Enhancing the luminescent efficiency of Y3Al5O12:Ce3+ by coating graphitic carbon nitride: Toward white light-emitting diodes

Journal of Alloys and Compounds 801 (2019) 10e18

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Enhancing the luminescent efficiency of Y3Al5O12:Ce3þ by coating graphitic carbon nitride: Toward white light-emitting diodes Hong-Ren Chen a, Chao Cai b, Zhong-Wei Zhang c, Li Zhang d, Hai-Peng Lu d, Xin Xu e, Hao Van Bui f, g, Ke-Hui Qiu a, **, Liang-Jun Yin h, * a

College of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology, Chengdu, PR China Chengdu Fine Optical Engineering Research Center, Chengdu, PR China Laboratory of Energy Storage and New Energy Materials Technology, Central Research Institute, Dongfang Electric Corporation, PR China d National Engineering Research Center of Electromagnetic Radiation Control Materials, University of Electronic Science and Technology of China, 2006, Xiyuan Road, Chengdu, PR China e Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, PR China f Phenikaa Institute for Advanced Study (PIAS), Phenikaa University, Yen Nghia Ward, Ha Dong District, Hanoi, 10000, Viet Nam g Phenikaa Research and Technology Institute (PRATI), A&A Green Phoenix Group, 167 Hoang Ngan, Hanoi, 10000, Viet Nam h School of Material and Energy, University of Electronic Science and Technology of China, 2006, Xiyuan Road, Chengdu, PR China b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 22 April 2019 Received in revised form 6 June 2019 Accepted 10 June 2019 Available online 11 June 2019

Achieving highly efficient Y3Al5O12:Ce3þ phosphor is desirable to fabricate bright white light-emitting diodes (wLED). Designing a synergistic effect of energy transfer from the guest materials to the host Y3Al5O12:Ce3þ is expected to favor the higher quantum efficiency of Y3Al5O12:Ce3þ phosphor. Here, we report a novel strategy to enhance the emission intensity and quantum efficiency of Y3Al5O12:Ce3þ phosphor by coating a thin g-C3N4 layer on the surface of Y3Al5O12: Ce3þ particles. Melamine was coated on the surface of Y3Al5O12: Ce3þ and heated at 580  C to develop the g-C3N4 layer. The phase, microstructure, photoluminescent properties and thermal stability are investigated in detail. It is demonstrated that g-C3N4 coated Y3Al5O12: Ce3þ has been successfully achieved, which shows higher emission intensity and quantum efficiency than the uncoated sample. This is due to the strong excitation energy absorption of g-C3N4, which is then transferred to luminescent Ce3þ ions. The synergistic effect of g-C3N4 coating-host materials shows potential applications to improve the quantum efficiency of other phosphors. © 2019 Elsevier B.V. All rights reserved.

Keywords: Y3Al5O12: Ce3þ G-C3N4 Coating wLED Quantum efficiency

1. Introduction In the past few decades, white light emitting diodes (wLED), as the new lighting revolution, have attracted much attention due to the unique characteristics including low power consumption, high brightness and excellent stability [1e3]. The first commercialized wLED was fabricated by coating a blue InGaN-LED chip with yttrium aluminum garnet yellow phosphor (Y3Al5O12: Ce3þ, YAG in abbreviation hereafter) [4]. Basically, the blue light emitted from the InGaN-LED chip is partly converted into yellow light by the down-converted YAG phosphor. This yellow light is mixed with the

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (K.-H. Qiu), [email protected] (L.-J. Yin). https://doi.org/10.1016/j.jallcom.2019.06.122 0925-8388/© 2019 Elsevier B.V. All rights reserved.

