Polymeric Micelle Assembly for Preparation of Large-Sized

Here we report the synthesis of mesoporous metal oxide materials with various ... The PEO corona helps the micelles to stay well dispersed in the prec...
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Polymeric Micelle Assembly for Preparation of Large-Sized Mesoporous Metal Oxides with Various Compositions Bishnu Prasad Bastakoti,† Shinsuke Ishihara,† Sin-Yen Leo,‡ Katsuhiko Ariga,† Kevin C.-W. Wu,‡ and Yusuke Yamauchi*,†,§,∥ †

World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ‡ Department of Chemical Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan § Faculty of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan ∥ Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan S Supporting Information *

ABSTRACT: Here we report the synthesis of mesoporous metal oxide materials with various compositions by assembly of spherical polymeric micelles consisting of triblock copolymer poly(styrene-b-2-vinyl pyridine-b-ethylene oxide) (PS-b-PVP-bPEO) with three chemically distinct units. The PVP block interacts strongly with the inorganic precursors for the target compositions. The hydrophobic PS block is kinetically frozen in the precursor solutions, enabling the spherical micelles to remain in a stable form. The frozen PS cores serve as templates for preparing robust mesoporous materials. The PEO corona helps the micelles to stay well dispersed in the precursor solutions, which plays a key role in the orderly arrangement of the micelles during solvent evaporation. This approach is based on assembly of the stable micelles using a simple, highly reproducible method and is widely applicable toward numerous compositions that are difficult for the formation of mesoporous structures.

1. INTRODUCTION The synthesis of mesoporous materials has attracted great interest due to their wide range of applications in drug delivery, catalysis, sensors, photovoltaic cells, and fuel cells.1,2 The highly porous structures enable large increases in surface area and access by guest species to the internal pore surface, which can significantly influence the physicochemical properties of mesoporous materials. Low-molecular-weight ionic and nonionic surfactants3,4 have normally been used to synthesize ordered mesoporous materials. The use of commercially available EOnPOmEOn-type block copolymers5 (F127, P123, and P108) leads to the formation of large-sized mesopores (around 10 nm) compared to the pores prepared with lowmolecular-weight surfactants (less than 5 nm). Further expansion of mesopores is achieved in the EOnPOmEOn-type block copolymer systems by using pore expanding agents.6 Mesoporous materials with large pores have become a subject of extensive research, because large-sized mesopores can accommodate large guest objects, leading to successful adsorption/immobilization of biomolecules and deposition of metal nanoparticles for catalysis without blocking the pore channels.7 In particular, transition metal oxides such as titania (TiO2) are favorable for the fabrication of photovoltaic cells8 and enable better catalytic performance.9 Solvent evaporation is very effective for the preparation of mesoporous metal oxides having different shapes such as film © 2014 American Chemical Society

and monolith. Various mesoporous metal oxides have been prepared by using the corresponding metal alkoxides or chlorides. Because the reactivities of metal alkoxides or chlorides is much higher than those of silicon alkoxides, controlling hydrolysis and condensation is necessary to prepare highly ordered mesostructures. As the solvent is gradually evaporated, unimers in the precursor solution start to assemble into micellar form. Therefore, solvent composition, evaporation rate, temperature, and humidity affect the mesostructural order in the final products. Also, various mesophase transitions have been frequently confirmed during solvent evaporation. Ogura et al. found that drying at a higher temperature promoted the phase transformation of not only hexagonal to cubic mesophases but also cubic to lamellar mesophases.10 In recent studies, several amphiphilic block copolymers containing hydrophobic PS blocks with high glass transition temperatures (Tg) have been used as templates,11 which coupled a solvent-pair-induced phase separation with sol−gel chemistry.12,13 Other amphiphilic diblock copolymers, such as poly(isoprene-b-ethylene oxide),14 poly(methyl methacrylate-bethylene oxide),15 and poly(ethylene oxide-b-acrylonitrile),16 are also applicable to synthesize mesoporous materials. Received: October 21, 2013 Revised: December 17, 2013 Published: January 6, 2014 651

