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Photoluminescence of CaAlSiN3:Eu2+-based fine red-emitting phosphors synthesized by carbothermal reduction and nitridation method
Optical Materials 38 (2014) 242–247
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Photoluminescence of CaAlSiN3:Eu2+-based fine red-emitting phosphors synthesized by carbothermal reduction and nitridation method Shuxing Li a,b, Xia Peng a,b, Xuejian Liu a,⇑, Zhengren Huang a a b
Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China University of the Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 14 August 2014 Received in revised form 9 October 2014 Accepted 22 October 2014 Available online 8 November 2014 Keywords: CaAlSiN3:Eu2+ Red phosphor Crystal phase Luminescence property
a b s t r a c t In this research, we have presented the synthesis and characterization of the various Ca1xEuxAl0.76Si1.18N3 (x = 0.01 0.1) red-emitting phosphors, which were successfully prepared by carbothermal reduction and nitridation (CTRN) method without the strict needs of high pressure. Here, raw materials were CaCO3, AlN, Si3N4, Eu2O3, and C. In particular, C was considered as efficient and robust reducing agent. The influences of reaction temperature, holding time, C content, and Eu2+ concentration were investigated in the crystal phase compositions and photoluminescence properties of the as-prepared phosphors. Importantly, CaAlSiN3:Eu2+-based red phosphors with interesting properties were obtained with reaction temperature at 1600 °C for 4 h by atmospheric N2–10%H2 pressure, and the C/O ratio of 1.5:1, respectively. The emission peak positions of as-prepared phosphors were red-shifted from 607 nm to 654 nm with Eu2+ concentration from 1 mol% to 10 mol%. Meanwhile the highest luminescence intensity was achieved with 2 mol% of Eu2+ concentration, which showed high external quantum efficiency up to 71%. Combining the phosphor blend of green-emitting b-sialon:Eu2+, yellow-emitting Ca-a-sialon:Eu2+, and red-emitting Ca0.98Eu0.02Al0.76Si1.18N3 with a blue LED (light emitting diodes), warm white LED can be generated, yielding the color rendering index (Ra) of 93 at correlated color temperature (CCT) of 3295 K. These results indicate that CaAlSiN3:Eu2+-based red-emitting phosphors prepared by facile CTRN are highly promising candidates for warm white LEDs. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction In recent years, rare earth doped (oxy)nitrides phosphors have been attracting much attention because of their nontoxicity, excellent thermal stability, and high absorption and conversion efficiency from near-UV to visible spectral region [1]. Typically, they are AlN:Eu2+ [2–4], and LaSi3N5:Ce3+ [5] blue-emitting phosphors, b-SiAlON:Eu2+ [6–9], CaSi2O2N2:Eu2+ [10,11], and c-AlON:Mn2+ [12] green-emitting phosphors, Ca-A-SiAlON:Eu2+ [13,14], and Y3Si6N11:Ce3+ [15] yellow-emitting phosphors, and SrAlSi4N7:Eu2+ [16], M2Si5N8:Eu2+ (M = Ca, Sr, Ba) [17–19], and CaAlSiN3:Eu2+ [20–23] red-emitting phosphors. Among these (oxy)nitrides, CaAlSiN3:Eu2+ red-emitting phosphors have been used as key materials for high color rendering index (CRI) and low correlated color temperature (CCT) solid illumination parts, which satisfy the requirements of medical and architectural lighting fields. Both excitation and emission bands of CaAlSiN3:Eu2+ showed very low energies due to the contributions by a combination of strong ⇑ Corresponding author at: Dingxi Road 1295, Shanghai, China. Tel.: +86 21 52414220; fax: +86 21 52413903. E-mail address: [email protected] (X. Liu). http://dx.doi.org/10.1016/j.optmat.2014.10.039 0925-3467/Ó 2014 Elsevier B.V. All rights reserved.
crystal-field splitting of the 5d states of Eu2+ ions and high covalence (nephelauxetic effect), especially for the first nearest coordination of Eu–N [24]. Up to now, various preparation methods for CaAlSiN3:Eu2+ redemitting phosphors have been reported. It included traditional high temperature solid state reaction (SSR) [19], spark plasma sintering (SPS) [25] with Ca3N2–AlN–Si3N4 as raw materials, self-propagating high-temperature synthesis (SHS) [22], direct nitridation [26], and ammonothermal synthesis [21,27] based on Ca1xEuxAlSi alloy. However, raw materials of all the mentioned methods were air-sensitive and oxygen-free metal nitrides or active metals, especially for expensive and deliquescent Ca3N2. Consequently, all the procedures before sintering must be taken in a glove box filled with N2 to avoid oxidation. Additionally, crushing and classification process must be taken on the phosphors prepared at relative high temperature (P1800 °C) and pressure (P0.9 MPa). At the same time, impurities would be inevitably introduced during crushing and classification process. Ammonothermal synthesis by nitridation of (Ca, Eu)AlSi alloy in supercritical ammonia (100 MPa) could never be industrially applicable because of extremely high pressure for use. Recently, Suehiro and co-workers [23] reported the synthesis of CaAlSiN3:Eu2+ phosphors by gas reduction nitridation (GRN)
