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The effect of pH on the physicochemical properties of silica aerogels prepared by an ambient pressure drying method
Materials Letters 61 (2007) 3130 – 3133 www.elsevier.com/locate/matlet
The effect of pH on the physicochemical properties of silica aerogels prepared by an ambient pressure drying method Seunghun Lee a , Young Chul Cha d , Hae Jin Hwang a,⁎, Ji-Woong Moon b , In Sub Han c a
d
Department of Materials Science and Engineering, Inha University, Incheon, Republic of Korea b Korea Institute of Ceramic Engineering and Technology, Seoul, Republic of Korea c Korea Institute of Energy Research, Daejeon, Republic of Korea School of Materials Science and Engineering, Inha University, 253 Yonghyun-dong, Nam-gu, Incheon 402-751, Korea Received 3 July 2006; accepted 2 November 2006 Available online 20 November 2006
Abstract Hydrophobic silica aerogels were successfully synthesized by an ambient pressure drying method from various silicic acids with different pH values which were prepared from water glass. In this study, we selected xylene as the solvent and performed the surface modification in a TMCS (trimethylchlorosilane)/xylene solution, in order to improve the reproducibility of the aerogels. The densities of the aerogels were about 0.10– 0.14 g/cm3 and the apparent porosities were in the range of 94.5–96%, depending on the processing conditions. Their specific surface area was in the range of 700–750 m2/g and their average pore size was around 20 nm. The transparency of the as-dried aerogels was enhanced as the pH value was increased. The transmittance of the heat-treated aerogels was slightly decreased with increasing heat-treatment temperature. On the other hand, an abrupt increase in the transmittance was observed when the temperature exceeded 400 °C and this phenomenon might be related to the change in the surface structure caused by the desorption of the methyl groups. © 2006 Elsevier B.V. All rights reserved. Keywords: Aerogel; Porous silica; Ambient drying; Surface modification
1. Introduction Nanoporous silica aerogels are materials which have a high specific surface area, a high porosity (80–99.8%), a low density (∼ 0.003 g/cm3), a high thermal insulation value (0.005 W/m K), an ultra low dielectric constant (k' = 1.0–2.0) and a low index of refraction (∼1.05) [1–5]. Because of their unique properties, silica aerogels have been extensively studied, not only for use as transparent thermal insulators but also as inter-metal dielectric materials (IMDs), optical and acoustic applications, and the space industry. Since Kistler described the first synthesis of an aerogel by supercritical drying in the early 1930s [6], various aerogel synthesis processes have been reported. In the 1970s, a silica aerogel was synthesized by the high temperature supercritical drying of a wet gel produced by the hydrolysis of TMOS (tetramethoxysilane) in methanol [7]. In the 1980s, researchers gained a new ⁎ Corresponding author. Tel.: +82 32 860 7521; fax: +82 32 862 4482. E-mail address: [email protected] (H.J. Hwang). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.11.010
understanding of the potential of aerogels, and TEOS (tetraethoxysilane) based silica aerogels, whose synthesis was less expensive and used less toxic sources compared to TMOS based aerogels, were developed. The low temperature supercritical drying technique, which uses liquid carbon dioxide, was introduced at the same time [8]. Nowadays, a few companies are commercially producing silica aerogel beads, powder or blankets. In addition, the NASA spacecraft called Stardust used an aerogel to sample the materials obtained from a comet. Transparent silica aerogel tiles have also been used as regenerative thermal boards. Generally, aerogels are synthesized by the supercritical drying method, in which metal alkoxides are hydrolyzed and condensed to form a silica wet gel. However, supercritical drying has certain limitations in terms of its cost efficiency, process continuity and safety, because alkoxides are expensive and a high temperature and pressure are needed to approach the critical point. If liquid carbon dioxide is used as a solvent in the low temperature supercritical drying process, the chemical durability of the aerogels in the atmosphere would be gradually decreased, since the aerogel particles are hydrophilic. To
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with the xylene being replaced every 6 h. After aging, the samples were chemically modified in 5% TMCS/xylene solution for 12 h at 50 °C. Finally, the surface-modified wet gels were washed in xylene repeatedly for 24 h at 50 °C, in order to remove the remaining surface modification agents and reaction products such as hydrochloric acid. The gels were kept in a beaker and dried under atmospheric pressure. Since the boiling point of xylene is 139–141.2 °C, the drying process was performed at 142 °C. The dried aerogels were subsequently heat-treated at 200, 300, 400 and 500 °C for 4 h. In this study, the aerogel before the heat-treatment is designated as the “as-dried aerogel.” 2.2. Characterization of the aerogels Fig. 1. Typical microstructure of the silica aerogel prepared from a water glass based silica sol with subsequent ambient drying.
