Synthesis and Luminescence Properties of YNbO4:A - ACS Publications

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Synthesis and Luminescence Properties of YNbO4:A (A = Eu3+ and/or Tb3+) Nanocrystalline Phosphors via a Sol−Gel Process Xiaoming Liu,†,‡ Ying Lü,†,‡ Chen Chen,†,‡ Shenglian Luo,*,†,‡ Yuehua Zeng,‡ Xiaoqian Zhang,‡ Mengmeng Shang,§ Chunxia Li,§ and Jun Lin*,§ †

Key Laboratory of Jiangxi Province for Persistant Pollutants Control and Resources Recycle, Nanchang Hangkong University, Nanchang 330063, P. R. China ‡ School of Environment and Chemical Engineering, Nanchang Hangkong University, Nanchang 330063, P. R. China § State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China ABSTRACT: Rare-earth (Eu3+ and/or Tb3+) ions doped YNbO4 phosphors were prepared though a Pechini-type sol−gel process. X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy, photoluminescence, and cathodoluminescence spectra were utilized to characterize the synthesized phosphors. XRD reveal that the samples begin to crystallize at 800 °C and pure YNbO4 phase can be obtained at 1000 °C. FE-SEM images indicate that the YNbO4:A (A = Eu3+ and/or Tb3+) samples consist of fine and spherical grains of around 40−80 nm. Under the ultraviolet light (around 255 nm) and low-voltage electron beams, the prepared YNbO4:A (A = Eu3+ and/or Tb3+) phosphors show the characteristic blue broadband emission (from 300 to 500 nm with a maximum around 403 nm) of the YNbO4 host lattice, the characteristic red emission of Eu3+ (5DJ → 7FJ′, J, J′ = 0,1,2,3 transitions) and the characteristic green emission of Tb3+ (5D4 → 7 F6,5,4,3 transitions). There exists an energy transfer from the YNbO4 host lattices to Eu3+ (Tb3+) ions. By tuning the relative doping concentration of Eu3+ and Tb3+, a singlecomposition white-light-emitting has been realized in YNbO4:Eu3+,Tb3+ phosphor. The obtained YNbO4:Eu3+, YNbO4:Tb3+, and YNbO4:Eu3+,Tb3+ phosphors have potential application in the areas of UV white-light-emitting diodes, field emission display devices, and vacuum fluorescent display devices, etc.

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INTRODUCTION Inorganic luminescence phosphors are playing a key role in applications of the lighting, image, and display fields, such as light-emitting diodes (LEDs), field emission displays (FEDs), vacuum fluorescent displays (VFDs), plasma display panels (PDPs), and X-ray computed tomography.1−7 Specially, whiteLEDs have received wide consideration in recent decades due to their properties of high brightness, environmental friendliness, energy saving, and long lifetime.8−12 Oxide phosphors have gained much interest due to their better thermal and chemical stability, and they are environmentally friendly compared with sulfides, which are currently used in the areas of LEDs, flat-panel displays (FPDs), FEDs, and VFDs.13−16 Therefore, more attention has been paid to the improvement of original oxide phosphors and developing new oxide phosphor materials with respect to the wide possible applications.17−19 Rare earth (RE) ion doped crystallites have been playing an important role in modern lighting and display fields owing to their excellent luminescent properties. Because the 4f electrons of rare earth ions are shielded by the outer 5s and 5p electrons, the intra-4f emission spectra of rare earth ions are characterized by narrow lines with high color purity.20 Usually, the luminescence efficiency of these materials is limited by the dynamics of the RE ions, which depend on the interaction between the RE ions and the host, such as the local © 2014 American Chemical Society

environment around the dopants, the doping concentration, the distribution of active ions in the host material, and the energy transfer from the host to the active ions.21 Therefore, it is necessary to choose a suitable host lattice for the RE ions to produce phosphors emitting a variety of colors. The typical rare earth ions, such as Eu3+ and Tb3+, are widely used in the luminescent materials. The trivalent europium ion (Eu3+) often shows characteristic red emission due to the transitions of 5D0 → 7FJ (J = 0, 1, 2, 3, 4) and the terbium ion (Tb3+) gives characteristic green emission originated from the transitions of 5 D4 → 7FJ (J = 3, 4, 5, 6).22−24 Furthermore, they are often codoped in the same host to obtain high luminous efficiency materials, such as Li3InB2O6 and NaYF4.25,26 Energy can be transferred from Tb3+ to Eu3+, thus enhancing the red emission intensity of Eu3+. The phenomenon has been proved in many other host lattices, such as SrGe4O9, CaYAlO4, Y2O3, and YBO3.27−31 Niobium-containing oxides comprise a large group of compounds with distinct structure that exhibit a wide range of interesting physical properties, such as high dielectric constants, photocatalytic behavior, and photoluminescence. Received: August 30, 2014 Revised: October 29, 2014 Published: October 31, 2014 27516

dx.doi.org/10.1021/jp508773t | J. Phys. Chem. C 2014, 118, 27516−27524

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gate = 50 ns) as the excitation source (Continuum Sunlite optical parametric oscillator (OPO)). All the measurements were performed at room temperature.

