Layered Perovskite Oxychloride Bi4NbO8Cl: a Stable Visible Light ...
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Layered Perovskite Oxychloride Bi4NbO8Cl: a Stable Visible Light Responsive Photocatalyst for Water Splitting Hironori Fujito,†‡ Hironobu Kunioku,†‡ Daichi Kato,† Hajime Suzuki,† Masanobu Higashi,† 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
Experimental (a) Preparation of catalyst Bi4NbO8Cl was prepared by a solid state reaction. Stoichiometric quantities of Bi2O3 (Wako, 99.99%), BiOCl (Wako, 99.5%) and Nb2O5 powders (Kojundo Chemicals, 99.9%) were weighed, mixed and heated in an evacuated silica tube at 1173 K for 20 h.1 For comparison, Bi4NbO8Br was synthesized in a similar manner using BiOBr. BiOBr was prepared by a soft liquid deposition method.2 During the preparation process of BiOBr, 5.0 mmol of Bi(NO3)3·5H2O was dissolved into ethanol (30 mL). 5 mmol KBr was dissolved in water (10 mL) and added dropwise into the above solution. The mixture was subsequently stirred for 5 h to complete the reaction. The resulting precipitate was filtered, washed with water several times and dried at 353 K for 5 h. Nanoparticulate platinum was employed as cocatalyst to enhance the photocatalytic activity for H2 evolution. Modification of Bi4NbO8Cl with nanoparticulate platinum was performed by an impregnation method from an aqueous solution containing H2[PtCl6] (0.5wt% as Pt), followed by heating under an H2 gas flow at 473 K for 1 h. Strontium titanate doped with rhodium species (SrTiO3:Rh)3 was prepared by solid state reaction. A mixture of SrCO3, TiO2 and Rh2O3 (Sr : Ti : Rh = 1.07 : 1 : 0.01) was calcined in air at 1073 K for 1 h and subsequently at 1273 K for 10 h. Ru species were loaded on SrTiO3:Rh by means of photodeposition method using RuCl3･nH2O as a Ru source, according to the method reported previously.4 A powder of SrTiO3:Rh was dispersed in an aqueous methanol solution (10vol%) containing RuCl3･nH2O (0.7wt% as Ru), and then the suspension was irradiated using 300 W Xe lamp (Cermax LF-300F, λ > 400 nm) for 5 h. (b) Characterization of catalyst The prepared samples were characterized by powder X-ray diffraction (XRD; D8 ADVANCE, S1
Bruker AXS; Cu Kα), scanning electron microscopy (SEM; NVision 40, Carl Zeiss-SIINT), UV-visible diffuse reflectance spectroscopy (V-650, Jasco), Raman spectroscopy (XploRA, Horiba) and X-ray photoelectron spectroscopy (XPS; JPC-9010MC, Jeol; Mg Kα). The binding energy determined by XPS was corrected with reference to the C 1s peak of impurity carbon (284.8 eV) for each sample. (c) Electrochemical measurement A paste was prepared by mixture of Bi4NbO8Cl (30 mg) and water (100 µL), coated on conductive substrate (FTO), and then dried at room temperature. The Mott-Schottky plots were measured using an electrochemical analyzer (PARSTAT2263, Princeton Applied Research). The electrochemical cell consisted of Bi4NbO8Cl/FTO electrode, a counter electrode (Pt wire), a Ag/AgCl reference electrode, and a Na2SO4 solution (0.1 M, adjusted to pH 2 by adding of H2SO4). AC amplitude and frequency were 10 mV and 1 kHz, respectively. (d) Calculation 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 300 eV. Geometry optimization calculation was performed before electronic structure calculation using the Broyden–Fletcher–Goldfarb–Shanno (BFGS) algorithm. (e) Photocatalytic reactions Photocatalytic reactions were carried out using a Pyrex glass reactor connected to a closed gas– circulation system. For the photocatalytic water oxidation (O2 evolution) in the presence of an