Freestanding carbon nanofoam papers with tunable porosity as lithium-sulfur battery cathodes.

To reach energy density demands greater than 3 mA h cm-2 for practical applications, the electrode structure of lithium-sulfur batteries must undergo an architectural redesign. Freestanding carbon nanofoam papers derived from resorcinol-formaldehyde aerogels provide a three-dimensional conductive mesoporous network while facilitating electrolyte transport. Vapor-phase sulfur infiltration fully penetrates >100 µm thick electrodes and conformally coats the carbon aerogel surface providing areal capacities up to 4.1 mA h cm-2 at sulfur loadings of 6.4 mg cm-2. Electrode performance can be optimized for energy density or power density by tuning sulfur loading, pore size, and electrode thickness.

Redox Cycling within Nanoparticle-Nucleated Protein Superstructures: Electron Transfer between Nanoparticulate Gold, Molecular Reductant, and Cytochrome c.

We previously described how thousands of the heme protein cytochrome c (cyt.c) self-organize into multilayered, roughly spherical superstructures as initiated by nucleation around one colloidal gold or silver nanoparticle. Within these superstructures, the protein is stabilized to unfolding in buffered media and survives superstructure encapsulation within silica gels and processing to form bioaerogels. We now report that Au∼cyt.c superstructures in buffered media are not simply static groupings of proteins, but that the Au core and protein corona exhibit dynamic electron-transfer reactions within the superstructure as verified by UV-visible and resonance Raman spectroscopy. Within the superstructure, hundreds to thousands of ferricytochrome c (FeIII-cyt.c) are reduced to ferrocytochrome c (FeII-cyt.c) following first-order kinetics with an average apparent forward rate constant of 1.9 ±0.4 × 10-5 s-1. The reducing power in the microheterogeneous medium is derived from two multielectron reductants: tannic acid used to stabilize the commercial gold sol and the Au nanoparticle at the center of the protein superstructure. Fluorescence monitoring of guanidinium chloride-induced unfolding reveals that superstructure-associated cyt.c is stabilized to unfolding before and after chemical reduction of FeIII-cyt.c to form FeII-cyt.c, indicating that the superstructures remain intact during microheterogeneous redox reactions. Smaller nucleating Au nanoparticles or lower ionic strength in the buffered medium yields a greater extent of cyt.c reduction. Partial oxidation of the cyt.c-associated nanoparticulate Au is verified by X-ray photoelectron spectroscopy. The Au nanoparticle at the heart of the superstructure functions as a direct electron donor to the heme with oxidized Au atoms being recycled back to Au(0) as long as residual tannic acid, derived from the Au sol mother liquor, is present in the aqueous microheterogeneous medium.

Enhancing Li-ion capacity and rate capability in cation-defective vanadium ferrite aerogels via aluminum substitution.

Cation-defective iron oxides have proven to be effective Li-ion charge-storage hosts in nonaqueous electrolytes, particularly when expressed in disordered, nanoscale forms such as aerogels. Replacing a fraction of Fe sites in ferrites with high-valent cations such as V5+ introduces cation-vacancy defects that increase Li-ion capacity. Herein, we show that compositional substitution with electroinactive Al3+ further increases Li-ion capacity by 30% when incorporated within a disordered VFe2Ox aerogel, as verified by electrochemical tests in a two-terminal Li half-cell. We use electroanalytical techniques to show that both Al-VFe2Ox and VFe2Ox aerogels exhibit many of the hallmarks of pseudocapacitive materials, including fast charge-discharge and surface-controlled charge-storage kinetics. These disordered, substituted ferrites also provide the high specific capacity expected from battery-type electrode materials, up to 130 mA h g-1 for Al-VFe2Ox. Our findings are discussed in the context of related Li-insertion hosts that blur the distinctions between battery-like and capacitor-like behavior.

Zinc-Sponge Battery Electrodes that Suppress Dendrites.

