by pH Value - American Chemical Society

Article pubs.acs.org/JPCC

Controlling the Morphology and Size of GdF3:RE3+ (RE = Dy, Tb, and Sm) by pH Value: Growth Mechanism, Energy Transfer, and Luminescent Properties Hongxia Guan,† Chengyi Xu,† Ye Sheng,† Yanhua Song,† Keyan Zheng,† Zhan Shi,‡ and Haifeng Zou*,† †

College of Chemistry and ‡State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, P.R. China ABSTRACT: In this paper, the synthesis of rare earth fluoride GdF3 multiform morphologies via a glutamic acid hydrothermal method has been fulfilled. The size and shape of the products could be tuned just by adjusting the pH values of the initial reaction solutions. The morphologies for the products include spindle-like, peanut-like, submicrometer flowers, and nanoflowers. Additionally, the possible formation mechanism of multiform morphologies was proposed. Furthermore, we systematically investigate the luminescence properties of different lanthanide ions (Dy3+, Tb3+, and Sm3+) in GdF3 host. In the GdF3:Tb3+,Sm3+system, the energy transfer process fromTb3+ to Sm3+ was demonstrated to be resonant type via a dipole−dipole mechanism. Most interestingly, white light and multicolor light have been obtained in Tb3+ and Sm3+ coactivated or Dy3+,Tb3+,Sm3+ tridoped GdF3 phosphors. The results show that the obtained nanophosphors have a promising application in display and lighting fields.

1. INTRODUCTION In recent years, there has been great interest in studying and developing rare earth (RE) luminescence materials on account of their potential applications, such as liquid lasers, photoelectric devices, nanolabels, multimodality bioimaging contrast agents, television tubes, mobile telephone screens, fluorescent lamps, and the promising white light-emitting diodes.1−8 Among all kinds of RE luminescence materials, RE fluoride (such as GdF3) luminescence materials have many excellent properties, for instance, long lifetime, lower energy consumption, high luminous efficiency, and environmentally friendliness, and so on.9−14 Therefore, GdF3 is considered to be a prominent luminescent host material. To date, numerous studies and efforts have been dedicated to control the size, phase, shape, and dimensionality of inorganic nano-/submicroparticles, which are of fundamental and technological importance on account of the strong correlation between these parameters and their chemical/physical characteristics, such as magnetic, luminescent, and catalytic properties, as well as their distinct effect on the particles’ potential applications in many fields including clinical therapy and bioimaging.15 Recently, many methodologies for synthesis of a range of nano-/submicroparticles have been reported, such as molten-salt synthesis, hydrothermal processing, and template synthesis and so on.16−20 Among these methods, the hydrothermal method is considered the most effective and convenient way to control the morphologies and architectures of inorganic materials. Moreover, the hydrothermal treatment is relatively mild, simple, and easy for obtaining a large amount of products. Hitherto, we synthesize and study different © XXXX American Chemical Society

morphology GdF3 materials by a facile hydrothermal process in this work, using nitric acid or ammonium hydroxide for adjusting the pH value. In order to generate multicolor light including white light from single-phase phosphors, doping with RE ions can be deemed to the first option.21 Among the RE ions, the asprepared GdF3:Tb3+ samples exhibit two characteristic emission bands, whose positions are at 487 and 542 nm, corresponding to the blue and green light, and the samarium ions (Sm3+) emit orange-red light based on the transition of 4G5/2 → 6H9/2. It is noteworthy that Dy3+ ions are usually used as efficient blue light emissive activators because of the4F9/2 → 6H15/2 transition. For multicolor light including white light, the simultaneous generation by phosphors of red, green, and blue (RGB) is necessary. Thereby, it is highly valuable to obtain multicolor light including white light through codoping Tb3+ and Sm3+into GdF3 host or by Dy3+,Tb3+,Sm3+ tridoped phosphors. Interestingly, the characteristic emissions of Tb3+ from the 5 D3 energy level overlap well with the 6H5/2 → 4I11/2(475 nm) transition of Sm3+.22 Remarkably, the energy transfer process from Tb3+ to Sm3+ has been investigated in some inorganic hosts, such as NaGd(WO4)2,23 Ca2Gd8Si6O26,24 NaGdF4,22 BaCeF5,25 and BaGdF5.26 Therefore, the energy transfer from Tb3+ to Sm3+ in GdF3 host is expected. Herein, we demonstrate a facile hydrothermal strategy for the synthesis of RE fluorides GdF3 nano-/microcrystals. Products Received: January 4, 2017 Revised: February 28, 2017

A

DOI: 10.1021/acs.jpcc.7b00048 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. XRD refinement results of (a) GdF3 (pH 2.3) and (b) GdF3:2%Tb3+,2%Sm3+(pH 2.3). XRD patterns of the as-prepared GdF3:2% Dy3+products using glutamic acid at 180 °C for 24 h under different pH values of pH 1.3, 2.3, 3.7, 4.7, 5.7, 6.3, 7.5, and 8.4 (c, d). The corresponding standard data of GdF3 (Powder Diffraction File No. 49-1804, Joint Committee on Powder Diffraction Standards, 1998) are given as reference.

