Preferential Adsorption of TiO2 Nanostructures on Functionalized Single-Walled Carbon Nanotubes: A DFT Study

The mechanism of attachment of nanocrystals (NCs) to curved carbonaceous species such as graphene nanoribbons and carbon nanotubes (CNTs) is of current scientific interest. In addition, we have observed anisotropic growth patterns of titania NCs from carbonaceous materials, for which there is no theoretical explanation. In this work, we use density functional theory (DFT) calculations for calculating the energy of adsorption of titania nanostructures to both armchair metallic and zigzag semiconducting single-walled carbon nanotubes (SWCNTs) in their pure and functionalized forms. Several adsorption sites are considered including top, bridge, and hollow sites for pure SWCNTs, while for functionalized SWCNTs epoxy, alcohol, and carboxylate are examined. Results from binding energy calculations were found to predict favorable adsorption of TiO2 NCs on the chemical adsorption sites of functionalized SWCNTs compared to the physical adsorption sites of pure SWCNTs. The rotation of anatase and rutile titania species on the physical adsorption sites showed interesting behavior particularly regarding binding strength and growth direction predictions. Partial density of states (PDOS) calculations examined the electronic structure of the assemblies. Charge density maps showed the importance of chemisorption sites for interactions between titania structures and SWCNTs. Electronic local potentials showed the difference in binding strengths for anatase titania on SWCNT physical adsorption sites. These results provide new theoretical evidence for controlled and oriented growth mechanisms on curved carbon-based substrates that have applications in various emerging applications from photovoltaic devices to nanomedicine.