Article pubs.acs.org/JPCC
The Influence of Oxygen Vacancies on Luminescence Properties of Na3LuSi3O9:Ce3+ Jianbang Zhou,† Jiuping Zhong,*,† Jingyuan Guo,‡ Hongbin Liang,† Qiang Su,† Qiang Tang,‡ Ye Tao,§ Federico Moretti,∥ Kheirreddine Lebbou,∥ and Christophe Dujardin∥ †
MOE Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, China ‡ Radiation Dosimetry Laboratory, School of Physics Science and Engineering, Sun Yat-sen University, Guangzhou 510275, China § Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, China ∥ Institut Lumière Matière, UMR5306 Université Lyon 1CNRS, Villeurbanne 69622, France S Supporting Information *
ABSTRACT: Oxygen vacancies play an important role in the luminescence processes of inorganic scintillator materials. In order to study the effects of oxygen vacancies on the luminescence properties of Na3LuSi3O9:Ce3+, these phosphors were prepared using a high temperature solid-state reaction method under different atmosphere and raw materials. It was found that oxygen vacancy had great influence on the absorption, photoluminescence and decay curves of Na3LuSi3O9:Ce3+. The luminescence intensity and peak position showed a regular change when synthesizing atmosphere changed. Na3LuSi3O9:Ce3+ with more oxygen vacancies showed much stronger luminescence intensity at high temperature than that without vacancies. And it was also found that the decreasing of oxygen vacancies can quicken the photoluminescence decay of Ce3+ in Na3LuSi3O9. The existence of oxygen vacancies in Na3LuSi3O9:Ce3+ was confirmed by Zr4+ doping and thermoluminescence emission spectra. At last, emission bands of Ce3+ and oxygen vacancies were well distinguished under X-ray excitation and probable cause of the formation of oxygen vacancies was discussed. scintillation decay of LYSO:Ce3+.11 The delayed emission (afterglow) due to defects can cause reduced contrast and image blurring in high speed X-ray imaging.12 Thus, annealing postprocessing treatment to decrease the concentration of defects is always needed for the optimization of scintillator material properties with fewer defects. In our previous work,9 it was found that strong and characteristic Ce3+ emission was observed with no defects detected in Na 3LuSi 3O 9 :Ce 3+ prepared in air atmosphere. While, plenty of oxygen vacancies were found in Na3LuSi3O9:Ce3+ phosphors when prepared in CO reducing atmosphere. These defects enhanced Ce3+ emission intensity and had shorter decay time than that of Ce3+. Therefore, it is necessary to investigate the effects of defects on the luminescence and scintillation behaviors of Na3LuSi3O9:Ce3+ and the probable formation mechanism of oxygen vacancies during synthesis processes. In this work, Na3LuSi3O9:Ce3+ phosphors were synthesized under different atmosphere with a high temperature solid-state reaction method. The role of oxygen vacancies in the luminescence process were discussed by determining the influence
1. INTRODUCTION Inorganic scintillators play an important role in radiation detection, such as nuclear medical diagnostics (positron emission tomography), high energy physics, and γ-ray detection.1 In many cases, basic scintillators require high density, high light yield, good energy resolution, and fast response time.2 To meet these requirements, many cerium-doped silicate based scintillators have been developed, such as, Lu2SiO5:Ce3+ (LSO),3 Lu2(1‑x)Y2xSiO5:Ce3+ (LYSO),4,5 Gd2SiO5:Ce3+ (GSO),6 and Lu2Si2O7:Ce3+ (LPS).7 These oxyorthosilicates and prosilicates show high light-yield and fast decay properties. But their high melting temperature (above 1900 °C)8 make the growth of such crystals difficult and costly for industry applications. To overcome this shortcoming, Na3LuSi3O9:Ce3+ with lower melting point (about 1500 °C) has been developed.9 Na3LuSi3O9:Ce3+ was found to have good scintillation properties under X-ray excitation. At the same time, it was also found that oxygen vacancies had great influence on the luminescence and decay properties of Na3LuSi3O9:Ce3+. Defects usually have a negative impact on the scintillation behaviors (light yield, decay time, afterglow, etc.) of scintillators.10 For example, the electron traps decrease the scintillation efficiency of the Ce-doped aluminum garnets and the presence of defects can prolong the slow components in the © 2016 American Chemical Society
Received: May 16, 2016 Revised: July 28, 2016 Published: July 28, 2016 18741
DOI: 10.1021/acs.jpcc.6b04964 J. Phys. Chem. C 2016, 120, 18741−18747
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The Journal of Physical Chemistry C of oxygen vacancies on luminescence and scintillation properties of Na3LuSi3O9:Ce3+.
5 to 450 °C and same amount of samples were put to ensure the results comparable.
