Mark J. MacLachlan

Periodic mesoporous organosilicas with organic groups inside the channel walls

Surfactant-mediated synthesis methods have attracted much interest for the production of inorganic mesoporous materials, which can, on removal of the surfactant template, incorporate polymeric, organic, inorganic and organometallic ‘guests’ in their pores. These materials—initially made of silica, but now also available in the form of other oxides, sulphides, phosphates and metals—could find application in fields ranging from catalysis, adsorption and sensing technology to nanoelectronics. The extension of surfactant-mediated synthesis to produce inorganic–organic hybrid material (that is, materials that contain organic groups as an integral part of their framework structure) promises access to an even wider range of application possibilities. Such hybrid materials have been produced in the form of amorphous silicates (xerogels) that indeed display unique properties different to those of the individual components, but their random networks with broad pore-size distributions severely limit the shape and size selectivity of these materials. Mesoporous hybrid materials with periodic frameworks have been synthesized, but the organic groups are all terminally bonded to the pore surface, rather than incorporated into the pore walls. Here we describe a periodic mesoporous organosilica containing bridge-bonded ethene groups directly integrated into the silica framework. We are able to solvent-extract and ion-exchange the surfactant templates to create a stable and periodic mesoporous ethenesilica with high surface area and ethene groups that are readily accessible for chemical reaction. Recent syntheses of similar periodic mesoporous organosilicas and the ability to incorporate a variety of bridging organic and organometallic species raise the prospect of being able to fuse organic synthesis and inorganic materials chemistry to generate new materials with interesting chemical, mechanical electronic, optical and magnetic properties.

Free-standing mesoporous silica films with tunable chiral nematic structures

Chirality at the molecular level is found in diverse biological structures, such as polysaccharides, proteins and DNA, and is responsible for many of their unique properties. Introducing chirality into porous inorganic solids may produce new types of materials that could be useful for chiral separation, stereospecific catalysis, chiral recognition (sensing) and photonic materials. Template synthesis of inorganic solids using the self-assembly of lyotropic liquid crystals offers access to materials with well-defined porous structures, but only recently has chirality been introduced into hexagonal mesostructures through the use of a chiral surfactant. Efforts to impart chirality at a larger length scale using self-assembly are almost unknown. Here we describe the development of a photonic mesoporous inorganic solid that is a cast of a chiral nematic liquid crystal formed from nanocrystalline cellulose. These materials may be obtained as free-standing films with high surface area. The peak reflected wavelength of the films can be varied across the entire visible spectrum and into the near-infrared through simple changes in the synthetic conditions. To the best of our knowledge these are the first materials to combine mesoporosity with long-range chiral ordering that produces photonic properties. Our findings could lead to the development of new materials for applications in, for example, tuneable reflective filters and sensors. In addition, this type of material could be used as a hard template to generate other new materials with chiral nematic structures.

Non-aqueous supramolecular assembly of mesostructured metal germanium sulphides from (Ge4S10)4- clusters

Microporous materials have found extensive application as catalysts, ion-exchange media and sorbents. The discovery of mesoporous silica has opened the path to selective catalysis and separation of large molecules and to the synthesis of inorganic–organic composite materials, polymer mesofibres and semiconducting quantum dots. Various oxide-based mesoporous materials, such as TiO2, ZrO2, SnO2, Al2O3, Nb2O5 and GeO2, have been reported. A challenge for materials research is now to expand the scope of mesoporous materials beyond the oxides. Only a few non-oxide mesostructured composites, such as CdS, SnS2 and CdSe, have been reported; they are usually synthesized by ad hoc hydrothermal methods or from aqueous solutions containing ill-defined species, and are often not well characterized. Herewe report the rational synthesis of a new family of metal germanium sulphide mesostructured materials prepared by a non-aqueous surfactant-templated assembly of adamantanoid [Ge4S10]4− cluster precursors. In the presence of quaternary alkylammonium surfactants, [Ge4S10]4− anions in formamide solution self-organize with metal cations (Co2+, Ni2+, Cu+ and Zn2+) to create well ordered hexagonal metal germanium sulphide mesostructures, some having fibre-like morphologies with channels running down the long axis of the fibre. Materials of this genre could prove effective in applications as diverse as detoxification of heavy metals in polluted water streams, sensing of sulphurous vapours, and the formation of semiconductor quantum ‘anti-dot’ devices.

