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

Abstract

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 (TiO2-x), and tungsten-doped (TiO2:W) films with varying dopant levels. X-ray photoelectron spectroscopy measurements showed the presence of both WTi″ and WTix type dopants that led to significantly reduced oxygen vacancy (VO) 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 (VO-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[TiIII]–[VO″] defect pair ɛ′ contribution, i.e., almost two orders of magnitude. Finally, this dielectrically observed reduction in VO was correlated to an increase in photocharge decay lifetimes. In other words, photocharge lifetimes increased in accordance with the reduction of VO defects brought on by tungsten doping.