X-ray absorption investigation of titanium oxynitride nanoparticles obtained from laser pyrolysis

International audienceThis work presents a structural study by X-ray Absorption Spectroscopy (XAS) and Electron Energy-Loss Spectroscopy (EELS) of complex titanium oxynitride nanoparticles (Ti(O,N)), synthesized by laser pyrolysis from titanium tetraisopropoxide and ammonia as precursors. Previous structural characterizations obtained by XRD and XPS have shown that the nanoparticles present a TiO type face-centered cubic (FCC) structure but with three different oxidation degree for titanium. The synthesis of this kind of titanium oxide or oxynitride nanoparticles is very unusual. Moreover, their properties are highly dependent of their structure. EELS spectrum-imaging data were therefore used for mapping the different chemical species. These measurements reveal that the nanoparticles are composed of a FCC solid solution of nitrogen and oxygen in titanium. The local structure around Ti was then studied. XANES measurements show an absorption threshold corresponding to a global valence state between Ti3+ and Ti4+, with a pre-edge structure characteristic of a mix between a face-centered cubic (FCC) structure and a disordered TiO2 structure whereas the EXAFS signal is dominated by the contribution of the FCC structure. Oxidative heat-treatments have been performed from 250 to 450 degrees C in order to follow the transition towards the dioxide phase. EELS measurements show that the oxidation occurs from the surface of the nanoparticles. XAS show that this transition does not involve any other crystallographic phase than TiO2, mainly in its anatase form, and seems to occur via a dioxide disordered phase. The FCC contribution completely disappears at 450 degrees C where the sample reaches a complete TiO2 structure. Moreover, a correlation is established between the structural evolution and optical properties