1：Experimental Design and Method Selection:

The study uses a hydrothermal treatment followed by vacuum freeze-drying to synthesize soluble g-C3N4 nanosheets, aiming to improve dispersibility without using acids. Photocatalytic hydrogen evolution tests are conducted to evaluate performance.

2：Sample Selection and Data Sources:

Bulk g-C3N4 is prepared from melamine precursor. SCN nanosheets are derived from this bulk material. Various SCN/g-C3N4 composites with different SCN weight percentages are prepared.

3：List of Experimental Equipment and Materials:

Equipment includes a muffle furnace for thermal polycondensation, hydrothermal reactor, centrifuge, vacuum freeze-dryer, ultrasonic disperser, transmission electron microscope (JEM-2100F, JEOL), atomic force microscope (MFP-3D, AR), X-ray diffractometer (D/MAXRBX, Rigaku), UV-vis spectrophotometer (UV-2450, Shimadzu), FTIR spectrophotometer (Nexus, Thermo Nicolet), XPS system (KRATOA XSAM800), thermogravimetric analyzer (STA-449F3, Netzsch), photocatalytic reactor with LEDs, and gas chromatograph (GC-2014C, Shimadzu). Materials include melamine (>99.9%, Alfa Aesar), deionized water, lactic acid, and Pt cocatalyst.

4：9%, Alfa Aesar), deionized water, lactic acid, and Pt cocatalyst. Experimental Procedures and Operational Workflow:

4. Experimental Procedures and Operational Workflow: Bulk g-C3N4 is synthesized by heating melamine at 550°C for 4 hours. SCN nanosheets are prepared by hydrothermal treatment of bulk g-C3N4 in water at 180°C for 12 hours, followed by centrifugation and vacuum freeze-drying. SCN/g-C3N4 composites are made by ultrasonic dispersion of SCN solution with bulk g-C3N4. Photocatalytic tests involve dispersing 50 mg catalyst in 80 mL lactic acid solution with Pt, purging with N2, irradiating with 420 nm LEDs, and measuring H2 production with GC.

5：Photocatalytic tests involve dispersing 50 mg catalyst in 80 mL lactic acid solution with Pt, purging with N2, irradiating with 420 nm LEDs, and measuring H2 production with GC. Data Analysis Methods:

5. Data Analysis Methods: Characterization techniques (XRD, TEM, AFM, FTIR, XPS, UV-vis, TG-DSC) are used to analyze material properties. Photocatalytic activity is assessed based on H2 evolution rates, with statistical analysis implied from repeated measurements.