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Valence band engineering of layered bismuth oxyhalides toward stable visible-light water splitting: Madelung site potential analysis Daichi Kato, Kenta Hongo, Ryo Maezono, Masanobu Higashi, Hironobu Kunioku, Masayoshi Yabuuchi, Hajime Suzuki, Hiroyuki Okajima, Chengchao Zhong, Kousuke Nakano, Ryu Abe, and Hiroshi Kageyama J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b11497 • Publication Date (Web): 06 Dec 2017 Downloaded from http://pubs.acs.org on December 6, 2017

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Journal of the American Chemical Society

Valence band engineering of layered bismuth oxyhalides toward stable visible-light water splitting: Madelung site potential analysis Daichi Kato,† Kenta Hongo,‡∥# Ryo Maezono,‡ Masanobu Higashi,† Hironobu Kunioku,† Masayoshi Yabuuchi,† Hajime Suzuki,† Hiroyuki Okajima, † Chengchao Zhong,† Kousuke Nakano,‡ Ryu Abe,*† § and Hiroshi Kageyama*†§ †

Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan

‡

Research Center for Advanced Computing Infrastructure, JAIST, Asahidai 1-1, Nomi, Ishikawa 923-1292, Japan

∥

Center for Materials research by Information Integration (CMI2), National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, JAPAN # PRESTO, Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-0012, Japan §

CREST, Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-0012, Japan

ABSTRACT: A layered oxychloride Bi4NbO8Cl is a visible-light responsive catalyst for water splitting, with its remarkable stability ascribed to the highly dispersive O-2p orbitals in the valence band, the origin of which, however, remains unclear. Here we systematically investigate four series of layered bismuth oxyhalides, BiOX (X = Cl, Br, I), Bi4NbO8X (X = Cl, Br), Bi2GdO4X (X = Cl, Br) and SrBiO2X (X = Cl, Br, I) and found that Madelung site potentials of anions capture essential features of the valence band structures of these materials. The oxide anion in fluorite-like blocks (e.g. [Bi2O2] slab in Bi4NbO8Cl) is responsible for the upward shift of the valence band, and the degree of electrostatic destabilization changes depending on building layers and their stacking sequence. This study suggests that the Madelung analysis enables a prediction and design of the valence band structures of bismuth and other layered oxyhalides and is applicable even to a compound where DFT calculation is difficult to perform.

INTRODUCTIONS Developing visible-light responsive photocatalysts for water splitting has been of technological challenge.1-5 Mixed anion compounds such as oxynitrides,6-14 oxysulfides15 and oxyhalides16 have been extensively investigated as promising candidates since the lower electronegativity of non-oxide anions locates their p orbitals at higher energies than O-2p orbitals, resulting in a reduced band gap. As shown in Figure 1a, this trend is exemplified by prototypical oxyhalides BiOX (X = Cl, Br, I) with alternating [Bi2O2] and [X2] layers (Figure 2a), where the band gap reduces from 3.42 eV (X = Cl), to 2.78 eV (X = Br) and to 1.84 eV (X = I), accompanied by negative shift of valence band maximum (VBM).17-20 However, most of mixed anion photocatalysts are prone to self-decomposition upon light irradiation primarily due to facile oxidation of non-oxide anions by photo-generated holes near VBM. Consequently, many efforts have been devoted to circumvent the oxidative deactivation by surface modifications such as loading some cocatalysts.8-14

Figure 1. Schematic valence and conduction band structures of (a) BiOX (X = Cl, Br, I) and (b) Bi4NbO8X (X = Cl, Br), where the data (Table S1) were taken from previous reports,17-20 except for the band position of BiOI. Here, flat band positions and band gaps are experimentally obtained, respectively, from Mott-Schottky measurements at pH = 2 and UV-vis diffuse reflectance spectra. Mott-Schottky data of BiOI, shown in Figure S5a, is in agreement with previous report.21 The red, blue, green and orange boxes respectively represent the bands from O-2p, Cl-3p, Br-4p and I-5p orbitals and the gray box shows a conduction band composed mainly of Bi-6p, according to DFT calculations.17-20

