Facile Morphological Modification of Ba5Nb4O15 Crystals Using

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Facile Morphological Modification of Ba5Nb4O15 Crystals Using Chloride Flux and in Situ Growth Investigation Tetsuya Yamada,† Yukinori Murata,‡ Hajime Wagata,‡ Kunio Yubuta,§ and Katsuya Teshima*,†,‡ †

Center for Energy and Environmental Science, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan Department of Environmental Science & Technology, Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan § Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan ‡

S Supporting Information *

ABSTRACT: The cation-deficient layered perovskite oxide Ba5Nb4O15 is one of the functional materials that exhibits a microwave-responsive dielectric property and an ultravioletactive photocatalytic property. Although systematic control of the morphology of Ba5Nb4O15 is beneficial for improving these properties, synthesized Ba5Nb4O15 usually has a plate-like shape owing to its crystal structure, with a particle size less than 5 μm. For systematic morphological control of Ba5Nb4O15, the crystal growth was studied by using a chloride-based flux method. Idiomorphic plate-like Ba5Nb4O15 crystals up to 50 μm in size and polyhedron ones ∼10 μm in size were obtained using a BaCl2 flux by changing the solute concentration to 5−20 mol % and 50 mol %, respectively. The growth of the Ba5Nb4O15 crystals was investigated by thermogravimetric and differential thermal analysis and in situ X-ray diffraction analysis. These analyses revealed the flux-growth manner of Ba5Nb4O15 as follows: (I) Ba5Nb4O15 was formed by a solid-state reaction above ∼650 °C. (II) After the melting of BaCl2 above ∼962 °C, the Ba5Nb4O15 crystals became larger and assumed idiomorphic shapes, indicating that they were somewhat dissolved in the flux and that the crystal growth was promoted. Increasing the holding time yielded an increased number of crystals larger than 28 μm. This indicates that Ostwald ripening effectively yields Ba5Nb4O15 crystals up to 50 μm in size. Chloride fluxes with different alkaline or alkaline earth cation fluxes did not produce such large crystals. It is assumed that the common ion effect of Ba2+ in the solute and flux provides an effective reaction field to facilitate Ostwald ripening.

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important issue. For microwave applications, fine crystals with a homogeneous size and shape and low aspect ratio are preferred for achieving a high-density sintered body,7 and a high crystallinity, large surface area, and high aspect ratio facilitate charge separation and electron transfer to improve the photocatalytic performance.8,9 Furthermore, large single crystals with a high crystallinity are suitable for studying the physical properties of materials, because of the ease of utilizing the crystal anisotropy and the absence of a grain boundary. Thus far, Ba5Nb4O15 crystals with a variety of shapes, such as rods,10 nanofibers,11 and plates,7 and sizes less than 5 μm have been prepared by several synthetic processes. Although these studies are interesting, there have been few systematic investigations for controlling the shapes and sizes of Ba5Nb4O15. Moreover, Ba5Nb4O15 crystals with unique morphologies are generally prepared by hydrothermal methods. Under low-temperature conditions, the lattice defects are not thermally relaxed, and hydroxide species are easily introduced in the lattice.12 Therefore, it is difficult to obtain single crystals

INTRODUCTION Perovskite-type compounds have long fascinated researchers because of their wide variety of physical properties, including conductivity, dielectricity, magnetism, thermal electricity, and piezoelectricity. Perovskite derivatives with the chemical formula A5B4O15 are classified as a cation-deficient layered perovskites.1−4 A5B4O15, which is regarded as AB0.8O3, shows a deficit of 0.2 B site ions compared with the cubic perovskite structure of ABO3. This deficit of B site ions yields a distortion from a cubic structure to a close-packed five-layered asymmetric structure with a hexagonal Bravais lattice belonging to a space group of P3m1. These cation defects and structural distortions of A5B4O15 induce some electronic states to exhibit specific physical properties similar to those of other deficient perovskite derivatives. Ba5Nb4O15, as represented in Figure 1, is a popular material in the A5B4O15 perovskite group1 because of its high dielectric properties, including a dielectric constant of εr = 40; high quality factor of Q × f = 53 000 at 16 GHz; resonant-frequency temperature coefficient of τf = 78 ppm·°C−1;5 and photocatalytic water-splitting ability with a quantum yield of 8% at 270 nm under ultraviolet irradiation.6 To improve these properties, the engineering of the crystal morphologies is an © XXXX American Chemical Society

