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Synthesis and characterization of silica aerogels by a novel fast ambient pressure drying process
Materials Letters 60 (2006) 3718 – 3722 www.elsevier.com/locate/matlet
Synthesis and characterization of silica aerogels by a novel fast ambient pressure drying process Fei Shi a,b,⁎, Lijiu Wang a , Jingxiao Liu b a
b
Building Materials Research Lab, Dalian University of Technology, Dalian 116024, China Department of Materials Science and Engineering, Dalian Institute of Light Industry, Dalian 116034, China Received 30 March 2005; accepted 26 March 2006 Available online 21 April 2006
Abstract Using cheap waterglass as silica source, silica aerogels were synthesized via a novel fast ambient drying by using an ethanol/ trimethylchlorosilane (TMCS)/Heptane solution for modification of the wet gel. One-step solvent exchange and surface modification were simultaneously progressed by immersing the hydrogel in EtOH/TMCS/Heptane solution, in which TMCS reacting with pore water and Si–OH group on the surface of the gel, with ethanol and heptane helping to decrease the rate of TMCS reacting with pore water and extrude water from gel pores. The synthesized silica aerogel was a light and crack-free solid, with the density of 0.128–0.136 g/cm3 and 93.8–94.2% porosity. The microstructure, morphology and properties of the aerogels were studied by FTIR, SEM, TEM and BET measurement. The results indicate that silica aerogels exhibit a sponge structure with uniform nano-particle and pores size distribution. The specific surface areas of silica aerogels are 559–618 m2/g. And there is an obvious Si–CH3 group on the surface of the silica aerogel. © 2006 Elsevier B.V. All rights reserved. Keywords: Waterglass; Ambient pressure drying; Silica aerogel; One-step solvent exchange/surface modification
1. Introduction Silica aerogels are unique porous materials consisting of more than 90% air and less than 10% solid silica in the form of highly cross-linked network structure. Because of their unique properties, i.e., large surface area, very low bulk density and very low thermal conductivity, silica aerogels have received significant attention in many fields such as catalysis [1,2], adsorption [3], thermal insulation [4–6] and drug delivery system [7] etc. Conventionally silica aerogels are prepared by supercritical drying of wet gels and usually using expensive TEOS (tetraethylorthosilicate) as silica source. Supercritical drying process can avoid capillary stress and associated drying shrinkage, which are usually prerequisite of obtaining aerogels structure. However, supercritical drying process is so energy intensive and dangerous that real practice and commercialization ⁎ Corresponding author. Department of Materials Science and Engineering, Dalian Institute of Light Industry, Dalian 116034, China. Tel.: +86 411 86323691 209. E-mail address: [email protected] (F. Shi). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.03.095
are difficult. So it is very necessary to synthesize silica aerogels by an ambient pressure drying technique at a reasonable cost. At present, the main methods adopted for ambient pressure drying include network strengthen [8] and solvent exchange/ surface modification [9,10] of wet gels. In order to solve problems of multi-step solvent exchange needing a very long diffusion time and consuming too many solvents, Schwertferger et al. [10] developed a process in which one-step solvent exchange and surface modification were simultaneously progressed using HMDSO (hexamethyldisiloxane)/TMCS solution for modification of the wet gels. Lately, Kim and Hyun [11] also synthesized silica aerogels via one-step solvent exchange/surface modification of wet gels using IPA (isopropyl alcohol)/TMCS/n-Hexane solution. However, few reports were found about other routes to synthesize silica aerogels via onestep solvent exchange/surface modification of wet gels and ambient pressure drying. In this study, we report upon our efforts to synthesize silica aerogels from cheap waterglass precursors by using ethanol (EtOH)/trimethylchlorosilane (TMCS)/Heptane solution for modification of wet gels via a novel fast ambient drying
