In Situ Preparation and Luminescent Properties of CeF3

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In Situ Preparation and Luminescent Properties of CeF3 and CeF3:Tb3+ Nanoparticles and Transparent CeF3:Tb3+/PMMA Nanocomposites in the Visible Spectral Range Ruitao Chai, Hongzhou Lian, Chunxia Li, Ziyong Cheng, Zhiyao Hou, Shanshan Huang, and Jun Lin* State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, and Graduate UniVersity of the Chinese Academy of Sciences, Beijing 100049, P. R. China ReceiVed: January 11, 2009; ReVised Manuscript ReceiVed: March 19, 2009

CeF3 and CeF3:Tb3+ nanoparticles were prepared by reverse microemulsion with a functional monomer, methyl methacrylate (MMA), as the oil phase, and CeF3:Tb3+/poly (methyl methacrylate) (PMMA) nanocomposites were obtained via polymerization of the MMA monomer. The nanoparticles and nanocomposites have been well characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), low- and high-resolution transmission electron microscope (TEM), selected-area electron diffraction (SAED), thermogravimetric analysis (TGA), UV/vis transmission spectra, photoluminescence excitation, and emission spectra and luminescence decays. The well-crystallized CeF3 and CeF3:Tb3+ nanoparticles are spherical with a mean diameter of 15 nm. They show the characteristic emission of Ce3+ 5d-4f (313 nm, 2D-2F5/2; 323 nm, 2D-2F7/2) and Tb3+ 5D4-7FJ (J ) 6-3, with 5D4-7F5 green emission at 541 nm as the strongest one) transitions, respectively. The energy transfer from Ce3+ to Tb3+ was investigated in CeF3:Tb3+ nanoparticles in detail. The obtained solid CeF3:Tb3+/PMMA nanocomposites are highly transparent (in the visible spectral range) and exhibit strong green photoluminescence upon UV excitation, due to the integration of luminescent CeF3:Tb3+ nanoparticles. The luminescent lifetime of Tb3+ is determined to be 4.1 ms in the nanocomposite. 1. Introduction Polymers are of profound interest to society and are replacing metals in diverse fields of life, which can be further modified according to modern application. “Nanocomposite polymers” are polymers that have a dispersed phase in them with ultrafine dimension typically on the order of nanometers.1 The introduction of inorganic nanoparticles into a polymer matrix has proved to be an effective method to improve the performance of polymer materials.2 Some unusual properties of the materials, such as superconductivity,3 thermal stability,4 and magnetic,5 nonlinear optical,6 and dynamic mechanical properties have been observed.7 As a consequence, the nanocomposite properties are strongly influenced by the nature of the interface between inorganic and polymer matrices. A strong interfacial interaction between the inorganic nanoparticles and the polymer matrix sometimes lead to unexpected new properties in these materials, which are often not exhibited by individual compounds and thus open a new avenue for chemists, physicists and materials scientists. The key issue is the development of synthesis methods allowing for particle generation in solution without aggregation and subsequent functionalization and transfer into the hydrophobic matrix without isolation of the particles. By now, several methods for preparing polymer nanocomposites such as sol-gel reactions,8 intercalative polymerization,2a,9 melt-processing,10 and in situ polymerization2b,11 have been applied depending on the nature of nanoparticles and the synthesis and processing of polymeric matrices. Polymerization of inverse microemulsions is an important and more specialized method for the preparation of nanocomposites.12 Inverse (water-in-oil) microemulsions are isotropic, transparent, thermodynamically stable media that * To whom correspondence should be addressed. E-mail: [email protected].

consist of small water droplets surrounded by a surfactant monolayer and dispersed in an oil-rich continuous phase. The dispersed aqueous droplets act as space confined synthesis of nanoparticles, while the surrounding oil phase consists of a polymerizable, liquid monomer. When the nanoparticles are formed and homogeneously dispersed in the micelles, the microemulsion is polymerized to transform the nanosuspension into solid nanocomposites. The advantage of this method for preparation of inorganic nanoparticles/polymer hybrids is to simplify the process of modifying inorganic nanoparticles with organic materials and to disperse these nanoparticles into the precursors. The choice of the polymer and the dispersed phase is determined by the properties required for the end product. Among the broad variety of available polymers, PMMA has been one of the most widely studied in the last decades due to its outstanding and promising mechanical and chemicophysical properties and has been applicated in many sectors such as aircraft glazing, signs, lighting, architecture, and transportation. Moreover, since PMMA is nontoxic, it could be also useful in dentures, medicine dispensers, food handling equipments, throat lamps, and lenses. PMMA embeddedd with inorganic or organically modified inorganic particles has been cast into films to yield enhanced functional properties such as electrical conductivity,13 photoconductivity,14 photoinduced chargetransfer,15 nonlinear optical properties,16 photoluminescence,11a,bd,17 mechanical,2b,12a,18 and magnetic properties.19 Compared with conventional oxide-based luminescent materials, fluorides have advantages as fluorescent host materials owing to their low vibrational energies and the subsequent minimization of the quenching of the excited-state of the rareearth ions.20 As a potential scintillator and tunable laser material, CeF3 is a luminescent material with 100% activator concentra-

