Scandium Molybdate Microstructures with Tunable Phase and

Jan 10, 2019 - The intensity between different transitions is dependent on the symmetry of local environment according to the Judd–Ofelt theory. The...
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Scandium Molybdate Microstructures with Tunable Phase and Morphology: Microwave Synthesis, Theoretical Calculations, and Photoluminescence Properties Hualan Xu,†,‡ Ran Liu,† Shiqi Zhang,† Meng Deng,† Kuangyi Han,† Bo Xu,‡ Chuying Ouyang,*,‡ and Shengliang Zhong*,† College of Chemistry and Chemical Engineering and ‡Department of Physics, Laboratory of Computational Materials Physics, Jiangxi Normal University, Nanchang 330022, PR China

Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF NEW ENGLAND on 01/14/19. For personal use only.

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ABSTRACT: In this paper, scandium molybdate microstructures have been prepared from solution via a microwave heating method. By controlling the experimental parameters such as molar ratio of reagent and reaction time, scandium molybdates with tunable phase and diverse morphologies including snowflakes, microflowers, microsheets, and branched spindles were obtained. The density of states and surface energies of Sc2Mo3O12 were primarily studied from first-principles calculations. An indirect band gap of 3.56 eV was observed for crystalline Sc2Mo3O12, and the surface energies of various facets were determined to be 0.27−0.91 J/m2. The influence of n(Sc3+):n(Mo7O246−) (short for Sc/Mo) molar ratio was systematically investigated and well-characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and UV−vis absorption spectroscopy (UV−vis). Results indicate that the Sc/Mo molar ratio has a great effect on the phase and morphology. Diffuse reflection spectra (DRS) revealed the Egap can be readily tuned from 3.69 to 4.16 eV, which is in accordance with the theoretical result. The photoluminescence (PL) properties of Eu3+-doped Sc2Mo3O12 were discussed. This facile synthesis strategy could be extended to the synthesis of other molybdates.

1. INTRODUCTION In the past decade, a growing interest in the development of micro-/nanocrystals with special morphologies and tunable sizes has been motivated by both fundamental science and potential application.1−5 The electronic, linear, and nonlinear optical properties are dependent on their composition, size, shape, and dimensions. Among these inorganic micro-/ nanomaterials, molybdates have received much research attention owing to their chemical stability, excellent optoelectronic properties, ion conductivity, ferroelectric properties, scintillating properties, negative thermal expansion, and ability to serve as a luminescence host.6,7 To date, a large number of reports exist on the controllable synthesis of molybdates. For example, hierarchical assemblies of CaMoO4 nano-octahedrons were synthesized by a microwave-assisted hydrothermal process, and their photoluminescence properties were investigated.8 PbMoO4 micro-octahedrons were prepared by the coprecipitation method and processed under hydrothermal conditions at different temperatures, and their PL behavior associated with the different distortions of the MoO4 and PbO8 clusters were explored.9 Hierarchical Sb2MoO6 architectures assembled from single-crystalline nanobelts showed superior electrochemical properties and can be used as anode materials for lithium-ion batteries.10 Rare earth molybdates are excellent matrixes for luminescent phosphor materials, and well-defined micro-/nanoparticles are especially attractive for bioimaging applications owing to the low toxicity of molybdenum and rare-earth cations.11 There© XXXX American Chemical Society

fore, much effort has recently been dedicated to the synthesis of both Y- and Ln (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu)-based molybdates with tunable phases, morphologies, and multicolor luminescence properties.12−15 As far as Sc2Mo3O12 is concerned, it is generally synthesized by a solid-state chemical reaction method.16−19 It belongs to a large family with the formula A2B3O12 (A = a variety of trivalent ions, B = Mo or W), which is able to exhibit as both an orthorhombic phase and monoclinic structure depending on the temperature. Interestingly, at lower temperature (4− 170 K), monoclinic Sc2Mo3O12 presents a positive thermal expansion coefficient of αv = +2.19 × 10−5 K−1, whereas at higher temperatures above 180 K, it displays a negative thermal expansion coefficient of αv = −6.3 × 10−6 K−1.20 To date, most attention has been paid to its phase transition and thermal expansion behaviors.16−23 There are very limited literature reports on the exploration of Sc2Mo3O12 as the luminescence host. However, compared with other rare-earth (Y3+ and Ln3+) based phosphors, Sc3+-based materials have superior chemical and optical properties, owing to their much smaller ion radius and distinct atomic electron configuration.24 Until recently, Li et al. synthesized a novel red-emitting phosphor, Eu3+-activated Sc2(MoO4)3, for white-light-emitting diodes at 800 °C by solid-state reaction.25 Zhao et al. prepared Sc2Mo3O12 via a hydrothermal method in the presence of PVP Received: October 30, 2018

