Oxidation State of Fe and Ti Ions Implanted in Yttria-Stabilized Zirconia Studied by XPS

The oxidation state of Fe and Ti ions implanted in yttria stabilized zirconia (YSZ) was studied by XPS (X-ray photoelectron spectroscopy) in combination with depth profiling using Ar + sputtering. In the "as-implanted" state of the sample Fe was found to be present as Fe 3 +, Fe 2+ and as metallic Fe °. This is in agreement with earlier conversion electron Mrssbauer Spectroscopy measurements. For Ti-implanted YSZ in the "as-implanted" state the majority of the Ti is present as TP +, Ti 3+, and Ti 2+ ions, while a part of the Zr cations is present in the divalent oxidation state (Zr 2 +). After oxidation in air, the Fe and Ti ions are present only in the valence three and four oxidation states, respectively. PACS: 79.60.Eq, 61.70.Tm X-ray photoelectron spectroscopy (XPS) is frequently used to study the chemical nature of surfaces [1]. It gives information on the chemical composition as well as the oxidation state of the elements present in the surface. Ion beams can be used for surface cleaning or depth profiling. Hence in principle it is possible to study the oxidation state of elements in thin films as a function of the depth. During sputtering preferential removal of elements may occur. These effects originate from differences in the surface binding energies and from mass differences [2]. In oxides the lattice oxygen generally is sputtered preferentially with respect to the metal ions [2, 3]. The remaining electrons of oxygen cause reduction of cations present in a thin surface layer, with a thickness comparable to the penetration depth of the bombarding ions [4]. It seems, however, not yet to be possible to correct sputtering depth profiles for these preferential sputtering effects. This study, which is part of a larger research effort [5], is intended to give information on the oxidation state of Fe and Ti ions in the "as-implanted" state of YSZ. These results are of importance for the study of the structure [6] and electrochemical properties [7] of these implanted layers. Most information available on the charge state has been obtained with conversion electron Mrssbauer spectroscopy (CEMS) [8-11]. With CEMS the oxidation state of the implanted ions, in a shallow surface layer of about 100 nm thickness, is analyzed in a non-destructive way. Few elements, however, have suitable isotopes for this technique. The charge state of 57Fe ions [11-18] can be studied very well, but the charge state of Ti cannot. In order to obtain results without any detailed description of the sputtering process at hand, a careful analysis of the evolution of the oxidation states of the cations as a function of the sputtering time is carried out with XPS. The data obtained from the samples in the "as-implanted" state are compared with the data, obtained under similar conditions, from samples with all cations in the highest oxidation state ("oxidized state"). From this analysis conclusions are drawn on the oxidation state of Fe and Ti ions in the "as-implanted" state of the YSZ sample. A disadvantage of this method is the destructive nature of the depth profiling technique and the inability to correct for changes in the oxidation state of the cations during ion bombardment, making it still impossible to quantify the results. An advantage of the method is that the oxidation state of implanted ions can be studied for elements which lack suitable isotopes for Mrssbauer spectroscopy.