Chiral Nematic Mesoporous Carbon Derived From Nanocrystalline Cellulose

Template synthesis based on the self-assembly of lyotropic liquid crystals offers access to mesoporous solids with high specific surface areas and periodic structures. Incorporating mesopores (i.e., pores ranging from 2 to 50 nm in diameter) into carbonaceous materials may be advantageous for certain applications, including the adsorption of large molecules, electrochemical double-layer capacitors, lithium ion batteries, catalyst supports, and field-effect transistors. Ordered mesoporous carbon materials were first synthesized by using ordered mesoporous silica as a hard template. 7] In the hardtemplating (also referred to as nanocasting) approach, mesoporous silica is repeatedly infiltrated with a suitable carbon precursor (e.g., sucrose) that is carbonized within the pores of the silica at elevated temperature. After sufficient pore loading and etching of the silica, mesoporous carbon with a structure that is the inverse of the original silica template is obtained. Despite the potential benefits of using mesoporous carbon over traditional activated carbon, the cost of making these materials may be prohibitive. Finding more economical synthetic routes, both in terms of the number of steps involved and precursors used, is important if mesoporous carbon is to be implemented in new technologies. Direct surfactant-templating approaches (soft templating) have also been developed for the synthesis of mesoporous carbon by condensing polymerizable carbon precursors (e.g., phenolic resins) around block copolymer templates. Soft templating requires fewer synthetic steps than hard templating and offers improved control over the morphology of the mesoporous carbon products. For example, free-standing mesoporous carbon membranes have been synthesized through evaporation-induced self-assembly coupled with soft templating. The specific surface areas of these films, however, are considerably less than those of mesoporous carbons produced by hard templating. The use of both hardand soft-templating approaches has enabled mesoporous carbon to be synthesized with cubic and hexagonal pore systems that are ultimately derived from the self-assembly of surfactants into ordered mesophases. The synthesis of mesoporous carbon templated by other liquid-crystal phases, for example nematic and chiral nematic phases, has been virtually unexplored. In particular, the incorporation of chiral organization into mesoporous carbon could open the door for applications that involve enantioselective adsorption. Chiral nematic liquid crystals, which consist of mesogens organized into a long-range helical assembly, exhibit unique properties, such as the selective reflection of circularly polarized light. The incorporation of chiral nematic organization into solid-state materials could give rise to novel properties. Kyotani and co-workers have synthesized graphitic carbon with chiral nematic ordering by first polymerizing polyacetylene within a thermotropic chiral nematic liquid crystal followed by doping with iodine and pyrolysis. It is expected that these materials will display interesting electromagnetic properties. As the major constituent of plant cell walls, cellulose is the most abundant biological material on the planet. Recently, there has been significant interest in the study of cellulose fibrils with nanometer dimensions that have high surface area and can behave as lyotropic liquid crystals. Stable suspensions of nanocrystalline cellulose (NCC) can be obtained through hydrolysis of bulk cellulosic material with sulfuric acid. In water, suspensions of NCC organize into a chiral nematic phase that can be preserved upon slow evaporation, thereby resulting in chiral nematic films. The unique physical properties and natural abundance of NCC make it attractive as a potential template for porous materials. Although bulk cellulosic materials are commonly used to generate activated carbon, to date there have been no studies on the use of NCC as a template for mesoporous carbon. Recently, our research group reported that evaporationinduced self-assembly of NCC with different silica precursors can result in composite films with chiral nematic structures, and that the removal of NCC from these films generates chiral nematic mesoporous silica. Herein we report that NCC– silica composite films may also be used to generate mesoporous carbon with a high specific surface area and excellent retention of the chiral nematic organization. This provides the first example of using nanocrystalline cellulose as a template for mesoporous carbon as well as the first demonstration of a mesoporous carbon with chiral nematic ordering. We demonstrate that the use of silica is necessary for both the introduction of mesoporosity and the preservation of the [*] K. E. Shopsowitz, Prof. Dr. M. J. MacLachlan Department of Chemistry, University of British Columbia 2036 Main Mall, Vancouver, BC, V6T 1Z1 (Canada) E-mail: mmaclach@chem.ubc.ca

