Patent Application
20250074786
The present invention relates to a two-dimensional (2D) bismuth material, also called bismuthene, with a sandwich-like sheet structure with at least two outer layers formed by organic molecules with sulphur atoms and at least one inner layer formed by a crystalline network of Bi(0) atoms. It also relates to a process of obtaining this material based on a colloidal approach wherein a photocatalytic reduction of a soluble Bi(III) organometallic complex is performed leading to the generation of the Bi(0) crystal lattice. Therefore, the present invention belongs to the field of nanomaterials based on pnictogens and their production processes.
Post-graphene sheet materials are of great scientific interest due to their unprecedented physical and chemical properties as they approach the two-dimensional (2D) limit. These systems could be promising not only for (opto)electronic applications, but also for catalysis, energy storage or biological applications. Among all ultra-thin sheet materials, 2D pnictogens occupy a prominent place since their properties can complement or even surpass graphene in some cases. Both P and As, Sb and Bi have sheet allotropes that can be exfoliated. However, the generation of anisotropic thin layers is increasingly complicated as we go down in the periodic table. This is due to the increasing inter-layer forces present on increasing atomic weight (P<As<Sb<<Bi). This results in a drastic reduction in lateral dimensions and poorly defined morphologies when exfoliations are performed in the liquid phase, as well as in very low or zero degrees of exfoliation when micromechanical exfoliation is used. Of all 2D pnictogens, bismuth is the most difficult to exfoliate. Hence, obtaining 2D pnictogens, especially from bismuth, represents a very significant challenge.
The scalable synthesis of high-quality 2D pnictogens in general, and bismuth in particular, represents a major challenge for the scientific community (Applied Physics Reviews 6, 021308 (2019)). Nowadays, the most common strategies are liquid phase exfoliation and epitaxial growth. Liquid phase exfoliation is the typical approach, although, in the case of bismuth, this technique causes a huge reduction in lateral dimensions mainly due to the strong interactions between the layers, as well as a pronounced oxidation, affecting both the final properties and the use in actual devices. The epitaxial growth technique is extremely expensive and does not allow independent crystals to be obtained, which prevents a mass production of these materials. In both cases, the processes mainly lead to the generation of small nanoparticles of different size and high polydispersity.
The document J. Phys. Chem. B, Vol. 110, No. 51, 2006, describes the synthesis of nanocubes, triangular nanoplates, nanobelts and Bi spheres using NaBiO4, polyvinylpyrrolidone and Fe(III), although these materials cannot be considered two-dimensional. The document Nanoscale, 2018, 10, 21106-21115, describes the acid exfoliation of coarse Bi using ammonium peroxydisulfate ((NH4)2S2O8), H2SO4 and H2O2, which results in particles without form below 500 nm, without control over morphology and highly defective.
Angew. Chem. Int. Ed. 2011, 50, 1363-1366, describes the synthesis of Bi nanocrystals by the reduction of bismuth dodecanethiolate, which was generated by the reaction of dodecanethiol and bismuth neodecanoate in octadecene, and the subsequent reduction with tri-n-octylphosphine (TOP). J. Phys. Chem. C 2014, 118, 2, 1155-1160, describes a process for producing bismuth nanospheres by thermolysis of bismuth acetate in oleylamine. But 2D materials are not obtained in any of these cases.
There is, therefore, a need to have efficient processes for the synthesis of 2D pnictogen crystals, and particularly Bi, which allow their growth and morphology to be controlled and which are easily scalable.
The present invention relates to 2D bismuthene material, with a sandwich-like sheet structure with at least two outer layers formed by organic molecules containing sulphur atoms and at least one inner layer formed by a crystalline network of Bi(0) atoms. These anisotropic crystals of Bi(0) have more than 200 nm of lateral dimension and a high size/thickness ratio. The formation of covalent bonds between the sulphur atoms of the outer layers and the most adjacent Bi(0) atoms induces a stability that results in resistance to degradation in the open atmosphere. The material characteristics given by this particular structure make it especially useful in catalytic, magnetic and optoelectronic applications.
Furthermore, the present invention relates to a process for producing the aforementioned 2D bismuthene material. Said process has a colloidal approach based on a photocatalytic reduction of a soluble organometallic complex Bi(III) leading to a generation of said crystals. This simple synthetic approach allows the manufacture of 2D bismuthene, 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.
