Light Absorption Properties and Electronic Band Structures of Lead

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Light Absorption Properties and Electronic Band Structures of Lead Titanium Oxyfluoride Photocatalysts Pb2Ti4O9F2 and Pb2Ti2O5.4F1.2 Haruki Wakayama,† Keishu Utimula,‡ Tom Ichibha,§ Ryo Kuriki,†,∥ Kenta Hongo,⊥,#,¶,∇ Ryo Maezono,§,∇ Kengo Oka,*,○ and Kazuhiko Maeda*,†

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Department of Chemistry, School of Science, Tokyo Institute of Technology, 2-12-1-NE-2 Ookayama, Meguro-ku, Tokyo 152-8550, Japan ‡ School of Materials Science, §School of Information Science, and ⊥Research Center for Advanced Computing Infrastructure, JAIST, Asahidai 1-1, Nomi, Ishikawa 923-1292, Japan ∥ Japan Society for the Promotion of Science, Kojimachi Business Center Building, 5-3-1 Kojimachi, Chiyoda-ku, Tokyo 102-0083, Japan # 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 ¶ 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 ○ Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, Bunkyo-ku, Tokyo 112-8551, Japan S Supporting Information *

ABSTRACT: Light absorption capability and electronic band structure are both fundamental information for the development of a new photocatalyst. Here, we investigated two oxyfluoride photocatalysts Pb2Ti4O9F2 and Pb2Ti2O5.4F1.2, which were active for H2 evolution in the presence of a sacrificial reagent, by means of X-ray diffraction, UV−visible diffuse reflectance spectroscopy, electrochemical impedance spectroscopy, and density functional theory calculations. Pb2Ti4O9F2 and Pb2Ti2O5.4F1.2 show absorption edges at around 410 and 510 nm, respectively, corresponding to band gaps of 3.0 and 2.4 eV. The different band gap values of the two materials are mainly due to their valence band maximum (VBM); the VBM of Pb2Ti4O9F2 is positioned at approximately 0.9 V more positive than that of Pb2Ti2O5.4F1.2. The significantly different VBM positions in these oxyfluorides could be explained in terms of the orbital interaction between Pb 6s/6p and O 2p in the valence band, where the shorter Pb−O bond in Pb2Ti2O5.4F1.2 reinforced the interaction, leading to more elevated VBM and a narrower band gap.

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INTRODUCTION

valence band maximum (VBM) more negative than the corresponding oxide material (i.e., band-gap narrowing). From the viewpoint of orbital energy, substitution of oxide ion by fluoride ion had been believed to be inappropriate for the band-gap narrowing, because of the highest electronegativity of fluorine. In fact, most of the oxyfluorides reported as photocatalysts have intrinsic band gaps of greater than 3.0 eV, which is too wide to efficiently absorb visible light,24−27 although some of them show visible light absorption due to electron transitions involving impurity levels.25,27 By contrast, we recently discovered that a pyrochlore oxyfluoride Pb2Ti2O5.4F1.2 has an unprecedented small band gap of 2.4

In order to meet the increasing demand for new energy, development of heterogeneous photocatalysts that can absorb visible light and split water to form H2 is of importance in recent years.1−4 So far, various kinds of materials including oxides,5−7 oxynitrides,8−10 oxysulfides,11−13 oxyhalides,14−16 and C/N-based organic polymers17−19 have been reported as photocatalysts for individual H2/O2 evolution from water and/ or pure water splitting. Upon suitable modification, some of them become good photocatalysts for visible-light CO2 reduction. 20−22 Of particular interest is mixed anion compounds, which could utilize visible lightthe main component of solar spectrumand show photocatalytic activity.23 The key concept of visible-light-absorbing mixed anion photocatalysts is the utilization of anion p orbitals having higher potential energies than O 2p orbitals, which makes the © XXXX American Chemical Society

Received: September 13, 2018 Revised: October 23, 2018 Published: October 29, 2018 A

DOI: 10.1021/acs.jpcc.8b08953 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

