Novel insight into the epitaxial growth mechanism of six-fold symmetrical β-Co(OH)2/Co(OH)F hierarchical hexagrams and their water oxidation activity

The six-fold symmetry widely presents in both natural and artificial architectures. Understanding the growth mechanism of six-fold symmetrical materials is of fundamental interest and significance. Herein, we report the formation process of β-Co(OH)2/Co(OH)F hierarchical hexagrams with a six-fold symmetrical arrangement. Our results demonstrate that hexagonal β-Co(OH)2 plates are first formed under the reaction condition. These hexagonal plates then act as templates for the growth of Co(OH)F nanorods. The intermediate material is therefore composed of plate-like β-Co(OH)2 hexagonal cores appended with six rod-like Co(OH)F branches, giving the β-Co(OH)2/Co(OH)F hybrid. After prolonged reaction, the β-Co(OH)2 hexagons can be completely converted, leading to authentic six-branched Co(OH)F nanorods as the final product. Consequently, for both intermediate and final materials, the Co(OH)F nanorods are arranged with a six-fold symmetry. Importantly, these Co(OH)F nanorods grow along β-Co(OH)2 hexagon edges as lateral branches instead of perpendicular to hexagons. This uncommon epitaxial growth mechanism is considered to be a result of the matching between the b-axis of Co(OH)F crystals and the a-axis of β-Co(OH)2 crystals, which is beneficial for the electrocatalysis. The β-Co(OH)2/Co(OH)F hierarchical hexagrams show enhanced water oxidation activity compared to the pure β-Co(OH)2 and Co(OH)F.