Article pubs.acs.org/Langmuir
Mesostructured Silica Containing Conjugated Polymers Formed within the Channels of Anodic Alumina Membranes from Tetrahydrofuran-Based Solution Avigail Keller, Saar Kirmayer, Tamar Segal-Peretz, and Gitti L. Frey* Department of Materials Engineering, Technion − Israel Institute of Technology, Haifa 32000, Israel S Supporting Information *
ABSTRACT: The synthesis of mesostructured silica from a tetrahydrofuran (THF)-based sol gel was carried out in the channels of an anodic alumina membrane (AAM) using the evaporation-induced self-assembly (EISA) method. Two different nonionic surfactants were used as structuredirecting agents, the triblock copolymer Pluronic P123 and the oligomer surfactant Brij56. The effect of the relative humidity and surfactant concentration on the type of mesophase and orientation of the in-channel mesostructures was studied using transmission electron microscopy (TEM) and grazing incidence small angel X-ray scattering (GISAXS). The inchannel structures obtained in this study were primarily of the 2D hexagonal phase with a circular orientation in which the hexagonally packed cylinders form a spiral-like shape from the channel wall inward. In addition, a columnar orientation of the hexagonal phase, in which the axes of the hexagonally packed cylinders are oriented parallel to the channel axes, was also observed. Finally, the use of the THF-based synthesis allowed the in situ incorporation of the highly hydrophobic yellow-emitting conjugated polymer poly[9,9-dioctylfluorene-co-benzothiadiazole] into the in-channel mesostructure upon its formation. The conjugated polymer was well distributed within the mesostructure and maintained its optical properties.
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INTRODUCTION Periodic mesostructured metal oxides have been the subject of extensive research in the past decade due to the variety of emanating applications such as separation,1 heterogeneous catalysis,2,3 drug release,4 and optoelectronics.5−10 Mesostructured metal oxide thin films are conventionally synthesized using the evaporation induced self-assembly (EISA) method, starting from an aqueous/alcohol solution of a metal oxide precursor and an amphiphilic surfactant, initially below its critical micelle concentration (cmc).11−18 Preferential evaporation of the alcohol during dip-coating results in two concurent processes: the self-organization of the surfactant species into liquid-crystal-like mesophase domains; the polymerization and cross-linking of the metal oxide network due to the increasing concentration of the hydrolyzed metal oxide species. Recently, mesostructured metal oxide films were processed from tetrahydrofuran (THF)-based solutions which allowed the direct coassembly of highly hydrophobic, high molecular weight guest molecules within the mesostructured metal oxide matrix.9,19 THF evaporation during film processing promotes surfactant self-organization into liquid crystal-like mesophases stabilized by high extents of metal oxide crosslinking into mesostructured composites.19,20 The type of mesostructure obtained from the THF-based precursor solution, cubic, hexagonal or lamellar, was found to depend on the chemical structure and concentration of the surfactant, the chemical composition of the precursor solution, and the relative humidity in the deposition chamber.20 However, © 2011 American Chemical Society
control over orientation and domain size of the mesostructured metal oxide films deposited from THF-based solutions has yet to be investigated. Importantly, preferential host orientation can induce alignment of coassembled functional guest molecules, endowing the composite materials with beneficial properties such as polarized emission,5,21,22 transport pathways, or control over energy transfer.23−25 To achieve control over domain orientation of EISAprocessed mesostructured metal oxides, mesostructured silica has recently been synthesized within the channels of polycarbonate 2 6 − 2 8 or anodic alumina membranes (AAM).29−46 Soaking the AAM in the aqueous/ethanol precursor sol solution, followed by solvent evaporation, resulted in the formation of a variety of mesostructures inside the membrane channels. The type of mesostructure obtained was found to critically depend on processing parameters such as humidity, precursor solution composition, and the surfactant used to direct the structure. Notably, carefully tuning the Si/ surfactant molar ratio in the precursor solution allowed the formation of unique channel-imposed arrangements including circular hexagonal, columnar hexagonal, and concentric lamellar.32,33,36,37,39−42 Here we show that conjugated polymer-incorporated mesostructured silica with preferential orientation and Received: October 2, 2011 Revised: December 5, 2011 Published: December 8, 2011 1506
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compositions from which the different in-channel silica mesostructures were prepared are summarized in Table 1.
channel-imposed arrangements can be deposited within the channels of an AAM from a THF-based solution. Two ethylene oxide-based surfactants, oligomer Brij56 and Pluronic block copolymer P123, were used to direct the mesostructures. Detailed understanding of the in-channel mesostructured silica products are obtained from transmission electron microscopy, TEM, and small-angle X-ray scattering measurements, SAXS, of the mesostructure within the AAM. Finally, the THF-based solution processing permitted the coassembly of highly hydrophobic, high molecular weight conjugated polymer guest molecules within the mesostructured silica matrixes during their formation inside the AAM channels. The incorporated conjugated polymers are furthermore shown to have stable semiconducting properties that may be exploited in electronic and optoelectronic applications.
