Eu 3+ -Doped Y 2 O 3 -SiO 2 Nanocomposite Obtained by a Sol-Gel Method

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1 Mat. Res. Soc. Symp. Proc. Vol Materials Research Society Eu 3+ -Doped Y 2 O 3 -SiO 2 Nanocomposite Obtained by a Sol-Gel Method Carla Cannas a,marianocasu a, Roberta Licheri a, Anna Musinu a, Giorgio Piccaluga a, Adolfo Speghini b and Marco Bettinelli b a Dipartimento di Scienze Chimiche, Università di Cagliari, SP Sestu-Monserrato, Km 0.700, I Monserrato, Cagliari, Italy b Dipartimento Scientifico e Tecnologico, Università di Verona, Ca' Vignal, Strada Le Grazie 15, I Verona, Italy ABSTRACT AY 2 O 3 -SiO 2 nanocomposite doped with Eu 3+ was obtained by a sol-gel method and characterized by X-ray diffraction, IR, 29 Si NMR and laser-excited luminescence spectroscopy. It was found that small (2-3 nm) yttria nanoparticles are homogeneously dispersed in, and interacting with, the amorphous silica matrix. Luminescence spectroscopy indicates that the Eu 3+ ion is preferentially located inside or at the surface of highly disordered Y 2 O 3 nanoparticles. These luminescent nanocomposites form a class of materials which could find applications in the field of phosphors. INTRODUCTION Lanthanide ions in insulating host matrix find uses in a wide variety of applications, such as phosphors for fluorescent lighting, display monitors, X-ray imaging, laser and amplifiers for fiber optic communication. Preparing nanoscale host materials can change the physical properties, which affect the luminescence and dynamics of the optically active dopant. Nanocrystalline Y 2 O 3 has been investigated as a host, due to its interesting applications in the field of the phosphors for lighting and for cathode ray tubes 1,2. In the present work, we decided to investigate the optical properties of Eu 3+ doped Y 2 O 3 nanoparticles dispersed in a transparent medium such as amorphous SiO 2, which can stabilize and protect the nanoparticles from aggregation 3.TheEu 3+ ion was chosen on the basis of its high emission quantum yield, its relatively simple energy level scheme, its applications as structural probe for the sites occupied by the Ln 3+ in a given material, 4 and the technological importance of Y 2 O 3 :Eu 3+ as a phosphor 2. The present study focuses on the preparation, the characterization of structural and optical properties of a Eu 3+ -doped Y 2 O 3 -SiO 2 nanocomposite, containing 10 wt% of Y 2 O 3, obtained by a sol-gel method which involves the use of the nitrate salt as precursor 5. This procedure in principle favors the formation of metal oxide nanoparticles hampering the embedding of metal ions in the silica matrix amorphous network 6. This work may also lead to an easy, effective preparation method for these materials. EXPERIMENTAL An Eu 3+ -doped Y 2 O 3 -SiO 2 nanocomposite containing 9.8 wt% of Y 2 O 3 and 0.2 wt% of Eu 2 O 3 (corresponding to an Eu/Si atomic ratio of ) was prepared by a sol-gel method following the procedure described elsewhere 7. An aqueous solution of weighted amounts of yttrium and europium nitrate (Y(NO 3 ) 3 6H 2 O and anhydrous Eu(NO 3 ) 3, Aldrich 99%) was mixed with an ethanolic solution of tetraethoxysilane (EtOH 95%, TEOS, Aldrich 98%) and acidified with nitric acid up to ph<1. The TEOS/EtOH/water molar ratio was 1/3/10. The clear sol was stirred for one hour and then poured into a Teflon beaker and allowed to gel at room temperature. The same procedure was used to prepare the silica matrix as reference sample. In order to minimize the presence of defects typical of sol-gel silica in the samples Y3.18.1

2 treated at low temperature, the gels were heated at T=900 C for 4 hours and then powdered prior the structural and optical characterization. The powder X-ray diffraction (XRD) spectra were collected by using a θ-2θ conventional equipment (Siemens D500) at Mo-Kα wavelength; yttria oxide nanoparticles were observed in electron micrographs obtained using a TEM (Jeol 200CX) operating at 200 KV. The CPMAS spectra were collected with a contact time of 4 ms and a recycle time of 5 s. The 29 Si chemical shift was referenced to that of tetramethylsilane. Mid-IR spectra, from 4000 to 400 cm -1, were obtained using a Bruker EQUINOX 55 spectrophotometer on KBr pellets of the samples. Room temperature luminescence and excitation spectra were measured using a Jasco FP777 conventional fluorimeter. The nm line of a Spectra-Physics 2017 Argon laser was also used to excite room temperature luminescence spectra. A fiber optic probe coupled to a Dilor Superhead, equipped with a suitable notch filter, was employed. The scattered signal was analyzed by a Jobin-Yvon HR460 monochromator and a Spectrum One CCD detector. A 150 lines/mm grating was used to collect the laser-excited luminescence spectra. RESULTS and DISCUSSION The XRD spectrum of the Eu 3+ -doped Y 2 O 3 -SiO 2 sample shows the typical amorphous character of the material and is very similar to the pattern of the corresponding silica matrix exhibiting a main large band at about 2θ=10, as it can be observed in Fig nm (uạ. ) I b a Θ Fig. 1 XRD spectra of silica (a) and nanocomposite (b) samples. Fig. 2. TEM bright field micrograph of the nanocomposite sample. The amorphous character of the Eu 3+ -doped yttria nanoparticles is confirmed by TEM measurements. In fact their presence is revealed only by the heavy absorption contrast in the bright field micrographs, while they are not visible in dark field mode; as an example the homogeneous dispersion of spherical nanoparticles in the silica matrix is shown in Fig. 2. The mean particle size is 2.5 nm with a very narrow distribution (+0.5 nm), as calculated on over 1500 nanoparticles. Y3.18.2

