Article pubs.acs.org/crystal
Morphology-Controlled Synthesis of Cubic Cesium Hydrogen Silicododecatungstate Crystals Sayaka Uchida,†,§ Yoshiyuki Ogasawara,‡ Toshiaki Maruichi,‡ Akihito Kumamoto,⊥ Yuichi Ikuhara,⊥ Teppei Yamada,∥ Hiroshi Kitagawa,# and Noritaka Mizuno*,‡ †
Department of Basic Sciences, School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan ‡ Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan § JST-PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan ⊥ Institute of Engineering Innovation, School of Engineering, The University of Tokyo, 2-11-16 Yayoi, Bunkyo-ku, Tokyo 113-8656, Japan ∥ Department of Chemistry and Biochemistry, School of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan # Division of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa-Oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan S Supporting Information *
ABSTRACT: Cubic particles of cesium hydrogen silicododecatungstate crystals are obtained for the first time by the use of spherical seed crystals and control of the Cs+ to SiW12O404− (Cs/POM) ratio in the synthetic solution. The morphology of the particles is controlled between rhombic dodecahedra faceted with {110} planes (thermodynamically stable morphology) and cubes faceted with {100} planes. Scanning transmission electron microscopy−energy-dispersive X-ray spectroscopy analysis of the cross-section shows that the cubes possess a core−shell structure, and the Cs/POM ratio of the shell (ave. 3.24) is larger than that of the core (ave. 2.76), suggesting the existence of anion (POM) vacancies in the shell. Solid state magic angle spinning NMR spectroscopy, nitrogen adsorption, and water vapor sorption measurements of the cubes show that the porous core is covered by the dense shell, and only water molecules can diffuse through the dense shell via the anion vacancies. Despite the small amounts of acidic protons, the cubes exhibit moderate proton conductivity (2.5 × 10−4 S cm−1) at room temperature under water vapor (298 K, P/P0 = 0.95), suggesting that mobile water molecules in the anion vacancies contribute to the proton conduction.
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INTRODUCTION Polyoxometalates (POMs) are nanosized anionic metal− oxygen clusters of early transition metals and have stimulated research in broad fields of science. The complexation of POMs with appropriate countercations forms solid compounds with unique properties.1 Especially, it is well-known that partial neutralization of acidic protons in H3PW12O40 or H4SiW12O40 with Cs+ forms insoluble solid particles of cesium hydrogen phosphododecatungstate or silicododecatungstate (Cs-POM), which function as heterogeneous acid catalysts and proton conductors.2,3 The crystal structure of Cs-POMs can be explained by a spherical approximation of anions: Anions (PW12O403− or HSiW12O403−) are packed in a body-centered cubic (bcc) cell, cations exist at the center of each plane and edge of the cell, and thus two anions and six cations exist in the unit cell, resulting in an anion to cation site ratio of 1:3 (Figure 1a).4 In a bcc cell, {110} planes are most densely packed and thus the most stable.5 Therefore, the rhombic dodecahedron faceted © XXXX American Chemical Society
with 12 {110} planes is a thermodynamically stable morphology.5 While spherical particles,2,6 rhombic dodecahedral particles faceted with {110} planes,2,6 and flat rhombic bipyramidal particles bisected by {110} planes7 have been reported for Cs-POMs, particles faceted with planes other than {110} have not yet been reported. Unique properties associated with facets and morphologies have been reported for various cubic metal and metal-oxide particles. For example, catalytic activities of the crystal faces of bcc iron in ammonia synthesis increase in the order of {110} < {100} < {111}.8 Octahedral particles of Cu2O9 show higher catalytic activity in N-arylation than those with cubic morphologies, and hexagonal platelet particles of Co3O410 exhibit higher capacitance performance than those with spherical morphologies. Received: October 24, 2014
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dx.doi.org/10.1021/cg501575x | Cryst. Growth Des. XXXX, XXX, XXX−XXX
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EXPERIMENTAL METHODS