10.1021/jp900269b CCC: $40.75  2009 American Chemical Society Published on Web 04/16/2009

CeF3 and CeF3:Tb3+ Nanoparticles

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TABLE 1: Salt Concentrations in Aqueous Phases and Ce0.85Tb0.15F3 Contents in Nanocomposites sample CeF3 Ce0.85Tb0.15F3 Ce0.85Tb0.15F3 Ce0.85Tb0.15F3 Ce0.85Tb0.15F3 Ce0.85Tb0.15F3 Ce0.85Tb0.15F3

NP1 NP2 NP3 NC1 NC2 NC3

concentration of Ce(NO3)3 (M)

concentration of Tb(NO3)3 (M)

concentration of NH4F (M)

Ce0.85Tb0.15F3 content (wt %)

0.5 0.255 0.425 0.85 0.255 0.425 0.85

0.045 0.075 0.15 0.045 0.075 0.15

1.5 0.9 1.5 3 0.9 1.5 3

0.072 0.115 0.231

tion.21 Doping Tb3+ in CeF3 resulted in a strong green emission from Tb3+ due to an efficient energy transfer from Ce3+ to Tb3+.22 However, so far no reports have appeared concerning the integration of luminescent CeF3:Tb3+ nanoparticles into the PMMA matrix. In this article, we describe in situ generation of CeF3 and CeF3:Tb3+ nanoparticles in reverse microemulsions containing methylmethacrylate (MMA) as the surrounding oil phase and CeF3:Tb3+/PMMA nanocomposites were prepared by in situ polymerization of the microemulsions. The morphology, structure, luminescence, and energy transfer properties of these nanoparticles and nanocomposites were investigated in detail. The obtained composite materials are highly transparent (in the visible spectral range) and show strong green photoluminescence. 2. Experimental Section Materials. Tb4O7 (99.999%) and Ce(NO3)3 · 6 H2O were purchased from Science and Technology Parent Company of Changchun Institute of Applied Chemistry. NH4F was purchased from Beijing Chemical Company. Cetyltrimethylammonium bromide (CTAB, 99%), methyl methacrylate (MMA, 99%), methanol (99.5%), and dichloromethane (99.5%) were purchased from Beijing Yili Fine Chemicals Co., Ltd. All chemicals are analytical grade and are used directly without further purification. Tb(NO3)3 were prepared by dissolving Tb4O7 in diluted nitric acid, and the water in the solutions was distilled off by heating. Butanediol monoacrylate (BDMA) and trimethylolpropane triacrylate (TMPTA) and photoinitiator 2,4,6-trimethylbenzoyldiphenylphosphine oxide (TPO) were obtained from TCI (Tokoy, Japan) and used as received. Preparation of CeF3 and Ce0.85Tb0.15F3 Nanoparticles. First, two organic solutions were prepared separately with 1.2 g (3.3 mmol) of CTAB, 22.5 g (0.225 mol) of MMA, and 1.6 g (0.011 mol) of BDMA, respectively. 0.5 M Ce(NO3)3 (1 mL) was added dropwise into one of the above-prepared organic solutions under magnetic stirring until clear microemulsion I was obtained. 1.5 M NH4F (1 mL) was added dropwise into another aboveprepared organic solution, and clear microemultion II was obtained. Microemultions I and II were mixed quickly and stirred vigorously at 50 °C for 1 h. The reaction products were centrifugated at 4800 rpm for 30 min, and then washed three times with a 1:1 (volume ratio) mixture of methanol and dichloromethane to remove organic phase and surfactant. The final products (CeF3 nanoparticles) were dried under an infrared lamp over a night. Ce0.85Tb0.15F3 nanoparticles were prepared by the similar procedure, except that different salt concentrations were selected to investigate the effect of the factor on the morphological, structural and optical properties. The salt concentrations in aqueous phase are given in Table 1. Preparation of Ce0.85Tb0.15F3/PMMA Nanocomposites. Compared with the preparation of nanoparticles, all of the components of microemulsions for nanocomposites decrease 19

