The surface structure of the La0.6Sr0.4CoO3 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.
Keywords: X-ray diffraction; in situ measurements; surface structure; thin films; transition metal oxide surfaces; water electrolysis.