Creative approaches to the design of catalytic nanomaterials are necessary in achieving environmentally sustainable energy sources. Integrating dissimilar metals into a single nanoparticle (NP) offers a unique avenue for customizing catalytic activity and maximizing surface area. Alloys containing five or more equimolar components with a disordered, amorphous microstructure, referred to as High-Entropy Metallic Glasses (HEMGs), provide tunable catalytic performance based on the individual properties of incorporated metals. Here, we present a generalized strategy to electrosynthesize HEMG-NPs with up to eight equimolar components by confining multiple metal salt precursors to water nanodroplets emulsified in dichloroethane. Upon collision with an electrode, alloy NPs are electrodeposited into a disordered microstructure, where dissimilar metal atoms are proximally arranged. We also demonstrate precise control over metal stoichiometry by tuning the concentration of metal salt dissolved in the nanodroplet. The application of HEMG-NPs to energy conversion is highlighted with electrocatalytic water splitting on CoFeLaNiPt HEMG-NPs.

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, Cp 2 Fe 0/+ ), 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 semiinfinite 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 ∼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.

Per-and polyfluoroalkyl substances (PFAS) are emerging as a hazardous class of environmental micropollutant, and robust, sensitive, and inexpensive sensing modalities are needed to detect the earliest onset of contamination of surface water. Here, we present a molecularly imprinted polymer (MIP)-modified microelectrode (r = 6.25 μm) sensor for the quantification of a pervasive environmental PFAS, GenX (HFPO-DA), in surface water obtained from the Haw River in North Carolina. A 20 nm film of ophenylenediamine was electropolymerized in the presence of GenX to generate a templated polymer adjacent to the electrode surface with subsequent solvent extraction resulting in GenX-specific recognition sites. The oxidation of ferrocene methanol was observed as a function of GenX concentration, and the current decreased linearly with the concentration of GenX. A linear dynamic range of 1−5000 pM with a limit of detection of 250 fM and excellent selectivity against environmental interferents, such as humic acid and perfluorooctanesulfonate, was achieved. The use of oxygen reduction as an additional ambient detection mechanism and the amenability of microelectrodes to relatively resistive environmental matrices are demonstrated to extend the applicability of MIP-modified microelectrodes to environmental waterways as deployable sensors.

We demonstrate a method to electrodeposit and observe the electrocatalysis of small platinum clusters and nanoparticles (NPs) in real time as they form on an ultramicroelectrode (UME). Water droplets ( r ∼ 700 nm), stabilized by sodium dodecyl sulfate (SDS), were suspended in a solution of dichloromethane (DCM) and tetrabutylammonium perchlorate ([TBA][ClO]), which was used to mitigate charge balance during droplet electrolysis. When droplets collided with an UME biased sufficiently negative to drive water reduction, large blips of current were observed. Droplets were synthesized with varying concentrations of HPtCl (from 24.4 mM to 32 nM), which can be reduced to Pt at 0.8 V more positive than water reduction on a Au or C UME. The observation of current blips synthesized with mM amounts of HPtCl indicated water droplets deliver HPtCl to the electrode surface, where a cathodic potential caused Pt NPs to form. The formation of clusters was observed by biasing the electrode potential more negative than water reduction on Pt but more positive than water reduction on Au, giving current blips for droplets containing μM to nM amounts of HPtCl. These blips corresponded to the electrocatalysis of thousands to tens of atoms (clusters). Droplet electrolysis allows for a large amplification such that the electrocatalysis of clusters can be observed in real time. Single, isolated clusters were further characterized voltammetrically on carbon fiber UMEs. The isolation step used the amperometric method, which unambiguously depicted cluster formation in real time. Carbon was chosen due to its large potential window and slow kinetics toward many inner-sphere reactions, such as proton reduction, used in this study. Voltammetric characterization of proton reduction in HClO and NaClO allowed for cluster size analysis using the limiting current. The reduction of proton on the clusters ( E ∼ -0.6 V vs Ag/AgCl) occurred at ca. 400 mV more negative than bulk, polycrystalline platinum.

Reactivity at phase boundaries is central to many areas of chemistry, from synthesis to heterogeneous catalysis. New tools are necessary to gain a more detailed understanding of processes occurring at these boundaries. We describe a series of experiments to visualize phase boundaries using electrogenerated chemiluminescence (ECL) on glassy carbon electrodes. By taking advantage of the solubilities of the ECL luminophore and the coreactant in different liquid phases, we demonstrate that the interface of various phases (i.e., the boundaries formed between a water microdroplet, 1,2-dichloroethane, and a glassy carbon electrode and the boundaries formed between an oxygen bubble, water, and a glassy carbon electrode) can be evaluated. We measured microdroplet contact radii, the three-phase boundary thickness, and growth dynamics of electrogenerated O 2 bubbles. These experimental tools and the fundamental knowledge they yield will find applications in biology, nanoscience, synthesis, and energy storage and conversion, where understanding phase boundary chemistry is essential.