Article pubs.acs.org/IC
Crystal Structure, Electronic Structure, and Photocatalytic Activity of Oxysulfides: La2Ta2ZrS2O8, La2Ta2TiS2O8, and La2Nb2TiS2O8 Yosuke Goto,† Jeongsuk Seo,† Kazunori Kumamoto,† Takashi Hisatomi,† Yoshikazu Mizuguchi,‡ Yoichi Kamihara,§ Masao Katayama,† Tsutomu Minegishi,†,∥ and Kazunari Domen*,† †
Department of Chemical System Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ‡ Department of Electrical and Electronic Engineering, Tokyo Metropolitan University, Hachioji 192-0397, Japan § Department of Applied Physics and Physico-Informatics, Faculty of Science and Technology, Keio University, Yokohama 223-8522, Japan ∥ JST, PRESTO, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan S Supporting Information *
ABSTRACT: The novel oxysulfides La2Ta2ZrS2O8 (LTZSO), La2Ta2TiS2O8 (LTTSO), and La2Nb2TiS2O8 (LNTSO) were synthesized, and their crystal structures, electronic structures, and photocatalytic activities for water splitting under visible light were investigated. Density functional theory calculations showed that these compounds are direct-band-gap semiconductors. Close to the conduction band minimum, the main contribution to the band structure comes from the d orbitals of Zr or Ti ions, while the region near the valence band maximum is associated with the 3p orbitals of S ions. The absorption-edge wavelength was determined to be 540 nm for LTZSO and 700 nm for LTTSO and LNTSO. An analysis of the crystal structure using synchrotron X-ray diffraction revealed that these compounds contained antisite defects at transition metal ion sites, and these were considered to be the origin of the broad absorption at wavelengths longer than that corresponding to band-gap excitation. LTZSO was revealed to be active in the oxygen evolution reaction from aqueous solution containing a sacrificial electron acceptor under visible-light illumination. This result was supported by the band alignment and flat-band potential determined by photoelectron spectroscopy and Mott− Schottky plots.
1. INTRODUCTION Water splitting over semiconductor photocatalysts is attractive for the large-scale production of renewable hydrogen using solar energy. Since the seminal work by Honda and Fujishima on photoelectrochemical (PEC) water splitting using TiO2,1 much attention has been paid to this subject. A number of semiconducting oxides containing transition metal ions with a d0 electronic configuration (such as Ti4+, Zr4+, Nb5+, and Ta5+) function as photocatalysts for overall water splitting under illumination by ultraviolet (UV) light.2−4 However, these oxides rarely absorb visible light because their valence band, which consists of O 2p orbitals, is too positive for water oxidation, resulting in a band-gap value of >3 eV. Because visible light is a major component of the solar spectrum, development of semiconductor photocatalysts that are active under visible light is one of the most fundamental goals for achieving high solar energy conversion efficiency at a reasonable quantum efficiency. However, because photocatalysis is triggered by photoexcited electrons and holes, which relax due to carrier recombination, it is more challenging to drive photocatalytic water splitting as the band-gap energy decreases. © XXXX American Chemical Society
Oxysulfides are promising materials for visible-light-driven photocatalysis. Close to the valence band maximum (VBM), the band structure is associated with S 3p orbitals, which have a more negative potential than O 2p orbitals. As a result, some oxysulfide semiconductors have a narrower band gap than the corresponding oxide semiconductors. For example, Sm2Ti2S2O55 and La5Ti2CuS5O76−8 have been found to act as semiconducting photocatalysts that evolve H2 or O2 from aqueous solutions containing electron donors or acceptors, respectively. The crystal structure of La2M3S2O8 (M = Ta, Nb) belongs to the orthorhombic Pnnm space group,9,10 which is characterized by vertex-sharing MO6 octahedra and edge-sharing MS4O2 octahedra along the c axis, as schematically shown in Figure 1. Bond valence sum calculations suggest a mixed valence of M5+ in the MO6 octahedra and M4+ in the MS4O2 octahedra, resulting in a formal valence state of La2MI5+2MII4+S2O8, where the suffixes I and II indicate the different transition metal ion sites. The reduced cations (Ta4+ or Nb4+), which have a d1 electronic Received: January 30, 2016
