Lamellar metal chalcogenide production, for use e.g. as lubricant or catalyst, by pyrolysis of liquid aerosol of solution of precursor compound(s) in carrier gas

@ Representative (s): CABINET ARMENGAUD AINE.

FR 2 863 265 - A1 (54) PROCESS FOR SYNTHESIS OF CHALCOGENIDE NANOPARTICLES HAVING A LAMELLAR STRUCTURE.

(ST) The invention relates to a process for the synthesis of metal chalcogenide nanoparticles with a lamellar structure, of general formula M_atX_b, in which M represents a metal and X a chalcogen, a and b representing the respective proportions of metal and of chalcogen, characterized in that it comprises pyrolysis of a liquid aerosol obtained from a solution of at least a precursor of a metal (M) and of a chalcogen (X), or of at least one precursor of said metal (M) and of at least one precursor of said chalcogen (X), dissolved in a solvent, said solution being sprayed into fine droplets suspended in a carrier gas.

"Process for the synthesis of chalcogenide nanoparticles having a lamellar structure"

The present invention relates to the synthesis of metal chalcogenide nanoparticles with a lamellar structure, of general formula M_atXb, in which M represents a metal and X a chalcogen, a and b representing the respective proportions of metal and chalcogen.

Such chalcogenides, in particular the dichalcogenides of transition metals (MX₂), are materials used industrially in the field of lubrication and catalysis (in particular for the hydrodesulfurization of oils).

As for the dichalcogenides, they have a hexagonal structure (for example 2H-MoS₂) and lamellar, i.e. made up of 001 oriented crystallographic planes which constitute an assembly of MX sheets₂ said by van der Waals.

An MX slip₂ consists of a plane of metal atoms (M) sandwiched by two planes of chalcogen atoms (X). Within the sheets, the atomic bonds between M and X are covalent and therefore solid. On the other hand, the sheets are linked to each other by weak atomic interactions (van der Waals forces between the chalcogen planes), thus allowing easy sliding perpendicular to the 001 planes, which is the source of their lubricating capacity at the l solid state.

The tribological properties of solid lubricants such as graphite and MX dichalcogenides₂ are of great industrial and technological interest in reducing friction. They are used in the event that liquid lubricants do not work, or as additives to improve the performance of liquid lubricants. For example MoS₂ is well known for its application in the technologies of the space industry and in a certain number of dispersions in grease, lacquers and as additives in engine oils, in particular in the automobile industry.

Under ultra-vacuum, MoS coatings₂ pure and stoichiometric, obtained by solid spraying, have an extremely low coefficient of friction, of the order of 10 “³. To enable MoS coatings₂ to slide relative to each other, the crystallites reorient themselves with their base planes (van der Waals plane) parallel to the sliding direction.

Although these materials possess remarkable anti-friction and slip properties with a long life when used in vacuum, these lubricating capacities become poor in the presence of moisture and / or oxygen, thus limiting their use. in the earth's atmosphere. MoS powder₂ thus exhibits a very short life time and a considerably higher coefficient of friction, greater than 0.1, when used in humid and / or oxygenated environments. In the ambient atmosphere, that is to say with a relative humidity of 50%, the pads of the polytype 2H tend to stick to the metal parts due to the presence of dangling links, which leads to deterioration and rapid oxidation.

The development of new lubricants of better quality and greater efficiency is both an economic and an environmental challenge.

Recently, it has been shown that MoS nanoparticles₂and WS₂, in their fullerene and nanotube form, exhibit tribological properties much superior to those of these materials in the form of conventional particles (platelets of hexagonal structure 2H) in a wide range of operating conditions (concentration, charge, charge / speed ratio, temperature and pressure ...).

The fullerenes of MoS₂ exhibit a low coefficient of friction close to 0.04 both under lubricating conditions, as additives in oils, and as a coating under ultra-vacuum conditions. The fullerene structure exhibiting very few pendant bonds provides great chemical stability even when the fullerenes are in contact with ambient air and in particular with oxygen. Their round and hollow shape is also a favorable factor because it allows deformation without breaking, due to a certain elasticity of the structure and thus better resistance over time.

