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Blue–green afterglow of BaAl2O4:Dy3+ phosphors
Materials Research Bulletin 75 (2016) 1–6
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Blue–green afterglow of BaAl2O4:Dy3+ phosphors Bao-gai Zhaia , Qing-lan Maa,b , Rui Xiongc , Xiazhang Lid , Yuan Ming Huanga,* a
School of Mathematics and Physics, Changzhou University, Jiangsu 213164, China School of Electronics and Information, Nantong University, Jiangsu 226019, China c School of Physics and Technology, Wuhan University, Hubei 430072, China d Analysis and Testing Center, Changzhou University, Jiangsu 213164, China b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 3 June 2015 Received in revised form 25 October 2015 Accepted 11 November 2015 Available online xxx
Dy3+ doped barium aluminate (BaAl2O4:Dy3+) phosphors were prepared via the sol–gel combustion route at the ignition temperature of 600 C. The phosphors were characterized with X-ray diffractometry, scanning electron microscopy, transmission electron microscopy, photoluminescence spectroscopy, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Regardless of the absence of Eu2+ luminescent centers, broadband blue–green afterglow with its peak at about 490 nm was recorded in the BaAl2O4:Dy3+ phosphors. The decay profile of the blue–green afterglow can be best fitted into a two-component exponential function with the two lifetime decay constants to be 8.81 and 45.25 s, respectively. The observation of blue–green afterglow from BaAl2O4:Dy3+ in the absence of Eu2+ provides unique opportunity in unveiling the afterglow mechanisms of rare-earth doped alkaline-metal aluminates. Possible mechanisms on the blue–green afterglow in BaAl2O4:Dy3+ phosphors are discussed in terms of the Dy3+ ions introduced trap centers as well as luminescent centers in the crystal lattice. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Optical materials Sol–gel chemistry Phosphors Luminescence Inorganic compounds Nanostructures Transmission electron microscopy
1. Introduction The long afterglows from rare-earth doped alkaline-earth aluminates have found wide applications in emergency signalization, micro-defect sensing, optoelectronics for image storage, and detectors of high-energy radiation. Compared to the intensively studied SrAl2O4:Eu2+,Dy3+, the afterglow from rare-earth doped barium aluminates (BaAl2O4) has attracted limited attention in spite of their excellent chemical stability, high luminescent intensity and long persistent time [1–6]. As documented in the literature, the studies on the afterglow of the BaAl2O4 system are primarily focused on Eu2+ doped BaAl2O4 with the second rareearth activator including La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Er, Tm [1,3]. For example, Rodrigues et al. reported identical blue–green persistent luminescence from the BaAl2O4:Eu2+,R3+ phosphors regardless of the difference in the auxiliary dopant R3+. Among these reports, it is generally agreed that the Eu2+ ion acts as the luminescent center while the auxiliary dopant (i.e., La3+, Ce3+, Pr3+, Nd3+, Sm3+, Gd3+, Tb3+, Dy3+, Er3+, Tm3+) act as the hole or electron traps [1–6]. It is hoped that proper trap energy levels can be formed in the BaAl2O4 phosphors via doping the host with the auxiliary
dopant, thereby achieving the aim of enhanced afterglow brightness and prolonged afterglow time. In spite of the work so far, the mechanism of afterglow in BaAl2O4 based phosphors needs further examination. Especially, what will happen to the afterglow in BaAl2O4 based phosphors if the luminescent center Eu2+ is absent? Generally speaking, trivalent rare earth dopant such as Dy3+ by itself can generate deep traps as well as luminescent centers in the phosphors, and afterglow behavior can be expected once the deep traps and the luminescent centers are present in phosphors. For example, broadband green afterglow could be achieved in the SrAl2O4:Dy3 + phosphors although Eu2+ luminescence centers were absent [7,8]. Therefore, it will be interesting if afterglow can be observed in BaAl2O4 doped with Dy3+ alone because such finding is strongly contrast to the generally accepted belief that the Eu2+ luminescent center is responsible for the blue–green afterglow in SrAl2O4:Eu2+, Dy3+ phosphors. In present work, we prepared BaAl2O4:Dy3+ phosphors via the sol–gel combustion route. Intense blue–green afterglow was discernible in the dark with naked eyes after the removal of the excitation source. 2. Materials and methods
* Corresponding author. Fax: +86 519 86056701. E-mail address: [email protected] (Y.M. Huang). http://dx.doi.org/10.1016/j.materresbull.2015.11.021 0025-5408/ ã 2015 Elsevier Ltd. All rights reserved.
BaAl2O4:Dy3+ phosphors were prepared by the sol–gel combustion method [7–9]. All reagents were provided by Sinopharm Chemical Reagents Co., Ltd. (Shanghai, China). The purity of Dy2O3
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was 99.99% while all other reagents were in analytical grade. Barium chloride (0.02 mol), aluminum nitrate (0.04 mol), urea (0.6 mol), Dy2O3 (0.0004 mol) and H3BO3 (0.002 mol) were dissolved in deionized water (100 ml) to form a transparent solution under the stirring of a magnetic bar. The solution was stored at room temperature for one week before the sol–gel combustion. The molar ratio of urea, barium chloride and aluminum nitrate was 30:1:2. With respect to the barium chloride, the molar fractions of Dy3+ ions and H3BO3 in the sol–gel were 4 and 10 mol%, respectively. After having been filled with 15 ml of the sol–gel, an alumina crucible in the capacity of 30 ml was transferred into an air-filled furnace for combustion at the ignition temperature of 600 C. X-ray diffraction (XRD) patterns were recorded on an X-ray diffractometer (D/max 2500 PC, Rigaku Corporation, Japan) using Cu Ka radiation (l = 1.5405 Å). The voltage applied to the Cu target in the XRD machine was 40 kV. The morphology of the phosphors was determined with a scanning electron microscopy (SEM) (JSM6360LA, Japan). The accelerating voltage applied to the electron gun in the SEM was 15 kV. The nanostructures and the selected area electron diffraction pattern of the phosphors were characterized on a transmission electron microscope (TEM) (JEOL JEM-2100, Japan Electronics Corp.) which was operated at 200 kV. Both the photoluminescence (PL) and afterglow spectra of the phosphors were taken at room temperature on a spectrophotometer (Tianjin Gangdong Inc., China). The excitation wavelength for the PL measurement was the 325 nm emission line from a heliumcadmium laser (Kimmon Koha Co., Ltd., Japan). In order to measure the afterglow decay curves, the phosphors were exposed to the irradiation of the UV laser for 10 min, and then the afterglow was collected with a focus lens into our spectrophotometer. The monitoring wavelength of the afterglow decay measurement was fixed at 490 nm, the data taking process for the afterglow decay was started once the UV irradiation from the laser was blocked off. The details on the PL and afterglow measurements were described in literature [7–9]. The excitation spectrum of our BaAl2O4:Dy3+ phosphors was measured with the fluorescence spectrometer LS45 (PerkinElmer) by fixing the monitoring wavelength at 575 nm. Fourier transform infrared (FTIR) spectroscopy was utilized to analyze the chemical bonding on the surface of the phosphors. The infrared absorption spectra of the materials were recorded by using the standard KBr pellet techniques with a model Nicolet iS5 FTIR spectrometer (Thermo Scientific) in the range of 400– 4000 cm1. The X-ray photoelectron spectroscopy (XPS) measurements were performed on an Escalab 250Xi spectrophotometer (Thermo Scientific) with Al Ka radiation (1486.6 eV). The XPS spectrometer was calibrated by recording the binding energy of Au 4f7/2 peak at 83.9 eV. C 1s peak at 284.6 eV was taken as an internal standard. 3. Results and discussions Fig. 1 shows the XRD curve of the BaAl2O4:Dy3+ phosphors. The concentration of Dy3+ ions in the phosphors was 4 mol%. The dominant peaks in the XRD curve are located at 19.60, 21.84, 28.28, 34.32, 40.10, 41.13, 45.10, 45.84, 53.63, 54.55, 57.79, 61.59, 67.23, 69.74, and 74.36 , respectively, which can be assigned to the Bragg reflections from the planes (2 0 0), (2 0 1), (2 0 2), (2 2 0), (2 2 2), (0 0 4), (4 0 2), (2 0 4), (4 2 0), (2 2 4), (4 2 2), (6 0 0), (2 0 6), (4 2 4), and (2 2 6) of hexagonal BaAl2O4. The standard diffraction data of the hexagonal BaAl2O4 (JCPDS card no. 34-0379) are depicted at the bottom of Fig. 1. A comparison shows that the diffraction peaks of the BaAl2O4:Dy3+ phosphors match with those of the standard hexagonal BaAl2O4 (a = 1.0447 nm, c = 0.8794 nm). It can be seen in Fig. 1 that the addition of a small amount of Dy3+ (4 mol%) has no obvious influence on the crystal structure of the host BaAl2O4. The
Fig. 1. XRD curve of the BaAl2O4:Dy3+ phosphors with the dopant concentration of 4 mol%. The standard data of hexagonal BaAl2O4 (JCPDS card no. 17-0306) are displayed at the bottom for comparison. Inset: the SEM micrograph of BaAl2O4:Dy3+ phosphor.