transmitted part of the blue light, forming white light. Recently, a new approach to fabricate wLED has been developed, which combines transparent ceramics/glasses and blue chips to provide white light [5e8]. These strategies using YAG phosphor still remains as the most widely applied technique for the fabrication of wLED. As the YAG phosphor is a core element of the wLED devices, enhancing its luminescent efficiency is crucial to achieve high-performance wLED. Various approaches have been developed to adjust and improve the optical properties of YAG. For instance, doping the YAG host lattice with metal ions such as Cuþ, La3þ, Tb3þ and Pr3þ allows to tailor the emission spectra of the phosphor. Particularly, by using different types of doping ions, blue or red shift of the emission spectra and improvement of color-rendering property can be achieved [9e11]. Dispersing YAG in ceramic or glass would improve the

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thermal stability and prevent luminescent degradation. However, this method may decrease the emission intensity of the phosphor [6e8]. Another approach is coating the YAG particles with a thin layer of other materials such as carbon, polystyrene, Al2O3 and SiO2. By doing so, the emission intensity and efficiency of YAG can be slightly improved as the coating layer could reduce surface defects that commonly act as excited electron trap centers [12e14]. The coating with a thin layer of Al2O3 can additionally improve the thermal stability of YAG phosphor [15]. Owing to its suitable band structure and chemical inert property, graphitic carbon nitride (g-C3N4) is a novel metal-free organic semiconductor that has been recently regarded as an appealing material for a variety of applications, including photocatalysis, bioimaging, and sensing [16e20]. The structure of g-C3N4 could be viewed as graphite with some of carbon atoms substituted by nitrogen atoms [21]. Ideal g-C3N4 has a two-dimensional structure with a band gap of about 2.7 eV. This value is nearly identical with the maximal excitation wavelength of YAG phosphor (i.e., ~460 nm) [22]. Therefore, the coupling of YAG phosphor with g-C3N4 may bring a synergistic effect that improves the excitation light absorption and consequently enhances the luminescent efficiency of YAG. This forms the motivation of this work. Here, we coated a thin g-C3N4 layer on the surface of YAG particles by thermal condensation of melamine. A thermal treatment was subsequently applied to optimize microstructure and composition of the g-C3N4 coating layer. The photoluminescent properties of the geC3N4ecoated YAG phosphors were investigated, which showed that the g-C3N4 coating could enhance the emission intensity and quantum efficiency of YAG phosphor. A mechanism of the enhancement was proposed, and the potential application of the synthesized phosphors in a wLED device was tested. 2. Materials and methods The synthesis of g-C3N4 (C3N4, in abbreviation hereafter) is similar to the method used by Zhang et al. [23]. Briefly, the C3N4 powders were synthesized through the thermal condensation of melamine (99.8 wt%, Meifeng Chemical Industry Co. Ltd. Sichuan, China). Melamine powders were contained in an alumina crucible, and heated to 580  C in air atmosphere for 3 h. After cooling down to room temperature, the product was taken out and ground into fine powers in an agate mortar. C3N4-coated YAG (YAG@C3N4, in abbreviation hereafter) was prepared by thermal condensation of melamine at surface of YAG. First, 1.5 g Melamine powders were dissolved in 20 mL ethylene glycol (99 wt%, Chron Chemicals Co. Ltd. Chengdu, China) at 70  C. Then 4.0 g YAG phosphor powders (Looking Long Technology Co. Ltd. Shenzhen, China) were added into the solution with vigorous stirring and dispersed in the solution. In next step, ethylene glycol was completely evaporated by heating at 70  C. Thereafter, the mixture was transferred to muffle furnace. The temperature of the furnace was controlled at 580  C for 3 h in air atmosphere. YAG@C3N4 was obtained after cooling down the sample to room temperature naturally in the furnace. To improve the crystallinity and remove impurities and defects in the C3N4 layer, the sample was heated up in N2 to 1500  C with a heating rate of 10  C/min, maintained at this temperature for 2 h and then cooled down naturally in N2 ambient to room temperature. X-ray diffraction measurements were performed using a SHIMADU diffractometer (Model 700, Japan) with Cu Ka radiation (l ¼ 1.54 Å). The XRD patterns were acquired at a scanning rate of 2 /min. The photoluminescence (PL) spectra of the phosphors were recorded at room temperature using a HITACHI fluorescence spectrophotometer (F-4600, Japan) equipped with a 200 W Xe lamp as the excitation source. The temperature dependent

