One-Step Exfoliation of Kaolinites and Their Transformation into

Jan 12, 2011 - Kaolinite nanoscrolls, rolled kaolinite sheets with a tubular form, were prepared by a one-step route in which intercalation of guest s...
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One-Step Exfoliation of Kaolinites and Their Transformation into Nanoscrolls Yoshiyuki Kuroda,† Kazuyuki Ito,† Kenichi Itabashi,† and Kazuyuki Kuroda*,†,‡ †

Department of Applied Chemistry, Faculty of Science and Engineering, Waseda University, 3-4-1 Ohkubo, Shinjuku-ku, Tokyo 169-8555, Japan, and ‡Kagami Memorial Research Institute for Materials Science and Technology, Waseda University, 2-8-26 Nishiwaseda, Shinjuku-ku, Tokyo 169-0051, Japan Received November 26, 2010. Revised Manuscript Received December 20, 2010

Kaolinite nanoscrolls, rolled kaolinite sheets with a tubular form, were prepared by a one-step route in which intercalation of guest species and swelling with solvent proceed at the same time. A methoxy-modified kaolinite was exfoliated by the intercalation of hexadecyltrimethylammonium chloride. The formation of nanoscrolls by the one-step route proceeded only by several alkyltrimethylammonium salts and 1-hexadecyl-3-methylimidazolium chloride. Intercalation of primary amines caused the formation of nanoscrolls by a two-step route in which the intercalation and swelling proceed separately. The successful one-step route is ascribed to the relatively weak interactions between the head groups of guest species and the interlayer surface of methoxy-modified kaolinite, and the interaction is thought to allow the formation of a flexible array of interlayer guest species for swelling. The tubular structure was mostly retained after the heat treatment at 600 °C to form hierarchically porous aluminosilicates with amorphous frameworks. The nanoscrolls intercalated organic guests species, which are not directly intercalated into methoxy-modified kaolinite, between the scrolled layers. The formation route to nanoscrolls is quite dependent not only on the surface modification of kaolinite but also on the structure of guest species.

1. Introduction Inorganic layered materials are quite useful as precursors of various nanomaterials, including nanohybrids,1 nanosheets,2 and nanoscrolls.3 A nanoscroll is a kind of multiwall nanotube with a scrolled form of nanosheets which have been prepared directly from layered materials or exfoliated nanosheets. Such materials possess inner cylindrical mesopores and lamellar walls retaining the crystal structures of parent layered materials. Nanoscrolls are useful because they are not only mesoporous materials but also layered materials that can intercalate various guest species.4 Nanoscrolls are promising as catalyst,5 photocatalyst,6 adsorbent,7 and so on. The crystal structures of some nanoscrolls can be transformed into other ones by the heat treatment, retaining nanotubular morphologies.8 The structural diversity of parent layered materials is important to obtain nanoscrolls, but not all *Corresponding author: e-mail [email protected]; Fax þ81-3-5286-3199; Tel þ81-3-5286-3199.

(1) (a) Ogawa, M.; Kuroda, K. Chem. Rev. 1995, 95, 399–438. (b) Alexandre, M.; Dubois, P. Mater. Sci. Eng. R 2000, 28, 1–63. (c) Carrado, K. A. Appl. Clay Sci. 2000, 17, 1–23. (2) (a) Sasaki, T. J. Ceram. Soc. Jpn. 2007, 115, 9–16.(b) Ma, R.-Z.; Sasaki, T. Adv. Mater. 2010, 22, 5082-5104. (3) (a) Schaak, R. E.; Mallouk, T. E. Chem. Mater. 2000, 12, 3427–3434. (b) Chen, Q.; Zhou, W.; Du, G.; Peng, L.-M. Adv. Mater. 2002, 14, 1208–1211. (c) Chen, D.-L.; Sugahara, Y. Chem. Mater. 2007, 19, 1808–1815. (d) Chuvilin, A. L.; Kuznetsov, V. L.; Obraztsov, A. N. Carbon 2009, 47, 3099–3105. (e) Ma, R.-Z.; Sasaki, T. Top. Appl. Phys. 2010, 117, 135–146. (4) Du, G.-H.; Chen, Q.; Yu, Y.; Zhang, S.; Zhou, W.-Z.; Peng, L.-M. J. Mater. Chem. 2004, 14, 1437–1442. (5) (a) Ntho, T. A.; Anderson, J. A.; Scurrell, M. S. J. Catal. 2009, 261, 94–100. (b) Kitano, M.; Nakajima, K.; Kondo, J. N.; Hayashi, S.; Hara, M. J. Am. Chem. Soc. 2010, 132, 6622–6623. (6) (a) Sun, X.; Li, Y. Chem.—Eur. J. 2003, 9, 2229–2238. (b) Maeda, K.; Eguchi, M.; Youngblood, W. J.; Mallouk, T. E. Chem. Mater. 2008, 20, 6770–6778. (7) Niu, H.; Zhang, S.; Zhang, X.; Cai, Y. ACS Appl. Mater. Interfaces 2010, 2, 1157–1163. (8) (a) Camerel, F.; Gabriel, J. P.; Batail, P. Chem. Commun. 2002, 1926–1927. (b) Kobayashi, Y.; Hata, H.; Salama, M.; Mallouk, T. E. Nano Lett. 2007, 7, 2142– 2145. (c) Zhang, H.; Li, G. R.; An, L. P.; Yan, T. Y.; Gao, X. P.; Zhu, H. Y. J. Phys. Chem. C 2007, 111, 6143–6148.

