1：Experimental Design and Method Selection:

The study employs a precursor-reforming strategy involving hydrothermal treatment of melamine to form melamine-cyanuric acid (MCA) precursors, followed by thermal polymerization under N2 atmosphere to produce g-C3N4 microtubes (mg-C3N4). The design rationale is to conquer hydrogen bonds during thermal exfoliation to form tubular structures with adjustable orifice closure and spatial anisotropic charge separation. Methods include hydrothermal synthesis, thermal polymerization, and photo-deposition for characterizing charge separation.

2：4). The design rationale is to conquer hydrogen bonds during thermal exfoliation to form tubular structures with adjustable orifice closure and spatial anisotropic charge separation. Methods include hydrothermal synthesis, thermal polymerization, and photo-deposition for characterizing charge separation. Sample Selection and Data Sources:

2. Sample Selection and Data Sources: Samples include bulk g-C3N4 (bg-C3N4), ultrathin g-C3N4 (ug-C3N4), and mg-C3N4 with varying hydrothermal times (16 h, 20 h, 24 h, 28 h). Precursors are melamine and MCA, selected based on hydrothermal time to study morphology evolution. Data sources are experimental characterizations and photocatalytic tests.

3：List of Experimental Equipment and Materials:

Equipment includes alumina crucible, Teflon stainless autoclave, Thermo ESCALAB 250XI (XRD, XPS), Nicolet Magna-IR 550 spectrometer (FT-IR), SEM (JSM-7001F, Jeol), TEM (JEM-2100F), UV-vis spectrophotometer (UV-2450, Shimadzu), BET/BJH surface area analyzer (3H-2000PS1), electrochemical workstation (Zahner Instruments), photoluminescence spectrometer (FLS1000, Hitachi), gas chromatograph (GC-2030), and Xe lamp with UV cut-off filter. Materials include melamine, deionized water, MnSO4·H2O, H2PtCl6, triethanolamine, and H2PtCl6·6H2O.

4：2O. Experimental Procedures and Operational Workflow:

4. Experimental Procedures and Operational Workflow: For bg-C3N4, melamine is heated at 550°C for 4 h. For ug-C3N4 and mg-C3N4, melamine is dissolved in water, hydrothermally treated at 180°C for varying times (16-28 h) to form MCA precursors, then thermally polymerized at 550°C for 4 h under N2. Photo-deposition of Pt and MnOx involves dispersing mg-C3N4 in solutions with metal precursors, stirring in dark, then UV illumination. Photocatalytic H2 evolution tests use 50 mg sample in 100 mL triethanolamine solution with Pt co-catalyst, under Xe lamp irradiation, with H2 concentration analyzed hourly by GC.

5：Photo-deposition of Pt and MnOx involves dispersing mg-C3N4 in solutions with metal precursors, stirring in dark, then UV illumination. Photocatalytic H2 evolution tests use 50 mg sample in 100 mL triethanolamine solution with Pt co-catalyst, under Xe lamp irradiation, with H2 concentration analyzed hourly by GC. Data Analysis Methods:

5. Data Analysis Methods: Data analysis includes XRD for crystal structure, FT-IR for chemical bonds, XPS for surface chemistry, SEM/TEM for morphology, UV-vis DRS for optical properties, BET for surface area, photocurrent and EIS for charge transfer, PL for carrier recombination, and GC for H2 quantification. Statistical techniques involve Tauc plots for band gap, Mott-Schottky for band alignment, and biexponential fitting for PL lifetime.