Highly Stable Modified Phosphors of Ba2SiO4

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Highly Stable Modified Phosphors of Ba2SiO4:Eu2+ by Forming a Robust Hydrophobic Inorganic Surface Layer of Silicon-Oxy-ImideCarbide Bi Zhang,† Jiang-Wei Zhang,‡ Hao Zhong,† Lu-Yuan Hao,† Xin Xu,*,† Simeon Agathopoulos,§ Chengming Wang,∥ and Liang-Jun Yin⊥

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Chinese Academy of Science Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China ‡ Hefei Guoxuan High-Tech Power Energy Co., Ltd., 599 Daihe Road, Hefei, Anhui, 230011, People’s Republic of China § Materials Science and Engineering Department, University of Ioannina, GR-451 10 Ioannina, Greece ∥ Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China ⊥ School of Energy Science and Engineering, University of Electronic Science and Technology of China, 2006 Xiyuan Road, Chengdu, 610051, People’s Republic of China ABSTRACT: A highly stable hydrophobic Ba2SiO4:Eu2+ (BSO) phosphor was produced by forming a robust hydrophobic inorganic silicon-oxy-imide-carbide layer on its surface. The process for developing this layer involved coating, ammonolysis, and pyrolysis of chlorosilane. The tightly adhered coating layer had a lotus, leaf-like micro- and nanohierarchical structure and contained CH3 groups, which enhanced the hydrophobicity of the modified phosphor significantly. The modified phosphors treated at 500 °C exhibited remarkable stability in their emission spectra, even after exposure to high-pressure water steam at 100 °C for 48 h, which is attributed to the high stability of the surface layer.

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INTRODUCTION Solid-state lighting technology based on the system InGaN attracts great attention due to specific advantages mainly related to energy efficiency, long lifetime, compactness, and environmentally friendly features.1,2 The emission of LED chips is often down-converted into useful green, yellow, or red light by phosphors. These phosphors must have the proper photoluminescence (PL) properties, high quantum efficiency, low thermal quenching, and high reliability.3−8 Nevertheless, thermally induced as well as moisture-induced luminescence degradation of phosphors reduces the reliability and shortens the lifetime of white LEDs significantly.9,10 For instance, many well-known phosphors, such as sulfides (CaS:Eu and SrS:Eu),11,12 silicates,13 aluminates,14,15 and even (oxy)nitrides,16 are moisture-sensitive materials. Green-emitting Ba2SiO4:Eu2+ (BSO) phosphor, which features high quenching temperature, has great potential, owing to being used on traffic signs.17−21 Nonetheless, on account of its poor moisture resistance, this phosphor suffers from a serious degradation in luminescence, which prevents its wide commercialization. However, BSO is an attractive material because it has a much lower production cost than the popular green-emitting phosphors of SrSi2O2N2:Eu2+ or β-Sialon.22−24 © 2017 American Chemical Society

The mechanisms of degradation in phosphors as well as possible methods for improving their stability have been widely studied.10,16,25−28 For instance, the PL properties of europiumdoped strontium aluminate nanoparticles were maintained by surface modification reaction with pyrophosphoric acid or 3allyl-2,4-pentanedione.29,30 The MAl2O4:Eu2+,Dy3+ phosphor coated with a MAl2B2O7 (M = Sr, Ca, Ba) layer exhibited improved water resistance.31 Heterocyclic compounds, βdiketones, and multi-carboxylic acids were used to achieve coordination with the active ions in Eu2+-activated strontium aluminates and improve water resistance.32 Moreover, it was reported that atomic layer deposition based coatings on BaMgAl10O17 phosphor can improve thermal stability.33,34 However, there is still poor documentation on minimizing the moisture-induced degradation at high temperatures, which is considerably more severe than that at room temperature. Moisture-induced degradation is caused by hydrolysis of the host and/or oxidation of the luminescent center. A suitable inorganic layer may effectively isolate a phosphor particle from Received: January 23, 2017 Revised: May 8, 2017 Published: May 8, 2017 11616

