Jeannine R. Szczech

Nanostructured silicon for high capacity lithium battery anodes

Nanostructured silicon is promising for high capacity anodes in lithium batteries. The specific capacity of silicon is an order of magnitude higher than that of conventional graphite anodes, but the large volume change of silicon during lithiation and delithiation and the resulting poor cyclability has prevented its commercial application. This challenge could potentially be overcome by silicon nanostructures that can provide facile strain relaxation to prevent electrode pulverization, maintain effective electrical contact, and have the additional benefits of short lithium diffusion distances and enhanced mass transport. In this review, we present an overview of rechargeable lithium batteries and the challenges and opportunities for silicon anodes, then survey the performance of various morphologies of nanostructured silicon (thin film, nanowires/nanotubes, nanoparticles, and mesoporous materials) and their nanocomposites. Other factors that affect the performance of nanostructured silicon anodes, including solvent composition, additives, binders, and substrates, are also examined. Finally, we summarize the key lessons from the successes so far and offer perspectives and future challenges to enable the applications of silicon nanoanodes in practical lithium batteries at large scale.

Enhancement of the thermoelectric properties in nanoscale and nanostructured materials

Thermoelectric materials can be used for solid state power generation and heating/cooling applications. The figure of merit of thermoelectric materials, ZT, which determines their efficiency in a thermoelectric device, remains low for most conventional bulk materials. Nanoscale and nanostructured thermoelectric materials are promising for increasing ZT relative to the bulk. This review introduces the theory behind thermoelectric materials and details the predicted and demonstrated enhancements of ZT in nanoscale and nanostructured thermoelectric materials. We discuss thin films and superlattices, nanowires and nanotubes, and nanocomposites, providing a ZT comparison among various families of nanocomposite materials. We provide some perspectives regarding the origin of enhanced ZT in nanoscale and nanostructured materials and suggest some promising and fruitful research directions for achieving high ZT materials for practical applications.

Synthesis and applications of metal silicide nanowires

Transition metal silicides represent an extremely broad set of refractory materials that are currently employed for many applications including CMOS devices, thin film coatings, bulk structural components, electrical heating elements, photovoltaics, and thermoelectrics. Many of these applications may be improved by making 1-dimensional nanomaterials. Chemical synthesis of silicide nanowires is more complicated compared to other classes of nanomaterials due to the complex phase behaviour between metals and silicon and the complex stoichiometries and structures of their resulting compounds. Recently, several synthetic strategies have been developed to overcome this challenge resulting in increasing reports of silicide nanowires in the literature. These strategies are highlighted in this feature article, along with future synthetic challenges and a review of the applications emerging from current silicide nanowires.

Mesoporous zirconium oxide nanomaterials effectively enrich phosphopeptides for mass spectrometry-based phosphoproteomics

This work represents the first use of mesoporous zirconium oxide nanomaterials for highly effective and selective enrichment of phosphorylated peptides.

Epitaxially-hyperbranched FeSi nanowires exhibiting merohedral twinning

We report the synthesis of epitaxially-hyperbranched FeSi nanowires via chemical vapor transport using FeSi2 as the source material and I2 as the transport agent. Scanning electron microscopy reveals that the nanowires have diameters ranging from 25 to 1000 nm, depending on the morphology. Structural characterization using electron diffraction, energy dispersive spectroscopy, and powder X-ray diffraction reveal that these nanowires are single-crystalline cubic FeSi with growth in the direction. X-Ray photoelectron spectroscopy shows that the thin, amorphous coating on these nanostructures is comprised primarily of silicon oxide. Interestingly, these FeSi nanowires exhibit merohedral twinning, an uncommon type of twinning in nanostructures that cannot be observed using electron diffraction, with the (001) twin plane parallel to the growth direction. Such merohedral twinning should generally be expected for all B20 silicide nanowires. In addition to nanowires, other morphologies including nanocombs, nanoflowers, and micron-sized crystals are also observed during the synthesis at various temperature zones of the growth substrates.

Stabilization of iron(VI) ferrate cathode materials using nanoporous silica coatings

Ferrate salts containing Fe(VI) have received attention as cathode materials in recent years due to their theoretical ability to accept three electrons while being reduced to Fe(III). Unfortunately, ferrate salts are also somewhat unstable, particularly when stored at elevated temperatures or in the presence of an alkaline electrolyte. In this paper, we document the stability of solid barium and potassium ferrate salts under various environmental conditions and report on the use of SiO 2 thin-film coatings to stabilize cathodes composed of solid barium ferrate. The nanoporous coatings are deposited from colloidal silica suspensions using sol-gel techniques. The enhanced stability of coated ferrates is demonstrated, and their discharge performance is characterized relative to uncoated materials. The coating technique employed may be applicable to other nanoparticulate metal oxide chemistries, thus presenting a possible method to modify ferrates and perhaps overcome their stability limitations.

Synthesis of mesoporous Si1−xGexO2 (0.10 ≤ x ≤ 0.31) using a nonionic block copolymer template

We have successfully synthesized mesoporous silica-germania mixed oxide (Si1−xGexO2) with x up to 0.31 by controlling the reaction to delay hydrolysis and condensation until the precursors have sufficiently ordered around the nonionic templating agent. Small-angle X-ray diffraction (SAXS) and transmission electron microscopy (TEM) reveal disordered worm-like mesopores for all germanium concentrations investigated, with pore periodicities of 9.8 and 10.5 nm for x ≈ 0.10 and 0.20 respectively. We confirm that the germanium and silicon are homogenous on the nanoscale using scanning transmission electron microscopy (STEM) with energy dispersive X-ray (EDX) mapping. Attempts to convert the mixed mesoporous oxides to mesoporous Si1−xGex alloys via magnesiothermic reduction resulted in phase segregation.