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DOI: 10.1021/acs.inorgchem.6b00247 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
within the generalized gradient approximation using the Perdew− Becke−Ernzerhof method.16 The Brillouin zone was sampled using a 3 × 3 × 3 Monkhorst−Pack grid,17 and a cutoff of 500 eV was chosen for the plane-wave basis set. The Hellmann−Feynman forces were reduced to 0.2 eV nm−1. Photocatalytic reactions were carried out using a closed-circulation system. Prior to the reactions, the prepared powder was modified with fine particles of IrO2, which was an model cocatalyst for oxygen evolution.5,6 IrO2 was loaded by impregnation from an aqueous Na2IrCl6 solution, followed by calcination at 300 °C for 1 h in air. The amount of cocatalyst was calculated to be 0.5 wt % on the basis of the Ir weight. Subsequently, 0.2 g of the sample was suspended in 200 mL of 10 mM aqueous AgNO3 solution containing 0.2 g of La2O3 as a pH buffer. The reaction vessel was evacuated to remove air before initiation of irradiation. The light source used was a 300 W Xe lamp equipped with a cutoff filter (Hoya Corp., L42, cutoff wavelength of 410 nm). The evolved gases were analyzed by gas chromatography (Shimadzu Corp., GC-8A, TCD, Ar carrier, MS5A column). Electrodes were fabricated from these oxysulfides using the particle transfer method.18 Ta contact and Ti conductive layers were deposited on the oxysulfide particle layer at 300 °C using radio frequency magnetron sputtering. A cobalt oxide cocatalyst, generally designated as CoPi, was loaded on the prepared photoelectrodes by electrodeposition at 1.1 V vs Ag/AgCl for 200 s in a 0.1 M potassium phosphate aqueous electrolyte containing 0.5 mM Co2+ ions.19,20 A three-electrode system with a potentiostat (Hokuto Denko, HSV-110) was used in the PEC experiments. Current−potential curves were measured in a stirred and Ar-saturated 0.5 M aqueous potassium borate (KBi) solution that was adjusted to pH 13 by KOH addition. The photoanodes were irradiated by chopped AM 1.5G simulated sunlight at 100 mW cm−2 (SAN-EI Electric Co., Ltd., XES-40S2-CE).
Figure 1. Crystal structure of La2MI5+2MII4+S2O8 (MI5+ = Ta, Nb; MII4+ = Zr, Ti). Antisite defects can be present at both types of transition metal ion sites (MI5+ and MII4+). Site occupancies are summarized in Table 1.
configuration, are considered to be detrimental to photocatalysis because of their metallic electronic properties, although the electrical resistance of these compounds has been reported to be as high as 100 kΩ at 300 K.11 In the present study, we synthesized the novel oxysulfides La 2 Ta 2 ZrS 2 O 8 (LTZSO), La 2 Ta 2 TiS 2 O 8 (LTTSO), and La2Nb2TiS2O8 (LNTSO) by substituting Zr4+ or Ti4+ for Ta4+ or Nb4+ in the MIIS4O2 octahedra, with the intention of conferring semiconducting properties as a result of the d0 electronic configuration of all the transition metal ions. Optical measurements established that absorption-edge wavelength of these compounds was the visible light region. The crystal structure was analyzed using synchrotron X-ray powder diffraction (SXRD). The electronic structure was computationally simulated using density functional theory (DFT). The photocatalytic activity for oxygen evolution from water was examined in the presence of a sacrificial electron acceptor.
