"Space—the final frontier." This preamble to a well-known television series captures the challenge encountered not only in space travel adventures, but also in the field of porous materials, which aims to control the size, shape and uniformity of the porous space and the atoms and molecules that define it. The past decade has seen significant advances in the ability to fabricate new porous solids with ordered structures from a wide range of different materials. This has resulted in materials with unusual properties and broadened their application range beyond the traditional use as catalysts and adsorbents. In fact, porous materials now seem set to contribute to developments in areas ranging from microelectronics to medical diagnosis.

Ordered porous materials for emerging applications

Pore size determination in modified micro- and mesoporous materials. Pitfalls and limitations in gas adsorption data analysis

Polyaniline nanofibers with uniform diameters between 30 and 50 nm can be made in bulk quantities through a facile aqueous/organic interfacial polymerization method at ambient conditions. The nanofibers have lengths varying from 500 nm to several micrometers and form interconnected networks. Thin films made of the nanofibers have superior performance in both sensitivity and time response to vapors of acid (HCl) and base (NH3).

Polyaniline nanofibers: facile synthesis and chemical sensors.

The concept of shape-controlled synthesis is discussed by investigating the growth mechanisms for silver nanocubes, nanowires, and nanospheres produced through a polymer-mediated polyol process. Experimental parameters, such as the concentration of AgNO(3) (the precursor to silver), the molar ratio between poly(vinylpyrrolidone) (PVP, the capping agent) and AgNO(3), and the strength of chemical interaction between PVP and various crystallographic planes of silver, were found to determine the crystallinity of seeds (e.g., single crystal versus decahedral multiply twinned particles). In turn, the crystallinity of a seed and the extent of the PVP coverage on the seed were both instrumental in controlling the morphology of final product. The ability to generate silver nanostructures with well-defined morphologies provides a great opportunity to experimentally and systematically study the relationship between their properties and geometric shapes.

Shape-controlled synthesis of metal nanostructures: the case of silver.

A solution-phase approach has been demonstrated for the large-scale synthesis of silver nanowires with diameters in the range of 30−40 nm, and lengths up to ∼50 μm. The first step of this process involved the formation of Pt (or Ag) nanoparticles by reducing PtCl2 (or AgNO3) with ethylene glycol (EG) heated to ∼160 °C. These Pt (or Ag) nanoparticles could serve as seeds for the heterogeneous nucleation and growth of silver (formed by reducing AgNO3 with EG) because of their close match in crystal structure and lattice constants. In the presence of poly(vinyl pyrrolidone) (PVP), the growth of silver could be directed into a highly anisotropic mode to form uniform nanowires with aspect ratios as high as ∼1000. UV−visible spectroscopy, SEM, TEM, XRD, and electron diffraction were used to characterize these silver nanowires, indicating the formation of a highly pure phase, as well as a uniform diameter and bicrystalline structure. Both morphology and aspect ratios of these silver nanostructures could be varie...

Uniform silver nanowires synthesis by reducing AgNO3 with ethylene glycol in the presence of seeds and poly(vinyl pyrrolidone)

Pure‐Silica Zeolite Low‐k Dielectric Thin Films

Conductive polyaniline nanowires in emeraldine form can be deposited by potentiodynamic electropolymerization from a reverse hexagonal liquid crystalline phase where one-dimensional (1D) aqueous channels can serve as space-confined reactors. Polyaniline nanowire bundles with single-wire diameter of 50–70 nm and length of several microns were obtained from surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reverse hexagonal liquid crystalline phase. Impedance measurement suggests improved ordering of the reverse hexagonal liquid crystal under external electric field during the electropolymerization process. This enhanced ordering appears essential for the nanowire growth. The nanowires obtained can be easily collected by a simple washing process.

Polyaniline nanowires by electropolymerization from liquid crystalline phases

Pure Silica Zeolite Films as Low‐k Dielectrics by Spin‐On of Nanoparticle Suspensions

