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C: Energy Conversion and Storage; Energy and Charge Transport

Template-Assisted Size Control of Polycrystalline BaNbON Particles and Effects of Their Characteristics on Photocatalytic Water Oxidation Performances 2

Tetsuya Yamada, Yukinori Murata, Sayaka Suzuki, Hajime Wagata, Shuji Oishi, and Katsuya Teshima J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12159 • Publication Date (Web): 20 Mar 2018 Downloaded from http://pubs.acs.org on March 21, 2018

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The Journal of Physical Chemistry

Template-assisted

Size

Control

of

Polycrystalline

BaNbO2N Particles and Effects of Their Characteristics on Photocatalytic Water Oxidation Performances Tetsuya Yamada,† Yukinori Murata,‡ Sayaka Suzuki,‡ Hajime Wagata,‡ Shuji Oishi, ‡§ Katsuya Teshima*,†, ‡

† Center for Energy and Environmental Science Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan ‡ Department of Materials Chemistry, Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan § Nagano Prefecture Nanshin Institute of Technology, 8304-190 Minamiminowa, Nagano 3994511, Japan

ABSTRACT The photocatalytic water oxidation using solar irradiation is a sustainable way to convert natural source to energy. The perovskite-type oxynitride BaNbO2N is a candidate photocatalyst for this process, because its long-range light absorbance up to ca. 740 nm leads to high ability of energy

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conversion. However, it is necessary to improve its poor performance by optimizing the crystallographic characteristics, chemical formula, depositions of co-catalysts, and so on. In this study, we aimed to identify the dominant factors of the photocatalytic performance of BaNbO2N. We controlled the particle characteristics by nitriding size-controlled Ba5Nb4O15 crystals in sizes of 0.2–50 µm as sacrificial templates. Porous BaNbO2N secondary particles of different sizes were achieved, and they exhibited distinctive photocatalytic performances for O2 evolution with rates between 14.1 and 113.9 µmol·h-1, depending on the precursor size and nitriding time. By correlating the performance with the basal particle characteristics, we assume the crystallinity and anion deficiency are the two dominant factors that competitively affect the photocatalytic performance of BaNbO2N.


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1. Introduction Nowadays, development of energy conversion system from natural sources is essential for construction of large-energy consumption but sustainable society. Light source is one of candidates of natural energy on this matter, because of its environmental-friendliness, ubiquitous, and practically unlimited natures. Light source can be converted to electric energy and storage in the form of carbon hydride, hydrogen gas by using artificial photosynthesis and photocatalytic water splitting. In these procedures, water oxidation is a dominant pathway to drive the reaction. Since the discovery of water splitting using TiO2 under ultraviolet light in 1972, the photocatalytic water oxidation has been studied in many ways: understanding its reaction mechanism,1,2,3 modifying the reaction pathway by adding electron donors as sacrificial regents,4,5,6,7 improving the performance by hybridization with other compounds (including metals8,9 and semiconductors10,11), improving material designs,12,13,14 and so on2,3,15,16. A recently highlighted issue is the utilization of sunlight as energy source for photocatalytic water oxidation. Solar light has only 5% of its energy in ultraviolet light (< 400 nm), but ca. 52 % in visible light (400 to 750 nm). Therefore, the development of visible-light responsive photocatalysts would be desirable for sustainable water oxidation through the effective use of solar energy. (Oxy)nitrides are promising candidates for visible-light response photocatalysts. With higher energy for the valence band from nitrogen atoms than that from oxygen atoms, (oxy)nitrides possess energy band gaps favorable for absorbing visible light and splitting water. Barium niobium oxynitride (BaNbO2N) is a perovskite-type compound consisting of cornersharing NbO4N2 octahedra and Ba ions at their centers to form a cubic crystal structure, which has a lattice parameter of a = 4.12845(8) Å and space group of Pm-3m.17 This structure is similar


