Moisture-induced degradation of the narrow-band red-emitting SrLiAl3N4:Eu2+ phosphor

Accepted Manuscript Moisture-induced degradation of the narrow-band red-emitting SrLiAl3N4:Eu phosphor

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Wenxia Li, Zhen Song, Dianpeng Cui, Zhiguo Xia, Quanlin Liu PII:

S1002-0721(17)30132-1

DOI:

10.1016/j.jre.2017.09.010

Reference:

JRE 87

To appear in:

Journal of Rare Earths

Received Date: 8 June 2017 Revised Date:

18 August 2017

Accepted Date: 5 September 2017

Please cite this article as: Li W, Song Z, Cui D, Xia Z, Liu Q, Moisture-induced degradation of the 2+ narrow-band red-emitting SrLiAl3N4:Eu phosphor, Journal of Rare Earths (2017), doi: 10.1016/ j.jre.2017.09.010. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Moisture-induced degradation of the narrow-band red-emitting SrLiAl3N4:Eu2+ phosphor

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Wenxia Li (李文霞)1, Zhen Song (宋振)1, Dianpeng Cui (崔殿鹏)1ZhiguoXia (夏志国)1, Quanlin Liu (刘泉林)1,* 1 The Beijing Municipal Key Laboratory of New Energy Materials and Technologies, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China * Corresponding author E-mail:[email protected] (Q. L. Liu). Tel. : +86-10-62334705; Fax. : +86-10-62334705

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Abstract： ：The degradation of materials plays an important role in their application and service process. In this work, the moisture-induced degradation of SrLiAl3N4:Eu2+ (SLAN), a very promising narrow-band red-emitting phosphor, was comprehensively investigated by treating it in two different moisture conditions in order to reveal the potential mechanism and optimize the luminescence properties. The degradation rate gradually slows down with the decreasing environmental humidity indicating that water plays a key role in the degradation. Moreover, we take the other option with 100% humidity at different temperatures for rapid degradation. In the rapid degradation, the luminescence of SLAN is quenched quickly and the phase and microstructure change obviously, with the phosphor being bleached. The host turns into NH3, Al2O3, Sr3Al2(OH)12 and LiAl2(OH)7 finally. It is further confirmed that the rapid degradation occurs with the help of water and the phosphor is oxidized during this process.

1. Introduction

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Key words: degradation; narrow-band; phosphor; mechanism

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Stability of materials plays an important role in materials application and service process. The degradation, as an irreversible transformation, will largely destroy the stability of materials, and thus make them lose their properties. Degradation of a material is influenced by its chemical compositions and crystal structure, as well as by a series of environmental factors such as temperature, humidity, pH, etc[1-7]. In addition, time is also an important factor of the degradation. The degradation usually turns to speed up or decelerates with time passing. White-light-emitting diodes (wLEDs) will definitely replace conventional lamps for general lighting in the near future as the result of their improved brightness and color-rendering properties[8]. Phosphors play very important roles in determining the luminescence efficacy, color temperature, color rendering, and reliability of the devices. Most commercial wLEDs are based on the blue-LED chips fabricated with yellow-light-emitting Y3Al5O12:Ce3+ (YAG:Ce3+) phosphor[9,10]. However, the performance is limited to cool white light (correlated color temperature of 4000-8000 K) and the color-rendering index (CRI < 75)[11,12]. Red-light-emitting phosphors, which can produce more saturated colors, larger color gamut and lower correlated color temperature have attracted extensive attention recently. To date, nitride red phosphors are widely applied commercially due to their excellent luminescence efficiency such as Sr2Si5N8:Eu2+ and CaAlSiN3:Eu2+[1,13-19]. However, both phosphors show broad-band emissions which limit the maximum achievable luminous efficiency for solid-state illumination and color gamut for

