Biparametric optical sensing of oxygen by titanium dioxide

Titanium dioxide (TiO2) is at the basis of many important applications which are decisively controlled by its photophysical characteristics. Photoluminescence (PL) analysis provides important information on these latter, while also representing a less explored and possibly multi-parametric route to chemical sensing. In the present work we show that oxygen exposure produces opposite responses in the PL activity of the two most common TiO2 polymorphs (rutile and anatase) and discuss the origin of such a phenomenon, focusing in particular on the near-infrared PL characteristic for rutile TiO2. We propose an interpretation of the experimental findings in terms of two possible O-2/TiO2 interaction routes, namely: (a) molecular chemisorption of oxygen and consequent dynamics of self-trapped hole states close to the sample surface and (b) dissociative chemisorption of oxygen, affecting the density of oxygen vacancies at (or close to) the surface. The remarkable result of an individual analyte producing different detectable effects on a same sensing material epitomizes the potentialities offered by an optical PL-based approaches toward multi-parametric chemical sensing, encouraging the exploration beyond the established chemoresistive approaches which are based on a single parameter. Moreover, it highlights interesting prospects for TiO2-based optical sensing.