Solvothermal Synthesis and Luminescence Properties of Yttrium

Oct 31, 2014 - Moreover, solvent exhibits a significant influence on the reaction process, luminescence intensity, and decay time of YAG:Ce crystallit...
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Solvothermal Synthesis and Luminescence Properties of Yttrium Aluminum Garnet Monodispersed Crystallites with Well-Developed Faces Meng M. Xu,†,‡ Zhi J. Zhang,*,§ Jun J. Zhu,†,‡ Jing T. Zhao,*,§ and Xiang Y. Chen†,‡ †

Shanghai Institute of Ceramics, Chinese Academy of Sciences, No. 1295 Dingxi Road, Shanghai 200050, China University of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing 100049, China § School of Materials Science and Engineering, Shanghai University, No. 99 Shangda Road, Shanghai 200072, China ‡

S Supporting Information *

ABSTRACT: Monodispersed yttrium aluminum garnet (YAG) crystallites with welldeveloped and controllable crystal faces were obtained by the solvothermal method. The effects of temperature, time, and solvents on the phase formation, morphology, and particle size distribution were investigated. YAG can be obtained at relatively lower temperature in water, while higher temperature and longer time are necessary with increasing amount of ethanol in the solvent. The presence of {100} faces is favored at lower temperature while the terminating faces are {110} and {211} at higher temperature in water and water− ethanol mixture (volume ratio = 1:1). Moreover, solvent exhibits a significant influence on the reaction process, luminescence intensity, and decay time of YAG:Ce crystallites. This study presents a novel way to obtain YAG crystallites with well-developed terminating faces which will be promising in the solid-state lighting and will lay a foundation to study the grain boundaries formed in the process of YAG transparent ceramics sintering.

lations.8,9,13 For experimental study, the single crystal cutting method was used to obtain the desired faces to get the specific grain boundaries;14,15 however, this method is different from the practical ceramic sintering process. If original particles with desired terminating faces can be obtained and the transparent ceramics can be fabricated from these particles, the study of grain boundaries will be more precise, which will in turn be helpful to improve the optical properties of YAG transparent ceramics.9 YAG powders are conventionally prepared by solid-state reaction with temperature higher than 1600 °C and repeated mechanical mixing to obtain pure phase YAG powders.16,17 These processes lead to large particle size and heavy agglomeration. Wet-chemical routes such as coprecipitation,18,19 sol−gel,20,21 combustion,22,23 and spray pyrolysis24,25 have been proposed to lower the temperature to obtain single phase YAG with fine particle size. However, the aggregation is still inevitable because of the post heat treatment in these methods. What is more, the terminating faces of YAG powders prepared by these methods are usually ambiguous. The hydrothermal or solvothermal method can be used to obtain crystal phase without post heat treatment, avoiding the problems aforementioned. Above all, it is easy to obtain particles with well-developed faces and modify the morphology of the crystallites to obtain the desire terminating faces by

1. INTRODUCTION Yttrium aluminum garnet (YAG) exhibits excellent optical and luminescence properties and has been considered as an important solid-state laser material, phosphor, and window material for a variety of lamps.1 Since Ikesue et al.2 prepared polycrystalline transparent YAG ceramics and found that the YAG transparent ceramics had nearly the same optical characteristics as single crystal YAG, considerable efforts have been devoted to fabricating high performance YAG ceramics. Furthermore, YAG-based phosphors doped with different rareearth ions are promising phosphor candidates in contrastenhanced display application and scintillation.3,4 YAG:Ce3+ is nonhygroscopic, emitting green light with very short decay time, and is attractive for X-ray imaging.3,5 Moreover, YAG:Ce3+ can convert blue light to green light and is the most commonly used phosphor in commercial white LEDs for solid-state lighting (SSL).6,7 Notably, the critical issue for preparation of YAG transparent optical ceramics and YAGbased phosphors is to synthesize high-quality powders with fine particle size and good dispersity. It is well-known that the grain boundaries show a strong influence on the mechanical, thermal, and optical properties of YAG transparent optical ceramics.8−12 Therefore, it is important to investigate the physical and chemical properties of the grain boundaries in YAG to understand the dependence of properties on the microstructure of grain boundaries and improve the optical properties of YAG transparent ceramics. Studies concerning the grain boundaries of YAG transparent ceramics are still rare except for some theoretical calcu© 2014 American Chemical Society

