# Zinc-Decorated and Nitrogen-Functionalized Hierarchical Porous Carbons for Carbon Dioxide Capture
## 3 Results and Discussion
### Characterization of ZIF-8 and Its Derived Carbons
The PSDs for all the samples are displayed in Figure 3f,g, where another two peaks at about 0.4 and 0.6 nm were revealed by incorporating the CO\({}_{2}\) data. The ZIF-8 polyhedrons can be transformed into Zn- and N-doped porous nanocarbons through direct carbonization. The existence of ultramicro-, micro-, meso-, and macropores was identified in all the carbonized samples (Figure 3h,i). Z8-650 and Z8-950 possessed relatively higher mesopore volumes, while Z8-750 and Z8-850 exhibited more uniform pore volume distributions. This hierarchical porous structure facilitates gas transport, thereby accelerating subsequent gas adsorption [32]. The surface and pore features of the samples are detailed in Table 1. The specific surface area (SSA) of ZIF-8 (1862 m\({}^{2}\)/g) was higher than that of the other ZDCs (Figure 3j), while ZIF-8 did not have ultramicropores, which are known as one of the key properties for CO\({}_{2}\) adsorption. Z8-750 and Z8-850 exhibited higher ultramicropore volume ratios (Figure 3k). Moreover, Z8-850 and Z8-750 also had higher S\({}_{\rm mi}\)/SSA and V\({}_{\rm mi}\)/V\({}_{\rm t}\) ratios (Figure 3l), which are indicative of microporosity. The XRD pattern of the as-prepared ZIF-8 crystals is shown in Figure 4 and was in good agreement with literature reports. With increasing temperatures, the ZIF-8 polyhedrons began to decompose [30]. The XRD patterns of the ZDCs had weak and broad peaks at 20 = 26.5\({}^{\circ}\), 35.4\({}^{\circ}\), and 43.3\({}^{\circ}\), which correspond to the C(002), ZnO(002), and Zn(101)/C(101) peaks [51; 52; 53], respectively. In the XRD pattern of Z8-650, no Zn or ZnO peaks were detected. The signal may have been covered by the detection noise. The ZnO (002) peak gradually increased with increasing carbonization temperature. This might be the reason why the particle sizes of carbonized ZIF-8 became larger as the carbonization temperature increased. The elemental compositions of the samples are shown in Figure 5a. Z8-650 had the lowest carbon content, which could be attributed to the decomposition of free methyl groups [2]. As the carbonization temperature increased above 650 \({}^{\circ}\)C, the decomposition of imidazolate rings could result in mass loss [2]. For Z8-850, a significant conversion of the Zn centers to crystalline ZnO was expected due to the increase in crystalline ZnO, which is consistent with the XRD results. Figure 3: (**a**,**b**) N\({}_{2}\) adsorption isotherms at 77 K. (**c**) CO\({}_{2}\) adsorption isotherms at 273 K. (**d**,**e**) Pore size distributions (N\({}_{2}\) vs. N\({}_{2}\) + CO\({}_{2}\)). (**f**,**g**) Pore size distributions resulting from combining N\({}_{2}\) and CO\({}_{2}\) adsorption isotherms. (**h**,**i**) Amounts and ratios of different pore size volumes. (**j**) Specific surface area (SSA) and micropore surface area (S\({}_{\rm mi}\)). (**k**) Ultramicropore (V\({}_{<\)0.7 nm) ratios. (**l**) Micropore surface (S\({}_{\rm mi}\)) and micropore volume (V\({}_{\rm mi}\)) ratios. V\({}_{\rm t}\)—total pore volume; V\({}_{\rm ma}\)—macropore volume obtained by subtraction. those observed in the elemental analysis. The ZIF-8 crystals had abundant amounts of N 1s (28.06 at. %) and Zn 2p (9.94 at. %). Since the boiling point of metal Zn is about 907 \({}^{\circ}\)C [54] and nitrogen-containing functional groups are unstable upon heating, with increasing temperature, the atomic ratios of N 1s and Zn 2p gradually decreased. Despite this, Zn still remained after ZIF-8 was heat treated at temperatures greater than 900 \({}^{\circ}\)C, which could be ascribed to N-Zn interactions [2]. The XPS C 1s spectrum of ZIF-8 (Figure 6a) could be decomposed into three peaks located at 285, 286.1, and 287.6 eV, which were assigned to C-C/C=C, C-N, and C=O bonds, respectively. A decrease in the peak area of C-C/C=C (285 eV) was observed after ZIF-8 was subjected to heat treatment, suggesting the decomposition of free methyl groups.