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Preparation of oxide with nano-scaled pore diameters using gel template
Journal of Non-Crystalline Solids 325 (2003) 124–132 www.elsevier.com/locate/jnoncrysol
Preparation of oxide with nano-scaled pore diameters using gel template Jinting Jiu, Ken-ichi Kurumada *, Masataka Tanigaki Department of Chemical Engineering, Faculty of Engineering, Kyoto University, Yoshida-Honmachi, Sakyo-Ku, Kyoto 606-8501, Japan Received 7 November 2002
Abstract Silica, MgO and ZnO with various nano-scaled pore diameters were prepared with template gel synthesized from hydroxyl ethyl methacrylate (HEMA) and ethylene glycol dimethacrylate (EGDMA), where the latter worked as the cross-linker. The effects of the molar ratio [HEMA]/[EGDMA] in the gel were discussed in relation to the resultant porous structure. The pore diameter of the oxide was shown to be varied by the molar ratio [HEMA]/[EGDMA] which directly affects the structure of gel, particularly the mesh size. From the different trend in the variation of the pore size in the oxide, a contrasting trend depending on the molar ratio [HEMA]/[EGDMA] has been found for silica, MgO and ZnO. The mesh-filling and hydrogen bonding mechanism has been considered to explain the formation process of gel templated nanoporous silica and ZnO or MgO, respectively. 2003 Elsevier B.V. All rights reserved. PACS: 81.05.R; 61.43.G; 82.70.G; 3.20.D
1. Introduction There have been intensive efforts to develop porous materials with a variety of prescribed pore sizes that open up new engineering possibilities such as filtrations, separation materials, catalyst supports, cell immobilizers, battery materials, and advanced biomaterials [1–4] since the first synthesis of mesoporous MCM-41 in 1992 [5]. Especially, * Corresponding author. Address: Graduate School of Environment and Information Science, Yokohama National University, Hodogaya, Yokohama 240-8501, Japan. Tel.: +81-45 339 4307; fax: +81-45 339 4305. E-mail addresses: [email protected] (J. Jiu), [email protected] (K. Kurumada).
the nanotemplating method has turned out to be effective for obtaining varied artificial nanoporous structures in solids. By using surfactant micellar systems, colloidal crystal, emulsions, latex spheres, and even bacteria as the nanostructural templates [6–8], it is possible to obtain porous solids which can be expected to have various pore sizes and distributions. Designing and synthesizing these nanostructure solids by the templating methods has been considered as a key preparation technique. Recently, use of polymers, especially block copolymer, as templates has been actively studied to prepare nanostructural solids with varied pore sizes and distributions by different polymerization methods for preparation of copolymer. Generally, the resultant nanoporous structure is determined
0022-3093/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0022-3093(03)00326-0
J. Jiu et al. / Journal of Non-Crystalline Solids 325 (2003) 124–132
by the micellar structure, which is formed by dissolving copolymers in solvent. The micellar structure can be kept by replication in the resultant materials. Typical examples can be found in the template structure of pluronic series [9]. In the present work, the authors are interested in using cross-linked gel structure as the nanotemplate from the viewpoint of obtaining bulk mesoporous solids materials. Properly shaped mesoporous materials will be considered for application in many fields. For this purpose, gel is a quite intriguing template due to its microscopic and mesoscopic structure which can give a rise to nanoporous structure in the templated solid materials, especially in keeping the shape of resultant solids. Moreover, authors are also interested in the possible formation mechanism of these nanoporous structure, which is estimated to be completely different from the replication mechanism in the micellar systems of copolymers or surfactants [10]. Here, solid sources are incorporated into the template gel using spontaneous swelling of the gel in liquid. Since the cross-linked gel structure is primarily responsible for the swelling behavior, the effect of the cross-link density was mainly focused on in the present work. Furthermore, the crosslink density is considered to affect the mesoporous structure of the templated solid, which is similar to the case of copolymer template. From the technical viewpoint, the variation in the cross-link density suggests the possibility of controling the pore size by their nano-scaled geometrical constraint. In the present work, siliceous and non-siliceous oxides were prepared using hydrophilic hydroxyl ethyl methacrylate–ethylene glycol dimethacrylate (HEMA–EGDMA) templating gels and the effects of the cross-link density varied by the molar ratio between HEMA and EGDMA on the resultant porous structure of the oxides were experimentally studied. The formation mechanism of the porous structure in oxide is also discussed in comparison with that in the normal templating methods.
