for Bi4TaO8Br and 2.948 Å for Bi4TaO8I. These Bi-O distances are longer than the regular Bi-O bond lengths shown in Figure 1 and Table 1, which indicates that the weak interaction between water molecule and (001) surface. The bonding strength of water adsorption on surfaces can be calculated using the adsorption energy defined as Eads= Etotal  Esurf  EH2O, in which Etotal is the total energy of the water adsorbed surface after geometry relaxation, Esurf is the energy of the surface slab without water adsorption, EH2O is the energy of one free water molecule. According to this definition, a more negative Eads value corresponds to more exothermic and stronger adsorption. The computed adsorption energy is -0.55 eV for Bi4NbO8Cl and Bi4NbO8Br, -0.53eV for Bi4NbO8I, -0.54 eV for Bi4TaO8X (X=Cl, Br and I), respectively. The results also suggest that the absorbing strength of water molecule is weak and quite similar in all the systems due to the molecular adsorption of water. To shed more light on the effect of water on the electronic structure of surface, we calculated DOS for every Bi4MO8X (001) surface adsorbed with water. Since they are very similar, we only displayed DOS figures of Bi4NbO8Br and Bi4TaO8I as instances in Figure 7. The results indicate that in the energy range of -10 eV to 6 eV, most of the contribution of water in DOS is located in valence band, which is composed of a localized energy state at about -8 eV and a broad energy state from -5 to -1 eV. VBM has seldom character of water. In other words, the adsorption of water has little effect on compositions of VBM and CBM. 3.5 Water oxidation mechanism During the process of overall water splitting, the oxygen evolution reaction (OER) is regarded as the bottleneck, because OER is a four-electron-transfer and energetically uphill reaction.54 Recently, Fujito et al. found that Bi4NbO8Cl is a stable O2-evoluting photocatalyst with the comparative performance to unmodified WO3 under the similar reaction condition.33 In order to explore the mechanism of OER in Bi4MO8X, we have performed the DFT calculations. Insights into the thermodynamics of the OER can be obtained by the scheme developed by Nørskov and co-workers,55 in which the



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molecular oxygen is formed through a surface HOO* intermediate and the reaction takes place at the coordinatively unsaturated surface sites. In the scheme, the OER is assumed to consist of four elementary reaction step, each involving the electron transfer accompanied by proton removal: H2O + *→HO* + H+ + e–

(1)

HO* → O* + H+ + e–

(2)

O* + H2O → HOO* + H+ + e–

(3)

HOO* → * + O2 + H+ + e–

(4)

where * denotes a surface site and X* represents an adsorbed X intermediate on the surface. We obtain the energy of H+ + e– implicitly by referencing it to the energy of H2 using the standard hydrogen electrode. This implies that at standard conditions (pH=0, p=1 bar and T=298 K) the free energy of H+ + e– can be taken equal to be half the formation energy of H2. The reaction free energy, ΔG = ΔE + ΔZPE - TΔS, is calculated as follows: The reaction energy ΔE is obtained from DFT calculations. The differences in zero point energies (ZPE), ΔZPE, and the change in entropic contribution ΔS are calculated using computed vibrational frequencies and standard tables for the reactants and products in the gas phase.56 The entropies for the atoms and molecules adsorbed to the surface active site are assumed to be zero. The temperature dependence of the enthalpy is neglected in these calculations. For simplicity, the effect of pH is not considered here and we restrict the calculations to pH=0. The free-energy change of the total reaction H2O  1/2 O2 + H2 is fixed at the experimental value of 2.46 eV per water molecule. This means that in the reaction step involving the formation of O2, we consider that ΔG(2H2OO2+2H2) = 4.92 eV =EO2+ 2EH2 2EH2O +(ΔZPE-TΔS)(2H2OO2+2H2) . The reaction overpotential can be calculated from the difference between the voltage at which all free-energy steps become downhill and the minimum voltage required for the OER. Figure 8 shows the calculated free energy diagrams of OER on all the Bi4MO8X surfaces at U = 0 V, pH = 0 and T = 298 K, respectively. The detailed free-energy diagrams of OER at different biases and the relaxed structures for intermediates on four surfaces are included in Fig. S2-S7. The computed results demonstrate that, for


