Langmuir 2004, 20, 4771-4773
4771
Notes Formation of Nanoscopic Foam in Instantaneously Heated Sol-Gel Derived Silica Ken-ichi Kurumada* School of Environment and Information Science, Yokohama National University, 79-7 Tokiwa-dai, Hodogaya, Yokohama 240-8501, Japan Noriyuki Kitao Department of Chemical Engineering, Kyoto University, Sakyo, Kyoto 606-8501, Japan Masataka Tanigaki International Innovation Center, Kyoto University, Sakyo, Kyoto 606-8501, Japan Kenzo Susa and Masahiko Hiro Hitachi Chemical Company Ltd., 48 Wadai, Tsukuba 300-4247, Japan Received December 1, 2003. In Final Form: March 1, 2004
Introduction Foaming is a typical phase transition phenomenon involving the gas-solid interface and has been a popularly used processing method applied for various plastic polymeric resins. In those systems, the targeted cell size of foams ranges from 1 to 100 µm, being large enough to be observed using an optical microscope. In those systems, obtaining foams below an order of 102 nm in diameter is believed to be technically difficult because foaming is normally implemented at relatively high temperatures where the resins become quite deformable in the vicinity of the melting point. Gaseous species undergo nucleation and generate foams as a type of phase separation. The extent of foaming, that is, the porosity, can be varied by temperatures at which foaming takes place. Foaming has been recognized as a useful method for reducing the weight density of materials for saving plastic resins. Besides these common practical backgrounds, foaming is an interesting interfacial and phase-separation behavior in that it involves such multiple elemental processes as nucleation, expansion of interfacial area (growth), and mutual coalescence. In particular, some attention has been paid to foaming in a quite small scale. In the technological standpoint, foaming has a possibility of being a production method for nanoporous materials on the basis of this distinctive interfacial phenomenon. Foaming of glassy matter by dissolved gaseous species has been reported.1-3 In those studies, the cell size of the foam is relatively large, since foaming is induced in the vicinity of the melting point of the matrix. Surfactants were revealed to be helpful for enhancing the nanopores in solid matter. Jansen et al. showed formation of a mesoporous foamlike structure (TUD-1) where triethanolamine functions as the nano* Corresponding author. Phone and fax: +81 45 3394307. E-mail:
[email protected].
foaming agent.4 Kim et al. have reported a similar result (MSU-H) using 1,3,5-trimethylbenzene which works as an expander of the nanopores replicated from pluronic assemblies.5 Nevertheless, these foams are distinct from the foamed structure in the present work in that their pores tend to be highly open to each other. Godey et al. observed helium nanobubbles formed in a silica matrix by transmission electron microscopy (TEM), where they arose during a long annealing process at 800 °C (103 to 6.1 × 104 s).6 Here, cellular bubbles within 10 nm were stably retained due to the hard matrix. Nogita et al. found xenon nanobubbles in heated uranium dioxide pellets from diffraction patterns of X-ray spectroscopy (EDX).7 Zbik et al. found nanobubbles entrapped in commercial kaolinite (clay) whose formation is triggered in the gap between two adjoining crystalline microplates.8 On the contrary, such small voids as nanobubbles are not so stably present in liquid phases. Bunkins et al. showed that nanoscale voids can be generated at spots where hydrophobic and hydrophilic surfaces coexist. They found that the dissolved amount of gaseous species has a great effect.9 Higashitani and co-workers thoroughly used atomic force microscopy to obtain experimental evidence of the nanobubbles in the presence of hydrophilic and hydrophobic surfaces and surfactant.10-13 They showed that the nanobubbles are formed only when a gaseous species is dissolved in the solvent by demonstrating that no nanobubbles are formed when the sample liquid is carefully degassed. All in all, nanoscale foamed structures are not easily realized and stabilized in various matrixes, and thus, nanofoaming has been more or less left intact. On the other hand, preparation of nanoporous hard solid materials has been extensively studied since the report of MCM 41, where the surfactant liquid crystal works as the directing agent of the nanoporous structure.14 In those systems, open-pore-type nanoporous materials are obtained, since the liquid crystalline structure has, in principle, tunnel-like or interconnected nanopores. In this regard, the authors have attempted to prepare nanoporous solids with closed nanopores from the technological (1) Sakaguchi, S. J. Non-Cryst. Solids 1995, 189, 43-49. (2) Kim, D. S.; Hrma, P. J. Am. Ceram. Soc. 1991, 74, 551-555. (3) Fujiu, Y.; Messing, G. L. J. Non-Cryst. Solids 1992, 143, 133139. (4) Jansen, J. C.; Shan, Z.; Marchese, L.; Zhou, W.; Puil, N. V. D.; Maschmeyer, Th. Chem. Commun. 