Method for preparing colored particle material by heterogeneous nucleation

Abstract

To provide a novel method for preparing a colored particle material, which is simple and economical, guarantees optimal stability of color, has high modularity, enables access to a very wide variety of colors and types of colored materials, and avoids solvent transfer as much as possible.SOLUTION: The present invention relates to a method for preparing a colored material by heterogeneous nucleation of metal nanoparticles, the nanoparticles exhibiting optical properties based on a surface plasmon phenomenon. The present invention also provides the resulting colored material and compositions containing them. The method for preparing a colored particle material by heterogeneous nucleation particularly includes mixing, at room temperature, a suspension including: a salt of at least one metal element, the metal element exhibiting a plasmon effect; at least one reducing agent; and at least one particle substrate. The mixing of the suspension forms a colored particle material.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to the field of material coloration. In particular, the present invention is directed to a method for preparing colored materials by heterogeneous nucleation of metal nanoparticles, said nanoparticles exhibiting optical properties based on the surface plasmon phenomenon. The present invention is also directed to the colored materials obtained by this method, as well as compositions containing them. [Background technology]

[0002] The use of metals in the form of nanoparticles can impart a color to a solid substrate or suspension containing the nanoparticles that differs from the original color of the bulk metal (i.e., the metal not in the form of nanoparticles). Indeed, when metal nanoparticles are subjected to an electromagnetic field with a wavelength much longer than their size, the free electrons in the conduction band located on the surface of the nanoparticles are subjected to the same electromagnetic field and collectively oscillate in phase. When the frequency of the incident wave matches the characteristic frequency of these oscillations, a resonance phenomenon called surface plasmon resonance occurs. This resonance can occur particularly in the visible, ultraviolet (UV), and infrared regions. These metal elements that exhibit the plasmonic effect are discussed, and the metal elements are in nanometer form.

[0003] The plasmon resonance frequency depends on various parameters, namely: - the properties of metals, - the size and shape of the nanoparticles, - nanoparticle distribution, especially interparticle distance, and - the optical properties of the substrate or surrounding medium, including in particular the refractive index; is affected by.

[0004] Indeed, the refractive index makes it possible to adjust the color, which is perceived differently depending on the refractive index, so that for example the color of an object will not be perceived the same if it is in air or in water.

[0005] Interestingly, by adjusting these various parameters, it is possible to change the color of the nanoparticles throughout the visible range, and even to shift the resonant frequency into the UV and near-infrared range.

[0006] To achieve this, it is known to color materials using so-called preformed nanoparticles that exhibit surface plasmon resonance. This technology is very promising and offers significant advantages over traditional coloring methods. In fact, this technology makes it possible to produce a wide variety of colors, especially vibrant colors, without using pigments or dyes that pose risks to human health and the environment, which is particularly suitable and useful for certain applications such as tableware, cosmetics, jewelry, watchmaking, food processing, or pharmaceuticals.

[0007] However, the use of these preformed nanoparticles has some disadvantages. For example, the production of powdered or particulate materials with a specific color requires precise control of the concentration of the preformed nanoparticles and the colloidal suspension. In fact, the size and interaction between the preformed nanoparticles in the suspension pose several constraints: - Increased viscosity, - reduced suspension stability, - Difficulty dispersing the material in the suspension.

[0008] For all these reasons, the concentration of preformed nanoparticles in the colloidal suspension needs to be closely monitored and controlled, thus limiting the concentration of preformed nanoparticles in the suspension and indirectly limiting the color intensity of the material, resulting in a good production yield.

[0009] The use of preformed nanoparticles means that complex methods must be implemented, involving many steps, using reagents that are often toxic to human health and / or the environment, and requiring expensive and energy-intensive infrastructure.

[0010] For example, French Patent No. 3096685 proposes a new method for preparing micrometric colored particulate materials (i.e. in powder form) from a substrate using at least one gold salt or at least gold nanoparticles, based on the principle of heterogeneous nucleation.

[0011] Heterogeneous nucleation allows for the nucleation and growth of nanoparticles to be particularly promoted on the surface of a solid substrate, with the solid substrate acting as a catalyst for the reaction. In this way, the solid substrate in particulate form is colored by the formation of colored nanoparticles on its surface, forming a colored particulate material. This method is particularly advantageous in that it reduces the possibility of demixing when shaping the material for its final use.

[0012] However, the use of this method (French Patent No. 3096685) has the disadvantage of being limited to a specific type of particle substrate, thus reducing its industrial applicability. Therefore, this method cannot be universally adapted or adapted to a wide range of supports. Indeed, the method is not suitable, for example, for coloring large quantities of particle substrates. In fact, the implementation of heterogeneous nucleation catalyzed by thermal treatment requires systematic adaptation of parameters between the method developed on a laboratory scale and the industrial scale, so as to maintain the same conditions for the entire volume of the particle substrate. These constraints, on the one hand, increase the development time of new types of coloring materials, and, on the other hand, limit their use to specific types of particle substrates. Furthermore, the need to heat the reaction medium inevitably involves expensive and energy-intensive infrastructure.

[0013] Furthermore, growing awareness of climate change issues, linked to the global goal of reducing greenhouse gas emissions, is encouraging manufacturers to explore simple, environmentally respectful, universal and effective alternative methods for coloring materials.