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Table 1. Synthesis Conditions Used in This Study To Prepare Silica Mesostructures from THF-Based Precursor Solutions in the Channels of AAMs surfactant
relative humidity, %
P123 P123 P123 Brij56 Brij56
45 90 90 90 90
surfactant/TEOS molar ratio 0.0165 0.0165 0.0197 0.138 0.166
Characterization. Samples were characterized by transmission electron microscopy (TEM) and glancing incidence small-angle X-ray scattering (GISAXS) measurements. TEM images were obtained with a Tecnai G2 T20 transmission electron microscope operating at 200 kV. Energy-filtered (EF) TEM measurements were performed using a FEI Titan 80−300 KeV FEG-S/TEM operating at 300 keV with filters set to sulfur L2/3 edge (165 eV). Samples for plane view TEM were prepared by dimple grinding followed by Ar ion polishing. Crosssection TEM samples were prepared using a focused ion beam (FIB, FEI Strata 400s dual-beam FIB) equipped with a Ga+ source for milling, thinning, and polishing. Glancing incidence small-angle X-ray scattering measurements (SAXS) were performed using a small-angle diffractometer (Rigaku SMAX-300) with Cu Kα radiation, λ = 1.5405 Å, and a pinhole collimation yielding a beam of 400 μm (fwhm). A two-dimensional position-sensitive gas detector (200 mm diameter) positioned 1500 mm behind the sample was used. The patterns were recorded using a microfocus X-ray tube (45 kV/0.9 mA) coupled to a side-by-side Kirkpatrick Baez multilayer monochromator. The substrates were horizontally located in the SAXS system, and the patterns were recorded in glancing incidence mode with a fixed incidence angle α (the angle between the incident beam and the sample surface) of approximately 2°. To ensure that the diffraction pattern originates from mesostructures in the membrane channels and not on the membrane surface, the substrates were polished prior to SAXS measurements, and about 20 μm were mechanically removed from the top and bottom surfaces of the membrane. The geometry of the SAXS measurements and possible diffraction patterns with their correlated in-channel mesostructure can be seen in Figure 1a. In principle, the cylinders can pack hexagonally inside the channels in two possible orientations. The first possible orientation is with the long axes of the cylinders oriented parallel to the surface of the membrane, as shown in Figure 1b and referred to as circular hexagonal orientation. This type of orientation would result in the type of GISAXS diffraction pattern shown in Figure 1b, which is similar to that obtained from a hexagonal mesostructure on a flat surface, Figure 1c, but rotated by 90°. Alternatively, the hexagonally packed cylinders can orient with the long axis perpendicular to the membrane surface, as shown in Figure 1d and referred to as having columnar hexagonal orientation. In this orientation, only the main axes of the cylinders contribute to the diffraction pattern showing only two diffraction spots.37,41 The mesostructure within the AAM channels can also form a circular lamellar phase as shown in Figure 1e, which also gives rise to two diffraction spots, similar to the spots obtained from the columnar hexagonal phase. A Varian Cary Eclipse spectrofluorometer was used to measure the photoluminescence (PL) spectra, with 420 nm excitation wavelengths for the F8BT polymer.
