Binary and Ternary Heterometallic (La3+, Gd3+, Y3+)–Eu3+ Functionalized SBA-15 Mesoporous Hybrids: Chemically Bonded Assembly and Photoluminescence
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A novel kind of organic–inorganic monomer SUASi has been achieved by modifying 5-sulfosalicylic acid (SUA) with 3-aminopropyltrimethoxysilane (APS), subsequently binary and ternary Eu3+ mesoporous hybrid materials with 5-sulfosalicylic acid (SUA)-functionalized SBA-15 and 1,10-phenanthroline (phen) are synthesized by co-condensation of SUASi and TEOS in the presence of Eu3+ complex and Pluronic P123 as a template. Finally, luminescent hybrid mesoporous materials consisting of active rare earth ions (Eu3+)—inert rare earth ions (Y3+, La3+, Gd3+) complex covalently bonded to the mesoporous materials network have been obtained via this sol–gel approach. The physical characterization and photoluminescence of all these resulting materials are studied in detail. Especially the luminescent behavior has been studied with the different ratios of Eu3+–(Y3+, La3+, Gd3+), which suggests that the existence of inert rare earth ions can enhance the luminescence intensity of Eu3+. This may be due to the intramolecular energy transfer between Y3+, La3+, Gd3+, and Eu3+ through the covalently bonded mesoporous framework.
KeywordsHeterometallic hybrid mesoporous materials Rare earth ions Covalently bonded Photoluminescence Energy transfer
Recently, rare earth hybrid inorganic/organic materials have attracted more attention due to their diverse potential applications in catalysis, gas adsorption, magnetism, optical devices or lasers [1, 2, 3, 4]. Many rare earth complexes have been investigated thoroughly and applied in the luminescence hybrid materials owing to the long-lived excited-states character and especially the efficient strong narrow-width emission band in the visible region of rare earth ions . Recently, the hybrid material systems of rare earth organic complexes introduced in silica gel have already been found to have superior characteristic emission intensities compared with simple rare earth ions in inorganic hosts. In particular, lots of investigations have been done on the sol–gel-derived host hybrid materials fabricated with rare earth complexes with β-diketones, aromatic, carboxylic acids, and heterocyclic ligands . These studies indicate that the thermal stabilities and mechanical properties of the hybrid materials can be improved by the silica matrices. Our research group has realized some modification path to design functional bridge to further assemble the hybrid materials in which luminescent rare earth complexes species are bonded with siloxane matrix through Si–C linkage [7, 8, 9].
Since early 1990s, ordered mesoporous materials with unique properties (high surface area, high pore volume, controlled pore structure, and uniform pore size distribution) are of great interest for adsorption, sensing, catalysis, and other applications [10, 11, 12, 13]. Recently, there are some research concerns on the use of ordered mesoporous silica materials as a support for rare earth complexes [14, 15, 16, 17]. SBA-15 is a typical mesoporous material with largest pore-size mesochannels, thick walls, adjustable pore size from 3 to 30 nm, and high hydrothermal, thermal, mechanical stability, so some works have been reported on the hybrids using SBA-15 as host to incorporate active molecules [18, 19, 20]. The large number of hydroxyl groups in SBA-15 provides it necessary qualification for the modification of inner face and self-assembly of huge guest molecules, namely, providing outstanding hosts for self-aggregation chemistry. Recently, some work brings out the mesoporous SBA-15 more extensive applications to functionalize its exterior and/or interior surfaces to prepare the organic/inorganic hybrids. Our research team has reported the synthesis and luminescence properties of SBA-15 mesoporous materials covalently bonded with rare earth complexes [21, 22, 23].
Some researches have found that the rare earth ions with stable electronic configuration (4f shells are empty, half-filled, and full), such as La3+, Y3+, Gd3+, and Lu3+ can enhance the luminescence of photoactive lanthanide ions (Eu3+, Tb3+) [24, 25, 26]. But it is hard to clearly prove the luminescent hybrid materials are homogenous at molecular level, so these researches are concentrated on the RE complexes in which luminescence enhancement mainly belongs to intermolecular energy transfer. We have carried some studies on the mechanism of co-luminescence, which is excited by intra-molecular energy transfer .
In this paper, we discuss the synthesis of a series of heterometallic hybrid mesoporous materials, in which inert rare earth, such as La3+, Gd3+, Y3+, and the active rare earth ion (Eu3+) are introduced through the covalently bonded SBA-15 framework, and the luminescence enhancement can be found through the intramolecular energy transfer within the hybrid mesoporous materials system.
