Co₃O₄@(Fe-Doped)Co(OH)₂ Microfibers: Facile Synthesis, Oriented-Assembly, Formation Mechanism, and High...

Cobalt oxide or hydroxide nanoarchitectures, often synthesized via solvothermal or electrodeposition or templated approaches, have wide technological applications owing to their inherent electrochemical activity and unique magnetic responsive properties. Herein, by revisiting the well-studied aqueous system of Co/NaBH₄ at room temperature, the chainlike assembly of Co₃O₄ nanoparticles is attained with the assistance of an external magnetic field; more importantly, a one-dimensional hierarchical array consisting of perpendicularly oriented and interconnected Co(OH)₂ thin nanosheets could be constructed upon such well-aligned Co₃O₄ assembly, generating biphasic core-shell-structured Co₃O₄@Co(OH)₂ microfibers with permanent structural integrity even upon the removal of the external magnetic field; isomorphous doping was also introduced to produce Co₃O₄@Fe-Co(OH)₂ microfibers with similar structural merits. The cobalt-chemistry in such a Co/NaBH₄ aqueous system was illustrated to reveal the compositional and morphological evolutions of the cobalt species and the formation mechanism of the microfibers. Owing to the presence of Co₃O₄ as the core, such anisotropic Co₃O₄@(Fe-doped)Co(OH)₂ microfibers demonstrated interesting magnetic-responsive behaviors, which could undergo macro-scale oriented-assembly in response to a magnetic stimulus; and with the presence of a hierarchical array of weakly crystallized thin (Fe-doped) Co(OH)₂ nanosheets with polycrystallinity as the shell, such microfibers demonstrated remarkable electrocatalytic activity toward oxygen evolution reactions in alkaline conditions.