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Synthesis, characterization, and luminescent properties of Lu2O3:Eu phosphors
ARTICLE IN PRESS
Journal of Luminescence 127 (2007) 469–473 www.elsevier.com/locate/jlumin
Synthesis, characterization, and luminescent properties of Lu2O3:Eu phosphors Xue-Jian Liua,b,, Hui-Li Lia, Rong-Jun Xieb, Naoto Hirosakib, Xin Xub, Li-Ping Huanga a
Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China Advanced Materials Laboratory, National Institute for Materials Sciences (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan
b
Received 20 April 2006; received in revised form 7 February 2007; accepted 8 February 2007 Available online 4 March 2007
Abstract Eu-doped lutetia (Lu2O3:Eu) nano-phosphors were synthesized by the sol–gel combustion process from a mixed aqueous solution of europium and lutetium nitrates, using organic glycine as the fuel. Powder X-ray diffraction shows that cubic Lu2O3:Eu crystallites are directly obtained by the sol–gel combustion process without further calcination. Electron microscopy reveals that the as-prepared phosphors are agglomerated and have a fluffy, fine, and porous morphology, consisting of primary particle size of 8–10 nm. The excitation spectrum is characterized by three dominant bands centered at 395, 466, and 534 nm, respectively. Both the photoluminescent and radioluminescent spectra are very similar and exhibit intense emission peaks centered at 612 nm due to 5D0-7F2 transition of Eu3+ ions. The energy transfer from Lu2O3 host to Eu3+ activator is more efficient in the case of calcined phosphors than for the as-prepared phosphors due to their improved lattice perfection. r 2007 Elsevier B.V. All rights reserved. Keywords: Lu2O3:Eu; Phosphors; Photoluminescence; Radioluminescence; Sol–gel combustion
1. Introduction Inorganic scintillators play an important role in radiation detection in many sectors of research concerning almost all medical diagnostic imaging modalities that use X-ray or gamma rays, dosimetry, nuclear medicine, highenergy physics, and also in many industrial measuring systems [1–3]. In the different applications, the scintillator is essentially a luminescent material that absorbs the highenergy photons (e.g., X-rays or gamma rays) and then emits visible light [1–3]. Recently, lutetium-based phosphor materials are of increasing interest as potential ionizing radiation detectors [4–13]. Their high density and high Z-number make them especially attractive for such applications. It has been recognized that the simple lutetium oxide, Lu2O3, could Corresponding author. Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China. Tel.: +86 21 52414220; fax: +86 21 52413903. E-mail address: [email protected] (X.-J. Liu).
0022-2313/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2007.02.058
serve as a convenient host lattice for activators to form promising scintillators or X-ray phosphors [4,5]. Indeed, Lu2O3 is one of the densest inorganic materials with a high density of 9.4 g cm3 and a band gap large enough to accommodate the energy levels of many luminescent activators, e.g., Eu, Tb, and so on [9]. In addition, Lu2O3 has the same cubic structure as Y2O3, a well-known host lattice for efficient, commercial phosphors. It has been recently reported that this host lattice activated with Eu3+ ion (Lu2O3:Eu) is expected to play a major role in digital X-ray imaging, with an X-ray absorptivity unsurpassed among current scintillator materials, a light output comparable to CsI:TI, and an emission wavelength much better matched to the spectral sensitivity of typical CCD arrays [4,5]. However, due to the physical nature of this sesquioxide, the growth of a high optical quality Lu2O3:Eu single crystal is an arduous process. As an alternative, polycrystalline Lu2O3:Eu, both in powder form and in sintered transparent ceramics, is a more practical approach if high density and transparency are made possible by synthesizing Lu2O3:Eu powders and using ceramic processing techniques.
