Article pubs.acs.org/IECR
Facile Synthesis of (Sr,Ca)2Si5N8:Eu2+-Based Red-Emitting Phosphor for Solid-State Lighting Takayuki Suehiro,* Rong-Jun Xie, and Naoto Hirosaki SiAlON Unit, Environment and Energy Materials Division, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan S Supporting Information *
ABSTRACT: A new facile synthetic route to produce a nitride-based red-emitting phosphor has been established. The (Sr,Ca)2Si5N8:Eu2+-based multicomponent phosphor was successfully synthesized from the stable SrCO3−CaCO3−Eu2O3− Si3N4 system by simple one-step heating at 1600 °C for 4 h in an unpressurized N2 atmosphere. The synthesized (Sr,Ca)2Si5N8:Eu2+-based red broadband emitting phosphor exhibited the peak wavelength as long as 661 nm with a practically high external quantum efficiency of 60% under the excitation at 450 nm, while the coexisting secondary phase was inactive under the blue-light excitation, showing no detrimental effects on the photoluminescent properties. The enhanced red emission compared to the unmodified Sr2Si5N8:Eu2+ phosphor enables further improvement of the color rendering properties of the white light-emitting diodes for solid-state lighting applications.
1. INTRODUCTION White light-emitting diodes (LEDs) have attracted increasing attention as a light source for the next-generation general illumination and automotive lighting applications due to the energy savings and positive environmental effects promised by the solid-state lighting (SSL).1−5 The warm-white LEDs with low correlated color temperatures (CCTs) of ∼3800−2600 K, required for primary important applications, e.g., residential and commercial lighting, have been attained by the use of the recently developed blue-light excitable, orange- to red-emitting nitride phosphors such as AE2Si5N8:Eu2+ (AE = Ca, Sr, Ba)6,7 and CaAlSiN3:Eu2+.8,9 These pure nitride phosphors are produced by using the expensive and air-sensitive raw materials, i.e., AE3N2 and EuN with complicated processing procedures, which is a major obstacle to their widespread applications in SSL. In this regard, we have developed a new synthesis route for Sr2Si5N8:Eu2+-based red-emitting phosphors without using any unstable starting materials,10 by utilizing the phase compatibility between Sr2Si5N8 and Sr2SiO4, expressed by the following reaction at 1600 °C in a N2 atmosphere:
red emission, which enables further improvement of the colorrendering properties of multichromatic white LEDs for general illumination.
2. EXPERIMENTAL SECTION The starting materials used were SrCO3 (99.9%), CaCO3 (99.99%), Eu2O3 (99.99%), and Si3N4 (SN-E10, UBE). The raw powder in a composition 0.800SrCO3−1.150CaCO3− 0.025Eu2O3−1.167Si3N4 was dry-mixed in a motar and heated at 1600 °C for 4 h under flowing N2 atmosphere using an alumina tube furnace. Phase assemblage of the synthesized powder was analyzed by X-ray diffractometry (XRD; RINT2200, Rigaku), and the particle morphology was analyzed by scanning electron microscopy (SEM; TM-1000, Hitachi) and transmission electron microscopy (TEM; HF-2000, Hitachi). Energy dispersive X-ray spectrometry (EDX) was conducted by using the TEM, for the compositional analysis of primary particles. Photoluminescence (PL) properties and quantum efficiencies of the as-synthesized powder were evaluated using a spectrofluorometer equipped with a 60-mm integrating sphere (FP-6500/ISF-513, Jasco). Spectral simulation of trichromatic white LEDs was carried out by integrating the emission spectra of the synthesized (Sr,Ca)2Si5N8:Eu2+based phosphor, a 450-nm InGaN LED and a SrSi2O2N2:Eu2+ green-emitting phosphor. 11 On this occasion, all the chromaticity coordinates of the simulation spectra were adjusted to the Planckian locus on the corresponding Commission Internationale de l’Eclairage (CIE) 1931 color diagram. The color rendering indices (CRI Ri) were calculated according to Japanese Industrial Standard (JIS) 8725/8726.