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to the well-known cubic-type perovskites represented by ABO3 (A, B = metallic cations). A notable optical feature of BaNbO2N is its superior visible-light absorption up to ca. 740 nm (band gap energy: ca. 1.7 eV) compared to other (oxy)nitrides.18,19,20 For example, the absorption edges are 700 nm for SrNbO2N,21 650 nm for LaTaO2N22 and BaTaO2N,23 and 600 nm for LaTiO2N24 and Ta3N5.25 Since the number of absorbed photons depends on the wavelength at the absorption edge, BaNbO2N is expected to exhibit superior energy conversion with solar irradiation than other (oxy)nitrides. So far, there are a few reports about the preparation and photocatalytic activity of BaNbO2N. Hisatomi et al. found that the Nb ion is easily reduced to inactive Nb4+ species during nitridation, and preparing BaNbO2N from Ba5Nb4O15 as a precursor satisfactorily inhibited the reduction.26 They also showed that the addition of Ba species during the transformation from Ba5Nb4O15 affected the photocatalytic O2 evolution performances of BaNbO2N, although the apparent quantum efficiency of this reaction remained at 0.04 % at 640 ± 30 nm.19 Seo et al. prepared BaNbO2N nanocrystals from another precursor (BaNbO3) to improve the surface and bulk crystallinity. The prepared material exhibited photocurrent over the order of mA/cm2.27 These previous reports indicate that the synthetic conditions can critically affect the photocatalytic performance of prepared BaNbO2N, and there is plenty of space to further improve the efficiency for practical applications. The photocatalytic performance is known to depend on a variety of particle characteristics. Specific surface area,28,29,30 crystal habits,31,32 qualities of crystallinity,30, 33,34 and deficiencies35,36 are the representative morphological factors. To improve the photocatalytic performance of BaNbO2N, a precise understanding of the contribution of each factor to the performance would be essential. Therefore, the preparation and


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testing of BaNbO2N samples with different crystallographic characteristics would help to evaluate these contributions. In our previous study, we grew layered perovskite-type Ba5Nb4O15 crystals of sizes between ca. 0.2–50 µm with a monodispersed, idiomorphic nature, by utilizing the molten flux of BaCl2.37 Nitriding these size-controlled Ba5Nb4O15 crystals as precursor templates could produce crystallographically different BaNbO2N products in terms of morphologies and crystal qualities. (I) In terms of morphologies, it was already known that layered perovskites A5B4O15 (A, B = metallic cations) can be nitrided to form cubic-type oxynitrides of ABO2N while maintaining their original appearances, in spite of the particles becoming porous and split into finer particles.38 As the particle size is related to the specific surface area, the nitridation of sizecontrolled Ba5Nb4O15 crystals could afford BaNbO2N with different specific surface areas. (II) In terms of crystal quality, products of poor crystallinity are expected to form initially in the transformation from Ba5Nb4O15 to BaNbO2N, and the crystallinity is improved as the nitridation continues. Simultaneously, the Nb5+ ion is easily reduced to form the anion-deficient species of BaNbO2-xN1-y (x < 2, y < 1) and/or byproducts containing Nb4+.19 Since the reactive areas may be different among the size-controlled Ba5Nb4O15 crystals, their nitridation should produce BaNbO2N with different amounts of crystallinity and anion deficiencies. Based on these considerations, herein we prepared crystallographically different BaNbO2N particles by nitriding the size-controlled Ba5Nb4O15 crystals as precursor templates. The samples’ particle characteristics, including specific surface area, crystallinity, and anion deficiency were investigated. We also examined the photocatalytic water oxidation abilities of the produced BaNbO2N samples by measuring the oxygen evolution under visible-light irradiation. The contributions of different particle characteristics to the photocatalytic


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performances were discussed, and a mechanism of the nitridation process is proposed. These results would provide important information to clarify the dominant factors in the photocatalytic performances.

2. Experimental BaNbO2N particles were prepared by nitridation of size-controlled Ba5Nb4O15 crystals. Based on a previous report,37 size-controlled Ba5Nb4O15 crystals were grown in molten BaCl2 flux at a solute concentration of 10 mol% at designated temperature for 10 h. For preparation of Ba5Nb4O15, BaCO3 (purity of 99.0%) and Nb2O5 (99.9%) were employed as solutes and BaCl2·2H2O (99.9%) was used as a flux. All reagents were purchased from Wako Pure Chemical Industries, Ltd., and were used without further purification. The reagents were mixed for 30 min and placed in a platinum crucible. They were heated to setting temperatures at a rate of 50 °C·h−1 and held for 10 h in an electric furnace. Then, the crucibles were cooled to 500 °C at 150 °C·h−1, followed by air cooling to room temperature. The crystal products were separated from the remaining flux in warm water. Finally, the crystals were sieved by a mesh with an inner diameter of 28 µm to isolate the larger crystals. The nitridation procedure was performed as follows. Approximately 1 g of the flux-grown Ba5Nb4O15 crystals was put in an aluminum crucible. The crucible was placed in a vertical tube furnace and heated to 950 °C at the rate of 600 °C·h−1, held for 10 h, then naturally cooled to room temperature in the furnace. The atmosphere in the furnace was set in N2 below 300 °C, and in NH3 above 300 °C at a flowing rate of 200 mL·min-1. The obtained products were washed with 0.1 M HCl to remove byproducts and dried at 100 °C.


