Neil Coombs

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.

Controlling Phase Separation and Optical Properties in Conjugated Polymers through Selenophene-Thiophene Copolymerization

Selenophene-thiophene block copolymers were synthesized and studied. The properties of these novel block copolymers are distinct from those of statistical copolymers prepared from the same monomers with a similar composition. Specifically, the block copolymers exhibit broad and red-shifted absorbance features and phase-separated domains in the solid state. Scanning transmission electron microscopy and topographic elemental mapping confirmed that the domains are either rich in selenophene or thiophene, indicating that the blocks of distinct heterocycles preferentially associate with one another in the solid state. This preference is surprising in view of the chemical similarities between repeat units. The overall results demonstrate a phase separation that is controlled by elemental differences. As a result of this phase separation, these novel conjugated block copolymers should find utility in a variety of studies and optoelectronics uses.

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.

Probing Dynamic Generation of Hot-Spots in Self-Assembled Chains of Gold Nanorods by Surface-Enhanced Raman Scattering

Further progress in the applications of self-assembled nanostructures critically depends on developing a fundamental understanding of the relation between the properties of nanoparticle ensembles and their time-dependent structural characteristics. Following dynamic generation of hot-spots in the self-assembled chains of gold nanorods, we established a direct correlation between ensemble-averaged surface-enhanced Raman scattering and extinction properties of the chains. Experimental results were supported with comprehensive finite-difference time-domain simulations. The established relationship between the structure of nanorod ensembles and their optical properties provides the basis for creating dynamic, solution-based, plasmonic platforms that can be utilized in applications ranging from sensing to nanoelectronics.

Iron nanoparticles catalyzing the asymmetric transfer hydrogenation of ketones.

Investigation into the mechanism of transfer hydrogenation using trans-[Fe(NCMe)CO(PPh(2)C(6)H(4)CH═NCHR-)(2)][BF(4)](2), where R = H (1) or R = Ph (2) (from R,R-dpen), has led to strong evidence that the active species in catalysis are iron(0) nanoparticles (Fe NPs) functionalized with achiral (with 1) and chiral (with 2) PNNP-type tetradentate ligands. Support for this proposition is given in terms of in operando techniques such as a kinetic investigation of the induction period during catalysis as well as poisoning experiments using substoichiometric amounts of various poisoning agents. Further support for the presence of Fe(0) NPs includes STEM microscopy imaging with EDX analysis, XPS analysis, and SQUID magnetometry analysis of catalytic solutions. Further evidence of Fe NPs acting as the active catalyst is given in terms of a polymer-supported substrate experiment whereby the NPs are too large to permeate the pores of a functionalized polymer. Final support is given in terms of a combined poisoning/STEM/EDX experiment whereby the poisoning agent is shown to be bound to the Fe NPs. This paper provides evidence of a rare example of asymmetric catalysis with nonprecious metal, zerovalent nanoparticles.

Synthesis of mesoporous silica spheres under quiescent aqueous acidic conditions

A gyroid-to-sphere shape transition has been unveiled in the growth of mesoporous silica morphologies that are synthesized under quiescent acidic aqueous conditions. It can be induced by a decrease of the acidity for a surfactant-based gyroid preparation. As the acidity is gradually lowered from the gyroid domain, the growth process changes from one involving a smooth continuous deposition of silicate–surfactant micellar solute species onto specific regions of an evolving silicate liquid crystal seed, to one in which deposition instead occurs on non-specific regions of the seed. This creates multigranular gyroid morphologies which at lower acidity emerge as sphere shapes. The gyroid-to-sphere metamorphosis appears to correlate with an acidity and/or temperature dependent switch in the mode of formation, from the gyroid involving fast and local polymerization of a growing silicate liquid crystal seed, to the sphere based upon a slower and global polymerization of a silicate liquid crystal droplet. Surface tension will cause such a droplet to adopt a spherical shape, ultimately to be rigidified in the form of a mesoporous silica sphere. Comparative gyroid and sphere information is presented on synthesis-size-shape-channel plan relations, degree of orientational order of the channels, extent of polymerization of the silica, thermal stability and nitrogen adsorption properties. The ability to synthesize 1–10 µm diameter mesoporous silica spheres with a narrow sphere size and pore size distribution portends a myriad of applications in large molecule catalysis, chromatographic separations and nanocomposites.

Ultrathin Gold Nanoframes through Surfactant-Free Templating of Faceted Pentagonal Silver Nanoparticles

Ultrathin gold nanoframes (up to 1.6 nm) were prepared via templating upon well-defined faceted silver morphologies. Starting with silver decahedra, small quantities of gold (1-10 mol% relative to the amount of silver) were selectively deposited on the nanoparticle edges under optimized reducing conditions. Silver dissolution in hydrogen peroxide yielded well-defined gold frames that retained their structural integrity in the ultrathin nanowire regime below 2 nm. The frame formation protocol was also successfully applied to other silver nanoparticle shapes featuring pentagonal twinning and (111) facets (e.g., pentagonal faceted rods and icosahedra). The demonstrated approach can be applied in the controlled preparation of ultrathin metal nanowires complementary to lithography and in the production of ultrafine noble-metal nanostructures for catalytic applications.

