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Effect of template-removing methods and modification to mesoporous blank silica and composited silica
Powder Technology 219 (2012) 271–275
Contents lists available at SciVerse ScienceDirect
Powder Technology journal homepage: www.elsevier.com/locate/powtec
Effect of template-removing methods and modification to mesoporous blank silica and composited silica Mina Guli a, b,⁎, Li Zhang a, Jianxi Yao a, b, Xiaotian Li c,⁎⁎ a b c
Beijing Key Laboratory of New and Renewable Energy, Renewable Energy School, North China Electric Power University, Beijing, China State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, Beijing, China Key Laboratory of Automobile Materials of Ministry of Education, Department of Material Science and Engineering, Jilin University, Changchun, China
a r t i c l e
i n f o
Article history: Received 1 September 2011 Received in revised form 12 December 2011 Accepted 26 December 2011 Available online 29 December 2011 Keywords: Mesoporous silica Modified Graft Dye
a b s t r a c t A pluronic template of rod-like mesoporous silica was removed by extraction, calcination, and microwave digestion methods. The results of X-ray diffraction and Fourier Transform Infrared Spectroscopy spectra analyses confirmed that microwave digestion was the most efficient method to remove the template because of its convenience, speed, and effectiveness. The pore surface of rod-like SBA-15 was modified by an organic silane with terminal amino groups. Also Rhodamine 6G dye molecules were grafted onto the mesopore walls of rod-like SBA-15 before and after modification. Blue-shifts in photoluminescence, pore features in N2 adsorption–desorption, transmission electron microscopy results of blank rod-like SBA-15, dye doping rodlike SBA-15 before and after modification confirmed that the mesoporous SBA-15 that had been modified by the organic silane with terminal amino groups was more effective for grafting of Rhodamine 6G molecules because the structures still remained as ordered mesostructures after grafting. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Mesoporous silica invented by the Mobil Corporation has attracted considerable interest since being announced in 1992 [1]. Nowadays, mesoporous materials [2] with variable compositions and morphologies have been investigated and received much attention because of their versatile uses in separation, catalysis, nanoreactors, and sensors [3,4]. There have been an increasing number of studies on the optical properties of dye-doped porous silica published in the past several years. The advantages of an inorganic matrix for the embedding of functional laser dyes are attributed to the more rigid environment as well as much higher migration stability and strongly increased photostability compared to organic polymer matrices. These properties render such composites attractive for second harmonic generation [5], solid-state tunable lasers [6] or delayed fluorescence [7]. The pore structure of silica enables dissolved ions to embed fluorescent dyes. This is the basis for the rapidly increasing research activities on dye-doped silica for dissolved analysis with a view to their application in industrial, environmental, and biomedical monitoring [8–10].
⁎ Correspondence to: Mina Guli, Beijing Key Laboratory of New and Renewable Energy, Renewable Energy School, North China Electric Power University, Beijing, 102206, PR China. Tel.: +86 10 61772816; fax: +86 10 61772816. ⁎⁎ Correspondence to: Xiaotian Li, Department of Material Science and Engineering, Key Laboratory of Automobile Materials of Ministry of Education, Jilin University, Changchun, 130012, PR China. Tel.: + 86 431 85168445; fax: + 86 431 85168444. E-mail addresses: [email protected] (M. Guli), [email protected] (X. Li). 0032-5910/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2011.12.061
Different organic and inorganic hosts have been extensively studied with the aim of increasing mechanical, thermal and photochemical stability of embedded guests with specific functionality [11–19]. This work mainly focused on the use of SBA-15, which has thicker wall and better ordered silica mesoporous structures with twodimensional hexagonal (P6mm) symmetry, instead of the M41S family that has been used for grafting dyes frequently. Due to its uniform particle size, larger surface area, and higher hydrothermal stability [20], SBA-15 has been used as a perfect host for assembling of organic dyes [21]. The chemical, thermal and mechanical stability of this kind of host-guest composite are particularly suitable for engineering stable photoluminescent devices [11,22–26]. Therefore, SBA-15 has a clear advantage over MCM-41 and zeolite for grafting of organic molecules. The first incorporation of dye molecules into mesostructured materials was aimed at monitoring the formation of the mesostructure in situ [27,28]. Recent efforts have been directed toward the goal of obtaining dye-doped structures for optical applications [29–32], especially for optical sensors, such as Comes Maria's work in 2008 [33] and Chang's work in 2011 [34], and for solid-state dye lasers such as Zhu's work about dye-doped mesostructured silica films [35]. Quite different approaches have been used before, ranging from the incorporation of phthalocyanins and specifically designed surfactants [36], or insertion of polymers into MCM-41-type materials after the surfactant has been removed [37], to the generation of photoluminescent silicon clusters on the walls of as-synthesized SBA-3 films [38]. Recently, the grafting process through covalent linkage of surface silanol groups and dye molecules has been regarded
