## 3 Results and Discussion
### Characterization of H\({}_{2}\) Production from Water Splitting
The optical bandgap values of these two samples are obtained from Figure S6b, which shows similar bandgap values of 2.39 and 2.42 eV for 10H\({}_{2}\)O-CN and 20H\({}_{2}\)O-CN, respectively. Figure S6c shows the UV-vis absorption intensities at different ratios of melamine to 2,5-thiophenedicarboxylic acid. The absorbance regions of 1:4 CN and 1:3 CN were redshifted compared to those of 1:1 CN, 1:2 CN and 1:5 CN, and remained at a 20% absorbance ratio even at 800 nm. The FCN sample prepared with further heating exhibited increases in the bandgap values compared to the other samples. The decrease in absorption and the larger bandgap may be due to the lighter color of the FCN sample reducing the light absorption following further heating. During photocatalysis using CN and FCN under visible-light irradiation, active oxygen species such as the superoxide radical (-O\({}^{2-}\)) and the hydroxyl radical (-OH) were detected as key initiators in methanol and water, respectively [1]. We have studied the relationships between the properties of the samples and the generation of -O\({}^{2-}\) and -OH under visible-light irradiation through ESR spectroscopy. CN and FCN samples were studied in the dark where they emitted very weak signals, indicating that the -O\({}^{2-}\) radical is very sensitive to the light emitted by these two samples (Figure 6a,b). Additionally, a sharp and narrow -O\({}^{2-}\) signal was recorded in situ when the samples were exposed to visible light, which confirms the visible-light-induced formation of the radical ligand [28]. -O\({}^{2-}\) radicals can be transferred out to the interface to reduce the O\({}^{2}\) into -O\({}^{2-}\). By comparison, it is clearly seen that CN and FCN samples can greatly enhance the generation of -OH radicals when irradiated by visible light compared to when left in darkness. The rate of generation of the -OH radical increases with the intensity of the incident radiation [29]. The -OH radicals can be transferred out to the interface to convert water or OH- into -OH. The rate of conversion is much higher under illumination than that without, suggesting that these three samples can use light to produce -O\({}^{2-}\) and -OH efficiently. Figure 5: (**a**) Photocatalytic H\({}_{2}\) evolution of CN and FCN. (**b**) The AQY against the light absorption wavelength of FCN. (**c**) The H\({}_{2}\) evolution cycling plots for FCN through 60 h of operation. (**d**) PL spectra for CN and FCN. (**e**) Transient photocurrent responses of CN and FCN. (**f**) UV-vis diffuse reflectance spectra and plots of \((ahv)^{2}\) vs. photon energy (\(hv\)) of CN and FCN. The energy gaps between the valence band maximum (VBM) and the conduction band minimum (CBM) were calculated from the VB-XPS spectra (Figure 6c) to be 2.36 and 1.91 eV for FCN and CN, respectively, which are lower than the band gap of 2.7 eV for BCN. The carbon vacancy reduces the CBM, thereby narrowing the band gap. By analyzing the UV-vis spectra, the CBM of FCN and CN were calculated to be \(-\)0.21 eV and \(-\)0.48 eV, respectively. The band structures of FCN and CN were proposed as illustrated in the schematic diagram in Figure 6d. The VBM and the CBM shifted to more positive potentials following further heating, which influenced the electronic structures of the conduction or valence bands. The redox potential for H\({}^{+}\)/H\({}_{2}\) increases with increasingly positive values of the CBM for FCN, which shortens the transition path of the photogenerated electrons [30]. In addition, the higher specific surface area provides a larger number of active sites for H\({}^{+}\) reduction during photocatalytic reactions, which expedites the photocatalytic production of H\({}_{2}\) for FCN.