Two-dimensional anisotropic bismuth materials and methods for obtaining same using colloidal synthesis - Patents.com

Abstract

The present invention relates to two-dimensional bismuth materials (also called "bismatenes") with a sandwich-type layered structure with at least two outer layers formed by organic molecules containing sulfur atoms forming Bi-S bonds and at least one inner layer formed by a crystal lattice of Bi(0) atoms. These materials are suitable for use in electronic, optoelectronic and catalytic applications, or in energy storage and conversion. The present invention also relates to a method for obtaining materials from bismuth salts that react with amines and subsequently with thiols under the effect of application of radiation and subsequent reduction. The method uses a colloidal approach to obtain Bi(0) crystals, which is based on the photocatalytic reduction of soluble Bi(III) organometallic complexes that lead to the generation of said crystals.

Description

Technical Field

[0001] The present invention relates to a two-dimensional (2D) bismuth material (also referred to as bismuthene) having a sandwich-like sheet structure with at least two outer layers formed by organic molecules having sulfur atoms and at least one inner layer formed by a crystal network of Bi(0) atoms. The present invention also relates to a method for obtaining this material based on a colloidal approach in which a Bi(0) crystal lattice is generated by photocatalytic reduction of a soluble Bi(III) organometallic complex. Thus, the present invention belongs to the field of nanomaterials based on group 15 elements (pnictogens) and methods for their production.

Background Art

[0002] Post-graphene sheet materials have attracted great scientific interest due to their unprecedented physical and chemical properties as they approach two-dimensional (2D) limits. These systems can also be promising not only for (opto)electronic applications but also for catalysis, energy storage, or biological applications. Among all ultrathin sheet materials, two-dimensional group 15 elements occupy a prominent place as their properties can complement or even in some cases exceed those of graphene. Both P and As, Sb and Bi have exfoliable sheet allotropes. However, the generation of anisotropic thin layers becomes increasingly complex as one descends the periodic table. This is due to the increasing interlayer forces present with increasing atomic weight (P < As < Sb << Bi). This results in a dramatic decrease in the lateral dimensions and ill-defined morphology when exfoliation is carried out in the liquid phase, as well as very low or zero exfoliation when micromechanical exfoliation is used. Among all two-dimensional group 15 elements, bismuth is the most difficult to exfoliate. Therefore, obtaining two-dimensional group 15 elements from bismuth in particular is a very important issue.

[0003] The scalable synthesis of two-dimensional group 15 elements, in general of high quality, and especially of bismuth, is a great challenge for the scientific community (Non-Patent Document 1). Today, the most common strategies are liquid phase exfoliation and epitaxial growth. Liquid phase exfoliation is the typical approach, but in the case of bismuth, this technique causes a significant reduction in lateral dimensions, mainly due to the strong interactions between layers and significant oxidation, affecting both the final properties and the use in practical devices. Epitaxial growth techniques are very expensive and do not allow the obtaining of independent crystals, preventing the mass production of these materials. In both cases, the methods mainly result in the generation of small nanoparticles of different sizes and high polydispersity.

[0004] The literature (Non-Patent Document 2) describes the synthesis of nanocubes, triangular nanoplates, nanobelts, and Bi spheres using NaBiO4, polyvinylpyrrolidone, and Fe(III), but these materials cannot be considered two-dimensional. The literature (Non-Patent Document 3) describes the acid exfoliation of crude Bi using ammonium peroxydisulfate ((NH4)2S2O8), H2SO4, and H2O2, which results in featureless particles smaller than 500 nm with no morphology control and high defects.

[0005] Non-Patent Document 4 describes the synthesis of Bi nanocrystals by reduction of bismuth dodecanethiolate, generated by reaction of dodecanethiol with bismuth neodecanoate in octadecene and subsequent reduction with tri-n-octylphosphine (TOP). Non-Patent Document 5 describes a method to produce bismuth nanospheres by pyrolysis of bismuth acetate in oleylamine. However, 2D materials are not obtained in any of these cases.

[0006] Therefore, there is a need to have efficient methods for their synthesis that allow control over the growth and morphology of two-dimensional group 15 element crystals, especially Bi, and that are easily scalable. [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] Applied Physics Reviews, Volume 6, 021308 (2019) [Non-Patent Document 2] "J.Phys.Chem.B", Volume 110, No. 51, 2006 [Non-Patent Document 3] Nanoscale, 2018, Vol. 10, pp. 21106-21115 [Non-Patent Document 4] Angew. Chem. Int. Ed., 2011, vol. 50, pp. 1363-1366 [Non-Patent Document 5] "J.Phys.Chem.C", 2014, Vol. 118, No. 2, pp. 1155-1160 Summary of the Invention

[0008] [Detailed Description of the Invention] The present invention relates to a two-dimensional bismuth material with a sandwich-like sheet structure with at least two outer layers formed by organic molecules containing sulfur atoms and at least one inner layer formed by a crystalline network of Bi(0) atoms. These anisotropic crystals of Bi(0) have lateral dimensions exceeding 200 nm and a large size / thickness ratio. The formation of covalent bonds between the sulfur atoms of the outer layer and the nearest neighboring Bi(0) atoms induces stability that provides resistance to degradation in open atmosphere. The material characteristics provided by this particular structure make it particularly useful in catalytic, magnetic and optoelectronic applications.

