Process for preparing a colored particulate material by heterogeneous germination

Abstract

The invention relates to a method for preparing colored materials by heterogeneous germination of metallic nanoparticles, said nanoparticles having optical properties based on the surface plasmon phenomenon. The invention also relates to said colored materials obtained, as well as compositions comprising them. In particular, the method for preparing the colored particulate material by heterogeneous germination, comprises in particular the mixing at room temperature of a suspension comprising: - at least one salt of a metallic element, said metallic element having a plasmonic effect, - at least one reducing agent, and - at least one particulate substrate, said mixture of the suspension forming a colored particulate material.

Description

Title of the invention: Process for preparing a colored particulate material by heterogeneous germination Technical field

[0001] The present invention relates to the field of coloring materials. The invention relates in particular to a process for preparing colored materials by heterogeneous germination of metal nanoparticles, said nanoparticles having optical properties based on the surface plasmon phenomenon. The invention also relates to the colored materials obtained by the process, as well as compositions comprising them. State of the art

[0002] The use of a metal in the form of nanoparticles can make it possible to give a suspension or a solid substrate comprising said nanoparticles a color different from the original color of the solid metal (i.e. a metal not in the form of nanoparticles). Indeed, when a metal nanoparticle is subjected to an electromagnetic field whose wavelength is much greater than its size, the free electrons of the conduction band located on the surface of said nanoparticle undergo the same field and oscillate collectively and in phase. When the incident background frequency corresponds to the natural frequency of these oscillations, a resonance phenomenon occurs, called surface plasmon resonance. This resonance can take place in particular in the visible, ultraviolet (UV) and infrared ranges.We then speak of metallic elements exhibiting a plasmonic effect, said metallic elements being in a nanometric form.

[0003] The plasmon resonance frequency is influenced by different parameters, namely: • the nature of the metal; • the size and shape of the nanoparticles; • the distribution of nanoparticles, in particular the inter-particle distance; and • the optical properties of the substrate or surrounding medium, including in particular the refractive index.

[0004] Indeed, the refractive index allows the color to be modulated. This is perceived differently depending on the refractive index. Thus, the color of an object will not be perceived in the same way, if the object is present in the air or in water, for example.

[0005] Interestingly, it is possible to modulate these different parameters in order to vary the color of the nanoparticles throughout the visible range, or even shift the resonance frequency into the UV or near infrared.

[0006] To achieve this, it is known to use so-called preformed nanoparticles exhibiting surface plasmon resonance in order to color materials. This technique is very promising and has significant advantages over traditional coloring methods. Indeed, it makes it possible to generate a variety of colors, including bright colors, without resorting to pigments or dyes that pose a risk to human health and the environment, which is particularly suitable and useful for certain applications such as tableware, cosmetics, jewelry, watchmaking, food processing or medicine.

[0007] However, the use of these preformed nanoparticles has certain drawbacks. For example, the production of a material in powder form, that is to say a particulate material, having specific colors requires precise control of the concentration of the colloidal suspensions as well as the preformed nanoparticles. Indeed, the size and the interactions between the preformed nanoparticles within the suspension generate several constraints, such as: • the increase in viscosity; • a decrease in suspension stability; • difficulties in dispersing the material within the suspension.

[0008] For all these reasons, the concentration of preformed nanoparticles in colloidal suspensions must be finely controlled and mastered, thus limiting the concentration of the suspension in preformed nanoparticles and indirectly the intensity of coloring of the material, consequently, obtaining a good production yield.

[0009] The use of preformed nanoparticles therefore requires the implementation of complex processes, comprising numerous steps, using reagents that are often toxic to human health and / or the environment and requiring costly and energy-intensive infrastructure.

[0010] By way of example, patent FR3096685 proposes a new method for preparing, from a substrate, a colored micrometric particulate material (i.e. in powder form) using at least one gold salt or at least gold nanoparticles, based on the principle of heterogeneous germination.

[0011] Heterogeneous germination specifically promotes the nucleation and growth of nanoparticles on the surface of a solid substrate, which acts as a catalyst for the reaction. In this way, the solid substrate in the form of particles 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 probability of demixing during the shaping of the material for the final application.

[0012] However, the use of the method (FR3096685) has the disadvantage of being limited to certain types of particulate substrates, thus reducing its potential for industrial application. Consequently, this method cannot be adapted or universally transposed to a wide range of supports. Indeed, said method is not, for example, suitable for coloring large volumes of particulate substrate. Indeed, the realization of heterogeneous germination catalyzed by a heat treatment systematically requires an adaptation of the parameters between the method developed on a laboratory scale and the method on an industrial scale so as to maintain the same conditions for the entire volume of particulate substrate. These constraints increase, on the one hand, the development time for new ranges of colored materials and, on the other hand, limit its implementation to certain types of particulate substrate.Furthermore, the need to heat the reaction medium necessarily involves costly and energy-intensive infrastructure.