A

DOI: 10.1021/acs.inorgchem.8b03056 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry by varying the amount of MoO42−, and the product showed strong morphology-dependent luminescence properties. The novel chocolate-like microcrystals exposed {001} facets showed the highest intensity and may find great potential applications in the areas of fluorescent lamps and color displays.26 Furthermore, by using glycine as surfactant, Zhao et al. synthesized Sc2Mo3O12 nanosheets by a one-step hydrothermal route, and the multicolor emission of green, yellow, and red was achieved in the Sc2Mo3O12 system.27 Despite these research efforts, the fundamental physical properties (e.g., the electronic properties and surface properties) of Sc2Mo3O12 are not well understood. At the same time, the phase selectivity has rarely been considered. It should be noted that critical experimental conditions, such as high temperature, long aging time, or organic surfactant, are required in these experiments. Therefore, preparation of Sc2Mo3O12 nano-/microparticles with controllable morphologies and uniform sizes through green, rapid, and energy saving routes is desirable. As one of the simplest, most environmental friendly, and cost-effective approaches in the inorganic synthetic field, microwave heating method has been widely used in the preparation of versatile materials such as oxides, chalcogenides, borides, carbides, oxynitrides, and so on.28 Rather than being heated on the exterior of the compounds with conventional methods, the microwave irradiation method can directly couple microwave energy to the molecules; thus, uniform rapid heating can be realized.29 Herein, we report for the first time on the synthesis of scandium molybdate microcrystals via the microwave heating method without the addition of any surfactants and templates. The effect of Sc/Mo molar ratios on the structural, morphological and absorption properties was investigated. Furthermore, by combining a first-principles calculation, the crystal structure, electronic properties, and surface energies of the Sc2Mo3O12 were studied to better understand the optical and morphology characteristics. Eu3+ doped Sc2Mo3O12 was also prepared and their temperature-dependent PL properties were investigated.

operating at 40 kV and 40 mA. The morphologies, microstructures, and elemental composition were measured by a SEM equipped with the energy dispersive spectroscopy EDS (Hitachi, S-3400N). Both the low- and high-resolution transmission electron microscopy (TEM) measurements were performed on a JEM-2100 (Japan) under an acceleration voltage of 200 kV. Thermogravimetric and differential thermal analysis (TG-DTA) curves were collected by a TA-50 thermal analyzer in the air condition with a heating rate of 10 °C min−1. The Eu3+ concentration was determined by inductive coupled plasma (ICP) atomic emission spectroscopy (POEMS, TJA). DRS were collected at a UV−visible−NIR spectrophotometer (Shimadzu UV-3600) attached to an integrating sphere. PL excitation and PL emission (PLE) spectra and lifetimes were recorded using an FLS980 (Edinburgh Analytical Instrument) equipped with a 450W xenon lamp and a high-energy microsecond flash lamp μF900H as the excitation sources. The low-temperature-dependent (77−300 K) fluorescence and decay curves were measured using the same FLS980 instrument equipped with nitrogen gas-flow cryostat (Oxford DN2), and the temperature accuracy was within ±0.5K. 2.3. Density Functional Theory Calculations. All calculations in the present study were performed with the local density approximation (LDA) using the Vienna Ab initio Simulation Package (VASP) 5.2 in the framework of the density functional theory (DFT).30 The valence electron configurations of Sc, Mo, and O elements used in the present study were 3s23p64s23d1, 3p64s24d55s1, and 2s22p4, with 11, 14, and 6 valence electrons, respectively. The energy cutoff of 550 eV was employed for the plane-wave expansion. A Monkhorst−Pack sampling method with 2 × 2 × 1 k-point grid was used for the integrals in reciprocal space. The atomic structures were relaxed, and the final force on each atom was less than 0.01 eVÅ−1.31 Generally, the LDA method typically provides good structural properties and is an effective tool to deal with the surface energies.32 To simulate the surfaces of orthorhombic Sc2Mo3O12, slab models were employed. The calculations were carried out using supercells with atomic layers and periodic boundary as the bulk in a and b directions, and separated in c direction by a vacuum of ∼15 Å, which is considered to be thick enough to minimize the interaction between the up and down surfaces of the slab. Furthermore, in order to reduce the calculation cost and keep charge neutrality, the stoichiometry of the slab models is the same as the bulk. Therefore, the calculation of the surface energy does not involve any references, which ensures that the relative surface energies of all considered surfaces are reasonable. The position of all atoms of the slab was relaxed.

2. EXPERIMENTAL AND THEORETICAL CALCULATION SECTIONS

3. RESULTS AND DISCUSSION 3.1. Composition and Structure Analysis. XRD was first used to examine the composition and phase of the products prepared at various Sc/Mo molar ratios after microwave heating 30 min. All the diffraction peaks in Figure S1a for the samples with molar ratios of 1:1, 2:1, and 5:1 could be indexed to orthorhombic phase Sc2Mo3O12 (PDF no. 21− 1329, JCPDS, 1967). EDS analysis on multipoints and areas confirm the presence of Sc, Mo, and O in the compounds, demonstrating that crystalline Sc2Mo3O12 could be synthesized by this fast precipitation reaction (Figure S2a). However, compared with the stoichiometric atomic ratio of Sc2Mo3O12, the atomic ratio of Sc to Mo varies from 1:1.80 to 1:1.91 (Table S1), indicating that these samples are Mo-rich. With the increased dosage of Sc(NO3)3, the atomic ratio of Sc to Mo decreased progressively. To further analyze the phase and crystal structure of the as-prepared samples, profile fitting via the Rietveld method was carried out. Figure 1 shows the experimental, calculated, and difference results of XRD profiles. On the basis of the Rietveld refinement results, it is found that the three samples are mostly composed of Sc2Mo3O12, but a secondary phase of MoO3 varies from 1.85 to 5.30 wt % can also be identified. The presence of MoO3 may be ascribed to

