Band Engineering of Double-Layered SillÃ©n-Aurivillius Perovskite

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Band Engineering of Double-Layered Sillén-Aurivillius Perovskite Oxychlorides for Visible-Light-Driven Water Splitting Akinobu Nakada, Masanobu Higashi, Takuma Kimura, Hajime Suzuki, Daichi Kato, Hiroyuki Okajima, Takafumi Yamamoto, Akinori Saeki, Hiroshi Kageyama, and Ryu Abe Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b00567 • Publication Date (Web): 02 Apr 2019 Downloaded from http://pubs.acs.org on April 3, 2019

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Chemistry of Materials

Band Engineering of Double-Layered Sillén-Aurivillius Perovskite Oxychlorides for Visible-Light-Driven Water Splitting Akinobu Nakada,a Masanobu Higashi,a Takuma Kimura,a Hajime Suzuki,b Daichi Kato,a Hiroyuki Okajima,a Takafumi Yamamoto,a Akinori Saeki,b Hiroshi Kageyamaa,c* and Ryu Abea,c* aDepartment

of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Nishikyoku, Kyoto 615-8510, Japan. bDepartment

of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan. cCREST,

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

ABSTRACT: Recently, Bi4MO8X (M = Nb, Ta; X = Cl, Br), Sillen-Aurivillius type single-layered perovskite oxyhalides, have been shown as promising visible-light-responsive photocatalysts with unique valence band structures. Here we report on the synthesis, structures and photocatalytic properties of a series of double-layered analogues A4A’M2O11Cl (A, A' = Bi, Pb, Ba, Sr; M = Ta, Nb, Ti). Fourteen compounds including ten unreported compounds were successfully synthesized via a twostep method with various combination of preliminary-prepared multi-metal oxide and oxyhalide precursors. In a marked contrast to Bi4MO8X with almost unvaried valence band maximum (VBM) and conduction band minimum (CBM), both VBM and CBM of A4A’M2O11X are variable to some degree, with a change of compositions. In Sr2Bi3M2O11Cl, for example, Srsite replacement by Pb and Ba resulted in a shift of the valence- and conduction-band edges toward narrowing the bandgaps. Structural characterizations and DFT/Madelung calculations revealed that the observed band edges are mainly understood in terms of the interaction between Bi/Pb 6s2 lone pair and O-2p orbitals and Madulung site potentials of Bi3+ cations. Furthermore, compounds with a higher occupancy of Bi3+ in the fluorite layer showed improved conductivity of photoexcited electrons, leading to better photoelectrochemical performance for water oxidation under visible light.

Photocatalytic splitting of water is regarded as one of the ideal ways for clean and sustainable production of hydrogen (H2) by harvesting abundant solar energy having about half the energy of visible region. Developing visiblelight-active photocatalysts is a main challenge toward establishing practical H2 production by effectively utilizing sunlight. Particulate inorganic semiconductors such as metal oxides,1–4 (oxy)nitrides,5–9 (oxy)sulfides10–12 and oxyhalides13–15 have been developed as photocatalysts, as well as photoelectrode materials.

stacking pattern providing negative Madelung site potentials of the oxygen atoms.18 The negative VBM endows Bi4MO8X with a bandgap suitable for visible light absorption while adequately maintaining the negative conduction-band minimum (CBM) for water reduction. Furthermore, the O-2p dominated VBM protects Bi4MO8X from self-oxidation (i.e., self-deactivation) by photogenerated holes, leading to stable photooxidation of water. This feature differentiates Bi4MO8X from typical mixed-anion photocatalysts such as oxynitrides and oxysulfides that suffer from the self-oxidation of N3- or S2anions by photogenerated holes.12,19–25

A series of bismuth-based layered perovskite oxyhalides of Sillén-Aurivillius (S-A) type have been shown recently as candidate photocatalysts for visible-light-induced water splitting. We found that Bi4MO8X (M = Nb, Ta; X = Cl, Br; Figure 1a)13,16 acts as a stable oxygen (O2)-evolving photocatalyst for Z-scheme type water splitting under visible-light irradiation. Most importantly, the valenceband maximum (VBM) of Bi4MO8X is located at a much higher potential (2.0-2.1 V vs. SHE), compared to conventional oxides (ca. 3.0 V vs. SHE), and is predominantly occupied by O-2p orbitals.16 This unique valence band structure can be explained by the interaction between the O-2p and Bi-6s2 lone pairs17 and the layer

All four Bi4MO8X compounds (M = Nb, Ta; X = Cl, Br) have almost identical band positions (−0.5 to −0.4 V for CBM and 2.0 to 2.1 V for VBM).16 As mentioned above, the unchanged VBM is interpreted by the dominant occupation of O 2p orbitals, not be Cl 3p and Br 4p orbitals. Likewise, CBM is predominantly occupied by Bi 6s orbitals, not by Nb 4d and Ta 5d orbitals. The present study aimed to extend the previous work by increasing the number of perovskite layers (n) in the Sillén-Aurivillius (S-A) layeredperovskite, with general formulae A4A’n-1MnO3n+5X having a staking sequence of [A2O2]-X-[A2O2]-[A’n-1MnO3n+1]. Namely, we here focus on the double layered (n = 2) system A4A’M2O11Cl (Figure 1b).26–28 The advantage of the n = 2

INTRODUCTION

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system over the previously studied n = 1 system (e.g., Bi4MO8X) is the compositional versatility of the cations (A, A’ and M) with different valences (e.g., A and A' = Sr2+, Ba2+, Pb2+, Bi3+; M = Ti4+, Nb5+, Ta5+) and the presence and absence of lone-pair electrons. Given the further layer contribution of Madelung potentials,18 the n = 2 system could offer an opportunity to systematically tune the band levels and the resultant photocatalytic activity. By employing the recently reported two-step synthesis, we prepared 14 compounds including 10 new ones (see Figure 1b). It is found that both valence- and conduction-band positions are tunable to a certain extent, which is rationalized by their crystal and electronic structures. Visible-light-induced oxidation of water and overall water splitting via photoelectrochemical and photocatalytic reactions were also demonstrated, and their properties are discussed from viewpoints of transient carrier mobility and surface atomic composition.

Figure 1. Crystal structures of Sillen-Aurivillius layered perovskite oxyhalides of (a) single-layered (n = 1) Bi4MO8Cl and (b) double-layered (n = 2) A4A’M2O11Cl.