EXPERIMENTAL SECTION
Materials. Analytical grade tetrahydrofuran (THF AR, BioLab, Israel), tetraethoxysilane (TEOS 98% GC, Aldrich, Germany), and hydrochloric acid (HCl, Carlo Erba, Italy) were used as received. Two amphiphilic nonionic block copolymers were used: a triblock poly(ethyleneoxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) was obtained as a gift from BASF, USA, and used as received, EO20PO70EO20 (Pluronic P123, Mn = 5750 g/mol) where ‘EOx’ and ’POy’ represent the lengths of the ethylene oxide and propylene oxide blocks, respectively; the second low-molecular-weight CnH2n+1(EO)m surfactant was purchased from Aldrich, Brij56 (n = 16, m = 10, Mw = 683 g/mol). Compared to the Pluronic triblock copolymer, the Brij surfactant composed of short hydrophilic ethylene oxide segments (10 EO monomer units) and C16H33 alkyl chains that are significantly more hydrophobic than the propylene oxide blocks of the Pluronic. The conjugated polymer poly[9,9-dioctylfluorene-co-benzothiadiazole] (F8BT, Mw = 22 000 g/mol) used in this study was purchased from ADS, Canada, and used as received. F8BT is useful as a TEM marker, due to its sulfur atoms that provide distinct signals in energyfiltered TEM images. Anodisc (anodic alumina membranes, Whatman) with a 13 mm diameter, an average channel diameter of 200 nm, and thickness of approximately 60 μm was used as received. In-Channel Mesostructure Deposition. A prehydrolyzed silica− sol was prepared by mixing 75 mL of THF, 15 mL of TEOS, and 6 mL of HCl (0.07 M), followed by sonication in an ultrasonic bath for 1 h. Variable amounts of structure directing agent, P123 or Brij56, in the range of 0.1−0.12 g, were dissolved in 1.5 mL of the THF silica−sol to obtain a surfactant−sol solution. Separately, 5 mg of F8BT was fully dissolved in 2.5 mL of THF, occasionally using mild heating conditions (up to 60 °C) and stirring. Prior to film deposition, 2.2 mL of the conjugated polymer solutions (filtered through 0.45 μm PTFE membrane) or 2.2 mL of pure THF (for the deposition of films without conjugated-polymer guest species) was added to the surfactant−sol solution. The anodic alumina membranes (AAMs) were positioned in a deposition chamber with controlled humidity at room temperature and soaked with the precursor mixtures by distributing 0.05 mL of the solutions over the whole membrane surface. The deposition chamber is a small glovebox (∼200 L), filled with air, in which the relative humidity is controlled by the addition of water vapors until the desired relative humidity is reached. The relative humidity is stable for at least 24 h and is monitored by a hygrometer. The solution penetrates into the membranes’ channels, and THF evaporation drives the selfassembly of the ordered mesostructure inside the channels (EISA method). The membranes appear dry after 3−5 h but were kept under controlled humidity conditions for an additional 20 h to ensure complete solvent evaporation. Finally, the membranes were placed on a hot plate at 70 °C for 2 h to remove all solvent traces. The effects of the relative humidity, surfactant type P123 or Brij56, and surfactant/Si molar ratio in the precursor solution on the inchannel mesostructure formation were studied. The synthesis
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RESULTS AND DISCUSSION The high solubility and nonselectivity of THF toward the hydrophobic and hydrophilic segments prevents the selforganization of surfactants into micellar aggregates in THF. The solution-phase behavior of the ternary THF−water− 1507
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water deficiency in the THF-based solution and affect the precursor hydrolysis and the surfactant self-assembly. Indeed, it was recently shown that the type of TiOx mesostructure deposited from THF-based precursor solutions on flat substrates is highly sensitive to the local water concentration.20 Therefore, we hypothesize that the type and orientation of the mesostructured silica structures formed within AAM channels from THF-based precursor solution will strongly depend on the RH in the deposition chamber. To study the effect of the humidity conditions on the type and orientation of the in-channel mesostructures formed from THF, the AAM was soaked in a THF-based precursor solution with a P123/TEOS molar ratio known to direct the hexagonal phase on a flat surface (P123/TEOS molar ratio = 0.0165),19 with the relative humidity in the deposition chamber either low (45%) or high (90%). We find that soaking the AAM in the THF-based precursor solution under low humidity conditions (45%) leads to the formation of mesostructured silica structures within >95% of the AAM channels; however, the structures are distorted and occupy about half of the channel volume, as shown in the top view TEM images in Figure 2a and 2b. Similar
Figure 1. (a) Schematic drawing of the SAXS measurements geometry. (b−e) Schematic diffraction patterns with their corresponding planner and in-channel mesostructures: (b) The circular hexagonal mesostructure yields two in-plane diffraction spots and four out-ofplane diffraction spots. (c) The hexagonal mesostructure on a flat surface yields a diffraction pattern similar to the one of circular hexagonal mesostructure but with a 90° rotation (d) The columnar hexagonal mesostructure yields only two in-plane reflections. (e) The in-channel circular lamellar mesostructure also shows two in-plane reflections but has a larger d-space than that of the columnar hexagonal mesostructure.