Received: August 22, 2014 Revised: October 14, 2014 Published: October 31, 2014 27000

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2.3. Characterization. X-ray powder diffraction (XRD) for phase identification was collected at ambient temperature on a HUBER G670 (Cu Kα1 radiation, λ = 1.540 56 Å, Ge monochromator). The 2θ ranges of all data sets were from 10° to 100° using a step scan with a step size of 0.005°. Fourier transform infrared (FT-IR) absorption spectra of the samples were measured on a spectrophotometer (FTIR-7600, Lambda Scientific, Australia) in the range from 400 to 4000 cm−1 using the KBr pellet (1 wt % sample) method. Particle sizes and morphology of the powders were examined by a scanning electronic microscope (SEM, FEI Magellan 400). The particle size distributions were detected by laser scattering particle size distribution analyzer on a zeta potential analyzer (Zetaplus, Brookhaven). Photoluminescence measurements (PL) were performed on a Hitachi F-4600 spectrometer equipped with a 150 W xenon lamp as excitation source. The decay curves were identified on a fluorescence spectrometer (FLS920, Edinburgh).

regulating the reaction condition. Since the continuous supercritical system was developed by Hakuta et al. for the rapid and continuous production of YAG:Tb particles,26 it is widely used in hydrothermal method to prepare YAG.27,28 Recently, batch reactor was also used to synthesize YAG particles in the hydrothermal method.29,30 However, high temperature and pressure (400 °C, 30 MPa) are necessary to get the single-phase YAG, no matter using continuous supercritical water system or batch reactor by the hydrothermal method.26,31 Inoue et al. developed the solvothermal method in glycol solvents, i.e., the glycothermal method.32 Using this method, YAG nanocrystals can be prepared at 300 °C from aluminum isopropoxide and yttrium acetate in 1,4-butylene glycol. In spite of the advantages of low temperature and pressure, the obtained YAG powders are aggregates of irregularly shaped grains, which is undesirable whether for YAG ceramics or for YAG-based phosphors. What is more, this method needs expensive alkoxide as starting materials. Li et al. have proved that monodispersed YAG nanopowders can be obtained by a mixed solvothermal method using inexpensive inorganic salts as starting materials and ethanol−water solution as the solvent.33 Unfortunately, the effect of solvent and other parameters on the phase formation, morphology, and luminescence properties of YAG powders is unclear. In this work, monodispersed YAG crystallites with welldeveloped and controllable terminating faces were synthesized by solvothermal method using water, ethanol−water mixture (volume ratio = 1:1), and ethanol as the solvent. We investigated the effect of reaction conditions such as solvent, reaction temperature, and time on the phase, morphology, and luminescence properties of the products. What is more, varying of the product as a function of reaction conditions provided an insight into the formation mechanism of the YAG crystallites. This work provides a new way to synthesize monodispersed YAG crystallites with well-developed and controllable faces and will lay a foundation for the study of grain boundaries in YAG transparent ceramics.

3. RESULTS AND DISCUSSION 3.1. Phase Formation and Morphology of YAG under Different Conditions. The type of solvent has a profound effect on the phase and morphology of the products. Below, the results in different solvents are described, and these results are also summarized in Table 1. Water. The X-ray diffraction patterns of the samples synthesized in water from 250 to 300 °C are shown in Figure 1a. YAG is obtained at 250 °C; however, a large number of impurities are present in the products. The amount of Table 1. Preparation Conditions and Characterization Results of YAG Powder