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The crystal phases of the final products were identified by X-ray diffraction (XRD, RIGAKU, MiniflexII) with Cu Kα radiation (λ = 0.154 nm) at 30 kV and 15 mA, using a scan speed of 20°·min-1 across 2θ = 10°–80°. The crystal morphologies were observed by scanning electron microscopy (SEM, JEOL, JCM-5700) at an acceleration voltage of 15 kV. Particle size distributions were evaluated using a nanoparticle size analyzer (SHIMAZU, SALD-7100). The specific surface areas were estimated by gas adsorption analysis. After degassing the products in vacuum at 100 °C for 5 h, nitrogen adsorption isotherm was measured at 77 K (BEL Japan, Inc., BELSORP-mini). The surface compositions were analyzed by X-ray photoelectron spectroscopy (XPS, JEOL, JPS-9010MX). XPS spectra were obtained by sweeping the energy using continuous X-ray with Mg Kα radiation at 10 kV and 10 mA. The carbon 1s peak was calibrated to 284.5 eV for charge correction. Thermal weight changes were examined by thermogravimetric analysis (TG, Rigaku, Thermo plus EVOII). Samples were set on an alumina pan, and then TG profiles were recorded during heating at the rate of 10 °C·min-1 up to 1000 °C in air atmosphere. To remove the contribution of absorbed impurity such as water molecules, we normalized the weight change by dividing it with the weight at 300 °C. The light absorption properties were examined by ultraviolet−visible spectroscopy (UV-vis, JASCO, V-630), recorded in the diffused reflex mode in the range between 200 and 800 nm. The photocatalytic water oxidation performance was evaluated from O2 gas evolution reactions with BaNbO2N particles under visible light irradiation. A co-catalyst was deposited by an impregnation process as follows. BaNbO2N particles were immersed in an aqueous solution of Co(NO3)2·6H2O (Kanto Chemicals) to reach 2wt% Co in CoOx/BaNbO2N.19 The mixture was continuously mixed while drying in hot water, and then heated at 500 °C for 1 h under NH3 atmosphere to obtain the co-catalyst deposited BaNbO2N. The photocatalytic O2 evolution was


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measured as follows. The photocatalyst of CoOx/BaNbO2N complex (0.10 g) was put in 300 mL of aqueous solution containing distilled water, 50 mM AgNO3 (Wako Pure Chemical Industries, Ltd.) as sacrifice agent, and 0.2 g of La2O3 (Wako Pure Chemical Industries, Ltd.) as a pH buffer. The reaction flask was covered by aluminum foil, placed in an isothermal bath at 20 °C, and connected to a closed-circulation system. Finally, the reactor was irradiated by a 300 W Xe lamp from the side, through a filter that removes UV light with wavelength less than 420 nm to induce photocatalytic O2 evolution. The evolved O2 was detected by gas chromatography (GC8A, TCD, Ar carrier, Shimadzu). During the photo-irradiation, the reactants in the reactor were continuously stirred to maintain homogeneity.

3. Results and discussion 3.1. Preparation BaNbO2N particles For convenience, we abbreviated the names of Ba5Nb4O15 crystals and nitrided products as BNO-Xs and BNON-Xs, respectively, X being the holding temperature (°C) under which Ba5Nb4O15 crystals were grown in the molten BaCl2 flux before conversion into nitrided products. Similar to reference,37 the BNO-X samples were grown in a controllable manner at temperatures between 600 and 1000 °C for 10 h (see Figure S1). The median particle size (D50) of BNO-600, -700, -800, and -900 are shown in Table 1, increasing from 1.5 to 3.29 µm as the holding temperature increased (see also Figure S2 for the size distribution of BNO-Xs). Heating at 1000 °C provided BNO-1000 with particle size around 50 µm, based on the SEM image in Figure S1(e). Subsequent nitridation of the size-controlled BNO-X samples led to the corresponding oxynitride products. Figure 1 shows the XRD profiles of the nitrided products


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prepared with a holding temperature of 950 °C for 10 h in NH3 gas flow. The XRD peaks of BNON-600, -700, -800, and -900 all exhibit a single phase, which is assigned to BaNbO2N [International Centre for Diffraction Data, Powder Diffraction File (ICDD PDF), 84-1749]. In contrast, the patterns of BNON-1000 show a slight amount of BaNbO2N and a majority of Ba5Nb4O15. In this case, the nitridation did not complete within 10 h, presumably due to the low reaction area on the very large Ba5Nb4O15 crystals. Further holding this sample up to 30 h successfully transformed BNON-1000 into the single phase of BaNbO2N, albeit in the form of fragments. Therefore, from this point on we focus on the particle characteristics and photocatalytic performances of other samples (BNON-600, -700, -800, and -900). In the SEM images (Figure 2), the sizes of BNON-X secondary particles were similar to those of the precursor primary particles. Their D50 values increase with X from 1.88 to 3.24 µm as shown in Table 1 (see also Figure S3 for the size distributions of BNON-Xs), which is clearly related to the morphologies of BNO-Xs as the sacrificial template. There are many submicron-sized pores on the surfaces of BNON-X originating from the stoichiometric change of Ba:Nb from Ba5Nb4O15 to BaNbO2N.19 The nitrogen gas adsorption properties of BNON-X were examined to estimate their specific surface areas. Fitting the N2 isotherms at 77 K in Figure S4 by using the Brunauer– Emmett–Teller (BET) equation, the estimated specific surface areas are shown in Table 1. Unexpectedly, there are a few differences of the specific surface areas of BNON-Xs that do not correspond to their morphologies, including the particle sizes (D50). Since surface area should decrease as particle size increases, these results cannot by explained only by size of particles. It is speculated that the appearances of submicron-sized pores in BNON-800 and -900 also contributed to increase the surface areas during the nitridation, resulting in similar surface areas among them.


