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.