NH3-Assisted Flux-Mediated Direct Growth of LaTiO2N Crystallites for

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An NH3-Assisted Flux-Mediated Direct Growth of LaTiO2N Crystallites for Visible-Light-Induced Water Splitting Kenta Kawashima, Mirabbos Hojamberdiev, Hajime Wagata, Kunio Yubuta, Junie Jhon M. Vequizo, Akira Yamakata, Shuji Oishi, Kazunari Domen, and Katsuya Teshima J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b03718 • Publication Date (Web): 15 Jun 2015 Downloaded from http://pubs.acs.org on June 25, 2015

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The Journal of Physical Chemistry

An NH3-Assisted Flux-Mediated Direct Growth of LaTiO2N Crystallites for Visible-Light-Induced Water Splitting Kenta Kawashima1, Mirabbos Hojamberdiev1, Hajime Wagata1, Kunio Yubuta2, Junie Jhon M. Vequizo3, Akira Yamakata3, Shuji Oishi1, 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

3

Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

Graduate School of Engineering, Toyota Technological Institute, 2-12-1 Hisakata, Tempaku, Nagoya 468-8511, 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: Photocatalytic overall water splitting on (oxy)nitrides under visible light is one of the interesting approaches to fulfill the growing demand for clean and renewable energy. The improvement of the fabrication method is however important for reducing the defect density of (oxy)nitride crystals. The present study aims to investigate the direct growth of the LaTiO2N (LTON) crystallites with less defect density by an NH3-assisted flux method and to demonstrate the visible-light-induced photocatalytic water oxidation activity in relation to their crystallite morphology. Single-phase LaTiO2N crystallites (average size of 120 ± 39 nm) in round shape with smooth surface and high crystallinity were grown by an NH3-assisted flux method using the KCl flux with the solute concentration of 5 mol % at 950 °C for 10 h. The photocatalytic water oxidation activity of bare and CoOx-loaded LaTiO2N crystallites grown directly by an NH3-assisted flux method (1-step-LTON) was evaluated under visible light by comparing with the LaTiO2N crystallites fabricated by a two-step method (2-step-LTON), converting La2Ti2O7 to LaTiO2N by high-temperature nitridation. Within the first 2 h of the photocatalytic water oxidation half-reaction, the O2 evolution rates of bare and CoOx-loaded 1-step-LTON crystallites were 82 µmol·h-1 and 204 µmol·h-1, respectively, which are much higher than that of bare and CoOx-loaded 2-step-LTON crystallites (37 µmol·h-1 and 177 µmol·h-1) due to less defect density of the LaTiO2N crystallites achieved by a direct fabrication route using KCl flux. An NH3-assisted flux growth is a promising route for the direct fabrication of the LaTiO2N crystallites with less defect density that is beneficial for the enhancement of photocatalytic water oxidation half-reaction.

KEYWORDS: LaTiO2N; Photocatalyst; Water splitting; Crystal growth; Flux method 2


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1. INTRODUCTION The rapid growth of population, depletion of fossil fuels, and increasing demand for energy supply are urging scientists to explore more sustainable energy resources. Hydrogen is considered as a viable alternative fuel for the substitution of fossil fuels because it is eco-friendly, cheap, abundant, and renewable. Photoelectrochemical or photocatalytic water splitting has been intensively studied as a clean and renewable hydrogen production technology using solar energy.1 Because

of

their

chemical

stability

and

innocuousness,

several

oxides

containing

early-transition-metal cations, namely TiO2,2 SrTiO3,3 La2Ti2O7,4 NaTaO3,5 K4Nb6O17,6 etc., have been demonstrated for the photocatalytic water splitting under UV light by using suitable cocatalysts and/or sacrificial reagents. However, these oxide photocatalysts with larger band gaps cannot be excited to photocatalytically split water under visible light (λ = 400 ~ 750 nm), which accounts for approximately 49% of solar energy penetrating the outer layer of the Earth’s atmosphere.7 Considering solar energy utilization, the development of a photocatalyst that can photocatalytically split water efficiently under visible light is essential. Hence, a number of oxides, oxynitrides, and nitrides, including WO3,8 Fe2O3,9 LaFeO3,10 LaTiO2N,11,12 TaON,13 LaTaON2,14 ATaO2N,15,16 ANbO2N (A = one or more alkali metal, alkaline earth metal, or rare earth metal)17–19 and Ta3N520 have been demonstrated as attractive visible-light-responsive photocatalysts for water splitting.

