Improved Energy Transfer between Ce3+ and Tb3+ Ions at the

Apr 22, 2009 - Improved Energy Transfer between Ce3+ and Tb3+ Ions at the Interface between Y2Sn2O7:Ce3+,Tb3+ Nanoparticles and Silica. Sandeep ... Fa...
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J. Phys. Chem. C 2009, 113, 8750–8755

Improved Energy Transfer between Ce3+ and Tb3+ Ions at the Interface between Y2Sn2O7:Ce3+,Tb3+ Nanoparticles and Silica Sandeep Nigam, Vasanthakumaran Sudarsan,* and Rajesh Kumar Vatsa Chemistry DiVision, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India

J. Ghattak and P. V. Satyam Institute of Physics, Bhubaneswar, India ReceiVed: February 2, 2009; ReVised Manuscript ReceiVed: March 26, 2009

Very small spherical nanoparticles (size in the range of 2-5 nm) of Y2Sn2O7 doped with both Ce3+ and Tb3+ were prepared by the urea hydrolysis of Y3+, Sn4+, Ce3+, and Tb3+ in ethylene glycol medium at 150 °C followed by heating at 700 °C. These nanoparticles exhibit very poor energy transfer between Ce3+ and Tb3+ ions and associated luminescence due to the combined effect of centrosymmetric D3d environment around the lanthanide ions and quenching of the lanthanide ion excited state due to the vibrations of surface hydroxyl groups present on the nanoparticles. However, the energy transfer and associated luminescence can be significantly improved after dispersing the nanoparticles in silica matrix. On the basis of the detailed steadystate and time-resolved luminescence studies, it has been established that a significant amount of Ce3+ and Tb3+ ions migrate from Y2Sn2O7 host to both the silica matrix and the interface region between silica and Y2Sn2O7 nanoparticles. Improved energy transfer from Ce3+ to Tb3+ and associated increase in Tb3+ luminescence from the nanoparticles dispersed in silica matrix has been attributed to the combined effect of lack of centrosymmetry around the lanthanide ions, removal of hydroxyl groups around the lanthanide ions, and the reduced distance between Ce3+ and Tb3+ ions at the interface regions between Y2Sn2O7 nanoparticles and silica. Introduction Oxides with pyrochlore structures (A2B2O7) are important functional materials due to their interesting physicochemical properties.1-4 Among different types of pyrochlores, the ones based on lanthanide ions (Ln3+) and Sn4+ ions are applicable as potential hosts for the incorporation of luminescent lanthanide ions.5,6 Such materials are necessary for developing phosphors, which can be extensively used in flat-panel displays such as plasma display panels (PDP), field emission displays (FED), and electroluminescent (EL) devices.5-8 It will be of interest to prepare the nanoparticles of these materials and study their luminescence properties as the nanoparticle-based phosphors are found to have many advantages over the bigger micron-sized particles. Important among them are the reduced electron penetration depth, subsequently lower excitation voltages, and higher luminescence intensity,9,10 which give rise to higher luminescent efficiency and better resolution for display devices. Very few reports are available regarding the luminescence properties of lanthanide ion doped Ln2Sn2O7 (Ln: represents lanthanide ions) nanoparticles. The reason being that conventionally such materials are prepared based on the solid-state reaction between the constituent metal oxides at relatively higher temperatures of around 1400-1500 °C. At this temperature, particle aggregation invariably takes place leading to the formation of particles having a size more than 100 nm. Of late, there are reports on the synthesis of Ln2Sn2O7 nanoparticles at relatively low temperatures using coprecipitation, sol-gel, and hydrothermal methods along with nanoparticle-mediated solid* To whom correspondence should be addressed. E-mail: vsudar@ barc.gov.in. Tel: +91-22-25590289. Fax: 91-22-25505151.