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RESULTS AND DISCUSSION Optimization of Synthetic Procedures Most of Sillén-Aurivillius (S-A) type perovskite oxyhalides such as Bi4MO8X have thus far been synthesized via single-step solid state reaction (SSR) of metal oxides (e.g., Bi2O3 and Ta2O5) and an oxyhalide (e.g., BiOCl) in a sealed quartz tube under vacuum. Compared with n = 1 type, reports on the synthesis of n = 2 type compounds are limited,26–28 probably due to the severity of required conditions. For instance, while the SSR synthesis of n = 1 Bi4TaO8Cl requires 973 K for 20 h, that of Sr2Bi3Ta2O11Cl involves calcination at a higher temperature of 1223 K for 35 h.29 Additionally, the SSR method did not yield a phasepure Ba2Bi3Ta2O11Cl (n = 2), although various conditions were employed such as 1123 K for 15 h and 1223 K for 50 h (see Figure S1). We have recently developed a two-step synthesis involving polymerized complex (PC) method,29 a method to obtain fine particles of a precursor oxide (e.g., SrBi2Ta2O9), followed by a heating with an oxyhalide (e.g., SrBiO2Cl). Importantly, this procedure via PC method lowers the final reaction temperature to access single phase synthesis, while retaining particles small enough and reducing charge-recombination centers. These features lead to highly active oxyhalide photocatalysts. In this work, we applied this synthetic method (with milder conditions) for preparing 14 double-layered (n = 2) S-A compounds, AA'Bi3M2O11Cl and ABi4TiMO11Cl (A: Ba, Sr, Pb; A': Ba, Sr; M: Nb, Ta), including unexplored ones. Briefly, precursor oxides, A’Bi2M2O9 and Bi3TiMO9 (A’: Sr, Ba; M: Ta, Nb), were prepared by PC method, in principle, following our previous report on SrBi2Ta2O9.29 Oxyhalide precursors ABiO2Cl (A: Sr, Ba, Pb) were synthesized by a SSR reported elsewhere.15 Reactions of various combination of these precursors at 1123 K for 15 h were performed and we successfully obtained purpose 14 double-layered S-A compounds, all of which are of tetragonal symmetry (Figure S2). Ta- and Ti-based ten compounds are obtained for the first time in this study. Note that all the previous syntheses of S-A oxyhalides have been carried out in an evacuated quartz tube, which was supposedly considered as crucial for avoiding halogen deficiency and/or oxidation. This study shows that pure phases can be prepared even in air. Interestingly, it turned out that the difference in synthetic atmosphere (i.e., vacuum or air) significantly affects physicochemical properties such as absorption spectra and resultant photocatalytic activity. Figure 2 shows UV-vis spectra of Ba2Bi3Ta2O11Cl as a representative example. The sample prepared under vacuum exhibits a broad absorption at wavelengths longer than the adsorption edge of bandgap transition (i.e.,  > 480 nm) whereas such broad absorption was absent for the sample with air calcination. Since this broad absorption is a signature of the presence of reduced species (e.g., cation interstitials and/or anion defects), resulting frequently in a charge recombination center,30 the S-A oxyhalides obtained the air calcination are more favorable in terms of application for photocatalysis.


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Chemistry of Materials Figure 3. Time courses of O2 generation from various Ba2Bi3Ta2O11Cl samples in aqueous FeCl3 solution (5 mM, pH = 2.5 adjusted by HCl) under visible-light irradiation ( > 400 nm).

Crystal Structures and Cation Distributions

Figure 2. UV-vis diffuse reflectance spectra of Ba2Bi3Ta2O11Cl samples prepared under various conditions. The samples are denoted as Xstep_Y(T) (X: the number of synthesis step, Y: calcination atmosphere (in air or in vacuum), T: calcination temperature). The inset shows expanded spectra in a longer wavelength region.

Figure 3 shows the results of photocatalytic activity test for water oxidation on these Ba2Bi3Ta2O11Cl samples with an FeCl3 electron acceptor, which constitutes a half reaction of Z-scheme water splitting. The sample via twostep synthesis in air showed by far the highest activity, which can be ascribed, as addressed earlier, to the higher quality with smaller signature of charge-recombination center of this specimen. Additionally, as confirmed by the SEM images (Figure S3), the two-step procedure provided smaller crystals compared to the one-step synthesis, naturally leading to higher specific surface areas (2.6 and 1.7 m2 g−1 for two- and one-step samples, respectively). The smaller particle size (as well as the higher surface area) obtained via two-step procedure is another reason for the higher activity. The same trend was observed for other compounds. Based on these observations, we hereafter show the results on the samples prepared via two-step procedure under aerobic condition unless otherwise specified.

Table S1 represents the tetragonal lattice parameters of all the compounds determined by Le Bail analysis31; they are dependent on incorporated cations in the structure. To obtain the precise structural information, we chose Ba2Bi3Ta2O11Cl, BaPbBi3Ta2O11Cl, SrPbBi3Ta2O11Cl and BaBi4TiTaO11Cl and performed Rietveld refinement of the synchrotron XRD (SXRD) patterns. Since all peaks could be indexed using the tetragonal cell without any reflection condition except for several small impurity peaks, we assumed the structure of Pb2Bi3Nb2O11Cl with the space group of P4/mmm.32 Different cations, if they share a same site, are assumed statistically disordered. Since Pb2+ and Bi3+ cannot be distinguished in the X-ray diffraction analyses because of their identical electronic configuration, we assumed Pb2+ as Bi3+. The refinement for Ba2Bi3Ta2O11Cl was converged successfully, yielding reliability factors of Rp = 7.87% and Rwp = 10.48% (Figure 4). Reliable values were also obtained for the other compounds (Figure S4). Table 1 lists the atomic coordinates for all four compounds, demonstrating notable site preferences. In Ba2Bi3Ta2O11Cl, the perovskite A' site is dominantly occupied by Ba2+. While Bi3+ is selectively occupied at the A2 site (next to the Cl layer) in the [A1A2O2] layer, the A1 site (next to the perovskite layer) is shared by about one-half each of Ba2+ and Bi3+. A similar cation (A and A’) distribution is observed for BaPbBi3Ta2O11Cl; Ba2+ favorably exists in the perovskite A' site. This is probably because the [A1A2O2] layer prefers lone-pair cations (Bi3+/Pb2+). The Sr-counterpart SrPbBi3Ta2O11Cl showed a similar cation distribution with BaPbBi3Ta2O11Cl, and the preferential occupation of Sr2+ at the A’ site is more pronounced. These trends are also seen in Nb-based ones.26



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Figure 4. Rietveld refinement of SXRD pattern of Ba2Bi3Ta2O11Cl using a proposed tetragonal model (space group: P4/mmm).

