Solution growth of anatase TiO2 nanowires from transparent conducting glass substrates

Sol–gel reaction conditions that enable the growth of one-dimensional (1D) anatase titanium dioxide (TiO2) nanostructures from fluorine-doped indium tin oxide (FTO) substrates are described. The generation of these linear nanostructures is achieved using acetic acid (HOAc) and titanium isopropoxide (Ti(OiPr)4) in anhydrous heptane in the absence of an external bias or template. The procedure requires the functionalization of base-treated substrates with Ti-oxide nucleation sites, which serve as a foundation for the growth of linear Ti-oxide macromolecules. Calcination of these macromolecules at 450 °C under an ambient atmosphere produce uniform films of randomly oriented anatase TiO2 nanowires. The nucleation and growth processes are both acutely sensitive to the relative molar ratio (R) of HOAc to Ti(OiPr)4. Optimal surface coverage of the nucleation sites is observed when the R value utilized for the nucleation phase (denoted Ri) is equal to 1.3. The highest quality nanowire films were obtained when the R value employed during the gelation phase (denoted Rf) was held between 8.5 and 14. Characterization of the films by electron microscopy revealed a uniform film of disordered anatase TiO2 nanowires with high aspect ratios. The dimensions of the nanostructures correspond to lengths of ca. 1–10 μm and widths of 54 ± 10 nm. High-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) techniques demonstrate that the anatase nanowires are a linear arrangement of crystallites ranging in size from 13 to 19 nm. A systematic evaluation of how reaction conditions (e.g., solvent volume, stoichiometry of reagents, substrate base treatment) affect the generation of these TiO2 films is presented.