J. Phys. Chem. C 2008, 112, 19901–19907
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Combustion Synthesis and Photoluminescence Properties of YNbO4-Based Nanophosphors Yuanyuan Zhou, Qian Ma, Mengkai Lu¨,* Zifeng Qiu, and Aiyu Zhang State Key Laboratory of Crystal Materials, Shandong UniVersity, Jinan 250100, China ReceiVed: July 15, 2008; ReVised Manuscript ReceiVed: October 8, 2008
YNbO4 and YNbO4:RE (RE ) Er3+, Sm3+, Tb3+, and Tm3+) nanophosphors have been prepared using a citrate-gel combustion procedure. The results of X-ray diffraction reveal that the YNbO4 samples crystallized in both monoclinic and tetragonal phases in the investigated temperature range. The excitation spectra of the undoped YNbO4 samples exhibit a red-shift as annealing temperature or time increases, and the emission spectra show a broadband around 400 nm, corresponding to the self-activated luminescence center of YNbO4. Upon UV excitation of the niobate host, characteristic emissions from f-f transitions of the RE ions were observed. The luminescence covers the blue (Tm3+), green (Er3+, Tb3+), and orange-red (Sm3+). The mechanism of the energy transfer between the niobate host and the RE ions was deduced. Introduction Blue, green, and red phosphors play an important role in our daily life due to their wide applications. Rare-earth-doped inorganic compounds form an important class of phosphors for many luminescence applications. The luminescence efficiency of these materials is often limited by the dynamics of the rareearth (RE) ions, which depend on the interaction between the RE ions and the host,1 such as the local environment around the dopants, the dopant concentration, the distribution of active ions in the host material, and the energy transfer from the host to the active ions. Therefore, it is necessary to choose a suitable host lattice for the RE ions to produce phosphors emitting a variety of colors. The family ABO4 (A ) Y, La, Lu; B ) P, V) is suggested to be an excellent host for luminescence materials, and the luminescence properties of the rare-earth-doped YVO4,2-5 YPO4,6,7 LaPO4,8,9 and LuVO410 have been extensively investigated. The compound YNbO4 also belongs to that family and is the subject of the present work. The host material, YNbO4, is a well-known self-activated phosphor. The bandgap of YNbO4 is reported to be 4.3 eV.11 In the YNbO4 host lattice, the niobium atom can be considered tetrahedrally coordinated to the oxygen atoms in a highly distorted site. All niobate groups may be considered as fluorescent centers. RE ions possess good luminescence characteristics (high color purity) due to their inner shell electronic transitions between the 4f n energy levels. Among many RE ions, Er3+,3,4,12-20 Sm3+,3,20-26 Tb3+,26-31 and Tm3+ 32-36 are well-known as active dopants for many different inorganic lattices. The RE ions can easily replace the yttrium ion. Because the properties of yttrium and rare-earth ions are similar, the concentration of the RE ions can be larger in yttrium oxide than in other hosts, leading to more efficient and special optical properties.20 The photoluminescence properties of YNbO4 activated by RE ions such as Eu3+, Dy3+, Sm3+, and Er3+ have been reported.37,38 Now, for good luminescence characteristics, the chemical processes to be adapted to obtain phosphor powders with good crystallinity, controlled chemical composition, fine size, and * Corresponding author. Tel.: +86-531-88364591. E-mail: mklu@ icm.sdu.edu.cn.
