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Eu3+ phosphors with enhanced red emission via energy transfer
Author’s Accepted Manuscript The synthesis and luminescent properties of morphology-controlled Gd2O3:Dy3+/Eu3+ phosphors with enhanced red emission via energy transfer Bin Liu, Jinkai Li, Guangbin Duan, Qinggang Li, Zongming Liu
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S0022-2313(18)31041-X https://doi.org/10.1016/j.jlumin.2018.10.035 LUMIN15983
To appear in: Journal of Luminescence Received date: 14 June 2018 Revised date: 8 September 2018 Accepted date: 8 October 2018 Cite this article as: Bin Liu, Jinkai Li, Guangbin Duan, Qinggang Li and Zongming Liu, The synthesis and luminescent properties of morphologycontrolled Gd2O3:Dy3+/Eu3+ phosphors with enhanced red emission via energy transfer, Journal of Luminescence, https://doi.org/10.1016/j.jlumin.2018.10.035 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 galley proof before it is published in its final citable 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.
The synthesis and luminescent properties of morphology-controlled Gd2O3:Dy3+/Eu3+ phosphors with enhanced red emission via energy transfer Bin Liu, Jinkai Li*, Guangbin Duan, Qinggang Li, Zongming Liu*
School of Materials Science and Engineering, University of Jinan, Jinan, Shandong 250022, China
[email protected] [email protected]
*
Corresponding authors. University of Jinan Jinan, China Tel.: +86-531-82765894
Abstract: The red phosphors of Gd2O3:Dy3+/Eu3+ system with nanorod and nanotube morphologies, have been obtained by calcining the precursor in the temperature range of 600-1300 oC for 4 h, and the precursors are synthesized via the hydrothermal method (pH=8.0-12.0, hydrothermal temperature: 120-180
o
C). The phase evolution, controlled-morphology and fluorescent properties were
systematically discussed by the various instruments of X-ray diffraction (XRD), field emission scanning
electron
microscope
(FE-SEM),
photoluminescence
excitation
(PLE)
and
photoluminescence (PL) spectroscopy and fluorescence decay analysis. By changing the synthesis pH and temperature, the phosphor morphology and size could be controlled, and the formation mechanism was analyzed together. The PL spectra of (Gd0.992-xDy0.008Eux)2O3 phosphors shows the strongest red emission (5D0→7F2 transition of Eu3+) under ~275 nm wavelength excitation (8S7/2→6IJ intra f-f transition of Gd3+). The color coordinates of (Gd0.992-xDy0.008Eux)2O3 phosphors were (~0.65, ~0.34), and thus emit the vivid red color emission. The intensity of Eu3+ emission increased with the calcined temperature (600-1300 oC) and Eu3+ content increasing to 4.0 at% (x=0.04), while the
intensity of Dy3+ emission steadily decreased proving the indirectly Dy3+→Eu3+ energy transfer. Compared to the Y2O3 or Gd2O3 doped singly with Eu3+ system, the Gd2O3:Dy3+/Eu3+ phosphor developed in this work displays the much higher red emission at 611 nm (5D0→7F2 transition of Eu3+) ascribed to the efficient Dy3+→Eu3+, Gd3+→Eu3+, Gd3+→Dy3+→Eu3+ energy transfer, and the former Dy3+→Eu3+ energy transfer efficiency was calculated. The luminescent properties (PLE/PL intensity, lifetime, quantum yield, etc.) are strongly dependent on the phosphor morphology, and the related discussion is performed in this work. The Gd2O3:Dy3+/Eu3+ phosphors with changed morphology, enhanced red emission and good thermal stability are expected to be widely used in light and displays area.
Keywords: hydrothermal method; Gd2O3:Dy3+/Eu3+ phosphor; luminescent property; energy transfer
1. Introduction The cubic-structured Ln2O3 (space group: Ia-3, Ln=La-Lu, and Y) oxides have been become one of the most important inorganic materials. Among which the Gd2O3 may be the best known due to its good physical and chemical properties that it is one of the widely studied as host matrix [1-3]. The Gd3+ in Gd2O3 can be easily replaced by activator ions, and thus displays colorful emission under ultraviolet (UV) light excitation [3-5]. Eu3+ and Dy3+ ions doped Gd2O3 matrix emit vivid red and yellow emission, respectively, which are widely used in field emission displays (FED), white light emitting diode (w-LED), cathode-ray tubes (CRT), electroluminescent (EL), plasma display panels (PDP), and so forth [5-10]. Gd2O3:Dy3+/Eu3+ solid solution was chosen according to the following main reasons: (1) the radius of Eu3+ (1.066 Å) and Dy3+ (1.027 Å) ions are similar to Gd3+ (1.053 Å) ion, Dy3+/Eu3+ codoped
Gd2O3 can decrease the lattice deformation compared to Y2O3 system [11]; (2) the smaller electronegativity of Gd3+ (1.20) than that of Y3+ (1.22) may result in an easier charge transfer (CT), and thus higher emission intensities [12]; (3) the Gd3+→Eu3+ and Dy3+→Eu3+ energy transfers reported widely can further enhance the red emission of Eu3+ in Gd2O3 system developed in this work and are also experimentally demonstrated in this paper [13-17]. Of course, the Gd3+→Dy3+→Eu3+ should not be neglected; (4) It is reported that the crystal structure change of Gd2O3 from cubic to monoclinic occurs above 1250 oC. The smaller Dy3+ addition to form (Gd,Dy)2O3 solid solutions can stabilize the crystal structure of Gd2O3 and increase the transition temperature, thus the development of Gd2O3:Dy3+/Eu3+ system is feasible [12]; (5) the particle size and morphology strongly affect the luminescent properties of phosphor which in turn relies on the synthesis route used [18,19]. The particle size and morphology have been controlled by the hydrothermal method through adjusting the reaction temperature and pH values in this work, and their effects on the luminescent properties of phosphor have been discussed in detail. In this paper, the Gd2O3:Dy3+/Eu3+ phosphors with controlled morphology have been successfully synthesized by hydrothermal method at different reaction pH and temperatures [20]. The phase evolution, morphology, fluorescent properties, energy transfer were discussed by the combined techniques of XRD, FE-SEM, PLE/PL spectroscopy and fluorescence decay analysis. The fluorescent properties dependence on the morphology/size were analyzed in detail. In the following, we reported the synthesis process, phase evolution, morphology, luminescent properties and energy transfer of Gd2O3:Dy3+/Eu3+ phosphors. 2. Experiment procedure 2.1 Materials
