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Preparation and some properties of organically modified layered alkali titanates with alkylmethoxysilanes
Journal of Colloid and Interface Science 296 (2006) 141–149 www.elsevier.com/locate/jcis
Preparation and some properties of organically modified layered alkali titanates with alkylmethoxysilanes Yusuke Ide a , Makoto Ogawa a,b,∗ a Graduate School of Science and Engineering, Waseda University, Nishiwaseda 1-6-1, Shinjuku-ku, Tokyo 169-8050, Japan b Department of Earth Sciences, Waseda University, Nishiwaseda 1-6-1, Shinjuku-ku, Tokyo 169-8050, Japan
Received 23 May 2005; accepted 26 August 2005 Available online 5 October 2005
Abstract The silylation of K2 Ti4 O9 ·nH2 O with organosilanes (methyl, n-butyl, n-octyl, n-dodecyl, n-octadecyltrimethoxysilanes and n-octadecyldimethylmethoxysilane) was conducted using the octylammonium-exchanged form as the intermediate. The surface coverage of the octadecylsilylated derivative was controlled by changing the employing amounts of octadecyltrimethoxysilane. The swelling behaviors of the octyl, dodecyl, and octadecylsilylated derivatives in organic solvents were investigated to show that the degree of the swelling varies depending on the kind of solvents, the alkyl chain length of the attached alkylsilyl groups, and the surface coverage. The octadecylsilylated derivative with the largest surface coverage was converted to film with a thickness of ca. 500 nm by casting the chloroform suspension on a substrate. The octadecylsilylated derivative showed a reversible thermoresponsive change of the basal spacing by ca. 0.5 nm in the temperature range between 15 and 60 ◦ C. © 2005 Elsevier Inc. All rights reserved. Keywords: Layered alkali titanate; Silylation; Swelling; Film; Phase transition
1. Introduction Materials design from layered solids through soft-chemical routes is a promising strategy, since the resulting layered inorganic–organic hybrids may have various nanostructures controlled by host–guest and guest–guest interactions [1–4]. A wide variety of layered inorganic–organic hybrids have been prepared by intercalation reactions and their properties and possible applications have been investigated so far [1–4]. Among possible intercalation reactions, the immobilization of organic units through covalent bonds, such as the silylation of layered solids bearing interlayer hydroxyl groups with organosilanes, is unique since the thermal and chemical stabilities of the hybrids can be expected and a more precise control of the nanostructure of the hybrids seems to be possible [9,12]. The silylation of layered silicates [5–14] resulted in hybrid materials with such functions as swelling ability in an organic solvent [8], film-forming ability [8], and adsorptive properties [9,12,13]. * Corresponding author. Fax: +81 3 3207 4950.
E-mail address: [email protected] (M. Ogawa). 0021-9797/$ – see front matter © 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.08.058
Accordingly, the preparations, structures, and properties of silylated derivatives of various layered solids are further worth investigating. Layered alkali titanates, niobates and titanoniobates are class of materials with ion exchange [15,16] and semiconducting properties [17,18]. Although the cation exchange reactions with alkylammonium ions [19–22] and cationic dyes [23–26] have been reported so far, there are few reports on the covalent attachment of functional units on the surfaces. The reaction of aminopropyltrimethoxysilane or tetraethoxysilane with alkylammonium-exchanged H2 Ti4 O9 ·nH2 O or H2 Ti3 O7 for the preparation of silica pillared titanates [27,28], that of aminopropyltrimethoxysilane with tetrabutylammonium-exchanged HCa2 Nb3 O10 for the attachment of oleic acid-ligated Fe3 O4 nanoparticles onto the surface of HCa2 Nb3 O [29], and that of hexamethylcyclotrisiloxane with tetraalkylammonium-exchanged HTiNbO5 for dispersing HTiNbO5 in polydimethylsiloxane [30] are reported examples. We have reported the interlayer silylation of K2 Ti4 O9 ·nH2 O [31] and K0.8 Ti1.73 Li0.27 O4 [32] with n-alkyltrimethoxysilanes using the octylammoniumexchanged forms as the intermediate. The octadecyl- and octylsilylated K2 Ti4 O9 were characterized to be thermally sta-
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ble if compared with the octylammonium-exchanged form and the attached octadecylsilyl groups did not leach out by the washing of the hybrid with water, aqueous hydrogen chloride, aqueous ammonia, acetone, and chloroform [31]. Moreover, the octadecylsilylated derivatives swelled in organic solvents [31, 32]. This property is worth mentioning as a characteristic of the materials, since there were several successful examples on the preparation and characterization of aqueous suspensions of layered alkali titanates [33], niobates [34] and titanoniobates [35]. This paper is a full version of our preliminary communication on the silylation of K2 Ti4 O9 ·nH2 O with alkyltrimethoxysilanes [31]. In the present study, alkylmethoxysilanes with different alkyl chain lengths and the numbers of methoxy groups were used. The employing amount of octadecyltrimethoxysilane was changed to vary the surface coverage. The swelling property was investigated systematically to show that the degree of the swelling correlates with the alkyl chain length, and the surface coverage, the kind of solvents. From the suspension, supported films of the octadecylsilylated derivative was prepared. Thermal nanostructural change of the octadecylsilylated derivative was also investigated. These results on the properties of the alkylsilylated K2 Ti4 O9 indicate the versatilities of the materials design of layered solids through the silylation. 2. Experimental 2.1. Materials K2 Ti4 O9 ·nH2 O was prepared by solid-state reaction between K2 CO3 (99.5%, Wako Chem. Ind.) and TiO2 (anatase, 98.5%, Kanto Chemical Co. Inc.) at the K2 CO3 :TiO2 molar ratio of 1:3.5 at 800 ◦ C in air for 24 h and for another 24 h after grinding [36,37]. n-Propylamine (98%) and n-octylamine (96%) were purchased from Kanto Chemical Co., Inc. and used as received. Organosilanes, methyltrimethoxysilane (96%, Kanto Chemical Co., Inc.), n-butyltrimethoxysilane (Gelest Inc.), n-octyltrimethoxysilane (96%, Aldrich), n-dodecyltrimethoxysilane (95%, Chisso), n-octadecyltrimethoxysilane (>85%, Tokyo Kasei Kogyo Co. Ltd.), and n-octadecyldimethylmethoxysilane (Chisso) were used as received. 2.2. Preparation of octylammonium-exchanged form The silylation was conducted by using the octylammoniumexchanged form of K2 Ti4 O9 ·nH2 O as the intermediate in a similar way used for the silylation of layered silicates where the dodecyltrimethylammonium-exchanged forms were used [7– 11]. The octylammonium-exchanged form of K2 Ti4 O9 ·nH2 O was prepared by one-pot process where K2 Ti4 O9 ·nH2 O (2.0 g) was allowed to react with an aqueous mixture (400 ml) of propylamine (1.2 ml) and octylamine (2.0 ml) whose pH was adjusted to 10 by the addition of 1 M HCl. Excess amounts of alkylamines (2.5 mol of propylamine and octylamine per K2 Ti4 O9 ) were used for the reaction. The reaction was conducted at room temperature for 1 h and the product was sep-
arated by centrifugation (4000 rpm, 20 min) and dried under reduced pressure. The octylammonium-exchanged form thus obtained was designated as C8 N+ -Ti4 O9 . 2.3. Silylation The C8 N+ -Ti4 O9 (0.5 g) was dispersed in a mixture of alkyltrimethoxysilanes (2.5–13.8 ml) and toluene (50 ml) and the mixture was refluxed at 60–80 ◦ C for 48 h. Excess amounts of alkyltrimethoxysilanes (ca. 30 mol per Ti4 O9 in the C8 N+ Ti4 O9 ) were used for the reactions. The products were separated by centrifugation (4000 rpm, 20 min) and washed with acetone. The silylated derivatives thus obtained are designated as Cn TMS-Ti4 O9 where n denotes the carbon number in the alkyl chain. As to the C18 TMS-Ti4 O9 , materials with different surface coverage were synthesized by changing the employing amount of the organosilane (ca. 5, 10, and 30 mol per Ti4 O9 ). They are designated as C18 TMSm -Ti4 O9 where m denotes the amount of the attached octadecylsilyl groups (mol) per Ti4 O9 unit. The silylation with n-octadecyldimethylmethoxysilane (ca. 30 mol per Ti4 O9 ) was also conducted under the same condition. The octadecyldimethylsilylated derivative thus obtained was designated as C18 DMMS-Ti4 O9 . 2.4. Preparation of suspensions The silylated derivatives were dispersed in benzene, chloroform, n-octanol, and ethyl acetate (0.13 g dm−3 (equivalent to 4.0 × 10−4 mol dm−3 ) of Ti4 O9 ) in glass ampoules and ultrasonicated for 1 or 5.5 h followed by stirring for 1–5 days at room temperature to give translucent suspensions. 2.5. Preparation of cast film An aliquot (0.3 ml) of the chloroform suspension of the C18 TMS-Ti4 O9 with the largest surface coverage, which was centrifuged at 4000 rpm for 20 min to remove non-swollen particles, was cast dropwise on quartz glass (27×10 mm) modified by n-octadecyltrimethoxysilane with a syringe. The temperature was kept at 0–3 ◦ C during the drying, since the rapid solvent evaporation resulted in rough surface. As a control experiment, the cast film of the parent titanate, K2 Ti4 O9 ·nH2 O, was attempted to prepare under the similar procedure except for repeating the cast (0.3 ml per a use). The chloroform dispersion of K2 Ti4 O9 ·nH2 O (which includes 13 wt% of the loaded titanate), prepared by the ultrasonication of K2 Ti4 O9 ·nH2 O and chloroform mixture (0.13 g dm−3 (equivalent to 4.0 × 10−4 mol dm−3 ) of Ti4 O9 ) for 2 h followed by decantation, was used for the film preparation. 2.6. Characterization X-ray diffraction patterns of the products were recorded on a Rigaku RAD IB powder diffractometer equipped with monochromatic CuKα radiation operated at 20 mA and 40 kV. Infrared spectra of KBr disks were recorded on a Shimadzu FT-8200 Fourier-transform infrared spectrophotometer at a res-
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olution of 1.0 cm−1 . CHN elemental analysis was performed on a Perkin Elmer 2400 II instrument. Inductively-coupled plasma atomic emission spectroscopy (ICP-AES) was performed on a Rigaku SPECTRO CIROS CCD. The products (10 mg) were decomposed for the ICP measurement by the alkali fusion with Na2 CO3 (0.5 g) and the following H2 SO4 treatment at 150–200 ◦ C for 2 h in a sealed container. Scanning electron micrograph (SEM) was performed on Hitachi S-2380N. X-ray diffraction patterns during heating and cooling were performed in situ on a Rigaku RINT-Ultima equipped with monochromatic CuKα radiation, operated at 40 mA, 40 kV with the heating/cooling rate of 2 ◦ C min−1 on vacuuming. Differential scanning calorimetric measurements were performed on Rigaku DSC 8230L with the heating/cooling rate of 2 ◦ C min−1 . Ultraviolet–visible absorption spectra of the suspensions (an optical pass length of 1 mm) and the cast film were recorded on a Shimadzu UV-3100PC spectrometer. X-ray diffraction patterns of slurries, which were prepared by putting the silylated derivatives (1.0 mg) in a glass holder and filling with organic solvents (0.1 ml, 390 g dm−3 (equivalent to 1.2 mol dm−3 ) of Ti4 O9 ), were performed on a Rigaku RINT 2000 MJ10013A equipped with monochromatic CuKα radiation, operated at 40 mA, 200 kV. In order to prevent evaporating of the solvents during the measurement, the slurries were covered with PET film.
