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Biomimetic Surface Patterns of Layered Aluminum Oxide Mesophases Templated by Mixed Surfactant Assemblies Mitsunori Yada,* Hirohumi Kitamura, Masato Machida, and Tsuyoshi Kijima Department of Materials Science, Faculty of Engineering, Miyazaki University, Miyazaki 889-21, Japan Received April 30, 1997. In Final Form: July 18, 1997X Aluminum-based mesophases templated by both dodecyl sulfate (DS) and alkyl alcohols (AA), CnH2n+1OH (n ) 4-18), or dilauryldimethylammonium (DDA) have been synthesized by the homogeneous precipitation method using urea. Coincorporation of DS and DDA or AA with n g 6 leads to the formation of such a stable lamellar mesophase as to hamper the transition from layer to hexagonal structure observed in the AA- or DDA-free system. The interlayer spacing of the lamellar mesophases increases to 3.9-5.0 nm from 3.5 nm for the AA- or DDA-free system. Biomimetic patterns such as cone-shaped or terraced hollows and domed-scales also occur on the surface of lamellar particles formed in the DS/AA with n g 12 or DS/DDA mixed systems, whereas no specific surface patterns are observed in the shorter chain alcohol mixed or DDA-free systems.
Introduction Much attention has been focused on a family of mesoporous materials such as MCM-411 and FSM-162 silicas, because of their great applicabilities as catalysts, molecular sieves, and host materials based on their large internal surface areas. The silica mesophase with a hexagonal structure is obtained mostly as microcrystals of hexagonal prism in shape. Recently, we synthesized aluminum-based dodecyl sulfate mesophases by the homogeneous precipitation method using urea.3 The surfactant-templated mesophases were observed to occur initially in the layer structure and grow into their hexagonal form with versatile morphologies such as winding-rod, spherical, tubular, and funneled shapes depending on the urea concentration.4 The bodies of most animals and plants are organized according to one of three types of symmetry: spherical, radial, or bilateral. Recently, Oliver et al.5 and Mann et al.6 reported that a surfactant-templating approach to aluminophosphates interestingly results in lamellar mesophases with a spherical or diatom-like surface pattern5,6 or a coaxial cylindrical bilayered structure.7,8 And they suggested that such biomimetic patterns would be formed by the adhesion of mesolamellar aluminophosphates vesicles onto the surface of spherical precipitates. When dodecyl sulfate is mixed with alkyl alcohol or dilauryldimethylammonium in water, highly ordered surfactant assemblies such as vesicles as well as spherical, hexagonal, or lamellar assemblies are formed depending on their concentration and/or mixing ratio.9 Such mixed X Abstract published in Advance ACS Abstracts, September 1, 1997.
(1) Kresge, C. T.; Lenowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) Inagaki, S.; Fukushima, Y.; Kuroda, K. J. Chem. Soc., Chem. Commun. 1993, 680. (3) Yada, M.; Machida, M.; Kijima, T. Chem. Commun. 1996, 769. (4) Yada, M.; Hiyoshi, H.; Ohe, K.; Machida, M.; Kijima, T. Proceedings of the 6th Tohwa Unversity International Symposium. Frontier Nanostructured Ceramics; Tohwa University Press: Fukuoka, 1996; pp 183188. (5) Oliver, S.; Kuperman, A.; Coombs, N.; Lough, A.; Ozin, G. A. Nature 1995, 378, 47. (6) Mann, S.; Ozin, G. A. Nature 1996, 382, 313. (7) Sayari, A.; Karra, V. R.; Reddy, J. S.; Moudrakovski, I. L. Chem. Commun. 1996, 411. (8) Chenite, A.; Page, Y. L.; Karra, V. R.; Sayari, A. Chem. Commun. 1996, 413. (9) Kondo, Y.; Uchiyama, H.; Yoshino, N.; Nishiyama, K.; Abe, M. Langmuir 1995, 11, 2380.