[0014] Thus, there is a need for a new method for coloring materials that is designed to minimize environmental impact while maintaining compatibility with a wide variety of substrates and optimal effectiveness, thus overcoming the disadvantages of the prior art. [Prior art documents] [Patent documents]

[0015] [Patent Document 1] French Patent Invention No. 3096685 Summary of the Invention [Problem to be solved by the invention]

[0016] In order to meet this need, the present invention proposes a novel method that makes it possible to overcome the above-mentioned disadvantages, in particular by being compatible with a wide variety of substrates, and that consumes little energy while maintaining an optimal coloring effect.In particular, the present invention provides a novel method for preparing colored particulate materials, which can change color, possibly under the influence of a stimulus, said method being simple, economical, guaranteeing optimal color stability, and having a high modularity, allowing access to a very large variety of colors and types of coloring materials, and avoiding solvent migration as much as possible. [Means for solving the problem]

[0017] In the present invention, the substrate is a particulate material, i.e. a powdery material, on which metal ions, which after nucleation form colored nanoparticles, preferably nucleate and grow to form a colored particulate material, which comprises at least one nanoparticle on its surface, said nanoparticle exhibiting plasmonic properties.

[0018] The present invention therefore relates to a method for preparing a colored particulate material by heterogeneous nucleation, comprising carrying out a single step a) of mixing, at room temperature, a suspension comprising: - a salt of at least one metal element, said metal element exhibiting a plasmonic effect, at least one reducing agent, and - at least one particle substrate;

[0019] According to one variant, the method according to the invention comprises a pretreatment step of the particulate substrate, located before step a), in order to prepare and condition the particulate substrate to be colored using the method according to the invention. This pretreatment step is preferably carried out by heat treatment and / or alkaline treatment and / or functionalization with coupling agents, which makes it possible to activate or add or modify the surface charge on said substrate.

[0020] Such a process allows for the formation of colored particle materials while overcoming the disadvantages of the prior art. Because the process is carried out at room temperature, the process according to the invention advantageously has low energy consumption while having acceptable reaction times, thus reducing operating costs and greenhouse gas emissions compared to comparable processes of the prior art.

[0021] Indeed, the inventors have surprisingly discovered that heterogeneous nucleation carried out at room temperature, i.e., at temperatures between 19 and 25° C., makes it possible to carry out heterogeneous nucleation at low energy costs while maintaining optimal coloring efficiency. To achieve this, the inventors have identified, inter alia, at least one interesting reducing agent.

[0022] Thus, according to one preferred embodiment, the reducing agent is selected from the group consisting of sodium tetrahydroborate (NaBH4), hydroquinone, tetrabutylammonium borohydride (TBH4), hydrazine, propanal, glucose, sucrose, citric acid, ascorbic acid, citrate, triethanolamine (TEA), hydroxylamine and mixtures thereof.

[0023] A particularly preferred reducing agent is triethanolamine (TEA), which is particularly suitable for carrying out heterogeneous nucleation at room temperature, i.e., the reaction does not need to be catalyzed by heat treatment, while providing acceptable reaction times. Alternatively, this reducing agent can be used at higher temperatures. Thus, TEA can be used at temperatures between 1°C and 100°C.

[0024] Thus, according to one variant, the present invention is also directed to a method for preparing a colored particulate material, comprising a single step a) of mixing, at a temperature between 1°C and 100°C, a suspension comprising: - a salt of at least one metal element, said metal element exhibiting a plasmonic effect, - Triethanolamine (TEA), and - at least one particle substrate;

[0025] The method according to the invention therefore makes it possible to obtain a wide spectrum of colours and is highly modular, thus offering the ability to colour a wide variety of materials with a wide variety of colours.

[0026] Advantageously, the method according to the invention is suitable for colouring any type of material with salts of any type of metallic element, said metallic element exhibiting a plasmonic effect.

[0027] According to one particularly advantageous embodiment of the invention, the salt of a metal element, which metal element exhibits a plasmonic effect, is selected from gold salts, silver salts, copper salts, aluminum salts, magnesium salts, indium salts, nickel salts, gallium salts, cobalt salts, iron salts, palladium salts, ruthenium salts, rhodium salts, platinum salts and mixtures thereof.

[0028] A salt of a metal element is a salt in which the metal element is in an oxidation state. For example, gold salt (+III) is a salt in which gold is in the oxidation state (+III).

[0029] According to one embodiment, the gold salt(+III) is chosen from tetrachloroauric acid HAuCl4, potassium tetrachloroaurate KAuCl4 and mixtures thereof, preferably KAuCl4.

[0030] According to one embodiment, the silver salt (+I) is chosen from AgNO3, AgClO4, Ag(acac), AgCl, Ag2SO4 and mixtures thereof, preferably AgNO3.

[0031] According to one embodiment, the copper salt (+II) is selected from copper chloride (CuCl), copper acetate (Cu(CHCOO)), copper sulfate (CuSO), Cu(acac), CuO(+I), Cu(OH), Cu(NO) and mixtures thereof, preferably CuCl.

[0032] In the present invention, any type of particle material can be used as the particle substrate for coloring, and therefore the particle substrate can be selected from organic materials, inorganic materials, and hybrid materials.

[0033] The method according to the invention is thus suitable for coloring any type of material, offering great versatility in the choice of material that one wishes to color. The method is thus versatile, simple and effective, while at the same time consuming little energy.