DOI: 10.1021/acs.jpcc.7b00751 J. Phys. Chem. C 2017, 121, 11616−11622

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The Journal of Physical Chemistry C moist air. However, most inorganic films (for instance, pyrophosphate, silicates, and aluminates) are intrinsically hydrophilic materials due to the presence of surface hydroxyl groups (−OH), which can easily absorb water molecules. These water molecules destroy the inorganic layer or diffuse through the surface layer, eventually leading to luminescence degradation. A thick film can merely slow down the degradation rate, but it does not permanently suppress it. Thus, decay in emission intensity will eventually occur. Hence, a stable hydrophobic surface layer might effectively improve moisture resistance. Our recent study presented a simple, yet feasible, surface modification method, whereby a hydrophobic surface layer was developed on the surface of phosphors through hydrolysis and polymerization of tetraethylorthosilicate (TEOS) and polydimethylsiloxane (PDMS).35 This hydrophobic surface layer gave the modified phosphor superior stability in high-pressure water steam conditions. However, that coating has a rather polymeric nature, which will eventually result in the reduction of the long-term stability of the phosphor. Therefore, polymer-derived ceramics obtained from pyrolysis of silicon-containing polymer precursors have gained increasing interest recently.36,37 The resultant ceramic materials consist of Si-C, Si-O, and Si-N units. They constitute a new class of amorphous materials derived from the parent structure of polysilazane and polysiloxane.36,38,39 Nanoporous inorganic fibers of silicon-oxy-imide-carbide display superhydrophobicity due to the presence of −CH3 groups on their surface.40 In our earlier study, a porous silicon nitride substrate was coated with a porous silicon-oxy-imide-carbide layer through a combination of ammonolysis and pyrolysis processes. The obtained membrane exhibited excellent stability after exposure to boiling water, aqueous solutions with pH ranging from 2 to 12, and organic solvent (benzene).41 This paper presents a feasible method for coating BSO phosphors with a hydrophobic inorganic layer through ammonolysis and pyrolysis of chlorosilane. The similarity between the Si−O bonds in BSO phosphors and the Si−N and Si−O bonds in the hydrophobic surface layer which will be formed is expected to lead to a strong interfacial bonding between the substrate and the hydrophobic layer, as well as to the modified phosphor displaying superior stability in highpressure water steam conditions.

Corporation, >98.5, GC) were dissolved in 25 mL of heptane (Sinopharm Chemical Reagent Co. Ltd., Shanghai, China, 97%). The mixture was stirred for 5 min and sealed with a plastic wrapper. Next, 5 g of the BSO phosphor was placed in a BN boat, which was soaked in the aforementioned solution, and it was then placed in a quartz tubular furnace. Then, the samples were calcined at various temperatures, ranging from 100 to 800 °C, for 1 h under flowing NH3 gas. The obtained surface modified BSO phosphor powder (denoted as HB-BSOT, where T represents the calcination temperature of the surface layer) was crushed gently to obtain a dispersed powder. For comparison purposes, as-synthesized BSO phosphors were also calcined using the same procedure (denoted as BSO-T, where T represents the calcination temperature). The moisture-induced degradation test (hereafter merely referred to as moisture test) was conducted in an autoclave with a Teflon lining.33 About 0.2 g of BSO (either modified or not) was put into a BN container, which was placed in an autoclave with a volume of 50 mL. Then, 5 mL of water was added to the autoclave in such a way that the BN container was immersed in the water but the phosphor inside the container did not come into direct contact with the water. The autoclave was sealed and then placed in an oven at a constant temperature of 100 °C (in order to secure the creation of water vapors) for 48 h. After the experiment, the sample was allowed to cool down to room temperature naturally inside the furnace. The powder was dried at 80 °C overnight. X-ray diffraction analysis (XRD, Philips PW 1700, Almelo, The Netherlands; Cu Kα1 radiation, λ = 1.54056 Å) was conducted to identify the crystalline phases using Cu Kα radiation at a scanning rate of 2°/min. The PL spectra of the produced phosphors were recorded at room temperature with a fluorescent spectrophotometer (F-4600, Hitachi, Japan, with a 200 W Xe lamp as an excitation source). The internal and external quantum efficiency was measured using a QE-2100 spectrophotometer from PHOTAL Otsuka Electronics Company, Ltd. (Osaka, Japan).The microstructure of the powders was observed with a scanning electron microscope (JEOL JSM6390, equipped with a field emission gun at an acceleration voltage of 20.0 kV). Their structure was also investigated by high-resolution transmission electron microscopy (ARM200F), equipped with a device for SAED analysis. Fourier transform infrared spectroscopy (FT-IR) was carried out in a PerkinElmer 580B IR spectrophotometer with the KBr pellet technique. The X-ray absorption spectra for the Eu LIII-edge were measured at a beamline of BL14W1 at the Shanghai Synchrotron Radiation Facility. Thermal analysis (DTA/TGA) was carried out in an air atmosphere with a Shimadzu DTG60H thermobalance (Kyoto, Japan) at temperatures of up to 1000 °C with a heating rate of 10 K/min.