similar ionic radii of Ba2+ (0.135 nm) and Dy3+ (0.0917 nm) may be responsible for the unchanged crystal structure. Thus, the XRD data have demonstrated that pure phase BaAl2O4:Dy3+ has been obtained via the sol–gel combustion. The inset of Fig. 1 represents the SEM micrograph of the BaAl2O4:Dy3+ phosphors. The lowmagnification SEM micrograph shows many aggregations of the BaAl2O4:Dy3+ nanoparticles. It is clear that the sizes of the BaAl2O4: Dy3+ aggregations are in the range of 5–60 mm. We noted that a lot of pores and voids are present in the BaAl2O4:Dy3+ phosphors. The voluminous gases (i.e., N2, NH3, CO2) generated in the combustion reaction are believed to be responsible for the formation of the pores. The exploration on the nanostructures of the synthesized BaAl2O4:Dy3+ phosphors requires the usage of TEM. Fig. 2 displays the low-magnification TEM micrograph, high-resolution TEM micrograph and the selected area electron diffraction pattern of the BaAl2O4:Dy3+ phosphors. As shown by the low-magnification TEM micrograph in Fig. 2(a), the typical sizes of BaAl2O4:Dy3+ phosphors are about 30 nm in diameter. Fig. 2(b) shows the highresolution TEM micrograph of BaAl2O4:Dy3+ phosphors to display the lattice structures of the BaAl2O4:Dy3+ nanocrystals. It can be seen in Fig. 2(b) that the synthesized phosphors are highly crystalline, and that the spacing between two adjacent planes is about 0.315 nm which is in good agreement with the distance between two (2 0 2) crystal planes of the hexagonal BaAl2O4:Dy3+ phosphors. The crystalline nature of the synthesized BaAl2O4:Dy3+ nanoparticles can also be evidenced with the selected area electron diffraction pattern of the BaAl2O4:Dy3+ phosphors, as shown in Fig. 2(c). Our TEM micrographs in Fig. 2 have verified that the BaAl2O4:Dy3+ phosphors are in hexagonal phase. Fig. 3 represents the PL spectra of the BaAl2O4:Dy3+ phosphors as the concentration of Dy3+ increases from 0 to 4 mol%. As shown by the bottom curve in Fig. 3, the host BaAl2O4 can give off strong PL upon the photoexcitation of the 325 nm photons. The bandgap of defect-free BaAl2O4 crystal is around 6.5 eV [1], so perfect hexagonal BaAl2O4 crystal has no absorption and emissions under our 325 nm UV excitation (3.8 eV). The broadband PL spectrum of the host, whose peak is located at about 400 nm, indicates that some defects are present in the host material. Intrinsic defects such as oxygen vacancies (VO), Ba vacancies (VBa) and some color centers are supposed to be responsible for the photon absorption and its subsequent emission in the host. Similar broadband PL was observed in SrAl2O4 [7,8]. After Dy3+ doping, each PL spectrum of
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Fig. 3. PL spectra of BaAl2O4:Dy3+ phosphors with different dopant concentrations. (For interpretation of the references to color in text, the reader is referred to the web version of this article.)
Fig. 2. Low-magnification TEM micrograph (a), high-resolution TEM micrograph (b) and the selected area electron diffraction pattern (c) of the BaAl2O4:Dy3+ phosphors.
the BaAl2O4:Dy3+ phosphors consists of two characteristic sharp emissions of Dy3+ ions in addition to the broad PL band from the host BaAl2O4. The two narrow PL bands are the characteristic emissions of Dy3+ due to its 4F9/2 ! 6H15/2 and 4F9/2 ! 6H13/2 transitions [7,8,10,11], indicating that Dy3+ ions act as the blue and yellow luminescent centers in the BaAl2O4 host. When compared to BaAl2O4:Eu2+,Dy3+ phosphors, our PL spectra of BaAl2O4:Dy3+ phosphors are dramatically different from those of BaAl2O4:Eu2+, Dy3+ phosphors [1,5,6,12,13]. Such difference can be attributed to the absence of Eu2+ in our BaAl2O4:Dy3+ phosphors. Fig. 4 illustrates the excitation spectrum of BaAl2O4:Dy3+ phosphors. The concentration of Dy3+ ions in the phosphors was 4 mol%. The monitoring wavelength was fixed at 575 nm. It is clear that five characteristic absorption peaks of Dy3+ are located at around 320, 350, 384, 450, and 475 nm in the excitation spectrum. As discussed in the literature, these absorption peaks can be assigned to the transitions from the ground state of Dy3+ (6H15/2) to its excited states 4L19/2,6P7/2+4M15/2, 4I13/2+4F7/2, 4I15/2, and 4F9/2, respectively [7,8,10,11]. These characteristic absorptions have demonstrated that neither Eu2+ nor Eu3+ ions are present in our
BaAl2O4:Dy3+ phosphors since no characteristic absorptions of Eu2+ and Eu3+ are detected in the excitation spectrum. In a similar way as reported in our recent work, our measurements on the L3 edge X-ray absorption spectrum and the time-resolved fluorescence spectroscopy confirmed the absence of Eu2+ ions in the BaAl2O4:Dy3+ phosphors [7]. After the UV irradiation of the laser is blocked off, the emissions of the BaAl2O4 host and the characteristic emissions of Dy3+ at 480 and 575 nm disappear immediately, leaving the BaAl2O4:Dy3+ phosphors glowing in dark for about several minutes. Fig. 5 shows the afterglow spectrum of the BaAl2O4:Dy3+ phosphors. It is
Fig. 4. Excitation spectrum of BaAl2O4:Dy3+ phosphors with the dopant concentration of 4 mol%.