[0013] In addition, the growing awareness of climate challenges associated with the global objective of reducing greenhouse gas emissions encourages manufacturers to explore alternative processes that are simple, environmentally friendly, universal and effective for coloring materials.

[0014] There is therefore a need for a new method of coloring materials designed to minimize its impact on the environment while maintaining optimal efficiency and adaptable to a wide range of supports and thus overcoming the drawbacks of the prior art. Summary of the invention

[0015] To meet this need, the invention proposes a new method making it possible to overcome the aforementioned drawbacks, in particular by being suitable for a wide range of substrates, and having low energy consumption while maintaining optimal coloring efficiency. In particular, the invention provides a new method for preparing a colored particulate material, possibly capable of changing color under the influence of a stimulus, said method is simple, economical, guarantees optimal color stability and has significant modularity, allowing access to a large palette of colors and types of colored materials, and avoiding solvent transfers as much as possible.

[0016] In the context of the invention, the substrate is a particulate material, that is to say a material in powder form, on which the metal ions forming colored nanoparticles after nucleation, will preferentially nucleate and grow, forming a colored particulate material, the latter comprising at least one nanoparticle on its surface, said nanoparticle having plasmonic properties.

[0017] Thus, the invention relates to a method for preparing a colored particulate material by heterogeneous germination, comprising the implementation of a single step a) of mixing at room temperature a suspension, said suspension comprising: • at least one salt of a metallic element, said metallic element having a plasmonic effect, • at least one reducing agent, and • at least one particulate substrate.

[0018] Such a method makes it possible to form a colored particulate material while overcoming the drawbacks of the prior art. Given the implementation of the method at room temperature, the method according to the invention advantageously has low energy consumption while having an acceptable reaction time, thus reducing operational costs and greenhouse gas emissions compared to an equivalent method of the prior art.

[0019] Indeed, the inventors have surprisingly discovered that heterogeneous germination carried out at room temperature, i.e. at a temperature between 19 and 25°C, makes it possible to achieve heterogeneous germination at low energy cost while maintaining optimal coloring efficiency. To achieve this, the inventors have in particular identified at least one reducing agent of interest.

[0020] Also, according to a preferred embodiment, the reducing agent is chosen from the group consisting of sodium tetrahydruroborate (NaBH4), hydroquinone, tetrabutylammoniumborohydride (TBH4), hydrazine, propanal, glucose, sucrose, citric acid, ascorbic acid, citrate, Triethanolamine (TEA), hydrolamine and mixtures thereof.

[0021] Particularly preferably, the reducing agent is Triethanolamine (TEA), this being particularly suitable for carrying out heterogeneous germination at room temperature, i.e. a reaction which does not need to be catalyzed by a heat treatment, whilst offering an acceptable reaction time. Alternatively, it can also be used at a higher temperature. Thus, TEA can be used at a temperature between 1°C and 100°C.

[0022] Thus, according to a variant, the invention also relates to a process 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: • at least one salt of a metallic element, said metallic element exhibiting a plasmonic effect; • Triethanolamine (TEA); and • at least one particulate substrate.

[0023] The method according to the invention thus makes it possible to obtain a spectrum of varied colors and presents significant modularity, thus offering the possibility of coloring a wide variety of materials with a wide variety of colors.

[0024] Advantageously, the method according to the invention is suitable for coloring all types of material with all types of salts of metallic elements, said metallic elements having a plasmonic effect.

[0025] According to a particularly advantageous embodiment of the invention, the salt of a metallic element, said metallic element having a plasmonic effect, is chosen from a gold salt, a silver salt, a copper salt, an aluminum salt, a magnesium salt, an indium salt, a nickel salt, a gallium salt, a cobalt salt, an iron salt, a palladium salt, a ruthenium salt, a rhodium salt, a platinum salt and mixtures thereof.

[0026] The metal element salt is a salt in which the metal element is in the oxidation state. For example, gold (+III) salt is a salt in which gold is in the oxidation state (+III).

[0027] According to one embodiment, the gold (+III) salt is chosen from tetrachloroauric acid HAuC14, potassium tetrachloroaurate KAuC14 and their mixture, and preferably KAuC14.

[0028] According to one embodiment, the silver salt (+1) is chosen from AgNO3, AgClO4, Ag(acac), AgCl, Ag2SO4 and their mixture, and preferably AgNO3.

[0029] According to one embodiment, the copper salt (+11) is chosen from copper chloride (CuCl2), copper acetate (Cu(CH3COO)2), copper sulfate (CuSO4), Cu(acac)2, Cu2O (+1), Cu(OH)2, Cu(NO3)2 and their mixture, and preferably CuCl2.

[0030] In the context of the invention, all types of particulate materials can be used as a particulate substrate in order to be colored. Thus, the particulate substrate can be chosen from organic materials, inorganic materials, and hybrid materials.

[0031] The method according to the invention is thus suitable for coloring all types of material, thus offering a great diversity in the choice of the material that one wishes to color. The method is thus versatile, simple, and efficient while having low energy consumption.