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EXPERIMENTAL SECTION The produced Eu2+-doped Ba2SiO4 phosphors had the chemical formula Ba1.94Eu0.06SiO4. Fine powders of BaCO3 (Sinopharm Chemical Reagent Co. Ltd., Shanghai, China, 99.0%), SiO2 (Sinopharm Chemical Reagent Co. Ltd., Shanghai, China, 99.99%), and Eu2O3 (Sinopharm Chemical Reagent Co. Ltd., Shanghai, China, 99.99%) were used. The amounts of raw powders were weighed according to the stoichiometry above. The powder mixtures were thoroughly wet ball-milled, with an appropriate amount of ethanol, for 48 h. After drying, the powder product was sieved. The resultant fine powder was placed in a BN crucible, which was put into an alumina tubular furnace. Firing took place at 1250 °C for 4 h under flowing N2 gas. The heating rate was 5 K/min. Cooling took place naturally inside the furnace under flowing N2 gas. Phase pure BSO phosphor powder was obtained. To produce the hydrophobic surface layer, 5 mL of methyldichlorosilane (Aladdin Industrial Corporation, 99%) and 5 mL of dimethyldichlorosilane (Aladdin Industrial

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RESULTS AND DISCUSSION The features of the hydrophobic layer formed on the surface of the particles of HB-BSO-500 powder were detected by TEM analysis (Figure 1). The surface of the phosphor particles was tightly covered by an amorphous layer with a thickness of 20− 40 nm, which had a vesicular structure with meso- and nanopores (Figure 1a). Many nanoparticles were formed in the surface layer, which was the hydrophobic layer. This lotus leaflike micro- and nanohierarchical hydrophobic layer (Figure 1b) significantly improves the hydrophobicity, as discussed later. The continuous rings and the weak, diffuse, halo rings of the electron-diffraction pattern (Figure 1c,e) suggest that the layer 11617

DOI: 10.1021/acs.jpcc.7b00751 J. Phys. Chem. C 2017, 121, 11616−11622

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The Journal of Physical Chemistry C

host. Hence, these results suggest that calcination only modifies the surface layer of phosphors. The information about the structure of the surface of the produced BSO and HB-BSO-T phosphors is provided by the FT-IR spectra shown in Figure 3. The bands of Si-O (at ∼820

Figure 3. FT-IR spectra of BSO and HB-BSO-T phosphors. Figure 1. TEM images of HB-BSO-500 at different magnifications from (a) to (c) (of the selected area) and the relevant results from SAED (d and e).