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Figure 1. XRD patterns of BNON-Xs (X = 600, 700, 800, 900, and 1000) prepared from the fluxgrown Ba5Nb4O15 crystals. The XRD pattern of the reference (ICDD PDF 84-1749) is also shown, with labeled Bragg peak of 110.

Figure 2. SEM images of BNON-Xs [X = (a) 600, (b) 700, (c) 800, and (d) 900] prepared from BNO-Xs.


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Table 1. Particle sizes and specific surface areas of BNO-X and BNON-X samples (X = 600, 700, 800, and 900). X 600 700 800 900

Particle size at D50 / µm BNO-X BNON-X 1.50 1.88 1.68 2.04 2.46 2.95 3.29 3.24

Specific surface area / m2·g-1 BNON-X 8.77 10.70 10.00 9.18

3.2. Particle characteristics The particle characteristics of BNON-Xs were investigated in terms of their crystal qualities, including crystallinity and anion deficiencies. The crystallinities were evaluated from the XRD data. The full width at half maximum (FWHM) of BNON-X was estimated using the Bragg peak of the (110) face in the XRD profiles of Figure 1. According to the results in Table 2, the FWHM of BNON-X slightly decreased with decreasing X, indicating an improvement of the crystallinity. The anion deficiencies were examined by analyzing the states of Nb on the BNONX surfaces using XPS analysis. Figure 3 shows the spectra corresponding to the binding energies of Nb 3d. The two identical peaks at ca. 209.6 and 206.8 eV are denoted as A1, A2, respectively. Referring to the XPS spectra of Nb2O5 (binding energy of Nb 3d5/2 is ca. 207 eV),39,40 in which the local structure of octahedral NbO6 units is similar to BaNbO2N, A1 and A2 should be assigned to 3d 3/2 and 3d 5/2 of the Nb5+ state, respectively. There is another peak at ca. 205.3 eV (denoted as B2), whose bonding energy corresponds to that of NbO2 (ca. 205 eV for Nb 3d5/2).41 Hence B2 is probably assignable to Nb 3d 5/2 of the Nb4+ state. Assuming two distinctive components in each peak, the spectra were deconvoluted into two pairs of peaks


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denoted as A (A1, A2), B (B1, B2) in the figure. Setting the peak intensities of A1/A2 and B1/B2 to 2/3 and their widths to 1 with 0.7 of the ratios of Gaussian to Lorentzian functions, the simulated curves could well describe the experimental spectra for all BNON-Xs. As the valence state of Nb in pure BaNbO2N is 5+, the peaks A1 and A2 correspond to BaNbO2N; and peaks B1, B2 indicate reduction products of BaNbO2N (including possible anion-deficient species and Nb4+ byproducts).19 The simulated peak areas of A1+A2 and B1+B2 are summarized in Table 2. As X increases from 600 to 800, the relative peak area of A in BNON-X gradually increases, suggesting a decrease of anion deficiency/Nb4+ byproducts in the BaNbO2N. BNON-900 and 800 exhibit similar relative peak areas of A, suggesting similar amounts of anion deficiency/Nb4+ byproducts. We further investigated the depth profiles of the XPS spectra of Nb by using Ar ion sputtering and angle-resolved analysis. However, the former did not produce reasonable data, because the Nb species are easily reduced during Ar ion sputtering. The angle-resolved analysis produced no difference in the canted angles between 0 and 45°, which indicate the valence state of Nb does not differ in the nanoscale-depth direction near the outmost layer in the XPS resolution. To qualitatively estimate the amount of nitrogen in each BNON-X, we performed TG analyses as shown in Figure S5(a). As the temperature increased, their weights all increased till around 700 °C, then decreased up to ca. 800 °C, and remained constant afterwards. We confirmed that Ba5Nb4O15, Nb2O5, and NbO were formed after heating at 1000 °C, as shown in Figure S5(b). If the BNON-X possesses anion deficiency, then its exact formula can be expressed as BaNbO2-xNy (-x+y

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