ABSTRACT
Making a measurement over millions of nanoparticles or exposed crystal facets seldom reports on reactivity of a single nanoparticle or facet, which may depart drastically from ensemble measurements. Within the past 30 years, science has moved toward studying the reactivity of single atoms, molecules, and nanoparticles, one at a time. This shift has been fueled by the realization that everything changes at the nanoscale, especially important industrially relevant properties like those important to electrocatalysis. Studying single nanoscale entities, however, is not trivial and has required the development of new measurement tools. This review explores a tale of the clever use of old and new measurement tools to study electrocatalysis at the single entity level. We explore in detail the complex interrelationship between measurement method, electrocatalytic material, and reaction of interest (e.g., carbon dioxide reduction, oxygen reduction, hydrazine oxidation, etc.). We end with our perspective on the future of single entity electrocatalysis with a key focus on what types of measurements present the greatest opportunity for fundamental discovery.
ABSTRACT
Emulsions are critical across a vast range of industries. Generally, emulsion synthesis is a complicated chemical process, requiring many mixed-phase systems. Here, we demonstrate that the flux of ions across the oil|water interface induces emulsification. Ion flux is achieved by a voltage-driven process, where an anode and a cathode are placed in each phase. When a current density of 2 mA cm-2 is reached across the interface, emulsification occurs. We demonstrate that emulsification can be tuned to occur in both phases, depending on the ions present. Droplet sizes are on the order of hundreds of nm and are stable for over an hour even without purposefully added surfactant. We demonstrate qualitative control over droplet size and charge based on salt content, current densities, and polarity of the interface. The process is 1000 times less energetic than ultrasonication. Our results introduce a robust and low-energy means of nanodroplet dispersion without the use of more than two phases and complex phase-transfer agents.
ABSTRACT
Emulsions are critical across a broad spectrum of industries. Unfortunately, emulsification requires a significant driving force for droplet dispersion. Here, we demonstrate a mechanism of spontaneous droplet formation (emulsification), where the interfacial solute flux promotes droplet formation at the liquid-liquid interface when a phase transfer agent is present. We have termed this phenomenon fluxification. For example, when HAuCl4 is dissolved in an aqueous phase and [NBu4][ClO4] is dissolved in an oil phase, emulsion droplets (both water-in-oil and oil-in-water) can be observed at the interface for various oil phases (1,2-dichloroethane, dichloromethane, chloroform, and nitrobenzene). Emulsification occurs when AuCl4- interacts with NBu4+, a well-known phase-transfer agent, and transfers into the oil phase while ClO4- transfers into the aqueous phase to maintain electroneutrality. The phase transfer of SCN- and Fe(CN)63- also produce droplets. We propose a microscopic mechanism of droplet formation and discuss design principles by tuning experimental parameters.
ABSTRACT
Nanodroplet-mediated electrodeposition is a reliable method for electrodepositing nanoparticles by confining a small amount of metal-salt precursor in water nanodroplets (radius â¼400 nm) suspended in an oil continuous phase. This technique provides a great advantage in terms of nanoparticle size, morphology, and porosity. For an electrochemical reaction to proceed in the aqueous nanodroplet, the electroneutrality condition must be maintained. Classically, [NB4][ClO4] or a comparable salt is added to the oil continuous phase to maintain charge balance. Unfortunately, the presence of this salt in the oil phase causes some metal salts, such as HAuCl4, to phase transfer, disallowing the formation of gold nanoparticles. Here, we demonstrate the partitioning of HAuCl4 is orders of magnitude lower using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) when LiClO4 is added to the nanodroplet phase and [NBu4][ClO4] is not added to the continuous phase. This simple change allows for the electrodeposition of gold nanoparticles. Scanning electron microscopy shows the morphology and size distribution of gold nanoparticles obtained at different concentrations of LiClO4. Transmission electron microscopy in selected diffraction mode was used and it determined the gold nanoparticles obtained are polycrystalline with miller indices of (222) and (200). This work widens the variety of nanoparticles that can be electrodeposited from nanodroplets for applications in energy storage and conversion, photoelectrochemistry, and biosensing.
