The role of structural defects in the growth of two-dimensional diamond from graphene

The paper was published in Nanomaterials.

The synthesis of a 2D diamond is the most challenging field since unlike graphene and many other two-dimensional materials, diamane cannot be cleaved from the bulk. Moreover, a thermodynamic analysis shows that a few-layered diamond film without coverage layer is simply unstable and decomposes into multilayered graphene because the diamond surface energy is higher than the one of graphite. This conclusion is well supported by experiment where the direct pressure in diamond anvil cells was used to induce conversion of the whole graphene-flake while the diamondization pressure was much higher than in the bulk case, and instability of the formed diamondized film was apparent after pressure release.

The most promising way to obtain a two-dimensional diamond seems to be the use of graphene as a precursor, by deposition of reference atoms (e.g. hydrogen) on its surface. In this case the thermodynamic stability of the material is reversed, the previously unstable diamond film becomes energy favorable and graphene layers tend to bond to each other. Despite a number of encouraging experimental results confirming such predictions, the question of diamane synthesis is far from being resolved. Indeed, the nucleation of the diamane in graphene is hindered by the high stability of the graphene π-system resisting attachment of reference atoms. As a result, only two layers of graphene can be connected relatively easily, and only in the case of using hydrogen plasma as a hydrogen source. In the case of using H2, we can expect the appearance of a significant nucleation barrier which can be overcome only by high pressure and temperature.

However, the real structure of graphene contains structural defects that can be used as nucleation centers which may allow the synthesis of diamane under less severe conditions. Here we investigated such an effect in detail. For this purpose, we studied ones of the most common structural defects in graphene and reveal their impact on diamond nucleation. We found that type and concentration of structural defects can sufficiently impact the initial stages of diamond nucleation. At the same time, it does not influence further diamond formation. Defects impact on C-H bonding strength is being disappeared at the second coordination sphere already. We show that vacancies agglomeration (that can be produced by low energy ion irradiation) can sufficiently expand the reactive region which vanishes the nucleation barrier for the first stages of nucleation (see the figure below).

Average binding energy as a function of numbers of atoms in H cluster for AB bilayer graphene contained 1 (blue line), 2 (red line), 3 (yellow line) and 4 (purple line) cross layer vacancies. In the inset top view of graphene with 4 vacancies agglomerate is presented. H bonding energies in the H2 molecule (εH2) and in infinite diamane (εb(∞)) are marked by dashed and solid horizontal lines, respectively. The εb(n) dependence for hydrogen on the surface of defectless AB bilayer graphene is shown by black

Stone-Wales defects impact is lower but still promotes the hydrogenation and bonding of the graphene layers. We show that 1D defect (dislocation) not only facilitates the diamondization but also may lead to the appearance of 2D diamond consisting of chemically connected grains of different crystallographic orientations. Therefore, polycrystalline graphene usually observed in the experiment can produce specific 2D diamond polycrystals containing different surfaces. Even hexagonal and cubic 2D diamonds can coexist together in the same film with grain boundary energy comparable with the same values for other two-dimensional carbon structures (see the figure below).

(left) the atomic structure of polycrystalline bilayer graphene containing symmetrically inclined grains by θ = 11.5° with 5|7 defects highlighted by blue and red colors corresponding to first and second layer, respectively; (right) atomic structure of polycrystalline diamane produced by hydrogenation of graphene presented in left consisting of grains with cubic diamond and lonsdaleite structures.

Scientists of LDMS and SRC IN proposed a promising sorbent for wastewater treatment from antibiotics

The article was published in Nanomaterials.

As a result of the constant increase in the use of drugs, the accumulation of antibiotics and their breakdown products in wastewater has become a serious problem for humans and the environment. Most often, antibiotics enter rivers and groundwater as waste from pharmaceutical companies, medical and pharmacy facilities, and agriculture. The presence of antibiotics in water leads to the growth of bacteria and microorganisms resistance to them, the development of allergic reactions, and even the multiplication of dangerous bacteria.

Currently, there are various methods of wastewater treatment. However, each method has its own limitations. One of the simplest and inexpensive methods of purification, which do not require complex production structures or additional chemical reactions is sorption. This is what the researchers at the Laboratory of Digital Materials Science and the Scientific Research Center "Inorganic Nanomaterials" have focused on. There is no need to create special expensive equipment or artificially introduce additional chemical or biologically active components that can disturb the ecological balance. It is enough just to pass contaminated water through a filter or suspension of boron nitride nanoparticles.

The sorbent created by the researchers based on hexagonal boron nitride is able to effectively clean antibiotic wastewater. Researchers chose three types of antibiotics, which are among the most common pollutants: ciprofloxacin, tetracycline and bicillin.

In the future, scientists are planning to increase the sorption capacity of nanoparticles by applying a polymer or metal ion deposition, as well as to expand the range of antibiotics studied.

Feature of the endohedral metallofullerene Y@C82 and Gd@C82 polymerization under high pressure

The paper was published in J. Phys. Chem. C (2022).

Fullerenes have attracted the attention of researchers since their discovery and further development of large-scale production. The unique symmetric structure, attractive physical and chemical properties make fullerenes promising for many fields of science and technology. An attractive feature of fullerenes is the possibility of tuning and modifying their structure in various ways. The accessibility of the outer surface allows functionalization, while the vast cavity inside the molecule can be used for introducing various guest atoms that can significantly change the physicochemical properties of the fullerene without essential structural distortion. For example, the endohedral metal atoms may lead to the appearance of magnetic moments in fullerenes and further apply them in quantum electronics and medicine. It is extremely interesting to study the physicochemical properties of endohedral metallofullerene (EMF), including their behavior under pressure. However, so far the lack of methods for their large-scale synthesis has not allowed such experimental studies.

The situation has changed since we developed a method for the synthesis of fullerene endohedral complexes in macroscopic quantities. In our recent work, we showed that introduction of Sc2C2 cluster inside a fullerene cavity significantly changes its behavior under pressure. To clarify the influence of metal ions on the polymerization behavior of fullerenes, it is important to investigate the behavior of EMF with only single guest metal ion inside, which would allow to exclude the anisotropic effects associated with the low symmetry of the Sc2C2 complex.

Here we present the first study of the properties under high pressures of the gadolinium and yttrium endohedral metallofullerenes as bulk material. The polymerization processes of both complexes were studied theoretically and experimentally. It was shown that the presence of the metal ions significantly affects the polymerization of endohedral complexes which are studied at high pressures up to 40 GPa. It was found that the process of polymerization of both classes of endohedral complexes is similar to each other but differs from the pure fullerene polymerization. DFT simulation results demonstrated that both metal ions dramatically change the fullerene bonding process by the polarization of the carbon bonds which leads to their increased chemical activity. The values of the bulk modulus were calculated for the resulting polymerized materials using the Raman measurements at a pressure range of 3-27 GPa. The moduli were equal to ~340 GPa for gadolinium and yttrium endohedral complexes which is lower than the B value of diamond (443 GPa) but comparable with Sc2C2@C82 (330 GPa).