April 5, 2022
Electron Energy Loss Spectroscopy Directly Observes Metal Oxidation in Highly Concentrated Graphene Liquid Cell TEM Solutions
Graphene liquid cell transmission electron microscopy is a popular technique to visualize nanoscale dynamics and transformations at atomic resolution. The solution in liquid cells is known to be affected by radiolysis from the electron beam, 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 of aqueous CeCl3. First, the ratio between the O K-edge and Ce M-edge was used to approximate the concentration of Ce salt in the graphene liquid cell. It was determined that the ratio between O and Ce in the graphene liquid cell was orders of magnitude lower than what would be expected for a dilute solution, indicating that the encapsulated solution is highly concentrated. To probe how this may affect the chemistry of a graphene liquid cell, we measured the oxidation of Ce3+ using time-resolved parallel EELS. It was determined that Ce3+ oxidizes faster under high electron fluences, but reaches the same steady state Ce4+ concentration regardless of the fluence. In order to fit the data to radiolysis models, rate constants and g-values of certain molecular species had to be changed, indicating that the traditional models used in the field are inadequate to model the chemistry in highly concentrated graphene liquid cells. Finally, dose dependent gold nanocrystal (AuNC) etching trajectories showed that AuNCs etch faster at higher electron fluences, indicating that the increase in the rate of oxidation controls the etch rate of the AuNCs in this system, rather than an increased steady state concentration of oxidant as has been previously postulated. EELS has proven to be a powerful technique to study the chemistry of the solution in graphene liquid cells. Understanding the effects of the highly concentrated solution in graphene liquid cells will provide new insight on previous studies and will open up opportunities to study systems in dense electrolyte solutions at high resolution.
Michelle Crook is a Ph.D. candidate working with Prof. Paul Alivisatos in the Department of Chemistry at the University of California, Berkeley. She received her B.S. degree in Chemistry with an emphasis in Materials Chemistry from Binghamton University in 2018. Her research focuses on using model systems to understand and define the solution chemistry occurring during electron beam irradiation in liquid cell transmission electron microscopy.