2. Experimental procedure As reported elsewhere, the template gel was prepared by the polymerization of HEMA and
125
EGDMA, where the latter works as the crosslinker [11]. A methanol solution of 11.72 g HEMA (Mw ¼ 130:14) and 0.038 g EGDMA (Mw ¼ 198:22) was initiated to be polymerized with 0.12 g ammonium persulfate as the polymerization initiator under nitrogen atmosphere. The polymerization lasted for 30 min at room temperature and then 0.72 ml N , N , N 0 , N 0 -tetramethylethylenediamine (TEMED) was added to promote the polymerization. The above solution was refrigerated at 3 C for 48 h for thorough gelation. The obtained gel was repeatedly rinsed with distilled water to remove the solvent methanol and unreacted monomer followed by drying at 40 C in a thermostated oven for a week. By varying the molar ratios [HEMA]/[EGDMA], template gels with different cross-link densities were prepared. Porous silica is prepared by complete swelling of the prepared gel in silica sol, which is freshly prepared by hydrolysis of tertraethoxysiliane (TEOS) under acidic condition. The molar composition was TEOS: 1, H2 O: 10, ethanol: 20 and HCl: 3.6 · 103 . The mixture was stirred for 2 h at room temperature to hydrolyze TEOS to silica sol. The swollen gel was dried again at 40 C for another week and calcined at 500 C for 5 h to remove the template gel. Finally, silica solid samples were obtained. For the preparation of porous ZnO and MgO, the condition was the same except substituting silica sol by Zn(NO3 )2 and Mg(NO3 )2 methanol solution, respectively. The samples were observed by transmission electron microscope (TEM, JEOL JEM-1010, operated at 100 kV). The nitrogen adsorption/desorption isotherms were measured at 77 K. Prior to the adsorption measurements, the samples were degassed in vacuum at 120 C for 1 h. Surface areas were calculated by the Brunauer–Emmett– Teller method. The pore-size distributions were determined from the adsorption isotherms by the Barrett–Joyner–Halenda (BJH) method. The pore volume was estimated from the adsorbed amount of N2 at P =P0 ¼ 0:9814, where P =P0 denotes the relative pressure. Thermogravimetric analysis (TGA) was performed. The samples were heated under flowing air from 30 to 600 C at 10 C/min. Infrared spectra were measured on the gels and
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gel/metal oxide hybrids by the Fourier transform infrared spectrometer from 700 to 4000 cm1 .
3. Results and discussion Fig. 1 shows TEM images of silica, ZnO and MgO templated from HEMA–EGDMA gels. They are commonly characterized by aggregation of particulate unit structures whose sizes range between 5 and 10 nm. It should be noted that the size of them are considerably uniform. Obviously, nanopores are formed as gaps among those particulate structures. The cross-linked gel structure seems to be responsible for these nanostructural commonalities. FT-IR spectroscopy was performed to obtain information of the gel/inorganic moieties hybrids. The spectra of the gels and hybrids are shown in Fig. 2(A). In curve 1, asymmetric and symmetric vibration modes of CH2 are seen at 2875 and 2960 cm1 , respectively. A broad band around 3400 cm1 is attributed to the residual water and AOH groups in the gel. It should be noted that this band broadens and shifts to lower wavenumber after inorganic moieties are incorporated into the gel network by swelling. Characteristic band located at 1550–1850 cm1 is associated with AC(@O)AOA groups (etheric oxygen groups) in HEMA and EGDMA [12]. The band also shifts to lower frequency side when the gel becomes hybridized with silica, Zn and Mg moieties as shown in curve 2, 3 and 4 respectively (Fig. 2(B)), indicating that the gel and inorganic moieties are mutually interactive in the molecular scale. As to the interaction between the gel and inorganic moieties, hydrogen bondings are expected to be formed and to contribute to the band shift toward the lower wavenumbers. The interaction is the driving force for inorganic moieties migrating from solution into the gel network. The same type of hydrogen bonding has been reported to act between metal salts and organic polymers [11,13]. It should be noted that the shift is more drastic in Zn or Mg/gel hybrids compared to silica/gel one, suggesting stronger attraction in the former case. Moreover, the bands in the range of 800–1300 cm1 are due to the fine vibration of C–H bonds.
Fig. 1. Typical TEM images of (A) Silica (B) ZnO and (C) MgO at [HEMA]/[EGDMA] ¼ 140, 1000 and 700, respectively.
J. Jiu et al. / Journal of Non-Crystalline Solids 325 (2003) 124–132
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Fig. 2. FT-IR spectra of gel (1) and hybrid of gel and silica (2), Zn (3) and Mg (4) moieties respectively. (A) the whole range of wavenumber; (B) a magnified view between 1550 and 1850 cm1 .
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However, the intensity of these bands clearly decreases after inorganic moieties migrate from solution into the gel. This decrease in FT–IR absorbance intensity has also been reported to be the hydrogen bonding interaction between inorganic moieties and organic functional groups [14]. TGA is an effective method to probe the interaction between gel and inorganic moieties. Fig. 3 shows the results of TGA of gel only (curve 1), gel/ silica, gel/ Zn and gel/Mg hybridized samples at [HEMA]/[EGDMA] ¼ 100 (curve 2), 500 (curve 3 and 4), respectively. The complete decomposition of the gel can be seen in the range of 200–350 C as shown by significant weight loss in TGA curve 1. Difference in curve 2–4 from curve 1 can be interpreted in terms of the interaction between the template gel and incorporated inorganic moieties. In the case of silica, only one stage of weight loss is seen in TGA curve (curve 2), which corresponds to the decomposition of the template gel in gel/silica hybrid. It should be noted that the decomposition peak temperature (maximum point of temperature at which the weight reduction is the largest) is noticeably higher than in the case of only gel (relevant derivative thermogravimetry curve 1). This peak shift is considered to be related to the silica–gel interaction in the molecular scale as mentioned in the results of FT-IR. In the case of gel/Zn and gel/Mg hybrids, the thermal history of the hybrids includes three clearly distinguished
1.6 1.2 0.8 0.4
4 3
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300 Temperture (0C)
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Fig. 3. Thermogravimety and relevant derivative thermogravimety curves of gel (1) and hybrid of gel and silica (2), Zn (3) and Mg (4) moieties respectively.