3 The 29 Si-MAS NMR spectra of the nanocomposite and of the corresponding silica matrix samples are shown in Fig. 3. The spectrum of the silica sample exhibits a peak at -110 ppm which, according to the literature, 8 can be easily ascribed to the highest condensation degree Q 4 groups (Q n represents the SiO 4 tetrahedron of the amorphous network which forms n bonds with neighboring tetrahedra) 9. c a b ppm ppm Fig Si MAS spectra of silica (a) and nanocpomposite (b) samples. 29 Si CPMAS spectrum of nanocomposite sample (c). The spectrum of the nanocomposite shows, in addition to the main signal assigned to Q 4 groups, a broadened shoulder at high frequency [(-70) (-100) ppm] suggesting the presence of Q n sites of low condensation degree. This observation is further confirmed by the 29 Si- CPMAS spectrum of the nanocomposite reported in Fig. 3c. In fact, three resolved peaks at about -110, -101 and -91 ppm can be respectively assigned to Q 4, Q 3 and Q 2 groups, according to the literature; 8 also a broad shoulder at about -84 ppm is present, which can be ascribed to Q 1 groups. The 29 Si-CPMAS spectrum of the corresponding silica matrix is not observable since the sample is free from silanol groups, being only formed by Q 4 groups. These results are in good agreement with IR measurements reported in the following. The IR spectra of the nanocomposite and the corresponding silica samples are reported in Figure 4. They show the bands typical of silica gel samples treated at high temperature 10.The intensity of the band at 795 cm -1, ascribed to the bending of Si-O-Si, is increased in the spectrum of the silica sample compared to that of the nanocomposite. This indicates the formation of a lower amount of bridging oxygen among SiO n groups in the nanocomposite. This observation is in agreement with the presence, in the spectrum of the nanocomposite, of a broad shoulder at 960 cm -1 due to the stretching of silanol groups, which is absent in the spectrum of silica matrix. Y3.18.3

4 7 F 0 7 F 3 Fig. 4. FTIR spectra of silica (a) and nanocomposite (b) samples. Another significative difference, between the spectra of the nanocomposite and that of the corresponding silica samples, is concerned with the presence of absorbed water. In fact, the spectrum of the nanocomposite exhibits two bands at about 1640 and 3440 cm -1, respectively due to the bending and the stretching of water molecules. This last band superimposes to that at 3500 cm -1, which can be ascribed to the stretching of silanol groups interacting through hydrogen bonds with water molecules. TGA measurements reveal that the weight losses registered up to 900 C for both nanocomposite and silica samples are less than 1%. This indicates that the water, visible in the IR spectrum of the nanocomposite is trapped inside the pores of the material. On the basis of these observations, it can be suggested that the silica polycondensation process has hampered in the nanocomposite by the small doped-yttria particles. These nanoparticles, with a great surface/volume ratio, form in the cavities of silica matrix and during the thermal treatment keep the water trapped at the nanoparticle/matrix interface. 5 D 0 7 F 2 Intensity (a.u.) 7 F 1 λ exc =488.0 nm 7 F Wavelength (nm) Fig. 5: Room temperature emission spectrum of the Eu 3+ -doped Y 2 O 3 -SiO 2 nanocomposite. Y3.18.4

