Lead Bismuth Oxyhalides PbBiO2X (X = Cl, Br) as Visible-Light

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Lead Bismuth Oxyhalides PbBiOX (X = Cl, Br) as VisibleLight-Responsive Photocatalysts for Water Oxidation: Role of Lone Pair Electrons in Valence Band Engineering Hajime Suzuki, Hironobu Kunioku, Masanobu Higashi, Osamu Tomita, Daichi Kato, Hiroshi Kageyama, and Ryu Abe Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01385 • Publication Date (Web): 10 Aug 2018 Downloaded from http://pubs.acs.org on August 11, 2018

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Chemistry of Materials

Lead Bismuth Oxyhalides PbBiO2X (X = Cl, Br) as Visible-Light-Responsive Photocatalysts for Water Oxidation: Role of Lone Pair Electrons in Valence Band Engineering Hajime Suzuki,† Hironobu Kunioku,† Masanobu Higashi,† Osamu Tomita,† Daichi Kato,† Hiroshi Kageyama†,‡ and Ryu Abe†,‡ †

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

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

ABSTRACT: We show that layered oxyhalides PbBiO2X (X = Cl, Br, I) with a Sillén-type structure possess band levels appropriate for visible light-induced water splitting. Under visible light, PbBiO2Cl and PbBiO2Br with bandgap (BG) of 2.51 and 2.48 eV, respectively, stably oxidized water to O2 in the presence of an Fe3+ electron acceptor. A comparison with structurally related SrBiO 2Cl and BaBiO2Cl (BG = 3.55 and 3.54 eV) combined with DFT calculations revealed a significant interaction between O 2p and Pb 6s orbitals leading to the upward shift of the valence band maximum in PbBiO 2X as compared with (Sr,Ba)BiO2Cl. Z-scheme water splitting into H2 and O2 has been demonstrated using PbBiO2Cl as an O2-evolving photocatalyst, coupled with an appropriate H2evolving photocatalyst in the presence of a Fe3+/Fe2+ redox mediator.

Introduction Water splitting using semiconductor photocatalysts is regarded as one of the promising technologies for clean production of hydrogen (H2) directly from water by harvesting abundant solar light energy.1-7 One of the important challenges to attain the sufficiently high efficiency in solar to hydrogen conversion is how to effectively utilize photons in visible light region of solar spectrum. Although metal oxides were extensively explored as photocatalysts in the early stages of research, 8-11 majority of them do not meet the requirements for water splitting under visible light. It is essentially difficult for the conventional metal oxides to possess both a smaller bandgap than 3.0 eV for visible light absorption and a higher (i.e., more negative) conduction band minimum (CBM) than the reduction potential of water. This fundamental issue basically arise from the fact that O 2p orbitals exclusively form the valence band of conventional metal oxides and consequently fix their valence band maxima (VBM) at deeply positive levels around +3.0 V vs. the standard hydrogen electrode (SHE) .12 Recently, mixed-anion semiconductors such as oxynitrides, oxysulfides, and oxyhalides have been recognized as alternative photocatalysts.13-18 Since the p orbitals with higher energy than O 2p (i.e., N 2p, S 3p, and I 5p) can make the VBMs higher compared to the oxide counterparts, a large portion of them possess the appropriate band levels (CBM and VBM) for visible light-induced water splitting.19-20 Unfortunately, however, most of these mixed-anion semiconductors are subject to deactivation through the self-oxidation of the non-oxide anions by the photo-generated holes (e.g., 2N3– + 6h+ → N2). Numerous efforts mainly involving surface modifications such as loading