2.1. Materials and Synthesis. All chemicals were analyticalgrade reagents and used directly without further purification. The starting materials were Sc(NO3)3·6H2O (99.99%), (NH4)6Mo7O24.4H2O (99.99%), and Eu(NO3)3·6H2O and were purchased from Shanghai Aladdin Industrial Corporation. In a typical procedure, the Sc2Mo3O12 micropowders were synthesized as follows: 1 mmol of (NH4)6Mo7O24·4H2O and 1 mmol of Sc(NO3)3·6H2O were added to 15 mL of deionized water, respectively. Subsequently, the above two solutions were mixed together in a 100 mL roundbottomed flask. After vigorous stirring for 30 min at room temperature, the as-obtained mixed solution was transferred into a household microwave oven equipped with a refluxing apparatus, the microwave power was set as low as 210 W, and the microwave radiation heating time was 30 min. After the system was cooled to room-temperature naturally, the product was collected, washed several times with water and ethanol, and finally dried at 60 °C in vacuum for 6 h. The Eu3+-doped Sc2Mo3O12 sample was prepared by the same procedure, except that the Sc(NO3)·6H2O was replaced with appreciated Eu(NO3)·6H2O at the initial stage. The commercial red-emitting Y2O3:Eu3+ was purchased from Shenzhen Looking Long technology Co., Ltd. 2.2. Measurement and Characterization. The XRD patterns of the samples were characterized by a D8 Advance diffractometer (Bruker Corporation, Germany) with Cu Kα radiation (λ = 1.5418 Å) B

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result, it is calculated to be approximately 71 wt % Sc2Mo3O12 and 29 wt % MoO3 in the compound, the value is similar to the EDS result of 77 wt % Sc2Mo3O12 and 23 wt % MoO3 that calculated from the atomic ratio of Sc/Mo 1:1.80. Besides, it can be seen from the TG curve that nearly no further weight loss above 1000 °C, it is reasonable to speculate that MoO3 can be excluded from the product after calcination. To verify this, Figure S4 also presents the XRD refinement of the sample with Sc/Mo 1:1 that calcined in air at 1000 °C, 4 h, it is confirmed a pure phase of orthorhombic Sc2Mo3O12 and the R factors were successfully converged at a satisfactory level. The EDS result in Table S1 show that the measured Sc/Mo atomic ratio is 1:1.58, which is close to the stoichiometric atomic ratio of 1:1.50 for Sc2Mo3O12. From this, it can also be seen that the Sc/Mo atomic ratio measured by EDS is a bit lower than the actual value. Interestingly, when changing the molar ratio of Sc/Mo to 1:2 or 1:5, XRD patterns in Figure S1b demonstrate that (NH4)Sc(MoO4)2 (PDF no. 52−0392, JCPDS, 1995) can be formed. It is pertinent to mention here that similar phenomenon was reported by Zhao et al.26 In their work, the product was prepared by hydrothermal method, but it was not indexed to a certain compound. The EDS patterns show that the two samples not only possess Sc, Mo, and O elements but also N element (Figure S2b,c), which is consistent with the XRD results. In an attempt to further explore these two samples, the compounds were calcined at 800 °C for 4 h in an air atmosphere. As expected, N element disappeared (Figure S 2d,e), and the products can be assigned to Sc2Mo3O12 (Figure S1b). The Rietveld plots of the two calcined samples are given in Figure S5; similarly, the refinement results reveal that they are mainly composed of Sc2Mo3O12 and a small amount of MoO3 (1−4.8 wt %). TG-DTA results also verified the Rietveld results. Figure S6 shows the TG-DTA curves of the sample prepared with Sc/Mo ratio of 1:5 before calcination, the weight loss (about 8%) before 280 °C corresponds to the loss of absorbed H2O and structured H2O from MoO3·nH2O. From the DTA curve, a broad endothermic peak can be observed, which may be due to the amorphous MoO3·nH2O losing its H2O molecules and turned into crystalline MoO3. The weight loss between 280 and 600 °C is mainly attributed to the lose of NH3 and H2O molecules from (NH4)Sc(MoO4)2. Also, absorbed NH4+ decomposes at this stage. The weight loss above 600 °C can attributed to the sublimation of MoO3. On the basis of the above results, the weight loss curve is basically consistent with conversion of NH4Sc(MoO4)2 to Sc2Mo3O12, and it may decompose via following process under high temperature:

Figure 1. XRD refinement results of the compounds prepared with different Sc/Mo molar ratio and performed using the Topas 5.0 program. The purple and pink short vertical lines show the Bragg reflections of the calculated pattern of Sc2Mo3O12 and MoO3, respectively. (a) 1:1; (b) 2:1, and (c) 5:1.

2(NH4)Sc(MoO4 )2

the hydrolysis of Mo7O246−, because under acidic condition the chemical equilibrium between Mo7O246− and H+ ions was established, which is expressed as follows: Mo7O24 6 − + 6H+ = 7MoO3 + 3H 2O

= Sc 2Mo3O12 + MoO3 + 2NH3↑ + H 2O↑

(2)

The compound of Sc2Mo3O12 showed the orthorhombic structure characterized by the space group (Pbcn) with four molecular formula per unit cell (Z = 4). The refined unit cell parameters and residual factors on the samples with different Sc/Mo molar ratios are shown in Table S2. Taking Sc/Mo 1:1 as an example, the unit cell parameters are a = 13.25 Å, b = 9.55 Å, c = 9.65 Å, and V = 1220.9 Å. The residual factors are Rwp = 14.31%, Rp = 10.69%, and χ2 = 2.33. Figure S7 displays the structural diagram for Sc2Mo3O12 modeled by the Visualization for Electronic and Structural Analysis (Vesta) program. As can be seen, Sc atoms were coordinated to 6 O

(1)

