1：Experimental Design and Method Selection:

The study used a facile thermal ammonolysis method to prepare Ni3N/g-C3N4 hybrid composites. This involved hydrothermal synthesis followed by annealing under NH3 gas to load Ni3N nanoparticles onto g-C3N4. The rationale was to utilize Ni3N as a cocatalyst to improve charge separation and transfer in g-C3N4 for enhanced photocatalytic hydrogen evolution.

2：The rationale was to utilize Ni3N as a cocatalyst to improve charge separation and transfer in g-C3N4 for enhanced photocatalytic hydrogen evolution. Sample Selection and Data Sources:

2. Sample Selection and Data Sources: Pristine g-C3N4 was prepared by thermal pyrolysis of precursors. Ni3N/g-C3N4 composites with different Ni3N loadings (labeled #1 to #4) were synthesized by varying the amounts of Ni(NO3)2·6H2O and hexamethylenetetramine (HMT) in aqueous solution with g-C3N4, followed by hydrothermal treatment and annealing. Pristine Ni3N was also prepared for comparison.

3：List of Experimental Equipment and Materials:

Materials included Ni(NO3)2·6H2O, HMT, g-C3N4, triethanolamine (TEOA), and other reagents from Aladdin Chemical Reagent Co., Ltd. Equipment included a powder X-ray diffractometer (PXRD, D/max-TTR III), scanning electron microscope (SEM, SIRION200), transmission electron microscope (TEM, JEM-2010), UV-Vis spectrometer (SOLID 3700), X-ray photoelectron spectrometer (XPS, ESCALAB 250), a 300-W xenon lamp with UV cut-off filter (λ > 420 nm), gas chromatograph (GC, SP6890 with TCD detector), and equipment for photoelectrochemical measurements (e.g., for PL spectra, photocurrent response, and EIS).

4：Experimental Procedures and Operational Workflow:

g-C3N4 was synthesized via thermal pyrolysis. For Ni3N/g-C3N4 composites, specific amounts of Ni(NO3)2·6H2O and HMT were dissolved in water, mixed with g-C3N4, subjected to hydrothermal treatment at 120°C for 12 h, washed, dried, and annealed at 380°C under NH3 flow for 3 h. Photocatalytic H2 production tests were conducted in a flask with magnetic stirring under visible-light irradiation using a xenon lamp, with H2 quantified by GC. Photoelectrochemical characterizations (PL, photocurrent, EIS) were performed in aqueous Na2SO4 solution.

5：Data Analysis Methods:

XRD patterns were analyzed for crystalline phases. SEM, TEM, and EDX were used for morphology and composition. XPS for chemical states. UV-Vis for optical properties. PL spectra for recombination rates. Photocurrent and EIS for charge separation efficiency. Hydrogen evolution rates were calculated from GC data, and AQY was calculated using the provided equation.