New Precursor Route Using a Compositionally Flexible Layered Oxide

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New Precursor Route Using a Compositionally Flexible Layered Oxide and Nanosheets for Improved Nitrogen-Doping and Photocatalytic Activity Kazuhiko Maeda, Yuki Tokunaga, Keisuke Hibino, Kotaro Fujii, Hiroyuki Nakaki, Tomoki Uchiyama, Miharu Eguchi, Daling Lu, Shintaro Ida, Yoshiharu Uchimoto, and Masatomo Yashima ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00256 • Publication Date (Web): 30 Mar 2018 Downloaded from http://pubs.acs.org on April 1, 2018

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ACS Applied Energy Materials

New Precursor Route Using a Compositionally Flexible Layered Oxide and Nanosheets for Improved NitrogenDoping and Photocatalytic Activity Kazuhiko Maeda,*a Yuki Tokunaga,a Keisuke Hibino,a Kotaro Fujii,a Hiroyuki Nakaki,b Tomoki Uchiyama,b Miharu Eguchi,c Daling Lu,d Shintaro Ida,e Yoshiharu Uchimoto,b and Masatomo Yashimaa a

Department of Chemistry, School of Science, Tokyo Institute of Technology, 2-12-1-NE-2 Ookayama, Meguro-ku, Tokyo 152-8550, Japan.

b

Graduate School of Human and Environmental Studies, Kyoto University, Nihonmatsu-cho, Yoshida, Sakyo-ku, Kyoto 606-8317, Japan.

c

Electronic Functional Materials Group, Polymer Materials Unit, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan. d

Suzukakedai Materials Analysis Division, Technical Department, Tokyo Institute of

Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8503, Japan. e

Department of Applied Chemistry and Biochemistry, Graduate School of Science and

Technology, Kumamoto University, 2-39-1 Kurokami, Chuo, Kumamoto 860-8555, Japan.

ACS Paragon Plus Environment

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KEYWORDS. Artificial photosynthesis, Heterogeneous photocatalysis, Layered materials, Mixed anion compounds, Nanosheets, Water splitting.

ABSTRACT. Nitrogen-doping into a metal oxide is a conventional method to prepare a visiblelight-responsive photocatalyst. However, the charge imbalance that results from aliovalent anion substitution (i.e., O2–/N3– exchange) generally limits the concentration of nitrogen that can be introduced into a metal oxide, which leads to insufficient visible-light absorption capability. Here we report an effective route to synthesize nitrogen-doped metal oxide using KTiNbO5, which is a compositionally flexible layered oxide and can be exfoliated into nanoscale sheets. KTiNbO5 has a unique layered structure, in which Ti4+ and Nb5+ coexist in the same two-dimensional sheet, and controllable Ti4+/Nb5+ ratios while maintaining the original KTiNbO5 structure. The use of a Nbrich oxide precursor could allow for the improvement in the introduction of nitrogen compared with stoichiometric KTiNbO5 during thermal ammonolysis with ammonia gas. Reassembled KTiNbO5 nanosheets with a larger surface area were determined to be more useful as a precursor than the layered precursor in terms of nitrogen introduction, and thus yielded more pronounced visible-light absorption and photocatalytic activity for water oxidation.

Introduction Mixed anion compounds, including nitrogen-doped metal oxides, have recently attracted attention as energy materials for applications in batteries, and as heterogeneous catalysts, photocatalysts, and photoelectrodes.1 Substitution of an O2– anion in a metal oxide for a N3– anion alters the local coordination geometries around the metal center, which results in new functionalities that are typically unattainable with single-anion compounds such as oxides and nitrides.


