Lamellar Micelles as Templates for the Preparation of Silica Nanodisks

Aug 23, 2011 - Subhasree Banerjee, Harekrishna Ghosh, and Anindya Datta*. Department of Chemistry, Indian Institute of Technology Bombay, Powai, ...
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Lamellar Micelles as Templates for the Preparation of Silica Nanodisks Subhasree Banerjee, Harekrishna Ghosh, and Anindya Datta* Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India

bS Supporting Information ABSTRACT: We have reported earlier that silica nanodisks are formed by the reverse micelle sol gel (RMSG) method, from ternary Aerosol OT (AOT)/nheptane/water mixtures. These nanodisks are formed from the ternary mixtures with higher water contents and containing a significant concentration of FeCl3. Such mixtures undergo a phase separation. The present work seeks to identify the origin of these nanodisks. They are found to be produced from the lower, water-rich layer and not from the upper, oil-rich layer or from the interfacial region. Further, upon increasing the water content by a factor of 10, the phase separation is achieved in the absence of the salt as well. In the next step, the naodisks are observed to form from concentrated aqueous AOT solutions, with no involvement of organic solvents. Polarized optical microscopy and infrared studies reveal the occurrence of lamellar AOT micelles in these media. These micelles act as the templates for the nanodisks. This phenomenon paves the way for the soft chemical preparation of nanodisks in the aqueous phase, without the need of using salt or organic solvents.

’ INTRODUCTION Template-based synthesis of nanomaterials is an established modality, in which the shape of the template dictates, to a large extent, the shape of the nanostructures produced. The template can be hard or soft in nature.1,2 A water droplet, with a specific shape, can itself act as a template. Such droplets, coated with surfactants and dispersed in a nonpolar solvent, are found commonly in reverse micelles and are used in the reverse micelle mediated sol gel (RMSG) method for production of large quantities of nanomaterials. Normal reverse micelles yield spherical nanoparticles,3 5 whereas one-dimensional nanotubes have been prepared using cylindrical reverse micelles, formed in the presence of high concentrations of FeCl3.6 In contrast, reports on two-dimensional nanostructures, prepared using this method, are rare.7 This is so despite the fact that such anisotropic nanostructures have myriad potential applications. Mitragotri and co-workers, for example, have reported that anisotropic drug delivery vehicles are likely to be more successful than spherical ones.8,9 Interspersed arrays of silica and gold nanodisks have been prepared by nanolithography.10,11 These nanosandwiches, encoded with chromophores, can be used for DNA detection. Weller and co-workers have demonstrated an outstanding photoconductivity in two-dimensional PbS sheets.12 A stack of two silicon nanodisks, interspersed by silica layers and connected in series by coupling three tunnel junctions, has been proposed to be a promising candidate for application in quantum-effect devices.13 Very recently, the optical band gap in silicon nanosheets has been found to be affected by the dimensionality of the nanomaterial.14 Graphene is yet another popular twodimensional nanomaterial, with applications in high-frequency r 2011 American Chemical Society

circuits.15 Thus, two-dimensional nanostructures are materials with great promise, so there is considerable interest in designing facile methods of preparation of such nanomaterials. Recently we have prepared flat silica nanodisks using the RMSG method, from ternary Aerosol OT/n-heptane/water systems with high water content and in the presence of FeCl3.16,17 This observation is surprising, as the formation of nanodisks would imply the existence of flat water pools in the system. In the present paper, we venture to identify the origin of the nanodisks and, then, to propose a straightforward method of preparation of such twodimensional nanostructures, without using FeCl3 and organic solvents. At this point, it would be worthwhile to discuss the different phases of the ternary microemulsions which act as templates for nanostructures, as discussed above. The size and the shape of the water core are governed by the choice of the surfactant and cosurfactant, the molar ratio of water to surfactant (w0), and the choice of the nonpolar dispersion medium. Sodium bis(2-ethylhexyl)sulfosuccinate (Aerosol OT, AOT) is the most popular surfactant in this context.7 This double-tailed surfactant with a small anionic headgroup has the right geometry for the formation of water-in-oil (w/o) microemulsion and can do so without help from a cosurfactant.18,19 The different phases that exist in AOT with Cu2+ counterions (Cu(AOT)2) at various w0 values have been reported by Pileni.20 The homogeneous L2 phase exists for w0 = 2 5.5. In this phase, the droplets of water Received: June 22, 2011 Revised: July 31, 2011 Published: August 23, 2011 19023

