Boosted Charge Transfer for Highly Efficient Photosynthesis of H2O2 over Z-Scheme I−/K+ Co-Doped g-C3N4/Metal–Organic-Frameworks in Pure Water under Visible Light

Abstract

Photocatalytic oxygen reduction reaction (ORR) is an environmentally friendly and cost-effective approach for H₂O₂ synthesis. However, the current photosynthesis system suffers from sluggish kinetics, rapid recombination of photoexcited charge carriers, and weak redox potentials, resulting in unsatisfactory solar-to-chemical conversion efficiency. Herein, a Z-scheme heterojunction (UiO/IKCN) is constructed through coupling I⁻/K⁺ co-doped g-C₃N₄ (IKCN) with NH₂-UiO-66, a typical metal-organic framework material. Under visible light irradiation, the optimal UiO/IKCN exhibits exceptional H₂O₂ production rates in pure water (13.3 mM g⁻¹ h⁻¹) and in isopropanol solution (72.6 mM g⁻¹ h⁻¹), that is 48.4 times higher than pristine CN in isopropanol (1.5 mM g⁻¹ h⁻¹). A high apparent quantum yield of 10.28% at 420 nm is achieved by UiO/IKCN, surpassing most previously reported values. The dominating role of two-electron ORR in H₂O₂ photosynthesis is elucidated in detail. The significantly enhanced photocatalytic activity can be attributed to the facilitated charge separation and Z-scheme charge transfer, which are unambiguously verified by stable-state surface photovoltage, transient photoluminescence, femtosecond transient absorption spectroscopy, in-situ irradiated X-ray photoelectron spectroscopy, and density functional theory calculations. This study represents the first exploration of H₂O₂ production using NH₂-UiO-66 and provides insights into the rational design of Z-scheme heterojunction for highly efficient photosynthesis.