Waterproof Narrow-Band Fluoride Red Phosphor K2TiF6:Mn4+ via

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Waterproof Narrow-band Fluoride Red Phosphor K2TiF6:Mn4+ via Facile Super-hydrophobic Surface Modification Yayun Zhou, Enhai Song, Ting-Ting Deng, and Qinyuan Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15503 • Publication Date (Web): 06 Dec 2017 Downloaded from http://pubs.acs.org on December 7, 2017

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ACS Applied Materials & Interfaces

Waterproof Narrow-band Fluoride Red Phosphor K2TiF6:Mn4+ via Facile Super-hydrophobic Surface Modification Ya-Yun Zhou, En-Hai Song, Ting-Ting Deng, and Qin-Yuan Zhang* State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510641, PR China. *Corresponding author E-mail: [email protected]

Abstract: As the unique and efficient narrow-band red emission and broadband blue light absorption characteristics, Mn4+ ions activated fluoride red phosphors have gained increasing

attention in warm white LEDs and liquid crystal display (LCD) backlighting applications, whereas the intrinsic hygroscopic nature of these phosphors have inevitably limited their practical applications. Herein, a waterproof narrowband fluoride phosphor K2TiF6:Mn4+ (KTF) has been demonstrated via a facile super-hydrophobic surface modification strategy. By using super-hydrophobic surface modification with Octadecyltrimethoxysilane (ODTMS) on KTF surfaces, the moisture resistance performances and thermal stability of the phosphor KTF can be significantly improved. Meanwhile, the absorption, quantum efficiency showed without obvious changes. The surface modification processes and mechanism, as well as moisture resistance performances and luminescence properties of the phosphors have been carefully investigated. It

was found that the luminous efficiency (LE) of the modified KTF was maintained at 83.9 % or 84.3 % after being dispersed in water for 2 h or aged at high temperature (85 °C) and high humidity (85%) atmosphere (HTHH) for 240 h, respectively. The WLEDs fabricated with modified KTF phosphor showed excellent color rendition with lower color temperature (2736 K), higher color rendering index (CRI, Ra = 87.3, R9 = 80.6), and high luminous efficiency (LE = 100.6 lm/W) at 300 mA. These results indicate that hydrophobic SCAs surface modification was a promising strategy for enhancing moisture resistance of humidity sensitive phosphors, exhibiting great potential for practical applications.

Keywords: K2TiF6:Mn4+, Fluoride phosphor, Waterproof, Hydrophobic surface modification, Warm WLEDs, High moisture resistance 1

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1. Introduction White lighting emitting diodes (WLEDs) are considered as the next generation solid-state lighting source, being widely applied in our daily lives, due to their prominent advantages of long lifetime, energy saving and environmental friendliness.1-3 However, the most popular and commercial WLEDs fabricated on the blue light emitting InGaN chips and yellow phosphor Y3Al5O12:Ce3+ (YAG) suffer from a low color rendering index (CRI, Ra < 80) and high correlated color temperature (CCT, Tc > 4000 K), and failed to be applied in indoor lighting or liquid crystal display (LCD) backlighting owing to the inherent deficient red light emission from YAG.4-6 To solve this issue, the suitable and efficient red-emitting phosphor should be added into the aforementioned design.7-9 As a great promising candidate, Mn4+-activated narrow-band red-emitting fluorides A2XF6 (A = Li, Na, K, Rb, and Cs; X = Si, Ge, Sn, Ti, Zr, and Hf) and A3MF6 (M = Al, Ga) phosphors have gained increasing attention because of their unique luminescence properties, good thermal stability, as well as high luminescence efficiency (LE).10-20 By using these Mn4+ related fluoride phosphors as red light component, warm white LEDs with luminous efficacy larger than 120 lm/W could be easily achieved.21 However, since the exposed MnF62− groups on the surface of these fluoride phosphors easily hydrolyze to hydrated MnO2 and simultaneously generate HF even in the working devices, their practical applications are limited.22-23 Despite substantial strides have been made in the research and development of Mn4+ activated fluoride red phosphors for WLEDs applications, little attention has been focused on improving the moisture resistance of these Mn4+ doped fluoride red phosphors. For moisture sensitive materials, coating with waterproof layer on the surface has demonstrated to be an efficient strategy to improve their stability. In order to minimize losses of the luminescence efficiency, the protective layer for phosphors should be usually transparent enough in the visible region, and easy to encapsulate on the phosphor particles.24 Inorganic protective layers have been proved to be good surface-coatings for enhancing the moisture resistance, whereas that may obviously decrease the LE of the phosphors.25-27 In contrast, organic compounds with a higher degree of transparency are more suitable for coating on the phosphor surfaces for bettering the moisture resistance.28 For improving the stability of the well-known fluoride red phosphor K2SiF6:Mn4+, Liu et al.,29 reported a method 2