The raw materials used in the synthesis process are Gd2O3 (99.99%, Huizhou Ruier Rare Chemical Hi-Tech Co. Ltd., Huizhou, China urea), Dy2O3 (99.99%, Huizhou Ruier Rare Chemical Hi-Tech Co. Ltd., Huizhou, China urea), Eu2O3 (99.999%, Huizhou Ruier Rare Chemical Hi-Tech Co. Ltd., Huizhou, China urea), ethylene glycol (EG, HOCH2CH2OH, >99%, Tianjin City Fuyu Fine Chemical Co. Ltd., Tianjin, China), and nitric acid (AR, Sinopharm Chemical Reagent Co. Ltd., Shanghai, China). All reagents need not to be purified and can be directly used in the study. 2.2. Preparation procedure The Gd2O3, Dy2O3 and Eu2O3 were dissolved in hot nitric acid to obtain the rare earth nitrates RE(NO3)3 (RE=Gd, Dy and Eu). The Gd(NO3)3, Dy(NO3)3 and Eu(NO3)3 were mixed as the mother salts according to the chemical formula (Gd0.992-xDy0.008Eux)2O3, and dissolved in pure water with a total volume of 50 mL. The pH of the mixed mother salts were adjusted by ammonia in the range of 8-12. Then, the mixed solutions were transferred to reaction kettles, put in the drying oven, and reacted for 24 h at the settled temperature from 120 to 180 oC. After the completion of precipitation, the turbid liquid was centrifuged and washed repeatedly with distilled water and alcohol to remove by-products. The wet precipitate was dried in air at 80 oC for 24 h, and finally calcined at 1300 oC for 4 h with the heating speed of 5 oC/min to produce oxides. 2.3. Characterization The XRD patterns for phase analysis were collected at room temperature using nickel-filtered CuKα radiation in the 2θ range 10-50° at a scan speed of 4.0° 2θ/min (Model D8 ADVANCE, BRUKER Co., Germany). The morphology of the precursor and resultant products were collected via FE-SEM (QUANTA FEG 250, FEI Co., America) with an acceleration voltage of 10 kV. FT-IR spectroscopy was performed by the standard KBr method (Spectrum RXI, Perkin-Elmer, Shelton,
CT). The PLE and PL spectra were obtained using a Fluorescence Spectrophotometer (FP-6500, JASCO Co., Japan) at room temperature equipped with a Φ60-mm integrating sphere (ISF-513, JASCO, Tokyo, Japan) and a 150-W Xe-lamp as the excitation source. The optical performances for all samples were conducted under identical conditions with the slit breadth of 5 nm. The phosphor powder was excited with a selected wavelength and the intensity of the intended emission was recorded as a function of elapsed time after the excitation light was automatically cut-off using a shutter. 3. Results and discussion Fig. 1 shows the XRD patterns of (Gd0.992-xDy0.008Eux)2O3 oxides calcined at 1300 oC. From which it can be seen that the Dy3+ and Eu3+ additions do not alter the crystal structure. All samples have the similar XRD diffraction behavior and are indexed to Gd2O3 (space group: Ia-3, JCPDS No. 43-1014). Fig. 2a and 2b show the PLE spectra of (Gd0.992-xDy0.008Eux)2O3 phosphors monitoring at 572 nm and 611 nm emission, respectively. The phosphors are synthesized by pH=9, 140 oC, and calcined at 1300 oC for 4 h. Monitoring at 572 nm (Fig. 2a), the PLE spectra of (Gd0.992-xDy0.008Eux)2O3 contains both the Gd3+ and Dy3+ ions excitation bands. The occurrence of PLE bands of Gd3+ indirectly proved the Gd3+→Dy3+ energy transfer. The typical excitation bands of Gd3+ ions at 275 nm, 309 nm and 314 nm are assigned to 8S7/2→6IJ, 8S7/2→6P5/2 and 8S7/2→6P7/2 transition, respectively, and the first one (275 nm) is the strongest [21-24]. As marked in the figure, other bands were observed corresponding to the 6H15/2→6P3/2 of Dy3+ ions at 326 nm, 6H15/2→6P7/2 at 351 nm, 6H15/2→6P5/2 at 363 nm and 6H15/2→4P13/2 at 385 nm, respectively [21-24]. The PLE intensity gradually decreased with the Eu3+ content increasing due to the efficient Dy3+→Eu3+ energy transfer. Monitoring at 611
nm (Fig. 2b), the PLE spectra of (Gd0.992-xDy0.008Eux)2O3 phosphors exhibits strongest excitation band at 262 nm ascribed to a CTB (charge transfer band) [25,26], namely, the electronic transition from the 2p orbital of O2- to the 4f orbital of Eu3+ activators. The appearance of typical Gd3+ and Dy3+ transitions marked in the Fig. 2b provides pervasive evidence of the Gd3+→Eu3+, Dy3+→Eu3+ efficient energy transfer. Thus, both the Dy3+ and Eu3+ ions can be excited by the wavelength at 275 nm. Under the optimal 275 nm excitation and same synthesized condition, the PL spectra consists both Eu3+ and Dy3+ emission (Fig. 2c). The former emission bands of Eu3+ were at 580 nm (5D0→7F0), 592 nm (5D0→7F1), 645 nm (5D0→7F3) and 692 nm (5D0→7F4), respectively, among which the red emission at 611 nm (5D0→7F2) is the strongest [25,26]. The later emission band of Dy3+ at 572 nm was attributed to the 4F9/2→6H13/2 of Dy3+ with weak emission intensity. The emission intensity of Eu3+ increased with the Eu3+ doped up to x=0.04 (x=4.0 at%), and then decreased owing to the concentration quenching. The optimal total concentration of Dy3+ and Eu3+ (4.8 at%) is closed to the reported results of Y2O3:Eu3+ (5.0 at%) or Gd2O3:Eu3+ (5.0 at%) system [27,28]. Further observation is that the emission intensity of the optimal Gd2O3:Dy3+/Eu3+ sample developed in this work is higher than the Y2O3 or Gd2O3 doped singly with Eu3+ system ascribed to the efficient Dy3+→Eu3+, Gd3+→Eu3+, Gd3+→Dy3+→Eu3+ energy transfer. The internal (εin) and external (εex) quantum efficiencies of (Gd0.952Dy0.008Eu0.04)2O3 (x=0.04) yield 72.1% and 40.6%, which are higher than Y2O3 (εin=59.3%, εex=32.2%) or Gd2O3 (εin=65.2%, εex=36.3%) system [29,30]. From the inset of Fig. 2c, it can be seen that the PL band intensity of Dy3+ at 572 nm gradually decreased as a function of Eu3+ addition which further proved the Dy3+→Eu3+ energy transfer. Fig. 2d shows the Commission International de L'Eclairage (CIE) chromaticity coordinates for the
emission of the (Gd0.952Dy0.008Eu0.04)2O3 samples under 275 nm excitation. The phosphors (Gd0.992-xDy0.008Eux)2O3 were analyzed to have color coordinates (x,y) of (~0.62, ~0.33), (~0.65, ~0.34), (~0.62, ~0.35), (~0.64, ~0.33) and (~0.63, ~0.33) for x=0.02, 0.04, 0.06, 0.08, and 0.1, respectively. Thus, the (Gd0.992-xDy0.008Eux)2O3 samples emit vivid red color, as seen from the digital picture (the inset of Fig. 2d) under 254 nm UV excitation from a hand-held UV lamp. In order to further elucidate the energy transfer, fluorescence decay kinetics were studied for the 4
F9/2→6H13/2 emission of Dy3+ and the 5D0→7F2 emission of Eu3+ under 275 nm excitation, and then
the lifetime and energy transfer efficiency were calculated. Fig. 3a and Fig. 3b show the decay curves of (Gd0.952Dy0.008Eu0.04)2O3 phosphors for the Dy3+ and Eu3+ emissions, respectively. It was found that the decay data can be well fitted with the following single exponential equation in each case:
I = Aexp(-t / τ R ) + B
(1)
where the I, t, τR, A and B are referred to relative fluorescence intensity, the decay time, fluorescence lifetime and the constant of A and B, respectively. According to the formula (1), the fitting results are obtained as follows: A=373937.6±3386.9 (au), B=372.52±81.23 (au), τR =0.22±0.02 ms (Fig. 3a) and A=172304.5±968.70 (au), B=377.96±120.92 (au), τR =1.24±0.09 ms (Fig. 3b), respectively. The lifetime value of Dy3+ (0.22 ms) for 572 nm is shorter than Eu3+ (1.24 ms) for 611 nm emission. The insets of Fig. 3a and Fig. 3b can be seen that the lifetime for Dy3+ and Eu3+ slowly yet continuously decreased at a higher Eu3+ content. The former was ascribed to the efficient Dy3+→Eu3+ energy transfer, and the later was understandable as follows: when the content of Eu3+ is relatively low, the distance between Eu3+ is long, thus the interactions among Eu3+ can be neglected. The further increasing of Eu3+ content leads to the formation of a resonant energy transfer net among the activators which can be an additional channel to the non-radiative centers on particle surfaces.
