Cubic and hexagonal mesoporous carbon in the pores of anodic alumina membranes

Cubic and circular hexagonal mesoporous carbon phases in the confined environment of the pores of anodic alumina membranes (AAM) were obtained by organic-organic self-assembly of a preformed oligomeric resol precursor and the triblock copolymer templates Pluronic F127 or P123, respectively. Casting and solvent evaporation were followed by self-assembly and the formation of a condensed wall material by thermopolymerization of the precursor oligomers, thus resulting in mesostructured phenolic resin phases. Subsequent thermal decomposition of the surfactant and carbonization were achieved through thermal treatment at temperatures up to 1000 °C under an inert atmosphere. The resulting hierarchical mesoporous composite materials were characterized by small-angle X-ray scattering and nitrogen-sorption measurements. The structural features were directly imaged in TEM cross-sections of the composite membranes. For both structures, the AAM pores were completely filled and no shrinkage was observed due to strong adhesion of the carbon-wall material to the AAM pore walls. As a consequence, the pore size of the mesophase system stays almost constant even after thermal treatment at 1000 °C.     

Formation Mechanism of Mesostructured Silica in Confined Space: An In Situ GISAXS Study

The structural evolution of periodic mesoporous material within the channels of anodic alumina membranes (AAMs) by evaporation‐induced self‐assembly (EISA) is investigated by a combination of in situ grazing‐incidence small‐angle X‐ray scattering (GISAXS) with parallel detection of solvent evaporation and ex situ transmission electron microscopy (TEM). Kinetically controlled and equilibrium‐controlled structural evolution can be distinguished for these EISA processes. A new mechanism for formation of mesostructures in the confined environment of AAMs is proposed. Data are presented for samples synthesized with nonionic surfactants at various surfactant:silica ratios and relative humidities. The formation of and transformations between circular or columnar 2D hexagonal and tubular lamellar structures are observed. The circular hexagonal phase is kinetically favored over the columnar hexagonal orientation. The TEM images provide evidence that phase transformations, depending on their type, either start preferentially at the channel wall or in the center of the mesostructured fibers.     

X-rays to study,induce, and pattern structures in sol–gel materials

X-rays investigations have been shown to reveal important information regarding material features and the formation mechanism of mesostructured materials. Small angle X-ray scattering (SAXS) analysis performed using a synchrotron source has been very important in the optimization of the organization of mesoporous coatings obtained by evaporation induced self-assembly (EISA). The interaction between X-rays and ordered mesoporous films has only recently been reported, and new knowledge has been developed to use this external radiation source to tune the local material properties. Here we discuss the recent developments in X-ray lithography combined with sol–gel synthesis to pattern mesostructured and hierarchical porous coatings including the ability to tailor functionalized surfaces.     

Peering into the self-assembly of surfactant templated thin-film silica mesophases

It is now recognized that self-assembly is a powerful synthetic approach to the fabrication of nanostructures with feature sizes smaller than achievable with state of the art lithography and with a complexity approaching that of biological systems. For example, recent research has shown that silica/surfactant self-assembly combined with evaporation (so-called evaporation induced self-assembly EISA) can direct the formation of porous and composite thin-film mesostructures characterized by precise periodic arrangements of inorganic and organic constituents on the 1-50-nm scale. Despite the potential utility of these films for a diverse range of applications such as sensors, membranes, catalysts, waveguides, lasers, nano-fluidic systems, and low dielectric constant (so-called low k) insulators, the mechanism of EISA is not yet completely understood. Here, using time-resolved grazing incidence small-angle X-ray scattering (GISAXS) combined with gravimetric analysis and infrared spectroscopy, we structurally and compositionally characterize in situ the evaporation induced self-assembly of a homogeneous silica/surfactant/solvent solution into a highly ordered surfactant-templated mesostructure. Using CTAB (cetyltrimethylammonium bromide) as the structure-directing surfactant, a two-dimensional (2-D) hexagonal thin-film mesophase (p6mm) with cylinder axes oriented parallel to the substrate surface forms from an incipient lamellar mesophase through a correlated micellar intermediate. Comparison with the corresponding CTAB/water/alcohol system (prepared without silica) shows that, for acidic conditions in which the siloxane condensation rate is minimized, the hydrophilic and nonvolatile silicic acid components replace water maintaining a fluidlike state that avoids kinetic barriers to self-assembly.     

