Next Generation Nanomaterials: Hierarchical Hybrid Architectures for Robust & Reusable Devices

S.M. Mukhopadhyay
Wright State University,
United States

Keywords: nanotube carpets, catalysts, antibacterial activity, tissue scaffolding, thermal transport

Summary:

Nanomaterials are known to offer significant advantages in a wide variety of applications ranging from sensing and catalysis to composite formation and bio-scaffolding. However, their use in engineering devices is often limited by difficulties of handling and confinement combined with environmental proliferation risks. Our goal is to address this dilemma by creating hierarchical hybrid materials comprising of tailored arrays of nanotubes and nanoparticles that remain strongly adhered to larger substrates having the necessary physical and chemical properties. This presentation will focus on novel architectures created in this laboratory, inspired by biological surfaces such as microvilli and capillaries. Such materials seen in nature often consist of a large unifying scaffold that supports progressively smaller and more specialized attachments to provide extremely high interaction area in very compact space. These architectures have been rarely used in engineered materials due to the complexities of bonding components of different sizes, shapes and compositions into one solid. However, recent advances in surface processing have made it possible design and fabricate these types of solids, which can significantly surpass current materials. In this study, graphitic carbon materials, ranging from rigid foam to flexible fabric, are selected as starting substrates. Successful attachment of strongly bonded carbon nanotube (CNT) arrays was achieved using covalently bonded intermediate layers. This type of hierarchical morphology provides the capability of increasing surface area by several orders of magnitude, as well as tuning the morphology for targeted applications by varying the packing density and/or the height of CNT arrays. We were able to control the permeation of CNT through the complex geometry of porous solids, which showed exceptionally high charge storage density, adsorption capacity, thermal transport and catalytic activity. Input from microstructural data was used to develop mathematical estimates of specific surface area, which correlated well with surface adsorption kinetics and BET measurements. These surfaces could be further activated with nanocatalyst particles to remove contaminants from water. For instance, silver nanoparticles were seen to enable rapid removal of pathogens from water, and attachment of palladium nanoparticles enabled degradation of emerging contaminants such as Atrazine and Trichloroethylene. An important factor of nanostructured surfaces in fluid-based applications is their fluid wettability, which can control the percolation of fluids through individual nanotubes. Wettability control has been investigated for hydrophobic CNT carpets: treatment with dry oxygen plasma is seen to induce reversible hydrophilicity, and sol-gel coating with silica is seen to impart permanent wettability. In yet another class of applications, these types of nanostructured scaffolds show increased proliferation of biological cells such as osteoblasts and myoblasts. It was seen that when the nanostructured CNT carpets interface with microscale aligned fibrous architecture of carbon fabric, significantly enhanced cell fusion into multinucleated mature fibrous tissue is observed, paving the path for synthetic biological fibers. These results underscore the versatility of hierarchical hybrid nanomaterial design that can utilize the synergy between nanoscale surface features and macroscale substrates to provide multi-functional advantages at different levels.