Hydrophobic Organic Skin as a Protective Shield ... - ACS Publications

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Hydrophobic Organic Skin as a Protective Shield for MoistureSensitive Phosphor-Based Optoelectronic Devices Paulraj Arunkumar, Yoon Hwa Kim, Ha Jun Kim, Sanjith Unithrattil, and Won Bin Im* School of Materials Science and Engineering and Optoelectronics Convergence Research Center, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 61186, Republic of Korea S Supporting Information *

ABSTRACT: A moisture-stable, red-emitting fluoride phosphor with an organic hydrophobic skin is reported. A simple strategy was employed to form a metal-free, organic, passivating skin using oleic acid (OA) as a hydrophobic encapsulant via solvothermal treatment. Unlike other phosphor coatings that suffer from initial efficiency loss, the OA-passivated K2SiF6:Mn4+ (KSF-OA) phosphor exhibited the unique property of stable emission efficiency. Control of thickness and a highly transparent passivating layer helped to retain the emission efficiency of the material after encapsulation. A moisture-stable KSF-OA phosphor could be synthesized because of the exceptionally hydrophobic nature of OA and the formation of hydrogen bonds (F···H) resulting from the strong interactions between the fluorine in KSF and hydrogen in OA. The KSF-OA phosphor exhibited excellent moisture stability and maintained 85% of its emission intensity even after 450 h at high temperature (85 °C) and humidity (85%). As a proof-of-concept, this strategy was used for another moisture-sensitive SrSi2O2N2:Eu2+ phosphor which showed enhanced moisture stability, retaining 85% of emission intensity after 500 h under the same conditions. White light-emitting devices were fabricated using surface-passivated KSF and Y3Al5O12:Ce3+ which exhibited excellent color rendering index of 86, under blue LED excitation. KEYWORDS: K2SiF6:Mn4+, moisture-resistant, oleic acid, solvothermal, and red phosphor

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INTRODUCTION Phosphor-converted white light-emitting diodes (pc-WLEDs) have revolutionized artificial lighting since the development of efficient InGaN-based blue LEDs by the 2014 Nobel laureates.1 Commercial pc-WLEDs based on the combination of blue LEDs and yellow-emitting Y3Al5O12:Ce3+ (YAG:Ce3+) phosphors are efficient, but have limitations because of their poor color rendering indices (CRI < 80) due to the lack of a red spectral region. Hence, the search for red phosphors has become crucial for attaining high CRI, color saturation, and reproducibility for WLEDs. Red-emitting phosphors based on a Eu3+-activator, inorganic quantum dots (CsPbI3), and nitride phosphors (CaAlSiN3:Eu2+) are among the most promising red components for pc-WLEDs.2−4 Nevertheless, poor absorption in the blue region and chemical instability of phosphor hindered the commercialization of former two candidates, whereas the commercialized nitride phosphor suffer from low CRI due to deep red emission, where the spectral sensitivity of human eye is poor.3,5 Recently, a new class of Mn4+-doped A2MF6 (A = Li, Na, and K; M = Si and Ti) fluoride phosphors has attracted increasing attention as red components because of their excellent optical properties, including narrow-band emission within the spectral sensitivity of human eye, high luminous efficiency, and excellent color rendering indices (80−86).6−8 Despite their exceptional luminescent properties, sensitivity to atmospheric moisture has © 2017 American Chemical Society

limited their applications. After long-term use under LED operating conditions of high temperature (85 °C) and humidity (85%), these fluoride phosphors turn brown because of the hydrolysis of MnF62− ions to hydrated MnO2, resulting in significant deterioration of their brightness and luminous efficiencies. 7 Fluoride phosphors that are resistant to moisture-induced degradation are highly desirable, as they can prolong the lifetimes of LEDs. Typically, moisture sensitive phosphors are encapsulated with hydrophobic materials to extend their lifetimes under LED operating conditions. Hydrophobic materials with high optical transparency in the visible region and controllable layer thicknesses are good at protecting moisture-sensitive phosphors from loss of quantum efficiency. Inorganic surface coatings such as oxides and metal phosphates, as well as encapsulation with glass and use of activator-free phosphor hosts significantly enhance the moisture stability of phosphors.7,9−11 Nguyen et al.,10 reported a moisture-resistant K2SiF6:Mn4+ (KSF) phosphor with an alkyl phosphate layer that used transition metals as cross-linkers. Setlur et al.,7 reported an improvement in the moisture stability of KSF phosphor through surface encapsulation with a Mn4+free phosphor host. Nevertheless, these inorganic coatings Received: November 2, 2016 Accepted: February 8, 2017 Published: February 8, 2017 7232