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for coating a hydrophobic alkyl phosphate layer onto this phosphor surface by using transition metals ions as cross-linkers. Arunkumar et al.,30 used oleic acid (OA) as a hydrophobic encapsulant to form an organic waterproof skin on the KSF surface through the formation of hydrogen bonds via a solvothermal procedure. Notably, both of surface-modification strategies can evidently improve the moisture resistance of the KSF phosphors, while the surface modification procedures are much complex. In addition, for forming the protective layers on the phosphor surface, a heating condition is required into the two cases, which may increase the valence change tendency of Mn4+ in the host lattice, as the valence sates of Mn are very sensitive to the temperature. Actually, besides to isolate the MnF62− of the phosphor surface to contact with moisture, reduce the hydroxyl groups (OH-) or absorbed-water molecules of the phosphors surface should also be necessary, because these hydrophilic groups can accelerate phosphors hydrolysis.

In this work, we present a facile and generic strategy for improving moisture resistance of Mn4+ activated fluoride phosphors via super-hydrophobic surface modification. For this purpose, we used silane coupling agents (SCAs) as coating layer. The SCAs are able to provide chemical bonding with surface hydroxyl groups of phosphors, which make of lower surface energy and manifest a large water contact angle (CA, >90 °).31-32 As a proof of concept, we chose K2TiF6:Mn4+ (KTF) as objective phosphor for their excellent luminescent properties. After modified with proper long chain alkyl SCAs on the phosphor surface, the existed surface hydrophilic groups would be dramatically reduced and the CA of phosphor surface tuned to super hydrophobic level (CA≥150 °), which leads to significantly improve the moisture resistance of fluoride phosphors. The super-hydrophobic modification processes and mechanism, as well as the effects of surface modification on luminescent properties and moisture resistance of KTF phosphor have been investigated in detail. These results show that the super-hydrophobic modified KTF phosphors are promising candidates for high power stable-color warm WLEDs.

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2. Experimental 2.1 Materials The raw materials KF, H2TiF6 (50 wt %), KHF2, HF (49 wt%), and n-Hexane were purchased from Aladdin Industrial Corporation (China). KMnO4, H2O2 (30 wt %), Ethyl Alcohol and Acetone were obtained from Guangzhou Chemical Reagent Factory (China), respectively. The silane coupling agent (SCA) include Octadecyltrimethoxysilane (90 %) (ODTMS), Hexadecyltrimethoxysilane (85 %) (HDTMS), Dodecyltrimethoxysilane (95 %) (DTMS), and n-Octyltrimethoxysilane (96 %) (OTMS) were purchased from Aladdin, TCI, J&K Chemical and Alfa Aesar, respectively. All the chemicals used in this study were of analytical grade without any further purification. 2.1 Preparation of KTF Firstly, the key manganese source K2MnF6 was synthesized by using a Bode’s method.33 In a typical process, 10 g KHF2 and 1 g KMnO4 were dissolved in HF (49 wt%) solution, then H2O2 (30 wt%) was added drop by drop to precipitate K2MnF6 yellow powders with magnetically stirring in an ice bath. The K2MnF6 powders was obtained after filtered carefully, washed with acetone several times, and dried at 70 °C for 2 hours. The preparation of KTF was optimized based on the methods from literatures 16 and 34. Specifically, 9.80 ml H2TiF6 solutions were added into 200 ml of H2O in a 500 ml beaker and stirred thoroughly. Then 1.23 g K2MnF6 and 5.81 g KF were gradually put into the above transparent colorless solution, respectively. After 60 min magnetically stirring, the precipitates were collected, washed with ethyl alcohol several times and dried at 70 °C for 2 hours. And finally the as-prepared KTF red phosphors were obtained (Figure 1a). 2.2 Surface modifications of KTF To change the surface properties of materials, the as-prepared KTF phosphor was modified with silane using a simple mixing method. Before the hydrophobic modification, the obtained KTF powder was pre-processed using an ultraviolet lamp for 30 min to increase hydroxyl (–OH) groups as chemical binding sites.35 In a typical modified procedure, 0.5 g as-prepared KTF was added in a plastic beaker with 50 ml 0.5 % ODTMS n-Hexane solution (0.5 %, the volume ratio of ODTMS to n-Hexane, similarly hereinafter), after vigorous stirred of 2 hours, the treated KTF phosphor was obtained after washed several times with n-Hexane 4


