Highly Stable K2SiF6:Mn4+@K2SiF6 Composite Phosphor with

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Functional Inorganic Materials and Devices

Highly stable K2SiF6:Mn4+@K2SiF6 Composite Phosphor with Narrow Red Emission for White LEDs Lin Huang, Yong Liu, Jinbo Yu, Yiwen Zhu, Fengjuan Pan, Tongtong Xuan, Mikhail G. Brik, Chengxin Wang, and Jing Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03893 • Publication Date (Web): 09 May 2018 Downloaded from http://pubs.acs.org on May 9, 2018

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ACS Applied Materials & Interfaces

Highly stable K2SiF6:Mn4+@K2SiF6 Composite Phosphor with Narrow Red Emission for White LEDs

Lin Huang a, Yong Liu a, Jinbo Yu a, Yiwen Zhu b, Fengjuan Pan c, Tongtong Xuan a, Mikhail G. Brik d, e, f, Chengxin Wang a, Jing Wang *, a

a.

Ministry of Education Key Laboratory of Bioinorganic and Synthetic Chemistry,

State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry, School of Materials Science and Engineering, Sun Yat-Sen University, Guangzhou, 510275, P. R. China. b.

College of Materials & Environmental Engineering, Hangzhou Dianzi University,

Hangzhou, 310018, P. R. China. c.

Beijing National Laboratory for Molecular Science (BNLMS), State Key Laboratory

of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing, 100871, P. R. China. d.

College of Science, Chongqing University of Posts and Telecommunications,

Chongqing, 400065, P. R. China. e.

Institute of Physics, University of Tartu, W. Ostwald Str 1, Tartu, 50411, Estonia.

f.

Institute of Physics, Jan Dtugosz University, Armii Krajowej 13/15, PL-42200

Częstochowa, Poland.

Keywords: white LEDs, fluoride phosphor, water resistant, green synthesis, 1/36

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composite structure

ABSTRACT: Poor water resistance and non-green synthesis remain great challenges for commercial narrow red emitting phosphor A2MF6:Mn4+ (A = alkali metal ion; M = Si, Ge, Ti) for solid state lighting and display. We develop here a simple and green growth route to synthesize homogeneous red emitting composite phosphor K2SiF6:Mn4+@K2SiF6 (KSFM@KSF) with excellent water resistance and high efficiency without the usage of toxic and volatile HF solution. After immersing into water for 6 hours, the as-obtained water-resistant products maintain 76% of the original emission intensity while the emission intensity of non-water-resistant ones steeply drops down to 11%. A remarkable result is that after having kept at 85% humidity and at 85 oC for 504 h (21 days), the emission intensity of the as-obtained water-resistant products is at 80-90%, from its initial value, which is 2-3 times higher higher than 30-40% for the non-water-resistant products. The surface deactivation enabled growth mechanism for these phosphors was proposed and investigated in detail. We found that non-toxic H3PO4/H2O2 aqueous solution promotes the releasing and decomposition of the surface [MnF6]2- ions and the transformation of the KSFM surface to KSF, which finally contributes to the homogeneous KSFM@KSF composite structure. This composite structure strategy was also successfully used to treat KSFM phosphor prepared by other methods. We believe that the results obtained in the present paper will open the pathway for the large scale environment-friendly synthesis of the excellent anti-moisture narrow red emitting A2MF6:Mn4+ phosphor to 2/36


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be for white LED applications.

1. Introduction The manganese (IV) doped fluorides are one of the most promising red phosphors for the white LEDs (WLEDs).1-6 Besides the A2MF6:Mn4+ prototype (A = alkali metal ion, M = Si, Ge, Ti, etc.),7-16 the A3BF6:Mn4+ (B = Al, Ga, etc.) phosphors,17-22 BaMF6:Mn4+

23, 24

or even NaHF2:Mn4+

25

have also been reported previously.

Compared with the rare-earth doped nitrides and oxides red-emitting materials, such as CaAlSiN3:Eu2+, LuVO4:Eu3+, etc.,26-29 the A2MF6:Mn4+ phosphors exhibit outstanding luminescence properties and better thermal stability than organic compounds or quantum dots, which also show excellent red emitting red emission at ambient conditions.30-32 Also, A2MF6:Mn4+ has cheap production cost because of the simple room temperature synthesis procedure that involves a use of non-rare-earth raw materials. Unfortunately, there are still several disadvantages of the manganese (IV) doped fluorides to be overcome yet. Firstly, all researches on the Mn4+-activated fluorides without exception had used the HF-rich solution for reduction of strong oxidizing permanganate chemicals. Extra safety precautions are necessarily needed when dealing with the HF-rich solution. Therefore, many efforts have been devoted to developing low-HF or HF-free green synthesis methods. Wang’s group demonstrated an optimized co-precipitation procedure where K2TiF6:Mn4+ was prepared by adding KF into K2MnF6/H2TiF6/CH3COOH mixture solution.33 It is very interesting that the 3/36



