Hydrolytically stable citrate capped Fe3O4@UiO-66-NH2 MOF: A hetero-structure composite with enhanced activity towards Cr (VI) adsorption and photocatalytic H2 evolution

Design and facile fabrication of a magnetically separable hetero-structure photocatalyst as well as an adsorbent having dual green benefits towards energy conversion and pollutant remediation are quite indispensable in the current scenario. In this regard, a composite of citrate capped Fe₃O₄ and UiO-66-NH₂ has been designed to remediate Cr (VI) by adsorption and harvest photons from visible light for clean energy (H₂) conversion. The material was prepared by the union of citrate capped Fe₃O₄ (CCM) and versatile aqueous stable Zr-based MOF (UiO-66-NH₂) through in-situ solvothermal method. The composite of CCM with MOF (MU-2) was studied through sophisticated analysis techniques; PXRD, FT-IR, BET, UV–Visible DRS, PL, TG, HRTEM and XPS etc. to reveal the inherent characteristics of the material. BET surface analysis revealed high specific surface area (572.13 m² g⁻¹) of MU-2 in comparison to its pristine MOF. Furthermore, the dual function composite MU-2′s VSM studies showed that its magnetic saturation is 3.07 emu g⁻¹ that is suitable for magnetic separation after desired reaction from aqueous media. The Cr (VI) sorption studies revealed that the composite adsorbent (MU-2) showed maximum monolayer adsorption capacity (Qₘ) of 743 mg g⁻¹ which followed pseudo second order kinetics. Moreover, the sorption thermodynamics revealed that the process was spontaneous and endothermic in nature. In addition to it, the synthesized composite material displayed enhanced activity towards photocatalytic H₂ evolution with a maximum evolution rate of 417 µmole h⁻¹ with an apparent conversion efficiency (ACE) of 3.12 %. Typically, MU-2 displays high adsorptions of Cr (VI) as well as some extent of Cr (VI) reduction owning to its populous active sites and free carboxylate groups respectively. Moreover, the synergistic effect of CCM and UNH in the composite resulted in Z scheme mediated charge transfer mechanism that showed enhanced H₂ photo-evolution rates. Hence, MU-2 can be readily utilized as magnetically retrievable dual function composite for Cr (VI) adsorption and photocatalytic H₂ evolution.