Browsing by Author "Horwood, Corie"

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    Electrochemical Formation and Optimization of Ta-based Nanomaterials
    (2015-09-28) Horwood, Corie; Birss, Viola
    The primary focus of this research has been to develop simple and precise methods for the formation of novel Ta-based nanostructures, including Ta oxide nanotubes (NTs) and Ta-supported Au nanoparticle (NP) arrays. These nanomaterials are very useful for applications in sensing, electrocatalysis/catalysis, spectroscopy, and more. The Ta oxide NTs are formed by the electrochemical anodization of Ta, a process which can be modified to produce ordered nanoscale dimples that cover the Ta surface. These dimples can then be used as templates for the formation of ordered Au nanoparticle arrays. An in-depth study of the anodization variables showed that NT growth can be slowed down by decreasing the anodization time and HF concentration in the solution, allowing the controlled formation of short (50-1000 nm) NTs in under one minute. The charge passed during anodization was found to be directly proportional to the length of the NTs formed, making cross-sectional imaging unnecessary. A novel two-step anodization method, interspersed with a thermal annealing or negative polarization step, produced stable and well-ordered NTs free of a problematic thin surface oxide layer. These vertically oriented arrays of short NTs were found to have interesting optical properties, with visible structural colours that depend on NT length. The colour of the NT arrays was also found to depend on the medium inside the NTs (air, water, other solids), used for the first time to monitor NT filling, and to determine the refractive index and porosity of the Ta oxide nanotubular array, properties that are otherwise very difficult to obtain. Dimpled Ta was used to create ordered Au NP arrays using two thin film dewetting methods, thermal annealing and pulsed laser-induced dewetting. These methods yield Au NPs of predictable size, shape, spacing, and surface density, with these parameters varying predictably with the initial thickness of the Au thin film and the technique used for dewetting. The Au NPs formed particularly by thermal dewetting were found to be electrochemically addressable, giving another measure of Au surface area and serving as promising materials for future electrochemical applications.