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SmVO4 nanocrystals with dodecahedral shape: Controlled synthesis, growth mechanism and photoluminescent properties
Accepted Manuscript Title: SmVO4 nanocrystals with dodecahedral shape: controlled synthesis, growth mechanism and photoluminescent properties Authors: Xianjin Ge, Youjin Zhang, Hai Wu, Maozhong Zhou, Tao Lin PII: DOI: Reference:
S0025-5408(17)31990-6 http://dx.doi.org/10.1016/j.materresbull.2017.08.037 MRB 9515
To appear in:
MRB
Received date: Revised date: Accepted date:
21-5-2017 13-8-2017 13-8-2017
Please cite this article as: Xianjin Ge, Youjin Zhang, Hai Wu, Maozhong Zhou, Tao Lin, SmVO4 nanocrystals with dodecahedral shape: controlled synthesis, growth mechanism and photoluminescent properties, Materials Research Bulletinhttp://dx.doi.org/10.1016/j.materresbull.2017.08.037 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
SmVO4 nanocrystals with dodecahedral shape: controlled synthesis, growth mechanism and photoluminescent properties
Xianjin Ge, Youjin Zhang*, Hai Wu, Maozhong Zhou, Tao Lin Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China
*Corresponding author: Youjin Zhang, [email protected] Tel: +86-551-63492145; Fax: +86-551-63492083
Graphical abstract
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Abstract: Dodecahedral SmVO4 nanocrystals with uniform size had been successfully synthesized by a facile hydrothermal process in the presence of potassium hydroxide and urea. The dodecahedron-like SmVO4 was characterized by X-ray diffraction, field-emission scanning electron microscopy, Fourier transform infrared, X-Ray photoelectron spectroscopy, high-resolution transmission electron microscopy and photoluminescence. The possible growth mechanism of the SmVO4 nanocrystals with dodecahedral shape was discussed. Under excitation at 315 nm, the SmVO4 sample exhibits strong orange emission while it as the host doped with suitable amount of Eu3+ ions shows strong reddish orange emission. The diversity in emission intensity detected from SmVO4: x% Eu3+ (x=1, 3, 5, 7) was compared. The hydrothermal method for preparing dodecahedral SmVO4 nanocrystals provides us an idea to synthesize other rare earth orthovanadate nanomaterials with special morphology and interesting optical property.
KEYWORDS: A. inorganic compounds, B. crystal growth, B. optical properties, C. electron microscopy, C. X-ray diffraction
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1 Introduction
Rare earth compounds as an important family of functional materials, which possess outstanding physicochemical properties, have been applied in many areas like electric, magnetic, optical and catalytic field. These useful functions derive from the distinctive 4f electron configurations, which are poorly affected by the coordination environment or crystal field owing to the shielding of the 4f orbitals by the filled 5s and 5p orbitals [1, 2]. When it comes to the extensive applications of rare earth compounds in different areas, it is inevitable to talk about rare earth orthovanadate (REVO4), which have been played an important role in various fields including laser hosts [3], catalysts [4], sensors [5], and phosphors [6], etc. In order to fabricate high quality REVO4 nanomaterials with uniform size and shape, plentiful synthetic methods have been established in the past decades, containing sol-gel technique [7], electrospinning technique [8], complexing agent-assisted precipitation [9], microwave-assisted synthesis [10] and hydrothermal synthesis [11] and so on. Among these approaches adopted to prepare REVO4 nanomaterials with well-controlled shapes and sizes, the 3 / 33
hydrothermal method as the relatively mild method which needs comparatively low reaction temperature, has become one of the most promising methods due to high purity, narrow particle size distribution and precise component of the final product. As for REVO4 nanomaterials, a large amount of papers about the preparation and properties had been published, however, most of them were concentrated on reporting YVO4 and LaVO4 nanomaterials. Various shapes of the YVO4 including spherical shape [12], polygonal shape [13], peanut-like shape, octahedral shape [14] had been prepared by hydrothermal process. Likewise, different shapes of the LaVO4 just like nanoparticles, nanorods [15], nanosheaves [16], and hollow microspheres [17] had been synthesized in the same method. Specifically, REVO4, as the catalysts for many organic reactions such as oxidative dehydrogenation of alkanes, olefins, methylbenzene, and oxidation of hydrogen sulfide to elemental sulfur [18–22], should be paid more attention on studying their preparations and properties. As we all know, only a few papers focused on the preparation and property researches of the SmVO4 nanomaterials. Do et al. [23] have reported that the SmVO4 nanocrystals with spherical and hexagonal shapes could be obtained by using oleylamine and oleic acid as the capping surfactant, respectively. Ramanan et al. [24] had directly precipitated lanthanide salts for the formation of the flake-shaped SmVO4 microcrystals. Sun et al. [25] had proved that SmVO4 nanorods can be formed in the absence of any surfactant or template using cheap and simple inorganic salts as raw materials. Zheng et al. [26] reported ultrathin SmVO4 nanosheets with uniform size and shape could be synthesized through a simple and facile ionic liquid-assisted hydrothermal approach. 4 / 33
Li et al. [27] revealed chrysanthemum-shaped SmVO4 nanorods can be prepared with a novel precursor decavanadate in the presence of EDTA (ethylene diamine tetra acetic acid) by hydrothermal method. SmVO4 nanocrystals with square-sheet morphology have been prepared by a precipitation reaction in the presence of oleic acid as ligand [28, 29]. Liu et al. [30] demonstrated a facile and effective solution-phase approach can be adopted to synthesize SmVO4 nanorod arrays. Despite these researched about SmVO4 nanomaterials have been carried out for many years, massive severe challenges still remained. Obviously, only flake-like, sphere-like or rod-like SmVO4 nanomaterials have been presented on the papers mentioned above. In the previous papers, dodecahedral LuVO4 and HoVO4 crystals had been successfully synthesized by our group in a simple and facile EDTA -assisted hydrothermal method [31, 32]. But to the best of our knowledge, up to now, there are no reports on preparing dodecahedral SmVO4 nanocrystals. In this paper, dodecahedral SmVO4 nanocrystal were prepared by a facile hydrothermal process in the presence of KOH and urea. The optimal conditions for preparing uniform dodecahedral SmVO4 nanocrystals and photoluminescence properties of the products, which doped with different amount of Eu3+ ions, were studied. The possible growth mechanism of the dodecahedral SmVO4 nanocrystals also had been discussed.
2 Experimental
2.1 Materials 5 / 33
Samarium nitrate hexahydrate (Sm (NO3)3·6H2O, 99.9%) and Europium nitrate hexahydrate (Eu(NO3)3·6H2O, 99.9%) were purchased from Aladdin Shanghai Biological Technology Co., Ltd. Ammonium metavanadate (NH4VO3, analytical reagent), potassium hydroxide (KOH, analytical reagent), and urea (CO(NH2)2, 99.99%) were attained from Sinopharm Chemical Reagent Co., Ltd. All reagents were used as received without further purification.