follows: First, 18.8 mL of Gd(NO3)3, 0.8 mL of Dy(NO3)3, 0.8 mL of Tb(NO3)3, and 0.8 mL of Sm(NO3)3 were poured into a 100 mL 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. Second, 0.8783 g of NaBF4 (1:4 molar ratio for RE(NO3)3/NaBF4) was slowly added into the above solution. Then, the above solution is subsequently stirred for another 20 min. Third, using nitric acid or ammonia, we adjust the pH value of the solution. The resultant milky colloidal suspension was transferred to a 50 mL stainless Teflon-lined 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 RE(NO3)3 and the pH values.

with diverse morphologies and sizes have been synthesized successfully by adjusting the pH value of the mother liquor. In addition, we reported the color-tunable luminescence behavior of trivalent RE ions (Dy3+, Tb3+, and Sm3+) activated GdF3 powders. Furthermore, the luminescence and energy transfer properties of Tb3+ and Sm3+ codoped GdF3 phosphors have been discussed in detail.

2. EXPERIMENTAL SECTION 2.1. Materials. All the chemicals were used directly as received without further purification. Gd(NO3)3, Dy(NO3)3, Tb(NO3)3, and Sm (NO3)3 were prepared by dissolving the corresponding Gd2O3, Dy2O3, Tb4O7, and Sm2O3 in HNO3 solution at elevated temperature followed by evaporating superfluous HNO3. Sodium fluoroborate (NaBF4) was of analytical grade. 2.2. Preparation. A series of RE-doped GdF3 nanocrystals were synthesized by a glutamic acid hydrothermal process without further sintering treatment. A typical procedure for preparing representative GdF3:2%Dy3+,2%Tb3+,2%Sm3+ is as B


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The Journal of Physical Chemistry C 2.3. Characterization. The morphology and size of the samples were observed by a field-emission electron microscope (FESEM, S-4800, Hitachi, Japan). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) micrographs were performed using a FEI Tecnai G2 S-Twin transmission electron microscope with a field-emission gun operating at 200 kV. X-ray powder diffraction (XRD) data for the prepared samples (GdF3:Dy3+,Tb3+,Sm3+) were taken using a Rigaku D/max-RA X-ray diffractometer with Cu Kα radiation (λ = 0.15406 nm) and Ni filter, operating at 20 mA and 30 kV; scanning speed, step length, and diffraction range were 10°/ min, 0.1°, and 20−70°, respectively. The measurements of photoluminescence (PL), photoluminescence excitation (PLE) spectra and the luminescence decay curves were carried out by a JobinYvon 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.

Table 4. Lattice Constants and Cell Volume of GdF3 (PDF No. 49-1804) and GdF3:2%Dy3+ (pH 1.3, 2.3, 3.7, 4.7, 5.7, 6.3, 7.5, and 8.4)

3. RESULTS AND DISCUSSION 3.1. Crystallization Behavior and Structure. Rietveld refinement is an effective method to analyze the position of

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 Rietveld refinement analysis indicated Rw (%) = 9.29% (GdF3) and Rw (%) = 9.66% (GdF3:2%Tb3+,2%Sm3+), values which are within the accepted error range, indicating that the refinement results and the above assumption are reliable. According to the literature, the Tb3+ and Sm3+ ions occupy the site of Gd3+ ions. In order to further evaluate the occupying sites of the doping ions of Tb3+ and Sm3+, structural refinement was also carried out on GdF3:2%Tb3+,2%Sm3+, as shown in Figure 1b. The resulting crystallographic data of GdF3 and GdF3:2%Tb3+,2% Sm3+ are summarized in Table 1. The atomic coordinates and site occupancy fraction (SOF) are presented in Tables 2 and 3, respectively. Figure 1c,d shows the XRD patterns of the samples synthesized with different pH values in the presence of Glu2− after hydrothermal treatment at 180 °C for 24 h. As shown in Figure 1c,d, it is obvious that all the diffraction peaks of the nanocrystal could be clearly indexed to the pure orthorhombic phase GdF3 crystal, agreeing well with the data reported in the Powder Diffraction File (PDF) No. 49-1804 (Joint Committee on Powder Diffraction Standards, 1998). No obvious shifting of diffraction peaks is observed at different pH values, and the lattice constants and the cell volume of the samples remain almost unchanged, as listed in Table 4.The above discussion indicates that pure GdF3 is obtained under these pH conditions. Notably, the relative intensities of the (101), (020), (111), and (210) peaks slightly differ from those of the standard PDF No. 49-1804. When the pH values are adjusted to 1.3 and 2.3, the (210) plane of the GdF3 is the preferential orientation growth direction. With the pH values continuing to increase to 3.7, GdF3 shows a preferential orientation growth along the [111] direction. As the pH values are controlled at 4.7, 5.7, 6.3, and 7.5, the (020) plane of the GdF3 is the preferential orientation growth direction. By further increasing the pH value to 8.4, the preferential orientation growth direction is the plane of (020) and (210).The growth rate of the plane of (011) at pH 4.7 and 5.7 significantly greater than that in the others. The above phenomenon indicates that the pH values of the mixed solvents in the hydrothermal reaction system potentially impact the preferential orientation growth direction. Moreover, sharp and strong peaks indicate that the

lattice constants samples GdF3standard card data GdF3:2%Dy (pH 1.3) GdF3:2%Dy (pH 2.3) GdF3:2%Dy (pH 3.7) GdF3:2%Dy (pH 4.7) GdF3:2%Dy (pH 5.7) GdF3:2%Dy (pH 6.3) GdF3:2%Dy (pH 7.5) GdF3:2%Dy (pH 8.4)

