# Zinc-Decorated and Nitrogen-Functionalized Hierarchical Porous Carbons for Carbon Dioxide Capture
## 1 Introduction  
One of the effective ways to mitigate climate change is to reduce carbon dioxide (CO\({}_{2}\)) emissions. During the transition period to reach a low-carbon society, CO\({}_{2}\) capture may be the most effective strategy to reduce atmospheric carbon dioxide levels. Among them, CO\({}_{2}\) adsorption is known as an available technique to achieve this goal. The synthesis of porous carbon materials using zeolitic imidazolate framework (ZIF) polyhedrons as the raw materials has attracted much attention recently for green energy and environmental applications [1, 2, 3]. In particular, porous carbons characterized with micropores [4, 5] and heteroatom surface functional groups [6, 7] have been recommended for CO\({}_{2}\) capture. As one of the most well-studied metal organic frameworks, ZIFs have received considerable research interest because of their unique features (permanent porosity, high thermal,and chemical stability) and potential applications in the fields of adsorption/separation and electrochemistry [1; 8]. Among all the ZIFs, ZIF-8, which possesses a sodalite (SOD) topology that is composed of four- and six-member ring ZnN\({}_{4}\) clusters, is of particular interest due to its high thermal and chemical stability [1; 9], characteristic porosity, and large specific surface area (1630-1700 m\({}^{2}\)/g) [10]. A structural feature of ZIF-8 is that its internal cavities have a diameter of 1.16 nm and are connected through small pores with a diameter of 0.34 nm. It has been reported that the kinetic diameter of nitrogen gas is 0.36 nm [11] and that of CO\({}_{2}\) gas is 0.33 nm [12]. Therefore, it is expected that molecules with different kinetic diameters can be separated using ZIF-8 due to its small apertures [13]. ZIF-8 polyhedrons can be readily manufactured using a variety of synthesis processes in water or organic solvents, such as conventional solvothermal [14], hydrothermal [15], colloidal precipitation [16; 17], mechanochemical [18], sonochemical [18], microwave-assisted [18], dry-gel [18], microfluidic [18], electrochemical [19], or soft template [20; 21] methods. The particle sizes of the samples prepared by sonochemical and dry-gel routes are significantly smaller than those prepared with other methods [18]. A higher ZIF-8 yield can be obtained using dry-gel and mechanochemical synthesis [18]. The hydrothermal or solvothermal processes are generally conducted with a reaction temperature of room temperature up to 200 \({}^{\circ}\)C and a reaction duration of hours to days [22]. The colloidal precipitation route is a simple and widely used approach [16] for the large-scale production of nano- to micro-size ZIF-8 crystals with controlled particle sizes and has been widely used in the literature. It has been reported that the yield, particle size, crystallinity, and porosity of ZIF-8 are dependent on the molar ratio of imidazole to Zn [23]. For instance, the particle size of ZIF-8 decreased with an increasing molar ratio of imidazole to Zn [24]. Cubic-shaped ZIF-8 is synthesized at low imidazole to Zn molar ratios, whereas at higher imidazole to Zn ratios, rhombic dodecahedron morphologies, or even truncated rhombic dodecahedron morphologies can be obtained [23]. In addition the ratio of imidazole to Zn, the solvent type [25] and the Zn precursor [26] also affect the crystallinity of ZIF-8. In addition to the above-mentioned parameters, the synthesis time also controls the structural evolution of ZIF-8. The first ten minutes are the nucleation stage of ZIF-8. Next, comes the growth phase, during which, it reaches optimal crystallinity after approximately 30-60 min. After that, the process enters a stable phase, and the structure remains unchanged [27]. Although ZIF-8 is considered thermally stable below 723 K in air and below approximately 823 K in N\({}_{2}\)[28], its CO\({}_{2}\) adsorption capacity relative to its specific surface area is not remarkable. Pyrolysis is an efficient and simple strategy to transform ZIF-8 polyhedrons into nitrogen-enriched carbons [29].