2.2 Synthesis of SmVO4 nanocrystals
SmVO4 nanocrystals were synthesized by a facile hydrothermal method. In a typical experiment, 1.0 mmol Sm (NO3)3·6H2O was dissolve in 5.0 mL distilled water in one beaker naturally and formed colorless solution. And in another beaker, 1.0 mmol NH4VO3 was dissolve in 10.0mL distilled water by heating and magnetic stirring for 15 minutes and formed faint yellow solution. Then the colorless Sm(NO3)3 solution was dropped into the NH4VO3 solution and the mixture turned into yellow emulsion immediately under continuous stirring. Subsequently, 2.5g solid KOH was introduced into yellow emulsion and the system was maintained stirring. After 3 minutes, the yellow emulsion change into white emulsion and 1.0g solid urea was poured in without visible color changed. After additional agitation for 3 minutes, the as-obtained white colloidal precipitate was transferred into a 40mL Teflon liner and some distilled water was filled up to 80% of the total volume. Then the autoclave was 6 / 33
sealed and treated at 240℃ for 2 days. After the autoclave was cooled to the room temperature naturally, the white precipitate was collected by filtration, and washed with distilled water for three times to remove any possible ionic remnants, and dried at 60℃ in air. The SmVO4 white powders were finally obtained and reserved for spare. A series of similar processes were implemented to synthesize SmVO4: x% Eu3+ (x=1, 3, 5, 7) samples by adding proper amount of Eu(NO3)3·6H2O into the Sm(NO3)3 solution at the primary stage as described above.
2.3 Characterization
Powder X-ray diffraction (XRD) was carried out with a Philips X'Pert PRO X-ray diffractometer equipped with graphite-monochromatized high-intensity Cu Kα radiation (λ= 0.15478 nm). The scanning rate was 0.05°s-1 in the 2θ range from 10° to 70°. The field-emission scanning electron microscopy (FESEM) images were obtained on a JEOL-6300F field-emission scanning electron microscope with an accelerating voltage of 15 kV. X-Ray photoelectron spectroscopy (XPS) analysis was performed on a Thermo-VG Scientific ESCALAB 250 X-ray photoelectron spectrometer, using Mg Kα radiation as the excitation source. The Fourier transform infrared (FTIR) spectroscopic study was performed on a Thermo Scientific Instrument Co., U.S.A Nicolet 8700 FTIR spectrometer. The transmission electron microscopy (TEM) image, high-resolution transmission electron microscopy (HRTEM) image and selected area electron diffraction (SAED) pattern were carried out on a JEOL 7 / 33
JEM-2010 high resolution transmission electron microscope working on 200 kV. The photoluminescence (PL) was carried out on a Jobin Yvon Fluorolog-3-Tou steady-state/lifetime spectrofluorometer at room temperature.
3 Results and discussion
3.1 Phase identification and morphology of the SmVO4 nanocrystals.
The XRD pattern of the prepared SmVO4 product shown in Fig. 1a indicated that all diffraction peaks for as-formed product can be readily indexed to pure tetragonal phase of SmVO4 [space group of I41/amd] with lattice constants a=b=7.268 Å and c=6.387 Å according to the Joint Committee on Powder Diffraction Standards (JCPDS) file No.86-0994. Obviously, no other peaks can be identified from the XRD pattern, revealing that the product is absolutely pure phase. The field-emission scanning electron microscopy (FESEM) micrograph shown in Fig.1b demonstrates the morphology of the prepared SmVO4 product seems like dodecahedron. The length of a SmVO4 nanocrystal grows to about 400 nanometers while its cross section is square with the length of side around from 200 nanometers, which can be seen from inset of Fig. 1b.
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Fig. 1. (a) XRD pattern of the SmVO4 product; (b) FESEM image of SmVO4 product.
The FTIR spectrum for the dodecahedron-like SmVO4 nanocrystals was recorded in the wave number range of 400-4000cm-1 (Fig. 2). As we have seen, a strong absorption band at around 821.5 cm-1 and a weak absorption band at around 447.4 cm-1 would be attributed to the adsorption of the V-O (from the VO43- group) and Sm-O bond respectively [33, 34]. And the peak at 1403.9 cm-1 could be assigned to the N-O bond from NO3- group [16]. The absorption bands located at 3430.7 cm-1 and 1633.4 cm-1 could be ascribed to the O-H symmetrical stretching and bending vibration of the adsorbed water molecules, which means the highly hydrated surface of the dodecahedron-like architecture [35].
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Fig. 2. FTIR spectrum of SmVO4 product obtained at 240℃ for 2 days with the molar ratio of Sm(NO3)3·6H2O to NH4VO3 of 1:1
The composition and metal oxidation states on the surface of the product was also characterized by XPS (Fig. 3). In Fig. 3a, the survey XPS spectrum of the sample in a wide binding energy range was depicted and core levels of Sm 3d, C 1s, V 2p and O 1s could be identified. The binding energies obtained in the XPS analysis are corrected for specimen charging by referencing the C 1s orbital to 284.8 eV. The peaks at 1083.0 eV and 1110.0 eV correspond to Sm 3d5/2 and Sm 3d3/2 respectively from Fig. 3b. Similarly, the peaks at 517.0 eV and 524.9 eV observed in Fig. 3c are attributed to V 2p3/2 and V 2p1/2 respectively, which are in good agreement with the reported studies [23, 31, 36, 37]. No characteristic peaks of other metals are detected. 10 / 33
In Fig. 3d, the O 1s peak at 529.9 eV is attributed to both O-Sm and O-V bonds in the tetragonal SmVO4 lattice [38], which consist of VO4 tetrahedra and SmO8 dodecahedra that share corners and edges with each other [39].
Fig. 3. XPS spectra of the as-prepared product: (a) survey spectrum; (b) Sm 3d region; (c) V 2p region; (d) O 1s region
3.2 Condition experiments for SmVO4 nanocrystals.
As we all know, the morphological uniformity and phase-purity of the product were confirmed to be highly correlative with the reaction temperature, reaction time and molar ratio of the reactants [40, 41]. In order to gain good-crystallinity and 11 / 33
uniform dodecahedral SmVO4 nanocrystals, a series of condition experiments were carried out, including reaction temperature, reaction time and molar ratio of Sm(NO3)3·6H2O to NH4VO3. First of all, the influence of reaction temperature on the crystal formation of dodecahedral SmVO4 was studied by XRD patterns and FESEM images, while other parameters were kept identical (Fig. 4 and Fig. 5). In view of Fig. 4, no matter what the reaction temperature it is, all the diffraction peaks of the products are matched with standard spectrum of the JCPDS file No.86-0994, which means pure SmVO4 crystals with tetragonal phase could be prepared without any other impurity phase at different reaction temperatures by hydrothermal method. And the conclusion could be confirmed by FESEM images. From Fig. 5, the products obtained at the different reaction temperatures are all dodecahedron-shaped nanocrystals. However, compared with other images, Fig. 5b showed us the dodecahedron-shaped nanocrystals with more uniform size and better crystallinity, which revealed that 240℃ would be the better condition for preparing dodecahedral SmVO4 nanocrystals.
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Fig. 4. XRD patterns of the products obtained at different reaction temperatures for 2 days with the molar ratio of the Sm(NO3)3·6H2O to NH4VO3 of 1:1 :(a) 220℃; (b) 240℃; (c) 260℃; (d) 280℃.
