Novel Anodized Metal Oxide Nanostructures on Non-Native Substrates

• Author / Creator

  Kisslinger, Ryan Kent

• Anodized metal-oxide nanostructures are of interest in a wide variety of applications, such as in solar energy harvesting, sensing, antifouling, and biomedical coatings. Anodization is a solution-processable method and presents a viable route to large-scale manufacturing of ordered nanostructures for real-world applications. Many devices require the nanostructured film to be fabricated on non-native substrates, which is typically produced by anodizing metal films deposited by vapour deposition processes; however, such configurations have received only a fraction of the attention that anodization of metal foils have. To date, studies have not sufficiently investigated the unique considerations inherent to anodizing on non-native substrates, nor fully leveraged the capabilities of anodizing to produce novel nanostructured morphologies. Thus, the purpose of this thesis is to address this knowledge gap. This is accomplished through three studies that consider TiO2 nanotubes, NiO nanopores, and Ta2O5 nanodimples, respectively.
  Preferentially oriented TiO2 nanotubes are considered in the first study, and this thesis demonstrates that they may be formed on non-native substrates. It is shown that optimal electrolyte water content plays the largest role in obtaining preferential orientation of the polycrystalline grains, with an optional cation adsorption step playing a secondary role. Unlike previous reports of preferential orientation on metal foils, the roughness of the anodized substrate is found to play no role on the resulting preferential orientation; however, the material identity of the non-native substrate is found to influence preferential orientation, which may be caused by the degree of lattice mismatch between TiO2 and the substrate. Photoconductivity characterizations and use of the preferentially oriented nanotubes in lead halide perovskite solar cells reveal that preferential orientation plays a significant role in charge trapping and charge transfer.
  Nanoporous NiO is considered in the second study, and this thesis demonstrates that it may be formed on non-natives substrates after annealing the as-anodized NiOx(OH)y. The relatively higher chemical etching rate of the NiOx(OH)y means that the nanostructure morphology is especially sensitive to substrate roughness, and remarkably differing morphologies are obtained across FTO-coated glass, ITO-coated glass, and Si wafer substrates. Furthermore, upon annealing, the high diffusivity between Ni and Si results in a NiSi layer. The as-produced NiO is demonstrated by electrochemical impedance spectroscopy and Mott-Schottky analysis to be p-type, with an acceptor charge carrier density of 2.85×1018 cm−3 and a flat band potential of 0.687 V versus Ag/AgCl.
  Ta2O5 nanotubes formed by anodization are demonstrated in the third study, which are subsequently delaminated to leave a nanodimpled Ta surface behind. It is shown that this surface may be oxidized to form Ta2O5 nanodimples. The role of the key synthesis parameters including anodizing voltage and anodizing time on the pore rate, pore depth, and resultant layer thickness are investigated in-depth, as well as the effects of annealing on the nanodimpled morphology. Stable dimple formation occurs after anodization within a range of voltages from 7–40 V. The bandgap of the Ta2O5 is found to be 4.5 eV through EELS and UV-Vis spectroscopy. The nanodimples are demonstrated as a platform for plasmonic photocatalysis in a model reaction monitored using surface enhanced Raman spectroscopy, and an increased photooxidation rate is observed when used in tandem with Au nanoparticles. FDTD simulations indicate that a notable red-shifting of the Au nanoparticle localized surface plasmon resonance of 75 nm may be attributable to partial embedding of the Au nanoparticles in the Ta2O5 film.
  

• Subjects / Keywords

  • nanotechnology
  • thin films
  • anodization
  • Tio2
  • NiO
  • Ta2O5
  • nanotubes
  • nanopores
  • nanodimples
  • renewable energy

• Graduation date

  Spring 2021

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-6fnq-k783

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.