Amorphous magnetic rust induces hole trapping and electron retention for efficient TiO₂ photocatalysis

Photogenerated charge carrier behavior at solid-liquid interfaces is fundamental to photocatalysis. Here, we report an approach to achieve superior charge separation by physically mixing TiO₂ with an amorphous magnetic rust (M-rust) upcycled from waste NdFeB magnets. The optimal TiO₂/M-rust hybrid exhibits significantly enhanced photocatalytic activity under UV light, while its transient photocurrent drops to merely one-eighth of that of pristine TiO₂. This stark "high-activity, low-photocurrent" contrast provides a unique platform to probe the interfacial charge transfer mechanism. X-ray photoelectron spectroscopy and in-situ electron paramagnetic resonance spectroscopy reveal the underlying charge transfer dynamics. Ti 2p shifts to lower binding energy, indicating electron retention on TiO₂, while Fe 2p shifts to higher binding energy, suggesting hole capture by the amorphous rust. The free Fe²⁺ main peak disappears, while the lattice Fe²⁺ satellite peak increases, providing evidence for an in-situ Fe³⁺/Fe²⁺ redox cycle. The lattice oxygen proportion increases, indicating evolution of surface chemical states. In-situ EPR reveals enhanced superoxide radical (•O₂^-) generation, identified as the dominant reactive species. Radical scavenging experiments further indicate that ·O₂^- is the essential initiating species, while ·OH acts as a downstream product that also participates in the degradation. Crystalline ordinary rust (O-rust) shows no such effects, indicating that the amorphous Fe-O-Fe network with abundant under-coordinated sites is the structural origin of hole trapping. This work provides spectroscopic evidence for the decoupling of photocurrent from catalytic activity at the amorphous/crystalline heterointerface, and demonstrates a sustainable route to upcycle electronic waste into functional catalytic materials.