YF3:RE3+ (RE = Dy, Tb, Eu) Sub-microstructures: Controllable

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YF:RE (RE= Dy, Tb, Eu) Submicro-Structures: Controllable Morphology, Tunable Multicolor and Thermal Properties Hongxia Guan, Ye Sheng, Yanhua Song, Chengyi Xu, Xiuqing Zhou, Keyan Zheng, Zhan Shi, and Haifeng Zou J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07879 • Publication Date (Web): 12 Sep 2017 Downloaded from http://pubs.acs.org on September 18, 2017

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The Journal of Physical Chemistry

YF3:RE3+ (RE= Dy, Tb, Eu) Submicro-Structures: Controllable Morphology, Tunable Multicolor and Thermal Properties Hongxia Guana, Ye Shenga, Yanhua Songa, Chengyi Xua, Xiuqing Zhoua, Keyan Zhenga, Zhan Shib, Haifeng Zoua,* a College of Chemistry, Jilin University, Changchun 130012, P. R. China. b State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, P. R. China Telephone: +86-0431-85155275; Fax: +86-0431-85155275; E-mail: [email protected] ABSTRACT: A series of emission-tunable YF3:RE3+ (RE= Dy, Tb, Eu) phosphors were firstly synthesized by glutamic acid assisted one-step hydrothermal process. The structure and morphology were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The results revealed that the samples were YF3 and Na(Y1.5Na0.5)F6 and the size and shape of the products could be tuned just by adjusting the pH values of the initial reaction solutions or by adjusting the RE3+/ NaF ratio or RE3+/ NaBF4 ratio. The morphologies for the products include bundle-like microstructures, fantails -like, micro-spindles, irregular bulk, walnut-like shape, irregular ellipsoidal micro-rods, the asymmetric hexagonal-prismatic tubular structures and irregular

micro-rods

and

irregular

nanoparticles

and

micro-sphere. Furthermore, the

photoluminescence properties of YF3:Dy3+, Tb3+ and YF3: Tb3+, Eu3+ were investigated in detail. Blue and yellow emissions corresponding to the 4F9/2→6H15/2 (475 nm), 4F9/2→6H13/2 (570nm) transition of Dy3+, blue and green corresponding to the 5D4→7F6 (488 nm), 5D4 → 7F5 (542 nm) transition of Tb3+ and red emissions corresponding to the 5D0 → 7F1 (591 nm), 5D0 → 7F2 (614 nm) and 5D0 → 7F4 (698 nm) transition of Eu3+ can be observed in YF3 host under 365 nm near-ultraviolet radiation. The energy transfer process from Dy3+ to Tb3+ and from Tb3+ to Eu3+ was demonstrated to be resonant type via quadrupole -quadrupole mechanism. Additionally, when co-doping Tb3+ with Dy3+ or Eu3+ or tri-doping Dy3+, Tb3+,Eu3+ ions in the single component, the colorful emitting can be obtained by giving abundant blue, green, yellow, orange, especially white-light-emission. The temperature-dependent photoluminescence for as-prepared phosphors have been investigated in detail. Importantly, it is found that fluorescent intensity ratio of

YF3:5%Dy3+, 2%Tb3+, 2%Eu3+ displays linear correlation with temperature in a wide range of 1

ACS Paragon Plus Environment


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298–398 K with high sensitivity of 0.24% K-1, indicating that it could be a good candidate for ratiometric optical thermometry. All these properties indicate that the developed phosphor may potentially be used as single-component multicolor-emitting phosphors. 1.

