pubs.acs.org/Langmuir © 2010 American Chemical Society
Ce:YAG Nanoparticles Embedded in a PMMA Matrix: Preparation and Characterization Maria Luisa Saladino,*,† Antonio Zanotto,† Delia Chillura Martino,† Alberto Spinella,‡ Giorgio Nasillo,† and Eugenio Caponetti‡,† † Dipartimento di Chimica Fisica “F. Accascina” and INSTM UdR di Palermo, Universit a di Palermo, Parco d’Orleans II, Viale delle Scienze pad. 17, Palermo 90128, Italy, and ‡Centro Grandi Apparecchiature, Universit a di Palermo, Via Marini 14, Palermo 90128, Italy
Received November 11, 2009. Revised Manuscript Received April 26, 2010 A Ce:YAG-poly(methyl methacrylate) composite was prepared using in situ polymerization by embedding the Ce: YAG nanopowder in a blend of methyl methacrylate (MMA) and 2-methacrylic acid (MAA) monomers and activating the photopolymerization using a radical initiator. The obtained nanocomposite was yellow and transparent. Its characterization was performed using transmission electron microscopy, small angle X-ray scattering, 13C crosspolarization magic-angle spinning nuclear magnetic resonance, and photoluminescence spectroscopy. Results showed that Ce:YAG nanoparticles are well dispersed in the polymeric matrix whose structure is organized in a lamellar shape. The luminescence properties of the nanocomposite do not show quenching or a significant spectral shift, indicating that the nanocomposite can be useful for advanced applications such as white LED construction.
1. Introduction Yttrium aluminum garnet (Y3Al5O12, YAG) doped with Ce(III) and combined with GaN blue-light-emitting diodes (LED) is applied to a white light solid-state LED (WLED).1,2 In addition, Ce:YAG can also be used in inorganic electroluminescence displays,3 X-ray scintillators,4,5 and fluorescence thermometers because fluorescence properties vary with temperature.6 WLED can be obtained by a combination of nonabsorbed blue emission from a blue LED and the broad yellow emission from Ce:YAG phosphor because blue and yellow are complementary colors.7-9 WLEDs have advantage such as higher energy efficiency, higher reliability, longer life, faster response, and lower pollution compared to traditional lighting. It is suggested that they have a prosperous future in the lamp market. However, at the moment, a limit is that the radiation diffusion on the particle surfaces reduces the WLED efficiency. It was suggested that nanophosphors be utilized to increase the transmission and to reduce the optical scattering loss.10,11 The particle scattering scales as the square of the particle mass; therefore, the reduction of particle size to the nanoscale range *Corresponding author. Tel: þ39 091 6459842. Fax: þ39 091 590015. E-mail:
[email protected]. (1) Murota, R.; Kobayashi, T.; Mita, Y. Jpn. J. Appl. Phys. 2001, 41, L887– L888. (2) Lee, S.; Seo, S. Y. J. Electrochem. Soc. 2002, 149, J85. (3) Wu, X.; Nakua, A.; Cheong, D. Proc. 10th Int. Display Workshops 2003, 1109–1112. (4) Cavouras, D.; Kandarakis, I.; Nikolopoulos, D.; Kalatzis, I.; Kagadis, G.; Kalivas, N.; Episkopakis, A.; Linardatos, D.; Roussou, M.; Nirgianaki, E.; Margetis, D.; Valais, I.; Sianoudis, I.; Kourkoutas, K.; Dimitropoulos, N.; Louizi, A.; Nomicos, C.; Panayiotakis, G. Appl. Phys. B: Lasers Opt. 2005, 80, 923. (5) Thinova, L.; Karasinski, C.; Tous, J.; Trojek, T. J. Phys.: Conf. Ser. 2006, 41, 573–576. (6) Allison, S. W.; Gillies, G. T.; Rondinone, A. J.; Cates, M. R. Nanotechnology 2003, 8, 859. (7) Huh, Y.-D.; Cho, Y.-S.; Do, Y. R. Bull. Korean Chem. Soc. 2002, 10, 1435. (8) Yoshinori, S.; Yasunobu, S.; Toshio, M. U.S. Patent 5,998,925, 1999. (9) Yoshinori, S.; Kensho, S.; Yasunobu, N.; Toshio, M. U.S. Patent 6,608,332, 2003. (10) Pan, Y. X.; Wang, W.; Liu, G. K.; Skanthakumar, S.; Rosenberg, R. A.; Guo, X. Z.; Li, K. K. J. Alloys Compd. 2009, 488, 638-642 (11) Yang, H.; Lee, D.-K.; Kim, Y.-S. Mater. Chem. Phys. 2009, 114, 665–668.
