363545 Site-Specific Placement of Phases in Porous Nanocomposites
Templating methods developed over the past two decades have enabled syntheses of nanoporous materials with well-controlled architecture, suitable for a variety of applications that include energy storage and conversion, catalysis, opto-electronics and photonics, sensing, sorption and separation, controlled storage and release, and more. Control over dimensions and morphology of both the pore space and the solid wall components is achieved by hard templating and/or soft templating. Hard templating involves confinement of precursors in a solid mold, such as colloidal crystals, anodic alumina membranes, track-etch membranes, lithographic patterns, and other porous solids. Soft templating relies on dynamic structures formed from emulsion droplets, surfactants, block-copolymers, ionic liquids and other micelle-forming substances to direct the organization of precursor components. Multiple templating methods can be combined to achieve hierarchical pore structures with defined pore size distributions and pore interconnectedness.
Next-generation nanostructured materials will require multiple components, including multiple active phases (catalytic, electroactive, conductive, magnetic, etc.) and phases that provide a structural backbone, stabilization, and perhaps transport pathways. It will be critical to have reliable control over component placement, ideally through self-organization and in a single step. Site-specific placement of component phases extends the level of achievable complexity in nanocomposite materials and governs the performance of porous nanocomposites, because it determines component spacing (dispersion, interactions), accessibility, and grain growth/aggregation. Positioning of phases also affects relative dimensions of components and relative amounts that are in contact with interactive species. For example, in catalyst materials, the location of active components on a supporting matrix impacts their accessibility and long-term stability. In electrode materials for electrical energy storage devices, such as rechargeable batteries and supercapacitors, it is often necessary to disperse active material throughout a conductive phase or coat it with a protective phase. The relative location of such multiple phases affects conductivity, accessibility to electrolyte, and the ability of active material to maintain nanosized dimensions during insertion or deinsertion of charged species.
This presentation will provide on overview of some methods that allow placing of individual components in porous nanocomposites at specific sites, such as the pore surfaces, within the pore walls, at specific nodal points of a porous skeleton, or uniformly distributed throughout the structure. These methods include capillary condensation of volatile precursors between closely spaced surfaces of the hard template, or modulating the interactions between precursors and a hard template. The interactions are tuned through precursor selection, choice of ligands that complex with the precursor and may influence polymerization of sol-gel precursors, and through surface modification of the hard template. Interactions between multiple components confined in a template can also be exploited to control the morphology of the porous microparticles and hierarchically structured extended porous solids.
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