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Fig. 5. FESEM images of the products obtained at different reaction temperatures for 2 days with the molar ratio of Sm(NO3)3·6H2O to NH4VO3 of 1:1 :(a) 220℃; (b) 240℃; (c) 260℃; (d) 280℃.
Next, the effect of reaction time on the formation of the SmVO4 nanocrystals was also researched by FESEM images (Fig. 6), while other parameters remained consistent. Apparently, when the reaction time was 1 day, it was not beneficial for the formation of the dodecahedral SmVO4 nanocrystals. Only a few regular dodecahedral SmVO4 nanocrystals were generated (Fig. 6a), which signified the beginning of the growing process for the dodecahedral SmVO4 nanocrystals. But when the reaction time was prolonged to 2 days, massive dodecahedral nanocrystals were produced with regular morphology and uniform size (Fig. 6b). As the reaction time was changed to 3 or 4 days, the crystal particles aggregated with each other (Fig. 6c and 6d). 14 / 33
Fig. 6. FESEM images of the products obtained at 240℃ with the molar ratio of Sm(NO3)3·6H2O to NH4VO3 of 1:1 for different reaction times: (a) 1d; (b) 2d; (c) 3d; (d) 4d.
Finally, the impact of the different molar ratio of Sm(NO3)3·6H2O to NH4VO3 on the morphologies of the products were investigated by FESEM images and XRD patterns under the same other conditions (Fig. 7 and Fig. 8). When the original molar ratio of Sm (NO3)3·6H2O to NH4VO3 was set as 1:0.5, the shape of the product included many rods and a few dodecahedra (Fig. 7a). Considering the XRD pattern in Fig. 8a, the rods could be regarded as Sm(OH)3 crystals and the left dodecahedra must be SmVO4 crystals because the XRD pattern only matched with pure hexagonal phase of Sm(OH)3 [space group of P63/m(176)] according to the JCPDS file No.83-2036 15 / 33
and tetragonal phase of SmVO4 [space group of I41/amd] according to the JCPDS file No.86-0994. When the molar ratio of Sm (NO3)3·6H2O to NH4VO3 was changed into 1:1 or 0.5:1, all XRD patterns of the products were totally matched with tetragonal phase of SmVO4. However, taking the FESEM images into consideration (Fig. 7b and Fig. 7c), the better molar ratio of Sm (NO3)3·6H2O to NH4VO3 should be 1:1 to obtain regular and uniform dodecahedral SmVO4 nanocrystals.
Fig. 7. FESEM images of the products obtained at 240℃ and remained for 2days with the different molar ratio of Sm(NO3)3·6H2O to NH4VO3 (a) 1:0.5; (b) 1:1; (c) 0.5:1.
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Fig. 8. XRD patterns of the products obtained at 240℃ and remained for 2 days with the different molar ratio of Sm(NO3)3·6H2O to NH4VO3 (a) 1:0.5; (b) 1:1; (c) 0.5:1.
In summary, considering these condition experiments, 240℃, 2 days and the molar ratio of Sm(NO3)3·6H2O to NH4VO3 of 1:1 are the preferable experiment parameters to prepare dodecahedral SmVO4 nanocrystals.
3.3 Possible growth mechanism of the dodecahedral SmVO4 nanocrystals
The formation process of the dodecahedral SmVO4 nanocrystals in the hydrothermal conditions could be concluded as the two stages: crystal nucleation stage and crystal growth stage. On the basis of the experimental results above, there is 17 / 33
no doubt that many factors influencing the formation of product are involved in the whole hydrothermal process such as reaction temperature, reaction time and molar ratio of the reactants. However, KOH and urea as important additives in the hydrothermal process should not be neglected. KOH and urea not only offered alkaline growing environment for the products in the whole process, but provided the NH4+ ions which acted as shape tailoring agents in the growth stage. According to the state of pentavalent vanadium ion in the solution with various pH from Fig. 9 [26], strongly alkaline system would have a beneficial effect on preparing SmVO4 crystal nuclei on account of conversion from VO3- to VO43-. With the hydrothermal process continuing, more and more VO43- ions were formed from VO3 - ions in alkaline solution. When [Sm3+] * [VO43-] ≥ Ksp(SmVO4) in the system, the precipitation process occurred and primitive SmVO4 crystal nuclei were generated. The nucleation stage could be depicted in the following equations.
+ 2
⟶
+
⇌
+
(1)
↓ (2)
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Fig. 9. The state of pentavalent vanadium ion in the solution with various pH.
As the time went on, the SmVO4 crystal nuclei began to grow up and the process proceeded to crystal growth stage. Fig. 10 showed the schematic representation of the possible formation process of the SmVO4 nanocrystals by two different hydrothermal methods, suitable amount of KOH and urea involved or not. As a contrast experiment, a similar hydrothermal process was carried out in the absence of KOH and urea. The outcome in two ways were apparently different while KOH and urea must be involved to synthesize dodecahedron-shaped SmVO4 nanocrystals (Fig. 10a). On the contrary, particle-shaped SmVO4 nanocrystals were prepared in the absence of KOH and urea (Fig. 10b). It proved that KOH and urea played a critical role in preparing dodecahedral SmVO4 nanocrystals. On the one hand, the NH4+ ions dissolved from NH4VO3 reactant and decomposed from urea in the hydrothermal system should be 19 / 33
the special shape tailoring agents due to their small size, tetrahedral symmetrical structure, and positive charges [42]. On the other hand, massive OH- ions from KOH would compete with VO43- during the crystal growth stage and prompted anisotropic growth [43]. Abundant NH4+ ions existed in the crystal growth stage, which may direct guide anisotropic growth of SmVO4 crystals and lead to dodecahedral morphologies [44]. In order to ascertain the growth direction and the bound facets of the dodecahedral structure, the TEM image, HRTEM image and SAED pattern (Fig. 11) were obtained by a JEOL JEM-2010 high resolution transmission electron microscope working on 200 kV. In Fig. 11a, the TEM image also confirm the sample with dodecahedron-shape, corresponding with the FESEM image. In Fig. 11b, the SAED pattern recorded at the marked area for the [010] zone axis reveals the singe-crystal nature of the dodecahedral sample. In Fig. 11c, the HRTEM image with the obvious lattice fringes demonstrates the high crystallinity of the sample. The _
lattice spacings of about 0.496 nm, 0.483 nm and 0.357 nm correspond to the (101) plane, (101) plane, and (200) plane of the tetragonal-phased SmVO4 structure (JCPDS No.86-0994, a=b=7.268 Å and c=6.387 Å), respectively. It means the bound facets consist of {100} and {101} facets. It is of considerable interest to note that the growth direction of the individual dodecahedron would be deduced in [001], parallel to the c axis [45, 46].
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Fig. 10. Schematic representation of the possible formation process of the SmVO4 nanocrystals.
Fig. 11. (a) TEM image of the SmVO4 product; (b) SAED pattern of SmVO4 product; (c) HRTEM image of the SmVO4 product.