2028 DOI: 10.1021/la1047134

layered materials can be exfoliated and transformed into nanoscrolls. Therefore, chemical insights into the weakening of the interlayer interactions are required for the generalization of formation methods of nanoscrolls. Kaolinite is a 1:1-type clay mineral whose layer consists of AlO2(OH)4 octahedral sheets and SiO4 tetrahedral sheets. Because the structure of kaolinite sheet is asymmetric along c axis, kaolinite can accommodate guest species anisotropically in the interlayer nanospace. For example, an acentric arrangement of p-nitroaniline (pNA) shows nonlinear optical properties.9 Therefore, nanoscrolls consisting of kaolinite layers are promising as host materials for hierarchical organization of guest species. A similar structure is observed for naturally occurring tubular halloysite. Tubular halloysite is expected to be applied as nanofillers replacing asbestos,10 drug carriers,11 anticorrosion coating materials,12 and nonlinear optical materials.13 The production of halloysite is, however, low in nature, and its morphology is not uniform. Therefore, the formation of well-defined nanoscrolls from exfoliated kaolinite nanosheets is an important issue. The delamination14 of kaolinite has been investigated by the intercalation of organic substances into kaolinite. Weiss reported (9) (a) Kuroda, K.; Hiraguri, K.; Komori, Y.; Sugahara, Y.; Mouri, H.; Uesu, Y. Chem. Commun. 1999, 2253–2254. (b) Takenawa, R.; Komori, Y.; Hayashi, S.; Kawamata, J.; Kuroda, K. Chem. Mater. 2001, 13, 3741–3746. (10) Du, M.; Guo, B.; Jia, D. Eur. Polym. J. 2006, 42, 1362–1369. (11) (a) Vergaro, V.; Abdullayev, E.; Lvov, Y. M.; Zeitoun, A.; Cingolani, R.; Rinaldi, R.; Leporatti, S. Biomacromolecules 2010, 11, 820–826. (b) Du, M.; Guo, B.; Jia, D. Polym. Int. 2010, 59, 574–582. (12) (a) Lvov, Y. M.; Shchukin, D. G.; M€ohwald, H.; Price, R. R. ACS Nano 2008, 2, 814–820. (b) Fix, D.; Andreeva, D. V.; Lvov, Y. M.; Shchukin, D. G.; M€ohwald, H. Adv. Funct. Mater. 2009, 19, 1720–1727. (c) Abdullayev, E.; Price, R.; Shchukin, D.; Lvov, Y. ACS Appl. Mater. Interfaces 2009, 1, 1437–1443. (d) Abdullayev, E.; Lvov, Y. J. Mater. Chem. 2010, 20, 6681–6687. (13) Yelleswarapu, C. S.; Gu, G. R.; Abdullayev, E.; Lvov, Y.; Rao, D. V. G. L. N. Opt. Commun. 2010, 283, 438–441. (14) The term “delamination” is used for the division of a layered material into pieces of multilamellars consisting of layers, and the term “exfoliation” is used for the division of a layered material into piecies of unilamellars.

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that an improved plasticity of kaolinite used for ceramic processing at an early age in China was due to delamination of kaolinite into thin multilamellars.15 The delamination of kaolinite is achieved by its intercalation of various organic species (e.g., urea, ammonium acetate, hydrazine, and potassium acetate).15-17 Curled layers similar to nanoscrolls in the edges of multilamellars were observed. Singh and Mackinnon reported that hydration of kaolinite by repeated intercalation and deintercalation of potassium acetate allows exfoliation of kaolinite to form nanoscrolls spontaneously. Kaolinite layers roll along either the a or b axis of parent kaolinite.18 Gardolinski and Lagaly reported the effectiveness of interlayer modification of kaolinite for the formation of nanoscrolls in good yield by a cycle of intercalation and deintercalation of alkylamines.19 The deintercalation was performed with toluene, and its affinity to the modifying groups is important for the formation of nanoscrolls. The swelling of organically modified kaolinite is supposed to occur during the deintercalation process. Matusik et al.20 investigated the effect of the structural order of parent kaolinite on the formation of nanoscrolls by the method reported by Gardolinski and Lagaly. They have reported that the yield of nanoscrolls depends mainly on the efficiency of the interlayer modification and the poorness of crystallinity of kaolinite. Letaief and Detellier reported that the protonation of ethanolamine groups immobilized on the layer surface of kaolinite followed by the reaction with poly(acrylic acid) causes partial formation of nanoscrolls.21 Nanoscrolls derived from kaolinite have been studied as catalyst supports.22 Although the principle of the formation of nanoscrolls from kaolinite is yet unrevealed, the interactions between organic guest species and the interlayer surface of kaolinite are suggested to be crucial. Therefore, more useful combinations of interlayer modifiers and guest species are necessary to be explored. As an intermediate to further intercalate large guest species, we have focused on methoxy-modified kaolinite whose surface AlOH groups are converted to AlOMe groups.23 Intercalation of kaolinite with poly(vinylpyrollidone),24a long-chain alkylamines,24b nylon-6,24c and pNA,9 and the surface modification with propanediols25a and butanediols25b by using methoxy-modified kaolinite as an intermediate have been achieved. Therefore, methoxy-modified kaolinite is a useful host material for the investigation of the effect of various guest species on the formation of nanoscrolls. Moreover, high intercalation reactivity of such nanoscrolls modified with methoxy groups is also expected. Herein, we report the effect of organic guest species on the formation of nanoscrolls from methoxy-modified kaolinite. (15) Weiss, A. Angew. Chem., Int. Ed. 1963, 2, 697–703. (16) (a) Tsunematsu, K.; Tateyama, H. J. Am. Ceram. Soc. 1999, 82, 1589–1591.  (b) Valaskova, M.; Rieder, M.; Matejka, V.; Capkov a, P.; Slíva, A. Appl. Clay Sci. 2007, 35, 108–118. (17) Hope, E. W.; Kittrick, J. A. Am. Mineral. 1964, 49, 859–866. (18) Singh, B.; Mackinnon, I. D. R. Clays Clay Miner. 1996, 44, 825–834. (19) (a) Gardolinski, J. E. F. C.; Lagaly, G. Clay Miner. 2005, 40, 537–546. (b) Gardolinski, J. E. F. C.; Lagaly, G. Clay Miner. 2005, 40, 547–556. (20) Matusik, J.; Gawel, A.; Bielanska, E.; Osuch, W.; Bahranowski, K. Clays Clay Miner. 2009, 57, 452–464. (21) Letaief, S.; Detellier, C. Langmuir 2009, 25, 10975–10979. (22) Nakagaki, S.; Machado, G. S.; Halma, M.; Marangon, A. A. D.; Castro, K. A. D. D.; Mattoso, N.; Wypych, F. J. Catal. 2006, 242, 110–117. (23) (a) Tunnery, J. J.; Detellier, C. J. Mater. Chem. 1996, 6, 1679–1685. (b) Komori, Y.; Sugahara, Y.; Kuroda, K. J. Mater. Res. 1998, 13, 930–934. (c) Komori, Y.; Enoto, H.; Takenawa, R.; Hayashi, S.; Sugahara, Y.; Kuroda, K. Langmuir 2000, 16, 5506–5508. (24) (a) Komori, Y.; Sugahara, Y.; Kuroda, K. Chem. Mater. 1999, 11, 3–6. (b) Komori, Y.; Sugahara, Y.; Kuroda, K. Appl. Clay Sci. 1999, 15, 241–252. (c) Matsumura, A.; Komori, Y.; Itagaki, T.; Sugahara, Y.; Kuroda, K. Bull. Chem. Soc. Jpn. 2001, 74, 1153–1158. (25) (a) Itagaki, T.; Kuroda, K. J. Mater. Chem. 2003, 13, 1064–1068. (b) Murakami, J.; Itagaki, T.; Kuroda, K. Solid State Ionics 2004, 172, 279–282.