ARTICLE IN PRESS 470
X.-J. Liu et al. / Journal of Luminescence 127 (2007) 469–473
In the past decade, a number of wet-chemical methods have been developed and successfully used to synthesize various rare-earth (RE)-doped phosphors [4–7,9–20]. One such method, called propellant synthesis, involves a combustion process between metal nitrates and an organic fuel and was pioneered by Pechini in 1967 [21]. This method explores an exothermic, generally very fast and self-sustaining chemical reaction between the desired metal salts and a suitable organic fuel, which is ignited at a temperature much lower than the actual phase formation temperature. Its key feature is that the heat required to drive the chemical reaction and accomplish the compound synthesis is supplied by the reaction itself and not by an external source. In the past several years, we synthesized nanoscaled YAG and LuAG powders and transparent optical ceramics by this method, using citric acid and glycine, respectivey, as the fuel [22–27]. Additionally, it has also been reported that Lu2O3:Ln (Ln ¼ Nd, Eu, and Er) powders were synthesized by this method, in which crystalline powders were generally obtained after calcinating the resultant precursors between 500 and 650 1C [9,10,14]. In the present work, nano-sized Lu2O3:Eu phosphors were prepared by the sol–gel combustion method from a mixed aqueous solution of RE nitrates. Organic glycine was used instead of citric acid as both a fuel and a chelating agent to form a stable complex with RE ions. The use of glycine, a smaller molecule than citric acid, is expected to result in reduction of the amount of carbon present and lowering of the temperature required for calcinations. Owing to the formation of stable complexes with RE cations, this method is a versatile tool for the synthesis of nanocrystalline multicomponent metal oxides, while retaining compositional homogeneity. The prepared Lu2O3:Eu phosphors were characterized by X-ray diffraction (XRD), electron probe microanalysis (EPMA), and transmission electron microscope (TEM). In addition, the excitation and emission spectra of the phosphors excited by both UV and X-ray were also investigated. 2. Experimental Crystalline Lu2O3 phosphors doped with 1.0 mol% Eu2O3 (Lu1.98Eu0.02O3) were prepared by a sol–gel combustion process using glycine as the fuel. The process involves the exothermic reaction between RE nitrates and organic fuel, e.g., glycine. The typical stoichiometric synthesis reaction is: 6REðNO3 Þ3 þ 10NH2 CH2 COOH þ 18O2 ! 3RE2 O3 þ 5N2 þ 18NO2 þ 20CO2 þ 25H2 O ðRE ¼ Lu; EuÞ The characteristics of the prepared powders are greatly influenced by the reaction temperature, which can be controlled by adjusting the glycine-to-RE nitrates molar ration. In the current study, a stoichiometric molar ration
of nitrate to glycine, i.e., 1.67, was adopted to prepare Lu2O3:Eu phosphors. Lutetium oxide (Lu2O3, 99.99%), europium oxide (Eu2O3, 99.99%), nitric acid (HNO3, excellent grade), glycine (NH2CH2COOH, analytical grade), and deionized water were used as starting materials in the present work. Firstly, a stoichiometric amount of Lu2O3 and Eu2O3 powders were combined together and dissolved simultaneously in nitric acid to yield a composition with a general formula Lu1.98Eu0.02O3. After being completely dissolved, a stoichiometric glycine was introduced to the nitrate solution. Then the mixed solution was heated at about 60 1C and continuously stirred using a magnetic agitator for several hours. On removal of the excess water, a transparent sol was formed. After being heated at about 80 1C and stirred constantly, the sol transformed into transparent sticky gel. The gel was then rapidly heated to 180 1C and an auto-combustion process, accompanied by a brown fume and bright flame, took place, finally yielding a white and fluffy precursor. The precursor was then heattreated at 800 1C for 2 h in a muffle furnace in air. For comparison, Lu2O3 phosphors doped with 1.0 mol% Eu2O3 was also prepared by the reverse-strike co-precipitation technique using ammonia water as precipitant, as described in the previous work [28]. The crystalline phases of products were identified by XRD (Model D/MAX-2550 V, Rigaku Co., Tokyo, Japan), using nickel-filtered Cu Ka radiation in the range of 2y ¼ 10–701. The microstructure and morphology of powders were examined using EPMA (Model JXA-8100, JEOL, Tokyo, Japan) and a transmission electron microscope (TEM, Model 200CX, JEOL, Tokyo, Japan) equipped with selected area electron diffraction (SAED), respectively. UV-excited photoluminescence and excitation spectra were recorded on a RF-5301 spectrofluorometer with a 1 nm resolution at room temperature, and a 450 W xenon lamp was used as an excitation source. X-ray-excited radioluminescence spectrum was recorded on an X-rayexcited spectrometer, FluorMain, where an F-30 movable X-ray tube (W anticathode target) was used as the X-ray source and operated under the condition of 50 kV and 4 mA at room temperature. 3. Results and discussion XRD patterns of the precursors prepared by the sol–gel combustion process and the powders calcined at 800 1C for 2 h are shown in Fig. 1. For comparison, the precursors prepared by co-precipitation and the powders calcined at 800 1C for 2 h are also shown in Fig. 1. It can be seen that the as-prepared precursors by co-precipitation technique remain amorphous until calcined at 800 1C. However, it is unexpected that the as-prepared precursors by the present sol–gel combustion process using glycine as the fuel is single phase and shows crystalline cubic Lu2O3 instead of an amorphous structure. No distinct difference can be observed between the diffraction patterns of the resultant
ARTICLE IN PRESS X.-J. Liu et al. / Journal of Luminescence 127 (2007) 469–473
Lu2O3:Eu phosphors, which results in drastic combustion due to more chemical energy being released from the exothermic reaction. This is another important factor that leads to the present results. Similarly, the use of glycine in stoichiometric ration results in white precursor because of a reduction in the amount of carbon present. Fig. 2 shows the representative EPMA and TEM/SAED micrographs of the precursors prepared by the sol–gel combustion process, respectively. It can be seen that the precursors have a very fluffy, fine, foamy, and porous morphology, as shown in Fig. 2(a), very similar to the one observed for the previously studied (Gd,Y)2O3 and LuAG powders [20,26]. The TEM micrograph, shown in Fig. 2(b), reveals the shape and size of primary particles. The asprepared powders are agglomerated and essentially consist
O
O Lu2O3
O
O O
D
O
O
O
O
O
O O O OO
O
C B A 10
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2 theta (degree) Fig. 1. XRD patterns of the precursors and powders calcined at 800 1C for 2 h prepared both by the sol–gel combustion process and co-precipitation technique: (A) precursors via co-precipitation; (B) 800 1C 2 h via coprecipitation; (C) precursors via sol–gel combustion; (D) 800 1C 2 h via sol–gel combustion.