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Figure 1. Formation mechanism of mesoporous TiO2 through a micelle assembly process by using the asymmetric triblock copolymer PS-b-PVP-bPEO.

domains of block copolymers, which often leads to irregular structures.31 Moreover, it is believed that PEO segments favor the formation of very flexible hybrid frameworks in which these groups are incorporated into the metal oxide framework walls. However, the original mesostructure is sometimes broken during calcination process due to poor thermal stability of block copolymers.32 Therefore, it is more interesting to use asymmetric triblock copolymer with high thermal stability with reactive domain. In this study, we propose a new strategy of “polymeric micelle assembly” for the synthesis of a wide variety of mesoporous materials with large mesopores (30−50 nm) using asymmetric triblock copolymers with a long hydrophobic segment and a reactive segment for strong interaction with inorganic precursors (Figure 1). Advances in living or controlled radical polymerization enable tailor-made block copolymers from several different monomers with well-defined compositions, controlled molecular weights, and low polydispersity.33−36 Here we used a poly(styrene-b-2-vinyl pyridineb-ethylene oxide (PS-b-PVP-b-PEO) asymmetric triblock copolymer having chemically distinct three blocks (hydrophobic PS, cationic PVP (at low pH), and hydrophilic PEO, respectively). It undergoes self-assembly to form highly stable polymeric micelles. The polymeric micelles exist in a frozen form in the precursor solutions, which can be used as porogens. Each block of an asymmetric triblock copolymer contributes significantly to the preparation of mesoporous metal oxide materials. The PEO block helps to disperse the polymeric micelles well in the precursor solutions. The hydrophobic PS and cationic PVP blocks play important roles as porogens and coordinating ligands, respectively. The strong interaction of inorganic precursors with the PVP unit and kinetically frozen PS core enable fabrication of large-sized mesoporous materials with crystallized frameworks. Framework compositions, mesopore sizes, and wall thicknesses can be rationally controlled by using the same chemistry. We believe that this strategy is far more versatile than previous approaches using low-molecularweight surfactants or other amphiphilic block copolymers. We mainly focused on the preparation of mesoporous TiO2 films and successfully extended to various compositions such as

Recently several research groups have used poly(ethylene-cobutylene-b-ethylene oxide) diblock copolymer as the structuredirecting agent for the fabrication of mesoporous strutcures with large pores.17,18 Such high-molecular-weight block copolymers with high carbon content can be carbonized inside the mesopores in situ during high temperature treatment prior to the combustion of carbon, which can support the crystallization of the inorganic framework with retention of the original mesostructures. Lee et al. have developed the combined assembly of soft and hard chemistry to synthesize the thermally stable and highly crystalline mesoporous transition metal oxides with uniform pores. The thermally stable polyisoprene block is converted to a sturdy and amorphous carbon material when heat treated under an inert environment. Highly crystalline transition metal oxides are obtained after removing the carbon residue heating at high temperaturte in air.19 In the above-mentioned strategies, either AB-type diblock copolymer or ABA-type triblock copolymer have been mostly utilized. Compared with AB- and ABA-type block copolymers, ABC-type asymmetric triblock copolymer have richer mesophase behaviors and more diverse components.20,21 The presence of the third compartment can control the copolymer properties, which deeply affects the self-assembly process. Very interesting and complicated hierarchical mesostructures can be formed by using unusual composition of the ABC type of block copolymer.22−24 Self-assembly platforms of triblock copolymer are used to synthesize mesoporous material with different compositions.25,26 Zhao et al. have used poly(ethylene oxide-bmethyl methacrylate-b-styrene) triblock copolymer for synthesis of ordered mesoporous carbons with large pores.27 The middle hydrophobic block polymethyl methacrylate greatly contributes the micropores on the carbon shell. These block copolymers lack the strong reactive block which can interact with a wide range of inorganic precursors. The interaction (hydrogen bonding) of the hydrophilic PEO blocks with the metal precursors is weaker than ionic bonding.28,29 Therefore, due to the high reactivity of most of the metal oxide precursors30 toward hydrolysis and condensation, these inorganic metal oxides cannot be deposited on the hydrophilic 652

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Figure 2. (a) 1H NMR spectra of (a-1) PS(14600)-b-PVP(16600)-b-PEO(36800) and (a-2) PS(17300)-b-PVP(18700)-b-PEO(24000). (b) Thermo gravimetric (TG) analysis of TiO2/PS-b-PVP-b-PEO nanocomposites, showing that the PS-b-PVP-b-PEO template can be burned out at around 500 °C.

