Figure 3: Materials characterizations of the NiCo₂S₄ ball-in-ball hollow spheres after annealing in N₂.

(a) XRD pattern, (b,c) FESEM images. Scale bars, 500 and 100 nm, respectively. (d,e) TEM images. Scale bars, 100 nm. (f) Enlarged TEM image of the NiCo₂S₄ mesoporous shell. Scale bar, 20 nm.