precursors and the powders further calcined at 800 1C. Indeed, it is well known that the precursors prepared by combustion synthesis are amorphous and that it is usually necessary to further calcine at higher temperature for crystallization, regardless of the types of fuel used [9,10,14]. Additionally, the as-synthesized powders by the combustion method commonly contain some carbon and nitrogen ligands that are caused by incomplete chemical reaction during the combustion process and generally appear black or fuscous in color. However, the appearance of the resultant precursors in the present work is rather white. It is believed that glycine plays a dual role in the present study. First, it serves as a chelating agent by complexing with RE cations, resulting in a simultaneous crystallization of Lu and Eu cations as the water in the precursor solution evaporates. Second, it serves as a fuel for the combustion reaction, being oxidized by nitrate anions. As is well known, the glycine molecule has a carboxylic acid group at one end and an amine group at the other end, both of which can participate in the complexation of RE ions. This zwitterionic character allows effective complexation with metal cations of varying ionic size. It has been recognized that alkali and alkaline-earth cations are most effectively complexed by the carboxylic acid group, while many transition metals are most effectively complexed by the amine group [29]. Therefore, it is reasonable to conclude that the ‘‘zwitterionic’’ character of glycine is one of the important factors that affect the phase formation of the final product. On the other hand, the chemical energy released from the exothermic reaction with various fuels and/or various molar ration of fuel to nitrate is different [30]. In the current study, a stoichiometric molar ration of glycine to nitrate, i.e., 1.67, was adopted to prepare
Fig. 2. EPMA (a) and TEM (b) micrographs of the precursors prepared by the sol–gel combustion process. SAED is inseted into (b).
ARTICLE IN PRESS X.-J. Liu et al. / Journal of Luminescence 127 (2007) 469–473
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of some slightly elongated crystallites with fairly uniform size of 8–10 nm estimated from TEM images. The SAED pattern of the precursors, inseted into Fig. 2(b), is typical of a cubic structure, which coincides with the XRD results. Fig. 3 shows the excitation and emission spectra of the as-prepared precursors via the sol–gel combustion process. We can see that the excitation spectrum of 612 nm emission for Eu3+ dominantly consisting of three bands centered at 395, 466, and 534 nm, respectively. In fact, monitoring various emission wavelengths, e.g., 581, 610, and 633 nm, all produce the same excitation spectra. These excitation bands result from the absorption of the incident radiation by Eu3+ ions and lead to the excitation of electrons from the Eu3+ 4f ground state to the different crystal field splitting components of excited 4f levels of Eu3+. According to the early researches [11,12], the excitations around 466 and 534 nm correspond to the 7F0-5D2 and 7F0-5D1 transition, respectively. However, it is difficult to identify the excitation around 395 nm and to clarify the origin of the comparative intensity for all transitions at this stage of the research. The peak at 250 nm results from the chargetransfer transitions from the oxygen ions to the Eu3+, which has been reported early [12]. In comparison with the results reported in Ref. [12], the charge-transfer band consists of two components: one located around 260 nm and another one positioned around 230 nm. The origin of the discrepancy in charge-transfer band is not clear at present, which is being studied in progress. The emission spectra of the as-prepared Lu2O3:Eu phosphors under various excitation wavelengths, presented in Fig. 3, reveal that the intense emission peak is centered at around 612 nm with a full-width at half-maximum of about 4 nm. This emission is undoubtedly attributed to the 5 D0-7F2 transition of the Eu3+ ions [9]. It has been recognized that when Eu3+ (4f6) is inside cubic Lu2O3, it tends to capture an oxygen 2p electron to move toward the
Emission
Excitation (λem=610nm
800°CX2h
Intensity (a.u.)
