Insight into Gold Nanoparticle–Hydrogen Interaction: A Way To Tailor Nanoparticle Surface Charge and Self-Assembled Monolayer Chemisorption

Abstract

The interaction of hydrogen with gold nanoparticles (Au NPs) and gold thin films also functionalized with thiols is investigated. Au NPs deposited on silicon substrates by radio frequency sputtering of a gold target and gold thin films have been exposed to a remote H2 plasma and subsequently functionalized by the aromatic (4-methoxyterphenyl-3″,5″-dimethanethiol) and aliphatic (dodecanethiol) thiols. The impact of hydrogenation on changes of the charge on gold surfaces and nanoparticles, on the kinetics of the thiol self-assembled monolayer (SAM) formation, and on the density of the resulting SAMs has been investigated combining spectroscopic ellipsometry (SE), Raman spectroscopy, and surface potential Kelvin probe microscopy (SP-KPM) in conjunction with noncontact atomic force microscopy (AFM). We found that remote H2 plasma pretreatments of gold surfaces are effective in improving thiolate adsorption, making SAMs more uniform and densely packed. We also demonstrate that hydrogenation of nanoparticles improves stability of thiol functionalized Au NPs, avoiding their aggregation. Additionally, we demonstrate that a remote H2 plasma processing is also effective in the selective removal of the carbon chain and of sulfur atoms from gold surfaces, therefore allowing tailoring of their optical and chemical properties.

Synthesis and Properties of Au Hydride

“…and amplitude and lineshape of the calculated imaginary part of the dielectric function are also seen to agree remarkably well with the measurements of Giangregorio et al. . Here, the differences with respect to Figure b are attributed to deviations in the nanoparticle thin films from an Au(111) surface.…”

“…Clearly, an overall 0.02 eV blue shift in ϵ 2 was detected during H 2 exposure. Incidentally, a similar blue shift upon hydrogenation of gold nanoparticles in a H 2 electrical discharge was reported by Giangregorio et al ., and a blue shift in the LSPR of Au nanorods was observed in the presence of H 2 and Pt nanoparticles, presumably the source of atomic H by spillover . Therefore, the presence of hydrogen significantly modifies the dielectric function, but nitrogen alone does not induce significant modifications.…”

“…This reaction also drives changes in the dielectric constant and the associated optical properties at the gold surface. Our study provides a new approach for exploring the bonding behavior between non‐reactive noble metals and H 2 in situ, without the need for extreme experimental conditions,, and opens up possibilities for novel methods of materials synthesis and low‐concentration sensing applications. Clearly, our work shows that gold hydride should be considered as an intermediate in the crucial area of photocatalytic reactions such as the artificial photosynthesis of ammonia, which mitigates the high energy consumption of the Haber‐Bosch process …”

“…We now discuss the robustness of our blue shift findings. Previous rationalizations of the blue shift utilize a model dielectric function focusing on the Drude part and a well‐defined charge transfer ,,,. However, band structure contributions are vital for an accurate description of the optical properties of gold nano‐structures and the AuH bond exhibits strong covalent character .…”

“…thermodynamics alone cannot drive the dissociation of H 2 given the large activation energy of 4.51 eV. Therefore pathways to gold‐hydride formation remain elusive, except possibly in some extreme conditions ,. The interaction of noble metals with photons whose energy is in resonance with the surface plasmons leads to the generation of hot electrons that can drive chemical reactions with a relatively low energy cost.…”