As previously indicated, given the recent nature of their discovery, the new forms of MX₂(fullerenes and nanotubes) are not used industrially. Only the classic wafer form (2H), of different dimensions (from 1 to a few hundred microns), is widely used in many fields and according to different packaging (powder, additives, coating, etc.).

In addition, there is currently only one method allowing the synthesis in appreciable quantity of pure phases of fullerenes or of MX nanotubes.₂. The marketing of these products (fullerenes) is very recent and is carried out for quantities of the order of a gram, under the name Nanolub ™ by the start-up "Applied Materials".

The method used consists in reacting at temperatures above 800 ° C nanoparticles of MO3 = (M = Mo, W) in the form of platelets or needles with gaseous hydrogen sulphide or selenide (H₂S and H₂Se). The developed reactor allows the continuous introduction into the furnace of MO3 nanoparticles transported by a carrier gas H₂/NOT₂. The carrier gas also has the role of reducing the metal from oxidation degree VI to oxidation degree IV. It is assumed that the mechanism of formation of the hollow particles of MX₂ involves the formation of an intermediate MX phase₃ amorphous.

Other methods are in development, but are still at the stage of laboratory experimentation.

For example, application WO 00/66485 relates to the synthesis of transition metal chalcogenide nanotubes from a transition metal or from a compound containing said transition metal, for example an oxide, water vapor and water. a gaseous chalcogen hydride or alternatively of hydrogen and chalcogen in the vapor phase.

US Pat. No. 6,217,843 describes a process for preparing, on the one hand, nanoparticles of metal oxides containing insertions of metal particles, and on the other hand structures of inorganic fullerene type, with interlayers and / or inclusions. metallic. These structures are metal chalcogenides obtained from the nanoparticles considered. This process consists in heating a group I metal with water vapor or by electron beam evaporation of the group I metal in question with water or any other suitable solvent, in the presence of a group II metal salt. . The process then consists either in recovering the group I metal oxide doped with a group II metal, or in continuing with sulphurization.

Patent application WO 0166676 relates to applications of metal chalcogenide fullerenes and only mentions as synthesis of the compounds the reaction of a chalcogen hydride with a transition metal oxide.

Patent application WO 0104382 relates to inorganic fullerene type metal chalcogenide nanoparticles comprising one or more layers of desired sizes and shapes. These nanoparticles are produced by sonoelectrochemical synthesis in the liquid phase.

Patent application WO 9744278 relates to a process for preparing nanoparticles of a metal chalcogenide, for example of transition metal, of inorganic fullerene type having a desired size and shape in high yields and in macroscopic quantities. The method comprises the steps of dispersing solid particles of at least one non-volatile metallic material having the preselected size and shape, heating the solid particles of the non-volatile metallic material in a reducing gas atmosphere containing at least one chalcogen for a time and at a temperature sufficient to allow the precursor of the metallic material and the chalcogen to react and form at least one layer of metallic chalcogenide coating the surface of solid particles to form the particles of the fullerene type, in recovering the macroscopic quantities thus obtained of the metal chalcogenide.

In general, the major drawbacks of the techniques described in the prior art are linked to the use of the highly toxic gases H2S and H₂Se and in the small quantities produced, of the order of a few grams per hour. Moreover, the synthesis of inorganic fullerenes of MoS₂ and WS₂ among others and of all the transition metal chalcogenide nanoparticles from a solid / gas chemical reaction does not make it possible to control the size and shape of the nanoparticles and in particular of the fullerenes, which affects the tribological performance of these materials especially under high load and sliding speed conditions.

It is thus necessary either to improve the reaction synthesis processes in the gas phase, or to develop new synthetic routes.

The inventors have developed, which is the subject of the present invention, a method for synthesizing nanoparticles of chalcogenides of metals M_atX_b which solves these drawbacks and in particular the problems of toxicity of the compounds necessary for this synthesis, while allowing production on a larger scale.

