Remarkable Birefringence in a TiO2–SiO2 Composite Film with an Aligned Mesoporous Structure

Mesostructured Materials with Controllable Long-Range Orientational Ordering and Anisotropic Properties

Inorganic-organic mesophase materials provide a wide range of tunable properties, which are often highly dependent on their nano-, micro-, or meso-scale compositions and structures. Among these are macroscopic orientational order and corresponding anisotropic material properties, the adjustability of which are difficult to achieve. This is due to the complicated transient and coupled transport, chemical reaction, and surface processes that occur during material syntheses. By understanding such processes, general criteria are established and used to prepare diverse mesostructured materials with highly aligned channels with uniform nanometer dimensions and controllable directionalities over macroscopic dimensions and thicknesses. This is achieved by using a micropatterned semipermeable poly(dimethylsiloxane) stamp to manage the rates, directions, and surfaces at which self-assembling phases nucleate and the directions that they grow. This enables mesostructured surfactant-directed silica and titania composites, including with functional guest species, and mesoporous carbons to be prepared with high degrees of hexagonal order, as well as controllable orthogonal macroscopic orientational order. The resulting materials exhibit novel anisotropic properties, as demonstrated by the example of direction-dependent photocurrent generation, and are promising for enhancing the functionality of inorganic-organic nanocomposite materials in separations, catalysis, and energy conversion applications.

Lithographically Formed Fine Wavy Surface Morphology for Universal Alignment Control of Mesochannels in Mesostructured Silica Films

In-plane orientation of mesochannels in mesostructured silica films is fully controlled by a lithographically formed anisotropic surface morphology of a substrate. The orientation is determined simply by elastic properties of a liquid crystal phase, which appears in the course of the formation of mesostructured silica films through the sol-gel process. When an array of linear microscopic grooves with a round cross section is closely formed on the substrate surface, the cylindrical mesochannels in the films are entirely aligned strictly perpendicular to the grooves, as a consequence of minimization of the total elastic energy. When the surface morphology geometrically fits to the hexagonal arrangement of the mesochannels, the orientation abruptly changes into the direction parallel to the long axis of the grooves. The alignment control based on the elastic property of the liquid crystal phase described in this report does not require any specific chemical interactions between the surfactant molecules and the substrate surface. Therefore, aligned mesostructured silica films with a large structural periodicity can successfully be formed using block copolymer surfactants, which hardly form an aligned mesostructure without the support of external fields. The vapor-phase synthesis, which enables considerable retardation of the solidification process of siliceous species, is the most favorable way, and totally aligned mesostructured silica films with significantly large thickness, more than 1 μm, can be obtained. Appropriate combination of the bottom-up and the top-down nanoprocesses reported in this paper, that is, self-assembly and photolithography, will enable the formation of highly anisotropic nanostructured materials, which will find various practical applications.

Macroscopically Oriented Porous Materials with Periodic Ordered Structures: From Zeolites and Metal-Organic Frameworks to Liquid-Crystal-Templated Mesoporous Materials

Porous materials with molecular-sized periodic structures, as exemplified by zeolites, metal-organic frameworks, or mesoporous silica, have attracted increasing attention due to their range of applications in storage, sensing, separation, and transformation of small molecules. Although the components of such porous materials have a tendency to pack in unidirectionally oriented periodic structures, such ideal types of packing cannot continue indefinitely, generally ceasing when they reach a micrometer scale. Consequently, most porous materials are composed of multiple randomly oriented domains, and overall behave as isotropic materials from a macroscopic viewpoint. However, if their channels could be unidirectionally oriented over a macroscopic scale, the resultant porous materials might serve as powerful tools for manipulating molecules. Guest molecules captured in macroscopically oriented channels would have their positions and directions well-defined, so that molecular events in the channels would proceed in a highly controlled manner. To realize such an ideal situation, numerous efforts have been made to develop various porous materials with macroscopically oriented channels. An overview of recent studies on the synthesis, properties, and applications of macroscopically oriented porous materials is presented.

Predicting Sizes of Hexagonal and Gyroid Metal Nanostructures from Liquid Crystal Templating

We describe a method to predict and control the lattice parameters of hexagonal and gyroid mesoporous materials formed by liquid crystal templating. In the first part, we describe a geometric model with which the lattice parameters of different liquid crystal mesophases can be predicted as a function of their water/surfactant/oil volume fractions, based on certain geometric parameters relating to the constituent surfactant molecules. We demonstrate the application of this model to the lamellar (Lα), hexagonal (H1), and gyroid bicontinuous cubic (V1) mesophases formed by the binary Brij-56 (C16EO10)/water system and the ternary Brij-56/hexadecane/water system. In this way, we demonstrate predictable and independent control over the size of the cylinders (with hexadecane) and their spacing (with water). In the second part, we produce mesoporous platinum using as templates hexagonal and gyroid phases with different compositions and show that in each case the symmetry and lattice parameter of the metal nanostructure faithfully replicate those of the liquid crystal template, which is itself in agreement with the model. This demonstrates a rational control over the geometry, size, and spacing of pores in a mesoporous metal.

Macroscopic ordering of helical pores for arraying guest molecules noncentrosymmetrically

Helical nanostructures have attracted continuous attention, not only as media for chiral recognition and synthesis, but also as motifs for studying intriguing physical phenomena that never occur in centrosymmetric systems. To improve the quality of signals from these phenomena, which is a key issue for their further exploration, the most straightforward is the macroscopic orientation of helices. Here as a versatile scaffold to rationally construct this hardly accessible structure, we report a polymer framework with helical pores that unidirectionally orient over a large area (∼10 cm(2)). The framework, prepared by crosslinking a supramolecular liquid crystal preorganized in a magnetic field, is chemically robust, functionalized with carboxyl groups and capable of incorporating various basic or cationic guest molecules. When a nonlinear optical chromophore is incorporated in the framework, the resultant complex displays a markedly efficient nonlinear optical output, owing to the coherence of signals ensured by the macroscopically oriented helical structure.