co-catalyst have been made to circumvent this oxidative deactivation. We have recently demonstrated that Bi4NbO8Cl exhibits appropriate VBM/CBM levels for water splitting as well as a narrow bandgap for visible light absorption, and can oxidize water to O2 efficiently and stably under visible light without observing any self-decomposition.21 Bi4NbO8Cl crystallizes in the Sillén–Aurivillius layered perovskite, with an alternate stack of NbO4 perovskite slabs and Bi2O2 fluorite-like layers (Figure S1). The stability against water oxidation is accounted for by the predominant occupation of O 2p orbitals in VBM, instead of Cl 3p orbitals. Madelung site potential analysis revealed that the upward shift of VBM in Bi4NbO8X (X = Cl, Br) originates from the electrostatic destabilization of the oxide anions in the fluorite layer.22 Furthermore, we found a significant interaction between O 2p and Bi 6s orbitals derived from the revised lone pair (RLP) model,23 which also leads to the upward shift of VBM.24 Unfortunately, it is difficult to elaborate on the contribution of these two factors largely due to the complex crystal structure with coherent octahedral rotations that generate eight inequivalent oxygen sites. In this study, we investigated structurally simpler lead bismuth oxyhalides PbBiO2X (X = Cl, Br, I), as potential materials for water splitting under visible light. PbBiO2X adopts a Sillén structure with fluorite-type PbBiO2 layers intervened by halide anions (Figure 1). It is worth noting that photocatalytic activity for dye degradation under visible light were reported in PbBiO2Cl and PbBiO2Br,28-29 but the studies of water oxidation behavior and electronic structures remain unexplored.

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Figure 1. Structure of ABiO2X: (a) metal-ordered AEBiO2X (Cmcm)25-26 and (b) metal-disordered PbBiO2X (I4/mmm).27 In addition to the simplicity of the crystal structure (vs. Bi4NbO8X), the presence of structurally related AEBiO2Cl (AE = Sr, Ba) offer a unique opportunity to clarify the role of the stereoactive lone-pairs of Pb2+ 6s orbitals in the valence band structure. The water oxidation behavior in PbBiO2X is discussed in relation with the valence band structures. We also use PbBiO2X as the O2-evolution photocatalyst, coupled with a H2evolving photocatalyst and a redox mediator, to achieve Zscheme water splitting. Experimental Catalyst preparation. All the five compounds in powder form were prepared by a solid-state reaction.27-28, 30 PbBiO2Cl and PbBiO2Br were prepared by calcining a stoichiometric mixture of PbO and BiOX (X = Cl and Br) at 973 K for 10 h in air. As for PbBiO2I, a 1:1 mixture of PbO and BiOI was pelletized and heated in the same condition, but in an evacuated silica tube. BiOCl was purchased from Wako Pure Industries, Ltd., while BiOBr and BiOI were synthesized by a soft liquid deposition method;31 A solution of 5 mmol of Bi(NO3)3·5H2O was prepared in ethanol and mixed with the solution of 5 mmol of KX (X = Br or I) dissolved in pure water. After stirring for 5 h at room temperature, the resulting precipitate was collected by centrifugation, washed several times with water, and finally dried in air at 353 K. In some cases, either Ru or Pt-based cocatalyst (0.5 wt% calculated as elemental metal) was loaded onto PbBiO2Cl to enhance the surface reactions. The loading was carried out by impregnation using either RuCl3·nH2O or H2PtCl6·6H2O as a precursor, followed by calcination in air at 573–973 K for 1 h. SrBiO2Cl (BaBiO2Cl) were prepared by the calcination of a stoichiometric mixture of BiOCl and SrCO 3 (BaCO3) in air at 1073 K for 20 h. A particulate sample of strontium titanate doped with rhodium cations (SrTiO3:Rh)32 was prepared by a solid state reaction and utilized as the H2-evolving photocatalyst. A mixture of TiO2, SrCO3, and Rh2O3 (Ti : Sr : Rh = 1 : 1.07 : 0.01) was calcined in air at 873 K for 2 h and subsequently at 1273 K for 10 h. A Ru-based cocatalyst (0.7 wt% calculated as metal) was loaded onto SrTiO3:Rh by means of photodeposition method using RuCl3·nH2O as a precursor.33 Characterization. The synthesized powder samples were characterized by X-ray diffraction (XRD; MiniFlex II, Rigaku, Cu Kα), scanning electron microscopy (SEM; VE-9800,