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Experimental

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display[20,21].The recently reported narrow-band red-emitting nitridoaluminate SrLiAl3N4:Eu2+ (SLAN) phosphor has been reported and drawn a lot of attention, which not only has excellent luminescence properties, but also shows a significant improvement in the color rendition and luminous efficacy of wLEDs[22,23]. However, SLAN is unstable when exposed to moisture and other atmospheric components mainly due to the presence of lithium, an alkali metal. Considering the potential application, the stability evaluation of SLAN is a primary requisite, because the degradation in moisture is an irreversible transformation. Notable examples include (Sr,Ca)AlSiN3:Eu2+[1],Sr2Si5N8:Eu2+[2], etc; however, moisture-induced degradation of SLAN has not been discussed yet. So this work aims to study the moisture-induced degradation of SLAN revealing the potential degradation mechanism. Once the degradation mechanism is properly understood, the next one can find a better way to treat the phosphor to guard against the degradation. In general, the stability of the phosphor for solid state lighting was evaluated by continuous damp heat (85 oC/85%) tests for at least 1000 hours. In order to quickly sense the reliability of the phosphor, the testing time should be significantly reduced. In other words, it is necessary to create a more serious environment for moisture-induced degradation in order to save time which will be beneficial to the application of the phosphor, although it is not an industry standard. In this paper, two methods are used for the moisture-induced degradation of SLAN. One is treating the phosphors in a condition in which the humidity is decreasing. The other is treating the phosphors under a much more serious condition with constantly 100% humidity to evaluate the rapid degradation behavior of SLAN. The photoluminescence, phase and microstructure of the treated samples are analyzed, and the rapid degradation mechanism is also discussed.

2.1 Material and characterization

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The SrLiAl3N4phosphor with the composition of Sr0.98LiAl3N4:Eu0.02 (SLAN) was prepared by firing the powder mixture of Sr3N2, Li3N, AlN and EuN at 1100 oCfor 4 h under a N2 atmosphere in a graphite furnace[24]. The photoluminescence (PL) spectra were measured using a Hitachi F-4600 spectrophotometer equipped with a 150 W xenon lamp as the light source. X-ray powder diffraction (XRD) patterns were recorded on a Philips X’Pert PW-3040 (Cu Kα radiation, 40 kV, 35 mA, λ = 0.15406 nm). The morphology of particles was observed using a scanning electron microscope (SEM, JEOLJSM-6510A) and the energy dispersive spectrometers (EDS) were also observed using the same equipment. X-ray photoelectron spectroscopy (XPS) was carried out using a photoelectron spectrometer (AXIS ULTRADLD) with Al Kα monochromatic X-ray radiation. All the measurements were performed at room temperature. 2.2 Experimental process for the moisture-induced degradation All the experiments were carried out in an autoclave with a Teflon lining of 50 mL. The degradation process is shown in Fig. 1. 0.1g SLAN powder was put into an alumina crucible that was placed in an autoclave. Then appropriate amount of water was added to the autoclave. Afterwards, the autoclave was lidded and loaded in an oven at a constant temperature for a certain period of time. In the first type of experiment, the amount of water was calculated according to the equation: pV= nRT (1)

ACCEPTED MANUSCRIPT whereV is the volume of the Teflon lining, n the mole of water, R the universal gas constant, T the absolute temperature, and p the saturated vapor pressure at a certain temperature. The experiment was carried out at 85 oC. However, it was necessary to add 10 mL water to the Teflon lining in the second type of experiment of rapid degradation. In this case, the crucible was immersed in water while the phosphor was not in direct contact with water. Results and discussion