Starting materials are purchased from Aldrich and are used as received. All organic solvents are distilled before utilization according to the literature procedures. Rare earth nitrates are obtained by dissolving their oxides in concentrated nitric acid (HNO3).
Synthesis of the Precursor SUASi from the Modification of 5-Sulfosalicylic Acid
Synthesis of SUA-Functionalized SBA-15 Mesoporous Material
The mesoporous material SUASi-SBA-15 is synthesized from acidic mixture with the following molar composition: 0.0172P123: 0.96TEOS: 0.04SUASi: 6HCl: 208.33H2O. P123 (1.0 g) is first dissolved in the deionized water (7.5 g) and 2 M HCl solution (30 g) at room temperature. After that the mixture of SUASi and TEOS is added into the above solution followed by 24 h of persistent stirring. Then it is heated at 100°C for 48 h in a Teflon bottle sealed in an autoclave. The solid product is filtered, washed thoroughly with deionized water, and dried at 60°C. The copolymer surfactant P123 is removed via Soxhlet extraction with ethanol under reflux for 2 days. After dried in vacuum, the material showed a light-yellow color (denoted as SUASi-SBA-15).
Synthesis of Hybrid Materials
The sol–gel-derived hybrid materials are prepared according to the similar method in ref. , denoted as Eu-SUASi and Eu-SUASi-phen. For binary hybrid mesoporous material: SUASi-SBA-15 is soaked in an appropriate amount of Eu(NO3)3 ethanol solution with stirring, the molar ratio of Eu3+:SUASi is 1:3. After being stirred for 12 h at room temperature, the mixture is filtrated and washed with ethanol solvent. Finally, the material is dried at 60°C under vacuum overnight, denoted as Eu-SUASi-SBA-15. For ternary hybrid mesoporous material: the synthesis procedure for phen-Eu-SUASi-SBA-15 is similar to the above except that the mixed ethanol solution of Eu(NO3)3 and 1,10-phenanthroline (phen) replace Eu(NO3)3, and the molar ratio of Eu3+:SUASi:phen is 1:3:1. The predicted structure of phen-Eu-SUASi-SBA-15 is shown in Fig. 1. The modified ligand is covalently bonded to the SBA-15 host through Si–O-Si by the hydrolysis-condensation of SUASi, denoted as phen-Eu-SUASi-SBA-15. Using the same method, we also have prepare active rare earth ions (Eu3+) and inert rare earth ions (La3+, Gd3+, Y3+) heterometallic hybrid materials with the different molar ratio (Eu3+:La3+ = 5:5; Eu3+:Gd3+ = 5:5; Eu3+:Y3+ = 1:9, 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, 8:2, 9:1).
1H NMR spectra are recorded on a BRUKER AVANCE-500 spectrometer with tetramethylsilane (TMS) as internal reference using CDCl3 as solvent. IR spectra are measured within the 4000–400 cm−1 region on an infrared spectrophotometer with the KBr pellet technique. The ultraviolet absorption spectra are taken with an Agilent 8453 spectrophotometer (CCl4 solution). X-ray powder diffraction patterns are recorded on a Rigaku D/max-rB diffractometer equipped with a Cu anode in a 2θ range from 0.6° to 6°. Nitrogen adsorption/desorption isotherms are measured at the liquid nitrogen temperature, using a Nova 1000 analyzer. Before the measurements, the samples are outgassed for 2 h in the degas port of the adsorption apparatus at 423 K. Surface areas are calculated by the Brunauer-Emmett-Teller (BET) method and pore size distributions are evaluated from the desorption branches of the nitrogen isotherms using the Barrett-Joyner-Halenda (BJH) model. Thermogravimetric analysis (TGA) is performed on a Netzsch STA 409 at a heating rate of 15°C/min under nitrogen atmosphere. The luminescence excitation and emission spectra are obtained on RF-5301 spectrophotometer. All spectra are normalized to a constant intensity at the maximum. Luminescence lifetime measurements are carried out on an Edinburgh FLS920 phosphorimeter using a 450 w xenon lamp as excitation source.
Results and discussion
The IR spectra of SBA-15 and SUASi-SBA-15 are shown in Fig. 3b. In the spectrum of SBA-15 material, the bands at 1078 cm−1 and 799 cm−1 corresponded to the asymmetric Si–O stretching vibration modes (νas, Si–O) and the symmetric Si–O stretching vibration (νs, Si–O), respectively. The bands at 452 cm−1 and 962 cm−1 are assigned to be the Si–O–Si bending vibration (δ, Si–O–Si) and the silanol (Si–OH) stretching vibrations of surface groups . The band appearing at 3449 cm−1 is the evidence of the presence of hydroxyl group. The spectrum of the SUASi-SBA-15 exhibits the parallel infrared absorption bands, proving the successful formation of the silica framework.