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Fig. 5. Afterglow spectrum of the BaAl2O4:Dy3+ phosphors with the dopant concentration of 4 mol%. Inset: the CIE chromaticity diagram to show the chromaticity coordinates of the afterglow and PL from BaAl2O4:Dy3+ phosphors. (For interpretation of the references to color in text, the reader is referred to the web version of this article.)
obvious that the afterglow spectrum of the BaAl2O4:Dy3+ phosphors is broad in profile with its peak at about 490 nm. The profile of afterglow spectrum of our BaAl2O4:Dy3+ phosphors is quite similar to those of BaAl2O4:Eu2+,Dy3+ phosphors [1,5,6,12,13]. For the purpose of comparison, Eu2+ (4 mol%) and Dy3+ (4 mol%) codoped BaAl2O4:Eu2+,Dy3+ phosphors were synthesized under identical experimental conditions. The afterglow intensity of BaAl2O4:Eu2+,Dy3+ phosphors, however, was much stronger than that of the BaAl2O4:Dy3+ phosphors. In order to quantitatively characterize the color of the afterglow, we calculated the CIE chromaticity coordinates of BaAl2O4:Dy3+ phosphors on the basis of their emission spectral data [14,15]. The CIE chromaticity coordinates of the afterglow were derived to be (0.1265, 0.3063) for the BaAl2O4:Dy3+ phosphors. The inset of Fig. 5 represents the CIE chromaticity diagram in which the chromaticity coordinates of afterglow and PL from BaAl2O4:Dy3+ phosphors are labeled. It is clear that the color of the afterglow is blue–green while the color of PL is purple. The data in Fig. 5 have demonstrated that long afterglow can be achieved in BaAl2O4:Dy3+ phosphors in spite of the absence of Eu2+ ions. Fig. 6 displays the semi-logarithmic plot of the afterglow decay curve of the BaAl2O4:Dy3+ phosphors. The monitoring wavelength was fixed at 490 nm. The hollow circles represent the experimental afterglow signals. As represented by the green solid line in Fig. 6, this decay curve was best fitted to the two-component exponential function IðtÞ ¼ I0 þ
2 X Ii expðt=t i Þ;
Fig. 6. Afterglow decay curve of BaAl2O4:Dy3+ phosphors with the dopant concentration of 4 mol%. (For interpretation of the references to color in text, the reader is referred to the web version of this article.)
description of the relation between the independent and dependent variables. We measured the FTIR absorption spectra of pristine BaAl2O4 as well as Dy3+ doped BaAl2O4 to understand the surface passivation of the BaAl2O4:Dy3+ phosphors. The concentration of Dy3+ ions in the BaAl2O4:Dy3+ phosphors was 4 mol%. Fig. 7 represents the FTIR spectra of the pristine BaAl2O4 nanocrystals (green curve) and BaAl2O4:Dy3+ nanocrystals (red curve), respectively. It can be seen clearly that the nine strong absorption bands are located at 430, 639, 843, 1360, 1416, 1612, 2851, 2922 and 3400 cm1 for the pristine BaAl2O4. As a contrast, only seven strong absorption bands are present in the FTIR spectrum of the BaAl2O4:Dy3+ phosphors, which are located at 430, 639, 843, 1360, 1416, 1612 and 3400 cm1, respectively. A comparison of the two FTIR spectra in Fig. 7 reveals that the pristine BaAl2O4 has two extra absorption bands at 2851 and 2922 cm1. It is safe for us to assign the two absorption bands at 2854 and 2925 cm1 to the symmetric and the asymmetric stretching frequencies of the methyl functional group (CH3). Therefore, the appearance of the two extra absorption bands at 2851 and 2922 cm1 points out that the surfaces of the pristine BaAl2O4 nanoparticles were contaminated by organics.
ð1Þ
i¼1
where I(t) is the afterglow intensity at time t after the block of the UV excitation, I0 is the back ground correction, Ii is the prefactor of the ith exponential component whose lifetime decay constant is t i (i = 1, 2). The background correction parameter I0 is found to be 25. The t 1 and t 2 extracted from the fitted curve are found to be 8.81 and 45.25, respectively. The prefactors I1 and I2 are derived to be 313.21 and 194.35, correspondingly. The green solid line in Fig. 6 represents the fitting of the experimental signals by the two exponential components plus one background correction with the above-listed five parameters. The multiple correlation coefficient, R, and the coefficient of determination, R2, are both measures of how well the regression model describes the data. In our case, the R is 0.9939 while the R2 is 0.9878, indicating that Eq. (1) is a good
Fig. 7. FTIR spectra of the pristine BaAl2O4 (green curve) and BaAl2O4:Dy3+ nanoparticles (red curve), respectively. The concentration of Dy3+ in the phosphors was 4 mol%. (For interpretation of the references to color in text and figure legend, the reader is referred to the web version of this article.)
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Such organic contamination is possibly due to long-time storage in the chemistry lab, which is supported by the disappearance of the two absorption bands in freshly prepared BaAl2O4:Dy3+ phosphors. Besides the surface contamination, Fig. 7 indicates that there is little difference in the FTIR spectra between the pristine BaAl2O4 and the BaAl2O4:Dy3+ phosphors because both of them exhibit strong absorptions at 430, 639, 843, 1360, 1416, 1612 and 3400 cm1. On one hand, it is well documented that the three absorption bands in the frequency range of 400–1000 cm1 can be assigned to the stretching vibrations of Al–O, Ba–O and Ba–O–Al bonds in the BaAl2O4 [3,5]. On the other hand, the strong absorption centered at the 3400 cm1 is obviously due to stretching mode of hydroxyl group (nO–H) whilst the absorption at 1360 cm1 can be attributed to the bending vibration of the hydroxyl group. As for the absorption peak at 1416 cm1, Stefani et al. assigned it to the stretching vibration of AlO4 tetrahedron unit in BaAl2O4, which indicates the formation of the BaAl2O4 phase [5]. The absorption band at 1612 cm1 can be most probably ascribed to either the bending vibration mode of adsorbed water on the
Fig. 8. XPS spectra of BaAl2O4:Dy3+ with the Dy3+ content of 4 mol%: (a) XPS survey spectrum of the BaAl2O4:Dy3+ nanoparticles; (b) high-resolution XPS spectrum of Ba 3d; (c) high-resolution XPS spectrum of O 1s; (d) high-resolution XPS spectra of Dy 3d and Cl.