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should essentially eliminate the scattering. For this reason, in recent times research has been focused on the preparation of Ce: YAG nanoparticles using solid-state reactions12-14 or on solution.15-19 Prolonged heating at high temperature (around 1600 °C) is required to obtain a pure garnet phase that overcomes the competitive formation of secondary phases. However, high temperature can increase the particle size and the aggregation grade and, in particular for Ce:YAG, can causes the oxidation of Ce(III) to Ce(IV).20 The great limit in the development and diffusion of Ce:YAG nanoparticles regards the enormous difficulty in finding a synthesis route that allows control of the particle morphology, size and distribution, and optical properties. In spite of this, some researchers at the University of Keio proposed a new procedure for the construction of high-efficiency LEDs using phosphor of 10 nm size.21,22 For the development of new preparation methods, only two authors report the construction of a device and the incorporation of doped YAG nanopowders into a solid matrix.23,24 The preparation of new transparent composites, consisting of polymers and nanopowders containing lanthanide ions, is a sector (12) Pan, Y.; Wu, M.; Su, Q. J. Phys. Chem. 2004, 65, 845. (13) Lu, C.; Hong, H.; Jagannathan, R. J. Mater. Chem. 2002, 12, 2525. (14) Na, Z.; Dajian, W.; Lan, L.; Yanshuang, M.; Xiaosong, Z.; Nan, M. J. Rare Earth 2006, 24, 294. (15) Katelnikovas, A.; Vitta, P.; Pobedinskas, P.; Tamulaitis, G.; Zukauskas, A.; Jørgensen, J.-E; Kareiva, A. J. Cryst. Growth 2007, 304, 361–368. (16) Pankratov, V.; Millers, D.; Grigorjeva, L; Chudoba, T. Radiat. Meas. 2007, 42, 679–682. (17) Xia, G.; Zhou, S.; Zhang, J.; Xu, J. J. Cryst. Growth 2005, 279, 357–362. (18) Li, X.; Liu, H.; Wang, J.; Cui, H.; Han, F. Mater. Res. Bull. 2004, 39, 1923– 1930. (19) Yuan, F.; Ryu, H. Mater. Sci. Eng. B 2004, 107, 14–18. (20) Saladino, M. L.; Caponetti, E.; Chillura Martino, D.; Enzo, S.; Ibba, G. Opt. Mater. 2008, 31, 261–267. (21) Kasuya, R.; Isobe, T.; Kuma, H.; Katano, J. J. Phys. Chem. B 2005, 109, 22126–22130. (22) Kasuya, R.; Isobe, T.; Kuma, H. J. Alloys Compd. 2006, 408-412, 820–823. (23) Ryszkowska, J. Mater. Sci. Eng., B 2008, 146, 54–58. (24) Nyman, M.; Shea-Rohwer, L. E.; Martin, J. E.; Provencio, P. Chem. Mater. 2009, 21, 1536–1542. (25) Wu, W.; He, T.; Chen, J.; Zhang, X.; Chen, Y. Mater. Lett. 2006, 60, 2410– 2415.
Published on Web 07/15/2010
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Figure 1. Preparation scheme of the Ce:YAG-PMMA nanocomposite.
of recent interest.25-29 In fact, inorganic-polymer nanocomposites are appealing because of their improved properties and unique combination of characteristics. The interface between the two different materials play a very important role in their luminescence properties.21,30,31 Ryszkowska prepared a nanocomposite of Tb:YAG and polyurethane using in situ polymerization.23 A correlation between the optical properties and the nanocomposite structure was performed, showing that a change in the content of the nanofiller affects the transparency of the polyurethane matrix and that large nanofiller agglomerates have a negative influence on the lifetime of luminescence and transmittance. Nyman et al., by a solventexchange process, were able to encapsulate Ce:YAG nanopowder into a transparent epoxy resin without any change in the Ce:YAG characteristic emission.24 This article reports the preparation of a new transparent composite material consisting of Ce:YAG nanopowder embedded in a poly(methyl methacrylate) (PMMA) matrix. PMMA was chosen because of its transparency and ductility characteristics. The present approach is divided into three distinct steps: (1) the synthesis of Ce:YAG nanopowder using the coprecipitation method; (2) the embedding of the synthesized material into a mixture of methyl methacrylate (MMA) and methacrylic acid (MAA) and its spatial confinement on the nanometer scale; and (3) in situ polymerization. The nanopowder and the composite were characterized using thermogravimetry and differential thermal analyses (TG-DTA), X-ray diffraction (XRD), high resolution-transmission electron microscopy (HR-TEM), small angle X-ray scattering (SAXS), 13C cross-polarization magic-angle spinning nuclear magnetic resonance (13C {1H} CP-MAS NMR), and photoluminescence spectroscopy.
