Direct growth of three-dimensional nanoflower-like structures from flat metal surfaces.

Here we report on a facile top-down approach for the direct growth of Co 3 O 4 hierarchical nanoflowers from a bulk Co surface via chemical etching and thermal annealing. The effect of the annealing temperature was investigated, showing that amorphous Co 3 O 4 was formed at 250 °C, while crystalline Co 3 O 4 with notable oxygen vacancies was created at 550 °C. The formed 3D nanostructures exhibited excellent oxygen evolution reaction (OER) activities with a low overpotential of 0.34 V at 10 mA cm -2 and high durability. The proposed novel approach was further demonstrated by the direct growth of 3D NiO and CuO nanostructures on Ni and Cu substrates.