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HF acid can be replaced by green CH3COOH acid to obtain K2TiF6:Mn4+ in the ion-exchange process. But HF is still inevitably used in the preparation process of the K2MnF6 precursor. Recently, our group developed a hydrothermal strategy based on low-toxic H3PO4/KHF2 liquid instead of high-toxic HF solution,34 which opens the opportunity for environment-friendly synthesis of the narrow red emitting A2MF6:Mn4+ phosphor.35, 36 Unfortunately, the active tetravalent manganese Mn4+ is easily reduced to Mn2+ in the hot-liquid system. Consequently, the inner quantum yield (IQY) previously reported by this hydrothermal method is only 28%. Therefore, there is still an urgent need to develop a novel green synthetic route for A2MF6:Mn4+ with high luminescence efficiency. Another issue for Mn4+-doped fluoride phosphor is its inherent poor moisture resistance. It is found that the dopant [MnF6]2- is extremely sensitive to humidity and consequently will be easily hydrolyzed into the mixed-valence Mn oxides and hydroxides in the degradation process of fluoride phosphor, which may lead to a deep body color of fluoride phosphors and weakened red emission intensity. Coating can be a general approach to prevent [MnF6]2- from hydrolyzing with simultaneous enhancement of the A2MF6:Mn4+ water-resistive properties. Nowadays, previously reported coating materials are, as a rule, organic polymers or inorganic insoluble substances.37-39 Liu’s research team reported that coating an alkyl phosphate layer on the K2SiF6:Mn4+ surface remarkably improve its water-resistant property and the coated sample keeps 50% of the original emission intensity while the uncoated sample only maintain 9% after immersed in water for 2 h.37 Im’s team reported a coating 4/36


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strategy using the oleic acid (OA) as a hydrophobic encapsulant via solvothermal treatment and demonstrated the coated K2SiF6:Mn4+ also exhibited excellent moisture stability.38 General Electric (GE) Company has developed a homogenous coating approaches and successfully synthesized high humidity-stable K2SiF6:Mn4+@K2SiF6 (KSFM@KSF) by treating original K2SiF6:Mn4+ phosphor with a mix solution of K2SiF6/HF/H2SiF6.40, 41 However, the fluoride phosphors made by these methods have high moisture resistance at the expenses of sacrificing its luminescence efficiency, extra consumption or usage of a HF-rich solution. Therefore, novel green synthetic route to obtain excellent anti-moisture and highly efficient narrow red emitting K2SiF6:Mn4+ phosphor is in great need, especially in large scale toward white LED industry adoptions. Herein, we reported a simple and green growth route to synthesize excellent water resistant and highly efficient homogeneous K2SiF6:Mn4+@K2SiF6 (KSFM@KSF) red emitting composite phosphor without the usage of toxic and volatile HF solution. After immersed into water for 6 hours, the as-obtained water-resistant products maintain 76% of the original emission intensity, while the emission intensity of the non-water-resistant ones steeply drops to 11%. A surface deactivation enabled growth mechanism was proposed and discussed in detail. This composite structure strategy was also successfully used to treat KSFM prepared by other methods. We believe that the main findings of the presented here research will open the opportunity for the large scale environment-friendly synthesis of the excellent anti-moisture narrow red emitting phosphor A2MF6:Mn4+ to be used in white LED with improved 5/36



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characteristics.

2. Results and discussion The X-Ray powder diffraction (XRD) patterns of the as-synthesized water resistant K2SiF6:Mn4+ (WR-KSFM) with various KMnO4 addition are shown in Figure 1a. All the sharp diffraction peaks of the samples can be indexed to the standard patterns of the cubic (space group = Oh-Fm-3m) K2SiF6 (JCPDS No. 85-1382). No distinct impurity peaks are observed in the range of 0.02 - 0.08 g of KMnO4 addition. When KMnO4 amount increases up to 0.1 g, an impurity peak at 27

o

from the SiO2 raw

material emerges, suggesting that the formations of [MnF6]2- and [SiF6]2- are competitive as the total KHF2 amount is fixed. Rietveld refinement of WR-KSFM with 0.08 g KMnO4 addition (labeled as WR-KSFM-8) was conducted, as seen in Figure 1c. The results indicate that the Rp = 2.189% and Rwp = 3.348%, with the crystal parameter a = 8.13588(5) Å belonging to cubic system (Figure 1b). Compared with the K2SiF6 matrix (a = 8.13463(4) Å, Figure S1, Supporting Information), WR-KSFM-8 shows a slight expansion of the lattice structure which is due to the replacement of the smaller Si4+ ion by the larger Mn4+ ion (ionic radius: Mn4+ = 0.53 Å, Si4+ = 0.4 Å, CN = 6). The refinement suggests the as-synthesized samples have high phase purity. Figure 2a shows typical room temperature photoluminescence excitation (PLE) and photoluminescence (PL) spectra of the as-synthesized WR-KSFM-8. Monitored at 630 nm, several broad excitation bands can be seen in the UV and blue region 6/36


