The growth of colloidal nanoparticles is simultaneously driven by kinetic and thermodynamic effects that are difficult to distinguish. We have exploited in situ scanning transmission electron microscopy in liquid to study the growth of Au nanoplates by radiolysis and unravel the mechanisms influencing their formation and shape. The electron dose provides a straightforward control of the growth rate that allows quantifying the kinetic effects on the planar nanoparticles formation. Indeed, we demonstrate that the surface-reaction rate per unit area has the same dose-rate dependent behavior than the concentration of reducing agents in the liquid cell. Interestingly, we also determine a critical supply rate of gold monomers for nanoparticle faceting, corresponding to three layers per second, above which the formation of nanoplates is not possible because the growth is then dominated by kinetic effects. At lower electron dose, the growth is driven by thermodynamic and the formation and shape of nanoplates are directly related to the twin-planes formed during the growth.

Alternative materials to platinum-based catalysts are required to produce molecular hydrogen from water at low overpotentials. Transition-metal chalcogenide catalysts have attracted significant interest over the past few years because of their activity toward proton reduction and their relative abundance compared with platinum. We report the synthesis and characterization of a new type of iron sulfide (FeS, pyrrhotite) nanoparticles prepared via a solvothermal route. This material can achieve electrocatalysis for molecular hydrogen evolution with no structural decomposition or activity decrease for at least 6 days at a mild overpotential in neutral water at room temperature.