1.4:0.6:2 for Pb2Ti2O5.4F1.2, respectively. The mixtures were treated at 823 K (Pb2Ti4O9F2) and 873 K (Pb2Ti2O5.4F1.2) for 12 h in an evacuated Pyrex tube. For comparison, PbTiO3 was synthesized through a solid-state reaction method by heating a stoichiometric mixture of PbO (99.5%, Kanto Chemical Co. Inc.) and TiO2 (rutile, 99.9%, Wako) powder at 1273 K for 15 h. Characterization of Materials. X-ray diffraction (XRD) was measured with a Rigaku MiniFlex 600 system. UV−vis absorption spectra were measured with a JASCO V-565 spectrophotometer. Specific surface area was measured with a BELSORP-mini II instrument (BEL Japan) at liquid nitrogen temperature and obtained by the Brunauer−Emmett−Teller (BET) theory. Scanning electron microscopy (SEM) image was acquired with a JEOL JSM-IT100LA scanning electron microscope. Electrochemical Impedance Spectroscopy. Pb2Ti4O9F2 electrodes were prepared by electrophoretic deposition on a conductive glass modified with a fluorine-doped tin oxide (FTO) layer. During the preparation, a pair of FTO glasses were immersed in acetone (50 mL) containing I2 (10 mg) and Pb2Ti4O9F2 powder (100 mg) and connected to a dc power supply (GW Instek PSW 80-13.5). Here, the FTO grasses were parallel to each other at a distance of 1.8 cm in the solution. Then, a bias of 30 V was applied between the FTO glasses for 4 min, resulting in Pb2Ti4O9F2-coated FTO glass. The coated area was fixed at 1.5 cm × 4.0 cm. The electrode was dried in air overnight. Impedance measurements were conducted using a BAS CHI760 electrochemical analyzer in a mixed solution of MeCN and TEOA (25 mL, 4:1 v/v). The as-prepared Pb2Ti4O9F2/FTO electrode was employed as a working electrode, in combination with an Ag/AgNO3 reference electrode (10 mM) and Pt wire as a counter electrode. Mott−Schottky plots were prepared at a frequency of 100 Hz. Photocatalytic Reactions. Photocatalytic H2 evolution reactions were conducted in a Pyrex top-irradiation type reaction vessel connected to a closed circulation system at around 298 K. The photocatalytic reaction was carried out using 200 mg of photocatalyst dispersed in 140 mL of MeCN− TEOA mixed solution (13:1 v/v) containing 1 mL of water. Pt (0.5 wt %) was deposited on oxyfluorides through in situ photodeposition using H2PtCl6 as a precursor. A 300 W xenon lamp (Cermax, PE300BF) was used as the light source, with an output current of 20 A. For visible light irradiation, an L42 cutoff filter and a CM-1 cold mirror were employed. The evolved gases were analyzed by gas chromatography (GC-8A with TCD detector and MS-5A column, argon carrier gas, Shimadzu). Density Functional Theory Calculations. First-principles density functional theory (DFT) simulations were carried out using BIOVIA Materials Studio Visualizer38 and CASTEP.39 Geometry optimizations of atomic positions with the experimental lattice constants and evaluation of total/partial density of states (DOS) were done at the generalized gradient approximation (GGA)/PBE40 level of theory with a plane wave cutoff of 340 eV and 4 × 4 × 2 Monkhorst−Pack k-point mesh. All ionic cores were replaced with ultrasoft pseudopotentials.41 Convergence criteria for SCF energy, optimization energy, and maximum force were set to be 1.0 × 10−6 eV/ atom, 1.0 × 10−5 eV/atom, and 0.01 eV/Å, respectively.

eV and functions as a stable visible-light-driven photocatalyst for water reduction/oxidation and CO2 reduction.28 It is considered that the visible-light-response of Pb2Ti2O5.4F1.2 originates from reinforced interaction between Pb 6s and O 2p orbitals, which is enabled by a short Pb−O bond in the pyrochlore structure stabilized by the fluorine substitution. This fact implies that structural modification by mixing heteroanions can be a strategy to achieve visible-light photocatalysis. Because phase-pure oxyfluoride could be a new class of visiblelight-responsive photocatalyst that can potentially be useful for various artificial photosynthetic reactions including H2/O2 evolution and CO2 reduction,28 it is of interest to investigate electronic band structure and light absorption property of other oxyfluorides, in particular, Pb(II)-containing ones. Oxyfluorides including fluorine-doped oxides have been studied for a wide range of applications including superconductors, ion conductors, electron conductors, antiferromagnets, dielectric materials, batteries, phosphors, and catalysts.29−36 However, information on light absorption property and electronic band structure of phase-pure oxyfluoride is limited. In this work, we examined Pb2Ti4O9F2 as a new photocatalyst for H2 evolution in comparison with the light absorption property and electronic band structure of previously reported Pb2Ti2O5.4F1.2.28 Because Pb2Ti4O9F2 and Pb2Ti2O5.4F1.2 adopt different crystal structures as shown in Figure 1, we consider that these two photocatalysts are good

Figure 1. Crystal structures of (a) Pb2Ti4O9F2 and (b) Pb2Ti2O5.4F1.2.

candidates to investigate the correlation among crystal structure, light absorption property, and electronic band structure in Pb(II)-containing oxyfluorides. Results of physicochemical analyses and density functional theory calculations showed that these two materials have different band gaps and hence possess different electronic band structures, which determine visible-light photocatalytic activity.