overcome these problems, Brinker introduced the ambient drying method. In this process, the hydroxyl groups of the wet gel surface, which can lead to shrinkage during the subsequent drying process at ambient pressure, are chemically substituted by hydrophobic functional groups. Because aerogels produced by the ambient drying method have hydrophobic surfaces, they can be used in the air for a long time without incurring any damage. In addition, since ambient drying can reduce the production cost of aerogels, their importance has changed from an area of purely scientific interests into one of practical usage. In this study, a novel ambient drying method is proposed for the purpose of enhancing the reproducibility of aerogel production, and water glass based silica aerogels with a porosity of around 95% were synthesized using this method. We focused our attention on the pH value of the silicic acids prepared from water glass and the xylene which is used as a solvent in the surface modification and subsequent ambient drying process. The physical/optical properties and chemical structures of the aerogels were investigated.
The bulk densities of the aerogels were calculated from their weight and volume, and the volumes were obtained from the diameters and thicknesses of the gels. FT-IR spectrometry (FTS165, Bio-Rad, USA) was used to confirm the chemical structure of the as-dried and heat-treated aerogels in the wave number range of 400 to 4000 cm− 1. The specific surface area of the aerogels was measured by BET (ASAP 2010, Micromeritics, USA). The transmittance of the aerogels was measured using UV–visible spectroscopy (UV-2401PC, Shimadzu, Japan) over the wavelength range of 300–800 nm. Thermal analysis of the as-dried gels was performed by TG/DSC (STA-409C, Netzsch, Germany) at a heating rate of 10 °C/min to determine the decomposition temperature of the surface hydrophobic groups. The microstructure of the aerogels was observed by FESEM (S-4200, Hitachi, Japan). 3. Results and discussion The FESEM image of the aerogel prepared from the silica sol at a pH value of 4.5 is shown in Fig. 1. Spherical particles with a size of a few tens of nm form a 3-dimensional network containing homogeneous pores. Although not shown in this report, the microstructures of the aerogels prepared at different pH values were similar. The average diameter of the pores obtained by BET analysis was about 20 nm. The
2. Experimental procedure 2.1. Sample preparation Sodium silicate solution (water glass) was used as the precursor to prepare the silica sol. 144 ml of water glass (Young Il Chemical, Korea; silica content 28–30 wt.%, SiO2:Na2O = 3.52:1) was mixed with 525 ml of distilled water to make an 8 wt.% silicate solution. Then, the diluted silicate solution was passed through a column filled with an ion exchange resin (Amberite, IR-120H, H. Rohm and Hass Co.) at a rate of 30 ml per minute. The prepared silica sol had a pH range of 2.4 to 2.8. A base catalyst (ammonia) was used to form the wet gel. 3 ml of silica sol was poured into polypropylene molds having a diameter of 20 mm. The molds were placed in an oven at 50 °C after being completely sealed. The process of gelation took from 30 s to 6 h depending on the pH value of the sols. Then, the wet gels were washed in ethanol four times within 24 h at 50 °C. The washed wet gels were aged in xylene for 24 h (GR, 99%, Samchun Chemical, Korea) at 50 °C,
Fig. 2. FT-IR spectra of the aerogels prepared at various pH values.