DOI: 10.1021/acsami.6b14012 ACS Appl. Mater. Interfaces 2017, 9, 7232−7240

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ACS Applied Materials & Interfaces

Figure 1. Structural and morphological characterization of the oleic acid-passivated K2SiF6:Mn4+ (KSF-OA) phosphor. X-ray powder diffraction patterns of (a) pristine KSF and (b) KSF-OA, alongside measured data and a profile calculated from the Rietveld refinement. SEM images of (c, d) pristine KSF and (e, f) KSF-OA. (g) TEM image of KSF-OA phosphor.

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suffer from initial loss of luminous efficiency at room temperature. Therefore, organic coatings may be considered as an alternative approach that provides precise control over coating thickness, without an initial loss of luminous efficiency. Organic polymers are reported to exhibit improved moisture resistance, but are less stable at high operating temperatures.10,12 A thermally stable organic molecule (temperature ≥200 °C) may be a promising candidate for hydrophobic coating of phosphors. Oleic acid, a fatty acid with a formula of CH3(CH2)7-CHCH(CH2)7COOH, is a high boiling point organic molecules (360 °C), which were used as surface-active agents for stabilization of quantum dots, nanocrystals, and cationic precursors. The polar (−COOH) group of oleic acid (OA) acts as a coordinating ligand, whereas the long alkyl tail (C18) serve as a hydrophobic moiety.13−15 OA is used to control particle growth, prevent nuclei agglomeration, and even bind and passivate inorganic CdSe QDs surfaces to maintain high quantum yield by minimizing surface bound electron traps.16,17 OA’s long alkyl chain, excellent optical transparency, and refractive index of 1.45 facilitate its use as a hydrophobic passivating layer for inorganic materials such as Fe2O3, upconversion NaYF4:Yb2+, Er3+ phosphors in biological applications.17−21 In this study, we demonstrate a facile method for fabricating moisture-resistant, red-emitting KSF phosphors using OA as a hydrophobic encapsulant via a solvothermal process. A homogeneous, hydrophobic passivating skin was formed on the KSF without any decrease in the initial luminous efficiency of the phosphor. The polar head (−COOH group) of the OA binds with fluorine from KSF through hydrogen bonding (F··· H) in the core. OA’s hydrophobic tail forms a shell with a thin passivating layer that shields the phosphor from atmospheric moisture, and prevents the hydrolysis of MnF62−. The OApassivated KSF phosphor exhibited excellent moisture resistance and may be a promising red component for color-stable pc-WLED lighting.

EXPERIMENTAL SECTION

Sample Preparation. Commercial KSF phosphor (denoted as KSF) without post treatment (coating) was purchased from Force4 Corporation, Gwangju, South Korea. The reference commercial KSF phosphor was purchased from Denka Company, Japan and used to evaluate the WLED device performance of KSF-OA phosphor, under high temperature (85 °C) and humidity (85%). The purchased Denka sample has a protective coating on the KSF phosphor which are sold for lighting applications. Oleic acid (Fluka, 4.3 mL, 13.5 mmol) was dissolved in 25 mL of anhydrous absolute ethanol (Daejung Chemicals, South Korea) and 1 g (4.5 mmol) of KSF powder was dispersed in the above solution via ultrasonication for 1 h. This mixture was loaded into a 100 mL hydrothermal reactor and heated at 140 °C for 6 h. After the reaction, the mixture was allowed to cool and centrifuged at 7000 rpm for 10 min. The product was washed repeatedly with ethanol to remove excess OA and dried at 70 °C for 4 h to produce OA-passivated KSF (KSF-OA) powder. A control reaction was performed to understand the effect of solvothermal (ST) activity on the emission property of KSF powders, using ethanol as solvent medium. This reaction was carried without OA and the resulting powders were denoted as solvothermally treated KSF (KSFST). Nonpolar solvents namely hexane (Sigma-Aldrich) and octadecene (Aldrich) were also used as solvent medium to monitor the formation of OA passivation layer on the KSF phosphor. Material Characterization. The phase identification of KSF, KSFOA, and KSF-ST were carried out via powder X-ray diffraction and synchrotron diffraction. X-ray diffraction results was collected using a Philips X’Pert diffractometer with Cu Kα radiation, in the range of 10° ≤ 2θ ≤ 100°, with a step size of 0.02°, at room temperature. Synchrotron diffraction was collected using radiation source with wavelength of 1.5183 Å in the range of 10° ≤ 2θ ≤ 150°, with a step size of 0.02°, at Pohang Accelerator Laboratory (PAL), South Korea. The Rietveld refinement was performed using the General Structure Analysis System (GSAS) program, with the split Pseudo-Voigt profile function.22 The particle size and morphology were analyzed via field emission scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscopy (HR-TEM). SEM images and elemental mapping were obtained using a Hitachi S-4700, and TEM images were recorded using a FEI Tecnai F20 from the Korea Basic Science Institute (KBSI), Gwangju. Surface analyses were performed 7233