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KEYENCE), UV-visible diffuse reflectance spectroscopy (V650, Jasco), and energy-dispersive X-ray spectroscopy (EDX; X-max, Oxford Instruments). The surface chemical states of samples were evaluated by X-ray photoelectron spectroscopy (XPS; ESCA MT-5500, ULVAC-PHI; Mg Kα), in which the binding energies were corrected with reference to the standard value of metallic Au (84.0 eV) deposited. Electrochemical measurement. The synthesized powder sample was mixed with a small amount of water. The obtained paste was coated on a fluorine doped tin oxide (FTO) conductive substrate via a squeezing method and dried in air at room temperature. The Mott-Schottky plots were recorded on an electrochemical analyzer (PARSTAT2263, Princeton Applied Research). Electrochemical measurements were performed in a three-electrode cell using a Pt wire counter-electrode, a Ag/AgCl reference electrode, and a phosphate buffer solution (0.1 M, pH = 6.0) with 10 mV amplitude and a frequency of 1 kHz. Calculation. The band structures of the oxyhalides were calculated using the Cambridge Serial Total Energy Package (CASTEP).34 The exchange and correlation energies were evaluated within the generalized gradient approximation (GGA) of density function theory (DFT), as proposed by Perdew, Burke, and Ernzerhof (PBE). The electrical states were expanded by using a plane-wave basis set with a cutoff energy of 630 eV. 48k-point was used. Geometry optimization was performed before calculating the electronic structures using the BroadenFletcher-Goldfarb-Shanno (BFGS) algorithm. Although there is a complete cation (Pb/Bi) disorder, a complete cation ordered structure model as in AEBiO2Cl was used in the band calculation of PbBiO2X. Photocatalytic reaction. Photocatalytic reactions were carried out in a closed gas-circulation system. For the photocatalytic water oxidation (O2-evolution) reaction in the presence of an electron accepter, the photocatalyst powder (0.2 g) was suspended in 250 mL of an aqueous AgNO3, Ag2SO4, Fe(NO3)3, Fe(ClO4)3, FeCl3, or Fe2(SO4)3 solution (5 mM) in a Pyrex glass reactor with magnetic stirring. The solution pH should be below about 2.5 for the O2-evolution reaction with the Fe3+ electron acceptor, because Fe3+ cations spontaneously form Fe(OH)3 and precipitate at higher pH values. Thus, a small amount of either aqueous HNO3, HClO4, HCl, or H2SO4 solution was added into the aqueous solutions of Fe(NO3)3, Fe(ClO4)3, FeCl3, or Fe2(SO4)3, respectively, to adjust the pH to 2.3 before the reaction. Apparent quantum efficiency (AQE) for O2 evolution was measured using a Xe lamp (MAX-302, Asahi Spectra Co. Ltd.) attached with a bandpass filter (central wavelength: 400 nm), and was estimated as follows. AQE (%) = (4 × R/I) × 100 R and I represent the O2 evolution rate and the rate of incident photons, respectively. The total number of incident photons was measured to be 25.3 mW. For the Z-scheme water-splitting reaction in the presence of Fe3+/Fe2+ redox couple, the Ru-loaded SrTiO3:Rh (0.1 g) and RuO2-PbBiO2Cl (0.2 g) photocatalysts were suspended together in an aqueous Fe(NO3)3 solution (5 mM, 250 mL), respectively, as the H2-evolving and O2-evolving photocatalysts. The solution pH was adjusted to 2.3 with diluted aqueous HNO3 solution. The suspension was irradiated using a 300 W Xe-arc lamp (Perkin-Elmer, Cermax PE300BF) equipped with both a cut-off filter (L-42, HOYA) and a cold mirror (CM-1, Kenko) that afford visible light irradiation with the wavelength range from 400 to 800 nm. Evolved gases were


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analyzed by an on-line gas chromatograph (thermal conductivity detector (TCD); 5 Å molecular sieve column packing; Ar carrier gas). Results and discussion Characterization of ABiO2X. ABiO2X has fluorite-like ABiO2 layers sandwiched by halide anions. For AE = Sr and Ba, AE and Bi cations are completely ordered in the orthorhombic lattice (space group (SG): Cmcm),25-26, 30 while PbBiO2Br and PbBiO2I are tetragonal (SG: I4/mmm) though a proposed cation-disorder is not obvious from X-ray diffraction.27 Two polymorphs with Cmcm and I4/mmm space groups are known for PbBiO2Cl.27,35 Figure S2 shows the XRD patterns of all the compounds. All the diffraction peaks agree with those in the standard powder diffraction file (PDF) databases. The PbBiO2Cl sample is assigned as the tetragonal system (Figure S3). The lattice constants of the samples (Table S1) calculated by Le Bail analysis36 using the Jana2006 program37 were in good agreement with the previously reported ones. 30 Figure S4 shows the SEM images of AEBiO2Cl and PbBiO2X (X = Cl, Br, and I). SrBiO2Cl, PbBiO2Cl, and PbBiO2Br exhibited aggregated primary particles with grain sizes of several hundred nanometers to several micrometers, whereas BaBiO2Cl and PbBiO2I have larger particles with grain sizes up to ~20 µm. Figure 2 shows the diffuse reflectance spectra of these samples. As for oxychlorides, the absorption edges of SrBiO2Cl and BaBiO2Cl were similar (~350 nm), but that of PbBiO2Cl was red-shifted (up to 500 nm). For Pb, PbBiO2Cl and PbBiO2Br have similar absorption edges of 500 nm, but PbBiO2I has a redshifted value of 530 nm. The Mott-Schottky plots of SrBiO2Cl, BaBiO2Cl, PbBiO2Cl, PbBiO2Br, and PbBiO2I in a phosphate buffer solution (pH = 6) gave flat-band potentials of –0.71, – 0.68, –0.49, –0.48, and –0.52 V (vs. the standard hydrogen electrode (SHE) at pH = 6) (Figure S5).