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EXPERIMENTAL SECTION Materials. All reagents were of reagent-grade quality and were used without further purification except for acetonitrile (MeCN) and triethanolamine (TEOA). MeCN was distilled two times using P2O5 (98.0%, Kanto Chemical Co. Inc.) as a dehydrating agent. TEOA was distilled under reduced pressure ( 300 nm). Table 1. Photocatalytic Activities of Pb2Ti4O9F2 and Pb2Ti2O5.4F1.2 for Hydrogen Evolutiona H2 evolution rate/μmol h−1 photocatalyst

>300 nm

>420 nm

Pb2Ti4O9F2 Pb2Ti2O5.4F1.2b

0.6 1.5

n.d. 0.2

a

Reaction conditions: catalyst, 200 mg (0.5 wt % Pt photodeposited in situ); MeCN/TEOA mixed solution (13:1 v/v), 140 mL containing 1 mL of water; light source, xenon lamp (300 W). b Data from ref 28 with permission. Copyright 2018 American Chemical Society.

Stable H2 evolution behavior was obtained (Figure S3). It was also confirmed that the crystal structure of Pb2Ti4O9F2 remained unchanged before and after the photocatalytic reaction (Figure S4), indicative of the stability of the material. Under >420 nm irradiation (or in the dark), however, no H2 was produced, because the band gap of Pb2Ti4O9F2 is too large to absorb photons with a wavelength longer than 420 nm (see Figure 4). As reported previously,28 Pb2Ti2O5.4F1.2 modified with a Pt cocatalyst was active under not only UV irradiation but also visible light with good stability. In this case, no noticeable change could be identified in XRD patterns and elemental compositions of Pb2Ti2O5.4F1.2 before and after the H2 evolution reaction.28

Scheme 1. Band Structure Diagrams of Pb2Ti4O9F2, Pb2Ti2O5.4F1.2, and PbTiO3a

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DISCUSSION The result of UV−visible diffuse reflectance spectroscopy indicated that Pb2Ti4O9F2 has a shorter absorption edge than Pb2Ti2O5.4F1.2 does; that is, the band gap of the former (ca. 3.0 eV) is larger than that of the latter (ca. 2.4 eV). Given that the CBM of Pb2Ti4O9F2 lies at a potential ca. 0.3 V more positive than that of Pb2Ti2O5.4F1.2, it is reasonable to conclude that the VBM of Pb2Ti4O9F2 is ca. 0.9 V more negative than that of Pb2Ti2O5.4F1.2. Band-edge potentials of Pb2Ti4O9F2 and Pb2Ti2O5.4F1.2 are summarized in Table 2, along with their

a

The diagram of PbTiO3 was depicted on the basis of a DFT study on PbTiO3, which has been reported by Piskunov et al.46

The VBM should correlate strongly with the Pb−O bond length in each material, where shorter Pb−O bonds can provide more pronounced RLP effect. Figure 6 compares the local coordination environments around Pb ion in Pb2Ti4O9F2, Pb2Ti2O5.4F1.2, and PbTiO3. In Pb2Ti4O9F2, there are two Pb− O bonds with different lengths: the shorter one is 2.534 Å, while the longer one is 2.947 Å, which are, respectively, 1 and 17% larger than the shortest Pb−O bond in PbTiO3 (2.510

Table 2. Band Gaps and Band-Edge Potentials of Pb2Ti4O9F2, Pb2Ti2O5.4F1.2, and PbTiO3 compound

band gapa/eV

CBMb/V

VBMb/V

Pb2Ti4O9F2 Pb2Ti2O5.4F1.2 PbTiO3

3.0 2.4 2.8

−1.3 −1.6 −1.5

+1.7 +0.8 +1.3

a

Estimated from the onset wavelength of UV−visible diffuse reflectance spectra. bVersus Ag/AgNO3. This may contain ±0.1 V uncertainty.