[0009] Furthermore, the present invention relates to a method for the production of the above-mentioned 2D bismuth materials, which has a colloidal approach based on the photocatalytic reduction of the soluble organometallic complex Bi(III) resulting in the generation of said crystals. This simple synthetic approach allows the production of 2D bismuth, paving the way to monolayer crystals, and can be easily scaled up to use these 2D crystals in the implementation of interesting medical applications, topological properties, and catalysis, to name a few.

[0010] More specifically, the present invention discloses a colloidal synthesis to obtain anisotropic two-dimensional crystals of Bi(0), also called bismatene, based on the photochemical reduction of an organometallic complex of Bi(III)-dodecanethiol. The role of reaction variables (dodecanethiol (DDT) concentration, temperature, intensity, and color of light), their influence on the kinetics, as well as crystal size and morphology, were evaluated, with the results presented below. After optimizing the conditions, micrometric crystals (>1.5 μm) of rhombohedral Bi(0) with a thickness of less than 10 nm, exhibiting preferentially the

[0001] face, can be easily obtained. Thorough structural, spectroscopic, and chemical characterizations have revealed that the crystals can be grown to a size of 1000 nm. 2 It becomes possible to determine on the larger surface that it is a single crystal without defects, that is free of oxidation, and that presents a thin layer of thiol (easily removable) that protects the crystal from oxidation.

[0011] While two-dimensional group 15 elements, especially phosphorene, tend to oxidize strongly and therefore decompose under ambient conditions, the material of the present invention has an extremely high stability against oxidation, as demonstrated by comparing the Raman spectra of individual hexagons in a silicon wafer with a layer of silicon oxide immediately after synthesis and after several days (Example 3). This stability is a direct result of the surface coating based on the synthesis method of the present invention.

[0012] [Table 1]

[0013] Photochemical reactions used to obtain Bi(0)

[0014] [Table 2]

[0015] It is known that two-dimensional group 15 elements tend to undergo severe degradation in open atmosphere. For example, the underlayer phosphorene and arsenene oxidize completely within a few hours. However, this limitation is overcome by the synthesis method of the present invention, since the underlayer bismuth is protected by a surface coating. The oxidative stability of the resulting material is demonstrated by the structural characterization of fresh individual bismuthene hexagons compared to individual hexagons characterized after several days without any signs of aging.

[0016] Thus, in a first aspect, the present invention provides a material comprising a two-dimensional crystal of Bi(0), comprising: - an organic molecule R2-SH, wherein R2 is an optionally substituted linear or branched C1-C 18 Alkyl or optionally substituted C5-C 10 At least two outer layers A which are aryl; - at least one inner layer B between layers A, which contains metal Bi(0) atoms forming a crystal structure; The present invention is characterized in that it is formed by an S atom of the organic molecule R2-SH of layer A is covalently bonded to an adjacent Bi(0) atom of layer B, and layers A and B are arranged to form a sandwich-like sheet structure with anisotropic ordering on the stacking axis; Regarding materials.

[0017] The crystals that form this material are two-dimensional hybrid macrocrystals (diameters on the order of microns, thickness on the scale of nanometers), where the outer layer A is functionalized with sulfur atoms of R2-SH and the inner layer B is rhombohedral bismuth, which understands functionalization as the incorporation of functional groups that can form covalent and other types of bonds with molecules present in the structure and facilitate the incorporation of other molecules or other functional groups into the structure, imparting new properties and / or functionality.

[0018] In a preferred embodiment, the layer A has a thickness of 0.5 to 3 nm, and in a more preferred embodiment, the layer A has a thickness of 1.5 nm.

[0019] In a preferred embodiment, Layer B has a thickness of 5 to 10 nm. In a more preferred embodiment, Layer B has a thickness of 7 nm.

[0020] In another preferred embodiment, the Bi(0) of layer B has a rhombohedral crystal structure (PDF#44-1246).

[0021] In another preferred embodiment, the material of the present invention forms crystals with hexagonal morphology, which preferably have a diameter greater than 1,000 nm, and more preferably may be up to 10-25 microns and / or less than 20 nm thick, and more preferably up to 3 nm and / or have an aspect ratio greater than 500.

[0022] In another preferred embodiment, the outer layer A has a lower density than the inner layer B.