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between 300 and 500 nm and are dominating at 355 nm and 450 nm, respectively, ascribed to the 4A2g → 4T1,2g transitions of Mn4+ ion. Under excitation at 450 nm, there are a series of sharp peaks around 580 - 680 nm in the PL spectrum, due to the spin-forbidden 2Eg → 4A2g transitions of Mn4+ associated with the [MnF6]2- vibronic modes. These results are identical with the previous works.1, 7 The emission intensity of WR-KSFMs gradually increases, reaching the maximum at 0.08 g KMnO4 and finally decreases, as shown in Figure 2b. The actual amount of Mn4+ dopant in series of the WR-KSFM samples was further evaluated by an inductively coupled plasma-atomic emission spectrometry (ICP-AES) technique (Figure 2b). Obviously, it shows similar varying trend as the dependence of the emission intensity on the nominal amount of KMnO4. The highest concentration of actual dopant Mn4+ ion is estimated to be 6.8%±0.1% for the sample with nominal amount of 0.08 g KMnO4 (WR-KSFM-8). As a representative, the WR-KSFM-8 sample is chosen for further discussion. Temperature-dependent emission performance is always considered as an important parameter of the Mn4+ doped fluoride phosphor when used for high-power WLEDs. WR-KSFM-8 exhibits acceptable thermal quenching behavior. For comparison, KSFM with approximate concentration was also synthesized via ionic exchange method (IE-KSFM) according to the earlier reports. The PL spectra and the temperature dependence of the WR-KSFM-8 integrated emission intensity in the temperature range of 300 - 500 K were measured (Figure S2, Supporting Information). As the temperature increases, the integrated emission 7/36



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intensity of Mn4+ ion in the 580 - 680 nm wavelength range initially increases slightly and then decreases significantly above 440 K (Figure S2b, Supporting Information). At 420 K (147 °C, close to the LED’s junction temperature) and 500 K (227 °C), Mn4+ ion has about 103% and 70% of its initial intensity at 300 K, respectively, which is much better than IE-KSFM that retains only 92% and 62%. The abnormal thermal quenching behavior of WR-KSFM-8 can also be observed in other Mn4+ ion doped phosphors,42 mainly ascribed to the temperature dependent competition between the Stokes and anti-Stokes emissions, which was fully discussed in previous work and will not be repeated here.7, 8 The color stability of phosphors at different temperatures can be quantitatively described by the chromaticity shift (∆E) using the following Equation 1:43 △E = (u 't −u'o )2 + (v't −v'o )2 + (w't − w'o )2

(1)

where u' = 4x/(3-2x+12y), v' = 9y/(3-2x+12y) and w' = 1-u'-v', while x and y are the chromaticity coordinates in the CIE 1931 color space, and o and t are the chromaticity shifts at 300 K and a given temperature, respectively. The inset of Figure S2b shows the color coordinates of WR-KSFM-8 at different temperature. As seen, KSFM shows almost 100% color purity at the whole temperature range of 300 - 500 K. The calculated ∆E of WR-KSFM-8 is about 2.7 × 10-2 at 500 K, smaller than 4.4 × 10-2 of the commercial red LED-used phosphor (Sr)CaAlSiN3:Eu2+.34 Therefore, it indicates that WR-KSFM-8 is a promising candidate for the high power WLEDs. Poor water-resistance property is one of main disadvantages that most of the Mn4+-doped fluoride phosphors. In view of this, the as-synthesized WR-KSFM-8 8/36


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exhibits remarkably excellent water resistance. Hereafter, we will systematically compare the water-resistant properties of WR-KSFM-8 to IE-KSFM synthesized by traditional ionic exchange method (Figure 3). Both WR-KSFM-8 and IE-KSFM samples show light-yellow body color and exhibit bright red emission under 365 nm UV lamp before immersed in water (Figure 3b). After immersing into deionized water with a solid-to-liquid ratio of 1 g / 10 mL, the red emission intensity of either WR-KSFM-8 or IE-KSFM declines as the immersion time increases. It is comparatively seen that IE-KSFM shows more serious degradation trend than WR-KSFM-8 (Figure S3, Supporting Information). As a quantitatively shown in Figure 3a, WR-KSFM-8 retains at least 70% of its original red emission intensity after 24 h’s immersion. As representative, the WR-KSFM-8 and IE-KSFM samples after 6 h immersion were compared in detail and the results were shown in Figure 3b-d. It is obviously seen in figure 3b that under a UV-light excitation, WR-KSFM-8 still maintains a light-yellow body color and emits bright red light while IE-KSFM turns from light-yellow to dark brown and shows much weaker red emission. Comparatively, WR-KSFM-8 retains 76% of its original red emission intensity, much higher than 11% only for IE-KSFM as shown in Figure 3c. Inner quantum yield (IQY) was also measured. The IQY of original WR-KSFM-8 and IE-KSFM are 51% and 59%, respectively, under the same testing conditions. After 6 h immersion in water, the IQY of WR-KSFM-8 maintains at 35% while that of IE-KSFM drops to only 1%. In addition, the water-resistance properties of both WR-KSFM-8 and IE-KSFM were also evaluated when they were exposed to 85% humidity and 85 oC for 504 h (21 days) 9/36