Received: April 6, 2016 Revised: May 24, 2016

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DOI: 10.1021/acs.cgd.6b00526 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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880 °C,19 which yields a far lower melting point than solo BaCl2 (m.p. 960 °C), indicating that the liquid phase appears at an early stage of the crystal growth in the system. The liquid phase promotes atomic diffusion in the reaction field, compared with solid-state synthesis. In addition, because Ba is a common element between Ba5Nb4O15 and BaCl2, there is no other cation contamination and no impurity phase formation. Considering these features, we expected the crystal growth of Ba5Nb4O15 to successfully occur by dissolution and reprecipitation in molten BaCl2. The morphological control of Ba5Nb4O15 crystals from the BaCl2 flux by changing the heating procedure and solute concentrations was investigated. The growth manner of the Ba5Nb4O15 crystals was also determined by in situ high-temperature X-ray diffraction (XRD) analyses. To investigate the effect of the cations of the chloride flux on the crystal growth of Ba5Nb4O15, other alkaline and alkaline earth chlorides, such as LiCl, NaCl, KCl, MgCl2, CaCl2, and SrCl2, were used as fluxes.

Figure 1. Graphical representation of the crystal structure of Ba5Nb4O15. Barium, niobium, and oxygen ions are indicated by green, yellow, and red balls, respectively. The octahedra represent NbO6 units composed of six oxygen ions at the top and niobium ions at the center.

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EXPERIMENTAL SECTION

BaCO3 (purity of 99.0%) and Nb2O5 (99.9%) were employed as solutes. As a flux, BaCl2·2H2O (99.9%), LiCl (99.0%), NaCl (99.5%,), KCl (99.5%), MgCl2·6H2O (97.0%), CaCl2 (95.0%), or SrCl2 (95.0%) was used. All reagents were purchased from Wako Pure Chemical Industries, Ltd., and were used without further purification. The experimental conditions for the BaCl2 flux are summarized in Table 1.

with few defects and a specific crystal shape. Developing a systematic procedure to control the morphologies of Ba5Nb4O15 crystals while achieving a higher crystallinity would be very helpful for more effective use of Ba5Nb4O15. The flux method, which is a liquid-phase method for crystal growth that uses high-temperature molten metals and molten salts as a solvent, enables us to grow crystals in a hightemperature solution without thermal strain, resulting in various idiomorphic crystal shapes and a high crystallinity. For decades, we have investigated various functional materials using the flux method. For example, polyhedron13 and platelike14 LiCoO2 crystals were systematically grown by changing the fluxes. The polyhedron crystals of LiCoO2 exhibit a superior battery capacity to plate-like crystals with a current rate of 0.1 C, suggesting the construction of effective pathways for Li ions.13 LaTiO2N was directly prepared as a visible lightresponsive photocatalyst in a flux with a NH3 flow and exhibited a higher oxygen-evolution rate from water with a sacrificial agent than photocatalysts prepared by a general method. This was attributed to the decrease in the defect concentration caused by the flux growth, as evidenced by timeresolve infrared spectroscopy, which showed an increase in the lifetime of excited carriers.15 Idiomorphic LiFePO4 crystals ∼1 mm in size were obtained by recrystallization driven by oversaturation due to the evaporation of the flux during heating.16 These studies show that the flux method provides flexible control of the crystal morphologies and improves the crystal quality. Regarding Ba5Nb4O15, the NaCl flux method has been applied, and plate-like Ba5Nb4O15 crystals ∼2 μm in size were prepared. However, the flux-grown Ba5Nb4O15 crystals were used only for a precursor of a BaNbO2N visible-lightactive photocatalyst; the growth manner was not investigated in detail.17,18 Until now, there have been no precise studies on the systematic control of the morphologies of Ba5Nb4O15 crystals by the flux method. Optimizing the flux-growth process of Ba5Nb4O15 crystals is beneficial not only for understanding the basics of crystal growth but also for applying the crystals to various functional materials. In this study, highly crystalline Ba5Nb4O15 crystals were grown from a BaCl2 flux by using BaCO3 and Nb2O5 as solutes. BaCO3 was decomposed to form BaO during heating. BaCl2 and BaO have a eutectic point of BaCl2: BaO = 15:85 mol % at