narrow size distribution are required. Liquid-phase synthesis of rare-earth-doped phosphors based on YVO42 and LaPO48,9 can yield well-dispersed nanoparticles since no calcination step is applied, but it is difficult to synthesize niobates using such a liquid-phase method. Combustion synthesis provides a very attractive route for making both simple and complex oxide phosphor materials such as Y2O3:Sm3+,21 Y3Al5O12:Sm3+,23 Lu2O3:Tb3+,28 Gd2O2S:Tb3+,29 SrAl2O4:Ce3+, Pr3+, Tb3+,30 and Gd3Ga5O12:Er3+, Tm3+,18,33 etc. It is an extremely time- and energy-efficient process. It also provides for thorough mixing of the starting components on an atomic scale, producing materials with uniformly distributed dopants.29 Thus, the convenient and low-cost combustion synthesis may be an excellent choice for preparing niobates except that agglomerated powders may be obtained after high-temperature treatment. In this paper, we have successfully prepared the YNbO4 and YNbO4:RE (RE ) Er3+, Sm3+, Tb3+, and Tm3+) phosphors by citrate-gel combustion method. This method can produce nanoparticles at low annealing temperature and short annealing time, compared to other ones.37,38 The relevant luminescent properties were studied in detail. To our knowledge, works of the luminescence behavior of the niobate formed by the combustion method are few. Experimental Section Yttrium oxide (Y2O3), niobium oxide (Nb2O5), erbium oxide (Er2O3), samarium oxide (Sm2O3), terbium oxide (Tb4O7), thulium oxide (Tm2O3), ammonium nitrate (NH4NO3), and citric acid (C6H8O7 · H2O) were used as starting materials. All the reagents were of analytical grade without further purification. Y(NO3)3 solution was obtained by dissolving Y2O3 into diluted nitric acid. RE(NO3)3 solution was obtained by dissolving the counterpart oxide into diluted nitric acid. Niobium citrate (Nb-CA) solution was prepared as follows: stoichiometric quantity of Nb2O5 was dissolved in HF acid (40%). Then, ammonia solution was added to this Nb-F solution to obtain Nb2O5 · nH2O. The Nb2O5 · nH2O precipitate was filtered, washed, and then dissolved in citric acid aqueous solution under heating at 80 °C. The molar ratio of the citric acid to the niobium oxide is 4:1. In a typical synthesis, stoichiometric Y(NO3)3, Nb-CA, and excessive NH4NO3 were mixed homogeneously and heated at
10.1021/jp806246t CCC: $40.75 2008 American Chemical Society Published on Web 11/21/2008
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Figure 1. XRD patterns of the YNbO4 samples.
100 °C. As water evaporated, the solution was turned into a yellow gel with high viscosity. The gel was then introduced into a muffle furnace preheated to 550 °C. As the ignition occurred, the reaction went on vigorously for a few seconds. Brown fluffy agglomeration was obtained after the combustion reaction. The combusted-agglomeration was annealed at 950, 1000, and 1050 °C, and then white product was obtained. The synthesis procedure of YNbO4:RE was adopted as is mentioned above except that RE(NO3)3 was introduced. The phase was characterized by X-ray diffraction (XRD) using a Germany Bruker Axs D8-advanced X-ray diffractometer system with graphite monochromatized Cu KR irradiation. The morphology was characterized with a JEM-100CX2 transmission electron microscope (TEM) and a Hitachi S-4800 scanning electron microscope (SEM), and the composition was characterized by an energy-dispersive X-ray (EDX) microanalyzer attached to the SEM. Excitation and emission spectra were measured on an F-4500 fluorescence spectrophotometer. All the measurements were taken at room temperature. Results and Discussion Figure 1 gives the XRD patterns of the YNbO4 samples annealed at various temperatures for different annealing times. The annealing temperatures of 950-1050 °C have been demonstrated to yield mixed-phase YNbO4 powders. All the tested samples exhibit biphasic solid solution: peaks from monoclinic (JCPDS No. 23-1486) and tetragonal (JCPDS No. 38-187) phases were observed, and the monoclinic phase dominated in the mixed phases. Furthermore, considering the relative intensity of the main peaks between two phases, the proportion of the monoclinic phase increased as the annealing temperature or time increased, indicating that the phase transition from tetragonal to monoclinic occurred. The variation of the relative amount of each phase as a function of annealing temperature or time can be explained by the homogenization of the composition and the enhancement of diffusion.39 Figure 2 shows the XRD patterns of the YNbO4:RE (RE ) Er3+, Sm3+, Tb3+, and Tm3+) nanophosphors annealed at 1000 °C for 1 h. Similarly, all the samples crystallized in both monoclinic and tetrahedral phases of YNbO4, and the monoclinic phase dominated. No diffraction peaks of possible impurities were detected; that is to say, the RE ions have incorporated into the YNbO4 lattice to substitute for yttrium. Figure 3 shows the TEM and SEM images of the YNbO4 samples annealed under different conditions. Because of the high-temperature treatment, all the samples exhibit some conglomeration among the crystalline grains. From Figure 3a,b,
Zhou et al.