3. RESULTS AND DISCUSSION Figure 2 shows an SXRD pattern for the LTZSO powder and the results of a Rietveld analysis. Almost all of the diffraction peaks
2. EXPERIMENTAL SECTION Polycrystalline samples were synthesized by a solid-state reaction in a sealed quartz tube. Typically, LTZSO was prepared using dehydrated La2O3, La2S3 (Kojundo Chemical Laboratory Co., Ltd., 99.9%), Ta2O5 (Rare Metallic Co., Ltd., 99.99%), ZrO2 (Kanto Chemical Co., Inc., 99.9%), and S (Kojundo Chemical Laboratory Co., Ltd., 99.99%) as starting materials. The dehydrated La2O3 was prepared by heating commercial La2O3 (Wako Pure Chemical Industries, Ltd., 99.99%) at 1000 °C for 10 h in air. These starting materials were mixed in amounts corresponding to a nominal chemical composition of La2Ta2TiS2.5O8 and heated at 1100 °C for 48 h in a sealed quartz tube. It should be noted that excess sulfur acts to suppress the formation of oxide impurities.7 All processes were carried out in a nitrogen-filled glovebox (MIWA MFG Co., Ltd.) at a dew point of less than −70 °C. For the synthesis of LTTSO and LNTSO, La2O3, Ta2O5, Nb2O5 (Kanto Chemical Co., Inc., 99.9%), TiS2 (Kojundo Chemical Laboratory Co., Ltd., 99.9%), and S were employed as starting materials. The SXRD experiments were performed at the BL02B2 beamline of SPring-8 using samples contained in borosilicate glass capillary tubes with an inner diameter of 0.1 mm. The wavelength of the radiation beam was determined to be 0.049575(8) nm using a CeO2 standard. Structural parameters were refined by Rietveld analysis using the RIETAN-FP code.12 The crystal structure was visualized using the VESTA software.13 Diffuse reflectance spectra (DRS) were collected using a spectrometer equipped with an integrating sphere (JASCO Corp., V-670) and were converted using the Kubelka−Munk function. The particle morphology in the samples was examined using scanning electron microscopy (SEM; Hitachi High Technologies, S-4700). Photoelectron spectroscopy in air (PESA) was performed using an open counter as an electron detector (Riken Keiki, AC-3). Electronic structure calculations based on DFT were performed using the VASP code.14,15 The exchange-correlation potential was treated
Figure 2. Synchrotron X-ray powder diffraction pattern for La2Ta2ZrS2O8 (LTZSO) and the results of Rietveld refinement. The crosses and solid curve represent the observed and calculated patterns, respectively, and the difference between the two is shown at the bottom. The vertical marks indicate the Bragg diffraction positions for LTZSO (upper) and ZrO2 (lower). The inset shows an expanded view in the 2θ range of 5−15°. Analogous data for La2Ta2TiS2O8 and La2Nb2TiS2O8 are shown in Figure S1 in the Supporting Information.
were assigned to LTZSO, indicating that this was the dominant phase in the sample. Several minor peaks attributable to a ZrO2 impurity phase (1.8 wt %) were also identified. Analogous data for LTTSO and LNTSO are shown in Figure S1 in the Supporting Information. The refined crystal structures, lattice parameters, and reliability factors are summarized in Tables 1 and 2. All of these compounds were characterized by a mixed B
DOI: 10.1021/acs.inorgchem.6b00247 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Table 1. Multiplicity and Wyckoff notation (WN), Site Occupancy (g), Fractional Coordinates (x, y, z), and Isotropic Displacement Parameters (Uiso) Refined by Rietveld Analysis for La2Ta2ZrS2O8, La2Ta2TiS2O8, and La2Nb2TiS2O8a
