A Pyrrole-Containing Surfactant as a Tecton for Nanocomposite SiO2

Sep 27, 2007 - Institute of Physical Chemistry, UniVersity of Giessen, Heinrich-Buff-Ring 58, D-35392 Giessen, Germany, and Inorganic and Materials ...
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Langmuir 2007, 23, 11273-11280

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A Pyrrole-Containing Surfactant as a Tecton for Nanocomposite SiO2 Films Helena Kaper,† Danielle Franke,† Bernd M. Smarsly,*,†,‡ and Charl F. J. Faul*,§ Max Planck Institute of Colloids and Interfaces, Research Campus Golm, D-14424 Potsdam, Germany, Institute of Physical Chemistry, UniVersity of Giessen, Heinrich-Buff-Ring 58, D-35392 Giessen, Germany, and Inorganic and Materials Chemistry, School of Chemistry, UniVersity of Bristol, Bristol BS8 1TS, United Kingdom ReceiVed June 27, 2007. In Final Form: August 10, 2007 A surfactant featuring a polymerizable pyrrole head group (dodecyl-dimethyl-(2-pyrrol-1-yl-ethyl)-ammonium bromide, DDPABr) was synthesized. The thermotropic behavior of the surfactant was investigated by differential scanning calorimetry (DSC) and X-ray scattering techniques, with small-angle X-ray scattering (SAXS) analysis revealing a highly ordered lamellar bilayer structure. After full characterization, DDPABr was used in the preparation of mesostructured SiO2 nanocomposite thin films via evaporation-induced self-assembly (EISA). Resulting thin SiO2DDPABr films were studied by 1D and 2D small-angle X-ray scattering (SAXS) techniques, indicating a lamellar nanocomposite structure. Suitable theoretical SAXS models were applied to fit the experimental 1D SAXS data. The surfactant could be chemically polymerized within the lamellar domains.

Introduction The production of nanostructured materials from a variety of starting materials (low molecular weight organic,1 polymeric,2 inorganic,3 biological in nature,4,5 or hybrid materials6) that rely heavily on noncovalent interactions7-10 has become less of a goal in itself in current research activities. Scientists active in modern supramolecular materials research and nanoscience, specifically those looking at finding applications of their research, have rather turned to the production of nanostructured materials that not only display function but moreover exhibit externally controllable and addressable functionality.11 It would therefore be very relevant to include the sought-after functionality within the starting materials/building blocks/tectons (i.e., a prefunctionalized preorganized state). However, the challenge would be to design and synthesize informationcontaining tectons, that is, codons,12 to self-assemble into functional supramolecular entities, even (dream) nanomachines!11 Here, the accurate placement of functional groups in all types of supramolecular assemblies13 (and the ability to control their interactions) becomes extremely important. The question of activity/function is very closely linked with the application of nanoscience in real technological applications. * To whom correspondence should be addressed. Fax: +49 641 99 34509 (B.M.S.); +44 117 929 0509 (C.F.J.F.). E-mail: bernd.smarsly@phys. chemie.uni-giessen.de (B.M.S.); [email protected] (C.F.J.F.). † Max Planck Institute of Colloids and Interfaces. ‡ University of Giessen. § University of Bristol. (1) Kato, T.; Mizoshita, N.; Kishimoto, K. Angew. Chem., Int. Ed. 2006, 45, 38. (2) Hamley, I. W. Soft Matter 2005, 1, 36. (3) Smarsly, B.; Antonietti, M. Eur. J. Inorg. Chem. 2006, 1111. (4) Davis, J. T. Angew. Chem., Int. Ed. 2004, 43, 668. (5) Zhang, S. G. Nat. Biotechnol. 2003, 21, 1171. (6) Sanchez, C.; Julian, B.; Belleville, P.; Popall, M. J. Mater. Chem. 2005, 15, 3559. (7) Ikkala, O.; ten Brinke, G. Chem. Commun. 2004, 2131. (8) Faul, C. F. J.; Antonietti, M. AdV. Mater. 2003, 15, 673. (9) Ward, M. D. Chem. Commun. 2005, 5838. (10) Hosseini, M. W. Chem. Commun. 2005, 5825. (11) Ozin, G. A.; Manners, I.; Fournier-Bidoz, S.; Arsenault, A. AdV. Mater. 2005, 17, 3011. (12) Lehn, J. M. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 4763. (13) Hawker, C. J.; Wooley, K. L. Science 2005, 309, 1200.