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method with complex process, however, the obtained product contained impurities of AlN and unreacted CaO. By comparison with mentioned methods, carbothermal reduction and nitridation (CTRN) method possesses the high advantage of relative low reaction temperature, cheap raw materials, and simple processing. At present, there are few reports of the synthesis of CaAlSiN3:Eu2+ red phosphors through CTRN method except for the research by Kim et al. [28,29] with hazardous CaCN2 as calcium and carbon sources. In this research, nominal compositions of Ca1xEuxAl0.76Si1.18N3 (x = 0.01 0.1) red phosphors were synthesized with stable and inexpensive CaCO3 as calcium source at relatively low temperature and by atmospheric pressure. The main effects of reaction temperature, holding time, C content and Eu2+ concentration on crystal phase compositions and photoluminescence properties were investigated in order to develop low-cost and high-quality CaAlSiN3:Eu2+-based red-emitting phosphors. 2. Experimental 2.1. Synthesis Nominal compositions of Ca1xEuxAl0.76Si1.18N3 (x = 0.01, 0.02, 0.03, 0.05, 0.08, and 0.1) red-emitting phosphors were synthesized according to the stoichiometric ratio. The raw materials of CaCO3 (99.99%), AlN (Tokuyama, H-Grade), a-Si3N4 (UBE, E-10), Eu2O3 (99.99%), and C powder (99.95%) were homogeneously mixed in a silicon nitride mortar. The resultant mixture was heated in atmospheric N2 at a constant flow rate of 200 mL/min at 850 °C for 1 h to decompose CaCO3 completely. Then, H2 gas was flowed inside the horizontal tube furnace by 200 mL/min N2 to 20 mL/min H2 gas mixture to guarantee the complete reduction of Eu3+ to Eu2+. Next, the temperature was increased to 1550–1650 °C, and maintained for 1–6 h. Finally, the fired samples were ground finely and prepared for measurements. 2.2. Characterization The crystal phases of sintered samples were determined by X-ray diffraction (XRD) (Bruker D8 Advanced) with Cu Ka1 radiation (k = 0.15406 nm) operating at 40 kV/40 mA with a step size of 0.02° and a scan speed of 5° min1. Electron micrograph of the typical sample was obtained by field-emission scanning electron microscope (Hitachi S-4800). Partial elemental analysis of the phosphors was performed using C measurement device (SICMMC-222) for C content, and O and N analyzer (LECO, Model TC600) for O and N content, respectively. The photoluminescence spectrum and quantum efficiency of powder phosphors were determined by fluorescence spectrophotometer (Jobin Yvon, Fluoromax-4) with 150W Xe lamp as excitation source. The quantum efficiency was estimated using an integrating sphere unit (Jobin Yvon, F-3029) with a monochromatic source of 460 nm. For the temperature-dependent photoluminescence measurement, the powder sample was loaded in a sample cavity and then heated to the desired temperature by a high-temperature fluorescence controller (Tianjin Orient KOJI Co., Ltd., TAP02). The sample was kept for 10 min to guarantee a uniform temperature both on the surface and in the interior of the sample. The optical properties of as-fabricated white LED was investigated by a UV– VIS-near IR Spectrophoto Colorimeter (PMS-80) with an integrating sphere under forward-bias current of 20 mA at room temperature.
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1650 °C with C/O ratio of 1.5:1 and the same holding time for 4 h, respectively. As CaCO3 was decomposed completely into CaO above 1000 °C, the O of C/O referred to oxygen element in CaO and Eu2O3 oxides. We suggested that the powders synthesized at temperature of 1550 °C contained a small amount of impurity phases Ca2SiO4 and unreacted C. The impurity phases of Ca2SiO4 and unreacted C gradually disappeared with an increasing temperature, and the CaAlSiN3 phase (JCPDS 39-0747) and a very small amount of CaAl0.54Si1.38N3 phase (ICSD 161796) [24] were formed without impurities at temperatures of 1600 °C and 1650 °C in Fig. 1. The excitation and emission spectra of Ca0.98Eu0.02Al0.76Si1.18N3 phosphors synthesized at different temperatures were presented in Fig. 2. Each phosphor showed a broad excitation spectra from 250 to 600 nm, matching effectively with UV (380 nm) and (or) blue (450 nm) LED chips. The strong orange–red emission spectra centered at about 630 nm were attributed to the characteristics of 4f65d1 ? 4f7 transition of Eu2+ ions [20–23]. The product synthesized at temperature of 1550 °C became gray because of the existence of unreacted C, which absorbed visible spectra intensively, leading to the large reduction of luminescence intensity. The phosphor heated at 1600 °C showed the best luminescence whose full width at half maximum (FWHM) of the emission band was about 100 nm. However, when we increased the reaction temperature up to 1650 °C, the luminescence intensity decreased, the reason for which was unclear in this case. 3.2. Holding time Under the atmosphere of C and H2 strong reducing agents, CaO and Eu2O3 were reduced to corresponding metals, and at the same time, they were nitrogenized and N needed to diffuse into metal particles in order to react completely. However, in the solid–solid reaction process N diffused very slowly into metal particles [2]. At the same time, low reactivity and small self-diffusion coefficient of silicon nitride and aluminum nitride also slowed the solid–solid diffusion process, but diffusion process was the rate-determining step in the whole reaction process [18]. Thus, the holding time was an important factor to produce pure crystal phase with good crystallization. As a result, the effect of holding time on the most typical XRD patterns of Ca0.98Eu0.02Al0.76Si1.18N3 phosphors synthesized at 1600 °C with C/O ratio of 1.5:1 was shown in Fig. 3. The intensity of diffraction peaks of unreacted C decreased without eliminating completely with holding time increasing from 1 h to 2 h, indicating that the C content decreased with longer holding
3. Results and discussion 3.1. Reaction temperature Fig. 1 showed typical X-ray diffraction (XRD) patterns of Ca0.98Eu0.02Al0.76Si1.18N3 powders synthesized at 1550 °C, 1600 °C, and
Fig. 1. X-ray diffraction patterns of Ca0.98Eu0.02Al0.76Si1.18N3 powders synthesized at different temperatures.