Figure 3. (a) Hydrodynamic diameter of PS-b-PVP-b-PEO micelles before and after adding TTIP. Microscopic images (b-1) TEM, (b-2) AFM, (c) SEM of micelles, and (d-1 and d-2) SEM images of TiO2/PS-b-PVP-b-PEO nanocomposites. Al2O3, tantalum ethoxide, niobium ethoxide, silicon ethoxide, and aluminum isopropoxide were used in place of TTIP, respectively. For the synthesis of mesoporous BTO, 0.5 g of barium acetate was dissolved into 1.5 mL of acetic acid solution at 50 °C. An equimolar amount of titanium butoxide was added to the solution and stirred for 1 h. The prepared solution was then added to the polymeric micellar solution. 2.2. Characterization. The DLS measurements were carried out using an Otsuka ELS Z zeta-potential and particle analyzer. All of the measurements were carried out at 25 °C. The correlation functions were analyzed by the contin method and used to determine the diffusion coefficient (D) of the particles. The hydrodynamic diameter (Dh) was calculated from D using the Stokes−Einstein equation. The morphology of the samples was observed under field emission scanning electron microscopy (SEM; Hitachi SU-8000) and transmission electron microscopy (TEM; JEOL JEM-1210). The crystalline phases and crystallinity were measured by X-ray powder diffraction (XRD; Shimadzu XRD-7000) analysis. Raman spectra were measured using a Raman spectrometer (T64000: Jobin-Yvon). Thermogravimetric analysis (TGA) was carried out using a Seiko-6300 TG/DTA instruments at a heating rate of 10 °C min−1 in air. N2 adsorption/ desorption was determined by a Quantachrom surface area analyzer. 1 H NMR spectra were recorded at 25 °C using a JEOL AL300 BX spectrometer. CDCl3 was used as solvent, and tetramethylsilane was used as the internal standard.

silicon oxide (SiO2), tantalum oxide (Ta2O5), niobium oxide (Nb2O5), aluminum oxide (Al2O3), titania-aluminum mixed oxide (TiO2−Al2O3), and barium titanium oxide (BTO), which evidence that the unique assembly of the PS-b-PVP-b-PEO block copolymer micelles are useful for the synthesis of highly robust mesoporous materials with various compositions.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Mesoporous Films by Micelle Assembly. PS-b-PVP-b-PEO with polydispersity index 1.15 (20 mg) was molecularly dissolved in 4 mL of THF. A 100 μL of HCl solution (37%) was slowly added into the magnetically stirred polymeric solution. The solution was stirred for 2 h before adding 100 μL of titanium tetraisopropoxide (TTIP). The resulting solution was stirred for another 4 h. The obtained precursor solution was casted or spincoated on substrates. Mesoporous films were prepared on Si(100) substrates by spin coating for 30 s using a spin coater at room temperature. The film thicknesses were controllable by changing the applied spinning speed. When the spinning speed was 3000 rpm, the film thickness was around 50 nm. The films were dried at room temperature for 6 h. After complete evaporation of the solvent, the samples were calcined at various temperatures to remove the micellar template and to induce crystallinity at a ramping rate of 1 °C in air for 4 h. For the preparation of mesoporous Ta2O5, Nb2O5, SiO2, and 653