[0034] When the particle substrate is an inorganic substrate, said substrate can be selected from all known mineral substances, preferably from the group consisting of silicates, glasses, metal oxides, rare earth oxides, metals, frits, enamels, glazes, ceramics, absorption pigments and mixtures thereof.

[0035] According to another aspect, the present invention also relates to a coloured particulate material obtainable by a method according to any one of the embodiments described above.

[0036] Finally, according to a last aspect, the present invention relates to a colouring composition comprising at least one colouring particulate material according to the invention and at least one solvent in which said colouring particulate material is dispersed. DETAILED DESCRIPTION OF THE INVENTION

[0037] Detailed Description of the Invention [Definition]

[0038] By "stabilizer" in the sense of the present invention is meant charged molecules that allow electrostatic stabilization when adsorbed onto the surface of nanoparticles, or polymers that allow steric repulsion between nanoparticles.

[0039] By "structuring agent" within the meaning of the present invention is meant any molecule that is capable of adsorbing to the surface of nanoparticles and thereby promoting the growth of specific crystal planes of the nanoparticles, thus allowing the formation of anisotropic particles.

[0040] "Heterogeneous nucleation" for the purposes of the present invention refers to the reduction of metal ions on the surface of a solid support, which acts as a catalyst for nucleation, as opposed to homogeneous nucleation, in which nuclei form spontaneously in the reaction medium.

[0041] "Particle substrate" or "particulate material" in the sense of the present invention means a finely divided material that can be equated with a powder containing an aggregate of particles. The substrate is in powder form, on which the salt of the metal element is deposited, forming nanoparticles by nucleation. Advantageously, the maximum dimension of the particle substrate is at most 1 mm.

[0042] In particular, the "largest dimension" of a nanoparticle or a particle substrate means, within the meaning of the present invention, the largest distance separating two points located on the outer contour of said nanoparticle or said particle substrate.

[0043] "Room temperature" in the sense of the present invention means a temperature between 19 and 25°C.

[0044] "Acceptable reaction time" in the sense of the present invention means a reaction time of 30 minutes or less, preferably less than 15 minutes. The reaction is considered complete when the observed color is stable.

[0045] [Method for preparing colored particle material]

[0046] The present invention is thus directed to a method for preparing colored particulate materials by heterogeneous nucleation, which is carried out in a single step and with low energy consumption.

[0047] The method according to the invention thus comprises a step a) of mixing at room temperature a suspension, said suspension comprising: - a salt of at least one metal element, said metal element exhibiting a plasmonic effect, at least one reducing agent, and - at least one particle substrate;

[0048] The one-step method forms a colored particulate material from the particulate substrate, the colored particulate material including at least one nanoparticle formed on a surface of the colored particulate material.

[0049] According to one object of the present invention, the particulate substrate used in the method according to the present invention can be pre-colored, either naturally or due to the presence of nanoparticles formed on its surface. The nanoparticles can be deposited according to the method of the present invention. The method according to the present invention can therefore be repeated multiple times and thus can include multiple cycles, each cycle allowing the formation of a colored particulate material. This is particularly advantageous for obtaining a colored particulate material exhibiting the desired coloration.

[0050] For example, the first cycle can be carried out from a gold salt, then the second cycle from a silver salt, and so on.

[0051] The method according to the invention thus makes it possible to prepare colored particulate materials by heterogeneous nucleation, while overcoming the disadvantages of the prior art. Indeed, the method according to the invention consumes less energy, thus reducing operating costs and greenhouse gas emissions compared to comparable methods of the prior art, while preserving a similar coloring effect. The colorimetric parameters of the colored particulate materials are thus similar to those obtained by methods originating from the prior art, in particular the method according to French Patent No. 3096685.

[0052] The method according to the invention is simple insofar as the formation of nanoparticles on the surface of the particulate material is carried out in a single step at room temperature. Indeed, carrying out the method at room temperature has many advantages, such as a better examination of the parameters related to nucleation, especially during heterogeneous nucleation. Conversely, if carried out at higher temperatures, nucleation may occur too quickly, which would require adjustments, such as the addition of an additional temperature stabilization step before reaching the final target temperature. As a result, these limitations are avoided at room temperature.

[0053] Furthermore, the method according to the invention is also suitable for coloring materials on both a laboratory and an industrial scale, regardless of the amount of material to be colored. Thus, the method according to the invention shows better reproducibility and time savings compared to the methods described in the prior art. Indeed, the method according to the invention avoids the influence of temperature on the nucleation and growth of nanoparticles on the surface of the particulate substrate, thus making it easier to transfer to an industrial scale.

[0054] According to one particular subject of the invention, the suspension of step a) comprises water and / or an organic solvent.

[0055] According to one particular object of the invention, the suspension of step a) is an aqueous suspension. Preferably, said suspension of step a) comprises at least 5% by weight of water relative to the total weight of said suspension, more preferably at least 10% by weight of water relative to the total weight of the suspension.

[0056] According to another object of the present invention, the suspension of step a) is an organic suspension. When the suspension of step a) is an organic suspension, said suspension preferably comprises an alcohol such as ethanol.

[0057] According to another embodiment of the invention, the suspension of step a) may comprise a mixture of water and organic solvent in a ratio of 5:95 to 95:5, preferably 60:40 to 40:60.

