Fig. 3. SEM images of the samples: (a) undoped, (b) 0.5 at%, (c) 1 at%, (d) 1.5 at% and (e) 2 at% Cu-doped Mn3O4 samples. (f) The 3D hierarchical structure of the 1.5 at% Cu-doped Mn3O4 grains growing on Ni foam. (g) TEM image of 1.5 at% Cu-doped Mn3O4. The inset shows the SAED pattern with indexed diffraction rings. (h) HRTEM image of 1.5 at% Cu-doped Mn3O4.