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according to the standard commercial degradation process of WLEDs. WR-KSFM keeps at least 80 % of its original red emission intensity, much higher than 30-40% for IE-KSFM (Figure S4, Supporting Information). We also investigated the water-resistance properties of KSFM samples synthesized by other method, for instance, traditional co-precipitation method (CO-KSFM). The as-synthesized CO-KSFM sample has IQY of 82% and 11% before and after 6 h immersion in water (Figure S5, Supporting Information). It only retains 35 % of its original red emission intensity. These results strongly indicate that the WR-KSFM phosphor has a remarkable water resistance performance compared with KSFMs synthesized by other methods, ionic exchange and co-precipitation, etc. In general, poor water resistance of luminescent materials are mainly ascribed to either the collapse of the matrix or the degeneration of the dopant, such as quantum dots and Eu2+-doped phosphors.44,

45

Hereafter, we will systematically compare

different degradation behaviors of WR-KSFM and IE-KSFM in water from the aspect of host and activator and try to propose a reasonable water-resistance mechanism for WR-KSFM to understand its excellent water-resistance properties. Firstly, the phase purity of all the WR-KSFM-8 and IE-KSFM samples before and after immersion in water for 0.5 h to 24 h were estimated (Figure S6, Supporting Information). The XRD results clearly show that all of them maintain the pure phase of K2SiF6 and there are no distinct impurity peaks to be observed, which suggests that the poor water resistance of IE-KSFM is not due to the decomposition of K2SiF6 matrix. 10/36


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To make sure whether the Mn4+ dopants decomposed or not and, if yes, its decomposition leads to the poor PL performance of KSFM, the composition, morphology, microstructure and valance of WR-KSFM-8 and IE-KSFM were systematically and comparatively investigated. Figure 4 shows the scanning electron microscope (SEM) images of the WR-KSFM-8 and IE-KSFM samples before and after immersion in water for 6 h. As presented in Figure 4a, the WR-KSFM-8 particles originally show a smooth surface and a size dispersion of about ten microns before immersion in water. After immersion in water for 6 h, it is obviously seen in Figure 4b, 3c that there is no significant difference in microstructure even in high magnification, compared to original WR-KSFM-8 particles. For original IE-KSFM particles, they also exhibit similar smooth surface before immersion in water (Figure 4d). The IE-KSFM surface becomes more and more coarse and there are many nanoneedles that cover the surface of KSFM particles after immersion, as obviously seen in Figure 4e and especially Figure 4f. Furthermore,

energy

dispersive

spectrometer

(EDS)

was

used

to

semi-quantitatively analyze surface elemental composition of the WR-KSFM-8 and IE-KSFM samples before and after immersion in water. Figure 5a, 5b show SEM images of WR-KSFM-8 before and after immersion with same magnification and the EDS data were estimated from the selected areas in SEM images of these particles. From the EDS data, the K/Si/F atomic ratios of WR-KSFM-8 before (area A) and after immersion (area B) in water were evaluated to be about 1.8/1/6.8 and 2.2/1/4.9, which is roughly close to the 2/1/6, which is the stoichiometric atomic ratio for 11/36



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nominally pure K2SiF6. Moreover, the content of the surface Mn ions almost keeps unchanged before and after immersion in water, while the O content at either area A or B is below the limit of detection. These results indicate that surface Mn ions do not involve distinctly in the dissolution process when the WR-KSFM-8 particles were immersed into water. For IE-KSFM (Figure 5c, 5d), it has almost the same K/Si/F atomic ratios to WR-KSFM-8 and contains no detective O element before immersion in water (area C). But, after immersion, the surface K/Si/F atomic ratios of IE-KSFM change greatly. The area D, where there mainly exist nanoneedles as obviously shown in Figure 4f, is selected for further analysis. The elemental composition data shows that area D contains no detective Si element but excess of Mn, F, K and especially element O which is barely existed in original IE-KSFM particles. These results suggest that Mn4+ ion in IE-KSFM particles may suffer a process of dissolution and hydrolysis, leading to the unwanted formation of the manganese oxide and manganese fluoride that cover on the surface of the IE-KSFM particles. To further prove the undesirable appearance of manganese oxide and manganese fluoride in IE-KSFM particles, the UV-vis diffuse reflection spectra (DRS) of WR-KSFM-8 and IE-KSFM before and after immersion in water for different time are compared in Figure 6. It is clearly seen for the two samples before immersion that there are two broad absorption bands centered at 450 nm and 355 nm, due to the Mn4+ 4

A2g → 4T2g and 4A2g → 4T1g transitions, respectively, which coincide with the PLE

spectra. After immersion, they enhance to different extent with prolonging time for WR-KSFM-8 and IE-KSFM, suggesting the Mn4+ ions in both samples underwent the 12/36


