NH3-Assisted Flux Growth of Cube-like BaTaO2N Submicron Crystals

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Crystal Growth & Design

NH3-Assisted Flux Growth of Cube-Like BaTaO2N Submicron Crystals in a Completely Ionized Nonaqueous High-Temperature Solution and Their Water Splitting Activity Mirabbos Hojamberdieva, Kunio Yubutab, Junie Jhon M. Vequizoc, Akira Yamakatac, Shuji Oishia, Kazunari Domend and Katsuya Teshimaa,e a

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

b

c

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

d

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

e

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 As the 600 nm-class photocatalyst, BaTaO2N is one of the promising candidates of the perovskite-type oxynitride family for photocatalytic water splitting under visible light. The oxynitrides are routinely synthesized by nitriding corresponding oxide precursors under a high-temperature NH3 atmosphere, causing an increase in the defect density and a decrease in photocatalytic activity. To improve the photocatalytic activity by reducing the defect density and improving the crystallinity, we here demonstrate an NH3-assisted KCl flux growth approach for the direct synthesis of the BaTaO2N crystals. The effects of various fluxes, solute concentration, and reaction time and temperature on the phase evolution and morphology transformation of the BaTaO2N crystals were systematically investigated. By changing the solute concentration from 10 to 50 mol %, it was found that phase-pure BaTaO2N crystals could only be grown with the solute concentrations of ≥ 10 mol % using the KCl flux, and the solute concentration of 10 mol % was solely favorable to directly grow cube-like BaTaO2N crystals with an average size of about 125 nm and exposed {100} and {110} faces at 950 °C for 10 h. The time- and temperature-dependent experiments were also performed to postulate the direct growth mechanisms of cube-like BaTaO2N submicron crystals. The BaTaO2N crystals modified with Pt and CoOx nanoparticles showed a reasonable H2 and O2 evolution, respectively, due to a lower defect density and higher crystallinity achieved by an NH3-assisted KCl flux method. KEYWORDS: Barium tantalum oxynitride; Perovskite; Flux growth; Water splitting

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INTRODUCTION

Perovskites have attracted much interest because of their interesting properties, including high dielectric properties, superconductivity, colossal magnetoresistance, and excellent electronic transport features.1-5 The partial replacement of O2– by N3– in the perovskite structure allows to narrow the band gap of parent oxide due to higher N 2p orbital energy than that of O 2p orbitals. The coexistence of O2– and N3– in the anion network results in intriguing physical and chemical properties that are different from their metal-oxide and nitride counterparts, making perovskite oxynitrides promising candidates particularly for the efficient utilization of solar energy and optoelectronic applications.6

BaTaO2N is a member of the family of alkaline-earth-based transition-metal oxynitrides with the formulae AB(O,N)3 which are generally formed by introducing nitrogen into the anionic sub-lattice of ABO3-type perovskites. As shown in Figure 1, BaTaO2N has a cubic perovskite crystal structure with the space group of (no. 221) and a = 4.11 Å.7 Ba is at the origin (0,0,0), Ta at the cube center (1/2, 1/2, 1/2) and three anions (one nitrogen and two oxygen) are randomly distributed at the face centers. Every Ba atom is 12-fold surrounded by anions with a Ba–(O,N) distance of 2.908 Å. Every Ta atom is octahedrally coordinated (Ta(O,N)6 coordination) with a Ta–(O,N) distance of 2.056 Å.7,8 However, due to the partial replacement of oxygen by nitrogen in the crystal lattice, the local symmetry at the origin and 3



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at the cube center should be lower, and local structure relaxation is expected because of the different ionic radii and valences of the oxygen and nitrogen ions.9 Previously, X-ray absorption fine structure,10 electron diffraction,11 neutron diffraction,12 and pair-distribution-function13 analyses have shown that the real structure of BaTaO2N is based on irregularly deformed Ta(O,N)6 octahedra and broadly distributed Ta–O/N bond distances. Although there is no contradiction with a macroscopic cubic description of the BaTaO2N crystal structure with the space group of , first-principles calculations and classical molecular dynamics simulations clearly revealed that all crystallographic unit cells were better described as orthorhombic with the space group of .14

Figure 1. Crystal structure of BaTaO2N. As the 600 nm-class photocatalyst, BaTaO2N is one of the promising candidates of the perovskite-type oxynitride family for visible-light-induced water splitting because it has a smaller band gap (Eg = 1.8 eV) than archetypal inorganic photocatalyst TiO2 and appropriate