state reactions.11-16 However, in all of the above synthetic methods, nanoparticles obtained have a size in the range of 20-30 nm or more. For example, Lu et al.12 prepared Eu3+ doped yttrium tin pyrochlore (Y2Sn2O7) having a minimum size of 40 nm based on cetyl trimethyl ammonium bromide (CTAB)assisted sol-gel technique followed by calcination at 600 °C. These nanoparticles displayed intense and prevailing emission at 589 nm (orange emission) characteristic of the 5D0-7F1 magnetic dipole transition from Eu3+ ions with a very weak red emission. Multiband orange-red luminescence has been observed from thin films of Eu3+-doped Y2SnO7 prepared by the sol-gel method followed by heating at 1000 °C.16 La2Sn2O7 nanoparticles were prepared by Wang et al.11 based on the coprecipitation of La3+ and Sn4+ ions using ammonium hydroxide solution followed by heating at 1000 °C. These nanoparticles were as low as 50 nm in size and they showed emission around 400 nm characteristic of the oxygen vacancies in the lattice. Except Eu3+ or Yb3+ ions,14 no studies have been reported on the luminescence properties of other lanthanide ions in these nanoparticles hosts. Conventionally, Ce3+ and Tb3+ co-doped phosphor materials are developed with a view to generate efficient green light due to energy transfer from Ce3+ to Tb3+ ions. Efficient green light emission along with blue and red light emission from the same material is essential for developing materials for white light displays. However, in hosts like Y2Sn2O7, which require higher heat treatment temperatures for long durations (∼5 to 6 h) for forming the phase, it is possible that site symmetry around the lanthanide ion can undergo significant change due to the defective nature of the pyrochlore lattice as well as the cation disorder taking place in the lattice. The effects are more pro-

10.1021/jp9009756 CCC: $40.75  2009 American Chemical Society Published on Web 04/22/2009

Energy Transfer between Ce3+ and Tb3+ Ions

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nounced when the particles’ sizes are very small. As a result of this, the energy transfer between Ce3+ and Tb3+ ions and associated luminescence intensities from doped nanoparticles are strong function of the parameters like preparation procedure, heat treatment temperatures, and extent of surface modification. In the present article, we investigate the effect of surface modification on the luminescence properties of very small (having size in the range of 2-5 nm) nearly spherical Y2Sn2O7 nanoparticles doped with lanthanide ions like Ce3+ and Tb3+ ions. To the best of the authors’ knowledge, this is the first time that a methodology has been developed to achieve improved energy transfer from Ce3+ to Tb3+ ions and associated Tb3+ luminescence from Y2Sn2O7:Ce3+,Tb3+ nanoparticles. Experimental Section Preparation of the Nanoparticles. For the preparation of Y2Sn2O7 and lanthanide ion (Tb3+ and Ce3+) doped Y2Sn2O7 nanomaterials, 0.5 g of Sn metal, 0.014 g of Tb4O7 (2.5 atom % Tb3+), 0.75 g of Y2CO3 3H2O (95 atom % Y3+), and 0.037 g of Ce(NO3)36H2O (2.5 atom % Ce3+) were used as the starting materials. These were dissolved in concentrated HCl in a beaker, and the excess acid was evaporated out repeatedly. To this solution, ethylene glycol (20 mL) was added and it was transferred into a two-necked RB flask. The solution was slowly heated up to 100 °C followed by the addition of 2 g of urea and the temperature was increased to 140 °C. At this temperature, the solution became turbid. The temperature was then raised to 150 °C and maintained at this value for 2 h. The precipitate was collected after the reaction by centrifugation and then washed two times with acetone and three times with ethyl alcohol followed by drying under ambient conditions. After drying, the sample was heated to 700 °C for 5 h. Schematic diagram of preparation procedure is shown in Figure 1 of the Supporting Information. For the preparation of Y2Sn2O7: Ce3+(2.5%),Tb3+(2.5%) by the solid-state method, appropriate amounts of Y2CO3, SnO2, Tb4O7/ Ce(NO3)36H2O, were ground well and heated in air at 900 and 1200 °C for 6 h. Incorporation of the Nanoparticles in Silica Matrix. Initially, Y2Sn2O7:Ce3+(2.5%),Tb3+(2.5%) nanoparticles were prepared in ethylene glycol medium at 150 °C starting from 0.5 g of Sn metal, 0.014 g of Tb4O7, 0.75 g of Y2 (CO3)33H2O, 0.037 g of Ce2(CO3)36H2O, and 2 g of urea. To this reaction mixture, 0.9 mL of tetraethylorthosilicate (TEOS) was added dropwise and the reaction was continued for one more hour and then cooled to room temperature. The precipitate was washed several times with acetone and ethanol followed by heating in air at 700 °C. A schematic diagram of the preparation of Y2Sn2O7:Ce3+(2.5%),Tb3+(2.5%) nanoparticles and the nanoparticles dispersed in silica is shown in Figure 1 of the Supporting Information. Characterization. X-ray diffraction (XRD) studies were carried out using a Philips powder X-ray diffracto-meter (model PW 1071) with Ni-filtered Cu KR radiation. The lattice parameters were calculated from the least-squares fitting of the diffraction peaks. The average crystallite size was calculated from the diffraction line width based on Scherrer relation D ) 0.9λ/β Cos θ, where D is the average particles size, λ is the wavelength of X-rays, and β is the full width at half-maximum (fwhm). All luminescence measurements were carried out at room temperature with a resolution of 3 nm, using Edinburgh Instruments’ FLSP 920 system having a 450 W Xe lamp as the excitation source. Lifetime measurements were carried out using a 100 W microsecond Xe flash lamp. TEM was performed on a Y2Sn2O7:Ce3+(2.5%),Tb3+(2.5%) sample using a JEOL JEM