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luminescence measurements were conducted on a fluorescent spectrophotometer equipped with a high temperature fluorescence controller (TAP-02, KOJI Instrument Co. Ltd., Tianjin, China). The internal and external quantum efficiencies were measured using a spectrophotometer (QE-2100, Otsuka Electronics Company, Ltd., Osaka, Japan). The cross-section processing was conducted by embedding powders in an epoxy resin and cutting them with an Arion cross-section polisher (Model-955, Gatan, USA). The microstructure and elemental distribution were analyzed using a JEOL transmission electron microscope (TEM) equipped with an energy dispersion X-ray detector (Model 2100 F, Tokyo, Japan). Composition of cross-section of coated phosphor particles was performed using EDX line scanning in TEM. The X-ray photoelectron spectroscopy (XPS) analysis of samples was performed using an ESCALAB250 X-ray photoelectron spectrometer (Thermo-VG Scientific, USA) with monochromatized Al Ka radiation (1486.92 eV). The deconvolution of XPS peaks was performed using Gaussian component after a Shirley background subtraction. Fourier transforms infrared spectroscopy (FTIR, Agilent Cary 630) was used to identify chemical group on surface of samples in the range of 400e4000 cm1. The optical parameters of the wLED device were measured using an integrating sphere spectroradiometer system (LHS-1000, Everfine Co., Hangzhou, China). 3. Results and discussions The XRD pattern of the as-synthesized C3N4 powders is shown in Fig. S1. The intense peak at 27.1 arises from the diffraction on the (002) plane of the graphitic structure. The diffraction peaks are consistent with the reported patterns of C3N4 [24,25], indicating the successful synthesis of C3N4 via the thermal condensation of melamine. The as-prepared C3N4 shows strong blue emission under 360 nm excitation (shown in Fig. S2). To investigate the lattice parameters changes after C3N4 coating and thermal treatment, XRD Rietveld refinements are performed on YAG, as-prepared C3N4-coated YAG (YAG@C3N4) and the YAG@C3N4 after annealing at 1500  C in N2 for 2 h (YAG@C3N41500) with the GSAS program [26]. Using the cubic structure of YAG as the starting model, all atomic position and lattice parameters are refined, converging to the residual factors Rwp ¼ 5.09% and Rp ¼ 2.56% for YAG, Rwp ¼ 5.21% and Rp ¼ 2.63% for YAG@C3N4, Rwp ¼ 4.98% and Rp ¼ 2.58% for YAG@C3N4-1500. The results are plotted in Fig. 1 and summarized in Tables S1eS2. A cubic single YAG phase remains in all samples. C3N4 phase is not observed in XRD patterns of YAG@C3N4 and YAG@C3N4-1500 because of its small amount and amorphous state. The lattice parameters of the three samples are shown in Fig. 1 (d) based on the refinement results, indicating that there is no significant change on the crystal lattice of YAG after coating and thermal treatment. The unchanged structure promises the stable coordination environment of Ce3þ and unshifted emission spectrum. The excitation and emission spectra are shown in Fig. 2(a). The excitation spectra recorded for the emission peak at 535 nm show two broad bands at 340 and 457 nm. The strongest excitation peak centered at 457 nm. Under 457 nm excitation, the PL spectra consist of a broad emission with a peak around 530e540 nm. This peak represents the 5d-4f electronic transitions of Ce3þ in YAG. The results demonstrate that the coating of YAG with C3N4 results in a decrease of emission intensity, however, the emission intensity of the phosphor after annealing (YAG@C3N4-1500) is significantly enhanced. This observation is consistent with the brightness of the powders appearance shown in Fig. 2(c). YAG@C3N4 exhibits low emission intensity because defects and impurities in coating layer could hinder energy transfer. These defects and impurities can be eliminated by thermal treatment, which explains the highest

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Fig. 1. Rietveld refinement plots for the powder X-Ray diffraction patterns of (a) YAG, (b) YAG@C3N4 and (c) YAG@C3N4-1500. (d) Corresponding lattice parameters in the samples.

