1：Experimental Design and Method Selection:

The study involves synthesizing g-C3N4 nanosheets via pyrolysis of melamine, UMOFNs nanosheets via a solvothermal method with modifications, and their composite via mechanical grinding. Characterization includes XRD, SEM, TEM, FTIR, XPS, UV-Vis DRS, fluorescence spectroscopy, photoelectrochemical tests, and photocatalytic hydrogen production experiments.

2：Sample Selection and Data Sources:

Samples include bulk g-C3N4, g-C3N4 nanosheets, UMOFNs, and g-C3N4/UMOFNs composites with varying UMOFNs content (0.1 wt% to 80 wt%). Data are obtained from laboratory experiments.

3：1 wt% to 80 wt%). Data are obtained from laboratory experiments. List of Experimental Equipment and Materials:

3. List of Experimental Equipment and Materials: Equipment includes muffle furnace, sonicator, centrifuge, XRD (D/MAX2500PC), SEM, TEM, FTIR (Perkin Elmer System 2000), XPS (Thermo ESCALAB 250Xi), UV-Vis spectrometer (UV1901), fluorometer (F-7000), electrochemical workstation (CHI660E), gas chromatography (GC-2014C), and xenon lamp (HPS-500XA). Materials include melamine, CoCl2·6H2O, NiCl2·6H2O, H2BDC, DMF, ethanol, TEA, lactic acid, H2PtCl6·2H2O, coumarin, and others.

4：Experimental Procedures and Operational Workflow:

g-C3N4 nanosheets are prepared by calcining melamine at 550°C and 520°C. UMOFNs are synthesized by sonicating DMF, ethanol, water with H2BDC, adding CoCl2·6H2O and NiCl2·6H2O, then TEA, and ultrasonicating for 8 h. Composites are made by grinding g-C3N4 with UMOFNs. Photocatalytic tests involve dispersing catalyst in lactic acid solution with Pt co-catalyst, irradiating with xenon lamp, and measuring H2 production with GC.

5：Data Analysis Methods:

Data are analyzed using XRD for crystal structure, SEM/TEM for morphology, FTIR for functional groups, XPS for composition, UV-Vis for optical properties, fluorescence for charge separation, electrochemical tests for photocurrent and impedance, and statistical methods for hydrogen production rates and quantum efficiency.