a
site
WN
g
La1 La2 Ta1 Zr1 Zr2 Ta2 S1 S2 O1 O2 O3 O4 O5
4g 4g 8h 8h 4e 4e 4g 4g 8h 8h 8h 4g 4g
1.0 1.0 0.875(2) 0.125(2) 0.750(2) 0.250(2) 1.0 1.0 1.0 1.0 1.0 1.0 1.0
La1 La2 Ta1 Ti1 Ti2 Ta2 S1 S2 O1 O2 O3 O4 O5
4g 4g 8h 8h 4e 4e 4g 4g 8h 8h 8h 4g 4g
1.0 1.0 0.883(2) 0.117(2) 0.766(2) 0.234(2) 1.0 1.0 1.0 1.0 1.0 1.0 1.0
La1 La2 Nb1 Ti1 Ti2 Nb2 S1 S2 O1 O2 O3 O4 O5
4g 4g 8h 8h 4e 4e 4g 4g 8h 8h 8h 4g 4g
1.0 1.0 0.809(4) 0.191(4) 0.618(4) 0.382(4) 1.0 1.0 1.0 1.0 1.0 1.0 1.0
x La2Ta2ZrS2O8 0.2089(3) 0.7147(2) 0.36383(11) 0.36383(11) 0 0 0.0107(12) 0.5130(11) 0.3030(12) 0.0627(12) 0.2854(13) 0.365(2) 0.135(2) La2Ta2TiS2O8 0.2190(3) 0.7173(3) 0.36246(12) 0.36246(12) 0 0 0.0086(12) 0.5276(13) 0.3100(15) 0.0560(15) 0.2861(16) 0.363(3) 0.155(3) La2Nb2TiS2O8 0.2143(3) 0.7189(3) 0.3639(2) 0.3639(2) 0 0 0.0169(11) 0.5152(11) 0.3121(13) 0.0603(13) 0.2849(16) 0.351(3) 0.121(3)
z
Uiso (10−4 nm2)
0.3376(2) 0.1586(2) 0.07972(8) 0.07972(8) 0 0 0.1445(9) 0.3503(8) 0.4754(12) 0.3994(10) 0.2147(11) 0.0467(18) 0.5613(19)
0 0 0.2493(12) 0.2493(12) 0.252(2) 0.252(2) 0 0 0.249(15) 0.261(5) 0.267(4) 0 0
1.42(8) 0.62(7) 0.214(19) 0.214(19) 1.11(6) 1.11(6) 1.37(15) 1.37(15) 1.16(19) 1.16(19) 1.16(19) 1.16(19) 1.16(19)
0.3421(3) 0.1579(3) 0.08040(11) 0.08040(11) 0 0 0.1359(10) 0.3572(10) 0.4742(15) 0.3950(12) 0.2246(14) 0.056(3) 0.550(3)
0 0 0.2450(9) 0.2450(9) 0.2647(12) 0.2647(12) 0 0 0.235(5) 0.247(8) 0.242(9) 0 0
0.84(8) 0.83(8) 0.38(2) 0.38(2) 1.33(11) 1.33(11) 0.80(16) 0.80(16) 1.3(2) 1.3(2) 1.3(2) 1.3(2) 1.3(2)
0.3423(2) 0.1539(3) 0.0837(2) 0.0837(2) 0 0 0.1323(9) 0.3513(10) 0.4792(11) 0.3941(11) 0.2234(11) 0.046(2) 0.553(2)
0 0 0.2498(10) 0.2498(10) 0.2266(9) 0.2266(9) 0 0 0.227(3) 0.265(4) 0.2745(3) 0 0
0.48(6) 0.83(7) 0.43(5) 0.43(5) 0.91(13) 0.91(13) 0.54(15) 0.54(15) 0.44(19) 0.44(19) 0.44(19) 0.44(19) 0.44(19)
Values in parentheses are standard deviations in the last digits.
that some of the Ta exists in the form of Ta4+ ions. Such ions have a d1 electronic configuration and are considered to be the origin of the broad absorption band at wavelengths longer than that corresponding to band-gap excitation, as described below. On the other hand, the 8h sites in the MIO6 octahedra are occupied by Ta and Zr with a ratio of 0.875(2):0.125(2). Although the valence state for Ta in these sites is considered to be 5+, Zr might have a 4+ state because the number of valence electrons in both the 5s and 4d orbitals of Zr is 4. It might be expected that the valence state for Ta in 4e sites would be 5+ in order to maintain electroneutrality. However, it can be expected that the Ta ions in the MIIS4O2 octahedra are partially reduced because of the lower electronegativity of sulfur. To fully elucidate the chemical bonding structure in these compounds, an element-specific spectroscopic technique such as X-ray absorption near edge
Table 2. Lattice Parameters and Reliability Factors (R) Obtained from Rietveld Analysisa a (nm) b (nm) c (nm) Rwp (%) RB (%) GOF a
y
La2Ta2ZrS2O8
La2Ta2TiS2O8
La2Nb2TiS2O8
1.002272(15) 1.166282(15) 0.774984(10) 6.643 1.317 6.6309
0.989726(12) 1.171271(14) 0.768451(8) 10.285 3.549 8.9820
0.981371(11) 1.174311(13) 0.764836(8) 9.046 4.250 8.5960
Values in parentheses are standard deviations in the last digits.
