A Stable, Narrow-Gap Oxyfluoride Photocatalyst for Visible-Light

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A Stable, Narrow-Gap Oxyfluoride Photocatalyst for VisibleLight Hydrogen Evolution and Carbon Dioxide Reduction Ryo Kuriki, Tom Ichibha, Kenta Hongo, Daling Lu, Ryo Maezono, Hiroshi Kageyama, Osamu Ishitani, Kengo Oka, and Kazuhiko Maeda J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b02822 • Publication Date (Web): 07 May 2018 Downloaded from http://pubs.acs.org on May 7, 2018

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Journal of the American Chemical Society

A Stable, Narrow-Gap Oxyfluoride Photocatalyst for VisibleLight Hydrogen Evolution and Carbon Dioxide Reduction Ryo Kuriki,1,2 Tom Ichibha,3 Kenta Hongo,4,5,6,7 Daling Lu,8 Ryo Maezono,3,7 Hiroshi Kageyama,9 Osamu Ishitani,1 Kengo Oka,*10 and Kazuhiko Maeda*1 1

Department of Chemistry, School of Science, Tokyo Institute of Technology, 2-12-1-NE-2 Ookayama, Meguro-ku, Tokyo 152-8550, Japan 2 Japan Society for the Promotion of Science, Kojimachi Business Center Building, 5-3-1 Kojimachi, Chiyoda-ku, Tokyo 102-0083, Japan 3 School of Information Science, JAIST, Asahidai 1-1, Nomi, Ishikawa 923-1292, Japan 4 Research Center for Advanced Computing Infrastructure, JAIST, Asahidai 1-1, Nomi, Ishikawa 923-1292, Japan 5 Center for Materials Research by Information Integration, Research and Services Division of Materials Data and Integrated System, National Institute for Materials Science, Tsukuba 305-0047, Japan 6

PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi-shi, Saitama 322-0012, Japan Computational Engineering Applications Unit, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan 8 Suzukakedai Materials Analysis Division, Technical Department, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan 9 Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan 7

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Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, Bunkyo-ku, Tokyo 112-8551, Japan KEYWORDS. Mixed anion compounds, Artificial photosynthesis, Pyrochlore, Solar fuels, Water splitting ABSTRACT: Mixed anion compounds such as oxynitrides and oxychalcogenides are recognized as potential candidates of visiblelight-driven photocatalysts since, as compared with oxygen 2p orbitals, p orbitals of less electronegative anion (e.g., N3–, S2–) can form a valence band that has more negative potential. In this regard, oxyfluorides appear unsuitable because of the higher electronegativity of fluorine. Here we show an exceptional case, an anion-ordered pyrochlore oxyfluoride Pb2Ti2O5.4F1.2 that has a small band gap (ca. 2.4 eV). With suitable modification of Pb2Ti2O5.4F1.2 by promoters such as platinum nanoparticles and a binuclear ruthenium(II) complex, Pb2Ti2O5.4F1.2 worked as a stable photocatalyst for visible-light-driven H2 evolution and CO2 reduction. Density functional theory calculations have revealed that the unprecedented visible-light-response of Pb2Ti2O5.4F1.2 arises from strong interaction between Pb-6s and O-2p orbitals, which is enabled by a short Pb-O bond in the pyrochlore lattice due to the fluorine substitution.

INTRODUCTION Inorganic compounds that consist of two or more different anions have recently attracted attention as materials that can exhibit emerging phenomena in various research disciplines, e.g., catalysis, energy conversion, and electronic materials.1 Heteroleptic coordination geometries around the metal center in mixed-anion compounds allow extensive crystal field splitting, local degree of freedom (cis vs. trans) and broken inversion symmetry. Visible-light-driven photocatalytic property is one of the respectful functions and has been extensively studied in recent years due to a growing need in solar-to-fuel energy conversion, called at times as “artificial photosynthesis”.2,3 Efficient solar energy conversion by photocatalysis necessitates a photocatalyst that is stable and has a band gap smaller than 3 eV to absorb visible light (> 400 nm), the main compo-