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surface of the nanoparticles or the stretching mode frequencies of some kinds of organics coming from the precursors. The chemical elements and their oxidation states on the surfaces of BaAl2O4:Dy3+ nanoparticles were investigated by using XPS analysis. Fig. 8 represents the XPS spectra of BaAl2O4:Dy3+ with the Dy3+ content of 4 mol%: (a) XPS survey spectrum of the BaAl2O4:Dy3+ nanoparticles; (b) Ba 3d3/2 and 3d5/2 spectra; (c) O 1s spectrum; (d) Dy 3d5/2 and 3d3/2 spectra. As shown in Fig. 8(a), the characteristic emission peaks of elements Al, Ba, O and C elements are identified in the BaAl2O4:Dy3+ nanoparticles. The presence of C, whose peak is located at 285.0 eV, can be attributed to the surface carbon adsorbents before the XPS test. Therefore, it is concluded that the sample is primarily composed of the Al, Ba and O elements, which is in good agreement with the above XRD and FTIR results. As to the XPS spectrum of Ba 3d, Fig. 8(b) depicts that the peaks of the spin-orbit component (3d5/2 and 3d3/2) are located at approximately 780.0 and 795.2 eV, respectively. As shown in Fig. 8(c), the XPS spectral profile of O 1s is nearly symmetric with its peak centered at 531.8 eV. More importantly, we recorded the characteristic emission peaks of Dy and Cl traces in the BaAl2O4: Dy3+ nanoparticles. As shown in Fig. 8(d), the peaks of Dy 3d5/2 and 3d3/2 are located at around 1296 and 1335 eV, respectively, whereas the peak at 1304 eV can be assigned to the Auger electrons from Cl. The data in Fig. 8(d) have pointed out the coexistence of Dy3+ and Cl in the phosphors although their intensities are very weak. The fact that Dy3+ doped BaAl2O4 can lead to green afterglow in the absence of Eu2+ luminescent centers provides a unique opportunity to explore the mechanisms on the afterglow of BaAl2O4 based phosphors. Previous results show that afterglows were observed in several hosts doped with Dy3+ alone. For example, Kuang et al. observed white afterglow in Dy3+ doped SrSiO3 [16], Huang et al. observed green afterglow in Dy3+ doped SrAl2O4 [7,8], Tshabalala et al. reported afterglow from Dy3+ doped Sr2SiO4 phosphor [17]. However, the mechanism on the blue– green afterglow in BaAl2O4:Dy3+ phosphors is not well understood. A possible afterglow mechanism of BaAl2O4:Dy3+ phosphor is illustrated in Fig. 9. Due to the presence of chemical and/or physical defects in the host, a series of discrete levels will be created within the forbidden band gap of BaAl2O4. The well-known defects in BaAl2O4 are VO and VBa. Color centers may be present in the host, too. Under the UV excitation, these defects in the host absorb a portion of the excitation energy, the subsequent radiative relaxations via the discrete levels in the band gap result in the weak emissions of the host (process 1). Upon doping, Dy3+ ions
Fig. 9. A possible afterglow mechanism of BaAl2O4:Dy3+ phosphors.
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enter into the crystal lattice with the substitution of Ba2+ cations to form luminescent centers or trap centers in the BaAl2O4. The blue and green luminescence centers introduced by Dy3+ ions in BaAl2O4 have been evidenced by the two sharp emissions in Fig. 3. Therefore, the first role of the Dy3+ in BaAl2O4 is to act as the luminescent centers (process 2). The second role of the Dy3+ in the BaAl2O4 is to form trap centers. When one Dy3+ ion substitutes the Ba2+ ion in the crystal lattice, one substitutional defect DyBa will be resulted. DyBa is positive and acts as an electron trap. Due to the requirement of charge neutrality, two Dy3+ ions replace three Ba2+ ions to balance the charge of these phosphors, which create two DyBa positive defects and one VBa negative defect [16,18–20]. VBa is negative and acts as the hole trap. Therefore, the second role is that Dy3+ ions introduce new electron traps and significantly increase the concentration of electron or hole traps (process 3). Such electron and hole traps are important for the afterglow. The electrons promoted to CB or the defect levels in the vicinity of Dy3+ can be trapped by electron traps DyBa, while the holes created in the VB or defect levels in the vicinity of VBa can be trapped by the hole traps VBa (process 4). The depths of these electron and hole traps are critically important to the afterglow. The thermoluminescence technique is one of the most suitable methods to probe the trap properties in BaAl2O4 [1,3,4]. Our thermoluminescence measurements showed that the BaAl2O4:Dy3+ phosphors possess high trap concentrations at temperatures of around 80 and 180 C. Once the trapped electrons and holes are thermally released from their traps, they can combine radiatively at a certain kind of luminescence center with the result of afterglow (process 5). Our observed blue–green afterglow in BaAl2O4:Dy3+ suggests that a certain kind of green luminescent center is present in the BaAl2O4: Dy3+ phosphors. The possible blue–green luminescent center can be provided by the intrinsic defects in the host itself or by the extrinsic defects introduced by the Dy3+ dopant. At current stage, the nature of the blue–green luminescence center for the afterglow is not known in detail, and further studies on the phosphors of BaAl2O4 doped with one rare earth alone (i.e., BaAl2O4:Nd3+) are under way. After having compared to the green afterglow of SrAl2O4:Dy3+ phosphors [7,8], the blue–green afterglow of BaAl2O4:Dy3+ phosphors in this work suggests that, although Eu2+ luminescent centers are absent, the afterglows achieved in MAl2O4:Dy3+ (M = Sr, Ba) possibly share quite similar mechanisms. 4. Conclusions Hexagonal BaAl2O4:Dy3+ phosphors were synthesized via the sol–gel combustion route at the ignition temperature of 600 C. Regardless of the absence of Eu2+ luminescent centers, broadband blue–green afterglow spectrum with its peak at about 490 nm has been recorded for the BaAl2O4:Dy3+ phosphors. The afterglow decay curve of the BaAl2O4:Dy3+ phosphors can be well fitted into a two-exponential decay function plus one background correction. The two decay time constants of the afterglow are derived to be 8.81 and 45.25 s, respectively. These results show that broadband afterglow can be achieved in BaAl2O4:Dy3+ phosphors in the
