Surface reconstruction in the CuO(001) surface: From square-planar to tetrahedral coordination for CuO₄ unit

Surface atomic structure is closely related to the physical and chemical properties of materials. Although cupric oxide (CuO) surfaces have been widely reported, to the best of our knowledge, the low Miller index and easily exposed CuO(0 0 1) surface has not been yet investigated in experiments at the atomic scale. Herein, for the first time, we report the atomic structures of CuO(0 0 1) surface combining aberration-corrected scanning transmission electron microscopy and first-principles calculations. Interestingly, akin to the (1 1 1) surfaces of FeO, CoO and NiO, a unusual tetrahedral reconstructed structure was revealed for the CuO(0 0 1) surface. The low energy barrier from square-planar to tetrahedral coordination for the surface CuO₄ unit was found by the climbing-image nudged-elastic band method.