2. EXPERIMENTAL SECTION Synthetic Procedures. The raw materials included Na2CO3 (analytical reagent, A.R.), SiO2 (4N), Lu2O3 (4N), Gd2O3 (4N), CeO 2 (4N), Ce(NO 3 ) 3 ·6H 2 O (3N), and ZrO 2 (4N). Na3LuSi3O9:Ce3+ samples for spectroscopic measurements were obtained through the following synthesis processes: A Samples prepared with CeO2 as raw material: Three series of Na3Lu(1‑x)CexSi3O9 phosphors were prepared with a high temperature solid-state reaction method under different atmosphere: synthesized in air, CO reducing atmosphere, synthesized in CO, and then recalcinated in air. (a) The starting materials according to the composition Na3Lu(1‑x)CexSi3O9 (x = 0, 0.005, 0.01, 0.03, 0.05, 0.07, 0.09), Na3Lu0.99Gd0.01Si3O9, and 0.01 at. % Zr4+-doped Na3Lu0.995Ce0.005Si3O9 were thoroughly ground and calcined at 1150 °C for 8 h under CO reducing atmosphere. (b) Na3Lu(1‑x)CexSi3O9 (x = 0, 0.005, 0.01, 0.03, 0.05, 0.07, 0.09) phosphors were also synthesized in air at 1150 °C for 8 h. (c) Part of the products obtained from procedure a were reground and recalcined in air at 1150 °C for 8 h. (d) To ensure the same calcination time as part c, the products of procedure a and b were reground and recalcined at 1150 °C for 8 h in CO and air atmosphere, respectively. The measurements and discussion later in this work concerned only about the products of procedures c and d. B Samples prepared with Ce(NO3)3·6H2O as raw material: The starting materials according to the composition Na3Lu0.99Ce0.01Si3O9 were thoroughly ground and calcined at 1150 °C for 8 h under air and CO atmosphere (both recalcinated), respectively. Measurements. The phase purity of the samples was examined by X-ray diffraction (XRD) using a BRUKER D8 ADVANCE powder diffractometer with Cu Kα radiation (λ = 0.15418 nm) operating at 40 kV and 40 mA. The UV−vis absorption spectra (converted from diffuse reflection spectra) were measured by a Cary 5000 UV−vis-NIR spectrophotometer equipped with a Cary diffuse reflectance accessory, using BaSO4 as the standard reference. The UV−visible excitation and emission spectra were recorded on an Edinburgh FLS920 spectrometer equipped with a CTI-cryogenics temperature control system and a 450 W xenon lamp was used as the excitation source. The decay curves were measured with a 150 W nF900 ns flash lamp with a pulse width of 1 ns and pulse repetition rate of 40−100 kHz. Radioluminescence (RL) spectra measurements were obtained by irradiating the samples with a Philips X-ray tube with tungsten anode set at 30 kV and 30 mA at room temperature. The emitted light was collected via an optical fiber and detected by an Andor Newton 970 CCD camera coupled to a monochromator (Andor Shamrock 500i) working in the 200−1000 nm interval. The thermally stimulated luminescence spectra were measured with a Risø TL/OSL-15-B/C spectrometer. The samples were irradiated with β-ray (90Sr as radiation source) for 100 s with a does rate of 0.1 Gy/s. The heating rate was 5 °C/s from
3. RESULTS 3.1. Phase Purity. To confirm the phase purity, XRD patterns of selected samples synthesized under different conditions are shown in Figure 1. Curve b showed XRD pattern of Na3-
Figure 1. XRD patterns of samples under different conditions: (a) standard card of Na3YSi3O9; (b) Na3Lu0.99Ce0.01Si3O9 with Ce(NO3)3· 6H2O as raw material synthesized in air; (c) Na3Lu0.99Ce0.01Si3O9 with CeO2 as raw material synthesized in CO; (d) 0.01 at. % Zr4+-doped Na3Lu0.995Ce0.005Si3O9 with CeO2 as raw material synthesized in CO atmosphere.