F. Shi et al. / Materials Letters 60 (2006) 3718–3722
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route. It was found that ethanol and heptane play an important role in obtaining monolithic and uniform silica aerogels. The pore water extrusion (solvent exchange) and surface modification mechanism of wet gels were discussed. This work may be expected to be favorable for real practice of silica aerogels in some fields such as thermal insulation, catalysis and drug delivery system etc. 2. Experimental 2.1. Preparation of silica hydrogels The silica hydrogels were prepared using waterglass (Be° = 40, Na2O:SiO2 molar ratio = 1:3.3) as starting materials. First the waterglass was diluted with deionized water (waterglass: deionized water (volume ratio) = 1:4) and ion exchanged with Amberlite (Strongly Acidic Styrene Type Cation Exchange Resin). The collected silica sol had a pH in the range of 2–3. For gelation, 1.0 N aqueous NaOH solution was used to modify the pH of silica sol. The obtained silica sols at pH = 5 were stirred for 1 min, and then transferred into plastic boxes (30 mm in diameter). Then the sols were aged until gelation occurred. 2.2. Solvent exchange/surface modification and drying of wet gels After gelation, the hydrogels were immersed into 50 vol% H2O/ethanol solution and aged for 24 h at room temperature so as to strengthen the gel network. Then EtOH/TMCS/Heptane solution was added to the aged gels. After the reaction between the wet gel and EtOH/TMCS/Heptane solution were complete and pore water were extruded (exchanged by heptane) thoroughly, the modified wet gels were aged again for 24 h and then were dried at 60, 80, 120 and 180 °C for 2 h respectively in the oven. 2.3. Characterization of aerogels The thermal behavior of the dried gel was examined using differential thermal analysis (TG/SDTA 851). Apparent density was calculated via formula of ρ = m / V by weighting samples of known dimensions. The microstructure and morphology of silica aerogels were observed by scanning electron microscopy (SEM, JEOL JSM-6460LV) and transmission electron microscopy (TEM, Philips Tecnai G20). The specific surface area and pore size distribution of aerogels were determined by Brunauer–Emmitt–Teller (BET) method (Beckman Coulter Sorption Analysis Instrument). Fourier transform infrared spectroscopy (FTIR, spectrum one-B FTIR) was employed to investigate the chemical bonding state of aerogels.
Fig. 1. SEM of SiO2 aerogels synthesized under different process parameters (A) EtOH/TMCS molar ratio = 1:1; (B) EtOH/TMCS molar ratio = 2:3.
synthesized silica aerogels exhibit porous network structure which contains 30–60 nm spherical solid clusters and pores below 100 nm between them, and the particle distribution of aerogels are uniform for using EtOH/TMCS (molar ratio) = 1:1 as the modification solution. TEM micrographs of the obtained aerogels are shown in Fig. 2. It can be observed that silica aerogels exhibit a sponge-like microstructure. The pores had a size in the range of 5–20 nm. For aerogels obtained using EtOH/TMCS (molar ratio) = 1:1, pore size distribution and network structure are more uniform than that of aerogels using EtOH/TMCS (molar ratio) = 2:3. The main reason for this phenomenon is that reaction between TMCS and pore water is very rapid, and the reaction between TMCS and EtOH can slower the reaction of TMCS with pore water. So using EtOH/TMCS (molar ratio) = 1:1 as modification for the wet gel is more favorable for obtaining uniform monolithic silica aerogels. Fig. 3 shows the nitrogen adsorption–desorption isotherm of silica aerogels. Both aerogels using different EtOH/TMCS molar ratios exhibit type-IV adsorption isotherms, which are considered to indicate the presence of mesopores. Obviously, the adsorption isotherms are very similar to that of the supercritical drying aerogels [12].
