A complete understanding of structure-function relationships in well-defined model catalysts can add new dimensions to our fundamental view of "real world" catalysis as well as rationally design catalysts at an atomic-level.In this work,perovskite ferroelectric KNbO3 nanowires with respective orthorhombic and monoclinic polymorph have been systematically compared in terms of the geometric and electronic structure.In particular,the surface photocatalytic active sites and local atomic displacements are probed by aberration-corrected scanning transmission electron microscopy(STEM),and the local ferroelectric polarization is quantified with the aid of first-principles calculations.