- Email: [email protected]
Control of the Porosity and Morphology of Ordered Mesoporous Silica by Varying Calcination Conditions
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 15 (2019) 546–554
www.materialstoday.com/proceedings
ICMAM-2018
Control of the Porosity and Morphology of Ordered Mesoporous Silica by Varying Calcination Conditions Mst. Rubaya Rashid, Farhana Afroze, Saika Ahmed, Muhammed Shah Miran, Md. Abu Bin Hasan Susan* Department of Chemistry, University of Dhaka, Dhaka-1000, Bangladesh
Abstract In this study, mesoporous silica (SBA-15) was prepared using pluronic P123 surfactant as the structure-directing template, HCl as catalyst and tetraethyl orthosilicate as the silica source. SBA-15 prepared were then dried at 110 oC and calcined at different temperatures (200-700 °C) and were characterized in detail by Fourier transform infrared spectrophotometer (FTIR), scanning electron microscope (SEM), thermogravimetric analysis (TGA) and BET surface analyzer. FTIR analysis shows that silica calcined at 110 and 200 oC show characteristic bands for the surfactant which diminishes at 300 oC; indicating incomplete removal of the surfactant. TGA analysis of as-synthesized, uncalcined SBA-15 shows that the degradation of the surfactant is completed at 300 oC. The surface area, total pore volume and mean pore radius increase with increasing calcination temperature up to 500 oC and then decrease at 700 oC. Silica dried at 110 oC and calcined at 200 oC shows almost no porosity, while porous nature starts to appear when silica is calcined at 300 oC. The surface area, mean pore radius and total pore volume of mesoporous silica also show dependence on the calcination time, where the sample calcined for 10.5 h show the highest surface area and pore volume. The condition to obtain highest surface properties and porosity of SBA-15 has been optimized as calcination temperature of 500 oC for a period of 10.5 h.
© 2019 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of INTERNATIONAL CONFERENCE ON MULTIFUNCTIONAL ADVANCED MATERIALS (ICMAM-2018).
Keywords: Mesoporous Silica; SBA-15; Pluronic P-123; Calcination temperature; Calcination time.
*Email address: [email protected]
2214-7853© 2019 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of INTERNATIONAL CONFERENCE ON MULTIFUNCTIONAL ADVANCED MATERIALS (ICMAM-2018).
M.S. Rashid et al. /Materials Today: Proceedings 15 (2019) 546–554
547
1. Introduction Mesoporous silica nanoparticles (MSNs), with average pore size between 2 to 50 nm, are fascinating materials of current age due to their porous channels and high surface area [1-4]. Santa Barbara amorphous-15 (SBA-15) is one of the most important hexagonal mesoporous materials of the recent days [5-7]. SBA-15 has uniform hexagonal pores with a narrow pore size distribution and tunable pore diameter of 5-15 nm. The thickness of the framework walls ranges between 3.1 to 6.4 nm, which gives the SBA-15 higher hydrothermal and mechanical stability compared to other forms of mesoporous silica. The high internal surface area of typically 400–900 m2/g makes it a well-suited material for various applications, which inter alia include: environmental analytics for adsorption and separation, advanced optics, as a support material for catalysts and as a template to produce nanostructured carbon or platinum replica. Pore size is an important property in practical applications of porous materials and has a precise meaning when the geometrical shape of the pores is well defined and known [8-10]. The properties and applications of MSNs significantly depend on the porosity related properties such as pore size, total pore volume as well as surface area. Research to date includes numerous studies to control the porous structure of mesoporous silica for obtaining optimum surface area and pore size/volume [11-14]. While these have mainly focused on the synthesis conditions to control the size and morphology of particles and their porosity, very few literatures, to our knowledge, reported the control of these structural parameters by having control over the removal of the surfactant template, especially for SBA-15. Also, justification of the time and temperature of the calcination process that are widely adopted for template removal should also have been understood properly; since optimization of the time or temperature at which calcination should be performed for obtaining the best surface properties and morphology of silica is a demanding question. In this work, we synthesized SBA-15 using pluronic P123 surfactant as the structure-directing template by sol-gel process and studied the surface properties, thermal behaviour and morphology. Main attempt has been made to study how the porosity, surface area and morphology of SBA-15 MSNs are influenced by calcination temperature and time. The aim has however been at optimizing the calcination conditions for enhanced surface properties of SBA-15 and proper understanding of the rationale for commonly used calcination time and temperature during synthesis of SBA-15. 2. Experimental 2.1. Materials Tetraethyl orthosilicate (TEOS) received from Merck was used as the source of silica. Pluronic P123 (BASF Corporation, Germany) surfactant was used as the pore-directing template without further purification. HCl (RCI LabScan, Australia) was used as the acid catalyst for hydrolysis of TEOS to silica. 2.2. Synthesis of SBA-15 4 g pluronic P123 was dissolved in 1.6 M 150.0 mL HCl aqueous solution with stirring at 35 oC; 8.5 g of TEOS was then added drop-wise into the solution with stirring at the same temperature [15]. Formation of silica could be observed immediately after the addition. The mixture was then heated at 35 oC overnight in static condition, and again at 100 oC for further 24 h. It was then filtered, washed with double distilled water several times to remove residual Cl- ions and the solid product was dried at 110 oC overnight. Finally, effect of calcination temperature on the structure of silica was studied by dividing the sample in to five different parts and then calcining those at 200, 300, 400, 500, and 700 oC in a muffle furnace for 16 h [16]. For analysis of the influence of time of calcination on the porosity, silica was synthesized following the similar protocol and the synthesized silica was then calcined for 6, 8, 10.5 and 16 h at 560 oC.
