Langmuir 2005, 21, 2545-2551
2545
Synthesis of Nanoporous Graphite-Derived Carbon-Silica Composites by a Mechanochemical Intercalation Approach Y.-H. Chu,†,§ Z.-M.Wang,*,†,‡ M. Yamagishi,‡ H. Kanoh,| T. Hirotsu,‡ and Y.-X. Zhang§ PRESTO, Japan Science and Technology Agency, Applied Interfacial Chemistry RG, Research Institute for Environmental Management Technology, National Institute of Advanced Industrial Science and Technology, 2217-14 Hayashi-cho, Takamatsu-shi, Kagawa 761-0395, Japan, Department of Chemical Engineering, Sichuan University, Chengdu, Sichuan 610065, China, and Center for Frontier Electronics and Photonics, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan Received August 11, 2004. In Final Form: December 30, 2004 A mechanochemical intercalation approach which applies a simple mechanical milling to induce intercalation reaction was applied to introduce controlled amount of tetraethoxylsilane (TEOS) into surfactant-preexpanded graphite oxide, and the relationships between the intercalation structure, the porosities of the calcined products, and the Si addition were examined. It was found that a small added amount of TEOS produced a more expanded ordered layer structure with the interlayer distance and silicon content increasing with the amount of TEOS added, although a large amount of added TEOS easily induces layer delamination, resulting in a less ordered structure. The silica structure in the composite is changed from a disordered structure having enhanced bond strain to a condensed silica network when the amount of TEOS added increases. The porosities of the final calcined samples increase with the increase of silicon content but then decrease slightly after reaching a maximum where silicon content starts to become constant, indicating that both silicon content and the composition state of silica particles and carbon layers play important roles in porosity formation.
Introduction A fundamental understanding and technical application in the fields of adsorption/separation, catalysis, and so forth require a variety of nanoporous materials with tailored pore geometries and surface specialties.1-3 The introduction of robust propping species into layered precursors by pillaring and intercalation techniques is a unique approach for obtaining these nanoporous materials.4-13 Utilization of layered precursors with a very thin layer thickness is crucial and challenging in this route because these materials permit the final nanoporous * Author to whom correspondence should be addressed. Tel: (+)81-87-869-3574; fax: (+)81-87-869-3550; e-mail: zm-wang@ aist.go.jp. † Japan Science and Technology Agency. ‡ National Institute of Advanced Industrial Science and Technology. § Sichuan University. | Chiba University. (1) Barton, T. J.; Bull, L. M.; Klemperer, W. G.; Loy, D. A.; McEnaney, B.; Misono, M.; Monson, P. A.; Pez, G.; Scherer, G. W.; Vartuli, J. C.; Yaghi, O. M. Chem. Mater. 1999, 11, 2633. (2) Schubert, U.; Hu¨sing, N. Synthesis of Inorganic Materials; WileyVCH: Weinheim, Germany, 2000. (3) Polarz, S.; Smarsly, B. J. Nanosci. Nanotechnol. 2002, 581. (4) Barrer, R. M. Zeolites and Clay Minerals as Sorbents and Molecular Sieves; Academic Press: New York, 1978. (5) Brindly, G. W.; Sempels, R. E. Clay Miner. 1977, 12, 229. (6) Pinnavaia, T. J. Science 1983, 220, 365. (7) Landi, M. E.; Aufdembrink, B. A.; Chu, P.; Jonhson, I. D.; Kirker, G. W.; Rubin, M. K. J. Am. Chem. Soc. 1991, 113, 3189. (8) Wong, S.-T.; Cheng, S. Inorg. Chem. 1992, 31, 1165. (9) Inagaki, S.; Fukushima, Y.; Kuroda, K. J. Chem. Soc., Chem. Commun. 1993, 680. (10) Pinnavaia, T. J. Mater. Chem. Adv. Chem. Ser. 1995, 245, 283. (11) Kosuge, K.; Tsunashima, A. J. Chem. Soc., Chem. Commun. 1995, 2427. (12) Yamanaka, S.; Kunii, K.; Xu, Z.-L. Chem. Mater. 1998, 10, 1931. (13) Jeong, S. Preparation of Molecular Sieves by Pillaring of Synthetic Clays. In Surfaces of Nanoparticles and Porous Materials; Schwarz, J. A., Contescu, C. I., Eds.; Marcel Dekker: New York; pp 15-30, 1999.
