Investigation of time variations of redox reactions in SrRuO3 through interface-sensitive resistance measurements

We used an interface-sensitive resistance measurement technique to observe the time evolution of the early stages of redox reactions at the interface between the metallic oxide SrRuO3 and a silver epoxy electrode at around room temperature ( similar to 280-320 K). On exposure to a reducing gas (CO or H-2), the interface resistance gradually increased. The time variation functions of the resistance increases for CO and H-2 were similar, although the magnitude was greater for H-2 than for CO. After substitution of O-2 for the reducing gas, the interface resistance decreased to almost the initial value, i.e., that before exposure to a reducing gas. The resistance variations can be well explained by the time variations of the oxygen deficiency at the SrRuO3 interface and subsurfaces as a result of reduction or oxidation. We regarded the reactions as pseudo-first-order reactions and evaluated the rate constants of the SrRuO3 redox reactions at a SrRuO3-Ag interface. For the reduction (resistance increase) process, a single exponential component was enough to fit the data, which suggests that the reduction proceeds successively from outside to inside the bulk. Adsorption of oxygen atoms in the oxidation (resistance decrease) process involved several rate constants, at least up to three exponential components, depending on the prior degradation by a reducing gas. The effective activation energy of each redox reaction was evaluated from an Arrhenius plot.