sample

2. EXPERIMENTAL METHODS 2.1. Preparation of the Precursor. CeO2 (99.99% purity) was dissolved in dilute nitrate acid under stirring and heating to obtain Ce(NO3)3 solution. Aluminum nitrate hydrate (Al(NO3)3·9H2O, A.R.), yttrium nitrate hexahydrate (Y(NO3)3· 6H2O, A.R.), and ammonium hydrogen carbonate (NH4HCO3, A.R.) were used as raw materials without further purification. Starting solution was prepared by dissolving these metal nitrates in distilled water in a ratio according to the formula (Y1−xCex)3Al5O12 (x = 0.01), while keeping the concentration of aluminum nitrate at 0.15 mol/L. The mixed nitrate solution was added to a 3 M ammonium hydrogen carbonate solution dropwise under mild agitation at room temperature. After aging for 12 h, the precipitate was repeatedly washed with distilled water until the pH is 7 and dried at 80 °C for 4 h. 2.2. Synthesis of YAG Particles. The dried precipitate was dispersed in different solvents and then placed in the autoclave. The autoclave was heated to the specific temperature (250− 300 °C) at a heating rate of 2 °C/min and kept at this temperature for 5−24 h. After the reaction at specific temperature, the autoclave was cooled to room temperature in the air. The resulting suspension was centrifugated, repeatedly washed with distilled water, and then dried in air at 80 °C for 12 h.

reaction time (h)

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water

250

5

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water

270

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water

280

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water

300

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water−ethanol mixture (volume ratio = 1:1) water−ethanol mixture (volume ratio = 1:1) water−ethanol mixture (volume ratio = 1:1) water−ethanol mixture (volume ratio = 1:1) water−ethanol mixture (volume ratio = 1:1) water−ethanol mixture (volume ratio = 1:1) ethanol ethanol ethanol ethanol ethanol ethanol

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8 9 10 11 12 13 14 15 16 17 18 27001

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product YAG,AlO(OH), undefined phase YAG,AlO(OH), undefined phase YAG, AlO(OH) (trace) YAG, AlO(OH) (trace) YAG, AlO(OH) (trace) YAG, AlO(OH) (trace) AlO(OH), undefined phase YAG, AlO(OH) undefined phase YAG, AlO(OH) undefined phase YAG, AlO(OH) (trace) YAG, AlO(OH) (trace) YAG, AlO(OH) (trace) amorphous amorphous amorphous amorphous YAG, amorphous YAG

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Figure 1. XRD patterns of the samples synthesized in water at different temperatures for 5 h (a) and for different reaction time at 300 °C (b).

obvious peaks corresponding to YAG can be found. Peaks corresponding to YAG appear at 270 °C and coexist with that of AlO(OH) and other unknown phases. The amount of impurities decreases with further increasing temperature, and only a trace of AlO(OH) remains coexisting with YAG in the powders synthesized at 300 °C. When water−ethanol mixture is used as solvent, YAG come into being at a higher temperature compared with that synthesized in water. This result indicates that the presence of ethanol inhibits the crystallization of YAG. Figure 3b shows the influence of reaction time on the phase of samples synthesized at 300 °C. The XRD results show similar tendency with that synthesized in water, implying that increasing time is beneficial to the improvement of the crystallinity of YAG. Figure 4 shows the SEM images of the samples synthesized in water−ethanol mixture. With increasing temperature, the mean particle size decreases and the terminating faces also undergo some changes, as shown in Figure 4a−c. Figures 4c−e are SEM images of samples synthesized at 300 °C for different times. It is found that reaction time shows no obvious influence on the size and morphology of particles, which is similar to that synthesized in water. Figures 4f−h are higher resolution SEM images corresponding to samples shown in Figures 4a−c, respectively. As shown in these higher resolution SEM images, YAG crystallites synthesized in water−ethanol mixture at 270 °C also exhibit the terminating faces of {110} and {100}; however, {211} faces occur, and the {100} is no longer predominant compared with that synthesized in water. This result indicates that the presence of ethanol increases the growth rate of {100} faces, while decreases the growth rate of {211} faces. With increasing temperature, {100} faces are growing smaller, and the evolution of the morphologies shows a similar tendency as that synthesized in water. Ethanol. XRD patterns of samples synthesized in ethanol are shown in Figure 5. The absence of any diffraction peaks indicates that the powders synthesized at 300 °C for 5 h are amorphous. Keeping the temperature at 300 °C and extending the reaction time to 12 h, the diffraction peaks of YAG arise, while the existence of a broad band in the range of 25°−35° indicates the amorphous phase in the product. Further increasing the reaction time to 24 h, the diffraction peaks become narrower and can be easily indexed to YAG, demonstrating that single phase YAG is successfully synthesized in ethanol at 300 °C for 24 h.