To this end, the process, according to the present invention, for the synthesis of metal chalcogenide nanoparticles with a lamellar structure is characterized in that it comprises pyrolysis of a liquid aerosol obtained from a solution of at least one precursor of a metal (M) and of a chalcogen (X), or of at least one precursor of said metal (M) and of at least one precursor of said chalcogen (X), dissolved in a solvent, said solution being sprayed into fine droplets suspended in a carrier gas.

More particularly, the method according to the present invention comprises the following steps:

- the formation of a solution of said at least one precursor of a metal and of a chalcogen, or of said at least one precursor of said metal and of said at least one precursor of said chalcogen in a solvent,

- the spraying of said solution in the form of liquid aerosol by a nebulizer, in particular of the pneumatic or ultrasonic type, in which the carrier gas circulates,

- injection of the aerosol into a heated oven to evaporate the solvent and react and / or decompose said precursor (s) of the metal and of the chalcogen so as to form the nanoparticles,

- transport by the carrier gas of the nanoparticles to the exit of the furnace, and

- recovery of the nanoparticles at the outlet of the oven.

Advantageously, said precursor of the metal and of the chalcogen contains both the metal and the chalcogen.

Such a precursor corresponds in particular to formula (A) _vs M (X)_din which A is a cation such that K⁺, N / A⁺ or NH₄⁺, M is a metal and X is a chalcogen, c and d representing the number of cations and chalcogens respectively.

In one embodiment of the invention, said metal is a transition metal. We will mention Ti, Zr, Hf, V, Nb, Ta, Mo, W, Re, Ta, Co, Ni, Pt, Pd, Cr and Ru.

In another embodiment of the invention, said metal is a metal from group III of the periodic table of the elements, such as Ga or In.

In yet another embodiment, said metal is a metal from group IV of the Periodic Table of the Elements. They are for example Sn, Pb, Bi and Ge.

The chalcogen is chosen from oxygen, sulfur, selenium and tellurium.

Particularly, at least one of the precursors of the transition metal and of said chalcogen is a tetrathiometallate or a tetraselenometallate.

The metal of the tetrathiometallate or of the tetraselenmetallate is advantageously chosen from molybdenum and tungsten.

Preferably, said carrier gas is a neutral gas such as nitrogen and argon, and / or hydrogen.

Advantageously, said solvent is a polar solvent.

More advantageously still, said solvent is chosen from water and ethanol, or a mixture thereof.

The method according to the invention consists firstly in spraying in the form of a liquid aerosol, that is to say in very fine droplets suspended in a gas, a solution containing one or more dissolved precursors of the metal and of the chalcogen, to using a nebulizer in which a neutral gas (argon or nitrogen) circulates. The liquid aerosol thus formed is then injected into an oven brought to a high temperature (typically several hundred degrees), causing the droplets on the one hand to evaporate the solvent and on the other hand to the reaction or decomposition of the particles. precursors to form fullerenes or nanotubes of MX2. These are transported by the neutral gas and finally collected at the exit of the furnace.

The process according to the invention is based on the pyrolysis of a liquid aerosol at high temperature. A solution of reaction precursors is sprayed with the aid of an inert gas into droplets of micron size which are suspended in the gas (liquid aerosol) and which each contain a certain quantity of dissolved precursors. This liquid aerosol is injected into a high temperature oven in which the droplets are converted into particles by direct pyrolysis, thus forming a solid aerosol, that is to say particles in suspension in a gas. The solid aerosol is transported by the carrier gas and collected at the outlet of the oven on a filter in powder form, or collected by bubbling in a solution which will then be centrifuged.

Many physical and chemical processes can take place as droplets, and then particles, are suspended in the gas phase. They include: the evaporation of the solvent from the droplets, the initiation of crystallization of the solute in the droplet at the solid / liquid interface, the reaction or decomposition of the precursors in the particle, diffusion processes in the particle which can lead, among other things, to changes in morphology, evaporation of volatile species such as metal oxides, condensation of particles on the walls of the furnace, the formation of new particles by coagulation of the initial particles.

