Depletable peroxidase-like activity of Fe3O4 nanozymes accompanied with separate migration of electrons and iron ions

Abstract As pioneering Fe3O4nanozymes, their explicit peroxidase (POD)-like catalytic mechanism remains elusive. Although many studies have proposed surface Fe2+-induced Fenton-like reactions accounting for their POD-like activity, few have focused on the internal atomic changes and their contribution to the catalytic reaction. Here we report that Fe2+within Fe3O4can transfer electrons to the surface via the Fe2+-O-Fe3+chain, regenerating the surface Fe2+and enabling a sustained POD-like catalytic reaction. This process usually occurs with the outward migration of excess oxidized Fe3+from the lattice, which is a rate-limiting step. After prolonged catalysis, Fe3O4nanozymes suffer the phase transformation to γ-Fe2O3with depletable POD-like activity. This self-depleting characteristic of nanozymes with internal atoms involved in electron transfer and ion migration is well validated on lithium iron phosphate nanoparticles. We reveal a neglected issue concerning the necessity of considering both surface and internal atoms when designing, modulating, and applying nanozymes.