Table 1. Atomic distributions in Ba2Bi3Ta2O11Cl, BaPbBi3Ta2O11Cl, SrPbBi3Ta2O11Cl and BaBi4TiTaO11Cl Ba2Bi3Ta2O11Cl Atom

BaPbBi3Ta2O11Cl

g

U / Å2

z

1a (0 0 0)

2g (0 0 z)

-

0.265 6(1)

(Ba) 0.007( 7)

0.013a

-

A2

(½ ½ z)

0.395 73(7)

0.099 (2) (Pb)

0.477 (4)

0.197( 4) (Ba)

(Ba) 0.523 (4)

0.023 8(6)

0.2668

M

Cl

(½ ½ z)

1b (0 0 ½)

U / Å2

z

0.602( 3) (Bi)

g

0.0217 (9)

-

0.098 (5) (Bi)

0.023 6(5)

0.2640

0.701( 3) (Bi)

0.75

0.75

(Ba)

(Bi)

0.976 (6)

0.25

0.012( 1)

-

0.065( 4) (Sr)

0.024( 6) 0.3980

z

0.033( 2) (Pb)

0.233( 1) (Pb)

0.009 7(4)

U

0.870( 7) (Sr)

0.201( 1) (Pb)

0.0051 (4)

0.3976

(Pb)

(Bi) 2h

0.296 (6) (Bi)

(Bi)

(Bi)

2h

g 0.606 (8) (Ba)

0.999 (7)

A1

BaBi4TiTaO11Cl

Site z

A’

SrPbBi3Ta2O11Cl

(Bi) 0.25

0.0241 (5)

0.2597( 2)

0.006 4(4)

0.3966( 1)

g

0.65 (2) (Ba) 0.35 (2) (Bi)

0.17 (1) (Ba) 0.83 (1) (Bi)

1 (Bi)

U / Å2

0.032( 2)

0.041( 1)

0.013a

(Pb)

0.1155 2(7)

1 (Ta)

0.003 6(5)

0.1173

1 (Ta)

0.006 6(5)

0.1144

1 (Ta)

0.005 8(4)

0.1167(2 )

-

1

0.013a

-

1

0.008 (3)

-

1

0.018( 4)

-


0.50 (Ta) 0.50 (Ti)

1

0.013a

0.013a

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O1

O2

1c (½ ½ 0)

4i (0 ½ z)

2h O3

O4

a

(½ ½ z)

4i (0 ½ z)

-

1

0.013a

-

1

0.013a

-

1

0.013a

-

1

0.013a

0.1074 (6)

1

0.013a

0.1026( 6)

1

0.013a

0.0969( 6)

1

0.013a

0.102(1)

1

0.013a

0.2041 (9)

1

0.013a

0.2141(9 )

1

0.013a

0.2134(9 )

1

0.013a

0.189(2)

1

0.013a

0.3373 (7)

1

0.013a

0.3396( 6)

1

0.013a

0.3379( 7)

1

0.013a

0.344(1)

1

0.013a

The parameters were not refined.



Optical Properties and Band Structures We previously reported an impact of A-site cation substitution on developing visible-light absorption by employing layered oxyhalides ABiO2X (A = Sr, Ba, Pb; X = Cl, Br, I) without perovskite slabs.15 Hence, the effects of expanded variation of cation substitution are investigated in the double perovskite system. As seen in Figures 5 and S5, all the compounds synthesized are capable of absorbing visible light ( > 400 nm), but their absorption edges significantly vary, depending on the composition. Sr2Bi3M2O11Cl exhibits the shortest absorption edge (black line in Figures 5 and S5). The bandgap energies (Eg) were determined by Tauc plots (Figure S6). Replacement of Sr2+ by Ba2+, Pb2+ or Bi3+ red-shifted the absorption edges (colored lines), meaning reduced the Eg’s.

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The energy levels of conduction band minimum (ECBM) and valence band maximum (EVBM) for each compound were estimated by the Eg’s and the ionization energies determined by means of photoelectron yield spectroscopy (PYS).33 The PYS provides a precise estimation of EVBM, as confirmed in well-known oxides, i.e., TiO2 (2.9 V vs SHE at pH 0), WO3 (3.0 V) and BiVO4 (2.5 V). As seen in Figure 6, the n = 2 S-A compounds possess negative EVBM’s than those of typical metal oxides. As demonstrated in n = 1 compounds Bi4MO8X,16 the upward shift of EVBM is due to the interaction of O-2p orbitals with Bi-6s2 lone pair (Figure S7)17 and with nearby cations/anions (Madelung site potentials).18 Indeed, O-2p orbitals in the doublelayered system are destabilized compared with Cl-3p orbitals (see DOS in Figure 7 and Madelung site potentials in Figure S8). In addition, the DOS data of all the compounds reveals sizable hybridization between O-2p (red) and Bi-6s (purple) at a lower energy region (−10 to −7 eV) (Figures 7 and S9). Importantly, the n = 2 compounds showed variable ECBM and EVBM, as opposed to Bi4MO8X with almost identical ECBM and EVBM (Figure S10). The contribution of Pb2+ to EVBM is recognized when one compares, e.g., Sr2Bi3Ta2O11Cl and SrPbBi3Ta2O11Cl. The Pbfor-Sr substitution shifts EVBM negatively while ECBM is unchanged. It was reported that the lone-pairs of Pb-6s can also interact with O-2p orbitals, similar to Bi-6s and O-2p.17 Indeed, further overlap of O-2p with Pb-6s orbitals can be found at −7 to −6 eV for SrPbBi3Ta2O11Cl, in addition to that with Bi-6s (Figures 7b and S9b).

Figure 5. UV-vis diffuse reflectance spectra of Sr2Bi3Ta2O11Cl (black), SrPbBi3Ta2O11Cl (green), SrBi4TiTaO11Cl (orange), Ba2Bi3Ta2O11Cl (yellow), BaPbBi3Ta2O11Cl (blue), BaBi4TiTaO11Cl (red) and PbBi4TiTaO11Cl (purple).