Several possibilities exist to address functionality within materials (through change in either the chemical or phase structure) and include (1) chemical doping (as is commonly found for conducting materials14 and inorganic materials such as WO3) and (2) external fields or influences (thermal,15 electrical (e.g., LC displays), or light (e.g., azobenzene materials16)). A feasible route for the production of robust functional materials can be found in the incorporation of functional starting materials into silica-based materials.17,18 In general, polymerizable pyrrole-containing surfactants are attractive to fabricate composites with diverse metal oxides, for instance, TiO2, allowing the development of novel types of solid-state solar cells. Recently, new strategies were developed for the preparation of inorganic/organic nanostructured hybrids, taking advantage of the self-organization of structure directors, especially surfactants. In essence, these methods are based on the self-assembly of surfactants in combination with sol-gel chemistry. In typical processes such as “nanocasting” and “evaporation-induced selfassembly” (EISA, see ref 3), low molecular weight precursors such as alkoxides are homogeneously mixed with water, the surfactant, and a volatile solvent. As this process takes place under moderate conditions (one of the main advantages of solgel chemistry), co-assembly of even fragile organic species is made possible. If a porous material is desired, the surfactant is finally removed by calcination or solvent extraction. Progress in new synthesis techniques was only possible because of the parallel development of suitable characterization techniques. In particular, detailed small-angle X-ray scattering (SAXS) analyses play an important role in gaining new insight and information about nanostructured systems, such as size, morphology, and order or disorder. Thus, to provide information about two-phase systems of alternating layers ABABAB of constant average electron density, a tool has recently been (14) Chiang, C. K.; Fincher, C. R.; Park, Y. W.; Heeger, A. J.; Shirakawa, H.; Louis, E. J.; Gau, S. C.; Macdiarmid, A. G. Phys. ReV. Lett. 1977, 39, 1098. (15) Wei, Z. X.; Laitinen, T.; Smarsly, B.; Ikkala, O.; Faul, C. F. J. Angew. Chem., Int. Ed. 2005, 44, 751. (16) Ikeda, T.; Tsutsumi, O. Science 1995, 268, 1873. (17) Hartmann, M. Chem. Mater. 2005, 17, 4577. (18) Ruland, W.; Smarsly, B. J. Appl. Crystallogr. 2004, 37, 575.

10.1021/la701911n CCC: $37.00 © 2007 American Chemical Society Published on Web 09/27/2007

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developed and refined.18,19 This model has been used20,21 for a variety of soft nanostructured materials produced by EISA3 or ionic self-assembly,8,22 and is applied in the current study to characterize lamellar mesostructures. In this investigation, we focus on the use of a pyrrole-based surfactant for the formation of silica hybrid materials and their structural characterization. Pyrrole species can be polymerized to provide poly(pyrrole), which is an attractive semiconductive macromolecule. Here, the surfactant possesses the ability to act as a multipurpose reagent, that is, both a structure-directing agent (i.e., in the inorganic templating process) as well as a reactive tecton to yield functional (i.e., conducting) hybrid materials after reaction. This idea was developed in 2003,22 with our groups recently continuing investigations into the phase behavior of such pyrrole-based surfactant systems.23 This follows on other investigations into the use of functional surfactants in hybrid materials from our groups.24

As a logical next step in these investigations, we proceeded to use dodecyl-dimethyl-(2-pyrrol-1-yl-ethyl)-ammonium bromide (DDPABr, see structure) for the production of functional nanostructured hybrid materials. To ensure that the surfactant used would induce the formation of well-defined, ordered hybrid mesophases, a counterion exchange procedure (from a tosylate to a bromide counterion) was performed on the material originally synthesized by us. In the present study we present the synthesis, phase and structural characterization (X-ray scattering and microscopy), and conducting properties of functional nanostructured hybrid films obtained from the characterized DDPABr surfactant. While this surfactant is most interesting in the form of hybrids with semiconducting oxides (TiO2, etc.), here the EISA approach was used to produce DDPABr/silica thin films as a model system with ordered mesostructure. In essence, our study thereby aims at a general understanding of the sol-gel templating behavior of such functional surfactants, which is a crucial precondition for their use in the generation of more complex nanostructured hybrids in future studies. Experimental Section Surfactant Synthesis and Ion Exchange. The modified synthesis strategy, interfacial properties, and aqueous phase behavior of the double-tailed polymerizable (pyrrolylalkyl) ammonium amphiphile dodecyl-dimethyl-(2-pyrrol-1-yl-ethyl)-ammonium tosylate (DDPAT) have recently been reported.23 DDPAT (0.5 g) was dissolved in ∼15 mL of a 1:1 (by volume) water/methanol mixture. A column was packed with a weakly basic ion-exchange resin (Amberlite IRA-96) to a volume of ∼30 cm3. The resin was conditioned by first passing a 1 M HBr solution through the column (reagent/resin volume ) ∼3:1; flow rate ) ∼1 cm min-1) and then washing with bidistilled water to remove excess acid until the solution collected from the column had a pH greater than 6 (rinse water/resin volume ) ∼4:1). The surfactant solution was added to the column and run with a 1:1 (by volume) water/ (19) Garnweitner, G.; Smarsly, B.; Assink, R.; Dunphy, D. R.; Scullin, C.; Brinker, C. J. Langmuir 2004, 20, 9811. (20) Kadam, J.; Faul, C. F. J.; Scherf, U. Chem. Mater. 2004, 16, 3867. (21) Zakrevskyy, Y.; Smarsly, B.; Stumpe, J.; Faul, C. F. J. Phys. ReV. E 2005, 71, 021701:1. (22) Ikegame, M.; Tajima, K.; Aida, T. Angew. Chem., Int. Ed. 2003, 42, 2154. (23) Franke, D.; Egger, C. C.; Smarsly, B.; Faul, C. F. J.; Tiddy, G. J. T. Langmuir 2005, 21, 2704. (24) Zhang, T. R.; Spitz, C.; Antonietti, M.; Faul, C. F. J. Chem.sEur. J. 2005, 11, 1001.

