Protonated Oxide, Nitrided, and Reoxidized K2La2Ti3O10 Crystals

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Protonated Oxide, Nitrided, and Re-oxidized K2La2Ti3O10 Crystals: Visible-Light-Induced Photocatalytic Water Oxidation and Fabrication of Their Nanosheets Kenta Kawashima, Mirabbos Hojamberdiev, Hajime Wagata, Kunio Yubuta, Kazunari Domen, and Katsuya Teshima ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01344 • Publication Date (Web): 26 Oct 2016 Downloaded from http://pubs.acs.org on October 30, 2016

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ACS Sustainable Chemistry & Engineering

Protonated Oxide, Nitrided, and Re-oxidized K2La2Ti3O10 Crystals: Visible-Light-Induced Photocatalytic Water Oxidation and Fabrication of Their Nanosheets

Kenta Kawashima1,2, Mirabbos Hojamberdiev1, Hajime Wagata1, Kunio Yubuta3, Kazunari Domen4, and Katsuya Teshima1,5,*

1

Department of Environmental Science and Technology, Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan

2

McKetta Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712, United States

3

Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan

4

Department of Chemical System Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

5

Center for Energy and Environmental Science, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan

1

ACS Paragon Plus Environment


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ABSTRACT: Protonated lanthanum titanium oxide H2La2Ti3O10 and oxynitride H2La2Ti3O10-3/2xNx crystals were synthesized from the oxide, nitrided, and re-oxidized layered K2La2Ti3O10 crystals prepared by solid state reaction through proton exchange. Here, we investigated the holding time of nitridation of oxide K2La2Ti3O10 crystals influencing their crystal structure, shape, and absorption wavelength and band-gap energy. The XRD and SEM results confirmed that the crystal structure and plate-like shape of the parent oxide were maintained after nitridation at 800°C for 10 h and the color of crystals was changed from white to dark-green. However, no clear absorption edges were observed in the UV-Vis diffuse reflectance spectra of the nitrided crystals due mainly to the reduced titanium species (Ti3+), which act as the recombination center of the photo-generated charge carriers. To decrease the amount of the reduced titanium species, the nitrided crystals were further re-oxidized at 400°C for 6 h. After partial re-oxidation, the absorption intensity in the longer wavelength region was reduced and the absorption edges appeared at about 449‒460 nm. The photocatalytic activity for water oxidation half-reaction was evaluated only for the protonated samples. The protonated re-oxidized K2La2Ti3O10 crystals showed the O2 evolution rate of 180 nmol·h-1 (for the photocatalytic water oxidation) under visible light irradiation, and the unexpected photocatalytic decomposition of N2O adsorbed onto the photocatalyst surfaces was observed for the protonated oxide and protonated nitrided layered K2La2Ti3O10 crystals. Furthermore, lanthanum titanium oxide [La2Ti3O10]2– and oxynitride [La2Ti3O10-3/2xNx]2– nanosheets were successfully fabricated by proton exchange and mechanical exfoliation (sonication) of the oxide, nitrided, and re-oxidized K2La2Ti3O10 crystals. The TEM results revealed that the lateral sizes of the fabricated nanosheets grown along the direction are 270‒620 nm. Apparently, the colloidal suspensions of the fabricated nanosheets showed a Tyndall effect, implying their good dispersion and stability for several weeks in water.

2


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KEYWORDS: Layered structure; Water oxidation; Oxynitride; Perovskite; Nanosheet; Visible light

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INTRODUCTION Since the first report of Honda-Fujishima effect,1 various types of photocatalytic materials have been researched for water and air purification, antifouling, antifogging, antibacterial, sterilization, and hydrogen production. Recently, layered perovskite oxides, such as K2La2Ti3O10,2 ABi2Nb2O9 (A = Ca, Sr, and Ba),3 RbLnTa2O7 (Ln = La, Pr, Nd, and Sm),4 and ABi2Ta2O9 (A = Ca, Sr, and Ba),5 were studied as photocatalysts for water splitting to generate hydrogen by utilizing solar energy. Moreover, their derivatives can easily be designed by exchanging the cations of the interlayer with other cations.6 The layered Ruddlesden-Popper phase K2La2Ti3O10 having triple perovskite layers made up of octahedral TiO6 is a tetragonal compound with space group I4/mmm and lattice parameters of a = b = 3.8767(1) Å, c = 29.824(1) Å, and α = β = γ = 90° (Figure 1, the crystal structure was drawn using VESTA 3).7-9 Having a band-gap energy (Eg) of about 3.5 eV,2,6 K2La2Ti3O10 has shown photoluminescence,10 up-conversion luminescence,11 and photocatalytic properties.2 Particularly, cocatalyst-loaded

