Electrochemical-Shock Synthesis of Nanoparticles from Sub-femtoliter Nanodroplets

Abstract

Conspectus Nanoparticles have witnessed immense development in the past several decades due to their intriguing physicochemical properties. The modern chemist is interested not only in methods of synthesizing nanoparticles with tunable properties but also in the chemistry that nanoparticles can drive. While several methods exist to synthesize nanoparticles, it is often advantageous to put nanoparticles on a variety of conductive substrates for multiple applications (such as energy storage and conversion). Despite enjoying over 200 years of development, electrodeposition of nanoparticles suffers from a lack of control over nanoparticle size and morphology. There have been heroic efforts to address these issues over time. With an understanding that structure–function studies are imperative to understand the chemistry of nanoparticles, new methods are necessary to electrodeposit a variety of nanoparticles with control over macromorphology and also microstructure. This Account details our group’s efforts in overcoming challenges of classical nanoparticle electrodeposition by electrodepositing nanoparticles from water nanodroplets. When a nanodroplet full of metal salt precursor is incident on the electrode biased sufficiently negative to drive electroplating, nanoparticles form at a fast rate (on the order of microseconds to milliseconds). We start with the general nuts-and-bolts of the experiment (nanodroplet formation and methods for electrodeposition). The deposition of new nanomaterials often requires one to develop new methods of measurement, and we detail new measurement tools for quantifying nanoparticle porosity and nanopore tortuosity within single nanoparticles. We achieve nanopore characterization by using Focused Ion Beam milling and Scanning Electron Microscopy. Owing to the small size of the nanodroplets and fast mass transfer (the contents of a femtoliter droplet can be electrolyzed in only a few milliseconds), the use of nanodroplets also allows the electrodeposition of high entropy alloy nanoparticles at room temperature. We detail how a deep understanding of ion transfer mechanisms can be used to expand the library of possible metals that can be deposited. Furthermore, simple ion changes in the dispersed droplet phase can decrease the cost per experiment by orders of magnitude. Finally, electrodeposition in aqueous nanodroplets can also be combined with stochastic electrochemistry for a variety of interesting studies. We detail the quantification of the growth kinetics of single nanoparticles in single aqueous nanodroplets. Nanodroplets can also be used as tiny reactors to trap only a few molecules of a metal salt precursor. Upon reduction to the zerovalent metal, electrocatalysis at very small metal clusters can be probed and evaluated with time using steady-state electrochemical measurements. Overall, this burgeoning synthetic tool is providing unexpected avenues of tunability of metal nanoparticles on conductive substrates.

Anodic Electrodeposition of IrOx Nanoparticles from Aqueous Nanodroplets

“…Studying electrochemistry in single nanodroplets has been previously shown to be very helpful in understanding the reactivity of single entities, studying their nucleation and growth properties, understanding the kinetics and thermodynamics of the process, and many more uses. − ,,, Here, we are using a reverse emulsion system (water nanodroplets in an organic phase) to achieve the electrodeposition of IrO x in anodic conditions by confining the reactions in nanoreactors (i.e., water nanodroplets). Scheme represents our reverse emulsion system, comprising water nanodroplets suspended in a 1,2-dichloroethane phase containing tetrabutylammonium perchlorate salt.…”

“…Our group has recently introduced nanodroplet-mediated electrodeposition for the controlled synthesis of nanoparticles with tunable size, shape, and composition. − In this method, nanoscale (∼100 s of nanometers) droplets of a precursor solution containing the desired metal ions are generated and then deposited onto a conductive substrate. To date, only cathodic electrodeposition has been achieved.…”

“…Amorphous materials (metallic glass high entropy alloy nanoparticles and cobalt oxyhydroxides) have been achieved by our group and Ahn’s group independently. − However, such materials were deposited cathodically and likely follow an electroprecipitation reaction, where the water droplet is reduced by itself, creating an alkaline environment to precipitate out oxyhydroxides. In all cases, the composition, size, microstructure (amorphous versus crystalline), and morphology of the resulting nanoparticles are determined by the precursor solution composition, deposition conditions, etc. ,, …”

“…Nanodroplet-mediated electrodeposition has emerged as a promising technique for the controlled synthesis of nanoparticles with tailored properties. Its advantages include size and shape control, composition control, scalability, versatility, and cost-effectiveness. ,, These make it highly attractive for various applications, including electronics, catalysis, energy storage, and biomedical devices.…”

Synthesis of metallic high-entropy alloy nanoparticles

“…The stochastic collision allows for the isolated delivery of individual nanodroplets containing a specific number of precursor metal cations to a localized nucleation and growth domain on the electrode. The contact between a droplet and the electrode is on the order of ∼10 nm in radius, and the electrochemical reduction of metal precursors takes a short time on the order of 100 ms. 71 The number of elements and compositions in the HEA nanostructures are conveniently controlled by tuning the concentrations of precursors in the aqueous solution. Fig.…”