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Article Scheme 1. Schematic Representatives of the Exfoliation and Transformation of Methoxy-Modified Kaolinite into Nanoscrolls by the One- and Two-Step Routes

We found that the method using some quaternary ammonium salts, such as hexadecyltrimethylammonium chloride (C16TMACl) and hexadecylmethylimidazolium chloride (C16mimCl), is more effective to form nanoscrolls than the methods developed previously18-20 because the intercalation and swelling of the interlayer of the intercalation compound proceeded at the same time (one-step route, Scheme 1). Nanoscrolls were also formed by the intercalation and the subsequent swelling process when several primary amines were used (two-step route, Scheme 1). Although Letaief and Detellier et al. recently reported the intercalation of various organic salts including ionic liquids26 and surface modification with 3-aminopropyltriethoxysilane (APTES),27 the reactivity of such molecules with methoxy-modified kaolinite has not been investigated. On the other hand, Dong et al. have reported the formation of silica nanotubes by the hydrothermal reaction of calcined kaolin clay with hexadecyltrimethylammonium bromide (C16TMABr),28 but the formation mechanism should not be related to the intercalation and exfoliation of kaolinite because the crystal structure of kaolinite must be destroyed by calcination.

2. Experimental Section Materials. Kaolinite used in this study was well-crystallized KGa-1b from Georgia obtained from the Source Clays Repository of Clay Materials Society (U.S.A.). This kaolinite contains a small amount of TiO2 (ca. 1.6%) as an impurity.29 The kaolinite was ground to pass a 100-mesh sieve before use. A methoxy-modified (26) (a) Letaief, S.; Detellier, C. J. Mater. Chem. 2005, 15, 4734–4740. (b) Letaief, S.; Elbokl, T. A.; Detellier, C. J. Colloid Interface Sci. 2006, 302, 254–258. (c) Letaief, S.; Detellier, C. J. Mater. Chem. 2007, 17, 1476–1484. (d) Letaief, S.; Detellier, C. Clays Clay Miner. 2008, 56, 82–89. (e) Letaief, S.; Diaco, T.; Pell, W.; Gorelsky, S. I.; Detellier, C. Chem. Mater. 2008, 20, 7136–7142. (f) Tonle, I. K.; Letaief, S.; Ngameni, E.; Detellier, C. J. Mater. Chem. 2009, 19, 5996. (g) Letaief, S.; Detellier, C. Clays Clay Miner. 2009, 57, 638–648. (27) Tonle, I. K.; Diaco, T.; Ngameni, E.; Detellier, C. Chem. Mater. 2007, 19, 6629–6636. (28) Dong, W.-J.; Li, W.-J.; Yu, K.-F.; Krishna, K.; Song, L.-Z.; Wang, X.-F.; Wang, Z.-C.; Coppens, M. O.; Feng, S.-H. Chem. Commun. 2003, 1302–1303. (29) Pruett, R. J.; Webb, H. L. Clays Clay Miner. 1993, 41, 514–519.

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Table 1. Reaction Conditions of the Intercalation Reactions guest species solution or liquid temperature/°C reaction time/days C16TMABr

0.7 M solution in methanol TBABr 3 M solution in methanol C6N liquid C8N liquid C12N liquid pNA 0.3 M solution in ethanol a r.t.: room temperature.

r.t.a

1

r.t.

1

r.t. r.t. 40 r.t.

1 1 1 3

kaolinite was synthesized according to our previous report.23c N-Methylformamide (NMF, Wako Pure Chemical Ind. Ltd.) was reacted with kaolinite to form a kaolinite-NMF intercalation compound. The kaolinite-NMF intercalation compound was reacted with methanol (Wako Pure Chemical Ind. Ltd.) repeatedly at room temperature and was used without any drying processes. C16TMACl was purchased from Wako Pure Chemical Ind. Ltd. CnTMACl (n = 12, 14, and 18), CnTMABr (n = 12, 14, and 16), tetrabutylammonium chloride (TBACl), tetrabutylammonium bromide (TBABr), and hexadecyldimethylamine (C16DMA) were purchased from TCI Co., Ltd. C16mimCl was purchased from Sigma-Aldrich Co. Ethanol was purchased from Junsei Chemical Co., Ltd. These reagents were used for the formation of nanoscrolls by the one-step route. Hexylamine (C6N), octadecylamine (C18N), and toluene were purchased from Wako Pure Chemical Ind. Ltd. 3-Aminopropyltrimethoxysilane (APTMS) was purchased from Sigma-Aldrich Co. 3-Aminopropylethyl ether (APEE) and APTES were purchased from TCI Co., Ltd. These reagents were used for the formation of nanoscrolls by the two-step route. Octylamine (C8N) was purchased from Sigma-Aldrich Co. Dodecylamine (C12N) and pNA were purchased from Wako Pure Chemical Ind. Ltd. These reagents were used for the intercalation of nanoscrolls. Formation of Nanoscrolls by the One-Step Route. A methoxy-modified kaolinite23c (ca. 0.5 g) was dispersed in a solution containing 1 M guest species in methanol (20 mL) and stirred for 1 day. After the reaction, the samples were washed with ethanol three times. The guest species used for the preparation of nanoscrolls are C12TMACl, C14TMACl, C16TMACl, C18TMACl, C12TMABr, C14TMABr, C16TMABr, TBACl, TBABr, C16DMA, and C16mimCl. The nanoscroll prepared by using C16TMACl is denoted as NS-C16TMACl. NS-C16TMACl was heat-treated at 600 °C for 3 h under ambient conditions to remove organic groups. Formation of Nanoscrolls by the Two-Step Route. A methoxy-modified kaolinite was reacted with the following guest species of C6N, C18N, APEE, APTMS, and APTES. When liquid guest species (C6N, APEE, APTMS, and APTES) were used, methoxy-modified kaolinite was directly dispersed in the neat liquids. When C18N was used, methoxy-modified kaolinite was reacted with a solution of C18N (10 g) in methanol (5 mL) at 50 °C for 72 h according to our previous report.24b The intercalation compounds were redispersed in toluene (20 mL) and sonicated for 10 min for the swelling. The process was repeated three times. The nanoscrolls prepared by using APTMS are denoted as NS-APTMS. NS-APTMS was also heat-treated at 600 °C for 3 h under ambient conditions.