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Precursors Intensity (a.u.)
250
highly stable 4f7 configuration. Consequently, a low-energy charge-transfer state is created by Eu3+, leading to the luminescence arising from the 5D0-7F2 transition [6]. On the other hand, on varying the excitation wavelength, there are no significant changes in the emission spectra except for their higher emission intensity, which seem to indicate that only one kind of Eu3+ site is present in the materials. However, it has been stated that Eu3+ enters both the C2 and S6 crystallographic sites of the Lu2O3 host and the emission results from two sites [12]. Perhaps there are two reasons responsible for the present emission. First, the resolution of the measurements is perhaps too low to clearly separate luminescence from Eu in C2 and S6 sites. It has been revealed that the luminescence from S6 site is weak since transitions in this site have more forbidden character, and especially, the luminescence lines are located close to the emission lines from C2 site, so that they are usually not been separated with low-resolution measurements [12]. Second, it was proved that energy transfer from Eu3+ in site S6 to Eu3+ in site C2 takes place when the dopant content reaches about 1% or higher [12]. In the present work, for 1% Eu3+-doped Lu2O3, the energy transfer from Eu3+ in site S6 to Eu3+ in site C2 may already happen to some extent. In addition, the fact that there are the same excitation spectra for various emission wavelengths gives an accessorial evidence for abovementioned experimental results. Although the excitation of 7 F2-5D0 is efficient for verifying whether one or more site is present in the crystals, it is regret that the experiment is not possible due to our laboratory constraints. Fig. 4 presents the radioluminescent spectrum of the asprepared Lu2O3:Eu precursors under X-ray excitation. For comparison, the radioluminescent spectrum of the powders calcined at 800 1C for 2 h is also shown. We noted that the radioluminescence excited by X-ray is very similar with the
λex=466nm λex=395nm 534 395
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Wavelength (nm) Fig. 3. Excitation (left) and emission (right) spectra of the as-prepared precursors via the sol–gel combustion process.
Fig. 4. Radioluminescent spectra of the as-prepared precursors via the sol–gel combustion process and the powders calcined at 800 1C for 2 h under X-ray excitation.
ARTICLE IN PRESS X.-J. Liu et al. / Journal of Luminescence 127 (2007) 469–473
photoluminescence excited by UV or visible light, as shown in Fig. 3. Comparing the radioluminescent spectra of the asprepared precursors and calcined powder, as shown in Fig. 4, we observed no noticeable shift in the transition energies between the two samples, indicating that the crystal fields surrounding the ions in the two materials are reasonably similar. However, the luminescent intensity is always lower in the as-prepared precursor, relative to the calcined powder. This behavior could be ascribed to the difference in lattice perfection. It is generally understood that, because of the rapid process, combustion synthesis does not favor formation of better crystallinity, and further calcinations will certainly perfect the host lattice. It has been documented that such imperfections are deleterious for carrier-mediated energy transfer [31]. Thus, the lower luminescent intensity in the precursors could be explained by the lattice imperfection, in comparison to the calcined powder. The excitation and emission spectra of calcined Lu2O3:Eu phosphors are also very similar with those of as-prepared precursors except for their higher intensity. Therefore, it can be concluded that the energy transfer from the Lu2O3 host lattice to the Eu3+ activator is more efficient in the case of the calcined phosphors than for the as-prepared phosphors due to their lattice perfection. 4. Conclusions
Technology Commission of Shanghai Municipality of China (no. 04DZ14002). One of the authors (X.J. Liu) would like to thank Non-Oxide Ceramics Group for the financial assistance for his stay at Advanced Materials Laboratory (NIMS, Japan) for carrying out a part of this work.