surfactant systems indicates that water molecules are absolutely necessary to promote the self-aggregation of the surfactant into micelles. In the case of the THF-based precursor sol solution, dilute water molecules produced in situ by silica condensation are sufficient to promote surfactant self-aggregation into micelles and ultimately liquid crystal-like inorganic−organic mesophases as the THF evaporates.19 In addition to water molecules produced by the silica condensation, water availability will also depend on the relative humidity and concentration of the hydrophilic segments, which are often saturated with water molecules.20 Therefore, the formation and orientation of metal oxide mesostructures from THF-based solutions in AAM channels is studied by examining the effect of two major parameters on the type and orientation of the mesophases deposited: the relative humidity in the deposition chamber, and the composition of the precursor solution. P123-Directed Mesostructured Silica from THF in AAM. The Effect of the Relative Humidity on the InChannel Phase and Orientation. The relative humidity (RH) in the deposition chamber is known to affect the type and ordering of silica mesostructures deposited from sol solutions on flat surfaces. This effect is moderate for aqueous/alcohol precursor solutions deposited on flat substrates or within AAM channels. For example, it was recently shown by Bein et al. that a precursor solution could direct the in-channel hexagonal phase at both low and high humidity conditions.37,39 In contrast, the RH in the deposition chamber has a dramatic effect on the type and ordering of mesostructures deposited from THF-based sol solutions where water deficiency limits the precursor hydrolysis and the surfactant self-assembly. Hence, at low RH, the THF-based process suffers from water-lean conditions, while at high RH conditions more water is absorbed from the environment into the solution. Under such conditions, the adsorbed chamber humidity could compensate for the
Figure 2. (a, b) Top view TEM images of an AAM soaked in a THFbased precursor solution with P123/TEOS molar ratio of 0.0165, known to result in the formation of a hexagonal mesostructure on flat substrates, at low humidity conditions (45%).
distorted in-channel mesostructures were previously reported by Bein et al. from an aqueous/ethanol precursor solution with P123/TEOS molar ratio = 0.017 at both 20% and 60% RH and identified using in situ GISAXS as degraded lamellar mesostructures.39 The distorted mesostructures formed from 1508
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resulting in the observed empty channels. The d-spacing evaluated from the TEM images, 9 nm, is in good agreement with that reported for hexagonal silica deposited on a flat surface from an identical THF-based precursor solution.19 However, using only top view TEM images, it is not possible to distinguish whether the observed circular mesostructures are the circular hexagonal phase (Figure 1b), or the circular lamellar phase (Figure 1e). To determine the phase and ordination of the obtained inchannel silica mesostructures, SAXS measurements were performed on the AAM in the configuration schematically illustrated in Figure 1a. The SAXS pattern obtained from this sample (P123/TEOS molar ratio of 0.0165, and 90% RH) is shown in Figure 3c. The pattern is noisy due to the low density of silica structures in the membrane. Nevertheless, the presence of several distinct spots in the pattern is indicative of long-range order in the sample. Importantly, these diffraction spots could not be associated with the circular lamellar orientation observed at low RH because the circular lamellar phase is expected to induce two in-plane spots only, as drawn schematically in Figure 1e. Therefore, the pattern indicates that the circle-like in-channel mesostructures obtained at high RH are of the circular hexagonal phase (Figure 1b). Hence, while the low humidity conditions direct a circular lamellar phase mesostructure, the high humidity conditions direct the deposition of the circular hexagonal phase from the same solution.19,39,47 The appearance of the hexagonal phase is associated with the swelling of the hydrophilic head groups by water adsorbed from the high RH environment, inducing the formation of higher curvature phases.19,39,47 In the case of THF-based precursor solutions, the water deficiency conditions endow the structure with high sensitivity to the relative humidity in the deposition chamber. At low RH, the water generated in the solution due to the metal oxide precursor condensation is sufficient to induce surfactant self-assembly, but insufficient to swell the hydrophilic head groups, resulting in low curvature phases in the channels of the AAM. In contrast, at high RH in the chamber, water from the chamber is adsorbed into the precursor solution, swells the hydrophilic headgroup, and induces the formation of mesophases with high curvature within the AAM channels. These results show that the RH in the deposition chamber plays a significant role in determining the type and orientation of the in-channel mesostructures deposited from THF-based precursor solutions. In addition to the circular hexagonal phase obtained in the AAM from the THF-based precursor solutions at high RH, the columnar hexagonal phase was also identified. The appearance of the columnar phase is rare and always in the center of and surrounded by the circular hexagonal phase, as shown in Figure 3b. The development of the columnar hexagonal phase in the center of the channel was also reported for in-channel mesostructures deposited from the aqueous/ethanol solutions.39 It was suggested that the columnar orientation is a result of a phase transformation from the initially formed circular phase to the more thermodynamically stable columnar phase.39,40 The appearances of the columnar phase in the inchannel mesostructures deposited from THF-based solution at high RH, but not at low RH, could support this mechanism. We speculate that under the THF-based conditions, the high humidity suppresses evaporation rates and allows the slowdried material in the channel centers to approach the more thermodynamically stable state, i.e., the columnar hexagonal phase.