Figure 1. XRD pattern corresponding to (a) as prepared (b) 500 °C heated (c) 700 °C heated and (d) 900 °C heated samples of Y2Sn2O7: Ce3+(2.5%),Tb3+(2.5%) nanoparticles.

3000F TEM machine. Fourier transformed infrared (FTIR) patterns were recorded with a resolution of 2 cm-1 for thin pellets of the samples made with KBr using a Bomem MB102 machine. UV-vis spectra were recorded for the nanoparticles dispersed in water using a Jasco V-650 spectrophotometer. Results and Discussion XRD and TEM Studies. Figure 1 shows the XRD patterns of as prepared Y2Sn2O7:Ce3+(2.5%),Tb3+(2.5%) sample along with the ones heated at different temperatures. As prepared sample is amorphous and heat treatment below 700 °C did not induce any crystallization. These amorphous materials are nothing but the yttrium tin hydroxides formed by the hydrolysis of Y3+, Sn4+, Ce3+, and Tb3+ by ammonia generated from urea. However, at 700 °C and above, thermal decomposition of the hydroxides leads to the formation of highly crystalline cubic Y2Sn2O7:Ce3+,Tb3+ nanoparticles as can be clearly seen from the XRD pattern shown in part c of Figure 1. For 900 °C heated sample, crystallinity has been found to improve as revealed by the decrease in line width of the diffraction peaks. On the basis of the line width of the diffraction peaks, average crystallite size is estimated to be around 5 nm for the 700 °C heated sample. Lattice parameters were calculated based on the leastsquares fitting of the diffraction peaks and are found to be a ) 10.553(1) Å for the 700 °C heated sample. For nanoparticles incorporated in silica and heated at 700 °C, the XRD pattern (not shown) revealed only a broad peak characteristic of amorphous silica, suggesting that the crystalline nanoparticles are very finely dispersed in amorphous silica matrix. TEM images of the nanoparticles obtained at 700 °C are shown in Figure 2. The image is characterized by very fine spherical particles having a diameter in the range of 2-5 nm. Very small spherically shaped particles are mainly formed due to the low concentration of the product nuclei (yttrium tin hydroxide) existing in the reaction medium after coprecipitation.17 A highresolution image corresponding to a particular nanoparticle is shown in the same figure as an inset. The distance between the planes (2.99 Å) matches well with that of (222) planes of cubic

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Figure 2. TEM image of Y2Sn2O7:Ce3+(2.5%),Tb3+(2.5%) nanoparticles obtained at 700 °C. A high-resolution image is shown as an inset.