3



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Lanthanum titanium oxynitride (LaTiO2N), which we mainly focus on in this study, is one of the visible-light-active photocatalysts that can photocatalytically generate hydrogen and oxygen from water with appropriate sacrificial reagents and cocatalysts under visible light irradiation due to its suitable band positions.21–25 LaTiO2N is an orthorhombic perovskite-type compound with space group of Imma and lattice constants of a = 5.5731(2) Å, b = 7.8708(3) Å, c = 5.6072(2) Å, and α = β = γ = 90°. As shown in Figure 1, it is composed of a TiOxNy octahedral skeletal structure (x + y = 6) and is an n-type semiconductor with a band gap of 2.1 eV.26 Also, LaTiO2N exhibits interesting ferroelectric,27 optical,28 insulating,29 and semiconducting30 properties, and has been investigated as an environmentally-safe pigment31 and water splitting photocatalyst.22 Compared with other visible-light-responsive photocatalysts, LaTiO2N has some advantages, including low cost, low toxicity, and low environmental load. To increase the quantum yield of photocatalytic crystals, it is important to improve their crystallinity because the formed defects in the crystals act as recombination centers for photogenerated electron-hole pairs.32 Therefore, the growth of highly crystalline LaTiO2N crystals is indispensable for achieving high quantum yield. LaTiO2N is generally synthesized by nitridation of the La2Ti2O7 precursor under a high-temperature NH3 atmosphere (two-step method).12,23,25 Recently, we have also fabricated LaTiO2N crystals by nitriding the flux-grown La2TiO5 crystals under a high-temperature NH3 atmosphere (two-step method).11 It was observed that the LaTiO2N crystals fabricated by nitriding the La2Ti2O7 precursor possessed a higher defect density due to the replacement of three O2– ions by two N3– ions in the 4


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lattice.33,34 Currently, our research group is intensively studying the direct growth of various (oxy)nitride crystals by an NH3-assisted flux method (one-step method) in order to grow high-quality crystals with specific morphology and size and to achieve higher quantum yield for photocatalytic water splitting under visible light. By employing this approach, the (oxy)nitride crystals can be directly grown through the dissolution of starting materials, the crystal nucleation, and growth process with the assistance of a flux under an NH3 atmosphere. In this work, we particularly emphasize on the growth of the LaTiO2N crystallites with a low defect density by an NH3-assisted direct flux growth method and visible-light-driven photocatalytic water oxidation activity. The growth process of the LaTiO2N crystallites was studied by controlling the holding temperature and time. Here we also comparatively demonstrate the photocatalytic O2 evolution of the LaTiO2N crystallites fabricated by one-step and two-step methods.

Figure 1. Schematic illustration of the idealized crystal structure of LaTiO2N.

5



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2. EXPERIMENTAL As one of the crystal growth techniques from a high-temperature solution, flux growth is an environmentally friendly, simple, and low-cost approach that allows to manipulate the morphology and size of the crystals with improved phase-purity, homogeneity, crystallinity, and well-defined faceted surfaces at temperatures below the melting points of solutes.35-37 LaTiO2N crystallites were directly grown by an NH3-assisted flux growth method using reagent-grade La2O3, TiO2, Na2CO3, K2CO3, KCl, NaCl, BaCl2, and KF (> 99%, Wako Pure Chemical Industries, Ltd.). A stoichiometric mixture of La2O3 and TiO2 was used as a solute, and Na2CO3, K2CO3, KCl, NaCl, BaCl2, and KF were used as a flux. To study the effect of solute concentration on the crystal growth and phase evolution, the solute (La2Ti2O7) concentration was changed from 0.1 to 50 mol %, and the total mass of a solute-flux mixture was approximately 3.0 g for each run. After manual dry mixing for 20 min, each solute-flux mixture was placed in a platinum cell with a capacity of 4.0 cm3. The mixture-containing platinum cell was placed in a horizontal tubular furnace, heated at 950 °C for 10 h at a heating rate of 600 °C·h-1 under an NH3 flow (200 mL·min-1) and then cooled naturally to room temperature. The flux-grown LaTiO2N crystallites were separated from the remaining flux by washing the final crystallite product with warm water and dried at 100 °C for 12 h.

The crystalline phases formed were identified using X-ray diffraction (XRD, MiniflexII, Rigaku) with Cu Kα radiation (λ = 0.15418 nm). The X-ray diffractometer was operated at 30 kV and 20 mA in the 2θ range from 10° to 70°. The morphology and size of the crystallite products were 6


