Eu3+

Article Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX

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Multichannel Luminescence Properties of Mixed-Valent Eu2+/Eu3+ Coactivated SrAl3BO7 Nanocrystalline Phosphors for Near-UV LEDs Xiaoming Liu,†,‡ Weijie Xie,† Ying Lü,† Jingchun Feng,† Xinghua Tang,† Jun Lin,*,‡ Yuhua Dai,† Yu Xie,†,§ and Liushui Yan*,† †

School of Environment and Chemical Engineering, Nanchang Hangkong University, Nanchang 330063, People’s Republic of China State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China § Jiangxi Province Engineering & Technology Research Center For Paper Chemicals, Nanchang Hangkong University, Nanchang 330063, People’s Republic of China ‡

ABSTRACT: Up to now, orchestrating the coexistence of Eu2+ and Eu3+ activators in a single host lattice has been an extremely difficult task, especially for the appearance of the characteristic emission of Eu2+ and Eu3+ in order to generate white light. Nevertheless, here we demonstrate a new Eu2+/Eu3+ coactivated SrAl3BO7 nanocrystalline phosphor with abundant and excellent multichannel luminescence properties. A series of Eu2+/Eu3+ coactivated SrAl3BO7 nanocrystalline phosphors were prepared through a Pechini-type sol−gel method followed by a reduction process. With excitation of UV/NUV light, the prepared SrAl3BO7:Eu2+,Eu3+ phosphors show not only the characteristic f−f transitions of Eu3+ ion (5DJ → 7FJ,J′, J, J′ = 0−3), but also the 5d → 4f transitions of Eu2+ ion with comparable intensity from 400 to 700 nm in the whole visible spectral region. The luminescence color of the SrAl3BO7:Eu2+,Eu3+ phosphor can be tuned from blue, bluegreen, white, and orange to orange-red by changing the excitation wavelength, the overall doping concentration of europium ions (Eu2+, Eu3+), and the relative ratio of Eu2+ to Eu3+ ions to some extent. A single-phase white-light emission has been realized in SrAl3BO7:Eu2+,Eu3+ phosphor. The obtained SrAl3BO7:Eu2+,Eu3+ phosphor has potential application in the area of NUV white-light-emitting diodes.

1. INTRODUCTION During the last couple of years, white light-emitting diodes (WLEDs) have emerged as superior candidates to replace conventional incandescent and fluorescent lamps because of their excellent properties, such as small size, high luminous efficiency, low energy cost, fast switching, environmental friendliness, and long lifetime.1−6 Recently, a significant number of papers have given detailed descriptions of different methods of creating white light.7,8 Currently, the most promising method employs phosphor materials as a medium converting the radiation from the primary source (LED) into visible light, which is called phosphor-converted white LED (pc-WLED).7 The present typical method to produce the white light is to combine a blue InGaN LED with yellow-emitting Y3Al5O12:Ce3+ (YAG:Ce3+) phosphor. However, due to lack of red emission, the low color-rendering index (CRI) and high correlated color temperature (CCT) has restricted some primary applications.9,10 An alternative way to improve the white-light quality of white LEDs and obtain a higher CRI, a UV LED chip (300−410 nm) coated with three emitting blue, green, and red phosphors has been introduced.11,12 However, owing to the strong reabsorption of blue light by red-/greenemitting phosphors and large Stokes shift, the luminous efficiency is low in this approach.5,13,14 In this regard, a single-phase full-color-emitting phosphor, which will overcome © XXXX American Chemical Society