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In aqueous solutions, the TEOS hydrolyses and polymerises to form a silica network. The possible chemical reactions occurring here are, [17-18] Hydrolysis: ≡Si-OR + H2O ⇌ ≡Si-OH + ROH Alcohol condensation: ≡Si-OR + HO-Si≡ ⇌ ≡Si-O-Si≡ + ROH Water condensation: ≡Si-OH + HO-Si≡ ⇌ ≡Si-O-Si≡ + H2O 2.3. Characterization Molecular characterization of the synthesized silica samples were carried out by Fourier transform infrared spectrophotometer (Frontier FT-IR/NIR, Perkin Elmer, USA) to record the spectra at the range of 4000-400 cm-1 by making pellets with KBr. Thermal analysis was performed at a heating rate of 10 °C per minute from ambient temperature to 550 °C under O2 atmosphere by a thermogravimetric-differential thermal analyzer (TG-DTA 7200, Hitachi, Japan). Surface area and pore size distribution of the synthesized silica samples calcined at different temperatures and for different time periods were measured by using BET surface area analyzer (Belsorp mini-II, BEL, Japan). N2 gas adsorption-desorption isotherms were obtained at different relative pressures, P/P0, ranging from 0.01-0.99, on the surface of solid silica nanoparticles and used to determine surface area and pore size distribution of the silica. N2 gas adsorption was performed at -196 oC, using liquid nitrogen as the cryogenic fluid and about 0.02 g of sample was taken in the sample cell for each measurement. Before each measurement, the samples were pre-treated at 130 oC for 2 h under N2 gas flow. The BET surface area was calculated from the adsorption branches in the relative pressure range of 0.05–0.25, and the total pore volume was evaluated at a relative pressure of about 0.99. The pore diameter and the pore size distribution were calculated from the adsorption branches using the Barrett–Joyner–Halenda (BJH) method. Morphological analyses of silica samples were carried out by JEOL analytical scanning electron microscope (Model JSM-6490LA) operating at 20 kV. Round sample stubs of aluminum were used throughout the investigation. Samples were mounted on a carbon tape attached on the round-shaped sample stage. 3. Results and discussion 3.1. Effect of calcination temperature A. Thermogravimetric analysis (TGA) Thermogram of dried SBA-15 sample is shown in Fig.1. The first step of the thermogram shows a weight loss of ~3.54% in the range of 30-100 oC due to the removal of surface adsorbed water. The second step of major weight loss commenced at 155 oC due to removal of the surfactant. About 32.1% weight loss was observed in this step (155-290 oC). This was followed by a small, steady weight loss from the silica sample. Analysis of TGA curve clearly reveals that surfactants are removed completely at higher temperatures [19]. This indicates that the porous structure of silica nanoparticles could be obtained only if calcined at higher temperatures.
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Fig. 1. TGA curve of as-synthesized silica dried at 110 oC.
B. FTIR spectral analysis The FTIR spectra of the silica nanoparticles dried at 110 oC followed by calcined at different temperatures from 200-700 oC are shown in Fig. 2. All the samples exhibit characteristic bands of Si-O-Si at 447 and 1100 cm-1 [2023]. The spectrum at around 3450-3500 cm-1 corresponds to the stretching vibration of intermolecular hydrogen bond (O-H) observed due to absorbed surface water molecules and also, for Si-OH groups present. The peak at 947 cm-1 is also due to the stretching vibration of Si-OH [24-27]. The stretching vibration of C-H vaguely appears at 2850-2950 cm-1 for silica calcined at 110 oC, possibly indicating the presence of remaining pluronic P123, which could not be completely removed at this lower temperature compared with the other calcined ones. Intensity of this band decreases for silica calcined at 200 oC and gradually diminishes with increasing calcination temperature. The stretching modes of vibrations of asymmetric and symmetric >C=O bonds of the surfactant P123 are observed at 1730 cm-1 only for the dried sample, which slowly diminishes in the calcined samples with increasing calcination temperature. No band is observed for samples calcined at temperatures higher than 400 oC at this position, possibly due to complete removal of surfactant.
Table 1: Assignment of bands in FTIR spectra of SBA-15 samples.
Fig. 2. FTIR spectra of SBA-15 samples calcined at different temperatures.
Wavenumber (cm-1)
Tentative band assignment
447
Si-O-Si
1100
Si-O-Si
3450-3500
O-H
947
Si-OH
1730 2850-2950
>C=O C-H
550
M.S. Rashid et al. / Materials Today: Proceedings 15 (2019) 546–554
C. Morphology analysis by scanning electron microscope (SEM) SEM images of the SBA-15 samples dried at 110 oC and calcined at 200-700 oC were studied for analysis of the variation in size and morphology of the particles occurred from different temperatures are shown in Fig. 3. [28]
Fig. 3. SEM images of SBA-15 samples calcined at temperatures (a) 110 oC, (b) 200 oC, (c) 400 oC, (d) 500 oC and (e) 700 oC.
The SEM images of the SBA-15 MSNs calcined at different temperatures show that they all have characteristic morphological shape of cylindrical particles as reported for SBA-15 in the literature [29]. The size of the particles is nearly homogeneous with diameter in the nano-size range. SBA-15 calcined at 700 oC seem slightly smaller in diameter compared to the other ones which might arise from the shrinkage of destroyed hexagonal pores of SBA-15 at such high temperature. D. N2 gas adsorption-desorption behavior N2 gas adsorption-desorption analyses of the SBA-15 samples calcined at various temperatures provide a clear picture of the variation of porosity and surface area owing to the change in temperature at which the samples were subjected during calcination.