2. Experimental Section 2.1. Materials. Y(NO3)3 3 6H2O (Aldrich, 99.9%), Al(NO3)3 3 9H2O (Aldrich, 98%), and Ce(NO3)3 3 6H2O (Aldrich, 99.99%) were the sources of Y(III), Al(III), and Ce(III) ions, respectively. Ammonia solution (25v/v%) was obtained from E. Merck, and 2-methacrylic acid (MAA, >98%) was obtained from Fluka. The aqueous solutions were prepared using all chemicals as received and adding conductivity-grade water. Methyl methacrylate (MMA, Aldrich, 99.0%,) was purified using a disposable column to eliminate the polymerization inhibitor. (26) Cao, Z.; Jiang, W.; Ye, X.; Gong, X. J. Magn. Magn. Mater. 2008, 320, 1499–1502. (27) Wang, Y.; Zhang, D.; Shi, L.; Li; Zhang, J. Mater. Chem. Phys. 2008, 110, 463–470. (28) Liu, F.-K.; Hsieh, S.-Y.; Ko, F.-H.; Chu, T.-C. Colloids Surf., A 2003, 231, 31–38. (29) Yuen, S.-M.; Ma, C.-C. M.; Chuang, C.-Y.; Yu, K.-C.; Wu, S.-Y.; Yang, C.-C.; Wei, M.-H. Compos. Sci. Technol. 2008, 68, 963–968. (30) Jeong, H. Thin Solid Films 2000, 363, 279–281. (31) Mai, Y. W.; Yu, Z. Z. Polymer Nanocomposites; CRC Press: Boca Raton, FL, 2006.
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2.2. Ce:YAG Nanopowder Preparation. The synthesis of aluminum, yttrium, and cerium hydroxides, the precursors of YAG doped with cerium at 5 atom %, was performed by the coprecipitation method reported in previous work.32 Briefly, yttrium, aluminum, and cerium nitrates were dissolved in deionized water in the appropriate stoichiometric molar ratio. The hydroxides were precipitated by the dropwise addition of 5 M ammonia to the nitrates solution, under constant stirring, until a pH of 8 was reached. The gelatinous precipitate thus obtained was filtered and washed with water several times to remove the residual ammonia and nitrate ions. The presence of ammonia and nitrate ions was checked using concentrated hydrochloric acid and the brown ring test, respectively. Then, the precipitate was washed with ethanol and dried at 50 °C. The precursors were calcined at different temperatures (500, 700, 800, 900, and 1000 °C) for 1 h to observe the phase evolution. 2.3. Ce:YAG-PMMA Nanocomposite Preparation. One of the challenges of preparing nanocomposite materials is avoiding the agglomeration of the nanofiller in the polymer matrix, which can lead to poor composite performance. Because the YAG reactivity is very low, it is not possible to functionalize the YAG surface to avoid particle aggregation. On this topic, Nyman et al. used the solvent-exchange process.24 The nano Ce:YAG dispersions were prepared in a mixture of butanediol and glycol and redispersed in tetrahydrofuran and then in EPON 815C epoxy resin. The Ce:YAG-PMMA nanocomposite was prepared using a modified procedure that was previously used by some of us to obtain the CdS-PMMA nanocomposite.32 The Ce:YAG nanopowder was added at 5 wt % to a mixture of MMA monomer and MAA (molar ratio 4:1). The dispersion was sonicated for 10 min, and 2,2-diethoxyacetophenone was added to start the photocuring process.33 The dispersion was maintained under shaking and continuously irradiated with a 256 nm lamp until polymerization occurred. This synthesis approach differs from that in previous work with respect to the presence of MAA, which was used by some authors.34 Moreover, MAA was able to improve the stability of the dispersion of YAG nanoparticles in pure MMA. With increased acidity and polarity of the solvent, hydrogen bonds and van der Waals forces occur between the organic matrix and the nanofiller, stabilizing the nanodispersion. The composite formation process is outlined in Figure 1. For the sake of comparison, pure PMMA was also prepared following the same procedure. A transparent yellow solid product, composed of the PMMA polymeric matrix containing Ce:YAG nanoparticles, was obtained. A portion was cut and lapped to obtain a disk having a diameter of 1 cm. The thicknesses of PMMA and Ce:YAGPMMA samples were 1.91 and 0.84 mm, respectively. A photograph of the Ce:YAG nanopowder, pure PMMA, and the Ce: YAG-PMMA nanocomposite is shown in Figure 2. (32) Caponetti, E.; Leone, M.; Militello, V.; Panto, V.; Pedone, L.; Polizzi, S.; Saladino, M. L. J. Colloid Interface Sci. 2005, 284, 495–500. (33) Fang, J.; Tung Kevin, L. D.; Stokes, L.; He, J.; Caruntu, D.; Zhou, W. L.; O’Connor, C. J. J. Appl. Phys. 2003, 91, 8816. (34) Khaled, S. M.; Sui, R.; Charpentier, P. A.; Rizkalla, A. S. Langmuir 2007, 23(7), 3988–3995.