We report two methods to create zinc-sponge electrodes that suppress dendrite formation and shape change for rechargeable zinc batteries. Both methods are characterized by creating a paste made of zinc particles, organic porogen, and viscosity-enhancing agent that is heated under an inert gas and then air. During heating under the inert gas, the zinc particles anneal together, and the porogen decomposes; under air, the zinc fuses and residual organic burns out, yielding an open-cell metal foam or sponge. We tune the mechanical and electrochemical properties of the zinc sponges by varying zinc-to-porogen mass ratio, heating time under inert gas and air, and size and shape of the zinc and porogen particles. An advantage of the reported methods is their ability to finely tune zinc-sponge architecture. The selected size and shape of the zinc and porogen particles influence the morphology of the pore structure. A limitation is that resulting sponges have disordered pore structures that result in low mechanical strength at low volume fractions of zinc (<30%). Applications for these zinc-sponge electrodes include batteries for grid-storage, personal electronics, electric vehicles, and electric aviation. Users can expect zinc-sponge electrodes to cycle up to 40% depth of discharge at technologically relevant rates and areal capacities without the formation of separator-piercing dendrites.

Differentiating Double-Layer, Psuedocapacitance, and Battery-like Mechanisms by Analyzing Impedance Measurements in Three Dimensions.

Electrochemical energy storage arises from processes that are broadly categorized as capacitive, pseudocapacitive, or battery-like. Advanced charge-storing materials that are designed to deliver high capacity at a high rate often exhibit a multiplicity of such mechanisms, which complicates the understanding of their charge-storage behavior. Herein, we apply a "3D Bode analysis" technique to identify key descriptors for fast Li-ion storage processes, where AC impedance data, such as the real capacitance (C') or phase angle (ϕ), are represented versus the frequency (f) and a third independent variable, the applied DC cell voltage. For double-layer processes, a near-constant C' or ϕ is supported across the entire voltage range, and the decrease in these values shows a near-linear decrease at higher f. For pseudocapacitance, an increase in C' is delivered, accompanied by high C' retention at higher f compared to double-layer processes. Interestingly, the lower ϕ values, where C' is highest, suggest that this is a key descriptor for pseudocapacitance, where high-rate charge storage is still facilitated within a kinetically limited regime. For battery-like processes, a high C' is only observed at the voltage at which the material stores charge, while outside that voltage, C' is negligible. The three-dimensional (3D) Bode analysis allows charge-storage dynamics to be mapped out in great detail with more delineation between mechanisms compared to the more frequently deployed kinetic analyses derived from cyclic voltammetry.

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.

Electroanalytical Assessment of the Effect of Ni:Fe Stoichiometry and Architectural Expression on the Bifunctional Activity of Nanoscale NiyFe1-yOx.

Electrocatalysis of the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) was assessed for a series of Ni-substituted ferrites (NiyFe1-yOx, where y = 0.1 to 0.9) as expressed in porous, high-surface-area forms (ambigel and aerogel nanoarchitectures). We then correlate electrocatalytic activity with Ni:Fe stoichiometry as a function of surface area, crystallite size, and free volume. In order to ensure in-series comparisons, calcination at 350 °C/air was necessary to crystallize the respective NiyFe1-yOx nanoarchitectures, which index to the inverse spinel structure for Fe-rich materials (y ≤ 0.33), rock salt for the most Ni-rich material (y = 0.9), and biphasic for intermediate stoichiometry (0.5 ≤ y ≤ 0.67). In the intermediate Ni:Fe stoichiometric range (0.33 ≤ y ≤ 0.67), the OER current density at 390 mV increases monotonically with increasing Ni content and increasing surface area, but with different working curves for ambigels versus aerogels. At a common stoichiometry within this range, ambigels and aerogels yield comparable OER performance, but do so by expressing larger crystallite size (ambigel) versus higher surface area (aerogel). Effective OER activity can be achieved without requiring supercritical-fluid extraction as long as moderately high surface area, porous materials can be prepared. We find improved OER performance (η decreases from 390 to 373 mV) for Ni0.67Fe0.33Ox aerogel heat-treated at 300 °C/Ar, owing to an increase in crystallite size (2.7 to 4.1 nm). For the ORR, electrocatalytic activity favors Fe-rich NiyFe1-yOx materials; however, as the Ni-content increases beyond y = 0.5, a two-electron reduction pathway is still exhibited, demonstrating that bifunctional OER and ORR activity may be possible by choosing a nickel ferrite nanoarchitecture that provides high OER activity with sufficient ORR activity. Assessing the catalytic activity requires an appreciation of the multivariate interplay among Ni:Fe stoichiometry, surface area, crystallographic phase, and crystallite size.

