Research Update: Triblock copolymers as templates to synthesize inorganic nanoporous materials , Yunqi Li, Bishnu Prasad Bastakoti, and Yusuke Yamauchi Citation: APL Materials 4, 040703 (2016); doi: 10.1063/1.4946885 View online: http://dx.doi.org/10.1063/1.4946885 View Table of Contents: http://aip.scitation.org/toc/apm/4/4 Published by the American Institute of Physics APL MATERIALS 4, 040703 (2016) Research Update: Triblock copolymers as templates to synthesize inorganic nanoporous materials Yunqi Li,1,2 Bishnu Prasad Bastakoti,1 and Yusuke Yamauchi1,2,a World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan Faculty of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan (Received 11 March 2016; accepted April 2016; published online 25 April 2016) This review focuses on the application of triblock copolymers as designed templates to synthesize nanoporous materials with various compositions Asymmetric triblock copolymers have several advantages compared with symmetric triblock copolymers and diblock copolymers, because the presence of three distinct domains can provide more functional features to direct the resultant nanoporous materials Here we clearly describe significant contributions of asymmetric triblock copolymers, especially polystyrene-block-poly(2-vinylpyridine)-block-poly(ethylene oxide) (abbreviated as PS-b-P2VP-b-PEO) C 2016 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4946885] I INTRODUCTION So far, many synthetic methods have been developed for the preparation of nanoporous inorganic materials with well-controlled shapes and pore sizes As compared with hard-templating methods using mesoporous silica (e.g., MCM-41, SBA-15, and KIT-6), soft-templating methods based on supramolecular templates show several advantages because self-assembly of amphiphilic molecules (e.g., surfactants and block copolymers) can be rationally tuned for designing final structures, depending on the synthetic conditions (Figure 1) Cationic surfactants (e.g., alkyl trimethylammonium, CnTMA), pluronic-type triblock copolymers, and other high molecular weight diblock copolymers (e.g., polystyrene-block-polyethylene oxide, PS-b-PEO) have been often used as templates CnTMA-type surfactants produce small-sized pores (less than 10 nm), which have very limited applications Symmetric triblock copolymers (e.g., PEO-b-PPO-b-PEO) are helpful for obtaining mesoporous materials with uniform-sized pores up to around 10 nm The further expansion of pores is possible by the addition of pore-expanding agents (e.g., 1,3,5-trimethylbenzene, 1,3,5-triisopropylbenzene) High molecular weight diblock copolymers (e.g., PS-b-PEO) enable to increase pore size larger than 20 nm without any additives Thus, cationic surfactants (e.g., CnTMA), low molecular weight block copolymers (e.g., P123, F127), and high molecular weight diblock copolymers (e.g., PS-b-PEO) have been utilized to control the resultant pore sizes in a wide range It has been known that the hydrophilic blocks of micelles (e.g., PEO block) serve as reservoirs of inorganic precursors When the inorganic precursors are adsorbed onto/into the hydrophilic blocks, micelles sometimes become unstable, leading to deformation of micelles and their subsequent aggregations To overcome this problem, triblock copolymer is the best choice as a new pore-directing agent Until now, various triblock copolymers, divided into symmetric and asymmetric types, have been utilized to prepare mesoporous materials, as listed in Tables I and II Especially, a E-mail: Yamauchi.Yusuke@nims.go.jp 2166-532X/2016/4(4)/040703/8 4, 040703-1 © Author(s) 2016 040703-2 Li, Bastakoti, and Yamauchi APL Mater 4, 040703 (2016) FIG Various mesoporous/nanoporous materials with different pore sizes and framework compositions Mesoporous silica materials are templated by PEO-b-PPO-b-PEO,1 cationic surfactant,4 anionic surfactant,5 and PEO-b-PMMA,9 respectively Mesoporous carbon materials are templated by PS-b-PEO,2 and PS-b-P4VP8 Mesoporous Au@PdPt and Pt materials are templated by PEO-b-PPO-b-PEO3 and Brij 58,6 respectively Mesoporous Lu2O3 is templated by PEO-b-PI.7 asymmetric triblock copolymer PS-b-P2VP-b-PEO possesses several advantages to prepare mesoporous/nanoporous materials with various morphologies, including monodispersed particles and continuous thin films Different from symmetric triblock copolymers reported previously, asymmetric ones can form monodispersed and stable spherical micelles, which are utilized as controlled templates The pore sizes and the wall thicknesses can be easily tuned by varying the block lengths of the PS and the amounts of the inorganic sources, respectively II SYMMETRIC TRIBLOCK COPOLYMERS Symmetric triblock copolymers are comprised of two distinct