In situ imaging and impedance measurements of titanium surfaces using AFM and SPIS

Surfaces of commercially pure titanium and titanium, 6-aluminum, 4-vanadium were subjected to simultaneous polarization/impedance testing and in situ electrochemical atomic force microscopy imaging to evaluate how the structure and properties of the passive oxide film is affected by varying potential and hydration. Current transients were acquired via a step polarization impedance spectroscopy technique: the voltage was stepped between -1 and 1V in 50mV increments, while current transients and surface morphology were digitally recorded. Numerical Laplace transformation applied to the current transient data provided frequency-dependent admittance (impedance^-1). Simultaneous AFM imaging of dry surfaces, initially hydrated surfaces, and surfaces immersed and changing with potential revealed that all sample surfaces were covered with protective titanium oxide domes that grew in area and coalesced due to hydration and as a function of increasing applied voltage and time. Reversal of dome growth did not occur upon voltage reduction, while impedance behavior was quasi-reversible, suggesting independence between structural and electrical properties. Oxide growth appeared to occur in part by lateral spreading and overgrowth of domes at the oxide-solution interface. Interfacial impedance data reflect oxide passivity and n-type semiconductor behavior. Non-linear Mott-Schottky fits specified multi-layer donor concentrations between 10¹⁸ and 10¹⁹cm^-3, depending on the surface.