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Visible-light-driven photocatalysis is one of the representative functions of nitrogen-doped metal oxides because the introduced nitrogen provides a new level above the valence band of the host metal oxide, which enables visible-light absorption without affecting the conduction band minimum.2 Nitrogen-doping into a metal oxide is a conventional means of producing a visiblelight-responsive photocatalyst, so that a wide variety of synthetic methods and host oxide materials have been developed to date, with various types of reactions/schemes available, including the decomposition of harmful organic compounds,2–8 water reduction/oxidation (i.e., H2/O2 evolution),9–16 CO2 reduction,17 and photoelectrochemical water splitting.18–21 Nitrogendoped metal oxides are also potentially useful for applications in ferroelectrics and oxygen reduction catalysis.22,23 However, one of the biggest problems of nitrogen introduction into a metal oxide is charge imbalance by the substitution of O2– in the host oxide for N3–, which limits the nitrogen concentration in the doped metal oxide host to lower levels and is unfavorable for visible-light absorption.9,10 Nitridation of oxides is typically conducted by thermal annealing of an oxide precursor under a flow of NH3 gas at high temperatures (>773 K in most cases).24,25 Nitridation at higher temperatures enhances the reactivity of NH3 with the precursor, which may produce a material that contains more nitrogen. However, high-temperature nitridation inevitably causes the formation of thermally stable byproduct phases and/or reduced metal species, which are generally unfavorable for heterogeneous photocatalysis.26 Therefore, it is difficult to synthesize a doped metal oxide that exhibits sufficient visible-light absorption. In addition, a recent transient absorption spectroscopy study revealed that the charge imbalance in a doped oxide can result in the formation of oxygen vacancies, which in turn produces trap states of photogenerated charge carriers and results in lower photocatalytic activity.15


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positive charge that compensates the negative charge resulting from O2–/N3– exchange during thermal ammonolysis with NH3 gas. This approach is clearly different from the aforementioned ordinary codoping strategy, because it does not rely on foreign cation doping. Another important point of using layered KTiNbO5 is that the compound can transform into nanosheets.29,30 Chemical exfoliation of KTiNbO5 (more precisely, its protonated form) results in a colloidal suspension of TiNbO5– polyanion nanosheets, which can be restacked by the addition of an acid, base, or salt to yield high-surface-area nanostructured solids.30 We consider that the nanostructured solid should be a good precursor for nitrogen-doping because it provides better a interface for NH3 gas to access. In a similar way, Liu et al. have demonstrated that efficient, homogeneous doping of nitrogen could be realized using a layered titanate, Cs0.68Ti1.83O4, as a precursor.7 They argued that the key to achieve efficient doping appeared to be the layered structure, which can provide a good pathway to introduce nitrogen. However, the use of metal oxide nanosheets as a precursor to nitrogen-doped materials has scarcely been applied to date. In this work, we first examine the validity of the concept to use a nonstoichiometric precursor route to synthesize nitrogen-doped metal oxide with KTiNbO5. An attempt is then made to apply nanosheets of KTiNbO5 and its variants as precursors to realize more efficient nitrogen-doping. To avoid topochemical dehydration that may cause undesirable structural change at higher temperatures,31,32 K+-restacked TiNbO5– sheets (not H+-restacked one) are employed as the precursor. The photocatalytic water oxidation activities of the synthesized nitrogen-doped materials are also examined. Experimental Section Materials and Reagents