The luminescent properties of niobates and rare earth ions doped niobates have been described a few years ago.32,33 YNbO4, as a well-known self-activated phosphor with a report bandgap of 4.3 eV, shows an efficient blue luminescence upon 254-nm excitation. Unfortunately, the luminescence properties of YNbO4 and RE ions doped YNbO4 (RE = Eu3+, Tb3+, Dy3+, Tm3+ etc.) have been neglected for a long time.32−36 As far as we know, only limited information is available on these classical phosphors.37−40 In those reports for Eu3+ or Tb3+ doped YNbO4, there is not a clear description about their photoluminescence (PL) and relevant luminescent mechanism. Furthermore, detailed investigations on PL of YNbO4:A (A = Eu3+ and/or Tb3+) or the cathodoluminesence (CL) of YNbO4:A (A = Eu3+ and/or Tb3+) have not been performed. Accordingly, in this paper, we report the synthesis of Eu3+, Tb3+ single-doped and Eu3+, Tb3+ codoped YNbO4 phosphors using a Pechini-tpye sol−gel process, and investigated their PL and CL properties in more detail. It is of great importance and interest to note that there is an efficient energy transfer from YNbO4 host lattice to Eu3+ and Tb3+, and white PL and CL emission have been realized in a single YNbO4 host lattice by doubly doping Eu3+ and Tb3+ ions for the first time.

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RESULTS AND DISCUSSION Crystallization Behavior and Morphology. The crystallization behavior and morphology of the studied samples were

