Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Selective Crystallization of Four Tungstates (La2W3O12, La2W2O9, La14W8O45, and La6W2O15) via Hydrothermal Reaction and Comparative Study of Eu3+ Luminescence Xiaofei Shi,†,‡,§ Zhihao Wang,†,‡,§ Toshiaki Takei,∥ Xuejiao Wang,⊥ Qi Zhu,†,‡ Xiaodong Li,†,‡ Byung-Nam Kim,§ Xudong Sun,†,‡,# and Ji-Guang Li*,†,‡,§ †
Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), Northeastern University, Shenyang, Liaoning 110819, China ‡ Institute for Ceramics and Powder Metallurgy, School of Materials Science and Engineering, Northeastern University, Shenyang, Liaoning 110819, China § Research Center for Functional Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ∥ Nanotechnology Innovation Station, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ⊥ College of New Energy, Bohai University, Jinzhou, Liaoning 121007, China # School of Environmental and Chemical Engineering, Dalian University, Dalian, Liaoning 116622, China S Supporting Information *
ABSTRACT: Hydrothermal reaction at 200 °C was systematically undertaken in wide ranges of solution pH (4−13) and W/La molar ratio (R = 0.5−2), without using any organic additive, to investigate the effect of hydrothermal parameter on product property and the underlying mechanism. Combined analysis by X-ray diffraction (XRD), inductively coupled plasma (ICP) spectroscopy, elemental mapping, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that either a decreasing pH or increasing R value yielded a product richer in W and, conversely, richer in La. The results were interpreted from the solution chemistry of La3+ and tungstate ions. As an outcome of our 40 well-designed experiments, four La tungstatesLa2W3O12, La2W2O9, La14W8O45, and La6W2O15were successfully obtained in a phase-pure form by calcining their hydrothermal precursors. Phase and morphology evolution, structure features, and properties of Eu3+ emission were, for the first time, comparatively investigated for the four compounds. Spectral analysis found that the 5 at. % Eu3+-doped La2W3O12 phosphor exhibits the highest quantum efficiency (∼47%), more red component, and the shortest fluorescence lifetime of luminescence (∼0.72 ms).
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INTRODUCTION Rare-earth (RE) tungstates have been a focus of great research interest, since they simultaneously possess the abundant functionalities of REs and optically active tungstate ligands. Particularly, the tungstate group may effectively compensate for the low absorption coefficient of most RE3+ activators through host sensitization, because of its unique capability of selfactivation, followed by efficient energy transfer to the activator.1−5 RE tungstates can be divided into multiple categories, such as RE 2 W 3 O 12 , RE 2 WO 6 , RE 6 WO 12 , RE2W2O9, RE10W22O81, and so forth, according to W/RE stoichiometry, and can be viewed as pseudo-binary compositions between RE2O3 and WO3.6 The compounds are generally used in luminescence, dielectrics, and high-temperature materials,7,8 and some specific phases have unique properties and applications. For example, RE2W2O9 exhibits a high oxygen conductivity, comparable to doped CeO2 at intermediate temperatures, and thus may serve as the electrolyte in solid oxide fuel cells (SOFCs), oxygen sensors, ceramic membranes, and oxygen pumps.9 RE tungstates are also known as a group of © XXXX American Chemical Society
photocatalysts for pollutant degradation and environment remediation.10−14 Several methodologies have been developed for the synthesis of RE tungstates, typically including solid reaction, sol−gel processing, hydrothermal reaction, and molten-salt-assisted crystallization.11,15−18 A careful literature survey found that La2W3O12 is mostly produced via solid reaction, sol−gel, the molten-salt technique, and hydrothermal reaction,11,19,20 and RE2W2O9 is produced via solid reaction of RE oxide (RE = La− Gd) and tungsten oxide or ammonium tungstate,13,21−23 while La6W2O15 and La14W8O45 have received much less investigation to date, possibly because of their complex structures and the difficulties in composition control. The hydrothermal route is generally advantageous to a better morphology, phase structure, and composition control of the product.16,24 For example, hydrothermally reacting a mixed solution of RE nitrate, Na2WO4, and glycol (molar ratio = 1:2:0.1) at pH 8 and 200 Received: March 26, 2018