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and dried in 120 °C for 2 hours (Figure 1a). The modified KTF phosphor was donated as OD-KTF (0.5 %). For comparison, the as-prepared KTF was also modified by different silanes including HDTMS, DTMS, and OTMS, which own various different lengths of alkyl chains. These modified KTF phosphors were marked as HD-KTF, D-KTF and O-KTF, respectively. 2.3 LEDs fabrications and performance measurement The commercial YAG:Ce3+ (YAG, Intematix, China) yellow phosphor, modified KTF red phosphors, and blue InGaN chips (~ 450 nm) were used to fabricate WLEDs. The phosphors were mixed with epoxy resin thoroughly (the mass ratio of YAG, epoxy resin and modified KTF are 1: 10: 2). The obtained phosphor epoxy resin mixture was used to coat the surface of the LED chips. These WLEDs were operated at a forward current of 300 mA. The photoelectric properties of the fabricated devices were measured by an auto-temperatured LED opto-electronic analyser (ATA-1000, Everfine, China).

3. Characterization The x-ray diffraction (XRD) patterns of the phosphors were characterized by an X-ray powder diffractometer (Philips PW1830, using Cu Kα radiation at λ= 1.5406 Å; tube voltage = 40 kV; tube current = 40 mA). The existence of C-H bonds on phosphors surfaces after modification was identified by fourier trans-form infrared spectrometer (FTIR, VERTEX 70, Bruker, Germany). The morphology and elemental composition of prepared phosphors was detected by a scanning electron microscope (SEM, NOVA NANOSEM 430, USA) with an attached energy-dispersive X-ray spectrometer (EDS). The thickness of the hydrophobic layer was observed through high resolution transmission electron microscopy (TEM, JEOL-1400, Japan). The surface element compositions and chemical change of samples were carried out by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Scientific, USA). The doping concentration of Mn4+ ions in phosphors was verified by inductively coupled plasma optical emission spectroscopy (ICP, Agilent Varian 720, USA). The distilled water contact angle (CA) was obtained by a video based contact angle measurement (Kino SL200KB, USA) at RT with 5 µl of the individual water droplet in all measurements. Accelerated ageing tests were experimented in a constant temperature and humidity chamber (BPS-50CL, Blue-pard, 5



China) maintaining at high temperature (85 °C) and high humidity (85%) atmosphere (HTHH), the moisture resistance of the phosphors were obtained by measuring their luminous intensity every 24 h. Moisture resistance of the phosphors (0.2 g) in distilled water (1 ml) were tested by measuring theirs in-situ luminous intensity (excited by 450 nm laser) with a high sensitive spectrometer (Nova, IdeaOptics Instruments, China). The optical properties at room temperature (RT) were measured by a fluorescence spectrophotometer (Edinburgh FL920, UK) using a 450 W xenon lamp or a 150 W µs flash lamp as an excitation source, respectively. Temperature-dependent fluorescent spectra and decay curves were measured in the temperature range of 25−250 °C using Edinburgh FL920 coupled with a temperature controller (TAP-02, Tianjin Orient-KOJI Instrument, China). Temperature-dependent absorption efficiency, internal and external quantum efficiencies (IQE and EQE) of the samples were measured by a QE-2100 spectrophotometer (Otsuka Electronics, Japan).

4. Results and discussion 4.1 Synthesis strategies of KTF with hydrophobic surface The SCA modification processes on KTF was illustrated in Figure 1b. There are two routes for this graft reaction, the first one was that the silane reacted with surface hydroxyl groups (-OH) of KTF and then formed OD-KTF directly (1). Meanwhile, for the second one, the silane was hydrolyzing with the absorbed water firstly and formed silanol (2), then the hydrogen bond was formed between the -OH in silanol and -OH on the surface of KTF. Finally, under more strenuous conditions at an evaluated temperature, the silane bond chemically links with the surface of phosphor, forming a stable chemical bond (Si-O) (3).36 Based on these, the long chain alkyl group of SCA have been grafted onto the phosphor surface successfully, and the organic layer formed on the phosphor surface are mainly ascribed to the result of above two routes. This modification processes can obviously reduce the hydroxyl groups (OH-) or absorbed-water molecules of the phosphor surface, which would be beneficial for improving the moisture resistance of the phosphors.

