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Luminescence of divalent europium activated spinels synthesized by combustion and the enhanced afterglow by dysprosium incorporation
Physica B 488 (2016) 8–12
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Luminescence of divalent europium activated spinels synthesized by combustion and the enhanced afterglow by dysprosium incorporation Haoyi Wu n, Yahong Jin School of Physics and Optoelectronic Engineering, Guangdong University of Technology, Waihuanxi Road, No.100, Guangzhou 510006, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 3 November 2015 Received in revised form 27 January 2016 Accepted 5 February 2016 Available online 8 February 2016
Herein we report a luminescent phenomenon of Eu2 þ in the spinel MgAl2O4 and ZnAl2O4 samples which are successfully synthesized via a combustion method. The XRD shows cubic spinel structure is obtained from the prepared samples. The mean crystal sizes estimated from XRD data are 30 and 10 nm for MgAl2O4 and ZnAl2O4 respectively, and the large grain particles are the agglomeration of crystallites. The Eu2 þ ions show a blue emission at around 480 nm and an afterglow phenomenon is observed after the removal of excitation. The afterglow spectrum of MgAl2O4: Eu2 þ , Dy3 þ shows two emissions at 480 and 520 nm while only one at 480 nm is observed in ZnAl2O4: Eu2 þ , Dy3 þ . The afterglow intensity and the persisting duration can be substantially enhanced by the Dy3 þ incorporation because the trapping ability of the electron traps is reinforced. This is confirmed by the TL curves of the samples. & 2016 Elsevier B.V. All rights reserved.
Keywords: Spinels Luminescence Afterglow
1. Introduction As a gemstone mineral, the natural spinels have been discovered for more than 1000 years, and the color center appeared in the crystal renders the materials various kinds of colors. This feature makes spinels famous in artwork of jewels. Besides, spinels are also well-known semi-conducting materials which have been widely used in modern science. Due to the band gap around 3– 4 eV, the materials can be easily excited by ultra–violet light and show transparent in visible range [1–3]. Therefore, they could be used as photocatalysts, optical ceramics and electronic materials [4–6]. Moreover, a spinel allows the appearance of energy level between the valence band (VB) and the conduction band (CB), by the introduction of some impurities. This energy level renders spinel additional optical features and extends its available applications, such as luminescence. By now, there has been numbers of literatures on lanthanides incorporated spinels, which show visible luminescence and potentially applied in light-emitting devices [7,8]. As the typical types of spinel, MgAl2O4 and ZnAl2O4 have been widely reported as the matrices for Cr3 þ , Eu3 þ , and Mn2 þ with the synthesis methods of solid-state reaction, sol–gel, co-precipitation, micro-emulsion and so on [9–20]. They exhibit red or green emission under a ultra–violet (UV) excitation. Based on the transition of 5d to 4f states, the emission of Eu2 þ shows different colors in different compounds because the energy levels of 5d states are n
Corresponding author. E-mail address: [email protected] (H. Wu).
http://dx.doi.org/10.1016/j.physb.2016.02.012 0921-4526/& 2016 Elsevier B.V. All rights reserved.
affected by the crystal fields. The coordinated environment of Eu2 þ determines the gap between the lowest 5d and highest 4f levels, and then the emission color is tunable in hosts. Therefore, Eu2 þ ions are widely employed as the luminescent center in many inorganic materials [21]. There were a few literatures reporting the luminescence of Eu2 þ in spinels [22,23]. Yet reports on their afterglow phenomenon appeared infrequently. In present work, the Eu2 þ incorporated spinels are synthesized through an urea-assisted combustion method. The X-ray diffraction (XRD) results demonstrate that this method is available for the synthesis of spinels at low temperature. Besides, the luminescence properties of Eu2 þ , including photoluminescence (PL) and thermoluminescence (TL, afterglow) in MAl2O4 (M: Mg, Zn) typed spinel are investigated. The Eu2 þ shows blue emission in the prepared samples as well as a visible afterglow. Moreover, the enhancement of afterglow has been found by simultaneously incorporating Dy3 þ ions. The intensity and the persisting duration of the afterglow can be substantially enhanced by incorporating Dy3 þ ions.