K2La2Ti3O10

(Ni‒K2La2Ti3O10

and

Cr‒Ni‒K2La2Ti3O10)

showed

high

photocatalytic activity for overall water splitting under ultraviolet (UV) light irradiation.2,6,12 The ultimate aim of the study of photocatalytic water splitting is to achieve efficient energy conversion from solar energy to chemical energy (H2) by using visible-light-active photocatalysts. Theoretically, the sun light (λ < 1100 nm) can be utilized for photocatalytic water splitting reaction.13 As K2La2Ti3O10 has a wide band-gap energy of 3.5 eV, it can only be excited under UV 4


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light (λ < 350 nm).2,6 To improve its photocatalytic activity by extending its response to visible light region, K2La2Ti3O10 was doped with different cations and anions, i.e., K2−xLa2Ti3−xNbxO10 (0 ⩽ x ⩽ 1),14 zinc- and vanadium-doped K2La2Ti3O10,15,16 K2La2Ti3-xMxO10+δ (M = Fe, Ni, and W),17 nitrogen-doped K2La2Ti3O10,18 and Sn2+ and N3--substituted K2La2Ti3O10),19 to engineer the band-gap of K2La2Ti3O10. Furthermore, K2La2Ti3O10 was also composited with narrow band-gap semiconductors, such as CdS,20 ZnIn2S4,21 and BiOBr,22 to improve its photocatalytic activity under visible light irrigation. Recently, the protonated layered compounds have also shown an enhanced photocatalytic water splitting activity compared with non-protonated counterparts due to the hydration of the interlayer cations and molecules.23 Protonated K2La2Ti3O10 (H2La2Ti3O10), which was composited with other semiconductors, such as TiO2, Fe2O3, and CdS, has also exhibited an enhanced photocatalytic activity because of the effective charge separation achieved by the coupling of two semiconductors having different band-gap energy levels and the short travel distance of the photo-generated charge carriers, diffusing to reach the interface.24-26 Although the nitrogen-doped K2La2Ti3O10 crystals have been fabricated by immersing in aqueous NH3 solution or heating with urea,18,19 the subsequent nitridation of the K2La2Ti3O10 crystals under an NH3 flow at high temperature in order to form the oxynitride crystals of K2La2Ti3O10-3/2xNx has not been reported yet. In this study, we have synthesized K2La2Ti3O10 crystals by solid-state reaction9 and investigated the nitridation behavior of K2La2Ti3O10 crystals 5



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under an NH3 flow at 800 °C for different times. Also, the nitrided K2La2Ti3O10 (K2La2Ti3O10-3/2xNx) crystals were further re-oxidized in order to decrease the excess amount of reduced species of Ti3+ to Ti4+. Here, we also discuss the effect of protonation on photocatalytic activity of the oxide, nitrided, and re-oxidized K2La2Ti3O10 crystals for water oxidation activity under visible light, and the exfoliation of the oxide, nitrided, and re-oxidized K2La2Ti3O10 crystals into their nanosheets.

Figure 1. Schematic illustration of the crystal structure of K2La2Ti3O10. EXPERIMENTAL Growth of K2La2Ti3O10 crystals. The layered K2La2Ti3O10 crystals were grown by a solid-state reaction using reagent-grade KNO3, La2O3, and TiO2 (> 98%, Wako Pure Chemical Industries, Ltd.) according to the experimental procedure reported previously elsewhere.9 A stoichiometric mixture of KNO3, La2O3, and TiO2 were dry mixed manually using an agate mortar and pestle for 15 min. An excess amount of KNO3 (25 mol%) was added to compensate potassium volatilized at 6


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high temperature. The mixture (10 g) was placed in a platinum crucible with a capacity of 30 cm3, and closed loosely with a platinum lid. The mixture-containing platinum crucible was heated at 1000°C for 6 h at a heating rate of 50°C·h-1, cooled to 500°C at a cooling rate of 150°C·h-1, and then cooled naturally to room temperature. Preparation of the protonated forms of oxide, nitrided, and re-oxidized K2La2Ti3O10 crystals. To obtain the layered oxynitride crystals, 0.5 g of the K2La2Ti3O10 crystals grown by a solid-state reaction was placed on an alumina plate and heated at 800°C for 3, 5, 7 and 10 h at a heating rate of 600°C·h-1 under an NH3 flow (200 mL·min-1) in a horizontal tubular furnace and cooled naturally to room temperature. The K2La2Ti3O10 crystals nitrided at 800°C for 10 h were then re-oxidized at 400°C for 6 h at a heating rate of 600°C·h-1. The protonated forms of oxide, nitrided, and re-oxidized K2La2Ti3O10 crystals were obtained by immersing 0.25 g of crystals in 50 mL of 0.1 M HCl solution for 7 days, washing with deionized water several times, and drying at room temperature. Fabrication of nanosheets. The nanosheets of oxide, nitrided (10 h), and re-oxidized K2La2Ti3O10 crystals were fabricated through a proton exchange process by immersing 0.25 g of crystals in 50 mL of 0.1 M HCl solution for 7 days, washing with deionized water several times, and drying at room temperature and exfoliation by sonicating 0.01 g of the protonated crystals in 5 mL of deionized water.

