Enhancement of Sm3+emission by SnO2nanocrystals in the silica matrix

Silica xerogels containing Sm3+ions and SnO2nanocrystals were prepared in a sol–gel process. The image of transmission electron microscopy (TEM) shows that the SnO2nanocrystals are dispersed in the silica matrix. The X-ray diffraction (XRD) of the sample confirms the tetragonal phase of SnO2. The xerogels containing SnO2nanocrystals and Sm3+ions display the characteristic emission of Sm3+ions (4G5/2 → 6HJ(J = 5/2, 7/2, 9/2)) at the excitation of 335 nm which energy corresponds to the energy gap of the SnO2nanocrystals, while no emission of Sm3+ions can be observed for the samples containing Sm3+ions. The enhancement of the Sm3+emission is probably due to the energy transfer from SnO2nanocrystals to Sm3+ions.


Introduction
Sm 3+ ions can exhibit strong emission in the orange spectral region. The silica gel has been known as an excellent host material for rare earth ions because of its high transparency, compositional variety and easy mass production [1]. Therefore, the silica gel containing Sm 3+ ions has a potential application for high-density optical memory [2,3]. However, the Sm 3+ -doped gel cannot emit strong fluorescence [4]. It is necessary to introduce a sensitizer into the gel containing Sm 3+ ions in order to obtain strong emission of Sm 3+ ions.
Our previous study [5] showed that there existed the interaction between Eu 3+ ions and CdS nanoparticles in the silica matrix. Furthermore, Franzo et al. [6], Brovelli et al. [7], Bang et al. [8] and Selvan et al. [9] investigated the energy transfer between Si, SnO 2 , ZnO and CdS nanoparticles and rare earth ions. The present work aims to understand whether the SnO 2 nanocrystals can sensitize the Sm 3+ emission in the silica matrix. The one-step synthesis of the silica xerogels containing SnO 2 nanocrystals and Sm 3+ ions was described in a sol-gel process. The energy transfer from SnO 2 nanocrystals to Sm 3+ ions was presumed to explain the enhancement of the Sm 3+ emission in the silica matrix.

Experimental
All of reagents were commercially available and used without further purification. Double-distilled water was used as solvent. The silica xerogels containing SnO 2 nanocrystals (10 wt%) and Sm 3+ ions (0.5 mol%) were prepared in the sol-gel process similar to the procedure described by Nogami et al. [1]. In a typical preparation, the tetraethyl orthosilicate (TEOS) (10 mL) was added in the flask containing ethanol (5 mL), HCl (0.1 mmol), and H 2 O (3.25 mL). After the mixture was stirred for 0.5 h at room temperature, Sm(NO 3 ) 3 aqueous solution (0.1 mol L -1 , 2.25 mL) was introduced into the solution and stirred for another 0.5 h. Subsequently, SnCl 2 Á 2H 2 O ethanol solution (0.15 g mL -1 , 5 mL) was introduced into the sol. After stirred for 2 h, the sol was kept at 313 K for about 2 weeks to form gel. The sample was further dried in air to form the stiff xerogel. Finally, the xerogel was annealed in air at 700°C for 5 h to obtain the silica xerogel having SnO 2 nanocrystals and Sm 3+ ions.
The X-ray diffraction (XRD) of the silica xerogel having SnO 2 nanocrystals and Sm 3+ ions was performed on a Rigaku D/Max 2550VB/PC X-ray diffractometer with Cu Ka radiation (k = 0.154056 nm). The transmission electron microscopy (TEM) images were taken with a JEOL JEM-100CX electron microscopy. The absorption spectra were carried on a Unico UV-2102 PCS UV-vis spectrophotometer. The emission and excitation spectra were measured at room temperature with a Shimadzu RF-5301PC spectrophotometer.