2Si3N4 + 4SrO → Sr2Si5N8 + Sr2SiO4
The synthesized Sr2Si5N8:Eu2+-based multicomponent phosphor exhibited a high external quantum efficiency of 64%, comparable to that attained by a single-phased sample, despite the inclusion of ∼36 wt % of the Sr2SiO4 secondary phase. The successful development of the highly efficient, multiphased Sr2Si5N8:Eu2+ phosphor has been achieved by the fact that both the absorption and the internal quantum efficiencies exhibit nonlinear relationships to the amount of an optically inert secondary phase (Supporting Information Figure S1), and the consequent external quantum efficiency scarcely depends on the phase purity of the phosphor. In the current work, we adopted the above synthetic strategy to the SrO−CaO−Si 3 N 4 multinary system to obtain (Sr,Ca)2Si5N8:Eu2+-based phosphors possessing an enhanced © XXXX American Chemical Society
Received: March 6, 2013 Revised: April 26, 2013 Accepted: May 16, 2013
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3. RESULTS AND DISCUSSION 3.1. Powder Properties. The (Sr,Ca)2Si5N8:Eu2+-based powder was synthesized from the composition AECO3−Si3N4 in the ratio of 2:1.167, with a slightly higher Si3N4 content compared to 2:1 adopted for the synthesis of pure Sr2Si5N8:Eu2+-based powder.10 The synthesis from the latter starting composition resulted in severe sintering of the product powder, which is attributable to the lower eutectic point in the current ternary reaction system.12 The fixed Ca to Sr ratio of 0.59: 0.41 was predetermined for maximizing the redshift of the emission wavelength. The XRD pattern of thus-obtained sample is shown in Figure 1. The synthesized powder was
confirmed to consist of (Sr,Ca)2Si5N8 (Pmn21, a = 5.6818(4) Å, b = 6.7608(5) Å, c = 9.2927(7) Å) and a new (Ca,Sr)−Si−O− N phase (Cm, a = 7.0719(13) Å, b = 23.819(4) Å, c = 4.8237(9) Å, β = 109.151(3)°), unlike the situation encountered in the SrCO3−Si3N4 binary reaction system, where strontium orthosilicate polymorphs were identified as secondary phases.10 This discrepancy might result from the different polymorphous transformations in Sr2SiO4 and Ca2SiO4;13 the latter possesses an orthorhombic γ-form as the low-temperature stable phase. The detailed structure of the secondary phase found in the current work will be reported elsewhere. The target phase, (Sr,Ca)2Si5N8 retained the crystal structure of pure Sr2Si5N8 (Pmn21, a = 5.7101(2) Å, b = 6.8213(2) Å, c = 9.3321(3) Å10), while showing an appreciable lattice shrinkage indicative of dissolution of smaller Ca2+ into the Sr2+ sites. The content of (Sr,Ca)2Si5N8 phase, expected from the starting composition and preliminary Rietveld refinement results, was found to be as low as ∼30 wt %, whereas the coexisting secondary phase possesses little effect on the optical properties of the product powder, as will be discussed later. Figure 2a shows the particle morphology of the synthesized (Sr,Ca)2Si5N8:Eu2+-based powder. The powder comprised of faceted and irregular-shaped primary particles of ∼1−50 μm, reflecting the ternary eutectic liquid-phase formation during the processing. The observation by TEM (Figure 2b and c) also confirmed that the powder consisted of only two kinds of primary particles, i.e., (Sr,Ca)2Si5N8 and the aforementioned new monoclinic (Ca,Sr)−Si−O−N phase. The Sr to Ca ratio in the (Sr,Ca)2Si5N8 phase determined by the TEM-EDX analysis
Figure 1. XRD pattern of the synthesized (Sr,Ca)2Si5N8:Eu2+-based powder.
