Activated Phase-Pure Oxonitridosilicate Phosphor in a Ba–Si–O–N

Aug 12, 2016 - O−N System via Facile Silicate-Assisted Routes Designed by First-. Principles Thermodynamic Simulation. Young Jun Yun,. †. Jin Kyu ...
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Eu2+-Activated Phase-Pure Oxonitridosilicate Phosphor in a Ba−Si− O−N System via Facile Silicate-Assisted Routes Designed by FirstPrinciples Thermodynamic Simulation Young Jun Yun,† Jin Kyu Kim,‡ Ji Young Ju,‡ Seul Ki Choi,‡ Woon Ik Park,§ Ha-kyun Jung,‡ Yongseon Kim,*,∥,⊥ and Sungho Choi*,‡,⊥ †

Materials & Components Research Institute/Convergence Composite Materials Team, Korea Testing & Research Institute, 98 Gyoyukwon-ro, Gwacheon, Gyeonggi-do, 13810, Republic of Korea ‡ Advanced Battery Materials Research Group, Korea Research Institute of Chemical Technology, 141 Gajeongro, Yuseong, Daejeon, 34114, Republic of Korea § Electronic Convergence Materials Division, Korean Institute of Ceramic Engineering and Technology, 101 Soho-ro, Jinju, Gyeongsangnam-do, 52851, Republic of Korea ∥ Department of Materials Science and Engineering, Inha University, Incheon, 22212, Republic of Korea S Supporting Information *

ABSTRACT: Eu2+-activated single phase Ba2+-oxonitridosilicate phosphors were prepared under a mild synthetic condition via silicate precursors, and their luminescent properties were investigated. Both the preferred oxonitridosilicate formation as for the available host compounds and thermodynamic stability within the Ba−Si−O−N system were elucidated in detail by the theoretical simulation based on the first-principles density functional theory. Those results can visualize the optimum synthetic conditions for Eu2+-activated highly luminescent Ba2+-oxonitridosilicates, especially Ba3Si6O12N2, as promising conversion phosphors for white LEDs, including Ba3Si6O9N4 and BaSi2O2N2 phases. To prove the simulated design rule, we synthesized the Ba3Si6O12N2:Eu2+ phosphor using various silicate precursors, Ba2Si4O10, Ba2Si3O8, and BaSiO3, in a carbothermal reduction ambient and finally succeeded in obtaining a phase of pure highly luminescent oxonitridosilicate phosphor without using any solid-state nitride addition and/or high pressure synthetic procedures. Our study provides useful guidelines for robust synthetic procedures for developing thermally stable rare-earth-ion activated oxonitridosilicate phosphors and an established simulation method that can be effectively applied to other multigas systems.

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Ca),7,8 M2Si5N8 (M = Ba, Sr, Ca),9 Ba3Si6O12N2,10−12 and SrSiAlON,13,14 have demonstrated remarkable potential capacity as host materials for phosphors used in pc-LEDs due to their excellent luminescent properties and high stabilities induced by the large crystal field splitting and nephelauxetic effect due to the coordinating nitrogen ions around the activated centers.15,16 Ba2+-oxonitridosilicates have been studied for many years. In the Ba−Si−O−N system, Ba 3Si6O12N 2, BaSi2O 2N2, and Ba3Si6O9N4 compounds have been suggested as promising pcWLEDs host materials.17−20 These compounds, containing the same metal element ratio (Ba/Si = 0.5) with different O/Ns, consist of a silicate network formed by a corner-sharing SiO4−xNx tetrahedron. Furthermore, due to the slightly different polymorphic crystal structures, these compounds are formed in a relatively narrow temperature range of 1300−1400 °C, so one or two compounds can be easily formed as byproducts in the synthetic procedure with an aimed compound; Ba3Si6O12N2 has

INTRODUCTION

For the recently developed lighting technology, solid-state lighting is one of the most promising light sources compared to the conventional gas-discharged fluorescent lamps: high energy efficiency with a desirable color rendering index, a long lifetime, compactness, and environmental friendliness.1,2 To get an efficient white-illuminating solid-state lighting, two types have been proposed for phosphor converted-white light emitting diodes (pc-WLEDs). One is to combine an InGaN-based blue light emitting diode overcoated with a yellow-emitting Y3Al5O12:Ce3+ (YAG:Ce3+) phosphor. The other is to use mixed red/green/blue tricolor phosphors combined with ultraviolet LEDs. This type of tricolor phosphor has the advantages of a higher color rendering index and higher thermal stability, which produce efficient WLEDs with a suitable color temperature.3,4 However, the conventional oxide phosphors have some inherent drawbacks, such as short excitation and emission wavelength and low stabilities. During the past decade, rare-earth-doped (oxo)nitridosilicate phosphors, such as α/β-SiAlON5,6, MSi2O2N2 (M = Ba, Sr, © XXXX American Chemical Society