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KCl (>99.5%, Kanto Chemicals), titanium tetra-isopropoxide (>97.0%, Kanto Chemicals), and NbCl5 (>95.0%, Wako Pure Chemicals) were used as the starting materials for the synthesis of K-Ti-Nb oxide precursors by the polymerized complex (PC) method, which was originally developed by Kakihana.33 Ethylene glycol (EG; 99.5%, Kanto Chemicals Co.), anhydrous citric acid (CA; 98.0%, Wako Pure Chemicals), and methanol (>99%, Kanto Chemical) were used as polymerization agents. HNO3 (69–70 wt%, Kanto Chemical), tetra(n-butyl)ammonium hydroxide (TBA+OH–; 40 wt% in H2O, Aldrich Chemical Co.), and KOH (>86.0%, Kanto Chemical) were used for synthesis of the nanosheet materials. All reagents were used as-received without further purification. AgNO3 (99.8%, Wako Pure Chemicals) and La2O3 (99.9%, Tokyo Chemical Industry) were used for the photocatalytic reactions. Prior to reaction, La2O3 was heated in air at 1273 K for 2 h to eliminate the La(OH)3 phase. Synthesis of Layered Oxide Precursors Oxide precursors containing K, Ti, and Nb were synthesized by the PC method. In a typical synthesis, KCl, titanium tetra-isopropoxide, NbCl5, EG, and CA were dissolved in 100 mL of methanol at a K:(Ti+Nb):EG:CA molar ratio of 1:2:60:15, where the ratio of Ti to Nb (Ti:Nb) was changed while maintaining the total molar amount of Ti and Nb. Here the deviation from Ti:Nb = 1:1 is represented using x. For example, x = 20% indicates a sample prepared with a 20 % excess of Nb by mole (i.e., Ti:Nb = 0.8:1.2). In a formula, x is defined as follows: x (%) = (Nb – 1)/(Ti + Nb) ! 200. The mixture was then heated at ca. 400 K on a hot-plate stirrer and then at ~723 K in a mantle heater to yield a gray powder. This material was calcined on an Al2O3 plate at 723 K for 5 h in air, which produced a white powder. This powder was again calcined in an Al2O3 crucible for 2 h in air at 1073 K.


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Exfoliation of Layered Oxides The as-prepared layered oxides were, if necessary, subject to exfoliation and restacking. First, the oxides (3 g) were stirred in HNO3 aqueous solution (1.4 M, 100 mL) for 3 days to replace the K+ in the interlayer with H+. The resultant powder was centrifuged and repeatedly washed with water until the pH of the supernatant became neutral, followed by drying at 343 K in an oven. The proton-exchanged materials were shaken at 160 rpm in aqueous TBA+OH– solution for 2 weeks at room temperature. The molar ratio of TBA+OH– to exchangeable cations in the layered solids was 1.0. The separation of unreacted layered solids was performed by spontaneous precipitation overnight and the supernatant was used as a nanosheet suspension. The nanosheet suspension was reacted with aqueous KOH solution (2 M) to yield restacked oxide nanosheets. The resulting precipitates were centrifuged and washed with water several times until the pH of the supernatant became neutral. After drying in an oven overnight, the sample was ground with a mortar and pestle. Nitridation of Oxide Precursors The as-prepared oxide precursors were heated at 963 K for 1 h under a flow of NH3 (20 mL min–1) using a horizontal tubular furnace. In a typical synthesis, the oxide precursor powder was placed on a SiO2 boat and inserted into an Al2O3 tubular reactor. After purging residual air with nitrogen gas, a flow of NH3 gas was started and the furnace was heated (ramp: 10 K min–1). After cooling to ca. 373 K, nitrogen was flowed again to purge the NH3 gas. Finally, the sample was collected at room temperature and ground into powder. Modification with RuO2 Cocatalyst RuO2 was loaded as a cocatalyst for water oxidation by a previously reported impregnation method.34 The as-synthesized nitrogen-doped sample was dispersed in RuCl3 aqueous solution


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containing 0.5 wt% of Ru as the metallic content. The mixture was stirred until the solution was completely evaporated. The solid sample was then heated in air at 573 K to convert the loaded Ru species to RuO2. Characterization of Materials The synthesized materials were characterized using X-ray diffraction (XRD; Rigaku MiniFlex 600), UV-visible diffuse reflectance spectroscopy (DRS; Jasco V-670), transmission electron microscopy (TEM; Hitachi High-Technologies, H-7650, JEM-2010F, Jeol), and scanning electron microscopy (SEM; JSM–IT100LA, Jeol, S-4700, Hitachi). The concentration of nitrogen in the prepared materials was determined using a J-SCIENCE JM10 elemental analyzer. The Brunauer-Emmett-Teller (BET) surface area was measured with a BEL Japan BELSORPmini apparatus at liquid N2 temperature (77 K). The thickness of the exfoliated nanosheets was measured using atomic force microscopy (AFM; Seiko, Nano-cute). X-ray absorption fine structure (XAFS) measurements were conducted using the BL01B1 beamline at the SPring-8 synchrotron facility (Hyogo, Japan) with a ring energy of 8 GeV and a stored current of 100 mA in the top-up mode to acquire Ru-K edge spectra (Proposal No. 2017B1919). XAFS spectra were acquired at room temperature in the transmission mode using a Si(311) double-crystal-monochromator. A pair of Rh-coated mirrors was used to eliminate higher harmonics. X-ray absorption near edge structure (XANES) spectra were processed using the Athena software package.35 Synchrotron X-ray Powder Diffraction Measurements Synchrotron X-ray powder diffraction (SXRD) data measurements of the nitridation products were conducted to investigate the crystal structures. SXRD data were measured at 297 K using a Debye–Scherrer camera with one-dimensional solid-state (MYTHEN) detectors installed at the