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examined by field-emission-type scanning electron microscopy (FE-SEM, JSM-7600F, JEOL) at an acceleration voltage of 5 kV. The crystallographic characteristics of the flux-grown LaTiO2N crystallites were analyzed by high-resolution transmission electron microscopy (HR-TEM, EM-002B, TOPCON) operated at 200 kV. The specific surface area (SBET) was obtained by using the Brunauer, Emmett, and Teller (BET) method from N2 gas adsorption−desorption isotherm at 77 K (BELSORP-mini, BEL Japan, Inc.) on the crystallite products degassed at 100 °C for 5 h in vacuum. The ultraviolet-visible (UV-Vis) diffuse reflectance spectra of the LaTiO2N crystallites were recorded on a JASCO V-630 spectrophotometer. The optical band gap energy was determined from the UV-Vis diffuse reflectance spectra. The nitrogen content in the flux-grown LaTiO2N crystallites was estimated by simultaneous thermogravimetry and differential thermal analysis (TG-DTA, Thermo plus EVO2, Rigaku). About 0.02 g of LaTiO2N crystallites was heated in a platinum crucible up to 1300 °C at a heating rate of 10 °C·min-1 under synthetic air flow (200 mL·min-1). The nitrogen content was estimated from the total weight change after the thermal reoxidation of LaTiO2N to La2Ti2O7.25 The photocatalytic O2 evolution reactions were performed with 0.10 g of photocatalyst (bare LaTiO2N or CoOx-loaded LaTiO2N) in 200 mL of 10 mM AgNO3 solution (> 99%, Wako Pure Chemical Industries, Ltd.) under visible light irradiation (300W Xe lamp, λ > 420 nm cutoff filter). The pH of the suspension was adjusted to 8 or 9 by adding 0.20 g of La2O3 (Wako Pure Chemical Industries, Ltd.). CoOx (2 wt% Co) as a cocatalyst for O2 evolution was deposited on the LaTiO2N 7



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crystallites fabricated by one-step and two-step methods. The LaTiO2N crystallites were first immersed in aqueous solution of Co(NO3)2·6H2O (Kanto Chemicals) and then heat treated at 700 °C for 1 h under an NH3 flow (100 mL·min-1) and subsequently at 200 °C for 1 h in air. The reaction vessel was connected to a closed-circulation system equipped with a vacuum pump and a gas chromatograph (GC-8A, TCD, Ar carrier, Shimadzu) to detect O2 gas evolved. To investigate the behavior and energy states of photogenerated charge carriers, time-resolved absorption spectroscopy (TAS) measurements were performed by using laboratory-build spectrometers. Water suspensions containing 1-step-LTON and 2-step-LTON crystallites (3 mg) were deposited on a calcium fluoride disc with a 16 mm diameter and dried at room temperature overnight. The LTON crystallites-deposited calcium fluoride disc was placed in a stainless-steel cell and evacuated at room temperature. In the IR region, IR light from a MoSi coil was focused on the sample, and the transmitted light was dispersed by the spectrometer. The monochromated light was detected by a photovoltaic MCT detector (Kolmar), and the output electric signal was amplified by ac-coupled amplifiers (Stanford Research Systems SR560 with 1 MHz bandwidth). The time resolution of these spectrometers is limited to 1 µs by the bandwidth of the amplifier. The transient absorption change was recorded by a digital oscilloscope after band gap photoexcitation using 355 nm pulses from a Nd:YAG laser (Continuum, Surelite I, 6 ns duration, 10-0.01 Hz). In the visible and near-IR (NIR) region, the probe light from a halogen lamp (50 W) was focused on the sample,

8


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and the transmitted or diffuse reflected light was dispersed by the spectrometer. The monochromated output was detected by a Si photodiode or InGaAs detectors.

3. RESULTS AND DISCUSSION Figure S1 in the SI shows the XRD patterns of the crystal products grown individually using different fluxes (K2CO3, Na2CO3, KCl, NaCl, BaCl2, and KF) with the solute concentration of 10 mol % at 950 °C for 10 h. As shown, LaTiO2N (ICDD PDF 48–1230) was formed as the main crystalline phase in the flux-grown crystal products. Using the carbonate fluxes (Na2CO3 and K2CO3), the main LaTiO2N phase was accompanied by minor phases, such as La(OH)3 (ICDD PDF 36–1481), K2La2Ti3O10 (ICDD PDF 87–1167), Na2La2Ti3O10 (ICDD PDF 86–1369) and other unidentified phases. The Na2La2Ti3O10 and K2La2Ti3O10 phases were additionally formed during the high-temperature NH3-assisted flux growth of the LaTiO2N crystallites according to the following reaction: Na2CO3 (K2CO3) + La2O3 + 3TiO2 → Na2La2Ti3O10 (K2La2Ti3O10) + CO2. The La(OH)3 phase was precipitated due to the reaction between La2O3 and water vapor molecules (La2O3 + 3H2O → 2La(OH)3) formed from the oxygen removed from the initial oxide crystals and surface hydrogen from the dissociation of NH3 gas at high temperature.38 Because of its higher reactivity, the KF flux provoked the formation of minor phases, KLaF4 (ICDD PDF 71–6482) and LaOF (ICDD PDF 89–3074), to the main LaTiO2N phase. Compared with the carbonate fluxes, each chloride flux (NaCl, KCl, and BaCl2) had a distinct impact on the direct formation of the LaTiO2N crystallites. The diffraction peaks in the XRD pattern of crystal products grown using the KCl flux 9



