Green Synthesis and Luminescent Properties of Mn⁴⁺ Doped Red Phosphor for WLED

Xiaoyi Liu^1,2, Guixia Liu^1,2*

¹College of Materials Science and Engineering, Changchun University of Science and Technology, Changchun, China.
²Key Laboratory of Applied Chemistry and Nanotechnology at Universities of Jilin Province, Changchun University of Science and Technology, Changchun, China.

DOI: 10.4236/opj.2023.136013                

Abstract

Herein, the K₃MoO₂F₅^.2H₂O:Mn⁴⁺ phosphor was synthesized by using low toxic NH₄HF₂ and HCl instead of highly toxic HF. The K₃MoO₂F₅^.2H₂O:Mn⁴⁺ phosphor has a blocky structure and exhibits sharp red emission at the range of 580 to 670 nm excited by the blue light at 470 nm. The fabricated WLED device at 20 mA current has low correlation color temperature (CCT = 3608 K) and high color rendering index (Ra = 90.1), which can significantly improve the electroluminescence performance of cold WLED devices. These results indicate that the K₃MoO₂F₅^.2H₂O:Mn⁴⁺ phosphor has potential application value in warm WLED excited by blue light chip.

Keywords

Mn⁴⁺, Green Synthesis, Phosphor, WLED

1. Introduction

Compared with traditional incandescent lamp and fluorescent lamp, white light emitting diode (WLED) has the advantages of low heat, low power consumption, fast response, long life and so on [1] [2]. However, for the traditional WLED device with yellow Y₃Al5O₁₂:Ce³⁺ (YAG: Ce) phosphor excited by blue InGaN chip, due to the lack of red light components, there are problems of high correlation color temperature(CCT, CCT > 4500 K) and low color rendering index (CRI, Ra < 80) [3]. Mn⁴⁺ activated phosphors have been reported for a variety of fluoride and oxide substrates [4] [5] [6] [7] [8]. However, Mn⁴⁺ doped oxide red phosphors usually require a high temperature solid phase method, which makes production expensive [9] [10]. Moreover, the strongest excitation peak of such phosphors is in the ultraviolet (UV) region rather than the blue region, resulting in a poor match with the blue LED chip. Compared with Mn⁴⁺ doped oxide phosphors, Mn⁴⁺ doped fluoride red phosphors can not only be synthesized under mild conditions, but also be effectively excited by blue LED chips. However, the synthesis of these phosphors requires highly toxic HF as a solvent, which will not only harm our bodies but also cause environmental pollution [11] [12]. Therefore, finding a green route is an important challenge in the synthesis of Mn⁴⁺ activated fluoride red phosphors. Wang et al. [13] synthesized K₂XF₆:Mn⁴⁺ (X = Ti, Si, Ge) series samples by partially replacing HF with acetic acid. However, the use of acetic acid reduced the solubility of K₂XF₆, and KMnO₄ was prone to decomposition under heat, resulting in a low effective doping concentration of Mn⁴⁺ in the matrix and a decrease in luminescence intensity. Kumar et al. [14] proposed a new HF free and environmentally friendly closed solid phase method for the preparation of K₂TiF₆:Mn⁴⁺ narrow band red luminescent material, which has higher color purity and lower color temperature, but its reaction conditions are harsh. In this work, K₃MoO₂F₅∙2H₂O:Mn⁴⁺ phosphor was synthesized with low toxicity (NH₄HF₂ + HCl) instead of highly toxic HF. The crystal phase structure, morphology and element composition of the phosphor was discussed, and the spectral characteristics of the phosphor were analyzed in detail. Finally, the prepared K₃MoO₂F₅∙2H₂O:Mn⁴⁺ red phosphors were assembled into WLEDs. The results show that K₃MoO₂F₅∙2H₂O:Mn⁴⁺ phosphor prepared by green route is an ideal material for improving the performance of WLEDs.

2. Experimental

2.1. Chemical Regents

KMnO₄ (99.5%), KF (99%), HF (40 wt%), H₂O₂ (30%), MoO₃ (99.5%), KOH (85%), NH₄HF₂ and HCl. All reagents were purchased from the Shanghai Macklin Biochemical Co. Ltd. and were used directly without further purification. K₂MnF₆ was prepared as a manganese source by the strategy shown in Figure 1 [15].

2.2. Maintaining Synthesis Process of K₃MoO₂F₅・2H₂O:Mn⁴⁺

The K₃MoO₂F₅∙2H₂O:Mn⁴⁺ phosphor was prepared by a simple co-precipitation method with low toxicity (NH₄HF₂ + HCl) instead of highly toxic HF. Typically, the preparation details of K₃MoO₂F₅∙2H₂O:Mn⁴⁺ phosphor are as follows (Figure 2). Firstly, 0.2822 g MoO₃ is completely dissolved in the 5 mL prepared KOH

Figure 1. Diagram of the preparation process of K₂MnF₆.

Figure 2. Diagram of the preparation process of K₃MoO₂F₅・2H₂O:Mn⁴⁺.