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process of hydrolysis. With increasing immersion time, the characteristic absorption bands of Mn4+ in IE-KSFM quickly vanish at 0.5 h and instead there is only a flatter broad absorption band appearing in wavelength range from ultra-violet to visible region, of which the absorption intensity further increases with increasing immersion time. These results are consistent with the increasing darkness of the body color (Figure 3b) and the significant change of surface compositions (Figure 4e and 5d) of IE-KSFM particles after immersion in water as discussed above. On the other hand, the characteristic absorption bands of Mn4+ in WR-KSFM-8 still exist even after immersion in water for 24 h, which reasonably explain why the WR-KSFM particles keep light yellow body color (Figure 3b) without obvious change of surface compositions (Figure 4b and 5b). Based on the above results and discussion, we can conclude that the poor water resistance of IE-KSFM is mainly caused by the contact between the Mn4+ dopant and H2O and consequent hydrolysis of Mn4+ ions: 1. Typically, dissolution of surface KSFM takes place when bare KSFM contacts with moisture or water (Equation 2). 2. Then, the dopant [MnF6]2- ions are released from the surface of KSFM particles. Furthermore, free active [MnF6]2- complex ions react with H2O and transfer into solid Mn(OH)4. (Equation 3). 3. Finally, Mn(OH)4 degrades into dark brown MnO2 (Equation 4). Accompanying with a rapid hydrolysis reaction between active [MnF6]2- and H2O, KSFM particles are probably covered by dark brown manganese oxide, which 13/36



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therefore enhances the absorption of visible light and decreases the emission efficiency of IE-KSFM. The whole process described can be expressed by Equations 2 - 4 as follows: K2Si1-xMnxF6 (s) ↔ 2K+ (aq) + (1-x)[SiF6]2- (aq) + x[MnF6]2- (aq) (2) [MnF6]2- (aq) + 4H2O (l) → Mn(OH)4 (s) + 4H+ (aq) + 6F- (aq) (3) Mn(OH)4 (s) → MnO2 (s, dark body color) + 2H2O (l) (4) To improve the water-resistant properties, many attempts were made to post-coat KSFM with insoluble layer to isolate it from moisture or water. For example, the post-coated insoluble layer are alkyl phosphate layer and organic matter with hydrophobic long chains.37,

38

In this paper, WR-KSFM-8 shows excellent

water-resistance properties. We also investigated whether WR-KSFM-8 also has layer coated on its surface and how and what kind of layer was coated. To answer this question, the Fourier transform infrared (FT-IR) spectra and X-ray photoelectron spectroscopy (XPS) spectra of the surface of WR-KSFM-8 particles were measured. The FT-IR spectra of WR-KAFM-8, IE-KSFM and K2SiF6 matrix show almost the same characteristics when compared with each other (Figure S7, Supporting Information). One may notice that WR-KSFM was obtained from H3PO4 liquid. Even though, there are no correlative vibration bands, such as νa P-O-(H) at 1000 cm-, νs P-O-(H) at 955 cm- and ν P=O at 1230 cm-.46 The absence of P was further confirmed quantitively by ICP-AES. Therefore, the above results indicate that instead of alkyl phosphate layer,37 the excellent water resistance performance of WR-KSFM may come from protection by other substance. 14/36


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The surface condition of WR-KAFM-8 was also evaluated by the XPS measurement. Figure 7 shows the surface elements composition of WR-KSFM-8. It can be seen that there are only K, Si and F elements, of which the atomic percentages were 20%, 9% and 57%, respectively, close to 2:1:6, i.e., the stoichiometric atomic ratio of K2SiF6. Surprisingly, there is no any signal of Mn to be detected while the actual molar concentration of Mn in WR-KSFM-8 is up to 6.8%, based on the ICP-AES measurement. It is well known that the penetration depth of XPS is only about 10 nm and its limit of detection is 0.1%. Therefore, these results above indicate that the surface of WR-KSFM-8 particles is mainly composed of homogeneous K2SiF6 without any Mn4+ ions that mainly exist in the inner core of WR-KSFM-8 particles. In other words, the as-synthesized WR-KSFM is probably a KSFM@KSF core-shell composite particles. The KSF shell is responsible for excellent water-resistance properties of WR-KSFM particle. GE had developed coating strategy for KSFM@KSF via post-treating KSFM phosphor in a saturated mix solution of K2SiF6/HF/H2SiF6, as seen in Figure 8a. HF/H2SiF6 may serve as inhibitor of hydrolysis of [MnF6]2-. Unfortunately, toxic HF acid was inevitably used in this treatment. Unlike the addition of a new KSF protecting layer on the original KSFM particles like GE’s coating method, we here propose a possible surface deactivation enabled green homogeneous coating mechanism for WR-KSFM, as seen in Figure 8b. In the process of post-treatment of our WR-KSFM, 40 mL 1.5% H2O2 solution and 10 mL distilled water were in turn used to wash the as-obtained precursor product 15/36