Lu0.99Ce0.01Si3O9 with Ce(NO3)3·6H2O as raw material synthesized in air and Curve c showed XRD pattern of Na3Lu0.99Ce0.01Si3O9 with CeO2 as raw material synthesized in CO. Curve d was XRD pattern of 0.01 at. % Zr4+-doped Na3Lu0.995Ce0.005Si3O9 with CeO2 as raw material synthesized in CO atmosphere. It must be noted that all the samples show the same XRD pattern, irrespective of dopant concentration and type, as well as synthesis atmosphere, and they were consistent with the standard card of Na3YSi3O9 (curve a; JCPDS number, 72-2455), which is isomorphic compound of Na3LuSi3O9 because of the close radii of Y3+ (90.0 pm) and Lu3+ (86.1 pm). 3.2. Influence of Oxygen Vacancies on Spectroscopic and Decay Properties of Na3LuSi3O9:Ce3+. 3.2.1. UV−Vis Absorption Spectra. To investigate the influence of oxygen vacancies on absorption spectra of Na3LuSi3O9:Ce3+, UV−vis absorption spectra of Na3Lu0.99Ce0.01Si3O9 with CeO2 as material prepared under different atmosphere (a, CO atmosphere; b, air; c, recalcined in air) are displayed in Figure 2. The absorption curves were determined from diffuse reflectance spectra using the Kubelka−Munk equation.13 For Na3Lu0.99Ce0.01Si3O9 synthesized in air (curve b) and recalcined in air (curve c), a rather intense absorption band was observed at about 250 nm, likely due to the presence of Ce4+.14 The intensity of this band was decreased by synthesizing the samples in CO (curve a). Curve a (synthesized in CO) clearly showed an intense absorption band centered at about 320 nm (labeled as band I) compared with curve b and curve c. Band I was considered to be caused by oxygen vacancies because all the samples synthesized in CO reducing atmosphere had this absorption band and this consideration was further confirmed in section 3.2.2 and 4.1. This band weakened after recalcined in air (curve c) 18742
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energy level. This band disappeared for Na3Lu0.99Ce0.01Si3O9 recalcined in air (Figure 3B) and synthesized in air (Figure 3C). The vacancy excitation band of Na3Lu0.99Ce0.01Si3O9 in vacuum ultraviolet (VUV) spectra behaved in the same way as mentioned above and the influence of oxygen vacancies on the split 5d states of Ce3+ were also determined in VUV−UV luminescence spectra (Figure S1). As shown in Figure 3A, excitation bands belonging to oxygen vacancies were observed when monitored at 390 nm and Ce3+ emission was observed under excitation of 310 nm, which indicated the energy transfer from oxygen vacancies to Ce3+. It can be noted that the maximum of the emission bands of Na3Lu0.99Ce0.01Si3O9 synthesized in CO (Figure 3A) was shifted from 375 to 380 nm and to 390 when excited at 310, 350, and 360 nm, respectively. For Na3Lu0.99Ce0.01Si3O9 recalcined in air, the emission band center shifted from 380 to 385 nm when excited at 350 and 360 nm (Figure 3B). And for Na3Lu0.99Ce0.01Si3O9 synthesized in air, the emission curves overlapped very well with center at 395 nm for excitation at 350 and 360 nm (Figure 3C). A proper consideration was that the emission spectra of the defect luminescence and the Ce3+ emission partly overlapped and their relative intensity determined the peak position. In Figure 3A, the emission of oxygen vacancies dominated when excitated at short excitation wavelength (310 nm) and the corresponding emission band was centered at 375 nm. The emission of oxygen vacancies became weak and Ce3+ emission dominated when excitated with longer wavelength (350 and 360 nm), and then the emission peak of Na3Lu0.99Ce0.01Si3O9 moved to longer wavelengths (up to 390 nm). However, in Figure 3C, the emission bands overlapped very well when excitated at different wavelength (350 and 360 nm) because of no oxygen vacancies for Na3Lu0.99Ce0.01Si3O9 synthesized in air. Another reason for the different excitation and emission positions of Na3LuSi3O9:Ce3+ under different synthesis atmosphere probably results from that the oxygen vacancies changed the crystal field. Thus, the energy level of the lowest 5d orbit of Ce3+ became higher and the excitation and emission spectra of Ce3+ shifted to shorter wavelength when synthesized in CO. As shown in Figure 3D, the emission intensity of Na3Lu0.99Ce0.01Si3O9 showed a decreasing trend as the synthesis atmosphere varied in the following sequence: CO atmosphere, recalcined in air, air. It was because when the atmosphere changed in this order, the defect luminescence became weaker and weaker and the energy transfer from oxygen vacancies to Ce3+ decreased due to the decreasing amount of oxygen vacancies. Characteristic emission of Ce3+ was observed in Figure 3D when synthesized in air and CeO2 used as raw material which meant that Ce4+ was reduced to Ce3+ (selfreduction). And it was proved that Ce4+ ions were reduced well by comparing with the luminescence spectra (Figure S2) of Na3Lu0.99Ce0.01Si3O9 with Ce(NO3)3·6H2O as raw material. The normalized emission spectra of Na3Lu1−xCexSi3O9 (x = 0.005, 0.01, 0.03, 0.05, 0.07, 0.09) synthesized in different atmosphere are presented in Figure 4. As shown, the emission peaks shifted from 380 to 390 nm for excitation at 350 nm (Figure 4A) when synthesized in CO reducing atmosphere. And for samples recalcined in air, the emission bands were centered at 383 nm for Ce3+ concentration below 3.0 at. % and 390 nm for concentration above 3.0 at. % (Figure 4B). All the results above indicated that at low Ce3+ doping concentration, oxygen vacancies had great influence on the emission spectra of Na3Lu1−xCexSi3O9, but the Ce3+ emission became stronger and
Figure 2. Absorption spectra of Na3Lu0.99Ce0.01Si3O9 with CeO2 as material prepared under different atmosphere: (a) CO atmosphere; (b) air; (c) recalcined in air.