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Fig. 1. X-ray diffraction patterns of (a) K2 Ti4 O9 ·nH2 O, (b) C8 N+ -Ti4 O9 , (c) C1 TMS-Ti4 O9 , (d) C4 NTMS-Ti4 O9 , (e) C8 TMS-Ti4 O9 , (f) C12 TMS-Ti4 O9 , (g) C18 TMS1.0 -Ti4 O9 , and (h) C18 DMMS-Ti4 O9 (a: H2 Ti4 O9 ·nH2 O).
3. Results and discussion 3.1. Formation of C8 N+ -Ti4 O9 The direct reaction between K2 Ti4 O9 ·nH2 O and alkyltrimethoxysilane was found to result in the silylation only at the external surface, so that the interlayer silylation was conducted by using the octylammonium-exchanged form as the intermediate [31]. This synthetic strategy was originally developed for the introduction of bulkyl organosilanes into layered silicates [7–11]. The basal spacing, d200 value, of 2.9 nm (0.87 for the K2 Ti4 O9 ·nH2 O, Fig. 1b) and the IR spectra, which shows absorption bands due to the C8 N+ , such as C–H stretching vibration at 2957, 2922, and 2852 cm−1 , C–H bending vibration at 1468 cm−1 , and C–N stretching vibration at 1379 cm−1 (Fig. 2b), indicate the formation of the C8 N+ -Ti4 O9 by the present one-pot process. From the elemental analysis (Table 1), the amount of the intercalated C8 N+ was determined to be 1.2 mol per Ti4 O9 , which was comparable to that prepared by the assistance with a macrocyclic compound [21]. Diffraction peaks due to the propylammonium-exchanged derivative with the d200 value of ca. 1.7 nm [20] were not detected in the XRD pattern. The C/N ratio (7.0) was consistent with that of the C8 N+ (6.9). These results indicate the absence of the co-adsorbed propylammonium ion (designated as C3 N+ ) in the C8 N+ -Ti4 O9 . Potassium ion was thought to be exchanged with the C3 N+ and the intercalated C3 N+ was subsequently exchanged with the C8 N+ during the process. From K and Ti contents, the amount of the remaining potassium ion was determined to be 0.36 mol per Ti4 O9 .
Fig. 2. IR spectra of (a) K2 Ti4 O9 ·nH2 O, (b) C8 N+ -Ti4 O9 , (c) C1 TMS-Ti4 O9 , (d) C4 TMS-Ti4 O9 , (e) C8 TMS-Ti4 O9 , (f) C12 TMS-Ti4 O9 , (g) C18 TMS1.0 Ti4 O9 , and (h) C18 DMMS-Ti4 O9 (P: C–N stretching vibration, F: OC–H bending vibration, E: SiO–C stretching vibration, ": Si–O–Si stretching vibration, Q: SiC–H bending vibration). Table 1 Chemical compositions and basal spacings of the octylammonium-exchanged form and the silylated derivatives Mass% C8 N+ -Ti4 O9 C1 TMS-Ti4 O9 C4 TMS-Ti4 O9 C8 TMS-Ti4 O9 C12 TMS-Ti4 O9 C18 TMS-Ti4 O9 C18 DMMS-Ti4 O9
Molar ratio
d200 value
C
N
Si
Ti
Si/Ti4 O9 N/Ti4 O9 (nm)
20.4 7.8 13.0 18.0 22.2 30.9 16.1
2.9 0.93 0.87 0.74 0.75 0.75 0.87
– 0.26 2.9 4.4 3.5 3.4 1.3
32.4 51.4 37.8 25.8 28.4 22.4 30.2
– 0.035 0.52 1.2 0.84 1.0 0.29
1.2 0.25 0.31 0.40 0.36 0.46 0.40
2.9 1.1 2.4 2.9 3.5 4.2 2.4
3.2. Formation of Cn TMS-Ti4 O9 The XRD patterns of the Cn TMS-Ti4 O9 are shown in Fig. 1, c–g. The d200 value of the C8 N+ -Ti4 O9 changed upon the
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silylation. The IR spectra of the Cn TMS-Ti4 O9 showed absorption bands due to the alkylsilyl groups [38], such as OC– H bending vibration at around 1180 cm−1 , SiO–C stretching vibration at around 1080 cm−1 , and Si–O–Si stretching vibration at around 1017 cm−1 (Fig. 2, c–g). The presence of these bands suggests that methoxy groups which are partially hydrolyzed and condensed are present in the Cn TMS-Ti4 O9 . It has been reported that the absorption band ascribable to Si– O–Ti stretching vibration appeared at around 960–920 cm−1 for silica–titania systems (titanosilicates [39] and silica–titania mixed oxides [40]). At such wavenumber region, the band could not be differentiated from the strong absorption band centered at around 935 cm−1 due to TiO6 octahedral framework of K2 Ti4 O9 ·nH2 O. In the IR spectrum of the octadecylsilylated derivative of K0.8 Ti1.73 Li0.27 O4 [32], the Si–O–Ti stretching vibration at 971 cm−1 was detected since this titanate dose not possess the absorption band due to TiO6 octahedral framework in this wavenumber region. From Si and Ti contents, the amounts of the attached alkylsilyl groups were determined. The deintercalation of the C8 N+ during the silylation was also confirmed by the decrease of N. Potassium ion was also deintercalated; the amount of the remained potassium ion in the C18 TMS-Ti4 O9 was calculated to be 0.078 mol per Ti4 O9 from K and Ti contents. These results are summarized in Table 1. The expanded interlayer space of the C8 N+ -Ti4 O9 made it possible to introduce bulkyl organosilanes in the interlayer space as reported for the silylation of layered silicates [7–11]. Both the Si content (Si/Ti4 O9 = 0.035 mol) and the C/N ratio (8.4) suggest that the C1 TMS-Ti4 O9 is mainly composed of the octylammonium-exchanged derivative where a slight amount of methylsilyl groups are co-adsorbed. Different intermolecular interaction between methyl groups compared with those between other alkyl groups may cause the intercalation of the lower amount of methyltrimethoxysilane. The alkylsilylated derivatives include the unreacted titanate, H2 Ti4 O9 ·nH2 O as shown by the presence of its diffraction peak (denoted as triangles in Fig. 1), however, it is difficult to differentiate quantitatively it from the silylated derivatives at present. It is possible to reduce the protonated phase by optimizing the silylation conditions [32]. In the SEM image of K2 Ti4 O9 ·nH2 O (Fig. 3a), needle-like particles with a length of ca. 4 µm were observed. The SEM of the C18 TMS-Ti4 O9 (Fig. 3b), as well as those of other Cn TMSTi4 O9 , showed that the morphology of K2 Ti4 O9 ·nH2 O did not change upon the silylation, indicating that the silylation occurred topochemically. Fig. 4 shows the variation of the d200 values of the Cn TMSTi4 O9 . There is a linear relationship between the carbon numbers in the alkyl chain and the d200 values in the Cn TMSTi4 O9 (n 4), indicating that the arrangements of the alkylsilyl groups are analogous in the four silylated derivatives. From the d200 values and the size of the alkylsilyl groups, two arrangements of the attached alkylsilyl groups are proposed for the four silylated derivatives; one is a bilayer paraffin-type coverage and the other is an interdigitated monolayer coverage. It is difficult to determine the alkyl chain arrangements in the interlayer