stages (curve 3 and 4). The first ones seen at 190 and 200 C in gel/Zn and gel/Mg, respectively, are related to evaporation and volatilization of residual water and organic species. Another possible cause of these peaks is decomposition of metal nitrate simultaneously followed by formation of bonding between these metal moieties and template gel [15]. The second decomposition peak from 200 to 480 C can be attributed to the decomposition of the template gel because of the overlap of the temperature range seen in curve 1. It should be
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noted that the decomposition temperature of the template gel is slightly higher in the hybrid samples than in the gel itself only. The increase in the decomposition temperature is to be related to the metal–gel interaction in the molecular scale as mentioned on the FT-IR measurement. Higher decomposition temperature is required to remove the template gel from the incorporated state in the hybrids as seen in the case of silica. Moreover, the decomposition temperature is also higher than in the case of silica/gel hybrid, which is consistent with the result of FT-IR, indicating the formation of more stable hybrids of the gel with Zn or Mg moieties. The third step of the weight loss seen at 490 and 510 C in gel/Zn and gel/Mg, respectively, indicated the formation of oxides. The completion of the thermal oxidation is shown to occur by these abrupt weight losses. Fig. 4 shows N2 adsorption–desorption isotherms of samples after the calcination. These oxides are predominantly nanoporous as evidenced by the type IV-like isotherms with clear hysteresis loops, indicating capillary condensation in the nanopores. The pore-size distributions of samples at different molar ratios [HEMA]/[EGDMA] of template gels were obtained according to the BJH method as shown in Fig. 4, respectively. In silica, the average pore size becomes larger as the molar ratio [HEMA]/[EGDMA] is increased. This trend reflects the mechanism of the pore formation where the unit particles in Fig. 1(A) becomes larger in a sparser gel network. The unit silica particulate structure is considered to be formed inside the free space of the gel mesh. As the result, the nanopores formed as gaps among these particulate structures will be larger in accordance with the larger particles formed at lower cross-link density, i.e. larger molar ratio [HEMA]/[EGDMA]. Therefore, as the template gel become more densely cross-linked, the pore size becomes smaller. On the contrary, the average pore-size decreases with [HEMA]/[EGDMA] in the case of ZnO and MgO (Fig. 4(B) and (C)). This completely opposite trend suggests a different formation mechanism of the nanopores and nano-sized unit structure seen in the TEM images (Fig. 1(B) and (C)). These different structuresÕ formation mechanism will be discussed in the following.
In the present work, it is gel that is used as the template, whose structure will drastically affect the pore structure of templated resultant materials. Gel templates have many advantages. The most important one is that the macroscopic shape of resultant materials can be constructed by using the macroscopic gel template, which is completely different from the structure of copolymer template dissolved in liquid to form microscopic micellar structure. Moreover, the structure of the template gel can be easily varied by cross-link density, i.e. the molar ratio [HEMA]/[EGDMA], which is an important reason for selecting cross-linked gel as the template in our works. Sparser cross-links, i.e. with higher moral ratio [HEMA]/[EGDMA] will give a looser template gel with a larger gel mesh size. On the contrary, a more confined template gel with a smaller mesh size will be prepared with denser cross-links. The formation of the nanoporous oxides is closely related to the structure of these gel templates. Here, an important difference in the formation mechanism of the nanoporous structure is discussed between silica and ZnO/MgO (Fig. 5). In the case of silica, the unit particulate structure seen in Fig. 1(A) is considered to be formed inside the free space in the gel mesh. Thus, the average size of the unit silica nanoparticles become smaller as the template gel becomes more confined with the decrease in the molar ratio [HEMA]/[EGDMA] (Fig. 5(A)). Because the resultant average size of the nanopores would be determined by the size of the unit particulate structure, that is, the average pore size would be comparable with them, the decrease in the pore size in Fig. 4(A) with the increase in the cross-link density can be ascribed to the smaller unit particulate structure formed in more confined gel network. Moreover, the opposite trend in the pore size seen in ZnO and MgO is quite indicative of a fundamental difference in the nanostructural formation mechanism from that occurring in silica/ template gel hybrids. Here, the formation of larger unit particulate structures of ZnO and MgO is to be linked with the more confined (densely crosslinked) gel at the smaller molar ratio [HEMA]/ [EGDMA]. The FT-IR and TGA measurements show the metal moieties are bound to the template gel via hydrogen bondings. Thus, the amount of
J. Jiu et al. / Journal of Non-Crystalline Solids 325 (2003) 124–132
A
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[HEMA] / [EGDMA] 27 40 100 140
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Fig. 4. N2 adsorption–desorption isotherms and BJH pore-size distribution of mesoporous silica (A), ZnO (B) and MgO (C), respectively.