Figure 3. FTIR patterns of (a) Y2Sn2O7:Ce3+(2.5%),Tb3+(2.5%) nanoparticles and (b) Y2Sn2O7:Ce3+(2.5%),Tb3+(2.5%) nanoparticles dispersed in silica. Both of the samples were heated at 700 °C.

Y2Sn2O7 lattice. It is worth mentioning here that preparation and characterization of Y2Sn2O7 nanoparticles with such small dimensions have not been reported so far. To check the purity of the prepared samples, FTIR spectra of 700 °C heated samples of Y2Sn2O7:Ce3+(2.5%),Tb3+(2.5%) nanoparticles and Y2Sn2O7:Ce3+(2.5%),Tb3+(2.5%) nanoparticles dispersed in silica were recorded and are shown in Figure 3. Y2Sn2O7:Ce3+(2.5%),Tb3+(2.5%) nanoparticles (part a of Figure 3) are characterized by broad peaks around 3400 and 1545 cm-1 along with relatively sharp peaks around 643 and 410 cm-1. On the basis of the previous FTIR studies on Y2O3 and SnO2 nanoparticles,18-21 the peaks around 3400 and 1545 cm-1 have been attributed to the stretching and bending vibrations respectively of OH groups attached with the surface Sn4+ and Y3+ ions in the nanoparticles.19,20 The peak around 643 cm-1 is relatively sharp and is arising due to the antisymmetric Sn-O-Sn stretching mode.21 The peak corresponding to the Sn-O-Sn symmetric stretching overlaps with that of the Y-O stretching mode18,21 and appears as a broad peak around 410 cm-1. The nanoparticles after incorporation in silica showed sharp peaks around 1016, 948, and 879 cm-1 (part b of Figure 3), which are characteristic of the stretching vibrations of Si-O-Si linkages,19,21 along with the peak around 643 cm-1 (antisymmetric Sn-O-Sn stretching vibrations). The fact that

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Figure 4. Emission (right) and excitation (left) spectra corresponding to 700 °C heated samples of (a) Y2Sn2O7:Tb3+(5%) nanoparticles; (b) Y2Sn2O7:Ce3+(2.5%),Tb3+(2.5%) nanoparticles; (c) Y2Sn2O7: Ce3+(2.5%),Tb3+(2.5%) nanoparticles dispersed in silica; and (d) SiO2:Ce3+(2.5%),Tb3+(2.5%) sample. The wavelength of excitation for Y2Sn2O7:Tb3+(2.5%) and Y2Sn2O7:Ce3+(2.5%),Tb3+(2.5%) nanoparticles is 370 nm, whereas for Y2Sn2O7:Ce3+(2.5%), Tb3+(2.5%) nanoparticles dispersed in silica and SiO2:Ce3+(2.5%),Tb3+(2.5%) samples it is 335 and 310 nm, respectively.

the peak maximum for antisymmetric Sn-O-Sn stretching vibrations at 643 cm-1 remained the same for both types of samples, confirms that the Sn-O structural units of Y2Sn2O7: Ce3+,Tb3+ nanoparticles are unaffected by the incorporation of the nanoparticles in silica. Peaks below 643 cm-1 have been attributed to the bending vibrations of Si-O-Si linkages, which are overlapping with the stretching modes of Sn-O-Sn/YO-Sn/Y-O linkages.19,21 Thus, the FTIR studies clearly revealed that the OH groups are stabilized on the surface of the nanoparticles and there is no interaction between tin and silicon structural units in the nanoparticle-incorporated silica sample. Luminescence Studies on Y2Sn2O7:Ce3+,Tb3+ Nanoparticles and Y2Sn2O7:Ce3+,Tb3+ Nanoparticles Dispersed in Silica. As crystalline nanoparticles were obtained only after 700 °C annealing, all luminescence measurements were carried out for 700 °C heated samples. For the purpose of comparison, emission and excitation spectra are also shown for 5 atom % Tb3+ alone doped Y2Sn2O7 nanoparticles and silica samples containing 2.5 atom % each of Ce3+ and Tb3+ ions (all of the samples were prepared by the same method and heated at 700 °C). Part a of Figure 4 shows the emission and excitation spectra corresponding to Y2Sn2O7:Tb3+(5%) nanoparticles. A weak emission characteristic of Tb3+ is observed from the sample (only a 545 nm peak is clearly seen). An excitation spectrum corresponding to a 545 nm emission from the sample revealed only peaks around 350 and 370 nm, which are characteristic of the intra-4f transitions (7F6 f 5G5 and 7F6 f 5G6) of Tb3+ ions. No peak characteristic of the 4f f 5d excitation transition of Tb3+ or excitation peak corresponding to the host could be observed form the sample. In bulk Tb3+ doped Y2Sn2O7 lattice, Tb3+ ions occupy the Y3+ site, which has D3d site symmetry. The D3d site is centrosymmetric, and for lanthanide ions in a centrosymmetric site all electric dipole-induced transitions are forbidden and only the magnetic dipole transition is allowed. Tb3+ in the centrosymmetric site is expected to give a weak Tb3+ emission, which is in accordance with our experimental observation. The extent of reduction in the luminescence