Intercalation of Guest Species between the Layers within Nanoscrolls. NS-C16TMACl or NS-APTMS (ca. 0.5 g in a wet state) was reacted with the guest solutions (15 mL) summarized in Table 1. NS-C16TMACl was washed with methanol three times before the reaction. NS-APTMS was used as-prepared. The reaction conditions in the cases of alkylamines24b and pNA9b were the same as those in our previous reports. Alkylamines were 2030 DOI: 10.1021/la1047134

Figure 1. (a) HRSEM image and (b, c) low- and high-magnification TEM images of NS-C16TMACl. Arrows A and B in (a) indicate the cross sections of nanoscrolls and curled layers, respectively. (d) TEM image of NS-APTMS. reacted without solvents. The intercalation compounds were then centrifuged. Because they were not washed after the reactions, the products contained remaining guest species. NS-C16TMACl reacted with C16TMABr and TBABr are denoted as NS-C16TMAClC16TMABr and NS-C16TMACl-TBABr, respectively, and NS-APTMS reacted with them are denoted as NS-APTMSC16TMABr and NS-APTMS-TBABr, respectively. Characterization. High-resolution scanning electron microscopy (HRSEM) images were recorded by a Hitachi S-5500 microsocpe at an accelerating voltage of 30 kV. Samples were observed without metal coating. Transmission electron microscopy (TEM) images, selective area electron diffraction (SAED) patterns, and energy-dispersive X-ray (EDX) spectra were obtained by a JEOL JEM-2010 microscope at an accelerating voltage of 200 kV. Low- and high-angle X-ray diffraction (XRD) patterns were measured by a Mac Science M03XHF22 diffractometer with Mn-filtered Fe KR radiation (40 kV, 20 mA) and by a Rigaku Ultima-III diffractometer using Cu KR radiation (40 kV, 40 mA), respectively. To obtain XRD patterns of slurries, samples were covered with a piece of polypropylene film on a sample holder. N2 adsorption-desorption isotherms were measured by a Quantachrome Autosorb-1 apparatus at -196 °C. Samples were evacuated at 120 °C for 3 h beforehand. The pore size distribution histrograms were calculated by the nonlocalized density functional theory (NLDFT). 29Si cross-polarization/magic angle spinning nuclear magnetic resonance (29Si CP/MAS NMR) spectra were recorded on a JEOL JNM-CMX 400 spectrometer at 79.42 MHz. Fourier transform infrared (FTIR) spectra were measured with a JASCO FT/IR 6100 spectrometer by a KBr disk.

3. Results and Discussion 3.1. Formation of Nanoscrolls by the One- and Two-Step Routes. i. Morphology. As a typical example of nanoscrolls prepared by the one-step route, NS-C16TMACl was mainly characterized. NS-C16TMACl was compared with NS-APTMS prepared by the two-step route. The cross-sectional HRSEM image of NS-C16TMACl clearly shows the formation of nanoscrolls (Figure 1a with an arrow A). The TEM image also shows Langmuir 2011, 27(5), 2028–2035

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Figure 2. Low-angle XRD patterns of (a) wet methoxy-modified kaolinite, (b) as-prepared NS-C16TMACl, (c) NS-C16TMACl after washing with ethanol, (d) methoxy-modified kaoliniteAPTMS intercalation compound before the dispersion process, and (e) as-prepared NS-APTMS. The asterisks indicate the peaks at 0.71 nm corresponding to unreacted kaolinite.

that the nanoscrolls possess a hollow cylindrical structure (Figure 1b). Because we have confirmed that nanoscrolls were formed before washing (Supporting Information, Figure S1), the methoxy-modified kaolinite was suggested to be swollen with methanol concurrently with the intercalation of C16TMACl (one-step route). Lamellar walls were also observed in the highly magnified TEM image (Figure 1c). Incompletely exfoliated and curled layers are also indicated in the HRSEM image (Figure 1a with an arrow B), which suggests that a kaolinite layer rolls concurrently with its exfoliation. The average inner and outer diameters of the nanoscrolls are ca. 17 nm (in the range of 1225 nm) and ca. 29 nm (21-39 nm), respectively. The sizes are similar to those prepared by the other methods.18-20 The length (100-850 nm) is smaller than the lateral size of parent methoxymodified kaolinite (200-1200 nm), implying that kaolinite layers are fragmented upon exfoliation. Most of the methoxy-modified kaolinite is transformed into nanoscrolls. When methoxy-modified kaolinite was reacted with APTMS, the morphology of the product is platelike (Supporting Information, Figure S2). After the swelling with toluene, the morphology changed into tubular (Figure 1d). Most of the methoxy-modified kaoliinte is also transformed into nanoscrolls, which is similar to the case by using C16TMACl. Thus, NS-APTMS is formed by the two-step route. The average inner and outer diameters of the nanoscrolls are ca. 21 and 40 nm, respectively, and these values are slightly larger than those of NS-C16TMACl. Moreover, although a few layers were mainly observed for the walls of NSC16TMACl, larger numbers of layers (typically 7-15 layers) were observed for the walls of NS-APTMS. These results imply that NS-APTMS are formed from larger kaolinite nanosheets. The size of exfoliated nanosheets probably depends on the degree of intercalation. Because NS-C16TMACl is formed by the concurrent intercalation and swelling, the exfoliation may occur before sufficient intercalation. On the other hand, NS-APTMS is formed by the exfoliation after sufficient intercalation. ii. Lamellar Structure in the Walls. The low-angle XRD pattern of wet methoxy-modified kaolinite shows a diffraction peak at 1.1 nm corresponding to the basal spacing (Figure 2a).23 That of NS-C16TMACl shows diffraction peaks at 3.9, 1.9, 1.3, and 0.95 nm without the diffraction at 1.1 nm (Figure 2b). Langmuir 2011, 27(5), 2028–2035

