Surface lattice enhancement of red-emitting fluorides enabled by embedding small cations

Abstract

The poor moisture resistance of Mn⁴⁺-activated fluoride red phosphors restricts their practical applications. Herein, this work proposes the embedment of small radius cations (Si⁴⁺ or Ge⁴⁺) into the inert shell (K₂TiF₆) constructed on the surface of K₂TiF₆:Mn⁴⁺ particles, which increases the covalence of the surface lattice and thus leads to a significant improvement of the stability of the fluoride. By using multidimensional microstructural characterization techniques, we confirmed the construction of a heterogeneous shell (K₂Ti_1−xSi_xF₆ and K₂Ti_1−yGe_yF₆) and systematically investigated the construction process. Compared to the untreated K₂TiF₆:Mn⁴⁺, the external quantum efficiencies of the K₂TiF₆:Mn⁴⁺@K₂Ti_1−xSi_xF₆ and K₂TiF₆:Mn⁴⁺@K₂Ti_1−yGe_yF₆ heterogeneous core–shell particles improved by 2–3%, and 99% and 88% of the initial luminescence intensity were maintained after boiling in water for 50 min, respectively, which are significantly better than that (25%) of the homogeneous K₂TiF₆:Mn⁴⁺@K₂TiF₆. The aging of red and white light-emitting diode devices at a high temperature (85 °C) and a high humidity (85%) shows that the heterogeneous core–shell structures have higher stability than their homogeneous counterparts. The surface lattice enhancement strategy proposed in this work is useful as a reference for improving the properties of other under-stable materials.

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1. Introduction

Mn⁴⁺ activated fluoride red phosphors have become the core light-emitting materials for wide-gamut light-emitting diode (LED) backlights, because they have special luminescence properties, such as efficient excitation by blue light, linear emission (<5 nm), a high internal quantum efficiency (IQE > 90%), a high thermal burst temperature, easy synthesis, etc.^1–4 The emission wavelength of fluorides (∼631 nm) is consistent with the red base color (630 nm, color coordinates x = 0.708, y = 0.292) requirement of the Rec. 2020 (B.T. 2020) standard.⁵ By combining red-emitting fluorides with green β-Sialon:Eu²⁺ powder and a blue LED chip,⁶ a white LED (WLED) with 96% NTSC (National Television Standards Committee) color gamut can be obtained, which is superior to nitride red powder packaged devices.⁷ A WLED device with 121% NTSC color gamut can be made by integrating a red LED (KSFM + blue LED chip) and a green perovskite quantum dot film as well.⁸ However, fluorides have a prominent problem of poor moisture resistance in practical applications due to the hydrolyzability of Mn⁴⁺ on the surface.^9–12 To enhance the moisture resistance of fluorides, a good strategy should start from avoiding the contact between Mn⁴⁺ and water.

Constructing core–shell structures is one of the most effective techniques to improve the luminescence efficiency and environmental stability of materials.^13–16 The construction of an inert shell on the surface of up-conversion nanoparticles can effectively eliminate the surface states and reduce the concentration quenching effect by cutting off the energy transfer from the luminescent center to the surface state as well.¹⁷ Furthermore, this technique can also be used to improve the luminescence efficiency and stability of quantum dots, such as CdSe, InP and perovskites.^18–24 For example, Huang et al. transformed the surface and grain boundaries of chalcogenide particles into a few nanometer thick, high mechanical strength and water-insoluble low-dimensional perovskite, which showed a significantly enhanced moisture-proof property and photothermal stability.²⁵ In recent years, a lot of work has been reported on the construction of core–shell structures to address the problem of low moisture resistance of Mn⁴⁺ doped fluorides.^26–31 The direct reduction method can remove Mn⁴⁺ from the surface and thus form a very thin Mn-free host shell on the fluoride particles. Although the fluorides treated with a reducing agent have good stability in cold water, the luminescence efficiency decreases sharply in a high temperature and high humidity environment.^32–34 The homogeneous core–shell structure K₂TiF₆:Mn⁴⁺@K₂TiF₆ is constructed by the reverse cation exchange method, which significantly improved the moisture resistance and the IQE of K₂TiF₆:Mn⁴⁺, especially for micron particles with high Mn⁴⁺ concentration doping.³⁵ However, due to the reversible nature of ion exchange, it is difficult to construct a truly inert shell by this method; the improvement in moisture resistance is therefore limited. Recently, our group reported a strategy of reduction-assisted surface recrystallisation (RSRC) to reconstitute an Mn⁴⁺-free inert shell (K₂SiF₆) on the K₂SiF₆:Mn⁴⁺ crystals, which improves the luminescence efficiency and stability. The luminescence intensity almost did not decrease after boiling in water for 20 min.³⁶ Moreover, with simple pyruvate treatment, both inorganic and organic bivalves were constructed on the fluoride surface, which significantly improved the water stability of the material.³⁷ In addition, lattice manipulation³⁸ and crystallinity enhancement³⁹ through ion doping engineering are also effective strategies to enhance the luminescence efficiency and stability.⁴⁰

