Luminescence Properties of SiO2@Eu(phen-Si) (sulphoxide) Core-Shell Nanometer Composite

A novel ternary europium complex Eu(Phen-Si)·(H 2 L) 2 ·(ClO 4 ) 3 ·6H 2 O was prepared using (phen)-N-(CONH(CH 2 ) 3 Si(OCH 2 CH 3 ) 3 ) 2 (Phen-Si) as the first ligand and 2-carboxyphenyl carboxymethyl sulphoxide (H 2 L) as the second ligand. The corresponding different thickness core-shell structure SiO 2 @Eu(phen-Si)·L nano composites were synthesized, with silica spheres as core, ternary europium complex (Eu(Phen-Si)·(H 2 L) 2 ·(ClO 4 ) 3 ) as shell. The ternary europium complex has been characterized by element analysis, molar conductivity, and IR spectra. The SiO 2 @Eu(phen-Si)·L core-shell structure composite was characterized by TEM and IR spectra. Core-shell structure composites exhibited stronger fluorescence properties than that of the ternary europium complex. The emission intensity of the two core-shell composites was 1.79, 3.99 times stronger than the ternary europium complex, respectively. The fluorescence lifetime of the core-shell structure composites were longer than the ternary europium complex.


Introduction
The core-shell nano composite is a nanoparticle of novel structure. It is an orderly assembly structure, which can be constituted of a nanomaterial coated with another nanomaterial by chemical bonds or physical adsorption. Therefore, it is a higher level nano composite structure. Because of its unique property, differing from any of its unary substance, many core-shell nano composites are intensely studied in the past decades [1][2][3].
Coating with other materials, the surface of nanoparticle changes the structure of the layer, size, or composition. It has the advantages that unary material does not have and open up a new approach for the new material design. Because of its easily controlling particle size, low cost, security, and low toxicity, SiO2 is the most widely used core-shell structure nanoparticles. As a hotspot in the field of nanometer materials, the study of SiO2 as core materials was reported more and more, with the rare earth inorganic materials used most as the coating layer [4][5][6][7]. Instead, research of rare earth organic complexes as cladding layer was reported few.
Silane coupling agent is a kind of bi-functional organic matter, generally formulated as Y(CH2)nSiX3. Y represents the organic functional groups, such as amino, carboxyl and double nitrogen, etc. X represents the alkoxy. Y group can combine with rare earth ions. X group can form Si-O-Si chemical bonds with hydroxyl groups of the SiO2 through hydrolysis. As a result, rare earth ions and SiO2 are connected together by silane coupling agent, as a "molecular bridge". Yan Bing and Zhang HongJie teams did a lot work in this area [8][9].
In this article, the phenanthroline of amino silane functionalization phen-Si, was used as a silane coupling agent. A novel ternary europium organic complex Eu(Phen-Si)·(H2L)2·(ClO4)3·6H2O has been synthesized for the first time, using Phen-Si as the first ligand, 2-carboxyphenyl carboxymethy sulphoxide(H2L) as the second ligand. The SiO2@Eu(phen-Si)·L core-shell structure nano composites have been prepared, using silica spheres as the core, the ternary europium organic complex Eu(Phen-Si)·(H2L)2·(ClO4) 3·6H2O as the shell, denoted as SiO2@Eu(phen-Si)·L.·Analytical and characterization tests were applied to the prepared complex and nano-particle composite.

Physical measurements
Elemental analysis was carried out on a HANAU analyzer. Conductivity measurement was carried out on a DDS-11D conductivity meter. The europium content of the complex was determined by EDTA titration. The infrared spectra was determined on a Nicolet NEXUS-670 FT-IR spectrophotometer. Fluorescence properties were determined on a FLS920 fluorescence spectrometer. Microstructures were recorded by Scanning electronic microscope (Hitachi S-4800), and transmission electron microscope (FEI Tecnai F20).

Synthesis of the SiO2@Eu(phen-Si)·L core-shell structure nano composites
Mono-disperse spherical SiO2 was synthesized by Stöber method. The average diameter of SiO2 was 250nm. Phen-Si was dissolved in 5 ml ethanol, adding to the solution which was a mixture contained SiO2, 5 ml alcohol and 5 ml water, and the pH was set to 9 by an appropriate amount of ammonia. Then the mixture was stirred 2h at room temperature.
Finally, the core-shell structure nano composites were successfully synthesized. 1 ml SiO2@(phen-Si) was dispersed in 20 ml ethanol. The molar ratio of SiO2@(phen-Si): sulfoxide: Eu 3+ was 1:2:1. The H2L and Eu(III) perchlorate were added into the silica solution sequentially. The mixture was stirred 0.5 h at 80℃. Then a white precipitate was obtained.