Table 1. Crystal Structural Data and Lattice Parameters formula crystal system space group Z a (Å) b (Å) c (Å) unit cell volume, V (Å3) RP RWP

GdF3 orthorhombic Pnma 4 6.537070 6.970705 4.404878 200.721 6.69% 9.29%

GdF3:2%Tb3+,2%Sm3+ orthorhombic Pnma 4 6.559845 6.995024 4.420182 202.826 7.11% 9.66%

Table 2. Atomic Coordinates and Site Occupancy Fraction (SOF) for GdF3 atom

x

y

z

SOF

Gd F1 F2

0.367265 0.523179 0.164455

0.250000 0.250000 0.062954

0.062831 0.575034 0.386682

1 1 1

Table 3. Atomic Coordinates and Site Occupancy Fraction (SOF) for GdF3 atom

x

y

z

SOF

F1 F2 Gd Tb Sm

0.519308 0.163755 0.367146 0.367146 0.367146

0.250000 0.062254 0.250000 0.250000 0.250000

0.580816 0.383431 0.062421 0.062421 0.062421

1 1 0.9603 0.0198 0.0199

atoms in a primitive cell. In order to evaluate the structural parameters of GdF3 (pH 2.3, with Glu2−) and GdF3:2%Tb3+,2% Sm3+ (pH 2.3, with Glu2−), structural refinement was carried out by the TOPAS program using the Rietveld method. Figure 1a,b gives the experimental and refined XRD patterns of the GdF3 and GdF3:2%Tb3+,2%Sm3+ 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 C

a (Å)

b (Å)

c (Å)

cell volume (Å3)

6.571

6.984

4.39

201.47

6.53864 6.54818 6.54917 6.53968 6.54876 6.5515 6.56242 6.53515

6.96849 6.98268 6.97217 6.962 6.97733 6.97247 6.9802 6.96486

4.39921 4.41335 4.40123 4.39259 4.39409 4.39455 4.4049 4.40248

200.45 201.8 200.97 199.99 200.78 200.74 201.78 200.39


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Figure 2. continued

D


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Figure 2. continued

E


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Figure 2. SEM images of GdF3:2%Dy3+ prepared at different pH values: (a) 1.3, (b) 2.3, (c) 3.7, (d) 3.9, (e) 4.1, (f) 4.3, (g) 4.5, (h) 4.7, (i) 5.7, (j) 5.9, (k) 6.1, (l) 6.3, (m) 7.5, (n) 8.4. TEM and HRTEM images of GdF3:2%Dy3+ prepared at different pH values: (b-1, b-2) 2.3, (m-1, m-2) 7.5. (o) PL spectra of GdF3:2%Dy3+ obtained with different pH values.

growth process of nanoparticles.27,28 It is worth pointing out that we can obtain different sizes and morphologies of particles just by varying the pH values of the initial reaction solution, although the crystal structures of the particles only have pure orthorhombic phase GdF3. From the SEM images in Figure 2a, we can obviously observe that when the pH value is adjusted to 1.3, the products are spindle-like structures with sheafing in the middle part and have a length about 600 nm. If we adjust an acid circumstance for the reaction (pH 2.3), then the morphology of the product transforms in a nonobvious manner; only the sheafing in the middle part disappeared. The morphology becomes a complete spindle-like structure with a length about 600 nm and a diameter about 250 nm, as shown in Figure 2b. With the pH values continuing to increase to 3.7, the morphology turns to a peanut-like architecture, taking on a length about 750 nm and a diameter about 450 nm (Figure 2c). If we slightly adjust an acid circumstance to the pH 3.9, then the morphology retains the peanut-like architecture, but the length and diameter decrease to 500 and 300 nm, respectively (Figure 2d). Notably, as the pH value is controlled at 4.1 and 4.3, the prepared samples grow into submicron flowers (Figure 2e,f). Adjusting the solution pH values at 4.5 and 4.7, the submicrometer flowers split to form microrods and nanoflowers, which are shown in Figure 2g,h. It is noteworthy that the nanoflowers gradually decrease with the increasing of pH values. However, when the pH values are fixed at 5.7, 5.9 and 6.1, the morphology is mainly nanoflowers. Simultaneously, the microrods become gradually smaller and smaller, as shown

Scheme 1. Possible Formation Mechanisms of GdF3 Nano-/ Submicrocrystals with Multiform Morphologies and Size

products synthesized at low temperature are still high crystallinity. 3.2. Morphology. The pH value of the mixed solvent in the hydrothermal reaction system is found to be an important synthetic parameter to influence the shapes and nano-/ submicroparticles of inorganic materials, because the degree of acidity or alkalinity of the hydrothermal reaction system plays an important role in the nano-/submicroparticles and F


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Figure 3. PLE (left) and PL (right) spectra of GdF3:2%Dy3+(a), GdF3:2%Tb3+(b), and GdF3:2%Sm3+(c). Inset presents the PL emission spectra of different Dy3+/Tb3+/Sm3+concentrations.