Received: May 31, 2016

A

DOI: 10.1021/acs.inorgchem.6b01278 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry the lowest formation temperature (∼1300 °C) compared with other oxonitridosilicates, 1350−1400 °C for BaSi2O2N2 and ∼1370 °C for Ba3Si6O9N4. Thus, a phase-pure rare-earthactivated Ba2+-oxonitridosilicate phosphor was rarely synthesized in previous studies. Green-emitting Ba3Si6O12N2:Eu2+ phosphors have been attracting much attention due to their narrow emission spectrum features and potential application in developing WLEDs.10−12 Up to now, the research on Ba3Si6O12N2:Eu2+ has focused on the effect of substitutional cations of the host lattice, and synthetic methods requiring high temperature/pressure with compulsory use of nitride materials like as Si3N4.11,12,21 Tang et al. reported luminescence properties that are sensitive to the cation exchange of the host lattice. Those results could also be used to explain how Ba2+ atoms are superior to other alkaline atoms in many oxonitridosilicate phosphors.21 Wang et al. suggested the effect of Mg2+ doping on the crystal structure, followed by the photoluminescence behavior of (Ba,Mg)3Si6O12N2:Eu2+. Unfortunately, synthesizing a rare-earth-ion doped Ba3Si6O12N2 phosphor often encounters problems with phase purity with orthosilicates formed as impurity phases that significantly decrease the thermal stability of the resultant oxonitridosilicate phosphor. Additionally, the overall synthesis of the given nitride compounds usually requires harsh process conditions, high temperature above 1800 °C, and a pressurized inert gas ambient. Therefore, it is critical to develop cost-effective procedures for rare-earth-ion doped (oxo)nitridosilicate phosphors that can be synthesized under ambient conditions with less thermal budget. In this work, we report the phase formation and luminescent properties of Eu2+-activated green-emitting Ba2+-oxonitridosilicates prepared with various silicates. As far as we know, there have been no reports on the formation of a single phase Ba−Si− O−N:Eu2+ phosphor without using any nitride raw materials under ambient conditions, followed by a systematic analysis based on the first-principles thermodynamic simulation. Both the oxynitrides formation mechanism and the thermodynamic stabilities of the Eu2+-activated silicate/oxynitride phases in the Ba−Si−O−N system were elucidated in detail by theoretical calculations based on the first-principles density functional theory (DFT). We demonstrate that the formation of a phasepure Ba3Si6O12N2:Eu2+ phosphor was observed by firing under a concentrated N2 ambient using (Ba1−xEux)Si2O5 silicates as a precursor, rather than mixing nitrides with the raw materials. Finally, the thermal stability and chip-in-package luminescence against the thermal/hydrolytic resistance of the given Ba3Si6O12N2:Eu2+ phosphor were performed with near-UVpumped LEDs.

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Characterization. The crystal structure of the prepared phosphors was identified by X-ray powder diffraction (XRD) with Cu Kα radiation. The measurement of the photoluminescence spectra for the phosphors was performed using a PSI photoluminescence system equipped with an Xe lamp. Additionally, thermal quenching and activation energy were measured using a heating apparatus, and the variation of the measurement temperature from room temperature to 180 °C was obtained using a photomultiplier optical fiber equipped thermostatic chamber. For the pc-WLED fabrication, the as-prepared phosphors were thoroughly mixed with the commercial silicone encapsulants (EG6301 A, B, Dowhitech Co.), and then the paste was mounted on the near-UV LED chip (purchased from SemiLEDs, λem = 395−400 nm). The pastecovered LED chip was dried in an oven at 150 °C for 1 h. A phase diagram of the Ba−Si−O−N quaternary system was simulated as a function of temperature and atmosphere based on the DFT calculations. The grand potential of the compounds with a general chemical formula of BaaSibOcNd, which is expressed as eq 1, was used as the measure of thermodynamic stability:22−24 d

Φ̅ (BaaSibOc Nd) =

G(T , P , BaaSibOc Nd) − 2 μO

2

a+b+d

(1)

