Abstract: The rational construction of a photocatalytic system that maximizes sunlight harnessing and facilitates its redox abilities is still an intriguing research domain in photocatalysis technology. The present study reports synthesising a novel S-scheme nanocomposite system combining CeO₂ and oxygen vacancies (OVs) modified self-doped Ti³⁺-doped TiO₂. A unique approach of incorporating active site engineering via OVs generation in self-doped Ti³⁺-TiO₂ and Ce³⁺/Ce⁴⁺ valency exchange in CeO₂ has synergistically endorsed the photoredox potential in the resulting heterostructure system. Typically, the OVs in Ti³⁺-TiO₂ serve as electron-rich centres, stimulating charge isolation and effective visible light absorption, while the Ce³⁺/Ce⁴⁺ valency exchange dynamics in CeO₂ facilitate effective electron shuttling and redox capabilities. This synergistic arrangement not only fosters interfacial charge transference but also expedites the overall redox potential, rendering superior catalytic activity in both oxidation and reduction reactions. As a result, the mid-state energy level and dual redox-active sites equipped Ti³⁺-TiO₂/CeO₂ system exhibit 84 % 4-nitrophenol photo-reduction to 4-aminophenol and 90.6 % photo-oxidative degradation of Sunset Yellow dye. Density Functional Theory (DFT) calculations and Bader charge analysis helped in identifying the exposed attacking sites that enabled selective photocatalytic interactions. Moreover, chromatography analyses (HPLC and LCMS-MS) further aided in understanding the reductive and oxidative mechanisms, respectively. The nanocomposite photocatalyst showed excellent stability under the experimental conditions and exhibited up to four cycles with no significant loss in efficacy. This study demonstrates the dual functionality of the S-scheme nanocomposites aimed at designing multifunctional photocatalytic materials to address critical environmental challenges.

Key words: Self-doped Ti³⁺-TiO₂, Photoredox reaction, Valency exchange, 4-Nitrophenol reduction, Azo dye oxidative degradation