High-frequency dielectric characterization of electronic defect states in co-sputtered W-doped TiO₂

Tungsten-doped titanium dioxide (TiO₂:W) has been reported to have increased photocatalytic performance as compared to undoped TiO₂. The exact mechanism behind this has been debated. Consequently, the purpose of this work is twofold: (i) synthesize TiO₂:W films with improved optoelectronic properties and (ii) refine the understanding of photocharge properties in tungsten-doped TiO₂. An in situ radio frequency magnetron-sputtering deposition process was used to fabricate undoped (TiO₂), oxygen deficient (TiO_2-x), and tungsten-doped (TiO₂:W) films with varying dopant levels. X-ray photoelectron spectroscopy measurements showed the presence of both W_Ti″ and W_Ti^x type dopants that led to significantly reduced oxygen vacancy (V_O) densities. These observations were corroborated by X-ray diffraction analysis, which revealed that the improved stoichiometry resulted in a marked enhancement of the rutile phase as compared to the sub-stoichiometric (V_O-doped) samples. Critically, high-frequency dielectric spectroscopy measurements revealed an optimal tungsten doping level of ∼2.5 at. %. This point showed the greatest tungsten induced reduction in the 2[Ti^III]–[V_O″] defect pair ɛ′ contribution, i.e., almost two orders of magnitude. Finally, this dielectrically observed reduction in V_O was correlated to an increase in photocharge decay lifetimes. In other words, photocharge lifetimes increased in accordance with the reduction of V_O defects brought on by tungsten doping.