Engineering the Atomic Layer of RuO₂ on PdO Nanosheets Boosts Oxygen Evolution Catalysis

We report an atomic-scale controllable synthesis of the face-centered cubic Ru overlayers on Pd nanosheets (Pd@Ru NSs) by a solution-based epitaxial growth method. The thickness of Ru overlayers can be accurately tuned at an atomic level, which has been confirmed by atomic force microscopy and high-angle annular dark-field scanning transmission electron microscopy. After annealing in air, the Pd@Ru NSs were transformed to PdO@RuO₂ NSs with rutile RuO₂ epitaxially grown on the PdO. The oxygen evolution reaction (OER) activity and stability strongly depend on the atomic layers of RuO₂ and ∼4 atomic layers of RuO₂ (PdO@RuO₂-4layers) exhibit superior stability and optimal activity for OER with only 257 mV of the overpotential to reach 10 mA cm^-2. Density functional theory calculations well reproduce the thickness dependence of OER activity and reveal that O* binds more weakly on the PdO@RuO₂-4layers that boosts the rate-determining step for formation of HOO*, assuring the best OER performance.