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as an alternative route to prepare dye-doped mesoporous materials [22,39], but these studies formed basic and preliminary work. Compared to other synthesis methods, in this work, a rational process was used in which the template of the mesoporous materials has been removed by the optimized method first, and then the mesoporous materials has been modified and postgrafted. The advantage of this process is that it can avoid introducing heteroatoms into the framework of the mesostructures and avoid the aggregation of the dye molecules. Three methods have been used to remove the template of rod-like SBA-15 (RSBA-15) including calcinations, extraction and microwave digestion. By using thorough characterizations of X-ray diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR), it was found that microwave digestion is the best method to remove the templates since it was rapid, convenient and thorough. After removing the template from RSBA-15, the dye has been grafted into the RSBA-15 before and after modification, respectively. The latter exhibited improved photoluminescence (PL) and characterization of N2 adsorption–desorption due to preservation of the microstructure. 2. Experimental 2.1. Synthesis of RSBA-15 powder Mesoporous RSBA-15 was synthesized using nonionic triblock copolymer poly (ethylene glycol)-block-poly (propylene glycol)-blockpoly (ethylene glycol) (Pluronic P123, molecular weight = 5800, EO20PO70EO20, BASF) as organic template according to the method reported in the literature [40,41]. In a typical synthesis process, 1 g of pluronic P123 and 2.1 g of Na2SO4 were added to 30 g of 2 mol/L HCl. After stirring for 0.5 h, a clear solution was obtained. Then, 2.1 g of tetraethyl orthosilicate (TEOS, 98%, Aldrich) was added to the solution with vigorous stirring for 10 min. The resulting mixture was subsequently kept under static conditions for 24 h at 311 K. Finally, the mixture was transferred into an autoclave and heated at 373 K for 24 h. The solid product obtained was filtered with deionised water, and dried overnight at 323 K. 2.2. Removing the Template from RSBA-15 Powder Three methods, including extraction, calcination, and microwave digestion [40] were used to remove the organic template from RSBA-15. For microwave digestion, the mixture of 1 g RSBA-15, 12 mL concentrated HNO3 and 8 mL H2O2 (30%) was treated by microwave energy of approximately 1200 W (2450 MHz and 220 V) under the pressure of 1.3 MPa for 2 min. Teflon sample vessels were used since it is transparent for microwave energy. The resulting mixture was filtered and dried at 323 K. For comparison, the template was also extracted by refluxing RSBA-15 in acidic ethanol for 2 days or calcined at 550 °C for 5 h. For extraction, the mixture of 1 g RSBA-15, 100 mL ethanol, and a few drops of 2 M HCl was refluxed for 2 days with stirring.
synthetic process 2.3 with stirring for further 30 min, the products were pale pink in color. A typical preparation of Rh6G/RSBA-15AF composites involved stirring a mixture of 0.25 g of TM550 modified RSBA-15 in 50 mL of Rh6G CHCl3 solution (1 × 10 − 3 mol/L) at room temperature for 10 h. The resulting mixture was washed several times with CHCl3 until no pink color was observed in the eluant. The products were dried overnight at room temperature and were pink in color. 2.5. Characterization XRD patterns were recorded on a Siemens D5005 diffractometer using Cu Kα radiation at 40 kV and 40 mA. FTIR spectra were measured on a Nicolet 5Dx-FT-IR spectrometer at room temperature. Analyses were performed using pressed KBr disks and recorded at a resolution of 4 cm − 1. The morphology of products was characterized by a JEOL JSM-6700F field emission scanning electron microscope (SEM). Transmission electron microscopy (TEM) experiments were performed on a model JEM-2010F electron microscope (JEOL, Japan) with an acceleration voltage of 200 kV. Fluorescence spectroscopy data were recorded on a perkin Elmer LS55 luminescence spectrometer. N2 adsorption measurements were performed at 77 K using a Micromeritics ASAP 2010 m instrument (Micromeritics Instrument Corp, Norcross, GA). 3. Results and disscussion Fig. 1 shows the IR spectra of RSBA-15 samples. An intense peak around 3373 cm − 1 appeared for all samples can be assigned to the Si–OH stretching vibration from RSBA-15, and the peak at 945 cm − 1 appeared almost for all samples can be assigned to the Si–OH bending vibration mode of RSBA-15. It shows that Si–OH remained after removal of the template, especially after extraction and microwave digestion, but decreased after calcination, which means extraction and microwave digestion methods can keep more Si–OH which benefit to the following reaction. The peak at 1633 cm − 1 observed in RSBA-15 can be attributed to the O–H bending vibration mode of the adsorbed water molecules, which showed a concomitant decrease in intensity after extraction and calcination but didn't show any decrease after microwave digestion, indicating that after microwave digestion the sample still remain large hydroxyl, but lose hydroxyl after extraction and calcination which is not good for the next reaction step. Several infrared absorption bands around 2850–3000 and 1350–1500 cm − 1 shown in spectrum Fig. 1 (a) can
2.3. Modification of RSBA-15 with silane coupling agent-(c2h5o)3si(ch2)3nh2 A mixture of 1 g RSBA-15 without template, 60 mL methanol and 3.2 mL (C2H5O)3Si(CH2)3NH2 (TM550) was refluxed for approximately 6 h. The resulting mixture was filtered and washed with methanol and dried at 323 K. 2.4. Formation of Rhodamine 6G/RSBA-15 (Rh6G/RSBA-15) Composites Before and After Modification (Referred to as Rh6G/RSBA-15BE and Rh6G/RSBA-15AF, Respectively) To prepare Rh6G/RSBA-15BE, 0.024 g Rh6G was added to the resulting mixture just after removing the template and before the
Fig. 1. FTIR spectra of (a) as-synthesized RSBA-15, (b) template removed by extraction, (c) by microwave digestion, and (d) by calcination.
