Processing, Structure, and Electrochemical Behavior of Anodically Formed Titanium Oxide Nanotubes for the Electrooxidation of Pathogens

Abstract

In the last few decades, there has been an upsurge in the development of electrooxidation (EO) disinfection technology as an alternative to conventional disinfection. EO is an electrochemically advanced oxidation process that provides an environmentally friendly, easy-to-operate, and highly scalable disinfection treatment route. Among the different factors influencing the efficacy of an EO disinfection system, the anode material has established itself to be the most critical factor. Currently, most electrochemical disinfection devices rely on using metal anodes for disinfection. However, the electronic structure of metals easily catalyzes water splitting (i.e., production of O2 and H2 gas) during operation, which can impede disinfection performance. Therefore, it is advantageous to develop anodes that favor oxidant generation over oxygen evolution. In this research, the band structure of titanium oxide nanotubes was engineered via annealing treatments at various temperatures and in different gases. An annealing profile was developed to create defect laden nanotubes that, when used as an anode, showed an enhanced electrochemical response, and enabled the generation of highly oxidizing species upon application of a voltage in an aqueous environment. A defect laden nanotubes based EO system was fabricated and found to provide efficient disinfection for an extended period of time (40 h). The extended performance had not been previously observed with other engineered band structures, demonstrating the unique effect the defect structure had on the stability of the electrochemical performance. In Chapter 1 of this dissertation, existing EO technologies are described, and the need for developing nanotubes based EO technology is discussed. Chapter 2 details the nanotubes synthesis procedure, the characterization techniques, and the electrochemical testing performed to examine both the electrochemical response and disinfection efficacy of the nanotubes. Chapter 3 reports the morphological and crystal phase changes observed within nanotubes while annealing in a 2% hydrogen balance nitrogen (2%H2) or oxygen (O2) atmosphere. Chapter 3 also discusses a new annealing treatment route that was developed to allow the formation of non-stoichiometric Magnéli phases while preserving the nanotubular morphology. Chapter 4 reports on a new characterization method for real-time investigation of the oxide/metal interface, enabling observation of the interface while the nanotubes were being annealed. This in-situ transmission electron microscopy analysis reveals the influence of the substrate and the role of the oxide/metal interface in anatase to rutile phase transformation. Chapter 5 discusses the changes to the band structure and shifts in the fermi energy level of nanotubes annealed in 2%H2 or O2. The nanotubes annealed in 2%H2 form an anatase dominant defect laden structure (i.e., D-NTs) and the nanotubes annealed O2 showed anatase-rutile mixed phase structure. The D-NTs with engineered band structure demonstrated better electrochemical response than anatase1rutile mixed phase nanotubes. Chapter 5 reports on the D-NTs-based EO disinfection system's performance against bacteria. The D-NTs showed a long effective lifetime and high disinfection efficacy while operating at small current densities.