Sol–gel synthesis of linear Sn-doped TiO2nanostructures

Abstract

The sol–gel synthesis of linear Sn-doped TiO₂ (TDT) nanostructures with high aspect ratios is reported. These binary metal-oxide nanostructures are readily accessed by treating titanium isopropoxide (Ti(OⁱPr)₄) with appropriate quantities of acetic acid (AcOH) and Sn(OAc)₄ in heptanes to generate linear macromolecules that form nanofibers upon calcination. While these linear nanostructures are isolated at low R values (0.05–0.20), where R is defined as the molar ratio of Sn(OAc)₄ to Ti(OⁱPr)₄, axially directional growth is not favored at higher R values (0.30–0.50). Scanning and transmission electron microscope (SEM and TEM) imaging of the nanofibers revealed diameters in the 10–20 nm range and lengths in excess of 1 mm. Elemental mapping by STEM-EDS techniques indicates a homogeneous distribution of Sn ions throughout the linear TDT structures. The molecular intermediates that form during the early stages of the reaction were monitored using electrospray-ionization mass spectrometry to confirm the presence of metal-oxide clusters containing both Sn and Ti ions. These intermediates then undergo polycondensation reactions to form the final linear product, thereby indicating the homogeneous incorporation of Sn into the TiO₂ lattice and rules out the possibility of independent SnO₂ and TiO₂ aggregates. Powder X-ray diffraction data indicate that pure TiO₂ nanostructures are anatase when calcined at 500 °C, but show a propensity to adopt the rutile phase at progressively higher Sn concentrations.