nent of solar spectrum. In contrast to conventional metal oxides with wide band gaps (> 3 eV), certain oxynitrides,4–8 oxysulfides9–11 and oxyhalides12,13 containing anions with lower electronegativity than oxygen have small band gaps (< 3 eV) and are hence able to absorb visible light, allowing photocatalytic activity. Here, the key concept of visible-light-response is to introduce differing anion (N3–, S2–, Cl–, Br–, etc.) orbitals to the upper side of valence band, thereby narrowing the band gap without essentially affecting the conduction band. Oxyfluorides (oxide-fluoride) exhibit a range of useful properties including high-Tc superconductivity,14,15 ion16 exchangeability, and large dielectric constants.17–20 They also have potential applications as high rate-capability cathode materials,21 multivalent battery electrodes22 and host materials of phosphors for white LED.23 In terms of visible-light-driven photocatalysis, however, oxyflorides are obviously unsuitable

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explained by the strong interaction between Pb-6s and O-2p orbitals derived from the oxygen deficient A2B2X6X!0.5 pyrochlore structure, which is reinforced by fluorine substitution for X1 site. This is unique feature of oxyfluoride pyrochlore Pb2Ti2O5.4F1.2, as the 2p orbital of fluorine did not directly contribute to the reduced band-gap, but undergoes structural modulation that leads to the distinct visible-light-absorption capability. The short A–O bonds in A2B2O7 pyrochlore has been used to provide strong magnetic anisotropy when A site is occupied by magnetic ion, leading to exotic phenomena based on the spin ice lattice.51 This study demonstrates that the short Pb–O bond in the oxyfluoride pyrochlore enhances Pb6s/O-2p hybridization, which results in the reduced band gap. We believe that the rigid pyrochlore structure induced by mixing hetero anions can serve as a useful platform in generating various functions associated with the short A–O bond, which may be beyond the known oxide pyrochlores. It is also considered that the creating a short A–O bond (A = Pb, Bi, etc.) is a general strategy to design new materials with a small band gap. At present, apparent quantum yield of the Pb2Ti2O5.4F1.2 photocatalyst was low: ca. 0.01% at 365 nm for H2 evolution reaction. The low activity of Pb2Ti2O5.4F1.2 would be at least in part due to the large size (i.e., low specific surface area) and irregular morphology of the synthesized Pb2Ti2O5.4F1.2 particles, as shown in Figure 2B, which are disadvantageous for heterogeneous photocatalysis.2,3,41 It is thus expected that the low activity can be addressed by refining the synthesis method of Pb2Ti2O5.4F1.2 and optimizing a cocatalyst, both of which in general have significant impacts on photocatalytic activity.2,3 This is currently under investigation in our group.

CONCLUSION In conclusion, we revealed that an oxyfluoride semiconductor, pyrochlore Pb2Ti2O5.4F1.2, is a stable visible-lightresponsive photocatalyst for H2 evolution and CO2 reduction by coupling with Pt nanoparticles and a Ru(II) binuclear complex, respectively. Even though Pb2Ti2O5.4F1.2 is an oxyfluoride, it has an unprecedented small band gap of ca. 2.4 eV, corresponding to ca. 500 nm visible light, as the result of strong interaction between Pb-6s and O-2p orbitals in the valence band, which is induced by fluorine incorporation in the pyrochlore structure. The results of the present study also indicate that the A site of pyrochlore (or defect-pyrochlore) structure with a lower coordination number is essentially suitable for strengthening the interaction between M-6s (M = Pb and Bi) and O-2p, thereby leading to more pronounced visible light absorption, which is perhaps unattainable with perovskite-type compounds as exemplified by PbTiO3 (see Figure S9). For developing a new visible-light-responsive photocatalyst, utilization of such lower coordination environment that maximizes the 6s–2p orbital interaction may be applicable not only to pyrochlores but also other structures, even oxyfluorides that may have been viewed as useless materials for synthesizing a narrowgap, visible-light-responsive photocatalyst. The discovery of an oxyfluoride that has a band gap smaller than 3 eV and works as a stable semiconductor photocatalyst, as presented in this work, will therefore open a new direction in the materials research on heterogeneous photocatalysis with visible light.

EXPERIMENTAL SECTION

General Procedures UV-vis absorption spectra were measured with a JASCO V-565 spectrophotometer. X-ray diffraction was measured with a Rigaku MiniFlex 600. FT-IR spectra were measured with a JASCO FT/IR610 spectrophotometer using diffusion reflection method. Specific surface area was measured with a BELSOEP-mini II instrument (BEL Japan) at liquid nitrogen temperature and obtained by the Brunauer#Emmett#Teller (BET) theory. SEM image was acquired with a JEOL JSM–IT100LA. Impedance measurements were conducted using a BAS CHI760 electrochemical analyzer.