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Fig. 3. FT-IR spectra of the aerogels heat-treated at various temperatures for 4 h. The aerogels were prepared from a silica sol with a pH value of 4.5.
BET surface area was about 700 to 750 m2/g. The highest volume ratio was observed for the pores with a diameter of 20 to 40 nm. As the pH value increased, the specific surface area was slightly decreased. Fig. 2 shows the FT-IR spectra of the aerogels prepared at various pH values. In Fig. 2, the absorption peaks near 2950 cm− 1 and 2900 cm− 1 are due to the terminal CH3 groups, and those at 1260 cm− 1 and 850 cm− 1 correspond to the Si–C bonding. The strong peak near 1090 cm− 1 shouldered at 1180 cm− 1 and the weak peak near 800 cm− 1 were assigned to the asymmetric and symmetric bending of the Si–O–Si bonds, respectively. The strong peak near 460 cm− 1 is attributable to the bending of the O–Si–O bonds. The spectra show almost the same patterns, irrespective of the pH value of the sol, which was varied between 3.0 and 5.0. Even though the gels are chemically identical, there are differences in their physical properties, such as their transmittance, density and thermal stability. When the aerogel was heat-treated at 400 °C, the IR spectra changed, as is evident in Fig. 3. It was found that the hydrophobic groups started to escape from the surface so that the surface became hydrophilic. In the spectra of the aerogel heat-treated at over 400 °C, the wide band near 3400 cm− 1 and the peaks at 1632 cm− 1 and 960 cm− 1 correspond to the hydroxyl groups adsorbed on the surface, the bending of the H–O–H bonds and the stretching of the Si–OH bonds, respectively [9–12].
Fig. 4. TG/DSC curves of the aerogel prepared from a silica sol with a pH value of 4.5.
Fig. 5. Transmittances of the silica aerogels in the visible light region. The aerogels were prepared at pH values of (a) 3.0, (b) 3.5, (c) 4.0, (d) 4.5 and (e) 5.0.
However, the aerogel heat-treated at 400 °C still has hydrophobic terminal groups. The aerogel heat-treated at 500 °C shows the same spectrum as the hydrophilic silica gel [13]. The TG/DSC curves of the aerogels prepared from the sol with a pH value of 4.5 are shown in Fig. 4. We observed a large exothermic peak in the DSC curve at around 400 °C. An abrupt weight loss was observed at the same temperature. From the results of the FT-IR spectra and TG/DSC curves, it can be inferred that hydroxyl groups are substituted for the hydrophobic methyl terminal groups in the silica network at around 400 °C. The change in the transmittances of the aerogels is also related to this reaction, as described below. The pH dependence of the transmittance of the aerogels with a thickness of 8 mm is shown in Figs. 5 and 6. Since the absorption in visible light is negligible in a silica aerogel, the observed decrease in the transmittance results from the scattering arising from the nanoporous aerogel network. As is evident in Fig. 5, the visible spectra of all of the samples demonstrate that with decreasing wavelength there is a decrease in transmittance. This is caused by the increase in the scattering intensity according to the Rayleigh relation [14]. For all pH values, the transmittances of the aerogels increased
Fig. 6. Transmittances of the silica aerogels prepared at a pH value of 4.5. The aerogels were heat-treated at different temperatures for 4 h: (a) as-prepared, (b) 200 °C, (c) 300 °C, (d) 400 °C and (e) 500 °C.