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distance of 2.9000 Å. Si4+ is surrounded by six fluorine ions at a distance of 1.6890 Å. These results are similar to the previously reported values.23,24 The Mn4+ activator occupies the Si4+ site in KSF, which is responsible for efficient red emission from the activator. SEM and TEM images of different KSF samples are shown in Figure 1c−g. SEM images of KSF (Figure 1c, d) and KSF-OA (Figure 1e, f) show particle sizes of 10−20 μm with highly crystallized edges. The KSF particles are sensitive to the highenergy electron beam used for TEM measurement. This is indicated by their instability to high-energy electron beam, as reported in the literature.10 Hence, TEM images were measured using short electron beam exposure periods. The surface passivating organic layers of the KSF-OA samples appear to be about 10−20 nm thick. The sensitivity of organic layers to the electron beam hinders the precise determination of the thickness of surface passivating layer. However, TEM image of SrSi2O2N2:Eu2+-OA phosphor revealed the exact thickness of OA layer of ∼37 nm (Figure S2). Elemental mapping of KSFOA exhibits a homogeneous distribution of K, Si, and F elements on the phosphor surface with the stoichiometric atomic composition of 2:1:6, respectively (Figure S3). The Mn4+ concentration in the KSF phosphor is ∼0.25%. Optical Studies of Passivated KSF Phosphor. Photoluminescence and excitation spectra of the pristine KSF, KSFOA, and KSF-ST samples were measured and presented in Figure 2a. A narrow red emission in the 550−650 nm range was observed under λex = 450 nm due to spin-forbidden 2Eg → 4A2g transitions of the Mn4+ activator. Multiple emission peaks at ∼596, 612, 630, and 647 nm correspond to the different vibronic modes of the MnF62−octahedrons ν3 (t1u), ν6 (t2u), ν6 (t2u), and ν3 (t1u), respectively.6 The dominant emission peaks at 612, 630, and 647 nm are typical emission bands observed with Mn4+-activated phosphors.25 The excitation spectra under λem = 630 nm exhibit two broad bands at 360 and 460 nm, which are attributed to the spin-allowed transitions of 4A2g→ 4 T2g and 4A2g→ 4T1g, respectively. The emission and excitation peak positions of KSF-OA were the same as those of pristine KSF, revealing the high optical transparency of the thin OA layer in the visible region. Likewise, KSF-ST samples show the same emission and excitation spectral profiles as the pristine KSF sample. Furthermore, UV−visible spectra reveal no significant change in the absorption of KSF-OA and KSF-ST, compared to the pristine sample (Figure 2b), which is in agreement with the excitation spectra (Figure 2a). The optical transparency of the OA molecule was confirmed from UV−vis spectra. The absorption band of OA is limited to the near-UV region (below 350 nm), thus, confirming its high optical transparency in the visible region range of 400−700 nm (Figure S4). OA does not alter the optical properties of the KSF phosphor, except by acting as a hydrophobic passivating layer. The PL spectra reveal that the emission intensities of KSFOA and KSF-ST are the same as that of pristine KSF under λex = 450 nm. Internal quantum efficiencies (IQEs) were measured to confirm that the emission intensity of encapsulated KSF-OA is unchanged. The KSF-OA phosphor exhibits an IQE value of 68.1, compared to 67.9 and 67.3 for pristine KSF and KSF-ST, respectively (Table 2). These results suggest that formation of an OA-based surface passivating layer and solvothermal treatment do not change or reduce the phosphors’ emission intensities. This unchanged luminous efficiencies of surface passivated phosphors is a unique property for the OA-based hydrophobic layer. It stands in contrast to the results reported

