Effect of electrolyte and electrical parameters on the anodic oxidation of ti to improve photocatalytic performance of TiO2 nanotube structures

Abstract

TiO2 is a semiconductor material that, when stimulated by photons with energy sufficient to overcome its band gap (3.0-3.2 eV), generates electron-hole pairs. By interacting with one another or through a succession of redox reactions, these charge carriers eventually have the capacity to mineralize a wide range of organic compounds, including dyes, surfactants, aromatics, and alkanes. This property makes various TiO2 forms (powder, nanotube, nanorod, etc.) appropriate for a variety of applications in the energy and environmental domains, such as solar cells, hydrogen production, water purification, air purification, and others. This thesis study was undertaken to investigate production, characterization and photocatalytic performance of TiO2 nanostructures as a very promising semiconducter photocatalyst. Anodic oxidation process utilized for production of TiO2 nanostructures from comercially pure titanium foils. Various electrolytes and elecrtrical parameretes explored to investigate their effect on the surface preperad. Produced nanostructures characterized by surface morphology analyses with optical and electron microscopy, surface phase and structure analyses with XRD, XRF and Raman, optical properties analyses with photoluminescence and UV-DRS, and with organic contaminant degradation tests. One of the anodic oxidation parameters studied in the scope the thesis was cyclic bipolar voltage. In this work group different duty cycle and frequency values was experimented and nano structures succesfully produced on titanium foils. Results showed that the increasing positive pulse duration, number of successive positive pulses and neutral period of the cycle could increase nanotube diameter. Also, application of negative voltage clearly deterioriated the nanotube morphology and produced a sponge like structure at the surface. Effect of pre-deformation mode and strain level on the photocatalytic performance of TiO2 nanotubes produced by anodic oxidation on titanium foils were also investigated. In this study set, three different amounts of tensile and compression strains applied to titanium foil prior to anodic oxidation. Results showed that pre-deformation on titanium foils could have a strong effect on photacatalytic performance of TiO2 nanotubes produced from these foils. Especially cold rolling deformation was quite successful in improving photacatalytic performance. On the other hand, critical amount of tensile deformation did improve photocatalytic performance, where further deformation amount detoriated it. One other anodic oxidation parameter studied in the thesis was effect of carbon structure, namely carbon nanotubes, addition to electrolyte. Different amounts of carbon nanotubes was added to anodic oxidation electrolyte since, in term of impoving xviii photocatalytic performance, carbon is a very effective dopant for TiO2 structutres. Results showed that deposition of carbon structures on TiO2 nanotubes was successful and photocatalytic performance of these doped samples were better than un-doped ones. However due to clustering and covering up effect of carbon structures, with the increase at the carbon concentration in electrolyte, photocatalytic performance decreased. Prior to anodic oxidation experiments, a surface preparation method named electropolsihing and a surface coating method named hot-dip aluminizing also studied. Electropolishing of titanium foils was experimented to produce smooth surfaces for further proceses. Succesfuly electropolished titanium surfaces were preperad after trials with various process parameters and these surfaces used in anodic oxidation trials. Alumizing of titanium foils were investigated to produce intermetalic surfaces for further procesess/applications. Intermetallics of aluminum and titanium succesfully produced with this method and these structures greatly improved high temperature oxidation resistance of titanium foils. Anodic oxidation of these surfaces was not undertaken in the scope of this thesis, however formation of nanostructures on intermetallics could be a very interesting further study. As a whole, thesis study investigated electropolishing, aluminizing and anodic oxidation of titanium foils. Succesfull parameters for electropolishing of commercially pure titanium was established. Additionally, various ways to improve photocatalytic performance of a TiO2 nanostructures demonstrated. In this context, outputs of the thesis study could be useful in two very strategically important and urgent fields namely, energy and envioroment.