Introduction

In the field of modern materials science and technology, to control the size, phase, shape and dimensionality of inorganic nano/microparticals remains the key issue and research focus, due to its potential applications in many fields including nano-electronics, information storage, catalysis, biosensors, clinical therapy, and bio-imaging.1-6 To date, a number of synthesis techniques have been developed to synthesize nano/microcrystals with uniform phase, dimension and morphology, such as the solid-state combinatorial chemistry method, the molten-salt approach, hydrothermal and solvothermal techniques, the sol–gel route, and chemical vapor synthesis.7-8 Among all the methods, hydrothermal method is considered as the most effective and convenient way to control the morphologies and architectures of inorganic materials.9 Moreover, the hydrothermal treatment is relatively mild, simple, and easy to obtain a large amount of products. Recently, the fabrication of nano or micro-scaled YF3 has attracted increasing attention. Some mild chemical procedures, such as microwave method, microemulsion method, a double-crucible method and solvothermal routes have been developed to prepare YF3 with different sizes and morphologies. Uniform and well-crystallized orthorhombic YF3:Eu3+ phosphors with different morphologies were prepared using a facile microwave-assisted hydrothermal method by Liu et al.10 YF3 particles obtained using the classical microemulsion method are found to be monodisperse amorphous spheres, with controllable diameters between 6 and 50 nm by Ritcey et al.11 Tyagi group report that redispersible YF3: Ln3+ and YF3: Ce3+/Ln3+ nanocrystals were obtained by a simple one step ethylene glycol mediated soft chemical synthesis.12 YF3:Eu3+ hollow nanofibers were successfully synthesized by fluorination of the as-prepared Y2O3:Eu3+ hollow nanofibers via a double-crucible method using NH4HF2 as fluorinating agent for the first time.13 Although a lot of methods have been used to prepare YF3 with diverse morphologies, these usually require catalysts, expensive and even toxic templates or surfactants, high temperature, and a series of complicated procedures.8 Therefore it would be significant to find an easy and environmentally-friendly method to fabricate various morphologies using only simple adjustments. In this work, using the NaF and NaBF4 as fluorine sources, we synthesize and study different morphology YF3 materials using nitric acid or 2


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ammonium hydroxide adjusting the pH value by glutamic acid one-step facile hydrothermal process. Yttrium trifluoride (YF3) is one of the most important host crystals for lanthanide-doped phosphors with interesting up/down conversion luminescent properties due to its wide band gap and the possibility of replacing Y3+ sites by trivalent RE ions with similar valence state and ionic radii.14-17 That is, Y3+ can be easily substituted by other trivalent RE ions without additional charge compensation.18 Hereby, many studies have been performed to obtain Ln3+-doped YF3 nanomaterials. Lin19 et al reported that under the excitation of low-voltage electron beams, the YF3:Ce3+, Tb3+ exhibited green emission with good chromaticity coordinates and YF3:0.001Pr3+ showed the quantum cutting process, which may have potential applications in FED devices. Han20 group present that YF3: Yb3+/Er3+/Tm3+ nanocrystals with optimized UC PL intensity and modulated color output show promise with respect to their use as effective imaging probes for bio-applications. In addition, Bi3+ and Eu3+ co-doped in the YF3 phosphor has been successfully synthesized, which the emitting color is mainly in the yellow area.21 Up to now, to the best of our knowledge, there are no reports for the development of Dy3+, Tb3+ and Tb3+, Eu3+ co-doped and Dy3+, Tb3+, Eu3+ tri-doped YF3 phosphors. Studies on the multicolor emitting properties and energy transfer mechanism of inorganic host materials (YF3) co-doped with Dy3+,Tb3+ and Tb3+, Eu3+ ions are limited. In this paper, we report a simple hydrothermal strategy to synthesize microstructures of YF3 phosphors with glutamic acid as chelating agent and using NaF or NaBF4 as F source. Additionally, the morphology of as-prepared YF3 can be easily adjusted by changing the pH value of the mother liquor or by adjusting the RE3+/ NaF ratio or RE3+/ NaBF4 ratio. Moreover, Dy3+, Tb3+, and Eu3+ were selected as co-activators or tri-activators to prepare single-phased multicolor-emitting phosphors. The photoluminescence properties and the energy transfer mechanism of Dy3+ to Tb3+ and Tb3+ and Eu3+ in the YF3 host under UV light have been studied in detail. The multicolor emission can be obtained by variation of the relative doping concentrations of rare earth (Dy3+, Tb3+, Eu3+) and adjusting the excitation wavelength. 2.