4.2 Structure, morphology and composition XRD patterns of KTF modified with different amount of ODTMS are shown in Figure 6


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2a. It can be observed that all diffraction peaks match well the hexagonal K2TiF6 (JCPDS No. 08-0488) with a space group of D3d3 -P 3m1 .21 No obvious characteristic diffraction peaks of ODTMS or other extra peaks were detected, indicating that the modified KTF phosphor were pure phases. In addition, the concentrations of ODTMS have no significant effect on crystal phases of as-prepared KTF phosphors. Meanwhile, the excess silane could be washed away easily by hexane. Furthermore, the influences of different silane, including ODTMS, HDTMS, DTMS and OTMS on phosphor were also discussed, and the corresponding XRD patterns are given in Figure S1. Apparently, the silane with various lengths of alkyl chains has no effect on crystal phases of as-prepared KTF phosphors in the modification processes. FTIR analysis was performed to determine the functional groups on the surface of modified KTF phosphors. The FTIR spectra of KTF modified with different amount of ODTMS are provided in Figure 2b. The KTF without any modification only has strong absorption band around at 3452 and 1633 cm-1, corresponding to the O-H stretching and H-O-H bending of the surface hydroxyl or absorbed water molecules, respectively.37 The existing -OH and H2O groups on KTF surface provide plenty of active positions for graft reaction. In the cases of 0.25-1.00 % samples, the variation of additional peaks except O-H stretching and H-O-H bending were appeared at 2925 and 2854 cm-1, which corresponding to the antisymmetric and symmetric stretching of the C-H bonds from -CH2- and -CH3 groups of the silane.38 Emerging absorption peaks of alkyl groups were also increased with the increasing of ODTMS concentration, indicated more alkoxy groups were modified onto KTF surface. The results show clearly that surface modification was chemically performed via the graft reaction. Meanwhile, the surface hydroxyl and absorbed water signal (3452, 1633 cm-1) became weaker when KTF was modified by 0.25 %, and nearly disappeared at 0.50 % as almost the whole functional groups -OH and H-O-H were grafted with ODTMS. However, when the concentration of ODTMS rose to 0.75% and 1.00 %, the excess ODTMS cause molecular condensation reaction with residual silanol groups and atmospheric water much easily, and resulted ineffective modification with KTF, which led to the hydrophilic groups still existed.39 Thus, to get modified KTF with less hydrophilic groups, 0.50 % was chose as the optimum SCA concentration in this study, which resulted the obtained OD-KTF (0.5 %) sample had an excellent waterproof property as shown in Figure 1a insert image. This 7



modification process has the advantage of small amounts of SCAs, simple techniques, and a lower cost, would have many practical applications for fluoride phosphors to improve their moisture resistance.

The morphology changes of KTF phosphors before and after modification are shown in Figure S2. The SEM image exhibits that KTF without modification adopt a hexagonal platelet-shaped morphology with diameters of 30-60 µm (Figure S2a). However, the hexagonal sheets were crushed into pieces with an average size of 10-30 µm after modification process (Figure S2b-e). With the increasing of ODTMS concentration from 0.25 % to 1.0 %, the average particle size of the samples were gradually decreased from ~40 to ~15µm (Figure S2f-j), which can be attributed to the excess long chain alkyl silane that plays a role as dispersing agent and break easily while mixing.40 EDS mapping provides an easy and direct way to analyze the element distribution of KTF samples. As shown in Figure 2c and 2d, signals for K, Ti, F and Mn were detected from both KTF and OD-KTF (0.5 %) phosphor, but enrichment of Si signal with homogeneous distribution only could be detected on the OD-KTF (0.5 %) surface. Obviously, the Si element was derived from ODTMS, which further confirmed that KTF surface was well modified by silane. Figure 2e-2i shows the TEM images of KTF modified by different amount of ODTMS. Obviously, a hydrophobic layer can be found on the surface of modified KTF phosphors. Moreover, with increasing ODTMS concentration from 0.25 % to 1.00%, the layer thickness gradually increases from 40 to 110 nm. Combined with FTIR spectra, OD-KTF (0.5 %) possesses a moderate layer thickness of ~ 80 nm and minimum surface hydroxyl.

