Porous metal oxides as gas sensors

Semiconducting metal oxides are frequently used as gas-sensing materials. Apart from large surface-to-volume ratios, well-defined and uniform pore structures are particularly desired for improved sensing performance. This article addresses the role of some key structural aspects in porous gas sensors, such as grain size and agglomeration, pore size or crack-free film morphology. New synthesis concepts, for example, the utilisation of rigid matrices for structure replication, allow to control these parameters independently, providing the opportunity to create self-diagnostic sensors with enhanced sensitivity and reproducible selectivity.     

Rigid nanoscopic containers for highly dispersed, stable metal and bimetal nanoparticles with both size and site control

We demonstrate a novel strategy for the preparation of mesoporous silica-supported, highly dispersed, stable metal and bimetal nanoparticles with both size and site control. The supporting mesoporous silica, functionalized by polyaminoamine (PAMAM) dendrimers, is prepared by repeated Michael addition with methyl acrylates (MA) and amidation reaction with ethylenediamine (EDA), by using aminopropyl-functionalized mesoporous silica as the starting material. The encapsulation of metal nanoparticles within the dendrimer-propagated mesoporous silica is achieved by the chemical reduction of metal-salt-impregnated dendrimer-mesoporous silica by using aqueous hydrazine. The site control of the metal or bimetal nanoparticles is accomplished by the localization of inter- or intradendrimeric nanoparticles within the mesoporous silica tunnels. The size of the encapsulated nanoparticles is controlled by their confinement to the nanocavity of the dendrimer and the mesopore. For Cu and Pd, particles locate at the lining of mesoporous tunnels, and have diameters of less than 2.0 nm. For Pd/Pt, particles locate at the middle of mesoporous tunnels and have diameters in the range of 2.0-4.2 nm. The Pd and Pd/Pt nanoparticles are very stable in air, whereas the Cu nanoparticles are stable only in an inert atmosphere.     

Hybrid Hydrogels of Porous Graphene and Nickel Hydroxide as Advanced Supercapacitor Materials

Graphene‐based hydrogels can be used as supercapacitor electrodes because of their excellent conductivity, their large surface area and their high compatibility with electrolytes. Nevertheless, the large aspect ratio of graphene sheets limits the kinetics of processes occurring in the electrode of supercapacitors. In this study, we have introduced in‐plane and out‐of‐plane pores into a graphene–nickel hydroxide (Ni(OH)₂) hybrid hydrogel, which facilitates charge and ion transport in the electrode. Due to its optimised chemistry and architecture, the hybrid electrode demonstrates excellent electrochemical properties with a combination of high charge storage capacitance, fast rate capability and stable cycling performance. Remarkably, the Ni(OH)₂ in the hybrid contributes a capacitance as high as 3138.5 F g^?1, which is comparable to its theoretical capacitance, suggesting that such structure facilitates effectively charge‐transfer reactions in electrodes. This work provides a facile pathway for tailoring the porosity of graphene‐based materials for improved performances. Moreover, this work has also furthered our understanding in the effect of pore and hydrogel structures on the electrochemical properties of materials.     

Fabrication of nanofibers with uniform morphology by self-assembly of designed peptides

Fabrication of controlled peptide nanofibers with homogeneous morphology has been demonstrated. Amphiphilic beta-sheet peptides were designed as sequences of Pro-Lys-X(1)-Lys-X(2)-X(2)-Glu-X(1)-Glu-Pro. X(1) and X(2) were hydrophobic residues selected from Phe, Ile, Val, or Tyr. The peptide FI (X(1)=Phe; X(2)=Ile) self-assemble into straight fibers with 80-120 nm widths and clear edges, as examined by transmission electron microscopy (TEM) and atomic force microscopy (AFM). The fiber formation is performed in a hierarchical manner: beta-sheet peptides form a protofibril, the protofibrils assemble side-by-side to form a ribbon, and the ribbons then coil in a left-handed fashion to make up a straight fiber. These type of fibers are formed from peptides possessing hydrophobic aromatic Phe residue(s). Furthermore, a peptide with Ala residues at both N and C termini does not form fibers (100 nm scale) with clear edges; this causes random aggregation of small pieces of fibers instead. Thus, the combination of unique amphiphilic sequences and terminal Pro residues determine the fiber morphology.     