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band-edge positions to facilitate the water reduction and oxidation, and is stable in aqueous solution and nontoxic. The presence of tantalum is additionally expected to increase efficiency.15 Recently, an IPCE value of 10% (at 1.2 VRHE) was achieved by Higashi et al.16 using the BaTaO2N photoanodes co-loaded with CoOx and RhOx nanoparticles, which is so far the highest among photoanode materials that can harvest light beyond 600 nm for water oxidation. Moreover, BaTaO2N was also demonstrated to exhibit an extremely high and temperature-independent dielectric constant both in polycrystalline as well as in thin film forms, making it suitable for applications in electronics industries.17,18 Generally, high crystallinity, low defect density, high dispersion, and large surface area are considered to improve the quality of photocatalytic materials that is essential for the enhancement of photocatalytic performance.19 To improve the quality of photocatalytic materials, various approaches, including ammonothermal synthesis,20 flux growth,21 cationic-deficient precursors,22 substitutions,23 and post-synthesis acid treatment,24 have been developed. Particularly, the higher photocatalytic activity of oxynitride photocatalysts was achieved by a flux growth method,25,26 one of the crystal growth techniques allowing to grow crystals with high crystallinity and idiomorphic shape from a supersaturated nonaqueous high-temperature solution with the assistance of a flux (molten salts or metals), due to the enhanced particle dispersion,27 increased surface area,28 and improved crystallinity.27,28

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The oxynitrides are routinely synthesized by a two-step method: (1) the synthesis of corresponding oxide precursor and (2) its high-temperature nitridation under an NH3 atmosphere, and are partially decomposed during the high-temperature nitridation, introducing nitrogen vacancies. Nitrogen vacancies cause the hindrance of the prompt migration of electrons from the bulk to the surface reaction sites.29 In this study, to upgrade the photocatalytic activity by reducing the defect density and improving the crystallinity, we demonstrate an NH3-assisted KCl flux growth of cube-like BaTaO2N crystals. The effects of flux type, solute concentration, and reaction time and temperature on the phase evolution and morphology transformation of the BaTaO2N crystals were systematically investigated. The photocatalytic H2 and O2 evolution from water, for solar-to-fuel conversion application, over the successfully flux-grown cube-like BaTaO2N crystals under irradiation with visible light was experimentally studied.

EXPERIMENTAL Direct Flux Growth of BaTaO2N Crystals Cube-like BaTaO2N crystals were directly grown by a flux growth method. As a solute, reagent-grade BaCO3 and Ta2O5 powders (> 99%, Wako Pure Chemical Industries, Ltd.) were manually dry mixed with the flux powders using a mortar and a pestle in the required stoichiometric ratio. To explore the influence of flux type on the growth of the BaTaO2N 6


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crystals with different morphologies, KCl, KI, KF, MgCl2, CaCl2, SrCl2, BaCl2·2H2O, K2SO4, K2MoO4, and K2CO3 (> 99%, Wako Pure Chemical Industries, Ltd.) were employed. Single-phase BaTaO2N crystals could be grown using only KCl due to the reactivity, instability, and melting-point mismatch of other fluxes under a high-temperature reductive NH3 atmosphere (Figures S1 and S2). Hence, further experiments were performed with KCl. After dry mixing, the mixtures with the solute concentration of 1-50 mol % were heated to 950 °C, maintained for 10 h under NH3 atmosphere, and cooled naturally. The resulting crystals were isolated by removing the flux with hot water and dried. The evaporation rate of flux (Evap. %) was calculated using the following formula: . (%) =

− ( + ) × 100 −

where – weight of starting mixture before the reaction; – weight of resulting product after the reaction; weight of flux; "

!

!

– weight of evaporated H2O from the mixture; " –

– weight of evaporated H2O from the flux.