2. Experimental procedures 2.1. Synthesis þ (deFour spinel-host luminescent samples, Mg0.99Al2O4: Eu20.01 2þ 3þ noted as MAE), Mg0.97Al2O4: Eu0.01, Dy0.02 (denoted as MAED), þ þ þ Zn0.99Al2O4: Eu20.01 (denoted as ZAE) and Zn0.97Al2O4: Eu20.01 , Dy30.02 (denoted as ZAED) were synthesized via a combustion method. Mg (NO3)2 6H2O (99%, Damao Chemical Reagent Factory, Tianjin), Zn (NO3)2 6H2O (99%, Damao Chemical Reagent Factory, Tianjin), Al
H. Wu, Y. Jin / Physica B 488 (2016) 8–12
9
(NO3)3 9H2O (99%, Damao Chemical Reagent Factory, Tianjin), Eu2O3 (99.99%, Guangzhou Research Institute of Non-ferrous Metals) and Dy2O3 (99.99%, Guangzhou Research Institute of Non-ferrous Metals) were employed as the raw materials. Additional 10 mol% of H3BO3 (99%, Damao Chemical Reagent Factory, Tianjin) was added as a flux. Initially, the raw materials were weighed according to the stoichiometric compositions. The Eu(NO3)3 and Dy (NO3)3 were obtained by dissolving Eu2O3 and Dy2O3 in dilute nitric acid. The Mg(NO3)2 6H2O/Zn(NO3)2 6H2O and Al(NO3)3 9H2O were dissolved in two cups of deionized water. After becoming transparent, two cups of solution were mixed with proper molar ratios and stirred for 1 h. After that, a proper amount of urea was added to form a precursor. The molar ratio for one sample to the urea was 1:20. After that, the precursor was stirred for 1 h, and then was put in a muffle furnace which was maintained at 620 °C. The precursor was boiled and then decomposed with the evolution of large amounts of gases. Then spontaneous ignition occurred and underwent smoldering combustion with enormous swelling, accompanied by the production of white foamy and voluminous ash. The entire process lasted for 5–10 min. Finally, the as-prepared MAE, MAED, ZAE and ZAED samples were obtained. 2.2. Characterization The phase identification of all obtained samples was carried out by XRD using a XD-2 diffractometer (Beijing PGENERAL) with Cu Kα irradiation (λ ¼ 1.5406 Å) at 36 kV tube voltage and 20 mA tube current. The scan range was 2θ from 10° to 70° with 6° min 1 speed. The scanning electron microscope (SEM) images were recorded using a Philips XL-30 apparatus. The PL and the afterglow spectra were investigated by an F-7000 Fluorescence Spectrophotometer (Hitachi) with a Xe lamp excitation. Prior to the afterglow spectra measurement, the samples were excited by a mercury lamp (254 nm, 60 W) for 1 min and the measurement was carried out without external excitation (the Xe lamp was closed). The decay and thermoluminescence (TL) curves were measurement using a FJ427A1 thermoluminescent reader (CNNC Beijing Nuclear Instrument Factory). 0.001 g powder for each sample was employed. During detection of decay curves, the temperature was maintained at 30 °C. Before the detection, each sample was excited by a mercury lamp for 1 min. For the TL curves measurement, the heating rate was 1 °C s 1. The range of the measurement was from room temperature to 300 °C. Prior to the measurement, each powder sample was also exposed to a mercury lamp for 1 min. The interval between the measurement and the removal of excitation was 10 min.
3. Results and discussions 3.1. Phase identification and morphology The XRD patterns of the obtained samples are shown in Fig. 1. As can be seen, the MAE and MAED samples exhibit the similar patterns with approximate peaks location. According to the JCPDF standard card No. 86-2258, the structure of the samples can be regarded as cubic MgAl2O4. The samples are face-centered type with the space group of Fd-3 m (No. 227) and the cell parameters are about a¼b¼c¼ 8.07 Å. No other structure is observed from the patterns. In addition, the ZAE and ZAED samples also exhibit cubic structure according to the JCPDF standard card No. 73-1961. The samples are also face-centered type and the space group is Fd-3 m (No. 227). The cell parameters are about a¼b¼c¼8.04 Å. The incorporated Eu2 þ and Dy3 þ ions tend to substitute the Mg2 þ and Zn2 þ sites due to the similar ionic sizes (Mg2 þ , fourth coordination, 57 pm; Zn2 þ , fourth coordination, 60 pm; Al3 þ , sixth coordination,
Fig. 1. XRD patterns of the samples.