materials to have a more exposed surface and higher specific surface area because of the thin walls. Graphite is a layered material having the thinnest atomic layer of all the layered materials. A nanoporous material with thin graphitic layers has been sought with expectations of high-efficiency adsorption and gas-storage capability.14-17 High-efficiency performance for catalysis and specific adsorption under moist conditions requires porous materials having active species coexisting in hydrophobic circumstances, such as carbonaceous material-supporting metal oxides, and so forth.18,19 Porous nanocomposites of thin hydrophobic carbon layer and nanosized metal oxide particles are the ideal form for these purposes. Because of its neutral wall nature, some species can be intercalated into graphite layers under controlled conditions to form the graphite-intercalated compounds typical of a nonporous sandwiched structure.20 Oxidation of graphite in a liquid phase containing strong oxidants donates oxygen to the basal planes of the graphite layers, leading to the production of graphite oxide (GO) with thin walls and a great minus charge density balanced by countercations such as protons in the interlayer galleys.21-28 GO shows rich intercalation chemistry owing (14) Matranga, K. R.; Myers, A. L.; Glandt, E. D. Chem. Eng. 1992, 7, 1569. (15) Tan. Z.; Gubbings, K. E. J. Phys. Chem. 1990, 94, 6061. (16) Dillon, A. C.; Jones, K. M.; Bekkedahl, T. A.; Kiang, C. H.; Bethune, D. S.; Heben, M. J. Nature 1997, 386, 377. (17) Dresselhaus M. S.; William, K. A. MRS Bull. 1999, 24, 45. (18) Radovic, L. R.; Rodriguez-Reinoso, F. Chem. Phys. Carbon 1997, 25, 243. (19) Yang, R. T. Adsorbents - Fundamentals and Applications; Wiley Inter-Science: Hoboken, NJ, 2003. (20) Pierso, H. O. Handbook of Carbons, Graphite, Diamond, and Fullerenes; Noyes Publication: Park Ridge, NJ, 1993. (21) Bailar, J. C., Jr.; Emele´us, H. J.; Nyholm, S. R.; TrotmanDickenson, A. F. Comprehensive Inorganic Chemistry; Pergamon Press: Oxford, UK, 1973. (22) Hoffman, U.; Frenzel, A.; Csalan, E. Liebigs Ann. Chem. 1934, 510, 1.
10.1021/la047983o CCC: $30.25 © 2005 American Chemical Society Published on Web 02/18/2005
2546
Langmuir, Vol. 21, No. 6, 2005
to its excellent swelling and intercalation properties, similar to clay minerals.29-38 Nanoporous materials containing thin graphite layers can thus be synthesized from GO by a soft chemical approach. We have reported that a nanoporous graphitic composite (NPGC) containing silica particles can be obtained by hydrolyzing tetraethoxylsilane (TEOS) in the expanded GO interlayer.39,40 The obtained novel porous nanocomposite of carbon and silica has a high surface area greater than 1000 m2/g and presents medium hydrophilicity falling between the typical activated carbon and hydrophilic silica particles. However, the present method requires a large amount of organic silicon sources and long hydrolyzing time for TEOS. A mechanochemical method, which applies mechanical energy in the form of grinding, milling, and so forth to bring about chemical reaction,41 can be a simple but very effective means to improve the TEOS intercalation reaction into layer materials. This method has been used for the solid-state ion exchange of zeolites and also intercalation chemistry of layered materials such as kaolinite-type minerals.42,43 Here, we applied the mechanochemical method at ambient temperature and atmospheric conditions to introduce a controlled amount of TEOS into the interlayer spaces of GO preexpanded by a long-chain surfactant. We have obtained new findings regarding the intercalation structure of silicon species after introduction and the resulting silica structure in the final nanoporous composites. The formation mechanism of the nanoporous composite by this method with respect to the relationship among porosities, silicon addition, and silica-carbon composition structures is discussed. Experimental Section Material Synthesis. GO was synthesized from natural graphite by Staudenmier’s method.44 Chemical analysis of carbon and hydrogen, and subtraction of physisorbed water content measured by thermal gravimetry (from the weight loss between 298 and 393 K), gives the chemical formula C8O4.8H1.4 (oxygen content was from subtraction of carbon and hydrogen), which is comparable to those reported in the literature prepared by Staudenmier’s method.23 The cation exchange capacity (CEC) of the prepared GO was measured by the following procedure: 60 (23) Scholz, W.; Boehm, H. P. Anorg. Allg. Chem. 1969, 369, 327. (24) Nakajima, T.; Matsuo, Y. Carbon 1994, 32, 469. (25) Hamwi, A.; Marchand, V. J. Phys. Chem. Solids 1996, 57, 867. (26) Matsuo, Y.; Sugie, Y. Carbon 1997, 35, 301. (27) Lerf, A.; He, H.; Forster, M.; Klinowski, J. J. Phys. Chem. 1998, 102, 4477. (28) He, H.; Klinowski, J.; Forster, M.; Lerf, A. Chem. Phys. Lett. 1998, 287, 53. (29) Kovtyukhova, N. I.; Karpenko, G. A.; Chuiko, A. A. Russ. J. Inorg. Chem. 1992, 37, 566. (30) Kotov, N. A.; De´ka´ny, I.; Fendler, J. H. Adv. Mater. 