Characterization The XRD patterns were recorded on a MiniflexII (Rigaku) in the 2θ range of 5-80°. The morphology was observed by using a JSM-7600F scanning electron microscope (JEOL). The crystallinity and developed facets of BaTaO2N crystals were examined by using an EM-002B transmission electron microscope (TOPCON). The UV-Vis diffuse reflectance spectrum was 7



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measured using a JASCO V-670 spectrophotometer and converted into the absorption spectrum. The chemical compositions of the crystals were analyzed by an SPS5510 inductively coupled plasma-optical emission spectrometer (SII), a JSM-7600F energy-dispersive X-ray spectrometer (JEOL), and a JPS-9010MC X-ray photoelectron spectrometer (JEOL). The peak positions were normalized by positioning the C 1s peak at 284.5 eV. The thermal reoxidation of the BaTaO2N crystals (about 50 mg) was studied by thermogravimetry-differential thermal analysis (TG-DTA, Thermo plus EVOII TG8120, Rigaku) from room temperature up to 1300 °C at heating and cooling rates of 10 °C·min−1 in synthetic air. Photocatalytic water splitting reactions were carried out by using the reactor connected to a closed circulation system equipped with a gas chromatograph and a vacuum pump. Prior to visible light irradiation (300W Xe lamp with a cutoff filter (λ > 420 nm)), BaTaO2N crystals were loaded with CoOx (2 wt% Co) and Pt (0.3 wt%) as O2 and H2 evolution cocatalysts by impregnation in aqueous solutions of Co(NO3)3 (Kanto Chemicals) and H2PtCl6·2H2O (Kanto Chemicals, 97% Pt) followed by heat treatment at 500 °C for 1 h under NH3 atmosphere and at 200 °C for 1 h under H2 atmosphere, respectively. BaTaO2N crystals (100 mg) were separately dispersed in aqueous solution of AgNO3 (100 mL, 10 mM) as a sacrificial electron acceptor with 100 mg of La2O3 (pH buffer) and aqueous methanol solution (100 mL, 10 vol.%).

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RESULTS AND DISCUSSION Within this study, an NH3-assisted KCl flux growth method was employed to directly grow cube-like BaTaO2N crystals in a completely ionized nonaqueous high-temperature solution. As the size, morphology, and crystal structure of photocatalysts often play a crucial role in evaluating their photocatalytic activity, we first studied the influence of solute concentration on phase evolution and morphology transformation of BaTaO2N crystals by changing the solute concentration (1 to 50 mol %). Figure 2 shows the XRD patterns of BaTaO2N crystals grown with different solute concentrations. As shown, all the diffraction lines in the XRD patterns of the crystals grown with 10, 20, and 50 mol % solute concentrations can be indexed as cubic perovskite BaTaO2N phase with space group of (no. 221) and unit cell dimensions of a = b = c = 4.11 Å and α = β = γ = 90° (ICDD PDF 84-1748). Neither the diffraction peak shift nor an impurity phase due to the possible incorporation of potassium from the KCl flux were observed, implying that phase-pure BaTaO2N crystals can be directly grown using the KCl flux at 950 °C for 10 h under an NH3 atmosphere with the solute concentration of ≥ 10 mol %. In contrast, the crystal products grown with the solute concentrations of 1 and 5 mol % additionally contain an orthorhombic Ta3N5 phase with the space group of Cmcm (no. 63) (ICDD PDF 79-1533) as an impurity phase to the BaTaO2N main phase because of lower supersaturation, particularly with the Ba2+ ions. As the

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crystallization kinetics is known to respond rapidly to changes in the supersaturation, it is thought that a lower supersaturation in a KCl flux-based nonaqueous high-temperature solution was favorable to initiate the crystallization of the Ta3N5 phase in addition to the BaTaO2N phase. Ta3N5

(e)

Intensity (arb. units)

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(d)

(c) (b)

(a) ICDD PDF 84-1748 BaTaO2N

20

30

40

50

60

70

2θ / degree Figure 2. XRD patterns of BaTaO2N crystals grown with different solute concentrations: (a) 1 mol %, (b) 5 mol %, (c) 10 mol %, (d) 20 mol %, and (e) 50 mol %. The SEM images of BaTaO2N crystals grown with different solute concentrations are shown in Figure 3. The BaTaO2N crystals gradually gained their cube-like shape with a clear edge and became more homogenous in size with the average crystal size of about 125 nm with an increase in solute concentration from 1 to 10 mol %. The BaTaO2N crystals grown with < 10 mol % solute concentration are inhomogeneous; that is, a number of larger crystals grown due to a less number of nuclei in the nonaqueous solution can also be seen. 10


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Figure 3. SEM images of BaTaO2N crystals grown with different solute concentrations: (a) 1 mol %, (b) 5 mol %, (c) 10 mol %, (d) 20 mol %, and (e) 50 mol %. In contrast, cube-like shape of the BaTaO2N crystals slowly turned into irregular shape with an increasing average crystal size with a further increase of solute concentration to > 10 mol %. Overall, no strong impact of a solute concentration on the crystal morphology could be noticed in comparison to our previous results.25,26 It can be deduced that the solute concentration of 10 mol % was satisfying to control the number of nuclei present in the nonaqueous solution that allowed to grow cube-like BaTaO2N crystals freely.

