1：Experimental Design and Method Selection:

The study employed a combining secondary sol-gel and facile hydrothermal method to prepare graphene-TiO2/g-C3N4 heterostructure photocatalysts.

2：Sample Selection and Data Sources:

Multi-walled carbon nanotubes (MWCNTs) were used to prepare graphene, and melamine was used to synthesize bulk g-C3N4. TiO2 precursor (P25) was used as the TiO2 source.

3：TiO2 precursor (P25) was used as the TiO2 source. List of Experimental Equipment and Materials:

3. List of Experimental Equipment and Materials: MWCNTs, KMnO4, H2SO4, HCl, H2O2, TiO2 precursor (P25), Melamine, NH3·H2O, and distilled water were used. Equipment included a tube furnace, Teflon-lined autoclave, and various characterization instruments like XRD, SEM, TEM, FT-IR, UV-vis-DRS, XPS, PL, and photoelectrochemical measurement systems.

4：Experimental Procedures and Operational Workflow:

Graphene was prepared by oxidative cutting method, g-C3N4 by thermal polycondensation, and graphene-TiO2/g-C3N4 by hydrothermal method followed by annealing.

5：Data Analysis Methods:

The photocatalytic performance was evaluated by degrading methylene blue (MB) under visible-light irradiation, and the degradation was monitored by UV-vis spectroscopy.