impurities decreases with increasing temperature, and the main product is YAG, with only a trace of AlO(OH) as an impurity at 300 °C; moreover, the sharp diffraction peaks corresponding to YAG indicate the highly crystallinity. Figure 1b shows the X-ray diffraction patterns of the samples synthesized at 300 °C for 5, 12, and 24 h. With increasing reaction time, the relative amount of AlO(OH) decreases, while the peaks of YAG become narrow, indicating an improvement of crystallinity of YAG. Figure 2 shows the SEM images of these samples. Two kinds of particles, the platelike particles and polyhedral particles which are respectively corresponding to AlO(OH) and YAG, can be found.34 It can be seen from Figure 2a−c that when the temperature increases from 270 to 300 °C, the mean particle size of YAG decreases from 450 to 210 nm; besides, the terminating faces are different as the temperature increases. Particles obtained at 300 °C for different reaction times are all monodispersed with similar size and exhibit well-defined crystal faces, as shown in Figure 2c−e. Reaction time has little effect on the particle size and morphology of YAG crystallites. The variation of the particle size with the temperature can be explained by reaction equilibrium theory,35 assuming that the reaction proceeds through a dissolving-recrystallization mechanism. As the solubility of the precursors increases while the density and dielectric constant of the medium decrease with increasing temperature, a higher supersaturation is obtained at higher temperature, resulting in the formation of more nuclei and smaller particles. The higher resolution SEM images of YAG obtained at different temperatures in water are shown in Figure 2f−h. It can be seen clearly that the crystallites obtained in water at 270 °C are terminated by {110} and {100} faces. With increasing temperature, {100} faces are growing smaller. The dominant faces of crystallites obtained at 280 °C are {110} and {211}, with only a small proportion of {100}, as shown in Figure 2g. When the temperature is 300 °C, {100} faces disappear and the terminating faces are {110} and {211}. As is reported, {110} and {211} faces are the most common terminating faces of YAG prepared by the hydrothermal method while {100} terminating faces are rare.36 From the analysis above, it can be inferred that the {100} faces show a relative lower growth rate and are favored in presence at lower temperature. Water−Ethanol Mixture (Volume Ratio = 1:1). Figure 3a shows the XRD patterns of the samples synthesized in water− ethanol mixture from 250 to 300 °C. At 250 °C, the powders are mixtures of AlO(OH), and some undefined phases and no 27002

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Figure 2. SEM images of the products synthesized in water under different temperatures and for different reaction time: (a) sample 2 (270 °C × 5 h), (b) sample 3 (280 °C × 5 h), (c) sample 4 (300 °C × 5 h), (d) sample 5 (300 °C × 12 h), (e) sample 6 (300 °C × 24 h), and higher resolution SEM images for (f) sample 2, (g) sample 3, and (h) sample 4.

is consistent with the XRD result. As shown in Figure 6b, extending the reaction time to 24 h, the gel-like particle can be hardly found, indicating the completion of reaction. The welldefined dodecahedra YAG crystallites shown in Figure 6b are terminated by {110} faces with mean size of 100 nm.

Figure 6 shows the SEM images of the particles synthesized in ethanol at 300 °C for different reaction times. Two different kinds of particles can be observed when the reaction time is 12 h, as shown in Figure 6a. The polyhedral particles are attributed to YAG, and the gel-like particles similar to the morphology of coprecipitation precursors indicate the amorphous phase, which 27003

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Figure 3. XRD patterns of the samples synthesized in water−ethanol mixture under different temperatures for 5 h (a) and at 300 °C for different reaction time (b).

and the broad band spilt into several bands with increasing temperature and reaction time, which are attributed to the O− H stretching in the AlO(OH), consistent with the XRD results. In addition, the absorption bands centered at about 1395, 1535, and 1640 cm−1 are contributed to CO32−, NH4+, and OH−, which are corresponding to the composition of precursors, NH4AlY0.6(CO3)1.9(OH)2·0.9H2O.38 These bands become weaker with increasing temperature and reaction time, indicating the decomposition of precursors. The FT-IR spectra of samples synthesized in water−ethanol mixture show similar changes with that synthesized in water, as shown in Figure 8b. However, samples synthesized in ethanol show distinct changes, as reaction temperature and time increase. Compared with the FT-IR spectra of precursors, several absorption bands show up after solvothermal reaction in ethanol, as shown in Figure 8c. The bands centered at about 1522, 1409, and 1079 cm−1 are attributed to C−O group, and the band centered at about 2968 cm−1 is attributed to the −CH3 group, both of which indicate the formation of organic intermediates. The intensity of these peaks decreases as the temperature increases, indicating the decomposition of the organic intermediates. The XRD patterns and FT-IR spectra provide an insight into the reaction process in different solvents. It is well-known that the reaction mechanism of the solvothermal synthesis of ceramic powders is the dissolution/precipitation or dissolution/ crystallization mechanism.39 The reaction process in the water can be concluded as follows: the precursor decomposes and dissolves as Al(OH)4− and Y(OH)6−, and then three reactions may occur in the process:

Figure 7 depicts the particle size distribution and average diameters of the samples synthesized at 300 °C for 24 h in different solvents evaluated by dynamic light scattering. From Figure 7a−c, it can be found that the particle size distribution becomes narrow with increasing amount of ethanol in the solvent. As shown in Figure 7d, the average diameters obtained by this method are larger than those observed in the SEM images, which can be attributed to the agglomeration of primary particles. With increasing amount of ethanol in the solvent, the average diameter of YAG crystallites decreases. According to the analysis above, it can be concluded that solvent shows a significant effect on phase formation and morphology of YAG. The condition to obtain YAG became harsher and the particle size decreased with increasing amount of ethanol; moreover, the terminating faces exhibit some changes. At 270 °C, YAG crystallites obtained in water are terminated by {110} and {100} faces. When water−ethanol mixture is used, {100} faces become smaller, while {211} faces occur. This phenomenon indicates that the presence of ethanol improves the growth rate of {100} faces while decreases the growth rate of {211} faces. At 300 °C, both of YAG crystallites synthesized in water and water−ethanol mixture are typically terminated by {110} and {211} faces, while the crystallites synthesized in ethanol are typically bounded by 12 {110} forms, conforming to the rhombic dodecahedral morphology commonly observed in synthetic garnets.37 The mechanism of the influence of solvent on the morphology of YAG crystallites is still unknown. However, the differences in the growth units formed in different reaction processes, which will be discussed later, may account for it. Moreover, the physical properties of solvents such as density, dielectric constants, and viscosity, which have an important influence on the diffusion, may be another important factor. Meanwhile, the decomposition of ethanol derivative formed in the reaction process may result in organic moiety on the interface of the crystallites, which probably decreases the size of crystallites synthesized in ethanol compared with that obtained in water. 3.2. Growth Mechanism of YAG Crystallites in Different Solvents. FT-IR spectra of samples synthesized in different solvents are shown in Figure 8 to analyze the effect of solvent on the reaction process. The spectra of samples synthesized in water are depicted in Figure 8a. In the spectra of precursors, the broad absorption band centered at 3450 cm−1 is attributed to moisture absorbed on the surface of the sample,

Al(OH)4 − → AlO(OH)

(1)

Y(OH)6 − → YO(OH)

(2)

Al(OH)4 − + Y(OH)6 − → Y3Al5Ox (OH)y

(3)

With the process of these reactions and the increase of concentration of the growth units to the critical value of nucleation, nucleation and growth of crystallites such as AlO(OH) and YAG will occur. It was reported that high temperature and pressure are necessary to obtain YAG single phase by the hydrothermal method,26,31 and in our work, AlO(OH) is present in the powders under lower temperature 27004

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Figure 4. SEM images of the products synthesized in a water−ethanol mixture at different temperatures and for different reaction times: (a) sample 8 (270 °C × 5 h), (b) sample 9 (280 °C × 5 h), (c) sample 10 (300 °C × 5 h), (d) sample 11 (300 °C × 12 h), (e) sample 12 (300 °C × 24 h), and higher resolution SEM images for (f) sample 8, (g) sample 9, and (h) sample 10.

and pressure. Allowing for these facts, it can be concluded that under lower temperature and pressure the active energies of reactions 1 and 3 are approximate and these two reactions are competitive, resulting in a mixture of YAG and AlO(OH). With further increasing temperature and pressure, reaction 3 is

favorable, and only a trace of AlO(OH) can be found in the powders synthesized at 300 °C. XRD patterns and FT-IR spectra of powders synthesized in water−ethanol mixture are similar to that synthesized in water, indicating a similar reaction process, while the distinction in the 27005

