Two-Dimensional Metal-Organic Frameworks with Unique Oriented Layers for Oxygen Reduction Reaction: Tailoring the Activity through Exposed Crystal Facets

As one of the most important families of porous materials, metal-organic frameworks (MOFs) have well-defined atomic structures. This provides ideal models for investigating and understanding the relationships between structures and catalytic activities at the molecular level. However, the active sites on the edges of two-dimensional (2D) MOFs have rarely been studied, as they are less exposed to the surfaces. Here, for the first time, we synthesized and observed that the 2D layers could align perpendicular to the surface of a 2D zeolitic imidazolate framework L (ZIF-L) with a leaf-like morphology. Owing to this unique orientation, the active sites on the edges of the 2D crystal structure could mostly be exposed to the surfaces. Interestingly, when another layer of ZIF-L-Co was grown heteroepitaxially onto ZIF-L-Zn (ZIF-L-Zn@ZIF-L-Co), the two layers shared a common b axis but rotated by 90 degrees in the ac plane. This demonstrated that we could control exposed facets of the 2D MOFs. The ZIF-L-Co with more exposed edge active sites exhibited high electrocatalytic activity for oxygen reduction reaction. This work provides a new concept of designing unique oriented layers in 2D MOFs to expose more edge-active sites for efficient electrocatalysis. [GRAPHICS] .

Automated summary

Metal-organic frameworks (MOFs) are a family of porous materials with well-defined atomic structures, which can be used to investigate the relationships between structures and catalytic activities. In this study, a two-dimensional zeolitic imidazolate framework L (ZIF-L) with a leaf-like morphology was synthesized, allowing for the exposure of active sites on the edges of the framework. By heteroepitaxially growing another layer of ZIF-L-Co onto ZIF-L-Zn, the exposed facets of the 2D MOFs could be controlled, resulting in higher electrocatalytic activity.