Formation of Ti(III) and Ti(IV) states in Ti₃O₅ nano- and microfibers obtained from hydrothermal annealing of C-doped TiO₂ on Si

In this work, it is investigated the formation of Ti(III) and Ti(IV) states at the surface and in the bulk of the Ti₃O₅ material grown as meshes of nano- and micro-fibers obtained from hydrothermal annealing of C-doped TiO₂ on Si. The topography and distribution of the fibers in the meshes were characterized by atomic force microscopy. When the fiber distribution was more compact, a higher photoluminescence signal at 850 nm (1.46 eV) was obtained, indicating the presence of a higher number of defects corresponding to the Ti(III) sites. From X-ray photoelectron spectroscopy, it was obtained a Ti(III)/Ti(IV) ratio much lower than the expected value for the Ti₃O₅ phase (2 Ti(III): 1 Ti(IV)). The discrepancy was mainly attributed to the reaction of surface Ti(III) states of the Ti₃O₅ fibers with water during the hydrothermal annealing, resulting in surface Ti(IV) with -OH radicals. On the other hand, X-ray photoelectron spectroscopy also indicated that substitutional and interstitial carbon atoms coexist, elemental carbon exists in the samples due to the co-deposition process and, as a result, the carbon inside of the TiO₂ rutile lattice is acting as one of the precursors for the formation of Ti₃O₅. - Highlights: • Ti(III) states are detected inside of Ti₃O₅ nano- and microfibers. • Ti(IV) states are predominantly detected on the surface of Ti₃O₅ nano- and microfibers. • Photoluminescence at 850 nm for Ti₃O₅ is due to defects associated to Ti(III). • Rutile possibly changes to C2/m Ti₃O₅ during the hydrothermal annealing of C-doped TiO₂