cm−1)42 are seen in the spectra of all phosphors, which is in broad agreement with the results from the X-ray diffractograms. In the spectra of the HB-BSO phosphors calcined at temperatures lower than 600 °C, the peak at ca. 1260 cm−1, which is attributed to the Si-CH3 group, is clearly seen, but it disappears in the spectra of the modified phosphors calcined at higher temperatures, evidently on account of the decomposition of the methyl group. The broad band due to N−H bonds (at ∼3400 cm−1)43 is also observed, which suggests that ammonolysis of chlorosilane took place during the calcination process. The results of the −Si−N− bonds (at ∼1040 cm−1) and −Si−O− bonds (at ∼1115 cm−1) can be seen in the spectra of the phosphors calcined at the high temperature of 700 °C, suggesting formation of SiNCO phase. The influence of calcination temperature on the maximum emission intensity of the BSO-T and HB-BSO-T phosphors, which was recorded in the PL spectra, is shown in Figure 4. A mainly single emission band in the region of 400−700 nm was recorded in all cases under excitation at 365 nm, which suggests

was amorphous, and fully covered the surface of the BSO particles. The dot pattern of the electron-diffraction pattern (Figure 1d) suggests that the Ba2SiO4 particle was well crystallized. To study the origin of the hydrophobic layer formed on the surface of the particles, the X-ray diffractograms of the produced BSO and the modified HB-BSO-T phosphors were recorded and are shown in Figure 2. The peaks of the Ba2SiO4

Figure 2. X-ray diffractograms of the samples (a) BSO, (b) BSO-500, (c) HB-BSO-300, (d) HB-BSO-400, (e) HB-BSO-500, (f) HB-BSO600, and (g) HB-BSO-700. The diffraction pattern of Ba2SiO4 (JCPDS No. 26-1403) is also plotted as (h).

host phase were clearly recorded in all cases. The increase of calcination temperature has little influence on the purity and the crystallinity of the produced phosphors. More specifically, evidence of the presence of some impurity phases was recorded in the diffractograms of the HB-BSO-T samples calcined at high temperatures, which may be ascribed to the small nanosized crystals that consisted of Si, N, C, O in the amorphous surface coating due to the reaction between the coating layer and the

Figure 4. Influence of calcination temperature on the maximum emission intensity of BSO-T and HB-BSO-T phosphors, recorded in the photoluminescence spectra under excitation at 365 nm, shown in the inset. 11618

DOI: 10.1021/acs.jpcc.7b00751 J. Phys. Chem. C 2017, 121, 11616−11622

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The Journal of Physical Chemistry C that the calcination had no influence on the emission wavelength. The weak emission peak of HB-BSO at around 430 nm should be attributed to the trace amount of SiNCO nanocrystals in the hydrophobic layer.44 Significant degradation in emission intensity was recorded in BSO-T after treatment at higher temperatures, which might be ascribed to the reaction between the surface of the BSO host and NH3, leading to a large amount of oxygen vacancies and the amorphous surface. On the other hand, the emission intensity of the HB-BSO phosphor increases with the increase of calcination temperature and reaches a maximum in the phosphors treated at 500 °C. Further increase of calcination temperature causes a decrease in emission intensity, probably owing to the reaction between the surface of BSO and the SiNCO coating. Similar emission spectra were recorded with the BSO and the HB-BSO-T phosphors before and after the moisture tests (not all of them presented). The influence of surface modification and calcination temperatures on the moisture resistance of these phosphors is observed in the values of the maximum emission intensity summarized in Table 1. The emission

Figure 5. (a) Emission intensity of BSO and HB-BSO-T samples treated in water steam at 100 °C for 48 h (moisture test) under excitation at 365 nm. (b) Photographs of BSO and HB-BSO-500 powders in water, and (c) water contact angle (∼110°) on the surface of a pressed HB-BSO-500 cake at room temperature.

hydrophobic surface. Indeed, the BSO particles formed a precipitate in the water (they were laid at the bottom of the container), whereas the HB-BSO-500 particles floated on top of the water surface (Figure 5b). Moreover, a high contact angle of ∼110° was measured at room temperature between the surface of a pressed HB-BSO-500 cake and water (Figure 5c), which suggests a poor wetting regime owing to the hydrophobic nature of the surface of the particles of this sample. Finally, it is worthy to note the poor moisture resistance in the sample HB-BSO-300 (Figure 5a). The X-ray diffractograms of BSO and HB-BSO-T powders after the moisture test are shown in Figure 6. The HB-BSO-T

