Hierarchical enhanced surface area structures and their associated applications with Titania

Hierarchical structures are comprised of nanoscale “building blocks”—varying in shape and size—assembled to produce micro/meso scale structures, which possess morphologies that depend highly on their arrangement. A structure that evolves into one containing building blocks arranged in a manner that increases the surface-to-volume ratio is referred to as a “hierarchical enhanced surface area structure (HESAS). This review showcases how specific structural features and surface characteristics of HESASs can provide performance augmenting benefits in a myriad of applications across several domains such as biology, medicine, environmental engineering, chemical engineering, energy conversion/storage, and sensors. Specific applications of HESASs can include, disinfection and antisepsis, water purification, photocatalytic degradation of pollutants (e.g., degradation of per- and polyfluoroalkyl substances), photocytotoxicity (e.g., photokilling of malignant cells), energy generation (e.g., solar cells), energy storage (e.g., Li-ion batteries), and sensors, just to name a few. As an important category of HESASs, flower-like structures will be defined and emphasized, and formations of titania will be highlighted to provide a robust and tangible discussion on their applicability. This review exposes that the energetic origins yielding these novel structures still need further investigation. For example, while process-structure correlations have been widely reported, the variation of processing techniques alone are not sufficient to enable large-scale, repeatable manufacturing of such structures. Density functional theory and molecular dynamics tools may provide interesting insight into the formations. A more robust understanding of the mechanisms causing these formations will enable development of tools and processes for their accurate and repeatable fabrication.