blocks, in which one block in the middle is connected to two similar blocks (i.e., ABA type) Pluronic-type triblock copolymers (e.g., PEO-b-PPO-b-PEO) have been mostly utilized as template to prepare mesoporous materials with different compositions, including a variety of metal oxides, carbon, silica, metals, as listed in Table I.1,10–16 Pluronic-type triblock copolymers behave like diblock amphiphilic block copolymer in aqueous solution They undergo self-assembly to form core-shell micelles with core consisting of hydrophobic PPO block surrounded by an outer shell of the hydrophilic PEO blocks above CMC (critical micelle concentration) Hydrophilic PEO blocks can form hydrogen bond interaction with inorganic precursors and hydrophobic PPO forms the micelle core to determine the resultant pore sizes However, commercially available pluronic-type triblock copolymers are limited, and mostly the PPO blocks are relatively short Sometimes, large pore size is beneficial for several applications Use of other symmetric triblock copolymers with high-molecular-weight hydrophobic block is interesting to realize the formation of large-sized pores A series of symmetric triblock copolymers of PEO-b-PB-b-PEO (PB; polybutadiene) designed based on pluronic-type triblock copolymer have been reported A high hydrophobic-hydrophilic contrast between the PEO and PB blocks results in a low CMC.17 Starting from either precursor salt (e.g., TiCl4) or TiO2 nanocrystals, it is possible to synthesize mesoporous crystalline TiO2 films through the assistance of symmetric triblock copolymer of PEO-b-PB-b-PEO The pore sizes are controllable by the molecular weight of hydrophobic PB block A laboratory-made symmetric triblock copolymer of PS-b-PEO-b-PS was also reported to synthesize the large- and tunable-sized nanoporous silica, and it is easy to enlarge pore size from to 40 nm linearly with the length of PS block without employing swelling agents.18 Another symmetric triblock copolymer of PMA-b-PEO-b-PMA (PMA; poly(methyl acrylate)) was also demonstrated to guide the preparation of highly ordered hexagonal mesoporous silica.19 040703-3 Li, Bastakoti, and Yamauchi APL Mater 4, 040703 (2016) TABLE I Examples of mesoporous/nanoporous materials prepared by various symmetric triblock copolymers Block copolymers Compositions Electron microscopic images References PEO-b-PPO-b-PEO TiO2, ZrO2, Al2O3, Nb2O5, Ta2O5, WO3, HfO2, SnO2, etc 10 PEO-b-PPO-b-PEO Silica PEO-b-PPO-b-PEO Carbon 11 PEO-b-PPO-b-PEO Pt 12 PMA-b-PEO-b-PMA SiO2 19 PS-b-PEO-b-PS SiO2 18 PEO-b-PB-b-PEO TiO2 17 III ASYMMETRIC TRIBLOCK COPOLYMERS Table II summarizes examples of mesoporous/nanoporous materials prepared by various asymmetric triblock copolymers Epps et al have already investigated the network phases of PI-bPS-b-PEO (PI; polyisoprene) and they have concluded that the presence of a third block enriched the network phases of triblock copolymer (ABC) as compared to diblock copolymer AB.20 ABC-type triblock copolymers provide great flexibility to design nanoarchitectures through their co-assemblies with inorganic sources Successful co-assembly of polymeric micelles with inorganic species relies on careful control of enthalpic, entropic, and kinetic parameters.21 Asymmetric triblock copolymer is composed of three distinct blocks and every block can contribute differently, which especially benefits to keep stable morphologies of micelles after accommodation of inorganic precursors Zhang et al utilized a laboratory-made asymmetric triblock copolymer, PS-b-PMMA-b-PEO (PMMA; poly(methyl methacrylate)) as a template for preparation ordered mesoporous carbon The long PMMA chain contributes well to generate large pore size and also interact with resol molecules Thus, the PS-b-PMMA-b-PEO is a potential template to create mesoporous nanomaterials with desirable structures and compositions.22 Distinct thermal degradation temperatures of each unit provide more opportunities Sun et al applied an asymmetric triblock copolymer of PDMS-b-PEO-b-PPO (PDMS; polydimethylsiloxane) to prepare functional mesoporous silica PEO and PPO blocks degrade at low temperature, but the hydrophobic PDMS block decomposes at higher temperature during calcination resulting in hydrophilic silanol groups decorated on the surface of pores It is very interesting to change 040703-4 Li, Bastakoti, and Yamauchi APL Mater 4, 040703 (2016) TABLE II Examples of mesoporous/nanoporous materials prepared by various asymmetric triblock copolymers Block copolymers Compositions Electron microscopic images References PS-b-P2VP-b-PEO TiO2, SiO2, Ta2O5, Al2O3, Nb2O5, etc 32 PS-b-P2VP-b-PEO Pt 33 PS-b-P2VP-b-PEO Carbon Unpublished PS-b-PDMA-b-PLA Polymer 29 PS-b-PMMA-b-PEO Carbon 22 PI-b-PS-b-PVP SiO2 31 