Figure 4. PLE spectra of GdF3:2%Tb3+, GdF3:2%Sm3+, and GdF3:2% Tb3+,2%Sm3+ phosphors. Figure 5. Spectral overlap between the PL spectrum of GdF3:Tb3+ and PLE spectrum of GdF3:Sm3+.

in Figure 2i−k. More important, by further increasing the pH values to 6.3 and 7.5, the microrods almost entirely disappear, and only the nanoflowers are formed (Figure 2l,m). It is interesting that when the pH value is further increased to 8.4, the morphology turns into irregular nanosphere, which is composed of many small nanoparticles. In our current system,

it is found that the pH value of mother liquor has an important effect on the shapes and sizes of the products (Figure 2).29,30 The morphology and size of the as-prepared GdF3 were also characterized by TEM and HRTEM, as shown in Figure 2b-1/ G


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Figure 6. PL spectra of the GdF3:2%Tb3+,x%Sm3+(x = 0, 0.25, 0.5, 1, 2, 3, 4, 5, and 6) (a) and GdF3:y%Tb3+,2%Sm3+ (y = 0, 1, 2, 3, and 4) (c) samples with different Tb3+ or Sm3+ doped concentrations (λex = 371 nm); emission intensity of Tb3+ and Sm3+ as a function of Tb3+ or Sm3+ concentration (b, d).

b-2 and m-1/m-2. From Figure 2b-1/b-2 and m-1/m-2, one can obviously observe that when the pH value is adjusted to 2.3 and 7.5, the products are spindle-like structures and nanoflowers. The corresponding HRTEM image clearly shows lattice fringes with interplanar spacing of 3.3 Å ascribed to the (111) plane of GdF3 (Figure 2b-2,m-2).31 In addition, the PL spectra of GdF3:2%Dy3+ obtained with different pH values are also given in Figure 2o. From Figure 2o, one can see that the emitting intensities of Dy3+ with different pH values are different. In fact, many factors can affect the luminescence properties of samples, such as crystallinity, the ratio of surface to volume, lattice defects, and surface chemistry. 3.3. Formation Mechanisms. To gain more information to reveal the growing process of GdF3 nano-/submicroparticles, pH-dependent experiments were carried out by keeping other reaction parameters unchanged. Lin et al. has reported that the pH value of the initial solution had an effect on the release rate and concentration of F− ions.15 When the mixed solvents were in an acid environment, the release of F− ions would be restrained, leading to the fact that the concentration of F−

decreases. When the mixed solvents were in an alkaline environment, the dissociative OH− in the mixed solvents could be beneficial to the release of F− ions, leading to the fact that the concentration of F− increases at the same time. The concentrations of F− ions are different; hence, the time of Gd3+ reacting with the F− ions to form the GdF3 nuclei are different. The shapes and sizes of the products are thus different, and Lin et al. also has reported that the different pH values will affect the existent form and complexing ability of organic additive (Glu2−) to Gd3+.15 When the pH values are adjusted to 7.5− 8.4, the capping ability of Glu2− restricted the crystal growth remarkably, leading to the smaller GdF3 nanoparticles. With decreasing pH value, Glu2− would partly combine with H+ in the solution and exist as HxGlux−2, directly leading its complexing ability. This will affect the selective adsorption of Glu2− on the different facets of the growing GdF3 crystallites, giving rise to the different growth rates between different crystallographic directions. A schematic illustration of the possible detailed formation process of the products with diversified sizes and morphologies is presented in Scheme H


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Figure 7. PL spectra of the GdF3:2%Tb3+,x%Sm3+(x = 0, 0.25, 0.5, 1, 2, 3, 4, 5, and 6) (a) and GdF3:y%Tb3+,2%Sm3+ (y = 0, 1, 2, 3, and 4) (c) samples with different Tb3+ or Sm3+ doped concentrations (λex = 271 nm); emission intensity of Tb3+ and Sm3+ as a function of Sm3+ or Tb3+ concentration (c, d).