the THF-based precursor solution in the AAM channels did not yield SAXS patterns, and hence the type and orientation of these mesostructures could not be determined unambiguously. Using the same P123/TEOS molar ratio (0.0165) but at high humidity conditions in the deposition chamber (90%) resulted in the formation of ordered undistorted mesostructures in the AAM channels, as shown in Figure 3a and 3b. In this case, the
Figure 3. Top view TEM images (a, b) and a SAXS pattern (c) of mesostructured silica formed in the channels of a AAM from a THFbased precursor solution with P123/TEOS molar ratio of 0.0165, at high humidity conditions (90%).
membrane is mostly empty with only ∼20% of the channels hosting silica structures, each with about 70% of the channel’s volume occupied and the structures adhering to one of the channel’s walls. Despite the observed order of the mesostructures, one can clearly see that an anisotropic shrinkage took place. The shrinkage seen in the TEM images could occur during TEM sample preparation or during the drying process. The shrunk mesostructures are either anchored to the channel wall only by a very limited surface or completely detached, 1509
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The Effect of the P123 Concentration on the In-Channel Phase and Orientation. In general, increasing surfactant concentration in a mesostructure precursor solution including the metal oxide precursor and surfactant leads to the formation of less curved phases on flat substrates.48 This principle is also reflected in the case of mesostructured silica deposited from aqueous/water precursor solutions within AAM channels, where an increase in surfactant concentration resulted in more abundance of the noncurved columnar hexagonal and circular lamellar phases, at the expense of the curved circular hexagonal phase.37,39 To examine the effect of surfactant concentration on the orientation of the in-channel hexagonal phase deposited from the THF-based system, the RH was kept high (90%) while the P123/TEOS molar ratio was increased slightly from 0.0165 to 0.0197. Such a minor change in surfactant concentration has no effect when the THF-based synthesis is carried out on a flat substrate and the hexagonal phase is formed with orientation parallel to the substrate. Increasing the P123 concentration in the THF-based precursor solution results in improved ordering of the inchannel circular hexagonal structures and higher abundance of the columnar hexagonal phase, as shown in the TEM plane view and cross-section images in Figure 4 a and Figure 4b, respectively. The images show that the hexagonal circular phase is predominantly formed, there is no shrinkage or distortion of the in-channel mesostructures, and over 85% of the channels are occupied by mesostructures. The SAXS pattern obtained from the same sample, shown in Figure 4c, confirms the predominance of a well-ordered circular hexagonal phase. The calculated d-spacing values from the SAXS pattern, d01 = 12.5 nm and d10 = d11 = 11.8 nm, correspond to a slightly distorted circular hexagonal structure and are in good agreement with average d-spacing calculated from the TEM images of the same sample, d ∼ 12.2 nm. This average d-space is ∼3 nm larger than that obtained from in-channel mesostructures synthesized with low P123 concentration at the same humidity conditions (Figure 3a,b). The TEM and SAXS results indicate, therefore, that the diameter of the hexagonally packed cylinders in the AAM depends on the concentration of the surfactant, with an increase in d-space when increasing P123/TEOS molar ratio. Notably, increasing the P123/TEOS molar ratio from 0.0165 to 0.0197 does not increase the d-space of hexagonal mesostructures deposited from THF-based solutions on flat substrates (not shown). A possible explanation for the increased diameter of the in-channel mesostructures is that the volume constraint imposed by the AAM channels leads to the incorporation of more P123 units into each of the hexagonally packed cylinders, thus increasing the cylinder diameter and observed d-space. Confinement of the hexagonally packed cylinders to the channels of an AAM also induces a change in the plane from which the mesostructure starts to nucleate. When comparing SAXS patterns of hexagonal mesostructures deposited a flat surface (Figure S1, Supporting Information) to those deposited in the channels of porous substrates such as AAM (Figure 4c), one can clearly see a 90° rotation of the diffraction pattern (see Supporting Information). This rotation indicates that while on a flat surface the most dense plane of the hexagonally packed cylinders (01) is parallel to the surface, in the case of the AAM the same dense plane is oriented in parallel to the channel wall, that is, perpendicular to the surface of the membrane (see TEM image and SAXS pattern in Figure 4b and 4c, respectively). This 90° rotation is associated with the mesostructures in the
Figure 4. Top view TEM image (a), cross-section TEM image (b), and SAXS pattern (c) of mesostructured silica formed in the channels of a AAM from a THF-based precursor solution with P123/TEOS molar ratio of 0.0197, at high humidity conditions (90%). (d) Schematic drawing of the circular hexagonal phase; the white circles represent the surfactant domains.