Table 1. Experimental Conditions for the Preparation of Ba5Nb4O15 Crystals under a BaCl2 Flux run no.

holding temperature/°C

holding time/h

cooling rate/°C·h−1

solute conc. /mol %

1 2 3 4 5 6 7 8 9 10 11 12 13 14

1000 700 800 900 1100 1000 1000 1000 1000 1000 1000 1000 1000 1000

10 10 10 10 10 10 10 10 10 10 10 10 10 0

150 150 150 150 150 150 150 150 150 150 150 5 rapid cooling 150

5 5 5 5 5 100 80 50 20 10 1 5 5 5

Stoichiometric amounts of BaCO3 and Nb2O5 were mixed with BaCl2· 2H2O to obtain solute concentrations of 1, 5, 10, 20, 50, 80, and 100 mol %. Note that the solute concentration is defined as the molar ratio of Ba5Nb4O15 for the fluxes. For all conditions, the total masses of the reagents were set to be ∼10 g. The reagents were mixed for 30 min and placed in a platinum crucible with a capacity of 30 cm3. After the lids were loosely closed, the crucibles were placed in an electric furnace. They were heated to setting temperatures at a rate of 50 °C· h−1 and held at these temperatures for a certain time. Then, the crucibles were cooled to 500 °C at an arbitrary setting rate, followed by air cooling to room temperature in the furnace, except for Run no. 13. In the case of Run no. 13, the crucible was rapidly cooled from 1000 °C to room temperature with water. 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. In the case where crystals larger than 28 μm were separated from smaller ones by the sieving, we mainly characterized the larger crystals. When almost all the crystals passed through the 28-μm mesh, we characterized these crystals without distinction. This is mentioned in the figure captions B

DOI: 10.1021/acs.cgd.6b00526 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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for each sample, and details are provided in the Supporting Information (see Figures S1 and S2). Ba5Nb4O15 crystals were also prepared using the other chloride fluxes under heating, with the following conditions: a solute concentration of 5 mol %, ramp rate of 50 °C·h−1, a holding temperature of 1000 °C, a holding time of 10 h, a cooling rate of 150 °C·h−1 to 500 °C, and air cooling to room temperature. The crystal morphologies of the obtained reactants were observed by scanning electron microscopy (SEM, JEOL, JCM-5700) at an acceleration voltage of 10 kV and transmission electron microscopy (TEM, TOPCON, EM-002B) at 200 kV. For the TEM observation, the crystallite was covered by epoxy, and then a crystal section was prepared by a focused ion beam. The chemical composition was analyzed by energy-dispersive X-ray spectroscopy (Ultra Dray, Thermo Fisher Scientific) at an acceleration voltage of 15 kV. The crystal phases were identified by XRD (RIGAKU, MiniflexII) with Cu Kα radiation (λ = 0.154 nm). The X-ray diffractometer was operated at 30 kV and 20 mA in a 2θ range of 10−80°. Thermogravimetric and differential thermal analysis (TG-DTA, Rigaku, Thermo plus EVOII) was performed to investigate the chemical reaction and phase transformation during the flux growth. A mixture of BaCO3 and Nb2O5 with BaCl2·2H2O as the flux at a solute concentration of 5 mol % was set on a platinum pan and heated to 1000 °C at a heating rate of 10 °C·min−1 in an air atmosphere. For further detailed examination during the growth, the phase transformation of the reactants was observed using an in situ high-temperature XRD instrument (Rigaku, SmartLab) with Cu Kα radiation (λ = 0.154 nm). The experimental conditions of the samples and the heating process were the same as those for the TG-DTA measurement. The sample was placed on a sample holder made of black silica glass covered by Au foil, and then the holder was set in a chamber equipped with an infrared heating system. During the heating, continuous XRD measurements were performed with a scan speed of 2θ = 20° min−1, and the temperature difference between the beginning (2θ = 10°) and the end (2θ = 40°) of each measurement was less than 30 °C.