S0743-7463(97)00446-0 CCC: $14.00
templating systems may therefore yield mesophases structurally and/or morphologically different from those obtained in the SDS-based systems.3,4 In this paper, we report the synthesis of an aluminumbased mesophase templated by both dodecyl sulfate and alkyl alcohol or dilauryldimethylammonium, indicating biomimetic patterns formed on the surface of the mesostructured particles. Experimental Section Aluminum nitrate nonahydrate (Al(NO3)3‚9H2O) was used as the aluminum source, and sodium dodecyl sulfate (SDS, CH3(CH2)11OSO3Na), alkyl alcohol (AA, CnH2n+1OH (n ) 4-18)), and dilauryldimethylammonium bromide (DDAB, (CH3(CH2)11)2(CH3)2NBr) were used as templating agents. The aluminum oxide/surfactant complexes were prepared as follows. Aluminum nitrate, AA or DDAB, SDS, urea, and water were mixed at a molar ratio of 1:x:2:30:60 (x ) 0-2.0) and stirred at 40 °C for 1 h to obtain the transparent mixed solution. Urea was used to gradually raise the pH of the reaction mixture because on heating at above 60 °C it is hydrolyzed to release ammonia. The mixed solution was thus heated at 80 °C and then kept at that temperature. The pH of the reaction mixture increased from 3.3-3.6 at its initial level to 7.0 or above, due to the enhanced hydrolysis of urea, while precipitation occurred and developed. Upon reaching a predetermined pH, the resulting mixture was immediately cooled to room temperature to prevent further hydrolysis of urea. After centrifugation, the resulting solid was washed with water a few times and then dried in air. Powder X-ray diffraction (XRD) measurement was made on a Simadzu XD-D1 diffractometer with Cu KR radiation. Thermogravimetric and differential thermal analyses (TG-DTAs) were carried out with a SEIKO TG/DTA320U. Scanning electron microscopy (SEM) was performed using a Hitachi H-4100M. X-ray microanalyses (XMAs) were carried out with a HORIBA X-ray microanalyzer EMAX-5770.
Results and Discussion Figure 1 shows the XRD patterns as a function of x for the resulting solids separated at pH 7.0 in the DS/lauryl alcohol (LA; n ) 12) system. The XRD pattern for x ) 0 is characterized by their major peaks attributable to the 100, 110, and 200 reflections based on a hexagonal unit (10) Yada, M.; Hiyoshi, H.; Machida, M.; Kijima, T. To be published.
© 1997 American Chemical Society
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Figure 2. TG curves as a function of x for of the lamellar mesophases with dodecyl sulfate/lauryl alcohol mixed surfactants.
Figure 1. XRD patterns as a function of x for aluminum-based surfactant mesostructured solids with dodecyl sulfate/lauryl alcohol mixed surfactants.
cell with a ) 4.3 nm (2d100/x3), as previously described.3 In contrast, the three diffraction peaks at 2θ ) 1-10° observed for the products with 0.5 e x e 1.5 are attributable to the 001, 002, and 003 reflections for a layered phase with an interlayer spacing of 4.4 nm. The x ) 2.0 product was explicitly observed to be a mixture of two layered phases including the 4.4 nm one. The coincorporation of DS and LA also led to a remarkable increase in intensity of the 001 and 002 reflections by a factor of ∼4 relative to those for the layered LA-free mesophase, along with the appearance of the 003 reflection. This would be attributable to an increase in structural order of the layered solids. In contrast to a halo band near 2θ ca. 20° for the LA-free layered and hexagonal mesophases, a week diffraction peak appears at 2θ ) 21° or d ) 0.42 nm, suggesting that the short range arrangement of constituent atoms becomes more ordered on coincorporation of LA and DS. The X-ray diffraction profile for the x ) 0.25 product is indicative of the intermediate stage during the lamellar to hexagonal transition. Furthermore, the x ) 1.0 product separated at pH 7.5 was found to have a typical lamellar structure. Figure 2 shows the TG curves as a function of x for the resulting mesophases with n ) 12 separated at pH 7.0. Each TG curve shows three or four weight losses until the mesostructured solid is totally converted into pure alumina at 900 °C or above. The first weight loss at below 100 °C is due to desorption of adsorbed water. The second weight loss around 200 °C would be attributable to desorption of water formed by condensation of hydroxyl groups in the
Figure 3. Plots of content of dodecyl sulfate and lauryl alcohol against x: b, dodecyl sulfate; O, lauryl alcohol; 4, LA-free lamellar mesophase.
aluminum-based layers as well as partial desorption of degradable surfactant, followed by the third weight loss due to complete desorption of the remaining surfactant moiety through combustion. With an increase of x from 0 to 1.25, the second weight loss remarkably increased and its offset temperature shifted from ca. 225 to 280 °C. It is also noted that the boiling temperature of free lauryl alcohol is as high as 260 °C. These facts mean that the rapid weight loss at below 225 °C is due to desorption of water and DS and that the additional weight loss in the temperature range 225-280 °C is mainly attributable to partial desorption of LA. The increase of coincorporated LA also led to a decrease from ca. 700 to 550 °C for the offset temperature at which the third weight loss is terminated, due to the combustion process enhanced by the remaining LA moiety. The fourth weight loss near 850 °C associated with the desorption of the remaining sulfated group10 remarkably decreased down to zero. The total weight loss increased from 72% for x ) 0 up to 88% for x ) 2, indicating that the alumina content in the resulting mesophases decreases from 28 to 12%. XMA data also showed that the DS-to-Al ratio for the single phases with x ) 0-1.25 exhibits a constant value of 0.45 (Figure 3) but that the value for the x ) 1.5 and 2.0 products varies widely from particle to particle. The marked decrease of the fourth weight loss would be therefore attributable to the decreased fraction of incorporated DS (11) Fukushima, Y.; Inagaki, S.; Kuroda, K. J. Chem. Soc. Jpn. 1995, 5, 327.
