Magnesiothermic Reduction of Thin Films: Towards Semiconducting Chiral Nematic Mesoporous Silicon Carbide and Silicon Structures

There is a growing demand for new methods to prepare porous Si‐based materials for applications in optoelectronic and microelectronic devices. In this work, the preparation of SiC and Si from magnesiothermic reduction of chiral nematic SiO₂/C composites and mesoporous SiO₂, respectively, is reported. The SiO₂/C composites are prepared by cocondensing SiO₂ with cellulose nanocrystals (CNCs) followed by pyrolysis. The magnesiothermic reduction of the composites produces SiC after prolonged solid‐state reaction, with mixed MgC₂/SiC intermediates. Iridescent mesoporous tetragonal MgC₂/SiC structures that retain the long‐range twisted organization of the starting composites transform to mesoporous cubic SiC with a chiral nematic hierarchical structure, but with some loss of order. On the other hand, the magnesiothermic reduction of the chiral nematic mesoporous SiO₂ templated from CNCs affords mesoporous Si materials with a layered hierarchical structure. The structural properties and the conductivity of the products, as well as the reaction pathways by analysis of the materials at intermediate stages, are investigated. These experimental results show that the magnesiothermic reduction is a promising way to obtain new porous semiconducting materials with chiral nematic structures.     

CdS Quantum Dots Encapsulated in Chiral Nematic Mesoporous Silica: New Iridescent and Luminescent Materials

Simultaneous integration of light emission and iridescence into a semiconducting photonic material is attractive for the design of new optical devices. Here, a straightforward, one‐pot approach for liquid crystal self‐assembly of semiconductor quantum dots into cellulose nanocrystal‐templated silica is developed. Through a careful balance of the intermolecular interactions between a lyotropic tetraalkoxysilane/cellulose nanocrystal dispersion and water‐soluble polyacrylic acid/mercaptopropionic acid‐stabilized CdS quantum dots, CdS/silica/nanocellulose composites that retain both chiral nematic order of the cellulose nanocrystals and emission of the quantum dots are successfully co‐assembled. Subsequent removal of the cellulose template and organic stabilizers in the composites by controlled calcination generates new freestanding iridescent, luminescent chiral nematic mesoporous silica‐encapsulated CdS films. The pores of these materials are accessible to analytes and, consequently, the CdS quantum dots undergo strong luminescence quenching when exposed to TNT solutions or vapor.     

Synthesis and Characterization of Silica Nanotubes with Radially Oriented Mesopores

Hierarchical silica nanotubes with radially oriented mesoporous channels perpendicular to the central axis of the tube were synthesized by using self‐assembled chiral anionic surfactant, co‐structure directing agent (CSDA) and silica precursor. The average inner diameter and the wall thickness were ∼94, ∼62, and ∼62 nm and to ∼27, ∼33, and ∼45 nm, respectively, by manipulating the synthesis gel composition, while the diameter of the wall mesopores was kept constant at ∼4 nm. These materials with such a unique structure were produced only with chiral surfactant and achiral or racemic surfactant did not give rise to mesoporous silica nanotubes. The existence of helicity in the lipid bilayer template was confirmed by means of circular dichroism spectroscopy. The mesoporous penetrating from outside to inside of silica nanotubes are thought to originate from the initial formation of self‐assembled lipid tubes with helical bilayers, which in turn re‐assemble to form the mesopores in the wall of the nanotubes upon addition of co‐structure directing agent and silica source.     

Hydrogen‐Bonded Liquid Crystals in Confined Spaces—Toward Photonic Hybrid Materials

A series of hybrid materials based on chiral nematic mesoporous organosilica (CNMO) films infiltrated with liquid crystalline hydrogen‐bonded assemblies is prepared and characterized with respect to the mutual manipulation of the photonic properties of the host and the liquid‐crystalline behavior of the guest. Detailed differential scanning calorimetry studies reveal the impact of confinement on the mesomorphic behavior of the liquid crystalline assemblies in the pores of the CNMO films. The photonic properties of the chiral nematic mesoporous host can be controlled by changing the temperature or irradiating the films with UV light. These stimuli‐induced phase transitions are accompanied by changes in the orientational order of the mesogens as revealed by ¹⁹F NMR spectroscopy. The combination of confinement and changes in the molecular orientation in a unique hybrid material based on hydrogen‐bonded liquid crystals and a porous host with a chiral nematic mesostructure is an interesting concept for the design of optical sensors, reflectors, or filters.     