Figure 2. Diffuse reflectance spectra of AEBiO2Cl (AE = Sr and Ba) and PbBiO2X (X = Cl, Br, and I).

Figure 3. Band edge positions of AEBiO2Cl (AE = Sr and Ba) and PbBiO2X (X = Cl, Br, and I) at pH = 6. Assuming that the flat-band potential was located just below the CBM, the VBMs were estimated to be 2.84, 2.86, 2.02, 2.00, and 1.83 V from their bandgaps. These results are summarized in Figure 3. Clearly, the VBM of PbBiO2Cl is much more negative compared to AEBiO2Cl. The remarkable upward shift represents a crucial role of the Pb 6s orbitals in elevating the VBM, as reported previously for some lead oxides.38-40 The CBMs are also affected by the divalent cations; the substitution of the alkali earth metals by Pb induces a positive shift of the CBM. Consequently, PbBiO2X have much smaller bandgaps than SrBiO2Cl and BaBiO2Cl. The VBM of PbBiO2Br is slightly more negative than PbBiO2Cl (by 0.03 V), a behavior which is different from conventional Bi-based oxyhalides BiOX, in which the VBM monotonically increases in order of BiOCl (3.1 V vs. SHE at pH 0) < BiOBr (2.5 V) < BiOI (1.6 V)21-22, 24, 41-42 due to the reduced electronegativity of halogen (Cl > Br > I). Together with our recent study on Bi4MO8X21 and SrBiO2Cl,22 these observations imply that the density of states (DOS) around the VBM in PbBiO2Cl and PbBiO2Br are predominantly formed by O 2p orbitals, not halide p orbitals. The VBM of PbBiO2I is appreciably more negative than that of PbBiO 2Cl and PbBiO2Br (by ca. 0.15 V), suggesting an occupation of I 5p orbitals at VBM. These arguments will be validated by DFT calculations in the next section. Electric structures. Figure 4 displays the ionic energy levels of oxygen and halogen anions in PbBiO2X and AEBiO2Cl calculated by the sum of Madelung potential and electron affinity. For comparison, the results of BiOX22 are also plotted. It is seen that the ionic energy levels of oxygen are located higher than X in PbBiO2X. This trend contrasts to the BiOX case with higher ionic energy levels of X.22 Reflecting the structural similarity and identical valence, ionic energy levels of oxide and halide anions for AEBiO2Cl are more or less similar to those of PbBiO2X. This in turn suggests that the origin of the higher VBMs of PbBiO2X (X = Cl, Br) than AEBiO2Cl is coming essentially from the stereoactive Pb cation. DFT calculations indeed showed this contrast.



Figure 4. The ionic energy levels of each anionic site in BiOX,22 PbBiO2X (X = Cl, Br, I), and AEBiO2Cl (AE = Sr,22 Ba) calculated by the sum of Madelung potential and electron affinity. Figures 5(a) and (b) show the DOS and the partial DOS (PDOS) for 6s and 6p orbitals of Bi and Pb. The band gap drastically decreases when Pb is introduced. The CBMs of AEBiO2Cl are composed predominantly of Bi 6p orbitals (pink) and partially of Sr or Ba orbitals, while in PbBiO2X (X = Cl, Br, and I) Pb 6p and Bi 6p orbitals hybridize at the CB (Figure 5(b)). The hybridization probably results in the highly dispersed CB shown in Figure S6, leading to the downward shift of CBM of PbBiO2X, compared to AEBiO2Cl. Regarding on the valence band, for SrBiO2Cl and BaBiO2Cl we found a significant contribution of Bi 6s/6p orbitals to the DOS around VBM, in addition to predominant O 2p orbitals (red). Most remarkably, the VBM in PbBiO2Cl is populated significantly by Pb 6s orbitals, along with O 2p orbitals. A certain contribution of Cl 3p orbitals is also seen. Furthermore, one can see a sizable overlap between the Pb 6s and O 2p orbitals at a deeper energy region from −7.5 to −5.5 eV (Figure S7).