The importance of shell lattice properties for the environmental stability of red-emitting core–shell fluorides is not well understood. The moisture resistance is closely related to the solubility of fluoride particles. At room temperature, the solubility of K₂TiF₆ (KTF), K₂GeF₆ (KGF) and K₂SiF₆ (KSF) in water is 1.36, 0.60 and 0.30 g/100 mL,⁴¹ respectively, indicating that the ionic bond component of these three compounds decreases and the covalence increases successively (Si has the strongest non-metallic properties among them). When the coordination number is 6, the ionic radii of Ti⁴⁺, Ge⁴⁺ and Si⁴⁺ are 0.60, 0.53 and 0.40 Å, respectively. In an octahedron, the bond length (1.92 Å) of Ti–F is significantly longer than those of Ge–F (1.77 Å) and Si–F (1.68 Å), so it has lower bond energy,⁴² resulting in the stability of KTFM being significantly lower than that of KSFM and KGFM. Therefore, the introduction of cations with a smaller radius, such as Si⁴⁺ or Ge⁴⁺, into the surface lattice of KTFM is expected to improve the covalency of the surface of KTFM and reduce its solubility, thus improving its moisture resistance. In this work, heterogeneous core–shell KTFM@KTSF and KTFM@KTGF were constructed by embedding small radius Si⁴⁺ or Ge⁴⁺ into the shell of KTFM particles. Compared with KTFM, KTFM@KTSF and KTFM@KTGF show excellent stability in water, even in boiling water (Video), and the external quantum efficiency (EQE) can be increased by 2–3%. The results of LED devices aged for 1000 h at a high temperature (HT, 85 °C) and a high humidity (HH, 85%) show that heterogeneous core–shell structures have higher environmental stability than their homogeneous counterparts. In addition, we confirmed the embedment of Si⁴⁺ or Ge⁴⁺ into the surface lattice of KTFM particles through multidimensional characterization, and the heterogeneous inert shell (KTSF and KTGF) has higher stability than the core KTFM. This strategy of surface lattice reinforcement could be a good reference for the stability improvement of other under-stable materials.

2. Experimental

2.1 Chemicals and materials

Glyoxalic acid (AR, 50 wt%), KHF₂ (99.0%), HF (49 wt%), K₂SiF₆ (99.0%), and K₂TiF₆ (99.5%) were purchased from Aladdin Reagent. KMnO₄ (99.5%), H₂O₂ (30 wt%), acetone (99.5 wt%) and alcohol (99.5 wt%) were bought from Sinopharm Chemical Reagent Co., Ltd. GeO₂ (99.999%) was purchased from Shanghai Titan Scientific Co., Ltd.