Materials All reagents were reagent-grade quality and were used without further purification except for MeCN and TEOA. MeCN was distilled two times using P2O5 (98.0%, Kanto Chemical Co. Inc.) as a dehydrating agent. TEOA was distilled under reduced pressure ( 400 nm) unless otherwise stated. The gaseous reaction products were analyzed using a gas chromatograph with a thermal conductivity detector (TCD–GC, GL Science, Model GC323). Formate generated in the liquid phase was analyzed using a capillary electrophoresis system (Otsuka Electronics Co., Model CAPI–3300).

Isotope Tracer Experiments Isotope–labeling experiment for CO2 reduction was performed using 10 mg of RuRu’(2.7 µmol g–1)/Pb2Ti2O5.4F1.2 dispersed in 2 mL of MeCN–TEOA mixed solution (4:1 v/v). 13CO2 (13C 99%; Aldrich Co.) was introduced into the photocatalyst suspension after degassing it by freeze-pump-thaw cycling (600 Torr). After 40 h of photoirradiation using a 400 W high-pressure Hg lamp (SEN) in combination with a NaNO2 solution as a filter, 1H–NMR (no-deuterium method) of the reactant solution was measured using a JEOL ECA400II (400 MHz) NMR spectrometer. Before the measurement, solid photocatalyst was removed by filtration. Photooxidation of water using 18O-enriched H2O (18O 97%; Aldrich Co.) was performed in a Pyrex test tube (8 mL capacity) by dispersing 10 mg of RuO2-loaded Pb2Ti2O5.4F1.2 in 1 mL of MeCN– H2O mixed solution (7:3 v/v) in the presence of 10 mM AgNO3 (99.8%, Wako Pure Chemicals Co.). After purging residual air in the test tube with He, irradiation was made using a 400 W high pressure Hg lamp (SEN) for 70 h. Then, the gaseous product was taken using a syringe through a septum, and was analyzed by means of a GCxMS (Shimadzu, QP-2010-Ultra). The same experiment was done using unlabeled water for comparison.

Density Functional Theory Calculations

Ryo Kuriki: 0000-0002-3843-2867 Tom Ichibha: 0000-0002-7455-4968 Kenta Hongo: 0000-0002-2580-0907 Daling Lu: 0000-0002-9084-480X Ryo Maezono: 0000-0002-5875-971X Hiroshi Kageyama: 0000-0002-3911-9864 Osamu Ishitani: 0000-0001-9557-7854 Kengo Oka: 0000-0002-1800-8575 Kazuhiko Maeda: 0000-0001-7245-8318

ACKNOWLEDGMENT This work was supported by a Grant-in-Aid for Scientific Research on Innovative Area “Mixed Anion (Project JP16H06439, JP16H06440, JP16H06441, JP17H05478 and JP17H05489)” (JSPS). It was also partially supported by a Grantin-Aids for Young Scientists (A) (Project JP16H06130), (B) (Project JP17K17762), the Photon and Quantum Basic Research Coordinated Development Program (MEXT, Japan), a PRESTO program (JPMJPR16NA) and a CREST program (Project JPMJCR13L1) (JST). K.M. acknowledges The Noguchi Institute and Murata Research Foundation financial support. R.K. wishes to acknowledge support by a JSPS Fellowship for Young Scientists (JP17J03705). K.H. is grateful for financial support from FLAGSHIP2020 (MEXT for the computational resources, Projects hp170269 and hp180175 at K-computer) and Starting Up Innovation Hub MI2I from JST. The computations in this work have been performed using the facilities of the Research Center for Advanced Computing Infrastructure at JAIST. R.M. is grateful for financial support from MEXT-KAKENHI (Project JP16KK0097), the FLAGSHIP2020 project, Toyota Motor Corporation, I#O DATA Foundation, and the Air Force Office of Scientific Research (AFOSR-AOARD/FA2386-17-1-4049).

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A Stable, Narrow-Gap Oxyfluoride Photocatalyst for Visible-Light

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