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Fig. 2. Excitation and emission spectra of Ca0.98Eu0.02Al0.76Si1.18N3 powders synthesized at different temperatures.
Fig. 4. Excitation and emission spectra of Ca0.98Eu0.02Al0.76Si1.18N3 powders synthesized at 1600 °C for different holding times.
Fig. 3. XRD patterns of Ca0.98Eu0.02Al0.76Si1.18N3 powders synthesized at 1600 °C for different holding times.
Fig. 5. XRD patterns of Ca0.98Eu0.02Al0.76Si1.18N3 powders synthesized at 1600 °C for 4 h with different C/O ratios.
time. The CaAlSiN3 phase (JCPDS 39-0747) and a very small amount of CaAl0.54Si1.38N3 phase (ICSD 161796) [24] were obtained without impurities with holding time for 4 h and 6 h. It meant that the complete reaction of the raw materials occurred. In addition, the diffraction peaks did not significantly show any changes between 4 h and 6 h. The excitation and emission spectra of Ca0.98Eu0.02Al0.76Si1.18N3 phosphors heated at various holding times were shown in Fig. 4. The products heated at 1600 °C for 1 h and 2 h exhibited the much weaker luminescence intensity because of the existing of unreacted C, while the product heated at 1600 °C for 4 h showed the strongest luminescence intensity due to its better crystallization of CaAlSiN3-type phase. The luminescence intensity of the product with a longer holding time for 6 h decreased, the reason for which was unclear. Therefore, a reaction temperature at 1600 °C and a holding time for 4 h were found to make Ca0.98Eu0.02Al0.76Si1.18N3 red-emitting phosphors with good luminescence.
(JCPDS 39-0747) and a very small amount of CaAl0.54Si1.38N3 phase (ICSD 161796) [24] were obtained with C/O ratio of 1.5:1. With increasing C/O ratio from 1.7 to 2.0:1, the diffraction peaks of residual C appeared, indicating the excess of C. The excitation and emission spectra of Ca0.98Eu0.02Al0.76Si1.18N3 phosphors synthesized with different C/O ratios were presented in Fig. 6. No obvious changes could be detected in the photoluminescence spectra profiles with different C/O ratios. The phosphor synthesized with C/O of 1.5:1 gave the best luminescence intensity due to the evidence of no other impurities except for CaAlSiN3 and a very small amount of CaAl0.54Si1.38N3 phase. Fig. 7 exhibited the mass fractions of C, N, and O in Ca0.98Eu0.02Al0.76Si1.18N3 red1-emitting phosphors synthesized with different C/ O ratios. With the ratio of C/O from 0.8 to 1.5:1, mass fraction of O sharply decreased with increasing C/O ratios, while C slowly increased. With the ratio of C/O from 1.5 to 2.0:1, mass fraction of O slowly decreased with increasing C/O ratios, while C sharply increased. The N content started to increase at low C/O ratios and reached its maximal of 26.68% at C/O ratio of 1.7:1 and then decreased, while the theoretical mass fraction of N in Ca0.98Eu0.02Al0.76Si1.18N3 was 30.45%. Thus, O and C contents were relatively
3.3. C content Fig. 5 showed the XRD patterns of Ca0.98Eu0.02Al0.76Si1.18N3 powders with different C/O ratios, reaction temperature at 1600 °C, and the same holding time for 4 h. When the ratio of C/ O was below 1.2:1, impurity of Ca2SiO4 existed. The CaAlSiN3 phase
1 For interpretation of color in Fig. 7, the reader is referred to the web version of this article.
S. Li et al. / Optical Materials 38 (2014) 242–247
Fig. 6. Excitation and emission spectra of Ca0.98Eu0.02Al0.76Si1.18N3 powders synthesized at 1600 °C for 4 h with different C/O ratios.
low and N content was relatively high with C/O ratio of 1.5:1 in our red phosphors. At the same time, the photoluminescence of the products was closely related to their C, N, and O content. Luminescence intensity was reduced significantly with increasing C content due to its strong absorption of visible spectra. When O content was high, crystal phase of oxide impurity was easily formed or the O atoms entered into the matrix lattice of CaAlSiN3 to replace the crystallographic site of N atoms and coordinate with the activator Eu2+ ions, leading to the formation of lattice distortion and point defects. All the above mentioned issues were unfavorable for a considerable improvement of luminescence. Nevertheless, nitrogen-rich coordination phenomenon inside red-emitting phosphors was formed more easily with higher N content, leading enhanced electronegativity and nephelauxetic effect [30]. Hence, the 5d excited states of Eu2+ ions caused a large splitting and thus owned lower energy levels. Therefore, electron transition between the 4f ground state and the lower energy levels of the 5d excited state was favorable and easier, resulting in an enhancement of emission intensity. 3.4. Europium concentration We demonstrated the influence of Eu2+ concentration on the XRD patterns of the nominal compositions of Ca1xEuxAl0.76Si1.18N3 (x = 0.01 0.1) phosphors heated at 1600 °C for 4 h with C/O ratio of 1.5:1 (Fig. 8a). The minor impurity phase of Ca2SiO4 was detected with Eu2+ concentration beyond 3 mol% because more