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3. RESULTS AND DISCUSSION 3.1. Preparation of Polymeric Micellar Solutions. The triblock copolymer was synthesized by living anionic polymerization.35 The composition of poly(styrene-b-2-vinyl pyridineb-ethylene oxide) was characterized by 1H NMR spectroscopy (Figure 2a). The 1H NMR signals around 3.7 ppm, 6.2−7.2 ppm, and 8.3 ppm are attributed to the PEO units, PS and P2VP units, and 6-position of pyridyl group in P2VP unit, respectively. Comparison of the areas of these 1H NMR peaks allowed the composition ratio of PS, P2VP, and PEO to be determined. In addition, the molecular weight (Mn) values were estimated by gel permeation chromatography (GPC). These results indicate that the compositions of the two block copolymers used in this study can be approximately formulated as PS(14600)-b-PVP(16600)-b-PEO(36800) and PS(17300)-bPVP(18700)-b-PEO(24000), respectively. The chemical structure of block copolymer is shown in Figure S1 of the Supporting Information. For the preparation of the precursor solution, THF was used as an organic solvent that can molecularly dissolve the PS-bPVP-b-PEO block copolymer, because THF is a good solvent for all blocks. A specific amount of HCl solution was slowly added dropwise in order to initiate the micellization. The HCl solution is a poor solvent for the polystyrene (PS) block. Therefore, the interfacial energy between the PS block and solvents is increased, leading to the formation of micelles in solution. The comprehensive light scattering experiment was performed to monitor the micelle formation in situ. The formation of micelles was immediately observed upon the addition of HCl solution (Figure 3a). In typical experimental, the mole of [H+] is much high rather than that of [PVP] unit. So, we assume that all of the PVP are protonated. The further increase in HCl concentration does not increase the zetapotential anymore. This fact also confirms the complete protonation of PVP. Here HCl has a dual role during micellization; the first role is to induce micellization as it is poor solvent for PS block and the second one is to change the conformation of PVP block form shrunken mode to extended mode due to the protonation. As seen in Figure 3a, the average hydrodynamic diameter (Dh) of the micelles was 80 ± 5 nm with polydispersity index 0.09, indicating the formation of monodispersed micelles. Figure 3b-1 shows the TEM image of PS-b-PVP-b-PEO micelles after staining with phosphotungstic acid. The average size of the PS core is estimated to be about 40 ± 5 nm. The discrepancy between the TEM and light scattering data is due to the thickness of the extended PVP shell and PEO corona in solution. The formation of micelles was further directly confirmed by AFM (Figure 3b-2). Polymeric micellar solution containing 100 μL of HCl solution was casted on silicon substrate. THF can be easily evaporated from the system due to its low boiling temperature. After drying, the spherical micelles were uniformly distributed (Figure 3c). The original spherical micelle structure was preserved even after solvent evaporation, indicating the formation of stable and robust micelles in the solution. In previous studies, the polymeric micellar solution was prepared by a dialysis method. The PS-b-PVP-b-PEO block copolymer was first dissolved in DMF and H2O was then added slowly to initiate the micellization. The organic solvent was removed by dialysis. It has been found that the PS-b-PVP-bPEO block copolymer undergoes micellization with different morphologies, depending on the preparative conditions.37,38

For example, in the presence of a solvent that is selective for the PS block, the micellar morphology exhibits a sphere-to-rod transition. In this study, the PS-b-PVP-b-PEO block copolymer forms the spherical micelles with a core−shell-corona architecture by addition of HCl (Figure 1). As the added amount of HCl solution increased, the micelle size gradually increased. The PS core became more rigid, and the PVP blocks showed extended conformation due to protonation. The latter effect is dominant in the system, resulting in the gradual increase in micelle size with the increase in added HCl amount, as shown in Figure 4a. When the amount of HCl solution added was over 100 μL, the Dh did not change so much, which can be explained by the following two

Figure 4. DLS data demonstrating the (a) micellization of PS-b-PVPb-PEO with respect to the amount of HCl(aq). (b) Interaction of different metal source with polymeric micelles TTIP (●), tantalum ethoxide (o), titanium butoxide + barium acetate (×) niobium ethoxide (□), and silicon ethoxide (■). (c) Zeta-potential of TiO2/ PS-b-PVP-b-PEO nanocomposites as a function of the amount of TTIP. 654