Figure 2. (a) SEM micrograph of the synthesized (Sr,Ca)2Si5N8:Eu2+-based powder, (b) TEM micrograph, and (c) corresponding SAED pattern of a (Sr,Ca)2Si5N8:Eu2+ primary particle. B
dx.doi.org/10.1021/ie400741u | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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unmodified Sr2Si5N8:Eu2+, which will be beneficial to the color rendering properties of white LEDs by minimizing the spectral gap between green and red region. The luminous efficacy of radiation (LER) estimated from the emission spectrum was 109 lm/W, significantly lowered compared to 235 lm/W for the unmodified Sr2Si5N8:Eu2+, due to the red-shifted spectral distribution. The synthesized (Sr,Ca)2Si5N8:Eu2+ exhibited a high absorption efficiency (Abs) of 81% considering the lower phase purity, which might be attained by the higher activator concentration (2.5%) compared to our previous Sr2Si5N8:Eu2+ (1%) sample. The internal quantum efficiency (IQE) showed only a slight decrease from 80 to 74%, and the resulting external QE (EQE) retained a high value of 60%. The emission intensity at 150 °C (I150) retained 76% of the intensity measured at room temperature under the 450 nm excitation, which was in between the values observed for stoichiometric Sr2Si5N8:Eu2+ (I150 ∼90%) and Ca2Si5N8:Eu2+ (I150 ∼40%).10,15 These results conclusively showed that the (Sr,Ca)2Si5N8:Eu2+-based red broadband emitting phosphor possessing the peak wavelength as long as 661 nm with a practically high EQE of ∼60% and moderate thermal stability can be attainable through the facile synthesis technique developed, without using any expensive/unstable starting materials and complicated processing procedures. 3.3. Application to White LEDs. Practical performance of the synthesized (Sr,Ca)2Si5N8:Eu2+-based red phosphor in SSL applications was evaluated by the spectral simulation of trichromatic warm-white LEDs for the CCTs of 3500 K (JIS class WW, “warm white”) and 3000 K (class L, “incandescent lamp color”). The emission spectra of the simulated white LEDs are shown in Figure 4, and the relevant color rendering
of the primary particles was found to be 0.58(1):0.42(3), which is consistent with the fact that the lattice volume of the synthesized (Sr,Ca)2Si5N8:Eu2+ (V = 356.97(4) Å3) is in between the values reported for (Sr0.5Ca0.5)2Si5N8:2% Eu (V = 356.94 Å3)14 and (Sr0.65Ca0.30Eu0.05)2Si5N8 (V = 360.16(1) Å).15 The analyzed Sr to Ca ratio in the (Sr,Ca)2Si5N8 phase is appreciably higher compared to that of the reaction system (0.41: 0.59), indicating the preferential dissolution of Ca into the secondary (Ca,Sr)−Si−O−N phase. 3.2. Photoluminescent Properties. Figure 3 shows the PL excitation and emission spectra of the synthesized
Figure 3. PL excitation (broken line) and emission (solid line) spectra of the synthesized (Sr,Ca)2Si5N8:Eu2+-based phosphor.
(Sr,Ca)2Si5N8:Eu2+-based powder. The relevant spectral parameters and the quantum efficiencies are listed in Table 1, together with the data for the previously reported Sr2Si5N8:Eu2+-based phosphor for comparison.10 As expected from the bright orange−red coloration under daylight, the synthesized (Sr,Ca)2Si5N8:Eu2+-based powder exhibited a broad excitation band extending from ultraviolet (UV) to visible spectral region of ∼550 nm, showing an equivalent excitation property observed for monophasic (Sr,Ca)2Si5N8:Eu2+.14,15 Under the blue-light excitation of 450 nm, the synthesized (Sr,Ca)2Si5N8:Eu2+ powder exhibited a markedly red-shifted emission band peaking at 661 nm, compared to 618 nm for the unmodified Sr2Si5N8:Eu2+, which is attributable to the aforementioned lattice shrinkage and the resulting stronger crystal field around Eu2+. An additional emission shoulder at the shorter wavelength side of ∼500 nm was observed only with the UV excitation at ∼360 nm or shorter, indicating that the secondary (Ca,Sr)−Si−O−N:Eu2+ phase is apparently inactive under the blue-light excitation, and its blue−green emission might be largely reabsorbed by the (Sr,Ca)2Si5N8:Eu2+ phase. The CIE 1931 chromaticity coordinates under the 450 nm excitation were (0.648, 0.339) with the dominant wavelength (λd) of 608 nm, which is the most reddish value attained so far for the (Sr,Ca)2Si5N8:Eu2+ system and comparable to the chromaticity of CaAlSiN3:Eu2+ with an activator concentration of around 1%.9 The full width at half-maximum (fwhm) of the emission band was broadened markedly compared to the
Figure 4. Simulated emission spectra of trichromatic white LED systems using the synthesized (Sr,Ca)2Si5N8:Eu2+- and Sr2Si5N8:Eu2+based phosphors as red illuminants. The broken line indicates the Planck curve of 3000 K.