Next, we studied the direct growth of BaTaO2N crystals at 950 °C for different reaction times under an NH3 atmosphere with 10 mol % solute concentration. The XRD patterns of the

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crystals grown at 950 °C for different reaction times are shown in Figure 4 against the evaporation rate of the KCl flux. Figures 4 and S3 in the ESI indicate that the BaTaO2N phase started to form at 950 °C for 0 h along with the hexagonal Ba5Ta4O15 (ICDD PDF18-0193), orthorhombic BaTa2O6 (ICDD PDF 20-0146), and not-yet-reacted Ta2O5 (ICDD PDF 25-0922) phases and eventually became a completely single phase at 950 °C for 4 h. Compared with the previously reported BaTaO2N crystals fabricated by high-temperature nitridation, an NH3-assisted direct flux growth approach was advantageous to directly grow phase-pure BaTaO2N crystals just within 4 h. Considering the sharp and intense diffraction lines, an extended period of reaction time up to 10 h resulted in the growth of highly crystalline BaTaO2N crystals. Ta2O5

Ba5Ta4O15

BaTa2O6

(g)

68.2%

Intensity (arb. units)

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(f)

67.6%

(e)

60.3% 44.4%

(d)

28.1% (c)

17.5% (b)

6.7% (a) ICDD PDF 84-1748 BaTaO2N

20

30

40

50

60

70

Evaporation rate of the KCl flux

2θ / degree

Figure 4. XRD patterns of crystals grown at 950 °C for (a) 0 h, (b) 1 h, (c) 2 h, (d) 4 h, (e) 6 h, (f) 8 h, and (g) 10 h. 12


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Figure 5 shows the SEM images of the crystals grown with 10 mol % solute concentration at 950 °C for different reaction times (0-10 h) under an NH3 atmosphere. As shown, the crystals grown at 950 °C for 0 h possess a vertically aligned plate-like crystal morphology. This sample contains BaTaO2N, Ba5Ta4O15, BaTa2O6, and Ta2O5 phases (Figure 4) but it was difficult to discriminate each phase’s specific morphology. With increasing the reaction time up to 4 h, plate-like crystals became thicker and turned into a completely irregular shape at 4 h. From the reaction time of 6 h, the BaTaO2N crystals commenced to gain their idiomorphic shapes with a clear edge. At the reaction time of 10 h, cube-like BaTaO2N crystals became more homogenous in size with average size of 125 nm. From the SEM observation, it can be concluded that the KCl flux and high-temperature NH3 atmosphere played an essential part for the growth of nonporous cube-like BaTaO2N crystals at the partial expenses of plate-like Ba5Ta4O15 and BaTa2O6 crystals.

Figure 6 shows the TEM and HRTEM images and SAED pattern of the BaTaO2N crystals grown. In Figure 6a, the bright-field TEM image indicates that the BaTaO2N crystals have a cube-like shape with a clear edge, and Figure 6b displays the high-resolution TEM image of an area shown with an arrow (Figure 6a). No obvious defects were observed in the lattice image, confirming the high crystallinity of the flux-grown BaTaO2N crystals. The highly ordered corresponding SAED pattern (Figure 6c) of the BaTaO2N crystal taken with the 13



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incident beam along the [110] direction evidences single-crystalline nature of cube-like BaTaO2N crystals. The presence of a typical cubic lattice structure was corroborated by indexing the SAED pattern. The #$$% and #%%$ spacings were determined to be 0.412 nm and 0.291 nm, respectively, which are consistent with the previously reported values of the #$$% = 0.411 nm and #%%$ =0.290 nm.8 The SEM and TEM results demonstrated that nonporous cube-like BaTaO2N crystals with single-crystalline nature could be directly grown with the exposed {100} and {110} faces by applying the present synthesis approach.