2.2 Synthesis of core–shell fluorides

K₂MnF₆ and KTFM were synthesized according to ref. 35 and 43 and homogeneous (KTFM@KTF) and heterogeneous (KTFM@KTSF and KTFM@KTGF) core–shell fluoride phosphors were synthesized through the RSRC method.³⁶ Firstly, certain amounts of K₂SiF₆ (or K₂GeF₆) and K₂TiF₆ were added to HF to form a saturated solution KTSF (or KTGF) at 100 °C. Subsequently, 1 mL of KTSF (or KTGF) and 1 mL of glyoxylic acid (GA) solution were mixed and then 0.5 g of KTFM was added to the mixed solution with stirring for 10 min at 100 °C. Finally, the yellow powders were centrifuged and dried at 60 °C for 3 h. In addition, K₂GeF₆ was synthesized by co-precipitation using GeO₂, KHF₂ and HF as raw materials. KTFM@KTF was obtained in a similar synthesis step, while the saturated solution in the first step was replaced with KTF.

2.3 LED packaging and aging

Red LEDs were fabricated by mixing red-emitting fluoride phosphor and epoxy resin (PS-7666A/B) uniformly and coating them on a blue chip (450 nm). WLEDs with multiple color temperatures were encapsulated with blue chips, yellow-emitting YAG:Ce³⁺ (YAP4533-H) or green-emitting β-sialon:Eu²⁺(SG-21098), fluoride phosphors and epoxy resin. The packaged LEDs were placed in an aging box (Hongjin Industrial Co., Ltd, China) with 85 °C and 85% humidity for 1000 h. The photoelectric parameters of the packaged devices were tested with an integrating sphere spectroradiometer (HAAS-2000, Everfine). The LEDs were operated at a voltage of 3.0 V.

2.4 Calculation of the quantum efficiency

Internal QE (IQE) = E_a/(R_b − R_a) (S1)

External QE (EQE) = E_a/R_b (S2)

where R_b is the peak area from incident light in the range 464–475 nm, R_a is the peak area of scattered light from the sample in the same wavelength range, and E_a is the peak area of the emission spectrum from the sample in the wavelength range of 585–706 nm.

2.5 Characterization

The crystal phases of the fluorides were tested using an X-ray powder diffractometer (XRD, Ultima IV) with Cu-Kα radiation. The photoluminescence excitation (PLE) and photoluminescence (PL) spectra of the samples were recorded using an F-4500 spectrometer (slit width: EX:1.0 nm, EM: 5.0 nm, response time: 2.0 s). The IQE was characterized using an FLS 1000 spectrometer. The morphology of the phosphors was imaged with a field-emission scanning electron microscope (FE-SEM, Zeiss Sigma 300). The surface elements of the fluorides were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB Xi+) and energy dispersive X-ray spectroscopy (EDX). The concentration of Mn⁴⁺ was determined by inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7700). Transmission electron microscopy (TEM) was carried out using a Talos F200. The structural information of the samples was obtained using a DXR laser micro-Raman spectrometer.

3. Results and discussion

3.1 Design and construction of a heterogeneous shell

Recently, we proposed for the first time an RSRC method to construct an inert shell KSF on KSFM particles (KSFM@KSF), which exhibit excellent moisture resistance.³⁶ When a reducing agent and a fluoride phosphor are added into saturated HF solution with a fluoride matrix, the solution-crystallization equilibrium of the fluoride phosphor exists in HF solution. The reducing agent immediately reduces the dissolved Mn⁴⁺ from the surface to Mn²⁺, effectively preventing the Mn⁴⁺ ions in the solution from entering the shell during the recrystallization process. As an ionic compound, KTF has higher solubility in water than KSF. Even if the homogeneous inert shell KTF is reconstructed on the KTFM particle, the problem of low moisture resistance still exists.³⁵ To solve this problem, an additional fluoride matrix containing cationic Si⁴⁺ or Ge⁴⁺, such as KSF or KGF, was added to the HF reaction medium. During the process of surface recrystallization, Si⁴⁺ or Ge⁴⁺ will be embedded into the shell of KTFM particles to form a heterogeneous shell K₂Ti_1−xSi_xF₆ (KTSF) or K₂Ti_1−yGe_yF₆ (KTGF), as shown in Fig. 1A. Due to the difference in the crystal structure of KSF (cubic), KGF (hexagonal) and KTF (hexagonal) (Fig. 1B), the embedding velocity and concentration of small radius cations would affect the moisture resistance of KTFM.