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Figure 3. High-angle XRD patterns of (a) kaolinite, (b) methoxymodified kaolinite, (c) NS-C16TMACl, and (d) NS-APTMS. The dotted lines indicate the positions of the diffractions of parent kaolinite.

These values are assignable to the basal spacing and the second-, third-, and fourth-order diffractions of the expanded lamellar structure in the walls of nanoscrolls, as shown in Figure 1c. It should be noted that the most part of the lamellar structures shrunk to show the distances from 2.0 to 2.5 nm in the TEM images. The decrease in the values is possibly due to the vacuum condition in the electron microscope and/or the damage by the electron beam irradiation. Considering the XRD patterns of methoxy-modified kaolinite intercalating C12TMACl, C14TMACl, or C18TMACl (Supporting Information, Figure S3), C16TMACl probably forms an tilted bilayer in the lamellar structure rather than a monolayer or an interdigitated bilayer (the detailed discussion is shown in the Supporting Information). The sharp peaks at 3.1 and 1.6 nm are attributable to excess C16TMACl forming crystals out of nanoscrolls. The XRD pattern of NSC16TMACl after washing with ethanol (Figure 2c) shows recovery of the diffraction at 0.86 nm, the value of which is same as that of dry methoxy-modified kaolinite. Therefore, the lamellar structure of NS-C16TMACl consists of methoxy-modified kaolinite layers. The low-angle XRD pattern of methoxy-modified kaoliniteAPTMS intercalation compound shows the basal spacing at 2.4 nm and the second- and third-order peaks at 1.2 and 0.79 nm (Figure 2d). After the formation of nanoscrolls by swelling with toluene, the basal spacing was shifted to 1.5 nm, and the secondorder peak almost disappeared (Figure 2e). The peak corresponds to the basal spacing of the lamellar structure within the walls of nanoscrolls. The decrease in the basal spacing suggests partial deintercalation of APTMS. The Si/Al ratio measured by EDX analysis was ca. 1.15, indicating that ca. 0.30 mol of APTMS per the unit of (Al2Si2O5(OH)4) remained in the nanoscrolls. Gardolinski and Lagaly reported that interlayer alkylamines were deintercalated after the formation of nanoscrolls,19b which is different from the present results. Because APTMS was polycondensed in the interlayer of methoxy-modified kaolinite, the deintercalation is probably suppressed due to the increase in molecular weight. High-angle XRD patterns show the changes in the lateral structure of the kaolinite sheet (Figure 3). The (020) and (110) diffractions are retained in the pattern of methoxy-modified kaolinite (Figure 3b), which indicates the retention of the lateral structure. Assuming that the crystal structure of kaolinite is elongated only along the c direction during the intercalation, DOI: 10.1021/la1047134

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Figure 5. N2 adsorption-desorption isotherms of (a) methoxyFigure 4. TEM images of (a, c) NS-C16TMACl whose long axes are parallel to the a and b axes of the parent kaolinite structure and (b, d) their corresponding SAED patterns.

the peaks at 0.442 and 0.426 nm are assignable to the (111) diffraction of wet methoxy-modified kaolinite and the (111) diffraction of the dry one. In the XRD patterns of NS-C16TMACl and NS-APTMS (Figure 3c,d), the (020) diffractions are slightly shifted to higher angle, which probably corresponds to the structural change due to the transformation into nanoscrolls. iii. Scrolling Directions. TEM images and the corresponding SAED patterns of NS-C16TMACl show two types of crystal orientations in the nanoscrolls, where the long axis of the nanoscrolls is parallel to the b or a axis of the parent kaolinite (Figure 4). In Figure 4a,b, the spots assignable to {020} of the parent kaolinite were observed in the direction of the long axis of nanoscroll, which indicates that the long axis is parallel to the b axis of the parent kaolinite structure. In Figure 4c,d, though the nanoscrolls were oriented aggregates, the spots assignable to {110} were observed in the direction tilted ca. 30° from the long axis, which indicates that the long axis is parallel to the a axis of the parent kaolinite structure. The spots assignable to (001) of the parent kaolinite structure were observed in the direction normal to the long axis of the nanoscrolls both in Figure 4b,d, which is characteristic of the structure of a nanoscroll. Nanoscrolls whose long axis is parallel to the b axis of parent kaolinite structure were predominantly observed. Such a tendency is also common to natural halloysite30 and experimentally prepared kaolinite nanoscrolls.18 The scrolling direction is explained by the crystal structure of parent kaolinite. Kaolinite is known to roll by the relaxation of its large misfit of the tetrahedral and octahedral sheets in the lateral dimension.31 Sato et al. have reported that kaolinite is slightly more flexible along the a direction than the b direction.32 In the present experiment, we also observed nanoscrolls whose long axis is mainly parallel to the b axis of parent kaolinite structure, which (30) (a) Honjo, G.; Kitamura, N.; Mihama, K. Clay Miner. Bull. 1954, 4, 133– 141. (b) Singh, B.; Gilkes, R. J. Clays Clay Miner. 1992, 40, 212–229. (31) (a) Bates, T. F.; Hildebrand, F. A.; Swineford, A. Am. Mineral. 1950, 35, 463–484. (b) Bailey, S. W. Sci. Geol., Mem. 1989, 86, 89–98. (c) Singh, B. Clays Clay Miner. 1996, 44, 191–196. (32) Sato, H.; Ono, K.; Johnston, C. T.; Yamagishi, A. Am. Mineral. 2005, 90, 1824–1826.