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A) silica
gel
precursory particles
swollen state
after calcination
larger pores
smaller pores
Fig. 5. The schemes of mesoporous oxides formed in the template gel (A) silica and (B) ZnO or MgO.
the incorporated metal moieties is determined by the number density of these hydrogen bondings in the template gel which increase with the crosslink density (Fig. 5(B)). It should be noted that the overall number density of these hydrogen bondings increase as the gel becomes more confined. Therefore, the higher cross-link density enables the template gel to hold the metal moieties more densely, which is more advantageous for the growth of those unit particulate structure of the ZnO/MgO seen in Fig. 1(B) and (C). As the result,
the average pore diameter becomes larger when the metal oxide is templated in the more densely cross-linked gel. In summary, in the case of silica, it is the size of mesh in the gel that is a more important factor for the migration of precursory particles, i.e. the mesh-filling process is the formation mechanism. In the case of MgO or ZnO, hydrogen bonding is the dominant factor for the migration of Zn or Mg moieties, which is completely different from the case of silica particles and determined the the porous structure of resul-
J. Jiu et al. / Journal of Non-Crystalline Solids 325 (2003) 124–132
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B) ZnO or MgO precursory metal moieties swollen state
less adhesion of primary moieties
more adhesion of primary moieties
in calcination (with formation of precursory oxide particles)
smaller precursory oxide particles
larger precursory oxide particles
after calcination
smaller pores
larger pores Fig. 5 (continued)
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tant solids. The above evaluated mechanism of the nanoporous structure are schematically represented in Fig. 5.
Ms K. Yamanaka for her thorough technical support in TEM observations.
4. Conclusion
References
Nanoporous silica, ZnO and MgO have been synthesized with [HEMA]/[EGDMA] gel as the nanopore-forming template. The nanoporous structure is formed from gaps among unit particulate structure with the diameter of 5–10 nm, for each case of silica, ZnO and MgO. In silica, the average pore diameter increases as the template gel is more sparsely cross-linked at the larger molar ratio [HEMA]/[EGDMA]. This trend is entirely opposite to that in ZnO and MgO, suggesting that the formation mechanism of the nanoporous structure is different between silica and ZnO or MgO. TGA and FT-IR measurements gave results indicative of the hydrogen bonding formation between the template gel and inorganic Zn or Mg moieties, which is to be linked with the opposite pore-size dependence on the cross-link density of the template gel. The present template method can be extended to a broader range of other organic polymer gels and inorganic materials and give a rise to porous materials with controled pore sizes.
Acknowledgements J.J. gratefully acknowledges Japan Society for Promotion of Science (JSPS) for her postdoctoral fellowship in Japan. The authors are grateful to
[1] R. Burch, N. Cruise, D. Gleeson, S.C. Tsang, Chem. Commun. (1996) 951. [2] J.L. Casci, Stud. Surf. Sci. Catal. 85 (1994) 329. [3] X.Z. Wang, T. Dong, Y.Z. Xiao, Chem. Commun. (1998) 1035. [4] K. Nakanishi, H. Minakuchi, N. Soga, N. Tanaka, J. SolGel Sci. Technol. 13 (1998) 163. [5] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 114 (1992) 10834. [6] P. Yang, T. Deng, D. Zhao, P. Feng, D. Pine, B.F. Chmelka, G.M. Whiteside, G.D. Stucky, Science 282 (1998) 2244. [7] A. Imhof, D. Pine, J. Adv. Mater. 10 (1998) 696. [8] B.R. Heywood, S. Mann, Adv. Mater. 6 (1994) 9. [9] P. Alexandridis, T.A. Hatton, Colloid Surf. A 96 (1995) 1; P. Yang, D. Zhao, D.I. Margolese, B.F. Chmelka, G.D. Stucky, Chem. Mater. 11 (1998) 2813. [10] A. Monnier, F. Schuth, Q. Huo, D. Kumar, et al., Science 261 (1993) 1299; V. Alfredsson, M. Keung, A. Monnier, et al., J. Chem. Soc., Chem. Commun. (1994) 921; Q. Huo, D.I. Margolese, et al., Nature 368 (1994) 317. [11] J. Jiu, K. Kurumada, M. Tanigaki, Mater. Chem. Phys. 78 (2002) 177. [12] J.F. Rabek, Experimental Methods in Polymer Chemistry, Physical Principles and Applications, Wiley, New York, 1980, p. 239. [13] Z.L. Hua, J.L. Shi, L. Wang, W.H. Zhang, J. Non-Cryst. Solids 292 (2001) 177; A. Fidalgo, L.M. Iiharco, J. Non-Cryst. Solids 283 (2001) 144. [14] S.H. Zhong, C.F. Li, X.F. Xiao, J. Membrane Sci. 199 (2002) 53. [15] J. Jiu, Y. Ge, X. Li, L. Nie, Mater. Lett. 54 (2002) 260.
