Oxygen vacancy and dual-field synergy in amorphous MnO2/B-doped tubular g-C3N4 heterojunction for efficient and selective photocatalytic oxidation of 5-hydroxymethylfurfural

Efficient charge separation and controlled generation of superoxide radicals (•O 2 − ) are critical for enhancing photocatalytic activity and selectivity in the oxidation of 5-hydroxymethylfurfural (HMF) to 5-formyl-2-furancarboxylic acid (FFCA). Herein, a crystalline-amorphous S-scheme heterojunction (A-MnO 2 /B-CN) was engineered through in situ hydrothermal growth of oxygen vacancy-rich amorphous manganese dioxide (A-MnO 2 ) nanoflowers on boron-doped tubular g-C 3 N 4 (B-CN). This unique crystalline-amorphous interface establishes a strong internal electric field (IEF) that drives directional charge transfer from A-MnO 2 to B-CN. Concurrently, topological disorder within the A-MnO 2 lattice promotes dynamic O 2 adsorption and generates a robust dipole field (DF), which steers photogenerated electrons toward active sites. These dual-fields synergistically promote S-scheme charge transfer, selectively enriching •O 2 − species and suppressing •OH byproducts. Crucially, atomic-scale reconstruction at crystalline-amorphous boundaries triggers Mn 3d-O 2p orbital hybridization, reducing the rate-determining step energy barrier by 41 %. Under visible light, the system achieves near-complete HMF conversion (99.04 %) with exceptional FFCA selectivity (95.67 %) and a record production rate of 157.9 µmol·g −1 ·h −1 (3.9-fold improvement over the crystalline MnO 2 /B-CN system). This work establishes a sustainable photocatalyst design paradigm that integrates amorphous-phase defect engineering with interfacial electric field control for efficient solar-driven valorization of biomass into value-added chemicals.