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Figure 5. SEM images of mesoporous TiO2 obtained after calcination at (a) 400 °C, (b and d) 500 °C, and (c) 600 °C and TEM images (e-1 and e2) after calcination at 500 °C.

reasons. The first one is that the association number of unimers does not change any more (i.e., the unimer concentration drops to zero), the exchange of unimer chains between micelles is stopped, and the micelles behave as kinetically frozen nanoparticles. The second reason is that the protonation of PVP units is complete, and the electrostatic repulsion between adjacent protonated PVP block remains the same. It is quite important to consider the dynamics of polymeric micelles, because it affects the stability of micelles significantly.11 Due to the high glass transition temperature of PS (Tg = 93 °C), the micelles in the precursor solution are in frozen conformation, in which the exchange of unimers between the micelles and the bulk solvent phase is kinetically frozen. The micelles effectively behave as inert PS spheres with reactive and hydrophilic polymer tethered to their surfaces. The micelles neither dissociate nor rearrange upon change in conditions. Thus, the micelles formed from PS-b-PVP-b-PEO block copolymers are very stable even in harsh conditions. The pyridine units in the micelles can serve as coordinating ligands with inorganic sources. Titanium tetraisopropoxide (TTIP) was added to the micellar solution containing 100 μL of HCl solution. In addition to HCl, the TTIP in the solution also influences the morphology of the micelles, because TTIP can be incorporated into the PVP domain. The Dh of micelles after TTIP addition was smaller than that of pure polymeric micelles, indicating shrinkage of the PVP shell (Figures 3a and 4b). The decrease in Dh indicates that the added TTIP binds strongly to the PVP domain. Here, HCl activates the TTIP for confined hydrolysis and condensation in protonated PVP shell.39 When we used different inorganic sources such as silicon ethoxide, tantalum ethoxide, and niobium ethoxide, a similar tendency was clearly observed (Figure 4b). The interaction of inorganic sources to micelles was examined by a comprehensive zeta-potential measurement for each micellar solution containing different amounts of TTIP (Figure 4c). The zeta-potential measurements proved that the pure polymeric micelles without TTIP had a net surface charge at around 30 mV. The successive addition of TTIP gradually decreased the zeta-potential, indicating the complexation of TTIP with polymer. Thus, the complexation masks the positively charged PVP block. As seen in Figure 4b, the micelle size change is attributed to the conformational change of PVP from an extended to shrunken form due to the weak electrostatic

repulsion between adjacent PVP groups. When the zetapotential tends to zero, no more electrostatic repulsion between the PVP blocks occurs, and the Dh is not largely changed. The outer hydrophilic PEO corona sterically stabilizes the nanocomposites in solution. The precursor solution containing spherical polymeric micelles with TTIP was coated on the substrates. During solvent evaporation, each micelle assembles spontaneously. After complete drying, the micelle morphology was verified by SEM observation (Figure 3d). The micelles were closely packed over the entire area. Figure S2 in the Supporting Information shows the TEM image of composites micelles. The dark shell indicated by yellow-colored ring on the spherical micelles further confirms that the inorganic precursors are effectively interacted with the PVP domains. 3.2. Mesoporous Titania Films by Micelle Assembly. The surface morphology of the calcined mesoporous titania film was observed by SEM (Figure 5a−c). The calcination removes the polymer, leaving the titania framework with interconnecting spherical mesopores with an average diameter of 40 nm. The pore size corresponds to the void space formed after combustion of the PS core. The wall thickness of the pores was around 25 nm when 100 μL of TTIP was used for synthesis. When the spin-coating speed was very high (over 3000 rpm), a monolayered mesopore arrangement was formed on the substrate (Figure 5d). The observed pore sizes were the same as those of mesoporous thick films, shown in Figures 5a− c. A typical TEM image of the calcined mesoporous titania structure is shown in Figure 5e. High resolution TEM was also used to investigate the crystallinity of the pore wall. The dspacing determined from the diffraction rings of the selected area’s electron diffraction (ED) pattern (Figure S3, Supporting Information) can be assigned to a crystallized anatase phase. N2 adsorption−desorption isotherms of the obtained samples calcined at different temperatures are shown in Figure S4 in the Supporting Information. The isotherm was of type IV, which was representative of mesoporous materials. The surface areas were 50, 58, and 62 m2·g−1 for the samples calcined at 400, 500, and 600 °C, respectively (Table S1, Supporting Information). The BET surface area increased from 50 (400 °C) to 58 m2·g−1 (500 °C) with the increase of the calcination temperature. According to TG analysis (Figure 2b), the polymer is not completely burnt out at 400 °C. Therefore, all the pores are not 655

