Simple protocol for generating TiO₂ nanofibers in organic media

Titania nanofibers in high yields can be accessed by treating titanium isopropoxide (Ti(OⁱPr)₄) with acetic acid (AcOH) in heptanes when R ≥ 4.2, where R = mol of AcOH/mol of Ti(O¹Pr) ₄. Electron microscopic (SEM and TEM) images of the samples confirmed that high-aspect-ratio nanofibers with diameters in the 10-20 nm range are produced under these conditions, whereas agglomerated spherical nanoparticles are produced at R ≤ 3.8. Powder X-ray diffraction and UV-vis data confirm the anatase crystalline phase after calcination at 400°C, with the progressive formation of the rutile phase upon heating to higher temperatures. N₂ physisorption analyses showed the calcined fibers prepared at R = 5.5 have surface areas of 95 m²/g. The self-assembly pathway leading to the nanofibers was delineated by in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy in tandem with electrospray ionization mass spectrometry (ESI-MS). It was found that the hexanuclear building block, Ti₆O₆(OAc)₆(O ⁱPr)₆ (TAC1), is formed during the initial stages of the reactions, and that the axially ligated isopropoxide ligands of this complex are subsequently hydrolyzed to facilitate the one-dimensional condensation of the macromolecules at R ≥ 4.2. Incomplete hydrolysis at lower acid ratios impedes this axial growth, resulting in spherical nanoparticles.