Fig. 1.

Graphical illustration of yolk–shell structure: a single shell with single yolk, b double shells with a single yolk, c multiple shells with a single yolk, and d single shell with multiple yolks. Reprinted with permission from Refs. [21, 55, 133]. TEM images of: e single shell with single yolk, f double shells with single yolk, and g single shell with multiple yolks. h SEM, i TEM, j HRTEM and k element mapping images of triple shells with single yolk. Reprinted with permission from Refs. [58, 60, 62, 68]