We have recently demonstrated that a single-layer Sillén− Aurivillius type perovskite Bi4NbO8X (X = Cl, Br), com-

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posed of 2[Bi2O2], [X] and [NbO4] layers (Figure 2b), can stably and efficiently oxidize water to O2 under visible light;17 without any surface modifications, no significant self-decomposition is observed after the reaction, with its apparent quantum efficiency comparable to well-known photocatalysts.22 Subsequently, the tantalum counterparts Bi4TaO8X (X = Cl, Br)20 and a structurally related Bi6NbWO4Cl23 were turned out to be also stable photocatalysts for water oxidation. The density functional theory (DFT) calculations of Bi4NbO8X revealed an utmost characteristic of the valence band structure with highly dispersive O-2p orbitals. This causes the VBM to be mainly composed of O-2p orbitals, rather than Cl-3p and Br-4p orbitals (Figure 1b). We attributed the stable photocatalytic activity to the occupation of O-2p orbitals around VBM that are expected to be robust against holes inserted, since many properties of oxides are described in terms of the ligand hole concept at the O-2p band as seen in high-Tc cuprate superconductors,24 SrFeO3,25 and cathode materials for lithium battery.26 Unfortunately, the origin of this unusual valence band structure in Bi4MO8X (M = Nb, Ta) is not clear yet. Our recent study20 has shown that Bi4NbO8X has a sizable interaction between O-2p orbitals and Bi-6s orbitals, which is derived from the revised lone pair (RLP) model,27 and this interaction appears to explain elevated (or destabilized) O-2p orbitals. However, we also found that such hybridization effect with a similar magnitude is present in BiOX with a conventional valence band structure (Figure 1a), indicating that there must be another key factor that differentiates the band structure of Bi4MO8X from that of BiOX. The difficulty in understanding the structure-property relation in Bi4MO8X lies in the complexity of its crystal structure; it has heavily tilted octahedral layers, leading to eight independent oxygen (O1-O8) sites (Figure 2b). Furthermore, DFT calculations show that the 2p orbitals for all the oxygen sites are highly dispersive, spreading from the bottom to the top of the valence band (Figure S1 taken from Ref 17), making it impossible to figure out which oxygen site(s) or other factors plays a crucial role in the unusual valence band structure.

Figure 2. Crystal structures of (a) BiOX (X = Cl, Br, I) and (b) Sillén-Aurivillius type perovskite Bi4NbO8X (X = Cl, Br). The purple, red and yellow-green balls, respectively, denote Bi, O and halogen (X) atoms, while NbO6 octahedra are shown in green. The BiOX structure consists of fluorite-like [Bi2O2] layers sandwiched between double halide [X2] layers, giving X−Bi−O−Bi−X sequence along the [001] direction. The Bi4NbO8X structure also

consists of fluorite-like [Bi2O2] layers, but sandwiched alternately by single [X] layer and perovskite [NbO4] layer. Dotted lines indicate the unit cell of each structure.

In this study, we investigate a series of layered bismuth oxyhalides, BiOX (X = Cl, Br, I), Bi4NbO8X (X = Cl, Br), Bi2GdO4X (X = Cl, Br) and SrBiO2X (X = Cl, Br, I), which have layers of various units such as [Bi2O2], [X2], [X], [NbO4], [SrBiO2], and [Bi2GdO4]. Remarkably, we found that Madelung site potentials, or the summation of electrostatic potential created at the selected crystallographic site by surrounding ions, can capture essential features of valence band structures of these materials including Bi4NbO8X. So far, such Madelung site potential analysis has been used to obtain a rough scheme of the band structures in ionic solids such as NaCl, ZnS, perovskite ABX3 semiconductors,28-30 and more recently to discuss the relative electronic energy levels of eight TiO2 polymorphs.31,32 However, as far as the authors are aware, this method has not been applied to a general and broad system, except for a number of studies on high-Tc cuprate superconductors and related systems.33-35 We show that the Madelung analysis enables us not only to understand how the layer sequence in the bismuth oxyhalides influences the valence band structures, but to design and predict band structures even when DFT calculation is difficult to perform.

