Highly Stable Modified Phosphors of Ba2SiO4:Eu2+ by Forming a

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Highly Stable Modified Phosphors of BaSiO:Eu by Forming a Robust Hydrophobic Inorganic Surface Layer of Silicon-Oxy-Imide-Carbide Bi Zhang, Jiang-Wei Zhang, Hao Zhong, Lu Yuan Hao, Xin Xu, Simeon Agathopoulos, Chengming Wang, and Liang-Jun Yin J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 08 May 2017 Downloaded from http://pubs.acs.org on May 8, 2017

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The Journal of Physical Chemistry

Highly Stable Modified Phosphors of Ba2SiO4:Eu2+ by Forming a Robust Hydrophobic Inorganic Surface Layer of Silicon-Oxy-Imide-Carbide Bi Zhang1, Jiang-Wei Zhang2, Hao Zhong1, Lu-Yuan Hao1, Xin Xu1,*, Simeon Agathopoulos3, Chengming Wang4, Liang-Jun Yin5

1

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, People’s Republic of China. 2

Hefei Guoxuan high-tech power energy co., LTD, 599 Daihe Road, Hefei Anhui,

People’s Republic of China. 3

Materials Science and Engineering Department, University of Ioannina, GR-451 10

Ioannina, Greece. 4

Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and

Technology of China, Hefei, Anhui 230026, P.R. China 5

School of Energy Science and Engineering, University of Electronic Science and

Technology of China, 2006 Xiyuan Road, Chengdu, People’s Republic of China. *Corresponding author: Tel: +86-551-63600824(o), +86-18655117978(m). Fax: +86-551-63601592, E-mail address: [email protected]

1 ACS Paragon Plus Environment


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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 nano-hierarchical structure and contained CH3 groups, which enhanced the hydrophobicity of the modified phosphor significantly. The modified phosphors treated at 500 oC, exhibited remarkable stability in their emission spectra, even after exposure to high-pressure water steam at 100 oC 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 reduce the reliability and shorten the lifetime of white LEDs significantly.9,10 For instance, many well-known phosphors, such as sulfides (CaS:Eu SrS:Eu),11,12

and

silicates,13

aluminates,14,15

even

(oxy)nitrides,16

are

moisture-sensitive materials. Green emitting Ba2SiO4:Eu2+ (BSO) phosphor, which features high quenching temperature, has great potential, owing for 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 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 europium-doped strontium aluminate nanoparticles were maintained by surface

modification

reaction

with

pyrophosphoric


acid

or

3-allyl-2,


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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, β-diketones, 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 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


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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 high-pressure water steam conditions.



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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 oC 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 methyldichlorosilane (Aladdin Industrial Corporation, 99%) and 5 ml dimethyldichlorosilane (Aladdin Industrial Corporation, >98.5, GC) were dissolved in 25 ml heptane (Sinopharm Chemical Reagent Co. Ltd, Shanghai, China, 97%). The mixture was stirred for 5 min and sealed with a plastic wrapper. 5 g of the BSO phosphor were 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 oC, for 1 h under flowing NH3 gas. The obtained surface modified BSO phosphor powder (denoted as HB-BSO-T, where T represents the calcination temperature of the surface layer) was crushed gently to obtain a dispersed powder. For


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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 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 were 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 oC (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 oC overnight. X-ray diffraction analysis (XRD, Philips PW 1700, Almelo, the Netherlands; CuKα1 radiation, λ = 1.54056 Å) was conducted to identify the crystalline phases using Cu Kα radiation at a scanning rate of 2 o/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 JSM-6390, 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 (ARM-200F), equipped with a device for SAED analysis. Fourier transform infrared



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spectroscopy (FT-IR) was carried out in a Perkin-Elmer 580B IR spectrophotometer with the KBr pellet technique. The X-ray absorption spectra for the Eu LIII-edge were measured at a beam-line of BL14W1 at the Shanghai Synchrotron Radiation Facility. Thermal analysis (DTA/TGA) was carried out in air atmosphere with a Shimadzu DTG-60H thermobalance (Kyoto, Japan) at temperatures of up to 1000 oC with a heating rate of 10 K/min.

Results & 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 nano-pores (Figure 1a). Many nanoparticles were formed in the surface layer, which was the hydrophobic layer. This lotus leaf-like micro- and nano-hierarchical 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, Figure 1e) suggest that the layer was amorphous, and fully covered the surface of the BSO particles. The dot pattern of electron-differaction pattern (Figure 1d) suggests that the Ba2SiO4 particle was well crystallized.


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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). 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 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 nano-sized crystals consisted of Si, N, C, O in the amorphous surface coating due to the reaction between the coating layer and the host. Hence, these results suggest that calcination only modifies the surface layer of phosphors.



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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-BSO-600, and (g) HB-BSO-700. The diffraction patterns of Ba2SiO4 (JCPDS No. 26-1403) are also plotted as (h). 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 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 oC, 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 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 10 ACS Paragon Plus Environment

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at the high temperature of 700 oC, suggesting formation of SiNCO phase.

Figure 3. FT-IR spectra of BSO and HB-BSO-T phosphors. 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 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 nano-crystals 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 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



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a maximum in the phosphors treated at 500 oC. 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.

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.

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 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 oC 12 ACS Paragon Plus Environment

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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 was determined to be 91.5% and 70.2%, respectively, while ηi and ηo of the HB-BSO-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 and in particular 700 oC 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 oC in the investigated system to create the optimum conditions for an effective improvement in moisture resistance. Table 1. Influence of moisture test on emission intensity of the produced phosphors. Note: The values have been normalized to BSO rescaling to a value of 1000. Sample

Emission intensity (before)

Emission intensity (after)

Emission intensity (after) / Emission intensity (before)

BSO

1000

26

2.6%

HB-BSO-100

999

34

3.4%

HB-BSO-200

996

68

6.8%

HB-BSO-300

977

155

16%

HB-BSO-400

1001

960

96%

HB-BSO-500

1053

1009

96%

HB-BSO-600

1030

896

87%

HB-BSO-700

650

85

13%

The PL spectra of BSO and HB-BSO-T before and after moisture test are plotted 13 ACS Paragon Plus Environment


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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 oC during the moisture test. On the other hand, the HB-BSO-500 sample featured remarkable moisture resistance, which should be attributed to its stable 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 ~110o 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).

Figure 5. (a) Emission intensity of BSO and HB-BSO-T samples treated in water steam at 100 oC for 48 h (moisture test) under excitation at 365 nm. (b) Photographs


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of BSO and HB-BSO-500 powders in water, and (c) water contact angle (~110o) on the surface of a pressed HB-BSO-500 cake at room temperature.

The X-ray diffractograms of BSO and HB-BSO-T powders after the moisture test are shown in Figure 6. The HB-BSO-T samples (for 400 – 600 oC) presented remarkable phase stability. The HB-BSO-T samples (for 600 oC) 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 H2O,O2 Ba2SiO4  →BaSiO3 ⋅ H2O + Ba(OH)2

(1)

Figure 6. X-ray diffractograms of BSO and HB-BSO-T after the moisture test (exposure to water steam at 100 oC for 48 h). (a) BSO, (b) HB-BSO-100, (c) HB-BSO-200,

(d)

HB-BSO-300,

(e)

HB-BSO-400,


(f)

HB-BSO-500,

(g)


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HB-BSO-600, and (h) HB-BSO-700. The diffraction pattern of Ba2SiO4 (JCPDS No. 26-1043) is also plotted as (i).

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

Highly Stable Modified Phosphors of Ba2SiO4:Eu2+ by Forming a

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