[0032] When the particulate substrate is an inorganic substrate, said substrate may be chosen from any known mineral substance. Preferably, said substrate is chosen from the group consisting of silicates, glasses, metal oxides, rare earth oxides, metals, frits, enamels, glazes, ceramics, absorption pigments and mixtures thereof.

[0033] According to another aspect, the invention also relates to a colored particulate material obtained by the method according to any of the embodiments previously described.

[0034] Finally, according to a last aspect, the invention relates to a colored composition comprising at least one colored particulate material according to the invention, and at least one solvent in which said colored particulate material is dispersed. Detailed description of the invention

[0035] Definitions

[0036] By "stabilizing agent" within the meaning of the invention, we mean charged molecules allowing electrostatic stabilization when they are adsorbed on the surface of the nanoparticles, or polymers allowing steric repulsions between the nanoparticles.

[0037] By "structuring agent" within the meaning of the invention, we mean any molecule capable of adsorbing on the surface of the nanoparticles and thus of promoting the growth of certain crystalline faces of the nanoparticle, thus allowing the formation of anisotropic particles.

[0038] By "heterogeneous germination" within the meaning of the invention, we mean the reduction of a metal ion on the surface of a solid support, acting as a catalyst for germination. Unlike homogeneous germination where the germs form spontaneously in the reaction medium.

[0039] By "particulate substrate" or "particulate material" within the meaning of the invention, is meant a finely divided material, comparable to a powder comprising a set of particles. Said substrate is in powder form on which the salts of metallic elements will be deposited forming nanoparticles by nucleation. Advantageously, the largest dimension of said particulate substrate is at most 1 mm.

[0040] By "largest dimension" in particular of a nanoparticle or of the particulate substrate, is meant, within the meaning of the invention, the greatest distance separating two points located on the external contour of said nanoparticle or of said particulate substrate.

[0041] For the purposes of the invention, “room temperature” means a temperature between 19 and 25°C.

[0042] For the purposes of the invention, the term "acceptable reaction time" means a reaction time of at most 30 minutes, preferably less than 15 minutes. The reaction is considered complete when the observed color is stabilized.

[0043] Process for preparing a colored particulate material

[0044] The present invention therefore relates to a process for preparing a colored particulate material by heterogeneous germination with low energy consumption, carried out in a single step.

[0045] The method according to the invention thus comprises a step a) of mixing at room temperature a suspension, said suspension comprising: • at least one salt of a metallic element, said metallic element exhibiting a plasmonic effect; • at least one reducing agent; and • at least one particulate substrate.

[0046] Said one-step method forming a colored particulate material, from said particulate substrate. Said colored particulate material comprising at least one nanoparticle formed on the surface of said colored material.

[0047] According to an object of the invention, the particulate substrate used in the method according to the invention may be previously colored either natively or by the presence of nanoparticles formed on its surface. Said nanoparticles may be deposited according to the method of the present invention. Also, said method according to the invention may be repeated several times, and thus comprise several cycles, each cycle allowing the formation of a colored particulate material. This is particularly advantageous in order to obtain a colored particulate material having the desired coloring.

[0048] For example, a first cycle can be implemented from a gold salt, then a second cycle from a silver salt, etc.

[0049] The method according to the invention therefore makes it possible to prepare a colored particulate material by heterogeneous germination while overcoming the drawbacks of the prior art. Indeed, the method according to the invention has low energy consumption, thus reducing operational costs and greenhouse gas emissions compared to an equivalent method of the prior art while maintaining similar coloring efficiency. The colorimetric parameters of the colored particulate material are thus similar to those of a colored particulate material obtained by a method from the prior art, in particular the method according to FR3096685.

[0050] The method according to the invention is simple, insofar as the formation of the nanoparticles on the surface of the particulate material is carried out in a single step, at room temperature. Indeed, implementation at room temperature has many advantages, in particular better control of the parameters linked to nucleation during heterogeneous nucleation. Conversely, when implemented at a higher temperature, nucleation may be too rapid, thus requiring adjustments, for example the addition of an additional temperature stabilization step, before reaching the target final temperature. Consequently, at room temperature, these constraints are overcome.

[0051] Furthermore, the method according to the invention is also suitable for the coloring of material on a laboratory as well as industrial scale, regardless of the volume of materials to be colored. In this way, the method according to the invention has better reproducibility and time savings compared to the methods described in the art. previous. Indeed, this makes it possible to overcome the influence of temperature on the nucleation and growth of nanoparticles on the surface of the particulate substrate, thus facilitating its transposition to industrial scales.

[0052] According to a particular object of the invention, the suspension of step a) comprises water and / or an organic solvent.

[0053] According to a particular object of the invention, the suspension of step a) is an aqueous suspension. Preferably, said suspension of step a) comprises at least 5% water by mass relative to the total mass of said suspension, more preferably at least 10% water by mass relative to the total mass of the suspension.

[0054] According to another object of the invention, the suspension of step a) is an organic suspension. When the suspension of step a) is an organic suspension, said suspension preferably comprises alcohol such as ethanol.

