Justin C. Lytle

Nickel ferrite aerogels with monodisperse nanoscale building blocks--the importance of processing temperature and atmosphere.

Using two-step (air/argon) thermal processing, sol-gel-derived nickel-iron oxide aerogels are transformed into monodisperse, networked nanocrystalline magnetic oxides of NiFe(2)O(4) with particle diameters that can be ripened with increasing temperature under argon to 4.6, 6.4, and 8.8 nm. Processing in air alone yields poorly crystalline materials; heating in argon alone leads to single phase, but diversiform, polydisperse NiFe(2)O(4), which hampers interpretation of the magnetic properties of the nanoarchitectures. The two-step method yields an improved model system to study magnetic effects as a function of size on the nanoscale while maintaining the particles within the size regime of single domain magnets, as networked building blocks, not agglomerates, and without stabilizing ligands capping the surface.

The importance of combining disorder with order for Li-ion insertion into cryogenically prepared nanoscopic ruthenia

Cryogenically prepared RuO2 (cryo-RuO2), a material known for its ability to “self-wire” into continuous, nanoscopic electronic pathways, is proposed as an electrode for Li-ion microbatteries with three-dimensionally interpenetrated components. We determined processing guidelines that optimize Li-ion uptake in cryo-RuO2 powders by varying the solid-state structure of cryo-RuO2 with thermal processing at 50–250 °C in flowing O2(g) or Ar(g). The highly disordered structure of as-prepared cryo-RuO2 is transformed to rutile RuO2 at 200 °C in O2(g), resulting in a 60% loss of Li-ion capacity (as-prepared: 214 mA h g−1; rutile: 84 mA h g−1). In contrast, thermal processing in Ar(g) preserves structural disorder in the cryo-RuO2, even up to 250 °C. The highest Li-ion capacity occurs for the treatment that mixes order (crystallinity) with disorder: >250 mA h g−1 for cryo-RuO2 heated in oxygen to 50 °C. This study provides processing guidelines to achieve fabrication of 3-D microbattery architectures containing a nanoscopic RuO2 electrode component.

Rewriting Electron-Transfer Kinetics at Pyrolytic Carbon Electrodes Decorated with Nanometric Ruthenium Oxide

Platinum is state-of-the-art for fast electron transfer whereas carbon electrodes, which have semimetal electronic character, typically exhibit slow electron-transfer kinetics. But when we turn to practical electrochemical devices, we turn to carbon. To move energy devices and electro(bio)analytical measurements to a new performance curve requires improved electron-transfer rates at carbon. We approach this challenge with electroless deposition of disordered, nanoscopic anhydrous ruthenium oxide at pyrolytic carbon prepared by thermal decomposition of benzene (RuOx@CVD-C). We assessed traditionally fast, chloride-assisted ([Fe(CN)6]3-/4-) and notoriously slow ([Fe(H2O)6]3+/2+) electron-transfer redox probes at CVD-C and RuOx@CVD-C electrodes and calculated standard heterogeneous rate constants as a function of heat treatment to crystallize the disordered RuOx domains to their rutile form. For the fast electron-transfer probe, [Fe(CN)6]3-/4-, the rate increases by 34× over CVD-C once the RuOx is calcined to form crystalline rutile RuO2. For the classically outer-sphere [Fe(H2O)6]3+/2+, electron-transfer rates increase by an even greater degree over CVD-C (55×). The standard heterogeneous rate constant for each probe approaches that observed at Pt but does so using only minimal loadings of RuOx.