Investigating the reorganization properties of partially charged ions at surfaces: A model study of Ag <i>δ</i> + adsorbed on Au(111)

Abstract

Metal–liquid interfaces host partially charged adsorbates whose solvent reorganization and polarization strongly influence electron-transfer kinetics, yet these quantities are difficult to extract from ab initio calculations because strong hybridization broadens and shifts the electronic levels of an adsorbate. Here, we combine the implicit continuum solvation model and explicit atomistic water molecular dynamics, using a combination of machine-learned interatomic potentials trained to density functional theory (DFT) and explicit DFT calculations, to quantify solvation potentials and reorganization energies for a model Agδ+ adsorbate on an Au(111) slab. Continuum solvation model calculations along the adsorption pathway yield bulk-like solvation shifts for fully solvated Ag+ and constrain the solvent polarization potential acting on adsorbed Agδ+ to roughly half this value. To separate nuclear from electronic contributions at finite temperature, we fine-tuned a machine-learned interatomic potential to ab initio molecular dynamics trajectories and generated 200 ps of explicit-water dynamics for both bulk Ag+ and surface Agδ+, with hybrid-functional DFT (HSE06) sampling of instantaneous eigenvalues. Gerischer–Hopfield analysis gives a bulk reorganization energy of near 1.4 eV and a lower bound at ∼30% of this value upon interfacial reorganization. Analysis of the solvation potential, non-vanishing reorganization energy (through tracking adsorbate core-level fluctuations), and persistent dipole polarization upon adsorption suggests that partially solvated surface species can retain an appreciable fraction of bulk-like solvation properties. Altogether, the theoretical findings presented imply that sufficiently resolved spectroscopic probes of core-level fluctuations could be essential to quantifying these properties. This, in turn, could have broad implications for understanding interfacial kinetics within many practical electrochemical systems.