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BL02B2 beamline of SPring-8, Japan (Proposal No. 2017B1265).36 The synchrotron X-ray was monochromatized using a double-crystal Si(111) monochromator, and the wavelength was determined to be 0.6000100(6) Å by Rietveld analysis of CeO2 powders (NIST SRM 674b) using the computer program FullProf.37,38 The obtained SXRD data were analyzed using the Rietveld method with the GSAS analysis software.39 Photocatalytic Reactions Photocatalytic reactions were conducted in a Pyrex top irradiation-type reaction vessel connected to a glass closed gas circulation system. Oxidation of H2O into O2 was performed in aqueous solutions containing silver nitrate as an electron acceptor. 50 mg of the photocatalyst powder was dispersed in aqueous AgNO3 (10 mM, 140 mL). The reactant solution was evacuated several times to remove air completely prior to irradiation under a 300 W xenon lamp (Cermax, PE300BF) fitted with a cutoff filter (! > 420 nm). The output current of the xenon lamp was 20 A unless otherwise stated. The temperature of the reactant solution was maintained at room temperature by a flow of cooling water during the reaction. Results and Discussion Nitrogen-Doping into Layered Oxides Figure 2 shows XRD patterns of the oxide precursors prepared with 0 " x " 20 %, i.e., Nb-rich materials. In all cases, the obtained products exhibited single-phase diffraction patterns assigned to KTiNbO5. The diffraction peaks became slightly narrower and weaker with an increase in x, which indicates a decrease in crystallinity. Further increase of x over 20% resulted in the production of impurity phases and poorer reproducibility. Therefore, x in the range from 0 to 20% was investigated in this work. It was also confirmed that the light absorption edges of these oxide materials were located at around 350 nm (Figure S1).


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nitrogen into the oxide precursors. Elemental analysis indicated that the concentration of nitrogen in the nitrided products increased with x, as listed in Table 1. However, the XRD patterns of the nitrided products remained unchanged from those of the oxide precursors, regardless of x, as shown in Figure 2. Rietveld patterns of synchrotron X-ray diffraction data showed that the nitrided product, even though it had the largest nonstoichiometry (x = 20%), was a single-phase KTiNbO5-like structure (Figure 3).

Figure 3. Rietveld pattern of the SXRD data for the nitridation sample from a bulk precursor (x = 20%). The experimental pattern (red + marks), calculated pattern (blue solid line), and difference profile (violet solid line) are shown. The tick marks indicate the peak positions. Structural analysis was performed for the KTiNbO5-type structure (crystal system: orthorhombic; space group: Pnma). The final reliability factor was Rwp = 0.0232, with lattice parameters of a = 6.4531(4) Å, b = 3.80529(19) Å, c = 18.5082(15) Å, and V = 454.49(7) Å3. SEM observations indicated that the nitrided products consisted of aggregated primary particles of 50–100 nm in size, which formed larger secondary particles of ca. 5 µm (Figure 4). However, no significant difference in the primary particle size was observed with respect to x (Figure S2, TEM images). This is consistent with the result of nitrogen adsorption experiments, which indicated that the specific surface areas of the nitrided products were almost the same, regardless of x (see Table 1). Energy dispersive X-ray spectroscopy (EDS), which is equipped


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with the SEM instrument, also confirmed homogeneous distribution of the constituent elements in the nitrided products, as shown in Figure 4. However, the existence of nitrogen dopant could not be visualized by EDS mapping, presumably due to the low concentration of nitrogen and/or overlapping with signals from Ti. These results indicate that nitrogen-doping occurred in all cases while maintaining the original layered crystal structure.