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Figure 4. Plots of interlayer spacing against the carbon number of the alkyl alcohol main chain for the lamellar mesophases with dodecyl sulfate and alkyl alcohol.
as well as the desorption of the sulfate group promoted by the combustion of the remaining LA moiety. On the basis of the above TG and XMA observations, we can reasonably assume that the weight loss due to the total desorption of incorporated LA is given by the total weight loss at temperatures above 225 °C less the loss associated with the remaining DS moiety in the same temperature range. The LA-to-Al ratios thus evaluated for the resulting mesophases are summarized in Figure 3. The LA content in the lamellar phases formed at x ) 0.25-1.25 shows a tendency to increase with increasing x, being in marked contrast to the DS content remaining unchanged. This is probably because DS molecules ∼20 Å long are arranged as a bilayer between any two aluminum oxide monolayers in such a manner that the terminal sulfate anions are ionically bonded to aluminum ions at regular spacings, while LA molecules are more weakly incorporated through the hydrogen bonding to Al(OH) groups together with the van der Waals attraction between the alkyl chains of both guest molecules to occupy the remaining interlayer space. More loading of LA in the solid would also make it structurally more ordered, in keeping with the above X-ray indications. Using various other alkyl alcohols, CnH2n+1OH (n ) 4-18), coupled with SDS as templates, and separating at pH 7.0 resulted in a disordered hexagonal phase for n ) 4 and a lamellar mesophase for n ) 6-18, while the AAfree system yielded a lamellar phase with an interlayer spacing of 3.5 nm at pH 6.0, followed by its conversion into a hexagonal structure at pH 7.0-7.5. Figure 4 shows the plots of the interlayer spacing of the lamellar mesophases against the number of CH2 units in the alkyl alcohol. The interlayer spacing of the mesophases remained nearly constant at 3.9 nm for 6 e n e 10, while it increased with an increase of n g 12. These results
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mean that dodecyl sulfate and alkyl alcohol molecules are coincorporated as a mixed bilayer in the lamellar mesophases and that the interlayer spacing of the mesophases is determined by the longer chain of both species, as schematically shown in Figure 5. The increase of 0.4 nm in interlayer spacing for the DS/AA mixed mesophase with n ) 6-10 relative to that for the AA-free mesophase3 indicates that the bilayered molecules in the former are placed with their alkyl chains less tilted relative to the inorganic layer than in the latter. The exclusive formation of the layered phases in the present system is in striking contrast to the AA-free system undergoing a layer-to-hexagonal transition. This is probably because the hydroxyl group of the AA molecule is less bulky than the sulfate group of DS, leading to the stabilization of the DS/AA mixed bilayer to prevent the transition from the layer to a hexagonal structure. A similar mechanism was suggested on FSM silica mesostructured materials synthesized by the intercalation of surfactant into layered polysilicate, kanemite, followed by a condensation reaction between the polysilicate layers.11 Typical scanning electron microscopic images are shown in Figure 6. In contrast to the DS-based layered mesophase indicative of an aggregate of lamellar particles, the DS/LA mixed mesophase displayed some unique biomimetic patterns on the surface of lamellar, spherical, or starfish-shaped particles, as shown in Figure 6b-h. Parts b and c of Figure 6 show a platy surface closely covered with circular or hexagonal scales ca. 0.8 µm in diameter. Some SEM images showed spherical particles with their surfaces packed with cone-shaped hollows regular in size (Figure 6d). The same area under high magnifications indicates that the hollows have a hexagonal edge and several circular steps on the inside (Figure 6e). Figure 6f shows a surface closely packed with domed scales. This surface pattern includes domains in which microdomes ca. 1 µm in diameter are arranged regularly with a spherical particle ca. 0.4 µm in diameter deposited on their tops (Figure 6g). The pattern in Figure 6h shows the growth of one terraced hollow surrounded by many smaller ones on the surface of a starfish-shaped particle. Some of the hollows are characterized by the cone-shaped growth on the inside bottom. Such biomimetic surface patterns as above were also observed for the lamellar mesophases templated by DS and AA with n > 12 (Figure 6i), while the layered phases with the alcohols with 6 e n e 10 showed no specific surface patterns. It is of interest that the lamellar mesophases indicative of biomimetic surface patterns correspond to those in which the interlayer spacing depends preferentially on the chain length
Figure 5. Schematic representation of the probable arrangement of dodecyl sulfate molecules (a), coincorporated dodecyl sulfate and alkyl alcohol molecules for n ) 6-10 (b), and those for n ) 12-18 (c) between the aluminum oxide layers.