[0058] According to a particularly preferred embodiment of the present invention, the mixing in step a) is carried out at room temperature, i.e., at a temperature between 19°C and 25°C. The heterogeneous nucleation carried out during step a) is therefore carried out without a heat treatment. In this way, the reaction can be carried out efficiently and independently of the amount of material to be pigmented.

[0059] Advantageously, the heterogeneous nucleation carried out during step a) does not require a heat treatment, which drastically reduces the energy consumption necessary for carrying out the method.

[0060] However, according to one variant of the invention, when the reducing agent is TEA, the method can be carried out at a temperature between 1° C. and 100° C. Preferably, the method is carried out at room temperature. However, for some specific particle substrates, the method can be carried out at a temperature above 25° C. The solvent is preferably aqueous, but according to one variant, it can also be a hydroalcoholic solvent.

[0061] Preferably, the mixing step a) is carried out, irrespective of the embodiment described above, by stirring, preferably by mechanical stirring, in particular using a paddle mixer or a bar magnet, thus facilitating the reaction of heterogeneous nucleation and the contact of the metal element salt with the particle substrate.

[0062] According to another particularly preferred object, the mixture of step a) comprises a salt of at least one metal element, said metal element exhibiting a plasmonic effect, said salt being selected from the group consisting of gold salts, silver salts, copper salts, aluminum salts, magnesium salts, indium salts, nickel salts, gallium salts, cobalt salts, iron salts, palladium salts, ruthenium salts, rhodium salts, platinum salts and mixtures thereof.

[0063] Even more preferably, the mixture of step a) comprises at least one metal element salt exhibiting a plasmonic effect, selected from among gold salts, silver salts, copper salts and mixtures thereof.

[0064] In the present invention, the metal element salt is a precursor of nanoparticles that will be formed by heterogeneous nucleation on the particle substrate or, optionally, on the particle material that has been colored according to the method of the present invention, in order to obtain the colored particle material according to the present invention.

[0065] According to another particular embodiment, the mixture of step a) comprises at least two distinct metal element salts, each of which exhibits a plasmonic effect. According to one variant of the invention, the mixture of step a) comprises at least two distinct metal element salts, each of which is added sequentially to the mixture, each of which exhibits a plasmonic effect. In other words, a first metal element salt is added to color the particle substrate, and a second metal element salt is added to color the colored particle material using the first salt, thereby obtaining a colored particle material, which comprises at least two distinct colored nanoparticles.

[0066] Preferably, the mixture of step a) comprises at least one gold salt and one silver salt, which can be added sequentially or simultaneously.

[0067] According to another particular embodiment, the mixture of step a) comprises at least three distinct metal element salts exhibiting the plasmonic effect. When the mixture of step a) comprises three distinct salts, it preferably comprises at least one gold salt, one silver salt, and one copper salt. The salts can be added sequentially or simultaneously.

[0068] Particularly advantageously, the association and combination of several distinct metal element salts, each exhibiting a plasmonic effect, allows for a wide variety of possible colors, and therefore for that matter, the wide variety of colored particulate materials obtainable using the method according to the invention.

[0069] According to one embodiment of the present invention, the mixture of step a) preferably comprises at least one reducing agent selected from the group consisting of sodium tetrahydroborate (NaBH4), hydroquinone, tetrabutylammonium borohydride (TBH4), hydrazine, propanal, triethanolamine (TEA), borane, organic acids, amines, sugars, and mixtures thereof.

[0070] More preferably, the mixture of step a) comprises at least one reducing agent selected from sodium tetrahydroborate (NaBH4), hydroquinone, tetrabutylammonium borohydride (TBH4), hydrazine, propanal, glucose, sucrose, citric acid, ascorbic acid, citrate salts, triethanolamine (TEA), hydroxylamine, and mixtures thereof.

[0071] When the reducing agent is selected from amines, it is preferably selected from triethanolamine (TEA) and hydroxylamine.

[0072] If the mixture of step a) comprises at least one reducing agent selected from sugars, it is preferably selected from glucose or sucrose.

[0073] If the mixture of step a) comprises at least one reducing agent selected from organic acids, it is preferably selected from ascorbic acid and its derivatives and citric acid and its derivatives.

[0074] According to another embodiment, step a) comprises citrate as reducing agent.

[0075] Preferably, the mixture of step a) comprises a molar / mass ratio of the reducing agent to the particle substrate of 10 to 6500 (mol / g), more preferably 50 to 820 (mol / g), otherwise a homogeneous nucleation reaction may occur.

[0076] Preferably, the maximum size of the particle substrate desired to be colored is between 10 nm and 1 mm. In fact, the heterogeneous nucleation carried out during step a) is particularly effective on such particle substrates having such dimensions. Otherwise, the nanoparticles would not be able to deposit on the surface of the substrate. Furthermore, if the substrate is larger than 1 mm, moderate stirring would not be able to keep the various elements in suspension.

[0077] The substrate on which the heterogeneous nucleation reaction takes place, i.e. the particulate substrate that it is desired to color, can be of any shape, in particular in the shape of platelets such as polyhedrons or flakes, or in the shape of beads. The method according to the invention is therefore universal and can color many types of particulate substrates.

[0078] The particle substrate of the mixture of step a) can be an organic or inorganic or hybrid material.