EXPERIMENTAL PROCEDURES We synthesized BiOX (X = Br, I), Bi2GdO4X (X = Cl, Br) and SrBiO2X (X = Cl, Br, I) to obtain information on the band structures. Bi2O3 (Wako, 99.99%), BiOCl (Wako, 99.9%), KBr (Wako, 99.9%), KI (Wako, 99.9%), Bi(NO3)3· 5H2O (Wako, 99.9%) SrO (Aldrich, 99.9%), Gd2O3 (Wako, 99.9%) powder specimens were used as received, while BiOX (X = Br, I) was prepared by a soft liquid deposition method as described elsewhere.17 Single phase of Bi2GdO4X (X = Cl, Br) was prepared by heating a 1 : 1 : 2.4 mixture of Bi2O3, Gd2O3 and BiOX under an Ar gas flow (30 mL/min) at 800 °C for 4 hours. Note that heating stoichiometric mixture gave small amount of Gd2O3, possibly resulting from a volatility of BiOCl. SrBiO2X (X = Cl, Br) were obtained by heating the stoichiometric mixture of SrO and BiOX (X = Cl, Br) at 800 °C for 20 hours. To prepare SrBiO2I, the stoichiometric mixture of SrO and BiOI was placed in an alumina crucible, sealed into an evacuated silica tube and heated at 800 °C for 5 days. To prepare Bi3Sr2Nb2O11X (X = Cl, Br), the stoichiometric mixture of SrO, Bi2O3, Nb2O5 and BiOX was placed in an alumina crucible, sealed into an evacuated silica tube and heated twice at 900 °C for 20 hours. The phase purity of all samples was monitored by X-ray powder diffractometer (MiniFlex II, Rigaku, Cu Kα) (Figure S2). The band gap energies of Bi2GdO4X (X = Cl, Br), SrBiO2X (X = Cl, Br, I) and Bi3Sr2Nb2O11X (X = Cl, Br) were estimated from Tauc plot of UV-visible diffuse reflectance spectra collected with UV-visible diffuse reflectance spectroscopy (V-650, Jasco). The Mott-Schottky measurements were carried out for BiOI, Bi2GdO4X and SrBiO2Cl to determine the flat-band potentials using an electrochemical analyzer (PARSTAT2263, Princeton Applied Research). We prepared a paste by mixing a target sample (30 mg) and water (100 µL), which was coated on a fluorine-doped tin oxide (FTO) substrate, and then dried at room temperature.


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Journal of the American Chemical Society The electrochemical cell was composed of sample/FTO electrode, a counter electrode (Pt wire), a Ag/AgCl reference electrode, and an electrolyte solution. As an electrolyte solution, we used a Na2SO4 solution (0.1 M, adjusted to pH 2 by adding H2SO4) for BiOI and Bi2GdO4X and a phosphate buffer solution (0.1 M, pH = 6.0) for SrBiO2Cl. AC amplitude and frequency applied were 10 mV and 1 kHz. To elucidate valence band structures of SrBiO2X (X = Cl, Br, I) and Bi2GdO4X (X =Cl, Br), we performed systematic first-principles simulations based on density functional theory (DFT).36,37 All the present computations were carried out using the CASTEP package38 based on plane-wave basis and pseudopotential approach. Their computational details were given in the caption of Figure S6 for density of states (DOS) and the footnote of Table S2 for fully optimized crystal structures. For comparison, electronic structures for BiOX (X = Cl, Br, I) and Bi4NbO8X (X = Cl, Br) were taken from literatures.17-20 Since DFT treatments of lanthanide compounds having partially-filled f electrons remain challenging,39,40 lanthanum or yttrium substituted correspondences are frequently used to model/approximate the original band structures.41,42 This will be validated in our study because these cations hardly affect valence band structures. In fact, the computed valence band structure of Bi2YO4Cl is quite similar to that of Bi2NdO4Cl,43 implying that it can also be a good approximation to Bi2GdO4Cl. The energy levels of each anion in all the compounds were determined by the sum of electron affinities and Madelung site potential. For the latter, we employed the reported crystal structures44-51 and calculated using VESTA.52 Radius of an ionic sphere was set at a value that is large enough but less than half the smallest interatomic distance in each structure and reciprocal-space range was set to 3 Å– 1 . We also checked convergence of the Madelung site potentials by variation of these parameters. The first electron affinities of chlorine, bromine, iodine are 3.61 eV, 3.36 eV and 3.06 eV, respectively.53 Although the second electron affinity of oxygen is ill-defined, it is usually taken to about 8.0 eV.54-56 We used 8.199 eV employed previously in Ref. 56 .

