Phase Transformations in the Pulsed Laser Deposition Grown TiO2 Thin Films as a Consequence of O-2 Partial Pressure and Nd Doping

摘要

In this work, we present the pronounced phase transformation in the TiO2 thin films grown in simultaneously varied oxygen (O-2) partial pressure and doping concentration of neodymium (Nd). Pulsed laser deposition (PLD) was employed for the synthesis of TiO2 films in varied O-2 partial pressures from 100 mTorr (O-2 rich) to 0 mTorr (O-2 absent) for each Nd doping concentration (1.0, 1.5, and 2.0 at. %). The phase and structural studies confirmed the systematic phase transformation from the anatase to the rutile to the metastable and finally to the amorphous phase as the O-2 content was reduced and Nd doping concentration was increased. A drastic reduction in the crystallite size was also observed. XPS confirmed the reduction of Ti (Ti4+ to Ti3+), creation of O vacancies, and consequently increased nonstoichiometry and short-range ordered TiO2 lattice. The experimental findings were complemented with the density functional theory calculations of surface energies and crystal facet areas of the anatase and rutile phases. The Wulff equilibrium particulate shapes of the anatase and rutile phases confirmed the possibility of transformation due to increased Nd doping and reduced oxygen. Thus, the obtained results suggested that the Nd doping and O-2 concentration during the growth were key parameters to precisely control the phase of the TiO2 films and its transformation from the anatase to rutile to a metastable phase and finally to the amorphous phase.