EELS Studies of Cerium Electrolyte Reveal Substantial Solute Concentration Effects in Graphene Liquid Cells

Graphene liquid cell transmission electron microscopy
is a powerful
technique to visualize nanoscale dynamics and transformations at atomic
resolution. However, the solution in liquid cells is known to be affected
by radiolysis, and the stochastic formation of graphene liquid cells
raises questions about the solution chemistry in individual pockets.
In this study, electron energy loss spectroscopy (EELS) was used to
evaluate a model encapsulated solution, aqueous CeCl3.
First, the ratio between the O K-edge and Ce M-edge was used to approximate
the concentration of cerium salt in the graphene liquid cell. It was
determined that the ratio between oxygen and cerium was orders of
magnitude lower than what is expected for a dilute solution, indicating
that the encapsulated solution is highly concentrated. To probe how
this affects the chemistry within graphene liquid cells, the oxidation
of Ce3+ was measured using time-resolved parallel EELS.
It was determined that Ce3+ oxidizes faster under high
electron fluxes, but reaches the same steady-state Ce4+ concentration regardless of flux. The time-resolved concentration
profiles enabled direct comparison to radiolysis models, which indicate
rate constants and g-values of certain molecular
species are substantially different in the highly concentrated environment.
Finally, electron flux-dependent gold nanocrystal etching trajectories
showed that gold nanocrystals etch faster at higher electron fluxes,
correlating well with the Ce3+ oxidation kinetics. Understanding
the effects of the highly concentrated solution in graphene liquid
cells will provide new insight on previous studies and may open up
opportunities to systematically study systems in highly concentrated
solutions at high resolution.