via X-ray photoelectron spectroscopy (XPS), using a VG Multilab 2000. Fourier transform infrared (FT-IR) spectra were recorded using a spectrometer (PerkinElmer Spectrum 400). The 1H magic angle spinning NMR spectra was measured using 400 MHz Avance III HD Bruker spectrometer (Korea Basic Science Institute; KBSI, Western Seoul center, South Korea) at Larmor frequency of 400.25 MHz. The samples were spun at 15 kHz, with a time interval of 10 s between successive scans and number of scans are fixed at 64. The 19F NMR was measured using 400 MHz Avance III HD Bruker spectrometer (Cooperative Center for Research Facility; CCRF, Gwangju, South Korea) at Larmor frequency of 376.55 MHz. Optical Characterization. Photoluminescence spectra were measured using a Hitachi F-4500 spectrophotometer. Temperaturedependent fluorescent spectra were measured in the temperature range of 25−200 °C using an integrated heater, temperature controller, and thermal sensor coupled to the Hitachi F-4500 fluorescent spectrometer. The internal quantum efficiency was measured via 450 nm excitation with a xenon laser (Hamamatsu C9920−02). The PL lifetimes of the phosphors were measured using a time-resolved photoluminescence system composed of a flash xenon lamp and a Hamamatsu C10627 streak camera. The lifetimes were evaluated under pulsed laser irradiation at 450 nm. Diffuse reflectance measurements were carried out using a Hitachi U-4100-Vis/NIR spectrometer. WLED Packaging and Moisture Tests. Commercial YAG:Ce3+ phosphors (Force4 Corporation, Gwangju, South Korea), KSF phosphors (KSF, KSF-OA, and KSF-ST) and blue LED chips (λmax = 450 nm) were used to fabricate WLEDs. Phosphors were mixed with silicone resin (Dow corning OE-6630 A and B) and coated over the surfaces of the LED chips. Moisture resistance tests were performed using LEDs fabricated with a KSF-OA phosphor, and they were subjected to high temperature (85 °C) and humidity (85%) conditions during measurement of the emission intensity. SrSi2O2N2:Eu2+ phosphor (Force4 Corporation, Gwangju, South Korea) was also used to demonstrate fabrication of moisture-resistant phosphors via encapsulation with a hydrophobic OA layer.

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RESULTS AND DISCUSSION Structural Characterization of Passivated KSF Phosphor. The X-ray diffraction patterns of uncoated KSF (KSF), KSF-OA, and solvothermally treated KSF (KSF-ST) phosphors were indexed to a cubic structure using the ICDD pattern 98− 002−9407 (Figure S1). No impurities were observed in any of these samples. In addition, Rietveld refinement of X-ray diffraction for pristine KSF and KSF-OA samples was performed using Fm3m cubic structure, and is presented in Figure 1a,b. A lattice constant of 8.1519 Å is obtained for coated KSF-OA. As shown in Table 1, this is slightly larger than for the pristine KSF samples. The K+ ions in the KSF structure are 12-fold coordinated to the fluorine ions with a K−F Table 1. Structural Parameters of KSF and KSF-OA Phosphors, Calculated Using the Rietveld Refinement formula

KSF

KSF-OA

X-ray source 2θ range (degree) T (K) symmetry space group Z a = b = c (Å) V (Å3) Rp Rwp χ2

