Mechanisms for the Formation of Active Sites in Single-Atom Alloys

TL;DR

This study uses density functional theory to understand how dopant atoms incorporate into metal surfaces to form single-atom alloys, revealing how surface features and element interactions influence active site formation.

Contribution

It provides a detailed mechanistic and periodic trend analysis of dopant incorporation in SAAs, guiding synthesis strategies for different transition metals.

Findings

Step edges and kink sites facilitate dopant incorporation.

Incorporation barriers vary with transition metal type and surface.

Adatom interactions influence island formation and incorporation efficiency.

Abstract

Reactive dopant atoms embedded in inert host metal surfaces define the active sites in single-atom alloys (SAAs), yet SAA synthesis remains challenging. To address this, we elucidate how dopant adatoms deposited on Cu and Ag surfaces become incorporated into the metal and identify periodic trends from early to late transition metals (TMs) using density functional theory. Adatoms diffuse nearly freely across terraces, as diffusion barriers are small, whereas direct incorporation into terraces is unfavourable. In line with conventional wisdom, step edges and kink sites strongly facilitate dopant incorporation, confirming their critical role in alloy formation. Attachment of adatoms to steps and kinks from the lower terrace is favoured. Incorporation then proceeds either from this attached state or when adatoms approach a step edge from above, where reactions often proceed without barrier. Incorporation barriers are generally lower for early and central TMs, increase towards late TMs, and are slightly higher on Cu than on Ag surfaces. Repulsive interactions between Pd adatoms and dopants explain the experimental observation that a dopant-rich brim on the upper terrace of Cu surfaces inhibits incorporation from above. In contrast, attractive interactions, as found for Ru, anchor diffusing adatoms (even on terraces) and promote the formation of adatom islands, yet hinder incorporation next to the dopant and may impede the growth of embedded dopant clusters. By rationalising periodic trends and experimental observations, we show how specific surface sites and adatom--dopant interactions shape dopant incorporation, offering guidance on the surface environments most conducive to SAA synthesis for different dopant elements.