Topotaxy in Layered Cobalt Oxide/Oxyhydroxide Thin Films

Inorganic metal hydroxides have found various applications in catalysis, batteries, sensors, and adsorbents. Epitaxial thin films offer well-defined crystallographic orientations and uniform surfaces, providing a prominent platform for the fundamental study of anisotropic and surface-driven functionalities. However, no reports have demonstrated the control of the crystallographic orientations of the metal hydroxide epitaxial thin films. In this study, using β-CoOOH as a model system, we demonstrate crystallographic orientation control of metal hydroxide epitaxial thin films using "topotaxy" reaction. We first fabricate the precursor β-Na_xCoO₂ epitaxial thin films through a reactive solid-state epitaxy using seven different single-crystal substrates: Al₂O₃(0001), SrTiO₃(100), and (110). (111), and MgAl₂O₄(100), (110), (111). Subsequently, the precursor films are immsersed in an acidic solution and transformed into successfully obtain β-CoOOH (001) and (012) epitaxial thin films on the Al₂O₃(0001) and SrTiO₃(100) substrates, respectively. Furthermore, we confirm that the surface morphology reflects the crystallographic orientations and that the coordination environment of Co ions is consistent with that of the bulk material. This study represents a significant step forward in fabricating metal hydroxide epitaxial thin films, paving the way for developing novel functionalities related to hydroxyl groups.