# Zinc-Decorated and Nitrogen-Functionalized Hierarchical Porous Carbons for Carbon Dioxide Capture
## 3 Results and Discussion
### Characterization of ZIF-8 and Its Derived Carbons
For all ZDCs, the XPS C 1s regions (Figure 6b-e) could be deconvoluted into five peaks: C-C/C=C (285 eV), C-N (286.1 eV), C=N/C-OH (286.6 eV), C=O (287.6 eV), and COOH (290.5 eV) [55; 56; 57; 58; 59]. Z8-750 had a higher percent of C=O, while Z8-850 had a relatively more uniform distribution of the functional groups (Figure 6f). Due to the nitrogen atoms in the organic linker, ZIF-8 showed a symmetrical XPS N 1s peak with a binding energy of approximately 398.4 eV (Figure 7a) [33]. This indicates that the major N groups in ZIF-8 were N=C. We further explored the N functional groups in the ZDCs and found that the left tail of the N 1s peak extended towards higher binding energies. This implies that the N=C bonds decomposed as the carbonization temperature increased. The XPS N 1s peaks of the ZDCs could be divided into, at most, four nitrogen-containing functional groups, including N=C (398.4 eV), O=C-NH (400.5 eV), quaternary or protonated N (401.2 eV), and oxidized species (404.6 eV), as shown in Figure 7b-e [52]. Z8-850 exhibited the highest percent of O=C-NH (pyrrole nitrogen), followed by Z8-750 and Z8-650, while only a little amount was detected in Z8-950 (Figure 7f). Curve fitting of the XPS O 1s spectra (Figure 8) showed the existence of up to three surface oxides [56; 57; 59], including C=O/O=C-N, C-OH, and COOH groups, which were located at 531.5, 532.7, and 533.7 eV, respectively. Similar to the N 1s peak results, ZIF-8 also exhibited a symmetrical XPS O 1s peak, and the C=O/O=C-N groups were the only type of oxygen-containing functional groups. The oxygen content on ZIF-8 increased upon heating. Two other surface oxides, C-OH and -COOH, were observed on all the ZDCs. Figure 6: X-ray photoelectron spectroscopy (XPS) C 1s spectra: (**a**) ZIF-8; (**b**) Z8-650; (**c**) Z8-750; (**d**) Z8-850; (**e**) Z8-950. (**f**) Proportions of functional groups. There was only one valence state of Zn (Zn\({}^{2+}\)) on ZIF-8, which was confirmed by the XPS Zn 2p peaks at 1022 eV (Zn\({}^{2+}\) 2p3/2) and 1045 eV (Zn\({}^{2+}\) 2p1/2) in Figure 9a [25]. As ZIF-8 was carbonized, some Zn\({}^{2+}\) was converted to metal Zn (Zn\({}^{0}\)) (Figure 9b-e). Although the atomic ratios of Zn decreased after being subjected to the heat treatment, the ratio of metal Zn to Zn\({}^{2+}\) was almost unchanged in the ZDCs, except for Z8-950 (Figure 9f). This finding suggests that the Zn chemical state is unchanged after thermal treatment between 650 and 850 \({}^{\circ}\)C [32], and only the total content decreased. Once the temperature is higher than 950 \({}^{\circ}\)C, Zn\({}^{2+}\) might be reduced to metal Zn with C and may lead to the release of Zn vapor [2]. Figure 8: X-ray photoelectron spectroscopy (XPS) O 1s spectra: (**a**) ZIF-8; (**b**) Z8-650; (**c**) Z8-750; (**d**) Z8-850; (**e**) Z8-950. (**f**) Proportions of functional groups. Figure 7: X-ray photoelectron spectroscopy (XPS) N 1s spectra: (**a**) ZIF-8; (**b**) Z8-650; (**c**) Z8-750; (**d**) Z8-850; (**e**) Z8-950. (**f**) Proportions of functional groups.