The composition the complex
Analytical data for the complex, presented in Table 1. The result of molar conductivity value of the complex in DMF was accordingly formulated as 1:2 electrolytes [13]. EDTA titration result showed that the content of the europium ion was 9.22%. According to the elemental analysis and EDTA titration data, the composition of the ternary complex was suggested as Eu(Phen-Si)·(H2L)2·(ClO4)3·6H2O.  According to the SEM (Fig.1a), it could be seen that the SiO2 had regular micro-structure with an average size of 250nm, smooth surface and good dispersity. Fig.1b-d showed the morphologies of core-shell structure SiO2@Eu(phen-Si)·L with different thickness. It showed that the size of SiO2 kernel was 250nm, the thickness of the coating layer was 20 and 30nm, which can be expressed as SiO2@Eu(phen-Si)·L(20), SiO2@Eu(phen-Si)·L(30). The ternary europium complex had been coated on SiO2 cores, the SiO2@Eu(phen-Si)·L core-shell structure had been formed successfully.  The IR of the two ligands and the Eu(Phen-Si)·(H2L)2·(ClO4)3·6H2O ternary complex could be seen in Fig.2a-c. In the phen-Si IR spectrum (Fig.2a), the νC=O and ν-CONHappeared at 1708cm -1 , 1694cm -1 and 1542 cm -1 respectively. After coordinating with Eu 3+ , they shifted to lower wave number which appeared at 1703 cm -1 , 1589cm -1 and 1552cm -1 (Fig.2c). It suggested that europium ion was bonded with phen-Si through the double nitrogen atoms of phen. Similarly, in the sulfoxide IR spectrum (Fig.2b), the νS=O, νC=O(COOH) and νC-OH(COOH) shifted from 1012cm -1 , 1695cm -1 and 1300 cm -1 to 989 cm -1 , 1589cm -1 and 1292cm -1 respectively. Which suggested that rare earth ion bonded with H2L through the COOH group [14]. When the perchlorate group wasn't coordinated, it was Td symmetry. There were two absorptions bands of 1084~1092 cm -1 and 621~629 cm -1 . When ClO4was coordinated, it showed C2v symmetry and there were five absorptions [15]. In the IR spectra of the complexes, five absorptions could be seen clearly at 1145, 1116, 1080, 920 and 627cm −1 . Combined with the molar conductivity, it could be induced that the one perchlorate group in the complex was bonded with the Eu(III) ion by the oxygen atoms, and the coordination was bidentate. Fig.3a-d was the IR of the SiO2, phen-Si, H2L and the SiO2@Eu(phen-Si)·L(20). The νC=O, ν-CONH-shifted from 1708cm -1 , 1694cm -1 and 1542 cm -1 to 1702 cm -1 , 1592cm -1 and 1504cm -1 respectively. It indicated that europium ion was bonded with phen-Si through the double nitrogen atoms of phen.

Infrared spectra
Similarly, in Fig.3c-d, the νS=O shifted from1013 cm −1 to 974cm −1 , which suggested that europium ion bonded with oxygen atom in sulfinyl group. These results suggest that the complex were formed in the shell layer of core-shell composite. The excitation and emission spectrum of the europium complex and the core-shell structure composites were measured in solid state at room temperature (Fig.4). The excitation spectrum of the products was obtained by monitoring their emissions at 612nm. The maximum peak was at 325nm (Fig.4 Insert). There were five emission peaks in the emission spectrum (Fig.4a-c), which were attributed to the characteristic 5 D0-7 FJ (J = 0, 1, 2, 3, 4) transitions of Eu(III) at 579, 592, 612, 650 and 700nm, and the 5 D0-7 F2 transition of Eu (III) was the strongest at 612nm. The emission intensity of the Eu(III) ion was 8220 a.u. in europium complex (Fig.4c). However, under the same condition, the emission intensity of SiO2@Eu(Phen-Si)·L(20) and SiO2@Eu(Phen-Si)·L(20) core-shell structure composites were 14341 and 32803 a.u. respectively. It was 1.79, 3.99 times greater of the ternary europium complex, respectively. The fluorescence of the core-shell composite was more enhanced than ternary europium complex. On the other hand, according to the Fig.4, the thickness of the coating layer could affect the emission intensity of composites. It meant that when the size of SiO2 core was fixed, with the increase of the emission intensity was increased with the increase of the thickness of the coating layer.

Luminescence properties
Where It was the fluorescence intensity varying with time t, and τ1 and τ2 were lifetime. The corresponding data were calculated. As the result, the lifetime of the europium ternary complex was 527.70 μs, but the lifetime of SiO2@Eu(Phen-Si)L(30) core-shell composites was 680.00 μs. The results were consistent with corresponding variation tendency of emission intensity.

The Conclusion
The novel complex of ternary europium complex Eu(Phen-Si)·(H2L)2·(ClO4)3·6H2O, two core-shell structure composites SiO2@Eu(phen-Si)·L(20) and SiO2@Eu(phen-Si)·L(30) were prepared. Core-shell structure composites exhibited superior fluorescence properties than that of the ternary europium complex. The emission intensity of the two core-shell composites was 1.79, 3.99 times than the ternary europium complex, respectively. The fluorescence lifetime of the core-shell structure composites was longer than the ternary europium complex. Because rare earth organic complexes were used as coating layer, thickness is nanometer order of magnitude, so the dosage of rare earth organic complexes can be decreased. As a result, the cost of materials will be greatly reduced. The complex and the core-shell structure composite had excellent luminescent properties.
The SiO2@Eu(phen-Si)·L core-shell structure nano composites had application potential as red luminescent materials and derived application. So this prepared approach provides an applicable new choice for future novel luminescent materials.