absence of Eu2+ center. The Dy3+ ions introduced trap centers in the crystal lattice are believed to be responsible for the afterglow in BaAl2O4:Dy3+ phosphors. Acknowledgment This work was supported by the Natural Science Foundation of China under the grant No. 11304025 and 11574036. References [1] L.C.V. Rodrigues, J. Hölsä, J.M. Carvalho, C.C.S. Pedroso, M. Lastusaari, M.C.F.C. Felinto, S. Watanabe, H.F. Brito, Co-dopant influence on the persistent luminescence of BaAl2O4:Eu2+,R3+, Physica B 439 (2014) 67–71. [2] D.B. Bem, F.B. Dejene, A.S. Luyt, H.C. Swart, Luminescence studies of a combustion-synthesized blue–green BaAlxOy:Eu2+,Dy3+ nanoparticles, Physica B 407 (2012) 1561–1565. [3] M.A. Lephoto, O.M. Ntwaeaborwa, S.S. Pitale, H.C. Swart, J.R. Botha, B.M. Mothudi, Synthesis and characterization of BaAl2O4:Eu2+ co-doped with different rare earth ions, Physica B 407 (2012) 1603–1606. [4] R. Sakai, T. Katsumata, S. Komuro, T. Morikawa, Effect of composition on the phosphorescence from BaAl2O4: Eu2+,Dy3+ crystals, J. Lumin. 85 (1999) 149–154. [5] R. Stefani, L.C.V. Rodrigues, C.A.A. Carvalho, M.C.F.C. Felinto, H.F. Brito, M. Lastusaari, J. Hölsä, Persistent luminescence of Eu2+ and Dy3+ doped barium aluminate (BaAl2O4:Eu2+,Dy3+) materials, Opt. Mater. 31 (2009) 1815–1818. [6] H. Aizawa, S. Komuro, T. Katsumata, S. Sato, T. Morikawa, Long afterglow phosphorescent characteristics of BaAl2O4:Eu,Dy films, Thin Solid Films 496 (2006) 179–182. [7] Y.M. Huang, Q.L. Ma, Long afterglow of trivalent dysprosium doped strontium aluminate, J. Lumin. 160 (2015) 271–275. [8] Q.L. Ma, B.G. Zhai, Y.M. Huang, Effect of sol–gel combustion temperature on the luminescent properties of trivalent Dy doped SrAl2O4, Ceram. Int. 41 (2015) 5830–5835. [9] B.G. Zhai, Q.L. Ma, Y.M. Huang, Combustion synthesis and luminescent properties of Eu3+ and Dy3+ co-doped amorphous BaAl2O4, Key Eng. Mater. 538 (2013) 58–62. [10] M.V. Vijaya Kumar, B.C. Jamalaiah, K. Rama Gopal, R.R. Reddy, Optical absorption and fluorescence studies of Dy3+-doped lead telluroborate glasses, J. Lumin. 132 (2012) 86–90. [11] M.G. Brik, T. Ishii, A.M. Tkachuk, I. Tanaka, Energy level structure of LiYF4:Dy3+: crystal field analysis, Mater. Trans. 45 (2004) 2026–2030. [12] F.B. Dejene, M.A. Kebede, M. Redi-Abshiro, B.V. Kgarebe, Structural and photoluminescence properties of Dy3+ co-doped and Eu2+ activated MAl2O4 (M = Ba, CaSr) nanophosphors, Opt. Mater. 35 (2013) 1927–1931. [13] H. Ryu, B.K. Singh, K.S. Bartwal, Synthesis and photoluminescence studies on Sr1xBaxAl2O4:Eu2+, Dy3+, J. Korean Ceram. Soc. 45 (2008) 146–149. [14] Q.L. Ma, B.G. Zhai, Y.M. Huang, Sol–gel derived ZnO/porous silicon composites for tunable photoluminescence, J. Sol–Gel Sci. Technol. 64 (2012) 110–116. [15] Y.M. Huang, Q.L. Ma, B.G. Zhai, Controlled morphology of ZnO nanostructures by adjusting the zinc foil heating temperature in an air-filled box furnace, Mater. Chem. Phys. 147 (2014) 788–795. [16] J. Kuang, Y. Liu, J. Zhang, White-light-emitting long-lasting phosphorescence in Dy3+-doped SrSiO3, J. Solid State Chem. 179 (2006) 266–269. [17] M.A. Tshabalala, F.B. Dejene, S.S. Pitale, H.C. Swart, O.M. Ntwaeaborwa, Generation of white-light from Dy3+ doped Sr2SiO4 phosphor, Physica B 439 (2014) 126–129. [18] D. Jia, X.J. Wang, E.V.D. Kolk, W.M. Wen, Site dependent thermoluminescence of long persistent phosphorescence of BaAl2O4:Ce3+, Opt. Commun. 204 (2002) 247–251. [19] D. Jia, X.J. Wang, W.M. Wen, Electron traps in Tb3+-doped CaA12O4, Chem. Phys. Lett. 363 (2002) 241–244. [20] B.F. Lei, Y.L. Liu, J. Liu, Z.R. Ye, C.S. Shi, Pink light emitting long-lasting phosphorescence in Sm3+-doped CdSiO3, J. Solid State Chem. 177 (2004) 1333–1337.
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Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Blue–green afterglow of BaAl2O4:Dy3+ phosphors Bao-gai Zhaia , Qing-lan Maa,b , Rui Xiongc , Xiazhang Lid , Yuan Ming Huanga,* a
School of Mathematics and Physics, Changzhou University, Jiangsu 213164, China School of Electronics and Information, Nantong University, Jiangsu 226019, China c School of Physics and Technology, Wuhan University, Hubei 430072, China d Analysis and Testing Center, Changzhou University, Jiangsu 213164, China b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 3 June 2015 Received in revised form 25 October 2015 Accepted 11 November 2015 Available online xxx
Dy3+ doped barium aluminate (BaAl2O4:Dy3+) phosphors were prepared via the sol–gel combustion route at the ignition temperature of 600 C. The phosphors were characterized with X-ray diffractometry, scanning electron microscopy, transmission electron microscopy, photoluminescence spectroscopy, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Regardless of the absence of Eu2+ luminescent centers, broadband blue–green afterglow with its peak at about 490 nm was recorded in the BaAl2O4:Dy3+ phosphors. The decay profile of the blue–green afterglow can be best fitted into a two-component exponential function with the two lifetime decay constants to be 8.81 and 45.25 s, respectively. The observation of blue–green afterglow from BaAl2O4:Dy3+ in the absence of Eu2+ provides unique opportunity in unveiling the afterglow mechanisms of rare-earth doped alkaline-metal aluminates. Possible mechanisms on the blue–green afterglow in BaAl2O4:Dy3+ phosphors are discussed in terms of the Dy3+ ions introduced trap centers as well as luminescent centers in the crystal lattice. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Optical materials Sol–gel chemistry Phosphors Luminescence Inorganic compounds Nanostructures Transmission electron microscopy
1. Introduction The long afterglows from rare-earth doped alkaline-earth aluminates have found wide applications in emergency signalization, micro-defect sensing, optoelectronics for image storage, and detectors of high-energy radiation. Compared to the intensively studied SrAl2O4:Eu2+,Dy3+, the afterglow from rare-earth doped barium aluminates (BaAl2O4) has attracted limited attention in spite of their excellent chemical stability, high luminescent intensity and long persistent time [1–6]. As documented in the literature, the studies on the afterglow of the BaAl2O4 system are primarily focused on Eu2+ doped BaAl2O4 with the second rareearth activator including La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Er, Tm [1,3]. For example, Rodrigues et al. reported identical blue–green persistent luminescence from the BaAl2O4:Eu2+,R3+ phosphors regardless of the difference in the auxiliary dopant R3+. Among these reports, it is generally agreed that the Eu2+ ion acts as the luminescent center while the auxiliary dopant (i.e., La3+, Ce3+, Pr3+, Nd3+, Sm3+, Gd3+, Tb3+, Dy3+, Er3+, Tm3+) act as the hole or electron traps [1–6]. It is hoped that proper trap energy levels can be formed in the BaAl2O4 phosphors via doping the host with the auxiliary
dopant, thereby achieving the aim of enhanced afterglow brightness and prolonged afterglow time. In spite of the work so far, the mechanism of afterglow in BaAl2O4 based phosphors needs further examination. Especially, what will happen to the afterglow in BaAl2O4 based phosphors if the luminescent center Eu2+ is absent? Generally speaking, trivalent rare earth dopant such as Dy3+ by itself can generate deep traps as well as luminescent centers in the phosphors, and afterglow behavior can be expected once the deep traps and the luminescent centers are present in phosphors. For example, broadband green afterglow could be achieved in the SrAl2O4:Dy3 + phosphors although Eu2+ luminescence centers were absent [7,8]. Therefore, it will be interesting if afterglow can be observed in BaAl2O4 doped with Dy3+ alone because such finding is strongly contrast to the generally accepted belief that the Eu2+ luminescent center is responsible for the blue–green afterglow in SrAl2O4:Eu2+, Dy3+ phosphors. In present work, we prepared BaAl2O4:Dy3+ phosphors via the sol–gel combustion route. Intense blue–green afterglow was discernible in the dark with naked eyes after the removal of the excitation source. 2. Materials and methods
* Corresponding author. Fax: +86 519 86056701. E-mail address: [email protected] (Y.M. Huang). http://dx.doi.org/10.1016/j.materresbull.2015.11.021 0025-5408/ ã 2015 Elsevier Ltd. All rights reserved.