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3.4 Photoluminescent (PL) properties of the dodecahedron-shaped SmVO4 and SmVO4: Eu3+
In view of the size of the SmVO4 crystals reached a few hundred nanometers, dodecahedron-shaped SmVO4 sample and SmVO4:Eu3+ samples might demonstrate remarkable photoluminescent properties because of intra-4f transitions independent of their shape [47]. The excitation and emission spectra of as-prepared SmVO4 sample are both shown in Fig. 12. It could be observed easily from the excitation spectrum (Fig .12, black curve) by monitoring the emission of the Sm3+ transition at 645 nm that the SmVO4 sample has a wide and strong absorption band around 315 nm in the range of 270-330 nm and several weak lines in the longer wavelength region (360-480 nm). The stronger one can be ascribed to the charge transfer from the oxygen ligands to the central vanadium ions inside the VO43- groups [48]. According to the molecular orbital theory, it coincides with transitions from the 1 A2 (1T1) ground state to 1A1 (1E) and 1E (1T2) excited state of the VO43- ions [49]. But the weaker one would be attributed to the intra-configurational 4f–4f transitions of Sm3+ ions [50]. And they were hardly detected compared with those of VO43− groups. This phenomenon could illustrate that the excitation of the Sm3+ ions is mainly through the energy transfer from VO43− groups to Sm3+ ions [51]. The emission spectrum is detected upon excitation into the vanadate group at 315 nm (Fig. 12, green curve). Obviously, the emission spectrum contains three emission bands mainly and their peaks are located at 564 nm, 601 nm and 645 nm respectively. Three emission bands correspond to the 22 / 33
transitions from the excited levels of 4G5/2 → 6HJ (J=5/2,7/2,9/2), arising from the f-f transition of Sm3+ ions, namely, green emission of Sm3+ at 564 nm (4G5/2 → 6H5/2), orange emission of Sm3+ at 601 nm (4G5/2 → 6H7/2) and red emission of Sm3+ at 645 nm (4G5/2 → 6H9/2). No emission from the VO43- group is observed, indicating that the energy transfer from VO43- to Sm3+ is quite complete and efficient [52].
Fig. 12. Photoluminescence excitation (black curve) and emission (green curve) spectra of SmVO4 sample.
There is no doubt that sizes and morphologies of the phosphors would have a significant impact on their PL properties [51]. Thus, dodecahedral SmVO4 would possess special PL properties because of its unique morphology and size compared 23 / 33
with other SmVO4 nanomaterials with other shapes and sizes. In fact, a comparison between dodecahedron-shaped SmVO4 and particle-shaped SmVO4 in PL properties were made and the outcome has proved that the dodecahedron-shaped SmVO4 and particle-shaped SmVO4 performed distinctly different excitation intensity and emission intensity (Fig. 13). In Fig. 13, green curves and black curves represent the excitation and emission spectra of SmVO4 samples with dodecahedron-shape and particle-shape, respectively. Conspicuously, SmVO4 sample with dodecahedron-shape performs stronger emission intensity than the other with particle-shape from emission spectra, which may be due to its high packing densities and low scattering of light [53]. In other words, dodecahedral SmVO4 nanocrystals get preference in applications in optical devices than the other one because of excellent PL properties.
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Fig. 13. Photoluminescence excitation (left) and emission (right) spectra of SmVO4 samples with dodecahedron-shape (green curves) and particle-shape (black curves).
The PL properties of the as-synthesized SmVO4: 5% Eu3+ sample was similarly characterized by the PL excitation and emission spectra. The excitation spectrum monitored at 618 nm consists of a broad and strong band around 315 nm and some weak lines in the range of 360-480 nm (Fig .14, black curve). Analogously, the strong one can be assigned to the charge transfer from the oxygen ligands to the central vanadium atom inside the VO43− groups. And the weak excitation peaks in the range of 360-480 nm would be attributed to the f–f transition within the 4f6 configuration of the Eu3+ ions [51]. The emission spectrum of SmVO4: 5% Eu3+ obtained by excitation into vanadate group at 315 nm consists of several sharp emission bands ranging from 550 to 720 nm. (Fig. 14, green curve). All emission bands come from the transitions of 5D0 → 7FJ (J=1, 2, 3, 4) levels of Eu3+ activators except three emission bands belongs to Sm3+ ions as described above. In other words, orange emission of Eu3+ at 592 nm and 618 nm are assigned to the 5D0 → 7F1 and 5D0 → 7F2 transition respectively, red emission of Eu3+ at 652 nm and 696 nm are attributed to the 5D0 → 7
F3 and 5D0 → 7F4 transition respectively. And this phenomenon indicates that SmVO4:
5% Eu3+ sample performs major emission bands from the transitions of Sm3+ ions and Eu3+ ions. Especially, the most intense emission band with the peak located at 618 nm could be assigned to the 5D0 → 7F2 electric-dipole allowed transition [51]. In addition, the crystal field splitting of the Eu3+ 5D0 → 7F2 and 5D0 → 7F4 transitions can clearly 25 / 33
be seen, indicating that the SmVO4: Eu3+ sample is well-crystallized [54, 55].
Fig. 14. Photoluminescence excitation (black curve) and emission (green curve) spectra of SmVO4: 5% Eu3+ sample.
Furthermore, the PL properties of the SmVO4 samples doped with different amount of Eu3+ ions were also characterized by emission spectra in Fig. 15. By varying the percentage of Eu3+ ions dopants, the emission intensity of samples exhibited significant diversity but the locations of emission bands remained identical. By comparison, with the increase of the doping Eu3+ ion concentration, the emission intensity of the SmVO4: x% Eu3+ (x=1, 3, 5) samples becomes stronger in transition of Eu3+ ions but weaker in transition of Sm3+ ions. It means that energy transfer from 26 / 33
VO43- groups to Eu3+ dopants becomes more efficient with the increase of the Eu3+ ion concentration and energy transfer from VO43- groups to Sm3+ hosts becomes less efficient with the decrease of the Sm3+ ion concentration. However, it is not hard to see SmVO4: 7% Eu3+ sample performs the strongest emission intensity among all SmVO4: x% Eu3+ (x=1, 3, 5, 7) samples in both transition of Eu3+ ions and Sm3+ ions. (Fig. 15, red curve), which indicates SmVO4: 7% Eu3+ sample might possess more extensive application in optical devices because of excellent luminescence properties.
Fig. 15. Photoluminescence emission spectra of SmVO4: x% Eu3+ samples: (x=1 green curve, x=3 blue curve, x=5 black curve, x=7 red curve)
4 Conclusions 27 / 33
In conclusion, the uniform dodecahedral SmVO4 nanocrystals were prepared by a hydrothermal method with the appropriate amount of potassium hydroxide and urea involving. The results of condition experiments show that 240℃, 2 days and the molar ratio of Sm(NO3)3·6H2O to NH4VO3 of 1:1 are the preferable experiment parameters to fabricate dodecahedral SmVO4 nanocrystals. The possible growth process of the SmVO4 nanocrystals was discussed, which might contain crystal nucleation stage and crystal growth stage, while potassium hydroxide and urea played an important role in the formation of dodecahedron-like SmVO4 nanocrystals. Under 315 nm excitation, SmVO4 and SmVO4: Eu3+ exhibit orange emission and reddish orange emission, respectively. Compared with SmVO4: Eu3+ samples doped with different amount of Eu3+ ions, SmVO4: 7% Eu3+ sample not only inherits major emission bands of the both Sm3+ ions and Eu3+ ions, but performs the strongest emission intensity among all SmVO4: x% Eu3+ (x=1, 3, 5, 7) samples. It might be applied in some optical devices that needed multiple emission bands and strong emission intensity. The method of preparing dodecahedral SmVO4 nanocrystals would be used to synthesize other rare earth orthovanadate nanocrystals.