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can be readily indexed as orthorhombic LaTiO2N without any indicative diffraction peaks assignable to impurity phases. In contrast, minor LaOCl phase (ICDD PDF 71–6482) and other unidentified phases were formed in addition to the main LaTiO2N phase using the NaCl flux. The formation of the LaOCl phase was promoted by the reaction between La2O3 and Cl– ions while the Na+ partially replaced La2+ in the LaTiO2N crystal lattice due to their similar ionic radii.39 When the BaCl2 flux was employed, minor phases formed along with the LaTiO2N phase could not be identified. Among all the fluxes used here, the KCl flux was found to be the most suitable one to directly grow single-phase LaTiO2N crystallites due to its less reactivity and higher stability under reductive high-temperature NH3 atmosphere. To study the effects of fluxes on crystal morphology, the SEM images of the crystal products grown separately using different fluxes (K2CO3, Na2CO3, KCl, NaCl, BaCl2, and KF) with the solute concentration of 10 mol % at 950 °C for 10 h are shown in Figure S2. The crystal products grown using the carbonate fluxes (Na2CO3 and K2CO3) have platelet (ca. 9.3 µm) and stack-layered structures (ca. 2.7 µm), respectively. The crystal products grown using the chloride and fluoride fluxes possess irregular shapes with the average size ranging from 100 nm to 2 µm. The difference in the shape and size of the LaTiO2N crystallites was presumably resulted from the difference in solute solubility in that specific flux and the chemical compositions of and interaction between the flux and the crystal.

10


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TiN

TiO0.48

La(OH)3

50 mol%

Intensity (arb. units)

??? ?

LaOCl

La2Ti2O7

? unknown

?

20 mol% 10 mol% 5 mol% 1 mol% 0.1 mol%

?

?

0 7

0 6

ICDD PDF#48-1230 LaTiO2N

0 5

0 4

0 3

0 2

0 1

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 LaTiO2N crystallites grown using the KCl flux with different solute concentrations at 950 °C for 10 h.

Figure 2 shows the XRD patterns of the LaTiO2N crystallites grown using the KCl flux with different solute concentrations at 950 °C for 10 h. Clearly, single-phase LaTiO2N crystallites were grown with the solute concentrations of 5-20 mol %. However, using the solute concentration of 0.1 mol %, the LaTiO2N crystallites were grown with minor phases, TiN (ICDD PDF 38–1420), TiO0.48 (ICDD PDF 89–3074), and some unidentified phases, due to less amount of La3+ ions present in the solution. An increase in the solute concentration to 1 mol % promoted the formation of other minor phases, such as La(OH)3 and LaOCl. At the highest solute concentration (50 mol %), the LaTiO2N phase was again accompanied by minor phases (TiO0.48, La(OH)3, La2Ti2O7 and unidentified phases) presumably because of a low solubility of the solute in the KCl flux. An excess amount of 11



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the La2Ti2O7 crystallites formed at the initial stage could not be completely nitrided to form the LaTiO2N crystallites at 950 °C for 10 h under high-temperature NH3 atmosphere. The effect of solute concentration on the morphology of LaTiO2N crystallites is visualized in Figure 3. With increasing the solute concentration from 0.1 to 50 mol %, the shape of the LaTiO2N crystallites gradually changed from rod-like to strongly joined, irregularly rounded and to platelet crystallites (Figure 3a-f). Apparently, the LaTiO2N crystallites grown with the solute concentration of 5 mol % have a regularly rounded shape with the size of 120 ± 39 nm (Figure 3c). At lower solute concentrations (0.1 and 1 mol %), Figures 3a and b, the solute was easily dissolved in the KCl flux and the anisotropic growth of rod-like LaTiO2N crystallites proceeded with a faster rate because of the increased mobility and the diffusion coefficient.40

Figure 3. SEM images of the LaTiO2N crystallites grown using the KCl flux with different solute concentrations at 950 °C for 10 h: (a) 0.1 mol%, (b) 1 mol%, (c) 5 mol%, (d) 10 mol%, (e) 20 mol%, and (f) 50 mol%. 12


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KCl

TiO2

La2O3

La2Ti2O7

? unknown

10 h

Intensity (arb. units)

7h 5h 3h 1h 10 min 0 min ?

?

0 7

0 6

ICDD PDF#48-1230 LaTiO2N

0 5

0 4

0 3

0 2

0 1

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 4. XRD patterns of the LaTiO2N crystallites grown using the KCl flux with different holding times at 950 °C.

