Everyone knows that water occurs in three states of aggregation. We put the kettle on, and the water begins to boil and evaporate, passing from liquid to gaseous. We put it in the freezer, and it begins to turn into ice, thereby moving from a liquid to a solid state. However, under certain circumstances, the water vapor present in the air can immediately pass into the solid phase, bypassing the liquid. We are familiar with this process by its result - beautiful patterns on the windows on a frosty winter day. Motorists, while scraping off a layer of ice from the windshield, often characterize this process using not very scientific, but very emotional and vivid epithets. One way or another, the details of the formation of two-dimensional ice have been shrouded in mystery for many years. And recently, for the first time, an international team of scientists was able to visualize the atomic structure of two-dimensional ice as it formed. What secrets are hidden in this seemingly simple physical process, how did scientists manage to uncover them, and why are their findings useful? The report of the research group will tell us about this. Go.
Research basis
To exaggerate, in fact, all the objects that surround us are three-dimensional. However, if we consider some of them more meticulously, we can also meet two-dimensional ones. A crust of ice that forms on the surface of something is a prime example of this. The existence of such structures is not a secret for the scientific community, because they have been analyzed many times already. But the problem is that it is quite difficult to visualize the metastable or intermediate structures involved in the formation of 2D ice. This is due to banal problems - the fragility and fragility of the structures under study.
Fortunately, modern scanning methods allow samples to be analyzed with minimal impact, which allows you to get the maximum data in a short period of time, due to the above reasons. In this study, the scientists used non-contact atomic force microscopy, while the tip of the microscope needle was coated with carbon monoxide (CO). The combination of these scanning tools makes it possible to obtain real-time images of the edge structures of two-dimensional two-layer hexagonal ice grown on the surface of gold (Au).
Microscopy has shown that during the formation of two-dimensional ice, two types of edges (segments connecting two vertices of a polygon) simultaneously coexist in its structure: zigzag (zigzag) and chair-like (armchair).
Armchair-shaped (left) and zigzag (right) ribs on the example of graphene.
At this stage, the samples were quickly frozen, which made it possible to examine the atomic structure in detail. A simulation was also carried out, the results of which largely coincided with the results of observations.
It was found that in the case of the formation of zigzag ribs, an additional water molecule is added to the already existing rib, and the whole process is regulated by the bridge formation mechanism. But in the case of the formation of chair-like ribs, additional molecules were not found, which contrasts strongly with the traditional ideas about the growth of two-layer hexagonal ice and two-dimensional hexagonal substances in general.
Why did scientists choose a non-contact atomic force microscope for their observations rather than a scanning tunneling microscope (STM) or a transmission electron microscope (TEM)? As we already know, the choice is related to the complexity of studying the short-lived and brittle structures of two-dimensional ice. STM has previously been used to study 2D ice grown on various surfaces, but this type of microscope is not sensitive to the position of the nuclei, and its needle can cause imaging errors. TEM, on the contrary, perfectly shows the atomic structure of the ribs. However, high-quality images require high-energy electrons, which can easily change or even destroy the edge structure of covalently bonded XNUMXD materials, not to mention the more loosely bonded edges in XNUMXD ice.
The atomic force microscope is free from such shortcomings, and the CO-coated needle makes it possible to study interfacial water with minimal effect on water molecules.
Results of the study
Image #1
Two-dimensional ice was grown on the surface of Au(111) at a temperature of about 120 K, and its thickness was 2.5 Γ (1a).
STM images of ice (1c) and the corresponding image of the fast Fourier transform (inset on 1a) show a well-ordered hexagonal structure with periodicity Au (111)-β3 x β3-30Β°. Although the cellular H-connected network of 2D ice is visible in the STM image, the detailed topology of the edge structures is difficult to determine. At the same time, AFM with a frequency shift (Ξf) of the same part of the sample gave better images (1d), which made it possible to visualize chair-like and zigzag sections of the structure. The total length of both options is comparable, but the average length of the precursor rib is slightly longer (1b). Zigzag ribs can grow up to 60 Γ in length, while armchair ribs become covered with defects during formation, which reduces their maximum length to 10β30 Γ .
Next, systematic AFM imaging was carried out at different needle heights (2a).
Image #2
At the highest needle height, when the AFM signal is dominated by a higher-order electrostatic force, two sets of β3 x β3 sublattices were distinguished in two-dimensional two-layer ice, one of which is shown in Fig. 2a (left).
At a lower needle height, the bright elements of this sublattice begin to show directivity, and the other sublattice turns into a V-shaped element (2a, centered).
At the minimum needle height, AFM shows a honeycomb structure with clear lines connecting two sublattices resembling H-bonds (2a, on right).
Density functional theory calculations show that two-dimensional ice grown on the Au(111) surface corresponds to an entangled two-layer ice structure (2s), consisting of two flat hexagonal layers of water. The hexagons of the two sheets are in conjugation, and the angle between the water molecules in the plane is 120Β°.
In each water layer, half of the water molecules lie horizontally (parallel to the substrate) and the other half lie vertically (perpendicular to the substrate), with one OβH pointing up or down. The vertically lying water in one layer donates an H-bond to the horizontal water in the other layer, resulting in a fully saturated H-shaped structure.
