Direct Probing of the Oxygen Evolution Reaction at Single NiFe2O4 Nanocrystal Superparticles with Tunable Structures

Due to the precisely controllable size, shape, and composition, self-assembled nanocrystal superlattices exhibit unique collective properties and find wide applications in catalysis and energy conversion. Identifying their intrinsic electrocatalytic activity is challenging, as their averaged properties on ensembles can hardly be dissected from binders or additives. We here report the direct measurement of the oxygen evolution reaction at single superparticles self-assembled from similar to 8 nm NiFe2O4 and/or similar to 4 nm Au nanocrystals using scanning electrochemical cell microscopy. Combined with coordinated scanning electron microscopy, it is found that the turnover frequency (TOF) estimated from single NiFe2O4 superparticles at 1.92 V vs RHE ranges from 0.2 to 11 s(-1) and is sensitive to size only when it is smaller than similar to 800 nm in diameter. After the incorporation of Au nanocrystals, the TOF increases by similar to 6-fold and levels off with further increasing Au content. Our study demonstrates the first direct single entity electrochemical study on individual nanocrystal superlattices with tunable structures and unravels the intrinsic structure-activity relationship that is not accessible by other methods.

Automated summary

The study directly measures the oxygen evolution reaction in single self-assembled superparticles and shows that the NiFe2O4 superparticles' TOF depends on their diameter, while the incorporation of Au nanocrystals significantly increases the TOF, leveling off after a certain concentration. This research provides insights into the intrinsic structure-activity relationship of individual nanocrystal superlattices with tunable structures through direct electrochemical analysis.