Syntheses. H4SiW12O40·24H2O (0.33 g, 0.1 mmol) was dissolved into 40 mL of 0.1 M nitric acid, and the solution was kept at 288 K with a water bath (solution A). CsNO3 (0.059 g, 0.3 mmol) was dissolved into 40 mL of 0.1 M nitric acid, and the solution was kept at 288 K with a water bath (solution B). Solution B was added to solution A (i.e., Cs/POM = 3), and the solution was stirred at 1140 rpm and 288 K for 1 h. The suspension was filtered with a membrane filter (pore size 0.2 μm) to remove large particles, and the solute was left to stand in an ice bath for 6 h. The solid was collected on a membrane filter, washed with 10 mL of water, and air-dried (CsHSiW1). Yield: 0.07 g. Elemental analysis calcd for Cs3HSiW12O40·9H2O: Cs 11.6, Si 0.82, W 64.2; found: Cs 12.2, Si 0.77, W 64.6. H4SiW12O40·4H2O (0.17 g, 0.05 mmol) was dissolved into 10 mL of 0.1 M nitric acid, and the solution was heated at 368 K (solution C). CsNO3 (0.029 g, 0.15 mmol) was dissolved into 10 mL of 0.1 M nitric acid, and the solution was heated at 368 K (solution D). Solution D was added to solution C (i.e., Cs/POM = 3) with stirring followed by the addition of 0.066 g of CsHSiW-1 as seed crystals, and the suspension was stirred at 800 rpm and 368 K for 48 h. The suspension was washed with 200 mL of water and centrifuged at 6000 rpm for 10 min. The solid was collected on a membrane filter, washed with 10 mL of water, and air-dried (CsHSiW-2). Yield 0.11 g. Elemental analysis calcd for Cs3HSiW12O40·4H2O: Cs 11.9, Si 0.84, W 65.9; found: Cs 12.0, Si 0.83, W 67.6. H4SiW12O40·24H2O (0.17 g, 0.05 mmol) was dissolved into 10 mL of water, and the solution was heated at 368 K (solution E). CsNO3 (0.049 g, 0.25 mmol) was dissolved into 10 mL of water, and the aqueous solution was heated at 368 K (solution F). Solution F was added to solution E (i.e., Cs/POM = 5) with stirring followed by the addition of 0.066 g of CsHSiW-1 as seed crystals, and the suspension was stirred at 800 rpm and 368 K for 48 h. The suspension was washed with 200 mL of water and centrifuged at 6000 rpm for 10 min. The solid was collected on a membrane filter, washed with 10 mL of water, and air-dried (CsHSiW-3). Yield 0.18 g. Elemental analysis calcd for Cs3.0H0.7[SiW12O40]0.926·5H2O: Cs 12.7, Si 0.83, W 64.8; found: Cs 12.7, Si 0.77, W 65.4. Characterizations. Thermogravimetric (TG) analyses were carried out with a thermogravimetry/differential thermal analyzer Thermo Plus Evo (Rigaku). Scanning electron microscope (SEM) images were obtained with S-4700 (Hitachi). Prior to the SEM measurements, the compounds were dispersed in n-hexane and the solution was dropped on a microgrid followed by platinum coating with ion beam sputtering. Powder X-ray diffraction (XRD) patterns were measured with MultiFlex (Rigaku Corporation) by using Cu Ka radiation. Diffraction data were collected in the range of 2θ = 5−40° at 0.01° point. High-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) imaging and energy dispersive X-ray spectroscopy (EDS) mapping were performed with JEM-ARM 200CF (JEOL) operating at 80 kV. Prior to the STEMEDS measurement of cross sections, STEM specimens were prepared by epoxy embedding followed by ion slicing with EM-09100IS (JEOL). Nitrogen adsorption and water vapor sorption isotherms were measured using a volumetric gas (ad)sorption apparatus ASAP-2010 (micromeritics) and Belsorp-max (BEL Japan Inc.), respectively. Prior to the measurements, the compounds (about 0.1 g) were pretreated in vacuo at 373 K for >3 h. Solid-state MASNMR spectra were recorded with CMX-300 Infinity spectrometer (JEOL), and the resonance frequency was 300.5 and 39.4 MHz for 1H and 133Cs, respectively. Single pulse excitation was used, MAS rate was 3 kHz, and adamantane (1H: 1.91 ppm) and 0.5 M CsNO3 aq. (133Cs: 0 ppm) were used as external standards for the calibration of chemical shifts. Fire-sealed glass cells and airtight vessels were used for the measurements of the compounds treated in vacuo at 373 K or exposed to saturated water vapor at 298 K, respectively. AC impedance measurements of the compounds were carried out in a thermo-hygrostat chamber (ESPEC) with a Solatron 1260 impedance gain/phase analyzer (Toyo-Technica) over the frequency range of 1 Hz to 1 MHz. About 5 mg of the compounds was compressed at 2 MPa into pellets of 2.5 mm in
Figure 1. (a) Structure of the unit cell of Cs-POM. Gray polyhedra and blue spheres show the [WO6] units of the silicododecatungstates and Cs+, respectively. Black cube indicate the unit cell. Red rectangle and blue square indicate one of the {110} and {100} planes, respectively. Arrangements of the ions in the (b) {110} and (c) {100} planes.