The XPS is especially sensitive to surfaces of samples, and which can probe to subsurface depths of 5-10 nm.41 Herein, the XPS spectra of some typical phosphor samples were measured and shown in Figure 3. Both XPS spectra of KTF and OD-KTF (0.5 %) were composed of clearly signal peaks of K, Ti, F, Mn, O, and C elements, without obvious shift of the binding energy had been observed after modification processes, indicating that the chemical variation on KTF surface was almost no change (Figure 3a). The relative intensity (K, Ti, F, Mn) of OD-KTF (0.5 %) was lower than that of KTF, which can be explained as 8


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less phosphor surface exposure after modification procedures. However, the C 1s and Si 2p peaks in OD-KTF (0.5 %) was much higher than KTF, corresponding to its surface was composed of main chains of SCAs (Figure 3b, c). The C 1s region could be divided into three Gaussian curves,42 the peaks locate at 286.0 ± 0.2, 284.6, and 283.1 ± 0.2 eVs were ascribed to C-O, C-C or C-H, C-Si, respectively.43 In addition, the Si 2p region also could be split into two Gaussian curves, the peaks reside at 103.1 and 101.9 eV were belonging to Si-C and Si-O, respectively.44 The absence of Si 2p peak from the XPS spectra of OD-KTF (0.5 %) might be attributable to the complete encapsulation of the alkoxy shell from ODTMS, and the long chain alkyl folded outward and covered Si atoms. The XPS analysis of phosphors surfaces showed that the methyl (-CH3) and methylene (-CH2-) really existing, being consistent with the results of FT-IR, EDS mapping, and TEM. These results strongly confirm that a hydrophobic layer with long chain alkyl from ODTMS was formed on the phosphor surface, and finally caused the significantly improvement of moisture resistance of KTF phosphor.

4.3 Moisture resistance performances Figure 4a shows the pictures that a drop of water onto the different modified KTF phosphor surfaces. Obviously, the KTF without modification was easily infiltrated by water, but moisture resistance became better after modified by silane with gradually increases the chain alkyl length. In the case of HD-KTF (0.5 %) and OD-KTF (0.5 %) samples, the water onto modified KTF phosphors just like a drop of water on the lotus leaf, vividly, showing a good hydrophobic property. To understand this phenomenon, the water contact angle (CA) was measured through distilled water meet the phosphors to quantify the wettability of modified KTF surface. It is observed that the CA was dramatically increased from near zero degree to 36.8°, 52.7°, 147.9°, and 155.4° after the surface modification of OTMS, DTMS, HDTMS, and ODTMS solutions (0.5 %), respectively (Figure 4b). This result illustrates that the super-hydrophobic performance (CA ≥ 150°) was modified on KTF surface, as well as the great improvement of moisture resistance after modification ODTMS.45 The actual results of CA values (the larger the CA, the stronger the hydrophobicity) were in agreement with the observed phenomenon (Figure 4a) and further proved excellent moisture resistance of 9



modified KTF.46 In order to evaluate the water resistance performance of phosphors in extreme situations, luminous efficiency (LE) of modified KTF phosphors that soaked in distilled water were real time detected by a highly sensitive spectrometer. The moisture resistance change of KTF phosphors over time are shown in Figure 4c. KTF without modification was susceptible to hydrolysis by water with a 43.7 % residue of original intensity after 30 min, whereas, the modified phosphors O-KTF (0.5 %), D-KTF (0.5 %), HD-KTF (0.5 %), and OD-KTF (0.5 %) were still maintaining 45.6, 46.6, 51.5, and 83.9 % after the same time, respectively. Emission intensity of modified KTF phosphors was decreasing with a trend of starts steep and later flattens. The decreasing rate has a positive relation with silane, the longer the carbon chain of silane, the more slowly the luminescence attenuates. Furthermore, as shown in Figure S1b, the KTF phosphors modified by OTMS, DTMS and HDTMS still had a few hydrophilic group (-OH and H2O), which can accelerate the phosphors hydrolysis, but the degree of hydrolysis was still less deep than KTF as the existence of long chain alkyl hydrophobic group. However, OD-KTF (0.5 %) with the least of the hydrophilic group (-OH) get the best of moisture resistance, the emission intensity decreases slowly at the beginning of 60 min and tends to be stable after that. Moreover, LE of KTF and OD-KTF (0.5 %) in HTHH conditions was recorded for 240 h, OD-KTF (0.5 %) have a relative luminous efficiency of 84.3 % to the initial intensity, much better than KTF (65.2 %). Moreover, TEM measurements were conducted to further reveal the variations onto the surface of modified samples (Figure S1c-S1g). The layer thickness of O-KTF, D-KTF, HD-KTF, and OD-KTF was about 220, 120, 100, and 80 nm, respectively. It can be deduced that the creating layer can effectively prevent KTF from water erosion; meanwhile, its thickness is highly related to the moisture resistance of these modified phosphors. In order to verify the universality of this approach, the commercial KSF (Grirem, China) was modified by ODTMS use the same method. The moisture resistance of OD-KSF (0.5 %) had got an obvious improvement, the relative LE of OD-KSF (0.5 %) still maintains 66.1 % after 30 min in the water, was much better than KSF (54.3 %) (Figure S3a). KSF phosphors promptly wetting by water droplet (Figure S3b) and turned brown after 5 min, whereas the OD-KSF (0.5 %) sample was still yellow and the water droplet still stand proudly on the surface after 5 min (Figure S3c). Thus, the ODTMS surface 10