Fig. 4. Left: N2 adsorption-desorption isotherms of SBA-15 calcined at different temperatures; Right: P/Po ranging from 0.75 to 1.00 magnified for clearer observation.
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The volume of N2 gas adsorbed per gram of SBA-15 increases gradually with higher calcination temperatures up to 500 oC, and then decreases at 700 oC (Fig. 4). Adsorption-desorption isotherm of the sample dried at 110 oC represents Type-II isotherm according to the IUPAC classification of the adsorption isotherms, with no hysteresis observed [30]. This is clearly indicative of the non-porous nature of this silica due to presence of the surfactants. Isotherms of the other SBA-15 samples, i.e. those calcined at 200-700 oC resemble adsorption isotherms of Type-IV. The presence of hysteresis in these isotherms indicate the porous nature of the corresponding samples [31] which arises from opening up of the pores. The steady increase in the hysteresis loop up to 500 oC arrives from the gradual removal of the surfactants. In addition, the shapes of the hysteresis loops (H1 type) match with the nature of mesoporous materials. The BJH pore size distribution plots of SBA-15 are shown in Fig. 5. The BJH pore size distribution plot of the SBA15 dried at 110 oC exhibits no peak, indicating absence of porous structure [32]. On the other hand, peaks start to appear for silica samples calcined at higher temperatures; i.e. 300-700 oC. The mean pore radius is found to be the smallest in the case of 300 oC (8 nm) and highest for higher temperatures (~12 nm) which is due to the removal of the template. The distribution of the pore size also becomes wider with increasing temperature, indicating increased inhomogeneity in the pore sizes due to calcination at high temperatures. Surface area and total pore volume of SBA-15 samples obtained from their respective BET plots and mean pore radius values from BJH plots are shown in Table 2. The surface area is the highest at the calcination temperature 200 oC and lowest for 700 oC. Total pore volume and mean pore radius also follow similar trend with increasing calcination temperature. The low values of pore volume, Vp and mean pore radius, rp at 110 oC arise from the incomplete removal of the surfactant from the pores of the synthesized silica. With increasing calcination temperature up to 500 oC, the Vp and rp also gradually increase to show a maximum value at 500 oC. This corresponds to the opening of pores due to more and more removal of surfactants as the temperature is increased, which is also evident in the FTIR spectra. The thermogram of as-synthesized silica (Fig.1) supports this assumption by showing that the surfactant is completely degraded at around 300 oC temperature. The highest Vp and rp at 500 oC might be an indication of complete surfactant removal by thermal degradation. Table 2. Surface area, pore volume and mean pore radius of SBA-15 nanoparticles calcined at different temperatures obtained from their BET and BJH plots.
Calcination temperature (oC)
Specific surface area, SBET (m2g-1)
Total pore volume, Vp (cm3g-1)
Mean pore radius, rp (nm)
110
375
0.36
4
200
581
0.71
5
300
453
0.72
8
400
464
0.72
12
500
488
0.74
12
700
315
0.55
12
Fig. 5. BJH plots of silica NPs calcined at different temperatures showing their pore size distribution.
After 500 oC, the SBET, Vp and rp again decrease. This phenomenon might be attributed to the damage of the porous structure at such high temperature and consequent coalescence and agglomeration of the pores, also evident from the wide distribution of BJH plot, giving rise to lower values of SBET and Vp.
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3.2. Effect of calcination time Effect of calcination time on the porosity and surface area of SBA-15 was analyzed from the N2 adsorptiondesorption analyses of samples calcined for different periods (0-16 h). Fig. 6 represents the adsorption-desorption isotherms of such samples, where the presence of hysteresis is indicative of the porous nature of the substances. The shapes of the isotherms refer to the existence of mostly mesopores, along with few amounts of micropores. Surface area calculated from the BET plots (not shown) shows the highest value for silica calcined for 10.5 h (Table 3). This can be attributed to the removal of all the templates (P123). The reduced surface area for 16 h might arise from the damage in the porous structure, also evident from the lower value of N2 adsorption in the isotherm.
Fig. 6. N2 adsorption-desorption isotherms of SBA-15 calcined for different time.
Typical BJH pore size distribution curves measured from the adsorption branch of the adsorption-desorption isotherms of SBA-15 silica nanoparticles calcined for different time (Fig. 7) show well-defined peaks for all the samples with average pore size of ~7 nm in the range of mesopores. As in Table 3, the total pore volume and mean pore radius increases as the calcination time increases and the values are highest for silica calcined for 10.5 h. Table 3. Surface area, pore volume and mean pore radius of SBA-15 calcined for different time (calculated from their BET and BJH plots).
Fig. 7. BJH pore size distribution plots of SBA-15 calcined for different time.