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Figure 2. (Left) Ce:YAG nanopowder, (middle) pure PMMA, and (right) Ce:YAG-PMMA nanocomposite.
2.4. Characterization Methods. The thermogravimetry and differential thermal analyses (TG-DTA) were conducted using a Setaram instrument from room temperature up to 1500 °C at a heating rate of 20 °C/min under an inert atmosphere. Once the final temperature was reached, specimens were immediately cooled. Powder XRD patterns were recorded with a Philips diffractometer in Bragg-Brentano geometry using Ni-filtered Cu KR radiation (λ = 1.54056 A˚) and a graphite monochromator in the diffracted beam. The X-ray generator worked at 40 kV and 30 mA; the instrument resolution (divergent and antiscattering slits of 0.5°) was determined using standards free from the effects of reduced crystallite size and lattice defects. The diffraction patterns were analyzed according to the Rietveld method35 using the program MAUD.36 A TEM study was performed using a JEOL 2010 operating at an accelerating voltage of 200 kV equipped with an X-ray energydispersive spectrometer (EDS, Oxford, model INCA ENERGY200T). The Ce:YAG nanopowder was dispersed in isopropanol and was sonicated to increase the homogeneity of the dispersion. A small drop of the suspension was deposited on a 300-mesh copper grid covered with a carbon film. The copper grid was introduced into the TEM analysis chamber after complete solvent evaporation. Ce:YAG-PMMA nanocomposite thin samples of about 150 nm in thickness were prepared using a Leica EM UC6 ultramicrotome equipped with a Leica EM FC6 cryocamera and a diamond blade. The thin samples thus obtained were deposited onto copper grids. The transmittance spectra were measured at room temperature using a double-beam Beckman DU-640 spectrometer. The emission (PL) spectra were measured using a Fluoromax 4 HORIBA Jobin Yvon spectrofluometer. Samples, placed at 45°, were excited by a Xe source operating at 150 W. The Ce:YAG nanopowders and the nanocomposite were excited at wavelengths of 450 and of 340 nm, respectively. Small angle X-ray scattering measurements were made using a Bruker AXS Nanostar-U instrument whose source was a Cu rotating anode working at 40 kV and 18 mA. The X-ray beam was monochromatized at a wavelength λ of 1.54 A˚ (Cu KR) using two G€ obel mirrors and was collimated using a series of three pinholes with diameters of 500, 150, and 500 μm. Samples were directly mounted on the sample stage to avoid additional scattering from the holder. Data were collected at room temperature for 1000 s and were recorded in a 2D multiwire proportional counter detector placed 24 cm from the sample. The measurements were carried out in two different portions of each sample to check the homogeneity. 13 C cross-polarization magic-angle spinning nuclear magnetic resonance (13C {1H} CP-MAS NMR) spectra were obtained at room temperature through a Bruker Avance II 400 MHz (9.4 T) spectrometer operating at 100.63 MHz for the 13C nucleus with a MAS rate of 13 kHz for 1024 scans, a contact time of 1.5 μs, and a repetition delay of 2 s. The optimization of the Hartmann-Hahn condition37 was obtained using an adamantane sample. All samples were placed in a 4 mm zirconia rotor with KEL-F caps using silica as a filler to avoid inhomogeneities inside the rotor. The proton spin-lattice relaxation time in the rotating frame T1F(H) was indirectly determined, with the variable spin lock (VSL) pulse sequence was determined by a carbon nucleus observation using a 90° τ-spin-lock pulse sequence prior to cross (35) Young, R. A. The Rietveld Method; University Press: Oxford, England, 1993. (36) Lutterotti, L.; Gialanella, S. Acta Mater. 1998, 46, 101. (37) Hartmann, S. R.; Hahn, E. L. Phys. Rev. 1962, 128, 2042.
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Figure 3. TG (---) and DTA (-) of the precursor mixture. polarization. The data acquisition was performed by 1H decoupling with delay times, τ, ranging from 0.1 to 7.5 ms and a contact time of 1.5 ms. The proton spin-lattice relaxation time in the laboratory frame T1(H) was determined, with the saturation recovery pulse sequence,38 by a carbon nucleus observation using a 90°-τ-90° pulse sequence prior to cross polarization with a delay time τ ranging from 0.01 to 3 s. The proton spin-lattice relaxation time in the rotating frame T1F(H) was indirectly determined, with the variable spin lock (VSL) pulse sequence, by the carbon nucleus observation using a 90°-τ spin-lock pulse sequence prior to cross polarization.39 Data acquisition was performed by 1H decoupling with a delay time τ ranging from 0.1 to 7.5 ms and a contact time of 1.5 ms. The TCH values for all of the PMMA signals were obtained through variable contact time (VCT) experiments.40 The contact times used in the (VCT) experiments were 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8, 1.0, 1.2, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 6.0, and 7.0 ms.