[0023] In a second aspect, the present invention relates to a method for producing the above-mentioned Bi(0) nanocrystalline material, comprising the steps of: The method comprises the following steps: (a) preparing a solution of a bismuth salt having the formula: [ka] In the formula, R1 is a linear or branched C1-C 18 is alkyl, [ka] In the formula, m is an integer value selected from 1 to 20, and adding an amine having the formula: [ka] In the formula, n1 and n2 are integer values ​​independently selected from 1 to 10. preparing a solution and adding an amine; (b) increasing the temperature of the reaction mixture obtained in (a) to a temperature of 150-250° C., subjecting it to a vacuum and applying radiation having a wavelength of 430-530 nm; (c) breaking the vacuum of the reaction medium of (b) by introduction of an inert gas; (d) adding to the reaction mixture a reducing agent having the formula: [ka] In the formula, R2 is an optionally substituted linear or branched C1-C 18 Alkyl or optionally substituted C5-C 10 adding a reducing agent selected from the group consisting of aryl; (e) stopping the reaction and isolating the resulting product; It is equipped with:

[0024] The term "alkyl", as used herein, refers to a straight or branched hydrocarbon chain radical having from 1 to 18 carbon atoms and attached to the remainder of the molecule by a single bond, such as, but not limited to, methyl, ethyl, n-propyl, i-propyl, butyl, tert-butyl, sec-butyl, n-pentyl, dodecyl, etc. These alkyl groups can be substituted with one or more substituents, such as halogen, hydroxyl, alkoxy, carboxyl, carbonyl, cyano, acyl, alkoxycarbonyl, amino, nitro, mercapto, and alkylthio.

[0025] The term "aryl" in the present invention refers to a single or multiple aromatic rings having 5-10 bonds, where a proton has been removed from the ring. Preferably, the aryl group has 5-7 carbon atoms. The aryl group is, for example, but not limited to, phenyl, naphthyl, diphenyl, indenyl, phenanthryl, or anthracyl. The aryl radical may be optionally substituted with one or more substituents, such as halogen, hydroxyl, alkoxy, carboxyl, carbonyl, cyano, acyl, alkoxycarbonyl, amino, nitro, mercapto, and alkylthio.

[0026] In a preferred embodiment, in the bismuth salt of formula (I), R1 is a linear or branched C8 alkyl.

[0027] In a more preferred embodiment, the bismuth salt of formula (I) is: [ka]

[0028] In another preferred embodiment, m is an integer value selected from 10 to 18 in the organic solvent of formula (II).

[0029] In a more preferred embodiment, the organic solvent of formula (II) is: [ka]

[0030] In another preferred embodiment, in the amine of formula (III), n1 and n2 are integer values ​​independently selected from 5 to 8.

[0031] In a more preferred embodiment, the amine of formula (III) is: [ka]

[0032] In another preferred embodiment, the temperature in step (b) is 200°C.

[0033] In another preferred embodiment, the radiation in step (b) is applied for a period of 10 to 30 minutes, preferably 15 minutes.

[0034] In another preferred embodiment, the inert gas in step (c) is a non-oxidizing gas, more preferably selected from argon, nitrogen, or carbon dioxide.

[0035] In another preferred embodiment, in the reducing agent of formula (IV), R2 is a linear C8-C 12 More preferably, the reducing agent of formula (IV) is selected from dodecanethiol or thiophenol.

[0036] In a preferred embodiment, the reaction is stopped in step (e) by a sudden decrease in temperature, which can be performed using an ice bath.

[0037] In another preferred embodiment, the product is separated in step (e) by centrifugation.

[0038] Further aspects of the invention relate to the use of the above-mentioned two-dimensional Bi(0) nanocrystal materials for energy storage and energy generation, e.g. in Li, Na or K batteries; for electro- or organo-catalysis; for obtaining (opto-)electronic materials; for photonics applications; in imaging agents for diagnostic tests. [Brief description of the drawings]

[0039] [Figure 1] (a) The synthetic procedure to obtain a few layers of 2D bismatene (2D). A solution of BiNeo and OA in ODE is degassed at 200 °C and then DDT is added to the reaction mixture. The formation of the organometallic complex Bi-DDT is demonstrated by the yellow color of the solution. Subsequently, Bi(III) is reduced to Bi(0) while the thiols are oxidized to disulfides. (b) BF-TEM of a single crystal of Bi(0) larger than 1.5 μm in diameter, (c) SAEP where the rhombohedral ordering along the axis