53.5 pm; Eu2 þ , eighth coordination, 125 pm; Dy3 þ , eighth coordination, 102.7 pm) [24]. A small amount of Eu2 þ and Dy3 þ incorporation do not have a notable effect on the XRD patterns. The mean crystal size of prepared samples can be calculated using Scherrer Formula [25]. The (220) and (311) crystal planes are selected to calculate the size and the formula is expressed as:
D=
0.89λ β cos(θ )
(1)
where D is the average crystal size for the samples. λ represents the wavelength of X-ray, which is 0.15406 nm in this work. β is the width at half intensity of the XRD peak and θ is the degree of the maximum intensity of the peak. The estimated results are listed in Table 1. The mean crystal size of the MgAl2O4 samples is around 30 nm and the one of ZnAl2O4 samples is around 10 nm. This result indicates that the obtained MgAl2O4 and ZnAl2O4 by combustion method have different crystal size although they have similar crystal structure. The MgAl2O4 seems crystalizing better than the ZnAl2O4. Fig. 2 shows the SEM images of MAED and ZEAD. The grain size of the particles is varying from a few microns to several tens of microns. The morphology of the particles seems to be formed from the agglomeration of crystallites. Pores can be observed on the surface of the bulk. This is caused by the vaporized gas in a rapid combustion process. The morphologies of MAE and ZAE are similar. 3.2. PL The emission and excitation spectra of the samples are shown in Fig. 3. The shapes of the spectra of samples are similar. A broad emission band at 400–570 nm with a peak around 480 nm can be Table 1 Estimated mean crystal size of the samples. SAMPLES
CRYSTAL PLANE
2θ (degree)
β (degree)
D (nm)
MAE
(220) (311) (220) (311) (220) (311) (220) (311)
31.384 36.956 31.290 36.862 31.611 37.133 31.454 36.902
0.23728 0.24417 0.26356 0.25845 0.86085 0.85401 0.70263 0.73407
33.11 32.17 29.81 30.40 9.13 9.20 11.18 10.70
MAED ZAE ZAED
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Fig. 4. (a) Decay curve of the samples. (b) Afterglow spectrum of MAED. (c) Afterglow spectrum of ZAED. (d) Decay curves of the samples from 100–400 s.
that Eu3 þ can be effectively reduced during the combustion process with urea, in which trivalent nitrogen ions could be oxidized and vaporize in a form of gas-phase oxynitrides. The oxidized nitrogen may result in the reduction of Eu3 þ to Eu2 þ . Similar phenomena have been reported in other compounds previously [28,29]. The Eu2 þ ions incorporate in spinels during the combustion and they absorb UV light and emit blue light. The incorporated Dy3 þ seems not showing a significant influence on the PL of the Eu2 þ in spinels. 3.3. Afterglow
Fig. 2. Morphology of (a) MAED and (b) ZAED.
Fig. 3. PL spectra of the samples.
observed. This band is ascribed to the 4f65d1 to 4f7 transition configuration of Eu2 þ ions [26,27]. No other emission is observed. The excitation spectra consist of several broad absorption bands from 250 nm to 400 nm. The maximal intensity of the excitation spectra locates around 320 nm. The spectral data of Eu2 þ indicates
In addition to the PL, the samples show visible afterglow after being excited by a UV light. The afterglow spectra are shown in Fig. 4(b) and (c). For MAED, an emission band centered at 480 nm is observed. Different from the emission spectra, a band centered at 520 nm is observed additionally. The band at 480 nm stems from the 4f65d1 to 4f7 transition configuration of Eu2 þ ions and the one at 520 nm may be attributed to the intrinsic defects of the lattice. According to previous researches [30–32], these intrinsic defects are attributed to vacancies (VK3 þ ) centers. The recombination of electrons in CB with trapped holes in VK3 þ centers results in a 520 nm emission [32]. In other word, VK3 þ centers exist in the spinel MgAl2O4 and it contributes to the afterglow emission. However, photoluminescence of which is not obvious because it may be concealed by the strong Eu2 þ emission. For ZAED, a broad afterglow band centered at 480 nm is observed. This band is similar with the PL emission, which is from the 4f65d1 to 4f7 transition configuration of Eu2 þ ions. No other emission is detected in this case. Decay curves of the samples are shown in Fig. 4(a). The samples show an obvious attenuation process. For the MgAl2O4 samples, the luminescent intensity of MAED maintains higher than the one of MAE within the whole measured interval, indicating that the afterglow duration of MAED persists longer than that of MAE. Similarly, the intensity of ZAED also maintains higher than the one of ZAE. This also indicates the longer afterglow duration of ZAED. As a result, the incorporation of Dy3 þ ions in both MgAl2O4: Eu2 þ and ZnAl2O4: Eu2 þ enhances the afterglow intensity as well as prolongs the afterglow duration. As compared to ZAED, the initial afterglow of MAED presents weaker during the first 100-second decaying, yet it turns to be stronger than that of ZAED after 100 s. This phenomenon suggests that the afterglow of MAED may show a longer duration than that of ZAED.
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Fig. 6. Proposed mechanism of the afterglow generation in spinels.
3.5. Discussion
Fig. 5. TL curves of the samples.