Thus, some of the Mo7O246− ions may inevitably hydrolyze to form MoO3. Due to the fast hydrolysis speed under microwave conditions, the obtained MoO3 is not well crystallized. The TG behavior of the representative sample of Sc/Mo molar ratio 1:1 was investigated and displayed in Figure S3, the weight loss of 6% before 600 °C correspond to the loss of absorbed H2O, NH4+, and structured H2O from MoO3·nH2O. While the weight loss of about 9% in the range of 600−1000 °C is assigned to the sublimation of MoO3. On the basis of the TG C

DOI: 10.1021/acs.inorgchem.8b03056 Inorg. Chem. XXXX, XXX, XXX−XXX

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1:5, and 1:2) before calcination was measured at room temperature. Figure 3 shows the variation of the absorption

atoms, forming distorted octahedral (ScO6) clusters with an average 2.09 Å Sc−O bond length. Mo atoms were coordinated to 4 O atoms, resulting in tetrahedral (MoO4) clusters with a shorter (1.74 Å) average Mo−O bond length. The LDA calculated equilibrium lattice parameters (a = 13.22 Å, b = 9.53 Å, c = 9.62 Å) and volume (1212.6 Å3) supporting the experimental values, in good agreement with standard Powder Diffraction File (PDF) no. 21−1329 (Joint Committee on Powder Diffraction Standards (JCPDS), 1967) (a = 13.26 Å, b = 9.55 Å, c = 9.65 Å) and volume (1222.3 Å3). 3.2. Band Structures and Density of States. In order to better understand the relationship between the electronic structure and optical properties, the band structures and density of states of Sc2Mo3O12 were calculated based on the fully relaxed lattice parameters and ionic positions. As can be seen in Figure 2a, the top of valence band (VB) is located at Y-

Figure 3. UV−vis DRS for the products prepared with various Sc/Mo molar ratios and 0.03 Eu3+ doped sample with Sc/Mo 1:1 before calcination.

coefficient as a function of the photon energy. The optical band gap can be calculated with following equations: [F(R ∞)hv]n = A(hv − Egap) F(R ∞) =

(1 − R ∞)2 2R ∞

(3)

(4)

where R∞ is the reflectance of the sample, F(R∞) is a Kubelka−Munk function defined as eq 4, hν is the photon energy, A is a proportional constant, and n is a constant that depends on the electronic transitions (n = 1/2 for an indirect transition ad n = 2 for a direct transition).34 From the above DFT obtained indirect gap, n is 1/2. On the basis of the F(R∞) value from eq 4 and the plot of [F(R∞)hν]1/2 againest hν as illustrated in Figure 3, the Egap values can be estimated by extrapolating the linear portion of DRS curves.35 It can be seen that the Egap steadily decreased from 4.16 to 3.69 eV with an increase of Sc/Mo molar ratio of 1:1, 2:1, and 5:1, respectively. Interestingly, when the ratio is changed from 1:5 to 1:2, the Egap moderately decreased to 4.12 and 4.05 eV, respectively. The difference between the two series of samples is due to they have different crystal phase. As is well-known, the exponential optical absorption edge and Egap are controlled by the degree of structural order− disorder in the lattice and local bond distortions. The widest Egap was detected for the sample with an equal molar ratio of 1:1. Increasing either the Sc or Mo ratio resulted in higher concentration of defects in the lattice and caused a reduction in energy levels between VB and CB. All these spectra exhibit a similar tendency except the Sc/Mo 5:1; it clearly shows an extra intense absorption band centered at about 3.85 eV (322 nm), which is mainly caused by oxygen vacancies. In fact, similar phenomena have also been reported by Zhou et al.

Figure 2. (a) Band structure of Sc2Mo3O12 host. (b) Total and partial density of state near the Fermi level.

point and the bottom of conduction band (CB) at X-point in the first Brillouin zone, with an indirect band gap of 3.56 eV. As is well-known, the optical properties are mostly determined by the electronic states near the Fermi level.33 Therefore, Figure 2b shows the total density of states (DOS) and the atomic projected partial DOS of the compound near the Fermi level. It was found that the top of VB predominantly originates from the O 2p orbital, and the bottom of CB mainly consists of Sc 3d and Mo 4d orbitals, indicating that the interaction between O 2p, Sc 3d, and Mo 4d orbitals play a major role in determining the band gap, which is strictly associated with the optical properties. The calculation verifies that Sc2Mo3O12 is a good host material and can provide a suitable band gap to contain the energy levels of rare earth element. 3.3. UV−Vis Diffuse Reflectance Spectroscopy. To verify the theoretical calculation above and further evaluate the effect of the Sc/Mo ratio on the optical properties, the UV−vis DRS of the powder sample with various ratios (1:1, 2:1, 5:1, D