Rechargeable nickel-3D zinc batteries: An energy-dense, safer alternative to lithium-ion.

The next generation of high-performance batteries should include alternative chemistries that are inherently safer to operate than nonaqueous lithium-based batteries. Aqueous zinc-based batteries can answer that challenge because monolithic zinc sponge anodes can be cycled in nickel-zinc alkaline cells hundreds to thousands of times without undergoing passivation or macroscale dendrite formation. We demonstrate that the three-dimensional (3D) zinc form-factor elevates the performance of nickel-zinc alkaline cells in three fields of use: (i) >90% theoretical depth of discharge (DODZn) in primary (single-use) cells, (ii) >100 high-rate cycles at 40% DODZn at lithium-ion-commensurate specific energy, and (iii) the tens of thousands of power-demanding duty cycles required for start-stop microhybrid vehicles.

CAD/CAM-designed 3D-printed electroanalytical cell for the evaluation of nanostructured gas-diffusion electrodes.

The ability to effectively screen and validate gas-diffusion electrodes is critical to the development of next-generation metal-air batteries and regenerative fuel cells. The limiting electrode in a classic two-terminal device such as a battery or fuel cell is difficult to discern without an internal reference electrode, but the flooded electrolyte characteristic of three-electrode electroanalytical cells negates the prime function of an air electrode-a void volume freely accessible to gases. The nanostructured catalysts that drive the energy-conversion reactions (e.g., oxygen reduction and evolution in the air electrode of metal-air batteries) are best evaluated in the electrode structure as-used in the practical device. We have designed, 3D-printed, and characterized an air-breathing, thermodynamically referenced electroanalytical cell that allows us to mimic the Janus arrangement of the gas-diffusion electrode in a metal-air cell: one face freely exposed to gases, the other wetted by electrolyte.

Manganese Oxide Nanoarchitectures as Broad-Spectrum Sorbents for Toxic Gases.

We demonstrate that sol-gel-derived manganese oxide (MnOx) nanoarchitectures exhibit broad-spectrum filtration activity for three chemically diverse toxic gases: NH3, SO2, and H2S. Manganese oxides are synthesized via the reaction of NaMnO4 and fumaric acid to form monolithic gels of disordered, mixed-valent Na-MnOx; incorporated Na(+) is readily exchanged for H(+) by subsequent acid rinsing to form a more crystalline H-MnOx phase. For both Na-MnOx and H-MnOx forms, controlled pore-fluid removal yields either densified, yet still mesoporous, xerogels or low-density aerogels (prepared by drying from supercritical CO2). The performance of these MnOx nanoarchitectures as filtration media is assessed using dynamic-challenge microbreakthrough protocols. We observe technologically relevant sorption capacities under both dry conditions and wet (80% relative humidity) for each of the three toxic industrial chemicals investigated. The Na-MnOx xerogels and aerogels provide optimal performance with the aerogel exhibiting maximum sorption capacities of 39, 200, and 680 mg g(-1) for NH3, SO2, and H2S, respectively. Postbreakthrough characterization using X-ray photoelectron spectroscopy (XPS) and diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS) confirms that NH3 is captured and partially protonated within the MnOx structure, while SO2 undergoes oxidation by the redox-active oxide to form adsorbed sulfate at the MnOx surface. Hydrogen sulfide is also oxidized to form a combination of sulfate and sulfur/polysulfide products, concomitant with a decrease in the average Mn oxidation state from 3.43 to 2.94 and generation of a MnOOH phase.

Retaining the 3D framework of zinc sponge anodes upon deep discharge in Zn-air cells.