A pyrrole-containing surfactant as a tecton for nanocomposite SiO2 films

A surfactant featuring a polymerizable pyrrole head group (dodecyl-dimethyl-(2-pyrrol-1-yl-ethyl)-ammonium bromide, DDPABr) was synthesized. The thermotropic behavior of the surfactant was investigated by differential scanning calorimetry (DSC) and X-ray scattering techniques, with small-angle X-ray scattering (SAXS) analysis revealing a highly ordered lamellar bilayer structure. After full characterization, DDPABr was used in the preparation of mesostructured SiO2 nanocomposite thin films via evaporation-induced self-assembly (EISA). Resulting thin SiO2-DDPABr films were studied by 1D and 2D small-angle X-ray scattering (SAXS) techniques, indicating a lamellar nanocomposite structure. Suitable theoretical SAXS models were applied to fit the experimental 1D SAXS data. The surfactant could be chemically polymerized within the lamellar domains.     

Synthesis of thermally stable highly ordered nanoporous tin oxide thin films with a 3D face-centered orthorhombic nanostructure

Thin films of nanoporous tin oxide with a 3D face-centered orthorhombic nanostructure have been synthesized by self-assembly that is controlled by post-coating thermal treatment under controlled humidity. In contrast to the conventional evaporation-induced self-assembly (EISA), the films here have no ordered nanostructure after dip-coating. However, the initial coatings are formed under conditions that inhibit significant hydrolysis and condensation for extended periods. This allows the use of postsynthesis thermal vapor treatments to completely control the formation of the nanostructure. With EO106-PO70-EO106 (Pluronic F127) triblock copolymer as the template, highly ordered nanostructures were generated by exposing the disordered films to a stream of water vapor at elevated temperature, which rehydrates the films and allows the formation of the thermodynamically favored phase. Further exposure to water vapor drives the condensation reaction through the elimination of HCl. The X-ray diffraction pattern from the nanostructure was indexed in the space group Fmmm as determined by analysis of 2D small-angle X-ray scattering patterns at various angles of incidence. The nanostructure is then stabilized and made nanoporous by extended controlled thermal treatments. After self-assembly and template removal, the films are thermally stable up to 600 degrees C and retain an ordered, face-centered orthorhombic nanostructure.     

Ordered mesoporous silicas and carbons with large accessible pores templated from amphiphilic diblock copolymer poly(ethylene oxide)-b-polystyrene

Highly ordered mesoporous carbons and silicas with ultralarge accessible pores have been successfully synthesized by using laboratory-made poly(ethylene oxide)-b-polystyrene (PEO-b-PS) diblock copolymers as templates via the evaporation-induced self-assembly (EISA) approach. Resols and tetraethyl orthosilicate (TEOS) serve as carbon and silica precursors, respectively. Small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) measurements show that the mesoporous carbons (denoted as C-FDU-18) possess face centered cubic closed-packing (fcc) mesostructure (Fm3m) with large-domain ordering. N2 sorption isotherms reveal a large mesopore at the mean value of 22.6 nm with a narrow pore-size distribution. Mesoporous silicas (Si-FDU-18) also display a highly ordered fcc closed-packing mesostructure with an ultralarge unit cell (a = 54.6 nm). A hydrothermal recrystallization was introduced for the first time to produce micropores in thick silica walls (approximately 7.7 nm) and thus to generate ultralarge accessible mesopores (30.8 nm). Notably, the amphiphilic diblock copolymer with high molecular weight (PEO125-PS230, 29700 g mol-1) in this report was prepared via a simple method of atom transfer radical polymerization (ATRP). It can be easily available for chemists even without any experience in polymer synthesis.     