Fig. 2. (a) Excitation and emission spectra of YAG, YAG@C3N4 and YAG@C3N4-1500. The excitation and monitoring wavelengths are 457 and 535 nm, respectively. (b) Excitation spectra when excitation peaks at 460 are normalized. (c) Photos of YAG, YAG@C3N4 and YAG@C3N4-1500 samples under UV light. (d) Quantum efficiency of YAG and YAG@C3N41500. The excitation wavelength is 460 nm.

emission of YAG@C3N4-1500. The mechanism will be discussed later. The excitation peaks at 457 nm of the samples are normalized and shown in Fig. 2(b). There is significant enhancement of excitation peaks at 340 nm after C3N4 coating, suggesting the absorbed energy of C3N4 is transferred to YAG instead of direct emission as

blue light. The quantum efficiency (QE) of phosphors is a critical factor in evaluating their application potential. The internal quantum efficiency (IQE) is the ratio of emitted photons by the number of only absorbed photons [27]. The external quantum efficiency (EQE) is the ratio of emitted photons by the number of incident photons

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[28]. Their relationships can be described as following:

hint ¼

Nem N h ¼ em Nabs ext Ninc

hext ¼ Ahint

(1)

where the Ninc, Nabs and Nem are the number of incident, absorbed and emitted photons, respectively, and the A is absorbance rate [29]. The QE of YAG:Ce and treated YAG:Ce are measured and shown in Fig. 2(d). Compared with uncoated YAG, the internal quantum efficiency (IQE) and the external quantum efficiency (EQE) of YAG@C3N4-1500 increases from 85.5% to 76.2%e86.7% and 78.6%, respectively. Comparably, Fig. S3 shows emission spectra of YAG and annealed YAG. PL intensity of YAG remains the same after annealing at 1500  C in N2. Besides, the XRD refinement results demonstrate the unaffected crystal structure of YAG after coating and annealing (Fig. 1). Therefore, it can be concluded that the enhancement of PL intensity and QE is due to the C3N4 coating instead of annealing effect or crystal structure change. The decay time is measured and shown in Fig. 3(a). The decay time of YAG@C3N4-1500 is slightly longer than that of YAG, implying that there is energy transfer from C3N4 layer to YAG particle. To further characterize optical properties of the samples, UVevis diffuse reflectance spectra are employed and shown in Fig. 3(b). Both YAG and YAG@C3N4-1500 have obvious light absorption at ~340 nm and ~460 nm. The C3N4 could absorb UV-blue light from 300 to 500 nm, as a consequence, YAG@C3N4-1500 shows higher absorption efficiency in this wavelength range than the uncoated YAG. It evidences that C3N4 coating layer could effectively enhance the absorption in the UV-blue region in YAG@C3N4-1500 phosphor. To get clear information about the microstructure and phase state of C3N4 layer in YAG@C3N4 and YAG@C3N4-1500 samples, STEM and HRTEM analyses are performed and shown in Fig. 4. The thin C3N4 layer can be distinguished from the YAG host. For YAG@C3N4 sample, the thickness of coating layer is found in the range of 40e50 nm (Fig. 4(a) and (b)). From the HRTEM image shown in Fig. 4(b), no sign of lattice fringes in the C3N4 layer is observed, suggesting its poor crystallinity. For YAG@C3N4-1500, the thickness of coating layer decreases to 3e7 nm (Fig. 4(c) and (d)). For this sample, the lattice fringes in the C3N4 layer appear, although they are not well defined. The FFT patterns in the inset of Fig. 4(b) and (d) reveal crystallinity of C3N4 layer before and after the thermal treatment. The results suggest that thermal treatment could improve the crystallinity of C3N4 layer. EDS mapping is used to investigate the elemental distribution of YAG@C3N4 and YAG@C3N4-1500. The patterns are shown in Fig. 5. Y, Al, O and Ce evenly distribute in phosphor particles. C and N distribute on surface of YAG@C3N4 and YAG@C3N4-1500 particles,