We fabricate three-dimensional zinc electrodes from emulsion-cast sponges of Zn powder that are thermally treated to produce rugged monoliths. This highly conductive, 3D-wired aperiodic scaffold achieves 740 mA h gZn(-1) when discharged in primary Zn-air cells (>90% of theoretical Zn capacity). We use scanning electron microscopy and X-ray diffraction to monitor the microstructural evolution of a series of Zn sponges when oxidized in Zn-air cells to specific depths-of-discharge (20, 40, 60, 80% DOD) at a technologically relevant rate (C/40; 4-6 mA cm(-2)). The Zn sponges maintain their 3D-monolithic form factor at all DOD. The cell resistance remains low under all test conditions, indicating that an inner core of metallic Zn persists that 3D-electrically wires the electrode, even to deep DOD.

Designing high-performance electrochemical energy-storage nanoarchitectures to balance rate and capacity.

The impressive specific capacitance and high-rate performance reported for many nanometric charge-storing films on planar substrates cannot impact a technology space beyond microdevices unless such performance translates into a macroscale form factor. In this report, we explore how the nanoscale-to-macroscale properties of the electrode architecture (pore size/distribution, void volume, thickness) define energy and power performance when scaled to technologically relevant dimensions. Our test bed is a device-ready electrode architecture in which scalable, manufacturable carbon nanofoam papers with tunable pore sizes (5-200 nm) and thickness (100-300 µm) are painted with ~10 nm coatings of manganese oxide (MnOx). The quantity of capacitance and the rate at which it is delivered for four different MnOx-C variants was assessed by fabricating symmetric electrochemical capacitors using a concentrated aqueous electrolyte. Carbon nanofoam papers containing primarily 10-20 nm mesopores support high MnOx loadings (60 wt%) and device-level capacitance (30 F g(-1)), but the small mesoporous network hinders electrolyte transport and the low void volume restricts the quantity of charge-compensating ions within the electrode, making the full capacitance only accessible at slow rates (5 mV s(-1)). Carbon nanofoam papers with macropores (100-200 nm) facilitate high rate operation (50 mV s(-1)), but deliver significantly lower device capacitance (13 F g(-1)) as a result of lower MnOx loadings (41 wt%). Devices comprising MnOx-carbon nanofoams with interconnecting networks of meso- and macropores balance capacitance and rate performance, delivering 33 F g(-1) at 5 mV s(-1) and 23 F g(-1) at 50 mV s(-1). The use of carbon nanofoam papers with size-tunable pore structures and thickness provides the opportunity to engineer the electrode architecture to deliver scalable quantities of capacitance (F cm(-2)) in tens of seconds with a single device.

Something from nothing: enhancing electrochemical charge storage with cation vacancies.

The performance of electrochemical energy storage devices (e.g., batteries and electrochemical capacitors) is largely determined by the physicochemical properties of the active electrode materials, such as the thermodynamic potential associated with the charge-storage reaction, ion-storage capacity, and long-term electrochemical stability. In the case of mixed ion/electron-conducting metal oxides that undergo cation-insertion reactions, the presence of cation vacancies in the lattice structure can enhance one or more of these technical parameters without resorting to a drastic change in material composition. Examples of this enhancement include the charge-storage properties of certain cation-deficient oxides such as γ-MnO2 and γ-Fe2O3 relative to their defect-free analogues. The optimal cation-vacancy fraction is both material- and application-dependent because cation vacancies enhance some materials properties at the expense of others, potentially affecting electronic conductivity or thermal stability. Although the advantages of structural cation vacancies have been known since at least the mid-1980s, only a handful of research groups have purposefully integrated cation vacancies into active electrode materials to enhance device performance. Three protocols are available for the incorporation of cation vacancies into transition metal oxides to improve performance in both aqueous and nonaqueous energy storage. Through a processing approach, researchers induce point defects in conventional oxides using traditional solid-state-ionics techniques that treat the oxide under appropriate atmospheric conditions with a driving force such as temperature. In a synthetic approach, substitutional doping of a highly oxidized cation into a metal-oxide framework can significantly increase cation-vacancy content and corresponding charge-storage capacity. In a scaling approach, electrode materials that are expressed in morphologies with high surface areas, such as aerogels, contain more defects because the increased fraction of surface sites favors the formation of cation vacancies. In this Account, we review studies of cation-deficient electrode materials from the literature and our laboratory, focusing on transition metal oxides and the impact cation vacancies have on electrochemical performance. We also discuss the challenges and limitations of these defective structures and their promise as battery materials.