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Photocatalytic water oxidation activity test. The photocatalytic O2 evolution reactions were carried out with 100 mg of protonated oxide, nitrided or re-oxidized K2La2Ti3O10 crystals and 200 mg of La2O3 (pH buffer) in 300 mL of 10 mM AgNO3 (> 99%, Wako Pure Chemical Industries, Ltd.) aqueous solution (sacrificial electron scavenger) in a glass cell connected to a closed-circulation system under visible light irradiation (300W Xe lamp with a λ > 420 nm cutoff filter and cold mirror), and the light intensity was 200 mW·cm-2. The evolved gases were detected by a gas chromatograph (GC-8A, TCD, Ar gas carrier, Shimadzu). Characterization. The X-ray diffraction (XRD) patterns of crystal samples were recorded on a MiniflexII X-ray diffractometer (Rigaku). The crystal morphology was observed by using a JSM-7600F scanning electron microscope (SEM, JEOL). The crystal orientations of the fabricated nanosheets were elucidated by an EM-002B high-resolution transmission electron microscope (HR-TEM, TOPCON) at an acceleration voltage of 200 kV. The ultraviolet‒visible (UV‒Vis) diffuse reflectance spectra of crystal samples were measured using a V-670 spectrophotometer (JASCO). Their absorption edges and band-gap energies were estimated from the UV‒Vis diffuse reflectance spectra by using Kubelka‒Munk theory. The re-oxidation temperature of K2La2Ti3O10 crystals nitrided at 800°C for 10 h was determined by simultaneous thermogravimetry and differential thermal analysis (TG-DTA, Thermo plus EVO2, Rigaku) by heating 0.02 g of sample from room temperature up to 1200°C at a heating rate of 10°C·min-1 under synthetic air flow. The surface chemical compositions of the protonated samples were analyzed by X-ray photoelectron 8


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spectroscopy (XPS, JPS-9010MC, JEOL) using a non-monochromatic Mg Kα X-ray source. The XPS profiles were fitted using a Gaussian–Lorentzian function, and the peak positions were normalized by positioning the C 1s peak at 284.5 eV. RESULTS AND DISCUSSION Figure 2 shows the XRD patterns of the layered K2La2Ti3O10 crystals synthesized by a solid state reaction and K2La2Ti3O10 crystals nitrided at 800°C for different holding times (3, 5, 7, and 10 h) under an NH3 flow (200 mL·min-1). The XRD pattern of the oxide crystals is identified as an anhydrous K2La2Ti3O10 phase (ICDD PDF 81-1167) with minor unknown phase. The XRD patterns of crystals nitrided at 800°C for 3 and 5 h contain diffraction peaks assignable to K2La2Ti3O10 and K2La2Ti3O10·1.6H2O (ICDD PDF 82-0208). It is thought that the formation of the layered K2La2Ti3O10·1.6H2O as an impurity phase may possibly be stemmed from the insertion of water molecules, which were formed during the nitridation process, into the interlayer spacing of the layered K2La2Ti3O10, resulting in the expansion of c-axis, as described below:6,8 K2La2Ti3O10 + 1/2xN2 + 3/2xH2 → K2La2Ti3O10-3/2xNx + 3/2xH2O

(1)

K2La2Ti3O10-3/2xNx + 1.6H2O → K2La2Ti3O10-3/2xNx·1.6H2O

(2)

In contrast, the XRD patterns of crystals nitrided at 800°C for 7 and 10 h have the diffraction peaks matching with those of anhydrous K2La2Ti3O10 and unknown phase. Interestingly, the K2La2Ti3O10·1.6H2O phase completely disappeared after prolonging the nitridation time to 7 and 10 h, implying that most of the interlayer water was evaporated. Compared to that of the 9



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as-synthesized K2La2Ti3O10 crystals, the 002 diffraction peak at 5.92° of the K2La2Ti3O10 crystals nitrided at 800°C for 7 and 10 h shifted toward lower 2θ angle due to the small amount of remaining interlayer water molecules. In the XRD patterns of crystals nitrided for 3 and 5 h, the 110 diffraction peak at 32.64° of the K2La2Ti3O10 crystals slightly shifted to higher 2θ angle compared to that of the as-synthesized K2La2Ti3O10 crystals, which might be associated with the superposition of the 110 diffraction peaks at 32.64 and 32.80° from the K2La2Ti3O10 and K2La2Ti3O10·1.6H2O crystals, respectively.27 On the other hand, the 110 diffraction peak in the XRD patterns of crystals nitrided for 7 and 10 h slightly shifted to lower 2θ angle owing to the expansion of the lattice volume toward a- and b-axes. As the crystal radius of N3– (157 pm) is larger than that of O2– (126 pm), this increase of the lattice volume may be caused by the presence of partially substituted nitrogen at oxygen sites as well as the insertion of nitrogen at interstitial sites in the lattice of triple perovskite sheets ([La2Ti3O10]2–).28,29 It should be noted that the layered structure of K2La2Ti3O10 was maintained even after high-temperature nitridaiton for longer period.

