Electronic properties of ion-implanted yttria-stabilized zirconia

Ion implantation of iron and titanium has been applied to modify the surface properties of polycrystalline yttria-stabilized zirconia ((ZrO₂)_0.87(YO_1.5)_0.13≙(YSZ)) discs in an attempt to prepare surfaces with a mixed conductivity and by this an enhanced surface oxygen exchange kinetics. Surface-sensitive spectroscopic techniques were applied to investigate the implanted layers as a function of different pretreatments such as oxidation, reduction and annealing. Depth profiles were recorded by Rutherford Backscattering Spectroscopy (RBS) and X-ray Photoelectron Spectroscopy (XPS) in combination with sputtering. Ion Scattering Spectroscopy (ISS) and XPS were used to investigate the surface composition and valency of implanted ions. Electronic properties like the band gap, the work function and the energy difference between the Fermi level and valence band edge (E_F-E_V) were obtained from Ultraviolet Photoelectron Spectroscopy (UPS) and Electron Energy Loss Spectroscopy (EELS). Overlayers of Fe₂O₃ or TiO₂ are formed during oxidation of as-implanted samples. The Fe- and Ti-oxides could be reduced in hydrogen to the oxidation states Fe²⁺, Fe⁰ or Ti³⁺. Annealing of the samples leads to decreased surface concentrations of the implanted ions due to in-diffusion. At the surface of the annealed iron-implanted samples, Fe²⁺ and metallic Fe could be generated after further reduction whereas at the surface of the annealed Ti-implanted samples only Ti⁴⁺ was detectable.