phases. The absence of the impurity phase for preparation at higher temperatures was caused by sieving, as there are large size differences between Ba5Nb4O15 crystals and impurity compounds under high-temperature conditions (see Supporting Information Figures S1 and S2). Figure 3 shows SEM images of the Ba5Nb4O15 grown at different temperatures. The size of the crystals increased up to ∼50 μm as the holding temperature increased from 700 to 1000 °C. There was almost no size difference between the crystals grown at 1000 and 1100 °C. Particulate crystals with undefined shapes were prepared at temperatures of 700 and 800 °C, whereas polyhedron and plate-like crystals with well-developed faces appeared at holding temperatures of 900, 1000, and 1100 °C. These results indicate that the holding temperature critically affected the flux growth. We also performed the preparation at 600 °C, which resulted in the incomplete formation of Ba5Nb4O15. Next, the solute-concentration dependence of the morphologies of Ba5Nb4O15 crystals grown at a holding temperature of 1000 °C was investigated. Figure 4 shows XRD patterns of the samples prepared at solute concentrations of 1, 10, 20, 50, 80, and 100 mol % in a high-temperature BaCl2 solution. The solute concentration of 100 mol % allowed a solid-state reaction without a BaCl2 flux. The XRD pattern of the sample prepared at the solute concentration of 100 mol % (Run no. 6) shows that the main crystal phase was Ba5Nb4O15 and that there were slight impurity phases. The XRD patterns of the impurities were not attributed to the starting materials but rather to reaction intermediates such as barium oxides (BaOx), niobium oxides (NbOx), and/or barium−niobium oxides (BaNbOx). At solute concentrations ranging from 5 (see Figure 2) to 80 mol %, the XRD patterns were attributed to Ba5Nb4O15 without impurity phases, indicating the successful preparation of the Ba5Nb4O15 crystals. The lattice parameters of the Ba5Nb4O15 crystals prepared at the solute concentration of 5 mol % were calculated from the XRD patterns to be a = 5.796 Å and c = 11.793 Å in the trigonal system, which agree with the values from the literature: a = 5.794 Å and c = 11.784 Å.1 At the solute concentration of 1 mol % (Run no. 11), Ba5Nb4O15 was not obtained, and the XRD patterns were not attributed to any raw materials or barium or niobium compounds. There are various barium−niobium oxides, depending on the atomic ratio of Ba and Nb in the phase diagram of BaO-Nb2O5.20 It is assumed that the excess Ba ions from BaCl2 contributed to the formation of various barium−niobium compounds at the solute concentration of 1 mol %. Figure 5 shows SEM images of Ba5Nb4O15 crystals grown at the different solute concentrations. At the solution concentration of 100 mol % (Run no. 6), aggregated particulate crystals with primary particles less than 1 μm in size were obtained (Figure 5a). This indicates that the solid-state reaction hardly grew Ba5Nb4O15 crystals at 1000 °C. At the solute concentration of 80 mol % (Run no. 7), the crystal faces were slightly developed to form polyhedron shapes, and the crystal size increased to ∼2 μm, as shown in Figure 5b. At the solute concentration of 50 mol % (Run no. 8), the crystal morphology apparently changed to a polyhedron-like shape with crystal sizes up to ∼10 μm (Figure 5c). At the solute concentrations of 20 mol % (Run no. 9) and 10 mol % (Run no. 10), there were face-developed plate-like crystals with an average size of 50 μm, as shown in Figure 5d,e, respectively. In these cases, the morphologies of the Ba5Nb4O15 crystals were similar to that of the sample prepared at a solute concentration of 5 mol % (Run no. 1), as shown in Figure 3d. At the solute concentration of 1 mol % (Run no. 11), the

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RESULTS AND DISCUSSION Figure 2 shows the XRD patterns of samples prepared at various holding temperatures of 700, 800, 900, 1000, and 1100

Figure 2. Powder XRD profiles of as-prepared Ba5Nb4O15 crystals prepared from a BaCl2 flux with a solute concentration of 5 mol % at holding temperatures of 700, 800, 900, 1000, and 1100 °C for 10 h (Run nos. 1−5, respectively). The arrows indicate the unidentified peaks. For the samples prepared at 1000 and 1100 °C, sieved crystals larger than 28 μm were characterized.