spaces only from the basal spacings, since the alkyl chains may take various conformations [41]. The co-absorbed C8 N+ may also affect the alkylsilyl group arrangement to some extent, however, further discussion is difficult at present since the distribution of the co-adsorbed C8 N+ is not clear. The co-adsorbed C8 N+ is also undesirable for a certain kind of applications, so that efforts are being made to deintercalate the C8 N+ completely by optimizing the silylation and washing conditions. As to the C18 TMS-Ti4 O9 system, the surface coverage was controlled as shown by the compositions of the C18 TMS-Ti4 O9 with different surface coverage (Table 2). This controlled surface coverage was achieved by simply changing the employing amount of the organosilane as reported for the silylation of magadiite with octylchlorosilanes [9,12]. Fig. 5 shows the variation of the d200 values of the C18 TMS-Ti4 O9 with different surface coverage. The d200 value increased linearly with the amounts of the attached octadecylsilyl groups, suggesting that the surface coverage are homogeneous and the silyl groups arrangement (a bilayer paraffin-type arrangement or an interdigitated monolayer one) are similar in the three derivatives with different surface coverage. If the packing of the octadecylsilyl groups is hexagonal closest one, the titanate surface occupied with a octadecylsilyl group is calculated from the compositions and the lattice constants of the parent titanate [15] to be 0.44, 0.68, and 0.87 nm2 for the C18 TMS1.0 -Ti4 O9 , the C18 TMS0.65 -Ti4 O9 , and the C18 TMS0.51 -Ti4 O9 (Table 2), respectively. In other word, mean distance between the adjacent octadecylsilyl groups are 0.66, 0.82, and 0.93 nm for the corresponding silylated derivatives, as shown in Table 3. Because the molecular size of the C8 N+ (1.0 × 0.4 nm) is relatively small, the effect of the remained C8 N+ on the octadecylsilyl groups arrangements is neglectable here. The XRD pattern of the C18 DMMS-Ti4 O9 is shown as Fig. 1h. The d200 value was 2.3 nm, which was smaller than that (4.2 nm) of the C18 TMS-Ti4 O9 . The IR spectrum showed the adsorption band ascribable to SiC–H bending vibration at 1249 cm−1 , while it did not show the absorption band due to Si–O–Si stretching vibration (Fig. 2h). From Si and Ti contents, the amount of the attached octadecyldimethylsilyl groups was determined to be 0.29 mol per Ti4 O9 (Table 1). These results indicate that the introduction of the C18 DMMS groups is different compared with that of the C18 TMS groups, suggesting that the formation of Si–O–Si bonds between the adjacent organosilanes is a driving force of the present silylation. The lower reactivity of this organosilane with the surface titanol groups under the present reaction condition can be another reason for the difference. 3.3. Properties of the Cn TMS-Ti4 O9 Fig. 6 shows the photographs of the benzene, chloroform, n-octanol, and ethyl acetate suspensions of the C18 TMS1.0 Ti4 O9 . In the UV–vis absorption spectra of these suspensions, absorption peaks appeared near absorption onsets and there was a linear relationship between the absorbance and the Ti4 O9 contents in the suspensions [31]. These spectral fea-
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Fig. 3. Scanning electron micrographs of (a) K2 Ti4 O9 ·nH2 O, (b) C18 TMS1.0 -Ti4 O9 , the C18 TMS1.0 -Ti4 O9 cast film from (c) top and (d) side views, and (e) the K2 Ti4 O9 ·nH2 O cast film. Scale bars: (a)–(c) and (e), 5 µm; (d), 2 µm.
Fig. 4. The variation of the basal spacing as a function of alkyl chain length of the alkylsilyl groups in the Cn TMS-Ti4 O9 .
tures are thought to be due to the molecular nature of the titania nanosheets and their size quantization, since a similar spectral characteristic was reported for the aqueous suspensions prepared by swelling of a layered protonic titanate, Hx Ti2−x/4 1x/4 O4 ·H2 O (1: vacancy) [42]. Fig. 7 shows the XRD patterns of the slurries. The XRD patterns of chloroform, benzene, and ethyl acetate slurries showed the expansion of the d200 value from 4.2 nm of the powder to 7.0, 7.7, and 4.5 nm, respectively, and that of n-octanol showed no expansion of the d200 value. These results indicate that the C18 TMS1.0 -Ti4 O9 incorporates organic solvents into the organically modified interlayer space to swell and the degree of the swelling varies depending on the kind of dispersants. The transparency of the
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Table 2 Amounts of octadecyltrimethoxysilanes used, chemical compositions, basal spacings, and surface coverage of the octadecylsilylated derivatives d200
Amounts of organosilane
Mass%
(mol per Ti4 O9 )
C
N
Si
Ti
Si/Ti4 O9
Molar ratio N/Ti4 O9
(nm)
Surface coverage with a C18 TMS group (nm2 )a
4.4 8.8 31.1
22.7 25.8 30.9
0.87 0.92 0.75
2.5 2.9 3.4
33.2 30.1 22.4
0.51 0.65 1.0
0.36 0.42 0.46
3.1 3.6 4.2
0.87 0.68 0.44
a Calculated as described in a footnote of Table 3.
Table 3 Relationship between the surface coverage and the expansion of the gallery height of the silylated derivatives upon the swelling
C8 TMS1.2 -Ti4 O9 C12 TMS0.84 -Ti4 O9 C18 TMS0.51 -Ti4 O9 C18 TMS0.65 -Ti4 O9 C18 TMS1.0 -Ti4 O9
Surface coverage with a Cn TMS group (nm2 )a
Mean distance between alkyl chains (nm)b
Expansion of gallery height (nm)c
0.37 0.54 0.87 0.68 0.44
0.61 0.73 0.93 0.82 0.66
2.9 → 3.2 (110) 3.5 → 4.2 (120) 3.1 → 3.9 (126) 3.6 → 5.2 (137) 4.2 → 7.0 (167)
a Calculated as bc/x, where x, b, and c denote the amount of the attached alkylsilyl groups per Ti4 O9 unit and lattice constants of the parent titanate, respectively. b Calculated as √bc/x, where x, b, and c denote the amount of the attached alkylsilyl groups per Ti4 O9 unit and lattice constants of the parent titanate, respectively. c Calculated by subtracting 0.5 nm (as the thickness of K Ti O single layer) 2 4 9 from the d200 value. Parentheses indicate a percentage of the expansion.
Fig. 6. Photographs of (a) chloroform, (b) benzene, (c) n-octanol, and (d) ethyl acetate suspensions of the C18 TMS1.0 -Ti4 O9 (0.13 g dm−3 (equivalent to 4.0 × 10−4 mol dm−3 ) of Ti4 O9 ). The vessel is quartz cell with an optical pass length of 5 mm.
Fig. 5. X-ray diffraction patterns of the C18 TMS-Ti4 O9 with different amounts of the attached alkylsilyl groups: (a) 0.51, (b) 0.65, and (c) 1.0 mol per Ti4 O9 (a: H2 Ti4 O9 ·nH2 O). Inset: variation of the basal spacing as a function of the amount of alkylsilyl groups.
suspensions was observed probably as a result of the difference in the expansion of the interlayer space (Fig. 6). Fig. 8 shows the change in the UV–vis absorption spectra of the C18 TMS1.0 -Ti4 O9 chloroform suspension during the ultrasonication and the subsequent stirring. The gradual decrease of the absorption in baseline region (800–350 nm) and the sharpening of the absorption bands at 267 nm indicate that the swelling was promoted during the present dispersing processes [43]. Ultrasonication prior to the stirring was found to be effective to promote the swelling, however, ultrasonication for the
Fig. 7. X-ray diffraction patterns of (a) the C18 TMS1.0 -Ti4 O9 and the slurry with (b) chloroform, (c) benzene, (d) n-octanol, and (e) ethyl acetate.
longer time (5.5 h) resulted in the fracture of the crystals in the lateral directions, as shown by the SEM of the deposit prepared by casting the suspensions on a substrate where particles with a size of several hundreds nm were observed (Fig. 9a). A portion of particles were precipitated from the chloroform suspension after standing for few days. However, as to
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Fig. 8. Change in UV–vis absorption spectra of the C18 TMS1.0 -Ti4 O9 chloroform suspension during the ultrasonication and the subsequent stirring and centrifugation. The Ti4 O9 concentrations were 0.13, 0.13, 0.13 and 0.11 g dm−3 , respectively. Inset: the variation of molar exciton coefficient at 267 nm as a function of time.
