1：Experimental Design and Method Selection:

The study employed a hydrothermal synthesis method to create Nb2O5/g-C3N4 heterostructures, chosen for its ability to produce porous structures with high surface areas, atomic-level mixing, and high reaction rates at low temperatures. The oxidant-peroxo method (OPM) was used for Nb2O5 synthesis, and in-situ formation was favored by electrostatic attraction between niobium peroxo complex and g-C3N

2：Sample Selection and Data Sources:

Samples included pure Nb2O5 (NBO), g-C3N4 (GCN), and heterostructures with different weight percentages of g-C3N4 (NBCN-X, where X=1 to 4 for 5, 10, 30, and 60 wt%). Materials were characterized using XRD, SEM, TEM, FTIR, BET, UV-vis absorption, and photoelectrochemical measurements.

3：List of Experimental Equipment and Materials:

Key equipment included autoclaves for hydrothermal synthesis, muffle furnaces for annealing, Bruker D8 Advance XRD, JEOL JSM-6700F SEM, FEI Tecnai G2 F20 TEM, Varian Cary 100 Bio UV-vis spectrophotometer, Bruker Vertex 80v FTIR spectrophotometer, Shimadzu 8A gas chromatograph for H2 measurement, CHI-660B electrochemical workstation with ZAHNER PECC-2 reactor for photoelectrochemical tests. Materials: NbCl5, melamine, H2O2, HNO3, Evonil Aeroxide TiO2 P25, H2PtCl6·6H2O, triethanolamine (TEOA), methanol.

4：Experimental Procedures and Operational Workflow:

Nb2O5 was synthesized by dissolving NbCl5 in HNO3 and H2O2, hydrothermal treatment at 110°C for 24h, annealing at 200-500°C. g-C3N4 was prepared by annealing melamine at 550°C. Heterostructures were made by adding g-C3N4 suspension to niobium peroxo complex solution, hydrothermal treatment, and annealing. Pt was photodeposited using H2PtCl6 and methanol. Photocatalytic H2 evolution was conducted in a reactor with 0.01g photocatalyst in 45mL water with 10 vol% hole scavenger (TEOA or methanol), degassed with Ar, irradiated with a 1000W Xenon lamp for 7h, and H2 measured hourly by GC.

5：01g photocatalyst in 45mL water with 10 vol% hole scavenger (TEOA or methanol), degassed with Ar, irradiated with a 1000W Xenon lamp for 7h, and H2 measured hourly by GC. Data Analysis Methods:

5. Data Analysis Methods: XRD for phase analysis, SEM/TEM for morphology, FTIR for chemical bonds, BET for surface area, UV-vis for bandgap, Mott-Schottky for band positions, electrochemical impedance spectroscopy (EIS) for charge transfer resistance, and statistical comparison of H2 evolution rates.