A Universal Platform for the Electrodeposition of Ligand-Free Metal Nanoparticles from a Water-in-Oil Emulsion System

Prodigious resources are currently being devoted to control the size and morphology of metal nanoparticles (NPs). Several homogeneous chemical and photochemical techniques exist for the synthesis of metal NPs; however, these synthetic methods generally leave a distribution of NP shapes and sizes and require a stabilizing ligand to prevent aggregation. Electrodeposition of metal NPs onto conductive surfaces is a versatile technique. However, spatial control on the conductive surface is difficult to attain, even on well-behaved materials like highly oriented pyrolytic graphite. Here, we achieve spatial control of Pt NPs on amorphous graphite by confining a precursor metal salt, such as hexachloroplatinic acid (HCPA), to a water droplet suspended in oil, such as dichloroethane. When a graphite electrode was placed in solution and biased at a mild potential (-0.7 V vs the ferrocene/ferrocenium couple, Cp2Fe0/+), droplet-mediated electrodeposition produced NPs characterized by the electrochemical collision method and scanning electron microscopy (SEM). The flux of droplets to the graphite surface followed the familiar Cottrell relationship for semi-infinite linear diffusion. Pt NP size selectivity can be directly modulated by tuning the initial concentration of HCPA in the droplet. Interestingly, the size, morphology, roughness, and coverage are shown to be influenced by the surfactant used to stabilize the water droplets, the concentration of HCPA, and the deposition potential. For instance, no surfactant, sodium dodecyl sulfate (SDS), and Span-20 generated NPs with relative roughness values of 46, 50, and 54%, respectively. Importantly, the incorporation of Span-20, a neutral emulsifier, facilitated homogeneously distributed Pt NP surface coverage on amorphous graphite, indicating the technique is apathetic to basal planes and edges of the graphite surface. The addition of SDS to droplets with large concentrations of HCPA resulted in conical and pillar-like NP morphologies, furthur enhancing surface area. The effect of deposition potential was also explored, which indicated that the roughness of the NPs can be increased by similar to 10% depending on the potential. We also demonstrate that the method can be extended to the deposition of several other metal NPs, including silver, gold, copper, tin, iron, and cerium onto various substrates such as gold, silicon, boron-doped diamond (BDD), and highly oriented pyrolytic graphite (HOPG). The advantage of this technique is that size-selective electrodeposition of ligand-free, uniformly distributed NPs can be achieved.