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be assigned to C–H stretching and bending vibrations of the template P123 that disappeared after microwave digestion and calcination, but were still visible after extraction, indicating that template can be completely removed by both microwave digestion and calcination. Comparing the three processes, microwave digestion was found to be the most rapid, effective and convenient method [42]. In the later experiments, the samples which using microwave digestion method for removing the RSBA-15 template had been employed. Fig. 2 shows the low-angle XRD patterns. All XRD patterns display three reflection peaks that are characteristic of hexagonal packing, indicating that the primary RSBA-15 silica was ordered in a hexagonal mesostructure (Fig. 2a), the structure was maintained after removal of the template by microwave digestion, extraction and calcination (Fig. 2b, c and d). Three methods of removing the template led to a decrease in the intensity of the XRD peaks, and broadening of the peaks [43]. The formation of Rh6G/RSBA-15AF composites (prepared with 1 × 10 − 3 mol/L Rh6G in CHCl3 solution, Fig. 2e) leads to a further decrease in the intensity and further broadening of the peaks, confirming that the pores of the host material have been filled with Rh6G [39,44] which can reduce the scattering contrast between the pores and the walls of mesoporous material [45]. Additionally, the peaks of Rh6G/RSBA-15AF were shifted to a higher angles, this indicates the d-spacing of Rh6G/RSBA-15AF became smaller than that of the blank sample. This suggests that mesoporous RSBA-15 had reacted with the amine groups, which subsequently further reacted with the dye Rh6G [46]. For Rh6G/RSBA-15BE, no strong reflections were visible, indicating the channels of the RSBA-15 had been damaged (Data not shown). In view of the IR and XRD results, the microwave digestion method was used for removing the template of the RSBA-15 and then dye was postgrafted into the RSBA-15 before and after modification. In order to characterize the change in pore features of RSBA-15, Rh6G/RSBA-15BE and RSBA-15AF, N2 adsorption-desorption isotherms were measured. As shown in Fig. 3, Rh6G/RSBA-15 composites and RSBA-15 give similar isotherm curves, implying that the mesostructure of Rh6G/RSBA-15 composites were similar to that of RSBA-15 [47]. However, some slight shifts of the inflection point towards the low pressure region can be observed in Fig. 3b and c, indicating that Rh6G molecules have been successfully grafted onto the pore surface of RSBA-15. The Rh6G/RSBA15AF composite (Fig. 3(c)) had a homogeneous pore diameter the same as blank samples, and it has a clearly lower desorption volume
Fig. 3. Nitrogen adsorption–desorption isotherm plots and pore size distribution curve (inset) of (a) RSBA-15, (b) Rh6G/RSBA-15BE composite and (c) Rh6G/RSBA-15AF composite.
Fig. 2. Small-angle XRD Spectra of RSBA-15 as synthesized (a), microwave digested RSBA-15 (b), extracted RSBA-15 (c), and calcined RSBA-15 (d) and Rh6G/RSBA-15AF composite (e).