2001, 713-714. (5) Kim, S.-S.; Pauly, T. R.; Pinnavaia, J. Chem. Commun. 2000, 1661-1662. (6) Godey, S.; Ntsoenzok, E.; Sauvage, T.; van Veen, A.; Labohm, F.; Beaufort, M. F.; Barbot, J. F. Mater. Sci. Eng. 2000, B73, 54-59. (7) Nogita, K.; Une, K. Nucl. Instrum. Methods Phys. Res., Sect. B 1998, 141, 481-486. (8) Zbik, M.; Smart, R. St. C. Miner. Eng. 2002, 15, 277-286. (9) Bunkins, N. F.; Kiseleva, O. A.; Lobeyev, A. V.; Movchan, T. G.; Ninham, B. W.; Vinogradova, O. I. Langmuir 1997, 13, 3024-3028. (10) Sakamoto, M.; Kanda, Y.; Miyahara, M.; Higashitani, K. Langmuir 2002, 18, 5713-5719. (11) Ishida, N.; Inoue, T.; Miyahara, M.; Higashitani, K. Langmuir 2000, 16, 6377-6380. (12) Ishida, N.; Sakamoto, M.; Miyahara, M.; Higashitani, K. Langmuir 2000, 16, 5681-5687. (13) Ishida, N.; Sakamoto, M.; Miyahara, M.; Higashitani, K. J. Colloid Interface Sci. 2002, 253, 112-116. (14) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710.
10.1021/la030427u CCC: $27.50 © 2004 American Chemical Society Published on Web 04/24/2004
4772
Langmuir, Vol. 20, No. 11, 2004
Notes
viewpoint because they are expected to be free of adsorption or condensation inside the materials. The authors have found that the foaming technique is quite interesting because it can give rise to distinctive spherical nanopores which cannot be obtained by the surfactant-templating method. In the present work, the closed-porosity-type nanoporous structure of the foam caused by residual volatile species in sol-gel derived silica is reported. Here, the dependence of the foamed structure on the temperature of the instantaneous heating process is focused on. Experimental Section Sample Preparation. Tetraethyl orthosilicate (TEOS) and hydrochloric acid (HCl) were purchased from Nacalai Tesque (Kyoto, Japan) and used as provided. Water for the sample preparation was purified with ion exchange followed by distillation. A 20.8 g (0.10 mol) portion of TEOS, 18.0 g (1.0 mol) of water, and 1.0 g of HCl (1.0 M) were mixed and stirred using a magnetic stirrer for 5 h at 23 °C (room temperature) at ∼300 rpm to hydrolyze TEOS. This solution was kept in a drying oven for 18 h. The solidification completed in the outside appeared ∼5 h after the drying at 50 °C started. The sample weight after 18 h of drying was ∼6.7 g, while the stoichiometrically expected weight is 6.0 g. Thus, the solidified silica after the drying contains ∼10% of volatile species. The main residual volatile species is considered to be water. This dried silica containing water was instantaneously heated at various temperatures between 400 and 950 °C in a thermostated muffle furnace. For effective instantaneous heating in a muffle furnace, the silica after the drying was contained in a quartz crucible preheated at the same temperature as that of the heating, which was followed by a further continuous heat treatment at the same temperature for 5 h. Sample Characterization. The morphology of the obtained sample was examined by TEM (JEOL JEM-1010, 100 kV). Finely ground samples were dispersed in ethanol and placed on a copper microgrid (Type B, Ohken Science, Japan). The TEM observation could capture sufficiently thin parts in the sample. The adsorption/desorption isotherms of nitrogen at 77 K were measured using a Coulter SA 3100 instrument to characterize the open pores in the sample. The closed porosity was evaluated from the apparent weight density of the sample using a gravity column filled with a mixture of ethanol and bromoform, whose specific weights are 0.79 and 2.89, respectively. Here, the open pores are assumed to be completely immersed by the dispersing mixture because of the high wettability of silica with both ethanol and bromoform. The apparent weight density of the sample can be evaluated by measuring the density of the mixture balanced with the sample immersed in it. (The density of the dispersing mixture of bromoform and ethanol can be arbitrarily adjusted due to the miscibility of these two moderately polar solvent species.)