A
DOI: 10.1021/acs.inorgchem.8b00807 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry °C directly produced NaGd(WO4)2:Ho/Yb upconversion phosphor,25 and hydrothermal reaction at 200 °C for 24 h of the aqueous solution of RE nitrate or acetate and sodium tungstate, followed by calcination at 900 °C, produced Ce2(WO4)3:Tb and Ce10W22O81:Tb phosphors.19 Besides, down-/up-converting Y2(WO4)3:Ln phosphors with threedimensional (3D) hierarchical architectures were synthesized via hydrothermal reaction of RE(NO3)3, Na(WO4)2, and sodium dodecyl benzenesulfonate (molar ratio = 1:1:0.5) at pH 7 and 200 °C, followed by calcination at 800 °C for 2 h.11 In addition, hydrothermally reacting RE(NO3)3 and Na2WO4 (1:1 molar ratio) at pH 10 and pH 13 produced precursors that can be transformed to Y2WO6:Eu red phosphors upon subsequent calcination at 1100 °C.24,26 The effects of hydrothermal parameter have also been addressed by other researchers. Byrappa et al.,27 for example, investigated the influences of pH (7.0−10.2) and pressure (P, 60−120 bar) on the hydrothermal crystallization of NaLa(WO4)2 at 240 °C, with 6.0 g of WO3 and 0.75 g of La(NO3)3 as reagents, and found that increasing pH resulted in a morphology change from well-developed bulk crystals to long needles, while increasing pressure led to a morphology transition from needles to bulk crystals, higher growth rate, and better crystal quality. They concluded that the optimal parameters are pH 8.4−8.8 and P = 120 bar. Using Y(NO3)3, Na2WO4, and sodium dodecyl sulfate (SDS) as raw materials (molar ratio = 1:1:1), Kaczmarek et al.12 studied the effect of pH on phase and morphology evolution at 200 °C. Results showed that the hydrothermal product obtained at pH 3 can be transformed to phase-pure Y2WO6 by calcination at 1100 °C for 3 h, while those of pH 7 and pH 10 produced unknown phases. Meanwhile, it was found that the primary crystallites evolved from irregular nanoblocks at pH 3 and pH 7 into regular microrods at pH 10. In their synthesis of RE(WO3)2(OH)3 and RE2(WO4)3 (RE = La and Y), the latter authors investigated the influences of RE source and dioctyl sodium sulfosuccinate (DSS) addition on the architecture of the product made via hydrothermal reaction of 1 mmol Na2WO4 and 1 mmol RE3+ at 200 °C.28 It was demonstrated that, in the absence of DSS, yttrium nitrate and acetate produced microspheres containing nanodisks while lanthanum nitrate and acetate produced stacks of nanodisks and erythrocyte-like microstructures assembled from nanodisks, respectively. DSS addition (0.5−1.25 mmol) was shown to promote the La product toward microspheres and, at the same time, change the primary crystallites from nanodisks into nanorods for both the La and Y products. It was concluded from previous studies that hydrothermal parameters, especially the RE/W molar ratio, solution pH, and reaction temperature, have remarkable influence on the direct and calcination product, but a systematic study has been rare for the factors and also their operating mechanism. This could possibly be hindered by the fact that tungstate anions present themselves as complicated species by polymerization and protonation in solution,29 and are readily influenced by tungstate concentration, the presence of other ionic species, and interaction of the tungstate anions with other anions.30 Lanthanum (La) is the most inexpensive RE and La3+ is optically inert, because of its vacant 4f shell; for thesse reasons, La compounds are frequently employed as host lattice for luminescence.31 In this work, we thoroughly investigate the crystallization of La tungstates through hydrothermal reaction at the highest available temperature of 200 °C, without using any organic additives, and the effects of W/La molar ratio and
solution pH on the characteristics of the products are unveiled. As an outcome, four types of La tungstates are successfully obtained as pure phases after subsequent calcination: La2W3O12, La2W2O9, La14W8O45, and La6W2O15. With this success, for the first time, we comparatively investigate the phase and morphology evolution and the emission properties of Eu3+ in them. We believe that the outcome of this work may have important implications to the phase-controlled synthesis of tungstates for other RE elements and even RE molybdates.