Fig. 1 (A) Intercalation of small-radius cations into the surface lattice of a K₂TiF₆:Mn⁴⁺ particle through the reduction-assisted surface recrystallization (RSRC) strategy. Colored balls represent different metal ions, blue: Ti⁴⁺, green: Si⁴⁺ or Ge⁴⁺, red: Mn⁴⁺, and gray: Mn²⁺, and the K⁺ and F⁻ ions are not given herein for simplicity. (B) Crystal structures of K₂TiF₆, K₂GeF₆ and K₂SiF₆.

3.2 Microstructure of the heterogeneous shells

To monitor the construction process of the heterogeneous shells KTSF and KTGF, we first tested the XRD pattern of the mixture of KTFM and KSF (Fig. 2A). Two phases, KTF and KSF, appeared in the pattern, indicating that the two compounds are not susceptible to the solid solution reaction under physical mixing only. The intensity of the diffraction peaks of KSF decreased significantly after 10 min of reaction in HF medium. After the reaction for 300 min, no diffraction peak of KSF was observed in the XRD pattern, suggesting that Si⁴⁺ had entered the KTFM surface to form the shell KTSF. The XRD patterns of the products under different reaction times are shown in Fig. S1.† In contrast, after mixing KTFM and KGF, which are both hexagonal structures, the reaction can form the heterogeneous core–shell structure KTFM@KTGF for 10 min (Fig. S2†), which is significantly faster than the formation of the shell KTSF.

Fig. 2 Structural and surface analysis of the KTFM@KTSF samples with the reaction time. (A) XRD patterns. (B) Raman scattering spectra. (C) Si 2p XPS survey spectra. (D) Mn 2p XPS survey spectra. (E) Elemental mapping.

Raman spectroscopy can be used to monitor changes in the metal-F bonds on the surface of KTFM. As shown in Fig. S3,† as the ionic radius decreases, the vibration frequencies of v₁ (Ti⁴⁺), v₁ (Ge⁴⁺) and v₁ (Si⁴⁺) increase successively. The vibration frequency of v₁ (Mn⁴⁺) is almost constant (595 cm⁻¹). Careful observation shows that the Raman peak of v₁ (Ti⁴⁺) gradually shifts towards a higher frequency with the increase of the RSRC reaction time (Fig. S4†), and even a vibrational peak of v₁ (Si⁴⁺) is observed at 650 cm⁻¹ (Fig. 2B), indicating that Si⁴⁺ is gradually embedded into the shell KTF. The Raman spectrum of KTFM@KTGF also shows similar changes (Fig. S5†), indicating that Ge⁴⁺ can also be embedded. The prolonged reaction time resulted in gradual weakening of the v₁ (Mn⁴⁺) signal of KTFM (Fig. 2B), indicating that the glyoxylic acid (GA) effectively reduced Mn⁴⁺ from the surface.

The embedding of Si⁴⁺ and Ge⁴⁺, as well as the reduction of Mn⁴⁺ on the surface, will lead to changes in the content of elements Si, Ge and Mn on the surface of KTFM, which was further confirmed by high-resolution XPS. Fig. 2C shows that a very weak Si 2p XPS signal is detected on the surface of KTFM when the reaction time is 10 min, and an obvious Si signal is recorded for the sample upon reaction for 300 min. As expected, very obvious Ge 3d XPS signals can be detected after within 10 minutes of the reaction (Fig. S6†), indicating that Ge⁴⁺ is more easily embedded on the surface of KTFM than Si⁴⁺, because the ionic radii of Ge⁴⁺ is closer to that of Ti⁴⁺. In addition, Mn⁴⁺ on the surface of KTFM can be quickly reduced to Mn²⁺ in the reaction solution by GA.⁴⁴ Therefore, the Mn 2p XPS signal on the surface of KTFM treated by RSRC is significantly inhibited (Fig. 2D and Fig. S7†).