1.The probable reaction processes for the formation of GdF3 when Glu2− is used as surfactant can be summarized as follows: 2Gd3 + + 3Glu 2 − → 2Ln 3 +3Glu 2 − (complex)

(1)

BF 4 − + H 2O → H3BO3 + 3HF + F−

(2)

2Gd3 +3Glu 2 − + 6F− → 2GdF3 + 3Glu 2 −

(3)

Therefore, the nucleation and growth of GdF3 would go through a longer process, which would be helpful in forming uniform crystals. 3.4. Luminescent Properties of GdF3:Ln3+ (Ln = Dy, Tb, and Sm). The samples used in PLE and PL measurements were obtained at pH 2.3. Figure 3a illustrates the roomtemperature PLE and PL spectra of GdF3:2%Dy3+.The PLE spectrum monitored with 475 nm emission (4F9/2 → 6H13/2) of Dy3+ consists of sharp excitation bands at 271 nm (8S7/2 → 6 I7/2, Gd3+) and several bands centered at 323, 348, 362, and 386 nm, corresponding to the transitions of Dy3+ from 6H15/2 to 6P3/2, 6P7/2, 6P5/2, and 4F7/2, respectively, which confirm the

Figure 8. Decay curves for the luminescence of Tb3+ in GdF3:2% Tb3+,x%Sm3+(x = 0, 0.25, 0.5, 1, 2, 3, 4, and 5) phosphors displayed on a logarithmic intensity (excited at 371 nm, monitored at 541 nm).

I


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the energy transfer between adjacent luminescent centers;33 thus, the optimum concentration of Dy3+ is 2%. As shown in Figure 3b, the PLE spectrum of GdF3:2%Tb3+ was recorded by monitoring the green emission (5D4 → 7F5) at 541 nm, which includes two parts: One is composed of some peaks range from 320 to 500 nm, which belong to f−f transitions of Tb3+.24 The other is a sharp peak at 271 nm due to the characteristic transition of Gd3+ (8S7/2 → 6IJ), implying that there is efficient energy transfer between Gd3+ and Tb3+. For the PL spectrum of GdF3:2%Tb3+, the peaks at 412, 437, 467, 489, 542, 581, and 619 nm are ascribed to the transition of Tb3+.34,35 The quenching concentration of GdF3:Tb3+ is 2% as shown in the inset of Figure 3b.The concentration quenching mechanism for Tb3+ is similar to that of Dy3+ ion. The PLE spectrum of GdF3:2%Sm3+ product monitored at 592 nm consists of an absorption peak of Gd3+ ions at 271 nm and some absorption peaks of Sm3+ in the longer wavelength region. The presence of the excitation bands of Gd3+ indicates the existence of energy transfer from Gd3+ to Sm3+. The PL spectrum of GdF3:2%Sm3+ product excited at 271 nm exhibits three main groups of emission lines at about 558, 592, and 647 nm, which are ascribed to 4G5/2 → 6H5/2, 4G5/2 → 6H7/2, and 4 G5/2 → 6H9/2.36,37 The optimum concentration of Sm3+ is 2% in GdF3:Sm3+ (inset of Figure 3c). The concentration quenching mechanism for Sm3+ is also similar to that of Dy3+ ion. The PLE spectra of GdF 3 :Tb 3+ , GdF 3 :Sm 3+ , and GdF3:Tb3+,Sm3+ phosphors at different emission wavelengths are exhibited in Figure 4. The PLE spectra of GdF3:Tb3+ and GdF3:Tb3+,Sm3+ by monitoring the Tb3+ emission wavelength at 541 nm consists of a series of lines at about 339, 349, and 376 nm, which are ascribed to the7F6 → 5G2, 7F6 → 5D2, and 7 F6 → 5D3transitions of Tb3+, respectively. The PLE spectra of GdF3:Sm3+ and GdF3:Tb3+,Sm3+ by monitoring the Sm3+ emission wavelength at 598 nm have 6H5/2 → 4D5/2 (371 nm), 6H5/2 → 4K11/2 (397 nm), 6H5/2 → 4F5/2 (463 nm), and 6 H5/2 → 4I11/2 (475 nm) transitions of Sm3+. Notably, the excitation position of Tb3+7F6 → 5D3 (376 nm) and the Sm3+6H5/2 → 4D5/2 (371 nm) transitions are very close. In view

Figure 9. Dependence of I0/I of Tb3+ on (a) CTb+Sm6/3 × 103, (b) CTb+Sm8/3 × 104, and (c) CTb+Sm10/3 × 105.

existing energy transfer from Gd3+ to Dy3+ in GdF3 host. At the excitation of 271 nm, the PL spectrum exhibits blue and yellow emission bands attributed to the 4F9/2 → 6H15/2 and 4F9/2 → 6 H13/2 transitions of Dy3+.32 The strong emission ascribed to the 4F9/2 → 6H15/2 magnetic dipole transition (475 nm), is about 2 times stronger than that of the electric dipole transition 4 F9/2 → 6H13/2 (570 nm), indicating that Dy3+ ions are positioned at the sites inversion symmetry in the GdF3 host. It can be seen that the emission intensity at 475 nm initially increases with the increase of Dy3+ doping concentration and reaches the maximum at 2%, then gradually decreases with the further increase in Dy3+ concentration. Below this value (2%), the emission intensity is weak because there are no sufficient luminescent centers. Above this value, the luminescent intensity decreases due to the concentration quenching effect based on

Figure 10. Energy transfer scheme inGdF3:Tb3+,Sm3+. J


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Figure 11. CIE chromaticity diagram for GdF3:Tb3+,Sm3+ and GdF3:Dy3+,Tb3+,Sm3+ samples.

