Hydrogen-induced suppression of the optical plasmonic response in ultra-small gold nanoparticles.

In this study, we investigate how hydrogen adsorption modifies the structure, electronic properties, and optical response of sub-3 nm gold nanoparticles (NPs) using DFT for structural relaxations and real-time TDDFT(+U) for the absorption spectra. Under vacuum, a full monolayer of adsorbed hydrogen drives pronounced surface restructuring into Au-H-Au-H chain motifs and induces a net electron transfer from Au to adsorbed H, stabilizing the hydrogenated shell. These changes shift and broaden the electronic density of states and strongly affect plasmonic features. Thus, while bare Au NPs display the localized surface-plasmon resonance (LSPR) for diameters larger than 2 nm, hydrogenation markedly diminishes the resonance, nearly quenching it for particles below 3 nm. Comparison with constrained geometries indicates that Au-H bonding and charge transfer primarily drive LSPR suppression, whereas structural distortion mainly contributes to spectral broadening. Our results suggest that hydrogen adsorption can profoundly change the plasmonic signature of ultrasmall Au NPs, providing important insight into the interpretation of their optical spectra under hydrogen-rich conditions.