The surface elemental distribution of a single KTFM@KTSF particle is analyzed by SEM/EDX. As shown in Fig. 2E, except for K, Ti and F, the Si element is also uniformly distributed in the whole particle, while the Mn signal is extremely weak, indicating that the shell contains very little Mn, which is consistent with the results of Mn 2p XPS. Various elements on the surface of the KTFM@KTGF particle also show a similar distribution to that of KTFM@KTSF (Fig. S8†).

Based on the above structure and surface analysis, we confirmed that small radius cations (Si⁴⁺ and Ge⁴⁺) were successfully embedded on the surface of KTFM. The embedding depth of impurity ions and the internal microstructure of fluoride particles were analyzed by TEM. The particle size of the core–shell fluoride prepared in this work is about 15 microns, and the electron beam is difficult to penetrate the whole particle. Therefore, we first sliced a KTFM@KTSF particle using the focused ion beam (FIB) technique and then analyzed the microstructure of the core/shell interface by scanning transmission electron microscopy (STEM)/energy dispersive X-ray (EDX) detector (Fig. 3A–D). Furthermore, the core and shell of the KTFM@KTSF section were characterized by selected electron diffraction (SAED) and high-resolution TEM (HRTEM). The results show that the KTSF shell (∼100 nm, Fig. 3E) presents more complete and bright diffraction spots (Fig. 3F and G), and the diffraction spots of the core and shell are consistent (Fig. 3H), indicating that the core and shell have the same crystal structure. The differences in components (core KTFM and shell KTSF) lead to different degrees of tolerance to electron beams between the core and shell. Fig. 3J presents the HRTEM of the lattice around the core–shell interface. The results exhibit that the KTSF shell can show a clear lattice fringe under electron beam irradiation (Fig. 3K), and the measured crystal plane spacing is about 0.32 nm, consistent with the (011) plane in the KTF structure. As the KTFM core is more unstable (Fig. 3L), its lattice fringes are difficult to identify. In the octahedral structure, the bond lengths of Si–F and Ti–F are 1.68 and 1.92 Å, respectively. Therefore, the embedding of a small number of Si⁴⁺ ions lead to the surface lattice shrinkage of KTFM (as shown in Fig. 3I), increases covalency between the metal and F atoms, and finally enhances the stability of the surface lattice.

Fig. 3 Small changes in the surface lattice of the KTFM particle. (A–D) HAADF-STEM images of KTFM@KTSF and main element (Ti, Mn and Si) mappings. (E) TEM image of a fluoride particle with a shell of 100 nm in thickness. (F–H) SAED patterns of the specific areas from image E. (I) Schematic shrinkage of the surface (shell) lattice of the KTFM particle for the introduction of some Si⁴⁺ ions. (J) A TEM image illustrating the core–shell interface of the KTFM@KTSF particle. (K) Magnified K area (shell) of the image in (J), (L) magnified L area (core) of the image in (J).

3.3 Luminescence properties of heterogeneous core–shell structures

As mentioned in the Introduction, the moisture resistance of fluoride depends on the content of Mn⁴⁺ ions on the surface and the covalence of the lattice. An important innovation of the RSRC strategy is to immediately reduce Mn⁴⁺ to Mn²⁺ in the process of surface lattice manipulation, so as to avoid the re-entry of Mn⁴⁺ dissolved in HF solution into the surface lattice of fluoride. Thus, the relative volume of the reducing agent GA determines the concentration of surface Mn⁴⁺ on the fluoride. When GA of different volumes was added into the reaction medium of 1 mL of HF, the relative PL intensity changes of as-prepared KTFM@KTSF are shown in Fig. 4A (blue line), and the luminescence spectra are shown in Fig. S9.† When the volume of GA is less than or equal to 2 mL, the luminescence intensity of surface reconstructed fluoride has no obvious change, but 3 mL of GA leads to a significant decrease in the luminescence intensity, and a larger volume (>4 mL) of GA even completely dissolves the fluoride, presumably due to a significant increase in the volume of the solvent (HF and GA). In order to confirm the moisture resistance of GA-treated KTFM@KTSF, the samples treated with different volumes of GA were placed in water and boiled for 50 min. It was found that the samples treated with 1 mL of GA had the most excellent stability against boiling. The luminescence intensity of the sample had no obvious change before and after boiling (Fig. 4A, red line).

