Color tunable Ba0.79Al10.9O17.14:xEu phosphor prepared in air via valence state control

A series of luminescent Ba0.79Al10.9O17.14:xEu (x = 0.005–0.12) phosphors were prepared by high-temperature solid-state reaction in air atmosphere. The coexistence of Eu2+ and Eu3+ was observed and verified by photoluminescence (PL) and photoluminescence excitation (PLE) spectra, X-ray photoelectron spectra (XPS), and diffuse reflection spectra. The band emission peaking at 430 nm was assigned to 4F65D–4F7 transition of Eu2+, and another four emissions peaking at 589, 619, 655, and 704 nm were attributed to 4F–4F transitions of 5D0–7FJ (J = 1, 2, 3, 4) of Eu3+. The related mechanism of self-reduction was discussed in detail. The color of the Ba0.79Al10.9O17.14:xEu phosphors could be shifted from blue (0.23, 0.10) to red (0.42, 0.27) by doping Li+ ions, and the temperature dependence properties were investigated.


Introduction
White light emitting diode (WLED) is considered as a new generation of solid-state lighting source due to the characteristics of high energy efficiency, long lifetime, low energy consumption, etc. There are three traditional approaches to generate WLED: (1) YAG phosphor excited by blue LED, (2) direct white phosphor excited by ultraviolet (UV) LED, and (3) tricolor phosphor excited by near ultraviolet (NUV) LED. The third approach pumping blue, green, and red emitting phosphors with NUV LED deserves more attention because it displays extensive spectral distribution over the whole visible range to obtain high quality white light [1][2][3][4][5]. Hexagonal aluminates have good thermal and chemical stabilities, which are widely used as tricolor phosphor host materials [6,7]. The luminescence property of Eu 2+ and Ce 3+ doped BaAl 12 O 19 was firstly studied by Verstegen and Stevels [8] in 1974. Xiao et al. [9] reported the effects of crystallization temperature, Eu 2+ concentration, and Al 3+ content on the occupation of Eu 2+ in BaAl 12 O 19 :Eu 2+ phosphor. The energy transfer mechanisms in BaAl 12 O 19 :Ce 3+ ,Eu 2+ phosphor were investigated in detail by Jeon et al. [10]. Deshmukh et al. [11] reported the effect of Ca 2+ and Sr 2+ ions on luminescence properties of BaAl 12 O 19 :Eu 2+ phosphor. BaAl 12 O 19 also can be used as long afterglow phosphor with co-doped Eu and Dy and shows high brightness, long afterglow time, and stable performance [12].
Generally, Eu 2+ ions are usually used as an activator  of the blue luminescent materials because of the predominant 4F 6 5D-4F 7 transition peaking from 400 to 550 nm [13,14]. With the wide use of Eu 2 O 3 as raw material in the synthesis of Eu-doped phosphors, the most common method to obtain Eu 2+ is preparing under reducing atmosphere, such as H 2 , H 2 /N 2 , or C. Since the reduction process from Eu 3+ to Eu 2+ in air was first found by non-equivalent substitution method in the 1990s [15], many reports pointed out that the reduction of Eu 3+ to Eu 2+ happens in some particular hosts in air atmosphere. In 1998, Zeng et al. [16] reported the reduction of Eu 3+ in SrB 6 O 10 prepared in air and the luminescence of SrB 6 O 10 :Eu 3+ . Next year, Pei et al. [17] discussed the mechanism of the abnormal reduction of Eu 3+ to Eu 2+ in Sr 2 B 5 O 9 Cl. Subsequently, Peng et al. [18,19]  :Eu phosphors were prepared in air by high-temperature solid-state reaction. The coexistence of Eu 2+ and Eu 3+ was proved by photoluminescence spectra, diffuse reflection spectra, and X-ray photoelectron spectra. The self-reduction mechanism was discussed in detail based on the charge compensation model, and the reduction process could be controlled by tuning synthesis temperature and doping concentration of Eu 3+ and Li + ions to obtain the colorful tunable phosphor.
The phase composition of synthesized samples was analyzed by X-ray diffraction (XRD) on a D8 Advance diffractometer with Cu Kα1 radiation (λ = 1.5406 Å) by the step of 4 (°)/min at room temperature. The emission and excitation spectra, and the temperature dependant luminescence properties were detected with a Hitachi F-4600 fluorescence spectrophotometer. The diffuse reflection spectra were measured via a Shimadzu UV-3600 UV-Vis-NIR spectrophotometer attached with an integral sphere. The photoluminescence decay curves were determined by a Horiba JOBIN YVON FL3-21 spectrofluorometer. Figure 1 shows the XRD patterns of Ba 0.79 Al 10.9 O 17.14 synthesized at 1450 ℃, Ba 0.79 Al 10.9 O 17.14 :0.04Eu synthesized at 1450 ℃ , 1500 ℃ , 1550 ℃ , and Ba 0.79 Al 10.9 O 17.14 :0.04Eu,0.04Li + synthesized at 1550 ℃ for 4 h in air. The reference pattern of the standard JCPDS Card No. 77-1522 for Ba 0.79 Al 10.9 O 17.14 was also listed in Fig. 1 in the bottom. It is indicated that the samples prepared in different conditions are all in pure Ba 0.79 Al 10.9 O 17.14 phase, and the changes of synthesis temperature and doping of Eu 3+ ions would not impact the host structure. When Li + ions were doped into the Ba 0.79 Al 10.9 O 17.14 host, the main diffraction peaks shifted slightly to higher angles. The change indicates that the Li + ions replace the ions with comparatively small radii in the Ba 0.79 Al 10.9 O 17.14 host lattice.