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[5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
Nano-sized, single phase, and crystalline cubic Lu2O3:Eu phosphors were successfully prepared via a sol–gel combustion process from a mixed solution of RE nitrates and glycine without further calcination. The as-prepared Lu2O3:Eu phosphors are agglomerated and have a fluffy, fine, and porous morphology, consisting of some slightly elongated crystallites with fairly uniform size of 8–10 nm. The excitation spectrum consists of three bands centered at 395, 466, and 534 nm, resulting from the absorption of the incident radiation by Eu3+ ions. Both the photoluminescent and radioluminescent spectra are very similar and are dominated by an intense emission peak centered at 612 nm due to 5D0-7F2 transition of Eu3+ ions. This emission coincides quite well with the sensitivity characteristics of scintillation sensors in digital applications. The spectroscopic characteristics of the calcined Lu2O3:Eu phosphors are similar with those of the as-prepared precursors except for their higher intensity. The energy transfer from Lu2O3 host to Eu3+ activator is more efficient in the case of the calcined phosphors than for the as-prepared phosphors due to their improved lattice perfection. Acknowledgments This work was partially funded by the National Nature Science Foundation of China (no. 50672113) and Science &
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Journal of Luminescence 127 (2007) 469–473 www.elsevier.com/locate/jlumin
Synthesis, characterization, and luminescent properties of Lu2O3:Eu phosphors Xue-Jian Liua,b,, Hui-Li Lia, Rong-Jun Xieb, Naoto Hirosakib, Xin Xub, Li-Ping Huanga a
Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China Advanced Materials Laboratory, National Institute for Materials Sciences (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan
b
Received 20 April 2006; received in revised form 7 February 2007; accepted 8 February 2007 Available online 4 March 2007
Abstract Eu-doped lutetia (Lu2O3:Eu) nano-phosphors were synthesized by the sol–gel combustion process from a mixed aqueous solution of europium and lutetium nitrates, using organic glycine as the fuel. Powder X-ray diffraction shows that cubic Lu2O3:Eu crystallites are directly obtained by the sol–gel combustion process without further calcination. Electron microscopy reveals that the as-prepared phosphors are agglomerated and have a fluffy, fine, and porous morphology, consisting of primary particle size of 8–10 nm. The excitation spectrum is characterized by three dominant bands centered at 395, 466, and 534 nm, respectively. Both the photoluminescent and radioluminescent spectra are very similar and exhibit intense emission peaks centered at 612 nm due to 5D0-7F2 transition of Eu3+ ions. The energy transfer from Lu2O3 host to Eu3+ activator is more efficient in the case of calcined phosphors than for the as-prepared phosphors due to their improved lattice perfection. r 2007 Elsevier B.V. All rights reserved. Keywords: Lu2O3:Eu; Phosphors; Photoluminescence; Radioluminescence; Sol–gel combustion
1. Introduction Inorganic scintillators play an important role in radiation detection in many sectors of research concerning almost all medical diagnostic imaging modalities that use X-ray or gamma rays, dosimetry, nuclear medicine, highenergy physics, and also in many industrial measuring systems [1–3]. In the different applications, the scintillator is essentially a luminescent material that absorbs the highenergy photons (e.g., X-rays or gamma rays) and then emits visible light [1–3]. Recently, lutetium-based phosphor materials are of increasing interest as potential ionizing radiation detectors [4–13]. Their high density and high Z-number make them especially attractive for such applications. It has been recognized that the simple lutetium oxide, Lu2O3, could Corresponding author. Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China. Tel.: +86 21 52414220; fax: +86 21 52413903. E-mail address: [email protected] (X.-J. Liu).
0022-2313/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2007.02.058
serve as a convenient host lattice for activators to form promising scintillators or X-ray phosphors [4,5]. Indeed, Lu2O3 is one of the densest inorganic materials with a high density of 9.4 g cm3 and a band gap large enough to accommodate the energy levels of many luminescent activators, e.g., Eu, Tb, and so on [9]. In addition, Lu2O3 has the same cubic structure as Y2O3, a well-known host lattice for efficient, commercial phosphors. It has been recently reported that this host lattice activated with Eu3+ ion (Lu2O3:Eu) is expected to play a major role in digital X-ray imaging, with an X-ray absorptivity unsurpassed among current scintillator materials, a light output comparable to CsI:TI, and an emission wavelength much better matched to the spectral sensitivity of typical CCD arrays [4,5]. However, due to the physical nature of this sesquioxide, the growth of a high optical quality Lu2O3:Eu single crystal is an arduous process. As an alternative, polycrystalline Lu2O3:Eu, both in powder form and in sintered transparent ceramics, is a more practical approach if high density and transparency are made possible by synthesizing Lu2O3:Eu powders and using ceramic processing techniques.