Preparation of oxide with nano-scaled pore diameters using gel template Jinting Jiu, Ken-ichi Kurumada *, Masataka Tanigaki Department of Chemical Engineering, Faculty of Engineering, Kyoto University, Yoshida-Honmachi, Sakyo-Ku, Kyoto 606-8501, Japan Received 7 November 2002
Abstract Silica, MgO and ZnO with various nano-scaled pore diameters were prepared with template gel synthesized from hydroxyl ethyl methacrylate (HEMA) and ethylene glycol dimethacrylate (EGDMA), where the latter worked as the cross-linker. The effects of the molar ratio [HEMA]/[EGDMA] in the gel were discussed in relation to the resultant porous structure. The pore diameter of the oxide was shown to be varied by the molar ratio [HEMA]/[EGDMA] which directly affects the structure of gel, particularly the mesh size. From the different trend in the variation of the pore size in the oxide, a contrasting trend depending on the molar ratio [HEMA]/[EGDMA] has been found for silica, MgO and ZnO. The mesh-filling and hydrogen bonding mechanism has been considered to explain the formation process of gel templated nanoporous silica and ZnO or MgO, respectively. 2003 Elsevier B.V. All rights reserved. PACS: 81.05.R; 61.43.G; 82.70.G; 3.20.D
1. Introduction There have been intensive efforts to develop porous materials with a variety of prescribed pore sizes that open up new engineering possibilities such as filtrations, separation materials, catalyst supports, cell immobilizers, battery materials, and advanced biomaterials [1–4] since the first synthesis of mesoporous MCM-41 in 1992 [5]. Especially, * Corresponding author. Address: Graduate School of Environment and Information Science, Yokohama National University, Hodogaya, Yokohama 240-8501, Japan. Tel.: +81-45 339 4307; fax: +81-45 339 4305. E-mail addresses: [email protected] (J. Jiu), [email protected] (K. Kurumada).
the nanotemplating method has turned out to be effective for obtaining varied artificial nanoporous structures in solids. By using surfactant micellar systems, colloidal crystal, emulsions, latex spheres, and even bacteria as the nanostructural templates [6–8], it is possible to obtain porous solids which can be expected to have various pore sizes and distributions. Designing and synthesizing these nanostructure solids by the templating methods has been considered as a key preparation technique. Recently, use of polymers, especially block copolymer, as templates has been actively studied to prepare nanostructural solids with varied pore sizes and distributions by different polymerization methods for preparation of copolymer. Generally, the resultant nanoporous structure is determined
0022-3093/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0022-3093(03)00326-0
J. Jiu et al. / Journal of Non-Crystalline Solids 325 (2003) 124–132
by the micellar structure, which is formed by dissolving copolymers in solvent. The micellar structure can be kept by replication in the resultant materials. Typical examples can be found in the template structure of pluronic series [9]. In the present work, the authors are interested in using cross-linked gel structure as the nanotemplate from the viewpoint of obtaining bulk mesoporous solids materials. Properly shaped mesoporous materials will be considered for application in many fields. For this purpose, gel is a quite intriguing template due to its microscopic and mesoscopic structure which can give a rise to nanoporous structure in the templated solid materials, especially in keeping the shape of resultant solids. Moreover, authors are also interested in the possible formation mechanism of these nanoporous structure, which is estimated to be completely different from the replication mechanism in the micellar systems of copolymers or surfactants [10]. Here, solid sources are incorporated into the template gel using spontaneous swelling of the gel in liquid. Since the cross-linked gel structure is primarily responsible for the swelling behavior, the effect of the cross-link density was mainly focused on in the present work. Furthermore, the crosslink density is considered to affect the mesoporous structure of the templated solid, which is similar to the case of copolymer template. From the technical viewpoint, the variation in the cross-link density suggests the possibility of controling the pore size by their nano-scaled geometrical constraint. In the present work, siliceous and non-siliceous oxides were prepared using hydrophilic hydroxyl ethyl methacrylate–ethylene glycol dimethacrylate (HEMA–EGDMA) templating gels and the effects of the cross-link density varied by the molar ratio between HEMA and EGDMA on the resultant porous structure of the oxides were experimentally studied. The formation mechanism of the porous structure in oxide is also discussed in comparison with that in the normal templating methods.