Figure 2. XRD patterns of the YNbO4:RE (RE ) Er3+, Sm3+, Tb3+, and Tm3+) nanophosphors.
it can be seen that the particle size increased with the annealing temperature increasing, and compared with Figure 3c,d, it is confirmed that annealing time also has an obvious effect on the particle size. Figure 4 shows the SEM images and EDX spectrum of the YNbO4:0.01 Er3+ sample annealed at 1000 °C for 1 h. The sample also exhibits some conglomeration. The EDX spectrum shows the erbium signal besides the Y and Nb signals, and the molar ratio of Er:Y:Nb is 0.011:1:1.029, consistent with the formula composition. That confirms the combustion method has an advantage in controlling chemical components. Figure 5a presents the excitation spectra of the YNbO4 samples monitored at 400 nm emission at room temperature. The maxima of the excitation bands are peaking at wavelengths of 240, 245, 250, and 260 nm for the samples annealed at 950 °C for 1 h, 1000 °C for 1 h, 1050 °C for 1 h, and 1000 °C for 3 h, respectively. The bands are associated with the direct excitation of the YNbO4 host itself, via the charge transfer transition between Nb and O of the tetrahedral [NbO4] groups. The red-shift of the excitation band may be related to the mixedphase structure which changed the crystal field.39 Figure 5b shows the PL spectra of the YNbO4 samples under 245 nm excitation. The emission spectra contain a broadband with a maximum at 400 nm, corresponding to the self-activated luminescence center of YNbO4. The luminescence is ascribed to the transition from the MLCT (metal to ligand charge-transfer) state to the ground state of the [NbO4] groups. Further more, the spectra are not shifted as the annealing temperature or time is varied; therefore, the influence of the heat treatment conditions on the shape and position of the emission band is imperceptible. Figure 6 shows the excitation and emission spectra of the YNbO4:0.01 Er3+ nanophosphor. The excitation spectrum exhibits a broadband peaking around 245 nm, and several sharp peaks at longer wavelengths. The broadband coincides with the undoped sample and is attributed to the charge transfer transition of the [NbO4] groups, whereas the sharp lines correspond to direct excitation from the erbium ground state into higher excited states of the erbium f-electrons, as shown in Figure 6a. The appearance of the niobate band in the excitation spectrum means that after optical excitation of the host, energy transfer to the Er3+ ions occurred. Under 245 nm excitation, the excitation energy is comparable to the charge-transfer gap energy of YNbO4; thus, emissions from both [NbO4] and Er3+ are possible. The broad blue emission in Figure 6b is ascribed to the niobate and quenches much compared with the undoped YNbO4. The green emission between 515 and 565 nm is ascribed to the
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Figure 3. TEM ((a) 950 °C, 1 h; (b) 1050 °C, 1 h; (c) 1000 °C, 3 h) and SEM ((d) 1000 °C, 1 h) images of the YNbO4 samples.
Figure 4. SEM image and EDX spectrum of YNbO4:0.01 Er3+.
transition from the thermalized (2H11/2, 4S3/2) excited states to the 4I15/2 ground state. The photoluminescence is purely green. No red emission between 630 and 700 nm which is ascribed to the 4F9/2 f 4I15/2 transition was observed; even the doping concentration increased to 6 mol %. The main emission peaks at 519, 524, and 533 nm are associated with the 2H11/2 f 4I15/2 transition, and the other two strong ones at 542 and 554 nm are associated with the 4S3/2 f 4I15/2 transition. The 2H11/2 state is located only slightly above the 4S3/2, and the 2H11/2 f 4S3/2 nonradiative relaxation should be very efficient. However, it
Figure 5. Excitation (λem ) 400 nm) (a) and emission (λex ) 245 nm) (b) spectra of the YNbO4 samples.
has been proved4,19 that the 2H11/2 state can be easily populated thermally at ambient temperature because of the thermal 2H11/2 f 4S3/2 backtransfer of energy, which allows for the 2H11/2 f 4 I15/2 radiative transition to appear. Under the direct excitation of Er3+ ions, the emission spectrum of Er3+ is similar to that under the host excitation
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Figure 6. Excitation (λem ) 554 nm) (a) and emission (λex ) 245 and 382 nm) (b) spectra of YNbO4:0.01 Er3+.