[0055] 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 between 5 / 95 and 95 / 5, preferably between 60 / 40 and 40 / 60.

[0056] According to a particularly preferred object of the invention, the mixing of step a) is carried out at room temperature, i.e. at a temperature between 19°C and 25°C. Thus, the heterogeneous germination carried out during step a) is carried out without heat treatment. In this way, the reaction can be carried out efficiently, regardless of the volume of materials to be colored.

[0057] Advantageously, the heterogeneous germination carried out during step a) does not require heat treatment, drastically reducing the energy consumption necessary for implementing the process.

[0058] However, according to a variant of the present invention, when the reducing agent is TEA, the process can be carried out at a temperature between 1°C and 100°C. Preferably, the process is carried out at room temperature. However, it can be carried out at a temperature above 25°C for certain specific particulate substrates. The solvent is preferably aqueous but, according to a variant, it can be a hydroalcoholic solvent.

[0059] Preferably, step a) of mixing, regardless of the embodiments previously described, is carried out with stirring, preferably with mechanical stirring, in particular by means of a paddle mixer or a magnetic bar, thus facilitating the heterogeneous germination reaction and the contact of the salts of metallic elements with the particulate substrate.

[0060] According to another particularly preferred object, the mixture of step a) comprises at least one salt of a metallic element, said metallic element having a plasmonic effect, said salt is chosen from the group consisting of a gold salt, a salt of silver, a copper salt, an aluminum salt, a magnesium salt, an indium salt, a nickel salt, a gallium salt, a cobalt salt, an iron salt, a palladium salt, a ruthenium salt, a rhodium salt, a platinum salt and mixtures thereof.

[0061] Even more preferably, the mixture of step a) comprises at least one salt of a metallic element exhibiting a plasmonic effect chosen from a gold salt, a silver salt, a copper salt and their mixtures.

[0062] In the context of the present invention, the salts of metallic elements are the precursors of the nanoparticles which will be formed by heterogeneous germination on the particulate substrate or possibly on a particulate material colored according to the method of the invention, in order to obtain a particulate material colored according to the present invention.

[0063] According to another particular embodiment, the mixture of step a) comprises at least two salts of distinct metal elements, each having a plasmonic effect. According to a variant of the invention, the mixture of step a) comprises at least two salts of distinct metal elements, each being added to said mixture successively and each having a plasmonic effect. In other words, a first salt of metal elements may be added in order to color the particulate substrate and a second salt of metal elements may be added in order to color the particulate material colored with the first salt, in order to obtain a colored particulate material, the latter comprising at least two distinct colored nanoparticles.

[0064] Preferably, the mixture of step a) comprises at least one gold salt and one silver salt. Said salts can be added successively or simultaneously.

[0065] According to another particular embodiment, the mixture of step a) comprises at least three salts of distinct metallic elements exhibiting a plasmonic effect. When it comprises three distinct salts, the mixture of step a) preferably comprises at least one gold salt, one silver salt and one copper salt. Said salts can be added successively or simultaneously.

[0066] Particularly advantageously, the association and combination of several salts of distinct metallic elements each exhibiting a plasmonic effect makes it possible to broaden the diversity of conceivable colors. Consequently, by extension, to broaden the diversity of colored particulate material obtained by means of the method according to the invention.

[0067] According to one embodiment of the invention, the mixture of step a) preferably comprises at least one reducing agent chosen from the group consisting of sodium tetrahydruroborate (NaBH4), hydroquinone, tetrabutylammoniumborohydride (TBH4), hydrazine, propanal, triethanolamine (TEA), boranes, organic acids, amines, sugars and mixtures thereof.

[0068] More preferably, the mixture of step a) comprises at least one reducing agent chosen from sodium tetrahydruroborate (NaBH4), hydroquinone, tetrabutylammoniumborohydride (TBH4), hydrazine, propanal, glucose, sucrose, citric acid, ascorbic acid, citrate, triethanolamine (TEA), hydrolamine and mixtures thereof.

[0069] When the reducing agent is chosen from amines, it is preferentially chosen from Triethanolamine (TEA) and hydrolamine.

[0070] When the mixture of step a) comprises at least one reducing agent chosen from sugars, this is preferably chosen from glucose or sucrose.

[0071] When the mixture of step a) comprises at least one reducing agent chosen from organic acids, this is preferably chosen from ascorbic acid and its derivatives and citric acid and its derivatives.

[0072] According to another embodiment, step a) comprises citrate as reducing agent.

[0073] Preferably, the mixture of step a) comprises a molar / mass ratio of said reducing agent and particulate substrate of between 10 and 6500 (mol / gram), more preferably of between 50 and 820 (mol / gram). Conversely, a homogeneous germination reaction is likely to occur.

[0074] Preferably, the largest dimension of the particulate substrate that one wishes to color is between 10 m and 1 mm. Indeed, the heterogeneous germination carried out during step a) is particularly effective on such particulate substrates having such dimensions. Conversely, the nanoparticles will not be able to deposit on the surface of the substrate. In addition, if the substrate is greater than 1 mm, it will not be possible to maintain the different elements in suspension with moderate agitation.