properties and the LER values are summarized in Table 2. The results for the system using the previously reported Sr2Si5N8:Eu2+-based phosphor10 are also shown for comparison. The general CRI Ra values attainable by the current (Sr,Ca)2Si5N8:Eu2+-system were in the range of 84−87 and
Table 1. PL Properties of the Synthesized (Sr,Ca)2Si5N8:Eu2+-Based Phosphor under the Excitation at 450 nm CIE coordinates sample 2+
(Sr,Ca)2Si5N8:Eu Sr2Si5N8:Eu2+ a a
quantum efficiencies
x
y
λd (nm)
λpeak (nm)
fwhm (nm)
Abs
IQE
EQE
LER (lm/W)
I150 (%)
0.648 0.621
0.339 0.378
608 599
661 618
114 88
0.81 0.80
0.74 0.80
0.60 0.64
109 235
76 86
Reference 10. C
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Notes
Table 2. CRI and LER Values for the Simulated Trichromatic White LED Systems Using the Synthesized (Sr,Ca)2Si5N8:Eu2+- and Sr2Si5N8:Eu2+-Based Red Phosphors
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful to Dr. T. Takeda and Dr. S. Funahashi of National Institute for Materials Science for fruitful discussions.
CRIs
a
red phosphor used
CCT (K)
Ra
R9
R13
R15
LER (lm/W)
(Sr,Ca)2Si5N8:Eu2+ (Sr,Ca)2Si5N8:Eu2+ Sr2Si5N8:Eu2+ a Sr2Si5N8:Eu2+ a
3500 3000 3500 3000
84 87 78 80
96 91 20 24
88 93 79 81
95 99 75 76
246 230 335 329
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Reference 10.
sufficiently high for general illumination, compared to 78−80 for the unmodified Sr2Si5N8:Eu2+-system. These results are attributed mainly to the improved R1 (light grayish red) and R8 (light reddish purple) values, both exceeding ∼90. The only appreciably decreased CRI compared to the unmodified Sr2Si5N8:Eu2+-system was R3 (strong yellow green), which might be caused by the spectral deficiency in the region ∼550− 600 nm, resulting from the red-shifted emission of (Sr,Ca)2Si5N8:Eu2+. The special CRIs of R9 (strong red), R13 (skin tone of European women), and R15 (skin tone of Asian women) were also improved significantly in the (Sr,Ca)2Si5N8:Eu2+-system indicating the suitability for highcolor rendering general lighting applications, which could be attained by the enhanced red emission covering the wavelength longer than ∼620 nm. The theoretical LER values for the simulated LED systems ranged from 230 to 335 lm/W, reflecting directly the LER values of the red phosphor used. Further optimization of the CRI and LER values will be attained easily via the processing method developed, by varying the Sr/Ca ratio in the starting composition, enabling a precise tuning of the red emission from (Sr,Ca)2Si5N8:Eu2+.
4. CONCLUSIONS We have established a facile method for producing the (Sr,Ca)2Si5N8:Eu2+-based red-emitting phosphor, from the stable and inexpensive SrCO3−CaCO3−Eu2O3−Si3N4 system by simple one-step heating under a N2 atmosphere. The synthesized (Sr,Ca)2Si5N8:Eu2+-based multicomponent phosphor possessed an enhanced red emission with the peak wavelength as long as 661 nm, compared to 618 nm for the unmodified Sr2Si5N8:Eu2+-based phosphor, along with a comparably high EQE of 60% under the blue-light excitation of 450 nm. The results of the spectral simulation of trichromatic warm-white LEDs demonstrated that the system using the synthesized (Sr,Ca)2Si5N8:Eu2+-based phosphor can attain the high CRI Ra values of 84−87, as well as the special CRIs of R9 = 91−96, R13 = 88−93, and R15 = 95−99, showing the promising applicability to the general illumination with a much improved light quality.
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REFERENCES
ASSOCIATED CONTENT
S Supporting Information *
Relationships between the quantum efficiencies and the phase purity. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. D
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