except for the emission intensity, as shown in Figure 6b. This further confirms that the energy transfer from the YNbO4 host to the Er3+ ions is efficient. Figure 7 shows the excitation and emission spectra of the YNbO4:0.02 Sm3+ nanophosphor. The excitation spectrum gives a strong broadband at 248 nm, which is caused by the Nb-O charge-transfer transition, and some weak peaks at longer wavelength corresponding to f-f transitions within Sm3+ 4f 5 electron configuration. The appearance of the niobate band in the excitation spectrum shows that, after optical excitation of the host, an efficient energy transfer occurs from niobate groups to Sm3+. Excitation into the niobate groups at 245 nm yields the characteristic yellow-red emission of Sm3+ at 550-575 nm (4G5/2 f 6H5/2), 575-625 nm (4G5/2 f 6H7/2), and 625-665 nm (4G5/2 f 6H9/2), respectively. In most matrixes such as YVO4,3 Y3GaO6,20 Y2O3,21 SrAl2O4,22 ZnAl2O4,24 CaTiO3,25 and Y2SiO5,26 the yellow emission is less intense than the red one. However, in this study, it is just opposite. The intensity of transitions between different J-number levels depends on the symmetry of the local environment of the Sm ion and can be described in terms of the Judd-Ofelt theory40,41 which states further that magnetic dipole (md) transitions are allowed if ∆J ) 0 or 1 (but not J ) 0 f J′ ) 0), regardless of environment. Then, according to the first condition (∆J ) 0), the transition 4G5/2 f 6H5/2 is an md in nature whose intensity remains without changing with the host matrix. The transition 4 G5/2 f 6H9/2 is totally an electric dipole (ed) one whose intensity increases as the symmetry of the environment of the luminescent
Figure 7. Excitation (λem ) 567 nm) (a) and emission (λex ) 245 nm) (b) spectra of YNbO4:0.02 Sm3+. Concentration dependence of the relative emission intensity at 565 nm (c) of Sm3+.
site decreases. However, the transition 4G5/2 f 6H7/2 is also an md one, but it is electric dipole (ed) dominated. After this, it is partly an md and also an ed natured one.24 Usually, the intensity ratio of ed to md is used to measure the symmetry of the local environment of the trivalent 4f ions. The greater the intensity of ed transition, the more the asymmetry. In our experiment, because of the coexistence of the two phases of YNbO4 (there are two different yttrium sites that can be substituted by Sm3+) and the 4G5/2 f 6H9/2 transition in the electronic levels of Sm3+ is less intense than the 4G5/2 f 6H5/2 transition, we suppose that
YNbO4-Based Nanophosphors Sm3+ prefers to occupy the Y site of the tetrahedral phase of YNbO4 in the mixed-phase lattice. It is well-known that the emission intensity is affected by the doping concentration of activator ions in the host materials. Figure 7c shows the emission intensity dependence of Sm3+ on its doping concentration. When the concentration of Sm3+ is varied, no difference in the spectral shape is observed, while the PL emission intensity of Sm3+ increases with the increase of its concentration first, reaching a maximum value when the doping concentration is 2 mol %, and then deceases because of the concentration quenching effect. Thus, the optimum concentration for Sm3+ in YNbO4 is 2 mol %. Figure 8 shows the excitation and emission spectra of the YNbO4:0.05 Tb3+ nanophosphor. Two strong bands in the excitation spectrum were observed: the one at 245 nm is caused by the Nb-O charge-transfer band, and the other one at 300 nm is due to the 4f 8 to 4f 75d1 transition. The weak peaks at longer wavelength in the excitation spectrum correspond to f-f transitions within the Tb3+ 4f 8 electron configuration. Excitation into the niobate groups at 245 nm yields the characteristic green emission of Tb3+ at 550 nm (5D4 f 7F5) and 582 nm (5D4 f 7 F4), respectively, while the host luminescence is quenched, suggesting that energy transfer from the host to Tb3+ occurs with high efficiency. Furthermore, under 300 nm excitation, the emission at 487 nm (5D4 f 7F6) can also be observed except for the one at 550 nm (the inset in Figure 8b). In the matrixes such as Y2SiO5,26 YPO4,27 and LaPO4,31 the emission of Tb3+ always contains 5D3 f 7FJ (J ) 6, 5, 4, 3); in this case, only 5D4 f 7F6,5,4 can be detected in the magnification. Figure 8c gives the emission spectrum of YNbO4:0.01 Tb3+, and it is clear that weak emission peaks from 5D3 f 7F3,4 can be detected. The quenching of the emission from the 5D3 level at higher doping concentration may be due to the crossrelaxation process of Tb3+: 5D3 + 7F6 f 5D4 + 7F0, which produces rapid population of the 5D4 level at the expense of 5 D3.27 Figure 9 shows the excitation and emission spectra of the YNbO4:0.02 Tm3+ nanophosphor. The strong band in the excitation spectrum is caused by the Nb-O charge-transfer transition, and the weak peaks at longer wavelength correspond to f-f transitions within Tm3+ 4f 12 electron