  Strontium cobaltite (SrCoOx) is known as a material showing fast topotactic
electrochemical Redox reaction so-called oxygen sponge. Although atomic scale
phenomenon of the oxidation of SrCoO2.5 into SrCoO3 is known, the macroscopic
phenomenon has not been clarified yet thus far. Here, we visualize the
electrochemical oxidation of SrCoOx macroscopically. SrCoOx epitaxial films
with various oxidation states were prepared by the electrochemical oxidation of
SrCoO2.5 film into SrCoO3-d film. Steep decrease of both resistivity and the
absolute value of thermopower of electrochemically oxidized SrCoOx epitaxial
films indicated the columnar oxidation firstly occurred along with the surface
normal and then spread in the perpendicular to the normal. Further, we directly
visualized the phenomena using the conductive AFM. This macroscopic image of
the electrochemical oxidation would be useful to develop a functional device
utilizing the electrochemical redox reaction of SrCoOx.