3. Results and discussion 3.2. Properties of silica aerogels 3.1. Morphology and microstructure of silica aerogels Fig. 1 shows the SEM morphology of aerogels obtained by using EtOH/TMCS/Heptane solution for modification of the wet gels. The
Table 1 shows the properties of aerogels synthesized by using EtOH/TMCS/Heptane solution with different EtOH/TMCS molar ratio as the modification solution. It can be seen that specific surface area of
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corresponding to –CH3 terminal groups are quite visible which are attributed to surface modification of wet gel by TMCS [15,16]. In this study, the obtained silica aerogels by TMCS surface modification are very hydrophobic, which is favorable for keeping its outstanding properties such as low density and thermal conductivity, and hydrophobicity of silica aerogel also facilitates its utility in composites with polymer. 3.3. Mechanism of one-step solvent exchange/surface modification In order to obtain high porous aerogel structure, elimination of capillary stress during drying is very important [17]. In this study, the elimination of capillary stress was performed by pore water solvent exchange and surface modification of wet gel before ambient drying. When adding ethanol, heptane and TMCS to the wet gel, reaction between the wet gel and EtOH/TMCS/Heptane solution occurred. It should be clear that TMCS would react with ethanol, pore water and Si–OH group in the wet gel. Practically, the reaction between TMCS and ethanol is helpful for decreasing the reacting rate of TMCS with pore water, which is favorable for obtaining crack-free aerogels. During the reaction process of EtOH/TMCS/ Heptane solution with wet gel, a similar phenomena as illustrated in the literature [10] can be observed: transparent yellow liquid (the aqueous HCl phase) coming out from the wet gel and staying under the heptane phase, while the resulting modified wet gels are floated on the top of the newly built solution of aqueous HCl phase which mainly consists of pore water extruded from the wet gels and the newly built HCl. So it is believed that one-step solvent exchange
Fig. 2. TEM photograph of SiO2 aerogel synthesized under different process parameters (A) EtOH/TMCS molar ratio = 1:1; (B) EtOH/TMCS molar ratio = 2:3.
silica aerogels is in the range of 559–618 m2/g. The porosity of silica aerogels is larger than 93% and the density is lower than 0.14 g/cm2. In addition, aerogels with lower density and larger specific surface area can be obtained by using EtOH/TMCS molar ratio = 2:3, while more monolithic aerogels can be obtained by using EtOH/TMCS molar ratio = 1:1 for modification of the wet gel. On the premise of pore water exchange and surface modification completely, the amount of TMCS should be controlled to be low as possible so as to obtain monolithic aerogels. Fig. 4 illustrates TG/DTA curve of 180 °C-dried silica aerogel that experienced modification by EtOH/TMCS/Heptane solution. It is believed that exothermic peak and weight loss around 260 °C are attributed to oxidation of –CH3 developed from modification [13], which indicates that hydrophobicity of silica aerogels can be maintained up to about 260 °C. And the exothermic peaks and weight losses around 570 and 750 °C are due to phase transition of silica respectively. FTIR spectra of silica aerogel that experienced surface modification by EtOH/TMCS/Heptane solution are indicated in Fig. 5. The peaks at 3435 and 1630 cm− 1 correspond to the O–H absorption band, which is caused by physically adsorbed water. The absorption peaks near 1091 and 463 cm− 1 are due to Si–O–Si vibrations, which will appear in any silica products [14]. It is obvious that apart from Si–O–Si and O–H absorption peaks, the absorption peaks at 2963, 1256 and 846 cm− 1
Fig. 3. Nitrogen adsorption–desorption isotherm for the SiO2 aerogel obtained by using EtOH/TMCS/Heptane as surface modification solution (A) EtOH/ TMCS molar ratio = 1:1; (B) EtOH/TMCS molar ratio = 2:3.
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Table 1 Properties of SiO2 aerogel obtained via ambient pressure drying by using EtOH/TMCS/Heptane solution for modification of wet gel Modification solution
Density/(g·cm− 3) a
Porosity/% a
Surface area/(m2 g− 1) b
Monolithic construction
EtOH/TMCS/Heptane (EtOH/TMCS molar ratio = 1:1) EtOH/TMCS/Heptane (EtOH/TMCS molar ratio = 2:3)
0.136
93.8
559.4
Monolithic
0.128
94.2
618.8
A few small cracks
a b
m The density was calculated from q ¼ ; Porosity(%)=100% 1− q q (ρSiO2 = 2.19 g/cm3). SiO 2 V Obtained from BET absorption measurement.