3. Results and Discussion 3.1. Ce:YAG Nanopowder Characterization. The TG and DTA thermograms of the mixed hydroxides, dried at 50 °C, are reported in Figure 3. The TG curve indicates an overall weight loss of approximately 45% up to 600 °C. No additional weight losses were observed. Two endothermic event and one exothermic event are observed at low temperature in the DTA curves: the two endothermic events at around 140 and 440 °C were assigned to the evolution of the physisorbed water and to the dehydroxilation, respectively. The exothermic event at 340 °C was attributed to the evolution of residues of nitrate ions.20 In the 800-1200 °C range of temperature, two additional exothermic effects are evident. These two effects are analogous to those previously reported20,41 and are attributed to the amorphous-to-crystalline transformation and to the subsequent transformation of the hexagonal metastable phase (YAlO3 and YAH), thus leaving mainly the YAG phase. The phase evolution of the powder calcined and treated at various temperatures was investigated using the X-ray diffraction technique. The sequence of patterns is reported in Figure 4A. The powder is completely amorphous up to 500 °C. Appreciable crystallization begins during the treatment at 700 °C. The pattern of the sample treated at 900 °C can be described in terms of two (38) Alamo, R. G.; Blanco, J. A.; Carrilero, I.; Fu, R. Polymer 2002, 43, 1857– 1865. (39) Lau, C.; Mi, Y. Polymer 2002, 43, 823–829. (40) Conte, P.; Spaccini, R.; Piccolo, A. Prog. Nucl. Magn. Reson. Spectrosc. 2004, 44, 215–223. (41) Caponetti, E.; Saladino, M. L.; Serra, F.; Enzo, S. J. Mater. Sci. 2007, 42, 4418–4427.
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Figure 4. (A) XRD patterns of the Ce:YAG nanopowder treated at different temperatures for 1 h. YAH phase peaks are indicated by open circles, and the remaining peaks are related to the YAG phase. (B) Dots represent XRD experimental data, and the continuous line is the Rietveld fit to the Ce:YAG nanopowder treated at 1000 °C. The bars mark the peak positions expected on the basis of the cubic garnet geometry. The curve of residuals (i.e., the difference between calculated and experimental intensity values) is displayed at the bottom.
Figure 5. TEM micrograph and a SAED and EDS spectrum of the Ce:YAG nanopowder treated at 1000 °C.
crystalline phases: the garnet (Y3Al5O12, YAG, 87((2) wt %) phase and the hexagonal phase (YAlO3, YAH, 13((2) wt %). The YAH peaks are indicated by open circles. The powder treated at 1000 °C is consists entirely of the YAG phase, indicating that the thermal treatment at 1000 °C promotes the conversion of YAH to YAG. The XRD experimental data of the Ce:YAG nanopowder treated at 1000 °C is reported in Figure 4B, together with the Rietveld fit, the curve of residuals, and bars of the garnet phase. The fit well reproduces the experimental data. The values of the lattice parameter, a, the crystallite dimension, D, and the lattice strain, ε, computed using the Rietveld method, are 12.0492((2) A˚, 45((2) nm, and 1.5((2) 10-3 A˚, respectively. The comparison with the lattice parameter of the pure YAG structure (between 12.002 and 12.016 A˚)42 indicates a small increase. A representative TEM micrograph of the sample treated at 1000 °C is reported in Figure 5 together with a select area electron diffraction (SAED) and an EDS spectrum. The micrograph reveals small aggregated grains having a spherical shape and a size ranging between 5 and 50 nm. The SAED pattern of the nanoparticles shows bright concentric rings without preferred orientation corresponding to the diffraction planes. The EDS spectrum, on the right of Figure 5, shows peaks of all elements present in the samples, including the copper of the grid, thus confirming the presence of cerium in the sample. (42) Inorganic Crystal Structure Database http://icsdweb.FIZ-Karlsruhe.de.
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XRD analysis has confirmed the presence of a unique crystalline phase, so cerium has to be in the YAG lattice and the small increase in the cell parameter can be explained by considering the partial substitution of Y(III) ions with Ce(III) ions. This finding can be rationalized in terms of the ionic radius of Ce(III) (1.034 A˚ ), which is larger than the ionic radius of Y(III) (0.893 A˚ ).43 3.2. Ce:YAG-PMMA Nanocomposite Characterization. A significant TEM micrograph of the Ce:YAG-PMMA nanocomposite is reported in Figure 6. Contrary to the Ce:YAG nanopowder, in the nanocomposite the nanoparticles are not aggregated but well distributed in the PMMA matrix. The observed nanoparticles are similar in size to those of the used powder. The local composition of the composite was investigated by EDS. The X-ray fluorescence spectra (not showed), acquired for some nanoparticles, show the peaks of all elements present in the sample, including the copper in the grid. The transmittance spectra of PMMA and the Ce:YAGPMMA composite were collected at room temperature. The transmittance values at different wavelengths, corrected for the thickness of the samples, are reported in Table 1. Both samples show a transparency that depends on the wavelength of the light and are more transparent in the red region of the spectrum, as shown by the TCe:YAG-PMMA/TPMMA ratio (43) CRC Handbook of Chemistry and Physics, 80th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1999.