Calcination time (h)
Specific surface area, SBET (m2g-1)
Total pore volume, Vp (cm3g-1)
Mean pore radius, rp (nm)
6
504
1.13
7.3
8
413
1.10
7.2
10.5
641
1.67
7.5
16
565
1.04
7.5
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4. Conclusions SBA-15 mesoporous silica nanoparticles synthesized by sol-gel method using P123 surfactant template and calcined at various temperatures, for various time periods were studied for understanding the effect of calcination on the particle and surface morphology. The SEM images of the SBA-15 calcined at different temperatures show the characteristic cylindrical particles for SBA-15. The particles are nearly homogeneous in size and shape with nanorange diameter for a specific temperature. SBA-15 particles calcined at 700 oC are slightly smaller in diameter that might arise from the shrinkage of destroyed hexagonal pores of SBA-15. The surface area and porosity were found to be significantly influenced by varying calcination conditions. The surface area, total pore volume and mean pore radius increase with increasing calcination temperatures up to 500 oC and then decrease at 700 oC. The silica dried at 110 oC and calcined at 200 oC show almost no porosity, while porous nature starts to appear when silica is calcined at 300 oC. FTIR analysis shows that silica calcined at 110 and 200 oC show characteristic band for the surfactant which diminishes at 300 oC; indicating incomplete removal of the surfactant, which causes lower surface area and pore size of silica, as shown by the N2 adsorption behavior. This is also supported by thermogravimetric analysis of as-synthesized, uncalcined SBA-15, which shows that the degradation of the surfactant is completed at 300 oC. Silica calcined at 700 oC shows a lower surface area and pore size compared to 500 oC, which might arise from the damage of the hexagonal ordered porous network, followed by coalescence of the pores. The surface area, mean pore radius and total pore volume of mesoporous silica also show dependence on calcination time, where the sample calcined for 10.5 h show the highest surface area and pore volume. These observations provide logical explanation of using 500 oC temperature and 10.5 h as the optimum condition to obtain highest surface properties and porosity of silica, SBA-15. Acknowledgement The authors gratefully acknowledge financial support from Higher Education Quality Enhancement Project (HEQEP) under the Sub-project, CPSF-231 from Bangladesh University Grants Commission financed by World Bank and the Ministry of Education, Bangladesh. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.
A. Fukuoka, J. Kimura, T. Oshio, T. Sakamoto, M. Ichikawa, J. Am. Chem. Soc., 129 (33) (2007) 10120-10125. D. H. Everett, Pure Appl. Chem., 58(7) (1986) 967-984. H. Meng, M. Xue, T. Xia, Z. Ji, D.Y. Tran, J.I. Zink, A.E. Nel, J. Am. Chem. Soc., 5 (5) (2011) 4131-4144. S. Ahmed, M.Y.A. Mollah, M.M. Rahman, M.A.B.H. Susan, “Mesoporous Silica: The Next Generation Energy Material” in “Innovations in Engineered Porous Materials for Energy Generation and Storage Applications”, CRC Press, (2018) 241-266. D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Science, 279 (1998) 548-552. D.Y. Zhao, Q.S. Huo, J.L. Feng, B.F. Chmelka, G.D. Stucky, J. Am. Chem. Soc., 120 (1998) 6024-6036. M. Raul−Augustin, B. Daniela, M. Cornel, M. Cristian, J. Phys. Chem. C, 119 (27) (2015) 15177-15184. J. Rouquerol, D. Avnir, C.W. Fairbridge, J.H. Haynes, K.S.W. Sing, Pure Appl. Chem., 66 (1994) 1739-1758. S.F. Hansen, B.H. Larsen, S.I. Olsen, A. Baun, Nanotoxicology, 1 (2007) 243-250. N. Jones, B. Ray, K.T. Ranjit, A.C. Manna, FEMS Microbio. Lett., 279 (2008) 71-76. G. Lawrence, A.V. Baskar, M.H. El-Newehy, W.S. Cha, S.S. Al-Deyab, A. Vinu, Sci. Technol. Adv. Mater. 16 (2015) 024806. A. M. Putz, K. Wang, A. Len, J. Plocek, P. Bezdicka, G.P. Kopitsa, T.V. Khamova, C. Ianăşi, L. Săcărescu, Z. Mitróová, C. Savii, M. Yan, L. Almásy, App. Sur. Sci., 424 (2017) 275-281. M. Barczak, New J. Chem., 42 (2018) 4182-4191. K. Zhuang, G. Yin, X. Pu, X. Chen, X. Liao, Z. Huang, Y. Yao, MRS Communications, 6 (2016) 449–454. R. Ottewill, C.C. Storer, T. Walker, Trans. Faraday Soc., 63 (1967) 2796-2802. D. Zhao, Y. Wan, W. Zhou, Ordered Mesoporous Materials, Wiley, (2013) 161. C.J. Brinker, J. Non-Cryst. Solids, 100 (1988), 31-50. L.L. Hench, J.K. West, Chem. Rev, 90 (1990) 33-72. V.B. Voitovich, V.A. Lavrenko, R.F. Voitovich, E.I. Golovko, Oxidation of Metals, 42 (1994) 223-237. S. Partyka, F. Roquerol, J. Roquerol, J. Colloid Interface Sci., 68 (1979) 21-31. W. Lili, W. Youshi, S. Yuanchang, W. Huiying, Rare Met., 25 (2006) 68-73. Y.T. Prabhu, K.V. Rao, V.S.S. Kumar, B.S. Kumari, Adv. Nanopart., 2 (2013) 45-50. H. Yongning, S. Xiaoliang, M.C. Lidun, J. Appl. Chem., 13 (1996) 92-94. K.S. Babu, V. Narayanan, Chem Sci Trans., 2 (S1) (2013) 33-36.
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25. 26. 27. 28. 29. 30. 31. 32.
D. Geeetha, T. Thilagavathi, J. Nanomater. Biostruct., 5 (2010) 297-301. J.P. Auffrkdic, A. Boultif, J.I. Langford, D. Louer, J. Am. Ceram. Soc., 78 (1995) 323-328. B.C. Lippens, J.H. Boer, J. Catalyst., 4 (1965) 319-323. D. McMullan, Scanning, 17 (3) (2006) 175-185. D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Science, 279 (5350) (1998) 548-552. K.S.W. Sing, D.H. Everett, R.A. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T. Siemieniewska, Pure Appl. Chem., 57 (1985) 603-619. F. Rouquerol, J. Rouquerol, K.S.W. Sing, Adsorption by Powder and Porous Solids, Academic Press, London, 1999. S. Brunauer, P.H. Emmett, E. Teller, J. Amer. Chem. Soc., 60 (1938) 309-319.