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modification substantially improved moisture resistance of water sensitive fluoride phosphors as we initially intended.

4.4 Luminescent properties The excitation and emission spectra of KTF and KTF modified by various SCAs are given in Figure 5a and 5b, both of them have no significant change compared with KTF. All of the samples exhibited obvious two broad absorption bands with maximum peaks at approximately 362 and 462 nm, originating from the spin-allowed transitions of 4A2g → 4T1g and 4A2g → 4T2g, respectively. The sharp red emission lines within the scope of 580~660 nm are due to spin-forbidden 2Eg → 4A2g transitions. The emission peaks at approximately 599, 608, 613, 621, 630, 635, and 646 nm are belonging to transitions of the v3 (t1u), v4 (t1u), v6 (t2u), zero phonon line (ZPL), v6 (t2u), v4 (t1u), and v3 (t1u) vibronic modes, respectively.47 The photoluminescence (PL) decay curves were fitted based on a single-exponential function, without obvious variation of the decay lifetimes after modified by different silane, as shown in Figure 5c and 5d. As comparison to un-modified one, the ICP results of Mn content (mol %) has slightly decline after modification (Table S1), which might be ascribed to the increased relative molecular mass after modification. This fact also indicates that the concentration of doped Mn did not change significantly after modification process. The luminescence intensity of O-KTF (0.5 %), D-KTF (0.5 %), HD-KTF (0.5 %), and OD-KTF (0.5 %) still maintained 87.9 %, 92.3 %, 93.6 %, and 95.7 % of the original intensity of KTF, respectively (Figure 5d). Absorptance (Abs) and external / internal quantum efficiency (EQE / IQE) of KTF and modified KTFs are given in Table S2. The Abs, EQE, and IQE values of OD-KTF (0.5 %) had been recorded of 78.2 %, 36.7 %, and 46.9 %, whereas that of KTF were of 80.6 %, 43.4 %, and 53.8 %, the relative Abs (RAbs), relative EQE (REQE), and IQE (RIQE) values of OD-KTF (0.5 %) to KTF were of 97.0 %, 85.4 %, and 87.2 %, respectively. The RAbs, REQE, and RIQE values of other modified KTF phosphors were also still maintained over 80 %, 80 %, and 90 %, respectively. Based on their relative PL intensities and QEs, an increasing layer thickness would degrade the emission of modified KTF samples. Notably, the layer with suitable thickness of ~ 80 nm realized the balance between the least reduction of the luminous efficacy and the less of existed hydrophilic groups. 29,48 Although 11



leading to a slight emission decrease, the organic layer effectively enhance the moisture resistance of KTF.

