Thomas Cottineau

One step synthesis of niobium doped titania nanotube arrays to form (N,Nb) co-doped TiO2 with high visible light photoelectrochemical activity

The chemical modification of aligned titanium dioxide nanotube (TiO2-NT) arrays provides new doping possibilities to improve their photoelectrochemical activity under visible light. Niobium doped TiO2-NTs containing up to 15% of Nb in the near-surface region are prepared by a flexible single step procedure using a fluoroniobate complex simultaneously acting as a source of the doping element and fluoride anions required for nanotube formation. This negatively charged complex allows an efficient insertion of Nb in the forming TiO2-NT structure during the anodization process. These nanotube arrays are further modified with nitrogen to achieve (Nb,N) co-doped nanotubes with noticeable visible light photoelectrochemical activity.

Intermediate band in the gap of photosensitive hybrid gel based on titanium oxide: role of coordinated ligands during photoreduction

UV light induced chemical modifications of a new hybrid organic–inorganic gel, based on titanium oxide and obtained by a soft chemistry method, are studied by XPS and UV-visible spectroscopy. XPS experiments, with an in situ UV illumination design, were performed in order to study the different chemical modifications of the gel. The Ti4+ to Ti3+ reduction, as well as the associated change in the ligands coordinating titanium cations, are accompanied by a deprotonation of the molecules in the vicinity of the inorganic part. In the meantime, an intermediate band arises in the gap just below the Fermi level. The appearance of this band correlates with the light absorption properties of the gel as measured by UV-visible spectroscopy. Following on from these experiments, a complete band diagram of the electronic structure is proposed in order to clarify the position of this intermediate band. These distinctive properties suggest that the nanostructured hybrid gel could be used as a new class of intermediate band material that can be synthesized at low cost for photovoltaic applications.

Temperature dependent photoluminescence of anatase and rutile TiO2 single crystals: Polaron and self-trapped exciton formation

We propose an analysis of the emission properties of anatase and rutile titanium dioxide (TiO2) that emphasizes the role of the strong electron-phonon interaction. We performed measurements of photoluminescence (PL) spectra of bulk monocrystals under continuous wave-laser excitation and of their temperature dependence. We show that in both anatase and rutile, weakly bound self-trapped excitons are actually made out from carrier polarons and give rise to a broad emission band in the visible spectral range. The thermal activation of carrier motion allows their hopping to distant sites that leads to the observed quenching of luminescence. In the specific case of rutile TiO2, the PL spectral shape and its intensity-quenching scenario reveal the presence of dark trap states. Moreover, an additional narrow line structure shows up at low temperatures. The latter is due to localized impurity states that can be attributed to oxygen vacancies and can be fitted with a large Huang-Rhys parameter S = 2.5 within a Franck-Condon model. Both phases show thus a very strong interaction between the photogenerated carriers and the lattice.