ScienceDirect Materials Today: Proceedings 15 (2019) 546–554
www.materialstoday.com/proceedings
ICMAM-2018
Control of the Porosity and Morphology of Ordered Mesoporous Silica by Varying Calcination Conditions Mst. Rubaya Rashid, Farhana Afroze, Saika Ahmed, Muhammed Shah Miran, Md. Abu Bin Hasan Susan* Department of Chemistry, University of Dhaka, Dhaka-1000, Bangladesh
Abstract In this study, mesoporous silica (SBA-15) was prepared using pluronic P123 surfactant as the structure-directing template, HCl as catalyst and tetraethyl orthosilicate as the silica source. SBA-15 prepared were then dried at 110 oC and calcined at different temperatures (200-700 °C) and were characterized in detail by Fourier transform infrared spectrophotometer (FTIR), scanning electron microscope (SEM), thermogravimetric analysis (TGA) and BET surface analyzer. FTIR analysis shows that silica calcined at 110 and 200 oC show characteristic bands for the surfactant which diminishes at 300 oC; indicating incomplete removal of the surfactant. TGA analysis of as-synthesized, uncalcined SBA-15 shows that the degradation of the surfactant is completed at 300 oC. The surface area, total pore volume and mean pore radius increase with increasing calcination temperature up to 500 oC and then decrease at 700 oC. Silica dried at 110 oC and calcined at 200 oC shows almost no porosity, while porous nature starts to appear when silica is calcined at 300 oC. The surface area, mean pore radius and total pore volume of mesoporous silica also show dependence on the calcination time, where the sample calcined for 10.5 h show the highest surface area and pore volume. The condition to obtain highest surface properties and porosity of SBA-15 has been optimized as calcination temperature of 500 oC for a period of 10.5 h.
© 2019 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of INTERNATIONAL CONFERENCE ON MULTIFUNCTIONAL ADVANCED MATERIALS (ICMAM-2018).
Keywords: Mesoporous Silica; SBA-15; Pluronic P-123; Calcination temperature; Calcination time.
*Email address: [email protected]
2214-7853© 2019 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of INTERNATIONAL CONFERENCE ON MULTIFUNCTIONAL ADVANCED MATERIALS (ICMAM-2018).
M.S. Rashid et al. /Materials Today: Proceedings 15 (2019) 546–554
547
1. Introduction Mesoporous silica nanoparticles (MSNs), with average pore size between 2 to 50 nm, are fascinating materials of current age due to their porous channels and high surface area [1-4]. Santa Barbara amorphous-15 (SBA-15) is one of the most important hexagonal mesoporous materials of the recent days [5-7]. SBA-15 has uniform hexagonal pores with a narrow pore size distribution and tunable pore diameter of 5-15 nm. The thickness of the framework walls ranges between 3.1 to 6.4 nm, which gives the SBA-15 higher hydrothermal and mechanical stability compared to other forms of mesoporous silica. The high internal surface area of typically 400–900 m2/g makes it a well-suited material for various applications, which inter alia include: environmental analytics for adsorption and separation, advanced optics, as a support material for catalysts and as a template to produce nanostructured carbon or platinum replica. Pore size is an important property in practical applications of porous materials and has a precise meaning when the geometrical shape of the pores is well defined and known [8-10]. The properties and applications of MSNs significantly depend on the porosity related properties such as pore size, total pore volume as well as surface area. Research to date includes numerous studies to control the porous structure of mesoporous silica for obtaining optimum surface area and pore size/volume [11-14]. While these have mainly focused on the synthesis conditions to control the size and morphology of particles and their porosity, very few literatures, to our knowledge, reported the control of these structural parameters by having control over the removal of the surfactant template, especially for SBA-15. Also, justification of the time and temperature of the calcination process that are widely adopted for template removal should also have been understood properly; since optimization of the time or temperature at which calcination should be performed for obtaining the best surface properties and morphology of silica is a demanding question. In this work, we synthesized SBA-15 using pluronic P123 surfactant as the structure-directing template by sol-gel process and studied the surface properties, thermal behaviour and morphology. Main attempt has been made to study how the porosity, surface area and morphology of SBA-15 MSNs are influenced by calcination temperature and time. The aim has however been at optimizing the calcination conditions for enhanced surface properties of SBA-15 and proper understanding of the rationale for commonly used calcination time and temperature during synthesis of SBA-15. 2. Experimental 2.1. Materials Tetraethyl orthosilicate (TEOS) received from Merck was used as the source of silica. Pluronic P123 (BASF Corporation, Germany) surfactant was used as the pore-directing template without further purification. HCl (RCI LabScan, Australia) was used as the acid catalyst for hydrolysis of TEOS to silica. 2.2. Synthesis of SBA-15 4 g pluronic P123 was dissolved in 1.6 M 150.0 mL HCl aqueous solution with stirring at 35 oC; 8.5 g of TEOS was then added drop-wise into the solution with stirring at the same temperature [15]. Formation of silica could be observed immediately after the addition. The mixture was then heated at 35 oC overnight in static condition, and again at 100 oC for further 24 h. It was then filtered, washed with double distilled water several times to remove residual Cl- ions and the solid product was dried at 110 oC overnight. Finally, effect of calcination temperature on the structure of silica was studied by dividing the sample in to five different parts and then calcining those at 200, 300, 400, 500, and 700 oC in a muffle furnace for 16 h [16]. For analysis of the influence of time of calcination on the porosity, silica was synthesized following the similar protocol and the synthesized silica was then calcined for 6, 8, 10.5 and 16 h at 560 oC.
