# Zinc-Decorated and Nitrogen-Functionalized Hierarchical Porous Carbons for Carbon Dioxide Capture
## 3 Results and Discussion  
### Characterization of ZIF-8 and Its Derived Carbons  
In order to understand the morphology and microstructure of ZIF-8 and its derived carbons, FESEM and TEM were performed. Figure 1 presents the FESEM micrographs of the samples. The ZIF-8 crystals showed a rhombic dodecahedron morphology [48; 49] with diameters of approximately 50-150 nm. The heat treatment did not change the morphology of ZIF-8. However, it was found that the smallest particles sizes occurred in the Z8-750 sample and the size of the particles gradually increased as the temperature increased. The average particle sizes were 105 \(\pm\) 16 nm (ZIF-8), 37 \(\pm\) 5 nm (Z8-650), 34 \(\pm\) 5 nm (Z8-750), 67 \(\pm\) 11 nm (Z8-850), and 70 \(\pm\) 10 nm (Z8-950), which were smaller than those reported by [PERSON] et al. [29] but similar to the results of [PERSON] et al. [6]. Similar results were observed in the TEM micrographs (Figure 2). The rhombic dodecahedron morphology was found to be maintained well after carbonization. The obvious decrease in particle size in Z8-650 and Z8-750 could be ascribed to the cleavage of methyl groups on the imidazole ligand and the decomposition of C-N bonds. However, the increase in particle size in Z8-850 and Z8-950 was attributed to the reduction of Zn\({}^{2+}\) into metal Zn with C, which occupied part of the space, as well as the release of Zn vapor, leading to volume expansion and pore structure development. This transition even induced significant shrinkage from the outside to the inside (Figure 2e) [30], which may be due to the release of a large amount of Zn vapor, leading to the formation of internal cavities. The STEM-EDX mapping of ZIF-8, Z8-750, and Z8-950 clearly showed that the elements were distributed evenly, but the Zn content decreased with the increase in treatment temperature (Figure 2f-h). The porous properties were further confirmed using measurements of the N\({}_{2}\) adsorption-desorption isotherms. Figure 3a displays the N\({}_{2}\) adsorption-desorption isotherms of the samples, revealing typical reversible type I isotherms. Nevertheless,  
Figure 2: TEM micrographs: (**a**) ZIF-8; (**b**) Z8-650; (**c**) Z8-750; (**d**) Z8-850; (**e**) Z8-950. STEM-EDX mapping: (**f**) ZIF-8; (**g**) Z8-750; (**h**) Z8-950. the isotherms of Z8-750 and Z8-850 still displayed a hysteresis loop, indicating the existence of capillary condensation. In addition, the isotherms near \(P/P_{0}=1\) produced a sharp rise, suggesting the existence of interparticle pores between particles. It is interesting that the nitrogen adsorption isotherms of ZIF-8 showed a two-stage jump at \(P/P_{0}=0.005\) and \(0.02\) (Figure 3b), similar to those reported in the literature [50]. The first jump could be attributed to the gate-opening effect, which is known as the gas-induced rotation of the imidazolate linkers under pressure; the second jump resulted from the reorganization of the nitrogen molecules due to strong electrostatic interactions with the organic framework. [PERSON] et al. [9] found that both the rotation of the imidazole ring and the bending of the methyl group cause the gate opening of ZIF-8. It is reported that the molar ratio of 2-MeIM to Zn\({}^{2+}\) does not have a significant impact on the gate opening of ZIF-8 [24]. Figure 3: _Cont._  
Using the N\({}_{2}\) adsorption isotherms at 77 K and CO\({}_{2}\) adsorption isotherms at 273 K, the PSDs were determined based on the non-local density functional theory (NLDFT) model (i.e., HS-2D-NLDFT). The CO\({}_{2}\) adsorption isotherms of all the samples at 273 K are shown in Figure 3c. The PSD data with/without CO\({}_{2}\) are compared in Figure 3d,e. It can be seen that for samples without ultramicropores (pore size \(<\) 0.7 nm), such as ZIF-8, there was no difference. However, if the samples were filled with ultramicropores, incorporating the CO\({}_{2}\) data into the N\({}_{2}\) isotherms was beneficial for determining the distributions in the low pore width range, such as in the isotherms of Z8-850.