band gaps. We have previously reported that the relatively high VBM of Pb2Ti2O5.4F1.2 is attributed to the elevated O 2p orbitals in the material.28 More concretely, it could be explained in terms of the revised lone pair (RLP) model, where the antibonding orbitals formed by Pb 6s and O 2p orbitals are stabilized through the interaction with the empty Pb 6p orbitals.44 Actually, DFT calculations visualized the Pb 6s/O 2p interaction in the lower part of the valence band in

Figure 6. Local coordination environments around the Pb ion in Pb2Ti4O9F2, Pb2Ti2O5.4F1.2, and PbTiO3 at room temperature. The shortest Pb−O bond lengths are marked in red. Data for the bonding distances are available from the literature studies.33,37,47 D

DOI: 10.1021/acs.jpcc.8b08953 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Å).47 This situation should cause weaker interaction between Pb 6s/6p and O 2p orbitals in Pb2Ti4O9F2 than in PbTiO3, resulting in the more positive VBM of Pb2Ti4O9F2. One may consider that the contribution of Pb−F bonds in Pb2Ti4O9F2 (2.417 and 2.563 Å) affects the more positive VBM at least to some extent, because F 2p orbitals are positioned more positively than O 2p. On the other hand, Pb2Ti2O5.4F1.2 has remarkably short Pb−O bond (2.248 Å) and three longer ones (2.590 Å), with three long Pb−O/F bonds (2.726 Å, the nominal occupation ratio is O/F = 0.6/0.4). Compared to the shortest Pb−O bond length in PbTiO3 (2.510 Å), the net Pb− O bond length in Pb2Ti2O5.4F1.2 is obviously shorter, leading to more pronounced RLP effect that gives an elevated VBM of Pb2Ti2O5.4F1.2. The different Pb−O bond lengths in the two oxyfluorides appear to originate from fluorine coordination and O/F anion order in each material. Accordingly, one can account for the difference in the VBM and the resulting light absorption property among Pb2Ti4O9F2, Pb2Ti2O5.4F1.2, and PbTiO3 in terms of the difference in the Pb−O bond length in these materials.

Kengo Oka: 0000-0002-1800-8575 Kazuhiko Maeda: 0000-0001-7245-8318 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research on Innovative Area “Mixed Anion (project JP16H06439, JP16H06441, JP17H05478, and JP17H05489)” (JSPS). It was also partially supported by a Grant-in-Aid for Young Scientists (B) (project JP17K17762) and a PRESTO program (JPMJPR16NA) (JST). R.K. wishes to acknowledge support by a JSPS Fellowship for Young Scientists (JP17J03705). K.O. is grateful for financial support from MEXT-KAKENHI (JP16K05731). 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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CONCLUSIONS In this work, we compared light absorption property and electronic band structure of two different oxyfluorides, Pb2Ti4O9F2 and Pb2Ti2O5.4F1.2. Although both materials worked as stable photocatalysts for H2 evolution in the presence of TEOA as an electron donor under band-gap irradiation, Pb2Ti4O9F2 responded only to UV light due to the large band gap of 3.0 eV, while Pb2Ti2O5.4F1.2 having a reduced band gap (2.4 eV) worked even under visible light. Density functional theory calculations indicated that the valence band maximum of the two materials consisted mainly of hybridized O 2p and Pb 6s/6p orbitals. However, the VBM of Pb2Ti4O9F2 was much more positive than that of Pb2Ti2O5.4F1.2, as revealed by electrochemical impedance spectroscopy and UV−visible diffuse reflectance spectroscopy. The difference in the VBM of the two materials can be understood in terms of RLP model arose from the Pb−O/F bonds in their crystal structures, where Pb2Ti4O9F2 possessed longer Pb−O bond length than Pb2Ti2O5.4F1.2, eventually leading to more positive VBM. Therefore, designing a crystal structure, which contains a shorter Pb−O bond and gives rise to stronger Pb/O orbital interaction, is concluded to be important for obtaining a narrow-gap Pb(II)-containing oxides that of course include mixed anion compounds.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b08953. EDS mapping results, Mott−Schottky plots, and XRD patterns for Pb 2Ti4O 9 F2 and time courses of H 2 evolution by Pt-loaded Pb2Ti4O9F2 under UV irradiation (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.O.). *E-mail: [email protected] (K.M.). ORCID

Tom Ichibha: 0000-0002-7455-4968 Kenta Hongo: 0000-0002-2580-0907 E

DOI: 10.1021/acs.jpcc.8b08953 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.8b08953 J. Phys. Chem. C XXXX, XXX, XXX−XXX