Energy Transfer between Ce3+ and Tb3+ Ions

Figure 5. Decay curve corresponding to 5D4 level of Tb3+ ions in 700 °C heated samples of (a) Y2Sn2O7:Tb3+(5%) nanoparticles (b) Y2Sn2O7:Ce3+(2.5%),Tb3+(2.5%) nanoparticles and (c)Y2Sn2O7: Ce3+(2.5%),Tb3+(2.5%) nanoparticles dispersed in silica. For all samples, emission was monitored at 545 nm. The excitation wavelength was 370 nm for Y2Sn2O7:Tb3+(2.5%) and Y2Sn2O7: Ce3+(2.5%),Tb3+(2.5%) nanoparticles and 335 nm for Y2Sn2O7: Ce3+(2.5%),Tb3+(2.5%) nanoparticles dispersed in silica.

intensity is further augmented by the very small size (2-5 nm) of the nanoparticles and OH groups stabilized on the surface of the nanoparticles. For the particles having size in the range of 2-5 nm, the surface area to volume ratio (4πr2/4πr3/3) is around 107 cm-1. Under these conditions, a significant number of Tb3+ ions will be on the surface of the nanoparticles and the excited state of the Tb3+ ions can decay nonradiatively by the OH group vibrations of Y/Sn-OH linkages present on the surface of the nanoparticles. This leads to the faster decay of the surface lanthanide ion excited state compared to the bulk lanthanide ion excited state. Similar excited-state quenching of the surface lanthanide ions by groups present on the surface has been reported for other nanoparticles also.22 This is indeed clear from the decay curve corresponding to the 5D4 level of Tb3+ ions in Y2Sn2O7:Tb3+(5%) nanoparticles shown in part a of Figure 5. The decay is found to have two components having lifetimes of 6 µs (38%) and 1.2 ms (62%). The lower lifetime component is due to the Tb3+ ions on the surface of the nanoparticles and the higher lifetime component is due to the ones present at the bulk of the nanoparticles. Part b of Figure 4 shows the emission spectrum from 2.5 atom % Ce3+ co-doped Y2Sn2O7:Tb3+(2.5%) nanoparticles. Generally, Ce3+ co-doping in Tb3+-doped sample leads to the improvement in Tb3+ luminescence intensity due to energy transfer from Ce3+ to Tb3+ ions. However, Tb3+ emission intensity is found to get reduced compared to that from Y2Sn2O7: Tb3+(5%) nanoparticles, as can be seen from the emission spectrum, possibly due to the low concentration of Tb3+ ions in the nanoparticles. Also, the excitation spectrum shown in part b of Figure 4 very clearly revealed that no energy transfer from Ce3+ to Tb3+ exists in the sample. Lack of energy transfer is understandable as it depends on the extent of overlap between donor emission peak (Ce3+ emission peak in the present case)

