Evolution of step morphology on vicinal sapphire (1 −1 0 2) surfaces accompanied with self-assembly of comb-shaped chemical domains

► We report morphological evolution on sapphire (1 −1 0 2) surfaces during annealing in air. ► Anisotropy of the surface occasionally appears as elongation of atomic-height islands and voids near the step edges. ► On another case, the anisotropy induces spontaneous formation of a comb-shaped surface. ► It has phase-separated surface with two domains characterized by different hydrophilicity.