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Figure 6. Typical scanning electron micrographs of the aluminum-based mesostructured solids with dodecyl sulfate/lauryl alcohol (a-h), stearyl alcohol (i), or dilauryldimethylammonium bromide (j) mixed surfactants: (a) lamellar aggregate; (b and c) surface networks of circular and hexagonal scales; (d and e) spherical particle with its surface covered with cone-shaped hollows and its enlarged pattern; (f and g) domed scales with or without spherical particles on their tops; (h) starfish-shaped particle with coneshaped hollows of different sizes; (i) domed scales; (j) surface covered with cone-shaped hollows.
of alkyl alcohol molecules coincorporated with the anionic surfactant species. Furthermore, aluminum hydroxide particles prepared with neither DS nor AA were observed to be much less regular in shape and to show no specific surface patterns. Most of the surface patterns shown above display a resemblance to those observed on mesolamellar aluminophosphates hydrothermally prepared in tetraethyleneglycol.5 The latter system showed bowl-shaped depressions and radiolarian microskeleton-like surface patterns. Oliver et al. suggested that those surface patterns resulted from the adhesion of mesolamellar aluminophosphate vesicles onto the surface on large spheres of the growing materials, followed by the fusion, fission, reshaping, and collapse of the vesicles.5 In contrast, we propose a more definite model for the surface
patterning in the present aluminum-based system, as illustrated in Figure 7. We assume that the surface patterning initiated by nucleation from a homogeneous bulk solution would yield flat, round nuclei to grow in size until the particle surface is closely covered, as demonstrated in Figure 6b and c. The coincorporation of dodecyl sulfate molecules together with longer chain alcohol molecules with n g 12 would be advantageous for the formation of stable round nuclei on the surface of lamellar particles. The moderate uniformity in size of the hexagonal scales observed in the above two patterns suggests that even if nuclei of different sizes occur, they would be cooperatively fused into thermodynamically more stable or relatively large grains during the nuclear growth process. If the hexagonal patterning is followed by the additional growth repeated convergently from the edge of
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Figure 7. Models proposed for the formation of (a, top) surface patterns with circular or hexagonal scales and cone-shaped hollows and (b, bottom) those with domed scales and domes with a spherical particle on their tops.
each hexagonal framework toward its center, the resulting surface pattern would be characterized by the cone-shaped, terraced hollows (Figure 7a). Such patterning on the surface of a mesolamellar sphere would lead to the pattern shown in Figure 6d and e. The surface pattern shown in Figure 6h might be formed by a similar mode in which the central nucleus grows in size in preference to all around on the surface of a curved, angular sheet. The regular arrangement of spherical particles on domed scales (Figure 6f and g), on the other hand, would be explainable by a similar nuclear growth mechanism but preceded by a different type of surface nucleation occurring locally from AA-free or DS-riched solution. The surface nucleation of this is likely based on the spatial effect of the large head molecules to form domed nuclei, as illustrated in Figure 7b. Additional nucleation by a similar mode would occur on the top of domed scales to deposit spherical particles. The loosely packed lamellae composed of both long and short chain molecules (Figure 5c) might also contribute to the occurrence of curved or spherical morphologies, compared with the closely packed ones shown in Figure 5a and b.
Figure 8. XRD pattern of aluminum-based surfactant mesostructured solid with dodecyl sulfate/dilauryldimethylammonium mixed surfactants.
When DDAB was used in place of AA at x ) 0.2, a lamellar mesophase with an interlayer spacing of 3.9 nm
Layered Aluminum Oxide Mesophase
formed to give the 001, 002, and 003 reflections, along with a halo band near 2θ ) 20°, as shown in Figure 8. Biomimetic surface patterns similar to those observed above appeared (Figure 6j), although most of the solids were composed of an aggregate of macroscopically lamellar particles. In summary, we synthesized aluminum-based lamellar mesophases templated with both dodecyl sulfate and alkyl alcohol or dilauryldimethylammonium bromide, observed biomimetic complex patterns on the surface of the mesostructured material, and proposed a model for the surface patterning process. Alumina is one of the most useful materials in the field of materials chemistry. The above surface-modified alumina may be expected to improve the mechanical, thermal conductive, electrical
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insulation, and catalyst properties characterized in alumina, since these properties are highly sensitive to its structural and morphological characteristics. The present method for the control of surface morphology by surfactant assemblies is promising as a new surface modification technique applicable to a variety of inorganic materials. The above observations for the biomimetic patterning may also be useful for clarifying the pathway of biomineralization in coral, shell, diatom, or other calcified tissues. Furthermore, the present approach and findings would contribute to the development of functional mesostructured materials. LA9704462
