1. Introduction

In the last decade, there has been a dramatic increase in the number of research on white light-emitting diodes (LEDs) as a new light source for general lighting and displays [1]. The conversional white light illumination are mostly composed of blue-emitting InGaN chip and yellow phosphor, typically Y₃Al₅O₁₂:Ce³⁺ (YAG) [2,3], which exhibits high luminescence efficiency and chemical stability. The combination of blue chip and YAG, however, show a lower color rendering index (Ra) of ~80 due to the lack of red color contribution. Thus, developing new red phosphors, which can be efficiently excited by either 460 or 405 nm LEDs is very crucial issue. For seeking suitable red phosphors, sulfides and oxysulfides, such as CaS:Eu²⁺, SrS:Eu²⁺, La₂O₂S:Eu³⁺, and Y₂O₂S:Eu³⁺, were reported to be efficiently excited under 460 or 405 nm. Nevertheless, the drawbacks of these compounds are sensitive to moisture, giving a poor chemical stability. Recently, nitrides and oxynitrides [4–8] demonstrate good potential as red phosphors due to their good thermal stability. Hence, it is urgent to search for new red phosphor with low cost and high chemical stability. From the points of view, the oxide-based hosts with Eu³⁺ have been widely investigated [9–12]. It has been well known that in some hosts, Bi³⁺ or Sm³⁺ was proven to be a very good sensitizer for Eu³⁺ with not only enhancing the luminescent performance but also broadening the excitation spectrum. Datta [13] investigated the role of Bi³⁺ in YVO₄:Eu³⁺ and showed an increase in luminescence intensity of almost 200% as Bi³⁺ is incorporated into the (Y_0.95Eu_0.05)VO₄ lattice. Neeraj et al. [14] reported the excitation band of Bi_xLn_1-xVO₄ system doped with Eu³⁺ or Sm³⁺ can be broaden by choosing suitable sensitizers. Park et al. [15] investigated the effect of Bi³⁺, Eu³⁺ codoping on the excitation of Eu³⁺ in YVO₄:Bi³⁺,Eu³⁺ and concluded that with increasing Bi³⁺ dopant content, the excitation range was found to shift to longer wavelength. Wang et al. [16] also demonstrated that the incorporation of Bi³⁺ and Sm³⁺ into NaEu(MoO₄)₂ host can both broadened the excitation band and enhance the emission intensity of Eu³⁺ under 395/405nm. Chi et al. [17] found that Bi³⁺-doped Y₂O₃:Eu is used as red phosphors with very high efficiency. Recently, Li et al [18] reported photoluminescence (PL) and thermoluminescence (TL) of Zn(BO₂)₂:Tb³⁺ and concluded that the phosphor exhibited potential application in gamma-rays TL dosimeter. To the best of our knowledge, the luminescence properties of Bi³⁺/Eu³⁺ – co-activated ZnB₂O₄ have not been reported yet. Hence, the aim of this study is to investigate and examine the luminescence properties of a new red phosphor, ZnB₂O₄:Bi³⁺,Eu³⁺. The improvement of luminescence intensity through energy transfer from co-doped sensitizer Bi³⁺ to activator Eu³⁺ in ZnB₂O₄ host will also be discussed in this work.

2. Experimental

Polycrystalline samples of ZnB₂O₄:Eu³⁺ and ZnB₂O₄:Bi³⁺,Eu³⁺ powders were synthesized by conventional solid state reaction, starting from ZnO, H₃BO₃, Eu₂O₃, and Bi₂O₃. The raw materials were weighed out in stoichiometric proportions and the mixtures were then fired at 850°C for 10 h under ambient atmosphere. The detailed measurements of photoluminescence (PL), photoluminescence excitation (PLE), Commission International de I’Eclairage (CIE) chromaticity, and diffuse reflectance (DR) spectra were carried out under ambient atmosphere and described in our previous work [19].

3. Results and discussion

3.1. XRD patterns and atom structure of synthesized ZnB₂O₄

As reported in the JCPDS card No. 39-1126, ZnB₂O₄, crystallizes in a cubic (space group: Im-3m (229)) structure with lattice constant: a = 7.473 Å and Z = 6. Figure 1 shows the XRD patterns of the as-synthesized ZnB₂O₄, Zn_0.9Eu_0.1B₂O₄, and Zn_0.8Eu_0.1Bi_0.1B₂O₄ samples. These XRD patterns were found to be consistent with that reported in JCPDS card No. 39-1126 and no peak shifting was observed in the XRD patterns of ZnB₂O₄:Eu or ZnB₂O₄:Bi,Eu samples.

 

Fig. 1 XRD patterns of (a) ZnB₂O₄, (b) (Zn_0.9Eu_0.1)B₂O₄ and (c) (Zn_0.8Eu_0.1Bi_0.1)B₂O₄. The standard XRD pattern of ZnB₂O₄ is taken from JCPDS Card No. 39-1126. The internal standard silicon is labeled with a star.