Textural data of hybrid mesoporous materials SUASi-SBA-15, Eu-SUASi-SBA-15 and phen-Eu-SUASi-SBA-15 materials
Photoluminescent data of the selected ternary homometallic and heterometallic hybrid mesoporous hybrid materials
Further, we compare the emission spectra for the Y3+–Eu3+ heterometallic hybrid mesoporous materials with different molar ratio of Y3+:Eu3+ (1:9, 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, 8:2, 9:1) is shown in Fig. 8b. All the hybrid mesoporous materials exhibit the characteristic emission lines of Eu3+. From the emission spectrum, when the molar ratio Y3+:Eu3+ = 4:6, the intensity of the spectrum is stronger than others. This is the best proportion for Y3+–Eu3+ heterometallic hybrid mesoporous materials with the best luminescent intensities.
where Ar and Anr are radiative and non-radiative transition rates, respectively. Ar can also be obtained by summing over the radiative rates A0J for each 5D0 → 7FJ (J = 0–4) transitions of Eu3+. The quantum efficiency data are shown in Table 2. From the equation of η, it can be seen the η value mainly depends on the values of lifetimes and I02/I01. As can be seen from Table 2, the quantum efficiency of phen-Eu-SUASi-SBA-15 (6.7%) is higher than that of phen-Eu-SUASi (3.4%), which can be ascribed to the substitution of the silanol with covalently bonded SUA groups in the pore channel of mesoporous SBA-15, resulted in the decrease in the level of non-radiative multiphonon relaxation by coupling to –OH vibrations and non-radiative transition rate. This clearly demonstrates the modifications in the Eu3+ ion local environment as phen-Eu-SUASi is covalently bonded to the mesoporous SBA-15. The results described above further provide the indirect formation on that phen-Eu-SUASi is successfully covalently bonded to the SBA-15 network. Furthermore, the quantum efficiencies of the heterometallic hybrid mesoporous materials have been increased, and among them the Y3+–Eu3+ heterometallic hybrid mesoporous materials have the maximum quantum efficiency compared with others. This phenomenon is another evidence for that the energy transfer efficient from the ligand to the Eu3+ is improved when the inert rare earth ions are introduced.
From the above results, we give the primary analyses on the luminescent enhancement. For the heterometallic covalently bonded hybrid mesoporous system, Eu3+ and inert rare earth ions (La3+, Gd3+, Y3+) coexist in the same huge molecular hybrids through the covalently bonded SBA-15 ordered Si–O framework. So it can be predicted that the intramolecular energy transfer process from the energy donor (SUASi-SBA-15) to Eu3+ can be enhanced for the inert RE3+ can strengthen the excitation of organic ligand unit, which take agreement with the similar phenomenon in other systems [20, 23]. But the proportion of inert ion exceeds one optimum value, as the concentration of inert RE3+ increase further, active luminescent center (Eu3+) reduces and the luminescence intensity of the system wanes.
In summary, we have assembled the luminescent organic–inorganic hybrid mesoporous materials by linking homometallic and heterometallic rare earth complexes (Eu3+, La3+–Eu3+, Gd3+–Eu3+, Y3+–Eu3+) to the functionalized ordered mesoporous SBA-15 through the modified SUASi bridge via co-condensation of TEOS in the presence of Pluronic P123 surfactant as a template. The luminescent behavior has been studied with the different ratios of active rare earth ions (Eu3+) and inert rare earth ions (La3+, Y3+, Gd3+), which suggests that the existence of inert rare earth ions can enhance the luminescence behavior (luminescent intensity, lifetime, and quantum yields), which may be due to the intramolecular energy transfer between inert rare earth ions and active rare earth ions. Among them Y3+:Eu3+ = 4:6 is the best ratio for Y3+–Eu3+ heterometallic hybrid mesoporous materials with the optimum luminescence property. Their luminescent property, thermal stability, highly ordered hexagonal channel structure and uniform pore sizes of the organic group functionalized SBA-15 mesoporous materials will be expected to expand their applications in both optical and electrical molecular devices.
This work is supported by the National Natural Science Foundation of China (20971100) and Program for New Century Excellent Talents in University (NCET-08-0398).
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