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Figure 6. Band alignments of n = 2 S-A compounds. The top of the valence-band potential (V vs. SHE at pH 0) is assumed to be the lowest ionization energy obtained by PYS (eV vs. vacuum) – 4.44.

Figure 7. Total and projected density of states (DOS) of (a) Sr2Bi3Ta2O11Cl, (b) SrPbBi3Ta2O11Cl, (c) Ba2Bi3Ta2O11Cl and (d) BaPbBi3Ta2O11Cl.

In contrast, the Ba-for-Sr substitution (i.e., Sr2Bi3M2O11Cl  Ba2Bi3M2O11Cl) positively shifted the ECBM, while maintaining EVBM. Here, the CBMs of these compounds are composed mainly of Bi-6p orbitals, with a small contribution from Ta-5d and O-2p (Figures 7a and 7c). There is a negligible contribution from alkaline-earth cations at CBM. To understand these behaviors, we calculated Madelung site potentials of cations in the [A1A2O2] layer for Sr2Bi3M2O11Cl and Ba2Bi3M2O11Cl (Table S2). Note that the previous study on the single-layered S-A system did not consider Madelung potentials at cationic sites.18 More positive potentials of Bi3+ were observed at the

A1 and A2 sites in Ba2Bi3Ta2O11Cl, which reasonably explains the difference in ECBM. A close look at the structures led us to recognize that Ba2Bi3Ta2O11Cl exhibits longer nearest A2−O (2.27 Å) and A2−Cl (3.43 Å) distances than Sr2Bi3Ta2O11Cl (2.25 and 3.37 Å), while the nearest A1-O distances are close to each other (2.40 Å) (Figure 8). The elongated interionic distances for the Ba system is probably due to their different ionic radius (1.26 and 1.42 Å for Sr2+ and Ba2+, respectively, in the octa-coordinate environment34), and likely results in stabilization of Bi-site potentials (i.e., ECBM), similar to the lattice expansion effect reported for cubic perovskites.35 On the other hand,



SrPbBi3Ta2O11Cl and BaPbBi3Ta2O11Cl possess almost the same band levels. Rietveld analyses (Table 1) revealed that the occupancy of alkali earth (AE) cations in the [A1A2O2] layer is much lower in the cases of AEPbBi3Ta2O11Cl compared to those of AE2Bi3Ta2O11Cl. In other words, the cation distribution in [A1A2O2] layer is similar for SrPbBi3Ta2O11Cl and BaPbBi3Ta2O11Cl, in which A1 and A2 sites are predominantly occupied by Pb/Bi. As a result, the distances of nearest neighboring atoms are similar (A1−O = 2.40 Å, A2−O = 2.25 and 2.26 Å, and A2−Cl = 3.37 and 3.38 Å for SrPbBi3Ta2O11Cl and BaPbBi3Ta2O11Cl, respectively), likely leading to almost identical band levels. This supports our hypothesis that the size of incorporated cation in the [A1A2O2] layer mainly affects the ECBM, while the substitution of the perovskite A’-site cations scarcely does.

Figure 8. Neighboring environment around the [A1A2O2] layer.

A co-substitution of Ta5+ and A2+ by Ti4+ and Bi3+ (ABi4TiTaO11Cl) resulted in a decrease of the bandgap. The bandgap of AA’Bi3Ta2O11Cl and ABi4TiTaO11Cl ranges from 2.86 to 2.57 eV for A = Sr and from 2.57 to 2.50 eV for A = Ba (see Figure 6). The negative shift of EVBM can reasonably be explained by the increased Bi content that leads to the enhanced interaction between O-2p and Bi3+ 6s2 lone pair electrons. The introduced Ti4+ was found to contribute CBM in addition to Bi-6p and Ta-5d orbitals (Figure S11). The Nb-counterparts show a similar trend upon cation substitutions, but they possess more negative band potentials (Figure 6). This is curious since simple Tacontaining oxides (e.g., Ta2O5) possess negative ECBM than Nb-containing ones (e.g., Nb2O5). Properties of Photogenerated Carriers and Photoelectrochemical Performances As shown above, all n = 2 S-A materials possess appropriate band alignments for visible-light-induced water splitting (i.e., more negative ECBM and positive EVBM than equilibrium potential for reduction and oxidation of water, respectively). We employed time-resolved microwave conductivity (TRMC) measurements that allow direct detection of the local mobility and lifetimes of photogenerated charge carriers.36 Even in a situation where the synthesized material does not show high activity due

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to problems in specific surface area, surface composition, defect content, and exposed facets, TRMC can tell us an intrinsic property of photocatalyst materials. Figures 9 and S12 show the transient photoconductivities upon photoexcitation at 355 nm by a Nd:YAG laser. In all cases, the ∑ value, where  is the quantum efficiency of charge carrier generation and ∑ is the sum of photogenerated carrier mobilities (hole and electron), increased within the instrumental time resolution (~10-7 s) and then gradually decreased due to charge recombination. Note that all n = 2 compounds are n-type semiconductors, as confirmed by the anodic photocurrent in the photoelectrochemical measurement (see Figure 10). Therefore, the ∑ values are likely attributed to the photogenerated mobile electrons in the conduction band. The maximum ∑ values and the averaged lifetimes are summarized in Table 2. Clearly, four compounds with AEBi4TiMO11Cl formula exhibited larger ∑ maximum (3.39 − 5.39 ⨯ 10–7 m2 (Vs)–1) than the others (0.54 −2.32 ⨯ 10–7 m2 (Vs)–1). From the structural analysis (Table 1), the Bi occupancy factors at the A1 and A2 sites in the Bi-rich ABi4TiTaO11Cl (0.92 for A = Ba and Sr, respectively) are higher than the Bi-poor compounds (0.68 − 0.75 for Ba2Bi3Ta2O11Cl, AEPbBi3Ta2O11Cl). Note again that, in the double perovskite system, the CBM is predominantly constructed by Bi-6p orbitals. Thus, it seems reasonable that the fluorite-type [A1A2O2] layers of ABi4TiMO11Cl with higher Bi3+ occupancies (i.e., less chemical randomness) exhibit relatively high mobility of photogenerated electrons. The higher photoconductivity in ABi4TiMO11Cl is also supported by the higher photocurrents generated on the photoanodes of BaBi4TiTaO11Cl (red) and SrBi4TiTaO11Cl (orange). The trend in photoanodic responses was reproduced even when a monochromatic light of 350 nm was used (Figure S13), to minimize the effect of light absorption properties as seen in the UV-vis DRS spectra in Figure 5. It was difficult to compare their onset potentials due to dark cathodic currents at a negative potential than −0.3 V. Note that the anodic photocurrents observed are not derived from unfavorable self-oxidation but water oxidation, as confirmed by the stoichiometric generation of H2 and O2 with almost 100% of Faradaic efficiency under visible light in the BaBi4TiTaO11Cl/FTO electrode (Figure 11). Another important finding by the TRMC measurements is drastically shortened carrier lifetimes by Pb2+ substitution, e.g., 318 and 10 ns in Sr2Bi3Ta2O11Cl and SrPbBi3Ta2O11Cl, respectively (Table 2). This strongly suggests the existence of a charge recombination center which might be the incorporated Pb cations themselves and/or defects unexpectedly formed during synthesis of the Pb-based compounds. The lower ∑max values for Pbincorporated materials are probably due to the prompt charge recombination within the time resolution. Similarly, the Nb-based compounds showed high photoconductivities for Bi-rich samples and short lifetimes for Pb-containing ones (Table 2).