[0001] can be observed. (d) High-resolution TEM image. (e) AFM performed in tapping mode with the scale indicated on the z-axis. [Figure 1A] (f) PXRD pattern (PDF#44-1246) which makes it possible to generally confirm the obtaining of the rhombohedral structure of Bi(0).(g) Single crystal Raman spectrum recorded with a red laser (633 nm) showing exclusively the peaks Eg and A1g (inset shows the optical micrograph). [Diagram 2] (a) Dependence of reaction rate on DDT concentration. The black curve is 3 equivalents of DDT relative to Bi(III), while the grey curve has an excess of DDT (1000 mM). Photographs of the initial (yellow, Bi-DDT) and final (black, Bi(0)NP) solutions of the reaction. (b) TEM image which makes it possible to observe the presence of Bi(0)NPs with a spherical morphology. [Diagram 3] Light and temperature dependence of the formation of Bi(0) by photochemical reduction. Representative BF-TEM images of the nanomaterial obtained at different temperatures evaluated with white LED light of different intensities: 807 lm (a) and 2000 lm (b). Dependence of the reaction rate on temperature and light intensity or brightness (c). [Figure 3A] Characteristic emission spectra of the used LED lamps: 807 lm (gray) and 2000 lm (black) (d). The grey box indicates the region of the electromagnetic spectrum 450–550 nm responsible for the photocatalytic reaction. [Figure 4] AFM image of a hexagonal particle (a) from which the thickness profile (b) was extracted. Typically, Raman spectra were obtained with signals Eg and A1g (c). [Figure 4A] See above discussion regarding FIG. [Diagram 5] BF-STEM images (a) and integrated EDS signals of Bi (b), S (c), and O (d). EDS spectrum signaling lines Bi-M, Bi-L, and SK (e). Adjustment of the EDS signal in the energy range included in the Bi-M and SK lines to take into account the sulfur signal. [Figure 6] XPS spectrum of the material obtained by the synthesis method (a), where a clear characteristic signal of S 2p superimposed with Bi 4f signal is observed. After removing this S-containing surface layer by Ar+ pickling, no S signal is observed, instead, Bi(0) signal can be exclusively observed (b). [Figure 7] AC-HAADF-HRSTEM image of Bi(0) where a defect-free surface can be observed on scales larger than 1000 nm2. The FFT of the image is shown in the box. [Figure 8] Characterization of organometallic precursor of Bi(III): PXRD pattern. Box: increase in the range of 2-10°. The first three peaks can be indexed with the 00l group with an interlayer distance of 31.7 Å (A). ATR-FTIR spectroscopy. The absence of a signal at 2500 cm-1 (SH vibration) ensures the absence of free thiols, which suggests a Bi-S bond (B). The diffuse UV-Vis reflectance spectrum has an absorption maximum at 417 nm (C). Thermogravimetric analysis carried out at 10 °C / min in an inert atmosphere (He, 20 mL / min) allows the observation of a single decomposition step at 305 °C (D). [Figure 8A] See above discussion regarding FIG. [Figure 8B] See above discussion regarding FIG. [Figure 9] UV-Vis absorption spectra of solutions containing mixtures of Bi-Neo, DDT, and amines: the pure reagents (A) and their mixtures (B). [Figure 9A] See above discussion regarding FIG. [Figure 10] Cross-section of a hexagonal particle obtained by the method of the invention. A sandwich structure based on a surface layer of about 1.5 nm and a core of approximately 7 nm. Experiment (left), simulation (center) and model (right). [Figure 11] Localized surface plasmons of colloidally synthesized few-layer bismatene. (a) STEM-EELS experimental data: (a1) HAADF-STEM data with indicated positions for extraction of EEL spectra. (a2)-(a5) Integrated EELS intensity showing four distinct plasmonic modes in the indicated energy range. (b) Simulation of the EELS intensity of individual hexagons. [Figure 12] Detailed schematic of the bismuth / bismatene sandwich hybrid structure. Electron diffraction patterns for each of the indicated areas. (bottom right panel) Examination of the cross section by means of HRTEM compared with the respective simulation. [Figure 13] Examination of a cross-section of a hexagonal particle obtained by the method of the invention by STEM coupled with an EDS / EDX probe. Top panel: Lines represent mass-normalized percentages of Bi-M, SK, CK and OK signals. Note: SK signal was increased by a factor of 10 to facilitate observation. Extraction of lines corresponding to the signal of each element from the HAADF image (bottom panel) then allows visual confirmation of the atomic distribution. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0040] The following describes the embodiment. EXAMPLES

[0041] [Example 1]: Synthesis of Bi 2D In this method, a soluble organic precursor of Bi(III) was reduced under the influence of light and temperature using dodecanethiol as a reducing agent. The resulting material was separated from the reaction mixture by centrifugation and washed with chloroform.