PDMS-b-PEO-b-PPO SiO2 23 PI-b-PS-b-PEO TiO2 26 PI-b-PS-b-PEO Carbon 27 the chemical character of mesopores to convert a hydrophobic functional group to a hydrophilic silanol.23 Since the pioneer work using PI-b-PS-b-PEO (PI; polyisoprene) by Wiesner et al., PI-b-PSb-PEO has been utilized to prepare multicomponent mesoporous materials Using the same asymmetric triblock copolymers, transition metal oxide nanocomposites,24,25 high-performance photoanodes of mesoporous TiO2 films,26 and ordered gyroid carbon electrodes27 have been fabricated Interestingly, continuous gyroid superstructures of metal nanoparticles (i.e., Pt and Au) can be realized from PI-b-PS-b-PDMAEMA (PDMAEMA; poly(N,N-(dimethylamino)ethylmethacrylate) triblock copolymer.28 Self-assembly of asymmetric triblock copolymers can also guide well-defined nanomaterials Self-assembly of PS-b-PDMA-b-PLA (PDMA; polydimethylacrylamide, PLA; polylactide) results in highly oriented nanostructured materials The selective etching of PLA forms nanoporous polystyrene matrix and the functional group of PDMA coated on the internal pore surface opens 040703-5 Li, Bastakoti, and Yamauchi APL Mater 4, 040703 (2016) diverse applications.29,30 In addition to selective etching of particular block, the remaining block opens an appropriate strategy to produce hybrid nanomaterials Lee and co-workers have reported self-assembled nanostructures prepared with PI-b-PS-b-P2VP The PI block was removed by sequential UV-ozone and oxygen plasma treatment After that, two different precursors, polydimethylsiloxane and HAuCl4 were introduced to obtain highly ordered nanoporous SiO2 with gold dots in each nanopore.31 IV SIGNIFICANT OF AN ASYMMETRIC PS-b-P2VP-b-PEO TRIBLOCK COPOLYMER Among asymmetric triblock copolymers, PS-b-P2VP-b-PEO shows very unique properties The hydrophobic PS block stabilizes the micelles and controls the pore size of porous materials, the P2VP block can interact with a wide range of inorganic sources, and the hydrophilic PEO block helps ordered packing of the micelles during the synthesis The simplicity, versatility, flexibility, and reproducibility of this synthetic approach allow the preparation of several porous materials with different compositions In addition, high thermal stability of PS-containing block copolymers is another advantage for obtaining crystalline mesoporous materials through calcination at high temperature A Mesoporous metals Mesoporous metals with well controlled pore size are promising materials for electrochemical applications Many mesoporous metals, especially platinum (Pt), with different pore sizes have been synthesized by using various templates (Figures 2(a)-2(d)) However, in most cases, the resulting particles show non-uniform sizes and their pore sizes are relatively smaller (at most 10 nm) As shown in Figures 2(e)-2(h), an asymmetric triblock copolymer, PS-b-P2VP-b-PEO, is effectively served as the pore-directing agent to prepare well-dispersed mesoporous Pt spheres.33 Negatively charged PtCl42− ions preferably interact with the protonated P2VP+ blocks, while the free PEO chains prevent the aggregation of the Pt/polymer nanocomposites In previous methods, hydrophilic block PEO accommodates inorganic salts through hydrogen bonds The stability of micelles is disturbed, and the resultant structure is deformed In the case of PS-b-P2VP-b-PEO, each block (PS, P2VP, and PEO) makes a significant contribution: (i) the size of the mesopores can be finely tuned by varying the length of the PS chain (Figures 2(f)-2(h)), (ii) the protonated P2VP+ blocks are FIG Examples of mesoporous Pt nanoparticles ((a)-(d)) Mesoporous/nanoporous Pt nanoparticles prepared by other surfactants and block copolymers ((a) Ref 3, (b) Ref 34, (c) Ref 35, and (d) Ref 36), and ((e)-(h)) shape- and size-controlled mesoporous Pt nanoparticles prepared by asymmetric block copolymer PS-b-P2VP-b-PEO as a new pore-directing agent Ref 33 040703-6 Li, Bastakoti, and Yamauchi APL Mater 4, 040703 (2016) binding sites for the anions, and (iii) hydrophilic PEO is essential for micelle stability and prevents particle aggregation Furthermore, a pseudo core-shell mesoporous particle with a Pd core and a Pt shell can also be successfully prepared using the same concept based on PS-b-P2VP-b-PEO micelles as templates Such metallic mesoporous particles are promising candidates for future applications in electrochemistry, such as methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR) B Mesoporous metal oxides Mesoporous metal oxides have been widely applied in several research fields, such as catalysis, energy storage/conversion, and drug delivery When normal surfactants (e.g., pluronic P123) are used, amorphous and/or semi-crystalline frameworks are usually