S. Lee et al. / Materials Letters 61 (2007) 3130–3133
with increasing pH over the entire wavelength range. The transmittance of the aerogel prepared at a pH value of 5.0 was about 80% at a wavelength of 800 nm, while it was only 45% for the aerogel prepared at a pH value of 3.0. This difference seems to emanate from variations in the microstructure, such as the morphological change of the pores in the aerogel network structure. Knoblich et al. and Emmerling et al. reported that at a given density of silica, the microstructure, especially the secondary particle (cluster) diameter, play an important role in determining the transmittance of the silica aerogel and that the concentration of the basic catalyst needs to be high to achieve an aerogel with high transmittance [15]. Similar results were observed in this study. On the other hand, most UV wavelengths were blocked by the aerogels, regardless of the pH value, so that over 99% of the waves were interrupted at a wavelength of around 300 nm. The transmittance also changed according to the modification of the surface chemical groups. The gels heat-treated at temperatures above 400 °C showed a transmittance of about 5–10% for the near-UV waves, whereas those heat-treated at less than 300 °C blocked over 99% of the near-UV waves. As is evident in Fig. 6, the transmittance gradually decreased as the heat-treatment temperature was increased up to 300 °C, abruptly increased at 400 °C, and remained constant thereafter. According to the FT-IR and TG/DSC results, the hydrophobic surface groups disappeared and the aerogel became hydrophilic at temperatures above 400 °C. Thus, the adsorption and desorption of the hydrophobic functional groups seems to be responsible for the optical behavior of the aerogels. Hydrophilic silica aerogels have better transparency than aerogels with methyl terminal groups. It is not clear why the transmittance increased when the heat-treatment was performed at temperatures above the hydrophobic to hydrophilic transition temperature. According to the results obtained by Tajiri et al. [16], it seems that the heat-treatment can modify the surface structure of the aerogel, thereby resulting in an increase in the transmittance, even though the microstructure such as the particle (cluster) and pore diameters are almost the same after the heat-treatment.
aerogels increases as the wavelength decreases, so that UV wavelengths are completely blocked. It seems that the silica particles and pore morphology affect the transparency of the aerogels. Considering the results of the FT-IR and DSC analyses, it can be concluded that heat-treatment at temperatures above 400 °C causes hydroxyl groups to be substituted for the methyl terminal groups. It was considered that the modification of the surface structure induced by the heat-treatment at temperatures above the hydrophobic to hydrophilic transition temperature of the aerogel might be related to the increase in the transparency of the aerogels. Acknowledgements This research was supported by a grant from the Energy Resources Technology Development Programs funded by the Korea Energy Management Corporation of the Ministry of Commerce, Industry and Energy of the Korean government. Portions of this work were supported by the Korea Research Foundation Grant (KRF-2005-202-D00243). References [1] [2] [3] [4] [5] [6] [7] [8] [9]
4. Conclusion Silica aerogels were successfully synthesized by surface chemical modification and ambient pressure drying for 5 days by solvent exchange with xylene. The reproducibility was significantly improved, which means that the ambient drying method can be used as a commercial process, as long as the problem concerning the long process time can be solved. From the results obtained in this study, it can be inferred that the amount of scattering caused by the nanoscale pores inside the
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[10] [11] [12] [13]
[14] [15] [16]
J. Fricke, J. Non-cryst. Solids 100 (1988) 169. J. Fricke, A. Emmerling, J. Am. Ceram. Soc. 75 (1992) 2027. G.S. Kim, S.H. Hyun, Ceramist 4 (2001) 5. C.E. Carraher Jr., Polym. News 30 (12) (2005) 386. J.M. Schultz, K.I. Jensen, F.H. Kristiansen, Sol. Energy Mat. Sol. Cells 89 (2005) 275. S.S. Kistler, J. Phys. Chem. 36 (1932) 52. G.A. Nicolaon, S.J. Teichner, Bull. Soc. Chim. Fr. 5 (1968) 1906. P.H. Tewari, A.J. Hunt, K.D. Lofftus, Springer Proc. Phys. 6 (1986) 31 (aerogels). J.G. Reynolds, P.R. Coronado, L.W. Hrubesh, J. Non-cryst. Solids 292 (2001) 127. A.A. Anappara, S. Rajeshkumar, P. Mukundan, P.R.S. Warrier, S. Ghosh, K.G.K. Warrier, Acta Mater. 52 (2004) 369. A.V. Rao, M.M. Kulkarni, D.P. Amalnerkar, T. Seth, J. Non-cryst. Solids 330 (2003) 187. A.V. Rao, E. Nilsen, M.-A. Einarsrud, J. Non-cryst. Solids 296 (2001) 165. R.A. Nyquist, R.O. Kagel, Handbook of Infrared and Raman Spectra of Inorganic Compounds and Organic Salts, vol. 4, Academic Press, Inc., San Diego, 1997, p. 94. C.G. Bohren, D.F. Huffmann, Absorption and Scattering of Light by Small Particles, Wiley, NewYork, 1983. B. Knoblich, Th. Gerber, J. Non-cryst. Solids 296 (2001) 81. K. Tajiri, K. Igarashi, T. Nishio, J. Non-cryst. Solids 186 (1995) 83.
