Nanoscale NEXAFS analysis of individual N-doped TiO2 nanoribbons
Aula: Sala Riunioni - Ore: 11.00
Via Musei 41, Brescia
prof. Luigi Sangaletti
Università Cattolica del Sacro Cuore
dott.ssa CARLA BITTENCOURT
Università di Mons, Belgio
Nitrogen doping titanium dioxide (TiO2), with N at substitutional sites has been reported to be indispensable for enhancing the use of TiO2 as a visible-light photocatalytic material. The main effect of the N-doping is the narrowing of the energy band gap of TiO2 due to the mixing of N 2p and O 2p states. Additionally, an isolated narrow band responsible for the visible light photoactivity is formed above the valence band. Nitrogen doping into the TiO2 lattice is rather challenging as N atoms must be accommodated geometrically and electronically. To address the fundamental issues we explored a chemical route and report here a comprehensive investigation of N-doped TiO2 nanoribbons. As a precursor material for N-doped TiO2 nanoribbons we used hydrogen titanate nanoribbons (HTiNRs) because upon heating HTiNRs easily transform first to the monoclinic TiO2 -phase (TiO2-B) and then to anatase (tetragonal phase) while retaining the morphology of the parent nanostructure intact.
The nitrogen doping of TiO2 nanoribbons during the thermal transformation of hydrogen titanate nanoribbons (HTiNRs) between 400 and 800 °C in a dynamic ammonia atmosphere was investigated using X-ray photoelectron spectroscopy (XPS) and transmission X-ray microscopy combined with near-edge X-ray absorption fine structure spectroscopy (NEXAFS-TXM). XPS results clearly reveal the nitrogen doping of TiO2 nanoribbons and that, depending on the calcination temperature, nitrogen atoms occupy interstitial and substitutional sites. Moreover, in samples calcined at 580 and 650 °C the presence of N2-like species in the HTiNRs was detected by NEXAFS-TXM. These species are trapped in the HTiNRs structure.