Ferrielectric Dipolar Ordering in a Donor–Acceptor Based Covalent–Organic Framework for Piezocatalytic Water Splitting

Abstract

Piezocatalysis has emerged as a promising technique for the production of green H₂ fuel by harvesting mechanical energy. A metal-free, highly porous covalent–organic framework (COF) as a piezocatalyst that produces H₂ at an ultra-high rate of 6.6 mmol g⁻¹ h⁻¹ under ultrasonication is reported. This activity originates from the electron–hole carriers generated in “nested” nearly degenerate conduction and valence bands driving ferrielectric ordering of dipoles whose coupling with soft torsional phonons facilitates absorption of energy from the mechanical stress fields. Conformationally flexible donor tris(4-aminophenyl)amine (TAPA) moiety in COF introduces soft torsional lattice modes that interact with pyromellitic dianhydride (PDA) acceptor to generate stress tunable dipoles and surface charges. As electron–hole pairs generated throughout the bulk ferrielectric are available at the pore surfaces of the COF, they are readily accessible to the water molecules to be split. The work provides a design concept based on donor–acceptor based frameworks showing conformational flexibility-driven symmetry breaking for piezocatalysis.