Fig. 4 Effect of surface lattice manipulation on the PL intensity and moisture-resistance of fluorides; blue color means the samples “before boiling”, and red color means those “after boiling”. (A) Volume of glyoxylic acid (GA). (B and C) The amount of additional KSF and KGF for the construction of KTFM@KTSF and KTFM@KTGF, respectively. (D–F) Relative PL intensities of the KTFM@KTSF, KTFM@KTGF and KTFM@KTF for different reaction times, respectively. Error bars represent the standard deviation from three repetitive experiments. (G) Relative PL intensities of the four fluoride samples (S1–S4) which are stored in water for different times at room temperature. (H) Pictures of the phosphors in water under natural light or UV light (365 nm).

The lattice covalency of the Mn⁴⁺-free inert shell is another important factor determining the stability of the ionic KTFM compound. By increasing the mass of KSF or KGF in HF medium, the surface lattice covalency is effectively increased, and the moisture resistance of the KTFM phosphor is thereby enhanced. When the masses of KSF and KGF in the reaction medium are 0.01 g and 0.02 g, respectively, the luminescence intensity of surface-reconstructed KTFM can basically maintain the initial value after boiling for 50 min (Fig. 4B and C).

The reaction time during RSRC can regulate the thickness of the inert shell and the luminescence properties of the KTFM. To verify that the intercalation of small radius cations is responsible for the increase of surface lattice covalency, we constructed three core–shell structures namely KTFM@KTSF, KTFM@KTGF and KTFM@KTF, and compared the dependence of their luminescence intensity and boiling resistance with the reaction time. When the time was less than 240 min, the PL intensities of KTFM@KTSF (Fig. 4D) and KTFM@KTGF (Fig. 4E) samples increased compared with that of KTFM, and the PL intensities of the core–shell samples obtained after reaction for 10 min increased by 18.24% and 23.58%, respectively. Compared with the pure KTFM, the PL intensity of KTFM@KTF increased by 9.58% (Fig. 4F). The increase in PL intensity is due to the formation of a thin inert shell on the particles, which prevents energy transfer from Mn⁴⁺ to the surface defects. If the time is prolonged, the PL intensity of KTFM@KTF decreases obviously, and for 240 min, it decreases by 24.46%. After boiling for 50 min, the PL intensity of KTFM is maintained at only 21% of the initial value. The PL intensity of KTFM@KTSF (reaction for 10 min) can maintain 99.19% of the initial value, and the luminescence intensity of the samples obtained during other reaction times did not decrease significantly. After boiling, the PL intensity of the KTFM@KTGF sample (10 min) can maintain 88.04%, and the luminescence intensity of the sample obtained after the reaction for 180 min almost did not decrease. In contrast, the PL intensity of the KTFM@KTF system (10 min) was reduced to 24.70% due to boiling, and the luminescence intensity of KTFM@KTF with the reaction for more than 240 min can only be maintained at the value before boiling. Insufficient concentration of Si⁴⁺ or Ge⁴⁺ ions in the HF medium, especially only the HF saturated solution of KSF or KGF, will cause a limited increase in the surface lattice covalency of KTFM, and thus a limited improvement in the moisture stability (Fig. S10 and S11†). By extending the reaction time, the thickness of the shell KTSF or KTGF with low Si⁴⁺ or Ge⁴⁺ concentrations can be increased to achieve the purpose of improving the moisture resistance of KTFM. However, the prolongation of the reaction time can severely reduce the Mn⁴⁺ content (number of luminescent centers) in the KTFM particles, causing a slight decrease in the PL intensity of the core–shell structure. Therefore, optimization of the relative mass of KSF or KGF in the reaction medium as well as the reaction time can enhance both the luminescence intensity and the moisture resistance of KTFM.