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which evidences the presence of C3N4 on the surface of YAG before and after the thermal treatment. Fig. 6 shows the EDX line-scanning analyses that were conducted on cross-section of YAG@C3N4 and YAG@C3N4-1500 particles. From surface to core, the content of C, N and O initially increases and then decreases, indicating that C3N4 only exists on surface of phosphor particle. This is consistent with the results obtained by EDS mapping shown in Fig. 5. It is believed that the oxygen impurity in coating layer comes from air atmosphere during the preparation of C3N4. In the process of thermal treatment, the carbon impurity in amorphous C3N4 could react with YAG and N2, which results in incorporated nitrogen into YAG lattice. The reaction formula could be expressed as follows:

x Y3 Al5 O12 þ xC þ N2 /Y3 Al5 O12x N2x þ xCO 3 3

(2)

Moreover, compared with YAG@C3N4, oxygen impurity content in coating layer of YAG@C3N4-1500 decreases because of the reaction between oxygen impurity and residual carbon at high temperatures. The decrease of carbon and oxygen impurities improves the light absorption and the energy transfer efficiency as consequences of better crystallinity of C3N4. Eventually, better photoluminescent properties of YAG@C3N4-1500 could be achieved after the thermal treatment. FTIR transmission spectra of YAG@C3N4 (Fig. 7(a)) and YAG@C3N4-1500 (Fig. 7(b)) reflect the chemical groups on the surface of phosphors. In the wavenumber range from 400 to 900 cm1, the absorption bands localized at 567, 697, 728, and 790 cm1 are ascribed to the metal-oxygen vibration characteristics of M  O (M ¼ Y, Al) stretches, confirming the existence of a YAG phase [30,31]. An absorption peak at 808 cm1 and the peaks in the range of 1200e1650 cm1 can be assigned to the bending and stretching vibrations of CeN heterocycle, respectively [32,33]. A broad absorption band at approximately 3170 cm1 is a characteristic of stretching vibration of OeH groups that exist on the surface of YAG@C3N4 [34]. This absorption band is not observed in the FITR spectrum of YAG@C3N4-1500. In addition, the CeN heterocycle signals become weaker for YAG@C3N4-1500 in comparison with YAG@C3N4. The weakened CeN signals in YAG@C3N4-1500 are caused by the reduced thickness of the C3N4 layer in YAG@C3N41500. X-ray photoelectron spectroscopy is used to investigate the surface chemical components and binding configuration of synthesized samples. The peaks of Y, Al, O, Ce, C and N elements can be observed in the XPS survey spectra of YAG@C3N4 and YAG@C3N41500 (Fig. 8(a)). The high-resolution XPS spectra of N1s are presented in Fig. 8(b) and (d). For YAG@C3N4 sample, three peaks can be distinguished at 398.5, 400.1 and 401.4 eV, corresponding to C]

Fig. 3. (a) Decay time of YAG and YAG@C3N4-1500. (b) UVevis diffuse reflectance spectra of YAG, C3N4 and YAG@C3N4-1500.

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Fig. 4. (a) STEM and (b) HRTEM of YAG@C3N4. (c) STEM and (d) HRTEM of YAG@C3N4-1500.

Fig. 5. EDS mappings of Y, Al, O, Ce, C and N for (a) YAG@C3N4 and (b) YAG@C3N4-1500 particles.

