Revealing Atomic Structure and Oxidation States of Dopants in Charge-Ordered Nanoparticles for Migration-Promoted Oxygen-Exchange Capacity

Doping of nanomaterials has become a versatile approach to tailoring their physical and chemical properties, leading to the emerging fields of solotronics and quantum-controlled catalysis. These extraordinary functionalities critically depend on the atomic arrangements and dynamic behaviors of dopants, which are however challenging to probe due to the ultrasmall volume of hosting nanomaterials and the even smaller scale of doping-induced structure variations. Here, we reveal the characteristic configurations of Ce dopants and their correlation with the remarkably enhanced oxygen-exchange capacity in <10 nm Mn₃O₄ nanoparticles. The element and oxidation-state sensitivity and quantification capability of atomic-resolution electron energy-loss spectroscopic mapping allow an unambiguous determination of substitutional solitary Ce dopants and CeO₂ nanoclusters inside the charge-ordered Mn₃O₄ matrix, as well as single-atomic-layer CeO_x on the surface. The observed high mobility of Ce dopants further illustrates an effective pathway for the conversion among various dopant nanophases. Our observation provides atomic-scale evidence of the oxygen-exchange mechanism through dopant migration in Ce-doped Mn₃O₄ nanoparticles, which rationalizes their superior redox efficiency and oxygen-exchange capacity for thermochemical synthesis of solar fuels. The demonstrated characterization strategy capable of directly probing local atomic and electronic structures of dopants can be widely applied to the investigation of structure-property interplay in other doping-engineered nanomaterials. © 2019 American Chemical Society.