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There also exists a hybridization between the Bi 6s and O 2p orbitals from − 10 to − 7.5 eV, as found in SrBiO2Cl and BaBiO2Cl. In the revised lone pair (RLP) model, two features should be seen in the band structure.23 The first one is that the 5s or 6s of post transition metals (e.g., Sn, Pb, Bi) significantly hybridize with O 2p at deeper potential region than the valence band. The second one is that the 5s or 6s (and 5p and 6p, to a lesser) obviously contribute to the DOS around VBM accompanied by the overlap with O 2p. Figures 5 and S6 clearly demonstrate that PbBiO2Cl has both features. Walsh et al. have also reported that the hybridization between the 6s electrons and O 2p electrons in PbO is greater than in Bi2O3, due to the smaller energy difference between Pb 6s and O 2p orbitals compared to that between Bi 6s and O2p. Therefore the significantly elevated VBM potential of PbBiO2Cl (even though it mainly consists of the O 2p orbitals) can be understood by the strong hybridization between the Pb 6s and O 2p orbitals based on the RLP model.23 As seen in Figure 4, the ionic energy levels of halogen in PbBiO2X elevate in order of Cl < Br < I, in accordance with the band levels (Figure 3). In accordance with the trend, the contribution of X in DOS around VBM increases in this order (Figure 5). Although the dominant contribution of O 2p is observed in both PbBiO2Cl and PbBiO2Br, PbBiO2I has a greater contribution of the I 5p orbitals than other orbitals (Figure 5). Photocatalytic water oxidation over ABiO2X. Figure 6 shows O2 evolution over PbBiO2X (X = Cl, Br, I) from an aqueous solution containing Fe(NO3)3 under visible light. As expected from the absorption property, SrBiO2Cl and BaBiO2Cl did not generate O2 under visible light (not shown). Both PbBiO2Cl and PbBiO2Br generated O2 at steady rates, with much higher O2 evolution rates observed in the chloride system. In the case of PbBiO2I, no O2 generation was observed, despite sufficient VBM/CBM levels.

Figure 6. O2 evolution over PbBiO2X (X = Cl, Br, I) from an aqueous solution containing Fe(NO3)3 (5 mM, 250 mL) under visible light (λ > 400 nm). Figure 5. DOS of AEBiO2Cl (AE = Sr and Ba) and PbBiO2X (X = Cl, Br, and I).


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Figure 7. XRD patterns of PbBiO2X (X = Cl, Br, I) before and after O2 evolution, as shown in Figure 6. Figure 7 shows the XRD patterns of PbBiO2X before and after the O2-evolution reactions, where no significant changes were observed. For all the samples, the peaks remain almost unchanged and no impurity peaks could be observed. Table 1 shows the X/Bi ratios of PbBiO2X before and after the O2-evolution reactions (Figure 6), as determined via EDX and XPS analyses. As expected from XRD, the X/Bi ratios determined by EDX stayed unchanged after the reactions. On the other hand, the X/Bi ratios determined by XPS, a surface-sensitive method, appreciably decreased after the reactions for all the samples. The reduced X/Bi ratio was also observed when the samples were stirred in the aqueous Fe(NO3)3 solution under dark conditions, indicating that the decreased ratio is caused by the occurrence of a surface chemical reaction (e.g., simple desorption or ion exchange) to some extent. In the case of PbBiO2Cl and