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EXPERIMENTAL SECTION The YNbO4:Eu3+, YNbO4:Tb3+, and YNbO4:Eu3+,Tb3+ samples were all prepared by a Pechini-type sol−gel process.32 The doping concentrations of Eu3+ and Tb3+ are 1−12 at. % of Y3+ in YNbO4. The stoichiometric amounts of Eu2O3 and Tb4O7 (99.99%, Shanghai Yuelong Non-Ferrous Metals Limited, China) were dissolved in dilute HNO3 (analytical reagent (A. R.), Beijing Fine Chemical Company, China) under stirring and heating, resulting in the formation of colorless solutions of Eu(NO3)3 and Tb(NO3)3; the final pH value of solution was tuned to 1 by using dilute ammonia. The Y(NO3)3 was dissolved in deionized water and NbCl5 was dissolved in absolute ethanol to get NbCl5 solution. The solutions of Y(NO3)3, Eu(NO3)3, and Tb(NO3)3 were mixed together, followed by the addition of the stoichiometric amounts of NbCl5 solution under stirring. Then citric acid and polyethylene glycol (PEG, molecular weight = 10 000) were dissolved in the above solution (CPEG = 0.01 M, citric acid/ metal ion = 2:1). The resultant mixtures were stirred for 2 h and condensed at 75 °C in a water bath until dry gels formed. After being dried in an oven at 110 °C for 10 h, the gels were ground and prefired at 450 °C for 4 h in air. Then, the samples were fully ground and fired at desired temperatures for 3 h to produce the final samples. X-ray diffraction (XRD) measurements were carried out on a Rigaku-Dmax 2500 diffractometer using Cu Kα radiation (λ = 0.15405 nm). The morphologies of the samples were inspected using a field emission scanning electron microscope (FE-SEM, JSF-6700) and a JEOL 2010 transmission electron microscopy (TEM) equipped with a field emission gun at a voltage of 200 kV for FE-SEM and TEM images. The PL measurements were performed on a Hitachi F-7000 spectrophotometer equipped with a 150-W xenon lamp as the excitation source. The CL measurements were carried out in an ultrahigh-vacuum chamber ( 0.03 in Y(1‑x)NbO4:xTb3+ phosphors), because most of the excitation energy is transferred from NbO4 group to Tb3+, so the emission is dominated by the characteristic red emission of Tb3+ (shown in Figure 8a), and the obtained phosphors give a strong green emission (for example, the CIE chromaticity coordinates are x = 0.2451 and y = 0.4413 for YNbO4:0.03Tb3+). When the doping concentration is moderate (0.015 < x < 0.03) in Y(1‑x)NbO4:xTb3+ phosphors, under the excitation of UV light, because a considerable part of the excitation energy is transferred from NbO4 group to Tb3+, the wide band blue emission of NbO4 group and characteristic green emission of Tb3+ have comparable intensities, and the obtained phosphors give strong blue−green emission (for example, the CIE chromaticity coordinates are x = 0.1821 and y = 0.2106 for YNbO4:0.15Tb3+). The luminescence color can be tuned from blue, to blue−green, to red by changing the doping concentration of Tb3+ ions in Y(1‑x)NbO4:xTb3+ phosphors. The energy transfer efficiencies from NbO4 group to Tb3+ in Y(1‑x)NbO4:xTb3+ phosphors are shown in Figure 8b. When the doping concentrations of Tb3+ x increase from 0.01 to 0.06, the energy transfer efficiency increases from 0.15 to 0.91.45 Figure 6b shows the corresponding CIE chromaticity diagram of Y(1‑x)NbO4:xTb3+ phosphors with different doping concentration of Tb3+ ions. The cross dots indicate the CIE chromaticity coordinates positions. The CIE chromaticity coordinates change from x = 0.1762 and y = 0.1341 (YNbO4:0.01Tb3+, blue), x = 0.2150 and y = 0.3043 (YNbO4:0.03Tb3+, blue−green), to x = 0.2723, y = 0.5582 (YNbO4:0.06Tb3+, green) by changing the doping concentration of Tb3+ from x = 0 to x = 0.06 in Y(1‑x)NbO4:xTb3+ phosphors. The corresponding luminescence color can change from blue, to blue−green, to green. Photoluminescence Properties of Eu3+ and Tb3+ Codoped YNbO4. It is known that white light can be realized by mixing red, green, and blue light in an appropriated ratio.3,11,22 So it is possible to obtain white emission in YNbO4 host lattice (blue) by codoping with Eu3+ (red) and Tb3+ (green) and appropriate tuning of the activator contents of Eu3+ and Tb3+. Fortunately, we have demonstrated this idea in Y(1‑x‑y)NbO4:xEu3+,yTb3+. Figure 9 shows the typical excitation and emission spectra of YNbO4:0.015Eu3+,0.02Tb3+. Under the excitation of 259 nm, YNbO4:0.015Eu3+,0.02Tb3+ phosphor shows the host lattice wide band blue emission from 300 to 550 nm with a maximum at 405 nm, and the characteristic emission of Tb3+, i.e., 5D4−7F6 (493 nm), 5D4−7F5 (550 nm), 5D4−7F4 (591 nm), and 5 D4−7F3 (623 nm),23 as well as the characteristic emission of Eu3+, i.e., the 5D3−7F1 (420 nm), 5D2−7F3 (448 nm), 5D2−7F0 (468 nm), 5D1−7F1 (541 nm), 5D1−7F2 (557 nm), 5D0−7F1 (597 nm), and 5D0−7F2 (616 nm).22 These emission lines cover the whole visible region with comparable intensity, resulting in a white luminescence emission. The corresponding CIE chromaticity coordinates of YNbO4:0.015Eu3+,0.02Tb3+ were determined to be x = 0.3275, y = 0.3382, which are in the white light zone in the CIE chromaticity diagram in Figure 6. To get high quality white light with excellent color purity, we have done a series of orthogonal experiments to optimize the relative content of Eu3+ to Tb3+ in Y(1‑x‑y)NbO4:xEu3+,yTb3+. When the concentration of Eu3+ is fixed at 0.15, the concentration of Tb3+ y increases from 0.005 to 0.04, the 27522

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CONCLUSIONS The YNbO4, YNbO4:Eu3+, YNbO4:Tb3+, and YNbO4:Eu3+,Tb3+ phosphors were prepared by a Pechini-type sol−gel process. Under the excitation of UV and low-voltage electron beams, the YNbO4 show strong blue luminescence. For YNbO4:Eu3+, YNbO4:Tb3+, and YNbO4:Eu3+,Tb3+ phosphors, their PL and CL color can be tuned from blue to red, green, and white, respectively, by changing the doping concentration of Eu3+, Tb3+, and the ratio of Eu3+ to Tb3+. Due to their good morphology, excellent PL, low-voltage CL properties, chromaticity diagram, and high color purity, the obtained YNbO4, YNbO4:Eu3+, YNbO4:Tb3+, and YNbO4:Eu3+,Tb3+ phosphors are promising candidates for applications in lighting and display areas, i.e., LEDs, VFDs, and FEDs devices, etc.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project is financially supported by the National Natural Science Foundation of China (NSFC 21161015, 21165013, 51332008, 51472234), the Joint Funds of the National Natural Science Foundation of China and Guangdong Province (Grant U1301242), the Natural Science Foundation of the Jiangxi Province of China (2009GZH0082), the Natural Science Foundation of the Jiangxi Higher Education Institutions of China (GJJ09180, GJJ14513), the open fund of Key Laboratory of Jiangxi Province for Persistant Pollutants Control and Resources Recycle, and Nanchang Hangkong University Doctoral Foundation.

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