Surface Plasmon Spectroscopy of Nanosized Metal Particles

TLDR

Optical measurements based on the Drude model are used to monitor electrochemical changes on nanosized metal particles, with the absorption spectrum of metal sols in water strongly influenced by cathodic/anodic polarization, chemisorption, adatom deposition, and alloying. Anion adsorption strongly damps free‑electron absorption, while cathodic polarization causes anion desorption; underpotential deposition of electropositive metals produces dramatic blue‑shifts of the surface plasmon band, allowing detection of as little as 0.1 ML, and can induce spontaneous alloying, as evidenced by gold on silver and silver on palladium where alloy formation and lattice strain affect the optical spectrum.

Abstract

The use of optical measurements to monitor electrochemical changes on the surface of nanosized metal particles is discussed within the Drude model. The absorption spectrum of a metal sol in water is shown to be strongly affected by cathodic or anodic polarization, chemisorption, metal adatom deposition, and alloying. Anion adsorption leads to strong damping of the free electron absorption. Cathodic polarization leads to anion desorption. Underpotential deposition (upd) of electropositive metal layers results in dramatic blue-shifts of the surface plasmon band of the substrate. The deposition of just 0.1 monolayer can be readily detected by eye. In some cases alloying occurs spontaneously during upd. Alloy formation can be ascertained from the optical absorption spectrum in the case of gold deposition onto silver sols. The underpotential deposition of silver adatoms onto palladium leads to the formation of a homogeneous silver shell, but the mean free path is less than predicted, due to lattice strain in the shell.