2. Experimental procedure As reported elsewhere, the template gel was prepared by the polymerization of HEMA and
125
EGDMA, where the latter works as the crosslinker [11]. A methanol solution of 11.72 g HEMA (Mw ¼ 130:14) and 0.038 g EGDMA (Mw ¼ 198:22) was initiated to be polymerized with 0.12 g ammonium persulfate as the polymerization initiator under nitrogen atmosphere. The polymerization lasted for 30 min at room temperature and then 0.72 ml N , N , N 0 , N 0 -tetramethylethylenediamine (TEMED) was added to promote the polymerization. The above solution was refrigerated at 3 C for 48 h for thorough gelation. The obtained gel was repeatedly rinsed with distilled water to remove the solvent methanol and unreacted monomer followed by drying at 40 C in a thermostated oven for a week. By varying the molar ratios [HEMA]/[EGDMA], template gels with different cross-link densities were prepared. Porous silica is prepared by complete swelling of the prepared gel in silica sol, which is freshly prepared by hydrolysis of tertraethoxysiliane (TEOS) under acidic condition. The molar composition was TEOS: 1, H2 O: 10, ethanol: 20 and HCl: 3.6 · 103 . The mixture was stirred for 2 h at room temperature to hydrolyze TEOS to silica sol. The swollen gel was dried again at 40 C for another week and calcined at 500 C for 5 h to remove the template gel. Finally, silica solid samples were obtained. For the preparation of porous ZnO and MgO, the condition was the same except substituting silica sol by Zn(NO3 )2 and Mg(NO3 )2 methanol solution, respectively. The samples were observed by transmission electron microscope (TEM, JEOL JEM-1010, operated at 100 kV). The nitrogen adsorption/desorption isotherms were measured at 77 K. Prior to the adsorption measurements, the samples were degassed in vacuum at 120 C for 1 h. Surface areas were calculated by the Brunauer–Emmett– Teller method. The pore-size distributions were determined from the adsorption isotherms by the Barrett–Joyner–Halenda (BJH) method. The pore volume was estimated from the adsorbed amount of N2 at P =P0 ¼ 0:9814, where P =P0 denotes the relative pressure. Thermogravimetric analysis (TGA) was performed. The samples were heated under flowing air from 30 to 600 C at 10 C/min. Infrared spectra were measured on the gels and
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gel/metal oxide hybrids by the Fourier transform infrared spectrometer from 700 to 4000 cm1 .
3. Results and discussion Fig. 1 shows TEM images of silica, ZnO and MgO templated from HEMA–EGDMA gels. They are commonly characterized by aggregation of particulate unit structures whose sizes range between 5 and 10 nm. It should be noted that the size of them are considerably uniform. Obviously, nanopores are formed as gaps among those particulate structures. The cross-linked gel structure seems to be responsible for these nanostructural commonalities. FT-IR spectroscopy was performed to obtain information of the gel/inorganic moieties hybrids. The spectra of the gels and hybrids are shown in Fig. 2(A). In curve 1, asymmetric and symmetric vibration modes of CH2 are seen at 2875 and 2960 cm1 , respectively. A broad band around 3400 cm1 is attributed to the residual water and AOH groups in the gel. It should be noted that this band broadens and shifts to lower wavenumber after inorganic moieties are incorporated into the gel network by swelling. Characteristic band located at 1550–1850 cm1 is associated with AC(@O)AOA groups (etheric oxygen groups) in HEMA and EGDMA [12]. The band also shifts to lower frequency side when the gel becomes hybridized with silica, Zn and Mg moieties as shown in curve 2, 3 and 4 respectively (Fig. 2(B)), indicating that the gel and inorganic moieties are mutually interactive in the molecular scale. As to the interaction between the gel and inorganic moieties, hydrogen bondings are expected to be formed and to contribute to the band shift toward the lower wavenumbers. The interaction is the driving force for inorganic moieties migrating from solution into the gel network. The same type of hydrogen bonding has been reported to act between metal salts and organic polymers [11,13]. It should be noted that the shift is more drastic in Zn or Mg/gel hybrids compared to silica/gel one, suggesting stronger attraction in the former case. Moreover, the bands in the range of 800–1300 cm1 are due to the fine vibration of C–H bonds.
Fig. 1. Typical TEM images of (A) Silica (B) ZnO and (C) MgO at [HEMA]/[EGDMA] ¼ 140, 1000 and 700, respectively.
J. Jiu et al. / Journal of Non-Crystalline Solids 325 (2003) 124–132
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Intensity ( a.u.)
Intensity (a.u.)
(A)
1
2 3 4
2 3 4 1000
1500
2000
2500
3000
3500
4000
1550
1600
1650
Wavenumber (cm-1)
1700
1750
1800
1850
Wavenumber (cm-1)
Fig. 2. FT-IR spectra of gel (1) and hybrid of gel and silica (2), Zn (3) and Mg (4) moieties respectively. (A) the whole range of wavenumber; (B) a magnified view between 1550 and 1850 cm1 .