times by weight. The data in detail are shown as follows. A total of 1.1 g (11 mmol) of methyl methacrylate (MMA) was mixed with 0.0603 g (0.16 mmol) of cetyltrimethylammonium (CTAB) and 0.08 g (0.55 mmol) of butanediol monoacrylate (BDMA) and then 50 µL of a mixed solution of Ce(NO3)3 and Tb(NO3)3 was added drop by drop to form a transparent microemulsion at 50 °C (ME 1). A second microemulsion (ME 2) was prepared similarly except the for solute in aqueous phase. The aqueous phase of ME 2 is a mixture of NH4F solution. The two microemulsions were mixed and stirred for 1 h at 50 °C. To this mixture were added 0.8 g (5.55 mmol) of BDMA, 0.8 g (2.70 mmol) of trimethylolpropane triacrylate (TMPTA), and 0.04 g (0.06 mmol) of 2,4,6-trimethylbenzoyldiphenylphosphine oxide (TPO) to form a photopolymerizable dispersion. Then, the particle dispersions in monomers were transferred into a glass mold formed by separating two glass plates with thick glass seals, and the glass templates were heated to 50 °C before use. Polymerization was carried out in a UV inert cube using UV radiation (200 W cm-1, LAMP PHILIPS) for 3 min. Different salt concentrations were selected to investigate the effect of the factors on the transparence and optical properties. The salt concentrations in aqueous phase and Ce0.85Tb0.15F3 contents in nanocomposites are given in Table 1. Characterization. X-ray diffraction (XRD) of the powder samples were examined on a D8 Focus diffractometer (Bruker) using Cu KR radiation (λ ) 0.15405 nm). Scanning electron microscopy (SEM) micrographs and energy-dispersive X-ray spectroscopy (EDS) were obtained with use of a field emission scanning electron microscope (FE-SEM, XL30, Philips). The nanocomposite samples were cryogenically broken after immersion in liquid nitrogen and the fractured surfaces were coated with Au and observed by scanning electron microscope (SEM). The plane surfaces were also observed in order to make clear whether there are CeF3:Tb3+ nanoparticles on the nanocomposite surface. Selected-area electron diffraction (SAED) studies were performed on a H-8100 transmission electron microscope operating at 200 kV. Low- and high-resolution transmission electron microscopy (TEM) was performed using FEI Tecnai G2 S-Twin with a field emission gun operating at 200 kV. Images were acquired digitally on a Gatan multiople CCD camera. Samples for TEM, SAED, and HRTEM were prepared by depositing a drop of a colloidal ethanol solution of the powder sample onto a carbon coated copper grid. The excess liquid was wicked away with filter paper, and the grids were dried in air. The thermogravimetric analysis (TGA) data were recorded with a thermal analysis instrument (Pyris Diamond, PerkinElmer) with a heating rate of 10 °C/min in air. The UV-vis transmission spectra were measured on a TU-1901 spectrophotometer, and the as-prepared bulk nanocomposites (40 × 30 × 2 mm) were measured without any processing. The excitation and emission spectra were taken on an F-4500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The PL lifetimes of the samples were measured with a Lecroy Wave Runner 6100 Digital Oscilloscope (1 GHz) using a tunable laser (pulse width ) 4 ns, gate ) 50 nm) as the excitation source (Contimuum Sunlite OPO). All of the measurements were performed at room temperature. 3. Results and Discussion 3.1. CeF3 and Ce0.85Tb0.15F3 Nanoparticles. CeF3 and Ce0.85Tb0.15F3 nanoparticles were prepared with reverse microemulsions, which consisted of MMA as the continuous oil phase and a surfactant/cosurfactant mixture of CTAB (cetyltrimethylammonium) and BDMA (butanediol monoacrylate). The aqueous solutions of Ce(NO3)3 or the mixture of Ce(NO3)3 and

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Figure 1. XRD patterns of CeF3 (a), Ce0.85Tb0.15F3-NP1 (b), Ce0.85Tb0.15F3-NP2 (c), and Ce0.85Tb0.15F3-NP3 (d) nanoparticles. The line spectrum corresponds to the standard data of CeF3 (JCPDS No. 08-0045).

