Atomic-Scale Observation of Oxygen Vacancy-Induced Step Reconstruction in WO₃

As the most common type of defects in WO3, surface oxygen deficiencies have been widely investigated in terms of their structure as well as physicochemical properties. In general, oxygen vacancy on the surface may lead to the reconstruction of the surface structure due to the need for surface energy minimization, especially for the area with high oxygen vacancy concentration. However, such a phenomenon has not been directly demonstrated or observed from the experiment. In the paper, we observed such surface reconstruction in the step area or high-index facets in WO3 using the spherical aberration-corrected scanning transmission electron microscope (Cs-STEM) with atomic-scale resolution. This surface reconstruction results in a structure of 5-fold symmetry pentagonal columns, which is first reported. According to the DFT calculation, the formation energy of oxygen vacancy in the step edge is much lower than the smooth surface. Furthermore, the formation energy of oxygen vacancy in the higher index (110) facet is lower than the low index (100) and (010) facets. Our experimental results support these findings, i.e., that the reconstruction is pronounced in the step or high-index facets with a high concentration of oxygen vacancy. Therefore, the high concentration of oxygen vacancy in the step or high-index facet area contributes to the formation of pentagonal column structures. This work could provide a novel insight into the correlation between intrinsic defect and surface structure formation in these materials.