occupancy of transition metal ions: namely, antisite defects. For example, in LTZSO, both Zr and Ta occupy the 4e sites in the MIIS4O2 octahedra with a ratio of 0.750(2):0.250(2), suggesting C
DOI: 10.1021/acs.inorgchem.6b00247 Inorg. Chem. XXXX, XXX, XXX−XXX
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for LTTSO and LNTSO. The band gap was determined to be 2.4 eV for LTZSO, 1.8 eV for LTTSO, and 1.8 eV for LNTSO, as shown in Figure S3 in the Supporting Information. The broad absorption at wavelengths longer than that corresponding to band-gap excitation is attributable to the existence of transition metal ions with a d1 electronic configuration (Ta4+ or Nb4+), as indicated by the SXRD results. Figure 5 shows the electronic density of states (DOS) obtained by DFT calculations. The valence band structures are
structure will be required. It is also possible that vacancies or antisite defects are present at anion sites, and this was not considered during the refinement in this study. The unit-cell volume reflects the ionic radius of the transition metal ions. For example, the unit-cell volume for LTZSO is larger than that for LTTSO because of the larger ionic radius of Zr4+ (Shannon’s sixcoordinate ionic radius,21 rZr = 72 pm) in comparison to that of Ti4+ (rTi = 60.5 pm). However, the difference in the lattice parameters a, b, and c for the two compounds is not so straightforward. Both a and c are larger for LTZSO than for LTTSO, while b is not. This is presumably because of the relatively complex crystal structure and/or the presence of antisite defects at transition metal ion sites. It should be noted that a nonisotropic change in lattice constants was also reported for chalcogenide solid solutions with complex mixed occupancy.,23 Figure 3 shows an SEM image of LTZSO, from which the primary particle size can be estimated to be about 1 μm.
Figure 5. Electronic density of states (DOS) for La2Ta2ZrS2O8, La2Ta2TiS2O8, and La2Nb2TiS2O8. The energy scale is aligned using the La 5s states at around −32.0 eV.
similar for all three compounds, with S 3p orbitals forming the region close to the VBM and O 2p orbitals located below this. The relatively high potential energy of S 3p orbitals allows these oxysulfides to absorb visible light. On the other hand, the conduction band structure is sensitive to the type of transition metal ions, and this is consistent with this band being formed by the d orbitals of these ions, as expected for an ionic bonding scheme. The region near the conduction band minimum (CBM) for LTZSO is formed by Zr 4d orbitals, while that for both LTTSO and LNTSO is formed by Ti 3d orbitals. The potential energy for Ti 3d orbitals is lower than that for Zr 4d orbitals, resulting in a narrower band gap for LTTSO and LNTSO, as shown in Figure 4. The band dispersion depicted in Figure S4 in the Supporting Information indicates that these compounds are direct-band-gap semiconductors. It should be noted that antisite defects at transition metal ion sites, which were investigated using SXRD as described above, were not considered in the DFT calculations. Figure 6 shows the reaction time course for photocatalytic oxygen evolution using LTZSO. As-grown LTZSO generated oxygen under visible light illumination (λ >410 nm), and the oxygen evolution rate was increased by deposition of an IrO2 cocatalyst. The gradual decrease in the oxygen evolution rate for the latter sample was due to photodeposition of Ag, which blocked the surface of the photocatalyst, thus shielding it from the incident light. In addition, LTZSO photoelectrodes fabricated using the particle transfer method generated an anodic photocurrent under illumination by simulated sunlight, as shown in Figure 7. This indicates that LTZSO is an n-type semiconductor and can be used for PEC water oxidation. The amount of oxygen evolved using LTTSO or LNTSO loaded with IrO2 was less than the detection limit (0.1 μmol h−1), presumably because of the narrower band gap of these compounds.
