Direct Optical Visualization of In Situ Photodegradation Dynamics in Metal-Organic Frameworks

Metal-organic frameworks (MOFs), such as HKUST-1, have been used in many applications such as catalysis, gas capture, and more. However, one major limitation hindering their application is inherent chemical instability, and conducting in situ studies on their degradation with sufficient spatial-temporal resolution remains a challenge. In this work, we employ optical microscopy to quantitatively monitor the degradation of HKUST-1 under alkaline and acidic reducing environments with video-rate temporal resolution. By color-mapping the degradation progress over different time intervals with alkaline hole (h⁺) scavengers (sodium ascorbate, NaAs), we observe a sigmoidal time-dependent degradation trend. The results reveal the presence of confined regions with faster degradation. It is discovered that degradation begins with the chemical reduction of HKUST-1 into Cu₂O nanoparticles, followed by self-photoreduction into Cu₂O/Cu. Furthermore, it is observed that there is a h⁺ scavenger concentration and laser-wavelength-dependent degradation. At higher concentrations and irradiation energy, there is faster degradation in the HKUST-1 framework. Under acidic reducing conditions with lactic acid (LA), the degradation rate constant is 22% higher than that under alkaline conditions, while the valence state of Cu remains unchanged. This can be attributed to distinct degradation mechanisms at different pH levels, in which acidolysis and metal-ligand disruption dominate in the presence of LA, while HKUST-1 degradation is primarily redox-driven in NaAs solution. These findings offer mechanistic insight into the degradation behavior of HKUST-1 and provide valuable guidance for optimizing MOF stability in practical applications.