Figure 1. Light microscopy pictures taken under crossed polarizers: (a) typical small domains observed after initial melting of the material (taken at 70 °C, cooling at 10 °C min-1) and (b) typical texture of DDPABr (taken at 130 °C, cooling at 1 °C min-1). Note here the large domains forming diagonally across the micrograph.

Figure 2. DSC plot of DDPABr (to 190 °C, heating and cooling at 10 °C min-1): (a) second heating curve and (b) second cooling curve. methanol mixture. The exchange was performed while applying Ar flow to the top of the column and to the collecting liquid to prevent oxidation of the pyrrole-based surfactant. Removal of the solvent from the collected liquid via rotary evaporation resulted in a white, viscous paste. This was dissolved in a minimum amount of a 1:1 (by volume) benzene/water mixture and freeze-dried, yielding the surfactant in the bromide form. 1H NMR (CDCl3, 25 °C, ppm): 6.85 (br s, 2H), 6.16 (br s, 2H), 4.52 (t, 2H), 4.20 (t, 2H), 3.28 (s, 6H), 1.88 (m, 2H), 1.59 (m, 2H), 1.23 (m, 18H), 0.86 (t, 3H). Preparation of Thin Films. The synthesis of the films was conducted in two steps: first, a prehydrolyzed stock solution containing tetraethyl orthosilicate (TEOS), ethanol, water, and hydrochloric acid (molar ratio ) 1.4:10:1:5 × 10-5) was prepared by refluxing at 60 °C for 90 min. The solution can be kept in the refrigerator for several months. For the preparation of the films, this prehydrolyzed solution was added to a solution containing DDPABr, ethanol, water, and hydrochloric acid so that the final molar composition of the solution was 1:29:17.5:0.14:3.7 × 10-3 TEOS/ EtOH/H2O/DDPABr/HCl. This solution was stirred at room temperature for 1 h. Films were prepared by dip-coating on silicon

DDPABr as a Tecton for Nanocomposite SiO2 Films

Langmuir, Vol. 23, No. 22, 2007 11275 Table 1. Parameters Obtained from the Fitting of the SAXS Data of the Crystalline DDPABra

Figure 3. Temperature-dependent WAXS plot of DDPABr at a heating/cooling rate of 10 °C min-1: (a) 110 °C, (b) 140 °C, and (c) 160 °C. s denotes the scattering vector with s ) 2/λ sin θ.

Figure 4. Investigation of the broad transition observed by DSC using temperature-dependent WAXS measurements (a) at room temperature and (b) at 60 °C. s denotes the scattering vector with s ) 2/λ sin θ.

Figure 5. SAXS measurements of the surfactant powder, performed using a diffractometer with slit-collimation. s ) 2/λ sin θ, where λ is the wavelength and 2θ is the scattering angle (open circles ) experimental data; solid line ) fitted model).

parameter

value

dL/nm σ(dL)/nm d1/nm σ(d1)/nm N σ(N) dz/nm

2.45 ( 0.01 0.05 ( 0.02 1.35 ( 0.01 0.12 ( 0.02 10 ( 1 2.5 ( 0.02 0.05 ( 0.05

a dL: repeat unit of the lamellae; σ(dL): variance of dL; d1: thickness of the lamellae d1; σ(d1): variance of d1; N: stack height of the lamellae; σ(N): variance of N; dz: interfacial width between the two domains.