°C at a solute concentration of 5 mol % (Run nos. 1−5, respectively). XRD patterns assigned to Ba5Nb4O15 [International Centre for Diffraction Data Powder Diffraction File (ICDD PDF) 01-070-9002] were confirmed for all the conditions, although they included a small amount of impurity C

DOI: 10.1021/acs.cgd.6b00526 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Figure 3. SEM images of as-prepared Ba5Nb4O15 crystals prepared from a BaCl2·2H2O flux with a solute concentration of 5 mol % at holding temperatures of (a) 700 (Run no. 2), (b) 800 (Run no. 3), (c) 900 (Run no. 4), (d) 1000 (Run no. 1), and (e) 1100 °C (Run no. 5) for 10 h. For the samples prepared at 1000 and 1100 °C, sieved crystals larger than 28 μm were characterized.

particles had different shapes and sizes (Figure 5f), similar to those shown in Figure S2a. The changes in the average crystal size are plotted in Figure 6 with respect to the solute concentration. The average size of the Ba5Nb4O15 crystals increased as the solute concentration decreased. The average crystal size suddenly increased by a factor greater than 20 between the solute concentrations of 50 and 20 mol %. There was a close relationship between the size and shape of the Ba5Nb4O15 crystals; the crystals grown at higher solute concentrations (50 and 80 mol %) had a relatively small size with a polyhedron shape, whereas the crystals grown at lower solute concentrations (5, 10, and 20 mol %) had a larger size with a plate-like shape, indicating that the dominant crystal-growth manner changed according to the solute concentration. The mechanism of crystal growth for the plate-like Ba5Nb4O15 under the BaCl2 flux was further investigated by changing the holding time and cooling rate. Figure S3a,b shows SEM images of as-prepared samples synthesized at different cooling rates: 5 °C·h−1 (Run no. 12) and rapid cooling (Run no. 13) from 150 °C·h−1 (Run no. 1), respectively. Here, the

Figure 4. Powder XRD profiles of sieved Ba5Nb4O15 crystals prepared using a BaCl2 flux with solute concentrations of 1, 10, 20, 50, 80, and 100 mol % at a holding temperature of 1000 °C for 10 h. The arrows indicate the unidentified peaks.

Figure 5. SEM images of sieved Ba5Nb4O15 crystals prepared using a BaCl2 flux at solute concentrations of (a) 100 mol % (Run no. 6), (b) 80 mol % (Run no. 7), (c) 50 mol % (Run no. 8), (d) 20 mol % (Run no. 9), (e) 10 mol % (Run no. 10), and (f) 1 mol % (Run no. 11) at a holding temperature of 1000 °C for 10 h. D

DOI: 10.1021/acs.cgd.6b00526 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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sample showed weight losses at ∼100, 650, and 950 °C. The first and second endothermic peaks with weight losses of 5.4 and 5.3 wt % corresponded to the removal of crystal waters from BaCl2·2H2O to BaCl2·H2O with a weight loss of 5.6 wt % and from BaCl2·H2O to BaCl2 with a weight loss of 5.5 wt %, respectively. The small endothermic peak with a weight loss of 3.2 wt % at ∼650 °C is attributed to the thermal decomposition of BaCO3 to BaO, corresponding to a calculated weight loss of 3.6 wt %. The endothermic peak without weight loss at ∼950 °C is attributed to the melting of BaCl2, which agrees with the value from the literature (962 °C). Little weight loss (0.3 wt %) occurred above 950 °C, indicating that BaCl2 was hardly vaporized around 1000 °C. The change of the crystal phase during heating was investigated using in situ high-temperature XRD, as shown in Figure 8. At room temperature, all the peaks can be assigned to Figure 6. Solute-concentration dependence of the size of Ba5Nb4O15 crystals grown from a BaCl2 flux at a holding temperature of 1000 °C for 10 h.

other experimental conditions were the same as those for Run no. 1. A comparison between Figures S3a,b and 3d shows that the crystals morphologies were similar, indicating that the cooling process did not affect the crystal growth, which reflects the poor solubility of the oxide solutes in the chloride molten salts.21,22 Subsequently, the crystal growth during the heating and holding was investigated. In the case of heating at 1000 °C without holding (Run no. 14), crystals ∼50 μm in size were obtained, as shown in Figure S3c, whose morphologies were almost the same as those of the crystals grown for 10 h (Run no. 1). In contrast, the mass of Ba5Nb4O15 crystals larger than 28 μm was doubled from ∼80 to ∼160 mg per 1 g of crystals by increasing the holding time. These results suggest that the crystal growth of Ba5Nb4O15 started above the melting point of BaCl2 and was driven by the enhancement of the atomic diffusion in the flux, probably leading to the dissolution of the Ba5Nb4O15 crystallite, subsequent crystallization in the larger crystals (Ostwald ripening), and the coalescence of the crystallite. To confirm the phase transition and chemical reaction during the flux growth, thermal analyses were performed. Figure 7 shows the TG-DTA curve of a mixture of Nb2O5, BaCO3, and BaCl2·2H2O fluxes at a solute concentration of 5 mol %. The