Long-term (360 h) immersion experiments at room temperature further verified the strength of the resistance of the four phosphors to moisture. As shown in Fig. 4G, after 360 h of water flooding, the luminescence intensity of the shell-constructed fluorides by the RSRC strategy maintained high initial values, with the PL intensity of KTFM@KTSF, KTFM@KTGF and KTFM@KTF maintaining 91%, 85% and 80%, respectively, which were much higher than that of the KTFM (28%), in agreement with the results of boiling water treatment. To further demonstrate the advances of heterogeneous core–shell fluorides, a selection of results reported in the literature were compared, as shown in Table S1,† which confirms a significant moisture resistance enhancement effect for this work. Fig. 4H shows that the body color of the core–shell fluoride remains bright yellow, while the untreated KTFM immediately turns brown upon exposure to water. The above results, on the one hand, indicate that prolonging the reaction time increases the thickness of the inert shell; on the other hand, the embedding of small radius cations significantly strengthens the surface lattice of KTFM.

Quantum efficiency further supports the advantages of constructing heterogeneous core–shell structures, as shown in Fig. S12† and Table 1. The EQE value is estimated by multiplying the absorption efficiency (AE) by IQE. The 6.29% (ICP data) Mn⁴⁺ doped KTFM phosphor exhibits an AE of 71.72%, IQE of 91.49% and EQE of 65.62%. The AE of KTFM@KTF prepared by the RSRC method is significantly decreased, due to the high solution-crystallization equilibrium reaction rate of KTFM in HF medium, and the internal Mn⁴⁺ is thus easily exchanged into the medium solution and immediately reduced by GA, resulting in a large reduction in the content of Mn⁴⁺. Due to the increase of covalency caused by the embedding of Si⁴⁺ or Ge⁴⁺, the dissolution–crystallization equilibrium reaction rate of the heterogeneous shell KTSF or KTGF in the HF medium is reduced, which makes it more difficult for Mn⁴⁺ in KTFM to be exchanged into the medium solution. Therefore, the KTFM@KTSF and KTFM@KTGF particles have higher AE. Because the reconstructed heterogeneous inert shell cuts off the energy transfer of Mn⁴⁺ from the KTFM core to the surface defects, the IQE of KTFM is thereby effectively increased, and the EQE of KTFM finally improved by 2–3%.

Table 1 The quantum efficiency data of four fluorides

Samples	AE(%)	IQE(%)	EQE(%)
KTFM	71.72	91.49	65.62
KTFM@KTSF	72.17	94.20	67.98
KTFM@KTGF	68.88	98.00	67.50
KTFM@KTF	68.69	96.62	66.37

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Thermal stability is an important parameter of phosphors for phosphor-converted LED applications.⁴⁵ In the temperature range of 25–250 °C, the emission spectra of KTFM and KTFM@KTSF are almost unchanged in shape (Fig. S13A and B†), and the emission intensity shows the same trend (Fig. S13C†). With the increase of temperature, the luminescence intensity of the two fluorides increases and reaches the maximum value at 150 °C, and their relative luminescence intensities are 116.1% and 110% at 25 °C, respectively. The main contribution to the increase in the PL intensity at elevated temperatures is due to the increased absorption of excitation light by fluoride phosphors.¹ Above 150 °C, the Mn⁴⁺ PL intensity decreased rapidly due to the markedly increased non-radiative transition probability with a further increase of temperature. In summary, the surface intercalation of Si⁴⁺ will not damage the thermal stability of fluoride.