For each phase, the composition and grand potential make a unique coordination in a 3-D space. That is, every composition is located in a Ba−Si−N ternary triangle on the x−y plane, and the grand potential on the z axis. Taking vertices and edges that form a convex hull25 to the −z direction and projecting them on the x−y plane, ternary phase diagrams of Ba−Si−N were obtained.24,26 The chemical potential of oxygen was included in the calculation of the grand potential as expressed in eq 1; thus, the Ba−Si−N ternary phase diagram of the Ba−Si−N−O quaternary system could be constructed. The Gibbs free energy of eq 1 was approximated with the energy at 0 K provided by the DFT calculation. The standard state chemical potential of oxygen and nitrogen was determined in a semiempirical way so that they minimized the discrepancy between the experimentally reported formation energy of the solid phases in the Ba−Si−N−O system and those obtained from the DFT calculation.24,27 Considering that the error of theoretical calculation is thus compensated for at 298 K with determination of the proper chemical potential of the gas phases, the simulation of phase equilibria using the 0 K energy is expected to provide reasonable data even at high temperatures because the change of the Gibbs free energy with temperature does not seem to be significantly different among the solid phases according to the Dulong−Petit law.28 For gas phases, the effects of temperature and pressure on the chemical potential were accommodated by referring to the JANAF Thermochemical Table.29 The energies of the phases reported to comprise the Ba−Si−O−N system were calculated using the DFT, based on the Perdew−Burke− Ernzerhof generalized gradient approximation and the projectoraugmented plane-wave pseudopotentials:30,31 QUANTUM-ESPRESSO32 code was used to apply kjpaw-type pseudopotentials with 40 Ry of kinetic energy cutoff for wave functions and 10−3 Ry/Bohr of the convergence threshold on forces for ionic minimization. The number of k-points was set as the nk ≈ 15 Å/lattice parameter for each lattice vector, and the equivalent density of k-points was applied for different phases. Full relaxation of atomic positions and lattice vectors was allowed during the DFT calculation.

EXPERIMENTAL SECTION

Sample Preparation. Ba2+-oxynitride phosphor samples were prepared using various silicates as a precursor: (Ba1−xEux)Si2O5, (Ba1−xEux)2Si3O8, and (Ba1−xEux)SiO3. Reagent grade BaCO3 (99%, High Purity Chemicals), SiO2 (99.9%, High Purity Chemicals), and Eu2O3 (99.99%, Rare Earth Co.) were used to synthesize the corresponding silicates without any further purification. The raw materials were weighed in the required amounts and thoroughly ground with acetone for mixing. The entire mixture was dried and fired at 600 °C for 1 h in air and 1200 °C for 5 h in 5% H2, respectively. The prefired samples were cooled, thoroughly pulverized, and finally put in an alumina crucible with overcoated graphite powder. Then, the samples were sintered at 1300 °C for more than 10 h under a flow of nitrogen containing 5% of H2. For reference, the given silicates were ground and mixed with Si3N4 (99.9%, Sigma-Aldrich) in a stoichiometric ratio and then fired again at 1200−1400 °C for 10 h in the same reducing gas flow.

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RESULTS AND DISCUSSION XRD analysis of the Eu2+-doped Ba2+-oxonitridosilicates was performed. First, we found the promising rare-earth-ion activated host compounds in the silica-based ternary systems, BaO−SiO2− Si3N4. Both the silicates and oxonitridosilicates of the given phase system have been described previously as shown in Figure S1.33 Various silicate polymorphs and BaSi2O xN y phases are thermodynamically stable with the addition of Si3N4, which illuminates the tunable emission in the blush-green to yellow color under UV excitation.11,18,20,34−36 B

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Inorganic Chemistry We first checked the degree of phase formation of Eu2+-doped Ba3Si6O12N2 compound. As shown in Figure 1, the XRD patterns

Figure 2. (a) Excitation and emission spectra of Eu2+-doped Ba2Si4O10 (blue), Ba3Si6O12N2 prepared by the conventional method (red), and Ba3Si6O12N2 using Ba2Si4O10 precursor (black), respectively. (b) CIE chromaticity diagram and illuminating images under excited at 365 nm of the corresponding samples.

