Temperature-Dependent Nanochemistry and Kinetics in Liquid Cell Electron Microscopy—Modeling and Nanomaterials Growth

Using the technique of liquid cell transmission electron microscopy (LC-TEM) we can apply the powerful capabilities of electron microscopy to imaging and controlling nanoscale phenomena in liquid media. The liquid cell itself is a microfabricated enclosure that is hermetically sealed from the vacuum of the microscope and is filled with a liquid sandwiched between two electron-transparent membranes. Within the liquid cell, it is possible to control the liquid temperature, apply an electrochemical bias using electrodes, and flow the liquid to change its composition. Temperature control is a particularly important parameter in the operation of battery materials and the kinetics of electrochemical processes such as corrosion and etching, as well as being a useful variable in understanding the physics of crystal growth, nanostructure evolution, and self-assembly.<br/>Here, we discuss the effect of temperature on the evolution of nanoscale morphological features during beam-induced solution-phase crystal growth. First, we build a robust model to calculate the equilibrium concentration of chemical species in the liquid medium under electron beam irradiation as a function of temperature. Such models need to include the complete radiolysis reaction set for the full set of chemical species in the initial solution. As an example, we consider metal ions such as Ag+ and conjugate ions such as NO3-. To include temperature effects, the model includes radiolysis reaction parameters that are temperature-dependent. We use an Arrhenius behavior for the reaction rates and G values (rates of generation of the primary products due to beam irradiation). With this model, we predict how temperature affects the radiolysis-driven equilibrium concentrations of the species. We then expand the model to help understand temperature-dependent kinetics of nanostructure evolution, by considering the diffusion and depletion of precursors. This involves modification of the Stokes-Einstein equation with temperature-dependent viscosity to calculate diffusion lengths. This complete model provides an opportunity to understand the radiolytic formation of nanocrystals at different temperatures under the combined effect of the important experimentally controllable variables for LC-TEM experiments: dose rate, initial concentration of the solution, pH, and aeration. We show the results of testing this model by comparing calculated results with experimental observations. The data is derived from experiments on nanoparticle generation from silver nitrate solution at different temperatures. Combined with the effect of higher diffusivity of atoms in a liquid medium at a higher temperature, the growth rate increases with temperature due to the faster kinetics at an elevated temperature. Moreover, the morphology changes with temperature - silver nanoparticles showed less regular shapes at higher temperatures, which can be attributed to the relatively slower particle rearrangement process (including surface diffusion and attachment) compared to the higher arrival rate of ions by diffusion. Growth of particles at room temperature showed time-dependent kinetics as well, attributable to the effect of diffusion of silver atoms from the beam boundary.<br/>These results pave the way to tune the growth kinetics and morphology evolution of nanoparticles through factors including temperature and dose, suggesting possibilities to control nanostructure formation and better understand growth mechanisms in LC-TEM. We are excited by the opportunities presented by liquid cell TEM to develop and test models to help us use temperature quantitatively to probe the physics of nanostructure evolution, with relevance to applications in energy storage, corrosion, and catalyst synthesis.