PbBiO2Br, the X/Bi ratios were found to be almost the same and independent of the light irradiation. On the contrary, in PbBiO2I light irradiation significantly promote the I/Bi ratio to be reduced, indicating that the decrease of I– proceeds both chemically and photochemically. The observed decrease in the I/Bi ratio upon photo irradiation is most likely due to the self-oxidation of I– to some oxidized species (e.g., I2) by the photo-generated holes and is reasonably explained by the dominant occupation of I 5p orbitals around the VBM. The photo-generated holes will be relaxed up to the VBM and thus localize on the I– anions, thereby oxidizing I– in the PbBiO2I preferentially instead of water. On the other hand, PbBiO2Cl and PbBiO2Br were more stable against photo-induced self-oxidation, probably as a result of the VBM of PbBiO2Cl and PbBiO2Br being predominantly formed by the O 2p orbitals. Such robust nature against self-oxidation is similar to what has been observed in Bi4MO8X (M = Nb, Ta; X = Cl, Br), where the DOS around VBM is predominantly occupied by the O 2p orbitals,21 accompanied by the strong hybridization with Bi 6s and Bi 6p orbitals.24 Considering the dominant contribution of the I 5p orbitals to the VBM in PbBiO2I, the VBM position should be much more sensitive to the decrease in X/Bi ratio when compared to PbBiO2Cl and PbBiO2Br. In other words, the loss of I– content near the surface made the surface energy state of PbBiO2I more positive as compared to the original sample, consequently making it difficult for the photo-generated holes to transport from the bulk to the surface. Therefore, both the preferential accumulation of holes on I–, which lead to self-oxidation, and the shift of VBM at the surface to positive levels by the loss of I–, are possible explanations for lack of O2 generation on PbBiO2I. On the other hand, the decrease of X/Bi ratios in PbBiO 2Cl and PbBiO2Br is expected to affect the surface energy only slightly since their VBMs are dominantly formed by O 2p orbital rather than X np, probably rationalizing the relatively stable O2 generation on them despite of obvious decrease in X/Bi during reaction. We hereafter focus on PbBiO2Cl because of its higher photocatalytic activity toward water oxidation, as compared to other AEABiO2X materials.

Table 1. X/Bi ratios of PbBiO2X (X = Cl, Br, I) before and after O2 evolution, as shown in Figure 6, along with those after stirring in the Fe(NO3)3 (aq.) under dark conditions. Cl

Br

I

Before

After

Dark

Before

After

Dark

Before

After

Dark

EDX

1.0

1.0

1.0

1.0

1.0

1.0

1.1

1.1

1.1

XPS

1.0

0.5

0.5

1.0

0.5

0.5

1.1

0.2

0.4



Figure 8. O2 evolution over PbBiO2Cl from an aqueous Fe3+ solution (5 mM, 250 mL) with different counter-anions (ClO4–, NO3–, Cl–, or SO42–) under visible light irradiation (λ > 400 nm), along with that from AgNO3 and Ag2SO4 (aq.).

Figure 8 shows O2 evolution over PbBiO2Cl from an aqueous Fe3+ solution with different counter-anions (ClO4–, NO3–, Cl–, or SO42–) under visible light irradiation, along with that from aqueous AgNO3 and Ag2SO4 solutions. The use of Fe2(SO4)3 resulted in negligible O2-evolution on PbBiO2Cl, although O2 generation was observed from Ag2SO4 (aq.) as well as AgNO3 (aq.). Similar detrimental effects of the co-presence of the SO42– anion and Fe3+/Fe2+ redox on the photocatalytic O2 evolution over WO343 or H2WO4.44 have been reported, where the O2-evolution rate increased as Fe2(SO4)3 < FeCl3 < Fe(NO3)3. Electrochemical measurements using WO3 and TiO2 substrates45 demonstrated that the Fe3+ complex ions formed in the aqueous Fe2(SO4)3 solution were less reactive compared to that formed in the aqueous FeCl3 solution, whereas the counter-anion type rarely affects photoelectrochemical water oxidation. This inertness is most likely due to the negative shift of redox potential of Fe3+/Fe2+ by the partial formation of a Fe3+ complex with SO42– anions in the solution, thereby lowering the efficiency of the reduction of the Fe3+ acceptor. Thus, the use of NO3– as counter-anions for the Fe3+/Fe2+ redox reaction is favorable for O2-evolution on the PbBiO2Cl photocatalyst, similar to the case of WO3 and H2WO4 photocatalysts. Although the H2-evolving ability of PbBiO2Cl was evaluated after loading of Pt species as cocatalyst for catalyzing proton reduction, it showed negligibly low activity for H2 evolution even from the aqueous solution containing a sacrificial electron donor (MeOH) under UV irradiation ( > 300 nm). This low activity is probably due to the insufficient electron transfer between oxyhalide PbBiO2Cl photocatalyst and loaded Pt cocatalyst. The surface modification of PbBiO2X with various cocatalysts are now under investigation to improve their H2-evolving abilities.