Figure 3. TEM (a-c) and HRTEM (d) images of Ce0.85Tb0.15F3 nanoparticles. (a) Ce0.85Tb0.15F3-NP1, (b) Ce0.85Tb0.15F3-NP2, (c) Ce0.85Tb0.15F3-NP3, and (d) Ce0.85Tb0.15F3- NP2. The inset of (b) is SAED pattern of Ce0.85Tb0.15F3-NP2. Figure 2. TEM image (a) and SAED pattern (b) of CeF3 nanoparticles.

Tb(NO3)3 (ME I) and NH4F (ME II) are dispersed as small droplets, stabilized by the surfactant-cosurfactant mixtures.11b CeF3 and Ce0.85Tb0.15F3 nanoparticles were prepared by mixing ME I and ME II. This double microemulsion-precipitation results in the formation of uniform, small nanoparticles. The nanoparticles can be collected by centrifuging the dispersion after reaction at 50 °C for 1 h. The phase structures of the nanoparticles were investigated by X-ray diffraction (XRD). Figure 1 shows XRD patterns of CeF3 (a) and Ce0.85Tb0.15F3 (b-d) nanoparticles and the standard data for CeF3 as well. The results of the XRD indicate that the as formed CeF3 and all of the Ce0.85Tb0.15F3 (prepared with different salt concentrations) are well crystallized, and the patterns are consistent with the hexagonal phase structure known from the bulk CeF3 crystal (JCPDS: 08-0045) with a space group of P63/mcm (193). Additionally, no other peaks can be found in the patterns, revealing that there is no impurity in the products. In general, the nanocrystallite size can be estimated from the Scherrer equation, D ) 0.941 λ/β cos θ, where D is the average grain size, λ is the X-ray wavelength (0.15405 nm), and θ and β are the diffraction angle and full-width at halfmaximum (fwhm) of an observed peak, respectively. 23 The average crystallite sizes of CeF3, Ce0.85Tb0.15F3-NP1, Ce0.85Tb0.15F3-NP2, and Ce0.85Tb0.15F3-NP3 nanoparticles were determined as 14.2, 15.5, 15.2, and 13.1 nm, respectively, by using the strongest peak (111) at 2θ ) 27.6°. It is noticed that the average crystallite size of Ce0.85Tb0.15F3 nanoparticles changes little with differentsaltconcentrationsofaqueousphaseinthemicroemulsions. Figure 2 shows the transmission electron microscopy (TEM) image (a) and selected area electron diffraction (SAED) patterns (b) of CeF3 nanoparticles. The TEM image illustrates that the well-separated CeF3 nanoparticles are slightly ellipsoidal with an average size of 15 nm. The SAED pattern shows a set of broad diffuse rings instead of spots due to the random orientation of the crystallites, corresponding to diffraction from different

planes of the nanocrystallites. The SAED patterns are consistent with a hexagonal phase structure of CeF3 with strong ring patterns due to (111), (112), (302), and (114) planes, in good agreement with XRD patterns and demonstrating its polycrystalline nature. Low-resolution (a-c) and high-resolution (d) TEM images and SAED patterns (the inset in Figure 3b) of Ce0.85Tb0.15F3 nanoparticles are depicted in Figure 3. The three kinds of Ce0.85Tb0.15F3 nanoparticles have similar morphology (ellipsoidal), average size (15 nm), and crystallinity, revealing that the influence of salt concentrations of aqueous phase in microemulsions on these properties is small. We can also see that such properties of Ce0.85Tb0.15F3 as morphology, size distribution, and SAED patterns are almost the same as those of CeF3, which indicates that the dopant ion Tb3+ has little effect on the morphology and crystallinity of CeF3. In other words, we can say that the Tb3+ ions are well-dissolved in CeF3 nanocrystals. The HR-TEM image in Figure 3d reveals that the as-prepared Ce0.85Tb0.15F3 nanoparticles are highly crystallized with the interplanar spacing of 0.367 nm corresponding to the (002) lattice planes. Figure 4 gives the excitation (a) and emission (b) spectra and the decay curve (c) for the luminescence of Ce3+ in CeF3 nanoparticles. The emission spectrum (Figure 4b) of CeF3 nanoparticles includes a broadband ranging from 290-400 nm with a maximum at 313 nm and a shoulder at 323 nm, which are assigned to the parity allowed transitions of the lowest component of the 2D state to the spin-orbit components of the ground state, 2F5/2 and 2F7/2 of Ce3+, respectively. Monitored with the emission wavelength of 313 nm, the obtained excitation spectrum (Figure 4a) consists of four broad peaks with maxima at 204, 239, 255 (strongest), and 276 nm, which correspond to the transitions from the ground state 2F5/2 of Ce3+ to the different components of the excited Ce3+ 5d states split by the crystal field.8d The above spectral properties (excitation and emission spectra) of CeF3 nanoparticles are basically consistent with those reported for the CeF3 bulk crystals21b,c and nanoparticles.24 Figure 4c shows the luminescence decay curve of Ce3+ in CeF3 nanoparticles. This curve can be well-fit into a single-exponential