J. Phys. Chem. C, Vol. 113, No. 20, 2009 8753 and acceptor absorption peak (Tb3+ excitation peak in the present case) and the distance between the donor and acceptor, as suggested by Dexter.23 As multipole interactions, which are proportional to the 1/Rn where R is the distance between donor and acceptor, are responsible for the through-space energy transfer processes, as shorter distances favor a higher extent of energy transfer. In the Y2Sn2O7 lattice, each YO8 polyhedron is separated by the SnO6 octahedron and the minimum distance between Y3+ ions is around 7.4 Å.24 At only 2.5 atom %, each of Tb3+ and Ce3+ ions are randomly distributed at the Y3+ site in the lattice, and the average distance between Ce3+ and Tb3+ ions will be significantly higher than 7.4 Å, thereby resulting in the increased distance between the donor and acceptor. Significant quenching of the lanthanide ion excited state taking place at the surface of the nanoparticles due to the vibrations of surface hydroxyl groups further prevents the energy transfer process. This explains the lack of energy transfer between Ce3+ and Tb3+ ions in the Y2Sn2O7 lattice. No Ce3+ emission also could be observed from the sample. The excited state of Tb3+ ions in the sample has been found to decay biexponentially with lifetime values 5 µs (46%) and 1.1 ms (54%) as can be seen from part b of Figure 5. By applying the same logic as in the case of Y2Sn2O7:Tb nanoparticles (part a of Figure 5), the faster decay component is attributed to the Tb3+ ions at the surface of the nanoparticles and the slower one to the ones at the bulk of the nanoparticles. To further substantiate the surface effect on the luminescence properties of the nanoparticles, following experiments were done where we covered the nanoparticles with silica. On the basis of previous studies,25-27 silica is known to modify the surface of the nanoparticles and hence it can be used as a probe to understand the surface effects on the luminescence properties of the nanoparticles. Emission and excitation spectra recorded from Y2Sn2O7:Ce3+(2.5 atom %),Tb3+(2.5%) nanoparticles incorporated in silica and heated at 700 °C are shown in part c of Figure 4. Strong Tb3+ emission after excitation at 335 nm is observed from the sample. An excitation spectrum corresponding to the 545 nm emission showed a broad peak around 335 nm with a shoulder peak around 310 nm. From the comparison of the emission and excitation spectrum with that of a silica sample containing Tb3+ and Ce3+ ions subjected to the same heat treatment temperature (part d of Figure 4), it is confirmed that the peak around 335 nm is arising due to the 4f f 5d excitation transition of Ce3+ ions present in the interface region between the Y2Sn2O7 lattice and the silica matrix and the shoulder peak around 310 nm is arising due to the Ce3+ ions in the silica matrix. Had it been due to the Ce3+ ions in the Y2Sn2O7 lattice, energy transfer between Ce3+ and Tb3+ would not have been observed (part b of Figure 4). At the interface between Y2Sn2O7: Ce3+(2.5%),Tb3+(2.5%) nanoparticles and silica, silicon structural units can interact with Y3+ structural units forming Y-O-Si type of linkages, which also reduces the symmetry around the lanthanide ions and the concentration of surface Sn-OH/Y-OH linkages. Cannas et al.25 have demonstrated the formation of such linkages by recording the 29Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectrum of Y2O3:Eu nanoparticles dispersed in silica. (Our previous studies have demonstrated that Sn4+ does not interact with silicon structural units up to a heat treatment temperature of 900 °C.27 FTIR studies discussed in the present manuscript (Figure 3) also supported that tin structural units do not interact with silica structural units). This leads to an entirely different environment around Ce3+ and Tb3+ions at the interface region between Y2Sn2O7:Ce3+(2.5%),Tb3+(2.5%) nanoparticles and

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Figure 6. UV-vis optical absorption spectrum of 700 °C heated samples of (a) Y2Sn2O7:Ce3+(2.5%),Tb3+(2.5%) nanoparticles and (b) Y2Sn2O7:Ce3+(2.5%),Tb3+(2.5%) nanoparticles dispersed in silica.