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Figure 9. Initial rates of O2 evolution over unmodified PbBiO2Cl (Bare), PbBiO2Cl adsorbed with Ru species without calcination (Ads.), and PbBiO2Cl loaded with Ru species at different temperatures from the aqueous Fe(NO3)3 solution under visible light irradiation. Although appreciable enhancement was achieved by simple impregnation of RuCl3, the rate of O2 evolution significantly increased with increasing calcination temperature. Note that calcination of the bare PbBiO2Cl in air had a negligible effect on the rate of O2 evolution. This result is reasonable because PbBiO2Cl was prepared by calcination in air at a higher temperature (973 K) for a longer time (10 h), as compared to the conditions for loading the co-catalysts. The electronic states of the Ru species on PbBiO2Cl were investigated by XPS. Figure 10 shows the spectra of Ru 3d for these samples, along with RuO2, RuO2·nH2O, and RuCl3 species as references for comparison, which suggest that the RuO2·nH2O species46-47 were loaded on PbBiO2Cl, as the case of WO3.47 The Ru 3d peak approaches that of anhydrous RuO2 on increasing the calcination temperature to above 673 K, at which the rate of O2 evolution on PbBiO2Cl is drastically increased. These findings suggest that RuO2 can more effectively promote the O2 evolution on PbBiO2Cl compared to RuO2·nH2O species. Although RuO2 is known as an effective catalyst for water oxidation, recent studies on photocatalytic water oxidation with various redox mediators also indicate the catalytic activity of RuO2 (or RuOx species) for the reduction of IO3– or Fe3+.48-51 Therefore, it is hard at present to identify the role of the RuOx cocatalsyts loaded on PbBiO2Cl, which will be left for future study.

Influence of loading co-catalyst on O2 evolution over PbBiO2Cl. Figure 9 shows the initial rates of O2 evolution from the aqueous Fe(NO3)3 solution over PbBiO2Cl loaded with Ru species at different temperatures.


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Figure 10. XP spectra of PbBiO2Cl absorbed adsorbed with Ru species without calcination (Ads.) and PbBiO2Cl loaded with Ru species at different temperatures. Apparent quantum efficiency of the O2 evolution over RuO2PbBiO2Cl (723 K) was determined to be 0.9% at 400 nm. Figure S8 shows the wavelength dependence of the O2 evolution over RuO2-PbBiO2Cl (723 K). The onset of O2 evolution (460–500 nm) was almost consistent with the absorption edge. Additionally, the O2 evolution rate increases drastically with a decrease in the cut-off wavelength. These findings clearly prove that the reaction proceeded by the photogenerated carriers through the band-gap excitation of PbBiO2Cl. Loading of the PtOx co-catalyst also enhanced the rate of O2 evolution though the rate was lower than that of Ru species (Figure S9 and S10). Z-scheme water splitting using PbBiO2Cl as an O2-evolution photocatalyst. Overall water splitting was attempted by using the mixture of the RuO2-PbBiO2Cl as a O2-evolving photocatalyst, an appropriate H2-evolving photocatalyst, and the Fe3+/Fe2+ redox mediator. In the present study, Rh-doped SrTiO3 (denoted as SrTiO3:Rh)32, 52-53 was used as the H2-evolution photocatalyst, which is a well-known photocatalyst with high activity for H2 evolution in the presence of the Fe2+ electron donor under visible light. Figure 11 shows time courses of gases evolution from the suspension of RuO2-PbBiO2Cl and Ru-SrTiO3:Rh under visible light irradiation. The initial rate of O2 evolution on RuO2-PbBiO2Cl is lower than that in the corresponding half reaction probably due to the light shielding by coexisting H2-evolving photocatalyst (Ru-SrTiO3:Rh).