BaAl2O4:Dy3+ phosphors were prepared by the sol–gel combustion method [7–9]. All reagents were provided by Sinopharm Chemical Reagents Co., Ltd. (Shanghai, China). The purity of Dy2O3
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was 99.99% while all other reagents were in analytical grade. Barium chloride (0.02 mol), aluminum nitrate (0.04 mol), urea (0.6 mol), Dy2O3 (0.0004 mol) and H3BO3 (0.002 mol) were dissolved in deionized water (100 ml) to form a transparent solution under the stirring of a magnetic bar. The solution was stored at room temperature for one week before the sol–gel combustion. The molar ratio of urea, barium chloride and aluminum nitrate was 30:1:2. With respect to the barium chloride, the molar fractions of Dy3+ ions and H3BO3 in the sol–gel were 4 and 10 mol%, respectively. After having been filled with 15 ml of the sol–gel, an alumina crucible in the capacity of 30 ml was transferred into an air-filled furnace for combustion at the ignition temperature of 600 C. X-ray diffraction (XRD) patterns were recorded on an X-ray diffractometer (D/max 2500 PC, Rigaku Corporation, Japan) using Cu Ka radiation (l = 1.5405 Å). The voltage applied to the Cu target in the XRD machine was 40 kV. The morphology of the phosphors was determined with a scanning electron microscopy (SEM) (JSM6360LA, Japan). The accelerating voltage applied to the electron gun in the SEM was 15 kV. The nanostructures and the selected area electron diffraction pattern of the phosphors were characterized on a transmission electron microscope (TEM) (JEOL JEM-2100, Japan Electronics Corp.) which was operated at 200 kV. Both the photoluminescence (PL) and afterglow spectra of the phosphors were taken at room temperature on a spectrophotometer (Tianjin Gangdong Inc., China). The excitation wavelength for the PL measurement was the 325 nm emission line from a heliumcadmium laser (Kimmon Koha Co., Ltd., Japan). In order to measure the afterglow decay curves, the phosphors were exposed to the irradiation of the UV laser for 10 min, and then the afterglow was collected with a focus lens into our spectrophotometer. The monitoring wavelength of the afterglow decay measurement was fixed at 490 nm, the data taking process for the afterglow decay was started once the UV irradiation from the laser was blocked off. The details on the PL and afterglow measurements were described in literature [7–9]. The excitation spectrum of our BaAl2O4:Dy3+ phosphors was measured with the fluorescence spectrometer LS45 (PerkinElmer) by fixing the monitoring wavelength at 575 nm. Fourier transform infrared (FTIR) spectroscopy was utilized to analyze the chemical bonding on the surface of the phosphors. The infrared absorption spectra of the materials were recorded by using the standard KBr pellet techniques with a model Nicolet iS5 FTIR spectrometer (Thermo Scientific) in the range of 400– 4000 cm1. The X-ray photoelectron spectroscopy (XPS) measurements were performed on an Escalab 250Xi spectrophotometer (Thermo Scientific) with Al Ka radiation (1486.6 eV). The XPS spectrometer was calibrated by recording the binding energy of Au 4f7/2 peak at 83.9 eV. C 1s peak at 284.6 eV was taken as an internal standard. 3. Results and discussions Fig. 1 shows the XRD curve of the BaAl2O4:Dy3+ phosphors. The concentration of Dy3+ ions in the phosphors was 4 mol%. The dominant peaks in the XRD curve are located at 19.60, 21.84, 28.28, 34.32, 40.10, 41.13, 45.10, 45.84, 53.63, 54.55, 57.79, 61.59, 67.23, 69.74, and 74.36 , respectively, which can be assigned to the Bragg reflections from the planes (2 0 0), (2 0 1), (2 0 2), (2 2 0), (2 2 2), (0 0 4), (4 0 2), (2 0 4), (4 2 0), (2 2 4), (4 2 2), (6 0 0), (2 0 6), (4 2 4), and (2 2 6) of hexagonal BaAl2O4. The standard diffraction data of the hexagonal BaAl2O4 (JCPDS card no. 34-0379) are depicted at the bottom of Fig. 1. A comparison shows that the diffraction peaks of the BaAl2O4:Dy3+ phosphors match with those of the standard hexagonal BaAl2O4 (a = 1.0447 nm, c = 0.8794 nm). It can be seen in Fig. 1 that the addition of a small amount of Dy3+ (4 mol%) has no obvious influence on the crystal structure of the host BaAl2O4. The
Fig. 1. XRD curve of the BaAl2O4:Dy3+ phosphors with the dopant concentration of 4 mol%. The standard data of hexagonal BaAl2O4 (JCPDS card no. 17-0306) are displayed at the bottom for comparison. Inset: the SEM micrograph of BaAl2O4:Dy3+ phosphor.
similar ionic radii of Ba2+ (0.135 nm) and Dy3+ (0.0917 nm) may be responsible for the unchanged crystal structure. Thus, the XRD data have demonstrated that pure phase BaAl2O4:Dy3+ has been obtained via the sol–gel combustion. The inset of Fig. 1 represents the SEM micrograph of the BaAl2O4:Dy3+ phosphors. The lowmagnification SEM micrograph shows many aggregations of the BaAl2O4:Dy3+ nanoparticles. It is clear that the sizes of the BaAl2O4: Dy3+ aggregations are in the range of 5–60 mm. We noted that a lot of pores and voids are present in the BaAl2O4:Dy3+ phosphors. The voluminous gases (i.e., N2, NH3, CO2) generated in the combustion reaction are believed to be responsible for the formation of the pores. The exploration on the nanostructures of the synthesized BaAl2O4:Dy3+ phosphors requires the usage of TEM. Fig. 2 displays the low-magnification TEM micrograph, high-resolution TEM micrograph and the selected area electron diffraction pattern of the BaAl2O4:Dy3+ phosphors. As shown by the low-magnification TEM micrograph in Fig. 2(a), the typical sizes of BaAl2O4:Dy3+ phosphors are about 30 nm in diameter. Fig. 2(b) shows the highresolution TEM micrograph of BaAl2O4:Dy3+ phosphors to display the lattice structures of the BaAl2O4:Dy3+ nanocrystals. It can be seen in Fig. 2(b) that the synthesized phosphors are highly crystalline, and that the spacing between two adjacent planes is about 0.315 nm which is in good agreement with the distance between two (2 0 2) crystal planes of the hexagonal BaAl2O4:Dy3+ phosphors. The crystalline nature of the synthesized BaAl2O4:Dy3+ nanoparticles can also be evidenced with the selected area electron diffraction pattern of the BaAl2O4:Dy3+ phosphors, as shown in Fig. 2(c). Our TEM micrographs in Fig. 2 have verified that the BaAl2O4:Dy3+ phosphors are in hexagonal phase. Fig. 3 represents the PL spectra of the BaAl2O4:Dy3+ phosphors as the concentration of Dy3+ increases from 0 to 4 mol%. As shown by the bottom curve in Fig. 3, the host BaAl2O4 can give off strong PL upon the photoexcitation of the 325 nm photons. The bandgap of defect-free BaAl2O4 crystal is around 6.5 eV [1], so perfect hexagonal BaAl2O4 crystal has no absorption and emissions under our 325 nm UV excitation (3.8 eV). The broadband PL spectrum of the host, whose peak is located at about 400 nm, indicates that some defects are present in the host material. Intrinsic defects such as oxygen vacancies (VO), Ba vacancies (VBa) and some color centers are supposed to be responsible for the photon absorption and its subsequent emission in the host. Similar broadband PL was observed in SrAl2O4 [7,8]. After Dy3+ doping, each PL spectrum of
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Fig. 3. PL spectra of BaAl2O4:Dy3+ phosphors with different dopant concentrations. (For interpretation of the references to color in text, the reader is referred to the web version of this article.)