all of the problems above by using a single emitting component phosphor with higher luminous efficiency and excellent CRI, is considered to be potentially useful because of small color aberration, high color rendering, and low cost.11 Therefore, highly efficient single-component phosphors which can be excited by UV and/or near-UV chips are of great interest to be explored for both fundamental research and practical applications.15−18 Rare-earth ions have been playing an significant role in modern lighting, displays, biological labels, fluorescent falsification prevention, and other optoelectronic devices owing to their excellent luminescence properties based on their 4f−4f or 5d−4f transitions.8,15,19,20 For example, the trivalent europium ion (Eu3+) is a famous orange-/red-emitting activator in phosphors owing to its 5D0−7FJ transitions (J = 0− 4).21−23 The excitation bands of Eu3+ ions usually consist of strong host lattice excitation bands, charge-transfer bands in the short-wavelength UV region ( 0.05), the emission is dominated by f−f transitions of Eu3+ ion with orange-red luminescence. The inset in Figure 7b shows the corresponding CIE chromaticity diagram of Sr1−xAl3BO7:x(Eu2+,Eu3+)-AR phosphors with different doping concentrations of europium ions (Eu2+ and Eu3+) excited by NUV light at 383 nm. From the inset in Figure 7b, it can be seen more visually that, with excitation of NUV light at 383 nm, the luminescence color of Sr1−xAl3BO7:x(Eu2+,Eu3+)-AR can change from blue-green (CIE chromaticity coordinates x = 0.2521, y = 0.3036), across the white zone (CIE chromaticity coordinates, x = 0.3256, y = 0.2887), ultimately to orange-red (CIE chromaticity coordinates x = 0.4443, y = 0.2991) by changing the doping concentration of europium ion (Eu2+ and Eu3+) from x = 0.01 to x = 0.09 in Sr1−xAl3BO7:x(Eu2+,Eu3+)AR phosphors. With excitation of NUV light at 395 nm, the Sr1−xAl3BO7:x(Eu2+,Eu3+)-AR phosphors show different luminescence. Basically, the emission is dominated by f−f transitions of Eu3+ ion with orange-red luminescence. Figure 7c shows the emission spectra of Sr1−xAl3BO7:x(Eu2+,Eu3+)-AR phosphors with different doping concentrations of europium ions (Eu2+ and Eu3+) excited by NUV light at 395 nm. From Figure 7c, it can be seen that the luminescence color of Sr1−xAl3BO7:x(Eu2+,Eu3+)-AR can be tuned from nearly white and orange to orange-red by changing the doping concentration of europium ion (Eu2+ and Eu3+) from x = 0.01 to x = 0.09 with the excitation of NUV light at 395 nm. The inset in Figure 7c shows the corresponding CIE chromaticity diagram of Sr1−xAl3BO7:x(Eu2+,Eu3+)-AR phosphors with different doping concentrations of europium ion (Eu2+ and Eu3+) ions excited by NUV light at 395 nm. The CIE chromaticity coordinates change from x = 0.3872 and y = 0.3185 (SrAl3BO7:0.01(Eu2+,Eu3+)-AR, white) to x = 0.5403 and y = 0.3316 (SrAl3BO7:0.09(Eu2+,Eu3+)-AR, orange-red) by changing the doping concentration of europium ion (Eu2+ and Eu3+) from x = 0.01 to x = 0.09 in Sr1−xAl3BO7:x(Eu2+,Eu3+)-AR phosphors. 2.2.3. Luminescence Mechanism of SrAl3BO7:x(Eu2+,Eu3+) Phosphor. The UV−vis diffuse reflectance spectra of SrAl3BO7:0.03(Eu2+,Eu3+)-BR and SrAl3BO7:0.03(Eu2+,Eu3+)AR phosphors are shown in Figure 8a. The SrAl3BO7:0.03(Eu2+,Eu3+)-BR phosphor shows two wide absorption bands ranging from 200 to 500 nm. One strong broad band ranging from 200 to 310 nm with a maximum at 250 nm is ascribed to the transition of Eu3+−O2− charge transfer, and the other band ranging from 310 to 500 nm peaking at 350 nm comes from the 4f → 5d transition of Eu2+ ion. After SrAl3BO7:0.03(Eu2+,Eu3+) phosphor was reduced under a flow of H2/Ar (H2 5%) gas mixture at 700 °C, more of the Eu3+ ions are reduced to Eu2+ ions in SrAl3BO7:0.03(Eu2+,Eu3+) phosphor. As shown in Figure 8a (blue line), the absorption spectrum of SrAl3BO7:0.03(Eu2+,Eu3+)-AR phosphor has a shape similar to that of SrAl3BO7:0.03(Eu2+,Eu3+)-BR phosphor but with an obviously wider and stronger absorption band ranging from 320 to 500 nm, which might be ascribed to the increased absorption of the 5d → 4f transition of Eu2+ ion in SrAl3BO7:0.03(Eu2+,Eu3+)-AR phosphor. Figure 8b shows the