Free-standing mesoporous carbon thin films with highly ordered pore architectures for nanodevices

We report for the first time the synthesis of free-standing mesoporous carbon films with highly ordered pore architecture by a simple coating-etching approach, which have an intact morphology with variable sizes as large as several square centimeters and a controllable thickness of 90 nm to ～3 μm. The mesoporous carbon films were first synthesized by coating a resol precursors/Pluronic copolymer solution on a preoxidized silicon wafer and forming highly ordered polymeric mesostructures based on organic-organic self-assembly, followed by carbonizing at 600 °C and finally etching of the native oxide layer between the carbon film and the silicon substrate. The mesostructure of this free-standing carbon film is confirmed to be an ordered face-centered orthorhombic Fmmm structure, distorted from the (110) oriented body-centered cubic Im3?m symmetry. The mesoporosity of the carbon films has been evaluated by nitrogen sorption, which shows a high specific BET surface area of 700 m(2)/g and large uniform mesopores of ～4.3 nm. Both mesostructures and pore sizes can be tuned by changing the block copolymer templates or the ratio of resol to template. These free-standing mesoporous carbon films with cracking-free uniform morphology can be transferred or bent on different surfaces, especially with the aid of the soft polymer layer transfer technique, thus allowing for a variety of potential applications in electrochemistry and biomolecule separation. As a proof of concept, an electrochemical supercapacitor device directly made by the mesoporous carbon thin films shows a capacitance of 136 F/g at 0.5 A/g. Moreover, a nanofilter based on the carbon films has shown an excellent size-selective filtration of cytochrome c and bovine serum albumin.     

Evaporation induced self-assembly of templated silica and organosilica thin films on various electrode surfaces

A new one-step method is reported for the deposition of hybrid mesoporous thin films on various electrode surfaces (gold, platinum, glassy carbon). Deposition was achieved by spin-coating sol–gel mixtures in the presence of a surfactant template to get mesostructured thin layers on the various conducting substrates. Film formation occurred by evaporation induced self-assembly (EISA) involving the hydrolysis and (co)condensation of silane and/or organosilane precursors on the electrode surface. Extraction of the surfactant from the ordered mesoporous films led to a large increase of mass transport rates into the materials and imparted high accessibility to the organic moieties in case of functionalized mesoporous overlayers. The electrochemical properties of the film-modified electrodes have been studied by cyclic voltammetry (CV), and also via the chemical accumulation of mercury ions prior to their stripping analysis by differential pulse voltammetry (i.e. for thiol-functionalized thin films). Some evidences to support the formation of self-assembled monolayers (SAMs) on electrodes, have been also discussed. The formation of well-adhering mesoporous thin films on solid electrode surfaces is expected to have a high impact on the development of new electrochemical sensors.     

Kinetics of polycondensation reactions during self-assembly of mesostructured films studied by in situ infrared spectroscopy

In situ synchrotron FTIR experiments have been performed during evaporation-induced self-assembly (EISA) of mesoporous films and the role of silica polycondensation in obtaining highly organized mesostructures has been illuminated.     

Self-assembly of an environmentally responsive polymer/silica nanocomposite

Thermoresponsive nanocomposite thin films composed of alternating layers of silica and polymerized N-isopropylacrylamide (NIPAM) or NIPAM plus dodecyl methacrylate (DM) hydrogels were prepared by surfactant-directed evaporation-induced self-assembly (EISA). During EISA, the organic monomers partition within the hydrophobic domains of a lamellar mesophase. In-situ polymerization via a free radical process results in a 1-2 nm thick hydrogel phase sandwiched between layers of silica oriented parallel to the substrate surface. The thermoresponsiveness of PNIPAM is preserved in this confined environment, and the polymeric layers reversibly swell and deswell by a factor of 2 in water upon temperature changes around the transition temperature of PNIPAM (32 degrees C). The composition, mesostructure, and environmental response were studied by detailed NMR, TGA, and SAXS analyses.     