10


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K2La2Ti3O101.6H2O

? unknown

(e)

(e)

?

Intensity (arb. units)

?

(c) (b) (a) ?

(c) (b) (a)

0 . 3 3

2θ / degree

8 . 2 3

6 . 2 3

4 . 2 3

0 6

0 5

0 4

0 3

0 2

K2La2Ti3O10 ICDD PDF # 82-0209

(d)

110


(d)

0 1 5

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2θ / degree

Figure 2. XRD patterns of the K2La2Ti3O10 crystals synthesized by solid state reaction (a) and the K2La2Ti3O10 crystals nitrided at 800ºC for 3 (b), 5 (c), 7 (d), and 10 h (e). The SEM images of the as-synthesized and nitrided K2La2Ti3O10 crystals are shown in Figure 3. As shown, the as-synthesized oxide crystals have a platelet shape and irregular lateral size ranging from 0.8 to 5.8 µm (with thickness of approximately 150‒230 nm). After nitridation, the nitrided crystals maintained the shape of oxide crystals, indicating that the K2La2Ti3O10 crystals are stable under high-temperature nitridation for long period.

11



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Figure 3. SEM images of the K2La2Ti3O10 crystals synthesized by solid state reaction (a) and the K2La2Ti3O10 crystals nitrided at 800ºC for 3 (b), 5 (c), 7 (d), and 10 h (e).

Figure 4a shows the UV–Vis diffuse reflectance spectra of the as-synthesized and nitrided K2La2Ti3O10 crystals. The color of the as-synthesized K2La2Ti3O10 crystals is pale-yellow with an absorption edge wavelength at about 364 nm and a narrower band-gap energy of about 3.41 eV compared with that of the previously reported K2La2Ti3O10 crystals (Eg = 3.63‒3.69 eV).18,19 This difference is possibly resulted from the incorporation of a small amount of nitrogen into the lattice during solid-state synthesis of K2La2Ti3O10. After nitridation, the absorption of crystals presumably 12


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shifted upward in the visible-light region (λ = 350‒800 nm), their absorption edge eventually disappeared, and the color gradually altered from pale-yellow to dark-green with increasing the nitridation time. This upward shift in the visible-light region and color change can be explained by the following three reasons: (i) negative shift of the valence band due to the partial substitution of oxygen by nitrogen forming the hybrid orbitals of O 2p and N 2p; (ii) absorption of interstitial nitrogen impurities having an isolated N 2p narrow band above the O 2p valence band in the forbidden band of K2La2Ti3O10-3/2xNx, and (iii) absorption of the d-d transition of the reduced Ti3+ species (oxygen vacancies) caused by nitrogen substitution.18,19,29-31 For instance, K2La2Ti3O10 crystals nitrided for 3 h has a broadened peak at 550 nm due to the absorption of interstitial nitrogen,30 and the absorption intensity in longer wavelength (λ = 600‒800 nm) became higher due to the d-d transition of the reduced Ti3+ species in

K2La2Ti3O10 crystals nitrided for 5, 7, and 10 h.

As known, the defects such as the reduced Ti3+ species may act as a recombination center for the photogenerated electrons and holes, causing a decrease in the photocatalytic activity.31

13



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Figure 4. (a) UV–Vis diffuse reflectance spectra of the K2La2Ti3O10 crystals synthesized by solid state reaction (blue) and the K2La2Ti3O10 crystals nitrided at 800ºC for 3 (green), 5 (orange), 7 (purple), and 10 h (red). (b) TG-DTA curves of the K2La2Ti3O10 crystals nitrided at 800ºC for 10 h.

To determine the optimum partial re-oxidation temperature for oxidizing Ti3+ into Ti4+ without destroying the layered structure of K2La2Ti3O10-3/2xNx, the re-oxidation behavior of the K2La2Ti3O10 crystals nitrided at 800°C for 10 h under an NH3 flow was studied by TG-DTA (Figure 4b) from 25 to 1300°C in synthetic air. After TG-DTA analysis, the color of the K2La2Ti3O10-3/2xNx crystals changed from dark-green to white because of re-oxidation.32 The weight loss from 25 to 300°C corresponds to the desorption of 0.56 wt% adsorbed water on the crystal surface (25‒100°C) and release of 0.45 wt% interlayer water (100‒300°C).8 A significant weight loss and an endothermic peak observed in the range of 300‒350°C can be attributed to the desorption of water contained in the oxide matrix of [La2Ti3O10]2‒.33 The weight change from 350 to 940°C is associated with the transformation from oxynitride to oxide (i.e. re-substitution of 3O2‒ for 2N3‒) through two processes:

(i)

the

formation

of

the

dinitrogen-containing

intermediate

compound

(K2La2Ti3O10(N2)1/2x) and oxidation of Ti3+ into Ti4+ (350‒570°C) and (ii) the release of dinitrogen (N2) gas from the K2La2Ti3O10(N2)1/2x crystals (570‒940°C), respectively:32 K2La2Ti3O10-3/2xNx + 3/4xO2 → K2La2Ti3O10(N2)1/2x

(3)

K2La2Ti3O10(N2)1/2x → K2La2Ti3O10 + 1/2xN2

(4) 14


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From 940 to 1300°C, another weight loss of 1.4 wt% and an exothermic peak are noted because of the partial decomposition of K2La2Ti3O10 to K2O and La2Ti3O9.9 From the TG-DTA results, 400°C was found to be an optimum partial re-oxidation temperature. Figure 5 shows the XRD patterns of the nitrided K2La2Ti3O10 crystals partially re-oxidized at 400°C for 6 h. For K2La2Ti3O10 crystals nitrided at 800°C for 3 and 5 h, the XRD patterns of the partially re-oxidized crystals are identified as K2La2Ti3O10 and K2La2Ti3O10·1.6H2O phase. After partial re-oxidation, the peak intensities of the (001) plane of the layered K2La2Ti3O10·1.6H2O crystals at 5.25° decreased owing to the release of interlayer water partially converting K2La2Ti3O10·1.6H2O into K2La2Ti3O10. In addition, the diffraction peaks of the (110) plane of the K2La2Ti3O10 crystals at 32.64° slightly shifted to lower 2θ angle compared to that of the as-nitrided K2La2Ti3O10 crystals probably because of the elimination of the superposition of the 110 diffraction peaks of the K2La2Ti3O10 and K2La2Ti3O10·1.6H2O crystals. In the case of K2La2Ti3O10 crystals nitrided at 800°C for 7 and 10 h, the XRD patterns of the partially re-oxidized crystals can be well-indexed as K2La2Ti3O10 without any diffraction peaks of impurity phases. After partial re-oxidation, the diffraction peaks of the (110) plane of K2La2Ti3O10 crystals at 32.64° in the XRD patterns slightly shifted to higher 2θ angle compared to that of the K2La2Ti3O10 crystals nitrided for 7 and 10 h (parents) because of the decrease in the lattice volume. The possible reason for the decrease in the lattice volume is a partial release of interstitial and substantial nitrogen existing in the [La2Ti3O10]2– lattice during partial re-oxidation process. The XRD results indicate that the 15



nitrogen introduced into the layered structure of K2La2Ti3O10-3/2xNx is maintained even after re-oxidation. K2La2Ti3O101.6H2O

? unknown

(b) (a)

(c) (b) (a)

K2La2Ti3O10 ICDD PDF # 82-0209

0 . 3 3

2θ / degree

8 . 2 3

6 . 2 3

4 . 2 3

0 6

0 5

0 4

0 3

0 2

110

(c)

(d)



(d)

0 1 5

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2θ / degree

Figure 5. XRD patterns of the re-oxidized K2La2Ti3O10 crystals nitrided at 800ºC for 3 (a), 5 (b), 7 (c), and 10 h (d).

16


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Figure 6. SEM images of the re-oxidized K2La2Ti3O10 crystals nitrided at 800ºC for 3 (a), 5 (b), 7 (c), and 10 h (d).

Figure 7. UV–Vis diffuse reflectance spectra of the K2La2Ti3O10 crystals synthesized by solid state reaction (blue) and the re-oxidized K2La2Ti3O10 crystals nitrided at 800ºC for 3 (green), 5 (orange), 7 (purple), and 10 h (red). 17



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Figure 6 shows the SEM images of the nitrided K2La2Ti3O10 crystals partially re-oxidized at 400°C for 6 h. As shown, the platelet shape and size of the K2La2Ti3O10-3/2xNx crystals are maintained without any significant changes after partial re-oxidation. However, the surfaces of the re-oxidized crystals became rougher due to the introduction and release of nitrogen during the nitridation and re-oxidation processes. The UV–Vis diffuse reflectance spectra of the partially re-oxidized K2La2Ti3O10-3/2xNx crystals are shown in Figure 7. As compared to the UV-Vis spectra of the nitrided K2La2Ti3O10 crystals, the absorption intensity of the partially re-oxidized K2La2Ti3O10-3/2xNx crystals was reduced in the longer wavelength region (λ = 440‒800 nm) and the color significantly changed from dark-green to yellow. The reduction in the absorption intensity in longer wavelength and color change are due to the re-oxidation of Ti3+ into Ti4+. The clear absorption edges of the partially re-oxidized crystals are found to be at about 449‒460 nm. Hence, the narrower band-gap energies of the partially re-oxidized crystals are approximately 2.70‒2.76 eV compared to that of the as-synthesized K2La2Ti3O10 crystals (Eg = 3.41 eV). The reduction in the optical band-gap was resulted from the negative shift of the valence band by partial substitution of nitrogen. Furthermore, the band-gap energies of the partially re-oxidized K2La2Ti3O10-3/2xNx crystals in this study are narrower than that of the nitrogen-doped K2La2Ti3O10 crystals (Eg = 3.44‒3.59 eV) reported previously.18,19 Presumably, this noticeable difference is accounted for by moving the valence band more upward because of the higher amount of nitrogen introduced in the K2La2Ti3O10 crystals. It is thought that 18