1 Crystal structures
The fragments of Ba 0.79 Al 10.9 O 17.14 unit cell are exhibited in Fig. 2. Ba 0.79 Al 10.9 O 17.14 is hexagonal structure with space group P6 3 /mmc, in which the Ba 2+ ions occupy nine-coordinated sites, and the Al 3+ ions form tetrahedrons and octahedrons with different coordinations [22]. The radius difference between Ba 2+ and Eu 2+ is much smaller than that of Ba 2+ and Eu 3+ as well as Ba 2+ and Al 3+ [23,24]. Thus, it is reasonable to consider the Eu ions are substituted in the Ba 2+ ion sites. Considering the charge mismatching between Ba 2+ and Eu 3+ , the Eu 3+ ions are introduced into the Ba 2+ sites in a non-equivalent compensation way.

2 Self-reduction of Eu 3+ to Eu 2+ in
Ba 0.79 Al 10.9 O 17.14 :xEu The photoluminescence excitation (PLE) and photoluminescence (PL) spectra of Ba 0.79 Al 10.9 O 17.14 : 0.04Eu are shown in Fig. 3. It is found that there is a coexistence of Eu 2+ and Eu 3+ in the host after sintering at 1550 ℃ for 4 h, though the source of Eu ions is Eu 2 O 3 . Monitoring the characteristic excitation of two ions, as shown in Fig. 3(a), the broad absorption bands monitored at 431 nm peaking at 259 and 314 nm originate from 4F 7 -4F 6 5D transitions of Eu 2+ , while the other absorption band monitored at 619 nm peaking at 254 nm is attributed to the 7 F J (J = 1, 2, 3, 4)-5 D 0 transitions of Eu 3+ [25]. The PL spectra under varying excitation factors are shown in Fig. 3  As we know, 5 D 0 -7 F 1,3 of Eu 3+ belong to the magnetic dipole transitions, whereas 5 D 0 -7 F 2,4 are electric dipole transitions [26]. The strongest emission peaking at 619 nm from 5 D 0 -7 F 2 transition of Eu 3+ reflects that the electric dipole transition is the dominant factor, which is greatly influenced by the lattice symmetry [27]. Figure 4 presents the diffuse reflection spectra of Ba 0.79 Al 10.9 O 17.14 :xEu (x = 0-0.10). It is observed that the energy absorption of Ba 0.79 Al 10.9 O 17.14 host appears in UV range, and the band gap is about 3.37 eV, which is estimated by the fitting line of the absorption edge. With Eu 3+ ions doping into the host, a broad absorption band occurs around 200-300 nm in the near-UV region because of the 4F 7 -4F 6 5D transition from Eu 2+ , and another one weak absorption peaking at 391 nm originates from the transition 7 F 3 -5 D 0 from Eu 3+ , which is in accordance with the excitation spectra of Eu 2+ and Eu 3+ , indicating that Eu 2+ and Eu 3+ can be singly or both excited with the various excitation wavelengths.
The coexistence of Eu 2+ and Eu 3+ ions is also pointed out by X-ray photoelectron spectra (XPS), as shown in Fig. 5. Only one broad band peaking at 1136.5 eV is observed when the preparation temperatures are 1450 ℃ and 1500 ℃, which is ascribed to Eu 3+ 3d 5/2 , because the self-reaction is incomplete and the amount of Eu 2+ is limited at this point. And another one peak at 1127.5 eV consistent with Eu 2+ 3d 5/2 is observed as the preparation temperature rises to 1550 ℃ [28]. The charge compensation model is an important theory for explaining the reduction of Eu 3+ to Eu 2+ in Ba 0.79 Al 10.9 O 17.14 :xEu in air [29][30][31]. It is known that the Eu 3+ ions occupy the Ba 2+ sites in a non-equivalent compensation way, and every three Ba 2+ ions are substituted by two Eu 3+ ions for balancing the charge neutrality. Thus one vacancy defect of Ba V  with two negative charges and two positively charged defects of Ba Eu are created. The vacancy works as the donor of electrons, while the defect works as the acceptor of electrons in the host. Thermal effect promotes the transition of negative charges from vacancy of Ba V  to Eu 3+ sites. When the negative charges get to the Eu 3+ sites, they will fill into the 4F orbit of Eu ions. As a consequence, the Eu 3+ ions are reduced to Eu 2+ ions in air. According to the electroneutrality principle [32,33], the doped Li + ions, working as the charge compensators, neutralize the negative charges and occupy the Ba vacancies. They prevent the formation of new vacancies, and