and narrower pore diameter than that of Rh6G/RSBA-15BE implying that the modification of organic silane groups in between the surface of the pores and the dye molecules may be the cause of the narrower pores [48,49]. The Rh6G/RSBA-15BE composite (Fig. 3(b)), has two kinds of pore diameter, perhaps due to loss of homogeneity since this sample was not modified. After grafting dye molecules Rh6G on the surface of the mesopores of RSBA-15 either before or after modification with TM550, the
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morphologies of RSBA-15 and Rh6G/RSBA-15 composites remained the same as shown in Fig. 4a, b and c, these exhibited a relatively uniform size of ca. 0.5 μm in width and 1–2 μm in length. The hexagonal mesostructure of RSBA-15 and Rh6G/RSBA-15AF has been confirmed by high resolution TEM images (Fig. 5a, b) that show well-ordered hexagonal arrays of mesopores with a 2D P6mm hexagonal structure [1,2,50]. The pore sizes measured from TEM images are 8 nm for RSBA-15 and 5 nm for the Rh6G/RSBA-15AF composite. For Rh6G/RSBA-15BE composite no channels were visible indicating the channel structure of RSBA-15 had been damaged. The absorption spectrum of the Rh6G/CHCl3 solution with the concentration of 1 × 10− 3 mol/L, room temperature PL spectra of Rh6G/ CHCl3 solution (a), Rh6G/RSBA-15BE (b) and Rh6G/RSBA-15AF (c) composites are shown in Fig. 6. The absorption and emission peaks of Rh6G/ CHCl3 solution are at 514 nm and 585 nm, respectively. It can be seen that the absorption and PL emission exhibit large Stoke shift, which is important to reduce self-absorption in solid-state dye lasers and optical sensors [51]. An 8 nm blue-shift can be observed in the PL spectrum of the Rh6G/RSBA-15BE (Fig. 6b) and a 46 nm blue-shift in Rh6G/RSBA15AF (Fig. 6c). These blue-shifts were due to confined Rh6G dye molecules in RSBA-15 mesostructure, and the interactions between the dye and the amino groups or Si–OH groups on the channel
Fig. 5. High resolution TEM images of (a) RSBA-15 showing hexagonal packed pores of 8 nm diameters; (b) Rh6G/RSBA-15AF showing 5 nm channels.
surface of RSBA-15 with or without modification. The confinement and interaction effect can be attributed to the dispersive of Rh6G dye molecules, which decreasing the interactions between neighboring Rh6G dye molecules trapped in the channels of the RSBA15. These could lower their excited state energy, and thus result in blue-shifted spectra [47]. The interactions between the Rh6G dye molecules and amino groups on the channel surface of RSBA-15 is stronger than that between the Rh6G dye molecules and Si–OH groups, therefore the blue-shift of the emission peak of Rh6G/RSBA-15AF was larger than that of Rh6G/RSBA-15BE. 4. Conclusions
Fig. 4. SEM micrographs of (a) RSBA-15, (b) Rh6G/RSBA-15BE composite and (c) Rh6G/ RSBA-15AF composite.
The investigation of three effective methods including extraction, calcination, and microwave digestion shows that the microwave digestion method was the most efficient and the most convenient method to remove the template of RSBA-15. Organic dye molecules Rh6G has been successfully grafted on the inner pore surface of the RSBA-15 material: Structural and spectroscopic analyses indicate that all the synthesized materials have RSBA-15 mesostructure character with high surface areas, and large pore sizes about 8 nm for RSBA-15, 5 nm for Rh6G/RSBA-15AF composite; In PL spectrum, a 46 nm blue-shift has been observed because of the interactions between the Rh6G dye molecules and the amino groups in RSBA-15; The Rh6G/RSBA-15AF composite had a homogeneous pore diameter the same as blank sample in N2 adsorption–desorption isotherm, and it has a clearly lower desorption volume and narrower pore diameter than that of RSBA-15 which confirmed that the interaction force between terminal amino groups of organic silane and dye molecules can greatly contribute to dispersal of Rh6G into the internal surface of modified RSBA-15. The results of this research indicate that Rh6G/ RSBA-15AF has the potential to be used in optical sensors and solidstate dye lasers.
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Fig. 6. Absorption of the Rh6G/CHCl3 solution with concentration of 1 × 10− 3 mol/L, and PL spectra of Rh6G/CHCl3 solution (a), Rh6G/RSBA-15BE (b) and Rh6G/RSBA-15AF (c).
