1：Experimental Design and Method Selection:

A hydrothermal method was developed to modify g-C3N4 with hydroxyl groups, involving pH adjustment of the treatment solution. Theoretical models included DFT simulations for energy calculations and adsorption studies.

2：Sample Selection and Data Sources:

g-C3N4 was synthesized from melamine, and modified samples were prepared with varying pH conditions. Data were obtained from experimental characterizations and simulations.

3：List of Experimental Equipment and Materials:

Equipment included a muffle furnace, autoclave, TEM, XRD, DRS, FTIR, zeta potential analyzer, HPLC, TOC analyzer, and computational tools like VASP. Materials included melamine, NaOH, HNO3, and phenol.

4：Experimental Procedures and Operational Workflow:

g-C3N4 was synthesized by heating melamine, then modified hydrothermally at 160°C for 4 hours with pH adjustment. Characterizations (TEM, XRD, XPS, DRS, FTIR, zeta potential) and photocatalytic tests (phenol degradation under visible light) were performed.

5：Data Analysis Methods:

Data were analyzed using standard techniques for each instrument (e.g., Tauc equation for band gap, fitting for XPS and FTIR peaks), with statistical analysis (mean and standard error from triplicates) for photocatalytic activity.