Redox deposition of nanoscale metal oxides on carbon for next-generation electrochemical capacitors.

Transition metal oxides that mix electronic and ionic conductivity are essential active components of many electrochemical charge-storage devices, ranging from primary alkaline cells to more advanced rechargeable Li-ion batteries. In these devices, charge storage occurs via cation-insertion/deinsertion mechanisms in conjunction with the reduction/oxidation of metal sites in the oxide. Batteries that incorporate such metal oxides are typically designed for high specific energy, but not necessarily for high specific power. Electrochemical capacitors (ECs), which are typically composed of symmetric high-surface-area carbon electrodes that store charge via double-layer capacitance, deliver their energy in time scales of seconds, but at much lower specific energy than batteries. The fast, reversible faradaic reactions (typically described as "pseudocapacitance") of particular nanoscale metal oxides (e.g., ruthenium and manganese oxides) provide a strategy for bridging the power/energy performance gap between batteries and conventional ECs. These processes enhance charge-storage capacity to boost specific energy, while maintaining the few-second timescale of the charge-discharge response of carbon-based ECs. In this Account, we describe three examples of redox-based deposition of EC-relevant metal oxides (MnO2, FeOx, and RuO2) and discuss their potential deployment in next-generation ECs that use aqueous electrolytes. To extract the maximum pseudocapacitance functionality of metal oxides, one must carefully consider how they are synthesized and subsequently integrated into practical electrode structures. Expressing the metal oxide in a nanoscale form often enhances electrochemical utilization (maximizing specific capacitance) and facilitates high-rate operation for both charge and discharge. The "wiring" of the metal oxide, in terms of both electron and ion transport, when fabricated into a practical electrode architecture, is also a critical design parameter for achieving characteristic EC charge-discharge timescales. For example, conductive carbon must often be combined with the poorly conductive metal oxides to provide long-range electron pathways through the electrode. However, the ad hoc mixing of discrete carbon and oxide powders into composite electrodes may not support optimal utilization or rate performance. As an alternative, nanoscale metal oxides of interest for ECs can be synthesized directly on the surfaces of nanostructured carbons, with the carbon surface acting as a sacrificial reductant when exposed to a solution-phase, oxidizing precursor of the desired metal oxide (e.g., MnO4(-) for MnO2). These redox deposition methods can be applied to advanced carbon nanoarchitectures with well-designed pore structures. These architectures promote effective electrolyte infiltration and ion transport to the nanoscale metal oxide domains within the electrode architecture, which further enhances high-rate operation.

Architectural integration of the components necessary for electrical energy storage on the nanoscale and in 3D.

We describe fabrication of three-dimensional (3D) multifunctional nanoarchitectures in which the three critical components of a battery--cathode, separator/electrolyte, and anode--are internally assembled as tricontinuous nanoscopic phases. The architecture is initiated using sol-gel chemistry and processing to erect a 3D self-wired nanoparticulate scaffold of manganese oxide (>200 m(2) g(-1)) with a continuous, open, and mesoporous void volume. The integrated 3D system is generated by exhaustive coverage of the oxide network by an ultrathin, conformal layer of insulating polymer that forms via self-limiting electrodeposition of poly(phenylene oxide). The remaining interconnected void volume is then wired with RuO(2) nanowebs using subambient thermal decomposition of RuO(4). Transmission electron microscopy demonstrates that the three nanoscopic charge-transfer functional components--manganese oxide, polymer separator/cation conductor, and RuO(2)--exhibit the stratified, tricontinuous design of the phase-by-phase construction. This architecture contains all three components required for a solid-state energy storage device within a void volume sized at tens of nanometres such that nanometre-thick distances are established between the opposing electrodes. We have now demonstrated the ability to assemble multifunctional energy-storage nanoarchitectures on the nanoscale and in three dimensions.