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Figure 7. Emission spectra of the Ce:YAG nanopowder (λex = 450 nm), the Ce:YAG-PMMA nanocomposite (λex = 450 and 340 nm), and PMMA (λex = 340 nm).
Figure 6. TEM micrograph of the Ce:YAG-PMMA nanocomposite.
Table 1. Transmittance (T) Values Corrected for the Thickness of the PMMA and Ce:YAG-PMMA Samples at Different Wavelengths wavelengths
TPMMA
TCe:YAG-PMMA
TCe:YAG-PMMA/TPMMA
350 400 450 500 600 700
0.70 0.81 0.84 0.85 0.87 0.88
0.56 0.69 0.74 0.78 0.82 0.86
0.80 0.85 0.88 0.91 0.94 0.98
reported in Table 1. The presence of Ce:YAG more drastically reduces the transparency in the blue region of the spectrum; in any case, the reduction is hardly larger than 20%. This small reduction cannot be ascribed to an agglomeration of nanoparticles because it has been excluded from TEM micrographs but can be attributed to the difference in the refractive index of the surrounding nanocrystal.44 The emission (PL) spectra of Ce:YAG nanopowders are reported in the upper part of Figure 7. Emission spectra were recorded for an excitation wavelength, λex, of 450 nm, which corresponds to excitation into the lowest-energy d level of Ce3þ. The PL spectrum consists of a characteristic broad asymmetric band having a maximum located at 530 nm. It is assigned to the 5d (2A1 g) f 4f (2F5/2 and 2F7/2) transitions of Ce(III). In fact, Ce(III) with a 4f1 electron configuration has two ground states of 2F5/2 and 2F7/2 because of the spin-orbit interaction.45 This constitutes further evidence for the presence of the Ce(III) ion in the YAG lattice, as reported in the literature.22 The PL spectrum of the Ce:YAG-PMMA composite is reported in the middle of Figure 7. The emission spectrum of the nanocomposite has a profile similar to that of the nanopowder, indicative of the same kind of emission centers except for a decrease in intensity due to their concentration in the PMMA matrix. The maximum emission wavelength does not significantly change, but a blue shift smaller than 5 nm is observed. This blue (44) Ryszkowska, J. Mater. Sci. Eng., B 2008, 146, 54–58. (45) Blasse, G.; Bril, A. Appl. Phys. Lett. 1967, 11, 53.
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shift is attributable to the different environment around the Ce: YAG nanopowders. Bhat et al.46 showed a small blue shift of the emission maximum with an increase in the refractive index of the medium that has been correlated to the refractive index term generally employed to describe solvent or medium effects on molecular electronic spectra. However, when the emission of the composite is registered using an excitation wavelength of 340 nm, the PL spectrum consists of two peaks, the first at 387 nm and the second at 407 nm (lower part of Figure 7). This profile is observed by some authors47 in the Ce0.85Tb0.15F3/ PMMA composite and was attributed to the emission of PMMA (instead of Ce3þ). As proof of this, a PL spectrum of PMMA is reported in the lower part of Figure 7. SAXS coupled with XRD measurements were performed in order to attempt a correlation between the optical properties and the structure of the nanocomposite. The SAXS 2D patterns of the Ce:YAG nanopowder and the Ce:YAG-PMMA nanocomposite, recorded at a 24 cm sample-to-detector distance, were isotropic. They were averaged around annular rings, thus providing a scattering intensity of I(Q) in the 0.02-0.8 A˚-1 Q range. Q, the momentum transfer, is equal to 4π sin θ/λ, with 2θ being the scattering angle. Measurements, carried out on different portions of the samples, were overlapped, thus showing that the samples are homogeneous. A comparison of the SAXS experimental data of the two samples, after corrections for the background and for the samples thickness, is shown in Figure 8. For both samples, the SAXS intensity increases in the region of high Q values. Moreover, the pattern of the composite shows a peak at 0.31 A˚-1 superimposed on the monotonic trend. The peak appearance indicates the presence of a repeat distance. This cannot be ascribed to some arrangements of nanoparticles. In fact, even by assuming that nanoparticles are in contact, the peak position, evaluated on the basis of the nanoparticle size from the TEM micrograph, should be 0.077 A˚-1, which is well below the observed value. It follows that it has to be ascribed to some structure induced on the polymer by the presence of Ce:YAG. To investigate this aspect, the XRD diffraction patterns were converted on the Q scale that ranges from 0.25 up to 5.3 A˚-1. This (46) Venkataprasad Bhat, S.; Govindaraj, A.; Rao, C. N. R. Chem. Phys. Lett. 2006, 422, 323–327. (47) Chai, R.; Lian, H.; Li, C.; Cheng, Z.; Hou, Z.; Huang, S.; Lin, J. J. Phys. Chem. C 2009, 113, 8070–8076.