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of large-sized mesoporous titania films, but in most cases the pore size and wall thickness were not uniform. The previous works are summarized in Table 1.47−54 The hydrolysis and

completely opened and not fully accessible (Figure 5a). As seen in Figure 5b,c, the opening of the pore is very clear when the calcination temperature is increased to 500 °C. That is the reason why the surface area is increased with the calcination temperature. Mesoporous titania films were obtained after condensation of the titania source, followed by removal of the polymer template by thermal treatment. The thermal treatment process induces crystallization of the network, and thermal stability of the polymer templates is critical for realizing both a well-ordered mesoporous structure and a well-developed crystallized framework. The used PS-b-PVP-b-PEO polymer was thermally stable compared with other block copolymers. The thermal stability was checked by using thermo gravimetric (TG) analysis (Figure 2b). In the case of commercially available Pluronic-type block copolymers (e.g., P123 and F127), the sharp weight loss starts at around 150 °C and completely burns out at 250 °C.40 At temperatures higher than 250 °C, there are no polymerderivatives inside the pores. Therefore, the crystal growth in the pore walls is not suppressed at all. Generally, the crystallization of titania into anatase is known to be initiated around 300 °C and is often accompanied by collapse of the porosity and destruction of the mesostructures.41,42 In contrast, the PS-bPVP-b-PEO polymer used in this study was completely burnt away at 500 °C, which is much higher than the crystallization temperature of titania. In the present study, the mesoporous architecture in the films was well retained up to 600 °C (Figure 5). The remaining template can act as a support to prevent rapid crystal growth, resulting in the retention of the original mesoporous structure. The crystallinity in the pore walls on heating was investigated by wide-angle XRD measurements (Figure S5, Supporting Information). As-prepared and calcined samples at different temperatures were measured. No peaks were observed for the as-prepared sample, indicating that the pore walls are in an amorphous phase. The peaks derived from the crystallized anatase phase gradually appeared when the calcination temperature was higher than 300 °C. All of the peaks can be readily indexed as an anatase crystal of TiO2 (JCPDS 00-0211272). These samples were also examined by Raman spectroscopy analysis. The Raman shift (Figure S6, Supporting Information) further confirms the anatase form of TiO2.43 Here, the Eg peak is mainly caused by the symmetric stretching vibration of O−Ti−O, the B1g peak is caused by the symmetric bending vibration of O−Ti−O, and the A1g peak is caused by the antisymmetric bending vibration of O−Ti−O in TiO2. The bands of anatase phase increased in intensity and decreased in line width when the sample was calcined at higher temperatures. The intensity of the lowest frequency Eg mode increased dramatically. This result suggests that the crystallinity of the anatase phase is greatly improved,44 which is consistent with the selected-area ED patterns and wide-angle XRD data (Figures S3 and S5, Supporting Information). As shown in Figure 5, the original mesostructure was well maintained without obvious mesopore contraction during condensation and crystallization of the titania framework at elevated temperature, in contrast to the previous system. In the case of mesoporous metal oxides prepared by P123 and F127 system, the original mesostructure totally collapsed, due to expansion of the wall thickness or merging of several mesopores during the crystallization process by calcination.45,46 3.3. Principle of ‘micelle assembly’. In a recent study, PS-b-PEO diblock copolymers were utilized for the preparation

