High-yield synthesis of silicon carbide nanowires by solar and lamp ablation.

We report a reasonably high yield (~50%) synthesis of silicon carbide (SiC) nanowires from silicon oxides and carbon in vacuum, by novel solar and lamp photothermal ablation methods that obviate the need for catalysis, and allow relatively short reaction times (~10 min) in a nominally one-step process that does not involve toxic reagents. The one-dimensional core/shell ß-SiC/SiOx nanostructures-characterized by SEM, TEM, HRTEM, SAED, XRD and EDS-are typically several microns long, with core and outer diameters of about 10 and 30 nm, respectively. HRTEM revealed additional distinctive nanoscale structures that also shed light on the formation pathways.

New high-temperature Pb-catalyzed synthesis of inorganic nanotubes.

A new procedure for the synthesis of MoS(2) nanotubes is reported, and additionally demonstrated for MoSe(2), WS(2), and WSe(2). Highly concentrated sunlight creates continuous high temperatures, strong temperature gradients, and extended hot annealing regions, which, together with a metallic (Pb) catalyst, are conducive to the formation of different inorganic nanotubes. Structural characterization (including atomic resolution images) reveals a three-step reaction mechanism. In the first step, MoS(2) platelets react with water-air residues, decompose by intense solar irradiation, and are converted to molybdenum oxide. Subsequently, the hot annealing environment leads to the growth of Pb-stabilized MoO(3-x) nanowhiskers. Shortly afterward, the surface of the MoO(3-x) starts to react with the sulfur vapor supplied by the decomposition of nearby MoS(2) platelets and becomes enveloped by MoS(2) layers. Finally, the molybdenum oxide core is gradually transformed into MoS(2) nanotubes. These findings augur well for similar syntheses of as yet unattained nanotubes from other metal chalcogenides.

Hollow V(2)O(5) nanoparticles (fullerene-like analogues) prepared by laser ablation.

Nanoparticles of materials with layered structure are able to spontaneously form closed-cage nanostructures such as nested fullerene-like nanoparticles and nanotubes. This propensity has been demonstrated in a large number of compounds such as WS(2), NiCl(2), and others. Layered metal oxides possess a higher ionic character and consequently are stiffer and cannot be evenly folded. Vanadium pentoxide (V(2)O(5)), a layered metal oxide, has received much attention due to its attractive qualities in numerous applications such as catalysis and electronic and optical devices and as an electrode material for lithium rechargeable batteries. The synthesis by pulsed laser ablation (PLA) of V(2)O(5) hollow nanoparticles, which are closely (nearly) associated with inorganic "fullerene-like" (NIF-V(2)O(5)) nanoparticles, but not quite as perfect, is reported in the present work. The relation between the PLA conditions and the NIF-V(2)O(5) morphology is elucidated. A new mechanism leading to hollow nanostructure via crystallization of lower density amorphous nanoparticles is proposed. Transmission electron microscopy (TEM) is used extensively in conjunction with structural modeling of the NIF-V(2)O(5) in order to study the complex 3-D structure of the NIF-V(2)O(5) nanoparticles. This structure was shown to be composed of facets with their low-energy surfaces pointing outward and seamed by defective domains. These understandings are used to formulate a formation mechanism and may improve the function of V(2)O(5) in its many uses through additional morphological control. Furthermore, this study outlines which properties are required from layered compounds to fold into perfectly closed-cage IF nanoparticles.

Core-shell PbI2@WS2 inorganic nanotubes from capillary wetting.

Multiwall WS(2) nanotube templates were used as hosts to prepare core-shell PbI(2)@WS(2) nanotubes by a capillary-wetting method. Conformal growth of PbI(2) layers on the inner wall of the relatively wide WS(2) nanotubes (i.d. ca. 10 nm) leads to nanotubular structures which were not previously observed in narrow carbon nanotube templates. Image simulation after structural modeling (see picture) showed good agreement with the experimental HRTEM image.

Solar Synthesis of PbS-SnS2 Superstructure Nanoparticles.

We report the synthesis and supporting density-functional-theory computations for a closed-cage, misfit layered-compound superstructure from PbS-SnS2, generated by highly concentrated sunlight from a precursor mixture of Pb, SnS2, and graphite. The unique reactor conditions created in our solar furnace are found to be particularly conducive to the formation of these nanomaterials. Detailed structural and chemical characterization revealed a spontaneous inside-out formation mechanism, with a broad range of nonhollow fullerene-like structures starting at a diameter of ∼20 nm and a wall thickness of ∼5 layers. The computations also reveal a counterintuitive charge transfer pathway from the SnS2 layers to the PbS layers, which indicates that, in contrast to binary-layered compounds where it is principally van der Waals forces that hold the layers together, polar forces appear to be as important in stabilizing superstructures of misfit layered compounds.

Stability Criteria of Fullerene-like Nanoparticles: Comparing V2O5 to Layered Metal Dichalcogenides and Dihalides.

Numerous examples of closed-cage nanostructures, such as nested fullerene-like nanoparticles and nanotubes, formed by the folding of materials with layered structure are known. These compounds include WS2, NiCl2, CdCl2, Cs2O, and recently V2O5. Layered materials, whose chemical bonds are highly ionic in character, possess relatively stiff layers, which cannot be evenly folded. Thus, stress-relief generally results in faceted nanostructures seamed by edge-defects. V2O5, is a metal oxide compound with a layered structure. The study of the seams in nearly perfect inorganic "fullerene-like" hollow V2O5 nanoparticles (NIF-V2O5) synthesized by pulsed laser ablation (PLA), is discussed in the present work. The relation between the formation mechanism and the seams between facets is examined. The formation mechanism of the NIF-V2O5 is discussed in comparison to fullerene-like structures of other layered materials, like IF structures of MoS2, CdCl2, and Cs2O. The criteria for the perfect seaming of such hollow closed structures are highlighted.