The efficiency of energy transfer (ηET) can be estimated from the lifetime for Dy3+ in the presence (τS) and absence (τS0) of Eu3+ using the formula [31-34]: ηET = 1-
τS τ S0
(2)
The results of calculation are shown in Fig. 3c. It is seen that when the concentration of Eu 3+ increases from x=0.02 to x=0.10, while the ηET gradually increases from 19.3% to 89%. Thus, the sensitizer of Dy3+ plays an important role in the effective luminescence of Eu3+, while these high efficiencies of energy transfer primarily originate from the significant spectral overlapping between the 4F9/2→6HJ emissions of Dy3+ and the 7F0,1→5D0,1 absorption of Eu3+ [35]. It is now the time to discuss the energy processes involved in the emission of (Gd0.992-xDy0.008Eux)2O3 phosphors. The illustration of the energy transfer processes for the (Gd0.992-xDy0.008Eux)2O3 phosphors was shown in Fig. 3d. Exciting the (Gd0.992-xDy0.008Eux)2O3 phosphor at 275 nm, it raises electrons from the 8S7/2 ground states to the 6IJ excited states of Gd3+, and at the same time excites the 6H15/2 electrons of Dy3+ to the 4F3/2 states. Since the 6IJ and 6PJ states of Gd3+ lie higher than the 4F3/2,9/2 emission states of Dy3+ and the 5D0,1 states of Eu3+ in the energy diagram, the electrons of the excited Gd3+ may be non-radiatively decay to the 5D1 and 4F3/2 states of both the Eu3+ and Dy3+ activators via Gd3+→Eu3+ and Gd3+→Dy3+ energy transfer. Meanwhile, the 4
F3/2,9/2 states of Dy3+ lie above the 5D0,1 states of Eu3+ ions, and the 5D4→7FJ emissions of Dy3+
significantly overlap the 7FJ→5D0,1 absorptions of Eu3+ ions, thus the Dy3+ ions could transfer their energy to the Eu3+ ions partly or totally, and the Dy3+ activators may intercept the energy transferred from Gd3+ to quench the Gd3+→Eu3+ transfer route. It should be noted that the Gd3+→Dy3+→Eu3+ energy transfer process may also happen in the (Gd0.992-xDy0.008Eux)2O3 matrix material. Then the electrons of 4F9/2 (Dy3+) and 5D1 (Eu3+) excited states relaxed to the 4F9/2 (Dy3+) and 5D0 (Eu3+),
respectively. Back jumping of the 4F9/2 (Dy3+) and 5D0 (Eu3+) electrons to the 2H13/2 (4F9/2→2H13/2 transition of Dy3+) and 7F2 (5D0→7F2 transition of Eu3+) levels finally yield the yellow (572 nm) and red (611 nm) emission. The above results may thus imply that multi-channel energy transfers exist in the present (Gd0.992-xDy0.008Eux)2O3 phosphor system including Gd3+→Eu3+, Gd3+→Dy3+, Dy3+→Eu3+, Gd3+→Dy3+→Eu3+. According to Dexter’s and Reisfeld’s energy-transfer expression for exchange and multipolar interactions, the relation can be given as follows [36-40]:
0 )C
(3)
0 C n / 3
(4)
ln(
where the C is the total concentration of Dy3+ and Eu3+, the η0 and η are the luminescence quantum efficiencies of the Dy3+ sensitizer in the absence and presence of Eu3+, respectively. The η0/η ratio can be approximated as the luminescence intensity ratio IS0/IS. The ln(IS0/IS)∞C would correspond to exchange interaction and (IS0/IS)∞Cn/3 with n=6, 8 and 10 correspond to dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole interactions, respectively. The plots of ln(Is0/Is)-C as well as (Is0/Is)-Cn/3 are illustrated in Fig. 4, from which the best linear relationship was found for n=8 by comparing the fitting factor values (R2). This clearly indicates that the mechanism of Dy3+→Eu3+ energy transfer in the (Gd0.952Dy0.008Eu0.04)2O3 phosphor is ascribed to electric dipole-quadrupole interactions. Fig. 5 shows the shape/size of (Gd0.952Dy0.008Eu0.04)2O3 precursors as the function of synthesized pH values (pH=8-12). From which it can be seen that all the particles possess good dispersion and uniform morphology. However, the particle shape/size changed with the synthesized pH. The particle changed from tubular (Fig. 5a) to rod-like morphology (Fig. 5b) when the synthesized pH value
increased from pH=8 to pH=9. As the pH value further increased, the size of precursor with rod-like morphology gradually decreased. The decreased particle size is mainly due to the high nucleation density derived from high pH. Fig. 6a shows the XRD patterns of nanotube and nanorod precursors fixed the reaction temperature at 140 oC, respectively. No impurity was identified along with the hydroxide phase in each case, and the diffraction can be well indexed with those of the Gd(OH)3 (space group: Ia-3d, JCPDS No. 38-1042). Further observation is that the diffraction intensity ratio of lattice plane (110)/(101) are 83.2% and 149% for nanorod and nanotube, respectively. However, the (110)/(101) diffraction intensity ratio in standard Gd(OH)3 card is 70.8%. Thus, it can be deduced that the crystal plane distribution of nanorod and nanotube is different, and the former displays directional growth. Mass transfer rate is a major factor affecting the morphology of the precursor, the growth mechanism of (Gd0.952Dy0.008Eu0.04)2O3 precursor nanotube and nanorod is shown in Fig. 6b. At low pH, the mass transfer speed is different between the inter part and outer region, and the former is lower than the later which lead to the nanotube formation. While the mass transfer speed of inter part is closed to the outer region, thus these particles formed the nanorod morphology. This growth mechanism is similar to the ZnO reported by Li et al [41]. Fig. 7 displays the particle morphology dependence on the synthesized temperature. Regardless of pH value fixed at 8 or 9, the precursor particles grow gradually as the synthesized temperature increasing from 120 oC to 180 oC. When the pH value was fixed at 8.0, the diameter of nanotube increased up to 200 nm with the synthesized temperature increasing to 180 oC. While the length of nanorod increased up to 1μm fixed the pH value at 9.0, the top of the nanorod particles become prominent and exhibit directional growth. The particle growth is mainly because the mass transfer
rate increases with the increase of hydrothermal temperature, thus strengthens the force of crystal growth. In order to investigation the shape/size effect on the luminescent properties, the PLE/PL behaviors as a functional pH values (8-12) using the sample (Gd0.952Dy0.008Eu0.04)2O3 (hydrothermal temperature: 140 oC, calcined temperature: 1300 oC) as an example were investigated (Fig. 8a and Fig. 8b). It can be seen that the pH value variety does not alter the shape and position of PLE and PL peak, but the intensity firstly decreased when the pH value increased from 8.0 to 9.0, and then increased gradually with the pH value increasing from 9.0 to 12.0 (Fig. 8c). This can be explained as follows: comparing to sample (pH=8.0) with nanotube morphology, the phosphors (pH=9.0) with nanorod morphology presented directional growth as seen from Fig. 5. Owing to the directional growth, the electronic transitions in other directions is limited, and the electric dipole transition probabilities of Eu3+ reduced, thus lead to the decreased fluorescent intensity. The pH value continually increased up to pH=12, the particle size decreased and the surface area increased, thus the number of luminescent centers on the particle surface increased resulting in the enhanced emission intensity. Fig. 8d is the lifetime of the 611 nm emission plotted against the synthesized pH value. A significant lifetime extended from 0.87 to 1.68 ms were observed from pH=8 to pH=12. The lifetime of the sample synthesized with pH=9 is close to that of the Y2O3:Eu3+ (1.39 ms) or Gd2O3:Eu3+ (1.32 ms) powder synthesized by the same hydrothermal method [29,30]. In some Eu3+ activated oxides, the lifetime decreased with the surface area of particle increasing, owing to the nonradiative transition rate increasing due to the enhancement of defects [42]. However, in other oxides, the lifetime decreases at a smaller surface area observed our present work. The increased lifetime with the pH value increasing can be arising from the effective refractive index which can be
expressed by the equation (5) [43]:
02 1 R ~ f ( ED) [ 1 (n 2 2)]2 n eff eff 3
(5)
where the f(ED) is the oscillator strength for dipole transition and λ0 the wavelength in vacuum. The neff is the effective refractive index and neff=nc·x+(1-x)·nmed, where x is the filling factor, nc and nmed are the refractive indices of the bulk material and the surrounding medium, respectively. For the intermediately sized particles of our samples, neff is affected by particle size and decreases for smaller particles. This would explain the increasing lifetime at a higher synthesized pH value. The effects of lattice defects on lifetime, however, can by no means be completely excluded. Deep traps are known to be able to temporarily arrest electrons and thus lead to longer lifetimes. Detailed defect analysis is underway. Fig. 9 and Fig. 10 show the hydrothermal temperature effects on the luminescent properties of samples (Gd0.952Dy0.008Eu0.04)2O3 fixed pH values of 8.0 and 9.0, respectively (hydrothermal temperature: 120-180 oC, calcined temperature: 1300 oC). The PLE/PL intensity of phosphors increased when the synthesized pH value was fixed at 8.0 while decreased for synthesized pH value of 9.0. The former increased luminescent intensity was mainly because the surface area of (Gd0.952Dy0.008Eu0.04)2O3 phosphors with tube morphology (Figs. 7a-7d) increased from 7.61 m2/g to 13.93 m2/g when the synthesized temperature increased from 120 to 180 oC. This leads to more luminescent center on the particle surface resulting in the increased emission intensity. The later was due to the directional growth of particle (Figs. 7e-7h) with the temperature increasing. This can reduce the electric dipole transition probabilities of the surrounding crystal field, and thus decreased fluorescent intensity was achieved. Regardless of the pH fixed at 8.0 or 9.0, the lifetime of samples is
almost the same as function of synthesized temperature, and the former sample (pH=8.0) is a little shorter than the later samples (pH=9.0). The lifetime may originate from defects or particle shape/size or the combined effects of the two [44,45]. Both the size of particle with nanotube and nanorod morphology increased as a function of synthesized temperature which lead to the increased neff in equation (5), thus the phosphors lifetime decreased. However, the lattice defects of particle surface (pH=8.0) would be increased due to the increased surface areas shown in Figs. 7a-7d as the synthesized temperature increasing. The directional growth shown in Figs. 7e-7h reduces the probability of electronic transitions. The increased defects and low electric dipole transition probabilities could increase the phosphor lifetime. Combination effects of the neff, lattice defect and electric dipole transition probabilities caused the unchanged phosphor lifetime. Fig. 11 shows the calcined temperatures effects on the morphology of (Gd0.952Dy0.008Eu0.04)2O3 particle (pH=8.0, hydrothermal temperature: 180 oC, calcined temperature: 600-1300 oC). Form which it can be seen that the particles gradually grow, still persist the tubular shape and good dispersion even the calcining temperatures increased up to 1300 oC (Figs. 11a-11e). Fig. 12a and Fig. 12b show the PLE and PL behaviors of the (Gd0.952Dy0.008Eu0.04)2O3 samples as the function of the calcining temperatures, respectively (pH=8.0, hydrothermal temperature: 180 oC, calcined temperature: 600-1300 oC). The calcining temperature does not change the shape and position of the emission peak of the Eu3+. However, the PLE/PL intensities increase gradually with temperatures increasing. This is mainly due to the improved crystallinity (Figs. 11a-11e) and reduced lattice defects. Compare to the PLE/PL spectra of (Gd0.952Dy0.008Eu0.04)2O3 phosphor, the excitation spectra of (Gd0.952Dy0.008Eu0.04)(OH)3 precursor (pH=8.0, hydrothermal temperature: 180
o
C)
contains only one sharp peak at 300 nm (8S7/2→6PJ) belong to the typical band of Gd3+ (Fig. 12a).
