1：Experimental Design and Method Selection:

The study involves synthesizing heterojunctions of BiOI with bulk and few-layered g-C3N4 using thermal polycondensation and hydrothermal methods, followed by characterization and photoelectrochemical testing to evaluate water splitting performance under visible light.

2：Sample Selection and Data Sources:

Samples include pristine BiOI, pristine g-C3N4, pristine g-C3N4-S, g-C3N4/BiOI composites, and g-C3N4-S/BiOI composites, synthesized from precursors like dicyandiamide, NH4Cl, Bi(NO3)3·5H2O, and KI.

3：List of Experimental Equipment and Materials:

Equipment includes a tube furnace for synthesis, CHI 600D potentiostat for photoelectrochemical measurements, JEOL 2200 FS TEM for imaging, Zeiss Sigma FESEM for surface topography, Bruker D8 X-Ray diffractometer for XRD, Perkin Elmer Lambda 1050 UV-Vis-NIR spectrophotometer for absorption spectra, Varian Cary Eclipse fluorimeter for PL, homemade single photon counting system for TRPL, Agilent FTS7000 FTIR Imaging System for FTIR, and Nicolet Omega XR Raman Microscope for Raman spectra. Materials include dicyandiamide, NH4Cl, Bi(NO3)3·5H2O, KI, ethylene glycol, ethanol, alpha-terpineol, FTO glass, TiO2, Pt electrode, Ag/AgCl reference electrode, and Na2SO4 electrolyte.

4：Experimental Procedures and Operational Workflow:

Synthesis involves thermal annealing of precursors, hydrothermal growth for composites, drop coating on FTO with TiO2 layer, and photoelectrochemical measurements using a three-electrode setup with linear sweep voltammetry, EIS, and light on-off experiments under AM1.5 and 425 nm LED illumination.

5：5 and 425 nm LED illumination. Data Analysis Methods:

5. Data Analysis Methods: Data analysis includes Tauc plots for bandgap estimation, fitting Nyquist plots to equivalent circuits for EIS, tri-exponential decay fitting for TRPL, and Mott-Schottky analysis for carrier concentration and flat band potential.