Fig. 2. Low-magnification TEM micrograph (a), high-resolution TEM micrograph (b) and the selected area electron diffraction pattern (c) of the BaAl2O4:Dy3+ phosphors.
the BaAl2O4:Dy3+ phosphors consists of two characteristic sharp emissions of Dy3+ ions in addition to the broad PL band from the host BaAl2O4. The two narrow PL bands are the characteristic emissions of Dy3+ due to its 4F9/2 ! 6H15/2 and 4F9/2 ! 6H13/2 transitions [7,8,10,11], indicating that Dy3+ ions act as the blue and yellow luminescent centers in the BaAl2O4 host. When compared to BaAl2O4:Eu2+,Dy3+ phosphors, our PL spectra of BaAl2O4:Dy3+ phosphors are dramatically different from those of BaAl2O4:Eu2+, Dy3+ phosphors [1,5,6,12,13]. Such difference can be attributed to the absence of Eu2+ in our BaAl2O4:Dy3+ phosphors. Fig. 4 illustrates the excitation spectrum of BaAl2O4:Dy3+ phosphors. The concentration of Dy3+ ions in the phosphors was 4 mol%. The monitoring wavelength was fixed at 575 nm. It is clear that five characteristic absorption peaks of Dy3+ are located at around 320, 350, 384, 450, and 475 nm in the excitation spectrum. As discussed in the literature, these absorption peaks can be assigned to the transitions from the ground state of Dy3+ (6H15/2) to its excited states 4L19/2,6P7/2+4M15/2, 4I13/2+4F7/2, 4I15/2, and 4F9/2, respectively [7,8,10,11]. These characteristic absorptions have demonstrated that neither Eu2+ nor Eu3+ ions are present in our
BaAl2O4:Dy3+ phosphors since no characteristic absorptions of Eu2+ and Eu3+ are detected in the excitation spectrum. In a similar way as reported in our recent work, our measurements on the L3 edge X-ray absorption spectrum and the time-resolved fluorescence spectroscopy confirmed the absence of Eu2+ ions in the BaAl2O4:Dy3+ phosphors [7]. After the UV irradiation of the laser is blocked off, the emissions of the BaAl2O4 host and the characteristic emissions of Dy3+ at 480 and 575 nm disappear immediately, leaving the BaAl2O4:Dy3+ phosphors glowing in dark for about several minutes. Fig. 5 shows the afterglow spectrum of the BaAl2O4:Dy3+ phosphors. It is
Fig. 4. Excitation spectrum of BaAl2O4:Dy3+ phosphors with the dopant concentration of 4 mol%.
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Fig. 5. Afterglow spectrum of the BaAl2O4:Dy3+ phosphors with the dopant concentration of 4 mol%. Inset: the CIE chromaticity diagram to show the chromaticity coordinates of the afterglow and PL from BaAl2O4:Dy3+ phosphors. (For interpretation of the references to color in text, the reader is referred to the web version of this article.)
obvious that the afterglow spectrum of the BaAl2O4:Dy3+ phosphors is broad in profile with its peak at about 490 nm. The profile of afterglow spectrum of our BaAl2O4:Dy3+ phosphors is quite similar to those of BaAl2O4:Eu2+,Dy3+ phosphors [1,5,6,12,13]. For the purpose of comparison, Eu2+ (4 mol%) and Dy3+ (4 mol%) codoped BaAl2O4:Eu2+,Dy3+ phosphors were synthesized under identical experimental conditions. The afterglow intensity of BaAl2O4:Eu2+,Dy3+ phosphors, however, was much stronger than that of the BaAl2O4:Dy3+ phosphors. In order to quantitatively characterize the color of the afterglow, we calculated the CIE chromaticity coordinates of BaAl2O4:Dy3+ phosphors on the basis of their emission spectral data [14,15]. The CIE chromaticity coordinates of the afterglow were derived to be (0.1265, 0.3063) for the BaAl2O4:Dy3+ phosphors. The inset of Fig. 5 represents the CIE chromaticity diagram in which the chromaticity coordinates of afterglow and PL from BaAl2O4:Dy3+ phosphors are labeled. It is clear that the color of the afterglow is blue–green while the color of PL is purple. The data in Fig. 5 have demonstrated that long afterglow can be achieved in BaAl2O4:Dy3+ phosphors in spite of the absence of Eu2+ ions. Fig. 6 displays the semi-logarithmic plot of the afterglow decay curve of the BaAl2O4:Dy3+ phosphors. The monitoring wavelength was fixed at 490 nm. The hollow circles represent the experimental afterglow signals. As represented by the green solid line in Fig. 6, this decay curve was best fitted to the two-component exponential function IðtÞ ¼ I0 þ
2 X Ii expðt=t i Þ;
Fig. 6. Afterglow decay curve of BaAl2O4:Dy3+ phosphors with the dopant concentration of 4 mol%. (For interpretation of the references to color in text, the reader is referred to the web version of this article.)
description of the relation between the independent and dependent variables. We measured the FTIR absorption spectra of pristine BaAl2O4 as well as Dy3+ doped BaAl2O4 to understand the surface passivation of the BaAl2O4:Dy3+ phosphors. The concentration of Dy3+ ions in the BaAl2O4:Dy3+ phosphors was 4 mol%. Fig. 7 represents the FTIR spectra of the pristine BaAl2O4 nanocrystals (green curve) and BaAl2O4:Dy3+ nanocrystals (red curve), respectively. It can be seen clearly that the nine strong absorption bands are located at 430, 639, 843, 1360, 1416, 1612, 2851, 2922 and 3400 cm1 for the pristine BaAl2O4. As a contrast, only seven strong absorption bands are present in the FTIR spectrum of the BaAl2O4:Dy3+ phosphors, which are located at 430, 639, 843, 1360, 1416, 1612 and 3400 cm1, respectively. A comparison of the two FTIR spectra in Fig. 7 reveals that the pristine BaAl2O4 has two extra absorption bands at 2851 and 2922 cm1. It is safe for us to assign the two absorption bands at 2854 and 2925 cm1 to the symmetric and the asymmetric stretching frequencies of the methyl functional group (CH3). Therefore, the appearance of the two extra absorption bands at 2851 and 2922 cm1 points out that the surfaces of the pristine BaAl2O4 nanoparticles were contaminated by organics.
ð1Þ
i¼1
where I(t) is the afterglow intensity at time t after the block of the UV excitation, I0 is the back ground correction, Ii is the prefactor of the ith exponential component whose lifetime decay constant is t i (i = 1, 2). The background correction parameter I0 is found to be 25. The t 1 and t 2 extracted from the fitted curve are found to be 8.81 and 45.25, respectively. The prefactors I1 and I2 are derived to be 313.21 and 194.35, correspondingly. The green solid line in Fig. 6 represents the fitting of the experimental signals by the two exponential components plus one background correction with the above-listed five parameters. The multiple correlation coefficient, R, and the coefficient of determination, R2, are both measures of how well the regression model describes the data. In our case, the R is 0.9939 while the R2 is 0.9878, indicating that Eq. (1) is a good
Fig. 7. FTIR spectra of the pristine BaAl2O4 (green curve) and BaAl2O4:Dy3+ nanoparticles (red curve), respectively. The concentration of Dy3+ in the phosphors was 4 mol%. (For interpretation of the references to color in text and figure legend, the reader is referred to the web version of this article.)
