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 (C8NH<sub>2</sub>), hexanethiolates (C6S), and mixed monolayers of C8NH<sub>2</sub> 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 Pd<sub>919</sub>(C6S)<sub>192</sub> or Pd<sub>919</sub>(C8NH<sub>2</sub>)<sub>177−<i>x</i></sub>(C6S)<sub><i>x</i></sub>, where <i>x</i> was varied to be 0, 3, 10, 16, 32, or 81 by the synthesis of pure C8NH<sub>2</sub> Pd MPCs and subsequent liquid-phase place exchange with a varied amount of C6SH. When <i>x</i> = 0−10, the Pd MPCs react strongly with H<sub>2</sub>, 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. Pd<sub>919</sub>(C6S)<sub>192</sub> MPCs are stable against significant aggregation in solution and do not exhibit large film morphology changes, but they are also not highly reactive to H<sub>2</sub>, 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 H<sub>2</sub>. Finally, when <i>x</i> is 32 and 81, the Pd MPCs are fairly stable, exhibit minimal aggregation or morphology changes, and readily react with H<sub>2</sub> based on the significant, reversible changes in film conductivity in the presence of H<sub>2</sub>. Pd MPCs with mixed monolayers have the benefit of high H<sub>2</sub> reactivity while maintaining the structural stability necessary for sensing and catalysis applications.