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high-temperature nitridation under an NH3 flow is more effective than immersing in aqueous NH3 solution or heating with urea to introduce higher amount of nitrogen into the K2La2Ti3O10 crystals. Figure 8 shows the TEM and lattice images, selected-area electron diffraction (SAED) patterns, and colloidal suspensions of the oxide, nitrided, and re-oxidized nanosheets fabricated thorough the proton exchange and exfoliation of the as-synthesized, nitrided (at 800°C for 10 h), and re-oxidized (at 400°C for 6 h) K2La2Ti3O10 crystals. As mentioned above, the K2La2Ti3O10 crystals have a platelet shape and irregular lateral size ranging from 0.8 to 5.8 µm (with thickness of approximately 150‒230 nm). According to the TEM results, the reduction in the lateral size of the fabricated nanosheets was observed (270‒620 nm) due to the mechanical shear stress under sonication.34 As a single set of diffraction spots in the SAED patterns was observed for the fabricated nanosheets, and no grain boundaries were found in their lattice images, evidencing that the fabricated nanosheets have a single-crystalline nature. Indexing of the SAED patterns obtained with the incident beam along the [001] zone axis direction reveals that the diffraction spots correspond to the (200), (110), (020), and (1-10) faces, indicating that the fabricated nanosheets were grown along the direction. The in-plane surfaces of the fabricated nanosheets lie on the (001) plane, and the side surfaces are the (100), (010), and (110) planes. As shown in Figure 8, all the suspensions of the fabricated nanosheets clearly showed a Tyndall effect, suggesting that the fabricated nanosheets have a good dispersion and are stable in water for several weeks.35

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Figure 8. TEM and HRTEM images, SAED patterns and the colloidal suspensions of the oxide (a), nitrided (b), and re-oxidized nanosheets (c) fabricated through proton exchange and exfoliation processes.

Figure 9 shows the XRD patterns of the protonated oxide, nitrided and re-oxidized K2La2Ti3O10 crystals. The XRD patterns of all the protonated oxide, nitrided, and re-oxidized crystals are identified as a H2La2Ti3O10 phase (ICDD PDF 48-0983) with minor unknown phase. This confirms that the interlayer K+ ions of the oxide, nitrided and re-oxidized K2La2Ti3O10 crystals were successfully exchanged with H+ ions during proton exchange. The structure of H2La2Ti3O10 in a layered perovskite-type is closely related to that of parent K2La2Ti3O10, and the space group of 20


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H2La2Ti3O10 crystal structure is the same as that of K2La2Ti3O10 (I4/mmm).9 However, the diffraction peak of the (002) plane of H2La2Ti3O10 shifted to lower 2θ angle compared to that of K2La2Ti3O10 because of the expansion of c-axis. An increase in the c-parameter is due to the presence of weaker interactions (hydrogen bonds) between the interlayers of [La2Ti3O10]2– unit layers in H2La2Ti3O10.25

Figure 9. XRD patterns and SEM images of the protonated oxide (a), nitrided (b) and re-oxidized K2La2Ti3O10 crystals (c).

Figure 9 shows the SEM images of the protonated oxide, nitrided and re-oxidized K2La2Ti3O10 crystals. After proton exchange, all the protonated crystals maintained the platelet shape of their parent oxide, nitrided and re-oxidized K2La2Ti3O10 crystals. The distribution of elements and 21



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chemical compositions of the protonated oxide, nitrided and re-oxidized K2La2Ti3O10 crystals were analyzed by energy-dispersive X-ray spectroscopy (EDS) and are shown in Figures S1 and S2 in the SI. The EDS elemental mapping images clearly show that the La, Ti, O, and N elements are uniformly distributed throughout the protonated oxide, nitrided, and re-oxidized crystals, implying the absence of any impurities. The EDS spectra shown in Figure S2 in the SI confirm that the La and Ti elements are present with nearly stoichiometric compositions in H2La2Ti3O10 and H2La2Ti3O10-3/2xNx, and potassium is present with a low concentration, indicating successful K+/H+ ion exchange. The La 3d and 4d, Ti 2p, O 1s, and N 1s XPS core-level spectra of the protonated oxide, nitrided and re-oxidized K2La2Ti3O10 crystals are shown in Figure S3. The binding energies of La 3d (835.0 and 839.1 eV for La 3d5/2, and 851.9 and 856.0 eV for La 4d3/2) for the protonated oxide, nitrided and re-oxidized K2La2Ti3O10 crystals indicate the presence of La3+.36 In all the protonated crystals, three components are required to fit the La 4d peak. The high intensity peaks at 102.5 and 105.7 eV are associated with La 3d5/2 and La 4d3/2, respectively and the low intensity broad peak at around 109.4 eV probably corresponds to a bonding satellite for La 4d3/2.36 The Ti 2p XPS core-level spectra of the protonated oxide, nitrided and re-oxidized K2La2Ti3O10 crystals showed the Ti 2p3/2 and Ti 2p1/2 peaks at around 458.7 and 464.4 eV, respectively, which can be recognized as the Ti4+ cations of the O ‒ Ti bond.37 Here, the peaks at 455.7 and 462.2 eV in the Ti 2p XPS core-level spectra of the protonated nitrided K2La2Ti3O10 crystals are also originated from Ti 2p3/2 22