the supernatant (which includes 83 wt% of the loaded titanate) obtained by the centrifugation of the suspension at 4000 rpm for 20 min, precipitation could not be observed for less than 4 weeks and the spectra scarcely changed (Fig. 8, inset). The centrifugation at 4000 rpm for 20 min of the benzene suspension also resulted in the precipitation of 37 wt% of the loaded titanate. On the contrary, almost all the loaded titanates were precipitated by the centrifugation for other suspensions. Such variation in the stability of the suspensions is thought to be due to the difference in the degree of the swelling (Fig. 7, b–e), that in the specific gravity of the dispersants (1.43, 0.88, 0.90, and 0.83 g cm−3 for chloroform, benzene, ethyl acetate, and n-octanol). The difference in the lateral size of the colloids may not be a reason for the variation, since the SEM of the deposits did not show the difference in lateral size of the crystals. The swelling behaviors of the chloroform suspensions of C18 TMS-Ti4 O9 with the smaller surface coverage, C8 TMSTi4 O9 and C12 TMS-Ti4 O9 were also investigated and the results and the calculated values from the geometrical consideration are summarized in Table 3 together with those of the C18 TMS1.0 -Ti4 O9 . The gallery height of the particles tends to
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increase with the increase of the alkyl chain length and the increase of the surface coverage, that is, the decrease of the mean distance between the alkyl chains. Taking the size of chloroform molecule (ca. 0.3 nm) into consideration, the swelling property is thought to correlate partly with the affinity of the modified titanate sheets with alkylsilyl groups to organic phases, rather than with the packing of alkyl chains attached on the surface where solvent molecules can penetrate between them [44]. Film-forming property was examined by using the C18 TMS1.0 -Ti4 O9 chloroform suspension where the swelling is most progressive among the present suspensions. The supernatant after the centrifugation was cast on a substrate to form a translucent film. The XRD pattern of the cast film is shown in Fig. 10b. The basal spacing (4.2 nm) was same as that of the powder and the peaks due to only h00 diffractions were assignable. There appeared a new weak diffraction at d = 0.31 nm denoted as filled circle in Fig. 10b though its assignment is difficult at present. In the SEM image of the cast film, oriented needle-like particles were observed (Fig. 3c). These results confirmed that the swollen C18 TMS1.0 -Ti4 O9 restacked on the substrate to form oriented aggregates with their basal plane parallel to the substrate. It is worth mentioning that the C18 TMS1.0 -Ti4 O9 experienced little morphological change in the lateral size upon the swelling. Contrastively, in the case of the swelling in aqueous solutions, substantial reduction in the lateral size of crystals have often been observed [45,46]. The dissociation of titanium ions from the titanate framework into organic solvents is implausible. The UV–vis absorption spectrum of the cast film is shown in Fig. 11b. An absorption peak at 266 nm, which is often observed in the nanosheet suspensions [31,32,42,43], was observed even after the nanosheets suspension was restacked into the film. A similar phenomenon was reported for the film obtained by casting the aqueous suspension of propylammoniumexchanged Na2 Ti3 O7 [47], where the optical anisotropy in the transition moment of the nanosheets between the directions perpendicular and parallel to the sheet may be an interpretation for the spectral difference between Na2 Ti3 O7 powder and the corresponding film. Such an anisotropy of the titania nanosheets has also been suggested from the reflectance and absorption
Fig. 9. Scanning electron micrographs of the deposits on a substrate of C18 TMS1.0 -Ti4 O9 chloroform suspensions prepared by stirring for 5d after pre-ultrasonication for (a) 5.5 and (b) 1 h (scale bar: 5 µm). Figure (b) is same to Fig. 3c.
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Fig. 10. X-ray diffraction patterns of (a) C18 TMS1.0 -Ti4 O9 and (b) the cast film of the C18 TMS-Ti4 O9 (a: H2 Ti4 O9 ·nH2 O, ": unknown). Fig. 12. DSC curve of the C18 TMS1.0 -Ti4 O9 on the two successive cycles with increasing and decreasing the temperature after the first cycle (heating/cooling rate: 2 ◦ C min−1 ). Open and filled circles denote the variation of basal spacing as a function of temperature during heating and cooling, respectively.
Fig. 11. UV–vis absorption spectra of (a) the C18 TMS1.0 -Ti4 O9 chloroform suspension used for the film preparation and (b) the C18 TMS1.0 -Ti4 O9 cast film.
spectral results of the multilayered film obtained by layer-bylayer deposition [48]. The SEM showed that the film thickness is ca. 500 nm and the film surface is rough with a maximum variation of several hundreds of nm (Fig. 3d). The roughness of the surface is ascribed from the presence of non-swollen particles and the boundaries of intricate particles as seen in Fig. 3c, bringing the turbidity of the film; the transmittance at 800–400 nm is 85– 73%. Utilizing the parent titanates with the smaller particle size is a way to improve the transparency of the cast film. The film preparation using the parent titanate, K2 Ti4 O9 · nH2 O, confirmed a merit of the present silylation. The SEM of the cast film prepared from chloroform dispersion of K2 Ti4 O9 · nH2 O (Fig. 3e) showed that needle-like particles less oriented and the coating on a substrate with particles was much more heterogeneous compared with that of the C18 TMS1.0 -Ti4 O9 cast film (Fig. 3c). In the XRD patterns, peaks not only due to 200 diffraction but also 311 diffraction were assignable (data not shown). These result showed that the unmodified titanate did not swell in the organic solvent not to form orient film. Another noticeable property of the C18 TMS1.0 -Ti4 O9 is the thermoresponse of the nanostructure. Fig. 12 shows the DSC curve of C18 TMS1.0 -Ti4 O9 on the successive heating and cool-
Fig. 13. X-ray diffraction patterns of the C18 TMS1.0 -Ti4 O9 recorded in situ from 15 to 60 ◦ C with the heating/cooing rate of 2 ◦ C min−1 on second cycle.
ing cycles from 5–75 ◦ C. A endothermic peak (45 ◦ C) and an exothermic peak (38 ◦ C) were detected, suggesting that the C18 TMS1.0 -Ti4 O9 showed reversible phase transition due to the conformational change of the octadecylsilyl groups. The basal spacing of C18 TMS1.0 -Ti4 O9 changed reversibly depending on temperature (Fig. 13), and the temperature dependence was corresponding to the DSC results (denoted as open and filed circles in Fig. 12), suggesting that the observed thermoresponse of the basal spacing is attributed to the conformational change of the attached octadecylsilyl groups. This thermoresponse is worth mentioning as a characteristic of the present material. There are several reports on the nanostructural change of layered inorganic–organic hybrids triggered by external stimuli. The conformational change triggered by temperature of alkylammoniums and alkanols co-intercalated in mica-type layered silicates [41], that triggered by temperature of dodecyl- or hexadecylammonium intercalated in zirconium phosphate [49], and that triggered by
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light of an azobenzene intercalated in magadiite [50] are reported examples. Recently, such nanostructural change has been applied to the control of the properties of the hybrids. For instance, the swelling and deswelling behaviors of poly(N -isopropylacrylamide)–silica hybrid film in water triggered by temperature and pH has been reported [51]. Our group has also reported the reversible intercalation and deintercalation of phenol for the azobenzene-intercalated montmorillonite upon UV and visible light irradiation [52]. The controllability of the surface coverage (see above) and the variation of alkylsilanes of the alkylsilylated derivatives may lead to tailor-made nanostructures and to modulate the degree of the structural change. These features make it attractive for such applications of the alkylsilylated derivatives as adsorbents and fillers. 4. Conclusions We have synthesized the silylated derivatives of K2 Ti4 O9 · nH2 O with alkylmethoxysilanes using the octylammonium-exchanged form as the intermediate. The surface coverage varied with the alkyl chain lengths and the numbers of methoxy groups of the alkylmethoxysilanes. The surface coverage of the octadecylsilylated derivative was controllable to some extent by changing the amounts of the organosilane used. The present synthetic strategy must be applicable to the modification of other layered alkali titanates [32], and layered alkali niobates and titanoniobates with a wide variety of organosilanes, taking the fact that the intercalating ability of K2 Ti4 O9 ·nH2 O is relatively low among such layered oxides into consideration [53]. The swelling behaviors of the alkylsilylated derivatives were found to vary depending on the kind of organic solvents, the alkyl chain length, and the surface coverage. Film of the octadecylsilane-derivatized titanate with a thickness of ca. 500 nm were obtained by casting the chloroform suspension on a substrate. Moreover, the octadecylsilylated derivative showed thermoresponsive change of the basal spacing. Acknowledgments This work was supported by a Grant-in-Aid for Scientific Research on Priority Areas (417) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government, CREST and Tokuyama Science Foundation. Waseda University also supported as financially as a special research projects (2004A-092, 2004A-093). References [1] S.M. Auerbach, K.A. Carrado, P.K. Dutta (Eds.), Handbook of Layered Materials, Dekker, New York, 2004. [2] G. Alberti, T. Bein (Eds.), Comprehensive Supramolecular Chemistry, vol. 7, Pergamon, Oxford, 1996. [3] M. Ogawa, K. Kuroda, Chem. Rev. 95 (1995) 399. [4] M. Ogawa, Ann. Rep. (Sec. C) 94 (1998) 209. [5] E. Ruiz-Hitzky, M. Rojo, Nature 287 (1980) 28.