[0075] The substrate on which the heterogeneous germination reaction will occur, i.e. the particulate substrate that it is desired to color, can be in any form, in particular in the form of platelets such as polyhedra or flakes, or in the form of beads. The method according to the invention is therefore universal and can color many types of particulate substrate.

[0076] The particulate substrate of the mixture of step a) may be an organic or inorganic or hybrid material.

[0077] When the particulate material is inorganic, it is chosen from silicates, glasses, metal oxides, rare earth oxides, metals, frits, enamels, glazes, ceramics, absorption pigments and mixtures thereof.

[0078] By way of non-limiting example, the mixture of step a) may comprise an inorganic particulate substrate such as silica, quartz, feldspar, limestone, kaolin, metal oxides, aluminate, 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, agent, steel and combinations thereof.

[0079] Thus, very advantageously, the method according to the invention is suitable for coloring a wide variety of materials.

[0080] According to another embodiment, the particulate substrate may be organic, preferably cellulose.

[0081] Advantageously, the method according to the invention is carried out at room temperature, thus making it possible to color organic and / or heat-sensitive materials, thus offering a versatile alternative method suitable for coloring heat-sensitive materials.

[0082] In the context of the present invention, the particulate substrate may be colorless, transparent, semi-transparent, opaque or colored. Indeed, said substrate may be, depending on a particular object, colored natively or by the method according to the invention. Such an embodiment is of particular interest when it is desired to repeat the method according to the invention. As such, the method according to the invention then comprises several successive cycles during which the colored material obtained during the first cycle is used as a substrate during the second cycle, etc.

[0083] Also, according to an object of the invention, the particulate material of step a) may comprise particles covered on the surface with a layer containing at least one metal oxide or silicon dioxide. Advantageously, said layer of metal oxide or silicon dioxide makes it possible to facilitate the attachment of the nanoparticles formed on the surface of the particulate material by heterogeneous germination.

[0084] According to another object, step a) is carried out in less than 30 minutes, more preferably in less than 15 minutes. Thus, the method according to the invention advantageously has lower energy consumption than the method of the prior art while having a similar completion time and coloring efficiency.

[0085] According to another embodiment of the invention, the mixture of step a) may also comprise at least one stabilizing agent. When the suspension also comprises at least one stabilizing agent, the latter is chosen 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-vynilpyrrolidone), polyelectrolytes, NMP-type monomers, and mixtures thereof.

[0086] According to another embodiment, the mixture of step a) may also comprise at least one structuring agent. This advantageously makes it possible to orient the growth on certain crystalline faces and thus to modulate the shape of the nanoparticles formed on the surface of the material. Controlling the shape of the nanoparticles is an important parameter insofar as this makes it possible to exploit the plasmon resonance of the metallic elements and therefore to control the final color of the colored material.

[0087] When the mixture of step a) comprises at least one structuring agent, the latter is preferably chosen from citrate, malate, succinate, polyvinylpyrrolidone (PVP), surfactants, and their mixture. More preferably, the surfactants are chosen from cetyltrimethylammonium bromide (CTAB), diethylamine (DEA), ethylenediaminetetraacetic acid (EDTA), and their mixture.

[0088] According to another object, the method according to the invention is implemented in a single step, but it may however comprise additional steps, in particular an isolation step. This makes it possible to separate the colored material in solid form from the liquid phase.

[0089] Thus, according to one embodiment of the invention, the method according to the invention comprises the implementation of the following steps: a. mixing at room temperature a suspension comprising: • at least one salt of a metallic element exhibiting a plasmonic effect; • at least one reducing agent, and • at least one particulate 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 germination.

[0090] Preferably, step b) of isolating the colored particulate material from step a) 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, said liquid phase comprising the free elements in suspension; and ii. drying to obtain the colored particulate material in dry form.

[0091] Preferably, sub-step i) of solid / liquid separation is carried out by means of at least one solid / liquid separation technique chosen from filtration, sedimentation, centrifugation, evaporation, freeze-drying and their combinations.

[0092] When sub-step i) of solid / liquid separation is carried out by means of at least two solid / liquid separation techniques, these are carried out successively, this advantageously reducing the duration of the isolation step.

[0093] 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 is preferably carried out after separation and before drying.

[0094] The washing or rinsing step is preferably carried out with water and / or using an organic solvent. When the step comprises washing with water and organic solvent, this can be carried out simultaneously or successively.

[0095] Thus, according to a particularly preferred embodiment, the method according to the invention comprises the following successive steps: a. mixture according to any of the embodiments described above, and b. isolation of the colored particulate material obtained at the end of step a) from the liquid phase, this advantageously comprising the following sub-steps: i. solid / liquid separation of the mixture of step a) so as to isolate the colored particulate material from the liquid phase, and ii. optionally, a step of washing the isolated particulate material with water and / or organic solvent; and iii. drying the particulate material and recovering a colored particulate material in dry and powder form.