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EXPERIMENTAL PROCEDURE
Hydrothermal Reaction and Calcination. The starting reagents are sodium tungstate dihydrate (Na2WO4·2H2O, analytical grade) and RE(NO3)3·6H2O (RE = La and Eu, 99.99% pure) from Kanto Chemical Co., Inc. (Tokyo, Japan). The aqueous solutions of 1.0 mol/ L for W and 0.1 mol/L for RE were prepared by dissolving the three salts in a proper amount of distilled water, respectively. In a typical procedure for hydrothermal reaction, a proper amount of Na2WO4·2H2O solution was added dropwise into 20 mL of a RE3+ solution under magnetic stirring at room temperature, followed by pH adjustment to a given value with dilute NaOH or HNO3 solution while keeping the volume of the mixture at 70 mL. After homogenization under magnetic stirring for 30 min, the mixture was transferred to a Teflon-lined stainless steel autoclave of 100 mL capacity for hydrothermal reaction at 200 °C for 24 h in an electric oven. After natural cooling to room temperature, the hydrothermal product was collected via centrifugation, washed with distilled water three times, rinsed with absolute ethanol once, and then dried in an air oven at 70 °C for 24 h. Both the solution pH (4.0−13.0) and W/La molar ratio (R = 0.5−2.0) were systematically varied to investigate their influences, and the corresponding hydrothermal product was denoted as (pH, R). Calcination of the hydrothermal product was performed in stagnant air at a predetermined temperature for 2 h, with a heating rate of 5 °C/ min at the ramp stage. The Eu3+ (5 at. %, relative to La)-doped phosphors were synthesized using the same procedures. Characterization. Phase identification was performed by X-ray diffractometry (XRD, Smart Lab3, Rigaku, Tokyo, Japan) under 40 kV/40 mA, using nickel filtered Cu Kα radiation (λ = 1.5406 Å) and a scanning speed of 4.0° 2θ min−1. Structure parameters of the products were derived from the XRD data, using the TOPAS software.32 Morphology and microstructure of the product were analyzed by fieldemission scanning electron microscopy (FE-SEM) (Model S-4800, Hitachi, Tokyo) under an acceleration voltage of 10 kV and transmission electron microscopy (TEM) (Model JEM-2100F, JEOL, Tokyo) under 200 kV. Elemental contents of the products were determined via inductively coupled plasma (ICP) spectroscopy for W, La, and Na on a Model SPS3520UV-DD instrument (SII Technologies, Inc., Tokyo) and for O via the He gas transportation fusion-thermal conductivity technique (Model TC-436, LECO, St. Joseph, MI). Photoluminescence and fluorescence decay kinetics of the Eu-doped phosphors were analyzed at room temperature with an FP-6500 fluorospectrophotometer (Jasco, Tokyo).