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oxygen atoms were introduced with increasing of raw materials Eu2O3, suggesting the inadequate reduction reaction. It was certain that the diffraction peak slightly shifted to low diffraction angle with increasing Eu2+ concentration, indicating expansion of crystal lattice, which was caused by the substitution of smaller Ca2+ ion (r = 1.14 Å, CN = 6) by larger Eu2+ ion (r = 1.31 Å, CN = 6) [31]. The typical particle morphology and photograph of synthesized Ca0.98Eu0.02Al0.76Si1.18N3 powder were shown in Fig. 8b and c, respectively. The large CaAlSiN3-type microstructures synthesized were confirmed in the crystal structure of phase CaAlSiN3 (JCPDS 39-0747) in the typical XRD patterns. The large CaAlSiN3-type microstructures had various shapes and morphologies, such as rods, plates, and polyhedral shapes. The particle sizes were about 10 lm. The excitation and emission spectra of Ca1xEuxAl0.76Si1.18N3 (x = 0.01 0.1) phosphors with different Eu2+ concentration were shown in Fig. 9. For all Ca1xEuxAl0.76Si1.18N3 (x = 0.01 0.1) phosphors, starting at low Eu2+ concentration, the emission intensity increased to a maximum at doping concentration of x = 0.02 with external quantum efficiency of 71%, and then fell again steadily to a minimum at x = 0.1. In this context, the decrease in emission intensity beyond critical concentration can be explained by concentration quenching [32], which was mainly caused by the effect of non-radiative energy transfer between Eu2+ ions. Increasing the concentration of Eu2+ ions caused the increase of probability of Eu– Eu pairs or clusters, which led to the non-radiative energy transfer between Eu2+ ions or between Eu2+ ion and the host resulting in the reduction of photoluminescence intensity. According to Blasse formula [32]:
RC ¼ 2
3V 4p X C N
13 ð1Þ
where N is the number of host cations in the unit cell, XC is the quenching concentration of Eu2+, and V is the volume of the unit cell. For Ca0.98Eu0.02Al0.76Si1.18N3 host, V is 275.73 Å3 according to lattice parameters of a = 5.6487 Å, b = 9.6754 Å, and c = 5.0450 Å. Additionally, N and XC are 4 and 0.02, respectively. Therefore, the critical distance (RC) for energy transfer from critical doping concentration was calculated to be 18.74 Å. In addition, as shown in Fig. 10, the emission peaks of Ca1xEuxAl0.76Si1.18N3 phosphors showed a shift from 607 nm to 654 nm with the Eu2+ concentration from 1 mol% to 10 mol%, which was ascribed to an increase in crystal field splitting. With increasing Eu2+ concentration and expansion of the crystal lattice, crystal field around Eu2+ ions created larger splitting and energy gap of 5d excited state became larger, and correspondingly the 5d excited
Fig. 7. Dependence of mass fraction of carbon, nitrogen, and oxygen on the C/O ratios.
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Fig. 8. (a) XRD patterns of Ca1xEuxAl0.76Si1.18N3 (x = 0.01 0.1) powders synthesized at 1600 °C for 4 h. (b) Typical SEM image of Ca0.98Eu0.02Al0.76Si1.18N3. (c) Photograph of Ca0.98Eu0.02Al0.76Si1.18N3.
Fig. 10. Dependence of peak emission intensity and wavelength on the Eu2+ concentration. Fig. 9. Excitation and emission spectra of Ca1xEuxAl0.76Si1.18N3 (x = 0.01 0.1) powders synthesized at 1600 °C for 4 h.
state possessed lower energy levels [33]. As a result, the emission peak shifted to the longer wavelength. 3.5. Application Fig. 11 illustrated normalized emission spectra of the white LED fabricated by combining the phosphor blend of green-emitting
b-sialon:Eu2+ (kem = 550 nm), yellow-emitting Ca-a-sialon:Eu2+ (kem = 580 nm), and as-prepared red-emitting Ca0.98Eu0.02Al0.76Si1.18N3 (kem = 630 nm) with a blue LED (kem = 450 nm). The obtained warm white LED gave high color rendering index (CRI) Ra of 93 at correlated color temperature (CCT) of 3295 K, and the R9 value was around 89. At the same time, the chromaticity coordinates located at x = 0.3871 and y = 0.3241, respectively. In addition, the relative emission intensity of Ca0.98Eu0.02Al0.76Si1.18N3 as a function of temperature was depicted in Fig. 12. The emission
S. Li et al. / Optical Materials 38 (2014) 242–247
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strongest emission intensity achieved at Eu2+ concentration of 2 mol% with external quantum efficiency up to 71%. Given correlated color temperature of 3295 K as well as the color rendering index of 93 for the obtained warm white LED, red-emitting phosphors prepared by CTRN can be very potential candidates for warm white LEDs. Acknowledgements The authors are very grateful to the financial support from the National Natural Science Fund of China (No. 51172263) and the Science and Technology Innovation Initiative of Shanghai Institute of Ceramics, Chinese Academy of Sciences (SICCAS). References
Fig. 11. Normalized emission spectra of the white LED fabricated by combining the phosphor blend of green-emitting b-sialon:Eu2+ (kem = 550 nm), yellow-emitting Ca-a-sialon:Eu2+ (kem = 580 nm), and as-prepared red-emitting Ca0.98Eu0.02Al0.76Si1.18N3 (kem = 630 nm) with a blue LED (kem = 450 nm). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 12. Temperature dependence of the emission intensity of Ca0.98Eu0.02Al0.76Si1.18N3 phosphor.