Chiral thiol-stabilized silver nanoclusters with well-resolved optical transitions synthesized by a facile etching procedure in aqueous solutions.

A novel approach of cyclic reduction in oxidative conditions has been developed to prepare a single dominant species of chiral thiol-stabilized silver nanoclusters (AgNCs). Such AgNCs, which are stable in solution for up to a few days, have been obtained for the first time. The generality of the established procedure is proven by using several enantiomeric water-soluble thiols, including glutathione, as protective ligands. The prepared AgNCs featured prominent optical properties including a single pattern of UV-vis absorption with well-resolved peaks. The chirality of the clusters has been investigated by circular dichroism (CD) spectroscopy. CD spectra displayed strong characteristic signatures in the visible range. Tentative identification of the cluster composition is discussed.

Formation of Hollow Helicoids in Mesoporous Silica: Supramolecular Origami

In the past, helical shapes in nature have inspired inventions such as the water screw for agriculture, the retaining screw for wine presses, and architectural designs for spiral staircases. Similarly, these days helix-shaped DNA, proteins and carbon nanotubes evoke great interest in biotechnology and nanotechnology. Also biomimetic synthesis of helical morphologies of calcium carbonate, barium sulfate, and silica provides insight into morphogenesis of mineralized spiral forms in biology and ideas for new opportunities in materials science. Herein we describe the synthesis of hollow helicoids made of hexagonal mesoporous silica, a remarkable topology in the materials world. They have a hierarchical architecture comprised of 5 nm diameter channels that coil in the form of a micrometerscale tubular spiral. A population analysis of helicoid shapes defines a surprisingly narrow distribution of pitch and flute widths, pitch angles, inside and outside diameters, and significantly an equal number of leftand right-handed forms. Evidence is presented that morphogenesis involves polymerization-induced differential contraction of a patch of hexagonal silicate liquid-crystal film formed at the air± water interface, which can fold into a hollow helicoid. A supramolecular Origami theoretical model explains the creation and observed narrow distribution of mesoporous silica, hollow helicoid shapes. Mesoporous silica hollow helicoids were prepared by using cetyltrimethylammonium chloride (CTACl) as the surfactant micellar template and tetraethylorthosilicate (TEOS) as the silica precursor. An aqueous solution of CTACl, hydrochloric acid and formamide was aged for 48 h before adding TEOS, and the material was formed after 3 days in a quiescent state. The use of formamide in the synthesis is intentional because upon acid hydrolysis it yields ammonium chloride and formic acid to give an ultimate solution ca. pH 1.9 and an ionic strength that favors hollow helicoid formation. This solution pH is notably higher than the one used in the synthesis of mesoporous silica curved shapes. Control experiments demonstrate that a high concentration of ammonium and formate ions is essential for the formation of mesoporous silica at a pH close to two, which borders on the isoelectric point of aqueous silica. We believe that a low acidity and high ionic strength medium favor a slow rate of silicification, and hence polymerization-induced differential contraction of silicate micelle rods in a patch of silicate liquid-crystal film formed at the air±water interface becomes influential in hollow helicoid formation. Powder X-ray diffraction (PXRD) patterns in Figure 1 clearly define as-synthesized and calcined

Graphene Oxide−Periodic Mesoporous Silica Sandwich Nanocomposites with Vertically Oriented Channels

This paper describes the synthesis and characterization of single-layer graphene oxide-periodic mesoporous silica sandwich nanocomposites. Through a comprehensive exploration of the synthesis conditions, it has proven possible to create the first example of a graphene oxide-periodic mesoporous silica nanocomposite in which hexagonal symmetry PMS film grows on both sides of the graphene oxide sheets with the mesoporous channels vertically aligned with respect to the graphene oxide surface. The formation of this novel architecture is found to be very sensitive to pH, the ratio of surfactant template to graphene oxide, the amount of silica precursor, and the temperature of the synthesis. On the basis of the collected data of a multi-technique analysis, it is proposed that the mode of formation of the nanocomposite involves the co-assembly of silicate-surfactant admicelles on opposite sides of graphene oxide platelets acting thereby as a template for growth of vertical mesopores off the platelet surface. These composites showed semiconductive behavior with electrical conductivity sensitively responding to analyte vapor exposure. The discovery of graphene oxide-periodic mesoporous silica sandwich nanocomposites will provide new opportunities for research that exploits the synergism of the graphene oxide and periodic mesoporous silica parts.