Hydrogen Reactivity of Palladium Nanoparticles Coated with Mixed Monolayers of Alkyl Thiols and Alkyl Amines for Sensing and Catalysis Applications

Palladium monolayer-protected clusters (MPCs) coated with octylamines (C8NH₂), hexanethiolates (C6S), and mixed monolayers of C8NH₂ and C6S exhibit significantly different reactivities with hydrogen gas, depending on the relative amounts of the two ligands coating the Pd nanoparticle surface, as determined by UV−vis spectroscopy of Pd MPCs in solution and electronic measurements of films of Pd MPCs as a function of exposure time to hydrogen. The average estimated composition of the ∼3.0 nm diameter Pd MPCs was Pd₉₁₉(C6S)₁₉₂ or Pd₉₁₉(C8NH₂)_177−x(C6S)_x, where x was varied to be 0, 3, 10, 16, 32, or 81 by the synthesis of pure C8NH₂ Pd MPCs and subsequent liquid-phase place exchange with a varied amount of C6SH. When x = 0−10, the Pd MPCs react strongly with H₂, leading to aggregated particles in solution and large irreversible changes in the morphology of films accompanied by an increase in film conductivity by 2−5 orders of magnitude. Pd₉₁₉(C6S)₁₉₂ MPCs are stable against significant aggregation in solution and do not exhibit large film morphology changes, but they are also not highly reactive to H₂, as determined by minimal changes in the optical properties of solutions and the small, irreversible changes in the conductivity of films in the presence of H₂. Finally, when x is 32 and 81, the Pd MPCs are fairly stable, exhibit minimal aggregation or morphology changes, and readily react with H₂ based on the significant, reversible changes in film conductivity in the presence of H₂. Pd MPCs with mixed monolayers have the benefit of high H₂ reactivity while maintaining the structural stability necessary for sensing and catalysis applications.