Functionalized Mesoporous SBA-15 with CeF3: Eu3+ Nanoparticle by Three Different Methods: Synthesis, Characterization, and Photoluminescence
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Luminescence functionalization of the ordered mesoporous SBA-15 silica is realized by depositing a CeF3: Eu3+ phosphor layer on its surface (denoted as CeF3: Eu3+/SBA-15/IS, CeF3: Eu3+/SBA-15/SI and CeF3: Eu3+/SBA-15/SS) using three different methods, which are reaction in situ (I-S), solution impregnation (S-I) and solid phase grinding synthesis (S-S), respectively. The structure, morphology, porosity, and optical properties of the materials are well characterized by X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy, N2 adsorption, and photoluminescence spectra. These materials all have high surface area, uniformity in the mesostructure and crystallinity. As expected, the pore volume, surface area, and pore size of SBA-15 decrease in sequence after deposition of the CeF3: Eu3+ nanophosphors. Furthermore, the efficient energy transfer in mesoporous material mainly occurs between the Ce3+ and the central Eu3+ ion. They show the characteristic emission of Ce3+ 5d → 4f (200–320 nm) and Eu3+5D0 → 7FJ(J = 1–4, with 5D0 → 7F1 orange emission at 588 nm as the strongest one) transitions, respectively. In addition, for comparison, the mesoporous material CeF3: Eu3+/SBA-15/SS exhibits the characteristic emission of Eu3+ ion under UV irradiation with higher luminescence intensity than the other materials.
KeywordsMesoporous material Nanoparticle Luminescence Cerium trifluoride doped with europium ion
In the past decades, inorganic luminescent materials with nanosale dimensions have been found many potential applications, such as light emitting devices, low-threshold lasers, optical amplifiers, biological fluorescence labeling, and so on [1, 2, 3, 4]. However, due to nonradiative decay from defects on the surface of the nanocrystals, the luminescence efficiency of nanostructural materials is usually lower than that of the corresponding bulk materials [5, 6]. To reduce these defects, the growth of a crystalline shell of a suitable inorganic material around each nanocrystal to form the core/shell structures has been regarded as an effective strategy to improve luminescent efficiency. Compared with the conventional oxide-based luminescent materials, fluorides are advantageous as fluorescent host materials owing to their low vibrational energies, and the subsequent minimization of the quenching of the excited state of the rare-earth ions . Hence, Lanthanide fluorides have attracted considerable attention because of their outstanding luminescent characteristics that originate from good coordination capability of hosted lanthanide ions in fluoride lattices and the wide band gap and very low vibrational energies induced by the high ionicity of the lanthanide to fluoride bond [8, 9, 10, 11, 12, 13]. A bulk crystal of CeF3 possesses a hexagonal phase structure with a space group of P 63/mcm (JSPDS 08-0045) and lattice constants a = 0.713 nm, c = 0.729 nm, and there are six formula weights per unit cell. The Ce3+ ion in the CeF3 crystal is coordinated by nine F− and has a C 2 site symmetry . As a potential scintillator and tunable laser material, CeF3 is a luminescent material with 100% activator concentration. Moreover, the material CeF3: Eu3+ has red luminescence property, the energy transfer processes from Ce3+ to Eu3+ can enhance the Eu3+ emission, which has gained a great deal of research interest.
Mesoporous materials with unique properties, such as high surface area, controlled pore structure and uniform pore size distribution, are of great interest for adsorption, sensing, catalysis, and other applications [14, 15, 16, 17, 18, 19]. Among them, SBA-15 is by far the largest pore size mesochannels, with thick walls, adjustable pore size from 3 to 30 nm, and high hydrothermal and thermal stability. Since the discovery of triblock copolymer-templated SBA-15 in 1998 , the adsorption and surface properties of this mesoporous material have been adjusted by anchoring a variety of functional groups onto the surface. Recently, ordered mesoporous SBA-15 has gained considerable attention as an ideal host for incorporation of active molecules because of their stable mesoporous structure, tunable pore size, and high specific surface area within abundant Si–OH active bonds on the pore walls, nontoxic nature, well-defined surface properties, and good biocompatibility [21, 22, 23, 24, 25, 26]. In addition, surface-functionalized mesoporous silica materials can be used as excellent hosts for a variety of guest molecules. Luminescence functionalization of inorganic porous materials (using YVO4: Eu3+, rare-earth complex, etc.) has been reported in several publications [27, 28, 29]. It is shown that the promising visible luminescent properties can be obtained by linking the rare-earth complexes to the mesoporous materials.