intensity decreased only about 9% at a fixed temperature of 250 °C compared to the intensity measured at room temperature, indicating good thermal stability. These results demonstrate that as-prepared red-emitting phosphors by facile CTRN method are good candidates for warm white LEDs. 4. Conclusions Nominal compositions of red-emitting phosphors, e.g. Ca1xEuxAl0.76Si1.18N3 (x = 0.01 0.1) were prepared by carbothermal reduction nitridation process. The interesting crystal phase of Ca1xEuxAl0.76Si1.18N3 was obtained at 1600 °C for 4 h by atmospheric N2–10% H2 pressure with C/O ratio of 1.5:1. With Eu2+ concentration from 1 mol% to 10 mol%, the emission peaks of the phosphors showed a red-shift from 607 nm to 654 nm. The
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Photoluminescence of CaAlSiN3:Eu2+-based fine red-emitting phosphors synthesized by carbothermal reduction and nitridation method Shuxing Li a,b, Xia Peng a,b, Xuejian Liu a,⇑, Zhengren Huang a a b
Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China University of the Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 14 August 2014 Received in revised form 9 October 2014 Accepted 22 October 2014 Available online 8 November 2014 Keywords: CaAlSiN3:Eu2+ Red phosphor Crystal phase Luminescence property
a b s t r a c t In this research, we have presented the synthesis and characterization of the various Ca1xEuxAl0.76Si1.18N3 (x = 0.01 0.1) red-emitting phosphors, which were successfully prepared by carbothermal reduction and nitridation (CTRN) method without the strict needs of high pressure. Here, raw materials were CaCO3, AlN, Si3N4, Eu2O3, and C. In particular, C was considered as efficient and robust reducing agent. The influences of reaction temperature, holding time, C content, and Eu2+ concentration were investigated in the crystal phase compositions and photoluminescence properties of the as-prepared phosphors. Importantly, CaAlSiN3:Eu2+-based red phosphors with interesting properties were obtained with reaction temperature at 1600 °C for 4 h by atmospheric N2–10%H2 pressure, and the C/O ratio of 1.5:1, respectively. The emission peak positions of as-prepared phosphors were red-shifted from 607 nm to 654 nm with Eu2+ concentration from 1 mol% to 10 mol%. Meanwhile the highest luminescence intensity was achieved with 2 mol% of Eu2+ concentration, which showed high external quantum efficiency up to 71%. Combining the phosphor blend of green-emitting b-sialon:Eu2+, yellow-emitting Ca-a-sialon:Eu2+, and red-emitting Ca0.98Eu0.02Al0.76Si1.18N3 with a blue LED (light emitting diodes), warm white LED can be generated, yielding the color rendering index (Ra) of 93 at correlated color temperature (CCT) of 3295 K. These results indicate that CaAlSiN3:Eu2+-based red-emitting phosphors prepared by facile CTRN are highly promising candidates for warm white LEDs. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction In recent years, rare earth doped (oxy)nitrides phosphors have been attracting much attention because of their nontoxicity, excellent thermal stability, and high absorption and conversion efficiency from near-UV to visible spectral region [1]. Typically, they are AlN:Eu2+ [2–4], and LaSi3N5:Ce3+ [5] blue-emitting phosphors, b-SiAlON:Eu2+ [6–9], CaSi2O2N2:Eu2+ [10,11], and c-AlON:Mn2+ [12] green-emitting phosphors, Ca-A-SiAlON:Eu2+ [13,14], and Y3Si6N11:Ce3+ [15] yellow-emitting phosphors, and SrAlSi4N7:Eu2+ [16], M2Si5N8:Eu2+ (M = Ca, Sr, Ba) [17–19], and CaAlSiN3:Eu2+ [20–23] red-emitting phosphors. Among these (oxy)nitrides, CaAlSiN3:Eu2+ red-emitting phosphors have been used as key materials for high color rendering index (CRI) and low correlated color temperature (CCT) solid illumination parts, which satisfy the requirements of medical and architectural lighting fields. Both excitation and emission bands of CaAlSiN3:Eu2+ showed very low energies due to the contributions by a combination of strong ⇑ Corresponding author at: Dingxi Road 1295, Shanghai, China. Tel.: +86 21 52414220; fax: +86 21 52413903. E-mail address: [email protected] (X. Liu). http://dx.doi.org/10.1016/j.optmat.2014.10.039 0925-3467/Ó 2014 Elsevier B.V. All rights reserved.
crystal-field splitting of the 5d states of Eu2+ ions and high covalence (nephelauxetic effect), especially for the first nearest coordination of Eu–N [24]. Up to now, various preparation methods for CaAlSiN3:Eu2+ redemitting phosphors have been reported. It included traditional high temperature solid state reaction (SSR) [19], spark plasma sintering (SPS) [25] with Ca3N2–AlN–Si3N4 as raw materials, self-propagating high-temperature synthesis (SHS) [22], direct nitridation [26], and ammonothermal synthesis [21,27] based on Ca1xEuxAlSi alloy. However, raw materials of all the mentioned methods were air-sensitive and oxygen-free metal nitrides or active metals, especially for expensive and deliquescent Ca3N2. Consequently, all the procedures before sintering must be taken in a glove box filled with N2 to avoid oxidation. Additionally, crushing and classification process must be taken on the phosphors prepared at relative high temperature (P1800 °C) and pressure (P0.9 MPa). At the same time, impurities would be inevitably introduced during crushing and classification process. Ammonothermal synthesis by nitridation of (Ca, Eu)AlSi alloy in supercritical ammonia (100 MPa) could never be industrially applicable because of extremely high pressure for use. Recently, Suehiro and co-workers [23] reported the synthesis of CaAlSiN3:Eu2+ phosphors by gas reduction nitridation (GRN)
