Conclusion The protective coatings were fabricated on

The morphologies and dimensions of the as-prepared precursors containing cobalt and zinc before calcination are found to strongly depend on the reaction parameters such as the amount of trisodium citrate and the hydrothermal time. First, we performed the controlled experiments through varying the amount of trisodium citrate when keeping other experimental conditions unchanged. Fig. 7a displays the irregular 2D plates with non-uniform sizes without addition of trisodium citrate. These plates have smooth surfaces and a thickness ranging from tens of nanometer to hundreds of nanometer (inset in Fig. 7a). When 0.3 g of trisodium citrate CFM 1571 added, well-fined flower-like microspheres with sizes of ca. 7 μm are formed, which are assembled by numerous nanosheets (Fig. 7b and inset). With increasing the amount of trisodium citrate to 0.5 g, ill-fined flower-like microspheres appear, characterized by the loose packing of nanosheets (Fig. 7c and inset). When the amount of trisodium citrate is further increased to 0.8 g, irregular assemblies with non-uniform sizes are generated (Fig. 7d and inset). Therefore, as described above, trisodium citrate plays a key role in tuning the morphology of the precursors based on the two aspects: on one hand, trisodium citrate can help to direct the anisotropic growth of the building blocks and further assemble it into the 3D hierarchitectures. On the other hand, a certain amount of trisodium citrate benefits the formation of the well-fined flower-like microspheres. Furthermore, a series of time-dependent experiments were carried out to explore the evolution of the structures and morphologies of the as-prepared precursors before calcination. When the hydrothermal time is as short as 1 h, the precursors are composed of well-fined microspheres with sizes of 5–8 μm (Fig. 8a and b). It is noted that a lot of shallow pits are distributed on the smooth surfaces of these microspheres (inset in Fig. 8b). Along with the extension of the hydrothermal time to 3 h, the microspheres are still formed (Fig. 8c and d), but their surfaces become very coarse. High-magnification SEM image (inset in Fig. 8d) shows that the coarse surfaces are composed of large amounts of nanosheets interweaving with each other. Increasing the hydrothermal time further to 6 h, the microspheres evolve into the open hierarchitectures from the packing of nanosheets (Fig. 8e). Finally, the well-fined flower-like microspheres assembled by nanosheets are formed when the hydrothermal time is enhanced to 10 h (Fig. 8f).