Preparation of Cu/CoFe-MOF nanosheets for photocatalytic degradation of methylene blue

Herein, Cu/CoFe-MOF nanosheets (NSs) were synthesized via a chemical reduction method, where metallic Cu nanoparticles were uniformly deposited onto ultrathin two-dimensional CoFe-MOF NSs. The as-prepared hybrid materials were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), atomic force microscopy (AFM), and ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS). Under visible light irradiation (λ ≥ 420 nm), the optimized 5.0 wt% Cu/CoFe-MOF NSs exhibited a degradation ratio of 68.1% toward methylene blue (MB), which is 1.38 times higher than that of pristine CoFe-MOF NSs (49.4%). Moreover, under the optimized conditions (pH = 4.5, catalyst dosage of 0.1 g, and MB concentration of 10 mg/L), the photocatalytic degradation ratio over 5.0 wt% Cu/CoFe-MOF was further enhanced to 96.3%. Radical trapping and EPR analysis revealed that photogenerated holes (h+) and superoxide radicals (O2•-) are the dominant reactive species. Notably, linear sweep voltammetry (LSV) measurements revealed a quantitative correlation that the enhancement (ca. 1.3-fold) in oxygen reduction current under O2 atmosphere than that of Ar atmosphere, matches almost exactly the improvement in photocatalytic activity (ca. 1.38-fold), identifying the activation of molecular oxygen to O2•- as the rate-determining step that is effectively addressed by Cu decoration. The catalyst also demonstrated excellent stability, retaining 95% of its activity after three cycles. This work provides a mechanistic paradigm for designing high-performance MOF-based photocatalysts by targeting the kinetic bottleneck of O2 activation.