One-dimensional (1D) nanowires and nanowire arrays of metal, semiconductor, and conductive polymers have attracted much attention because of their unique electrical, magnetic, optical, and mechanical properties, and their potential application in nanodevices. Although other successful approaches such as vapor±solid and liquid±solid phase methods exist for silicon and other semiconductor nanowires, the most widely used method for fabrication of nanowires and nanowire arrays is still chemical or electrochemical deposition guided by an appropriate porous ahardo template such as anodized alumina, track-etched polycarbonate and mica, carbon nanotubes, zeolite/mesoporous silica, and diblock copolymers. Step edges of freshly cleaved graphite and a conducting quantum well in a semiconductor multilayer prepared by molecular beam epitaxy also have been used as nanowire growth guides. The hard template approach is an effective route to metal, semiconductor, and polymer nanowires; however, most of the templates are tedious to fabricate and dissolution of the template in corrosive media is required to retrieve the nanowires if separate single wires are desired. Surfactant mesophases have proved to be a useful and versatile asofto template for the synthesis of nanostructured materials. For example, syntheses of porous zeolite-type materials using normal (oil in water) surfactant mesophases as a template are well established. Reverse (water in oil) micelles and microemulsions with spherical aqueous microdomains have been used for nanoparticle and sometimes nanowire synthesis (mainly compounds such as BaSO4 and BaCrO4). [21±24] All of the aforementioned syntheses use low surfactant concentrations where liquid-crystalline phases are not present. In contrast, at high surfactant concentrations, homogeneous normal or reverse hexagonal liquid-crystalline phases can be obtained. Attard et al. and others have demonstrated that normal hexagonal liquid crystals can template the synthesis of bulk porous materials and porous metal films by electrodeposition. Very recently, a reverse hexagonal liquid crystal was used as a template to synthesize ZnS nanowires by gamma ray irradiation. The ZnS nanowires so obtained are fairly short (<2 lm) probably due to the random orientation of the reverse hexagonal liquid crystal. In this communication, we demonstrate for the first time fabrication of crystalline silver nanowire arrays with a high aspect ratio by electrodeposition from a reverse hexagonal liquid-crystalline phase containing one-dimensional aqueous channels. We provide evidence that a high electric field during electrodeposition helps align the reverse hexagonal liquidcrystalline phase, which is believed to be essential for production of high aspect ratio crystalline metal nanowires. For growth of silver nanowires, a reverse hexagonal liquidcrystalline phase was first prepared according to the wellknown ternary phase diagram consisting of the anionic surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT), an oil phase of p-xylene, and water. For our synthesis the water phase was substituted by an aqueous 0.1 M AgNO3 solution. The resulting mixture has the characteristic birefringence of a hexagonal liquid crystal when viewed under a polarized light microscope. The electrodeposition was conducted by using a potentiostat in a two-electrode configuration with the two electrodes narrowly spaced (0.5±1.0 mm) and the aforementioned reverse liquid-crystalline phase as electrolyte. The nanowire product deposited on the cathode substrate (polished stainless steel) was thoroughly washed with ethanol. High-density nanowire arrays were obtained over a short deposition time of 15 min with the nanowires roughly perpendicular to the cathode surface (Fig. 1A). The slight disorder in the nanowire array may arise from the washing process after deposition. After a deposition time of 2 h, nanowires tens of micrometers long were obtained (Fig. 1B). The nanowires were almost parallel (no longer perpendicular) to the electrode surface, most probably due to the washing process after deposition. The high-density nanowire array can be separated into single wires after ultrasonic dispersion in ethanol (Fig. 1C), indicating that the nanowires can be used in a bundle form or as single wires for nanodevices. Figure 2 shows the X-ray diffraction (XRD) pattern of the silver nanowire arrays deposited on a stainless steel substrate. It shows the diffraction peaks with d-spacings of 2.36 , 2.04 , 1.44 , 1.23 , and 1.18 , which are consistent with that of face centered cubic (fcc) Ag metal phase. Energy dispersive X-ray (EDX) analysis shows that the wires are made of pure silver, suggesting that surfactant molecules can be totally removed from the nanowire arrays by a simple washing process. Transmission electron microscopy (TEM) images and electron diffraction of nanowires are shown in Figure 3. Figures 3A and 3B show a bright field (BF) and dark field (DF) image, respectively, of a fragment of a single silver nanowire. The entire crystal is bright in DF mode with operating reflection (11Å1), confirming the single crystal nature of the nanowire. The individual wire is either a single crystal or consists of symmetrically oriented domains that are twinned on {111} planes. The twinned nanowire crystals are made of multiple

Nanowire Arrays Electrodeposited from Liquid Crystalline Phases

Cuprite (Cu2O) nanowires with diameter of 25−100 nm were electrodeposited from anionic surfactant sodium bis(2-ethylhexyl) sulfosuccinate reverse hexagonal liquid crystalline phase. The nanowires can be grown to up to tens of micrometers in length by simply changing the electrodeposition time. Resistivity measurements suggest improved alignment of the reverse hexagonal liquid crystalline phase under electric field during electrodeposition. The enhanced alignment of the liquid crystalline phase appears essential for the growth of nanowires with high aspect ratio. This new synthesis approach is fast and simple, and a great variety of organized assemblies are potentially available by simply adjusting the liquid crystal type and composition.

Anupam Mitra

Papers

Pure‐Silica Zeolite Low‐k Dielectric Thin Films

Polyaniline nanowires by electropolymerization from liquid crystalline phases

Pure Silica Zeolite Films as Low‐k Dielectrics by Spin‐On of Nanoparticle Suspensions

Nanowire Arrays Electrodeposited from Liquid Crystalline Phases

Cuprite Nanowires by Electrodeposition from Lyotropic Reverse Hexagonal Liquid Crystalline Phase