More specifically, the present invention discloses the colloidal synthesis for obtaining anisotropic 2D crystals of Bi(0), also called bismuthene, based on the photochemical reduction of an organometallic complex of Bi(III)-dodecanethiol. The role of reaction variables (dodecanethiol (DDT) concentration, temperature, intensity, and colour of light), the effect on kinetics, and crystal size and morphology were evaluated and the results are shown below. After optimising the conditions, micrometric crystals (>1.5 μm) of rhombohedral Bi(0), with a thickness less than 10 nm, preferentially exhibiting the [001] plane, can be easily obtained. The meticulous structural, spectroscopic and chemical characterisation makes it possible to assert that the crystals are single crystals, without defects on surfaces greater than 1000 nm2, with the absence of oxidation and presenting a thin layer of thiol (easily removable) that protects it against oxidation.
While two-dimensional pnictogens, especially phosphorene, tend to oxidise strongly and, therefore, to decompose under ambient conditions, the material of the invention has enormous stability to oxidation that is demonstrated by comparing the Raman spectra of individual hexagons in silicon wafers with a layer of silicon oxide directly after synthesis and after several days (Example 3). This stability is the direct result of a surface coating based on the synthesis process of the present invention.
3C12H26SH+Bi(C10H19O2)3→Bi(C12H26S)3+3C10H20O2
It is known that 2D pnictogens tend to undergo severe degradation in an open atmosphere. For example, low-layer phosphorene and arsenene oxidise completely within hours. However, this limitation is overcome with the synthesis process of the invention, since the low-layer bismuthene is protected by a surface coating. The oxidation stability of the resulting material is demonstrated by the structural characterisation of the fresh individual bismuthene hexagons compared to the individual hexagons characterised a few days later, without any signs of ageing.
Therefore, in a first aspect the present invention relates to a material comprising two-dimensional crystals of Bi(0) characterised in that it is formed by:
wherein the S atoms of the organic molecules R2—SH of layers A are covalently bonded to the adjacent Bi(0) atoms of layer B and wherein said layers A and B are arranged forming a sandwich-like sheet structure with anisotropic order on the stacking axis.
The crystals that form this material are two-dimensional hybrid macrocrystals (diameter in the order of microns, thickness on the nanometer scale). The outer layers A are functionalised with the sulphur atoms of R2—SH and the inner layer B is rhombohedral bismuth, understanding functionalisation as the incorporation of functional groups that can form covalent bonds and other types of bonds with the molecules present in the structure and which favours the incorporation of other molecules or other functional groups to said structure, giving it new properties and/or functionalities.
In a preferred embodiment, layers A have a thickness of between 0.5 to 3 nm. In a more preferred embodiment, layers A have a thickness of 1.5 nm.
In a preferred embodiment, layer B has a thickness of between 5 to 10 nm. In a more preferred embodiment, layer B has a thickness of 7 nm.
In another preferred embodiment, the Bi(0) of layer B has a rhombohedral crystal structure (PDF #44-1246).
In another preferred embodiment, the material of the invention forms crystals of hexagonal morphology. These crystals preferably have a diameter greater than 1,000 nm, and may be more preferably up to 10-25 microns, and/or a thickness of less than 20 nm, and may be more preferably up to 3 nm, and/or an aspect ratio greater than 500.
In another preferred embodiment, the outer layers A have lower density than the inner layer B.
In a second aspect, the present invention relates to a process for producing the above-described Bi(0) nanocrystal material comprising the following steps:
[R1—COO—]Bi3+ (I)
CH3—(CH2)m—CH═CH2 (II)
NH2—(CH2)n1—CH═CH—(CH2)n2—CH3 (III)
R2—SH (IV)
The term “alkyl” refers, in the present invention, to hydrocarbon chain radicals, linear or branched, having between 1 and 18 carbon atoms, being bound to the rest of the molecule by a single bond, for example, 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 by one or more substituents such as halogens, hydroxyl, alkoxy, carboxyl, carbonyl, cyano, acyl, alkoxycarbonyl, amino, nitro, mercapto and alkylthio.
The term “aryl” refers, in the present invention, to single or multiple aromatic rings, having from 5 to 10 bonds wherein a proton has been removed from the ring. Preferably, the aryl group has 5 to 7 carbon atoms. The aryl groups are, for example, but not limited to phenyl, naphthyl, diphenyl, indenyl, phenanthryl or anthracyl. The aryl radicals may be optionally substituted by one or more substituents such as halogens, hydroxyl, alkoxy, carboxyl, carbonyl, cyano, acyl, alkoxycarbonyl, amino, nitro, mercapto, and alkylthio.