Results and Discussion Figure 1 shows TEM images of the foamed silica formed at 500, 600, 700, 800, 900, and 950 °C. No foamed structure was observed when the sample was prepared under basic conditions with ammonia. This is because of the formation of the highly microporous structure of sol-gel derived silica normally observed under basic conditions. Such a structure allows volatile, low-molecular-weight species to have an easy way out. The foaming behavior is clearly characterized by a spherical cellular structure with the diameter ranging from 5 to 100 nm in the order of magnitude. No foam was observed when the sample was heated at temperatures lower than 500 °C. It should be noted that the foamed structure is much finer than conventional ones in plastic resins, whose sizes below 1 µm are normally quite difficult to obtain. The nanofoam formation in the present work is due to the hard and deformable matrix of silica supported by strong Si-O-Si bonds. They are ubiquitously generated in the silica matrix, indicating that the vaporized species which gives
Figure 1. TEM micrographs of the foamed silica: heated (a) at 500 °C; (b) at 600 °C; (c) at 700 °C; (d) at 800 °C; (e) at 900 °C; (f) at 950 °C. (g) Example prepared under basic conditions (pH 10). Silica matrix is seen as the white parts on the black background.
rise to the foam is mainly residual water distributed in the silica matrix. The extent of foaming clearly increases with the temperature of the heating. At lower temperatures, the cells are isolated from each other, maintaining their highly spherical shape, which indicates that the cells are generated like balloons blown up by the inner pressure due to the vaporized volatile species. The isotropic shape of the cell of the foam is formed due to the isotropic mechanical properties of the silica matrix. It is likely that, in a matrix with an anisotropic microstructure, needlelike cracks tend to grow one-dimensionally, leading to overall destruction of the sample. As the temperature of the heating is increased, the geometry of the cells deviates from the spherical shape because of the interference by the neighboring cells, which finally leads to the intercellar coalescence. Figure 2 shows nitrogen adsorption/desorption isotherms at 77 K. The amount of adsorbed nitrogen clearly increases with the temperature of the heating process. Adsorption at the very small relative pressure (P/P0 ∼ 0) is scarcely seen, indicating that micropores are not formed as known for sol-gel derived silica under acidic conditions. Therefore, the microstructure of the silica matrix itself is not affected by the foaming. Adsorption takes place mainly at the larger relative pressure P/P0 g 0.8. Therefore, the pores captured by nitrogen adsorption should be classified as macropores. This adsorption behavior can be attributed to the condensation of nitrogen inside the cells of the foam. They are accessible only in the case when they are exposed on the sample surface and allow nitrogen to condense inside themselves. As the temperature of the heating process is increased, the adsorbed amount of nitrogen increases, reflecting that more nitrogen can permeate into the inner region of the foamed silica. This trend is to be related to the increase in the interconnected cells due to the enhanced foaming with the temperature of the heating.
Notes
Langmuir, Vol. 20, No. 11, 2004 4773
Figure 3. Dependence of the closed porosity (Φclosed pore) of foamed silica on the temperature of instantaneous heating (Tinstantaneous heating).
Figure 2. Dependence of the adsorbed amount of nitrogen (Vad) on the relative pressure (P/P0) at 77 K of foamed silica prepared by instantaneous heating at different temperatures (Tinstantaneous heating); Tinstantaneous heating is indicated in the righthand side of the figure.
However, the total amount of the adsorbed nitrogen is significantly smaller than that of the nanoporous materials obtained by replication of the liquid crystalline phase of surfactants.14 Therefore, foaming will be advantageous for the formation of closed-porosity-type porous structures. Figure 3 shows the dependence of the closed porosity on the temperature of the heating varied from 500 to 950 °C. The adsorbed volume of nitrogen at 77 K is plotted together. The closed porosity has a peak at 750 °C, at which the adsorbed volume begins to increase steeply with the temperature. This coincidence can be easily understood by the gradual transition of the closed cells to relatively open ones due to the coalescence of the neighboring cells as observed in the TEM images. However, 35% of the closed porosity was obtained even at 950 °C, showing that the isolated pores are still largely maintained. The present work shows that such nanoscale fine foaming can be induced by instantaneous heating of sol-
gel derived silica containing some residual volatile species (water) in hard matrix like silica and that foaming of solgel derived silica by instantaneous heating is advantageous for obtaining closed nanopores. Such closed cellular foamed structures do not appear under basic conditions where the silica matrix is formed by the aggregation of unit nanoscale particulate structures to lead to a highly microporous structure. The heating temperature is an important factor affecting the extent of foaming. The closed porosity was enhanced with the heating temperature up to 750 °C, above which it decreased due to the transition of the closed cells to open ones. Further studies of nanoporous foamed structures are now ongoing in our group to determine various factors of closed porosity. Acknowledgment. The authors gratefully acknowledge Ms. Kuniko Yamanaka for her thorough technical support in our TEM observations. Financial aid for our research activity by Hitachi Chemical Co. Ltd. is greatly appreciated. The authors also express gratitude to Mr. K. Abe, Hitachi Cemical Co. Ltd., for many useful suggestions and great collaborations in the present work. K.K. is very grateful to Prof. Atsushi Suzuki (Yokohama National University) for many useful critical comments on the present work. LA030427U