Herein, we propose a novel design on the synthesis and characterization of the mesoporous luminescence material by incorporating CeF3: Eu3+ nanophosphors into the surface of mesoporous SBA-15 using three different methods: reaction in situ (I-S), solution impregnation(S-I), and solid phase grinding synthesis (S-S), respectively. The obtained composite materials were well characterized by SAXRD (Small Angle X-rays Diffraction), FTIR (Fourier Transform Infrared Spectroscopy), N2 adsorption/desorption analysis, TEM (Transmission Electron Microscopy), HRTEM (High-Resolution Transmission Electron Microscopy), and luminescence spectra. Comparative studies on the luminescence properties of all these synthesized materials were investigated in relation to guest–host interactions between the guest molecule and the silica matrix.
Pluronic P123 (EO20PO70EO20, Aldrich), tetraethoxysilane (TEOS, Aldrich). Europium nitrate was prepared by dissolving Eu2O3 in concentrated nitric acid.
Method I: Solution Impregnation (S-I) Synthesis of Mesoporous SBA-15 and Functionalized Mesoporous SBA-15 by CeF3: Eu (Denoted as CeF3: Eu3+/SBA-15/SI)
SBA-15 was synthesized according to the procedure as follows: 1.0 g P123 was dissolved in 7.5 g of H2O and 30.0 g of dilute HCl solution (2.0 M) with stirring at 35°C. Then, 2.125 g of TEOS was added dropwise to the solution with stirring, and the mixed solution (15 mL) was transferred into a Teflon bottle sealed in an autoclave (2 MPa, 25 mL), which was heated at 100°C for 48 h. The obtained material was filtered, washed, and dried in air at room temperature. The as-synthesized material was calcined from room temperature to 550°C at a heating rate of 1°C min−1 and kept at 550°C for 6 h to remove the templates.
Deposition of the CeF3: Eu3+ phosphor layer onto the surface of the template-free SBA-15 was prepared using solution impregnation (S-I) by a sol–gel process . The doping concentration of Eu3+ was 5 mol % of Ce3+ in CeF3. In a typical process, 1.9 mmol Ce (NO3)3·6H2O and 0.1 mmol of Eu(NO3)3·6H2O were dissolved in 12 mL N, N-dimethyl-formamide (DMF), and then a solution of diethylene glycol (DEG 25 mL) containing 6 mmol of NH4F was injected into the DMF solution. The mixture was stirred at 200°C for 1 h to form a stable sol. Then desired amount of the template-free SBA-15 powder was added into the sol with stirring. The suspension was stirred for another 3 h, and then the resulting material was separated by centrifugation, washed several times with distilled water and absolute ethanol, and finally dried at 60°C.
Method II: Reaction In situ (I-S) (Denoted as CeF3: Eu3+/SBA-15/IS)
In this process, 1.0 g P123 was dissolved in 7.5 g of H2O and 30.0 g of dilute HCl solution (2.0 M) with stirring at 35°C (Solution 1). At the same time, 1.9 mmol Ce(NO3)3·6H2O and 0.1 mmol of Eu(NO3)3·6H2O were dissolved in 12 mL N, N-dimethyl- formamide (DMF), and then a solution of diethylene glycol (DEG 25 mL) containing 6 mmol of NH4F was injected into the DMF solution. The mixture was stirred for 1 h to form a stable sol (solution 2). Then, 2.125 g of TEOS was added dropwise to the solution 2 with stirring, and the mixed solution was soaked in about 25 mL of Solution 1. After stirring 1 h at the room temperature, the mixture was transferred into a Teflon bottle sealed in an autoclave (2 MPa, 25 mL), which was heated at 120°C for 48 h. The obtained material was filtered, washed, and dried in air at room temperature. The as-synthesized material was calcined from room temperature to 550°C at a heating rate of 1°C min−1 and kept at 550°C for 6 h to remove the templates.
Method III: Solid Phase Grinding Synthesis (S-S) (Denoted as CeF3: Eu3+/SBA-15/SS)
CeF3: Eu3+ solid precipitate was prepared in a typical procedure as follows: 1.9 mmol Ce (NO3)3·6H2O and 0.1 mmol of Eu(NO3)3·6H2O were dissolved in 25 mL of diethylene glycol (DEG, 98.0%) in a round-bottomed flask at 100°C under stirring to form a clear solution. The solution was then heated in a silicon oil bath under vigorous stirring with a flow of N2 atmosphere, and the temperature of the solution was further increased to 200°C. At this temperature, a solution of DEG (25 mL) containing 6 mmol of NH4F was injected into the solution, and the mixture was kept at 200°C for 1 h. Then, the obtained suspension was cooled to room temperature and diluted with 50 mL of ethanol. The solid particles were separated by centrifugation.