3.4 Stability of core–shell fluorides in WLED devices

To further evaluate the potential of heterogeneous core–shell fluoride phosphors in WLED devices for lighting and displays, four fluoride phosphors, KTFM (S1), KTFM@KTSF (S2), KTFM@KTGF (S3) and KTFM@KTF (S4), were packaged with blue chips and other phosphors into three series of LED devices. Series 1: combined with blue chip to make red LED 1, LED 2, LED 3 and LED 4, respectively. Series 2: with yellow Y₃Al₅O₁₂:Ce³⁺ (YAG) and blue chips to fabricate white LED 5, LED 6, LED 7 and LED 8, respectively. Series 3: with green β-sialon:Eu²⁺ (SIALON) phosphors and blue chips to package white LED 9, LED 10, LED 11 and LED 12, respectively. All LEDs were tested at 60 mA. For the convenience of comparison, the peak value of the blue chip in the electroluminescence (EL) spectrum was set as 1, and the relative intensity at 631 nm (characteristic emission of Mn⁴⁺) in the EL spectrum of each series of LEDs was compared and is shown in Fig. 5.

Fig. 5 Electroluminescence (EL) properties of LED devices under HTHH conditions with aging time. EL spectra of the LED 2 (A), LED 6 (C) and LED 10 (E) with normalized luminous intensity of blue chips, inset shows pictures of the lighted LEDs. EL height at 631 nm of the LED devices as a function of aging time, (B) LED 1-4, (D) LED 5-8, and (F) LED 9-12.

Fig. 5A, C and E respectively show the normalized EL spectra of LED 2, LED 6 and LED 10 after aging under HTHH conditions for different times. With the increase of aging time, YAG and SIALON showed good stability, while the intensity of the narrow band red emission peak of fluoride continued to decline, indicating that the stability of red fluoride phosphors under harsh conditions remains a challenge. By comparing the changes of the EL intensity of S1–S4 in the three series of LEDs with aging time under harsh conditions (Fig. 5B, D and F), one can find that the stability of the four fluorides in descending order is S2, S3, S4 and S1. Additionally, the luminous efficacy (LE) of the WLED with S2 and S3 remains 90% and 81% of the starting value after 1000 h of aging (Fig. S14†), respectively. The performance of the devices proves that reconstructing heterogeneous shells by embedding Si⁴⁺ or Ge⁴⁺ has a more significant advantage in improving the moisture resistance of the KTFM phosphor.

4. Conclusions

A heterogeneous inert shell (KTSF or KTGF) was reconstructed on the surface of KTFM particles by the RSRC method. The surface lattice of the inert shell was strengthened by the embedding of small radius cations Si⁴⁺ or Ge⁴⁺. The FIB-STEM-EDX results showed that the inert shell KTSF is more tolerant than the core KTFM under electron beam irradiation, as the former has higher covalency, thus forming a distinct core–shell interface. The optimization of the relative mass of KSF or KGF, the concentration of the reducing agent in the reaction medium and the reaction time can improve the stability of the inert shell and the luminescence intensity of KTFM. The EQE of the heterogeneous core–shell structures KTFM@KTSF and KTFM@KTGF increases by 2–3% relative to the non-core–shell structure K₂TiF₆:Mn⁴⁺. After boiling for 50 min, the luminescence intensity of both could maintain 99.19% and 88.04% of the initial values, respectively, while that of KTFM@KTF decreases to 24.70%. The aging of LED devices at a high temperature (85 °C) and a high humidity (85%) for 1000 h shows that the fluoride with a heterogeneous inert shell has higher stability than that with a homogeneous shell. Accordingly, the strategy using the chemical equilibrium principle to reinforce the surface lattice can be used as a reference to improve the properties of under-stable ionic compounds.

Author contributions

Wenli Zhou supervised the study. Pingping Wan contributed to the synthesis and characterization of the materials, the PL measurements and analysis. Chen Yang and Aolin Wang conducted spectral measurements. Wenli Zhou and Pingping Wan wrote the manuscript. All authors discussed the results and contributed to writing the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 21501058) and the Changsha Natural Science Foundation (No. kq2202235).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3qi01088k

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