of the synchronous emissions of these two ions, it is believed that the near-UV wavelength around 371 nm might be used to efficiently excite these codoped nanoparticles, which properly fits the requirements for WLEDs.24 Moreover, the excitation intensity of 371 nm in the GdF3:2%Tb3+,2%Sm3+ phosphors monitored with 598 nm is stronger than the excitation peak of 371 nm in the GdF3:2%Sm3+ phosphors when monitored with 598 nm. More importantly, the Tb3+ and Sm3+ codoped GdF3 phosphors show a weak excitation peak (7F6 → 5D3, 376 nm) compared with the only Tb3+ doped GdF3 phosphors, which indicates that the energy transfer from the Tb3+ to Sm3+ is expected. Generally speaking, one of the essential conditions commonly required for energy transfer is the overlap between the emission spectrum of sensitizer and the excitation spectrum of activator.38 As shown in Figure 5, the comparison of the PL spectrum of GdF3:Tb3+ and the PLE spectrum of GdF3:Sm3+

reveals a significant spectral overlap between the emission of Tb3+ (5D4 → 7F6, 488 nm) and the excitation of Sm3+ (6H5/2 → 4 I11/2, 475 nm). Therefore, an effective resonance-type energy transfer from Tb3+ to Sm3+ in a GdF3 host is expected. Taking into consideration the possible existence energy transfer between Tb3+ and Sm3+ ions, a series of GdF3:2% Tb3+,x%Sm3+ (x = 0, 0.25, 0.5, 1, 2, 3, 4, 5, and 6) and GdF3:y% Tb3+,2%Sm3+ (y = 0, 1, 2, 3, and 4) samples have been synthesized. The PL spectra of GdF3:2%Tb3+,x%Sm3+ (x = 0, 0.25, 0.5, 1, 2, 3, 4, 5, 6) and GdF3:y%Tb3+,2%Sm3+ (y = 0, 1, 2, 3, 4) samples are given in Figure 6a,c, respectively. From Figure 6a,c, one can see that the PL spectrum simultaneously contains the 487 nm (5D4 → 7F6), 541 nm (5D4 → 7F5), 586 nm (5D4 → 7 F4), and 618 nm (5D4 → 7F3) of Tb3+ and the 557 nm (4G5/2 → 6H5/2), 598 nm (4G5/2 → 6H7/2), and 648 nm (4G5/2 → 6 H9/2) of Sm3+.21,39 As shown in Figure 6b, the emission intensities of Tb3+ ions are gradually decreasing, although the K


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nm. As shown in Figure 7a,c, we can observe the characteristics of both Tb3+ and Sm3+ ions. It is noteworthy that the emitting intensities of Tb3+ and Sm3+ ions vary with the doped concentrations of Tb3+ and Sm3+ ions. This is beneficial to obtain the multicolor emitting. From Figure 7b, the intensities of Tb3+ ions continue to decrease with increasing Sm3+ ions concentration, while the emitting intensities of Sm3+ ions first increase and then decrease with increasing Sm3+ ions concentration. In addition, as shown in Figure 7d, the intensities of Tb3+ and Sm3+ increase at first and then decrease when the concentration of Tb3+ ion is gradually increasing. The conclusions further support the results that different color hues of green emission can be realized by modulating the content of Tb3+ and Sm3+ ions when excited by a single excitation wavelength. To better demonstrate the energy transfer phenomena from Tb3+ to Sm3+, the fluorescent decay curves of Tb3+ ion emission in the GdF3:2%Tb3+,x%Sm3+(x = 0, 0.25, 0.5, 1, 2, 3, 4, and 5) samples monitored at 541 nm with irradiation of 371 nm were measured. As shown in Figure 8, the luminescent decay times of Tb3+ in GdF3:2%Tb3+,x%Sm3+ can be fitted well via a single exponential function such as40

Table 5. CIE Chromaticity Coordinates for GdF3:Dy3+,Tb3+,Sm3+ Samples labels

samples

excitation (nm)

1 9 2 10 3 11 4 12 5 13 6 14 7 15 8 16 20 17 21 22 18 23 19 24 25 26 27 28 29 30 31 32 33 34