and surface modification can be simultaneously accomplished during the reaction process. The following seven reactions occurred during the solvent exchange/surface modification of wet gel. They could explain the phenomena mentioned above: 2ðCH3 Þ3 Si–ClðTMCSÞ þ H2 OðporewaterÞ →ðCH3 Þ3 –Si–O–Si–ðCH3 Þ3 ðHMDSOÞ þ 2HCl
ð1Þ
ðCH3 Þ3 Si–Cl þ CH3 CH2 OHðEtOHÞ→ðCH3 Þ3 Si–O–CH2 CH3 þ HCl ð2Þ 2ðCH3 Þ3 Si–O–CH2 CH3 þ H2 OðporewaterÞ →ðCH3 Þ3 –Si–O–Si–ðCH3 Þ3 þ 2CH3 CH2 OH
ð3Þ
ðCH3 Þ3 –Si–O–Si–ðCH3 Þ3 þ 2HCl→2ðCH3 Þ3 Si–Cl þ H2 O
ð4Þ
ðCH3 Þ3 Si–O–CH2 CH3 þ HCl→ðCH3 Þ3 Si–Cl þ CH3 CH2 OH
ð5Þ
ðCH3 Þ3 Si–Cl þ ≡Si–OH→≡Si–O–SiðCH3 Þ3 þ HCl
ð6Þ
ðCH3 Þ3 Si–O–CH2 CH3þ≡Si–OH→≡Si–O–SiðCH3 Þ3þCH3 CH2 OH: ð7Þ Because the reaction between the TMCS and pore water in the wet gel was so rapid, it would easily cause the gel to crack. Thus controlling the reacting rate of the TMCS with the pore water is very significant. In the experiment, when adding ethanol and heptane to the wet gel firstly, cracks were prohibited effectively. In fact, during the solvent exchange/ surface modification process, ethanol and heptane help to decrease the reacting rate of TMCS with pore water and extrude (or exchanged with) pore water from the wet gels. Due to the low surface tension of heptane, capillary stress was eliminated and associated drying shrinkage and cracking was avoided. If the operation of adding ethanol, heptane
Fig. 4. TG/DTA curve of 180 °C-dried silica aerogel that experienced modification by EtOH/TMCS/Heptane (EtOH/TMCS molar ratio = 2:3).
solvent and TMCS modification reagent is appropriate, then monolithic silica aerogels with low density could be obtained.
4. Conclusion From our research on the synthesis of silica aerogels via ambient drying by using EtOH/TMCS/Heptane solution for modification of wet gel, the following conclusions can be drawn: 1. Monolithic and hydrophobic silica aerogels were successfully prepared from waterglass via ambient drying by modifying wet gel using EtOH/TMCS/Heptane solution. The obtained silica aerogels showed properties of 0.128– 0.136 g/cm3 density, 93.8–94.2% porosity and 559–618 m2/ g specific surface area. 2. One-step solvent exchange and surface modification of wet silica gels were simultaneously accomplished during the reaction of EtOH/TMCS/Heptane solution with the wet gels. After the hydrogels aged in EtOH/water solution, first adding ethanol and heptane to the wet gels is favorable for obtaining crack-free silica aerogels. 3. The obtained aerogels using EtOH/TMCS/Heptane with EtOH/TMCS molar ratio = 1:1 as a modification solution were more monolithic and microstructure showed more uniform particle and pore size distribution. By contrast, aerogels with lower density and larger surface area were obtained by using EtOH/TMCS molar ratio = 2:3 for modification of the wet gels.
Fig. 5. FTIR spectra of 180 °C-dried silica aerogel modified by EtOH/TMCS/ Heptane solution with different EtOH/TMCS molar ratio (r); (a) r = 1:1, (b) r = 2:3.
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Acknowledgement The authors gratefully acknowledged the financial support from Liaoning Province Ministry of Education Grant (2020701096). References [1] C.A. Muller, M. Maciejewski, T. Mallat, A. Baiker, J. Catal. 184 (1999) 280. [2] S. Maury, P. Buisson, A. Perrard, A.C. Pierre, J. Mol. Catal., B Enzym. 29 (2004) 133. [3] L.W. Hrubesh, P.R. Coronado Jr., J.H. Satcher, J. Non-Cryst. Solids 285 (2001) 328. [4] G.S. Kim, S.H. Hyun, J. Non-Cryst. Solids 320 (2003) 125. [5] W.C. Ackerman, M. Vlachos, S.R. Rouanet, J. Fruendt, J. Non-Cryst. Solids 285 (2001) 264.