Figure 3. SEM image of La2Ta2ZrS2O8. SEM images of La2Ta2TiS2O8 and La2Nb2TiS2O8 are shown in Figure S2 in the Supporting Information.
However, the particles are highly aggregated because of the relatively high synthesis temperature of 1100 °C. The secondary particle size is roughly estimated to be 5−10 μm. SEM images of LTTSO and LNTSO are shown in Figure S2 in the Supporting Information. Although the three compounds are isostructural, the primary particles in LTTSO and LNTSO are slightly larger than those in LTZSO and are rodlike. This is probably due to faster thermal diffusion of Ti in comparison to Zr during the solid-state reactions. Figure 4 shows DRS for the three compounds in the UV− visible region. The absorption edge due to band-gap excitation was estimated to be about 540 nm for LTZSO and about 700 nm
Figure 4. UV−visible DRS of La2Ta2ZrS2O8 (LTZSO), La2Ta2TiS2O8 (LTTSO), and La2Nb2TiS2O8 (LNTSO). D
DOI: 10.1021/acs.inorgchem.6b00247 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 6. Reaction time courses for oxygen evolution under visible light irradiation (λ >410 nm) using as-grown and 0.5 wt % IrO2-loaded La2Ta2ZrS2O8 (LTZSO). Conditions: photocatalyst, 0.20 g; solution, 200 mL of 10 mM AgNO3 aqueous solution containing 0.20 g of La2O3; light source, 300 W Xe lamp.
Figure 8. Band alignment and flat-band potential for La2Ta2ZrS2O8 determined by PESA, DRS, and ac impedance measurements. CBM, VBM, and Efb denote the conduction band minimum, valence band maximum, and flat-band potential, respectively. Ionization potentials, i.e., the VBM energies, were obtained from the PESA measurements. The bandgaps were determined using DRS measurements. Efb was deduced from Mott−Schottky plots on the basis of the ac impedance measurements. Analogous data for La2Ta2TiS2O8 and La2Nb2TiS2O8 are shown in the Supporting Information.
The band alignment for LTTSO and LNTSO is presented in Figure S8 in the Supporting Information. In this case, Efb is not shown because of the metallic nature of these compounds. We note that the DOS calculated using DFT (Figure 5) indicated that both LTTSO and LNTSO should be semiconductors. This suggests that their metallic properties are the result of antisite defects. This might be clarified by performing a theoretical analysis of defect physics using computational simulations.24−26 However, this is beyond the scope of the present study because of the negligible photocatalytic activity of these compounds.
Figure 7. Current−potential curve for a CoPi/La2Ta2ZrS2O8 photoelectrode prepared by the particle transfer method with Ta contact layer and Ti conductor layer. Conditions: light source, solar simulator (SANEI, XES-40S2-CE); electrolyte solution, 0.5 M aqueous KBi solution at pH 13 adjusted using KOH; scan rate, 10 mV s−1.
4. CONCLUSION The novel oxysulfides LTZSO, LTTSO, and LNTSO were prepared by substituting d0-state ions (Zr4+ or Ti4+ ion) for d1state ions (Ta4+ and Nb4+ ion). DFT calculations showed that all of these compounds were direct-band-gap semiconductors and that the region of the conduction band close to the CBM was associated mainly with the d orbitals of Zr or Ti ions. The absorption edge wavelengths were determined to be 540 nm for LTZSO and 700 nm for LTTSO and LNTSO. A crystal structure analysis using SXRD indicated the presence of antisite defects at transition metal ion sites, resulting in a broad absorption band at wavelengths longer than that corresponding to band-gap excitation. LTZSO was found to generate oxygen from an aqueous AgNO3 solution under illumination by visible light. We believe that the strategy used in the present study will prove useful for identifying novel oxysulfide photocatalysts with the potential for sunlight-to-fuel conversion.