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S0025-5408(17)31990-6 http://dx.doi.org/10.1016/j.materresbull.2017.08.037 MRB 9515
To appear in:
MRB
Received date: Revised date: Accepted date:
21-5-2017 13-8-2017 13-8-2017
Please cite this article as: Xianjin Ge, Youjin Zhang, Hai Wu, Maozhong Zhou, Tao Lin, SmVO4 nanocrystals with dodecahedral shape: controlled synthesis, growth mechanism and photoluminescent properties, Materials Research Bulletinhttp://dx.doi.org/10.1016/j.materresbull.2017.08.037 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
SmVO4 nanocrystals with dodecahedral shape: controlled synthesis, growth mechanism and photoluminescent properties
Xianjin Ge, Youjin Zhang*, Hai Wu, Maozhong Zhou, Tao Lin Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China
*Corresponding author: Youjin Zhang, [email protected] Tel: +86-551-63492145; Fax: +86-551-63492083
Graphical abstract
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Abstract: Dodecahedral SmVO4 nanocrystals with uniform size had been successfully synthesized by a facile hydrothermal process in the presence of potassium hydroxide and urea. The dodecahedron-like SmVO4 was characterized by X-ray diffraction, field-emission scanning electron microscopy, Fourier transform infrared, X-Ray photoelectron spectroscopy, high-resolution transmission electron microscopy and photoluminescence. The possible growth mechanism of the SmVO4 nanocrystals with dodecahedral shape was discussed. Under excitation at 315 nm, the SmVO4 sample exhibits strong orange emission while it as the host doped with suitable amount of Eu3+ ions shows strong reddish orange emission. The diversity in emission intensity detected from SmVO4: x% Eu3+ (x=1, 3, 5, 7) was compared. The hydrothermal method for preparing dodecahedral SmVO4 nanocrystals provides us an idea to synthesize other rare earth orthovanadate nanomaterials with special morphology and interesting optical property.
KEYWORDS: A. inorganic compounds, B. crystal growth, B. optical properties, C. electron microscopy, C. X-ray diffraction
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1 Introduction
Rare earth compounds as an important family of functional materials, which possess outstanding physicochemical properties, have been applied in many areas like electric, magnetic, optical and catalytic field. These useful functions derive from the distinctive 4f electron configurations, which are poorly affected by the coordination environment or crystal field owing to the shielding of the 4f orbitals by the filled 5s and 5p orbitals [1, 2]. When it comes to the extensive applications of rare earth compounds in different areas, it is inevitable to talk about rare earth orthovanadate (REVO4), which have been played an important role in various fields including laser hosts [3], catalysts [4], sensors [5], and phosphors [6], etc. In order to fabricate high quality REVO4 nanomaterials with uniform size and shape, plentiful synthetic methods have been established in the past decades, containing sol-gel technique [7], electrospinning technique [8], complexing agent-assisted precipitation [9], microwave-assisted synthesis [10] and hydrothermal synthesis [11] and so on. Among these approaches adopted to prepare REVO4 nanomaterials with well-controlled shapes and sizes, the 3 / 33
hydrothermal method as the relatively mild method which needs comparatively low reaction temperature, has become one of the most promising methods due to high purity, narrow particle size distribution and precise component of the final product. As for REVO4 nanomaterials, a large amount of papers about the preparation and properties had been published, however, most of them were concentrated on reporting YVO4 and LaVO4 nanomaterials. Various shapes of the YVO4 including spherical shape [12], polygonal shape [13], peanut-like shape, octahedral shape [14] had been prepared by hydrothermal process. Likewise, different shapes of the LaVO4 just like nanoparticles, nanorods [15], nanosheaves [16], and hollow microspheres [17] had been synthesized in the same method. Specifically, REVO4, as the catalysts for many organic reactions such as oxidative dehydrogenation of alkanes, olefins, methylbenzene, and oxidation of hydrogen sulfide to elemental sulfur [18–22], should be paid more attention on studying their preparations and properties. As we all know, only a few papers focused on the preparation and property researches of the SmVO4 nanomaterials. Do et al. [23] have reported that the SmVO4 nanocrystals with spherical and hexagonal shapes could be obtained by using oleylamine and oleic acid as the capping surfactant, respectively. Ramanan et al. [24] had directly precipitated lanthanide salts for the formation of the flake-shaped SmVO4 microcrystals. Sun et al. [25] had proved that SmVO4 nanorods can be formed in the absence of any surfactant or template using cheap and simple inorganic salts as raw materials. Zheng et al. [26] reported ultrathin SmVO4 nanosheets with uniform size and shape could be synthesized through a simple and facile ionic liquid-assisted hydrothermal approach. 4 / 33
Li et al. [27] revealed chrysanthemum-shaped SmVO4 nanorods can be prepared with a novel precursor decavanadate in the presence of EDTA (ethylene diamine tetra acetic acid) by hydrothermal method. SmVO4 nanocrystals with square-sheet morphology have been prepared by a precipitation reaction in the presence of oleic acid as ligand [28, 29]. Liu et al. [30] demonstrated a facile and effective solution-phase approach can be adopted to synthesize SmVO4 nanorod arrays. Despite these researched about SmVO4 nanomaterials have been carried out for many years, massive severe challenges still remained. Obviously, only flake-like, sphere-like or rod-like SmVO4 nanomaterials have been presented on the papers mentioned above. In the previous papers, dodecahedral LuVO4 and HoVO4 crystals had been successfully synthesized by our group in a simple and facile EDTA -assisted hydrothermal method [31, 32]. But to the best of our knowledge, up to now, there are no reports on preparing dodecahedral SmVO4 nanocrystals. In this paper, dodecahedral SmVO4 nanocrystal were prepared by a facile hydrothermal process in the presence of KOH and urea. The optimal conditions for preparing uniform dodecahedral SmVO4 nanocrystals and photoluminescence properties of the products, which doped with different amount of Eu3+ ions, were studied. The possible growth mechanism of the dodecahedral SmVO4 nanocrystals also had been discussed.
2 Experimental
2.1 Materials 5 / 33
Samarium nitrate hexahydrate (Sm (NO3)3·6H2O, 99.9%) and Europium nitrate hexahydrate (Eu(NO3)3·6H2O, 99.9%) were purchased from Aladdin Shanghai Biological Technology Co., Ltd. Ammonium metavanadate (NH4VO3, analytical reagent), potassium hydroxide (KOH, analytical reagent), and urea (CO(NH2)2, 99.99%) were attained from Sinopharm Chemical Reagent Co., Ltd. All reagents were used as received without further purification.
