**4.3 Multishelled hollow spheres**

ZnO with multishelled hollow spheres of 5 mm in diameter were prepared by a facile solvothermal process in a ternary solvent system. The anisotropic singlecrystalline nanorod building blocks with uniform diameter of about 40 nm are highly directional in the as-synthesized products. The evolution process of hollow ZnO core-shell structures consists of a two-step self-assembly process and symmetric Ostwald ripening starting from the inner core and at the interface of the core and shell. The hollow spheres and core-shell structures could be obtained by simply adjusting the composition of the mixed solvents, as is critical to the packing design [28]. In comparison of the hollow structures and nanorods at all working temperatures, the double-wall hollow structures exhibited the highest sensitivity to formaldehyde gas due to the high donor-related (DL) and the low acceptor-related

#### **Figure 11.**

*(a and b) Typical top view and (c) cross-sectional SEM images of the vertically aligned ZnO rods with flowerlike structures. Inset of* **Figure 10(a)** *shows magnified image of one of the flowers.*

(AL) intrinsic defects. ZnO semiconductor with high response and selectivity to H2S as well as good stability was obtained by controlling the shape-defined building units and their assembly structure.