and the samples synthesized in air (curve b) did not show any absorption band at this wavelength range. Of course, it was also possible that band I was covered by much more intense absorption band of Ce4+. 3.2.2. UV−Visible Luminescence Properties of Na3LuSi3O9:Ce3+. To investigate the influence of oxygen vacancies on photoluminescence properties of Na3LuSi3O9:Ce3+, the normalized excitation and emission spectra of Na3Lu0.99Ce0.01Si3O9 (CeO2 as raw material) prepared in CO (A), recalcined in air (B), and synthesized in air (C) are given in Figure 3. Obviously,
Figure 3. Emission spectra (D) and normalized excitation and emission spectra of Na3Lu0.99Ce0.01Si3O9 (CeO2 as raw material): (A) synthesized in CO reducing atmosphere; (B) recalcined in air; (C) synthesized in air (measured at room temperature).
the broad excitation band from 280 to 340 nm of Na3Lu0.99Ce0.01Si3O9 is stronger than that transition from 2F5/2 to 5d1 within Ce3+ (centered at 350 nm) when Na3Lu0.99Ce0.01Si3O9 was prepared under CO reducing atmosphere (Figure 3A). The intensity of this band (280−340 nm) decreased significantly after recalcination in air (Figure 3B), and it nearly disappeared (as the red arrow showed) when synthesized in air (Figure 3C). It further verified our assumption that the broad excitation band (280−340 nm) was caused by oxygen vacancies. Another excitation band from 250 to 280 nm can be observed in Figure 3A which also belonged to oxygen vacancies absorption at higher 18743
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Figure 4. Normalized emission spectra of Na3Lu1−xCexSi3O9 (x = 0.005, 0.01, 0.03, 0.05, 0.07, 0.09) synthesized in CO reducing atmosphere (A, λex = 350 nm); recalcined in air (B, λex = 350 nm) and synthesized in air (C, λex = 360 nm) measured at room temperature.
stronger with increasing Ce3+ doping concentration and the Ce3+ emission dominated at high Ce3+ content. As for Na3Lu1−xCexSi3O9 synthesized in air, all the normalized emission spectra overlapped with center at 395 nm (Figure 4C) because of no defect luminescence. It was also found that oxygen vacancy had an effect on the luminescence concentration quenching of Na3Lu1−xCexSi3O9 (Figure S3). 3.2.3. X-ray Excited Luminescence (XEL) Properties of Na3LuSi3O9:Ce3+. To investigate the influence of oxygen vacancies on X-ray excited luminescence properties, the X-ray excited luminescence spectra of Na3Lu0.99Ce0.01Si3O9 synthesized with CeO2 (a, prepared in CO; b, prepared in air) and Ce(NO3)3·6H2O (c, prepared in CO; d, prepared in air) as raw materials are given in Figure 5. As mentioned in the photo-
By comparing curves a and b (c and d), it can be known that the intensity of oxygen vacancies’ emission decreased obviously when synthesized in air. It can also be noted that from curves a and c (b and d), the defect luminescence was also strongly impaired when Ce(NO3)3·6H2O was used as raw material which means that the reduction of Ce4+ (CeO2) could promote the formation of oxygen vacancy when synthesized in CO. According to the above results, it can be concluded that Ce4+ in the raw material and CO reducing atmosphere were two key factors to the formation of oxygen vacancy. 3.2.4. Thermal Quenching Properties of Na3LuSi3O9:Ce3+. Figure 6 shows the temperature dependence on the PL spectra
Figure 5. X-ray excited luminescence spectra of Na3Lu0.99Ce0.01Si3O9 with CeO2 (a, in CO; b, in air) and Ce(NO3)3·6H2O (c, in CO; d, in air) as raw materials.
luminescence spectra, the emission of oxygen vacancies overlapped with Ce3+ emission partly and it could not be distinguished in the PL emission spectra (Figure 3). However, the emission of oxygen vacancies became discernible in the XEL spectra (shoulder peaks from 330 to 367 nm of curve b and curve c). And this emission band can be reabsorbed by Ce3+ (absorption wavelength range of Ce3+ was from 330 to 380 nm, Figure 3). It can also be observed that Na3Lu0.99Ce0.01Si3O9 with CeO2 as raw material synthesized in CO (curve a) showed the sgrongest emission intensity under X-ray radiation due to intense emission of oxygen vacancies and energy transfer from oxygen vacancies to Ce3+. Curve d showed characteristic double bands emission of Ce3+ ions centering at about 395 and 427 nm without emission from oxygen vacancies, which is consistent with Figure 3D.
