Synthesis and optical performance of a new red-emitting ZnTiF6·6H2O:Mn4+ phosphor for warm white-light-emitting diodes
Introduction
Over the past decade, white-light-emitting diodes (WLEDs) have gained extensive attention to scientists and engineers due to their admirable merits of energy-saving, long lifetime, high luminous efficacy and environment friendliness [1], [2], [3]. There are several approaches to produce phosphor-conversion LEDs. For UV-LED chips pumping phosphors, both doping a single rare earth ion such as Dy3+ and co-doping a sensitizer and activator into appropriate single-phase hosts to produce white light can provide good color rendering. However, problems in the luminescence efficiency and manufacture cost are still exists. There is a tradeoff between the luminescence and the color rendering [4], [5], [6]. Currently, blue LED chips pumping phosphors as the most widely adapted approach to obtain WLEDs, suffers from some technical weaknesses in practical application due to lack of red emitting components, which leads to high correlated color temperature (CCT) and low color rendering index (CRI) [7], [8], [9]. In order to solve this issue, enormous research efforts have been devoted to developing rare-earth (RE) activated sulfide and nitride red phosphors [10], [11]. Unfortunately, most nitride-type phosphors require critical preparation conditions and sulfide-type ones are not stable, which limits their application [12]. Therefore, it is highly desirable to develop new phosphors to meet the practical requirements of WLEDs [13].
As an alternative, the non-rare-earth metal ion Mn4+, as an efficient activator for red phosphor, offers several important advantages due to its spectroscopic properties and economic feasibility [14], [15]. Compared to the parity-forbidden f–f transition of RE ions, the 3 d3 electron configuration of Mn4+ is easily affected by differently coordinated environments due to the electrons located in an outer orbit [15], [16]. Generally, Mn4+-activated phosphor materials exhibit broad adsorption bands spanning from 380 to 490 nm and sharp red emission between 600 and 780 nm [17]. A typical example of Mn4+-doped oxide phosphor is Sr4Al14O25 [14], [18], [19]. However, its emission is located in the wavelength region between 650 and 680 nm, which is too far red-shifted for efficient warm WLEDs. Meanwhile, most of the Mn4+-activated oxide phosphors are synthesized at a higher temperature, which requires special equipment and thus causes rather high cost [20]. To circumvent the above-mentioned drawbacks, a series of Mn4+-doped fluorides A2BF6:Mn4+ (A = K, Na, Ba, Rb, Cs and BSi, Ti, Ge) have been reported, which have a strong and broad absorption band in blue region and emit red luminescence with a emission peak at ∼630 nm [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38]. For example, Adachi et al. prepared a series of A2BF6:Mn4+ red phosphors using wet chemical etching by mixing metal wafer in a HF/AMnO4 (A = K, Na) solution [26], [27], [28]. Pan et al. synthesized K2SiF6:Mn4+, K2TiF6:Mn4+, BaSiF6:Mn4+ and BaTiF6:Mn4+ by hydrothermal methods and studied their optical properties [29], [30], [31], [32], [33]. However, wet chemical etching of metal wafer has some shortcomings such as expensive cost of raw materials and low yield. In addition, high temperature, high pressure and long reacting time are generally required during the hydrothermal synthesis [24]. More recently, Chen et al. demonstrated a convenient chemical route based on cation exchange strategy to fabricate highly efficient K2TiF6:Mn4+ red phosphor [34]. Liu et al. reported a series of A2BF6:Mn4+(A = K, Na, Rb and B= Ge, Si, Ti) with high thermal stability and extremely high emission intensity synthesized by chemical co-precipitation at room temperature [35], [36], [37], [38].
Herein, a new ZnTiF6·6H2O:Mn4+ red phosphor is fabricated through a mild chemical co-precipitation method. The synthetic technique is facile and the amount of HF used in our experiment is much lower than those adopted in the previously reported works. The optimum contents of K2MnF6 for preparation of ZnTiF6·6H2O:Mn4+ reach to 9.5 mol%. By encapsulating the as-obtained ZnTiF6·6H2O:Mn4+ red phosphor with YAG:Ce3+ yellow phosphor on a InGaN chip, a high-performance warm WLED with low CCT (3987 K) and high CRI (83.1) is achieved.
Section snippets
Raw materials
ZnF2 (AR, 99.0%), TiO2 (AR, 99.8%), KHF2 (AR, 99.0%), H2O2 (AR, 30 wt.% in H2O), KMnO4 (AR, 99.5%) and HF (GR, 40%) were purchased from Aladdin Chemistry Co. Ltd. (Shanghai, China) and the commercial YAG: Ce3+ was purchased from XinLi Illuminant Co. LTD. All the chemicals were used without further purification.
Synthesis of ZnTiF6·6H2O:Mn4+ phosphor
A two-step chemical co-precipitation method was used to synthesize ZnTiF6·6H2O:Mn4+ red phosphor according to the procedures described in the reports [22], [34], [36] with some
Results and discussion
XRD patterns of the synthesized ZnTiF6·6H2O with different K2MnF6 contents are shown in Fig. 2(a), along with its corresponding standard diffraction cards (ZnTiF6·6H2O JCPDS card no.87-1595 and K2MnF6 JCPDS card no.34-0733). When the doping of K2MnF6 concentration is less than 11 mol%, all the diffraction peaks can be indexed to hexagonal ZnTiF6·6H2O phase (JCPDS card no.87-1595). No traces of K2MnF6 residual and other impurity phases are found, which indicates that the doping of K2MnF6 does
Conclusions
In summary, a new red phosphor ZnTiF6·6H2O:Mn4+ has been successfully prepared via a simple two-step chemical co-precipitation method. The phosphor has a trigonal crystal structure, and no impurities or other fluoride phases are detected when the Mn4+ doping concentration is less than 11 mol%. ZnTiF6·6H2O keep micro-rod morphology before and after the introduction of Mn4+ ions. Mn4+ ion is substituted for the Ti site in the ZnTiF6·6H2O host, resulting in the strong blue-light excitation
Acknowledgments
This work was supported by Zhejiang Province Natural Science Foundation of China (LQ14E020006, LQ13E020003), the Natural Science Foundation of Zhejiang for Distinguished Young Scholars (LR15E020001), 151 talent's projects in the second level of Zhejiang Province, and National Nature Science Foundation of China (21271170, 61372025, 51372172, 51502066, and 51572065).
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