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before drying it in vacuum at 40 oC for 24 h. One of the main purposes of the usage of H2O2 is to remove the excessive and unreacted MnO(OH)2 according to Equation 5. Consequently, insoluble MnO(OH)2 raw materials were transferred into soluble Mn2+ ions and finally separated from the solid WR-KSFM. In the meantime, the main motivation of the use of H2O is to provide the HF free aqueous solution environment for KSFM. At the solid-liquid interface between the WR-KSFM surface and H2O, free [MnF6]2- ions were released from the WR-KSFM’s surface immediately, reacted with H2O2 and were reduced to Mn2+ ions, according to Equation 6. Due to small solubility of K2SiF6 (0.084 g/100 mL H2O),47 it is easy for the ionic compound K2SiF6 to reach its solid-liquid equilibrium. Consequently, some K2SiF6 began to deposit on the surface of WR-KSFM according to Equation 7. Finally, the core/shell KSFM@KSF composite structure was obtained. Much different from GE’s method based on concentrated HF solution, this surface deactivation enabled coating strategy to be simple and green. MnO2 (s) + H2O2 (l) + 2H+ (aq) → Mn2+ (aq) + 2H2O (l) + O2 (g) (5) [MnF6]2- (aq) + H2O2 (l) + 4H+ (aq) → Mn2+ (aq) + 6HF (l) + O2 (g) (6) K2Si1-xMnxF6 (s) + (1-x) [SiF6]2- (aq) → K2SiF6 (s) + x[MnF6]2- (7) We also treated both IE-KSFM and CO-KSFM by this surface deactivation enabled green homogeneous growth strategy. It is very interesting that they also have excellent water resistance property like WR-KSFM after treating with H3PO4/H2O2 mixed aqueous solution. It is obviously seen that after immersing in water for 6 h, red emission intensity of 16/36


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either p-IE-KSFM or p-CO-KSFM (IE-KSFM or CO-KSFM after post-treatment) slightly declines to different extent (Figure S8, Supporting Information). P-IE-KSFM retains 73% of its original red emission intensity, much better than 9% of IE-KSFM. Comparatively, p-CO-KSFM shows better water-resistance property. 86% of its original red emission intensity remains, much better than 35% of CO-KSFM. Besides immersion in water, the water-resistance property of p-IE-KSFM as a representative was evaluated by exposing it to 85% humidity and 85 oC environment for 504 h (21 days) according to the commercial standard degradation process of white LEDs (Figure S9, Supporting Information). P-IE-KSFM maintains 70% of its original red emission intensity, much higher than 40% of IE-KSFM. The XPS curves of IE-KSFM and p-IE-KSFM show that the deactivation process can significantly reduce the surface Mn4+ content, seen in Figure S10, which actually improved the water-resistance property of the phosphor. To demonstrate the potential application of WR-KSFM in solid state lighting, the binary and ternary WLEDs were fabricated by combining a blue LED chip, yellow Y3Al5O12:Ce3+ (YAG: Ce3+) and red WR-KSFM or without this red phosphor. Figure 9a shows the electroluminescence (EL) spectra of the as-fabricated WLEDs, operated at 3.0 V with drive current 20 mA. The binary WLED gives white light with the correlated color temperature (CCT) of 5661 K, the color render index (Ra) of 69.8 and luminous efficacy of 168 lm/W. Its chromaticity coordinates are (0.3288, 0.3388), approximately close to the equal energy white light point of (0.333, 0.333). The ternary WLED exhibits white light (0.3339, 0.3093) with the CCT of 5398 K, the Ra 17/36



Page 18 of 37

and R9 of 80.5 and 63.8 and luminous efficacy of 96 lm/W. Comparatively, the ternary WLED has higher color render ability for red color than binary one, due to the increasing Ra and especially R9 values that are caused by the addition of red emitting WR-KSFM. Moreover, the ternary WLED shows good color stability as the drive current increases from 20 to 200 mA (Figure 8b). The chromaticity coordinates move slightly and the emission keeps good color stability. The calculated chromaticity shift (∆E) of ternary WLED from 20 mA to 200 mA is only 1.2 × 10-2.

3. Conclusion In summary, we developed a simple, green, homogeneous growth route to synthesize

excellent

water

resistant

and

highly

efficient

red

emitting

K2SiF6:Mn4+@K2SiF6 composite phosphor without the usage of toxic and volatile HF solution. The surface deactivation enabled growth mechanism was proposed and it was studied in detail. We found that surface [MnF6]2- ion was released and immediately degraded by acid H2O2, and meanwhile homogeneous KSF shell was in-situ deposited on the KSFM core during the dissolution-precipitation processes of surface KSFM in non-toxic H3PO4/H2O2 solution mixture, which finally contribute to the homogeneous KSFM@KSF composite core-shell structure. This composite structure strategy was also successfully used to treat the KSFM prepared by other methods. We believe that this new method of the red phosphor fabrication will open a new pathway for the large scale environment-friendly synthesis of excellent anti-moisture narrow red emitting A2MF6:Mn4+ phosphor for applications in the white 18/36


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LED with considerably improved characteristics.