To study the direct growth process of the LaTiO2N crystallites in the KCl flux under a high-temperature NH3 atmosphere, a series of crystal products were grown at 950 °C for different reaction times with the solute concentration of 5 mol % and subject to characterization without removing the KCl flux with warm water because La2O3 is easily hydrated to form La(OH)3 in water. The XRD results are shown in Figure 4. The diffraction peaks assignable to the La2O3, TiO2, KCl, and some unidentified phases confirmed that the raw materials slightly reacted at 950 °C for < 10 min. During the reaction period from 10 min to 5 h, the main LaTiO2N phase was gradually formed 13



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along with the La2Ti2O7 phase (ICDD PDF 28–0517), which was eventually converted to the LaTiO2N with the evaporation of the KCl flux. Finally, single-phase LaTiO2N crystallites were grown using the KCl flux with the solute concentration of 5 mol % at 950 °C for > 7 h. In addition, the results from the temperature-dependent experiments revealed that phase-pure LaTiO2N crystallites can be directly grown at > 900 °C for 10 h, and the LaTiO2N crystallites gained their rounded shape with smooth surface at 950 °C for 10 h (Figures S3 and S4 in the SI). Figure 5 shows the SEM images of the LaTiO2N crystallites grown using the KCl flux at 950 °C for different reaction times with the solute concentration of 5 mol %. At the reaction period from 10 min to 5 h (Figures 5a-d), the flux-grown LaTiO2N crystallites have two different morphologies: large rod-like and small rounded. According to the EDS results (not shown here), large rod-like crystallites belong to the La2Ti2O7 phase, whereas small rounded crystallites to the LaTiO2N phase. With increasing the reaction time from 1 to 7 h (Figures 5b-e), the number of small rounded crystallites and their sizes were increased due to Ostwald ripening,41 evidencing the formation of single-phase LaTiO2N crystallites with a rounded shape, and the size of the La2Ti2O7 crystallites became slightly larger. At the reaction time of 10 h (Figure 5f), homogenous regularly rounded LaTiO2N crystallites with the average size of 120 ± 39 nm were grown at the expense of the La2Ti2O7 crystallites via a dissolution-precipitation process and due to the Gibbs-Thomson effect.42 It is believed that the amount of active atomic nitrogen generated from the dissociation of NH3 gas at lower temperatures (< 950°C) and shorter reaction times (< 7 h) was not sufficient enough to 14


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directly form the single-phase LaTiO2N, causing the simultaneous formation of the La2Ti2O7 phase additionally, which was later completely converted to LaTiO2N. La2O3 + 2TiO2 → La2Ti2O7

(1)

La2Ti2O7 + 2N + 3H2 → 2LaTiO2N + 3H2O

(2)

Figure 5. SEM images of the LaTiO2N crystallites grown using the KCl flux with different holding times at 950 °C: (a) 10 min, (b) 1 h, (c) 3 h, (d) 5 h, (e) 7 h, and (f) 10 h.

Figure 6 displays the TEM and HRTEM images and fast Fourier transform (FFT) pattern of the LaTiO2N crystallites grown using the KCl flux at 950 °C for 10 h. The TEM image in Figure 6a confirms that the LaTiO2N crystallites have a rounded shape and a strong connection. The corresponding FFT pattern in the inset of Figure 6b suggests that the round-shaped LaTiO2N crystallites are single crystalline in nature. The HRTEM image in Figure 6b shows parallel lattice

15



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fringes with spacings of 0.394 nm, which correspond to the (002) interplanar distance of the LaTiO2N,43 with no clear defects in this range.

Figure 6. TEM (a) and HRTEM (b) images and FFT pattern (inset) of the LaTiO2N crystallites grown using the KCl flux at 950 °C for 10 h.

Figure 7 shows the TG-DTA curves of the LaTiO2N crystallites grown using the KCl flux at 950 °C for 10 h. The brown-colored LaTiO2N crystallites were reoxidized to white La2Ti2O7 crystallites by heating from room temperature to 1300 °C in air.44 The composition of LaTiO2.3N0.8 was approximated from the TG data using the following formula:25,45 LaTiO3.5-1.5yNy, y = (∆m / 3/2(M(O2) – M(N2)))·(M(La2Ti2O7) / m(La2Ti2O7))

(3)

The weight change ∆m was estimated from the transformation of LaTiO2N to La2Ti2O7. The cations (lanthanum and titanium) were presumed to have their highest oxidation state (La3+ and Ti4+) before and after oxidation. Using this calculation, the amount of nitrogen estimated for the LaTiO2N crystallites grown directly in this study is found to be similar (y = 0.8) compared with the 16


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previously reported values,45,46 and is expected to endow it with higher photocatalytic water splitting activity. Thus, it can be deduced that the LaTiO2N crystallites with the relative stoichiometric composition can be grown directly using the KCl flux at 950 °C for 10 h.