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The decrease in crystallinity of ZnB₂O₄:Eu,Bi with increasing Eu³⁺ and Bi³⁺ dopant content was observed from the XRD patterns, which can be attributed to the variation of charges of dopants that results in the defect formation in the lattice. Little impurity phases, such as H₃BO₃ and B₂O₃ were observed at 2θ = 29.7°, 30.2°, 30.9°, 31.3°, 31.6° and 32.1° in Fig. 1b and 1c, respectively. The intensity of impurity phases are too small that the effect on luminescence of ZnB₂O₄:Eu or ZnB₂O₄:Bi,Eu could be neglected. The XRD data shown in Fig. 1 indicated the as-prepared ZnB₂O₄ samples were almost pure phase with highly crystallinity. It is known that the ionic radii (r) of Zn²⁺ (CN = 4) and B³⁺ (CN = 3) are 0.60 Å and 0.21 Å [20], respectively. As a result of the ionic radius of B³⁺ is too small, it is difficult for these elements to replace B³⁺ in the ZnB₂O₄. Hence, in this study, it is believed that the Zn²⁺ sites are substituted of Eu³⁺ and Bi³⁺ in the lattice.

3.2. The UV excitation spectrum under Bi³⁺/Eu³⁺ activators

Figure 2 shows the reflectance spectra of pure ZnB₂O₄ and tri-valance ions (such as Bi, Eu or Bi-Eu) activated–ZnB₂O₄. The spectrum of pure ZnB₂O₄ host (Fig. 2(a)) exhibited the fundamental absorption edge at ~380 nm. For doping 10 mol% Eu³⁺ in ZnB₂O₄, two absorption bands peaks at ~280 nm and 393 nm are observed in the spectrum. The former is attributed to the O^2--Eu³⁺ charge transfer band (CTB); the latter is resulting from f-f transition of Eu³⁺ (⁷F₀→⁵L₆). While solely doping 1 mol % Bi³⁺, a very broad absorption band between 250 nm to 390 nm is observed, which could be responsible for the 6s6p excitation state of Bi³⁺ [21].

 

Fig. 2 Reflectance spectra of as-synthesized samples: (a) ZnB₂O₄; (b) Zn_0.9B₂O₄:Eu_0.1; (c) Zn_0.89B₂O₄:Bi_0.01Eu_0.1; (d) Zn_0.8B₂O₄:Bi_0.1Eu_0.1; and (e) Zn_0.99B₂O₄:Bi_0.01.

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For Zn_0.89B₂O₄:Bi_0.01Eu_0.1 and Zn_0.8B₂O₄:Bi_0.1Eu_0.1, the spectra show that with increasing the Bi³⁺ content, the absorption band shifts to longer wavelength. It was found that the absorption edges were almost overlapping in ZnB₂O₄, ZnB₂O₄:1%Eu, ZnB₂O₄:3%Eu and ZnB₂O₄:5%Eu. The unobserved absorption edge for ZnB₂O₄:10%Eu may be due to its relatively strong luminescence. The detector of reflector received higher luminescence while excitation wavelength in the range of 350~400 nm.

The PL and PLE spectra of ZnB₂O₄:0.1Eu³⁺ are shown in Fig. 3 . A broad excitation band observed at 250~280 nm can be attributed to the O^2--Eu³⁺ charge transfer (CT) transition. The sharp excitation peaks between 300 and 550 nm are due to the typical f-f transition of Eu³⁺. The strongest line absorption in the excitation spectrum is located at 393 nm, which is resulting from the ⁷F₀→⁵L₆ transition. The PL spectrum exhibits typically Eu³⁺ line emission at 578 nm (⁵D₀→⁷F₀), 585, 590, 600 nm (⁵D₀→⁷F₁), 610, 621 nm (⁵D₀→⁷F₂), 651 nm (⁵D₀→⁷F₃) and 689 nm (⁵D₀→⁷F₄). It has been reported that the highly intense line at ⁵D₀→⁷F₁ is due to magnetic dipole transition, while strong emission at ⁵D₀→⁷F₂ is attributed to the electric dipole transition which is observed at 610 nm (⁵D₀→⁵F₂) indicating that Eu³⁺ ion occupied the site of non-inversion symmetry [22]. The ⁵D₀→⁷F₀ transition at 578 nm is observed in (Zn_0.9Eu_0.1)B₂O₄ revealing that Eu³⁺ occupied a site with C_v, C_nv or C_s symmetry [23]. The inset displays that the PL intensity of Eu³⁺ excited at 393 nm as a function of Eu³⁺ concentration. It can be seen that the optimal dopant concentration of Eu³⁺ is 10 mol %. Below 10 mol%, the PL intensity of the emission peaks were found to increase with increasing Eu³⁺ content, while above 10 mol%, it decreased with the increasing Eu³⁺ content. The former observation could be attributed to the distance between Eu³⁺ ions is too far away that the intensity is proportional to the content of Eu³⁺. The latter observation is presumably due to the concentration quenching of Eu³⁺ ions.

 

Fig. 3 PL/PLE spectra of as-synthesized (Zn_0.9Eu_0.1)B₂O₄ excited at 393 nm. The inset represents the effect of Eu³⁺ concentration on the PL intensity.