1996, 8, 637. (31) Matsuo, Y.; Tahara, K.; Sugie, Y. Carbon 1997, 35, 113. (32) Kyotani, T.; Moriyama, H.; Tomita, A. Carbon 1997, 35, 1185. (33) De´ka´ny, I.; Kru¨ger-Grasser, R.; Weiss, A. Colloid Polym. Sci. 1998, 276, 570. (34) Kovtyukhova, N. I.; Ollivier, P. J.; Martin, B. R.; Mallouk, T. E.; Chizhik, S. A.; Buzaneva, E. V.; Gorchinskiy, A. D. Chem. Mater. 1999, 11, 771. (35) Matsuo, Y.; Hatase, K.; Sugie, Y. Chem. Commun. 1999, 43. (36) Liu, P.; Gong, K.; Xiao, P.; Xiao, M. J. Mater. Chem. 2000, 10, 933. (37) Cassagneau, T.; Fendler, J. H.; Jonhson, S. A.; Mallouk, T. E. Adv. Mater. 2000, 12, 1363. (38) Xu, J.; Hu, Y.; Song, L.; Wang, Q.; Fan, W.; Chen, Z. Carbon 2002, 40, 445. (39) Wang, Z.-M.; Hoshinoo, K.; Xue, M.; Kanoh, H.; Ooi, K. Chem. Commun. 2002, 1696. (40) Wang, Z.-M.; Hoshinoo, K.; Shishibori, K.; Kanoh, H.; Ooi, K. Chem. Mater. 2003, 15, 2926. (41) Laszlo, T. J. Metals 2000, 52 (1), 12. (42) Karge, H. G. Stud. Surf. Sci. Catal. 1997, 105, 2043. (43) Yariv, S.; Lapides, I.; Michaelian, K. H.; Lahav, N. J. Therm. Anal. Calorimetry 1999, 56, 865. (44) Staudenmaier, L. Ber. Dtsch. Chem. Ges. 1989, 31, 1481.
Chu et al. mg of GO was immersed in a 10-mL mixture of 0.1 N NaOH and 0.1 N NaCl solutions. After vibration in a shaker at 298 K for 4 days, the mixture was filtrated by aspirator and 5 mL of the filtrate was added to a 0.1 N HCl solution with adequate volume to allow an excess amount of protons to remain in the solution. The solution was then back-titrated by 0.1 N NaOH to give the amount of exchangeable protons in GO. The CEC of this GO was determined as 4.92 meq-[H+]‚g-1, being 4 to 5-fold that of montmorillonite (0.8 to 1 meq‚g-1).45 The conventional method for preparing NPGC from a GO precursor was described in detail in our previous papers.39,40 Preexpansion of GO interlayers with long-chain surfactant molecules is one of the critical steps and the preexpanded sample is the base material for further mechanochemical intercalation of silicon species. To achieve this preexpansion, GO was dispersed in a 0.05 N NaOH solution by ultrasonic treatment, after which 600 mL of 1.8 mM hexadecyltrimethylammonium (C16TMA) bromide aq was added dropwise. GO layers then reassemble by adsorbing surfactant molecules between the layers and precipitate from the solution. After filtration with an aspirator and sufficient washing with distilled water, the surfactant-intercalated GO precipitate (denoted GOC16) was collected and dried in air at 333 K overnight. The surfactant intercalation amount was determined to be 1.48 mmol‚(g-GOC16)-1 or 4.86 mmol‚(g-GO-C)-1 by chemical analysis of the nitrogen content, which is equivalent to 55% of the total ion exchange sites. Generally, the subsequent hydrolysis of TEOS in GO interlayers is done by bringing the GOC16 sample in contact with a large amount of TEOS liquid (in a TEOS/GOC16 molar ratio of 64:1) and continuing to vibrate the mixture at 298 K for 1 week.40 This ordinary method consumes an excessive amount of TEOS (most of which is thrown away after centrifugation) and requires a very long reaction time. Here, we use the following mechanochemical procedure to add the appropriate amount of TEOS into GOC16 and promote a fast hydrolysis reaction of TEOS in GO interlayers: At first, the required amount of GOC16 sample was measured and placed into an agate mortar. A predetermined volume of TEOS was accurately measured by a Finn pipet and added, resulting in samples with TEOS molar ratios of 0.5, 1.0, 3.0, 6.0, 9.0, 9.7, 12, 15, and 22-fold of the amount of the total ion-exchangeable sites on GO, respectively. The impregnated solid and TEOS liquid was then quickly mixed with a pestle mill, after which the sample was exposed to air overnight. The