TG

3 -

Exo. Exo. Endo.

6 -

DTA

Heat Flow / μV

0

Weight change (%)

La2Ti2O7

3

LaTiO2N

0 0 0 0 0 5 0 5 0 0 5 2 2 1 1 5 0 0 5 2 1

6 0 0 0 1

0 5 7

0 0 5

0 5 2

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


Temperature / ℃ Figure 7. TG-DTA curves of the LaTiO2N crystallites grown using the KCl flux at 950 °C for 10 h.

The UV-Vis diffuse reflectance spectra of the LaTiO2N crystallites grown directly using the KCl flux at 950 °C for 10 h (1-step-LTON) and fabricated through the nitridation of the La2Ti2O7 precursor (2-step-LTON) are shown in Figure 8. The absorption edge wavelength was found to be approximately 600 nm (Eg = 2.06 eV), which is consistent with the previously reported data.21 In the longer wavelength region (over 600 nm), a strong background absorption was observed due to the presence of defects associated with the reduced Ti species and anion vacancies (O2− or N3−) as well as amorphous defect by-products.47 17


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1-step-LaTiO2N

2-step-LaTiO2N

0 0 8

0 0 7

0 0 6

0 0 5

0 0 4

0 0 3

0 0 2

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

Absorbance (arb. unit)


Wavelength / nm Figure 8. UV-Vis spectra of the LaTiO2N crystallites grown directly using the KCl flux at 950 °C for 10 h (1-step-LTON) and fabricated through the nitridation of the La2Ti2O7 precursor (2-step-LTON).

To compare the photocatalytic O2 evolution of the LaTiO2N crystallites grown directly using the KCl flux at 950 °C for 10 h (1-step-LTON) and fabricated through the nitridation of the La2Ti2O7 precursor (2-step-LTON)21, we have performed the water oxidation experiments on two different LaTiO2N samples (1-step-LTON and 2-step-LTON) under visible light irradiation (λ > 420 nm) in the presence of AgNO3 as an electron acceptor, and the results are summarized in Figure 9. Within the first 2 h of the photocatalytic water oxidation half-reaction, the O2 evolution rate of bare 1-step-LTON was 82 µmol·h-1 which was approximately two-fold greater than that of bare 18


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2-step-LTON (37 µmol·h-1). It is thought that the difference in the O2 evolution rates of these bare LaTiO2N crystallite samples was related to several factors, including the specific surface area, the nitrogen content, the amount of defects acting as recombination centers of photogenerated electrons and holes, the crystallinity, and the crystal morphology. The specific surface area of bare 1-step-LTON (8.6 m2·g-1), which was determined by N2 gas adsorption at 77 K, was slightly lower than that of bare 2-step-LTON (10.5 m2·g-1) due to the presence of a wide range of pores in the 2-step-LTON sample stemmed from a lattice condensation process caused by the replacement of O2− with N3− in the anionic network.25 No profound difference was found in the nitrogen contents of bare 1-step-LTON (y = 0.80) and bare 2-step-LTON (y = 0.86). However, in the UV-Vis diffuse reflectance spectra (Figure 8), a strong background absorption attributable to the defects46,47 beyond the absorption edge wavelength (> 600 nm) was noted for both LaTiO2N crystallites. As the intensity of background absorption of bare 1-step-LTON is lower than that of bare 2-step-LTON, the test results for photocatalytic O2 evolution over bare LaTiO2N crystallite samples seem quite reasonable because bare 1-step-LTON crystallites were expected to have lower defects, as compared to bare 2-step-LTON crystallites, due to the direct growth of bare 1-step-LTON crystallites by an NH3-assisted flux method using the KCl flux. To investigate the effect of a CoOx cocatalyst on the activity of photocatalytic O2 evolution, we have also conducted the experiments of water oxidation on two different CoOx-loaded LaTiO2N crystallites (1-step-LTON-CoOx and 2-step-LTON-CoOx) under the identical experimental 19