AAM channels nucleating at the channel walls. Indeed, a model proposing mesostructure nucleation at random locations on the channel walls was reported by Bein et al.39 Therefore, is it expected that the chemical character of the channel walls will have a significant influence on both the mesostructure phase and its orientation. The increased surfactant concentration leads to a more abundant formation of the columnar hexagonal orientation in the AAM channels. The columnar hexagonal mesostructures generally appear in the center of the channels surrounded by the circular hexagonal phase as shown in Figure 5a, occupy less than 50% of the channel volume, and have an average diameter of 12 nm. This value is smaller than that of the circular hexagonal cylinders in the same sample which have a diameter of approximately 14.1 nm, corresponding to the d-space of 12.2 nm. The reduced diameter of the columnar cylinders can be 1510
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phases in AAM channels from aqueous/ethanol solutions.39 The THF-based precursor solution with the low Brij56 concentration directed the formation of well-ordered circular and columnar hexagonal mesostructures within the AAM channels, as can be seen in the top view TEM images in Figure
Figure 5. Top view TEM images of mesostructured silica formed in the channels of a AAM formed from a THF-based precursor solution with P123/TEOS molar ratio of 0.0197, at high humidity conditions (90%). (a) The columnar phase is located in a small area at the center of the channel surrounded by the circular phase. (b) The columnar phase extends over a large channel area and can be found also near the channel wall.
attributed to the extreme confinement conditions in the center of the channel during the formation of the columnar phase which compresses the cylinders. The abundance of similar inchannel mesostructures obtained from aqueous/ethanol-based sol solutions has been associated with a phase transformation from the circular hexagonal phase to the more thermodynamically stable columnar hexagonal phase.39,40 Occasionally columnar hexagonal mesostructure domains are also found near the AAM walls and extend over large domains, as shown in Figure 5b. The cylinder diameter in such domains is typically larger than that found in the central domains, ∼14.2 nm, and is comparable to that of the circular hexagonal phase in the same sample. The variety of cylinder diameters in the columnar hexagonal phase might reflect different projections of the in situ transformation. However, it could also indicate that wide cylinders formed from high P123 solutions are not stable in the bent state imposed by the circular orientation and form the columnar orientation directly from solution. Brij56-Directed Mesostructured Silica from THF in AAM. The THF-based synthesis is versatile, and many structure-directing agents can be used to direct a variety of phases with different lattice parameters on flat substrates.19,20,47 To study the influence of the surfactant type on the obtained in-channel mesostructures deposited from THF-based precursor solutions, samples were synthesized using Brij56 as the structure-directing agent. This surfactant, like P123, is a commercially available nonionic surfactant in which the hydrophilic segment is PEO. However, Brij56 is only a small oligomer, and its hydrophobic segment consists of an alkyl chain that is significantly more hydrophobic than the PPO block of P123. In addition, Brij56 was recently used in both the THF-based synthesis of mesostructured metal oxides on flat surfaces47 and to direct mesostructured silica from an aqueous/ ethanol solution in AAM channels.37,39,40,49 Interestingly, it was shown by Bein and co-workers that the hexagonal circular-tocolumnar transformation of in-channel Brij-directed mesostructures is more rapid than that observed for the in-channel P123-directed mesostructures.39 Brij56-directed in-channel silica mesostructures from THFbased solutions were deposited at high RH (90%) and Brij56/ TEOS molar ratios of 0.138 and 0.166. The selected low Brij56 concentration is known to direct the hexagonal phase from a THF-based solution on a flat surface.47 A similar ratio, 0.133, was shown to direct the circular and columnar hexagonal
Figure 6. (a, b) Top view TEM image, and (c) SAXS pattern of inchannel mesostructures synthesized with low Brij56 concentration (Brij56/TEOS molar ratio = 0.138) at high humidity conditions (90%).