2 1 3
2.4 1.6
4 Weight (mg)
However, the intensity of these bands clearly decreases after inorganic moieties migrate from solution into the gel. This decrease in FT–IR absorbance intensity has also been reported to be the hydrogen bonding interaction between inorganic moieties and organic functional groups [14]. TGA is an effective method to probe the interaction between gel and inorganic moieties. Fig. 3 shows the results of TGA of gel only (curve 1), gel/ silica, gel/ Zn and gel/Mg hybridized samples at [HEMA]/[EGDMA] ¼ 100 (curve 2), 500 (curve 3 and 4), respectively. The complete decomposition of the gel can be seen in the range of 200–350 C as shown by significant weight loss in TGA curve 1. Difference in curve 2–4 from curve 1 can be interpreted in terms of the interaction between the template gel and incorporated inorganic moieties. In the case of silica, only one stage of weight loss is seen in TGA curve (curve 2), which corresponds to the decomposition of the template gel in gel/silica hybrid. It should be noted that the decomposition peak temperature (maximum point of temperature at which the weight reduction is the largest) is noticeably higher than in the case of only gel (relevant derivative thermogravimetry curve 1). This peak shift is considered to be related to the silica–gel interaction in the molecular scale as mentioned in the results of FT-IR. In the case of gel/Zn and gel/Mg hybrids, the thermal history of the hybrids includes three clearly distinguished
1.6 1.2 0.8 0.4
4 3
0.0
2 1 0
100
200
300 Temperture (0C)
400
500
600
Fig. 3. Thermogravimety and relevant derivative thermogravimety curves of gel (1) and hybrid of gel and silica (2), Zn (3) and Mg (4) moieties respectively.
stages (curve 3 and 4). The first ones seen at 190 and 200 C in gel/Zn and gel/Mg, respectively, are related to evaporation and volatilization of residual water and organic species. Another possible cause of these peaks is decomposition of metal nitrate simultaneously followed by formation of bonding between these metal moieties and template gel [15]. The second decomposition peak from 200 to 480 C can be attributed to the decomposition of the template gel because of the overlap of the temperature range seen in curve 1. It should be
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noted that the decomposition temperature of the template gel is slightly higher in the hybrid samples than in the gel itself only. The increase in the decomposition temperature is to be related to the metal–gel interaction in the molecular scale as mentioned on the FT-IR measurement. Higher decomposition temperature is required to remove the template gel from the incorporated state in the hybrids as seen in the case of silica. Moreover, the decomposition temperature is also higher than in the case of silica/gel hybrid, which is consistent with the result of FT-IR, indicating the formation of more stable hybrids of the gel with Zn or Mg moieties. The third step of the weight loss seen at 490 and 510 C in gel/Zn and gel/Mg, respectively, indicated the formation of oxides. The completion of the thermal oxidation is shown to occur by these abrupt weight losses. Fig. 4 shows N2 adsorption–desorption isotherms of samples after the calcination. These oxides are predominantly nanoporous as evidenced by the type IV-like isotherms with clear hysteresis loops, indicating capillary condensation in the nanopores. The pore-size distributions of samples at different molar ratios [HEMA]/[EGDMA] of template gels were obtained according to the BJH method as shown in Fig. 4, respectively. In silica, the average pore size becomes larger as the molar ratio [HEMA]/[EGDMA] is increased. This trend reflects the mechanism of the pore formation where the unit particles in Fig. 1(A) becomes larger in a sparser gel network. The unit silica particulate structure is considered to be formed inside the free space of the gel mesh. As the result, the nanopores formed as gaps among these particulate structures will be larger in accordance with the larger particles formed at lower cross-link density, i.e. larger molar ratio [HEMA]/[EGDMA]. Therefore, as the template gel become more densely cross-linked, the pore size becomes smaller. On the contrary, the average pore-size decreases with [HEMA]/[EGDMA] in the case of ZnO and MgO (Fig. 4(B) and (C)). This completely opposite trend suggests a different formation mechanism of the nanopores and nano-sized unit structure seen in the TEM images (Fig. 1(B) and (C)). These different structuresÕ formation mechanism will be discussed in the following.