Figure 5. SEM images of crystals grown at 950 °C for (a) 0 h, (b) 1 h, (c) 2 h, (d) 4 h, (e) 6 h, (f) 8 h, and (g) 10 h. 14


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Figure 6. (a) TEM and (b) HRTEM images and (c) SAED pattern of BaTaO2N crystals. (d) Possible growth mechanisms of cube-like BaTaO2N crystals.

According to the results obtained from the time- and temperature-dependent experiments, the growth of cube-like BaTaO2N crystals had possibly the following reaction steps: (i) the decomposition of BaCO3 and dissolution of BaO and Ta2O5 in the KCl flux; (ii) reactant diffusion through the molten KCl flux; (iii) nucleation and growth of the plate-like BaTa2O6 and Ba5Ta4O15 crystals; (iv) the dissolution of the BaTa2O6 and Ba5Ta4O15 crystals, and (v) crystallization and growth of cube-like BaTaO2N crystals under an NH3 flow, as expressed by the following reactions: BaCO3 → BaO + CO2

(1)

BaO + Ta2O5 → BaTa2O6

(2)

2BaTa2O6 + 3BaO → Ba5Ta4O15

(3)

2Ba5Ta4O15 + 2BaTa2O6 + 12N → 12BaTaO2N + 9O2

(4) 15



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Briefly, the growth of cube-like BaTaO2N crystals is schematically illustrated in Figure 6d. At temperatures < 950 °C, BaCO3 was gradually decomposed, and the BaTa2O6 phase was initially formed through the reaction between BaO and Ta2O5, as shown in Figure S3. At 950 °C for 0 h, the BaTa2O6 phase was accompanied by the Ba5Ta4O15 phase due to the additional Ba2+ ions supplied by the complete decomposition of BaCO3, and BaTaO2N phases. By extending the reaction time to 2 h at 950 °C, the BaTa2O6 phase completely disappeared leaving behind the Ba5Ta4O15 phase as a still impurity phase to the BaTaO2N. An increase in the reaction time to 4 h at 950 °C resulted in the single-phase BaTaO2N, and the reaction time was further prolonged up to 10 h to explore the complete crystal growth. Generally, the dissociation of NH3 gas into NH2, NH, N2, and H2 occurs at high temperatures (> 700 °C).30,31 Due to the increased adsorption sites, the surfaces of the Ba5Ta4O15 and BaTa2O6 crystals adsorbed nitrogen, hydrogen, and ammonia radicals. However, an inappreciable amount of active atomic nitrogen produced at < 950 °C (< 4 h) induced the simultaneous formation of the Ba5Ta4O15 and BaTa2O6 phases additionally along with BaTaO2N phase. A further decomposition of ammonia species at higher temperature and longer reaction time provided a surplus of active atomic nitrogen on the surface. To form the BaTaO2N crystals through the nitridation process, the oxide anions in the Ba5Ta4O15 and BaTa2O6 crystals were gradually exchanged with the abundantly adsorbed active nitrogen atoms to give nitride anions because

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of similarity in ionic radius of N3– ion (0.132 nm) with an O2– ion (0.124 nm)32 and the released oxygen reacted with surface hydrogen to form water vapor molecules. The different anionic charges of the nitride and oxide anions induced a change of valence state of the tantalum cation.33 Along with the partial replacement of oxide anions with nitride anions under high-temperature NH3 atmosphere, the morphological phase transformation from plate-like Ba5Ta4O15 and BaTa2O6 crystals to cube-like BaTaO2N crystals with single-crystalline nature was succeeded with the assistance of the KCl flux. The Ostwald ripening mechanism can be applied to plausibly understand the formation of the Ba5Ta4O15 and BaTa2O6 plate-like crystals because the heat treatment temperature (950 °C) was higher than the melting temperature of KCl (770 °C). After the dissolution of BaO and Ta2O5, the KCl flux solution was supersaturated to a critical level, and the temperature was high enough to nucleate the Ba5Ta4O15 and BaTa2O6 crystals through a heterogenous mechanism under high-temperature NH3 atmosphere, and larger plate-like Ba5Ta4O15 and BaTa2O6 crystals grew at the expenses of the smaller crystallites via dissolution and precipitation.34,35 Meantime, under high-temperature NH3 atmosphere, the BaTaO2N crystals with the cubic structure tend to grow at the partial expenses of the Ba5Ta4O15 and BaTa2O6 crystals in the KCl flux. Considering the high evaporation rate of the KCl flux, shown in Figure 4, the growth of cube-like BaTaO2N crystals was driven by a cooling process. With increasing the