Acknowledgments This work was supported by the National Science Foundation of China (Grant no. 51102092, 21076094, 51072051, 20876040), the Fundamental Research Funds for the Central Universities (No. 11QG24), and Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20110036120002). References [1] J.S. Beck, J.C. Vartuli, Journal of the American Chemical Society 114 (1992) 10834–10843. [2] C.T. Kresge, M.E. Leonowicz, Nature 359 (1992) 710–712. [3] C. McDonagh, B.D. MacCraith, Chemical Analysis 70 (1998) 45–50. [4] M.E. Davis, Nature 417 (2002) 813–821. [5] A. Iwata, N. Nakashima, Y. Izawa, C. Yamanaka, Analytical Sciences 9 (1993) 807–810. [6] R. Reisfeld, Optical Materials 4 (1994) 1–3. [7] S.K. Lam, D. Lo, Chemical Physics Letters 281 (1997) 35–43. [8] B.D. MacCraith, G.O. Keeffe, C.M. McDonagh, J.F. McGilp, B.O. Kelly, M. Cavanagh, Optical Engineering 33 (1994) 3861–3866. [9] S.A. Yamanaka, D.H. Charych, D.A. Loy, D.Y. Sasaki, Langmuir 13 (1997) 5049. [10] A. Flamini, A. Panusa, Sensors and Actuators B 42 (1997) 39–46. [11] P. Proposito, M. Casalboni, Handbook of Organic–inorganic Hybrid Materials and Nanocomposites, American Scientific Publisher, 2003, I, XXX. [12] D. Avnir, D. Levy, R. Reisfeld, Journal of Physical Chemistry 88 (1984) 5956–5959. [13] N. Wang, X.X. Zhang, N. Han, H.H. Liu, Journal of Composite Materials 44 (2010) 27–39. [14] L.T. Canham, Applied Physics Letters 63 (1993) 337–339. [15] G.E. Malashkevich, Physics of the Solid State 40 (1998) 427–431. [16] L.F. Vieira Ferreira, M.J. Lemos, M.J. Reism, Langmuir 16 (2000) 5673–5680. [17] A.V. Aristov, I.V. Kovaleva, Optics and Spectroscopy 70 (1991) 1025–1029. [18] V.I. Zemskii, I.K. Meshkovskii, I.A. Sokolov, Pis'ma V Zhurnal Tekhnicheskoi Fiziki 12 (1986) 331–337. [19] J.M. McKiernan, S.A. Yamanaka, B. Dunn, J.I. Zink, Journal of Physical Chemistry 94 (1990) 5652–5654. [20] Z.T. Zhang, Y. Han, L. Zhu, Angewandte Chemie International Edition 40 (2001) 1258–1262. [21] T. Seckin, A. Gultek, Journal of Applied Polymer Science 90 (2003) 3905–3911. [22] G. Schulz-Ekloff, D. Wo¨ hrle, B. Duffel, R.A. Schoonheydt, Microporous and Mesoporous Materials 51 (2002) 91–138.
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Contents lists available at SciVerse ScienceDirect
Powder Technology journal homepage: www.elsevier.com/locate/powtec
Effect of template-removing methods and modification to mesoporous blank silica and composited silica Mina Guli a, b,⁎, Li Zhang a, Jianxi Yao a, b, Xiaotian Li c,⁎⁎ a b c
Beijing Key Laboratory of New and Renewable Energy, Renewable Energy School, North China Electric Power University, Beijing, China State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, Beijing, China Key Laboratory of Automobile Materials of Ministry of Education, Department of Material Science and Engineering, Jilin University, Changchun, China
a r t i c l e
i n f o
Article history: Received 1 September 2011 Received in revised form 12 December 2011 Accepted 26 December 2011 Available online 29 December 2011 Keywords: Mesoporous silica Modified Graft Dye
a b s t r a c t A pluronic template of rod-like mesoporous silica was removed by extraction, calcination, and microwave digestion methods. The results of X-ray diffraction and Fourier Transform Infrared Spectroscopy spectra analyses confirmed that microwave digestion was the most efficient method to remove the template because of its convenience, speed, and effectiveness. The pore surface of rod-like SBA-15 was modified by an organic silane with terminal amino groups. Also Rhodamine 6G dye molecules were grafted onto the mesopore walls of rod-like SBA-15 before and after modification. Blue-shifts in photoluminescence, pore features in N2 adsorption–desorption, transmission electron microscopy results of blank rod-like SBA-15, dye doping rodlike SBA-15 before and after modification confirmed that the mesoporous SBA-15 that had been modified by the organic silane with terminal amino groups was more effective for grafting of Rhodamine 6G molecules because the structures still remained as ordered mesostructures after grafting. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Mesoporous silica invented by the Mobil Corporation has attracted considerable interest since being announced in 1992 [1]. Nowadays, mesoporous materials [2] with variable compositions and morphologies have been investigated and received much attention because of their versatile uses in separation, catalysis, nanoreactors, and sensors [3,4]. There have been an increasing number of studies on the optical properties of dye-doped porous silica published in the past several years. The advantages of an inorganic matrix for the embedding of functional laser dyes are attributed to the more rigid environment as well as much higher migration stability and strongly increased photostability compared to organic polymer matrices. These properties render such composites attractive for second harmonic generation [5], solid-state tunable lasers [6] or delayed fluorescence [7]. The pore structure of silica enables dissolved ions to embed fluorescent dyes. This is the basis for the rapidly increasing research activities on dye-doped silica for dissolved analysis with a view to their application in industrial, environmental, and biomedical monitoring [8–10].