In a preferred embodiment, in the bismuth salt of formula (I), R1 en is a linear or branched C8 alkyl.
In a more preferred embodiment, the bismuth salt of formula (I) is as follows:
In another preferred embodiment, in the organic solvent of formula (II) m is an integer value selected from between 10 and 18.
In a more preferred embodiment, the organic solvent of formula (II) is as follows:
CH3(CH2)15CH═CH2
In another preferred embodiment, in the amine of formula (III) n1 and n2 are an integer value independently selected from between 5 and 8.
In a more preferred embodiment, the amine of formula (III) is as follows:
In another preferred embodiment, the temperature in step (b) is 200° C.
In another preferred embodiment, the radiation of step (b) is applied for a time of between 10 and 30 minutes, preferably 15 minutes.
In another preferred embodiment, the inert gas of step (c) is a non-oxidising gas. More preferably, it is selected from argon, nitrogen or carbon dioxide.
In another preferred embodiment, in the reducing agent of formula (IV), R2 is selected from a linear C6-C12 alkyl or a phenyl. More preferably, the reducing agent of formula (IV) is selected from dodecanethiol or thiophenol.
In a preferred embodiment, the reaction is stopped in step (e) by a sudden decrease in temperature that can be performed with an ice bath.
In another preferred embodiment, the product is separated in step (e) by centrifugation.
Another aspect of the invention relates to the use of the two-dimensional Bi(0) nanocrystal material described above for energy storage and generation, for example in Li, Na or K batteries; for electrocatalysis or organic catalysis; for obtaining (opto)electronic materials; for photonic applications; in contrast agents for diagnostic tests.
In this process, a soluble organic precursor of Bi(III) was reduced under the influence of light and temperature, using dodecanethiol as a reducing agent. The obtained material was separated from the reaction mixture by centrifugation and washed with chloroform.
As a first step, a stock solution of 100 mM bismuth neodecanate (BiNeo) in 1-octadecene (ODE) was prepared by dissolving 3.614 g BiNeo in 25 mL ODE. In the particular case of colloidal synthesis, 125 μmol (1.25 mL) of this solution were pipetted and placed in a 50 mL two-necked flask. Subsequently, 22.22 mL of ODE and 1.235 mL of oleylamine (3.75 mmol, 30 eq.) were added. The system was subjected to vacuum, while being heated in oil bath (200° C.) under constant magnetic stirring (400 RPM) and continuously illuminated using an LED lamp. After 13 minutes of degassing, it was changed to argon atmosphere, while the reaction mixture remained at 200° C. for a further 2 minutes. Through a septum, and using a syringe, 0.3 mL (1.25 mmol, 10 eq.) of dodecanethiol (DDT) was injected. The resulting mixture was allowed to react for 30-40 seconds. Upon addition of DDT, the reaction mixture immediately turned yellow, indicating formation of Bi-DDT. The organometallic complex of Bi(III) was photochemically reduced to Bi(0) while DDT was oxidised 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 colour (see colour evolution in the scheme of
After separation of the 2D crystals of Bi(0), the solid was redispersed in chloroform and washed by centrifugation 3 times (10 k RPM, 10 minutes). In this way, we sought to exchange the remaining ODE with chloroform, thus allowing the solid to dry more easily.
The final product was allowed to dry in the dry box, or stored as a suspension in chloroform, to be used in subsequent characterisations. In the case of microscopies, the suspensions were sonicated before use (TEM, SEM, AFM, Raman), while in other assays the final dry solid was used (PXRD, TGA).
The organometallic complex Bi-DDT was identified as the active species (precursor) in the reduction reaction from Bi(III) to Bi(0), which can be monitored even at room temperature (see Example 3 below). In
Taking into account the behaviour at room temperature, the dependence of the reaction rate and the resulting morphology on both the light intensity and the reaction temperature was also evaluated. The results are shown in
As can be seen in
After this analysis, the parameters 200° C. and 2000 lm were chosen as the optimised synthetic conditions.
STEM-EDS mappings reveal the elemental composition of the hexagonal crystals of Bi(0) with a small amount of sulphur (S) on the surface.
XPS measurements confirm the presence of sulphur on the surface of the Bi crystals.