obtained TEOS-added samples were calcined under vacuum at 823 K for 2 h to give the nanoporous carbon/silica composites. The samples before and after calcination are denoted GOC16S-n and GOC16S-n-823, respectively, where n stands for the molar ratio of TEOS added to the amount of the total ion-exchangeable sites in GO. Characterization. X-ray diffraction patterns (XRD) of samples were measured by a Rigaku 1200 or 2100 system using Cu KR radiation (λ ) 0.15418 nm) in the 2θ range of 1.5-50°. The operating tube voltage and current were 40 kV and 30 mA, respectively. Data were collected at a scanning speed of 2°‚min-1 and a sampling angle interval of 0.02°. The weight loss changes of GOC16S-n in air were monitored at 303 K by a MacScience-made TG/DTA 2000-type thermal gravimetry (TG)/differential thermal analysis (DTA) apparatus. The silicon contents in GOC16S-n-823 were determined from the retained weights after burning the carbon components in the samples by a TG program which raised temperature from room temperature (RT) to 1073 K at a ramp rate of 10 K‚min-1 under an air stream. Commercially available R-Al2O3 was used as the reference material in TG/DTA measurements. The surface species of samples and their changes after TEOS addition were examined by diffuse reflectance infrared Fourier transform (DRIFT) spectra, which were measured by a Nicolet NEXUS 470-type FT-IR spectrometer with a MCT (Hg1-xCdxTe) detector under a continuous flow of nitrogen gas of 99.999% purity. DRIFT spectra were recorded from 256 scans at a resolution of 2 cm-1 after CO2 species completely disappeared from the spectra. The nitrogen adsorption isotherms were measured at 77 K by a commercial volumetric apparatus (Belsorp 18A, Belsorp Co. or (45) Yamanaka, S.; Hattori, M. Catal. Today 1988, 2, 261.
Synthesis of Nanoporous Composites
Langmuir, Vol. 21, No. 6, 2005 2547
Figure 1. XRD patterns of (a) GOC16 and GOC16S-n with n ) (b) 3, (c) 6, (d) 9, (e) 9.7, (f) 12, (g) 15, and (h) 22. Autosorp-1, Quantachrome Co.). Samples were calcined at 823 K for 2 h under vacuum before adsorption. The microscopic features of samples were observed by a JEOLmade JSM 6330F type field emission scanning electron microscope (FE-SEM) at an accelerating electronvoltage of 2 kV.
Results and Discussion Intercalation Properties of TEOS into SurfactantPreexpanded GO from XRD Measurement. Figure 1 shows the XRD patterns of GOC16 and GOC16S-n about 3 h after TEOS introduction. The prepared GO presented a sharp (001) peak at d ) 0.831 nm (2θ ) 10.64°) and a (002) peak at d ) 0.415 nm (2θ ) 21.38°) with less intensity (not shown for simplicity), indicating a higher order repeating layered structure. Surfactant intercalation generally expands the interlayer distance of GO but the value of the c-axis repeating distance (Ic) of GO after surfactant intercalation varies with the intercalated conditions (amount of surfactant, the drying state of the sample, etc.). In this preparation, 55% replacement of ion exchange sites by surfactant molecules leads to a diffraction peak at 2θ ) 4.98° which corresponds to an Ic value (1.77 nm) greater than 2-fold that of GO. This Ic value can be explained by an intercalation structure in which surfactant molecules are arranged either tilted at an angle of 32° or by a pseudo-trilayer stacking46,47 where the surfactant length and head diameter and GO layer thickness are considered to be 2.15 nm,6 ∼0.4 nm,48 and 0.58 nm (from subtraction of 0.831 and one water layer with diameter 0.25 nm), 31 respectively. In the conventional approach, further treatment of GOC16 with an excess amount of TEOS for a long time leads to the complete disappearance of the ordered layer structure.39 However, as shown in Figure 1, samples after TEOS addition up to n ) 15 present an evident peak at a smaller diffraction degree, indicating that the mechanochemical approach is very effective at introducing TEOS molecules into GO layers, resulting in an expanded interlayer with a confirmed regular layered structure. The Ic value of GOC16S-n increases with the amount of TEOS added, manifesting a gradually enhanced TEOS inclusion. However, the diffraction peak with Ic ) 3.59 nm becomes (46) Lagaly, G. Solid State Ionics 1986, 22, 43. (47) Ogawa, M.; Kuroda, K. Bull. Chem. Soc. Jpn. 1997, 70, 2593. (48) The surfactant head size (∼0.4 nm) was estimated from the molecular structure.