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reaction time, the crystal shape of the oxide precursors gradually changed from plate-like to larger irregular and then to ultimately submicron-sized cube-like BaTaO2N crystals with single-crystalline nature (Figure 5). Compared to the BaTaO2N crystals grown at 4-8 h, the BaTaO2N crystals grown at 10 h possessed more homogenous shape and smaller size. It is thought that the extended reaction time of 10 h probably caused a higher dissolution rate, resulting in a greater number of nuclei in the high-temperature nonaqueous solution. As a result, cube-like BaTaO2N crystals the average size of about 125 nm were grown. The {100} and {110} faces are believed to have lower surface energy for the preferential development in the equilibrium shape. Additionally, the lowered surface energy of these faces might also be stemmed from the abundantly adsorbed active nitrogen atoms along with the K+ ions having a capping effect for the formation of these faces.36 Under NH3 atmosphere, high-temperature KCl flux solution allowed the crystal faces of BaTaO2N with lower surface energy to grow gradually and hindered the formation of polycrystalline porous structures that are generally formed during the nitridation process of the oxide precursors because of lattice strain accumulation upon structural transformation.25,26 We have also studied the reoxidation of the flux-grown cube-like BaTaO2N crystals in synthetic air, and Figure 7a shows the TG-DTA traces from room temperature up to 1300 °C. The first exothermic event combined with a 1.82% weight gain centered at 764 °C is mainly

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attributed to the evolution of N2, and additionally, other gases such as O2, H2O, and CO2 from impurities. A dinitrogen retention phenomenon and the corresponding intermediate phases formed during the oxynitride-to-oxide transformation were represented by this weight increase behavior.37,38 The second exothermic event can be noted in a small peak at 1004 °C with a continuous weight loss that is probably assignable to the phase transformation from Ba5Ta4O15 to Ba3Ta5O15 and BaO2. A change in the valence of tantalum from Ta4+ to Ta5+ is also expected occur during this exothermic reaction.33 The XRD patterns shown in Figures 7b clearly indicate the reoxidation of the BaTaO2N crystals to the Ba4Ta5O15, Ba3Ta5O15, and BaO2 crystals. This oxynitride-to-oxide phase transformation was also followed by the morphological change. Namely, cube-like BaTaO2N crystals were converted into irregular pseudo-plate Ba4Ta5O15, Ba3Ta5O15, and BaO2 crystals, and the powder color was shifted from brown to white.

Figure 7. (a) TG-DTA curves, (b) XRD patterns and SEM images of BaTaO2N crystals before and after reoxidation up to 1300 °C. 19



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In order to gain more information on the chemical composition, elemental distribution, and impurities present in the cube-like BaTaO2N crystals, X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDS), and inductively coupled plasma optical emission spectroscopy (ICP-OES) analyses were carefully performed, and the results are shown in Figure 8. From the full XPS spectrum shown in Figure 8a, only Ba, Ta, O, N, and impurity carbon were observed, and no noticeable peaks assignable to other impurities were present on the surface, confirming the purity of the flux-grown BaTaO2N crystals. Nevertheless, further results from the ICP-OES analysis of the completely dissolved BaTaO2N crystals in aqua regia clearly show that 0.47 at% potassium was unintentionally introduced into the crystals. Figure 8b also shows the mapping images of five elements (Ba, Ta, O, N, and K) present in the BaTaO2N crystals. These evenly distributed colorful points of these elements further prove the homogenous composition of the BaTaO2N crystals. The signal of potassium can also be easily noticed in the corresponding EDS spectrum shown in Figure 8h, implying that the potassium was in fact unintentionally introduced into the BaTaO2N crystals, which is consistent with the ICP-OES results. Despite the fact that the potassium was introduced, the atomic ratios of elements are nearly close to their feeding molar ratios.

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Figure 8. (a) Wide XPS spectrum and (inset) ICP-OES result of BaTaO2N crystals. EDS analysis: (b) grey image, (c) Ba mapping image, (d) Ta mapping image, (e) O mapping image, (f) N mapping image, (g) K mapping image, and (h) corresponding spectrum.