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method with complex process, however, the obtained product contained impurities of AlN and unreacted CaO. By comparison with mentioned methods, carbothermal reduction and nitridation (CTRN) method possesses the high advantage of relative low reaction temperature, cheap raw materials, and simple processing. At present, there are few reports of the synthesis of CaAlSiN3:Eu2+ red phosphors through CTRN method except for the research by Kim et al. [28,29] with hazardous CaCN2 as calcium and carbon sources. In this research, nominal compositions of Ca1xEuxAl0.76Si1.18N3 (x = 0.01 0.1) red phosphors were synthesized with stable and inexpensive CaCO3 as calcium source at relatively low temperature and by atmospheric pressure. The main effects of reaction temperature, holding time, C content and Eu2+ concentration on crystal phase compositions and photoluminescence properties were investigated in order to develop low-cost and high-quality CaAlSiN3:Eu2+-based red-emitting phosphors. 2. Experimental 2.1. Synthesis Nominal compositions of Ca1xEuxAl0.76Si1.18N3 (x = 0.01, 0.02, 0.03, 0.05, 0.08, and 0.1) red-emitting phosphors were synthesized according to the stoichiometric ratio. The raw materials of CaCO3 (99.99%), AlN (Tokuyama, H-Grade), a-Si3N4 (UBE, E-10), Eu2O3 (99.99%), and C powder (99.95%) were homogeneously mixed in a silicon nitride mortar. The resultant mixture was heated in atmospheric N2 at a constant flow rate of 200 mL/min at 850 °C for 1 h to decompose CaCO3 completely. Then, H2 gas was flowed inside the horizontal tube furnace by 200 mL/min N2 to 20 mL/min H2 gas mixture to guarantee the complete reduction of Eu3+ to Eu2+. Next, the temperature was increased to 1550–1650 °C, and maintained for 1–6 h. Finally, the fired samples were ground finely and prepared for measurements. 2.2. Characterization The crystal phases of sintered samples were determined by X-ray diffraction (XRD) (Bruker D8 Advanced) with Cu Ka1 radiation (k = 0.15406 nm) operating at 40 kV/40 mA with a step size of 0.02° and a scan speed of 5° min1. Electron micrograph of the typical sample was obtained by field-emission scanning electron microscope (Hitachi S-4800). Partial elemental analysis of the phosphors was performed using C measurement device (SICMMC-222) for C content, and O and N analyzer (LECO, Model TC600) for O and N content, respectively. The photoluminescence spectrum and quantum efficiency of powder phosphors were determined by fluorescence spectrophotometer (Jobin Yvon, Fluoromax-4) with 150W Xe lamp as excitation source. The quantum efficiency was estimated using an integrating sphere unit (Jobin Yvon, F-3029) with a monochromatic source of 460 nm. For the temperature-dependent photoluminescence measurement, the powder sample was loaded in a sample cavity and then heated to the desired temperature by a high-temperature fluorescence controller (Tianjin Orient KOJI Co., Ltd., TAP02). The sample was kept for 10 min to guarantee a uniform temperature both on the surface and in the interior of the sample. The optical properties of as-fabricated white LED was investigated by a UV– VIS-near IR Spectrophoto Colorimeter (PMS-80) with an integrating sphere under forward-bias current of 20 mA at room temperature.
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1650 °C with C/O ratio of 1.5:1 and the same holding time for 4 h, respectively. As CaCO3 was decomposed completely into CaO above 1000 °C, the O of C/O referred to oxygen element in CaO and Eu2O3 oxides. We suggested that the powders synthesized at temperature of 1550 °C contained a small amount of impurity phases Ca2SiO4 and unreacted C. The impurity phases of Ca2SiO4 and unreacted C gradually disappeared with an increasing temperature, and the CaAlSiN3 phase (JCPDS 39-0747) and a very small amount of CaAl0.54Si1.38N3 phase (ICSD 161796) [24] were formed without impurities at temperatures of 1600 °C and 1650 °C in Fig. 1. The excitation and emission spectra of Ca0.98Eu0.02Al0.76Si1.18N3 phosphors synthesized at different temperatures were presented in Fig. 2. Each phosphor showed a broad excitation spectra from 250 to 600 nm, matching effectively with UV (380 nm) and (or) blue (450 nm) LED chips. The strong orange–red emission spectra centered at about 630 nm were attributed to the characteristics of 4f65d1 ? 4f7 transition of Eu2+ ions [20–23]. The product synthesized at temperature of 1550 °C became gray because of the existence of unreacted C, which absorbed visible spectra intensively, leading to the large reduction of luminescence intensity. The phosphor heated at 1600 °C showed the best luminescence whose full width at half maximum (FWHM) of the emission band was about 100 nm. However, when we increased the reaction temperature up to 1650 °C, the luminescence intensity decreased, the reason for which was unclear in this case. 3.2. Holding time Under the atmosphere of C and H2 strong reducing agents, CaO and Eu2O3 were reduced to corresponding metals, and at the same time, they were nitrogenized and N needed to diffuse into metal particles in order to react completely. However, in the solid–solid reaction process N diffused very slowly into metal particles [2]. At the same time, low reactivity and small self-diffusion coefficient of silicon nitride and aluminum nitride also slowed the solid–solid diffusion process, but diffusion process was the rate-determining step in the whole reaction process [18]. Thus, the holding time was an important factor to produce pure crystal phase with good crystallization. As a result, the effect of holding time on the most typical XRD patterns of Ca0.98Eu0.02Al0.76Si1.18N3 phosphors synthesized at 1600 °C with C/O ratio of 1.5:1 was shown in Fig. 3. The intensity of diffraction peaks of unreacted C decreased without eliminating completely with holding time increasing from 1 h to 2 h, indicating that the C content decreased with longer holding
3. Results and discussion 3.1. Reaction temperature Fig. 1 showed typical X-ray diffraction (XRD) patterns of Ca0.98Eu0.02Al0.76Si1.18N3 powders synthesized at 1550 °C, 1600 °C, and
Fig. 1. X-ray diffraction patterns of Ca0.98Eu0.02Al0.76Si1.18N3 powders synthesized at different temperatures.