Cu Kα 15−100 295 cubic Fm3m 4 8.1358(1) 538.52(1) 5.25% 7.70% 3.20

8.1519(0) 541.72(1) 5.62% 8.96% 3.48 7234


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Figure 2. Optical properties of the KSF-OA phosphor. (a) Excitation and emission spectra, (b) Kubelka−Munk derived absorption spectra, and (c) decay time measured under 450 nm excitation and monitoring at 630 nm emission for pristine KSF, KSF-OA, and KSF-ST phosphors. Temperaturedependent emission spectra in the range of 25−200 °C, in terms of (d) emission intensity at 630 nm and (e) the integral area of the emission spectra in the range of 550−700 nm.

does not significantly alter the lifetime or PL properties of the KSF phosphor. This is an indirect verification of the stability of the luminescence efficiency after coating with a hydrophobic OA layer. Generally, a change in the luminescent lifetime is strongly correlated with a change in a phosphor’s luminescence intensity.10,27 Some instances of increasing lifetimes have been reported, which result in an increase in the emission intensity due to the suppression of surface traps which cause nonradiative transitions.28 Therefore, these results further confirm that the OA surface passivation process does not reduce the emission intensity, unlike other systems that suffer from emission loss after surface coating. The temperature-dependent emission spectra of various KSF phosphors under λex = 450 nm in the temperature range of 25− 200 °C are shown in Figure 2d and 2e. The temperature dependence of the integrated area (550−700 nm) and emission peak intensities (630 nm) suggest a considerable improvement in the thermal stability of the KSF-OA phosphor. The relative emission intensity of the KSF-OA phosphor remains at 80%, similar to pristine KSF at 200 °C. The OA-passivated phosphor clearly exhibits reasonably good thermal stability than pristine KSF due to high boiling point of OA (360 °C). Nevertheless, the thermal stability of KSF-OA phosphor is limited to 200 °C, which is attributed to the decomposition reaction and thermal

Table 2. Internal Quantum Efficiencies (IQEs) of KSF Phosphors at Room Temperature phosphor

IQE (%)

KSF KSF-OA KSF-ST

67.9 68.1 67.3

with inorganic metal oxide coatings and alkyl phosphates with metal linkers, both of which show significant losses of luminous efficiency after coating.10 The luminous efficiency was sustained via the use of a thin organic passivating layer made from a hydrophobic material chosen for its optical transparency, good thickness control, and ease of processing. The OA encapsulation may also suppress the surface trap states, which could slightly improve the IQE of KSF-OA phosphor. Generally, OA acting as a ligands effectively passivates the surface trap states of quantum dots and improves their QE by suppressing the trapassisted nonradiative recombination.26 The PL decay characteristics of different KSF samples were monitored at 630 nm under λex = 450 nm, and are shown in Figure 2c. The decay time was determined using a single exponential decay mode, and the lifetime is estimated to be 9.1, 9.0, and 8.9 ms for KSF, KSF-OA, and KSF-ST, respectively. These results indicate that the OA-based hydrophobic layer 7235


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ACS Applied Materials & Interfaces instability of organic OA layer at temperatures above 200 °C (Figure S5). Surface Analysis of the KSF-OA Phosphor. FT-IR spectra were used to confirm the presence of OA (CH3(CH2)7-CHCH(CH2)7COOH) on the surface of KSF particles, and are presented in Figure 3a. The FT-IR