promote the reaction going towards the reactants. The reaction process is conducted as follows: When Eu 3+ ions are doped into the host and reduced to Eu 2+ ions, they will be surrounded by the framework structure of AlO 4 tetrahedron, which has considerable inhibitory effects on oxidation of Eu 2+ [34,35]. The framework consists of six-membered network structures formed by corner-shared AlO 4 tetrahedrons, whose centers are occupied by Eu 2+ ions working as the charge compensation cations. The Eu 2+ ions in the hollow structures of AlO 4 tetrahedrons are protected from oxidation, so that they can exist stably in the host.

3 Luminescence of Ba 0.79 Al 10.9 O 17.14 :xEu
The amount ratio of Eu 2+ and Eu 3+ ions in the host is defined as η, which can approximately be equal to the   Figure 6 presents the PL spectra (λ ex = 254 nm) of Ba 0.79 Al 10.9 O 17.14 :xEu (x = 0.005-0.12) prepared at 1550 ℃ for 4 h and the variation of η with the variety of the Eu 3+ ion concentration. The emissions of Eu 2+ and Eu 3+ enhance with the increase of Eu 3+ ion concentration and reach the maximum at 2 mol% due to the concentration quenching [36,37]. The value of η has a similar tendency with emission intensity which indicates that the Eu 2+ emission of blue light plays a leading role when the concentration of Eu 3+ ions is changed from 0.5 to 12 mol%, and the small amount doping of Eu 3+ ions can promote the self-reduction to some extent.
The decay curves of the Eu 2+ and Eu 3+ luminescence in Ba 0.79 Al 10.9 O 17.14 :xEu (x = 0.04-0.12) prepared at 1550 ℃ upon excitation at 359 and 375 nm are measured and depicted in Fig. 7. The corresponding lifetime can be well fitted to a first-order exponential equation [20]: where I is the luminescence intensity, A is a constant, t is the time, and τ is the lifetime for the exponential component. As shown in Fig. 7(a) where 0  is the relative lifetime, nr A is the non-radiative rate due to multiphonon relaxation, and t P is the energy transfer rate between Eu 3+ ions. With increasing of Eu 3+ ions, the distance between Eu 3+ ions decreases, and the energy transfer rate among Eu 3+ and the probability of energy transfer to luminescent killer sites increase. Therefore, the lifetime is shorten with increasing Eu 3+ concentration [40].
With the increasing doping of Li + ions, as shown in Fig. 8, the calculated lifetime of Eu 2+ decreases and the reaction goes towards the reactants. It can be indicated that the content of Eu 3+ increases, and the energy transfer process between Eu 2+ and Eu 3+ is enhanced. Figure 9 displays the PL spectra of Ba 0.79 Al 10.9 O 17.14 : 0.04Eu prepared at 1450 ℃, 1500 ℃, and 1550 ℃ for 4 h, and the variation of η with different preparation temperatures is shown in the inset. It is found that η is increased with the rise of synthesis temperature, and the emission intensity of Eu 2+ increases while the emission intensity of Eu 3+ decreases obviously. When the synthesis temperature is less than 1500 ℃, Eu 3+ emission of red light is the dominant effect. Eu 2+ emission becomes dominant when the synthesis temperature increases above 1500 ℃. The result indicates that the preparation temperature rise can be an advantage for self-reduction of Eu 3+ .
The PL spectra of Ba 0.79 Al 10.9 O 17.14 :0.04Eu,yLi + (y = 0.04-0.14) prepared at 1550 ℃ for 4 h and the variation of η are illustrated in Fig. 10. It is found the blue emissions of Eu 2+ are very sensitive with the doping of Li + ions which play a role of charge compensation. Both the emission intensity of Eu 2+ and the value of η decrease with the increase of Li + ion doping concentration, because self-reduction of Eu 3+ goes towards the reactants. Figure 11 shows the CIE chromaticity diagram of Ba 0.79 Al 10.9 O 17.14 :0.04Eu,yLi + (y = 0.04-0.14) and the relative digital images upon excitation of 254 nm. It can be concluded that the doping of Li + ions promotes the self-reduction of Eu 3+ ions, and the color of phosphors can be shifted from blue (0.23, 0.10) to red (0.42, 0.27).