DOI: 10.1021/acs.inorgchem.8b03056 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry where the absorption band is observed at 320 nm.36 Combining the experimental observation and the DFT calculation, we can conclude that the theoretical Egap value 3.56 eV corresponded to the experimental value of 3.69−4.16 eV, as it is generally accepted that the DFT calculation at LDA level has an underestimation. To better examine whether the presence of small amount of MoO3 will affect the band gap plots and luminescent properties of the products, Sc2Mo3O12: 0.03Eu3+ prepared with Sc/Mo ratio 1:1 was selected. The XRD pattern in Figure S8 show that nearly all diffraction peaks of the sample before calcination can be indexed to Sc2Mo3O12, and the DRS show that the corresponding Egap is 3.90 eV (Figure 3). In order to eliminate MoO3 from the Sc2Mo3O12: 0.03Eu3+ sample, the powder was calcined in air at 1000°C for 4 h, and its corresponding Rietveld structure refinement was plotted in Figure S9. The fitting result confirmed that the product is a pure phase. DRS reveals that the Egap is 3.93 eV (Figure S10), which is very close to the value before calcination, indicating that the Egap of Sc2Mo3O12:0.03Eu3+ is not considerably affected by the impurity of MoO3, owing to the fact that MoO3 has a much lower band gap of 2.8−3.2 eV.37,38 3.4. Morphological Analysis. It was found that the Sc/ Mo molar ratio was the dominant factor in the determination of the morphologies.39,40 The SEM images of the samples with different molar ratios are displayed in Figure 4. When the ratio was 1:1, it was composed of snowflakelike microstructures with an average diameter of 8−10 μm (Figures 4a,b). These snowflakelike flowers were constructed of 8 petals with rough surfaces, and each petal was composed by a large number of tiny nanosheets that were linked together and extended outward from the center of the snowflakes. When the molar ratio was increased to 2:1, as illustrated in Figure 4c, the product had irregularly flowerlike morphology with a size of approximately 9−12 μm. It can be clearly observed from Figure 4d that the microflower is constructed of multilayerintersected flat nanosheets, and the nanosheets are tightly interwoven and self-assembled with each other. Increasing the molar ratio to 5:1, as can be seen in Figure 4e,f, the flower became more regular and turned to peony-like with an average size of 10 μm. The peony-like flowers were composed of several petals which were constructed by a large number of nanosheets and extended outward from the center. The size and shape of the products are remarkably different when the Sc/Mo molar ratio is 1:2 or 1:5, as shown in Figure 4g,h, When the molar ratio was 1:2, the prepared sample was built from large thin slices with about 2 μm in width and 6 μm in length. A further decrease of the molar ratio to 1:5 results in branched spindle and strawlike morphology, as shown in Figures 4i,j. It is obvious that the Sc/Mo molar ratio has great impact on the morphology of the product. As identified by XRD analysis in Figure S1b, these two samples are (NH4)Sc(MoO4)2. After annealing the two samples at 800 °C for 4 h both of them transformed to Sc2Mo3O12, but the morphology and size varied greatly, as shown in Figure S11. It was composed of irregular nanoparticles. To gain a better understanding of the crystal growth and morphology evolution, time-dependent experiments were performed with other conditions unchanged. It can be observed from Figure 5 that the morphology of the sample evolved from anomalous polyhedrons to microflowers. After 10 min of microwave heating (Figure 5a), the product was composed of some irregular nanoparticles and nanorods with a

Figure 4. SEM images for the products prepared at different Sc/Mo molar ratio before calcination: (a,b) 1:1 (low and high magnification), (c, d) 2:1, (e,f) 5:1, (g,h) 1:2, and (i, j) 1:5.

Figure 5. SEM and TEM images of the products after different microwave heating times when the Sc/Mo ratio was 1:1. (a−c) 10 min, (d−f) 15 min, (g) 20 min, (h) 25 min, and (i) 40 min.

length of about 1.7 μm and width of 0.6−1 μm and several randomly oriented and monodispersed nanorods with a length E

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Inorganic Chemistry of 2.4 μm. According to high-resolution TEM (HRTEM) images in Figure 5b,c, the lattice fringes are distinguished, and one observed lattice spacing is 0.41 nm, which is identified to the (211) plane of Sc2Mo3O12. Increasing the heating time to 15 min, the irregular nanoparticles/rods grew and evolved into octahedron with a diameter of 1−3 μm, the nanorods showed a tendency to agglomerate as show in Figure 5d. From the TEM images in Figure 5e,f, the polyhedra have a smooth surface, and the lattice fringe of 0.69 nm in HRTEM image can be assigned to the (002) plane. Upon extending the heating time to 20 min, the formation of the microflowers appeared (Figure 5g), and it can be seen that the microflowers were constructed from a large number of nanorods with a diameter of approximately 4 μm. When the reaction time was further extended to 25 min, the morphology of the product was obviously changed, as it was composed of disordered particles and nanoflakes. Interestingly, when the time was 30 min, the crystals gave a unique snowlike structure composed of nanoflakes, as mentioned in Figure 4a,b. Extending the reaction time to 40 min, the morphology was similar to that of 30 min, but some irregular particles were observed simultaneously (Figure 5h,i), indicating that a longer reaction time is disadvantageous to form uniform Sc2Mo3O12 microcrystals. The XRD patterns of the product obtained after different reaction time were presented in Figure S12. From these data, it can be seen that all of the products are roughly attributed to orthorhombic phase Sc2Mo3O12, which is the same as that of the product prepared after 30 min. The above results reveal that reaction time has some influence on the morphologies, but it does not affect the crystal phase of the final product. 3.5. Surface Energies. To evaluate the stability of various surfaces of the compound, the surface properties should be taken into account because morphology is intrinsically determined by the surface energies of the facets.41,42 So far, to the best of our knowledge, there are no accurate calculations or experimental determinations of the surface energies of Sc2Mo3O12 in the literature. To fill this gap, we carried out an extensive first-principles investigation on the low index surfaces of (100), (001), (110), (101), (211), and (013) with periodic slab models at stoichiometric composition; the models are illustrated in Figure S13. After all atoms are fully relaxed and the system reaches the minimized energy as show in Table S3, the surface energies could be calculated using the following expression:43 γ = (Eslab − E bulk )/2S

Figure 6. Theoretical determined surface energy for various faces of Sc2Mo3O12.