Figure 6. Temperature dependence on luminescence intensity of Na3Lu0.99Ce0.01Si3O9 synthesized in CO reducing atmosphere (A, λex = 350 nm) and air (B, λex = 360 nm) and corresponding Arrhenius plot of temperature dependence on luminescence intensity of Na3Lu0.99Ce0.01Si3O9 synthesized in CO (C, λex = 350 nm) and air (D, λex = 360 nm).
of Na3Lu0.99Ce0.01Si3O9 synthesized in CO reducing atmosphere (A, λex = 350 nm) and in air (B, λex = 360 nm). It can be noted that the emission intensity decreased and the full width at half-maximum (fwhm) broadened when the temperature increased from 300 to 500 K. To fully understand the temperature dependence of the emission intensity and to determine the activation energy for thermal quenching, corresponding 18744
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Figure 7. Decay curves of Na3Lu0.99Ce0.01Si3O9 synthesized in CO reducing atmosphere (A), recalcined in air (B) and synthesized in air (C). The curve in part C was fitted with a single exponential equation.
Table 1. Fitting Results of the Decay Curves of Figure 7 CO recalcined in air air
λex = 310 nm, λem = 350 nm
λex = 310 nm, λem = 385 nm
λex = 350 nm, λem = 385 nm
τ1 = 7.0 ns, τ2 = 27.9 ns τ1 = 4.0 ns, τ2 = 23.2 ns
τ1 = 8.7 ns, τ2 = 36.7 ns τ1 = 5.3 ns, τ2 = 29.4 ns
35.1 ns 32.7 ns 29.8 ns
Arrhenius equation15 eq 1 was fitted to the thermal quenching data of Na3Lu0.99Ce0.01Si3O9 and the graph of ln[(I0/IT) − 1] vs 1/T was given in Figure 6, parts C and D. IT = I0/(1 + c exp( −ΔE /KT ))
where It and I0 are the luminescence intensity, A is a constant, t is the time, and τ is the decay time. The values of τ were calculated to be 35.1, 32.7, and 29.8 ns (as shown in Table 1) for Na3Lu0.99Ce0.01Si3O9 synthesized in CO atmosphere (A, λex = 350 nm, λem = 385 nm), recalcined in air (B, λex = 350 nm, λem = 385 nm) and synthesized in air (C, λex = 350 nm, λem = 385 nm), respectively, which means that the decreasing of the oxygen vacancies can quicken the lifetime of Ce3+. Pulsed X-ray decay measurements were also performed on Na3Lu0.99Ce0.01Si3O9 prepared in different atmosphere in our previous work and they all showed a decay time of about 30 ns.9 When samples were excited at 310 nm and monitored at 385 nm, it could not be fitted with a single component because of the energy transfer from the oxygen vacancies to Ce3+. The lifetime of defect luminescence (Figure 7, parts A and B; λex = 310 nm, λem = 350 nm) was relatively short and the curves can be fitted with a second order exponential equation:16
(1)
In eq 1, I0 is the initial emission intensity, IT is the intensity at a given temperature T, c is a constant for a certain host, K is Boltzmann constant (8.629 × 10−5 eV/K), ΔE is activation energy of thermal quenching. The activation energy for thermal quenching was calculated to be 0.1828 eV (Figure 6C) and 0.3386 eV (Figure 6D), respectively. Obviously, Na3Lu0.99Ce0.01Si3O9 synthesized in CO and excited at 350 nm has lower activation energy which meant it was easier to be thermal quenched than that of synthesized in air. However, the emission intensity of Na3Lu0.99Ce0.01Si3O9 synthesized in CO at 500 K was 53.9% of its original value at 300 K (Figure 6A), which was much higher than that of 26.5% synthesized in air (Figure 6B). It was probably due to that there were many traps located between the lowest 5d energy level of Ce3+ and the conduction band for Na3Lu0.99Ce0.01Si3O9 synthesized in reduction atmosphere. On the one hand, electrons at the lowest 5d level of Ce3+ ions can be easily trapped by defects when the sample was heated which led to low thermal activation energy. On the other hand, these defects showed emission and the energy transfer rate from vacancies to Ce3+ increased when the sample was heated which made it look like that the decrease of the intensity of Ce3+ was impaired (compared with that of air treated one). So there were some deviations for the result of the activation energy of Ce3+ with the Arrhenius equation for Na3Lu0.99Ce0.01Si3O9 synthesized in CO reducing atmosphere. The thermal activation energy for Ce3+ should be 0.3386 eV which was calculated from Na3Lu0.99Ce0.01Si3O9 synthesized in air. 3.2.5. Luminescence Decay Properties of Na3LuSi3O9:Ce3+. As shown in Figure 7, when Na3Lu0.99Ce0.01Si3O9 samples synthesized in CO reducing atmosphere, recalcined in air and synthesized air were excited with Ce3+ absorption peaks, Ce3+ were excited directly, so the curve can be well fitted with a single exponential equation:16 It = A + I0 exp( −t /τ )
It = A1 exp( −t /τ1) + A 2 exp(−t /τ2)
(3)
In eq 3, It is the luminescence intensity, t is time, τ1 and τ2 are the slow and fast components of the decay lifetimes, and A1 and A2 are the fitting parameters, respectively.
