Fine-Tuning Porosity and Time-Resolved Observation of the Nucleation and Growth of Single Platinum Nanoparticles.

Porous metal nanoparticles (NPs) are important to a variety of applications; however, robust control over NP porosity is difficult to achieve. Here, we demonstrate control over NP porosity using nanodroplet-mediated electrodeposition by introducing glycerol into water droplets. Porosity approached 0 under viscous conditions (>6 cP), and intermediate viscosities allowed the fine-tuning of NP porosity between 0 and 15%. This method also allowed for control over average pore radius (1 to 5 nm) and pore density (2 to 6 × 1015 pores per square meter). Reduced mass transfer within water droplets was validated by studying single chloroplatinate-filled water droplet (droplet radius of ∼450 nm) collisions on a platinum ultramicroelectrode (UME, rUME = 5 μm). Collision transient lifetimes in the i- t response increased with increasing viscosity, and the total charge per event was conserved. The change in shape was consistent with the nucleation and growth of a platinum NP within the droplet, which was confirmed by fitting transients to classical nucleation and growth theory for single centers as a function of over-potential. This analysis allowed electrokinetic growth and diffusion-controlled growth to be distinguished and semi-quantified at the single NP level.