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conditions applied for bare LaTiO2N crystallite samples. Within the first 2 h of the photocatalytic water oxidation half-reaction, the O2 evolution rates of CoOx-loaded 1-step-LTON (204 µmol·h-1) and 2-step-LTON (177 µmol·h-1) crystallites were approximately three- and five-fold higher than that of bare 1-step-LTON and 2-step-LTON crystallites, respectively, owing to a rapid hole capture by CoOx cocatalyst. Probably, the different progress rates of O2 evolution over the two CoOx-loaded LaTiO2N crystallite samples were stemmed from the difference in crystallinity and dispersion of CoOx cocatalyst nanoparticles loaded onto the LaTiO2N crystallite samples with different morphologies (1-step-LTON is dense with smooth surface and 2-step-LTON is porous). With the extension of the photocatalytic water oxidation half-reaction time to 5 h, the amount of O2 evolved over the 2-step-LTON sample gradually reached that of 1-step-LTON sample. As Maegli et al.25 described, the 2-step-LTON crystallites with porous morphology (Figures S5 and S6 in the SI) were thought to have more dangling bonds compared with the 1-step-LTON crystallites with smooth surfaces, and therefore, the density of nucleation centers in the form of dangling bonds was higher in the 2-step-LTON crystallites. Thus, the CoOx cocatalyst nanoparticles were loaded onto the 2-step-LTON crystallites with better dispersion (Figure S7), which resulted in a higher progress rate of O2 evolution in the prolonged reaction time compared with the 1-step-LTON crystallites. Due to the photooxidation of nitrogen anion (N–3 species) present in the vicinity of the LaTiO2N crystallite surface, a little amount of N2 gas (5.7-7.2 µmol) was generated during the photocatalytic reaction period (0-5 h), which corresponds to the previously reported data for other (oxy)nitride 20


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photocatalysts.16,19 The N2 gas evolution rate was drastically diminished with the prolongation of reaction time, confirming the stability of the LaTiO2N crystallite samples fabricated in this study.

0 0 6

: CoOx-loaded LaTiO2N (1-step)

0 0 5

: CoOx-loaded LaTiO2N (2-step) : Bare LaTiO2N (2-step)

: Bare LaTiO2N (1-step)

0 0 4 0 0 3 0 0 2 0 0 1

O2 gas evolved / μmol

0 0 7 5

4

3

2

1

0 0

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


Time / h Figure 9. Time courses of O2 evolution of bare and CoOx-loaded LaTiO2N crystallites grown directly using the KCl flux at 950 °C for 10 h (1-step-LTON) and fabricated through the nitridation of the La2Ti2O7 precursor (2-step-LTON).

Here we also compared the O2 evolution rates of the LaTiO2N crystallites grown by one-step and two-step methods in this study with the previously reported data for LaTiO2N crystals although the different systems were applied. In case of bare LaTiO2N crystallite samples, the O2 evolution rate of our bare sample (1-step-LTON) was 82 µmol·h-1 which is higher than the previously reported rates (ca. 50 µmol·h-1 and 53 µmol·h-1)23,25 but slightly lower than the rate (84 µmol·h-1) achieved by 21



Matsukawa et al.48 through the removal of defects near the surface of the LaTiO2N crystals using aqua regia. As shown above, bare 2-step LTON crystallite sample showed a lower O2 evolution rate. The LaTiO2N crystallites converted from the La2Ti2O7 precursor (2-step LTON) by nitridation under a high-temperature NH3 atmosphere were thought to contain more defects compared with the LaTiO2N crystallites grown directly by an NH3-assisted flux method using the KCl flux.

(a)

5 µs 10 µs 20 µs 50 µs 100 µs 500 µs 1 ms

∆Abs = 2 x 10-3

0

(b)

∆Absorbance

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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1 0-2 10-3

1-step-LTON

2-step-LTON 17200 cm-1

10-4 1 µs 10 µs 10 0 µs 1 m s

Time Delay

0 2 0000

1 5000

10000

5000

1000

Wavenumber / cm-1

22


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Figure 10. Transient absorption spectra of bare LaTiO2N crystallites fabricated through nitridation of the La2Ti2O7 crystallites (2-step-LTON) (a) and grown directly using the KCl flux at 950 °C for 10 h (1-step-LTON) (b), irradiated by UV (355 nm) laser pulses under a vacuum. Pump energy is 0.5 mJ/pulse, and repetition rate is 5 Hz. Decay curves of transient absorption intensity at 17200 cm-1 of 1-step-LTON and 2-step-LTON, irradiated by UV (355 nm, 0.1 Hz, 0.5 mJ/pulse) laser pulses under vacuum (inset).

To investigate the effects of defects on the behavior of photogenerated charge carriers, time-resolved absorption spectroscopy (TAS) measurements were performed by using laboratory-build spectrometers.49 The measured transient absorption (TA) spectra of bare 1-step-LTON and 2-step LTON crystallites after UV-laser pulse irradiation are shown in Figure 10. As shown in Figure 10a, the TA spectra of bare 2-step-LTON crystallites contain two main absorption peaks at 17200 cm-1 and 4000 cm-1. The absorption peak at 17200 cm-1 is ascribed to the photogenerated holes, whereas the broad absorption peak at 4000 cm-1 is attributed to the optical transitions of deeply trapped electrons from midgap states to the conduction band.50,51 The intensity of the absorption peak at 17200 cm-1 of 1-step-LTON crystallites is almost four-fold higher than that of 2-step-LTON, implying that the reactive holes are higher in 1-step-LTON crystallites compared with 2-step-LTON crystallites. Interestingly, the intensity of the broad absorption peak at 4000 cm-1 is almost unnoticeable in Figure 10b. This is not due to the fast recombination of electrons but the 23



