How biological antifreeze protein smelts against ice, the relative slip between glaciers, and the degradation and catalysis of atmospheric ozone are closely related to the structure of ice and nucleation growth. After nearly a hundred years of exploration, 18 three-dimensional crystal structures of ice have been discovered, the most common of which is hexagonal ice. However, whether there is a stable existence of two-dimensional ice, academic save has been very controversial.
Two-dimensional ice is stacked with two layers of hexagonal ice, which are connected by hydrogen bonds.
As early as the 1920s, Prague, a famous British physicist and X-ray discoverer, and several other scientists used X-rays to “depict” the structure of ice crystals, beginning the study of three-dimensional ice structures. In 2015, a team led by graphene discoverer Andrew Gam discovered a quadrifold two-dimensional ice phase between double-layergraphenes, which was a sensation, but the two-dimensional ice was later questioned as a crystal structure of sodium chloride, and its existence remained a mystery.
(a) thick ice in the Ross Sea, Antarctica; (b) molecular models of ice Ih, the most common ice phase in nature; and (c) two-dimensional ice found in this study (3D effect of the results). Respondent’s Map
In this study, the researchers precisely controlled temperature and water pressure, and grew a single-crystal two-dimensional ice structure on the gold substrate of hydrophobic water. They used non-intrusive atomic force microscopy technology to distinguish imaging at the submolecular level of two-dimensional ice, and combined theoretical calculations to determine its atomic structure.
“The results show that two-dimensional ice is stacked by two layers of hexagonal ice without rotation, connected by hydrogen bonds, each water molecule and the same layer of water molecules form three hydrogen bonds, and the upper and lower layers of water molecules form a hydrogen bond, so all hydrogen bonds are saturated, the structure is very stable, is a self-contained ‘self-saturation’ two-dimensional ice.” Jiang Ying, a professor at Peking University’s Quantum Materials Center, said.
The two-dimensional ice found in this study is no longer a traditional tetrahexon structure, but a two-dimensional plane structure of hexagonal, with a very flat surface. In 1997, American scientists Guga, Zeng Xiaocheng and others used molecular dynamics simulations to predict this “interlocking” double-layer edgy ice for the first time, but there has been a lack of definitive experimental evidence. Therefore, the study is also the first two-dimensional ice structure confirmed by the experiment, the researchers officially named it “two-dimensional ice I phase.”
How can I see the formation of two-dimensional ice? The researchers cleverly moved two-dimensional ice from -153 degrees Celsius to “frozen” to -268 degrees Celsius, froze a series of intermediate states during ice growth, and made a steady imaging of it, eventually seeing the dynamic growth of two-dimensional ice at the atomic scale.
At the same time, combined with theoretical calculation and simulation, they put forward the “bridge” growth of two-dimensional Icelandic jagged border and the “seeding” growth mechanism of the chair-shaped boundary. Moreover, the relative stability of the sub-stable state of the two-dimensional ice boundary has little to do with the specific structure of the substrate.
Submolecular-level resolution imaging of two-dimensional Icelandic internal structures. A, b graph from left to right, in turn, from high to low at different point heights of atomic force microscope experiment and simulation, c is a two-dimensional ice structure model schematic top and side view. Respondent’s Map
It is important for the development of anti-icing and lubrication materials
For a long time, how ice becomes nuclear and grows, mostly confined to the macro-scale research, lack of micro-scale images. For the first time, the study achieved atomic scale representation of two-dimensional ice nuclei, helping to understand the shape and growth of ice under low-peacekeeping and restricted conditions.
The discovery of two-dimensional ice not only challenges the traditional understanding of ice phase for more than 100 years, but also has broad application prospects. “For example, we recently found that two-dimensional ice has an important effect on the growth of three-dimensional ice. If there is two-dimensional ice, the three-dimensional ice will grow on the surface, very stable. But without two-dimensional ice, the resulting three-dimensional ice is very small and can easily be blown away by the wind. So we can design and develop anti-icing materials more specifically based on the structure of two-dimensional ice. Jiang Ying believes that all the hydrogen bonds of water molecules in two-dimensional ice are saturated, so the interaction with the surface is minimal and can act as super-lubricated. Two-dimensional ice can reduce friction between materials.
The “bridge” and “seeding” growth patterns corresponding to the jagged (a) and armchair (b) boundaries of two-dimensional Iceland. Respondent’s Map
At the same time, 2D ice can be used as a special two-dimensional material, providing a new platform for high temperature superconductivity, deep ultraviolet detection, frozen mirror imaging and other research.
“This study opens the door to a series of studies in the two-dimensional ice family, and the growth mechanism of this two-dimensional ice is similar to the growth mechanism of previously revealed honeycomb two-dimensional materials, such as graphene and boron nitride, in nucleation and growth dynamics. Guo Wanlin, a member of Nanjing University of Aeronautics and Astronautics, commented that the study has changed people’s traditional understanding of two-dimensional ice nucleation and growth, and is of great significance to the research of materials science, tribology, biology, atmospheric science and planetary science.
Yang Jinlong, academician of China University of Science and Technology, believes that this research breaks the experimental challenge, captures the boundary atomic structure in the process of two-dimensional ice growth, combined with theoretical calculation simulation, reveals the growth mechanism of two-dimensional ice, and provides a new perspective for people to understand the growth and morphology of ice in restricted space, which is of great scientific significance.