Pitch‐black surface stemming from self‐standing ZnFe ₂ O ₄ nanowalls

Combinations of surface nanostructures with the intrinsic photo‐interactivity of constituent materials could enhance photoabsorption properties. Although the intrinsic photoabsorption properties of ZnFe 2 O 4 in a broad wavelength range have been reported, the photoabsorption properties of ZnFe 2 O 4 with surface nanostructures have not been investigated yet. In this study, a pitch‐black surface, which is covered with a ZnFe 2 O 4 nanowall array, is generated via a facile synthesis method, that is, heating a pre‐electrodeposited surface layer comprising Zn and Fe in air. The key factors for generating the nanowall array are the Zn content in the pre‐electrodeposited ZnFe layer and the heating temperature. The generation mechanism of the nanowall array is proposed based on the thermodynamics of oxides and the dynamics of crystal growth. The black surface absorbs greater than 94% and 97% of the incident light over wavelength ranges of 200–2500 nm and 200–1100 nm, respectively. Surfaces with the ZnFe 2 O 4 nanowall array are promising for various optical, environmental and energy applications owing to their unique nanostructure and the intrinsic functional properties of ZnFe 2 O 4 .