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The Journal of Physical Chemistry C are coated by a single layer of surfactants, as has been established by light scattering and small-angle neutron scattering experiments.21,22 At w0 = 4, the shape of the water droplet changes from spherical to cylindrical. When more water is added to homogeneous L2 phase, it separates into two phases: one contains pure nonpolar solvent and the other one is a more concentrated reverse micellar solution, known as the L2* phase. L2* is a bicontinuous network of cylinders, which exists in reverse micellar solution at 5.6 < w0 < 11. Structural studies and smallangle X-ray scattering experiments indicate that the number of cylinders increases with the increase in the water content where the length of the cylinders remains constant. The Lα phase appears at w0 = 11. It is a mixture of planar lamellae and spherulites. This system of nomenclature was originally proposed by Luzzati and co-workers.23 In this system, the capital letter denotes the dimensionality: “L” for lamellar, “H” for two-dimensional hexagonal, etc. The Greek letters denote the short-range conformations of the nonpolar chains of the surfactant. “α”, for example, denotes a liquidlike system, while subsequent Greek letters denote systems with higher orders. At w0 = 20, an isotropic phase appears. Within 20 < w0 < 29, there is an appearance of a new phase consisting of nonpolar solvent and an isotropic phase containing a mixture of interconnected cylinders and sponge. At 30 < w0 < 35, the interconnected cylinders give way to an isotropic phase. Despite the existence of different-shaped self-assembled aggregates, the product obtained from the reverse micellar solutions are mainly spherical nanoparticles. Anisotropic structures are rarely observed. This is so because colloidal templates are highly dynamic and the energy required to form spherical nanoparticles is smaller than that required to form anisotropic nanostructures.24 For this reason, reverse micelles by themselves can be used to tune the size, but not the shape, of spherical nanoparticles. Anisotropic nanostructures are usually obtained only in the presence of high concentrations of salt.7,25,26

’ EXPERIMENTAL METHODS Silica nanostructures have been synthesized by the method reported earlier.16 Purified and dried AOT27,28 (Aldrich 98%) is dissolved in freshly distilled n-heptane (HPLC grade, Spectrochem, Mumbai, India), to which saturated aqueous solutions of FeCl3 (98%, AR grade from Merck) are added. Tetraethyl orthosilicate (TEOS; >99%, Aldrich) is used as the silica precursor. NaOH (AR grade from Merck) is used to co-condense the silica gel and product is soaked in HCl to remove ionic salts. Finally, the product is washed repeatedly with water and ethanol and dried in an oven at 100 °C. The silica nanostructures are imaged by a scanning electron microscope (SEM; HITACHI, S-3400N). AOT gels prepared at different experimental conditions are imaged by a Leica DM4500 P microscope with a Leica DFC 420 camera. A Perkin-Elmer Spectrum One FT-IR spectrometer is used to record the IR spectra of AOT. ’ RESULTS AND DISCUSSION Silica nanotubes (SNTs) are examples of anisotropic, onedimensional nanostructures, prepared using cylindrical AOT reverse micelles in the presence of high concentrations of FeCl3.6,29,30 We have found that nanodisks, not nanotubes, are formed upon addition of tetraethyl orthosilicate (TEOS) to the phase-separated water/AOT/heptane mixtures when the w0 is increased to 22 and the FeCl3 concentration is maintained

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at 17 M.16 The first question to be addressed in the present work concerns the origin of these nanodisks. Previously, we hypothesized that the nanodisks might have been formed at flat water pools, in the interface between the upper and the lower layers. In order to test this hypothesis, the phase-separated mixtures have been taken in a separating funnel and the phases have been allowed to settle, by letting the apparatus stand overnight. The upper phase is transparent and lemon yellow in color, while the lower layer is viscous, brown, and opaque. Since water is denser than n-heptane, it is expected to be the principal component of the lower layer, while the upper layer is expected to have a high content of n-heptane. The three different regions;the upper, oil-rich layer; the lower, water-rich layer; and the interfacial region;have been separated carefully. TEOS has been added separately to each of these solutions, and the RMSG method has been carried out in the usual manner.16 Scanning electron micrographs have been recorded for the silica nanostructures obtained from all three phases. The upper, oil-rich layer is found to yield spherical nanoparticles (Figure S1A in the Supporting Information). As expected, nanodisks are not obtained from this layer. However, nanodisks are not formed from the interfacial region either, which is contrary to our earlier hypothesis. Instead, spherical nanoparticles, similar to those obtained from the upper layer, are obtained from the interfacial layer as well (Figure S1B in the Supporting Information). Flat nanodisks, on the other hand, are obtained in ample abundance from the lower, waterrich layer (Figure 1A). Thus, the region of origin of the nanodisks is identified. Formation of two-dimensional disklike nanostructures from the lower, water-rich layer indicates the existence of two-dimensional surfactant aggregates in this layer. Such planar or lamellar surfactant aggregates are known to exist in water-rich ternary dispersions of AOT.21,31 Lamellar mesophases have been also reported for aqueous solutions of 10 60 wt % (w0 = 247 41) AOT.32 34 They have been investigated using time-resolved spectroscopic techniques.35,36 Considering the partition of AOT in water to be almost complete, the concentration of AOT in the lower, water-rich phase of the present experiment is estimated to be 50% by weight (w0 = 22). At this concentration, the lower layer is expected to contain the lamellar mesophase of AOT and this mesophase is what appears to be the template for the silica nanodisks. Next, we have attempted to devise a “cleaner” method of preparation of the nanodisks without using FeCl3. The phase separation of the ternary system does occur even without the addition of FeCl3, but significantly higher water content is required to bring it about. The different layers of two such saltfree mixtures, with w0 = 100 and 200, have been isolated in the fashion of the experiments with the mixtures containing FeCl3. Silica nanostructures have been prepared from each of these layers. The result of this experiment is similar to that obtained for the systems containing FeCl3. The upper layer and the interface yield spherical nanoparticles (Figures S2 and S3 in the Supporting Information), while nanodisks are found to be formed from the opalescent lower layer (Figure 1B,C). Thus, the lower layers of these phase-separated ternary water/AOT/n-heptane systems are found to provide templates for salt-free preparation of silica nanodisks. This is rather remarkable, as spherical nanoparticles generally predominate over anisotropic ones in the absence of salts. This is so because the colloidal templates are dynamic in nature. They collide with each other and exchange surfactant molecules constantly. Hence, the memory of the shape of the 19024