the required temperature range (-40 to 190 °C). Measurements were generally carried out at a heating/cooling rate of 10 °C min-1, but at the phase transition regions a much slower rate (∼1 °C min-1) was used. Wide-angle X-ray scattering (WAXS) measurements were performed using a Nonius PDS120 powder diffractometer in transmission geometry. A FR590 generator was used as the source of Cu KR radiation (λ ) 0.154 nm). Monochromatization of the primary beam was achieved by means of a curved Ge crystal. Scattered radiation was measured using a Nonius CPS120 positionsensitive detector. The resolution of this detector in 2θ is 0.018°. Temperature-dependent measurements were performed using a heating stage and temperature control unit from Anton Paar (Austria). Small-angle X-ray scattering (SAXS) measurements were carried out with a Nonius rotating anode (U ) 40 kV; I ) 100 mA; λ ) 0.154 nm) using a 2D MAR charge-coupled device (CCD) detector. The detector was placed at a distance of 740 mm from the sample. With this setup, the pure surfactant as well as the thin films prepared on ultrathin Si wafers were investigated. Conductivity measurements were performed on a Keithly 2000 multimeter utilizing a four probe setup. Transmission electron microscopy (TEM) micrographs were obtained with a Zeiss EM 912 OMEGA instrument operating at an acceleration voltage of 120 kV. Samples were prepared by removal of tiny amounts of the film using TEM tweezers and sliding the carbon coated 400-mesh copper grid over the film. Tapping mode atomic force microscopy (AFM) images were recorded with a multimode atomic force microscope from Vecco Instruments employing Olympus microcantilevers (resonance frequency ) 300 kHz; force constant ) 42 N m-1). A Bio-Rad 6000 Fourier transform infrared (FT-IR) spectrometer equipped with a single reflection diamond attenuated total reflection (ATR) was employed for recording IR spectra.

Results and Discussion wafers at a relative humidity of 30% and a speed of 35 cm min-1. Shortly after drying the film in the dip-coat chamber, the films were further treated at 80 °C for 2 h in air. For the GISAXS experiments, the films were dip-coated on 40 µm thin films but were otherwise treated analogously. Polymerization in Thin Films. The polymerization of the surfactant was carried out in a closed iodine chamber. Elemental iodine was put into a petri dish with a lid. The silica film and the pure surfactant were kept in the chamber for ∼14 h. Complete polymerization of the surfactant was confirmed by a color change (from white to black) as well as by IR spectroscopy. Instrumentation. Thermogravimetric analysis (TGA) was performed on a Netzsch TG 209 instrument at a scanning rate of 20 °C min-1. The phase behavior of the materials was investigated by differential scanning calorimetry (DSC). All DSC measurements were performed on a Netzsch DSC 204 instrument. The samples were examined at a scanning rate of 10 °C min-1 by applying several heating and cooling cycles. Polarized light microscopy (PLM) measurements were performed with an Olympus BX50 microscope equipped with a Linkam THMS 600 heating/cooling stage and a TP92 temperature controller over

Temperature-Dependent Surfactant Phase Behavior (TGA, PLM, and DSC). The thermal properties of DDPABr were studied by thermogravimetric analysis (TGA), DSC, temperaturedependent WAXS, and PLM experiments. TGA shows decomposition above 207 °C (Figure S1). PLM, DSC, and temperaturedependent WAXS experiments reveal the complexity of the thermotropic phase behavior of DDPABr, as discussed below. Investigation of the phase behavior by PLM was performed from -40 to 190 °C, with heating and cooling rates of both 10 and 1 °C min-1.25 In both cases (slow and fast cooling rates), similar textures (see Figure 1a) and behavior were observed: initial melting of the sample starting at lower temperatures, followed by a broad transition to the isotropic state. In the case of the faster heating rate, the onset of this phase transition to the isotropic state was observed in the range of 154-164 °C. The slower heating rate confirmed this transition, with a slight shift (25) For the slower cooling rate, the range investigated was from -40 to 170 °C. This was done to reduce the time the samples were left at high temperatures so as to avoid possible polymerization.

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Figure 6. Schematic representation of the lamellar structure of crystalline DDPABr.

Figure 7. Atomic force microscopy image (in tapping mode) of a silica-DDPABr hybrid film obtained via EISA.

to higher temperatures, that is, from 159 to 170 °C (this transition was optically more difficult to follow at the slower heating rate). After ensuring that the sample equilibrated in the isotropic state, the sample was cooled (at rates of 10 and 1 °C min-1). There was, however, no appreciable difference in the textures observed in both cases, except for larger domains in the case of the lower cooling rate (see Figure 1b). A very abrupt change into a birefringent state was observed at 150 °C (10 °C min-1) and 157 °C (1 °C min-1). It was interesting to note that on the transition from the isotropic state mostly large bladelike domains could be observed in the samples (see Figure 1b), with isotropic areas between these large domains. A very subtle change in these isotropic areas (to very slightly birefringent domains) could be observed at approximately 62 °C. However, the exact temperature and nature of this transition were hard to characterize further in detail (see Figure 4 and discussion below). DSC investigations to 190 °C (Figure 2) revealed only two transitions in the heating and cooling curves, with one transition at 26 °C and another endothermic, broad transition centered around 140 °C (it can be excluded that this transition is due to polymerized DDPABr, since the DSC trace of fully polymerized DDPAB shows no phase transitions). The corresponding exothermic transitions can be found at 129 and 22 °C on the cooling curve, thus showing that all transitions are reversible. Differences were found to exist between the data from the DSC and PLM investigations. Various reasons for the discrepancies between these data sets are possible and are briefly mentioned: (1) partial polymerization of the samples (especially