Figure 4. Excitation (a, λem ) 313 nm) and emission (b, λex ) 255 nm) spectra and the decay curve (c) for the luminescence of Ce3+ in CeF3 nanoparticles.

function as I ) I0 exp(-t/τ) (I0 is the initial emission intensity at t ) 0 and τ is 1/e lifetime of the Ce3+ ion). The lifetime of Ce3+ is determined to be 13.2 ns, a value which is in good agreement with the reported lifetimes of Ce3+.21b,22d,25 The short lifetime of Ce3+ is due to the allowed character for its 5d-4f transition. The Tb3+ ion doped CeF3 nanoparticles show a strong green emission under UV excitation. Ce0.85Tb0.15F3-NP1, Ce0.85Tb0.15F3-NP2, and Ce0.85Tb0.15F3-NP3 have similar spectral properties, so we just give the excitation (a) and emission (b) spectra and the decay curve (c) for the luminescence of Tb3+ in Ce0.85Tb0.15F3-NP3 nanoparticles in Figure 5 to account for the photoluminescence properties of Ce0.85Tb0.15F3 nanoparticles. The excitation spectrum (Figure 5a) recorded at 541 nm (5D47 F5) of Tb3+ is composed exclusively of the excitation bands of Ce3+, which is similar to the excitation spectrum of Ce3+ in CeF3 nanoparticles (Figure 4a). Excitation into the Ce3+ band at 255 nm yields both the very weak emission of Ce3+ (300-350 nm) and the strong emission of Tb3+ (450-650 nm). This indicates that an energy transfer from Ce3+ to Tb3+ occurs in the Ce0.85Tb0.15F3 nanoparticles, as observed in the reported bulk26 and nanocrystalline22d-g materials. The emission of Tb3+ is due to transitions between the excited 5D4 state and the 7FJ (J ) 6, 5, 4, 3) ground states of Tb3+ ions. No emission from the higher 5D3 level is observed due to a cross relaxation effect at the high Tb3+ concentration.8d Figure 5c shows the luminescence decay curve of Tb3+ in Ce0.85Tb0.15F3-NP3 nanoparticles.


Figure 5. Excitation (a) and emission (b) spectra and the decay curve (c) for the luminescence of Tb3+ in Ce0.85Tb0.15F3-NP3 nanoparticles.

This decay curve of Tb3+ can also be fitted into a singleexponential function as I ) I0 exp(-t/τ) (I0 is the initial emission intensity at t ) 0 and τ is 1/e lifetime of the Tb3+ ion). The lifetime of Tb3+ is determined to be 5.0 ms in Ce0.85Tb0.15F3 nanoparticles. The luminescence lifetime of Tb3+ (5D4) is in the range of milliseconds due to the forbidden nature of the f-f transition. 3.2. CeF3:Tb3+/PMMA Nanocomposites. The three complex liquid mixtures including Ce0.85Tb0.15F3 nanoparticles were transformed into solid transparent (in the visible spectral range) polymer materials by in situ polymerizations of these microemulsions. TMPTA and TPO were added for the photopolymerization process serving as cross-linker and radical initiator, respectively. However, not all microemulsions in the MMA/ CTAB/BDMA/water system remain transparent during polymerization. It is found that further addition of BDMA (20 wt %) is essential for increasing the stability in this step. While being necessary for the polymerization reaction, higher BDMA concentrations in the starting solution work against the particle generation. Therefore, additional BDMA was added after the precipitation and crystallization steps. Then, these liquid mixtures were transferred into glass molds and polymerized subsequently under UV irradiation. The glass templates must be heated to 50 °C before use to avoid the microemulsions turning into aggregated phase upon cooling. Later, the solid and transparent (in the visible spectral range) polymers were obtained. Figure 6 gives the photographs of Ce0.85Tb0.15F3-NC1 (a), Ce0.85Tb0.15F3-NC2 (c), and 0.85Tb0.15F3 -NC3 (e) under

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Figure 6. Photographs of the as-prepared Ce0.85Tb0.15F3-NC1 (a, b), Ce0.85Tb0.15F3- NC2 (c, d), and Ce0.85Tb0.15F3-NC3 (e, f). (a, c, e): under daylight; (b, d, f): under UV excitation in dark (λ ) 254 nm).