In the UV-Vis diffuse reflectance spectrum of cube-like BaTaO2N crystals (Figure 9), the onset of light absorption of BaTaO2N was observed at around 660 nm, which also agrees with the previous report.39 The background absorption beyond the absorption edge wavelength, which is generally considered as an indicator of defect density, was not observed, suggesting that an NH3-assisted flux method was advantageous to directly grow cube-like BaTaO2N crystals with high crystallinity and less defects related to reduced tantalum species and anion vacancies.

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Figure 9. UV-Vis diffuse reflectance spectrum of BaTaO2N crystals. The reaction time courses of H2 and O2 evolution over cube-like BaTaO2N crystals under visible light are presented in Figure 10. In the absence of cocatalysts, cube-like BaTaO2N crystals did not promote photocatalytic activity for H2 and O2 evolution under visible light. In contrast, as shown in Figures 10a and b, the BaTaO2N crystals generated H2 and O2 under visible light (λ > 420 nm) after modification with 0.3 wt% Pt and CoOx (2 wt% Co) nanoparticles, respectively, suggesting that cube-like BaTaO2N crystals can be activated by modifying with appropriate cocatalysts. Considering the amount of evolved gases in this study, the BaTaO2N particles treated with a H2 stream and post-necking with TaCl5, pre-loaded with CoOx and post-loaded with RhOx previously showed much higher photocatalytic activity for the generation of H2 and O2 because of decreased electrical resistance and improved the photocurrent and stability under visible light irradiation.16 22


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Compared to the previously reported data for the BaTaO2N modified with 1.5 wt% IrO2 and 0.3 wt% Pt cocatalyst nanoparticles,39,40 cube-like BaTaO2N crystals, in this study, modified with Pt and CoOx nanoparticles showed a relatively higher H2 and O2 evolution which is thought to be attributed to the decreased defect density and higher crystallinity achieved by an NH3-assisted flux method,41 which otherwise would have acted as recombination centers

Amount of H2 evolved (µmol)

between photogenerated electrons and holes. 18 16 (a) 14 12 10 8 6 4 2 0 0 2

4

6

8

10 12 14

Time / h

Amount of O2 evolved (µmol)

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


(b)

20 15 10 5 0

0

1

2

3

4

5

6

Time / h

Figure 10. The reaction time courses of (a) H2 and (b) O2 evolution over cube-like BaTaO2N crystals under visible light irradiation.

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The effects of defects on the behavior of photogenerated charge carriers in 1-step-BTON and 2-step-BTON were investigated by transient absorption spectroscopy (TAS) from visible to mid-infrared region (20000 – 1000 cm-1). The details of TAS measurements were reported elsewhere.42,43 Figure 11 shows the TA spectra of 1-step-BTON and 2-step-BTON. The observed transient absorption from 20000 to 1500 cm-1 can be divided into three parts: a relatively sharp peak at ~ 16000 cm-1, broad absorption at 13000-6000 cm-1, and 3000-1500 cm-1. Absorption in these three regions is assigned to trapped holes, deeply trapped electrons at the defects, and free or shallowly trapped electrons, respectively.44,45 It is clearly shown that the intensity of holes giving an absorption peak at 16000 cm-1 is much higher in 1-step-BTON than in 2-step-BTON, suggesting that the number of surviving holes in 1-step-BTON is greater than in 2-step-BTON. Furthermore, the absorption intensity of free or shallowly trapped electrons in 1-step-BTON is also higher than in 2-step-BTON. These results clearly indicate that 1-step-BTON possesses much higher amount of photogenerated charge carriers which can be efficiently utilized for the photocatalytic reactions; therefore, 1-step-BTON is expected to show higher photocatalytic water splitting activity than 2-step-BTON. In addition, the absorption intensity of deeply trapped electrons (13000-6000 cm-1) is much larger in 2-step-BTON than in 1-step-BTON, implying that the number of defects in 2-step-BTON is higher than in 1-step-BTON. Since free-electrons or electrons in the

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shallow traps are highly reactive than those electrons in the deep traps, it is expected that 1-step-BTON should exhibit higher photocatalytic water splitting activity than 2-step-BTON.