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In aqueous solutions, the TEOS hydrolyses and polymerises to form a silica network. The possible chemical reactions occurring here are, [17-18] Hydrolysis: ≡Si-OR + H2O ⇌ ≡Si-OH + ROH Alcohol condensation: ≡Si-OR + HO-Si≡ ⇌ ≡Si-O-Si≡ + ROH Water condensation: ≡Si-OH + HO-Si≡ ⇌ ≡Si-O-Si≡ + H2O 2.3. Characterization Molecular characterization of the synthesized silica samples were carried out by Fourier transform infrared spectrophotometer (Frontier FT-IR/NIR, Perkin Elmer, USA) to record the spectra at the range of 4000-400 cm-1 by making pellets with KBr. Thermal analysis was performed at a heating rate of 10 °C per minute from ambient temperature to 550 °C under O2 atmosphere by a thermogravimetric-differential thermal analyzer (TG-DTA 7200, Hitachi, Japan). Surface area and pore size distribution of the synthesized silica samples calcined at different temperatures and for different time periods were measured by using BET surface area analyzer (Belsorp mini-II, BEL, Japan). N2 gas adsorption-desorption isotherms were obtained at different relative pressures, P/P0, ranging from 0.01-0.99, on the surface of solid silica nanoparticles and used to determine surface area and pore size distribution of the silica. N2 gas adsorption was performed at -196 oC, using liquid nitrogen as the cryogenic fluid and about 0.02 g of sample was taken in the sample cell for each measurement. Before each measurement, the samples were pre-treated at 130 oC for 2 h under N2 gas flow. The BET surface area was calculated from the adsorption branches in the relative pressure range of 0.05–0.25, and the total pore volume was evaluated at a relative pressure of about 0.99. The pore diameter and the pore size distribution were calculated from the adsorption branches using the Barrett–Joyner–Halenda (BJH) method. Morphological analyses of silica samples were carried out by JEOL analytical scanning electron microscope (Model JSM-6490LA) operating at 20 kV. Round sample stubs of aluminum were used throughout the investigation. Samples were mounted on a carbon tape attached on the round-shaped sample stage. 3. Results and discussion 3.1. Effect of calcination temperature A. Thermogravimetric analysis (TGA) Thermogram of dried SBA-15 sample is shown in Fig.1. The first step of the thermogram shows a weight loss of ~3.54% in the range of 30-100 oC due to the removal of surface adsorbed water. The second step of major weight loss commenced at 155 oC due to removal of the surfactant. About 32.1% weight loss was observed in this step (155-290 oC). This was followed by a small, steady weight loss from the silica sample. Analysis of TGA curve clearly reveals that surfactants are removed completely at higher temperatures [19]. This indicates that the porous structure of silica nanoparticles could be obtained only if calcined at higher temperatures.
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Fig. 1. TGA curve of as-synthesized silica dried at 110 oC.
B. FTIR spectral analysis The FTIR spectra of the silica nanoparticles dried at 110 oC followed by calcined at different temperatures from 200-700 oC are shown in Fig. 2. All the samples exhibit characteristic bands of Si-O-Si at 447 and 1100 cm-1 [2023]. The spectrum at around 3450-3500 cm-1 corresponds to the stretching vibration of intermolecular hydrogen bond (O-H) observed due to absorbed surface water molecules and also, for Si-OH groups present. The peak at 947 cm-1 is also due to the stretching vibration of Si-OH [24-27]. The stretching vibration of C-H vaguely appears at 2850-2950 cm-1 for silica calcined at 110 oC, possibly indicating the presence of remaining pluronic P123, which could not be completely removed at this lower temperature compared with the other calcined ones. Intensity of this band decreases for silica calcined at 200 oC and gradually diminishes with increasing calcination temperature. The stretching modes of vibrations of asymmetric and symmetric >C=O bonds of the surfactant P123 are observed at 1730 cm-1 only for the dried sample, which slowly diminishes in the calcined samples with increasing calcination temperature. No band is observed for samples calcined at temperatures higher than 400 oC at this position, possibly due to complete removal of surfactant.
Table 1: Assignment of bands in FTIR spectra of SBA-15 samples.
Fig. 2. FTIR spectra of SBA-15 samples calcined at different temperatures.
Wavenumber (cm-1)
Tentative band assignment
447
Si-O-Si
1100
Si-O-Si
3450-3500
O-H
947
Si-OH
1730 2850-2950
>C=O C-H
550
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C. Morphology analysis by scanning electron microscope (SEM) SEM images of the SBA-15 samples dried at 110 oC and calcined at 200-700 oC were studied for analysis of the variation in size and morphology of the particles occurred from different temperatures are shown in Fig. 3. [28]
Fig. 3. SEM images of SBA-15 samples calcined at temperatures (a) 110 oC, (b) 200 oC, (c) 400 oC, (d) 500 oC and (e) 700 oC.
The SEM images of the SBA-15 MSNs calcined at different temperatures show that they all have characteristic morphological shape of cylindrical particles as reported for SBA-15 in the literature [29]. The size of the particles is nearly homogeneous with diameter in the nano-size range. SBA-15 calcined at 700 oC seem slightly smaller in diameter compared to the other ones which might arise from the shrinkage of destroyed hexagonal pores of SBA-15 at such high temperature. D. N2 gas adsorption-desorption behavior N2 gas adsorption-desorption analyses of the SBA-15 samples calcined at various temperatures provide a clear picture of the variation of porosity and surface area owing to the change in temperature at which the samples were subjected during calcination.
Fig. 4. Left: N2 adsorption-desorption isotherms of SBA-15 calcined at different temperatures; Right: P/Po ranging from 0.75 to 1.00 magnified for clearer observation.