Experimental

2.1 Materials 3



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All of the chemicals were used directly as received without further purification. Y(NO3)3, Dy(NO3)3, Tb(NO3)3 and Eu(NO3)3 were prepared by dissolving the corresponding Y2O3, Dy2O3, Tb4O7 and Eu2O3 in HNO3 solution at elevated temperature followed by evaporating superfluous HNO3. Sodium fluoroborate (NaBF4) and sodium fluoride (NaF) were of analytical grade and were purchased from Beijing Chemical Corporation. 2.2 Preparation A series of rare earth-doped YF3 nanocrystals were synthesized by glutamic acid assisted hydrothermal process without further sintering treatment. In a typical procedure of preparing representative YF3: 5%Dy3+, 2%Tb3+, 2%Eu3+: Firstly, 18.2 mL Y(NO3)3 (0.1M), 2 mL Dy(NO3)3 (0.05M) and 0.8 mL Tb(NO3)3 (0.05M) and 0.8 mL Eu(NO3)3 (0.05M) were poured to a 100mL flask with 0.5885 g (1:2 molar ratio for RE3+/ glutamic acid) of glutamic acid ( hereinafter shortened form Glu2- ). And the solution is stirred for 20 min. Secondly, 0.6587 g of NaBF4 (1:3 molar ratio for RE(NO3)3/ NaBF4) was slowly added into the above solution. Then the above solution is sostenuto stirred for another 20 min. Thirdly, using the nitric acid or ammonia adjusts the pH values of the solution. Then the resultant milky colloidal suspension was transferred to a 50 mL stainless Teflon-loined autoclave, sealed, and kept at 180 °C for 24h. Finally, the autoclave was naturally cooled to room-temperature and the products were deposited at the bottom of the vessel. The precipitates were separated by centrifugation, washed with deionized water and ethanol in sequence each several times to remove citric acid and other remnants, and then dried in air at 60 °C for 12 h. Other samples were prepared in a similar procedure, except for changing the value of the NaBF4, NaF, RE(NO3)3 and the pH values. 2.3 Characterization 4


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The morphology and size of the samples were observed by a field emission electron microscope (FESEM, S-4800, Hitachi, Japan). X-ray powder diffraction (XRD) data for the prepared samples (YF3: Dy3+, Tb3+ ,Eu3+) were taken using a Rigaku D/max-RA X-ray diffractometer with Cu K α radiation (λ= 0.15406 nm) and Ni filter, operating at 20 mA, 30 kV, scanning speed, step length and diffraction range were 10°/min, 0.1° and 20-70°, respectively. The measurements of photoluminescence (PL) and photoluminescence excitation (PLE) spectra were carried out by a Jobin Yvon FluoroMax-4 using a 150W xenon lamp as the excitation source. The excitation and emission slits were set to 2.5 and 2.5 nm. All of the measurements were performed at room temperature. 3. Results and Discussion 3.1 Crystallization Behavior and Structure

a1500

b

Measured Calculated Difference Positions

Positions

1500

900

Rp = 7.18% Rwp = 9.88%

600

Measured Calculated Difference

2000

Relative Intensity(a.u.)

1200

Relative Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Rp = 5.27% Rwp = 6.88%

1000

300

0

500

0

-300 20

30

40

50

2 Theta (degree)

60

70

20

30

40

50

60

70

2 Theta (degree)

5



c 800

3+

3+

3+

YF3:5%Dy ,2%Tb ,3%Eu

700 3+

3+

YF3:,3%Eu ,2%Tb

600

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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500

3+

3+

3+

3+

3+

3+

YF3:3%Tb ,4%Eu

400

YF3:2%Tb ,4%Dy

300

YF3:2%Dy ,3%Tb

200 100

JCPDS No. 70-1935

0 10

20

30

40

50

60

70

2-Theta (degree)