Table 1. Influence of Moisture Test on Emission Intensity of the Produced Phosphorsa

a

sample

emission intensity (before)

emission intensity (after)

emission intensity (after)/emission intensity (before)

BSO HB-BSO-100 HB-BSO-200 HB-BSO-300 HB-BSO-400 HB-BSO-500 HB-BSO-600 HB-BSO-700

1000 999 996 977 1001 1053 1030 650

26 34 68 155 960 1009 896 85

2.6% 3.4% 6.8% 16% 96% 96% 87% 13%

The values have been normalized to BSO rescaling to a value of 1000.

intensity values of the BSO and the HB-BSO-T produced at low calcination temperatures decreased dramatically after the moisture test. However, the HB-BSO-T samples calcined at 400 and 500 °C maintained 96% of the initial intensity (i.e., before the moisture test). The internal quantum efficiency (ηi) and external quantum efficiency (ηo) of the BSO were determined to be 91.5% and 70.2%, respectively, while ηi and ηo of the HBBSO-500 reached 92.1% and 70.9%, and they were maintained as 90.9% and 70.1% after the moisture test, respectively. Nevertheless, higher calcination temperatures at 600 °C and in particular 700 °C resulted in phosphors which displayed a considerable degradation in moisture resistance. These results suggest that surface modification at a suitable temperature can improve moisture resistance substantially. In this regard, the calcination temperature should be fixed at 500 °C in the investigated system to create the optimum conditions for an effective improvement in moisture resistance. The PL spectra of BSO and HB-BSO-T before and after moisture test are plotted in Figure 5a. A mainly single emission band in the region of 400−700 nm was recorded in all cases under excitation at 365 nm. Thus, the moist atmosphere does not alter the emission spectrum. However, in the BSO sample, a significant decay in emission intensity was recorded after its exposure to water steam at 100 °C during the moisture test. On the other hand, the HB-BSO-500 sample featured remarkable moisture resistance, which should be attributed to its stable

Figure 6. X-ray diffractograms of BSO and HB-BSO-T after the moisture test (exposure to water steam at 100 °C for 48 h): (a) BSO, (b) HB-BSO-100, (c) HB-BSO-200, (d) HB-BSO-300, (e) HB-BSO400, (f) HB-BSO-500, (g) HB-BSO-600, and (h) HB-BSO-700. The diffraction pattern of Ba2SiO4 (JCPDS No. 26-1043) is also plotted as (i).

samples (for 400−600 °C) presented remarkable phase stability. The HB-BSO-T samples (for 600 °C) and the BSO samples were remarkably unstable, since new phases of BaSiO3·H2O (PDF No. 00-018-0179) and Ba(OH)2 (PDF No. 00-021-0073) were developed, whose formation is apparently ascribed to the hydrolysis of Ba2SiO4, via an oxidizing gas penetration mechanism:16 H 2O,O2

Ba 2SiO4 ⎯⎯⎯⎯⎯⎯⎯→ BaSiO3·H 2O + Ba(OH)2 11619

(1)

DOI: 10.1021/acs.jpcc.7b00751 J. Phys. Chem. C 2017, 121, 11616−11622

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The Journal of Physical Chemistry C These findings are in broad agreement with the PL behavior plotted in Figure 5a as well as with the results summarized in Table 1. The poor stability in the HB-BSO-T samples calcined at temperatures lower than 400 °C may be attributed to the unstable organic matter, although the samples exhibited similar contact angles of 110°. On the other hand, very high calcination temperatures should destroy the hydrophobicity of the surface layer (the water droplet permeated rapidly into the phosphor powder) and lead to a decrease in moisture resistance. The influence of moisture on the microstructure of the produced phosphors is summarized in the typical SEM images in Figure 7, which generally agree with the results above. More

Figure 8. Normalized Eu LIII-edge XAFS of (a) BSO, and (b) HBBSO-500 samples after exposure to water steam at 100 °C for 48 h (moisture test).