ARTICLE IN PRESS 470
X.-J. Liu et al. / Journal of Luminescence 127 (2007) 469–473
In the past decade, a number of wet-chemical methods have been developed and successfully used to synthesize various rare-earth (RE)-doped phosphors [4–7,9–20]. One such method, called propellant synthesis, involves a combustion process between metal nitrates and an organic fuel and was pioneered by Pechini in 1967 [21]. This method explores an exothermic, generally very fast and self-sustaining chemical reaction between the desired metal salts and a suitable organic fuel, which is ignited at a temperature much lower than the actual phase formation temperature. Its key feature is that the heat required to drive the chemical reaction and accomplish the compound synthesis is supplied by the reaction itself and not by an external source. In the past several years, we synthesized nanoscaled YAG and LuAG powders and transparent optical ceramics by this method, using citric acid and glycine, respectivey, as the fuel [22–27]. Additionally, it has also been reported that Lu2O3:Ln (Ln ¼ Nd, Eu, and Er) powders were synthesized by this method, in which crystalline powders were generally obtained after calcinating the resultant precursors between 500 and 650 1C [9,10,14]. In the present work, nano-sized Lu2O3:Eu phosphors were prepared by the sol–gel combustion method from a mixed aqueous solution of RE nitrates. Organic glycine was used instead of citric acid as both a fuel and a chelating agent to form a stable complex with RE ions. The use of glycine, a smaller molecule than citric acid, is expected to result in reduction of the amount of carbon present and lowering of the temperature required for calcinations. Owing to the formation of stable complexes with RE cations, this method is a versatile tool for the synthesis of nanocrystalline multicomponent metal oxides, while retaining compositional homogeneity. The prepared Lu2O3:Eu phosphors were characterized by X-ray diffraction (XRD), electron probe microanalysis (EPMA), and transmission electron microscope (TEM). In addition, the excitation and emission spectra of the phosphors excited by both UV and X-ray were also investigated. 2. Experimental Crystalline Lu2O3 phosphors doped with 1.0 mol% Eu2O3 (Lu1.98Eu0.02O3) were prepared by a sol–gel combustion process using glycine as the fuel. The process involves the exothermic reaction between RE nitrates and organic fuel, e.g., glycine. The typical stoichiometric synthesis reaction is: 6REðNO3 Þ3 þ 10NH2 CH2 COOH þ 18O2 ! 3RE2 O3 þ 5N2 þ 18NO2 þ 20CO2 þ 25H2 O ðRE ¼ Lu; EuÞ The characteristics of the prepared powders are greatly influenced by the reaction temperature, which can be controlled by adjusting the glycine-to-RE nitrates molar ration. In the current study, a stoichiometric molar ration
of nitrate to glycine, i.e., 1.67, was adopted to prepare Lu2O3:Eu phosphors. Lutetium oxide (Lu2O3, 99.99%), europium oxide (Eu2O3, 99.99%), nitric acid (HNO3, excellent grade), glycine (NH2CH2COOH, analytical grade), and deionized water were used as starting materials in the present work. Firstly, a stoichiometric amount of Lu2O3 and Eu2O3 powders were combined together and dissolved simultaneously in nitric acid to yield a composition with a general formula Lu1.98Eu0.02O3. After being completely dissolved, a stoichiometric glycine was introduced to the nitrate solution. Then the mixed solution was heated at about 60 1C and continuously stirred using a magnetic agitator for several hours. On removal of the excess water, a transparent sol was formed. After being heated at about 80 1C and stirred constantly, the sol transformed into transparent sticky gel. The gel was then rapidly heated to 180 1C and an auto-combustion process, accompanied by a brown fume and bright flame, took place, finally yielding a white and fluffy precursor. The precursor was then heattreated at 800 1C for 2 h in a muffle furnace in air. For comparison, Lu2O3 phosphors doped with 1.0 mol% Eu2O3 was also prepared by the reverse-strike co-precipitation technique using ammonia water as precipitant, as described in the previous work [28]. The crystalline phases of products were identified by XRD (Model D/MAX-2550 V, Rigaku Co., Tokyo, Japan), using nickel-filtered Cu Ka radiation in the range of 2y ¼ 10–701. The microstructure and morphology of powders were examined using EPMA (Model JXA-8100, JEOL, Tokyo, Japan) and a transmission electron microscope (TEM, Model 200CX, JEOL, Tokyo, Japan) equipped with selected area electron diffraction (SAED), respectively. UV-excited photoluminescence and excitation spectra were recorded on a RF-5301 spectrofluorometer with a 1 nm resolution at room temperature, and a 450 W xenon lamp was used as an excitation source. X-ray-excited radioluminescence spectrum was recorded on an X-rayexcited spectrometer, FluorMain, where an F-30 movable X-ray tube (W anticathode target) was used as the X-ray source and operated under the condition of 50 kV and 4 mA at room temperature. 3. Results and discussion XRD patterns of the precursors prepared by the sol–gel combustion process and the powders calcined at 800 1C for 2 h are shown in Fig. 1. For comparison, the precursors prepared by co-precipitation and the powders calcined at 800 1C for 2 h are also shown in Fig. 1. It can be seen that the as-prepared precursors by co-precipitation technique remain amorphous until calcined at 800 1C. However, it is unexpected that the as-prepared precursors by the present sol–gel combustion process using glycine as the fuel is single phase and shows crystalline cubic Lu2O3 instead of an amorphous structure. No distinct difference can be observed between the diffraction patterns of the resultant
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Lu2O3:Eu phosphors, which results in drastic combustion due to more chemical energy being released from the exothermic reaction. This is another important factor that leads to the present results. Similarly, the use of glycine in stoichiometric ration results in white precursor because of a reduction in the amount of carbon present. Fig. 2 shows the representative EPMA and TEM/SAED micrographs of the precursors prepared by the sol–gel combustion process, respectively. It can be seen that the precursors have a very fluffy, fine, foamy, and porous morphology, as shown in Fig. 2(a), very similar to the one observed for the previously studied (Gd,Y)2O3 and LuAG powders [20,26]. The TEM micrograph, shown in Fig. 2(b), reveals the shape and size of primary particles. The asprepared powders are agglomerated and essentially consist
O
O Lu2O3
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2 theta (degree) Fig. 1. XRD patterns of the precursors and powders calcined at 800 1C for 2 h prepared both by the sol–gel combustion process and co-precipitation technique: (A) precursors via co-precipitation; (B) 800 1C 2 h via coprecipitation; (C) precursors via sol–gel combustion; (D) 800 1C 2 h via sol–gel combustion.
