Imaging defects and their evolution in a metal-organic framework at sub-unit-cell resolution.

Area of Science:

• Materials Science
• Crystallography
• Catalysis

Background:

• Defect engineering in metal-organic frameworks (MOFs) is crucial for tuning material properties.
• Characterizing defects in MOFs is challenging due to their fragile crystal structures and the difficulty in probing local atomic arrangements.

Purpose of the Study:

• To directly observe and characterize structural defects in the catalytic MOF UiO-66 with sub-unit-cell resolution.
• To understand the evolution of these defects during MOF crystallization and their impact on catalytic activity.

Main Methods:

• Utilized low-dose transmission electron microscopy (TEM) combined with electron crystallography for high-resolution imaging.
• Employed density functional theory (DFT) calculations to rationalize experimental observations.
• Assessed catalytic performance for glucose isomerization.

Main Results:

• Direct, real-space observation of ordered 'missing linker' and 'missing cluster' defects in UiO-66.
• Three-dimensional reconstruction revealed missing-linker defects terminated by formate groups.
• Observed defect evolution during Ostwald ripening, with missing-linker defects persisting.
• Missing-cluster defects exhibited higher catalytic activity for glucose to fructose isomerization compared to missing-linker defects.

Conclusions:

• Sub-unit-cell resolution imaging and DFT calculations provide unprecedented insight into MOF defect structures and formation mechanisms.
• Defect type significantly influences catalytic performance, highlighting the potential of targeted defect engineering for enhanced functionality.