electron accepter (i.e., half reaction of water splitting) catalyst powder (0.1 g) was suspended in 250 mL of an aqueous AgNO3, FeCl3 or Fe(NO3)3 solution (5 mM) in the reactor by using a set of magnetic stirrer and bar. As for the O2 evolution in the presence of Fe3+ electron acceptor, the reaction must be carried out in acidic condition with pH below ca. 2.5, because Fe3+ cations easily precipitate as Fe(OH)3 with higher pH values. In order to maintain the pH values below 2.5 stably through the reactions, a small amount of HCl or HNO3aq. was added into FeCl3 or Fe(NO3)3aq. to adjust the pH to 2.5 before reaction. For the photocatalytic water reduction (H2 evolution) in the presence of an electron donor, Pt-loaded Bi4NbO8Cl catalyst powder (0.1 g) was suspended in 250 mL of an aqueous methanol solution (20vol%). For the two-step water-splitting reaction with Fe3+/Fe2+ redox couple, the Ru-loaded SrTiO3:Rh (0.1 g) and Bi4NbO8Cl (0.15 g) were suspended in an aqueous FeCl3aq. (2 mM, 250 mL) as H2-evolving and O2-evolving photocatalysts, respectively. The pH value of the solution was adjusted to be ca. 2.5 by adding a small amount of aqueous HClaq. The suspension was irradiated using 300 W Xe lamp fitted with L-42 cut-off filter. The evolved gases were analyzed by on-line gas chromatography (detector; TCD, column packing; molecular sieve 5 A, Ar carrier). S2
Figure S1. SEM images of prepared Bi4NbO8Cl before and after the photocatalytic O2 evolution in an aqueous FeCl3 solution (5 mM) under visible light irradiation.
Figure S2. XRD pattern of prepared Bi4NbO8Cl and Bi4NbO8Br before and after the photocatalytic O2 evolution in an aqueous FeCl3 solution (5 mM) under visible light irradiation.
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Figure S3. Mott-Schottky plots of Bi4NbO8Cl in 0.1 M Na2SO4aq (pH 2). The Result for BiOCl is also shown for comparison.
Figure S4. Time course of H2 evolution over Pt(0.5 wt%)/Bi4NbO8Cl in an aqueous methanol solution under UV light (λ > 300 nm) irradiation.
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Figure S5. Raman spectra of prepared Bi4NbO8Cl before and after the photocatalytic O2 evolution in an aqueous FeCl3 solution (5 mM) under visible light irradiation.
Figure S6. Time course of O2 evolution over Bi4NbO8Br in an aqueous FeCl3 solution (5 mM) under visble light (λ > 400 nm) irradiation.
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Figure S7. Time course of Z-scheme water splitting coupled with Ru/SrTiO3:Rh photocatalyst via Fe3+/Fe2+ redox mediator under visble light (λ > 400 nm) irradiation.
Figure S8. Electronic band structure of Bi4NbO8Cl. The VBM is situated at the Z point, while the conduction band minimum (CBM) is located in the region of G–Z point.
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Figure S9. PDOS and TDOS of (a) Bi4NbO8Cl and (b) Bi4NbO8Br. The Fermi energy, defined as the highest occupied energy level, was taken as VBM. S7
Figure S10. TDOS of (a)BiOCl, (b)BiOBr, and PDOS projected onto each constituent element.
Table S1. Ratios of Cl/Bi and Cl/Nb for Bi4NbO8Cl before and after the photocatalytic reaction.
Table S2. Bond valence sum (BVS) of Bi4NbO8Cl.
References (1)Kusainova, A. M.; Zhou, W. Z.; Irvine, J. T. S.; Lightfoot, P. J. Solid State Chem. 2002, 166, 148-157. (2)He, Y.; Zhang, Y. H.; Huang, H. W.; Tian, N.; Guo, Y. X.; Luo, Y. Colloid Surface A 2014, 462, 131-136. (3)Konta, R.; Ishii, T.; Kato, H.; Kudo, A. J. Phys. Chem. B 2004, 108, 8992-8995. (4)Sasaki, Y.; Iwase, A.; Kato, H.; Kudo, A. J. Catal. 2008, 259, 133-137.
