Defects promoted topotactic transformation from Co(OH)2 nanodiscs to Co3O4 nano octahedra

Abstract

Topotactic transformation was one of the most frequently related processes in the synthesis process of cobalt oxide nanomaterials. Microstructural evolution studies of this process inform and enable the process of controlling the shape of the final products. One such topotactic transformation was investigated here, the transition from Co(OH)2 hexagonal nano platelets to Co3O4 octahedra by a hydrothermal method in an ammonia solution. Evolution of phase and microstructure was studied during different stages of the reaction to reveal the criteria which prompt this transformation. A phase transformation from Co(OH)2 to CoOOH and hence to Co3O4 was observed. High resolution transmission electron microscopy and image simulation were utilised to disclose the role of defects in the topotactic transformation. A high density of defects including edge dislocations and displacements was found on the surface of dried Co(OH)2 nano platelets. The coexistence of CoOOH domains embedded in the Co(OH)2 surface indicates the absence of the hydrogen bonds of the starting materials. The breaking of hydrogen bonds leads to bonding variation and the further creation of defects, finally prompting the transformation from Co(OH)2 to CoOOH. The breaking of hexagonally arranged hydrogen bonds creates seeds for the formation of Co3O4.