In the present work, it is gel that is used as the template, whose structure will drastically affect the pore structure of templated resultant materials. Gel templates have many advantages. The most important one is that the macroscopic shape of resultant materials can be constructed by using the macroscopic gel template, which is completely different from the structure of copolymer template dissolved in liquid to form microscopic micellar structure. Moreover, the structure of the template gel can be easily varied by cross-link density, i.e. the molar ratio [HEMA]/[EGDMA], which is an important reason for selecting cross-linked gel as the template in our works. Sparser cross-links, i.e. with higher moral ratio [HEMA]/[EGDMA] will give a looser template gel with a larger gel mesh size. On the contrary, a more confined template gel with a smaller mesh size will be prepared with denser cross-links. The formation of the nanoporous oxides is closely related to the structure of these gel templates. Here, an important difference in the formation mechanism of the nanoporous structure is discussed between silica and ZnO/MgO (Fig. 5). In the case of silica, the unit particulate structure seen in Fig. 1(A) is considered to be formed inside the free space in the gel mesh. Thus, the average size of the unit silica nanoparticles become smaller as the template gel becomes more confined with the decrease in the molar ratio [HEMA]/[EGDMA] (Fig. 5(A)). Because the resultant average size of the nanopores would be determined by the size of the unit particulate structure, that is, the average pore size would be comparable with them, the decrease in the pore size in Fig. 4(A) with the increase in the cross-link density can be ascribed to the smaller unit particulate structure formed in more confined gel network. Moreover, the opposite trend in the pore size seen in ZnO and MgO is quite indicative of a fundamental difference in the nanostructural formation mechanism from that occurring in silica/ template gel hybrids. Here, the formation of larger unit particulate structures of ZnO and MgO is to be linked with the more confined (densely crosslinked) gel at the smaller molar ratio [HEMA]/ [EGDMA]. The FT-IR and TGA measurements show the metal moieties are bound to the template gel via hydrogen bondings. Thus, the amount of
J. Jiu et al. / Journal of Non-Crystalline Solids 325 (2003) 124–132
A
300
[HEMA] / [EGDMA] 27 40 100 140
0.12 Dv / Dp (cm3 g- 1 nm-1)
Volume Adsorbed cc/g (STP)
A
[HEMA] / [EGDMA] 27 40 100 140
400
129
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100
0.08
0.04
0 0.00 0.0
0.2
0.4
0.6
0.8
2
1.0
4
6
8
[HEMA] / [EGDMA] 1000 700 500 100
B 100
14
16
18
20
[HEMA] / [EGDMA] 1000 700 500 100
B 0.020
80
Dv / Dp (cm3 g- 1 nm-1)
Volume Adsorbed cc/g (STP)
12
0.025
120
60 40 20
0.015
0.010
0.005
0
0.000
-20 0.0
0.2
0.4
0.6
0.8
0
1.0
10
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30
40
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[HEMA] / [EGDMA] 1000 700 500
C
20 Pore Diameter (nm)
Relative Pressure Ps /P0
C
[HEMA] / [EGDMA] 500 700 1000
0.05
250
Dv / Dp (cm3 g- 1 nm-1)
Volume Adsorbed cc/g (STP)
10
Pore Diameter (nm)
Relative Pressure Ps /P0
200 150
0.04
0.03
0.02
100 0.01
50 0
0.00 0.0
0.2
0.4
0.6
Relative Pressure Ps /P0
0.8
1.0
0
10
20
30
40
50
Pore Diameter (nm)
Fig. 4. N2 adsorption–desorption isotherms and BJH pore-size distribution of mesoporous silica (A), ZnO (B) and MgO (C), respectively.
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A) silica
gel
precursory particles
swollen state
after calcination
larger pores
smaller pores
Fig. 5. The schemes of mesoporous oxides formed in the template gel (A) silica and (B) ZnO or MgO.
the incorporated metal moieties is determined by the number density of these hydrogen bondings in the template gel which increase with the crosslink density (Fig. 5(B)). It should be noted that the overall number density of these hydrogen bondings increase as the gel becomes more confined. Therefore, the higher cross-link density enables the template gel to hold the metal moieties more densely, which is more advantageous for the growth of those unit particulate structure of the ZnO/MgO seen in Fig. 1(B) and (C). As the result,
the average pore diameter becomes larger when the metal oxide is templated in the more densely cross-linked gel. In summary, in the case of silica, it is the size of mesh in the gel that is a more important factor for the migration of precursory particles, i.e. the mesh-filling process is the formation mechanism. In the case of MgO or ZnO, hydrogen bonding is the dominant factor for the migration of Zn or Mg moieties, which is completely different from the case of silica particles and determined the the porous structure of resul-
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B) ZnO or MgO precursory metal moieties swollen state
less adhesion of primary moieties
more adhesion of primary moieties
in calcination (with formation of precursory oxide particles)
smaller precursory oxide particles
larger precursory oxide particles
after calcination
smaller pores
larger pores Fig. 5 (continued)
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tant solids. The above evaluated mechanism of the nanoporous structure are schematically represented in Fig. 5.
Ms K. Yamanaka for her thorough technical support in TEM observations.
4. Conclusion
References
Nanoporous silica, ZnO and MgO have been synthesized with [HEMA]/[EGDMA] gel as the nanopore-forming template. The nanoporous structure is formed from gaps among unit particulate structure with the diameter of 5–10 nm, for each case of silica, ZnO and MgO. In silica, the average pore diameter increases as the template gel is more sparsely cross-linked at the larger molar ratio [HEMA]/[EGDMA]. This trend is entirely opposite to that in ZnO and MgO, suggesting that the formation mechanism of the nanoporous structure is different between silica and ZnO or MgO. TGA and FT-IR measurements gave results indicative of the hydrogen bonding formation between the template gel and inorganic Zn or Mg moieties, which is to be linked with the opposite pore-size dependence on the cross-link density of the template gel. The present template method can be extended to a broader range of other organic polymer gels and inorganic materials and give a rise to porous materials with controled pore sizes.
Acknowledgements J.J. gratefully acknowledges Japan Society for Promotion of Science (JSPS) for her postdoctoral fellowship in Japan. The authors are grateful to
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