2.2 Synthesis of SmVO4 nanocrystals
SmVO4 nanocrystals were synthesized by a facile hydrothermal method. In a typical experiment, 1.0 mmol Sm (NO3)3·6H2O was dissolve in 5.0 mL distilled water in one beaker naturally and formed colorless solution. And in another beaker, 1.0 mmol NH4VO3 was dissolve in 10.0mL distilled water by heating and magnetic stirring for 15 minutes and formed faint yellow solution. Then the colorless Sm(NO3)3 solution was dropped into the NH4VO3 solution and the mixture turned into yellow emulsion immediately under continuous stirring. Subsequently, 2.5g solid KOH was introduced into yellow emulsion and the system was maintained stirring. After 3 minutes, the yellow emulsion change into white emulsion and 1.0g solid urea was poured in without visible color changed. After additional agitation for 3 minutes, the as-obtained white colloidal precipitate was transferred into a 40mL Teflon liner and some distilled water was filled up to 80% of the total volume. Then the autoclave was 6 / 33
sealed and treated at 240℃ for 2 days. After the autoclave was cooled to the room temperature naturally, the white precipitate was collected by filtration, and washed with distilled water for three times to remove any possible ionic remnants, and dried at 60℃ in air. The SmVO4 white powders were finally obtained and reserved for spare. A series of similar processes were implemented to synthesize SmVO4: x% Eu3+ (x=1, 3, 5, 7) samples by adding proper amount of Eu(NO3)3·6H2O into the Sm(NO3)3 solution at the primary stage as described above.
2.3 Characterization
Powder X-ray diffraction (XRD) was carried out with a Philips X'Pert PRO X-ray diffractometer equipped with graphite-monochromatized high-intensity Cu Kα radiation (λ= 0.15478 nm). The scanning rate was 0.05°s-1 in the 2θ range from 10° to 70°. The field-emission scanning electron microscopy (FESEM) images were obtained on a JEOL-6300F field-emission scanning electron microscope with an accelerating voltage of 15 kV. X-Ray photoelectron spectroscopy (XPS) analysis was performed on a Thermo-VG Scientific ESCALAB 250 X-ray photoelectron spectrometer, using Mg Kα radiation as the excitation source. The Fourier transform infrared (FTIR) spectroscopic study was performed on a Thermo Scientific Instrument Co., U.S.A Nicolet 8700 FTIR spectrometer. The transmission electron microscopy (TEM) image, high-resolution transmission electron microscopy (HRTEM) image and selected area electron diffraction (SAED) pattern were carried out on a JEOL 7 / 33
JEM-2010 high resolution transmission electron microscope working on 200 kV. The photoluminescence (PL) was carried out on a Jobin Yvon Fluorolog-3-Tou steady-state/lifetime spectrofluorometer at room temperature.
3 Results and discussion
3.1 Phase identification and morphology of the SmVO4 nanocrystals.
The XRD pattern of the prepared SmVO4 product shown in Fig. 1a indicated that all diffraction peaks for as-formed product can be readily indexed to pure tetragonal phase of SmVO4 [space group of I41/amd] with lattice constants a=b=7.268 Å and c=6.387 Å according to the Joint Committee on Powder Diffraction Standards (JCPDS) file No.86-0994. Obviously, no other peaks can be identified from the XRD pattern, revealing that the product is absolutely pure phase. The field-emission scanning electron microscopy (FESEM) micrograph shown in Fig.1b demonstrates the morphology of the prepared SmVO4 product seems like dodecahedron. The length of a SmVO4 nanocrystal grows to about 400 nanometers while its cross section is square with the length of side around from 200 nanometers, which can be seen from inset of Fig. 1b.
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Fig. 1. (a) XRD pattern of the SmVO4 product; (b) FESEM image of SmVO4 product.
The FTIR spectrum for the dodecahedron-like SmVO4 nanocrystals was recorded in the wave number range of 400-4000cm-1 (Fig. 2). As we have seen, a strong absorption band at around 821.5 cm-1 and a weak absorption band at around 447.4 cm-1 would be attributed to the adsorption of the V-O (from the VO43- group) and Sm-O bond respectively [33, 34]. And the peak at 1403.9 cm-1 could be assigned to the N-O bond from NO3- group [16]. The absorption bands located at 3430.7 cm-1 and 1633.4 cm-1 could be ascribed to the O-H symmetrical stretching and bending vibration of the adsorbed water molecules, which means the highly hydrated surface of the dodecahedron-like architecture [35].
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Fig. 2. FTIR spectrum of SmVO4 product obtained at 240℃ for 2 days with the molar ratio of Sm(NO3)3·6H2O to NH4VO3 of 1:1
The composition and metal oxidation states on the surface of the product was also characterized by XPS (Fig. 3). In Fig. 3a, the survey XPS spectrum of the sample in a wide binding energy range was depicted and core levels of Sm 3d, C 1s, V 2p and O 1s could be identified. The binding energies obtained in the XPS analysis are corrected for specimen charging by referencing the C 1s orbital to 284.8 eV. The peaks at 1083.0 eV and 1110.0 eV correspond to Sm 3d5/2 and Sm 3d3/2 respectively from Fig. 3b. Similarly, the peaks at 517.0 eV and 524.9 eV observed in Fig. 3c are attributed to V 2p3/2 and V 2p1/2 respectively, which are in good agreement with the reported studies [23, 31, 36, 37]. No characteristic peaks of other metals are detected. 10 / 33
In Fig. 3d, the O 1s peak at 529.9 eV is attributed to both O-Sm and O-V bonds in the tetragonal SmVO4 lattice [38], which consist of VO4 tetrahedra and SmO8 dodecahedra that share corners and edges with each other [39].
Fig. 3. XPS spectra of the as-prepared product: (a) survey spectrum; (b) Sm 3d region; (c) V 2p region; (d) O 1s region
3.2 Condition experiments for SmVO4 nanocrystals.
As we all know, the morphological uniformity and phase-purity of the product were confirmed to be highly correlative with the reaction temperature, reaction time and molar ratio of the reactants [40, 41]. In order to gain good-crystallinity and 11 / 33
uniform dodecahedral SmVO4 nanocrystals, a series of condition experiments were carried out, including reaction temperature, reaction time and molar ratio of Sm(NO3)3·6H2O to NH4VO3. First of all, the influence of reaction temperature on the crystal formation of dodecahedral SmVO4 was studied by XRD patterns and FESEM images, while other parameters were kept identical (Fig. 4 and Fig. 5). In view of Fig. 4, no matter what the reaction temperature it is, all the diffraction peaks of the products are matched with standard spectrum of the JCPDS file No.86-0994, which means pure SmVO4 crystals with tetragonal phase could be prepared without any other impurity phase at different reaction temperatures by hydrothermal method. And the conclusion could be confirmed by FESEM images. From Fig. 5, the products obtained at the different reaction temperatures are all dodecahedron-shaped nanocrystals. However, compared with other images, Fig. 5b showed us the dodecahedron-shaped nanocrystals with more uniform size and better crystallinity, which revealed that 240℃ would be the better condition for preparing dodecahedral SmVO4 nanocrystals.
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Fig. 4. XRD patterns of the products obtained at different reaction temperatures for 2 days with the molar ratio of the Sm(NO3)3·6H2O to NH4VO3 of 1:1 :(a) 220℃; (b) 240℃; (c) 260℃; (d) 280℃.
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Fig. 5. FESEM images of the products obtained at different reaction temperatures for 2 days with the molar ratio of Sm(NO3)3·6H2O to NH4VO3 of 1:1 :(a) 220℃; (b) 240℃; (c) 260℃; (d) 280℃.