NeC, Ne(C)3, and CeNeH bonds, respectively [35,36]. The relevant peaks for the YAG@C3N4-1500 sample are much sharper. It can be observed that the chemical states of N for YAG@C3N is more complex than YAG@C3N4-1500, suggesting C3N4 layer is purified by thermal treatment. Fig. 8(c) and (e) show the spectra of the C1s spectra of YAG@C3N4 and YAG@C3N4-1500 samples, respectively. For YAG@C3N4, three peaks at 284.8, 285.7 and 288.6 eV are found, which are ascribed to the CeC from the carbon impurity, the CeN] C groups and sp3 bonded carbon in C3N4, respectively [35]. The intensity of peak ascribed to CeN]C groups increases in YAG@C3N4-1500. On the other hand, the intensity of peaks located

at 284.8 eV decreases. These results indicate decreased impurity C content in C3N4 layer. Therefore, thermal treatment further brings in optimization of composition of C3N4 layer in YAG@C3N4-1500. Thermal stability of phosphors is an important parameter for industrial application. Fig. 9(a) displays relative maximum PL intensities of coated and uncoated YAG phosphor under 457 nm excitation at different temperature. It is observed that the intensity decreases for both YAG and YAG@C3N4-1500 as the temperature increases from room temperature to 300  C, exhibiting a typical thermal quenching characteristic. The two curves in Fig. 9(a) are almost the same. In thermal quenching, the influence of thin C3N4 layer on the host is small. Activation energy is calculated by using

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Fig. 6. (a, c) Cross-section images of YAG@C3N4 and YAG@C3N4-1500, respectively. (b, d) EDX line-scanning analyses for cross-section of YAG@C3N4 and YAG@C3N4-1500, respectively.

Fig. 7. FTIR spectra of (a) YAG@C3N4 and (b) YAG@C3N4-1500.

the Arrhenius equation [37,38].

IT ¼

I0 DE

1 þ Ae kT

(3)

Where the I0 is emission intensities at room temperature, IT is emission intensities at a given temperature, A is a constant, and k is the Boltzmann constant. DE is the activation energy for thermal quenching. The diagram of ln [(I0/IT)-1] versus 1/kT is plotted and shown as an inset in Fig. 9(a). DE is calculated as 0.251 eV for YAG@C3N4-1500. To further investigate thermal stability of the samples, YAG and YAG@C3N4-1500 are heated in air at 600  C for 2 h and then cooled down to room temperature naturally. The results show that the PL intensities of YAG and YAG@C3N4-1500 after the heat treatment remain at 90.2% and 96.0% of their original PL intensities,

respectively. The small improvement of thermal stability can be attributed to C3N4 coating layer, which prevents the phosphor particles from directly exposure to high temperature environment and Ce3þ oxidation. To determine the chemical state in Ce ions of the samples after heating at 600  C in the air, XPS is employed and Ce 3d spectra are shown in Fig. 9(b). The peaks belonging to Ce3þ and Ce4þ are shown in magenta and blue, respectively. The peaks around 917.6 eV, 908.8 eV, 903.6 eV and 899.4 eV come from 3d3/2, whereas the peaks around 897.4 eV, 888.9 eV, 886.0 eV, 882.9 eV and 880.7 eV come from 3d5/2 [39,40]. The peak around 917 eV is a fingerprint of Ce4þ, but the intensity of this component is not proportional to the amount of Ce4þ states [41]. This peak appears both in YAG and YAG@C3N4-1500, indicating the existence of Ce4þ. In the Ce 3d spectrum of YAG@C3N4-1500, the peaks belonging to Ce3þ have lager area than that in YAG. In the process of heating in air at 600  C,

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Fig. 8. (a) Typical XPS survey. N1s, C1s of (b, c) YAG@C3N4 and (d, e) YAG@C3N4-1500, respectively.