with the slower cooling rates and more time spent at higher temperatures); (2) slightly different thermal histories of samples (a parameter seen to have an influence on the behavior as observed in the PLM investigations); and (3) differences in the local measurement environment (e.g., orientation effects on the different sample substrates (glass versus aluminum), supercooling effects in the sample, etc.). Such effects, even when attempts were made to keep them to a minimum, were still observed in this investigation. Temperature-Dependent Surfactant Phase Structure. To understand these observed phenomena, temperature-dependent WAXS experiments were performed. In general, the data obtained from these measurements could not be compared directly to the DSC data after various cooling and heating cycles. The reason for this is that large quantities of DDPABr had to be exposed to high temperatures (190 °C) for long periods of time in air to obtain high quality WAXS data, which invariably led to polymerization, easily observed by the brown coloration of the material. Heating was therefore not performed to 190 °C but only to lower temperatures. However, quantitatively, similar behavior was observed as found in the DSC investigations. Figure 3 shows the behavior of a sample upon heating to 160 °C. The disappearance of the strong reflections, indicative of crystalline surfactant tails, clearly shows that melting has started to occur. A peak is still present in the small-angle region, which could be due to residual order within the system at 160 °C (i.e., not a fully melted system). To study the phase transition around 45 °C observed by DSC, WAXS measurements were carried out at room temperature and 60 °C (Figure 4). According to these experiments, the transition indicates a change in the alkyl order, since very subtle changes in the position of the peaks in the wide-angle range can be observed. Such small changes have before been ascribed to the formation of so-called alkyl rotator phases.26 SAXS Evaluation of the Pure Surfactant Phase Structure. SAXS measurements were performed on the surfactant powder, using a SAXS setup with a slitlike collimation (Figure 5). The SAXS pattern revealed a series of equidistant reflections with a repeat unit of d ) 2.45 nm, thus being in agreement with a highly ordered and regular lamellar structure. To elucidate the distribution of the surfactant’s moieties (head group and tail) within the lamellar domains, the SAXS data were analyzed based on the stacking model discussed in ref 18. In our case, slit smearing had to be taken into account, leading to the (26) Muller, A. Proc. R. Soc. London, Ser. A 1932, 138, 514.

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Figure 8. 2D SAXS pattern obtained from a silica-DDPABr hybrid film treated at 80 °C, attributable to a lamellar mesostructure. The scale bar is given in units of s ) 2/λ sin θ. The arrow indicates a signal originating from the crystallized surfactant.

Figure 9. (A) Experimental SRSAXS curve, measured in symmetric reflection, of the lamellar SiO2 film treated at 80 °C (a), together with modeling based on a two-phase system (b). (B) SAXS curve after polymerization induced by iodine.

observed asymmetric peak shape. Hence, the theoretical model function is given by

Ism(s) )

∫0∞I1(xs2 + y2)W(y) dy

where I1(s) represents the theoretical scattering using an ideal pointlike collimation and W(y) denotes the beam profile which was also experimentally determined for our setup. The main parameters to be fitted are the repeat unit of the lamellae, dL, the thicknesses of the constituting layers, d1 and d2, with d1 + d2 ) dL, the stack height of the lamellae, N, as well as the corresponding variances, σ(dL), σ(d1), and σ(N), and also the interfacial width, dz. In Table 1, the fitted parameters are given. It is seen that the