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no applied bias, U = 0 V, all steps in all the surfaces are uphill. Even at standard equilibrium potential for oxygen evolution at U = 1.23 V, some of the steps become downhill but some still remain uphill. So it is necessary to impose a bias (or overpotential) on all the surfaces to have every step downhill. As shown in Fig. 8, for all the Bi4MO8X, the rate-limiting step is the first step, the process of generating the OH* species, which may possibly be explained by the fact that the dissociative adsorption of water on the surfaces is energetically unfavorable. The generated value of overpotential is 0.73 V for Bi4NbO8Cl, 0.77 V for Bi4NbO8Br, 0.70 V for Bi4NbO8I, 0.72 V for Bi4TaO8Cl, 0.72 V for Bi4TaO8Br and 0.69 V for Bi4TaO8I. The second step, the transfer of a proton from the surface adsorbed OH to the electrolyte, has the slight lower ΔG value than the first step. In the third step, the dissociation of a second water molecule and the loss of a proton-electron pair result in the formation of HOO* structure. As shown in Figure 8, the ΔG value of this step is 1.17 eV for Bi4NbO8Cl, 1.21 eV in Bi4NbO8Br, 1.51 eV in Bi4NbO8I, 1.34 eV in Bi4TaO8Cl, 1.34 eV in Bi4TaO8Br, 1.32 eV in Bi4TaO8I. The relaxed structures of OOH adsorbed surfaces indicate that the shortest distance between H atom and the surface X atom is 1.999 Å in Bi4NbO8Cl, 2.170 Å in Bi4NbO8Br, 3.265 Å in Bi4NbO8I, 2.184 Å in Bi4TaO8Cl, 2.372 Å in Bi4TaO8Br, 2.633 Å in Bi4TaO8I. The obvious longer M-X distance in Bi4NbO8I makes the configuration less stable than other cases, which possibly explains the apparent increase in height of the third step for Bi4NbO8I. Rossmeisl et al. investigated three widely used catalysts for OER, and showed that the calculated overpotential for (110) surface of RuO2, IrO2 and TiO2 is 0.37 V, 0.56 V and 1.19 V, respectively.57 Liao and co-workers reported the calculated overpotential of OER for α-Fe2O3 (0001) surface is 0.77 V, which is proven to be a good photoanode material for the water oxidation process.58 As to the stable termination of Co3O4 (110) surface, the overpotential is theoretically predicted to be 0.390.66 V.59 Among (200), (020) and (002) surfaces of WO3, the smallest overpotential is computed to be 1.04 V.60 Our previous work indicated that (010) and (011) surface of BiVO4 need overpotential of 1.35 V and 1.33 V for OER, respectively.53 In other words, based on the computed overpotential of OER, the ability of water oxidation for



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Bi4MO8X is supposed to be higher than TiO2, WO3 and BiVO4, comparable to α-Fe2O3, and lower than RuO2, IrO2 and Co3O4. 4. Conclusions The crystal structures, electronic structures, favorable adsorption configurations of water and OER mechanisms of Bi4MO8X (M=Nb and Ta, X=Cl, Br and I) have been studied using GGA-PBE and HSE06 methods. Our results show that the calculated crystal structures are consistent with the experimental observations and the lattice parameters gradually expand with the increase of ionic size of X- for both Bi4NbO8X and Bi4TaO8X. All Bi4MO8X systems possess indirect band gaps, which is favorable for the separation of photogenerated electron-hole pairs. The values of band gap predicted by HSE06 functional are in excellent with experimental data and more accurate than those computed by GGA-PBE method. The density of states and charge distribution reveal that the VBM of Bi4MO8X predominately consists of O 2p states, which may be one of the reasons for their high stabilities in photocatalytic reactions. The favorable adsorption of water on Bi4MO8X (001) surface is molecular configuration, which is the main reason that the first step in OER is the rate-determining step. The calculated overpotentials of OER for Bi4MO8X are in the range between 0.69 V and 0.77 V, which are comparable to and even lower than those computed for other widely used water oxidation photocatalysts. Our results show that Bi4MO8X (M=Nb and Ta, X=Cl, Br and I) is a series of promising photocatalysts for splitting water under visible light irradiation and further experiments are highly demanded. Acknowledgements This work is financially supported by National Natural Science Foundation of China under Grant 21473183 and 21303079. References 1.

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Table 1. Theoretical and experimental (in parentheses) crystal parameters and selected local structures (Å) for Bi4MO8X (M=Nb and Ta, X=Cl, Br, and I). parameter

Bi4NbO8Br

Bi4NbO8I

5.4960 (5.4472)26 5.5390 (5.4901) 28.5383 (28.8125)

5.5211 (5.4846)27 5.5676 (5.5334) 28.9225 (29.0951)

5.5557

Bi4TaO8Br

Bi4TaO8I

5.4770 (5.4589)26 5.5166 (5.5044) 28.5658 (28.6998)

5.4942 (5.4710)27 5.5356 (5.5172) 29.0375 (29.2241)

5.5279

M-O1 M-O2 M-O3 M-O4

1.892(1.94) 1.891(1.97) 1.996(1.96) 2.159(2.02)

1.892(1.89) 1.892(1.92) 1.992(2.03) 2.177(2.18)

1.892 1.892 1.984 2.196

1.913 (1.93) 1.914 (1.97) 1.989 (1.98) 2.083 (2.06)

1.913(1.83) 1.913(1.91) 1.987(1.95) 2.092(2.15)

1.911 1.912 1.980 2.112

M-O5 M-O6 Bi1-O1

2.159(2.01) 1.996(2.05) 2.304(2.27)

2.176(2.08) 1.991(2.01) 2.300(2.23)

