Optoelectronic and photocatalytic properties of in situ platinum-doped TiO₂ films deposited by means of pulsed laser ablation technique

We report on the effect of Pt incorporation on the microstructural, optoelectronic and catalytic properties of TiO₂ thin films deposited by means of the pulsed-laser deposition (PLD) method. The structural, morphological, optoelectronic and catalytic properties of the deposited TiO₂:Pt films were systematically investigated, as a function of their Pt content, by means of various techniques including X-ray diffraction, X-ray photoelectron spectroscopy (XPS), atomic force microscopy, Fourier-transform infra-red spectroscopy and ellipsometry. The in situ doping of the PLD-TiO₂ films, at different Pt contents, was achieved by the concomitant laser ablation of a TiO₂ target uniformly covered with Pt wires. We show that by increasing the number of Pt wires positioned on the TiO₂ target from 1 to 4, the Pt concentration of the TiO₂:Pt films increases from 0.05 to 0.35 at.%. While the bandgap of room-temperature (RT) deposited films was found to be insensitive to their Pt content (remaining around the 3.2 eV anatase value), the TiO₂:Pt films deposited at high temperature (T_d = 500 °C) were found to exhibit a significant narrowing of their bandgap (from 3.3 to 2.45 eV), when their Pt content is increased (from 3.3 to 2.45 eV). From a structural point of view, while the RT-deposited TiO₂:Pt films deposited are mainly amorphous with the presence of some TiO₂ nanocrystallites whose density increases as Pt content increases, while those deposited at 500 °C were found to crystallize in the anatase phase with their crystallinity improving with increasing Pt contents. XPS analyses have revealed that the incorporated Pt mainly exists in a metallic state, likely as nano-inclusions in an otherwise TiO₂ matrix. By being able to monitor the optoelectronic properties of the annealed TiO₂:Pt films through their Pt content, their photocatalytic activity was significantly improved with respect to that of their undoped TiO₂ counterpart. The enhancement of the photocatalytic activity is believed to be due to an enhanced photoabsorption of photons because of bandgap narrowing along with a better harvesting of the separated photogenerated charges.