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Article 2 F5/2 and 2F7/2 due to the spin−orbit interaction. Two main excitation peaks are observed in the wavelength range of 300− 520 nm with the peaks at about 340 and 450 nm, associated with the 4f (2F7/2) → 5d transitions. There is no significant change in the position and shape of the excitation and emission peaks with changing solvent, which suggests that the covalency of the Ce3+−O2− bonds and crystal field strength around Ce3+ are very similar. However, the luminescence intensity of samples synthesized in different solvents exhibits a significant difference. The powders synthesized in water show very low emission intensity, and the intensity increases significantly with increasing amount of ethanol in the solvent. The powders obtained in ethanol exhibit the maximum emission intensity which is almost 10 times higher than that synthesized in water. Generally, the difference in the emission intensity can be explained from crystallinity and surface properties.40−44 As shown in Figure 10a, precise XRD measurement reveals that the (400) peak for YAG:Ce synthesized in water locates at 2θ = 29.655°, lower than 2θ = 29.715° for YAG:Ce synthesized in ethanol. The unit cell parameter and volume of YAG:Ce synthesized in water are 12.0417(13) Å and 1748.3(3) Å3, respectively, lager than 12.0347(11) Å and 1743.1(3) Å3 for that synthesized in water−ethanol mixture, and the values decrease further to 12.0039(13) Å and 1729.7 (3) Å3 for that synthesized in ethanol, as depicted in Figure 10b. It was reported that the enlargement of the unit cell parameter can be attributed to the substitution of Al ions in the 16a sites with RE ions and the presence of many voids.44 The smaller lattice parameter and volume of YAG:Ce synthesized in ethanol indicate fewer disorders compared with that synthesized in water, which may result in increasing PL and PLE intensity. The decreasing peak width in the XRD patterns with increasing amount of ethanol also indicates an improvement of the crystallinity, which tends to improve the luminescence intensity of sample synthesized in ethanol. The surface properties may also contribute to the improvement of luminescence intensity of YAG:Ce synthesized in the presence of ethanol. It has been proved that the oxidation of Ce3+ to Ce4+ occurring near the surface led to a decrease of luminescence intensity, while the surface modification by organics can prevent the oxidation and improve the luminescence properties.45−47 According to the above analysis, the organics formed during the reaction process in the presence of ethanol may be absorbed on the surface of YAG:Ce crystallites and inhibit the oxidation of Ce3+, which

Figure 5. XRD patterns of the samples synthesized in ethanol at 300 °C for different reaction times.

results of XRD and FT-IR of samples synthesized in ethanol indicates a different process. From the FT-IR spectra, it can be concluded that ethanol derivative such as AlO(OH)m(OCH2CH3)n (m + n = 1) and YO(OH)k(OCH2CH3)l (k + l = 1), similar to the result reported by Inoue,32 may form following the decomposing of the precursors. The cleavage of the C−O bond in the ethanol derivative is necessary to form crystallites such as AlO(OH) and YO(OH), and from the absence of aluminum and yttrium compound in the XRD patterns, it is assumed that these processes may be unfavorable in thermodynamics than the reaction of AlO(OH)m(OCH2CH3)n and YO(OH)k(OCH2CH3)l forming the Y3Al5Ox(OH)y(OC2H5)z (x + y + z = 12), which may act as the growth units of YAG. The cleavage of C−O bond in Y3Al5Ox(OH)y(OC2H5)z is necessary to form YAG, and it is more difficult compared with the cleavage of −OH in Y3Al5Ox(OH)y formed in water, which result in higher temperature and longer time to obtain YAG in ethanol than that in water. 3.3. Photoluminescence. The excitation and emission spectra of samples synthesized at 300 °C for 24 h in different solvents are shown in Figure 9. A broad emission band in the wavelength range of 470−700 nm with the peak at 527 nm and a shoulder at the longer wavelength side can be observed in the PL spectra. The former is assigned to the 5d → 4f (2F5/2) transition and the latter to the 5d → 4f (2F7/2) transition, since Ce3+ with a 4f1 electron configuration has two ground states of

Figure 6. SEM images of the products synthesized in ethanol at 300 °C for different reaction time: (a) sample 17 (300 °C × 12 h); (b) sample 18 (300 °C × 24 h). 27006

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Figure 7. Particle size distribution of the samples synthesized in different solvents: (a) sample 6 (water), (b) sample 12 (water−ethanol mixture), (c) sample 18 (ethanol), and (d) average diameters of samples synthesized in different solvents.

