Electrochemical preparation of defect-engineered titania: Bulk doping versus surface contamination

Defect-engineered or substoichiometric TiO_x is of interest for use in photo- and electrocatalytic processes both as active material and catalyst support. Electrochemical doping of TiO₂ via cathodic polarization is an appealing preparation method and frequently employed. Here, we explored the electrochemical preparation of TiO_x in an undivided cell using iridium-based (iridium mixed-metal-oxide) and boron doped diamond (BDD) counter electrodes. Cyclic voltammetry and impedance spectroscopy revealed superior charge transfer properties of crystalline TiO_x electrodes prepared with BDD (TiO_x-BDD). It is shown that the electrochemical properties correlate well with intensities of the H-signals determined using Time of Flight - Secondary Ion Mass Spectrometry (ToF-SIMS). Thus, it is concluded that electrochemical preparation using BDD causes favourable H⁺ intercalation and/or H diffusion into the sub-surface layers of TiO_x. Our extensive analysis using a combination of electrochemical and surface characterization (LEIS and XPS) techniques, additionally suggests that cathodic deposition of Ir, originating from the Ir-based counter electrode, present in sub-ppm concentrations only results in less-efficient doping. Instead in the presence of sub-ppm level Ir contamination hydrogen evolution is favoured during cathodic polarization. The results presented within this study highlight the necessity to use inherently stable counter electrodes for electrochemical preparation and reveal the pronounced influence of trace contamination in electrochemistry in general and the doping mechanism of TiO_x electrodes in particular.