RESULTS AND DISCUSSIONS

Figure S3 adapted from P. A. Cox28 illustrates the concept of how bands in extended solids are formed from isolated atoms. In short, band structures can be determined by considering (1) electron affinity of anion or ionization potential of cation, (2) Madelung potential at each crystallographic site, (3) polarization effect and (4) band width coming from orbital hybridizations. For each anion sites of four series of layered bismuth oxyhalides (BiOX, Bi4NbO8X, Bi2GdO4X and SrBiO2X), we calculated the sum of (1) and (2), that is, the ionic orbital energy levels of each anion with neglected covalent interaction. The calculated ionic energy levels of oxygen and halogen in BiOX and Bi4NbO8X are shown in Figure 3. Surprisingly, they all well reproduce essential features of valence band structures in Figure 1a, b. In the case of BiOX, the energy level of halogen is higher than that of oxygen regardless of X. As expected from the electronegativity, the energy level of X becomes higher as the halide anion changes from Cl– to Br–, and to I–, while the oxygen anion becomes electrostatically more stabilized. In a marked contrast, the oxide anions in Bi4NbO8X are broadly higher than the X anion. What is striking here is that eight oxygen sites (O1-O8) have distinct energies, ranging from 10.7 V to 17.3 V for Bi4NbO8Cl and from 6.85 V to 14.4 V for Bi4NbO8Br, as opposed to the DFT calculations that yielded highly dispersive projected density of states (PDOS) of the eight oxygen sites, all spreading from the bottom to the top of the valence band (Figure S1).17 There is an overall tendency that the O1-O4 sites in the [Bi2O2] slab are less stable than the O5O8 sites in the perovskite [NbO4] slab, strongly suggesting that the former takes a leading role in providing the final valence band structures with the upward shift of VBM. A comparison with BiOX furthermore reveals that the energy levels of some oxygen sites of Bi4NbO8X are higher than that of BiOX, whereas the opposite dependence is seen for the halogen site. This means that the unique band structure of Bi4NbO8X is caused not only by electrostatic destabilization of oxide anions (in particular in the [Bi2O2] layer), but also by stabilization of halide anions. It is noted that the O6 site of the perovskite layer in Bi4NbO8Br is located higher than any other anions, the reason of which is not clear but may be attributed to the ignorance of the orbital hybridization in our Madelung analysis.29



Figure 3. The ionic energy levels of each anionic site in BiOX, Bi4NbO8X, Bi2GdO4X and SrBiO2X calculated by the sum of Madelung potential and electron affinity. The red, blue, green and orange symbols represent oxygen, chlorine, bromine and iodine anions, respectively.

gether by removing one M3+ layer. The Gd ion is coordinated by the eight oxygens at the vertices of a pseudo-cube.

Figure 4. Crystal structures of (a) Bi2GdO4X (X = Cl, Br) and (b) SrBiO2X (X = Cl, Br, I) (non-standard setting of Amma for comparison). The purple, red, orange, yellow and yellow-green balls indicate Bi, O, Gd, Sr and X atoms, respectively. Dotted lines indicate the unit cell of each structure.

Although BiOX and Bi4NbO8X are structurally related, we are not able to readily unveil the origin of the difference in the band structures of the two. This is mainly due to the complex crystal structure of the latter (space group: P21cn) involving heavily tilted octahedra resulting in eight crystallographic sites for oxygen (Figure 2b). In this context, SrBiO2X (X = Cl, Br, I) and Bi2GdO4X (X = Cl, Br) with simpler and less distorted lattices (Figure 4a, b) offer an ideal route to overall and detailed comparison, and possibly provide an answer as to the origin. As shown in Figure 4b, SrBiO2X (space group: Cmcm) consists of alternate stacking of [SrBiO2]+ and [X]– layers,57 and is related to BiOX (P4/mmm) with [Bi2O2]2+ and [X2]2– layers (Figure 2a) by the elimination of one [X]– layer from [X2]2– as a result of the aliovalent Sr-for-Bi replacement. Note that Bi3+ and Sr2+ cations are fully ordered. The Bi2GdO4X structure has a space group of P4/mmm (Figure 4a), with alternate stacking of [Bi2GdO4]+ and [X]− layers.57 The [M3O4]+ slab is constructed from double 2[MIII2O2]2+ layers that are joined to-

