A new device enables high-resolution monitoring of dynamic processes of the liquid phase at the nanoscale

In situ monitoring and recording of important liquid-phase electrochemical reactions in energy devices is critical to the advancement of energy science.

A research team led by a scientist from the City University of Hong Kong (CityU) has recently developed a new small device to contain liquid samples for Transmission electron microscope (TEM), which opens the door to visualizing and recording complex electrochemical reactions at the nanoscale in real time and at high resolution.

The research team believes that this innovative method will shed light on strategies for fabricating a powerful research tool for unraveling mysteries of electrochemical processes in the future.

The use of conventional TEM is limited to thin, stable, and solid samples due to the vacuum environment (vacuum environment prevents absorption or deflection of electrons along their paths and affecting observation) in the chamber to hold the samples. Liquid samples are not compatible with vacuum, so they cannot be examined directly in conventional TEM.

Fortunately, with the advent of the more advanced “liquid cell TEM” on site, study has become possible liquid phase In situ dynamic processes, such as observing crystal nucleation and growth in solution, electrochemical reactions in power devices, and life activities of living cells.

The “liquid cell” is an essential component of TEM for sample retention electron beam pass through, thus enabling on-site monitoring. But it is difficult to manufacture a high-quality liquid cell for TEM because it involves incorporating electrodes, encapsulating the electrolytes in a small “sealed” liquid cell to prevent leakage, and connecting it to an external power source at the same time.

A research team led by Dr. Zeng Quan, Assistant Professor in the Department of Materials Science and Engineering at CityU, and Professor Li Guo of the Massachusetts Institute of Technology (MIT) has succeeded in developing an efficient and novel method for fabricating “closed” electrochemical liquid cells, which can significantly improve the accuracy of TEM. with liquid samples.

“The newly developed closed liquid cell performs two main functions: (1) enclosing liquid samples in a closed container, thus separating them from the vacuum environment of the microscope; and (2) confining liquid samples in a sufficiently thin liquid layer using two-electron-transparent silicon nitride (SiN)._x), so that electrons can travel through the liquid layer and visualize the interactions.”

To fabricate the high-performance “sealed” electrochemical liquid cells in this protocol, the research team used advanced nanofabrication techniques, including photolithography, to fabricate in situ the core component of liquid TEM – the liquid cell. Photolithography is a process used UV light To transfer an engineered design from an optical mask to a photosensitive (photoresist) chemical coated on a substrate.

The team fabricated the bottom and top bracket separately, then pieced them together. Gold or titanium electrodes were deposited on the bottom slide during the metal deposition process. The electrolyte was then loaded and sealed inside the liquid cell.

Using this innovative liquid cell with transmission electron microscopy, the dynamic electrochemical reactions of the liquid sample on the electrode surface can be recorded in real time with high resolution through the integrated TEM operating system with a high spatio-temporal resolution camera.

“The electrochemical liquid cell designed with our custom nanofabrication method has thinner SiN_x “Imaging windows (35 nm) compared to commercial windows (50 nm),” explained Dr. Zeng. It also has a thinner liquid film (150 nm) than commercial ones (1000 nm). SiN thinner_x Imaging windows and a thin liquid film ensure that our fabricated liquid cell can capture electrochemical reactions with TEM spatial resolution better than commercial ones.”

The team believes that many opportunities and applications for in situ TEM monitoring of electrochemical reactions will emerge soon after the development of the liquid electrochemical cell with the choice of patterned metal electrodes and liquid electrolytes encapsulated in the liquid cell.

The newly proposed fabrication protocol can also be used in other in situ techniques beyond TEM. For example, a suitable modification of this protocol would be suitable for the fabrication of electrochemical liquid cells for in situ X-ray characterizations of electrochemical reactions (X-ray absorption spectroscopy, X-ray diffraction, etc.).

The results are published in Nature Protocols.