Fabrication Of Hierarchically Structured Oxides

Hierarchically structured materials are usually composite materials, constructedfrom building blocks of different length scales and different chemical entities throughmultiple types of chemical interactions including both chemical bonding and weakinteractions. These materials often exhibit high specific surface area, hierarchicalporosity, low density and optimized functional activities rendering them fit forapplications ranging from catalysis, sensing, energy storage, optoelectronics torenewable energy. Developing more sustainable approaches for constructinghierarchically structured materials and understanding the structure-functionrelationship are the primary objectives of hierarchically structured materials research.Common approaches used for preparing hierarchically structured material arebased on templating method, post-processing method and self-generation method.Post-processing method employs etching agents, such as acids and bases, to removeselectively parts of the precursors as a means to render structural hierarchy, createporosity and increase specific surface area with an ultimate goal of enhancingmaterial’s functional performance. For example, alkali treatment of TiO2proceeds in anumber of steps involving:(1) breaking the Ti-O-Ti bonds forming Ti-O-Na linkages;(2) partial/complete hydrolysis of the Ti-O-Na linkages using water and/or acid suchas HCl results in Ti-O-H linkages;(3) formation of the Ti-O-H linkages isaccompanied by generation of linear fragments that, upon rearrangement, formsheets/ribbons of the edge-sharing octahedra with Na+and OH-intercalating adjacentsheets;(4) the sheets self-assemble/fold to create tubular nanostructures.One-dimensional nanostructures in the form of fibers, wires and rods have attracted considerable attention due to their special properties and applications inmany areas. Electrospinning has emerged as a general and low-cost technique forfabricating high aspect ratio fibers of wide-ranging compositions including bothorganic fibers and inorganic fibers. Fibers with varying degree of structural hierarchycan be obtained by adjusting processing parameters or the chemistry ofelectrospinning solutions.TiO2is one of the most attended metal oxide semiconductor. Under theirradiation of UV lights, crystalline TiO2(anatase, rutile) can catalyze the degradationof organic pollutants effectively. ZrO2is a ceramic material with good thermalstability and has been used as catalyst support. ZrO2is also the only transition metaloxide having both acid and base centers on its surface. So far, there has been verylimited report on the fabrication and application of TiO2-ZrO2composite fibers. In thecurrent project, we intended to introduce ZrO2into TiO2and investigated the thermalstability and photocatalytic activity of the TiO2-ZrO2composite fibers. Specifically,following work has been carried out:1. TiO2-ZrO2composite fibers have been fabricated using single-spineretelectrospinning technique from electrospinning solutions containing zirconiumalkoxide and titanium alkoxide. By modulating the relative humidity (RH) ofelectrospining environment, solid fibers and mesoporous fibers of TiO2-ZrO2havebeen obtained at RH <30%and RH>30%, respectively;2. Hydrothermal alkaline treatment of TiO2-ZrO2mesoporous fibers gives rise tourchin-like microspheres;3. Mesoporous fibers of ZrO2have been obtained from electrospinning solutioncontaining surfactant P123that exhibit high porosity and good thermal stability.