Results and discussion
The TEM image of the silica xerogel containing Sm 3+ ions and SnO 2 nanocrystals is shown in Fig. 1. It can be clearly observed that a lot of nanoscale particles are dispersed in the silica matrix. These particles ought to be assigned to SnO 2 nanocrystals (see the discussion below). From the UV-Vis spectrum of the silica xerogel containing Sm 3+ ions and SnO 2 nanocrystals (Fig. 3), it can be observed that there exists a relatively steep shoulder around 300 nm, which may be assigned to the direct electron transition of the SnO 2 nanocrystals [10]. Furthermore, the shoulder red-shifts with increasing   These results further confirm that the SnO 2 nanocrystals are incorporated in the silica matrix, and the network of silica and SnO 2 is not formed. Figure 4 shows the emission spectra of the silica xerogels under the excitation of 335 nm (3.7 eV) corresponding to the energy gap of the SnO 2 nanocrystals. The peaks before 500 nm should be ascribed to the emission of silica gels. No characteristic emission of Sm 3+ ions can be observed for the silica xerogel containing Sm 3+ ions (curve a), while the sample containing SnO 2 nanocrystals and Sm 3+ ions shows strong characteristic emission of Sm 3+ ions (curve b). The emission peaks are assigned to the 4 G 5/2 fi 6 H J (J = 5/2, 7/2, 9/2) transitions of Sm 3+ ions [11]. These results indicate that the SnO 2 nanocrystals can sensitize the emission of Sm 3+ ions in the silica matrix. Meanwhile, it is possible that there exists effective energy transfer between SnO 2 nanocrystals and Sm 3+ ions in the silica matrix. The SnO 2 nanocrystals may act as light-harvesting antennas to sensitize emission of Sm 3+ ions.
It is well known that the energy transfer occurs unless the energy gap of the donor is equal to that of the acceptor in resonance condition. The emission band centered at 400 nm of SnO 2 nanocrystals in the SiO 2 gel which is ascribed to the electron transition mediated by defect levels [12] overlaps the dominating absorption line at 404 nm of Sm 3+ ions [13]. Therefore, it is possible that the energy transfers from SnO 2 nanocrystals to Sm 3+ ions. The proposed mechanism of the energy transfer between SnO 2 nanocrystals and Sm 3+ ions is shown in Scheme 1. When the sample is excited, the energy is harvested by the SnO 2 nanocrystals and transmitted from the defect levels of the SnO 2 nanocrystals to the Sm 3+ ions. The excited Sm 3+ ions emit the characteristic fluorescence via radiative relaxation. The surface states of the SnO 2 nanocrystals play an important role in the energy transfer. In our materials, these defect sites would be at the interface between the nanocrystals and the silica matrix. The results reported previously [14] shows that the energy transfer is not observed for SnO 2 nanoparticles doped with rare earth ions. Furthermore, the Sm 3+ ions cannot be doped into the lattice of SnO 2 nanoparticles in our experiments because the size of Sm 3+ ions (0.096 nm) is much bigger than that of Sn 4+ ions (0.076 nm). Meanwhile, the energy transfer between SnO 2 nanoparticles and Sm 3+ ions absorbed on the SnO 2 nanoparticles are not observed. Therefore, it is reasonable to deduce that the energy transfer takes place between the SnO 2 nanocrystals and the Sm 3+ ions near the nanocrystals.
The excitation spectra of the silica xerogel containing Sm 3+ ions and SnO 2 nanocrystals are monitored at 567 nm, 606 nm and 654 nm, respectively, as shown in Fig. 5. It can be seen that the sample displays a broad peak at 325 nm and a narrow peak at 404 nm for all of emission. The narrow peak can be assigned to the direct excitation of the Sm 3+ ions, and the broad peak corresponds to the electron transition in the SnO 2 nanocrystals [15]. This result further confirms that the energy can transfer from the SnO 2 nanocrystals to the Sm 3+ ions when the sample is excited.  5 Excitation spectra of the silica xerogel containing Sm 3+ ions (0.5 mol%) and SnO 2 nanocrystals (10 wt%). Curve a, monitored at 567 nm ( 4 G 5/2 fi 6 H 5/2 ); curve b, monitored at 606 nm ( 4 G 5/2 fi 6 H 7/2 ); curve c, monitored at 654 nm ( 4 G 5/2 fi 6 H 9/2 )