Chai et al.

Figure 8. SEM images (a) and EDS spetrum (b) of the plane surfaces of the Ce0.85Tb0.15F3/PMMA nanocomposite containing 0.231 wt % of Ce0.85Tb0.15F3 nanoparticles (Ce0.85Tb0.15F3-NC3).

Figure 9. TEM images (a) and SAED pattern (b) of the prepared Ce0.85Tb0.15F3/PMMA nanocomposite (thin cut) containing 0.231 wt % of Ce0.85Tb0.15F3 nanoparticles (Ce0.85Tb0.15F3-NC3).

Figure 7. SEM images (a-c) and EDS spetrum (d) of the fractured surface of Ce0.85Tb0.15F3/PMMA nanocomposites. Ce0.85Tb0.15F3-NC1 (a), Ce0.85Tb0.15F3-NC2 (b), and Ce0.85Tb0.15F3-NC3 (c and d).

daylight. It is found that the transparency of these nanocomposites decreases gradually with the increase of incorporating concentration of CeF3:Tb3+ nanoparticles. To establish the degree of nanoparticles dispersion within the polymeric matrix, nanocomposite samples were cryogenically broken after immersion in liquid nitrogen and the fractured surfaces were coated with Au and observed by scanning electron microscope (SEM). Figure 7 shows the SEM micrographs of the fractured surfaces of the nanocomposites containing 0.072 (a), 0.115(b), and 0.231 wt % (c) of Ce0.85Tb0.15F3 nanoparticles, respectively. From the figures, fine nanoparticles dispersed in the polymeric matrix can be seen clearly, and nanopowder agglomeration has been noted in the case of high content (0.231 wt %, see Figure 7c). Figure 7d shows the EDS pattern for the Ce0.85Tb0.15F3/PMMA nanocomposite, which confirms the presence of F, Ce, and Tb. The Au peak and Si peak in the figure come from the coating for the measurement and the glass template, respectively. The plane surface of nanocomposite were also observed in order to make clear whether there are CeF3:Tb3+ nanoparticles on the nanocomposite surface. Figure 8a shows the SEM micrographs of the plane surface of the Ce0.85Tb0.15F3/PMMA nanocomposite containing 0.231 wt % of Ce0.85Tb0.15F3 nanoparticles. The image establishes the presence of CeF3:Tb3+ nanoparticles on the nanocomposite surface. The CeF3:Tb3+ nanoparticles dispersed in the PMMA matrix with a little

agglomeration. The EDS (Figure 8b) pattern confirms the presence of F, Ce and Tb. The Pt peak and Al, Si peak in the figure come from the coating for the measurement and glass template, respectively. All of these phenomena of the plane surface are similar to those of the fractured surface of the nanocomposite. For further confirming the structure of CeF3:Tb3+ in nanocomposites, the TEM image (a) and SAED pattern (b) of the nanocomposite (thin slices cut by ultramicrotome) were shown in Figure 9. The average size of CeF3:Tb3+ in nanocomposite is 15 nm, which is in agreement with that of nanoparticles centrifugated from microemulsions. SAED pattern shows a set of strong rings due to (111), (112), (302), and (114) planes which are consistent with a hexagonal phase structure of CeF3. The thermal stabilities of Ce0.85Tb0.15F3/PMMA nanocomposites were investigated by TGA with pure PMMA polymer as a comparison, and the corresponding thermograms are illustrated in Figure 10. For the pure polymer, there are three degradation steps in the curve, (1) 280-450 °C, (2) 450-530 °C, and (3) 530-600 °C. The first one, which takes place by heat absorption, is ascribed to the degradation of the polymer’s unsaturated groups, in contrast the second reaction is due to unzipping initiated by the random scission produced by monomer volatilization, beyond 530 °C PMMA is destroyed completely.27 Ce0.85Tb0.15F3/PMMA nanocomposites show slightly less thermal stability at 230 °C probably due to physisorbed water evaporating at this temperature. The decomposition steps and the causation of weight loss of Ce0.85Tb0.15F3/PMMA nanocomposites are similar to those of pure polymer beyond 330 °C. It is noteworthy that the temperature for polymer degradation increases with the increasing of Ce0.85Tb0.15F3 nanoparticle contents during 320-460 °C, which indicates that the interaction between the CeF3:Tb3+ nanoparticles and the PMMA polymer matrix are so strong that the thermal degradation of the PMMA polymer is more difficult.27 The UV-vis transmittance spectra of pure polymer and nanocomposites containing different Ce0.85Tb0.15F3 concentra-


