Prof. Oren Regev
Mesophase templated nanostructures, polymer-nanoparticles interactions.

The main emphasis of Prof. Regev research group is on the creation of controlled nanostructures and inclusion or trapping of active ingredients in these structures. These include mesoporous materials with applications in chemical biochemical catalysis, interaction between nanotubes and their surroundings and use of surface active molecules to form solid nanowires. The most salient activities of Prof. Regev group are briefly summarized below:

1. Gold nanoparticles integrated in onion-type multilamellar vesicles
Gold nanoparticles have been synthesized in onion-type multilamellar vesicles using vesicles components as the reductant. Particles size and shape depend on whether the gold salt is introduced directly in the vesicles or by diffusion from the external dispersion medium (see Fig. 1). A close coupling between nanoparticle morphology and lamellar phase is evidenced by cryo-TEM imaging.

Fig. 1: Cryo-TEM image of gold nanoparticles grown in onions dispersed in gold salt for 12 hours. Black arrows: lamellar defects around nanoparticles, White arrows: bending of bilayers. Bar = 200 nm.

2. Mesoporous material
A novel family of mesoporous molecular sieves (M41-S) has recently been reported by scientists of Mobil Oil Research and Development. During its synthesis a base- or acid-catalyzed polymerization of inorganic compounds (silica in most cases) takes place around surfactant micelles that serve as a templating agent. At the end of the synthesis, a silica matrix imbedded by micelles namely, a mesoporous material, is obtained. Its mechanism of formation is not yet completely understood. We have studied the mechanism of formation and the phase transition sequence occurring during the synthesis of bicontinuous cubic mesoporous phase. It was found that the changes in the surfactant packing parameter, dictated by the time and temperature-dependent reaction parameters, result in a hexagonal-lamellar-hexagonal-cubic phase transition sequence. In this study, we presented a small angle X-ray scattering (SAXS) in-situ study of a new preparation procedure of the bicontinuous cubic (Ia3d) phase in which an organic base is used as a reaction catalyst. The coexisting hexagonal and lamellar phases detected at an early reaction stage could indicate their importance in the formation of a cubic (Ia3d) mesophase. We found that the final cubic phase is formed only when heat is applied. The cubic phase is formed by the collapse of a preceding hexagonal phase, where a given hexagonal plane (1 0 0) evolves to a cubic plane (2 1 1) having the same interplanar distance.

3. Conductive nanotube - polymer composites using latex technology
Nanotubes are promising fillers for a polymer matrix and are expected to enhance both electrical and mechanical properties of composites at extremely low loading due to their high aspect ratio and inherent properties. We describe experiments with composites consisting of individual, or bundle of few single-wall, exfoliated nanotubes in a highly viscous polymer matrix. Using latex technology we compatibilize the nanotube and the polymer by surfactant molecules, which disperse both the nanotubes and the latex, hence avoiding the need for direct attraction between the nanotubes and the polymer matrix. The latter have previously limited the spectrum of polymer type of the matrix, or required nanotube functionalization (impairing their properties) to assure their dispersion in the matrix. We show that a homogeneous network of nanotubes is integrated within a polymer matrix of choice, giving rise to low resistivity with a conductivity percolation threshold as low as 0.28 wt% nanotubes (see Fig. 2).

Fig. 2: Resistivity measurements of composite films containing SDS-dispersed SWNT in PS (open squares) and in PMMA (solid squares) matrix. GA-dispersed SWNT in PS (solid triangles).
PS/SDS, PMMA/SDS=30:1 w/w, PS/GA=30:5 w/w

4. Hierarchically Ordered CdS Nanowires dispersed in Aqueous Solution
This work demonstrates the formation of cadmium sulfide (CdS) nanowires. These nanowires are templated by mesoporous SBA-15 and their connectivity is tuned by the number of nanoconnectors (templated by micropores) (cf. Fig. 3). They form stable dispersions in aqueous sodium dodecyl sulfate (SDS) solutions.

 

Fig 3. TEM image of silica-free CdS nanowires calcined at 773K. The dashed zone in (a) is magnified in (b) and further in (c). The black circles indicate the presence of nanoconnectors between the crystalline nanowires. Inset in (c) shows EDS pattern recorded to determine the chemical composition of the CdS nanowires