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
K. Kamiuto, T. Miyamoto, S. Saitoh, Appl. Energy 62 (1999) 113. I. Smirnova, A.W. Suttiruengwong, J. Non-Cryst. Solids 350 (2004) 54. M.A. Einarsrud, E. Nilsen, J. Non-Cryst. Solids 226 (1998) 122. A. Venkateswara Rao, E. Nilsen, M.A. Einarsrud, J. Non-Cryst. Solids 296 (2001) 165. F. Schwertfeger, D. Frank, M. Schmidt, J. Non-Cryst. Solids 225 (1998) 24. G.S. Kim, S.H. Hyun, J. Mater. Sci. 38 (2003) 1961. J.H. Lee, S.Y. Choi, C.E. Kim, G.D. Kim, J. Mater. Sci. 32 (1997) 3577. Y.G. Kwon, S.Y. Choi, E.S. Kang, S.S. Baek, J. Mater. Sci. 35 (2000) 6075. F. Shi, L.J. Wang, J.X. Liu, S.H. Li, New Building Materials, vol. 6, 2004, p. 19, (in Chinese). A.Y. Jeong, S.M. Goo, D.P. Kim, J. Sol–Gel Sci. Technol. 19 (2000) 483. A. Venkateswara Rao, M.M. Kulkarni, D.P. Amalnerkar, T. Seth, Appl. Surf. Sci. 206 (2003) 262. V.D. Land, T.M. Harris, D.C. Teeters, J. Non-Cryst. Solids 283 (2001) 11.
Synthesis and characterization of silica aerogels by a novel fast ambient pressure drying process Fei Shi a,b,⁎, Lijiu Wang a , Jingxiao Liu b a
b
Building Materials Research Lab, Dalian University of Technology, Dalian 116024, China Department of Materials Science and Engineering, Dalian Institute of Light Industry, Dalian 116034, China Received 30 March 2005; accepted 26 March 2006 Available online 21 April 2006
Abstract Using cheap waterglass as silica source, silica aerogels were synthesized via a novel fast ambient drying by using an ethanol/ trimethylchlorosilane (TMCS)/Heptane solution for modification of the wet gel. One-step solvent exchange and surface modification were simultaneously progressed by immersing the hydrogel in EtOH/TMCS/Heptane solution, in which TMCS reacting with pore water and Si–OH group on the surface of the gel, with ethanol and heptane helping to decrease the rate of TMCS reacting with pore water and extrude water from gel pores. The synthesized silica aerogel was a light and crack-free solid, with the density of 0.128–0.136 g/cm3 and 93.8–94.2% porosity. The microstructure, morphology and properties of the aerogels were studied by FTIR, SEM, TEM and BET measurement. The results indicate that silica aerogels exhibit a sponge structure with uniform nano-particle and pores size distribution. The specific surface areas of silica aerogels are 559–618 m2/g. And there is an obvious Si–CH3 group on the surface of the silica aerogel. © 2006 Elsevier B.V. All rights reserved. Keywords: Waterglass; Ambient pressure drying; Silica aerogel; One-step solvent exchange/surface modification
1. Introduction Silica aerogels are unique porous materials consisting of more than 90% air and less than 10% solid silica in the form of highly cross-linked network structure. Because of their unique properties, i.e., large surface area, very low bulk density and very low thermal conductivity, silica aerogels have received significant attention in many fields such as catalysis [1,2], adsorption [3], thermal insulation [4–6] and drug delivery system [7] etc. Conventionally silica aerogels are prepared by supercritical drying of wet gels and usually using expensive TEOS (tetraethylorthosilicate) as silica source. Supercritical drying process can avoid capillary stress and associated drying shrinkage, which are usually prerequisite of obtaining aerogels structure. However, supercritical drying process is so energy intensive and dangerous that real practice and commercialization ⁎ Corresponding author. Department of Materials Science and Engineering, Dalian Institute of Light Industry, Dalian 116034, China. Tel.: +86 411 86323691 209. E-mail address: [email protected] (F. Shi). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.03.095
are difficult. So it is very necessary to synthesize silica aerogels by an ambient pressure drying technique at a reasonable cost. At present, the main methods adopted for ambient pressure drying include network strengthen [8] and solvent exchange/ surface modification [9,10] of wet gels. In order to solve problems of multi-step solvent exchange needing a very long diffusion time and consuming too many solvents, Schwertferger et al. [10] developed a process in which one-step solvent exchange and surface modification were simultaneously progressed using HMDSO (hexamethyldisiloxane)/TMCS solution for modification of the wet gels. Lately, Kim and Hyun [11] also synthesized silica aerogels via one-step solvent exchange/surface modification of wet gels using IPA (isopropyl alcohol)/TMCS/n-Hexane solution. However, few reports were found about other routes to synthesize silica aerogels via onestep solvent exchange/surface modification of wet gels and ambient pressure drying. In this study, we report upon our efforts to synthesize silica aerogels from cheap waterglass precursors by using ethanol (EtOH)/trimethylchlorosilane (TMCS)/Heptane solution for modification of wet gels via a novel fast ambient drying