Figure 2. (A) Time course of Ic values from XRD measurements of GOC16S-n samples at n ) (a) 1, (b) 3, (c) 6, (d) 9, and (e) 12, and (B) the weight ratio change of GOC16S-6 measured by a TG balance in air at RT.
broader and more ambiguous when n is higher than 22. We can estimate the structure of intercalated surfactant molecules from the Ic value using the above-mentioned parameters of the molecular size and the GO layer thickness. Thus, the increase of Ic from 1.77 to 2.52 nm causes a change in the tilting angel of the surfactant molecules from 32° to 63.9°, and Ic ) 2.81 nm represents an intercalated structure where one monolayer of the surfactant molecules is arranged completely vertically to the GO layers. Therefore, the inclined surfactant molecules intercalated in GO layers are gradually forced upright by inclusion of TEOS molecules between their chains and the GO layers. Further adsorption of TEOS into GO interlayers by a condition at n > 9 gives rise to a gallery height greater than one monolayer of surfactant (the surfactant length), thus weakening the attraction forces between the GO layers. Accordingly, excessive TEOS addition easily induces delamination of the GO layers, which accounts for the less ordered structure of the GOC16S-22 sample. When exposing the GOC16S-n samples to air at room temperature (RT), the (001) diffraction angles gradually increase and the peaks become broader with increasing exposure time. To examine the change process, Ic values and weights of samples were measured at a determined time interval after TEOS addition by X-ray diffractometer and TG balance, respectively. Figure 2 shows the time course of Ic values of GOC16S-n samples and the change in the ratio of the weight of the GOC16S-6 sample, W, over the initial weight (the weight immediately after TEOS addition), Winitial. Ic values decrease with the increase in air exposure time, becoming almost a constant value after 9 h. A greater TEOS addition (n value) gives a greater constant Ic value, each of which is still higher than that of GOC16 (1.77 nm). Corresponding to the Ic change, the
2548
Langmuir, Vol. 21, No. 6, 2005
Figure 3. DRIFT spectra of (a) GO, (b) GOC16, and GOC16S-n at n ) (c) 3, (d) 6, (e) 9, (f) 9.7, (g) 12, and (h) 15.
sample suddenly loses weight at the initial time and becomes stable after about 5 h; the stable weight ratio is also higher than that of GOC16 over the initial GOC16S-6 sample (18.6 wt %). Therefore, these behaviors indicate that, while the added TEOS molecules may evaporate when exposed to air, some of the Si species is still stable between the GO interlayers after a long time. DRIFT Spectra. Figure 3 shows the DRIFT spectra of GO, GOC16, and GOC16S-n samples after being exposed to air for 1 day. The characteristic peaks in the spectra of GO are those at 1723 cm-1 and at 3624 cm-1, which can be ascribed, respectively, to the CdO vibration of carboxyl or carbonyl species and the O-H stretching band of alcohol species.49,50 These two species disappear upon intercalation of the surfactant, indicating that they are the ion-exchange centers for surfactant molecules. Some of the peaks at 1000-1500 cm-1 in GO, which are absent in that of GOC16, can be ascribed to the bending or stretching bands of the alcohol or carboxyl groups,49 while others still remaining after surfactant intercalation (e.g., at 967 cm-1, 1077 cm-1, etc.), are ascribed to the absorption peaks due to the epoxy and other oxygen-containing groups.49 The spectrum of GOC16 also shows a peak at 3026 cm-1, which is ascribed to the cyclic -CH2 or epoxy-related -CH2 group possibly existing in the vicinity of the edge. The peak at around 1480 cm-1, and those at 2850-2980 cm-1, are ascribed to the asymmetric bending band of the -CH3 group or the scissoring band of the -CH2 group, and the asymmetric and symmetric stretching bands of -CH3 and -CH2 groups from the intercalated surfactant. Spectra of samples after TEOS addition exhibit obvious bands in the 1000-1300 cm-1 range, which can be ascribed to the Si-O-Si bonding of silica structure,51 a conclusion also supported by the appearance of the bands at 970 and 3691 cm-1 after TEOS addition, which are assigned to the (49) Fanning, P. E.; Vannice, M. A. Carbon 1993, 31, 721. (50) Zawadzki, J. Chem. Phys. Carbon 1989, 21, 147. (51) Iler, R. K. The Chemistry of Silica - Solubility, Polymerization, Colloid and Surface Properties, and Biochemistry; Wiley-Interscience: New York, 1979.
Chu et al.