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Chemistry of Materials Table 2. Maximum φ∑ values and averaged lifetimes of the TRMC signals. Compound

Figure 9. Transient conductivities of Sr2Bi3Ta2O11Cl (black), SrPbBi3Ta2O11Cl (green), SrBi4TiTaO11Cl (orange), Ba2Bi3Ta2O11Cl (yellow), BaPbBi3Ta2O11Cl (blue), BaBi4TiTaO11Cl (red) and PbBi4TiTaO11Cl (purple) excited by 355 nm laser.

φ∑max / 10−7 m2(Vs)−1

Lifetimea / ns

Sr2Bi3Ta2O11Cl

1.76

318

SrPbBi3Ta2O11Cl

0.98

10

SrBi4TiTaO11Cl

3.39

143

Ba2Bi3Ta2O11Cl

1.32

251

BaPbBi3Ta2O11Cl

1.74

44

BaBi4TiTaO11Cl

5.39

189

PbBi4TiTaO11Cl

2.32

14

Sr2Bi3Nb2O11Cl

2.14

120

SrPbBi3Nb2O11Cl

0.63

8

SrBi4TiNbO11Cl

4.08

76

Ba2Bi3Nb2O11Cl

0.54

71

BaPbBi3Nb2O11Cl

0.97

22

BaBi4TiNbO11Cl

3.58

167

PbBi4TiNbO11Cl

1.28

5

aEstimated by fitting of TRMC decays shown in Figures 9 and S12 using a stretched exponential function, α exp(–(kt)β), where α, k, and β are the coefficient, the rate constant, and the power factor of the exponent, respectively. β was fixed at 0.20 to secure the consistency in the comparison of decays. The lifetime () is defined by the inverse of k ( = 1/k).

Figure 10. Current-potential curves using photoelectrodes of Sr2Bi3Ta2O11Cl (black), SrPbBi3Ta2O11Cl (green), SrBi4TiTaO11Cl (orange), Ba2Bi3Ta2O11Cl (yellow), BaPbBi3Ta2O11Cl (blue), BaBi4TiTaO11Cl (red) and PbBi4TiTaO11Cl (purple) in a Na2SO4 (0.1 M, pH 6.8) solution under intermittent visible light ( > 400 nm). Scan rate: 50 mV s−1.



Figure 11. Time courses of H2 and O2 evolution in photoelectrochemical water splitting on BaBi4TiTaO11Cl/FTO photoanode coupled with a Pt counter electrode at an applied potential of 1.23 V vs. RHE in a sodium phosphate buffer (0.1 M, pH 6.0) under visible-light irradiation (λ > 400 nm).

Performance as O2-Evolving Photocatalyst for Z-Scheme Water Splitting We evaluated the photocatalytic activity for water oxidation (O2 evolution) using a reversible Fe3+ electron acceptor, which is a typical half reaction of Z-scheme overall water splitting. Table 3 summarizes initial O2 evolution rates over the present photocatalyst materials, along with their specific surface areas and the Cl/Bi ratios as estimated by means of energy dispersive X-ray spectrometry (EDX) and X-ray photoelectron spectroscopy (XPS). Before discussing the photocatalytic activity, we outline the results of EDS and XPS. Specific surface area, which is related to the particles size, is one of the important factors affecting the photocatalytic performance in terms of the number of catalytic active sites and the diffusion length of carriers from bulk to surface. Our samples exhibited similar values (1.1 - 2.6 m2 g−1), except for Sr2Bi3M2O11Cl series (7.7 and 4.4 m2 g−1). Our recent study on Bi4MO8X (M = Nb, Ta; X = Cl, Br) has revealed that as the number of halide defects near surface increases, the photocatalytic activity for water oxidation decreases markedly.29,37 For this reason, all the present samples were synthesized using 5% excess A'BiO2Cl precursor to compensate a loss of chlorine species during calcination. Most of the samples thus prepared showed nearly ideal Cl/Bi values (estimated by EDX), but low Cl/Bi values are observed in SrPbBi3M2O11Cl (e.g., 0.26 in M = Ta vs. the ideal value of 0.33). Although EDX analysis includes inaccuracy to some extent, such a difference in SrPbBi3M2O11Cl is indicative of Cl defects. The XPS experiments revealed that the Cl/Bi ratios near the surface varies greatly compared to the results of EDX. In particular, Ba2Bi3M2O11Cl exhibited markedly higher Cl/Bi values (0.43 and 0.59 for M = Ta and Nb) than the ideal value of 0.33. Additionally, lower Cl/Bi values are found for the Pb-containing compounds, except BaPbBi3Nb2O11Cl. For Ti-containing compounds, the combination Ti with Sr tends to show lower Cl/Bi values than ideal one.