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Fig. 2. Excitation and emission spectra of Ca0.98Eu0.02Al0.76Si1.18N3 powders synthesized at different temperatures.
Fig. 4. Excitation and emission spectra of Ca0.98Eu0.02Al0.76Si1.18N3 powders synthesized at 1600 °C for different holding times.
Fig. 3. XRD patterns of Ca0.98Eu0.02Al0.76Si1.18N3 powders synthesized at 1600 °C for different holding times.
Fig. 5. XRD patterns of Ca0.98Eu0.02Al0.76Si1.18N3 powders synthesized at 1600 °C for 4 h with different C/O ratios.
time. The CaAlSiN3 phase (JCPDS 39-0747) and a very small amount of CaAl0.54Si1.38N3 phase (ICSD 161796) [24] were obtained without impurities with holding time for 4 h and 6 h. It meant that the complete reaction of the raw materials occurred. In addition, the diffraction peaks did not significantly show any changes between 4 h and 6 h. The excitation and emission spectra of Ca0.98Eu0.02Al0.76Si1.18N3 phosphors heated at various holding times were shown in Fig. 4. The products heated at 1600 °C for 1 h and 2 h exhibited the much weaker luminescence intensity because of the existing of unreacted C, while the product heated at 1600 °C for 4 h showed the strongest luminescence intensity due to its better crystallization of CaAlSiN3-type phase. The luminescence intensity of the product with a longer holding time for 6 h decreased, the reason for which was unclear. Therefore, a reaction temperature at 1600 °C and a holding time for 4 h were found to make Ca0.98Eu0.02Al0.76Si1.18N3 red-emitting phosphors with good luminescence.
(JCPDS 39-0747) and a very small amount of CaAl0.54Si1.38N3 phase (ICSD 161796) [24] were obtained with C/O ratio of 1.5:1. With increasing C/O ratio from 1.7 to 2.0:1, the diffraction peaks of residual C appeared, indicating the excess of C. The excitation and emission spectra of Ca0.98Eu0.02Al0.76Si1.18N3 phosphors synthesized with different C/O ratios were presented in Fig. 6. No obvious changes could be detected in the photoluminescence spectra profiles with different C/O ratios. The phosphor synthesized with C/O of 1.5:1 gave the best luminescence intensity due to the evidence of no other impurities except for CaAlSiN3 and a very small amount of CaAl0.54Si1.38N3 phase. Fig. 7 exhibited the mass fractions of C, N, and O in Ca0.98Eu0.02Al0.76Si1.18N3 red1-emitting phosphors synthesized with different C/ O ratios. With the ratio of C/O from 0.8 to 1.5:1, mass fraction of O sharply decreased with increasing C/O ratios, while C slowly increased. With the ratio of C/O from 1.5 to 2.0:1, mass fraction of O slowly decreased with increasing C/O ratios, while C sharply increased. The N content started to increase at low C/O ratios and reached its maximal of 26.68% at C/O ratio of 1.7:1 and then decreased, while the theoretical mass fraction of N in Ca0.98Eu0.02Al0.76Si1.18N3 was 30.45%. Thus, O and C contents were relatively
3.3. C content Fig. 5 showed the XRD patterns of Ca0.98Eu0.02Al0.76Si1.18N3 powders with different C/O ratios, reaction temperature at 1600 °C, and the same holding time for 4 h. When the ratio of C/ O was below 1.2:1, impurity of Ca2SiO4 existed. The CaAlSiN3 phase
1 For interpretation of color in Fig. 7, the reader is referred to the web version of this article.
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Fig. 6. Excitation and emission spectra of Ca0.98Eu0.02Al0.76Si1.18N3 powders synthesized at 1600 °C for 4 h with different C/O ratios.
low and N content was relatively high with C/O ratio of 1.5:1 in our red phosphors. At the same time, the photoluminescence of the products was closely related to their C, N, and O content. Luminescence intensity was reduced significantly with increasing C content due to its strong absorption of visible spectra. When O content was high, crystal phase of oxide impurity was easily formed or the O atoms entered into the matrix lattice of CaAlSiN3 to replace the crystallographic site of N atoms and coordinate with the activator Eu2+ ions, leading to the formation of lattice distortion and point defects. All the above mentioned issues were unfavorable for a considerable improvement of luminescence. Nevertheless, nitrogen-rich coordination phenomenon inside red-emitting phosphors was formed more easily with higher N content, leading enhanced electronegativity and nephelauxetic effect [30]. Hence, the 5d excited states of Eu2+ ions caused a large splitting and thus owned lower energy levels. Therefore, electron transition between the 4f ground state and the lower energy levels of the 5d excited state was favorable and easier, resulting in an enhancement of emission intensity. 3.4. Europium concentration We demonstrated the influence of Eu2+ concentration on the XRD patterns of the nominal compositions of Ca1xEuxAl0.76Si1.18N3 (x = 0.01 0.1) phosphors heated at 1600 °C for 4 h with C/O ratio of 1.5:1 (Fig. 8a). The minor impurity phase of Ca2SiO4 was detected with Eu2+ concentration beyond 3 mol% because more