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Figure 4 shows the PL spectra of ZnB₂O₄ with different content of Eu³⁺. The inset in Fig. 4 displayed the intensity ratio of 583 nm and 609 nm. In this study, the dominant emission peak was varied with the content of Eu³⁺. The ratio of ⁵D₀ – ⁷F₁/⁵D₀ – ⁷F₂ was different with the increase of Eu³⁺ concentrations. The results indicate that there are more than one Eu sites in the lattice. The excitation and emission wavelengths of Bi³⁺ observed at 380 nm and 485 nm are corresponded to the ¹S₀→¹P₁ and ³P₁→¹S₀ transitions [24]. The emission band of Bi³⁺ was found to overlap with the excitation bands of Eu³⁺ ions, especially at 393 nm (⁷F₀→⁵L₆ transition) and 464 nm (⁷F₀→⁵D₂ transition), respectively. It is well known that for effective energy transfer to occur, an overlap of the emission region of the sensitizer, said Bi³⁺ in this study, and the absorption region of the activator, namely Eu³⁺ was necessary. We also investigated the emission intensity of (Zn_0.9Eu_0.1)B₂O₄ doped with 1, 3, 5, 10, 15 and 20 mol % of Bi³⁺. Similar to Eu³⁺ doping in ZnB₂O₄, an optimal doping concentration of Bi³⁺ was also found to be 10 mol %. With Eu³⁺/Bi³⁺ codoping in ZnB₂O₄, the optimal composition was determined to be (Zn_0.8Bi_0.1Eu_0.1)B₂O₄. Thus, it can be concluded that in ZnB₂O₄, Bi³⁺ is a sensitizer for the luminescence of Eu³⁺ and with the Bi³⁺ codoped in ZnB₂O₄:Eu³⁺, the PL intensity and brightness can be significantly improved by an increment of 16% and 52%, respectively. As compared to the red-emitting phosphor La₂O₂S:Eu³⁺ the intensity and integrated area of ZnB₂O₄:Eu³⁺,Bi³⁺ were found to be 95% and 104% of those of the commodity, respectively.

 

Fig. 4 shows the PL spectra of ZnB₂O₄ with different content of Eu³⁺. The inset in Fig. 4 displayed the intensity ratio of 583 nm and 609 nm. In this study, the dominant emission peak was varied with the content of Eu³⁺.

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3.3 PL spectra and relative emission intensity dependence of temperature effect

For the application of high power LEDs, the thermal stability tests of phosphors are shown in Fig. 5 . In general, the luminescence intensity decreases with increasing temperature. The thermal quenching phenomena observed in phosphors are generally attributed to (a) the degradation of host structure, (b) the oxidation of activators, and (c) the interruption of energy transfer processes between host and activator. Figure 5 displays the PL spectra of (Zn_0.8Bi_0.1Eu_0.1)B₂O₄ excited at 393 nm with varied temperatures ranging from 25°C to 300°C. The PL spectra of (Zn_0.8Bi_0.1Eu_0.1)B₂O₄ were found to show little change with increasing temperature except the intensity.

 

Fig. 5 PL spectra of (Zn_0.8Eu_0.1Bi_0.1)B₂O₄ excited at 393 nm with different temperatures. The inset shows the comparison of PL intensity vs. temperature relationship for (a) (Zn_0.8Eu_0.1Bi_0.1)B₂O₄ and (b) La₂O₂S:Eu³⁺ commodity.

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In comparison, the luminescence of La₂O₂S:Eu³⁺ commodity was also carried out under the same condition. The inset in Fig. 5 shows the comparison of the temperature-dependent PL intensity or thermal quenching behavior for (Zn_0.8Bi_0.1Eu_0.1)B₂O₄ and La₂O₂S:Eu³⁺. The PL intensity of La₂O₂S:Eu³⁺ was firstly observed to increase before reaching its maximum value at 100°C and dramatically decreased with increasing temperature. The thermal behavior of (Zn_0.8Bi_0.1Eu_0.1)B₂O₄ is a “irreversible” process, meaning that while the temperature returns to room temperature, the PL intensity cannot go back to its original value. The increase of PL intensity at lower temperature was probably due to the red shift of excitation band of La₂O₂S:Eu³⁺ at ~395 nm[25]. As for (Zn_0.8Bi_0.1Eu_0.1)B₂O₄, the inset of Fig. 5 shows a linear decay in PL intensity with increasing temperature, which may result from the interruption of energy transfer processes between host and activator. The study on the thermal quenching of luminescence indicates that ZnB₂O₄:Eu,Bi can serve as a potential candidate as a red-emitting phosphor in the application high power phosphor-converted LEDs.

4. Conclusion

A new red-emitting phosphor, ZnB₂O₄:Eu³⁺,Bi³⁺, was synthesized by solid state reactions for the first time. The optimized compositions of ZnB₂O₄ co-doping Eu³⁺ and Eu³⁺/Bi³⁺ are ZnB₂O₄:10%Eu and ZnB₂O₄:10%Eu,10%Bi, respectively. The introduction of Bi³⁺ ions successfully enhances both emission intensity and quantum efficiency by 14% and 6%, respectively. The present results indicate that the novel red emitting phosphor is a suitable candidate for the application on NUV white LEDs.