4. Experimental Section 4.1 Synthesis of WR-KSFM: Silicon dioxide (SiO2, 99%), potassium hydrogen fluoride (KHF2, 99%), potassium permanganate (KMnO4, 99.5%), potassium formate (CH2O2·K, 99%) and phosphoric acid liquid (H3PO4, 3 M) were taken as the starting materials to obtain the WR-KSFM. 0.08 g KMnO4 was added into 32 mL 0.5 M CH2O2·K solution in ultrasonic vibration environment. After reacting for 30 mins, the active brown MnO(OH)2 was produced. The precipitate was collected by centrifugation and washed by distilled water. 0.12 g SiO2, 3.75 g KHF2 and 10 mL 3 M H3PO4 were then added and mixed thoroughly with the dark brown solid on a vigorous stirring for 6 h. The obtained product was washed in turn by 40 mL 1.5% H2O2 and 10 mL distilled water orderly and dried under vacuum at 40 oC for 24 h. Serial KSFMs with different dopant were prepared a following similar procedure by adjusting KMnO4 and CH2O2·K addition proportionally, during which KMnO4 were weighted 0.02 g, 0.04 g, 0.06 g, 0.08 g and 0.10 g, respectively. 4.2 Synthesis of Reference KSFM by different synthetic methods: 4.2.1 Synthesis of dopant precursor K2MnF6: According to the previous report,7 K2MnF6 was prepared by dissolving 9.00 g KHF2 and 0.45 g KMnO4 in 30 mL 40% HF solution and put on a continuous stirring for 2 h. Then 0.3 mL 30% H2O2 was added into the mixture dropwise and the yellowish powder precipitates. It was washed 19/36



Page 20 of 37

by acetone several times and dried in vacuum at 40 oC for 24 h, K2MnF6 was obtained. 4.2.2 Synthesis of IE-KSFM: 0.036 g of the as-synthesized K2MnF6 was dissolved in 1 mL 40% HF solution and 1.00 g K2SiF6 (99.999%) was added into the mixed solution. The mixture was continuously stirred for 2 h and the product was collected by centrifugation and washed by ethanol several times. After drying in vacuum at 40 o

C for 24 h, light yellow KSFM was ready for the next experiments. 4.2.3 Synthesis of CO-KSFM: According to the previous patent,40 20.0 g K2SiF6

was dissolved in 300 mL 40% HF solution and then heated to 80 oC. Afterward, 2.24 g K2MnF6 was added and stirred under thermal environment for 2 h. The solution was then transferred to a cold bath at -30 oC. After cooling the HF solution, the phosphor slurry was collected by centrifugation, washed by methanol and dried in vacuum at 40 o

C for 24 h. 4.2.4 Post-treatment of reference KSFM: Both IE-KSFM and CO-KSFM were

post-treated to acquire similar water resistance property as WR-KSFM. 0.4 g IE-KSFM was weighted and put into a mixed solution containing 1 mL 3M H3PO4, 1 mL 30% H2O2 and 10 mL H2O. The mixture was stirred for 6 h. The post-treated product, labeled as p-IE-KSFM, was collected by centrifugation, washed by 5 mL deionized water once and dried in vacuum at 40 oC. P-CO-KSFM was obtained following a similar procedure, while the stirring time was adjusted to 1 h. 4.3 LEDs fabrication: WLEDs with different correlated color temperatures (CCTs) 20/36


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and color rendering index average (Ra) were fabricated by combining blue-chip (450 460 nm, VF:3 V, IF:350 mA, purchased from Shenzhen VANQ Technology Co., Ltd) and YAG:Ce3+ yellow phosphor (purchased from Jiangmen Kanhoo Industry Co., Ltd) with/without WR-KSFM red phosphor. A phosphor-silicone mixtures were obtained by mixing up 0.10 g silicone and 0.01 g YAG:Ce3+ with/without 0.09 g WR-KSFM. After vacuum treatment to remove air bubbles, the mixture was coated on the lead frame and solidified under 150 oC for 4 h to produce WLEDs. The WLEDs were operated at 3.0 V with drive current 20 - 200 mA. 4.4 Analysis and Characterization: The solid products were characterized by using X-Ray powder diffraction (XRD) studies on D8 ADVANCE powder diffractometer (Bruker AXS, Germany) with Cu-Kα radiation (λ = 1.54059 Å) at room temperature in the range of 10 °< 2θ < 70 °. The high quality XRD data for Rietveld refinement was collected over a range 15 °< 2θ < 110 ° at an interval of 0.02 ° with a counting time of 8 sec per step on a silicon wafer.