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Layered Perovskite Oxychloride Bi4NbO8Cl: a Stable Visible Light Responsive Photocatalyst for Water Splitting Hironori Fujito,†‡ Hironobu Kunioku,†‡ Daichi Kato,† Hajime Suzuki,† Masanobu Higashi,† 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
Experimental (a) Preparation of catalyst Bi4NbO8Cl was prepared by a solid state reaction. Stoichiometric quantities of Bi2O3 (Wako, 99.99%), BiOCl (Wako, 99.5%) and Nb2O5 powders (Kojundo Chemicals, 99.9%) were weighed, mixed and heated in an evacuated silica tube at 1173 K for 20 h.1 For comparison, Bi4NbO8Br was synthesized in a similar manner using BiOBr. BiOBr was prepared by a soft liquid deposition method.2 During the preparation process of BiOBr, 5.0 mmol of Bi(NO3)3·5H2O was dissolved into ethanol (30 mL). 5 mmol KBr was dissolved in water (10 mL) and added dropwise into the above solution. The mixture was subsequently stirred for 5 h to complete the reaction. The resulting precipitate was filtered, washed with water several times and dried at 353 K for 5 h. Nanoparticulate platinum was employed as cocatalyst to enhance the photocatalytic activity for H2 evolution. Modification of Bi4NbO8Cl with nanoparticulate platinum was performed by an impregnation method from an aqueous solution containing H2[PtCl6] (0.5wt% as Pt), followed by heating under an H2 gas flow at 473 K for 1 h. Strontium titanate doped with rhodium species (SrTiO3:Rh)3 was prepared by solid state reaction. A mixture of SrCO3, TiO2 and Rh2O3 (Sr : Ti : Rh = 1.07 : 1 : 0.01) was calcined in air at 1073 K for 1 h and subsequently at 1273 K for 10 h. Ru species were loaded on SrTiO3:Rh by means of photodeposition method using RuCl3･nH2O as a Ru source, according to the method reported previously.4 A powder of SrTiO3:Rh was dispersed in an aqueous methanol solution (10vol%) containing RuCl3･nH2O (0.7wt% as Ru), and then the suspension was irradiated using 300 W Xe lamp (Cermax LF-300F, λ > 400 nm) for 5 h. (b) Characterization of catalyst The prepared samples were characterized by powder X-ray diffraction (XRD; D8 ADVANCE, S1
Bruker AXS; Cu Kα), scanning electron microscopy (SEM; NVision 40, Carl Zeiss-SIINT), UV-visible diffuse reflectance spectroscopy (V-650, Jasco), Raman spectroscopy (XploRA, Horiba) and X-ray photoelectron spectroscopy (XPS; JPC-9010MC, Jeol; Mg Kα). The binding energy determined by XPS was corrected with reference to the C 1s peak of impurity carbon (284.8 eV) for each sample. (c) Electrochemical measurement A paste was prepared by mixture of Bi4NbO8Cl (30 mg) and water (100 µL), coated on conductive substrate (FTO), and then dried at room temperature. The Mott-Schottky plots were measured using an electrochemical analyzer (PARSTAT2263, Princeton Applied Research). The electrochemical cell consisted of Bi4NbO8Cl/FTO electrode, a counter electrode (Pt wire), a Ag/AgCl reference electrode, and a Na2SO4 solution (0.1 M, adjusted to pH 2 by adding of H2SO4). AC amplitude and frequency were 10 mV and 1 kHz, respectively. (d) Calculation 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 300 eV. Geometry optimization calculation was performed before electronic structure calculation using the Broyden–Fletcher–Goldfarb–Shanno (BFGS) algorithm. (e) Photocatalytic reactions Photocatalytic reactions were carried out using a Pyrex glass reactor connected to a closed gas– circulation system. For the photocatalytic water oxidation (O2 evolution) in the presence of an electron accepter (i.e., half reaction of water splitting) catalyst