ZIF-67-derived NiCo2O4/ZIF-8 heterostructure for enhanced solar photocatalytic degradation of Reactive Blue-222

Reactive Blue-222 (RB-222) is resistant to biodegradation because of its complex molecular structure and high chemical stability, so conventional wastewater treatment is largely ineffective at removing it. In this study, a NiCo 2 O 4 /ZIF-8 nanocomposite was fabricated via a ZIF-67-templated synthesis of NiCo 2 O 4 nanorods followed by in-situ growth of ZIF-8. This structural design aims to address limitations in light absorption, charge recombination, and surface accessibility. Structural and surface characterization (XRD, FTIR, FE-SEM/EDX, XPS and BET) indicates the successful integration of spinel NiCo 2 O 4 with the porous ZIF-8 framework, resulting in approximately six-fold increase in surface area (377.86 m 2 g − 1 ) compared to pristine NiCo 2 O 4 . Optical analysis revealed enhanced visible-light absorption with an effective bandgap of 1.88 eV for the composite. Under natural visible light irradiation, the optimized 10 wt% NCO/ZIF-8 achieved 96.7% degradation of RB-222 within 120 min (20 mg L − 1 , pH 7). The pseudo-first-order rate constant was 0.0296 min − 1 , which represents approximately two-fold and four-fold enhancements over pristine NiCo 2 O 4 and ZIF-8, respectively. The pH-dependent performance correlated with the measured point of zero charge (pHpzc = 9.31), indicating that electrostatic interactions and reactive oxygen species (ROS) generation both drive the degradation process. Radical scavenging experiments identified superoxide radicals ( • O 2 − ) as the dominant reactive species, with photo-generated holes (h + ) providing secondary contributions and hydroxyl radicals ( • OH) playing a minor role. Band structure analysis reveals a Type-I (straddling-gap) heterojunction in which NiCo 2 O 4 acts as the sole photoactive component; ZIF-8 (Eg = 4.42 eV) is not photoexcited under solar irradiation. All photogenerated electrons and holes originate from NCO upon visible-light absorption. Contact-induced band bending at the NCO/ZIF-8 interface suppresses bulk recombination and concentrates photocarriers at the surface, enabling efficient • O 2 ⁻ generation (dominant) and secondary hole-mediated oxidation, while ZIF-8 contributes through dye adsorption and interfacial charge localization rather than photocarrier generation. This interpretation is supported by photoluminescence quenching, radical scavenging experiments identifying superoxide radicals as the dominant reactive species, and kinetic analysis showing enhanced degradation rates.