F. Shi et al. / Materials Letters 60 (2006) 3718–3722
3719
route. It was found that ethanol and heptane play an important role in obtaining monolithic and uniform silica aerogels. The pore water extrusion (solvent exchange) and surface modification mechanism of wet gels were discussed. This work may be expected to be favorable for real practice of silica aerogels in some fields such as thermal insulation, catalysis and drug delivery system etc. 2. Experimental 2.1. Preparation of silica hydrogels The silica hydrogels were prepared using waterglass (Be° = 40, Na2O:SiO2 molar ratio = 1:3.3) as starting materials. First the waterglass was diluted with deionized water (waterglass: deionized water (volume ratio) = 1:4) and ion exchanged with Amberlite (Strongly Acidic Styrene Type Cation Exchange Resin). The collected silica sol had a pH in the range of 2–3. For gelation, 1.0 N aqueous NaOH solution was used to modify the pH of silica sol. The obtained silica sols at pH = 5 were stirred for 1 min, and then transferred into plastic boxes (30 mm in diameter). Then the sols were aged until gelation occurred. 2.2. Solvent exchange/surface modification and drying of wet gels After gelation, the hydrogels were immersed into 50 vol% H2O/ethanol solution and aged for 24 h at room temperature so as to strengthen the gel network. Then EtOH/TMCS/Heptane solution was added to the aged gels. After the reaction between the wet gel and EtOH/TMCS/Heptane solution were complete and pore water were extruded (exchanged by heptane) thoroughly, the modified wet gels were aged again for 24 h and then were dried at 60, 80, 120 and 180 °C for 2 h respectively in the oven. 2.3. Characterization of aerogels The thermal behavior of the dried gel was examined using differential thermal analysis (TG/SDTA 851). Apparent density was calculated via formula of ρ = m / V by weighting samples of known dimensions. The microstructure and morphology of silica aerogels were observed by scanning electron microscopy (SEM, JEOL JSM-6460LV) and transmission electron microscopy (TEM, Philips Tecnai G20). The specific surface area and pore size distribution of aerogels were determined by Brunauer–Emmitt–Teller (BET) method (Beckman Coulter Sorption Analysis Instrument). Fourier transform infrared spectroscopy (FTIR, spectrum one-B FTIR) was employed to investigate the chemical bonding state of aerogels.
Fig. 1. SEM of SiO2 aerogels synthesized under different process parameters (A) EtOH/TMCS molar ratio = 1:1; (B) EtOH/TMCS molar ratio = 2:3.
synthesized silica aerogels exhibit porous network structure which contains 30–60 nm spherical solid clusters and pores below 100 nm between them, and the particle distribution of aerogels are uniform for using EtOH/TMCS (molar ratio) = 1:1 as the modification solution. TEM micrographs of the obtained aerogels are shown in Fig. 2. It can be observed that silica aerogels exhibit a sponge-like microstructure. The pores had a size in the range of 5–20 nm. For aerogels obtained using EtOH/TMCS (molar ratio) = 1:1, pore size distribution and network structure are more uniform than that of aerogels using EtOH/TMCS (molar ratio) = 2:3. The main reason for this phenomenon is that reaction between TMCS and pore water is very rapid, and the reaction between TMCS and EtOH can slower the reaction of TMCS with pore water. So using EtOH/TMCS (molar ratio) = 1:1 as modification for the wet gel is more favorable for obtaining uniform monolithic silica aerogels. Fig. 3 shows the nitrogen adsorption–desorption isotherm of silica aerogels. Both aerogels using different EtOH/TMCS molar ratios exhibit type-IV adsorption isotherms, which are considered to indicate the presence of mesopores. Obviously, the adsorption isotherms are very similar to that of the supercritical drying aerogels [12].