formation. Metal-oleate complex could be formed through the potassium in KSF and −COO− group in OA, resulting in the −COOK bond.31,33 Hydrogen bonds may be formed between F−, with its strong partial negative charge and the proton from the −COOH group in OA. Hydrogen bonding is a relatively stronger interaction between atoms or molecules. The nature of interaction between the KSF and OA can be identified from the vibration band of FT-IR spectra. The characteristic vibration bands of metal-oleates are in the range of 1650−1510 cm−1 (precisely at 1610 and 1520 cm−1) for the bidentate coordination.31 In the KSF-OA sample, the absence of peaks in the 1650−1510 and 1410 cm−1 corresponding to the −COO− stretching, completely precludes the metal-oleate formation (−COOK). In addition, the major band at 1710 cm−1, can be assigned to the carbonyl group (CO) of OA on the surface of KSF particles.31 Moreover, KSF phosphor having a highly electronegative fluorine as anion on its crystal structure, fluorine would be the most favorable site for interaction with reactive ligands. These suggests that the hydrogen bond formation between the fluorine in KSF and hydrogen in OA might be the most favorable interaction. Therefore, the interaction between OA encapsulant and KSF phosphor may be via strong hydrogen bond formation. To further examine the chemical nature and surface composition of the KSF-OA phosphor, the XPS spectra of the C 1s, K 2p, Si 2p, and F 1s core levels were measured. The C 1s spectrum is shown in Figure 3b. XPS is a surface-sensitive technique, and can primarily probe the outermost 5−10 nm of a sample.34 The strong C 1s peak from the KSF-OA sample reveals the presence of a carbon-containing hydrophobic OA shell at 284.6 eV, which corresponds to aliphatic carbon chains (C−C).33 The absence of a C1s peak from the carboxylate group (−COOH) may be ascribed to complete encapsulation of the group in the core layer, alongside KSF. Only the shell, which is composed of long, aliphatic carbon chains from OA, is present on the surface. Moreover, a shift in the F 1s peak to a lower binding energy (0.9 eV) in the KSF-OA sample suggests that fluorine from the KSF phosphor is linked to the electropositive hydrogen in the OA carboxyl group (Figure S7). Thus, fluorine-terminated KSF may be linked to the electropositive hydrogen, through hydrogen bonding. These results further confirm the absence of metal-oleate, for which a shift in K 2p peak to a lower binding energy is expected. Therefore, it could be confirmed that OA forms a hydrophobic skin over the KSF phosphor through the formation of hydrogen bond in the core. The long alkyl chain from OA forms a hydrophobic sheath on the phosphor surface, resulting in the moisture-resistant KSF phosphor. The formation of hydrogen bond between KSF and OA encapsulant in KSF-OA was further substantiated through 1H MAS and 19F NMR spectroscopy (Figure S8). A feature of 1H MAS NMR resonance of KSF-OA, shifting downfield (7.7 ppm) compared to pristine KSF (7.5 ppm) exposes the deshielding effect of electronegative fluorine of KSF coordinated to the hydrogen nuclei of pendant − COOH group of OA, suggesting the formation of hydrogen bond (−OH···F). Similarly, the 19F NMR resonance of KSF-OA is shifted upfield to −130.15 ppm compared to that of pristine KSF (−130.25 ppm), describing the shielding effect of hydrogen of pendant −COOH of OA coordinated to the fluorine nuclei of KSF. These results further confirms the formation of intermolecular hydrogen bonding between the fluorine of KSF and hydrogen of (−COOH) of OA layer. Li et al. has reported a similar

Figure 3. Nature of bonding between KSF and the OA encapsulant. (a) FT-IR spectra of pristine KSF, KSF-OA, KSF-ST, and OA-ST. (b) XPS C 1s spectra of pristine KSF, KSF-OA, and KSF-ST phosphors.

spectra of pristine KSF, KSF-OA, and KSF-ST samples were measured in the solid form, whereas other OA samples, particularly pure OA, OA dissolved in ethanol (OA), and solvothermal-treated OA dissolved in ethanol (OA-ST) were measured in their liquid forms. All of the KSF samples exhibited typical bands at 714 and 483 cm−1, corresponding to the vibrations of Si−F bonds.29 The OA vibration modes in the KSF-OA sample were identified using the reference OA samples. The FT-IR spectra of pure OA, OA, and OA-ST were measured to monitor the influence of solvothermal treatment on OA, since the liquid changed from colorless to yellow during solvothermal reaction (Figure S6). However, there are no significant differences between the FT-IR spectra of different OA samples, thus, indicating that the chemical nature of the OA is retained even after solvothermal treatment. The only spectral difference noted is the presence of a wider −OH vibration band from carboxyl groups at 3500 cm−1 in the OA and OA-ST samples and also due to the association of −OH groups of ethanol and OA.30 This strongly refutes the possibility of side or decomposition reactions during solvothermal process. Moreover, the absence of sharp band at 3590 cm−1 in OA and OA-ST sample, corresponding to free −OH of ethanol suggests possible formation of intermolecular hydrogen bonding with OA through −OH of ethanol.31 The FT-IR spectrum of the KSF-OA sample shows prominent bands at 2927, 2857, and 1710 cm−1, corresponding to antisymmetric and symmetric stretching of the C−H bonds of long alkyl chains, and the CO bond from the carboxyl group in OA, respectively confirming the presence of OA on the phosphor surface. This is similar to the spectrum of the OAST sample.32 This indicates that the surfaces of the KSF particles are encapsulated by the OA ligands. The interaction between OA and the KSF phosphor may be presumed to occur through two possible mechanisms. Two of the mechanisms involve chemisorption: metal−oleate and hydrogen bond 7236