The effect of pH on the physicochemical properties of silica aerogels prepared by an ambient pressure drying method Seunghun Lee a , Young Chul Cha d , Hae Jin Hwang a,⁎, Ji-Woong Moon b , In Sub Han c a
d
Department of Materials Science and Engineering, Inha University, Incheon, Republic of Korea b Korea Institute of Ceramic Engineering and Technology, Seoul, Republic of Korea c Korea Institute of Energy Research, Daejeon, Republic of Korea School of Materials Science and Engineering, Inha University, 253 Yonghyun-dong, Nam-gu, Incheon 402-751, Korea Received 3 July 2006; accepted 2 November 2006 Available online 20 November 2006
Abstract Hydrophobic silica aerogels were successfully synthesized by an ambient pressure drying method from various silicic acids with different pH values which were prepared from water glass. In this study, we selected xylene as the solvent and performed the surface modification in a TMCS (trimethylchlorosilane)/xylene solution, in order to improve the reproducibility of the aerogels. The densities of the aerogels were about 0.10– 0.14 g/cm3 and the apparent porosities were in the range of 94.5–96%, depending on the processing conditions. Their specific surface area was in the range of 700–750 m2/g and their average pore size was around 20 nm. The transparency of the as-dried aerogels was enhanced as the pH value was increased. The transmittance of the heat-treated aerogels was slightly decreased with increasing heat-treatment temperature. On the other hand, an abrupt increase in the transmittance was observed when the temperature exceeded 400 °C and this phenomenon might be related to the change in the surface structure caused by the desorption of the methyl groups. © 2006 Elsevier B.V. All rights reserved. Keywords: Aerogel; Porous silica; Ambient drying; Surface modification
1. Introduction Nanoporous silica aerogels are materials which have a high specific surface area, a high porosity (80–99.8%), a low density (∼ 0.003 g/cm3), a high thermal insulation value (0.005 W/m K), an ultra low dielectric constant (k' = 1.0–2.0) and a low index of refraction (∼1.05) [1–5]. Because of their unique properties, silica aerogels have been extensively studied, not only for use as transparent thermal insulators but also as inter-metal dielectric materials (IMDs), optical and acoustic applications, and the space industry. Since Kistler described the first synthesis of an aerogel by supercritical drying in the early 1930s [6], various aerogel synthesis processes have been reported. In the 1970s, a silica aerogel was synthesized by the high temperature supercritical drying of a wet gel produced by the hydrolysis of TMOS (tetramethoxysilane) in methanol [7]. In the 1980s, researchers gained a new ⁎ Corresponding author. Tel.: +82 32 860 7521; fax: +82 32 862 4482. E-mail address: [email protected] (H.J. Hwang). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.11.010
understanding of the potential of aerogels, and TEOS (tetraethoxysilane) based silica aerogels, whose synthesis was less expensive and used less toxic sources compared to TMOS based aerogels, were developed. The low temperature supercritical drying technique, which uses liquid carbon dioxide, was introduced at the same time [8]. Nowadays, a few companies are commercially producing silica aerogel beads, powder or blankets. In addition, the NASA spacecraft called Stardust used an aerogel to sample the materials obtained from a comet. Transparent silica aerogel tiles have also been used as regenerative thermal boards. Generally, aerogels are synthesized by the supercritical drying method, in which metal alkoxides are hydrolyzed and condensed to form a silica wet gel. However, supercritical drying has certain limitations in terms of its cost efficiency, process continuity and safety, because alkoxides are expensive and a high temperature and pressure are needed to approach the critical point. If liquid carbon dioxide is used as a solvent in the low temperature supercritical drying process, the chemical durability of the aerogels in the atmosphere would be gradually decreased, since the aerogel particles are hydrophilic. To