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modified kaolinite, (b) NS-C16TMACl, and (c) NS-C16TMACl heat-treated at 600 °C and the pore size distribution histograms of (d) NS-C16TMACl and (e) NS-C16TMACl heat-treated at 600 °C, calculated by the NLDFT method.

is consistent with the mechanism. In this manner, a kaolinite layer should mainly roll with its tetrahedral sheet on the outside of nanoscroll.31 iv. Characterization of the Porosity. The N2 adsorptiondesorption isotherms of methoxy-modified kaolinite and NSC16TMACl show a large increase in the porosity of kaolinite by the transformation into nanoscrolls (Figure 5a,b). The BET surface area increased from ca. 17 m2/g (methoxy-modified kaolinite) to ca. 71 m2/g (NS-C16TMACl). The isotherm of NSC16TMACl is similar to type II, although a gradual uptake and hysteresis at 0.80 < P/P0 < 0.95 are attributed to capillary condensation in large cylindrical mesopores of the nanoscrolls. The pore size distribution histogram also shows a monodispersed distribution around ca. 13 nm (Figure 5d), which is consistent with the inner diameter of the nanoscrolls estimated by the TEM observations. When NS-C16TMACl was heated at 600 °C, crystallinity was lowered (Supporting Information, Figure S4). The sharp peaks at 0.35 and 0.19 nm are due to TiO2 anatase present originally in KGa-1b kaolin, though the amount is quite low.29 The tubular morphology of the nanoscrolls is well retained (Figure 6a), whereas the lamellar structure in their walls disappeared (Figure 6b). The N2 adsorption-desorption isotherm of NSC16TMACl treated at 600 °C is like type II with a gradual uptake and the hysteresis at 0.80 < P/P0 < 0.95 (Figure 5c). The pore size distribution calculated by the NLDFT method shows the retention of large mesopores ca. 13 nm in diameter (Figure 5e). The BET surface area is ca. 170 m2/g, and the value is larger than that of uncalcined NS-C16TMACl. Because the mass loss of NSC16TMACl during the heat treatment is ca. 25%, the BET surface area is calculated to be ca. 95 m2/g, if the increase is caused only due to the mass loss of the pore walls. Consequently, it seems that this increase should be ascribed to the opening of cylindrical mesopores which were occluded with organic substances before the heat treatment. The N2 adsorption-desorption isotherms and the pore size distribution histograms of NS-APTMS and NS-APTMS heattreated at 600 °C are shown in Figure S5 in the Supporting Information, which also shows the presence of mesopores in Langmuir 2011, 27(5), 2028–2035

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Figure 6. (a) HRSEM and (b) TEM images of NS-C16TMACl

after the heat treatment at 600 °C.

Table 2. Effect of Guest Species on the Formation of Nanoscrolls guest species

degree of the transformationa

method

C12TMACl nob C14TMACl partialc one-step one-step C16TMACl abundantd C18TMACl nob C12TMABr abundantd one-step one-step C14TMABr partialc C16TMABr nob TBACl nob TBABr nob C16DMA nob C16mimCl abundantd one-step two-step C6N partialc C18N nob c APEE partial two-step two-step APTMS abundantd two-step APTES abundantd a The degree was estimated by a large number of TEM observations. b Nanoscrolls were not observed. c Both nanoscrolls and platelike kaolinite were observed with similar frequency (a typical TEM image is shown in Figure S10 in the Supporting Information). d Most of the samples were nanoscrolls, and platelike kaolinites were rarely observed.

nanoscrolls. The BET surface areas of NS-APTMS and NSAPTMS treated at 600 °C were ca. 49 and ca. 64 m2/g, respectively. These values are lower than those of NS-C16TMACl, which can be explained by the larger number of layers in the walls of NS-APTMS. The crystallinity of NS-APTMS was also lowered by the heat treatment. Because the crystallinity of methoxy-modified kaolinite is mostly retained in nanoscrolls, the nanoscrolls are promising as crystalline mesoporous hybrid materials with large mesopores which are useful to incorporate nanoparticles, polymers, biomolecules, and so on. Though the heat-treated nanoscrolls became amorphous, they are valuable as mesoporous aluminosilicates. The use of such nanoscrolls as building blocks of hierarchically porous materials like other aluminosilicate nanotubes are also expected.33 3.2. Effect of Guest Species on the Formation Routes. The reason why nanoscrolls are formed by different routes was investigated by using various organic guest species. Table 2 summarizes the effect of guest species. The results clearly show that the formation route depends on the head groups of guest species. Some alkyltrimethylammonium salts and C16mimCl lead to the formation of nanoscrolls via the one-step route, and primary amines except C18N lead to the formation via the twostep route. (33) (a) Kuroda, Y.; Tamakoshi, M.; Murakami, J.; Kuroda, K. J. Ceram. Soc. Jpn. 2007, 115, 233–236. (b) Kuroda, Y.; Kuroda, K. Sci. Technol. Adv. Mater. 2008, 9, 025018.

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29 Si CP/MAS NMR spectra of (a) methoxy-modified kaolinite, (b) NS-C16TMACl, and (c) NS-APTMS.