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The volume of N2 gas adsorbed per gram of SBA-15 increases gradually with higher calcination temperatures up to 500 oC, and then decreases at 700 oC (Fig. 4). Adsorption-desorption isotherm of the sample dried at 110 oC represents Type-II isotherm according to the IUPAC classification of the adsorption isotherms, with no hysteresis observed [30]. This is clearly indicative of the non-porous nature of this silica due to presence of the surfactants. Isotherms of the other SBA-15 samples, i.e. those calcined at 200-700 oC resemble adsorption isotherms of Type-IV. The presence of hysteresis in these isotherms indicate the porous nature of the corresponding samples [31] which arises from opening up of the pores. The steady increase in the hysteresis loop up to 500 oC arrives from the gradual removal of the surfactants. In addition, the shapes of the hysteresis loops (H1 type) match with the nature of mesoporous materials. The BJH pore size distribution plots of SBA-15 are shown in Fig. 5. The BJH pore size distribution plot of the SBA15 dried at 110 oC exhibits no peak, indicating absence of porous structure [32]. On the other hand, peaks start to appear for silica samples calcined at higher temperatures; i.e. 300-700 oC. The mean pore radius is found to be the smallest in the case of 300 oC (8 nm) and highest for higher temperatures (~12 nm) which is due to the removal of the template. The distribution of the pore size also becomes wider with increasing temperature, indicating increased inhomogeneity in the pore sizes due to calcination at high temperatures. Surface area and total pore volume of SBA-15 samples obtained from their respective BET plots and mean pore radius values from BJH plots are shown in Table 2. The surface area is the highest at the calcination temperature 200 oC and lowest for 700 oC. Total pore volume and mean pore radius also follow similar trend with increasing calcination temperature. The low values of pore volume, Vp and mean pore radius, rp at 110 oC arise from the incomplete removal of the surfactant from the pores of the synthesized silica. With increasing calcination temperature up to 500 oC, the Vp and rp also gradually increase to show a maximum value at 500 oC. This corresponds to the opening of pores due to more and more removal of surfactants as the temperature is increased, which is also evident in the FTIR spectra. The thermogram of as-synthesized silica (Fig.1) supports this assumption by showing that the surfactant is completely degraded at around 300 oC temperature. The highest Vp and rp at 500 oC might be an indication of complete surfactant removal by thermal degradation. Table 2. Surface area, pore volume and mean pore radius of SBA-15 nanoparticles calcined at different temperatures obtained from their BET and BJH plots.
Calcination temperature (oC)
Specific surface area, SBET (m2g-1)
Total pore volume, Vp (cm3g-1)
Mean pore radius, rp (nm)
110
375
0.36
4
200
581
0.71
5
300
453
0.72
8
400
464
0.72
12
500
488
0.74
12
700
315
0.55
12
Fig. 5. BJH plots of silica NPs calcined at different temperatures showing their pore size distribution.
After 500 oC, the SBET, Vp and rp again decrease. This phenomenon might be attributed to the damage of the porous structure at such high temperature and consequent coalescence and agglomeration of the pores, also evident from the wide distribution of BJH plot, giving rise to lower values of SBET and Vp.
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3.2. Effect of calcination time Effect of calcination time on the porosity and surface area of SBA-15 was analyzed from the N2 adsorptiondesorption analyses of samples calcined for different periods (0-16 h). Fig. 6 represents the adsorption-desorption isotherms of such samples, where the presence of hysteresis is indicative of the porous nature of the substances. The shapes of the isotherms refer to the existence of mostly mesopores, along with few amounts of micropores. Surface area calculated from the BET plots (not shown) shows the highest value for silica calcined for 10.5 h (Table 3). This can be attributed to the removal of all the templates (P123). The reduced surface area for 16 h might arise from the damage in the porous structure, also evident from the lower value of N2 adsorption in the isotherm.
Fig. 6. N2 adsorption-desorption isotherms of SBA-15 calcined for different time.
Typical BJH pore size distribution curves measured from the adsorption branch of the adsorption-desorption isotherms of SBA-15 silica nanoparticles calcined for different time (Fig. 7) show well-defined peaks for all the samples with average pore size of ~7 nm in the range of mesopores. As in Table 3, the total pore volume and mean pore radius increases as the calcination time increases and the values are highest for silica calcined for 10.5 h. Table 3. Surface area, pore volume and mean pore radius of SBA-15 calcined for different time (calculated from their BET and BJH plots).
Fig. 7. BJH pore size distribution plots of SBA-15 calcined for different time.
Calcination time (h)
Specific surface area, SBET (m2g-1)
Total pore volume, Vp (cm3g-1)
Mean pore radius, rp (nm)
6
504
1.13
7.3
8
413
1.10
7.2
10.5
641
1.67
7.5
16
565
1.04
7.5
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4. Conclusions SBA-15 mesoporous silica nanoparticles synthesized by sol-gel method using P123 surfactant template and calcined at various temperatures, for various time periods were studied for understanding the effect of calcination on the particle and surface morphology. The SEM images of the SBA-15 calcined at different temperatures show the characteristic cylindrical particles for SBA-15. The particles are nearly homogeneous in size and shape with nanorange diameter for a specific temperature. SBA-15 particles calcined at 700 oC are slightly smaller in diameter that might arise from the shrinkage of destroyed hexagonal pores of SBA-15. The surface area and porosity were found to be significantly influenced by varying calcination conditions. The surface area, total pore volume and mean pore radius increase with increasing calcination temperatures up to 500 oC and then decrease at 700 oC. The silica dried at 110 oC and calcined at 200 oC show almost no porosity, while porous nature starts to appear when silica is calcined at 300 oC. FTIR analysis shows that silica calcined at 110 and 200 oC show characteristic band for the surfactant which diminishes at 300 oC; indicating incomplete removal of the surfactant, which causes lower surface area and pore size of silica, as shown by the N2 adsorption behavior. This is also supported by thermogravimetric analysis of as-synthesized, uncalcined SBA-15, which shows that the degradation of the surfactant is completed at 300 oC. Silica calcined at 700 oC shows a lower surface area and pore size compared to 500 oC, which might arise from the damage of the hexagonal ordered porous network, followed by coalescence of the pores. The surface area, mean pore radius and total pore volume of mesoporous silica also show dependence on calcination time, where the sample calcined for 10.5 h show the highest surface area and pore volume. These observations provide logical explanation of using 500 oC temperature and 10.5 h as the optimum condition to obtain highest surface properties and porosity of silica, SBA-15. Acknowledgement The authors gratefully acknowledge financial support from Higher Education Quality Enhancement Project (HEQEP) under the Sub-project, CPSF-231 from Bangladesh University Grants Commission financed by World Bank and the Ministry of Education, Bangladesh. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.