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K, Si, and F are in stoichiometry, within the KSF samples. In addition, a significant proportion of carbon (∼14−16%) and oxygen (∼3%) were observed even in the pristine KSF and KSF-ST, which were attributed to the absorbed CO2from the atmosphere, as reported earlier.36,37 A sharp decrease in the concentration of core elements were apparent in the KSF-OA, attributed to the presence of OA sheath on the phosphor surface. The atomic% of core elements in KSF-OA reduced up to 30, 47, and 40%, for K, Si, and F, respectively. The total sum of carbon and oxygen content in the KSF-OA also increased to 50 from 20% of the pristine KSF. These results strongly evidence the presence of sheath of OA encapsulant (∼30%) on the phosphor surface. The robustness and water tolerance of KSF-OA phosphor was investigated under harsh water condition at room temperature and presented in the Figure 4a, b. An extremely harsh condition was employed in which phosphor was dispersed in water at a phosphor concentration of 2.50 × 105 ppm, where phosphor comes in direct contact with water as shown in Figure 4a. The pristine KSF particles form a brown solution in water within 5 min. In contrast, the KSF-OA sample remains unchanged after 30 min, suggesting that OAencapsulated KSF has enhanced moisture stability. After 4 h, the orange color of the pristine KSF becomes completely

hydrogen bond formation between halide ion in the hybrid perovksite material and hydrogen in the phosphonic acid crosslinker, for solar cell applications.35 An improved stability of perovskite solar cells was attained due to the strong hydrogen bonding and effective passivation of perovskite surface, rendering immunity to moisture which proved to be beneficial for the long-term stability of solar cells. This further corroborates the possibility of hydrogen bond formation in the current work, between hydrogen of carboxylic acid in OA and fluorine in KSF phosphor rendering excellent moisture stability to the KSF-OA-based optoelectronic devices. The surface elemental composition was also calculated by integrating the XPS peaks to estimate the content of OA encapsulant on the phosphor surface, and the result is given in Table 3. The atomic compositions of the core elements namely Table 3. Surface Elemental Compositions, As Calculated from XPS Results surface composition (atomic %) sample

K 2p

Si 2p

F 1s

C 1s

O 1s

KSF KSF-OA KSF-ST

14.8 9.7 13.7

10.7 5.7 10.7

54.7 32.4 57.8

16.1 41.8 14.1

3.8 10.5 3.7

Figure 4. Moisture stability of the KSF-OA phosphor. (a) Images of pristine KSF and the KSF-OA phosphor in deionized water at a concentration of 2.50 × 105 ppm, over 4 h. (b) Emission spectra of pristine KSF and KSF-OA samples in deionization water at a concentration of 6.25 × 103 ppm after 15 days. Inset shows the image of the KSF and KSF-OA samples in the deionized water. A brown colored MnO2 was formed in the KSF sample due to hydrolysis of MnF62− in aqueous medium, while orange color of KSF powders are retained for KSF-OA sample even after 15 days. (c) Images of pristine KSF, KSF-OA, and KSF-ST phosphors under visible (top layer) and blue light excitation (bottom layer). (d) Schematic illustration of the nature of bonding between KSF and OA in the KSF-OA phosphor. The core elements, namely K, Si, and F from the KSF phosphor and shell elements (C, O, and H) of the OA encapsulant are represented as pale blue, red, royal blue, gray, red, and white spheres, respectively. The KSF phosphor and OA encapsulant exhibit hydrogen bonding (F···H) between the fluorine in KSF and the hydrogen atom of the carboxyl group in OA. 7237