powder (0.1 g) was suspended in 250 mL of an aqueous AgNO3, FeCl3 or Fe(NO3)3 solution (5 mM) in the reactor by using a set of magnetic stirrer and bar. As for the O2 evolution in the presence of Fe3+ electron acceptor, the reaction must be carried out in acidic condition with pH below ca. 2.5, because Fe3+ cations easily precipitate as Fe(OH)3 with higher pH values. In order to maintain the pH values below 2.5 stably through the reactions, a small amount of HCl or HNO3aq. was added into FeCl3 or Fe(NO3)3aq. to adjust the pH to 2.5 before reaction. For the photocatalytic water reduction (H2 evolution) in the presence of an electron donor, Pt-loaded Bi4NbO8Cl catalyst powder (0.1 g) was suspended in 250 mL of an aqueous methanol solution (20vol%). For the two-step water-splitting reaction with Fe3+/Fe2+ redox couple, the Ru-loaded SrTiO3:Rh (0.1 g) and Bi4NbO8Cl (0.15 g) were suspended in an aqueous FeCl3aq. (2 mM, 250 mL) as H2-evolving and O2-evolving photocatalysts, respectively. The pH value of the solution was adjusted to be ca. 2.5 by adding a small amount of aqueous HClaq. The suspension was irradiated using 300 W Xe lamp fitted with L-42 cut-off filter. The evolved gases were analyzed by on-line gas chromatography (detector; TCD, column packing; molecular sieve 5 A, Ar carrier). S2
Figure S1. SEM images of prepared Bi4NbO8Cl before and after the photocatalytic O2 evolution in an aqueous FeCl3 solution (5 mM) under visible light irradiation.
Figure S2. XRD pattern of prepared Bi4NbO8Cl and Bi4NbO8Br before and after the photocatalytic O2 evolution in an aqueous FeCl3 solution (5 mM) under visible light irradiation.
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Figure S3. Mott-Schottky plots of Bi4NbO8Cl in 0.1 M Na2SO4aq (pH 2). The Result for BiOCl is also shown for comparison.
Figure S4. Time course of H2 evolution over Pt(0.5 wt%)/Bi4NbO8Cl in an aqueous methanol solution under UV light (λ > 300 nm) irradiation.
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Figure S5. Raman spectra of prepared Bi4NbO8Cl before and after the photocatalytic O2 evolution in an aqueous FeCl3 solution (5 mM) under visible light irradiation.
Figure S6. Time course of O2 evolution over Bi4NbO8Br in an aqueous FeCl3 solution (5 mM) under visble light (λ > 400 nm) irradiation.
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Figure S7. Time course of Z-scheme water splitting coupled with Ru/SrTiO3:Rh photocatalyst via Fe3+/Fe2+ redox mediator under visble light (λ > 400 nm) irradiation.
Figure S8. Electronic band structure of Bi4NbO8Cl. The VBM is situated at the Z point, while the conduction band minimum (CBM) is located in the region of G–Z point.
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Figure S9. PDOS and TDOS of (a) Bi4NbO8Cl and (b) Bi4NbO8Br. The Fermi energy, defined as the highest occupied energy level, was taken as VBM. S7
Figure S10. TDOS of (a)BiOCl, (b)BiOBr, and PDOS projected onto each constituent element.
Table S1. Ratios of Cl/Bi and Cl/Nb for Bi4NbO8Cl before and after the photocatalytic reaction.
Table S2. Bond valence sum (BVS) of Bi4NbO8Cl.
References (1)Kusainova, A. M.; Zhou, W. Z.; Irvine, J. T. S.; Lightfoot, P. J. Solid State Chem. 2002, 166, 148-157. (2)He, Y.; Zhang, Y. H.; Huang, H. W.; Tian, N.; Guo, Y. X.; Luo, Y. Colloid Surface A 2014, 462, 131-136. (3)Konta, R.; Ishii, T.; Kato, H.; Kudo, A. J. Phys. Chem. B 2004, 108, 8992-8995. (4)Sasaki, Y.; Iwase, A.; Kato, H.; Kudo, A. J. Catal. 2008, 259, 133-137.
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