1：Experimental Design and Method Selection:

The study uses metal-assisted chemical etching (MACE) for Si nanowire fabrication and liquid exfoliation for g-C3N4 nanosheets, followed by spin coating to create the hybrid heterostructure. PEC measurements are conducted to evaluate water reduction performance.

2：Sample Selection and Data Sources:

Boron-doped p-type Si (100) wafers are used as substrates. Bulk g-C3N4 is synthesized from melamine powder. Electrolyte is 0.5 M Na2SO4 at pH~

3：5 M Na2SO4 at pH~List of Experimental Equipment and Materials:

7. 3. List of Experimental Equipment and Materials: Equipment includes XRD (PAN analytical X' Pert PRO), AFM (Agilent 5500), FESEM (Carl Zeiss Sigma), HRTEM (LEBRA 200 TEM), UV-Vis spectrometer (SHIMADZU 2450), electrochemical analyzer (Biologic SP 150), and light source (Oriel with AM 1.5G filter). Materials include AgNO3, HF, H2O2, melamine, isopropanol, Na2SO4, In-Ga alloy, epoxy resin, Ag/AgCl reference electrode, Pt counter electrode.

4：5G filter). Materials include AgNO3, HF, H2O2, melamine, isopropanol, Na2SO4, In-Ga alloy, epoxy resin, Ag/AgCl reference electrode, Pt counter electrode. Experimental Procedures and Operational Workflow:

4. Experimental Procedures and Operational Workflow: Si nanowires are fabricated via MACE with Ag deposition and etching. Bulk g-C3N4 is synthesized by heating melamine. g-C3N4 nanosheets are exfoliated in isopropanol. Hybrid structure is formed by spin coating g-C3N4 nanosheets onto Si nanowires. Characterization involves XRD, AFM, FESEM, TEM, UV-Vis, and PEC measurements including LSV, EIS, and Mott-Schottky analysis.

5：Data Analysis Methods:

Data is analyzed using electrochemical techniques; EIS data is fitted with equivalent circuits, and carrier lifetime is calculated from Bode plots. Photocurrent density and efficiencies are derived from J-V curves.