Figure 7. Emission spectra of Sr1−xAl3BO7:x(Eu3+,Eu2+)-AR with different europium ion concentrations (Eu3+ and Eu2+) with NUV light excitation at 363 nm (a), 383 nm (b), and 395 nm (c), respectively. The insets are the corresponding CIE chromaticity diagrams for a series of Sr1−xAl3BO7:x(Eu3+,Eu2+)-AR excited by NUV light at 363 nm (a), 383 nm (b), and 395 nm (c), respectively.

changes in the doping concentration of europium ions (Eu2+ and Eu3+) from x = 0.01 to x = 0.09 in Sr1−xAl3BO7:x(Eu2+,Eu3+)-AR phosphors. Accordingly, the corresponding luminescence color can change from bluegreen to white. Figure 7b shows the emission spectra of Sr1−xAl3BO7:x(Eu2+,Eu3+)-AR phosphors with different doping concentrations of europium ions (Eu2+ and Eu3+) excited by NUV light at 383 nm. From Figure 7b, it can be seen that the luminescence color of Sr1−xAl3BO7:x(Eu2+,Eu3+)-AR can be tuned from blue-green and white to orange-red by changing the doping concentration of europium ions (Eu2+ and Eu3+) with the excitation of NUV light at 383 nm. Basically, with the excitation of NUV light at 383 nm, the f−f transitions of Eu3+ H

DOI: 10.1021/acs.inorgchem.7b01938 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 8. UV/vis diffuse reflectance spectra of SrAl3BO7:0.03(Eu3+,Eu2+)-BR and SrAl3BO7:0.03(Eu3+,Eu2+)-AR nanocryataline phosphors (a) and Δ Absorbance of SrAl3BO7:0.03(Eu3+,Eu2+) nanocryataline phosphors (b) (Δ Absorbance = AAR − ABR).

net variation in absorbance (Δ Absorbance) before and after reduction of SrAl3BO7:0.03(Eu2+,Eu3+) phosphors. Here, Δ Absorbance = AAR − ABR (AAR = absorbance of sample after reduction, ABR = absorbance of sample before reduction).48,49 The Δ Absorbance of SrAl3BO7:0.03(Eu2+,Eu3+) phosphors (shown in Figure 8b) shows a wide band from 230 to 500 nm with peaks at 285 and 340 nm, which is similar in shape to the excitation spectrum of SrAl3BO7:0.03(Eu2+,Eu3+)-AR phosphor shown in Figure 6a (pinkish red line) monitoring the 5d → 4f transition of Eu2+ ion at 475 nm. Therefore, the Δ Absorbance of SrAl3BO7:0.03(Eu2+,Eu3+) phosphors is ascribed to variation in the 4f → 5d transition of Eu2+ ion introduced during the course of reduction under a flow of H2/Ar (H2 5%) gas mixture.20 Figure 9 shows the luminescence decay curves of the SrAl3BO7:0.03(Eu2+,Eu3+)-AR phosphor. As discussed in section 3.2.1, there are four luminescence centers in SrAl3BO7:0.03(Eu2+,Eu3+)-AR phosphor and therefore the luminescence decay curves of the SrAl3BO7:0.03(Eu2+,Eu3+)AR phosphor were obtained by monitoring these four luminescence centers: e.g., the 5d → 4f transition of Eu2+ at 410 and 475 nm, the 5D0−7F1 transition of Eu3+ at 594 nm, and the 5D0−7F2 transition of Eu3+ at 617 nm, respectively. As shown in Figure 9a, the decay curve of the 5d → 4f transition emission of Eu2+ in SrAl3BO7:0.03(Eu2+,Eu3+)-AR phosphor at 410 nm can be well fitted to a single-exponential function as I = I0 exp(−t/τ), where I0 is constant, t is the time, and τ is the decay time. The lifetime of the 5d → 4f transition emission of Eu2+ in SrAl3BO7:0.03(Eu2+,Eu3+)-AR phosphor at 410 nm was determined to be 0.43 μs, which is basically of the same magnitude as the 5d → 4f transition emission of Eu2+ in Ca3Y2Si3O12:Eu2+,Eu3+ and LiBaPO4 Eu2+ phosphor.7,50 The