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Preparation and some properties of organically modified layered alkali titanates with alkylmethoxysilanes Yusuke Ide a , Makoto Ogawa a,b,∗ a Graduate School of Science and Engineering, Waseda University, Nishiwaseda 1-6-1, Shinjuku-ku, Tokyo 169-8050, Japan b Department of Earth Sciences, Waseda University, Nishiwaseda 1-6-1, Shinjuku-ku, Tokyo 169-8050, Japan
Received 23 May 2005; accepted 26 August 2005 Available online 5 October 2005
Abstract The silylation of K2 Ti4 O9 ·nH2 O with organosilanes (methyl, n-butyl, n-octyl, n-dodecyl, n-octadecyltrimethoxysilanes and n-octadecyldimethylmethoxysilane) was conducted using the octylammonium-exchanged form as the intermediate. The surface coverage of the octadecylsilylated derivative was controlled by changing the employing amounts of octadecyltrimethoxysilane. The swelling behaviors of the octyl, dodecyl, and octadecylsilylated derivatives in organic solvents were investigated to show that the degree of the swelling varies depending on the kind of solvents, the alkyl chain length of the attached alkylsilyl groups, and the surface coverage. The octadecylsilylated derivative with the largest surface coverage was converted to film with a thickness of ca. 500 nm by casting the chloroform suspension on a substrate. The octadecylsilylated derivative showed a reversible thermoresponsive change of the basal spacing by ca. 0.5 nm in the temperature range between 15 and 60 ◦ C. © 2005 Elsevier Inc. All rights reserved. Keywords: Layered alkali titanate; Silylation; Swelling; Film; Phase transition
1. Introduction Materials design from layered solids through soft-chemical routes is a promising strategy, since the resulting layered inorganic–organic hybrids may have various nanostructures controlled by host–guest and guest–guest interactions [1–4]. A wide variety of layered inorganic–organic hybrids have been prepared by intercalation reactions and their properties and possible applications have been investigated so far [1–4]. Among possible intercalation reactions, the immobilization of organic units through covalent bonds, such as the silylation of layered solids bearing interlayer hydroxyl groups with organosilanes, is unique since the thermal and chemical stabilities of the hybrids can be expected and a more precise control of the nanostructure of the hybrids seems to be possible [9,12]. The silylation of layered silicates [5–14] resulted in hybrid materials with such functions as swelling ability in an organic solvent [8], film-forming ability [8], and adsorptive properties [9,12,13]. * Corresponding author. Fax: +81 3 3207 4950.
E-mail address: [email protected] (M. Ogawa). 0021-9797/$ – see front matter © 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.08.058
Accordingly, the preparations, structures, and properties of silylated derivatives of various layered solids are further worth investigating. Layered alkali titanates, niobates and titanoniobates are class of materials with ion exchange [15,16] and semiconducting properties [17,18]. Although the cation exchange reactions with alkylammonium ions [19–22] and cationic dyes [23–26] have been reported so far, there are few reports on the covalent attachment of functional units on the surfaces. The reaction of aminopropyltrimethoxysilane or tetraethoxysilane with alkylammonium-exchanged H2 Ti4 O9 ·nH2 O or H2 Ti3 O7 for the preparation of silica pillared titanates [27,28], that of aminopropyltrimethoxysilane with tetrabutylammonium-exchanged HCa2 Nb3 O10 for the attachment of oleic acid-ligated Fe3 O4 nanoparticles onto the surface of HCa2 Nb3 O [29], and that of hexamethylcyclotrisiloxane with tetraalkylammonium-exchanged HTiNbO5 for dispersing HTiNbO5 in polydimethylsiloxane [30] are reported examples. We have reported the interlayer silylation of K2 Ti4 O9 ·nH2 O [31] and K0.8 Ti1.73 Li0.27 O4 [32] with n-alkyltrimethoxysilanes using the octylammoniumexchanged forms as the intermediate. The octadecyl- and octylsilylated K2 Ti4 O9 were characterized to be thermally sta-
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ble if compared with the octylammonium-exchanged form and the attached octadecylsilyl groups did not leach out by the washing of the hybrid with water, aqueous hydrogen chloride, aqueous ammonia, acetone, and chloroform [31]. Moreover, the octadecylsilylated derivatives swelled in organic solvents [31, 32]. This property is worth mentioning as a characteristic of the materials, since there were several successful examples on the preparation and characterization of aqueous suspensions of layered alkali titanates [33], niobates [34] and titanoniobates [35]. This paper is a full version of our preliminary communication on the silylation of K2 Ti4 O9 ·nH2 O with alkyltrimethoxysilanes [31]. In the present study, alkylmethoxysilanes with different alkyl chain lengths and the numbers of methoxy groups were used. The employing amount of octadecyltrimethoxysilane was changed to vary the surface coverage. The swelling property was investigated systematically to show that the degree of the swelling correlates with the alkyl chain length, and the surface coverage, the kind of solvents. From the suspension, supported films of the octadecylsilylated derivative was prepared. Thermal nanostructural change of the octadecylsilylated derivative was also investigated. These results on the properties of the alkylsilylated K2 Ti4 O9 indicate the versatilities of the materials design of layered solids through the silylation. 2. Experimental 2.1. Materials K2 Ti4 O9 ·nH2 O was prepared by solid-state reaction between K2 CO3 (99.5%, Wako Chem. Ind.) and TiO2 (anatase, 98.5%, Kanto Chemical Co. Inc.) at the K2 CO3 :TiO2 molar ratio of 1:3.5 at 800 ◦ C in air for 24 h and for another 24 h after grinding [36,37]. n-Propylamine (98%) and n-octylamine (96%) were purchased from Kanto Chemical Co., Inc. and used as received. Organosilanes, methyltrimethoxysilane (96%, Kanto Chemical Co., Inc.), n-butyltrimethoxysilane (Gelest Inc.), n-octyltrimethoxysilane (96%, Aldrich), n-dodecyltrimethoxysilane (95%, Chisso), n-octadecyltrimethoxysilane (>85%, Tokyo Kasei Kogyo Co. Ltd.), and n-octadecyldimethylmethoxysilane (Chisso) were used as received. 2.2. Preparation of octylammonium-exchanged form The silylation was conducted by using the octylammoniumexchanged form of K2 Ti4 O9 ·nH2 O as the intermediate in a similar way used for the silylation of layered silicates where the dodecyltrimethylammonium-exchanged forms were used [7– 11]. The octylammonium-exchanged form of K2 Ti4 O9 ·nH2 O was prepared by one-pot process where K2 Ti4 O9 ·nH2 O (2.0 g) was allowed to react with an aqueous mixture (400 ml) of propylamine (1.2 ml) and octylamine (2.0 ml) whose pH was adjusted to 10 by the addition of 1 M HCl. Excess amounts of alkylamines (2.5 mol of propylamine and octylamine per K2 Ti4 O9 ) were used for the reaction. The reaction was conducted at room temperature for 1 h and the product was sep-
arated by centrifugation (4000 rpm, 20 min) and dried under reduced pressure. The octylammonium-exchanged form thus obtained was designated as C8 N+ -Ti4 O9 . 2.3. Silylation The C8 N+ -Ti4 O9 (0.5 g) was dispersed in a mixture of alkyltrimethoxysilanes (2.5–13.8 ml) and toluene (50 ml) and the mixture was refluxed at 60–80 ◦ C for 48 h. Excess amounts of alkyltrimethoxysilanes (ca. 30 mol per Ti4 O9 in the C8 N+ Ti4 O9 ) were used for the reactions. The products were separated by centrifugation (4000 rpm, 20 min) and washed with acetone. The silylated derivatives thus obtained are designated as Cn TMS-Ti4 O9 where n denotes the carbon number in the alkyl chain. As to the C18 TMS-Ti4 O9 , materials with different surface coverage were synthesized by changing the employing amount of the organosilane (ca. 5, 10, and 30 mol per Ti4 O9 ). They are designated as C18 TMSm -Ti4 O9 where m denotes the amount of the attached octadecylsilyl groups (mol) per Ti4 O9 unit. The silylation with n-octadecyldimethylmethoxysilane (ca. 30 mol per Ti4 O9 ) was also conducted under the same condition. The octadecyldimethylsilylated derivative thus obtained was designated as C18 DMMS-Ti4 O9 . 2.4. Preparation of suspensions The silylated derivatives were dispersed in benzene, chloroform, n-octanol, and ethyl acetate (0.13 g dm−3 (equivalent to 4.0 × 10−4 mol dm−3 ) of Ti4 O9 ) in glass ampoules and ultrasonicated for 1 or 5.5 h followed by stirring for 1–5 days at room temperature to give translucent suspensions. 2.5. Preparation of cast film An aliquot (0.3 ml) of the chloroform suspension of the C18 TMS-Ti4 O9 with the largest surface coverage, which was centrifuged at 4000 rpm for 20 min to remove non-swollen particles, was cast dropwise on quartz glass (27×10 mm) modified by n-octadecyltrimethoxysilane with a syringe. The temperature was kept at 0–3 ◦ C during the drying, since the rapid solvent evaporation resulted in rough surface. As a control experiment, the cast film of the parent titanate, K2 Ti4 O9 ·nH2 O, was attempted to prepare under the similar procedure except for repeating the cast (0.3 ml per a use). The chloroform dispersion of K2 Ti4 O9 ·nH2 O (which includes 13 wt% of the loaded titanate), prepared by the ultrasonication of K2 Ti4 O9 ·nH2 O and chloroform mixture (0.13 g dm−3 (equivalent to 4.0 × 10−4 mol dm−3 ) of Ti4 O9 ) for 2 h followed by decantation, was used for the film preparation. 2.6. Characterization X-ray diffraction patterns of the products were recorded on a Rigaku RAD IB powder diffractometer equipped with monochromatic CuKα radiation operated at 20 mA and 40 kV. Infrared spectra of KBr disks were recorded on a Shimadzu FT-8200 Fourier-transform infrared spectrophotometer at a res-
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olution of 1.0 cm−1 . CHN elemental analysis was performed on a Perkin Elmer 2400 II instrument. Inductively-coupled plasma atomic emission spectroscopy (ICP-AES) was performed on a Rigaku SPECTRO CIROS CCD. The products (10 mg) were decomposed for the ICP measurement by the alkali fusion with Na2 CO3 (0.5 g) and the following H2 SO4 treatment at 150–200 ◦ C for 2 h in a sealed container. Scanning electron micrograph (SEM) was performed on Hitachi S-2380N. X-ray diffraction patterns during heating and cooling were performed in situ on a Rigaku RINT-Ultima equipped with monochromatic CuKα radiation, operated at 40 mA, 40 kV with the heating/cooling rate of 2 ◦ C min−1 on vacuuming. Differential scanning calorimetric measurements were performed on Rigaku DSC 8230L with the heating/cooling rate of 2 ◦ C min−1 . Ultraviolet–visible absorption spectra of the suspensions (an optical pass length of 1 mm) and the cast film were recorded on a Shimadzu UV-3100PC spectrometer. X-ray diffraction patterns of slurries, which were prepared by putting the silylated derivatives (1.0 mg) in a glass holder and filling with organic solvents (0.1 ml, 390 g dm−3 (equivalent to 1.2 mol dm−3 ) of Ti4 O9 ), were performed on a Rigaku RINT 2000 MJ10013A equipped with monochromatic CuKα radiation, operated at 40 mA, 200 kV. In order to prevent evaporating of the solvents during the measurement, the slurries were covered with PET film.