Figure 4. (A) SEM images of the nitrided product (x = 20 mol%), and (B) SEM-EDS mapping analysis for the nitrided product (x = 20 mol%). The white scale bar in panel (B) corresponds to 5.0 µm.


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The higher concentration of nitrogen in the doped material directly reflects the light absorption profiles. As shown in Figure 5, nitridation of the stoichiometric oxide precursor (i.e., 0% sample) resulted in the generation of new absorption in the visible-light region, in addition to the intrinsic absorption of the oxide precursor at ca. 350 nm. This is a typical feature found in nitrogen-doped metal oxides.3,9,10,15 Increasing x resulted in more pronounced visible-light absorption, which could be explained in terms of the increased concentration of nitrogen in the nitrided material (see Table 1). In addition, absorption at longer wavelengths became weaker with an increase in x. This is attributed to the reduction of the density of reduced metal species (most likely Ti3+ and/or Nb4+ species) that were generated during the nitridation process.40–42 Therefore, the excess Nb precursor was found to be useful, not only to increase the nitrogen concentration but also to reduce the density of such reduced metal species. 2.0

Kubelka-Munk / a.u.

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x x x x

1.5

= = = =

0% (Before) 0% (After) 10% (After) 20% (After)

1.0

0.5

0.0 300

400

500

600

700

Wavelength / nm

Figure 5. DRS of nitrogen-doped KTiNb1+xO5 oxides prepared with 0 " x " 20%. Data for KTiNbO5 (x = 0%) are also shown for comparison. Nitrogen-Doping into Metal Oxide Nanosheets Thus, it was demonstrated that the use of composition-modified lamellar oxide precursors enabled preparation of the corresponding KTiNbO5-type layered oxynitrides that possess more visible-light absorption. Such effective nitrogen-doping may also be possible by using a suitable


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materials. The specific surface areas of the restacked materials were determined to be 40–50 m2 g–1, which is significantly larger than that of the precursors (10–15 m2 g–1). Nitridation was then conducted using these nanosheet materials as the precursors. XRD analysis showed that the position of the diffraction peaks remained unchanged, even after nitridation, as shown in Figure 6. Rietveld analysis using SXRD data again confirmed the production of a single-phase KTiNbO5-like structure for the nitrided nanosheet material (Figure S4). However, the diffraction peaks became sharper and narrower upon nitridation, which indicates that the nitridation process at 963 K restored the periodic layered structure. This is supported by a decrease in the specific surface area after nitridation (see Table 1); there was approximately a 50% drop in specific surface area after nitridation. Nevertheless, the nitrided nanosheet materials in general had larger specific surface areas than the corresponding layered materials, although the extent became lower at larger x values. The specific surface areas of the nitrided nanosheet materials tended to decrease with increasing x. Because the precursor oxide nanosheets had similar specific surface area of 40–50 m2 g–1 as mentioned above, the observed decrease in the nitrided materials would be associated with the difference in how these oxide nanosheet precursors underwent structural changes upon nitridation. At present, however, we could not fully elucidate it. More importantly, the products obtained by nitridation of the oxide nanosheet precursors contained higher concentrations of nitrogen than those obtained with the layered precursors (Table 1), even though the nitridation experiments were conducted under the same conditions. As a result, visible-light absorption of the nitrogen-doped nanosheet materials was more pronounced than that for the materials prepared from the layered precursors, as shown in Figure 7. The nitrogen concentration in the nanosheet-derived materials was also increased with x. Even