[0042] As a first step, a stock solution of 100 mM bismuth neodecanoate (BiNeo) in 1-octadecene (ODE) was prepared by dissolving 3.614 g of BiNeo in 25 mL of ODE. In the specific case of colloid synthesis, 125 μmol (1.25 mL) of this solution was pipetted into a 50 mL two-neck flask. Then, 22.22 mL of ODE and 1.235 mL of oleylamine (3.75 mmol, 30 equiv.) were added. The system was subjected to vacuum and continuously irradiated with an LED lamp while heating in an oil bath (200 °C) under constant magnetic stirring (400 RPM). After degassing for 13 min, the atmosphere was changed to argon while the reaction mixture was kept at 200 °C for another 2 min. Through the septum, 0.3 mL (1.25 mmol, 10 equiv.) of dodecanethiol (DDT) was injected using a syringe. The resulting mixture was allowed to react for 30-40 seconds. Upon addition of DDT, the reaction mixture immediately turned yellow, indicating the formation of Bi-DDT. The organometallic complex of Bi(III) was photochemically reduced to Bi(0), while DDT was oxidized to disulfide. The reduction of Bi(III) to Bi(0), and consequently the end of the reaction, was confirmed by the appearance of a black color (see color evolution in scheme a of Figure 1), showing a pronounced nematic texture (the first evidence of high anisotropy of the crystals, i.e., bidimensional (2D) crystals). The reaction was stopped by immersing the flask in an ice bath for 5 minutes. The solution was then centrifuged (10k RPM, 10 minutes) in an argon dry box to avoid oxidation of the Bi(0) crystals.

[0043] After isolation of the 2D crystals of Bi(0), the solid was redispersed in chloroform and washed three times by centrifugation (10k RPM, 10 min). In this way, we attempted to exchange the remaining ODE with chloroform and thus make the solid easier to dry.

[0044] The final products were either dried in a dry box or stored as suspensions in chloroform for subsequent characterization. For microscopy, the suspensions were sonicated before use (TEM, SEM, AFM, Raman), while for other assays the final dry solids were used (PXRD, TGA).

[0045] [Table 3]

[0046] Example 2: Characterization of two-dimensional bismuth crystals Figures 1 and 1A show the microscopic and structural characterization of the obtained two-dimensional hexagonal crystals of Bi(0). In Figure 1(b), a low magnification image of the BF-TEM allows observing crystals with a diameter of approximately 1.5 μm, which exhibit a hexagonal morphology. In Figure 1(c), the electron diffraction pattern (SAEP) provides clear evidence of a rhombohedral crystal structure, with the crystallite sizes oriented on the axis 0001 along the stacking axis of the layered crystals. HRTEM (d) and AFM (e) confirm the hexagonal morphology of the obtained layered crystals. The PXRD pattern in Figure 1A(f) can be fully indexed with the crystal structure reported for rhombohedral Bi (PDF#44-1246), providing a final test of the overall rhombohedral structure of the obtained crystals.

[0047] The organometallic complex Bi-DDT was identified as the active species (precursor) in the reduction reaction of Bi(III) to Bi(0), which can be monitored even at room temperature (see Example 3 below). In Figure 2(a) the dependence of the reaction rate on the concentration of DDT is observed. When using a stoichiometric amount of DDT (3 equivalents relative to Bi(III), black curve), the reaction is much slower than when using more than 1000 mM DDT (gray curve). Over the concentrations of DDT used, in all cases evaluated, spherical nanoparticles with sizes smaller than 20 nm are obtained at room temperature (see Figure 2b). It is important to mention that in order for the reduction of Bi-DDT to Bi(0) to occur, the reaction unavoidably requires visible light. If the Bi-DDT precursor or the reaction mixture is stored in the dark, the reduction does not occur.

[0048] Considering the behavior at room temperature, the dependence of the reaction rate and the resulting morphology on both light intensity and reaction temperature was also evaluated. The results are shown in Figures 3 and 3A. Overall, a clear trend of reaction rate vs. temperature can be observed. At higher temperatures, the time required to reduce Bi(III) to Bi(0) is significantly reduced, as can be seen in Figure 3(c) (note that the time axis is in logarithmic scale). In addition to the effect of temperature on the reaction rate, a significant increase in the reaction rate can also be observed when using brighter / stronger LED lamps (black squares, 2000 lm vs. grey circles, 807 lm). Figure 3A(d) shows the emission spectrum of the lamps, where it can be observed that, beyond the change in intensity, both emissions are similar, thus ruling out the effect of changes in the emission spectrum. Next, experiments were performed with red and blue light, and it could be concluded that only the blue emission is responsible for the increase in the reaction rate (grey box, Figure 3A(d)). Interestingly, red light can even slightly slow down the reaction rate.

[0049] As can be seen in Figure 3(a), temperature plays an important role in the morphology of the Bi(0) crystals. Spheroidal grains dominate at low temperatures, while an increase in spheroidal grains promotes grain faceting that eventually develops into a highly anisotropic two-dimensional material at temperatures above 200 °C. Interestingly, when the highest temperatures and most intense light are used, the size of the crystals decreases again and the most pronounced growth along the stacking axis (thickness) is observed.

