Temperature control during liquid-phase electron microscopy experiments opens up new perspectives of studying the dynamic of nanoparticles in liquid environments mimicking their formation or application media. Using recently developed heating liquid cells, we directly observed the influence of temperature on the nucleation and growth processes of gold nanoparticles in water.
Temperature control is a recent development that provides an additional degree of freedom in studying nanochemistry by liquid cell transmission electron microscopy, notably the formation of gold nanoparticles in solution. This methodology allows imaging of the dynamics of individual nanostructures in liquid with great control over the composition and temperature of the environment under realistic synthetics conditions. Interestingly, this method can be used to study the effects of temperature on the structural evolution of soft or biological nanoobjects in liquid environments by mimicking their formation or application medium.
The key success factors for liquid TEM experiments are a clean sample preparation and a consideration for the electron beam effects on the nanoparticle dynamics. For liquid cell preparation, first fill one glass Petri dish with acetone and another with methanol in a fume hood. Place one small and one large E-Chip into the acetone for two minutes before moving both chips into the methanol for two minutes.
After the methanol wash, use an air pistol and tweezers to dry the cells and use a binocular magnifier or an optical microscope to verify the integrity of the silicon nitride window. If the chips are intact, plasma clean the E-Chips with a mixture of argon and oxygen gas for two minutes and load the gasket O-rings into the liquid cell holder. Place the small E-Chip into the liquid cell holder and drop approximately two microliters of the liquid sample of interest onto the chip.
Using a sharply cut piece of filter paper, remove any excess liquid from the chip until the liquid droplet forms a flat dome and place the big E-Chip onto the small E-Chip front side facing down. Slide the lid back onto the liquid cell holder and gradually tighten each screw. Use filter paper to remove any excess liquid from the chips, rotating the liquid cell holder around its axis to make sure that all of the liquid is captured.
Test the vacuum sealing of the liquid cell in a pumping station. If the vacuum level of the pump reaches five times 10 to the negative two pascals, verify the integrity of the silicon nitride window one last time and load the liquid cell holder onto the microscope. To set up the flow mode, load one syringe with the solution of interest and connect two external peak tubes to the syringe.
Place the syringe onto the syringe pump and insert the external peak tubes into the entries of the liquid cell holder. Insert one additional external peak tube for the output of the liquid cell holder. Then inject the solution into each inlet at a flow rate of five microliters per minute.
To heat the liquid environment, open the heating software and power up the power supply. Click the Device Check button and open the Experiment tab. Click Manual to activate the manual heating mode and select the targeted temperature to change the temperature rate as appropriate to the experiment.
Then click Apply to heat the E-Chips to the targeted temperature. To image the radiolysis-driven formation of gold nanoparticles with a good signal-to-noise ratio, in STEM-HAADF mode, identify a pristine area of the sample near a corner of the observation window in which the liquid thickness is at a minimum. Note the imaging conditions, including the spot size, the condenser aperture size and the magnification to allow subsequent calibration of the electron dose rate and the cumulative electron dose irradiating the analyzed area.
Then acquire videos of the nanoparticle growth at different temperatures using the same imaging conditions. For a single nanoparticle nanodiffraction, acquire a STEM-HAADF image of several nanoobjects and use STEMx software to acquire the diffraction pattern of individual nanoparticles within the image. As observed in these two STEM-HAADF image series, the growth of a very dense assembly of small nanoparticles can be observed at low temperatures.
While at high temperatures, a few large and well-faceted nanostructures are obtained. As the contrast of STEM-HAADF images is proportional to the gold nanoparticle thickness, two populations of objects formed during these growth experiments can be observed:highly contrasted 3D nanoparticles and large 2D nanostructures with a triangular or hexagonal shape and a lower contrast. Automated video processing as demonstrated in this method allows measurement of the nucleation and growth rates of nanoparticles.
At low temperatures, more than 800 nanoparticles are formed within a few tens of seconds of observation, while only 30 nanoparticles are formed in the same amount of time at a high temperature. Conversely, the mean surface area of the nanoparticles increases 40 times faster at 85 degrees Celsius than at 25. Here, the diffraction pattern of two gold nanoparticles that have been selected directly from a typical STEM image can be observed.
The face-centered cubic structure of gold-oriented long view 001 and 112 zone axes can be identified. Studying the effects of temperature on the nucleation and growth of nanoparticles by liquid cell TEM requires a comparison of videos acquired with the same electron dose rate because radiolysis has also an impact on nanoparticles formation. Ex situ SEM or TEM characterizations can be performed after unsealing the liquid cell to further analyze the nanoobject structures.
Temperature-controlled liquid cell TEM provides an opportunity to investigate the effect of temperature on the many other chemical reactions that occur at the interface between solids and liquids, opening many avenue in materials, life, and Earth sciences.
