## 1 Introduction  
Many scientific results have shown that the surface structure of materials, such as the fine features of the surface, shape, and morphology, can affect physical, chemical, and electrochemical properties to some extent [1; 2; 3]. An understanding of the controlling factors for the fabrication of materials with the desired crystal structure, shape, and morphology is important for many applications, such as electrocatalysts, electrode materials for energy storage, and drug delivery. Manganese oxide (MnO\({}_{2}\)) can be widely applied in many areas--particularly as electrode materials for batteries and electrochemical supercapacitors [4; 5; 6; 7; 8], in water treatment [9], and in the demetallization of petroleum residues [10]. The liberated tunable physical and chemical properties come from different structure forms, such as \(\alpha\)-MnO\({}_{2}\) (Hollandite), \(\beta\)-MnO\({}_{2}\) (Pyrolusite), \(\gamma\)-MnO\({}_{2}\) (Nsutite), as well as \(\delta\)-MnO\({}_{2}\) (Birnesite). Each structure form favours more specific properties than the others. For example, as an electrode material, \(\alpha\)-MnO\({}_{2}\) displays a better electrochemical activity than do other forms [11; 12; 13; 14; 15; 16]. It is generally accepted that the morphology and crystal structure of MnO\({}_{2}\) play an important role in improving the performance of many activities, including electrochemical activities [17; 18; 19; 20]. Besides the interesting aspects of its crystal structure, the development of nanotechnologies makes it possible to design MnO\({}_{2}\) with different shapes, such as nano-belts, wires, fibres, and rods [21; 22; 23; 24]. Such nanomaterials can act as nano-building blocks that construct microstructures with sophisticated shapes, such as those of flowers, cubes, bird's nests, and sea urchins, which influence physical and chemical properties [25; 26]. Nevertheless, the variety of crystal forms and requirements for the well-defined surface morphology and crystallographic geometry demand a high-level controlling synthetic process for fabricating MnO\({}_{2}\) substances. Accordingly, many chemistry routes have been validated in producing the nanostructured manganese oxide with the desired shapes, morphologies, and crystal structures [27; 28]. For example, [PERSON] et al. [9] reported the synthesis of MnO\({}_{2}\) hollow nanostructures with either a spherical or cubic shape via a template-assistant route and claimed better efficiency in removing organic waste from water. [PERSON] et al. [10] reported a morphology-controlled synthesis of doped MnO\({}_{2}\) via the reflux method. A so-called bird's nest structure has been produced on BET surface areas in a range of 90-280 m\({}^{2}\)/g, varying in introduced metals, such as Co, Cu, and Fe, and in concentrations of the metals involved in the synthesis. In efforts to produce different crystal structures of MnO\({}_{2}\), [PERSON] et al. [29] demonstrated that synthesis using a hydrothermal method might be favourable for producing \(\alpha\)-, \(\beta\)-, and \(\delta\)-MnO\({}_{2}\), while \(\gamma\)-MnO\({}_{2}\) was fabricated through a sol-gel method. With an acidic aqueous solution, [PERSON] et al. [30] synthesized hollow-structured \(\gamma\)-MnO\({}_{2}\) assembled by nanorods. The formation of such a structure has been attributed to the presence of Mn\({}^{2+}\) ions in solution that promote the crystallization of \(\gamma\)-MnO\({}_{2}\). This process also involved a solid-state reaction, performed by grinding the starting materials before the wet chemistry process. Considering mass production, a simple and inexpensive synthetic route is the most desirable process and can be easily adopted for industrial applications [31]. In this regard, the materials produced via a solid-state reaction are easy to scale up at low cost. Many inorganic oxides can be obtained via the solid-state reaction. The resulting materials, however, need further treatments at high temperatures to produce a well-defined crystal structure with smooth surfaces.