The Development of Chiral Nematic Mesoporous Materials

Cellulose nanocrystals (CNCs) are obtained from the sulfuric acid-catalyzed hydrolysis of bulk cellulose. The nanocrystals have diameters of ~5-15 nm and lengths of ~100-300 nm (depending on the cellulose source and hydrolysis conditions). This lightweight material has mostly been investigated to reinforce composites and polymers because it has remarkable strength that rivals carbon nanotubes. But CNCs have an additional, less explored property: they organize into a chiral nematic (historically referred to as cholesteric) liquid crystal in water. When dried into a thin solid film, the CNCs retain the helicoidal chiral nematic order and assemble into a layered structure where the CNCs have aligned orientation within each layer, and their orientation rotates through the stack with a characteristic pitch (repeating distance). The cholesteric ordering can act as a 1-D photonic structure, selectively reflecting circularly polarized light that has a wavelength nearly matching the pitch. During CNC self-assembly, it is possible to add sol-gel precursors, such as Si(OMe)4, that undergo hydrolysis and condensation as the solvent evaporates, leading to a chiral nematic silica/CNC composite material. Calcination of the material in air destroys the cellulose template, leaving a high surface area mesoporous silica film that has pore diameters of ~3-10 nm. Importantly, the silica is brilliantly iridescent because the pores in its interior replicate the chiral nematic structure. These films may be useful as optical filters, reflectors, and membranes. In this Account, we describe our recent research into mesoporous films with chiral nematic order. Taking advantage of the chiral nematic order and nanoscale of the CNC templates, new functional materials can be prepared. For example, heating the silica/CNC composites under an inert atmosphere followed by removal of the silica leaves highly ordered, mesoporous carbon films that can be used as supercapacitor electrodes. The composition of the mesoporous films can be varied by using assorted organosilica precursors. After removal of the cellulose by acid-catalyzed hydrolysis, highly porous, iridescent organosilica films are obtained. These materials are flexible and offer the ability to tune the chemical and mechanical properties through variation of the organic spacer. Chiral nematic mesoporous silica and organosilica materials, obtainable as centimeter-scale freestanding films, are interesting hosts for nanomaterials. When noble metal nanoparticles are incorporated into the pores, they show strong circular dichroism signals associated with their surface plasmon resonances that arise from dipolar coupling of the particles within the chiral nematic host. Fluorescent conjugated polymers show induced circular dichroism spectra when encapsulated in the chiral nematic host. The porosity, film structure, and optical properties of these materials could enable their use in sensors. We describe the development of chiral nematic mesoporous silica and organosilica, demonstrate different avenues of host-guest chemistry, and identify future directions that exploit the unique combination of properties present in these materials. The examples covered in this Account demonstrate that there is a rich diversity of composite materials accessible using CNC templating.

Chiral nematic assemblies of silver nanoparticles in mesoporous silica thin films.

Silver nanoparticles (NPs) have been synthesized inside mesoporous silica films with chiral nematic structure. Circular dichroism measurements of the silver NP-loaded silica films show NP-based optical activity in the vicinity of the surface plasmon resonance. These materials, with an optical response associated with the chiral assembly of metal NPs, may be useful for developing new sensors.

Functional materials from cellulose-derived liquid-crystal templates.

Cellulose nanocrystals (CNCs), known for more than 50 years, have attracted attention because of their unique properties such as high specific strength and modulus, high surface area, and fascinating optical properties. Just recently, however, their potential in supramolecular templating was identified by making use of their self-assembly behavior in aqueous dispersions in the presence of compatible precursors. The combination of the mesoporosity, photonic properties, and chiral nematic order of the materials, which are available as freestanding films, has led to a significant number of interesting and promising discoveries towards new functional materials. This Review summarizes the use of cellulose derivatives, especially CNCs, as novel templates and gives an overview of the recent developments toward new functional materials.

Flexible and iridescent chiral nematic mesoporous organosilica films.

Nanocrystalline cellulose (NCC) has been used to template ethylene-bridged mesoporous organosilica films with long-range chirality and photonic properties. The structural color of the organosilica films results from their chiral nematic ordering, can be varied across the entire visible spectrum, and responds to the presence of chemicals within the mesopores. To synthesize these materials, acid hydrolysis was used to remove the NCC template without disrupting the organosilica framework. The resulting mesoporous organosilica films are much more flexible than brittle mesoporous silica films templated by NCC. These materials are the first of a novel family of chiral mesoporous organosilicas with photonic properties.

Hard Templating of Nanocrystalline Titanium Dioxide with Chiral Nematic Ordering

Anatase TiO(2) nanocrystals have been organized into high-surface-area (150-230 m(2) g(-1)) mesoporous films with long-range chiral nematic ordering. The chiral structure of the anatase films causes them to selectively reflect circularly polarized light and appear iridescent. These materials show replication of structural features found in the silica template on nanometer to millimeter length scales.

Oriented Periodic Mesoporous Organosilica (PMO) Film with Organic Functionality Inside the Channel Walls

The first examples of an oriented periodic mesoporous organosilica (PMO) film, containing a variety of organic groups (ethane, ethene, benzene, thiophene) inside the channel walls, are reported. The mesostructure of the PMO film appears oriented with respect to the surface of the underlying glass substrate. Liquid-crystal topological defects in the precursor gels are replicated in the resulting PMO film and are evident in polarized optical microscopy images, recorded between crossed-polarizers, which show fan-type optical birefringence texture characteristic of the mesostructure.