⁎ Correspondence to: Mina Guli, Beijing Key Laboratory of New and Renewable Energy, Renewable Energy School, North China Electric Power University, Beijing, 102206, PR China. Tel.: +86 10 61772816; fax: +86 10 61772816. ⁎⁎ Correspondence to: Xiaotian Li, Department of Material Science and Engineering, Key Laboratory of Automobile Materials of Ministry of Education, Jilin University, Changchun, 130012, PR China. Tel.: + 86 431 85168445; fax: + 86 431 85168444. E-mail addresses: [email protected] (M. Guli), [email protected] (X. Li). 0032-5910/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2011.12.061
Different organic and inorganic hosts have been extensively studied with the aim of increasing mechanical, thermal and photochemical stability of embedded guests with specific functionality [11–19]. This work mainly focused on the use of SBA-15, which has thicker wall and better ordered silica mesoporous structures with twodimensional hexagonal (P6mm) symmetry, instead of the M41S family that has been used for grafting dyes frequently. Due to its uniform particle size, larger surface area, and higher hydrothermal stability [20], SBA-15 has been used as a perfect host for assembling of organic dyes [21]. The chemical, thermal and mechanical stability of this kind of host-guest composite are particularly suitable for engineering stable photoluminescent devices [11,22–26]. Therefore, SBA-15 has a clear advantage over MCM-41 and zeolite for grafting of organic molecules. The first incorporation of dye molecules into mesostructured materials was aimed at monitoring the formation of the mesostructure in situ [27,28]. Recent efforts have been directed toward the goal of obtaining dye-doped structures for optical applications [29–32], especially for optical sensors, such as Comes Maria's work in 2008 [33] and Chang's work in 2011 [34], and for solid-state dye lasers such as Zhu's work about dye-doped mesostructured silica films [35]. Quite different approaches have been used before, ranging from the incorporation of phthalocyanins and specifically designed surfactants [36], or insertion of polymers into MCM-41-type materials after the surfactant has been removed [37], to the generation of photoluminescent silicon clusters on the walls of as-synthesized SBA-3 films [38]. Recently, the grafting process through covalent linkage of surface silanol groups and dye molecules has been regarded
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as an alternative route to prepare dye-doped mesoporous materials [22,39], but these studies formed basic and preliminary work. Compared to other synthesis methods, in this work, a rational process was used in which the template of the mesoporous materials has been removed by the optimized method first, and then the mesoporous materials has been modified and postgrafted. The advantage of this process is that it can avoid introducing heteroatoms into the framework of the mesostructures and avoid the aggregation of the dye molecules. Three methods have been used to remove the template of rod-like SBA-15 (RSBA-15) including calcinations, extraction and microwave digestion. By using thorough characterizations of X-ray diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR), it was found that microwave digestion is the best method to remove the templates since it was rapid, convenient and thorough. After removing the template from RSBA-15, the dye has been grafted into the RSBA-15 before and after modification, respectively. The latter exhibited improved photoluminescence (PL) and characterization of N2 adsorption–desorption due to preservation of the microstructure. 2. Experimental 2.1. Synthesis of RSBA-15 powder Mesoporous RSBA-15 was synthesized using nonionic triblock copolymer poly (ethylene glycol)-block-poly (propylene glycol)-blockpoly (ethylene glycol) (Pluronic P123, molecular weight = 5800, EO20PO70EO20, BASF) as organic template according to the method reported in the literature [40,41]. In a typical synthesis process, 1 g of pluronic P123 and 2.1 g of Na2SO4 were added to 30 g of 2 mol/L HCl. After stirring for 0.5 h, a clear solution was obtained. Then, 2.1 g of tetraethyl orthosilicate (TEOS, 98%, Aldrich) was added to the solution with vigorous stirring for 10 min. The resulting mixture was subsequently kept under static conditions for 24 h at 311 K. Finally, the mixture was transferred into an autoclave and heated at 373 K for 24 h. The solid product obtained was filtered with deionised water, and dried overnight at 323 K. 2.2. Removing the Template from RSBA-15 Powder Three methods, including extraction, calcination, and microwave digestion [40] were used to remove the organic template from RSBA-15. For microwave digestion, the mixture of 1 g RSBA-15, 12 mL concentrated HNO3 and 8 mL H2O2 (30%) was treated by microwave energy of approximately 1200 W (2450 MHz and 220 V) under the pressure of 1.3 MPa for 2 min. Teflon sample vessels were used since it is transparent for microwave energy. The resulting mixture was filtered and dried at 323 K. For comparison, the template was also extracted by refluxing RSBA-15 in acidic ethanol for 2 days or calcined at 550 °C for 5 h. For extraction, the mixture of 1 g RSBA-15, 100 mL ethanol, and a few drops of 2 M HCl was refluxed for 2 days with stirring.
