Substituent-dependent photocatalytic hydrogen evolution over functionalized MIL-101(Cr) metal–organic framework

Functional group engineering is an effective strategy for regulating the electronic structure and photocatalytic activity of metal–organic frameworks (MOFs). In this study, MIL-101(Cr) and three functionalized derivatives, MIL-101(Cr)-NH₂, MIL-101(Cr)-NO₂, and MIL-101(Cr)-Br, were synthesized to investigate the effect of functional group substitution on photocatalytic hydrogen evolution. Among the synthesized materials, MIL-101(Cr)-NH₂ exhibited the highest photocatalytic hydrogen production, achieving 630.57 μmol g⁻¹ after 5 h under visible-light irradiation, and maintained its activity over four consecutive photocatalytic cycles. Comprehensive structural, photophysical, and electrochemical characterizations were performed using X-ray diffraction (XRD), scanning electron microscopy (SEM), infrared spectroscopy (IR), UV–Vis diffuse reflectance spectroscopy (UV–Vis DRS), photoluminescence (PL) spectroscopy, electrochemical impedance spectroscopy (EIS), and photocurrent response measurements. The results revealed that the introduction of amino functional groups significantly enhanced the light-harvesting capability of MIL-101(Cr), extended its visible-light absorption range, promoted the separation and transfer of photogenerated charge carriers, and effectively suppressed electron–hole recombination. Furthermore, MIL-101(Cr)-NH₂ exhibited the largest specific surface area and excellent thermal stability, which further contributed to its superior photocatalytic performance. This work provides valuable insights into the rational design and development of high-performance MOF-based photocatalysts for hydrogen production.