3. Results and discussion 3.2. Properties of silica aerogels 3.1. Morphology and microstructure of silica aerogels Fig. 1 shows the SEM morphology of aerogels obtained by using EtOH/TMCS/Heptane solution for modification of the wet gels. The
Table 1 shows the properties of aerogels synthesized by using EtOH/TMCS/Heptane solution with different EtOH/TMCS molar ratio as the modification solution. It can be seen that specific surface area of
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corresponding to –CH3 terminal groups are quite visible which are attributed to surface modification of wet gel by TMCS [15,16]. In this study, the obtained silica aerogels by TMCS surface modification are very hydrophobic, which is favorable for keeping its outstanding properties such as low density and thermal conductivity, and hydrophobicity of silica aerogel also facilitates its utility in composites with polymer. 3.3. Mechanism of one-step solvent exchange/surface modification In order to obtain high porous aerogel structure, elimination of capillary stress during drying is very important [17]. In this study, the elimination of capillary stress was performed by pore water solvent exchange and surface modification of wet gel before ambient drying. When adding ethanol, heptane and TMCS to the wet gel, reaction between the wet gel and EtOH/TMCS/Heptane solution occurred. It should be clear that TMCS would react with ethanol, pore water and Si–OH group in the wet gel. Practically, the reaction between TMCS and ethanol is helpful for decreasing the reacting rate of TMCS with pore water, which is favorable for obtaining crack-free aerogels. During the reaction process of EtOH/TMCS/ Heptane solution with wet gel, a similar phenomena as illustrated in the literature [10] can be observed: transparent yellow liquid (the aqueous HCl phase) coming out from the wet gel and staying under the heptane phase, while the resulting modified wet gels are floated on the top of the newly built solution of aqueous HCl phase which mainly consists of pore water extruded from the wet gels and the newly built HCl. So it is believed that one-step solvent exchange
Fig. 2. TEM photograph of SiO2 aerogel synthesized under different process parameters (A) EtOH/TMCS molar ratio = 1:1; (B) EtOH/TMCS molar ratio = 2:3.
silica aerogels is in the range of 559–618 m2/g. The porosity of silica aerogels is larger than 93% and the density is lower than 0.14 g/cm2. In addition, aerogels with lower density and larger specific surface area can be obtained by using EtOH/TMCS molar ratio = 2:3, while more monolithic aerogels can be obtained by using EtOH/TMCS molar ratio = 1:1 for modification of the wet gel. On the premise of pore water exchange and surface modification completely, the amount of TMCS should be controlled to be low as possible so as to obtain monolithic aerogels. Fig. 4 illustrates TG/DTA curve of 180 °C-dried silica aerogel that experienced modification by EtOH/TMCS/Heptane solution. It is believed that exothermic peak and weight loss around 260 °C are attributed to oxidation of –CH3 developed from modification [13], which indicates that hydrophobicity of silica aerogels can be maintained up to about 260 °C. And the exothermic peaks and weight losses around 570 and 750 °C are due to phase transition of silica respectively. FTIR spectra of silica aerogel that experienced surface modification by EtOH/TMCS/Heptane solution are indicated in Fig. 5. The peaks at 3435 and 1630 cm− 1 correspond to the O–H absorption band, which is caused by physically adsorbed water. The absorption peaks near 1091 and 463 cm− 1 are due to Si–O–Si vibrations, which will appear in any silica products [14]. It is obvious that apart from Si–O–Si and O–H absorption peaks, the absorption peaks at 2963, 1256 and 846 cm− 1
Fig. 3. Nitrogen adsorption–desorption isotherm for the SiO2 aerogel obtained by using EtOH/TMCS/Heptane as surface modification solution (A) EtOH/ TMCS molar ratio = 1:1; (B) EtOH/TMCS molar ratio = 2:3.