data can be reasonably fitted over a wide range of scattering vectors s ) 2/λ sin θ. However, the analysis was only carried out until the interference of the fourth order, as the assumptions of SAXS are no longer fulfilled at larger scattering vectors. The good agreement regarding the relative intensities provides reliable values for the relative thicknesses of the single layers. Interestingly, we obtained d1 ) 1.35 nm and d2 ) 1.1 nm with rather low values for the variance σ (σ(d1) < 0.02 nm) and dz (see Table 1). Hence, the thicknesses d1 and d2 were surprisingly similar, which is not typical for other surfactants. However, these values are fully reasonable in the light of the chemical structure of DDPABr, with the head group being composed of both the pyrrole and the ammonium unit. The larger value (d2 ) 1.35 nm) therefore corresponds to the alkyl tail, while the smaller one (d1 ) 1.1 nm) corresponds to the rest of the molecule (Figure 6). In essence, the SAXS analysis demonstrates the suitability of the recently developed approach, even for smeared data. Silica-Surfactant Hybrids: Synthesis and Characterization. Thin films of nanostructured hybrids with SiO2 were prepared by EISA.27,28 Starting from a dilute solution containing prehydrolyzed TEOS, the template, and a volatile solvent, films were dip-coated on Si wafers and studied by TEM, AFM, and SAXS. Figure 7 shows an AFM image revealing a very homogeneous, crack-free surface with a roughness on the nanometer scale. Furthermore, an apparent lamellar nanostructure is observed with an average repeat unit of ∼3 nm. However, evidently AFM does not allow an unambiguous conclusion about the nature of the mesostructure, that is, lamellar, 2D hexagonal, and so forth. Therefore, SAXS experiments were carried out on thin films (treated at 80 °C) prepared on ultrathin Si wafers as substrates, using a rotating anode setup and a 2D MAR CCD detector (Figure 8). Such patterns, obtained under an angle of incidence below 1°, revealed equidistant reflections in the s3 direction (i.e., perpendicular to the film surface), which corresponded to a repeat unit of d ) 3.2 nm. The fact that no off-specular reflections were observable proved the presence of a lamellar mesostructure with (27) Brinker, C. J.; Lu, Y. F.; Sellinger, A.; Fan, H. Y. AdV. Mater. 1999, 11, 579. (28) Ogawa, M. Langmuir 1997, 13, 1853.

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Figure 10. Stacking scheme of the lamellar SiO2-DDPABr nanocomposite with the layer sequence ABAB. Table 2. Parameters Obtained from the Fitting of the SAXS Data of the SiO2 Films parameter

value

dL/nm σ(dL)/nm d1/nm σ(d1)/nm N σ(N) dz/nm

3.2 ( 0.05 0.14 ( 0.02 1.9 ( 0.05 0.1 ( 0.02 20 ( 3 3.0 ( 1.0 0.55 ( 0.05

a d : repeat unit of the lamellae; σ(d ): variance of d ; d : length L L L 1 of the lamellae d1; σ(d1): variance of d1; N: stack height of the lamellae; σ(N): variance of N; dz: width of the transition zone between the two domains.

the lamellae being oriented parallel to the substrate. Also, the narrow width of the two spotlike signals indicated a high degree of mesostructural perfection and orientation with respect to the surface. In addition, a distinct signal was observed at s3 ) 0.42 nm-1, which can be attributed to crystallized surfactant, as the apparent repeat unit was on the order of the observed d-spacing of the pure surfactant and the position did not change upon changing the angle of incidence. Furthermore, SAXS measurements were performed in symmetric reflection (SRSAXS) to characterize the lamellar mesostructure in detail. The 1D SRSAXS pattern (Figure 9) shows two equidistant maxima at the same positions as those in the 2D SAXS measurements and is therefore in full agreement with a lamellar mesostructure as well. Using the stacked model approach with the same parameters as described above, the data could be reasonably evaluated (see Figure 10 for a schematic representation). In contrast to powderlike materials, such analysis has to take into account the preferred orientation with respect to the substrate. It should be noted that, in spite of the use of slitlike collimation, the reflections are not asymmetric, in contrast to the surfactant powder measured on the same instrument. It was demonstrated18 that a high degree of orientation of the mesostructure relative to the substrate results in profiles without asymmetry even if wide slits are used. As a main result, the features of the experimental data could be modeled over the entire range of scattering vectors, thus proving the feasibility of the analytical approach. From this analysis, the thickness of the transition zone between the two layers was ∼0.5 nm, and for the two layer thicknesses we obtained d1 ) 1.9 nm and d2 ) 1.3 nm with a variance of only (0.05 nm at maximum for both layers (see Table 2). Since the fully extended surfactant already has a length of 2.3 nm, the result shows that (a) the surfactant tails are highly interdigitated and/or (b) the surfactant tail is tilted in relation to the head group. TEM images of the mesostructure were obtained from films heat-treated at 80 °C (also used for the SAXS experiments).

Figure 11. IR spectra of DDPABr before polymerization (solid line, top) and after polymerization (dotted line, bottom).

