## 3 Results and Discussion  
### Characterization of Physical and Chemical Properties  
Figure 1 shows the proposed schematic for the synthesis of CTF-derived carbon nitride. CTF is a framework structure formed by organic functional groups connected by a triazine ring. The designed triazine ring is used as the precursor to generate carbon nitride following calcination. In the first step, melamine and 2,5-thiophenedicarboxylic acid are hydrothermally dissolved in the mixed solution. Covalent-organic frameworks (COFs) can be designed using topology diagrams to guide their structure because their skeletons and pores are designable. The combination of C\({}_{3}\)\({}^{-}\) symmetric(e.g., Melamine) knots and C\({}_{2}\)-symmetric(e.g., 2,5-thiophenedicarboxylic acid) linkers (C\({}_{3}\)+C\({}_{2}\)) leads to the formation of hexagonal COFs [17]. Calication in an Ar atmosphere is an effective method for creating carbon vacancies in C\({}_{3}\)N\({}_{4}\)[10; 18]. Argon molecules absorb large amounts of energy at high temperatures, causing some molecules to move irregularly. Argon molecules in irregular motion will impact the surface of C\({}_{3}\)N\({}_{4}\) and transfer energy to the surface atoms. When the energy transferred exceeds the binding energy of the target atom, the atom detaches from the surface, leaving a vacancy. Finally, a highly active photocatalyst modified with carbon vacancy was obtained by two-step calcination under an argon and air atmosphere. XRD analyses were performed to investigate the phase structure of the samples. The XRD patterns of CN having different precursor ratios are shown in Figure S1a,b. The diffraction peaks observed at 13.3\({}^{\circ}\) (indexed as (100)) and at 27.2\({}^{\circ}\) (indexed as (002)) of CN originate from the in-plane structural packing motif (tri-s-triazine structures) and interplane stacking of the aromatic systems of C\({}_{3}\)N\({}_{4}\), respectively [19]. As the fraction of water increased, the peaks at 13.0\({}^{\circ}\) remained unchanged, but the peaks at 27.6\({}^{\circ}\) shifted to lower angles, implying that the addition of water to the precursor maintained the in-plane structure but increased the interplanar spacing of the crystal. The sharpness of the peak indicates the high crystallinity of the sample that exhibits good photocatalytic performance. This is mainly attributed to the further calcining that can increase the degree of polymerization of CN and thereby increase its photocatalytic performance. Figure 2a  
Figure 1: Schematic for the preparation of FCN. shows the two distinct diffraction peaks exhibited by FCN, located at \(2\theta=13.2^{\circ}\) and \(27.6^{\circ}\), which indicate that calcination increased the spacing between the in-plane structure and decreased the interplanar spacing of the crystal [4]. FT-IR spectroscopy can identify the functional groups present in the samples (Figures 2b and S1c-f). The intense band in the vicinity of 812 cm\({}^{-1}\) corresponds to the tri-s-triazine ring mode of CTF [20]. The FCN spectrum contained several peaks between 1215 and 889 cm\({}^{-1}\) that were attributed to the stretching vibrations of the C-O-C group [13]. Furthermore, the peaks appearing at 1508 and 1357 cm\({}^{-1}\) were assigned to the triazine unit in CN, demonstrating that the chemical framework of CN was retained following the calcination of CTF. CN presented two peaks in the vicinities of 1637 and 1215 cm\({}^{-1}\) that were attributed to C-N stretching. The peaks between 3000 and 3400 cm\({}^{-1}\) were attributed to the residual N-H groups and the O-H band. The range is related to the partial hydrogenation of nitrogen on the edges of the layers [21]. Other vibration bands shifted significantly following further heating, indicating that CN crystallized and functional groups branches were generated [22]. Figure 2c shows the N\({}_{2}\) sorption isotherm with an inflection point at low \(P/P_{0}\) values and a type IV character typical with H3 adsorption hysteresis [23].