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though these are doped materials, the relatively steep absorption profiles of the nitrogen-doped, restacked nanosheet materials suggest the nature of band-to-band transitions, as reported by Liu et al., where similar absorption profiles were observed for a layered titanate, Cs0.68Ti1.83O4, doped with nitrogen.7 Regarding chemical state of the bulk nitrogen species, Yoshida et al. reported that there are at least two types of chemical state of nitrogen species in nitrogen-doped TiO2: (1) photocatalytically active nitrogen that substitutes the oxygen sites and (2) inactive NOx (1 < x < 2) species.43 According to that report, it is claimed that the substituted nitrogen species are responsible for the band gap narrowing of TiO2. Although we could not exactly identify the chemical state of nitrogen species existing in the nitrogen-doped KTiNbO5, it is likely that the doped nitrogen species that substitutes lattice oxygen are responsible for the visible-light absorption capability. More effective nitrogen-doping was thus demonstrated using oxide nanosheets as the precursors. The Nb-rich oxide nanosheets enabled more nitrogen to be introduced. Combining the two factors (i.e., nanosheet precursor and Nb-enrichment), it is concluded that more effective nitrogen-doping into a metal oxide was achievable. 2.5

Kubelka-Munk / a.u.

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x = 20% (Layered) x = 0% (Nanosheet)

2.0

x = 10% (Nanosheet) x = 20% (Nanosheet)

1.5 1.0 0.5 0.0 300

400

500

600

700

Wavelength / nm

Figure 7. DRS of nitrogen-doped KTiNbO5-based oxides (restacked nanosheets), prepared with 0 " x " 20%. Data for nitrogen-doped layered material (x = 20%) are also shown for comparison.


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Photocatalytic Water Oxidation Using the as-prepared nitrogen-doped KTiNbO5-based materials, water oxidation reaction was first examined with AgNO3 as a sacrificial electron acceptor under visible light (! > 420 nm). The water oxidation reaction was selected as a test reaction because oxynitride-type materials including nitrogen-doped oxides generally exhibit high photocatalytic activity for the water oxidation reaction, even without the aid of a cocatalyst such as RuO2 or IrO2.44 However, the as-prepared nitrogen-doped oxide materials exhibited negligible activity without loading a cocatalyst under the present reaction conditions. Nevertheless, modification of the doped materials with RuO2 cocatalyst resulted in clearly observable O2 evolution upon visible light. Therefore, the photocatalytic activities were compared using RuO2-loaded samples. XAFS measurement for the RuO2-loaded sample showed that the Ru-K edge X-ray absorption nearedge structure (XANES) data, the extended XAFS (EXAFS) oscillation, and the Fourier transform (FT) spectrum are all consistent with reference data for RuO2 (see Figure S5). However, the EXAFS oscillation and the associated FT of the sample are relatively weak at longer distances, primarily due to the small particle sizes. Thus, it was shown that the RuO2loaded sample contained RuO2 having smaller particle size. This was supported by TEM observations, which indicated the production of RuO2 nanoparticles with sizes in the range of ca. 2–5 nm (Figure S6). As listed in Table 2, all of the synthesized materials produced O2 upon visible-light irradiation. Although the activity did not largely depend on the x value both in layered and nanosheet series, nanosheet samples showed higher activity than layered samples, regardless of x. Among the tested samples, nitrogen-doped nanosheet materials with smaller x values exhibited the highest activity. N2 evolution, which has been observed in water oxidation in some oxynitride


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photocatalysts,40–42,44 was negligible. The activity became almost zero under >480 nm irradiation (Table 2, entry 7). Of course, the precursor oxides showed no activity under the present reaction condition, due primarily to the lack of visible light absorption. These results indicate that the water oxidation reaction was driven by light absorption of the nitrogen-doped nanosheet photocatalyst. Table 2. Photocatalytic activities of nitrogen-doped KTiNbO5-based materials (loaded with 0.5 wt% RuO2) for water oxidation under visible light (! > 420 nm)a Entry

Type of precursor

x/%

Amount of O2 evolved / µmol

1

Bulk

0

2.2

2

10

2.4

3

20

2.5±0.4

0

4.0

5

10

3.8

6

20

3.7

7b

10

N.D.