In ionic crystals, charge neutralization as well as atomic density is an important parameter to discuss the stability of facets and morphologies.11 As for face centered cubic (fcc) metals such as Au, Ag, and Pd, {111} planes are the most densely packed, and an octahedron faceted with eight {111} planes is a thermodynamically stable morphology.12 On the other hand, NaCl with fcc packing of Na+ and Cl− prefers a cube faceted with six {100} planes, since charge neutralization is attained in {100} planes while {111} planes are composed of alternate arrangements of oppositely charged planes.13 In this context, synthesis of cubic particles of Cs-POM faceted with {100} planes seems possible, since Cs-POM is an ionic crystal and charge neutralization is attained in a {100} planem while charge density in a {110} plane is ±0.0203 e A−2 (Figure 1a,b).14 In this work, cubes as well as rhombic dodecahedra of cesium hydrogen silicododecatungstate were obtained from spherical seed crystals by the control of the Cs+ to SiW12O404− ratio (Cs/POM) in the synthetic solution. Scanning transmission electron microscopy−energy-dispersive X-ray spectroscopy (STEM-EDS) analysis of the cross-section showed that the cubes possessed a core−shell structure with anion vacancies in the shell. Spectroscopic and sorption measurements of the cubes showed that the porous core was covered by the dense shell, and only water molecules could diffuse through the dense shell via the anion vacancies. Despite the small amounts of acidic protons, the cubes exhibited moderate proton conductivity at room temperature under water vapor, suggesting that mobile water molecules in the anion vacancies contributed to the proton conduction. B
dx.doi.org/10.1021/cg501575x | Cryst. Growth Des. XXXX, XXX, XXX−XXX
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diameter and ca. 0.4 mm in thickness. Gold electrodes were attached on both ends of the pellets to form four end terminals (quasi-fourprobe method). Bulk conductivities were estimated by semicircle fitting of Nyquist plots.
suspensions were stirred at 368 K, and the respective CsHSiW2 and -3 were formed. In method 1, the spheres started to transform into an angular form after 24 h, and rhombic dodecahedral particles (size: 368 ± 113 nm) were obtained after 48 h (Figure 3a, CsHSiW-2). On the other hand, in method 2, the spheres have completely transformed into a cubic form (size: 349 ± 41 nm) after 24 h (Figure 3b, CsHSiW-3).15,16 The powder XRD measurements (Figure S2, Supporting Information) showed that all compounds crystallized in the cubic Pn3̅m space group in the same manner as that of (NH4)3PW12O40,4 and the lattice constant was 11.79 Å. Elemental analysis of the bulk solid showed that the Cs/ POM ratios of CsHSiW-1 and -2 were 3.0 (chemical formula: Cs3HSiW12O40), and these values (3.0) agreed with the cation to anion site ratio (3). On the other hand, the Cs/POM ratio of CsHSiW-3 was 3.24 (chemical formula: Cs3.00H0.70[SiW12O40]0.926), and the value (3.24) was larger than the cation to anion site ratio (3) and smaller than the anion to cation charge ratio (4). Notably, Cs/POM ratio of the flat rhombic bipyramidal particles of cesium hydrogen silicododecatungstate was 3.6, and the large value is ascribed to the existence of anion vacancies in the crystal lattice.7 STEM imaging and EDS mapping were performed to further analyze the compositions and structures. Figure 4 shows the
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RESULTS AND DISCUSSION Schematic illustration of the syntheses is shown in Figure 2. CsHSiW-1 was grown in an ice bath from particles of