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with the xylene being replaced every 6 h. After aging, the samples were chemically modified in 5% TMCS/xylene solution for 12 h at 50 °C. Finally, the surface-modified wet gels were washed in xylene repeatedly for 24 h at 50 °C, in order to remove the remaining surface modification agents and reaction products such as hydrochloric acid. The gels were kept in a beaker and dried under atmospheric pressure. Since the boiling point of xylene is 139–141.2 °C, the drying process was performed at 142 °C. The dried aerogels were subsequently heat-treated at 200, 300, 400 and 500 °C for 4 h. In this study, the aerogel before the heat-treatment is designated as the “as-dried aerogel.” 2.2. Characterization of the aerogels Fig. 1. Typical microstructure of the silica aerogel prepared from a water glass based silica sol with subsequent ambient drying.
overcome these problems, Brinker introduced the ambient drying method. In this process, the hydroxyl groups of the wet gel surface, which can lead to shrinkage during the subsequent drying process at ambient pressure, are chemically substituted by hydrophobic functional groups. Because aerogels produced by the ambient drying method have hydrophobic surfaces, they can be used in the air for a long time without incurring any damage. In addition, since ambient drying can reduce the production cost of aerogels, their importance has changed from an area of purely scientific interests into one of practical usage. In this study, a novel ambient drying method is proposed for the purpose of enhancing the reproducibility of aerogel production, and water glass based silica aerogels with a porosity of around 95% were synthesized using this method. We focused our attention on the pH value of the silicic acids prepared from water glass and the xylene which is used as a solvent in the surface modification and subsequent ambient drying process. The physical/optical properties and chemical structures of the aerogels were investigated.
The bulk densities of the aerogels were calculated from their weight and volume, and the volumes were obtained from the diameters and thicknesses of the gels. FT-IR spectrometry (FTS165, Bio-Rad, USA) was used to confirm the chemical structure of the as-dried and heat-treated aerogels in the wave number range of 400 to 4000 cm− 1. The specific surface area of the aerogels was measured by BET (ASAP 2010, Micromeritics, USA). The transmittance of the aerogels was measured using UV–visible spectroscopy (UV-2401PC, Shimadzu, Japan) over the wavelength range of 300–800 nm. Thermal analysis of the as-dried gels was performed by TG/DSC (STA-409C, Netzsch, Germany) at a heating rate of 10 °C/min to determine the decomposition temperature of the surface hydrophobic groups. The microstructure of the aerogels was observed by FESEM (S-4200, Hitachi, Japan). 3. Results and discussion The FESEM image of the aerogel prepared from the silica sol at a pH value of 4.5 is shown in Fig. 1. Spherical particles with a size of a few tens of nm form a 3-dimensional network containing homogeneous pores. Although not shown in this report, the microstructures of the aerogels prepared at different pH values were similar. The average diameter of the pores obtained by BET analysis was about 20 nm. The
2. Experimental procedure 2.1. Sample preparation Sodium silicate solution (water glass) was used as the precursor to prepare the silica sol. 144 ml of water glass (Young Il Chemical, Korea; silica content 28–30 wt.%, SiO2:Na2O = 3.52:1) was mixed with 525 ml of distilled water to make an 8 wt.% silicate solution. Then, the diluted silicate solution was passed through a column filled with an ion exchange resin (Amberite, IR-120H, H. Rohm and Hass Co.) at a rate of 30 ml per minute. The prepared silica sol had a pH range of 2.4 to 2.8. A base catalyst (ammonia) was used to form the wet gel. 3 ml of silica sol was poured into polypropylene molds having a diameter of 20 mm. The molds were placed in an oven at 50 °C after being completely sealed. The process of gelation took from 30 s to 6 h depending on the pH value of the sols. Then, the wet gels were washed in ethanol four times within 24 h at 50 °C. The washed wet gels were aged in xylene for 24 h (GR, 99%, Samchun Chemical, Korea) at 50 °C,
Fig. 2. FT-IR spectra of the aerogels prepared at various pH values.