However, these images are not in conformity with the very high degree of mesostructural order as seen by SAXS and AFM (see the Supporting Information for a TEM micrograph). Although the TEM images clearly revealed a mesostructured framework with a periodicity on the order of 3-4 nm, no typical lamellar structures could be observed. On the contrary, only wormlike domains were observable with local mesoscopic order but with the same mesoscopic dimension (∼3 nm). The apparent discrepancy to the diverse AFM and SAXS experiments can be explained by the distinct properties of the surfactant in combination with the experimental TEM conditions. It is possible that the electron beam results in the decomposition or uncontrolled further polymerization of the surfactant, both resulting in the disruption of the lamellar mesostructure. Evidently, a noninvasive technique such as SAXS or AFM is favorable in this case, but the presence of local mesoscopic order seen by TEM clearly confirmed the results from other techniques. It should be emphasized that the preparation of inorganic nanocomposites was not possible using the corresponding tosylate. Even under carefully chosen conditions, we always observed phase separation between the surfactant phase and silica; that is, no hybrid structure on the nanometer scale could be obtained. This interesting result points to the necessity of well-chosen surfactants for the generation of self-assembled inorganic/organic nanocomposites. The tosylate surfactant is not suitable, probably due to its hydrophobic character and the inability to mediate the interfacial tension between the silica phase and the surfactant. In this respect, our study again shows the need for well-chosen surfactants. Polymerization. The films were polymerized by exposing them to iodine vapor. To confirm that polymerization took place, pure DDPABr was also polymerized inside the chamber. The pure surfactant turned black after a short period of time, as was expected. The polymerization was further confirmed by IR spectroscopy (Figure 11). A significant change in the IR spectrum is observed at 1044 and 1088 cm-1. This double band corresponds to the vibration of the pyrrole ring29 and disappears in the IR spectra of the polymerized sample, thus indicating a high degree of polymerization of DDPABr. The broad and very characteristic pyrrole band at around 700 cm-1 also disappears upon polymerization. As for the films, it is not possible to confirm the polymerization of DDPABr inside the pores with IR spectroscopy, since SiO2 shows very strong vibration bands at around 1000 cm-1. However, since the pyrrole ring C-H stretch vibration can be found at approximately 3000 cm-1 and is present in the film as well as in the pure surfactant, it can be shown that the films do contain DDPABr. A marked color change (white to black) was observed after polymerization. (29) The Aldrich Library of Infrared Spectra, 3rd ed.; 1981.

DDPABr as a Tecton for Nanocomposite SiO2 Films

In principle, frequencies for the CH2 antisymmetric νas(CH2) and symmetric νs(CH2) stretching bands are sensitive to the conformation of the alkyl chains. When the alkyl chains are highly ordered (all-trans conformation), the antisymmetric and symmetric bands usually appear near 2918 and 2850 cm-1, respectively.24 When the alkyl chains are highly disordered, the frequencies may shift upward (near 2927 and 2856 cm-1). Hence, νas(CH2) and νs(CH2) can be used as practical indicators of the degree of order for alkyl chains. In the present case, indeed both signals are observable in the as-prepared state and after polymerization. The two signals shift from 2929 and 2890 cm-1 (before polymerization) to 2918 and 2846 cm-1 (after polymerization). Thus, the FTIR data suggest a higher order in the mutual arrangement of the alkyl chains upon polymerization. However, we believe that such interpretation has to be made with care, since the local environment (silica nanocomposite) also influences such vibrations and it cannot be easily quantified. In conclusion, the changes in the CH2 vibrations also prove a high turnover rate during polymerization. As evidenced by SAXS, the mesostructure is preserved upon polymerization (see Figure 9B). The pattern is not as distinct, and the weak intensity of the higher order reflections indicates a certain distortion of the lamellar structure. Also, the first order interference is superimposed by a broad maximum, which could originate from nontemplated, polymerized surfactant (see Figure 8). Nevertheless, the presence of well-defined SAXS signals proves that the polymerization does not lead to mesostructural collapse. Conductivity Measurements. The volume resistivity of pure polymerized DDPABr was measured by the four point method and was found to be around 12 MΩ. It is important to note that unpolymerized surfactant was nonconductive. The volume resistivity of the polymerized hybrid films was even higher (∼18 MΩ), probably because of the lamellar structure of the films. Evidently, the SiO2 lamellae interrupt the conductivity of the polymerized DDPABr. The films are conductive only in the direction of the lamellae, which could not be measured with the experimental setup used in this investigation. However, measurements on SiO2, prepared without the DDPABr template, showed a very high volume resistivity being at the limit of resolution of the setup (>25 MΩ). Although these results indicated that the polymer was indeed semiconductive within the silica nanocomposite, different types of conductivity measurements will be needed to clarify the conductivity parallel to the lamellae. These issues will be addressed in our future exploration of this area.

