title: TiO2 nanotubes: interdependence of substrate grain orientation and growth rate.
abstract: Highly ordered arrays of TiO2 nanotubes can be produced by self-organized anodic growth. It is desirable to identify key parameters playing a role in the maximization of the surface area, growth rate, and nanotube lengths. In this work, the role of the crystallographic orientation of the underlying Ti substrate on the growth rate of anodic self-organized TiO2 nanotubes in viscous organic electrolytes in the presence of small amounts of fluorides is studied. A systematic analysis of cross sections of the nanotubular oxide films on differently oriented substrate grains was conducted by a combination of electron backscatter diffraction and scanning electron microscopy. The characterization allows for a correlation between TiO2 nanotube lengths and diameters and crystallographic parameters of the underlying Ti metal substrate, such as planar surface densities. It is found that the growth rate of TiO2 nanotubes gradually increases with the decreasing planar atomic density of the titanium substrate. Anodic TiO2 nanotubes with the highest aspect ratio form on Ti(-151) [which is close to Ti(010)], whereas nanotube formation is completely inhibited on Ti(001). In the thin compact oxide on Ti(001), the electron donor concentration and electronic conductivity are higher, which leads to a competition between oxide growth and other electrochemical oxidation reactions, such as the oxygen evolution reaction, upon anodic polarization. At grain boundaries between oxide films on Ti(hk0), where nanotubes grow, and Ti(001), where thin compact oxide films are formed, the length of nanotubes decreases most likely because of lateral electron migration from TiO2 on Ti(001) to TiO2 on Ti(hk0).
authors.affiliation: Department of Physics E19 and Institute for Advanced Study, Technische Universität München , James-Franck-Strasse 1, 85748 Garching, Germany.
authors.forename: Silvia
authors.initials: S
authors.lastname: Leonardi
authors.forename: Valeria
authors.initials: V
authors.lastname: Russo
authors.forename: Andrea
authors.initials: A
authors.lastname: Li Bassi
authors.forename: Fabio
authors.initials: F
authors.lastname: Di Fonzo
authors.forename: Thomas M
authors.initials: TM
authors.lastname: Murray
authors.forename: Harry
authors.initials: H
authors.lastname: Efstathiadis
authors.forename: Andrea
authors.initials: A
authors.lastname: Agnoli
authors.forename: Julia
authors.initials: J
authors.lastname: Kunze-Liebhäuser
keywords: EBSD; anodic TiO2 nanotubes; growth rate; organic electrolyte; self-organized anodic growth