Si-O stretching band of silanol groups and to the -O-H stretching band of terminal silanol groups (or vicinal -OH species with H-bonding), respectively.52,53 Thus, DRIFT spectra indicate formation of a silica structure even at the preparatory stage when the n value is small. More detailed discrimination of the band species at around 1000- 1300 cm-1 will provide further knowledge concerning the changing process of the silica structure. It is noticeable that the band position in this range varies in accordance with the amount of n values: only bands with smaller wavenumbers (1100-1205 cm-1) are observed for samples prepared at a small n value (n less than 9.7) while these bands are gradually blue shifted to 1233 cm-1 together with the development of the band at around 480 cm-1 when the n value surpasses 9.7. According to the extensively reported results in the literature,53 the initially evolved broad bands around 1100-1205 cm-1 at n less than 9.7 can be assigned to the composed bonds of disorderinduced LO4-TO4 pair modes at ∼1170 cm-1 (LO4) and ∼1200 cm-1 (TO4), which correlate to a disordered SiO2 structure because of the enhanced bond strain. This bond strain is often reported in research on thin silica films having a large surface area53,54 and may also be easily produced by confinement of the silica species between GO layers. In addition, these bands, together with a small band at around 603 cm-1, may also be responsible for some small cyclic siloxane ring structures (fourfold rings at around 1080 and 1200 cm-1) which are often observed as a form of oligomers of silicon tetrahedron linkage in the beginning stage of the sol-gel-derived silica formation process.54 On the other hand, the peak at around 1233 cm-1 can be ascribed to the LO3 mode of Si-O-Si, the enhanced intensity of which is indicative of silica network condensation.54 Thus, the above results manifest that the silica structure and the resultant composition with GO layers are different, depending on the added TEOS amount. To confirm the evaporation of TEOS from the sample after addition and to differentiate whether the peaks at around 1100-1205 cm-1 are from TEOS molecules remaining in the samples, the change of DRIFT spectra with time and temperature after TEOS addition were monitored and the spectra are shown in Figure 4. The main difference between the spectrum of the sample immediately after preparation (Figure 4a) and that shown in Figure 3e is the clear appearance of sharp peaks in the former, for example, at 799, 967, 1112, 1171, 1398, and 2975 cm-1, which are ascribed to the combined Si-O and C-O stretching, the H3CC bending, the combined asymmetric C-C and C-O stretching, the -CH3 bending, the -CH2 bending, and the asymmetric -CH3 stretching modes, respectively, from TEOS molecules.53,55 Since the -CH3 asymmetric stretching band has a position slightly higher than that of -CH2, the enhanced relative intensity of the shoulder peak at 2975 cm-1 is attributed to the increase in the fraction of -CH3 over -CH2 in the sample by TEOS addition (-CH3:-CH2 ) 1 in molar ratio in TEOS). The intesities of these peaks from TEOS molecules dramatically decrease after being exposed to air at RT for only 1 h (Figure 4b) and the peaks almost completely disappear by heat treatment at 393 K, indicating that the added TEOS molecules easily evaporate even though their evaporation temperature (442 K at ambient atmospheric (52) Muroya, M. Colloids Surf. 1999, 157, 147. (53) Innocenzi, P. J. Non-Cryst. Solids 2003, 316, 309. (54) Innocenzi, P.; Falcaro, P.; Grosso, D.; Babonneau, F. J. Phys. Chem. B 2003, 107, 4711. (55) Matos, M. C.; Ilharco, Almeida, R. M. J. Non-Cryst. Solids 1992, 147-148, 232.
Synthesis of Nanoporous Composites
Figure 4. DRIFT spectra of GOC16S-9 measured (a) immediately after preparation, (b) after being exposed to air at RT for 1 h, and after treatment at (c) 393 K, (d) 433 K, and (e) 498 K, each for 10 min.
pressure) is higher. Because the Si-OH species at 3691 cm-1 are observable from Figure 4b and the spectrum after heat treatment at 433 K is quite similar to that shown in Figure 3e, the retained Si species can be confirmed to be the silica species, not the molecular TEOS. Heat treatment at 498 K, beyond which GO layers chemically decompose, leads to discrimination of two peaks: one at 1609 cm-1 is assigned to the CdC stretching band of aromatic rings which are easily formed by GO decomposition,26 and another one at 1320 cm-1, although not confirmed, may be considered to be the combined peaks from both silica structures and the retained oxygencontaining groups after GO decomposition, for example, lactone species, acetate species, and so forth.49 Thus, the above behavior indicates that not all TEOS molecules added by the mechanochemical approach transform to silica in the GO interlayer. Some of the TEOS molecules are intercalated by hydrophobic interaction with the surfactant chain, expanding the GO layers and creating the regular structure, whereas others are rapidly hydrolyzed by interacting with the retained surface H+ sites on GO or the slight amount of water contained around the H+ sites on GO under an acid-catalytic condition. Gradual disordering of the layer structure with the exposure to air is partially related to the evaporation of molecularly intercalated TEOS molecules which are weakly bonded on the hydrophobic sites in GOC16. Porosity. Figure 5 shows the N2 adsorption isotherms on GOC16-823 and GOC16S-n-823 at 77 K. In contrast to the small N2 adsorption on GOC16, N2 adsorption on GOC16S-n-823 increases with the increase of added TEOS, reaching a maximum at n ) 6 and 9, but reversing and decreasing at n > 9. All the N2 adsorption isotherms present the shape typical of type IV or a composite shape of types I and IV.56-58 The evident adsorption hysteresis, which is closed in the vicinity of P/P0 ) 0.45, indicates the existence of mesoporosity in these samples. There is a (56) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603. (57) Gregg, S. J.; Sing, K. S. W. Adsorption, surface area, and porosity; Academic: New York, 1982. (58) Rouquerol, F.; Rouquerol, J; Sing, K. S. W. Adsorption by powders & porous solids, Academic: San Diego, CA, 1999.
