# A Dual‐Piezoelectric Metal–Organic Cage/COF Z‐Scheme Heterojunction for Solar‐Mechanical Water Splitting

## TL;DR

A new solar-mechanical system splits water efficiently using a dual-piezoelectric material to produce hydrogen and hydrogen peroxide.

## Contribution

Introduces a dual-piezoelectric Z-scheme heterojunction using a metal–organic cage and COF for enhanced water splitting.

## Key findings

- The heterojunction achieves 1297.6 µmol g⁻¹ h⁻¹ H₂ and 1304.2 µmol g⁻¹ h⁻¹ H₂O₂ under solar and ultrasound.
- Piezoelectric polarization and Z-scheme charge transfer synergistically enhance performance.
- Pd and C2–C3 sites are identified as active for H₂ and H₂O₂ production, respectively.

## Abstract

Developing efficient solar‐mechanical energy conversion systems is crucial for sustainable H2/H2O2 production. We report an unprecedented piezoelectric metal–organic cage (MOC‐FA3), integrated with piezoelectric β‐ketoenamine COF (S‐COF) into a Z‐scheme heterojunction via supramolecular interactions. The non‐centrosymmetric piezoelectricity of MOC‐FA3 is confirmed by multimodal characterization (KPFM, piezoelectric coefficient measurement, and in situ spectroscopy) and piezocatalytic H2/H2O2 production (334.3/203.6 µmol g−1 h−1). Under ultrasound (60 W, 40 kHz) and AM 1.5G, the heterojunction achieves exceptional synergistic pure water splitting at 1297.6 H2 and 1304.2 H2O2 µmol g−1 h−1, among the highest for COF‐based piezo‐photocatalysts, with 1.95‐fold and 2.06‐fold enhancements over S‐COF. This stems from Z‐scheme charge transfer coupled with piezoelectric‐field‐enhanced separation. Experimental results and DFT calculations identify Pd sites (H2 evolution) and C2‐C3 sites (2e− water oxidation → H2O2), with in situ EPR confirming •OH intermediates. This work introduces MOCs as a new piezoelectric material class, demonstrating synergistic Z‐scheme/piezoelectric coupling for mechano‐photonic energy conversion.

A dual‐piezoelectric Z‐scheme heterojunction is constructed by integrating a piezoelectric metal–organic cage (MOC‐FA3) with a β‐ketoenamine‐linked covalent organic framework (S‐COF). This design enables efficient solar‐mechanical energy conversion, achieving remarkable piezo‐photocatalytic pure water splitting for simultaneous H2 and H2O2 production without sacrificial agents, leveraging synergistic piezoelectric polarization and Z‐scheme charge transfer.