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Figure 8. SAXS intensities vs scattering vector Q of PMMA (O) and the Ce:YAG-PMMA composite (b).
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Figure 10. Ce:YAG-PMMA intensity after subtracting the PMMA contribution. The points are the experimental data, and the straight line is the best fit using Nallet’s model.48
concentration fluctuation is negligible. It follows that the original equation reduces to IðqÞ ¼
Figure 9. SAXS (O) and XRD intensities (b) vs scattering vector Q of PMMA and the Ce:YAG-PMMA composite.
allows an overlapping region from 0.25 up to 0.8 A˚-1 in the SAXS and XRD experiments. Obviously, the XRD intensities were scaled to the SAXS ones and the overlap is roughly good as shown in Figure 9. The first indication that clearly emerges is that the observed increase in the SAXS intensity in the high-Q region has to be ascribed to the first amorphous peak detected for both samples. By comparing the two patterns, a small displacement of the first amorphous maximum toward a higher Q value is a consequence of the Ce:YAG presence. The positions of other maxima remain unchanged, and the resolution of the peaks improves. This, together with the presence of the diffraction peak at 0.31 A˚-1, indicates that Ce:YAG induces local order with small effects in the amorphous structure. On this basis, as a first approximation, it was assumed that in the SAXS range the amorphous contribution could be considered to be unvaried. Therefore, the PMMA intensity was subtracted from that of Ce:YAG-PMMA. The I(Q) versus Q curve thus obtained is reported in Figure 10. Because the low-angle region of the pattern is dominated by a single diffuse band, the presence of a cubic arrangement was excluded and it was considered that the polymer backbones could arrange to form lamellae. On this basis, the model proposed by Nallet et al.48 was considered. The model was modified by considering that the diffuse scattering arising from a polymer Langmuir 2010, 26(16), 13442–13449
A ðQ - Qmax Þ2 ξl 2 þ 1
þB
ð1Þ
where the first term represents the quasi-Bragg peak due to the stacking of the lamellar layers and the last term is a background constant term. A is the intensity scale factor, ξl is the spatial correlation length of the lamellar layers, and Qmax is the Q value corresponding to the peak. The intensity computed by this model well reproduces the experimental one. The calculated A and ξl values were 81((1) au and 46((1) A˚, respectively. The Qmax value was 0.308((2) A˚-1. From the peak position, using Bragg’s equation, a repeated distance d of about 20.4((1) A˚ was evaluated. These findings confirm that the Ce:YAG nanoparticles induces local structure in the amorphous polymer that arranges to form domains of this size. The obtained ξl, and d values, following the model, suggest the presence of two repeating and correlated layers of polymer. A pictorial representation of the local structure is shown as the inset in Figure 11. Notwithstanding, detailed information on the physical-chemical environment of each component (nanoparticles and polymer) cannot be obtained by the above experiment. Thus, to gain insight on the organization of the polymer molecules in contact with the nanoparticle surface, 13C {1H} CPMAS NMR experiments were performed. The 13C {1H} CP-MAS NMR spectrum of PMMA is reported in the lower part of Figure 11. Five peaks are present, and their assignments, according to the literature,49 are indicated in Figure 11. The peak positions are reported in Table 2. The 13C {1H} CP-MAS NMR spectrum of the Ce:YAG-PMMA nanocomposite is reported in the upper part of Figure 11. No modification in the chemical shift and in the band shape is observed, indicating that the interaction between polymer and nanoparticles occurs without the formation of chemical bonds. Thus, the spin-lattice relaxation time in the laboratory frame T1(H) and in the rotating frame T1F(H) and the cross-polarization time TCH were (48) Nallet, F.; Roux, D.; Milner, S. T. J. Phys. (Paris) 1990, 51, 2333. (49) Eijkelenboom, A. P. A. M.; Maas, W. E. J. R.; Veeman, W. S. Macromolecules 1992, 25, 4511–4518.
DOI: 10.1021/la9042809
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Figure 11.