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Fig. 1. X-ray diffraction patterns of (a) K2 Ti4 O9 ·nH2 O, (b) C8 N+ -Ti4 O9 , (c) C1 TMS-Ti4 O9 , (d) C4 NTMS-Ti4 O9 , (e) C8 TMS-Ti4 O9 , (f) C12 TMS-Ti4 O9 , (g) C18 TMS1.0 -Ti4 O9 , and (h) C18 DMMS-Ti4 O9 (a: H2 Ti4 O9 ·nH2 O).
3. Results and discussion 3.1. Formation of C8 N+ -Ti4 O9 The direct reaction between K2 Ti4 O9 ·nH2 O and alkyltrimethoxysilane was found to result in the silylation only at the external surface, so that the interlayer silylation was conducted by using the octylammonium-exchanged form as the intermediate [31]. This synthetic strategy was originally developed for the introduction of bulkyl organosilanes into layered silicates [7–11]. The basal spacing, d200 value, of 2.9 nm (0.87 for the K2 Ti4 O9 ·nH2 O, Fig. 1b) and the IR spectra, which shows absorption bands due to the C8 N+ , such as C–H stretching vibration at 2957, 2922, and 2852 cm−1 , C–H bending vibration at 1468 cm−1 , and C–N stretching vibration at 1379 cm−1 (Fig. 2b), indicate the formation of the C8 N+ -Ti4 O9 by the present one-pot process. From the elemental analysis (Table 1), the amount of the intercalated C8 N+ was determined to be 1.2 mol per Ti4 O9 , which was comparable to that prepared by the assistance with a macrocyclic compound [21]. Diffraction peaks due to the propylammonium-exchanged derivative with the d200 value of ca. 1.7 nm [20] were not detected in the XRD pattern. The C/N ratio (7.0) was consistent with that of the C8 N+ (6.9). These results indicate the absence of the co-adsorbed propylammonium ion (designated as C3 N+ ) in the C8 N+ -Ti4 O9 . Potassium ion was thought to be exchanged with the C3 N+ and the intercalated C3 N+ was subsequently exchanged with the C8 N+ during the process. From K and Ti contents, the amount of the remaining potassium ion was determined to be 0.36 mol per Ti4 O9 .
Fig. 2. IR spectra of (a) K2 Ti4 O9 ·nH2 O, (b) C8 N+ -Ti4 O9 , (c) C1 TMS-Ti4 O9 , (d) C4 TMS-Ti4 O9 , (e) C8 TMS-Ti4 O9 , (f) C12 TMS-Ti4 O9 , (g) C18 TMS1.0 Ti4 O9 , and (h) C18 DMMS-Ti4 O9 (P: C–N stretching vibration, F: OC–H bending vibration, E: SiO–C stretching vibration, ": Si–O–Si stretching vibration, Q: SiC–H bending vibration). Table 1 Chemical compositions and basal spacings of the octylammonium-exchanged form and the silylated derivatives Mass% C8 N+ -Ti4 O9 C1 TMS-Ti4 O9 C4 TMS-Ti4 O9 C8 TMS-Ti4 O9 C12 TMS-Ti4 O9 C18 TMS-Ti4 O9 C18 DMMS-Ti4 O9
Molar ratio
d200 value
C
N
Si
Ti
Si/Ti4 O9 N/Ti4 O9 (nm)
20.4 7.8 13.0 18.0 22.2 30.9 16.1
2.9 0.93 0.87 0.74 0.75 0.75 0.87
– 0.26 2.9 4.4 3.5 3.4 1.3
32.4 51.4 37.8 25.8 28.4 22.4 30.2
– 0.035 0.52 1.2 0.84 1.0 0.29
1.2 0.25 0.31 0.40 0.36 0.46 0.40
2.9 1.1 2.4 2.9 3.5 4.2 2.4
3.2. Formation of Cn TMS-Ti4 O9 The XRD patterns of the Cn TMS-Ti4 O9 are shown in Fig. 1, c–g. The d200 value of the C8 N+ -Ti4 O9 changed upon the
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silylation. The IR spectra of the Cn TMS-Ti4 O9 showed absorption bands due to the alkylsilyl groups [38], such as OC– H bending vibration at around 1180 cm−1 , SiO–C stretching vibration at around 1080 cm−1 , and Si–O–Si stretching vibration at around 1017 cm−1 (Fig. 2, c–g). The presence of these bands suggests that methoxy groups which are partially hydrolyzed and condensed are present in the Cn TMS-Ti4 O9 . It has been reported that the absorption band ascribable to Si– O–Ti stretching vibration appeared at around 960–920 cm−1 for silica–titania systems (titanosilicates [39] and silica–titania mixed oxides [40]). At such wavenumber region, the band could not be differentiated from the strong absorption band centered at around 935 cm−1 due to TiO6 octahedral framework of K2 Ti4 O9 ·nH2 O. In the IR spectrum of the octadecylsilylated derivative of K0.8 Ti1.73 Li0.27 O4 [32], the Si–O–Ti stretching vibration at 971 cm−1 was detected since this titanate dose not possess the absorption band due to TiO6 octahedral framework in this wavenumber region. From Si and Ti contents, the amounts of the attached alkylsilyl groups were determined. The deintercalation of the C8 N+ during the silylation was also confirmed by the decrease of N. Potassium ion was also deintercalated; the amount of the remained potassium ion in the C18 TMS-Ti4 O9 was calculated to be 0.078 mol per Ti4 O9 from K and Ti contents. These results are summarized in Table 1. The expanded interlayer space of the C8 N+ -Ti4 O9 made it possible to introduce bulkyl organosilanes in the interlayer space as reported for the silylation of layered silicates [7–11]. Both the Si content (Si/Ti4 O9 = 0.035 mol) and the C/N ratio (8.4) suggest that the C1 TMS-Ti4 O9 is mainly composed of the octylammonium-exchanged derivative where a slight amount of methylsilyl groups are co-adsorbed. Different intermolecular interaction between methyl groups compared with those between other alkyl groups may cause the intercalation of the lower amount of methyltrimethoxysilane. The alkylsilylated derivatives include the unreacted titanate, H2 Ti4 O9 ·nH2 O as shown by the presence of its diffraction peak (denoted as triangles in Fig. 1), however, it is difficult to differentiate quantitatively it from the silylated derivatives at present. It is possible to reduce the protonated phase by optimizing the silylation conditions [32]. In the SEM image of K2 Ti4 O9 ·nH2 O (Fig. 3a), needle-like particles with a length of ca. 4 µm were observed. The SEM of the C18 TMS-Ti4 O9 (Fig. 3b), as well as those of other Cn TMSTi4 O9 , showed that the morphology of K2 Ti4 O9 ·nH2 O did not change upon the silylation, indicating that the silylation occurred topochemically. Fig. 4 shows the variation of the d200 values of the Cn TMSTi4 O9 . There is a linear relationship between the carbon numbers in the alkyl chain and the d200 values in the Cn TMSTi4 O9 (n 4), indicating that the arrangements of the alkylsilyl groups are analogous in the four silylated derivatives. From the d200 values and the size of the alkylsilyl groups, two arrangements of the attached alkylsilyl groups are proposed for the four silylated derivatives; one is a bilayer paraffin-type coverage and the other is an interdigitated monolayer coverage. It is difficult to determine the alkyl chain arrangements in the interlayer