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ACS Applied Materials & Interfaces brown due to hydrolysis of MnF62− to MnO2, whereas only 25% of the KSF-OA sample becomes brown. PL after 4h in deionized water was measured to further monitor the water stability of KSF-OA sample in comparison with pristine KSF (Figure S9). The pristine KSF exhibited only 4% of the emission intensity with respect to KSF-OA sample after 4 h in deionized water. This shows that the KSF-OA sample is highly water-tolerant, because of the presence of a hydrophobic, passivating OA sheath over the KSF phosphor. Therefore, OA surface passivation could be a promising approach for obtaining hydrophobic coating of phosphors. Water tolerance test for pristine KSF and KSF-OA samples were conducted by measuring PL in water after 15 days, using a phosphor concentration of 6.25 × 103 ppm as shown in Figure 4b. After 15 days, the red emission of the KSF-OA sample still remained. The retention of orange color of the encapsulated phosphor reveals its excellent moisture resistance. In contrast, no emission is observed for pristine KSF because of the complete hydrolysis of Mn4+ to hydrated MnO2, which precipitates as a brown powder. These results suggest that the KSF-OA phosphor with a hydrophobic, passivating OA skin exhibits good stability in water. Images of KSF and KSF-OA samples under visible and blue light excitation (λex = 450 nm) are shown in Figure 4c. The orange KSF samples exhibit a strong red emission under blue light excitation. The ionic conductivity were measured for the pristine KSF and KSF-OA in deionized water to monitor the change in conductivity with respect to time (Table S1). A large increase in the ionic conductivity (28% increase, 40 mS/m) was detected for uncoated KSF sample, compared to the negligible increase in the ionic conductivity value (∼1% increase, 2 mS/m) for KSFOA. These results strongly suggest the effective passivation of KSF phosphor by the hydrophobic OA layer from the surrounding moisture. A schematic illustration of the nature of the interaction between the core KSF phosphor and the OA passivating shell is presented in Figure 4d. Oleic acid is an excellent organic hydrophobic material with a high boiling point and the formula CH3(CH2)7-CHCH(CH2)7COOH, where COOH is the polar headgroup. The remaining long alkyl chain behaves as a strongly hydrophobic tail. The polar KSF molecule, which contains fluorine-terminated moieties, interacts with the polar headgroup (−COOH) of OA through the formation of F···H via hydrogen bonding. This has been confirmed via FT-IR and XPS (Figure 4a, b). The core undergoes polar interactions between fluorine and hydrogen, forming F···H bonds. The shell, which is composed of long alkyl chains, provides strong resistance to atmospheric moisture. In addition, the bending configuration of the alkyl chain of the cis-form of OA ensures complete encapsulation of KSF particles with a hydrophobic, passivating skin. These results have implications for strong interactions between the KSF phosphor and the organic passivating OA skin that enhances the moisture stability of the phosphors. The OA encapsulation of KSF phosphor were also carried out by simple mixing of OA and KSF without solvent, and in the presence of ethanol as solvent. The resulting KSF powder exhibit moisture instability owing to improper adherence of OA on the phosphor surface. It is to be noted that a sticky powder was obtained because of the presence of large amount of OA on the phosphor surface, making it unsuitable for commercial applications. Hence, extensive ethanol washing were employed to remove excess OA, for obtaining a free-flowing KSF powder.

In addition, OA encapsulation was carried out under refluxing condition, also exhibited poor moisture stability caused by nonuniform encapsulation. An homogeneous OA encapsulation layer was achieved only through the solvothermal method in ethanol medium. This suggests that solvothermal treatment is necessary for the formation of homogeneous, thin hydrophobic OA layer, which could improve the moisture stability of KSF phosphor. Moreover, polar solvent especially ethanol was identified as an ideal solvent medium for initiating an interaction between the polar group of OA (−COOH) and KSF (F− anion). The use of nonpolar solvents such as hexane and octadecene in solvothermal condition, suffered serious loss in the initial emission intensity due to thick formation of OA layer (not shown). Therefore, solvothermal processing using ethanol medium is mandatory for the fabrication of moistureresistant KSF phosphor with a thin hydrophobic, passivating OA layer. WLED Fabrication. The electroluminescence (EL) spectra of WLEDs made using red-emitting KSF-OA and yellowemitting YAG:Ce3+ under blue LED light (λmax = 450 nm) are presented in Figure 5a. The WLEDs cover the entire visible spectrum, including a white emission which lies within the sensitivity of the human eye (