Black Titania with Nanoscale Helicity

The fabrication of mesoporous black titanium dioxide (TiO_2?x ) with a chiral nematic organization of core–shell nanorods is reported. Chiral templating of TiO₂ nanoparticles onto gelatin‐functionalized cellulose nanocrystals (CNCs) followed by calcination yields carbonized TiO₂/CNC helical materials that recover white TiO₂ replicas after carbon removal. The hydrothermal surface reduction of the traditional white TiO₂ by ascorbic acid affords freestanding chiral nematic black TiO_2?x. The black TiO_2?x is a visible light active semiconducting mesoporous structure constructed by chiral nematic crystalline–amorphous TiO₂ core–shell nanorods. The chiral black TiO_2?x nanoparticles supported on mesoporous nanocarbon networks are evaluated as lithium‐ion battery anode electrodes. Beyond the current efforts, these black TiO_2?x materials and their composites may be useful in the fields of energy storage and catalysis.     

Silica–Carbon Nanocomposites—A New Concept for the Design of Solar Absorbers

To create materials that are composites or hybrids structured on the nanometer scale or the meso‐domain, respectively, is one of the major tasks in modern materials science. In this paper, we demonstrate general strategies on how to obtain these nanocomposites founded on the knowledge about ordered mesoporous materials. One strategy involves the formation of the composite by performing a chemical reaction in the pores of a pre‐formed ordered mesoporous silica while the other strategy uses compounds that first mold their porous environment in the silica and in a succeeding step react to the final composite. As a model system, here, we present the formation of porous silica–carbon hybrid materials. Besides this more general question, we also tackle the task of finding a suitable application for the obtained nanocomposites. We chose an application as selective solar‐absorber materials.     

Sequential Hydroboration–Alcoholysis and Epoxidation–Ring Opening Reactions of Vinyl Groups in Mesoporous Vinylsilica

We report the sequential transformation of vinyl groups into hydroborate and alcohol as well as vinyl into epoxide and diol functional groups in hexagonal mesoporous vinylsilica materials, denoted meso‐vinyl‐SiO₂. The first transformation proceeds quantitatively through the hydroborylated derivative. After appropriate quenching, the hydroborylated materials are stable at ambient conditions and can undergo transformation into alcohols and various other functional groups. The pore volume and pore size uniformity were found not to be greatly affected by quenching of the hydroboranes with methanol, but they were reduced by quenching with water due to the deposition of boron‐containing species in the channels. Complete conversion of hydroborylated materials to alcohol‐functionalized materials required basic conditions and treatment time of several hours. Although this led to a significant structural shrinkage, decrease in pore volume, and decrease in ordering, there was no evidence of a partial collapse or removal of substantial parts of the pore walls under optimized synthesis conditions. This is the first successful conversion of organic groups of a functionalized ordered mesoporous silica host in alkaline solution, conditions known to be detrimental for silica frameworks. Epoxidation of the vinyl groups and subsequent conversion of the resulting epoxides into diols are also briefly described. The chemical transformation through epoxidation affords ordered mesoporous silica materials functionalized with potentially chiral organic groups, which could find applications in asymmetric catalysis and chiral separations. It was found that the epoxidation was slower than hydroboration and resulted in a lower degree of conversion. These two examples of hydroboration–alcoholysis and epoxidation–ring opening reactions of terminally bonded vinyl groups in meso‐vinyl‐SiO₂ demonstrate the novel concept of sequential organic chemical transformations harbored inside the ordered channels of mesoporous organosilica materials.     

Evaporation‐Induced Coating and Self‐Assembly of Ordered Mesoporous Carbon‐Silica Composite Monoliths with Macroporous Architecture on Polyurethane Foams

A facile approach of solvent‐evaporation‐induced coating and self‐assembly is demonstrated for the mass preparation of ordered mesoporous carbon‐silica composite monoliths by using a polyether polyol‐based polyurethane (PU) foam as a sacrificial scaffold. The preparation is carried out using resol as a carbon precursor, tetraethyl orthosilicate (TEOS) as a silica source and Pluronic F127 triblock copolymer as a template. The PU foam with its macrostructure provides a large, 3D, interconnecting interface for evaporation‐induced coating of the phenolic resin‐silica block‐copolymer composites and self‐assembly of the mesostructure, and endows the composite monoliths with a diversity of macroporous architectures. Small‐angle X‐ray scattering, X‐ray diffraction and transmission electron microscopy results indicate that the obtained composite monoliths have an ordered mesostructure with 2D hexagonal symmetry (p6m) and good thermal stability. By simply changing the mass ratio of the resol to TEOS over a wide range (10–90%), a series of ordered, mesoporous composite foams with different compositions can be obtained. The composite monoliths with hierarchical macro/mesopores exhibit large pore volumes (0.3–0.8 cm³ g^?1), uniform pore sizes (4.2–9.0 nm), and surface areas (230–610 m² g^?1). A formation process for the hierarchical porous composite monoliths on the struts of the PU foam through the evaporation‐induced coating and self‐assembly method is described in detail. This simple strategy performed on commercial PU foam is a good candidate for mass production of interface‐assembly materials.     

