Hierarchically structured MOF-on-MOF photocatalysts with engineered charge dynamics for sustainable green fuel generation

Abstract

The limited photocatalytic performance of individual metal–organic frameworks (MOFs) restricts their practical use. To address this, the integration of distinct MOFs into MOF-on-MOF heterostructures can create well-defined charge-transfer interfaces, significantly enhancing photocatalytic efficiency. Motivated by this, we investigated the in situ assembly of ZIF-67 with NH₂-MIL-125(Ti), resulting in a binary ZIF-67/NH₂-MIL-125(Ti) all-solid-state Z-scheme heterostructure. Comprehensive characterisation through PXRD, BET, FTIR, UV-Vis spectroscopy, contact angle analysis and electrochemical studies confirmed enhanced structural and optoelectronic properties. Elemental profiling was carried out by ICP-OES, CHNO evaluation, and EDX spectroscopy. The hybrid catalyst exhibited an impressive H₂O₂ formation efficiency of 1345 µmol g⁻¹ h⁻¹, accompanied by a quantum yield of 3.64% under 400 nm irradiation, and also delivered a H₂ evolution output of 215 µmol h⁻¹, each showing a fourfold improvement compared to the individual pristine MOFs. The synergistic interaction between the well-designed MOF-on-MOF heterostructure and the Z-scheme charge transfer mechanism effectively minimised charge recombination, as evidenced by XPS, PL spectra, TRPL, and EIS analyses. Furthermore, free radical trapping experiments and ESR studies confirmed the critical role of ˙O₂⁻ radicals in the photocatalytic formation of H₂O₂. This study provides valuable insights for developing advanced MOF-based heterojunction photocatalysts tailored for efficient solar-to-chemical energy conversion.