AFM simulation using a quadrupole (dz 2) needle (2b) based on the above model is in good agreement with the experimental results (2a). Unfortunately, the similar height of horizontal and vertical water makes it difficult to identify them during STM imaging. However, when using atomic force microscopy, the molecules of both types of water are clearly distinguishable (2a ΠΈ 2b right) because the higher order electrostatic force is very sensitive to the orientation of the water molecules.
It was also possible to further define the OH directionality of horizontal and vertical water through the interaction between higher order electrostatic forces and Pauli repulsive forces, as shown by the red lines in the figure. 2a ΠΈ 2b (center).
Image #3
On the pictures 3a ΠΈ 3b (step 1) shows enlarged AFM images of zigzag and armchair ribs, respectively. It was found that the zigzag edge grows with the preservation of its original structure, and with the growth of the chair-shaped edge, the edge is restored in the periodic structure of 5756-rings, i.e. when the structure of the edges periodically repeats the sequence pentagon - heptagon - pentagon - hexagon.
Density functional theory calculations show that the non-reconstructed zigzag rib and chair rib of type 5756 are the most stable. The 5756-rib is formed as a result of combined effects that minimize the number of unsaturated hydrogen bonds and reduce the strain energy.
Scientists recall that the basal planes of hexagonal ice usually end in zigzag ribs, and chair-shaped ribs are absent due to the higher density of unsaturated hydrogen bonds. However, in small systems or where space is limited, the armchair fins can reduce their energy through proper reconstruction.
As mentioned earlier, when ice growth at 120 K was stopped, the sample was immediately cooled to 5 K to try to freeze metastable or transitional fin structures and provide a relatively long sample life for detailed STM and AFM studies. It was also possible to reconstruct the growth process of two-dimensional ice (image No. 3) thanks to a CO-functionalized microscope needle, which made it possible to detect metastable and transitional structures.
In the case of zigzag ribs, individual pentagons were sometimes found attached to straight ribs. They could line up in a row, forming an array with a frequency of 2 x aice (aice is the lattice constant of two-dimensional ice). This observation may indicate that the growth of zigzag edges is initiated by the formation of a periodic array of pentagons (3a, step 1-3), which involves adding two water pairs for the pentagon (red arrows).
Next, the array of pentagons is connected, forming a structure like 56665 (3a, step 4) and then restores the original zigzag shape by adding more water vapor.
With armchair-shaped ribs, the situation is opposite: there are no arrays of pentagons, and instead, short gaps of the 5656 type are often observed on the rib. The length of the Type 5656 Rib is significantly shorter than that of the 5756. This is possibly due to the fact that the Type 5656 Rib is highly stressed and less stable than the 5756 Rib. water vapor (3b, stage 2). Further, the 656-rings grow in the transverse direction, forming a rib of the 5656 type (3b, step 3), but with a limited length due to the accumulation of strain energy.
If one water pair is added to the hexagon of the rib type 5656, then the deformation can be partially weakened, and this again leads to the formation of the rib type 5756 (3b, step 4).
The above results are very indicative, but it was decided to support them with additional data obtained from molecular dynamics calculations of water vapor on the Au(111) surface.
It was found that two-dimensional two-layer ice islands are successfully and freely formed on the surface, which is consistent with our experimental observations.
Image #4
On the image 4a The mechanism of collective formation of bridges on zigzag ribs is shown step by step.
Below are media materials on this study with a description.
Media material No. 1
It is worth noting that a single pentagon attached to a zigzag edge cannot act as a local growth promoting nucleation center.
Media material No. 2
Instead, a periodic but unconnected network of pentagons is initially formed on the zigzag edge, and subsequent incoming water molecules collectively try to connect these pentagons, which leads to the formation of a structure of 565-type chains. Unfortunately, during practical observations, such a structure was not observed, which is explained by its extremely short lifespan.
Media material No. 3 and No. 4
The addition of one pair of water connects the 565-type structure and the adjacent pentagon, resulting in the formation of the 5666-type structure.
The 5666 type structure grows transversely to form the 56665 type structure and eventually develops into a fully connected hexagonal lattice.
Media material No. 5 and No. 6
On the image 4b growth is shown in the case of a chair-shaped rib. The conversion from type 575 rings to type 656 rings starts from the bottom layer, forming a composite structure 575/656, which cannot be distinguished from the type 5756 rib in experiments, since only the upper layer of two-layer ice can be imaged during experiments.
Media material No. 7
The resulting bridge 656 becomes the nucleation site for the growth of the rib type 5656.
Media material No. 8
The addition of one water molecule to a 5656-type fin results in a highly mobile unpaired molecular structure.
Media material No. 9
Two of these unpaired water molecules can subsequently combine into a more stable heptagonal structure, completing the transformation from 5656 to 5756.
For a more detailed acquaintance with the nuances of the study, I recommend looking at .
Finale
The main conclusion of this study is that the observed behavior of structures during growth may be common to all types of two-dimensional ice. Two-layer hexagonal ice forms on various hydrophobic surfaces and under hydrophobic confinement conditions, and therefore can be considered as a separate 2D crystal (2D ice I), the formation of which is insensitive to the underlying structure of the substrate.
Scientists honestly say that their imaging technique is not yet suitable for working with three-dimensional ice, but the results of studying two-dimensional ice can serve as a basis for explaining the formation of its bulk cousin. In other words, understanding how two-dimensional structures are formed is an important foundation for studying three-dimensional structures. It is for this purpose that the researchers plan to improve their methodology in the future.
Thank you for your attention, stay curious and have a good week everyone guys. π
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Source: habr.com