silica compared to either Y2Sn2O7 or silica. As the nanoparticles of the present study have a large surface to volume ratio, it is expected that a significant amount of lanthanide ions can exist at the interface region between the nanoparticles and silica. The relative intensity of the excitation peaks corresponding to Ce3+ ions in silica and the Y2Sn2O7-silica interface is obtained by deconvoluting the excitation spectrum in part c of Figure 4. It is found that the relative intensity for Ce3+ ions in silica to Ce3+ ions at the interface region between Y2Sn2O7 and silica is in the ratio 3:1. From the relative intensity ratio, it is inferred that a significant amount of Ce3+ and Tb3+ ions have migrated from the nanoparticles to silica matrix. Unlike in Y2Sn2O7 nanoparticles, the distance between Ce3+ and Tb3+ ions in silica or at the interface region between silica and Y2Sn2O7 will be smaller and this favors an enhanced energy transfer from the 5D5/2 and 5 D3/2 energy levels, which are the levels corresponding to the excited 5d1 configuration of Ce3+ ions, to the 5L7 and 5L9 levels of Tb3+ ions. To further confirm the existence of energy transfer, lifetime measurements corresponding to the 5D4 level of Tb3+ ions in the sample have been carried out. The decay curve corresponding to the 5D4 level of Tb3+ ions in Y2Sn2O7: Ce3+(2.5%),Tb3+(2.5%) nanoparticles dispersed in silica and heated at 700 °C is shown in part c of Figure 5. The 5D4 level has been found to decay biexponentially with lifetime values around 8 µs (4%) and 2.9 ms (96%). The lifetime values are found to be significantly higher for the nanoparticles dispersed in silica compared to Ce3+, Tb3+ co-doped, or Tb3+doped Y2Sn2O7 nanoparticles, supporting the energy transfer taking place between Ce3+ and Tb3+ ions in Y2Sn2O7: Ce3+(2.5%),Tb3+(2.5%) nanoparticles dispersed in silica. A schematic diagram of the energy transfer is shown in Figure 2 of the Supporting Information. It is possible that the poor luminescence from Y2Sn2O7: Ce3+(2.5%),Tb3+(2.5%) nanoparticles could be due to the conversion of Ce3+/Tb3+ to Ce4+/Tb4+ brought about by heat treatment. To verify this aspect, UV-vis optical absorption studies were carried out for 700 °C heated samples of both Y2Sn2O7:Ce3+(2.5%),Tb3+(2.5%) nanoparticles and Y2Sn2O7: Ce3+(2.5%),Tb3+(2.5%) nanoparticles dispersed in silica and are shown in Figure 6. The onset of absorption corresponds to a wavelength of 250 nm for Y2Sn2O7:Ce3+(2.5%),Tb3+(2.5%) nanoparticles and 335 nm for Y2Sn2O7:Ce3+(2.5%),Tb3+(2.5%) nanoparticles dispersed in silica. These results are in accordance

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Figure 7. TEM image of 700 °C heated sample of Y2Sn2O7: Ce3+(2.5%),Tb3+(2.5%) nanoparticles dispersed in silica. A highresolution image is shown as an inset.

