Synergistic Porosity and Defect Engineering to Construct S-Scheme Porous Carbon-Modified CeO 2 /Graphitic Carbon Nitride Heterojunctions for Enhanced Photocatalytic Degradation of Ciprofloxacin

Although CeO2/graphite-like carbon nitride (CeO2/CN) heterojunctions exhibit good photocatalytic potential, the low specific surface area and poor interfacial contact of conventional CeO2 substrates limit the charge separation efficiency of the system. To this end, this study proposes a strategy combining substrate premodification with interface optimization: first, a porous, carbon-modified CeO2 substrate (PC-CeO2) was prepared via a polyvinylpyrrolidone (PVP)-assisted hydrothermal calcination process; this was then combined with g-C3N4 to successfully construct a PC-CeO2/CN S-Scheme heterojunction. Characterization results indicate that the in situ pyrolysis of PVP effectively suppressed the agglomeration of CeO2 nanoparticles, endowing the substrate with a high specific surface area (114.25 m2/g) and inducing initial oxygen vacancy on the surface. Following composite formation with g-C3N4, the concentration of oxygen vacancies in the system increased further; this porous defect structure facilitated close interfacial contact for the assembly of the two phases. Combined spectroscopic and photoelectrochemical analyses indicate that charge transfer in the PC-Ce/CN composite system follows an S-Scheme mechanism. The optimized porous interface reduces interfacial charge transfer resistance, while oxygen vacancies act as effective charge capture centers; the synergistic effect of these two factors promotes the spatial separation of photogenerated carriers and maintains a strong redox potential. Using ciprofloxacin (CIP) as a probe molecule, the PC-Ce2/CN heterojunction at the optimal molar ratio achieved a degradation efficiency of 97% within 30 min of light activation (k = 0.1069 min–1). ESR characterization and quenching experiments confirmed that h+, ·O2– and ·OH all actively participate in the reaction system. This work provides a reliable strategy for structural and defect engineering in the design of highly efficient S-Scheme photocatalytic systems.