Figure 8. FT-IR spectra of the products synthesized at different temperature and for different time in (a) water, (b) water−ethanol, and (c) ethanol.

found that the time constants exhibit little change when the solvent changes from the water to water−ethanol mixture; however, when ethanol was used, both τ1 and τ2 become larger. It is well-known that the lifetime is described as (γr + γnr)−1, where γr and γnr are the radiative rate and the nonradiative (by quenching) rate. The improvement of the crystallinity of YAG:Ce synthesized in ethanol as shown in the XRD pattern and the passivated surface by the organic compound as illuminated in the FT-IR spectra lead to the decrease of γnr and finally result in increasing lifetime.49

4. CONCLUSIONS Monodispersed YAG:Ce3+ crystallites with well-developed crystal faces were obtained by solvothermal method in water, water−ethanol mixture, and ethanol. The effects of reaction temperature and time on the phase formation and morphologies of the samples obtained in different solvents were investigated, and it was found that the amount of impurity phase decreased and the crystallinity of YAG improved with increasing temperature and time. The particle size of YAG decreases with increasing temperature while the reaction time shows little influence on the particle size. It was also proved that temperature exhibits an influence on the terminating faces of YAG; low temperature is in favor of the presence of {100}. The solvents have a significant influence on the phase formation, morphology, and size distribution of YAG. YAG arises at 250 °C in water and 270 °C in water−ethanol mixture, while it requires 300 °C and 24 h to obtain YAG in ethanol. The average size of YAG:Ce crystallites decreases, and the size

Figure 9. Excitation and emission spectra of YAG:Ce synthesized at 300 °C for 24 h in different solvents.

also accounts for the improvement of the luminescence intensity. The photoluminescence decay curves of YAG:Ce synthesized at 300 °C for 24 h in different solvents are shown in Figure 11a−c. The curves can be fitted using a double-exponential function I = A1 exp(−t/τ1) + A2 exp(−t/τ2)+ I0, and the time constants, τ1 and τ2, are listed in Table S1 (in the Supporting Information). The fast portion of the decay is likely due to the quenching of Ce3+ by the defects at the surface, while the longer lifetime is assigned to Ce3+ inside the particle.48 It can be 27007

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Figure 10. (a) Precisely measured XRD peaks of (400) planes and (b) cell parameters and volumes of YAG:Ce synthesized in different solvents at 300 °C for 24 h.

U1332202 Innovation program of Shanghai Institute of Ceramics under Grant No. Y34ZC130G, and the Open Fund of Key Laboratory of Transparent Opto-functional Inorganic Materials, Shanghai Institute of Ceramics of Chinese Academy of Sciences.

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ABBREVIATIONS YAG, yttrium aluminum garnet; YAG:Ce, cerium-doped yttrium aluminum garnet.

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Figure 11. PL decay curves of YAG:Ce synthesized at 300 °C for 24 h in different solvents: (a) sample 6, (b) sample 12, and (c) sample 18.

distribution becomes narrow with increasing amount of ethanol in the solvent. The possible reaction mechanism in different solvents was deduced from the results of XRD and FT-IR. The influence of solvent on the luminescence properties was also investigated, and it was found the monodispersed YAG crystallites obtained in ethanol show relatively good photoluminescence properties.

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AUTHOR INFORMATION

REFERENCES

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* Supporting Information S

S1: the decay time constants, τ1 and τ2, of samples synthesized in different solvents at 300 °C for 24 h; S2: author informations of refs 3 and5. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Authors

*E-mail [email protected]; Ph +86 021 52412620 (Z.J.Z.). *E-mail [email protected]; Ph +86 021 66138033 (J.T.Z.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China under Grant No. 11104298 and 27008

dx.doi.org/10.1021/jp508507s | J. Phys. Chem. C 2014, 118, 27000−27009

The Journal of Physical Chemistry C

Article

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