(1.6 mol∙L⁻¹) solution. Subsequently, 0.4563 g NH₄HF₂ was added into 10 mL HCl solution (4 mol∙L⁻¹), and this solution was added dropwise to the above solution. After 30 min of magnetic stirring, 0.0099 g K₂MnF₆ was added to the reacted solution and continued magnetic stirring for 30 min, a yellow precipitate was formed. Finally, the target product was obtained by centrifugation, washing with ethanol three times and drying at 60˚C for 6 h.

2.3. Fabrication of LED Devices

The warm white LED device was fabricated by mixing the as-synthesized red phosphor, yellow (YAG:Ce³⁺) emitting commercial phosphor and organic silica gel A and B (a mass ratio of 1:4) with a blue LED chip (0.4 W, 460 nm), the mass ratio of yellow phosphor to red phosphor is 1:8. The integrating sphere LED photoelectric parameter comprehensive test system (SSP6612) was used to measure the photoelectric performances including the color rendering index, correlation color temperature electroluminescence (EL) spectra and CIE color coordinates of the fabricated LED devices.

2.4. Materials Characterization

The crystal structure of the phosphor was characterized via RigakuD/max - RA X-ray diffraction (XRD) with Cu Kα radiation (λ = 0.15406 nm) in the scanning range from 10˚ to 90˚ and the scanning speed was 6˚ min⁻¹. The morphology and composition of the sample was identified via JEOL JSM-7610F field emission scanning electron microscope (FE-SEM) and OXFORD ISIS-300 energy dispersive spectrometer (EDS). Using barium sulfate as the substrate, the UV-visible diffuse reflectance spectrum (DRS) of the phosphor was measured by Shimadzu UV-2550 spectrophotometer, in which the mode of the integrating sphere was set to external. Using the light source of 150 W xenon lamp as excitation source, the photoluminescence excitation (PLE) spectra, photoluminescence (PL) spectra of the sample was collected by HITACHI F-7000 fluorescence spectrophotometer in natural environment.

3. Results and Discussion

3.1. Structure, Morphology and Composition

The crystal phase structure and purity of the sample can be examined by XRD. Figure 3 shows the XRD pattern of the as-prepared K₃MoO₂F₅∙2H₂O:Mn⁴⁺ red

Figure 3. XRD pattern of K₃MoO₂F₅・2H₂O:Mn⁴⁺.

phosphor by the co-precipitation method. It can be seen that the as-obtained diffraction peaks can be better corresponded with the standard card (PDF# 31-1117). The Mo⁶⁺ ion and Mn⁴⁺ ion have similar ionic radius and the same coordination number ((R_Mo⁶⁺ = 0.59 Å, R_Mn⁴⁺ = 0.53 Å and CN = 6), thus rendering it possible the doping of Mn⁴⁺ does not influence significantly on the crystal structure of K₃MoO₂F₅∙2H₂O. SEM image (a) and EDS spectrum (b) of K₃MoO₂F₅∙2H₂O:Mn⁴⁺ can be seen from Figure 4. The image shows that the phosphor is irregular lump with an average particle size of 70 - 100 μm and has a rough surface. As show in Figure 4(b), the presence of K, Mo, O, F, Mn elements can be clearly observed. The above results show that the K₃MoO₂F₅∙2H₂O:Mn⁴⁺ phosphor was successfully synthesized by the co-precipitation method.

3.2. Photoluminescence Property

Figure 5(a) shows the excitation and emission spectra of phosphor respectively. When 632 nm was used as the monitoring wavelength, there were two obvious wide absorption bands in the wavelength range of 300 - 550 nm, and the centers of the absorption bands were located at 374 nm (UV region) and 470 nm (blue region), respectively. These two absorption bands are derived from the spin-allowing ⁴A_2g → ⁴T_1g and ⁴A_2g → ⁴T_2g energy level transitions of Mn⁴⁺, respectively. The DRS of K₃MoO₂F₅∙2H₂O:Mn⁴⁺ (Figure 5(b)) shows that the phosphor has strong absorption in the blue region. In addition, the absorption peak also appears in the ultraviolet region at 270 nm, which can be attributed to the O/F → Mo charge transfer band in the matrix. At 470 nm excitation, the emission spectrum of K₃MoO₂F₅∙2H₂O:Mn⁴⁺ phosphor consists of seven typical emission peaks of Mn⁴⁺, 600 nm, 611 nm and 615 nm correspond to anti-Stokes v₃ (t_1u), v₄ (t_1u) and v₆ (t_2u) vibration models of Mn⁴⁺, respectively. Stokes vibration model v₆ (t_2u), v₄ (t_1u) and v₃ (t_1u) of Mn⁴⁺ correspond to emission peaks at 632 nm, 636 nm and 649 nm in the spectrum, among which 632 nm is the strongest emission peak. The characteristic emission peak at 624 nm is strong zero phonon line (ZPL) emission. This is due to the octahedral distortion caused by the non-equivalent substitution and the low symmetry of the crystal structure, so the strength of the ZPL is strong. Figure 5(c) is the CIE coordinate diagram of K₃MoO₂F₅∙2H₂O:Mn⁴⁺ phosphor excited by 470 nm blue light. It can be seen that its coordinate is located at (0.6808, 0.3190), close to the NTSC ideal red coordinate point (0.67, 0.33). The illustration shows the phosphor under sunlight and UV-light irradiation respectively. It can be seen from the figure that the phosphor emits bright red light under UV-light irradiation. The color purity of phosphor is an important parameter to evaluate its color characteristics. The color purity of K₃MoO₂F₅∙2H₂O:Mn⁴⁺ red phosphor is calculated as 98% by Equation (1) [16].

