Tunable nanomechanical performance regimes in ceramic nanowires.

At the macroscopic size regime, ceramic materials exhibit brittle fracture and catastrophic failure when they are subjected to mechanical loads that exceed their characteristic strength. In this report, we present recoverable plasticity in alpha-phase, potassium stabilized manganese dioxide nanowire (α-K0.13MnO2 NW) crystals when they are subjected to atomic force microscopy (AFM) based three-point bending tests at very low loading rates. The force-deflection curves and AFM scans obtained from these measurements reveal yielding and extended plasticity in the NWs during the loading process, while the large plastic deformation is recovered spontaneously during the unloading process. However, the same material system exhibits failure via fracture at substantially higher strengths when it is subjected to bending tests at nearly an order of magnitude higher loading rates. These results highlight an important new pathway to controllably tune the nanomechanical performance of these technologically important nanoceramics for application-specific needs: either achieve self-reversible and ultra-large plasticity, or achieve substantially higher fracture strengths that approach the intrinsic limits of the material system.

A 3D nanoelectrokinetic model for predictive assembly of nanowire arrays using floating electrode dielectrophoresis.

Floating electrode dielectrophoresis (FE-DEP) presents a promising avenue for scalable assembly of nanowire (NW) arrays on silicon chips and offers better control in limiting the number of deposited NWs when compared with the conventional, two-electrode DEP process. This article presents a 3D nanoelectrokinetic model, which calculates the imposed electric field and its resultant NW force/velocity maps within the region of influence of an electrode array operating in the FE-DEP configuration. This enables the calculation of NW trajectories and their eventual localization sites on the target electrodes as a function of parameters such as NW starting position, NW size, the applied electric field, suspension concentration, and deposition time. The accuracy of this model has been established through a direct quantitative comparison with the assembly of manganese dioxide NW arrays. Further analysis of the computed data reveals interesting insights into the following aspects: (a) asymmetry in NW localization at electrode sites, and (b) the workspace regions from which NWs are drawn to assemble such that their center-of-mass is located either in the inter-electrode gap region (desired) or on top of one of the assembly electrodes (undesired). This analysis is leveraged to outline a strategy, which involves a physical confinement of the NW suspension within lithographically patterned reservoirs during assembly, for single NW deposition across large arrays with high estimated assembly yields on the order of 87%.

Prediction of optimal structural water concentration for maximized performance in tunnel manganese oxide electrodes.

Crystal water has been shown to stabilize next-generation energy storage electrodes with structural tunnels to accommodate cation intercalation, but the stabilization mechanism is poorly understood. In this study, we present a simple physical model to explain the energetics of interactions between the electrode crystal lattice, structural water, and electrochemically cycled ions. Our model is applied to understand the effects of crystal water on sodium ion intercalation in a tunnel manganese oxide structure, and we predict that precisely controlling the crystal water concentration can optimize the ion intercalation voltage and capacity and promote stable cycling. The analysis yields a critical structural water concentration by accounting for the interplay between elastic and electrostatic contributions to the free energy. Our predictions are validated with first-principles calculations and electrochemical measurements. The theoretical framework used here can be extended to predict critical concentrations of stabilizing molecules to optimize performance in next-generation battery materials.

Brittle fracture to recoverable plasticity: polytypism-dependent nanomechanics in todorokite-like nanobelts.

Atomic force microscopy (AFM) based nanomechanics experiments involving polytypic todorokite-like manganese dioxide nanobelts reveal varied nanomechanical performance regimes such as brittle fracture, near-brittle fracture, and plastic recovery within the same material system. These nanobelts are synthesized through a layer-to-tunnel material transformation pathway and contain one-dimensional tunnels, which run along their longitudinal axis and are enveloped by m × 3 MnO6 octahedral units along their walls. Depending on the extent of material transformation towards a tunneled microstructure, the nanobelts exhibit stacking disorders or polytypism where the value for m ranges from 3 to up to ∼20 within different cross-sectional regions of the same nanobelt. The observation of multiple nanomechanical performance regimes within a single material system is attributed to a combination of two factors: (a) the extent of stacking disorder or polytypism within the nanobelts, and (b) the loading (or strain) rate of the AFM nanomechanics experiment. Controllable engineering of recoverable plasticity is a particularly beneficial attribute for advancing the mechanical stability of these ceramic materials, which hold promise for insertion in multiple next-generation technological applications that range from electrical energy storage solutions to catalysis.

Ion Removal Performance, Structural/Compositional Dynamics, and Electrochemical Stability of Layered Manganese Oxide Electrodes in Hybrid Capacitive Deionization.

Hybrid capacitive deionization (HCDI) is a derivative of capacitive deionization (CDI) method for water desalination, in which one carbon electrode is replaced with a redox-active intercalation electrode, resulting in substantial improvements in ion removal capacity over traditional CDI. The search for high-performing intercalation host compounds is ongoing. In this study, two-layered manganese oxides (LMOs), with sodium (Na-birnessite) and magnesium (Mg-buserite) ions stabilizing the interlayer region, were for the first time evaluated as HCDI electrodes for the removal of ions from NaCl and MgCl2 solutions to understand structural/compositional dynamics and electrochemical stability of LMO electrodes over extended cycling. Both materials demonstrated excellent initial ion removal performance with the highest capacities of 37.2 mg g-1 (637 µmol g-1) exhibited by Mg-buserite in NaCl solution and 50.2 mg g-1 (527 µmol g-1) exhibited by Na-birnessite in MgCl2 solution. The performance decay observed over the course of 200 ion adsorption/ion release cycles was attributed to two major phenomena: oxidation of carbon electrode and evolution of the structure/composition of LMO electrodes. The latter involves disorder in stacking of Mn-O layers and changes in the interlayer spacing/interlayer ions reflecting the composition of the solution being desalinated. This work highlights the importance of understanding the interactions between the HCDI electrodes and solutions containing different ions and the structural analysis of redox-active material in intercalation electrodes over the course of operation for gaining insight into the fundamental processes governing desalination performance and developing next-generation HCDI systems with long-term electrochemical stability.