An unusual temperature gradient crystallization process: facile synthesis of hierarchical ZnO porous hollow spheres with controllable shell numbers

In this study, three types of hierarchical ZnO porous hollow spheres consisting of ZnO nanoparticles with controllable shell numbers were synthesized through different temperature gradient processes using a simplified hard template method. Compared with the traditional hard template method, we facilitated the experiment by the fabrication of zinc precursor-carbon microsphere composites during the formation of carbon microspheres. The shell numbers of the spheres could be controlled by varying the temperature gradient rate during the calcination procedure. Importantly, a different formation mechanism is proposed in this experiment. That is, the synthesis of hierarchical multi-shelled porous hollow spheres might be based on not only the inside-out Ostwald ripening process but also on an unusual temperature gradient crystallization mechanism. Further investigation also revealed that the samples exhibited a varied photocatalysis performance with the efficiency decreasing from the hierarchical triple-shelled porous hollow spheres to the single-shelled spheres progressively. What is more, these hierarchical hollow materials have a good recycle performance, which will be a great potential material in real applications. Our present work not only provides a method, but also, more importantly, provides a theoretical guide to fabricate the multi-shelled porous hollow materials with hierarchical structures, which can be used in many industrial applications.