Surface electronic and structural properties of nanostructured titanium oxide grown by pulsed laser deposition

Titanium oxide nanostructured thin films synthesized by pulsed laser deposition (PLD) were here characterized with a multi-technique approach to investigate the relation between surface electronic, structural and morphological properties. Depending on the growth parameters, these films present characteristic morphologies ranging from compact to columnar and to an extremely open structure. As-deposited films have a disordered structure both in the bulk and on the surface, as shown by Raman spectroscopy and by the fine structure of X-ray absorption spectra near the Ti and O edge (NEXAFS). The surface reactivity towards the atmosphere, with consequent formation of surface hydroxyl terminal groups, turns out to be dependent not only on the effective surface but also on the surface structure. By ultraviolet photoemission spectroscopy, we observed that, depending on the sample structure and morphology, defect states at 1 eV binding energy in the valence band can be induced by exposing the samples to the intense synchrotron photon beam. After annealing in air at 673 K, the structural order increases towards a mainly anatase phase in which the presence of rutile increases in films with a more open morphology. Such structural modifications influence the surface stability since the defect formation in the valence band is strongly reduced in all the annealed films, and it is completely hindered in the most compact films.