Conclusions In the present contribution, a novel structure-directing agent, acting both as surfactant and additionally as monomer for the polymerization of pyrrole, was studied with respect to its phase behavior and use to generate nanocomposites with silica as a model system. Dodecyl-dimethyl-(2-pyrrol-1-yl-ethyl)-ammonium bromide (DDPABr) was characterized in detail with regard to behavior in the solid state. In particular, suitable SAXS analysis approaches were applied to elucidate the structure on the nanometer scale. At room temperature, DDPABr possesses a lamellar mesostructure of two alternating types of layers, with the thickness being in accordance with the molecule’s particular feature, namely, a relatively large head group. In situ temperaturedependent X-ray scattering, DSC, and polarized microscopy were performed to study the phase evolution upon temperature changes. Via EISA, the suitability of DDPABr as a structure-directing agent was used to generate nanocomposites with silica. Here, as

Langmuir, Vol. 23, No. 22, 2007 11279

a model system, we succeeded to prepare nanocomposites with SiO2 using well-established templating strategies such as EISA, resulting in crack-free, homogeneous thin films. As evidenced by diverse SAXS 1D and 2D experiments, the films consisted of a well-ordered lamellar mesostructure of alternating sheets of SiO2 and the surfactant, thus proving the feasibility of this surfactant as a structure-directing tecton. A novel, recent SAXS analysis approach18 allowed the fitting of the SAXS data in terms of such a bilayer model providing precise and accurate values for the thicknesses of both layers. Our results demonstrated that a laboratory X-ray setup is sufficient to obtain 2D SAXS data of high quality even on thin films when suitable, ultrathin substrates are utilized. A further point to note is that the current study also provided insight into the general parameters determining the self-assembly and formation of silica/surfactant nanostructures in the EISA process, particularly the influence of different head groups. While the surfactant used in the present study contains a pyrrole moiety, numerous studies applied CTAB, and recent studies reported the templating behavior of ionic surfactants containing imidazolium head groups, that is, the ionic liquid C16mimCl.30-33 Compared to CTAB, C16mimCl showed certain peculiarities (e.g., a special thermotropic behavior and abnormally high degree of mesostructural order of lamellar silica/C16mimCl nanocomposites), which were partly attributed to the strong mutual interaction of the imidazolium groups and the resulting preference for a mutual parallel alignment. Evidently, DDPABr, also possessing a heterocyclic head group, features certain similarities compared to C16mimCl, for example, the presence of a defined thermotropic liquid-crystalline phase and also a certain preference for lamellar mesostructures, probably also owing to the particular head group and enhanced interaction between them. However, in the case of DDPABr, this interaction seems to be even stronger than that for C16mimCl, which in turn impedes the formation of welldefined SiO2-DDPABr nanocomposites. In general, the EISA process is based on the compatibility of the siliceous species with the surfactant head group; that is, the interfacial energy between the SiO2 oligomers and the surfactant head group has to be sufficiently low to allow for the integration of the oligomers between two layers of head groups (e.g., for a lamellar structure). If the interaction between head groups is too strong, SiO2 oligomers are expelled from the surfactant, finally resulting in phase separation. Taking into account the problems in preparing SiO2-DDPABr mesostructured films, the head group interaction in DDPABr seems to be already of a critical strength regarding the EISA process. Although these results do not allow for an “a priori” prediction of the EISA templating behavior, our studies suggest that pyrrole-containing surfactants have to be further optimized regarding their templating performance. Finally, the pyrrole-containing surfactant could be easily polymerized in the pure form and even in the silica nanocomposites with SiO2 acting as a model system for an organic/ inorganic hybrid structured on the nanometer scale. While the polymerization in such mesostructured composites might eventually be accompanied by a moderate distortion of the mesostructure, we believe that these results show a promising route toward the fabrication of more complex materials utilizing such (30) Wang, T. W.; Kaper, H.; Antonietti, M.; Smarsly, B. Langmuir 2007, 23 (3), 1489. (31) Kaper, H.; Smarsly, B. Z. Phys. Chem. 2006, 220 (10-11), 1455. (32) Sel, O.; Kuang, D. B.; Thommes, M.; Smarsly, B. Langmuir 2006, 22, 2311. (33) Kuang, D. B.; Brezesinski, T.; Smarsly, B. J. Am. Chem. Soc. 2004, 126, 10534.

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polymerizable surfactants. The combination with TiO2 or other metal oxides may have the potential to create interesting materials for novel areas of applications. In particular, such nanocomposites, for example, prepared by using preformed TiO2 nanoparticles, could be promising as solid-state solar cells. So far, such materials suffer from a difficult mixing on the nanometer scale, but our results suggest that the “bottom-up” approach, using selforganizing, polymerizable functional monomers, has the potential to be an alternative methodology to create such functional composites.

Kaper et al.

Acknowledgment. This work, as part of the European Science Foundation EUROCORES Programme SONS, was supported by funds from the DFG and the EC Sixth Framework Programme. The MPG and the University of Bristol are acknowledged. Supporting Information Available: TEM image of a silica hybrid heat-treated at 80 °C, and TGA of the surfactant. This material is available free of charge via the Internet at http://pubs.acs.org. LA701911N