Thin ice secret exposed


Rebecca Pool

Monday, January 6, 2020 - 19:45
Image: Atomic force microscopy reveals exactly how ice grows in two dimensions. [Joseph Francisco]
Using atomic force microscopy and computer simulations, atmospheric chemists from the US-based University of Pennsylvania and colleagues have identified a new way that ice grows in two dimensions.
The latest atomic resolution images and results may one day inform the design of materials that make ice removal a simpler and less costly process.
“Removing ice is critical when it comes to things like wind turbines, which cannot function when they are covered in ice,” says Dr Chongqin Zhu from the School of Arts and Sciences, Pennsylvania. “If we understand the interaction between water and surfaces, then we might be able to develop new materials to make this ice removal easier.”
On extremely cold days, water vapour in the air can transform directly into solid ice, depositing a thin layer on surfaces.
But though commonplace, this process is one that has kept physicists and chemists busy figuring out the details for decades.
However, working with Professor Joseph S. Francisco, Zhu and colleagues have now described the first-ever visualisation of the atomic structure of two-dimensional ice as it formed, in Nature.
The results build on previous research into two-dimensional ice, using computational methods and simulations, in which Francisco, Zhu, and colleagues showed that ice grows differently depending on whether a surface repels or attracts water, and the structure of that surface. 
In the latest work, the researchers joined forces with Professor Ying Jiang from the International Center for Quantum Materials, Peking University, China, and colleagues, to verify simulations and see if they could obtain actual images of two-dimensional ice. 
Using noncontact atomic-force microscopy with a CO-terminated tip on a 2D bilayer ice grown on a Au(111) surface, Jiang and colleagues captured structural information with a minimum of disruption to the ice itself, even allowing the identification of unstable intermediate structures that arise during ice formation.
“We’re interested in the chemistry of ice at the transition with the gas phase, as that’s relevant to the reactions that are happening in our atmosphere,” says Francisco. “One of the things that I find very exciting is that this challenges the traditional view of how ice grows.”
Virtually all naturally occurring ice on Earth is known as hexagonal ice for its six-sided structure. 
One plane of hexagonal ice has a similar structure to that of two-dimensional ice and can terminate in two types of edges - “zigzag” or “armchair.” Usually this plane of natural ice terminates with a zigzag edge.
However, when ice is grown in two dimensions, researchers have discovered that the pattern of growth is different.
The latest results reveal, for the first time, that the armchair edges can be stabilised and that their growth follows a novel reaction pathway. 
“This is a totally different mechanism from what was known,” highlights Zhu.
Although the zigzag growth patterns were previously believed to only have six-membered rings of water molecules, the researchers' calculations and the atomic force microscopy revealed an intermediate stage where five-membered rings were present.
Zhu and colleagues believe that this result may help to explain past experimental observations that indicated ice could grow in two different ways on a surface, depending on the properties of that surface. 
“Knowing the structure is very important,” says Zhu. “Low-dimensional water is ubiquitous in nature and plays a critical role in an incredibly broad spectrum of sciences, including materials science, chemistry, biology, and atmospheric science.
“Looking for features of three-dimensional ice will be the next step and should be very important in looking for applications of this work,” he adds.
In addition to lending insight into future design of materials conducive to ice removal, the techniques used in the work are also applicable to probe the growth of a large family of two-dimensional materials beyond two-dimensional ices, opening the door to visualising the structure and dynamics of low-dimensional matter.
Research is published in Nature.
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