Ultrafine Nanoclusters Unlocked 3d-4f Electronic Ladders for Efficient Electrocatalytic Water Oxidation.

The vast extensional planes of two-dimensional (2D) nanomaterials are recognized as desirable ground for electrocatalytic reactions. However, they tend to exhibit catalytic inertia due to their surface-ordered coordination configurations. Herein, an in situ autoxidation strategy enables high-density grafting of ultrafine CeO 2 nanoclusters on 2D Co(OH) 2 . Affluent active units were activated at the inert interface of Co(OH) 2 via the formation of Co-O-Ce units. The optimized catalyst exhibits oxygen evolution reaction activity with an overpotential of 83 mV lower than that of Co(OH) 2 at 10 mA cm -2 . The cascade orbital coupling between Co (3d) and Ce (4f) in Co-O-Ce units drives electron transfer by unlocking a "d-f electron ladder". Meanwhile, the bond-order theorem analyses and the d-band center show that the occupancy of Co-3d- e g is optimized to balance the adsorption-desorption process of active sites to the key reaction intermediate *OOH, thereby making it easier to release oxygen. This work will drive the development of wider area electron modulation methods and provide guidance for the surface engineering of 2D nanomaterials.