Shape-Memory Photonic Thermoplastics from Cellulose Nanocrystals

Responsive materials prepared using shape-memory photonic crystals have potential applications in rewritable photonic devices, security features, and optical coatings. By embedding chiral nematic cellulose nanocrystals (CNCs) in a polyacrylate matrix, a shape-memory photonic crystal thermoplastic (CNC-SMP) allows reversible capture of different colored states is reported. In this system, the temperature is used to program the shape-memory response, while pressure is used to compress the helical pitch of the CNC chiral nematic organization. By increasing the force applied ( ≈ 140–230 N), the structural color can be tuned from red to blue. Then, on-demand, the CNC-SMP can recover to its original state by heating it above the glass transition temperature. This cycle can be performed over 15 times without any loss of the shape-memory behavior or mechanical degradation of the sample. In addition, multicolor readouts can be programmed into the chiral nematic CNC-SMP by using a patterned substrate to press the sample, while the glass transition temperature of the CNC-SMP can be tuned over a 90  ° C range by altering the monomer composition used to prepare the polyacrylate matrix.     

Generation of Monodisperse Mesoporous Silica Microspheres with Controllable Size and Surface Morphology in a Microfluidic Device

A one‐step in situ method, termed microfluidic diffusion‐induced self‐assembly, for the synthesis of monodisperse ordered mesoporous silica microspheres, is reported. The method combines microfluidic generation of uniform droplets and subsequent in situ rapid solvent diffusion‐induced self‐assembly within the microfluidic channel. The mesoporous silica microspheres prepared in this way reveal well‐ordered 2D hexagonal mesostructures with unprecedented corrugated surface morphology of disordered mesopores that are larger than 15 nm. It is speculated that the formation of an interfacial subphase and rapid diffusion of solvent to oil are attributed to the formation of the unique surface morphology. It is also shown that the surface morphology and the particle size of the mesoporous silica microspheres can be systematically controlled by adjusting fluidic conditions.     

Self‐Assembly and Characterization of Mesostructured Silica Films with a 3D Arrangement of Isolated Spherical Mesopores

We report the self‐assembly and characterization of mesoporous silica thin films with a 3D ordered arrangement of isolated spherical pores. The preparation method was based on solvent‐evaporation induced self‐assembly (EISA), with MTES (CH₃–Si(OCH₂CH₃)₃) as the silica precursor and a polystyrene‐block‐poly(ethylene oxide) (PS‐b‐PEO) diblock copolymer as the structure‐directing agent. The synthetic approach was designed to suppress the siloxane condensation rate of the siloxane network, allowing co‐self‐assembly of the silica and the amphiphile, followed by retraction of the PEO chains from the silica matrix and matrix consolidation, to occur unimpeded. The calcined films retained the methyl ligands and exhibited no measurable microporosity, thereby indicating that the 3D‐ordered spherical mesopores are not interconnected. A solvent‐mediated formation mechanism is proposed for the absence of microporosity. Due to their closed porosity and hydrophobicity, the MTES‐based films and MTES‐TEOS (Si(OCH₂CH₃)₄)‐based hybrid films we describe should be promising for applications such as low‐k dielectrics.     

Cover Picture: Generation of Monodisperse Mesoporous Silica Microspheres with Controllable Size and Surface Morphology in a Microfluidic Device (Adv. Funct. Mater. 24/2008)

A one‐step in situ method, termed microfluidic diffusion‐induced self‐assembly, for the synthesis of monodisperse ordered mesoporous silica microspheres, is reported. The method combines microfluidic generation of uniform droplets and subsequent in situ rapid solvent diffusion‐induced self‐assembly within the microfluidic channel. The mesoporous silica microspheres prepared in this way reveal well‐ordered 2D hexagonal mesostructures with unprecedented corrugated surface morphology of disordered mesopores that are larger than 15 nm. It is speculated that the formation of an interfacial subphase and rapid diffusion of solvent to oil are attributed to the formation of the unique surface morphology. It is also shown that the surface morphology and the particle size of the mesoporous silica microspheres can be systematically controlled by adjusting fluidic conditions.     