2.192 1.984 2.298

2.081 (2.05) 1.990 (2.00) 2.318(2.17)

2.092(2.05) 1.988(2.08) 2.314(2.20)

2.113 1.981 2.311

Bi1-O2 Bi1-O3 Bi1-O4

2.324(2.37) 2.373(2.38) 2.271(2.21)

2.332(2.24) 2.369(2.46) 2.269(2.13)

2.349 2.363 2.263

2.331(2.31) 2.373(2.50) 2.268(2.24)

2.339(2.31) 2.371(2.42) 2.265(2.08)

2.353 2.358 2.263

Bi1-O5 Bi1-O6 Bi2-O1

2.602(2.71) 2.598(2.58) 2.236(2.21)

2.607(2.76) 2.613(2.61) 2.244(2.21)

2.606 2.652 2.251

2.606(2.76) 2.572(2.65) 2.229 (2.22)

2.611(2.64) 2.588(2.70) 2.235(2.28)

2.610 2.621 2.241

Bi2-O2 Bi2-O3 Bi2-O4 Bi2-X1 Bi2-X2 Bi2-X3 Bi2-X4

2.223(2.29) 2.285(2.27) 2.216(2.18) 3.283(3.25) 3.368(3.40) 3.414(3.45) 3.295(3.32)

2.229(2.10) 2.285(2.41) 2.219(2.14) 3.386(3.45) 3.443(3.56) 3.456(3.46) 3.402(3.45)

2.240 2.282 2.216 3.490 3.575 3.577 3.530

2.216 (2.16) 2.279 (2.23) 2.219 (2.21) 3.276 (3.35) 3.358 (3.38) 3.406 (3.37) 3.294 (3.40)

2.222(2.24) 2.281(2.27) 2.218(2.17) 3.392(3.46) 3.453(3.51) 3.463(3.39) 3.366(3.47)

2.232 2.277 2.218 3.513 3.554 3.576 3.520

a b c

Bi4NbO8Cl

5.6001 29.3496

Bi4TaO8Cl


5.5656 29.5001


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Figure 1. (a) Crystal structure of Bi4MO8X (M=Nb and Ta, and X=Cl, Br and I). (b) Coordination environment around M, Bi1 and Bi2 with relative bond lengths listed in Table 1. The blue, purple, green and red balls represent metal, bismuth, halide and oxygen atoms, respectively.


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Figure 2. The band structure of (a) Bi4NbO8Cl, (b) Bi4NbO8Br, (c) Bi4NbO8I, (d) Bi4TaO8Cl, (e) Bi4TaO8Br, and (f) Bi4TaO8I. The horizontal dashed lines represent the Fermi level.



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Figure 3. The density of states of (a) Bi4NbO8Cl, (b) Bi4NbO8Br, (c) Bi4NbO8I, (d) Bi4TaO8Cl, (e) Bi4TaO8Br, and (f) Bi4TaO8I. The vertical dashed line represents the Fermi level.


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Figure 4. The charge density distribution of the highest valence bands and lowest conduction bands of (a) Bi4NbO8Cl and (b) Bi4NbO8I. This displays the isosurface of density difference at a value of about 0.002 electrons per Å3. The purple, cyan, red, green and dark purple balls represent Bi, Nb, O, Cl and I atoms, respectively.



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-3.5 H+/H2

-4.5 -5.5

2.40 eV

2.45 eV

2.51 eV

2.49 eV

2.53 eV

2.74 eV

O2/H2O

-6.5 -7.5

Bi4NbO8Cl Bi4NbO8Br

Bi4NbO8I

Bi4TaO8Cl

Bi4TaO8Br

Bi4TaO8I

Figure 5. Schematic representation of the calculated VBM and CBM positions of Bi4MO8X (M=Nb and Ta, and X=Cl, Br and I).


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Figure 6. Optimized structures (side and top views) of water adsorption on (001) surface of (a) Bi4NbO8Cl, (b) Bi4NbO8Br, (c) Bi4NbO8I, (d) Bi4TaO8Cl, (e) Bi4TaO8Br, and (f) Bi4TaO8I. The purple, red, green, blue, orange, grey, brown and white balls represent Bi, O, Nb, Ta, Cl, Br, I and H atoms, respectively.



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Figure 7. The density of states of water adsorption on (001) surface of (a) Bi4NbO8Br, and (b) Bi4TaO8I.


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Figure 8. Free energy diagram for oxygen evolution reaction on the (001) surface of (a) Bi4NbO8Cl, (b) Bi4NbO8Br, (c) Bi4NbO8I, (d) Bi4TaO8Cl, (e) Bi4TaO8Br, and (f) Bi4TaO8I at U=0 V, pH=0 and T=298 K. The ΔG value (vertical solid line with arrows) and the corresponding external potential needed (in parentheses) of the rate-determining step for energy surface are shown.



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Table of Contents


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Density Functional Studies on Layered Perovskite Oxyhalide

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