CAREER: Facilitated Ion Transport in Nanostructured Titanosilicates

普渡大学的半导体纳米线显示出量子尺寸效应，预计将具有革命性的热电输运特性，可能导致固态冷却装置的效率高于蒸汽压缩制冷技术。然而，为了实现这种器件，必须合成直径远小于其热德布罗意波长的连续导线，以达到量子约束状态。目前正在研究的一种合成这种纳米线的技术是模板辅助方法，其中有序的纳米孔材料，如微孔沸石、介孔二氧化硅（即SBA-15、MCM-41）和阳极氧化铝作为被动载体来模板线的直径和形状。然而，具有足够小直径的纳米线尚未实现，并且由于质量传输的限制，在最小孔隙材料中尝试失败。这一限制可以通过电化学生长技术与一类新的二氧化钛硅酸盐，其纳米结构的框架有利于阳离子的传输来克服。这种新型的硅酸钛，被称为ETS，于1989年首次报道。已经确定了几种具有从0.4 nm到0.8 nm范围的微孔的框架拓扑结构。其中一种结构（Sr-ETS-4）最近被证明对N2/CH4、N2/O2和Ar/O2的分离具有非凡的气体分离性能。所有ETS结构的一个关键特征是存在与微孔平行的连续二氧化钛链（- o - ti - o - ti -）。链上的每个钛单元携带两个负电荷单元，必须通过微孔中的阳离子来平衡。这种独特的纳米结构与传统的沸石有着根本的不同，它被认为可以通过绳子将阳离子拉过框架，从而促进阳离子在结构中的传输。这种“绳子”可以通过减少孔一端的框架阳离子而被电化学地“拉动”。这在结构中产生了电荷不平衡的状态，并诱导了一系列相关的阳离子跳跃，这些阳离子跳跃将额外的阳离子拉入孔的另一端的框架中。在这个过程中，阴极通过框架生长，形成一排纳米线来模拟孔隙。这一现象将在一系列拟议的研究中进行研究，重点是：合成高质量的ETS-10、ETS-4单晶及其相关结构。通过使用复杂阻抗谱来检测IA族、IIA族、金、铅、铋和碲阳离子的离子电导率、活化能和弛豫频率，了解这些独特纳米结构材料中的基本离子传输。亚纳米线在ETS框架孔隙中的电化学生长，用于热电器件的开发。这项多学科研究处于工程、化学、物理和材料科学的交叉路口，是综合教育和研究计划的一部分，旨在：(1)在多学科环境中培养和指导研究生，使其成为具有在纳米技术领域发现和发展新思想的背景和技能的创造性独立研究人员；(2)积极鼓励和支持本科生参与研究；(3)开发并实施一种新的教学方法，利用学生自主创作的基于网络的内容来促进终身学习，并在纳米结构材料化学新课程的背景下吸引学生参与。

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.