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Table 1 Properties of SiO2 aerogel obtained via ambient pressure drying by using EtOH/TMCS/Heptane solution for modification of wet gel Modification solution
Density/(g·cm− 3) a
Porosity/% a
Surface area/(m2 g− 1) b
Monolithic construction
EtOH/TMCS/Heptane (EtOH/TMCS molar ratio = 1:1) EtOH/TMCS/Heptane (EtOH/TMCS molar ratio = 2:3)
0.136
93.8
559.4
Monolithic
0.128
94.2
618.8
A few small cracks
a b
m The density was calculated from q ¼ ; Porosity(%)=100% 1− q q (ρSiO2 = 2.19 g/cm3). SiO 2 V Obtained from BET absorption measurement.
and surface modification can be simultaneously accomplished during the reaction process. The following seven reactions occurred during the solvent exchange/surface modification of wet gel. They could explain the phenomena mentioned above: 2ðCH3 Þ3 Si–ClðTMCSÞ þ H2 OðporewaterÞ →ðCH3 Þ3 –Si–O–Si–ðCH3 Þ3 ðHMDSOÞ þ 2HCl
ð1Þ
ðCH3 Þ3 Si–Cl þ CH3 CH2 OHðEtOHÞ→ðCH3 Þ3 Si–O–CH2 CH3 þ HCl ð2Þ 2ðCH3 Þ3 Si–O–CH2 CH3 þ H2 OðporewaterÞ →ðCH3 Þ3 –Si–O–Si–ðCH3 Þ3 þ 2CH3 CH2 OH
ð3Þ
ðCH3 Þ3 –Si–O–Si–ðCH3 Þ3 þ 2HCl→2ðCH3 Þ3 Si–Cl þ H2 O
ð4Þ
ðCH3 Þ3 Si–O–CH2 CH3 þ HCl→ðCH3 Þ3 Si–Cl þ CH3 CH2 OH
ð5Þ
ðCH3 Þ3 Si–Cl þ ≡Si–OH→≡Si–O–SiðCH3 Þ3 þ HCl
ð6Þ
ðCH3 Þ3 Si–O–CH2 CH3þ≡Si–OH→≡Si–O–SiðCH3 Þ3þCH3 CH2 OH: ð7Þ Because the reaction between the TMCS and pore water in the wet gel was so rapid, it would easily cause the gel to crack. Thus controlling the reacting rate of the TMCS with the pore water is very significant. In the experiment, when adding ethanol and heptane to the wet gel firstly, cracks were prohibited effectively. In fact, during the solvent exchange/ surface modification process, ethanol and heptane help to decrease the reacting rate of TMCS with pore water and extrude (or exchanged with) pore water from the wet gels. Due to the low surface tension of heptane, capillary stress was eliminated and associated drying shrinkage and cracking was avoided. If the operation of adding ethanol, heptane
Fig. 4. TG/DTA curve of 180 °C-dried silica aerogel that experienced modification by EtOH/TMCS/Heptane (EtOH/TMCS molar ratio = 2:3).
solvent and TMCS modification reagent is appropriate, then monolithic silica aerogels with low density could be obtained.
4. Conclusion From our research on the synthesis of silica aerogels via ambient drying by using EtOH/TMCS/Heptane solution for modification of wet gel, the following conclusions can be drawn: 1. Monolithic and hydrophobic silica aerogels were successfully prepared from waterglass via ambient drying by modifying wet gel using EtOH/TMCS/Heptane solution. The obtained silica aerogels showed properties of 0.128– 0.136 g/cm3 density, 93.8–94.2% porosity and 559–618 m2/ g specific surface area. 2. One-step solvent exchange and surface modification of wet silica gels were simultaneously accomplished during the reaction of EtOH/TMCS/Heptane solution with the wet gels. After the hydrogels aged in EtOH/water solution, first adding ethanol and heptane to the wet gels is favorable for obtaining crack-free silica aerogels. 3. The obtained aerogels using EtOH/TMCS/Heptane with EtOH/TMCS molar ratio = 1:1 as a modification solution were more monolithic and microstructure showed more uniform particle and pore size distribution. By contrast, aerogels with lower density and larger surface area were obtained by using EtOH/TMCS molar ratio = 2:3 for modification of the wet gels.
Fig. 5. FTIR spectra of 180 °C-dried silica aerogel modified by EtOH/TMCS/ Heptane solution with different EtOH/TMCS molar ratio (r); (a) r = 1:1, (b) r = 2:3.
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