[0096] According to another embodiment of the invention, step b) of isolating the colored particulate material can be repeated several times, preferably from 1 to 5 times, more preferably from 1 to 3 times. Each repetition forms a cycle.

[0097] Thus, the method preferably comprises a step a) and a step b) of isolation, said isolation step comprising the following successive sub-steps: i. solid / liquid separation of the mixture of step a) so as to isolate the colored particulate material from the liquid phase; ii. washing of the particulate material from the previous step; iii. solid / liquid separation of the mixture from the previous step; and iv. drying to obtain the colored particulate material comprising at least one nanoparticle formed on its surface by heterogeneous germination.

[0098] The successive washing steps advantageously make it possible to remove an excess of organic compounds, for example the reducing agent, the stabilizing agent, the structuring agent present in the suspension, which are likely to cause problems with the final coloring.

[0099] Preferably, the drying is carried out using an oven.

[0100] According to a particular object of the invention, at least one nanoparticle is formed on the surface of the colored particulate material, the latter having a larger dimension between 2nm and 100nm.

[0101] Advantageously, the nanoparticle formed on the surface of the colored particulate material has a substantially hemispherical shape. However, the substantially hemispherical shape is the thermodynamically most stable shape, and is therefore preferred to improve the stability of the coloring. However, according to certain embodiments, it is possible to modulate the shape of the nanoparticles formed on the surface of the colored particulate material according to the knowledge of a person skilled in the art.

[0102] According to another object of the invention, the particulate substrate may be porous. The use of such porous substrates advantageously makes it possible to increase the accessible and available surface area for depositing the salts of metallic elements forming the nanoparticles by nucleation. In this context, the nanoparticle formed on the surface of the colored particulate material advantageously has a longitudinal shape, such as the shape of a rod, in particular when the size of the pores allows it. The use of porous particulate material thus makes it possible to modulate the shape of the nanoparticles to obtain a wider range of colors.

[0103] According to another embodiment, the method according to the invention comprises the implementation of the following steps: a. mixture at room temperature of a suspension comprising: • at least one salt of a metallic element exhibiting a plasmonic effect; • at least one reducing agent; and • at least one particulate material, forming the colored particulate material; b. optionally, washing the mixture, c. addition of at least one salt of a metallic element distinct from the mixture of step a) into the mixture resulting from step a) or b); d. isolation of the colored particulate material from the previous step, said colored particulate material having at least two distinct nanoparticles formed on its surface by heterogeneous germination.

[0104] According to another embodiment of the invention, the method according to the invention comprises a step of pre-treatment of the particulate substrate, located before step a), in order to prepare, condition the particulate substrate to be colored by means of the method according to the invention. This pre-treatment step is carried out by means of a heat treatment and / or an alkaline treatment, which makes it possible to activate the surface charges of the substrate, making it possible to improve the nucleation of the nanoparticle, as well as the fixing of the latter on the substrate. This pre-treatment step is followed by cooling to room temperature then filtration, followed by drying to obtain a powder, said powder comprising a multitude of particulate substrates, capable of be colored using the method according to the invention.

[0105] The activation of the charges on the surface of the particulate substrate makes it possible in particular to promote the electrostatic interactions between the particulate substrate and the nanoparticles during the heterogeneous germination of step a), but also to increase the grafting rate of the metal element salts having a plasmonic effect on the particulate substrate.

[0106] According to another object of the invention, the method according to the invention consists of implementing the following successive steps: a. Mixture at room temperature of a suspension comprising: • at least one salt of a metallic element, said metallic element exhibiting a plasmonic effect; • at least one reducing agent; and • at least one particulate substrate, b. isolation, and optionally washing, of the colored particulate material resulting from step a), said colored particulate material having at least one nanoparticle formed on its surface by heterogeneous germination.

[0107] According to another object of the invention, the method may also comprise another step, after obtaining the colored particulate material, during which a stimulus is applied to said colored particulate material. Such a stimulus makes it possible in particular to modify the color of the colored particulate material.

[0108] Preferably, said stimulus is an external stimulus such as heat treatment, exposure to UV radiation, the use of a laser, spontaneous or induced rehydration. This will make it possible to act on the shape, the size of the nanoparticles and the surrounding optical index and consequently their color, thus modifying the color of the colored particulate material.

[0109] Colored particulate material

[0110] According to another aspect, the invention relates to a colored particulate material obtained by the method according to any of the embodiments previously described.

[0111] Preferably, the colored particulate material according to the invention comprises on its surface at least one nanoparticle formed by heterogeneous germination, preferably a multitude of nanoparticles formed by heterogeneous germination.

[0112] According to another embodiment, the colored particulate material comprises at least one nanoparticle on its surface, said nanoparticle having a spherical, spheroidal or anisotropic shape such as rods, cubes, triangles, bipyramids, etc.

[0113] According to a particular embodiment, at least one nanoparticle formed on the surface of the colored particulate material according to the invention has a largest dimension of between 2 and 100nm, preferably between 10 and 50nm.

