Surface Structure Modulation of La_0.6Sr_0.4CoO₃ Films on SrTiO₃ (001) Substrate under Electrochemical Conditions

The surface structure of the La_0.6Sr_0.4CoO₃ film, a typical model water-splitting catalyst, is examined under vacuum and electrochemical conditions using surface X-ray diffraction. The pristine sample has a two-unit-cell-thick strongly polarized SrCoO₃ layer at the surface, and the surface termination is predominantly a CoO₂ layer with apical oxygen atoms. After electrochemical treatment, the surface was covered with an additional edge-shared CoO₆ double layer. The polarization in the SrCoO₃ region was greatly reduced. These structural changes were accompanied by an increase in the working electrode current, suggesting a strong relationship between surface structure modulation and catalytic activity. Reversible structural modulation induced by the film's electric potential was observed and was qualitatively understood as atomic displacements caused by the local electric field and change in the Co ionic radii.