Ultraviolet irradiation penetration depth on TiO2

High-energy ultraviolet (UVC) irradiation of metal oxides (MOs, e.g., TiO₂) results in photoinduced surface oxygen vacancies (PI-SOVs), which can change the charge carrier (e.g., electrons and holes) migration dynamics. Although PI-SOVs alter the electronic and chemical properties of MOs, there is no consensus on the penetration depth of the UVC irradiation, which induces PI-SOVs and is an important variable for the design and operation of MO-based systems. Here, we performed optical transmission and time-resolved atomic force microscopy measurements on back-illuminated TiO₂ samples. Our experiments show that the effect of UVC irradiation on MOs can be observed hundreds of micrometers across the bulk, i.e., orders of magnitude larger than previously postulated values. We believe that our findings would be important both for the fundamental understanding of UVC irradiation/penetration and for device design/fabrication processes.