(CIE) coordinates and illuminating images of the given phosphors (Figure 2a) under 365 nm excitation. The two Ba3Si6O12N2:Eu2+ phosphors appear considerably different from the silicate phosphor used as a precursor, while the silicateassisted oxynitride phosphor exhibits a much greener color emission under the given excitation conditions with the CIE value (0.285, 0.640). Thus, it could be applicable to the supplemental luminescent materials for the typical YAG:Ce3+based WLEDs solid-state lighting. To confirm our preliminary results, we checked the phase formation of Ba3Si6O12N2:Eu2+ with various synthetic conditions using only the Ba2Si4O10:Eu2+ precursor in N2 gas with a carbothermal reduction ambient. Figure S2 shows that the single phase Ba3Si6O12N2 synthesized using the Ba2Si4O10 as a precursor was obtained with a longer firing time (>24 h), while that of the sample conventionally mixed with BaCO3, SiO2, and Eu2O3 raw materials had some subsidiary phases. Note that there were also corresponding PLE and PL spectra. From a chemical point of view, the oxygen-rich Ba3Si6O12N2 phase seems to be easily formed compared to the other Ba-oxynitrides, such as Ba3Si6O9N4 and BaSi2O2N2, with less nitride addition. Nevertheless, the nitride raw materials inevitably form the phase-pure oxynitride phase. Using the simulated thermodynamic calculations, those results will be clear from the discussion in the next section. Oxonitridosilicate phosphors illuminate a rather longer wavelength radiation ascribed to the stronger splitting of 5d orbitals with an enhanced nephelauxetic effect on rare-earth ions caused by the higher effective charges of N3− ions.41 Furthermore, the crystalline structure networks of these compounds show more complex and flexible manners owing to the presence of N atoms. Unlike the oxygen bridge that links two [Si-O] tetrahedra in oxosilicates, the nitrogen bridge in nitridosilicates may connect three or even four neighboring [Si-N] tetrahedra.42,43 Thus, as an intermediate family of compounds between oxosilicates and nitridosilicates, oxonitridosilicates are supposed to have structural diversity and excellent luminescence properties. There has been a report for the formation of a single phase Ba3Si6O12N2 compound with regard to the polymorphic barium oxosilicates, Ba5Si8O21, BaSi2O5, and BaSiO3, respectively.37 That finding can be easily accepted because the overall crystal structures of those Ba−Si−O(−N) compounds are similar but have different oxygen/nitrogen coordination states: corner-sharing [SiO3N] tetrahedra for Ba3Si6O12N2 and corner-sharing [SiO4] tetrahedra for BaSi2O5

Figure 1. XRD patterns of Eu2+-doped Ba2Si4O10 and Ba3Si6O12N2 samples prepared by the given synthesis conditions.

of Eu2+-doped silicate (used for oxonitridosilicates precursor hereafter) and Ba3Si6O12N2 prepared with or without Si3N4 were indexed to the standard diffraction patterns of Ba2Si4O10 (JCPDS 01-083-1445) and Ba3Si6O12N2 (JCPDS 97-042-1322). As expected, even with some unidentified phases, both Ba3Si6O12N2 forms with the addition of Si3N4 and Ba2Si4O10 compounds matched the corresponding phases with regard to the given phase diagram. Interestingly, however, the formation of phase-pure Ba3Si6O12N2 was fully conducted using the Ba2Si4O10:Eu2+ precursor in a N2 gas ambient (without any Si3N4 addition). Furthermore, the crystallinity and phase purity of the silicateassisted synthesized Ba3Si6O12N2 compounds were significantly enhanced over the conventionally prepared one using the solidstate reaction with the Si3N4 mixture. There are some reports regarding the Ba 2+ -oxonitridosilicates, BaSi 2 O 2 N 2 and Ba3Si6O12N2, prepared via a method using silicates as a precursor.33,37 However, the addition of Si3N4 in the subsequent firing was inevitable and understandable silicate composition concerning the resultant oxonitridosilicates is ambiguous to support the luminescence and phase formation behavior. The excitation and emission spectra of the corresponding Eu2+-doped Ba2Si4O10 and Ba3Si6O12N2 phosphors are presented in Figure 2a. For Ba3Si6O12N2:Eu2+, the overall excitation spectra had a very broad excitation band with the right tail extending to 425 nm, which absorbs UV or near-blue light strongly and matches well with the blue LEDs. Under 310, 360, or 390 nm excitation, the emission spectra are all symmetric bands centered at 525 nm, which is attributed to the transition from 4f65d1 to the 4f7 levels of the Eu2+ occupying the unique Ba2+ sites.37−39 For comparison, we also denoted the luminescence spectra of the Ba2Si4O10:Eu2+ with a broader and shorter wavelength shifted bluish-green emission bands, which also well matched the emission property of Eu2+-activated orthorhombic BaSi2O5.40 The emission intensity of the phosphor prepared by the silicateassisted method was about 2 times higher than that of the one synthesized by the solid-state reaction with Si3N4 addition. Thus, the higher PL intensity in the phosphor prepared by the precursor method is related to the phase-pure Ba3Si6O12N2 formation with high crystallinity (Figure 1). Figure 2b shows the corresponding Commission Internationale de l’Eclairage C