3.4. TL The long afterglow of luminescent materials is effectively modulated by the traps which are induced by lattice defects. When the materials are excited by external excitation, electrons in lanthanides ions can be promoted from ground state to excitation state, followed by trapping process of charge carriers (electrons and/or holes). The thermal liberation of carriers from trap(s) to luminescent centers accompanied by charge recombination defers the emission and generates afterglow after the removal of excitation. In other words, liberation of trapped carriers is determined by the ambient temperature around the materials. Thus a temperature-dependence glow curve can partial reveals the status of traps since trapped carriers can be liberated rapidly under heating. This temperature-dependence is the TL curve of the materials. The TL curves of MAE, MAED, ZAE and ZAED are shown in Fig. 6. Deconvolution of the glow curves demonstrates that the curves consist of three TL band centered around 79, 115 and 200 °C, respectively. According to the previous work, the bands at 79 °C can be associated to the electron traps while the one at 200 °C is associated to the hole traps [20,32]. Compared to the band at high temperature, the one at 79 °C is more probable to liberate trapped carriers due to the lower activated energy. Thus electron traps may have more contribution to the afterglow of spinels samples. After incorporating Dy3 þ to the lattices, the band at 115 °C is substantially enhanced. Since the afterglow properties are enhanced by Dy3 þ incorporation accordingly, this band at 115 °C is deduced as the lattice defects that strengthen electron traps. Compared to the ZAED, this TL band of MAED shows a slight movement toward high temperature, indicating a small enhancement of the activated energy of the trapped. This may be the reason that MAED shows a longer afterglow duration than ZAED.
By now, there have been several models for long afterglow phosphors, including hole trap model [33] and electron trap model [34]. Based on the hole and electron traps, some models proposed by Clabau et al. [35,36] and Aitasaol et al. [37] provide an explanation to the generation of afterglow of the long afterglow phosphors. Their models involve electron traps served by oxygen vacancies which are reinforced by the incorporated Ln3 þ ions (i. e. Nd3 þ , Dy3 þ ) in vicinity. Hole traps which are acted by cation vacancies are also the possible trapping carriers. In addition, trapping dynamic model of MgAl2O4 incorporated with Ce3 þ was proposed by Jia et al. [32]. Since the emission of Eu2 þ stems from the 5d to 4f transition, which is similar with the emission of Ce3 þ , dynamic model for MAl2O4: Eu2 þ , Dy3 þ (M: Mg, Zn) can be conceived. Theoretical studies suggest the band gaps of MgAl2O4 and ZnAl2O4 are around 4.3 and 5.8 eV, respectively [38,39]. Yet the difference of band gaps seems not affecting the luminescence of the Eu2 þ . As the energy level of Eu2 þ and Dy2 þ may locate between VB and CB of spinels, the mechanism of long afterglow can be proposed and shown in Fig. 6. When the phosphors are excited by an excitation, electrons in 4f ground state of Eu2 þ are promoted to 5d excited state, which is close to the bottom of CB. Thus the excited electrons can partially migrate in CB and then are captured by electron traps, which may be induced by oxygen vacancies. On the other hand, holes in 4f ground state of Eu2 þ may be promoted to the VB and then captured by hole traps which may be induced by cation vacancies. After the removal of excitation, thermal liberation of trapped carriers from traps to VB and CB leads to the recombination of carriers in Eu2 þ centers accompanied by a 480 nm photo-emission. This is the generation of afterglow. When Dy3 þ ions are incorporated in the matrix, they tend to occupy the Mg2 þ /Zn2 þ sites. Both Dy3 þ ions and oxygen vacancies act as electron traps cluster via the aggregation of cation vacancies which are negative charged, otherwise the two electron traps repulse each other [37]. The Dy3 þ ions in the cluster reinforce the trapping ability of the electron traps so that more electrons can be trapped during the excitation process. That is why the TL band at 115 °C is enhanced. For the MgAl2O4 matrix, the substitution of Dy3 þ to Mg2 þ appears to deepen the traps so that the afterglow duration is prolonged. This is confirmed by the small movement of the 115 °C TL band. From the afterglow emission spectra we find that a VK3 þ also acts as a luminescent center in MgAl2O4. During the excitation process, the VK2 þ centers which are impurities in crystal can capture holes to form VK3 þ centers, in which holes recombine with electrons from CB, resulting in the 520 nm afterglow emission. However, this emission is not observed at ZnAl2O4 matrix. Further work is necessary to identify the nature
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H. Wu, Y. Jin / Physica B 488 (2016) 8–12
of VK3 þ centers so as to explain this luminescence phenomenon.
4. Conclusions The MAl2O4 (M: Mg, Zn) spinels with Eu2 þ and Dy3 þ incorporation have been successfully synthesized by a combustion method. The mean crystal size estimated by XRD patterns are around 30 and 10 nm for MgAl2O4 and ZnAl2O4 and the small crystallites agglomerate to form the bulk materials. The Eu2 þ ions show a blue emission centered at 480 nm, as well as afterglow phenomenon. The afterglow intensity and duration can be substantially enhanced by the Dy3 þ incorporation. The TL curves of the samples suggest that the trapping ability of the electron traps is reinforced by Dy3 þ ions in the cluster. MgAl2O4: Eu2 þ , Dy3 þ shows a longer afterglow duration than ZnAl2O4: Eu2 þ , Dy3 þ due to the deepening of the electron traps by Dy3 þ incorporation. This report about the long afterglow emission of Eu2 þ in spinels may assist in further exploring the interesting luminescence in minerals.