Mesoscopically structured nanocrystalline metal oxide thin films

This review describes the main successful strategies that are used to grow mesostructured nanocrystalline metal oxide and SiO₂ films via deposition of sol-gel derived solutions. In addition to the typical physicochemical forces to be considered during crystallization, mesoporous thin films are also affected by the substrate-film relationship and the mesostructure. The substrate can influence the crystallization temperature and the obtained crystallographic orientation due to the interfacial energies and the lattice mismatch. The mesostructure can influence the crystallite orientation, and affects nucleation and growth behavior due to the wall thickness and pore curvature. Three main methods are presented and discussed: templated mesoporosity followed by thermally induced crystallization, mesostructuration of already crystallized metal oxide nanobuilding units and substrate-directed crystallization with an emphasis on very recent results concerning epitaxially grown piezoelectric structured α-quartz films via crystallization of amorphous structured SiO₂ thin films.

Aromatic-aromatic interactions enhance interfiber contacts for enzymatic formation of a spontaneously aligned supramolecular hydrogel

Anisotropy or alignment is a critical feature of functional soft materials in living organisms, but it remains a challenge for spontaneously generating anisotropic gel materials. Here we report a molecular design that increases intermolecular aromatic-aromatic interactions of hydrogelators during enzymatic hydrogelation for spontaneously forming an anisotropic hydrogel. This process, relying on both aromatic-aromatic interactions and enzyme catalysis, results in spontaneously aligned supramolecular nanofibers as the matrices of a monodomain hydrogel that exhibits significant birefringence. This work, as the first example of monodomain hydrogels formed via an enzymatic reaction, illustrates a new biomimetic approach for generating aligned anisotropic soft materials.

Strict in-plane alignment control of block-copolymer-templated mesostructured silica films using an alignment-controlling agent

An alkoxysilane with an alkyl chain is introduced as an alignment-controlling agent of a block-copolymer-templated mesostructured silica film. Use of the alkylalkoxysilane achieves the alignment of the mesochannels of a triblock-copolymer-templated film by an intermolecular interaction with a rubbing-treated polyimide film. Co-use of an alkoxysilane with a hydroxymethyl group as a hydrophobicity reducing agent improves the alignment close to that of the film prepared using an alkyl surfactant. This concept widens the range of the structural period of aligned mesoporous films and thus widens the useful range of the anisotropic optical properties.

Optically anisotropic thin films by shear-oriented assembly of colloidal nanorods

Device-scale thin films of highly oriented (in-plane) colloidal nanorods are made available by using a simple coating process involving thixotropic rod gel suspensions. Application of this process to LaPO₄ nanorods leads to films exhibiting outstanding anisotropic optical properties, such as a remarkably large birefringence (Δn = 0.13) associated with high transparency, and sharply polarized fluorescence spectra when doped with europium.

Oxide-based inorganic/organic and nanoporous spherical particles: synthesis and functional properties

This paper reviews the recent progress in the preparation of oxide-based and heteroatom-doped particles. Surfactant-templated oxide particles, e.g. silica and titania, are possible candidates for various potential applications such as adsorbents, photocatalysts, and optoelectronic and biological materials. We highlight nanoporous oxides of one element, such as silicon or titanium, and those containing multiple elements, which exhibit properties that are not achieved with individual components. Although the multicomponent nanoporous oxides possess a number of attractive functions, the origin of their properties is hard to determine due to compositional/structural complexity. Particles with a well-defined size and shape are keys for a quantitative and detailed discussion on the unique complex properties of the particles. From this viewpoint, we review the synthesis techniques of the oxide particles, which are functionalized with organic molecules or doped with heteroatoms, the physicochemical properties of the particles and the possibilities for their photofunctional applications as complex systems.

Critical aspects in the production of periodically ordered mesoporous titania thin films

Periodically ordered mesoporous titania thin films (MTTF) present a high surface area, controlled porosity in the 2-20 nm pore diameter range and an amorphous or crystalline inorganic framework. These materials are nowadays routinely prepared by combining soft chemistry and supramolecular templating. Photocatalytic transparent coatings and titania-based solar cells are the immediate promising applications. However, a wealth of new prospective uses have emerged on the horizon, such as advanced catalysts, perm-selective membranes, optical materials based on plasmonics and photonics, metamaterials, biomaterials or new magnetic nanocomposites. Current and novel applications rely on the ultimate control of the materials features such as pore size and geometry, surface functionality and wall structure. Even if a certain control of these characteristics has been provided by the methods reported so far, the needs for the next generation of MTTF require a deeper insight in the physical and chemical processes taking place in their preparation and processing. This article presents a critical discussion of these aspects. This discussion is essential to evolve from know-how to sound knowledge, aiming at a rational materials design of these fascinating systems.

Engineering the formation of secondary building blocks within hollow interiors

Secondary building blocks within the cavities of primary silica-architecture building blocks are successfully engineered. The immobilized surfactant directs the selective dissolution and reassembly of dissolved silicate species for the formation of secondary building blocks (hollow nanospheres/nanorods; see figure). Supported TiO(2) on nanostructures with multilevel interiors is shown to exhibit significantly enhanced activity in photocatalytic H(2) production.