Nanotubes Patterned Thin Films of Barium-strontium Titanate

A novel, low-temperature synthesis method for producing Ba_xSr_(1−x)TiO₃ (BST) thin films patterned in the form of nanotubes (“honeycomb”) on Ti substrates is reported. In this two-step method, the Ti substrate is first anodized to produce a surface layer (∼300 nm thickness) of amorphous titanium oxide nanotube (∼100 nm diameter) arrays. In the second step, the anodized substrate is subjected to hydrothermal treatment in aqueous Ba(OH)₂ + Sr(OH)₂ at 200 °C, where the nanotube arrays serve as templates for their topotactic (shape-preserving) hydrothermal conversion to polycrystalline BST nanotubes. A simple geometrical model is proposed to elucidate the mechanism of the hydrothermal growth of BST nanotubes. This opens the possibility of tailoring the titanium oxide nanotube arrays and of using various precursor solutions and their combinations in the hydrothermal bath to produce ordered, patterned thin-film structures of various Ti-containing ceramics. These could find use not only in a variety of electronic, optoelectronic, and sensor device applications but also in biomedical and catalysis applications, where patterned thin films are desirable.