4. DISCUSSION From above results, it can be observed that the oxygen vacancies have obvious influence on the luminescence properties of Na3LuSi3O9:Ce3+ samples. For further understanding the effects of oxygen vacancies on Na3LuSi3O9:Ce3+ synthesized in different ambient condition, the confirmation and roles of oxygen vacancies will be discussed below. 4.1. Confirmation of Oxygen Vacancies by Thermoluminescence (TL) Spectra. To confirm the existence of oxygen vacancies in Na3LuSi3O9:Ce3+, wavelength resolved TL glow curves of Na3Lu0.995Ce0.005Si3O9 prepared under different atmosphere are displayed in Figure 8 (A, CO; B, recalcined in air; C, air). It was found that Na3Lu0.995Ce0.005Si3O9 showed the strongest TL intensity when prepared in CO (Figure 8A) and the TL intensity became weak after recalcined in air (Figure 8B) and no thermoluminescence was observed for Na3Lu0.995Ce0.005Si3O9 prepared in air (Figure 8C). According to the above all, it
(2) 18745
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Figure 9. Normalized excitation (λem = 385 nm) and emission (λex = 350 nm) spectra of Na3Lu0.99Ce0.01Si3O9 with CeO2 (a, c) and Ce(NO3)3·6H2O (b, d) as raw materials synthesized in CO reducing atmosphere measured at room temperature.
menon indicates that the reduction of Ce4+ to Ce3+ promoted the formation of the oxygen vacancies. One can note that the peak positions of the Ce3+ excitation (curve b, 360 nm) and emission (curve d, 395 nm) bands of Na3Lu0.99Ce0.01Si3O9 with Ce(NO3)3·6H2O as raw material had obvious redshift compared with that using CeO2 as raw material (curve a, 350 nm; curve c, 380 nm). It can be concluded that the amount of oxygen vacancies decreased when Ce(NO3)3· 6H2O was used as raw material and the lowest 5d orbit of Ce3+ became lower. And the suppressed defect luminescence due to the decreasing oxygen vacancies (Ce(NO3)3·6H2O as raw material) also contributed to the redshift.
Figure 8. TL emission spectra of Na3Lu0.995Ce0.005Si3O9 synthesized in (A) CO atmosphere, (B) recalcined in air and (C) air. (D) emission spectrum of TL at 150 °C (a) and emission spectrum of Na3Lu0.995Ce0.005Si3O9 excited at 350 nm (b) synthesized in CO atmosphere.