where the presence of water, due to the reaction, generates the hydrolysis reaction. Then, −Si-O−, −Si-N−, or −Si-C− forms and cross-links with the Si−O bonds in the BSO phosphor, leading to the formation of the hydrophobic layer.46 The mixture of CCl2HCH3 and CCl2(CH3)2 could lead to diverse structures, favoring the formation of the lotus leaf-like microand nanohierarchical structure.41 The thermal stability in the powder of HB-BSO-500 was determined by thermal analysis. The thermographs of the TGA and DTA curves, shown in Figure 9, suggest that the surface

Figure 7. SEM images of BSO powder (a) as-synthesized and (b) after moisture test, and HB-BSO-500 powder (c) as-synthesized and (d) after moisture test (exposure to water steam at 100 °C for 48 h).

specifically, prismatic particles with well-formed edges, which suggest high crystallinity, are observed in both the BSO and the HB-BSO-500 powders. It is noteworthy that many nanosized grains are also clearly seen, which form a lotus leaf-like microand nanohierarchical layer on the surface of the particles of HBBSO-500 powder, apparently produced by the ammonolysis and pyrolysis of chlorosilane. The grain size of the BSO was about 1−2 μm, and ∼5% weight gain in the HB-BSO-500 sample was observed after surface modification. After the moisture test, in the powder of HB-BSO, the particles maintained their initial shape, whereas big particles appeared in the BSO powder, which can be ascribed to the newly formed phases (Figure 6). The formation of agglomerates is observed in both samples, but is more pronounced in the BSO powder. The results of XANES measurements for the BSO and HBBSO-500 powders after the moisture test are shown in Figure 8. The two peaks at ca. 6977 and 6984 eV are attributed to the divalent and trivalent oxidation states of Eu, respectively.45 After the moisture test, the Eu2+ in BSO was oxidized into Eu3+ completely, whereas there is no evidence of oxidation of the Eu2+ in HB-BSO-500. These results suggest that HB-BSO is highly stable under severe conditions of high-pressure water steam. The formation of this hydrophobic layer can be evidently ascribed to the ammonolysis and the pyrolysis of chlorosilane,39 according to the chemical eqs 2 and 3

Figure 9. Thermal analysis (TGA and DTA curves) of BSO and HBBSO-500 in air atmosphere.

layer is stable up to 550 °C. The small weight loss might be attributed to a release of low-molecular-weight oligomers. An exothermic peak was recorded at ca. 550 °C, which can be ascribed to the breaking of the Si−CH3 bonds. This can provide an explanation for the loss of hydrophobicity in the HB-BSO samples calcined at temperatures higher than 600 °C. The results of Figure 9 suggest that the hydrophobic surface layer features adequate stability as far as applications of the HBBSO phosphors in white LEDs are concerned. 11620

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CONCLUSIONS Significant luminescence degradation of BSO phosphors occurred at 100 °C under high-moisture conditions, owing to the hydrolysis of the host and to the oxidation of the activator (Eu2+), via an oxidizing gas penetration mechanism. A modified BSO phosphor was successfully produced, where a hydrophobic and stable surface layer was formed. This layer improved the resistance against oxidation and hydrolysis substantially, as well as the thermal stability of the produced phosphors when they were exposed to severe conditions of high-pressure water steam. The modified phosphors heat-treated at 500 °C displayed the best behavior, since a negligible decay in their PL properties was recorded after the moisture test. The proposed method exhibits great potential for being applied to other phosphors, as well, such as silicate, (oxy)nitride, and aluminate phosphors, in order to increase their stability in high humidity and high temperature conditions.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86-551-63600824(o), +86-18655117978(m). Fax: +86551-63601592. E-mail: [email protected]. ORCID

Xin Xu: 0000-0001-6547-255X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (Grant Nos. 51372238, 11435012), the CNPC-CAS Strategic Cooperation Research Program (2015A4812), and the Provincial Teaching Research Project of Anhui Province (2014jyxm010).

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DOI: 10.1021/acs.jpcc.7b00751 J. Phys. Chem. C 2017, 121, 11616−11622