5 The room temperature laser-excited luminescence spectrum of powders of the Eu 3+ doped nanocomposite, measured with excitation wavelength nm, consists of relatively broad 5 D 0 7 F J emission transitions (Figure 5). The highly forbidden 5 D 0 7 F 0 transition peaks at nm (17320 cm -1 ), the magnetic dipole 5 D 0 7 F 1 transition is resolved into a shoulder at ~586.5 nm (17050 cm -1 ) and two peaks at and nm (16920 and cm -1, respectively), whilst the hypersensitive 5 D 0 7 F 2 transition is only very weakly structured and peaks at nm (16310 cm -1 ). The luminescence spectrum of the Eu 3+ doped nanocomposite is typical of a disordered medium. In the sample under investigation, the Eu 3+ could be present inside the Y 2 O 3 nanoparticles, or dispersed in the SiO 2 glass matrix. It is therefore useful to compare the present spectra with the ones reported for Eu 3+ doped sol-gel SiO 2 glasses. Thorough studies on well characterized samples of these materials have been carried out by Ferrari et al. 11 and Campostrini et al. 12. The luminescence spectrum of the SiO 2 :Eu 3+ gel-derived glass heat treated at 1000 C is characterized by the 5 D 0 7 F 0 transition peaking at cm -1,whilst the 5 D 0 7 F 2 transition peaks around cm -1. These results are significantly different from the ones obtained for the doped nanocomposite under investigation, whose spectrum does not appear to be dominated by Eu 3+ centers located in the SiO 2 network. λ em =613.0 nm Intensity (a.u.) Wavelength (nm) Fig. 6: Room temperature excitation spectrum of the Eu 3+ -doped Y 2 O 3 -SiO 2 nanocomposite. It is therefore conceivable that the Eu 3+ ions are mainly located inside or at the surface of the Y 2 O 3 nanoparticles. The presence of a significant inhomogeneous broadening in the luminescence spectrum, indicates that presumably these particles are highly disordered, in agreement with the absence of well-defined peaks in the X-ray diffraction pattern. On the other hand, it has already been shown that nanoparticles of Y 2 O 3 and other materials activated by Eu 3+, obtained by solution techniques, can be characterized by luminescence spectra affected by a strong inhomogeneous broadening Y3.18.5

6 The excitation spectrum shown in Fig. 6 is characterized by the onset of a strong band starting around 300 nm and peaking close to 200 nm. This band is assigned to a charge transfer transition involving the excitation of an electron from the 2p orbitals of oxygen to the 4f orbitals of europium 1. It has to be noted that the present spectrum appears to be very similar to the one reported for a Y2O3:Eu3+@silica sample, obtained by chemical vapor synthesis of the doped yttria nanopowder followed by filling of the pores of a porous silica aerogel. This observation seems to confirm that Eu3+ is indeed located inside or at the surface of the yttria nanoparticles. CONCLUSIONS The sol-gel technique has been successful in obtaining a Y 2 O 3 -SiO 2 nanocomposite containing 10 wt% of Y 2 O 3 doped with Eu 3+. We have shown that under the synthetic conditions described here small (2-3 nm) yttria nanoparticles are formed strongly interacting with the SiO 2 matrix through water molecules at the interface. Excitation and laser-excited luminescence spectra of these materials indicates that presumably the Eu 3+ ion is preferentially located inside or at the surface of highly disordered Y 2 O 3 nanoparticles. These findings show that it is possible to disperse Ln 3+ doped yttria nanoparticles in a transparent host. The resulting nanocomposites, in which Ln 3+ is a luminescent ion, such as Eu 3+,Tb 3+ and Er 3+, are candidates for the development of efficient phosphors for cathode ray tubes and for upconversion. ACKNOLEDGEMENTS This work was carried out within the MURST PRIN project _005. REFERENCES 1. G. Blasse, and B.C. Grabmaier, Luminescent Materials (Springer-Verlag, Berlin, 1994). 2. T. Ye, Z. Guiwen, Z. Weiping and X. Shangda, Mater. Res. Bull. 32, 501 (1997). 3.Q.Li,L.GaoandD.Yan,Chem.Mater.11, 533 (1999). 4. J.-C. G. Bünzli, in: J.-C. G. Bünzli and G. R. Choppin (Eds.), Lanthanide Probes in Life, Chemical and Earth Sciences: Theory and Practice, (Elsevier, Amsterdam, 1989, p. 219). 5. G. Piccaluga, A. Corrias, G. Ennas and A. Musinu, Mater. Sci. Foundation ISSN, Ed. M. Magini and F. H. Wohlbier (Trans Tech. Publ.Inc NH, USA) Vol. 13, p. 1, (2000). 6.S.RoyandD.Chakravorty,J.Mater.Res.9, 2314 (1994). 7. C. Cannas, D. Gatteschi, A. Musinu, C. Sangregorio, G. Piccaluga, J. Phys. Chem. B, 102, 7721 (1998). 8. G. Engelhardt and D. Michel, High Resolution Solid State NMR of Solids and Zeolites, (J. Wiley & Sons, New York, 1987), chapter E. Lippmaa, M. Magi, A. Samoson, G. Engelhardt and A. R. Grimmer, J. Amer. Chem. Soc. 102, 4889 (1980). 10. S. Bruni, F. Cariati, M. Casu, A. Lai, A. Musinu, G. Piccaluga, S. Solinas, NanoStrucur. Mater. 11, 573 (1999). 11. M. Ferrari, R. Campostrini, G. Carturan and M. Montagna, Phil. Mag. B 65, 251 (1992). 12.R.Campostrini,G.Carturan,M.Ferrari,M.MontagnaandO.Pilla,J.Mater.Sci.7, 745 (1992). 13. P. K. Sharma, R. Nass and H. Schmidt, Opt. Mater. 10, 161 (1998). 14. P. K. Sharma, M. H. Jilavi, H. Schmidt and V. K. Varadan, Int. J. Inorg. Mater. 2, 407 (2000). Y3.18.6