###### Abstract  
In this work, mesoporous nickel oxide (NiO) hierarchical nanostructures were synthesized by a facile approach by hydrothermal reaction and subsequent calcination. The phase structure, microstructure, element composition, surface area, and pore size distribution of the as-prepared products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and the Brunauer-Emmett-Teller (BET) technique. The precursor of Ni\({}_{3}\)(NO\({}_{3}\))\({}_{2}\)(OH)\({}_{4}\) nanosheet, Ni\({}_{3}\)(NO\({}_{3}\))\({}_{2}\)(OH)\({}_{4}\) microsphere, and Ni(HCO\({}_{3}\))\({}_{2}\) sub-microsphere was obtained by hydrothermal reaction at 160 \({}^{\circ}\)C for 4 h when the ratio of Ni\({}^{2+}\)/HMT (hexamethylenetetramine) was 2:1, 1:2, and 1:3, respectively. After calcination at 400 \({}^{\circ}\)C for 2 h, the precursors were completely transformed to mesoporous NiO hierarchical nanosheet, microsphere, and sub-microsphere. When evaluated as additives of the thermal decomposition of ammonium perchlorate (AP), these NiO nanostructures significantly reduce the decomposition temperature of AP, showing obvious catalytic activity. In particular, NiO sub-microsphere have the best catalytic role, which can reduce the high temperature decomposition (HTD) and low temperature decomposition (LTD) temperature by 75.2 and 19.1 \({}^{\circ}\)C, respectively. The synthetic approach can easily control the morphology and pore structure of the NiO nanostructures by adjusting the ratio of Ni\({}^{2+}\)/HMT in the reactants and subsequent calcination, which avoids using expensive templates or surfactant and could be intended to prepare other transition metal oxide.  
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Footnote †: journal: Applied Physics Letters  
_Applied_  
developed a sol-gel method to prepare porous NiO nanostructures using Tween-80 as a non-ionic surfactant and demonstrated excellent photocatalysis and sensing properties. [PERSON] et al. [18] reported NiO hierarchical flower-like nanostructures prepared by the solvothermal method, in which cetyltrimethylammonium bromide was used as a surfactant. [PERSON] et al. [19] synthesized hierarchical NiO tremella-like and pinecone-like microspheres by the solvothermal method. The NiO microspheres demonstrated good supercapacitor properties. [PERSON] et al. [20] used \(\alpha\)-Ni(OH)\({}_{2}\) as a precursor to prepare chrysanthemum-like NiO microspheres which exhibited superior lithium storage properties. [PERSON] et al. [21] prepared porous NiO hierarchical microtubes by a novel template-free method, which exhibited superior electrochemical performance due to their high surface area and pore volume.  
Although some progress has been made, most of the reported methods require surfactants or templates, and thus it is difficult to obtain mesoporous NiO hierarchical materials with multi-morphologies and controllable pore structure. Therefore, it is highly desirable to develop a facile approach to mesoporous NiO hierarchical materials with multi-morphologies.  
In this paper, we developed a rapid and easy approach to mesoporous NiO hierarchical nanostructures by hydrothermal reaction and subsequent calcination. By adjusting the ratio of Ni\({}^{2+}\)/HMT (hexamethylenetramine) in a hydrothermal reaction, the precursors of Ni\({}_{3}\)(NO\({}_{3}\))\({}_{2}\)(OH)\({}_{4}\) nanosheet, Ni\({}_{3}\)(NO\({}_{3}\))\({}_{2}\)(OH)\({}_{4}\) microsphere, and Ni(HCO\({}_{3}\))\({}_{2}\) sub-microsphere were prepared, which transformed to mesoporous NiO nanosheets, microspheres, and sub-microspheres by simple calcination. These mesoporous NiO nanostructures can effectively reduce the thermal decomposition temperature of ammonium perchlorate (AP), thereby indicating the potential application as additives for AP thermal decomposition. The approach reported here could be intended to prepare other transition metal oxide.