Visualizing Eigen/Zundel cations and their interconversion in monolayer water on metal surfaces.

Area of Science:

• Surface Science
• Physical Chemistry
• Nanotechnology

Background:

• The behavior of hydrated protons on solid surfaces is critical for fields like electrochemistry and hydrogen fuel cells.
• Understanding these hydrated protons at an atomic level is essential but challenging due to limitations in characterization techniques.

Purpose of the Study:

• To directly visualize and characterize Eigen- and Zundel-type hydrated protons on Au(111) and Pt(111) surfaces.
• To elucidate the structural organization and stability of these hydrated protons within water networks.

Main Methods:

• Utilized cryogenic qPlus-based atomic force microscopy (AFM) under ultrahigh vacuum conditions.
• Applied atomic-scale imaging to resolve hydrated proton structures on metal surfaces.

Main Results:

• Directly visualized Eigen cations forming ordered monolayers and Zundel cations forming long-range ordered structures.
• Observed that nuclear quantum effects stabilize Zundel cation structures.
• Identified a transformation where two Eigen cations form one Zundel cation with proton transfer to the surface.
• Found a preference for Zundel over Eigen configurations on Pt(111), which was not observed on Au(111).

Conclusions:

• Provided unprecedented atomic-scale insights into hydrated proton structures on metal surfaces.
• Demonstrated the distinct self-assembly behaviors and surface preferences of Eigen and Zundel cations.
• Highlighted the role of nuclear quantum effects in stabilizing specific hydrated proton configurations.