Programming Charge Dynamics in Photocatalytic Covalent Organic Frameworks through Heterometallic Symmetry Breaking

Area of Science:

• Materials Science
• Photocatalysis
• Nanotechnology

Background:

• Covalent organic frameworks (COFs) possess symmetric potential landscapes that hinder exciton dissociation and promote charge recombination, limiting their photocatalytic performance.
• Existing strategies like donor-acceptor architectures and single-metal incorporation inadequately address long-range directional charge separation in COFs.

Purpose of the Study:

• To develop a general strategy for breaking the intrinsic electronic symmetry of COFs.
• To engineer a heterogeneous distribution of metal sites within COF pores to improve charge separation and carrier mobility.
• To establish heterometallic symmetry breaking as a viable design principle for advanced porous crystalline materials.

Main Methods:

• Utilized an atomically precise salen-based zinc COF for partial substitution of zinc with cobalt.
• Employed density functional theory (DFT) calculations and photoelectrochemical analyses to study electronic asymmetry.
• Investigated heterometallic combinations including Cu/Zn and Ni/Zn alongside Co/Zn.

Main Results:

• Partial substitution of Zn with Co in a ZnCOF created an asymmetric metal-site distribution, inducing a built-in electric field.
• A Co/Zn ratio of approximately 1:5 was found to generate the most significant asymmetry and in-plane electric field.
• The optimized heterometallic COF exhibited a CO2-to-CO conversion rate of 6917 μmol·g-1, a 124-fold enhancement over pristine ZnCOF.

Conclusions:

• Heterometallic symmetry breaking is a powerful approach to engineer charge dynamics in COFs.
• This strategy significantly enhances photocatalytic efficiency for solar energy conversion applications.
• The developed method is versatile and applicable to various metal combinations in porous crystalline materials.