Figure 10. TGA diagrams of pure polymer and Ce0.85Tb0.15F3/PMMA nanocomposites. The lines along the arrow are representative of pure polymer (a), Ce0.85Tb0.15F3-NC1 (b), Ce0.85Tb0.15F3-NC2 (c), and Ce0.85Tb0.15F3-NC3 (d).

Figure 11. Transmission spectra of pure polymer (a) and Ce0.85Tb0.15F3/ PMMA nanocomposites (b-d). The Ce0.85Tb0.15F3 contents for lines b-d are 0.072, 0.115, and 0.231 wt %, respectively.

tions are shown in Figure 11. Pure polymer has a transmittance of 90% (Figure 11a). The loss of 10% is caused by reflection on the two surfaces of the block (2 mm thickness). The transmittance of Ce0.85Tb0.15F3-NC1 (0.072 wt % Ce0.85Tb0.15F3) is very close to that of pure polymer, and the loss in transparency is detected only below λ ) 500 nm (Figure 11b). Compared to that of Ce0.85Tb0.15F3-NC1, the transmittance of Ce0.85Tb0.15F3-NC2 (0.115 wt % Ce0.85Tb0.15F3) reduces a little but is still as high as 85% (800 nm) (Figure 11c). Ce0.85Tb0.15F3-NC3 containing 0.231 wt % Ce0.85Tb0.15F3 appears optically transparent even though the transmittance is only 62% at 800 nm (Figure 11d). Upon UV excitation Ce0.85Tb0.15F3/PMMA nanocomposites show green emission, whose photographs under UV excitation in dark and the excitation and emission spectra are shown in Figures 6, panels b, d, and f, and 12, respectively. From the photographs, we can see that the nanocomposite exhibits bluegreen emission when the incorporating content of Ce0.85Tb0.15F3 is 0.072 wt % (Figure 6b), and that the strong green emission could be observed in Ce0.85Tb0.15F3-NC2 and Ce0.85Tb0.15F3-NC3, in which the contents of Ce0.85Tb0.15F3 are 0.115 and 0.231 wt %, respectively. The excitation and emission spectra of Ce0.85Tb0.15F3/PMMA nanocomposites and Ce0.85Tb0.15F3


Figure 12. Excitation (a, c, and e, λem ) 541 nm) and emission (b, d, f, and g, λex ) 255 nm) spectra of the as-prepared Ce0.85Tb0.15F3-NC1 (a and b), Ce0.85Tb0.15F3-NC2 (c and d), Ce0.85Tb0.15F3-NC3 (e and f), and pure polymer (g).

Figure 13. Decay curve of Tb3+ luminescence in Ce0.85Tb0.15F3-NC3.

nanoparticles have similar profiles, indicative of the same kind emission centers in them. But there are also two differences in them. First, compared with CeF3:Tb3+ nanoparticles, the excitation spectra of Ce0.85Tb0.15F3/PMMA nanocomposites is not well resolved, and only the strongest band around 255 nm of Ce3+ is present (other shoulders present in that of the CeF3:Tb3+ nanoparticles cannot be seen clearly here) due to the noncrystalline nature (amorphous) PMMA matrix. Second, besides the typical emission of Tb3+ ions, the broad emission band at 350-400 nm can be clearly seen in the Ce0.85Tb0.15F3/PMMA nanocomposites. The profiles of these emission bands at 350-400 nm in nanocomposites are similar to that of pure polymer (Figure 12g), so these emission bands could be attributed to the emission of PMMA (instead of Ce3+). In other words, the Ce3+ f Tb3+ energy transfer behaves similarly in Ce0.85Tb0.15F3/PMMA nanocomposites as in Ce0.85Tb0.15F3 nanoparticles, i.e., there exists an efficient energy transfer from Ce3+ to Tb3+ in both cases. 22 From Figure 12, it is realized that with the increase of Ce0.85Tb0.15F3 content, the photoluminescence intensity of nanocomposites are enhanced gradually, which is in good agreement with the results of their photographs (Figure 6). Figure 13 exhibits the decay curve for the luminescence of Tb3+ in Ce0.85Tb0.15F3-NC3 nanocomposite. It can be well fitted