Analogous to the case of EDS measurements, the energies of Bi 4f and S 2p overlap, preventing an unequivocal characterisation of the surface chemistry of the crystals (
In particular, more detailed research was performed using spectroscopic techniques, namely XPS and STEM-EDS in combination with Raman spectroscopy. In
From the EDS it follows that Bi and S are equally distributed, while the oxygen and carbon signal can be clearly correlated with the underlying support film of the TEM. As shown in the inset of
Under vacuum conditions in the XPS, a brief sputtering of Ar (30 s) was performed to remove this coating layer with the result shown in
The combination of surface sensitive XPS and spatially resolved EDS thus allows concluding the equally distributed surface coating with the protective sulphur species which, while separating Bi from environmental degradation, can be easily removed by ion beam sputtering.
Finally, a cross-sectional inspection of the hexagonal microparticles has been performed to unequivocally demonstrate the sulphur-based protective layer.
Further analysis of this sandwich structure was performed by electron diffraction (ED).
After clarifying the functionalisation/protection of the surface by spectroscopic techniques, monochromatic and aberration-corrected STEM-EELS spectrum images were used to investigate the plasmonic behaviour of the material in the low energy loss regime. Electronic excitations in unoccupied orbitals are usually probed in the EELS. Here the low-energy region of the material was investigated. The observed low energy modes can give access to phononic modes (<1 eV), localised surface plasmon resonances (1-10 eV), as well as volume plasmon modes (˜20-50 eV). These optical modes indicative of a metal surface are especially interesting for photonics and catalysis.
The observation of localised surface plasmons is a proof of the high quality of the surface, usually the quality of the plasmonic emission in bismuth systems is deteriorated by the accumulation of defects and distortions in the crystalline order. As shown in
This exceptional quality of the material allows viewing different plasmonic modes in an energy range below 10 eV.
Specifically, four different plasmonic modes could be displayed at the indicated energies. Comparison with the simulated results reveals an excellent match of the localisation near the surfaces. To extract the localised EELS spectra, a logarithmic Fourier deconvolution and a subsequent subtraction of the vacuum-acquired deconvoluted ZLP were performed.
Transmission (scanning) electron microscopy and diffraction have been carried out on Bi(0) crystals placed on TEM sample holders covered with a lacey carbon film of a CHCl3 solution. A JEOL-ARM200F was used for the AC-HRSTEM, operated at 200 kV. TEM and ED measurements were carried out using a Tecnai F20 (200 kV) or a JEOL JEM1010 (100 kV). EDXS mappings were recorded on a Fei Titan Themis300 (300 kV). Scanning Electron Microscopy data were acquired on a Hitachi S4800 FEG, operated at 10 kV. The samples for the AFM, SEM and Raman measurements were prepared by pouring a Bi(0) solution onto silicon wafers with a standard silicon oxide layer and allowing the CHCl3 to evaporate.
Atomic force microscopy was carried out using a Veeco Nanoscope IVa, operated in Si cantilever strike mode.
Raman spectroscopic measurements were performed with a Horiba LabRam HR Evolution, using a HeNe laser (632.8 nm) with a final power of 7.6 μW and a 100× objective (NA 0.9).
Powder X-ray diffraction (PXRD) patterns were obtained using a PANalytical Empyrean X-ray platform with a capillary platform (diameter: 0.7 mm) and copper radiation (Cu Kα=1.541 78 Å). Measurements were made in triplicate in the 2-theta 2-70° range using a step size of 0.02°/step with an integration time of 1 s.
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. For all elements, more than 100 spectra were recorded using a 0.1 eV step with a focused point greater than 400 μm. XPS data were analysed with Thermo Avantage v5.9912 software.
The FIB was carried out with a FEI Helios G4 dual-beam FIB-SEM. A representative crystallite was chosen for the cross-section and a protective layer of Pt was deposited over the crystallite. Two trenches were then cut into the silicon substrate with 30 kV Ga ions. The sheet was attached to the needle of a micromanipulator and lifted from the silicon wafer and welded to an Omniprobe copper grid using Pt. Then, the sheet was thinned until reaching electronic transparency (thickness of about 80 nm) with Ga ions. The foil sample was then investigated on a Philips CM200 FEG TEM, operated at 200 kV.
| Number | Date | Country | Kind |
|---|---|---|---|
| P202130722 | Jul 2021 | ES | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/ES2022/070492 | 7/26/2022 | WO |
