CAREER: Facilitated Ion Transport in Nanostructured Titanosilicates

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AbstractCTS-0134255Hillhouse, Hugh W.Purdue UnivesitySemiconductor nanowires that exhibit quantum-size effects are predicted to have revolutionary thermoelectric transport properties, potentially resulting in solid-state cooling devices with efficiencies greater than vapor-compression refrigeration technology. However, in order to realize such devices, continuous wires must be synthesized with diameters much smaller than their thermal de Broglie wavelength to reach the quantum confinement regime. One technique being investigated to synthesize such nanowires is a template assisted approach in which ordered nanoporous materials such as microporous zeolites, mesoporous silica (i.e. SBA-15, MCM-41), and anodic alumina are used as passive hosts to template the diameter and form of the wire. However, nanowires with a sufficiently small diameter have not yet been realized, and in the smallest pore materials attempts have failed due to mass transport limitations. It is proposed that this limitation can be overcome by using electrochemical growth techniques with a new class of titanosilicate whose nanostructured framework facilitates cation transport. This new class of titanosilicate, designated as ETS, was first reported in 1989. Several framework topologies have been identified that have micropores ranging from just under 0.4 nm up to 0.8 nm. One structure in particular (Sr-ETS-4) has recently been shown to be possess extraordinary gas separation properties for N2/CH4, N2/O2, and Ar/O2 separations. A key feature that is unique to all of the ETS structures is the presence of continuous titania chains (-O-Ti-O-Ti-) that run parallel to the micropores. Each titanium unit in the chain carries two units of negative charge and must be balanced by cations in the micropore. This unique nanostructure is fundamentally different from classical zeolites, and is hypothesized to facilitate cation transport though the structure by acting as a rope to pull cations through the framework. This 'rope' may be 'pulled' electrochemically by reducing a framework cation at one end of the pore. This creates a state of charge imbalance in the structure and induces a series of correlated cation hops that pull an additional cation into the framework at the opposite end of the pore. In the process, the cathode grows through the framework forming an array of nanowires that mimic the pores. This phenomenon will be investigated in a body of proposed research that focuses on:Synthesizing high quality single crystals of ETS-10, ETS-4, and related structures.Understanding the fundamentals ion transport in these unique nanostructured materials by using complex impedance spectroscopy to examine ion conductivities, activation energies, and relaxation frequencies for Group IA, Group IIA, gold, lead, bismuth, and tellurium cations.Electrochemical growth of sub-nanometer wires in the pores of ETS frameworks for the development of thermoelectric devices. This multidisciplinary research is at the cross roads of engineering, chemistry, physics, and materials science, and is one part of an integrated education and research plan that seeks to: (1) train and mentor graduate students in a multidisciplinary environment to become creative independent researchers who have the background and skills to discover and develop new ideas in the area of nanotechnology, (2) actively encourage and support undergraduate research participation, and (3) develop and implement a new teaching approach that utilizes student authorship of web based content to facilitate lifelong learning and engage student participation in the context of a new course on nanostructured materials chemistry.