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Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Luminescence of divalent europium activated spinels synthesized by combustion and the enhanced afterglow by dysprosium incorporation Haoyi Wu n, Yahong Jin School of Physics and Optoelectronic Engineering, Guangdong University of Technology, Waihuanxi Road, No.100, Guangzhou 510006, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 3 November 2015 Received in revised form 27 January 2016 Accepted 5 February 2016 Available online 8 February 2016
Herein we report a luminescent phenomenon of Eu2 þ in the spinel MgAl2O4 and ZnAl2O4 samples which are successfully synthesized via a combustion method. The XRD shows cubic spinel structure is obtained from the prepared samples. The mean crystal sizes estimated from XRD data are 30 and 10 nm for MgAl2O4 and ZnAl2O4 respectively, and the large grain particles are the agglomeration of crystallites. The Eu2 þ ions show a blue emission at around 480 nm and an afterglow phenomenon is observed after the removal of excitation. The afterglow spectrum of MgAl2O4: Eu2 þ , Dy3 þ shows two emissions at 480 and 520 nm while only one at 480 nm is observed in ZnAl2O4: Eu2 þ , Dy3 þ . The afterglow intensity and the persisting duration can be substantially enhanced by the Dy3 þ incorporation because the trapping ability of the electron traps is reinforced. This is confirmed by the TL curves of the samples. & 2016 Elsevier B.V. All rights reserved.
Keywords: Spinels Luminescence Afterglow
1. Introduction As a gemstone mineral, the natural spinels have been discovered for more than 1000 years, and the color center appeared in the crystal renders the materials various kinds of colors. This feature makes spinels famous in artwork of jewels. Besides, spinels are also well-known semi-conducting materials which have been widely used in modern science. Due to the band gap around 3– 4 eV, the materials can be easily excited by ultra–violet light and show transparent in visible range [1–3]. Therefore, they could be used as photocatalysts, optical ceramics and electronic materials [4–6]. Moreover, a spinel allows the appearance of energy level between the valence band (VB) and the conduction band (CB), by the introduction of some impurities. This energy level renders spinel additional optical features and extends its available applications, such as luminescence. By now, there has been numbers of literatures on lanthanides incorporated spinels, which show visible luminescence and potentially applied in light-emitting devices [7,8]. As the typical types of spinel, MgAl2O4 and ZnAl2O4 have been widely reported as the matrices for Cr3 þ , Eu3 þ , and Mn2 þ with the synthesis methods of solid-state reaction, sol–gel, co-precipitation, micro-emulsion and so on [9–20]. They exhibit red or green emission under a ultra–violet (UV) excitation. Based on the transition of 5d to 4f states, the emission of Eu2 þ shows different colors in different compounds because the energy levels of 5d states are n
Corresponding author. E-mail address: [email protected] (H. Wu).
http://dx.doi.org/10.1016/j.physb.2016.02.012 0921-4526/& 2016 Elsevier B.V. All rights reserved.
affected by the crystal fields. The coordinated environment of Eu2 þ determines the gap between the lowest 5d and highest 4f levels, and then the emission color is tunable in hosts. Therefore, Eu2 þ ions are widely employed as the luminescent center in many inorganic materials [21]. There were a few literatures reporting the luminescence of Eu2 þ in spinels [22,23]. Yet reports on their afterglow phenomenon appeared infrequently. In present work, the Eu2 þ incorporated spinels are synthesized through an urea-assisted combustion method. The X-ray diffraction (XRD) results demonstrate that this method is available for the synthesis of spinels at low temperature. Besides, the luminescence properties of Eu2 þ , including photoluminescence (PL) and thermoluminescence (TL, afterglow) in MAl2O4 (M: Mg, Zn) typed spinel are investigated. The Eu2 þ shows blue emission in the prepared samples as well as a visible afterglow. Moreover, the enhancement of afterglow has been found by simultaneously incorporating Dy3 þ ions. The intensity and the persisting duration of the afterglow can be substantially enhanced by incorporating Dy3 þ ions.