[0050] After this analysis, the parameters 200 °C and 2000 lm were chosen as the optimized synthesis conditions. Figures 4 and 4A show an AFM image (a) indicating the sector where the thickness measurements (b) were taken. The Raman spectrum of an 8 nm thick crystal is shown in Figure 4A (c). Here, the typical doubly degenerate in-plane and out-of-plane modes are at 74.8 cm, respectively. -1 and 99.8 cm -1 Symmetry E with Raman transpose of g and A 1g The spectrum was measured using a HeNe laser with an emission wavelength of 632.8 nm. The characterization performed by Raman microscopy was performed using the Raman spectrum of Bi2O3 (approximately 320 cm -1 This allows to exclude the presence of a signal from the Bi(0) crystal (Fig. 1A) and ensures the absence of oxidation in the Bi(0) crystal.

[0051] STEM-EDS mapping reveals the elemental composition of hexagonal crystals of Bi(0) with a small amount of sulfur (S) at the surface. Figure 5(b)-Figure 5(d) show the mapping of element Bi, element S, and element O. Bi and S are equally distributed, while the absence of O in the crystal ensures the purity of the obtained crystal (native surface that is not oxidized). Figure 5(e) shows the integrated EDS spectrum pointing out the SK lines. As the sulfur signal highly overlaps with the Bi-M line, a tuning of the EDS signal was performed in the energy range 2.20-2.65 keV. The results are shown in Figure 5f-g. It is thus clear that a quantitative tuning of the EDS signal can only be performed if the intensity of the SK lines is taken into account.

[0052] XPS measurements confirm the presence of sulfur at the surface of the Bi crystals. As in the case of EDS measurements, the energies of Bi 4f and S2p overlap, preventing a clear characterization of the interface chemistry of the crystals (Figure 6a). However, after pickling with Ar+ ions to remove the first surface layer, the characteristic Bi(0) signal can be clearly observed (Figure 6b). This first surface layer(s) is rich in sulfur and acts as a barrier against oxidation, preserving the identity of the material.

[0053] FIG. 7 shows the image of HAADF-HRSTEM with aberration correction and then Fast Fourier Transformation (FFT) is observed. As mentioned above, the material is large scale (>1000 nm 2 ) and is a single crystal without any major defects.

[0054] [Example 3]: Surface chemistry In particular, more detailed studies were carried out using spectroscopic techniques, namely XPS and STEM-EDS in combination with Raman spectroscopy. In Figure 5, the STEM-EDS mapping shows integrated Bi-M, SK, and OK signals adjacent to the BF-STEM image. High magnification TEM data contained in the ED shows a thin amorphous rim around the Bi crystallites, indicating the thickness of the coating layer.

[0055] From the EDS, Bi and S are equally distributed, while the oxygen and carbon signals become clearly correlated with the underlying support film in the TEM. Only the additional contribution of sulfur allows for a precise adjustment of the obtained integrated signal, as shown in the inset of Figure 5e. The surface sensitivity of X-ray photoelectron spectroscopy helps to clarify the presence of a sulfur-rich surface layer, as can be seen in Figure 6. Interestingly, the strong contribution of S 2p at 165-170 eV points to a strongly oxidized sulfur species (Bi-S bond test), pointing to oxygen protection.

[0056] A brief sputtering of Ar (30 s) was performed under XPS vacuum conditions to remove this coating layer, the results of which are shown in Figures 6a and 6b. After etching, one can trace the almost complete loss of the XPS intensities related to carbon (285 eV) and oxygen (532 eV), as well as the disappearance of the S 2p contribution between 165 and 170 eV (Figure 6b). Focusing on the Bi 4f region, the 4f 7 / 2 and 4f 5 / 2 A shift in the contributions of both Ar and Ar is evident, which gives rise to a pure Bi(0) surface. Thus, Ar sputtering allows for a simple and effective removal of the protective layer and gives easy access to pure unoxidized Bi(0).

[0057] The combination of surface-sensitive XPS and spatially resolved EDS therefore makes it possible to conclude an equally distributed surface coating with protective sulfur species that can be easily removed by ion beam sputtering while isolating Bi from environmental degradation.

[0058] Finally, a cross-sectional examination of the hexagonal microparticles was performed to clearly demonstrate the sulfur-based protective layer. Figure 10 shows a TEM image recorded on a hexagonal particle. As can be seen, the material shows atomic ordering over the entire examined area. Interestingly, two different sections can be observed, namely the surface exhibits a different atomic distribution compared to the inner part. In the core of the microparticles the structure corresponds to rhombohedral Bi (as demonstrated by PXRD in Figure 1), while the surface consists of a highly ordered arrangement of Bi-S atoms, which accounts for the lack of surface oxidation (Figure 5d).