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Inorganic Chemistry

control of oxygen partial pressure, if possible. The simulation result showed good agreement with our experimental result: we succeeded in synthesizing the Ba3Si6O12N2:Eu2+ phosphor at 1300 °C (1573 K) with carbothermal reduction and a nitridation process, whereas Ba3Si6O9N4:Eu2+ was not obtained. The simulation result may seem to be unreliable considering that oxynitride phases were synthesized at 1400−1500 °C with hydrogen reduction in some studies,11,39 but those studies used Si3N4 as the starting material. As presented in Figure 4a, Si3N4

as shown in Figure S3. In this regard, Li et al. suggested the “dissolution−diffusion−precipitation” multistep phase transformation from orthosilicate to Ba3Si6O12N2. The orthosilicates first melted with an increasing temperature and then dissolved thermally stable Si3N4 into the glassy Ba−Si−O phase, and finally the Ba3Si6O12N2 phase precipitates from the nitrogen saturated liquid phase accompanying the residual barium orthosilicates.11 Thus, phase-pure Ba3Si6O12N2 can be only achieved by the careful control of the Si/Ba and/or O/Ba molar ratios of silicates through Si3N4 addition, which is substantially different from what we found. To clearly understand the transformation behavior and to propose sophisticated synthetic procedures, we conducted a firstprinciples thermodynamic simulation of silicate-assisted Ba3Si6O12N2 phosphor formation (especially using Ba2Si4O10 under gaseous N2 ambients) under various synthetic conditions. The simulation results of phase diagrams for the Ba−Si−O−N system are presented in Figure 3. It appeared that nitrides or

Figure 4. Schematic presentation of synthetic route of Ba3Si6O12N2: (a) at the condition that oxygen partial pressure is not low enough for stabilization of Ba3Si6O12N2 phase ((A) and (B) indicate the thermodynamic and kinetic routes, respectively), (b) at the condition Ba3Si6O12N2 is thermodynamically stable.

becomes unstable with an increase in temperature in a hydrogen atmosphere and tends to be oxidized into SiO2. Therefore, when the firing process starts with Si3N4 and an oxide of Ba-Si as the raw materials, Si3N4 would be oxidized and react with the Ba-Si oxide, resulting in an increase of Si content in the oxide (route (A) in Figure 4a). This reaction would finish with formation of Ba2Si4O10 if the Ba/Si ratio is set at 1/2 in the overall composition. However, this is a pure thermodynamic viewpoint: If it is assumed that a direct reaction between Si3N4 and Ba-Si oxide is favorable by some kinetic reason, the route (B) in Figure 4a may not be ignored; thus, oxynitride phases that are thermodynamically unstable may be synthesized. Thus, we suggest that synthesis of oxynitrides from Si3N4 under hydrogen conditions is explained by the kinetic route of reaction, and it is expected that the number of impure oxide phases would change according to the relative strength of the thermodynamic/kinetic driving forces. When the temperature and oxygen partial pressure reach conditions appropriate for oxynitride phases to become stable as presented in Figure 4b, the kinetic route (B) of Figure 4a then acts as a thermodynamically favorable reaction route and formation of oxynitrides is promoted (solid arrow in Figure 4b). Oxide phases formed through route (A) of Figure 4a may also be gradually converted to oxynitrides, provided there is sufficient reaction time for reduction and nitridation (dotted line arrow in Figure 4b). This strong reducing condition is not expected to be achieved under a hydrogen atmosphere; therefore, oxynitrides may only be able to be obtained from a kinetic route, as in Figure 4a, mixed with oxide impurities without carbon. In this study, we considered the synthesis of the Ba 3 Si 6 O 1 2 N 2 :Eu 2 + phosphor using (Ba 1 − x Eu x )Si 2 O 5 , (Ba1−xEux)2Si3O8, and (Ba1−xEux)SiO3 as an intermediate precursor, which were synthesized in advance. They were mixed with SiO2 as necessary and fired under carbothermal reduction and nitridation. The synthesis mechanism can be interpreted from Figure 5: During the temperature rising period,

Figure 3. Phase diagrams of Ba−Si−O−N system simulated by firstprinciples DFT calculations: (a) 1300−1800 K under hydrogen atmosphere, (b, c) 1600 and 1800 K under carbothermal reducing atmosphere, (d) 1600 K under carbothermal reduction, but the oxygen partial pressure is assumed to be increased by 3% from the equilibrium condition.