Highly Ordered Mesoporous Carbonaceous Frameworks from a Template of a Mixed Amphiphilic Triblock‐Copolymer System of PEO–PPO–PEO and Reverse PPO–PEO–PPO

A series of highly ordered mesoporous carbonaceous frameworks with diverse symmetries have been successfully synthesized by using phenolic resols as a carbon precursor and mixed amphiphilic surfactants of poly(ethylene oxide)‐b‐poly(propylene oxide)‐b‐poly(ethylene oxide) (PEO–PPO–PEO) and reverse PPO–PEO–PPO as templates by the strategy of evaporation‐induced organic–organic self‐assembly (EISA). The transformation of the ordered mesostructures from face‐centered (Fd m) to body‐centered cubic (Im m), then 2D hexagonal (P6mm), and eventually to cubic bicontinuous (Ia d) symmetry has been achieved by simply adjusting the ratio of triblock copolymers to resol precursor and the relative content of PEO–PPO–PEO copolymer F127, as confirmed by small‐angle X‐ray scattering (SAXS), transmission electron microscopy (TEM), and nitrogen‐sorption measurements. The blends of block copolymers can interact with resol precursors and tend to self‐assemble into cross‐linking micellar structures during the solvent‐evaporation process, which provides a suitable template for the construction of mesostructures. The assembly force comes from the hydrogen‐bonding interactions between organic mixed micelles and the resol‐precursor matrix. The BET surface area for the mesoporous carbonaceous samples calcined at 600 °C under nitrogen atmosphere is around 600 m² g^?1, and the pore size can be adjusted from 2.8 to 5.4 nm. An understanding of the organic–organic self‐assembly behavior in the mixed amphiphilic surfactant system would pave the way for the synthesis of mesoporous materials with controllable structures.     

Ordered mesoporous ceramics stable up to 1500 degrees C from diblock copolymer mesophases

In the present study, a poly(isoprene-block-dimethylamino ethyl methacrylate) diblock copolymer (PI-b-PDMAEMA) is used to structure-direct a polysilazane pre-ceramic polymer, commercially known as Ceraset. To the polymer was added a 2-fold excess in weight of the silazane oligomer (Ceraset). The resulting composite was cast into films, and after cooperative self-assembly of block copolymer and Ceraset, the structure was permanently set in the hexagonal columnar morphology, as evidenced by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Cross-linking of the silazane oligomer was achieved with a radical initiator at 120 degrees C. Upon heating of the composite to 1500 degrees C under nitrogen, the structure is preserved and a mesoporous ceramic material is obtained, as demonstrated by SAXS and TEM. The pores are open and accessible, as evidenced by nitrogen sorption/desorption measurements indicating a surface area of about 51 m2 g-1 and a pore diameter of 13 nm, consistent with TEM analysis. These results suggest that the use of block copolymer mesophases may provide a simple, easily controlled pathway for the preparation of various high-temperature ceramic mesostructures.     

Quantitative SAXS analysis of oriented 2D hexagonal cylindrical silica mesostructures in thin films obtained from nonionic surfactants

Oriented mesostructured surfactant-silica nanocomposite thin films with a 2D hexagonal mesostructure of cylindrical micelles were prepared by evaporation-induced self-assembly using two different nonionic Brij surfactants and studied by small-angle X-ray scattering in symmetric reflection (SRSAXS) and grazing incidence (GISAXS) geometries. A novel SRSAXS evaluation approach was applied that allowed a good fitting of the SRSAXS data over almost the whole range of scattering vectors. Aside from the cylinder radius and the lattice parameter, the approach provided accurate values for the polydispersity of the micelles, lattice distortions, and preferred orientation. These analyses revealed a significant rise of the micelle radius and accordingly the lattice parameter upon an increase in the ratio surfactant/SiO(2), attributable to a decrease in the solubilization of the poly(ethylene oxide) (PEO) chains by water, in agreement with Monte Carlo simulations. Furthermore, the SRSAXS analysis was successfully applied to the corresponding mesoporous films for the determination of pore sizes.     