Dual‐Cargo Selectively Controlled Release Based on a pH‐Responsive Mesoporous Silica System

A pH‐controlled delivery system based on mesoporous silica nanoparticles (MSNs) was constructed for dual‐cargo selective release. To achieve a better controlled‐release effect, a modified sol–gel method was employed to obtain MSNs with tunable particle and pore sizes. The systems selectively released different kinds of cargo when stimulated by different pH values. At the lower pH value (pH 2.0) only one kind of cargo was released from the MSNs, whereas at a higher pH value (pH 7.0) only the other kind of cargo was released from the MSNs. The multi‐cargo delivery system has brought the concept of selective release to new advances in the field of functional nanodevices and allows more accurate and controllable delivery of specific cargoes, which is expected to have promising applications in nanomedicine.     

Borate‐Driven Gatelike Scaffolding Using Mesoporous Materials Functionalised with Saccharides

We report the development of an MCM‐41 mesoporous support that is functionalised with saccharides at the pore outlets and contains the dye [Ru(bipy)₃]²⁺ in the pores (solid S1 ; bipy=2,2′‐bipyridyl). For this hybrid system, the inhibition of mass transport of the dye from the pore voids to the bulk solution in the presence of borate is demonstrated in water at neutral pH. The formation of the corresponding boroester derivative is related to the selective reaction of borate with the appended saccharides. This control is selective and only anion borate, among several anions and cations, can act as a molecular tap and inhibit the delivery of the entrapped guest. Additionally, the S1 –borate system behaves as pH‐controlled gatelike scaffolding. This pH‐responsive release can be achieved in an acidic pH (due to hydrolysis of the boroester), whereas the system remains closed at neutral pH. Molecular dynamic simulations using force‐field methods have been made to theoretically study the open/close borate‐driven mechanism. A mesoporous silica structure was constructed for this purpose, taking the plane (1?11) of the β‐cristobalite structure as a base on which hexagonal nanopores and anchored saccharide derivatives were included. The final model shows a highly flexible nanopore diameter of approximately 12.5 Å of similar size to the [Ru(bipy)₃]²⁺ complex (ca. 12 Å). However, the anchoring of borate to the appended saccharides results in a remarkable reduction of the pore size (down to ca. 6.4 Å) and a significant constraint in the flexibility and mobility of the saccharides. The theoretical calculations are in agreement with the experimental results and enable visualisation of the functional borate‐driven dye‐delivery‐inhibition outcome.     

Controllable Synthesis of Mesoporous Co3O4 Nanostructures with Tunable Morphology for Application in Supercapacitors

Flower power : Various mesoporous Co₃O₄ architectural structures (see figure) have been successfully prepared through a facile binary‐solution route and sequential thermal decomposition at atmospheric pressure. The electrochemical experiments showed that the specific capacitance of Co₃O₄ nanosheets was higher than that of Co₃O₄ microspheres in a KOH electrolyte.

     

Supercritical fluid synthesis of metal and semiconductor nanomaterials

In the near future physical and economic constraints are expected to limit the continued miniaturisation of electronic and optical devices using current "top-down" lithography-based methods. Consequently, nonlithographic methods for synthesising and organising materials on the nanometre scale are required. In response to these technological needs a number of research groups are developing new supercritical fluids methodologies to synthesise and self-assemble "building blocks" of nanomaterials, from the "bottom-up", into structurally complex device architectures. This concept paper highlights some of the recent advances in the synthesis of metal and semiconductor nanoparticles and nanowires by using supercritical fluids. In addition, we describe an efficient supercritical fluid approach for constructing ordered arrays of metal and semiconductor nanowires within mesoporous silica templates.     

When Mesoporous Silica Meets the Alkaline Polyelectrolyte: A Controllable Synthesis of Functional and Hollow Nanostructures with a Porous Shell

An efficient and environment friendly surface‐protected etching method by using mesoporous silica as a template and alkaline polyelectrolyte as both the protecting and etching agent was developed to prepare a SiO₂ nanotube with a porous shell. The polyelectrolytes attached to the template not only create a localized alkaline environment, but also effectively protect the silica surface, whereas the mesopore channels accelerate the diffusion of etchant throughout the template, all of which facilitate the formation of hollow structures in a fully controllable way. By tuning the etching power and protecting ability of the polyelectrolyte, the rigidity and porosity of products can be precisely manipulated. It is inspiring that various alkaline polyelectrolytes including polypeptide and dextran derivative can be used for the etching process, so that the porous and hollow nanostructures are born with positively charged and biocompatible surface as well as abundant amino groups for further coupling, which make them potential capsules for drug delivery and probes for imaging and detection. The protective etching process can also be extended to the preparation of yolk‐shell super structures with functional cores, or porous nanoparticle assemblies with their individual characteristics maintained.