A typical procedure for the preparation of pure mesoporous SBA-15 powder was synthesized according to the literature by using a triblock copolymer as the template . The result material CeF3: Eu3+/SBA-15/SS was obtained by the mixture of CeF3: Eu3+ solid precipitate and SBA-15 powder using solid phase grinding at the room temperature for 2 h and dried at 60°C.
X-ray powder diffraction patterns were recorded on a Rigaku D/max-rB diffractometer equipped with a Cu anode in a 2θ range from 0.6° to 6°. IR spectra were measured within the 4,000–400 cm−1 region on an infrared spectrophotometer with the KBr pellet technique. Nitrogen adsorption/desorption isotherms were measured at the liquid nitrogen temperature, using a Nova 1000 analyzer. Surface areas were calculated by the Brunauer–Emmett–Teller (BET) method, and pore size distributions were evaluated from the desorption branches of the nitrogen isotherms using the Barrett–Joyner–Halenda (BJH) model. The fluorescence excitation and emission spectra were obtained on a RF-5301 spectrophotometer. Luminescence lifetime measurements were carried out on an Edinburgh FLS920 phosphorimeter using a 450 w xenon lamp as excitation source. Transmission electron microscope (TEM) experiments were conducted on a JEOL2011 microscope operated at 200 kV or on a JEM-4000EX microscope operated at 400 kV.
Results and Discussion
FTIR (Fourier Transform Infrared Spectroscopy)
Power SAXRD (Small Angle X-rays Diffraction) and WAXRD (Wide Angle X-rays Diffraction)
Textural data of SBA-15, CeF3: Eu3+/SBA-15/SI, CeF3: Eu3+/SBA-15/SS and CeF3: Eu3+/SBA-15/IS
High-Resolution Transmission Electron Microscopy (HRTEM)
Photoluminescent data of all mesoporous materials
Luminescence Decay Times (τ) and Emission Quantum Efficiency (η)
On the basis of the above discussion, the quantum efficiencies of the four kinds of europium mesoporous hybrid materials can be determined, as shown in Table. 2. As can be clearly seen from Table 2, the quantum efficiencies of CeF3: Eu3+/SBA-15/SS (η = 22.77%) is higher than that of CeF3: Eu3+/SBA-15/SI and CeF3: Eu3+/SBA-15/IS, which is in agreement with the intensity of the emission spectra. Furthermore, CeF3: Eu3+/SBA-15/IS (η = 4.61%) exhibits much lower emission quantum efficiency than CeF3: Eu3+/SBA-15/SS (η = 22.77%), which indicates that the deposition of CeF3: Eu3+ onto the mesoporous framework of SBA-15 using in situ method has greatly influenced the luminescence of Eu3+ ion in CeF3: Eu3+. Further, we selectively determined the energy transfer efficiency for the three kinds of mesoporous materials. For these functionalized mesoporous SBA-15 by CeF3: Eu3+ mesoporous materials, S-I, I-S, and S-S systems, CeF3 plays two roles: both the host and the energy donor for Eu3+(energy acceptor).The energy transfer efficiency from CeF3 to Eu3+ of CeF3: Eu3+/SBA-15/SS (76%) is higher than that of CeF3: Eu3+/SBA-15/SI (55%) and CeF3: Eu3+/SBA-15/IS (38%). This is due to the different reaction condition, the contribution of energy transfer efficiency of CeF3 decreases in the total luminescence quantum efficiency of Eu3+.
Ultraviolet–Visible Diffuse Reflection Absorption Spectra
In summary, we have designed novel luminescent mesoporous materials by the deposition of a CeF3: Eu3+ phosphor layer onto the channel surface of mesoporous SBA-15 using three different methods: reaction in situ (I-S), solution impregnation (S-I), and solid phase grinding synthesis (S-S), respectively. The corresponding composite materials were denoted as CeF3: Eu3+/SBA-15/IS, CeF3: Eu3+/SBA-15/SI, and CeF3: Eu3+/SBA-15/SS. The synthesis of this system provides a convenient approach of tailoring the surface properties of mesoporous silicates by nanophosphor functionalization, and the resulting materials all retain the ordered mesoporous structures. Further investigation into the luminescence properties of CeF3: Eu3+/SBA-15 mesoporous materials shows that the characteristic luminescence of the corresponding Eu3+ through the energy transfers from the Ce3+ to the central Eu3+ ions. As a result, more Eu3+ ions have occupied the inversion sites in CeF3: Eu3+ nanoparticles, resulting in that magnetic dipole transition 5D0 → 7F1(588 nm) are the dominant bands for the materials. Meantime, the CeF3: Eu3+/SBA-15/SS exhibits the higher luminescence intensity than the other two materials. So it can be concluded that the obtained samples using different synthesis methods show different luminescence properties, which can be related to the guest–host interactions between the guest molecule and the silica matrix.
This work was 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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