The Rietveld analysis of all the samples’

structure were carried out using the Topas Academic software on the laboratory XRD data. The morphology and elemental composition of the as-prepared products were measured by SEM (FEI Quanta 400). The UV-vis diffuse reflection spectra (DRS) were recorded with a Cary 5000 UV-Vis-NIR spectrophotometer (Varian) equipped with double out-of-plane Littrow monochromator. The photoluminescence excitation (PLE) and photoluminescence (PL) spectra within the temperature range 293 - 500 K were measured using an Edinburgh Instruments FSP920 Time Resolved and Steady State Fluorescence Spectrometers equipped with a 450 W Xe lamp, a 60 W µF900 21/36



Page 22 of 37

microsecond flash lamp, TM300 excitation monochromator and double TM300 emission monochromators and thermo-electric cooled red-sensitive PMT. The spectral resolution of the steady measurements was about 0.05 nm in UV-Vis. The sample was mounted in an Oxford OptistatDN2 nitrogen cryostat for PLE and PL measurements above room temperature. The room temperature inner quantum yield (IQY) of the samples was measured using a barium sulfate coated integrating sphere (150 mm in diameter) attached to the FSP920. The IQY is defined as the ratio of the number of emitted photons (Iem) to the number of absorbed photons (Iabs), and can be calculated by the following equation 8:48 =

=

(8)

Where ER is the emission spectrum of the excitation light, recorded with the equipment blank sample in place, and collected using an integrating sphere, ES is the spectrum of the light used for exciting the sample, and LS is the luminescence emission spectrum of the sample. The actual doping concentration of Mn4+ in products was measured by using an Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) on PerkinElmer Optima 8300 and each sample was retested 3 times. The Fourier transform infrared (FT-IR) spectra were measured with a resolution 1 cm-1 in the range 400 - 4000 cm-1 using Bruker EQUINOX 55 spectrometer. The electroluminescence (EL) spectra and photoelectric properties including LE, CCT, CIE chromaticity coordinates and CRI of the fabricated WLEDs were evaluated using a Labsphere CDS2100 spectrometer and Labsphere LPS-100-0260 power supply. X-ray photoelectron spectroscopy (XPS, ESCALAB 22/36


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250, Thermo Fisher Scientific, USA) measurements were performed to characterize the chemical compositions of the obtained product.

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Photoluminescence Properties of Red-Emitting Li2ZnSn2O6:Mn4+ Phosphor for Solid-State Lighting. J. Lumin. 2018, 197, 169-174.

ASSOCIATED CONTENT Supporting Information SI contents XRD refinement data of KSF matrix; thermal quenching curves of WR-KSFM and IE-KSFM; FT-IR spectra of different products; XRD, photographs, PL spectra and intensity curves of water corrosion & HH/HT experiment products, XPS spectra of IE-KSFM and p-IE-KSFM. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected], Tel: +86-20-84112112, Fax: +86-20-84111038.

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

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Figure 1 (a) XRD patterns of the as-synthesized WR-KSFM phosphors with various KMnO4 addition, (b) cubic structure of KSFM and (c) XRD refinement results of WR-KSFM-8.

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Page 32 of 37

Figure 2 (a) Room temperature PL and PLE spectra of WR-KSFM-8 and (b) the dependences of actual molar ratio of Mn and the integrated emission intensity of WR-KSFMs on the nominal addition of KMnO4.

Figure 3 (a) The emission intensity of WR-KSFM-8 and IE-KSFM after immersion in water for t hours (t = 0 ~ 24; normalized at t = 0), (b) photographs of WR-KSFM-8 (A, C) and IE-KSFM (B, D) before and after immersion in water for 6 hours, taken under 32/36


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natural light and 365-nm UV-light, and comparisons of (c) the emission intensity and (d) inner quantum yield between WR-KSFM-8 and IE-KSFM before and after immersion in water for 6 hours.

Figure 4 SEM images of WR-KSFM-8 (a, before immersion; b and c, after immersion for 6 h) and IE-KSFM (d, before immersion; e and f, after immersion for 6 h).

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Figure 5 SEM images and EDS of WR-KSFM-8 (a and A, before immersion; b and B, after immersion for 6 h) and IE-KSFM (c and C, before immersion; d and D, after immersion for 6 h).

Figure 6 DRS of (a) WR-KSFM-8 and (b) IE-KSFM before and after immersion in water for different time (t = 0.5, 6, 12 and 24 h).

Figure 7 XPS spectra and high-resolution XPS of K2p, Si2p, F1s and Mn2p of WR-KSFM-8.

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Figure 8 Designing strategies of KSFM@KSF composite: (a) Coating; (b) Surface deactivation.

Figure 9 (a) The EL spectra of the binary and ternary WLEDs fabricated by combining blue LED chip, yellow YAG:Ce3+ and red WR-KSFM or not, under a 35/36



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drive current of 20 mA, (b) the chromaticity coordinates of ternary WLED under drive current from 20 - 200 mA and (c) enlarged figure of selected white light area in (b).

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Highly Stable K2SiF6:Mn4+@K2SiF6 Composite Phosphor with

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