Figure 8. In situ high-temperature XRD patterns of a BaCO3, Nb2O5, and BaCl2·2H2O mixture with a solute concentration of 5 mol %. For the assignment of the peaks, the following ICDD PDF data were used: BaCO3 01-076-2826, Nb2O5 00-027-1313, BaCl2·2H2O 00-025-1135, BaCl2 00-024-0094, and Ba5Nb4O15 00-70-9002.

Nb2O5, BaCO3, BaCl2·2H2O, and Au (sample stage), as indicated by the symbols in the figure. At 221 °C, the patterns assigned to BaCl2·2H2O disappeared, and BaCl2 patterns appeared because of the removal of crystal waters. At 618 °C, the BaCO3 patterns disappeared owing to the thermal decomposition to BaO and CO2, coinciding with the TGDTA results. XRD peaks from BaO were not detected, which is attributed to the formation of a liquid phase between BaO and BaCl2 (discussed later). Patterns from Ba5Nb4O15 appeared at 932 °C, accompanied by patterns from BaCl2. This indicates that the formation of Ba5Nb4O15 was derived from a solid-state reaction. With further heating at 999 °C, the BaCl2 patterns almost disappeared because of melting, whereas the Ba5Nb4O15 patterns were detected. It is assumed that Ba5Nb4O15 did not dissolve much but grew in the flux. Note that the in situ XRD system analyzed the chemical reaction with increasing temperature; therefore, it showed slightly different results from Figures 2−5, where the samples were prepared by heating, holding, and cooling. Considering the aforementioned experimental results, the crystal-growth manner of Ba5Nb4O15 under the BaCl2 flux is schematically explained as shown in Figure 9. Through the removal of crystal waters in BaCl2·2H2O and the decomposition of BaCO3 to BaO, a solid-phase reaction between BaO

Figure 7. TG-DTA profiles of the preparation of Ba5Nb4O15 under a BaCl2·2H2O flux at a solute concentration of 5 mol %. E

DOI: 10.1021/acs.cgd.6b00526 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Figure 9. Schematic of the crystal-growth mechanism of Ba5Nb4O15 in a BaCl2 flux.

and Nb2O5 to form Ba5Nb4O15 occurred above 600 °C with the existing solid BaCl2. Considering the phase diagram of BaO and BaCl2,19 it is assumed that BaO and BaCl2 formed a liquid phase in local areas in the reaction field above 900 °C below the melting point of BaCl2 (962 °C). This could have assisted the atomic diffusion of the crystals and developed their facets, yielding polyhedron Ba5Nb4O15 crystals at a relatively low temperature (Figure 3c). Above 962 °C, Ba5Nb4O15 started to grow in the BaCl2 melt because of the dissolution of the crystallite and the reprecipitation of the larger crystals (Ostwald ripening), as well as the coalescence of crystals. Therefore, crystals several tens of microns in size were obtained. During the holding in the BaCl2 melt, we assume that Ostwald ripening was promoted, yielding far more micron-sized crystals. The flux-grown micron-sized crystals were further investigated to evaluate their crystallinity and morphology. Figure 10

Figure 11. XRD patterns of products prepared using various chloride fluxes with a solute concentration of 5 mol % at a holding temperature of 1000 °C for 10 h.