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Fig. 3. FT-IR spectra of the aerogels heat-treated at various temperatures for 4 h. The aerogels were prepared from a silica sol with a pH value of 4.5.
BET surface area was about 700 to 750 m2/g. The highest volume ratio was observed for the pores with a diameter of 20 to 40 nm. As the pH value increased, the specific surface area was slightly decreased. Fig. 2 shows the FT-IR spectra of the aerogels prepared at various pH values. In Fig. 2, the absorption peaks near 2950 cm− 1 and 2900 cm− 1 are due to the terminal CH3 groups, and those at 1260 cm− 1 and 850 cm− 1 correspond to the Si–C bonding. The strong peak near 1090 cm− 1 shouldered at 1180 cm− 1 and the weak peak near 800 cm− 1 were assigned to the asymmetric and symmetric bending of the Si–O–Si bonds, respectively. The strong peak near 460 cm− 1 is attributable to the bending of the O–Si–O bonds. The spectra show almost the same patterns, irrespective of the pH value of the sol, which was varied between 3.0 and 5.0. Even though the gels are chemically identical, there are differences in their physical properties, such as their transmittance, density and thermal stability. When the aerogel was heat-treated at 400 °C, the IR spectra changed, as is evident in Fig. 3. It was found that the hydrophobic groups started to escape from the surface so that the surface became hydrophilic. In the spectra of the aerogel heat-treated at over 400 °C, the wide band near 3400 cm− 1 and the peaks at 1632 cm− 1 and 960 cm− 1 correspond to the hydroxyl groups adsorbed on the surface, the bending of the H–O–H bonds and the stretching of the Si–OH bonds, respectively [9–12].
Fig. 4. TG/DSC curves of the aerogel prepared from a silica sol with a pH value of 4.5.
Fig. 5. Transmittances of the silica aerogels in the visible light region. The aerogels were prepared at pH values of (a) 3.0, (b) 3.5, (c) 4.0, (d) 4.5 and (e) 5.0.
However, the aerogel heat-treated at 400 °C still has hydrophobic terminal groups. The aerogel heat-treated at 500 °C shows the same spectrum as the hydrophilic silica gel [13]. The TG/DSC curves of the aerogels prepared from the sol with a pH value of 4.5 are shown in Fig. 4. We observed a large exothermic peak in the DSC curve at around 400 °C. An abrupt weight loss was observed at the same temperature. From the results of the FT-IR spectra and TG/DSC curves, it can be inferred that hydroxyl groups are substituted for the hydrophobic methyl terminal groups in the silica network at around 400 °C. The change in the transmittances of the aerogels is also related to this reaction, as described below. The pH dependence of the transmittance of the aerogels with a thickness of 8 mm is shown in Figs. 5 and 6. Since the absorption in visible light is negligible in a silica aerogel, the observed decrease in the transmittance results from the scattering arising from the nanoporous aerogel network. As is evident in Fig. 5, the visible spectra of all of the samples demonstrate that with decreasing wavelength there is a decrease in transmittance. This is caused by the increase in the scattering intensity according to the Rayleigh relation [14]. For all pH values, the transmittances of the aerogels increased
Fig. 6. Transmittances of the silica aerogels prepared at a pH value of 4.5. The aerogels were heat-treated at different temperatures for 4 h: (a) as-prepared, (b) 200 °C, (c) 300 °C, (d) 400 °C and (e) 500 °C.