Under 300 nm wavelength excitation, the PL spectra of (Gd0.952Dy0.008Eu0.04)(OH)3 precursor was dominated by 592 nm (5D0→7F1) rather than 611 nm (5D0→7F2), ascribed to the relatively high symmetry of Gd3+ in Gd(OH)3 (the inset of Fig. 12b). As for phosphor, the thermal stability is very important for its potential application. The temperature effects on emission intensity was studied with the range of 298-573 K, and the activation energy of thermal quenching was determined in this work. The results are presented in Fig. 13a. The shape and location of characteristic peaks of (Gd0.952Dy0.008Eu0.04)2O3 (pH=8.0, hydrothermal temperature: 180 oC, calcined temperature: 1300 oC) do not be changed. However, the emission intensity decreased with the temperature increased gradually. The reason can be explained that the thermal activation at a certain intersection of the ground state and the excited state causes thermal quenching, leading to the intensity decreased. In order to further explore the impact of temperature on thermal quenching, the explanation can be acquired by Arrhenius equation:
ln(
I0 E 1) ln A a I kT
(6)
where Ea, T, A and k refer to activation energy, temperature (K), constant and Boltzmann constant, respectively, the I0 and I represent the emission intensity at room temperature and differently operated temperature, respectively. The relationship between ln(I0-I/I) and 1/kT for the thermal quenching is depicted in Fig. 13b. From which it can be seen that the slope of fitting curve is -0.2118, and thus, the Ea can be calculated to be 0.2118 eV being closed to the reported value of 0.2397 eV for Gd2O3:Tb3+/Eu3+ system [46]. The higher activation energy indicates that the phosphor has good thermal stability and is expected to be widely used in lighting and display areas.
Conclusions
In this paper, Gd2O3 codoped with Dy3+/Eu3+ phosphors were successfully obtained through hydrothermal method, followed by calcination at 1300 oC. The combination techniques of XRD, FE-SEM, PLE/PL, fluorescence decay and quantum efficiency on (Gd0.992-xDy0.008Eux)2O3 phosphors have been performed. The conclusions can be summarized as follows: (1) The controlled morphology of the resultant phosphors can be achieved by adjusting the synthesized pH and hydrothermal temperatures. The particles changed from nanotube to nanorod with the pH value increasing from 8.0 to 9.0. The particle grows gradually as the synthesis temperatures increasing while the particle size decreased as the pH value increasing. The formation mechanism of phosphor with controlled morphology has been discussed in detail by analyzing the mass transfer and nuclear formation rate; (2) The (Gd0.992-xDy0.008Eux)2O3 phosphors display strong red emission at 611 nm ascribed to the 5
D0→7F2 transition of Eu3+. The optimal doped concentration of Eu3+ was determined to be 4.0%
(x=0.04). Due to the efficient Dy3+→Eu3+, Gd3+→Eu3+, Gd3+→Dy3+→Eu3+ energy transfer, the luminescent properties of the optimal Gd2O3:Dy3+/Eu3+ sample developed in this work are better than the Y2O3 or Gd2O3 doped singly with Eu3+ system. The energy transfer efficiency, mechanism, process and thermal stability were analyzed in detail; (3) The luminescent properties (PLE/PL, lifetime, etc.) of resultant phosphors are strongly dependent on the calcined temperature, particle shape, size, surface defect, disorder degree and symmetry. These above factors effects on luminescent properties have been studied.
Acknowledgements This work was supported in part by the National Natural Science Foundation of China (No.
51402125), China Postdoctoral Science Foundation (No. 2017M612175), the Research Fund for the Doctoral Program of University of Jinan (No. XBS1447), the Natural Science Foundation of University of Jinan (No. XKY1515), the Science Foundation for Post Doctorate Research from the University of Jinan (No. XBH1607), the Special Fund of Postdoctoral innovation project in Shandong province (No. 201603061).
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Fig. 1 the XRD patterns of (Gd0.992-xDy0.008Eux)2O3 oxides calcined at 1300 oC as a function of Eu3+ content are shown in Fig. 1 (pH=9, 140 oC). Fig. 2 a comparison of the PLE behaviors of the (Gd0.992-xDy0.008Eux)2O3 phosphors calcined at 1300 o
C, with parts (a) and (b) for the 572 nm emission of Dy3+ and the 611 nm emission of Eu3+,
respectively. (c) shows the emission spectra of the (Gd0.992-xDy0.008Eux)2O3 phosphors under 275 nm excitation. The Y2O3 and Gd2O3 were included for comparison. (d) is the CIE chromaticity diagram for the emission of (Gd0.992-xDy0.008Eux)2O3 phosphors (x=0.02-0.1) under 275 nm excitation. The inset in (c) is the expanded emission peak of Dy3+ at 572 nm. The inset in (d) shows the appearances of luminescence for the x=0.04 sample under 254 nm excitation from a handheld UV lamp. Fig. 3 the decay curves of Dy3+ and Eu3+ emission monitored at 572 nm (a) and 611 nm (b) for the phosphors (Gd0.952Dy0.008Eu0.04)2O3 under excitation at 275 nm. The insets of Fig. 3a and 3b are the fluorescence lifetime of (Gd0.992-xDy0.008Eux)2O3 with Eu3+ contents (x=0.02-0.10) for the Dy3+ (572 nm) and Eu3+ (611 nm) emissions, respectively. Fig. 3c shows the energy transfer efficiency of the activators as a function of the Eu3+ content. Fig. 3d illustrates the energy transfer processes for the (Gd0.952Dy0.008Eu0.04)2O3 phosphors. Fig. 4 dependence of ln(IS0/IS) on C3/3 (a) and (IS0/IS) on C6/3 (b), C8/3 (c), and C10/3 (d) for the Dy3+ emission, respectively. Fig. 5 the FE-SEM morphologies of (Gd0.952Dy0.008Eu0.04)2O3 precursors with synthesized pH values of 8 (a), 9 (b), 10(c), 11(d), 12(e), respectively. Fig. 6 the XRD patterns of nanotube (a) and nanorod (b) precursors under the hydrothermal pH value of 8.0 and 9.0 are shown in Fig. 6a, respectively. Fig. 6b illustrates the schematic diagram of growth mechanism of (Gd, Dy, Eu)(OH)3 precursor (140 oC). Fig. 7 (a-d) show the FE-SEM morphologies of (Gd0.952Dy0.008Eu0.04)2O3 precursors with synthesized temperatures of 120 oC (a), 140 oC (b), 160 oC (c), 180 oC (d), respectively (pH=8). (e-h) show the FE-SEM morphologies of (Gd0.952Dy0.008Eu0.04)2O3 precursors with synthesized temperatures of 120 o
C (e), 140 oC (f), 160 oC (g), 180 oC (h), respectively (pH=9). The insets in Fig. 7c and 7d are the
enlarged view of the FE-SEM morphology. Fig. 8 PLE (a, λem=611 nm) and PL (b, λex=275 nm) behaviors of the phosphors (140 oC) calcined at 1300 oC as a function of pH values. (c) is the relative Eu3+ emission intensity of 611 nm normalized
to the sample synthesized by pH=8. (d) shows the lifetime values of (Gd0.952Dy0.008Eu0.04)2O3 sample as a function of pH values. Fig. 9 PLE (a, λem=611 nm) and PL (b, λex=275 nm) behaviors of the phosphors (pH=8) calcined at 1300 oC as a function of synthesized temperature. (c) is the relative Eu3+ emission intensity of 611 nm normalized to the sample synthesized by 120 oC. (d) is the decay behavior of sample (Gd0.952Dy0.008Eu0.04)2O3. The inset in Fig. 9d shows the lifetime values of sample (Gd0.952Dy0.008Eu0.04)2O3 as a function of synthesized temperatures. Fig. 10 PLE (a, λem=611 nm) and PL (b, λex=275 nm) behaviors of the phosphors (pH=9) calcined at 1300 oC as a function of synthesized temperature. (c) is the relative Eu3+ emission intensity of 611 nm normalized to the sample synthesized by 120 oC. (d) shows the lifetime values of sample (Gd0.952Dy0.008Eu0.04)2O3 as a function of synthesized temperatures. Fig. 11 the FE-SEM morphologies of the (Gd0.952Dy0.008Eu0.04)2O3 phosphors with the calcining temperatures of 600 oC (a), 800 oC (b), 1100 oC (c), 1200 oC (d), 1300 oC (e) (pH=8, 180 oC), respectively. The insets in Figs. 11a-11e are the enlarged view of the FE-SEM morphology. Fig. 12 (a) and (b) are the PLE and PL behaviors of (Gd0.952Dy0.008Eu0.04)2O3 phosphors as a function of the calcining temperature, respectively, the precursor was also included. The inset of Fig. 12b shows the expanded emission peak of Eu3+ for (Gd0.952Dy0.008Eu0.04)(OH)3 precursor. Fig. 13 the temperature-dependence of PL intensity is shown in (a), and (b) displays the relationship between ln(I0/I-1) and 1/kT.