Figure 11. Amount of H2 and O2 evolved with time using RuO2-PbBiO2Cl (723 K) as an O2 evolution photocatalyst and Ru-SrTiO3:Rh as a H2 evolution photocatalyst from the aqueous Fe(NO3)3 solution (5 mM, 250 mL) under visible light irradiation (λ > 400 nm, Xe lamp). At the initial stage when the concentration of Fe3+ was much higher than that of Fe2+, O2 was predominantly produced on the RuO2-PbBiO2Cl photocatalyst. The gradual increase in H2 evolution was observed with increasing irradiation time due to the increased concentration of Fe2+, which acted as an electron donor for the Ru-SrTiO3:Rh photocatalyst. On the other hand, O2 evolution gradually decreased because of the decreased concentration of Fe3+ and/or backward reaction, i.e. re-oxidation of Fe2+, as often observed in other Z-scheme systems.53, 54 As shown in Figure S11, the rate of O2 evolution on RuO2PbBiO2Cl obviously decreased with increasing Fe2+ concentration in solution, indicating the occurrence of backward reaction. Note that RuO2-PbBiO2Cl can generate O2 even at a considerably high Fe2+ concentration (5 mM), enabling this photocatalyst to evolve O2 in the Z-scheme reaction. Subsequently, H2 and O2 generated in a stoichiometric ratio (H2/O2 = 2) with a steady rate. The total number of electrons required for the O2 evolution from RuO2-PbBiO2Cl was 1.03 mmol. This value is larger than the amount of PbBiO2Cl (0.41 mmol), indicating that PbBiO2Cl functions as a stable photocatalyst. These results thus represent the first report on Z-scheme water splitting using a simple Sillén phase oxyhalide as an O2-evolving photocatalyst under visible light. Conclusion We demonstrated that a Sillén phase oxyhalide PbBiO2Cl can function as a stable O2-evolving photocatalyst in a Z-scheme water splitting system under visible light. Both experimental and theoretical results revealed that all the PbBiO2X oxyhalides satisfy required band levels for water splitting upon visible light irradiation, i.e., a more negative CBM than H+/H2 and a more positive VBM than O2/H2O. Unusually negative VBMs dominantly composed of O 2p in PbBiO2Cl and PbBiO2Br were explained in terms of destabilization of oxygen 2p orbital in the fluorite layer, as demonstrated in the previous study on Bi4NbO8X (X = Cl, Br). In addition, with the simpler structure



of PbBi2OX and reference system AEBiO2X, we were able to present clearly the strong hybridization of Pb 6s and O 2p orbitals based on the revised lone pair model. The elevated VBM consisting of the O 2p orbital allowed the material to absorb visible light up to 510 nm while placing their CBM at higher than that of H+/H2, and also accounted for its stability against water oxidation under light irradiation. That is, the photo-generated holes preferentially occupied the stable O2– anions and not the halides (Cl– or Br–), resulting in stable water oxidation without self-oxidative deactivation. On the other hand, the negligible photocatalytic activity of PbBiO2I, in which VBM were dominantly composed of I 5p, was possibly due to the accumulation of holes on the unstable I– anions, which contributed significantly to the DOS around the VBM. This issue can probably be resolved by surface modification with an effective co-catalyst such as CoOx, which could consume the holes efficiently and also catalyze the chemical reactions (e.g., water oxidation) on the surface. These results strongly suggest that an extensive control of VBM is possible by utilizing various post-transition metal cations (e.g., Sn2+, Sb3+, Pb2+, and Bi3+). A large number of Sillén oxyhalides have been reported to date, some of which with stereoactive lone pairs from those cations may function as visiblelight-responsive photocatalysts in Z-scheme water splitting and/or overall water splitting via one-step photoexcitation using a single photocatalyst. Tuning of anion orbitals in mixed-anion compounds by utilizing hybridization derived from stereoactive lone pairs (RLP model) as well as layer stacking sequence (Madelung site potential analysis) will open a new avenue toward achieving an efficient water splitting by harvesting a wider range of the solar light spectrum.

SUPPORTING INFORMATION XRD patterns, Le Bail analysis, lattice constants, SEM images, Mott-Schottky plots, XPS results of prepared samples, band structures, DOS of ABiO2X, and O2 evolution on PbBiO2Cl under various conditions.

ACKNOWLEDGMENT This work was financially supported by the JST-CREST project, the JSPS KAKENHI Grant Number 17H06439 in Scientific Research on Innovative Areas “Innovations for Light-Energy Conversion (I4LEC)”, and the JSPS Grant-in-Aid for Scientific Research (B) (Grant Number 15H03849) and for JSPS Research Fellow (Grant Number 16J11397). 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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Lead Bismuth Oxyhalides PbBiO2X (X = Cl, Br) as Visible-Light

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