Figure 1 XRD patterns of YF3 (a), YF3:2%Dy3+, 3%Tb3+ (b) and YF3:Dy3+, Tb3+, YF3:Tb3+, Eu3+, and YF3:Dy3+, Tb3+, Eu3+ (c) samples (1:3 molar ratio for RE(NO3)3/ NaBF4); the corresponding standard data of YF3 (JCPDS No. 70-1935) is given as reference；structural models of the orthorhombic YF3 structures undoped (d) and doped with Dy3+, Tb3+ ions (e). Rietveld refinement is an effective method to analyze the position of atoms in a primitive cell. In order to evaluate the structural parameters of YF3 (pH=2.31, with Glu2-, RE(NO3)3/ NaBF4) and YF3:2%Dy3+, 3%Tb3+ (pH=2.31, with Glu2-, RE(NO3)3/ NaBF4), structural refinement was carried out by the TOPAS program using the Rietveld method. Figure 1 (a) and Figure 1 (b) give the experimental and refined XRD patterns of the YF3 and YF3:2%Dy3+, 3%Tb3+ samples. The red solid line and black crosses represent the calculated and experimental patterns, respectively. The pink vertical lines show the positions of the simulated diffraction patterns. The difference between the experimental and calculated results is plotted by the blue line at the bottom. By comparing the calculated data with the experimental spectra, we found that each peak is in good agreement. There is no impurity phase found in the samples, which reveals that it is a good single-phase. The 6


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Rietveld refinement analysis indicated that Rw (%) = 9.88% (YF3) and Rw (%) = 6.88% (YF3:2%Dy3+, 3%Tb3+), which are within the accepted error range, indicating that the refinement results and the above assumption are reliable. According to the literature, the Dy3+ and Tb3+ ions occupy the site of Y3+ ions. In order to further evaluate the occupying sites of the doping ions of Dy3+ and Tb3+, the structural refinement was also carried out on YF3:2%Dy3+, 3%Tb3+, as shown in Figure 1(b). The resulting crystallographic data of YF3 and YF3:2%Dy3+, 3%Tb3+ are summarized in Table 1. The atomic coordinates and site occupancy fraction (SOF) are presented in Table 2 and Table 3, respectively. In addition, the phase identification of YF3:Dy3+,Tb3+, YF3:Tb3+,Eu3+, and YF3:Dy3+,Tb3+,Eu3+ phosphors characterized by XRD is also shown in Figure1 (c). As can be seen, the powder diffraction patterns are in good agreement with the standard card (JCPDS Card No. 70-1935) and there is no detectable impurity phase present, which reveal the high phase purity of the as-prepared products. And the lattice constants and the cell volume of the samples remain almost unchanged, as listed in Table 4. In addition, the structural models of the orthorhombic YF3 structures undoped and doped with Dy3+, Tb3+, Eu3+ ions are also given in Figure 1(d, c).

Table 1 Crystal structural data and lattice parameters YF3:2%Dy3+, 3%Tb3+

Formula

YF3

Crystal system

Orthorhombic

Orthorhombic

Space group

Pnma

Pnma

Z

4

4

a/Å

6.345981

6.354991

b/Å

6.869964

6.876776

4.432710

4.431479

c/Å 3

Unit cell volume ( V Å )

193.251

193.664

RP

7.18%

5.27%

RWP

9.88%

6.88%

Table 2 Atomic coordinates and site occupancy fraction (SOF) for YF3. x

y

z

Y

Atom

0.367457

0.250000

0.441404

SOF 1

F1

0.169433

0.250000

0.590386

1

F2

0.255564

0.065486

0.124387

1

Table 3 Atomic coordinates and site occupancy fraction (SOF) for YF3:2%Dy, 3%Tb. Atom

x

y

z

SOF

F1

0.024824

0.250000

0.592123

1

7



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F2

0.174594

0.064737

0.124186

1

Y

0.366551

0.250000

0.441105

0.9507

Dy

0.366551

0.250000

0.441105

0.0194

Tb

0.366551

0.250000

0.441105

0.0299

Table 4 Lattice constants and cell volume of YF3 (JCPDS No. 70-1935) and YF3:Dy3+, Tb3+, YF3:Tb3+, Eu3+, and YF3:Dy3+, Tb3+, Eu3+ samples. Samples