∆Abs = 1 x 10

-3

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

∆Abs = 0.5 x 10

-4

0 60 00 5 00 0 40 00 30 00 2 00 0 Wavenumber / cm-1

(a)

0 ∆Abs = 0.5 x 10

-3

0 60 00 50 00 400 0 3 00 0 20 00 Wavenumber / cm-1

(b)

0 2 0000

1 5000

1 0000

5000

Wavenumber / cm

1000

-1

Figure 11. Transient absorption spectra of bare (a) 1-step-BTON and (b) 2-step-BTON samples. Insets represent the expanded spectra obtained from 6000 – 1500 cm-1. Highly crystalline 2D structures of visible-light-active oxides and oxynitrides are expected to exhibit an improved photocatalytic activity compared to their bulk (large 3D) counterparts because the travel distance of photoexcited carriers in the 2D structures can be shortened, many photons can be absorbed in a short time under low photon flux density due to a large section area, and cocatalyst nanoparticles can be homogenously deposited on the exposed 25



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surface.46 In this context, we have also attempted to directly grow the 2D structures of the BaTaO2N crystals by an NH3-assisted flux method using various fluxes in this study. As the morphology control of the BaTaO2N crystals using various fluxes was not satisfactorily fulfilled in this work due mainly to the high reactivity, instability, and melting point-mismatch of the employed fluxes under a high-temperature reductive NH3 atmosphere, our study on the growth of plate-like BaTaO2N crystals via a two-step fabrication method and their effect on the photocatalytic H2 and O2 evolution under visible light is currently in process and will be reported in the forthcoming publication. CONCLUSIONS In summary, cube-like BaTaO2N submicron crystals were directly grown by an NH3-assisted flux method using the KCl flux. By changing the solute concentration, it was deduced that the single-phase BaTaO2N crystals could be grown with the solute concentrations of ≥ 10 mol %, and the solute concentration of 10 mol % was solely favorable to directly grow cube-like BaTaO2N crystals with average size of about 125 nm and exposed {100} and {110} faces at 950 °C for 10 h. The rigorous chemical analyses revealed that 0.47 at% potassium from the KCl flux was unintentionally introduced into the BaTaO2N crystals. The BaTaO2N crystals modified with Pt and CoOx nanoparticles showed a reasonable H2 and O2 evolution due to the lower defect density and higher crystallinity. The absence of

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significant background absorption in the UV-Vis spectrum also confirmed that an NH3-assisted flux method was advantageous to directly grow cube-like BaTaO2N crystals with higher crystallinity and lower defects. ASSOCIATED CONTENT Supporting Information

XRD patterns and SEM images of the crystals grown with KCl, BaCl2, K2CO3 and SrCl2 fluxes. Note that KI, KF, MgCl2, CaCl2, SrCl2, BaCl2, K2SO4, K2MoO4, and K2CO3 were found to be not suitable fluxes for the direct growth of the BaTaO2N crystals under a high-temperature reductive NH3 atmosphere. The XRD patterns of the crystal products grown using the KCl flux at different temperatures for 0 h under an NH3 atmosphere. Synthesis procedure, XRD pattern, SEM images, and UV-Vis diffuse reflectance spectrum of BaTaO2N crystals fabricated by a two-step method for comparison. Decay curves of transient absorption intensity at 15600 and 15200 cm-1 of 1-step-BTON and 2-step-BTON, respectively. This material is available free of charge via the Internet at http://pubs.acs.org.

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

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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 partially supported by the Japan Technological Research Association of Artificial Photosynthetic Chemical Process (ARPChem). The authors thank Drs. Takashi Hisatomi and Qian Wang of the University of Tokyo, Japan, for the H2 and O2 evolution experiments.

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For Table of Contents Use Only

NH3-Assisted Flux Growth of Cube-Like BaTaO2N Submicron Crystals in a Completely Ionized Nonaqueous High-Temperature Solution and Their Water Splitting Activity Mirabbos Hojamberdieva, Kunio Yubutab, Junie Jhon M. Vequizoc, Akira Yamakatac, Shuji Oishia, Kazunari Domend and Katsuya Teshimaa,e

Cube-like BaTaO2N crystals with average size of about 125 nm and exposed {100} and {110} faces were directly grown by a one-step NH3-assisted flux method using the KCl flux and solute concentration of 10 mol % at 950°C for 10 h. A reasonable H2 and O2 evolution was achieved due to lower defect density and higher crystallinity.

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NH3-Assisted Flux Growth of Cube-like BaTaO2N Submicron Crystals

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