GdF3:0.02Tb3+ GdF3:0.02Tb3+ GdF3:0.02Tb3+,0.0025Sm3+ GdF3:0.02Tb3+,0.0025Sm3+ GdF3:0.02Tb3+,0.005Sm3+ GdF3:0.02Tb3+,0.005Sm3+ GdF3:0.02Tb3+,0.01Sm3+ GdF3:0.02Tb3+,0.01Sm3+ GdF3:0.02Tb3+,0.03Sm3+ GdF3:0.02Tb3+,0.03Sm3+ GdF3:0.02Tb3+,0.04Sm3+ GdF3:0.02Tb3+,0.04Sm3+ GdF3:0.02Tb3+,0.05Sm3+ GdF3:0.02Tb3+,0.05Sm3+ GdF3:0.02Tb3+,0.06Sm3+ GdF3:0.02Sm3+,0.01Tb3+ GdF3:0.02Sm3+,0.01Tb3+ GdF3:0.02Sm3+,0.02Tb3+ GdF3:0.02Sm3+,0.02Tb3+ GdF3:0.02Sm3+,0.03Tb3+ GdF3:0.02Sm3+,0.04Tb3+ GdF3:0.02Sm3+,0.04Tb3+ GdF3:0.02Sm3+,0.05Tb3+ GdF3:0.02Dy3+,0.02Tb3+,0.005Sm3+ GdF3:0.02Dy3+,0.02Tb3+,0.01Sm3+ GdF3:0.02Dy3+,0.02Tb3+,0.02Sm3+ GdF3:0.02Dy3+,0.02Tb3+,0.03Sm3+ GdF3:0.02Dy3+,0.02Tb3+,0.04Sm3+ GdF3:0.02Dy3+,0.02Tb3+,0.05Sm3+ GdF3:0.02Dy3+,0.02Tb3+,0.02Sm3+ GdF3:0.02Dy3+,0.02Tb3+,0.02Sm3+ GdF3:0.02Dy3+,0.02Tb3+,0.02Sm3+ GdF3:0.02Dy3+,0.02Tb3+,0.02Sm3+ GdF3:0.02Dy3+,0.02Tb3+,0.02Sm3+

271 371 271 371 271 371 271 371 271 371 271 371 271 371 271 271 371 271 371 371 271 371 271 371 371 371 371 371 371 341 357 399 463 475

CIE (x, y) (0.287, (0.260, (0.304, (0.317, (0.313, (0.327, (0.324, (0.351, (0.308, (0.337, (0.294, (0.326, (0.278, (0.292, (0.270, (0.303, (0.338, (0.321, (0.359, (0.349, (0.324, (0.352, (0.330, (0.263, (0.290, (0.241, (0.277, (0.265, (0.251, (0.280, (0.292, (0.371, (0.392, (0.442,

0.513) 0.417) 0.512) 0.417) 0.504) 0.388) 0.496) 0.381) 0.435) 0.336) 0.403) 0.403) 0.363) 0.274) 0.346) 0.402) 0.316) 0.460) 0.370) 0.377) 0.478) 0.397) 0.485) 0.358) 0.403) 0.262) 0.320) 0.285) 0.254) 0.346) 0.354) 0.380) 0.479) 0.531)

I = I 0 + A e −t / τ

(4)

where I and I0 are the luminescence intensities at times t and 0, respectively, and τ is the luminescence lifetime. From Figure 8, the corresponding luminescence decay times of Tb3+ in GdF3:2%Tb3+,x%Sm3+(x = 0, 0.25, 0.5, 1, 2, 3, 4, and 5) are determined to be 8.02, 7.31, 7.02, 6.29, 5.17, 4.30, 3.45, and 2.64 ms, respectively. The lifetimes for Tb3+ ions were found to drastically decrease with increasing Sm3+ concentration. The decrease in the fluorescence decay cure further demonstrated that the existing energy transfer from Tb3+ to Sm3+. Based on Dexter’s energy transfer formula of multipolar interaction and Reisfeld’s approximation, the following equation can be used to analyze the potential mechanism:41,42 IS0/IS ∝ C n /3

(5) 3+

where IS0 is the intrinsic luminescence intensity of Tb and IS is the luminescence intensity of Tb3+ in the presence of the Sm3+. C is the sum of the content of Tb3+ and Sm3+; n= 6, 8, and 10 correspond to dipole−dipole, dipole−quadrupole, and quadrupole−quadrupole interactions, respectively. Figure 9 has shown the plots of IS0/IS of Tb3+ on n = 6, 8, and 10. From Figure 9, a line relation is well-fitted at n = 6, indicating that the energy transfer mechanism is via dipole−dipole interaction between the Tb3+ and Sm3+ ions. The process of energy transfer is shown in the Figure 10. During the whole excitation process, the Gd3+ ions absorb the photon energies of UV/n-UV light first. Then, the energy transfer processes occur from Gd3+ groups to Tb3+ and Sm3+ ions. The as-prepared GdF3:Tb 3+ samples exhibit two characteristic emission bands, whose positions are at 487 and 542 nm, corresponding to blue and green light. The asprepared GdF3:Sm3+ samples exhibit red light, so the whitelight-emission can be observed in the GdF3:Tb3+,Sm3+ sample. The second part is the energy transfer from Tb3+ ions to Sm3+ ions. Under the excitation at 371 nm, electrons are excited from the ground to the 5D3 level. Then, nonradiative transitions occur, and the 5D4 level is populated. Thus, the transitions from 5 D4 to 7F6, 7F5, 7F4, and 7F3 can be observed. At the same time, the 4G5/2 levels of Sm3+also can be populated via the energy