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PII: DOI: Reference:
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S0022-2313(18)31041-X https://doi.org/10.1016/j.jlumin.2018.10.035 LUMIN15983
To appear in: Journal of Luminescence Received date: 14 June 2018 Revised date: 8 September 2018 Accepted date: 8 October 2018 Cite this article as: Bin Liu, Jinkai Li, Guangbin Duan, Qinggang Li and Zongming Liu, The synthesis and luminescent properties of morphologycontrolled Gd2O3:Dy3+/Eu3+ phosphors with enhanced red emission via energy transfer, Journal of Luminescence, https://doi.org/10.1016/j.jlumin.2018.10.035 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 galley proof before it is published in its final citable 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.
The synthesis and luminescent properties of morphology-controlled Gd2O3:Dy3+/Eu3+ phosphors with enhanced red emission via energy transfer Bin Liu, Jinkai Li*, Guangbin Duan, Qinggang Li, Zongming Liu*
School of Materials Science and Engineering, University of Jinan, Jinan, Shandong 250022, China
[email protected] [email protected]
*
Corresponding authors. University of Jinan Jinan, China Tel.: +86-531-82765894
Abstract: The red phosphors of Gd2O3:Dy3+/Eu3+ system with nanorod and nanotube morphologies, have been obtained by calcining the precursor in the temperature range of 600-1300 oC for 4 h, and the precursors are synthesized via the hydrothermal method (pH=8.0-12.0, hydrothermal temperature: 120-180
o
C). The phase evolution, controlled-morphology and fluorescent properties were
systematically discussed by the various instruments of X-ray diffraction (XRD), field emission scanning
electron
microscope
(FE-SEM),
photoluminescence
excitation
(PLE)
and
photoluminescence (PL) spectroscopy and fluorescence decay analysis. By changing the synthesis pH and temperature, the phosphor morphology and size could be controlled, and the formation mechanism was analyzed together. The PL spectra of (Gd0.992-xDy0.008Eux)2O3 phosphors shows the strongest red emission (5D0→7F2 transition of Eu3+) under ~275 nm wavelength excitation (8S7/2→6IJ intra f-f transition of Gd3+). The color coordinates of (Gd0.992-xDy0.008Eux)2O3 phosphors were (~0.65, ~0.34), and thus emit the vivid red color emission. The intensity of Eu3+ emission increased with the calcined temperature (600-1300 oC) and Eu3+ content increasing to 4.0 at% (x=0.04), while the
intensity of Dy3+ emission steadily decreased proving the indirectly Dy3+→Eu3+ energy transfer. Compared to the Y2O3 or Gd2O3 doped singly with Eu3+ system, the Gd2O3:Dy3+/Eu3+ phosphor developed in this work displays the much higher red emission at 611 nm (5D0→7F2 transition of Eu3+) ascribed to the efficient Dy3+→Eu3+, Gd3+→Eu3+, Gd3+→Dy3+→Eu3+ energy transfer, and the former Dy3+→Eu3+ energy transfer efficiency was calculated. The luminescent properties (PLE/PL intensity, lifetime, quantum yield, etc.) are strongly dependent on the phosphor morphology, and the related discussion is performed in this work. The Gd2O3:Dy3+/Eu3+ phosphors with changed morphology, enhanced red emission and good thermal stability are expected to be widely used in light and displays area.
Keywords: hydrothermal method; Gd2O3:Dy3+/Eu3+ phosphor; luminescent property; energy transfer
1. Introduction The cubic-structured Ln2O3 (space group: Ia-3, Ln=La-Lu, and Y) oxides have been become one of the most important inorganic materials. Among which the Gd2O3 may be the best known due to its good physical and chemical properties that it is one of the widely studied as host matrix [1-3]. The Gd3+ in Gd2O3 can be easily replaced by activator ions, and thus displays colorful emission under ultraviolet (UV) light excitation [3-5]. Eu3+ and Dy3+ ions doped Gd2O3 matrix emit vivid red and yellow emission, respectively, which are widely used in field emission displays (FED), white light emitting diode (w-LED), cathode-ray tubes (CRT), electroluminescent (EL), plasma display panels (PDP), and so forth [5-10]. Gd2O3:Dy3+/Eu3+ solid solution was chosen according to the following main reasons: (1) the radius of Eu3+ (1.066 Å) and Dy3+ (1.027 Å) ions are similar to Gd3+ (1.053 Å) ion, Dy3+/Eu3+ codoped
Gd2O3 can decrease the lattice deformation compared to Y2O3 system [11]; (2) the smaller electronegativity of Gd3+ (1.20) than that of Y3+ (1.22) may result in an easier charge transfer (CT), and thus higher emission intensities [12]; (3) the Gd3+→Eu3+ and Dy3+→Eu3+ energy transfers reported widely can further enhance the red emission of Eu3+ in Gd2O3 system developed in this work and are also experimentally demonstrated in this paper [13-17]. Of course, the Gd3+→Dy3+→Eu3+ should not be neglected; (4) It is reported that the crystal structure change of Gd2O3 from cubic to monoclinic occurs above 1250 oC. The smaller Dy3+ addition to form (Gd,Dy)2O3 solid solutions can stabilize the crystal structure of Gd2O3 and increase the transition temperature, thus the development of Gd2O3:Dy3+/Eu3+ system is feasible [12]; (5) the particle size and morphology strongly affect the luminescent properties of phosphor which in turn relies on the synthesis route used [18,19]. The particle size and morphology have been controlled by the hydrothermal method through adjusting the reaction temperature and pH values in this work, and their effects on the luminescent properties of phosphor have been discussed in detail. In this paper, the Gd2O3:Dy3+/Eu3+ phosphors with controlled morphology have been successfully synthesized by hydrothermal method at different reaction pH and temperatures [20]. The phase evolution, morphology, fluorescent properties, energy transfer were discussed by the combined techniques of XRD, FE-SEM, PLE/PL spectroscopy and fluorescence decay analysis. The fluorescent properties dependence on the morphology/size were analyzed in detail. In the following, we reported the synthesis process, phase evolution, morphology, luminescent properties and energy transfer of Gd2O3:Dy3+/Eu3+ phosphors. 2. Experiment procedure 2.1 Materials
The raw materials used in the synthesis process are Gd2O3 (99.99%, Huizhou Ruier Rare Chemical Hi-Tech Co. Ltd., Huizhou, China urea), Dy2O3 (99.99%, Huizhou Ruier Rare Chemical Hi-Tech Co. Ltd., Huizhou, China urea), Eu2O3 (99.999%, Huizhou Ruier Rare Chemical Hi-Tech Co. Ltd., Huizhou, China urea), ethylene glycol (EG, HOCH2CH2OH, >99%, Tianjin City Fuyu Fine Chemical Co. Ltd., Tianjin, China), and nitric acid (AR, Sinopharm Chemical Reagent Co. Ltd., Shanghai, China). All reagents need not to be purified and can be directly used in the study. 2.2. Preparation procedure The Gd2O3, Dy2O3 and Eu2O3 were dissolved in hot nitric acid to obtain the rare earth nitrates RE(NO3)3 (RE=Gd, Dy and Eu). The Gd(NO3)3, Dy(NO3)3 and Eu(NO3)3 were mixed as the mother salts according to the chemical formula (Gd0.992-xDy0.008Eux)2O3, and dissolved in pure water with a total volume of 50 mL. The pH of the mixed mother salts were adjusted by ammonia in the range of 8-12. Then, the mixed solutions were transferred to reaction kettles, put in the drying oven, and reacted for 24 h at the settled temperature from 120 to 180 oC. After the completion of precipitation, the turbid liquid was centrifuged and washed repeatedly with distilled water and alcohol to remove by-products. The wet precipitate was dried in air at 80 oC for 24 h, and finally calcined at 1300 oC for 4 h with the heating speed of 5 oC/min to produce oxides. 2.3. Characterization The XRD patterns for phase analysis were collected at room temperature using nickel-filtered CuKα radiation in the 2θ range 10-50° at a scan speed of 4.0° 2θ/min (Model D8 ADVANCE, BRUKER Co., Germany). The morphology of the precursor and resultant products were collected via FE-SEM (QUANTA FEG 250, FEI Co., America) with an acceleration voltage of 10 kV. FT-IR spectroscopy was performed by the standard KBr method (Spectrum RXI, Perkin-Elmer, Shelton,