The temperature dependent emission spectra of various KTF and OD-KTF (0.5 %) phosphors under 462 nm light excitation are shown in Figure S4 and the temperature dependent of the relative integrated PL intensity (IPL(T) / IPL(25°C)) of the KTF phosphors is shown in Figure 6a. Their emission intensities rise up in a short paragraph following enhanced vibronic absorption then decrease by intense non-radiative transition at higher temperature, which are agree well with the result of Zhu’s report. 21 The relative integrated PL intensity of OD-KTF (0.5 %) at 150 °C remained at 107.9 % compared with 25 °C, which exhibits better thermal stability than KTF (106.0 % at 150 °C). The activation energy (Ea) of KTF and OD-KTF (0.5 %) phosphors for thermal quenching were also determined. The Ea can be fitted to IT / I0 = [1 + G ⋅ exp(-Ea / κT )]-1 , where I0 and IT is the integrated PL intensity of initial and different temperatures, while G and the Ea are the refined variables, and κ is the Boltzmann constant (8.617 ×10-5 eV K-1).49 Based on above equation and Figure 6a, the Ea for the OD-KTF (0.5 %) sample was determined to be 0.73 eV, which shows slightly higher than that of KTF (~0.71 eV). Different from the emission intensity, the PL lifetime of KTF and OD-KTF (0.5 %) declines gradually at the temperature range from 25 to 250 °C (Figure S4b, 4d) due to the serious non-radiative transition processes.50 It is noted that the OD-KTF (0.5 %) decreases slower than of KTF (Figure 6b) with increasing temperature. These results clearly demonstrated that the super-hydrophobic surface modification would not only improve moisture resistance, but also enhance thermal quenching performance of KTF phosphor. The temperature dependent RAbs are listed in Table S2 and shown in Figure 6c. It can be observed that the absorbance of both KTF and OD-KTF (0.5 %) phosphors were slightly decreased before 100 °C, and then maintained 98.3 % and 97.3 % of original intensity after that, respectively. The absorbance of OD-KTF (0.5 %) was less than KTF which could be ascribed to the isolation of the surface organic layer on KTF. As the PL quantum efficiency are proportional to the PL intensity, the variation trends of temperature dependent RIQE, REQE were similar to that of the temperature dependent PL intensity, as shown in Figure 6d and listed in Table S3.21 However, the loss of non-radiative transition with increasing 12


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temperature of OD-KTF (0.5 %) was less than KTF, the thermal quenching stability of OD-KTF (0.5 %) was still much better, though it’s absorbance slightly less than KTF. At the temperature of 150 °C, KTF and OD-KTF (0.5 %) have a RIQE of 107.9 % and 109.5 % to their initial IQE under the 460 nm excitation, respectively. Meanwhile, the corresponding REQE was 106 % for KTF and 106.5 % for OD-KTF (0.5 %). These phenomena were again suggested that the thermal stability of modified KTF was much better than KTF, the modified organic layer might improve the heat dissipation capacity.

4.5 WLEDs performances To further investigate the potential application of modified KTF, WLED devices were fabricated based on the commercial YAG with or without as-prepared OD-KTF (0.5 %) phosphors on blue LED chips (~450 nm). Figure 7a shows the EL spectra of the two LEDs under a drive current of 300 mA, the related chromaticity parameters are listed in Table S4. WLED without red phosphor (WLED-1) exhibits cold bright white light with the Ra, R9, Tc, and luminous efficiency (LE) of 73.9, -10.1, 6560 K, and 127.8 lm/W, respectively. After addition of OD-KTF (0.5 %) (WLED-2), the sharp red emission can be observed from the spectra, WLED-2 shows warm bright white light with the Ra, R9, Tc, and LE of 87.3, 80.6, 2736 K, and 100.6 lm/W, respectively. Commission Internationale de L'Eclairage (CIE) chromaticity coordinates of WLED-1 (0.3131, 0.3192) and WLED-2 (0.4581, 0.4121) are shown in Figure 7b, both of them located closely to the black body radiation curve in CIE 1931 color spaces. WLEDs were further measured under different currents to investigate the high power WLED application. With increasing current from 60 to 300 mA, the LE of WLED-1 and WLED-2 gradually falls together from 173.7 to 127.8 lm/W, and 139.5 to 100.6 lm/W, separately (Figure 7c). Moreover, as the current is improved, Ra value of WLED-2 shows a slight drop, and CCT does not change significantly (Table S4). Besides, temperature dependent photoelectric characteristics of WLEDs were measured under extreme conditions to verify the reliability of phosphors (Table S5). Here, temperature dependent CCT of WLED was easy to evaluate the thermal stability of the added phosphors. As shows in (Figure 7d), Tc of WLED-1 was increased significantly with a rise of temperature because the thermal quenching effect of YAG. However, Tc of WLED-2 was decreased gradually due to relative 13



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increased the red component in the spectra. The increased absorption of the excitation light by OD-KTF (0.5 %) phosphor makes the red emission of OD-KTF (0.5 %) increased, which devote more considerable contribution to the LE, even though the yellow emission of YAG declined.21 This result is consistent with the temperature dependent luminescence properties. The package results indicate that the modified red phosphor OD-KTF (0.5%) is a promising candidate for improving the color rendering performance of the high power warm WLEDs.