Page 10 of 16

Turning now to photocatalytic activity, one may first notice the O2 evolution rates is apparently high for Ba2Bi3M2O11Cl series (12.8 and 17.6 mol h−1). This is probably related to the much higher Cl/Bi ratio around the surface. As seen in Table 2, the ABi4TiMO11Cl series has a considerably larger ∑max (3.39 − 5.39 ⨯ 10–7 m2 (Vs)–1) compared to Ba2Bi3M2O11Cl (1.32 and 0.54 ⨯ 10–7 m2 (Vs)–1). While such a larger ∑max is suggestive of higher photocatalytic activity of ABi4TiMO11Cl, their O2 evolution rate is obviously lower than those of Ba2Bi3M2O11Cl. As mentioned above, the surface Cl/Bi ratio of ABi4TiMO11Cl was lower than the ideal value of 0.25, except for BaBi4TiTaO11Cl. It is therefore reasonable to conclude that the surface Cl/Bi ratio significantly affects the photocatalytic activity, similar to n = 1 Bi4MO8X.29,37 Although we tried to analyze the Cl/Bi ratio after photocatalytic reaction, adsorption of Cl anions, which is derived from FeCl3 in the reaction solution, makes it difficult to precisely determine the Cl/Bi ratio after photocatalytic reaction. Despite highly efficient carrier generation and subsequent balk migration in ABi4TiMO11Cl, the presence of surface halogen defects functioning as recombination centers would result in lower activity than expected. Conversely, the ∑max values in Ba2Bi3M2O11Cl are moderate, but a large surface Cl/Bi ratio would promote the surface reactions, allowing a higher O2 evolution rate. The activity of the lead-containing compounds is basically low, and some showed no activity. It can reasonably be explained by the rapid recombination of carriers as revealed by the smaller lifetimes of TRMC signals (see Table 2). Although surface properties (surface area and Cl/Bi ratios) of Sr2Bi3M2O11Cl appear adequate, the photocatalytic activity was relatively low. This low performance of Sr2Bi3M2O11Cl stems from a very small number of photons available under visible-light irradiation ( > 400 nm), as seen in Figures 5 and S5. In fact, the photocatalytic activity of Sr2Bi3Ta2O11Cl became remarkably improved by using a light source of  > 350 nm. In contrast, SrPbBi3Ta2O11Cl with low surface properties did not show such improved photocatalytic activity (Figure S14). These findings indicate the importance of surface composition, in particular the amount of halogen anions, and the band alignment, in order to establish highly efficient photocatalysis on these oxyhalide semiconductors. In the present study, the n = 2 type S-A compounds were synthesized under the same conditions (i.e., the calcination temperature, atmosphere, the molar ratio of precursors). Hence there is a room for optimization of synthetic conditions for each compound, as has done in our recent study on Bi4NbO8Cl (c.f., flux synthesis).38 The n = 2 S-A compounds also showed activity for H2 evolution, as expected by the negative ECBM than the proton reduction potential. However, the photocatalytic activity is quite low even under UV light ( > 300 nm) with methanol electron donor (Figure S15).


Page 11 of 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3. Surface Properties and Photocatalytic Water Oxidation Activities of n = 2 S-A Compounds.

Compound

Initial rate of

Cl/Bi ratio

Specific surface area

O2 generationa

/ m2 g−1

ideal

EDX

XPS

Sr2Bi3Ta2O11Cl

7.7

0.33

0.33

0.35

1.2

SrPbBi3Ta2O11Cl

1.1

0.33

0.26

0.18

1.1

SrBi4TiTaO11Cl

2.1

0.25

0.22

0.16

1.5

Ba2Bi3Ta2O11Cl

2.6

0.33

0.30

0.43

12.8

BaPbBi3Ta2O11Cl

1.6

0.33

0.32

0.22

1.6

BaBi4TiTaO11Cl

1.5

0.25

0.24

0.28

2.7

PbBi4TiTaO11Cl

1.5

0.25

0.25

0.17

n.d.

Sr2Bi3Nb2O11Cl

4.4

0.33

0.32

0.38

2.5

SrPbBi3Nb2O11Cl

1.3

0.33

0.27

0.30

n.d.

SrBi4TiNbO11Cl

2.4

0.25

0.24

0.18

0.5

Ba2Bi3Nb2O11Cl

1.8

0.33

0.33

0.59

17.6

BaPbBi3Nb2O11Cl

1.6

0.33

0.33

0.34

1.0

BaBi4TiNbO11Cl

1.8

0.25

0.23

0.21

2.8

PbBi4TiNbO11Cl

1.4

0.25

0.22

0.20

n.d.

/ mol h−1

aReaction

condition: Photocatalyst (0.2 g) dispersed in aqueous FeCl3 solution (5 mM, 250 mL), Light source: Xe lamp (300 W) fitted with L42 cutoff filter for visible-light irradiation ( > 400 nm).

Ba2Bi3Nb2O11Cl exhibited the best photocatalytic activity for water oxidation, so it was applied for the Z-scheme water splitting system. Figure 12 shows the result on the overall water splitting using Ba2Bi3Nb2O11Cl as an O2evolving photocatalyst, Rh-doped SrTiO3 with Ru cocatalyst as a H2-evolving photocatalyst, and a Fe3+/Fe2+ redox mediator under visible-light irradiation ( > 400 nm). H2 and O2 simultaneously evolved at the stoichiometric ratio (2:1), and no noticeable deactivation was observed. A slightly larger amount of O2 from the stoichiometric ratio at the initial stage because FeCl3 (oxidized form) was used as the starting redox reagent. The apparent quantum efficiency for water splitting was 0.7% at 420 nm monochromatic light irradiation, which is comparable to that using an optimized Bi4TaO8Cl as an O2 evolving photocatalyst (0.9%).29 Figure 12. Time courses of photocatalytic water splitting into H2 and O2 from a mixture of Ba2Bi3Nb2O11Cl and Ru/SrTiO3:Rh in aqueous FeCl3 solution (2 mM, pH = 2.5 adjusted by HCl) under visible-light irradiation ( > 400 nm).



CONCLUSION In this work, we demonstrated that valence- and conduction-band potentials of double-layered SillenAurivillius (S-A) type perovskite oxychlorides are tunable by various cation substitution. In particular, the cation compositions in the [A1A2O2] layer is found to be a key for the band engineering. As a general trend, 6s2 lone pair cations (i.e., Bi3+ and Pb2+) act to shift VBM potentials toward negative direction, while Ba2+ cations act to shift CBM potentials positively. Like the single-layered S-A oxychlorides, the double-layered system is active photoelectrode and photocatalyst materials for visiblelight-driven water splitting. The higher occupation of Bi3+ in the fluorite-based [A1A2O2] layer is responsible for improving conductivity of photogenerated electrons, leading to better photoelectrochemical performance for water oxidation. We believe that extended cation substitution and improving surface properties will enable to achieve optimized photocatalytic activities of the double- and triple-layered S-A oxyhalides.

