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-NaxCoO2 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.