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difference in the energy states and absorption coefficient of electrons since the number of holes in 1-step-LTON crystallites is much higher than in 2-step-LTON crystallites. From the obtained data, we can speculate that the number of deeply trapped electrons is much less in 1-step-LTON crystallites than in 2-step-LTON crystallites, and the number of shallow-trapped electrons is much larger in 1-step-LTON crystallites than in 2-step-LTON crystallites, implying that the 1-step-LTON crystallites have a less defect density. Further, we also studied the decay kinetics of photogenerated charge carriers on bare 1-step-LTON and 2-step-LTON crystallites. As shown in the inset of Figure 10b, the lifetimes of photogenerated charge carriers on both samples with different defect densities are longer than milliseconds, evidencing the decisive contribution of the surface defects on the prolongation of the lifetimes of charge carriers.52 The signal intensity of 1-step-LTON in the range of 1 µs to 1 ms is higher than that of 2-step-LTON due to the difference in the structure of the surface defects (energy states and the lifetimes of charge carriers are influenced by trapping).53 Comparing the cocatalyst-loaded LaTiO2N samples, our CoOx-loaded sample (1-step-LTON-CoOx) showed the O2 evolution rate of 204 µmol·h-1, which is higher than the rate (170 µmol·h-1) previously reported by Maegli et al.25 but lower than the rate (736 µmol·h-1) achieved by Zhang et al.21. In addition to the defect density, the difference in the O2 evolution rate was also resulted from the difference in crystallinity, the specific surface area, and the morphology and size of the crystals of the photocatalyst support as well as the system applied. Moreover, the crystallization and dispersion of CoOx cocatalyst nanoparticles on the photocatalyst support have also been considered 24


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to be the key factors. As reported earlier,25 the LaTiO2N crystallites with complex and skeletal morphology fabricated by a two-step method had a better dispersion of CoOx cocatalyst nanoparticles, leading to the higher O2 evolution rate. Our attempt to achieve a higher O2 evolution rate by tailoring the morphology and size, reducing the defect density, and doping of the LaTiO2N crystallites by an NH3-assisted direct flux method is currently in process.

4. CONCLUSIONS In summary, phase-pure LaTiO2N crystallites with the average size of 120 ± 39 nm and high crystallinity were grown by an NH3-assisted flux method using the KCl flux with the solute concentration of 5 mol % at 950 °C for 10 h. It was observed that within the first 2 h of the photocatalytic water oxidation half-reaction, bare and CoOx-loaded 1-step-LTON samples showed the O2 evolution rates of 82 µmol·h-1 and 204 µmol·h-1, respectively, which were higher than that of bare and CoOx-loaded 2-step-LTON samples (37 µmol·h-1 and 177 µmol·h-1). The enhanced O2 evolution rate of the LaTiO2N crystallites was achieved by reducing their defect density by applying an NH3-assisted direct flux growth approach.

ASSOCIATED CONTENT

AUTHOR INFORMATION Corresponding Author 25



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*E-mail: [email protected]; Phone: +81 26 269 5556

Notes The authors declare no competing financial interest.

AUTHORS CONTRIBUTION The manuscript was written through the equal contribution of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGEMENTS This research was supported in part by the Japan Technological Research Association of Artificial Photosynthetic Chemical Process (ARPChem). The authors thank Drs. Takashi Hisatomi and Riyo Niishiro of the University of Tokyo, Japan, for the water oxidation half-reaction experiments.

Supporting Information Available The XRD patterns and SEM images of the crystal products grown using the KF, BaCl2, NaCl, KCl, Na2CO3, and K2CO3 fluxes. The XRD patterns and SEM images of the LaTiO2N crystallites grown using the KCl flux at different temperatures for 10 h. The XRD patterns and SEM images of the La2Ti2O7 (Figure 6a) and LaTiO2N (Figure 6b) crystallites (two-step method). SEM images and EDS spectra of CoOx-loaded LaTiO2N crystallites fabricated by one-step and two-step methods. This information is available free of charge via the Internet at http://pubs.acs.org

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For Table of Contents Only

Single-phase LaTiO2N crystallites with high crystallinity were directly grown by an NH3-assisted flux method using the KCl flux at 950 °C for 10 h. Bare and CoOx-loaded 1-step-LTON crystallites showed higher O2 evolution rates compared with 2-step-LTON crystallites due to less defect density of the LaTiO2N crystallites achieved by a direct fabrication route.

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NH3-Assisted Flux-Mediated Direct Growth of LaTiO2N Crystallites for

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