precursors and the powders further calcined at 800 1C. Indeed, it is well known that the precursors prepared by combustion synthesis are amorphous and that it is usually necessary to further calcine at higher temperature for crystallization, regardless of the types of fuel used [9,10,14]. Additionally, the as-synthesized powders by the combustion method commonly contain some carbon and nitrogen ligands that are caused by incomplete chemical reaction during the combustion process and generally appear black or fuscous in color. However, the appearance of the resultant precursors in the present work is rather white. It is believed that glycine plays a dual role in the present study. First, it serves as a chelating agent by complexing with RE cations, resulting in a simultaneous crystallization of Lu and Eu cations as the water in the precursor solution evaporates. Second, it serves as a fuel for the combustion reaction, being oxidized by nitrate anions. As is well known, the glycine molecule has a carboxylic acid group at one end and an amine group at the other end, both of which can participate in the complexation of RE ions. This zwitterionic character allows effective complexation with metal cations of varying ionic size. It has been recognized that alkali and alkaline-earth cations are most effectively complexed by the carboxylic acid group, while many transition metals are most effectively complexed by the amine group [29]. Therefore, it is reasonable to conclude that the ‘‘zwitterionic’’ character of glycine is one of the important factors that affect the phase formation of the final product. On the other hand, the chemical energy released from the exothermic reaction with various fuels and/or various molar ration of fuel to nitrate is different [30]. In the current study, a stoichiometric molar ration of glycine to nitrate, i.e., 1.67, was adopted to prepare
Fig. 2. EPMA (a) and TEM (b) micrographs of the precursors prepared by the sol–gel combustion process. SAED is inseted into (b).
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of some slightly elongated crystallites with fairly uniform size of 8–10 nm estimated from TEM images. The SAED pattern of the precursors, inseted into Fig. 2(b), is typical of a cubic structure, which coincides with the XRD results. Fig. 3 shows the excitation and emission spectra of the as-prepared precursors via the sol–gel combustion process. We can see that the excitation spectrum of 612 nm emission for Eu3+ dominantly consisting of three bands centered at 395, 466, and 534 nm, respectively. In fact, monitoring various emission wavelengths, e.g., 581, 610, and 633 nm, all produce the same excitation spectra. These excitation bands result from the absorption of the incident radiation by Eu3+ ions and lead to the excitation of electrons from the Eu3+ 4f ground state to the different crystal field splitting components of excited 4f levels of Eu3+. According to the early researches [11,12], the excitations around 466 and 534 nm correspond to the 7F0-5D2 and 7F0-5D1 transition, respectively. However, it is difficult to identify the excitation around 395 nm and to clarify the origin of the comparative intensity for all transitions at this stage of the research. The peak at 250 nm results from the chargetransfer transitions from the oxygen ions to the Eu3+, which has been reported early [12]. In comparison with the results reported in Ref. [12], the charge-transfer band consists of two components: one located around 260 nm and another one positioned around 230 nm. The origin of the discrepancy in charge-transfer band is not clear at present, which is being studied in progress. The emission spectra of the as-prepared Lu2O3:Eu phosphors under various excitation wavelengths, presented in Fig. 3, reveal that the intense emission peak is centered at around 612 nm with a full-width at half-maximum of about 4 nm. This emission is undoubtedly attributed to the 5 D0-7F2 transition of the Eu3+ ions [9]. It has been recognized that when Eu3+ (4f6) is inside cubic Lu2O3, it tends to capture an oxygen 2p electron to move toward the
Emission
Excitation (λem=610nm
800°CX2h