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In the first type of experiment, we measured a series of emission spectra of SLAN at 85 oC for different periods of time as shown in Fig. 2(a). It can be seen that the luminous intensity decreases more and more slowly as time goes on. The reason for this is that the humidity is 100%at the beginning of the reaction and the humidity gradually reduced with the progress of the reactiondue to a very slow leak of vapor. The luminescence intensity is basically unchanged in the later stages of the experiment and the humidity is the same as that before the experiment. Moreover, the phase remains unchanged as shown in Fig. 2(b). Therefore, it can be concluded that the water plays a significant role in the decline of the luminous intensity. Under these circumstances, we designed to add more water of 10 mL in the second program to further study the effect of water on the aging of the phosphor. To determine the variation of photoluminescence properties of the samples in rapid degradation with 100 % humidity, we studied the influence of reaction time on the degradation (Fig. 3).It shows the typical emission spectra of SLAN phosphor with narrow bands covering the spectral range of 575 to 725 nm, peaking at 649 nm. The peak positions do not change with increasing treating time while the peak intensity of the PL decreases. This result may be due to the gradual entry of water into the interior of the sample. Also, it can be reflected from the phosphor color that the phosphors bleach out gradually as shown in the insets of Fig. 5(a). Also, the insets of Fig. 3 exhibit some pictures of the phosphors under the ultraviolet light radiation. We can see that the untreated phosphor emits a dazzling red light, and with the increase of time, the color of the emitted light gradually dimmed. In particular, the luminescence intensities of the samples almost disappear after treatment for 72 h at 85 oC accompanied with the color of the sample becoming white. On the other hand, the influence of reaction temperature on the rapid degradation was also studied (Fig. 4). It is shown that as the temperature increases, the luminescence intensity decreases faster with increasing treating time, especially at 85 oC. The luminescence loss is 93.47% at 85 o Cfor samples treated for 4 h in rapid degradation (vs. 13.02% at 55 oC). The degradation originates from the decomposition of SLAN. XRD patterns were performed for the samples after treatment for different time at 85 oC, as shown in Fig. 5(a). It shows that the untreated sample is of good crystallinity. When the samples are treated for 1 h, the reduction in the peak intensity clearly suggests that the phosphor starts to degrade. As the treating time increases, the peak intensity continues to decrease. At the same time, some new diffraction peaks appear, signifying that SLAN is destroyed gradually. For the samples treated for 72 h, we can see that the remains are almost composed of Al2O3, Sr3Al2(OH)12, LiAl2(OH)7, which indicates that the degradation is carried out thoroughly. Moreover, the insets vividly show the color change of the phosphor during the experiment: from pink to pale pink, and finally white. This is also consistent with the observed morphological changes in Fig. 6.