[0079] When the particulate material is inorganic, it is selected from silicates, glasses, metal oxides, rare earth oxides, metals, frits, enamels, glazes, ceramics, absorbing pigments and mixtures thereof.

[0080] As non-limiting examples, the mixture of step a) can include inorganic particulate substrates such as silica, quartz, feldspar, limestone, kaolin, metal oxides, aluminates, alumina, zirconium dioxide, non-oxides, ultra-refractory ceramics such as borides, carbides, nitrides of refractory metals, silicon or magnesium reinforced ceramics, metals such as aluminum, copper, silver, steel, and combinations thereof.

[0081] Very advantageously, the method according to the invention is therefore suitable for coloring a wide variety of materials.

[0082] According to another embodiment, the particulate substrate may be organic, preferably cellulose.

[0083] Advantageously, the method according to the invention is carried out at room temperature and thus makes it possible to colour organic and / or heat-sensitive materials, thus offering a versatile alternative method suitable for colouring heat-sensitive materials.

[0084] In the present invention, the particle substrate can be colorless, transparent, translucent, opaque, or colored. In fact, the substrate can be colored naturally or by the method according to one particular object. Such an embodiment is particularly useful when it is desired to repeat the method according to the present invention. For this reason, the method according to the present invention in this case includes multiple successive cycles, such as using the colored material obtained in the first cycle as the substrate in the second cycle.

[0085] Thus, according to one object of the present invention, the particulate material of step a) may comprise particles covered with a layer comprising at least one metal oxide or silicon dioxide, said metal oxide or silicon dioxide layer advantageously making it possible to facilitate the attachment of nanoparticles formed by heterogeneous nucleation to the surface of the particulate material.

[0086] According to another object, step a) is carried out in less than 30 minutes, more preferably less than 15 minutes.Thus, the method according to the invention advantageously consumes less energy than the prior art methods, while showing similar coloring results and execution times.

[0087] According to another embodiment of the present invention, the mixture of step a) can also contain at least one stabilizer. If the suspension also contains at least one stabilizer, it is selected from citrate, malate, succinate, citric acid, polyvinyl alcohol, polyacrylic acid, poly(ethylene glycol) (PEG), amino derivatives such as diethylamine, sulfur derivatives such as thiols, triphenylphosphine-based ligands, dendrimers, amino surfactants such as cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), PVP (poly-n-vinylpyrrolidone), polyelectrolytes, NMP-based monomers, and mixtures thereof.

[0088] According to another embodiment, the mixture of step a) can also contain at least one structuring agent. This advantageously allows for directing growth on specific crystal planes, and thus allows for the shape of the nanoparticles formed on the surface of the material to be adjusted. Controlling the shape of the nanoparticles is an important parameter, since it allows for the utilization of the plasmon resonance of the metal element, and thus allows for the control of the final color of the colored material.

[0089] If the mixture of step a) contains at least one structuring agent, it is preferably selected from citrate, malate, succinate, polyvinylpyrrolidone (PVP), surfactants, and mixtures thereof. More preferably, the surfactant is selected from cetyltrimethylammonium bromide (CTAB), diethylamine (DEA), ethylenediaminetetraacetic acid (EDTA), and mixtures thereof.

[0090] According to another object, the process according to the invention is carried out in a single step, but can include additional steps, in particular an isolation step, which makes it possible to separate the coloring material in solid form from the liquid phase.

[0091] Thus, according to one embodiment of the present invention, the method according to the present invention comprises the implementation of the following steps: a. Mixing at room temperature a suspension containing: - at least one metal element salt exhibiting a plasmonic effect, at least one reducing agent, and - at least one particle substrate, b. Isolation of the colored particulate material obtained in step a), said colored particulate material having at least one nanoparticle formed on its surface by heterogeneous nucleation.

[0092] Preferably, step b) of isolating the colored particle material obtained from step a) comprises the following substeps: i. solid / liquid separation of the mixture of step a) to isolate the colored particulate material from a liquid phase, said liquid phase containing free elements in suspension; and ii. Drying to obtain the colored particulate material in dry form.

[0093] Preferably, the solid / liquid separation substep i) is carried out using at least one solid / liquid separation technique selected from filtration, sedimentation, centrifugation, evaporation, freeze-drying and combinations thereof.

[0094] If the solid / liquid separation substep i) is carried out using at least two solid / liquid separation techniques, these steps are carried out sequentially, which advantageously reduces the duration of the isolation step.

[0095] According to one embodiment, step b) of isolating the colored particulate material may further comprise at least one additional sub-step of washing or rinsing the colored particulate material, said step being preferably carried out after separation and before drying.

[0096] The washing or rinsing step is preferably carried out using water and / or an organic solvent. If the step involves washing with water and an organic solvent, this can be carried out simultaneously or sequentially.

[0097] Thus, according to one particularly preferred embodiment, the method according to the invention comprises the following sequential steps: a. a mixture according to any one of the embodiments described above, and b. Isolation of the colored particulate material obtained at the end of step a) from the liquid phase, said step advantageously comprising the following substeps: i. solid / liquid separation of the mixture of step a) to isolate the colored particulate material from the liquid phase; and ii. Optionally, a washing step of the isolated particulate material with water and / or an organic solvent; and iii. Drying the particulate material and recovering the colored particulate material in dry and powder form.