CT). The PLE and PL spectra were obtained using a Fluorescence Spectrophotometer (FP-6500, JASCO Co., Japan) at room temperature equipped with a Φ60-mm integrating sphere (ISF-513, JASCO, Tokyo, Japan) and a 150-W Xe-lamp as the excitation source. The optical performances for all samples were conducted under identical conditions with the slit breadth of 5 nm. The phosphor powder was excited with a selected wavelength and the intensity of the intended emission was recorded as a function of elapsed time after the excitation light was automatically cut-off using a shutter. 3. Results and discussion Fig. 1 shows the XRD patterns of (Gd0.992-xDy0.008Eux)2O3 oxides calcined at 1300 oC. From which it can be seen that the Dy3+ and Eu3+ additions do not alter the crystal structure. All samples have the similar XRD diffraction behavior and are indexed to Gd2O3 (space group: Ia-3, JCPDS No. 43-1014). Fig. 2a and 2b show the PLE spectra of (Gd0.992-xDy0.008Eux)2O3 phosphors monitoring at 572 nm and 611 nm emission, respectively. The phosphors are synthesized by pH=9, 140 oC, and calcined at 1300 oC for 4 h. Monitoring at 572 nm (Fig. 2a), the PLE spectra of (Gd0.992-xDy0.008Eux)2O3 contains both the Gd3+ and Dy3+ ions excitation bands. The occurrence of PLE bands of Gd3+ indirectly proved the Gd3+→Dy3+ energy transfer. The typical excitation bands of Gd3+ ions at 275 nm, 309 nm and 314 nm are assigned to 8S7/2→6IJ, 8S7/2→6P5/2 and 8S7/2→6P7/2 transition, respectively, and the first one (275 nm) is the strongest [21-24]. As marked in the figure, other bands were observed corresponding to the 6H15/2→6P3/2 of Dy3+ ions at 326 nm, 6H15/2→6P7/2 at 351 nm, 6H15/2→6P5/2 at 363 nm and 6H15/2→4P13/2 at 385 nm, respectively [21-24]. The PLE intensity gradually decreased with the Eu3+ content increasing due to the efficient Dy3+→Eu3+ energy transfer. Monitoring at 611
nm (Fig. 2b), the PLE spectra of (Gd0.992-xDy0.008Eux)2O3 phosphors exhibits strongest excitation band at 262 nm ascribed to a CTB (charge transfer band) [25,26], namely, the electronic transition from the 2p orbital of O2- to the 4f orbital of Eu3+ activators. The appearance of typical Gd3+ and Dy3+ transitions marked in the Fig. 2b provides pervasive evidence of the Gd3+→Eu3+, Dy3+→Eu3+ efficient energy transfer. Thus, both the Dy3+ and Eu3+ ions can be excited by the wavelength at 275 nm. Under the optimal 275 nm excitation and same synthesized condition, the PL spectra consists both Eu3+ and Dy3+ emission (Fig. 2c). The former emission bands of Eu3+ were at 580 nm (5D0→7F0), 592 nm (5D0→7F1), 645 nm (5D0→7F3) and 692 nm (5D0→7F4), respectively, among which the red emission at 611 nm (5D0→7F2) is the strongest [25,26]. The later emission band of Dy3+ at 572 nm was attributed to the 4F9/2→6H13/2 of Dy3+ with weak emission intensity. The emission intensity of Eu3+ increased with the Eu3+ doped up to x=0.04 (x=4.0 at%), and then decreased owing to the concentration quenching. The optimal total concentration of Dy3+ and Eu3+ (4.8 at%) is closed to the reported results of Y2O3:Eu3+ (5.0 at%) or Gd2O3:Eu3+ (5.0 at%) system [27,28]. Further observation is that the emission intensity of the optimal Gd2O3:Dy3+/Eu3+ sample developed in this work is higher than the Y2O3 or Gd2O3 doped singly with Eu3+ system ascribed to the efficient Dy3+→Eu3+, Gd3+→Eu3+, Gd3+→Dy3+→Eu3+ energy transfer. The internal (εin) and external (εex) quantum efficiencies of (Gd0.952Dy0.008Eu0.04)2O3 (x=0.04) yield 72.1% and 40.6%, which are higher than Y2O3 (εin=59.3%, εex=32.2%) or Gd2O3 (εin=65.2%, εex=36.3%) system [29,30]. From the inset of Fig. 2c, it can be seen that the PL band intensity of Dy3+ at 572 nm gradually decreased as a function of Eu3+ addition which further proved the Dy3+→Eu3+ energy transfer. Fig. 2d shows the Commission International de L'Eclairage (CIE) chromaticity coordinates for the
emission of the (Gd0.952Dy0.008Eu0.04)2O3 samples under 275 nm excitation. The phosphors (Gd0.992-xDy0.008Eux)2O3 were analyzed to have color coordinates (x,y) of (~0.62, ~0.33), (~0.65, ~0.34), (~0.62, ~0.35), (~0.64, ~0.33) and (~0.63, ~0.33) for x=0.02, 0.04, 0.06, 0.08, and 0.1, respectively. Thus, the (Gd0.992-xDy0.008Eux)2O3 samples emit vivid red color, as seen from the digital picture (the inset of Fig. 2d) under 254 nm UV excitation from a hand-held UV lamp. In order to further elucidate the energy transfer, fluorescence decay kinetics were studied for the 4
F9/2→6H13/2 emission of Dy3+ and the 5D0→7F2 emission of Eu3+ under 275 nm excitation, and then
the lifetime and energy transfer efficiency were calculated. Fig. 3a and Fig. 3b show the decay curves of (Gd0.952Dy0.008Eu0.04)2O3 phosphors for the Dy3+ and Eu3+ emissions, respectively. It was found that the decay data can be well fitted with the following single exponential equation in each case:
I = Aexp(-t / τ R ) + B
(1)
where the I, t, τR, A and B are referred to relative fluorescence intensity, the decay time, fluorescence lifetime and the constant of A and B, respectively. According to the formula (1), the fitting results are obtained as follows: A=373937.6±3386.9 (au), B=372.52±81.23 (au), τR =0.22±0.02 ms (Fig. 3a) and A=172304.5±968.70 (au), B=377.96±120.92 (au), τR =1.24±0.09 ms (Fig. 3b), respectively. The lifetime value of Dy3+ (0.22 ms) for 572 nm is shorter than Eu3+ (1.24 ms) for 611 nm emission. The insets of Fig. 3a and Fig. 3b can be seen that the lifetime for Dy3+ and Eu3+ slowly yet continuously decreased at a higher Eu3+ content. The former was ascribed to the efficient Dy3+→Eu3+ energy transfer, and the later was understandable as follows: when the content of Eu3+ is relatively low, the distance between Eu3+ is long, thus the interactions among Eu3+ can be neglected. The further increasing of Eu3+ content leads to the formation of a resonant energy transfer net among the activators which can be an additional channel to the non-radiative centers on particle surfaces.
The efficiency of energy transfer (ηET) can be estimated from the lifetime for Dy3+ in the presence (τS) and absence (τS0) of Eu3+ using the formula [31-34]: ηET = 1-
τS τ S0
(2)
The results of calculation are shown in Fig. 3c. It is seen that when the concentration of Eu 3+ increases from x=0.02 to x=0.10, while the ηET gradually increases from 19.3% to 89%. Thus, the sensitizer of Dy3+ plays an important role in the effective luminescence of Eu3+, while these high efficiencies of energy transfer primarily originate from the significant spectral overlapping between the 4F9/2→6HJ emissions of Dy3+ and the 7F0,1→5D0,1 absorption of Eu3+ [35]. It is now the time to discuss the energy processes involved in the emission of (Gd0.992-xDy0.008Eux)2O3 phosphors. The illustration of the energy transfer processes for the (Gd0.992-xDy0.008Eux)2O3 phosphors was shown in Fig. 3d. Exciting the (Gd0.992-xDy0.008Eux)2O3 phosphor at 275 nm, it raises electrons from the 8S7/2 ground states to the 6IJ excited states of Gd3+, and at the same time excites the 6H15/2 electrons of Dy3+ to the 4F3/2 states. Since the 6IJ and 6PJ states of Gd3+ lie higher than the 4F3/2,9/2 emission states of Dy3+ and the 5D0,1 states of Eu3+ in the energy diagram, the electrons of the excited Gd3+ may be non-radiatively decay to the 5D1 and 4F3/2 states of both the Eu3+ and Dy3+ activators via Gd3+→Eu3+ and Gd3+→Dy3+ energy transfer. Meanwhile, the 4
F3/2,9/2 states of Dy3+ lie above the 5D0,1 states of Eu3+ ions, and the 5D4→7FJ emissions of Dy3+
significantly overlap the 7FJ→5D0,1 absorptions of Eu3+ ions, thus the Dy3+ ions could transfer their energy to the Eu3+ ions partly or totally, and the Dy3+ activators may intercept the energy transferred from Gd3+ to quench the Gd3+→Eu3+ transfer route. It should be noted that the Gd3+→Dy3+→Eu3+ energy transfer process may also happen in the (Gd0.992-xDy0.008Eux)2O3 matrix material. Then the electrons of 4F9/2 (Dy3+) and 5D1 (Eu3+) excited states relaxed to the 4F9/2 (Dy3+) and 5D0 (Eu3+),
respectively. Back jumping of the 4F9/2 (Dy3+) and 5D0 (Eu3+) electrons to the 2H13/2 (4F9/2→2H13/2 transition of Dy3+) and 7F2 (5D0→7F2 transition of Eu3+) levels finally yield the yellow (572 nm) and red (611 nm) emission. The above results may thus imply that multi-channel energy transfers exist in the present (Gd0.992-xDy0.008Eux)2O3 phosphor system including Gd3+→Eu3+, Gd3+→Dy3+, Dy3+→Eu3+, Gd3+→Dy3+→Eu3+. According to Dexter’s and Reisfeld’s energy-transfer expression for exchange and multipolar interactions, the relation can be given as follows [36-40]:
0 )C
(3)
0 C n / 3
(4)
ln(
where the C is the total concentration of Dy3+ and Eu3+, the η0 and η are the luminescence quantum efficiencies of the Dy3+ sensitizer in the absence and presence of Eu3+, respectively. The η0/η ratio can be approximated as the luminescence intensity ratio IS0/IS. The ln(IS0/IS)∞C would correspond to exchange interaction and (IS0/IS)∞Cn/3 with n=6, 8 and 10 correspond to dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole interactions, respectively. The plots of ln(Is0/Is)-C as well as (Is0/Is)-Cn/3 are illustrated in Fig. 4, from which the best linear relationship was found for n=8 by comparing the fitting factor values (R2). This clearly indicates that the mechanism of Dy3+→Eu3+ energy transfer in the (Gd0.952Dy0.008Eu0.04)2O3 phosphor is ascribed to electric dipole-quadrupole interactions. Fig. 5 shows the shape/size of (Gd0.952Dy0.008Eu0.04)2O3 precursors as the function of synthesized pH values (pH=8-12). From which it can be seen that all the particles possess good dispersion and uniform morphology. However, the particle shape/size changed with the synthesized pH. The particle changed from tubular (Fig. 5a) to rod-like morphology (Fig. 5b) when the synthesized pH value