Figure 5. Schematic valence and conduction band structures of (a) Bi2GdO4X (X =Cl, Br) and (b) SrBiO2X (X =Cl, Br, I). The band gaps and CBM positions (see Table S1) were obtained, respectively, from UV-vis spectra (Figure S4) and Mott-Schottky plot at pH = 2 for Bi2GdO4X and pH = 6.0 for SrBiO2Cl (Figure S5). The red, blue, green and orange boxes, respectively, represent the bands from O-2p, Cl-3p, Br-4p and I-5p orbitals and the gray box shows a conduction band, derived from DTF calculations (Figure S6).

The band gaps and CBM positions of Bi2GdO4X and SrBiO2X, experimentally obtained from UV-vis spectrum and Mott-Schottky plot at pH = 2, are shown in Figure 5. VBMs of Bi2GdO4X are as high as those of Bi4NbO8X, and the band gaps are nearly the same between X = Cl and Br (2.43 and 2.42 eV), the same trend as found in Bi4NbO8X (2.39 and 2.48 eV).17 The observed positions of VBM in Bi2GdO4X with nearly no X- dependence imply that the DOS around VBM is mainly composed of oxygen orbitals not halogen orbitals (Figure 5), as in the case of Bi4NbO8X.17 This interpretation is confirmed by DFT calculations for Bi2YO4X (Figure S6) and Bi2LnO4Cl (Ln = Eu and Nd).43 It means that the unusual valence band structures


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Journal of the American Chemical Society in the Sillen-Aurivillius phase Bi4MO8X (and its derivative Bi6NbWO4Cl23) can be further expanded to other layered bismuth oxyhalides. Interestingly, the SrBiO2X system shows characteristics somewhat intermediate between BiOX and Bi4NbO8X. First, the band gaps of SrBiO2Cl and SrBiO2Br are close to each other (3.38 eV for Cl and 3.15 eV for Br), similarly to Bi4NbO8X (2.39 eV and 2.48 eV). Second, these values are much larger than Bi4NbO8X and rather close to BiOX (3.42 eV and 2.78 eV). Lastly, the band gap of SrBiO2I is 1.76 eV, which is much smaller than those of SrBiO2Br and SrBiO2Cl. The DFT calculations on SrBiO2Cl (Figure S6) are coherent with these observations; The VBM of SrBiO2Cl is mainly composed of O-2p orbitals while the contribution of halogen orbitals is significant in SrBiO2I (Figure 5). The Madelung site potentials of anions in Bi2GdO4X and SrBiO2X in Figure 3 again well reproduce essential features of PDOS in the valence band obtained from DFT calculations (Figure 5). In Bi2GdO4X, the energy level of the oxygen is much higher as compared with halogen (Cl, Br). There is no X-site dependence. In SrBiO2X, the oxygen is located slightly lower than those of Bi2GdO4X and Bi4NbO8X, but higher than BiOX. On the other hand, halogen is located higher than those of Bi2GdO4X and Bi4NbO8X, while is close to BiOX. Now we are in possession of four sets of layered bismuth oxyhalides that have distinct valence band structures, along with X-site dependence. This allows one to systematically comprehend the band structures of these systems and finally to reveal the origin of the unusual valence band structure in Bi4NbO8X. As mentioned earlier, from the DFT calculations on Bi4NbO8X,17 one can only recognize equally dispersive O-2p orbitals of all the oxygen sites (O1-O8) within and between fluorite [Bi2O2] and perovskite [NbO4] slabs. The Madelung analysis for Bi4NbO8X and other systems led us to conclude that the oxide anions (O1-O4) in the [Bi2O2] block rather than the [NbO4] block (O5-O8) play a vital role in providing the final unusual valence band structure with the upward shift of VBM. This conclusion is further reinforced by the similar valence band structures and Madelung site potentials in Bi2GdO4X free from a perovskite block. Also, a structural distortion, which can often tune a band gap of many compounds,58-60 is not a main driving force since, unlike Bi4NbO8X (P21cn), Bi2GdO4X has a non-distorted tetragonal lattice. The energy level of the oxide anion in the Bi2O2-based layer rises in order of BiOCl < SrBiO2Cl < Bi4NbO8Cl ≈ Bi2GdO4Cl (Figure 3). We emphasize that this dependence cannot be accounted for by the local (i.e., nearest-neighbor) coordination environment around the oxide anion. This is readily rationalized when the coordination geometry around the oxide anion is compared between BiOX and Bi2GdO4X (Figure S7). For both compounds, the oxide anion is tetrahedrally coordinated by four trivalent cations to form OBi4 in BiOX and OBi2Gd2 in Bi2GdO4X with very similar bond lengths. Accordingly, these nearest-neighbor cations give little difference in the Madelung potential, underlining the necessity to pay attention to further neighboring coulomb interactions. Thus, the next attempt would be to see the contribution of further neighboring individual ions to electrostatic poten-