Next, the effect of reaction time on the formation of the SmVO4 nanocrystals was also researched by FESEM images (Fig. 6), while other parameters remained consistent. Apparently, when the reaction time was 1 day, it was not beneficial for the formation of the dodecahedral SmVO4 nanocrystals. Only a few regular dodecahedral SmVO4 nanocrystals were generated (Fig. 6a), which signified the beginning of the growing process for the dodecahedral SmVO4 nanocrystals. But when the reaction time was prolonged to 2 days, massive dodecahedral nanocrystals were produced with regular morphology and uniform size (Fig. 6b). As the reaction time was changed to 3 or 4 days, the crystal particles aggregated with each other (Fig. 6c and 6d). 14 / 33
Fig. 6. FESEM images of the products obtained at 240℃ with the molar ratio of Sm(NO3)3·6H2O to NH4VO3 of 1:1 for different reaction times: (a) 1d; (b) 2d; (c) 3d; (d) 4d.
Finally, the impact of the different molar ratio of Sm(NO3)3·6H2O to NH4VO3 on the morphologies of the products were investigated by FESEM images and XRD patterns under the same other conditions (Fig. 7 and Fig. 8). When the original molar ratio of Sm (NO3)3·6H2O to NH4VO3 was set as 1:0.5, the shape of the product included many rods and a few dodecahedra (Fig. 7a). Considering the XRD pattern in Fig. 8a, the rods could be regarded as Sm(OH)3 crystals and the left dodecahedra must be SmVO4 crystals because the XRD pattern only matched with pure hexagonal phase of Sm(OH)3 [space group of P63/m(176)] according to the JCPDS file No.83-2036 15 / 33
and tetragonal phase of SmVO4 [space group of I41/amd] according to the JCPDS file No.86-0994. When the molar ratio of Sm (NO3)3·6H2O to NH4VO3 was changed into 1:1 or 0.5:1, all XRD patterns of the products were totally matched with tetragonal phase of SmVO4. However, taking the FESEM images into consideration (Fig. 7b and Fig. 7c), the better molar ratio of Sm (NO3)3·6H2O to NH4VO3 should be 1:1 to obtain regular and uniform dodecahedral SmVO4 nanocrystals.
Fig. 7. FESEM images of the products obtained at 240℃ and remained for 2days with the different molar ratio of Sm(NO3)3·6H2O to NH4VO3 (a) 1:0.5; (b) 1:1; (c) 0.5:1.
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Fig. 8. XRD patterns of the products obtained at 240℃ and remained for 2 days with the different molar ratio of Sm(NO3)3·6H2O to NH4VO3 (a) 1:0.5; (b) 1:1; (c) 0.5:1.
In summary, considering these condition experiments, 240℃, 2 days and the molar ratio of Sm(NO3)3·6H2O to NH4VO3 of 1:1 are the preferable experiment parameters to prepare dodecahedral SmVO4 nanocrystals.
3.3 Possible growth mechanism of the dodecahedral SmVO4 nanocrystals
The formation process of the dodecahedral SmVO4 nanocrystals in the hydrothermal conditions could be concluded as the two stages: crystal nucleation stage and crystal growth stage. On the basis of the experimental results above, there is 17 / 33
no doubt that many factors influencing the formation of product are involved in the whole hydrothermal process such as reaction temperature, reaction time and molar ratio of the reactants. However, KOH and urea as important additives in the hydrothermal process should not be neglected. KOH and urea not only offered alkaline growing environment for the products in the whole process, but provided the NH4+ ions which acted as shape tailoring agents in the growth stage. According to the state of pentavalent vanadium ion in the solution with various pH from Fig. 9 [26], strongly alkaline system would have a beneficial effect on preparing SmVO4 crystal nuclei on account of conversion from VO3- to VO43-. With the hydrothermal process continuing, more and more VO43- ions were formed from VO3 - ions in alkaline solution. When [Sm3+] * [VO43-] ≥ Ksp(SmVO4) in the system, the precipitation process occurred and primitive SmVO4 crystal nuclei were generated. The nucleation stage could be depicted in the following equations.
+ 2
⟶
+
⇌
+
(1)
↓ (2)
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Fig. 9. The state of pentavalent vanadium ion in the solution with various pH.
As the time went on, the SmVO4 crystal nuclei began to grow up and the process proceeded to crystal growth stage. Fig. 10 showed the schematic representation of the possible formation process of the SmVO4 nanocrystals by two different hydrothermal methods, suitable amount of KOH and urea involved or not. As a contrast experiment, a similar hydrothermal process was carried out in the absence of KOH and urea. The outcome in two ways were apparently different while KOH and urea must be involved to synthesize dodecahedron-shaped SmVO4 nanocrystals (Fig. 10a). On the contrary, particle-shaped SmVO4 nanocrystals were prepared in the absence of KOH and urea (Fig. 10b). It proved that KOH and urea played a critical role in preparing dodecahedral SmVO4 nanocrystals. On the one hand, the NH4+ ions dissolved from NH4VO3 reactant and decomposed from urea in the hydrothermal system should be 19 / 33
the special shape tailoring agents due to their small size, tetrahedral symmetrical structure, and positive charges [42]. On the other hand, massive OH- ions from KOH would compete with VO43- during the crystal growth stage and prompted anisotropic growth [43]. Abundant NH4+ ions existed in the crystal growth stage, which may direct guide anisotropic growth of SmVO4 crystals and lead to dodecahedral morphologies [44]. In order to ascertain the growth direction and the bound facets of the dodecahedral structure, the TEM image, HRTEM image and SAED pattern (Fig. 11) were obtained by a JEOL JEM-2010 high resolution transmission electron microscope working on 200 kV. In Fig. 11a, the TEM image also confirm the sample with dodecahedron-shape, corresponding with the FESEM image. In Fig. 11b, the SAED pattern recorded at the marked area for the [010] zone axis reveals the singe-crystal nature of the dodecahedral sample. In Fig. 11c, the HRTEM image with the obvious lattice fringes demonstrates the high crystallinity of the sample. The _
lattice spacings of about 0.496 nm, 0.483 nm and 0.357 nm correspond to the (101) plane, (101) plane, and (200) plane of the tetragonal-phased SmVO4 structure (JCPDS No.86-0994, a=b=7.268 Å and c=6.387 Å), respectively. It means the bound facets consist of {100} and {101} facets. It is of considerable interest to note that the growth direction of the individual dodecahedron would be deduced in [001], parallel to the c axis [45, 46].
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Fig. 10. Schematic representation of the possible formation process of the SmVO4 nanocrystals.
Fig. 11. (a) TEM image of the SmVO4 product; (b) SAED pattern of SmVO4 product; (c) HRTEM image of the SmVO4 product.