Table 1. Pore Size of Mesoporous Materials Using PS-b-PEO Diblock Copolymer as Template poly(styrene-b-ethylene oxide)

pore size (nm)

PS(2k)-b-PEO(5k) PS(5k)-b-PEO(5k) PS(12k)-b-PEO(5k) PS(3k)-b-PEO(10k) PS(7.5k)-b-PEO(10k) PS(10k)-b-PEO(10k) PS(12k)-b-PEO(10k) PS(12.5k)-b-PEO(5.5k) PS(18k)-b-PEO(7.5k) PS(24k)-b-PEO(5.5k) PS(20k)-b-PEO(6.5k) PS(1.6k)-b-PEO(1.8k) PS(20k)-b-PEO(1.8k) PS(40k)-b-PEO(53k) PS(58.6k)-b-PEO(71k) PS(100k)-b-PEO(150k)

8 11 19 11 16 20 22 12 30 30 30 7 20 40 60 100

compositions

refs

SiO2

47

TiO2 TiO2 SiO2, carbon TiO2 SiO2 SiO2/Nb2O5 TiO2

48 49 50 51 52 53 54

condensation of metal alkoxides (e.g., TTIP) make the PEO blocks chemically reactive, because the metal species on the surface of the PEO domains are capable of forming hydrogen bonds with other metal species on the surface of adjacent PEO domains.30 However, the adhesion between the inorganic materials and PEO block is usually poor.55 Many efforts toward optimization of the precursor compositions are required for well-organized mesoporous films. In contrast, the use of a PS-b-PVP-b-PEO triblock copolymer can simplify the synthesis of well-organized mesoporous metal oxides. The protonated PVP blocks interact strongly with the metal species, as mentioned above.56,57 Therefore, the concentration of metal species in the PEO blocks is smaller. The PEO corona can prevent micelle aggregation and can disperse the micelles uniformly. Once the polymeric micelles are formed in solution, even after the addition of metal alkoxides, the micelles remain in a highly stable conformation with kinetically frozen PS cores (Figure 3a). Assembly of the well-dispersed micelles induces ordered micelle packing, which helps to form well-defined mesoporous metal oxides. As another comparison, we used a PS-b-PVP diblock copolymer to fabricate mesoporous titania under the same conditions as those of the PS-b-PVP-b-PEO triblock copolymer. The strong interaction of the added Ti species with the PVP shell and absence of a PEO outer shell induced micelle aggregation. Therefore, the distribution of mesopores was random, although the mesopore sizes seemed almost uniform (Figure S7, Supporting Information). The BET surface area was very low (only 29 m2 g−1). Unlike PS-b-PEO and PS-b-PVP diblock copolymer systems, the PS-b-PVP-b-PEO triblock copolymer system enables rational design of mesoporous metal oxides with various compositions. The presence of a pH-responsive PVP block along with a hydrophilic PEO and hydrophobic PS block in PSb-PVP-b-PEO introduces interesting multiple functionalities. The effects of HCl and the metal source on the polymeric micelle properties, which are important for getting the final micelle structures, can be observed in situ. The molecularly 656

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Figure 6. SEM image of mesoporous TiO2 synthesized using (A) different concentrations of TTIP ((a) 25, (b) 50, (c) 100, (d) 150, and (e) 200 μL) and (B) different polymers ((a) PS(14600)-b-PVP(16600)-b-PEO(36800) and (b) PS(17300)-b-PVP(18700)-b-PEO(24000)). The role of the PS block length in forming larger pores is also shown schematically.

Figure 7. SEM images of mesoporous films of various compositions (a) Ta2O5, (b) BTO, (c) Nb2O5, (d) SiO2, (e) Al2O3−TiO2, and (f) Al2O3.

