On the performance of a hierarchically porous Ag2S-CuxS electrode in Li-ion batteries.

A new binder- and carbon-free electrode for lithium-ion batteries was prepared using a hierarchically porous Ag-based current collector. The latter was produced by applying the method of selective dissolution of the less noble metals from the Cu60Ag30Al10 master alloy tape. The current collector was reaction-coated with an electrochemically active Ag2S-CuxS coating. The metallic structure provided a mechanically stable conductive scaffold on the walls of which the Ag2S-CuxS skin material was directly deposited. The ordered porosity - hierarchical and directional - provided easy penetration of the liquid electrolyte as well as short Li+ ion diffusion paths. The as-prepared electrodes were tested in a half-cell configuration vs. Li/Li+ at various current rates to study the cycling and rate performances of the electrode. The first cycling capacity of ∼1250 mA h g-1 was measured at 0.4 A g-1 current rate. After a rapid decrease, a stable reversible capacity of ∼230 mA h g-1 was established at a current rate of 0.4 A g-1 (calculated vs. the weight of the incorporated sulphur). Excellent charge/discharge cycling and rate properties were observed for over 1000 cycles at higher rates of 1.0 and 2.0 A g-1, in the potential window of 0.15-2.8 V vs. Li/Li+. The observed cycling stability was ascribed to the mechanism of a "displacement" reaction with Li ions. Additional capacity is also available from alloying-dealloying with Ag (and Cu to some extent) and S redox reactions. These results open up a new opportunity for using a Cu-Ag alloy as the precursor for making electrodes for thin Li-ion and Li-S batteries with high cycling stability at relatively high current rates.

Stabilization of boron carbide via silicon doping.

Boron carbide is one of the lightest and hardest ceramics, but its applications are limited by its poor stability against a partial phase separation into separate boron and carbon. Phase separation is observed under high non-hydrostatic stress (both static and dynamic), resulting in amorphization. The phase separation is thought to occur in just one of the many naturally occurring polytypes in the material, and this raises the possibility of doping the boron carbide to eliminate this polytype. In this work, we have synthesized boron carbide doped with silicon. We have conducted a series of characterizations (transmission electron microscopy, scanning electron microscopy, Raman spectroscopy and x-ray diffraction) on pure and silicon-doped boron carbide following static compression to 50 GPa non-hydrostatic pressure. We find that the level of amorphization under static non-hydrostatic pressure is drastically reduced by the silicon doping.

Unusual infrared-absorption mechanism in thermally reduced graphene oxide.

Infrared absorption of atomic and molecular vibrations in solids can be affected by electronic contributions through non-adiabatic interactions, such as the Fano effect. Typically, the infrared-absorption lineshapes are modified, or infrared-forbidden modes are detectable as a modulation of the electronic absorption. In contrast to such known phenomena, we report here the observation of a giant-infrared-absorption band in reduced graphene oxide, arising from the coupling of electronic states to the asymmetric stretch mode of a yet-unreported structure, consisting of oxygen atoms aggregated at the edges of defects. Free electrons are induced by the displacement of the oxygen atoms, leading to a strong infrared absorption that is in phase with the phonon mode. This new phenomenon is only possible when all other oxygen-containing chemical species, including hydroxyl, carboxyl, epoxide and ketonic functional groups, are removed from the region adjacent to the edges, that is, clean graphene patches are present.

Suspended multiwalled carbon nanotubes as self-aligned evaporation masks.

We describe the nanofabrication study of self-aligned electrodes on suspended multiwalled carbon nanotube structures. When metal is deposited on a suspended multiwalled carbon nanotube structure, the nanotube acts as an evaporation mask, resulting in the formation of discontinuous electrodes. The metal deposits on the nanotubes are removed with lift-off. Using Al sacrificial layers, it was possible to fabricate self-aligned contact electrodes and control electrodes nanometers from the suspended carbon nanotubes with a single lithography step. It was also shown that the fabrication technique may also be used to form nano-gapped contact electrodes. The technique should prove useful for the fabrication of nano-electromechanical systems.

Synthesis of carbon 'onions' in water.

The fabrication of carbon nanomaterials usually calls for expensive vacuum systems to generate plasmas and yields are disappointingly low. Here we describe a simple method for producing high-quality spherical carbon nano-'onions' in large quantities without the use of vacuum equipment. The nanoparticles, which have C60 cores surrounded by onion-like nested particles, are generated by an arc discharge between two graphite electrodes submerged in water. This technique is economical and environmentally benign, and produces uncontaminated nanoparticles which may be useful in many applications.

Thin films of fullerene-like MoS2 nanoparticles with ultra-low friction and wear

The tribological properties of solid lubricants such as graphite and the metal dichalcogenides MX2 (where M is molybdenum or tungsten and X is sulphur or selenium) are of technological interest for reducing wear in circumstances where liquid lubricants are impractical, such as in space technology, ultra-high vacuum or automotive transport. These materials are characterized by weak interatomic interactions (van der Waals forces) between their layered structures, allowing easy, low-strength shearing. Although these materials exhibit excellent friction and wear resistance and extended lifetime in vacuum, their tribological properties remain poor in the presence of humidity or oxygen, thereby limiting their technological applications in the Earth's atmosphere. But using MX2 in the form of isolated inorganic fullerene-like hollow nanoparticles similar to carbon fullerenes and nanotubes can improve its performance. Here we show that thin films of hollow MoS2 nanoparticles, deposited by a localized high-pressure arc discharge method, exhibit ultra-low friction (an order of magnitude lower than for sputtered MoS2 thin films) and wear in nitrogen and 45% humidity. We attribute this 'dry' behaviour in humid environments to the presence of curved S-Mo-S planes that prevent oxidation and preserve the layered structure.