EXPERIMENTAL Synthesis As the precursors for synthesizing n = 2 S-A compounds via the two-step procedure, fine particles of ABi2M2O9 and Bi3TiMO9 (A: Sr, Ba; M: Ta, Nb) were initially prepared by a polymerized complex (PC) method. Bi(NO3)3∙5H2O (Wako Pure Chemicals), ACO3 (Wako Pure Chemicals), MCl5 (High Purity Chemicals) and Ti(IV) isopropoxide (Nakarai Tesk) were used as the raw metal sources. These metal sources were mixed with a stoichiometric ratio (A : Bi : M = 4 mmol : 8 mmol : 8 mmol for ABi2M2O9 and Bi : Ti : M = 9 mmol : 3 mmol : 3 mmol for Bi3TiMO9) in methanol (20 mL) with continuous stirring at room temperature, followed by addition of citric acid (0.13 mol, Wako Pure Chemicals). The mixture was further stirred at an elevated temperature of 423 K until a transparent solution was formed. After ethylene glycol (32 mL, Wako Pure Chemicals) was added, the solution was further heated to ca. 593 K on a hot stirrer with continuous stirring to accelerate polymerization. The obtained brown gel was then heated at 623 K in air by using a mantle heater, which gave a black solid mass. Finally, ABi2M2O9 and Bi3TiMO9 were obtained by calcining the black solid mass on an Al2O3 plate at 773 K for 2 h in air. Another precursor A'BiO2Cl (A': Sr, Ba, Pb) was prepared by SSR, i.e., calcination of a mixture of SrCO3, BaCO3 or PbO (10 mmol, Wako Pure Chemicals), and BiOCl (10 mmol, Wako Pure Chemicals) at 1073 K for 20 h (A’ = Sr, Ba) or 973 K for 12 h (A’ = Pb) in air.15 The target compounds, AA'Bi3M2O11Cl and ABi4TiMO11Cl, were synthesized by SSR of ABi2M2O9/Bi3TiMO9 and A'BiO2Cl. These precursors with a ratio of 1.0 mmol : 1.05 mmol were thoroughly mixed and heated in air or in an evacuated silica tube, at 1123 K for 15 h. For all, a 5% excess of A'BiO2Cl was used to compensate a loss of chlorine species due to volatilization during the thermal treatment.29,37 For comparison, we prepared several

Page 12 of 16

AA'Bi3M2O11Cl samples via conventional one-step SSR in air or under vacuum using commercially available precursors ACO3 (Wako Pure Chemicals), Bi2O3 (Wako Pure Chemicals), BiOCl (Wako Pure Chemicals), TiO2 (Wako Pure Chemicals) and M2O5 (High Purity Chemicals) under appropriate heating conditions between 1123 K for 15 h and 1223 K for 50 h. As a H2-evolving photocatalyst, Rh-doped SrTiO3 (SrTiO3:Rh) was prepared by a hydrothermal synthesis.7,39 Sr(OH)2·8H2O (22 mmol, Kanto Chemicals), TiO2 (19.6 mmol, JRC-TIO-10), and Rh(NO3)3 (0.5 mmol, Kanto Chemicals) were mixed in water (50 mL). The mixture was placed in a Teflon-lined stainless steel autoclave and heated at 433 K for 42 h. The precipitate obtained was washed with hot water and then with ambient water, followed by drying at 363 K in an oven. The hydrothermal product was mixed with an additional amount of Sr(OH)2·8H2O (7 mol%), followed by heating at 1273 K for 10 h in air. The resulting product was washed with ambient water and dried at 363 K. Ru cocatalyst was loaded by a photodeposition method in methanolic (10 vol%) aqueous dispersion of SrTiO3:Rh containing RuCl3 (0.7 wt% based on metal) with visible-light irradiation ( > 400 nm) for 2 h. Characterization Powder X-ray diffractometry (XRD; MiniFlex II, Rigaku, X-ray source: Cu K), scanning electron microscopy (SEM; NVision 40, Carl Zeiss-SIINT), the Brunauer-EmmettTeller surface area (BET; BELSORP-mini, BEL Japan), UVvisible diffuse reflectance spectroscopy (V-650, JASCO) and X-ray photoelectron spectroscopy (XPS; JPC-9010MC, JEOL, X-ray source: Mg K) were used for characterization of the samples. The binding energies determined by the XPS measurements were corrected with reference to the C 1s peak of extraneous carbon (284.8 eV). The lattice parameters were determined by Le Bail analysis31 with JANA2006 pragram.40 Synchrotron XRD (SXRD) patterns were collected at the BL02B2 in SPring-8, Japan ( = 0.41866 Å) and were analyzed by the Rietveld method using RIETAN-FP program.41 Photoelectron yield spectroscopy (PYS) measurements were conducted by using a Bunko Keiki BIP-KV202GD. Theoretical Calculation The electronic structures of the materials were calculated using the Cambridge Serial Total Energy Package (CASTEP).42 The energy was calculated using the generalized gradient approximation (GGA) of DFT proposed by Perdew, Burke, and Ernzerhof (PBE). The electronic states were expanded using a plane wave basis set with a cutoff of 571.4 eV. The k-point set was 9⨯9⨯3. Geometry optimization calculation was performed before electronic structure calculation using the Broyden– Fletcher–Goldfarb–Shanno (BFGS) algorithm. The Madelung site potentials of cation and anion sites in AE2Bi3Ta2O11Cl were calculated using VESTA.43 Energy levels of each anion in Sr2Bi3Ta2O11Cl were determined by