Electroless deposition of conformal nanoscale iron oxide on carbon nanoarchitectures for electrochemical charge storage.

We describe a simple self-limiting electroless deposition process whereby conformal, nanoscale iron oxide (FeO(x)) coatings are generated at the interior and exterior surfaces of macroscopically thick ( approximately 90 microm) carbon nanofoam paper substrates via redox reaction with aqueous K(2)FeO(4). The resulting FeO(x)-carbon nanofoams are characterized as device-ready electrode structures for aqueous electrochemical capacitors and they demonstrate a 3-to-7 fold increase in charge-storage capacity relative to the native carbon nanofoam when cycled in a mild aqueous electrolyte (2.5 M Li(2)SO(4)), yielding mass-, volume-, and footprint-normalized capacitances of 84 F g(-1), 121 F cm(-3), and 0.85 F cm(-2), respectively, even at modest FeO(x) loadings (27 wt %). The additional charge-storage capacity arises from faradaic pseudocapacitance of the FeO(x) coating, delivering specific capacitance >300 F g(-1) normalized to the content of FeO(x) as FeOOH, as verified by electrochemical measurements and in situ X-ray absorption spectroscopy. The additional capacitance is electrochemically addressable within tens of seconds, a time scale of relevance for high-rate electrochemical charge storage. We also demonstrate that the addition of borate to buffer the Li(2)SO(4) electrolyte effectively suppresses the electrochemical dissolution of the FeO(x) coating, resulting in <20% capacitance fade over 1000 consecutive cycles.

Making the most of a scarce platinum-group metal: conductive ruthenia nanoskins on insulating silica paper.

Subambient thermal decomposition of ruthenium tetroxide from nonaqueous solution onto porous SiO(2) substrates creates 2-3 nm thick coatings of RuO(2) that cover the convex silica walls comprising the open, porous structure. The physical properties of the resultant self-wired nanoscale ruthenia significantly differ depending on the nature of the porous support. Previously reported RuO(2)-modified SiO(2) aerogels display electron conductivity of 5 x 10(-4) S cm(-1) (as normalized to the geometric factor of the insulating substrate, not the conducting ruthenia phase), whereas RuO(2)-modified silica filter paper at approximately 5 wt % RuO(2) exhibits approximately 0.5 S cm(-1). Electron conduction through the ruthenia phase as examined from -160 to 260 degrees C requires minimal activation energy, only 8 meV, from 20 to 260 degrees C. The RuO(2)(SiO(2)) fiber membranes are electrically addressable, capable of supporting fast electron-transfer reactions, express an electrochemical surface area of approximately 90 m(2) g(-1) RuO(2), and exhibit energy storage in which 90% of the total electron-proton charge is stored at the outer surface of the ruthenia phase. The electrochemical capacitive response indicates that the nanocrystalline RuO(2) coating can be considered to be a single-unit-thick layer of the conductive oxide, as physically stabilized by the supporting silica fiber.

Multifunctional 3D nanoarchitectures for energy storage and conversion.

The design and fabrication of three-dimensional multifunctional architectures from the appropriate nanoscale building blocks, including the strategic use of void space and deliberate disorder as design components, permits a re-examination of devices that produce or store energy as discussed in this critical review. The appropriate electronic, ionic, and electrochemical requirements for such devices may now be assembled into nanoarchitectures on the bench-top through the synthesis of low density, ultraporous nanoarchitectures that meld high surface area for heterogeneous reactions with a continuous, porous network for rapid molecular flux. Such nanoarchitectures amplify the nature of electrified interfaces and challenge the standard ways in which electrochemically active materials are both understood and used for energy storage. An architectural viewpoint provides a powerful metaphor to guide chemists and materials scientists in the design of energy-storing nanoarchitectures that depart from the hegemony of periodicity and order with the promise--and demonstration--of even higher performance (265 references).

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.