can be known that Na3LuSi3O9:Ce3+ prepared in CO had the most oxygen vacancies and the amount of vacancies decreased after recalcined in air and there was no oxygen vacancy when prepared in air. Na3Lu0.995Ce0.005Si3O9 synthesized in CO reducing atmosphere and recalcined in air both had a TL emission band centered at about 390 nm which was in line with the Ce3+ emission ((Figure 8D). There may be two reasons contributed to this, one was that the energy of the recombination of holes and electrons could be transferred to luminescence centers (i.e., Ce3+) and then the Ce3+ became excited. And the other one was that cerium itself behaved as a recombination center.17 The existence of oxygen vacancy was also confirmed by the suppression of the intensity of oxygen vacancy excitation band in Zr4+-doped Na3Lu0.995Ce0.005Si3O9 (Figure S4). It was because Zr4+ ions in Lu3+ sites will introduce an excess positive charge. Thus, the introduction of Zr4+ would decrease the number of oxygen vacancies18 and it can be reflected by the suppression of the defect absorption in the excitation spectra. 4.2. Probable Formation Mechanism of Oxygen Vacancies: The Reduction of Ce4+ to Ce3+. From the XEL spectra of Na3Lu0.99Ce0.01Si3O9 (Figure 5), it can be noted that the raw materials (CeO2 and Ce(NO3)3·6H2O) had an effect on the formation of oxygen vacancies. To interpret how raw materials influenced the formation of oxygen vacancies, the normalized excitation and emission spectra of Na3Lu0.99Ce0.01Si3O9 with different raw materials (curves a and c, CeO2 as raw material; curves b and d, Ce(NO3)3·6H2O as raw material) synthesized in CO reducing atmosphere were displayed in Figure 9. It was observed that the relative intensity of the oxygen vacancy excitation band decreased obviously when Ce(NO3)3·6H2O was used as raw material (curve b) compared that CeO2 as raw material (curve a). The ratio of the intensity of oxygen vacancy excitation band to that of Ce3+ excitation band was 1.21 to 1 for CeO2 as raw material and it is 0.60 to 1 for Ce(NO3)3·6H2O as raw material. This pheno-
5. CONCLUSIONS In order to investigate the role of oxygen vacancies in the luminescence process of a new scintillator phosphor Na3LuSi3O9:Ce3+, a series of Na3Lu(1‑x)CexSi3O9 phosphors were prepared with a high temperature solid-state reaction method under different atmosphere (air, CO reducing atmosphere and recalcined in air) and different raw materials. It was found that the absorption spectra, luminescence excitation and emission spectra, thermoluminescence and decay properties of Na3Lu(1‑x)CexSi3O9 were greatly influenced by the synthesis atmosphere. Emission band from 250 to 340 nm belonging to oxygen vacancies were observed in the UV luminescence spectra of Na3LuSi3O9:Ce3+ synthesized in CO and recalcined in air, respectively. However, no such bands were observed for phosphors prepared in air. Obvious redshift caused by oxygen vacancy in the emission spectra of Na3LuSi3O9:Ce3+ with increasing Ce3+ content was observed when synthesized in CO and recalcined in air. Na3LuSi3O9:Ce3+ had fast decay time (∼30 ns) and the photoluminescence decay of Ce3+ can be quickened by the decreasing of oxygen vacancies. Na3LuSi3O9:Ce3+ synthesized in CO showed much better thermal quenching properties than that synthesized in air and the thermal quenching activation energy for Ce3+ (from lowest 5d level to conduction band) was calculated to be 0.3386 eV. The presence of oxygen vacancies was confirmed by the thermoluminescence spectra and excitation spectra of Zr4+ codoped Na3Lu0.995Ce0.005Si3O9. It was also found that Na3LuSi3O9:Ce3+ prepared in CO atmosphere showed the highest PL and XEL intensity due to vacancies’ luminescence and the energy transfer from oxygen vacancies to Ce3+. At last, it was proved that the reduction of Ce4+ to Ce3+ could promote the formation of the oxygen vacancies. 18746