Conclusion SCAs with long chain alkyl were successfully modified on the KTF surface by grafting with the existence hydroxy groups through a facile and mild hydrophobic modification procedure.

The

moisture

resistance

performances

of

the

phosphors

have

been significantly improved after modification, the water CA of KTF were obviously increased from near zero degree to 36.8°, 52.7°, 147.9° and 155.4° after modified by OTMS, DTMS, HDTMS and ODTMS, respectively. The SCAs with long chain alkyl can significantly decrease surface tension of KTF, and obviously improve the moisture resistance of the phosphor. The moisture resistance of modified KTF was much better than KTF, either in water or in harsh conditions of HTHH environment. The thermal stability of modified KTF, especially the PL intensity and QE were exhibited slightly better than KTF at the same temperature. Using the modified KTF phosphor as red light component, warm white LEDs with lower color temperature (2736 K), higher color rendering index (Ra = 87.3, R9 = 80.6), and high LE (100.6 lm/W, 300mA) can be easily achieved. These results indicated that super-hydrophobic surface modification with silanes can be applied for enhancing moisture resistance of fluoride phosphors, showing high promoting use in WLED applications.

ASSOCIATED CONTENT Supporting Information XRD patterns and FTIR spectra of KTF phosphors with or without modification of different SCAs (0.5 %) (Figure S1). SEM images and particle size distribution of KTF and KTF modified by different amount of ODTMS (Figure S2). The time-dependent relative LE of KSF and OD-KSF (0.5 %) soaked in water and images of water droplet on KSF and OD-KSF 14


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(0.5 %) (Figure S3). Temperature-dependent emission spectra, PL decay curves of KTF and OD-KTF (5 %) phosphors (Figure S4). ICP results of phosphors samples (Table S1). Absolute and relative of Abs, EQE, and IQE of KTF and KTF modified by various SCAs phosphors (Table S2). Temperature-dependent RAbs, REQE, and RIQE of KTF and OD-KTF (0.5 %) (Table S3). Important photoelectric parameters of WLEDs under different drive current and temperature (Table S3, S4). The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author: Qinyuan Zhang E-mail: [email protected]

ACKNOWLEDGMENTS This research was financially supported by the National Science Foundation of China (Grant Nos. 51472088, 51602104, and U1601205) and Fundamental Research Funds for the Central Universities (No. D2174710).

15



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Figure 1. (a) Synthesis diagram of KTF red phosphor via a cation exchange method; (b) grafting diagram of alkyl trimethoxy-silane on KTF surfaces.

21



Figure 2. (a) X-ray diffraction patterns, (b) FTIR spectra of KTF phosphors modified with different amount of ODTMS; EDS mapping images of (c) KTF and (d) OD-KTF (0.5 %) phosphors; TEM images of KTF modified by different amount of ODTMS, (a) 0.00 %, (b) 0.25 %, (c) 0.50 %, (d) 0.75 %, and (e) 1.00 %.

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Figure 3. (a) XPS spectra and high resolution XPS of (b) C 1s, (c) Si 2p of KTF and OD-KTF (0.5 %).

23



Figure 4. (a) Images of a drop of water onto the modified KTF phosphors with different; (b) CA of KTF phosphors with or without modification of 0.5 % SCA solution; (c) Relative luminous efficiency of KTF and modified KTF (0.5 %) in water of 2h; (d) Relative luminous efficiency of KTF and OD-KTF (0.5 %) under a HTHH environment over the time for 240h.

24


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Figure 5. (a) Excitation and (b) emission spectra, (c) luminescence decay curves, and (d) relative PL and lifetime intensity of KTF phosphors modified with various of silane (0.5 %).

25



Figure 6. (a) Relative PL intensity, (b) lifetime, (c) absorbance, and (d) QE of KTF and OD-KTF (0.5 %) phosphors as a function of temperature.

26


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Figure 7. (a) Electroluminescence spectra (EL) spectra, (b) CIE 1931 color spaces chromaticity

coordinates,

(c)

luminous

efficiency

at

different

current,

and

(d)

temperature-dependent CCT of WLEDs based on YAG (WLED-1) and YAG & OD-KTF (0.5 %) (WLED-2).

27



TOC

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Waterproof Narrow-Band Fluoride Red Phosphor K2TiF6:Mn4+ via

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