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and Ti 2p1/2, which can be attributed to the reduced Ti species, such as Ti3+ cations of the O ‒ Ti ‒ N bond.37 This difference indicates that only the protonated nitrided K2La2Ti3O10 crystals may contain oxygen vacancies on the crystal surface. In the O 1s spectra of all the protonated crystals, the two components centered at around 530.7 and 532.7 eV can be assigned to the O2‒ anions (O ‒ Ti linkage) and carbonate-like species (O ‒ C linkage), respectively.38 Regarding the N 1s XPS core-level spectra of all the protonated crystals, the deconvoluted peaks can be classified into four: (i) the substituted nitrogen of the O ‒ Ti ‒ N bond (around 396.8 eV), (ii) surface-adsorbed NH2 (399.5 eV), (iii) surface-adsorbed NHx bond (400.3 eV), and (iv) nitrogen species of the Ti – O – N or Ti – N – O bonds or surface-adsorbed NOx (400.8 eV), respectively.19,39‒41 The XPS results confirmed that the protonated nitrided K2La2Ti3O10 crystals contain higher amount of the substituted nitrogen compared to the protonated oxide and protonated re-oxidized K2La2Ti3O10 crystals. The photocatalytic water oxidation activities of the protonated oxide, nitrided and re-oxidized K2La2Ti3O10 crystals were evaluated under visible light irradiation (λ > 420 nm) in an aqueous AgNO3 solution, and the reaction time courses of O2 evolution are plotted in Figure 10. It is worth to note that this is the first report on the photocatalytic sacrificial O2 evolution over layered H2La2Ti3O10 and H2La2Ti3O10-3/2xNx crystals. After 5 h of the photocatalytic water oxidation half-reactions with the protonated oxide, nitrided and re-oxidized K2La2Ti3O10 crystals, the total amounts of evolved gases are 21.5, 16.7, and 2.7 µmol for O2 gas and 40.6, 33.8, and 3.6 µmol for 23



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N2 gas, respectively. Even though the protonated oxide K2La2Ti3O10 crystals with an absorption edge wavelength at about 364 nm could not act as a photocatalyst under the present conditions, and the protonated nitrided K2La2Ti3O10 crystals having the greater amount of reduced titanium species (Ti3+), which act as the recombination center for the photo-generated charge carriers, should have lower photocatalytic activity,42 they have exhibited the O2 and N2 gas evolution after 5 h of the reactions. First, the partial photolysis of AgNO3 occurred immediately, and silver nanoparticles were formed on the surfaces of the protonated crystals under light irradiation because AgNO3 has a photosensitivity.43 According to the previous report, the TiO2 crystals coated with silver nanoparticles have exhibited the narrower bang-gap energy (2.75 eV) compared to the pure TiO2 crystals.44 Therefore, the protonated oxide Ag/H2La2Ti3O10 is also expected to show a narrower band-gap energy. Furthermore, the photocatalytic activity of the protonated nitrided K2La2Ti3O10 crystals with silver nanoparticles can be improved due to the effective charge separation between H2La2Ti3O10-3/2xNx and Ag nanoparticles.45 Interestingly, the ratio of the evolved N2/O2 gases for the protonated oxide and protonated nitrided K2La2Ti3O10 crystals were found to be 1.9 and 2.0, respectively. It is known that the photocatalytic decomposition of nitrous oxide (N2O) can take place in the presence of silver.46,47 From the XPS results, it was confirmed that the adsorbed NOx species existed on the surfaces of all the protonated crystals. Therefore, it is believed that regarding the protonated oxide and nitrided K2La2Ti3O10 crystals, the entire reactions can mainly be assigned as the photocatalytic decomposition of N2O, as given by the following reaction: 24