formed When the applied temperature is increased to improve the crystallinity in the framework, the ordering of mesoporous structures is largely decreased Hard templates (e.g., mesoporous silica, mesoporous carbon) are able to preserve well-ordered mesoporous structures and realize highly crystallized frameworks; however, hard-templating methods involve several complicated steps Another important aspect, mesoporous oxides with ultra-large pores are the subjects of extensive research Simplifying the method of preparing large-sized mesoporous metal oxides with highly crystallized frameworks has been a hot topic, recently As mentioned above, pore expanders (e.g., 1,3,5-trimethylbenzene, 1,3,5-triisopropylbenzene) contribute to enlarge the pore sizes, but some optimization is required to localize such expanders inside the micelles To realize both well-ordered mesoporous structures and highly crystalized frameworks at the same time, the use of PS-b-P2VP-b-PEO is critical.37 The above two strengths for the preparation of mesoporous metal oxides are highlighted When the PS-b-P2VP-b-PEO is employed as a pore-directing agent, the hydrophobic PS core of PS-b-P2VP-b-PEO determines the resultant pore size (Figure 3) A maximum of appropriately 50 nm pore has been realized in obtained metal oxides, and it is easy to control the pore sizes in a wide range, from nm to 50 nm, by adjusting the molecular weights of PS blocks The PS-b-P2VP-b-PEO has a high combustion temperature (>500 ◦C), which is higher than the crystallization temperatures of general metal oxides Therefore, the remaining derivatives inside the pores can retard the serious distortion of mesopores during the crystallization process of frameworks Some crystalized mesoporous TiO2, Ta2O5, Nb2O5, Al2O3, and BaTiO3 have been prepared by changing the inorganic precursor FIG Schematic presentation for preparation of mesoporous/nanoporous metal oxides by an asymmetric block copolymer, PS-b-P2VP-b-PEO, as a new pore-directing agent The spherical polymeric micelles of PS-b-P2VP-b-PEO can be observed by TEM with highlighted PS core stained by 0.1 wt % phosphotungstic acid 040703-7 Li, Bastakoti, and Yamauchi APL Mater 4, 040703 (2016) V CONCLUSION AND PERSPECTIVES This review highlights the contribution of triblock copolymers as templates to direct the preparation of inorganic nanoporous materials in recent years, especially, pointing out the great advantages of asymmetric triblock copolymers Every block of asymmetric triblock copolymers can be precisely designed and functionalized in order to share different responsibilities In the case of PS-b-P2VP-b-PEO, the hydrophobic PS units decide the resultant pore size, the protonated P2VP units serve as the binding sites of inorganic sources, while free hydrophilic PEO provides stability of polymeric micelles Furthermore, chemical and/or physical characters, such as different thermal decomposition temperatures, provide more variation to obtain well-established inorganic mesoporous materials As you know, the synthesis of block copolymer is time-consuming, costly and sometimes requires complicated synthetic steps, which is the main challenge of the polymeric micelle-templating method However, it is possible to provide many new and easy ways to mitigate the production cost of block copolymers with the huge investigation in this field Currently, PS-b-P2VP-b-PEO is commercially available and surely will yield several porous inorganic materials that will be useful in daily life.38–43 When general block copolymers are used as pore-directing agents, homogeneous elemental distributions are observed, due to a single binding site of the micelles In contrast, PS-b-P2VPb-PEO still has more mysterious characteristics that can be exploited The protonated P2VP+ favorably combines with anions, and the PEO also interacts with some inorganic species through hydrogen bonding Through a controllable deposition by optimizing reaction times and/or precisely designed experimental processes, different components can be distributed in distinct areas of the micelles Thus, it is possible to realize the preparation of double-shell nanomaterials in a simple one-pot reaction by the incorporation of more than one type of inorganic source on the polymeric micelles Double-shell nanomaterials 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A laboratory-made symmetric triblock copolymer of PS-b-PEO-b-PS was also reported to synthesize the large- and tunable-sized nanoporous silica, and it is easy to enlarge pore size from to 40 nm... templates to synthesize nanoporous materials with various compositions Asymmetric triblock copolymers have several advantages compared with symmetric triblock copolymers and diblock copolymers, because