Langmuir, Vol. 21, No. 6, 2005 2549
Figure 5. N2 adsorption isotherms at 77 K on GOC16-823 (9, 0) and GOC16S-n-823 with n ) 0.5 (solid right-pointing triangle, open right-pointing triangle), 1 (solid left-pointing triangle, open left-pointing triangle), 3 ([, ]), 6 (b, O), 9 (2, 4), 12 (1, 3), and 22 (square with dot, small open square). Filled and unfilled symbols represent the adsorption and desorption branches, respectively. Table 1. Porous Parameters of GOC16S-n-823 Obtained from N2 Adsorption n
ABET/ m2-g-1
V0, 0.95/ mL-g-1
V0,meso/ mL-g-1
V0,micro/ mL-g-1
RP/nm
0 0.5 1 3 6 9 9.7 12 22
60 155 314 460 1030 1040 950 945 780
0.17 0.27 0.34 0.45 0.73 0.70 0.60 0.65 0.55
0.22 0.32 0.33 0.41 0.48 0.50 0.35 0.46 0.40
∼0 ∼0 0.01 0.04 0.25 0.20 0.25 0.19 0.15
2.1 2.1 2.1 2.0 1.9 2.1 2.1 2.0 2.0
sharp uprising at P/P0 < 0.1 in N2 adsorption isotherms on samples with a higher adsorption amount, manifesting formation of microporosities in these samples. Although several methods such as Rs comparison plot can be applied to the detailed analysis of porosities,59 here, for the purpose of relative comparison, we only approximate the total specific surface area using that determined by the Brunauer-Emmett-Teller (BET) method at the range of P/P0 ) 0.05-0.1,57 ABET, the total pore volume using that determined from the adsorption amount at P/P0 ) 0.95,57 V0,0.95, the mesoporous volume, V0, meso, and the mesoporous radius, Rp, using the Berret-Joyner-Halenda (BJH) method, 57,58 and the micropore volume, V0,micro, from the difference of V0,0.95 and V0,meso. These porous parameters are shown in Table 1. All the samples have mesopores with an average pore radius around 2.0 nm. GOC16 and GOC16S-n-823 with small n values (not more than 3) almost have only mesoporosities, indicating the importance of the house-of-card structure of carbon layers in the formation of porosities. On the other hand, GOC16S-n-823 samples with n not less than 6 present high porosities with a specific surface area greater than or near 1000 m2/g, indicating the increased importance of the interstitial spaces between silica nanoparticles in the formation of their porosities. We have further checked the reproducibility of values of ABET and Si contents in GOC16 and GOC16S-n-823 by (59) Kaneko, K.; Ishii, C.; Kanoh, H.; Hanzawa, Y.; Setoyama, N.; Suzuki, T. Adv. Colloid Interface Sci. 1998, 76-77, 295.
2550
Langmuir, Vol. 21, No. 6, 2005
Figure 6. Relationships between TEOS addition and amounts of silicon in GOC16S-n (0) and GOC16S-n-823 (9), and SBET values of GOC16S-n-823 (b).