[0114] Colored composition

[0115] Finally, according to a last aspect, the invention relates to a colored composition comprising at least one colored particulate material according to the invention, and at least one solvent in which said colored particulate material is dispersed.

[0116] Preferably, the solvent is chosen from alcohols such as ethanol, esters such as ethyl acetate, or an aqueous solvent. Examples

[0117] Example 1: Coloring process according to the invention using gold salts

[0118] In this example, the inventors colored a particulate substrate with gold nanoparticles.

[0119] To achieve this, the inventors implemented the method comprising the following steps: a. Mixing with stirring, at room temperature, a suspension in a 50 mL flask, the suspension comprising: • 2 g of substrate, namely glass frit; • 10 mL of distilled water; • 1.037 mL of TEA concentrated at 50 mM; and • 0.518 mL of KAuC14 concentrated to 10 mM The mixing of step a) was carried out until the color of the suspension was stabilized, i.e. for 15 min. a. isolation of the colored particulate material comprising the following substeps: i. Centrifugation at 4000rpm for 3 minutes of the mixture from step a) so as to isolate the colored particulate material and the liquid phase, ii. Washing with water the colored material from the previous step, iii. Centrifugation at 4000rpm for 3 minutes of the mixture from the previous step so as to isolate the colored particulate material and the liquid phase, iv. Drying by means of an oven at 80°C of the colored particulate material resulting from the previous step, so as to obtain a colored particulate material comprising at least one nanoparticle formed on its surface by heterogeneous germination, in powder form.

[0120] The colored particulate material obtained is then heat treated (800°C) in order to obtain the final color. Thus, the glass frit (in powder form) comprising the gold nanoparticles formed on its surface is thus melted, then cooled in order to obtain colored enamel thanks to the colored particulate material (i.e. the glass frit).

[0121] The colorimetric parameters are measured using a spectrophotometer, according to the Lab color model. This model, well known to those skilled in the art, called Lab, makes it possible to measure and quantify colors more precisely over a very broad spectrum. Thus, L is the initial of Luminosity and a and b designate the chromatic components.

[0122] The colorimetric parameters of the colored enamel according to example 1 are as follows: • L* = 44.15; • a = 40.18 ; and • b = 12.90.

[0123] This example thus demonstrates that the method according to the invention makes it possible to color a particulate material by heterogeneous germination at room temperature.

[0124] Example 2: Coloring process according to the invention using gold salt and silver salt

[0125] In this example, the inventors colored a particulate substrate with gold nanoparticles, then silver nanoparticles. The method is as follows: a. Mix with stirring, at room temperature, a suspension in a 50 mL flask, for 15 minutes, the suspension comprising: • 2 g of substrate, namely the glass frit • 10 mL of distilled water; • 1.037 mL of TEA concentrated at 50 mM; and • 0.518 mL of concentrated KauCl4 at 10 mM b. Mix with stirring, at 80°C, for 15 minutes, a suspension comprising: • the colored particulate material having at least one gold nanoparticle, serving as a new substrate; • 0.375mL of 50mM concentrated citrate; • 0.187mL of AgNO3 concentrated at 10mM; and • 600pL of 50mM concentrated ascorbic acid. c. isolation of the resulting colored particulate material, said isolation comprising the following substeps: i. Centrifugation at 4000rpm for 3 minutes of the mixture obtained in order to isolate the colored particulate material and eliminate the liquid phase; ii. Washing the colored particulate material with water; iii. Centrifugation at 4000rpm for 3 minutes of the mixture from the previous step in order to isolate the colored particulate material and eliminate 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.

[0126] Here again, as for example 1, a heat treatment step at a temperature of 800°C makes it possible to transform the glass frit into colored enamel.

[0127] 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

[0128] Example 3: Process for coloring a large volume with gold salts according to the process of the invention

[0129] This example aims to demonstrate that the process can be implemented with a larger volume, without affecting the coloring efficiency of the process according to the invention.

[0130] The method used in this example comprises the implementation of the following steps: a. Mixing with stirring, at room temperature, for 15 minutes, a suspension in a 500 mL flask, the suspension comprising: i. 10g of substrate, namely the glass frit ii. 100 mL of distilled water; iii. 5.183mL of 50mM concentrated TEA; and iv. 2.592 of concentrated KauCl4 at OrnM b. isolation of the colored particulate material from the previous step by implementing the following sub-steps: i. Centrifugation at 4000rpm for 3 minutes of the mixture obtained in order to isolate the colored particulate material and elimination of the liquid phase; ii. Washing the colored material with water; iii. Centrifugation at 4000rpm for 3 minutes of the mixture from the previous step in order to isolate the colored particulate material and elimination of the liquid phase; iv. Drying the colored particulate material comprising at least one gold nanoparticle formed on its surface by heterogeneous germination.

[0131] Here again, a step of heat treatment of the colored particulate material is implemented in order to obtain the colored enamel.