Lattice constants: a (Å)

Lattice constants: b (Å)

Lattice constants: c (Å)

Cell volume ( Å3 )

YF3 standard card data

6.3537

6.8545

4.3953

191.42

YF3 :2%Dy,3%Tb

6.35015

6.86832

4.41381

192.51

YF3 :2%Tb,4%Dy

6.33654

6.85154

4.40241

191.13

YF3 :3%Tb,4%Eu

6.34197

6.85719

4.40409

191.53

YF3 :3%Eu,2%Tb

6.35553

6.8729

4.40767

192.53

YF3 :5%Dy,2%Tb,3%Eu

6.35052

6.87593

4.42385

193.17

3.2 Morphology and Structure

8


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g 1100

1:11

1000

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


900

1:9

800

JCPDS No.16-0334

700

♣

♣

1:7

♣

1:5

600

♣ 500

♣

JCPDS No.77-2042

♣

400 300

1:3

200

1:1

100

JCPDS No.70-1935 0 10

20

30

40

50

60

70

2-Theta (degree)

Figure 2 SEM images of samples prepared with glutamic acid at different RE3+/ NaF ratio: 1:1(a), 1:3 (b), 1:5 (c), 1:7 (d), 1:9 (e), 1:11 (f); XRD patterns of samples prepared with glutamic acid at different RE3+/ NaF ratio (g). The corresponding standard data of YF3 (JCPDS No. 70-1935), NaYF4 (JCPDS No. 77-2042) and Na(Y1.5Na0.5)F6 (JCPDS No. 16-0334) are given as reference. Generally, the successful synthesis of the materials in a solution-based system not only depends on the intrinsic structure of the target compounds but also requires more fastidious control of the growth parameters. The effect of NaF on the morphology of the products was investigated, as shown in Figure 2. When the RE3+/ NaF ratio is 1:1, the product is composed of a large scale of irregular nanoparticles with the average size of around 40 nm (Figure 2a). If we slightly adjust the RE3+/ NaF ratio to 1:3, then the morphology retains the irregular nanoparticles architecture, but the average size increases to 80 nm (Figure 2b). With increasing the RE3+/ NaF ratio to 1:5 and 1:7, the morphology consists of asymmetric hexagonal-prismatic tubular structures and irregular nanoparticles, as shown in Figure 2 c, d. It is worth that the irregular nanoparticles gradually decrease with the increasing of RE3+/ NaF ratio. However, when the RE3+/ NaF ratio are fixed at 1:9 and 1:11, the irregular nanoparticles almost entirely disappear, and only the irregular microrods are formed (Figure 2e, f). In our current system, it is found that the RE3+/ NaF ratio has 9



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an important effect on the shapes and sizes of the products. From Figure 2 (g), When the RE3+/ NaF ratio is 1:1 and 1:3, the resulted product could be indexed as a pure orthorhombic phase YF3, which coincides well with the data reported in the JCPDS standard card (JCPDS No. 70-1935). With the RE3+/ NaF ratio increase to 1:5 and 1:7, The XRD pattern shows that the products are composed of mixed crystal phases of NaYF4 and Na(Y1.5Na0.5)F6. By further increasing the RE3+/ NaF ratio to 1:9 and 1:11, it is obvious that all the diffraction peaks of the nanocrystal could be clearly indexed to the pure orthorhombic phase Na(Y1.5Na0.5)F6 crystal, agreeing well with the data reported in the JCPDS standard card (JCPDS No. 16-0334).