tials of anions in choice. Recall that electrostatic destabilization of the oxygen in fluorite-based Bi-O slabs (e.g. [Bi2O2] in Bi4NbO8Cl and [Bi2GdO4] in Bi2GdO4Cl) is responsible for the unique valence band structures. Let us therefore focus on the oxide anion in the oxygen sub-layer, “basal layer (BL)”, of the fluorite-based Bi-O slabs, and examine electrostatic potentials from each cations/anions within 4 Å from BL and 1st nearest- and 2nd nearestneighbor sub-layers (1NNL and 2NNL), as shown in Figure 6 and Figure S8. In BiOCl, cations/anions within 4 Å from a selected oxygen of BL are Bi(1) × 4 (2.31 Å) in 1NNL, O(2) × 4 (2.75 Å) in BL, Cl(3) × 4 (3.24 Å) in 2NNL and O(4) × 4 (3.88 Å) in BL (Figure 6a), where the number in parenthesis (e.g. Bi(1)) means it is the Nth nearest neighbor of the selected oxygen. As shown in Figure 6b, Bi2GdO4Cl has Bi(1) × 2 (2.24 Å) in 1NNLu, Gd(2) × 2 (2.39 Å) in 1NNLd, O(3) × 4 (2.74 Å) in BL, O(4) × 1 (2.80 Å) in 2NNLd, Cl(5) × 2 (3.62 Å) in 2NNLu, O(6) × 4 (3.88 Å) in BL, and O(7) × 4 (3.92 Å) in 2NNLd. The electrostatic potentials from these cations/anions are given in Table 1.

Figure 6. Layer-by-layer description of structures of (a) BiOCl (a = 3.89 Å, c = 7.35 Å), (b) Bi2GdO4Cl (a = 3.88 Å, c = 8.92 Å). (c) Bi4NbO8Cl and (d) SrBiO2Cl. For simplicity, the idealized structure without tilting is presented in (c).

Table 1. Electrostatic potential given to the oxygen in the BL layer from surrounding ions (< 4 Å). BiOCl

BL

1NNLu

Bi2GdO4Cl

Ion

Potential (eV)

Ion

Potential (eV)

O(2)* × 4

O(3) × 4

O(4) × 4

–10.5 –7.4

O(6) × 4

–10.5 –7.4

Bi(1) × 2

18.7

Bi(1) × 2

19.3



1NNLd

Bi(1) × 2

18.7

Gd(2) × 2

18.1

2NNLu

Cl(3) × 2

–4.5

Cl(5) × 2

–4.0

2NNLd

Cl(3) × 2

–4.5

O(4) × 1

–10.4 –7.4

O(7) × 4

* The number in parenthesis means that it is Nth nearest neighbor ions of the oxygen in BL. Regarding the subscripts, u and d, of 1NNL/2NNL, see Figure 6a, b.