2. Experimental procedures 2.1. Synthesis þ (deFour spinel-host luminescent samples, Mg0.99Al2O4: Eu20.01 2þ 3þ noted as MAE), Mg0.97Al2O4: Eu0.01, Dy0.02 (denoted as MAED), þ þ þ Zn0.99Al2O4: Eu20.01 (denoted as ZAE) and Zn0.97Al2O4: Eu20.01 , Dy30.02 (denoted as ZAED) were synthesized via a combustion method. Mg (NO3)2 6H2O (99%, Damao Chemical Reagent Factory, Tianjin), Zn (NO3)2 6H2O (99%, Damao Chemical Reagent Factory, Tianjin), Al
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(NO3)3 9H2O (99%, Damao Chemical Reagent Factory, Tianjin), Eu2O3 (99.99%, Guangzhou Research Institute of Non-ferrous Metals) and Dy2O3 (99.99%, Guangzhou Research Institute of Non-ferrous Metals) were employed as the raw materials. Additional 10 mol% of H3BO3 (99%, Damao Chemical Reagent Factory, Tianjin) was added as a flux. Initially, the raw materials were weighed according to the stoichiometric compositions. The Eu(NO3)3 and Dy (NO3)3 were obtained by dissolving Eu2O3 and Dy2O3 in dilute nitric acid. The Mg(NO3)2 6H2O/Zn(NO3)2 6H2O and Al(NO3)3 9H2O were dissolved in two cups of deionized water. After becoming transparent, two cups of solution were mixed with proper molar ratios and stirred for 1 h. After that, a proper amount of urea was added to form a precursor. The molar ratio for one sample to the urea was 1:20. After that, the precursor was stirred for 1 h, and then was put in a muffle furnace which was maintained at 620 °C. The precursor was boiled and then decomposed with the evolution of large amounts of gases. Then spontaneous ignition occurred and underwent smoldering combustion with enormous swelling, accompanied by the production of white foamy and voluminous ash. The entire process lasted for 5–10 min. Finally, the as-prepared MAE, MAED, ZAE and ZAED samples were obtained. 2.2. Characterization The phase identification of all obtained samples was carried out by XRD using a XD-2 diffractometer (Beijing PGENERAL) with Cu Kα irradiation (λ ¼ 1.5406 Å) at 36 kV tube voltage and 20 mA tube current. The scan range was 2θ from 10° to 70° with 6° min 1 speed. The scanning electron microscope (SEM) images were recorded using a Philips XL-30 apparatus. The PL and the afterglow spectra were investigated by an F-7000 Fluorescence Spectrophotometer (Hitachi) with a Xe lamp excitation. Prior to the afterglow spectra measurement, the samples were excited by a mercury lamp (254 nm, 60 W) for 1 min and the measurement was carried out without external excitation (the Xe lamp was closed). The decay and thermoluminescence (TL) curves were measurement using a FJ427A1 thermoluminescent reader (CNNC Beijing Nuclear Instrument Factory). 0.001 g powder for each sample was employed. During detection of decay curves, the temperature was maintained at 30 °C. Before the detection, each sample was excited by a mercury lamp for 1 min. For the TL curves measurement, the heating rate was 1 °C s 1. The range of the measurement was from room temperature to 300 °C. Prior to the measurement, each powder sample was also exposed to a mercury lamp for 1 min. The interval between the measurement and the removal of excitation was 10 min.
3. Results and discussions 3.1. Phase identification and morphology The XRD patterns of the obtained samples are shown in Fig. 1. As can be seen, the MAE and MAED samples exhibit the similar patterns with approximate peaks location. According to the JCPDF standard card No. 86-2258, the structure of the samples can be regarded as cubic MgAl2O4. The samples are face-centered type with the space group of Fd-3 m (No. 227) and the cell parameters are about a¼b¼c¼ 8.07 Å. No other structure is observed from the patterns. In addition, the ZAE and ZAED samples also exhibit cubic structure according to the JCPDF standard card No. 73-1961. The samples are also face-centered type and the space group is Fd-3 m (No. 227). The cell parameters are about a¼b¼c¼8.04 Å. The incorporated Eu2 þ and Dy3 þ ions tend to substitute the Mg2 þ and Zn2 þ sites due to the similar ionic sizes (Mg2 þ , fourth coordination, 57 pm; Zn2 þ , fourth coordination, 60 pm; Al3 þ , sixth coordination,
Fig. 1. XRD patterns of the samples.