product varied with the extending of reaction time at early stage (Figure 5). 3.6. PL Properties of Eu3+-Doped Sc2Mo3O12. Figure 7 shows the PL properties of Sc2Mo3O12: 0.03Eu3+ before calcination. The ratio of n(Eu3+) to n(Sc3+) was measured by ICP and was found to be 0.032, which agrees well with the starting reagent used, indicating that the Eu3+ ions have been doped into Sc2Mo3O12 host successfully. By monitoring the most intense emission band of 5D0 → 7F2 transition at 611 nm, the excitation spectrum (Figure 7a) is composed of a broad band centered at 279 nm associated with the ligand to metal charge transfer (CT) O2−−Eu3+ transition, and several narrow bands at 319, 363, 383, and 417 nm correspond to the intraconfigurational 4f−4f transitions of Eu3+ ions, which could be referred to the transitions from ground state 7F0 to the excited states of 5H6, 5D4, 5G2, and 5D3, respectively. The strongest band peaked at 394 nm was ascribed to the 7F0 → 5 L6 transition. The f−f electron transitions were well protected by the external shell, they were weakly affected by the ligand field.45 Figure 7b exhibits the emission spectrum obtained in the spectral range 500−750 nm at room temperature. It shows the typical Eu3+ emission bands upon the excitation at 394 nm, which were attributed to the transitions from the first excited metastable 5D0 level to the 7Fi (i = 1−4) ground states. As can be seen, the electric dipole transition 5D0−7F2 (peak at 611 nm) were considerably stronger than other transitions. This band split into 3 sublevels, suggesting the Eu3+ ion occupied the Sc3+ site in D2 symmetry, while the maximum of magnetic dipole transition 5D0−7F1 was located at 593 nm, and other peaks from 5D0−7F3 and 5D0−7F4 transitions were at 652 and 701 nm, respectively. The intensity between different transitions is dependent on the symmetry of local environment according to the Judd−Ofelt theory. The analyses of the emission band can provide direct information about the crystallographic sites occupied by the Eu3+ ion. The presence of much more intense 5D0−7F2 transitions in comparison to the 5D0−7F1transition indicated a low local symmetry around Eu3+ in Sc2Mo3O12 host, as shown in Figure 7b. The integral ratio of 5D0−7F2 and 5D0−7F1 was calculated to be ∼8.89, which was beneficial to achieve red phosphors with high color purity.46 To estimate the PL lifetime for the Sc2Mo3O12:0.03Eu3+ phosphors, the dynamic decay curve is displayed in Figure 8. It was well-fitted with a second-order exponential decay using the following equation:47

(5)

where Eslab and Ebulk are the total energies of the slab and the bulk with the same number of atoms and S represents the surface area of the slab model. As displayed in Figure 6 and Table S3, the surface energy is determined to be 0.27, 0.76, 0.45, 0.79, 0.75, and 0.91 J/m2 for the facets (100), (001), (110), (101), (211), and (013), respectively. According to previous calculations,32,42 the average surface energy of (100) facets is usually lower than that of (001) facets, and a similar result was obtained in this work. In general, crystal facets with higher surface energies will diminish during the growth process to minimize the surface energy according to the Gibbs−Wulff theorem.44 Interestingly, the (211) and (002) facets with high surface energy were observed in the products after 10 and 15 min heating times, but they may disappear with the extending of reaction time, as we can see that the morphology of the F

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Figure 7. PL properties of Sc2Mo3O12: 0.03Eu3+ prepared when the Sc/Mo molar ratio was 1:1 monitored at λex = 394 nm and λem = 611 nm at room temperature. (a) Excitation spectrum and (b) emission spectrum.

Figure 9 presents the calculated CIE chromaticity coordinates (0.657, 0.341) for Sc2Mo3O12:0.03Eu3+ from the

Figure 8. Room-temperature decay curve of Sc2Mo3O12:0.03Eu3+ with Sc/Mo molar ratio of 1:1 monitored at 611 nm upon excitation of 394 nm.

I(t ) = A1 e(−t / τ1) + A 2 e(−t / τ2)

(6)

where I(t) is the PL intensity, A1 and A2 are fitting constants, τ1 and τ2 represent the rapid and slow lifetime for exponential functions. Here, we can find that the lifetimes of τ1 and τ2 have little difference with 0.31 and 0.58 ms, respectively, which means the decay component originates from the same emission center. On the basis of the fitting parameters, the average lifetime (τavg) can be defined by τavg = (A1τ21 + A2τ22)/(A1τ1 + A2τ2), and the calculated τavg value was determined to be 0.46 ms. Furthermore, in order to study the influence of MoO3 on the luminescent properties of Sc2Mo3O12: 0.03Eu3+, the excitation and emission spectra of the calcined sample were measured. As can be seen in Figure S14, both of the spectra exhibit the same behavior with the product before calcination, and the average decay time is 0.55 ms (Figure S15), meaning MoO3 has slight effect on the luminescent properties of the Sc2Mo3O12:0.03Eu3+. In fact, Eu3+ preferred to replace Sc3+ site in Sc2Mo3O12 host, due to the charge imbalance and big difference between the radius of Eu3+(0.947 Å) and Mo6+ (0.59 Å), Eu3+ ions are hard to be doped into MoO3 host. Besides, owing to the band gap of Sc2Mo3O12 and MoO3 being quite different, energy transfer between them can be omitted. On the basis of these findings, it is safe to draw a conclusion that the presence of small amount of MoO3 has slight effect on the luminescent properties of Sc2Mo3O12:0.03Eu3+.

Figure 9. Calculated PL emission color coordinates for Sc2Mo3O12:Eu3+, commercial red-emitting Y2O3:Eu3+, and NSTC red illuminate in the CIE 1931 chromaticity diagram.