[0098] According to another embodiment of the present invention, the step b) of isolating the colored particulate material can be repeated several times, preferably 1 to 5 times, more preferably 1 to 3 times, each repetition forming one cycle.

[0099] Thus, the method preferably comprises a step a) and an isolation step b), said isolation step comprising the following sequential sub-steps: i. solid / liquid separation of the mixture of step a) to isolate the colored particulate material from the liquid phase; ii. Washing of particulate material from previous process; iii. Solid / liquid separation of the mixture obtained from the previous step; and iv. Drying to obtain a colored particulate material comprising at least one nanoparticle formed on its surface by heterogeneous nucleation.

[0100] The successive washing steps advantageously make it possible to remove excess organic compounds present in the suspension, such as reducing agents, stabilizing agents, structuring agents, which may cause problems in the final coloration.

[0101] Preferably, drying is carried out using a dryer.

[0102] According to one particular object of the invention, at least one nanoparticle is formed on the surface of the colored particulate material, said nanoparticle having a maximum dimension between 2 nm and 100 nm.

[0103] Advantageously, the nanoparticles formed on the surface of the colored particulate material have a substantially hemispherical shape, which is the most thermodynamically stable shape and is therefore preferred for improving color stability. However, according to certain embodiments, the shape of the nanoparticles formed on the surface of the colored particulate material can be adjusted according to the knowledge of a person skilled in the art.

[0104] According to another object of the present invention, the particle substrate may be porous. The use of such a porous substrate advantageously makes it possible to increase the accessible and available surface area for the deposition of metal element salts that form nanoparticles by nucleation. In this situation, the nanoparticles formed on the surface of the colored particle material advantageously have an elongated shape, such as the shape of small rods, especially if the size of the pores allows. The use of porous particle materials thus makes it possible to adjust the shape of the nanoparticles and obtain a wider range of colors.

[0105] According to another embodiment, the method according to the invention comprises the implementation of the following steps: a. Mixing at room temperature a suspension containing: - at least one metal element salt exhibiting a plasmonic effect, at least one reducing agent, and - at least one particulate material, which forms a colored particulate material; b. optionally, washing the mixture; c. adding at least one metal element salt separate from the mixture of step a) to the mixture obtained from step a) or b); d. Isolating the colored particulate material obtained from the previous step, said colored particulate material having at least two distinct nanoparticles formed on its surface by heterogeneous nucleation.

[0106] According to another embodiment of the present invention, the method according to the present invention comprises a pretreatment step of the particulate substrate prior to step a) in order to prepare and condition the particulate substrate to be colored using the method according to the present invention. This pretreatment step is carried out by heat treatment and / or alkali treatment, which makes it possible to activate the surface charge of the substrate and improve the nucleation of nanoparticles and their fixation to the substrate. This pretreatment step is followed by cooling to room temperature, followed by filtration and subsequent drying to obtain a powder containing a large number of particulate substrates suitable for coloring using the method according to the present invention.

[0107] In another embodiment, the method according to the present invention includes a pretreatment step of the particle substrate prior to step a) to prepare and condition the particle substrate to be colored using the method according to the present invention. This pretreatment step is carried out by functionalizing the particle substrate, which allows for adding and / or modifying the surface charge on the substrate, thereby improving the nucleation of nanoparticles and their attachment to the substrate. Such functionalization is carried out with a coupling agent compatible with the particle substrate. Advantageously, the coupling agent is selected from organosilanes. For example, if the substrate is silica, the coupling agent can be a silane with amine, aldehyde, acrylate, isocyanate, thiol, or carboxylic acid functional groups. This pretreatment step can be carried out at a temperature between 15 and 85°C, followed by filtration and drying to obtain a powder containing a large number of particle substrates suitable for coloring using the method according to the present invention.

[0108] The activation or addition of charges on the surface of the particle substrate not only makes it possible to promote electrostatic interactions between the particle substrate and the nanoparticles during the heterogeneous nucleation of step a), but also to increase the grafting rate of the metal element salts exhibiting the plasmonic effect on the particle substrate.

[0109] According to another object of the invention, the method according to the invention comprises the implementation of the following successive steps: a. Mixing at room temperature a suspension containing: - a salt of at least one metal element, said metal element exhibiting a plasmonic effect, at least one reducing agent, and - at least one particle substrate, b. Isolating and optionally washing the colored particulate material obtained from step a), said colored particulate material having at least one nanoparticle formed on its surface by heterogeneous nucleation.

[0110] According to another object of the invention, the method can also comprise a further step of applying a stimulus to the colored particulate material after obtaining said colored particulate material, such stimulus making it possible in particular to change the color of the colored particulate material.

[0111] Preferably, the stimulus is an external stimulus such as heat treatment, exposure to UV radiation, the use of a laser, spontaneous or induced rehydration, which allows to influence the shape, size and optical index of the surrounding nanoparticles and thus their color, thus changing the color of the colored particulate material.

[0112] [Colored particle material]

[0113] According to another aspect, the present invention relates to a coloured particulate material obtainable by a method according to any one of the embodiments described above.

[0114] Preferably, the coloured particulate material according to the invention comprises at least one nanoparticle formed by heterogeneous nucleation on its surface, preferably a multiplicity of nanoparticles formed by heterogeneous nucleation.

[0115] According to another embodiment, the colored particulate material comprises at least one nanoparticle on its surface, said nanoparticle having a spherical shape, a spheroidal shape, or an anisotropic shape such as a small rod, a cube, a triangle, a bipyramid, etc.