synthetic process 2.3 with stirring for further 30 min, the products were pale pink in color. A typical preparation of Rh6G/RSBA-15AF composites involved stirring a mixture of 0.25 g of TM550 modified RSBA-15 in 50 mL of Rh6G CHCl3 solution (1 × 10 − 3 mol/L) at room temperature for 10 h. The resulting mixture was washed several times with CHCl3 until no pink color was observed in the eluant. The products were dried overnight at room temperature and were pink in color. 2.5. Characterization XRD patterns were recorded on a Siemens D5005 diffractometer using Cu Kα radiation at 40 kV and 40 mA. FTIR spectra were measured on a Nicolet 5Dx-FT-IR spectrometer at room temperature. Analyses were performed using pressed KBr disks and recorded at a resolution of 4 cm − 1. The morphology of products was characterized by a JEOL JSM-6700F field emission scanning electron microscope (SEM). Transmission electron microscopy (TEM) experiments were performed on a model JEM-2010F electron microscope (JEOL, Japan) with an acceleration voltage of 200 kV. Fluorescence spectroscopy data were recorded on a perkin Elmer LS55 luminescence spectrometer. N2 adsorption measurements were performed at 77 K using a Micromeritics ASAP 2010 m instrument (Micromeritics Instrument Corp, Norcross, GA). 3. Results and disscussion Fig. 1 shows the IR spectra of RSBA-15 samples. An intense peak around 3373 cm − 1 appeared for all samples can be assigned to the Si–OH stretching vibration from RSBA-15, and the peak at 945 cm − 1 appeared almost for all samples can be assigned to the Si–OH bending vibration mode of RSBA-15. It shows that Si–OH remained after removal of the template, especially after extraction and microwave digestion, but decreased after calcination, which means extraction and microwave digestion methods can keep more Si–OH which benefit to the following reaction. The peak at 1633 cm − 1 observed in RSBA-15 can be attributed to the O–H bending vibration mode of the adsorbed water molecules, which showed a concomitant decrease in intensity after extraction and calcination but didn't show any decrease after microwave digestion, indicating that after microwave digestion the sample still remain large hydroxyl, but lose hydroxyl after extraction and calcination which is not good for the next reaction step. Several infrared absorption bands around 2850–3000 and 1350–1500 cm − 1 shown in spectrum Fig. 1 (a) can
2.3. Modification of RSBA-15 with silane coupling agent-(c2h5o)3si(ch2)3nh2 A mixture of 1 g RSBA-15 without template, 60 mL methanol and 3.2 mL (C2H5O)3Si(CH2)3NH2 (TM550) was refluxed for approximately 6 h. The resulting mixture was filtered and washed with methanol and dried at 323 K. 2.4. Formation of Rhodamine 6G/RSBA-15 (Rh6G/RSBA-15) Composites Before and After Modification (Referred to as Rh6G/RSBA-15BE and Rh6G/RSBA-15AF, Respectively) To prepare Rh6G/RSBA-15BE, 0.024 g Rh6G was added to the resulting mixture just after removing the template and before the
Fig. 1. FTIR spectra of (a) as-synthesized RSBA-15, (b) template removed by extraction, (c) by microwave digestion, and (d) by calcination.
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be assigned to C–H stretching and bending vibrations of the template P123 that disappeared after microwave digestion and calcination, but were still visible after extraction, indicating that template can be completely removed by both microwave digestion and calcination. Comparing the three processes, microwave digestion was found to be the most rapid, effective and convenient method [42]. In the later experiments, the samples which using microwave digestion method for removing the RSBA-15 template had been employed. Fig. 2 shows the low-angle XRD patterns. All XRD patterns display three reflection peaks that are characteristic of hexagonal packing, indicating that the primary RSBA-15 silica was ordered in a hexagonal mesostructure (Fig. 2a), the structure was maintained after removal of the template by microwave digestion, extraction and calcination (Fig. 2b, c and d). Three methods of removing the template led to a decrease in the intensity of the XRD peaks, and broadening of the peaks [43]. The formation of Rh6G/RSBA-15AF composites (prepared with 1 × 10 − 3 mol/L Rh6G in CHCl3 solution, Fig. 2e) leads to a further decrease in the intensity and further broadening of the peaks, confirming that the pores of the host material have been filled with Rh6G [39,44] which can reduce the scattering contrast between the pores and the walls of mesoporous material [45]. Additionally, the peaks of Rh6G/RSBA-15AF were shifted to a higher angles, this indicates the d-spacing of Rh6G/RSBA-15AF became smaller than that of the blank sample. This suggests that mesoporous RSBA-15 had reacted with the amine groups, which subsequently further reacted with the dye Rh6G [46]. For Rh6G/RSBA-15BE, no strong reflections were visible, indicating the channels of the RSBA-15 had been damaged (Data not shown). In view of the IR and XRD results, the microwave digestion method was used for removing the template of the RSBA-15 and then dye was postgrafted into the RSBA-15 before and after modification. In order to characterize the change in pore features of RSBA-15, Rh6G/RSBA-15BE and RSBA-15AF, N2 adsorption-desorption isotherms were measured. As shown in Fig. 3, Rh6G/RSBA-15 composites and RSBA-15 give similar isotherm curves, implying that the mesostructure of Rh6G/RSBA-15 composites were similar to that of RSBA-15 [47]. However, some slight shifts of the inflection point towards the low pressure region can be observed in Fig. 3b and c, indicating that Rh6G molecules have been successfully grafted onto the pore surface of RSBA-15. The Rh6G/RSBA15AF composite (Fig. 3(c)) had a homogeneous pore diameter the same as blank samples, and it has a clearly lower desorption volume
Fig. 3. Nitrogen adsorption–desorption isotherm plots and pore size distribution curve (inset) of (a) RSBA-15, (b) Rh6G/RSBA-15BE composite and (c) Rh6G/RSBA-15AF composite.
