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

•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.

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.