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Figure 1. Scanning electron micrographs of silica nanodisks prepared in the lower layer of the ternary AOT/n-heptane/H2O mixture (A) with w0 = 22 in the presence of FeCl3, (B) with w0 = 200 in the absence of FeCl3, (C) with w0 = 100 in the absence of FeCl3, (D) in AOT solution containing 60 wt % AOT (w0 = 41), (E) in AOT solution containing 10 wt % AOT (w0 = 247), and (F) in AOT solution containing 10 wt % AOT (w0 = 247) in the presence of 17 M FeCl3.

template is lost. This phenomenon further disfavors the formation of the anisotropic nanostructures, the energy requirement of whose formation is higher than that for spherical ones anyway. Generally, the stability of the aggregated templates is enhanced by the presence of salts, thereby tilting the equilibrium toward the anisotropic nanostructures that are formed from these templates. The fact that salts are not indispensible in the present experiments may be attributed to the extremely high viscosity of the aqueous AOT solution, which hinders the dynamic collision and consequent rapid disruption and re-formation of the lamellar micelles, thereby enhancing the memory of their shapes and favoring the formation of the nanodisks. In order to test this contention, TEOS has been added to relatively dilute aqueous solutions of AOT at w0 = 247 (10 wt %), where AOT is known to form lamellar micelles in even in the absence of salt. No nanodisk is found to form from this solution (Figure 1E). However, nanodisks are formed from another portion of the AOT solution that contains 17 M FeCl3 (Figure 1F).

Now that it is established that the lower, water-rich phase is where the nanodisks are prepared, the situation can be simplified further. It is clear by now that n-heptane has no real role to play here. Therefore, it should be possible to replace the ternary n-heptane/AOT/water system by aqueous AOT solutions. Indeed, upon addition of TEOS to a 60 wt % (w0 = 41) aqueous AOT solution containing no salt, nanodisks are formed in abundance (Figure 1D). Thus, we achieve a method of obtaining two-dimensional silica nanodisks without using a salt or an organic solvent. This provides a method of preparation that is more environmentally benign than the conventional RMSG method. So far in this work, the involvement of the lamellar micelles has been manifested indirectly, in the shape of the nanodisks produced from the medium. In the concluding part of this work, we seek to obtain more direct evidence of the presence of the lamellar mesophase in the lower, water-rich layer, in order to fortify our contention. This is attempted by two experiments: 19025

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Figure 2. Crossed polarized optical micrographs (POMs) of a portion of lower layer of a microemulsion at w0 = 200: (A) when polarizers are at 0° and (B) when polarizers are crossed at 90°.