the C3N4 layer on the surface of YAG weakens the oxidation degree. Therefore, PL intensity of YAG@C3N4-1500 remains more stable than that of the uncoated YAG phosphor. A wLED device is fabricated by using a 460 nm emitted blue-LED chip, YAG@C3N4-1500 and CaAlSiN3:Eu2þ(lem ¼ 630 nm) phosphors. The optical properties of prepared wLED device is presented in Fig. 9 (c). The wLED device produces a bright white light with chromatic coordinates (0.368, 0.3367) and a color temperature of 4056 K. It achieves a color rendering index (Ra) as high as 89. These results validate the suitability of the C3N4 coated YAG phosphor in a blue-LED chip pumped white luminescence device. Given that the band gap of C3N4 matches the excitation spectrum of YAG phosphor, C3N4 layer could absorb light, generating electrons and holes in conduction band and valence band, transferring energy to YAG eventually. As a consequence, C3N4 and YAG phosphor can exhibit a synergistic effect, enhancing PL intensity and quantum efficiency of YAG phosphor. A schematic picture is drawn in Fig. 10 to illustrate the proposed mechanism. For the asprepared YAG@C3N4, the residual carbon, oxygen impurity and

defects in C3N4 layer can absorb light and directly reduce the excitation light energy, and facilitates the electron-hole recombination inside the layer. Consequently, a low PL intensity is exhibited in YAG@C3N4 sample. The thermal treatment purifies the C3N4 layer and improves its crystallinity. Because of the lower defect density in C3N4, the generated electrons and holes can transfer to luminescent Ce3þ ions in YAG lattice through the C3N4-YAG interface. An intense emission at ~540 nm is observed, ascribed to the 5d-4f electronic transitions of Ce3þ in YAG. Therefore, the C3N4 coating plays a positive role to achieve a higher PL intensity and QE in YAG@C3N41500. Note that the PL properties of YAG are strongly dependent on the structure and composition of C3N4 layer. It can be concluded that continuous optimization of crystallization and purity of C3N4 layer can further improve the PL properties in YAG@C3N4-1500. 4. Conclusions In this work, g-C3N4 is successfully synthesized and coated on the surface of YAG: Ce3þ phosphor through the thermal

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Fig. 9. (a) Normalized PL intensities of YAG phosphor and YAG@C3N4-1500 as a function of temperature. The inset of (a) shows the Arrhenius plot to calculate the activation energy for thermal quenching of YAG@C3N4-1500. (b) Ce 3d XPS spectra of YAG and YAG@C3N4-1500 after heating at 600  C in air for 2 h and cooling down to room temperature. (c) Electroluminescence spectra of wLED based on YAG@C3N4-1500 and CaAlSiN3:Eu2þ phosphors and photo of the fabricated wLED device in a dark environment.

Fig. 10. A schematic picture showing the synthesis, structure, composition and mechanism of the enhanced QE in YAG@C3N4-1500.

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condensation of melamine, which is proved by HRTEM, EDS, FTIR and XPS analyses. XRD Rietveld refinement confirms that coating has no obvious influence on crystal structure of YAG. An optimization of composition and crystallinity of C3N4 coating layer is realized through the thermal treatment. Compared with original YAG: Ce3þ phosphor, the coated samples exhibit higher emission intensity and quantum efficiency as well as thermal stability. YAG: Ce3þ could absorb light in a wider spectral range due to energy transfer from C3N4 layer to YAG: Ce3þ core, which explains the enhanced quantum efficiency. The present research suggests that coating C3N4 on the surface of YAG is an effective strategy to improve the luminescent properties of YAG, which can be also applied for other phosphors.

[15]

[16]

[17]

[18] [19]

[20]

Acknowledgments This research was supported by the National Natural Science Foundation of China (Grant No. 51302029, 51802294), Sichuan Science and Technology Program (Grant No. 2018FZ0100), Key Scientific and Technological Research and Development Program (Grant No. 2017GZ0400), Sichuan Province, P.R. China.

[21]

[22]

[23]

Appendix A. Supplementary data [24]

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.06.122.

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