Electronic Structure of 1D Lepidocrocite TiO₂ as Revealed by Optical Absorption and Photoelectron Spectroscopy

We recently demonstrated scalable, one-pot syntheses of one-dimensional, titania lepidocrocite microfilaments by reacting Ti-containing water-insoluble, earth-abundant compounds such as TiC, TiB2, TiN, etc., with tetraalkylammonium hydroxide, TMAOH, for a few days at 85 °C under ambient pressure. The resulting one-dimensional lepidocrocite (1DL) titania-based nanofilaments, NFs, tend to self-align along the [100] growth direction to form microfilaments that sometimes self-align into pseudo-two-dimensional (2D) sheets. With sub-square-nanometer cross sections, the resulting band gap energy, Eg, at 4.0 eV is one of the highest ever reported for a titania material. Despite a large band gap, the nanofilaments exhibit significant absorbance throughout the visible spectrum ascribable to intra-gap defect states based on UV–Vis absorbance data and ultraviolet photoelectron spectroscopy (UPS). UP spectra demonstrate work functions of 4.0 ± 0.3 eV vs vacuum and Fermi energies of 3.8 ± 0.1 eV with respect to the valence band edge. Transient absorption (TA) spectroscopy of the 1DL nanofilament thin films with sub-band-gap, visible-light illumination reveals photoexcitations with lifetimes in excess of nanoseconds. In combination with the established oxidative stability, long-lived visible photoexcitations bring forward possible applications of 1DL nanofilaments in photocatalysis and optoelectronics.