repeating experiments several times under the same conditions, the results of which are shown in Figure 6. The ABET values of GOC16S-n-823 basically increase with an increase of their silicon contents at n < 6, reaching more than 1000 m2‚g-1 at n ) 6 and 9, and then slightly decreasing with the increase of n at n > 9. The change in silicon content in GOC16S-n-823 correlates with that in GOC16S-n, both gradually increasing with the increase of TEOS added and becoming constant after n ) 6. Therefore, the increased silicon content in GOC16S-n-823 at n < 6 is responsible for the enhancement of porosities of the composites. This relationship stands, even for the samples showing less reproducibility in this range, that is, the specific surface area is proportional to the silicon content in each batch. In contrast to the high specific surface area and well-reproducible data obtained for the samples prepared at n ) 6-9, samples prepared at n > 9 demonstrate comparatively unstable values for porosities, despite reproducible data on silica content. The decreased porosity and the greater variances for these samples can be attributed to an unstable composition state of carbon layers and Si species because of GO layers more easily delaminating under these conditions. FE-SEM Images. Figure 7 shows the FE-SEM images of GO, GOC16, GOC16S-n, and GOC16S-n-823 samples. In accordance with the previous results,40 the stacked GO layers with wrinkled morphology (Figure 7a) are expanded and build a loosely packed structure by surfactant intercalation (Figure 7b). The loose morphology of GOC16 is not seriously changed by addition of TEOS at n ) 1 (Figure 7c), whereas explicit morphologies of thicker stacking plates can be observed for the GOC16S-6 sample
Chu et al.
Figure 7. FE-SEM images of (a) GO, (b) GOC16, GOC16S-n with n ) (c) 1, (d) 6, and (e) 22, and (f) GOC16S-6-823.
(Figure 7d) because of the formation of long-range layered structure by TEOS intercalation at this condition. The morphology of GOC16S-22 is similar to that of GOC16S-6, but its layered plates look more disordered in agreement with XRD results. Unlike that prepared by the conventional approach where carbon layers composed with silica particles present ambiguous morphology coming from both carbon layers and nonplate particles,40 the image of GOC16S-6-823 exhibits only a clear curved layered structure. This difference is indicative of a quite uniform composite structure between carbon layers and silica particles brought about by the mechanochemical intercalation approach under these conditions. Intercalation and Porosity Formation Mechanism. From the above results, we can summarize the mechanism of intercalation and porosity formation by this approach as depicted in Figure 8. The long-range layered structure of GO is expanded by surfactant intercalation, giving a structure where surfactant molecules are arranged in an inclined direction or by pseudo-trilayer stacking. Surfactant molecules replace about 55% of ionexchange sites, leaving the other 45% of ion-exchange sites retained in GO layers. These sites form a microscopic hydrophilic environment among the surfactant-modified hydrophobic GO interlayers and can provide sites for TEOS hydrolysis. Simple mechanochemical interaction of TEOS with surfactant-preexpanded GO samples leads to intercalation of TEOS, initially by the driving force of hydrophobic attraction in the interlayer, forming an expanded regularly layered structure. Some of the intercalated TEOS molecules then contact the hydrophilic microspace and are quickly hydrolyzed by interacting with the H+ sites and water molecules adsorbed around the
Synthesis of Nanoporous Composites
Langmuir, Vol. 21, No. 6, 2005 2551
proportional to their silicon content because of the silica bridging roles. However, a large amount of TEOS addition (n > 12) leads to a weakened interlayer attraction causing GO layers to delaminate easily. These induce a more condensed silica network and produce an unstable composition of silica with carbon layers, giving rise to the comparatively unstable acquisition of porosity by the calcined products. Conclusion
Figure 8. Schematic diagrams of intercalation and pore formation mechanism.
exchangeable H+ sites. As TEOS hydrolysis and successive Si-O-Si polymerization proceed, the physisorbed TEOS molecules gradually evaporate by being exposed to air. Both the expansion of the silica network and the loss of hydrophobic attraction by TEOS evaporation in GO layers induce disordering of the intercalated structure. Porosity formation is dependent not only on silica amount (TEOS addition) but also on the composition state of silica and carbon layers. Satisfactory composition of TEOS and GO layers is realized by the addition of a small amount of TEOS (n < 9), the porosity of the calcined products being
The mechanochemical approach, which applies mechanical milling to improve TEOS intercalation chemistry, is a simple but very effective way to introduce a controlled amount of TEOS into interlayers of surfactant-preexpanded GO. XRD results show that an intercalated structure with ordered layers can be achieved with a small amount of TEOS added but the intercalated structure becomes less ordered because of easy delamination of GO layers when a large amount of TEOS is added. While some of the added TEOS molecules weakly bonded by hydrophobic interaction evaporate, some of the TEOS molecules are quickly hydrolyzed by interacting with the H+ sites or the localized water molecules around H+ sites under an acid-catalytic condition. DRIFT results indicate that the silica structure is dependent on the amount of TEOS added: it presents as a more disordered structure or a cyclic tetragon structure by confinement in GO layers when a small amount of TEOS is added but becomes a more condensed network structure when a large amount of TEOS is added. Microscopic observation confirms the uniform composition of silica species and carbon layers at a smaller TEOS addition. The porosity of the calcined composites increases with the increase of silicon content at first and slightly decreases with the increase of TEOS addition after silicon contents reach a constant value, indicating the important role of the composition state of silica particles and carbon layers in the formation of porosities. LA047983O