oxynitrides of Ba-Si are not thermodynamically stable with a reducing condition controlled by hydrogen (Figure 3a). On the other hand, some oxynitride phases such as Ba3Si6O12N2 and Ba3Si6O9N4 are expected to be obtained at over 1600 K with an equilibrium oxygen partial pressure of carbothermal reduction (Figure 3b,c). However, the oxygen partial pressure may not be able to be controlled as low as that of the thermodynamic equilibrium condition in actual experiments due to insufficient oxidation of carbon, slight leakage of air, local inhomogeneity of the atmosphere, etc. Considering these experimental limitations, we modified the oxygen partial pressure to be slightly (3−5%) higher than that of the equilibrium condition at 1600 K and simulated the phase diagram (Figure 3d). Only the Ba3Si6O12N2 phase appeared stable on the phase diagram with this modification, indicating that Ba3Si6O9N4 would be difficult to synthesize by the general carbothermal reduction process, requiring high enough temperatures far over 1600 K with strict D

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Figure 5. Schematic presentation of the synthetic process of Ba3Si6O12N2 using barium silicate precursors: (a) during the temperature rising period, (b) at high enough temperature for the stabilization of the Ba3Si6O12N2 phase.

only oxides are stable because the temperature is not high enough for formation of oxynitrides, so the reaction proceeds to the final destination of the reaction, that is, toward for formation of Ba2Si4O10 when the Ba/Si ratio is set at 1/2 as presented in Figure 5a. Provided that the Ba2Si4O10 phase is formed and the temperature reaches a high enough temperature for the stabilization of the Ba3Si6O12N2 phase, the phase transition proceeds from oxide to oxynitride along the Ba2Si4O10 − Ba3Si6O12N2 tie line (Figure 5b). If the intermittent Ba2Si4O10 phase is not fully synthesized, which may occur when the heating rate is not slow enough or there is not sufficient reaction time for the above-mentioned reaction, the final product is expected to be a mixture of Ba3Si6O12N2 and impure phases such as SiO2, Ba2Si3O8, and BaSiO3. In addition, the possibility that the alternative reaction route of the remaining SiO2 to be reduced into Si2ON2 or Si3N4 emerges and delays synthesis of Ba3Si6O12N2 may not be excluded. In summary, the most efficient way for synthesis of Ba3Si6O12N2 using oxide raw materials is the route of [SiO2 + Ba-Si oxide] → [Ba2Si4O10] → [Ba3Si6O12N2]. Our simulation results suggests that increasing the degree of completion of Ba2Si4O10 formation may be the key factor in the synthesis of the Ba3Si6O12N2 phase. This conclusion of the simulation study is supported by the experimental results in Figure 6 and Figure S2: The crystal and luminescence quality of Ba3Si6O12N2:Eu2+ phosphor was better with the (Ba1−xEux)Si2O5-assisted method and longer firing time. As we can see in the BaO−SiO2 binary system, there are multiple promising silicate phases for Eu2+-activated emission for which the emission wavelength depends on the polymorphic composition. To confirm the simulation results for the proper intermittent silicate phase to achieve desirable single phase oxonitridosilicates, we studied the formation degree of the Eu2+-doped Ba3Si6O12N2 phosphors using the silica-rich silicates, BaSiO3, Ba2Si3O8, and Ba2Si4O10, as the precursor and thoroughly investigated their photoluminescence properties. The phase formation and corresponding luminescence behavior of the Ba3Si6O12N2 phosphors with various silicate precursors is summarized in Figure 6. Note that the crystal phase and luminescence properties of the individual silicate as a precursor are presented in the Supporting Information (Figure S4). Even under the same conditions, we found that the phase purity, crystallinity, and luminescence intensity of the resultant Ba3Si6O12N2 phosphors are totally different for the types of silicates that we used. The oxonitridosilicate obtained using either Ba2Si3O8 or Ba2Si4O10 as the precursor was mainly composed of hexagonal Ba3Si6O12N2, while the sample using the BaSiO3 precursor was nearly unable to form the single phase one

Figure 6. (a) XRD patterns of Eu2+-doped Ba3Si6O12N2 samples prepared using different Ba-silicates as a precursor. (b) Corresponding excitation and emission spectra of Ba3Si6O12N2:Eu2+ phosphors; Ba2Si4O10 (black), Ba2Si3O8 (red), and BaSiO3 (blue), respectively. (c) SEM and particle size distribution of Ba3Si6O12N2:Eu2+ phosphors prepared by using Ba2Si4O10 (left) and Ba2Si3O8 (right) precursors.