10


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g 800

1:11

700

1:9

600

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


500

1:7

400

1:5

300

1:3

200

1:1

100

JCPDS No. 70-1935 0 10

20

30

40

50

60

70

2-Theta (degree)

Figure 3 SEM images of samples prepared with glutamic acid at different RE3+/ NaBF4 ratio: 1:1(a), 1:3 (b), 1:5 (c), 1:7 (d), 1:9 (e), 1:11 (f); XRD patterns of samples prepared with glutamic acid at different RE3+/ NaBF4 ratio (g). The corresponding standard data of YF3 (JCPDS No. 70-1935) is given as reference. The effect of NaBF4 on the morphology of the products was investigated, as shown in Figure 3. From Figure 3(a), we can clearly see that monodisperse micro-sphere with the diameter of 2 µm can be prepared at RE3+/ NaBF4 ratio=1:1. If the RE3+/ NaBF4 ratio was increased to 1:3 and 1:5, the nanoparticles can be obtained. When the RE3+/ NaBF4 ratio was further increased to 1:7, the nanoparticles began to self-assembly. It is interesting that when the RE3+/ NaBF4 ratio was further increased to 1:9 and 1:11, the nanoparticles self-assemble into ellipsoidal micro-rods. From Figure 3 (g), it can be seen that only the pure YF3 is obtained under these RE3+/ NaBF4 ratio conditions. Note that when the NaBF4 does fluorine source, the amount of NaBF4 changed, only the pure YF3 is obtained. In aqueous solution, NaBF4 is slowly hydrolyzed to produce Na+，BO33- and F- anions. Due to the strong interactions between Na+ and BO33- anions, there two cations can be selectively adsorb on specific facets of the initial YF3 crystals and change their surface energy.

11



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Figure 4 SEM images of the synthesized samples (1:3 molar ratio for RE(NO3)3/ NaBF4), with the pH values of the initial solution: pH=1.10 (a), pH=2.31 (b), pH=3.22 (c), pH=4.10 (d), pH=5.73 (e), pH=6.65 (f), pH=7.37 (g), pH=8.15 (h), pH=9.31 (i), pH=10.11 (j), and pH=11.19 (k). The pH of the initial solution is an important factor in determining the mode of growth and the final product.22 The pH of the initial solution was adjusted by adding nitric acid or ammonia and samples with various morphologies and changing phases could be obtained, as shown in Figure 4 and Figure 5. The SEM images of the products synthesized under the condition that NaBF4 as F- source and different pH value are shown in Figure 4. After the pH of the initial solution adjusted for 1.10, it was found that the shape of the obtained sample was a mixture of bundle-like microstructures and nanoparticles, with the bundle-like microstructures also being constructed from nanoparticles. When the pH of the initial solution was extended to 2.31 and 3.22, the bundle-like microstructures continued to grow into fantails, which are composed of many nanoparticles. When further increasing the pH of the initial solution to 4.10, the micro-spindles can be obtained, which was assembled by nanoparticles. As the pH of the initial solution was then increased to 5.73, a fast growth process occurred and large numbers of irregular bulk were formed. And some small particles are attached on the surfaces of the irregular bulk, which suggests these small particles may serve as building blocks for the growth of the irregular bulk. By further adjusting the pH of the initial solution to 6.65, the morphology remains irregular bulk, but the size 13



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is obviously increased. Notably, as the pH of the initial solution is controlled at 7.37, the shape of the obtained sample was a mixture of walnut-like shape and irregular bulk, which the walnut -like shape was assembled by nanoparticles. Adjusting the initial solution pH value at 8.15, the walnut -like shape grow up the irregular ellipsoidal micro-rods. With increasing the pH value of initial solution to 9.31, the morphology consists of asymmetric hexagonal-prismatic tubular structures and irregular bulks. More important, by further increasing the initial solution pH value to10.11, the irregular bulks entirely disappear, and the asymmetric hexagonal-prismatic tubular structures are formed. It is interesting that when the pH value is further increased to 11.19, the surfaces of the asymmetric hexagonal-prismatic tubular structures are very coarse. It is clear that the pH value of initial solution can influence the morphology of products. The glutamic acid has amino and carboxyl which the complexing ability is very strong. Then it can coordinate with rare earth ions and affect the crystal nucleation. So there are many possibilities for the nucleation process of crystal nucleus of anisotropic growth. The different pH values will affect the existent form and complexing ability of organic additive Glu2- to Y3+. When the PH > 7, the glutamic acid mainly in the form of Glu2-. Whereas, when the PH

YF3:RE3+ (RE = Dy, Tb, Eu) Sub-microstructures: Controllable

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