Multifunctional Interphase Tailored by the Perylene Diimide/Graphene Oxide Heterojunction in Carbon Fiber/Epoxy Composites: A Route for Simultaneous Reinforcement and Dual-Mode Damage Monitoring

Structural health monitoring (SHM) of carbon fiber-reinforced polymer (CFRP) composites requires sensing systems that are both highly sensitive and structurally compatible. Herein, a dual-mode self-sensing strategy is developed by constructing a perylene diimide/graphene oxide (PDI/GO) heterojunction-engineered interphase in carbon fiber (CF-PDI/GO)/epoxy (EP) composites through synergistic covalent bonding and π-π interactions, thereby simultaneously tailoring π-electron transfer channels and interfacial properties. Formed via π-π stacking between GO and PDI, the PDI/GO heterojunction shows a mechano-responsive fluorescence "off-on" behavior driven by efficient electron transfer via long-range electron delocalization. In contrast to CF-PDI, CF-PDI/GO exhibits quenched fluorescence alongside enhanced conductivity and chemical activity, causing enhancements of 222.9% in electrical conductivity, 82.5% in interfacial shear strength, and 123.9% in transverse fiber bundle tensile strength of CF-PDI/GO/EP composites (vs CF-desized/EP). Under mechanical stimuli, an obvious electrical resistance change is observed with fluorescence "switching-on" signals at damaged regions, which is attributed to breakage of long-range π-electron channels between CF and the PDI/GO heterojunction. Benefiting from its inherent self-sensing mechanism, electrical impedance tomography (EIT) is used for in-situ and real-time damage visualization with higher sensitivity compared to infrared thermography (IRT). This work presents a heterojunction-enabled interphase engineering strategy for simultaneously reinforcing and sensing CFRP composites, offering a pathway toward next-generation smart structural materials.