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highly stable 4f7 configuration. Consequently, a low-energy charge-transfer state is created by Eu3+, leading to the luminescence arising from the 5D0-7F2 transition [6]. On the other hand, on varying the excitation wavelength, there are no significant changes in the emission spectra except for their higher emission intensity, which seem to indicate that only one kind of Eu3+ site is present in the materials. However, it has been stated that Eu3+ enters both the C2 and S6 crystallographic sites of the Lu2O3 host and the emission results from two sites [12]. Perhaps there are two reasons responsible for the present emission. First, the resolution of the measurements is perhaps too low to clearly separate luminescence from Eu in C2 and S6 sites. It has been revealed that the luminescence from S6 site is weak since transitions in this site have more forbidden character, and especially, the luminescence lines are located close to the emission lines from C2 site, so that they are usually not been separated with low-resolution measurements [12]. Second, it was proved that energy transfer from Eu3+ in site S6 to Eu3+ in site C2 takes place when the dopant content reaches about 1% or higher [12]. In the present work, for 1% Eu3+-doped Lu2O3, the energy transfer from Eu3+ in site S6 to Eu3+ in site C2 may already happen to some extent. In addition, the fact that there are the same excitation spectra for various emission wavelengths gives an accessorial evidence for abovementioned experimental results. Although the excitation of 7 F2-5D0 is efficient for verifying whether one or more site is present in the crystals, it is regret that the experiment is not possible due to our laboratory constraints. Fig. 4 presents the radioluminescent spectrum of the asprepared Lu2O3:Eu precursors under X-ray excitation. For comparison, the radioluminescent spectrum of the powders calcined at 800 1C for 2 h is also shown. We noted that the radioluminescence excited by X-ray is very similar with the
λex=466nm λex=395nm 534 395
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Wavelength (nm) Fig. 3. Excitation (left) and emission (right) spectra of the as-prepared precursors via the sol–gel combustion process.
Fig. 4. Radioluminescent spectra of the as-prepared precursors via the sol–gel combustion process and the powders calcined at 800 1C for 2 h under X-ray excitation.
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photoluminescence excited by UV or visible light, as shown in Fig. 3. Comparing the radioluminescent spectra of the asprepared precursors and calcined powder, as shown in Fig. 4, we observed no noticeable shift in the transition energies between the two samples, indicating that the crystal fields surrounding the ions in the two materials are reasonably similar. However, the luminescent intensity is always lower in the as-prepared precursor, relative to the calcined powder. This behavior could be ascribed to the difference in lattice perfection. It is generally understood that, because of the rapid process, combustion synthesis does not favor formation of better crystallinity, and further calcinations will certainly perfect the host lattice. It has been documented that such imperfections are deleterious for carrier-mediated energy transfer [31]. Thus, the lower luminescent intensity in the precursors could be explained by the lattice imperfection, in comparison to the calcined powder. The excitation and emission spectra of calcined Lu2O3:Eu phosphors are also very similar with those of as-prepared precursors except for their higher intensity. Therefore, it can be concluded that the energy transfer from the Lu2O3 host lattice to the Eu3+ activator is more efficient in the case of the calcined phosphors than for the as-prepared phosphors due to their lattice perfection. 4. Conclusions
Technology Commission of Shanghai Municipality of China (no. 04DZ14002). One of the authors (X.J. Liu) would like to thank Non-Oxide Ceramics Group for the financial assistance for his stay at Advanced Materials Laboratory (NIMS, Japan) for carrying out a part of this work.
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Nano-sized, single phase, and crystalline cubic Lu2O3:Eu phosphors were successfully prepared via a sol–gel combustion process from a mixed solution of RE nitrates and glycine without further calcination. The as-prepared Lu2O3:Eu phosphors are agglomerated and have a fluffy, fine, and porous morphology, consisting of some slightly elongated crystallites with fairly uniform size of 8–10 nm. The excitation spectrum consists of three bands centered at 395, 466, and 534 nm, resulting from the absorption of the incident radiation by Eu3+ ions. Both the photoluminescent and radioluminescent spectra are very similar and are dominated by an intense emission peak centered at 612 nm due to 5D0-7F2 transition of Eu3+ ions. This emission coincides quite well with the sensitivity characteristics of scintillation sensors in digital applications. The spectroscopic characteristics of the calcined Lu2O3:Eu phosphors are similar with those of the as-prepared precursors except for their higher intensity. The energy transfer from Lu2O3 host to Eu3+ activator is more efficient in the case of the calcined phosphors than for the as-prepared phosphors due to their improved lattice perfection. Acknowledgments This work was partially funded by the National Nature Science Foundation of China (no. 50672113) and Science &
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