spaces only from the basal spacings, since the alkyl chains may take various conformations [41]. The co-absorbed C8 N+ may also affect the alkylsilyl group arrangement to some extent, however, further discussion is difficult at present since the distribution of the co-adsorbed C8 N+ is not clear. The co-adsorbed C8 N+ is also undesirable for a certain kind of applications, so that efforts are being made to deintercalate the C8 N+ completely by optimizing the silylation and washing conditions. As to the C18 TMS-Ti4 O9 system, the surface coverage was controlled as shown by the compositions of the C18 TMS-Ti4 O9 with different surface coverage (Table 2). This controlled surface coverage was achieved by simply changing the employing amount of the organosilane as reported for the silylation of magadiite with octylchlorosilanes [9,12]. Fig. 5 shows the variation of the d200 values of the C18 TMS-Ti4 O9 with different surface coverage. The d200 value increased linearly with the amounts of the attached octadecylsilyl groups, suggesting that the surface coverage are homogeneous and the silyl groups arrangement (a bilayer paraffin-type arrangement or an interdigitated monolayer one) are similar in the three derivatives with different surface coverage. If the packing of the octadecylsilyl groups is hexagonal closest one, the titanate surface occupied with a octadecylsilyl group is calculated from the compositions and the lattice constants of the parent titanate [15] to be 0.44, 0.68, and 0.87 nm2 for the C18 TMS1.0 -Ti4 O9 , the C18 TMS0.65 -Ti4 O9 , and the C18 TMS0.51 -Ti4 O9 (Table 2), respectively. In other word, mean distance between the adjacent octadecylsilyl groups are 0.66, 0.82, and 0.93 nm for the corresponding silylated derivatives, as shown in Table 3. Because the molecular size of the C8 N+ (1.0 × 0.4 nm) is relatively small, the effect of the remained C8 N+ on the octadecylsilyl groups arrangements is neglectable here. The XRD pattern of the C18 DMMS-Ti4 O9 is shown as Fig. 1h. The d200 value was 2.3 nm, which was smaller than that (4.2 nm) of the C18 TMS-Ti4 O9 . The IR spectrum showed the adsorption band ascribable to SiC–H bending vibration at 1249 cm−1 , while it did not show the absorption band due to Si–O–Si stretching vibration (Fig. 2h). From Si and Ti contents, the amount of the attached octadecyldimethylsilyl groups was determined to be 0.29 mol per Ti4 O9 (Table 1). These results indicate that the introduction of the C18 DMMS groups is different compared with that of the C18 TMS groups, suggesting that the formation of Si–O–Si bonds between the adjacent organosilanes is a driving force of the present silylation. The lower reactivity of this organosilane with the surface titanol groups under the present reaction condition can be another reason for the difference. 3.3. Properties of the Cn TMS-Ti4 O9 Fig. 6 shows the photographs of the benzene, chloroform, n-octanol, and ethyl acetate suspensions of the C18 TMS1.0 Ti4 O9 . In the UV–vis absorption spectra of these suspensions, absorption peaks appeared near absorption onsets and there was a linear relationship between the absorbance and the Ti4 O9 contents in the suspensions [31]. These spectral fea-
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Fig. 3. Scanning electron micrographs of (a) K2 Ti4 O9 ·nH2 O, (b) C18 TMS1.0 -Ti4 O9 , the C18 TMS1.0 -Ti4 O9 cast film from (c) top and (d) side views, and (e) the K2 Ti4 O9 ·nH2 O cast film. Scale bars: (a)–(c) and (e), 5 µm; (d), 2 µm.
Fig. 4. The variation of the basal spacing as a function of alkyl chain length of the alkylsilyl groups in the Cn TMS-Ti4 O9 .
tures are thought to be due to the molecular nature of the titania nanosheets and their size quantization, since a similar spectral characteristic was reported for the aqueous suspensions prepared by swelling of a layered protonic titanate, Hx Ti2−x/4 1x/4 O4 ·H2 O (1: vacancy) [42]. Fig. 7 shows the XRD patterns of the slurries. The XRD patterns of chloroform, benzene, and ethyl acetate slurries showed the expansion of the d200 value from 4.2 nm of the powder to 7.0, 7.7, and 4.5 nm, respectively, and that of n-octanol showed no expansion of the d200 value. These results indicate that the C18 TMS1.0 -Ti4 O9 incorporates organic solvents into the organically modified interlayer space to swell and the degree of the swelling varies depending on the kind of dispersants. The transparency of the
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Table 2 Amounts of octadecyltrimethoxysilanes used, chemical compositions, basal spacings, and surface coverage of the octadecylsilylated derivatives d200
Amounts of organosilane
Mass%
(mol per Ti4 O9 )
C
N
Si
Ti
Si/Ti4 O9
Molar ratio N/Ti4 O9
(nm)
Surface coverage with a C18 TMS group (nm2 )a
4.4 8.8 31.1
22.7 25.8 30.9
0.87 0.92 0.75
2.5 2.9 3.4
33.2 30.1 22.4
0.51 0.65 1.0
0.36 0.42 0.46
3.1 3.6 4.2
0.87 0.68 0.44
a Calculated as described in a footnote of Table 3.
Table 3 Relationship between the surface coverage and the expansion of the gallery height of the silylated derivatives upon the swelling
C8 TMS1.2 -Ti4 O9 C12 TMS0.84 -Ti4 O9 C18 TMS0.51 -Ti4 O9 C18 TMS0.65 -Ti4 O9 C18 TMS1.0 -Ti4 O9
Surface coverage with a Cn TMS group (nm2 )a
Mean distance between alkyl chains (nm)b
Expansion of gallery height (nm)c
0.37 0.54 0.87 0.68 0.44
0.61 0.73 0.93 0.82 0.66
2.9 → 3.2 (110) 3.5 → 4.2 (120) 3.1 → 3.9 (126) 3.6 → 5.2 (137) 4.2 → 7.0 (167)
a Calculated as bc/x, where x, b, and c denote the amount of the attached alkylsilyl groups per Ti4 O9 unit and lattice constants of the parent titanate, respectively. b Calculated as √bc/x, where x, b, and c denote the amount of the attached alkylsilyl groups per Ti4 O9 unit and lattice constants of the parent titanate, respectively. c Calculated by subtracting 0.5 nm (as the thickness of K Ti O single layer) 2 4 9 from the d200 value. Parentheses indicate a percentage of the expansion.
Fig. 6. Photographs of (a) chloroform, (b) benzene, (c) n-octanol, and (d) ethyl acetate suspensions of the C18 TMS1.0 -Ti4 O9 (0.13 g dm−3 (equivalent to 4.0 × 10−4 mol dm−3 ) of Ti4 O9 ). The vessel is quartz cell with an optical pass length of 5 mm.
Fig. 5. X-ray diffraction patterns of the C18 TMS-Ti4 O9 with different amounts of the attached alkylsilyl groups: (a) 0.51, (b) 0.65, and (c) 1.0 mol per Ti4 O9 (a: H2 Ti4 O9 ·nH2 O). Inset: variation of the basal spacing as a function of the amount of alkylsilyl groups.
suspensions was observed probably as a result of the difference in the expansion of the interlayer space (Fig. 6). Fig. 8 shows the change in the UV–vis absorption spectra of the C18 TMS1.0 -Ti4 O9 chloroform suspension during the ultrasonication and the subsequent stirring. The gradual decrease of the absorption in baseline region (800–350 nm) and the sharpening of the absorption bands at 267 nm indicate that the swelling was promoted during the present dispersing processes [43]. Ultrasonication prior to the stirring was found to be effective to promote the swelling, however, ultrasonication for the
Fig. 7. X-ray diffraction patterns of (a) the C18 TMS1.0 -Ti4 O9 and the slurry with (b) chloroform, (c) benzene, (d) n-octanol, and (e) ethyl acetate.
longer time (5.5 h) resulted in the fracture of the crystals in the lateral directions, as shown by the SEM of the deposit prepared by casting the suspensions on a substrate where particles with a size of several hundreds nm were observed (Fig. 9a). A portion of particles were precipitated from the chloroform suspension after standing for few days. However, as to
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Fig. 8. Change in UV–vis absorption spectra of the C18 TMS1.0 -Ti4 O9 chloroform suspension during the ultrasonication and the subsequent stirring and centrifugation. The Ti4 O9 concentrations were 0.13, 0.13, 0.13 and 0.11 g dm−3 , respectively. Inset: the variation of molar exciton coefficient at 267 nm as a function of time.