polarized optical microscopy and infrared spectroscopy. The polarized optical micrographs (POMs), especially with crossed polarizers, are useful to identify lamellar structures, because these structures are birefringent.21 This is manifested in the POMs of 60 wt % (w0 = 41) aqueous AOT, in which the lamellar mesophase is known to exist (Figure S4 in the Supporting Information).29 POMs of the lower, water-rich layer of the AOT/water/n-heptane mixture, in which the molar ratio of water to AOT is 200, indicate the existence of a similar birefringent mesophase in this layer (Figure 2). These lamellar nanostructures are not observed in the POMs of the upper layer, thus confirming that they occur exclusively in the lower layer (Figures S5A and S6A in the Supporting Information). A control experiment has been performed to ensure that the birefringence does not have its origin in some aggregate that is not lamellar. This experiment is based on the knowledge that the lamellar mesophase occurs in solutions with 10 60 wt %, whereas onionlike vesicles exist at concentrations below 10 wt % (w0 = 247).37 This mesophase does not polarize light. POMs recorded for 5 wt % (w0 = 497) aqueous AOT do not reveal any structure (Figures S5B and S6B in the Supporting Information), thus indicating that the POMs recorded for the more concentrated AOT solutions are reliable. The infrared spectroscopic investigation is based on the earlier study of water layers confined in the lamellar micelles of AOT, by Prouzet and co-workers, using the OH stretching mode (3000 3800 cm 1) of water in the mid-IR region.28,29 This band can sense subtle differences in intermolecular hydrogen bonding interactions among water molecules.38,39 It can be resolved into three Gaussian bands, each of which is attributed to a different kind of water cluster. The band at 3320 cm 1 is due to “network water” (NW), where each water molecule is bound to four other water molecules; that at 3465 cm 1 is due to “intermediate water” (IW) molecules, with three hydrogen bonds with their neighbors; and that at 3585 cm 1 is due to “multimer water” (MW), with an average of two hydrogen bonds between a water molecule and its neighbors. The area under each of the component spectra yields the relative contribution of the corresponding water cluster. NW predominates in bulk water, with a contribution of approximately 70%, from Figure 3 in ref 29. Similarly, the contributions of IW and MW have been ascertained to be approximately 30 and 10%, respectively. This is so because in isotropic bulk water most of the water molecules are hydrogen bonded to four neighboring molecules, forming

Figure 3. OH stretching mode of water (solid black line) in the lower layer of the ternary AOT/n-heptane/H2O mixture at w0 = 200. The fit to three Gaussian functions is shown in gray, and the components are shown in dashed (network water, NW), dotted (intermediate water, IW), and dashed dotted (multimer water, MW) lines.

the tetrahedral NW clusters. Upon confinement in lamellar AOT micelles, the contribution of MW decreases linearly with the thickness of the water layer below 50 Å, while that of IW increases linearly in the same range. The values converge to about 46 and 42%, respectively, at a very low thickness of the water layer. This is so because a decrease in dimensionality imposes anisotropy along one of the three spatial axes. This disfavors the tetrahedral clusters of five water molecules (NW) and favors the planar clusters, where the central water molecule is hydrogen bonded to three neighbors. In our experiment, the treatment of Prouzet and co-workers has been extended to the systems of our interest. The IR spectra of bulk water (Figure S7 in the Supporting Information), water in 60 wt % (w0 = 41) aqueous AOT solution (Figure S7 in the Supporting Information), and the lower layer of the w0 = 200 solution (Figure 3) have been recorded. In our experiments, the relative contributions of NW, IW, and MW turn out to be 77, 18, and 5%, respectively, for bulk water. This is in good agreement with the estimates of Prouzet and co-workers. The relative contributions are found to be 50, 38, and 12% for 60 wt % (w0 = 41) aqueous AOT solution and 44, 44, and 12% for the lower layer of the w0 = 200 solution. This is a manifestation of the existence of two-dimensional layers of water, confined in lamellar micelles, in the lower, water-rich layer. The thickness of the water layers is estimated to be 5 10 nm, using the plot in Figure 3 in ref 29. 19026

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’ CONCLUSIONS To conclude, the lower, water-rich layer in the AOT/water/nheptane system is identified as the origin of the nanodisks. This layer is found to contain lamellar micelles of AOT, which are the species that act as the templates for these two-dimensional nanostructures. Finally, an environmentally benign salt-free method of synthesis of nanodisks, which also does not require the use of organic solvents, has been formulated. This method has the potential of emerging as a clean, soft synthetic procedure of the two-dimensional nanostructures, which are perceived to emerge as key components in potential application of nanostructures in drug delivery, biomolecular sensing, and nanoelectronics. ’ ASSOCIATED CONTENT

bS

Supporting Information. Additional SEM images, POMs, and IR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone:+91 22 2576 7149. Fax: +91 22 2570 3480. E-mail: [email protected].

’ ACKNOWLEDGMENT This work is supported by CSIR Project No. 01(2277)/08/ EMR-II. S.B. thanks CSIR for a Senior Research Fellowship. The SEM images have been recorded at the Department of Metallurgy and Materials Science, IIT Bombay, with the support and guidance of Dr. S. L. Kamath. The POM images have been recorded in the Department of Earth Sciences, IIT Bombay, with help from Ms. Priyanka Bhatta and Prof. Santanu Banerjee. The authors thank Mr. E. Siva Subramaniam Iyer for insightful suggestions and Mr. Arunasish Layek for his help with the figure for the table of contents.

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