[0116] According to one particular embodiment, the at least one nanoparticle formed on the surface of the colored particulate material according to the invention presents a maximum dimension of between 2 and 100 nm, preferably between 10 and 50 nm.

[0117] [Coloring composition]

[0118] Finally, according to a last aspect, the present invention relates to a colouring composition comprising at least one colouring particulate material according to the invention and at least one solvent in which said colouring particulate material is dispersed.

[0119] Preferably, the solvent is selected from among an alcohol, such as ethanol, an ester, such as ethyl acetate, or an aqueous solvent. [Example]

[0120] Example 1: Coloring method according to the present invention using gold salts

[0121] In this example, we used gold nanoparticles to color the particle substrate.

[0122] To achieve this, the inventors have implemented a method comprising the following steps: a. Mixing by stirring at room temperature of a suspension in a 50 mL round-bottom flask, the suspension containing: - 2 g of substrate, i.e. glass frit, - 10 mL of distilled water, - 1.037 mL of TEA at a concentration of 50 mM, and - 0.518 mL of KAuCl4 at a concentration of 10 mM. The mixing of step a) was carried out until the color of the suspension had stabilized, ie for 15 minutes. b. Isolation of colored particulate material, comprising the following sub-steps: i. centrifuging the mixture of step a) for 3 minutes at 4000 rpm to isolate the colored particulate material and the liquid phase; ii. Washing the colored material from the previous process with water; iii. Centrifuging the mixture from the previous step at 4000 rpm for 3 minutes to separate the colored particulate material and the liquid phase; iv. Drying the colored particulate material obtained from the previous step using a dryer at 80°C to obtain a colored particulate material in powder form comprising at least one nanoparticle formed on the surface by heterogeneous nucleation.

[0123] The resulting colored particulate material is then heat treated (800°C) to obtain the final color: the glass frit (in powder form) with gold nanoparticles formed on its surface is thus melted and then cooled, obtaining an enamel colored by virtue of the colored particulate material (i.e., the glass frit).

[0124] Colorimetric parameters are measured using a spectrophotometer according to the Lab color model. This model, known to those skilled in the art, allows for more accurate measurement and quantification of colors across a very wide spectrum. Thus, L stands for luminosity, and a and b refer to the color components.

[0125] The colorimetric parameters of the colored enamel according to Example 1 are as follows: -L * =44.15, - a=40.18, and - b=12.90.

[0126] This example thus shows that the method according to the invention makes it possible to colour particulate materials by heterogeneous nucleation at room temperature.

[0127] Example 2: Coloring method according to the present invention using gold and silver salts

[0128] In this example, we colored a particle substrate with gold nanoparticles and then silver nanoparticles, as follows: a. Mixing by stirring for 15 minutes at room temperature of a suspension in a 50 mL round-bottom flask, the suspension containing: - 2 g of substrate, i.e. glass frit, - 10 mL of distilled water, - 1.037 mL of TEA at a concentration of 50 mM, and - 0.518 mL of KAuCl4 at a concentration of 10 mM. b. Mixing by stirring for 15 minutes at 80°C of a suspension containing: a colored particulate material having at least one gold nanoparticle, which serves as a new substrate; - 0.375 mL of citrate at a concentration of 50 mM, - 0.187 mL of AgNO3 at a concentration of 10 mM, and - 600 μL of ascorbic acid at a concentration of 50 mM. c. Isolation of the resulting colored particulate material, said isolation comprising the following sub-steps: i. centrifuging the resulting mixture at 4000 rpm for 3 minutes to isolate the colored particulate material and remove the liquid phase; ii. Washing the colored particulate material with water; iii. Centrifuging the mixture from the previous step at 4000 rpm for 3 minutes to isolate the colored particulate material and remove the liquid phase; iv. Drying the colored particulate material separated from the liquid phase, said colored particulate material comprising at least one gold nanoparticle and at least one silver nanoparticle.

[0129] Again, as in Example 1, a heat treatment step at a temperature of 800° C. makes it possible to transform the glass frit into a colored enamel.

[0130] The colorimetric parameters of the enamel are then measured using a spectrophotometer according to the Lab color model: -L * =29.32, - a=35.96, and - b=20.61.

[0131] Example 3: Large-scale coloring method using gold salts according to the present invention

[0132] This example aims to demonstrate that the method according to the invention can be carried out using larger amounts without adversely affecting the colouring effect of the method.

[0133] The method used in this example involves performing the following steps: a. Mixing by stirring for 15 minutes at room temperature of a suspension in a 500 mL round bottom flask, the suspension containing: i. 10 g of substrate, i.e., glass frit; ii. 100 mL of distilled water; iii. 5.183 mL of TEA at a concentration of 50 mM, and iv. 2.592 mL of KAuCl4 at a concentration of 10 mM. b. Isolation of the colored particle material obtained from the previous step by carrying out the following sub-steps: i. centrifuging the resulting mixture at 4000 rpm for 3 minutes to isolate the colored particulate material and remove the liquid phase; ii. Washing the coloring material with water; iii. Centrifuging the mixture from the previous step at 4000 rpm for 3 minutes to isolate the colored particulate material and remove the liquid phase; iv. Drying the colored particulate material comprising at least one gold nanoparticle formed on the surface by heterogeneous nucleation.

