Mechanism-Guided Design of a PPy-TiO₂-CNT Nanocomposite for Visible-Light Photodegradation

Visible-light dye photodegradation is often constrained by band-structure limitations, weak interfacial coupling, and slow charge transport, which collectively accelerate electron-hole recombination and limit durability. To address these bottlenecks, a polypyrrole-titanium dioxide-carbon nanotube (PPy-TiO₂-CNT) heterojunction was designed in which PPy serves as a visible-light photosensitizer, TiO₂ provides a stable oxidative scaffold and electron-accepting phase, and CNTs create conductive pathways that strengthen interfacial coupling and accelerate electron extraction and transfer. Unlike previously reported binary PPy-oxide or PPy/CNT systems, the present work develops a continuously coupled ternary PPy-TiO₂-CNT interface that integrates visible-light sensitization, oxide redox functionality, and CNT-mediated electron conduction within a single heterojunction while also linking the material design to an LC-MS-verified degradation pathway. The synthesized PPy-TiO₂-CNT heterojunction nanocomposite was evaluated by systematic screening of key operating variables (pH 3-11; 5-50 mg L^-1; 0.10-1.00 mg mL^-1). Under optimal conditions, the nanocomposite achieved a rate constant of 0.053 min^-1 and 99.4% MB removal under visible light, while retaining 92.3% activity after five cycles. The targeted bottlenecks were addressed experimentally by (i) extended visible absorption and an apparent optical gap of ≈2.10 eV (DRS/Tauc) to enhance photon harvesting, (ii) strong PL quenching, which provides evidence of suppressed radiative recombination, and (iii) the smallest Nyquist semicircle (lowest Rct) to confirm accelerated interfacial charge transfer. These characterization results collectively indicate improved interfacial charge separation and transport, which is reflected in the favorable quantitative performance of the optimized CTP-3 sample, namely an apparent optical gap of ≈2.10 eV, a rate constant of 0.053 min^-1, 99.4% MB removal, and 92.3% retained activity after five cycles. Under visible irradiation, PPy acts as the primary photoactive component (HOMO → LUMO), while the CNT network promotes electron extraction and delivery to interfacial O₂, supporting a reactive oxygen species (ROS) sequence from superoxide (^•O₂ ^-) to hydroxyl radical (^•OH) that oxidizes MB. Liquid chromatography-mass spectrometry reveals sequential N-demethylation, aromatic oxidation, and ring opening toward mineralization. Overall, the continuously coupled PPy-TiO₂-CNT interface mitigates recombination and transport limitations while maintaining durability, establishing a reproducible route to mechanism-guided materials-process codesign for efficient visible-light photocatalysis.