Figure 7.

i. One-Step Route. The one-step route was achieved by using guest species containing trimethylammonium groups probably because the interactions between the guest species and the interlayer surface of methoxy-modified kaolinite are relatively low. The interactions were characterized by the 29Si CP/MAS NMR spectra (Figure 7). The spectrum of NS-C16TMACl shows a shoulder at -92.8 ppm in addition to the main Q3 signal at -91.1 ppm, whereas those of methoxy-modified kaolinite and NSAPTMS show only the main signals at -91.3 and -91.9 ppm, respectively. Such an upfield signal is known for the intercalation of bulky organic molecules into kaolinite34 or the interlayer surface modification of kaolinte with bulky molecules.25a Several factors, such as shielding due to weakening of hydrogen bonds of the tetrahedral sheets,34 deshielding of Si atoms by interlayer cation,35-37 and the change in the Si environment due to rolling, should be combined to affect the chemical shift of the signal. The hydrogen bonding of the tetrahedral sheets affects the deshielding of Si atoms whose signals appear in downfield. The varied chemical shifts should be determined by the balance of the shielding due to weakening of the hydrogen bonding by the interlayer expansion and the deshielding due to positive charge of C16TMACl molecules and/or their hydrogen bonding. Because signals should shift to downfield by the interactions between the tetrahedral sheets and C16TMACl molecules, the signal at -92.8 ppm is possibly attributable to a part of Si atoms less interacted with guest species. In the case of NS-APTMS, the signal at -91.9 ppm can be explained by relatively strong hydrogen bonding which causes more deshielding of Si atoms in the tetrahedral sheets than the case of NS-C16TMACl. The rolling of kaolinite layer may also affect the Si environment, and the effect should depend on the diameter and the number of layers in the nanoscrolls, whereas there are no reports on the NMR spectroscopic data of kaolinite nanoscrolls. The signal at -67 ppm due to the T3 environment in the spectrum of NS-APTMS is attributed to polycondensed APTMS molecules in the interlayer. The relatively broader OH stretching bands in the FTIR spectrum of NS-C16TMACl than those of methoxy-modified kaolinite and NS-APTMS also suggest weak interactions between kaolinite layers and C16TMACl (Figure 8a,b). Therefore, these results suggest that tetrahedral sheets interact weakly with C16TMACl due to its bulky headgroup. The weak interaction may lead to the formation of a flexible array of interlayer guest species, which (34) Thompson, J. G. Clays Clay Miner. 1985, 33, 173–180. (35) Fitzgerald, J. J.; Hamza, A. I.; Dec, S. F.; Bronnimann, C. E. J. Phys. Chem. 1996, 100, 17351–17360. (36) Thompson, J. G. Clay Miner. 1984, 19, 229–236. (37) The 29Si NMR signal due to Q3 Si atoms in the tetrahedral sheets of pyrophillite, a 2:1-type clay mineral without interlayer cation, is observed at -95.0 to -96.7 ppm (ref 35). The signals of montmorillonite with interlayer cations are observed at -93 ppm (ref 36). The downfield shift of the signal is probably due to the deshielding of Si atoms by the interlayer cations.

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Figure 8. FTIR spectra of (a) methoxy-modified kaolinite, (b) NS-C16TMACl, and (c) NS-APTMS.

allows concurrent swelling of the intercalation compound with methanol. To understand the effect of the head groups of guest species on the formation of nanoscrolls, TBACl, TBABr, or C16DMA, which can be regarded as guest species with modulated head groups, were used. In these cases, nanoscrolls were not formed (Table 2) because the guest species were hardly intercalated into methoxy-modified kaolinite (Supporting Information, Figure S6a-c). Because the ammonium group of tetrabutylammonium cation is bulkier than that of C16TMACl, the interactions between tetrabutylammonium cation and the interlayer surface of methoxy-modified kaolinite are smaller than those in the cases using alkyltrimethylammonium salts. Although the bulkiness of C16DMA is similar to that of C16TMACl, the headgroup of C16DMA is neutral with less polarity than that of C16TMACl. The interaction between dimethylamino group and the interlayer surface is probably too weak for the intercalation. The effect of the head groups is summarized as follows. The strong interaction of amino group leads to the formation of nanoscrolls via the two-step route because the guest species form a rigid array of interlayer guest species. The relatively weak interaction of trimethylammonium group leads to the formation of nanoscrolls via the one-step route probably because of a flexible array of interlayer guest species that allow concurrent swelling. Too weak interaction of tetrabutylammonium salts or C16DMA suppresses the intercalation. C16mimCl acts almost in the same manner as C16TMACl (Supporting Information, Figure S6d). Therefore, the formation route probably depends only on the charge and the size of the head groups. The local structure of alkylammonium salts (chain length of long alkyl group and counteranion) significantly influences the formation of nanoscrolls (Table 2). When the counteranion is chloride, nanoscrolls were formed only when the chain length was 14 or 16. When the chain length was 12 or 18, nanoscrolls were not obtained, whereas they were intercalated (Supporting Information, Figure S3). The results imply that the interlayer spaces of the intercalation compounds were not swollen with methanol. Thus, the chain length is suggested to be influential to the affinity (e.g., hydrophobic interaction) of the interlayers of the intercalation compounds with methanol. When the counteranion is bromide, nanoscrolls were formed only when the chain length was 12 or 14. Because the appropriate chain length is shortened when the counteranion is enlarged from chloride to bromide, the total size and/or hydrophilicity of the guest species are considered to be important parameters. The surface-modifying group of the interlayer surface of kaolinite should also be influential to the swelling behavior because we have confirmed that kaolinites without surface modification (kaolinite, kaolinite-NMF intercalation 2034 DOI: 10.1021/la1047134

Figure 9. Low-angle XRD patterns of (a) NS-C16TMAClC16TMABr, (b) NS-APTMS-C16TMABr, (c) NS-C16TMAClTBABr, and (d) NS-APTMS-TBABr. The asterisks indicate the peaks at 0.71 nm corresponding to unreacted kaolinite. The filled circles indicate the peaks due to excess guest species.