6a,b. The d-space of the circular hexagonal phase calculated from the TEM is 5.6 nm, slightly larger than the d-space of Brij56-directed hexagonal mesostructures deposited from similar THF-based solutions on a flat surface.47 Interestingly, most of the in-channel structures are damaged (see for example Figure 6b), probably due to the tedious TEM sample 1511
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analysis. The d-space, calculated from the SAXS pattern (Figure 7c) and evaluated from the TEM images (Figure 7a,b) is 6.3 nm, which is 0.7 nm lager than that of the same mesostructures prepared with low Brij56 concentration. A similar increase in the d-space with increasing the surfactant concentration was observed for the P123 system and was associated to the higher incorporation of surfactant molecules in each hexagonally packed cylinder imposed by the channel volume constraint. The increased Brij56 concentration also leads to more abundance of the columnar hexagonal phase. The domains of the columnar hexagonal phase are significantly larger in this sample, occupying 30% to 100% of the channel volume. Large domains of the Brij56-directed columnar hexagonal phase were also obtained from aqueous/alcoholic precursor solutions in AAM channels, although with Brij56 concentrations substantially higher than those used in this study. Using in situ SAXS, Bein et al. identified a phase transformation from the circular hexagonal phase to the more thermodynamically stable columnar hexagonal and circular lamellar phases. Such phase transformation could also be responsible for the high abundance of the columnar phase directed by Brij56 from THF precursor solutions in this study. Coassembly of Mesostructured Silica−SDA-Conjugated Polymer Films. When guest species, organic or otherwise, are to be incorporated during self-assembly into the in-channel structure, they must be compatible with the synthesis solution so that they do not macroscopically phase-separate from the synthesis mixture during deposition. The water−ethanol synthesis conditions therefore are entirely unfeasible for the incorporation of highly hydrophobic guest molecules. Processing silica mesostructures from THF-based solutions, on the other hand, allows the coassembly of highly hydrophobic macromolecules such as conjugated polymers into the hydrophobic regions of the mesostructured host during film formation.19,47 To demonstrate the usefulness of the THF-based synthesis for the incorporation of conjugated polymers into mesostructured silica within the AAM channels, poly(9,9-dioctylfluoreneco-benzothiadiazole) (F8BT) was introduced into the THFbased precursor solution. The green-emitting F8BT is a highly hydrophobic, highly luminescent conjugated polymer with one sulfur atom per monomer (see Figure 8a) and high molecular weight (22 000 g/mol). The intense photoluminescence (PL) spectrum of F8BT offers an internal probe sensitive to aggregations and conjugation length, providing more information about the in-channel structures. The location of the conjugated polymers within the inchannel mesostructure can be measured from the difference in chemical composition between the silica, surfactant, and conjugated polymer. More specifically, the elementally distinct signals measured at sub-eV-resolution in electron energy loss spectroscopy (EELS) are spatially resolved by energy-filtered TEM (EFTEM), allowing a distribution map of elements within a material to be established. Sulfur atoms, for example, have an EELS signature centered at 165 eV, corresponding to the L2/3 loss edge and hence can be used to map the distribution of sulfur-bearing conjugated polymer guest molecules in the mesostructured silica within the alumina channels. Samples with the sulfur-containing conjugated polymer F8BT were prepared by adding the conjugated polymer to a synthesis solution with P123/TEOS molar ratio of 0.0197 and carried out at 90% relative humidity. The SAXS pattern
preparation procedure and accumulation of radiation under the electron beam during TEM analysis. The sensitivity of the Brij56-directed mesostructures to the electron beam obscures the quantitative analysis, and only a rough estimation of the pre-TEM occupied channel volume is possible. About 35% of the channels are occupied with hardly damaged or nondamaged structures with an in-channel volume filling of >90%. The SAXS pattern obtained from this sample is very noisy due to the evident damage of the mesostructures but still demonstrates the long-range order in the sample as well as the formation of the circular hexagonal phase, as shown in Figure 6c. Increasing the Brij56 concentration to Brij56/TEOS molar ratio = 0.166 results in the formation of highly ordered circular and columnar hexagonal mesostructures, as can be seen in the TEM images and SAXS pattern presented in Figure 7.
Figure 7. (a, b) Top view TEM image, and (c) SAXS pattern of samples synthesized with Brij56/TEOS molar ratio = 0.166 at high humidity conditions of 90%.