A. Fukuoka, J. Kimura, T. Oshio, T. Sakamoto, M. Ichikawa, J. Am. Chem. Soc., 129 (33) (2007) 10120-10125. D. H. Everett, Pure Appl. Chem., 58(7) (1986) 967-984. H. Meng, M. Xue, T. Xia, Z. Ji, D.Y. Tran, J.I. Zink, A.E. Nel, J. Am. Chem. Soc., 5 (5) (2011) 4131-4144. S. Ahmed, M.Y.A. Mollah, M.M. Rahman, M.A.B.H. Susan, “Mesoporous Silica: The Next Generation Energy Material” in “Innovations in Engineered Porous Materials for Energy Generation and Storage Applications”, CRC Press, (2018) 241-266. D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Science, 279 (1998) 548-552. D.Y. Zhao, Q.S. Huo, J.L. Feng, B.F. Chmelka, G.D. Stucky, J. Am. Chem. Soc., 120 (1998) 6024-6036. M. Raul−Augustin, B. Daniela, M. Cornel, M. Cristian, J. Phys. Chem. C, 119 (27) (2015) 15177-15184. J. Rouquerol, D. Avnir, C.W. Fairbridge, J.H. Haynes, K.S.W. Sing, Pure Appl. Chem., 66 (1994) 1739-1758. S.F. Hansen, B.H. Larsen, S.I. Olsen, A. Baun, Nanotoxicology, 1 (2007) 243-250. N. Jones, B. Ray, K.T. Ranjit, A.C. Manna, FEMS Microbio. Lett., 279 (2008) 71-76. G. Lawrence, A.V. Baskar, M.H. El-Newehy, W.S. Cha, S.S. Al-Deyab, A. Vinu, Sci. Technol. Adv. Mater. 16 (2015) 024806. A. M. Putz, K. Wang, A. Len, J. Plocek, P. Bezdicka, G.P. Kopitsa, T.V. Khamova, C. Ianăşi, L. Săcărescu, Z. Mitróová, C. Savii, M. Yan, L. Almásy, App. Sur. Sci., 424 (2017) 275-281. M. Barczak, New J. Chem., 42 (2018) 4182-4191. K. Zhuang, G. Yin, X. Pu, X. Chen, X. Liao, Z. Huang, Y. Yao, MRS Communications, 6 (2016) 449–454. R. Ottewill, C.C. Storer, T. Walker, Trans. Faraday Soc., 63 (1967) 2796-2802. D. Zhao, Y. Wan, W. Zhou, Ordered Mesoporous Materials, Wiley, (2013) 161. C.J. Brinker, J. Non-Cryst. Solids, 100 (1988), 31-50. L.L. Hench, J.K. West, Chem. Rev, 90 (1990) 33-72. V.B. Voitovich, V.A. Lavrenko, R.F. Voitovich, E.I. Golovko, Oxidation of Metals, 42 (1994) 223-237. S. Partyka, F. Roquerol, J. Roquerol, J. Colloid Interface Sci., 68 (1979) 21-31. W. Lili, W. Youshi, S. Yuanchang, W. Huiying, Rare Met., 25 (2006) 68-73. Y.T. Prabhu, K.V. Rao, V.S.S. Kumar, B.S. Kumari, Adv. Nanopart., 2 (2013) 45-50. H. Yongning, S. Xiaoliang, M.C. Lidun, J. Appl. Chem., 13 (1996) 92-94. K.S. Babu, V. Narayanan, Chem Sci Trans., 2 (S1) (2013) 33-36.
554
M.S. Rashid et al. / Materials Today: Proceedings 15 (2019) 546–554
25. 26. 27. 28. 29. 30. 31. 32.
D. Geeetha, T. Thilagavathi, J. Nanomater. Biostruct., 5 (2010) 297-301. J.P. Auffrkdic, A. Boultif, J.I. Langford, D. Louer, J. Am. Ceram. Soc., 78 (1995) 323-328. B.C. Lippens, J.H. Boer, J. Catalyst., 4 (1965) 319-323. D. McMullan, Scanning, 17 (3) (2006) 175-185. D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Science, 279 (5350) (1998) 548-552. K.S.W. Sing, D.H. Everett, R.A. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T. Siemieniewska, Pure Appl. Chem., 57 (1985) 603-619. F. Rouquerol, J. Rouquerol, K.S.W. Sing, Adsorption by Powder and Porous Solids, Academic Press, London, 1999. S. Brunauer, P.H. Emmett, E. Teller, J. Amer. Chem. Soc., 60 (1938) 309-319.