## 3 Results and Discussion
### Characterization of Physical and Chemical Properties
A narrow distribution of micropores centered at approximately 1 nm was observed (inset in Figure 2c), which suggests that a large number of micropores exist in these samples. The BET surface area of CN was 31.3 m\({}^{2}\)g\({}^{-1}\) and further heating greatly increased its specific surface area (159 m\({}^{2}\)g\({}^{-1}\)). The surface area of FCN can be increased 5 times by calcining, and the increased specific surface area can provide a large number of active sites, which improves the catalytic activity (Table 1). XPS was performed to further confirm the chemical states and surface elements. The survey XPS spectrum shown in Figure 2d-f verified the presence of C, N, and O. For the C 1s core level of CN, two peaks were identified at 284.5 and 288.0 eV, corresponding to \(sp^{3}\) C-C bonds and C-N=C coordination, respectively [24]. While identifying the vacancy of carbon in C\({}_{3}\)N\({}_{4}\), the areal ratio between the two C 1s XPS peaks was found to have decreased compared to that for the NC sample due to the reduction of tertiary carbon content. A lower C/N ratio compared to the NC sample could also indicate a higher carbon vacancy. The peak of FCN at 288.0 eV shifted to lower binding energies than in CN, implying that the structure of C-N=C formed from melamine changes during the decomposition of the organic framework and finally forms FCN. Figure 2e presents the N 1s XPS spectra for CN and FCN. The peak of FCN can be deconvoluted into four peaks at 398.6, 399.8, 401.0, and 404.2 eV, corresponding to C-N=C, N-(C)\({}_{3}\), C-NH, and charge effects on the FCN surface, respectively. The peak of CN at 398.4 eV was lower than that of FCN, which implies that the process of calcination can affect the C-N=C coordination. In the O 1s spectra, the samples produced peaks at 531.9 and 531.0 eV, respectively, indicating the formation of the C-O, N-O, or C-O-C bonds [25]. The peaks at 531.9 and 533.0 eV shifted to higher binding energies and lower peak areas in CN, which suggests that the bond between carbon and oxygen breaks during the reaction to generate water or gas. \begin{table}
\begin{tabular}{c c c c c c} \hline  & \(\mathbf{S_{BET}(m^{2}/g)}\) & **C(Atomic\%)** & **N(Atomic\%)** & **O(Atomic\%)** & **S(Atomic\%)** \\ \hline CTF & 6.0 & 42.5 & 40.0 & 14.1 & 3.4 \\ CN & 31.3 & 44.5 & 52.2 & 3.0 & 0.3 \\ FCN & 159 & 42.6 & 54.7 & 2.4 & 0.2 \\ \hline \end{tabular}
\end{table} 
Table 1: Specific surface areas and XPS elemental compositions of the samples. The differences between the morphologies of CN produced under different conditions were investigated by SEM (Figures 3 and S2). When the water content in the precursor was controlled, 10H\({}_{2}\)O-CN exhibited the finest fiber morphology. As the water content increased, the fibrous appearance gradually devolved into a blocky structure. When regulating the ratio of the two organic components, the samples exhibited similar fibrous morphologies. Figure S3 shows the SEM images and the corresponding spectra obtained from the energy dispersive X-ray spectroscopy (EDS)of CTF and CN. CTF exhibited a parallelogram-like fibrous structure. EDS element mapping revealed the presence of C, N, O, and S elements. Following calcination, the amount of S in CN decreased significantly. This is attributed to the generation of sulfur dioxide when CTF is calcined, which increases the specific surface area of CN and facilitates the catalytic reaction. Figure 4a,b show that the structure of the calcined sample changes from the parallelogram structure of CTF into a fibrous morphology along a certain crystal plane. To further observe the microstructure of the nanofibers, TEM was performed on the FCN sample, and the results are shown in Figure 4c,d, attesting to the thinness and porosity of each fiber. HRTEM  
Figure 2: XRD, FT-IR, \(N_{2}\) adsorption-desorption isotherms, and XPS characterizations of CN and FCN. (**a**) XRD patterns. (**b**) FT-IR spectra. (**c**) \(N_{2}\) adsorption-desorption isotherms and the corresponding pore-size distribution curves (inset).