[0134] In this case too, a heat treatment step of the colored particulate material is carried out to obtain the colored enamel.

[0135] The colorimetric parameters of the pigmented enamel according to the Lab model, measured using a spectrophotometer, are as follows: -L * =40.15, - a=45.01, and - b=14.74.

[0136] Example 4: Large-scale coloring method using gold and silver salts according to the method of the present invention

[0137] In this example, we colored a particle substrate using gold nanoparticles followed by silver nanoparticles in large quantities, as follows: a. The suspension in a 500 mL round bottom flask is mixed by stirring for 15 minutes at room temperature, the suspension containing: - 10 g of substrate, i.e. glass frit - 100mL of distilled water, - 5.183 mL of TEA at a concentration of 50 mM, and - 2.592 mL of KAuCl4 at a concentration of 10 mM. b. Mix by stirring a suspension containing the following for 15 minutes at 80°C: a colored particulate material having at least one gold nanoparticle, which serves as a new substrate; - 1.873 mL of citrate at a concentration of 50 mM, - 0.937 mL of AgNO3 at a concentration of 10 mM, and - 600 μL of ascorbic acid at a concentration of 50 mM. c. Isolation of the resulting colored particulate material, said isolation comprising the following sub-steps: i. centrifuging the resulting mixture at 4000 rpm for 3 minutes to isolate the colored particulate material and remove the liquid phase; ii. Washing the colored particulate material with water; iii. Centrifuging the mixture from the previous step at 4000 rpm for 3 minutes to isolate the colored particulate material and remove the liquid phase; iv. Drying the colored particulate material separated from the liquid phase, said colored particulate material comprising at least one gold nanoparticle and at least one silver nanoparticle.

[0138] Finally, a supplementary heat treatment step at a temperature of 800° C. makes it possible to color the support with said coloring particulate material.

[0139] The colorimetric parameters of the pigmented enamel according to the Lab model, measured using a spectrophotometer, are as follows: -L* =44.94, - a=42.58, and - b=27.53.

[0140] [Example 5: Verification of reproducibility of the method according to the present invention]

[0141] The purpose of this example is to demonstrate that the method according to the invention makes it possible to obtain reproducible results, for which purpose the method described in Example 3 was carried out four times and the measurements of the colorimetric parameters of the resulting colored materials were measured using a spectrophotometer.

[0142] The results are shown in Table 1 below.

[0143] [Table 1]

[0144] The results demonstrate that the method according to the invention makes it possible to obtain reproducible coloring results.

Claims

1. 1. A method for preparing a colored particulate material by heterogeneous nucleation, comprising carrying out a mixing step a) at room temperature of a suspension, said suspension comprising: a salt of at least one metal element, said metal element exhibiting a plasmonic effect; at least one reducing agent, and at least one particle substrate A preparation method comprising:

2. 2. The method of claim 1, wherein the suspension comprises at least 5% by weight of water relative to the total weight of the suspension.

3. The reducing agent is sodium tetrahydroborate (NaBH ４ ), hydroquinone, tetrabutylammonium borohydride (TBH ４ 3. The method according to claim 1 or 2, characterized in that the activator is selected from the group consisting of hydrazine, propanal, glucose, sucrose, citric acid, ascorbic acid, citrate, triethanolamine (TEA), hydroxylamine and mixtures thereof.

4. 4. The method according to claim 1, wherein the salt of a metal element is selected from the group consisting of gold salts, silver salts, copper salts, aluminum salts, magnesium salts, indium salts, nickel salts, gallium salts, cobalt salts, iron salts, palladium salts, ruthenium salts, rhodium salts, platinum salts, and mixtures thereof.

5. 5. The method according to claim 1, wherein the particle substrate has a maximum dimension of from 10 nm to 1 mm.

6. 6. The method according to any one of claims 1 to 5, characterized in that the particulate substrate is selected from the group of inorganic particulate substrates consisting of silicates, glasses, metal oxides, rare earth oxides, metals, frits, enamels, glazes, ceramics, absorption pigments, and mixtures thereof.

7. 7. The method according to claim 1, wherein the duration of step a) is not more than 30 minutes.

8. 8. The method according to claim 1, further comprising a step b) of isolation of the colored particulate material obtained in step a), said colored particulate material having at least one nanoparticle formed on its surface by heterogeneous nucleation.

9. Step b) of isolating the colored particle material comprises the following sub-steps: i. solid / liquid separation of the mixture of step a) to isolate the colored particulate material from the liquid phase; and ii. Drying to obtain the colored particulate material in dry form 9. The method of claim 8, comprising sequentially:

10. 10. The method according to claim 9, characterized in that the solid / liquid separation is carried out using at least one solid / liquid separation technique selected from filtration, sedimentation, centrifugation, evaporation, freeze-drying and combinations thereof.

11. 11. A method according to any one of claims 8 to 10, characterized in that step b) comprises the additional sub-step of washing the coloured particulate material.

12. 12. A method according to any one of claims 1 to 11, characterized in that the nanoparticles present on the surface of the coloured particulate material have a maximum dimension of between 2 and 100 nm.

13. 13. The method according to any one of claims 1 to 12, characterized in that the method comprises a step of pre-treatment of the particulate substrate by heat treatment and / or alkaline treatment and / or functionalization treatment prior to step a).

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