with the results obtained from the emission and excitation spectra of the samples (Figure 4). These results also rule out the existence of Ce4+ or Tb4+ ions in the samples. Ce4+ and Tb4+ ions if present in the sample will be characterized by a strong absorption peak in the UV region due to the charge transfer transition between the 2p orbital of O2- and the 4f orbital of Ce4+/Tb4+ ions,28,29 which is not observed in the present study. Hence, on the basis of UV-vis optical absorption, steady-state, and time-resolved emission and excitation measurements, it is confirmed that the existence of lanthanide ions in the centrosymmetric site (D3d symmetry) and significant quenching of the lanthanide ion excited state by surface hydroxyl groups present with the Y2Sn2O7:Ce3+(2.5%),Tb3+(2.5%) nanoparticles are responsible for its poor luminescence properties. However, covering the nanoparticles with silica leads to the formation of an interface that lacks centrosymmetry. In addition to this, the surface hydroxyl groups get removed because the surface is now covered with silica. These two effects together improve the luminescence properties of the Y2Sn2O7:Ce3+(2.5%),Tb3+(2.5%) nanoparticle dispersed in silica. Incorporation of nanoparticles in silica matrix is confirmed from the TEM images shown in Figure 7. Silica is amorphous and in the amorphous matrix crystalline regions of Y2Sn2O7 can be clearly seen. The inset shows the high-resolution image from a representative region. Experiments were also done to compare the luminescence properties of the Ce3+, Tb3+-doped nanoparticles and the nanoparticles dispersed in silica with that of bulk Y2Sn2O7 samples doped with Ce3+, Tb3+ ions. The bulk samples were prepared by a solid-state reaction between stoichiometric amounts of tin metal, Y2O3, Ce2O3, and Tb4O7 at high temperatures of mainly 900 and 1200 °C. The XRD pattern shown in Figure 3 of the Supporting Information very clearly shows the formation of the Y2Sn2O7 phase at both the temperatures. However, no luminescence could be observed from the sample (Figure 4 of the Supporting Information) even after exciting the sample over a number of wavelengths. Because of the centrosymmetric environment around the lanthanide ions in the bulk Y2Sn2O7 sample, emission intensity from the lanthanide ions is very poor and usually gets buried under the scattering background arising from the sample. On the basis of these studies, it is established that dispersion of the nanoparticles in silica is a convenient and suitable method for achieving the energy transfer between Ce3+-Tb3+ ions and associated improvement in the Tb3+ luminescence from Ce3+ co-doped Y2Sn2O7:Tb3+ nanoparticles.

Energy Transfer between Ce3+ and Tb3+ Ions Conclusions Very small spherical nanoparticles having 2-5 nm size of Y2Sn2O7 doped with both Ce3+ and Tb3+ ions were prepared by the urea hydrolysis of Y3+, Sn4+, Ce3+, and Tb3+ in ethylene glycol medium at 150 °C followed by heating at 700 °C. Strong energy transfer and associated improvement in the Tb3+ luminescence has been observed from the nanoparticles only when the nanoparticles are dispersed in silica matrix. A significant amount of lanthanide ions migrate to the silica matrix and the remaining occupy the interface between the nanoparticles and silica matrix as revealed by the deconvolution of the excitation spectrum corresponding to the silica-dispersed sample. Removal of the centrosymmetry and lack of surface hydroxyl groups with the nanoparticles dispersed in silica reduces the extent of excited-state quenching of the lanthanide ions and this leads to the improved luminescence properties of the nanoparticles dispersed in silica. Further, compared to Y2Sn2O7:Ce3+, Tb3+ nanoparticles, reduction in the distance between the Ce3+ and Tb3+ ions existing in both silica matrix and in the interface between the nanoparticles and silica is also responsible for the improved energy transfer between Ce3+ and Tb3+ ions and associated luminescence of Y2Sn2O7:Ce3+(2.5%),Tb3+(2.5%) nanoparticles dispersed in silica. This is further supported by the increased lifetime values of the 5D4 level of Tb3+ ions in the sample. Acknowledgment. Authors thank Dr. T. Mukherjee, Director, Chemistry Group, BARC and Dr. D. Das, Head, Chemistry Division, BARC for their encouragement during this work. Supporting Information Available: Schematic diagram of the preparation of Y2Sn2O7:Ce3+(2.5%),Tb3+(2.5%) nanoparticles and the nanoparticles dispersed in silica; schematic diagram of the energy transfer between Ce3+ and Tb3+ ions; XRD patterns of the Y2Sn2O7:Ce3+(2.5%),Tb3+(2.5%) sample prepared at 900 and 1200 °C by solid-state reaction; and emission spectrum from Y2Sn2O7:Ce3+(2.5%),Tb3+(2.5%) samples prepared by solid-state reaction. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Lian, J.; Wang, L. M.; Wang, S. X.; Chen, J.; Boatner, L. A.; Ewing, R. C. Phys. ReV. Lett. 2001, 87, 1459011–4.

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