53.5 pm; Eu2 þ , eighth coordination, 125 pm; Dy3 þ , eighth coordination, 102.7 pm) [24]. A small amount of Eu2 þ and Dy3 þ incorporation do not have a notable effect on the XRD patterns. The mean crystal size of prepared samples can be calculated using Scherrer Formula [25]. The (220) and (311) crystal planes are selected to calculate the size and the formula is expressed as:
D=
0.89λ β cos(θ )
(1)
where D is the average crystal size for the samples. λ represents the wavelength of X-ray, which is 0.15406 nm in this work. β is the width at half intensity of the XRD peak and θ is the degree of the maximum intensity of the peak. The estimated results are listed in Table 1. The mean crystal size of the MgAl2O4 samples is around 30 nm and the one of ZnAl2O4 samples is around 10 nm. This result indicates that the obtained MgAl2O4 and ZnAl2O4 by combustion method have different crystal size although they have similar crystal structure. The MgAl2O4 seems crystalizing better than the ZnAl2O4. Fig. 2 shows the SEM images of MAED and ZEAD. The grain size of the particles is varying from a few microns to several tens of microns. The morphology of the particles seems to be formed from the agglomeration of crystallites. Pores can be observed on the surface of the bulk. This is caused by the vaporized gas in a rapid combustion process. The morphologies of MAE and ZAE are similar. 3.2. PL The emission and excitation spectra of the samples are shown in Fig. 3. The shapes of the spectra of samples are similar. A broad emission band at 400–570 nm with a peak around 480 nm can be Table 1 Estimated mean crystal size of the samples. SAMPLES
CRYSTAL PLANE
2θ (degree)
β (degree)
D (nm)
MAE
(220) (311) (220) (311) (220) (311) (220) (311)
31.384 36.956 31.290 36.862 31.611 37.133 31.454 36.902
0.23728 0.24417 0.26356 0.25845 0.86085 0.85401 0.70263 0.73407
33.11 32.17 29.81 30.40 9.13 9.20 11.18 10.70
MAED ZAE ZAED
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Fig. 4. (a) Decay curve of the samples. (b) Afterglow spectrum of MAED. (c) Afterglow spectrum of ZAED. (d) Decay curves of the samples from 100–400 s.
that Eu3 þ can be effectively reduced during the combustion process with urea, in which trivalent nitrogen ions could be oxidized and vaporize in a form of gas-phase oxynitrides. The oxidized nitrogen may result in the reduction of Eu3 þ to Eu2 þ . Similar phenomena have been reported in other compounds previously [28,29]. The Eu2 þ ions incorporate in spinels during the combustion and they absorb UV light and emit blue light. The incorporated Dy3 þ seems not showing a significant influence on the PL of the Eu2 þ in spinels. 3.3. Afterglow
Fig. 2. Morphology of (a) MAED and (b) ZAED.
Fig. 3. PL spectra of the samples.
observed. This band is ascribed to the 4f65d1 to 4f7 transition configuration of Eu2 þ ions [26,27]. No other emission is observed. The excitation spectra consist of several broad absorption bands from 250 nm to 400 nm. The maximal intensity of the excitation spectra locates around 320 nm. The spectral data of Eu2 þ indicates
In addition to the PL, the samples show visible afterglow after being excited by a UV light. The afterglow spectra are shown in Fig. 4(b) and (c). For MAED, an emission band centered at 480 nm is observed. Different from the emission spectra, a band centered at 520 nm is observed additionally. The band at 480 nm stems from the 4f65d1 to 4f7 transition configuration of Eu2 þ ions and the one at 520 nm may be attributed to the intrinsic defects of the lattice. According to previous researches [30–32], these intrinsic defects are attributed to vacancies (VK3 þ ) centers. The recombination of electrons in CB with trapped holes in VK3 þ centers results in a 520 nm emission [32]. In other word, VK3 þ centers exist in the spinel MgAl2O4 and it contributes to the afterglow emission. However, photoluminescence of which is not obvious because it may be concealed by the strong Eu2 þ emission. For ZAED, a broad afterglow band centered at 480 nm is observed. This band is similar with the PL emission, which is from the 4f65d1 to 4f7 transition configuration of Eu2 þ ions. No other emission is detected in this case. Decay curves of the samples are shown in Fig. 4(a). The samples show an obvious attenuation process. For the MgAl2O4 samples, the luminescent intensity of MAED maintains higher than the one of MAE within the whole measured interval, indicating that the afterglow duration of MAED persists longer than that of MAE. Similarly, the intensity of ZAED also maintains higher than the one of ZAE. This also indicates the longer afterglow duration of ZAED. As a result, the incorporation of Dy3 þ ions in both MgAl2O4: Eu2 þ and ZnAl2O4: Eu2 þ enhances the afterglow intensity as well as prolongs the afterglow duration. As compared to ZAED, the initial afterglow of MAED presents weaker during the first 100-second decaying, yet it turns to be stronger than that of ZAED after 100 s. This phenomenon suggests that the afterglow of MAED may show a longer duration than that of ZAED.
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Fig. 6. Proposed mechanism of the afterglow generation in spinels.
3.5. Discussion
Fig. 5. TL curves of the samples.