Owing to the nearly identical oxide network of the BL in BiOCl (a = 3.89 Å) and Bi2GdO4Cl (a = 3.88 Å), oxide anions at the BL (O(2), O(4) in BiOCl and O(3), O(6) in Bi2GdO4Cl) give almost the same electrostatic potential (Table 1). Similarly, 1NNLs in both compounds are composed of trivalent cations (Bi3+, Gd3+) of almost equidistant Bi–O/Gd–O bonds, thereby causing little difference in Madelung potential. A prominent difference occurs for the anions in 2NNL. In BiOCl, 2NNLu and 2NNLd are equivalent, each having two chlorine anions (Cl(3) × 2) within 4 Å. A similar chlorine layer is found in 2NNLd (Cl(5) × 2) of Bi2GdO4Cl, but 2NNLu has one O(4) and four O(7) anions within 4 Å. As a result, the electrostatic potential from 2NNL amount to –18.0 eV in BiOCl and –40.0 eV in Bi2GdO4Cl. Namely, the more negative Madelung potential of oxygen site in Bi2GdO4Cl than BiOCl (Figure 3) originates from the higher valent (O2– vs Cl–) and denser anion arrangement (O × 5 vs. Cl × 2) in 2NNLu. The energy levels of relevant oxygen sites of Bi4NbO8Cl and SrBiO2Cl can be interpreted in a similar manner. Figure 6c shows the idealized non-distorted crystal structure of Bi4NbO8Cl for the sake of comparison, and Table S3 provides the electrostatic potentials at oxygen in Bi2O2 slab, where the result of O4 site is given among four oxygen sites in this slab. It is seen that 1NNLu, 1NNLd and 2NNLu of Bi4NbO8Cl are structurally similar to BiOCl and Bi2GdO4Cl. As in Bi2GdO4Cl, 2NNLd is composed of oxide anions, resulting in a lower total Madelung potential of – 20.3 eV from 2NNLd (Table S3) than that of BiOCl (9.0 eV from 2NNLd (Table 1)) and hence the upward shift of VBM. In SrBiO2Cl, the 1NNL and 2NNL sequence (Figure 6d) resembles that of BiOCl, but half of cations in 1NNL are divalent (Sr2+), and this difference causes the corresponding Madelung potential to become lower. As a consequence, the electrostatic potential of SrBiO2Cl from cations of 1NNL becomes smaller than that of BiOCl, which accounts for the observed smaller band gap. With respect to the halogen, its dependence of the energy level is in reverse order to oxygen (Bi4NbO8Cl ≈ Bi2GdO4Cl < SrBiO2Cl ≈ BiOCl), implying an intimate correlation between oxygen and halogen. Hence the energy level of chlorine will be discussed in the similar way. In fact, it is seen that the chlorine layer of Bi4NbO8Cl and Bi2GdO4Cl is sandwiched by cationic (Bi3+) layers, while in BiOCl it is adjacent to the Cl– layer as well as Bi3+ layer, a difference that gives rise to the lowered Madelung potential of chlorine in BiOCl. In SrBiO2Cl, the chlorine layer is sandwiched from both sides by cation layers like Bi2GdO4Cl. However, since the Bi3+ cations are half replaced by Sr2+ cations, the Madelung potential of Cl is reduced.

Figure 7. (a) Crystal structure of n = 2 Sillén-Aurivillius phase of ((Bi0.75Sr0.25)2O2)2XSrNb2O7 (X = Cl, Br). (b) Ionic energy levels of anions. The purple, red, orange, yellow-green and dark-green balls indicate Bi, O, Sr, X and Nb atoms, respectively. O4 corresponds to the oxygen in the Bi-O layer (see Figure S9).

