1：Experimental Design and Method Selection:

A direct polymerization strategy involving heating thiourea in air without templates to synthesize ultrathin g-C3N4 nanosheets.

2：Sample Selection and Data Sources:

Thiourea, dicyandiamide, and melamine were used as precursors; samples were synthesized at different temperatures (540, 560, 580°C).

3：List of Experimental Equipment and Materials:

Alumina crucible, muffle furnace, agate mortar, transmission electron microscopy (TEM, Tecnai G2 F20, FEI), atomic force microscopy (AFM, JPK NanoWizard?ULTRA Speed, Germany), X-ray diffraction (XRD, X'Pert PRO MPD, PANalytical Instrument), Fourier transform infrared spectrometer (FT-IR, Nexus 470, Thermo Electron Corporation), X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI, America), gas/vapor adsorption instrument (BELSORP-max, Japan), UV spectrophotometer (Lambda 365, America), electrochemical workstation (CHI-760E and CHI-852C, Chenhua Instruments), time-resolved photoluminescence spectrometer (QuantaMaster? 40, Photon Technology International, Inc.), Xe lamp with optical filter, gas chromatography (GC-7920, China Education Au-Light).

4：Experimental Procedures and Operational Workflow:

Precursors were heated in a muffle furnace at specified temperatures, cooled, ground, and characterized. Photocatalytic tests involved suspending catalysts in triethanolamine solution with Pt co-catalyst, irradiating with visible light, and measuring H2 evolution.

5：Data Analysis Methods:

Data were analyzed using techniques such as XRD for crystal structure, FT-IR for functional groups, XPS for chemical states, BET for surface area, UV-vis for optical properties, EIS and photocurrent for electrochemical properties, PL for charge carrier behavior, and GC for H2 quantification.