compound, and kaolinite-dimethyl sulfoxide intercalation compound) are not transformed into nanoscrolls by the reaction with C16TMACl. ii. Two-Step Route. When primary amines were used as guest species (the two-step route), their tail groups show an influence on the swelling of the intercalation compound. All of the primary amines used in these experiments (alkylamines, APEE, and 3-aminopropylalkoxysilanes) were intercalated into methoxymodified kaolinite. However, in the swelling process, nanoscrolls were not formed by using C18N, and the degree of the formation of nanoscrolls depended on the kind of guest species. Nanoscrolls were obtained when C6N was used, whereas they were not obtained when C18N was used, which is possibly due to the difference in the flexibility of the guest arrays and/or the affinities between the intercalation compounds with toluene. On the other hand, the effect of APEE was similar to that of C6N, even though a relatively hydrophilic ether group is incorporated in the tail group. Therefore, we suppose that the flexibility of the guest arrays is more influential than their affinity with toluene. In the literature by Gardolinski and Lagaly,19b nanoscrolls were obtained in the both cases of C6N and C18N. In these cases kaolinite was modified with bulky groups, such as the 1,3-butanedioxy group. Thus, the conformation of C18N may be disturbed by the presence of the modifying groups, which may lead to the formation of a slightly flexible interlayer guest species. Abundant nanoscrolls were obtained when APTMS and APTES were used as guest species. These guest species are polycondensed in the interlayer of methoxy-modified kaolinite, which is probably effective to weaken the interlayer interactions between kaolinite layers. Consequently, a small difference in the structure of primary amines is not influential to the formation of nanoscrolls, unlike the one-step route. In the one-step route, the interactions among kaolinite layers, guest species, and methanol should be balanced to achieve both sufficient intercalation and swelling. On the other hand, in the two-step route, only the interaction between the intercalation compound and toluene should be adjusted to achieve sufficient swelling. 3.3. Intercalation in Nanoscrolls. Both NS-C16TMACl and NS-APTMS are useful as host materials for further intercalation of organic molecules which are not intercalated into methoxy-modified kaolinite (C16TMABr and TBABr, Supporting Information, Figures S6b and S7). Figure 9 shows low-angle XRD patterns of the NS-C16TMACl and NS-APTMS intercalated with C16TMABr or TBABr. NS-C16TMACl-C16TMABr Langmuir 2011, 27(5), 2028–2035

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Table 3. Basal Spacings of the Nanoscrolls-Organic Species Intercalation Compounds basal spacing/nm NSguest species C16TMACl C16TMABr TBABr C6N C8N C12N pNA

4.0 1.5 2.6 3.2 4.2 1.5

NSAPTMS

methoxy-modified kaolinite9,24b

4.0 1.7 2.6 3.2 4.2 1.6

2.69 3.20 4.23 1.49

and NS-APTMS-C16TMABr and NS-C16TMACl-TBABr and NS-APTMS-TBABr show similar d-values. Alkylamines and pNA were also successfully intercalated into both NSC16TMACl and NS-APTMS (Supporting Information, Figures S8 and S9). The basal spacing of each intercalation compound does not depend on the host material (Table 3). Because the values of basal spacing of the nanoscrolls intercalated with alkylamines are almost the same as those of the intercalation compounds prepared from methoxy-modified kaolinite, alkylamines are concluded to form bilayers with their alkyl groups arranged almost perpendicularly to the kaolinite layers.24b The Si/Al ratio of NSAPTMS was changed into ca. 1.0 in every cases (data not shown); therefore, APTMS oligomers present in the interlayer of the nanoscrolls were probably exchanged with the guest species. The improved intercalation reactivity of the nanoscrolls is probably due to the distorted layer stacking of kaolinite layers within the lamellar walls of nanoscrolls, which efficiently suppresses the formation of hydrogen bonds between kaolinite layers. Some of the intercalation compounds show broad peaks at 0.85-0.89 nm that are not attributable to high-order diffractions but the diffraction due to methoxy-modified kaolinite (Supporting Information, Figures S8 and S9), suggesting somewhat incomplete intercalation due to the distorted layer stacking. The asymmetric orientation of pNA9 is also expected to be in the lamellar structures of nanoscrolls. Such an intercalation compound is promising as a colloidally utilizable nonlinear optical material. The intercalation of C16TMACl or C16TMABr in the nanoscrolls is promising for further materials design because hexadecyltrimethylammonium cations are often used to control interlayer environments of clay minerals or as templates to form

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mesoporous materials38 and porous clay heterostructures from layered silicates.39

4. Conclusion We have achieved the facile one-step route for the formation of nanoscrolls from methoxy-modified kaolinite, in which intercalation and exfoliation proceed at the same time. Weakening of the interaction between head groups of guest species and the interlayer surfaces of kaolinite was found to be critical for the one-step route. Modification with methoxy groups is quite useful for both the formation of nanoscrolls and the intercalation reaction of the obtained nanoscrolls. Because the exfoliation behavior of kaolinite is found to be dependent on the guest species and the surface modification, the present system will contribute to the understanding of the exfoliation mechanisms of layered materials, which will lead to the creation of advanced nanomaterials with controlled compositions and structures by using various nanosheets and their derivatives as building blocks. Acknowledgment. The authors are grateful to Mr. Minekazu Fuziwara (Kagami Memorial Research Institute for Materials Science and Technology, Waseda Univ.) for TEM observation and Dr. Hideo Hata (Shiseido Co., Ltd., and Waseda Univ.) and Mr. Junnosuke Murakami (Waseda Univ.) for their helpful advice. This work was supported by the Elements Science and Technology Project “Functional Designs of Silicon-OxygenBased Compounds by Precise Synthetic Strategies” and the Global COE program “Practical Chemical Wisdom” from MEXT, Japan. Y.K. is grateful for financial support via a Grant-in-Aid for JSPS Fellows from MEXT. Supporting Information Available: Discussion, Figures S1-S10 (TEM and SEM images, XRD patterns, and N2 adsorption-desorption measurements), and list of abbreviations. This material is available free of charge via the Internet at http://pubs.acs.org. (38) (a) Yanagisawa, T.; Shimizu, T.; Kuroda, K.; Kato, C. Bull. Chem. Soc. Jpn. 1990, 63, 988–992. (b) Kimura, T.; Kamata, T.; Fuziwara, M.; Takano, Y.; Kaneda, M.; Sakamoto, Y.; Terasaki, O.; Sugahara, Y.; Kuroda, K. Angew. Chem., Int. Ed. 2000, 39, 3855–3859. (39) Galarneau, A.; Barodawalla, A.; Pinnavaia, T. J. Nature 1995, 374, 529– 531.

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