Sub-stoichiometric titanium dioxide (TiO_x) is known for its corrosion resistance and electrical conductivity.¹ Thus, it is recognized as promising material for (photo-)electrochemical applications and used as protective layer in PEC water splitting or electrode material for water treatment.^1,2 In general, its properties strongly depend on the synthesis and the conductivity of TiO_x can be tuned by its Ti/O ratio.^1,3 TiO_x is commonly prepared by hydrogenation;³ however, the requirements for high pressure, high temperature and hydrogen atmosphere are cumbersome imposing a potential safety risk. Over the past years, electrochemical preparation of TiO_x via cathodic polarization of TiO₂ substrates became an interesting alternative.⁴

Pt is widely used in electrochemistry as counter electrode⁵ and frequently applied for TiO_x preparation;⁴ however, the suitability of Pt as (anodic) counter electrode (CE) is questionable since the stability of Pt is often overestimated.⁵ As Pt contamination can severely influence the electrocatalytic properties this study focuses on the preparation of TiO_x using three different CE (Pt, Iridium mixed metal oxide, Boron doped diamond) and the influence of the CE will be discussed in detail.

It will be shown using state of the art techniques such as SEM, XPS and highly surface sensitive low energy ion scattering (LEIS) spectroscopy that indeed Pt contamination dominates the properties of TiO_x (Figure 1a). In contrast, for Iridium mixed metal oxide and Boron doped diamond CEs no related contaminations were observed allowing to determine the intrinsic properties of TiO_x electrodes. Using Fe^2+/3+ redox couple experiments and electrochemical impedance spectroscopy we determined the charge transfer properties of the TiO_x electrodes prepared with BDD and IrMMO (Figure 1b). Although both BDD and IrMMO appear to be suited for the preparation of TiO_x differences in the doping are observed that will be discussed in detail in this contribution.

References:

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2. B. Xu, H. Y. Sohn, Y. Mohassab, and Y. Lan, RSC Adv., 6, 79706–79722 (2016), doi: 10.1039/c6ra14507h.
3. X. Yan, L. Tian, X. Tan, M Zhou, L. Liu and X. Chen, MRS Commun., 6, 192–203 (2016), doi: 10.1557/mrc.2016.33.
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