PS(17300). Similar trends were also reported for PS-containing diblock copolymers (Table 1), as the PS block only participates in mesopore formation. The pore sizes and wall thicknesses reported here are much higher than those of previously reported mesoporous TiO2 synthesized using a PS-b-PEO diblock copolymer. Recently, Kimura et al. reported extremely large-sized mesoporous titania films, but the pore size and wall thickness were not uniform.54 Nevertheless, the wall thicknesses and mesopore sizes can be effectively controlled by changing the precursor concentration and PS block length. We also tried this concept of micelle assembly for the synthesis of mesoporous materials with other compositions (Figure 7). Instead of TTIP, tantalum ethoxide, niobium ethoxide, silicon ethoxide, aluminum isopropoxide, and a mixture of barium acetate and titanium butoxide in acetic acid were used as precursor sources for Ta2O5, Nb2O5 SiO2, Al2O3, and BTO, respectively. In order to investigate potentials and limits of our micelle assembly approach, we deliberately selected several compositions that are known to be difficult for the formation of mesoporous structures. Similar to the titania system with PS-b-PVP-b-PEO, the PVP can serve as a coordinating ligand for different metal and metal oxide precursors (Figure 4b). The added inorganic precursor interacted strongly with micelles, and the spherical micelles were in a highly stable form. After drying of the solvent, the

dissolved unimers are assembled to form micelle composites with the target inorganic sources. Highly stable micelles with a reactive PVP shell are formed in the precursor solutions, and the inorganic sources can interact strongly with the micellar surface. Therefore, unlike the previously reported systems, strict control of solvent composition, temperature, and humidity is not required. These stable micelles are thus directly used as the porogens. Based on our “micelle assembly” concept, we tried to tune wall thickness by changing the TTIP concentration. A thicker wall was observed at higher concentrations of TTIP (Figure 6A). When 25 μL TTIP was used, a very thin wall thickness of 10 nm for the mesoporous TiO2 was obtained. It was increased to 50 nm by increasing the TTIP volume to 200 μL. More deposition of TTIP took place around the PS core at higher concentrations. Hence we can tune the thickness of the TiO2 wall according to our own purpose and desire. Another way to tune the morphological parameters of mesoporous TiO2 is to use a block copolymer having different block lengths. The sizes of the core-forming polystyrene and the shell-forming PVP have direct influences on the pore size and wall thickness, respectively. Larger pores were obtained when the polymer with a longer PS core was used as template (Figure 6B). We were able to increase the pore size from 40 to 50 nm by changing the molecular weight of PS from PS(14600) to 657

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films were calcined at 500 °C. Well-ordered mesoporous films were observed under SEM (Figure 7). We believe that our synthetic approach will potentially become a versatile method for the fabrication of mesoporous materials with different compositions.

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4. CONCLUSION We have demonstrated a polymeric micelle assembly approach for the synthesis of a wide variety of mesoporous materials with larger pores and robust walls by using an asymmetric triblock copolymer, PS-b-PVP-b-PEO. The molecularly dissolved block copolymer undergoes self-assembly upon addition of HCl solution. Three different blocks in the obtained stable micelles each contribute toward the formation of the targeted mesoporous materials. The kinetically frozen PS block stabilizes the micelles and controls the pore size of mesoporous materials. The strong interaction of inorganic precursors with the PVP shell enables fabrication of highly robust walls, and the hydrophilic PEO helps orderly packing of the micelles during solvent evaporation. Several metal sources can be easily deposited on the PVP shell with simple chemistry. The simplicity, versatility, flexibility, and reproducibility of this synthetic approach allows fabrication of several mesoporous materials with different compositions. High thermal stability of block copolymers is another advantage toward obtaining ordered crystalline mesoporous materials at elevated temperatures. We have found that the pore sizes and wall thicknesses can be easily tuned to several nanometers by varying the block lengths of PS and the metal source concentrations, respectively. The fine-tunability of the nanoarchitecture with respect to crystallinity, pore size, and wall thickness can provide remarkable insight into the performance of mesoporous materials for diverse applications in the future.

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ASSOCIATED CONTENT

* Supporting Information S

Additional material as discussed 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]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This research is supported by the Japan Society for the Promotion of Science (JSPS).

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