configuration. Excitation into the niobate groups at 245 nm yields the characteristic emission of Tm3+ at 362 nm (1D2 f 3H6) and 457 nm (1D2 f 3F4), respectively. Tm3+ has three emitting levels, that is, 3P0 (35 000 cm-1), 1D2 (27 500 cm-1), and 1G4 (21 200 cm-1). The excited energy level of [NbO4] is at 40 810 cm-1, higher than the top emitting level for Tm3+. However, the emission from 3P0 or 1G4 is too weak to be detected as compared to the one from 1D2. So the host lattice has most energy to populate the 1D2 emitting level, and the population of the 1D2 state results in the radiative emission of the UV (1D2 f 3H6) and blue (1D2 f 3F4) transition. This is different from Tm-doped YVO4 in which nonradiation of 1D2 f 1G4 occurs and results in the 1G4 f 3H6 transition.32 Figure 10 depicts a simple model11,42 illustrating the blue emission process in YNbO4, the energy transfer from [NbO4] to the RE ions, and the emission process of the RE ions in YNbO4, taking Tm3+ for example. For the YNbO4 sample, under 245 nm excitation, a charge-transfer transition from the O2ion to the Nb5+ metal ion occurs. The possible excited states of Nb5+ are 3T1 (MLCT state), 3T2, 1T1, and 1T2 (charge-transfer (CT) state), and the ground state is 1A1. The order of these levels is 1T2 > 1T1 > 3T2 g 3T1.11 After excitation, the transitions from the 3T2, 3T1 excited level to the ground state 1A1 occur and give
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Figure 8. Excitation (λem ) 550 nm) (a) and emission (λex ) 245 nm) (b) spectra of YNbO4:0.05 Tb3+; emission spectrum (λex ) 245 nm) (c) of YNbO4:0.01 Tb3+ (the inset in panel b is the emission spectrum under 300 nm excitation).
the broad blue emission band. For the Tm3+ ions, the excitation spectrum consists of only f-f transitions from 300 to 400 nm; both the charge transfer band and 4f12-4f115d1 excitation band are located below 200 nm. Therefore, the Tm3+ ions have no absorption at the excitation wavelength of ∼245 nm; the characteristic emissions of the Tm3+ ions in the YNbO4 host lattice originate from the energy transfer of the niobate-group excitation.
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Zhou et al. transfer from a higher level to the MLCT state induced by the excitation in the [CeO6] groups. However, in our experiment, if the energy transfer occurs from the MLCT state of YNbO4 to intra4f transition state of Tm3+, then the emission originating from the 1 D2 f 3H6 transition should not appear because the MLCT state lies lower than the 1D2 emitting level. In addition, from the excitation spectrum of YNbO4:0.02 Tm3+, it can be seen that Tm-O CT state lies higher than the Nb-O CT state of the [NbO4] groups, and it is impossible for the energy transfer to occur from the Nb-O CT state to the Tm-O CT state. So the emission mechanism for YNbO4:RE is that first, energy transfers from [NbO4] to every intra-4f transition state of the doped RE ions, which is higher than the MLCT state of YNbO4, then nonradiative transition occurs between the intra-4f transition state to populate the emitting level which leads to the characteristic luminescence of the RE ions. Conclusions
Figure 9. Excitation (λem ) 457 nm) (a) and emission (λex ) 245 nm) (b) spectra of YNbO4:0.02 Tm3+.
YNbO4 and YNbO4:RE (RE ) Er3+, Sm3+, Tb3+, and Tm3+) nanophosphors were prepared by the citric-gel combustion method using cheap inorganic compounds as raw materials. This method produces a mixture of phases caused by a low annealing temperature and a short annealing time. The excitation spectra of undoped YNbO4 show a red-shift as the proportion of the monoclinic phase increases, while the influence of the heat treatment conditions on the peak shape and peak positions of the PL emissions is imperceptible. Upon excitation into the niobate groups at 245 nm, the doped RE ions show their characteristic emissions in the YNbO4 host lattice, because of the efficient energy transfer from the [NbO4] groups. The strong emissions in the visible region show that YNbO4 is a suitable host for RE ion doped phosphor materials. The energy transfer mechanism deduced is that the energy transfer process occurs from the host to the intra-4f transition state which is higher than the MLCT state of YNbO4, then nonradiative transition occurs between the intra-4f transition state to populate the emitting level, which leads to the characteristic luminescence of the RE ions. References and Notes
Figure 10. Simple model illustrating the blue emission process in YNbO4, the energy transfer from YNbO4 to Tm3+, and the emission process of Tm3+ in YNbO4.
Hirai and Kawamura34 supposed that the emission mechanism for Sr2CeO4:Ln3+ in which the energy transfer occurs to every intra4f transition state of the doped Ln3+ions, which is a little lower than the MLCT state of Sr2CeO4, after the nonradiative energy
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