Approximately 75% of the channels in the sample are occupied with mesostructures, and the in-channel volume filling is >90%. The mesostructures in this sample are also unstable, and accumulative radiation damage is observed during the TEM 1512
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Figure 8. (a) Chemical structure of poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT), (b) SAXS pattern of the AAM with P123-directed F8BTincorproated silica mesostructures, (c) top view HRTEM image of in-channel P123-directed F8BT-incorproated silica mesostructures, and (d) energy-filtered TEM image of the same area shown in panel c, constructed from electrons that interacted with sulfur atoms.
obtained from the AAM with in-channel P123-directed F8BTincorprated mesostructured silica can be seen in Figure 8b. The diffraction pattern verifies the formation of the circular hexagonal mesostructure within the alumina channels. The dspacing of this sample is about 11.5 nm, slightly smaller than the corresponding d-spacing of the sample synthesized in the same conditions without the conjugated polymer (12.2 nm). A top view bright-field TEM image of an in-channel P123directed F8BT-incorprated silica mesostructure is shown in Figure 8c. The image shows contrast between the highly scattering alumina membrane (black frame), the less scattering silica mesostructures (dark gray features), and weakly scattering surfactant domains (light gray features) which reflect as circular hexagonal mesostructure ordering. By comparison, the energyfiltered TEM image in Figure 8d of the same area shows intensity contrast between light regions containing sulfur and dark regions that do not. Comparing the two images indicates a clear correlation between the light areas in Figure 8c associated with the organic surfactant domains and those in Figure 8d corresponding to the sulfur-rich regions. Such correlated and complementary bright-field and energy-filtered electron scattering intensities indicate that the highly hydrophobic conjugated polymer is indeed incorporated and well distributed within the surfactant regions of the hexagonal in-channel structure. (Note that the energy-filtered TEM image in Figure 8d also shows white dots in the alumina membrane associated with phosphor contamination). The photoluminescence spectra of the AAM with P123directed F8BT-incorproated silica mesostructures (red line in Figure 9) is generally similar to that of corresponding pristine F8BT films, confirming that the F8BT-conjugated polymer maintains its optical properties within the mesostructured film. The spectrum is however slightly red-shifted compared to that of a pristine F8BT polymer film (blue line in Figure 9). A similar red shift in the PL spectrum was also obtained for a P123-directed F8BT-incorproated silica film cast from the same solution on a flat substrate (green line in Figure 9) and is in agreement with a red-shifted emission of other conjugated
Figure 9. Photoluminescence (PL) spectra of a pristine F8BT film (blue line), P123-directed F8BT-incorproated mesostructured silica film prepared by drop casting (green line), and P123-directed F8BTincorproated silica mesostructures in an AAM (red line).
polymers incorporated into mesostructured silica films from THF-based solutions during their formation.19,47 The redshifted PL spectrum is consistent with a conjugated polymer chain morphology that is on average extended, compared to the pristine film, due to confinement in the cylindrical mesostructured silica channels, which tend to favor extended polymer conformations and thus increased conjugation lengths. Extended conjugation enhances carrier mobility and is therefore desirable in optoelectronic applications.
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CONCLUSION A novel THF-based sol−gel synthesis of mesostructured silica was used for the first time in a confined environment of a porous alumina membrane, and the influence of synthesis conditions on the obtained in-channel mesostructure were examined. It was found that the relative humidity, surfactant/ precursor molar ratio, and type of surfactant play a significant role in determining both the mesophase type and orientation of the in-channel silica mesostructures. In general, hexagonal silica 1513
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mesostructures were directed in the AAM channels by two distinct nonionic structure directing agents: a triblock copolymer, Pluronic P123, and an oligomer surfactant, Brij56. The obtained hexagonal mesostructures are either in the circular or columnar orientation. The columnar mesophase is located either at the center of the channels surrounded by a circular phase or at large domains close to the channel walls. This columnar orientation was previously reported for hexagonal mesophases deposited in AAM channels from aqueous/ethanol solutions and associated with the circular-tocolumnar phase transformation. Such phase transformation could also be responsible for the abundance of the columnar phase in mesostructures directed by Bri56 and P123 from THF solutions, although formation of the columnar phase directly from the precursor solution could also be considered. Finally, using the THF-based synthesis allowed the incorporation of a highly hydrophobic conjugated polymer into the in-channel mesostructures and its good distribution in the organic domain, while maintaining its optical properties.
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ASSOCIATED CONTENT
S Supporting Information *
Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
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ACKNOWLEDGMENTS The authors thank the Russell Berrie Nanotechnology Institute in the Technion for financial and technical support, and the Electron Microscopy Center in the Technion for scientific and technical assistance.
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