Fig. 2. Small-angle XRD Spectra of RSBA-15 as synthesized (a), microwave digested RSBA-15 (b), extracted RSBA-15 (c), and calcined RSBA-15 (d) and Rh6G/RSBA-15AF composite (e).
and narrower pore diameter than that of Rh6G/RSBA-15BE implying that the modification of organic silane groups in between the surface of the pores and the dye molecules may be the cause of the narrower pores [48,49]. The Rh6G/RSBA-15BE composite (Fig. 3(b)), has two kinds of pore diameter, perhaps due to loss of homogeneity since this sample was not modified. After grafting dye molecules Rh6G on the surface of the mesopores of RSBA-15 either before or after modification with TM550, the
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morphologies of RSBA-15 and Rh6G/RSBA-15 composites remained the same as shown in Fig. 4a, b and c, these exhibited a relatively uniform size of ca. 0.5 μm in width and 1–2 μm in length. The hexagonal mesostructure of RSBA-15 and Rh6G/RSBA-15AF has been confirmed by high resolution TEM images (Fig. 5a, b) that show well-ordered hexagonal arrays of mesopores with a 2D P6mm hexagonal structure [1,2,50]. The pore sizes measured from TEM images are 8 nm for RSBA-15 and 5 nm for the Rh6G/RSBA-15AF composite. For Rh6G/RSBA-15BE composite no channels were visible indicating the channel structure of RSBA-15 had been damaged. The absorption spectrum of the Rh6G/CHCl3 solution with the concentration of 1 × 10− 3 mol/L, room temperature PL spectra of Rh6G/ CHCl3 solution (a), Rh6G/RSBA-15BE (b) and Rh6G/RSBA-15AF (c) composites are shown in Fig. 6. The absorption and emission peaks of Rh6G/ CHCl3 solution are at 514 nm and 585 nm, respectively. It can be seen that the absorption and PL emission exhibit large Stoke shift, which is important to reduce self-absorption in solid-state dye lasers and optical sensors [51]. An 8 nm blue-shift can be observed in the PL spectrum of the Rh6G/RSBA-15BE (Fig. 6b) and a 46 nm blue-shift in Rh6G/RSBA15AF (Fig. 6c). These blue-shifts were due to confined Rh6G dye molecules in RSBA-15 mesostructure, and the interactions between the dye and the amino groups or Si–OH groups on the channel
Fig. 5. High resolution TEM images of (a) RSBA-15 showing hexagonal packed pores of 8 nm diameters; (b) Rh6G/RSBA-15AF showing 5 nm channels.
surface of RSBA-15 with or without modification. The confinement and interaction effect can be attributed to the dispersive of Rh6G dye molecules, which decreasing the interactions between neighboring Rh6G dye molecules trapped in the channels of the RSBA15. These could lower their excited state energy, and thus result in blue-shifted spectra [47]. The interactions between the Rh6G dye molecules and amino groups on the channel surface of RSBA-15 is stronger than that between the Rh6G dye molecules and Si–OH groups, therefore the blue-shift of the emission peak of Rh6G/RSBA-15AF was larger than that of Rh6G/RSBA-15BE. 4. Conclusions
Fig. 4. SEM micrographs of (a) RSBA-15, (b) Rh6G/RSBA-15BE composite and (c) Rh6G/ RSBA-15AF composite.
The investigation of three effective methods including extraction, calcination, and microwave digestion shows that the microwave digestion method was the most efficient and the most convenient method to remove the template of RSBA-15. Organic dye molecules Rh6G has been successfully grafted on the inner pore surface of the RSBA-15 material: Structural and spectroscopic analyses indicate that all the synthesized materials have RSBA-15 mesostructure character with high surface areas, and large pore sizes about 8 nm for RSBA-15, 5 nm for Rh6G/RSBA-15AF composite; In PL spectrum, a 46 nm blue-shift has been observed because of the interactions between the Rh6G dye molecules and the amino groups in RSBA-15; The Rh6G/RSBA-15AF composite had a homogeneous pore diameter the same as blank sample in N2 adsorption–desorption isotherm, and it has a clearly lower desorption volume and narrower pore diameter than that of RSBA-15 which confirmed that the interaction force between terminal amino groups of organic silane and dye molecules can greatly contribute to dispersal of Rh6G into the internal surface of modified RSBA-15. The results of this research indicate that Rh6G/ RSBA-15AF has the potential to be used in optical sensors and solidstate dye lasers.
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Fig. 6. Absorption of the Rh6G/CHCl3 solution with concentration of 1 × 10− 3 mol/L, and PL spectra of Rh6G/CHCl3 solution (a), Rh6G/RSBA-15BE (b) and Rh6G/RSBA-15AF (c).
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