CaCl2, and SrCl2 fluxes with a solute concentration of 5 mol % at a holding temperature of 1000 °C for 10 h. The XRD patterns of the samples prepared using NaCl and KCl were attributed to the Ba5Nb4O15, which had a few impurity phases. They exhibited plate-like crystals approximately 1−3 μm in size, as shown in Figure 12, which were far smaller than those

Figure 12. SEM images of Ba5Nb4O15 crystals grown in (a) NaCl and (b) KCl fluxes with a solute concentration of 5 mol % at a holding temperature of 1000 °C for 10 h.

prepared using the BaCl2 flux. The formation of smaller Ba5Nb4O15 crystals is probably attributed to the lower solubility of the NaCl and KCl solutes compared with BaCl2. In contrast, Ba5Nb4O15 was not obtained using the other chloride fluxes. The XRD patterns of the samples prepared using LiCl, MgCl2, CaCl2, and SrCl2 differ from that for Ba5Nb4O15, and their main phases are assigned as follows: Li3NbO423 from the LiCl flux, Mg4Nb2O924 from the MgCl2 flux, Ca4Nb2O925 from the CaCl2 flux, and Sr5Nb4O1526 from the SrCl2 flux. The generation of different products was due to the existence of phases more stable than Ba5Nb4O15 under the reactions of Nb2O5 and the other chloride fluxes.

Figure 10. (a) TEM image and corresponding SAED pattern and (b) lattice image of a cross section of a plate-like Ba5Nb4O15 crystal prepared at a solute concentration of 5 mol % at a holding temperature of 1000 °C for 10 h (Run no. 1).

shows a bright-field TEM image, a selected-area electron diffraction (SAED) pattern, and a lattice image of a cross section perpendicular to the hexagonal surface of the plate-like Ba5Nb4O15 crystal shown in Figure 3d. The SAED pattern shown in Figure 10a suggests that the plate-like crystal was single-crystalline and that there was a strong diffraction line of (005), indicating that the hexagonal surface aligned with the ab plane. The lattice image in Figure 10b shows almost no defects of the crystallite in this range. The anisotropic XRD pattern of the plate-like crystals also follows the assignment of the facet, as explained in Figure S4. To confirm the effect of the cations in the fluxes, preparations of Ba5Nb4O15 using other alkaline and alkaline earth chloride fluxes were investigated. Figure 11 shows XRD patterns of samples prepared from LiCl, NaCl, KCl, MgCl2,

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CONCLUSION The crystal growth of Ba5Nb4O15 in chloride fluxes was schematically investigated. Using a BaCl2 flux, facile morphological control of the Ba5Nb4O15 crystals was achieved while maintaining a high crystallinity. By changing the holding temperature between 700 and 1100 °C and the solute concentration between 5 and 100 mol %, polyhedron and plate-like Ba5Nb4O15 crystals between 2 and 50 μm in size were F

DOI: 10.1021/acs.cgd.6b00526 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Article

(12) Zhu, X.; Zhu, J.; Zhou, S.; Liu, Z.; Ming, N.; Hesse, D. Solid State Phenom. 2005, 106, 41−46. (13) Teshima, K.; Lee, S.; Mizuno, Y.; Inagaki, H.; Hozumi, M.; Kohama, K.; Yubuta, K.; Shishido, T.; Oishi, S. Cryst. Growth Des. 2010, 10, 4471−4475. (14) Mizuno, Y.; Zettsu, N.; Yubuta, K.; Sakaguchi, T.; Saito, T.; Wagata, H.; Oishi, S.; Teshima, K. Cryst. Growth Des. 2014, 14, 1882− 1887. (15) Kawashima, K.; Hojamberdiev, M.; Wagata, H.; Yubuta, K.; Vequizo, J. J. M.; Yamakata, A.; Oishi, S.; Domen, K.; Teshima, K. J. Phys. Chem. C 2015, 119, 15896−15904. (16) Yamada, T.; Zettsu, N.; Handa, N.; Oishi, S.; Teshima, K. Cryst. Growth Des. 2015, 15, 3922−3928. (17) Zhou, H.; Chen, X.; Fang, L.; Hu, C.; Wang, H. J. Mater. Sci.: Mater. Electron. 2010, 21, 939−942. (18) Hisatomi, T.; Katayama, C.; Moriya, Y.; Minegishi, T.; Katayama, M.; Nishiyama, H.; Yamada, T.; Domen, K. Energy Environ. Sci. 2013, 6, 3595−3599. (19) Neumann, B.; Kroeger, C.; Juettner, H. Z. Elektrochem. Angew. Phys. Chem. 1935, 41, 725−736. (20) Spitsyn, V. I.; Ippolitova, E. A.; Kovba, L. M.; Lykova, L. N.; Leshchenko, P. P. Russ. J. Inorg. Chem. (Engl. Transl.) 1982, 27, 464− 467. (21) Hayashi, Y.; Kimura, T.; Yamaguchi, T. J. Mater. Sci. 1986, 21, 2876−2880. (22) Kimura, T. Molten Salt Synthesis of Ceramic Powders. In Advances in Ceramics - Synthesis and Characterization, Processing and Specific Applications; Sikalidis, C., Ed.; INTECH: Rijeka, 2011. (23) Ukei, K.; Suzuki, H.; Shishido, T.; Fukuda, T. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1994, 50, 655−656. (24) Kumada, N.; Taki, K.; Kinomura, N. Mater. Res. Bull. 2000, 35, 1017−1021. (25) Levin, I.; Chan, J. Y.; Geyer, R. G.; Maslar, J. E.; Vanderah, T. A. J. Solid State Chem. 2001, 156, 122−134. (26) Weiden, M.; Grauel, A.; Norwig, J.; Horn, S.; Steglich, F. J. Alloys Compd. 1995, 218, 13−16.

