Photoactivated defect engineering and nanostructure functionalization of MoS₂via a photochemical Fenton process

Molybdenum disulfide (MoS₂) is a promising 2D material for (photo)catalysis. However, its performance in (photo)catalytic applications is usually limited by a small amount of catalytically active defects. Here, we developed a novel large-scale, rapid, green, low-cost photoetching technique to transform multilayer MoS₂ into a few-layer MoS₂ with high defect density and simultaneous spatial functionalization of MoS₂ with magnetic nanostructures using a photo-driven Fenton reaction. The photoetching process and resulting nanostructures were characterized by optical microscopy, atomic force microscopy, photoluminescence, and Raman spectroscopy. We elucidated the reaction mechanism driven by the Fenton reaction in which photogenerated charge carriers in MoS₂ play a dual role: reducing Fe³⁺ and Cu²⁺ ions and generating hydrogen peroxide (H₂O₂) from water and dissolved O₂. In this Fenton reaction, Fe²⁺ ions react with H₂O₂ to generate hydroxyl (˙OH) radicals, oxidizing MoS₂ and forming metal oxide nanostructures at the reaction sites. This dual pathway, triggered by MoS₂ photon absorption even at low-intensity illumination, ensures in situ generation of Fenton reactants (Fe²⁺ and H₂O₂), generating ˙OH, to achieve on-demand thinning and functionalization of MoS₂ in a single step. Electron paramagnetic resonance spectroscopy confirmed the generation of ˙OH radicals as the main reactive oxygen species. This photochemical approach enables the photo-driven creation and growth of defects from submicrometer regions up to a dozen micrometers, both at native defects and predefined defective region seeds, by photochemical processing of MoS₂ in FeCl₃ and CuSO₄ solutions. The presence of metal oxide nanostructures on MoS₂ was verified using magnetic force microscopy, scanning electron microscopy with elemental mapping by energy dispersive X-ray spectroscopy and Raman spectroscopy. The simultaneous photoetching and metal oxide deposition improves the catalytic performance of MoS₂ in the electrical hydrogen evolution reaction, evidenced by a potential shift from -0.7 V (graphite electrode) to -0.47 V (MoS₂ sample photoetched in FeCl₃ solution under a halogen lamp illumination) at a current density of 10 mA cm^-2.