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In addition, there is ammonia odor in the degradation process. Therefore, we studied the pH change of the solution in rapid degradation (Fig. 5(b)). The pH of treating for 0h, 1h, 2h, 3h, 4h, 5h, 6h, 72h are 7.00, 10.22, 10.50, 10.79, 11.07, 11.14, 10.73, 9.42 respectively. Obviously, the pH value first increases and then decreases with time. It reaches the maximum when the sample is treated for 5 h, at which time the rapid degradation is essentially complete, which is in good correspondence with the XRD patterns. After that the volatilization of NH3 resulted in a decrease in pH value. Figure 6 gives the SEM images exhibiting the surface morphology of the samples before and after degradation. It is found that all the as-prepared particles have an even size distribution of 1–2 µm with some agglomerates and exhibit smooth surfaces (Fig 6(a)). The particles begin to break down after treating for 2 h with some fine debris on the surface which coincides with the reduction of peak intensity in XRD patterns. Also, the tiny phosphor particles form some larger aggregates of 5-7 µm(Fig 6(b)). After a 4-hour treatment, large clusters with the size of about 10 µm can be observed withmany debris (Fig 6(c)). Afterwards, some novel lamellar crystals grow from the phosphor powderafterdegradingfor 72 h and the morphology of the sample is completely different from the beginning(Fig 6(d))which corresponds to the disappearance of the luminescence in Fig. 3. In fact, there is still a lot of water in the reactor when it is opened and it can be clearly seen that some water goes into the phosphor. So it is believed that H2O infiltrates the interior of the particles and causes the degradation. It is very important to investigate the luminescence degradation of nitride phosphor in order to better understand the factors controlling the luminescence loss and provide some suggestions on design and applications of materials. Honda et al. confirmed that the supply of oxygen via moisture was faster than in dry atmosphere[3]. Also, it is believed the aging of the phosphor is mainly due to the presence of water from the above analysis. Herein, we guess that the phosphor is oxidized during the degradation with the help of water which can be proved from the following analysis. The EDS data for SLAN of untreated and treated for different times are shown in Fig. 7. As can be seen, the peak related to N is getting weaker as time increases while the peak related to O is getting stronger. The measurements indicate that the phosphor is gradually oxidized during the degradation. To further confirm the oxidation of the phosphor, the valence of the Eu ion was determined by the XPS measurements of the untreated and 72 h-treated SLAN phosphors. XPS analysis was conducted on the Eu3d5/2 core level, as shown in Fig 8(a). The chemical state of Eu ions can be clearly identified on the basis of the energy position and the shape of the Eu3d5/2 core level in the XPS spectra of the Eu3+ and Eu2+ states of SLAN. The measurements suggest that the conversion of Eu3+ is higher than that of Eu2+, indicating that the material is oxidized leading to the conversion from Eu2+ to Eu3+. In addition, Bizarriet al. confirmed the oxidation process for the electron transfer from dopant ions to adsorbed oxygen ions occurs when the divalent europium ions are close to adsorbed oxygen ions[4]. To obtain further proof for the existence of oxygen, the O/N ratio for the samples was determined by XPS analysis as shown in Fig. 8(b). The binding energy from 402 to 392 eV is assigned to the N 1s level and 536 to 526 eV is assigned to the O 1s level. We can see a stronger signal of the oxygen peak in the sample after treatment for 72 h. The degradation process makes an increase in the mobility of oxygen to flow more quickly and diffuse into the lattice with the help of water[5].

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Moreover, another thing worthy to be mentioned is the understanding of the structure-property relationships depending on their chemical compositions. Comparing SLAN to CaAlSiN3:Eu2+ (SCASN), SCASN has arelatively stable and rigid crystal structure, and the coordinated tetrahedrons in the six-ring of Ca2+ ions contain three [AlN4] tetrahedrons and three [SiN4] tetrahedrons. However, Sr sites are coordinated by eight N atoms, forming a strand in every second channel in SLAN. The channels are comprised of edge- and corner-sharing AlN4 and LiN4 tetrahedrons and these tetrahedrons also build a highly condensed, rigid framework. Also, we consider the bond length in SLAN (Al-N: 0.187–0.200 nm, Li-N: 0.198–0.210 nm and Sr-N: 0.269–0.291 nm)[22] and in SCASN (Si-N: 0.165–0.175 nm and Al-N: 0.175–0.185 nm)[25]. On the basis of the above-mentioned information of the structure and bond length, it is believed that SCASN is more stable than SLAN which also explains why SLAN is so easy to break down when encountering water. So it is believed during the degradation that the introduction of hot water can result in the consumption of nitrogen and lithium components in the SLAN bulk and oxidize the phosphor. It is also believed that the oxidation starts at the phosphor particle surface, and then occurs inside the particle through the cracks with the help of moisture. And N in SLAN finally turns into NH3 because of the existence of H2O. From the above XRD analyses, SLAN phosphor finally turns into NH3, Al2O3, Sr3Al2(OH)12 and LiAl2(OH)7. So that we can put forward the following reaction equation: 6SrLiAl3N4(s) + 69H2O(g) →6LiAl2(OH)7(s) +2Sr3Al2(OH)12(s)+Al2O3(s)+24NH3(g)(2) 4 Conclusions

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In summary, we investigated the moisture-induced degradation of the narrow-band red-emitting phosphor SrLiAl3N4:Eu2+. As the humidity of the environment decreases from 100%, the degradation of the SLAN gradually slows down indicating that the water plays a key role in the degradation. Under these circumstances, we took the other option with 100% humidity for rapid degradation. It is confirmed that the phosphor cracks gradually when treated under a condition with 100% humidity, and at higher temperature cracks more rapid. Ultimately, the host turns into NH3, Al2O3, Sr3Al2(OH)12 and LiAl2(OH)7. Moreover, this work reveals the potential degradation mechanism. Once the degradation mechanism is properly understood, the next one can find a better way to treat the phosphor to guard against the degradation.