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13
C {1H} CP-MAS NMR spectra of pure PMMA (lower) and of the Ce:YAG-PMMA composite (upper). Table 2. T1G(H) Values for All of the Peaks in the 13C Spectra of PMMA and Ce:YAG-PMMA Samples T1(H), s
TCH, μs
T1F(H), ms
peak
ppm
PMMA
Ce:YAG-PMMA
PMMA
Ce:YAG-PMMA
PMMA
Ce:YAG-PMMA
1 2 3 4 5
16.4 45.5 52.7 55.1 178.8
0.71 ( 0.02 0.72 ( 0.02 0.69 ( 0.02 0.68 ( 0.02 0.64 ( 0.02
0.86 ( 0.02 0.85 ( 0.02 0.84 ( 0.02 0.86 ( 0.02 0.80 ( 0.02
6.6 ( 0.2 7.0 ( 0.2 7.6 ( 0.3 7.2 ( 0.2 6.4 ( 0.4
9.0 ( 0.4 12.8 ( 0.2 13.6 ( 0.3 15.0 ( 0.4 24.8 ( 0.7
306 ( 21 577 ( 21 136 ( 12 63 ( 4 1788 ( 452
314 ( 22 641 ( 32 290 ( 40 62 ( 9 2440 ( 147
determined through solid-state NMR measurements in order to evaluate the dynamic modifications occurring in the polymeric chain of the PMMA matrix after composite formation. It is known that the spin-lattice relaxation time in the rotating frame (T1F) is sensitive to the molecular motions that occur in the kilohertz region. In particular, it is inversely proportional to the spectral density of motion. Kilohertz-region motions are typically associated with cooperative polymer backbone rearrangements that envelop the collective motions of a large number of monomer units.50 Long-range cooperative motions such as these are considered to be the motions that define a polymer’s response to mechanical perturbations.51,52 The T1(H), T1F(H), and TCH values obtained from each peak in the 13C spectra of PMMA and Ce:YAG-PMMA samples are reported in Table 2. In PMMA, each peak has constant values of T1(H) and T1F(H) around 0.7 s and 7 ms, respectively. These values increase when the Ce:YAG nanoparticles are present in the PMMA matrix. The higher T1(H) values obtained for the composite are evidence of the polymer stiffness increase that can be caused by the higher order of the polymer chains, which is attributable to the interactions of the polymeric matrix with the filler. The increase in the T1F(H) values confirms the T1(H) finding on a smaller scale. In (50) Boyer, R. F. Polym. Eng. Sci. 1968, 8, 161–185. (51) Farrar, T. C.; Becker, E. D. Pulse and Fourier Transform NMR; Academic Press: New York, 1971; pp 46-65. (52) Campbell, I. D.; Dwek, R. A. Biological Spectroscopy; Benjamin Cummings Pub. Co.: Menlo Park, CA, 1984; pp 127-177.
13448 DOI: 10.1021/la9042809
addition, it allows us to establish that the nucleus is directly involved in the interaction with the nanoparticles. In fact, the high T1F(H) value for the carbonyl groups (peak 5) is evidence that the interaction between PMMA and the nanofiller is principally localized in this nucleus environment. Furthermore, the TCH value trend demonstrates that there is a decrease in the dipolar interactions of the carbonyl carbon, confirming the rearrangement of the PMMA according to the proposed model in SAXS analysis.
4. Conclusions A new transparent polymeric composite was prepared by embedding Ce:YAG nanopowder in the PMMA matrix by in situ polymerization. The preparation method ensure a more homogeneous dispersion compared to other methods such as melting and turns out to be very versatile in obtaining a material with different shapes and thicknesses. The intensive characterization of the material, performed using a multitechnique approach, was necessary to gain better knowledge of its structure and properties. The composite transparency indicates good nanopowder dispersion in the polymer, which was confirmed by TEM observations. The nanocomposite consists of Ce:YAG nanoparticles surrounded by a polymeric PMMA matrix whose local structure is organized in lamellar mode as inferred by SAXS combined with XRD. The interaction between the two components occurred at the interface through the carbonylic groups of the polymer, as proven by solid-state NMR measurements. The optical properties of the obtained nanocomposite are similar to the ones observed in the Ce:YAG nanopowder. The present Langmuir 2010, 26(16), 13442–13449
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finding suggests that PMMA is an optimal support for the Ce: YAG nanopowder because it does not modify its PL spectrum. All of the results show that the nanocomposite is an ideal yellow light complementary to the blue light emitted, for example, by GaN LED to give a white LED. Acknowledgment. We thank the MIUR for supporting this research through the PRIN 2007 prot. 20077R3PXF_002
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“New nanocomposites preparation for optical, electric and magnetic applications” and the Fondazione Banco di Sicilia, which cofinanced the Fluoromax 4 (Jobin-Yvon) spectrofluorimeter (Convenzione PR 19.b/06). NMR, SAXS, and HR-TEM experimental data were provided by Centro Grandi Apparecchiature - UniNetLab - Universita di Palermo funded by POR Sicilia 2000-2006, Misura 3.15 Azione C Quota Regionale.
DOI: 10.1021/la9042809
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