Micro-Raman spectroscopy study of surface transformations induced by excimer laser irradiation of TiO2

Pressed disks of TiO2 powder particles ( approximate to1 mum in size) have been irradiated with a pulsed KrF (248 nm) excimer laser source at fluences between 0.1 and 1 J cm(-2). Surface films (1.5-2 mum thick) have been studied by Raman microprobe spectroscopy and atomic force microscopy (AFM). The Raman study reveals a three-layer structure for the irradiated anatase powders. A dark layer of reduced oxide is sandwiched between a top coating of molten/resolidified rutile and an underlying defective, slightly oxygen-deficient mixed-phase of rutile and anatase. AFM measurements indicate that a smooth surface layer coexisting with the initial rough grain morphology gradually appears with increasing fluence. At low fluence, anatase is reduced in a dark film and further transformed into rutile. At intermediate fluence, a shiny coating of resolidified stoichiometric rutile forms on the dark film. It gets thicker as the fluence increases while darkening of the sublayer intensifies up to a maximum approximately 700 mJ cm(-2). At high fluence, however, melting and re-oxidation (and eventually ablation) prevail over reduction; the whole layer turns into a greyish crust of mostly resolidified rutile in non-ablated regions. A physico-chemical mechanism is proposed to explain the in-depth distribution of the various components as a function of fluence. (C) 2003 Elsevier Science B.V. All rights reserved.