The present systematic analysis on four sets of layered bismuth oxyhalides has revealed that valence band structures can be tuned in a rational way. Additionally, we can predict valence band structures of compounds that have not been synthesized yet. Akin to high-Tc cuprates and iron arsenides, one may be able to construct (at least conceive) a variety of building layers with compositional variance and their stacking sequences. The Madelung analysis of such rough structural models allows a judgment of whether they are worth preparing for further research. Importantly, it does not require any precise crystal structural information that is desired for DFT calculations. The Madelung analysis is also a powerful tool when DFT computation is difficult to perform, for example in a compound involving site deficiency, solid solution, and atomic displacement, which requires a large supercell for DFT and one often encounters convergence problems. As a proof-ofconcept, we employed Bi3Sr2Nb2O11Cl reported by Charkin and co-workers.61 This compound belongs to n = 2 member of Sillén-Aurivillius-type perovskite with a general formula of (M4O4)X(An-1BnO3n-1). Although the A site of Bi3Sr2Nb2O11Cl are mostly occupied with Sr cations (94%, thus its composition is written approximately as (Bi0.75Sr0.25)4O4Cl(SrNb2O7), Bi and Sr are partly disordered over the two sites in (Bi0.75Sr0.25)4O4 slabs with Sr occupying 40%/13% of M1/M2 site, as shown in Figure S9. In addition, there is a local displacement of O2 site. DFT calculation is clearly quite hard. On the contrary, the Madelung analysis can be done in a straightforward way and it suggested a destabilization of the O4 site in the fluorite block (Figure 7b), as in Bi4NbO8Cl. UV-vis reflectance spectrum (Figure S10) indeed shows a reasonably small bad gap of 2.76 eV. As a second example, we studied a bromine analogue, Bi3Sr2Nb2O11Br reported in Ref. 58. Unexpectedly, the Madelung site potentials of Br and O4 became negative (– 4.1 eV and –3.1 eV), indicating the instability of the proposed structure. The reason may be related to the fact that all the Sr ions were assumed, without refinement, to occupy the fluorite layer site,62 giving the formal composition of ((Bi0.5Sr0.5)4O4Br)BiNb2O7. Accordingly, we calculated the


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Journal of the American Chemical Society Madelung site potential by assuming the Bi3Sr2Nb2O11Cl structure (Figure S9), successfully yielding positive values (Figure 7b), similar to those of the Cl counterpart. Our structural model is also supported by the UV-vis diffuse spectrum (Figure S10) that is nearly identical to Bi3Sr2Nb2O11Cl, indicating the dominant occupation of O2p orbitals at VBM. The higher energy levels of O4 than that of halogen (and other oxygen sites) are understood in terms of the similar stacking sequence up to 2NNL (Figure 7a).

CONCLUSION We have demonstrated that Madelung site potential can reproduce the essential feature of valence band structures in a series of layered bismuth oxyhalides. In particular, this analysis is successful in revealing the origin of unusual occupation of oxygen orbitals at VBM in Bi4NbO8Cl; it originates from the small Madelung potential at the oxygen site in the fluorite-based [Bi2O2] layer. Further analysis revealed that the increased density of oxygens causes from stacking the [Bi2O2] and [NbO4] slabs (without an intervening halide) that yields the upward shift of the oxygen bands. Moreover, the comparison with other layered systems BiOX, Bi2GdO4X and SrBiO2X revealed that building layers and their stacking sequence allows for tuning and controlling of the valence band structures to improve photocatalytic activities as well as predicting new structures of layered bismuth oxyhalides that are deserved to be prepared in future. We believe that this strategy can be a general guide to be used in other (non-bismuth) layered semiconductors to rationally design stable photocatalysts or solar cell components.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Computational and experimental details (PDF)

AUTHOR INFORMATION Corresponding Author *[email protected], *[email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the Grant-in-Aid for Scientific Research on Innovative Areas “Mixed Anion” project (JP16H06439, JP16H06440, and JP16H06441) from MEXT and CREST (JPMJCR1421). K.H. is grateful to KAKENHI (17K17762), PRESTO (JPMJPR16NA) and Support Program for Starting Up Innovation Hub MI2I from JST, Japan for their financial supports. R.M. is grateful to MEXT-KAKENHI (17H05478) and USAFOSR-AOARD for their financial supports. R.M. and K.M. are also grateful to MEXT-FLAGSHIP2020 (hp170269, hp170220) for their computational resources. The computation in this work has been performed using the facilities of the Center for Information Science in JAIST.

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