[0132] The colorimetric parameters of the colored enamel, according to the Lab model measured using a spectrophotometer, are as follows: • L* = 40.15; • a = 45.01; and • b = 14.74

[0133] Example 4: Process for coloring a large volume with gold and silver salts according to the process of the invention

[0134] In this example, the inventors colored a particulate substrate with gold nanoparticles, then silver nanoparticles in a large volume. The method is as follows: a. Mix with stirring, at room temperature, a suspension in a 500 mL flask for 15 minutes, the suspension comprising: • 10 g of substrate, namely • 100 mL of distilled water; • 5.183 mL of TEA concentrated at 50 mM; and • 2.592 mL of concentrated KauCl4 at 10 mM b. Mix with stirring, at 80°C, for 15 minutes, a suspension comprising: • the colored particulate material having at least one gold nanoparticle, serving as a new substrate; • 1.873 mL of 50mM concentrated citrate; • 0.937 mL of concentrated AgNO3 at 10 mM; and • 600 pL of 50mM concentrated ascorbic acid. c. isolation of the resulting colored particulate material, said isolation comprising the following substeps: i. Centrifugation at 4000rpm for 3 minutes of the mixture obtained in order to isolate the colored particulate material and eliminate the liquid phase; ii. Washing the colored particulate material with water; iii. Centrifugation at 4000rpm for 3 minutes of the mixture from the previous step in order to isolate the colored particulate material and eliminate 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.

[0135] Finally, an additional step of heat treatment at a temperature of 800°C makes it possible to color a support with said colored particulate material.

[0136] The colorimetric parameters of the colored enamel, according to the Lab model measured using a spectrophotometer, are as follows: L* = 44.94; a = 42.58; and • b = 27.53

[0137] Example 5 - Verification of the reproducibility of the method according to the invention

[0138] The objective of this example is to demonstrate that the method according to the invention allows to obtain reproducible results. For this, the process described in Example 3 was carried out 4 times, the measurement of the colorimetric parameters of the colored material obtained was measured using a spectrophotometer.

[0139] The results are presented in Table 1 below:

[0140] [Tables 1] L* ab Sample 1 40.15 45.01 14.74 Sample 2 43.9 41.15 11.59 Sample 3 44.83 42.81 10.93 Sample 4 45.38 43.66 10.56

[0141] The results demonstrate that the method according to the invention makes it possible to obtain reproducible coloring results.

Claims

Claims

1. Process for preparing a colored particulate material by heterogeneous germination, comprising the implementation of a step a) of mixing at room temperature a suspension, said suspension comprising: • at least one salt of a metallic element, said metallic element having a plasmonic effect, • at least one reducing agent, and • at least one particulate substrate.

2. Method according to the preceding claim, characterized in that the suspension comprises at least 5% water by mass relative to the total mass of said suspension.

3. Method according to one of the preceding claims, characterized in that the reducing agent is chosen from the group consisting of sodium tetrahydroborate (NaBH4), hydroquinone, tetrabutylammoniumborohydride (TBH4), hydrazine, propanal, glucose, sucrose, citric acid, ascorbic acid, citrate, triethanolamine (TEA), hydrolamine and mixtures thereof.

4. Method according to one of the preceding claims, characterized in that the salt of a metallic element is chosen from the group consisting of a gold salt, a silver salt, a copper salt, an aluminum salt, a magnesium salt, an indium salt, a nickel salt, a gallium salt, a cobalt salt, an iron salt, a palladium salt, a ruthenium salt, a rhodium salt, a platinum salt, and mixtures thereof.

5. Method according to one of the preceding claims, characterized in that the largest dimension of the particulate substrate is between 10 mm and 1 mm.

6. Method according to one of the preceding claims, characterized in that the particulate substrate is chosen from an inorganic particulate substrate from the group consisting of silicates, glasses, metal oxides, rare earth oxides, metals, frits, enamels, glazes, ceramics, absorption pigments, and mixtures thereof.

7. Method according to one of the preceding claims, characterized in that the duration of step a) is at most 30 minutes.

8. Method according to one of the preceding claims, characterized in that that the method also comprises a step b) of isolating the colored particulate material obtained in step a), said colored particulate material having at least one nanoparticle formed on its surface by heterogeneous germination.

9. Method according to the preceding claim, characterized in that step b) of isolation of the colored particulate material successively 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.

10. Method according to the preceding claim, characterized in that the solid / liquid separation is carried out by means of at least one solid / liquid separation technique chosen from filtration, sedimentation, centrifugation, evaporation, lyophilization and their combinations.

11. Method according to one of claims 8 to 10, characterized in that step b) comprises an additional sub-step of washing the colored particulate material.

12. Method according to one of the preceding claims, characterized in that the largest dimension of the nanoparticle present on the surface of the colored particulate material is between 2 and 100nm.

13. Method according to one of the preceding claims, characterized in that the method comprises a step prior to step a), of pretreatment of the particulate substrate by means of a heat treatment and / or an alkaline treatment.