Thus and by way of example, depending on the process conditions and the properties of the material, dense or porous particles can be formed.

The implementation of the above provisions, alone or in combination, leads to nanoparticles of the nanotubes, fullerenes or nanoboxes type.

Said nanoboxes constitute new products and, as such, also fall within the scope of the invention. They are straight and rectangular parallelepipeds, closed, generally hollow.

Such nanoboxes, and in general the nanoparticles obtained, are of great interest in the development of various industrial products, such as catalysts, lubricants, in the medical field, or as intercalating agents, for example for the storage of hydrogen.

The invention will be better understood on reading the detailed description given below given by way of example and with reference to FIGS. 1 and 2 which represent fullerenes, and to FIGS. 3 and 4 which respectively represent nanoboxes in profile or front.

A solution of ammonium tetrathiometallate or ammonium tetraselenometallate MX₄(NH₄)₂ with M = molybdenum (Mo) or tungsten W in oxidation degree VI; X = sulfur (S) or selenium (Se), at a concentration of 4 x 10⁴ M, is sprayed using a commercial nebulizer and an inert gas (Ar or N₂). A liquid aerosol consisting of droplets of micron size is thus formed and transported through a tubular furnace heated between 600 and 1050 ° C., by the neutral gas of the liquid aerosol which then serves as carrier gas. The solution can be made from deionized water or from an alcohol of the ethanol type, for example.

The advantage of using an alcoholic solvent comes from its low surface tension. Its viscosity facilitates the formation of very fine droplets, while its low boiling point allows easy evaporation. At high temperature, a pyrolytic decomposition / reaction of the solution containing the precursor MX takes place₄(NH₄)₂.

The sprayer is a glass chromatography spray head as marketed by Bioblock Scientific. The optimum gas pressure circulating in the sprayer is 1.2 to 1.4 bar. The Thermolyne brand tube furnace can rise to a temperature of 1200 ° C in steps of 10 ° C. Different solvents such as water, ethanol, methanol or propanol have been used, in which the tetrathiomolybdate salt (NH₄)₂MoS₄ or tetrathiotunsgtate (NH₄)₂WS₄ (Aldrich).

Spherical fullerenes of MoS₂ were obtained using the ethanol solvent when the spraying is carried out at 750 and 900 ° C.

Spherical particles 200 nm in diameter are observed with SEM. They have a granular appearance. These same particles observed with the METHR are hollow. They are formed from sheets of MoS₂ (shots by van der Waals) closed in on themselves. When the solvent used is distilled water sprayed at 750 ° C., the particles appear to be formed from an assembly of spherical fullerenes of a few nm to several tens of nm in diameter.

When spraying is carried out at a higher temperature, for example at 1050 ° C, the MoS fullerenes₂ are larger in size with a diameter of about 100 nm.

By way of illustration, Figures 1 and 2 show fullerenes synthesized at 900 ° C from an ethanolic (Figure 1) and aqueous (Figure 2) solution of (NH₄)₂MoS₄, observed by high resolution transmission electron microscopy (METHR).

In the case of the WS₂ formed from (NH₄)₂ WS₄ dissolved in ethanol sprayed at 750 and 900 ° C, the SEM images show the formation of parallelepipedal particles and, where appropriate, spherical particles.

The study at the METHR shows that these particles, as illustrated by Figures 3 and 4, are entirely surrounded by sheets of WS₂ well aligned. They contain sheets of WS₂ curved.

The present invention has the advantage of allowing the manufacture of a wide variety of materials in an advantageous manner, and at low cost given the reduced number of operations to be implemented. The process of the invention makes it possible to obtain particles of well-defined dimensions by controlling the size of the droplets generated by the sprayer and also to form dense or porous particles by controlling the dynamics of evaporation of the droplets.

The experimental conditions of the process according to the invention therefore offer control over the size, morphology and the degree of crystallinity of the nanoparticles formed and therefore their physical, mechanical, optical, electronic and catalytic properties.