Figure 9. Luminescence decay curves for SrAl3BO7:0.03(Eu3+,Eu2+)BR nanocrystalline phosphor monitored by emission of europium ions (Eu3+,Eu2+) at 410 nm (a), 475 nm (b), 594 nm (c), and 617 nm (d), respectively.

same situation holds for the 5d → 4f transition emission of Eu2+ in SrAl3BO7:0.03(Eu2+,Eu3+)-AR phosphor at 475 nm, as shown in Figure 9b; its lifetime is determined to be around 0.32 μs. The luminescence decay curves of the 5D0−7F1 I


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Inorganic Chemistry transition of Eu3+ (594 nm, Figure 9c) and the 5D0−7F2 transition of Eu3+ (617 nm, Figure 9d) in SrAl3BO7:0.03(Eu2+,Eu3+)-AR phosphor can be fitted to a single-exponential function also, and the corresponding lifetimes of the 5D0−7F1 transition at 594 nm and the 5D0−7F2 transition at 617 nm in SrAl3BO7:0.03(Eu2+,Eu3+)-AR phosphor were determined to be 3.72 and 0.96 ms, respectively, which are on the same order of magnitude as reported in previous studies.51,52 Since the 5d → 4f transition emission of Eu2+ is parity allowed and spin selection is not appropriate, the emission transition is a fully allowed one. On comparison with the 5d → 4f transition emission of Eu2+, the parity of f−f transitions of Eu3+ does not change; thus, the lifetime of the excited state of Eu3+ ion is longer than that of Eu2+ ion. As shown in Figure 9, the decay time of the 5d → 4f transition emission of Eu2+ ion (on the order of microseconds) is much shorter than the that of f−f transitions of Eu3+ ion (on the order of milliseconds) in SrAl3BO7:0.03Eu3+,2+-AR phosphor, which also favors the energy transfer from Eu2+ ions to Eu3+ ions in the SrAl3BO7 host lattice, accordingly.20 It is well-known that the 5D0−7FJ emission is very sensitive to the symmetry of the surrounding environment of the Eu3+ ion. If a Eu3+ ion occupies a site with inversion symmetry in the host lattice, the optical transitions between levels of the 4fn configuration are strictly forbidden as an electric-dipole transition (parity selection rule). Only magnetic-dipole transitions (5D0−7F1) that obey the selection rule ΔJ = 0, ±1 (J = 0 to J = 0 forbidden) can be observed; thus, the the 5 D0−7F1 transition will have a longer decay time (around 3−7 ms).20,51 If a Eu3+ ion occupies a site without inversion symmetry in the host lattice, the uneven crystal field components can mix opposite-parity state to the 4f n configuration level, and parity selection rules are partially relaxed. Therefore, the so-called forced electric-dipole transitions (5D0−7F2) are no longer strictly forbidden and can be observed with short decay time (around 0.1−1 ms).20,21,23 The 5 D0−7F1 and 5D0−7F2 transitions of Eu3+ in SrAl3BO7:0.03(Eu2+,Eu3+)-AR phosphor having different decay times further illustrate that there are two types of Eu3+ ion sites in the SrAl3BO7 host lattice.20 Table 2 show the decay lifetimes of Eu2+ in SrAl3BO7:x(Eu2+,Eu3+)-AR phosphor (x = 0.01−0.06) monitored by the