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3.4 Photoluminescent (PL) properties of the dodecahedron-shaped SmVO4 and SmVO4: Eu3+
In view of the size of the SmVO4 crystals reached a few hundred nanometers, dodecahedron-shaped SmVO4 sample and SmVO4:Eu3+ samples might demonstrate remarkable photoluminescent properties because of intra-4f transitions independent of their shape [47]. The excitation and emission spectra of as-prepared SmVO4 sample are both shown in Fig. 12. It could be observed easily from the excitation spectrum (Fig .12, black curve) by monitoring the emission of the Sm3+ transition at 645 nm that the SmVO4 sample has a wide and strong absorption band around 315 nm in the range of 270-330 nm and several weak lines in the longer wavelength region (360-480 nm). The stronger one can be ascribed to the charge transfer from the oxygen ligands to the central vanadium ions inside the VO43- groups [48]. According to the molecular orbital theory, it coincides with transitions from the 1 A2 (1T1) ground state to 1A1 (1E) and 1E (1T2) excited state of the VO43- ions [49]. But the weaker one would be attributed to the intra-configurational 4f–4f transitions of Sm3+ ions [50]. And they were hardly detected compared with those of VO43− groups. This phenomenon could illustrate that the excitation of the Sm3+ ions is mainly through the energy transfer from VO43− groups to Sm3+ ions [51]. The emission spectrum is detected upon excitation into the vanadate group at 315 nm (Fig. 12, green curve). Obviously, the emission spectrum contains three emission bands mainly and their peaks are located at 564 nm, 601 nm and 645 nm respectively. Three emission bands correspond to the 22 / 33
transitions from the excited levels of 4G5/2 → 6HJ (J=5/2,7/2,9/2), arising from the f-f transition of Sm3+ ions, namely, green emission of Sm3+ at 564 nm (4G5/2 → 6H5/2), orange emission of Sm3+ at 601 nm (4G5/2 → 6H7/2) and red emission of Sm3+ at 645 nm (4G5/2 → 6H9/2). No emission from the VO43- group is observed, indicating that the energy transfer from VO43- to Sm3+ is quite complete and efficient [52].
Fig. 12. Photoluminescence excitation (black curve) and emission (green curve) spectra of SmVO4 sample.
There is no doubt that sizes and morphologies of the phosphors would have a significant impact on their PL properties [51]. Thus, dodecahedral SmVO4 would possess special PL properties because of its unique morphology and size compared 23 / 33
with other SmVO4 nanomaterials with other shapes and sizes. In fact, a comparison between dodecahedron-shaped SmVO4 and particle-shaped SmVO4 in PL properties were made and the outcome has proved that the dodecahedron-shaped SmVO4 and particle-shaped SmVO4 performed distinctly different excitation intensity and emission intensity (Fig. 13). In Fig. 13, green curves and black curves represent the excitation and emission spectra of SmVO4 samples with dodecahedron-shape and particle-shape, respectively. Conspicuously, SmVO4 sample with dodecahedron-shape performs stronger emission intensity than the other with particle-shape from emission spectra, which may be due to its high packing densities and low scattering of light [53]. In other words, dodecahedral SmVO4 nanocrystals get preference in applications in optical devices than the other one because of excellent PL properties.
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Fig. 13. Photoluminescence excitation (left) and emission (right) spectra of SmVO4 samples with dodecahedron-shape (green curves) and particle-shape (black curves).
The PL properties of the as-synthesized SmVO4: 5% Eu3+ sample was similarly characterized by the PL excitation and emission spectra. The excitation spectrum monitored at 618 nm consists of a broad and strong band around 315 nm and some weak lines in the range of 360-480 nm (Fig .14, black curve). Analogously, the strong one can be assigned to the charge transfer from the oxygen ligands to the central vanadium atom inside the VO43− groups. And the weak excitation peaks in the range of 360-480 nm would be attributed to the f–f transition within the 4f6 configuration of the Eu3+ ions [51]. The emission spectrum of SmVO4: 5% Eu3+ obtained by excitation into vanadate group at 315 nm consists of several sharp emission bands ranging from 550 to 720 nm. (Fig. 14, green curve). All emission bands come from the transitions of 5D0 → 7FJ (J=1, 2, 3, 4) levels of Eu3+ activators except three emission bands belongs to Sm3+ ions as described above. In other words, orange emission of Eu3+ at 592 nm and 618 nm are assigned to the 5D0 → 7F1 and 5D0 → 7F2 transition respectively, red emission of Eu3+ at 652 nm and 696 nm are attributed to the 5D0 → 7
F3 and 5D0 → 7F4 transition respectively. And this phenomenon indicates that SmVO4:
5% Eu3+ sample performs major emission bands from the transitions of Sm3+ ions and Eu3+ ions. Especially, the most intense emission band with the peak located at 618 nm could be assigned to the 5D0 → 7F2 electric-dipole allowed transition [51]. In addition, the crystal field splitting of the Eu3+ 5D0 → 7F2 and 5D0 → 7F4 transitions can clearly 25 / 33
be seen, indicating that the SmVO4: Eu3+ sample is well-crystallized [54, 55].
Fig. 14. Photoluminescence excitation (black curve) and emission (green curve) spectra of SmVO4: 5% Eu3+ sample.
Furthermore, the PL properties of the SmVO4 samples doped with different amount of Eu3+ ions were also characterized by emission spectra in Fig. 15. By varying the percentage of Eu3+ ions dopants, the emission intensity of samples exhibited significant diversity but the locations of emission bands remained identical. By comparison, with the increase of the doping Eu3+ ion concentration, the emission intensity of the SmVO4: x% Eu3+ (x=1, 3, 5) samples becomes stronger in transition of Eu3+ ions but weaker in transition of Sm3+ ions. It means that energy transfer from 26 / 33
VO43- groups to Eu3+ dopants becomes more efficient with the increase of the Eu3+ ion concentration and energy transfer from VO43- groups to Sm3+ hosts becomes less efficient with the decrease of the Sm3+ ion concentration. However, it is not hard to see SmVO4: 7% Eu3+ sample performs the strongest emission intensity among all SmVO4: x% Eu3+ (x=1, 3, 5, 7) samples in both transition of Eu3+ ions and Sm3+ ions. (Fig. 15, red curve), which indicates SmVO4: 7% Eu3+ sample might possess more extensive application in optical devices because of excellent luminescence properties.
Fig. 15. Photoluminescence emission spectra of SmVO4: x% Eu3+ samples: (x=1 green curve, x=3 blue curve, x=5 black curve, x=7 red curve)
4 Conclusions 27 / 33
In conclusion, the uniform dodecahedral SmVO4 nanocrystals were prepared by a hydrothermal method with the appropriate amount of potassium hydroxide and urea involving. The results of condition experiments show that 240℃, 2 days and the molar ratio of Sm(NO3)3·6H2O to NH4VO3 of 1:1 are the preferable experiment parameters to fabricate dodecahedral SmVO4 nanocrystals. The possible growth process of the SmVO4 nanocrystals was discussed, which might contain crystal nucleation stage and crystal growth stage, while potassium hydroxide and urea played an important role in the formation of dodecahedron-like SmVO4 nanocrystals. Under 315 nm excitation, SmVO4 and SmVO4: Eu3+ exhibit orange emission and reddish orange emission, respectively. Compared with SmVO4: Eu3+ samples doped with different amount of Eu3+ ions, SmVO4: 7% Eu3+ sample not only inherits major emission bands of the both Sm3+ ions and Eu3+ ions, but performs the strongest emission intensity among all SmVO4: x% Eu3+ (x=1, 3, 5, 7) samples. It might be applied in some optical devices that needed multiple emission bands and strong emission intensity. The method of preparing dodecahedral SmVO4 nanocrystals would be used to synthesize other rare earth orthovanadate nanocrystals.
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