PL emission in Figure 7b; it was easy to see that the color tone is near that of the available commercial red phosphor Y2O3:Eu3+ (0.634, 0.365) and approached the standard National Television System Committee (NSTC) red illuminant (0.670, 0.330), 48 demonstrating that Sc 2 Mo 3 O 12 possesses a high color purity and shows its great potential application in the areas of WLEDs and color displays. However, the intensity is lower than that of the commercial Y2O3:Eu3+ (Figure S16), which may be due to how it is prepared at low temperature and the product’s low crystallinity. 3.7. Temperature-Dependent PL Properties Analysis. The crystal structure of negative thermal expansion materials has a close relationship with the temperature, and the transition usually occurs at low temperatures. 16 The luminescence properties, in particular, are strongly dependent on the environment temperature.49 Recently, our group also prepared the negative thermal expansion (NTE) ScF 3 submicroparticles, and their temperature-dependent PL G

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factor 0.992, and the activation energy Ea was determined to be 0.04 eV. Figure 11a illustrates the corresponding decay curves excited at 394 nm and monitored at 611 nm for different temperatures.

properties were investigated. The luminescence intensity and decay time varied with the temperature increase, while the R value denotes for the intensity ratio of I(5D0 → 7F2)/I(5D0 → 7 F1) was almost constant.50 As mentioned above, Sc2Mo3O12 exhibited a NTE coefficient at 180−300 K,20 and our aim was to comprehensively understand the temperature dependence of luminescence properties. Thus, the emission spectra and decay curves for the Sc2Mo3O12:0.03Eu3+ were measured at 77−300 K. As can be seen in Figure 10a, the emission spectra

Figure 11. Decay curves of Sc2Mo3O12:0.03Eu3+ excited at 394 nm, monitored at 611 nm and (b) corresponding lifetime in temperature range 77−300 K.

All curves followed second-order exponential, and the fitting parameters are presented in Table S4. The decay time gradually decreases from 828.57 to 766.96 μs with a 93% drop from 77 to 270 K as displayed in Figure 11b. Upon further increasing the temperature to 300 K, the decay time dropped to 643.76 μs.

Figure 10. (a) Temperature-dependent PL spectra (the inset shows the spectra in wavelength 500−750 nm) and (b) the activation energy (ΔE) of Sc2Mo3O12:0.03Eu3+ excited at 394 nm.

4. CONCLUSIONS In summary, scandium molybdate microstructures with tunable phase and morphology were prepared by a facile microwave heating method. Results show that the molar ratio of Sc/Mo had a great effect on the morphology and phase of the final products. High Sc/Mo molar ratio leads to snowlike Sc2Mo3O12, while low Sc/Mo molar ratio results in (NH4)Sc(MoO4)2 with different morphology. By employing the DFT calculation, the band gap was estimated to be 3.56 eV, close to the Egap value obtained from DRS experiments. The surface stability of several low index surfaces was studied, and the (100) facet was found to be most stable with the surface energy of 0.27 J/m2. Under the excitation of 394 nm, the phosphor exhibited typical red emission, peaking at 611 nm and having high purity, which is comparable to the commercial Y2O3:Eu3+ phosphor and NSTC red illuminate. Temperaturedependent PL results revealed that the luminescence intensity increased with the decrease of temperature and that the Ea was 0.04 eV. Furthermore, the presence of small amount of MoO3 in the compounds insignificantly affected the PL properties of Sc2Mo3O12. The present work has provided an effective way to

almost overlapped with each other, except for the slight decrease in the intensity. In general, the decrease of PL intensity with the rise of temperature usually occurs in the rareearth-doped phosphors, which is caused by the larger nonradiative transition at higher temperature.51 The relative PL emission intensities fell to 65.5% (at 300 K) with respect to the original values at 77 K. Furthermore, the quenching behavior can be calculated by the following Arrhenius equation:48 I0 It = −E 1 + A exp k Ta

( ) B

(7)

where I and I0 represent the relative PL emission intensities at different operating temperatures and initial temperature, A is a constant, and kB is the Boltzmann’s constant with 8.617 × 10−5 eV/K. Ea denotes the activation energy for the electron from the emitting state to the quenching state. Figure 10b shows the plot of ln[(I0/I) − 1] versus 1/Kb for the Sc2Mo3O12:0.03Eu3+ phosphors. The experimental data can be fitted linearly with R H

