Buried electrostatic modulation enables size-dependent reactivity in Pd-based nanocatalysts

The catalytic reactivity of metal nanocatalysts is generally assumed to plateau beyond tens of nanometers as their electronic structure and Fermi level (<i>E</i> _F) converge with bulk properties. Surprisingly, our catalytic experiments reveal that Au nanocubes retain pronounced and switchable size-dependent reactivity even above 30 nm, while Pd nanocubes remain size-invariant in this regime. This intriguing contrast parallels their plasmonic behavior, where Au's sp-electron dominance enables robust plasmonic activity despite Pd's strong d-electron damping. Crucially, unlike prior Au-Pd studies focused on static d-band shifts (ligand effects) or light-induced hot electron effects (plasmonics), we propose that Au's sp-electron dominance enables strong <i>E</i> _F responsiveness to transient surface charge, whereas Pd's d-states near <i>E</i> _F likely buffer these electrostatic perturbations. Leveraging this fundamental contrast, we designed Au-Pd core-shell nanocubes where the buried Au core serves as an internal electrostatic modulator. By sensing and amplifying surface charge, the Au core dynamically tunes Pd shell reactivity, enabling reversible and size-dependent modulation under reductive <i>versus</i> oxidative conditions-a behavior unattainable with Pd alone and unprecedented in this large size regime. This rational design introduces a new electrostatic degree of freedom in nanocatalysis, establishing <i>E</i> _F responsiveness-not d-band energetics or surface coordination-as a foundational principle for charge- and size-tunable catalysis.