(Invited) Local Redox Reaction By Ion Transport Probed By in-Situ PES Measurements

Wednesday, 4 October 2017: 15:40
Camellia 4 (Gaylord National Resort and Convention Center)
T. Tsuchiya (NIMS)
It is now evident that ion migration in oxides governs the memristive behavior of oxide thin film. After various approaches, still there are some uncertainties about the relation between ion migration and memristive behaviors. Complexities lie in the the ion migration behavior and redox reaction in oxides and at interface between oxide electrolyte and electrode. The mode change from frost-like to filament growth of reduced domains are of another issue to be discussed. In the present study, the author employed in-situ measurement of photoemission spectroscopy (PES) using synchrotron radiation to look at the redox reaction in oxide just underneath the electrode. The author employed thin Au electrode with 3-5 nm in thickness, using hard X-ray, for the excitation of photoelectrons 3-5nm underneath the Au electrode. Various type of “electrolytes,” Gd-doped ceria(GDC), amorphous TaOx(a-TaOx), amorphous silica(a-SiO2), and so on, are employed to look at the electrochemical redox reaction in the vicinity of electrode. In case of GDC, XAS measurements have been also made to look at the redox reaction in deeper region from the electrode/oxide interface.

In-situ PES measurements on various oxide thin film systems examined with MIM-type two-electrode cells revealed the variation of Fermi level upon electrochemical polarization, suggesting variations of local electrochemical redox state by the ion migration. Surprisingly narrow electrochemical windows were suggested limited by a pinning at the donor state of component cations and a chemical pinning due to oxygen evolution, in contrast to the ultra-wide band gap of these almost insulating oxides systems. The importance of the reversibility and how to control the kinetics at the anodic electrode should be emphasized. One important conclusion is the system is identical to the simple two-electrode cell composed of the electrolyte that shows variation of Fermi level within an electrochemical window of oxide system by ion migration or electrochemical reaction and modulated overpotential at electrolyte/electrode interfaces, the latter of which gives huge influence to the cell behaviors. These suggest that oxide memristor is governed by the electrochemical properties of the cell. Finally, the author proposes both a new Pourbaix-type E-pO diagram to understand the redox and acid/base reaction in addition to new proposal of the definition of basicity of complex oxide based on electronic band structure, as a necessary framework of solid state electrochemistry for oxide systems.