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(6) Zhai, Y.; Zhang, W.; Yin, Y.; Han, Y.; Zhao, X.; Ding, H.; Li, N. Morphology tunable synthesis and luminescence property of NaGd(MoO4)2:Sm3+ microcrystals. Ceram. Int. 2017, 43, 841−846. (7) Namvar, F.; Beshkar, F.; Salavati-Niasari, M. Novel microwaveassisted synthesis of leaf-like MnMoO 4, nanostructures and investigation of their photocatalytic performance. J. Mater. Sci.: Mater. Electron. 2017, 28, 7962−7968. (8) Longo, V. M.; Cavalcante, L. S.; Paris, E. C.; Sczancoski, J. C.; Pizani, P. S.; Li, M. S.; Andres, J.; Longo, E.; Varela, J. A. Hierarchical assembly of CaMoO4 nano-octahedrons and their photoluminescence properties. J. Phys. Chem. C 2011, 115, 5207−5219. (9) Sczancoski, J. C.; Bomio, M. D. R.; Cavalcante, L. S.; Joya, M. R.; Pizani, P. S.; Varela, J. A.; Longo, E.; Li, M. S.; Andrés, J. A. Morphology and blue photoluminescence emission of PbMoO4 processed in conventional hydrothermal. J. Phys. Chem. C 2009, 113, 5812−5822. (10) Lu, X.; Wang, Z. Y.; Lu, L.; Yang, G.; Niu, C.; Wang, H. Synthesis of hierarchical Sb2MoO6 architectures and their Electrochemical behaviors as anode materials for Li-Ion batteriey. Inorg. Chem. 2016, 55, 7012−7019. (11) Abtmeyer, S.; Pa̧zik, R.; Wiglusz, R. J.; Małecka, M.; Seisenbaeva, G. A.; Kessler, V. G. Lanthanum molybdate nanoparticles from the bradley reaction: factors influencing their composition, structure, and functional, characteristics as potential matrixes for luminescent phosphors. Inorg. Chem. 2014, 53, 943−951. (12) Deng, H.; Zhao, Z.; Wang, J.; Hei, Z.; Li, M. X.; Noh, H. M.; Jeong, J. H.; Yu, R. J. Photoluminescence properties of a new orange− red emitting Sm3+ -doped Y2Mo4O15 phosphor. J. Solid State Chem. 2015, 228, 110−116. (13) Zhou, Y.; Yan, B. RE2 (MO4)3:Ln3+ (RE = Y, La, Gd, Lu; M = W, Mo; Ln = Eu, Sm, Dy) microcrystals: controlled synthesis, microstructure and tunable luminescence. CrystEngComm 2013, 15, 5694−5702. (14) Baur, F.; Glocker, F.; Jüstel, T. Photoluminescence and energy transfer rates and efficiencies in Eu3+ activated Tb2Mo3O12. J. Mater. Chem. C 2015, 3, 2054−2064. (15) Zhou, L.; Hu, S.; Zhou, X.; Tang, J.; Yang, J. One-step surfactant-free synthesis of Eu3+ activated NaTb(MoO4)2 microcrystals with controllable shape and their multicolor luminescence properties. CrystEngComm 2016, 18, 7590−7600. (16) Wu, M.; Liu, X. Z.; Chen, D.; Huang, Q.; Wu, H.; Liu, Y. Structure, Phase transition, and controllable thermal expansion behaviors of Sc2−xFexMo3O12. Inorg. Chem. 2014, 53, 9206−9212. (17) Li, J.; Cheng, X.; Zhu, J. Preparation of Sc2Mo3O12 and characterization of its negative thermal expansion property. Appl. Mech. Mater. 2013, 320, 181−184. (18) Yamamura, Y.; Ikeuchi, S.; Saito, K. Characteristic phonon spectrum of negative thermal expansion materials with frame work structure through calorimetric study of Sc2M3O12 (M = W and Mo) and non-hydrolytic sol-gel (NHSG). Chem. Mater. 2009, 21, 3008− 3016. (19) Young, L.; Alvarez, P. T.; Liu, H.; Lind, C. Extremely low temperature crystallization in the A2M3O12family of negative thermal expansion materials. Eur. J. Inorg. Chem. 2016, 2016, 1251−1256. (20) Evans, J. S. O.; Mary, T. A. Structure phase transition and negative thermal expansion in Sc2(MoO4)3. Int. J. Inorg. Mater. 2000, 2, 143−145. (21) Varga, T.; Moats, J. L.; Ushakov, S. V.; Navrotsky, A. Thermochemistry of A(2)M(3)O(12) negative thermal expansion materials. J. Mater. Res. 2007, 22, 2512−2521. (22) Maczka, M.; Souza Filho, A. G.; Paraguassu, W.; Freire, P. T.C.; Mendes Filho, J.; Hanuza, J. Pressure-induced structural phase transitions and amorphization in selected molybdates and tungstates. Prog. Mater. Sci. 2012, 57, 1335−1381. (23) Varga, T.; Wilkinson, A. P.; Lind, C.; Bassett, W. A.; Zha, C. S. High pressure synchrotron x-ray powder diffractionstudy of Sc2Mo3O12 and Al2W3O12. J. Phys.: Condens. Matter. 2005, 17, 4271−4283.

synthesize scandium molybdate microstructures, which can extend to other phosphor for WLEDs and FEDs.

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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.inorgchem.8b03056.

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Powder XRD patterns and EDS analysis for the samples prepared with varies Sc/Mo ratios; TG-DTA curves for Sc/Mo 1:5 and XRD Rietveld refinement result of Sc/ Mo 1:1, 1:2, 1:5 after calcination; structure parameters and crystal model of Sc2MO3O12; UV−vis DRS for Sc2Mo3O12:0.03Eu3+ after calcination; time-dependent XRD patterns for Sc/Mo 1:1; slab models and DFT calculated surface energies for the low index surfaces of Sc2Mo3O12; emission comparison between the commercial red-emittingY2O3:Eu3+phosphor and as-prepared Sc2Mo3O12:Eu3+; temperature-dependent life times and fitting parameters for Sc2Mo3O12:0.03Eu3+ with Sc/Mo ratio 1:1 (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.Y.O.). *Tel.: +86 791 88120386; fax: +86 791 88120386. E-mail: [email protected] (S.L.Z.) ORCID

Bo Xu: 0000-0002-6896-0409 Chuying Ouyang: 0000-0001-8891-1682 Shengliang Zhong: 0000-0002-6660-6465 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 91622105), Jiangxi Provincial Department of Science and Technology (Nos. 20161BAB203083 and 20172BCB22008), Jiangxi Provincial Education Department (No. GJJ160327) and the Scientific Research Foundation of Graduate School of Jiangxi Normal University (No. JYS2016005).

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