Hybrid Cholesteric Films with Tailored Polarization Rotation

Incorporation of metal nanoparticles in chiral (Ch) films of cellulose nanocrystals (CNCs) enhances nanoparticle plasmon resonances, due to the coupling of the intrinsic properties of the plasmonic guest and the photonic properties of the Ch‐CNC host. In contrast with previous reports, the properties of the Ch‐CNC host are focused and an effective strategy is developed for tuning the optical polarization rotation of the composite films formed by the CNCs and gold nanoparticles. A twofold enhancement in the polarization rotation power of the composite Ch‐CNC films, with an insignificant change in the incurred optical losses, is achieved by varying the density and dimensions of gold nanoparticles embedded in the Ch‐CNC films. For such films, a new approach is developed to obtain broadband circular dichroism by fabricating films from mixtures of CNC suspensions ultrasonicated for different time intervals. These new findings enable fine‐tuning of the power and spectral range of the polarization rotation and offer a novel strategy for the fabrication of broadband reflectors and polarizers, smart solar windows, and detectors for circularly polarized light.     

Tessellation of Chiral‐Nematic Cellulose Nanocrystal Films by Microtemplating

In biological architectures, material properties are optimized by the hierarchical structuring of components with a multiscaled order, from the nano‐ to the macroscales. Such designs enable, for instance, programmed yield points that maximize toughness. However, research efforts in biomimetic materials have focused on the assembly of nano‐ or macrostructures individually. In this study, high strength cellulose nanocrystals (CNCs), assembled into chiral‐nematically ordered structures, are tiled into a higher level, macro‐sized, architecture by topographical templating. As templates, two meshed architectures with distinct feature sizes are evaluated, and the optomechanical properties of the resulting films are compared to featureless, flat, CNC films. Controlling capillary stresses arising during CNC assembly is shown to enable control over the orientation of the chiral‐nematic director across the topography of the template. Tuning the specific reflections and multiscaled fracture propagation is demonstrated for the microtemplated CNC films. The latter phenomenon contributed to enhancing the toughness of the material through a high tortuosity of fracture propagation in all (x, y, z) directions. The presented findings are expected to pave the way towards the incorporation of current research in cellular metamaterials with the research focusing on the generation of nanoscaled biomimetic constructs.     

Preparation of Hollow Zeolite Spheres and Three‐Dimensionally Ordered Macroporous Zeolite Monoliths with Functionalized Interiors

A novel and flexible strategy involving hydrothermal transformation of guest‐incorporated zeolite‐seeded mesoporous silica spheres was proposed to prepare guest‐encapsulated hollow zeolite spheres and three‐dimensionally (3D) ordered macroporous zeolite monoliths. The guest species that were pre‐incorporated into the mesopores of silica spheres could be spontaneously encapsulated inside the formed hollow zeolite shells by consuming silica nutrition of the original mesoporous silica cores during the hydrothermal process. A wide range of guest materials with a size ranging from nanometers to micrometers, e.g., Ag and PdO nanoparticles, and mesoporous spheres of carbon and polymer of micrometer size were successfully encapsulated into both discrete hollow zeolite spheres and 3D ordered macroporous zeolite monoliths. Such materials are expected to find a variety of applications such as catalysis, adsorption, and novel microreactors for their special structures with active species inside and zeolitic porous shell outside.     