Figure 8 shows the band alignment and flat-band potential (Efb) determined on the basis of PESA, DRS, and Mott− Schottky (M-S) plots for LTZSO. From the M-S plot, Efb is about −1.6 V vs Ag/AgCl and is almost independent of pH in an alkaline solution, as shown in Figure S6 and Table S1 in the Supporting Information. This suggests that the band-gap potential for LTZSO is almost independent of pH, as is the case for Sm2Ti2S2O5.5 The VBM for LTZSO is more positive than the equilibrium potential for water oxidation at pH 8, indicating that, thermodynamically, LTZSO can generate oxygen from water. Furthermore, the CBM for LTZSO is more negative than the hydrogen reduction potential, which suggests that LTZSO can also generate hydrogen with appropriate surface modifications. However, hydrogen production by these compounds was not observed in an aqueous solution containing electron donors, even after loading with Pt as a hydrogen evolution cocatalyst. This is presumably because of the presence of d1-state transition metal ions associated with antisite defects. Such defects could possibly be suppressed by aliovalent ion substitution or the use of a different synthesis method. It might also be necessary to reduce the particle size, because it is more difficult for excited carriers in an n-type semiconductor to migrate to the surface due to the Schottky barrier between the photocatalyst and a cocatalyst such as Pt.
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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.inorgchem.6b00247. SXRD patterns, SEM images, (αhν/s)2−hν plots, band dispersion plots, PESA spectra, M-S plots, flat-band potentials at various pHs, and band alignment diagrams for LTZSO, LTTSO, and LNTSO (PDF) E
DOI: 10.1021/acs.inorgchem.6b00247 Inorg. Chem. XXXX, XXX, XXX−XXX
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(23) Goto, Y.; Naito, F.; Sato, R.; Yoshiyasu, K.; Itoh, T.; Kamihara, Y.; Matoba, M. Inorg. Chem. 2013, 52, 9861−9866. (24) Hiramatsu, H.; Kamiya, T.; Tohei, T.; Ikenaga, E.; Mizoguchi, T.; Ikuhara, Y.; Kobayashi, K.; Hosono, H. J. Am. Chem. Soc. 2010, 132, 15060−15067. (25) Zakutayev, A.; Tate, J.; Schneider, G. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 82, 195204. (26) Goto, Y.; Tanaki, M.; Okusa, Y.; Shibuya, T.; Yasuoka, K.; Matoba, M.; Kamihara, Y. Appl. Phys. Lett. 2014, 105, 022104−022107.
Crystallographic data for La2Ta2ZrS2O8 (CIF) Crystallographic data for La2Ta2TiS2O8 (CIF) Crystallographic data for La2Nb2TiS2O8 (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail for K.D.:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Drs. Y. Kuroiwa, C. Moriyoshi, and E. Magome of Hiroshima University and and Dr. A. Miura of Hokkaido University for assistance with the SXRD measurements and crystal structure analysis. Synchrotron radiation experiments were performed at SPring-8 with the approval of the Japan Synchrotron Research Institute (Proposal No. 2015A1441). This work was supported by a Grant-in-Aid for Specially Promoted Research (#23000009), the international exchange program of the A3 Foresight Program of the Japan Society for the Promotion of Science (JSPS), and Companhia Brasileira de Mmetallurgia e Mineraçaõ (CBMM). This work was also supported in part by the Artificial Photosynthesis Project of the Ministry of Economy, Trade and Industry (METI) of Japan, Grant-in-Aids for Young Scientists (A) (No. 15H05494) and Young Scientists (B) (No. 5K17895) of JSPS, and JST, PRESTO.
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DOI: 10.1021/acs.inorgchem.6b00247 Inorg. Chem. XXXX, XXX, XXX−XXX