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(11) Nikl, M.; Laguta, V. V.; Vedda, A. Complex Oxide Scintillators: Material Defects and Scintillation Performance. Phys. Status Solidi B 2008, 245, 1701−1722. (12) Blacklocks, A. N.; Chadwick, A. V.; Jackson, R. A.; Hutton, K. B. Investigation into Thallium Sites and Defects in Doped Scintillation Crystals. Phys. Status Solidi C 2007, 4, 1008−1011. (13) Zhang, C. L.; Guo, D. L.; Xu, W. N.; Hu, C. G.; Chen, Y. X. Radiative/Nonradiative Recombination Affected by Defects and Electron−Phone Coupling in CdWO4 Nanorods. J. Phys. Chem. C 2016, 120, 12218−12225. (14) Koroleva, T. S.; Kidibaev, M. M.; Nehari, A.; Pedrini, C.; Lebbou, K.; Belsky, A. N.; Tcherepanov, A. N.; Ishchenko, A. V.; Ivanov, V. Y.; Sedunova, I. N.; et al. Ce-Doped Li6Ln(BO3)3 (Ln = Y, Gd) Single Crystals Fibers Grown by Micro-Pulling Down Method and Luminescence Properties. Opt. Mater. 2013, 35, 868−874. (15) Peng, M. Y.; Yin, X. W.; Tanner, P. A.; Brik, M. G.; Li, P. F. Site Occupancy Preference, Enhancement Mechanism, and Thermal Resistance of Mn4+ Red Luminescence in Sr4Al14O25: Mn4+ for Warm Wleds. Chem. Mater. 2015, 27, 2938−2945. (16) Yu, H.; Deng, D. G.; Zhou, D. T.; Yuan, W.; Zhao, Q. G.; Hua, Y. J.; Zhao, S. L.; Huang, L. H.; Xu, S. Q. Ba2Ca(PO4)2: Eu2+ Emission-Tunable Phosphor for Solid-State Lighting: Luminescent Properties and Application as White Light Emitting Diodes. J. Mater. Chem. C 2013, 1, 5577−5582. (17) Bos, A. J. J. Theory of Thermoluminescence. Radiat. Meas. 2006, 41, S45−S56. (18) Krasnikov, A.; Savikhina, T.; Zazubovich, S.; Nikl, M.; Mares, J. A.; Blazek, K.; Nejezchleb, K. Luminescence and Defects Creation in Ce3+-Doped Aluminium and Lutetium Perovskites and Garnets. Nucl. Instrum. Methods Phys. Res., Sect. A 2005, 537, 130−133.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b04964. VUV luminescence properties of Na3LuSi3O9:Ce3+, selfreduction of Ce4+ in Na3LuSi3O9:Ce3+, luminescence quenching of Na3Lu1−xCexSi3O9, and luminescence spectra of Zr4+-doped Na3Lu0.995Ce0.005Si3O9 (PDF)
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AUTHOR INFORMATION
Corresponding Author
*(J.Zhong) E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the “973” Programs (2014CB643801), National Natural Science Foundation of China (No.11375278), Science & Technology Project of Guangdong Province (No. 2015A050502019), Science & Technology Project of Guangzhou (No. 201510010296), Science & Technology Project of Jiangxi Province (No. 20141BDH80038), State Key Laboratory of Optoelectronic Materials and Technologies (Sun Yat-sen University), and China Scholarship Council (No. 201406385019).
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REFERENCES
(1) Feng, H.; Xu, W. S.; Ren, G. H.; Yang, Q. H.; Xie, J. J.; Xu, J.; Xu, J. Y. Optical, Scintillation Properties and Defect Study of Gd2Si2O7:Ce Single Crystal Grown by Floating Zone Method. Phys. B 2013, 411, 114−117. (2) Van Eijk, C. W. E. Inorganic-Scintillator Development. Nucl. Instrum. Methods Phys. Res., Sect. A 2001, 460, 1−14. (3) Liu, B.; Gu, M.; Qi, Z. M.; Shi, C. S.; Yin, M.; Ren, G. H. LaserExcited Spectra of Lu2SiO5:Ce Scintillator. J. Lumin. 2007, 127, 645− 649. (4) Ricci, P. C.; Carbonaro, C. M.; Casu, A.; Cannas, C.; Corpino, R.; Stagi, L.; Anedda, A. Optical and Structural Characterization of Cerium Doped LYSO Sol−Gel Polycrystal Films: Potential Application as Scintillator Panel for X-Ray Imaging. J. Mater. Chem. 2011, 21, 7771−7776. (5) Qin, L. S.; Li, H. Y.; Lu, S.; Ding, D. Z.; Ren, G. H. Growth and Characteristics of LYSO (Lu2(1−x−y)Y2xSiO5:Cey) Scintillation Crystals. J. Cryst. Growth 2005, 281, 518−524. (6) Bondar, V.; Grigorjeva, L.; Kärner, T.; Sidletskiy, O.; Smits, K.; Zazubovich, S.; Zolotarjovs, A. Thermally Stimulated Luminescence of Undoped and Ce3+-Doped Gd2SiO5 and (Lu,Gd)2SiO5 Single Crystals. J. Lumin. 2015, 159, 229−237. (7) Pauwels, D.; Le Masson, N.; Viana, B.; Kahn-Harari, A.; van Loef, E. V. D.; Dorenbos, P.; van Eijk, C. W. E. A Novel Inorganic Scintillator: Lu2Si2O7:Ce3+ (LPS). IEEE Trans. Nucl. Sci. 2000, 47, 1787−1790. (8) Yan, C. F.; Zhao, G. J.; Hang, Y.; Zhang, L. H.; Xu, J. Comparison of Cerium-Doped Lu2Si2O7 and Lu2SiO5 Scintillators. J. Cryst. Growth 2005, 281, 411−415. (9) Zhou, J. B.; Pan, F. J.; Zhong, J. P.; Liang, H. B.; Su, Q.; Moretti, F.; Lebbou, K.; Dujardin, C. Luminescence Properties of Na3luSi3O9:Ce3+ as Potential Scintillator Material. RSC Adv. 2015, 5, 102477−102480. (10) Blahuta, S.; Bessière, A.; Viana, B.; Ouspenski, V.; Mattmann, E.; Lejay, J.; Gourier, D. Defects Identification and Effects of Annealing on Lu2(1‑x)Y2xSiO5 (LYSO) Single Crystals for Scintillation Application. Materials 2011, 4, 1224−1237. 18747
DOI: 10.1021/acs.jpcc.6b04964 J. Phys. Chem. C 2016, 120, 18741−18747