4 Temperature dependence properties
It is known that the thermal quenching property is an important factor for white light output in white LED application [41,42]. The temperature-dependent emission spectra (λ ex = 254 nm) of Ba 0.79 Al 10.9 O 17.14 : 0.04Eu,0.04Li + prepared in air are shown in Figs. 12(a) and 12(b). The emission intensity decreases to 83.84% at 100 ℃ and 70.73% at 150 ℃ compared with the intensity at room temperature, which indicates that the Ba 0.79 Al 10.9 O 17.14 :0.04Eu,0.04Li + phosphor shows good thermal stability. The decrease of emission intensity is due to the probability of molecule collision and nonradiative transition is enhanced with the temperature rise [43].
The activation energy from the temperature quenching can be calculated with the Arrhenius equation [44,45]: where 0 I is the emission intensity of Ba 0.79 Al 10.9 O 17.14 : 0.04Eu,0.04Li + phosphor at room temperature, T I is the emission intensity at different temperature, c is a constant, ΔE is the activation energy for temperature quenching, k is the Boltzman's constant (8.62×10 5 eV), and T is the temperature. As Fig. 13 shows, the relationship between 1/(kT) and   0 ln / 1 I I  presents a relative linearity, and the activation energy for thermal quenching of Ba 0.79 Al 10.9 O 17.14 :0.04Eu,0.04Li + is calculated as 0.232 eV. The slope of the straight line is 0.232 which equals to ΔE.

Conclusions
Tunable luminescent Ba 0.79 Al 10.9 O 17.14 :xEu (x = 0.005-0.12) phosphors have been prepared successfully in air by high-temperature solid-state reaction. The coexistance of Eu 2+ and Eu 3+ in the host has been observed, and the mechanism of self-reduction has been discussed in detail based on the charge compensation model. The critical quenching concentration of Eu 2+ in Ba 0.79 Al 10.9 O 17.14 :xEu (x = 0.005-0.12) is about 2 mol%, which is verified to be the dipole-dipole interaction. The colors of phosphors can be easily tuned from blue (0.23, 0.10) to red (0.42, 0.27) by adjusting the valance state through controlling the concentration of Li + , and the emission intensity maintains at a high level with 70.73% at 150 ℃ compared with the intensity at room temperature. The prepared Ba 0.79 Al 10.9 O 17.14 :xEu (x = 0.005-0.12) phosphors can be regarded as an alternative to obtain color tunable emission for white LED.