A Direct Z-Scheme Single-Atom MOC/COF Piezo-Photocatalytic System for Overall Water Splitting

Overall water splitting into H₂ and H₂O₂ via Z-scheme piezo-photocatalytic systems is an ideal method for renewable energy production. Herein, we have synthesized a triangular prism-shaped metal-organic cage (MOC-Q3) integrating three catalytic Pd²⁺ centers and two photosensitive ligands, which is successfully immobilized on a highly crystalline β-ketoenamine-linked covalent organic framework (EA-COF) to form a Z-scheme single-atom photosystem. The optimized MOC-Q3/EA-COF achieves a high H₂ yield (26.17 mmol g^-1 h^-1) with a TON_Pd of 118,521 with ascorbic acid as sacrificial agent due to broad light absorption, effective carrier separation, and widely distributed Pd active sites, which is among the highest for COF-based solar H₂ evolution photocatalysts. Interestingly, EA-COF is found to be a piezoelectric material and its piezoelectric performance is mainly due to the in-plane polarization of the 2,4,6-trihydroxybenzene-1,3,5-tricarbaldehyde groups in the COF, which is confirmed by experimental observations and density functional theory calculations. The EA-COF shows H₂ and H₂O₂ production rates of 239.94 and 400.38 μmol g^-1 h^-1, respectively, in pure water when excited by ultrasound coupled with light irradiation. The integration of MOC-Q3 can further enhance the efficiency of EA-COF in piezo-photocatalytic water splitting. The superior MOC-Q3/EA-COF exhibits H₂ and H₂O₂ generation rates of 426.38 and 535.14 μmol g^-1 h^-1, respectively, outperforming pure EA-COF by 1.8 and 1.3 times. This is a pioneering work to construct a Z-scheme MOC/COF piezo-photocatalytic system, which provides an efficient way to use mechanical and solar energy to produce H₂ and H₂O₂ through overall water splitting.