Synthesis of Self‐Supported Ordered Mesoporous Cobalt and Chromium Nitrides

Herein, we demonstrate an ammonia nitridation approach to synthesize self‐supported ordered mesoporous metal nitrides (CoN and CrN) from mesostructured metal oxide replicas (Co₃O₄ and Cr₂O₃), which were nanocastly prepared by using mesoporous silica SBA‐15 as a hard template. Two synthetic routes are adopted. One route is the direct nitridation of mesoporous metal oxide nanowire replicas templated from SBA‐15 to metal nitrides. By this method, highly ordered mesoporous cobalt nitrides (CoN) can be obtained by the transformation of Co₃O₄ nanowire replica under ammonia atmosphere from 275 to 350 °C, without a distinct lose of the mesostructural regularity. Treating the samples above 375 °C leads to the formation of metallic cobalt and the collapse of the mesostructure due to large volume shrinkage. The other route is to transform mesostructured metal oxides/silica composites to nitrides/silica composites at 750–1000 °C under ammonia. Ordered mesoporous CrN nanowire arrays can be obtained after the silica template removal by NaOH erosion. A slowly temperature‐program‐decrease process can reduce the influence of silica nitridation and improve the purity of final CrN product. Small‐angle XRD patterns and TEM images showed the 2‐D ordered hexagonal structure of the obtained mesoporous CoN and CrN nanowires. Wide‐angle XRD patterns, HRTEM images, and SAED patterns revealed the formation of crystallized metal nitrides. Nitrogen sorption analyses showed that the obtained materials possessed high surface areas (70–90 m² g^?1) and large pore volumes (about 0.2 cm³ g^?1).     

Hierarchic Nanostructure for Auto‐Modulation of Material Release: Mesoporous Nanocompartment Films

The preparation of mesoporous nanocompartment films composed of both hollow silica capsules and silica particles by using layer‐by‐layer (LbL) adsorption is described. The resultant nanocompartment films exhibit stepwise release of encapsulated water molecules without application of external stimuli. The hollow hierarchic pore structure of the silica capsules, including their internal void and mesoporous walls, is a key factor for the regulation and stepwise release of water, and is probably caused by the non‐equilibrated concurrent evaporation of material from the mesopore and capillary penetration into the mesopores. The number of release steps and rate of release can be tuned by variation of several parameters including water content, ambient temperature, layer multiplicity, and co‐adduct particle size. Application of the mesoporous nanocompartment films for the release of substances, including therapeutic agents and fragrances, indicates that the stepwise material release can be applied for a wide range of liquid substances. The films should lead to a novel material release system useful even for biomedical applications capable of controlled and sustained delivery of drug molecules.     

Ordered Mesoporous Carbon/Fused Silica Composites

A series of novel, dense, and interesting ordered mesoporous carbon (OMC)/fused silica composites with different carbon contents has been prepared by a controllable but simple sol‐gel method followed by hot‐pressing. In the as‐sintered OMC/fused silica composites the carbon particles still exist in the form of perfectly ordered carbon nanowires. Conductivity measurements on the composites indicate that these novel composites are electrically conductive and have a typical percolation threshold of 3.5–5 vol% OMC. The electromagnetic interference (EMI) shielding efficiency (SE) of an OMC/fused silica composite containing 10 vol% OMC is as high as 40 dB in the X band which is higher than that of a carbon nanotube (CNT)/ fused silica composite with the same carbon content (～30 dB). This indicates that these conductive OMC/fused silica composites are very suitable for an application as EMI shielding materials. Upon increasing the volume content of OMC in the composite the overall contribution as well as the increase rate of the microwave absorption are larger than those of the microwave reflection, which suggest that OMC/fused silica composites may also be promising electromagnetic (EM) wave absorbing materials. Based on the promising properties of these composites this work will hopefully lead to the development of new low‐cost and highly efficient EMI shielding or EM wave absorbing materials.     

Inside Front Cover: Three‐Dimensionally Ordered Gold Nanocrystal/Silica Superlattice Thin Films Synthesized via Sol–Gel Self‐Assembly (Adv. Funct. Mater. 7/2006)

The synthesis of three‐dimensionally ordered, transparent gold‐nanocrystal (NC)/silica superlattice thin films using the self‐assembly (by spin‐coating) of water‐soluble gold nanocrystal micelles and soluble silica is reported by Fan and co‐workers on p. 891. The robust, 3D NC/silica superlattice films are of interest for the development of collective optical and electronic phenomena, and, importantly, for the integration of NC arrays into device architectures. Nanocrystals and their ordered arrays hold many important applications in fields such as catalysis, surface‐enhanced Raman spectroscopy based sensors, memory storage, and electronic and optical nanodevices. Herein, a simple and general method to synthesize ordered, three‐dimensional, transparent gold nanocrystal/silica superlattice thin films by self‐assembly of gold nanocrystal micelles with silica or organosilsesquioxane by spin‐coating is reported. The self‐assembly process is conducted under acidic sol–gel conditions (ca. pH 2), ensuring spin‐solution homogeneity and stability and facilitating the formation of ordered and transparent gold nanocrystal/silica films. The monodisperse nanocrystals are organized within inorganic host matrices as a face‐centered cubic mesostructure, and characterized by transmission electron spectroscopy and X‐ray diffraction.