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

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.