Atomic-scale Imaging of Iodide-Gold Interactions in Nanoconfined Liquid-Solid Interfaces

Functionalization of nanoporous metallic materials enables the tailoring of surface chemistry and morphology in nanostructured materials, optimising their performance for electrocatalytic and sensor applications. Liquid phase chemical functionalization is governed by liquid solid interfaces. Yet, these interfaces remain poorly understood due to the challenges of characterising the liquid phase at high spatial and chemical resolutions. To elucidate pathways for functionalizing nanoscale metals, it is crucial to measure the distribution of species, including light elements, across the liquid solid interface, capturing both reactants and products. Here, we employ cryogenic atom probe tomography to directly analyse frozen liquid solid reaction interfaces at near atomic resolution. Focusing on the interaction of iodide and sodium ions with nanoporous gold, we observe the formation of iodine containing complexes on gold nanoligament surfaces and subsurfaces. These findings reveal aspects of the gold iodide system that were previously hidden, including the reaction mechanism between iodide and gold atoms on the surface, and the multiple gold iodide complexes forming. Our work demonstrates that cryogenic atom probe tomography can provide unprecedented visualisation and characterisation of nanoscale interfaces during chemical and electrochemical reactions, with potential implications for modern manufacturing, energy technologies, and sustainable materials development.

💡 Research Summary

The authors present a pioneering study that applies cryogenic atom probe tomography (cryo‑APT) to directly image and chemically characterize the liquid‑solid interface between nanoporous gold (NPG) and an aqueous sodium iodide (NaI) solution at near‑atomic resolution. Nanoporous gold, produced by dealloying, offers a high surface‑area, conductive scaffold that is widely used in electrocatalysis and sensing, yet the exact nature of its interaction with ionic species in solution has remained elusive because conventional techniques (electrochemical measurements, scanning probe microscopy, cryo‑TEM) either average over macroscopic areas or lack sufficient chemical specificity for light elements and complex ions.

In the experimental workflow, a 15 nm ligament NPG sample was immersed in 50 mM NaI, rapidly plunge‑frozen, and then shaped into an APT tip using a focused‑ion‑beam “satellite‑dish” milling protocol that avoids a separate cryo‑lift‑out step. The final specimen consists of a ~3 µm thick ice cap covering a sub‑micron NPG tip with a radius below 100 nm. This geometry preserves the native distribution of ions while providing the sharp geometry required for field evaporation.

APT analysis was performed with a base evaporation field of 24 V nm⁻¹ and laser‑assisted pulsing. The mass spectrum (0–550 Da) revealed a rich set of ions: Au⁺, AuI⁺/2⁺, AuI₂⁺/2⁺, Au₂I⁺/2⁺, AuI(H₂O)⁺/2⁺, I⁺, I²⁺, I(H₂O)⁺, Na⁺, and Na(H₂O)ₓ⁺ (x ≤ 4). Notably, several gold‑iodide complexes (AuI, AuI₂, Au₂I) were detected for the first time in an APT study of a gold‑halide system, confirming that iodide ions bind directly to gold atoms rather than merely adsorbing as simple anions. Sodium, by contrast, appeared only as hydrated Na⁺ clusters, indicating negligible specific interaction with the gold surface. This observation aligns with hard‑soft acid‑base (HSAB) theory: Na⁺ (hard acid) has low affinity for gold (soft base), whereas I⁻ (soft base) strongly coordinates to gold.

Three‑dimensional reconstructions show that the NPG ligaments retain a solid gold network while a thin (~4 nm) Au‑I shell envelops the ligaments, containing roughly 40 at.% Au and 40 at.% I. This shell is significantly thicker than the Au‑I layers reported in dealloyed Ag‑Au systems (≈2 nm), suggesting that the gold‑iodide reaction proceeds not only at the outer surface but also penetrates into the interior of the ligaments. Concentration profiles across the liquid‑solid boundary demonstrate that iodide concentration within the pores is markedly higher than that of sodium, reflecting the higher detection efficiency of I⁻ (lower detachment energy) and its stronger electrostatic attraction to the gold surface.

The authors propose a mechanistic picture in which gold atoms at the surface undergo partial dissolution during dealloying, immediately capture nearby I⁻ to form Au‑I complexes, and then re‑incorporate into the ligament structure, generating a mixed Au‑I shell. Sodium ions remain hydrated and are largely excluded from the gold lattice, contributing only a weak background signal.

Beyond the specific chemistry of the Au‑I system, the work showcases cryo‑APT as a uniquely powerful tool for probing buried liquid‑solid interfaces. Unlike cryo‑TEM, which suffers from electron scattering in thick liquid layers and limited elemental sensitivity, cryo‑APT provides quantitative mass spectra for both heavy and light elements, enabling precise stoichiometric analysis of complex ionic species. The ability to detect hydrated clusters, differentiate charge states, and map three‑dimensional distributions opens new avenues for studying electrochemical interfaces in batteries, fuel cells, and catalytic reactors where interfacial composition dictates performance.

In conclusion, this study delivers the first atomic‑scale, three‑dimensional chemical map of a gold‑iodide liquid‑solid interface, revealing multiple Au‑I complexes, a distinct Au‑I shell, and the inert behavior of Na⁺. It validates cryogenic atom probe tomography as a breakthrough analytical platform for elucidating reaction pathways at nanoscale interfaces, with broad implications for the design of next‑generation electrocatalysts, sensors, and sustainable material technologies.