nonradiative energy transfer from Eu2+ to Eu3+ in SrAl3BO7: (Eu2+,Eu3+)-AR phosphor.20,53,54 Unfortunately, due to the fact that Eu2+ and Eu3+ ions always coexist in the SrAl3BO7 host lattice, it is very difficult to ascertain the exact concentration of Eu2+ and Eu3+ ions in the SrAl3BO7 host lattice; therefore, currently we can not calculate quantitatively the energy transfer efficiency from Eu2+ and Eu3+ in SrAl3BO7:(Eu2+,Eu3+)-AR phosphor.6,20,53−55 XPS survey spectra of SrAl3BO7:0.03(Eu2+,Eu3+)-BR and SrAl3BO7:0.03(Eu2+,Eu3+)-AR are shown in Figure 10. All XPS

Figure 10. XPS survey spectra of the SrAl3BO7:0.03(Eu3+,Eu2+)-BR and SrAl3BO7:0.03(Eu3+,Eu2+)-AR phosphors (a) and high-resolution XPS spectra at the Eu 3d5 position of the SrAl3BO7:0.03(Eu3+,Eu2+)BR and SrAl3BO7:0.03(Eu3+,Eu2+)-AR phosphors (b).

Table 2. Decay Lifetimes of Eu2+ in SrAl3BO7:x(Eu2+,Eu3+)AR Phosphor Monitored by the 5d → 4f Transition Emission at 410 and 475 nm, Respectively

spectra were corrected for instrument and sample charges considering the C 1s position at 284.6 eV. As shown in Figure 10a, both SrAl3BO7:0.03(Eu2+,Eu3+)-BR and SrAl3BO7:0.03(Eu2+,Eu3+)-AR phosphors revealed photoelectron peaks corresponding to Sr 4p, Al 2p, O 1s, C 1s, and Eu 3d5 emissions.35 The high-resolution XPS spectra at Eu 3d5 emission position for both SrAl3BO7:0.03(Eu2+,Eu3+)-BR and SrAl3BO7:0.03(Eu2+,Eu3+)-AR phosphors are shown in Figure 10b. From Figure 10b, it can be seen clearly that europium ions in both Eu2+ and Eu3+ oxidation states are present in SrAl3BO7:0.03(Eu2+,Eu3+)-BR and SrAl3BO7:0.03(Eu2+,Eu3+)AR phosphors. The XPS signal of Eu2+ ions in SrAl3BO7:0.03(Eu2+,Eu3+)-AR is much higher than that of Eu2+ ions in SrAl3BO7:0.03(Eu2+,Eu3+)-BR. This means that, before reduction, Eu2+ ion exists in the SrAl3BO7:0.03Eu phosphor. After reduction under a flow of H2/Ar (H2 5%) gas mixture at 700 °C, the XPS signal of Eu2+ ions in SrAl3BO7:0.03(Eu2+,Eu3+)

τ (μs) x

λex 284 nm, λem 410 nm

λex 347 nm, λem 475 nm

0.01 0.02 0.03 0.04 0.05 0.06

0.52 0.47 0.43 0.36 0.31 0.28

0.43 0.39 0.32 0.27 0.22 0.19

5d → 4f transition emission at 410 and 475 nm, respectively. From Table 2, it can be observed that with an increase in the concentration of doping europium ions (Eu2+, Eu3+), the decay times (τ) of Eu2+ in SrAl3BO7:x(Eu2+,Eu3+)-AR phosphor monotonously decrease from 0.52 to 0.25 μs and from 0.43 to 0.16 μs for the monitored 5d → 4f transition emission at 410 and 475 nm, respectively, which can well certify the existence of J


