Rapid Determination of the Optimum Thickness of Doped TiO₂ Nanotubes Layer by Local Photoelectrochemistry and Operando Properties Extraction by Data Modeling

Herein, the optimal thickness of TiO 2 , M‐doped, N‐doped, and (M:N) codoped TiO 2 (with M = Nb or Ta) nanotubes (NTs) for photoelectrochemical (PEC) water splitting is determined, in one step through local PEC analysis of electrodes with variable thickness of the photoactive material. A simple anodization method allows the growth, on a single electrode, of aligned doped TiO 2 NTs (TiO 2 ‐NTs) whose length gradually changes from 0 to 15 μm. The external quantum efficiency as a function of the NTs film thickness is measured using a small beam of UV or visible light to scan the surface and locally trigger the oxygen evolution reaction. The results provide the optimal NT layer thickness for each type of doped TiO 2 ‐NTs and demonstrate that if the (M:N) codoped TiO 2 ‐NTs have the best activity in the visible region, the optimal layer thickness is reduced compared to TiO 2 or M:doped TiO 2 ‐NTs. Additionally, by fitting the data with a model that incorporates charge carrier transfer and light absorption mechanisms in the film, it becomes possible to evaluate in operando the interfacial charge transfer, absorption coefficient, or majority carrier mean path which are important properties of semiconducting materials for PEC applications.