Defect and Strain Engineering to Functionalize TiO2 through Mechanical and Electrochemical Means


Student thesis: Doctoral Thesis

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  • Christopher Michael LEE


Awarding Institution
Award date26 Aug 2016


Strain and defect engineering has found a niche in materials science, with strain engineering shifting from being a key component to improving the mechanical properties of materials to being central in the creation of functional materials. Through this newer application of strain engineering, we have seen improvements in the functional performance of materials (e.g., photocatalysis, charge storage) through easier and more economical means. Within the umbrella of strain engineering is also the aspect of doping materials—creating lattice strain while also providing an additional charge or chemical augmentation—resulting in improved functional performance as well. Both of these methods have been used to provide an easier and cheaper method of modifying materials further improving their functionality in relation to their standardized predecessors.
Taking anodic TiO2 as the exemplary nanomaterial system, this thesis will show that, through the creation of strain/defects, the material performance can be improved. Both bottom up (i.e., pulsed anodization) and top down (i.e., SMAT) modification processes will be explored.
Through the novel use of a classical process, surface mechanical attrition treatment (SMAT), TiO2 can be placed under a substantial strain—facilitating the creation of defects and trapping sites within the material, improving both the capacitance and the photocatalytic properties of the material (Chapters 2 and 3). As such, SMAT—a process normally reserved for the treatment of bulk material—can be extended to provide treatment to nanostructured materials, leading to augmented performance on already active materials. Moreover, SMAT has the unique ability to, through the many collisions involved in the process, supplant oxides and other dopants to the surface of the material, leading to even more improved performance of the material from its original fabricated state. This allows SMAT to forge a new pathway to provide strain engineering—capable of being tailored from bulk modification to delicate functionalization. Furthermore, SMAT can be applied to the substrates, which are then anodized to generate robust 3-D structures and devices.
Besides the top-down modification method of SMAT, a bottom up method of using a pulsed anodization was investigated (Chapter 5). By modifying the anodization parameters, defect-engineered nanostructures can be generated, capable of displaying enhanced functional performance, e.g., higher energy storage capabilities. It was found that, besides the anodization voltage profiles, the electrolyte also plays an important role in controlling the material’s morphology (Chapter 4).