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into a singly exponential function as I ) I0 exp(-t/τ) (I0 is the initial emission intensity at t ) 0 and τ is the 1/e lifetime of the emission center). Through this fitting, the lifetime of Tb3+ in the Ce0.85Tb0.15F3-NC3 nanocomposite is determined to be 4.1 ms. The luminescence lifetime of Tb3+ is in the range of milliseconds due to the forbidden nature of the f-f transition of the Tb3+ (5D4) ion. Note that the lifetime of Tb3+ decreases greatly in Ce0.85Tb0.15F3/PMMA nanocomposites with respect to that in the pure Ce0.85Tb0.15F3 nanoparticles. This results from the abundant organic groups with high phonon energy in PMMA that will quench the 5D4 emission of Tb3+ (thus decrease its lifetime) by the multiphonon relaxation process.21a 4. Conclusions In summary, well-crystallized CeF3 and CeF3:Tb3+ nanoparticles and a series of transparent CeF3:Tb3+/PMMA nanocomposites have been successfully prepared by in situ generation and in situ polymerization routes. The as-prepared CeF3 and CeF3:Tb3+ nanoparticles are all ellipsoidal with an average size of 15 nm, and they can show characteristic emission of Ce3+ (5d-4f) and Tb3+ (f-f), respectively. In the CeF3:Tb3+/PMMA nanocomposites, CeF3:Tb3+ nanoparticles are well dispersed in the polymer matrix. These nanocomposites exhibit high transparency for visible light and show strong green emission of Tb3+ (dominate at 541 nm) upon UV excitation. This method may be extended to prepare other functional nanocomposites. Acknowledgment. This project is financially supported by National Basic Research Program of China (2007CB935502), and the National Natural Science Foundation of China (NSFC 50702057, 50872131, 00610227). References and Notes (1) Qutubuddin, S.; Fu, X. In Nano Surface Chemistry Rosoff, M. Ed.; Marcel Dekker Inc.: New York, 2002; p 653. (2) (a) Okamoto, M.; Morita, S.; Taguchi, H.; Kim, Y. H.; Kotaka, S.; Tateyama, H. Polymer 2000, 41, 3887. (b) Avella, M.; Errico, M. E.; Martuscelli, E. Nano Lett. 2001, 1, 213. (3) Nagaoka, K.; Naruse, H.; Shinohara, I. J. Polym. Sci.: Polym. Lett. Ed. 1984, 22, 659. (4) Gotoh, Y.; Igarashi, R.; Ohkoshi, Y.; Nagura, M.; Akamatsu, K.; Deki, S. J. Mater. Chem. 2000, 10, 2548. (5) Palacio, F.; Maron, M.; Garin, J.; Reyes, J.; Fontcuberta, J. Mol. Cryst. Liq. Cryst. Sci. Technol. 1989, 176, 415. (6) Shiraishi, Y.; Toshima, N. Colloid Surf A 2000, 169, 59. (7) (a) Sawai, D.; Miyamoto, M.; Kanamoto, T.; Ito, M. J. Polym. Sci., Polym. Phys. Ed. 2000, 38, 2571. (b) Hong, S. U.; Jin, J. H.; Won, J.; Kang, Y. S. AdV. Mater. 2000, 12, 968. (8) (a) Huang, H.; Orler, B.; Wilkes, G. L. Macromolecules 1987, 20, 1322. (b) Huang, H.; Wilkes, G. L. Polym. Bull. 1987, 18, 455. (c) Noell, J. L. W.; Wilkes, G. L.; Mohanty, D. K.; McGrath, J. E. J. Appl. Polym. Sci. 1990, 40, 1177. (d) Yu, M.; Lin, J.; Fu, J.; Zhang, H. J.; Han, Y. C. J. Mater. Chem. 2003, 13, 1413. (e) Buissette, V.; Moreau, M.; Gacoin, T.; Boilot, J.-P. AdV. Funct. Mater. 2006, 16, 351.

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JP900269B

In Situ Preparation and Luminescent Properties of CeF3

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