Article

Inorganic Chemistry has increased greatly, indicating some parts of the Eu3+ ions have been reduced to Eu2+ ions in SrAl3BO7:0.03(Eu2+,Eu3+) phosphors. The results of XPS spectra of SrAl3BO7:0.03(Eu2+,Eu3+)-BR and SrAl3BO7:0.03(Eu2+,Eu3+)-AR phosphors are basically consistent with the luminescence spectra of SrAl3BO7:0.03(Eu2+,Eu3+)-BR and SrAl3BO7:0.03(Eu2+,Eu3+)AR phosphors shown in Figures 4 and 6, respectively. Figure 11 shows a simple model illustrating the characteristic 5d → 4f transition emission of Eu2+ ions, the f−f transition

4. CONCLUSIONS In summary, a series of Eu2+-/Eu3+-coactivated strontium aluminoborate nanocrystalline phosphors, e.g., SrAl3BO7:Eu2+,Eu3+, have been prepared by a Pechini-type sol−gel method followed by a reduction process. With the excitation of UV light, the SrAl3BO7:Eu2+,Eu3+ phosphor shows not only the characteristic f−f transitions of Eu3+ ions but also the 5d → 4f transition emission of Eu2+ ions with comparable i n t en s it y . T he pho t o l u m in es c en ce co l o r o f t he SrAl3BO7:Eu2+,Eu3+-AR nanocrystalline phosphor can be tuned from blue, blue-green, white, and orange to orange-red by changing the excitation wavelength of UU/NUV light, the overall doping concentration of europium ions (Eu2+ and Eu3+), and the relative luminescence ratio of Eu2+ to Eu3+ ions to some extent. A single-phase white-light emission with high quality has been realized in SrAl3BO7:Eu2+,Eu3+-AR phosphor. Due to its abundant and excellent luminescence properties, the obtained SrAl3BO7:Eu2+,Eu3+-AR phosphor has potential applications in the areas of NUV white-light-emitting diodes.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for J.L.: [email protected]. *E-mail for L.Y.: [email protected]. ORCID

Jun Lin: 0000-0001-9572-2134 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was financially supported by the National Natural Science Foundation of China (NSFC 51762035, 21161015, 51468043, 51672265, 91433110, U1301242), the Natural Science Foundation of the Jiangxi Province of China (20164BCD40098, 20152ACB20011, 2009GZH0082), the Natural Science Foundation of the Jiangxi Higher Education Institutions of China (GJJ09180, GJJ14513), and Nanchang Hangkong University Doctoral Foundation.

Figure 11. Simple model illustrating the characteristic 5d → 4f transition emission of Eu2+ ions, the f−f transition emission of Eu3+ ions, and energy transfer from Eu2+ ions to Eu3+ ions in SrAl3BO7:Eu2+,Eu3+ phosphor.

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emission of Eu3+ ions, and energy transfer from Eu2+ ions to Eu3+ ions in SrAl3BO7:Eu2+,Eu3+ phosphor. As discussed in the Introduction section, the strontium-based aluminoborate with inherent displacive flexibility of the stuffed tridymite-type tetrahedral framework has provided different sites for doping Eu2+ ions to Eu3+ ions.28−36 The 5d1 excited state configuration of Eu2+ ion is very sensitive to the crystal structures and covalency of the host lattice, and it is split by crystal field into two components. The stronger the crystal field, the farther away the split two components. There are two types of sites of Eu2+ ions in the SrAl3BO7 host lattice: one is in a weak crystalline field environment, and the other lies in a stronger crystalline field environment. Due to the match of energy level, there is an energy transfer from Eu2+ to Eu3+ in SrAl3BO7:Eu2+,Eu3+ phosphor. As discussed in section 3.2.1, there are four kinds of luminescence centers in SrAl3BO7:0.03(Eu2+,Eu3+) phosphor: that is, two types of Eu2+ ions and two types of Eu3+ ions. The UV (NUV) light in this range (250−420 nm) can excite not only the divalent europium ions (Eu2+) but also the trivalent europium ions (Eu3+). All of these elements have led to the versatile luminescent properties of SrAl3BO7:0.03(Eu2+,Eu3+) phosphor.20

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