[0059] Further analysis of this sandwich structure was carried out by electron diffraction (ED). Figure 12 shows a more detailed description of the bismuth / bismuth hybrid structure by ED simulation of each layer. The perfect correlation of the obtained ED pattern with the simulation reinforces the hypothesis of obtaining a structure that protects the invention. Finally, the examination of a cross section of this hexagonal material obtained by the method of the invention was carried out by means of a stem coupled to an EDS / EDX probe that allows to spatially locate each type of element (Figure 13). As can be seen in the top panel, it is composed only of Bi in the center of the particle, but on the surface where the presence of S and C atoms typical of DDT molecules is observed. No O atoms are observed either inside or on the surface of the material, thus confirming the existence of Bi. x O y It is important to note that the formation of bismuth oxide type 1 can be excluded. These results therefore confirm that a sandwich structure is obtained consisting of a rhombohedral Bi core and two protective outer layers of functionalized Bi covalently linked to DDT molecules (Bi-S bonds).

[0060] [Example 4]: Surface plasmon mode After clarifying the surface functionalization / passivation by spectroscopic techniques, monochromatic and aberration-corrected STEM-EELS spectral images were used to investigate the plasmonic behavior of the material in the low energy loss regime. Electronic excitations of unoccupied orbitals are usually probed with EELS. Here, the low energy regime of the material was investigated. The observed low energy modes can give access to phononic modes (<1 eV), localized surface plasmon resonances (1-10 eV), and volume plasmon modes (~20-50 eV). These optical modes, which point to metallic surfaces, are of particular interest for photonics and catalysis.

[0061] The observation of localized surface plasmons is a testament to the high quality of the surface; typically, the quality of plasmonic emission in bismuth systems is degraded by the accumulation of defects and distortions in the crystalline order. As shown in Figure 1, colloidally synthesized hexagonal bismuth microparticles show no notable crystalline defects, either locally (HR-STEM) or globally (PXRD), highlighting the advantage of the colloidal approach of the method of the present invention.

[0062] This exceptional quality of the material allows us to see different plasmon modes in the energy range below 10 eV. Figure 11a shows STEM images of the investigated hexagonal, resolved modes (a2-a5) as well as the calculated magnetic field distribution (b2-b5). Thus, the exceptional surface quality in combination with next-generation electron microscopy allows the direct measurement of these surface modes without the need for any pre-treatment.

[0063] Specifically, four different plasmon modes can be seen at the indicated energies. Comparison with simulated results reveals excellent agreement of the near-surface localization. To extract the localized EELS spectra, log-Fourier deconvolution and subsequent subtraction of the vacuum-acquired deconvolution ZLP were performed.

[0064] [Explanation of measurement methodology] Electron microscopy: Transmission (scanning) electron microscopy and diffraction were performed on Bi(0) crystals placed on a TEM sample holder covered with a lacy carbon film in CHCl3 solution. A JEOL-ARM200F was used for AC-HRSTEM operated at 200 kV. TEM and ED measurements were performed with a Tecnai F20 (200 kV) or a JEOL JEM1010 (100 kV). EDXS mapping was recorded on a Fei Titan Themis300 (300 kV). Scanning electron microscopy data were acquired on a Hitachi S4800 FEG operated at 10 kV. Samples for AFM, SEM, and Raman measurements were prepared by injecting Bi(0) solution onto a silicon wafer with a standard silicon oxide layer and evaporating the CHCl3.

[0065] Atomic Force Microscopy: Atomic force microscopy was performed using a Veeco Nanoscope IVa operating in strike mode with a Si cantilever.

[0066] Raman spectroscopy: Raman spectroscopic measurements were performed on a Horiba LabRam HR Evolution using a HeNe laser (632.8 nm) with a final power of 7.6 μW and a 100× objective lens (NA 0.9).

[0067] Powder X-ray diffraction: Powder X-ray diffraction (PXRD) patterns were obtained using a PANalytical Empyrean X-ray platform equipped with a capillary platform (diameter: 0.7 mm) and copper radiation (Cu Kα = 1.54178 Å). Measurements were performed in triplicate in the range 2θ–70° using a step size of 0.02° / step, and the integration time was 1 s.

[0068] X-ray photoelectron spectroscopy (XPS): XPS measurements were recorded on a Thermo Scientific™ K-Alpha X-ray photoelectron spectrometer. Al Kα X-ray radiation was used as the X-ray source. More than 100 spectra were recorded for all elements using 0.1 eV steps with a focal spot larger than 400 μm. XPS data were analyzed with Thermo Avantage v5.9912 software.