increased from pH=8 to pH=9. As the pH value further increased, the size of precursor with rod-like morphology gradually decreased. The decreased particle size is mainly due to the high nucleation density derived from high pH. Fig. 6a shows the XRD patterns of nanotube and nanorod precursors fixed the reaction temperature at 140 oC, respectively. No impurity was identified along with the hydroxide phase in each case, and the diffraction can be well indexed with those of the Gd(OH)3 (space group: Ia-3d, JCPDS No. 38-1042). Further observation is that the diffraction intensity ratio of lattice plane (110)/(101) are 83.2% and 149% for nanorod and nanotube, respectively. However, the (110)/(101) diffraction intensity ratio in standard Gd(OH)3 card is 70.8%. Thus, it can be deduced that the crystal plane distribution of nanorod and nanotube is different, and the former displays directional growth. Mass transfer rate is a major factor affecting the morphology of the precursor, the growth mechanism of (Gd0.952Dy0.008Eu0.04)2O3 precursor nanotube and nanorod is shown in Fig. 6b. At low pH, the mass transfer speed is different between the inter part and outer region, and the former is lower than the later which lead to the nanotube formation. While the mass transfer speed of inter part is closed to the outer region, thus these particles formed the nanorod morphology. This growth mechanism is similar to the ZnO reported by Li et al [41]. Fig. 7 displays the particle morphology dependence on the synthesized temperature. Regardless of pH value fixed at 8 or 9, the precursor particles grow gradually as the synthesized temperature increasing from 120 oC to 180 oC. When the pH value was fixed at 8.0, the diameter of nanotube increased up to 200 nm with the synthesized temperature increasing to 180 oC. While the length of nanorod increased up to 1μm fixed the pH value at 9.0, the top of the nanorod particles become prominent and exhibit directional growth. The particle growth is mainly because the mass transfer
rate increases with the increase of hydrothermal temperature, thus strengthens the force of crystal growth. In order to investigation the shape/size effect on the luminescent properties, the PLE/PL behaviors as a functional pH values (8-12) using the sample (Gd0.952Dy0.008Eu0.04)2O3 (hydrothermal temperature: 140 oC, calcined temperature: 1300 oC) as an example were investigated (Fig. 8a and Fig. 8b). It can be seen that the pH value variety does not alter the shape and position of PLE and PL peak, but the intensity firstly decreased when the pH value increased from 8.0 to 9.0, and then increased gradually with the pH value increasing from 9.0 to 12.0 (Fig. 8c). This can be explained as follows: comparing to sample (pH=8.0) with nanotube morphology, the phosphors (pH=9.0) with nanorod morphology presented directional growth as seen from Fig. 5. Owing to the directional growth, the electronic transitions in other directions is limited, and the electric dipole transition probabilities of Eu3+ reduced, thus lead to the decreased fluorescent intensity. The pH value continually increased up to pH=12, the particle size decreased and the surface area increased, thus the number of luminescent centers on the particle surface increased resulting in the enhanced emission intensity. Fig. 8d is the lifetime of the 611 nm emission plotted against the synthesized pH value. A significant lifetime extended from 0.87 to 1.68 ms were observed from pH=8 to pH=12. The lifetime of the sample synthesized with pH=9 is close to that of the Y2O3:Eu3+ (1.39 ms) or Gd2O3:Eu3+ (1.32 ms) powder synthesized by the same hydrothermal method [29,30]. In some Eu3+ activated oxides, the lifetime decreased with the surface area of particle increasing, owing to the nonradiative transition rate increasing due to the enhancement of defects [42]. However, in other oxides, the lifetime decreases at a smaller surface area observed our present work. The increased lifetime with the pH value increasing can be arising from the effective refractive index which can be
expressed by the equation (5) [43]:
02 1 R ~ f ( ED) [ 1 (n 2 2)]2 n eff eff 3
(5)
where the f(ED) is the oscillator strength for dipole transition and λ0 the wavelength in vacuum. The neff is the effective refractive index and neff=nc·x+(1-x)·nmed, where x is the filling factor, nc and nmed are the refractive indices of the bulk material and the surrounding medium, respectively. For the intermediately sized particles of our samples, neff is affected by particle size and decreases for smaller particles. This would explain the increasing lifetime at a higher synthesized pH value. The effects of lattice defects on lifetime, however, can by no means be completely excluded. Deep traps are known to be able to temporarily arrest electrons and thus lead to longer lifetimes. Detailed defect analysis is underway. Fig. 9 and Fig. 10 show the hydrothermal temperature effects on the luminescent properties of samples (Gd0.952Dy0.008Eu0.04)2O3 fixed pH values of 8.0 and 9.0, respectively (hydrothermal temperature: 120-180 oC, calcined temperature: 1300 oC). The PLE/PL intensity of phosphors increased when the synthesized pH value was fixed at 8.0 while decreased for synthesized pH value of 9.0. The former increased luminescent intensity was mainly because the surface area of (Gd0.952Dy0.008Eu0.04)2O3 phosphors with tube morphology (Figs. 7a-7d) increased from 7.61 m2/g to 13.93 m2/g when the synthesized temperature increased from 120 to 180 oC. This leads to more luminescent center on the particle surface resulting in the increased emission intensity. The later was due to the directional growth of particle (Figs. 7e-7h) with the temperature increasing. This can reduce the electric dipole transition probabilities of the surrounding crystal field, and thus decreased fluorescent intensity was achieved. Regardless of the pH fixed at 8.0 or 9.0, the lifetime of samples is
almost the same as function of synthesized temperature, and the former sample (pH=8.0) is a little shorter than the later samples (pH=9.0). The lifetime may originate from defects or particle shape/size or the combined effects of the two [44,45]. Both the size of particle with nanotube and nanorod morphology increased as a function of synthesized temperature which lead to the increased neff in equation (5), thus the phosphors lifetime decreased. However, the lattice defects of particle surface (pH=8.0) would be increased due to the increased surface areas shown in Figs. 7a-7d as the synthesized temperature increasing. The directional growth shown in Figs. 7e-7h reduces the probability of electronic transitions. The increased defects and low electric dipole transition probabilities could increase the phosphor lifetime. Combination effects of the neff, lattice defect and electric dipole transition probabilities caused the unchanged phosphor lifetime. Fig. 11 shows the calcined temperatures effects on the morphology of (Gd0.952Dy0.008Eu0.04)2O3 particle (pH=8.0, hydrothermal temperature: 180 oC, calcined temperature: 600-1300 oC). Form which it can be seen that the particles gradually grow, still persist the tubular shape and good dispersion even the calcining temperatures increased up to 1300 oC (Figs. 11a-11e). Fig. 12a and Fig. 12b show the PLE and PL behaviors of the (Gd0.952Dy0.008Eu0.04)2O3 samples as the function of the calcining temperatures, respectively (pH=8.0, hydrothermal temperature: 180 oC, calcined temperature: 600-1300 oC). The calcining temperature does not change the shape and position of the emission peak of the Eu3+. However, the PLE/PL intensities increase gradually with temperatures increasing. This is mainly due to the improved crystallinity (Figs. 11a-11e) and reduced lattice defects. Compare to the PLE/PL spectra of (Gd0.952Dy0.008Eu0.04)2O3 phosphor, the excitation spectra of (Gd0.952Dy0.008Eu0.04)(OH)3 precursor (pH=8.0, hydrothermal temperature: 180
o
C)
contains only one sharp peak at 300 nm (8S7/2→6PJ) belong to the typical band of Gd3+ (Fig. 12a).
