FUNCTIONAL MATERIALS FROM LAYERED OXIDES
Open Access
- Author:
- Kobayashi, Yoji
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- March 04, 2008
- Committee Members:
- Thomas E Mallouk, Committee Chair/Co-Chair
Harry R Allcock, Committee Member
John V Badding, Committee Member
Digby D Macdonald, Committee Member - Keywords:
- layered double hydroxide
Nb2O5
nanotube
proton conductivity
Dion-Jacobson
layered perovskite
layered oxide
spinel
superparamagnetism
oxyfluoride
WIEN2k - Abstract:
- In Chapter 2, the layered Dion-Jacobson perovskites HLaTa2O7, HLaNb2O7, and structural variants of HLaNb2O7 were prepared and their AC and DC conductivities were measured under various temperatures and atmospheres. The layered structure in HLaNb2O7 was modified by pillaring, exfoliation-restacking, and further annealing. The modification in structure strongly affected the grain boundary conduction, rather than grain bulk conduction. Grain boundary conduction is thought to be strongly dependent on adsorbed water, whereas grain conduction would be more dependent on movement of interlayer protons. HLaNb2O7 was prone to reduction under hydrogen, yielding a mixed ionic-electronic conductor. In-plane conductivity measurements on an oriented film of HLaNb2O7 showed similar behavior. Part of this reduction was suppressed by addition of water vapor to the ambient atmosphere and by replacing the B-site niobium with tantalum. In Chapter 3, the nanoscroll-to-nanotube thermal transformation was studied for H4Nb6O17.4.4H2O scrolls, prepared by exfoliation of K4Nb6O17. Thermal dehydration of the scrolls produces Nb2O5 nanotubes at 400-450oC. The non-topochemical transformation results in polycrystalline nanotubes; however, significant texturing with respect to the tube axis is observed. Substituting Ta for Nb in the precursor compound led to a lower yield of scrolls, most likely because there is less built-in lattice strain to drive scrolling of the unilamellar colloidal sheets. Chapter 4 reports the synthesis of a wide range of single crystal spinel platelets with exposed (111) faces, lateral dimensions in the micron range, and thicknesses of 20-50nm, prepared by soft chemical dehydration of well crystallized layered precursors. This method enables the synthesis of the metastable composition NiCoAlO4, which cannot be prepared by conventional solid state synthesis. Because of their nanoscale thickness, mosaic structure, shape anisotropy, and surface porosity, NiCoAlO4 and NiCo2O4 platelets exhibit room temperature superparamagnetism (TB = 40K, 250K respectively) despite the fact that they have micron-size lateral dimensions. The accessibility of a wide range of superparamagnetic spinel platelets (as opposed to cubes or spheres) should be useful for studies in ferrofluids and related composite magnetic materials. The final chapter examines the fluorination of the layered perovskites NaYTiO4, RbLaNb2O7, and KCa2Nb3O10 to yield the fluorine-exchanged perovskite products NayTiO4-xFx, RbLaNb2O7-xFx, and KCa2Nb3O10. Such fluorination under an inert atmosphere preserves the layered perovskites structure but slightly modifies the unit cell. Fluorination reduces the B-site cation, inducing a mixed valency at the B-site. Electronic structure calculations on RbLaNb2O6F show that in the absence of structural disorder, the resulting material is a 2-D metallic conductor. RbLaNb2O6F is air stable, and is capable of ion-exchange with aqueous acid. By exfoliating these layered materials, it may be possible to obtain metallic conducting oxide nanosheets, which have not been extensively reported upon in the past.