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oxygen atoms were introduced with increasing of raw materials Eu2O3, suggesting the inadequate reduction reaction. It was certain that the diffraction peak slightly shifted to low diffraction angle with increasing Eu2+ concentration, indicating expansion of crystal lattice, which was caused by the substitution of smaller Ca2+ ion (r = 1.14 Å, CN = 6) by larger Eu2+ ion (r = 1.31 Å, CN = 6) [31]. The typical particle morphology and photograph of synthesized Ca0.98Eu0.02Al0.76Si1.18N3 powder were shown in Fig. 8b and c, respectively. The large CaAlSiN3-type microstructures synthesized were confirmed in the crystal structure of phase CaAlSiN3 (JCPDS 39-0747) in the typical XRD patterns. The large CaAlSiN3-type microstructures had various shapes and morphologies, such as rods, plates, and polyhedral shapes. The particle sizes were about 10 lm. The excitation and emission spectra of Ca1xEuxAl0.76Si1.18N3 (x = 0.01 0.1) phosphors with different Eu2+ concentration were shown in Fig. 9. For all Ca1xEuxAl0.76Si1.18N3 (x = 0.01 0.1) phosphors, starting at low Eu2+ concentration, the emission intensity increased to a maximum at doping concentration of x = 0.02 with external quantum efficiency of 71%, and then fell again steadily to a minimum at x = 0.1. In this context, the decrease in emission intensity beyond critical concentration can be explained by concentration quenching [32], which was mainly caused by the effect of non-radiative energy transfer between Eu2+ ions. Increasing the concentration of Eu2+ ions caused the increase of probability of Eu– Eu pairs or clusters, which led to the non-radiative energy transfer between Eu2+ ions or between Eu2+ ion and the host resulting in the reduction of photoluminescence intensity. According to Blasse formula [32]:
RC ¼ 2
3V 4p X C N
13 ð1Þ
where N is the number of host cations in the unit cell, XC is the quenching concentration of Eu2+, and V is the volume of the unit cell. For Ca0.98Eu0.02Al0.76Si1.18N3 host, V is 275.73 Å3 according to lattice parameters of a = 5.6487 Å, b = 9.6754 Å, and c = 5.0450 Å. Additionally, N and XC are 4 and 0.02, respectively. Therefore, the critical distance (RC) for energy transfer from critical doping concentration was calculated to be 18.74 Å. In addition, as shown in Fig. 10, the emission peaks of Ca1xEuxAl0.76Si1.18N3 phosphors showed a shift from 607 nm to 654 nm with the Eu2+ concentration from 1 mol% to 10 mol%, which was ascribed to an increase in crystal field splitting. With increasing Eu2+ concentration and expansion of the crystal lattice, crystal field around Eu2+ ions created larger splitting and energy gap of 5d excited state became larger, and correspondingly the 5d excited
Fig. 7. Dependence of mass fraction of carbon, nitrogen, and oxygen on the C/O ratios.
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Fig. 8. (a) XRD patterns of Ca1xEuxAl0.76Si1.18N3 (x = 0.01 0.1) powders synthesized at 1600 °C for 4 h. (b) Typical SEM image of Ca0.98Eu0.02Al0.76Si1.18N3. (c) Photograph of Ca0.98Eu0.02Al0.76Si1.18N3.
Fig. 10. Dependence of peak emission intensity and wavelength on the Eu2+ concentration. Fig. 9. Excitation and emission spectra of Ca1xEuxAl0.76Si1.18N3 (x = 0.01 0.1) powders synthesized at 1600 °C for 4 h.
state possessed lower energy levels [33]. As a result, the emission peak shifted to the longer wavelength. 3.5. Application Fig. 11 illustrated normalized emission spectra of the white LED fabricated by combining the phosphor blend of green-emitting
b-sialon:Eu2+ (kem = 550 nm), yellow-emitting Ca-a-sialon:Eu2+ (kem = 580 nm), and as-prepared red-emitting Ca0.98Eu0.02Al0.76Si1.18N3 (kem = 630 nm) with a blue LED (kem = 450 nm). The obtained warm white LED gave high color rendering index (CRI) Ra of 93 at correlated color temperature (CCT) of 3295 K, and the R9 value was around 89. At the same time, the chromaticity coordinates located at x = 0.3871 and y = 0.3241, respectively. In addition, the relative emission intensity of Ca0.98Eu0.02Al0.76Si1.18N3 as a function of temperature was depicted in Fig. 12. The emission
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strongest emission intensity achieved at Eu2+ concentration of 2 mol% with external quantum efficiency up to 71%. Given correlated color temperature of 3295 K as well as the color rendering index of 93 for the obtained warm white LED, red-emitting phosphors prepared by CTRN can be very potential candidates for warm white LEDs. Acknowledgements The authors are very grateful to the financial support from the National Natural Science Fund of China (No. 51172263) and the Science and Technology Innovation Initiative of Shanghai Institute of Ceramics, Chinese Academy of Sciences (SICCAS). References
Fig. 11. Normalized emission spectra of the white LED fabricated by combining the phosphor blend of green-emitting b-sialon:Eu2+ (kem = 550 nm), yellow-emitting Ca-a-sialon:Eu2+ (kem = 580 nm), and as-prepared red-emitting Ca0.98Eu0.02Al0.76Si1.18N3 (kem = 630 nm) with a blue LED (kem = 450 nm). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 12. Temperature dependence of the emission intensity of Ca0.98Eu0.02Al0.76Si1.18N3 phosphor.
intensity decreased only about 9% at a fixed temperature of 250 °C compared to the intensity measured at room temperature, indicating good thermal stability. These results demonstrate that as-prepared red-emitting phosphors by facile CTRN method are good candidates for warm white LEDs. 4. Conclusions Nominal compositions of red-emitting phosphors, e.g. Ca1xEuxAl0.76Si1.18N3 (x = 0.01 0.1) were prepared by carbothermal reduction nitridation process. The interesting crystal phase of Ca1xEuxAl0.76Si1.18N3 was obtained at 1600 °C for 4 h by atmospheric N2–10% H2 pressure with C/O ratio of 1.5:1. With Eu2+ concentration from 1 mol% to 10 mol%, the emission peaks of the phosphors showed a red-shift from 607 nm to 654 nm. The
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