$\text{Colorpurity}=\frac{\sqrt{{\left(x-x_i\right)}^2+{\left(y-y_i\right)}^2}}{\sqrt{{\left(x_d-x_i\right)}^2+{\left(y_d-y_i\right)}^2}}$ (1)

where, (x_i, y_i) is the color coordinate of equal energy white light (0.33, 0.33), and (x_d, y_d) is the color coordinate of the strongest emission wavelength of the light source (0.6851, 0.3148). (x, y) represents the color coordinate of K₃MoO₂F₅∙2H₂O: Mn⁴⁺ sample (0.6808, 0.3190). Obviously, all the emission peaks of oxyfluorides are located in sensitive areas that can be observed by the human eye. The excellent

Figure 4. SEM image (a) and EDS spectrum (b) of K₃MoO₂F₅・2H₂O:Mn⁴⁺.

Figure 5. PLE and PL spectra (a), DRS (b) and CIE color coordinate and digital photos under natural light and ultraviolet light irradiation (c) of K₃MoO₂F₅・2H₂O:Mn⁴⁺.

optical properties of phosphor indicate that K₃MoO₂F₅∙2H₂O:Mn⁴⁺ red phosphor has potential application value in warm white LED.

3.3. Application of K₃MoO₂F₅∙2H₂O:Mn⁴⁺ Phosphor in Warm WLED

In order to explore the value of K₃MoO₂F₅∙2H₂O:Mn⁴⁺ red phosphor in practical applications, a series of WLED devices were packaged by combining blue chip (a) with yellow YAG:Ce³⁺ phosphor (b), red K₃MoO₂F₅∙2H₂O:Mn⁴⁺ phosphor (c) yellow YAG:Ce³⁺ phosphor + red K₃MoO₂F₅∙2H₂O:Mn⁴⁺ phosphor (d) mixed phosphor. Figures 6(a)-(d) show the electroluminescence spectra of the corresponding LED devices at 20 mA driving current. The characteristic emission at 430 - 470 nm is that of blue chip. The 470 - 590 nm range belongs to the emission peak of YAG:Ce³⁺ yellow phosphor. In the range of 590 - 660 nm, there is obvious red emission, indicating that the K₃MoO₂F₅∙2H₂O:Mn⁴⁺ phosphor can be well excited by blue chip and emit strong red light. Due to the lack of red light component, the cold WLED device has a higher relative color temperature (CCT = 5190 K) and a lower color rendering index (Ra = 70.4), and its LE is 120.07 lm∙W⁻¹. In order to improve this problem, the red light component was introduced into the WLED device, and the CCT of the device was reduced to 3608 K, the CRI was increased to 90.1, and the LE was 50.17 lm∙W⁻¹. At the same time, CIE coordinate of the cold WLED was transferred from (0.3536, 0.3960) to (0.4069, 0.3681) in the warm WLED region (Figure 6(e)). The above results show that the device obtained is more suitable for the application of warm WLED device in indoor lighting field. Table 1 lists the photoelectric performance parameters of each device in detail.

Figure 6. Electroluminescence (EL) spectra and digital photos and CIE color coordinates (e) excited by the current of 20 mA of GaN chip (a), GaN chip + red K₃MoO₂F₅・2H₂O:Mn⁴⁺ phosphor (b), GaN chip + yellow YAG:Ce³⁺ phosphor (c), GaN chip + yellow YAG:Ce³⁺ phosphor + red K₃MoO₂F₅・2H₂O:Mn⁴⁺ phosphor (d).

Table 1. Important photoelectric parameters of the LED devices under 20 mA.

4. Conclusion

K₃MoO₂F₅∙2H₂O:Mn⁴⁺ phosphor was prepared using (NH₄HF₂ + HCl) instead of highly toxic HF. Under 470 nm blue light excitation, K₃MoO₂F₅∙2H₂O:Mn⁴⁺ phosphor shows narrow-band red emission. The emission peak ranges from 580 nm to 670 nm, which is attributed to the ²E_g → ⁴A_2g level transition prohibited by the spin of Mn⁴⁺. Efficient warm WLED with low CCT (3608 K), high CRI (Ra = 90.1) and LE of 50.17 lm∙W⁻¹ were obtained using K₃MoO₂F₅∙2H₂O:Mn⁴⁺ as red light component. These results indicate that phosphor K₃MoO₂F₅∙2H₂O:Mn⁴⁺ has potential application value in warm WLED.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (51802027).

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

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