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2N2O → 2N2↑ + O2↑ After 30 min of the reaction, the O2 and N2 gas evolution rate gradually decreased due possibly to the coverage of the surfaces of the photocatalysts with silver nanoparticles with increasing the reaction time.46 In contrast, the ratio of the evolved N2/O2 gases for the protonated re-oxidized K2La2Ti3O10 crystals was 1.3, indicating that its gas evolution process is different from those of the protonated oxide and protonated nitrided K2La2Ti3O10 crystals. As the protonated re-oxidized K2La2Ti3O10 crystals have the higher absorption edge wavelength at about 460 nm and lesser amount of Ti3+, the photocatalytic O2 evolution reaction for this sample can be expressed as below: 4AgNO3 + 2H2O → 4Ag + O2↑ + 4HNO3 Meantime, the partial photolysis of AgNO3 and the photocatalytic decomposition of N2O are also expected to occur. However, the amounts of evolved O2 and N2 gases of the protonated re-oxidized K2La2Ti3O10 crystals were much lower than those of the protonated oxide and protonated nitrided K2La2Ti3O10 crystals. This difference is presumably related to the deposition of more silver nanoparticles on the photocatalyst surfaces in early period of the reaction owing to the acceleration of the deposition rate of silver nanoparticles by the photocatalytic water oxidation reaction. However, the total amount of evolved O2 gas (2.7 µmol) of the protonated re-oxidized K2La2Ti3O10 crystals was contributed by the photocatalytic water oxidation (0.9 µmol) and the photocatalytic decomposition of N2O (1.8 µmol). It was already reported that the LaTiO2N crystals with a visible 25



light absorption edge of 600 nm could not show a high photocatalytic activity under the near bandgap light irradiation (λ = 580 nm).47 Thus, the protonated re-oxidized K2La2Ti3O10 crystals with an absorption edge of 460 nm could not show high amount of evolved O2 gas from the photocatalytic water oxidation half-reaction under the light irradiation used in this study (λ > 420 nm). From the above results, it is clear that only the protonated re-oxidized K2La2Ti3O10 crystals is active during the photocatalytic water oxidation half-reaction because of the modified band structure (Eg = 2.70 eV) and less defects associated with reduced titanium species and anion vacancy.

0 1 N O T L H

_

_

(b)

_

(c)

5

4

3

2

1

5

4

3

2

1

0 0

Time / h

Time / h

Figure 10. Reaction time courses for O2 and N2 evolution from aqueous AgNO3 solution over the protonated oxide (a), nitrided (b) and re-oxidized K2La2Ti3O10 crystals (c). CONCLUSIONS

26


o e r

0 1 N O T L H

0 1 N O T L H

(c)

0 0 1 1 N N O O ToT Le L Hr H

(a)

0 1 N O T L H

5

(b)

0 1 O T L H

0 1 O T L H

0 1 O T L H

0 1

N2 gas evolved / µmol

5 1

O2 gas evolved / µmol

0 2 (a)

o 0 e r 5 0 5 0 5 0 5 0 5 0 4 4 3 3 2 2 1 1

5 2

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In

summary,

protonated

lanthanum

titanium

oxide

H2La2Ti3O10

and

oxynitride

H2La2Ti3O10-3/2xNx crystals were synthesized from the oxide, nitrided, and re-oxidized layered K2La2Ti3O10 crystals. Although partial nitridation and re-oxidation resulted in a noticeable change in absorption wavelength and band-gap energy of K2La2Ti3O10 crystals, their crystal structure and morphology were maintained. Also, the oxide [La2Ti3O10]2– and oxynitride [La2Ti3O10-3/2xNx]2– nanosheets were fabricated from the oxide, nitrided, and re-oxidized K2La2Ti3O10 crystals through proton exchange and mechanical exfoliation. Among three protonated samples (oxide, nitrided, and re-oxidized), the protonated re-oxidized K2La2Ti3O10 crystals exhibited high stability and O2 evolution rate of 180 nmol·h-1 after 5 h of the photocatalytic water oxidation half-reaction. Unexpectedly, the protonated oxide and protonated nitrided K2La2Ti3O10 crystals did not exhibit any photocatalytic activity for O2 evolution due to the photolysis of AgNO3 and photo-decomposition of N2O during the reaction. Introducing nitrogen into the crystal lattice and partial re-oxidizing of Ti3+ were found to be effective for enhancing the photocatalytic water oxidation activity of the layered K2La2Ti3O10 crystals.

AUTHOR INFORMATION Corresponding Author *Phone: +81-26-269-5541. Fax: +81-26-269-5550. E-mail: [email protected]

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ACKNOWLEDGMENT This research was supported in part by the Japan Technological Research Association of Artificial Photosynthetic Chemical Process (ARPChem).

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxxxx. The EDS element mapping images, EDS spectra and XPS core-level spectra of the protonated oxide, nitrided and re-oxidized K2La2Ti3O10 crystals.

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The protonated oxide, nitrided, and re-oxidized layered K2La2Ti3O10 (H2La2Ti3O10 and H2La2Ti3O10-3/2xNx) are studied for sustainable production of hydrogen from water splitting.

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Protonated Oxide, Nitrided, and Reoxidized K2La2Ti3O10 Crystals

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