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RESULTS AND DISCUSSION Characterization of Hydrothermal Product and Tungstate Speciation upon Calcination. Under the fixed hydrothermal conditions of 200 °C and 24 h, the effects of solution pH and W/La molar ratio (R) were investigated in the wide ranges of pH 4−13 and R = 0.5−2, and the XRD patterns and FE-SEM morphologies of all the products are shown in Figures S1 and S2, respectively, in the Supporting Information. It is seen from Figure S1 that the (13, 0.5) sample matches well with the hexagonal structured La(OH)3 phase (JCPDS File No. 01-077-3934). However, the other products cannot be indexed to any single La or W compound after a thorough survey of the currently available JCPDS standard files and reported literature. B
DOI: 10.1021/acs.inorgchem.8b00807 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Table 1. Phase Composition of the Hydrothermal Producta Phase Composition pH 4 5 6 7 8 9 10 11 12 13 a
R = 0.5 X X+A Y+A Y+A Y+A La(OH)3 La(OH)3 La(OH)3 La(OH)3 La(OH)3
+ + + +
A A A A
R=1
R = 1.5
R=2
X+Z X+Z X+Z+A Y+A Y+A Y+A Y+A A A La(OH)3 + A
X+Z X+Z+A X+Z+A X+Z+A NLW + Y + A NLW + Y Y+A Y+A A La(OH)3 + A
X+Z X+Z X+Z NLW + X + Z NLW + Y NLW + Y Y+A Y+A A La(OH)3 + A
The symbols of X, Y, and Z denote three different unknown crystalline phases; NLW denotes NaLa(WO4)2; and A represents an amorphous mass.
Figure 1. TEM morphologies and the results of elemental mapping for the representative samples of (a) (4, 0.5), (b) (6, 0.5), (c) (9, 0.5), and (d) (11, 2). The selected area electron diffraction (SAED) patterns taken for the two different types of objects are included in panel (d).
known that tungstate anions present themselves as monomeric [WO4]2− in a solution of sufficient alkalinity (pH ≥ 8) while the protonated and polymerized forms of [H18(WO4)12]6−, [H10(WO4)6]2−, [H7(WO4)6]5−, and [HWO4]− prevail in acidic and neutral solutions of pH 4−7 by the reaction29,30,34−40
Phase compositions of the products, deduced from Figure S1, are summarized in Table 1. It can be concluded that (1) in most cases, the hydrothermal product contains unknown phases and an amorphous mass (mostly seen via enlarged view of the XRD profile), (2) La(OH)3 appeared in the products of the higher pH (9− 13) at the smallest R of 0.5, whose formation expanded to the wide R range of 0.5−2 at the highest pH (13), and (3) NaLa(WO4)2 has a tendency to crystallize in the ranges of pH 7−9 and R = 1.5−2. Note that single-phased NaLa(WO4)2 can be successfully crystallized at 200 °C under pH 8 and the larger R value of 3.33 Table 1 shows that the (13, 0.5) product is composed of pure La(OH)3, while those of (9−12, 0.5) and (13, 1−2) were additionally accompanied by an amorphous mass. The amorphous mass surely contains W, as evidenced by the fact that La tungstates appeared after calcination (see Table 3, presented later in this work). Such results may be understood from the solution chemistry of La and tungstate ions. It is
p[WO4 ]2− + qH+ + r H 2O ⇄ [Hq(WO4 )p (H 2O)r ]q − 2p (1) 3+
On the other hand, La undergoes hydrolysis in aqueous solution, according to the reaction La 3+ + OH− + H 2O ⇄ [La(OH)x (H 2O)y ]3 − x
(2)
and a higher pH produces more hydroxyls in the complex ion.41,42 The protonated tungstate ions are acidic species, which react more readily with the hydrolyzed La ions (a base) than [WO4]2− via neutralization reaction. The crystallization of pure La(OH)3 at (13, 0.5) conforms to the above discussion and suggests that [WO4]2− exhibits negligible reactivity toward La(OH)3 under highly alkaline conditions. It can also be C
DOI: 10.1021/acs.inorgchem.8b00807 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry inferred that, under a fixed R, the hydrothermal product would have decreasing W content with increasing solution pH. Elemental mapping via STEM of the (4, 0.5) and (6, 0.5) hydrothermal products and phase analysis of the (9, 0.5) calcination product indeed found the decreasing W/La molar ratios of ∼1.63 (Figure 1a; close to that of La2W3O12 in Table 3), ∼1.08 (Figure 1b; close to that of La2W2O9 in Table 3), and ∼0.57 (Table 3), respectively. The (4, 0.5) and (6, 0.5) products, simultaneously containing La and W, were observed to be solely composed of microplates (see Figure 1a and Figure S2 in the Supporting Information) and rounded agglomerates formed by finer particles/crystallites (Figure 1b and Figure S2), respectively. The (9, 0.5) product is composed of microrods containing La and no W and agglomerated nanoparticles containing both La and W (Figure 1c and Figure S2); therefore, the former can be assigned to the La(OH)3 phase found via XRD (see Figure S1a). The above-discussed influence of solution pH under a fixed R was further supported by the results of elemental analysis for the R = 2 series, where it is seen that the W/La ratio gradually decreased from ∼2.07 to 0.01 with the solution pH increasing from 4 to 13 (see Table 2).