Page 13 of 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the sum of electron affinities and Madelung site potential as we previously reported.18 Time-Resolved Microwave Conductivity (TRMC) Measurement TRMC experiments were conducted with the third harmonic generator (THG; 355 nm) of a Nd:YAG laser (Continuum Inc., Surelite II, 5–8 ns pulse duration, 10 Hz) as the excitation source (4.6 × 1015 photons cm−2 pulse−1) and X-band microwave (~9.1 GHz) as the probe. The photoconductivity Δσ was obtained by applying the formula ΔPr/APr, where ΔPr, A, and Pr are the transient power change of the reflected microwave power, the sensitivity factor, and the reflected microwave power, respectively. The photoconductivity transient Δσ was converted to the product of the quantum yield (φ) and the sum of charge carrier mobilities ∑  (= μ+ + μ–) by the formula ∑ = Δσ(eI0Flight)−1, where e and Flight are the unit charge of a single electron and a correction (or filling) factor. The experiments were performed in air at room temperature (298 K). Photoelectrochemical Measurement A series of photoelectrodes were prepared by electrophoresis deposition of particulate oxyhalide samples on a conductive glass modified with a fluorinedoped tin oxide (FTO). In the electrophoresis deposition procedure, a pair of FTO glasses was immersed parallel to each other with a distance of 0.8 cm in dispersions of a series of oxyhalide powder, and connected to a DC power supply (PS60-6.6A, TEXIO). A bias of 25 V was applied between the FTO glasses resulting in a photocatalystcoated FTO glass. The oxyhalide-coated area was fixed at ca. 1.5 cm ⨯ 4.0 cm, where 3-4 mg of oxyhalides were included. Subsequently, post-necking treatment was conducted to improve the interparticle resistance of electrodes.44,45 A methanolic TiCl4 solution (10 mM, 50 L) was added dropwise on the oxyhalide electrodes three times. After drying, the electrodes were heated at 523 K under Ar flow (20 mL min–1) for 1 h. Photoelectrochemical measurements were conducted in an aqueous solution of Na2SO4 (0.1 M, pH 6.8) under Ar atmosphere. Linear sweep voltammograms were recorded by a potentiostat (VersaSTAT 4, AMETEK) connected to the photoelectrode, an Ag/AgCl reference electrode, and a Pt-wire counter electrode. Intermittent visible light ( > 400 nm) was irradiated to the oxyhalide electrodes using a Xe lamp (Cermax, 300 W) fitted with a CM-1 cold mirror and a L-42 cut-off filter (light intensity is ca. 360 mW cm−2, output current: 20 A). Photoelectrochemical water splitting was carried out in an O-ring sealed Pyrex cell. The BaBi4TiTaO11Cl electrode (geometrically effective surface area was ca. 6 cm2), a Ptwire electrode, and a Ag/AgCl reference electrode were installed into the reaction cell filled with a sodium phosphate buffer solution (0.1 M, 100 mL, pH 6.0), and connected to a potentiostat (HA-151A, Hokuto Denko). The solution was purged with Ar for 1 h before reaction. During photoelectrolysis, visible light (λ > 400 nm) was

illuminated to the Bi4TiTaO11Cl electrode using a Xe lamp (Cermax, 300 W) fitted with a CM-1 cold mirror and a L-42 cut-off filter (light intensity is ca. 360 mW cm−2, output current: 20 A) under Ar flow (10 mL min−1). The gaseous products (i.e., H2 and O2) were measured using gas chromatography (Inficon 3000 MicroGC), directly connected to the reaction cell. Photocatalytic Reaction Photocatalytic reactions were carried out in a Pyrex reaction vessel connected to a glass closed gas circulation system. For the photocatalytic water oxidation, a suspension of photocatalyst (0.2 g) in 250 mL aqueous FeCl3 solution (5 mM, pH 2.5 adjusted by HCl) was purged with Ar and irradiated at λ > 400 nm using a Xe lamp (Cermax, 300 W) fitted with a CM-1 cold mirror and a L-42 cut-off filter (light intensity is ca. 360 mW cm−2, output current: 20 A). The evolved gases were analyzed by a gas chromatography (GC-8A, Shimadzu, TCD detector, MS 5A column, Ar carrier) connected to the closed gas circulation system. Overall water splitting was carried out with the same experimental setting for a reaction suspension of the oxyhalide photocatalyst and Ru/SrTiO3:Rh (0.1 g each) in an aqueous FeCl3 solution (1 mM, pH 2.5 adjusted by HCl). H2 evolution reaction was carried out using Ba2Bi3Nb2O11Cl (0.2 g) in 250 mL methanol-water mixed solution (1:4, v/v) under UV-vis light irradiation ( > 300 nm). Pt cocatalyst (3 wt% as the Pt metal) was modified by in situ photodeposition method using H2PtCl6 precursor during photocatalytic reaction. To measure the external quantum efficiency, the similar experimental setting was used with a monochromatic light irradiation ( = 420 nm) using a Xe lamp (Cermax, 300 W) fitted with a CM-1 cold mirror and a band pass filter. The apparent quantum yield (AQY), equivalent to external quantum yield, of Z-scheme water splitting was determined based on eq. 1. AQY(%) = 100 × 𝐴 × 𝑅/𝐼

(1)

where A is the number of photons required to generate one molecule of products (i.e., 4 for H2 and 8 for O2 evolution), R is the rate of gas generation (mol s−1) and I is the incident photon flux (2.2  10−7 einstein s−1).

ASSOCIATED CONTENT This material is available free of charge via the Internet at http://pubs.acs.org. XRD patterns, SEM images, lattice parameters, UV-vis DRS, calculated ionic energy levels, Madelung site potentials and DOS, band diagrams, TRMC, photocurrent responses, and photocatalytic water splitting.

AUTHOR INFORMATION Corresponding Author *[email protected] (H.K.) *[email protected] (R.A.)

ORCID Akinobu Nakada: 0000-0001-6670-5044 Masanobu Higashi: 0000-0002-1265-2191



Hajime Suzuki: 0000-0002-8891-2033 Daichi Kato: 0000-0002-1600-4095 Takafumi Yamamoto: 0000-0002-7960-1014 Akinori Saeki: 0000-0001-7429-2200 Hiroshi Kageyama: 0000-0002-3911-9864 Ryu Abe: 0000-0001-8562-076X

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The authors declare no competing financial interest.

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Notes

ACKNOWLEDGMENT This work was supported by the CREST (JPMJCR1421), JSPS KAKENHI (JP17H06439, JP16H06439, JP16H06441, and JP15H03849), and JSPS Core-to-Core Program (A) Advanced Research Networks. The authors are also indebted to the technical division of Institute for Catalysis, Hokkaido University for their help in building the experimental equipment.

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Band Engineering of Double-Layered SillÃ©n-Aurivillius Perovskite

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