concentration of Tb3+ ions is fixed at 2%. While the intensities of Sm3+ ions increase until a maximum is reached at the concentration of around 1% and then decrease with increasing Sm3+ concentration. From Figure 6d, one can see that with the increasing Tb3+ ions concentration, the emission intensities of the Tb3+ first increase to an optimum at 2% and then decrease, which is attributed to the cross relaxation from 5D3 to 5D4 of Tb3+: 5D3 + 7F6 → 5D4 + 7F0. Although the concentration of Sm3+ ions is fixed at 2%, the emission intensities of Sm3+ ions first increase and then decrease. The concentration quenching of the Sm3+ emission is mainly due to the cross relaxation between neighboring Sm3+ ions which are in resonance of their energy levels due to the Sm3+ (4G5/2) + Sm3+(6H5/2) → Sm3+ (6F9/2) + Sm3+ (6F9/2) transitions. The above-mentioned phenomenon powerfully demonstrated that the occurrence of the energy transfer from Tb3+ to Sm3+ ions. Taking into consideration the characteristic emissions of terbium (Tb3+) and samarium (Sm3+) ions, it is expected that the different color hues of green emissions could be achieved in the GdF3 system by properly designed activator contents. Therefore, Figure 7a,c, respectively, gives the PL spectrum of GdF3:2%Tb3+,x%Sm3+ (x = 0, 0.25, 0.5, 1, 2, 3, 4, 5, and 6) and GdF3:y%Tb3+,2%Sm3+ (y = 0, 1, 2, 3, 4) samples excited at 271 L



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transfer process from Tb3+ to Sm3+. The characterized red emission of Sm3+ ions can be observed. RE-doped GdF3 samples not only have energy transfer processes but also realize the adjustment of the emitting light color. Figure 11 gives the Commission Internationale de L’Eclairage (CIE) chromaticity coordinates of GdF3:Tb3+,Sm3+ and GdF3:Dy3+,Tb3+,Sm3+ samples. From Figure 11A (points 1−8), it is seen that the colors of the GdF3:2%Tb3+,x%Sm3+ (x = 0, 0.25, 0.5, 1, 2, 3, 4, 5, and 6) phosphors under excitation at 271 nm shift gradually from the green region (0.287, 0.513) to the yellowish-green region (0.324, 0.496) and eventually to the bluish-green region (0.270, 0.346) with an increase of the doping content of Sm3+, confirming that their emission colors are tunable. Upon excitation with 371 nm UV light, the colors of the GdF3:2%Tb3+,x%Sm3+ (x = 0, 0.25, 0.5, 1, 2, 3, 4, 5, 6) phosphors shift gradually from the green region (0.260, 0.417) to the warm white region (0.337, 0.336) and eventually to the cool white region (0.292, 0.274) with an increase of the doping content of Sm3+, as presented in Figure 10A (points 9−15). We also give the CIE chromaticity diagram of GdF3:y%Tb3+,2% Sm3+excited at 271 nm (points 16−19) and that excited at 371 nm (points 20−23) in Figure 11B. From the above discussion, we know that only white and green light are obtained when Tb3+ and Sm3+ ions are doped in the GdF3. In order to obtain more colors of light, Dy3+ ions are doped into GdF3:Tb3+,Sm3+. Figure 11C (points 24−29) also presents the CIE diagram of GdF3:2%Dy3+,2%Tb3+,z%Sm3+ (z = 0.5, 1, 2, 3, 4, and 5) phosphors excited at 371 nm. In Figure 11C (points 30−34), the CIE diagram of GdF3:2%Dy3+,2%Tb3+,2%Sm3+ under different excitation wavelengths (341, 357, 399, 463, and 475 nm) are also given. The CIE chromaticity coordinates for points 30−34, respectively, were determined to be (0.80, 0.346), (0.292, 0.354), (0.371, 0.380), (0.392, 0.479), and (0.442, 0.531), which are located in green, yellow-green, white, light yellow, and yellow regions, respectively. Finally, the specific CIE chromaticity coordinates of GdF3:Dy3+,Tb3+,Sm3+ samples are shown in the Table 5.

ACKNOWLEDGMENTS This present work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51272085 and 21671078), and the Opening Research Funds Projects of the State Key Laboratory of Inorganic Synthesis and Preparative Chemistry and College of Chemistry, Jilin University (2016-06). Project Supported by Graduate Innovation Fund of Jilin University (2016145 and 2016154).

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REFERENCES

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4. CONCLUSIONS In summary, we proposed to synthesize the GdF3:Dy3+,Tb3+,Sm3+ by a facile hydrothermal method. The influence of the pH values of the mixed solvents in the hydrothermal reaction system has been investigated in detail. In the GdF3:Tb3+,Sm3+ system, the energy transfer process from Tb3+ to Sm3+ was demonstrated to be resonant type via dipole−dipole mechanism. Most interestingly, the white light and multicolor light have been obtained in Tb3+ and Sm3+ coactivated or Dy3+, Tb3+, and Sm3+ tridoped GdF3 phosphors. The results show that the obtained nanophosphors have a promising application in display and lighting fields.

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AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: +86-0431-85155275. E-mail: [email protected]. ORCID

Zhan Shi: 0000-0001-9717-1487 Haifeng Zou: 0000-0002-1331-2738 Notes

The authors declare no competing financial interest. M


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