Scientists from the SLAC National Accelerator Laboratory of the Department of Energy and Stanford University have captured the first images of carbon dioxide molecules in a molecular cage – part of a highly porous nanoparticle known as the MOF or metal-organic framework with great potential for separating and storing Gases and liquids.
The images taken in the Stanford SLAC cryo-EM equipment show two configurations of the CO2 molecule in its cage, which scientists call a guest-host relationship. reveal that the cage expands slightly when the CO2 enters; and increase jagged edges where MOF particles can grow by adding more cages.
"This is a breakthrough achievement that will undoubtedly provide unprecedented insight into how these highly porous structures perform their extraordinary functions, demonstrating the power of cryo-EM to solve a particularly difficult problem in MOF chemistry," said Omar Yaghi, a professor at the University of California at Berkeley and a pioneer in chemistry, who was not involved in the study.
The Research Team The study was described today by the SLAC / Stanford professors Yi Cui and Wah Chiu in the journal Matter .
Tiny patches with enormous surfaces
MOFs have the largest surface areas of any known material. A single gram, or three-hundredths of an ounce, can have a surface that is nearly the size of two football fields and provides enough room for guest molecules to invade millions of host cages.
Despite their enormous commercial potential and their two decades Due to intensive and accelerating research, MOFs are only just reaching the market. Scientists around the world are developing more than 6,000 new types of MOF particles each year to find the right combinations of structure and chemistry for specific tasks, such as: For example, increasing the storage capacity of gas tanks or separating and separating CO2 from chimneys to combat climate change.
"The Intergovernmental Panel on Climate Change requires some sort of carbon capture technology to limit global temperature rise to 1.5 degrees Celsius," said Yuzhang Li, a Stanford postdoctoral researcher and lead author of the report. "These materials have the potential to bind large volumes of CO2, and understanding where the CO2 is bound in these porous frameworks is really important in designing materials that make it cheaper and more efficient."
One of the most powerful methods of observing materials is Transmission Electron Microscopy (TEM), which allows images to be taken atomically. However, many MOFs and the bonds that contain guest molecules melt into blobs when exposed to the intense electron beams required for this type of imaging.
Several years ago, Cui and Li used a method that was used for many years to study biological samples: freeze samples to better hold them under electron bombardment. They used a state-of-the-art TEM instrument at Stanford Nano Shared Facilities to investigate atomically detailed, flash-frozen samples containing dendrites-finger-like lithium-metal proliferations that can penetrate and damage lithium-ion batteries.
Atomic images, one electron after another
For this latest study, Cui and Li used instruments in the Stanford SLAC cryo-EM systems, which have much more sensitive detectors that pick up signals from single electrons can go through a sample. This allowed scientists to image atomic details while minimizing electron beam exposure.
The investigated MOF is called ZIF-8. It came in particles with a diameter of only 100 billionths of a meter; They would need to line about 900 of them to match the width of a human hair. "It has a high commercial potential because it is very cheap and easy to synthesize," said Stanford's postdoctoral fellow, Kecheng Wang, who played a key role in the experiments. "It's already used to capture and store toxic gases."
Cryo-EM not only allowed them to create super-sharp images with minimal damage to the particles, but also to prevent the CO2 gas from escaping during ingestion. By imaging the specimen from two angles, the researchers were able to confirm the positions of two of the four sites where CO2 is believed to be poorly captured in the cage.
"I was very excited when I saw the pictures, it's a brilliant piece of work," said Stanford Professor Robert Sinclair, an expert on using TEM to study materials that helped interpret the team's results. "Photographing the gas molecules in the MOFs is an incredible advance."
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