Under 300 nm wavelength excitation, the PL spectra of (Gd0.952Dy0.008Eu0.04)(OH)3 precursor was dominated by 592 nm (5D0→7F1) rather than 611 nm (5D0→7F2), ascribed to the relatively high symmetry of Gd3+ in Gd(OH)3 (the inset of Fig. 12b). As for phosphor, the thermal stability is very important for its potential application. The temperature effects on emission intensity was studied with the range of 298-573 K, and the activation energy of thermal quenching was determined in this work. The results are presented in Fig. 13a. The shape and location of characteristic peaks of (Gd0.952Dy0.008Eu0.04)2O3 (pH=8.0, hydrothermal temperature: 180 oC, calcined temperature: 1300 oC) do not be changed. However, the emission intensity decreased with the temperature increased gradually. The reason can be explained that the thermal activation at a certain intersection of the ground state and the excited state causes thermal quenching, leading to the intensity decreased. In order to further explore the impact of temperature on thermal quenching, the explanation can be acquired by Arrhenius equation:
ln(
I0 E 1) ln A a I kT
(6)
where Ea, T, A and k refer to activation energy, temperature (K), constant and Boltzmann constant, respectively, the I0 and I represent the emission intensity at room temperature and differently operated temperature, respectively. The relationship between ln(I0-I/I) and 1/kT for the thermal quenching is depicted in Fig. 13b. From which it can be seen that the slope of fitting curve is -0.2118, and thus, the Ea can be calculated to be 0.2118 eV being closed to the reported value of 0.2397 eV for Gd2O3:Tb3+/Eu3+ system [46]. The higher activation energy indicates that the phosphor has good thermal stability and is expected to be widely used in lighting and display areas.
Conclusions
In this paper, Gd2O3 codoped with Dy3+/Eu3+ phosphors were successfully obtained through hydrothermal method, followed by calcination at 1300 oC. The combination techniques of XRD, FE-SEM, PLE/PL, fluorescence decay and quantum efficiency on (Gd0.992-xDy0.008Eux)2O3 phosphors have been performed. The conclusions can be summarized as follows: (1) The controlled morphology of the resultant phosphors can be achieved by adjusting the synthesized pH and hydrothermal temperatures. The particles changed from nanotube to nanorod with the pH value increasing from 8.0 to 9.0. The particle grows gradually as the synthesis temperatures increasing while the particle size decreased as the pH value increasing. The formation mechanism of phosphor with controlled morphology has been discussed in detail by analyzing the mass transfer and nuclear formation rate; (2) The (Gd0.992-xDy0.008Eux)2O3 phosphors display strong red emission at 611 nm ascribed to the 5
D0→7F2 transition of Eu3+. The optimal doped concentration of Eu3+ was determined to be 4.0%
(x=0.04). Due to the efficient Dy3+→Eu3+, Gd3+→Eu3+, Gd3+→Dy3+→Eu3+ energy transfer, the luminescent properties of the optimal Gd2O3:Dy3+/Eu3+ sample developed in this work are better than the Y2O3 or Gd2O3 doped singly with Eu3+ system. The energy transfer efficiency, mechanism, process and thermal stability were analyzed in detail; (3) The luminescent properties (PLE/PL, lifetime, etc.) of resultant phosphors are strongly dependent on the calcined temperature, particle shape, size, surface defect, disorder degree and symmetry. These above factors effects on luminescent properties have been studied.
Acknowledgements This work was supported in part by the National Natural Science Foundation of China (No.
51402125), China Postdoctoral Science Foundation (No. 2017M612175), the Research Fund for the Doctoral Program of University of Jinan (No. XBS1447), the Natural Science Foundation of University of Jinan (No. XKY1515), the Science Foundation for Post Doctorate Research from the University of Jinan (No. XBH1607), the Special Fund of Postdoctoral innovation project in Shandong province (No. 201603061).
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Fig. 1 the XRD patterns of (Gd0.992-xDy0.008Eux)2O3 oxides calcined at 1300 oC as a function of Eu3+ content are shown in Fig. 1 (pH=9, 140 oC). Fig. 2 a comparison of the PLE behaviors of the (Gd0.992-xDy0.008Eux)2O3 phosphors calcined at 1300 o
C, with parts (a) and (b) for the 572 nm emission of Dy3+ and the 611 nm emission of Eu3+,
respectively. (c) shows the emission spectra of the (Gd0.992-xDy0.008Eux)2O3 phosphors under 275 nm excitation. The Y2O3 and Gd2O3 were included for comparison. (d) is the CIE chromaticity diagram for the emission of (Gd0.992-xDy0.008Eux)2O3 phosphors (x=0.02-0.1) under 275 nm excitation. The inset in (c) is the expanded emission peak of Dy3+ at 572 nm. The inset in (d) shows the appearances of luminescence for the x=0.04 sample under 254 nm excitation from a handheld UV lamp. Fig. 3 the decay curves of Dy3+ and Eu3+ emission monitored at 572 nm (a) and 611 nm (b) for the phosphors (Gd0.952Dy0.008Eu0.04)2O3 under excitation at 275 nm. The insets of Fig. 3a and 3b are the fluorescence lifetime of (Gd0.992-xDy0.008Eux)2O3 with Eu3+ contents (x=0.02-0.10) for the Dy3+ (572 nm) and Eu3+ (611 nm) emissions, respectively. Fig. 3c shows the energy transfer efficiency of the activators as a function of the Eu3+ content. Fig. 3d illustrates the energy transfer processes for the (Gd0.952Dy0.008Eu0.04)2O3 phosphors. Fig. 4 dependence of ln(IS0/IS) on C3/3 (a) and (IS0/IS) on C6/3 (b), C8/3 (c), and C10/3 (d) for the Dy3+ emission, respectively. Fig. 5 the FE-SEM morphologies of (Gd0.952Dy0.008Eu0.04)2O3 precursors with synthesized pH values of 8 (a), 9 (b), 10(c), 11(d), 12(e), respectively. Fig. 6 the XRD patterns of nanotube (a) and nanorod (b) precursors under the hydrothermal pH value of 8.0 and 9.0 are shown in Fig. 6a, respectively. Fig. 6b illustrates the schematic diagram of growth mechanism of (Gd, Dy, Eu)(OH)3 precursor (140 oC). Fig. 7 (a-d) show the FE-SEM morphologies of (Gd0.952Dy0.008Eu0.04)2O3 precursors with synthesized temperatures of 120 oC (a), 140 oC (b), 160 oC (c), 180 oC (d), respectively (pH=8). (e-h) show the FE-SEM morphologies of (Gd0.952Dy0.008Eu0.04)2O3 precursors with synthesized temperatures of 120 o
C (e), 140 oC (f), 160 oC (g), 180 oC (h), respectively (pH=9). The insets in Fig. 7c and 7d are the
enlarged view of the FE-SEM morphology. Fig. 8 PLE (a, λem=611 nm) and PL (b, λex=275 nm) behaviors of the phosphors (140 oC) calcined at 1300 oC as a function of pH values. (c) is the relative Eu3+ emission intensity of 611 nm normalized
to the sample synthesized by pH=8. (d) shows the lifetime values of (Gd0.952Dy0.008Eu0.04)2O3 sample as a function of pH values. Fig. 9 PLE (a, λem=611 nm) and PL (b, λex=275 nm) behaviors of the phosphors (pH=8) calcined at 1300 oC as a function of synthesized temperature. (c) is the relative Eu3+ emission intensity of 611 nm normalized to the sample synthesized by 120 oC. (d) is the decay behavior of sample (Gd0.952Dy0.008Eu0.04)2O3. The inset in Fig. 9d shows the lifetime values of sample (Gd0.952Dy0.008Eu0.04)2O3 as a function of synthesized temperatures. Fig. 10 PLE (a, λem=611 nm) and PL (b, λex=275 nm) behaviors of the phosphors (pH=9) calcined at 1300 oC as a function of synthesized temperature. (c) is the relative Eu3+ emission intensity of 611 nm normalized to the sample synthesized by 120 oC. (d) shows the lifetime values of sample (Gd0.952Dy0.008Eu0.04)2O3 as a function of synthesized temperatures. Fig. 11 the FE-SEM morphologies of the (Gd0.952Dy0.008Eu0.04)2O3 phosphors with the calcining temperatures of 600 oC (a), 800 oC (b), 1100 oC (c), 1200 oC (d), 1300 oC (e) (pH=8, 180 oC), respectively. The insets in Figs. 11a-11e are the enlarged view of the FE-SEM morphology. Fig. 12 (a) and (b) are the PLE and PL behaviors of (Gd0.952Dy0.008Eu0.04)2O3 phosphors as a function of the calcining temperature, respectively, the precursor was also included. The inset of Fig. 12b shows the expanded emission peak of Eu3+ for (Gd0.952Dy0.008Eu0.04)(OH)3 precursor. Fig. 13 the temperature-dependence of PL intensity is shown in (a), and (b) displays the relationship between ln(I0/I-1) and 1/kT.
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