(Figure 6a). Moreover, the degree of phase-pure Ba3Si6O12N2 formation increased using the silicate with a smaller Ba/O ratio from BaSiO3 through Ba2Si3O8 to Ba2Si4O10. The emission intensity of the phosphor prepared by the Ba2Si4O10 silicateassisted sample was about 2.5 times higher than that of the phosphor synthesized using Ba2Si3O8, indicating that the silicarich phase is thermodynamically favorable to forming the phasepure and highly crystalline Ba3Si6O12N2 phase even with carbothermal/N2 annealing. As shown in Figure 6c, we have measured the particle morphology and size distribution of the corresponding samples, Ba2Si4O10- and Ba2Si3O8-assisted Ba3Si6O12N2 samples, since the overall luminescence property of powder phosphors fairly depends on the morphology of the particles, i.e., size, size distribution, shape, and so on. However, we can hardly find out the morphology effect on the emission intensity of the E

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Inorganic Chemistry Ba3Si6O12N2:Eu2+ samples prepared with different silicate precursors. Both samples are much similar to large particle size and irregular morphologies. This result is also in line with the synthetic experiments of Moriga et al., who reported that a higher Si/Ba ratio was beneficial for the synthesis of Ba3Si6O12N2 and (Ba1−(x+y)SrxEuy)2Si6O12N2 phases.44,45 Additionally, as shown in Figure S5, we do check the phase formation and luminescent properties of the corresponding Ba3Si6O9N4:Eu2+ phosphor prepared using a silicate precursor to verify the DFT calculation. From the photoluminescent spectra and other luminescence behavior (such as temperature PL and decay), only the sample synthesized with Si3N4 addition exhibits the bluish-green-emitting phase-pure Ba3Si6O9N4:Eu2+ phosphor. Thus, it can clearly demonstrate the calculation results that we propose; Ba3Si6O9N4 might be difficult to synthesize by N2feed carbothermal reduction condition that we found in Ba3Si6O12N2 formation, requiring high enough temperatures far over 1600 K under controlled oxygen partial pressure. The energy transfer from the host (energy absorber) to the activator (energy emitter) through the thermal activation over the energy barrier plays a key role in high power WLEDs.46 Therefore, the energy transfer efficiency is strongly dependent on the energy barrier ΔEa and the induced temperature T. Generally, high temperature will increase the probability of nonradiative transition, which is caused by the thermal activation point between the excited state and the ground state. The activation energy of nonradiative transition increases with an increase in the crystal stiffness and a decrease in the Eu2+ mobility.47 The temperature variable emission spectra for the given phosphors are presented in Figure 7a. Interestingly, our results clearly show that the degree of thermal quenching of the two oxynitrides is substantially different with varying the silicate

composition, which might be related to the phase-pure oxynitride formation with the suppressed Eu2+ vibration by the neighboring nitrogen coordination. As can be seen in Figure 7b, the thermal quenching data were fitted to the Arrhenius eq 2 ⎛I ⎞ ΔEa ln⎜ 0 ⎟ = ln A − ⎝I⎠ kT

(2)

where I0 and I are the emission intensities measured at room and the given temperature, respectively. A is a constant, and k is the Boltzmann constant. The activation energy (ΔEa) for thermal quenching was 0.279 eV for the Ba2Si4O10-assisted oxonitridosilicates, which is larger than that for the Ba2Si3O8-assisted sample (0.157 eV). Thus, there was still some of the residual silicate phase in the Ba2Si3O8assisted synthesized Ba3Si6O12N2:Eu2+ phosphor. Temperaturedependent luminescence decay was measured after being excited at 365 nm and was monitored at 525 nm as shown in Figure 8.

Figure 8. Temperature variable luminescence decay spectra of Ba2Si4O10-assisted Ba3Si6O12N2:Eu2+ phosphors. The first-order exponential decay function is also denoted.