S. Lee et al. / Materials Letters 61 (2007) 3130–3133
with increasing pH over the entire wavelength range. The transmittance of the aerogel prepared at a pH value of 5.0 was about 80% at a wavelength of 800 nm, while it was only 45% for the aerogel prepared at a pH value of 3.0. This difference seems to emanate from variations in the microstructure, such as the morphological change of the pores in the aerogel network structure. Knoblich et al. and Emmerling et al. reported that at a given density of silica, the microstructure, especially the secondary particle (cluster) diameter, play an important role in determining the transmittance of the silica aerogel and that the concentration of the basic catalyst needs to be high to achieve an aerogel with high transmittance [15]. Similar results were observed in this study. On the other hand, most UV wavelengths were blocked by the aerogels, regardless of the pH value, so that over 99% of the waves were interrupted at a wavelength of around 300 nm. The transmittance also changed according to the modification of the surface chemical groups. The gels heat-treated at temperatures above 400 °C showed a transmittance of about 5–10% for the near-UV waves, whereas those heat-treated at less than 300 °C blocked over 99% of the near-UV waves. As is evident in Fig. 6, the transmittance gradually decreased as the heat-treatment temperature was increased up to 300 °C, abruptly increased at 400 °C, and remained constant thereafter. According to the FT-IR and TG/DSC results, the hydrophobic surface groups disappeared and the aerogel became hydrophilic at temperatures above 400 °C. Thus, the adsorption and desorption of the hydrophobic functional groups seems to be responsible for the optical behavior of the aerogels. Hydrophilic silica aerogels have better transparency than aerogels with methyl terminal groups. It is not clear why the transmittance increased when the heat-treatment was performed at temperatures above the hydrophobic to hydrophilic transition temperature. According to the results obtained by Tajiri et al. [16], it seems that the heat-treatment can modify the surface structure of the aerogel, thereby resulting in an increase in the transmittance, even though the microstructure such as the particle (cluster) and pore diameters are almost the same after the heat-treatment.
aerogels increases as the wavelength decreases, so that UV wavelengths are completely blocked. It seems that the silica particles and pore morphology affect the transparency of the aerogels. Considering the results of the FT-IR and DSC analyses, it can be concluded that heat-treatment at temperatures above 400 °C causes hydroxyl groups to be substituted for the methyl terminal groups. It was considered that the modification of the surface structure induced by the heat-treatment at temperatures above the hydrophobic to hydrophilic transition temperature of the aerogel might be related to the increase in the transparency of the aerogels. Acknowledgements This research was supported by a grant from the Energy Resources Technology Development Programs funded by the Korea Energy Management Corporation of the Ministry of Commerce, Industry and Energy of the Korean government. Portions of this work were supported by the Korea Research Foundation Grant (KRF-2005-202-D00243). References [1] [2] [3] [4] [5] [6] [7] [8] [9]
4. Conclusion Silica aerogels were successfully synthesized by surface chemical modification and ambient pressure drying for 5 days by solvent exchange with xylene. The reproducibility was significantly improved, which means that the ambient drying method can be used as a commercial process, as long as the problem concerning the long process time can be solved. From the results obtained in this study, it can be inferred that the amount of scattering caused by the nanoscale pores inside the
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[10] [11] [12] [13]
[14] [15] [16]
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