Foundation item:

Project supported by the National Natural Science Foundation of China (51602019, 51472028), and Science and Technology Support Program of Jiangsu Province (BE2014047).

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The table and figure captions are as follows,

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Fig. 1 Schematics of the degradation experiments under conditions: (a) where the humidity is decreasing and (b) with constantly 100% humidity. Fig. 2 Luminescence intensity (a) and XRD patterns (b) of SLAN in the first type of experiment. Samples were excited at λex= 460 nm.

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Fig. 3Emission spectra of SLAN treated in rapid degradation at 85 oC for different time periods. Samples were excited at λex= 460 nm. The insets show some pictures of the phosphors under the ultraviolet light radiation. Fig. 4Luminescence intensity of SLAN treated in rapid degradation at different temperatures for different time periods. Samples were excited at λex= 460 nm.

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Fig. 5 (a) XRD patterns of SLAN treated in rapid degradation at 85 oC; the insets show the change in the color of the phosphor during the experiment and (b) profiles of pH changes as a function of time in the rapid degradation.

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Fig. 6 SEM images of the as-prepared SLAN (a) and of SLAN after treatment in rapid degradation at 85 oC for 2 h (b), 4 h (c), and 72 h (d). Fig. 7 EDS of the as-prepared SLAN (a) and of SLAN after treatment in rapid degradation at 85 o C for 72 h (b). Fig. 8 XPS spectra of the Eu3d5/2 core level (a) and wide scan ESCA spectra (b) for the untreated SLAN and SLAN after treatment in rapid degradation at 85 oC for 72 h.

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Fig. 1 Schematics of the degradation experiments under conditions: (a) where the humidity is decreasing; (b) with constantly 100% humidity.

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Fig. 2 Luminescence intensity (a) and XRD patterns (b) of SLAN in the first type of experiment. Samples were excited at λex= 460 nm.

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Fig. 3Emission spectra of SLAN treated in rapid degradation at 85 oC for different time periods. Samples were excited at λex= 460 nm. The insets show some pictures of the phosphors under the ultraviolet light radiation.

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Fig. 4Luminescence intensity of SLAN treated in rapid degradation at a certain temperature for different time periods. Samples were excited at λex= 460 nm.

Fig. 5 (a) XRD patterns of SLAN treated in rapid degradation at 85 oC; the insets show the change in the color of the phosphor during the experiment and (b) profiles of pH changes as a function of time in the rapid degradation.

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Fig. 6 SEM images of the as-prepared SLAN (a) and of SLAN after treatment in rapid degradation at 85 oC for 2 h (b), 4 h (c), and 72 h (d).

Fig. 7 EDS of the as-prepared SLAN (a) and of SLAN after treatment in rapid degradation at 85 C for 72 h (b).

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Fig. 8 XPS spectra of the Eu3d5/2 core level (a) and wide scan ESCA spectra (b) for the untreated SLAN and SLAN after treatment in rapid degradation at 85 oC for 72 h.

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Wenxia Li1, Zhen Song1, Dianpeng Cui1ZhiguoXia1, Quanlin Liu1,*

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Graphic Abstract

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Moisture-induced degradation of the narrow-band red-emitting SrLiAl3N4:Eu2+ phosphor

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The change of XRD patterns for SrLiAl3N4:Eu2+ phosphor during the rapid degradation.