Table 1. Fitting Parameters of Temperature Variable Decay Spectra of Ba2Si4O10-Assisted Ba3Si6O12N2:Eu2+ Phosphors decay constants temperature (K)

τl (ms)

A1

298 184 100

0.304 0.305 0.307

1.115 1.303 1.116

The detailed fitting parameters are listed in Table 1. The corresponding luminescence decay curves can be fitted into an exponential decay behavior48 ⎛ t⎞ I(t ) = y0 + A1 exp⎜ − ⎟ ⎝ τ1 ⎠

(3)

where I(t) is the luminescence intensity at the given temperature and times and t is the time. τ1 is the luminescence decay time that we measured determined by the time taken by electrons to stay in the excited (and/or metastable) state, followed by transition to the ground state. All the measured decay spectra are well matched to the single logarithmic plot (first-order kinetics simulated curve in Figure 8),

Figure 7. (a) Temperature variable photoluminescence emission spectra for Ba3Si6O12N2:Eu2+ phosphors using different silicates precursors. (b) The dependence of emission intensity, ln(I0/I), on temperatures for the given samples. F

DOI: 10.1021/acs.inorgchem.6b01278 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry and the resultant luminescence decay times, τ1, are measured to be 0.305 ms (298 K), 0.305 ms (184 K), and 0.307 ms (101 K) with a decreasing temperature. Generally, the binding state between the activator and coordinated oxygen(nitride) anions also influences the luminescence lifetime through nonradiative relaxation.49 Thus, the results demonstrated that the residual traps and/or recapturing sites for electrons were not very active for the silicate-supported Ba3Si6O12N2:Eu2+ phosphors that we prepared. To check the chip-in-package luminescence behavior of a pcLED using the silicate-assisted Ba3Si6O12N2:Eu2+ phosphor, we finally fabricated LED lamps using a near-UV-LED chip overcoating with the given phosphor compositions. The detailed LED chip fabrication procedure and the emission spectra of a bare nUV-chip under the applied power were shown in our previous results.50 Figure 9a shows the EL spectra and a captured

shows the stability of the electroluminescence intensity of the given LED lamps. It can be seen that the LED lamps using the corresponding phosphor composition stably illuminate without any remarkable degradation on either emission intensity or CIE value. We checked the thermal stability of a UV LED chip without phosphor coating (data not shown here). Thus, the longterm stability data of the phosphor-in-package LED lamp as well as the thermal quenching data are obviously meaningful for guaranteeing the thermal stability of the proposed phosphor composition.

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CONCLUSION Promising green-emitting Eu2+-activated Ba2+-oxonitridosilicate phosphors for pc-WLEDs were successfully prepared without using any nitride raw materials. To understand the formation behavior and develop sophisticated synthetic procedures, the Ba3Si6O12N2:Eu2+ phosphor using various Ba2+-oxosilicates as a precursor under carbothermal reduction nitridation was predicted by first-principles simulations, and we succeeded in synthesizing the corresponding oxynitride phosphor using all oxide raw materials only under gaseous N2 ambients. The resultant Ba3Si6O12N2:Eu2+ phosphor exhibited the PLE spectra spanned the UV to the blue region and the PL spectra showing green-yellow emission under 390−400 nm excitation with CIE (x, y) = (0.285, 0.640). Furthermore, the formation degree and thermal quenching behavior of the given Ba3Si6O12N2:Eu2+ phosphor was strongly dependent on the silicate composition used as precursors, which can be understood by the different covalent bonding degree with BaO/SiO2 content. Bright and green color pure conversed LED lamps can be obtained by combining the as-prepared Ba3Si6O12N2:Eu2+ phosphor with ultraviolet illuminated LEDs, demonstrating the potential green-emitting phosphors for ultraviolet-pumped pcWLEDs. This study will provide useful guidelines for robust synthetic procedures of thermally stable oxynitride/nitride phosphors, and the established simulation method can effectively be applied to other multigas systems.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01278. Phase diagram, XRD, and additional luminescence data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.K.). *E-mail: [email protected] (S.C.).

Figure 9. (a) Emission spectra with increasing the applied power of the pc-LED lamps using the Ba2Si4O10-assisted Ba3Si6O12N2:Eu2+ phosphors. Inset is the representative chip image with power on. (b) 35 days long-term stability of the corresponding LED lamps.

Author Contributions ⊥

The manuscript was written through contributions of all authors. These authors contributed equally.

Notes

image of the given LED lamp using the Ba3Si6O12N2:Eu2+ phosphor. Apparently, the overall emission spectra are mainly composed of green-emitting activated components centered at 530 nm, which were well matched to that of the powder samples as shown in Figure 2a. Moreover, the emission intensity gradually increased up to 1 W applied power, which can be applicable to the power LEDs like head lamps and outdoor lighting. Figure 9b

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work is a joint research project. The authors appreciate the financial support from the ISTK (Korea Research Council for Industrial Science and Technology). G

DOI: 10.1021/acs.inorgchem.6b01278 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.6b01278 Inorg. Chem. XXXX, XXX, XXX−XXX