also accounts for the appearance of W-containing amorphous mass when the R value increased from 0.5 to 1, even under the highest solution pH of 13 (see Table 1). It can thus be said that either a lower pH or a higher R ratio would enhance the reactivity of tungstate species, and simultaneously decreasing pH and increasing R have a more remarkable effect. As a result, W-rich and W-lean products resulted for the top-right and bottom-left corners of Tables 1 and 3, respectively. The unknown compound Y crystallized in the products of a wide range of (pH, R) combinations (see Table 1). Our extensive efforts to produce Y in a phase-pure form unfortunately failed. TEM analysis with the (11, 2) product, for example (Figure 1d), indicated that the Y phase crystallized as single-crystalline microplates and an amorphous mass presented as nanoparticles, with their analyzed W/La molar ratios of ∼1:1 and 0.8:1, respectively. The overall W/La ratio determined via EDS for the (11, 2) hydrothermal product appears slightly larger than that for the calcination-derived phase mixture of La2W2O9 (W/ La = 1:1 molar ratio) and La14W8O45 (W/La ≈ 0.57:1 molar ratio; see Table 3). The difference could be due to localized sample detection by energy-dispersive spectroscopy (EDS). XRD analysis of the products calcined from the hydrothermal precursors (Table 1) found either a pure phase or a phase mixture for a specific (pH, R) combination, as shown in Figure S3 in the Supporting Information. The phase constituents derived from Figure S3 are tabulated in Table 3 for each of the (pH, R) combinations used for hydrothermal reaction. It is seen that the various La tungstates of La10W22O81, NaLa(WO4)2, La 2 W 3 O 12 , La 2 W 2 O 9 , La 14 W 8 O 45 , La 2 WO 6 , La 22 W 9 O 60 , La6W2O15 and La6WO12, which have the decreasing W/La molar ratios of 2.2, 2, 1.5, 1, 8/14 (∼0.57), 0.5, 9/22 (∼0.41), 2/6 (∼0.33), and 1/6 (∼0.17), respectively, may crystallize under a certain (pH, R) conditions. It is clear from Table 3 that the product from the precursor synthesized under a lower pH and a higher R has a tendency to have more W. Notably, the four La tungstatesLa2W3O12 (JCPDS No. 01-082-2068), La2W2O9 (JCPDS No. 01-070-7873), La14W8O45 (JCPDS No. 00-032-0502), and La6W2O15 (JCPDS No. 00-037-0124) were successfully crystallized as pure phases upon calcining their precursors hydrothermally synthesized under the (pH, R) combinations of (4, 0.5), (6, 0.5) and (8−9, 1), (9, 0.5), and (13, 1.5), respectively. The phase and morphology evolutions of these four single-phased La tungstates were further discussed in detail in the following section. Structure/Morphology Evolution of the Phase-Pure La Tungstates and Investigation of Eu3+ Luminescence.
Table 2. Results of ICP Analysis and Derived Molar Ratios for the Hydrothermal Products of R = 2 Composition (wt %) sample
W
La
O
Na
Na:La:W:O molar ratio
4, 2 5, 2 6, 2 7, 2 8, 2 9, 2 10, 2 11, 2 12, 2 13, 2
56.9 56.1 56.5 55.8 55.2 52.1 47.1 40.9 33.7 0.56
21.2 21.1 21.4 21.7 22.5 24.6 32.4 36.4 41.5 72.5
22 21 21 19 18 19 19 21 22 26