4

Nanosheet

a

Reaction conditions: catalyst, 50 mg (La2O3 200 mg); reactant solution, aqueous AgNO3 (10 mM, 140 mL); light source, 300 W xenon lamp fitted with a CM-1 cold mirror and a L-42 cutoff filter. Reaction time: 10 h. b Under > 480 nm irradiation. Reproducibility in the O2 evolution activity was within ~20%. Thus, in contrast to our initial expectation (i.e., improved light absorption capability of a semiconductor photocatalyst will lead to enhanced activity), the water oxidation activity of the nitrogen-doped layered materials was largely insensitive to the x value (Table 2), even though the visible light absorption capability was improved upon an increase in x (Figure 5). Nevertheless, the nitrogen-doped nanosheet materials exhibited obviously higher activity than the layered ones. This might result from distinctly more pronounced visible light absorption of the nanosheet materials (Figure 7). Here larger specific surface area of the nanosheet materials


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may seem to have a positive impact on activity to some extent (in particular, lower x cases), because it in principle provides more reaction sites.45 However, the activity and the extent of specific surface area of the x = 20% nanosheet sample were almost identical to those of the corresponding layered one (Tables 1 and 2). Therefore, more pronounced visible light absorption of the nanosheet materials compared to the layered analogues is likely to lead to the enhanced photocatalytic activity for water oxidation. It is also noted that the absolute activities of the present nitrogen-doped nanosheet materials were low, compared to other oxynitride-type materials.44 This might be at least in part due to the lower degree of crystallinity of the parent layered oxide materials, which can increase the probability of electron/hole recombination.45 Improvement of crystallinity of nanosheet materials can be readily made upon calcination of the parent layered solids at elevated temperatures.46 However, high temperature calcination of our Nb-rich oxide precursor at >1073 K resulted in the generation of impurity phases such as K4Nb6O17 and its hydrated form. Improvement of nitrogen doping amount in nanosheet materials while satisfying high crystallinity of the material is thus the next challenge toward enhanced visible-light photocatalysis. Conclusions In this work, an effective route to obtain nitrogen-doped metal oxide was developed using compositionally flexible layered KTiNbO5 and its nanosheets as precursors. A Nb-rich oxide nanosheet was found to be the best precursor in terms of nitrogen-doping, which produced a steep absorption edge at around 460 nm after nitridation while maintaining the original KTiNbO5 structure, which was confirmed by synchrotron X-ray powder diffraction measurements. More pronounced visible-light absorption of the nanosheet material was concluded to result in enhanced photocatalytic activity for water oxidation under visible light. We believe that the


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present finding in the synthesis of nitrogen-doped metal oxides with higher nitrogen concentrations using compositionally flexible metal oxides (in particular, nanosheets) is applicable to other mixed metal oxides and nanosheets. ASSOCIATED CONTENT Supporting Information. Additional characterization data. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author [email protected] Author Contributions K.M. designed the project and wrote a draft of the manuscript. Y.T. conducted most of the experiments and analyses with K.M. K.H., K.F. and M.Y. performed Rietveld analysis. H.N., T.U. and Y.U. performed XAFS measurement. M.E. and D.L. conducted TEM measurement. AFM observation was done by S.I. All of the authors discussed and provided comments on the experiments and the manuscript during preparation. ACKNOWLEDGMENTS This work was supported by Grants-in-Aid for Scientific Research on Innovative Area “Mixed Anion (Projects JP16H06440, JP16H06441, JP17H05484)” from the Japan Society for the Promotion of Science (JSPS). This work was also partially supported by Grant-in-Aids for Young Scientists (A) (Project JP16H06130) and for Grant-in-Aid for Challenging Research (Exploratory) (Project JP17K19169) from JSPS. K.M. acknowledges the Noguchi Institute, The


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Nukumizu, K.; Nunoshige, J.; Takata, T.; Kondo, J. N.; Hara, M.; #Kobayashi, H.; Domen, K. TiNxOyFz as a Stable Photocatalyst for Water Oxidation in Visible Light (

New Precursor Route Using a Compositionally Flexible Layered Oxide

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