[0069] FIB cross section: FIB was performed using an FEI Helios G4 dual beam FIB-SEM. A representative crystallite was selected for cross sectioning and a protective layer of Pt was deposited on top of the crystallite. Two trenches were then cut in the silicon substrate with Ga ions at 30 kV. The sheet was attached to a micromanipulator needle, lifted off the silicon wafer and welded to an Omniprobe copper grid with Pt. The sheet was then thinned until it reached electron transparency by Ga ions (thickness approx. 80 nm). The foil sample was then investigated in a Philips CM200 FEG TEM operated at 200 kV. [Explanation of symbols]

[0070] (none)

Claims

1. A material comprising a two-dimensional crystal of Bi(0), - an organic molecule R ２ -SH, wherein R ２ is a linear or branched C １ to C １８ alkyl or optionally substituted C ５ to C １０ aryl, at least two outer layers A, and - at least one inner layer B between the layers A, comprising Bi(0) atoms forming a crystal structure, formed by The S atom of the organic molecule R ２ -SH of the layer A is covalently bonded to adjacent Bi(0) atoms of the layer B, and the layers A and B are arranged to form a sandwich-like sheet structure having an anisotropic order on the stacking axis. A material characterized by this.

2. The material according to claim 1, wherein the layer A has a thickness between 0.5 and 3 nm.

3. The material according to claim 2, wherein the layer A has a thickness of 1.5 nm.

4. The material according to claim 1, wherein the layer B has a thickness between 5 and 10 nm.

5. The material according to claim 4, wherein the layer B has a thickness of 7 nm.

6. The material according to claim 1, wherein the Bi(0) of the layer B has a rhombohedral crystal structure (PDF#44-1246).

7. The material according to claim 1, wherein the material forms crystals in a hexagonal form.

8. The material according to claim 7, wherein the crystals have a diameter greater than 1,000 nm.

9. The material according to claim 1, wherein the crystals have a thickness of less than 20 nm.

10. The material according to claim 1, wherein the crystal has an aspect ratio greater than 500.

11. The material according to claim 1, wherein the outer layer A is functionalized with sulfur atoms forming Bi—S bonds, and the inner layer B is rhombohedral bismuth.

12. A method for producing the material according to any one of claims 1 to 11, the method comprising the following steps, namely: (a) preparing a solution of a bismuth salt having the following formula (I) in an organic solvent having the following formula (II) and adding an amine having the following formula (III), 【Chemical Formula 1】 wherein R １ is linear or branched C １ - C １８ alkyl, 【Chemical Formula 2】 wherein m is an integer value selected from 1 to 20, 【Chemical Formula 3】 wherein n1 and n2 are integer values independently selected from 1 to 10, the step of preparing the solution and adding the amine, (b) raising the temperature of the reaction mixture obtained in (a) to a temperature of 150 to 250 °C, subjecting it to a vacuum, and applying radiation having a wavelength of 430 to 530 nm; (c) breaking the vacuum of the reaction medium in (b) by introducing an inert gas; (d) adding a reducing agent having the following formula (IV) to the reaction mixture, 【Chemical Formula 4】 wherein R ２ is optionally substituted linear or branched C １ - C １８ alkyl or optionally substituted C ５ - C １０ aryl, the step of adding the reducing agent, (e) a step of stopping the reaction and separating the obtained product; A method comprising the steps of:

13. R in the bismuth salt of the formula (I) １ is linear or branched C ８ alkyl, the method according to claim 12.

14. The bismuth salt of the formula (I) is 【Chemical Formula 5】 as shown in, the method according to claim 13.

15. m in the organic solvent of the formula (II) is an integer value selected from 10 to 18, the method according to claim 12.

16. The organic solvent of the formula (II) is 【Chemical Formula 6】 as shown in, the method according to claim 15.

17. n1 and n2 in the amine of the formula (III) are integer values independently selected from 5 to 8, the method according to claim 12.

18. The amine of the formula (III) is 【Chemical Formula 7】 as shown in, the method according to claim 17.

19. The temperature of the step (b) is 200 ° C, the method according to claim 12.

20. The radiation of the step (b) is applied for a time of 10 to 30 minutes, preferably 15 minutes, the method according to claim 12.

21. The inert gas of the step (c) is a non-oxidizing gas, preferably a non-oxidizing gas selected from argon, nitrogen, or carbon dioxide, the method according to claim 12.

22. R in the reducing agent of the formula (IV) ２is a linear C ８ ~C １２ alkyl or phenyl, the method according to claim 12.

23. The method according to claim 22, wherein the reducing agent of the formula (IV) is selected from dodecanethiol or thiophenol.

24. In step (e), the reaction is stopped by suddenly lowering the temperature of the mixture obtained in (d), preferably by means of an ice bath, the method according to claim 12.

25. In step (e), the product is separated by centrifugation or filtration, the method according to claim 12.

26. Use of the material according to any one of claims 1 to 11 for energy storage or energy generation.

27. Use of the material according to any one of claims 1 to 11 for catalysis.

28. Use of the material according to any one of claims 1 to 11 for applications in photonics.

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