3.4. TL The long afterglow of luminescent materials is effectively modulated by the traps which are induced by lattice defects. When the materials are excited by external excitation, electrons in lanthanides ions can be promoted from ground state to excitation state, followed by trapping process of charge carriers (electrons and/or holes). The thermal liberation of carriers from trap(s) to luminescent centers accompanied by charge recombination defers the emission and generates afterglow after the removal of excitation. In other words, liberation of trapped carriers is determined by the ambient temperature around the materials. Thus a temperature-dependence glow curve can partial reveals the status of traps since trapped carriers can be liberated rapidly under heating. This temperature-dependence is the TL curve of the materials. The TL curves of MAE, MAED, ZAE and ZAED are shown in Fig. 6. Deconvolution of the glow curves demonstrates that the curves consist of three TL band centered around 79, 115 and 200 °C, respectively. According to the previous work, the bands at 79 °C can be associated to the electron traps while the one at 200 °C is associated to the hole traps [20,32]. Compared to the band at high temperature, the one at 79 °C is more probable to liberate trapped carriers due to the lower activated energy. Thus electron traps may have more contribution to the afterglow of spinels samples. After incorporating Dy3 þ to the lattices, the band at 115 °C is substantially enhanced. Since the afterglow properties are enhanced by Dy3 þ incorporation accordingly, this band at 115 °C is deduced as the lattice defects that strengthen electron traps. Compared to the ZAED, this TL band of MAED shows a slight movement toward high temperature, indicating a small enhancement of the activated energy of the trapped. This may be the reason that MAED shows a longer afterglow duration than ZAED.
By now, there have been several models for long afterglow phosphors, including hole trap model [33] and electron trap model [34]. Based on the hole and electron traps, some models proposed by Clabau et al. [35,36] and Aitasaol et al. [37] provide an explanation to the generation of afterglow of the long afterglow phosphors. Their models involve electron traps served by oxygen vacancies which are reinforced by the incorporated Ln3 þ ions (i. e. Nd3 þ , Dy3 þ ) in vicinity. Hole traps which are acted by cation vacancies are also the possible trapping carriers. In addition, trapping dynamic model of MgAl2O4 incorporated with Ce3 þ was proposed by Jia et al. [32]. Since the emission of Eu2 þ stems from the 5d to 4f transition, which is similar with the emission of Ce3 þ , dynamic model for MAl2O4: Eu2 þ , Dy3 þ (M: Mg, Zn) can be conceived. Theoretical studies suggest the band gaps of MgAl2O4 and ZnAl2O4 are around 4.3 and 5.8 eV, respectively [38,39]. Yet the difference of band gaps seems not affecting the luminescence of the Eu2 þ . As the energy level of Eu2 þ and Dy2 þ may locate between VB and CB of spinels, the mechanism of long afterglow can be proposed and shown in Fig. 6. When the phosphors are excited by an excitation, electrons in 4f ground state of Eu2 þ are promoted to 5d excited state, which is close to the bottom of CB. Thus the excited electrons can partially migrate in CB and then are captured by electron traps, which may be induced by oxygen vacancies. On the other hand, holes in 4f ground state of Eu2 þ may be promoted to the VB and then captured by hole traps which may be induced by cation vacancies. After the removal of excitation, thermal liberation of trapped carriers from traps to VB and CB leads to the recombination of carriers in Eu2 þ centers accompanied by a 480 nm photo-emission. This is the generation of afterglow. When Dy3 þ ions are incorporated in the matrix, they tend to occupy the Mg2 þ /Zn2 þ sites. Both Dy3 þ ions and oxygen vacancies act as electron traps cluster via the aggregation of cation vacancies which are negative charged, otherwise the two electron traps repulse each other [37]. The Dy3 þ ions in the cluster reinforce the trapping ability of the electron traps so that more electrons can be trapped during the excitation process. That is why the TL band at 115 °C is enhanced. For the MgAl2O4 matrix, the substitution of Dy3 þ to Mg2 þ appears to deepen the traps so that the afterglow duration is prolonged. This is confirmed by the small movement of the 115 °C TL band. From the afterglow emission spectra we find that a VK3 þ also acts as a luminescent center in MgAl2O4. During the excitation process, the VK2 þ centers which are impurities in crystal can capture holes to form VK3 þ centers, in which holes recombine with electrons from CB, resulting in the 520 nm afterglow emission. However, this emission is not observed at ZnAl2O4 matrix. Further work is necessary to identify the nature
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of VK3 þ centers so as to explain this luminescence phenomenon.
4. Conclusions The MAl2O4 (M: Mg, Zn) spinels with Eu2 þ and Dy3 þ incorporation have been successfully synthesized by a combustion method. The mean crystal size estimated by XRD patterns are around 30 and 10 nm for MgAl2O4 and ZnAl2O4 and the small crystallites agglomerate to form the bulk materials. The Eu2 þ ions show a blue emission centered at 480 nm, as well as afterglow phenomenon. The afterglow intensity and duration can be substantially enhanced by the Dy3 þ incorporation. The TL curves of the samples suggest that the trapping ability of the electron traps is reinforced by Dy3 þ ions in the cluster. MgAl2O4: Eu2 þ , Dy3 þ shows a longer afterglow duration than ZnAl2O4: Eu2 þ , Dy3 þ due to the deepening of the electron traps by Dy3 þ incorporation. This report about the long afterglow emission of Eu2 þ in spinels may assist in further exploring the interesting luminescence in minerals.
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