obtained. Such large morphologic changes are explained by different crystal-growth manners. The crystal-growth manner of large Ba5Nb4O15 crystals ∼50 μm in size comprised a solid-state reaction to form Ba5Nb4O15 nuclei, followed by crystal growth in the BaCl2 melt. NaCl and KCl yielded smaller Ba5Nb4O15 crystals than BaCl2, whereas other chloride fluxes did not yield Ba5Nb4O15. It is widely known that chloride fluxes provide poor solubility for metal oxides. However, our results show that it is possible to grow large crystals using Ostwald ripening by exploiting the common-ion effect. If the growth conditions are properly selected, chloride fluxes are a powerful tool for obtaining high-quality crystals with modified morphologies, with the advantages of a low cost, easy removal by water, a low reactivity, and a small environmental load.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b00526. Powder XRD profiles (Figure S1) and SEM images (Figure S2) of as-prepared and after sieving samples prepared using BaCl2 flux at a solute concentration of 5 mol % with a holding temperature of 1000 °C for 10 h. SEM images of as-prepared Ba5Nb4O15 crystals prepared using BaCl2 flux with holding temperatures of 1000 °C under different cooling rates and holding times (Figure S3). XRD profiles of plate-like single crystals aligned at the direction of hexagonal plates on a sample stage (Figure S4) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was partially supported by the Japan Technological Research Association of Artificial Photosynthetic Chemical Process (ARPChem) and JSPS KAKENHI Grant Number 25249089.

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REFERENCES

(1) Galasso, F.; Katz, L. Acta Crystallogr. 1961, 14, 647−650. (2) Vineis, C.; Davies, P. K.; Negas, T.; Bell, S. Mater. Res. Bull. 1996, 31, 431−437. (3) Sreemoolanadhan, H.; Sebastian, M. T.; Mohanan, P. Mater. Res. Bull. 1995, 30, 653−658. (4) Kamba, S.; Petzelt, J.; Buixaderas, E.; Haubrich, D.; Vanek, P.; Kuzel, P.; Jawahar, I. N.; Sebastian, M. T.; Mohanan, P. J. Appl. Phys. 2001, 89, 3900−3906. (5) Kim, J.; Kim, D.; Jung, H.; Hong, K. J. Eur. Ceram. Soc. 2006, 26, 2105−2109. (6) Miseki, Y.; Kato, H.; Kudo, A. Chem. Lett. 2006, 35, 1052−1053. (7) Wu, S.; Chen, X.; Chen, L. J. Electroceram. 2008, 21, 810−814. (8) Zhang, X.; Qin, J.; Xue, Y.; Yu, P.; Zhang, B.; Wang, L.; Liu, R. Sci. Rep. 2014, 4, 4596. (9) Amano, F.; Nogami, K.; Tanaka, M.; Ohtani, B. Langmuir 2010, 26, 7174−7180. (10) Hsiao, Y.; Chang, Y.-H.; Chang, Y.-S.; Fang, T.-H. J. Am. Ceram. Soc. 2007, 90, 2287−2290. (11) Hildebrandt, N. C.; Soldat, J.; Marschall, R. Small 2015, 11, 2051−2057. G

DOI: 10.1021/acs.cgd.6b00526 Cryst. Growth Des. XXXX, XXX, XXX−XXX