Impact of film thickness on the morphology of mesoporous carbon films using organic-organic self-assembly

Mesoporous polymer and carbon thin films are prepared by the organic-organic self-assembly of an oligomeric phenolic resin with an amphiphilic triblock copolymer template, Pluronic F127. The ratio of resin to template is selected such that a body-centered cubic (Im3m) mesostructure is formed in the bulk. However, well-ordered mesoporous films are not always obtained for thin films (<100 nm), and this behavior is found to be directly correlated with the initial phenolic resin to template ratio. Furthermore, the symmetry of ordered phases is highly dependent on the number of layers of spheres in the film: Monolayers and bilayers are characterized by hexagonal close-packed (HCP) symmetry, while films with approximately 5 layers of spheres exhibit a mixture of HCP and face-centered orthorhombic (FCO) structures. Ultrathick films having more than 30 layers of spheres are similar to the bulk body-centered cubic symmetry with a preferential orientation of the closest-packed (110) plane parallel to the substrate. Film thickness and initial composition of the carbonizable precursors in the template are critical factors in determining the morphology of mesoporous carbon films. These results provide insight into why difficulties have been reported in producing ultrathin ordered mesoporous carbon films using cooperative organic-organic self-assembly.     

挥发诱导自组装法制备结构有序的有机-无机纳米复合材料*

挥发诱导自组装方法（EISA）可以用来模拟自然界中的有机-无机杂化材料的纳米有序结构的形成过程。这是一种新兴的、有效的合成结构有序纳米复合材料的方法,其具有快速、简便、高效的特点。本文就挥发诱导自组装方法的原理及利用该方法制备结构有序的有机-无机纳米复合材料(MSC材料)的机理、有关影响MSC材料有序结构形成的因素作了详细探讨,并对利用EISA方法制备MSC材料的研究和发展前景进行了展望。     

Combined emulsion and solvent evaporation (ESE) synthesis route to well-ordered mesoporous materials

Control over morphology and internal mesostructure of surfactant templated silicas remains a challenge, especially when considering scaling laboratory syntheses up to industrial volumes. Here we report a method combining emulsification with the evaporation-induced self-assembly (EISA) method for preparing spherical, mesoporous silica particles. This emulsion and solvent evaporation (ESE) method has several potential advantages over classic precipitation routes: it is easily scaled while providing superior control over stoichiometric homogeneity of templating surfactants and inorganic precursors, and particle sizes and distributions are determined by principles developed for manipulating droplet sizes within water-in-oil emulsions. To demonstrate the method, triblock copolymer P104 is used as a templating amphiphile, generating unusually well-ordered 2D hexagonal (P6mm) mesoporous silica, while particle sizes and morphologies were controlled by varying the type of emulsifier and the method for emulsification.     

Slow drying of a spray of nanoparticles dispersion. In situ SAXS investigation

Nanoparticles confined in droplets of less than a picoliter are forced to organize in submicronic dry grains through solvent evaporation. The evolution of structures of the grains and the constituent nanoparticles during the slow drying process are investigated in situ with small-angle X-ray scattering (SAXS) for the first time. The scattering results have been explained on the basis of the equilibrium thermodynamics of the droplets in the drying tube. We demonstrate that this technique is really efficient in describing the internal arrangement of the nanoparticles inside the drying droplets. Distinction between an almost homogeneous repartition of the nanoparticles in droplets and formation of core shell like particles even in strongly polydispersed droplets can be made using SAXS.     

Recent Advances in Polymerization-Induced Self-Assembly (PISA) Syntheses in Non-Polar Media

It is well-known that polymerization-induced self-assembly (PISA) is a powerful and highly versatile technique for the rational synthesis of colloidal dispersions of diblock copolymer nanoparticles, including spheres, worms or vesicles. PISA can be conducted in water, polar solvents or non-polar media. In principle, the latter formulations offer a wide range of potential commercial applications. However, there has been just one review focused on PISA syntheses in non-polar media and this prior article was published in 2016. The purpose of the current review article is to summarize the various advances that have been reported since then. In particular, PISA syntheses conducted using reversible addition-fragmentation chain-transfer (RAFT) polymerization in various n-alkanes, poly(α-olefins), mineral oil, low-viscosity silicone oils or supercritical CO₂ are discussed in detail. Selected formulations exhibit thermally induced worm-to-sphere or vesicle-to-worm morphological transitions and the rheological properties of various examples of worm gels in non-polar media are summarized. Finally, visible absorption spectroscopy and small-angle X-ray scattering (SAXS) enable in situ monitoring of nanoparticle formation, while small-angle neutron scattering (SANS) can be used to examine micelle fusion/fission and chain exchange mechanisms.