B.- Zhai et al. / Materials Research Bulletin 75 (2016) 1–6
Such organic contamination is possibly due to long-time storage in the chemistry lab, which is supported by the disappearance of the two absorption bands in freshly prepared BaAl2O4:Dy3+ phosphors. Besides the surface contamination, Fig. 7 indicates that there is little difference in the FTIR spectra between the pristine BaAl2O4 and the BaAl2O4:Dy3+ phosphors because both of them exhibit strong absorptions at 430, 639, 843, 1360, 1416, 1612 and 3400 cm1. On one hand, it is well documented that the three absorption bands in the frequency range of 400–1000 cm1 can be assigned to the stretching vibrations of Al–O, Ba–O and Ba–O–Al bonds in the BaAl2O4 [3,5]. On the other hand, the strong absorption centered at the 3400 cm1 is obviously due to stretching mode of hydroxyl group (nO–H) whilst the absorption at 1360 cm1 can be attributed to the bending vibration of the hydroxyl group. As for the absorption peak at 1416 cm1, Stefani et al. assigned it to the stretching vibration of AlO4 tetrahedron unit in BaAl2O4, which indicates the formation of the BaAl2O4 phase [5]. The absorption band at 1612 cm1 can be most probably ascribed to either the bending vibration mode of adsorbed water on the
Fig. 8. XPS spectra of BaAl2O4:Dy3+ with the Dy3+ content of 4 mol%: (a) XPS survey spectrum of the BaAl2O4:Dy3+ nanoparticles; (b) high-resolution XPS spectrum of Ba 3d; (c) high-resolution XPS spectrum of O 1s; (d) high-resolution XPS spectra of Dy 3d and Cl.
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surface of the nanoparticles or the stretching mode frequencies of some kinds of organics coming from the precursors. The chemical elements and their oxidation states on the surfaces of BaAl2O4:Dy3+ nanoparticles were investigated by using XPS analysis. Fig. 8 represents the XPS spectra of BaAl2O4:Dy3+ with the Dy3+ content of 4 mol%: (a) XPS survey spectrum of the BaAl2O4:Dy3+ nanoparticles; (b) Ba 3d3/2 and 3d5/2 spectra; (c) O 1s spectrum; (d) Dy 3d5/2 and 3d3/2 spectra. As shown in Fig. 8(a), the characteristic emission peaks of elements Al, Ba, O and C elements are identified in the BaAl2O4:Dy3+ nanoparticles. The presence of C, whose peak is located at 285.0 eV, can be attributed to the surface carbon adsorbents before the XPS test. Therefore, it is concluded that the sample is primarily composed of the Al, Ba and O elements, which is in good agreement with the above XRD and FTIR results. As to the XPS spectrum of Ba 3d, Fig. 8(b) depicts that the peaks of the spin-orbit component (3d5/2 and 3d3/2) are located at approximately 780.0 and 795.2 eV, respectively. As shown in Fig. 8(c), the XPS spectral profile of O 1s is nearly symmetric with its peak centered at 531.8 eV. More importantly, we recorded the characteristic emission peaks of Dy and Cl traces in the BaAl2O4: Dy3+ nanoparticles. As shown in Fig. 8(d), the peaks of Dy 3d5/2 and 3d3/2 are located at around 1296 and 1335 eV, respectively, whereas the peak at 1304 eV can be assigned to the Auger electrons from Cl. The data in Fig. 8(d) have pointed out the coexistence of Dy3+ and Cl in the phosphors although their intensities are very weak. The fact that Dy3+ doped BaAl2O4 can lead to green afterglow in the absence of Eu2+ luminescent centers provides a unique opportunity to explore the mechanisms on the afterglow of BaAl2O4 based phosphors. Previous results show that afterglows were observed in several hosts doped with Dy3+ alone. For example, Kuang et al. observed white afterglow in Dy3+ doped SrSiO3 [16], Huang et al. observed green afterglow in Dy3+ doped SrAl2O4 [7,8], Tshabalala et al. reported afterglow from Dy3+ doped Sr2SiO4 phosphor [17]. However, the mechanism on the blue– green afterglow in BaAl2O4:Dy3+ phosphors is not well understood. A possible afterglow mechanism of BaAl2O4:Dy3+ phosphor is illustrated in Fig. 9. Due to the presence of chemical and/or physical defects in the host, a series of discrete levels will be created within the forbidden band gap of BaAl2O4. The well-known defects in BaAl2O4 are VO and VBa. Color centers may be present in the host, too. Under the UV excitation, these defects in the host absorb a portion of the excitation energy, the subsequent radiative relaxations via the discrete levels in the band gap result in the weak emissions of the host (process 1). Upon doping, Dy3+ ions
Fig. 9. A possible afterglow mechanism of BaAl2O4:Dy3+ phosphors.
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enter into the crystal lattice with the substitution of Ba2+ cations to form luminescent centers or trap centers in the BaAl2O4. The blue and green luminescence centers introduced by Dy3+ ions in BaAl2O4 have been evidenced by the two sharp emissions in Fig. 3. Therefore, the first role of the Dy3+ in BaAl2O4 is to act as the luminescent centers (process 2). The second role of the Dy3+ in the BaAl2O4 is to form trap centers. When one Dy3+ ion substitutes the Ba2+ ion in the crystal lattice, one substitutional defect DyBa will be resulted. DyBa is positive and acts as an electron trap. Due to the requirement of charge neutrality, two Dy3+ ions replace three Ba2+ ions to balance the charge of these phosphors, which create two DyBa positive defects and one VBa negative defect [16,18–20]. VBa is negative and acts as the hole trap. Therefore, the second role is that Dy3+ ions introduce new electron traps and significantly increase the concentration of electron or hole traps (process 3). Such electron and hole traps are important for the afterglow. The electrons promoted to CB or the defect levels in the vicinity of Dy3+ can be trapped by electron traps DyBa, while the holes created in the VB or defect levels in the vicinity of VBa can be trapped by the hole traps VBa (process 4). The depths of these electron and hole traps are critically important to the afterglow. The thermoluminescence technique is one of the most suitable methods to probe the trap properties in BaAl2O4 [1,3,4]. Our thermoluminescence measurements showed that the BaAl2O4:Dy3+ phosphors possess high trap concentrations at temperatures of around 80 and 180 C. Once the trapped electrons and holes are thermally released from their traps, they can combine radiatively at a certain kind of luminescence center with the result of afterglow (process 5). Our observed blue–green afterglow in BaAl2O4:Dy3+ suggests that a certain kind of green luminescent center is present in the BaAl2O4: Dy3+ phosphors. The possible blue–green luminescent center can be provided by the intrinsic defects in the host itself or by the extrinsic defects introduced by the Dy3+ dopant. At current stage, the nature of the blue–green luminescence center for the afterglow is not known in detail, and further studies on the phosphors of BaAl2O4 doped with one rare earth alone (i.e., BaAl2O4:Nd3+) are under way. After having compared to the green afterglow of SrAl2O4:Dy3+ phosphors [7,8], the blue–green afterglow of BaAl2O4:Dy3+ phosphors in this work suggests that, although Eu2+ luminescent centers are absent, the afterglows achieved in MAl2O4:Dy3+ (M = Sr, Ba) possibly share quite similar mechanisms. 4. Conclusions Hexagonal BaAl2O4:Dy3+ phosphors were synthesized via the sol–gel combustion route at the ignition temperature of 600 C. Regardless of the absence of Eu2+ luminescent centers, broadband blue–green afterglow spectrum with its peak at about 490 nm has been recorded for the BaAl2O4:Dy3+ phosphors. The afterglow decay curve of the BaAl2O4:Dy3+ phosphors can be well fitted into a two-exponential decay function plus one background correction. The two decay time constants of the afterglow are derived to be 8.81 and 45.25 s, respectively. These results show that broadband afterglow can be achieved in BaAl2O4:Dy3+ phosphors in the
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