the supernatant (which includes 83 wt% of the loaded titanate) obtained by the centrifugation of the suspension at 4000 rpm for 20 min, precipitation could not be observed for less than 4 weeks and the spectra scarcely changed (Fig. 8, inset). The centrifugation at 4000 rpm for 20 min of the benzene suspension also resulted in the precipitation of 37 wt% of the loaded titanate. On the contrary, almost all the loaded titanates were precipitated by the centrifugation for other suspensions. Such variation in the stability of the suspensions is thought to be due to the difference in the degree of the swelling (Fig. 7, b–e), that in the specific gravity of the dispersants (1.43, 0.88, 0.90, and 0.83 g cm−3 for chloroform, benzene, ethyl acetate, and n-octanol). The difference in the lateral size of the colloids may not be a reason for the variation, since the SEM of the deposits did not show the difference in lateral size of the crystals. The swelling behaviors of the chloroform suspensions of C18 TMS-Ti4 O9 with the smaller surface coverage, C8 TMSTi4 O9 and C12 TMS-Ti4 O9 were also investigated and the results and the calculated values from the geometrical consideration are summarized in Table 3 together with those of the C18 TMS1.0 -Ti4 O9 . The gallery height of the particles tends to
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increase with the increase of the alkyl chain length and the increase of the surface coverage, that is, the decrease of the mean distance between the alkyl chains. Taking the size of chloroform molecule (ca. 0.3 nm) into consideration, the swelling property is thought to correlate partly with the affinity of the modified titanate sheets with alkylsilyl groups to organic phases, rather than with the packing of alkyl chains attached on the surface where solvent molecules can penetrate between them [44]. Film-forming property was examined by using the C18 TMS1.0 -Ti4 O9 chloroform suspension where the swelling is most progressive among the present suspensions. The supernatant after the centrifugation was cast on a substrate to form a translucent film. The XRD pattern of the cast film is shown in Fig. 10b. The basal spacing (4.2 nm) was same as that of the powder and the peaks due to only h00 diffractions were assignable. There appeared a new weak diffraction at d = 0.31 nm denoted as filled circle in Fig. 10b though its assignment is difficult at present. In the SEM image of the cast film, oriented needle-like particles were observed (Fig. 3c). These results confirmed that the swollen C18 TMS1.0 -Ti4 O9 restacked on the substrate to form oriented aggregates with their basal plane parallel to the substrate. It is worth mentioning that the C18 TMS1.0 -Ti4 O9 experienced little morphological change in the lateral size upon the swelling. Contrastively, in the case of the swelling in aqueous solutions, substantial reduction in the lateral size of crystals have often been observed [45,46]. The dissociation of titanium ions from the titanate framework into organic solvents is implausible. The UV–vis absorption spectrum of the cast film is shown in Fig. 11b. An absorption peak at 266 nm, which is often observed in the nanosheet suspensions [31,32,42,43], was observed even after the nanosheets suspension was restacked into the film. A similar phenomenon was reported for the film obtained by casting the aqueous suspension of propylammoniumexchanged Na2 Ti3 O7 [47], where the optical anisotropy in the transition moment of the nanosheets between the directions perpendicular and parallel to the sheet may be an interpretation for the spectral difference between Na2 Ti3 O7 powder and the corresponding film. Such an anisotropy of the titania nanosheets has also been suggested from the reflectance and absorption
Fig. 9. Scanning electron micrographs of the deposits on a substrate of C18 TMS1.0 -Ti4 O9 chloroform suspensions prepared by stirring for 5d after pre-ultrasonication for (a) 5.5 and (b) 1 h (scale bar: 5 µm). Figure (b) is same to Fig. 3c.
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Fig. 10. X-ray diffraction patterns of (a) C18 TMS1.0 -Ti4 O9 and (b) the cast film of the C18 TMS-Ti4 O9 (a: H2 Ti4 O9 ·nH2 O, ": unknown). Fig. 12. DSC curve of the C18 TMS1.0 -Ti4 O9 on the two successive cycles with increasing and decreasing the temperature after the first cycle (heating/cooling rate: 2 ◦ C min−1 ). Open and filled circles denote the variation of basal spacing as a function of temperature during heating and cooling, respectively.
Fig. 11. UV–vis absorption spectra of (a) the C18 TMS1.0 -Ti4 O9 chloroform suspension used for the film preparation and (b) the C18 TMS1.0 -Ti4 O9 cast film.
spectral results of the multilayered film obtained by layer-bylayer deposition [48]. The SEM showed that the film thickness is ca. 500 nm and the film surface is rough with a maximum variation of several hundreds of nm (Fig. 3d). The roughness of the surface is ascribed from the presence of non-swollen particles and the boundaries of intricate particles as seen in Fig. 3c, bringing the turbidity of the film; the transmittance at 800–400 nm is 85– 73%. Utilizing the parent titanates with the smaller particle size is a way to improve the transparency of the cast film. The film preparation using the parent titanate, K2 Ti4 O9 · nH2 O, confirmed a merit of the present silylation. The SEM of the cast film prepared from chloroform dispersion of K2 Ti4 O9 · nH2 O (Fig. 3e) showed that needle-like particles less oriented and the coating on a substrate with particles was much more heterogeneous compared with that of the C18 TMS1.0 -Ti4 O9 cast film (Fig. 3c). In the XRD patterns, peaks not only due to 200 diffraction but also 311 diffraction were assignable (data not shown). These result showed that the unmodified titanate did not swell in the organic solvent not to form orient film. Another noticeable property of the C18 TMS1.0 -Ti4 O9 is the thermoresponse of the nanostructure. Fig. 12 shows the DSC curve of C18 TMS1.0 -Ti4 O9 on the successive heating and cool-
Fig. 13. X-ray diffraction patterns of the C18 TMS1.0 -Ti4 O9 recorded in situ from 15 to 60 ◦ C with the heating/cooing rate of 2 ◦ C min−1 on second cycle.
ing cycles from 5–75 ◦ C. A endothermic peak (45 ◦ C) and an exothermic peak (38 ◦ C) were detected, suggesting that the C18 TMS1.0 -Ti4 O9 showed reversible phase transition due to the conformational change of the octadecylsilyl groups. The basal spacing of C18 TMS1.0 -Ti4 O9 changed reversibly depending on temperature (Fig. 13), and the temperature dependence was corresponding to the DSC results (denoted as open and filed circles in Fig. 12), suggesting that the observed thermoresponse of the basal spacing is attributed to the conformational change of the attached octadecylsilyl groups. This thermoresponse is worth mentioning as a characteristic of the present material. There are several reports on the nanostructural change of layered inorganic–organic hybrids triggered by external stimuli. The conformational change triggered by temperature of alkylammoniums and alkanols co-intercalated in mica-type layered silicates [41], that triggered by temperature of dodecyl- or hexadecylammonium intercalated in zirconium phosphate [49], and that triggered by
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light of an azobenzene intercalated in magadiite [50] are reported examples. Recently, such nanostructural change has been applied to the control of the properties of the hybrids. For instance, the swelling and deswelling behaviors of poly(N -isopropylacrylamide)–silica hybrid film in water triggered by temperature and pH has been reported [51]. Our group has also reported the reversible intercalation and deintercalation of phenol for the azobenzene-intercalated montmorillonite upon UV and visible light irradiation [52]. The controllability of the surface coverage (see above) and the variation of alkylsilanes of the alkylsilylated derivatives may lead to tailor-made nanostructures and to modulate the degree of the structural change. These features make it attractive for such applications of the alkylsilylated derivatives as adsorbents and fillers. 4. Conclusions We have synthesized the silylated derivatives of K2 Ti4 O9 · nH2 O with alkylmethoxysilanes using the octylammonium-exchanged form as the intermediate. The surface coverage varied with the alkyl chain lengths and the numbers of methoxy groups of the alkylmethoxysilanes. The surface coverage of the octadecylsilylated derivative was controllable to some extent by changing the amounts of the organosilane used. The present synthetic strategy must be applicable to the modification of other layered alkali titanates [32], and layered alkali niobates and titanoniobates with a wide variety of organosilanes, taking the fact that the intercalating ability of K2 Ti4 O9 ·nH2 O is relatively low among such layered oxides into consideration [53]. The swelling behaviors of the alkylsilylated derivatives were found to vary depending on the kind of organic solvents, the alkyl chain length, and the surface coverage. Film of the octadecylsilane-derivatized titanate with a thickness of ca. 500 nm were obtained by casting the chloroform suspension on a substrate. Moreover, the octadecylsilylated derivative showed thermoresponsive change of the basal spacing. Acknowledgments This work was supported by a Grant-in-Aid for Scientific Research on Priority Areas (417) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government, CREST and Tokuyama Science Foundation. Waseda University also supported as financially as a special research projects (2004A-092, 2004A-093). References [1] S.M. Auerbach, K.A. Carrado, P.K. Dutta (Eds.), Handbook of Layered Materials, Dekker, New York, 2004. [2] G. Alberti, T. Bein (Eds.), Comprehensive Supramolecular Chemistry, vol. 7, Pergamon, Oxford, 1996. [3] M. Ogawa, K. Kuroda, Chem. Rev. 95 (1995) 399. [4] M. Ogawa, Ann. Rep. (Sec. C) 94 (1998) 209. [5] E. Ruiz-Hitzky, M. Rojo, Nature 287 (1980) 28.
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